Selection of a Universal Method for Measuring Nitrogen Oxides in Underground Mines: A Literature Review and SWOT Analysis

Abstract

Workstations in deep underground mines are among the most dangerous in the world. Workers are exposed to various hazards such as water hazards, climate hazards, and gas hazards. In this article, the authors proposed the most suitable method for measuring nitrogen oxides, such as nitric oxide(NO) and nitrogen dioxide (NO₂), under actual underground mine conditions. The selection of the method was based on a literature review, in which the authors presented a brief characterization of available measurement methods and proposed their classification into four categories: chemical methods, electrochemical methods, chemiluminescence methods, and analytical methods. A SWOT analysis was used to select the appropriate method for NOx determination. The authors focused on identifying the most universal method that can handle measurements in the harsh conditions of underground mines, with an emphasis on ease of use in the field. Due to the mine atmosphere being rich in harmful substances, the selectivity of the method was also taken into account. The method chosen by the authors is intended for measuring both low concentrations of NOx (in the atmosphere) and high concentrations (diesel exhaust emissions). Because of the versatility of the method and its potential application in both small and large laboratories, the cost criterion was also considered.

1. Introduction

The environmental conditions in underground mines are extremely demanding and pose a serious challenge to both workers and the technologies used. The situation is particularly difficult in deep mines, where the depth of exploitation exceeds 1200 m below the surface. At such depths, extreme conditions prevail, including high geostatic pressure, significantly elevated rock mass temperature, and difficult-to-control humidity and ventilation levels. In addition, access to deep excavations is logistically complicated and involves additional technical and organizational difficulties [1,2,3,4]. The hazards listed above may result from the depth, the type of mineral being mined, the type of adjacent rock, or may be related to mining technology [5]. One of the most dangerous hazards is gas. Gas hazards primarily include gases such as carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), and nitrogen oxides (NOx) [6,7]. Of the gases listed above, NOx will be characterized in this article.

This study focuses on gas hazards, with particular emphasis on NOx hazards. The occurrence of gas hazards in underground mines, including nitrogen oxides in particular, is greatly influenced by a number of geological and technological factors [8]. One of the key factors influencing the presence of NOx in the mine atmosphere is the depth of mining operations. At greater depths, natural ventilation is limited and the temperature of the rock mass is significantly higher, which can promote the accumulation of gases and accelerate chemical reactions, including the oxidation of nitrogen oxide to the more toxic nitrogen dioxide [9,10]. Equally important are the type of mineral being extracted and the properties of the surrounding rock mass. Some geological formations can be natural sources of gas emissions, including nitrogen oxides, especially when they contain organic matter or when oxidation processes occur [11]. The mineral composition of the rock can also affect the retention or migration of gases in pores and fractures, making it difficult to remove them effectively from the mining area. Mining technology plays a key role in the formation of NOx in the working environment [12,13]. The largest sources of NOx emissions in underground workings are diesel-powered machines, which are commonly used for material transport, drilling, and auxiliary operations [14,15,16]. Under conditions of limited ventilation, engine exhaust gases can cause a rapid increase in NO and NO₂ concentrations to levels that are hazardous to human health. In addition, processes such as blasting also contribute to NOx formation as a result of the reaction of atmospheric nitrogen with the combustion products of explosives [17,18]. All of the above factors make the hazards associated with NOx gases in underground mines complex, dynamic, and difficult to predict. Effective management of this hazard requires continuous monitoring of concentration levels, adaptation of working technologies to local geological conditions, and implementation of appropriate detection and ventilation systems.

Similar to the commonly used term NOx, Varshney et al. [19] and Clapp et al. [20] define this group as consisting of nitrogen monoxide and nitrogen dioxide, reflecting their chemical relationship: NO is rapidly oxidized to NO₂ [21,22,23]. Due to its physicochemical properties, NO is an important air pollutant, especially in industrial environments such as underground mines. Short-term exposure to elevated NO concentrations can lead to symptoms such as coughing; irritation of the nasal mucosa, throat, and eyes; headaches; and discomfort when breathing. These effects are mainly due to the irritating effect of NO and its oxidation products on the respiratory tract epithelium. Long-term exposure to nitric oxide is associated with the risk of more serious health effects, including chronic bronchitis, impaired lung function, and recurrent headaches [24]. In individuals with pre-existing respiratory diseases, such as bronchial asthma or chronic obstructive pulmonary disease (COPD), exposure to NO may exacerbate symptoms and increase the frequency of disease exacerbations. As reported by Sonker et al. [25] and Khan et al. [26], nitrogen dioxide at room temperature is a brown, highly toxic gas with a characteristic pungent odor. NO₂ is distinguished by its high chemical reactivity, which means it can easily initiate a variety of chemical reactions, including, under certain conditions, explosive reactions involving nitro compounds. Short- or medium-term exposure to elevated NO₂ concentrations may cause acute symptoms such as irritation of the conjunctiva, nasal and throat mucous membranes; coughing; shortness of breath; and shallow breathing. These effects mainly result from the irritating action of NO₂ and its oxidation products on the respiratory epithelium, leading to local inflammation and increased bronchial reactivity. In individuals with pre-existing respiratory conditions, such as bronchial asthma or chronic obstructive pulmonary disease (COPD), exposure to NO₂ can further impair lung function, exacerbate symptoms, and increase the frequency of disease exacerbations. Long-term exposure to elevated NO₂ levels poses a significant health risk. Studies have demonstrated an increased likelihood of developing chronic bronchitis, bronchial asthma, and cardiovascular diseases. Additionally, there is evidence linking chronic exposure to NO₂ with a higher incidence of respiratory cancers [27,28]. Consequently, NO₂ is recognized as a major air pollutant with a substantial impact on human health, particularly in urban and industrial environments. For this reason, it is particularly important to conduct systematic monitoring of the workplace atmosphere, with special emphasis on the presence and concentration levels of NOx, namely, nitric oxide and nitrogen dioxide. Accurate knowledge and continuous control of these gas concentrations enable a rapid response in the event of exceeding permissible exposure limits, thereby enhancing the safety of personnel working in high-risk environments such as underground mines. Continuous monitoring of gas hazards not only helps to minimize the risk of acute poisoning and chronic diseases but also facilitates more effective occupational safety management and the implementation of preventive measures to reduce emissions of these compounds [29,30]. According to Oluwoye et al. [31] and Ghose et al. [32], NOx in the atmosphere of underground mines comes from both natural processes (e.g., emissions from rocks, oxidation of atmospheric nitrogen) and technological activities. Of these, diesel engine machinery has the greatest impact on NOx levels, especially under conditions of limited ventilation [33,34]. In addition, occupational exposure to NOx is regulated in many countries, including through limits on permissible concentrations in mine air, mandatory ventilation standards, and regular monitoring of worker exposure in accordance with occupational safety regulations. Compliance with these regulations is critical to minimizing health risks and ensuring safe working conditions in mining and industry.

Currently, many different methods are used worldwide to determine the concentration of nitrogen oxides. These methods differ in terms of their operating principles, technological sophistication, and practical applicability. The choice of the appropriate method depends on many factors, such as the required measurement accuracy, device response time, purchase and maintenance costs, availability of specialized equipment, and, most importantly, the specific measurement environment and the expected concentration range. This article provides a comprehensive overview and classification of currently available methods for measuring NO and NO₂ concentrations, including both direct and indirect techniques, as well as laboratory methods and devices designed for real-time monitoring. Each of these methods is characterized in detail, taking into account parameters such as sensitivity, selectivity, susceptibility to environmental interference (e.g., dust, humidity), and practical aspects related to operation and maintenance. Subsequently, all reviewed methods are compared in terms of their suitability for application under the specific and demanding conditions of underground mines, where key challenges include limited space, variable ventilation, high humidity, and significant dust levels. The aim of this analysis is to identify—based on the available literature and previous research experience—the most suitable measurement method that can effectively operate both at low NOx concentrations typical of the mine atmosphere and at high concentrations, as found in diesel engine exhaust gases used in mining operations. To enable an objective assessment and facilitate comparison, the authors propose the use of a SWOT (strengths, weaknesses, opportunities, threats) analysis, which helps to systematically identify the main strengths and weaknesses of each method, as well as potential opportunities and threats related to their implementation under real-world conditions [35]. This approach will make it possible to select the most adequate solution from the perspective of occupational safety, monitoring effectiveness, and technical feasibility in underground mining environments.

According to Leiber et al. [36] and Benzaghta et al. [37], SWOT analysis (strengths, weaknesses, opportunities, threats) is one of the most widely used tools for strategic analysis, applied both in management and scientific research. Its essence lies in identifying and assessing internal factors (strengths and weaknesses) as well as external factors (opportunities and threats) and then comparing them. This approach makes it possible to develop a strategy that maximizes the advantages and potential of a given method while simultaneously minimizing risks associated with its limitations and external threats. The strengths of available NOx determination methods may include, among others, high accuracy and repeatability of measurement results, short response times of measuring instruments, simplicity of the analytical procedure itself, and the low cost of purchasing or manufacturing the equipment. In some cases, an additional advantage may be the widespread availability of necessary reagents and the possibility of applying the method in continuous or real-time monitoring. The weaknesses of these methods, on the other hand, may stem from high leasing, servicing, or calibration costs of specialized measuring instruments. Moreover, some methods require the use of toxic or hard-to-obtain chemical reagents, which generate additional costs and necessitate the implementation of strict safety procedures. Another limitation may be the need to employ qualified personnel for equipment operation and result interpretation, as well as constraints regarding the types of samples or working environments in which the method can be applied (e.g., under conditions of high humidity or dust levels typical of underground mines). The possibilities for further development and wider application of NOx measurement methods are mainly related to dynamic technological progress, including the emergence of modern sensors and monitoring systems that enable more precise, faster, and more automated measurements. Another important supporting factor is the development of legal regulations and environmental standards, including EU directives and WHO guidelines, which require accurate monitoring of nitrogen oxide concentrations both in ambient air and in workplaces. In underground mines and other industrial settings, these regulations take into account close proximity, elevated concentrations, and long exposure times. Regulatory requirements mandate air pollution measurements in the workplace to ensure that worker exposure remains at a safe level, which in turn may influence the choice of measurement methods, with more complex technologies being less preferred due to higher costs or operational challenges.

To protect workers from the risks of exposure to hazardous chemicals, the European Commission proposes indicative occupational exposure limit values (IOELVs). On 31 January 2017, the European Commission published Directive (2017/164), establishing a fourth list of indicative occupational exposure limit values, including values for nitrogen oxide (2.5 mg/m³) and nitrogen dioxide (0.96 mg/m³) related to an eight-hour working day (TWA) [38,39]. These values are intended to reduce the health risks associated with long-term exposure to these harmful substances, which are mainly emitted by diesel engines and other sources of exhaust fumes in industries such as mining. In Poland, in accordance with the regulations on underground mining, the occupational exposure limit values for nitrogen oxides are specified in the Regulation of the Minister of Energy of 23 November 2016 [40]. The permissible concentration of nitrogen monoxide in mining excavations cannot exceed 2.6 ppm, which corresponds to approximately 4.8 mg/m³, while for nitrogen dioxide, the requirements are based on the EU directive and amount to approximately 0.96 mg/m³. This approach is consistent with international practice, where exposure standards for NO and NO₂ are set based on toxicological and epidemiological studies to protect workers’ health from the adverse effects of inhaling these gases.

All these aspects have been taken into account in the following sections of this article, where individual NOx determination methods are analyzed and compared within the framework of a SWOT analysis. This approach makes it possible to identify the solution best suited to the specific needs of monitoring in underground mining environments. The SWOT analysis presented in this article will enable a comprehensive evaluation of various methods used to determine NOx concentrations. Thanks to this multi-faceted approach, it will be possible to objectively compare the available methods and identify the one that best meets the specific requirements of underground mining environments. This includes both technical aspects and practical considerations related to the implementation and everyday use of these methods for routine monitoring of gas hazards.

2. Methods of Measuerement NOx Concentrations

Currently, there are many different methods used worldwide for determining nitrogen oxide concentrations. These methods have been developed based on the specific chemical and physical properties of the analyzed compounds, such as dipole moment, chemical reactivity, and ultraviolet (UV) absorption capacity [41]. By exploiting these properties, it has been possible to design both simple and rapid detection techniques, as well as more advanced and highly precise laboratory methods.

A classification of the currently available NOx determination methods proposed by the authors is presented in Figure 1. The classification was made on the basis of detection principles. The authors divided the methods into four main groups:

• chemical methods—based on specific chemical reactions of NO and NO₂;
• chemiluminescence methods—which utilize the phenomenon of light emission during chemical reactions;
• electrochemical methods—relying on the reaction of the target gas with an electrode;
• instrumental methods—including, among others, chromatography and spectrometry techniques.

Although chemiluminescence belongs to the subgroup of chemical methods, it has been distinguished by the authors due to the use of luminescence intensity as an indicator of concentration.

In the following part of the article, each of these groups of methods will be characterized in detail. The authors will present the principle of operation, key advantages and limitations, and assess their suitability for use in the specific and challenging conditions typical of underground mines, such as high humidity, dust concentration, and limited space. This approach aims to facilitate an objective comparison and help select the method best adapted to the requirements of gas monitoring in mining environments.

Table 1. Overview of gas detection methods with descriptions and references.

Category	Method/Technique	Description	Examples and References
Chemical		Chemical methods rely on reactions between the target gas and specific reagents. The resulting chemical changes are analyzed to determine gas concentration.	Kain et al. (1967) [42]; Salem et al. (2009) [43]; Karlsson & Torstensson (1974) [44]; Pankivskyi et al. (2022) [45]; Sun et al. (2017) [46]
Chemiluminescence		Measures light emitted during a chemical reaction. The intensity of emitted light correlates with gas concentration.	Alexa et al. (2025) [47]; García-Robledo et al. (2014) [48]; Tzani et al. (2021) [49]; Dunlea et al. (2007) [50]; Wang et al. (2005) [51]; Navas et al. (1997) [52]; Oh & Woo (2011) [53]; Dickerson et al. (2019) [54])
Electrochemical		Detects changes in voltage or current caused by the presence of target gases.	Khan et al. (2019) [26]; Ryu et al. (2020) [55]; Bedioui et al. (2012) [56]; Schmitz et al. (2025) [57]
Instrumental	Gas Chromatography	Separates and identifies components in gas mixtures. Accurate, but often limited to labs.	Coskun (2016) [58]; Marley et al. (2004) [59]; Li et al. (2012) [60]; Althannas et al. (2023) [61]
	UV–Vis Spectrophotometry	Uses UV and visible light absorption to quantify gases. Suitable for field use.	Pratiwi & Nandiyanto (2021) [62]; Schneider et al. (1987) [63]; Platt & Stutz (2007) [64]
	FTIR	Detects gases via infrared absorption spectra. Allows multi-gas detection.	Vojtíšek & Pechout (2022) [65]; Bacsik et al. (2004) [66]; Smith et al. (2011) [67]
	NDIR	Non-dispersive IR sensors measure gas absorption of IR light, commonly used for CO2.	Hussain et al. (2018) [68]; Niklas et al. (2019) [69]; Xu et al. (2022) [70]; Park et al. (2019) [71]
	NDUV	Uses UV absorption to detect gases like NO2; high selectivity but can be interfered by other UV-absorbing gases.	Platt & Stutz (2007) [64]; Zhang et al. (2016) [72]; Park et al. (2019) [71]; Liu et al. (2023) [73]; Favre et al. (2019) [74]

below presents a concise overview of selected methods for the determination of NOx, applied both in laboratory settings and field measurements. The listed techniques include chemical, electrochemical, and instrumental approaches, each briefly described to allow for quick comparison of their key features. In addition, for each method, representative references are provided, covering both recent scientific publications and foundational studies from earlier decades. This selection offers the reader a starting point for exploring more in-depth information on each technique.

Further details regarding the principles, advantages, limitations, and practical applications of the methods are discussed in the following subsections. These sections also include an extended list of references, offering broader theoretical and practical context for the NOx measurement techniques described.

2.1. Chemical

Chemical methods for determining NOx include a range of analytical techniques that use specific chemical reactions to detect and quantitatively measure these gases. The most commonly used methods include titration methods and methods based on chemical absorption, in which NOx is absorbed into reactive solutions and interacts with chemical reagents, leading to changes in the composition of the solution that can then be quantified using other methods such as titration or volumetric analysis [42,47]. The advantages of these methods include relatively high accuracy and precision, as well as relatively low reagent and equipment costs compared to more advanced instrumental techniques. In addition, these methods are well established and widely documented in the scientific literature, which facilitates their adaptation and implementation in laboratory conditions [43]. However, the main limitations include the time-consuming nature of the analyses and the necessity of conducting them under laboratory conditions, which requires gas sampling and appropriate sample preparation. Furthermore, the execution of these methods demands skilled personnel capable of performing the chemical reactions correctly and accurately interpreting the results. Despite these limitations, chemical methods remain valuable, particularly in situations where precise analysis is required, but more complex continuous monitoring equipment is unavailable. In the context of mine monitoring, they can serve as reference methods or support measurements conducted by instrumental techniques.

2.1.1. Iodometric (Titration) Method

The iodometric method is a classical analytical technique used for determining nitrogen dioxide (NO₂) concentrations in air. It relies on the oxidation of iodide ions (I⁻) to free iodine (I₂) by NO₂ (or by nitrite, ${\mathrm{NO}}_2^{-}$, formed after NO₂ absorption) in an acidic medium. In practice, air containing NO₂ is passed through an acidified solution of potassium iodide (KI) or through filters soaked in KI, where NO₂ reacts with iodide, releasing iodine [44,45]. This reaction causes a yellow coloration due to the presence of free iodine, which can be further enhanced by adding starch to form a dark blue iodine–starch complex [75]. The released iodine is then quantified by titration with sodium thiosulfate (Na₂S₂O₃) until the blue color disappears, indicating that the iodine has been completely reduced back to iodide. Alternatively, the amount of iodine can be measured by colorimetric or spectrophotometric detection. The measured iodine (or color intensity) is directly proportional to the NO₂ (or nitrite) content in the sample, allowing calculation of the NO₂ [76].

2.1.2. Absorption Method in Sulfuric Acid

The sulfuric acid absorption method is based on the efficient absorption of NO_x (i.e., NO and NO₂) in a sulfuric acid solution, resulting in the formation of nitrate (NO₃^–) and nitrite (NO₂^–) ions or, depending on reaction conditions, more complex compounds such as sulfuric acid nitrate (HNO₃HSO₄) or sulfate nitrate (HNO₃SO₄). In practice, mine air samples are passed through an absorber or scrubbers filled with dilute sulfuric acid (typically 0.1 M H₂SO₄). In the acidic environment, NO₂ undergoes hydrolysis and disproportionation to form nitrite and nitrate ions, while NO may be pre-oxidized (e.g., with ozone or another oxidant) to ensure more complete capture. Quantitative determination of these ions is then performed, which can involve classical iodometric titration, colorimetric analysis (e.g., the Griess reaction), or instrumental techniques such as ion chromatography [46]. In some variants, as described by Joshi et al. [77], after absorption, NO is separated by distillation or diffusion into a NaOH solution, where it reacts with a KMnO₄ solution to form sodium nitrate (NaNO₃). The amount of KMnO₄ consumed allows for calculation of the NO_x concentration in the sample [78,79]. This method demonstrates good sensitivity—with detection limits reaching the micromolar level—and is suitable for both field and laboratory studies, although it requires careful optimization of reaction conditions and control of potential interferences.

2.2. Chemiluminescence

As reported by Gamiz-Graci et al. [48] and Tzani et al. [49], chemiluminescence is a process in which the energy released by a chemical reaction causes the emission of light. This phenomenon occurs when the energy released during a chemical reaction is large enough to exceed the energy threshold required to excite a molecule and bring it to an excited state. When this molecule returns to its ground state, it emits energy in the form of light [80].

Chemiluminescence is one of the main methods for measuring nitrogen oxides. Nitrogen oxide in the gas phase oxidizes with ozone to form an excited unstable nitrogen dioxide molecule [81]. According to Alam et al. [82], this molecule emits radiation in the wavelength range of 600–3000 nm during its return to the ground state. The intensity of the radiation is proportional to the concentration of nitric oxide. The reactions that take place are shown below [83]:

$$\mathrm{NO}+{\mathrm{O}}_3\to {\mathrm{NO}}_2^{*}+{\mathrm{O}}_2$$

(1)

$${\mathrm{NO}}_2^{*}\to {\mathrm{NO}}_2+\mathrm{hv}$$

(2)

Determining nitrogen dioxide concentration using the chemiluminescent method is only possible after it has been converted to nitric oxide. The NO₂ concentration is calculated based on the difference between the emission rate for the sample that passed through the converter and the emission rate for the sample that was not converted [50]. Chemiluminescence of NO and NO₂ is a very sensitive and precise method for determining these gases, making it frequently used in air quality research and industry.

A disadvantage of this method is that it requires complex reaction and detection systems, as well as ozone, which can impact its cost and complexity [51,52]. Chemiluminescent NOx analyzers are devices that utilize the chemiluminescence phenomenon to measure nitrogen oxides concentrations in air or other gases. They are widely used in industry, scientific research, and air quality control. Chemiluminescent NOx analyzers consist of several basic components, including a sampling system, a reagent system, a reaction chamber, a detector and signal processing system, and an ozone generator [53,84]. The sampling system is responsible for collecting the gas sample for analysis. The gas sample is fed into the analyzer through a sample tube and is processed to remove any impurities such as moisture, oxygen, and organic compounds. The reagent system contains ozone, which is used as a reactant to react with NOx in the reaction chamber. The reaction chamber is where the reaction between ozone and NOx takes place, and this reaction results in the emission of light, which is then detected by the detector. The detector is responsible for detecting and measuring the intensity of light emitted during the reaction of NOx with ozone. This intensity is directly proportional to the concentration of NOx in the sample [54]. The signal processing circuit converts the signal detected by the detector into a numerical value, allowing the NOx concentration to be read on the display. NOx chemiluminescence analyzers are very accurate and sensitive, making them ideal for use in various fields such as industry, scientific research, and air quality control [85].

2.3. Electrochemical

Electrochemical methods for the determination of nitric oxide and nitrogen dioxide rely on measuring the electric current generated by the electrochemical reaction occurring between the electrode and the target gases NO and NO₂ [55,86]. This technology uses specially designed sensors in which the analyzed gas diffuses into the measuring chamber and undergoes an electrochemical reaction on the surface of a suitably selected working electrode. The generated electrical signal (current or potential) is proportional to the actual concentration of the target component in the analyzed sample. According to Privett et al. [87], some of the most commonly used electrodes for NO measurement are platinum electrodes coated with a layer of palladium oxide or porous carbon. These materials are characterized by high catalytic activity and good electrical conductivity, which enables effective oxidation of NO molecules on the electrode surface. In this reaction, NO is oxidized to NO⁺, and the resulting electric current flow is a direct measure of NO presence in the analyzed sample [56]. Modern designs increasingly utilize nanomaterials, such as carbon nanotubes or graphene, which enlarge the active surface area of the electrodes, enhancing sensor sensitivity and selectivity [88].

In the case of NO₂, according to Hoherčáková et al. [89], the most commonly used electrodes are platinum or gold electrodes coated with a layer of lead dioxide (PbO₂). During electrochemical detection, NO₂ acts as an oxidizing agent. It is reduced on the electrode surface, according to the following reaction:

$${\mathrm{NO}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \mathrm{NO}+{\mathrm{H}}_2\mathrm{O}.$$

(3)

This results in an electric current proportional to the NO₂ concentration [90,91]. The use of PbO₂ as the electrode material improves the selectivity of the method towards NO₂ and helps to minimize interferences from other gases, although it does not completely eliminate them.

Modern electrochemical NOx sensors are compact and operate at low voltage, which allows them to be integrated into portable systems, IoT networks, and smart city devices [57,92]. The advantages of these methods include ease of use, fast response times (from a few to several tens of seconds), low sample consumption, and relatively low operating costs, both for stationary and mobile monitoring. The use of new electrode materials and advanced calibration algorithms further improves measurement accuracy and helps to reduce the impact of interference [88,93]. Nevertheless, as comparative studies with reference analyzers show, the accuracy of electrochemical sensors can be limited by the presence of interfering gases (such as CO, SO₂, O₃), as well as by temperature and humidity fluctuations and electrode material aging [57,92]. Therefore, regular sensor calibration and the use of appropriate environmental compensation algorithms are crucial.

Electrochemical methods have found wide application both in research laboratories and in industry, for monitoring air quality and measuring NO and NO₂ concentrations in automotive exhaust gases, industrial emissions, or workplace environments [92,93]. The increasing availability and miniaturization of these sensors further promote their use in environmental applications and modern warning systems.

2.4. Instrumental Method

2.4.1. Gas Chromatography

Gas chromatography (GC) is a separation technique that enables the resolution of mixtures into their individual components using a stationary phase and a gaseous mobile phase. In GC, the sample mixture is injected into a column containing a stationary phase (e.g., a sorbent) and separated utilizing a carrier gas (such as nitrogen or helium) that flows through the column [58,59].

GC is one of the recognized analytical methods used to determine the concentration of nitrogen oxides (NOx), which are harmful air pollutants. In this approach, the collected air sample flows through a chromatographic column, where NO and NO₂ are separated based on their physicochemical properties. After elution from the column, the components are detected, usually by electron capture detectors (ECD), thermal conductivity detectors (TCD), or flame ionization detectors (FID) after preliminary conversion of NOx to more detectable derivatives, as NO and NO₂ have low detectability using traditional FID or TCD methods. To increase the selectivity and sensitivity of NOx analysis, pre-conversion techniques are often used, such as reducing nitrogen oxides to more reactive forms, including N₂O or NO, using a catalyst. The resulting mixture can be subjected to precise quantitative analysis using appropriate detection systems [59,94]. The typical detection limit of this method for NOx in air is between a few and several tens of ppb, which allows it to be used for atmospheric air quality assessment as well as for industrial emission analysis [95].

High chromatographic resolution and the possibility of coupling GC with additional detection techniques (e.g., mass spectrometry, MS) enable unequivocal identification and highly precise quantification of both forms of nitrogen oxides, even in the presence of various other air constituents [58]. Although GC requires specialized laboratory equipment, it remains a reference technique in many environmental laboratories and enables trace gas analysis even in the presence of complex matrix backgrounds.

2.4.2. Absorption Spectroscopy

Instrumental methods for the determination of nitrogen oxides use phenomena related to the absorption or scattering of light by NOx molecules to measure the concentration of these compounds. Instrumental methods for the determination of NOx are accurate and often used in industry but require expensive equipment and appropriate technical expertise.

• UV–Vis spectrophotometry
  UV–Vis spectrophotometry is an analytical technique that utilizes ultraviolet (UV) and visible (VIS) radiation to determine the quantity and identity of substances present in a sample. This method is based on measuring the absorption of light by the analyte across a specific wavelength range [62,96]. The phenomenon of light absorption is described by the Lambert–Beer law, according to which the absorbance of a sample is proportional to the concentration of the absorbing species and the optical path length [97]. A typical UV–Vis spectrophotometer consists of a light source, a monochromator, a sample chamber, and a detector. Light sources may include deuterium, xenon, or LED lamps emitting radiation in the UV–Vis range, while the monochromator allows for the selective choice of a specific wavelength of light that illuminates the sample. The sample chamber usually contains a solution or gas sample, and a detector, such as a photodiode or photomultiplier, records the amount of light absorbed by the sample. The measurement data is presented in the form of an absorption spectrum, which allows the identification and quantification of analytes [62,97]. UV–Vis spectrophotometry is widely used for the determination of NOx, both in the gas phase and in solution. The analysis of NO and NO₂ is particularly important for air quality monitoring, as these compounds exhibit characteristic absorption bands in the 200–400 nm range [63,98]. Nitrogen dioxide exhibits an intense absorption maximum around 400 nm, while nitrogen monoxide absorbs in the lower UV range, near 214 nm [63,64].
  Accurate quantification of NOx using UV–Vis spectrophotometry often requires calibration with standard solutions and selection of appropriate wavelengths, depending on the analyte being measured. The sensitivity of this method allows for the detection of very low NOx concentrations, even at parts per billion (ppb) levels, making it particularly useful in environmental and atmospheric analyses [96]. This technique can be successfully used for direct gas analysis, as well as after preliminary absorption of gases in solution, where the resulting nitrate and nitrite ions can be determined spectrophotometrically after a colorimetric reaction [62,64]. The high sensitivity, repeatability, automation potential, and relatively low operational costs contribute to the widespread use of UV–Vis spectrophotometry as one of the most common analytical methods for NOx determination in air and other environmental matrices [64,96].
• Fourier transform infrared (FTIR)
  Fourier transform infrared spectroscopy (FTIR) is a type of infrared absorption spectroscopy that, unlike classical dispersive methods, measures the intensity of light from a broad-spectrum source passing through a scanning interferometer as a function of the optical path difference. Applying a Fourier transform to the recorded interferogram yields a complete absorption spectrum. The development of faster computational speeds and advanced interferometer technology has significantly improved the signal-to-noise ratio and shortened measurement times, leading to FTIR spectroscopy increasingly replacing traditional infrared absorption techniques in laboratories and industrial applications [65]. When measuring NO and NO₂, a gas sample is passed through an optical gas cell, where infrared radiation passes through the sample and a portion of it is absorbed by NO and NO₂ molecules at characteristic wavelengths. The transmitted light is then analyzed by an FTIR spectrometer, yielding an absorption spectrum unique to the molecular structure of the sampled gases [66]. Specific absorption bands in the infrared region correspond to the fundamental vibrational transitions of NO and NO₂, enabling characteristic identification as well as quantification of concentration.
  Calibration of FTIR analyzers is achieved by analyzing reference gas mixtures of known NO and NO₂ concentrations, creating precise relationships between absorption band intensity and analyte concentration. Advanced software solutions facilitate spectral fitting and multicomponent analysis even in complex gas mixtures, providing highly selective and robust results even in the presence of water vapor, CO₂, or other common interferences [67].
  The FTIR method is considered one of the most precise and selective methods for detecting trace amounts of NO and NO₂ in atmospheric, flue gas and process gas streams. It is widely used for air quality monitoring, laboratory analysis and emission compliance due to its multi-component capability, fast response and non-destructive nature [65,67]. However, its implementation requires specialized, high-quality instrumentation, careful calibration and operator expertise, resulting in higher operating costs and technical complexity compared to simpler analytical methods [67].
• Non-dispersive infrared (NDIR)
  The non-dispersive infrared (NDIR) method is one of the most widely used optical techniques for the determination of gaseous nitrogen oxides, relying on the selective absorption of infrared (IR) radiation by gas molecules containing more than two different atoms [68,70]. The spectral characteristics of such gases are due to their permanent or induced dipole moments, which enable them to undergo vibrational or bending transitions upon absorption of IR radiation of the appropriate wavelength, resulting in the absorption of photon energy [70]. As described by Tang et al. [99], a typical NDIR setup involves passing a gas sample through a measurement chamber containing an IR radiation source. An appropriately selected detector and optical filter enable selective measurement of light intensity reduction at a wavelength characteristic of the target gas that is proportional to its concentration. This approach is particularly useful for gases such as CO₂, CO, N₂O, SO₂, NO and NO₂ and is used in both laboratory analysis and environmental monitoring [69,70,100].
  Significant technical advances have been made in NDIR sensors in recent years, including the integration of advanced filters, miniaturization of detectors, and the use of multi-channel detection. These innovations have led to lower detection limits and better compensation of interfering species—such as water vapor or CO₂ in exhaust gases—through multiplexing or implementation of advanced signal processing algorithms [69,70,100]. For example, the use of plasmonic metamaterial absorbers or piezoelectric arrays improves the detection and selectivity of many components [100]. For NOx quantification, important challenges include the separation of NO and NO₂ signals and potential interference from other gases with overlapping infrared absorption bands. Consequently, the efficiency of calibration, the choice of optical filters, and the development of appropriate signal-processing algorithms are all critical to obtaining accurate results [69,70,101]. The relative simplicity and miniaturization potential of NDIR configurations contribute to the widespread use of these sensors as distributed monitoring systems in industry, meteorology, transportation, and environmental research [69,100]. Review articles indicate that the NDIR technique offers a good compromise among sensitivity, rapid response, and constructional simplicity, remaining one of the most economical and convenient methods for monitoring trace gases such as NOx [69,70]. At the same time, the fast-paced development of new detectors, IR sources, and algorithms for compensation of interfering factors ensures the increasing significance of this method in the coming years [70,100].
• Non-dispersive ultraviolet (NDUV)
  The non-dispersive ultraviolet (NDUV) method takes advantage of the fact that various gaseous compounds selectively absorb ultraviolet (UV) light at specific wavelengths. By passing UV light through a gas sample and measuring attenuation at the corresponding wavelengths, individual components can be identified and quantified based on their unique absorption signatures [72]. In NDUV, UV absorption by target gas molecules—such as NO and NO₂—is measured over carefully selected wavelength ranges. The gas sample is introduced into a measuring cell, where it is irradiated with ultraviolet light. The absorption profile is determined by comparing the intensity of the light before and after it passes through the gas, allowing both qualitative identification and quantification of NO and NO₂ levels.
  A typical NDUV system uses two detection channels; one measures the absorption of UV light by the gas sample, while the other monitors the absorption by a reference gas, which is a chemically defined composition free of the analyte(s) of interest. Comparing the two signals allows for accurate compensation for baseline shifts, lamp fluctuations, and interference from other absorbing species [102,103]. Using the measured absorption values for both the sample and the reference, along with known absorption coefficients for the target analytes, the concentration of NOx can be calculated according to the Beer–Lambert law. Advanced implementations can utilize multi-wavelength analysis and spectral deconvolution algorithms to further reduce interference and improve selectivity for NO and NO₂, even in complex or humid gas mixtures.
  NDUV analyzers are widely used for online and real-time monitoring of NO and NO₂ in emissions (e.g., vehicle exhaust, industrial stacks), ambient air, and process gases because of their high precision, fast response, and low maintenance requirements. Limitations primarily relate to potential spectral overlap with other UV-absorbing compounds and the need for regular calibration, but the development of advanced optics and algorithms has greatly mitigated these challenges [104]. The NDUV method operates on the principle of

  $$N_2+O_2\to N{O}_2^{*}\to NO+hv$$

  (4)

  $$2NO\to N2+O_2$$

  (5)
  Equation (4) shows NO/NO₂ generation and photon emission, while Equation (5) shows NO decay. These processes form the basis of detection in the NDUV method.
  The NDUV method offers many advantages, such as speed of measurement, low equipment cost, and high selectivity of measurements. However, the method is susceptible to the influence of interferences, such as changes in temperature, humidity, or the presence of other gas absorbance in similar wavelength ranges, which can affect measurement results.
  The NDUV method is widely used in industry, air quality monitoring, and laboratories to measure NO and NO₂ concentrations in air and industrial gases.

3. Comparison of Methods

The purpose of this article is to select, based on a literature review, a suitable method for measuring nitrogen oxides, i.e., NO and NO₂. The measurement method proposed in the article should be suitable for measuring both low and high concentrations. The choice of method will be made using SWOT analysis and point system analysis. Parameters such as measurement range, ease of use under field conditions, sensitivity, selectivity, stability, measurement precision, ease of measurement, accuracy, ease of validation, and cost will be considered during the analysis.

A flowchart predemonstrating the selection of a method for the determination of nitrogen oxides under actual mine conditions is shown in Figure 2.

3.1. SWOT Analysis—Methods for Measuring NOx

A comprehensive SWOT analysis was conducted to identify the most optimal method for measuring both high and low concentrations of NOx under underground mining conditions. The selection of the most effective solution was especially important due to the specific challenges inherent to the mining environment, including the presence of various NOx sources (e.g., engine exhaust), the need to detect a wide range of concentrations (from single-digit ppm to several hundred ppm), spatial constraints, and demanding operational conditions. The analysis considered eight key technical and practical factors, relevant to both routine monitoring and occupational safety:

• Sensitivity: The minimum detectable concentration of NOx and the method’s linear working range.
• Price: Both the investment cost of devices/analytical systems and ongoing operational expenses.
• Selectivity: The ability to distinguish NOx from other gases typically present in the mine atmosphere.
• Stability: The resilience of the method/instrument to changing environmental conditions and long-term use.
• Precision: The repeatability and reliability of measurement results.
• Validation: Availability of standards and the possibility of regular verification of analytical performance.
• Ease of use in the field: Mobility, device size, and suitability for operation by non-specialist personnel.
• Ease of sampling/Measurement: Directness, potential for continuous monitoring, and minimization of errors related to sample collection.

The analysis included four main groups of analytical techniques: chemical, electrochemical, chemiluminescent, and instrumental methods (NDIR, NDUV, FTIR, UV–Vis). Each technology was evaluated against the above criteria, and its advantages and disadvantages were compared from the perspective of routine mining practice and safety requirements. The SWOT approach made it possible to identify methods that offer a compromise between high sensitivity, selectivity, and stability, while being practical and convenient for mobile use in the harsh and often unpredictable conditions of underground mines. Moreover, the analysis highlighted the potential risks associated with implementing each technique and identified those most suitable not only for detecting trace amounts of NOx, but also for analyzing the high concentrations present in diesel exhaust from mining machinery.

Initially, a detailed analysis of four NOx measurement methods was conducted based on eight predefined evaluation criteria: sensitivity, cost, selectivity, stability, precision, validation capability, field usability, and sampling or measurement approach. Each method was evaluated individually, allowing a preliminary assessment of its suitability for use in an underground mining environment (

Table 2. SWOT analysis—chemical methods (e.g., Griess method, colorimetry).

Criteria	Strengths	Weaknesses
Sensitivity	Sufficient for standard applications (ppm)	Low for trace levels (ppb)
Cost	Very low; cheap reagents and equipment	–
Selectivity	Good with proper reagent selection	Matrix interferences
Stability	Stable in laboratory conditions	Sensitive to field conditions (light, temperature)
Precision	Good in optimized procedures	Degrades at low concentrations
Validation	Well described in the literature; standards available	Requires matrix/sample-specific adaptation
Field usability	Simple equipment; no power supply needed	Manual process; time-consuming
Measurement	Quick sample collection possible	No continuous monitoring; off-line analysis

,

Table 3. SWOT analysis—chemiluminescence methods.

Criteria	Strengths	Weaknesses
Sensitivity	Very high; meets EPA/EU reference levels (ppb)	–
Cost	-	Very high (purchase/maintenance)
Selectivity	Excellent for NO/NOx; reference method	Ozone interference; NO2 needs converters
Stability	Excellent in lab/stationary use	Challenging in field use; filters/ozone systems required
Precision	Excellent; reference-grade	–
Validation	Fully validated; meets EPA/EN norms	–
Field usability	Fully automated; continuous	Heavy, non-portable
Measurement	Automatic gas measurement	Requires drying and filtration systems

,

Table 4. SWOT analysis—electrochemical methods.

Criteria	Strengths	Weaknesses
Sensitivity	Very good (ppb–ppm), fast response	Sensor saturation at high levels
Cost	Low to moderate; compact devices	Sensor replacement cost; calibration needed
Selectivity	Improving with better electrodes and algorithms	Interference (e.g., SO2, CO); needs compensation
Stability	Good in the short to mid term	Drift and degradation under harsh conditions
Precision	Good after calibration	Requires frequent recalibration in the field
Validation	Factory/field validation, norm-compliant	Strongly affected by environmental conditions
Field usability	Highly convenient – portable sensors	Needs regular calibration/maintenance
Measurement	Real-time, direct gas detection	–

and

Table 5. SWOT analysis—instrumental methods.

Criteria	Strengths	Weaknesses
Sensitivity	Very good (to ppb); wide range	High cost of advanced instruments
Cost	Medium (NDIR/UV–Vis); high (FTIR/GC)	Spare parts, service costs
Selectivity	Excellent; multi-gas capability (NDIR, FTIR, GC)	Background interferences (e.g., water vapor, CO2)
Stability	High after calibration	Requires regular maintenance
Precision	Very good with calibration	–
Validation	ISO/EPA standards; widely used	Needs environment-specific validation
Field usability	Portable versions in development	Advanced versions need trained operators
Measurement	Fast/continuous (NDIR, UV–Vis); offline (GC)	FTIR/GC mostly lab-based, less mobile

).

In the next stage of the study, a comprehensive SWOT analysis was conducted for the four main categories of NOx determination methods—chemical, electrochemical, chemiluminescent, and instrumental techniques (NDIR, NDUV, UV–Vis, FTIR). The analysis considered the main strengths and weaknesses of each method, as well as the opportunities and risks associated with their practical application in demanding underground mine conditions. The goal was not only to identify technical advantages and limitations but also to identify implementation barriers and optimization opportunities, especially for mobile measurements over a wide range of NOx concentrations (from 10 ppm to 750 ppm). Figure 3, Figure 4, Figure 5 and Figure 6 below present the main results of this SWOT analysis, allowing a clear comparison of the analytical advantages and limitations of each technique in the context of underground mining. This graphical overview helps to quickly identify the factors affecting the practical suitability, efficiency, and implementation potential of each method, thus supporting decision makers in selecting optimal NOx-monitoring strategies in actual mining conditions.

A comprehensive SWOT analysis was conducted for the four main groups of NOx determination methods—chemical, electrochemical, chemiluminescent, and instrumental (NDIR, NDUV, UV–Vis, FTIR). The aim was to evaluate their suitability for mobile measurements in the harsh environment of underground mines, where detection of both low (0–10 ppm and up to 750 ppm) [29,33,105,106,107]. Chemical methods are among the oldest analytical techniques. They offer relatively high selectivity due to specific reactions with nitrogen oxides and are inexpensive, mainly due to the low cost of reagents. However, they are subject to relatively high measurement errors, use potentially harmful chemicals, and require laboratory facilities, making field measurements impossible. As a result, these methods are not suitable for operational use in underground mines, and their importance in practical applications is steadily declining. Chemiluminescence methods provide very high sensitivity and precision, delivering results quickly. Although theoretically well suited for field measurements, these techniques require expensive, specialized equipment and expert handling. Their main limitation is poor selectivity. In the confined spaces of underground mines, where the air can contain various interfering compounds, the risk of cross-sensitivity is high, which can significantly reduce the reliability of the measurement. This makes their practical use underground problematic. Electrochemical methods are fast, user-friendly, and cost-effective. They allow easy monitoring in the field, including tracking personal exposure. However, they require frequent maintenance or electrode replacement and have low selectivity compared to chemiluminescence. Moreover, when measuring directly in the exhaust stream of mining machinery, a common risk is sensor damage due to high temperatures. Instrumental methods—including NDIR, NDUV, UV–Vis, and FTIR—have proven to be the most suitable group for use in underground mine conditions. They combine high sensitivity, precision, selectivity, and stability with the capacity for continuous real-time monitoring. Portable analyzers based on these techniques offer fast response times, environmental robustness, and increasing market availability due to the continued miniaturization and automation of analytical systems. Although they require more advanced handling and have higher operating costs compared to simpler methods, their overall performance and reliability are unparalleled. Sample preparation is often necessary, but this is offset by their ability to be used in both laboratory and field settings. Based on the SWOT analysis, instrumental methods clearly stand out as the most suitable solution for measuring NOx in underground mining. Compared to electrochemical and chemiluminescent methods, they offer much better selectivity, which is a key factor in complex mine atmospheres containing interfering compounds such as H₂S, CO, CO₂, and SO₂. This is especially important when monitoring low concentrations of NOx, where even small disturbances can seriously distort results. Another important advantage is their suitability for actual underground operations. While some methods require sample conditioning, modern portable analyzers allow for effective deployment directly in the mine. As a result, instrumental methods are not only technically superior but also practically feasible for reliable NOx monitoring in underground mining environments.

In conclusion, instrumental methods should be regarded as the optimal solution for mobile and accurate NOx measurements in underground mines.

3.2. SWOT Analysis—Instrumental Methods for NOx Determination

The next stage of the study involved a detailed SWOT analysis of individual techniques grouped under instrumental methods that had previously been identified—based on a multi-criteria analysis—as the most promising approaches for use in underground mining conditions. These techniques, including FTIR, NDUV, NDIR, gas chromatography, and UV–Vis spectrophotometry, were comprehensively evaluated in terms of their sensitivity, selectivity, operational stability, investment and maintenance costs, validation capabilities, precision, field usability, and ease of sampling and measurement.

Table 6. GC (gas chromatography).

Parameter	Strengths	Weaknesses
Sensitivity	Very good (ppb–ppm), reference method	Analysis time typically in the order of tens of minutes; quasi-real-time possible but not continuous
Cost	Compatible with various detectors, standardized	High cost, requires carrier gases
Selectivity	Excellent for multi-component analysis	Time-consuming, less suitable for continuous online use
Stability	Excellent in lab conditions	Needs stable power, carrier gases, regular service
Precision	Very high after calibration	Column degradation possible in complex matrices
Validation	Easy via gas standards and protocols	Temperature/pressure fluctuations may require recalibration
Field usability	Low—lab-based, skilled operation required	Limited portability for mining/field environments
Measurement	Very selective with small samples	Sample collection and preparation may be time-consuming

,

Table 7. UV–Vis spectrophotometry (with or without absorption scrubber).

Parameter	Strengths	Weaknesses
Sensitivity	Can detect very low (ppb/ppm) and high concentrations	Depends on scrubber efficiency and colorimetric reaction
Cost	Low to medium – analyzers and reagents affordable	Cost increases with automation or flow-integration
Selectivity	Good due to selective chemical reactions	Possible interferences from substances absorbing at same wavelength
Stability	Good post-calibration, long-term use possible	Reagents need regular replacement; scrubbers need cleaning
Precision	Very good with repeatable reactions and automation	Affected by dosing precision and reagent degradation
Validation	Easy using reference solutions, standard protocols	Limited validation for online/portable non-automated setups
Field usability	Available in lab/portable analyzers	Manual versions are time-consuming; reagent handling needed
Measurement	Automated (flow analyzers) or manual, multiparameter	Manual sampling may introduce measurement errors

,

Table 8. FTIR (Fourier transform infrared spectroscopy).

Parameter	Strengths	Weaknesses
Sensitivity	Very high, detection down to ppb	May lose sensitivity in presence of water vapor and CO2
Cost	Multi-gas capability justifies investment	High purchase and service costs
Selectivity	Spectral analysis allows simultaneous detection of many gases	Overlapping IR bands may complicate results, matrix calibration needed
Stability	Good after calibration, stable optical components	Requires cleanliness of optics (e.g., window contamination)
Precision	Very good, enables multipoint calibration	Environmental conditions may affect readings
Validation	International standards, easy auditability	Needs periodic calibration validation
Field usability	Portable versions available, fast analysis	Weight and power demand reduce mobility
Measurement	Direct, continuous measurement	Sample prep required under high dust conditions

,

Table 9. NDIR (non-dispersive infrared).

Parameter	Strengths	Weaknesses
Sensitivity	Good (ppm), fast detection, continuous	Less sensitive than FTIR for low concentrations
Cost	Affordable, wide range of devices, low operation cost	Requires regular calibration and replacement of parts
Selectivity	Adequate for common gases (NO, NO2)	Interference from strong IR absorbers
Stability	Good, temperature-resistant	Gradual detector drift, optical contamination effects
Precision	High for medium/high concentrations	Average at very low concentrations
Validation	Standard procedures, in-house calibration easy	Mixed gas validation requires reference testing
Field usability	Portable versions, easy to use	Requires regular maintenance
Measurement	Direct, fast reading	Dust filtration needed in high-dust environments

and

Table 10. NDUV (non-dispersive ultraviolet).

Parameter	Strengths	Weaknesses
Sensitivity	High, ppb–ppm detection, fast response	May be affected by UV-absorbing species
Cost	Lower than FTIR, portable analyzers available	More expensive than electrochemical sensors
Selectivity	Very good for NO, NO2, can distinguish them	Potential interference under high dust or other UV-absorbing compounds
Stability	Stable across wide temperatures and conditions	Requires stable light sources and regular optics maintenance
Precision	Very high in continuous measurements	–
Validation	Accredited procedures, easy gas standard calibration	Must be validated against potential interferences
Field usability	Small, compact analyzers, fast installation	Sensitive to difficult optical conditions
Measurement	Direct, continuous, online monitoring possible	Periodic optics cleaning required

summarize the strengths and weaknesses of each method in relation to eight selected criteria.

After conducting a SWOT analysis of the groups of methods selected by the authors, analytical methods were chosen as the best for a longer analysis. In this section, a SWOT analysis of the methods belonging to the group of analytical methods (gas chromatography, NDIR, NDUV, FTIR, UV–Vis spectrophotometry) was carried out. The analysis also addressed the subsequent application of the method to measurements in underground mines. Parameters such as measurement range, measurement selectivity, ease of use in the field, and measurement precision were also mainly considered. The SWOT analysis for each of the five methods that make up the analytical methods group is shown below in Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11.

The SWOT analyses presented above apply to instrumental methods such as gas chromatography, infrared methods (FTIR, NDIR), NDUV, and UV–Vis spectrophotometry. Gas chromatography has the highest selectivity and sensitivity but requires significant investment costs, specialized handling, and expensive columns for separating nitrogen compounds. Despite achieving the best results in the SWOT analysis of measurement parameters, due to cost and technical requirements, this method is more suitable for large laboratories.

Infrared methods (FTIR and NDIR) differ in selectivity and measurement range. Determining NO and NO₂ with FTIR can be difficult due to overlapping absorption bands and the need for complex algorithms, while NDIR is more selective and accurate but typically has a smaller measurement range. For detecting low concentrations of NOx, chemiluminescence or UV–Vis spectrophotometry may be a better choice than IR methods. The NDUV method stands out for its ease of use and high sensitivity but is less selective and highly susceptible to interference from air pollutants such as dust, smoke, and water vapor. In addition, sensitivity to temperature changes of the UV lamp can affect the stability of the measurement. UV–Vis spectrophotometry is the most optimal method for measuring NOx in underground mine conditions, combining high sensitivity, versatility, and ease of use with relatively low acquisition and operating costs. It enables the determination of both very low (ppb/ppm) and high concentrations of nitrogen oxides, thanks to different sample preparation procedures and a wide dynamic range of the detector. This method offers high selectivity and precision due to the use of specific colorimetric reactions, such as the Griess reagent, allowing accurate and repeatable measurements even in samples with complex matrices. The method’s stability and ease of validation are ensured by its broad standardization, facilitating calibration and quality control across various conditions. An additional advantage of UV–Vis spectrophotometry is its suitability for field use; compact, automated analyzers adapted for harsh industrial and mining environments are available, enabling rapid and repeatable measurements without the need for highly skilled personnel. With appropriate automation and filtration systems (e.g., scrubbers), the method performs well despite interferences caused by dust, moisture, or other gases. In mine air conditions, the risk of interference is significant due to the complex gas composition; in addition to oxygen and nitrogen, mine air may also contain carbon dioxide, carbon monoxide, methane, and organic and inorganic dust. Some of these components absorb radiation in the UV–Vis region, which can potentially interfere with measurements. For example, carbon monoxide and carbon dioxide have characteristic absorption bands that can partially overlap with the absorption of other gases. Despite this limitation, UV–Vis spectrophotometry is considered the optimal method for monitoring mine air due to its high sensitivity, measurement speed, and suitability for use in challenging field conditions. This allows for effective and safe monitoring of gas concentrations, which is crucial for the safety of mine workers.

NDIR and NDUV methods are well suited for rapid continuous monitoring but require more frequent calibration and are more prone to interference from other gases. In contrast, FTIR and gas chromatography are more laboratory- or reference-based solutions; they are costly, requiring specialized personnel, and they are less mobile.

Table 11. Comparison of NOx measurement methods in terms of sensitivity, selectivity, cost, mobility, and practical application.

Method	Sensitivity	Selectivity	Cost	Mobility	Comment
UV–Vis	High	High	Low/Med	High	Balanced across all criteria.
NDIR/NDUV	Med/High	Good/Very good	Medium	Very high	Good for mobile use, IR/UV-sensitive.
FTIR	Very high	Very high	Very high	Low	Reference method; expensive, less field-suitable.
GC	Very high	Very high	Very high	Very low	Lab use only; not for the field.

presents a comparison of NOx measurement methods in terms of the most important parameters, i.e., sensitivity, selectivity, cost, mobility, and practical application.

4. Conclusions

• The UV–Vis spectrophotometry method was identified, based on a literature review and SWOT analysis, as the most advantageous technique for measuring NOx concentrations under real underground mine conditions. Based on the SWOT assessment, two methods were distinguished: gas chromatography and UV–Vis spectrophotometry. Although gas chromatography offers superior selectivity and sensitivity, its significant capital and operating costs, requirements for specialized personnel, and the need for expensive columns for separation of nitrogen compounds limit its use, especially in smaller laboratories and field measurements. UV–Vis spectrophotometry, on the other hand, combines high sensitivity, a wide measurement range (suitable for both low and high NOx concentrations), simplicity of operation, and relatively low acquisition and maintenance costs, making it a more versatile and practical choice in underground mining environments.
• Accurate knowledge and control of NOx concentration levels are essential to ensure safe working conditions underground and to effectively manage gas hazards. By precisely monitoring emissions from mining machinery along with average NOx levels in the mine atmosphere and taking into account the volume of air flow through underground workings, site managers can reliably assess workers’ exposure to harmful gases. This makes it possible to make timely interventions to reduce risks, such as improving ventilation or limiting the duration of exposure, thereby reducing the incidence of occupational diseases and accidents associated with long-term exposure to toxic atmospheres.
• Real-world measurement data indicate that UV–Vis spectrophotometry enables direct measurement of NO and NO₂ concentrations with high accuracy and stability. These methods rely on the Beer–Lambert law, which reduces the frequency of calibration. For example, Hu et al. [108] applied dual-channel cavity ring-down spectroscopy for simultaneous measurement of NO and NO₂, achieving precision on the order of 1 ppb. Field and laboratory studies [109] demonstrate that UV–Vis performs well in various environments, including low-concentration scenarios, although environmental factors such as humidity, particulates, and temperature fluctuations may affect signal stability. In underground mines, advanced measurement systems such as MONOKS [107] enable monitoring of a wide range of NOx concentrations, from low ambient air levels to high exhaust levels from mining machinery. These systems account for challenging mine conditions, including dust, temperature variations, and humidity, confirming the practical applicability of UV–Vis spectrophotometry in field conditions.
• Future research will focus on practical testing and comparative evaluation of selected measurement methods under actual deep underground mine conditions. The next phase involves the implementation of these methods for measuring NO and NO₂ concentrations in both ambient mine air (characterized by low concentrations) and in mining machinery exhausts (where concentrations are much higher). Special emphasis will be placed on field testing the UV–Vis spectrophotometry method, which has shown the greatest practical potential in theoretical analyses. The study will evaluate the method’s accuracy, stability, ease of use, and robustness under the harsh environmental conditions typical of deep underground mines, including factors such as humidity, the presence of particulates, and temperature fluctuations. The results will enable optimization of measurement procedures and provide practical recommendations for the mining industry in the selection and application of NOx-monitoring techniques.