Proof of Principle of the Lunar Soil Volatile Measuring Instrument on Chang’ e-7: In Situ N Isotopic Analysis of Lunar Soil

Abstract

The nitrogen isotopic compositions of lunar soil have important implications for the sources of lunar volatiles and even the evolution of the moon. At present, the research on the lunar nitrogen isotopic compositions is mainly based on the lunar meteorites and the samples brought back by the Apollo and Luna missions. However, volatiles adsorbed on the surface of the lunar soil may be lost due to changes in temperature and pressure, as well as vibration and shock effects when the sample is returned. At the same time, in the case of low N content in the sample, since N is the main component of the earth’s atmosphere, it is easily affected by the atmosphere during the analysis process. Therefore, in situ nitrogen isotopic analysis of lunar soil on orbit is necessary to avoid the problems mentioned above and is one of the primary science goals for the Lunar Soil Volatile Measuring instrument on Chang’e-7 spacecraft. After the nitrogen purification procedure, the volatiles in lunar soil that are released through single-step or stepped heating techniques diffuse to the quadrupole mass spectrometer to obtain the N contents and isotopic compositions of the lunar soil. This paper introduces the ground test for N isotopic analysis of lunar soil in orbit according to the Lunar Soil Volatile Measuring Instrument. After long-term repeated measurements, the background and CO-corrected Air-STD ¹⁴N/¹⁵N ratio is 268.986 ± 4.310 (1SD, n = 35), and the overall reproducibility of measurements is 1.6%. The accuracy of N isotopic compositions is calculated to be better than 5%, which can distinguish different sources of N components in lunar soil.

1. Introduction

The widely accepted theory for the formation of the moon is the Giant Impact Hypothesis [1,2]. This hypothesis suggested that the moon was formed approximately 4.5 billion years ago from debris ejected into an Earth-orbiting disk by the collision of a Mars-sized planet (Theia) with the early Earth [3]. The huge heat generated during this process and the degassing process in the subsequent magmatic ocean stage would result in a lack of volatile substances on the moon. However, the results of the samples brought back from the Apollo and Luna missions show that the nitrogen content of lunar soils and breccia can reach 10–130 ppm. Moreover, compared with other similar volatile elements (such as C and S), the nitrogen isotopic composition of lunar soils varies greatly. Whether the volatile is released from the different whole rock samples [4,5,6,7] or from the different heating stages of the same sample [8,9], the ¹⁵N/¹⁴N ratio varies more than 300‰, which indicates that the nitrogen reservoir in lunar soil is not entirely intrinsically lunar but rather reflects an extralunar origin [10].

The variations of the N isotopic compositions can reach 500% on a solar system scale (Figure 1) [11]. The Sun and, by inference, the protosolar nebula from which the solar system formed are highly depleted in the heavier ¹⁵N isotope compared with that of Earth’s atmosphere. The inner planets and meteorites have similar or slightly heavier N isotopic compositions than those of Earth, while the comets are obviously enriched in ¹⁵N. Thus, [11] suggests that the protosolar nebula, inner solar system, and cometary materials represent three distinct isotopic reservoirs and that the ¹⁵N enrichment generally increases with distance from the Sun.

Furthermore, the N isotopic signature in a meteorite from the Pesyanoe parent body is -33‰ and not as extreme as the Jupiter δ¹⁵N value, which may suggest that in addition to changes in N isotopic composition based on distance from the accreting sun, changes with time in the solar wind N composition are also a possibility [28]. As for the material accreted by the Earth, Moon, and Mars, the isotopic compositions might be dominated by the volatile-rich enstatite, carbonaceous, and Pesyanoe-like material. So the change in N isotopic composition needed with time is from ~−30‰ to the heavy signatures seen on Earth and Mars atmospheres today. The fact that the 4 Ga old ALH84001 has a δ¹⁵N value similar to ~−33‰ [16] is consistent with this argument. Therefore, the starting N signatures in the inner solar system could as well have been close to ~−33‰.

The huge difference in ¹⁵N/¹⁴N ratios among the reservoirs in the solar system can help us distinguish the sources of components from different N isotopic compositions in the lunar soil, which is of great significance for understanding the source of lunar volatiles and even the evolution of the moon.

There are currently several viewpoints on the source of lunar nitrogen reservoirs (Figure 2).

1.

Implantation of solar wind ions

Although the Moon may have had an early magnetic field [30,31], it has been strongly irradiated by the Sun for billions of years, and a large number of solar wind ions are implanted into the lunar soils. Early studies suggest that the main source of nitrogen in lunar soils is the solar wind (SW). This conclusion derives from such observations as the nitrogen in lunar soils mainly implies a surficial component [32,33,34], a good correlation between nitrogen content and SW ³⁶Ar [5,35], as well as the good correlation between nitrogen isotopic compositions and ⁴⁰Ar/³⁶Ar ratios, which is taken as a proxy for the time a soil was exposed at the lunar surface [4,5,7]. The variations in δ¹⁵N values observed in lunar soils are interpreted to reflect a secular evolution of SW nitrogen [4,5,7]. Recent direct measurements of SW nitrogen implanted into the Genesis Solar Wind Concentrator of SiC target yielded a ¹⁵N/¹⁴N ratio of (2.178 ± 0.024) × 10⁻³ for present-day solar wind, corresponding to δ¹⁵N = −407 ± 7‰ [13].

2.

Implantation of solar energetic particles (SEP)

Early studies suggested that mineral grains on the lunar surface are not only irradiated by the SW but also by more SEP [6]. A typical N release pattern of an ilmenite fraction separated from an Apollo 17 soil showed that the N in the lunar soils consists of at least two components originating in the Sun, as these two components differ in release temperature and therefore probably in implantation energy [6]. The ¹⁵N/¹⁴N ratio of these implanted nitrogens decreases with increasing energy of implantation, which shows that the N derived from SEP is enriched in the light ¹⁴N relative to the N derived from SW [36].

3.

A planetary nitrogen component

However, [37] pointed out that there is no known solar system process that can lead to a 300‰ change in the N isotopic compositions of the solar wind. Meanwhile, increasing evidence suggests that the variations in δ¹⁵N values observed in lunar soils are the result of varying degrees of mixing between a heavy ‘planetary’ N component and a light solar wind N component [38,39,40,41,42,43].

Detailed ion-probe measurement of the N and H isotopic compositions through the outer rims of lunar soil grains shows that samples with δ¹⁵N values as low as −250‰ together with δD values down to −900‰ indicate the presence of pure SW H, highly depleted in D due to D-burning in the Sun. However, the ¹⁵N-enriched grains have δD values as high as +600‰, highlighting the contribution of a nonsolar (planetary) component [39]. Ref. [42] further limited the δ¹⁵N value of this ‘planetary’ N component to from +100‰ to +160‰ through binary mixing calculations. Accordingly, the potential sources of this ¹⁵N-enriched planetary N component include primitive carbonaceous chondrites, micrometeorites, interplanetary dust particles (IDPs), and cometary materials [40,42,43]. During the late heavy bombardment period of the lunar formation, the micro-impactors of these chondritic and cometary materials represented the major contributors of planetary N components to the lunar surface [40,42,43].

4.

Implantation of Earth wind (EW) ions

Later, [10] pointed out that the putative planetary component assumed by [40] is not well constrained other than isotopically, and the N and H isotopic compositions of this planetary component are close to those of the terrestrial atmosphere, so that the mixing trend between SW and the putative planetary component can also be attributed to mixing between SW and the terrestrial component. They suggested that when the earth had no essential geomagnetic field, the terrestrial atmospheric gases would escape and implant into lunar soil grains. A recent study has also shown that Earth’s atmospheric ions have been implanting into both farside and nearside lunar soil since Earth began to have a dipolar magnetosphere, and the lunar soil could continuously preserve Earth’s atmospheric ions from 3.5 Ga to present [44].

5.

Indigenous lunar nitrogen

By analyzing the volatiles released by stepwise heating of a recently exposed (~2 Ma) lunar rock, [45] proposed that there exists an indigenous N component. In contrast to lunar soils, lunar rocks with short exposure times usually contain 1 ppm or less N [45,46,47,48,49,50,51]. At such low concentrations, the analyses of N isotopic compositions are easily affected by terrestrial atmospheric contamination. In addition, any contribution of N from SW radiation injection [43,52] and cosmogenic ¹⁵N produced in situ [53,54] will also mask the indigenous N isotopic composition. Previous studies have shown that the δ¹⁵N values of the indigenous lunar nitrogen component range from −46 to +27‰ [49,50,55,56,57], which encompasses the range of isotope signatures recorded by Earth’s primordial mantle (δ¹⁵N ≤ −40 to −5‰) [58], enstatite chondrites (δ¹⁵N = −30 ± 10‰) [59], and carbonaceous chondrites (δ¹⁵N = +20 ± 20‰) [20,60].

6.

Cosmogenic nitrogen

By analyzing the volatiles released by stepwise heating of lunar soils, it was found that the nitrogen released at high temperature steps (≥1200 °C) was significantly enriched in ¹⁵N [4,8,41,50]. This ¹⁵N enrichment has been attributed to the production of cosmogenic ¹⁵N through the spallation reactions of ¹⁶O (p, 2p)¹⁵N and ¹⁶O (p, pn)¹⁵O [53,54], which may explain some high δ¹⁵N values but cannot explain the significantly light N isotopic compositions of lunar soils. The content of cosmogenic ¹⁵N of lunar samples can be estimated if the shielding depths and irradiation are known, so that this component can be corrected accordingly [43].

The above-mentioned studies of the lunar N isotopic compositions are mainly based on the lunar meteorites and the samples brought back by the Apollo and Luna missions. However, the volatiles adsorbed on the surface of the lunar soil may be lost due to changes in temperature and pressure, as well as vibration and shock effects when the sample is returned. At the same time, in the case of low N content in the sample, since N is the main component of the earth’s atmosphere, it is easily affected by the atmosphere during the analysis process. Therefore, in situ N isotopic analysis of lunar soils on orbit is necessary to obtain the lunar N isotopic compositions accurately. In situ N isotopic analysis of lunar soils on orbit can help us to clarify the influence of the terrestrial atmosphere on the δ¹⁵N values of lunar soil and characterize the sources of N and other volatiles delivered to the lunar surface, which is crucial for a broader consideration of how N is transported around the solar systems and how it is added to the surface and atmosphere of other rocky objects in the solar system after initial planetary formation.

2. In Situ N Isotopic Analysis on Orbit

2.1. Development Status of In Situ N Isotopic Analysis on Orbit

Viking 2, launched by the National Aeronautics and Space Administration (NASA), landed on Mars in 1976 (

Table 1. Overview of spacecrafts related to nitrogen isotopic compositions.

	Viking Lander	Galileo Probe Mass Spectrometer (GPMS)	Rosetta Orbiter Spectrometer for Ion and Neutral Analysis Mass Spectrometer (ROSINA)	The Sample Analysis at Mars (SAM)
	NASA	NASA	ESA	NASA
Research object	Martian atmosphere	Jovian atmosphere	Jupiter family comet 67P/Churyumov-Gerasimenko	Martian atmosphere
Type of mass spectrometer	Double-focusing mass spectrometer	Quadrupole Mass spectrometer	Double-focusing mass spectrometer	Quadrupole Mass spectrometer
Measurement of nitrogen isotopic compositions	Remove the interfering substances, CO and CO2, by a chemical filter, which employs silver oxide to oxidize CO to CO2, lithium hydroxide-oxide mixture to absorb all the CO2, and magnesium perchlorate to remove H2O.	Obtain from the doubly charged ratio of 15NH3++/14NH3++ at 9 and 8.5 amu. The contribution of 14NDH2++ at 9 amu will be negligible, as will the signal from H2O++.	The mass spectrometer has a high mass resolution of m/Δm about 3000 at the 1% level at atmoic mass per unit of charge 28 u/e, allowing the separation of N2 from CO (Δm = 0.011 u) by numerical peak fitting.	Derive 14N/15N from direct atmospheric experiments using the m/z 14/14.5 count ratio. In enrichment experiments, chemical scrubbers remove CO2, H2O, and other species with chemical affinity to the scrubber material.
Mass range	12–200 amu	2–150 amu	from 1 amu to >300 amu	2–535 amu
Result	14N/15N ratio of 168 ± 17	15N/14N ratio of (2.3 ± 0.3) × 10−3	N2/CO ratio of (5.70 ± 0.66) × 10−3	14N/15N ratio of 173 ± 11

). It is equipped with three furnaces, in which the samples will simply be expelled by heating to 500 °C. The released volatiles can be separated by a gas chromatograph or directly entered into the mass spectrometer for N isotopic composition analysis [61]. For the interference of CO⁺ (from CO or CO₂) with N₂⁺ at m/e = 28 and 29 during nitrogen measurements, a separate mass spectrometeric analysis is performed, from which CO and CO₂ have been removed by a chemical filter. The filter employs Ag₂O to oxidize all CO to CO₂, LiOH to adsorb CO₂, and Mg (ClO₄)₂ to remove H₂O. This mission confirmed the presence of nitrogen in the Martian atmosphere, and a ¹⁴N/¹⁵N ratio of 168 ± 17 for the Martian atmosphere was determined [62].

As the first spacecraft dedicated to Jupiter launched by NASA, the Galileo Probe Mass Spectrometer (GPMS) arrived at Jupiter in 1995 (Table 1). This spacecraft is equipped with a quadrupole mass spectrometer to detect the isotopic composition of the Jovian atmosphere. Considering that the nitrogen isotope measurements are difficult to obtain from the singly charged ratio of ¹⁵NH₃⁺/¹⁴NH₃⁺ at m/e = 18 and 17 because the contribution of water cannot easily be independently constrained, the ¹⁵N/¹⁴N ratio was established from the doubly charged ¹⁵NH₃⁺⁺/¹⁴NH₃⁺⁺ signals at m/e = 9 and 8.5 because the contributions of ¹⁴NDH₂⁺⁺ and H₂O⁺⁺ at m/e = 9 and 8.5 are negligible, yielding ¹⁵N/¹⁴N = (2.3 ± 0.3) × 10⁻³ [24,63].

The primary measurement objective of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer (ROSINA), launched by the European Space Agency (ESA) in 2004, is to determine the elemental, isotopic, and molecular composition of the atmosphere and ionosphere of comets (Table 1). The Double Focusing Mass Spectrometer (DFMS) it used has very high mass resolution (m/Δm ≈ 3000), allowing the separation of CO from N₂ [64]. A direct in situ measurement of N₂ in the comet 67P/Churyumov-Gerasimenko was made by this spectrometer, and a N₂/CO ratio of (5.70 ± 0.66) × 10⁻³ was obtained [65].

The Sample Analysis at Mars (SAM) investigation of the Mars Curiosity Rover launched by NASA in 2011 is designed to find chemical and mineral evidence of past habitable environments on Mars (Table 1). Two ovens were designed to bring the solid sample to a maximum temperature of 900–1100 °C to release volatiles for N isotopic composition analysis by a quadrupole mass spectrometer [66]. Since there is no CO⁺⁺ contribution at m/e = 14, an approach similar to [24] was used to derive the ¹⁴N/¹⁵N ratio from direct atmospheric experiments using the count ratios of m/e = 14 and 14.5. While in enrichment experiments, the interfering substances, CO₂, H₂O, and other species, were removed by chemical scrubbers [18]. This spacecraft measured a Mars atmospheric ¹⁴N/¹⁵N ratio of 173 ± 11 [18].

So far, there has been no relevant work on the in situ N isotopic analysis or even the volatile analysis of lunar soil on orbit. However, Chang’ e-7, which is scheduled to launch in 2026, will carry the Soil Volatile Measuring Instrument to analyze nitrogen isotopes in situ.

2.2. Process Design of In Situ N Isotopic Analysis on Orbit

Characterizing the sources of nitrogen delivered to the lunar surface is one of the three primary science goals of the Lunar Soil Volatile Measuring Instrument on the Chang’ e-7 spacecraft of China’s fourth lunar exploration phase. A direct in situ N isotopic analysis of lunar soil made using this instrument can help to clarify the influence of the terrestrial atmosphere on the δ¹⁵N values of lunar soil and to investigate the sources of N in the lunar soil. The schematic diagram of the in situ N isotopic analysis on orbit is shown in Figure 3.

Based on the environment and conditions of lunar polar regions, a volatile extraction method coupling sampling and volatile extraction was proposed by the Lunar Soil Volatile Measuring Instrument [67]. A kind of microquantitative sampler was designed with some sampling pieces assembled in a sampling tube to complete lunar soil sampling. After sampling, the sample was transferred to the volatile preparation unit, which can heat the sample through induction heating. The volatiles in lunar soil are converted into gases and released according to different temperatures [68].

N isotopic compositions are usually obtained by measuring the signal of N₂ at mass numbers of 28 (¹⁴N¹⁴N) and 29 (¹⁴N¹⁵N). The major interferences at masses 28 and 29 are CO and hydrocarbons. However, it is difficult to separate these compounds from N₂ during the process of N isotope analysis because the mass resolution of the quadrupole mass spectrometer is too low. To solve this problem, a separation and purification technique is needed to separate N₂. At first, the gases reacted with pure O₂ produced by heating the CuO_(1−x) (where x ranges from 0 to 0.5) at 850 °C. During this time, carbonaceous gases, such as CO, CH₄ and other hydrocarbons, are oxidized to CO₂, hydrogen is converted to H₂O, and sulfur is converted to SO₂. The condensable gas formed by these reactions can be adsorbed in a glass tube used for a cold trap held at a liquid-nitrogen temperature, thus separating it from N₂ [69,70,71,72]. Previous studies have demonstrated that the purification procedures mentioned above are capable of totally removing the interferences [69].

On the basis of the above, the process of in situ N isotopic analysis on orbit by the Lunar Soil Volatile Measuring Instrument is as follows: A small amount of CuO_(1−x) powder can be pre-loaded in the microquantitative sampler. After sampling, the lunar soil will be transferred to the volatile preparation unit, which will heat the sample through induction heating. When the heating temperature reaches 850 °C, the CuO_(1−x) powder loaded in the microquantitative sampler will release large amounts of O₂, which will react with the volatiles released by the lunar soil for 10 min to promote the oxidization of carbonaceous gases to CO₂, hydrogen to H₂O, and sulfur to SO₂. The gas formed after the oxidation reaction is adsorbed by the getter and separated from N₂. Then the purified nitrogen diffuses through the pipeline to the mass spectrometer by opening Valve 1 for the N isotope analysis.

3. Devices and Procedures for the Ground Verification Test

In order to verify the possibility of in situ N isotopic analysis on orbit, a high vacuum line used for nitrogen extraction, purification, separation, and analysis was built for the ground verification test (Figure 4).

3.1. Test Devices

The vacuum line used for the ground verification test is basically separated into four parts by all-metal valves (Va, Vb, Vc, and Vd), including two gas inlet sections, a gas purification section, and a gas measurement section (Figure 4).

The nitrogen extraction and purification vacuum lines are basically made of stainless steel. A Pyrex glass tube is used for cold trap as the stainless steel will adsorb a large amount of N at liquid-nitrogen temperatures. Quartz glass tubes are used for the CuO_(1−x) finger and rock sample finger, considering the operating temperatures of 850 °C or higher.

Among them, the rock sample furnace and the CuO_(1−x) furnace in the ground verification test system correspond to the volatile extraction and N₂ purification systems in the vacuum line of the Lunar Soil Volatile Measuring Instrument. Valve Vc corresponds to Valve 1 in the on-orbit analysis system. The gas inlet sections and gas purification section are evacuated by a turbo molecular pump (TP) and a rotary pump down to (1–4) × 10⁻⁷ Pa, while the gas measurement section is evacuated by an ion pump (IP) down to (3–6) × 10⁻⁷ Pa to simulate the vacuum conditions of extra vehicular in the on-orbit analysis.

3.1.1. Gas Inlet Sections

Stepped heating analyses of lunar soils have consistently revealed a similar release profile with low temperature, ¹⁵N-enriched nitrogen preceding a mid-temperature, ¹⁵N-depleted nitrogen, which is then followed at the highest temperature step by a second ¹⁵N-enriched nitrogen [8,41]. Therefore, in order to characterize the different sources of N in the lunar soil, the solid rock samples can be heated stepwise or totally by loading them into a detachable high-vacuum quartz glass tube placed in a heating device to release the volatiles (gas inlet Section 1). The mass of the rock samples, heating temperature, and heating time can be adjusted according to the rock type and total volatile content of the sample.

The heating devices used to heat the CuO_(1−x) finger and rock samples are designed as a cylindrical tube furnace, which is heated by alloy wire resistance. It consists of a cylindrical chamber surrounded by the Pt-Rh alloy resistance with a melting point higher than 1700 °C that is embedded in the spiral grooves of a thermally insulating matrix, which can avoid the deformation of the resistance wire due to the stress. The chamber is made of high-temperature ceramics and two semi-cylindrical elements brought together to form a cavity. The S-type thermocouple is set on the side wall of the cylindrical chamber so that the temperature of the heating device can be monitored and controlled by an Al-518P temperature controller. A thermal insulation structure is designed outside the cylindrical chamber to improve the heating efficiency. The maximum working temperature of the heating device is 1200 °C, and the control accuracy is within 0.1 °C. The temperature in the cylindrical chamber can be raised from room temperature (~23.5 °C) to 500 °C within 1 min.

The gas sample and nitrogen standard (Air-STD) are reserved in a large stainless steel reservoir with a capacity of 5L (gas inlet Section 2). The gas is introduced into the gas purification section by using a pipette valve (0.5 mL), which can adjust the amount of gas.

3.1.2. Gas Purification Section

The nitrogen purification and separation vacuum lines used in this ground verification test is mainly composed of a CuO_(1−x) furnace, valves, pipelines, and cold trap.

The CuO_(1−x) furnace consists of a detachable high-vacuum quartz glass tube filled with 1 g CuO_(1−x) particles. The glass tube is placed in the heating device, as mentioned in Section 3.1.1. CuO_(1−x) is used for the production, transfer, and resorption of oxygen. When first introduced into the vacuum system, copper oxide is stoichiometric. As the oxidation proceeds, CuO_(1−x) will gradually transform into Cu and cannot produce enough O₂. In this case, CuO_(1−x) in the Pyrex glass tube needs to be refilled. Anytime a new CuO_(1−x) furnace is installed, it must be purified by extensive cycling between 850 and 450 °C with non-resorbed gases being pumped away [69].

3.1.3. Gas Measurement Section

The purified gases are introduced into the quadrupole mass spectrometer (QMS) through a 59.0 UHV all-metal variable leak valve produced by the VAT company by opening Vc, and the N isotopic compositions and contents are analyzed in dynamic mode. The leak valve used in the ground verification test allows the precise control of very small gas flows from 1 × 10⁻¹⁰ mbar·m³/s to 500 mbar·m³/s. By adjusting the manual actuator of the leak valve, the pressure of the gases introduced into the QMS can be controlled between 4 × 10⁻⁸ Pa and 0.01 Pa in order to meet the relevant requirements of the Lunar Soil Volatile Measuring Instrument for mass spectrometer measurement.

The QMG700 Analytical Mass Spectrometer used for N isotope and content analysis in the ground verification test is a high-end quadrupole mass spectrometer produced by Pfeiffer Company, corresponding to the mass spectrometer used in the on-orbit analysis, which can realize online analysis for isotopic compositions and contents of volatiles. The QMG700 is equipped with a Faraday collector. The sensor length is 441.5 mm. The rod diameter is 8 mm, and the rod material is Mo. More than nine decades of dynamic range and ppb detection limits make it the ideal instrument for ultra-high vacuum measurements. It provides unit resolution across its entire mass range from 1–512 amu. The maximum operating pressure is 1 × 10⁻⁴ mbar, while the minimum detectable partial pressure is 1 × 10⁻¹⁵ mbar. The resolution is 0.3–7 amu (adjustable), and the measurement speed is between 0.125 ms and 60 s/amu (adjustable). The relevant parameters meet the requirements of the Lunar Soil Volatile Measuring Instrument for mass spectrometer measurement.

The data analysis system is Pfeiffer Vacuum’s QUADERA mass spectrometer software (version 4.62), which is installed with different Measurement Modes. In the ground verification test, the commonly used modes are Scan Measurement (in a scan-analog measurement, the ion current is measured as a function of m/z) and Multiple Ion Detection Measurement (MID; in this mode, only the ion current of the specified mass number is scanned, and the changes in the signal of the specified mass number with time can be easily recorded and displayed).

3.2. Test Procedural

Before the whole ground verification test, the whole vacuum lines used for nitrogen extraction, purification, separation, and analysis need to be baked at 120 °C for 2–3 days and evacuated by TP by opening v4 in order to remove the N adsorbed in the vacuum lines. During this process, IP needs to remain closed. Before the experiment, the vacuum lines are cooled down to room temperature. Then, Vc and Vd were closed, and the whole vacuum line was evacuated by TP and IP down to 10⁻⁷ Pa.

When the devices are in standby mode, keep Vc, Vd, and V4 open. The gas inlet sections and gas purification section are evacuated by TP and a rotary pump, while the gas measurement section is evacuated by IP.

3.2.1. Nitrogen Extraction

The Air-STD is housed in a 5 L electro-polished stainless cylinder, which can be introduced into the gas purification section using a pneumatically actuated pipette valve with an internal volume of 0.5 mL.

The detailed steps for nitrogen extraction are as follows:

(1)

Open v5 so that the air in the stainless cylinder can diffuse into valve v6. Maintain for 2 min until the diffused gas reaches an equilibrium state;

(2)

Close v5 and open v6 to make the gas between valves v5 and v6 diffuse into the gas purification section. Maintain for 2 min; and

(3)

Close v6.

The N₂ contents entering the gas measurement section can be changed and calculated by multiple diffusions and pumpings of the gas in the gas purification section, as well as by entering different-sized aliquots of Air-STD.

3.2.2. Nitrogen Purification

The detailed steps for nitrogen purification are as follows:

(1)

Once the gases from the gas inlet Section 2 have been introduced to the gas purification section, v1 is closed so that part of the gas is introduced into the CuO_(1−x) furnace while the remains are directly introduced into the CT.

(2)

Open v3 and adsorb CO₂, H₂O, and other condensable gases for 10 min at liquid-nitrogen temperature. This step can be used to evaluate the effect of the nitrogen purification section.

(3)

Open Vc and then introduce the gas into the gas measurement section for nitrogen analysis.

(4)

Simultaneously with steps (2) and (3), the gases in the CuO_(1−x) furnace are reacted for 10 min with pure O₂, produced by heating the CuO_(1−x) particle at 850 °C. During this process, carbonaceous gases, such as CO, CH_4, and other hydrocarbons, are oxidized to CO₂, hydrogen is oxidized to H₂O, and sulfur is oxidized to SO₂.

(5)

After 10 min, excess O₂ needs to be reabsorbed back onto the CuO_(1−x) particle, first at 600 °C for 15 min and then at 450 °C for 15 min, since in the mass spectrometer oxygen will react with carbon on the filament [73] so that the result will be the same as if the nitrogen is contaminated by CO.

(6)

The oxidized gases can then be introduced into the CT by opening v1 to let the condensable gases adsorbed in a glass tube CT held at liquid-nitrogen temperature for 10 min.

(7)

The purified nitrogen is then admitted to the mass spectrometer through a leak valve by opening Valve C for the N isotope analysis.

Previous studies have demonstrated that this purification procedure is totally effective in removing O₂, CO, CH_4, and CO₂ from N₂ and is also effective in decomposing N₂O to the elements [69].

3.2.3. Nitrogen Isotope Analysis

The N isotopic compositions of the sample are obtained by repeatedly measuring the m/z = 28 (¹⁴N¹⁴N) and 29 (¹⁴N¹⁵N) on the single Faraday collector. During the analysis, the mass resolution was 1 amu. The measurement speed was 200 ms/amu. The working current of the ion source is 2.0 mA.

In addition to m/z = 28 (¹⁴N¹⁴N) and 29 (¹⁴N¹⁵N), the major interference of CO is monitored at m/z = 30 (¹²C¹⁸O) to check m/z = 28 and 29 for ¹²C¹⁶O and ¹³C¹⁶O. The ¹⁵N¹⁵N at m/z = 30 is so rare as to produce only a small signal below the background level. Therefore, the signal of ¹⁵N¹⁵N at m/z = 30 can be ignored. At the same time, the signals of m/z = 2, 12, 16, and 44 were monitored. Hydrogen is monitored because excess H will react with N to form N₂H, which can affect the signal at m/z = 29. The signals at m/z = 12, 16, and 44 are monitored to check the interference of CO and hydrocarbons at m/z = 28 and 29. In order to easily observe the changes in the signal of the mass number mentioned above with time, the MID mode in the data analysis system was used.

In order to obtain a more accurate ion current signal, the average of the 10 data points after the gases in the QMS reach an equilibrium state is chosen as the final signal. It takes about 2 min to scan all of the peaks.

4. Standards, Blanks, and Corrections

4.1. Blank Contribution and Correction

Since lunar soils contain 10–100 ppm N while lunar rocks with short exposure times contain 1 ppm or less N [45,46,47,48,49,50,51], the procedural blanks generated by the whole vacuum line used in the ground verification test may represent a significant contribution to nitrogen-poor samples. Thus, it is critical to characterize blanks as accurately as possible, and a test of a procedural stepped-heating blank was carried out. The results obtained are shown in Figure 5. As shown in Figure 5, in the case of stepped heating, the ion current signal of the gases without purification in the CuO_(1−x) furnace at m/z = 28 and 29 has a significant increase at temperatures above 800 °C, while the ion current signal of the gases purified in the CuO_(1−x) furnace at m/z = 29 is basically consistent with the background level at room temperature. Although the ion current signal of the purified gases at m/z = 28 has a little bit of an increase at high temperature steps, it is not significantly different from the background level at room temperature.

The minimum acceptable level for the blank depends on the sample being analyzed. For 200 mg of lunar soil with an N content of 10 ppm, the ion current signal of the Faraday cup at m/z = 28 will reach 10⁻⁸ A (according to the sensitivity of the Faraday cup at m/z = 28 calculated in 4.2) when all the volatiles are released by total combustion. The current signal at m/z = 28 generated by the volatiles released at each temperature step should also reach 10⁻¹¹ A, which is significantly higher than the procedural blank.

To this end, our analytical procedure includes a procedural blank run prior to each individual sample run. The ¹⁴N¹⁴N/¹⁴N¹⁵N ratio of the measured samples (including Air-STD) can be simply corrected by the following equation:

$${\left({}^{14}N{}^{14}N/{}^{14}N{}^{15}N\right)}_{corrected}=\frac{{\left({}^{14}N{}^{14}N\right)}_{sample}-{\left({}^{14}N{}^{14}N\right)}_{blank}}{{\left({}^{14}N{}^{15}N\right)}_{sample}-{\left({}^{14}N{}^{15}N\right)}_{blank}}$$

(1)

4.2. Air Standards

For expressing the N isotopic composition, the stable isotope delta notation is used as follows:

$$\delta {}^{15}N=[\frac{{({}^{15}N/{}^{14}N)}_{sample}}{{({}^{15}N/{}^{14}N)}_{air}}-1]\times 1000$$

(2)

where δ¹⁵N expresses the deviation of the ¹⁵N/¹⁴N ratio of the sample relative to that of the standard in parts per thousand (‰). Air is the most commonly used standard of N isotopic composition (¹⁵N/¹⁴N = 3.676 × 10⁻³, δ¹⁵N = 0‰) [12]. Thus, in order to verify the possibility of in situ N isotopic analysis on orbit and to check the analytical precision of N isotopic compositions measured by QMS, the isotopic ratio of Air-STD was measured repeatedly according to the procedure mentioned in 3.2. The ¹⁵N/¹⁴N ratio was calculated by the measured ¹⁴N¹⁴N/¹⁴N¹⁵N ratio.

The Air-STD is housed in a 5L electro-polished stainless cylinder and is introduced into the gas purification section. After a series of decompression diffusion processes, the partial pressure of the purified N₂ in the vacuum line is between 0.1 and 1 Pa, and the amount of N₂ is between 7.3 × 10⁻⁹–6.9 × 10⁻⁸ mol before passing through the leak valve.

The partial pressure of the measured gas in the vacuum line before passing through the leak valve is defined as P1. These gases are introduced into the mass spectrometer through the leak valve, and the partial pressure of the gas in the mass spectrometer chamber is defined as P2. In addition, the inlet flow conductance of the mass spectrometer is defined as U1, and the exhaust flow conductance through the ion pump is defined as U2. As the gas entering the mass spectrometer chamber through the leak valve is very small relative to the gases in the vacuum line in the process of air pressure equilibrium between the two, it can be approximated that the partial pressure P1 is constant.

P₁U₁ = P₂U₂

(3)

where the inlet flow conductance of the mass spectrometer U₁ = 1.04 × 10⁻³ mL/s, while the exhaust flow conductance through the ion pump U₂ = 106 mL/s. Thus, the partial pressure of the purified N₂ in the mass spectrometer chamber is between 9.8 × 10⁻⁷–9.8 × 10⁻⁶ Pa.

According to the working principle of the mass spectrometer, the correlation coefficient between the partial pressure of the purified N₂ in the mass spectrometer chamber (P₂) and the ion current signal measured by the mass spectrometer (I) is defined as the sensitivity. The sensitivity of the Faraday collector at ¹⁴N¹⁴N is about 3 × 10⁻⁴ A/Pa.

As part of the initial test of the system, we repeatedly measured the ¹⁴N¹⁴N/¹⁴N¹⁵N ratio of Air-STD. The results after CO and blank correction are shown in

Table 2. Repeated measurements of N isotopic compositions of Air-STD.

Date	Run Number	m/z = 28 Ion Current (A)	14N14N/14N15N Blank Corrected	14N14N/14N15N CO and Blank Corrected	14N/15N Corrected	δ15N Corrected (‰)	Blank Contribution on Mass 28 (%)
31 October 2022	1	5.61 × 10−10	133.103	135.654	271.308	2.7	0.32
	2	5.57 × 10−10	133.596	138.524	277.048	−18.1	0.32
1 November 2022	1	5.54 × 10−10	135.318	137.773	275.546	−12.7	0.31
2 November 2022	1	6.46 × 10−10	133.719	135.050	270.099	7.2	0.27
3 November 2022	1	7.36 × 10−10	134.882	136.337	272.673	−2.3	0.28
	2	1.41 × 10−9	133.393	133.798	267.596	16.6	0.16
4 November 2022	1	1.43 × 10−9	135.550	136.342	272.684	-2.4	0.15
	2	2.47 × 10−9	132.353	135.314	270.628	5.2	0.08
7 November 2022	1	1.12 × 10−9	128.589	138.649	277.299	−19.0	0.12
	2	1.12 × 10−9	134.518	135.632	271.264	2.8	0.12
9 November 2022	1	1.10 × 10−9	133.795	134.364	268.728	12.3	0.20
	2	1.14 × 10−9	135.243	135.960	271.921	0.4	0.16
10 November 2022	1	1.17 × 10−9	135.208	137.012	274.024	−7.3	0.13
14 November 2022	1	1.17 × 10−9	133.583	133.781	267.561	16.7	0.11
15 November 2022	1	1.15 × 10−9	135.558	135.139	270.278	6.5	0.11
	2	1.21 × 10−9	134.442	134.342	268.684	12.5	0.12
16 November 2022	1	1.70 × 10−9	135.002	135.153	270.306	6.4	0.11
17 November 2022	1	1.18 × 10−9	136.147	136.026	272.053	-0.1	0.11
6 February 2023	1	9.52 × 10−10	130.584	130.990	261.979	38.4	0.23
7 February 2023	1	5.08 × 10−10	123.434	130.040	260.080	46.0	0.17
	2	1.00 × 10−9	131.562	131.766	263.533	32.3	0.22
2 March 2023	1	4.84 × 10−10	127.530	131.564	263.128	33.8	1.01
	2	2.85 × 10−10	128.692	131.286	262.571	36.0	1.34
6 March 2023	1	3.49 × 10−10	131.492	131.100	262.201	37.5	0.94
6 April 2023	1	3.46 × 10−10	132.830	132.429	264.858	27.1	0.56
7 April 2023	1	3.43 × 10−10	132.010	132.224	264.448	28.7	0.60
14 April 2023	1	3.24 × 10−10	135.831	135.409	270.819	4.5	0.42
17 April 2023	1	3.25 × 10−10	135.401	134.983	269.966	7.7	0.68
18 April 2023	1	2.96 × 10−10	136.247	135.822	271.645	1.4	0.55
	2	3.06 × 10−10	136.278	135.854	271.707	1.2	0.45
21 April 2023	1	3.10 × 10−10	135.092	134.675	269.351	10.0	0.79
5 May 2023	1	3.06 × 10−10	132.941	132.539	265.078	26.2	2.80
9 May 2023	1	3.01 × 10−10	135.335	134.917	269.834	8.2	0.50
19 May 2023	1	3.07 × 10−10	133.922	133.513	267.026	18.8	0.66
23 May 2023	1	2.88 × 10−10	133.707	133.300	266.600	20.4	0.44
mean			133.340	134.493	268.986	11.6

, Figure 6 and Figure 7. It can be seen from the results that the blank contribution of the Air-STD at m/z = 28 is generally within 1%, and the ¹⁴N/¹⁵N ratio without CO correction is 266.679 ± 5.588 (1SD, n = 35; Figure 6). The overall reproducibility of multiple measurements is 2.1%. The ¹⁴N/¹⁵N ratio after CO correction [72] was 268.986 ± 4.310 (1SD, n = 35; Figure 7), and the overall reproducibility of multiple measurements was 1.6%. Notably, the accuracy of the ¹⁴N/¹⁵N ratios of CO-corrected air-STDs is considerably higher. Together with the improved reproducibility, the necessity of CO interference corrections when making high-precision nitrogen isotope analyses is demonstrated. Compared with the canonical δ¹⁵N value (0‰) [12] of Air-STD, the accuracy of N isotopic compositions obtained in this ground verification test is generally better than 5%, which can distinguish different sources of nitrogen in lunar soils.

To test for the mass discrimination effect, different amounts of N₂ were inlet and measured by adjusting the manual actuator of the leak valve or by inleting different-sized aliquots of the Air-STD. By linear regression fitting of the data, we obtained that the R² of the linear regression line between the ion current signal at m/z = 28 and the ¹⁴N/¹⁵N ratio is 0.0709 (Figure 8), indicating that there is no obvious linear relationship between the two; that is, any pressure-induced discrimination effects in the mass spectrometer can be ignored over the ion current signals at m/z = 28 measured in this study.

5. Conclusions

One of the primary science goals for the Lunar Soil Volatile Measuring instrument on Chang’e-7 spacecraft is to characterize the source(s) of nitrogen being delivered to the lunar surface. In order to clarify the influence of the terrestrial atmosphere on the δ¹⁵N values of lunar soils and determine the N isotopic compositions of lunar soils, it is proposed by the Lunar Soil Volatile Measuring instrument to remove the interfering substances of nitrogen released by the stepped-heating of lunar soils through a microquantitative sampler pre-loaded with CuO_(1−x) powder, and the N isotopic compositions of lunar soils are determined by the quadrupole mass spectrometer. After a long period of repeated measurements, the ¹⁴N/¹⁵N ratios of Air-STD obtained in the ground verification test after blank and CO correction are 268.986 ± 4.310 (1SD, n = 35). The overall external reproducibility of multiple measurements is 1.6%, and the accuracy of the N isotopic compositions test is generally better than 5%, which is sufficient to distinguish different sources of nitrogen in lunar soils.