Tropical Weathering Effects on Neat Gasoline: An Analytical Study of Volatile Organic Profiles

Abstract

Gasoline is the most common ignitable liquid used to initiate fires, making its detection and identification in fire debris crucial for determining incendiary origins. Fire debris is typically collected after extinguishment and safety clearance, often resulting in gasoline weathering, especially when delayed. Most research on gasoline weathering has been conducted in controlled laboratory settings in temperate climates. However, the effects of tropical conditions on the rate of gasoline weathering and the resulting chemical composition of volatiles remain largely unexplored. Understanding how tropical environmental factors alter gasoline weathering is essential for accurate fire debris interpretation in such regions. This study investigates how tropical climates impact gasoline weathering indoors and outdoors. Weathered samples were prepared by volume reduction method, gradually evaporating gasoline from 10% to 95%. Indoor samples were exposed to room temperature, while outdoor samples were left in open space under natural tropical conditions. Gas Chromatography/Mass Spectrometry (GC-MS) analysis revealed chromatographic shifts in heavier compounds (C₃–C₄ alkylbenzenes) compared to lighter ones like toluene as weathering progressed. Correlation between indoor and outdoor samples was high (>0.970) at 10–50% weathering but declined (<0.600) at 90–95%, indicating differing patterns. All target compounds remained detectable across all samples.

1. Introduction

Arson is a legal term used to describe the intentional or reckless setting of fire to destroy or damage a property with malicious intent [1]. The aftermath of arson is usually catastrophic, affecting not only physical property but also potentially leading to the loss of innocent life. According to the report by Comité Technique International de Prévention et d’Extinction du Feu (CTIF) World Fire statistics for the years 2021–2022, arson accounted for 64,818 fires out of a total of 923,515 fires reported [2]. The United States Bomb Data Center reported 6,465 arson-related incidents in 2021, which accounted for approximately 28% of all fires reported. Out of the 74 fatalities linked to “Incendiary” incidents, the majority were classified as “Victims,” accounting for 84 percent of the total population [3]. Noumeur et al. (2024) in his research on fire incidents and risk analysis in Malaysia found that between 2000 and 2019, arson was the predominant cause of fires throughout the country. The severe disruption caused by arson to the lives of victims necessitates meticulous fire investigations to ensure that justice is served. By understanding the predominant causes of fires and implementing effective preventive measures, a significant reduction in deliberate fire-like arson could protect communities from its devastating effects [4].

In forensic fire, physical evidence discovered at the scene is crucial for the determination of fire origin and cause [5]. A fire occurs under the circumstances of bringing together fuel, ignition source, and oxidizer, resulting in combustion. The presence of these factors is crucial in confirming the cause of fire. Past cases have occurred where fire investigators declared a fire was intentionally set when no accidental cause of the fire was found [6]. This type of declaration is referred to as a “negative corpus” determination and relies on a lack of evidence of one of the factors [7]. According to the National Fire Protection Association (NFPA) 921 (2021 Edition, Section 18.6.5), this approach is inconsistent with the scientific method and should not be used, as it generates untestable hypotheses and may lead to incorrect conclusions in fire investigations [8]. This further reinforces the necessity for fire investigations to be conducted with methodological rigor, ensuring that the collection and interpretation of evidence are grounded in scientific principles. Such an approach is crucial to support reliable conclusions, particularly in cases where arson is suspected.

Deliberate settings of fire usually involve the usage of ignitable liquid as accelerant to intensify the rate of fire [9]. In most fire incidences, the presence of an ignitable liquid strongly suggests a fire of suspicious cause [10]. However, the presence of ignitable liquid residues in a sample from a fire scene is not enough to confirm incendiary fire or arson. Hence, it is important to look out for evidence that can support the actual cause of the fire.

Based on statistics, most arson cases involve the usage of ignitable liquid accelerants such as gasoline, diesel, or high-concentration alcohol [11]. Among the well-known ignitable liquids, gasoline is the most used accelerant due to its high flammability, low cost, and ease of accessibility [12,13]. However, identifying gasoline can be challenging due to factors such as compound volatilization and the presence of other substances that may produce pyrolytic products which can disrupt accurate identification [14,15,16]. Therefore, accurate analysis and characterization of gasoline residues, following certain fire investigation protocols and the use of validated standards (such as ASTM), are crucial [17].

Weathering is a degradation process where volatile compounds evaporate faster than less volatile ones, and this results in the changes in gasoline’s chemical profile [18]. This process is influenced by temperature, pressure, light, and convection [19,20]. When fires are initiated, gasoline residues are exposed to weathering, which alters their chemical composition. Neat gasoline comprises organic compounds ranging from C₄ to C₁₂, which possess high evaporation rates and low boiling points [21,22]. The changes in gasoline’s chemical profile caused by weathering pose a challenge for fire investigators in confirming that residues are gasoline used by perpetrators [18]. Matrix interferences from hydrocarbons, petroleum-based products, and pyrolysis products complicate ignitable liquid residue (ILR) identification [15,16]. Comparing pristine and weathered ignitable liquids adds complexity to the investigation, and microbial degradation further impacts identification of ignitable liquid [17].

Numerous studies have investigated how the weathering process affects gasoline’s volatile organic profile [19,23]. These studies observed how temperature, time, and different mediums affect gasoline compounds’ weathering. For instance, Willis et al. demonstrated the impact of elevated temperatures on the distribution of weathered residues, and Hodalik et al. studied the effect of weathering time on gasoline residue composition in cotton carpets [17,23]. All studies emphasized the importance of prompt evidence collection to avoid significant changes in gasoline’s chemical profile caused by weathering.

The National Academy of Sciences highlights the need for statistical evaluation of forensic evidence, recommending objective decision-making tools for fire debris analysis [24,25]. Statistical methods like PPMC provides quantitative assessments, helping to classify and associate ILRs with neat liquid by enhancing interpretation of GC-MS data in fire debris analysis. By measuring the strength and direction of linear relationships between chromatographic peak intensities, PPMC helps classify ignitable liquid residues (ILRs) and associate them with known neat liquids. This approach supports objective decision-making by reducing subjectivity in pattern recognition and improving the reproducibility of results [26]. Chemometric approaches are effective for classifying pristine ignitable liquids, but it can be a struggle with weathered residues [27]. Statistical analysis of the weathered compound can provide an understanding of the significant difference in weathering condition towards the distribution of gasoline residue.

This study’s findings will offer important insights into the behavior of volatile organic compounds in tropical climates, contributing to a better understanding of their behavior in varied environmental settings. By examining the changes in concentration through statistical and correlation analysis, we anticipate uncovering patterns that could predict future trends and inform effective management practices for these compounds in regions with tropical climate.

2. Materials and Methods

2.1. Sample Preparation

Neat RON95 gasoline was acquired from a local brand gasoline station (Petronas) for indoor and outdoor weathered samples. A 10 mL volume was measured into a 15 mL graded measuring cylinder. The neat gasoline was allowed to undergo weathering by volume reduction from 10 mL to 9 mL, 7.5 mL, 5 mL, 2.5 mL, 1.0 mL, and 0.5 mL to obtain 10%, 25%, 50%, 75%, 90%, and 95%, respectively. The samples for indoor weathering took place under a continuous stream of nitrogen inside a fume hood at room temperature 25 °C. Meanwhile, the samples for outdoor weathering were left exposed to the tropical environment naturally without any air flow assistance. All weathered samples were prepared in duplicates. Data loggers (R.S. Pro-USB-2) were placed indoors and outdoors to record the temperature and humidity of the environment where the weathering occurred.

As the sample reached its respective weathering percentage, 20 μL of the weathered sample was pipetted onto a Kimwipes cloth (Kimberly-Clark) which was placed inside an aluminum can to imitate sampling in real conditions. An activated charcoal tablet was suspended in the headspace of the aluminum can following an ignitable liquid extraction technique recommended by ASTM E1412 [28]. The aluminum can was then tightly shut and put in an oven for 4 h at 85 °C [29]. The volatiles adsorbed on activated charcoal tablet was desorbed using 5 mL pentane solvent containing internal standard (1% v/v tetrachloroethylene). Next, the desorbed pentane was transferred into a 2 mL GCMS vial. A control sample was prepared with similar procedures without the presence of gasoline. All samples were analyzed in duplicate using gas chromatography-mass spectroscopy to obtain 28 chromatography results.

2.2. Gas Chromatography/Mass Spectrometry (GC-MS)

All samples were analyzed using a Shimadzu (Kyoto, Japan) GC system coupled with Hewlett-Packard (Palo-Alto, CA, USA) 5973 (MSD) selective mass detector. The GC column was a 128-0122 DB-1MS column with 25 m length × 0.22 mm diameter × 0.33 μm thickness [15]. The parameter of GC/MS was set as follows: 1 μL sample injection at 250 °C with a 5:1 split ratio. The oven was set at 40 °C and was held for 5 min before ramping it to 280 °C at 15 °C/min and held again for 2 min. The carrier gas was helium with a flow rate of 1 mL/min, and the transfer line temperature was set at 250 °C. The MS quad temperature was set at 150 °C and the ion source at 230 °C, respectively. Total ion chromatograms were integrated using the post-run analysis software Version 4.20 and integrated with the National Institute of Standards and Technology (NIST) version 2017 for compound identification and peak area abundance. Peaks in TIC were identified based on the mass spectrum NIST database, and the respective peak areas were recorded for subsequent statistical analysis.

2.3. Data Collection and Analysis

Prominent peaks from neat gasoline samples were identified and marked as target compounds as listed by the ASTM E1618 under Section 10.2.1 [21]. These compounds include toluene, C₂–C₄-alkylbenzene, naphthalene, and long-chain alkane. To ensure consistency across samples, internal standard normalization was first applied by adjusting the abundance of each peak relative to the internal standard within the chromatogram. This step standardized the scale of the highest peak across chromatograms. To further minimize variation between replicates due to differences in injection volumes, total area normalization was subsequently performed. In this process, the total area of each chromatogram was calculated, and the average area across all chromatograms was determined. Each abundance value was then normalized by dividing it by the total area of its respective chromatogram and multiplying by the average area, thereby harmonizing the data for comparative analysis. All these normalization calculations were performed using Microsoft Excel (Excel 2024, Microsoft).

3. Results and Discussion

3.1. Identification of Compounds in Weathered Gasoline

GC-MS analysis of gasoline neat and weathered showed consistent detection of target compounds (

Table 1. List of gasoline target compounds detected in all samples.

Retention Time	Target Compound	Chemical Group
3.429	toluene	C1-alkylbenzene
6.091	ethylbenzene	C2-alkylbenzene
6.313	p-xylene	C2-alkylbenzene
6.818	o-xylene	C2-alkylbenzene
8.056	propylbenzene	C3-alkylbenzene
8.193	1-ethyl-3-methylbenzene	C3-alkylbenzene
8.224	1-ethyl-4-methylbenzene	C3-alkylbenzene
8.325	1,3,5-trimethylbenzene	C3-alkylbenzene
8.469	1-ethyl-2-methylbenzene	C3-alkylbenzene
8.708	1,2,4-trimethylbenzene	C3-alkylbenzene
9.104	1,2,3-trimethylbenzene	C3-alkylbenzene
9.25	indane	benzocyclopentane
9.56	1-methyl-4-propylbenzene	C4-alkylbenzene
9.61	1-methyl-3-propylbenzene	C4-alkylbenzene
9.754	1-methyl-2-propylbenzene	C4-alkylbenzene
10.227	1,2,3,4-tetramethylbenzene	C4-alkylbenzene
10.378	1,2,4,5-tetramethylbenzene	C4-alkylbenzene
10.418	1,2,3,5-tetramethylbenzene	C4-alkylbenzene
11.01	naphthalene	naphthalene
11.476	dodecane	alkane
11.641	tridecane	alkane
12.199	1-methylnaphthalene	methylnaphthalene
12.352	2-methylnaphthalene,	methylnaphthalene
13.376	2,3-dimethylnaphthalene,	dimethylnaphthalene
13.537	1,3-dimethylnaphthalene	dimethylnaphthalene

). These compounds are also aligned to ASTM 1618–19 list of target compound for gasoline [21]. Lighter compounds, such as toluene, were seen to elute earlier compared to heavier compounds such as C₃-alkylbenzenes and naphthalene. This is due to its lower molecular weight, lower boiling point (110.6 °C), and weaker intermolecular interactions with the stationary phase [30]. These properties allow it to vaporize more readily and interact less with the column, resulting in a shorter retention time. This elution behavior aligns with established chromatographic principles and is well-documented in the literature examining the structural influence on retention characteristics of VOCs [31]. This principle explains why toluene, with its lower boiling point and higher volatility, consistently elutes before the heavier C₃-alkylbenzenes residues.

3.2. Effect of Evaporation on Chemical Composition of Weathered Gasoline

Figure 1 illustrates the chromatographic profiles of gasoline samples subjected to indoor evaporation. In the chromatogram of unweathered gasoline, prominent peaks correspond to toluene, C₂-alkylbenzenes, and C₃-alkylbenzenes, with additional contributions from C₄-alkylbenzenes and naphthalenes. As evaporation progresses toward the 95% threshold, the chromatographic landscape undergoes a marked transformation. Toluene, being highly volatile, is no longer detectable, and the signal intensity of C₂-alkylbenzenes is substantially diminished. In contrast, compounds with lower volatility, namely, C₃-alkylbenzenes, C₄-alkylbenzenes, and methylnaphthalenes, exhibit relatively higher peak intensities.

This apparent increase is not indicative of actual enrichment but rather a consequence of the preferential loss of lighter compounds. The depletion of volatile constituents such as toluene and C₂-alkylbenzenes leads to a relative concentration shift, whereby the remaining heavier compounds dominate the chromatographic profile. This shift is further accentuated by the analytical behavior of the chromatogram, where the absence of early-eluting compounds enhances the visibility of later-eluting compounds. The persistence of these less volatile compounds, including C₃-alkylbenzenes and naphthalenes as seen in Figure 1D, is attributed to their higher boiling points and resistance to evaporation under indoor conditions. Consequently, the chemical signature of extensively weathered gasoline is characterized by a predominance of these thermally stable constituents.

Figure 2 presents the chromatograms of gasoline samples subjected to outdoor weathering. Unlike the more predictable indoor environment, outdoor conditions introduce greater variability in temperature, humidity, and airflow, which significantly influence the evaporation dynamics. At the 50% weathering stage, a pronounced increase in the peak intensities of several target compounds, including toluene, C₂-alkylbenzenes, and C₃-alkylbenzenes, is observed. This transient spike may reflect a momentary concentration effect due to uneven evaporation rates or compound redistribution within the sample matrix, likely driven by fluctuating environmental factors.

However, as weathering progresses to the 95% stage, a substantial reduction in the abundance of these target compounds is evident. The decline is consistent with the cumulative loss of volatile constituents over time, exacerbated by the erratic nature of outdoor exposure. Unlike indoor samples, which benefit from controlled temperature and limited airflow, outdoor samples are subject to continuous temperature shifts, direct sunlight, relative humidity, and wind currents, all of which accelerate and destabilize the evaporation process.

This pattern aligns with findings from previous studies that investigated weathering under variable thermal conditions, where inconsistent temperature profiles led to non-linear evaporation behavior and unpredictable compound retention [17]. The chromatographic profile at 95% weathering, as shown in Figure 2D, is dominated by less volatile compounds such as C₃-alkylbenzenes and naphthalenes, which persist due to their higher boiling points and lower susceptibility to environmental loss.

To further illustrate compound-specific trends, Figure 3 and Figure 4 compare the summed peak areas of C₂–C₄ alkylbenzenes and naphthalene compounds across gasoline samples subjected to progressive weathering under indoor and outdoor conditions, respectively. Due to the substantial variation in peak areas between the alkylbenzene and naphthalene groups, the data are presented in separate charts with adjusted y-axis scales to enhance clarity.

The visual data in Figure 3 reinforce the chromatographic observations, showing a consistent decline in lighter compounds such as ethylbenzene and xylenes, while heavier constituents like tetramethylbenzenes, naphthalenes, and methylnaphthalenes become increasingly dominant. This quantitative representation enhances interpretive clarity and highlights the progressive simplification of the chemical profile under stable indoor conditions.

Figure 4 complements this observation by presenting normalized abundance data for outdoor samples across the same weathering stages. Unlike the smooth decline seen indoors, outdoor profiles exhibit transient spikes, particularly around 50%, followed by a sharp drop in volatile compounds at 95%. The erratic pattern reflects the influence of tropical environmental factors and underscores the need for rapid sampling and adaptive forensic interpretation. The increasing prominence of heavier compounds such as trimethylbenzenes and naphthalene derivatives at later stages further supports the chromatographic findings and highlights the accelerated chemical evolution under outdoor conditions.

Despite the qualitative differences observed in the chromatograms and bar chart between indoor and outdoor conditions, the fluctuation of the target compounds based on molecular weight follows a similar trend. As the weathering percentage increases, the abundance of lighter compounds decreases, while heavier compounds become more prominent. These weathering patterns have been consistently observed in past research, supporting the similarity of the current study’s weathering patterns [18,23]. The increase in abundance of heavily weathered counterparts is due to chromatographic shift effect during the separation process. Lighter compounds in gasoline evaporate quickly and elute earlier in chromatography, resulting in a decrease in their abundance as weathering increases. On the other hand, heavier compounds evaporate more slowly, are retained longer during chromatography, and thus become more prominent with progressive weathering. This chromatographic separation enhances weathering patterns by reducing lighter compounds and increasing the prominence of heavier compounds [32].

A detailed comparison of the chemical composition of weathered gasoline between temperate and tropical regions reveals significant differences, particularly from a fire debris analysis perspective. Gasoline samples weathered in tropical environments, as demonstrated in the chromatographic data collected in this study, show a much faster and more abrupt chemical evolution compared to those weathered under temperate conditions reported in past studies [19,20].

In tropical regions, consistently high ambient temperatures (typically 28–32 °C), combined with elevated humidity and natural airflow, contribute to the rapid evaporation of volatile components. This is especially evident in the early disappearance of compounds such as toluene and C₂-alkylbenzenes, which are visibly prominent in fresh samples but become undetectable within 12–24 h. In contrast, temperate climate studies, both controlled temperature simulations [19] and ambient lab observations [20] have shown that these volatiles persist for a much longer duration, often remaining detectable even after several days or up to 70% weathering by mass.

Compounds with intermediate volatility, such as C₂- and C₃-alkylbenzenes, also demonstrate differing behavior across climates. In tropical samples, these components decline steadily and contribute to a transitional fingerprint that stabilizes within the first 72 h of exposure. In temperate settings, these same compounds exhibit greater resilience, forming part of the dominant profile well into the mid- and late-stage weathering processes. This prolonged detectability under cooler conditions allows for a broader analytical window during forensic analysis.

Heavier aromatic compounds, such as naphthalene, C₁-alkylnaphthalene, and C₂-alkylnaphthalene, show consistent persistence across both climatic conditions due to their low volatility. In both tropical and temperate samples, these compounds remain detectable throughout the entire weathering timeline. However, under tropical conditions, these heavy aromatics begin to dominate the chromatographic profile much earlier, often by the third day of exposure. This early shift from a mixed to a heavy-end profile can obscure the presence of light or mid-weight compounds that are often used to assess sample freshness or identify accelerants.

Another key difference lies in the timeline of fingerprint simplification. In tropical climates, the chromatographic profile becomes significantly flattened and simplified within 2–3 days, with distinct early peaks disappearing and the TIC signal skewing toward high-boiling components. In contrast, gasoline weathered under temperate conditions retains a more complex and balanced profile over an extended period, offering more diagnostic markers for source identification and classification.

These observations carry important implications for forensic interpretation. The faster weathering kinetics in tropical environments reduce the time window for detecting volatile markers, necessitating quicker evidence collection and adjusted interpretive frameworks. Analytical tools such as the ratio of persistent to volatile compounds (e.g., naphthalene/toluene) may be used as diagnostic indicators of both weathering stage and environmental exposure. Furthermore, chemometric models developed in temperate studies (e.g., PCA clusters in Hodálik or evaporation simulations in Birks) may require recalibration before being applied in tropical forensic contexts.

Overall, the comparison underscores that gasoline weathering is highly climate-sensitive, and forensic protocols should account for regional environmental factors. In tropical climates, where chemical transformation is accelerated, fire debris analysis must prioritize early sampling, higher-resolution detection of mid-weight compounds, and context-aware interpretation of compound ratios. Without such adjustments, there is a heightened risk of underestimating weathering extent or misclassifying the ignitable liquid residue type.

3.3. Effect of Environment Temperature on Weathering Rate of Gasoline

Table 2. Environmental conditions recorded for indoors and outdoors on sample collection day.

Environment Condition	Indoor						Outdoor
Weathering Percentage (%)	10	25	50	75	90	95	10	25	50	75	90	95
Heat Index (°C)	25	25	24	24	24	24	48	36	36	25	33	28
Temperature (°C)	24.5	24.5	23.5	23.5	23.5	24	36.5	30	29.5	24	27.5	26
Humidity (%rh)	78	78	77.5	82.5	78.5	77.5	57.5	74	77.5	95	90	92
Dewing Point (°C)	20.4	20.4	19.3	20.3	19.5	19.8	26.7	24.9	25.1	23.1	25.7	24.6
Sampling Day	Day 1				Day 2		Day 1		Day 2	Day 5	Day 9	Day 11

shows the differences between the environmental conditions for both indoor and outdoor weathered gasoline (in which heat factor was absent). Indoor weathering from 10 to 95% were completed within just two days, while outdoors requires 11 days to achieve complete weathering percentage. The significant disparity in weathering time can be attributed to the variations in heat index and humidity recorded during collection. The indoor environment exhibited a heat index fluctuation of ± 1 °C, while the outdoor environment experienced a much larger fluctuation of ± 12 °C. Similarly, the humidity levels exhibited notable differences. Indoors, fluctuations were confined to ± 5 °C, whereas outdoors, the fluctuations reached ± 37.5 °C. This discrepancy is likely due to the controlled indoor environment, where the temperature was constant, while the outdoor conditions were exposed to unpredictable natural environmental changes.

There are two main hypothesized factors that contribute to faster indoor weathering compared to outdoor. Indoor weathering was performed in a controlled environment under a fume hood with the help of a continuous nitrogen stream, while outdoor weathering was exposed to natural tropical weather without any temperature control. A nitrogen stream is widely used in laboratories to expedite the evaporation of liquids. This technique works by decreasing the partial vapor pressure on the surface of the liquid. As nitrogen displaces the ambient air, it reduces the concentration of vapor molecules in the surrounding environment, creating a pressure gradient that facilitates the escape and evaporation of liquid molecules. Consequently, this leads to a more efficient reduction in the volume of the sample, making the process invaluable for various analytical and preparative procedures, including liquid evaporation process. Previous research utilizing nitrogen blowdown has demonstrated that this technique enhances the evaporation rate compared to other methods [33]. This further supports the faster indoor weathering rate observed in this study.

For outdoor weathering, the heat and humidity showed inconsistencies. Despite higher outdoor temperatures, the weathering process took longer to reach its final stage. This raises the question of whether, in addition to temperature, humidity also contributes to a slower rate of evaporation. Previous research has addressed this, indicating an inverse relationship between relative humidity and evaporation rates, with higher evaporation occurring at lower humidity levels [32]. In the present study, outdoor conditions showed distinctly high relative humidity, which correlates with the observed slower evaporation rate. This helps explain why outdoor weathering may have taken longer compared to indoor conditions.

3.4. Effect of Evaporation on Association of Weathered Gasoline Using PPMC Coefficients

Recent research efforts have demonstrated the value of statistical models in enhancing fire debris analysis, particularly in interpreting gasoline weathering patterns. For example, Allen et al. developed and validated likelihood ratio models using GC-MS data aligned with ASTM E1618-19 classifications. Their work showed that statistical classifiers like quadratic discriminant analysis (QDA) and support vector machines (SVM) significantly improved evidentiary interpretation, achieving high accuracy in distinguishing ignitable liquid residues from substrate pyrolysis product [34] The Pearson product–moment correlation coefficients (PPMC) presented in

Table 3. Pearson product–moment correlation coefficients (PPMC) of neat and weathered gasoline in different environment and evaporation levels. Outdoor weathered samples are denoted as PO (petrol outdoor) and indoor weathered samples as PI (petrol indoor). The number indicates the level of evaporation (10–95%).

	Neat	PO10	PO125	PO50	PO75	PO90	PO95	PI10	PI25	PI50	PI75	PI90	PI95
Neat	1.000
PO10	0.928	1.000
PO125	0.911	0.995	1.000
PO50	0.872	0.951	0.957	1.000
PO75	0.906	0.993	0.993	0.966	1.000
PO90	0.898	0.993	0.995	0.961	0.996	1.000
PO95	0.906	0.994	0.996	0.967	0.997	0.998	1.000
PI10	0.907	0.977	0.983	0.970	0.988	0.988	0.986	1.000
PI25	0.919	0.982	0.987	0.972	0.990	0.990	0.989	0.999	1.000
PI50	0.874	0.950	0.961	0.970	0.970	0.968	0.965	0.993	0.990	1.000
PI75	0.741	0.837	0.867	0.922	0.877	0.874	0.870	0.925	0.916	0.960	1.000
PI90	0.346	0.522	0.595	0.646	0.580	0.588	0.583	0.631	0.616	0.682	0.827	1.000
PI95	0.187	0.393	0.476	0.505	0.450	0.463	0.458	0.490	0.474	0.537	0.695	0.975	1.000

reveal several insightful relationships among different gasoline weathering conditions. Notably, there are high correlations between neat and 10% outdoor weathered gasoline (0.928) and between neat and 10% indoor weathered gasoline (0.907). This indicates that gasoline still shares similar compound patterns despite the reduction in toluene in the 10% weathered samples.

Moreover, comparisons between indoor and outdoor weathered compounds for 10%, 25%, and 50% gasoline also show high correlations at 0.977, 0.987, and 0.970, respectively. As the weathering percentage reaches 90%, the correlation is moderately high at 0.588, and it becomes moderately low at 0.458 when weathering reaches 95%. This demonstrates that while early stages of weathering exhibit similar target compound patterns, later stages show divergent patterns, likely due to varying environmental conditions.

Interestingly, stark differences are observed in the correlation values between neat gasoline and 95% weathered gasoline under indoor and outdoor conditions. The correlation for indoor weathering is weak at 0.187, whereas the outdoor correlation is substantially stronger at 0.906. This contrast highlights the pronounced influence of environmental factors, particularly those associated with tropical climates such as elevated temperatures, high humidity, and prolonged sunlight exposure, on the weathering process. The outdoor environment appears to facilitate a more uniform and predictable degradation pattern, while indoor conditions yield more variable compound distributions. These findings underscore the importance of contextualizing weathering environments in fire debris analysis and reinforce the relevance of region-specific models when interpreting VOC profiles in tropical settings.

Furthermore, the observed divergence in compound patterns between indoor and outdoor weathering has direct implications for forensic scene reconstruction. Accurate interpretation of ignitable liquid residues requires consideration of the environmental conditions under which weathering occurred. Without such context, there is a risk of misclassifying or overlooking key compounds, particularly in advanced weathering stages. This study provides foundational data to support more reliable reconstruction of fire scenes in tropical regions.

Overall, the PPMC data elucidate the interrelationships between various gasoline weathering conditions. By analyzing these correlations, we can better understand how different environmental factors impact gasoline weathering both indoors and outdoors.

3.5. Comparing Target Compound Concentration Between Indoor and Outdoor Gasoline Using t-Test

To determine statistical significance quantitatively, a t-test was conducted to compare specific compound concentrations between the PI and PO groups. The results of the independent t-test for PO vs. PI, shown in

Table 4. p-values for each of gasoline’s target compound.

Compound	p-Value	Mwt (g/mol)
Toluene	0.0265	92.41
Ethylbenzene	0.2026	106.17
p-xylene	0.4437	106.17
o-xylene	0.3441	106.17
Propylbenzene	0.5647	120.19
1-ethyl-3-methylbenzene	0.0183	120.19
1-ethyl-4-methylbenzene	0.0136	120.19
1,3,5-trimethylbenzene	0.0016	120.19
1-ethyl-2-methylbenzene	0.0407	120.19
1,2,4-trimethylbenzene	0.0316	120.19
1,2,3-trimethylbenzene	0.3441	120.19
Indane	0.1846	118.18
1-methyl-4-propylbenzene	0.1042	134.22
1-methyl-3-propylbenzene	0.1471	134.22
1-methyl-2-propylbenzene	0.1309	134.22
1,2,3,4-tetramethylbenzene	0.3037	134.22
1,2,4,5-tetramethylbenzene	0.1446	134.22
1,2,3,5-tetramethylbenzene	0.1853	134.22
Dodecane	0.2779	170.34
Tridecane	0.1982	184.37
Naphthalene	0.4471	128.17
1-methylnaphthalene	0.3110	142.20
2-methylnaphthalene,	0.5097	142.20
2,3-dimethylnaphthalene,	1.783	156.22
1,3-dimethylnaphthalene	1.814	156.22

, provide the p-values for each target compound. Compounds with p-values below 0.05 indicate statistically significant differences between the PO and PI groups. These compounds, which elute earlier due to their lower molecular weight, are likely key contributors to the observed differences between indoor and outdoor weathering effects.

However, samples from the scene are usually exposed to high temperatures or prolonged heat, causing lighter compounds to volatilize while heavier compounds remain detectable. Therefore, comparison of ignitable liquid residue from fire scenes (regarded as outdoor burning) to reference database (created under controlled indoor condition) is permissible (or regarded as acceptable) because there is no significant difference between gasoline residue made under the two weathering conditions.

4. Conclusions

This study elucidates three critical observations on the effects of tropical weathering on gasoline evaporation. Firstly, there are notable differences in weathering rates between indoor and outdoor settings, influenced by distinct temperature and humidity levels. Secondly, the correlation of target compounds between indoor and outdoor conditions is strong at initial weathering stages (10%, 25%, and 50%), becoming moderate at 90% and 95%. Neat and 95% weathered gasoline exhibit weak correlation indoors but strong correlation outdoors. Thirdly, t-test results reveal significant differences in the concentrations of specific target compounds, such as toluene and C₃-alkylbenzene, between indoor and outdoor settings. These findings have direct implications for forensic fire scene analysis. The ability to detect and interpret gasoline residues depends heavily on understanding the environmental context in which weathering occurred. In tropical climates, outdoor weathering may preserve identifiable compound patterns even at advanced stages, improving the reliability of residue detection. These findings suggest that while tropical climate may influence gasoline weathering rates and compound distribution, target compounds can still be identified in similar settings, aiding in fire scene analysis. This supports the development of region-specific forensic models and enhances the accuracy of scene reconstruction, especially when distinguishing between neat and weathered samples.

Future studies could focus on the influence of additional environmental variables, such as rainfall, wind speed, and soil types, on gasoline weathering residues. Conducting comparative analyses across different months can help observe seasonal variations in weathering patterns. Developing and validating new analytical approaches, such as humidity-adjusted chemometric models for field analysis, would enhance the detection and analysis of weathered gasoline residues in various forensic scenarios and improve practical applications. These avenues will provide a comprehensive understanding of gasoline weathering under diverse conditions. Together, these efforts will advance the practical application of fire debris analysis in tropical regions.