Characterization of a headspace sampling method with a five-component diesel fuel surrogate☆
Graphical abstract
Introduction
Arson remains a challenging crime to investigate because most of the physical evidence is destroyed by the fire and by fire-fighting efforts. National arson rates are unknown due to irregular reporting, but 45% of reported arson offenses involved structures (2017 data) [1]. For structural fires, fire investigations often involve the collection of solid debris for subsequent laboratory analysis to extract and identify ignitable liquid (IL), which can indicate arson. The Ignitable Liquids Reference Collection Database provides chromatographic data and classification according to ASTM Standard Test Method E1618 for neat, weathered, and biologically degraded ILs [2], [3]. There have also been efforts to infer the specific source of neat and weathered gasolines [4].
Analyzing fire debris for IL residue requires extracting and concentrating IL compounds from debris into a form suitable for analytical instruments. ASTM Standard Practice E1412 describes the use of activated charcoal strips (ACSs) for passive headspace concentration and was originally approved in 1991; the method is well established in current forensic practice and is commonly used by forensic laboratories in the United States. The typical procedure is to sample headspace vapors by suspending an ACS above the fire debris in a steel can for 16 h to 24 h at 60 °C to 80 °C. Analytes are then desorbed from the ACS with solvent and analyzed by gas chromatography with mass spectrometry (GC–MS). The effects of time, temperature, and strip size on the recovery of IL compounds have been investigated over more than 20 years [5], [6]. If the results suggest displacement has occurred, the fire debris headspace may be re-sampled under different conditions, for example, for a shorter time and/or at a lower temperature. Displacement refers to the substitution of high-volatility compounds with low-volatility compounds at adsorbent sites. This process is driven by equilibration of each compound between the adsorbent and the headspace and occurs when the ACS becomes saturated due to high IL concentration (not easily controlled, depends on the debris) and/or small adsorbent capacity (controlled, depends on the analyst) [7].
ACSs have a high affinity for hydrocarbons, which makes them effective adsorbents but demands a strong elution solvent. Carbon disulfide is usually the solvent employed, because its extreme affinity for carbon allows it to effectively displace IL compounds. The significant drawback of carbon disulfide is its high toxicity and flammability. Small exposures to carbon disulfide can cause cardiovascular, neurological, and reproductive toxicity. Accelerated solvent extraction with acetone is a proposed alternative to carbon disulfide for volatile organic compounds captured during air quality measurements in occupational environments; however, this approach has not been applied to less volatile target compounds found in ILs such as gasoline or diesel fuel [8]. Other solvents have been evaluated for their ability to desorb target compounds found in gasoline and diesel fuel towards the goal of improving laboratory safety [9]. ASTM Standard Practice E1412 permits diethyl ether and pentane as alternatives to carbon disulfide; however, both solvents are less effective at desorbing gasoline and diesel fuel compounds than carbon disulfide and are not commonly employed by forensic laboratories [5], [10], [11]. Dichloromethane has been proposed as a safer alternative to carbon disulfide, despite both its carcinogenicity and inferior desorption of target compounds from gasoline and diesel fuel [11].
There have been efforts to develop passive headspace concentration methods that avoid carbon disulfide in favor of thermal desorption [12]. Solid-phase microextraction (SPME) utilizes fibers coated with an adsorbent phase, often polydimethylsiloxane, that are thermally desorbed directly into a gas chromatograph for analysis [13]. ASTM Standard Practice E2154 describes SPME passive headspace concentration as suitable for screening fire debris samples and was originally approved in 2001 [14]. SPME fibers are more prone to displacement than ACSs due to the lower number of adsorption sites, and SPME fibers may preferentially collect aliphatic or aromatic compounds depending on the adsorbent phase and temperature [15], [16] because of competitive adsorption. The relative concentration of a compound equilibrated between two phases is described by its partition coefficient, but even if this value can be predicted at the sample temperature [17], relating adsorbent concentration to vapor concentration is impractical for complex mixtures. Headspace sorptive extraction (HSSE) is similar in concept but utilizes polydimethylsiloxane stir bars to provide approximately 100x the adsorbent volume [18]. The method appears promising and should be less prone to displacement, but it requires additional validation. Common among these passive concentration methods is the issue of competitive adsorption or displacement. Capillary microextraction of volatiles (CMV), utilizing glass fiber filters modified with a sol–gel adsorbent phase [19], was recently employed to extract IL compounds from simulated fire debris [20]. CMV fibers are like SPME fibers in that both can be thermally desorbed; however, CMV filters are robust and can be used for dynamic headspace concentration.
Displacement is less of a concern for dynamic headspace concentration methods because they can be monitored for breakthrough. PLOT (porous layer open tubular)-cryoadsorption (PLOT-cryo) was developed at NIST in 2008 to concentrate vapors from low-volatility explosives and identify compounds in the headspace of these materials [21], [22]. This headspace concentration method sweeps headspace vapor through a capillary vapor “trap” coated with a porous alumina adsorbent layer that is cooled to promote adsorption. Breakthrough may be a sign that the adsorbent is becoming saturated (leading to displacement), but it can also indicate that the flow rate is too high to permit adsorption of all compounds onto the porous alumina. In either scenario, breakthrough can be detected by bubbling the vapor exiting the capillary through a vial of solvent and subsequently analyzing this solvent for the presence of analytes. Capillaries are easily eluted with solvent and the eluate can be analyzed by GC–MS or other analytical techniques. Over the last decade, PLOT-cryo has been applied to detection of grave soil, poultry spoilage, and natural gas contaminants, among other applications [23], [24], [25]. The portable PLOT-cryo instrument developed in 2014 was first applied to detect diesel fuel spiked on glass beads (with a detection limit below 1 ppm) and more recently used to detect explosives- and decomposition-related compounds inside a simulated shipping container [26], [27], [28].
In a preliminary investigation of its application to fire debris, PLOT-cryo was shown to extract representative IL compounds from gasoline and diesel fuel from laboratory-generated fire debris [29]. Several advantages stood out from that work. Unlike passive headspace concentration methods in which the distribution of compounds recovered may not represent the distribution of compounds in the headspace vapor due to competitive adsorption or displacement, headspace flow through a capillary vapor trap cooled to sub-ambient temperatures promoted adsorption of all IL compounds by the alumina adsorbent. Because alumina is a weaker adsorbent than carbon, capillaries could be completely eluted with acetone, which is a respiratory irritant, but still a much safer solvent than carbon disulfide. Solvent flow through the capillary also promotes complete desorption compared to equilibrium desorption with the same solvent. Alumina’s ability to be reused is another advantage, while carbon is considered disposable because of the difficulty of verifying complete sample removal. In our 2014 paper, PLOT-cryo was directly compared to the ACS method and found to collect a larger number of compounds on the low-volatility end of each fuel; however, we must note that due to safety concerns, the student researcher eluted ACSs with acetone instead of carbon disulfide. This choice surely had a negative impact on ACS performance, and additional head-to-head experiments are needed before any definitive conclusion can be made.
PLOT-cryo shares features with ASTM Standard Practices E3189-19 and E1413-19 for static and dynamic headspace concentration of IL residues onto adsorbent tubes [30], [31]. In the 2014 fire debris work, we used a flow rate of 1.3 mL/min, which is close to, but still less than, the recommendation for static headspace concentration: 2 mL/min to 80 mL/min. Since then, however, we have employed carrier gas flow rates of up to 10 mL/min in other work with pure compounds and rates up to 100 mL/min using the portable instrument [26], [43]. On the other hand, the collection volume may be greater than 10% of the headspace volume (guidance that separates static from dynamic headspace concentration), although this depends on the sample container. In the 2014 paper, we used a 2 mL sample vial and collected approximately 40 mL of headspace. This collection volume represents a small fraction of the volume of a quart (less than 5%) or gallon-sized steel can (~1%), which are commonly used to collect fire debris. However, this calculation does not account for the volume occupied by the debris, which typically occupies one-quarter to one-half of the can volume. PLOT-cryo therefore falls somewhere between the ASTM definitions for static and dynamic headspace concentration regarding flow rate and collection volume. Regarding temperature, samples in the 2014 paper were heated to 175 °C, which is higher than recommended in any ASTM standard for headspace concentration of IL residues. Although high sample temperatures generate rapid results, temperatures above 90 °C are unsafe for moist samples because the pressure exerted by water vapor can cause sample containers to violently fail. PLOT-cryo also utilizes a chilled adsorbent to enhance trapping, which is not recommended in any ASTM standard and is a key feature of the PLOT-cryo method.
Our primary goal here is to determine to what extent the PLOT-cryo method might be optimized with a surrogate mixture or “artificial” diesel fuel by examining the influence of flow rate and temperature on the composition and spatial distribution of the collected headspace compounds. Surrogate or artificial mixtures have been used in the past primarily for engine testing and have been designed to approximate kinetics, combustion, and certain thermophysical characteristics important in that application, such as volatility, viscosity, and energy content [32], [33]. In a recent application to forensic science, researchers used a seven-component artificial gasoline mixture to study the temperature dependence of IL weathering behavior [34], [35]. To investigate variables contributing to distortion of the collected headspace compounds related to ACS saturation, Williams et al. employed a simple hydrocarbon mixture [7]. The mixture was composed of five hydrocarbons selected for their relative volatilities and was an effective choice for the questions being investigated, but it was not intended to be a surrogate (i.e., it did not represent the thermophysical properties of any fuel). We propose that by selecting a thermodynamically representative surrogate mixture we will not only simplify the analysis of trends, but the method development will also be transferable to the more complex target fuel.
To investigate the fitness of surrogate mixtures for this purpose, we first selected a five-component diesel surrogate comprised of compounds with a range of volatilities (described in the next section). While varying the flow rate (0.5 scc/min – 2.5 scc/min) and sample temperature (60 °C – 120 °C), we examined PLOT-cryo collections from neat diesel surrogate and neat diesel fuel to compare the total quantity of collected headspace as well as trends in the volatility and hydrocarbon class. Our findings indicate that the surrogate was a useful tool for predicting the effects of variable sampling parameters on the diesel fuel collections, but it had limitations associated with its simplicity.
Section snippets
Materials
Table 1 shows the composition of the diesel surrogate. It contains n-octadecane (OD) to represent alkanes, 2,2,4,4,6,8,8-heptamethylnonane (HMN) to represent isoalkanes, 1,2,3,4-tetrahydronaphthalene or tetralin (THN) to represent naphthoaromatics, 1,2,4-trimethylbenzene (TMB) to represent alkylbenzenes, and 1-methyl naphthalene (MN) to represent polynuclear aromatics [33]. We chose this five-component surrogate because its distillation curve has the lowest average difference from the target
Influence of flow rate
In general, dynamic headspace concentration cannot be assumed to be an equilibrium process, because vapor–liquid equilibrium (VLE) cannot be established if the vapor is too rapidly removed from the headspace. However, it is possible under certain conditions. We have previously demonstrated that at flow rates up to 10 scc/min, vapor–liquid or vapor–solid equilibrium can be established for pure compounds, permitting the determination of vapor pressure as a function of temperature [22], [42], [43]
Conclusion
Surrogate mixtures designed to represent complex fluids with fewer components have long been used to simplify fuels research, but their potential contributions to forensic science have not yet been fully explored. We demonstrated here that a five-component diesel fuel surrogate was a valuable tool for developing PLOT-cryoadsorption for headspace concentration of ILs from fire debris. The thousands of components in diesel fuel lead to data analysis challenges, especially when looking for subtle
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
M. E. Harries was supported by a National Research Council (NRC) Postdoctoral Research Associateship and S. S. Wasserman was supported by a Summer Undergraduate Research Fellowship (SURF). This research was supported by funding from the NIST Special Programs Office and by funding from the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice through an interagency agreement (DJO-NIJ-19-RO-0007). The funders had no role in study design, data collection and
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