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MS carrier and buffer gas for use in forensic laboratories
SCIJUS-00502; No of Pages 6 Science and Justice xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Science and Justice journal homepage: www.elsevier.com/locate/scijus
Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories Chinyere N. Nnaji, Kristina C. Williams, Jonathan M. Bishop, Guido F. Verbeck ⁎ Department of Chemistry, University of North Texas, Denton, TX 76201, USA
a r t i c l e
i n f o
Article history: Received 10 August 2014 Received in revised form 9 January 2015 Accepted 12 January 2015 Available online xxxx Keywords: GC–MS Hydrogen Carrier gas Buffer gas Illicit drugs Energetic materials
a b s t r a c t A custom set of ion volumes was manufactured in order to investigate the gain and byproducts using hydrogen as a buffer gas following electron ionization in a quadrupole ion trap mass spectrometer as compared with helium. Analyses of illicit drugs such as cocaine, codeine, and oxycodone, and explosives such as TNT, RDX, and HMTD with ion volume exit orifices of 1 mm, 2 mm, 4 mm, 6 mm, 8 mm and 10 mm were performed using GC/MS. Strong similarities between hydrogen and helium spectra of illicit drugs and explosives provide evidence that hydrogen can be used effectively as a buffer gas in an ion trap mass spectrometer. © 2015 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Helium is quickly becoming an expensive and limited commodity, especially with the closing of helium plants across the globe (Amarillo Natural Gas, Amarillo, TX). While the Ras Laffan Helium 2 plant in Qatar is said to produce 1.3 billion ft3/year, it is still just a temporary solution. With a projected capacity of 900 ton ft3, the Qatar-based plant contains only enough to sustain our needs for another 30 years. Limitations on helium usage may soon take effect, as it is used in several areas including magnetic resonance imaging (MRI), magnetocephalography (MEG), welding, and as a buffer gas for many chemistry and biology related analysis techniques. Currently, gas chromatography–quadrupole ion trap mass spectrometry (GC–QITMS) uses helium as both a carrier gas for GC and a buffer gas for the ion trap. Since GC–MS is the primary workhorse for analysis of illicit drugs [1–11] and explosives [12–14] in forensic laboratories, a review and comparison are presented here in order to illustrate the reduction of our need for helium as a carrier and buffer gas. Hydrogen is a highly effective carrier gas because it increases the speed of analysis time [15] and the resolution in GC [16,17]. Since the viscosity of hydrogen is lower than that of helium, there is less gas flow required to maintain the same linear velocity, achieving a faster chromatographic separation [18]. While the lower viscosity of hydrogen can lead to a lower column head pressure, the use of smaller diameter or ⁎ Corresponding author at: University of North Texas, Department of Chemistry, P.O. Box 305070, Denton, TX 76203, USA. Tel.: +1 940 369 8423. E-mail address: [email protected] (G.F. Verbeck).
longer length columns will resolve the issue of increased solvent expansion [19]. Resolution increases about 150% when using hydrogen as a carrier gas. Hydrogen has been used as a carrier gas with a variety of detection techniques including electron capture detection (GC–ECD) [20], flame ionization detection (GC–FID) [21], and GC–MS [16]. Ion traps utilize a background or buffer gas to induce collisional damping and thus increase mass resolution and sensitivity [22]. A lower molecular weight buffer gas helps prevent significant momentum changes, such as smaller displacement and velocities, upon collision with an analyte ion, minimizing the loss of trapped ions [22]. Helium and hydrogen are both ideal buffer gases in this case. Hydrogen has also been briefly explored showing mechanisms of ion–ion and ion–molecule interactions, including the tendency of many molecular ions to present as an M+1 peak due to the addition of a hydrogen atom [23]. The use of low RF amplitude was employed to trap low mass gases to facilitate ionization to show that the ionization mechanisms are different between hydrogen and helium as buffer gases. Helium was shown to react rapidly with background gases by facilitating the cleavage of diatomic nitrogen bonds. Monovalent nitrogen underwent charge-exchange reactions with water, initially giving an intense signal at m/z 18. As this peak grew in intensity, m/z 19 also grew due to self-protonation of the water ion. In comparison, hydrogen also reacted with background ions by self-protonation and ion exchange, producing signals at m/z 3, 28, and 29 corresponding to H+ 3 , + N+ 2 , and N2H respectively. Hydrogen has also recently been effectively utilized as a buffer gas in a field-deployable ion trap, though atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) were utilized rather than electron ionization (EI) [24].
http://dx.doi.org/10.1016/j.scijus.2015.01.003 1355-0306/© 2015 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
Please cite this article as: C.N. Nnaji, et al., Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories, Sci. Justice (2015), http:// dx.doi.org/10.1016/j.scijus.2015.01.003
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Another factor to consider when changing buffer gases in an external ionization source is the ion volume exit orifice. The ion volume is a region before the ion trap where the analyte is introduced, ionized, and diffusively ejected towards the trap. The most common ion volumes have an exit orifice that is approximately 1 mm in diameter and is often referred to as a closed or chemical ionization ion volume. Cascading ionization and interaction occur since the volume has such a small exit orifice. However, closed ion volumes pose a problem as they produce high energy ions and increase reactivity with non-inert buffer gases [25]. Another disadvantage of a closed ion volume is that fewer ions make it to the trap due to collisions with the ion volume near the exit orifice. Using an open ion volume, or an ion volume that does not have any walls perpendicular to exit flow, will have an increased gain according to calculations by Slobodenyuk [26]. This increased gain in conjunction with the absence of high-energy ion formation can facilitate the analysis of energetic materials. Lower energy ions exiting the ion volume will exhibit reduced side reactions with self-protonated species, and increased gain will improve spectra intensities. The use of compressed hydrogen in the laboratory comes with safety concerns; however, these have been greatly reduced with the advent of small, portable hydrogen generators which produce hydrogen and oxygen from water via electrolysis. The generators can often produce enough hydrogen to run up to 3 GC–MS devices with standard splitflow values without the concern of having compressed hydrogen in the lab. Current generators can run 10 years on a single catalyst with minimal water filter maintenance. The ability to switch from a helium-dependent lab to one that utilizes hydrogen will save forensic laboratories money as well as time in the event of a potential helium-shortage crisis in the future, thus investigation of relevant analytes has been performed. Here, custom ion volumes were machined in order to monitor ion fragmentation and intensity changes as a function of exit orifice diameter. Illicit drugs are investigated with both gases to showcase the utility of hydrogen in a forensic laboratory, a setting in which lack of variation between samples is key. High-energy materials, though typically difficult to analyze due to high reactivity and instability, were also analyzed using a combination of high gain and low-energy byproducts.
Fig. 1. CAD schematic of the ion volume (a) and ion volume array (b), 10 mm (left) to 1 mm (right) exit orifice.
the hydrogen gas. The modification of several ion volumes was performed by boring out 1 mm diameter ion volumes into diameters of 2 mm, 4 mm, 6 mm, and 8 mm, while 1 mm and 10 mm diameters were unmodified (Fig. 1). After switching gases, 12 h was allotted for the equilibration of the system.
2.3. GC–MS methods The GC–MS temperature program for explosives was 40 °C (3 min hold) to 300 °C (3 min hold) at 10 °C/min, with an inlet temperature of 250 °C, a transfer line temperature of 300 °C, and a split flow of 20 mL/min. The GC–MS temperature program for drugs was 70 °C (3 min hold) to 300 °C (5 min hold) at 20 °C/min, with an inlet temperature of 250 °C, and a transfer line of 300 °C and was run in
2. Material and methods 2.1. Materials Cocaine, codeine, phenobarbitol, diazepam, and oxycodone QuikChek standards (1 mg/mL in methanol) were purchased from Grace (Deerfield, Illinois). Methyl ethyl ketone peroxide (MEKP), trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), cyclotrimethylenetrinitramine (RDX), ethylene glycol dinitrate (EDGN), hexamethylenetriperoxidediamine (HMTD), triacetonetriperoxide (TATP), and pentaerythritoltetranitrate (PETN) were synthesized in our lab and characterized via mass spectrometry. Optima methanol (99.9%) was purchased from Fisher (Fisher Scientific, Hampton, New Hampshire). Acetonitrile (99.8%) was purchased from Sigma Aldrich (Sigma-Aldrich, St. Louis, Missouri). Solid explosive samples were synthesized and characterized in house, then prepared by dissolving 1 mg of each solid in 1 mL of acetonitrile. Liquid explosives were diluted in acetonitrile in a 1:100 (v:v) ratio. The drug standards were 1 mg/mL, then diluted 1:4 (v:v) in methanol. 1 μL of each sample was manually injected into the GC–MS. 2.2. Instrumentation and modifications A Thermo Focus GC was coupled to a Polaris-Q MS equipped with a 1 cm quadrupole ion trap (Thermo Scientific, Waltham, MA). The Polaris-Q was fitted with a gate valve in order to quickly and easily change ion volumes without breaking vacuum. For the experiments using hydrogen as a buffer gas, a Peak Precision Hydrogen Trace generator (Peak Scientific Instruments, Inchinnan, UK) was used to generate
Fig. 2. Air/water spectrum with helium (a) and hydrogen (b). The hydrogen spectra have a higher ratio of water, which is mostly protonated. Nitrogen is also only present in protonated form in the hydrogen spectrum.
Please cite this article as: C.N. Nnaji, et al., Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories, Sci. Justice (2015), http:// dx.doi.org/10.1016/j.scijus.2015.01.003
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Fig. 3. Mass spectra obtained of cocaine (a), phenobarbital (b), codeine (c), and oxycodone (d) using a 2 mm ion volume with hydrogen and using a 1 mm ion volume with helium.
splitless mode. Each injection volume was 1 μL. A Restek RTX-1 (Restek, Bellefonte, PA) column was used (15 m × 0.25 mm ID × 0.1 μm film thickness). The ionization source was set to 200 °C with a maximum ion time of 25 ms. All drug analyses were performed using a mass range of m/z 40–350. Analyses of explosives had varying mass ranges of m/z 28–650 for PETN, TATP, EGDN, RDX (helium), and HMTD; m/z 50–650 for TNT, DNT, and RDX (hydrogen); and m/z 33–350 for MEKP. All spectra presented are the summed scan average of the peak, with a 25 μs ionization time, and 3 summed microscans per data point.
1 mm for helium and 2 mm for hydrogen were selected because they exhibited the highest intensities in the air/water spectra. When hydrogen is used as a buffer gas there is an order of magnitude increase for water (m/z 19) in comparison to the oxygen peak at m/z 32. This is due to excess H3O+ that is created from the air/water in the hydrogen gas. In the hydrogen spectrum nitrogen shows a more abundant M+1 peak. Evidence has been shown that the N2H+ ion forms using EI [27,28], but with excess hydrogen present during the ionization process, nitrogen shows a more abundant M+1 peak at m/z 29. This is in contrast to the presence of N2 and N2H+ at m/z 28 and 29 with the use of helium as a buffer gas.
3. Results and discussion 3.2. Drug sample comparison 3.1. Background Background spectra using both helium gas and hydrogen gas were collected. This was done to show the effects of the gases used on the peak shown. Fig. 2 shows the air/water spectra using 1 mm exit orifice for helium gas (a) and 2 mm exit orifice for hydrogen gas (b) to evaluate the changes due solely to changing of the gas. Exit orifice diameters of
A wide range of controlled and abused substances were run on the GC/MS using all six ion volumes with hydrogen and with helium. Fig. 3 shows that there is no difference between the spectral patterns for cocaine, phenobarbital, codeine, and oxycodone. Because EI is a particularly hard ionization technique, it is not uncommon to produce little to no molecular ion [29]. For this reason, common fragment
Fig. 4. Comparison of absolute intensity and the six ion volumes of cocaine's fragment peak, m/z 182.04, while using helium as the carrier gas.
Fig. 5. Comparison of absolute intensity and the six ion volumes of cocaine's fragment peak, m/z 182.04, while using hydrogen as the carrier gas.
Please cite this article as: C.N. Nnaji, et al., Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories, Sci. Justice (2015), http:// dx.doi.org/10.1016/j.scijus.2015.01.003
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times. When the ion volume has a 10 mm exit orifice, absolute intensity with hydrogen is greater than with helium by 2 times. 3.3. Energetic materials
Fig. 6. Comparison of helium and hydrogen's absolute intensities in relation to the six ion volumes.
peaks for cocaine (m/z 182) and phenobarbital (m/z 204) are shown as well as the parent mass peaks of codeine (m/z 299) and oxycodone (m/z 315). Even though the ions themselves have electron rich oxygen or nitrogen species, there is no appreciable reaction with hydrogen, therefore no shift in the mass spectrum or alternative fragmentation patterns in any of these drugs is observed. Cocaine's primary fragment peak at m/z 182.04 was used to compare intensities resulting from each of the six ion volumes while using helium as the carrier and buffer gas (Fig. 4). An exit orifice of 10 mm produced the highest intensity while the 6 mm exit orifice produced the lowest. The same results were observed when hydrogen was used as the carrier and buffer gas (Fig. 5). For both gases, the 10 mm exit orifice ion volume was found to be optimal. However, exit orifices of 1, 2, 4, and 8 mm showed a different ordering of intensities (Fig. 6). In a closed ion volume, 1 mm, absolute intensity due to helium is greater than that due to hydrogen by 1.25
Eight different explosives (TNT, DNT, MEKP, PETN, RDX, EGDN, TATP, and HMTD) were synthesized and run on the GC–MS with both hydrogen and helium using the six ion volumes. As with the illicit drugs, the 10 mm exit orifice ion volume had the highest intensities with both buffer gases. This is especially necessary for analyses of energetic materials; since many of them are unstable compounds, the intensities tend to be much lower. Similar to the previously discussed illicit materials, most of the explosive material spectra were unchanged. Fig. 7 shows the relevant ion peaks that the PETN, TATP, EGDN, and RDX spectra produce independent of the gases which only differ in intensity. However, some explosives, such as TNT, DNT, MEKP, and HMTD, did show some differences in fragmentation and fragment ratios (Fig. 8). When helium was used as a carrier and buffer gas, 2,4-DNT shows a significant peak at m/z 119 that is absent in spectra collected using hydrogen. The peak at m/z 119 corresponds to a fragment with a single oxygen which is absent in the hydrogen spectrum. The fragment ratios of DNT and TNT are also different. HMTD also exhibits fragment peaks associated with hydrogen that are not observed with helium at m/z 55, 57, 61, 72, 77, 84 and 88. In the MEKP spectrum, both hydrogen and helium produce fragments at m/z 73 and 89. The interaction of hydrogen with the [C4OH+ 9 ] fragment causes loss of a hydroxyl or methyl group with subsequent protonation. This additional fragmentation could be advantageous to molecules that have hydroxyl groups, helping to further characterize a molecule. 4. Conclusions Hydrogen has been shown to be an effective replacement for helium as a carrier and buffer gas in GC–MS. Hydrogen has already been studied
Fig. 7. PETN (a), TATP (b), EGDN (c), and RDX (d) spectra using both helium and hydrogen as the carrier gas. No appreciable difference between the positions of the major peaks is observed.
Please cite this article as: C.N. Nnaji, et al., Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories, Sci. Justice (2015), http:// dx.doi.org/10.1016/j.scijus.2015.01.003
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Fig. 8. TNT (a), MEKP (b), 2,4-DNT (c), and HMTD (d) spectra using helium gas and hydrogen gas. Notice the m/z 1 shift of 2,4-DNT with hydrogen compared with helium. There is a different fragmentation ratio for TNT with hydrogen between the spectra of TNT. MEKP has more fragmentation with hydrogen. HMTD was undetectable with helium except at m/z 88.
and confirmed to be a superior carrier gas compared to helium in GC, and our experiments show that hydrogen is an equally adequate buffer gas for mass spectrometry. With the use of hydrogen generators, both gases are safe for laboratory use; however, hydrogen is renewable, abundant, and significantly more inexpensive. The balance between reactivity and sensitivity will influence the choice of ion volume. Changing from a 1 mm to a 2 mm diameter exit orifice ion volume would enable an immediate transition, and the use of a 10 mm exit orifice would provide optimal intensities when switching to hydrogen for illicit drug analysis and energetic materials in forensic laboratory settings. The more delicate samples require open ion volumes, whereas hardier samples can be optimized in order to achieve higher sensitivity. Hydrogen gas may cause some slight variations in the mass spectra, usually as an M+1 peak or increased ion–molecule reaction chemistry; the formation of N2H+ ion supports this claim. With the increasing cost and depletion of helium, hydrogen is a suitable replacement for GC–QITMS as both a carrier gas and a buffer gas. This will be particularly true with the advent of databases built around using hydrogen in these systems rather than helium, furthering the ability to use hydrogen in a forensic laboratory setting. Acknowledgments The authors would like to thank Peak Scientific for supplying the hydrogen generator, Kurt Weihe (UNT Physics Machine Shop) for machining the ion volumes, Roberto Aguilar for the CAD drawings, and John Beatty for synthesizing all of the explosives. References [1] H. Schütz, J.C. Gotta, F. Erdmann, M. Riße, G. Weiler, Simultaneous screening and detection of drugs in small blood samples and bloodstains, Forensic Sci. Int. 126 (3) (2002) 191–196. [2] J.S. Chiang, S.D. Huang, Simultaneous derivatization and extraction of amphetamine and methylenedioxyamphetamine in urine with headspace liquidphase microextraction followed by gas chromatography–mass spectrometry, J. Chromatogr. A 1185 (1) (2008) 19–22.
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Contents lists available at ScienceDirect
Science and Justice journal homepage: www.elsevier.com/locate/scijus
Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories Chinyere N. Nnaji, Kristina C. Williams, Jonathan M. Bishop, Guido F. Verbeck ⁎ Department of Chemistry, University of North Texas, Denton, TX 76201, USA
a r t i c l e
i n f o
Article history: Received 10 August 2014 Received in revised form 9 January 2015 Accepted 12 January 2015 Available online xxxx Keywords: GC–MS Hydrogen Carrier gas Buffer gas Illicit drugs Energetic materials
a b s t r a c t A custom set of ion volumes was manufactured in order to investigate the gain and byproducts using hydrogen as a buffer gas following electron ionization in a quadrupole ion trap mass spectrometer as compared with helium. Analyses of illicit drugs such as cocaine, codeine, and oxycodone, and explosives such as TNT, RDX, and HMTD with ion volume exit orifices of 1 mm, 2 mm, 4 mm, 6 mm, 8 mm and 10 mm were performed using GC/MS. Strong similarities between hydrogen and helium spectra of illicit drugs and explosives provide evidence that hydrogen can be used effectively as a buffer gas in an ion trap mass spectrometer. © 2015 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Helium is quickly becoming an expensive and limited commodity, especially with the closing of helium plants across the globe (Amarillo Natural Gas, Amarillo, TX). While the Ras Laffan Helium 2 plant in Qatar is said to produce 1.3 billion ft3/year, it is still just a temporary solution. With a projected capacity of 900 ton ft3, the Qatar-based plant contains only enough to sustain our needs for another 30 years. Limitations on helium usage may soon take effect, as it is used in several areas including magnetic resonance imaging (MRI), magnetocephalography (MEG), welding, and as a buffer gas for many chemistry and biology related analysis techniques. Currently, gas chromatography–quadrupole ion trap mass spectrometry (GC–QITMS) uses helium as both a carrier gas for GC and a buffer gas for the ion trap. Since GC–MS is the primary workhorse for analysis of illicit drugs [1–11] and explosives [12–14] in forensic laboratories, a review and comparison are presented here in order to illustrate the reduction of our need for helium as a carrier and buffer gas. Hydrogen is a highly effective carrier gas because it increases the speed of analysis time [15] and the resolution in GC [16,17]. Since the viscosity of hydrogen is lower than that of helium, there is less gas flow required to maintain the same linear velocity, achieving a faster chromatographic separation [18]. While the lower viscosity of hydrogen can lead to a lower column head pressure, the use of smaller diameter or ⁎ Corresponding author at: University of North Texas, Department of Chemistry, P.O. Box 305070, Denton, TX 76203, USA. Tel.: +1 940 369 8423. E-mail address: [email protected] (G.F. Verbeck).
longer length columns will resolve the issue of increased solvent expansion [19]. Resolution increases about 150% when using hydrogen as a carrier gas. Hydrogen has been used as a carrier gas with a variety of detection techniques including electron capture detection (GC–ECD) [20], flame ionization detection (GC–FID) [21], and GC–MS [16]. Ion traps utilize a background or buffer gas to induce collisional damping and thus increase mass resolution and sensitivity [22]. A lower molecular weight buffer gas helps prevent significant momentum changes, such as smaller displacement and velocities, upon collision with an analyte ion, minimizing the loss of trapped ions [22]. Helium and hydrogen are both ideal buffer gases in this case. Hydrogen has also been briefly explored showing mechanisms of ion–ion and ion–molecule interactions, including the tendency of many molecular ions to present as an M+1 peak due to the addition of a hydrogen atom [23]. The use of low RF amplitude was employed to trap low mass gases to facilitate ionization to show that the ionization mechanisms are different between hydrogen and helium as buffer gases. Helium was shown to react rapidly with background gases by facilitating the cleavage of diatomic nitrogen bonds. Monovalent nitrogen underwent charge-exchange reactions with water, initially giving an intense signal at m/z 18. As this peak grew in intensity, m/z 19 also grew due to self-protonation of the water ion. In comparison, hydrogen also reacted with background ions by self-protonation and ion exchange, producing signals at m/z 3, 28, and 29 corresponding to H+ 3 , + N+ 2 , and N2H respectively. Hydrogen has also recently been effectively utilized as a buffer gas in a field-deployable ion trap, though atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) were utilized rather than electron ionization (EI) [24].
http://dx.doi.org/10.1016/j.scijus.2015.01.003 1355-0306/© 2015 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.
Please cite this article as: C.N. Nnaji, et al., Hydrogen as a GC/MS carrier and buffer gas for use in forensic laboratories, Sci. Justice (2015), http:// dx.doi.org/10.1016/j.scijus.2015.01.003
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Another factor to consider when changing buffer gases in an external ionization source is the ion volume exit orifice. The ion volume is a region before the ion trap where the analyte is introduced, ionized, and diffusively ejected towards the trap. The most common ion volumes have an exit orifice that is approximately 1 mm in diameter and is often referred to as a closed or chemical ionization ion volume. Cascading ionization and interaction occur since the volume has such a small exit orifice. However, closed ion volumes pose a problem as they produce high energy ions and increase reactivity with non-inert buffer gases [25]. Another disadvantage of a closed ion volume is that fewer ions make it to the trap due to collisions with the ion volume near the exit orifice. Using an open ion volume, or an ion volume that does not have any walls perpendicular to exit flow, will have an increased gain according to calculations by Slobodenyuk [26]. This increased gain in conjunction with the absence of high-energy ion formation can facilitate the analysis of energetic materials. Lower energy ions exiting the ion volume will exhibit reduced side reactions with self-protonated species, and increased gain will improve spectra intensities. The use of compressed hydrogen in the laboratory comes with safety concerns; however, these have been greatly reduced with the advent of small, portable hydrogen generators which produce hydrogen and oxygen from water via electrolysis. The generators can often produce enough hydrogen to run up to 3 GC–MS devices with standard splitflow values without the concern of having compressed hydrogen in the lab. Current generators can run 10 years on a single catalyst with minimal water filter maintenance. The ability to switch from a helium-dependent lab to one that utilizes hydrogen will save forensic laboratories money as well as time in the event of a potential helium-shortage crisis in the future, thus investigation of relevant analytes has been performed. Here, custom ion volumes were machined in order to monitor ion fragmentation and intensity changes as a function of exit orifice diameter. Illicit drugs are investigated with both gases to showcase the utility of hydrogen in a forensic laboratory, a setting in which lack of variation between samples is key. High-energy materials, though typically difficult to analyze due to high reactivity and instability, were also analyzed using a combination of high gain and low-energy byproducts.
Fig. 1. CAD schematic of the ion volume (a) and ion volume array (b), 10 mm (left) to 1 mm (right) exit orifice.
the hydrogen gas. The modification of several ion volumes was performed by boring out 1 mm diameter ion volumes into diameters of 2 mm, 4 mm, 6 mm, and 8 mm, while 1 mm and 10 mm diameters were unmodified (Fig. 1). After switching gases, 12 h was allotted for the equilibration of the system.
2.3. GC–MS methods The GC–MS temperature program for explosives was 40 °C (3 min hold) to 300 °C (3 min hold) at 10 °C/min, with an inlet temperature of 250 °C, a transfer line temperature of 300 °C, and a split flow of 20 mL/min. The GC–MS temperature program for drugs was 70 °C (3 min hold) to 300 °C (5 min hold) at 20 °C/min, with an inlet temperature of 250 °C, and a transfer line of 300 °C and was run in
2. Material and methods 2.1. Materials Cocaine, codeine, phenobarbitol, diazepam, and oxycodone QuikChek standards (1 mg/mL in methanol) were purchased from Grace (Deerfield, Illinois). Methyl ethyl ketone peroxide (MEKP), trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), cyclotrimethylenetrinitramine (RDX), ethylene glycol dinitrate (EDGN), hexamethylenetriperoxidediamine (HMTD), triacetonetriperoxide (TATP), and pentaerythritoltetranitrate (PETN) were synthesized in our lab and characterized via mass spectrometry. Optima methanol (99.9%) was purchased from Fisher (Fisher Scientific, Hampton, New Hampshire). Acetonitrile (99.8%) was purchased from Sigma Aldrich (Sigma-Aldrich, St. Louis, Missouri). Solid explosive samples were synthesized and characterized in house, then prepared by dissolving 1 mg of each solid in 1 mL of acetonitrile. Liquid explosives were diluted in acetonitrile in a 1:100 (v:v) ratio. The drug standards were 1 mg/mL, then diluted 1:4 (v:v) in methanol. 1 μL of each sample was manually injected into the GC–MS. 2.2. Instrumentation and modifications A Thermo Focus GC was coupled to a Polaris-Q MS equipped with a 1 cm quadrupole ion trap (Thermo Scientific, Waltham, MA). The Polaris-Q was fitted with a gate valve in order to quickly and easily change ion volumes without breaking vacuum. For the experiments using hydrogen as a buffer gas, a Peak Precision Hydrogen Trace generator (Peak Scientific Instruments, Inchinnan, UK) was used to generate
Fig. 2. Air/water spectrum with helium (a) and hydrogen (b). The hydrogen spectra have a higher ratio of water, which is mostly protonated. Nitrogen is also only present in protonated form in the hydrogen spectrum.
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Fig. 3. Mass spectra obtained of cocaine (a), phenobarbital (b), codeine (c), and oxycodone (d) using a 2 mm ion volume with hydrogen and using a 1 mm ion volume with helium.
splitless mode. Each injection volume was 1 μL. A Restek RTX-1 (Restek, Bellefonte, PA) column was used (15 m × 0.25 mm ID × 0.1 μm film thickness). The ionization source was set to 200 °C with a maximum ion time of 25 ms. All drug analyses were performed using a mass range of m/z 40–350. Analyses of explosives had varying mass ranges of m/z 28–650 for PETN, TATP, EGDN, RDX (helium), and HMTD; m/z 50–650 for TNT, DNT, and RDX (hydrogen); and m/z 33–350 for MEKP. All spectra presented are the summed scan average of the peak, with a 25 μs ionization time, and 3 summed microscans per data point.
1 mm for helium and 2 mm for hydrogen were selected because they exhibited the highest intensities in the air/water spectra. When hydrogen is used as a buffer gas there is an order of magnitude increase for water (m/z 19) in comparison to the oxygen peak at m/z 32. This is due to excess H3O+ that is created from the air/water in the hydrogen gas. In the hydrogen spectrum nitrogen shows a more abundant M+1 peak. Evidence has been shown that the N2H+ ion forms using EI [27,28], but with excess hydrogen present during the ionization process, nitrogen shows a more abundant M+1 peak at m/z 29. This is in contrast to the presence of N2 and N2H+ at m/z 28 and 29 with the use of helium as a buffer gas.
3. Results and discussion 3.2. Drug sample comparison 3.1. Background Background spectra using both helium gas and hydrogen gas were collected. This was done to show the effects of the gases used on the peak shown. Fig. 2 shows the air/water spectra using 1 mm exit orifice for helium gas (a) and 2 mm exit orifice for hydrogen gas (b) to evaluate the changes due solely to changing of the gas. Exit orifice diameters of
A wide range of controlled and abused substances were run on the GC/MS using all six ion volumes with hydrogen and with helium. Fig. 3 shows that there is no difference between the spectral patterns for cocaine, phenobarbital, codeine, and oxycodone. Because EI is a particularly hard ionization technique, it is not uncommon to produce little to no molecular ion [29]. For this reason, common fragment
Fig. 4. Comparison of absolute intensity and the six ion volumes of cocaine's fragment peak, m/z 182.04, while using helium as the carrier gas.
Fig. 5. Comparison of absolute intensity and the six ion volumes of cocaine's fragment peak, m/z 182.04, while using hydrogen as the carrier gas.
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times. When the ion volume has a 10 mm exit orifice, absolute intensity with hydrogen is greater than with helium by 2 times. 3.3. Energetic materials
Fig. 6. Comparison of helium and hydrogen's absolute intensities in relation to the six ion volumes.
peaks for cocaine (m/z 182) and phenobarbital (m/z 204) are shown as well as the parent mass peaks of codeine (m/z 299) and oxycodone (m/z 315). Even though the ions themselves have electron rich oxygen or nitrogen species, there is no appreciable reaction with hydrogen, therefore no shift in the mass spectrum or alternative fragmentation patterns in any of these drugs is observed. Cocaine's primary fragment peak at m/z 182.04 was used to compare intensities resulting from each of the six ion volumes while using helium as the carrier and buffer gas (Fig. 4). An exit orifice of 10 mm produced the highest intensity while the 6 mm exit orifice produced the lowest. The same results were observed when hydrogen was used as the carrier and buffer gas (Fig. 5). For both gases, the 10 mm exit orifice ion volume was found to be optimal. However, exit orifices of 1, 2, 4, and 8 mm showed a different ordering of intensities (Fig. 6). In a closed ion volume, 1 mm, absolute intensity due to helium is greater than that due to hydrogen by 1.25
Eight different explosives (TNT, DNT, MEKP, PETN, RDX, EGDN, TATP, and HMTD) were synthesized and run on the GC–MS with both hydrogen and helium using the six ion volumes. As with the illicit drugs, the 10 mm exit orifice ion volume had the highest intensities with both buffer gases. This is especially necessary for analyses of energetic materials; since many of them are unstable compounds, the intensities tend to be much lower. Similar to the previously discussed illicit materials, most of the explosive material spectra were unchanged. Fig. 7 shows the relevant ion peaks that the PETN, TATP, EGDN, and RDX spectra produce independent of the gases which only differ in intensity. However, some explosives, such as TNT, DNT, MEKP, and HMTD, did show some differences in fragmentation and fragment ratios (Fig. 8). When helium was used as a carrier and buffer gas, 2,4-DNT shows a significant peak at m/z 119 that is absent in spectra collected using hydrogen. The peak at m/z 119 corresponds to a fragment with a single oxygen which is absent in the hydrogen spectrum. The fragment ratios of DNT and TNT are also different. HMTD also exhibits fragment peaks associated with hydrogen that are not observed with helium at m/z 55, 57, 61, 72, 77, 84 and 88. In the MEKP spectrum, both hydrogen and helium produce fragments at m/z 73 and 89. The interaction of hydrogen with the [C4OH+ 9 ] fragment causes loss of a hydroxyl or methyl group with subsequent protonation. This additional fragmentation could be advantageous to molecules that have hydroxyl groups, helping to further characterize a molecule. 4. Conclusions Hydrogen has been shown to be an effective replacement for helium as a carrier and buffer gas in GC–MS. Hydrogen has already been studied
Fig. 7. PETN (a), TATP (b), EGDN (c), and RDX (d) spectra using both helium and hydrogen as the carrier gas. No appreciable difference between the positions of the major peaks is observed.
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Fig. 8. TNT (a), MEKP (b), 2,4-DNT (c), and HMTD (d) spectra using helium gas and hydrogen gas. Notice the m/z 1 shift of 2,4-DNT with hydrogen compared with helium. There is a different fragmentation ratio for TNT with hydrogen between the spectra of TNT. MEKP has more fragmentation with hydrogen. HMTD was undetectable with helium except at m/z 88.
and confirmed to be a superior carrier gas compared to helium in GC, and our experiments show that hydrogen is an equally adequate buffer gas for mass spectrometry. With the use of hydrogen generators, both gases are safe for laboratory use; however, hydrogen is renewable, abundant, and significantly more inexpensive. The balance between reactivity and sensitivity will influence the choice of ion volume. Changing from a 1 mm to a 2 mm diameter exit orifice ion volume would enable an immediate transition, and the use of a 10 mm exit orifice would provide optimal intensities when switching to hydrogen for illicit drug analysis and energetic materials in forensic laboratory settings. The more delicate samples require open ion volumes, whereas hardier samples can be optimized in order to achieve higher sensitivity. Hydrogen gas may cause some slight variations in the mass spectra, usually as an M+1 peak or increased ion–molecule reaction chemistry; the formation of N2H+ ion supports this claim. With the increasing cost and depletion of helium, hydrogen is a suitable replacement for GC–QITMS as both a carrier gas and a buffer gas. This will be particularly true with the advent of databases built around using hydrogen in these systems rather than helium, furthering the ability to use hydrogen in a forensic laboratory setting. Acknowledgments The authors would like to thank Peak Scientific for supplying the hydrogen generator, Kurt Weihe (UNT Physics Machine Shop) for machining the ion volumes, Roberto Aguilar for the CAD drawings, and John Beatty for synthesizing all of the explosives. References [1] H. Schütz, J.C. Gotta, F. Erdmann, M. Riße, G. Weiler, Simultaneous screening and detection of drugs in small blood samples and bloodstains, Forensic Sci. Int. 126 (3) (2002) 191–196. [2] J.S. Chiang, S.D. Huang, Simultaneous derivatization and extraction of amphetamine and methylenedioxyamphetamine in urine with headspace liquidphase microextraction followed by gas chromatography–mass spectrometry, J. Chromatogr. A 1185 (1) (2008) 19–22.
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