The strong adsorption of amines to the stationary phase
The strong adsorption of amines to the stationary phase of gas chromatography (GC) columns causes issues such as tailing, ghosting and low reproducibility , . A common practice to overcome this problem is the chemical derivatisation of amines prior to GC analysis . Comprehensive reviews of the various derivatisation reactions used in conjunction with amines are available in the literature , . Gas-chromatography-mass spectrometry (GC–MS) is commonly used for forensic drug analysis and is often combined with derivatisation to increase sensitivity, improve reproducibility, and prevent the formation of extraneous artifacts during analysis , . With the advent of microextraction techniques such as solid phase microextraction (SPME), on-sorbent derivatisation processes, which combine the sampling, pre-concentration and derivatisation into a single step, have streamlined and enhanced forensic analytical capability by lowering limits of detection and providing timely results , , , .
Methamphetamine can undergo many derivatisation reactions, including silylation, acylation, carbamate formation and Schiff ACY-241 formation; however, many of these are not feasible for on-sorbent derivatisation as they require strict reaction conditions such as high temperatures or the total absence of moisture . The formation of carbamate derivatives are favourable, since they occur rapidly, at room temperature, and the precursors and products are relatively stable. The general scheme for the formation of the carbamate derivative of a primary or secondary amine via reaction with a chloroformate is shown in Scheme 1.
Although carbamate derivatives of methamphetamine have been studied extensively both in-solution and on-sorbent, the use of pentafluorobenzyl chloroformate as a derivatising agent for methamphetamine has not been reported previously, to the best of our knowledge , , , . However, pentafluorobenzyl carbamates have been reported to be easily formed, stable derivatives, with good peak shape, and structurally informative fragments .
A microextraction technique called capillary microextraction of volatiles (CMV) that was recently developed by Almirall et al. has been successfully used for the sampling and subsequent analysis of drugs , , explosives , , and volatile organic compounds . The devices allow for active gas sampling, and we reported that the devices were 30 times more sensitive than a similar dynamic SPME method, when airborne methamphetamine sampling was coupled with subsequent GC–MS analysis , . Furthermore, the devices could be stored for up to 3 days post-sampling of methamphetamine, without any significant loss of the analyte. This would allow for remote in-field sampling followed by subsequent laboratory analyses and could have applications in clandestine laboratory investigations.
In this study, we report the use of an on-sorbent derivatisation process using CMV devices, for the sampling of airborne methamphetamine for analysis by GC–MS. While we had previously demonstrated that the increased sensitivity of CMV sampling allowed for lower detection limits for underivatized airborne methamphetamine, the significant tailing of the methamphetamine chromatographic peak prevented accurate integration and subsequent quantitation, particularly for low amounts of analyte . We hypothesise that sampling airborne methamphetamine using CMV devices preloaded with pentafluorobenzyl chloroformate would result in the in-situ formation of the stable methamphetamine derivative, yielding an amenable, characteristic chromatographic peak and enabling lower limits of detection for airborne methamphetamine.
Materials and methods The CMV devices used in this study were prepared by forming a polymerised siloxane sol-gel coating on glass fibre filters as has been previously described , . Glass fiber filters were obtained from Sartorius Stedium Biotech. (diameter 4.7 cm) and were treated with a piranha solution, which consisted of a 2:1 mixture of conc. H2SO4 (J.T. Baker, 95–98%) and 30% H2O2 (Univar), prior to coating with siloxanes. Vinyl terminated polydimethylsiloxane (vt-PDMS), polymethylhydrosiloxane (PMHS), trimethoxymethylsiloxane (MTMOS) and acetonitrile were obtained from Sigma Aldrich. The glass housings for the CMV devices consisted of 2 cm sections of NMR tubes (Norell Inc., i.d. 2.4 mm). Dichloromethane (99.9%) was obtained from Scharlau and sodium hydroxide was purchased from ECP Ltd. Trifluoroacetic acid (Applichem) was used to prepare a 95% (v/v) solution in deionised water.