Gas Chromatography-Mass Spectrometry Paired with Total Vaporization Solid-Phase Microextraction as a Forensic Tool

Summary

Total Vaporization Solid Phase Microextraction (TV-SPME) completely vaporizes a liquid sample whilst analytes are sorbed onto a SPME fiber. This allows for partitioning of the analyte between only the solvent vapor and the SPME fiber coating.

Abstract

Gas Chromatography – Mass Spectrometry (GC-MS) is a frequently used technique for the analysis of numerous analytes of forensic interest, including controlled substances, ignitable liquids, and explosives. GC-MS can be coupled with Solid-Phase Microextraction (SPME), in which a fiber with a sorptive coating is placed into the headspace above a sample or immersed in a liquid sample. Analytes are sorbed onto the fiber which is then placed inside the heated GC inlet for desorption. Total Vaporization Solid-Phase Microextraction (TV-SPME) utilizes the same technique as immersion SPME but immerses the fiber into a completely vaporized sample extract. This complete vaporization results in a partition between only the vapor phase and the SPME fiber without interference from a liquid phase or any insoluble materials. Depending upon the boiling point of the solvent used, TV-SPME allows for large sample volumes (e.g., up to hundreds of microliters). On-fiber derivatization may also be performed using TV-SPME. TV-SPME has been used to analyze drugs and their metabolites in hair, urine, and saliva. This simple technique has also been applied to street drugs, lipids, fuel samples, post-blast explosive residues, and pollutants in water. This paper highlights the use of TV-SPME to identify illegal adulterants in very small samples (microliter quantities) of alcoholic beverages. Both gamma-hydroxybutyrate (GHB) and gamma-butyrolactone (GBL) were identified at levels that would be found in spiked drinks. Derivatization by a trimethylsilyl agent allowed for conversion of the aqueous matrix and GHB into their TMS derivatives. Overall, TV-SPME is quick, easy, and requires no sample preparation aside from placing the sample into a headspace vial.

Introduction

Solid-Phase Microextraction (SPME) is a sampling technique in which a liquid or solid sample is placed into a headspace vial and a SPME fiber, coated with a polymeric material, is then introduced into the sample headspace (or immersed in a liquid sample). The analyte is sorbed onto the fiber and then the fiber is placed inside the GC inlet for desorption^1,2. Total Vaporization Solid-Phase Microextraction (TV-SPME) is a similar technique as immersion SPME but completely vaporizes a liquid sample before analytes are adsorbed onto the fiber. This allows for partitioning of the analyte between only the solvent vap....

Protocol

1. General TV-SPME sample preparation and GC-MS analysis

NOTE: If the sample is already dissolved in a matrix, skip to Step 1.2.

1. Extract or dissolve the solid sample in enough solvent (water, methanol, acetone, etc.) to reach the desired concentration. Liquid samples can be used “as is”.
   NOTE: The amount of solid sample used depends on the desired concentration of the sample. Concentrations below 1 ppm (w/v) are recommended to avoid overloading the GC column. Analyte should be soluble in chosen organic solvent.
   1. Ensure that the sample has fully dissolved.
2. Calculate the volume....

Results

A GBL volume study was performed to demonstrate the sensitivity of TV-SPME compared to headspace and immersion SPME. A 100ppm_v sample of GBL in water was prepared and placed into 20 mL headspace vials with volumes of 1, 3, 10, 30, 100, 300, 1000, 3000, and 10,000 µL. The phase ratio of the samples allowed for TV-SPME (1-3 µL), Headspace SPME (10 – 3,000 µL) and Immersion SPME (10,000 µL). All samples were analyzed in triplicate and the average peak area was plotted against the sample .......

Discussion

TV-SPME has some benefits over liquid injection GC in that large sample sizes (e.g., 100 µL) may be used without instrument modifications. TV-SPME also has some of the same benefits as headspace SPME. Headspace SPME does not require any extraction or filtration because any nonvolatile compounds will remain in the headspace vial and will not be adsorbed onto the fiber, yielding a clean sample. This method also helps to eliminate matrix effects due to this being a two-phase system (headspace and fiber) as opposed to a.......

Materials

Name	Company	Catalog Number	Comments
10 µL Syringe	Gerstel	100111-014-00
BSTFA + 1% TMCS (10 x 1 GM)	Regis Technologies Inc.	50442882
eVol XR Sample Dispensing System Kit	ThermoFisher Scientific	66002-024
-Butyrolactone (GBL)	Sigma-Aldrich	B103608-26G
-Hydroxy Butyric Acid (GHB)	Cayman Chemicals	9002506
Headspace Screw-Thread Vials, 18 mm	Restek	23083
Magnetic Screw-Thread Caps, 18 mm	Restek	23091
Optima water for HPLC	Fisher Chemical	W71
SPME Fiber Assembly Polydimethylsiloxane (PDMS)	Supelco	57341-U
SPME Fiber Assembly Polydimethylsiloxane/Divinylbenzene (PDMS/DVB)	Supelco	57293-U
Topaz 2.0 mm ID Straight Inlet Liner	Restek	23313