Abstract
An evaluation of static headspace–multicapillary column-gas chromatography–ion mobility spectrometry (SHS-MCC-GC-IMS) has been undertaken to assess its applicability for the determination of 32 volatile compounds (VCs). The key experimental variables of sample incubation time and temperature have been evaluated alongside the MCC-GC variables of column polarity, syringe temperature, injection temperature, injection volume, column temperature and carrier gas flow rate coupled with the IMS variables of temperature and drift gas flow rate. This evaluation resulted in six sets of experimental variables being required to separate the 32 VCs. The optimum experimental variables for SHS-MCC-GC-IMS, the retention time and drift time operating parameters were determined; to normalise the operating parameters, the relative drift time and normalised reduced ion mobility for each VC were determined. In addition, a full theoretical explanation is provided on the formation of the monomer, dimer and trimer of a VC. The optimum operating condition for each VC calibration data was obtained alongside limit of detection (LOD) and limit of quantitation (LOQ) values. Typical detection limits ranged from 0.1ng bis(methylthio)methane, ethylbutanoate and (E)-2-nonenal to 472ng isovaleric acid with correlation coefficient (R2) data ranging from 0.9793 (for the dimer of octanal) through to 0.9990 (for isobutyric acid). Finally, the developed protocols were applied to the analysis of malodour in sock samples. Initial work involved spiking an inert matrix and sock samples with appropriate concentrations of eight VCs. The average recovery from the inert matrix was 101±18% (n=8), while recoveries from the sock samples were lower, that is, 54±30% (n=8) for sock type 1 and 78±24% (n=6) for sock type 2. Finally, SHS-MCC-GC-IMS was applied to sock malodour in a field trial based on 11 volunteers (mixed gender) over a 3-week period. By applying the SHS-MCC-GC-IMS database, four VCs were identified and quantified: ammonia, dimethyl disulphide, dimethyl trisulphide and butyric acid. A link was identified between the presence of high ammonia and dimethyl disulphide concentrations and a high malodour odour grading, that is, ≥ 6. Statistical analysis did not find any correlation between the occurrence of dimethyl disulphide and participant gender.
Highlights
Ion mobility spectrometry (IMS) was developed in the latter half of the last century in response to the need by the military agencies for a fast and sensitive technique for the detection of chemical warfare agents, explosives, hazardous chemicals and drugs [1,2,3,4]
The overall aim of this article was to evaluate the performance of SHS-multicapillary columns (MCC)-GC-IMS for the analysis of volatile compounds in complex matrices
The chemistry of the ionization process is generally not well presented in the literature. This discussion will attempt to rectify that situation by providing a logical series of essentially balanced chemical equations which adequately describe the formation of the reactant ion or the formation of dimers and trimers
Summary
Ion mobility spectrometry (IMS) was developed in the latter half of the last century in response to the need by the military agencies for a fast and sensitive technique for the detection of chemical warfare agents, explosives, hazardous chemicals and drugs [1,2,3,4]. Ion mobility spectrometry and time-of-flight mass spectrometry (TOFMS) are similar in the sense that ionized compounds are separated on the basis of their charge and size by passage along a tube, the drift tube, under the influence of an electric field. In the case of TOFMS, it separates fragment ions whereas IMS being a soft ionization technique only generates and separates molecular ions. In contrast to TOF-MS, IMS is an atmospheric technique and as soft ionization is used, only molecular ions of volatile organic compounds need to be resolved. An IMS instrument is compact, ideal for use in the field, relatively simple to operate, sensitive and produces the results very rapidly. Many instruments are used by the security operations at airports in addition to the military [5]
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