M Wooding PhD Abstract

Chemical profiling of the human skin surface for malaria vector control using combined chromatography and mass spectrometry techniques

Odour mosquito lures are currently used as part of integrated vector  control strategies in the fight against malaria. Variation in inter-human attractiveness to mosquitoes, and the preference of mosquitoes to bite certain regions on the human host, are possible avenues for identifying potential mosquito attractants and repellants. The chemical complexity of the human skin surface and ethical considerations when sampling humans call for non-intrusive sampling solutions that enables the detection of a wide range of chemical compounds, whilst not impeding on the dignity of the individual sampled, and as well as for sophisticated analytical techniques that enable the detection of low concentration chemicals in a complex matrix.

To address challenges associated with human skin sampling, a practical noninvasive sampling approach was investigated to sample the wrist and ankle skin surface area of human volunteers. In-house developed polydimethylsiloxane (PDMS) sorptive samplers were fashioned into anklets and bracelets and placed in direct contact with the skin surface for ease of sampling. A barrier, namely a Mylar® sheet, was used to prevent atmospheric contamination of the skin surface and the PDMS sampler. An internal standard was added to the PDMS samplers, using solvent modifications, before sampling to compensate for any variations during sampling and analysis. Solvent modification of the sampler with isopropanol did not bring about the expected improvement of recovery of more polar compound, however, a possible increase in analyte capacity with solvent modification was noted. 

To enable a more comprehensive chemical profiling of the human skin surface, two complementary analytical approaches were followed namely direct thermal desorption of the PDMS sampler in the GC inlet liner with comprehensive gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS) and solvent desorption of the PDMS sampler in an LC vial with ultra-performance liquid chromatography with ion mobility spectrometry coupled high-resolution mass spectrometry (UPLC-IMS-HRMS). Both approaches provided reliable data sets to be used in chemometric techniques such as principal component analysis (PCA), orthogonal partial least squares discriminant analysis (OPLS-DA) and S-plots in the investigation of inter- and intra-human differences for skin surface chemical profiles. Mosquito biting site preference, ankle vs wrist, as well as perceived mosquito attractiveness between individuals, were investigated. 

Compounds from a broad range of chemical classes were detected and tentatively or unequivocally identified as contributing to the differences in human skin surface chemical profiles using GC×GC-TOFMS and UPLC-IMS-HRMS. Known mosquito semiochemicals were also detected on the human skin surface. In addition to known human skin compounds three compounds, (-)-carvone, (E)-2-decenal and caffeine, not previously reported were unequivocally identified. Limits of detection, using GC×GC-TOFMS, ranged from 1 pg (carvone) to 362 pg (indole). Method limits of detection ranged from 8.7 ng (sulfadimethoxine) to 95 ng (taurine) for the UPLCIMS- HRMS method.  These complementary approaches enabled the detection and identification of as yet unknown human skin surface chemicals consisting of highly-volatile to nonvolatile compounds from endogenous and exogenous sources. As very few have been reported in the literature, collision cross section (CCS) values of all observed adducts are presented and the benefit of CCS libraries and modelling is demonstrated.

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