Versatile silicone rubber samplers for trace organic analysis in a chromatography-mass spectrometry laboratory

Abstract

Extraction is required to separate and concentrate trace level analytes from the sample matrix prior to gas chromatography (GC). Classical extraction procedures utilise large amounts of hazardous solvents, generate waste, and sensitivity limitations are associated with the injection of microlitre amounts of the final solvent extract. In response to real world challenges, and to overcome the problems associated with solvent extraction, novel silicone rubber (polydimethylsiloxane (PDMS)) samplers were developed for solvent free enrichment of trace level analytes from indoor air, contaminated soil, desert soil, ultra high temperature (UHT) milk and Pinotage wine. Versatile PDMS samplers as a loop, a multichannel trap, or a denuder for trace environmental forensics, geochemical and aroma investigations are presented. A unique off-line multidimensional GC approach involving heart-cut gas chromatographic fraction collection is described, as is off-line olfactory assessment of recombined heart-cuts for synergistic odour effects. PDMS loop samplers were used for the extraction of DDT (1,1,1- trichloro-2,2-bis(p-chlorophenyl)ethane) and its associated environmental pollutants from soil samples. Miniature denuder samplers accomplished separate concentration of vapour phase and of particulate phase fractions of DDT and its associated environmental pollutants from indoor air, in a single step. Ratios of airborne p,p’-DDD/p,p’-DDT and of o,p’-DDT/p,p’-DDT are unusual and do not match the ideal certified ingredient composition required of commercial DDT. Results suggest that commercial DDT used for indoor residual spraying may have been compromised with regards to insecticidal efficacy, demonstrating the power of this new environmental forensics tool. Multichannel PDMS trap samplers were used in a unique heart-cut multidimensional GC approach for off-line enantiomeric separation of o,p’-DDT and o,p’-DDD in air and soil. This alternative multidimensional method is compared to the complementary technique of comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry detection (GCxGC-TOFMS). PDMS loop samplers were also employed for the solvent free extraction of hydrocarbons from desert soil to investigate, for the first time, a possible geochemical origin of the enigmatic fairy circles of Namibia. It is proposed that microseepages of natural gas and low volatility hydrocarbons are expressed at the surface as a geobotanical anomaly of barren circles and circles of altered vegetation. Multichannel PDMS trap samplers were utilised for sampling of the headspace of UHT milk and of Pinotage wine, and to study off-line, using a portable olfactometer, synergistic effects between recombined heart-cut aroma compounds. Olfactory results show that a synergistic combination of 2- heptanone and 2-nonanone was responsible for a pungent cheese-like odour in UHT milk, while a synergistic combination of furfural and 2-furanmethanol was responsible for a roast coffee bean-like odour in coffee style Pinotage wine. The small, low cost samplers are quick and easy to assemble and they fit commercial thermal desorber systems. The PDMS samplers are reusable. Solvent extraction of the sampling materials, extract clean-up and pre-concentration are not required. Thus, potential loss of analyte, introduction of contaminants and waste disposal are minimised. Solvent free thermal desorption permits transfer of the entire sample mass to the cooled injection system (CIS) inlet of a GC resulting in greater sensitivity when compared to injection of microlitre amounts of a solvent extract. This allowed for sampling of smaller sized soil samples, shorter air sampling times and lower air sampling flow rates when compared to solvent based methods.