Hydrology: Current Research

Dehydration of 2-methylisoborneol to 2-methyl-2-bornene in the Trace Analysis of Taste-Odorants in Water by Purge-and-Trap Sampling with Gas Chromatography (GC) -Mass Selective (MS) Detection

Abstract

Thavrin Manickum and Wilson John

A “salt-free”, automated purge-and-trap sampling method, with gas chromatographic-mass spectrometric detection for simultaneous detection of 2-methylisoborneol (2-MIB) and geosmin (GSM) was developed. The preliminary procedure involved purging a 25 mL water sample, at 80^°C for 11 min, with helium gas, and trapping onto a Tenax sorbent. Target analytes were desorbed with helium gas onto the gas chromatograph, coupled to a mass spectrometer, via a transfer line. Chromatographic separation was performed on HP5-MS capillary column (30 m x 0.25 mm x 0.25 μm), followed by mass spectrometric detection in the selected ion monitoring mode. During the preliminary method optimization and development stages, inconsistencies were noted regarding the relative ratios of ions m/z 95 and 107, two ion fragments frequently used for identification and subsequent quantitation of 2-MIB. NIST reference mass spectra, liquid auto sampler injection of commercial standards of the target analytes, and gas chromatographic retention times were used to confirm peak identification on the chromatograms of aqueous standards analyzed by purge-and-trap, with mass selective detection. The effect of mass spectrometer source temperature and use of a 6 mm drawout plate in the source body, on the relative ratios of m/z 95, 107, 108, for 2-MIB, and m/z 111, 112, 125, for GSM, and precision, was investigated. It was found that use of the latter had no significant effect on the ratios. The observed data indicated that 2-MIB dehydrates to 2-methyl-2-bornene (2-M-2-B) during the purge-and-trap cycle conditions used, presumably when the sample is initially purged at 80^°C. The relative retention times were ± 9.2min, ± 12.0min and ± 15.2min, for 2-M-2-B, 2-MIB and GSM, respectively, under the gas chromatographic conditions. Preliminary results indicate suitability of the technique for trace analysis of GSM in water samples over the 5-100 ng/L analytical range.