The highly cited method of Lloyd and Grimm1 for off-flavor analyte detection in fish has seen wide-spread use over the past two decades. The method relies on microwave heating of fish tissue to generate steam; analytes with high vapor pressures are distilled with the steam, which is cooled and captured as an aqueous sample. The quick sample preparation method, coupled with solid-phase microextraction (SPME), is an attractive option for multiple reasons. SPME use with gas chromatography-mass spectrometry (GCMS) provides enhanced sensitivity compared to traditional liquid injections. Automated SPME interfaces the sample concentration step with introduction into the GC inlet.
Despite its popularity, certain aspects of the microwave-SPME-GCMS method are not optimal. Mass spectral detection of sub-ppb concentrations of MIB and GSM requires the use of selected ion mode (SIM) sampling; this provides sensitivity for targeted compounds but excludes information from other taste and odor compounds that may be present in a sample. Furthermore, the microwave and glassware are configured for single-sample processing, limiting the ability to perform multiple analyses and analytical replicates.
Building upon the benefits of the Lloyd and Grimm method, an improved method was developed which provides the option of analyzing the MIB and GSM content of 20 samples simultaneously. The method uses a repurposed GC oven as a heating source, with PTFE tubing and septa connecting gas flow to distillation and collection vessels. The opportunity to perform concurrent distillations of multiple samples facilitates quality assurance measures to be taken (such as blanks, standards, and spiked replicates). The SPME Arrow, which offers a stronger core and thicker adsorbent phase, was used in place of traditional SPME. Additionally, synchronous SIM/scan detection provided sensitive signal with complete scan data from each acquisition.
GSM levels in fish tissue (trout and catfish), fish feed, and fish feces were determined using the improved method. Recovery values of 45(±7)% and 21(±6)% were obtained for GSM distilled from water and trout, respectively. Linear analyte recovery (r2 = 0.991) over a wide concentration range (0.1 to 10 ppb) was achieved through simultaneous distillation of 20 samples of spiked trout. Semi-quantitative information from over 30 volatile compounds (aldehydes, ketones, and alcohols) was also obtained during off-flavor analysis by SPME-GCMS.
1. Lloyd, S. W.; Grimm, C. C., Analysis of 2-Methylisoborneol and Geosmin in Catfish by Microwave Distillation−Solid-Phase Microextraction. Journal of Agricultural and Food Chemistry 1999, 47 (1), 164-169.