Direct Immersion Versus Headspace SPME Sampling of Whiskey Samples

Solid phase microextraction (SPME) is a nonexhaustive sampling technique in which a coated fiber is exposed to a sample, the analytes of the sample adhere to the fiber and the fiber is then desorbed onto a gas chromatograph coupled to a detector for separation and analysis. There are two types of SPME sampling techniques. The first entails bringing a sample to equilibrium and exposing the SPME fiber to the headspace of the sample. The second involves placing the SPME fiber directly into the liquid phase of the sample and allowing the analytes to adhere to the fiber directly from the sample. This study examines both SPME sampling techniques using whiskey samples.

Whiskey is comprised of both volatile and nonvolatile flavor components. To fully understand the complexities of a whiskey sample, distilleries often use different sampling techniques. Solid phase microextraction (SPME) is one of those techniques. Since SPME involves extracting flavors onto a fiber, the fiber extraction coating is integral to separating the analytes of interest out of the matrix. Furthermore, the SPME sampling method plays a role in obtaining an accurate flavor profile.

Experimental

The EST Analytical Flex Robotic Sampling System was installed on an Agilent 7890A GC and 5975 inert XL MS. A 50:30 μm divinylbenzene–carboxen–polydimethylsiloxane (DVB–CAR–PDMS) coated fiber was fitted on the Flex for analyte extraction. For analyte separation, a Restek Stabilwax DA 30 m × 0.25 mm × 0.25 μm column was mounted in the GC.

In order to perform the headspace SPME sampling, 1 g of sodium chloride was added to each sample vial along with 5 mL of whiskey. The samples were then sealed in a 20 mL headspace vial. For the direct immersion SPME, 10 mL of whiskey was added to the sample vial and sealed. The Flex method builder software enabled method development for both types of analyses. Several different fiber coatings were tried in order to establish the best fiber for the application. Furthermore, optimum method parameters for each type of extraction were established.

Figure 1: SPME technique comparison.

Results

Although the same SPME fiber was used for both techniques, the direct immersion SPME was able to extract the heavier compounds in the matrix much more readily than the headspace SPME. Since the heavier compounds do not separate out into the headspace as readily as the lighter analytes, this was expected. On the other hand, headspace SPME extraction is not as hard on the fiber as direct immersion SPME, thus the number of extractions that can be done with one fiber is much greater when using headspace SPME.

Conclusions

The Flex with the SPME option provided an excellent platform to automate both types of SPME extractions. SPME provided an impressive amount of information on the analytes in the whiskey samples whether using headspace or immersion extraction. Thus, the technique for sampling the whiskey sample would be dependent upon the analytes of interest in the sample and the number of extractions required of the fiber.

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