Selectable 1D or 2D GC-MS in Combination with Olfactometry, Preparative Fraction Collection, and Aroma Office 2D Database for Odor Analysis

GC-Olfactometry (GC-O) is a valuable method for the selection of odor components from a complex mixture. In particular, GC-O in combination with MS (GC-O/MS) allows not only evaluation of the odor compounds, but also identification with mass spectral information. However, many key odor compounds can occur at very low concentrations. Therefore, the identification of odor compounds remains a hard task even with GC-O/MS because some compounds co-elute with other analytes or sample matrix, which leads to difficulties when correlating the detected aroma with the right compound.

Heart-cutting two-dimensional (2D) GC-MS with simultaneous olfactometry can significantly improve the identification capability as well as the resolution of complex regions. However, for conventional 2D GC-MS analysis, one usually requires a dedicated 2D GC-MS system, which is then not available for routine 1D GC-MS analysis as it is configured. Therefore, one has to have two different GC-MS systems (1D GC-MS and 2D GC-MS) or one has to re-setup a single GC-MS system for heart-cutting 2D GC-MS applications. Also, the first dimensional chromatogram obtained by heart-cutting 2D GC-MS is normally generated with a monitor detector such as flame ionization detector (FID). Thus, selection and confirmation of the heart-cut region for heart-cutting 2D GC-MS analysis has to be done without mass spectral information. In this case, preliminary tests with the injection of an authentic compound or known compound prior to each heart-cutting 2D GC-MS application would be required.

In the past few years, in our laboratory, we developed a novel selectable 1D/2D GC-MS system for simple and fast operation of both 1D GC-MS and 2D GC-MS with simultaneous olfactometry or element-specific detection using a single GC-MS system. The selection of 1D/2D GC-MS operation is only performed by a mouse click, without any instrumental set-up change. Also, this system can eliminate preliminary analysis with a “monitor FID” for the selection of the heart-cut region, and can provide a “monitor total ion chromatogram (TIC)” for the 1D column separation on 2D GC-MS analysis. Moreover, simultaneous mass spectrometric and olfactometry or element-specific detection can be performed for both the 1D column separation and the 2D column separation under a constant split ratio.

2D GC-O/MS does not obtain mass spectrum for the olfactory detected compounds in some cases (no peaks on the second dimensional TIC at the corresponding retention times), particularly when analyzing highly complex aromas. In this case, it is essential to have an enrichment step before final MS detection. Recently, we developed a simple preparative fraction collection device called “single PFC” with an adsorbent bed packed tube for subsequent thermal desorption (TD). The single PFC can be combined with the TD-selectable 1D/2D GC-O/MS system. After the fraction collection and the enrichment of the olfactory detected compound over dozens of injections, the trapped compound is thermally desorbed into the same system. Finally, the desorbed compound is analyzed by 2D GC-O/MS [5].

The use of retention indices (RI) in conjunction with GC–MS in odor analysis is well established and many researchers use such procedures in their routine analysis to identify odor compounds. Novel RI database called “Aroma Office 2D” (GERSTEL K.K./Nishikawa Keisoku Co. Ltd, Tokyo, Japan) is the searchable database contains RI information for a wide array of odor compounds (72,120 entries) from many literature references. The key features of Aroma Office 2D is as follows;

• An RI search can be performed on conventional 1D data.
• A cross search can be used for two RIs obtained on different columns.
• A cross search can be performed by using two different RIs acquired from both 1D and 2D  chromatograms, as well as a combined search with a single RI and mass spectraum.

In this study, a combined approach consisting of selectable 1D/2D GC-MS, olfactometry, single PFC, and Aroma Office 2D database for odor analysis is presented. The feasibility and benefits of the approach is demonstrated with analysis of trace odor compounds in complex samples, e.g. beer, wine, and whiskey. Also, novel sample preparation-introduction technique, e.g. stir bar sorptive extraction (SBSE) –TD is combined with the proposed approach to perform miniaturized and solvent-free method.