Because GC and GC-MS are not always sufficient for olfactory analysis, chemists turn to gas chromatography–olfactometry (GC-O) for the detection of low-level odor-active compounds. Though powerful, GC-O is a time-consuming technique.
Brechbuehler, Inc. (Houston, TX) has a long history of developing hardware and software for GC-O. The Odor Search tool is designed to facilitate the transition between GC-MS and GC-O experiments. By comparing the library search results to a database of odoractive compounds, the authors were able to predict the area of interest to the GC-O panelist. In addition, the software may aid in identifying odor-active compounds without having to perform GC-O. This article describes the analysis of a soft drink odor profile.
Identification of odor-active compounds
Food, beverage, and other consumer product analyses can lead to complex chromatograms. A challenge facing the analytical chemist is the identification of odor-active compounds. GC-MS can easily identify compounds at low levels. However, library searches do not yield odor information. A link between the library and the odor database is needed. Odor Search is such a link.
Odor Search takes the results from a library search performed from a GC-MS run and compares each GC-MS library match to the internal odor database. It then lists all common compounds found both in the chromatogram and in the odor database. Xcalibur software (Thermo Fisher Scientific, Waltham, MA) automatically processes the data file, integrating the chromatograms and a library search of the peaks found. It produces a results file (filename.rst) containing the information on the data processed (i.e., peak data, library matches, etc.).
Odor Search reads the results file and compares each library match based on the CAS number. It displays the chromatogram and raw spectrum as well as the library spectrum. Once a search is completed, it is possible to search for a specific odor in the results. The software's odor library consists of 5889 compounds (some duplicates) created from data gathered from the Internet.
Fragrance and flavor companies usually create their own libraries, and it is possible to import these data into Odor Search. While the number of fields is unlimited, there are some restrictions to the entries. The software requires at least the compound name, CAS number, and associated odor. Additionally, specific information includes the structure file (jpg, bmp, or gif) and retention indexes. The remaining data, such as molecular weight, formula, and boiling point, are treated as custom information.
Comparing chromatograms
Figure 1 - Chromatogram library. Left: list of chromatograms. Right: compounds from the selected chromatograms.
Odor Search also allows comparisons of chromatograms. A chromatogram database can be created from the results file. For example, both acceptable and unacceptable samples can be entered. The current chromatogram can be searched against this library of chromatograms. The common peaks between the best match and the sample can then be removed from the list of odor-active compounds, leaving the compounds of interest. Figure 1 shows the chromatogram library and compounds from the selected chromatograms.
Figure 2 - Results from chromatogram search. Top left: list of matches—number of compounds from the sample found in the oil, total number of compounds in the oil from the chromatogram library, percent found. Bottom left: list of compounds (RT, CAS, and ratio) in the sample matched with the reference chromatogram. Right: list of compounds from the reference chromatogram. Items shown in red are the matched compounds.
Another application is to enter a database of essential oils. The sample can then be compared to the chromatogram database; if it contains an essential oil, it will show as a match. Results from a chromatogram search are given in Figure 2.
Analysis of an energy drink
Figure 3 - GC-MS run of a soft drink analyzed on a Trace DSQII.
An energy drink was analyzed by GC-MS using the ITEX (in-tube extraction) headspace technique (CTC Analytics, Zwingen, Switzerland). The GC-MS was run on a Trace DSQII system (Thermo Fisher Scientific) (see Figure 3). Conditions were as follows—40 °C for 1 min to 100 °C at 5 °C/min to 250 °C at 10 °C/min, hold for 5 min; injector—250 °C, split (25:1); MS—scan 1: 10–150 AMU for 0.2 sec, scan 2: 34–365 AMU for 0.16 sec; sampling by ITEX.
Figure 4 - Odor Search results from the peak at 19.90 min.
Figure 5 - Chromatogram results from GC-O. a) Volatile section of the chromatogram, b) middle section, c) high-temperature section. Shown in green is the peak detected by the majority of panelists during the GC-O experiments.
Figure 6 - Summary report showing the total ion chromatogram (TIC) and list of compounds detected with the following information: peak number (sorted by area), retention time, compound name, odor from the odor library, similarity index from the NIST search, formula from NIST, and CAS number from NIST.
Figure 7 - Single peak report (first match shown only) showing the library match information, the odor and synonyms from the odor library, the formula, chromographic peak (from the data file), and the library spectrum for the displayed compound.
Odor Search results
The search results from the above chromatogram are shown in Figure 4. The GC-MS library used was the NIST and FFNSC (Flavor and Fragrances of Natural and Synthetic Compounds). For improved accuracy of the library search, the chromatogram was processed with ClearView™ GC-MS data reprocessing software (ALMSCO International, Llantrisant, U.K.) for background subtraction. Results from the GC-O experiment using Odor Search are given in Figure 5. The software reports either a summary (Figure 6) or a single peak report (Figure 7) of all peaks identified.
Conclusion
Odor Search is a powerful tool for determining odor-active compounds. For some of the samples, the software was able to analyze taste. For other samples, a combination of GC-MS, Odor Search, and GC-O was used. The chromatogram search function compares the sample to the stored chromatograms. This can be used to determine essential oils contained in the samples.
Mr. Mottay is with Brechbuehler, Inc., 3845 FM1960 RD W #275, Houston, TX 77068, U.S.A.; tel.: 281-880-6963; fax: 281-880-9496; e-mail: [email protected]. Dr. Stoop, Mr. Picher, and Dr. Hofstetter are with Brechbühler AG, Schlieren, Switzerland.