A case study
Figure 2 presents the total ion chromatogram obtained from the MS detector operating in scan mode during a multidimensional
GC analysis. Not only does this result demonstrate the complexity of the GC analysis, but it also shows a crucial aspect of
odor testing. The numerous peaks in the chromatogram represent the volatile and semivolatile compounds collected from the
headspace of the sample. Both odorless and odorous compounds are represented by these peaks, but identifying the compounds
associated with each of the peaks is an enormous task. From the list of compounds generated, determining which compounds are
responsible for the off odor from this sample is an even larger task. Scientists ultimately determined that this sample was
contaminated with TBA. Under these experimental conditions, TBA elutes at approximately 30 min.
Figure 2: Total ion chromatogram from a TBA contaminated sample.
By combining olfactometry with the GC–MS analysis, the retention times at which odor-causing compounds elute can be identified
quickly. Figure 3 is an aromagram, which is a graphical presentation of the sample's aroma and intensity versus retention
time in the GC–MS chromatogram format. Each aroma detected at the sniff port by the scientist during the olfactory analysis
is assigned descriptors and a relative intensity. The aromagram shows numerous aroma notes that indicate the presence of compounds
whose concentrations have exceeded their odor-threshold concentrations. Although numerous aroma compounds were sensed during
the analysis, a single, large, character-defining odorant peak occurred at a retention time of approximately 30 min. It should
be noted that the mass spectrometer operating in scan mode detected nothing but baseline noise at this retention-time region.
Figure 3: Aromagram from a TBA contaminated sample.
In Figure 4, the gas chromatogram and the aromagram have been combined, and the retention-time axis has been expanded from
24 to 32 min. Both the mass spectrometer trace from the gas chromatogram and the aromagram show several compounds in this
region. However, the retention times for these compounds do not coincide with each other. Thus, the compounds indicated in
the GC trace are odorless or below their odor-threshold concentration. The aromas indicated in the aromagram trace have exceeded
their odor-threshold concentration, but are below the detection limit of MS. The same situation obtains for TBA, where the
aromagram shows an intense peak that cannot be identified on the MS scan.
Figure 4: An expanded combination plot of the gas chromatogram (red curve) with the aromoagram (black curve).
This retention-time area must be investigated closely to confirm the identification of the compound with the mass spectrometer.
Two methods may provide compound verification. The first method involves sample preconcentration of the headspace before analysis.
The second method is to operate the mass spectrometer in selective ion monitoring (SIM) mode for ions representative of a
short list of compounds. Nonetheless, in this example, the olfactory analysis that is carried out simultaneously during the
GC–MS analysis was crucial in identifying the retention time of the compound responsible for the major odor effect.
Once the aroma or odor compounds are identified, method development can occur based on the results. Early researchers in the
wine industry focused on the analysis of TCA using SPME followed by GC–MS analysis (12). The MS analysis was performed in
the selective-ion mode. Quantification is based on an internal standard with a quantitation limit of 5 ng/L.
The odorous mixed trihalogenated anisoles have also been evaluated in water samples using a headspace–SPME and GC–MS method
(13). By optimizing the parameters affecting collection efficiency, the detection limit for TCA was lowered to 0.25 ng/L.
This same technology has been applied to the analysis of food packaging (14). TCA, TBA, and pentachloroanisole were simultaneously
analyzed in packaging samples by SPME and GC. The detection limits ranged from 0.43 to 1.32 ng/g.
The determination of haloanisoles, specifically TBA, presents a difficult analytical challenge. Due to their extremely low
odor thresholds, these compounds generate odors that are smelled long before their concentrations are large enough to be identified
instrumentally. Combining olfactometry analysis with multidimensional GC–MS creates an extremely useful analytical methodology
for identifying aroma or odor notes from a sample. The chemical compounds responsible for the aroma notes can be verified
using the mass spectrometer detector. In many cases, identification of the compound causing the off odor is the ultimate goal,
so that the manufacturing process can be scrutinized to determine the compound's origin. However, the origin of a compound
within a manufacturing process can be determined knowing the retention time and the aroma character of a compound alone. Finally,
analytical protocols can be developed that monitor these low-concentration compounds as part of a quality-control program.
Roger J. Bleiler* is a senior scientist, Fred Kuhrt is an operations manager, and Don Wright is a senior scientist and consultant, all at MOCON, 2011 A Lamar Blvd., Round Rock, TX 78664, tel. 512.218.9873, fax 512.218.9875,
*To whom all correspondence should be addressed.