High-speed comprehensive two-dimensional gas chromatography (GC×GC) is performed, in which a polar second column performs separations every half second on portions of the effluent from a nonpolar first column. Chemometric techniques that are traditionally used on chromatographic separations with multichannel detection are applied to two-dimensional chromatographic data, for the purpose of quantifying incompletely resolved peaks. Generalized rank annihilation method (GRAM) is evaluated in the quantification of varying amounts of selected overlapped analytes in a GC×GC analysis of modified white gasoline. GRAM requires a sample and standard data set for quantification, and the high retention time precision arising from use of shortened GC columns aids in the analysis. Results from GRAM analysis of GC×GC data are compared with a reference GC method. The test analytes ethylbenzene and m-xylene, existing in various proportions in white gasoline samples, were successfully deconvoluted despite having resolutions of 0.46 and 0.20 on the first and second dimensions of separation, respectively. Like other second-order techniques, GRAM was able to reliably quantify m-xylene despite the presence in the analytical sample of an overlapping compound not present in the calibration standard. Because GRAM can be successfully applied to GC×GC data, full resolution of all the analytes of interest is not necessary. As a result, GC×GC run times can be dramatically shortened, which has significant implications for analyses in which short cycle times are critical, such as in process analysis.

No abstract

and as shown in Figures 6 and 7, peak broadening increases with capacity factor. The key to reduced peak broadening is the use of thinner claddings. If an analyte is in the presence of a large interferent that has a RI different from the analyte, the cladding RI can be matched to that of the interferent so that the analyte will be selectively detected. Alternately, a short section of the annular column can be used as a detector only, in conjunction with capillary GC or microbore LC. Since the mode-filtered light detector is an optical detector, it can be used in hazardous areas where flame-based detectors are a concern.

Comprehensive two-dimensional (2-D) separations are emerging as powerful tools for the analysis of complex samples. The substantially larger peak capacity for a given length of time relative to 1-D separations is a well-known benefit of comprehensive 2-D separation methods. Unfortunately, with complex samples, the probability of peak overlap in 2-D separations is still quite high. This is especially true if one desires to speed up the analysis by reducing the run time and, thus, by reducing the resolving power along the first dimension separation. Chemometric methods hold considerable promise to overcome the limitations brought upon by the likelihood of peak overlap. Thus, chemometric methods should be able to effectively extend the resolving power of 2-D separation methods. In this paper, the theoretical enhancement provided by application of the generalized rank annihilation method (GRAM) for the analysis of unresolved peaks in comprehensive 2-D separations is carefully modeled and critically evaluated. First, Monte Carlo simulations are used to determine the conditions where the use of GRAM results in the successful analysis of unresolved peaks. A wide range of experimental conditions and performance criteria are modeled, typical to many available 2-D separation methods, including analyte/interference peak height ratio, first- and second-dimension resolutions, signal-to noise ratio, injection volume reproducibility, and run-to-run retention time reproducibility. Essentially, a wide range of experimental conditions and performance criteria are found to provide reliable data amenable to GRAM analysis. The information gleaned from this first set of simulations is then used in conjunction with Monte Carlo simulations of comprehensive 2-D separations. For these simulated 2-D separations, the total number of analyzable peaks when using GRAM was determined and found to be substantially better than using only traditional quantitative methods such as peak integration or height. For example, it was determined that the use of GRAM increases the average number of analyzable peaks by a factor of 2 for 2-D separations in which the peak capacity is 67% occupied by randomly distributed peaks. The results of the studies are general, and the use of GRAM should increase the number of analyzable peaks for all forms of comprehensive 2-D separations.

A parallel gas chromatographic instrument with time-offlight mass spectrometric detection (GC/TOF-MS) is reported. An injected sample is first split between two GC columns that provide complementary separations. The effluent from the two columns is recombined prior to detection with a single TOF-MS. Switching from single to parallel columns increases the chemical selectivity of a GC/TOF-MS data set without increasing analysis time, by doubling the number of peaks, or features, in the chromatographic dimension. The resulting analyzer can be used to reduce analysis times for partially resolved peaks. Simulations compare the quantitative precision of paralleland single-column instruments using the generalized rank annihilation method (GRAM). Results indicate that a parallel column GC/TOF-MS should substantially improve the chemical selectivity and quantitative precision of the analysis relative to a single-column instrument. For a column at half its peak capacity, for example, a singlecolumn instrument met the target precision less than 75% of the time, while a parallel-column instrument achieved 95% success. Parallel-column analyses of methyl tertbutyl ether (MTBE) and benzene in gasoline samples were also performed to support the simulation studies. An objective chromatographic standardization technique corrected for retention time shifts before GRAM was applied. Although MTBE and benzene were poorly resolved in the 40-s runs, chemometric techniques successfully quantitated them.