In a comprehensive two-dimensional gas chromatograph, a thermal modulator serially couples two columns containing dissimilar stationary phases. The secondary column generates a series of high-speed secondary chromatograms from the sample stream formed by the chromatogram eluting from the primary column. This series of secondary chromatograms forms a two-dimensional gas chromatogram with peaks dispersed over a retention plane rather than along a line. The method is comprehensive because the entire primary column chromatogram is transmitted through the secondary column with fidelity. One might expect that a two-dimensional separation in which both dimensions are basically the same technique, gas chromatography, would be inefficient because the two dimensions would behave similarly, generating peaks whose retentions correlate across dimensions. Applying a temperature program to the two columns, however, can tune the separation to eliminate this inefficiency. The temperature program reduces the retentive power of the secondary column as a function of progress of the primary chromatogram such that the retention mechanism of the primary column is eliminated from the second dimension. Retention of a substance in the second dimension is then determined by the difference in its interaction with the two stationary phases. Retention times in the second dimension then fall within a fixed range, and the whole retention plane is accessible. In a properly tuned comprehensive two-dimensional chromatogram, retention times in the two dimensions are independent of each other, and the two-dimensional chromatogram is orthogonal. Orthogonality is important for two reasons. First, an orthogonal separation efficiently uses the separation space and so has either greater speed or peak capacity than nonorthogonal separations. Second, retention in the two dimensions of an orthogonal chromatogram is determined by two different and independent mechanisms and so provides two independent measures of molecular properties.

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Abstract. Comprehensive two-dimensional gas chromatography is applicable to the analysis of complex petroleum mixtures. A kerosene sample analyzed by this method generates over six thousand chromatographic peaks. The chromatogram is orthogonal such that the second dimension separation is independent of the first. Aliphatics, aromatics, and naphthalene derivatives form distinct bands of peaks on the chromatographic plane with further subdivisions within each band based on chemical structure. With a nonpolar first column and a moderately polar second column, the first dimension separation is based largely on substance volatility while the second dimension separation is based on polarity. Each chromatographic peak has a pair of characteristic retention times, providing a more reliable identification. Peak capacity is much greater than that obtainable from any one-dimensional separation.

A simple approach to two-dimensional liquid chromatography has been developed by coupling columns of different selectivity using a 12-port, dual-position valve and a standard HPLC system. The valve at the junction of the two columns enables continuous, periodic sampling (injection) of the primary column eluent onto the secondary column. The separation in the primary dimension is comparable to conventional HPLC, whereas the secondary column separation is fast, lasting several seconds. The high-speed separation in the secondary dimension enables the primary column eluent to be sampled with fidelity onto the secondary column throughout the chromatographic run. One might expect a coupled column liquid chromatography system operating in reverse-phase mode to be strongly correlated and, hence, inefficient. However, by applying a solvent gradient in the primary dimension and by progressively incrementing the solvent strength in the secondary dimension (tuning), the inefficiency or cross correlation between the two dimensions is minimized. In a tuned two-dimensional system, the influence of primary column retention (usually hydrophobicity) is minimal on secondary column retention. This enables subtle differences in component interaction with the two stationary phases to dominate the secondary column retention. The peaks are randomly dispersed over a retention plane rather than along a diagonal, resulting in an orthogonal separation. The peak capacity is multiplicative, and each component has a unique pair of retention times, enabling positive identification. In addition, the location of the component provides two independent measures of molecular properties. The 2D-LC system was evaluated by analyzing a test mixture made of some aromatic amines and non-amines on different secondary columns (ODS-AQ/ODS monolith, ODS/amino, ODS/cyano). The relative location of sample components in the two-dimensional plane varied significantly with change in secondary column. Among the secondary columns, the amino and cyano columns offered the most complementary separation, with the retention order of several components reversed in the secondary dimension. The theoretical peak capacity of the 2D-LC system was around 450 for a separation lasting 30 min. A 2D-LC system involving amino and cyano columns resulted in a high-speed separation of the test mixture, with most of the chemical components resolved within a few minutes.

A comprehensive 2-D-LC-MS method has been developed by coupling columns of different selectivity. The primary column eluate is alternately trapped and sampled onto the secondary columns through a guard column interface. When one guard column traps the eluate, the other injects the previously trapped components onto a secondary column. This cycle is repeated throughout the chromatogram. The use of dual secondary columns provides the secondary columns with additional time to generate high-speed chromatograms. Each secondary column generates alternate chromatograms which when combined generate the entire chromatogram. The primary column separation is comparable to conventional HPLC, whereas the secondary column separation is fast. With both the columns operating in reverse phase mode, one would expect strong correlation in the two-dimensional retention and hence inefficiency in separation. However, differences in column operation modes, interaction mechanisms, and vendor silica result in a complementary separation. The system was evaluated by comparing it to one-dimensional counterparts and coupled column chromatography. Although some correlations were observed in 2-D-LC-MS, peaks do show two-dimensional distribution with superior UV and MS data as co-elution is minimized. Also, the ease of converting conventional systems to 2-D-LC-MS is discussed.

The development work toward the robust and efficient manufacturing process to vismodegib, the active pharmaceutical ingredient (API) in Erivedge, is described. The optimization of the four-stage manufacturing process was designed to produce the API with the required critical quality attributes: (1) the selective catalytic hydrogenation reduction of the nitro compound 3 to the corresponding aniline 4 while minimizing the formation of potential genotoxic (mutagenic) impurities; (2) the control of the polymorphic phase and multipoint specification for particle size distribution.

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