Analytical study of comprehensive and targeted multidimensional gas chromatography incorporating modulated cryogenic trapping

This paper reports the results of an analytical study comparing capillary gas chromatography (GC) operated in the normal mode with 2 new GC techniques, comprehensive GC (GC x GC) and targeted (or selective) multidimensional GC, which use a longitudinally modulated cryogenic system (LMCS), recently developed in our laboratory. A high-temperature application of derivatized sterols, of interest in fecal pollution monitoring, was chosen for this work. A directly connected coupled-column ensemble was used, comprising a nonpolar column and a moderately polar column. With LMCS, effluent from the first column is zone-compressed in a cryogenic trap and then pulsed to a short second column, producing narrower peaks with sharp, tall peak responses at the detector. The modulator is operated at a constant frequency, e.g., 0.25 s(-1), to produce the GC x GC result, or is moved in a predefined manner so that whole peaks are selectively trapped and subsequently pulsed through to the second column in the targeted mode. Standard solutions containing a mixture of 7 sterols and 5-alpha-cholestane internal standard were used. Detection sensitivity is increased by a factor of >25 with the use of LMCS. The estimated limit of detection was about 0.1 microg/mL when normal GC with flame ionization detection (GC/FID) and a 1.0 microL splitless injection volume were used, compared with 0.02 and 0.004 microg/mL for the LMCS operated in GC x GC and selective modes, respectively. Calibration curves for GC/FID were linear over the 0.1-2.0 microg/mL range tested. Reproducibilities for the GC x GC and normal GC modes were comparable; generally, relative standard deviations (RSD) were on the order of 3-4%, based on raw peak responses. Improved reproducibility was found for selective LMCS operation, at an RSD of around 2%; with internal standardization, better results were achieved. The coupled-column arrangement allowed complete separation of sterol peaks from overlapping impurity peaks in a number of instances with LMCS modes, and its use should improve data quality over that of normal GC operation, in which the overlapping peaks interfere with measurement of peak response in the normal mode.