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Letters to the Editor |
1 Research Center for Drug Metabolism, College of Life Science, Jilin University, Changchun, China
2 School of Pharmacy, University of Otago, Dunedin, New Zealand
aAddress correspondence to this author at: Research Center for Drug Metabolism, College of Life Science, Jilin University, Changchun, China. Fax 86-431-5619955; e-mail gujk{at}mail.jlu.edu.cn.
To the Editor:
Liquid chromatography/mass spectrometry (LC/MS) has rapidly assumed an important role in the clinical laboratory because of its speed and inherently superior sensitivity and selectivity. It has been applied extensively in clinical research in areas such as therapeutic drug monitoring, drug metabolism, pharmacokinetics, and clinical toxicology (1)(2)(3). In recent years, considerable effort has been devoted to increasing the throughput of LC/MS analyses. Strategies include the use of short columns, "ballistic gradient" chromatography(4), and coupling HPLC systems in parallel to a single mass spectrometer(5)(6). Despite these improvements in sample throughput, the time required to complete an LC/MS analysis is generally limited by the speed of conventional autosamplers to a minimum of 3–5 min per sample. Here we describe how the software controlling injection of one commercially available LC/MS autosampler can be reprogrammed to markedly increase sample throughput. This was possible because the autosampler in question, the Agilent G1313A, is controlled by step commands, the sequence of which can be changed by the operator(7).
A chromatographic run can be described in terms of various times: e.g., idle time, dead time, chromatography time, and cycle time. The definitions of these times as they apply to a standard autosampler injection program are shown in Fig. 1
. The idle time of the Agilent G1313A autosampler, which includes all preparation time before an injection, is 
1.0 min for an injection volume of 20 µL.
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We have established that the cycle time of the Agilent G1313A autosampler can be reduced considerably by incorporating both the dead time and idle time into the chromatography time. This is done by changing the order of step commands from 1–draw sample, 2–needle into seat, and 3–inject in the standard program to 1–inject, 2–wait 1.5 min, 3–draw sample, 4–needle into seat, and 5–valve mainpass in the new program. By making the "inject" command come first followed by a wait of arbitrary length before the other commands, the function of the inject command becomes simply to trigger the mass spectrometer to acquire data, and the actual injection is carried out by the "valve mainpass" command. This means that the autosampler preparation steps occur after the inject command rather than before it. The next sample is injected on the column before analysis of the current sample is completed, and both the idle time and the dead time are incorporated into the chromatography time (Fig. 1
). The only shortcoming of the new program is that the first injection is wasted because, during this time, only the mobile phase is in the sample loop.
As an illustration of the improved efficiency of the new program, we compared it with the standard program in a clinical pharmacokinetic study of metformin (8). We found that the new program reduced the cycle time from 4.0 min to 2.4 min and nearly doubled sample throughput without requiring modification of the LC/MS interface or compromising data quality. Experience with the new program suggests that it can increase sample throughput in almost any LC/MS assay except those in which matrix effects are present after the last peak and may interfere with the next sample. In this situation, the new program can still be used, but the chromatography time needs to be extended.
Acknowledgments
We thank the National Natural Sciences Foundation (Grant 30070879) and 863 project of China (Grant 2003AA2Z347C) for financial support.
References
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