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analytical chemistry
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β- Pinene
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a-Terpinene
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Figure 7. Separation of a terpene mixture (total ion chromatogram) and correspond ing mass spectra Column, 300 X 0.22 mm, 3-μΓη ODS; eluent, methanol:water (80:20). Reproduced with permission from Reference 23
spray interface (21). The advent of capillary and open-tubular LC col umns has allowed direct liquid injec tion (22) (DLI) to be more readily de veloped. Essentially, the flux of mate rial due to the eluent flow for these chromatographic systems is found to be small enough to allow for DLL LC/MS in such systems is quite favor able for thermally labile species that could not survive a GC/MS analysis. If total ion current is measured, LC/MS offers universal detection at subnanogram levels. DLI from a capillary LC system produced the separation of a terpene mixture (23) shown in Figure 7. Also, the real-time mass spectra of a-terpinene and /3-pinene are provided to show the system's ability to provide infor mation for definitive molecular identi fication. Detectabilities for state-ofthe-art LC/MS systems are on the or der of 1-10 pg for open-tubular LC with DLI. Summary It is apparent that substantial ad vances in detection methods for LC have been made in recent years. The use of lasers to reduce detector vol umes and to provide added selectivity and sensitivity and the development of small columns to minimize contri butions from the solvent and to allow cost-effective use of exotic mobile and stationary phases are two important
1254 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
reasons. The development of separa tion processes has in turn benefited from this new generation of detectors. This trend is expected to continue. It is up to our imaginations to seek out new frontiers for the next generation of detection methods. The key, per haps, is to consider LC in the broader context of measurement science and not simply to try to adapt known mea surement principles to known LC ef fluents. Acknowledgment The authors thank the many co workers who have contributed to vari ous parts of this work, particularly M. J. Sepaniak, W. D. Pfeffer, S. D. Woodruff, J. C. Kuo, D. R. Bobbitt, B. H. Reitsma, K. J. Skogerboe, and S. A. Wilson. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University un der Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences. References (1) Synovec, R. E.; Yeung, E. S. Anal. Cham. 1983,54, 1599-1603. (2) Scott, R.P.W. Liquid Chromatography Detectors, 2nd éd.; Elsevier: Amsterdam, 1986. (3) Liquid Chromatography Detectors; Vickrey, T. M., Ed.; Marcel Dekker: New York, 1983. (4) Woodruff, S. D.; Yeung, E. S. Anal. Chem. 1982,54, 2124-25. (continued)
