New LC clean-up technique is simpler and faster. Rheodyne's Technical Note 2 tells how to prepare samples with greater speed and lowercostthan purification steps using liquid/liquid partition, column chromatography or commer cial separation cartridges. The rapid clean-up technique uses Rheodyne's Model 7125 Syringe Loading Sample Injector with a short column in the sample loop. It allows complex samples to be injected directly into the chromatographic system. This technique can be used to: • Eliminate interferences from complex matrices • Concentrate samples to improve detection limits • Shorten analysis time by trapping late-eluting peaks
Send for Tech Note # 2 The,simplified procedures are fully described in this well-illustrated 4-page technical note. Contact Rheodyne. Inc., RO. Box 996, Cotati, Calif. 94928, U.S.A. Phone [707] 664-9050.
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and chromomycin molecules are close enough for the Hoechst excited state to transfer its energy to the chrom omycin before it can be radiated. Vari ations in energy transfer efficiency among chromosome types also en hance observed fluorescence differ ences. The measurements just discussed dealt with properties characteristic of the whole chromosome. Other tech niques allow measurement of some as pects of chromosome morphology. Using a narrowly focused excitation beam, the distribution of dye along a chromosome can be scanned as it flows past and the pulse profile ana lyzed {10,11). Measurements with scanning microscope systems have shown that there is less DNA content per unit length at the centromere than at other points. Figure 6 shows the profiles of two chromosomes obtained in flow, and the centromeric dips are clearly visible. The time for the scan is about 1 μβ, approximately 50 points 20 ns apart being sampled for a typical 10-jum-long chromosome. The centro mere can be resolved in chromosomes as small as 3.5 μια in length. On the right is a normal chromosome with a single centromere. On the left is a chromosome from a cell exposed to X-rays. An error was made in rejoin ing breaks caused by the radiation, and the result is a dicentric chromo some with two centromeres. Chromo some aberrations, like the dicentrics, can be used as biological dosimeters, since their frequency of occurrence de pends on the dose received. This is particularly useful in cases of inadver tent human exposure, where no other dosimeters are present. Searching for aberrations visually is time-consum ing, since they are rare for low expo sures. The high analysis rates possible in flow may find application in speeding this process. Conclusion Among the additional applications of flow cytometry are blood analysis, bacterial classification, tumor classifi cation and diagnosis, monitoring of cancer therapy, and immunological studies. All of these exploit the high speed and accuracy of the technique, which allow collection of information on a sufficient number of individual particles to describe the distribution of characteristics in a population with good statistical precision. Many re quire the ability to sort subpopula tions for further study or use. Applica tions outside of biology are waiting to be explored. References (1) Herzenberg, L. Α.; Sweet, R. G.; Herzenberg, L. A. Sci. Am. 1976, 234 (3), 108. (continued on page 519 A)
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 ·
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