Focus
How to select LC filters to maximize resolution. A column inlet filter is usually necessary to capture contaminating particles, yet this filter may easily impair resolution if not wisely selected. Rheodyne's Tech Note 6 reports experiments measuring how much filters of various sizes and flow geom etry affect the resolution achieved by columns of several sizes. The'newer microbore columns and short 4.6-mm columns prove to be most sensitive to filter performance. If their resolution is to be preserved, an inlet filter with very little sample dispersion must be used. The Tech Note helps the reader select the optimum filter for his appli cation: one with little enough sample dispersion to preserve resolution, yet large enough capacity to prevent a rapid rise in backpressure.
Send for Tech Note # 6 For a copy free of charge contact Rheodyne, Inc., PO. Box 996, Cotati, California 94928, U.S.A. Phone (707) 664-9050.
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have synthesized several highly selec tive nonalkyl bonded phases designed for normal-phase chromatography; for example, a recently developed pyrene bonded phase can easily separate nitro-polynuclear aromatic hydrocar bons from other PAHs. A sensitive new detector for microbore LC was presented by L. B. Rog ers, L. A. Carreira, and their col leagues at the University of Georgia. The photothermal deflection (PTD) spectrometer, one of several new de tector designs based on laser-induced effects, indirectly measures the heat deposited in a sample subsequent to laser light absorption by deflection of the probe beam onto a diode. It exhib ited detection limits three to four or ders of magnitude lower than those obtained with a conventional UV-VIS absorption detector when tested with three acid sulfonate dyes. Because of their "variable solvating powers and gaslike diffusion coeffi cients and viscosities," supercritical fluids can provide efficient new meth ods for sample extraction, cleanup, and fractionation, said Richard Smith of Battelle Northwest Laboratories. Supercritical fluid chromatography (SPC) is applicable to wide classes of thermally labile and less volatile pol lutants not usually amenable to tradi tional gas and liquid chromatography, with new small-diameter (25-50 μτη) fused-silica capillary columns provid ing chromatographic efficiencies near ly equal to those obtained in conven tional GC. SFC/MS can readily deter mine pesticides (such as Silvex) that yield poor detection limits by both di rect liquid injection and thermospray LC/MS. Although capillary SFC is not yet commercially available, Smith pre dicted that one or two capillary instru ments will be introduced this year. Tandem mass spectrometry, which has previously been used mostly for structure elucidation of unknowns, is finding new uses in environmental analysis. Richard Yost of the Universi ty of Florida explained that because MS/MS monitors daughter ions of a specific parent ion or vice versa, it is a highly selective technique; this selec tivity often makes it possible to ana lyze complex samples with minimal cleanup and no preconcentration. Be cause chromatographic separation is eliminated, MS/MS is extremely rap id—Yost reports that he can analyze up to 100 samples per hour. And de tection limits can be as low as 100 fg. (For more information on MS/MS in trace analysis, see Yost's R E P O R T in the June 1985 issue of ANALYTICAL CHEMISTRY.)
Microbeam surface techniques are
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also finding new applications in envi ronmental analysis because they make it possible to study pollutants in situ. David Hercules of the University of Pittsburgh is using laser MS to study organophosphorus pesticides directly on plant tissues with a unique thinpolymer-film sampling system that al lows spectra to be obtained at near at mospheric pressure. Hercules is also developing a modified laser microprobe that he hopes will be able to map organic species on a surface much as an Auger microprobe produces an elemental map. Richard Linton and his group at the University of North Carolina at Chap el Hill are using microbeam tech niques for single-particle analysis. They have used photoacoustic spec troscopy to study the reactivity of polynuclear aromatic adsorbates on aerosol surfaces and recently coupled digital imaging techniques with ion microprobe MS to study the three-di mensional distribution of trace ele ments in individual atmospheric parti culates. According to Linton, these three-dimensional distribution data are of particular interest because "the intra- and inter-particle elemental dis tribution may influence the extent to which toxic species are available in en vironmental or biological systems." Typical element detection limits are less than 10 - 1 5 g in analytical volumes of 0.1 μτη3. Finally, the environmental applica tions of multiparameter luminescence techniques were discussed by Isiah Warner of Emory University. Accord ing to Warner, because luminescence measurements directly measure emit ted photons above background rather than a small difference between two very large signals, these techniques typically provide detection limits more than three orders of magnitude lower than absorption measurements do. They also have the potential for high selectivity because of the many analytical parameters that can be var ied; for example, lifetimes, selective excitation, phosphorescence, fluores cence, polarization, and combinations of these parameters can all be used to achieve increased selectivity. (For a detailed discussion of multiparameter luminescence measurements, see Warner's INSTRUMENTATION in the March 1985 issue of ANALYTICAL CHEMISTRY.)
The 16th Annual Symposium on the Analytical Chemistry of Pollutants will be held March 17-19,1986, in Lausanne, Switzerland. For further in formation, contact Ernest Merian, Im Kirsgarten 22, CH-4106 Therwil, Switzerland. M.D.W.
