How to simplify HPLC using low-pressure valves. Rheodyne's Technical Note 3 shows eight ways of using lowpressure switching valves to simplify method development and routine analysis in high-pressure liquid chromatography These economical 2-position and 6-position Teflon valves can be inserted before the pump or after the column Among uses described in detail are switching between reservoirs to select the correct mobile phase for a particular routine analysis. Switching between several different solvents during method devel opment when seeking maximum selectivity. Switching to a flushing solvent. Switching effluent back to the reservoir to conserve solvent. And switching effluent to a fraction collector.
Send for Tech Note #3 All techniques are fully described in this well-illustrated 6-page tech nical note. Contact Rheodyne. Inc.. PO. Box 996. Cotati. Calif. 94928. USA. Phone (707) 664-9050.
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ly useful in the AFS analysis of com plex samples. This atomizer should also be particularly advantageous for reducing light scatter from particles. The direct analysis of solids is also a high priority in trace element analysis, and the plasma should be very useful in this context. Observation of fluores cence at the usual height of 15 to 25 mm above the torch is not possible be cause of strong background emission. A special torch was therefore devel oped with an extension on the length of the outer quartz tubing to allow a constriction in the volume occupied by the atoms at greater heights in the at omizer (45 to 65 mm was employed). Scatter problems due to 10 000 ppm interferences were assessed. Measure ments generally gave negative values (up to 15%) suggesting the absence of scatter, but the presence of physical or chemical interferences. Electrothermal Atomizers. Elec trothermal devices theoretically more ideally suit the requirements of AFS. Relatively high temperatures can be obtained with the atoms contained in an inert gas atmosphere, and such an atmosphere greatly reduces quenching problems. However, in practice, in tense background emission from the atomizer surface can significantly re duce the signal-to-noise ratio, unless it is effectively blocked out. There are two classes of electrothermal atomiz ers used in AFS, the conventional ali quot injection type and the continu ous nebulization variety. In the for mer, the sample is injected using a mi croliter pipet followed by a multistep thermal program. In the latter, sample is force-fed into a desolvation cham ber and thence into the vitreous car bon tube atomizer (23). The following are typical forms of the electrothermal atomizer used in AFS: carbon filament (24), graphite braid (25), wire loop (26), graphite tube (27), tungsten-coated crucible (28) and graphite cup (29). Others. Cathodic sputtering atom izers are particularly suited to the analysis of solid metal samples. In this approach, scatter can be corrected by time resolution of the scatter and fluo rescence signal (30). Cathodic sputter ing has not received widespread ac ceptance to date, but, because of its great potential, it should in the near future. The cold vapor Hg method, com monly employed in atomic absorption, is readily applicable to AFS. For ex ample, Caupeil et al. (31 ) used a nondispersive instrument. The Hg vapor passes out of the reaction vessel in an argon carrier gas contained in a glass tube. The gas stream exits through a small aperture in the end of a glass tube into the optical beam. The hydride generation method has frequently been applied to atomic flu
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orescence. Most commonly the hy drides are injected into an H2 flame atomizer. The H2 flame has the desir able properties of low background emission and low temperature. It readily atomizes hydrides and thus has been found to be an excellent choice of atomizer for this application. For example, Azad et al. (32) deter mined Se at submicrogram levels using a nondispersive AFS unit and an argon-hydrogen-air-entrained flame.
Radiation Sources Hollow cathode lamps, electrodeless discharge lamps, lasers, and xenon arc lamps are the sources most commonly employed in AFS. Since fluorescence intensity is proportional to source in tensity, it is desirable to employ high luminosity sources. Hollow Cathode Lamps. Conven tional, commercially available, hollow cathode lamps have been widely used as sources for AFS. As operated in AAS equipment, they do not possess sufficient intensity to be of great use in AFS. It is, however, possible to pulse these lamps momentarily with higher currents as long as the average current during the duty cycle does not exceed the manufacturer's recom mended maximum value. Thus, Paler mo et al. (33) and Salin and Ingle (34) used the time division multiplex mode of operation of the equipment in nondispersive and dispersive AFS respec tively. Boosted-output hollow cathode lamps suitable for AFS were first pro posed by Sullivan and Walsh (35). In this design, a secondary low voltage high current discharge was placed in front of the hollow cathode, resulting in more efficient excitation of the atoms produced in the primary dis charge. Van Gelder (36) modified this design so the secondary discharge passes coaxially through the center of a cylindrical cathode in the direction of propagation of the radiation beam. On the average, boosted-output lamps have intensities at least one order of magnitude greater than hollow cath ode lamps operated conventionally. Boosted-output hollow cathode lamps are not, unfortunately, now produced commercially, and hence these sources are only infrequently used in AFS at present. Xenon Arc Lamp. The xenon, pressure-broadened arc is a contin uum source. An Eimac-type arc lamp has been found by most workers to be the most satisfactory design for AFS. The discharge in this lamp occurs in a parabolic mirror cavity, and thus the radiation produced is collected over approximately 4π steradians and emitted as a collimated beam. The continuum source used with a monochromator (37) or interference filters (15) can be readily used for multiele-
