FROM THE LEADER IN POWDER TECHNOLOGY
Microscan • f o r * » mud. accurate measurement mated operation with user-piGmpting of particle size detnbutnns using the • Printer, plotter, video terninai and universally-accepted sedimentation keyboard included technique • Full data reduction—percent rnas, sur • Wide range of particle ciamcter.s- face area and number distributions and 300 tc 0 1 micrometer histograms in printed and plotteo torn ι • Cwnptale. compuler-contioHeJ auto
• Buill-m RS-232 interface
QUANTACHROME
CORP.
POWDER TECHNOLOGY INSTRUMENTATION
5 Aerial Way, Syosset, NY. 11791 · (516) 935-2240 · TWX 510-221-2239 CIRCLE 135 ON READER SERVICE CARD
PHOTOIONIZATION DETECTOR The O.I. Model 4430 PID
• "Hidden" window does not allow sample contact with window • LampSaver circuit prolongs lamp life • Utilizes GC electrometer for unitized operation • Write or call O.I. today for more details
O•I•CORPORATION Graham Rd. at Wellborn Rd. • P.O. Box 2980 • College Station, Texas 77841-2980 Telephone (409) 690-1711 CIRCLE 120 ON READER SERVICE CARD 962 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
ally kept in excess of about 50. Because resolution increases with a decrease in the channel thickness w (7), there is a continuing effort to design channels thinner than those with a w of around 250 jiim, the size used most extensively to date. In reducing w, however, one must pay increased attention to the smoothness of the major walls as well as to their structural integrity. Rough ness and bowing will lead to progres sively larger departures from ideal be havior, and therefore to reduced accu racy in the retention-derived sample characteristics. The open-channel geometry causes only moderate resistance to flow, and no extraordinary demands are placed on the pressures to be generated by the pumping system responsible for carrier delivery. However, pulsing flows are poorly dampened by these channels, and without additional dampeners, peristaltic and other pulsing pumps should be avoided in high-resolution FFF work. Detectors and recorders are the same as those used in regular high-perfor mance liquid chromatography (HPLC) systems. As more and more FFF sys tems are computer controlled, there is a tendency to adapt the configurations used in modern HPLC, which entails substituting the recorder with a video monitor for real-time display of the course of the separation and a printer for producing a hard copy of the fractogram. In addition to its data display function, the computer normally pro cesses the detector signal interactively to give particle diameter and molecular weight information for selected posi tions in the fractogram. The strength of the applied field is inversely related to the zonal layer thickness £ (see Figure 1), which deter mines the retention time for a given sample. The introduction of computer ized field control has, more than any other FFF improvement, enabled the operator to perform highly reproduc ible fractionations under optimal con ditions of analysis speed and compo nent resolution. Although much of the methods development has been done using a constant field, sometimes re ferred to as "isocratic" conditions, it was realized early on (8) that process ing of complex sample mixtures, whose components spanned a large mass range, might profitably involve a pro grammed reduction in field strength, as shown in Figure 4 (9). Recent theoreti cal analysis by Williams and Giddings (10) has led to the formulation of some general guidelines for the choice of pro gram form, which can be easily imple mented through the computer-guided operation. Sample. Injections into the FFF channels, whose volumes tend to range from 1 to 5 mL, are performed either
