Sample preconcentration pressurization

As mentioned before, when pressure-based sample injection is employed, the maximum injection volume must be less than the effective length of the capillary. At least 10% of the effective capillary length must remain available for separation. To inject a larger sample volume, electrokinetic injection must be employed. In most online sample preconcentration techniques, the maximum amount that can be injected without loss of separation efficiency is certainly less than the capillary volume. However, with a large volume sample injection under cathodic EOF conditions in SDS MEKC, a sample devoid of the micelle can be continuously electrokinetically injected for a volume equivalent to seven times the capillary volume without significant loss of separation efficiency under favorable... 

The experiments were carried out in 200 dm Teflon bag reactors at room temperature (297 3 K) and at atmospheric pressure (725 5 Torr). The decay of the VOC concentration and build up of the product concentrations were monitored during the course of the experiment. The concentration of VOC was measured by GC-FID. The formation of organic nitrates and carbonyls were measured by GC with sample preconcentration using adsorption tubes filled with Tenax TA. Detection was performed with FID and BCD coupled in series. The carbonyl... 

FIG. 8. Sample preconcentration using two pumps—a single high-pressure switching valve and a concentrator column. EP, eluent pump SP, sample pump C, analytical column D, detector. The concentrator column is indicated by the hatched area and flow paths are shown by the solid lines. Note that the sample pump is turned off for the final analysis stage. 

Van Os et al. [41] achieved complete separations in 6min of l-30mg L concentrations of bromide, chloride, nitrite, nitrate and sulphate using a Zipax SAX separation column, with eluent suppression and electrical conductivity detection [42]. The necessary high pressure packing techniques for packing the separation column have been described [43]. With sample preconcentration, detection limits were reduced to about 5pg L but calibration graphs for chloride and nitrate were not Unear. Sodium adipate and 1.4 X 10 disodium succinate are used as eluants, both at pH7. 

Preconcentration will only give precise results for quantitative work if the initial extraction technique gives high, or at least known and reproducible, recoveries of the desired compounds from the initial sample. As typically, several cycles are needed, and the solvent containing the extracted compounds must be concentrated to a small volume. This is normally carried out by evaporation under reduced pressure. Volatile compounds may be lost in this procedure. However, for many applications the compounds of interest... 

Joannon and Pin.50 A low detection limit was achieved in quadrupole ICP-MS when the pressure in the interface was reduced from approx. 2 to 0.85 mbar. Lariviere et al.51 developed a selective extraction procedure for the preconcentration of 226Ra from uranium ores and biological samples. The measurements were performed by ICP-QMS with a hexapole collision cell in order to reduce possible interferences. An absolute detection limit of 0.02 fg (0.75 mBq) was obtained using less than 4 mg of solid sample or 25 ml of liquid sample. 

A programmed temperature-vaporization (PTV) injector (with a sorbent-packed liner) was used to preconcentrate and inject the sample. Thermal desorption was performed and the analytes were passed to a primary column (16 m X 0.32 mm i.d., film thickness 5 p.m, 100% methyl polysiloxane) and separated according to analyte vapour pressure. Selected heart-cuts were transferred to a second column (15 m X 0.53 mm i.d., Al203/Na2S04 layer, open tubular column with 10 (im stationary phase) where final separation was performed according to chemical functionality. 

Figure 13.14 LC-diode-array detection (DAD) chromatogram (at 220 nm) obtained after preconcentration of 50 ml of ground water sample spiked with various pollutants at levels of 3 p.g l-1 passed through (a) a PLRP-S cartridge and (b) an anti-isoproturon cartridge. Peak identification is as follows 1, chlortoluron 2, isoproturon plus diuron 3, linuron 4, diben-zuron , water matrix. Reprinted from Journal of Chromatography, A 777, I. Ferrer et al. Automated sample preparation with extraction columns by means of anti-isoproturon immunosorbents for the determination of phenylurea herbicides in water followed by liquid chromatography diode array detection and liquid chromatography-atmospheric pressure chemical ionization mass spectrometry , pp. 91-98, copyright 1997, with permission from Elsevier Science.

ICP-MS dominates the field of environmental assay most metallic and amphoteric elements are susceptible of analysis, and it is often a great convenience (to say nothing of being relatively economical) to be able to assay for all elements of interest in a single analysis. Another feature of ICP-MS, however, has been exploited perhaps even more tellingly than multielement analysis. This is that the sample introduction system lends itself to a wide variety of enhancement schemes, in part because the sample is introduced to the instrument at atmospheric pressure and in part because samples are most often in a water-based (dilute acid) medium. These attributes combine to allow various separation and preconcentration schemes to be implemented on-line or nearly so. 

Fig. 5.8. (A) General scheme of a dynamic focused microwave-assisted extractor. (B) Experimental set-up used to integrate microwave-assisted extraction with the subsequent steps of the analytical process. (1) Leaching step CT controller, MO microwave oven, S sample, R condenser, WR water reservoir, TCPP two-channel piston pump, ER extract reservoir, SV switching valve. (2) Clean-up/preconcentration step M methanol, A air, B buffer, PP peristaltic pump, F filter, EL elution loop, MC mini-column, R retention direction, E elution direction, 1V1-1V3 injection valves, W waste. (3) Individual separation-detection step HPIV high-pressure injection valve, AC analytical column, DAD diode array detector, SR solvent reservoirs.

Porphyrin complexes, as oxidation catalysts 1201, 1205,1211 Porphyrins—see also meso-Tetraphenylporphyrin reactions of 1082 Potential energy surface 3 Potentials, double-minima 371, 377 symmetrical OH stretching 382 Potential weU, shape of 349 Preconcentration, of analytical samples 930, 932, 934, 936, 942, 944-949, 955, 962-964, 971, 972, 983-987, 995 Prenylphenols, C chemical shifts for 338 Pressure transducers 851 Prestegane A 1244... 

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