Precolumns

In contrast, short guard columns are used as a protection between the injector and 

On the other hand, short guard columns are used as a protection between the injector and colunrn. They are packed with the same or a chemically similar stationary phase as the main column and prevent impurities such as strongly 

The particle size distribution should be as narrow as possible (with a ratio of diameters of the smallest and the largest particles of 1 1.5 or 1 2, as already mentioned). This is because the smallest particles determine the column permeability and the largest particles fix the plate number. Small particles produce a high flow resistance and large particles are responsible for a high degree of band broadening. For this reason, a particle size analysis accompanies many products. 

10% of the material is less than 4 pm 50% of the material is less than 5 pm 90% of the material is less than 7 pm 

The properties of the packing materials may vary from hatch to batch of the same product, so a large amount of one batch should be bought for series analysis over a long period of time. 

Various types of stationary phases are in use Porous particles, non-porous particles of small diameter, porous layer beads, perfusive particles, and monolithic materials. 

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An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

Liquid Ghromatography/Mass Spectrometry. Increased use of Hquid chromatography/mass spectrometry (Ic/ms) for stmctural identification and trace analysis has become apparent. Thermospray Ic/ms has been used to identify by-products in phenyl isocyanate precolumn derivatization reactions (74). Five compounds resulting from the reaction of phenyUsocyanate and the reaction medium were identified two from a reaction between phenyl isocyanate and methanol, two from the reaction between phenyl isocyanate and water, and one from the polymerisation of phenyl isocyanate. There were also two reports of derivatisation to enhance either the response or stmctural information from thermospray Ic/ms for linoleic acid hpoxygenase metabohtes (75) and for cortisol (76). 

A range of preparative and semipreparative soft gel systems with an improved mechanical stability and thus the chance to run them with increased flow rates were tested for their potential on the separation of starch glucans. For each of these systems a Sephacryl S-200 precolumn proved to be a perfect shock absorber for sample application, improved reproducibility of separations, and increased lifetime of soft gel systems. 

Another TSK combination (precolumn -I- PWM -I 6000 -I 5000 -I- 4000 -I-3000) was tested on differences in separation performance between individual narrow distributed samples and mixtures of several narrow distributed samples. The result is summarized in Eig. 16.31 within experimental error the summed chromatograms (theory) of four narrow distributed glucans (dextran) match perfectly with the experimentally determined chromatogram of the mixture. The (theory/experimental) ratio, plotted for quantification of the match, in-... 

In Fig. 16.32, application of a TSK PW SEC system consisting of a combination of precolumn + PWM + 6000 + 5000 + 4000 + 3000 demonstrates a possibility for analytical purposes to change from DMSO-dissolved glucans to an aqueous solution. An initially DMSO-dissolved potato starch sample was applied to the TSK PW system and because separated with an aqueous... 

FIGURE 4.22 HPLC chromatogram of amino acids employing precolumn derivatiza-tion with OPA. Chromatography was carried out on an Ultrasphere ODS column using a complex tetrahydrofuran methanol 0.05 M sodium acetate (pH 5.9) 1 19 80 to methanol 0.05 M sodium acetate (pH 5.9) 4 1 gradient at a flow rate of 1.7 mL/min. 

In this way, the liquid can be transferred at a speed corresponding to the evaporation speed. The fraction to be analysed is contained in a loop (see Eigure 2.5), connected to a switching valve. By opening the valve, the sample in the loop is driven by the carrier gas into the GC unit (8), instead of the LC pump. An early vapour exit is usually placed after a few metres of the deactivated precolumn (9) and a short piece (3-4 m) of the main column (retaining precolumn). This valve is opened during solvent evaporation in order to reduce the amount of solvent that would reach the detector, and at the same time, to increase the solvent evaporation rate (6). 

Figure 2.6 Gas cluotnatograni of a 10 ml test sample containing C I4 C26 alkanes in -hexane (about 1 ppb each) the earner gas (H2) inlet pressure was 2.5 bar for a 22 m X 0.32 mm id separation column coupled with a 2 m X 0.32 mm id uncoated precolumn (no vapour exit). Reprinted from Journal of High Resolution Chromatography, 9, K. Grob et al., Concunent solvent evaporation for on-line coupled HPLC-HRGC , pp. 95-101, 1986, with peimission from Wiley-VCH.

DIRECT INTRODUCTION OE WATER VIA A VAPORIZER CHAMBER/PRECOLUMN SOLVENT SPLIT/GAS DISCHARGE INTEREACE... 

Figure 2.10 Schematic representation of a vaporizing chamber/precolumn solvent split/gas discharge interface, where the vaporizer is packed and heated at a suitable temperature for solvent evaporation. The vapour exit can be positioned at the end of the retention gap.

Figure 2.12 Schematic representation of an on-line SPE-GC system consisting of three switching valves (VI-V3), two pumps (a solvent-delivery unit (SDU) pump and a syringe pump) and a GC system equipped with a solvent-vapour exit (SVE), an MS instrument detector, a retention gap, a retaining precolumn and an analytical column. Reprinted from Journal of Chromatography, AIIS, A. J. H. Eouter et al, Analysis of microcontaminants in aqueous samples hy fully automated on-line solid-phase extraction-gas chromatography-mass selective detection , pp. 67-83, copyright 1996, with permission from Elsevier Science.

K. Grob and Z. Li, Intr oduction of water and water-containing solvent mixtures in capillary gas clir omatogr aphy. I. Eailure to produce water-wettable precolumns (retention gaps) , J. Chromatogr. 473 381-390 (1998). 

T. Hyotylainen, K. Grob, M. Biedermann and M-L. Riekkola, Reversed phase HPLC coupled on-line to GC by the vaporizer/precolumn solvent split/gas dischar ge analysis of phthalates in water , 7. High Resolut. Chromatogr. 20 410-416 (1997). 

A fundamental parameter characterizing the usefulness of a given precolumn for enrichment purposes is the breakthrough volume, Vg. This volume can be determined by monitoring continuously or discretely the detector signal at the outlet of the precolumn (35-37). The breakthrough volume can be defined by the following expression (37) ... 

Step 2) Introduce heart-cut to the analytical column and detector. At the predetermined time interval, which was previously calculated by eluting analyte standards without the analytical column, i.e. the onset of the heart-cut, valve B is closed to divert the precolumn effluent to the analytical column. 

Step 3) Bypass the precolumn and detection of the analyte of interest. When all of the analytes of interest have been eluted from the precolumn, valve A is opened so that the eluent stream is diverted to valve B, which is immediately opened to allow eluent from valve A to flow into the analytical column, thus bypassing the precolumn. 

Step 4) Precolumn clean-up not shown in Figure 5.4. After the heart-cut analytes have been transferred to the analytical column, a step-gradient programme is used to flush the precolumn of the more strongly retained compounds. An additional pump configuration makes precolumn clean-up possible while the analysis is running. 

Z. Yu and D. Westerlund, Char-acterization of the precolumn biortap 500 C g for direct injection of plasma samples in a column-switching system , Chromatographia, 47 299-304(1998). 

A. Faijam, A. E. Brugman, A. Soldaat, P. Timmerman, H. Fingerman, G. J. de Jong, R. W. Frei and U. A. Th Brinkman, Immunoaffinity precolumn foi selective sample preti eatment in column liquid cliromatography immunoselective desoiption , Chromatographia 31 469-477 (1991). 

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