Solute focusing injector

In addition, solute focusing is possible by maintaining a low initial temperature (e.g. 40 °C) for a long period of time (8-12 min ) to allow the mixture of decompressed carbon dioxide, helium gas and the solutes to focus on the GC column. The optimization of the GC inlet temperature can also lead to increased solute focusing. After supercritical fluid analysis, the SF fluid effluent is decompressed through a heated capillary restrictor from a packed column (4.6 mm i.d.) directly into a hot GC split vaporization injector. 

An alternative approach is to use a splitless injection system. If the valve in Fig. 1 is closed, then all the sample passes into the column and there is no split ipso facto, the device is a splitless injector. When used in the splitless mode, however, it is usual to employ a somewhat wider capillary column, which will allow the penetration of a small-diameter injection syringe and thus permit on-column injection. Under these circumstances, there can be no differential sampling of the form described. This procedure, however, introduces other injection problems that can affect both resolution and quantitative accuracy that need to be addressed (See the entries Retention Gap Injection Method and Solute Focusing Injector Method). 

With packed HPLC columns, conventional HPLC injectors can be used. With open capillary columns, split or splitless injection is needed in order not to overload the columns. Special injection techniques have been developed for these purposes, since standard GC techniques cannot be used at high pressures. With knowledge about the critical data of the sample solvent, a retention gap can be used to separate solvent from solutes and remove the solvent prior to solute focusing on the analytical column (Figure 5.8). 

The quantity and volume of samples required for impurity determination by CZE are very small probably less than 5 uL of volume is required for a well-designed injector, and only a few nanoliters (i.e., a few nanograms) are actually injected. However, it is experimentally simpler if that sample is present in a relatively concentrated solution, 0.05-2 mg/mL, when UV detection is being used. Our focus was not to achieve ultra-low detection limits such as might be required for trace level contaminants or for quantitation of trace levels of natural products. For those applications, the most common approach has been the use of a laser-based detector, preferably combined with a fluorescent label on the analyte. With this combination, extremely low limits of detection can be achieved (9, 22-25). 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

The sample introduction system must be capable of introducing a known and variable volume of sample solution reproducibly into the pressurized mobile phase as a sharp plug without adversely affecting the efficiency of the column. The superiority of valve injection has been adequately demonstrated for this purpose and is now universally used in virtually all modern instruments for both manual and automated sample introduction systems [1,2,7,31,32]. Earlier approaches using septum-equipped injectors have passed into disuse for a several reasons, such as limited pressure capability, poor resealability, contamination of the mobile phase, disruption of the column packing, etc., but mainly because they were awkward and inconvenient to use compared with valves. For dilute sample solutions volume overload restricts the maximum sample volume that can be introduced onto the column without a dramatic loss of performance. On-column or precolumn sample focusing mechanisms can be exploited as a trace enrichment technique to enhance sample detectability. Solid-phase extraction and in-column solid-phase microextraction provide a convenient mechanism for isolation, concentration and matrix simplification that are easily interfaced to a liquid chromatograph for fully or semi-automated analysis of complex samples (section 5.3.2). 

As the name indicates, the cold on-column injector allows the injection of the sample directly as a liquid onto the column, of which the inlet and/or outlet section is maintained at a lower temperature than the oven. After the solutes are focused in the inlet section of column, the sample is then vaporized as the oven temperature is increased. In this way, all of the sample components are transferred into the column, so that the sample discrimination attributed to the syringe needle heating during... 

An optimum solution would be the separation of the analytes during the injection process from the high boiling matrix material. The more volatile low molecular weight analytes, for example, pesticides, are able to travel quickly into the analytical column, while high boilers move slowly and can be kept in the insert liner and a pre-column. Analytes that reach the analytical column stop motion at the beginning of the column and get focused by the column film. At this point, the pre-column can be swept backwards during the complete analysis run. For maintenance purposes, the pre-column as a cheap consumable can be replaced easily like the injector insert liner (Munari, 2000). 

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