Backflush techniques, column switching

Valves can also be used for column switching to achieve unique configurations for specific separations. A review by Willis [4] contains many different valving arrangements. Backflushing can also be achieved with proper valving and it is a commonly applied technique in gas analysis. 

Trace enrichment is a sample preparation procedure that is performed by passing a crude sample through a special collection column, but with a couple of added features. It is often done online, one at a time, just prior to the analytical procedure. It usually involves a minicolunm very similar to the familiar off-line SPE tube, but packed into a stainless steel column and attached to the injection valve. This is called a trace enrichment column in many published procedures. Another name that is often given to this technique in the title or keywords of published entries is column switching. This is because this technique involves valves that must be switched from one solvent flow path to another. The valve, in its initial position, allows the crude sample to be passed through the TE column with pump 1, whereas the mobile phase is passed only through the analytical column with pump 2 (Fig. 1). After the TE column has collected the sample components of interest, these components are backflushed onto the analytical column by switching the valve to its alternative position (Fig. 2). 

Figure 39. Arrangements for column switching technique A) Heart-, front-, and end-cut techniques B) Backflush technique C) Multidimensional chromatography a)-0 Column 1 -6 g) Detector h) Injection valve...

Direct-injection techniques into either GC or HPLC instruments bypass the extraction step and can offer a very rapid analytical process. In GC, solid-phase microextraction (SPE) can be used, while HPLC tends to require use of pre-columns that are backflushed with the use of column-switching valves. 

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