Heart techniques, column switching

Figure 5.4 Schematic diagrams of a heait-cut valve configuration system. Reprinted from Journal of Chromatography, 602, S. R. Villasenor, Matrix elimination in ion cliromatography by heart-cut column switching techniques , pp. 155-161, copyright 1992, with permission from Elsevier Science.

S. R. Villasenor, Heart-cut column switching techniques for the determination of an aliphatic amine in an organic matrix and for low levels of sulfate in an anion matrix , J. Chromatogr. 671 11-14(1994). 

With on-line techniques, the column switching operations are done using valves. Fig. 5.3d shows a simple arrangement for zone cutting that could be used for sample clean-up. The zone marked Y is to be determined and all other zones are to go to waste (this type of cut is called a heart cut). Initial separation takes place on column Cl so that early zones (X) are routed to waste. When zone Y is eluted from Cl, valve V2 is switched to elute this zone onto column C2. After complete transfer of Y onto C2, valve VI is switched to prevent further elution of unwanted zones (Z, for instance). Zone Y is eluted to the detector and Cl can be cleaned and re-equilibrated with mobile phase. 

A schematic diagram of a heart-cut LC-LC system is depicted in Figure 5.4. The column switching technique was developed by employing two high-pressure four-way pneumatic valves inserted before and after the precolumn (39). The front-cut and the end-cut of the sample eluted from the first column were vented to waste. The valves were manipulated to transfer only the heart-cut of the analyte of interest to the analytical column. The detailed operational conditions for the four-step sequence of this system can be described as follows ... 

When column switching is used for sample cleanup, the technique is known as zone cutting. If the fraction of effluent to be transferred from the precolumn to the analytical column is at the front of the precolumn chromatogram, the technique is known as front cutting. Heart-cut technique... 

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...

A commonly used system in environmental analysis is the heart-cutting technique which uses the separation power of the first column to obtain a higher selectivity than with the previously described precolumn enrichment. The two columns are coupled via a switching valve, as shown in Figure 13.5. 

The design has been well proved in quality assurance and origin control of flavours and fragrances. A double-oven system is shown in the Fig. 17.3, with two independent temperature controls and two detectors (DM 1, DM 2). A live switching coupling piece is used to switch the effluent flow to either the first detector or the chiral column. With optimum pneumatic adjustment of the MDGC system, certain fractions are selectively transferred onto the chiral main column as they are eluted from the precolumn (heart-cutting technique) [15]. 

A sample is usually preseparated on a short precolumn and then directed onto an analytical column (with the same or different selectivity) where actual separation occurs. The heart-cut technique is most often used when the solute coelutes with another sample or when it is situated on the flank of another peak. In this case the interesting part of the chromatogram is cut out and parked on the analytical column, while the rest of the sample is eluted from the precolumn into the waste. Thereafter, the valve is switched back to the analytical column position, and the parked sample... 

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