Column-cutting techniques

Making the proper connections, especially in the installation of a capillary column, is critical to the good practice of gas chromatography and to avoiding problems. The two most important considerations are that the capillary column be properly cut and that the column end be inserted the correct distance into the inlet. Proper column-cutting techniques are reviewed in the literature provided by the column manufacturers, and the inlet manufacturer provides the proper installation procedure. Note that these procedures may vary from manufacturer to manufacturer, so they should be studied carefully prior to installing a column. 

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. Villasenoi, Matrix elimination in ion chromatography by heart-cut column switcliing techniques , 7. Chromatogr. 602 155-161 (1992). 

Another application of SFC-GC was for the isolation of chrysene, a poly aromatic hydrocarbon, from a complex liquid hydrocarbon industrial sample (24). A 5 p.m octadecyl column (200 cm X 4.6 mm i.d.) was used for the preseparation, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The direct analysis of whole samples transferred from the supercritical fluid chromatograph and selective and multi-heart-cutting of a particular region as it elutes from the SFC system was demonstrated. The heart-cutting technique allows the possibility of separating a trace component from a complex mixture (Figure 12.21). 

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. 

Mangani, F., Luck, G., Fraudeau, C., Verette, E. (1997). Online column-switching high-performance liquid chromatography analysis of cardiovascular drugs in serum with automated sample clean-up and zone-cutting technique to perform chiral separation. J. Chromatogr. A 762, 235-241. 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

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]. 

On-column injection is preferred, because it occurs at room temperature. Hot injector ports can lead to decomposition of the sample, and to racemization of chiral components. If a single oven system is used, then split injection offers some advantages because it is important not to expose the chiral column to large amounts of solvents. With an MDGC system, the heart-cutting technique removes the solvent from the chiral column, so on-column injection is preferred. 

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 ... 

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). 

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... 

For single separation duty, Bernot et al. (1991) presented a method to estimate batch sizes, operating times, utility loads, costs, etc. for multicomponent batch distillation. The approach is similar to that of Diwekar et al. (1989) in the sense that a simple short cut technique is used to avoid integration of a full column model. Their simple column model assumes negligible holdup and equimolal overflow. The authors design and, for a predefined reflux or reboil ratio, minimise the total annual cost to produce a number of product fractions of specified purity from a multicomponent mixture. 

Furthermore, enantio-MDGC, employing heart-cutting techniques from DB-1701 as the preseparation column on to heptakis (3-0-acetyl-2,6-di-0-pentyl)-P-cyclodextrin as the chiral main column, was described by Mosandl et al. /2S7 as a powerful tool in the direct enantiomer separation of chiral y-lactones from complex matrices without any further clean-up or derivatization procedures. 

High-speed chromatography combines column selectivity and special selectivity adjustment techniques with conventional 2DGC to improve the speed of analysis in what is essentially a rapid, heart-cutting technique. " Rapid chromatography requires cryofocusing of analytes in... 

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