Open tubular columns comparison with

Figure 2.4 Comparison of a typical separation on a packed and a open tubular column. The seunple (solvent extract of river water) is the same in both cases. (Reproduced with permission from ref. 135. Copyright Ann Arbor Science Publishers).

Extensive comparisons between GC and SFC have been reported in chiral separation [63-66]. Zoltan investigated the performance of SFC and GC using the same chiral capillary columns coated with cyclodextrin-based stationary phases. It was observed that chiral selectivity was higher in GC than in SFC using the same open tubular column at the identical temperature (e.g., >100°C). However, the selectivity in SFC was significantly increased at low temperatures, especially for polar compounds [67]. 

Marine lipids with their diversity of unsaturated and branched chain acid moieties are a difficult class of materials to analyze. Ruminants (sheep, goats, cows, etc.) have a bacterial "factory" in the rumen which is able to produce branched-chain partially-hydrogenated lipids from ingested plant lipids. These lipids are incorporated into the milk and meat of the animals and eventually into animals which feed upon the ruminants. As a rule animal lipids are highly complex in comparison to plant materials. Although the branched chain materials are usually present in low concentration when compared to the common fatty acid moieties, complete description of these fats requires more sophisticated GC and thus long open tubular columns in tandem with mass spectrometry and computer analysis of the data has become an important approach. Even with a 100-m column, subcutaneous lipids of barley-fed lambs were so complex that prior fractionation with urea adducts was necessary (17). 

Separation columns are the heart of any CEC techniques. In order for CEC to become a versatile analytical technique, it is important to have columns with good stability, high EOF velocity generation, and affinity for different analytes. To this end, several different types of capillary columns have been developed for the separation of biomolecules. In this section, a summary of recent advances in column technology, including open tubular columns, duplex columns, and monolithic columns, will be presented. A comparison of different column configurations is shown in Figure 13. 

Also important is the use of the modem chromatographic methods to separate and identify individual components in commercial cmde shale oil. The use of efficient gas chromatographic columns coupled with ancillary techniques such as mass spectrometry and vapor-phase IR spectroscopy allows the identification of individual shale oil components. A principal part of this study is the comparison of diflFerent types of open tubular columns for the separation of the alkane-alkene fraction of shale oil WCOT, SCOT, and PLOT columns are examined. Gas chromatographic separation of shale oil acids and bases also is performed allowing the identification of these components. The potential utility of subtractive pre-columns in HPLC analysis is illustrated also. 

Note that the order of elution in the GC chromatogram of Fig. 4.8 is from lower to higher molecular weight. In other words, the capacity factor, k, for benzene is much smaller than for ortho-xylene. From Eq. (4.31), this suggests that ortho-xylene has a much larger partition constant, K, into the column stationary phase in comparison to benzene. Values of k can also be correlated with increases in boiling point for these substituted aromatics. This particular open tubular column lacks the necessary selectivity, a, to separate the meta- from the para-xylene isomer. Also note that resolution is more than adequate for all BTEX compounds except for ethylbenzene from... 

The overall method for gasoline requires no sample preparation other than the addition of internal references. The method is fast since samples may be run at 3-min intervals, interference-free and specific for MMT. Analysis time comparison is favourable with respect to atomic absorption analysis which typically required about 15 min per sample when run in batches. Further, the procedure is readily adapted for use with open tubular columns. Porous layer open tubular columns (PLOY) are best suited to accommodate the direct injection of gasoline samples. The improvement in analysis time for MMT determination by the latter method may outweigh the somewhat poorer limit of detection resulting from limitations on the sample size. 

Figure 8.39 Comparison of a straight and knitted open tubular reactor of equal length for preserving column resolution. (Reproduced with permission from ref. 6S6. Copyright Dr Alfred Huethig Publishers).

Figure 1.19 shows a typical normal acid profile [358] and for comparison one from a patient with propionic acidemia [362]. Greatly improved separations are possible by using open tubular capillary columns [40] and it is likely that such columns will be widely used for this purpose in future. 

A variable-wavelength UV-absorbance detector (Model Ovldec 100-V, Jasco Inc., Tokyo, Japan) was modified to permit "on-column" detection with packed and open tubular fused-slllca microcolumns, as described previously (3,45,47). The UV—absorbance detector was placed In series before the laser fluorescence detector (see Figure 1), and was used for comparisons of sensitivity, selectivity, and dead volume (44). 

W. Som-Aum, S. Liawruangrath, and E.H. Hansen. Flow injection on-line preconcentration of low levels of Cr(VI) with detection by ETAAS comparison of using an open tubular PTFE knotted reactor and a column reactor packed with PTFE beads. Analytica Chimica Acta 463 99-109,2002. 

---