Column and eluent selection

I. Column and Eluent Selection in Suppressed Ion Exchange Systems... 

Table 7.87 shows the main features of on-line micro LC-GC (see also Table 7.86). The technique allows the high sample capacity and wide flexibility of LC to be coupled with the high separation efficiency and the many selective detection techniques available in GC. Detection by MS somewhat improves the reliability of the analysis, but FID is certainly preferable for routine analysis whenever applicable. Some restrictions concern the type of GC columns and eluent choice, especially using LC columns of conventional dimensions. Most LC-GC methods are normal-phase methods. This is partly because organic solvents used as eluents in NPLC are compatible with GC, making coupling simpler. RPLC-GC coupling is demanding water is not a suitable solvent for GC, because it hydrolyses the siloxane bonds in GC columns. On-line RPLC-GC has not yet become routine. LC-GC technology is only applicable to compounds that can be analysed by GC, i.e. volatile, thermally stable solutes. LC-GC is appropriate for complex samples which are difficult or even impossible to analyse by a single chromatographic technique. Present LC-GC methods almost exclusively apply on-column, loop-type or vaporiser interfaces (PTV).

Table 10.3 Columns and eluent systems used for the analysis of steryl ferulates in selected studies. 

A dependency on the type of ligand, its density, the eluent used and temperature is found when evaluating hydrophobicities of stationary phases. This property can be assessed by the retention factor of a hydrophobic solute or by the ratio of the retention factors of two non-polar solutes. The latter is called selectivity for example, when the components differ only in one methyl group, the term methylene selectivity coefficient is applied. Hence, hydrophobic properties describe the polarity of a column and its selectivity towards solutes with only small differences in polarity. This becomes rather important when endcapped stationary phases are compared (Section 3.2.3.1) as some new types of adsorbents allow separation with 100% water as eluent. 

The simultaneous analysis of alkali and alkaline earth metals is another important ion chromatographic application in the field of drinking water and surface water analysis. Environmental samples with low levels of ammonium, in matrices with high concentration of sodium, are a typical case. Unfortunately, ammonium and sodium ions have similar selectivities for the common stationary phases. This problem can be solved by a column-switching technique or by applying appropriate columns and eluents. Owing to the strong environmental impact, trace heavy metal ion determination and speciation have received particular attention in recent years. 

As a rule, this is a more economical way to improve peak shape than to continue to improve the selectivity (change of chemistry, which means a change of columns and eluents). 

Here, the resolution is not sufficient and hence there is a need for action. In this case, the answer to the first question, if the run time is okay, is yes . As regards the selectivity, the decision is less clear-cut. If one were to try to improve the separation in this case by changing the column or eluent (selectivity, therefore chemistry ), this would be unlikely to be a quick solution. 

Three different types of columns packed with gels of different pore sizes are available. Columns should be selected that are suitable for the molecular weight range of specific samples, as each type has a different exclusion limit (Fig. 6.41, page 215). Bovine serum albumin (BSA), myoglobin, and lysozyme show good peak shapes using only 100 mM of sodium phosphate buffer as an eluent. There is no need to add any salt to the eluent to reduce the ionic interaction between protein and gel. 

Another way to improve the analysis of complex matrices can be the combination of a multidimensional system with information-rich spectral detection (31). The analysis of eucalyptus and cascarilla bark essential oils has been carried out with an MDGC instrument, coupling a fast second chromatograph with a matrix isolation infrared spectrometer. Eluents from the first column were heart-cut and transferred to a cryogenically cooled trap. The trap is then heated to re-inject the components into an analytical column of different selectivity for separation and subsequent detection. The problem of the mismatch between the speed of fast separation and the... 

Finally, the data in Table 8-6 show the elution of the lead column. The eluent is H,0. The driving force for the elution in this case is the lack of C10 present to act as an anion in the binding of the ammonium perchlorate salt pair. The D-enantiomer versus L-enantiomer ratio in the elution is slightly greater than 6 1, as expected by the inherent selectivity of the ligand. For this separation system, LiClO is then added back to the eluent and the eluent is sent on as load to the next purification stage. 

Several methods are applied to reduce the separation time. The best way is the selection of a suitable column and an eluent using isocratic elution. However, much skill and knowledge are required to make such a system. A flow rate gradient, step-wise elution, or eluent composition gradients are commonly applied to reduce separation times. 

An improvement in the resolution can also be achieved by increasing the separation of the peak (A/R) or narrowing the peak width (increased N). An increase in AtR depends on the selection of stationary phase materials and eluent a reduction of the peak width depends on improving the column... 

Coupled columns packed with different stationary phases can be used to optimize the analysis time (71, 75). In this approach the different columns are connected in a series or in parallel. liie sample mixture is first fractioned on a relatively short column. Subsequently the fractions of the partially separated mixture are separated on other columns containing the same or other stationary phases in order to obtain the individual components. Columns differing in length (number of theoretical plates), adsorptive strength or phase ratio (magnitude of specific surface area), and selectivity (nature of the stationary phase) can be employed, whereas, the eluent composition remains unchanged. Identification of the individual sample components via coupled column technique requires a careful optimization of each column and precise control of each switching step. 

In the method proposed by van Staden for the determination of three halides, these are separated in a short colunm packed with a strongly basic ion-exchange resin (Dowex i-X8) that is placed in an FI manifold. A laboratory-made tubular silver/silver halide ion-selective electrode is used as a potentiometric sensor. Van Staden compared the response capabilities of the halide-selective electrodes to a wide concentration range (20-5000 pg/mL) of individual and mixed halide solutions in the presence and absence of the ion-exchange column. By careful selection of appropriate concentrations of the potassixun nitrate carrier/eluent stream to satisfy the requirements of both the ion-exchange column and the halide-selective electrode, he succeeded in separating and determining chloride, bromide and iodide in mixed halide solutions with a detection limit of 5 /xg/mL [130]. 

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