Temperature-programmed elution

Cehte or firebrick packing for glc columns is often treated with TMCS, DMCS, or other volatile silylating agents (see Table 1) to reduce tailing by polar organic compounds. A chemically bonded methyl siUcone support is stable for temperature programming to 390°C and allows elution of hydrocarbons up to C q (20). 

Temperature programming was introduced in the early days of GC and is now a commonly practiced elution technique. It follows that the temperature programmer is an essential accessory to all contemporary gas chromatographs and also to many liquid chromatographs. The technique is used for the same reasons as flow programming, that is, to accelerate the elution rate of the late peaks that would otherwise take an inordinately long time to elute. The distribution coefficient of a solute is exponentially related to the reciprocal of the absolute temperature, and as the retention volume is directly related to the distribution coefficient, temperature will govern the elution rate of the solute. 

Figure 6. Graphs of Elution Temperature of (R) 4-Benzyl-2-oxazol-idinone against Program Rate for Three Different Initial Temperatures...

Peak capacity can be very effectively improved by using temperature programming in GC or gradient elution in LC. However, if the mixture is very complex with a large number of individual solutes, then the same problem will often arise even under programming conditions. These difficulties arise as a direct result of the limited peak capacity of the column. It follows that it would be useful to derive an equation that... 

This equation, although originating from the plate theory, must again be considered as largely empirical when employed for TLC. This is because, in its derivation, the distribution coefficient of the solute between the two phases is considered constant throughout the development process. In practice, due to the nature of the development as already discussed for TLC, the distribution coefficient does not remain constant and, thus, the expression for column efficiency must be considered, at best, only approximate. The same errors would be involved if the equation was used to calculate the efficiency of a GC column when the solute was eluted by temperature programming or in LC where the solute was eluted by gradient elution. If the solute could be eluted by a pure solvent such as n-heptane on a plate that had been presaturated with the solvent vapor, then the distribution coefficient would remain sensibly constant over the development process. Under such circumstances the efficiency value would be more accurate and more likely to represent a true plate efficiency. 

The ability of a GC column to theoretically separate a multitude of components is normally defined by the capacity of the column. Component boiling point will be an initial property that determines relative component retention. Superimposed on this primary consideration is then the phase selectivity, which allows solutes of similar boiling point or volatility to be differentiated. In GC X GC, capacity is now defined in terms of the separation space available (11). As shown below, this space is an area determined by (a) the time of the modulation period (defined further below), which corresponds to an elution property on the second column, and (b) the elution time on the first column. In the normal experiment, the fast elution on the second column is conducted almost instantaneously, so will be essentially carried out under isothermal conditions, although the oven is temperature programmed. Thus, compounds will have an approximately constant peak width in the first dimension, but their widths in the second dimension will depend on how long they take to elute on the second column (isothermal conditions mean that later-eluting peaks on 2D are broader). In addition, peaks will have a variance (distribution) in each dimension depending on... 

Temperature programmed GC (Fig. 2) separates these components as well as a cyclic formal. The mono, di and tri brominated products of 1 require higher temperatures to elute in a reasonable time more than the column can withstand. TMS derivatives do not require temperatures quite so high (Fig. 3). Using this technique for quantitation, however, is complicated by the decreasing sensitivity of the FID to increasing bromine content. 

To ensure operation under reproducible conditions, the column is enclosed in a thermostatically controlled oven whose temperature can be held constant to within 0.1°C. Operating temperatures range from ambient to over 400°C and may remain constant during a separation - isothermal operation - or automatically increased at a predetermined rate to speed the elution process - temperature programming (p. 106). The latter is a form of gradient elution. Rapid temperature equili-... 

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