Chromatographic processes column efficiency

The physical properties of the mobile phase, mainly viscosity, diffusivity and solubility, affect the flow characteristics, column efficiency (kinetics), and retention (thermodynamics) in the chromatographic process. These physical properties are affected by temperature. Chromatographic techniques, although basically simple in... 

As readily observed in most chromatograms, peaks tend to be Gaussian in shape and broaden with time, where W, becomes larger with longer This is caused by band-broadening effects inside the column, and is fundamental to all chromatographic processes.The term, plate number (N), is a quantitative measure of the efficiency of the column, and is related to the ratio of the retention time and the standard deviation of... 

Solute equilibrium between the mobile and stationary phases is never achieved in the chromatographic column except possibly (as Giddings points out) at the maximum of a peak (1). As stated before, to circumvent this non equilibrium condition and allow a simple mathematical treatment of the chromatographic process, Martin and Synge (2) borrowed the plate concept from distillation theory and considered the column consisted of a series of theoretical plates in which equilibrium could be assumed to occur. In fact each plate represented a dwell time for the solute to achieve equilibrium at that point in the column and the process of distribution could be considered as incremental. It has been shown that employing this concept an equation for the elution curve can be easily obtained and, from that basic equation, others can be developed that describe the various properties of a chromatogram. Such equations will permit the calculation of efficiency, the calculation of the number of theoretical plates required to achieve a specific separation and among many applications, elucidate the function of the heat of absorption detector. 

The chromatographic process begins by injecting the solute onto the top of the column. The solvent need not be the mobile phase, but frequently it is appropriately chosen to avoid detector interference, column/analyte interference, loss in efficiency, or all of these. Sample introduction can be accomplished in various ways. The simplest method is to use an injection valve. In more sophisticated LC systems, automatic sampling devices are incorporated where sample introduction is done with tire help of autosamplers and microprocessors. It is always best to remove particles from the sample by filtering, or centrifuging since continuous injections of particulate material will eventually cause blockage of injection devices or columns. 

Because a chromatographic peak is assumed to result from the spreading of the sample zone with a Gaussian distribution of sample concentrations in the mobile and stationary phases, the calculation of the column efficiency is related to the associated variance, a2. The efficiency of the process is thus the ratio t2/a2 a2 is the variance in time units (in this case). When dr is measured on the recorder chart, the ratio d2la2 is considered, with a in distance units. 

The need for reliable methods for selecting liquid phases is greatly diminished when highly efficient OT columns are used. Indeed, it is often the case that a 15-m column of DB-5 (or the equivalent nonpolar bonded phase) will separate most of an analyst s samples. Nevertheless, the following methods have contributed to our understanding of the chromatographic process and are worthy of consideration. 

The efficiency at which the column or plate count is packed initially in chromatography is important because (1) it determines the shape of the individual band profiles, and (2) it can effect the void fraction and thus the operating pressure. For chromatographic processes where the plate count is important (see Section 7.8.1 on flow rate), developing methods to measure the plate count of the column after it is packed or of a prepacked column is important. There are several methods for measuring the plate count of a packed column, as given below. 

The same compounds have also been examined by Urasa et al. [50], who coupled an ion chromatograph to a DCP atom emission spectrometer2 by connecting the column end capillary with the nebulizer of the spectrometer, which was slightly modified [51]. A principle problem of DC plasma detection is the relatively low sensitivity, which may be attributed to the inadequate efficiency of the nebulization and to the sample dilution during the chromatographic process. The dilution effect alone accounts for a reduction... 

Displacement chromatography A chromatographic process in which, after the sample is injected, the mobile phase is replaced by a solution of a compoxmd more strongly adsorbed than any feed component, the displacer. Eventually, the feed components are separated into a series of successive zones, forming an isotachic train which propagates without further change. If the column efficiency, the sepa-... 

Frontal analysis A chromatographic process in which a feed solution is abruptly substituted for the mobile phase and pumped through the column. Each component has its own breakthrough curve, but only the least retained one gives a pure zone. This method is used by chemical engineers for bulk purification requirements. In chromatography, it is an acciurate method of isotherm measurement because the retention time of the fronts is related to the amounts adsorbed and is independent of the column efficiency. Also called breakthrough analysis. 

---