Liquid chromatography dispersion

Katz et fl/.[l] searched the literature for data that could be used to identify the pertinent dispersion equation for a packed column in liquid chromatography. As a result of the search, no data was found that had been measured with the necessary accuracy and precision and under the sufficiently diverse solute/mobile phase conditions required to meet the second criteria given above. It became obvious that a... 

The small peak volumes typical of samples eluted from small bore columns and short small diameter particle columns used in high-speed liquid chromatography place severe demands on the dispersion characteristics of all components of the liquid chromatograph. The standard deviation of a peak eluting from a column is given by... 

This is an important source of dispersion in gas chromatography, less so in practice in liquid chromatography. Why do you think this is so ... 

Liquid-junction potential, 14 26, 27 Liquid junctions, 14 30 Liquid-like interface model, 14 464 Liquid-liquid chromatography, 6 374 Liquid-liquid dispersions... 

The chromatographic parameters discussed above were calculated for packed beds (increased dispersion and bed length), and for expanded beds with moderate, higher and lower dispersion. The results indicated that the corresponding separation parameters of packed beds and expanded beds are commensurable, therefore, expanded beds can be successfully employed in liquid chromatography even in the case of trace analysis of synthetic dyes in waste water and sludge [75],... 

Willoughby, R.C. Browner, R.F. Mono-disperse Aerosol Generation Interface for Combining Liquid Chromatography with Mass Spectroscopy. Anal. Chem. 1984,56, 2625-2631. 

The Van Deemter equation (1) was the first rate equation to be developed and this took place as long ago as 1956. However, it is only relatively recently that the equation has been validated by careful experimental measurement (2). As a result, the Van Deemter equation has been shown to be the most appropriate equation for the accurate prediction of dispersion in liquid chromatography columns, The Van Deemter equation is particularly pertinent at mobile phase velocities around the optimum velocity (a concept that will shortly be explained). Furthermore, as all LC columns should be operated at, or close to, the optimum velocity for maximum efficiency, the Van Deemter equation is particularly important in column design. Other rate equations that have been developed for liquid chromatography will be discussed in the next chapter and compared with the Van Deemter equation... 

In previous chapters, liquid chromatography column theory has been developed to explain solute retention, band dispersion, column properties and optimum column design for columns that are to be used for purely analytical purposes. The theories considered so far, have assumed that solute concentrations approach (for all practical purposes) infinite dilution, and, as a consequence, all isotherms are linear. In the specific design of the optimum preparative column for a particular preparative separation, initially, the same assumptions will be made. 

Following initial sample extraction, the primary extract must frequently be subjected to some kind of further cleanup including liquid-liquid partitioning, diphasic dialysis, solid-phase extraction, matrix solid-phase dispersion, immunoaffinity chromatography cleanup, liquid chromatography cleanup, or online trace enrichment. In some instances, some of these procedures are used in combination in order to attain higher purification levels. 

Tire aqueous or organic extract obtained at this point may be a very dilute solution containing interfering compounds and making it difficult to determine trace level concentrations of the analyte(s) of interest. To reduce interferences and concentrate the analyte(s), the primary sample extract is furiher subjected to various types of sample cleanup procedures such as conventional liquid-liquid partitioning, solid-phase extraction, matrix solid-phase dispersion, online trace enrichment, liquid chromatography, online dialysis and subsequent trace enrichment, and supercritical fluid extraction. In most cases some of Urese procedures are used in combination to obtain highly purified extracts. 

In the previous relationships used to express the characteristics of a separation, the speed of the mobile phase in the column does not appear. Obviously, the speed has to affect the progression of the solutes, hence their dispersion within the column, and must have an effect on the quality of the analysis. These kinetic considerations are collected in a famous equation proposed by van Deemter. First used in gas chromatography, this equation has been expanded to liquid chromatography and relates H (HETP) to the mean linear velocity of the mobile phase in the column, it (see Fig. 1.9). The simplified form of this equation is given below ... 

C. Crescenzi, S. Bayoudh, P. A. G. Cormack, T. Klein, and K. Ensing, Determination of Clenbuterol in Bovine Liver by Combining Matrix Solid-Phase Dispersion and Molecularly Imprinted Solid-Phase Extraction Followed by Liquid Chromatography/Mass Spectrometry, Anal. Chem. 2001, 73, 2171 ... 

At equilibrium in water at 20°, gas-liquid chromatography indicates that there is 76% -D-fructopyranose, 20% -D-fructofuranose, and 4% of an unknown compound, which has a specific rotation of about +122° (if the value of +17° assigned by Hudson (7) to / -D-furariose is correct). We deduced that the furanose form is void of sweetness for at least two reasons. As an example of hydrogen bonded hydroxyl groups, both hydroxy-methyl substituents are so dispersed as to be (perhaps) completely bonded to the ring oxygen atom (8). 

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