Microbore column discussion

It is seen that the maximum value for the standard deviation of all the processes that contribute the extracolumn dispersion must be less than 4.5 microliters and gives an indication of the difficulties involved in designing detectors that can function well with microbore columns. It is also seen that equation (8) can be extremely useful in detector design and provides the necessary data that would allow a detecting system to be constructed to suit a particular range of column sizes. It is obvious that, although the maximum value for (a ) is now known, it will be necessary to examine quantitatively the contribution of the various extracolumn dispersion processes to the overall value of (Og). These details will be discussed later in this book. 

The importance of the systematic errors on the isotherm data that can be introduced by the use of an erroneous column diameter in the numerical calculations is illustrated by data from Zhou et al. [178] who compared the isotherm data acquired on three different columns (i.d. 1.07, 4.57, and 10.1 mm) packed with the same stationary phase. The data were eventually shown to be in close agreement, with the band profiles on the large column matching closely the profiles calculated with the isotherm data measured with the microbore column. However, the initial comparison of the isotherm data suggested a marked disagreement. This was explained by a difference of 0.07 mm between the nominal column i.d. of the microbore column supplied by the manufacturer and used in the initial calculations, and the true i.d., measured later with an electronic caliper. A quantitative discussion of the importance of the errors caused by a small error on the column diameter is available [178]. 

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

Other detectors. The above discussions have been concerned with those detectors most commonly employed in routine HPLC analysis and which are commercially available. Many other detectors have been developed to monitor specific solute properties in column effluents and new detection systems continue to be reported in the literature, many directed to meeting the requirements imposed by microbore and capillary separation technologies. The interested reader is directed to the reviews of Yeung [46] and Fielden [47] for a more detailed discussion of detector types employed in HPLC. 

There have been thousands of articles published on the application of column-switching HPLC method. Today, column-switching HPLC method is being used around the world in all areas of chemistry, environmental problem solving, medical research, and so forth. In clinical and pharmaceutical analysis, column switching has been mainly apphed to onhne sample cleanup and trace enrichment using microbore analytical column. Examples of column-switching HPLC method not discussed in any detail are presented in Table 2. 

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