Total chromatographic system

The variance in volume units of the output function of the chromatographic system can be simply obtained by combining eqns. fl,9 and 11, and converting time to volume units by multiplying by the flow rate, which results in  

It can be seen that short, narrow columns of Tow theoretical plate height make high demands upon the dynamic characteristics of the system especially with 

From eqn. 12 it can be seen that, in instrument design, the sum of the terms, for the cell volume, the mixing in the sampling and the detector unit, and the effect of the detector dynamics, 

In geometrically irregular conduits a more complicated behaviour must be expected, although the general line will be the same. Fig. 6 shows an exaraple[9] of the dispersion outside the column in a well designed chromatograph, including all the effects described by eqn. 12. It can be seen that the volume standard deviation indeed approaches the value [(Vp + 7 ) / 12].  

Sternberg, Advances in Chromatography, Vol. 2, Marcel Dekker, New York, 1965, p.205. 

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The analytical specifications must prescribe the ultimate performance of the total chromatographic system, in appropriate numerical values, to demonstrate the performance that has been achieved. 

The analytical specifications must prescribe the ultimate performance of the total chromatographic system, in appropriate numerical values, to demonstrate the performance that has been achieved. The separation of the critical pair would require a minimum column efficiency and the number of theoretical plated produced by the column should be reported. The second most important requisite is that the analysis should be achieved in the minimum time and thus the analysis time should also be given. The analyst will also want to know the maximum volume of charge that can be placed on the column, the solvent consumption per analysis, the mass sensitivity and finally the total peak capacity of the chromatogram. The analytical specifications can be summarized as follows. 

The first chromatogram obtained on a new column may be viewed as the birth certificate of a column and defines column performance at time t = 0 in the laboratory a test mix should also be analyzed periodically to determine any changes in column behavior occurring with age and use. For example, a column may acquire a pronouncedly basic character if it has been employed routinely for amine analyses. /Mother important but often overlooked aspect is that a test mixture serves to monitor the performance of the total chromatographic system, not just the performance of the column. If separations gradually deteriorate over time, the problem may not always be column-related but could be due to extra-column effects, such as a contaminated or activated inlet liner. Commonly used components, their accepted abbreviations, and functions are listed in Table 3.16. 

The second and third peaks will be the pair of peaks in the mixture that are eluted closest together and, thus, the most difficult to separate (usually given the term the critical pair as they define the severity of the separation). Finally, the fourth peak will be that which is eluted last from the mixture and will determine when the analysis is complete and establishes the total analysis time. The chromatographic system must be designed to separate the critical pair and, as this is the pair that is eluted closest together, all other peaks should also be resolved... 

Another critical instrument specification is the total extra-column dispersion. The subject of extra-column dispersion has already been discussed in chapter 9. It has been shown that the extra-column dispersion determines the minimum column radius and, thus, both the solvent consumption per analysis and the mass sensitivity of the overall chromatographic system. The overall extra-column variance, therefore, must be known and quantitatively specified. 

Unfortunately, some of the data that are required to calculate the specifications and operating conditions of the optimum column involve instrument specifications which are often not available from the instrument manufacturer. In particular, the total dispersion of the detector and its internal connecting tubes is rarely given. In a similar manner, a value for the dispersion that takes place in a sample valve is rarely provided by the manufactures. The valve, as discussed in a previous chapter, can make a significant contribution to the extra-column dispersion of the chromatographic system, which, as has also been shown, will determine the magnitude of the column radius. Sadly, it is often left to the analyst to experimentally determine these data. 

The ideal interface is rare. Table 7.3 lists the qualities required for an ideal interface from a chromatographic point of view [3]. Nowadays, hyphenation goes a long way towards total analysis systems (e.g. HPLC-UV-NMR-MS), especially in the pharmaceutical industry. Such magic-wand systems are by no means a panacea for all analytical problems they are more likely to be confined to niche applications. Multihyphenation and multidetector monitoring set their own... 

Using the same chromatographic system as given in Fig. 17.19 we were able to investigate modified polyamide 6.6 by 2DLC. The samples were modified by amidation with propionic acid, introducing additional functionality fractions to the system. In total, seven different species were expected to be present in the samples (see Table 17.1). 

After an evaluation of all of the components of the total analyzer system, the instrument (s) can be calibrated or standardized as necessary. For many types of analyzers (e.g. gas chromatographs, oxygen... 

The total extra column dispersion that takes place in a liquid chromatographic system places a limit on the minimum column radius that can be employed for a given separation. The effect of extra column dispersion on the minimum column radius was examined by Reese and Scott (13) who derived an equation that allows the minimum column radius to be calculated for any particular separation. 

It Is seen from equation (30) that the optimum flow-rate Is also proportional to the extra column dispersion and, as a consequence, the total volume of mobile phase employed in an analysis will also depend on the extra column dispersion. It follows that the economy of the analysis lies in the hands of the designer of the chromatograph, a responsibility for which, many instrument makers are not aware. Steps taken in the design of the chromatographic system that would reduce the extra column dispersion by a factor of two would also halve the volume and cost of solvent used in the... 

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