The Maximum Permissible Detector Dispersion

The expression for the maximum permissible detector dispersion, given by equation (21), also shows it s strong dependance on the product of the solute diffusivity and the viscosity of the mobile phase together with the inverse of the fourth power of (or1) A graph relating (op) to the separation ratio of the critical pair is shown in figure (6) 

Maximum Detector Dispersion against Separation Ratio 

The results in figure (6) emphasizes the stringent demands that open tubular columns place on system design, Even when separating solute pairs having an (a) value of 1.01. and operating at an in et pressure of only 1 p.s.t, the maximum value for (op) will only be about 0.16 microlitres If it is assumed that in practice (ap) can not be reduced to much less than 5 nanolitres (and this would be extremely difficult) the use of open tubular 

The basic design equations for the design of open tubular columns in LC can be summarized as follows 

PRINT Enter Separation Ratio of the Critical Pair INPUT A PRINT Enter Capacity Ratio of the First Peak of the Pair INPUT k I PRINT Enter Capacity Ratio of the Last Eluted Peak INPUT K2 PRINT Enter Diffusivity of Solute in Mobile Phase UNPUT D1 PRINT Enter Viscosity of Mobile Phase (Poises) .INPUT M PRINT Enter Column Inlet Pressure (p.s i) INPUT P PRINT BASIC CHROMATOGRAPH IC DATA  

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It is seen from equation (12) that the system concentration sensitivity is also directly (and this time solely) dependent on the detector sensitivity and is entirely independent of the column dimensions and efficiency. Consequently, in contrast to the system mass sensitivity, the system concentration sensitivity is not dependent on the detector dispersion. It should be noted, however, that the maximum permissible sample volume has to be employed to realize the maximum concentration sensitivity... 

It follows that the maximum permissible time constant will be only 10% of the peak standard deviation if sufficient extra column dispersion is retained for sample volume and detector cell volume... 

The maximum allowable dispersion will include contributions from all the different dispersion sources. Furthermore, the analyst may frequently be required to place a large volume of sample on the column to accommodate the specific nature of the sample. The peak spreading resulting from the use of the maximum possible sample volume is likely to reach the permissible dispersion limit. It follows that the dispersion that takes place in the connecting tubes, sensor volume and other parts of the detector must be reduced to the absolute minimum and, if possible, kept to less than 10% of that permissible (i.c.,1 % of the column variance) to allow large sample volumes to be used when necessary. 

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