Liquid Chromatography Detectors - Dispersion in Detector Sensors > Dispersion in Connecting Tubes > Page 6

Table 1  Standard Deviation of Connecting Tubes of Different Sizes

Connecting Tubes for Liquid  Chromatography

  Standard Deviation of Tube Dispersion
Tube Diameter l=1 cm l=2 cm l=5 cm l=10 cm l=15 cm  
0.001 in, 0.00254 cm 22.3 nl 31.5 nl 49.9 nl 70.5 nl 86.4 nl  
0.002 in, 0.00508 cm 47.6 nl 67.3 nl 106.4 nl 150 nl 184.4 nl  
0.003 in, 0.00762 cm 107 nl 151.3 nl 239.2 nl 0.34 ml 0.41 ml  
0.005 in, 0.01270 cm 298 nl 421 nl 0.67 ml 0.94 ml 1.15 ml  
0.010 in, 0.02540 cm 1.19 ml 1.68 ml 2.66 ml 3.76 ml 4.61 ml  


 (Dm) is taken as  2 x 10-5 cm2sec-1 and the flow rate at 0.5 ml/min. All values are fairly typical for the normal operation of the chromatographic system near optimum conditions.

It is seen from table 1 that the effect of dispersion in connecting tubes is large due to the very low diffusivity of solutes in liquids. It will be shown in book 8 that for the successful use of microbore columns (columns less than 2 mm I.D.) tube dispersion needs to reduced to about 80 nl. Again assuming that to minimize the chance of tube blocking, the limiting minimum I.D. for the connecting tube is made to be 0.003 in (and tubes of this diameter will still easily block) then  the connecting tube must be less than 1 cm long. It is clear that the length of the connecting tube between column and detector must be reduced to an absolute minimum. If the tubing diameter is reduced further and the column diameter is increased, then longer tubing lengths may be possible. Alternatively, the resolution of the early peaks can be sacrificed in favor of later eluting peaks which will also allow longer connecting tubes to be used. These techniques to reduce the effect of connecting tube dispersion in LC are common with most manufacturers. The simple solution of designing the chromatographic system such that the detector sensor is situated very close to the end of the column does not appear to be considered a practical option.