Plate Theory and Extensions - The Peak Capacity of a Chromatographic Column > Page 99

Equation (80) shows that the capacity ratio of the last eluted peak is inversely proportional to the detector sensitivity (minimum detectable concentration). Consequently, it is the detector sensitivity that determines the maximum peak capacity attainable from the column. Using equation (80), peak capacities was calculated for three different detector sensitivities and a column having an efficiency of 10,000 theoretical plates, a dead volume of 6.7 ml and a sample concentration of 1%v/v. The results are shown in Table 3.

Table 3. Capacity Ratios and Peak Capacities for Detectors of Different Sensitivities


 Detector Sensitivity Maximum (k') Retention Time (min.) Peak Capacity
10-6 (g/ml) 220 73.6 134
10-7 (g/ml) 2,200 736.0 177
10-8 (g/ml) 22,000 7360.0 194

Table 1 shows that increasing the minimum detectable concentration by an order of magnitude (e.g., from 10-7 g/ml to 10-8 g/ml) the maximum capacity ratio is also increased by an order of magnitude. However, this only produces an increase in peak capacity of 10%. In addition, the small increase in peak capacity is realized at the expense of a retention time that is increased from twelve hours to about five days.

Attempting to increase the peak capacity of a chromatographic system by using higher detector sensitivity is very costly in time. Increased peak capacities are best achieved by constructing columns of high intrinsic efficiency.