Summary of detectors used in HPLC

For the majority of analyses of drugs in formulations, variable wavelength UV or diode array UV detectors are used. A typical UV dector has a narrow cell about 1 mm in diameter with a length of 10 mm, giving it an internal volume of about 8.  

The linear range of such detectors is between 0.0001 and 2 absorbance units and samples have to be diluted sufficiently to fall within the range. Although the exact concentration of a sample passing through the flow cell is not known, a suitable concentration can be approximated as shown in Calculation example 12.3. 

A typical elution volume of chromatographic peak volume is 400 //I. If 20 ft (0.02 ml) of a solution containing paracetamol at a concentration of 1 mg/100 ml is injected into an HPLC system with a flow cell with a pathlength of 10 mm  

If the peak has a Gaussian shape, the maximum absorption for the peak would be co 1.5 times the mean absorption, i.e. in this case 0.05 or 50 milliabsorbance units (mAU). 

Selective detectors tend to be employed where the analyte is present in small amounts in a complex matrix such as in bioanalytical procedures where components extracted from the biological matrix along with the analyte can cause interference. Some formulated compounds have only very poor chromophores - these include sugars, lipids, surfactants, amino acids and some classes of drugs, e.g. a number of anticholinergic drugs lack chromophores. In these cases an alternative to UV detection has to be employed. 

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