Analyte concentration:

In practice, thus, a single calibration does not serve the reasons. We required taking 3-4 standards of various concentrations and a bank to acquire an appropriate calibration plot. As you could see in Figure, a single standard calibration plot does not hold good. In additional, if the analyte concentration happens to be outside the limits of the standards used for calibration then the analyte sample should be concentrated or suitably diluted.

As the test solution is frequent a complex whose all constituents are not known, it becomes almost not possible to prepare standard solutions having a same composition to the analyte sample to acquire a calibration plot. Within such cases we have to use internal standard method and the standard further method.