Internal standard method:

Within this method, a fixed amount of an internal standard that is chemically same to the analyte being determined and absorbs at same wavelength, is added to the standard solution and the test sample. An intensity ratio of the analyte and internal standard is plotted as a function of the analyte concentration within the standard solution. For instance, although determining Na or K in blood serum Li is used as internal standard. A classic plot acquired in an atomic absorption spectrophotometric determination using internal standard method is given in Figure.

Figure: A typical calibration plot for internal standard method in AAS

If the aspiration fluctuates then every signal is affected to the similar extent and the ratio at a provided analyte concentration remains constant.