2. Method
Standard addition is important for successful
determination of the elements present in gall bladder
stones if analyte is present at low levels and
no suitable standards are available, especially if
the analyst is interested in only one analyte element.
The method involves the addition of known
quantities of the analyte to the specimen. The
intensity of an analyte is given as
IisMiWi (1)
where Wi is the weight fraction of element i, Mi is
the sensitivity and is expressed in counts per
second per unit concentration. Adding a known
amount of analyte to the unknown sample gives
an increased intensity,
IiqDIisMiŽWiqDWi. (2)
assuming that linear calibration is adequate
throughout the range of additions. An increase in
the concentration of analyte by DWi will increase
the intensity by MiDWi. To check the linearity of
the calibration, the process can be repeated by
adding different amounts of the analyte to the
sample and plotting the intensity measured vs. the
concentrations added. The intercept on the concentration
axis equals Wi. The intensities must be
corrected for background and line overlap. The
method is mainly suitable for determination of
trace and minor concentration levels because the
increase in the concentration, DWi, must be similar
to the unknown concentration, Wi, in the sample
itself. The extrapolation error can be quite large if
the slope of the line is not known accurately.
Adding significant amounts to the sample may
lead to non-linearity, because it alters the matrix
effect w13x. The method is applicable only to
samples to which additions can be made — principally
powders, fusion products and solutions.