Instant Notes: Analytical Chemistry

Detailed calculations of the correlation coefficient, r, and the slope and intercept

values have not been given, but should be set out as in the previous example if a

suitable calculator or computer program is not available.

The amount of calcium in the sample can be read from the extrapolated graph

or calculated from the slope, b, and the intercept, a

Calcium concentration in sample =a/b=0.2569/0.005349 =48 ppm

For some analytical techniques, particularly chromatography, variations in

experimental conditions can adversely affect the precision of the data. A

calibration procedure whereby a constant amount of a selected substance,

the internal standardis added to all samples and analyte standards alike

compensates for variations in sample size and other parameters. The ratio of

the detector response for the analyte in each standard to the corresponding

response for the added internal standard is plotted on the y-axis of a calibration

graph against the mass or concentration of the analyte on the x-axis. The corre-

lation coefficient, slope and intercept values can be computed as shown previ-

ously, and response ratiosfor the analyte and added internal standard in the

samples can then be used to determine the amount of analyte in the samples by

interpolation on the graph. If only one or two analyte standards are prepared,

the amount of analyte in a sample can be calculated by simple proportion, i.e.

=

For some purposes, only the relative amounts of the analytes in a multicomponent

mixture are required. These are normalized to 100 or 1 by expressing each as a

Internal
normalization

sample

standard

for

for

ratio

ratio

response

response

sample

standard

in

in

analyte

analyte

Internal
standardization

B4 – Calibration and linear regression 47

–60 –50 –40 –30 –20 –10 0

0

10 20 30 40 50 60

Sample concentration

48 ppm

Emission intensity at 423 nm

Slope = 0.005349

Spiked samples

Unspiked

sample

Intercept = 0.2569

Calcium (ppm)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Fig. 4. Standard addition calibration graph, regression line, slope and intercept values for the

flame emission determination of calcium in a clinical sample.