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4.3
Calibration: PFP7
It is important to understand that the principles of flame photometry are such that, over certain
concentration ranges, light emitted from the flame is directly proportional to the concentration
of the species being aspirated.
The graph below shows that the direct relationship between the flame emission and
concentration is only true at relatively low concentrations. Above these low levels the flame
begins to saturate and the flame emission ceases to increase in a linear relationship to
concentration.
10
10
Figure 4.3.1: Relationship between sample concentration and flame emission.
If the samples being analysed lie on the linear part of the curve then the user can take direct
concentration readings from the digital display. If, however, the concentration of samples are
above the levels shown on the graph then the user has the choice of either:
a. diluting the samples so that they lie on the linear part of the curve, or
b. constructing a calibration curve and relating the digital display reading to the
concentration by cross-reference to the curve.
A calibration curve is prepared using standard solutions containing known concentrations of the
elements to be determined and if necessary, other materials to ensure that the standard and
sample backgrounds match. The concentration range covered by the calibration curve will
depend upon the expected concentration of the samples so that the sample readings fall
somewhere in the middle of the calibration curve.
Once the calibration curve has been plotted, the readings for the sample solutions are
compared with the curve to allow the sample concentrations to be established.
It is important to realise that each element has its own characteristic curve and separate
calibration curves must be constructed.
If the same estimation is performed on a routine basis, the calibration curve need only be
prepared once and checked periodically. Instrument re-calibration is easily achieved by setting
the blank solution to read zero and the top standard to read the same value as it did when the
calibration curve was initially prepared. The graph in Figure 4.3.2 shows a typical curve
obtained when measuring in parts per million (ppm).
8
8
6
6
4
4
2
2
0
0
2
2
4
4
6
6
ppm
ppm
17
8
8
10
10
12
12
PFP7/REV C/01-08
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