Metrohm 757 VA Computrace - Manual (Page 130)

5 Determination mode
757 VA Computrace – Software
120
Rules for standard addition
Standard addition is the usual calibration method for the major-
ity of the applications possible with the 757 VA Computrace. Its
advantage lies in its high dependability as the calibration in the
sample takes place under real matrix conditions and all meas-
urement parameters remain unchanged. With regard to optimum
accuracy and scatter, several
rules must be observed in stan-
dard addition:
• Check linearity range
In development of the method, the linearity range should be
checked for each substance. This involves performing several
standard additions over a wide concentration range. Using
the calibration curve shown on
DETERMINATION CURVES win-
dow, you can then determine the region in which the standard
addition is linear and that in which it is nonlinear.
• Addition procedure
If the substance content lies in the linear range, multiple stan-
dard addition is meaningful only if you wish to check the line-
arity in every determination. To reduce the scatter, it is better
to spike the solution once only and to choose as many repli-
cations as possible.
• Standard addition ratio 1:2 to 1:5
The optimum standard addition ratio for the entire standard
addition is from 1:2 to 1:5, i.e. the sum of all standard addition
amounts should be 2 to 5 times the amount of sample in the
measuring vessel. This can be easily checked later with the
parameters Mass and Add.mass put out in the
RESULTS
window.
• Consideration of blank values
Any blank values must be determined separately and sub-
tracted by means of the formula in the
CALCULATION window.