Metrohm 797 VA Computrace - Manual (Page 157)

5.8 Data processing and evaluation
797 VA Computrace – Software
145
Rules for standard addition
Standard addition is the usual calibration technique for the ma-
jority of the applications possible with the 797 VA Computrace. Its
advantage lies in its high dependability as the calibration in the
sample takes place under real matrix conditions and all measure-
ment parameters remain unchanged. With regard to optimum ac-
curacy and scatter, several rules must be observed in standard ad-
dition:
• 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 window,
you can then determine the region in which the standard addi-
tion 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 linear-
ity in every determination. To reduce the scatter, it is better to
spike the solution once only and to choose as many replications
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 pa-
rameters
Amount
and
Add.amount
put out in the RESULTS
window.
• Consideration of blank values
Any blank values must be determined separately and subtrac-
ted by means of the formula in the
CALCULATION window.
Calibration curve calculation
The content determination using a calibration curve is performed in
two steps:
• First the relation between the mass concentration c of a sub-
stance and the evaluation quantity
EV
is determined by mea-
suring different reference solutions.
Note: "Minimum number" as well as "Recommended number"
of calibration points which have to be determined for a reliable
calibration depend on the chosen
Regression technique on the
tab Substances of the window EDIT WORKING METHOD PARA-
METERS :
Minimum
number
Recommended
number