Instruction Manual

5 Determination mode
797 VA Computrace – Software
148
EV
Sample
c
(s)
mean(s)
c
(eff)
5. Calculation of result deviation Conc.dev.
The total deviation of the calculated substance concentration
Conc. is determined using a linear error calculation which takes
into account both the error contribution from the measure-
ment and that from the calibration. Independent of the number
of measurements, the total deviation Conc.dev. is always cal-
culated in a way that Conc. ± Conc.dev. gives the range in
which the mass concentration may be expected with a prob-
ability of 68.3%.
Rules for calibration curves
The result determination with the aid of a calibration curve sa-
ves time compared with standard additions, but is reliable only
if the matrix of all samples and calibration solutions is identical
or has no influence on the measurement.
if all measurement parameters (capillary, temperature, etc.) re-
main unchanged during measurements.
if the accuracy of the results obtained is checked regularly with
the standard addition method.
With regard to optimum accuracy and scatter, a number of rules
must be observed with calibration curves:
Check linearity range
In development of the method, the linearity range of the cali-
bration curve should be checked for each substance by recor-
ding the curve over a wide concentration range. Using the cali-
bration curve shown in the DETERMINATION CURVES window,
you can then determine the region in which the curve is linear
and that in which it is nonlinear.
Working in the linear range
Performing determinations in the linear range, to keep the scat-
ter as low as possible it is advisable to calibrate above all in the
lower and upper part of this range and select as many re-
plications as possible.
Checking the offset
The size of the offset indicates a possible systematic error or
blank value. To convert this error into the effective mass con-
centration in g/L, Y.offset must be divided by Slope.