Metrohm 746 VA Trace Analyzer - Manual (Page 489)

6 Data processing and evaluation
746 VA Trace Analyzer / 747 VA Stand
6-26
EV
a
0
linear
nonlin
std.add.
The non-linear curve shown in the new coordinate grid is not de-
scribed completely by the parameters a, b and c determined for the
original curve.
6 Calculation of the mass concentration ρρ
s s
The found, effective mass concentration ρ
eff,s eff,s
is converted into the sought
mass concentration ρ
ss
by taking the dilution into account (see section 6.9.2):
Mass conc. = ρ
ss
= ρ
eff,s eff,s
/ f
VV
7 Calculation of the total scatter σσ
x x
As measurement of the sample is part of the calibration in standard addition,
for the determination of the total scatter σ
xx
only the error contribution from
the calibration must be taken into account (see section 6.9.3):
σ
xx
= σ
x,C x,C
As the total scatter σ
xx
refers to the effective mass concentration ρ
eff,s eff,s
, it must
also be converted by taking the dilution into account:
MC.dev. = σ
xx
/ f
VV
To be able to calculate the total scatter MC.dev. , with the linear curve type at
least 3, with the nonlinear curve at least 4 measured value pairs must be
available, otherwise MC.dev. = 0 will be set and the message no
meas.deviation put out on the "RESULTS/Evaluations" page.
Standard addition is the usual calibration method for the majority of the applications
possible with the 746 VA Trace Analyzer. Its advantage lies in its high dependability
as the calibration in the sample takes place under real matrix conditions and all
measurement parameters remain unchanged. With regard to optimum accuracy
and scatter, several rules must be observed in standard 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 the "CURVES"
page, you can then determine the region in which the standard addition is
linear and that in which it is nonlinear.
• Working in the linear range
If the substance content lies in the linear range, multiple standard addition is
meaningful only if you wish to check the linearity in every determination. To
reduce the scatter, it is better to spike the solution once only and to choose
as many replications as possible.
ρρ
eff eff
Sample
Standard solution