Dilution calculation, Standard addition calculation – Metrohm 757 VA Computrace User Manual

Page 128

5 Determination mode

757 VA Computrace – Software

118

Dilution calculation

In all cases in which the sample volume is diluted in the measur-
ing vessel (e.g. by addition of buffer) before the start of the first
sweep, this must be taken into consideration by entering the two
parameters

Sample amount

and

Cell volume

on the

Determination

tab of the

EDIT WORKING METHOD PARAMETERS

window (see sec-

tion 5.2).
If the sample is additionally diluted after the start of the first sweep
(e.g. by standard addition solutions), the dilution is recalculated
continuously for every dilution step so that the effective mass
concentration of the analyte in the measuring vessel is shown in
the calibration curve for each measurement solution.
If an auxiliary solution is added by a Dosimat using the

Predose

function, this volume must be taken into consideration by modify-
ing the

Cell volume

manually.

Standard addition calculation

In the standard addition method, a known amount of the analyte
is added once or several times to the sample. The addition may
be performed manually or automatically using a Dosimat. The fol-
lowing procedure is used to calculate the sought mass concen-
tration of the sample:
1.

Measurement of sample solution
The sample solution with the unknown mass concentration
c(s) of the sample is measured one or more times (defined
by

No. of replications

). This gives:

EV(s)

Evaluation quantity of a single measure-
ment for the sample

mean(s)

Mean value of all evaluation quantities
for the sample

Std.dev.(s) Standard deviation of the individual value

EV(s) = s(s)

Measurement of spiked sample solutions
The sample solution is spiked

n times (defined by

No. of addi-

tions

) with a standard solution of known mass concentration

c(st). Each of these spiked solutions is measured one or
more times (defined by

No. of replications

). This gives:

EV(n)

Evaluation quantity of a single measure-
ment for the spiked sample

mean(n)

Mean value of all evaluation quantities
for spiked sample