Metrohm 746 VA Trace Analyzer - Manual (Page 294)

5.6.3 SUBSTANCES
746 VA Trace Analyzer / 747 VA Stand
5-143
Parameter Range; Default value / Meaning
ADD... Standard addition with the constant, preset volume
V.add .
PADD... Standard addition with a volume that is proportional to a
signal (
EV: ) measured in the original sample,
which originates from any substance present in the
sample.
A standard addition is normally programmed with the following basic
structure:
:
SMPL>M or SMPL/M
Sample addition to measuring vessel
(ADD Loop start for standard additions
(REP Loop start for replication
SEGMENT Call-up of segment (measurement)
REP)n Loop end, n = number of replications
ADD>M or PADD>M Constant or proportional standard addition
ADD)n Loop end, n = number of standard additions
:
Standard addition is the usual calibration method for the majority of
the applications possible with the 746 VA Trace Analyzer. Its advan-
tage lies in its high level of reliability as the calibration is performed in
the sample under real matrix conditions and all measurement pa-
rameters remain unchanged.
With regard to optimum accuracy and deviation, a number of rules
must be observed in standard additions:
• 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 non-linear (see also
Curve type ).
• 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 deviation, it is better to spike
the solution once only and to choose as many replications as
possible.
• Working in the non-linear range
If the substance content is in the non-linear range, several stan-
dard additions have to be performed to determine the nonlinear
curve shape exactly.
• Standard addition ratio 1:2 to 1:5
The optimum standard addition ratio for the entire standard addi-
tion lies between 1:2 and 1:5, i.e. the sum of all standard addition
amounts should be 2 to 5 times greater than the amount of sam-
ple in the measuring vessel. This can be easily checked later us-
ing the parameters
Mass and Add.mass put out on the dialog
page "RESULTS/Evaluations".
• Consideration of blank values
Any blank values must be determined separately and subtracted
in the formula calculation on the dialog page "CALCULATIONS".