Metrohm 746 VA Trace Analyzer - Manual (Page 306)

5.6.3 SUBSTANCES
746 VA Trace Analyzer / 747 VA Stand
5-155
Parameter Range; Default value / Meaning
The result determination with the aid of an internal standard saves
time compared with standard additions (particularly when several
substances are determined with the aid of the same internal stan-
dard), but is reliable only
• if analyte and reference substance can be measured in
the same segment
• if the requirement of a linear relation between analyte
and reference substance is met
• if the matrix of all samples and calibration solutions is
identical or has no influence on the measurement, or if
the influence of the matrix is the same for all substances
(reference and analyte)
• if all measurement parameters (capillary, temperature,
etc.) remain unchanged during measurements
• if the accuracy of the results obtained is checked regu-
larly with the standard addition method.
With regard to optimum accuracy and deviation, a number of rules
must be observed with the internal standard method (see also section
6.9.6):
• Check linearity range
Check the linearity range of the calibration curve for each sub-
stance.
• Working in the linear range
In determinations in the linear range, calibration must be per-
formed above all in the upper and lower parts of this range and
as large a number of replications as possible chosen.
• Determining working range
Extrapolations above the range determined with calibration solu-
tions are not allowed. Thus, enter the maximum admissible
working range on the dialog page "SUBSTANCES".
• Keep temperature constant
Owing to the large temperature dependence of the measured
values (
≥ 2%/°C), it is advisable to work with the 6.1418.220 ther-
mostatted measuring vessel.
• Consideration of blank values
Any blank values must be determined separately and subtracted
in the formula calculation on the dialog page "CALCULATIONS".
Mode analyte, referen.; analyte
Mode for parameter display
Selection of the parameter display for the measured substance or the
reference substance:
analyte Measured substance (analyte)
referen. Reference substance
(internal standard)
Analyte and reference substance must be assigned to the
same segment.
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