Metrohm 746 VA Trace Analyzer - Manual (Page 498)

6.9 Content calculation
746 VA Trace Analyzer / 747 VA Stand
6-35
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 standard), 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 sub-
stance is met
– 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.) remain un-
changed during measurements
– if the accuracy of the results obtained is checked regularly with the stan-
dard addition method.
With regard to optimum accuracy and scatter, a number of rules must be observed
with the internal standard method:
• Check linearity range
In development of the method, the linearity range of the calibration curve for
the internal standard should be checked for each substance by recording the
curve over a wide concentration range. Using the calibration curve shown on
the "CURVES" page, you can then determine the region in which the curve
EV
a
/EV
r
vs ρ
eff,a,s eff,a,s
/ ρ
eff,r,s eff,r,s
is linear and hence where the method of internal
standards may be used.
• Working in the linear range
To keep the scatter as low as possible, calibrate above all in the lower and
upper part of the linear range and select as many replications as possible.
• Determining the working range
The calibration curve is defined only for the range between the calibration
solutions with the lowest and highest mass concentrations. Extrapolations
outside this range are not allowed. Hence, enter the maximum possible
working range on the dialog page "SUBSTANCES" with the two parameters
Range min
and
Range max
. If a mass concentration
Mass conc.
outside the
defined working range is found for the substance, an error message is put
out and conc. out of range displayed on the results page.
• 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 thermostatted measur-
ing vessel.
• Consideration of blank values
Any blank values must be determined separately and subtracted in the for-
mula calculation on the dialog page "CALCULATIONS".