Metrohm 746 VA Trace Analyzer - Manual (Page 494)

6.9 Content calculation
746 VA Trace Analyzer / 747 VA Stand
6-31
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".
6.9.6 Internal standard
In the method of internal standards a known quantity of a reference substance (r =
reference) is added to the analysis solution. The mass concentration ρ
eff,a eff,a
of the
analyte (a = analyte) is calculated as follows from the ratio of the two evaluation
quantities EV
a
/ EV
r
and the proportionality factor Prop.fact. determined before-
hand by measurement of different reference solutions:
ρ
aa
= ρ
rr
×
EV
a
/ EV
r
× 1/ Prop.fact.
As in the case of the calibration curve, the content is determined with an internal
standard in two steps:
– First the relation between the ratio of the mass concentrations ρ
eff,a eff,a
/ ρ
eff,r eff,r
of
analyte and reference substance and the ratio of the evaluation quantities
EV
a
/ EV
r
is determined by measurement of different reference solutions.
– Finally, the ratio of the mass concentrations ρ
eff,a eff,a
/ ρ
eff,r eff,r
in the sample is meas-
ured and the mass concentration ρ
eff,a eff,a
determined using the recorded cali-
bration curve.
The method of internal standards is programmed on the dialog page "OP.SE-
QUENCE" with the following instructions and parameters (see section 5.7.3):
(CAL...CAL)n Loop for recording the calibration curve
n Number of calibration solutions – 1
CSOLN... Addition of the calibration solutions
RADD... Addition of the internal standard
Soln.name Name of the solution with the internal standard
V.add Addition volume of the solution with the internal standard
The remaining parameters needed for the internal standards method are entered
on the dialog page "SUBSTANCES/Calibration" (see section 5.6.3). The most im-
portant of these are:
Technique
Selection of the calibration technique
int.std. Internal standard
Mode Mode for parameter display
analyte
Measured substance (analyte)
referen. Reference substance (internal standard)