Metrohm 746 VA Trace Analyzer - Manual (Page 490)

6.9 Content calculation
746 VA Trace Analyzer / 747 VA Stand
6-27
• Working in the nonlinear range
If the substance content is in the nonlinear range, several standard 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 addition is from
1:2 to 1:5, i.e. the sum of all standard addition amounts should be 2 to 5
times the amount of sample in the measuring vessel. This can be easily
checked later with 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 for-
mula calculation on the dialog page "CALCULATIONS".
6.9.5 Calibration curve
The content determination using a calibration curve is performed in two steps:
– First the relation between the mass concentration ρ
eff eff
of a sub-
stance and the evaluation quantity EV is determined by measur-
ing different reference solutions.
– Finally, the sample is measured and its mass concentration ρ
eff,s eff,s
determined using the recorded calibration curve.
The recording of a calibration curve is programmed with the following instructions
and parameters on the "OP.SEQUENCE" page (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
The remaining parameters needed for the calibration curve are entered on the dia-
log page "SUBSTANCES/Calibration" (see section 5.6.3). The most important of
these are:
Technique Selection of the calibration technique
cal.crv. Calibration curve
Curve type Curve type
linear Straight line of type y = a + bx
0/linear Straight line of type y = bx
nonlin Curve of type y = a + bx + cx 4
0/nonlin Curve of type y = bx + cx 4
V0.CSoln.
Addition volume of the calibration solutions
CSoln.name Name of the calibration solutions
Mass conc. Mass concentration of the calibration solutions
In the content determination using a calibration curve, the following procedure is
used to calculate the sought mass concentration ρ
eff,s eff,s
of the sample (see also ex-
ample in section 4.3):