Metrohm 746 VA Trace Analyzer - Manual (Page 299)

5.6 Method Specifications
746 VA Trace Analyzer / 747 VA Stand
5-148
Parameter Range; Default value / Meaning
Determinations with the aid of a calibration curve are normally pro-
grammed with the following basic structure:
:
D SMPL>M or SMPL/M
Sample addition to measuring vessel
C (CAL Loop start for calibration
C CSOLN>M Addition of calibration solution to measuring vessel
(REP Loop start for replication
SEGMENT Call-up of segment (with sweep)
ØMEAS Switch off voltage (solution change)
REP)n Loop end, n = number of replications
C RINSE Resetting of the volume bookkeeping to 0
C CAL)n Loop end, n = number of calibration solutions – 1
:
First, this program must be used to record the calibration curve. For
this,
Run mode = calibration must be set on the "MONITORING"
page. The instructions marked by D are ignored. The calibration
solutions (max. 10) are measured in sequence and the coefficients of
the calibration curve determined. To store these permanently, the
method must then be copied from the working memory into the
method memory. This occurs automatically with
Auto.batch = on ; an
unsecured method with the same name in the method memory will be
overwritten. A switch is then made to
Run mode = determination and
the sample measured. Instructions marked by C are ignored.
The result determination with the aid of a calibration curve saves time
compared with standard additions, but is reliable only
• 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.) re-
main unchanged during measurements
• if the accuracy of the results obtained is checked regularly with
the standard addition method.
With regard to optimum accuracy and deviation, a number of rules
must be observed with calibration curves (see also section 6.9.5):
• 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.
• Working in the non-linear range
In determinations in the non-linear range, as many calibration
solutions as possible with concentrations evenly distributed
throughout the entire range must be recorded.
• Checking the offset
With curves, check with linear or nonlin whether the offset
Y.offset may be neglected. The size of the offset provides in-
formation on a possible systematic error or blank value.