Metrohm 746 VA Trace Analyzer - Manual (Page 138)

4.4 Lead determination with internal standard
746 VA Trace Analyzer / 747 VA Stand
4-53
• The status display in the header of the dialog page "MONITORING"
switches to * BUSY * , the start time of the measurement is entered un-
der Determination and the determination is given a standard name,
also formed from the start time. Further, during the measurement the
measurement window is superimposed which shows the current pro-
gram line being executed and the remaining line time.
• Shortly after the start of the determination, the screen shows the prompt
to add 10 mL sample to the measuring vessel.
5 Addition of sample
• Remove stopper 25 25 on 747 VA Stand and use a pipette to add 10 mL of
drinking water to the measuring vessel through the pipetting opening.
Press the <ENTER> key.
• The screen shows the prompt to add 1 mL acetate buffer to the measur-
ing vessel.
6 Addition of acetate buffer
• Use a pipette to add 1 mL acetate buffer (1 mol/L, pH 4.6) to the
measuring vessel through the pipetting opening. Then close the pipet-
ting opening using stopper 25 25 and press the <ENTER> key.
• The screen shows the prompt to add 100 µ L of the solution CdRefSol
with the internal standard to the measuring vessel.
7 Addition of the internal standard
• Use a pipette to add 100 µµL of the cadmium standard solution with a
mass concentration of 1 mg/L to the measuring vessel through the pi-
petting opening. Then close the pipetting opening using stopper 25 25 and
press the <ENTER> key.
8 Measurement of the sample solution
• Measurement of the sample solution runs as follows (see also section
4.4.3):
− Purging and stirring for 180 s.
− Call-up of the segment DPASV for measured value recording.
Lead and cadmium are enriched with stirring at the hanging mer-
cury drop at a measurement voltage of –800 mV and then deter-
mined anodically without stirring from –800 ... –200 mV. The se-
lected measurement mode DP leads to a current peak for lead
whose maximum lies at ca. –400 mV and a current peak for cad-
mium with a maximum at ca. –580 mV.
− The DPASV segment is then repeated twice. Both the three single
measurements for the substance lead and also the three single
measurements for cadmium are allocated the VR numbers 00 ,
01 and 02 .
• On completion of the last measurement, the lead mass concentration is
calculated using the proportionality factor stored in the method. The pro-
gram is now at an end. The status display in the header of the dialog
page "MONITORING" switches back to
* READY *
, the instrument is
ready for a new determination.