Metrohm 746 VA Trace Analyzer User 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.

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 key.

•

The screen shows the prompt to add 1 mL acetate buffer to the measur-

ing vessel.

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 key.

•

The screen shows the prompt to add 100

L of the solution

CdRefSol

with the internal standard to the measuring vessel.

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 key.

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

and

•

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.