Buchi DuMaster D-480, Руководство по эксплуатации

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Operating instructions DuMaster D-480 ©BÜCHI Labortechnik AG
7 - Using the instrument 135
Values for oxygen dosing
Meaning of oxygen dosing
The following illustrate the meaning of oxygen dosing:

If oxygen supply is insufficient, the catalysts are reduced and thus limited in their effect and
durability. The unburnt sample substance remains in the ash finger and distorts the results of
analysis of subsequent samples.

If your sample matrix is always the same, it is advisable to optimize oxygen dosing for reasons of
costs.
How oxygen dosing takes place
Oxygen dosing takes place by allocation of the corresponding dosing parameters via a method.
By choosing a method you specify values for the following parameters:

O
2
dosing time

O
2
dosing flow

O
2
cut-off threshold
Comments:

The O
2
dosing time is a minimum dosing time.

The O
2
dosing flow is based upon the weight and the carbon and/or fat content of the sample.
Rule of thumb: 500 mg organic substance requires 0.4 - 0.5 litres oxygen for combustion. At a
present O
2
dosing time of 120 sec, a flow value should be set to 200 - 250 ml/min.

The percentage of the O
2
cut off threshold is referred to the peak maximum (e.g. 10% - 30%).
The software automatically scans the course of the peak; the peak maximum will be registered.

The O
2
dosing cuts off after the current peak value has dropped below the cut off threshold
(percent of the maximum peak value) and if the O
2
dosing time has expired.
Further method parameters
By choosing a method you also specify values for the following parameters in addition to O
2
dosing
time:

Autozero delay

Peak anticipation

Integrator reset delay

Base weight

Kjeldahl factor
Viewing the settings
Information can be found under
Viewing method settings
(on page 129).
Optimize O
2
dosing
If your sample matrix is always the same, it is advisable to optimize oxygen dosing for reasons of
costs. Proceed as follows:

Measure a typical sample of known element content several times with the same sample weight
(max. ± 2 mg fluctuation).

Systematically increase O
2
cut off threshold until you get a false low reading or a substantial
widening of the N peak.

Set the O
2
cut off threshold of the last successful measurement before the false low reading
occurred. These settings are optimal for this matrix and sample weight.

Check whether oxygen dosing is generally usable. To do so, measure several known samples of
similar matrix.