BUCHI DuMaster D-480 - Manual (Page 137)

Operation Manual D-480 ©BÜCH Labortechnik AG
7 - Using the instrument 137
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 oxygen supply is excessive, oxygen is bound to the copper contact of the reduction tube, thus
drastically reducing the service life of the reduction tube.
 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. It has to be set to 110 - 120 sec.
 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
 Kjeldahl factor
Viewing the settings
Information can be found under Viewing method settings (on page 131).
Setting O
2
dosing (method)
You can fine-tune settings for the different oxygen requirement for combustion of samples by
allocating an O
2
dosing flow (method). The following table shows some examples:
Sample type Method O
2
dosing
Test substance for daily factor (e.g. 250 mg
glutamic acid) 250 mg standard 170 ml/min, 80 sec
Test substance for large sample weights
(aspartic acid) CalibrationHigh 250 ml/min, 150 sec
Test substance for low sample weights (aspartic
acid) CalibrationLow 170 ml/min, 100 sec