SCP SCIENCE DigiPREP, Service Manual

Page: 57 / 88

KeyPad
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SCP SCIENCE
57
Example:
On the second ramp and soak cycle, heat the
Digi
PREP HT
to 300
o
C, assuming that the fi rst
Setpoint temperature was 200
o
C.
Solution:
Calculate
Δ
T.
Δ
T=T
fi nal
-T
initial
=second set point - fi rst set point
=300
o
C - 200
o
C=100
o
C
To determine the rate from the HT Typical Heating
Rates chart, use the '25 to 200' row because
Δ
T is
within this range. Use the '225-350' column since
the second set poitn is 300
o
C.
The table recommends a heating rate of at least
4
o
C/min.
Time to Temperature =
Δ
T ÷ rate
=100 / 4 = 25 min
Therefore, a minimum of 25 minutes is required.
3rd Step
Example:
On the third ramp and soak cycle, heat the
Digi
PREP HT
to 380
o
C. The second Setpoint
temperature was 300
o
C (from the previous example)
Solution:
Calculate
Δ
T.
Δ
T=T
fi nal
-T
initial
=third set point - second set point
=380
o
C - 300
o
C=80
o
C
To determine the rate from the ‘
HT
Heating Rates’
chart, use the ‘25 to 200’ row because
Δ
T
is within this range. Use the ‘350+’ column
since the second setpoint is 380
o
C.
The table recommends a heating rate of 3
o
C/min
Time to Temperature =
Δ
T ÷ rate
=80 / 3 = 26.6 min
Therefore, use a 'time to temperature' of 27 minutes.
METHOD DEVELOPMENT
FOR DIGESTING SAMPLES
A typical H
2
SO
4
digestion program is as follows.
Suitable boiling chips or the supplied boiling rods
should be used to prevent bumping during the
digestion procedure.
Step 1
Set point Temperature is 150
o
C
Time to Temperature 45 minutes
Time at Temperature is 60 minutes.
Step 2
Set Point Temperature is 380
o
C
Time to temperature is 45 minutes
Time at Temperature is 30 minutes
If the sample contains water, the water must be
evaporated off prior to proceeding to Step 2. If this is
not done then severe bumping may occur.
The amount of vacuum pulled on the manifold
should be suffi cient to prevent fume from escaping
from the tube. The incoming air will promote refl ux,
as this is a necessary step to wash down the walls
of the tubes to ensure complete digestion of the
sample.
If bumping occures at the set point temperature
then the temperature should be lowered by 5
o
C.
If bumping does not occur at set point
temperature then the temperature may be raised by
5
o
C to help speed up the rate of evaporation.
The amount of TKN digestion reagent in the sample
will have an impact on the boiling point of the
sample. Blank samples will not evaporate at the
same rate as sample with solids.
At the end of Step 1 the sample should resemble
a black liquid. It should contain about 10-15% of the
volume of the starting sample if it was a liquid sample
to start with.
If the sample is dry at this step. More sulphuric acid
needs to be added to the TKN digestion reagent.