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behrotest
®
K8 / K12 / K20 / K16 / K24 / K40
0591
11
Avoid bumping!
With liquid samples, bumping may occur so that liquid
will splash out of the sample. With liquid samples, add
some boiling chips. Boiling chips do not contain nitrogen.
Nevertheless, treat the blank in the same way as the
samples.
When digesting liquids like water or milk, the water con-
tained in them must be evaporated before digesxtion
really starts. During this time, heating must be adjusted
in such a way that the sample is boiling steadily but does
not boil over.
Foaming
Some kinds of samples will develop a foam when react-
ing with concentrated sulfuric acid, which will rise up in
the vessels, creep over the rims or even rise into the
suction nozzles. Textbooks have various advices what to
do against foam – but it will depend on the kind of sam-
ple which technique will work.
Heating slowly so the foam won’t rise so fast, maybe
even preparing the sample in the evening and letting
it rest overnight before starting the digestion. That
will sometimes work, but it won’t always work. Some
sample materials will start reacting with the sulfuric
acid in the cold, and the longer it takes the sample to
reach digestion temperature, the higher the foam will
rise.
In these cases it is better to have the sample come
to digestion temperature as quick as possible so the
foam will collapse soon. Which means: preheat the
block and prepare the sample immediately before
starting the digestion.
An anti-foam agent can neutralize the foaming com-
ponents of the sample. Add some drops of paraffine
oil or silicone oil to the sample. Make sure to treat the
blank in the same way as the samples.
If you have an infrared digestion device (e. g. behr
InKjel) too, the easiest way to digest high-protein
samples will be the infrared digestion. In an infrared
digestion, the samples reach digestion temperature
very quickly so the foam will collapse soon.
After Digestion: Take the Samples Out
During digestion, the block and the sample vessels will
get more than 400 °C hot. If you let the vessels cool
down in the digestion block, it can happen that they get
jammed or even crushed, due to different thermal expan-
sion coefficients of aluminium and glass.
Take the sample rack out of the block right after dige-
stion and hang it on the hooks. The scrubber must still
be left working.
For liquid samples,
use boiling chips
