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General Information
sample. The analyte must have some vapor pressure at the boiling
point of water (100 °C). When an aqueous sample is boiled, the
steam and analyte distill over together. When the steam and
analyte condense, a clean aqueous distillate solution is collected
which can be determined free of matrix effects or interferences
present in the original sample matrix.
For at least the last 200 years, analytical distillations have been
performed in large-scale distillation glassware which was designed
more for preparative distillations, particularly of organic compounds
and solvents. The large volume and numerous joints in this type of
apparatus made it difficult to recover very small amounts of analyte.
Because of this, large volumes of sample were necessary to distill
and transfer a sufficiently precise amount of analyte into the
receiving flask.
The Micro Dist system strips analytical distillation down to its bare
essentials. The sample is pipetted into a sample tube and sealed to
a collector tube. This assembly is placed into a special heating
block which fits it very closely, allowing the poor-conducting
polypropylene to be heated rapidly yet locally. As the sample boils,
its vapors rise within the tube and pass through a hydrophobic,
porous membrane. The vapors condense above the membrane to
form droplets, which pool over the membrane, but cannot pass
back through it. The short vertical path of distillation is possible
because of the physical separation of distilling sample and distillate
by the hydrophobic membrane. As little as 10 ng of analyte can be
precisely distilled.
The heating block temperature is set well above the boiling point of
water. This keeps the headspace above the boiling sample
sufficiently hot to prevent the condensation of the distillate below
the lower membrane. The portion of the Micro Dist collector tube
above the block cools off rapidly because of the very slow thermal
conductivity of polypropylene. This allows the tube to be efficiently
air-cooled as there is little heat transferred to the cool portion of the
collector tube.
When the distillation is complete, the hot assembly is pulled from
the heating block. Because the cooling of the air in the sample tube
will soon cause a vacuum which will aspirate back the distillate
through the membrane, the sample tube is pulled off immediately.
This is easy to do when the plastic tube is hot. The collector tube is
set aside to cool for about 10 minutes.
The distillate and droplets are then easily coaxed from the
non-wettable walls to the bottom of the collector tube. The top is
then broken off and the distillate sample is diluted to the original
volume with deionized water. After capping and mixing, the distillate
sample is ready to be determined by a suitable method, automated
or manual.
2.3 Distilling Calibration Standards with the Micro Dist System
Analytically speaking, calibration standards should be distilled in
the same manner as samples. Such sample preparation steps are a
vital and inseparable part of the whole analytical method used to
determine the analyte in the original sample. Examples of such
sample preparations are a Kjeldahl digestion, an extraction of a
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