Dialysis
is a method for separating low-molecular substances
from higher-molecular ones («colloids»)
by means of a semi-permeable membrane.
Example: This method
is also known under the name of «blood washing» and
is used by patients suffering from chronic kidney insufficiency.
Low-molecular substances in blood are ions which affect
the electrolyte
equilibrium. If the kidney is not functioning properly
then the concentration of the ions in the blood will
increase and the metabolic functions will be adversely
affected. This means that the concentration of the
ions must be reduced at regular intervals.
This is
carried
out by dialysis using the «continuous flow» method:
An acceptor solution with a very low ionic strength – usually
ultrapure water – is pumped past the semi-permeable
membrane. The blood flows past the other side of the
membrane. As the passage of the ions through the membrane
is virtually unhindered, they diffuse from the blood – which
has a high ionic strength – into the acceptor
solution with its lower ionic strength. As the acceptor
solution is permanently renewed and is in «continuous
flow», the concentration gradient is very steep
and the efficiency is relatively high. Care is deliberately
taken that the two solutions do not form a concentration
equilibrium.
Sample preparation by dialysis
tries to achieve the exact opposite: the acceptor solution and sample solution
must have the same concentration in order to achieve
correct results. For this reason Metrohm has developed
the patented «stopped flow» method. As
in the «continuous flow» method, the sample
solution is continuously pumped past the semi-permeable
membrane. The difference is that the acceptor solution
is at rest. This means that an equilibrium can be established
between sample solution and acceptor solution. Depending
on the sample matrix and the amount of higher molecular
substances this normally takes place is less than ten
minutes in the new Metrohm dialysis cell.
Once the equilibrium has been achieved the acceptor
solution is transferred to the sample loop and injected.
After a total of four steps – cleaning the system,
dialysis of the sample, transferring the acceptor solution,
injecting the sample – the dialysis of a sample
is complete. This requires about 10 minutes. The valve
control of the Metrohm IC system allows one sample
to be measured while the next one is already being
dialyzed. This saves time.
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