Metrohm Inline-Ultrafiltration - Manual (Page 24)

4.1 Operation
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
18 ￭￭￭￭￭￭￭￭
IC equipment: Inline-Ultrafiltration
4 Operation and maintenance
4.1 Operation
4.1.1 Service life of the filtration membrane
One of the most commonly encountered problems in filtration is that solid
substances in the sample are deposited on the filtration membrane, caus-
ing it to become blocked over time. The ultrafiltration cell has been
designed so as to prevent this effect to the extent possible. It has a sym-
metrical design and is placed horizontally in the Sample Processor or in the
ion chromatograph. The sample flows through the lower chamber of the
ultrafiltration cell and the filtrate is aspirated from the top. In this way,
solid substances adhere less to the membrane.
Nevertheless, depending on the level and type of contamination in the
sample, the filtration process must be monitored and the filtration mem-
brane replaced if necessary.
A declining recovery rate in standard solution analyses is an indicator for
an imminent blockage of a filtration membrane. These standard solutions
should ideally be prepared with the sample matrix to be analyzed.
If a large number of samples is analyzed, we recommend measuring check
standards regularly (in the case of samples with high particle loads, after
every 5th to 10th sample). No general prediction regarding the number of
filtration cycles can be made. Also, the change in the recovery rate may be
different with more samples being analyzed. Whereas the recovery rate
with one sample matrix may remain constant over many samples and then
suddenly drop off severely, its decline may be slow and continuous with a
different sample composition.
At which time a filtration membrane needs to be replaced depends on the
sample matrix and the specifications of the analysis method applied. Expe-
rience has shown that minuscule particles and suspended substances in
the sample matrix will lead to blockage of the filtration membrane faster
than coarser particles will, because the latter will be more readily propel-
led past the membrane in the flow of sample material.
The following table lists some sample types that were filtered with the
ultrafiltration cell (6.2729.110) and a filtration membrane (6.2714.020)
with a pore size of 0.2 µm and subsequently analyzed on a Metrohm ion
chromatograph. The concentration of the following seven anions was
determined for each sample type: F
– , Cl – , NO
2
– , Br – , NO
3
– , HPO
4
2– , SO
4
2– .