Fixing:
plugged frits
If you suspect a
partially-plugged inlet frit:
1. Confirm that
the problem occurs consistently: all the peaks in the chromatogram
show a similar pattern, and the problem occurs with every injection. A
partially-plugged frit distorts the flow profile at the column inlet.
Because all the sample components are still "together" at this point,
it is likely to affect all of the peaks in the same manner. If only
some peaks are distorted while others are well-formed, the problem is
far more likely to be chemical in nature. A partially-plugged frit is
unlikely to spontaneously unplug; therefore, a transient problem is far
more likely to be caused by air bubbles, overload, sample diluent
incompatibility, etc.
2. Replace the
guard column or cartridge (if you were using one). A
partially-plugged frit or headspace in a guard cartridge will generate
the same symptoms as one at the head of the analytical column. Guard
cartridges are designed to be replaced.
3. Reverse-flush
the column. Disconnect the column and reconnect it in the
reverse-flow direction (i.e., what was the outlet end of the column
should now be the inlet). Before doing this, check with the
column manufacturer to confirm that the column can be reverse-flushed
(most columns -- but by no means all -- can be reversed-flushed; Newer,
ultra-small particle columns are more likely to have different frit
configurations at the inlet and outlet; these columns generally cannot
be reverse-flushed). Pump about 10 column volumes of mobile phase
through the column at the regular flow rate, then disconnect the column
and reconnect it in the original flow direction and test the column. If
the problem has been solved, consider appropriate preventive
maintenance steps. If the problem persists, change the inlet frit or
replace the column.
4. Change the
inlet frit. This is a "last resort". It is time-consuming and
carries a substantial risk of further damaging the column. Disconnect
the column from the system and mount it in a vise. Using appropriate
wrenches, carefully remove the inlet end fitting and lift it away from
the column. Examine the exposed column bed. The packing should be white
and level with the column tube walls. If there is a head space or void
at the column inlet, the column should usually be replaced. Head spaces
can be filled, but this is not usually
cost-effective unless the column is very expensive or no replacement
column is available.
In most cases, the
inlet frit is a separate element and can be replaced. If you are lucky,
the frit will remain on the end of the column when the inlet end
fitting is lifted off. In most cases, the frit will remain in the end
fitting. It can often be removed by tapping the fitting on the bench
top. A stuck frit can usually be forced out by pumping mobile phase
through the fitting. The old frit should be discarded. Obtain a new
frit from the column manufacturer (this is important; frits vary in
thickness and the replacement should exactly match the original). Place
the new frit on the end of the column, rinse with a small amount of
organic solvent to remove any residual packing dust, and re-install the
end fitting. Re-equilibrate the column with mobile phase and test the
column. If the problem has been solved, consider appropriate preventive
maintenance steps. If the problem persists, replace the column.
If the frit is an
integral part of the end fitting (so-called "pressed in" frit), then
the entire end fitting must be replaced.
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LC Resources, Inc. all rights reserved