Metrohm 750 Autosampler - Manual (Page 46)

4.3 Remedying malfunctions
750 Autosampler
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4.3 Remedying malfunctions
4.3.1 Non-reproducible injections
Non-reproducible injections are usually due to problems such as fitting
leaks, plugged tubing, poorly swaged fittings, or submerged waste
tubes. In some instances, non-reproducibility may appear to be injector
related when, in fact, it is caused by other components in the chroma-
tographic system. Systematically analyze the entire system to assure
that efforts are expended on the proper component before undertaking
any repair.
Leaks
Plumbing leaks are the simplest causes of non-reproducibility to detect.
Leaks affect reproducibility by diverting solvent from the column in an
unknown amount and requiring a longer time to elute the sample be-
cause of reduced solvent flow. Places to check for leakage are the
transfer tubing between the transfer needle and the injection valve, the
transfer needle, sample loop, and injector seals. Once the leak has
been found, either tighten the fitting or replace the fitting, should tight-
ening not be sufficient.
Plugged tubing
Plugs and partial plugs due to debris may occur in the transfer needle,
the transfer tubing, the sample loop, or the injector body. Plugs prevent
proper loading of the sample loop.
Disconnect both ends of the transfer tubing 88 and use a syringe to
force solvent through to check for blockage. There should be no diffi-
culty in forcing solvent freely through the tubing. Force solvent through
the transfer needle in the same fashion. There should be no difficulty in
forcing solvent freely through the needle. Replace the needle or the
tubing if either is plugged and resume operation.
Remove the sample loop from the injector valve body of the 733 IC
Separation Center and check it for blockage with a syringe if the prob-
lem persists. Partial blockage of the loop will prevent accurate and con-
sistent sample loading. Follow the procedures in the Instruction for Use
for the valve if loop replacement is necessary.
Poorly swaged fittings
Non-reproducibility due to dead volume caused by poorly swaged fit-
tings usually occurs after incorrect installation of new loops or other
connections or after changing previously swaged loops. A gap as small
as 0.05 mm between the end of the tubing and the shoulder inside the
female portion of the fitting will have a dead volume of 0.1 µ L with 1 / 16
"
OD tubing. This type of irreproducibility always exhibits one of two peak
height patterns. The patterns are observed by setting the 750 Auto-
sampler to make three injections per vial. In the first pattern, the heights
of the peaks in the first of the three injections tends to be smaller than
the heights of the peaks for the second two injections which are usually
about the same size. In the second pattern, larger peak heights on the