Metrohm 750 Autosampler - Manual (Page 47)

4 Notes – Maintenance – Faults
750 Autosampler
44
first injection followed by smaller but equal peak heights for the second
two injections are observed. Observation of either pattern requires
complete replacement of the loop.
The correct way to mount seals is described in detail in chap. 2.5 of the
732/733 Instructions for Use.
When using a previously swaged steel capillary, be sure to use it with
the fitting in which its ferrules were originally set. Small inconsistencies
in the machining of the fitting are a source of dead volume.
Submerged waste tube
A waste tube that has been submerged below the liquid level in the
waste container can cause the waste solution to siphon into the sample
loop. The type of non-reproducibility this causes exhibits random peak
heights. Do not allow the waste tube to be submerged below the liquid
level in the waste container.
Injection valve leak
The probable cause of injection valve leakage on the 733 IC Separation
Center is a worn or damaged seal. In this case new seals must be fit-
ted.
4.3.2 Missed injections
Skipped vials
The torque of the stepper motor which drives the transfer needle is lim-
ited so as to prevent the vial from bursting under pressure in the event
that a plug occurs in the transfer needle, transfer tubing, or injection
valve. If more pressure is required to push a cap in than is provided, the
needle motor will stall. While a method is running, if the motor stalls on
two consecutive vials, the Autosampler will stop the method and issue
an error message.
However, if the motor stalling is intermittent, the Autosampler will con-
tinue with the method. A list of the skipped vials will be shown on the
display once the method is complete.
Plugs in the plumbing between Autosampler and injection valve are the
major source for motor stalls and skipped vials (see section 4.3.1).
However, the same symptom can occur with highly viscous samples or
when FilterCaps  are used.
FilterCaps  due to job they are ask to do can pose some minor plug-
ging problems. In some instances, the sample may be too "dirty" for
use with FilterCaps  . A suggested work around for the problem of us-
ing FilterCaps  with really "dirty" samples is to limit the number of injec-
tions per vial to 1 and to set the Needle Depth to " POS1 " or " POS2 " using
the system settings key. Should this latter suggestion fail to provide
satisfactory results, an alternative sample filtering procedure should be
used.