John V. Hinshaw
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Periodic maintenance is the mainstay of keeping laboratory instruments running at the peak of performance. Analysts can avoid
many problems by proactive evaluation, repair, and replacement of critical system components on a regular basis. The alternative
of allowing contamination to build up, syringes to wear out, or septa to leak only results in increased down-time compared
to planned maintenance outages. Unplanned maintenance and repair is not predictable and in my experience, most such incidents
occur at the worst time.
Table I: GC system maintenance items and suggested performance intervals
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Periodic maintenance procedures fall into two categories. First are those that should occur on a regular basis, for example,
every six months. Second are those for which the frequency is determined by system usage characteristics (for example, to
be performed every 200 injections) and usually set by the type of sample and its level of contamination; dirtier samples impose
more frequent maintenance on syringes, inlets, and the column entrance or retention gap. This installment of "GC Connections"
presents some recommended maintenance intervals and related suggestions for a number of the major components of a gas chromatography
(GC) system. Table I lists maintenance items and both regular intervals as well as sample-based intervals. The suggestions
in this column are not intended as a set of rules to be followed rigidly. Rather, the information should be considered as
a partial list of items that should be subject to periodic maintenance. Each laboratory has different requirements, not all
of which appear here. It's a good idea to review maintenance procedures as laboratory requirements and usage patterns change
over time.
Regular Timed Maintenance
Procedures that should be performed on a regular time basis include such simple things as straightening up the laboratory
area, dusting behind instruments, archiving instrument logbooks, or defragmenting disk drives. Most laboratories run a fairly
clean operation, although I've seen some remarkable exceptions in my travels. Usually, some dirt and grime will accumulate
beneath instruments and computers or become trapped around gas lines and electrical cords. More seriously, a buildup of dirt
on instrument cooling air intake vents and filters, on internal cooling fans and heat sinks, and around computer components
eventually can compromise instrument operation. While it's simple enough to clean around the outside of instruments and computers,
opening up the panels and clearing out the internal dust is another matter. Doing so exposes the inside of the instrument
or computer to potential harm from static discharges, especially as found at the end of a vacuum cleaner hose, and also exposes
untrained personnel to high voltages that might be present, in some cases even if the power is off and the AC line unplugged.
Such operations are best left to trained service persons.
Certain pieces of equipment should be checked — and checked functionally where appropriate — every so often. Gas tank regulators,
external gas lines and their fittings, gas generation equipment, and the types or grades of gas in use should all be examined
at least every three months. Verify that the carrier and detector gases are of the correct identity and grade. I once came
upon a laboratory where the helium carrier gas cylinder had mistakenly been replaced with a nitrogen cylinder (in the United
States both have the same style high-pressure fitting), which resulted in some apparently mysterious chromatographic behavior
until the problem was discovered.
Leaks can develop in gas fittings as they are opened and resealed — for example, while replacing in-line gas filters. And
speaking of gas filters, it's a good idea to label new filters as they are installed with the date and type of filter being
put into use. Check each in-line gas union with a high sensitivity helium leak detector — they also react to hydrogen. Such
a device won't pick up leaks in nitrogen or air lines, but I don't recommend using any kind of liquid leak solution. In these
cases, a pressure drop test will reveal any gross leaks. Turn off the air or nitrogen carrier, detector, or makeup gas flow
at each connected gas chromatograph, then close the gas tank high-pressure side regulator valve and wait 10–20 min. When the
high-pressure valve at the tank is reopened, observe whether the high-pressure gauge reading jumps upward significantly. If
so, then there is a downstream leak somewhere. I sometimes will repressurize the lines with helium just so that I can use
the helium leak detector to pinpoint such a leak. Just remember to reconnect to the correct tank when you are ready to put
the instrument back into service.
Gas generation systems are easy to install but sometimes, especially when installed out of direct view, they can be ignored
for longer than desirable. I have seen several instances of gas generation equipment running with the "service required" and
operational indicators facing the wall, presumably to get better access to the inlet and outlet fittings, so that it was impossible
to see the status of the generator. Eventually, such situations will manifest themselves with increased noise and drift. It's
preferable to check and perform the necessary maintenance in advance.
The high-pressure diaphragm in the dual-stage pressure regulators used in chromatography laboratories operates under a high
stress level at the intermediate pressure stage of up to 500 psi (345 N/cm2). With time and many pressure/flow changes during normal use the high-pressure diaphragm can suffer metal fatigue, as can
the counter-spring. This gradual loss of performance can take years, but then again, I've seen many pressure regulators that
must be more than 25 years old still in routine use in GC laboratories. Early symptoms of regulator degradation can be a loss
of regulating ability, so I like to test regulators every year or so by turning off the gas at the regulator outlet valve
and then disconnecting the supply tubing somewhere convenient toward the instrument. Then I gradually turn on the gas at the
regulator outlet valve to increase the flow as I observe the pressure on the outlet gauge. As the flow rate increases, the
outlet pressure should drop slightly but remain within 5 psi or so of a 90-psig (620-kPa) set-point for a high-quality, dual-stage
laboratory regulator. Another sign of a problem is when the outlet gauge does not return to zero when the regulator is depressurized,
or if the needle is bent. Such a broken regulator should be replaced immediately.
Pure inert gas supply tanks generally don't have an expiration date, but certified gas standards do. I sometimes will keep
expired gas standards around for a quick test of a setup, but I always make sure that in-certification tanks are employed
when making quantitative measurements. The same goes for liquid standards. Also, any gas blending equipment in use will have
a fixed expiration date for its accuracy certification and will need to be recalibrated regularly, usually once a year. Other
equipment in use in the laboratory such as precision thermometers, volt-ammeters, and data-acquisition systems also require
recalibration on a regular basis. GC oven temperatures sometimes drift out of specification over a long time period — more
so on older instruments — and many laboratory periodic maintenance procedures call for checking and recalibration. Also, electronic
pressure control (EPC) systems will require regular zeroing and accuracy checks as the electronic pressure gauges contained
in them undergo normal aging and drifting.
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