How to Restore an Idle GC Column to Operating Condition - - Chromatography Online
How to Restore an Idle GC Column to Operating Condition


LCGC Europe
Volume 25, Issue 3, pp. 148-153

At some point, you may need to bring an idle capillary GC inlet, column and detector back to operating condition. Here is what to do — including column installation, electronic pneumatic control calibration, system bakeout and basic test mix performance.

A few months ago I needed to use a split–splitless inlet, column and flame ionization detection (FID) system that had been idle for more than a year. The gas chromatography (GC) system had been in continuous use for valved applications, but the inlet carrier gas controller had been disconnected from the carrier gas supply the entire time, and the column oven connections and gas inlets had been capped. The intended column — 20 m 180 m, 0.12-m d f methylsilicone fused silica — had been stored in its box with the inlet and outlet stuck into a septum. The GC instrument itself was apparently in excellent working condition, so I optimistically anticipated a quick install and performance checkout. Fortunately, that prediction proved accurate. Here are the steps that I followed.

Instrument Setup

My initial task was to set up the pneumatics, inlet and detector. First, with the GC system powered on, I checked the carrier gas configuration for the split–splitless inlet. Although the set column dimensions did not match the column I would use, the basic configuration would allow carrier to flow to the inlet once a supply was connected. I also made sure that carrier gas leak detection was disabled. I planned to purge the pneumatics initially and did not want the system to shut off the flow. I then doublechecked that the inlet and FID heaters were turned off and also turned off the detector and pneumatic controllers that had been in use.

Pneumatics

Next, I turned off the GC system power, allowed its zones to cool, removed the existing columns and capped off the detector and carrier gas source that had been in use. I then removed the interconnecting tubing at the gas switching valves while making a note of how they were connected. The new application would require oven temperatures of up to 300 C, well above the maximum limits for the gas sampling valves, so I disconnected the valves from their actuators and removed them as well.

For my next step, I inspected the gas tanks. I found suitable gas purities: 99.9999% research-grade helium carrier gas, zero grade air with < 1 ppm total hydrocarbons, and highpurity 99.998% hydrogen. The carrier-gas regulator was a stainless steel diaphragm, high-purity dual-stage device and the fuel gas regulators were brass dual-stage types. Moving down the installed copper gas lines, I found a suitable set of multistage gas filters that were in apparent good condition with littletono indicated filter exhaustion.

I turned off the carrier gas at the tank, moved the carrier-gas supply over to the pneumatic controller for the split–splitless inlet system, but did not tighten the connection completely, and capped the carrier-gas connection that had been used for the valved application. While making the carriergas connection, I noted that the existing tubing and ferrules appeared to not have been overtightened or scratched and that the nut screwed onto the fitting smoothly with no binding.

Before fully tightening the new carriergas connection, I turned the gas on at the tank and allowed some helium to purge through the line. This would protect the upstream filters by removing the air that had inevitably diffused in while the tubing was disconnected. Of course, any additional air present in the pneumatic controller would enter the line as well, so after tightening the fitting onequarter turn past finger tight I immediately turned on the GC system and set a split flow of 50 mL/min at a 10-psig (70-kPa) inlet pressure.

I also turned on the air and hydrogen supplies, verified that the regulator output pressures were correct and then enabled the FID fuel gas flows and checked that they were set to the correct values as well, according to the instrument manual.

After I was satisfied that the gas lines were purged sufficiently, I turned off the gas flows at the instrument pneumatic controllers and then closed the tank valves to commence a gross leak check. After 10 min, I turned the tank valves back on one at a time and observed any motion of the highpressure gauges on the pressure regulators. An observable change in the high-pressure gauges that is greater than the wiggle produced by lightly tapping the gauges may indicate a serious leak that requires further investigation. I saw no discernible movement, so I proceeded to do a fine leak check of the hydrogen and helium lines with a leak-check device. On its high-sensitivity setting, the leakchecker probe picked up a small leak at the hydrogen filter fitting, but I was able to seal it completely by tightening the fitting one-eighth turn. Otherwise, I would have chosen to cut the tubing an inch shorter and remake the fitting with a new nut and ferrules instead of tightening the leaking fitting further.

At this point, the instrument had been powered on for the better part of an hour, so it was time to zero the pressure transducers, which need some time to stabilize after powering on. The largest drift error in the pressure transducers is their zero reading. Span errors normally are not corrected, as that would require a very accurate (and expensive) external gauge, and in doing so one would be just as likely to introduce errors as to correct them.

Following the procedure in the manual, I turned off the gas supplies and allowed the line pressures to decay to zero, which took another 10 min or so. Then I accessed the pneumatic zeroing section of the instrument firmware and set the zeros. I finished by turning the gas supplies back on.


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