Baseline and noise slowly increase over multiple injections

Discussions about GC and other "gas phase" separation techniques.

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Hi everyone!

I’ve been having issues over the past several weeks with a Trace 1310 dual-column GC, using an FID with two ZB-624plus columns. The baseline for the back column - but not the front - slowly but steadily rises from injection to injection, and the noise rises proportionately. There isn't normally any real problem during the temperature ramp of any particular injection, but over a few days of injections the baseline will rise and noise will increase until the column is basically unusable.

Here’s a summary of chromatographic conditions:

Column: Zebron ZB-624plus
Length: 30 m
ID: 0.32 mm
Stationary phase: G43, 1.8 µm.
Split ratio: 10:1
Injection volume: 1 µL
Column temperature: 40°C for 5 min, then ramp 30°C/min for 6 min, then hold at 220°C for 5 min
Injection port temperature: 200°C
Detector: Flame ionization with temperature of 280°C
Carrier gas: Helium

I work at a plant making (normally) high-purity ethanol, and this is almost exclusively the only thing we run on this GC. There is very little in the way of non-volatiles in the samples we run – we’re limited to 25 mg/L total nonvolatile residue – so I do not believe the issue is likely to come down to nonvolatile or semi-volatile residue in the samples. The front column of the same instrument is working normally, as well, and receives the same type of samples.

My boss and I have tried replacing the septum, liner, and needle multiple times with no noticeable change. Changing the split vent filter didn’t do anything either. We also tried replacing the column with a new one of the same type, only to have the issue reappear within a few days on the new column as well.

The only thing we have found that works in the short run, not counting the column change, is to bake out the column around its isothermal maximum of 300 C for 2-3 hours, with the inlet and FID temperature also increased to 300 C. This causes the baseline to decline to near-normal levels and reduces noise temporarily, but both climb back to where they had been after a few injections with the normal method.

I’ve attached a screenshot of the issue. Each injection is just a solvent blank using the same high-purity ethanol. The date is just the date of calibration – these injections actually took place over the course of about three weeks, ending a couple of weeks ago. It’s hard to see the noise on this scale except for the last injection, but it steadily crept up along with the baseline. Lately it has gotten even worse and we’ve had to take the column out of service.

Image

Has anyone else run into an issue like this? Thanks in advance for the help!
Have you tried running the back column to the front detector? Would allow you to separate the inlet/column from the detector as possible sources.

Best regards,

aicmm
As mentioned, if you switch so the front column goes to the back detector and back column to front detector and the problem follows the column, then I would suspect you have an air leak into the rear inlet somewhere.

As the column oxidizes the bleed increases but baking it out removes the damaged phase for a short time. There could be a damage fitting somewhere in the gas flow pathway to the rear injection port allowing a small amount of air into the system, especially when the temperature is low that is oxidizing the stationary phase. I have had leaks that occur at only low temperatures and seem to seal closed as the oven ramps, which could be why you can remove the bleed by baking at high temperatures, but it returns after normal use.
The past is there to guide us into the future, not to dwell in.
AICMM wrote:
Have you tried running the back column to the front detector? Would allow you to separate the inlet/column from the detector as possible sources.

Best regards,

aicmm

That's a good idea. My boss is currently reluctant to do this because we are pretty dependent on the front column and he's worried about introducing issues that would either persist or at least cause us to have to recalibrate - which would be an issue because we are currently out of our calibration standards and are waiting on a new shipment to arrive. It's definitely something to consider once we get the new standards though.

James_Ball wrote:
As mentioned, if you switch so the front column goes to the back detector and back column to front detector and the problem follows the column, then I would suspect you have an air leak into the rear inlet somewhere.

As the column oxidizes the bleed increases but baking it out removes the damaged phase for a short time. There could be a damage fitting somewhere in the gas flow pathway to the rear injection port allowing a small amount of air into the system, especially when the temperature is low that is oxidizing the stationary phase. I have had leaks that occur at only low temperatures and seem to seal closed as the oven ramps, which could be why you can remove the bleed by baking at high temperatures, but it returns after normal use.

That's interesting - I didn't realize that it was possible to have a an air leak that sealed at higher temperatures. If this were the problem, how would we diagnose it or fix it? We have looked for leaks with a leak detector and not detected anything, but perhaps a small enough leak wouldn't be easily detectable.
Try to inject air couple of times.
If baseline raises - wash the FID. If not - wash or replace liner.
Stibnut wrote:
AICMM wrote:
Have you tried running the back column to the front detector? Would allow you to separate the inlet/column from the detector as possible sources.

Best regards,

aicmm

That's a good idea. My boss is currently reluctant to do this because we are pretty dependent on the front column and he's worried about introducing issues that would either persist or at least cause us to have to recalibrate - which would be an issue because we are currently out of our calibration standards and are waiting on a new shipment to arrive. It's definitely something to consider once we get the new standards though.

James_Ball wrote:
As mentioned, if you switch so the front column goes to the back detector and back column to front detector and the problem follows the column, then I would suspect you have an air leak into the rear inlet somewhere.

As the column oxidizes the bleed increases but baking it out removes the damaged phase for a short time. There could be a damage fitting somewhere in the gas flow pathway to the rear injection port allowing a small amount of air into the system, especially when the temperature is low that is oxidizing the stationary phase. I have had leaks that occur at only low temperatures and seem to seal closed as the oven ramps, which could be why you can remove the bleed by baking at high temperatures, but it returns after normal use.

That's interesting - I didn't realize that it was possible to have a an air leak that sealed at higher temperatures. If this were the problem, how would we diagnose it or fix it? We have looked for leaks with a leak detector and not detected anything, but perhaps a small enough leak wouldn't be easily detectable.


Does the Trace have a seal at the bottom of the injection port similar to the Agilent? I have seen leaks there that will seal when the oven temperature increases. The change in temperature differential between the hot injection port and oven decreases so the expansion of the seal closes the gap and it reopens as the oven cools down. Any other crack at the bottom of the inlet would expand and contract with oven temperature too, even though the inlet is hot, the tip where the column nut is acts as a heat sink when the oven is cooler.
The past is there to guide us into the future, not to dwell in.
James is right - you have a leak somewhere between where the gas feeds to the inlets separate and the back column, or concievably along the split line. Probably a very tiny leak. If you increase total flow to that column (increase the split) you will diute the incoming air and the deterioration will be slower, but that confirms the diagnosis - the only cure is to find and fix the leak.

Peter
Peter Apps
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