Within 0.1 for a gcms is pretty normal. Is autoinjector being used? I would presume that auto injector was in use as the elusion timing is quite consistent. Maybe the purity of the gas? May want to condition the gas after it was just being switched on.

SP-2560 is a highly polar column, very sensitive to the presence of water and polar compounds in both samples and carrier gas (essentially water). As you are using gas generators, check the efficiency of water removal systems and try using a higher standby temperature during the weekend.

By higher standby temperature you are referring to the oven? Would it need to be higher than 180C? We have tried to leave our ovens at this temperature due to this is where our GC method start and this has no effect and we still see this shift happening.

As for water in the system we have had this issue in the past and we see the phase degrade super fast. This will cause constant shifting of retention times but to lower times and never back to higher times. We also have Moisture/Oxygen indicator traps on our Hydrogen Lines and have saw no change in the indicator to alert the presence of either oxygen or water.

Highly polar column phases are intrinsically less stable, also in terms of RT reproducibility. All in all your shifts are very small - please consider that, since you are operating against atmospheric pressure, RT varies accordingly. I suggest you to use internal standards as a RT reference/correction, which most integrating softwares can easily handle.

It looks like we could identify peaks according to a RT reference/correction. However, the integration parameters for the integration of peaks would not change so peaks would not be integrated correctly (or at all). I'm not seeing anywhere within the integration program I can adjust the times set for integration of each peak based upon a RT reference/correction and they are locked in without manual manipulation. We are using LabSolutions from Shimadzu.

I agree that we should see small differences in RT (in thousandths of a minute) which we do once things stabilize. Once things stabilize we are able to run ~300 samples (or more) consecutively with a shift in RT by a hundredth or maybe two hundredths of a minute. We are trying to figure out why we are seeing a RT shift by a half a tenth to a tenth of a minute change upon starting a GC again either from being off or fully on when it sits for ~1 day or more which is throwing off the peak integration time parameters set within the integration method. We can shift retention times according to a RT reference/correction but cannot shift the integration parameters set within the integration program.

RT seems to stabilize with repeated injections. Every Monday, there might be a way to inject a few times as a blank and then proceed with the injection for actual analysis.

It's been a while since I used the shimadu GC, so I don't remember,

Instead of using RT dependent settings for peak integration, there will also be a way to do so with other settings.

There may be several reasons for the RT difference, which depends on the GC setup and method and the sample.

If you let us know more information about GC method and sample properties, it would be helpful to point out the problem.

I agree with Salvatore Chiavarini

In my opinion, you should check:

• A micro-leak in the carrier gas circuit (air and/or moisture entering on Monday morning before pressure stabilization).
• The filter at the carrier gas inlet, which must be changed regularly.
• Is there a temperature change in the laboratory on Monday morning?
• If the laboratory is pressurized, be cautious of pressure variations, as the pressure, flow (and temperature) control of the chromatographs is calibrated based on measurements from the laboratory environment.

Another solution to automatically manage these retention time variations:

• Automatic integration of all peaks using the method in your CDS. For optimal integration, the peak integration start threshold should be adjusted to be more sensitive.
• Use a software, like GC-LC Concordance, that automatically handles random retention time shifts (for working with norms, in QC and for quantification). Additionally, the software will automatically remove small peaks or background noise and keep only those that are relevant to you.

Thanks for the responses and sorry for the late replies, things have been little busy in the the lab.

To answer Jaemin Lee question about the method and sample properties. The samples are mainly DBS or RBC analyzed for FAMEs using BF3-MeOH for the methylation of the fatty acids. We then extract these into hexane and inject onto our GC. The GC inlet is at 230C, FID is at 290C and column starts out at 180C with hydrogen as carrier gas at constant velocity of 55cm/sec. There is an oven temperature program.

To answer Raymond Loyer post: We have a moisture/oxygen trap on our Hydrogen line (carrier gas) which we only change if we see a change in the indicators within them. We have not changed these in over a year. If we did get a leak in the line downstream from this trap and moisture/oxygen got introduced into the carrier gas we would see a much quicker deterioration of the column phase, which we have not seen. Not to say that the leak is minute enough that this deterioration is not happening but is causing this shift though, we would just have no way to identify a leak like this. Our laboratory is not pressurized and we will see this shift in retention times anytime the GC sit idle for >1-2 day, it does not have to be a Monday or be in the morning. There is a way to adjust the retention times based upon our ISTD peak however, this doe not adjust any of the integration parameters within our integration method (where we tell the software to start looking to integrate a peak at this time and stop at this time). This is the spot the software we currently have breaks down as with the shift that happens at the start none of the peaks get integrated.