No GC experience. Having problems.

[tleek]

As the title states, I have no experience with GC or chromatography at all beyond a couple lectures ages ago. I got a new job about six weeks ago that requires I use one, however due to some big changes I have been left to my own devices. I was experiencing poor linearity and some other issues so I decided it was time to change consumables that had not been changed in ages. New liner, septum, gold seal, ferrules and column. Now when I run a standard that contains 3 components, all I get is a large, rounded peak that pops up way too early compared to the separate components. This also happened with a methanol blank. I conditioned the column and made sure to clear it of oxygen beforehand. However, I do not have a leak detector. Did I perhaps ruin the column? Is this why my components are mashing up at the beginning and not separating? Perhaps because of a leak? Any help would be appreciated. 30m column. 0.25mm id and 0.25um thickness. FID. HP 5890 series II chromatograph.

[rb6banjo]

We are going to need some more information on your method so we can help you. Please provide as many of the method conditions that you can:

inlet pressure (column flow)
split flow/split ratio
Temperatures (inlet, oven program, detectors, etc.)

The more detail you can provide, the better will be the suggestions we can give you. Perhaps an image of a chromatogram could help us too. Read one of the top messages on this board and it will instruct you on easy ways to show us images.

[dblux_]

stationary phase ?
what are the analytes ?
Did your instrument resolve analytes before your manipulations ? (show chromatogram)
BTW - it's not true that you need leak detector to be a chromatographer

[tleek]

We were getting results before my manipulations, but started to have issues with calibration, and that is why we changed things. The new column is the same as the old, just longer. Went from 15m to 30m. Restek Rxi-35Sil MS fused silica column. Midpolarity crossbond phase. I am testing for cannabinoid concentrations in medical cannabis. Specifically THC, CBD and CBN. The method already existed before I got here. I still don't know how to properly create one, but here is some info: Hydrogen carrier gas. Under inlet pressure programs- (calculated values) 19.8 psi @ temp 50c. flow 2 ml/min. Velocity 65cm/sec. split flow 2 ml/min. split ratio 1:1. Under oven- inlet A at 250c. Detector A at 310c. Oven Equib. Time 0.20 min. Oven Program- initial temp 225c. Initial time 0.5 min. Rate 20c/min. Final Temp 280c. Final time 1 min. Detector is FID. Signal- Source detector A. peak width 0.053 min. Data Rate 5 Hz.
Here is a chromatogram from before my tinkering, using a cannabinoid standard from restek at a concentration of 0.01 mg/ml (My calibration is off, which is a whole different struggle for me.)

And here is a chromatogram after my tinkering using the same diluted standard.

As you can see the components are not even showing up at all.

[Peter Apps]

You need to go at this step by step.

First step. Cool down the oven and the detector and turn off all the detector gasses. Disconnect the column from the detector and put the end of the column into some methanol or ethanol. If you see bubbles you at least have carrier gas flow. If there are no bubbles you have either a complete blockage, a massive leak in the inlet, or a failure of gas supply to the inlet. A leak that big will usually make the GC shut gas flow off because inlet pressure will not be maintained. Check that there is pressure in the inlet from the GC readout. If there is no pressure check all the settings and that there actually is gas supply to the GC. If there is pressure but there are no bubbles redo the column installation and inlet liner installation completely, follow the GC manufacturer's instructions to the letter. Check again for flow.

If you have bubbles on the first check, redo the column and liner installation in case something is in the wrong place / upside down or whatever. Re-check for bubbles. If you have bubbles, re-install the column to the detector, heat up the detector and light the flame. Leave the oven cool.

Inject some gas from a cigarette lighter. Do you see a peak ? What is the retention time ?

Peter

[James_Ball]

Looking at the two chromatograms what I see is they have equivalent baselines. Seems both are in the 1000 to 2000 range on the left hand scale, so it seems you are at the same sensitivity.

You said you went from 15m to 30m column length. Did you keep the same pressure on the inlet?

Plugging those numbers in the Restek pressure/flow calculator it would seem that a 30m column at 19psi would give 2.32ml/min if you are using helium carrier, but gives 5.15 ml/min(123cm/sec linear velocity) if using hydrogen carrier. If that was on the 15m column it would be much faster flows.

Try your setup but adjust the inlet pressure to 7psi if you are using Hydrogen carrier gas or 17psi if using Helium carrier gas, and see how that changes things.

Also how far did you install the column into the inlet? The tip of the column should only be about 7mm above the top of the ferrule when using a split/splitless injection port on a 5890. One trick I have used to make it easy to get proper placement is to insert the column with the top of the port open and the liner removed, tighten the column nut just enough that you have to apply pressure to make the column move up and down in the inlet but that it won't just fall out on its own. Next take a cotton swab and tap the end on the counter a few times to flatten it and drop it into the inlet, this will allow you to see column movement even when it is at the bottom of the inlet. Pull the column down just until you stop seeing the swap stop moving downwards, then push the column back up until you see the end of the swab move up about 1-2mm, then tighten the column nut and remove the swab and reinstall the inlet liner and cap. That will have you are the proper height.

When you change the length of the column you have to also change the pressure in the inlet to maintain the same flow rate through the column since the flow is controlled by head pressure. If you have electronic pressure control the software should calculate it for you, but you have to have the correct column dimensions and carrier gas put in the software for it to calculate correctly. Also if you get the column too high or low in the inlet, you will not get good peak shapes, so be sure you are at the proper height as above, 1-2mm inside the inlet, which is about 7mm above the tip of the ferrule. On the detector side, make sure the column is at the right depth also. I will need to look that up to be sure what it is(I am thinking 7cm for some reason but someone else here might know for certain), but if you measured from the old column you should have it close.

[Bigbear]

Not to be contrary James, but Agilent has told me the preferred injection depth is 5mm above the tip of the ferrule. The way I do it is to push the column through a septa trying to make it perpendicular. Then put on the nut then ferrule. Clip the column making sure the cut is square with no burrs.
Have the column nut sitting on the septa when you adjust the column depth to 5mm. Then the septa will hold the proper depth when you install the column.

[James_Ball]

Not to be contrary James, but Agilent has told me the preferred injection depth is 5mm above the tip of the ferrule. The way I do it is to push the column through a septa trying to make it perpendicular. Then put on the nut then ferrule. Clip the column making sure the cut is square with no burrs.
Have the column nut sitting on the septa when you adjust the column depth to 5mm. Then the septa will hold the proper depth when you install the column.

5mm is probably the correct depth, I haven't installed one that way in so long I could be off on the 7mm. I do know that 1-2mm above the gold seal gives the best peak shapes, too high and you get split peaks, too low, especially below the top of the gold seal and you get really bad tailing.

Now I am working with the MMI which is 10-12mm above the ferrule and the PTV which is no more than 17mm(14-15 seems to work best). Gets all kinds of confusing when jumping between instruments now

[tleek]

I tried the butane, same result. One large, early eluting, tailing peak. This is the same with methanol, a diluted standard, etc. I made sure my column begins 2mm above the seal. Proper distance from FID.

[Peter Apps]

I tried the butane, same result. One large, early eluting, tailing peak. This is the same with methanol, a diluted standard, etc. I made sure my column begins 2mm above the seal. Proper distance from FID.

On the chromatogram you posted on Tuesday there are no peaks at all - and certainly no large tailing ones.

Did you do the bubble test ?. Did you re-install the column at the inlet ?

Peter

[tleek]

Pardon my technical inabilities. The chromatogram that looks like it has no peaks, if I zoom out in chemstation, becomes a large rounded peak. It doesn't taper off or flatten out. I have done the bubble test and reinstalled. I will post more images afterwards if I get a chance. I am going through all the basic trouble shooting again today, and if I still have issues I am reinstalling the old 15m column and seeing what happens.

[Peter Apps]

Please post a chromatogram of the whole run from an injection of butane.

Peter

[Hopane]

Hello everyone,

I am new to the forum and just wanted to introduce myself before I reply to the topic. I am a PhD student working for the most time with GC - Qtof and GC - C - IRMS for petroleum related applications. I was reading a lot here and I now decided to be a little bit more active

To the topic: I can only see baseline on the chromatogram that you postet. Let's assume that your system is leak free and everything is correct installed! You have put in a double length column (30 m instead of 15 m) and I guess you haven't changed the flow or pressure setup. These means, that you will experience a way longer retention time. So maybe the peaks you want to see are not recorded at all. Have you installed the column correctly in the GC Software? Have you tried to record for a longer time?

Since you have to adjust the settings anyway you should also try to think about your split. A 1:1 split doesn't make much sense and can lead to poor performance. Maybe you can try split less injection and adjust the vent time until you have stable peak areas.

I hope I could help at least a bit with my first comment here in this forum.

Cheers

[tleek]

Got a tech in. New everything. Column, consumables, good method, good curve etc. New problems though. will post new topic. Thank you all.