Disturbing pattern on baseline during sample injection

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

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Hi,

As the title says I see a weird pattern in my baseline, please see attached image: Image.
I tried uninstalling and reinstalling my column multiple times, but only once did i get a good baseline and THF peak, see next image:
Image
My other attempts at reinstalling still gave me this weird pattern. I tried different liners but to no avail. So my conclusion is that there must be something wrong with the EFC. It is splitless all the way through, could it be that it puts too much strain on the GC (varian 3800)? It seems as if it can't hold the pressure constant when 1 ul liquid is injected (tried with air as well where this pattern is missing). This has only happened in one analysis so far, I have no problem with other analysis. I guess my problem is that I don't understand what is wrong, do you have any idea what might be causing this?

I appreciate your help :)
Can you list your splitless injection parameters?

- Inlet pressure
- Inlet temperature
- Liner volume
- Splitless time

That chromatogram, and the fact that the pressure is fluctuating while injecting, points towards overloading of the inlet in my opinion.

Consider injecting less, or inject in split mode - however, this will obviously lower the signals for your compounds of interest.
The upper chromatogram is a nice view of what happens when all that solvent slowly bleeds out of the inlet. Switch to split at 0.3 or so and throw all the residual solvent away and the baseline should drop quite nicely.

Best regards,

AICMM
I agree with the previous post, either change to a split injection or at least purge the inlet 20:1 after about 0.3min. I'm more curious about the defined column parameters. Are you certain the dimensions are correct and the EPC isn't trying to control column dimensions that are way off? That sawtooth pattern could be detector related or pressure related.

If those two chromatograms are supposed to look the same, the first may have had the column installed too far into the inlet.

If you could post all your conditions it would help.
Regards,

Christian
Hej,

Thanks for your reply, I tried opening the split after 2 minutes and the baseline dropped and gave me a really nice chromatogram :) I will try opening it even earlier than that, perhaps at 0,3 as you suggested. I did try injecting less at first but it didn't help, the column parameters are correct, I think opening the split should do the trick.

Angelika
The time between splitless injection and opening the split valve is the "splitless time". It's quite an important parameter: too short and you will lose analytes, too long and you can have problems like in the OP.

You can use a rule of thumb if you have no idea about this setting. For this, you need your liner internal volume and your column flow. You want to aim at staying splitless for about 2 liner volumes swept with carrier gas before opening the split valve. For example:

- Liner volume = 800µL
- Carrier flow = 1.2 mL/min

It takes 0.8/1.2 min = 0.67min to sweep the liner once, and 1.33min to sweep it twice, which should be a good splitless time to start with.
Rndirk wrote:
The time between splitless injection and opening the split valve is the "splitless time". It's quite an important parameter: too short and you will lose analytes, too long and you can have problems like in the OP.

You can use a rule of thumb if you have no idea about this setting. For this, you need your liner internal volume and your column flow. You want to aim at staying splitless for about 2 liner volumes swept with carrier gas before opening the split valve. For example:

- Liner volume = 800µL
- Carrier flow = 1.2 mL/min

It takes 0.8/1.2 min = 0.67min to sweep the liner once, and 1.33min to sweep it twice, which should be a good splitless time to start with.


Hej,

Thanks for the tip i didn't know about it :) if my liner volume is 0,986 ml and carrier flow is 7 ml/min it should take 0,14 min to sweep through the liner once and 0,28 min twice. So opening it at 0,3 min should be fine, right?
nyfiken wrote:
Rndirk wrote:
The time between splitless injection and opening the split valve is the "splitless time". It's quite an important parameter: too short and you will lose analytes, too long and you can have problems like in the OP.

You can use a rule of thumb if you have no idea about this setting. For this, you need your liner internal volume and your column flow. You want to aim at staying splitless for about 2 liner volumes swept with carrier gas before opening the split valve. For example:

- Liner volume = 800µL
- Carrier flow = 1.2 mL/min

It takes 0.8/1.2 min = 0.67min to sweep the liner once, and 1.33min to sweep it twice, which should be a good splitless time to start with.


Hej,

Thanks for the tip i didn't know about it :) if my liner volume is 0,986 ml and carrier flow is 7 ml/min it should take 0,14 min to sweep through the liner once and 0,28 min twice. So opening it at 0,3 min should be fine, right?


Yes
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