Wide solvent peak GC-fid

[rikkert]

Hello all,

I just started up an old Chrompack GC (CP9001) with FID Detector and capillary column (0,32ID).

Now the Injection is Split/Splitless, when I inject in Split mode the solvent peak is a sharp (narrow) peak. (for example: hexane).

But now I tried the Splitless mode and the solvent peak lasts 30 second's approximately.

What causes this? Any tips or any help is welcome!

Greetings, Rik.

[skunked_once]

When you inject in split mode, only a small portion of the solvent enters the column. The rest is split out to the split vent.
In splitless mode, all of the solvent enters the column, hence the wide solvent peak.

[rikkert]

When you inject in split mode, only a small portion of the solvent enters the column. The rest is split out to the split vent.
In splitless mode, all of the solvent enters the column, hence the wide solvent peak.

Is there any possibility to prevent this? (for example: install another glass insert or whatever)

[Forse]

Prevent what?

The point of a splitless injection is that all the injected sample goes on the column.

[rikkert]

Prevent what?

The point of a splitless injection is that all the injected sample goes on the column.

Prevent a wide peak ( it also has a massive tail which I like to prevent as well ) and other components interfere with the solvent peak.

[Forse]

You need to provide some information for us to help you.

Liner: Volume and temp?
Solvent: Hexane?
Oven program: ?


Here is some information on splitless injections.
http://www.sge.com/support/training/inj ... -injection

[rikkert]

You need to provide some information for us to help you.

Liner: Volume and temp?
Solvent: Hexane?
Oven program: ?


Here is some information on splitless injections.
http://www.sge.com/support/training/inj ... -injection

The liner volume is 1mL and the temperature (injection temperature) is 250 degrees (celcius). The solvent isn't really relevant (it happens with cyclohexane, toluene, dichloromethane), it gives a wide peak (lasting 30 seconds) and has a massive tail. The oven program depends on the sample.

We have a calibration standard with dichloromethane as solvent, it contains C-8 to C-40 components so the oven program is set; 50 degrees (initial phase, for 5 minutes), rise with 15 degrees/min to 280 degrees (final phase, 30 minutes). After 50 minutes all the components are off the column with this program (all the Carbon components give 'normal' peaks).

[LiVD]

You should also set a splitless time. After this time your GC will open its split valve and purge the remaining solvent from the injector. This should cut the tail of your solvent peak.

[CE Instruments]

From first description your injection sounded fine. In Splitless mode you should have a solvent peak the width of your splitless time. 30 seconds sounded fine , a minute is the most common time. However this solvent peak should be pretty much square with almost no tail. You either are not opening the bottom split valve after the splitless time or you have another issue. Any chance of a chromatogram. BTW there will be a big defference between different solvents. For splitless to work properly your oven temperature should be between 10 and 20C below the boiling point of your solvent to allow sample focusing.

[AICMM]

rikkert,

If you are standing in front of the instrument, do you hear the splitless/split valve click? It's not the perfect troubleshooting tool but it is a really good start. If no click, then you valve may be bad.

Second, make sure the right valve is turning on and off in your method. Third, try a much shorter split time, say 10 seconds splitless then split and see if your solvent peak blocks off much quicker.

Best regards,

AICMM

[rikkert]

rikkert,

If you are standing in front of the instrument, do you hear the splitless/split valve click? It's not the perfect troubleshooting tool but it is a really good start. If no click, then you valve may be bad.

Second, make sure the right valve is turning on and off in your method. Third, try a much shorter split time, say 10 seconds splitless then split and see if your solvent peak blocks off much quicker.

Best regards,

AICMM

Thank you for your response, I do hear the valve click and I'm sure I switch the right valve (V3). I also notice that peaks are higher in splitless mode (obviously because there is no split ratio).

Is there a way to set the splitless time? I can switch manually on the CP9001 (by pressing VLV and then entering 3, and to turn it off again I pres VLV and then enter 20) but is there a way to set the splitless mode for a certain time (so after I press start and inject it automatically switches back to split mode after the set time).

[James_Ball]

The solvent, liner volume, inlet temperature and valve closure times will all matter when doing splitless injections.

If the expansion volume of your solvent is greater than the volume of your inlet liner for the given pressure then it will overload the liner and cause a lot of the tailing problems you are describing.

This calculator if from Agilent but it should give you an idea of how your port is performing if you select the pressure, liner volume and solvent, http://www.chem.agilent.com/en-US/Techn ... ators.aspx

We had an analyst here trying to inject 2ul of DCM with too low of a head pressure and he was getting very erratic area counts on his internal standards, once I calculated that the 2ul was overloading the injection port, we switched it to 1ul and now it performs great.

[Peter Apps]

Is there a way to set the splitless time? I can switch manually on the CP9001 (by pressing VLV and then entering 3, and to turn it off again I pres VLV and then enter 20) but is there a way to set the splitless mode for a certain time (so after I press start and inject it automatically switches back to split mode after the set time).

The users manual, or the instrument help in the software is your friend. The "splitless time" goes by several names; purge time, vent time, split on time etc etc. I can heartily recommend that you read through the manual to get an overview of how to use the basic functions of the hardware and the software. It will save you a lot of trial and error.

Peter

[marlenl]

Yes, You can programing the splitless from the GC. First You need to activate the Page3 on the Page1, than You can program the splitless on the page3. 0 min VLV3 and 0.75min VLV-3. After save the methode on the Page5.

L

[mrussell]

Hello all,

I just started up an old Chrompack GC (CP9001) with FID Detector and capillary column (0,32ID).

Now the Injection is Split/Splitless, when I inject in Split mode the solvent peak is a sharp (narrow) peak. (for example: hexane).

But now I tried the Splitless mode and the solvent peak lasts 30 second's approximately.

What causes this? Any tips or any help is welcome!

Greetings, Rik.

Try this. Start at 1uL injection or even a half or less if your instrument will do it. Start your oven temp at 20C below the boiling point of your solvent. Make sure you have the right liner in the inlet.(they are often different for split or splitless). Set your purge time to 5 minutes and the purge flow to 60ml/min. Set your temperature program to hold 1 minute,then ramp at 15c/minute to the maximum column temp. You should get a long tail on your solvent peak and your analyte peak will probably be in that. Now back that purge time down to say .75 minutes. You will notice that tail gets cut off and your analyte peak is nicely separated and has a good gaussian shape.

Split injections are easier to do,becuause they are much more tolerant of the instrument settings,but of course,becuase they "split" the sample,putting,say,1 part on the column to roughly 20 (or 50 or 100) out the split vent (remember,this is not exact,its very approximate,the number reall refers to gas flows,not analyte amounts) they are less sensitive than splitless injections.