GC-MS Split Peaks

[Banzo]

Hi everyone,
We have an Agilent 7820A GC with 5975 MS and 7693A ALS and we have been analyzing drug samples for over a year without any problems. Also see the chromatogram pictures and files in this zip. The sample is a mixture of amphetamine, methamphetamine, MDMA, Paracetamol, Cocaine and Tribenzylamine (IS) (in this order). The two spectra are made from the same vial of methanolic solution.
But after the last maintenance when I replaced the liner, O-ring, septum and gold plated inlet seal, changed diffusion and foreline pump oil, cleaned MSD Source and changed split vent traps, the Methamphetamine peak showed not 1 but 2 peaks with almost identical mass spectra.
First I thought it was an inlet issue because of the inlet maintenance I did. But after changing the liner, O-ring, septum and gold plated inlet seal while ensuring the correct column depth, the split peaks were still there. Also I noticed only two compounds exhibiting this behavior these are Methamphetamine and Dromostanolone enanthate. All other compounds are either fine or they have a bit more tailing than before.
So I started looking at other possible causes.

So far I’ve done the following things which didn’t help:
(I always wear clean lint free gloves)
1. Checked the analysis method it was almost identical except the MSD transferline was higher when the split peaks occurred but lowering the MSD transfer line to the original temp didn’t change this. Also the tune parameters changes because of the maintenance of MSD.
2. Changed liner + O-ring
3. Changed septum
4. Changed gold plated inlet seal
5. Recut the column with ceramic wafer and reinstalled it
6. Did a manual injection (to rule out the ALS)
7. Solvent rinsed the column.
8. Changed the installed column (HP-5MS approx.. 28 m x 0,250 mm x 0,25 µm) for a new column (DB-5MS 30 m x 0,250 mm x 0,25 µm). (conditioned before installing the MS side) also made sure the column depth in the inlet and MS where right.
9. Cleaned the septum nut
10. Cleaned the needle tip holder on the ALS since it was a bit dirty.
11. Changed ALS syringe.
12. Lowered the voltage of the MS electron multiplier from 1 to 0.5 this only lowered the peak area a bit.
13. Injecting 0.1 µl instead of the regular 1 µl. The peak of methamphetamine was good when injecting 0.1 µl. But it seems strange that injecting 1 µl like I always did would give split peaks and 0.1 µl would not.

I’ll keep updating this list so it will all be in one place what I’ve done so far.
I’m not sure what is causing these split peaks, all information I’ve found and received from different sources suggest it is an inlet issue of some sort.
Can anyone help me with this? Or does anyone have any ideas on what I could try next?

[Don_Hilton]

I'm miossing a link to your pictures - but.
If the split peak is along the chromatographic trace the problem is in getting the compunds to move through the GC column in a single band. This rules out the detector.

If the two compounds are a couple of the earlier compunds, the fact that you are using methanol waves a flag at me. Have you, by chance changed the initial column temperature? Methanol will not wet a 5% methyl silicone column very well, so if it is flashed from the inlet to a cold column, the methanol beads up and scatters on the column - taking your analytes with it. If your column is warmer than the boiling point of methanol, it will not condense. And if you use a very small injection volume, you may be able to avoid this issue because there is not enough methanol to bead up and move around.

Second set of thougths. Have you changed to a new type of inlet liner or changed temperature there?

[Banzo]

Hello Don_Hilton,

I've added the link in the first line now I forgot about that.

The methamphetamine is one of the erlier compounds at about 4 minutes just after amphetamine (at which I cannot find a second peak even when injecting a higher concentration) but the other substance with two peaks is Dromostanolone enanthate which eludes at 23 min but has a completely different method.
The injector temperature is at 250 °C and the initial column temperature is 80 °C (I haven't changed this) so this is above the boiling point of methanol.
Your point may be valid but we have used methanol for almost all our analysis the past year including for methamphetamine and never had a problem until now.

I've changed the liner but its the same type of liner and I've not changed the temperatures except the MSD transfer line but I already rulled this out.

[Peter Apps]

This sounds familiar - I am almost sure that there has been a previous thread on it. Try a search of the archives.

Peter

[Banzo]

Hello Peter Apps,

I've searched the archives but I couldn't find anything which maches our problem.
Can you please provide a link to it if such post exists?

[Rick]

I suspect a trouble with split valvle....verify your split vent trap and if the split vent valve is operational..dilute your sample and try splitless mode.

[Peter Apps]

Hello Peter Apps,

I've searched the archives but I couldn't find anything which maches our problem.
Can you please provide a link to it if such post exists?

Sorry, but I have more pressing calls on my time than doing internet searches for other people's problems ! What search terms did you use ?

Have you tried a Scopus or Google search ?

Peter

[James_Ball]

Hello Don_Hilton,

I've added the link in the first line now I forgot about that.

The methamphetamine is one of the erlier compounds at about 4 minutes just after amphetamine (at which I cannot find a second peak even when injecting a higher concentration) but the other substance with two peaks is Dromostanolone enanthate which eludes at 23 min but has a completely different method.
The injector temperature is at 250 °C and the initial column temperature is 80 °C (I haven't changed this) so this is above the boiling point of methanol.
Your point may be valid but we have used methanol for almost all our analysis the past year including for methamphetamine and never had a problem until now.

I've changed the liner but its the same type of liner and I've not changed the temperatures except the MSD transfer line but I already rulled this out.

It sounds like an inlet problem and the only thing you haven't rechecked is the split vent trap installation it seems. It could be a problem there, since the gas flow must go through that and to the back pressure regulator and sensors so that the proper flows can set by the electronics. If you have a leak or blockage there it can cause problems by not giving you proper split ratio or column flow. Another thing to check and clean is the actual split vent line. On most Agilent GCs it is the copper 1/8" line from the inlet to the split vent trap. We found that cleaning that can solve many problems. What I usually do now is just replace it with a new piece of copper tubing, easier than cleaning it.

[Banzo]

Hello everyone thanks for the tips you gave.
We had a mechanic come by and fix the problem and it was indeed the split vent line because substances condense in the split vent line.
He also cleaned the piece of fixed tubing from the splitvent line to the inlet body with a cotton swab soaked in the most used solvent.
And he changed the whole piece of copper tubing.
Also he trimmed a bit from the column as he though there might be a piece of cotton wool stuck in the front end of the column.

This is now fixed so the topic can be closed.