We have a Superlcowax 10 GC column (30m, 0.2mm, 0.2um) and I am using it for essential oil chemicals analysis as recommended.
I have a-Pinene, beta-Pinene, camphene, careen, limonene, terpinolene and so on as my target chemicals.
I am trying to develop a quantification method of this chemicals on thermos ISQ, currently inject pure standard to identify the peak and looking for SIM ions for later on use.
I have found all other chemicals by full scan, but I cannot find a-Pinene anyhow.
Is this one elute two early?
My GCMS conditions are:
GC injection (PTV), 60°c- 280°C, oven: 50 °C(2min), 10 °C/min to 240, 30 °C/min to 260 (5min). carrier gas (He) flow 1mL/min.
MS transfer line temp 260 °C, Ion source temp: 260, Scan for 40-400.
With this condition, I end up with RT as follow:
Camphene: 4.97
Beta-Pinene 5.23
Carene 5.65
Limonene 6.27
Terpinolene 7.37
…
I suppose a-Pinene will come out before camphene, but I cannot find it, even with very short solvent delay, see the TIC and EIC (attachment):
Any idea?
thanks.
Chang'er
Try increasing the hold time at 50C from 2 minutes to 5 minutes using the same short solvent delay as before. Isothermal conditions will provide the highest resolution; the alpha pinene may be coeluting with the camphene.
Using published kovat retention indices and your retention times a quick plot and extrapolation crudely estimates that alpha pinene should be eluting around 4.01mins on your GC column (see attached chart, kovat index values obtained from websites wwwchemspider.com, www.flavornet.org and www.pherobase.com). I wouldn't have thought alpha pinene would co-elute with the camphene as their respective kovat indices are quite different, 1002 & 1077 respectively.
When you say you are injecting pure standards to identify their retention times, what solvent are you diluting these pure standards in? I am assuming these are all single pure compounds around 95% or better? From your chromatogram it looks like there is a messy residual solvent peak from 3.5-4.9 minutes, what is your solvent delay/cut time set to? If this is indeed a solvent peak then alpha pinene is eluting in the middle of it.
I agree with Bruce that you could try increasing the hold time as you are running a moderately fast ramp at 10degC/min. Another option would be to drop the initial column temperature to 40 deg C keeping the increased hold time.
All the best.
Hi guys, I tried increasing the holding time and also lowered down the initial oven temp. to 40, no help.
any idea on how to separate a-Pinene from toluene?
thanks.
Hi Chang'er
What concentration of alpha Pinene are you injecting? I see you have a PTV injector, any chance you can inject your standard into a hot injector and analyse a concentrated version in split mode? That would help sharpen up the large solvent peak and allow space for alpha pinene. Are you able to not use Toluene? It is not the cleanest of solvents so may pay to avoid using it if you can.
thanks Jason, I am injecting a-Pinene at 1ppm (ug/mL), do you mean I can set PTV injector as a hot injector or try another GC with hot injector?
I did try inject 10ppm with split ratio of 10, seems no help for sharpening the solvent peak.
most of my standards are in Toluene, adapted from previous person. I will try to dilute it with Hex.
Hi Chang
I would inject a more concentrated standard to obtain a large peak of alpha pinene so it is more obvious where it is eluting. In the past I have injected a 4g/L concentration of alpha pinene at a 80:1 split on a 60m wax column 0.25mm ID x 0.25um film thickness. This equates to a splitless injection (1uL) of a 50ppm standard. I know you will eventually be looking for low concentrations but first you need to locate where it elutes. I have attached a chromatogram of S-alpha pinene (the 4g/L standard as mentioned above), ignore the retention time here as I was using an MDGC system so the peak was heart cut from one column to the next.
If you still can't see alpha pinene I would try a GC with a standard injection port and go from there to get something going and then look to port this method back the GC with the PTV injector.
All the best
Jason
I agree with Jason, Toluene typically contains higher boiling (and higher mass) related impurities like xylenes and mesitylenes. Also be sure to use a high purity grade of hexane (suitable for residue analysis). You might try the isothermal step at 35C (or even 30C if your GC will go that low) and hold for a longer time (10 minutes) just to see if you can resolve the toluene and alpha pinene.
Inject the headspace of pure a-pinene to find its retention time - this is very likely to verify that it is co-eluting with the toluene. Eliminating the toluene is a straightforward procedural change that will completely solve the problem.
Why are you using such a dilute standard ? At 1 ug/ml you are putting only 1 ng onto the column splitless. This is a reasonable sized peak, but 10 - 50 ng would be much easier to see while you are setting up the method.
many thanks guys. the problem solved now, indeed, the a-Pinene was co-eluted with the toluene, I did a split injection at 10:1, increase the initial oven T to 70 and inject phase evap T 120. then a-pinene came out earlier than toluene.
the toluene peak was to big.
cheers.
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