Chromatography Forum

Hello all,


My lab is moving from a Rxi-5ms 20m column to a BPX5 30m column for analysis of ignitable liquids (both petroleum-based and water-soluble). We use helium as a carrier gas with a Shimadzu GC-2010 Plus gas chromatograph fitted with a Shimadzu QP2010 Ultra mass spectrometer detector and Shimadzu AOC-20i autosampler.

The internal diameter of the column is 0.25mm and the film thickness is 0.25 micrometers. All analyses look fine except the diesel chromatograms do not resolve pristane and phytane. Our method includes an injector temperature of 240 Celsius, injection volume of 1 microliter, split mode of 15:1, and column flow of 1.5 mL/min.

The temperature program is 37 Celsius for 2.5 minutes, ramped to 290 Celsius at 10 degrees/min and held for 5 minutes.

I looks like pristane and phytane are coeluting with the n-alkanes and was hoping anyone could help me improve my resolution to separate the peaks.



Thanks!

My first question is why would you move away from a shorter column that was working to a longer column that doesn't work as well?

The best way is to use DB1 or equivalent 30m. Your peaks would be resolved. Our lab use it for many years.

My first question is why would you move away from a shorter column that was working to a longer column that doesn't work as well?

Our lab does fire debris analysis looking for traces of ignitable liquids. That includes compounds like ethanol and acetone. In our shorter column, we could not look at those compounds because they eluted at the same time as the solvent. We use a carbowax polar column for those type of analyses but changing columns over take even more time than extra length on a column.

Wow! I was very surprised to read this post today. Finally, something I an contribute to.

GC3PO, I am a fire debris analyst as well. In our lab, we use almost exactly the same setup as you. We currently run a 30m Rxi-5ms column in the same GC-MS system. We have only small variations in the method - we inject at 250 C with a column flow of 1 ml/min.

Pristane and phytane should resolve clearly on this type of column. Your first Rxi column should have been able to resolve them even though it was slightly shorter. Here is a chromatogram from one of my analyses: http://i.imgur.com/L3wmVfF.jpg.

You can see pristane and phytane adjacent to C17 and C18 (third and fourth tallest peaks). Remember that some distillate products do not have much pristane and phytane in them at all. In some diesels, it is difficult to tell phytane is there as it is usually present in much lower quantities than pristane. Obviously, these kinds of differences depend on your geographic location.

As for a solution, I'm not sure sure. I would have thought you would see them on your first column. Perhaps you can try lowering the column flow rate and running a slightly slower oven temperature ramp rate. Give it a try.

Which solvent are you using? You could try using a different solvent if your current one gets in the way of lighter compounds.

Where are you based? Feel free to PM me - happy to chat.

Hi cene,

I'm having trouble starting a private message on the site. I am doing work at a lab in Auckland. The previous method for our fire debris analysis involved using the 20m Rxi-5ms column for petroleum-based ignitable liquids and if there was a need to test for any oxygenated solvents, we had to change the column to a more polar one. This takes time and with other casework it seemed like it was time to create a new method with a column that could separate both petroleum-based and oxygenated ignitable liquids. The BPX5 column has shown proficiency in separation of most of our solvent standards and all of our petroleum-based standards with the exception of diesel. The polarity of the BPX5 and Rxi-5ms is almost identical so the issue of pristane and phytane co-eluting should be solved by boiling point gradients in the temperature program.

I have tried many temperature programs and even the one you suggested with no change in the shape of the peaks, so I'm not even sure if something is improving or worsening the resolution. Here is a TIC of our diesel standard run with the program you use. There is also an image zoomed into C17 and C18, There is no indication of pristane and phytane is that bump on the right of the C18 peak.
http://imgur.com/a/Afy6X

Do you perform any analysis with oxygenated solvents on the Rxi-5ms column? Perhaps we just needed to get a longer column to so that type of analysis. We use DCM for a carrier solvent.



Thank you for your time.

Hi GC3PO,

The separation on your chromatogram looks pretty good, so I'm not sure why those compounds aren't resolving properly. As I understand it, the BPX-5 column actually has a different stationary phase to the Rxi-5ms. It's more equivalent to an Rxi-5Sil MS, which is based on a silarylene phase. All I can think of is that this type of phase doesn't separate pristane and phytane out as well as a standard dimethyl polysiloxane phase. In any case, its odd.

I use an Rxi-5ms column and find that it is generally quite good. You should be able to identify ethanol, acetone, and other common oxygenates without issue, but you'll be blind to compounds that elute just before C6 when using DCM. What were the issues with analysing oxygenates on your old Rxi column?

You're not far away - I'm in Australia. Send me an email (link) or let me know yours and we can chat.

Hi Alexander

I fully appreciate your comment

Wow! I was very surprised to read this post today. Finally, something I an contribute to.

I have often been in the same position.

It would be nice to share your private chat with the rest of the forum so that others may contribute, learn and also hopefully help

Kind regards

Ralph

GC3PO, how did you go with this?

Hi everybody.... In these case I think pristane and phytane don't resolve becasuse the change of column, we use Rxi 5MS and only phytane is seen. The lenght of the column is a factor that should contribute to resolve those peaks so I think is just the polarity of the column what is promoting not resolving the peaks.

I'd like to be in touch with everybody that is doing fire debris analysis because I have some issues.

Hi everybody.... In these case I think pristane and phytane don't resolve becasuse the change of column, we use Rxi 5MS and only phytane is seen. The lenght of the column is a factor that should contribute to resolve those peaks so I think is just the polarity of the column what is promoting not resolving the peaks.

I'd like to be in touch with everybody that is doing fire debris analysis because I have some issues.

Hi,

I use a 30m Rxi-5MS and both pristine and phytane resolve quite well. Shorter columns have issues with this. Send me a PM if you'd like to discuss the discipline, always happy to help if I can

Hi everybody.... In these case I think pristane and phytane don't resolve becasuse the change of column, we use Rxi 5MS and only phytane is seen. The lenght of the column is a factor that should contribute to resolve those peaks so I think is just the polarity of the column what is promoting not resolving the peaks.

I'd like to be in touch with everybody that is doing fire debris analysis because I have some issues.

Hi,

I use a 30m Rxi-5MS and both pristine and phytane resolve quite well. Shorter columns have issues with this. Send me a PM if you'd like to discuss the discipline, always happy to help if I can

Hi Cene, helping is good, but doing it by private message deprives the rest of the forum of the knowledge you share.

Peter