How to set solvent delay?

[Sharon]

Hello all,

I am newbie in GCMS. I have a question about how to set up the solvent delay time. Usually we set 3 min. Is it according to the bp of solvent? If the solvent does not coming out at the beginning of the run, how can we decide the time? Thank you very much!

[Peter Apps]

My approach is to switch the filament on a couple of minutes before the first peak of interest comes out.

Peter

[Johnny Rod]

You need to find the solvent retention time without overloading the MS, so you can set the solvent delay accordingly. One way to do this is to inject an empty vial, with no solvent delay set, and make sure the wash vial has the same solvent as your sample. You'll get a small wash solvent peak, enough to see but not enough to overload. If you're using HS or something then use a similar approach, a low conc to give you an RT marker.

[Sharon]

You need to find the solvent retention time without overloading the MS, so you can set the solvent delay accordingly. One way to do this is to inject an empty vial, with no solvent delay set, and make sure the wash vial has the same solvent as your sample. You'll get a small wash solvent peak, enough to see but not enough to overload. If you're using HS or something then use a similar approach, a low conc to give you an RT marker.

Thank you for your reply! Another question is if I can not use the same solvent in my sample as the wash vial, is there a way to decide the solvent delay time which I used to dissolve my sample? Is it OK to decrease the injection volume and increase the split ratio with no solvent delay set?
Thanks a lot.

[dblux_]

Hello all,

I am newbie in GCMS. I have a question about how to set up the solvent delay time. Usually we set 3 min. Is it according to the bp of solvent? If the solvent does not coming out at the beginning of the run, how can we decide the time? Thank you very much!

If your instrument allows, you can monitor high vacuum pressure in MSD chamber.
When vacuum drops substantially and then rises again it's right time to turn off filament (just before vacuum begins to drop).

[Johnny Rod]

Put in a wash vial of your sample solvent temporarily and see what its RT is, by the same method, it's not like it's a lot of trouble. Even a high split with smaller injection is going to be a massive solvent peak otherwise.

[Peter Apps]

You need to find the solvent retention time without overloading the MS, so you can set the solvent delay accordingly. One way to do this is to inject an empty vial, with no solvent delay set, and make sure the wash vial has the same solvent as your sample. You'll get a small wash solvent peak, enough to see but not enough to overload. If you're using HS or something then use a similar approach, a low conc to give you an RT marker.

Thank you for your reply! Another question is if I can not use the same solvent in my sample as the wash vial, is there a way to decide the solvent delay time which I used to dissolve my sample? Is it OK to decrease the injection volume and increase the split ratio with no solvent delay set?
Thanks a lot.

If changing a wash vial is not possible, then put one small drop of solvent into a sample vial and inject from the headspace above it so that you are only injecting solvent vapour. Set the split to 100:1.

Remember that the solvent peak from a real injection is much wider than the peaks you will see with this technique, so add 2 minutes at least to the peak retention time to get the filament on time.

Peter