Reducing sample concentration

[dendira]

Hi all.

I am a relatively novice chemist so would appreciate some help with what I think should be a simple issue

I am trying to analyse some liquid samples using a Shimadzu QP2010 Ultra GC/MS. The liquids are various commercial products (mostly hydrocarbon-based). I would like to inject these samples without diluting them in a solvent because I want to see everything that is in the sample, including compounds that come off the column before the solvent. The problem is, as the samples are not diluted, they are far too concentrated and keep overloading the detector. Can anyone advise how I can best reduce their concentration, or reduce the sensitivity of the MS? I have already tried using a very high split ratio (400:1) but some compounds are still too concentrated.

So far I am thinking of the following options but am not sure which would be best for the instrument:
- Lower detector voltage
- Lower filament current
- Lower injection volume
- Higher injection split

Any advice is appreciated. Thank you!

[Forse]

Dilute the sample.

If your solvent comes of the column at 3-4 min turn the detector of at this point. The Shimadzu software must be able to do this. If you think something is comming of at the same time as the solvent use another solvent and make two dilluted samples.

[dendira]

Dilute the sample.

If your solvent comes of the column at 3-4 min turn the detector of at this point. The Shimadzu software must be able to do this. If you think something is comming of at the same time as the solvent use another solvent and make two dilluted samples.

Hi Forse,

Thanks for the advice. The software can turn the detector off however I would prefer to not use a solvent at all because it is important the sample is as pure as possible. I could use the two solvents I suppose but I would prefer to deal with a single chromatogram. I will keep that in mind.

[Peter Apps]

It will be far easier to make sensible suggestions if you tell us what you are doing currently; injection volume, flow rates, temperatures etc. All we know is the 400:1 split ratio.

Peter

[DReggio]

Since you have already have saturated runs, you know when the earliest eluters are. With this information find a solvent that elutes sooner.

Alternatively is the detector saturated for those early compounds? It may be possible to combine diluted and undiluted runs and gain all the information you need.

Spend some time thinking about if it is possible at all to dilute, concentrated hydrocarbons are hard on a gc/ms system.

[dendira]

It will be far easier to make sensible suggestions if you tell us what you are doing currently; injection volume, flow rates, temperatures etc. All we know is the 400:1 split ratio.

Peter

Hi Peter.

Sorry, first post on the forums, so not sure what info you might have needed

Injection volume is 1ul. Flow rate is 1ml/min. Carrier gas is helium. Oven program goes from 50 - 280 C. Let me know if there is any other info that would be helpful.

Since you have already have saturated runs, you know when the earliest eluters are. With this information find a solvent that elutes sooner.

Alternatively is the detector saturated for those early compounds? It may be possible to combine diluted and undiluted runs and gain all the information you need.

Spend some time thinking about if it is possible at all to dilute, concentrated hydrocarbons are hard on a gc/ms system.

Hi DReggio,

Thanks for the suggestions. My earliest eluters come off the column quite soon - at about 1.5 minutes. My current solvent is DCM. Can you suggest any solvents which come out earlier but are just as effective as DCM?

Combining the data is an option. Similar to Forse's idea. At this point it seems like the only real option if I can't do it via the hardware.

[dendira]

Bit of an update.

I found that with a split ratio of 850:1 (maximum possible with the given inlet pressure) the concentrations were brought down to acceptable levels. However the resolution did not appear to be as good. So at this point I will go ahead with some dilutions and see how it goes.

[Peter Apps]

Try reducing injection volume - you might need a different syringe.

Do you by any chance have a headspace sampler ?

Do you need qualitative results (identifications), or quantitative ?

Peter

[dendira]

Try reducing injection volume - you might need a different syringe.

Do you by any chance have a headspace sampler ?

Do you need qualitative results (identifications), or quantitative ?

Peter

Hi Peter.

I might look at getting a different syringe. However for the moment the results I have will do. I don't have a headspace sampler. And I am only after qualitative results.

[DReggio]

Thanks for the suggestions. My earliest eluters come off the column quite soon - at about 1.5 minutes. My current solvent is DCM. Can you suggest any solvents which come out earlier but are just as effective as DCM?

While not as effective as DCM, Methanol does a good job with hydrocarbons and should (depending on your column type) come off considerably sooner. Hexane is an option to consider if you need a non-polar solvent.

Since you are worried about the solvent masking sample analytes, you could perform dilutions using two different solvents. Different portions of the chromatography would be masked in each run giving you the full range when both are considered.

[dendira]

Thanks for the suggestions. My earliest eluters come off the column quite soon - at about 1.5 minutes. My current solvent is DCM. Can you suggest any solvents which come out earlier but are just as effective as DCM?

While not as effective as DCM, Methanol does a good job with hydrocarbons and should (depending on your column type) come off considerably sooner. Hexane is an option to consider if you need a non-polar solvent.

Since you are worried about the solvent masking sample analytes, you could perform dilutions using two different solvents. Different portions of the chromatography would be masked in each run giving you the full range when both are considered.

Hi DReggio,

I might give methanol a try. Hexane comes out a bit later than DCM so is not as ideal.

In the end the dilutions will work. I just wanted to know if there was an easy way to reduce sensitivity using instrument parameters rather than doing dilutions. However it looks like that isn't the preferred way of doing things.

[osp001]

I don't have a headspace sampler.

Vials, lids with septa, crimper, sand bath, thermometer/thermocouple, and a hotplate. Don't overload your vials with sample (10 uL or so), lest they explode on you.

[PeakDetector]

Try reducing injection volume - you might need a different syringe.

Do you by any chance have a headspace sampler ?

Do you need qualitative results (identifications), or quantitative ?

Peter

Agreed, and you can use a smaller sample loop or try a partial fill.