Pentane solvent Injection Issue

[Darraghcls]

Not sure is this a dumb question or not; but have any of you guys had an issue with using Pentane for the extraction of TPH from water?

I'm trying to set up a method for analyzing TPH C6-C36 in water, I'm using Pentane for the extraction solvent, the problem I'm having is when it comes to injecting the sample into the GC, the Pentane won't stay in the autosampler syringe, as soon as the needle leaves the vial the solvent just appears to flow straight out of the syringe.

We use a Gerstel MPS with a 10ul syringe and aiming for an injection volume of arounf 1-2 ul into a standard split/splitless injector on an Agilent 7890a system with ZB-1 column to a FID.

Any suggestions how to get around this problem? I'm not tied to using Pentane but I have been asked not to use DCM if possible.

Any and all help would be greatly appreciated

[Peter Apps]

Pentane boils at 36 C, is any hot air form ovens or detectors blowing over your syringe ? What is the room temp, do you have air conditioning ?

Peter

[AICMM]

Darraghcls,

Try it at altitude, it's even worse. The TNRCC method uses pentane but most of them work at sea level so it's not nearly as bad for them.

Cool your tray, fan your injector tower, cool your lab as much as possible, run your samples at night, all things I tried when I had to use pentane.

Best regards,

AICMM

[Darraghcls]

Cheers for the replies guys.

I was thinking its probably the heat in the lab, we do have air-con but its useless , so I've been using fans to try keep the system as cool as possible.

If I want to get away from Pentane, and still be able to analyze C6-C36 is DCM the only other real alternative

[AICMM]

Darraghcls,

I think the DCM will wipe out the C6 even with a split injection (been a while.)

Can you find a spare chiller (or even run tap water for a test) to cool the tray itself?

Best regards,

AICMM

[MSCHemist]

Not my area of expertise but I recall Carbon disulfide is invisible on the FID could you use that?

[dblux_]

Not my area of expertise but I recall Carbon disulfide is invisible on the FID could you use that?

Not true when used as a solvent.

[Don_Hilton]

CS2 is pretty close to invisible in the FID when used as a solvent, and is used for that purpose, but if the lab is warm enough to give problems with pentane, CS2 may still be a bit of a problem.

[dblux_]

CS2 is pretty close to invisible in the FID when used as a solvent, and is used for that purpose, but if the lab is warm enough to give problems with pentane, CS2 may still be a bit of a problem.

I don't want to enclose my chromatograms, so please see two chromatograms at the bottom of the OSHA method:
https://www.osha.gov/dts/sltc/methods/o ... rg091.html
Even the highest purity CS2 when used as solvent gives such peak. Take it for granted

Situation worsenes even more when GC is equipped with MSD. Due to the nature of CS2 the source has to be switched off for relatively long time to extent the lifetime of filaments. And that are so called "advantages" of CS2 in the lab.

[Peter Apps]

CS2 is pretty close to invisible in the FID when used as a solvent, and is used for that purpose, but if the lab is warm enough to give problems with pentane, CS2 may still be a bit of a problem.

I don't want to enclose my chromatograms, so please see two chromatograms at the bottom of the OSHA method:
https://www.osha.gov/dts/sltc/methods/o ... rg091.html
Even the highest purity CS2 when used as solvent gives such peak. Take it for granted

Situation worsenes even more when GC is equipped with MSD. Due to the nature of CS2 the source has to be switched off for relatively long time to extent the lifetime of filaments. And that are so called "advantages" of CS2 in the lab.

An FID has an extremely low response to CS2, which often allows it to be used as a solvent for analytes that would be riding on, or obscured by, the tail of the peak from a more conventional solvent. If the analyte co-elutes with the centre of mass of the CS2 peak then obviously this is not going to work, partly because there is some signal from the large quantities of CS2, and also because the CS2 is likely to affect the flame properties. CS2, like any solvent, has impurities in it that might coelute with analytes.

An MS works by a completely different mechanism to an FID, and so it is no surprise that it responds to CS2.

Peter

[dblux_]

An FID has an extremely low response to CS2, which often allows it to be used as a solvent for analytes that would be riding on, or obscured by, the tail of the peak from a more conventional solvent.

Have you really (!) seen chromatograms I mentioned ?

Main application of CS2 (in GC) is desorption of organic solvents from charcoal in industrial hygiene investigation. I mean evaluation of exposition of workers to organic solvents at worplaces.

If the analyte co-elutes with the centre of mass of the CS2 peak then obviously this is not going to work, partly because there is some signal from the large quantities of CS2, and also because the CS2 is likely to affect the flame properties. CS2, like any solvent, has impurities in it that might coelute with analytes.

An MS works by a completely different mechanism to an FID, and so it is no surprise that it responds to CS2.

Peter

The main mechanism of so called CS2 peak (on FID) is not signal from impurities but sudden formation of SO2 and as a consequence fuel lean flame which gives "peak".
You won't observe such peak if CS2 is not a solvent but a constituent of analised mixture.

As to MS detector. You don't get what I mean. Nobody aquires mass spectra of CS2 as a solvent. Nobody wants to deliberately burn out filaments.
What I wanted to emphasise is that due to the fact that MS source has to be switched of for relatively long time until the concentration of CS2 in MS chamber is suffitiently low, then your instrument is "blind" for longer time than in case of FID. And it affects peaks of interest with RT close to RT of CS2.

BTW - every lab which can escapes from CS2 and substitutes it with different solvents. Thats why I strongly discourage to apply it.
It makes sense to take into account TVL value for CS2 as compared to organic solvents.
If there is no choice stay with CS2.

[dblux_]

And an advice for Darraghcls: check your syringe if it isn't worn out.
Pentane won't escape from syringe at least up to 29 degC.
Experiment with different types of syringes - I mean check if teflon tipped plungers are better for you.

[Peter Apps]

An FID has an extremely low response to CS2, which often allows it to be used as a solvent for analytes that would be riding on, or obscured by, the tail of the peak from a more conventional solvent.

Have you really (!) seen chromatograms I mentioned ? Yes, I see a peak for CS2, a peak that is a lot narrower and sharper than it would be for a solvent that the FID is sensitive to, and which, as a direct consequence, gives better resolution between solvent and methanol, which is the whole point of using CS2 as the solvent

Main application of CS2 (in GC) is desorption of organic solvents from charcoal in industrial hygiene investigation. I mean evaluation of exposition of workers to organic solvents at worplaces. If CS2 is such a poor choice for analysing volatile solvents, why do the official methods specify it ?

If the analyte co-elutes with the centre of mass of the CS2 peak then obviously this is not going to work, partly because there is some signal from the large quantities of CS2, and also because the CS2 is likely to affect the flame properties. CS2, like any solvent, has impurities in it that might coelute with analytes.

An MS works by a completely different mechanism to an FID, and so it is no surprise that it responds to CS2.

Peter

The main mechanism of so called CS2 peak (on FID) is not signal from impurities but sudden formation of SO2 and as a consequence fuel lean flame which gives "peak". That peak is a signal for CS2, why do you put in in quotation marks ?. "Have you really (!)" injected CS2 and looked at the impurities ? I have, it was very dirty.
You won't observe such peak if CS2 is not a solvent but a constituent of analised mixture. Of course not, the FID's response to CS2 is too low to detect it at less than microgram quantities

As to MS detector. You don't get what I mean. Nobody aquires mass spectra of CS2 as a solvent. Nobody wants to deliberately burn out filaments.
What I wanted to emphasise is that due to the fact that MS source has to be switched of for relatively long time until the concentration of CS2 in MS chamber is suffitiently low, then your instrument is "blind" for longer time than in case of FID. And it affects peaks of interest with RT close to RT of CS2.But there would be no point in using CS2 as a solvent in GC-MS because there would be no advantage to the low detector response seen with the FID, so what is your point ?

BTW - every lab which can escapes from CS2 and substitutes it with different solvents. Thats why I strongly discourage to apply it.
It makes sense to take into account TVL value for CS2 as compared to organic solvents. Also the vile stench of CS2 (although that makes it safer to work with because you know when you are being exposed) and its extremely low flash point. But neither of these are directly to do with is utility as a GC-FID injection solvent
If there is no choice stay with CS2.

[dblux_]

If CS2 is such a poor choice for analysing volatile solvents, why do the official methods specify it ?

The answer is quite simple. Just imagine an industrial painter who applies paint consisting of a mixture of different organic solvents. Your goal is to evaluate his exposition to solvents - all solvents, unknown for you so far (you have always to evaluate exposition to all harmful substances !).
First step is to collect them on the charcoal in adsorbent tubes.
And now the most interesting, what solvent would you choose for desorption ?
Almost all official methods are based on CS2, because you can't choose anything that can be in the paint (you can't choose toluene, xylenes, alcohols, ketones to name the few). But there is no CS2 in the paint so you must choose CS2. FYI - there are only few exception.

"Have you really (!)" injected CS2 and looked at the impurities ? I have, it was very dirty.

Circa 15 years as chromatographist in industrial hygiene laboratory is sufficient ? I hope so. Solvent blanks accompanied all GC determinations.
There was only one problem with commercial CS2 - it contained benzene. And CS2 low in benzene is much more expensive. That's why laboratories purify CS2 on their own.

But there would be no point in using CS2 as a solvent in GC-MS because there would be no advantage to the low detector response seen with the FID, so what is your point ?

But official methods oblige you to use CS2 for the reason clearified earlier. Typically FID and MSD are used in parallel. And MSD is used for identification of peaks while FID for quantitation.

[MSCHemist]

Still 36 degrees is ~96-97 deg F that is pretty darn hot for a lab even close to an instrument. I'd first ensure I had a high quality gas tight syringe. If you are doing manual injection you can put the syringe in the refrigerator/freezer.

I used to make standards including neat acetaldehyde and that was a pain as it boils at 20 deg C and 68 deg F. I put both the chemical and syringe in the freezer for ~ 20 minutes before hand and worked quick.