Analysis of hexane by HS-SPME -- what is the suitable IS?

[angeldevil]

I would like to quantify the hexane in sample using HS-SPME.
I selected PDMS SPME fiber and Wax-GC column. I had tried Chloroform as Internal standard, but tailing was obviously observed in the chromatogram. Anyone know what is the problem? Any suggestion for IS? or other method?

Angeldevil

[krickos]

Hi

First, we lack the details about your procedure/instrument settings to help you in a good manner.

Some general comments though:

A. As IS the rule of thumb is to pick a substance that is as equivalent as possible as your target analyte, in this case n-heptane might have been a better choice.

B. Column choice. Again lack method parameters but if you start at a GC oven temp of 40°C, hexane will hardly be retained on the column. A DB-1 or DB-624 type of colum for example would have been my choice.

[zokitano]

Regarding your problem we have to have a little bit more information that you've provided in your post. Tailing of chloroform is probably chromatography related problem or fiber desorption related problem. However, since you said that you're using PDMS fiber for quantification of hexane, I must ask you at what concentrations you're trying to measure hexane. I hope that you've read the manufacturer recommendation regarding the usage of the PDMS fiber. Is it clearly stated there that non-polar solvents or samples that contain percent levels of for e.g hexane, methylene chloride, diethyl ether... should be avoided when doing DI or HS SPME with PDMS fiber. Moreover, you should avoid any chlorinated solvents (such chloroform) when using any type of SPME fiber coating. Remember that using the fiber with these solvents can cause fiber swelling and stripping of the coating when the fiber is retracted in the fiber assembly needle.
I hope your emulsion is water based and you're seeking for low ppm conc of hexane, and you're using 30um or 7um PDMS fibers which are more resistant to swelling than 100um PDMS type. If this is your actual situation then avoid the usage of chloroform as IS and use another IS in (same) low conc as your analyte. You could try using heptane (as suggested by Krickos) or pentane as IS. But be sure that you're quantifying low conc of your analyte since fiber swelling can be a real problem in your subsequent analysis. If you can't avoid using the fiber with higher concentrations of hexane or other non-polar solvents (present in the emulsion) consider putting the fiber into a clean water for e.g 5 seconds after the extraction, before you retract the fiber back into the needle. In this way, any swelling that can occur to the fiber can be minimized by putting it into clean water before fiber retraction.

Hope this helps,
Best regards.

[angeldevil]

Dear Friends,

Thanks for the reply. The informations are indeed very helpful.

Sorry for not providing adequete information while putting forward my problem.Please allow me to clarify.
Yes, I am using 100um PDMS, HS-SPME GC analysis. You are absolutely right that I wanted to quantify trace amount of hexane in my oil-in-water emulsion system.

To get the RRF, I had spike 20ul of chlorofom and 20 ul of hexane in 4ml of emulsifier containing (1%) water phase in a 20ml vial. The vial was condition in 60degC water bath for 20mins and fiber was exposed for 10min.

I used DB-Wax as my GC column, temperature program begin at 40degC hold for 3mins, then ramp at 4degC/min to 60deg C and hold for 2 mins. Significant Tailing was observed.

I had tried before both pentane and Heptane as IS, however, the retention time is just too close (overlapping) therefore I opt for other IS like Chloroform.

my question:
- with the above procedure I had done, Is that mean that my 100um PDMS fiber is damage? or how can I recover its function?
-Is the above method appropriate for hexane quantification (except I shd not use chloroform as IS)? I have other fiber like PDMS-DVB; carboxen-PDMS; PDMS-DVB-carboxen -- are there better fiber to be use?
- Do you know any establish method for hexane analysis?

Thanks for the expert advice. You are so nice.

Rgds
AngelDevil

[chromatographer1]

I would stay with an alkane, perhaps try C9, C10, or even C12. A alkene could also be used to increase the separation of hexane from the IS. Even so, lighter alkanes won't be separated by much using a wax column.

Another column as mentioned before would be helpful.

best wishes,

Rodney George
consultant

[chromatographer1]

I would stay with an alkane, perhaps try C9, C10, or even C12. A alkene could also be used to increase the separation of hexane from the IS. Even so, lighter alkanes won't be separated by much using a wax column.

Another column as mentioned before would be helpful.

best wishes,

Rodney George
consultant

[zokitano]

AngelDevil,

I agree with previous posters that more suitable GC columns for your present purposes are -1, -5 or -624 phase.

If you noticed a reduction in the extraction performance of your 100um PDMS fiber (you can test it with the standard solution and compare the areas under peak of the analyte that you've got previously and now) you may observe degradation of the fiber, but this degradation is regulary dependent on the number of SPME done, temperatures of desorption, quality of used samples/standards etc. Don't believe that your fiber's coating is stripped as long as it is still in use and you haven't observed any coating material removed from the fiber. Remember that if your fiber coating is physically damaged it cannot be restored. You will need a new fiber!

You should try to search the web for any articles describing a method alike yours. You can use all your other fibers (PDMS-DVB, CAR-PDMS...) and see if they could serve your needs. Remember that with the PDMS-only coating you'll extract your analytes through liquid-liquid partitioning mechanism (absorption), while when using the "hybrid" coatings (e.g PDMS-DVB) the mechanism of extraction is absorption and adsorption. The hybrid coatings can show limited linear range in the upper concentrations (adsorbent saturation) so you should also consider this fact when you'll try to quantify your analytes in high conc. range.
Hybrid coatings can work better for analysis of low mass analytes in very low (ppb or less) concentrations, which are more efficiently extracted via adsorption mechanism than with absorption (compared to the PDMS-only fibers).

Best regards.