Methanol at low ppb level

[danicrd]

Hi
for a new project we need to detect methanol at low ppb level: 5 to 50ppb, in drinking water.
We have two configuration available: Varian 3800 with fid, equipped with S/SL inj and wax column and PE turbomatrix connected to 3800 with fid and MS. We have two colum in the GC, a db-wax 30m*0.25*1.2df and a PONA.
Currently we are able to reach 100ppb, using headspace.
We incubate samples for 12' at 85°C, and we use 20mL vials with 10mL of water.
Fid is held at "normal" condition, with 30ml H2, 300 air, 38 makeup.
In your opinion is possible to reach 5ppb with fid?
Now we get microvolts of signal with 100 ppb, but the peak is very, very little
What are parameters to optimize? Detector or sampler?
Thank's to all for suggestion.

[chromatographer1]

I was able to see much less than 1ng of methanol in 25 microliters of water in 1997 or 40 ppb using FID. I actually could see perhaps 5-10 as the lower limit. That was with 400 times less sample than the 10,000 microliters you are now using.

I could have reduced the LOD by half by doubling the loop but that would have been very close to the maximum possible with those conditions. The PE HS40
uses a timed injection system which should be able to duplicate the injection.

See
1 June 1997 issue of Analytical Chemistry, Vol 69 No. 11 last article p2221

I suspect that using a column set similar to what I used or at least the 5 meter attachment to the analytical column you will be able to reach your goal in your reduce your sample size down to 1mL and use a 6mL vial and use the GC conditions I used. PE has the adapters you will need.

I think you will be able to meet your goal. Note that benzene will elute very close to methanol with the column you presently use and benzene is everywhere at very low levels.

best wishes,

Rodney George
chromatographer1 @ aol

[oscarBAL]

Hello Dani.
I think SPME is more sencitive that HS; but you would have to select very well the Fiber.
I am not sure but you could try in case HS doesn't work.

[zokitano]

I would suggest you to try what chromatographer1 have previously said. If you have HS sampler than get the Anal.Chem. article and try to reproduce the conditions given there.

If you want to spare extra money or already have the SPME equipment you may try what oscarbal said (personally I think that 65um Carbowax/divinylbenzene fiber would do the job for methanol).

Although SPME is very sensitive technique it requires basic SPME knowledge and experience. If you're more familiar with conventional (static) headspace sampling, then try to do what previously was suggested by chromatographer1.

Regards

[zokitano]

If you are considering SPME for methanol analysis I think you'll find these articles useful:

High-speed gas chromatographic analysis of solvents in pharmaceuticals using solid phase microextraction, Journal of Pharmaceutical and Biomedical Analysis, Volume 29, Issue 3, 1 July 2002, Pages 507-518
A. R. Raghani

Headspace SPME method development for the analysis of volatile polar residual solvents by GC-MS, Journal of Pharmaceutical and Biomedical Analysis, Volume 23, Issue 1, 1 August 2000, Pages 197-210
Costin C. Camarasu

Residual solvents determination in pharmaceutical products by GC-HS and GC-MS–SPME, Journal of Pharmaceutical and Biomedical Analysis, Volume 18, Issues 4-5, 1 December 1998, Pages 623-638
Costin C. Camarasu, Mária Mezei-Szuts, Gábor Bertók Varga

[chromatographer1]

Zokitano's suggestions are excellent and I agree that your goal may be possible and perhaps even more easily attained using SPME. The reproducibility may be difficult to equal that capable in static HS. There are not always clear cut choices and this is one occasion where circumstances can have a great bearing upon the success of the effort.

Good luck and please share your results with the Forum.

chromatographer1 at Aol

[fsistere]

Hi

You can add 20% of NaCl to increment the extraction, and you can try study the incubation temperature. Perhaps 85ºC is too high. Sometimes there is better response at slow temperature. Study incubation time too.

[danicrd]

thanks to all for suggestion. I will let you know my results...