I am collecting samples on Quartz fiber filters n then doing quantification with TD-GC/MS.I got Levoglucosenone,Benzene dicarboxylic acid ,diols,Ketones instead of alkanes.Rather I want to see alkanes and Levoglucosan.Please help how can i increase my quality of result?
Thank you Sir,
Could you please tell me the procedure of derivatization in detail?
In my experience of TD GCMS, never had problem of alkane to be oxidized. The major problems for direct TD GCMS for samples collected on filter are (1) retention time shift and (2) lose separation due to polar components in the samples.
Why not to test "Carbopack" filling for absorption of alkanes and levoglucosane, Sarika?
As for sililation of glass wool: put a few gams of wool into the small flask and add 1 ml of a mixture of TMS and HMDS /1:4/. Close hermetically the flask and put iit n oven preheated to 100 oC. The reaction is completed within 10 to 20 min.
Let the contents to cool to ambient temperature, add carefully a few ml of anhydrous ethanol to decompose sililation mixture.
Dry the wool for 1 h at 80 oC, and then keep it in an exicator. All steps of derivatization should be performed in a digestorium /sil. agent sare strong irritants!/.
Test derivatized glass wool for residual traces of ethanol and possible by-products via GLC.
Good luck.
The oxygenates that you see are very very unlikely to be formed by oxidation of alkanes in air. You are using the wrong kind of filter; alkanes will not stick to bare quartz, follow Mario's suggestion.
But Peter Sir and Mario sir,many people are doing the same alkane analysis with quartz fiber filters and getting good results.Please suggest .........
Sarika,
Are you using the filters to sample suspended solids, and then analyzing the solids, or are you hoping that alkanes will adsorb onto the quartz ?. Are you filtering a liquid or a gas ? You give so few details that it is very difficult to make suggestions.
Physical filtration of solids should work. Adsorption will not work. Alkanes are hydrophobic, quartz is hydrophillic therefore alkanes adsorb poorly onto quartz.
It is very unlikely that alkanes are dissolved in an aqueous sample, if they are present they will be adsorbed onto hydrophobic particles. Alkanes might be present as vapor in an air sample, but there also they tend to partition onto particles.
Why do you expect to see alkanes rather than the compounds that you actually do see ?
I suggest the alkenes generate ketones in air, alkanes not so much unless really hot. Alkanes are much less likely to oxidize, which is why we get long chain oil molecules out of the ground in petroleum. As Peter says above, alkanes are hydrophobic. This is why you should use an organic-coated solid adsorbent surface. Alternatively, I successfully analyzed many branched hydrocarbons, alkanes and alkenes from individual 100 year old tsetse flies from the London Museum of Natural History by simple hexane extraction and GC/MS.
If Sarika is looking for alkanes in gas phase, quartz fiber filter is a very poor choice for sample collection. If Sarika is looking for alkanes in particulate matters, direct TD GCMS can only work for a few samples. If you keep desorbing quartz fiber filter with particulate matters and run GCMS, you will see that the retention time and baseline shift as well as peak shape will become unacceptable very quickly. The alkane oxidation is not a issue. If you did not find alkanes in your direct TD GCMS, it is not because alkanes are oxidized, it is because the alkanes peaks are under other background. The better way to run this kind of samples for alkanes (C16 and up) is to extract the samples collected on to quartz fiber filter using DCM, remove the polar components, then run GCMS. If Sarika is looking for alkanes (C15 and under), those alkanes are presenting primarily in gas phase or at least significantly portion in gas phase for ambient samples. You will have to design the sample collection to collect both gas and partculate matters.
DCM (dichloromethane) should be good for solvent if it is research grade...that is: distilled in glass to remove high boilers. I recently checked by GC/MS some mixed hexane reagent grade, which had a significant amount of C12, C14 and C16 alkanes.
Thank you Wentai luo,Yes i am looking for alkanes in particulate matter i.e.PM 10 in ambient air,by collecting them on Quartz filter paper with the help of medium volume sampler.I like your suggestion very much.could you please assure me ,If DCM extraction method would be great suitable for analysis of higher alkanes as it has many losses during extraction then direct TD-GC/MS.
Thanks,
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