I am new to handling a GC-MS and am trying to analyze the headspace from meat packets. I am using a SPME filament PDMS (non-polar) and carb/pdms (bi-polar) filaments as most of the volatiles I am concerned with are sulfides, hydrocarbons, esters, ketones and alcohols. I would like to know the proper method for sampling the headspace using the filament. The meat samples are placed in vials and are left outside (30 C). I heat the raw meat (in vials) at 50C in a hotplate for 20 minutes and then sample the headspace at the same temperature for 3 minutes.
But I find very small peaks or no peaks at all, even though I know the meat is spoiled.
I am using a 30m long and 0.25 mm dia column coated with 0.25 microns 5% phenyl and 95 dimethyl polysiloxane. Its a non-polar column.
Can anyone give me suggestions as to how I should proceed in sampling the headspace, transfer it to the GC inlet and also if I should go for a longer column with a bigger diameter and more micron thickness?
I also feel that I have to apply more pressure to puncture the vial top. Will this damage the filament?
Can anyone please help me out with your valuable suggestions??
By Anonymous on Thursday, August 7, 2003 - 03:43 pm:
I have done a lot of spme, but not, unfortunately, of meat. You should probably collect for more than 3 minutes. For some applications, an hour is necessary. Changing the column will not help increase your peak area and your column should be suitable for a wide range of compounds. I wouldn't go out and buy a new one until I had improved my sample collection and found that I required better resolution.
I have never damaged a filament by puncturing a septum as long as the fiber was retracted into the holder. If you are using screw top vials, it may help to loosen the caps a bit.
By Anonymous on Friday, August 8, 2003 - 07:43 am:
Thanks a lot for your advice. I have another doubt. I tried collecting my sample for 15 minutes at 60 C. Once I retracted the filament and removed the filament from the septum, I heard a loud "whooosh " sound, like some pressure is being released. Do you think all the sample which I collected will get pushed out because of this?
The run which I did after collecting that sample, did not show me proper results.
Can you suggest the proper method for sample collection ? I would appreciate your response.
By Anonymous on Friday, August 8, 2003 - 07:47 am:
Perhaps the gases you wish to discover are more volatile than you expect or more sensitive to the human nose than the collection fibers you are using.
I would try using static headspace instead of SPME and see if you get better results.
Also be certain that you are measuring the masses of components generated by the spoiled meat. If you don't scan the masses being produced then you will see nothing.
By Anonymous on Friday, August 8, 2003 - 08:49 am:
Thanks again. I had tried static headspace sampling before, but because the concentration of gases emitted was in the range of parts per billion or even lower, I couldn't detect any appreciable quantity.
So I reverted to SPME. I saw some papers on the use of SPME and also on the use of Tenax trap to sample the volatiles from meat.
Is there any suggestions you can offer on how to use an SPME fiber with the maximum efficiency?
By Anonymous on Friday, August 8, 2003 - 09:44 am:
With regards to your "whooshing". Since you have already adsorbed your sample to your fiber, you shouldn't lose anything upon withdrawal. However if this pressure build up is worrying you then you can release it by inserting a narrow bore syringe needle through your septum for a second or two. Your loss of sample from the headspace should be insignificant and if you treat all your samples exactly the same way, it shouldn't matter.
Keep increasing your collection time beyond 15 minutes, Also, to increase the sensitivity of the technique, you could try varying the sample/headspace ratio or experiment with different sized collection vessels, to allow a larger sample size.
Protocol development is a struggle sometimes, but I feel sure spme will work for you. Don't give up!
By Anonymous on Monday, August 11, 2003 - 06:15 am:
In addition to the above, I use a soft septum and pre-pierce with an old SPME needle to avoid accidentally bending the needle. Also, make sure you are using a narrow bore SPME liner to sharpen the peaks.
I don't have experience of tainted meat but from my background reading I recall that microbial taints will vary with type of meat, the micro-organisms involved and the storage (vacuum packed or aerobically stored). Have you tried selective ion monitoring for the expected sulphur species, short chain acids and esters? Your nose should give you some further clues. For this analysis I would prefer to use a polar column.
Initially, you may need to use a dynamic purge and trap onto Tenax to get enough sample to identify - then you can use SPME with SIM. - but as mentioned in the previous reply don't give up on the SPME yet.
Out of interest, what hydrocarbons would you expect?
By Anonymous on Monday, August 11, 2003 - 11:57 am:
Thanks a lot for the encouraging replies. I expect to see decane, undecane, methyl sulfides, nonanal, ethyl esters, and some other aldehydes and ketones. Mostly I expect to see higher aliphatic hydrocarbons and their derivatives. From most literature, I find that people have used a non-polar column for their analysis.
Now comes another question, does the film thickness in the column affect the analysis? Do I need a thicker film (more than 1 microns thick stationary phase in the column)??
By Anonymous on Tuesday, August 12, 2003 - 04:28 am:
Phase selection is sometimes a matter of personal choice (and what you have available!). In the malodours I have looked at a non-polar column can give tailing peaks.
Briefly, with respect to your analysis, stick with a narrow film to keep the peaks sharp.
Thicker films are useful, for example:
1.olefactometry where broader peaks give you more time to sniff.
2. reduced adsorption for certain compounds
3. higher sample capacity before overloading
You have more experience than me on meat taints but I would suspect that most of your microbiological metabolite taint odours are volatile short chain acids, esters, sulphur species and some short chain alkene