I am a graduate student working on headspace analysis of fresh meat. I am trying to comeup with a method in which I can use a syringe for static headpsace anlysis (instead of SPME technique). However I am not seeing any distinct peaks inspite of the fact that the meat is spoiled.
I am interested in gases like Decane, undecane, ethyl acetate, pentanol, nonanol, methyl-butanone, xylenes, hydrogen sulfide, etc.
The column I am using is a 30 m long , 0.25 mm dia column with 0.25 micron thick coating of 5% phenyl (XTi-5 , Restek Corp capillary column).
Can anyone give me some suggestions as to how I should conduct my experiments to notice some distinct peaks (any peak is fine)? How should I draw the sample from the headspace vials? Should I boil the meat piece?
I would appreciate your response.
By Anonymous on Friday, April 25, 2003 - 03:00 am:
The crucial point is getting enough analytes into you syringe. The key to this is increasing the temperature.
Maybe you should look for a lab that has a HS autosampler with a closed HS system (e.g. HP or PE) autosampler and ask them to do a run for you.
By Ralph Calvert on Friday, April 25, 2003 - 03:27 am:
What detector will you be using? Are you hoping to analyse the volatiles from spoilt/tainted meat rather than fresh meat?
My reason for asking is because we have evolved a very sensitive nose to odours which come from microbially contaminated meat as a protection mechanism. This means that we can detect amines and sulphides (for example)at very low levels and why we all find them disgusting. Our nose can detect contamination even at a level which would not be harmful to us. This also makes detection more difficult and requires sensitive or selective detectors.
Assuming it is taint volatiles you are after -some taint amines (apart from indole) are perhaps better analysed by HPLC. In GC sulphides are better detected with a sulphur selective detector or MS. For short chain acids, especially acetic acid I would also prefer to use a polar column e.g. Innowax.
You may need to increase the amount getting onto the column. This means
a)increasing the spoilage before sampling
b)injecting larger volume or injecting several lots from a few vials and allow them to condense onto a cold column before commencing your temperature progam.
c) consider flushing the headspace onto a tube packed with an adsorbent such as Tenax (say 20mg). You can even suspend the adsorbent in a closed headspace vial (static sampling) for several hours.Then transfer the adsorbent into your injection port liner and desorb. I have done this quite successfully many times for other low level taints and can give you details.
A good background to taints can be found here
Microbiology of food taints
INTERNATIONAL JOURNAL OF FOOD SCIENCE AND TECHNOLOGY
33 (1): 31-51 FEB 1998
Document type: Review Language: English Cited References: 169 Times Cited: 6
Fresh and processed foods are often spoilt by the presence of undesirable flavours and odours caused by microbial action. The aim of this paper is to review the current knowledge of microbiologically induced taints that occur in a wide range of foodstuffs, including meats, poultry, fish, crustaceans, milk, dairy products, fruits, vegetables, cereals and cereal products. Examples have been chosen where the compounds responsible for the taint have been identified and sufficient data obtained to demonstrate the involvement of microorganisms. However, in some cases the full identity of the causative organism may not have been elucidated. The types of microorganisms covered by this review include bacteria, fungi, yeasts, actinomycetes and cyanobacteria. Although cyanobacteria do not in general infect foods, their presence in aqueous systems and water supplies can lead to off-flavours in aquatic organisms and processed foodstuffs. Several examples of each of these processes are discussed. Wherever possible, the likely biosynthetic pathway used by the microorganism to produce the offending compound in a foodstuff is indicated.
By sundar on Friday, April 25, 2003 - 09:33 am:
Ralph and Mr.Anonymous, Thanks a lot for your suggestions. It gives me some hope. I will try injecting a larger volume of sample headspace. So far I am using only 0.3 mL.
The detector I am using is a Mass spec. We decided on using the syringe for convenience sake. But it looks like I will not be able to see any response by using a syringe for sampling.
Can you help me out and let me know if a larger volume and higher temperature (during sampling) will help me out?
By Ralph on Sunday, April 27, 2003 - 09:17 am:
A larger volume and a higher temperature will help. Use selected ion monitoring of the compounds of interest to improve sensitivity e.g. I seem to recall that m/z 47 is found in most sulphides
By Anonymous on Monday, April 28, 2003 - 03:32 am:
You did not mention that you have tried using any standards yet! Do these work? You can get some results just by equipping an empty clean 2.5 litre solvent bottle with a septum cap and injecting a microlitre or two of liquid in the bottle and allowing it to evaporate. Then inject some of the gas from the bottle into your GC. If you can't see anything from these standards then you certainly wil not see the same substances from your samples.
It's possible that the analytes are condensing inside your cold syringe and not being transferred to the GC. This is why headspace is usually done with more sophisticated sytems
By Anonymous on Monday, April 28, 2003 - 03:35 am:
Also, try it with GC-FID first. If you have an old mass spectrometer, it will be a lot less sensitive than an FID, although modern systems can be more sensitive. Eliminate this possibility as soon as possible.
By ralph on Monday, April 28, 2003 - 07:00 am:
that's three good suggestions from David. Why don't you want to use SPME? Additionally, if you want to try solid phase macro extraction e.g tenax tubes I can e-mail you a presentation showing various applications I have tried with a simple 0.8ul autosampler vial with 20mg Tenax as adsorbent used in static headspace analysis.
By Anonymous on Monday, April 28, 2003 - 04:09 pm:
You should be using SPME. It was designed for applications such as yours and is adaptable to any GC. You can collect for an extended period of time and thereby concentrate the volatiles onto the fiber. (Do not boil your meat sample, this will create totally different volatiles than you find in raw meat!)
By Anonymous on Monday, May 12, 2003 - 01:52 pm:
An additional problem not yet mentioned is that most syringes are poorly suited to headspace work. An "off-the-rack" syringe, particularly one that has been used for other work before trying it for headspace, is usually extrememly adsorptive with gas samples. I understand the desire to keep costs down in your situation, but at least spring for a new syringe. Then use a deactivation reagent on it. Finally, the syringe should be as close to your sample temperature as possible. However, most syringes have a temperature limit lower than I suspect you'll want to use. Just get as close as you can with the temperature, and take the sample and inject it quickly, before the syringe cools significantly.
By Kevin on Wednesday, May 14, 2003 - 08:52 am:
If you wouldn't mind I'd like to see a copy of that presentation. Could you please email that to me ??
If not, I understand.. Thanks