I am extracting very volatile compounds from solid foods using Headspace Solid Phase Microextraction then analyse the extracts using GC-FID. So currently I'm trying to develop a method but struggling in-terms spiking in order to calculate recoveries.
How you prepare your sample? Any homogenisation or milling prior to analysis is made?
Generally it will be hard task - somw volatiles remain in solid material and their availability to be emitted to the headspace during short thermostating is limited.
Regards
G
This will be very hard to do! 1st to have to ensure that your HS-GC works by using a standard in blank (no food). 2nd you have to spike your food sample with your VOCs standard and keep the sample cold in order to reduce volatility. 3rd you have to determine your %Recovery (spike sample - unspiked sample) in your food samples.
A possible approach for spiking is to use standard substances diluted in high boiling solvents and to apply them with a syringe on the food matrix. For this purpuse we use some high boiling alkanes for lipophilic substances or dimethylacetamide for more polar substances. Details may vary depending on the target compounds....
SPME is a very complex method if you use it for solid food. The partition coefficient between SPME phase and food may vary dramatically.
Grzegorz Boczkaj yes there is grinding of the samples prior to analysis.
Bruce Neagle how do i spike them?
Ludwig Gruber thank you so much will definitely try that approach.
The approach given by Ludwig is the simplest one. The main issue- volatiles in solid particles are somewhow "trapped" and will be emitted slower and not quantitatively. Thus the recovery determined on the basis of injected standard will be overestimated.
It depends! You should have determined your linear range and your LOQ, thus you select the concentration at the midpoint. Using the highest concentration of the standard (example 100 ppm), you spike (add) this standard to the sample as to obtain the expected or spiked concentration using your method. Thus;
% Recovery =[(found concentration using method - spiked concentration) x 100]
(all divided by spiked concentration) Should be 100.0% if no VOCs are present!
Difference = 102.3% - 100.0% The difference of 2.3% was the concentration of your original sample. before spiking.
My previous answer has a MAJOR flaw! What happens when your %Recovery (as an example) is only 85.0% (or 85.0/100.0)! This can occur if your VOC is bound or your method is bad. Then;
found unspiked concentration of sample/0.85 = calculated concentration
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