Chromatography Forum

such as volatile compounds in cereal. Is it possible to quantify them? is it so difficult to get the same matrix effect on external standards and samples.

It depends...

SPME is all about partitioning: The ratio of component concentrations between phases. If you have a component that will parition strongly into the SPME fiber - you have a chance. But, even there if you have a lot of some other component in the mix that also partitions into the fiber - that other component can sufficiently saturate the fiber that its nature changes - and the partition coefficient of your analyte of interest between the fiber and the matrix also changes. I've seen this with tobacco products. SPME of a non-menthol tobacco can give a very intersting profile. But of mentholated tobacco -- well, you see mostly menthol.

Now the matrix - how homogenious is it. If the fat content changes from one cerial sample to the next - the mass of fat changes and while the ratio of conentrations may be constant - the concentrations change because the "solvent volume" provided by the fat (or protien, or starch or...) has changed. (and the volume of the SPME fiber remains the same) Or there may be large pieces of crushed grain in the product - but differing masses from sample to sample.

You must consider the analyte, the matrix, and the fiber. And you must consider other volatiles which can saturate the fiber.

For the specific analyte and specific matrix, you have to do some testing to see. Personally, If I were to go with SPME for quantificaiton, I would keep in mind isotope dilution for quantification. Using the 13C labeled compounds, you will have little difference in partition coefficients between native and labeled compounds.

And keep in mind equilibration time. If you are going to put solid particles in to the sample vial, how long does it take to reach equilibrium through to the center of the cerial particles. (And if you grind it really small, how much of the volatiles are left at the exit from the mill?)

If you add water, you may improve diffusion and partitioning for some compounds.

What are the materials you want to determine in cereal?

Quantitation with SPME can be troublesome because the matrix has such a large effect on what and how much is binding to the fiber as everything is competing with eachother for space on the fiber, also the condition of the fiber and the adsorption equilibrium of different compounds for the fiber varies.

That said I did quantitation of compounds in oatmeal via SPME. Some compounds had low precission and linearity (hexanal). Others work very well such as pyrazines. Use a good internal standard and also get something that resembles a blank matrix for your calibration.