Chromatography Forum

Hi all
I have issue with my method on HS-SPME, the ISTD that I use shows inconsistent results! I spiked 3 samples to check recovery of them and ISTD is different in all those 3! sample matrix and every thing is the same. I desorb the fiber for 8min on column and after have 5min post desorb conditioning.
Is it the fiber? what I'm missing?
Your help is highly appreciated.
Thanks in advance.

Liquid or solid sample? What analyte(s)? Chromatographic conditions?

If sample is liquid, what happens if you sample just water and your internal standard? I always try to go back and start as simply as possible to rule out the GC system itself as a problem. After I'm sure the instrument is performing as it should, I try to figure out the issues I have with the sample or sampling.

Liquid or solid sample? What analyte(s)? Chromatographic conditions?

If sample is liquid, what happens if you sample just water and your internal standard? I always try to go back and start as simply as possible to rule out the GC system itself as a problem. After I'm sure the instrument is performing as it should, I try to figure out the issues I have with the sample or sampling.

Hi
Liquid samples. It's not the instrument, I am mot sure if its the sample matrix, splash of sample on fiber or what. Lets say if I inject 20 samples 3 of them, randomly show higher conc. this is happening in wine samples ( I'm assuming it's due to the ethanol in the sample as same method with grape juice is doing ok) but how can I cancel the effect of ethanol?

I feel your pain. Beer is my matrix and ethanol is always there. In beer, however, it's more dilute so I don't see it as being as big a problem.

Are you looking for trace levels of your analytes or could you live with some dilution of the sample? If wine is ~13%, 2:5 dilution gets you to 5%. Something that's 100 ppb in the wine is still 40 ppb in the dilution.

Sometimes, if you dilute the solvent-based sample (like beer, wine, and spirits) with water, you improve the partitioning behavior of your analyte - favoring the headspace. The effects of the dilution are not as detrimental to your ability to detect your analyte as they might seem at first thought.

Are all the samples the same % alcohol or do some have higher and some lower and does the enhanced ISTD recovery equate to either?

If the high recovery equates to the high %alcohol, do you make the standards in a similar %alcohol?

As above, dilution can help make the matrix more equal or you have to make the standard matrix match the sample matrix.