I have developed a method for analysis of Isovaleric acid absorption capacity for various dry powder materials using headspace sampling GCMS. External calibration standards are prepared from 20-120 uls isovaleric acid in 5%NaCl in water. A small sample of the material in question is then dispersed in 5%NaCl and loaded with isovaleric acid. The residual isovaleric acid measured from the linear calibration curve is subtracted from the amount added to determine the absorption capacity.
So, my question is this: I would like to add an internal standard to the method to help cope with MS detector drift. The internal standard has to be something which would not be absorbed by the material under examination - most oxygenated compounds, esters, alcohols, ketones tend to be absorbed.
I've experimented with adding a small amount (1ul) of CCl4 to each vial, but peak area for 1ul CCl4 under the headspace conditions is huge. I can't think of a way to reproducibly add 0.1ul to each vial as internal standard.
Anybody got any ideas?
By Anonymous on Friday, December 13, 2002 - 07:12 am:
How about using something like acetonitrile and adding it to the 5%NaCl solution, keeping the volume of the NaCl solution in the headspace vials constant of course. Just a thought if I understood your method.
By Anonymous on Friday, December 13, 2002 - 08:56 am:
That's a good idea, would eliminate a step in sample prep. Right now, I have a solution of 50ul CCl4 in 20mls acetone. I add 20uls of this internal standard to each vial and ratio the isovaleric acid area to CCl4 area. I guess I picked CCl4 because I wouldn't expect it to interact with test materials. It seems to work but it is an extra step. The thing to do would be to make up the 5%NaCl with the appropriate amount of this solution added to it.
Thanks for the idea.
By Anonymous on Friday, December 13, 2002 - 11:12 am:
We've used something similar here for a different application, with n-propyl alcohol internal standard in water, which combines our internal standard with the dilution solvent. We pre-mix that, then use a Hamilton dispenser to dispense exactly the same amount of into each sample.
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