By ves on Wednesday, October 22, 2003 - 12:47 pm:
I want to analyze the acids in powdered milk - acetic, propionic, butyric, lactic, etc...the small acids. I have a rp-amide-c16 column to do the separation, PDA and/or elsd to do the detection. How might I do an extraction for this sample?
By Uwe Neue on Wednesday, October 22, 2003 - 03:40 pm:
I would do this via SPE with a mixed-mode anion exchanger. Dissolve the sample, adjust the pH to alkaline (best with ammonia). Take an Oasis MAX cartridge. Load the sample onto the cartridge after wetting the cartridge with water or better water with ammonia. Wash with maybe 50/50 methanol/water at alkaline pH. Then elute with 50/50 methanol/acidified water. You can find the background for this suggestion on the Waters website.
By Einar Pontén - SeQuant AB on Wednesday, October 22, 2003 - 04:31 pm:
Is it really that simple. I would suspect that the SPE is clogged by milk proteins and fat.
Don't you need an initial centrifugation to get a clear solution between the fat on the surface and precipitated proteins on the bottom? My experience is that in some cases a filtration through a centrifugation filter-tube could do the job.
More comments on this subject would be interesting.
By ves on Thursday, October 23, 2003 - 05:45 am:
I have oasis hlb and have coming Discovery DSC-NH2. Might one of these work?
By Anonymous on Thursday, October 23, 2003 - 10:32 am:
Could the sample be contaminated with impurities in the solvents used for SPE? I am concerned that slvents might contain some acetic acid etc, which could contribute significantly to the analytical results.
By Uwe Neue on Thursday, October 23, 2003 - 02:56 pm:
ves,
I don't think that it will work on Oasis HLB, since there is no specific mechanism to retain your analytes. The Discovery DSC NH2 is not a quaternary amine, thus it may be more tricky to get the analytes retained. Also, I would think that it is silica-based, so I would not recommend the protocol that I suggested.
Einar,
we have been doing this type of sample prep with blood samples, food samples, environmental samples. I can send you an article on this, if you want me to.
SPE can be very simple if you think it through.
By ves on Friday, October 24, 2003 - 05:27 am:
Uwe,
please send the article.
shirleyv@ncat.edu
By Markus Laeubli, Metrohm on Monday, October 27, 2003 - 10:51 pm:
Metrohm uses Inline Sample Dialysis for such samples. We did anions, cations and carbohydrates in milk and milk powder without any problem of clogging the system.
For more details see:
http://www.metrohm.com/applications/ic/misp_index.html
The following IC Application Notes show these examples:
S-44, S-162, N-18, C-28, P-5