Choosing a silylating reagent

[MSCHemist]

I have a wide variety of analytes I'd like to do on the GC by silyating them. I am a bit bewildered by the wide assortment of different reagents HMDS, BSA, BSTFA.

Here is a list of possible analytes
Inosinate and guanylate
retinol and other vitamins
sugars

Which one should I choose and what is the best way of getting extracting the analytes out of an aqueous solution.

[Consumer Products Guy]

I'd contact Regis, Pierce, or Supelco tech service and ask them. We tend to use BSTFA for most of our silylation reactions here.

[chromatographer1]

My opinion: depending whether you are trying to measure trace amounts accurately, or to determine purity of each analyte (probably the former, not the latter)

from aqueous solutions silylation is not recommended with any reagent unless you can be certain that there is an excess of reagent to react with analyte AND with the water.

Inosinate and guanylate

These would be better analyzed by HPLC or TLC instead of GC. Stability issues are paramount.

retinol and other vitamins

These would be better analyzed by HPLC or TLC instead of GC.

sugars

Sweeley reagent is the historical preference for analysis by GC. But MSTFA might be the better solution. If the amount of sugars are at very low levels in water then drying the water rather than extracting the sugars might be the best route.

best wishes,

Rod

[MSCHemist]

I know I can't do it in aqueous solution I was asking what is the best way to get the polar analytes out, extract with chloroform, dry it in an oven, Solid phase cleanup.

I'm one of those people who are trying to do stuff on a GC that would be better done on an HPLC but I don't have an HPLC and won't anytime in the near future but I'd like to have as much capability as possible.

Sweeley is HMDS +TMCS catalyst I read that is recommended for sugars but is one of the weaker silylating reagents.

Thanks for the guidance BTW

[chromatographer1]

I would dry the sample under a flow of nitrogen until almost all of the water is evaporated, then add MSTFA. It depends upon how much water is left in your sample and the levels you wish to measure of the analytes you seek.

The Sweeley reagent is good for sugars.

The other analytes I would use TLC unless exact measurement is needed, but 0.1% of an analyte dissolved in a 5% (50mg/mL) aqueous solution is easily detected with the analytes you have given, including the sugars. This is a low cost analysis.

But you have not mentioned the amounts you wish to measure..... 1 ppm or 1% by wt or by vol ?

I know when all you have is a hammer that everything looks like a nail, but sometimes things just do not work out.

best wishes,

Rod

[MSCHemist]

These aren't trace levels. I am doing glutamate/MSG with ethyl chloroformate derivitization and my samples are averaging 10% I am diluting 1000 fold. I imagine the other anaytes will be in the ppt range at least.

The main tricky point is 80% of my samples are starch encapsulated and most organic solvents including MeCl2, alkanes, ethyl acetate won't extract the analytes. Warm water is the only thing that disperses it so I need to dissolve first in water to disperse the encapsulation. It is possible DMF might work I've never tried it otherwise I'd dissolve right in pyridine.

[chromatographer1]

Both of these are excellent solvents for silylation.

ppt you say !

good luck with that.

Rod

[MSCHemist]

yep a lot of my work is in supporting duplications of concentrated meat flavors. MSG is a big one. Inosinate and guanylate are also big. Then the typical what's in it and approximately how much scan samples.

I'm working on capsaicinoids and piperine. I thought I might need to silylate them but it appears unnecessary.

I do a lot of BHA and BHT quantitation now on the GC/MS. Not sure why everyone does it on the HPLC it works just fine diluted heavily in ethanol and the runtime is just 20 min.

The only real trace I do I 3-monochloropropanediol that method is a big pain for the derivitization. Dissolve in water, put through a diatomaceous earth SPE column, blow down, derivitize with HFBI, extract the reagent with 3Xwater, then run on the GC and the sample peak shape and separation between 2 and 3 mcpd sucks.

[MSCHemist]

I think I will go with BSTFA + TMCS in pyridine. Fisher recommends TMCI, Sigma states HMDS + TMCS, BSTFA + TMCS, or BSA + TMCS all work for nucleotides. It also looks good for sugars if I decide to get into it.

It looks like retinol has been done by GCMS by derivitizing it to its tert-butyl silyl ester rather than its trimethylsilyl ester so MTBSTFA. It is reported retinol/vitamin A is stable up to a year after derivitization.
http://www-infocris.iaea.org/ididas/w3. ... 39&ID=1749

[chromatographer1]

If you use your own pyridine, be sure to distill it under basic conditions and keep it as dry as possible before you add the silylating reagents.

We used to do a once a year distilllation in January (cold and dry air) before making the Sweeley reagent for commercial sales to avoid spurious peaks we would otherwise produce in the reagent.

Good luck,

Rod

[MSCHemist]

Wouldn't it just be easier to toss in a little anhydrous Na2SO4 into the reaction media to trap the water or store the reagents in small containers with some at the bottom.

I may need to use some excess because it will react with the starch from the encapsulation and that might consume a fair amount of the reagent.

[Consumer Products Guy]

We use 5 gram bottles of BSTFA containg TMCS. Typically we use DMF or pyridine as the sample solvent.

We used NaOH pellets in the bottle back when we made Tri-Sil in pyridine, but that was back in the 1970s. I changed us to BSTFA with TMCS decades ago. We have used MTBSTFA occasionally.

[Don_Hilton]

Na2SO4 is not as strong a dessicant as you want for this.