Chromatography Forum

Hello,

Does anyone know what is the use of acidic or basic additives (DEA or TFA etc) in normal phase chiral chromatography?
Everywhere i looked , i read that the use or the mechanism is not well defined.
Do you know why we use them? It is a matter of PH or maybe "masking agent" of some short.
Could someone explain to me why we add these solvents ?

They are added for the same reasons we sometimes use them for achiral chromatography. To limit the ionized forms of the molecule. For NP chiral MD, always start with neutral conditions.

BTW: Be careful when using them and just as with ion-pairing agents, label any chiral column which has been used with additives. You should not use them for other methods, only those with the same additives as their surface chemistry may be changed, forever. *To insure that we can always reproduce any chiral method developed, our lab maintains three sets of each type of chiral column (neutral, acidic and basic). We never mix them.

Thanks for the answer!

In the future i will have to seperate the chiral enantiomers of sertraline as per EP, using a Chiral AD column (tris-amylose). The MP contains Hexane/Isopropanol plus about 0.7% Diethylamine.
So the additive , the analyte and the stationary phase are potentially basic.
How in your opinion the DEA acts?

Your question about DEA was answered above. These are very basic HPLC questions. Most any good chromatography text can provide more details.

As also noted before: If you use the Chiral AD 'coated' phase column with DEA, then that column will need to be labeled as for use with DEA only. It will never provide reliable data again for other sample types which do not require DEA in the mobile phase. Do not use that column under acid or neutral conoditions as it will provide unreliable methods (That is why we dedicate them to specific analysis methods). By a new column for any other work.
Be very careful when eqilibrating the AD in the Hex/IPA mix. It can take a very long time to stabilize (normal) and make sure the sample is fully dissolved in the injection solution before analysis.

Thanks again for the answer.

I'm confused about some things anyway.

We add DEA or TEA to mask the acidic silanols, to elevate the PH so the basic analyte will be non ionized or both?
We add TFA to drop the PH so the acidic analyte will be non ionized, to protonate the acidic silanols, or both?
I understand TFA is an ion pair reagent too, isn't it. Has something to do with that?
In addition, i wasn't sure that the same sort of mechanisms with these additives, are similar when working RP methods with a C18 and NP chiral method with chiral amylose phase.
I would be very happy if someone spends 5 min to answer to me these questions.

These maybe basic HPLC questions but i'm a bit confused that's why i ask.

Hi jcwagner,

jc wagner asked

Does anyone know what is the use of acidic or basic additives (DEA or TFA etc) in normal phase chiral chromatography?

This website may help answer these questions:

http://chiraltech.com/global-faqs/

FAQ #4: To aid in analyte solubility in the event that they are not soluble in hexane-alcohol based eluents.

From FAQ #4, " If your sample is an acid salt of a base, then (the) addition of 0.1% DEA to the sample solvent may help solubility by converting the material to the free base, which usually is more soluble. Conversely, If your sample is a salt of an acid, then (the) addition of 0.1% trifluoroacetic acid may improve solubility."

FAQ #9: For acidic and basic analytes, to allow for better interaction with the chiral (the amylose tris (3,5-dimethylphenyl carbamate) portion of the stationary phase. [As you and multidimensional both note above, to limit the ionized form(s) of the analytes-said another way, to act as ion-pairing agents.]

From FAQ #9, "For basic and acidic samples, it may be necessary to incorporate an additive in the mobile phase in order to allow the recognition and optimize the separation. Thus, basic samples may require a basic additive (DEA, butylamine, ethanolamine, ethylenediamine…) and acidic compounds the addition of an acid (TFA, acetic or formic acid…). The percentage needed is typically 0.1% and should normally not exceed 0.5%."

Best Wishes!

Acids for acidic compounds and bases for basic compounds. While the literature frowns on it, I haven't encountered any adverse effects of going from TFA and DEA mobile phases.