what is the use of pKa value in mobile phase preparation in HPLC analysis

I don't understand your question clearly, but I think you mean the following:

If the pKa value for you component is too close with the pH value for the aqueous phase of you mobile phase, you get unstable retention times, non-robust methods,...

You have to be aware for bringing the mobile phase pH too close to the pKa value.

I think pH of the mobile phase should be less than the pKa value , this is to supress the ionisation there by avoiding of getting the split peaks.

How to determine pKa value of drug substance which is either acid or base?

HOW TO CALCULATE RESPONSE FACTOR FOR DRUG SUBSTANCE

Given several of the postings above, I believe the bit of clarification is in order. In the first place, there is no rule as to whether or not you should operate the above or below the pKa value of the analyte. This choice is dependent upon a number of factors including: whether or not the ionizable group is an acid or a base or if both are present, the magnitude of the capacity factor for the analyte in protonated state relative to the deprotonation state and the retention time of the analyte relative to other components present. For analytes with acidic functional groups, operating below the pKa of the acidic group will increase retention whereas operating above the pKa of the acidic group will decrease retention. It depends on the analyte, of course, whether or not it is desirable to increase or decrease retention. Furthermore, although operating near the pKa will reduce the reproducibility of the analytical system, operating with a properly buffered mobile phase (preferably prepared via weight rather than using a pH meter) will greatly minimize this issue. Sometimes, the best selectivity for a given analytical system requires operating near the pKa of the analyte. Of course, if the pKa value refers to a basic functional group, most of the above still applies although in this case the retention characteristics are reversed (i.e. operating above the pKa gives maximum retention).

As far as calculating pKa is concerned, the first strategy is to see if you can find a reference containing the necessary information. There are a number of references including the Merck Index and Lange's Handbook of Chemistry which have quite a few pKa values. Also, there are a number of ways to measure the pKa value including: titration, NMR and UV-Vis.

Hi Chris,

I often wonder why the pKa and titration curves for ion exchange stationary phases are not available at the vendor's web sites. Specificly, I'm interested in data on the WCX-10 column. Thanks.

Anon (Monday September 23rd),

My comments above were specifically directed to reversed phase separations. However, matters are considerably more complex when it comes to weak ion exchange phases. Recently, I had another inquiry on this topic and sent the following in order to clarify why the WCX-10 could be useful for chromatofocusing in spite of its apparently limited pH range:

As to the reason for the relatively broad pKa range for this type of material (we are utilizing a copolymer produced from acrylic acid and a neutral spacer monomer), this stems from two factors common to polymerized ionic monomers: electrostatic effects which reduce acidity of neighboring ionizable groups and a statistical effect which increases the acidity of the first to be ionized group.

In case you are not aware of these two effects, the first effect (electrostatic) arises from the general tendency of an ionized group to reduce the probability of ionization of immediate neighbor groups. Accordingly, polymerized ionic monomers such as acrylic acid show a rather broad range of functional group acidity. On the basis of this effect alone, however, one would expect the minimum pKa to be approximately 4.8 and the maximum pKa to be 3-4 larger. The fact that titration data reveals the lowest pKa values to be significantly less than 4.8 is due to the statistical effect. The statistical effect can be seen in a number of systems but the best documented cases are for carbohydrates and for certain polycarboxylic acids (reference benzenecarboxylic acids with 1-6 acid groups). The latter system shows quite nicely both phenomenon at the same time. Benzoic acid has a pKa of 4.2. Benzenepentacarboxylic acid has pKa values ranging from 1.8 to 6.46. Benzenehexacarboxylic acid has pKa values ranging from 0.68 to 7.49. As you can see, even setting aside inductive effects, there is a general tendency toward increasing ionization as the multiplicity of ionizable groups increases. This is true not only for specific compounds but also for polymers of the sort utilized in our weak cation exchange phase (the ionizable groups are present on the surface of the resin particle has individual polymer strands propagating outward from the resin surface: a so-called brush polymer).

From the above, I think you can see why publishing the pKa of the phase wouldn't be very useful. Regarding the absence of any titration data on our web site, I'm afraid I can't offer any excuse other than you are the first I'm aware of who requested this information. I will see to it that this information is posted as soon as possible.

I want determine limit of detection
and limit of quantitation
. In pratical, I dont know , You can help me , please.
My job, I am analysis carbonyl compound in wine by HPLC.

ANON

You want to know about determination of limit of quantitation & limit of Detection. First tell me which software you are using for HPLC analysis. if it is of Jasco Company. then I can help you in solving your problem.

for details you contact me at rachananandre@rediffmail.com

ANON: make a new topic, you are going off-topic!