What is the selectivity of DEAE resins to inorganic anions?

[iyanachk]

I am trying to find out information about the selectivity (relative affinity) of various inorganic anions toward DEAE weak anion exchange resins or gels. Can anyone help, or point to information source?

Thanks!

[HW Mueller]

Ask the manufacturer.

[iyanachk]

This is the reply of GE Healthcare (formerly Amersham Biosciences) regarding DEAE Sephadex:

We measure the capacity in terms of mmol of Cl- per gram of media. We do
not have data for other anions. For Cl-, the value for DEAE Sephadex A-50
is 3.0-4.0 mmol per gram, and for DEAE Sephadex A-25 is also 3.0-4.0 mmol
per gram.

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What is the selectivity (strength of binding or retaining) of DEAE
Sephadex to various inorganic anions?

[HW Mueller]

A cursory look at the booklet "Ion Exchange Chromatography, Principals and Methods", by (still) Pharmacia seems to indicate that their DEAE stuff is recommendet for proteins, etc. So you are out of luck unless someone else used that for inorganic ions (and published it) instead of something like Bio-Rad´s AG, Dionex material and, and...

[Chris Pohl]

iyanachk,

For the most part, ion exchange selectivity follows the same general pattern regardless of the ion exchange material although there are definitely changes in elution order for certain species as the matrix varies. So, if you're willing to start with a reasonably accurate albeit qualitative characterization of selectivity, you can obtain this information from a number of available sources. For example, the selectivity coefficients for ion exchange materials can be found in Lange's Handbook of Chemistry for SAX media based on styrenic monomers. You can also find this information in the number of older references on analytical uses of ion exchange materials. Another source of elution order information is the Hoffmeister series. Quite a number of papers refer to this selectivity series. Although there are quite a few different selectivity orders in the published literature which are attributed to Hoffmeister, the selectivity order is at least qualitatively consistent for most of the ions.

The one distinction between WAX and SAX is the high selectivity of WAX for hydroxide ion. In contrast, with SAX phases hydroxide is generally one of the lowest affinity ions (with the exception of some of the phases that we have developed here at Dionex).

In general, the WAX order is (from lowest affinity to highest affinity): fluoride, acetate, formate, chloride, bromide, nitrate, iodide, thiocyanate and perchlorate with hydroxide having a higher affinity than any of the previous ions in the list. If you are interested in other ions than the ones I've mentioned above, please let me know as we have an extensive database of selectivity information regarding inorganic ions with various ion exchange phases. In the interest of brevity, I only included some of the more commonly studied anions.

[Victor]

Chris,

I would be interested if you could make a connection for us between the Hofmeister series (which is a series of anions in order of their ability to precipitate hen egg protein) and the order of elution of anions on an anion exchange resin. Personally I find the rationalisation of the Hofmeister series in terms of biological effects quite complex, perhaps even more complex than the order of elution of anions from a resin......

[HW Mueller]

These phenomena seem all related to heats of hydration of the ions, also, surface tension influence of the ions appear to run parallel. In other words, interactions with water and interactions with other ions are determining most of this behavior. Some inorganic texts are good in relating these properties, in turn, to charge "concentration", etc.

[iyanachk]

Chris,

The ions, I am particularly interested in are carbonate, bicarbonate and acetate in relation to the phosphate, pyrophosphate, and other polyphosphates. I am also interested in any other anions, which might have weak affinity to WAX. I agree, that generally the selectivity order for WAX would follow that for SAX, for which data is widely available. Yet, I am afraid, that for those particular ions, especially in the pH region, I am interested in (pH 7.5-9.0) there might be significant deviation. That is why I very much appreciate your advise.

[Chris Pohl]

Victor,

The reason behind the similarities in the Hoffmeister series and ion exchange selectivity stems from the common elements affecting both of these properties: hydration (as mentioned above by Hans). Hoffmeister's original publications dealt with the effect of different electrolytes on the solubility of eggwhite proteins. Unfortunately, I haven't been able to lay my hands on the original publication (there was a series of papers published in the 1890s) so I can't elaborate on the details of his experiments. But, if one considers the effects of different ions on the solubility of the proteins in aqueous solution (without using such a high ionic strength that the proteins are salted out), then the tendency of the ion to result in a water insoluble protein salt is connected to the extent of hydration of the ion producing a salt with the protein. The lower the hydration of the ion, the lower the solubility of the protein.

In ion exchange, to a first approximation, the driving force is quite similar. Highly hydrated ions (which more effectively become incorporated into the water structure with minimum disruption of the water structure) prefer to stay in the external solution outside the ion exchange material while poorly hydrated ions (which are minimally incorporated into the water structure with maximum disruption of the water structure) prefer to be in the ion exchange phase. Of course, in the case of ion exchange there are other factors providing the secondary retention mechanisms which can cause significant deviations in selectivity from the Hoffmeister order. For example, there are ion exchange materials capable of generating all of the following elution orders: bromide, chlorate, nitrate; chlorate, bromide, nitrate; bromide, nitrate, chlorate and chlorate, nitrate, bromide. Although I was never able to determine unambiguously what the Hoffmeister order is for this set of anions (a search of the published literature reveals at least six different Hoffmeister orders, but I have no way of knowing if any of them are the actual order in his publications since I was unable to obtain copies of the original publications), it's safe to say that the Hoffmeister order would be only one of these orders.

In general, differences in selectivity between ion exchange and protein solubility are connected to differences in entropy and enthalpy of hydration. For species in solution, entropy of hydration tends to be the dominant factor. For cross-linked ion exchange materials, enthalpy of hydration tends to be the dominant factor with the extent of cross-linking influencing the relative effect of enthalpy and entropy.

[Chris Pohl]

iyanachk,

The answer to your question: the selectivity order for carbonate, bicarbonate, acetate, phosphate, pyrophosphate and polyphosphates is complicated by the fact that the items of interest are of different valency. For ions of the same valency, the order is invariant for a given material. For example, the selectivity of most materials is greater for acetate than for bicarbonate. But the selectivity of the ion exchange material for these two ions relative to the rest of the ions in your list depends upon the concentration. At low concentrations, the selectivity is charge dependent with the highest charged ions having the highest selectivity. At higher concentrations, selectivity parameters are intermingled and must be determined empirically. In general, though, a homologous series of ions will invariably elute in the order monovalent first, divalent second, trivalent third, etc.

Theoretically, under the right conditions it should be possible to reverse this order if the charge density of the stationary phase is too low to support retention of highly charged species. However, in my experience, one cannot use weak ion exchange materials to accomplish this. This should only be achievable with uniform dilution of charge density with fixed ion exchange sites. My guess as to the reason that weak ion exchange materials won't work for this application is that polyvalent ions induce ionization necessary to support retention. Even if the pH is adjusted so that only 1 in 10 ion exchange sites are in ionic form, in the presence of a highly charged counterion, the ionization is shifted to favor multiple ionization sites in the proximity of the highly charged counterion, thus retaining the selectivity order mentioned above.

[Victor]

Chris-thank you for this nice answer.

I still have problems with the Concept of the Hofmeister series and protein precipitation. For anions, I thought that strongly hydrated ions (or kosmotropes if you want to use that name) were good protein precipitants. Thus highly hydrated anions like sulfate, citrate and fluoride are good protein precipitants. I saw this in simple terms as the removal of water from the surface of the protein allowing closer interaction of protein molecules and precipitation. I am not sure how this ties in with your statement "the lower the hydration of the ion, the lower the solubility of the protein". However, the reverse is apparently true for cations; weakly hydrated cations are good protein precipitants..... Can you help to clarify any of this?

Explaining the retentivity in WAX chromatgraphy would seem to be a simpler problem. However, the reduced retention of highly hydrated ions might be explained by two effects: the increased distance between point charges caused by the hydration, or perhaps the size exclusion of larger hydrated ions from the pores of the resin. Can you clarify how you envisage the mechanism of the reduced retention of hydrated ions?

[iyanachk]

Chris,

Thank you for the information. In the past I was puzzled by the fact, that even at very high pH WAX retains quite a capacity for polyphosphates. Your point that polyvalent ions can induce additional charge on WAX explains that.

My lesson home from this discussion is that this area – mixture of mono and polyvalent ions interacting with WAX is quite empirical. I am not interested in the theoretical models, that can explain the observations. My interest in all that is quite practical. Do you know any published studies about WAX selectivity toward the ions of interest (phosphates and polyphosphates, carbonates etc.)?

Thank you!

[HW Mueller]

Sorry, iyanachk, but a little theory is neccessary to make an "educated" guess, and, along this line Chris´s statement on "the lower the hydration..., the lower the solubility..." also cought my special attention. For instance, Ba is way toward chaotropy (low energy of hydration) on the series at my disposal. It certainly likes to cling to carboxylic acids (more than to water, apparently). Most likely it does that at carboxylates of proteins also, but does that cause precipitation? Chris, do you have some refs/examples on this? Normally, a chaotropic cation, as well as chaotropic anion solvates proteins. But lyotropic (high hydration energy) salts at low concentration (~~0.1M or lower) also has solubilizing qualities. Quite complex . . ., the likes of Debye and Hückel apparently spend a lot of time on this. Also, Chris pointed to enthalpy and entropy....not easy for "over the thumb" predictions.

(On the other hand, iyanachk is certainly not off track with his desire to be practical)

[Chris Pohl]

Victor,

In answer to your statement: "I am not sure how this ties in with your statement "the lower the hydration of the ion, the lower the solubility of the protein". However, the reverse is apparently true for cations; weakly hydrated cations are good protein precipitants..... " perhaps I am a bit confused but it appears that your statement is identical to mine as opposed to conflicting with mine. My statement was: the lower hydration of the ion the lower the solubility of the protein. This is identical to saying that weakly hydrated ions are good protein precipitants. So I'm not sure I understand the conflict between these two statements.

As to the question about how both poorly hydrated and highly hydrated ions can post induced precipitation of proteins, this can be explained as follows: poorly hydrated ions form poorly hydrated salts with proteins resulting in significant decreases in the water solubility of the protein while highly hydrated ions tie up most of the available water and thus render the protein less soluble due to a shortage of available water for solvation of the protein. Trichloracetic acid protein precipitation is an example of the case where precipitation is induced by formation of a poorly hydrated salt. Ammonium sulfate precipitation is an example where precipitation is induced because the salt ties up enough water to render the protein insufficiently solvated to stay in solution. Of course, this latter case only works when the concentration of the precipitant salt is fairly high. In general, low concentrations of ammonium sulfate will have no effect on protein solubility.

[Chris Pohl]

iyanachk,

I'm afraid that I haven't published anything regarding the selectivity of polyvalent ions retained on WAX phases. What I reported in my earlier post was based on experimental observations in our labs here but none of it has been published. Frankly, when it comes to separating this class of compounds (condensed phosphates) we mainly make use of hydroxide selective SAX materials in conjunction with hydroxide eluent, electrolytic eluent suppression and conductivity detection. This retention mode gives better dynamic capacity and high-resolution separations.