Dissolution of HILIC Columns in Neutral pH Buffers

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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Dear readers, Would you like to share your experience with commercial HILIC columns in terms of retention time stability using high pH buffers say ~ 6.8 with a strong HILIC mobile phase such as > 25% water content? By retention time stability, I mean drifts (regular increase or decrease). My understanding is that HILIC columns are inherently less stable than reversed phase silica phases because the stagnant water layer helps to dissolve the silica surface. We also know that using high buffer concentration helps to dissolve silica in HILIC mode.

For instance, a famous zwitterionic column on extended usage developed a few mm deep void. It is a polymer coated silica; in this case the retention times were relatively stable but most the underlying silica dissolved with time.

I regularly see new HILIC phases in chromatography journals but most of them are silent about their stability with few exceptions for reliable names. What is your experience for HILIC stability? It might be appropriate to discuss generic stationary phase chemistry rather than company names to maintain neutrality.

Thanks.
I'm not sure whether this problem can be discussed considering only the surface chemistry. I can imagine that the silica underneath, its production process, and its final purity, will have the utmost impact on reproducibility and ruggedness, and will be different from brand to brand.
Dear M_Farooq,

The first HILIC phases on the market had different bondings like Carbamoyl, Amino and Diol without endcapping. With buffers at pH4,5 it is possible to achieve 1000 injections. High pH changes are always stress for packing materials both with reversed phase and HILIC.
It is always the question how a column is treated. With high pH buffers it is even more stress for the column packing material. If you have a mixed mode material as you mentioned it is difficult to say which mode is responsible that you find a void volume on the top of the column. Latest developments on HILIC phases mostly have an endcapping. Please check out the book High Performance Liquid Chromatography from Uwe Neue, the chapter about HILIC columns.
In my opinion HILIC columns show a better stability than most RP columns, but I personally don't count Amino columns to HILIC. But they are sensitive to big changes in buffer salts and pH.
Talking about stationary phase chemistry you want, but than we need to talk also about the basic silica, before the bonding. You can get cheap basic silica and high tech silica for your product, and with a cheap silica, no matter if RP or HILIC, a column material after bonding will be not so stable than a high tech silica.
Gerhard Kratz, Kratz_Gerhard@web.de
This article is not about column robustness, however it mentions it in the last paragraph
Systematic evaluation of acetone and acetonitrile for use in hydrophilic interaction liquid chromatography coupled with electrospray ionization mass spectrometry of basic small molecules
http://onlinelibrary.wiley.com/doi/10.1 ... .5271/full
Dear G. Kratz,
I totally agree with your view point esp. with the pH. Even unstable chemistries behave better around pH 4-5. My opinion is that hydrophilic ligands leach much more quickly than reversed phase. You must have read Kirkland's work where he showed that TMS capped silica (essentialy just C1) just lasted for few hours. As the carbon chain becomes longer and longer, the stationary phase become more and more stable, just like C18. When polar embedded phases were introduced, they were much hydrolytically unstable than C18.

Now the problem with a HILIC phase is that it has to be hydrophilic, that's why if you dig a little deeper, most successful HILIC columns have a highly cross-linked polymer coatings rather than silane chemistry. You can see the literature on ZIC-HILIC. However in publications you will still see small linker attached on silica and used for HILIC. Not surprisingly, nobody talks about their stability.
Hello Carlo, Thanks for the article. http://onlinelibrary.wiley.com/doi/10.1 ... .5271/full

The authors use ethylene bridged bare silica. So it is no surprise that those bare silica columns were ultra-stable. The only drawback with such hybrid silica is that the there is a lack of silanols like typical HPLC grade silica. Silanols are beneficial and perhaps acceptable in HILIC mode.
If the Trimethoxy groups are changed to larger groups who will give a better steric hindrance you will get also with C1 a good stability. Monomeric bonded phases are more stable under alkaline conditions than polymeric, or trifunctional bonded phases. C18 with a spacer, or polar embedded group were designed to give a much better peak shape with basic or polar analytes, mostly with TFA at more acidic conditions.
Gerhard Kratz, Kratz_Gerhard@web.de
Gerhard Kratz wrote:
If the Trimethoxy groups are changed to larger groups who will give a better steric hindrance you will get also with C1 a good stability. Monomeric bonded phases are more stable under alkaline conditions than polymeric, or trifunctional bonded phases. C18 with a spacer, or polar embedded group were designed to give a much better peak shape with basic or polar analytes, mostly with TFA at more acidic conditions.


I agree. Isopropyl side-chains make the ligand very stable wrt hydrolysis. This raises a question. Trimethylsilane was a popular end-capping agent. Why did it not show bleeding? Although it is well known that TMS bonded alone will leach away in a day or so. Did the C18 chains protect the TMS somehow, like a large tree protecting the grass growing underneath from sun?
On the other hand, this makes a poor HILIC phase, because this makes a hydrophobic surface. I once tried this approach and got a poor phase. You comment is interesting that monomeric phases are more stable than polymeric ones. Do you have any reference in your mind? I always had the opposite notion.
M_Farooq wrote:
This raises a question. Trimethylsilane was a popular end-capping agent. Why did it not show bleeding? Although it is well known that TMS bonded alone will leach away in a day or so. Did the C18 chains protect the TMS somehow, like a large tree protecting the grass growing underneath from sun?


TMS-endcapping actually does show bleeding under acidic conditions. If you use a TMS-endcapped C18 column under (strongly) acidic conditions, the endcapping will say goodbye first, long before C18 is washed off. That's why it's still a good idea to use some sort of old-school, non-endcapped C18 column for separations under acidic conditions (e.g. of acidic compounds).
Just as an example, Zorbax SB-C18, which is an isopropyl-side-chain C18 column meant for strongly acidic conditons (SB = "stable bond") is not endcapped.
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