HILIC question

[mardexis]

Surface silicon hydride is considerably different from surface silanol. There may still be some accessible silanol along with the hydride but the surface properties are very much different.

Think of it as a hybrid particle.

[Uwe Neue]

Mardexis... how is this material behaving differently from a silica? This is my basic question, and I have not gotten a valid answer.

From my standpoint, if the animal has a trunk like an elephant, has feet like an elephant, a fat body like an elephant, with a nearly hairless skin, and a tail like an elephant, I am calling it "elephant".

If the hydride-bonded particle behaves like silica, it must not have just a few, but plenty of silanols on the surface. So there is no magic about the hydride, but we observe just old HILIC on silanols, just as on straight silica.

[zoom]

On silica, the surface is populated by silanols (Si-OH) whereas on type c silica, the surface is mostly silicon hydride (Si-H). On regular silica, you have the capacity for more hydrogen bonding -- you can have polar hydrogens on the analyte bonding with the Si-OH oxygen, and you can have the Si-OH hydrogen bonding with oxygen and/or nitrogen atoms on the analyte. On type c silica, that is not the case. Silicon is not electronegative enough to allow the attached hydrogen to form hydrogen bonds with oxygens and/or nitrogens on the analyte. Likewise, the H is smaller than the OH and causes less shielding of the underlying Si-O-Si oxygens, so analytes with polar hydrogens can form hydrogen bonds with the Si-O-Si oxygen.

Based on what I've seen, type c silica has a different selectivity from normal silica in aqueous normal phase. I've seen some compounds that bond strongly to regular silica (yielding broad, tailing peaks) give much better results on type c silica. Likewise, I've seen some compounds that work well on regular silica shoot through type c silica with very little retention.

The difference between regular and type c silicas could be compared to the difference between a regular C18 column and a polar-embedded C18 column -- they will behave the same way for some compounds, but differently for others.

[HW Mueller]

Now what is it zoom? Above you state that C type silica has no -OH (+ similar statements), now it is merely mostly SiH? Roughly how many SiOH and how many SiH?

[Uwe Neue]

Packings for HILIC all contain very polar functions, starting with SiOH for silica, NH2 for amino phases, CONH2 for amide phases, O-CH2-HCOH-CH2OH for diol. Si-O-Si is not very polar, it actually has some mild hydrophobic properties. So unless SiH is very polar (which is not a very rational assumption, not dissimilar to a claim that CH is polar), the retention of hydride phases in HILIC is due to silanols.

One does get selectivity differences between the different types of HILIC phases, but the basic retention mechanism is the same and requires a polar surface.

[zoom]

I don't know the total number of silanol sites on the surface of type c silica. What I know is based on what I've read in a number of chromatography articles about this stationary phase and from my results in the lab. Some articles say that there are no silanol sites while others state that there are very few. I've worked with one compound in particular that binds to ordinary bare silica, but it gives a tight peak with type c silica under the same mobile phase conditions.

Perhaps some of you who work at column makers should do some reverse engineering.

[HW Mueller]

zoom, I crtainly would like to see what the exact conditions were that yielded a "compact" peak only with type C, also what was the analyte, and the other C-18 column, how old (heavy use or not?) was it?

[cheminista]

Type-C columns behave differently because they ARE different. These are NOT HILIC columns. [/b]

[HW Mueller]

OK, I checked google in the hope to get another view of this matter. A curious article:

http://chromatographyonline.findanalyti ... ail/310847

has, apparently, the creator of silica-C as co-author. Obviously they are trying to emphasize that their creation is great. Thus, they give HILIC with silica-C a new name (forgot already as I think it´s not important to remember). In other words, there is some difference due to the hydride, but not basically. My impression is that the difference is less than between silica and Zic-HILIC, which has been discussed extensively here. It has also been claimed that Zic-HILIC is free of SiOH/SiO- even though it can very easily be demonstrated that this is not the case. As a matter of fact, some proponents of Zic-HILIC have apparently expounded on the advantages of this column by using the properties of SiO- for part of the retention.
Not to be misunderstood, I am glad that these different columns exist, but why all this hype?

But, for someone who crew up with Stahl´s bible on TLC, a statement indicating that nonpolar compounds can not be retained on silica seems strange inddeed.

[cheminista]

Mueller seems to have a beef with Type-C columns-have you used them in your lab? It doesn't sound like you have.
Maybe you should challenge your own assumptions, and instead of guesswork and "impressions," actually work with these columns to learn about them-you may end up as impressed as I am with their capabilites.[/b]

[HW Mueller]

Cheminista, send me one and I will show you in a few minutes that
22-Na+ will stick to these columns at a pH of 5 or above. Thse mentioned search clearly indicated that the C-type still has SiO-!! If one were to attach negative adjectives to persons, than please do it to those who state that C-type has no SiOH/SiO- as this clearly appears not to be true.
Why did you choose to ignore my statement: "Not to be misunderstood . . . ."? Where did I say that the columns are of no use, or anythuing akin to this?
What is your qualification to ask me to challenge my "assumption" regarding differences between different columns? Have you used Zic-HILIC so that you can present us with a comparison?

I really was somewhat peefed when I found out that the Zic-HILIC behaves almost exactly like a plain silica column regarding positively charged ions at higher mobile phase pH. It turned out that it was quite useful for some other applications, though.

[Vlad Orlovsky]

We did not see big difference between bare silica and Zic-HILIC either...big enough to justify 4 times higher price.
Here is comparison, as you can see all three columns provide separation and retention, just differnet selectivity and question is "How much particular selectivity cost and how critical it is?
http://www.sielc.com/application_182.html

While ago we provided one of the pharma companies with several experimental HILIC columns, without telling them what it is and requested feed back. When we got data back and compare them against chemistries, the diffrence was attributed to residual silanols (mostly), and that is when we design a phase where charges are separated by long hydrophilic link...but that a different story.

[cheminista]

What negative adjectives?
Everyone I know who has or is using HILIC complains about them-I have found the Type-C columns are easy to use and do not take forever to equilibrate and are extremely precise and reproducible.
It is my job to investigate all new technologies, and this means having an open mind and realizing that not all columns behave the same. Type-C columns are unique, no doubt in my mind, and in my opinion, represent a technology everyone should be interersted in learning about.

[Uwe Neue]

Cheminista: it is nice to hear your advertisement. However, you have not provided any statement at all about the nature of the surface functions on type C silica that make HILIC (also called aqueous normal phase) possible. You need a polar function on the surface for this to happen. SiH is not polar, bonded C18 ligands are not polar. So what is left appears to be silanols, as the functions enabling HILIC on type C packings. This is plain and simple logic, right?

[cheminista]

I guess you have made up your mind. If someone sees an elephant and not a tree I guess there is no use arguing with them.
I know that we use the Type c C18 in both RP and ANP modes. This illustrated to me that they are different and that the mechanisms are different.
I heard at Pittcon that Klaus Albert is studying the TC.