polymeric c18 columns

[olivia]

Hi, columns prepared from polymerisation reactions such as c18(2) will have lower efficiencies than the corresponding monomeric column prepared by the same manufacturer with the same base silica gel. However, I think they will also have higher retention.

For my USP testing, where the method states use C18 column for the analysis of say lidocaine, would it be inappropriate to use a c18(2) column because of the bonding process, or is the definition about using any c18 column so vague that it is allowed.
Can someone please advise
Many thanks
Liv

[Uwe Neue]

C18s are made in more ways than you can imagine. The consequence is that you can get a lot of different selectivities and retention patterns. To understand what the USP column is that is used in a particular assay, you can subscribe to the USP or buy their reagent book, all of which will cost you a few hundred dollars.

If the column dimension prescribed by the USP is a length of 30 cm and an i.d. of 3.9 mm, the column is a microBondapak C18 column. I checked this out a few years ago, and at that time a good 70 to 80% of the procedures were run on microBondapak C18. The proportion may have changed a bit over time.

The USP also has a website where one can compare the selectivity of different columns. This was discussed here, and the webaddress was given at the forum. I do not have it on this computer, so you need to look for yourself, or maybe somebody else can advise you.

From the regulatory standpoint, if the column fulfills the often very loose criteria of the USP assay, the column is suitable for your assay. With other words, if it works, it works...

[Bruce Hamilton]

The USP uses generic coding for HPLC columns, with brief definitions, eg L1 = Octadecylsilane chemically bonded to porous silica or ceramic microparticles, and most methods use that code.

Many USP methods use "L1" columns, and several C18 columns from a manufacturer may exceed L1 specifications.

GC and HPLC column manufacturers' catalogues usually include a table of the equivalancy codes of their columns to the USP column specifications.

The column databases Uwe referred to are freely available at:
http://www.usp.org/USPNF/columns.html

The EP also has a free "knowledgebase" that lists the specific columns used to validate their methods often with a chromatogram, which can sometimes be helpful..
http://extranet.pheur.org/publications/ ... s_sw.shtml

Note that this chromatography forum has a search engine, and there has been discussion on many USP column topics and issues over last couple of years.

Please keep having fun.

Bruce Hamilton

[rick1112]

Hi

I would like to discuses the issue regarding monomeric Vs. polymeric little further…

Is is true that polymeric columns are much better than monomeric columns??? If so why??

thanks

[Uwe Neue]

First of all, we need to be clear on the terms. My definition of "polymeric" C18 packings is packings with a surface coating of 4 micromoles/m^2 and higher. This can be accomplished only with trifunctional silanes. Such packings are commonly used for the analysis of aromatic hydrocarbons, and as there is not need for endcapping, they are not endcapped either. Maybe there are some other endcapped packings around that fall into the same category, but nothing comes to my mind at the moment. Thus the polymeric packings that I can think of are not terribly good for other analyses, especially the analysis of bases.

Lower coating levels are then monomeric coatings. Monomeric coatings can be made from mono-functional, di-functional or trifunctional silanes, at various coating levels, wih or without endcapping, on silica or on hybrid packings as the base material. Among these, you find the best performers for retention, lack of silanol activity or combinations of both. You also find some of the worst performers, due to lack of endcapping and the use of an older, less pure silica. So the classification of "better" is even in this group not simple at all.

Here is my definition of "better": a high purity silica or a hybrid packing with a C18 coating and good endcapping. And my personal definition of "better than better": a coating with an embedded polar group created in a one-step process on a high-purity silica or hybrid packing.

[AdrianF]

I have used the following method for Lidocaine:

Buffer: pH 8 phosphate buffer(0.69% K2HP04 adjusted to pH 8 with 0.4% KH2PO4)
Mobile phase: 35% Buffer, 65% Methanol
Column Spherisorb ODS2 5u 150mm
Wavelength: 230nm
Flow rate: 1ml min

Retention time about 7min

[Sallybeetle]

Dear Uwe Neue:

I am going to post your definitions on my office wall. This crystallized what I have been thinking for a long time.