trace metal effects on LC columns

[Charles A. Burger]

What effect does the metals content in stationary phase and/or mobile phase have on gradient chromatography, and why?

[Mark Tracy]

It depends on your analyte and its concentration. The effect can be nothing or severe. The usual effect is peak tailing for some compounds, especially those that strongly bind to metals. Do you have a specific situation?

[pawan ratra]

A charged compound may irreversible bind with the metal ions in the column and the peak usually present may not elute at all.

[Charles A. Burger]

I was thinking more along the lines of having the best gradient profile I can get.

2 examples are USP Clindamycin PO4 assay, and a Morphine SO4 related comp method. Both are reverse phase and Both are gradient and both have had issues with the gradient profile having baseline gaff's, not including the natural slope up that is brought on by the increase of organic. The Clindamycin wavelength is 214 and the morphine wavelength is 280. The one thing I've discovered is that you want solvents that are as pure as you can get.

A seminar on column technology plus some bits and pieces of things I've read moved me to ask about the metals in Silica. A rep from Zorbax explained that they try and purify the silica as much as possible and he noted that metals are a definate concern in the process.

My goal is to understand what makes gradient baselines become less than stable. Any reference material would be appreciated.

Thanks

Chuck

[Uwe Neue]

I do not think that metals would influence the issues that you describe.

There are two distinct different types of issues and problem causes.

1. The first one is associated with metals (such as Al or Fe) buried in the silica. This is common with older "low purity" silicas. The metal ions are in the matrix and acidify surface silanols. This is why the older types of silica are more active towrds retention of bases and often give tailing. Since these metals are inside the matrix, they can not be removed.

Modern silicas are made from high-purity raw materials (i.e. organosilanes) that do not contain such impurities any more. With a very well designed and run process, metal impurities in these modern "high-purity" silicas practically do not exist any more. This is the primary reason why you get a much better peak shape for bases on these modern silicas.

2. However, there is another source of metal, and this is the column hardware and your stainless steel HPLC instrument. Actually, also your brown solvent bottles are a nice source of metals. These metal contaminants accumulate on the column during the operation. Contrary to the metals buried in the matrix, they sit on the surface and can be the cause of complexation, if you have analytes that are prone to do such things.

The only way to avoid this second type of metal contamination is to use a metal free system and to take the proper precautions with your source of mobile phase.

Unfortunately, there are lot of myths on this, but I hope that this clarifies things.