What are some of the circumstances that would cause sample to "stick" to a column (C18). I am running into a situation where I have to inject my standard 10 or more time to achieve a repeatable area. What are some of the causes of this and what are some possible solutions

Thanks

Ricardo

Ricardo,

There are a few possibilities that I can think of:

1). The compound is easily oxidized. In this case adsorbed oxygen needs to be consumed before reproducible peak heights will be observed. Use of thoroughly degassed eluents and possibly the addition of a reducing agent to the eluent should help.

2). The compound has an extremely high affinity for surface silanols (generally this is due to cation exchange of cationic analytes with surface silanols). In this case, multiple injections will saturate the highest affinity sites. If this is the cause, lowering the eluent pH or adding a competing cationic eluent component will eliminate the problem.

3). The compound is a chelating agent. Since there is a tendency for metals to contaminate buffers commonly used in mobile phases, ultimately the column will become contaminated as metals bind to surface silanols. Treatment of the column with a potent chelating agent such as EDTA will temporarily eliminate the problem but addition of a chelating agent to the eluent is necessary to minimize the problem on a more permanent basis.

To localise if it is a silica problem try a non-silica column with the same analyte (e.g. polymeric or Hypercarb ....)
PETER