lc-ms peak

[jiang295]

if i use uv and ms in series, tubing between uv and ms is 1m long with od at 0.005'', then i got twice of peak width at ms than in uv.
is it only tubing problem?

You can try this all out by yourself, and confirm or dismiss my proposition that the tubing is the possible source of the problem. Disconnect the column and inject the same sample under the same mobile phase conditions and flow rate with the same setup, just without the column.

[jiang295]

if i use uv and ms in series, tubing between uv and ms is 1m long with od at 0.005'', then i got twice of peak width at ms than in uv.
is it only tubing problem?

You can try this all out by yourself, and confirm or dismiss my proposition that the tubing is the possible source of the problem. Disconnect the column and inject the same sample under the same mobile phase conditions and flow rate with the same setup, just without the column.

[Uwe Neue]

I thought you had them in parallel rather than in series... ???

[jiang295]

i did another test.

I thought you had them in parallel rather than in series... ???

[jiang295]

you are absolutely right about this.

Well, that assumption would depend on the analyte and the MS instrument conditions. If you are talking about your own method, it is possible that the mass spec is just simply more sensitive than the UV and that's why the analyte tailing becomes apparent.

[Uwe Neue]

Hmmh...

I don't know about that... At equal injection (which is what we are talking about here), the peak should look the same. Unless your noise in the UV is horrendous, and you have a very slow signal-to-noise ratio, I can't see this as a valid explanation.

[jiang295]

it is really just senstivity issue.
the fact that ms is far more senstive than uv make peak in ms much wider than those in uv.

I injected very small amount of sample, peak width become very similar to both detectors.

Hmmh...

I don't know about that... At equal injection (which is what we are talking about here), the peak should look the same. Unless your noise in the UV is horrendous, and you have a very slow signal-to-noise ratio, I can't see this as a valid explanation.

[Uwe Neue]

That makes no sense... On a one to one scale, comparing peaks of the same height, they must look the same.

I think we are getting closer to the true explanation...
If your MS peak looks wider at the bottom at equal height at a high sample concentration, and if both peaks look more similar at equal height at low concentration, your MS signal is strongly non-linear at the higher concentration, i.e. with a downward curvature of the response. Did you look at the linearity of the MS response?