quinine in tonic water

[WK]

Hi All,
I have been trying to develop a robust method for this analysis with UV-Vis detection. I always fall short of the desired result (50%-70%). I have tried several different wavelengths. Why is fluorimetry so desirable for this analysis?
Thanks in advance
WK

[tom jupille]

Fluorescence is more sensitive (probably not an issue in this case) and much more selective (lots of structures absorb in the UV range, but only a few of them will fluoresce (get rid of the absorbed energy by emitting a photon rather than dribbling it away as vibrational energy).

[Mark Tracy]

This is how I did quinine in tonic water:
http://www1.dionex.com/en-us/acclaim_li ... 29720.html

[Victor]

Wk Your post makes no sense to me.

What is the desired result? I presume you mean that your quantitative results are lower than you expected-but I am just guessing. How stable is quinine in tonic water anyway-maybe you are getting the right result but the samples have degraded.

Despite this there is easily enough quinine in tonic water to do it with UV detection-as Mark has shown. I'm sure there are simpler ways of doing this, without gradients, or diethylamine or MSA-unless you have to use USP methods that is.

[Mark Tracy]

I used a gradient because this particular brand contained benzoate, and that elutes much later than quinine. The gradient actually saves time.

[WK]

Thanks for your replies.
I was trialing a Discovery RP-Amide C16 at 95% 0.1%TFA (A) and 5% acetonitrile (B). Then to 60% A after 5mins.
When I use distilled water as diluent for std (40ppm) and samples (1/10v/v) I get approx. 25ppm quinine in tonic.
When I use 0.1% TFA as diluent for std (40ppm) and samples (1/10v/v) I get approx. 55ppm quinine in same tonic sample.
The discovery gives me sharp peak shape whereas my XDB and BDS are very tailing even with diethylamine.

[Uwe Neue]

is the peak area of either the standard or the sample remaining constant between both sample preparation techniques? For example, if the peak area of the sample remained constant, and if the same sample has been used, then the error might be in the preparation of the standard. If the peak area of the standard remained constant, then the two samples are not of the same quality (for whatever reason...).