[dimidola12]

Here's a question: I am trying to monitor the disappearance of a peak at 260 nm as a result of UV irradiation. The way I am currently doing this is by taking samples, diluting them and collecting the spectra. The problem is that my sample doesn't have any other characteristic peak, therefore I make triplicates of my dilutions just to be safe. This would be enough assuming that my solvent doesn't evaporate and the refs are nice people that trust me. I thought I could add some sort of a dye that is stable under UV irradiation (and heat), does not absorb anywhere near 260 nm, is water soluble and is not a divalent salt.
Any ideas?

[JGK]

so if I read this correctly, you are:

1. Exposing a sample of material to UV.
2. Taking a sub-sample and diluting it.
3. Placing the diluted material into a UV-Spectrophotometer (where it will be exposed to more UV).
4. Taking a scan.

Surely you would be better off not doing step 1, merely diluting it and  placing it in the spectrophotometer and performing scans at timed intervals

[curiouscat]

Surely you would be better off not doing step 1, merely diluting it and  placing it in the spectrophotometer and performing scans at timed intervals

Not necessarily. I suspect his first UV is of a much higher intensity.

I might be wrong.

[Arkcon]

What you're doing is a specific case of a general process -- determining a substance's photo-decomposition.  There is a defined method for the this specific procedure according to NIST, and a pharmaceutical would be required to follow it according to USP (EUP, JP, RP whatever) regulations.  You can look it up, but to generally describe it:

Your sample and a standardized sample of quinine would be exposed side by side to a calibrated light source for varying periods of time.  It is known that quinine decomposes and loses it absorbance at it's own max wavelength at a certain rate on exposure to light.  You can compare the rate at which your sample degrades to the rate quinine degrades to make a determination as to how light unstable you sample is, for USP purposes.

Now, if you're not trying to submit your sample for FDA approval, you don't have to be that rigid.  However, you can use the same idea: you purchase a broad spectrum lamp that is calibrated with a certain light output, and/or you monitor its output with a light meter (preferable, since all light sources wear out over time.)  You expose your sample and a sample of quinine, to see how the sample behaves over time relative to the quinine.  You of course need a control for both, a sample of your product and quinine that is wrapped in foil or black cardboard, because focusing a light on a sample is also going to cause thermal decomposition apart from the photo-decomposition.  But you can find the complete FDA protocol online, and adapt it to your needs.

[dimidola12]

Not necessarily. I suspect his first UV is of a much higher intensity.

Yes that's right.

To clarify: I am irradiating a sample with a relatively high energy UV. The sample does not decompose. Its chemical structure changes. As a result of that change, a peak at 260 nm slowly disappears. I cannot take a spectrum of the sample as is because it is too concentrated. That's why I take small samples and dilute them 50 times before I collect the UV spectra. I try to keep the concentrations of my samples the same, but it is not that easy. That's why I need to add a control. Something that absorbs far from the 260 region. Then I can normalize my spectra to the intensity of the standard.

Arkcon, quinine won't do. It absorbs at 250.

[curiouscat]

Potassium Dichromate?

nicotinic acid?

[dimidola12]

Potassium Dichromate?

Is divalent. I have SDS and it will crash out.

nicotinic acid?

Absorbs at the 250 region.

Thanks for the effort. I am sure there must be SOMETHING out there...