[newooub]

I scanned 18.1Mohm DI water at 400-190nm range, and the spectra around 200nm kept changing. The maximum absorption variation was around 0.007A when I tried scanning DI water vs air reference.

I poured the DI water directly from the filter into a quartz cuvette that had been cleaned with 0.1M HCl. There was big (up to 0.005A) absorption change in multiple scans over the first 10-20 minutes, followed by smaller drift (~0.002A) over the next 40 minutes. Then, when I changed the water, there starting spectra changed by around 0.001. Changing the water several times seemingly changed the spectra up & down randomly.

Taking the cuvette in and out of the holder only changed the absorbance by around 0.0003A, so cuvette position alone was not a likely cause.

The spectra drift of air vs air or empty cuvette vs air over several hours only changed the spectra by 0.0005. So, the environment, cuvette or electronic drift alone didn't seemed to have caused it.

I was wondering if anyone who might have worked with UV spectrometry at very low chemical concentration or absorbance could say what happened. Thanks in advance.

[Borek]

Atmospheric CO₂? Something else airborn?

[newooub]

The scan of air vs air reference only showed up to 0.0005A variation, but water vs air reference had .007A variation, so it's almost 10 times higher.

I also put the lid on the cuvette, so the CO2 should not get in... I think.

[marquis]

Dumb question.

The reference cell is often used to blank out solvent effects.  What kind of variation do you get if you put the same water solution into both the reference and sample cells?

I don't know what quartz cell you are using, but the one I had used a white teflon cover. It sat on the cell and didn't tightly seal the cell. It was not airtight.  Does this sound like the cover you are using?

I apologize for asking obvious questions.  Hope they help.

[newooub]

For DI water vs DI water scans, I also got around 0.007A maximum variation. The reason I used air as reference was because air looked more stable than DI water, and I wanted to see the variation of the DI water alone.

Yea, the lid was not air tight (I was hoping it would reduce O2 or CO2 diffusion a bit). Do you think O2 or CO2 diffusion can cause that much change within one hour?

[newooub]

Just want to clarify. Different DI water sample from same filter seemed to have some spectra variation, so if I have water in my reference holder, the reference water spectra might be different from the sample water spectra.

Also, the spectra of each sample drifted overtime.

[marquis]

Ok.  Time for a list of possibles.  Some of these you have probably already tried.

First, most of these UV/VIS are supplied with a holmium oxide or holmium chloride standard.  These are usually run daily.  To eliminate instrument malfunction, try running this standard three times and see how the absorbance varies.  If it is very close, the instrument is not the issue.

Next question- what is the wavelength where you see this variation?  The UV cutoff wavelength for water is in the 200 nm range.  If you are seeing 0.007A variation at 200 nm, it probably isn't an issue. 

On the other hand, the lowest concentration standards at higher wavelengths are typically around 0.010A.  These standards always have some noise in them, but still.
It might show a problem.

Most purified water requires more than just conductivity.  Frequently, Total organic carbon (TOC) is also required.  If you suspect the water, see if TOC was also run.  While the test is for total organic carbon, the instruments often give values for total carbon and total inorganic carbon to derive the total organic carbon value.  If you suspect CO2, this test might help answer that question.

Good luck.  Hope this helps.