UV Spectroscopy

[MedPR]

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Everything I've read has discussed UV spec as a method primarily for distinguishing between things with double bonds, especially conjugated double bonds. However, in reading about HPLC, this site (which I've found to be pretty credible thus far) discusses UV absorptions of methanol and water..

http://www.chemguide.co.uk/analysis/chromatography/hplc.html#top

You might wonder why the solvents used don't absorb UV light. They do! But different compounds absorb most strongly in different parts of the UV spectrum.

Methanol, for example, absorbs at wavelengths below 205 nm, and water below 190 nm. If you were using a methanol-water mixture as the solvent, you would therefore have to use a wavelength greater than 205 nm to avoid false readings from the solvent.

What's up with that?

 

[chiddler]

what exactly is your question?

are you asking why solvent is not detected? or why different compounds are detected with different UV frequencies?

 

[MedPR]

what exactly is your question?

are you asking why solvent is not detected? or why different compounds are detected with different UV frequencies?

I thought that UV spec only detected things with double bonds. Methanol doesn't have any double bonds so why is it recognized by UV?

 

[BerkReviewTeach]

I thought that UV spec only detected things with double bonds. Methanol doesn't have any double bonds so why is it recognized by UV?

Technically, single bonds also have UV absorbances, but they're in a range where there is ambient noise, so it's generally practical to use UV for conjugation and pi-bonds. I'd assume the MCAT is limiting the UV-vis spectrum of interest to 200-800 nm, which includes pi-bonds.

 

[MedPR]

Technically, single bonds also have UV absorbances, but they're in a range where there is ambient noise, so it's generally practical to use UV for conjugation and pi-bonds. I'd assume the MCAT is limiting the UV-vis spectrum of interest to 200-800 nm, which includes pi-bonds.

"Ambient noise" meaning too many absorbances to accurately identify something within that range?