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Topic: Which compounds are UV inactive?  (Read 11565 times)

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Offline goldy

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Which compounds are UV inactive?
« on: June 05, 2010, 05:02:53 AM »
HI all,
I have a question that what kind of coumpounds are UV inactive. Also, why +ve spectrum in MS analysis is more important during their analysis?
If anyboby knows about this, I expect the answer.
Thank you so much.

Offline bromidewind

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Re: Which compounds are UV inactive?
« Reply #1 on: June 09, 2010, 03:06:52 AM »
Technically speaking, nothing is truly entirely UV inactive. However, some compounds absorb light far below the cutoff points of most UV spectrometers (typically around 200 nm) and therefore register as inactive. These tend to be compounds that are heavy in sigma-bonds, as they are very stable and relatively unaffected by wavelengths longer than 200 nm. This is why you often dissolve compounds to be tested via UV in ethanol because EtOH doesn't absorb above 200 nm.

As to your second question, I'm not sure what you mean. I haven't heard of +ve spectrum in mass spec... but then again, I've dealt very little with MS.

Offline moov

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Re: Which compounds are UV inactive?
« Reply #2 on: June 09, 2010, 03:33:21 AM »
i need to give u small information what i know,

The molecular ions are energetically unstable, and some of them will break up into smaller pieces. The simplest case is that a molecular ion breaks into two parts - one of which is another positive ion, and the other is an uncharged free radical.

(M+.) -----> X+  (+) Y.

The uncharged free radical won't produce a line on the mass spectrum. Only charged particles will be accelerated, deflected and detected by the mass spectrometer. These uncharged particles will simply get lost in the machine - eventually, they get removed by the vacuum pump.

The ion, X+, will travel through the mass spectrometer just like any other positive ion - and will produce a line on the stick diagram
 
Moovendan

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