UV-Vis Amine Problem: Solutions & Explanations

[acvtre]

Good day to everybody, I immediately expose my problem.
I'm doing some measurements with the UV-vis spectroscopy and I'm having problems with the amines in the non visible region.

This is what I do: put my amine in a quartz cuvette -> do the background (reference) -> analyze the same solution that I'm using as reference from 600nm to 200nm.
The result that I should get should be a totally flat line, instead I get a negative absorption below 230nm which is a non-sense for Lambert-Beer law.

What's the problem? Is there any resonance in the non visible region for the amines?

 

[Kevin McHugh]

Are you using a dual beam spectrophotometer? Is it a peak or is it the baseline going negative? What is your solvent? There aren't to many optically transparent below 210 nm.

 

[acvtre]

Yes, it is a double beam, infact I've got one space for the for the reference and another one for the sample to analyze. For what I understand the baseline is constant and what I see is a peak that goes below it.
I've got this problem only with the amines. I've tried with pure ethanolamine and ethanoldiamine and they both give me the same problem.
Is it possible that the region below 250nm is unabsorbed and since I have the background the sum is a negative absorption?

 

[Kevin McHugh]

Are these your solvents or your analytes? I've done a ton of UV-vis work on hindered amines (-NH2 surrounded by two t-butyl groups). There lambda max was 224 nm. Try isooctane or acetonitrile as a solvent. These are optically clear down to 200 nm and 190 nm respectively.

 

[acvtre]

I encountered this problem with a solution that only had 0.1% of ethanolammine. In particular, my analyte was composed by: GIZO nanoparticles dispersed in ethylene glycol, pentanol, PVA and ethanolammine.
As I had the negative absorbance I analyzed the ethanolamine as analyte on itself, with ethanolamine as background, and found out that it is the culprit for the weird result. Moreover, if I repeat the measurement I always get different results below 230nm till 200nm, sometimes negative peaks, sometimes absorbance over 10...

My guess is that the UV-vis equipment has some problems in analyzing the molecules that hybridize. Something like, the non-bonded electrons in the hybridized p-orbitals mess up with the light. Does it have any sense in your opinion?

 

[Kevin McHugh]

Acvtre, what is the lambda max for the ethanolamine? Ethylene glycol has a UV cut off around 210 nm, so your spectra won't go to 200 nm. What is the particle size distribution for the nanoparticles? They may be scattering the shorter wavelengths. IIRC, a particle diameter larger than 2x the wavelength will scatter the wavelength.

 

[acvtre]

All the solutions give me a peak around 210nm, which is the nanoparticle peak, I alwasy use the background. Only the ethanolamine gives me a weird result, even though I use the background, which is a non-sense.
As I've written, also when I use only ethanolamine I get the same weird results, this means that it's not a problem of lambda max IMHO.

 

[Kevin McHugh]

What are you trying to do? You didn't answer the question what is lambda max of the diethanolamine? Also give PSD specs for the nanoparticles. What are you trying to quantify? The EG has a cut off at 210 nm. Please pay attention to my responses.

 

[acvtre]

What are you trying to do? You didn't answer the question what is lambda max of the diethanolamine? Also give PSD specs for the nanoparticles. What are you trying to quantify? The EG has a cut off at 210 nm. Please pay attention to my responses.

- I use the UV-vis spectrophotometer in order to find out if I got GIZO nanoparticles in different solutions, to do so, I always use a reference background, wavelength scan from 600nm to 200nm and what I need to see is a peak between 205-210nm.

- I really don't know what is the lambda max of the diethanolamine
- what are the PSD specs?

- what is really weird in my opionion is that, even though I analyse the ethanolamine with ethanolamine as background itself I don't get a flat response. Do you have any idea why this happens? In my opinion, as far as I have a background I should have a perfectly flat result regardless of cut-off wavelengths or similar stuff.

P.S.: sorry for the late reply

 

[Kevin McHugh]

PSD is particle size distribution. So, you are using UV-vis to determine if your nanoparticles are in solution? I could not find the UV cut-off for ethanolamine, you can do it yourself. Zero the spec down to 200 nm in acetonitrile. Then run a fairly concentrated sol'n of ethanolamine as a sample. Note any absorbances or where abs goes to >2. Make sure they are miscible first. Sometimes refractive indices of solvents will do funny things to baselines, especially in that region.

 

[acvtre]

PSD is particle size distribution. So, you are using UV-vis to determine if your nanoparticles are in solution? I could not find the UV cut-off for ethanolamine, you can do it yourself. Zero the spec down to 200 nm in acetonitrile. Then run a fairly concentrated sol'n of ethanolamine as a sample. Note any absorbances or where abs goes to >2. Make sure they are miscible first. Sometimes refractive indices of solvents will do funny things to baselines, especially in that region.

Aaaaaand here it is:

I've done the analyses as you suggested with a solution of water + ethanolamine with a 20% and 40% concentration and acetonitrile as background. As you can see there's a big absorbance at 225nm. So I think this one is the cut-off wavelength of the ethanolamine.

In my analytes I should get a peak from the nanoparticles around 210nm. Do you know if it's possible to see it with the UV-vis in any way even if there's ethanoalmine in the solution?

 

[Kevin McHugh]

Are you using the ethanolamine as a suspension agent for the nano particles?

 

[acvtre]

I'm trying different solutions and one of these has 0.1% of ethanolamine as a suspension agent, right.