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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: 08roselx on December 02, 2019, 12:06:31 PM

Title: Uv-Vis Spectrophotometry Gaps in spectra
Post by: 08roselx on December 02, 2019, 12:06:31 PM

Hi

I am trying to calculate the SPF of sunscreens using the Mansur equation. I am using a method directly from a paper.

The solutions are made up in ethanol and the UV-Vis is blanked with the ethanol prior to analysis. 

I weigh 0.25g sunscreen and make up to 50ml then take 1ml and make to 25ml.

I took a spectrum of one formulation (made in triplicate) and I get breaks in the spectra where there is no absorption value?

I diluted the sample further as at some wavelengths I was getting absorption values of 4 and still the same issue. I also got negative absorption values. 

Can anyone help as to what this may be as I have never seen this before?

Title: Re: Uv-Vis Spectrophotometry Gaps in spectra
Post by: Corribus on December 02, 2019, 01:50:55 PM

Can you show a sample spectrum? An OD of 4 is way too high.  You should try to be below 1 if possible.

Title: Re: Uv-Vis Spectrophotometry Gaps in spectra
Post by: wildfyr on December 04, 2019, 10:35:15 AM

I think your detector might be saturated because the concentration is way too high. You mean Absorbance values, not OD right?

I tend not to trust absorbance values over 2.

Title: Re: Uv-Vis Spectrophotometry Gaps in spectra
Post by: 08roselx on December 04, 2019, 05:04:35 PM

The solvent that I had been using was contaminated which is why further dilution didn't help me. Ran samples again today using a different batch of solvent and all was fine :) I knew the absorbance was too high that's why I diluted the samples further (even though I knew it shouldn't have been high as I was following a standard protocol) but dilution didn't reduce the abs values as it was contaminated with some other substance.