I am doing molecular-imprinted experiments, in UV-vis measurement, my sample solvent is methanol and acetone(9:1). I have scanned the baseline, but when I scanned the sample, it showed a negative peak? Why? Thanks.
In what range are you getting the negative peak?
Are you measuring the baseline with the solvent?
I suggest the following procedure:
1: Measure the base line using no cuvette.
2: Measure the absorption spectra of the cuvette with no solvent.
3: Measure the absorption spectra of the cuvette with the solvent.
4: Measure the absorption spectra of the cuvette with your sample.
With this method you will find if the cuvette or solvent is absorbing in the region you want to measure your compound.
See the Solvent Cut-off wavelengths and the cuvette cut-off for example at:
http://www.chem.fsu.edu/~shatruk/docs/Solvent-UV-cutoffs.pdf
http://www.sigmaaldrich.com/analytical-chromatography/analytical-products.html?TablePage=104902943
What you are probably using is a commercial UV-vis spectrophotometer. It works by comparing two different signals, the blank (in your case, MeOH/acetone) and the the bulk (MeOH/acetone/sample). A negative peak means your sample is more transparent than your blank.
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