I have taken absorption spectra of muscovite mica in UV-visible spectrometer. The absorption is constant (flat) from 200 nm to 300 nm and then falls. What could be the possible reason for this flatness? Does this depend on the material used or is there any limitation for the instrument?
Did you measure in absorbance? How large is absorbance, maybe around 3? If yes, transmission is too low and you get a plateau which could show some spikes due to detector noise. If you can, post a spectrum.
Thank you Prof. Thomas. I have measured absorbance. Abosorption is in the range of 1 to 10. Please see the image (wavelength range 200 to 300, the absorption is constant with wavelength).
I presume the y-axis is Absorbance - which is a unitless parameter, and does not have arbitrary units.
Absorbance is -1*log10(Transmission)
Transmission = Iout/Iin
An Absorbance of 1 corresponds to a transmission of 0.1 (10%)
An Absorbance of 2 corresponds to a transmission of 0.01 (1%)
An Absorbance of 3 corresponds to a transmission of 0.001 (0.1%)
Increasing Absorbance means decreasing Iout, to the stage where the transmitted light is comparable to the noise levels, and is no longer representative of the sample. The Absorbance value at which this happens depends on the spectrometer, and the wavelength range.
In general, there is less input light in the UV region, and an Absorbance of 2 or greater is the highest that can be trusted.
I fully agree with Hugh's answer. How do your samples look like? Do you use any substrate? Looking at the spectra, I would recommend to you to make the mica thinner if you can and to record the spectra again. Ideally absorbance should be between 0.5 and 1, in this range the instrumental errors should minimal.
Mica sample is already thin. The value of absorbance observed for mica in literature around 2-3. I am giving two references here. My question is why the absorbance is flat near 300 nm wavelength?
when the absorbance is too high, the transmitted light is at or below the noise level, so the instrument calculates
Abs = -1*log10 (Noise/Iin)
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