Multiple Choice
In UV-Vis spectrophotometry, how is the absorbance value displayed on a spectrophotometer defined in terms of incident light intensity and transmitted light intensity ?
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17. Fundamentals of Spectrophotometry
Measuring Absorbance
Multiple Choice
In UV-Vis spectrophotometry, what are the percent transmittance and absorbance of a properly prepared blank sample (used to zero the instrument) at the wavelength of interest?
A
and
B
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C
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D
and
Verified step by step guidance1
Understand that in UV-Vis spectrophotometry, a blank sample is used to set the baseline or zero point for the instrument at the wavelength of interest. This blank contains all components except the analyte, so it should ideally not absorb any light at that wavelength.
Recall the definition of percent transmittance (%T), which is the ratio of transmitted light intensity (I) to the incident light intensity (I_0), multiplied by 100: \(\%T = \frac{I}{I_0} \times 100\).
Since the blank does not absorb light, all the incident light passes through it, meaning \(I = I_0\). Substituting this into the %T formula gives \(\%T = 100\).
Absorbance (A) is related to transmittance by the formula \(A = -\log_{10}\left(\frac{I}{I_0}\right) = -\log_{10}\left(\frac{\%T}{100}\right)\). For the blank, since \(\%T = 100\), absorbance becomes \(A = 0\).
Therefore, the properly prepared blank sample at the wavelength of interest has a percent transmittance of 100% and an absorbance of 0, which sets the zero baseline for subsequent measurements.
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