Podcast
Questions and Answers
What is the primary function of a monochromator in a spectrophotometer?
What is the primary function of a monochromator in a spectrophotometer?
- To amplify the light signal
- To measure the irradiance of the emergent light
- To select a narrow band of wavelengths (correct)
- To contain the sample
What is measured in a single-beam spectrophotometer?
What is measured in a single-beam spectrophotometer?
- The molar absorptivity of the analyte
- The transmittance of the sample
- The irradiance of the incident light
- The irradiance of the emergent light (correct)
Under what condition does Beer's law fail?
Under what condition does Beer's law fail?
- When the concentration of the analyte is very high
- When the analyte undergoes a chemical change (correct)
- When the molar absorptivity is very low
- When the pathlength of the cuvet is very short
What is the purpose of a cuvet in a spectrophotometer?
What is the purpose of a cuvet in a spectrophotometer?
Why is glass not suitable for ultraviolet measurements?
Why is glass not suitable for ultraviolet measurements?
What is the common pathlength of a cuvet in a spectrophotometer?
What is the common pathlength of a cuvet in a spectrophotometer?
What is the purpose of recording a baseline spectrum in a double-beam spectrophotometer?
What is the purpose of recording a baseline spectrum in a double-beam spectrophotometer?
Why is it important to choose the wavelength of maximum absorbance?
Why is it important to choose the wavelength of maximum absorbance?
What is the ideal range of absorbance values?
What is the ideal range of absorbance values?
What is the main requirement for a compound to be analyzed by spectrophotometry?
What is the main requirement for a compound to be analyzed by spectrophotometry?
What is the unit of molar absorptivity?
What is the unit of molar absorptivity?
What is the purpose of wiping off fingerprints and dust from cuvets?
What is the purpose of wiping off fingerprints and dust from cuvets?
What is the product of frequency times wavelength equal to?
What is the product of frequency times wavelength equal to?
What happens to the energy of a molecule when it absorbs a photon?
What happens to the energy of a molecule when it absorbs a photon?
What is the relationship between irradiance and transmittance?
What is the relationship between irradiance and transmittance?
What is the unit of pathlength in Beer's law?
What is the unit of pathlength in Beer's law?
What is the unit of molar absorptivity?
What is the unit of molar absorptivity?
What is the limitation of Beer's law?
What is the limitation of Beer's law?
Study Notes
Measuring Absorbance
- In a double-beam spectrophotometer, light is split alternately between sample and reference cuvets.
- A baseline spectrum must be recorded first, which will be subtracted from the sample's absorbance.
- The wavelength of maximum absorbance should be chosen because it has the greatest sensitivity and least variation in absorbance versus wavelength.
- Optimal absorbance is between 0.4-0.9, as too much or too little light absorbed makes measurements difficult.
Beer's Law in Chemical Analysis
- A compound must absorb light, and this absorption should be distinguishable from other substances in the sample.
- A compound's molar absorptivity can be calculated from its absorbance and concentration.
Properties of Light
- Photons are particles of light with energy (E = hν), where h is Planck's constant (6.626 x 10^-34 J-s).
- The frequency (ν) multiplied by the wavelength (λ) equals the speed of light (c, approximately 3.0 x 10^8 m/s).
- The energy of a molecule increases when it absorbs a photon.
Absorption of Light
- When light is absorbed by the analyte, the irradiance (P) decreases.
- Transmittance (T) is the fraction of original light that passes through the sample, while absorbance (A) is the logarithm of the reciprocal of transmittance.
- Beer's law states that absorbance is proportional to the concentration of light-absorbing molecules in the sample.
- A chromophore is the part of a molecule responsible for light absorption.
Measuring Absorbance (continued)
- A spectrophotometer requires a continuous source, a monochromator, and a detector to measure the irradiance of emergent light.
- Cuvets are typically made of fused-silica (SiO2) and have a pathlength of 1.000 cm.
- For IR spectroscopy, cells are constructed of NaCl or NaBr, and solid samples are ground into a fine powder and dispersed in mineral oil (Nujol).
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Description
Learn about the principles of spectroscopy and absorbance measurement, including the use of double-beam spectrophotometers, recording baseline spectra, and selecting the optimal wavelength for analysis.
