Week 4
SPECTROPHOTOMETRY
INTRODUCTION TO SPECTROPHOTOMETRY METHODS
by Tan Ching Mun (16002662)
Spectroscopy or spectrochemical method is a study of the interactions of radiation and matters (absorption, emission, and fluorescence).
What is Electromagnetic Radiation?
Information that we can get from the wave:
- Amplitude ( A ): Length of the electric vector at a maximum in the wave.
- Period ( p ): The time in seconds required for the passage successive maxima or minima through a fixed point in space.
- Frequency ( ѵ ): The number of oscillations of the field that occur per second and is equal to 1/ p . (s 1 or Hz).
- Wavelength ( λ ): The linear distance between any two equivalent points on successive waves (successive maxima or minima). (Å, nm, etc.).
Example:
note: c=3x10^10 cm/s
λ= 147nm=1.47x10^-5cm
1 nm=1x10^-7 cm
Electromagnetic Spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.
Absorption of Radiation
- A process in which electromagnetic energy is transferred to the atoms, ions, or molecules of the sample.
- Promotes the particles of sample excited from ground states (lower energy) to one or higher energy states.
- The energy of the exciting photon must exactly match the energy differences between ground state and excited state of the absorbing species.
- has very sharp spectra
- Absorption of a few well-defined frequencies (simpler spectrum)
- Small number of possible energy states for absorbing particles - relative simplicity of spectra
- Excitation occurs by an electronic process
- has very broad spectra
- More complex than atomic spectra
- An enormous number of energy states of molecules
- Energy associated with the bands of molecules made up of three components.
(electronic> vibrational> rotational)
- Rotational
- Vibrational
- Electronic
- Line Spectrum
-sharp, well-defined peaks
-by individual atoms
- Band Spectrum
-group of lines
-closely spaced
- Continuum spectrum
-increase in the background
Characteristics of Infrared Spectra
- Absorption peaks are sharp (easy to identify)
- A ''Fingerprint", a complete absorption spectrum unique to that molecule can be obtained.
- Shows combined spectra of compounds
Quantitative Analysis
Requires two power measurements: one before a beam (incident radiation) has passed through the medium that contains the analyte (Po) and the other after (P).
Terms widely used in absorption spectrometry related to Po & P:
Transmittance & Absorbance.
Transmittance,T - fraction of incident radiation transmitted by the medium, or the amount of radiation not absorbed, or that was allowed to pass through the molecule (ranges from 0 to 1)
Absorbance, A - capacity of a substance to absorb light, reciprocal of transmittance
- When no light is absorbed, Po=P
- When the concentration of a solution increases, absorbance increases, transmittance decreases.
Beer's Law
- absorbance is proportional to the concentration of absorbing species
- works very well for a dilute solution ( </=0.01 M)
absorptivity, a (L/g cm)
b (cm)
c (g/L)
A =εbc
molar absorptivity
ε (L/mol cm)
c (mol/L or M)
For two absorbing species, if c is in g/ L:
Reflection
This week, I was introduced to the topic of spectrophotometry which includes electromagnetic radiation, absorption and emission process, qualitative and quantitative analysis, and analysis of mixture. This chapter reminds me of the knowledge I learned back in my secondary school. However, this chapter is still hard as there is a lot of new knowledge being taught. I learned the electromagnetic spectrum which varies from gamma rays to long radio waves. I also learned the visible spectrum which shows that light has many different wavelengths and each range of wavelength represents a different colour in the rainbow. These light can be absorbed and transmitted in another colour which is their complementary colours depending on the wavelengths of the light. Calculations are vastly involved in this chapter, the wavelength and frequency using the speed of light, the energy transferred, absorbance and, transmittance.
The topics regarding spectrophotometry will be continued next week.
Stay tuned!
Tutorial 3
As usual, the tutorial questions were posted prior to the tutorial session. I have gone through the questions by myself before being discussed. After attending the tutorial session, below are the correct answer for each question.
1. Define the following:
(a) Beer’s Law
Beer's Law states that the concentration of a chemical solution is directly proportional to its absorption of light. The premise is that a beam of light becomes weaker as it passes through a chemical solution.
(b) Absorbance
Absorbance is a measure of a quantity of light absorbed by a sample. It is also known as optical density, extinction, or decadic absorbance.
2. Calculate the wavelength of
(a) the sodium line at 589 nm in an aqueous solution with a refractive index of 1.35.
(b) the output of a ruby laser at 694.3 nm when it is passing through a piece of quartz
that has a refractive index of 1.55.
3. Identify factors that cause the Beer’s law relationship to be nonlinear.
Absorbance (A) is the negative logarithm of transmittance (T).
A= -log T
4. Calculate the frequency in hertz of
(a) an X-ray beam with a wavelength of 2.65 Å.
(c) the line at 694.3 nm produced by a ruby laser.
5. In a solution of pH 5.3, the indicator bromocresol purple exhibits a yellow color, but when the pH is 6.0, the indicator solution changes to purple. Discuss why these colors are observed in terms of the wavelength regions and colors absorbed and transmitted.
The yellow color comes about because the solution absorbs blue light in the wavelength region 435-480nm and transmits its complementary color (yellow).
The purple color comes about because green radiation (500-560nm) is absorbed and its complementary color (purple) is transmitted.
6. What is the relationship between absorbance and transmittance?
The absorbance has a logarithmic relationship to transmittance with an absorbance of 0 corresponds to a transmittance of 100% and an absorbance of 1 corresponds to a transmittance of 0%.
A = -log T
A = log ( Io/I)
7. Calculate the wavelength and the energy in joules associated with a signal at 220 MHz?
8. Calculate the wavelength in centimeters of
(a) an airport tower transmitting at 118.6 MHz
(b) an NMR signal at 105 MHz
9. Convert the accompanying transmittance data to absorbances.
(a) 27.2%
(b) 0.579%
10. Differentiate between Line spectrum and Continuous spectrum?
That's all for Week 3! See you next week, Spectrophotometry Part 2 for a better understanding of the application and instrumentation of spectrophotometry.
