1. Introduction to Spectrochemical Methods
Chapter 7

2. Introduction Spectrochemical methods Atomic spectroscopy
Absorption or emission of light
More than half or all instrumental methods of analysis
Spectroscopy or spectrometry
Science that deals with light
Its absorption and emission by solutions
Other material substances
Instrument used
Spectrometer
When using a light sensor/phototube = spectrophotometer
Spectrochemical analysis
Degree which light absorbed or the primary light emitted
Related to the amount of analyte present in the sample
Critical measurements!
Atomic spectroscopy
Spectral differences between atoms
Molecular spectroscopy

3. Characterizing Light Dual nature of light Particles Waves
Photons or quanta
Particle theory of light
Waves
Electromagnetic disturbances or electromagnetic waves
Wave theory of light
Dual nature not unlike modern description of electrons
Described as particles
To explain aspects of their behavior
For more accurate description
Must be described as entities of energy and NOT particles

4. Characterizing Light Wave theory of light
Light travels in a fashion similar to that of a series of repeating waves of water
Wave pool at an amusement park
Electromagnetic waves
They are wave disturbances that have an electrical component and a magnetic component
Do NOT require matter to exist
Can travel through a vacuum

5. Characterizing Light
Wavelength, speed, frequency, energy, and wavenumber
Wavelength (λ)
The physical distance from a point on one wave, to the same point on the next wave
Measured in metric units

6. Characterizing Light
Wavelength, speed, frequency, energy, and wavenumber
Speed of light (c)
Speed in which electromagnetic waves move
Speed of light in a Vacuum ≈ 3.00 x 1010 cm/sec
Accounts for instantaneous speed that light fills a room when switch turned on
ALL ELECTORMAGNETIC WAVES TRAVEL AT THE SAME SPEED IN A VACUUM REGARDLESS OF THEIR WAVELENGTH

7. Characterizing Light
Wavelength, speed, frequency, energy, and wavenumber
Frequency (ν = nu)
Number of moving electromagnetic waves past a fixed point in 1 second
Expressed in waves or cycles per second
= hertz (Hz)
Units = sec-1
Wavelength, speed, frequency can be expressed mathematically
C = λν units are cm x sec-1 or cm/sec
Wavelength and frequency are inversely proportional
As one increases
Other decreases

8. Characterizing Light
Wavelength, speed, frequency, energy, and wavenumber
Energy
B/c light is a form of energy
Each wavelength or frequency has certain amount of energy
Considered to be the energy associated with a single photon of light
.: particle theory and wave theory linked via energy
E = hν
E = energy, h = proportionality constant called Planck’s constant
depends on units used-metric = 6.63 x J/sec
.: E = hc/λ

9. Characterizing Light
Wavelength, speed, frequency, energy, and wavenumber
Wavenumber (ν)
Wavelength expressed in centimeters
Characterized by the reciprocal of this wavelength
ν = 1/ λ (cm) Used in conjunction with infrared light.

10. The Electromagnetic Spectrum

11. The Electromagnetic Spectrum
So broad broken down into regions
Visible light
That portion of the spectrum we see with our eyes
≈ 350 nm to ≈750 nm
Very narrow region
UV, infrared, x-ray, radio, and television

12. The Electromagnetic Spectrum
UV, visible, and infrared regions
Mostly ones emphasized
Nanometer and micrometer units used for wavelength
Something to remember
Long wavelength = low energy
Infrared region
Wavelengths extremely short
Have higher energy than radio or television
Cause no harm
Remotes for TVs, VCRs, etc
UV, x-rays, and gamma rays
Very short wavelengths
Very high energy
Very dangerous!

13. Interaction of Light With Matter
Light striking matter causes different events
Transmitted
Pass without interaction through the material
Light passing through glass
Reflected
Changes directions
Light in a mirror
Scattered
Deflected into many different directions
Occurs when light strikes a substance composed of many individual, small particles
Absorbed
Light fives up some or all of its energy to the material

15. Absorption Spectra Instruments used to measure absorption
Some in the UV and visible regions
Others for the infrared region
Methods used
Beam of light formed
Sample measured contained so that light passes through
Absorption of the wavelengths present in light beam measured by a sensor and signal processor

17. Absorption Spectra An absorption spectrum
Plot of the amount of light absorbed by a sample vs. the wavelength of the light
Light absorbed called the absorbance (A)
Obtained by using a spectrometer to
Scan a particular wavelength region
To observe amount of light absorbed by the sample along the way
It’s a continuous spectrum (fig. 7.13, pg. 189)
The spectrum is an unbroken pattern
Does not display breaks or sharp peaks

18. Absorption Spectra Absorption vs. wavelength
Can be displayed as a transmission spectrum
Plotting the amount of light transmitted by a sample
Rather than the light absorbed
y-axis is transmittance (T) or percent transmittance instead of the absorbance (fig. 7.16, pg. 191)
High transmittance = low absorbance and vice versa
Absorption pattern
Differs from compound to compound
“molecular fingerprint”
Often useful for identification
Detecting impurities
Other sample components

19. Absorption Spectra Light Emission Matter will emit light Fluorescence
Molecular and ionic analytes
Useful for qualitative and quantitative analysis
Called an emission spectrum
Plot of emission intensity vs. wavelength
Product of the change in the energy level of an electron
From excited state to ground state (lowest energy level)
Fluorescence
When molecules/complex ions emit light under certain conditions
When absorption of light in the UV region is followed by emission of light in the visible region
Involves the loss in energy from an excited state to a lower state

20. Absorbance, Transmittance, and Beer’s Law
A = εbc
Beer’s Law
ε = extinction coefficient or absorptivity
Units depends on other parameters
Absorbance is dimensionless quantity
b = path length
Distance the light travels through the measured solution
Inside diameter of the sample container
Usually centimeters or millimeters
c = concentration
Expressed in any concentration unit
Usually expressed in molarity, ppm, or grams/100ml.
.: c= molarity, b = cm, then absorptivity = L mol-1 cm-1

21. Absorbance, Transmittance, and Beer’s Law
Container to be used
Varies according to the method
UV-VIS
Small test tube or square tube with an inside path length of 1 cm
Called a cuvette IR
Container called the IR liquid sampling cell
Sample contained in a space between two salt plates
Created with a thin spacer between the plates
Path length is the thickness of the spacer

22. Absorbance, Transmittance, and Beer’s Law
Quantitative analyses by Beer’s law
Prepared series of standard solutions
Measure absorbance of each in identical containers
Plotting the measured absorbance vs. concentration
Creates a standard curve
Absorbance of an unknown solution then measured and concentration determined from the standard data