Instrumental ChemistryInstrumental Chemistry

Chapter 7 and 8

Optical Spectroscopy-

Components and Introduction

Brief IntroductionBrief Introduction Spectroscopic instruments were developed for use

in the visible region are optical instruments. Optical spectroscopic methods are based upon six

phenomena namely:

(1) absorption

(2) fluorescence,

(3) phosphorescence

(4) scattering

(5) emission,

(6) chemiluminescene

ABSORPTIONIn atoms, electron transitions of outer-shell electrons correspond to the absorption or emission of electromagnetic radiation that is in the ultraviolet and visible regions. Because vibrational and rotational energy levels are not possible in atoms, the absorption and emission of radiation that occurs in molecules when an electron travels between the numerous vibrational and rotational energy states of one electron level are not possible. Only a single transition between each set of electron levels is possible. As a consequence, the bands of emitted or absorbed radiation are narrow in atoms.

Absorption (cont.)Absorption (cont.) Atomic-absorption (AA) spectroscopy uses

the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption.

Light SourceLight Source The light source is usually a hollow-

cathode lamp of the element that is being measured. Lasers are also used in research instruments. Since lasers are intense enough to excite atoms to higher energy levels, they allow AA and atomic fluorescence measurements in a single instrument. The disadvantage of these narrow-band light sources is that only one element is measurable at a time.

AtomizerAtomizer AA spectroscopy requires that the analyte atoms be in the gas

phase. Ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source such as a flame or graphite furnace. Flame AA can only analyze solutions, while graphite furnace AA can accept solutions, slurries, or solid samples.

Flame AA uses a slot type burner to increase the path length, and therefore to increase the total absorbance (see Beer-Lambert law). Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small droplets before entering the flame.

The graphite furnace has several advantages over a flame. It is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.

FLUORESCENCE

The absorption of energy from a radioactive source by atoms can result in atoms in an excited electron level. Atomic fluorescence occurs when the excited atoms emit radiation after initially being excited by absorption of photons.

FluorescenceFluorescence Atomic fluorescence is the optical emission from gas-phase atoms that

have been excited to higher energy levels by absorption of electromagnetic radiation. The main advantage of fluorescence detection compared to absorption measurements is the greater sensitivity achievable because the fluorescence signal has a very low background. The resonant excitation provides selective excitation of the analyte to avoid interferences. AFS is useful to study the electronic structure of atoms and to make quantitative measurements. Analytical applications include flames and plasmas diagnostics, and enhanced sensitivity in atomic analysis. Because of the differences in the nature of the energy-level structure between atoms and molecules, discussion of laser-induced fluorescence (LIF) from molecules is found in a separate document.

Types of FluorescenceTypes of Fluorescence Resonant fluorescence: Occurs when the fluoresced radiation is of the

same wavelength as the absorbed radiation. Resonant fluorescence is the type that is used most often for quantitative analysis.

Direct-line fluorescence: Occurs when an electron in an excited state emits radiation upon falling, to an electron level that is above the level that is above from which the electron originally absorbed radiation. The wavelength of the emitted radiation is longer than the wavelength of the absorbed radiation.

Stepwise fluorescence: is preceded by absorption and collision deactivation to a lower excited electron level. Fluorescence occurs when the atom emits a photon as the electron or by collision deactivation.

Types of Fluorescence (cont)Types of Fluorescence (cont) Stokes fluorescence: Is fluorescence in which the fluoresced radiation

has a wavelength that is greater than the absorbed radiation. Further loss of energy to the around state can occur either by emission of another photon or by collision deactivation.

Anti-stokes fluorescence: It is a form of fluorescence in which the emitted radiation has a shorter wavelength than the absorbed radiation. A fifth type of fluorescence can also occur.. After an atom becomes electronically excited by absorption, the excited atom transfers some or all of its energy to an atom of a different element. The atom of the ground state (or some other lower energy level). Atomic fluorescence of that type, which is rarely encountered, is sensitized fluorescence.

The The SpectrofluorometerSpectrofluorometer

The sensitivity of fluorescence is dependent on both the fluorophore and the instrument. The response of a fluorophore will depend on the molar absorptivity and the quantum yield. These factors are, in general, beyond the control of the analyst.

PolarizationPolarization

Molecule of interest is randomly oriented in a rigid matrix (organic solvent at low temperature or room temperature polymer). Plane polarized light is used as the excitation source.

PhosphorescenceExcitation during both fluorescence and phosphorescence occurs when radiation is absorbed by an electron in the ground state of a molecule causing the electron to be excited to a higher electron state. During the absorption the electron does not reverse its spin. Generally excitation occurs from a singlet ground state to an excited singlet state.

Phosphorescence (cont)Phosphorescence (cont) The transitions between the excited singlet state and the

excited triplet state or between the excited triplet state and the ground singlet state are examples of intersystem crossing. The considerable barrier to spin reversal that exists in a molecule prevents intersystem crossing from occurring, as rapidly as singlet transitions. Because of that barrier, phosphorescence occurs on a much longer time scale than fluorescence.

SCATTERING

Radioactive Scattering is the change in direction of motion of an incident photon as it strikes a particle of the sample.

CHEMILUMINESCENCE:

Chemiluminescence occurs after excitation of a molecule or ion by the energy emitted during the chemical or biochemical reaction in which the excited species is a product.

In many cases, the chemical excited energy level of a molecule is identical to the energy level that could be attained by absorption of electromagnetic radiation.

Chemiluminescence (cont)Chemiluminescence (cont)

Chemiluminescence can occur in the ultraviolet, visible, or near-infrared regions. The majority of chemiluminescent reactions occur in the visible region. Bioluminescence (BL) is chemiluminescence that occurs in biological systems. Perhaps the best-known example of bioltiminescence is that which occurs when fireflies emit light.

Components of Spectroscopic Components of Spectroscopic InstrumentsInstruments

A stable source of radiant energy A transparent container for holding the sample A device that isolates a restricted re-ion of the spectrum

for measurement A radiation detector, which converts radiant energy to a

usable signal (usually electrical) A signal (usually electrical) A signal processor and readout, which displays the

transduced signal on a meter scale, a cathode-ray tube, a digital meter or a recorder chart

LASERS

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.

A laser is a device that emits high-intensity coherent (in-phase) radiation over a narrow (typically 0.001 to 0.0 I nm) bandwidth.

Components of LasersComponents of Lasers

MECHANISM OF LASER ACTION

Laser action can be understood by considering the four processes:

(a) Pumping

(b) Spontaneous emission (fluorescence)

(c) Stimulated emission

(d) Absorption

Atomic Line WidthsAtomic Line Widths

The widths of atomic lines are of considerable importance in atomic spectroscopy. Narrow lines are highly desirable for both absorption and emission work because they reduce the possibility of interference due to overlapping spectra. Line widths are of prime importance in the design of instruments for atomic absorption spectroscopy.

Atomization MethodsAtomization Methods In order to obtain optical and atomic mass

spectra, the constituents of a sample must be converted to gaseous atoms or ionized, which then can be determined by emission, absorption, fluorescence, or mass spectral measurements. The process by which the sample is converted into an atomic vapor is called atomization. The precision and accuracy of atomic methods are critically dependent upon the atomization step and the method of introduction of the sample into the atomization region.

AbsorptionAbsorption

Types of Designs of Types of Designs of Optical InstrumentsOptical Instruments

Temporal Design Spatial Design Multiplex Design

References www.anachem.umu.se/cgi/jumpstation.exe?

OpticalMolecularSpectroscopy www.anachem.umu.se/jumpstation.htm www.anachem.umu.se/cgi/jumpstation.exe?

AtomicSpectroscopy www.minyos.its.rmit.edu.au/~rcmfa/

mstheory.html http://www.chemsw.com http://www.scimedia.com/chem-ed/spec/

atomic/aa.htm