UV – VISIBLE SPECTROMETRY

Jatin DewanDivya GuptaLakshata AggarwalManishaKrishna Panicker

SPECTROMETRY

•The sample absorbs a portion of incident radiation, the remainder is transmitted to a detector, where it is changed into an electrical signal and displayed, usually after amplification, on a meter, chart recorder or some other type of readout devices.

ANALYTICAL APPLICATIONS

•Analytical applications of the absorption of radiations by matter can be either qualitative or quantitative.

•A display of absorption v/s wavelength is called an absorption spectrum of that molecular species and serves as a “fingerprint” for identification.

RADIATION SOURCE

Radiation source have two basic requirements:-

1. High intensity

2. Stable.

TYPES OF RADIATION SOURCES

1. Hydrogen or deuterium discharge lamp

2. Tungsten halogen lamp

3. Xenon flash lamp

SOURCE STABILITY

High short term stability of an tungsten and discharge source is required for single beam spectrophotometers. The intensity of radiation from an tungsten or discharge source is proportional to the lamp voltage raised to some power that is larger than unity. Source stability is achieved by using constant voltage tranformers and electronic voltage regulators.

SELECTION OF WAVELENGTH

•Isolation of discrete bands of radiation.

•The narrower the bandwidth the better the performance.

•To isolate a narrow band of wavelength, filters or monochromators, or both, are used.

FILTERS

•ABSORPTION FILTERS

•INTERFERENCE FILTERS

Disadvantages •Not good wavelength selection.•May deviate from Beer’s Law.•They might also absorb the required

radiation.

MONOCHROMATORS•They are used for spectral scanning i.e.

varying the wavelength of radiation over a considerable range.

•All monochromators are similar in mechanical construction. They all have slits, mirror, lens, grating or prism.

GRATING MONOCHROMATOR SYSTEM

A grating is, in essence, an array of a very large number of equidistant slits that reflect or transmit radiation. Only at certain definite angles is radiation of any given wavelength in phase. At other angles the waves from the slits destructively interfere.

Detector

•A transducer that converts electromagnetic radiation into an electron flow and, subsequently, into a current flow or voltage in the readout circuit.

•Example: Photomultiplier tube

Photo-multiplier tube

•Combination of photo-emissive cathode, several dynodes and an anode.

Advantages

•Stable except after exposure to high light

levels• Sensitive•Reasonable cost•Rapid response

(on the order of nanoseconds)

Methods of detection• UV- Vis spectroscopy is applied to both

Qualitative as well as Quantitative analysis.

• Qualitative analysis concerns identification of an unknown substance and is frequently achieved through comparison of the spectrum of a solution of the unknown with a reference spectrum.

• Quantitative analysis : Determination of the concentration of given species. It is achieved through BEER-LAMBERT’S LAW.

Beer- Lambert’s law

•According to law:Absorbance is directly proportional to:

ConcentrationPath length

Absorbance= E.C.LWhere,

E= absorptivity constantC= concentration

L= path length

Instrumentation

Types of Uv- vis spectrophotomet

er

Single Beam Double Beam

Single Beam

•Earliest design.•The light beam do not split. All of it

passes through the sample or reference.•First a reference is used to calibrate the

instrument.•Then sample is measured, replacing the

reference.

Single Beam

Double Beam•The light beam is split into 2; one passes

through sample and other through reference.•They consists of rotating discs called Beam

choppers or Chopper mirrors.•Some instruments contain only a single

detector and the measurement is done alternatively.

•Some instruments contain 2 detectors and both the sample and reference are measured simultaneously.

Double Beam

Comparison of double beam and single beam

•DB Increases the speed with which measurements can be taken hence simplifies spectrometry.

•Reduces chances of human error.•More precise and consistent result.•SB needs to be calibrated regularly

between readings.•SB has a simple construction and is low

cost while DB is complex and expensive.