Question

1)

Write a brief introduction on the theory behind IR spectroscopy and gas chromatography. Explain what each instrumentation technique is doing and how it works. Also address the

purpose of each part of the experiment.

Answer 1

IR spectroscopy

Theory -

Adsorption of infra red radiations (IR) is due to the changes in the rotational and vibrational levels of a molecule. The adsorption of IR creates varoius bands in a molecule to strech and bend wth respect to one another. Adsorption of radiation frequency less than 100 cm^-1 causes molecular rotation. again when radiation with frequency 10000 to 100 cm^-1 is absorbed molecular vibrations take place.

Thus vibrational changes occurs with and rotaional changes. Hence IR spectra appears as Vibrational-rotational bands. The refion of IR spectra is 2.5 to 15 .

In IR spectra no two compounds other than enantiomers show similar spectra.

when IR light is absorbed by a molecule vibrational and rotational energies are increased. Two main vibrations are 1. stretching and bending.

Stretching vibrations are two types a) symmetric b) asymmetric

Bending vibrations are four types a) Scissoring b) Rocking c) Wagging d) Twisting

Bending vibrations need lesser energy than stretching.

Instrument technique:

NERNST GLOWER is the source of IR light. In Nernst filament IR light is produced when a rod of oxides of Zirconium, Erbium and Ytterbium is electrically heated to 1000 to 18000 cm^-1.

Another filament is used a GLOBER filament. It is more effective in lower frequencies. now to get a monochromatic light an optical prism is used. Glass and quartz can not be used as they absorb IR loght.

Sodium chloride is commonly used as cell containers as it gives very good resolution in the 1350 to 600 cm^-1 range.

thereafter the light source is splitted into two beams . One is passed through the sample and another beam called reference beam . When the beam is passed through the sample it is then less intense due to absorption of certain frequencies of light. Now there is difference between the intensity of reference (I₀) and sample beam (I). The absorbanmce of the sample is then calculated by

A = log (I₀/I) .

Gas chromatography

Theory

In gas chromatography sample is vapourised ( without decomposition) by adding it into a heated system by eluting through a column by inert gases (mobile phase) and then detected.

Gas chromatograpy are three types

Gas-Solid

Gas-liquid

Gas-Bonded phase

The separation of compounds is done on the basis of different strength of affinity of the compounds with the stationary phase. Longer the compound interacts with the stationary phase longer time it will take to elute from the column.

parameters used in GC

Retention time

It is the difference of time required for 50% elution of a component from a column.

Retention volume

It is the volume of a gas whioch is required to elute 50% elution of a component from a column..

Height equivalent to a theoritical plate (HETP) - Efficiency of a column is expressed by HETP

HETP = length of the colum / No of theoritical plates

Instrumentation

For all other chromatography techniques a mobile and a stationary phase is required. Mobile phase is composed of inert gases an stationary phase is a microscopic layer of liquid or polymer in a solid support in a glass tube, which is called column .

GC contains different type of detectors

Mass spectrometer- Uses mass spectroscopy to detect organic compound by their fragments.

Flame ionisation detector - very sensitive to all organic compounds

Thermal conductivity detector - low sensitivity , used earlier.

Electron capture detector - Response well more compounds containaing electronegetive groups or atoms. But it is radioactive as sample passes over beta emitter and highly selective.