Introduction to Chromatography

Chromatography, ohh the editor showing me this as a error !! But i am sure u must be known to aleast to this word that u can say such a word exist. This is a common and simple instrument in analytical instrumentation.

The technique is used for analysis of available chemical components in a fluid(gas or liquid) mixture. The technique adopt several methods to separate the components and then detecting the concentration of each component.

A Sample Chromatogram

The output is graph with time in x-axis and concentarion of different components on the Y-axis

What is the Principle and the method ?

when a fluid is allowed to pass through some another fluid or solid tunnel, the rate of flow for the components of mixture will be different. So at the other end of the tunnel the components will reach at different time.(the medium or the column called separator).

OK now our job turned easy becoz we know which component will come out at which time( its called retention time). we have to detect the gas coming out at different time.

Why different retention time?

Now the questions is why these components(our analyte is a mixture of different molecules and ions). The prime cause is the average speed. Acording to kinetic theory the average speed of gas molecules will depend upon its molecular mass.

except this there are different processes invloved in side the chromatography column/channel. The molecules may be adsorbed on the columns inner surface temporarily and then released. this stick and release may happen many times before it comes out of the column.

The terminology

Now its time to get some terminology things. i hate these from the begining as these may confuse the reader.

The gas (or liquid) is passed through the tunnel(chromatography column) which contains some material called the stationary phase. Again the analyte mixture may not pass through itself so we need some truck(we call it carrier gas) it acts as the mobile phase.

Simply we may say the mobile phase+analyte mixture is passed through the stationary phase which causes the separation of components. Separation caused by differntial partiotioning, selective adsorption, selective ion exchange...etc.

Scintillation, luminescence, Fluorescence & Phosphorescence

All these three are used in instrumentation engineering. But is this all are the same, what is the relation, similarity or difference between these processes.

florescence means absorbing energy in some higher energy or higher frequency band end emit in lower band.

luminescence is the type radiation from a cold body. here the excitation of the molecules must be by some other method than heating.

Scintillation is when the material is excited by some radiation and give out some emission in lower band(visible range)

X-Ray Detectors

These Detectors are categorised into two main classes depending on their application 1) Imaging, 2) Dose measurement, except this some Analytical instruments use these detectors.
Photographics plates are the simplest type of detector. Plates Coated with photosensitive chemical (silver halides) when exposed to Xray decomposes to form silver Metal. Now photographic plate type detectors replaced by semiconductor detectors both for imaging and analytical instruments (XRD).

X-Ray Gas Detectors are same as the radiation detectors used for detection of neutral (chargeless particle) radiation detectors.

These are categorised as following

• Gas Ionization Chamber
• Solid State detectors
• Scintillators
• Scintillator Solid state Detectors

Gas Filled Detectors / Gas Ionization Chamber
Gas ionisation chambers are generally a tubular structure in which a gas placed between two electrons allowed to be exposed to the radiation. which in turn ejects out an electron from the gas atoms shells giving a charge particle (ionised gas atom). This ion can now be drifted by the electric field. The ion is collected at the anode(+ve) so producing a pulse of current in circuit.

Geiger Muller Tube

Ionisation Chamber, proptional Counter and Geiger Muller Couter are usable for Xray detection.

X-Ray Methods in Analytical instruments

What is X-Ray ?

X-Ray is an electromagnetic wave with frequency 3x10E16Hz to 3x10E19 Hz or in wavelength scale 0.01 to 10 nano meters i.e. shorter than UV and longer than gamma radiation. In energy scale 120eV to 120 KeV. these are em waves with High penetrating Power.

What are the Properties useful for Us?

Penetration power: this ray can penetrate deep into metals atteneuting throughout. So can used probe into solid blocks.

optical Resolution: as per raleigh criterion the resloution of an optical microscope increases with decrease in wavelength and these are em waves with lowest wavelength.

Diffraction:Wavelength suitable for crystalography. The planes of crystals produce a diffraction pattern when radiation of suitable wavelength target on it. the xray wavelength suitable for this.

as pern barggs equation

2dsin(theta)=n x Lambada

Spectroscopic Analysis: The emission / absorption is not just a single line or just a single band. It is a set of quantized bands. Now you can say then sodium(Na) has only two lines 589.0 and 589.6 nm , yes it is true if you are looking only at the visible spectra.

there are more spectras which belongs to transition other than 3p1/2->3s1/2(589nm) and 3p3/2->3s1/2(589.6nm).

We have more no of spectra for most of the elements in X-ray region.

What are the classes ?

These rays are categorised according to penetrability through matter. Hard Xrays (12ev to 120Kev)/0.1 to 0.01nm wavelength. and soft xrays (0.12 to 12KeV)/(10->0.1nm).

These are sometimes also classified as per the method of generation or its spectrum.

Continuous / white X-ray

Discrete / Characteristic X-ray

Generation Methods:

There are two methods two generate x-ray based on two physics processes.

Flouroscence: when a high energy photon stikes a electron it ejects out the electron from the shell leaving a blank space. That blank space filled by a electron from upper shell which means dexcitation of the electron so there is a emission of a photon.

Bremhallstrahlung: This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube.

So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines.

X-ray Generation Techniques:

X-ray is generated by X-ray tubes. I have diagram of the two oldest tubes and the commercial tube. The basic design is same as we have to achieve the process "bombardment of high eneergy electron beam over some metal surface". The enhancements in this tube is

1) Enhancement of the cathode (use of filaments, use of isolated filament)

2) Use of cooling systems for anode.

3) Replaceable anode as the anode is continuously corroded.

4) Rotating anode tube

The output of the X-Ray tube is dependent on the enrgy of the electron beam, which in turn dependent on the electric field provided by the applied potential.

So the applied potential will be one key parameter for the output energy.

Another Key factor is the anode temperature, how much heat the anode can sustain,

only 0.05% of the electron beam is converted to x-Ray then what happen to rest part is converted to heat. So this amount of heat should be dissipiated in anyway otherwise our anode will melt down.

Figure 1. Schematic of an X-ray tube for X-ray diffraction analysis: (1) metallic anode block (usually grounded); (2) beryllium window for exit of X-radiation; (3) hot cathode; (4) glass envelope insulating the anode part of the tube from the cathode part; (5) cathode leads, to which the filament voltage and high (with respect to the anode) voltage are supplied; (6) electrostatic electron-focusing system; (7) anode (anticathode); (8) inlet and outlet pipes for the flowing water that cools the anode block