Department of Chemical and Biomolecular Engineering
Faculty of Engineering
INSTRUMENTAL CHEMICAL ANALYSIS:
BASIC PRINCIPLES AND TECHNIQUES
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Department of Chemical and Biomolecular Engineering Faculty of Engineering
INSTRUMENTAL CHEMICAL ANALYSIS:
BASIC PRINCIPLES AND TECHNIQUES
PREFACE
This manual has been prepared for the final year undergraduate students to educate the basics in the instrumental chemical analysis and techniques. It explains the classification of instruments based on the interactions of the molecules with the matter and energy, the principles involved in the individual technique and their applications in various fields. As this work is intended for quick and easy learning all the descriptions are kept at concise and simple, though appropriate references are given for advanced and detailed descriptions.
We hope that this could be a good initiative and guidance for the students who as a part of their study programme should pursue with short research works. We strongly advice the students to go through this manual completely before handling the instruments as this may give some confidence and familiarisation over the techniques.
The details explained in this manual have been collected from various sources cited at the reference section and any suggestions or modifications for this manual from the staff members and students are welcome.
Good Luck! DR D RAJARATHNAM
- 1 INTRODUCTION
- 2 CLASSIFICATION OF THE ANALYTICAL TECHNIQUES
- 3.1 Processes in Spectroscopy 3 ANALYSIS THROUGH SPECTROSCOPY
- 3.1.1a Atomic Absorption Spectroscopy ( AAS ) 3.1.1 Atomic Level
- 3.1.1b Atomic Emission Spectroscopy ( AES )
- 3.1.1c Plasma Emission Spectroscopy
- 3.1.1d Fluorometry : Atomic Fluorescence
- 3.1.2a Ultraviolet - Visible Spectroscopy ( UV/Vis ) 3.1.2 Molecular Level
- 3.1.2b Fluorometry : Molecular Fluorescence
- 3.1.2c Fourier Transform Infrared Spectroscopy ( FT-IR )
- 3.1.2d Fourier Transform Raman Spectroscopy ( FT-RAMAN )
- 3.1.2e Microwave Spectroscopy
- 3.1.2f Nuclear Magnetic Resonance Spectroscopy ( NMR )
- 3.1.2g Electron Spin Resonance Spectroscopy ( ESR )
- 4.1 LIQUID CHROMATOGRAPHY (LC/HPLC) 4 ANALYSIS THROUGH CHROMATOGRAPHY
- 4.1.1 Adsorption Chromatography
- 4.1.2 Partition Chromatography
- 4.1.3 Ion-Exchange Chromatography
- 4.1.4 Size Exclusion Chromatography
- 4.2 GAS CHROMATOGRAPHY (GC)
- 5.1 THERMAL ANALYSIS 5 ANALYSIS THROUGH THERMAL ENERGY
- 5.1.1 Thermogravimetric Analysis ( TGA )
- 5.1.2 Thermomechanical Analysis ( TMA )
- 5.1.3 Differential Thermal Analysis ( DTA )
- 5.1.4 Differential Scanning Coloriemetry ( DSC )
- 6.1 Generation of X-Rays 6 ANALYSIS THROUGH X-RAY TECHNIQUES
- 6.2 X-Ray Fluorescence Spectrometry ( XFS )
- 6.3 X-Ray Photo-emission Spectrometry ( XPS )
- 6.4 X-Ray Diffractometry ( XRD )
- 7.1 Scanning Electron Microscopy ( SEM ) 7 ANALYSIS THROUGH MICROSCOPY
- 7.2 Transmission Electron Microscopy ( TEM )
- 7.3 Scanning Probe Microscopy ( SPM )
- 8.1 Polarography 8 ANALYSIS THROUGH ELECTRO-CHEMICAL TECHNIQUES
- 8 .2 Electrophoresis
- 9 .1 Total Organic Carbon Analyzer ( TOC ) 9 ANALYSIS THROUGH MISCELLANEOUS TECHNIQUES
- 9.2 Elemental Analyzer ( CHN/S Analyzer )
- 9.3 Polarimetry
- 9.4 UV/Visible Spectropolarimetry
- 9.5 Vibrational Circular/Linear Dichroism ( VCD/VLD )
- 9.6 Mass Spectrometry ( MS )
- 9.7 Laser Light Scattering System (Particle Sizing and Analysis)
- 10 SUMMARY
- 11 REFERENCE
1. INTRODUCTION
The need of the sophisticated analytical instruments and determinations using them is almost a routine process for the modern chemical laboratories. It has been a vast expanding area of knowledge as the instrument and computer manufacturers are producing analytical machines, which are in ever-increase of power and scope. Further, all the manual techniques in the line of the analytical studies had steadily been transferred to the instrumental techniques. Basically, chemical analysis can be divided into three broad categories as given below, which are almost invariably applied to major areas such as Fundamental Research, Product Development, Product Quality Control, Monitoring & Control of Pollutants, Medical & Clinical Studies, etc:
QUALITATIVE ANALYSIS: Chemical analysis which just identifies one or more species present in a sample
QUANTITATIVE ANALYSIS: Chemical analysis which finds out the total amount of the particular species present in a sample
STRUCTURAL ANALYSIS: Chemical analysis which helps in finding the spatial arrangement of atoms in a molecule and the presence or position of certain organic functional groups in a given compound
In addition, ‘Surface Analysis’ , plays an important role in material studies to obtain surface related physical properties such as topography, depth profiling, orientation of molecules, etc.
Chemical analysis has some basic steps like, choice of method, sampling, preliminary sample treatment, separations, final measurement and assessment of results. It is with the first step viz. choice of method, care should be exercised to select the proper instrument to carry out fruitful analysis. A wrong selection at this point will lead to a meaningless analysis. Selection of the instrument is such important criteria!
In order to compete with this type of situation some basic knowledge of instruments and analytical techniques are required. This may give the person the ability, with some confident, to choose and operate a varied range of instruments which would be much required at the advanced research laboratories. The following pages will give an insight into the theory, principles and applications of various analytical instruments.
3. ANALYSIS THROUGH SPECTROSCOPY
3.1 Processes in Spectroscopy The interaction of the light (electro-magnetic radiation) with a substance and the subsequent energy transfer ends with three main processes namely:
Absorption: The process by which the energy of the light (in the form of photons) is transferred to the atom or molecule raising them from the ground state to an excited state
Fluorescence: The absorbed energy is rapidly lost to the surroundings by collisions within the system and relax back to the ground state. Sometimes the energy is not lost in this way but is re-emitted a few milli seconds later, which is referred as fluorescence
Emission : If the substances (atoms or molecules) are heated to high temperatures (in a flame or in an electric discharge) the electrons are exited to higher energy levels. Later, they relax to the ground state with the emission of radiation, the magnitude of which is more or less equivalent to absorbed energy
Most of the analytical techniques are based on the light interactions with the substances and utilise any of the above three associated processes. Substances interact with light differently at various wavelengths and hence different types of analysis & instruments. The entire spectrum of light can be represented as below. Since, light has both electrical and magnetic components, this representation is referred as an ‘Electro-Magnetic Spectrum’ :
Fig: 1 g p λ = 0.1 nm 10nm 200nm 400nm 800nm 0.4cm 25cm
Nuclear Inner orbital Ionization of Valence Molecular vibrations/ Spin orientation Reactions electron transition atoms/molecules electron transition rotations in magnetic field
High energy Low energy High frequency Low frequency Short wavelength Long wavelength
Gamma Rays X-Rays^ UV^ Visible^ IR^ Microwaves^
Radio and TV waves
The following is short comparison between Ultra Violet (UV), Visible (Vis) and Infra Red (IR) ranges for the energy, frequency and wavelength: Energy: UV > Vis > IR Frequency: UV > Vis > IR Wavelength: UV < Vis < IR
The symbol for the wavelength is “lambda” (λ) and the unit is either nanometer (nm) or
micrometer (or micron, μm). The symbol for frequency is “nu” (μ) and the unit is either hertz
or sec-1. A parameter closely related to frequency is the wave number, which has the symbol “nu bar” ( υ ) and the unit is cm-^1.
There are two levels by which the substances can interact with the light as, atomic level and molecular level and hence the corresponding techniques:
3.1.1 Atomic Level 3.1.1a Atomic Absorption Spectroscopy (AAS): Principle: The sample is vaporized by aspiration of solution into a flame or evaporation from electrically heated surface (temperature range: 1800 – 3100^0 K). At this condition where the individual atoms co-exist, a beam of light is passed through them. The atoms will absorb in the visible and ultraviolet region resulting in changes in electronic structure (excited state). So, the resultant light beam coming out of the sample will be missing the light in the corresponding wave length, which is a measure of the characteristics of the sample.
Instrumentation: Sources emitting radiation characteristic of element of interest (hollow - cathode lamp), flame or electrically heated furnace, monochromator, detector (photomultiplier) and recorder. The following is the simplified outline of the instrumentation:
Fig: 2
Applications: This is the most widely used technique for the quantitative determination of metals at trace levels (0.1 to 100ppm), which present in various materials. It utilizes Beer - Lambert Law for the analysis and a standard curve is obtained by plotting absorbance vs concentration of the samples taken. The usual procedure is to prepare a series of standard solutions over a concentration range suitable for the sample to be analysed. Then, the standards and the samples are separately aspirated into the flame, and the absorbances are read from the instrument. The plot will give the useful linear range and the concentrations of the samples can be found out from the plot.
Disadvantages: Sample must be in solution or at least volatile. Individual source lamp and filters needed for each element, since, each metal has its own characteristic absorption.
3.1.1b Atomic Emission Spectroscopy (AES): Principle: This is simply called as ‘Flame Photometry’, and measures the atoms excited by a flame (temperature range: 2000 – 3100 0 K) and not by light source as in the atomic absorption case. After excitation, atoms will readily lose the gained energy and revert back to the ground state and the emission occurs. It is that emission that actually being measured. The wavelengths of the emitted light will almost be similar as those that were absorbed in the atomic absorption, since exactly the same energy transitions occur, except in the order of reverse!
A plasma source or jet is a flame-like system of ionized, very hot flowing argon gas. At high temperatures (≈ 6000 K) a gas such as argon will contain a high proportion of ions and free electrons constituting plasma (This ionisation is initiated by “Tesla” coil). Additional energy may be supplied to the electrons in the plasma by the application of an external electromagnetic field through RF coil. By collisions between the electrons and other species in the plasma this additional energy is uniformly distributed. As the collisions increase, the energy transfer becomes more efficient, which leads to a substantial temperature enhancement to a range of 8000 - 10000 K. It is the temperature at which the samples are introduced and analysed.
Applications: Similar to atomic emission spectroscopy but it covers very widespread for both qualitative and quantitative analysis of metals and some non-metals too, at trace levels. Because of the high temperature and homogeneity of the source, it offers better signal stability and hence the analytical precision. The technique when utilising an optical emission detector is termed as Inductively Coupled Plasma – Optical Emission Spectrometer ( ICP- OES ) and if it utilises a mass spectrometer ( refer section 9.6 ) as detector then it is termed as Inductively Coupled Plasma – Mass Spectrometer ( ICP-MS ).
Disadvantages: Samples require dissolution before analysis. Instrumentation is complex and requires high operator’s skill and is very expensive.
3.1.1d Fluorometry: Atomic Fluorescence This technique is not widely used though its counterpart - the molecular fluorescence is applied well to the analytical studies. The principle of atomic fluorescence is that when atoms are elevated to higher energy levels, they sometimes return to the ground state through a pathway, which has several intermediate electronic states, before reaching to the actual ground state. Such series of fall through the electronic levels accompany by light emission - which is atomic fluorescence. The intensity of this emitted light is measured at right angles to the incident light and related to concentration. Uses are similar to AAS and AES.
3.1.2 Molecular Level 3.1.2a Ultraviolet - Visible Spectroscopy ( UV/Vis ): Principle: It involves the absorption of electromagnetic radiation by the substances in the ultraviolet and visible regions of the spectrum. This will result in changes in the electronic structure of ions and molecules through the excitations of bonded and non-bonded electrons.
Instrumentation: It consists of a dual light source viz., tungsten lamp for visible range and deuterium lamp for ultraviolet region, grating monochromator, photo-detector, mirrors and glass or quartz cells.
NOTE: For measurements to be made under visible region both glass and quartz cells can be used. For the measurements under ultraviolet region, only quartz cell should be used, since, glass cells absorb ultraviolet rays.
There are two types of instrumental designs for this technique as single beam and double beam spectrophotometers. However double beam spectrophotometers are widely used and following is the outline of the instrument:
Fig: 5
Applications: It is the most widely used technique for quantitative molecular analysis, for this Beer-Lambert law is applied. Sometimes it is used in conjunction with other techniques such as NMR, IR, etc., in the identification and structural analysis of organic compounds. For qualitative analysis it provides valuable information through the absorption spectrum which is unique for a given compound.
Disadvantages: Samples should be in solution. Mixture of substances poses difficult to analyse and requires prior separation. Interference from the sample’s matrix makes the measurement difficult.
3.1.2b Fluorometry: Molecular Fluorescence Principle: This technique utilises the phenomenon of molecular fluorescence, the theory behind this is exactly the same that has been discussed under atomic fluorescence but through the excitation of bonded electrons. Here, most often the irradiating light is in the range of ultraviolet and visible.
Instrumentation: The instrumental set-up comprises of a UV/Visible source, two monochromators, detector and recorder. The fluorescence exhibited by the sample is measured at right angles to the incident beam. The following is the basic set-up:
Fig: 6
Applications: The applications of this technique are limited and it offers quantitative estimations of those compounds like benzene and fused benzene ring systems. Inorganic metals can also be analysed by the ability of them to form complexes with the ligands. It finds uses in the analysis of foods for vitamin content, since vitamins like riboflavin, niacin, etc., exhibit fluorescence. Only limited compounds show the fluorescence hence this technique is relatively free of any interference and is very sensitive.
3.1.2d Fourier Transform Raman Spectroscopy (FT-Raman): Principle: This technique is complementary to FT-IR and is a scattering technique, whereby a laser beam (near-IR region) is directed to the sample and the scattered radiation is collected. Most of the scattered radiation has the same wavenumber as that of the incident laser beam, however a fraction will be having a different wavenumber. This is the Raman signal and characteristic of particular functional group. Raman spectroscopy finds applications in identifying organic compounds containing non-polar bonds such as carbon - carbon double bonds or aromatic rings (weak dipoles).
Instrumentation: The instrumentation comprises of exciting laser normally in near-IR region, Rayleigh filter, beam splitter, detector, etc. Data collection and processing are akin to IR including the Fourier transformations.
Fig: 8
Collector Lens Sample
Laser Deflector
Rayleigh Interferometer Filter Laser Tube
Detector
PC System
Applications: The applications are similar to FT-IR and gives useful information on the non- polar bonds, i.e. bonds with null or reduced dipole moment. Water is a good solvent for FT- Raman.
Disadvantages: Signal strength is normally weak, and liquid samples give poor signals. Heat sensitive samples can’t be analyzed, since local heating will damage the samples. Dark colored samples can’t be analyzed.
3.1.2e Microwave Spectroscopy: This technique is actually an extension to IR spectroscopy. Microwave region lies at the far infra-red region of the electromagnetic spectrum and its absorption by molecules give rise to change in the rotational energies of the molecules. In IR spectroscopy, the molecules are subjected to changes in vibrational energies; the energy required for making changes at rotational levels is lesser than that for vibrational levels. Though the principles are same to that of IR, the instrumentation is slightly different and it requires samples in gaseous state for the analysis. Its applications are limited to smaller and simpler molecules since larger molecules will have interactions between the rotational energy levels within the molecule through various bonds they have. Besides qualitative analysis, this technique can be applied for conformational analysis of simpler compounds (study of stereo chemistry of the compounds).
3.1.2f Nuclear Magnetic Resonance Spectroscopy (NMR): Principle: In NMR substances absorb energy in the radio frequency region of the electro- magnetic spectrum under influence of a strong magnetic field. It is a well known fact that the nuclei of the atoms bonded to each other in molecules spin on an axis like a top. Since nuclei are positively charged, this spin will create a small magnetic field. If an external magnetic field is applied to these nuclei this magnetic field will split into two energy levels. The energy difference is very small and corresponds to radiofrequency energy which is unique for every molecule and will give the information regarding the nature of the compounds and the presence of various functional groups and their environment.
Since this technique is mostly measures the spinning of the hydrogen nuclei (almost all the organic compounds contain hydrogen atoms!), it is sometimes referred as Proton Magnetic Resonance (PMR) spectroscopy.
Instrumentation: The instrumentation for this technique includes powerful magnet, radio- frequency signal generator, amplifier, detector, etc. The following is the outline of the instrument:
Fig: 9
Applications: The application lies mostly in the identification and structural analysis of organic compounds and thus, it is mostly a tool for qualitative analysis. It gives valuable information regarding the position of the functional groups in a molecule and provides distinguished spectra for the isomer. Much precise information on the structure of the compounds can be obtained using the same technique with other magnetic nuclei like C 13 , O 17 , the instrumentation being the same except that the sweep of the magnetic field is varied.
Disadvantages: Very expensive and the instrumentation is complex and needs exceptional skills to operate. Its sensitivity ranges from moderate to poor, however, can get clear information using C^13 or O 17 NMR. The usage of the solvents is limited and in most of the situations deuterated solvents are required.
3.1.2g Electron Spin Resonance Spectroscopy (ESR): The basic principle of electron spin resonance spectroscopy is that, electrons always have a spin and thus have a magnetic moment. Thus, the magnetic resonance theory applies to electrons too like that of nuclei, as in NMR. Especially this technique is of high value when it comes to the compounds which contain odd electrons, i.e. those substances which have paramagnetic behaviour (if electrons are paired as in bonded orbital then their mutual spinning will cancel each other and there will be no response for the applied magnetic field,
4.1.1 High-Performance Adsorption Chromatography: Here the analyte species (components to be analysed) are adsorbed onto the surface of a polar packing. The stationary phase consists of finely divided solid particles packed inside a steel tube. If the component mixture is eluted through this tube with the mobile phase, different components present in the mixture adsorb to different degrees of strength and they become separated as the mobile phase moves steadily through the column. The nature of the adsorption involves the interaction of polar molecules with a very polar solid stationary phase. The stationary phase could be silica gel or alumina. This method is extensively used for the separations of relatively non-polar, water- insoluble organic compounds (since polar molecules will be adsorbed on to the column momentarily). One particular application is in resolving isomeric mixtures such as meta- and para -substituted benzene derivatives.
4.1.2 High-Performance Partition Chromatography: It is the most widely used liquid chromatographic procedures to separate most kinds of organic molecules. Here the components present in the analyte mixture distribute (or partition) themselves between the mobile phase and stationary phase as the mobile phase moves through the column. The stationary phase actually consists of a thin liquid film either adsorbed or chemically bonded to the surface of finely divided solid particles. Of these the latter is considered more important and has a distinct stability advantage. It is not removed from the solid phase either by reaction or by heat and hence it is more popular. It finds wide applications in various fields, viz., pharmaceuticals, bio-chemicals, food products, industrial chemicals, pollutants, forensic chemistry, clinical medicine, etc.
4.1.3 High-Performance Ion-Exchange Chromatography: This method is used to separate mixtures of ions (organic or inorganic), and finds its application mostly in protein separations. The stationary phase consists of very small polymer resin “beads” which have many ionic bonding sites on their surface, termed as Ion Exchange Resins. This resin can be either an anion exchange resin, which possesses positively charged sites to attract negative ions, or a cation exchange resin, which possesses negatively charge sites to attract positive ions. If the analyte mixture which contains mixture of ions is introduced into the column packed with suitable ion-exchange resin, selected ions will be attached or bonded on to the resin, thus being separated from other species that do not bond. Later, these attached ions can be dislodged from the column by repeated elution with a solution that contains an ion that competes for the charged groups on the resin surface, in other words, which has high affinity for the charged sites on the resin than the analyte ions. Thus the analyte ions get exchanged and separated from the column.
4.1.4 High Performance Size Exclusion Chromatography: This technique is for separating dissolved species on the basis of their size and particularly applicable to high-molecular- weight species like oligomers and polymers to determine their relative sizes and molecular weight distributions. Here, the stationary phase is polymer resin, which contains small pores. If the components to be separated are passed through the column the small sized particles can easily enter into these pores and their mobility is retarded. Whereas the large sized particles, which can’t enter into these pores can come out of the column fast and elude first. Thus the separation of various sized particles is possible through variations in the elution time. It is classified into two categories based on the nature of the columns and their packing as:
Gel Filtration Chromatography - which uses hydrophilic packing to separate polar species and uses mostly aqueous mobile phases. This technique is mostly used to identify the molecular weights of large sized proteins & bio-molecules.
Gel Permeation Chromatography - which uses hydrophobic packing to separate nonpolar species and uses nonpolar organic solvents. This technique is used to identify the molecular weights of polymers.
Instrumentation: The basic HPLC system consists of a solvent (mobile phase) reservoir, pump, degasser, injection device, column and detector. The pump draws the mobile phase from the reservoir and pumps it to the column through the injector. At the end of the column (effluent end), a detector is positioned. Mostly UV absorption detector is used. In the case of analytical studies, after the detection the eluents are collected in waste bottles. In the case of preparative studies the eluents are fractionally collected for further studies. Most of the HPLC design will be the same as described for all the four main groups previously described. However, there can be differences in selecting the specific detectors for particular type of analysis, say for example, with ion-exchange chromatography, detectors commonly used are conductivity detectors for obvious reasons. Other important detectors for HPLC separations include refractive index detector, fluorescence detector and mass selective detector. The following is the most generalised outlay of the HPLC system:
Fig: 10
Pump Degasser Column Compartment
Mobile Phase PC System Detector
Injector Valve
Sample Injection Degasser Pump
Column Compartment
Detector
Injector Valve
PC System
Solvent Tray
Disadvantages: Column performance is very sensitive, which depends on the method of packing. Further, no universal and sensitive detection system is available.
4.2 GAS CHROMATOGRAPHY (GC) Principle: Here an inert carrier gas (Helium or Nitrogen) acts as the mobile phase. This will carry the components of analyte mixture and elutes through the column. The column usually contains an immobilized stationary phase. The technique can be categorised depending on the type of stationary phase as follow:
Gas Solid Chromatography (GSC) - here the stationary phase is a solid which has a large surface area at which adsorption of components of the analyte takes place. The separation is possible based on the differences in the adsorption power and diffusion of gaseous analyte molecules. The application of this method is limited and is mostly used in the separation of the low-molecular-weight gaseous species like carbon monoxide, oxygen, nitrogen and lower hydrocarbons.
negative. While positively charged ions are drawn to a collector, negatively charged ions are attracted to positively charged burner head, this creates an electric circuit and the signal is amplified. The FID detector is very sensitive, but destroys the sample by burning. It only detects organic substances that burn and fragment in a hydrogen flame (e.g. hydrocarbons). Hence its usage is restricted for preparative GC and for inorganic substances which do not burn.
Electron Capture Detector (ECD) - this is another type of ionization detector which utilises the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the carrier gas molecules, thus generating electrons which constitute an electrical current. This detector is used for environmental and bio-medical applications. It is especially useful for large halogenated hydrocarbons and hence in the analysis of halogenated pesticide residues found in environmental and bio-medical samples. It is extremely sensitive. It does not destroy the sample and thus may be used for the preparative work.
Nitrogen/Phosphorus Detector (NPD) - the design of the detector is same to that of the FID detector except that a bead of alkali metal salt is positioned just above the flame. It is also known as ‘Thermionic Detector’. It is useful for the phosphorus and nitrogen containing pesticides, the organophosphates and carbamates. The sensitivity for these compounds are very high since the fragmentation of the other organic compounds are minimized.
Flame Photometric Detector (FPD) - here a flame photometer is incorporated into the instrument. The principle is that the sulfur or phosphorus compounds burn in the hydrogen flame and produce light emitting species. This detector is specific for organic compounds containing sulphur or phosphorus. It is very selective and very sensitive.
Electrolytic Conductivity Detector (ECD Hall) - this otherwise known as ‘Hall detector’, converts the eluting gaseous components into ions in liquid solution and then measures the electrolytic conductivity of the solution in a conductivity cell. The conversion to ions is done by chemically oxidizing or reducing the components with a “reaction gas” in a small reaction chamber. This detector is used in the analysis of organic halides and has excellent sensitivity & selectivity, but is a destructive detector.
The recent developments allow the GC to be coupled with other analytical techniques like Infra Red Spectrometry (Gas Chromatography-Infrared Spectrometry, GC-IR) and Mass Spectrometry (Gas Chromatography- Mass Spectrometry, GC-MS). These are termed as ‘hyphenated techniques’, and are very efficient for qualitative analysis as very accurate and precise information like mass or IR spectrum of the individual sample components are readily obtained as they elute from the GC column. It saves time and reduces the steps involved for a component to be separated and analysed.
Disadvantages: Samples must be volatile and thermally stable below about 400 0 C. No single universal detector is available and most commonly used detectors are non-selective. One should take much care in the analytical steps starting from the selection of the column, the detector and must define the temperatures of all the three ports viz., injection port, column oven and detector. An improper programming on these will lead to erratic results.
5. ANALYSIS THROUGH THERMAL ENERGY
5.1 THERMAL ANALYSIS
The technique of thermal analysis actually comprises of a series of methods, which detect the changes in the physical and mechanical properties of the given substance by the application of heat or thermal energy. The physical properties include mass, temperature, enthalpy, dimension, dynamic characteristics, etc. It finds its application in finding the purity, integrity, crystallinity and thermal stability of the chemical substances under study. Sometimes it is used in the determination of the composition of complex mixtures. This technique has been adopted as testing standard in quality control in the production field, process control and material inspection. It is applied in wide fields, including, polymer, glass, ceramics, metals, explosives, semiconductors, medicines and foods. The following are the popular methods under this technique:
5.1.1 Thermogravimetric Analysis (TGA): In this technique the change in sample weight is measured while the sample is heated at a constant rate (or at constant temperature), under air (oxidative) or nitrogen (inert) atmosphere. This technique is effective for quantitative analysis of thermal reactions that are accompanied by mass changes, such as evaporation, decomposition, gas absorption, desorption and dehydration. The following is the simplified diagram for the instrumentation:
Fig: 12
The micro-balance plays a significant role, during measurement the change in sample mass affects the equilibrium of the balance. This imbalance is fed back to a force coil, which generates additional electromagnetic force to recover equilibrium. The amount of additional electromagnetic force is proportional to the mass change. During the heating process the temperature may go as high as 1500 0 C inside the furnace.
5.1.2 Thermomechanical Analysis (TMA): Thermomechanical analysis is the measurement of a material’s behaviour, ie. expansion or contraction, when temperature and a load is applied. A scan of dimensional changes related to temperature (at constant load) or load (at constant temperature) provides valuable information about the sample’s mechanical properties. One of the most important applications of TMA is the characterization of composite and laminate materials, through the study of glass transition temperature and the expansion coefficient. Another application is in the quantitative measurement of extension and contraction observed in textile fibres, thin films and similar materials.
5.1.3 Differential Thermal Analysis (DTA): This technique measures the temperature difference between a sample and a reference material as a function of temperature as they are heated or cooled or kept at a constant temperature (isothermal). Here the sample and reference material are simultaneously heated