IR Spectroscopy.pptx

Explains in detail principle, instrumentation, advantages, disadvantages and applications of IR Spectroscopy.
As per PCI B. Pharmacy syllabus for BP701T, Instrumental Methods of Analysis.

1. 1 Prof. V. M. Patil Associate Professor & PG Teacher Department of Pharmaceutical Chemistry Ashokrao Mane College of Pharmacy, Peth Vadgaon Infra Red (IR) Spectroscopy

2. Infrared spectroscopy (IR spectroscopy or Vibrational Spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum.

3. A basic IR spectrum is essentially a graph of infrared light  absorbance  (or Transmittance) on the vertical axis vs. frequency or wavelength on the horizontal axis. Typical units of frequency used in IR spectra are reciprocal centimeters  (sometimes called  wave numbers ), with the symbol cm−1. Units of IR wavelength are commonly given in  micrometers  (formerly called "microns"), symbol μm , which are related to wave numbers in a  reciprocal  way. 

4. WAVELENGTH / WAVENUMBER/ FREQUENCY/ VELOCITY Wavelength ( λ ) : Difference between two successive maxima and minima and expressed as nm, mm, cm, m. Frequency (v) : Number of waves per unit time and expressed as Hertz or cycles per second as s -1 . Wavenumber (v̅): Number of waves per unit length and expressed as cm -1 . Velocity (c): Velocity of light in air i.e. 3× 10 8 m/s. Relationship λ = 1/ v̅ = c/v

5. The infrared portion of the electromagnetic spectrum is usually divided into four regions; Photographic region: ranges from visible to 1.2 μ associated with higher energy Very near IR region- overtone region- ranges from 1.2 to 2.5μ (approx. 14000–4000 cm −1 ) Near IR region- vibrational region- ranges from 2.5 to 25μ (approx. 4000–400 cm −1 ) Far IR region (named for their relation to the visible spectrum) - rotation region- ranges from 25 to 300/400μ (approx. 400–10 cm −1 ) lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy.

6. Definition of Infrared Spectroscopy A chemical substance shows marked selective absorption in the IR region. After absorption of IR radiation, the molecules of the chemical substance vibrate at many rates of vibration, giving rise to close-packed absorption bands called an IR absorption spectrum which may extend over a wide wavelength range. Various bands will be present in IR spectrum which will correspond to the characteristic functional groups & bonds present in the chemical substance. Thus, IR spectrum of a chemical substance is a fingerprint for its identification.

7. Infrared Spectroscopy Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels. The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible.

8. Theory of IR Absorption Spectroscopy Correct wavelength of radiation- Molecule absorbs radiation only when natural frequency of vibration of some part of molecule (atom or group of atoms) is same as frequency of incident radiation e.g. frequency of vibration of HCl molecule is 8.7 × 10 13 sec -1 (2890 cm -1 ) Electric dipole- When absorbed radiation causes change in electric dipole of molecule, molecule will show IR spectrum (radiation causes slight positive and negative charge) . e.g. HCl ( H + Cl - ) Higher change in dipole: Higher rate of vibration: Strong band in IR spectrum and Lower change in dipole: Lower rate of vibration: Weak band in IR spectrum.

9. Symmetrical diatomic molecule like O 2 , H 2 , N 2 CH 2 =CH 2 etc. will not show change in electric dipole hence will not show IR absorption. The water vapors and CO 2 will absorb IR radiation but it will solved in double beam spectrophotometer and these will show weak bands.

10. In general, For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field , perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

11. Different types of molecular energies E int = E elec + E vib + E rot

12. Types of Molecular Energies: Translational : Associated with uniform motion of molecule as whole. Rotational : Associated with overall rotation of molecule with atoms considered as a fixed point masses. Vibrational : Associated with oscillations of atoms (of molecule) which are considered as point masses about equilibrium. Electronic : Associated with motions of electrons while considering nuclei of atoms as fixed points.

13. Molecular Rotations Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions.

14. Vibrational -Rotational Transitions In general, a molecule which in an excited vibrational state will have rotational energy and can loose energy in a transition which alters both the vibrational and rotational energy content of the molecule. The total energy content of the molecule is given by the sum of the vibrational and rotational energies. For a molecule in a specific vibrational and rotational state, denoted by the pair of quantum numbers ( v , J ), we can write its energy as: E ( v , J ) = E vib ( v ) + E rot ( J )

15. The energies of these three transitions form a very distinctive pattern. If we consider the lower vibrational state to be the initial state, then we can label the absorption lines as follows. Transitions for which the J quantum number decreases by 1 are called P-branch transitions , those which increase by 1 are called R-branch transitions and those which are unchanged are called Q-branch transitions . Vibrational -Rotational Transitions (contd.)

16. Vibrational Motion Subdivided into so-called normal modes of vibration which rapidly increases with the number of atoms in the molecule. Each of these normal vibrational modes contributes RT to the average molar energy of the substance and is a primary reason why heat capacities increase with molecular complexity. If there are X vib modes of vibration, then the vibrational energy contributes X vib (RT) to the average molar energy of the substance.

17. Type of Molecular Vibrations

18. Molecular Vibrations A molecule essentially resembles a system of balls of varying masses corresponding to the atoms of the molecule and springs of varying lengths corresponding to various chemical bonds. There are two kinds of fundamental vibrations: Stretching: in which the distance between two atoms increases or decreases but the atoms remain in the same bond axis. Bending: in which position of the atom changes relative to the bond axis.

19. Stretching and Bending

20. Infrared radiation λ = 2.5 to 17 μm υ = 4000 to 600 cm -1 These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules. IR is used to tell: 1. what type of bonds are present 2. some structural information

21.  IR source è sample è prism è detector   graph of % transmission vs. frequency => IR spectrum   4000 3000 2000 1500 1000 500 v (cm -1 ) 100 %T

22. Some characteristic infrared absorption frequencies   BOND COMPOUND TYPE FREQUENCY RANGE, cm -1   C-H alkanes 2850-2960 and 1350-1470   alkenes 3020-3080 (m) and   RCH=CH2 910-920 and 990-1000   R2C=CH2 880-900   cis -RCH=CHR 675-730 (v)   trans -RCH=CHR 965-975   aromatic rings 3000-3100 (m) and   monosubst . 690-710 and 730-770   ortho -disubst . 735-770   meta - disubst . 690-710 and 750-810 (m)   para - disubst . 810-840 (m)   alkynes 3300     O-H alcohols or phenols 3200-3640 (b)     C=C alkenes 1640-1680 (v)   aromatic rings 1500 and 1600 (v)     C≡C alkynes 2100-2260 (v)     C-O primary alcohols 1050 (b)   secondary alcohols 1100 (b)   tertiary alcohols 1150 (b)   phenols 1230 (b)   alkyl ethers 1060-1150   aryl ethers 1200-1275(b) and 1020-1075 (m)   all abs. strong unless marked: m, moderate; v, variable; b, broad

23. IR spectra ALCOHOLS & ETHERS C—O bond 1050-1275 (b) cm -1 1 o ROH 1050 2 o ROH 1100 3 o ROH 1150 ethers 1060-1150 O—H bond 3200-3640 (b) 

24. 1-butanol CH 3 CH 2 CH 2 CH 2 -OH C-O 1 o 3200-3640 (b) O-H

25. IR spectra of ALKANES C—H bond “saturated” (sp 3 ) 2850-2960 cm -1 + 1350-1470 cm -1 -CH 2 - + 1430-1470 -CH 3 + “ and 1375 -CH(CH 3 ) 2 + “ and 1370, 1385 -C(CH 3 ) 3 + “ and 1370(s), 1395 (m)

26. n -pentane CH 3 CH 2 CH 2 CH 2 CH 3 3000 cm -1 1470 &1375 cm -1 2850-2960 cm -1 sat’d C-H

27. IR spectra BENZENE =C—H bond, “unsaturated” “aryl” (sp 2 ) 3000-3100 cm -1 + 690-840 mono-substituted + 690-710, 730-770 ortho -disubstituted + 735-770 meta - disubstituted + 690-710, 750-810(m) para -disubstituted + 810-840(m) C=C bond 1500, 1600 cm -1

28. ethylbenzene 690-710, 730-770 mono- 1500 & 1600 Benzene ring 3000-3100 cm -1 Unsat’d C-H

29. BRUKE TENSOR TM Series Perkin Elmer TM Spectrum One Instrumentation

30. Infrared Instruments An infrared spectrophotometer is an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample. Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data.

31. Dispersive instruments : with a monochromator to be used in the mid-IR region for spectral scanning and quantitative analysis. Fourier transform IR (FTIR) systems: widely applied and quite popular in the far-IR and mid-IR spectrometry. Nondispersive instruments: use filters for wavelength selection or an infrared-absorbing gas in the detection system for the analysis of gas at specific wavelength.

32. Dispersive IR spectrophotometers Simplified diagram of a double beam infrared spectrometer Modern dispersive IR spectrophotometers are invariably double-beam instruments , but many allow single-beam operation via a front-panel switch.

33. Double-beam operation compensates for atmospheric absorption, for the wavelength dependence of the source spectra radiance, the optical efficiency of the mirrors and grating, and the detector instability, which are serious in the IR region  single-beam instruments not practical . Double-beam operation allows a stable 100% T baseline in the spectra.

34. What is FTIR Fourier-transform infrared spectroscopy is a vibrational spectroscopic technique, meaning it takes advantage of asymmetric molecular stretching, vibration, and rotation of chemical bonds as they are exposed to designated wavelengths of light. Fourier transform is to transform the signal from the time domain to its representation in the frequency domain.

35. The Fourier transform method provides an alternatives to the use of monochromators based on dispersion. In conventional dispersive spectroscopy, frequencies are separated and only a small portion is detected at any particular instant, while the remainder is discarded. The immediate result is a frequency-domain spectrum . Fourier transform infrared spectroscopy generates time-domain spectra as the immediately available data, in which the intensity is obtained as a function of time. Direct observation of a time-domain spectrum is not immediately useful because it is not possible to deduce, by inspection, frequency-domain spectra from the corresponding time-domain waveform ( Fourier transform is thus introduced). Fourier Transform Infrared Spectrometer (FTIR)

36. Theory and Instrumentation Light enters the spectrometer and is split by the beam splitter. The above figure shows Michelson interferometer.

37. Theory and Instrumentation ( contd.) The light originates from the He-Ne laser Half of the light is reflected 90 degrees and hits a fixed mirror, while the other half passes through the beam splitter and hits the moving mirror The split beams are recombined, but having traveled different distances, they exhibit an interference pattern with each other As they pass through the sample, the detector collects the interfering signals and writes a plot of response V s mirror displacement known as an interferogram.

38. Fourier Transform Interferometer

39. In one arm of the interferometer, the IR source radiation travels through the beam splitter to the fixed mirror back to the beam splitter through the sample and to the detector. In the other arm, the IR source radiation travels to the beam splitter to the movable mirror, back through the beam splitter to the sample and to the detector. The difference in pathlengths of the two beams is the retardation  . An He-NE laser is used as a monochromatic reference source. The laser beam is sent through the interferometer in the opposite direction to that of the IR beam. Single-beam FTIR Spectrometer

40. Double-beam FTIR Spectrometer

41. Interferometer Michelson interferometer If moving mirror moves 1/4 l (1/2 l round-trip) waves are out of phase at beam-splitting mirror - no signal If moving mirror moves 1/2 l (1 l round-trip) waves are in phase at beam-splitting mirror – signal ...

42. Interferograms

43. Difference in pathlength called retardation  Plot d vs. signal - cosine wave with frequency proportional to light frequency but signal varies at much lower frequency One full cycle when mirror moves distance l /2 (round-trip = l ) Frequency of signal: Substituting l = c/ n If mirror velocity is 1.5 cm/s Bolometer, pyroelectric , photoconducting IR detectors can "see“ changes on 10 -4 s time scale! V MM velocity of moving mirror

44. Computer needed to turn complex interferograms into spectra.

45. Measuring processes

46. • Very high resolution (< 0.1 cm –1 ) Two closely spaced lines only separated if one complete "beat" is recorded. As lines get closer together, d must increase. Dn (cm - 1 ) = 1/ d Mirror motion is 1/2 d Resolution governed by distance at which movable mirror travels • Very high sensitivity ( nanogram quantity) can be coupled with GC analysis (–> measure IR spectra in gas-phase ) • High S/N ratios - high throughput Few optics, no slits mean high intensity of light • Rapid (< 10 sec) • Reproducible and Advantages of FTIR

47. For routine instrument calibration, run the spectrum of polystyrene film (or indene) at resolution 2 cm -1 . Band positions are available in the literature. Higher resolution calibrations may be made from gas-phase spectra (e.g. HCl gas). Spectrum calibration

48. Non interferometric IR spectroscopy using no moving parts

49. Dispersive IR Spectrophotometer Designs Null type instrument

50. Components of dispersive spectrophotometers Incandescent lamp Used in near IR, but fails in far IR b’cos it is glass enclosed & low spectral emissivity Nernst Glower heated rare earth oxide hollow rod e.g. zirconia , yttria , thoria (2 X 30 mm); 1000-1800 o C 1-50 µm (mid- to far-IR) 7100 cm -1 Globar heated sintered Silicon Carbide rod (4 X 50 mm); 1300-1700 o C 1-50 µm (mid- to far-IR) 650-5200 cm -1 Hg arc lamp (Beckmann designed quartz Hg lamps) Shorter wavelength- heated quartz envelope Longer wavelength- Hg plasma 50 - 300 µm (far-IR) Filament lamp 1100 K 0.78-2.5 µm (Near-IR) CO2 laser stimulated emission lines 9-11 µm 1. IR source

51. 2. Optical system / Monochromators Prism Monochromator Glass or quartz; contains sodium chloride as common prism salt Single pass monochromator Double pass monochromator

52. b)Reflection gratings (made from various plastics): The groove spacing is greater (e.g. 120 grooves mm -1 ). To reduce the effect of overlapping orders and stray radiation, filters or a preceding prism are usually employed. Two or more gratings are often used with several filters to scan a wide region. Mirrors but not lenses are used to focus and collimate the IR radiation. Generally made from Pyrex or another material with low coefficient of thermal expansion. Front surfaces coated with a vacuum-deposited thin metal film of Al, Ag, or Au .

53. Windows are used for sample cells and to permit various compartment to be isolated from the environment. transparent to IR over the wavelength region inert to the various chemicals analyzed  capable of being shaped, ground, and polished to the desired optical quality

54. 3. Sample preparation techniques The preparation of samples for infrared spectrometry is often the most challenging task in obtaining an IR spectrum. Since almost all substances absorb IR radiation at some wave length, and solvents must be carefully chosen for the wavelength region and the sample of interest. Infrared spectra may be obtained for gases, liquids or solids (neat or in solution)

55. A gas sample cell consists of a cylinder of glass or sometimes a metal. The cell is closed at both ends with an appropriate window materials ( NaCl / KBr ) and equipped with valves or stopcocks for introduction of the sample. Long pathlength ( 10 cm) cells – used to study dilute (few molecules) or weakly absorbing samples. Multipass cells – more compact and efficient instead of long- pathlength cells. Mirrors are used so that the beam makes several passes through the sample before exiting the cell. (Effective pathlength  10 m). To resolve the rotational structure of the sample, the cells must be capable of being evacuated to measure the spectrum at reduced pressure. Sampling of Gases

56. For quantitative determinations with light molecules, the cell is sometimes pressurized in order to broaden the rotational structure and all simpler measurement.

57. Pure or soluted in transparent solvent – not water (attacks windows) The sample is most often in the form of liquid films (“sandwiched” between two NaCl plates) Adjustable pathlength (0.015 to 1 mm) – by Teflon spacer Sampling of Liquids

58. Sample thickness is so selected that transmittance lies between 15-20% For most of liquids thickness lies between 0.01-0.05 mm. The rectangular cells are made up of NaCl , KBr , or ThBr If cell possesses good quality windows, flat & parallel, its thickness ‘t’ can be calculated as, 2t = N/w1-w2 Where, t = thickness N = no. of fringes between wave no.s w1 & w2

59. Sampling of Solid • Spectra of solids are obtained as alkali halide discs ( KBr ), mulls (e.g. Nujol , a highly refined mixture of saturated hydrocarbons) and films (solvent or melt casting ) Solid run in solution : Solid dissolved in non-aqueous solvent, a drop of solution is placed on alkali metal disc & solvent allowed to evaporate leaving thin film of solute or entire solution is placed in thin liquid sample cell. Reference beam is kept with cell containing solvent only. Solid films: For amorphous solids, sample is deposited on the surface of KBr or NaCl cell by evaporation of solution of the solids.

60. Pressed pellet technique (Alkali halide discs) : A milligram or less of the fine ground sample mixed with about 100 mg of dry KBr powder in a mortar or ball mill. The mixture compressed in a die (pressure at least 25000 psig) to form transparent disc or small pellet (1-2 mm thick & 1 cm in diameter). Shows band at 3450cm-1 due to OH from moisture.

61. Mull Technique : Grinding a few milligrams of the powdered sample with a mortar or with pulverizing equipment. A few drops of the Nujol oil (mineral oil) added (grinding continued to form a smooth paste). The IR of the paste can be obtained as the liquid sample. Shows bands at 2915, 1462, 1376 & 719 cm-1

62. Thermocouple thermoelectric effect -dissimilar metal junction cheap, slow, insensitive Bolometer Ni, Pt resistance thermometer (thermistor) Highly sensitive <400 cm -1 Pyroelectric Thin dielectric flake (0.25 to 12 mm 2 )- as temp. changes electrostatic charge changes creating voltage Pieroelectric material e.g. DLATGS ( Deuterated L- Alanine TriGlycene Sulphate), LiTaO3 (Lithium tantalate ), PZT (Lead Zirconium Titnate ) fast and sensitive (mid IR); usually employed in FTIR Golay cell Metal cylinder with diaphragm & metal plate, filled with xenon Sensitive as thermocouple Thermisters Fused mixture of metal oxides slow 4 . Detector / transducer

63. Photoconductivity cell Non thermal detector, PbS , CdS, PbSe light sensitive cells fast and sensitive (near IR) Semiconductor detector Non thermal detector High sensitivity Fourier Transform Syetem Use of two beams out of phase Rapid scanning

64. Bolometer with wheatstone bridge Electrical resistance of metal increases (0.4 %) with every degree celsius rise in temp. Response time – 4 m sec

65. Thermocouple Electrical current flows two dissimilar wire connected to both ends & exists with temp. difference. Electricity flows is directly proportional to energy difference between two connections. Response time – 60 m sec

66. Thermisters Fused mix. of metal oxides, As the temp. of mix. increases , its electrical resistance decreases (opposite to bolometers ) Thermisters typically changes resistance by 5% per degree celsius Response time is slow. Golay Cell Consist of small metal cylinder which is closed by blackened metal plate at one end & by flexible metalized diaphragm at other. Cylinder is filled with xenon & then sealed. IR radiation falls on blackened metal plate, which heats the gas & expands results into deformation of metalized diaphragm which seperates two chambers. Response time is 10 -2 sec

67. Photoconductivity Cell Consist of thin layer of lead sulphide or lead telluride supported on glass & enclosed into evacuated glass envelope. when IR radiations fall on lead sulphide , its conductance increases & causes more current flow. Response time is 0.5 msec

68. Non dispersive IR Spectrophotometer

69. Single beam IR Spectrophotometer

70. Double beam IR Spectrophotometer

71. Near-infrared and Far-infrared absorption The techniques and applications of near-infrared (NIR) and far-infrared (FIR) spectrometry are quite different from those discussed above for conventional, mid-IR spectrometry. Near-infrared : 0.8 -2.5  m, 12500 - 4000 cm -1 Mid-infrared : 2.5 - 50  m, 4000 - 200 cm -1 Far-infrared : 50 - 1000  m, 200 - 10 cm -1

72. Near-infrared spectrometry NIR shows some similarities to UV-visible spectrophotometry and some to mid-IR spectrometry. Indeed the spectrophotometers used in this region are often combined UV-visible-NIR ones. The majority of the absorption bands observed are due to overtones (or combination) of fundamental bands that occur in the region 3 to 6 m, usually hydrogen-stretching vibrations. NIR is most widely used for quantitative organic functional-group analysis. The NIR region has also been used for qualitative analyses and studies of hydrogen bonding, solute-solvent interactions, organometallic compounds, and inorganic compounds.

73. Far-infrared spectrometry Almost all FIR studies are now carried out with FTIR spectrometers. The far-IR region can provide unique information. The fundamental vibrations of many organometallic and inorganic molecules fall in this region due to the heavy atoms and weak bonds in these molecules. Lattice vibrations of crystalline materials occur in this region, Electron valence/conduction band transition in semiconductors often correspond to far-IR wavelengths .

74. FT – IR can take wavelength readings across the whole IR region simultaneously and smoothly, making this a very rapid technique. The technique is non-invasive and non-destructive. Its resolution of 125 cm -1 is not spectacular in comparison to other vibrational techniques and it will not give the same detailed structural information that NMR, MS, or X-ray crystallography. IR spectroscopy is disgracefully sensitive to the absorption of water, and it has the tendency to overwhelm all of the other peaks. If there is significant moisture in the sample the penetration distance of the light decreases. It may be advantageous to go with Raman in place of IR in the case of excess moisture. Advantages and Disadvantages:

75. Advantages and Disadvantages(contd.) Spectra in the frequency domain can never be eyeballed conclusively. They are always subject to some sort of manipulation, leading some to believe that the data can say whatever the experimenter wants it to say depending on how it is manipulated. Much higher signal to noise ratio . Felgett’s advantage = Greater throughput of power. Hard to do samples having low transmission and weak spectra can be done with FTIR. Greater wavenumber accuracy. Most FT instruments have an accuracy of +/- 0.01 cm -1 .

76. Limitations: Molecular wt. can not be determined Does not provides information about relative positions of functional groups of molecule From single IR spectrum, it is not possible to know whether it is pure compound or mixture. e.g. mix. of paraffin & alcohol gives same spectra as high mole. wt. alcohols

77. 1. Fundamental chemistry Determination of molecular structure/geometry. e.g.  Determination of bond lengths, bond angles of gaseous molecules 2.  Qualitative analysis – simple, fast, nondestructive Monitoring trace gases: NDIR . Rapid , simultaneous analysis of GC, moisture, N in soil. Analysis of fragments left at the scene of a crime Quantitative determination of hydrocarbons on filters, in air, or in water Applications

78. 3. Identification of substances 4. Studying the progress of reaction 5. Detection of impurities 6. Isomerism in organic chemistry 7. Quantitative analysis

79. Applications Applications are vast and diverse . Some of them are, Determination of purity Shape of symmetry of molecule Presence of water in sample Measurements of paints & varnishes In pharmaceutical industry Compositional analysis of organic, inorganic and polymers Biological and biomedical fields like detection of water in biological membranes

80. Analysis of aircraft exhausts Measurement of toxic gas in fuels Combustion studies Gas analysis And many more

81. Questions….....