Analytical Instrumentation
Dr C Ghanshyam, DU-1
CSIR-CSIO, Chandigarh
1
ANALYTICAL TECHNIQUE/METHOD
 A fundamental scientific phenomenon that has proved useful
for providing information on the ...
• UV-Vis
• FT-IR/NIR
• AAS
• Fluorescence
• HPLC
• GC
• LC-MS/MS
07/20/13 3
Spectroscopic Chromatographic
PRINCIPAL TYPES OF CHEMICAL INSTRUMENTATION
Spectroscopic
techniques
Electrochemical
techniques
Chromatographic
techniques...
Applications
AnalyticalAnalytical
ScienceScience
Organic Chemistry
PhysicalChemistry
Physics
Materials Science
Medicine
Bi...
Radiation Sources
 Two types of Radiation sources.
Non-ionizing:
This Radiation does not create ions when it interacts wi...
109
107
105
103
101
10-1
10-3
10-5
10-7
10-9
10-11
gamma
X-rays
Ultra Violet
Infra red
microwave
Radio
waves
500
600
700
V...
E = h *c / λ
F = 1 / λ
h = 6.62606896 x 10
-34
Js
c = 3 X 10
8
m/s
Where:
E = Photon Energy
h = Planck’s Constant
c = Spee...
Introduction to spectroscopy
 Spectroscopy is the measurement of electromagnetic radiation
absorbed, scattered, or emitte...
Radiation Sources
Radiation Sources in absorption spectrometry have two basic requirements:
 Must provide sufficient radi...
Discharge Lamps
Hydrogen or Deuterium Discharge Lamps:
A Hydrogen or Deuterium discharge lamp is a low-
pressure gas-disch...
Incandescent Filament Lamps:
It is a source of electric light that works by incandescence (a general
term for heat-driven ...
Light source
• Distribution of energy through spectrum is function of temperature.
For Visible region-
• Tungsten filamen...
For Ultra Violet region-
 Hydrogen discharge lampHydrogen discharge lamp
• Consist of two electrode contain in deuterium...
• Deuterium lamps:-
• Radiation emitted is 3-5 times more than the hydrogen
discharge lamps.
• Xenon discharge lamp:-
• Xe...
 Prisms-
• Prism bends the monochromatic light.
• Amount of deviation depends on wavelength.
• Quartz prism used in UV-re...
The monochromator consists of five elements
• An entrance slit
• Collimating lens (disperse the ‘white radiation’ into par...
Dispersion Elements
Filters
Absorption Filters:
• Absorption Filter is colored glass consisting of dye molecules that
absorbs the wavelength w...
Filters
Interference Filters:
 These filters rely on optical interference to provide narrow bands of radiation.
 Dielect...
Filters
Comparison between Interference and Absorption filters
 Interference filters can provide superior bandwidth defin...
Wavelength Selection
 Spectrophotometric methods usually require the isolation of discrete bands of
radiation.
 Basis of...
Emission Profile of a Tungsten and Deuterium Lamp
Spectroscopy requires all materials in the beam path other than the
analyte should be as transparent to the radiation as ...
Detectors
• Photo Multiplier Tubes
• Photo Diodes
•Photo diode Arrays
•
Materials
Materials commonly used to produce photodiodes include[2]
:
Material Wavelength range (nm)
• Silicon 190–1100
• ...
PHOTOMULTIPLIER
• Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of
vacuum tubes, and mo...
• These electrons are directed by the focusing electrode toward the electron multiplier,
where electrons are multiplied by...
Photovoltaic mode
• When used in zero bias or photovoltaic mode, the flow of photocurrent out of the
device is restricted ...
• A p–i–n photodiode, also called PIN photodiode, is a photodiode with an intrinsic (i) (i.e.,
undoped) region in between ...
PHOTODIODE
A photodiode is a type of photo detector capable of converting light into either current or
voltage, depending ...
Important Properties of Photo detectors
Depending on the application, a photo detector has to fulfil various requirements:...
Photo detectors or Photo sensors
Photo detectors are sensors of light or other electromagnetic energy. There are several
v...
Mass Spectrometry
• Introduction
– General overview
• Mass Spectrometry is the generation, separation and characterization...
Mass Spectrometry
II. The Mass Spectrometer
– General Schematic
• A mass spectrometer needs to perform three functions:
• ...
Mass Spectrometry
II. The Mass Spectrometer
A. Single Focusing Mass Spectrometer
• A small quantity of sample is injected ...
Mass Spectrometry
II. The Mass Spectrometer
A. The Single Focusing Mass Spectrometer
4. Ions (+) are accelerated using a (...
Mass Spectrometry
II. The Mass Spectrometer
A. Single Focusing Mass Spectrometer
6. At a given potential, only one mass wo...
Mass Spectrometry
II. The Mass Spectrometer
A. Single Focusing Mass Spectrometer
8. By varying the applied potential diffe...
Mass Spectrometry
II. The Mass Spectrometer
C. Double Focusing Mass Spectrometer
4. Here, the beam of sorted ions from the...
Mass Spectrometry
II. The Mass Spectrometer
D. Quadrupole Mass Spectrometer
• Four magnets, hyperbolic in cross section ar...
Mass Spectrometry
II. The Mass Spectrometer
D. Quadrupole Mass Spectrometer
3. The compact size and speed of the quadrupol...
Mass Spectrometry
III. The Mass Spectrum
– Presentation of data
• The mass spectrum is presented in terms of ion abundance...
Mass Spectrometry
III. The Mass Spectrum
B. Determination of Molecular Mass
5. The Nitrogen Rule is another means of confi...
Mass Spectrometry
III. The Mass Spectrum
B. Determination of Molecular Mass
5. Some molecules are highly fragile and M+ pe...
Mass Spectrometry
II. The Mass Spectrometer
C. Double Focusing Mass Spectrometer
• Resolution of mass is an important cons...
FTIR Spectrometer
• FTIR (Fourier Transform InfraRed) spectrometer is used to
obtain an infrared spectra by first collecti...
FT-IR method
• Fourier transform infrared (FTIR) spectroscopy is a measurement technique for
collecting infrared spectra.
...
Continuous wave Michelson or Fourier transform spectrograph
The Fourier transform spectrometer is just a Michelson interfe...
• Light from the source is split into two beams by a half-silvered mirror, one is reflected
off a fixed mirror and one off...
• This allows multiple samples to be collected and averaged together resulting in an
improvement in sensitivity. Virtually...
2. The Interferometer: The beam enters the interferometer where the “spectral encoding”
takes place. The resulting interfe...
So, what information can FT-IR provide?
• • It can identify unknown materials
• • It can determine the quality or consiste...
What is FT-IR?
• FT-IR stands for Fourier Transform InfraRed, the preferred method of infrared
spectroscopy. In infrared s...
A Simple Spectrometer Layout
The Sample Analysis Process
ADVANTAGES OF FT-IR
Some of the major advantages of FT-IR over the dispersive technique include:
• Speed : Because all of ...
07/20/13 59
Analytical instrumentation
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Analytical instrumentation

  1. 1. Analytical Instrumentation Dr C Ghanshyam, DU-1 CSIR-CSIO, Chandigarh 1
  2. 2. ANALYTICAL TECHNIQUE/METHOD  A fundamental scientific phenomenon that has proved useful for providing information on the composition of the substances.  UV-VIS, IR spectrophotometry.  Specific application of a technique to solve an analytical problem.  IR analysis of some compound is an example of an instrumental method 2
  3. 3. • UV-Vis • FT-IR/NIR • AAS • Fluorescence • HPLC • GC • LC-MS/MS 07/20/13 3 Spectroscopic Chromatographic
  4. 4. PRINCIPAL TYPES OF CHEMICAL INSTRUMENTATION Spectroscopic techniques Electrochemical techniques Chromatographic techniques Miscellaneous techniques Hyphenated techniques • Ultravoilet and visible spectrophotometry • Fluorescence and phosphorescence spectrophotometry • Atomic spectrometry ( emission and absorption) • Infrared spectrophotometry • Raman spectroscopy • X-Rays spectroscopy • Nuclear magnetic resonance spectroscopy • Electron spin resonance spectroscopy • Potentiometry (PH and ion selective electrodes) • Voltametry • Voltammetric techniques • Stripping techniques • Amperometric technique • Coulometry • Electrogravemetry • Conductance technique • Gas Chromatography • High Performance liquid chromato- graphic techniques •Thermal Analysis •Mass spectrometry •Kinetic Techniques •GC-MS (gas chromatography- mass spectrometry) • ICP-MS(inductively coupled plasma – mass spectrometry) • GC-IR (gas chromatography – infrared spectroscopy) • ICP-AES
  5. 5. Applications AnalyticalAnalytical ScienceScience Organic Chemistry PhysicalChemistry Physics Materials Science Medicine Biochemistry Forensic Science
  6. 6. Radiation Sources  Two types of Radiation sources. Non-ionizing: This Radiation does not create ions when it interacts with matter but dissipates energy generally in the form of heat. Ultraviolet, visible, infrared, microwaves, radio Ionizing: This energy can knock electrons out of molecules with which they interact, thus creating ions. x-rays, alpha, beta, gamma, cosmic rays
  7. 7. 109 107 105 103 101 10-1 10-3 10-5 10-7 10-9 10-11 gamma X-rays Ultra Violet Infra red microwave Radio waves 500 600 700 Violet, indigo, blue Green, yellow Orange, red Electro Magnetic Spectrum
  8. 8. E = h *c / λ F = 1 / λ h = 6.62606896 x 10 -34 Js c = 3 X 10 8 m/s Where: E = Photon Energy h = Planck’s Constant c = Speed of Light λ = Wavelength F = Frequency When radiation enters into the matter its velocity decreases but its frequency remains constant. Energy and Frequency
  9. 9. Introduction to spectroscopy  Spectroscopy is the measurement of electromagnetic radiation absorbed, scattered, or emitted by atoms, molecules, or other chemical species. Absorption or emission associated with changes in the energy states of the interacting chemical species and each specie has characteristic energy states.  By exposing these atoms to such temperatures they are able to “jump” to high energy levels and in return, emit light.
  10. 10. Radiation Sources Radiation Sources in absorption spectrometry have two basic requirements:  Must provide sufficient radiation energy over the wavelength region where absorption is measured.  Maintain constant intensity over the time interval during measurement. Readout device Readout device Instrument modules for measuring absorption of radiation
  11. 11. Discharge Lamps Hydrogen or Deuterium Discharge Lamps: A Hydrogen or Deuterium discharge lamp is a low- pressure gas-discharge light source often used in spectroscopy when a continuous spectrum in the ultraviolet region is needed. • operate under low pressure (~ 0.2-0.5 Torr) • operate under low voltage (40V dc) • The deuterium lamp emits radiation extending from 112 nm to 900 nm, although its continuous spectrum is only from 180 nm to 370 nm • Deuterium use in place of hydrogen brightness is 3 to 5 times more.
  12. 12. Incandescent Filament Lamps: It is a source of electric light that works by incandescence (a general term for heat-driven light emissions). An electric current passes through a thin filament, heating it to a temperature that produces light. • Measured above 350nm to 2.5µm (Near IR). • Wire filament generally tungsten. • Filament is enclosed in a Glass bulb with an inert gas or vacuum. • To increase emissivity ,efficiency and Luminance filaments are coiled. • Tungsten-Halogen lamps • Iodine used as filling gas.
  13. 13. Light source • Distribution of energy through spectrum is function of temperature. For Visible region- • Tungsten filament lamp Use for region 350nm to 2000nm. Problem- • Due to evaporation of tungsten life period decreases. • It is overcome by using tungsten-halogen lamp. • Halogen gas prevents evaporation of tungsten.
  14. 14. For Ultra Violet region-  Hydrogen discharge lampHydrogen discharge lamp • Consist of two electrode contain in deuterium filled silica envelop. • Gives continuous spectrum in region 185-380nm. • Above 380nm emission is not continuous.  UV-Vis spectrophotometer have both deuterium & tungsten lamps.  Selection of lamp is made by moving lamp mounting or mirror to cause the light fall on monochromator.
  15. 15. • Deuterium lamps:- • Radiation emitted is 3-5 times more than the hydrogen discharge lamps. • Xenon discharge lamp:- • Xenon stored under pressure in 10-30 atmosphere. • It possesses two tungsten electrode separated by 8 cm. • Intensity of UV radiation more than hydrogen lamp. • Mercury arc:- • Mercury vapour filled under the pressure . • Excitation of mercury atom by electric discharge
  16. 16.  Prisms- • Prism bends the monochromatic light. • Amount of deviation depends on wavelength. • Quartz prism used in UV-region. • Glass prism used in visible region spectrum. Function – • They produce non linear dispersion.
  17. 17. The monochromator consists of five elements • An entrance slit • Collimating lens (disperse the ‘white radiation’ into parallel streams) • Prism on which the incoming radiation is dispersed at both surfaces. • Another lens, this time to focus the collimated beam to a rectangular points along the focal plane. • A exit slit which can be narrowed or enlarged as required, depending on required resolution. Monochromators - Prisms All the wavelength elements are present on the focal plane, however, only those selected to exit the unit are positioned the exit slit.
  18. 18. Dispersion Elements
  19. 19. Filters Absorption Filters: • Absorption Filter is colored glass consisting of dye molecules that absorbs the wavelength we wish to reject. • Absorption filters are produced in host materials ( glass, gelatin, liquid and plastic). • Bandwidths are extremely large (30 to 250 nm). • Combining two absorption filters of different λmax can yield a bandpass filter. Comparison of various types of filters for visible radiation
  20. 20. Filters Interference Filters:  These filters rely on optical interference to provide narrow bands of radiation.  Dielectric layer thickness determines the wavelength of transmitted radiation  A simple interference filter consists of two-interfaced dielectric spacer film (CaF2, MgF2) sandwiched between two parallel partially reflected metal films (silver). Bandwidth of 10-15 nm, FWHM; Max trans ~40% Dielectric layer =1.35, 185 nm provides filter atȠ central λ=500 nm
  21. 21. Filters Comparison between Interference and Absorption filters  Interference filters can provide superior bandwidth definition over an absorption filter.  Greater the bandwidth definition the lower the %transmittance through the filter. Effective bandwidths for two types of filters
  22. 22. Wavelength Selection  Spectrophotometric methods usually require the isolation of discrete bands of radiation.  Basis of quantitative analysis is based on the assumption of monochromatic radiation.  In an emission mode, the most favorable signal ratio between background and the analytical emission lines must be selected.  To isolate a narrow band of wavelengths, filters or monochromators or both are used.  In particular, narrow bands are required with linear systems (such as a monochromator) to ensure linearity of response as a function of sample (analyte). Filters:  Absorption Filters  Interference Filters Monochromators:  Prism monochromators  Grating monochromators Output of a typical wavelength selector
  23. 23. Emission Profile of a Tungsten and Deuterium Lamp
  24. 24. Spectroscopy requires all materials in the beam path other than the analyte should be as transparent to the radiation as possible. The geometries of all components in the system should be such as to maximize the signal and minimize the scattered light. The material from which a sample cuvette is fabricated controls the optical window that can be used. Some typical materials are: Optical Glass - 335 - 2500 nm Special Optical Glass – 320 - 2500 nm Quartz (Infrared) – 220 - 3800 nm Quartz (Far-UV) – 170 - 2700 nm Sample cell (cuvette)
  25. 25. Detectors • Photo Multiplier Tubes • Photo Diodes •Photo diode Arrays •
  26. 26. Materials Materials commonly used to produce photodiodes include[2] : Material Wavelength range (nm) • Silicon 190–1100 • Germanium 400–1700 • Indium gallium arsenide 800–2600 • Lead(II) sulfide <1000-3500 P-N vs. P-I-N photodiodes • Due to the intrinsic layer, a PIN photodiode must be reverse biased (Vr). The Vr increases the depletion region allowing a larger volume for electron-hole pair production, and reduces the capacitance thereby increasing the bandwidth. • The Vr also introduces noise current, which reduces the S/N ratio. Therefore, a reverse bias is recommended for higher bandwidth applications and/or applications where a wide dynamic range is required. • A PN photodiode is more suitable for lower light applications because it allows for unbiased operation.
  27. 27. PHOTOMULTIPLIER • Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. • These detectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is very low. • The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners. Structure and operating principles • Photomultipliers are constructed from a glass envelope with a high vacuum inside, which houses a photocathode, several dynodes, and an anode. • Incident photons strike the photocathode material, which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect.
  28. 28. • These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission. • The electron multiplier consists of a number of electrodes called dynodes. Each dynode is held at a more positive voltage than the previous one. Schematic of a photomultiplier tube coupled to a scintillator
  29. 29. Photovoltaic mode • When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. • The diode becomes forward biased and "dark current" begins to flow across the junction in the direction opposite to the photocurrent. • This mode is responsible for the photovoltaic effect, which is the basis for solar cells— in fact, a solar cell is just a large area photodiode. Photoconductive mode • In this mode the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. • This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. • The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. Other modes of operation • Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. • This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsively of the device.
  30. 30. • A p–i–n photodiode, also called PIN photodiode, is a photodiode with an intrinsic (i) (i.e., undoped) region in between the n- and p-doped regions. • Most of the photons are absorbed in the intrinsic region, and carriers generated therein can efficiently contribute to the photocurrent. • Compared with an ordinary p–n photodiode, a p–i–n photodiode has a thicker depletion region, which allows a more efficient collection of the carriers and thus a larger quantum efficiency, and also leads to a lower capacitance and thus to higher detection bandwidth.
  31. 31. PHOTODIODE A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. • Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. • Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction. Principle of operation • A photodiode is a PN junction or PIN structure. • When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. • If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. • Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.
  32. 32. Important Properties of Photo detectors Depending on the application, a photo detector has to fulfil various requirements: • It must be sensitive in some given spectral region (range of optical wavelengths). In some cases, the responsivity should be constant or at least well defined within some wavelength range. • The detector must be suitable for some range of optical powers. The maximum detected power can be limited e.g. by damage issues or by a nonlinear response, whereas the minimum power is normally determined by noise. • In some cases, not only a high responsivity, but also a high quantum efficiency is important, as otherwise additional quantum noise is introduced. • The active area of a detector can be important e.g. when working with strongly divergent beams from laser diodes. • The detection bandwidth may begin at 0Hz or some finite frequency, and ends at some  maximum frequency which may be limited by internal processes (e.g. the speed of electric carriers in a semiconductor material) or by the involved electronics (e.g. introducing some RC time constants). • Finally, the size, robustness and cost are essential for many applications.
  33. 33. Photo detectors or Photo sensors Photo detectors are sensors of light or other electromagnetic energy. There are several varieties: • Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated • Photodiodes which can operate in photovoltaic mode or photoconductive mode • Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes. • Photo resistors or Light Dependent Resistors (LDR) which change resistance according to light intensity • Phototubes containing a photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity. • Phototransistors, which act like amplifying photodiodes.
  34. 34. Mass Spectrometry • Introduction – General overview • Mass Spectrometry is the generation, separation and characterization of gas phase ions according to their relative mass as a function of charge • Previously, the requirement was that the sample be able to be vaporized (similar limitation to GC), but modern ionization techniques allow the study of such non-volatile molecules as proteins and nucleotides • The technique is a powerful qualitative and quantitative tool, routine analyses are performed down to the femtogram (10-15 g) level and as low as the zeptomole (10-21 mol) level for proteins • Of all the organic spectroscopic techniques, it is used by more divergent fields – metallurgy, molecular biology, semiconductors, geology, archaeology than any other
  35. 35. Mass Spectrometry II. The Mass Spectrometer – General Schematic • A mass spectrometer needs to perform three functions: • Creation of ions – the sample molecules are subjected to a high energy beam of electrons, converting some of them to ions • Separation of ions – as they are accelerated in an electric field, the ions are separated according to mass-to-charge ratio (m/z) • Detection of ions – as each separated population of ions is generated, the spectrometer needs to qualify and quantify them 2. The differences in mass spectrometer types are in the different means to carry out these three functions 3. Common to all is the need for very high vacuum (~ 10-6 torr), while still allowing the introduction of the sample
  36. 36. Mass Spectrometry II. The Mass Spectrometer A. Single Focusing Mass Spectrometer • A small quantity of sample is injected and vaporized under high vacuum • The sample is then bombarded with electrons having 25-80 eV of energy • A valence electron is “punched” off of the molecule, and an ion is formed
  37. 37. Mass Spectrometry II. The Mass Spectrometer A. The Single Focusing Mass Spectrometer 4. Ions (+) are accelerated using a (-) anonde towards the focusing magnet 5. At a given potential (1 – 10 kV) each ion will have a kinetic energy: ½ mv2 = eV As the ions enter a magnetic field, their path is curved; the radius of the curvature is given by: r = mv eH If the two equations are combined to factor out velocity: m = mass of ion v = velocity V = potential difference e = charge on ion H = strength of magnetic field r = radius of ion path
  38. 38. Mass Spectrometry II. The Mass Spectrometer A. Single Focusing Mass Spectrometer 6. At a given potential, only one mass would have the correct radius path to pass through the magnet towards the detector 7. “Incorrect” mass particles would strike the magnet
  39. 39. Mass Spectrometry II. The Mass Spectrometer A. Single Focusing Mass Spectrometer 8. By varying the applied potential difference that accelerates each ion, different masses can be discerned by the focusing magnet 9. The detector is basically a counter, that produces a current proportional to the number of ions that strike it 10. This data is sent to a computer interface for graphical analysis of the mass spectrum
  40. 40. Mass Spectrometry II. The Mass Spectrometer C. Double Focusing Mass Spectrometer 4. Here, the beam of sorted ions from the focusing magnet are focused again by an electrostatic analyzer where the ions of identical mass are separated on the basis of differences in energy 5. The “cost” of increased resolution is that more ions are “lost” in the second focusing, so there is a decrease in sensitivity
  41. 41. Mass Spectrometry II. The Mass Spectrometer D. Quadrupole Mass Spectrometer • Four magnets, hyperbolic in cross section are arranged as shown; one pair has an applied direct current, the other an alternating current • Only a particular mass ion can “resonate” properly and reach the detector The advantage here is the compact size of the instrument – each rod is about the size of a ball-point pen
  42. 42. Mass Spectrometry II. The Mass Spectrometer D. Quadrupole Mass Spectrometer 3. The compact size and speed of the quadrupole instruments lends them to be efficient and powerful detectors for gas chromatography (GC) 4. Since the compounds are already vaporized, only the carrier gas needs to be eliminated for the process to take place 5. The interface between the GC and MS is shown; a “roughing” pump is used to evacuate the interface Small He molecules are easily deflected from their flight path and are pulled off by the vacuum; the heavier ions, with greater momentum tend to remain at the center of the jet and are sent to the MS
  43. 43. Mass Spectrometry III. The Mass Spectrum – Presentation of data • The mass spectrum is presented in terms of ion abundance vs. m/e ratio (mass) • The most abundant ion formed in ionization gives rise to the tallest peak on the mass spectrum – this is the base peak base peak, m/e 43
  44. 44. Mass Spectrometry III. The Mass Spectrum B. Determination of Molecular Mass 5. The Nitrogen Rule is another means of confirming the observance of a molecular ion peak 6. If a molecule contains an even number of nitrogen atoms (only “common” organic atom with an odd valence) or no nitrogen atoms the molecular ion will have an even mass value 7. If a molecule contains an odd number of nitrogen atoms, the molecular ion will have an odd mass value 8. If the molecule contains chlorine or bromine, each with two common isotopes, the determination of M+ can be made much easier, or much more complex as we will see
  45. 45. Mass Spectrometry III. The Mass Spectrum B. Determination of Molecular Mass 5. Some molecules are highly fragile and M+ peaks are not observed – one method used to confirm the presence of a proper M+ peak is to lower the ionizing voltage – lower energy ions do not fragment as readily 6. Three facts must apply for a molecular ion peak: 1) The peak must correspond to the highest mass ion on the spectrum excluding the isotopic peaks 2) The ion must have an odd number of electrons – usually a radical cation 3) The ion must be able to form the other fragments on the spectrum by loss of logical neutral fragments
  46. 46. Mass Spectrometry II. The Mass Spectrometer C. Double Focusing Mass Spectrometer • Resolution of mass is an important consideration for MS • Resolution is defined as R = M/∆M, where M is the mass of the particle observed and ∆M is the difference in mass between M and the next higher particle that can be observed 4. If higher resolution is required, the crude separation of ions by a single focusing MS can be further separated by a double-focusing instrument
  47. 47. FTIR Spectrometer • FTIR (Fourier Transform InfraRed) spectrometer is used to obtain an infrared spectra by first collecting an interferogram of a sample signal using an interferometer, then performs a Fourier Transform on the interferogram to obtain the spectrum. • An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them. The FTIR spectrometer uses a Michelson interferometer.
  48. 48. FT-IR method • Fourier transform infrared (FTIR) spectroscopy is a measurement technique for collecting infrared spectra. • Instead of recording the amount of energy absorbed when the frequency of the infra- red light is varied (monochromator), the IR light is guided through an interferometer. • Interferometry is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. The instrument used to interfere the waves together is called an interferometer. • After passing through the sample, the measured signal is the interferogram. • Performing a Fourier transform on this signal data results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy. • FTIR spectrometers are cheaper than conventional spectrometers because building an interferometer is easier than the fabrication of a monochromator. • In addition, measurement of a single spectrum is faster for the FTIR technique because the information at all frequencies is collected simultaneously.
  49. 49. Continuous wave Michelson or Fourier transform spectrograph The Fourier transform spectrometer is just a Michelson interferometer but one of the two fully- reflecting mirrors is movable, allowing a variable delay (in the travel-time of the light) to be included in one of the beams.
  50. 50. • Light from the source is split into two beams by a half-silvered mirror, one is reflected off a fixed mirror and one off a moving mirror which introduces a time delay. • The beams interfere, allowing the temporal coherence of the light to be measured at each different time delay setting, effectively converting the time domain into a spatial coordinate. • By making measurements of the signal at many discrete positions of the moving mirror, the spectrum can be reconstructed using a Fourier transform of the temporal coherence of the light. • Michelson spectrographs are capable of very high spectral resolution observations of very bright sources. • The Michelson or Fourier transform spectrograph was popular for infra-red applications at a time when infra-red astronomy only had single pixel detectors. Extracting the spectrum • The intensity as a function of the path length difference in the interferometer p and wavenumber is:
  51. 51. • This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Virtually all modern infrared spectrometers are FTIR instruments. Conceptual introduction (for FTIR and other absorption spectroscopy) • The goal of any absorption spectroscopy (FTIR, Ultraviolet-visible ("UV-Vis") spectroscopy, etc.) is to measure how well a sample absorbs or transmits light at each different wavelength. • The most straightforward way to do this is to shine a monochromatic light beam through a sample, measure how much of the light is absorbed, and repeat for each different wavelength. • Fourier transform spectroscopy is a less intuitive way to get the same information. Rather than passing a monochromatic beam of light through the sample, this technique passes a beam containing many different frequencies of light at once, and measures how much of that beam is absorbed by the sample. • Next, the beam is modified to contain a different combination of frequencies, giving a second data point. • This process is repeated many times. Afterwards, a computer takes all this data and works backwards to infer what the absorption is at each wavelength.
  52. 52. 2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer. 3. The Sample: The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed. 4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal. 5. The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.
  53. 53. So, what information can FT-IR provide? • • It can identify unknown materials • • It can determine the quality or consistency of a sample • • It can determine the amount of components in a mixture Fourier transform infrared spectroscopy is preferred over dispersive or filter methods of infrared spectral analysis for several reasons: • It is a non-destructive technique • It provides a precise measurement method which requires no external calibration • It can increase speed, collecting a scan every second • It can increase sensitivity – one second scans can be co-added together to ratio out random noise • It has greater optical throughput • It is mechanically simple with only one moving part The Sample Analysis Process The normal instrumental process is as follows: 1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).
  54. 54. What is FT-IR? • FT-IR stands for Fourier Transform InfraRed, the preferred method of infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. • Some of the infrared radiation is absorbed by the sample and some of it is passed through (transmitted). • The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. • Like a fingerprint no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis.
  55. 55. A Simple Spectrometer Layout
  56. 56. The Sample Analysis Process
  57. 57. ADVANTAGES OF FT-IR Some of the major advantages of FT-IR over the dispersive technique include: • Speed : Because all of the frequencies are measured simultaneously, most measurements by FT-IR are made in a matter of seconds rather than several minutes. This is referred to as Felgett advantage as explained before. • Sensitivity : It is dramatically improved with FT-IT for many reasons. The detector employed are much more sensitive, the optical throughput is much higher (referred to as the Jacquinot Advantage) which results in much lower noise levels and the fast scans enable the coaddition of several scans in order to reduce the random measurement noise to any desired level referred to as signal averaging. • Mechanical simplicity : The moving mirror in the interferometer is the only continuously moving part in the instrument. Thus, there is very little possibility of mechanical breakdown. • Internally Calibrated : These instruments employ a HeNe laser as an internal wavelength calibration standard(referred to as the Connes advantage). These instruments are self- calibrating and never need to be calibrated by the user. These advantages make measurements made by FT-IR extremely accurate and reproducible.
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