3.Sample handling techniques used in IR
- 1. Sample Handling Techniques in INFRARED SPECTROSCOPY Dr. Tambe V.S. PES Modern College of Pharmacy (For Ladies)
- 2. The desirable characteristics of cell materials are: • Inertness towards the sample • High transmission in the wavelength range to be scanned • Should be available in pure state that is suitable for spectroscopic studies (Silver salts: have limited water solubility, but are photosensitive and are easily darkened over prolonged use.) • Should be safe to handle ,i.e, non-toxic • For transmission mode: Should have low refractive index in the wavelength region of interest so that reflection loss is minimal and there are few interference fringes • For reflectance mode: Should have high refractive index • Should not be hygroscopic as this can lead to fogging of windows • Should not be too hard or too soft for convenience of polishing • Cost
- 3. Thallium bromide-iodide (KRS 5) Zinc Sulphide Silver bromide Polyethylene
- 4. SOLIDS • Pelleting technique • Mulling technique • Solid run in solution thin films or cast films • Reflectance technique (specular, diffuse, total internal reflection, attenuated total reflectance) • Photoacoustic spectroscopy • FT-IR microscopy LIQUIDS • Neat liquids • Solutions • Reflectance technique (specular, diffuse, total internal reflection, attenuated total reflectance) • Photoacoustic spectroscopy GASES • Absorbance in multiple pass cell • Photoacoustic spectroscopy
- 5. Photoacoustic Spectroscopy (PAS) It is a non-invasive reflectance technique with penetration depths in the range from microns down to several molecular monolayers. Gaseous, liquid or solid samples can be measured in particular for highly absorbing samples. The photoacoustic effect occurs when intensity-modulated light is absorbed by the surface of a sample located in an acoustically isolated chamber filled with an inert gas. A spectrum is obtained by measuring the heat generated from the sample due to a re- absorption process. The sample absorbs photons of the modulated radiation, which have energies corresponding to the vibrational states of the molecules. The absorbed energy is released in the form of heat generated by the sample, which causes temperature fluctuations and, subsequently, periodic acoustic waves. A microphone detects the resulting pressure changes, which are then converted to electrical signals. Fourier-transformation of the resulting signal produces a characteristic infrared spectrum.
- 6. PAS (Solids, Liquids, Gases) Modulated light Sent to sample surrounded by inert gas like Helium, Nitrogen with good thermal conductivity Sample absorb energy and is vibrationally excited and it releases the energy Released energy is reabsorbed by gas and is heated Pressure fluctuation in gas Sensed by a sensitive microphone
- 7. Reflectance Technique (Solids, Liquids) Reflectance techniques may be used for samples that are difficult to analyse by the conventional transmittance methods. 1. Specular reflectance 2. Diffuse reflectance 3. Attenuated total reflectance/ Multiple internal reflectance
- 8. Specular Reflectance • Seen when reflecting medium is a smooth polished surface. • The angle of reflection is identical to the incident angle of the radiation. • If the surface is made up of an IR absorber, the relative intensity of reflection is less for wavelengths that are absorbed than for wavelengths that are not. • Thus, a plot of reflectance R, which is the fraction of the incident radiant energy reflected, versus wavelength or wavenumber provides a spectrum for a compound similar in general appearance to a transmission spectrum for the species.
- 9. Diffuse-reflectance IR Fourier transform spectrometry (DRIFTS) Effective way of directly obtaining IR spectra on powdered samples with a minimum of sample preparation. Uses FTIR instruments offer adapters that fit in cell compartments and permit diffuse- reflectance measurements. The collimated beam from the interferometer is directed to an ellipsoidal mirror and then to the sample. The sample is usually ground and mixed with KBr or KCI as a diluent. The mixture is then placed in a sample cup 3-4 mm deep and about 10-15 mm in diameter. A complex combination of reflection, absorption, and scattering occurs before the beam is directed to the detector.
- 10. Attenuated Total Reflectance (ATR) Internal-reflection spectroscopy is a technique for obtaining IR spectra of samples that are difficult to deal with, such as solids of limited solubility, films, threads, pastes, adhesives, and powders
- 11. When a beam of radiation passes from a more dense to a less dense medium, reflection occurs. The fraction of the incident beam reflected increases as the angle of incidence becomes larger; beyond a certain critical angle, reflection is complete. During the reflection process, the beam penetrates a small distance into the less dense medium before reflection occurs. The depth of penetration depends on – the wavelength, the index of refraction of the two materials, and the angle of the beam with respect to the interface. The penetrating radiation is called the evanescent wave. At wavelengths where the less dense medium absorbs the evanescent radiation, attenuation of the beam occurs, which is known as attenuated total reflectance.
- 12. • The sample is placed on opposite sides of a transparent crystalline material of high refractive index. • By proper adjustment of the incident angle, the radiation undergoes multiple internal reflections before passing from the crystal to the detector. • Absorption and attenuation take place at each of these reflections. This adapter fits into the cell area of most IR spectrometers and permits ATR measurements. Cells for liquid samples are also available.
- 13. Multiple internal reflectance (MIR) • Similar to ATR • but MIR produces more intense spectra from multiple reflections. • While a prism is usually used in ATR work, MIR uses specially shaped crystals which cause many internal reflections, typically 25 or more. •
- 14. • Solids Most organic compounds exhibit numerous absorption bands throughout the mid-IR region, and finding a solvent that does not have overlapping peaks is often impossible. Because of this, spectra are often obtained on dispersions of the solid in a liquid or solid matrix. Generally. in these techniques. the solid sample must be ground until its particle size is less than the wavelength of the radiation to avoid the effects of scattered radiation.
- 15. 3. Pelleting: A milligram or less of the finely ground sample is intimately mixed with about 100 mg of dried potassium bromide powder. (alkali-metal halides) Halide salts have the property of cold flow, in which they have glasslike transparent or translucent properties when sufficient pressure is applied to the finely powdered materials. Being ionic, KBr transmits throughout most of the lR region with a lower cutoff of about 400 cm-1. Mixing is carried out with a mortar and pestle or, better. in a small ball mill. The mixture is then pressed in a special die at 10000 to 15000 pounds per square inch to yield a transparent disk. Best results are obtained if the disk is formed in a vacuum to eliminate occluded air. The resulting spectra may exhibit bands at 3450 and 1640 cm -I (2.9 and 6.1 /lm) due to absorbed moisture. Ion exchange can occur with some samples such as amine hydrochlorides or inorganic salts. With the former, bands of the amine hydrobromide are often found. Polymorphism can also occur because of the forces involved in grinding and pressing the pellet. These can convert one polymorph into another. CsI and CsBr are sometimes used for pelleting. Cesium iodide has greater transparency at low frequencies than KBr.
- 17. 4. Mulls IR spectra of solids that are not soluble in an IR-transparent solvent or are not conveniently pelleted in KBr are often obtained by dispersing the anaIyte in a mineral oil or a fluorinated hydrocarbon mull. Mulls are formed by grinding 2 to 5 mg of the finely powdered sample (particle size <2 micrometer) in the presence of one or two drops of a heavy hydrocarbon oil (Nujol). If hydrocarbon bands are likely to interfere, Fluorolube, a halogenated polymer can be used. Mull is then examined as a film between flat salt plates.
- 18. Limitations: • Too little sample and there will be no sign of the sample in the spectrum. • Too much sample and a thick paste will be produced and no radiation will be transmitted. • If crystal size of the sample is too large, it leads to a scattering of radiation which gets worse at the high-frequency end of the spectrum. • If the mull is not spread over the whole plate area, part of the beam of radiation passes through the mull and consequently only part through the plate, thus producing a distorted spectrum. • Too little mull leads to a very weak spectrum showing only the strongest absorption bands. Too much mull leads to poor transmission of radiation so that the base.
- 19. 5. Cast film used mainly for polymeric materials. Solvent Casting: The sample is first dissolved in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. Care is important to ensure that the film is not too thick otherwise light cannot pass through. This technique is suitable for qualitative analysis.
- 20. • You need to choose a solvent which not only dissolves the sample, but will also produce a uniform film. The solution is poured on to a leveled glass plate (such as a microscope slide) or a metal plate and spread to a uniform thickness. The solvent is then evaporated in an oven and, once dry, the film can be stripped from the plate. Alternatively, it is possible to cast a film straight on to the infrared window to be used.
- 21. Melt casting: Solid samples which melt at relatively low temperatures without decomposition can be prepared by melt caşting. A film is prepared by hot-pressing the sample in a hydraulic press between heated metal plates.
- 22. Microtomy • cut a thin (20–100 μm) film from a solid sample. • This is one of the most important ways of analysing plastic products as the integrity of the solid is preserved.
- 23. Liquids A common problem encountered in obtaining good quality spectra from liquid films is sample volatility. When the spectrum of a volatile sample is recorded it progressively becomes weaker because evaporation takes place during the recording period. Liquids with boiling points below 100°C should be recorded in solution or in a short-pathlength sealed cell, but cannot be taken apart for cleaning. Before producing an infrared sample in solution, a suitable solvent must be chosen. In choosing a solvent for your sample you need to consider the following points: (a) it has to dissolve the compound; (b) it should be as non-polar as possible to minimise solute-solvent interactions; (c) it should not strongly absorb infrared radiation.
- 24. Semi-permanent cells demountable so that the windows can be cleaned. The spacer is usually made of polytetrafluoroethylene (PTFE) and is available in a variety of thicknesses, allowing one cell to be used for various pathlengths. All these cell types are filled by using a syringe. An important consideration in the choice of infrared cells is the type of window material. The material must be transparent to the incident infrared radiation and therefore alkali halides are normally used in transmission methods. The cheapest material is sodium chloride (NaCl).
- 25. Gaseous Sample
- 26. • Used for Low boiling point liquid and gases • Cylindrical cells has reflecting internal surface which achieves pathlength of few cm to 10 m • For adequate measurement sensitivity, closed-path optical instruments that sense trace gases need long optical path lengths. • for large instruments, multipass gas cells are used which are bulky. • For smaller instruments, a small multipass cell with a small, circular mirror that folds the sensing laser beam into a dense star pattern with an optical path length of up to 4 m in a volume of only 30 ml is used.
- 27. (GC-IR) allows the identification of the components eluting from a gas chromatograph. GC-IR can easily distinguish structural isomers. This technique is also particularly useful for pollution studies or gas purity determinations. Thermogravimetric analysis (TGA) is a technique which involves measuring the change of mass of a sample when it is heated. Identification of the decomposition products is done with IR. TGA and infrared spectroscopy have been combined to provide a complete qualitative and quantitative characterization of thermal decomposition processes.
- 28. Thank You….