Submitted By:

HEMA BANSAL M.Pharma(SEM-1)

Section-B

SAMPLE HANDLING IN

IntroductionIR transmitting material Sampling of solidsSampling of gases Sampling of liquidsProblems

CONTENTS

In 1965 Miller has developed appropriate method to handle sample in the gas, liquid and solid phase. However a no. of problems exit with regard to sample handling.

The main problems- Is that almost all substances absorb IR radiation. This restricted choice of the materials that may be

used for construction of the sample cell.

IR spectroscopy used for the characterization of: Solid Liquid Gas

INTRODUCTION

Sample must be transparent to IR radiation.

Conditions restricts selection of salts like NaCl or KBr.

Final choice of salt depends on the wavelength range.

INFRARED-TRANSMITTING MATERIALS

Materials Wavelength Refractive index range(um) at 2um

NaCl 0.25-17 1.52KBr 0.25-25 1.53KCl 0.30-20 1.5AgBr 0.40-23 2.2CaF2 0.15-9 1.40BaF2 0.20-11.5 1.46ZnS 0.57-14.7 2.26Germanium 0.50-16.7 4.0 Silicon 0.20-6.2 3.5Polyethylene 16-300 1.54

Various techniques for preparing solid sample:

Solids Run in Solution Solid Films KBr Pellete Technique Mull Technique Cast Film Technique

Sampling of SOLIDS

Solids may also be dissolved in a non-aqueous solvent.A drop of the solution is placed on a alkali metal disk.The solvent allowed to evaporate leaving a thin film of

the solute.The entire solution is placed in a sample cell.

Limitation:Cannot be used for all solids (limited number of

suitable solvents)No single solvent which is transparent throughout IR

region.

Solids Run in Solution

Solid films

For amorphous solids

Sample is deposited on the surface of a KBr or NaCl cell evaporation of a solution of the solid.

Useful for rapid qualitative analysis.

Not carried out for quantitative analysis.

KBr Pellet Technique:A sample in KBr powder can be compressed under pressure with or without vacuum to form transparent disks.

A solid sample of approximately 2-3 mg is allowed to mix with about 0.5- 1 g of KBr.

Thoroughly grind the mixture in a mortar, then press the mixture in a pellet die under a pressure of about 6000-10000 psi to obtain a transparent disk.

This powder mixture is then pressed in a mechanical press to form a translucent pellet through which the beam of the spectrometer can pass.

Advantages:KBr pellets stored for long periods of time.Resolution of the spectrum is superior.

Disadvantages:High pressure causes polymorphic changesNot successful for some polymer(difficult to grind)

Substances that transmit a wide range of IR frequency

Help to minimize scattering by surrounding the analyte with a medium whose refractive index more closely matches to that of the sample than does air.

Examples: NUJOL Fluorolube mulls Refined mineral oil Perfluro kerosene Hexafluro butane

MULLING AGENTS

Mull techniqueThe mull technique is probably one of the most common and easiest ways to prepare a solid sample. The sample is prepared by first grinding about 5 mg of a solid sample in a mortar.Adding 2 drops of the mulling oil (Nujol or Fluorolube mulls) to the sampleGrinding the mixture thoroughly. The suspended particles obtained must be less than 2m in order to avoid excessive scattering of radiation. Paste the suspended particles on one of the IR salt plates, and cover with the matching salt plate.Mounted in a path of infrared beam and spectrum is run.

Cast Film Technique It is used mainly for polymeric materials.

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.

The final method is to use microtomy to cut a thin (20–100 µm) film from a solid sample.

This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.

GASESGas sample cell –Cylindrical glass bodySurfaces in the light path are made of KBr,NaCl.Cell length-10cmTo end of the cell are attached disk of appropriate window material with wax epoxy cement or pressure plate.

The beam is directed to 90degree angle from its normal direction and thus longer path length is provide.

The use of longer cell even with effective path length as long as 1km.

For some special application- Example: Corrosive gases or vapors metals such as stainless steel, nickel are used for cell bodies.

The gas sample is carried from the vessel to the detector site, where it flows through a tube only to return, unchanged, to the sterilizer.

The IR source is placed at one end of the tube and the IR detector at the other.

The IR beam passes through the gas sample, and the detector collects the spectral reading and transmits the data to a microprocessor .

Gas can be conducted through the beam path, or the beam can be reflected through a window in the sterilizer so that it crosses the headspace gas and returns to the detector.

LIQUIDSLiquid samples are used at room temperature.

There IR spectra are obtained directly

To obtain the transmittance in the range of 15%-70% ,select- Sample concentration Path length should be selected

Parameters:Rectangular cell(NaCl,KBr)Cell thickness-0.02mm in the case of most neat liquidsConcentration-10% Cell length-0.1 mm The solvent selected must be transparent

Solvents used:Acetone MethaneToluene HexaneBenzene DioxeneCS2 EtherCCl2 Tetrachloro ethylene

Neat liquids can be analyzed between salt plates made of NaCl or KBr.Non- or low volatility liquids can be analyzed by placing a drop of the sample ontospecially prepared thin polyethylene polymer substrates.

Use a fixed pathlength cell. Determine pathlength when empty by counting interference fringes

Cells for liquids

Fixed pathlength

Some problems with spectraAsymmetric, sloping bands. Badly ground.

Sample does not cover beam.

Sample (mull) too thick

Also for air bubble in liquid cell; polymer film with hole or crack

Liquid evaporated between KBr plates

Wet sample. Sloping to high energy. Water bands.

Sample too thin

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