ANALYTICAL

INSTRUMENT

Presented by :- Pranjit Sharmah 2013-2014 M.Sc Sem I

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ANALYTICAL INSTRUMENTS

In mineral science, there are several analytical instruments used for various purpose, viz…

Scanning electron microscopy X-ray diffraction Transmission electron microscopy X-ray fluorescence Flame atomic absorption spectroscopy Electron microprobe analysis Secondary ion mass spectrometry Atomic force microscopy

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SCANNING ELECTRON MICROSCOPY (SEM)

The scanning electron microscope has an electron optical column in which a finely focused electron beam can be scanned over a specified area of the specimen.

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FUNDAMENTAL PRINCIPLES OF SCANNING ELECTRON MICROSCOPY (SEM)

Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD) that are used to determine crystal structures and orientations of minerals.

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SCHEMATIC DRAWING OF FINELY FOCUS ELECTRON BEAM IMPINGING ON A MATERIAL SURFACE :

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APPLICATIONS OF SEM

The SEM is routinely used to – To generate high-resolution images of shapes

of objects.

Identify phases based on qualitative chemical analysis and/or crystalline structure.

SEMs equipped with diffracted backscattered

electron detectors can be used to examine microfabric and crystallographic orientation in many materials.

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STRENGTHS AND LIMITATIONS OF SEM

Strength:- Most SEM are comparatively easy to operate.

Modern SEMs generate data in digital format which are highly portable.

For many applications, data acquisition is rapid.

Limitations- . Samples must be solid and they must fit into the

microscope chamber An electrically conductive coating must be applied to

electrically insulating samples for study in conventional SEM.

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X-RAY DIFFRACTION TECHNIQUES (XRD)

The first application of an X-ray experiment to the study of crystalline material in 1912 by Max Von Lou. There are two different X-ray diffraction techniques –

i) Single crystal techniques

ii)X-ray powder diffraction techniques

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SINGLE CRYSTAL TECHNIQUES……

What is Single-crystal X-ray Diffraction???

Single-crystal X-ray Diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell dimensions, bond-lengths, bond-angles, and details of site-ordering.

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FUNDAMENTAL PRINCIPLES OF SINGLE-CRYSTAL X-RAY DIFFRACTION

Single crystal X-ray diffraction concern with the interaction of an X-ray beam with a very small single crystal.

The X-ray beam and the crystalline structure produce that can be recorded on film or measured by electronic device called X-ray counter.

The most commonly used single crystal method is precession method.

The modern approach to data acquisition by “single crystal diffractometer” .

The most commonly used automated technique in structure analysis is four circle diffractometer.

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APPLICATIONS

Specific applications of single-crystal diffraction include:

For precise determination of a unit cell, including cell dimensions and positions of atoms within the lattice.

New mineral identification, crystal solution and refinement.

Determination of crystal-chemical vs. environmental control on mineral chemistry etc.

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STRENGTHS AND LIMITATIONS OF SINGLE-CRYSTAL X-RAY DIFFRACTION

Strengths:- Non-destructive.

Detailed crystal structure, including unit cell dimensions, bond-lengths, bond-angles and site-ordering information

Limitations Must have a single, stable sample, generally between 50

—250 microns in size.

Optically clear sample.

Data collection generally requires between 24 and 72 hours

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X-RAY POWDER DIFFRACTION (XRD)

X-ray powder diffraction (XRD) is a rapid

analytical technique primarily used for phase identification of a crystalline material.

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FUNDAMENTAL PRINCIPLES OF X-RAY POWDER DIFFRACTION (XRD)

The sample of powder diffrractometer analysis spread uniformly over a surface of a glass slide. The instrument is so constructed that the sample, when clamped in place, rotates in the path of a collimated X-ray beam, an X-ray detector, mounted on a arm rotates about it to pick up the diffracted X-ray signals. X-ray detector maintains the appropriate geometrical relationship to receive each diffraction separately. Once the diffractometer tracing is obtained and various diffraction peaks have been tabulated in sequence of decreasing interplanar spacing together with their relative intensities.

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SCHEMATIC ILLUSTRATION OF A SINGLE-CRYSTAL DIFFRACTOMETER

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APPLICATIONS

X-ray powder diffraction is most widely used for:-

Characterization of crystalline materials

Identification of fine-grained minerals that are difficult to determine optically.

Measurement of sample purity.

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STRENGTHS AND LIMITATIONS OF X-RAY POWDER DIFFRACTION (XRD)?

Strength :- Powerful and rapid (< 20 min) technique for

identification of an unknown mineral.

Minimal sample preparation is required.

Limitation :- Requires tenths of a gram of material which must be

ground into a powder.

For mixed materials, detection limit is ~ 2% of sample

For unit cell determinations, indexing of patterns for non-isometric crystal systems is

complicated.

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X-RAY FLUORESCENCE(XRF)

An X-ray fluorescence (XRF) spectrometer is an x-ray instrument used for routine, relatively non-destructive chemical analyses of rocks, minerals, sediments and fluids.

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FUNDAMENTAL PRINCIPLES OF X-RAY FLUORESCENCE (XRF)

An XRF spectrometer works if a sample is illuminated by an intense X-ray beam, known as the incident beam, some of the energy is scattered, but some is also absorbed within the sample in a manner that depends on its chemistry. When the primary X-ray beam illuminates the sample, excited sample in turn emits X-rays along a spectrum of wavelengths characteristic of the types of atoms present in the sample. Various types of detector are used to measure the intensity of the emitted beam. The intensity of the energy measured by these detectors is proportional to the abundance of the element in the sample.

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SCHEMATIC CROSS SECTION OF XRF

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APPLICATION

 X-Ray fluorescence is used in a wide range of applications, including:-

Research in igneous, sedimentary, and metamorphic petrology

Mining (e.g., measuring the grade of ore)

Metallurgy (e.g., quality control)

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STRENGTHS AND LIMITATIONS……..

Strengths:- Bulk chemical analyses of major elements (Si,

Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment.

Limitations:- XRF analyses cannot distinguish variations among

isotopes of an element.

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ELECTRON MICROPROBE ANALYSIS(EMPA)

An electron probe micro-analyzer is a micro beam instrument used primarily for the in situ non-destructive chemical analysis of minute solid samples. EPMA is also informally called an electron microprobe, or just probe. It is fundamentally the same as an SEM, with the added capability of chemical analysis.

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FUNDAMENTAL PRINCIPLES OF ELECTRON PROBE MICRO-ANALYZER (EMPA)

An electron microprobe operates under the principle that if a solid material is bombarded by an accelerated and focused electron beam, the incident electron beam has sufficient energy to liberate both matter and energy from the sample. These electron-sample interactions mainly liberate heat, but they also yield both derivative electrons and x-rays. These quantized x-rays are characteristic of the element. EPMA analysis is considered to be "non-destructive" so it is possible to re-analyze the same materials more than one time.

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APPLICATIONS OF EMPA

Quantitative EMPA analysis is the most commonly used method for chemical analysis of geological materials at small scale.

EPMA is also widely used for analysis of synthetic materials such as optical wafers, thin films, microcircuits, semi-conductors, and superconducting ceramics.

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STRENGTHS AND LIMITATIONS OF ELECTRON PROBE MICRO-ANALYZER (EPMA)

Strength's:- An electron probe is the primary tool for chemical analysis

of solid materials at small spatial scales .

Spot chemical analyses can be obtained in situ, which allows the user to detect even small compositional variations within textural context or within chemically zoned materials.

Limitations:- Electron probe unable to detect the lightest elements (H,

He and Li).

Probe analysis also cannot distinguish between the different valence states of Fe.

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TRANSMISSION ELECTRON MICROSCOPY (TEM)

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through.

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FUNDAMENTAL PRINCIPLES OF TEM

A transmissions electron microscope consist of a finely focused electron beam that impinges on a very thin foil of an object. Transmission through the object, can be used to display electron diffraction patterns, and high resolution transmission electron microscope images. The thin foil is produced by ion bombardment or sputter-etching method for non conducting materials and conducting material is done by electrochemical methods. The thin foil is held n place with a sample holder centered in a electron beam.

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APPLICATIONS OF TEM

Transmission electron microscope provide information about symmetry, distance among indexed diffraction provide information about unit cell size. This in turns allows for phase identification.

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STRENGTHS AND LIMITATIONS OF TEM

Strength:- The TEM technique is especially powerful in

elucidating structural features that range in size from 100 to 10,000 Ao .

TEM studies allow for phase identification of extremely

small particles or intergrowths of minerals.

Limitation:- TEMs are large and very expensive.

TEMs require special housing and maintenance.

Images are black and white.

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FLAME ATOMIC ABSORPTION MICROSCOPY

This analytical technique is commonly considered a “wet” analytical procedure because the original sample must be completely dissolved in a solution before it can be analysed.

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FUNDAMENTAL PRINCIPLES OF FAA

The energy source in this technique is a light source. When the light energy is equivalent to the energy required to raise the atom from its low energy level to high energy levels, it is absorbed and causes excitation of atom. In order to determine element concentrations by this method, the atoms must be completely free of any of the bonding. The amount of light radiation that is measured by spectrometer is expressed by –

A= log lo/ l, where A is absorbance, l0 is the incident light intensity and l the transmitted light intensity.

The incident light intensity is supplied by hollow cathode lamp. The reduction in intensity is measured by the photomutiplier.

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STRENGTH AND LIMITATION OF FAA

Strength:- Greater sensitivity and detection limits than other methods.

Direct analysis of some types of liquid samples.

Very small sample size.

Limitation:- The method cannot be used to detect non-metallic elements.

It also suffers from ionic interference.

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SECONDARY ION MASS SPECTROMETRY(SIMS)

Secondary ion mass spectrometry (SIMS) is a technique used to analyse the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam.

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FUNDAMENTAL PRINCIPLE OF SIMS

The instruments employs a focused beam of ions that impinges on the solid surface of the sample. These atoms of the surface are extracted as secondary ions for analysis by mass spectrometer. In this method particles are ejected in ionized state. The instruments consists of a source of bombarding primary ions that are focused onto the specimen sample by lens systems. In most of the instruments used for mineralogical analysis the extracted secondary ions focused into a double focusing mass spectrometer which separates ions on the basis of energy and mass.

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SCHEMATIC DEPICTION OF SIMS SOURCE 

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APPLICATIONS OF SIMS It is commonly applied to the determination of the

abundance and distribution of the rare earth material.

The SIMS technique also allows collecting isotropic

composition for age dating and diffusion studies.

Instrument can be used to determine the

concentration of any chemical element.

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STRENGTH AND LIMITATIONS OF SIMS

Strength :- This technique offers very high sensitive quantitative

elemental analysis with detection limits in the parts per million to parts per billion range.

It can analyze most chemical elements from H to U.

Limitations:- Not all elements in all substrates (matrices) can

be analysed quantitatively.

SIMS instrumentation tends be expensive.

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