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Spark Optical Emission Spectroscopy

Dr Heath Bagshaw CMA, Trinity College Dublin

Analytical

Workshop 2012

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Why Use Spark OES Analysis?

Quick, reliable and reproducible analysis technique

Can analyse wide range of elements

Usually used for metals\alloy analysis, such as :-

Steel, cast iron and high alloyed steels

Non-ferrous metals and their alloysAI: wrought alloys, casting alloys, etc.

Cu: bronze, brass, cupronickel, etc.

Mg, Zn alloys, solders

Nitrogen in steel

P in aluminiumUltra low carbon analysis

Elements such as Se, La, Te, etc.

Good limit of detection for most materials - generally less then 50 ppm although 10

ppm detection limits are typical.

Higher resolution spectrometers (using PMTs) have limits of detection of

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Analytical

Workshop 2012

When an element is burned it burns with a distinct colour

Colours - Flame Testing

Barium Strontium Potassium

These colours are due to the emission spectrum of the element.

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Basic Theory

Analytical

Workshop 2012

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The emitted radiation\light is split using a prism or diffraction grating to produce a

spectrum.

More Basic Theory

Analytical

Workshop 2012

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modern equipment uses a slightly different set up:-

A diffraction grating is used to produce the spectrum and mirrors focus the radiation

onto detectors.

Basics cont

Analytical

Workshop 2012

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Analytical

Workshop 2012

Emission Spectra

In the case of optical emission spectroscopy the EM radiation is in the visible\optical

region of the EMS which we see as different colours depending on the

wavelength\energy of the emitted photon.

The energy of a photon and its wavelength are related by the following equation:-

where h is Planck's constant (6.626 10-34 joules) and c is the

speed of light (2.998 108 m/s).

hcE=

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Analytical

Workshop 2012

Examples of Emission Spectra

Emission spectrum of Fe

Emission spectrum of H

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Emitted Light

Polychromatic light

ComputerCalculation of results

Comparison to Standards

Data output and storage

Analytical

Workshop 2012

The System

CCD measures thelight intensity at

each wavelength

Diffraction Grating,

Produces a

dispersion spectrum

Samplesparked to

produce a

burn

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Entrance Slit

Grating

130 800 nm

Analytical

Workshop 2012

The Spectrometer (Schematic) The diffraction grating Is like a concave mirror with lines ruled on it.

Modern Gratings have up to 4000 rules/mm.

The incoming light is reflected and diffracted, producing the spectrum

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Analytical

Workshop 2012

The Spectrometer (Real)

Entrance Slit

130 800 nm

The CCD chips simultaneously detect all incident light and determine the intensity of

each wavelength.

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The Sample

Generally speaking the sample is metallic, flat and freshly ground.

The sample is clamped in place and sparked to generate a spectrum

this leaves burn marks on the sample:-

If the sample does not burn correctly spurious results can be produced.

Poor burning can be caused by poor grinding and rough\uneven surfaces.

Analytical

Workshop 2012

Burn marks

Sample

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Analytical

Workshop 2012

Analysis The Intensity of an emission line (colour) is proportional to concentration allows

measurement of how much of each element is present.

A number of standards are run first to set up a calibration curve, these take into

account any matrix matching difficulties (i.e. overlap of elements in some materials).

Take the example of Fe

Requires six standards to correctly calibrate the instrument

for Fe analysis.

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AnalyticalWorkshop 2012

Analysis Continued

Once calibration is completed numerous samples can be analysed.

The sample is simply clamped into place, sparked and a spectrum collected.

The spectrometer collects the intensity of light at all wavelengths and compares this

to the values for the calibration standard. This gives an accurate value of the elements

present in the sample.

Multiple sparks are collected until concordant results are obtained within an

acceptable standard deviation.

Further samples of the same alloy type can then be analysed.

Different alloys require re-calibration before analysis can occur.

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System Types

Spark Optical Emission Spectrometers come in all shapes and sizes for a multitude of

uses.

Foundry\steel making applications usually floor standing to analyse composition of

metals during production quality control.

Portable units often used in manufacturing plants to assess

metal composition.

Portable units to analyse large immovable materials

Hand held units to assist in scrap sorting

The smaller the unit the less accuracy In the measurements

so convenience over quality still useful as analysis can be

Performed anywhere.