What is Optical Emission Spectroscopy (OES)?

What is a Spectrometer?

At the very outset, it is important to understand the term “spectrometer”. A spectrometer is a device that separates and analyses the individual spectral components of a physical phenomenon to produce analytical results of interest. The spectrum – while most naturally associated with light by most – could be also be mass, magnetic, electron etc. leading to a large variety of types of spectrometry, such as optical spectrometry, photoelectron spectrometry, mass spectrometry etc.

What’s an Optical Emission Spectrometer (OES)? or What’s an Atomic Emission Spectrometer (AES)

Optical spectrometry refers to the analysis of a light spectrum separated by wavelengths. It can be of two types – absorption or emission. An Atomic Optical/Emission Spectrometer (AES / OES) is one that analyses an optical (light) spectrum emitted by an excited sample. The excitation could be by a number of means, such as application of a spark, plasma, flame etc. That said, the term “OES” is now almost ubiquitously used by people to refer to the arc-spark OES technique.

What principles does arc/spark Optical Emission Spectrometer work on?

The key principles of spectrometry used in an arc/spark Optical Emission Spectrometer (OES) are:

This means that once we know the wavelength of the photon emitted, we know which element is emitting it!

Understanding OES
Understanding OES
Understanding OES

How does an arc/spark Optical Emission Spectrometer(OES) work?

In an arc/spark OES, the principles outlined above are leveraged to analyze metallic (by and large – but more on this later) samples to assess exactly which elements are present in it – and in what proportion. The output of the OES is a detailed assessment of the elemental composition of the sample in weight percentages.

First up, there is a need to “spark” the sample. The sample is therefore first prepared, i.e. one face of the sample is made absolutely uniform, clean, flat and as free from surface flaws as possible. Suitable methods of sample preparation must be used for this. The prepared sample is then placed on the sample stand as shown below. The sample stand has a hole in it which the sample must cover. Below this, there is an electrode at a fixed distance from the sample’s exposed surface. This entire spark enclosure is filled with Argon when analysis is to be done. Then, a high current is applied to the sample.

The extremely high levels of DC current create a plasma in the Argon-purged atmosphere of the spark chamber, and a rapid series of high-energy sparks is therefore created between the electrode and the sample. Application of these sparks causes a part of the sample to vaporise. The vaporised atoms in the plasma absorb energy and their electrons move to higher energy-states with each spark. With each removal, the electrons move back into ground state and emit photons. Given the large number of elements simultaneously emitting photons, a composite emission is generated. This composite light is made to fall upon a diffraction grating.

The diffraction grating separates each individual wavelength and creates a spectrum inside what is called the “optical chamber”.

The spectrum can now clearly be analysed! The basis for analysis is of course, simplicity itself. We know the wavelengths that characterize each element. Further, the stronger the element, the higher its concentration. Were we to therefore measure the intensity of the emission at each wavelength of interest to us, we could compare with a database of knowledge and say with conviction what the concentration of individual elements is.

Understanding OES

How has have Optical Emission Spectrometers(OES) evolved? What are PMTs and what are CCDs, for instance?

How the first spectrometers worked

The first instruments (very early) had to work without photoemitters. The earliest researchers therefore had to rely on more mundane analog methods! They simply placed a photographic plate upon which the diffracted spectrum would fall. This plate was then developed and studied to arrive at the required results.

The first move to automation – the introduction of PMT detectors

In the 1930s however, there emerged the photomultiplier tube (PMT), a vacuum tube that emits electrons when light is incident upon it. Spectrometers therefore rapidly moved to using PMTs. A PMT was therefore placed inside the optical chamber in precise position for each wavelength that the user wished to analyse. Along with this, there was also a computer connected to the spectrometer. The computer stored the database against which the PMTs’ outputs were compared to arrive at the elemental composition required. This automated the process and not only made it far more rapid and convenient, but also far more accurate and error-free.

Understanding OES

This worked very well for decades – but, as ever, technology moved on. PMTs clearly had a load of drawbacks:

Flexibility was absent – once bought, that’s that!

The number of elements analysed was limited

Cost and tedium was still very high

The revolution – the death of PMTs OES and rise of CCD OES and then CMOS technology:

The introduction of CCD (Charged Coupled Device) and now CMOS (Complementary metal-oxide semiconductor) detectors solved literally every issue that the PMT devices posed and also offered several more upsides to spectrometer makers and users. Just a few of these are:

Unmatched flexibility

Instruments became smaller and less expensive

Low tedium and low running costs!

Spectrometers therefore rapidly shifted towards using these devices and today, the modern OES exclusively comprises of optics with these devices.

What are the types of Optical Emission Spectrometers available today, then?

While modern OES design focuses exclusively on CMOS / CCD detectors, there remain some legacy instrument models in the market which still feature PMT detectors. Just as when the shift to DSLRs began, it didn’t immediately see all analog SLRs immediately withdrawn, so too, while the fall in PMT OES marketshare has been precipitous, there are still a handful of models with this technology that remain in the market. For the most part, the OES of today can therefore be classed into three types as shown below.
Understanding OES

What do the results appear like?

The results of an Optical Emission Spectrometer(OES) are completely quantitative and appear as weight percentages.
A typical analytical result would appear as below:

 

 

Analysis of Low Alloy Steels Samples

Base: Fe | Matrix: MS/CS/LAS | Sample Name: IARM 28 G |

Elements

Burn 1

Burn 2

Burn 3

Mean

Cert. Val.

SD

C (%)

0.1894

0.1882

0.1889

0.1888

0.1890

0.0006

Si (%)

0.2521

0.2519

0.2530

0.2523

0.2520

0.0006

Mn (%)

0.7896

0.7915

0.7904

0.7905

0.7900

0.0010

P (%)

0.0207

0.0217

0.0215

0.0213

0.0210

0.0005

S (%)

0.0241

0.0229

0.0235

0.0235

0.0230

0.0006

O (%)

0.0031

0.0030

0.0031

0.0031

0.0030

0.0001

Cr (%)

0.0496

0.0494

0.0494

0.0495

0.0490

0.0001

Mo (%)

0.0095

0.0097

0.0095

0.0096

0.0090

0.0001

La (%)

<0.0001

<0.0001

<0.0001

<0.0001

Ta (%)

<0.0005

<0.0005

<0.0005

<0.0005

Ni (%)

0.0445

0.0445

0.0435

0.0442

0.0440

0.0006

Al (%)

0.0026

0.0020

0.0020

0.0022

0.0020

0.0003

Cu (%)

0.1822

0.1821

0.1805

0.1816

0.1780

0.0010

Ti (%)

0.0010

0.0011

0.0009

0.0010

0.0011

0.0001

V (%)

0.0013

0.0014

0.0015

0.0014

0.0014

0.0001

Nb (%)

0.0011

0.0013

0.0012

0.0012

0.0013

0.0001

W (%)

0.0033

0.0031

0.0032

0.0032

0.0032

0.0001

Co (%)

0.0028

0.0029

0.0027

0.0028

0.003

0.0001

B (%)

0.0001

0.0001

0.0001

0.0001

0.0001

0.0000

Sn (%)

0.0140

0.0142

0.0145

0.0142

0.0160

0.0003

As (%)

0.0029

0.0028

0.0033

0.0030

0.0031

0.0003

Pb (%)

0.0028

0.0029

0.0028

0.0028

0.0031

0.0001

Zr (%)

0.0008

0.0009

0.0009

0.0009

0.0010

0.0001

N (%)

0.0088

0.0090

0.0087

0.0088

0.0089

0.0002

Mg (%)

<0.0010

<0.0010

<0.0010

<0.0010

Te (%)

<0.0010

<0.0010

<0.0010

<0.0010

Ag (%)

<0.0002

<0.0002

<0.0002

<0.0002

Sb (%)

0.0020

0.0020

0.0024

0.0021

0.0020

0.0002

Bi (%)

<0.0001

<0.0001

<0.0001

<0.0001

Se (%)

<0.0001

<0.0001

<0.0001

<0.0001

Zn (%)

<0.0001

<0.0001

<0.0001

<0.0001

Ce (%)

<0.0002

<0.0002

<0.0002

<0.0002

H (%)

<0.0001

<0.0001

<0.0001

<0.0001

Fe (%)

98.3921

98.3916

98.3932

98.3923

Is there anything more to know about OES?

Of course! As with any field, this is merely the beginning. For more, do refer to:

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