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Spark Optical Emission Spectrometer

• satyendra
• August 6, 2022
• 2 Comments
• Charge coupled device, CID, CMOS, Detectors, Diffraction grating, HEPS, LDR, LODs, Optical emission spectrometer, PAD, Photo multiplier tube, Spectrochemical analysis, Spectrometry, Spectroscopy,

Spark Optical Emission Spectrometer

Chemical analysis and the identification of metals are used for a wide variety of purposes. These have become necessary to control the incoming goods or to ensure that the manufacturing process produces the product with the correct elemental composition. It is also used in failure investigation to determine if the correct material was chosen. Further, in cases where national / international standards are to be adhered to, the determination of chemical composition is self-evident.

For understanding the general importance of the chemical composition in the metal industry, one is required to know that there are thousands of metal grades already invented and each grade has its own set of specific metallurgical properties. For example, in steel there are already more than 3,500 steel grades known, and each steel grade can distinguish itself from other steel grades through a different melting point, corrosion / acid resistance, and tensile strength etc. This is why, choosing the correct grade is absolutely imperative and using an incorrect grade can lead to manufacturing failures with heavy damages and costs.

A chemical analysis is normally performed with a spectrometer. But a spectrometer on its own is not enough. It is to be remembered that spectrometers need an extensive know-how, they need high quality argon / helium to work, and that the right surface preparation is needed for correct analysis. Every organization needs the budget and the qualified people to integrate the spectrometer in the organization.

Optical emission spectrometry is an analytical technique, universally considered and used to determine the chemical composition of metal and alloys. Because of its method precision and the short analysis time, it represents a valid control system of production in all the fields regarding metal industry in any time during production cycle, in case, it is necessary to have a fast and accurate chemical analysis of material.

Optical emission spectrometry is considered as one of the old optical analytical methods. Flame was used as a light source for the optical emission spectrometry in the beginning. In the 1950s and the early 1960s, spark (also known as arc) optical emission spectrometry prevailed since it has better excitation capability. Of the spark optical emission spectrometers in service today, 99 % are used for the routine spectrochemical analysis of metals of which 80 % of them consists of alloys of iron, aluminum, and copper. The remaining applications concern alloys of nickel, cobalt, zinc, titanium, magnesium, and the lead /tin /solder group and 1 % consists of refractories, precious metals, steel-making slags and geological materials.

Spectrochemical analysis is a type of chemical analysis used to determine the arrangement of atoms and electrons within molecules of chemical compounds. It observes the quantity of energy absorbed during changes in motion or structure. The wavelength and intensity of electromagnetic radiation is measured to produce quantifiable results which are primarily used for quality assessment.

Brief history

In the spectroscopic history, spark excitation was the leading technique for elemental analysis for a long time. In 1752, Thomas Melville observed bright yellow light emitted from a flame produced by burning a mixture of alcohol and sea salt. In 1776, Alessandro Volta reported for the first-time, uses of sparks for chemical analysis. He was able to identify certain gases by the colours emitted when he applied a spark to them. In the late 18th and early 19th centuries, Fraunhofer and others compared the spectra emitted by flames and sparks with that from the sun and planets. In 1826, W. H. Talbot reported a series of experiments in which he observed the colouring of flames by a variety of salts.

Nature of emission spectra was beginning to be understood in1859, when Kirchhoff and Bunsen observed that the sharp line spectra from flames were produced by atoms and not molecules. In the beginning of the 20th century, the sharp lines which appeared in the light emitted from electrical arcs and sparks were a driving force for science. During the middle of the 20h century, quantitative spark spectroscopy was the best tool which the analysts had to probe trace concentrations for a wide range of elements.

There are a handful of spectrometric technologies available for determining the chemical composition of metals. The four most common used techniques in the metal industry are (i) x-ray fluorescence (XRF) spectrometry, (ii) optical emission spectrometry, (iii) spark optical emission spectrometry, and (iv) glow discharge optical emission spectrometry.

Spark excitation was the leading technique for elemental analysis for a long period in the spectroscopic history. Presently, other techniques like ICP-OES (inductively couple plasma – optical emission spectrometry), ICP-MS (inductively couple plasma – mass spectrometry) and MIP-OES (microwave induced plasma – optical emission spectrometry) are getting more attention because of their almost perfect analytical performance. However, spark optical emission spectrometry still retains the super solid sampling capability, in comparison with ICP or MIP techniques. For spark excitation source, electrical discharge excitation sources were developed in late 19th century. Later on, direct current, alternating current, and high voltage excitation sources were developed in the first two decades of the 20th century.

Spark optical emission spectrometry is frequently used in metallurgical industry, geological field and for monitoring of the environment. Numerous advances in terms of evaluation of the instrument performance, reproducibility, limits of detection (LODs), and linear dynamic ranges (LDR) have been done and reported. Advances related to optimization of experimental conditions, interferences by spectrometry molecular spectral band or by coexisting elements or background have also been done. Improved sensitivity have been reported even for some hard-to-excite elements such as Hf (hafnium) and W (tungsten).

A low-cost and compact spark optical emission spectrometer has been achieved by using a hand-held spectrometer with a charge-coupled device (CCD) as a detector.

Spectroscopy and the spectrometer

Spectroscopy and spectrometer are the terms which are frequently being used when discussing spectrochemical analysis. In simple terms, spectroscopy is the study of energy in relation to a sample material, and a spectrometer is the instrument used during spectrometry, the act of spectroscopy. Spectrometer is a device which 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, can also be mass, magnetic, electron etc. leading to a large variety of types of spectrometry, such as optical spectrometry, photoelectron spectrometry, and mass spectrometry etc.

Spectroscopy – Spectroscopy is the study of the interaction between radiated energy and a sample material. This interaction produces electro-magnetic waves in the form of visible light, typically seen as sparks. Spectroscopy was introduced in the 17th century when Isaac Newton discovered that white light could be separated into component colours using a prism, and these components could be recombined to form white light. He realized that the prism is not what creates the colours, but it instead works to separate colour components of white light. In the early 1800s, Joseph von Fraunhofer performed experiments which further evolved spectroscopy into a more precise and quantitative scientific technique. However, it was not until the 19th century that the quantitative measurement of dispersed light was standardized and recognized as a sound method of testing. Joseph von Fraunhofer performed experiments with the spectrometer which led spectroscopy to become a more precise and quantitative scientific technique.

The advantages and disadvantages of the optical emission spectroscopy are (i) fast quantitative determination of elements (typically less than one minute), (ii) low capital investment and operating costs, (iii) easy sample preparation, (iv) quick analysis of carbon, nitrogen, oxygen, phosphorus, and sulphur in steel, (v) calculates carbon content (%) of stainless steels or low alloy steels, (vi) provides input data to carbon equivalent calculation, (vii) not completely ‘non-destructive’ (slight surface damage can be expected), (viii) cannot test small parts, (ix) difficult to test in tightly confined spaces, (x) needs constant calibration and maintenance, and (xi) routine third party certification of results can be needed.

Optical emission spectroscopy can be employed on a range of materials from pure to alloyed metals. Several industries benefit from spectrometers for process and quality control.

Spectrometer – A spectrometer is the instrument used in spectroscopy which produces spectral lines and measures their wavelengths and intensities. It is a scientific device which separates particles, atoms, and molecules by their mass, momentum, or energy. Spectrometers are integral to chemical analysis and particle physics. There are two types of spectrometers namely (i) optical spectrometer, and (ii) mass spectrometer.

An optical emission spectrometer, or simply ‘spectrometer’, is able to separate white light and measure individual narrow bands of colours (spectrum). It shows the intensity of light as a function of wavelength or frequency and the deflection is created by refraction in a prism, or by diffraction in a diffraction grating. Optical emission spectrometer uses the concept of optical dispersion, and since each element in a sample leaves a unique spectral signature, spectral analysis can determine the composition of the sample itself. Optical emission spectrometer is common in metal production, astronomy, solar power, and semiconductor industries.

Optical emission spectrometer is user-friendly and widely accepted in the metal manufacturing industry. Although a popular instrument, it still comes with a few limitations including minor surface damage to the sample material and the need for constant maintenance.

Spectrometers are frequently the instrument of choice for laboratories in the iron and steel industry since they need only minimal intervention from the operators when used for inspection, quality control, and alloy identification. Stationary and portable versions are available, both with a high level of accuracy. Routine calibration and maintenance are needed, and third-party certification of results are frequently needed for spectrometer results to retain their validity. Spectrometers allow for metal analysis throughout the metal life cycle from metal production to processing, as well as at the end of its service life at the recycling plants

Optical emission spectrometers use the concept of optical dispersion. Each element in a sample leaves a unique spectral signature, allowing spectral analysis to decipher the composition of the sample.

Components of optical emission spectrometer

Optical emission spectrometry involves applying electrical energy in the form of spark generated between an electrode and a metal sample, whereby the vapourized atoms are brought to a high energy state within a so-called ‘discharge plasma’.

These excited atoms and ions in the discharge plasma create a unique emission spectrum specific to each element. Hence, a single element generates several characteristic emission spectral lines.

Hence, the light generated by the discharge can be said to be a collection of the spectral lines generated by the elements in the sample. This light is split by a diffraction grating to extract the emission spectrum for the target elements. The intensity of each emission spectrum depends on the concentration of the element in the sample. Detectors (e.g., photo multiplier tubes, charge coupled devices) measure the presence or absence or presence of the spectrum extracted for each element and the intensity of the spectrum to perform qualitative and quantitative analysis of the elements.

In the broader sense, optical emission spectrometry includes, inductively coupled plasma (ICP) optical emission spectrometry, which uses an inductively coupled plasma as the excitation source. The terms ‘optical emission spectrometry’ and ‘photo-electric optical emission spectrometry’, however, normally refer to optical emission spectrometry using spark discharge, direct-current arc discharge, or glow discharge for generating the excitation discharge.Some optical emission spectrometers feature ‘pulse distribution analysis’ (PDA) to improve the measurement reproducibility (accuracy). This method involves statistical processing of the spark pulse-generated emission spectra obtained from spark discharges in an argon atmosphere. Fig 1 shows Important parts of spectrometer.

Fig 1 Important parts of spectrometer

Sample preparation – Paper grinding is possible in most of the cases, but for lowest carbon and oxygen levels, milling is desired. Simple and reasonable care is to be taken during and after sample preparation, e.g., (i) the prepared surface is to be flat in order to avoid air penetrating in the stand during the analysis, (ii) grinding paper change is to be at well-defined time intervals, (iii) exposure of sample surface to dust is to be avoided, and (iv) touching the prepared surface is to be avoided.

Optical emission spectrometer provides quantitative analysis using three key components namely (i) an electrical source, (ii) an optical system, and (iii) a computer system.

Electrical source – An electrical source is needed to excite atoms into an active state within a metal sample. A small portion of the sample is heated to thousands of degrees centigrade using a high voltage electrical source in the spectrometer through an electrode. An electrical discharge is produced due to the difference in electrical potential between the electrode and the sample metal. This electrical discharge causes the sample metal to heat up and vapourize at the surface.

The electrode heats up the sample, exciting the atoms into an active state. This also produces an electrical discharge, causing the sample metal to vapourize at the surface. During this process, the activated atoms produce emission lines which are distinct to each element. Two types of electrical discharges exist namely (i) an electric arc, or (ii) a spark. An electric arc produces an ongoing electrical discharge, much like lightning. An electric spark is more of an abrupt electric discharge, a brief emission of light frequently accompanied by a sharp snapping sound. Fig 2 shows schematic of metal surface vapourization by the electric discharge.

Fig 2 Schematic of metal surface vapourization by the electric discharge

Optical system – The optical system is composed of five main parts namely (i) the primary slit (20 micro-metre), (ii) the grating to diffracts the light on the secondary slits, (iii) the secondary slits to narrow down the wavelength bands, (iv) the mirrors to reflect the light for focusing on the photo multiplier tubes, and (v) the photo multipliers tubes. The primary lens is made of calcium fluoride (CaF2).

The optical system transfers the emission lines from the vapourized sample, known as plasma, into the spectrometer. The diffraction grating in the spectrometer works to separate the incoming light into element-specific wavelengths. The intensity of light of each wavelength is then measured by a corresponding detector. The intensity measured during this process is proportional to the concentration of the element in the sample metal being tested. Since each element emits a specific set of wavelengths based on its electronic structure, the elemental composition can be determined by observing these wavelengths. The intensity of light of each wavelength is measured by a corresponding detector which determines the elemental composition. Fig 3 shows schematic of the optical system of a spectrometer.

Fig 3 Schematic of the optical system of a spectrometer

Computer system – A computer system is needed in the spectrometer to process the data. The measured intensities are processed through a pre-defined calibration to produce elemental concentrations. Present day technology has advanced the user interface to offer clear results with minimal operator intervention. The analysis time, from the start of the analysis to the display of the result, is normally around 22 s (seconds) to 25 s.

Principle

Spark optical emission spectrometer works on two very basic principles of physics (Fig 4). The first is that the electrons in atoms absorb energy (get ‘excited’) and move into higher energy states (also called orbits) when energy is applied. When this energy source is removed, the electrons fall into the ground state and release the absorbed energy in the form of photons. The second principle is that no two atoms of different element can emit photons at the same wavelength. Hence, every wavelength is unique to a single element alone. This means that once the wavelength of the photon emitted is known, then the element, which is emitting it, is known.

Fig 4 Principles of physics for spark spectrometer

Optical emission spectrometry is based on application of the electrical energy in the form of spark generated by interaction of an electrode and a metal sample, whereby the vapourized atoms are brought to a high energy state within a so-called ‘discharge plasma’. These excited atoms and ions in the discharge plasma create a unique emission spectrum specific to each element. The radiation emitted by element is directly fed to the spark spectrometer optics or through an optical fibre, where it is dispersed into its spectral components. Hence, the light generated by the discharge can be said to be a collection of the spectral lines generated by the elements in the sample. This light is split by a diffraction grating to extract the emission spectrum for the target elements. Fig 5 shows working principle of a spectrometer.

Fig 5 Working principle of a spectrometer

Excitation in spark spectrometer is produced by the energy of the electrical discharge between the sample and the electrode. In all early instruments electrical discharge was carried out in air but today it is carried out in an argon atmosphere. For this type of spectrometer using electrical excitation, the sample is to be electrically conductive. Hence, this instrumentation is very popular in the metal industry.

Direct current arc excitation source has been used in connection with a multi-channel spectrometer. Advantages of direct current spark are (i) simple handling, (ii) no / minimum sample preparation, (iii) no / minimum use of special atmospheres to achieve, (iv) complete evaporation of the analytes, (v) low contamination by the method itself, and (vi) rapid and cost-effective qualitative method for semi quantitative overall analysis of unknown materials.

Alternating current arc has used as excitation source and reported in 2000 by Goldik. The arc was made from two electrodes with 10 mm gap between them. The applied open circuit voltage was 13 kV and the closed-circuit current was 30 mA with source temperature of 3,500 K using iron Fe (iron) spectral lines. This system has advantage of its low acquisition and operation cost. Its atmospheric operation without additional gas is another advantage of this new emission source.

Spark-like excitation refers to high-current discharges which come in short bursts, similar to gun shots. Some typical electrical parameters for a ‘spark-like’ discharge, are (i) frequency – 200 Hz (hertz) to 1,000 Hz, (ii) operating voltage – 400 V (volt) to 1,000 V, (iii) peak current – 50 A (ampere) to 150 A, and (iv) FWHM (full width at half maximum) – 50 micro-seconds to 100 micro-seconds. Arc-like excitation refers to a lower current, drawn-out, almost continuous discharge. Arc like discharges can be either direct current (DC), or alternating current (AC) with a frequency like the spark-like discharges. The DC arc is a low-current discharge of the order of perhaps 3 A to 10 A. The AC arc can provide currents as high as 20 A to 30 A with a frequency of 100 Hz to 200 Hz and a total duration for each cycle of 500 micro-seconds to 1,000 micro-seconds. Analytically, the differences between the arc and spark excitations can be summarized as in Tab 1.

The precision numbers listed in Tab 1 are at concentrations well above the limit of detection. The primary function of the excitation source is to generate spark-like or arc-like electrical discharges. These discharges vapourize sample material and produce excitation of the electrons of the atoms for the measurement of spectral line light intensities.

Yamamoto and Takimoto invented a device composed of a sample holder having a through hole, a spark electrode, and an inert gas inlet tube, suited for use in the measurement of oxygen in a metal sample, wherein the size, i.e., the diameter, of the metal sample was appropriately selected so that the electric spark was generated from the whole discharged area to eliminate the influence of element segregation in the sample. The analytical precision was improved by using an optimized threshold value obtained by measuring the relationship between the maximum analytical precision and spectral intensity of matrix element. The elemental detection was performed by measuring the spectral ratio of the analyte element to the matrix element within the threshold range.

A typical spark stand (electrode and sample geometry) diagram is shown in Fig 1. It shows the conventional ‘point-to-plane’ configuration (electrode to sample). The electrode typically is made of tungsten, although graphite electrodes have been used in the past with non-ferrous materials. The spark stand in modern instrumentation is flushed with argon to prevent oxidation effects and allow transmission of UV (ultraviolet) wavelengths absorbed by oxygen.

The voltage in the circuit is normally between around 400 V and 1,000 V which is not enough to cross the 3 mm to 4 mm spark gap, a much higher voltage, on the order of 10,000 V, is needed, which is provided by an ignitor circuit. In present day spark excitation sources, the discharge is unidirectional, from the electrode to the sample. The switch is opened and closed electronically many times per second. This is referred to as the ‘frequency’ of the discharge. In ‘spark gap’ temperature of 5,000 K to 15,000 K is generated. It is in this region where analytical processes important for spectrochemical analysis occur.

The detectors like charge coupled devices or photo multiplier tubes, which are the most suitable line for the application, are then used to measure the range of wavelengths emitted by each element. The concentration of the element in the sample is directly proportional to the radiation intensity, which is calculated internally from a stored set of calibration curves and can be shown directly as percent concentration. Fig 6 shows structure of optical emission spectrometers

Fig 6 Structure of optical emission spectrometers

High energy pre-spark – The excitation source has another very important function. It is to homogenize the sample surface in preparation for the actual measurement of light intensities. The homogenization is achieved by first subjecting the sample surface to a HEPS (high energy pre spark). The action of HEPS is comparable to that of a remelt furnace, i.e., on subjecting the sample surface to the electrical discharge the sample is effectively ‘re-melted’. This technique is important in removing any effects of the metallurgical history or structure of the sample (for example, cast versus wrought, heat treatment, and so forth).

Typical pre-spark currents can be from around 120 A to 150 A with durations from around 10 s to 20 s. Depending upon the electrical parameters used, the crater produced in the sample surface during the high-energy pre-spark is around 10 mm in diameter with a maximum depth (penetration at the centre) on the order of 50 micro-metre.

The sample is ready after the pre-spark period for the actual measurement or integration of the light intensity signals. This technique is used primarily when very small quantities of elements present in sample are to be detected. The excitation is carried out by spark-like discharge for around a 5 s time period. The complete source cycle during the sparking of a sample is (i) HEPS (homogenization) – 10 s, (ii) spark (measurement) – 5 s and (iii) arc (measurement) – 5 s.

The most important part of the protocol is excitation of the sample. The sample is excited on a sample stand by using an electronic device which provides high voltage spark.  A low voltage arc between the surface of the sample under test and a counter electrode is used to analyze the sample. A classical source is of the RLC (resistor, inductor, and capacitor) circuit discharge type and its frequency can rise up to 400 Hz. A spark is generated by applying high voltage to the sample and the emitted light is used to determine the concentrations of the element within the sample.

The purpose of sample slide helps in placing the sample in a reproducible way relative to the optical device in the spectrometer. The argon is circulated in the enclosure of the sample stand at a rate of 0.2 litres per minute (l/m) to 0.5 l/min which increases automatically during sparking. A water circuit is used to cool the table. The process involves production of two sparks, one is of high voltage while other is of low voltage. The high-energy spark prepares (melt and homogenize) the sample surface and then the light emitted from lower energy spark is measured. The shutter is of the instrument unit opens during the measuring phase by an electromagnetic valve.

The optical dispersion system of the spectrometer is placed in a mechanical housing. To avoid any deformation in the spectrometer by dilatation, it is placed in a thermo-controlled (at 38 deg C +/- 0.1 deg C) insulated cabinet called oven. The light path is around 2 m (metre) long. As air absorbs ultraviolet light, it is necessary to put the spectrometer under vacuum to avoid this effect.

As regards detection system, photo multiplier tube is most commonly used as the detector for direct-reading optical emission spectrometer. However, this system has limitations of single channel and bulkiness. With the fast development in detectors, other detectors like semi-conductor, solid-state detectors including the charge-coupled device, photo-diode-array detector (PAD), and charge-injection device (CID) are used in spark optical emission spectrometer.

In its broadest definition, a charge-coupled device image sensor is an analogue integrated circuit which converts an optical image into an electronic output. As with several semi-conductor components, the evolution of charge-coupled device from a laboratory concept has been extremely rapid, boosted by their potential for a wide range of applications in both consumer products and sophisticated professional equipment. Today, this advanced technology is used in the spectrometers. In the present-day optical emission spectrometers, charge-coupled device has the equivalent of 8,044 detectors of 7 micrometres by 7 micrometres. Because of this, all the wavelengths (from 170 nanometres to 410 nanometres) necessary for multi-element, multi-base analysis are available without the need for additional hardware. Their integrated nature provides their best-known features, namely unlimited lifetime and miniaturization. To these can be added some more specific characteristics, including image analysis in discrete elements (pixels) with exact field registration. Each spectrum is stored, and a retrospective analysis for ‘extra’ elements is possible without the need to spark the sample another time.

Solid-state detectors have an advantage of multi-channel detection for simultaneous multi-element measurements. Whereas charge-coupled device detectors have made possible to detect elements simultaneously in polymer materials within 5 s, the use of miniaturized   spectrometer led to a possible portable spark optical emission spectrometer instrument. But quick quantitative analysis of 1.8 (Zn) mg/g (milli grams per gram) to 9.3 (Mg) mg/g direct solid sampling has been made possible by LODs (limits of detection).

Coates and co-workers proposed an on-site analyzer for analyzing lubricant oils and functional fluids. The optical emission spectrometer analyzed light captured from a spark emission stand through which the fluid sample has flown. Diagnostic text for an operator is generated by an expert which operates system according to a set of rules based on the information collected from the optical emission spectrometer and other measurement devices about the fluid sample. The size, weight, and cost of instrument has reduced and made it suitable for on-site analysis.

Charge injection device, a solid-state detector similar to charge coupled device, has also been used in optical analytical instrumentation. It has same main advantages as the charge coupled device. The entire spectral range has been recorded with the use of a direct current arc spectrometer using echelle grating and a charge injection device camera as a detector. Simultaneous precision in measurement can be achieved in any particular emission line. The technique has been applied for the determination of molybdenum in tungsten compounds.

The optical emission spectrometers can be factory calibrated for iron and steel utilizing a very sophisticated multi-variable regression tool which corrects for matrix effects as well as spectral interferences. The tool provides an immediate ‘turnkey’ system which gives the user the highest accuracy possible. The calibrations are available for the different qualities. For each quality, the supplier use certified material as standard samples and setting-up samples are delivered with the instrument to maintain the accuracy of the calibration.

In a spark optical emission spectrometer, the principles outlined above are leveraged to analyze metallic samples to assess exactly which elements are present in it, and in what proportion. The output of the optical emission spectrometer is a detailed assessment of the elemental composition of the sample in weight percentages.

Test procedure

First up, there is a need to ‘spark’ the sample. Hence, the sample is 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 are to be used for this. The prepared sample is then placed on the sample stand. 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 direct current create a plasma in the argon-purged atmosphere of the spark chamber, and a rapid series of high-energy sparks is hence created between the electrode and the sample. Application of these sparks causes a part of the sample to vapourize. The vapourized 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 analyzed. The basis for analysis is of course, simplicity itself. The wavelengths which characterize each element are known. Further, the stronger the intensity of the emission at an element’s wavelength, the higher is its concentration. A database containing the concentration levels that different intensity values correspond to for each wavelength of interest is available which simply look up the emission intensity against this database and say with conviction what the concentration of individual elements is.

New developments

The first instruments (very early) had to work without photo-emitters. The earliest procedures hence had to rely on more mundane analog methods. As per these procedures, a photographic plate is simply placed, upon which the diffracted spectrum would fall. This plate was then developed and studied to arrive at the required results.

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

This worked very well for decades, but, as the technology moved on, photo multiplier tubes clearly had a number of drawbacks. Photo multiplier tubes have medium high resolution, and very high focal length. Further, they were very large and expensive. Flexibility was absent, once manufactured, there was no possibility of any modification. It did not have any ability at all to modify. Even a single element increase meant a new optical emission spectrometer. Cost and tedium were very high. Detectors, cards etc. were extremely expensive. Regular profiling was needed. It had vacuum which was required to be maintained. This led to the fading away of photo multiplier tubes and increase in the popularity of charge-coupled devices and complementary metal oxide semi-conductor (CMOS) detectors. Fig 7 shows developments in optical emission spectrometers.

Fig 7 Developments in optical emission spectrometers

The introduction of charge-coupled device and now complementary metal-oxide semiconductor (CMOS) detectors solved literally every issue which the photo multiplier tube devices posed and also offered several more upsides to spectrometer makers and users. Just a few of these are (i) unmatched flexibility since every wavelength is captured and so can be analyzed, (ii) no limitation of space as charge coupled devices are small, (iii) instruments became smaller and less expensive since high-resolution grating and charge coupled devices result in shorter focal lengths, (iv) fewer detectors mean fewer cards and lower cost with low tedium and low running costs, (vi) no need for profiling etc. since all this is automated, and (vii) no vacuum and efficient electronic which results in lower running costs.

Hence, spectrometers rapidly shifted towards using these devices and today, the modern optical emission spectrometer exclusively comprises of optics with these devices. Present day spectrometers are of two types. Type A use single / fewer (2 to 3) charge coupled devices / complementary metal oxide semi-conductor detectors closer to the grating. These spectrometers have lower resolution, low focal length, very compact, and very economical. Type B use several charge-coupled devices / complementary metal oxide semi-conductor detectors (5 to 20+) further away from grating. These spectrometers have very high resolution, medium focal length, compact compared to spectrometer with photo multiplier tube, and economical compared to spectrometer with photo multiplier tube.

While modern optical emission spectrometer design focuses exclusively on charge coupled device / complementary metal oxide semi-conductor detectors, there remain some legacy instrument models which still feature photo multiplier tube detectors. Just as when the shift to DSLRs (digital single lens reflex) began, it did not immediately see all analog SLRs (single lens reflex) immediately withdrawn, so too, while the fall in photo multiplier tube optical emission spectrometer’s market share has been precipitous, there are still a handful of models with this technology which remain in the market.

Application

Optical emission spectrometer is capable of solid sampling. It is widely applied for the analysis of a broad range of samples. It is applied for the analysis of steel, metals and alloys, geological samples, biological samples, environmental samples, and other kinds of samples and special techniques. It has a very high up-time, delivers dependable performance. The optical emission spectrometer offers rapid elemental analysis of solid metal samples, making it indispensable for quality control in iron and steel plants and aluminum metallurgy processes.

The spark optical emission spectrometer is able to determine all the elements necessary in the present and future applications, in all possible qualities of iron and steel such as white or grey cast iron, alloyed cast iron, low alloy steel and high alloy steel. It is the answer to the analytical needs, whether for incoming goods control, metal sorting, process quality control, final product quality control, certification or investigation.

In case of sampling, it is used to analyze the inclusions of micro impurities and the macro components. It is used to analyze the standard reference materials and real steel samples. It is also used in the analysis of non-metal elements (e.g., carbon, nitrogen, sulphur, phosphorus, oxygen, silicon, and boron) in steel samples. Quantitative or qualitative analysis of nitrogen in steel is also determined through this technique.

Considerable progress has been made in the analysis of low concentration carbon, nitrogen, and oxygen in steels by optical emission spectrometry. The concentration of these elements, as well as those of phosphorus, sulphur, and hydrogen is to be reduced to obtain so-called ‘clean steels’, or controlled, as they have, individually or together, dramatic effects on steel properties, such as strength, formability, toughness, weldability, fatigue resistance, etc. The constant amelioration of steel cleanness being necessary to produce always improved and more competitive steel products, there is a demand for quantitative analysis at lower and lower levels. The new performance obtained with the latest models of optical emission spectrometer fulfills the latest analytical requirements of steel producers regarding carbon, nitrogen, and oxygen.

In case of metals and alloys, the technique is used (i) for the analysis of impurity of elements in metals or alloy, (ii) for the analysis of primary and commercial-purity of aluminum, (iii) for the analysis of molybdenum powder in chromium, iron, and nickel, (iv) for the analysis of oxygen in copper, and (v) for the detection of gases in titanium and for the detection of impurities (iron, silicon, copper, manganese, aluminum, and nickel) in pure magnesium.

In case of geological and related samples, the technique is used for the simultaneous determination of gold, platinum, palladium, rhodium, iridium, and ruthenium in geological samples and for the analysis of rock, ores and minerals.

In case of biological samples, direct analysis of micro-volume of biological samples like liver, heart, brain, kidney, bone, serum, and muscles of white albino rats has been reported by this method.

In case of environmental and waste samples, the technique is used for the analysis of chromium in air and micro quantities of elements in waste samples.