CALIBRATION AND THE ROLE OF CALIBRATION SAMPLES IN METAL OPTICAL EMISSION SPECTROMETER.

THEORY OF CALIBRATION:

Concentration vs Intensity calibration curve

Calibration comprises measurement of calibration samples and determination of the functional relationship between the intensity ‘I’ of the line of an analyte and its concentration c in these samples. The functional relationship is the calibration function or calibration curve. It includes relationships between vaporisation, excitation, radiation offtake, dispersion and the measured value. Since spectrochemical analysis is a process of analysis is a process of analysis by comparison ( in contrast to absolute methods such as weighing ), it is necessary to carry out calibration with samples of accurately known concentration, the calibration samples.

The calibration function must not be confused with the function inverse to it-the read out or evaluation function. In the case of the calibration function I = f1 (c), the concentrations of the calibration samples are assumed to be free of error, and the errors (deviations from a best fit curve after correction of the intensities for systematic errors) are imputed entirely to the spectrometer method, so that the preconditions for regression calculations showing correlation coefficients as a quality index are useless. With the evaluation function c = f2 = ( I ) the concentration c of an analyte in an analytical sample is determined, which is accordingly subject to error, f2 = 1/f1.

For optical emission spectrometry there is no theory of calibration curves which can be used for practical purposes. There are formulae for which it is assumed that it is possible to represent the relationship between line intensity and concentration as a power function : I = I0 ck. The calibration function can be represented mathematically in various ways :

linear calibration function : I = f(c) = a0 + a1 c

non-linear calibration function : I =f(c) = a0 + a1 c +a2 c2+...+an cn

The extent to which the regression approaches the true course of the calibration

curve can be discerned from the residual scatter, namely at the point when the

addition of further terms to the approximation function does not produce any

further improvement in the residual scatter.

CALIBRATION SAMPLES

Fundamental role of the calibration samples is attested by international community and by International Standardisation Organization (ISO), which delivered the following definitions :

Reference Materials (RM) : they are Materials or substances whose properties are so well defined that they can be used to calibrate the instrument, verify the measure or assign values to the materials.

CRM sample with Spark analysis spots

Certified Reference Materials (CRM) : they are Materials whose values concerning one or more properties are certified by means of a valid technical procedure and equipped by a Certificate or other documents from a qualified technical Body ( public or private Organization or Society., which deliver a certificate for the Reference Material )

Calibration samples present three disadvantages :

1) They are expensive

2) Their dimensions and shapes are not always available for the sample-holder stand of the spectrometer.

3) They are available only for some elements and concentrations

In some cases calibration samples can be synthesised, for example by alloying or diluting part of a charge. Because of this manipulation, the calculated values are rarely reliable and their composition should be confirmed by chemical analysis.

RECALIBRATION SAMPLES

When calibrating spectrometers with calibration samples (reference samples)

Recalibration samples are measured a number of times in order to obtain a reliable nominal value suitable for calibration. The additive and/or multiplicative changes in the sensitivity of the spectrometer bring about displacements of the calibration curves in the linear scale of the co-ordinate system. In order to trace (calculate) the actual intensity values at any later time back to the nominal intensity values submitted at the time of calibration a low (LP) and a high (HP) intensity is required for each analyte channel. In metal analysis with spark discharge the low points of all the analyte channels are usually measured with the pure base (Fe, Al, Cu,...). The high points are usually measured from synthetic samples having as many elements as possible with good homogeneity and precision.

The synthetic composition is given as a guide analysis and the samples often do not lie on the calibration curves. Mathematical procedure of calibration is a automated process.

In emission spectrometry recalibration samples run out, because of the polishing of the surface before recalibration. When recalibration samples are replaced there is no guarantee that, even with the same sample number, the new sample concentrations will correspond exactly to the sample being replaced. For this reason when calibrating a spectrometer for metal analysis, a minimum supply of recalibration samples should be available, for example five recalibration samples for each type.

The frequency of recalibration depends on the instrument and its use.

Interdependence with the instrument means that devices of the same kind, specially because of different phototubes stability, must be recalibrated at different intervals. Interdependence with use means that, even if stability is the same, recalibration frequency depends on the kind of analysis (traces analysis, sorting analysis).

(Note: The above post is written in context to calibration of Spark Optical emission spectrometer for metal and alloy analysis.)

LASER INDUCED BREAKDOWN SPECTROSCOPY

LIBS, is a spectroscopy technique in which a short laser pulse beam is focused on a target sample. Laser energy ionizes the sample material by heating it,  creating small area of plasma. Excited ions in the plasma state emits light waves which are collected and the spectrum is resolved by a spectrometer and analyzed by suitably calibrated  photon or light detector. Each chemical element has a unique wavelength or signature which can be optically resolved from the obtained spectrum. As  result, the composition of the elements which constitutes in the target sample can be determined. Below provided some of the general information about the technique : i Advantages ii Considerations Spectral coverage vs. resolution Light sensitivity iii. General Applications I. Advantages LIBS is considered one of the most  efficient and user friendly analytical techniques for trace elemental analysis in gases, solids, and liquids. Some of its major advantages include: Real-time measurements: online monitoring and quality control of industrial processes Noninvasive, nondestructive technique: valuable samples can be reused, sensitive materials can be analyzed, suitable for in-situ biological analysis Remote measurements can be done from up to 50 meters distance: can be used in hazardous environments and for space exploration missions on other planets Compact and inexpensive equipment: can be widely used in industrial environments, perfect for field measurements High-spatial resolution: can obtain 2D chemical and mechanical profiles of virtually any solid material with up to 1 µm precision Non or very little sample preparation is required: reduced measurement time, greater convenience, less opportunity for sample contamination Samples can be in virtually any form: gas, liquid, or solids Analysis can be performed with a very small amount of sample (nanograms): very useful in chemistry for characterization of new chemicals and in material science for characterization of new composite materials or nanostructures Virtually any chemical element can be analyzed, such as heavier elements unavailable for X-ray fluorescence Analysis can be done on extremely hard materials like ceramics and superconductors; these materials are difficult to dissolve or sample to perform other types of analysis In aerosols both particle size and chemical composition can be analyzed simultaneously II. Considerations  Spectral Coverage vs. Resolution Compact echelle spectrometers designed for LIBS applications are offered by several manufacturers. In the rare occasion that an application requires even higher resolution, the Acton Series of spectrometers with their long focal lengths are extremely useful. The latest models  use toroid mirrors with improved spectral quality. For a  detector with 1024 horizontal pixels, each of which is 26 m wide, the theoretical field of view is 26.6 mm. But since a standard 25 mm intensifier is used, the field of view is 25 mm. For example, if you decided to utilize a 2400 groove/mm grating in the Acton Series 2500 in order to enhance resolution, the linear dispersion will be 0.6 nm/mm while the spectral coverage will be 0.6*25 = 15 nm. To cover a spectral range between 300 and 600 nm (for example), you will need to perform at least 20 laser shots each time, moving the spectrometer grating to a new position and "gluing" all 20 spectra together. This is a very standard procedure which can be done painlessly and automatically. The only disadvantage to this is that acquisition of one spectrum could take up to a few dozen seconds or longer, which is why the echelle spectrometer has become extremely popular, especially in industrial and field applications where real-time measurements such as online quality control is a must.  Light sensitivity Typically, the laser pulse in LIBS applications lasts for femto- to nanoseconds (10-15 to 10-9 s). Especially in applications where non-invasive and non-destructive analysis is required, a relatively small amount of laser energy is transferred to the sample. Therefore, one laser pulse produces a weak emission signal which is hard or impossible to collect with conventional CCD detectors. That is why intensified CCDs (ICCDs) are widely used in LIBS. To improve the emitting signal on the order of 10-30 times, a scheme with two orthogonal lasers beams is often used. In this dual-scheme, the first and usually more powerful laser pulse ablates and atomizes sample material while the second one heats the ablated material even further, allowing it to improve the intensity of atomic or ionic lines. Factors such as the level of laser excitation energy for both pulses and the time delay between the pulses play a crucial role in achieving signal intensity enhancement. This technique increases the sensitivity of LIBS by  at least one order of magnitude and allows for a greater possible number of applications. If measurement time duration is not an issue, a regular CCD, (1024x1024 pixels, 13 µm pixel size), can be used together with the an spectrometer for LIBS applications. To obtain the reasonable light level required for a non-intensified CCD, long exposure time measurements should be performed. In this case, plasma emission signal is accumulated on the CCD from a multiple laser pulse. However, one should be careful about excessive accumulation of background noise and low signal-to-noise ratio. It is especially important when performing measurements in the open air without an enclosed sample chamber. Since the CCD stays open for a long period of time, all sources of stray light in the room should be eliminated and measurements should be conducted in darkness.  CCD usually proves a more sophisticated system than the  ICCD because intensified CCDs are prone to permanent damage by excessive light levels. Extra care should be taken so as not to expose ICCDs to the bright sources of light like laser reflections. In the case of a regular CCD, it is difficult to damage with excessive light. III. General Applications The fact that LIBS generally requires little-to-no sample preparation, simple instrumentation, and can easily be performed on-the-field in hazardous industrial environments in real-time, it is a very attractive analytical tool. The following are a few examples of real life applications, where LIBS is successfully used: Express-analysis of soils and minerals (geology, mining, construction) Exploration of planets (such as projects using LIBS for analyzing specific conditions on Mars and Venus to understand their elemental composition) Environmental monitoring (Real-time analysis of air and water quality, control of industrial sewage and exhaust gas emissions) Biological samples (non-invasive analysis of human hair and teeth for metal poisoning, cancer tissue diagnosis, bacteria type detection, detection of bio-aerosols and bio-hazards, anthrax, airborne infectious disease, viruses, sources of allergy, fungal spores, pollen). Replacing antibody, cultural, and DNA types of analysis Archeology (analysis of artifacts restoration quality) Architecture (quality control of stone buildings and glasses restoration) Army and Defense (detection of biological weapons, explosives, backpack-based detection systems for homeland security) Forensic (gun shooter detection) Combustion processes (analysis of intermediate combustion agents, combustion products, furnace gases control, control of unburned ashes) Metal industry (in-situ metal melting control, control of steel sheets quality, 2D mapping of Al alloys) Nuclear industry (detection of cerium in U-matrix, radioactive waste disposal)

TOTAL REFLECTION X-RAY FLUORESCENCE ANALYSIS

INTRODUCTION

Total reflection x-ray fluorescence (TXRF) analysis is a powerful analytical tool with respect to detectable elemental range, simplicity of quantification and detection limits. This includes the capacity to detect almost all elements of the periodic system, namely from boron to uranium. Even the highest-Z elements of the actinides can be detected. Quantitatively, the dynamic range covers several orders of magnitude, so ultra-trace element levels to major elemental concentrations can be determined. In terms of detection limits, the levels of femtogram absolute detectable masses under optimized excitation–detection conditions can be reached. Some of these features can be topped with additional properties such as rapid analysis time of a few seconds and simultaneous detection of the elements present. In some applications, non-destructiveness is of importance, e.g. while dealing with precious substances of cultural values from fine arts or also in cases of forensic investigations if only small amounts of sample are available. TXRF is an energy-dispersive XRF (EDXRF) technique, and excitation geometry with angles below the critical angle of total reflection is perfectly suited for these investigations.

The above statements emphasize the analytical power, and in addition to these arguments one can add the large number of applications that has led to the revival of x-ray fluorescence analysis for ultra-trace element analysis. The applications range from the interesting fields of medicine, techniques and environment to forensic, fine arts, extra-terrestrial samples and fundamental research. With new physical and technical ideas leading to modifications of the physical properties of the primary radiation, e.g. monoenergetic, linearly polarized, highly intense, or on the detector side high resolution, high counting capacity, large area or even arrays of detectors, new perspectives are opening up for TXRF

In Fig. 1 the experimental set-up of conventional EDXRF and TXRF is schematically shown. As TXRF is basically an energy-dispersive analytical technique, the main difference to conventional EDXRF is neither the source nor the detector but the geometry of excitation at small incidence angles below the critical angle of total reflection.

     Figure 1 Comparison between conventional (left) and total reflection mode of excitation (right).

THEORY

The theory of x-ray total reflection is based on the phenomenon that at an incident angle below the critical angle the narrow collimated primary beam is totally reflected. A beam gets reflected from a flat polished surface of any material at the same angle as the incident one and has almost the same intensity as the primary beam (total intensity is reflected), except for a small portion that is refracted and penetrates the reflecting medium. This evanescent wave loses intensity exponentially as it penetrates deeper into the medium. In Fig. 2, the fundamental formalism of x-ray total reflection is shown, based on the Fresnel formalism and the complex index of refraction for x-rays, which is given below. The index of refraction for x-rays differs only slightly from 1, which is described theoretically by the value of υ which is in the range of 10-5 . For many materials the angles involved are small, typically a few milli radians or a tenth of a degree.

      Figure 2 Sketch of the theoretical conditions for x-ray total

Applications to chemical analysis:

The incident radiation in TXRF is a fine, collimated, almost parallel beam with typical dimensions of 8 mm width and only 50 μm height. This narrow beam impinges at an angle below the critical angle of total reflection (1.8 mrad or 0.1 degrees in the case of a monoenergetic Mo K˛ radiation and Si reflector) on the surface of a flat polished material. that serves as the sample carrier. In the case of chemical analysis of different samples, such as from the environment, medicine or technical products, different procedures apply. The sample has to be transferred into the liquid form by chemical digestion procedures. A small volume of 1–20 μl of the dissolved sample is taken using a pipette. The acidic or aqueous solution is dropped at the center of the reflector and dried by infrared heating, on a hot plate or in vacuum.

Possible applications of TXRF.

Environment	Water: sea, rain, pore water, river, mineral, spring water, drinking water, chemicals and deionized water
	Air: aerosols, vapour, air dust, airborne particles, fresh air
	Soil: sewage sludge, sediments
	Plant material: Algae, fine roots, cucumber plants, pollen.
	Foodstuff: fish, flour,  fruits, crab, mussel, mushrooms, nuts, vegetables, wine, tea, soft drinks, onion
	Drinks: Coffee,  spirits and beverages, honey.
Medicine/biology	Body fluids: blood, serum, urine, amniotic fluid,  cerebrospinal fluid
	Tissue: hair, kidney, lung, liver, stomach, nails, colon
	Various enzymes, polysaccharides, glucose, proteins, cosmetics, biofilms, human bones
Industrial/technical applications	Surface analysis: Si wafer surfaces, GaAs wafer surfaces
	Implanted ions: Depth and profile variations
	Thin films: single layers, multilayers
	Oil: Crude oil,  fuel oil, grease, pure fuel oil, waste oil, petroleum, oil-shale ash, diesel
	Chemicals: Acids,  bases,salts, solvents
	Fusion/fission research: trans mutational elements in Al C Cu, iodine in water
Mineralogy	Ores, rocks, minerals, rare earth elements, quartz, mineral sands, diamond, crystals
	Geological materials, bio-mineralisation
Fine arts/archaeological/forensic	Pigments, paintings, varnish21
	Bronzes, pottery, jewellery, manuscripts, Egyptian masks
	Textile fibres, glass, cognac, dollar bills, gunshot residue, drugs, tapes, sperm, finger prints.

Sample preparation

The samples can be from many scientific disciplines and thus the physical state will be different. The best-suited samples for TXRF are in the liquid state—either in the form of aqueous or acidic solutions—so if solids or powders are to be analysed, these samples must be transferred into the liquid state. The presented procedures are typical but can of course be adapted to the sample type, and the world of chemistry is fully open to new ideas, given in detail in the respective literature. Sample preparation has been discussed by several authors and many publications deal with special methods. Even so, the complex subject leaves a lot of ideas open to get the sample prepared in a way that the elements present can be detected even if they are at the lowest concentrations. Pre-concentration methods or selective enrichment techniques by chelation or electrochemical methods can be introduced to ensure that adequate masses are present on the sample reflector in the range of absolute picograms from very low-concentration samples, e.g. sea water in which the salt extraction also is an important preparation step. Recently, the collection of fine dust and also aerosols directly on the sample carrier in a short time of a few minutes to several hours by multi-stage samplers and the direct analysis of the minute masses collected by TXRF were successfully proven in large-scale experimental series in different parts of the world at locations of interest.

INSTRUMENTATION IN TXRF

Various combinations of x-ray sources, spectral modification elements, reflector materials and detectors can be used to optimize excitation and detection conditions. Measurements can be performed in air, in a He atmosphere or in a vacuum chamber. Vacuum conditions are mandatory for the detection of light elements to avoid absorption, but it is also advantageous because the scattering of primary x-rays from air is avoided. The components of a TXRF spectrometer are shown in the below figure

        Figure 3 Components of a TXRF spectrometer

 Sources of x-rays for TXRF are mainly x-ray tubes, ranging from low power (50–75W) to high-power standing anodes of up to 3000W and finally rotating anodes up to 18 kW. The appropriate focus is a line focus with dimensions 8 or 12mm long and 40 μm wide. Everything is done to get a high flux of photons onto the sample. There is radiation from the ultimate source with best properties of having a naturally collimated beam characteristic, an extremely high flux and a linearly polarized beam—the SR. Even though it is difficult to get access to SR, the results achievable show the importance of this effective combination of the source and TXRF, from which ultimate low detection limits of a few femtogram have been achieved.

The beam from an x-ray tube is unpolarised and has a continuous spectral distribution of the characteristic radiation. In many cases, a monoenergetic excitation is the preferred one because the background is optimally low as only scattered photons of single energy are present and will appear as two lines elastically and Compton-scattered in the spectrum. The first low-cost approach to modify the spectrum was an optical flat in the beam path and taking advantage of the energy dependence of the critical angle to suppress the high-energy part of the bremsstrahlung spectrum. This leads to reduced background, in particular in the low-energy region, as the Compton edge produced by the backscattered high-energy photons disappears. Versions with two reflectors attached togetherwith a spacing of 50 mm lead to a double-reflector monochromator. Efficient monochromators are nowadays available using multilayers with reflectivities in the range of up to 80% of the characteristic radiation of the anode material of choice. Typically in use are combinations of layered structures made of W–C, Ni–C andMo–Si with a d spacing of 2–3 nm. Thus, they are adjusted to fulfill the Bragg equation and the full spectrum is modified to be monoenergetic. Using modern technology to bend these structures, it is possible to design x-ray optical components that focus the beam with a small divergence, thereby increasing the flux of photons in the focal spot which is designed to be at the sample position. The result is an increase in intensity on the sample several times more than that with the non-focussing flat multilayer. These optical components are commercially available and can be inserted in the beam path of the TXRF spectrometer to improve further detection limits. 

Quantitative TXRF

The conversion of the measured intensities into concentrations is one of the most important steps in analytical XRF. In the special case of TXRF, the complications are rather completely removed, as the approach for the thin-film sample can be applied, which leads to a simple and linear relation between the intensity, I, and concentration, C, of the element considered. The addition of an internal standard with known concentration leads to a simple quantification procedure, as follows:

• Choose or add to the multi-element standard one element as internal standard, which is the reference for calibration of the spectrometer.
• Establish the intensity vs concentration curve as the regression curve with reference to the internal standard to determine the sensitivity, Sstd/Si, from multi-element standard.
• Add the internal standard of known concentration Cstd to the unknown sample.
• Measure the intensity of element Ii and intensity of internal standard Istd.
• Determine the concentration of unknown element Ci using the relation:

Advantages of TXRF

• Double excitation by direct and reflected beams
• Almost no penetration of the primary radiation into the substrate, resulting in low background Large solid angle, as the detector can be placed close to the reflector surface 
• Large solid angle, as the detector can be placed close to the reflector surface
• The consequent improved signal/background and improved detection limits
• Very low detection limits: femtogram levels, picograms per gram concentrations, 108 atoms/cm2 of metal contamination detectable on wafer surfaces

Environment

Water:

sea, rain, pore water, river, mineral, spring water, drinking water, chemicals and deionized water

Air:

aerosols, vapour, air dust, airborne particles, fresh air

Soil:

sewage sludge, sediments

Plant material:

Algae, fine roots, cucumber plants, pollen.

Foodstuff:

fish, flour,  fruits, crab, mussel, mushrooms, nuts,

vegetables, wine, tea, soft drinks, onion

Drinks:

Coffee,  spirits and beverages, honey.

Medicine/biology

Body fluids:

blood, serum, urine, amniotic fluid,  cerebrospinal fluid

Tissue: hair, kidney, lung, liver, stomach, nails, colon

Various enzymes, polysaccharides, glucose, proteins,

cosmetics, biofilms, human bones

Industrial/technical

applications

Surface analysis:

Si wafer surfaces, GaAs wafer surfaces

Implanted ions:

Depth and profile variations

Thin films:

single layers, multilayers

Oil:

Crude oil,  fuel oil, grease, pure fuel oil, waste oil, petroleum, oil-shale ash, diesel

Chemicals:

Acids,  bases,salts, solvents

Fusion/fission research:

trans mutational elements in Al C Cu, iodine in

water

Mineralogy

Ores, rocks, minerals, rare earth elements, quartz,

mineral sands, diamond, crystals

Geological materials, bio-mineralisation

Fine

arts/archaeological/forensic

Pigments, paintings, varnish21

Bronzes, pottery, jewellery, manuscripts, Egyptian masks

Textile fibres, glass, cognac, dollar bills, gunshot residue, drugs, tapes, sperm, finger prints.

OPTICAL EMISSION SPECTROSCOPY

Optical Emission Spectrometry (OES) is a fast, accurate and reliable method for quantitative analysis of  metals to about 1–3% relative precision.Optical emission spectroscopy methods are among the most useful and flexible means of performing elemental analysis of Steel ,Copper , Aluminium,Magnesium, Lead etc and its alloys. Accurate analysis of metals and alloys requires that :

• The melt or bulk sample is homogeneous;
• Samples are representative of the melt or bulk sample  and
• The metallurgical structure of the sample is similar to that of the standards used. The sample requirements are met by chill-cast disk samples collected as described in ASTM E 716-94 (or equivalents AS2612-83 or JISH 1305-1976). For the point-to-plane method of spectrochemical analysis  , samples are machined on a lathe to give a smooth, clean surface just prior to analysis.
The sample is mounted in the spectrometer and a spark is generated between the sample and a tungsten electrode. The material is volatilized from the surface of the sample resulting in the emission of light. This light is diffracted into its component wavelengths inside the spectrometer. Fixed detectors simultaneously measure light intensity at wavelengths characteristic of each element. OES requires calibration of the spectrometer
with certified chill-cast standards, which must have the same matrix as the alloys to be tested. Therefore
quantitative OES can only be performed on chillcast samples collected from the melt according to
standard methods. Other techniques, such as Induction Coupled Plasma (ICP), can be used for
quantitative analysis of alloys in other forms, for example billet slices, ingots, extrusions and forgings.

OES PRINCIPLE


The OES system consists of four main parts :
• Sample stand.
• Spark generator.
• Optics assembly.
• Data Acquisition Electronics assembly.

The clean sample is mounted in the stand and a spark is generated between the sample and a tungsten electrode. An example of this setup is illustrated in Figure 1. High purity argon is used as a discharge atmosphere to prevent any interaction between the atmosphere and the sample surface.

The material is violated from the surface of the sample resulting in the emission of light. When atoms are excited by an external energy source, some electrons move into higher energy levels. As these electrons drop back to their original levels, the atoms emit light (photons) of characteristic wavelength. The spectrometer collects the light emitted from the sample and splits it into its component wavelengths using a diffraction grating. Fixed detectors (photo multiplier tubes in above figure) simultaneously measure light intensity at wavelengths characteristic of each element. This is illustrated in Figure above. The intensity of the signals depends on the number of photons produced per unit time. The spectrometer is programmed for fixed sample types, elements and concentration ranges and only give results within the limits of calibration .