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.)

DIFFERENT NANO PARTICLES AND THEIR APPLICATIONS

Nanoparticle type	Applications
Alumino silicate nano particles	It can be used to reduce bleeding in trauma patients with external wounds by activating the blood clotting mechanism, causing blood in a wound to clot quickly. Z-Medica is producing a medical gauze that uses aluminosilicate nano particles for use on external wounds. For trauma patients with internal bleeding another way to reduce the blood loss is needed.  Also the researchers at Chase Western Reserve University are developing polymer nanoparticles that act as synthetic platelets. Lab tests have shown that injection of these synthetic platelets significantly reduces blood loss.
Polyethylene glycol-hydrophilic carbon clusters (PEG-HCC)	They have been shown to absorb free radicals at a much higher rate than the proteins out body uses for this function. This ability to absorb free radicals may reduce the harm that is caused by the release of free radicals after a brain injury.
Iron oxide nano particles	It can be used to improve MRI images of cancer tumours. The nanoparticle is coated with a peptide that binds to a cancer tumour, once the nanoparticles are attached to the tumour the magnetic property of the iron oxide enhances the images from the Magnetic Resonance Imagining scan.
Gold nano particles	A method being developed to fight skin cancer uses gold nanoparticles to which RNA molecules are attached. The nanoparticles are in an ointment that is applied to the skin. The nanoparticles penetrate the skin and the RNA attaches to a cancer related gene, stopping the gene from generating proteins that are used in the growth of skin cancer tumours.
Gold nano particles embedded in a porous manganese oxide	Using gold nanoparticles embedded in a porous manganese oxide as a room temperature catalyst to breakdown volatile organic compounds in air.
Palladium nano particles	A layer of closely spaced palladium nanoparticles that detect hydrogen. When hydrogen is absorbed the palladium nanoparticles swell, causing shorts between nanoparticles which lowers the resistance of the palladium layer.
Quantum Dots (crystalline nano particles)	Quantum Dots (crystalline nanoparticles) that identify the location of cancer cells in the body.
Gold nano particles with organic molecules	Combining gold nanoparticles with organic molecules to create a transistor known as a NOMFET (Nanoparticle Organic Memory Field-Effect Transistor).
Iron nano particles	Iron nanoparticles used to clean up carbon tetrachloride pollution in ground water.
Silicon nano particles coating	Silicon nanoparticles coating anodes of lithium-ion batteries to increase battery power and reduce recharge time.
Gold nano particles	Gold nanoparticles that allow heat from infrared lasers to be targeted on cancer tumours.
Silicate nano particles	Silicate nanoparticles used to provide a barrier to gasses (for example oxygen), or moisture in a plastic film used for packaging. This could reduce the possibly of food spoiling or drying out.
Zinc oxide nano particles	Zinc oxide nano particles dispersed in industrial coatings to protect wood, plastic and textiles from exposure to UV rays.
Silicon dioxide crystalline nano particles	Silicon dioxide crystalline nano particles filling gaps between carbon fibres strengthen tennis racquets.
Silver nano particles	Silver nano particles in fabric that kills bacteria making clothing odour-resistant.
Porous silica  Nano particles used to deliver chemotherapy drugs to cancer cells.	Porous silica nano particles used to deliver chemotherapy drugs to cancer cells.
Semiconductor nano particles	Semiconductor nano particles applied in a low temperature printing process that results in low cost solar cells.
Iron oxide nano particles	Iron oxide nano particles used to clean arsenic from water wells.
	Nano particles, when activated by x-rays, that generate electrons that cause the destruction of cancer cells to which they have attached themselves. This is intended to be used in place radiation therapy with much less damage to healthy tissue.
	A nano particle cream that releases nitric oxide gas to fight staph infections.
Gold-palladium	That can replace expensive and potentially toxic reagents that promote oxidation of aromatic primary alcohols to aldehydes, which is one of the crucial processes in the perfume production.

DIFFERENCE BETWEEN PHOTOMULTIPLIER (PMT) & CHARGED COUPLED DEVICE (CCD) DETECTORS

Some of the most common differences between Photo multiplier (PMT) detectors & Charged Coupled Device detector (CCDs)..


1.       Photomultiplier tubes (PMTs) and Charged coupled devices (CCDs) both give spectra. The difference is the PMT is used with a small slit in front of it to control the bandwidth of light being detected. The CCD takes advantage of the dispersed light fully. The pixel columns will each correspond to a wavelength (resolution and range depend on the grating used). A PMT requires scanning of the Monochromator to collect a spectra. The CCD takes a single snap shot and you have a spectrum. The CCD sensitivity and dynamic range is lower than a PMT.

2.     A photomultiplier tube is a detection device that is made from a glass vacuum tube with a series of metal plate electrodes. A CCD is a solid state detector made from semiconductor materials.

3.     The main difference is one of sensitivity. Generally speaking the better the spectral resolution of the instrument the lower the amount of light reaching the detector and so you need more sensitivity in your detector. A PMT measures a single point in the spectrum at a time whereas with a CCD the complete spectrum is imaged across the CCD and so can be measured all at the same time. 

4.     An instrument with a CCD is usually much faster and cheaper but will not have as good a spectral resolution (the ability to resolve absorbance peaks very close to each other).

5.     CCDs and photomultipliers vary in a number of aspects. One difference is gain, a photomultiplier has gain whereas a CCD does not (hence the multiplier bit of PMT). The PMT gain may be up to 10,000,000 and is available at high speeds and for large area detectors, which means that one can usually get close to the theoretical noise floor. On the other hand, PMTs have poor quantum efficiency compared to CCDs (25% typ against 85% typ) so you can sometimes get better performance with a CCD if you can go slowly enough.

6.     PMTs are also typically single channel devices, although 16 channel linear arrays are available. CCDs are usually linear or 2D arrays.

7.     In a dispersive spectrometer a linear CCD array can capture the entire spectrum in one measurement. A single channel PMT must have the spectrum scanned across it sequentially to produce the entire spectrum.

8.     PMT's are typically preferable to CCD's on spectroscopic application for several reasons. The ability to adjust the gain of each PMT allows a manufacturer to adjust the response of each PMT to the specific signal being measured, so every element you are trying to detect can be analyzed at optimum conditions. Solid state CCD's are a compromise. Every element detected has the same conditions, so most are compromised. 

9.     Also, PMT's can be heated and held at constant temperature (in well made instruments) to prevent drift caused by variation in temperature. If you try to heat a CCD, the noise level will go up, and the signal to noie ratio will degrade as a result. CCD's are sometimes cooled to try to improve their s/n ratio, but usually not cooled enough to really help much due to condensation issues that arise. 

10. A third advantage of PMT's is that they can be used in a vacuum chamber without long term degradation for decades of use. The surface of a CCD will degrade under vacuum over a few (8-15) years. Most manufacturers making CCD based instruments opt for a Nitrogen or Argon flush, rather than vacuum to displace the oxygen from the detector chamber. This method results in decreased performance compared to PMT's, and is used in lower performance less expensive spectrometers.

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)