Atomic absorption spectroscopy

Atomic absorption spectrometry (AAS ) is a belonging to the group of atomic spectrometry analytical method. In analytical chemistry it is a proven and fast method for quantitative and qualitative analysis of many elements ( metals, semi- metals) in mostly aqueous solutions and solids. The AAS is based on the attenuation (absorption ) of radiation by interaction with free atoms. Because each chemical element has a characteristic line spectrum can be made to a reference measurement without the sample statements of the elements contained in a sample on the evaluation of the difference spectrum. Atomic absorption spectrometry is divided in terms of introducing a sample into the gas phase in the following sub- processes:

• F- AAS (short for engl. Flame atomic absorption spectrometry, dt, flame atomic absorption spectrometry ', also called flame technique )
• GF- AAS or eM - AAS ( graphite furnace atomic absorption spectrometry german, German, AAS with electrothermal heating ', also graphite furnace technique)
• CV- AAS ( atomic absorption spectrometry engl. cold vapor, dt, AAS cold vapor technique ', also called hydride )
• HR-CS AAS (german high-resolution continuum source atomic absorption spectrometry -, dt, atomic absorption spectrometry with continuum source and high-resolution echelle double monochromator ')

It was developed by Alan Walsh in the 1950s in Australia.

• 7.1 deuterium background correction
• 7.2 Zeeman background correction
• 7.3 Background correction in HR-CS AAS
• 7.4 matrix modifier in GF-AAS

Principle

A light source emitting light of different wavelengths at a certain intensity. In the beam path is a atomizing unit in which atomizes the components of a sample to be tested, that is, be converted into individual excited atoms. The atomization of the elements is done either through a gas flame ( Ethin/Luft- or ethyne / nitrous oxide mixture) in the solution to be analyzed is atomised or by rapid, intense heating in an electrically heated graphite tube into which previously had a small amount of the solution was placed therein.

After attenuation of the light beam in the atom cloud ( absorption), its intensity is measured downstream of the atomizing unit and compared with the intensity of the unattenuated light. It is detected how much of the incident light of a specific wavelength is absorbed by the element to be measured (in most cases, the AAS is Einelementtechnik ). It is the Lambert- Beer law. With increasing concentration of the analyte in the sample, the attenuation of the irradiated light ( absorbance ) increases proportionally.

The absorbed light energy is emitted again from the thus excited atomic on the same wavelength, thus showing the atomic fluorescence. Getting an intensity attenuation, the absorption signal can be measured, has a geometric reason: The incident light is focused by the optics of the instrument to a very small solid angle. However, the re- emission occurs when the spherical wave on the whole solid angle of 4π. Only a negligible proportion of them come with the light from the lamp through the exit slit.

The AAS is a relative measurement method. According to the Lambert -Beer law (valid for low concentrations ) is added, the absorbance of the calibration standard of known concentration, and a calibration curve generated with samples of unknown concentration was added to this calibration and the concentration read (nowadays evaluated by computer software). A great advantage of the AAS against other spectroscopic methods is the selectivity of the process. The lamps used as light sources to emit a specific element electromagnetic spectrum that is selectively absorbed by the same, to be examined, due to the composition of its light emitting element means ( hollow-cathode material salt in an electrodeless discharge lamp ( EDL) ). Spectral interferences in AAS come very rarely. Latest on the market developments such as high-resolution continuum source AA spectrometer, however, work with only one light source. A xenon short arc lamp as a continuous radiation source covers all elements and all available wavelengths. This source of radiation opens the entire relevant for AAS wavelength range in a single step. Therefore, the sequential multi-element routine is possible if the elements to be determined to be determined from the same dilution. A new feature is useful for the evaluation of molecular absorption bands, where additional elements, such as sulfur or phosphorus, can be analyzed. An HR -CS AAS measures therefore independent of hollow cathode lamps. This offers benefits such as reduced lead times, no lengthy curing time of the light source, as drifts can be corrected simultaneously. The gain of time put into perspective again by the fact that a much longer initialization of the device is required due to the complex optics, which nullifies the main advantage of the AAS, a quick measurement readiness for rapid analysis fewer samples again.

Construction

The light source is a line source (eg, a hollow cathode lamp HKL). A nebulizer is used to form fine droplets of the analyte to atomize them effectively in the heat of a gas flame. To protect the detector, a dispersion unit ( monochromator ) is connected. The detector is typically a photomultiplier.

Light sources

In the AAS element-specific lamps. A distinction is made between

• Hollow cathode lamps with a cathode consisting of the element of the analyte. Alternatively,
• Super lamps ( additional cathode) or
• Electrodeless discharge lamps ( EDL principle gas discharge tube) can be used.

Both types of lamps latter offer a higher light intensity, which in particular elements that absorb in the UV range (As, Cd, Pb, Sb, Se, Bi, Tl, Hg), better detection limit by better signal -to-noise ratio ( engl. signal to noise ratio, SNR) brings. Normal hollow cathode lamps indicate below about 300 nm, a significant intensity deterioration of their emission lines. Both types of lamps require a separate power supply, but which is already installed in modern equipment for T..

In HR-CS AAS using only a single radiation source, a specially designed xenon short arc lamp as a continuous radiation source for all elements and all wavelengths over the entire spectral range of 190-900 nm, the lamp has a modified electrode shape and works under high pressure. Under these conditions, a hot focal spot is formed which reaches a temperature of about 10,000 K. The emission intensity of this lamp is intense throughout the spectral region of at least a factor of 10, in the deep UV by more than a factor of 100 than that of conventional short arc xenon lamps. AAS also important for the more intensive on average by a factor of 100 emission intensity is compared over the entire spectrum with conventional hollow cathode lamps. The radiation intensity has in AAS while not affecting the sensitivity, but probably on the signal / noise ratio.

Atomization

The goal of atomic absorption spectroscopy is to convert a high proportion of atoms in the gaseous state and to produce as little excited or ionized atoms. For this, the sample is evaporated ( free of solvents and volatile components ) and be incinerated and dissociate into free atoms. For atomization in AAS mainly flames and Graphitrohröfen be used. Distinction must be made between F- AAS and GF- AAS. Wherein the F- AAS, the sample is fed continuously at a constant speed, from which it receives time constant signal. In the GF- AAS only once a known amount of sample is placed. Ideally, the spectral signal having a maximum, and then decreases to zero when the atom cloud is carried out of the atomizer. In HR-CS AAS the same atomizers are used as in the classical line source AAS. Because of the visibility of the spectral environment of the analytical line can be facilitated and simplified in the HR-CS AAS method development and optimization for an experienced analyst, a lowbrow user can also be confused by the additional information.

Flame Atomic Absorption Spectroscopy

In the flame atomic absorption spectroscopy ( F - AAS), also known as flame technique, the dissolved sample is first converted into an aerosol., The sample is atomized with a pneumatic atomizer at a mixing chamber and fluidized with fuel gas and oxidant (oxidizing agent ). It forms a fine mist, aerosol. To make the drop size even smaller and more uniform, the aerosol first hits a baffle ceramic sphere and then, if necessary, a mixing blade that leaves only fine droplets happen. A small part of the original aerosol finally passes from the mixing chamber into the flame. Where first the solvent is evaporated and the solid components of the sample to melt, evaporate, and subsequently dissociate. At high flame temperatures can especially in alkali and alkaline earth elements lead to some Ionisationsinterferenzen that can be controlled by adding a Ionisationspuffers (cesium or potassium chloride). Too low flame temperatures lead to chemical interference. In flame AAS, the flame can alternatively be operated with two different gas mixtures.

• Air - acetylene flame: In general, these flame is used, it uses air as the oxidant, and acetylene as the fuel gas.
• Nitrous oxide - acetylene flame: The compounds of some of the elements (e.g., aluminum, silicon, titanium, as well as calcium and chromium) require higher temperatures for dissociation. In this case, the gas nitrous oxide ( laughing gas) is used as the oxidant in place of air pressure. This flame is about 2800 ° C to about 500 ° C hotter than the air -acetylene flame., Oxides of, for example, chromium, calcium, and aluminum may be atomized by its reducing action.

"Normal" nitrous oxide flame

Graphite furnace atomic absorption spectrometry

In the graphite furnace atomic absorption spectrometry ( GF- AAS), graphite furnace technique or atomic absorption spectrometry called with electrothermal heating ( EtA - AAS) is made use of the fact to exploit the fact that graphite conducts electricity and heated by its electrical resistance when an electrical voltage.

First 5 to 50 microliters of sample solution is introduced into a graphite tube furnace and heated in several steps. The program largely depends on the element to be analyzed and its chemical environment. It also plays a big role in what kind of a device and what is working for a graphite furnace system ( along heated / transverse heated graphite furnace ). In general it can be said that in the transverse heated graphite furnace about 200 ° C lower pyrolysis temperatures and 200 to 400 ° C lower atomizing temperatures are used. As a guide for choosing the right Temperatur-/Zeitprogramms the "recommended conditions " of the graphite furnace manufacturer should be used. On this basis, temperatures and times should be optimized so that the measured signal receives a maximum signal area with minimal background signal. The sample composition can make a deviation from standard program required.

Each step involves a rise time ( ramp) within which the specified temperature is reached. The slower the heating rate is selected, the lower is the risk of splashing of the sample and the better the precision of several repeated measurements. For a gentler drying step may be " dry 1" split, for example, in one step at 110 ° C and a step at 130 ° C. Step " drying 2" can also be omitted for simple samples ( drinking water); it is used more for samples with complex matrices ( body fluids or highly saline waste water). For the atomization is usually selects a ramp of 0 seconds, in this case the maximum power of the power supply is applied to the graphite tube, to achieve the maximum heating rate. Thus, the atomic cloud of analyte reaches a maximum density and results in a maximum sensitivity. The temperatures are of course dependent on the analyte and may differ greatly. Advantageous with respect to the flame technique is that the sample can be quantitatively accommodated in the optical path, where longer (up to 7 s ) remains. Further can be separated by different evaporation temperatures often interfering matrix components; either they vaporize before, or they are left behind. The detection limits are, therefore, up to three orders of magnitude better than that of the flame technique, or ICP-OES. However, it may get interference, when not working under specific test conditions. The summary of all measures that lead to an interference-free analysis in graphite furnace AAS is as STPF concept (English: stabilized temperature platform furnace ) refers.

STPF concept

• Pyrolytic coated graphite tube ( better durability and sensitivity)
• Platform in the graphite tube ( atomization in a constant-temperature gas phase in the graphite tube )
• Peak area evaluation ( less dependence on the time of maximum atomisation of the analyte )
• Use of modifiers (stabilization of the analyte at higher pyrolysis temperatures or reduce the decomposition temperature of the matrix components )
• Gas stop during atomization ( atomic cloud remains longer in the tube )
• Fast waveform capture
• Transversely heated graphite furnace ( temperature stability over the entire length of the tube, no condensation effects and recombination to molecules )

Glowing platform along the heated graphite furnace

Transversely heated graphite furnace Zeeman

Hydride and cold vapor techniques

Definition of Terms

• HG- AAS: Hydride generation - AAS - hydride
• CV- AAS: Cold Vapour AAS cold vapor technique
• HG -ET- AAS: Hydride Generation Electrothermal AAS enrichment of hydrides in a graphite tube furnace (also: FIAS - Furnace, FIFU )

Monochromator

The monochromator shares originating from the lamp and shining light on atomizing unit into its spectrum and isolated from a particular wavelength. With modern instruments to be used solely holographic gratings that diffract the incident light. The higher the number of grooves on the grating used, the better the resolving power of the optics of a spectrometer. In contrast to the emission techniques are used in the line source AAS as radiation sources, which produce not a continuous spectrum but only the spectral lines of the element contained therein. For this reason, the demands on the resolution of the optical system of an AAS are lower than the look of an ICP- OES.

On the choice of the width of the exit slit of the wavelength range is limited, which reaches the detector. The use of a continuum radiation in high-resolution continuum source AA spectrometers necessarily requires the use of high-resolution monochromator. Classic monochromators this way they were used in the optical emission, have a large amount of space and have a strong tendency to wavelength drift. Neither is acceptable in the HR-CS AAS. The problem was solved with the construction of a compact double monochromator with active wavelength stabilization. Both monochromators are in Littrow mounting with a focal length of 30 or 40 cm. The radiation reflector of the continuum passes through the entrance slit in the monochromator and is deflected by the first parabolic mirror to the prism. The prism is reflective on the back, so that the radiation passes through the prism twice before now spectrally dispersed, falls back onto the parabolic mirror. This results in the radiation over a deflecting mirror for intermediate gap. The prism is thereby rotated so that the radiation passes through the clearance gap to the second region of the monochromator in the analytical line. The second parabolic mirror directs radiation to the echelle grating, where the selected spectral range is now resolved high. The entire high-resolution portion of the spectrum is then imaged by the parabolic mirror to the detector. The resolution of the double monochromator is traditionally at a value that is better by a factor of 100 than the resolution of classical AAS instruments.

Basic arrangements of monochromators:

• Czerny-Turner grating monochromator: Light (A) is focused on the entrance slit (B) and is collimated by a concave mirror ( parallelized ) (C). The collimated beam is diffracted by a rotatable grid (D) and the dispersed beam back through a second mirror (E) to the exit slit (F) is focused. Each wavelength of light is focused onto a different position of the gap. The wavelength is allowed to pass through the gap (G) depends on the angle of rotation of the grating (D).
• Echelle arrangement: echelle monochromators in atomic spectrometry combining the advantages of a very compact design with high light throughput with very good optical resolution. By combining an echelle grating ( splitting the light into its diffraction orders ) with a prism ( splitting of the diffraction orders in the wave lengths) is a two-dimensional arrangement of the spectrum, with a suitable choice of the detector ( semiconductor detector with segmentation of the light-sensitive areas ) results in a simultaneous detection of several wavelengths permits. Is implemented, this arrangement in many OES and AAS in a few.
• Littrow configuration

Detector

To measure the attenuation of light are employed secondary electron multiplier ( SEM ) or - increasingly nowadays - a semiconductor detectors. The latter show a more homogeneous and more effective light yield ( quantum efficiency ) over the relevant wavelength range ( 190-900 nm ) and thus a better signal / noise ratio, which is reflected in better detection limits. As a detector, a CCD array is used in the HR-CS AAS. Each pixel is thereby evaluated independently, so that the device works in principle with independent detectors. All pixels are simultaneously exposed and read out simultaneously. While the signal processing is carried out already, the next exposure, which allows a very rapid measurement sequence. The absorption line is measured substantially five central pixels and the remaining pixels, only the statistical fluctuations of the baseline zeigen.Diese can be used for correction purposes. After all pixels are simultaneously exposed and read out, all the intensity fluctuations that are not wavelength dependent, such as fluctuations in the lamp emission, are determined with the aid of corrective pixels and eliminated. This results in an extremely stable and low-noise system, which leads to a significant improvement of the signal-to- noise ratio. The same correction system automatically eliminates any continuous background absorption. The detector registers not only the radiation on the analysis line, but their whole environment. Thus, for example, spectral interferences are recognizable and can be easily avoided.

Interference in AAS

Due to the presence of foreign substances in the sample may cause interference ( interference ) can occur. We distinguish:

• Spectral interferences
• No spectral interferences

Spectral interferences are to a certain degree adjusted by background correction or at least reduced. To this end, connected in the AAS in addition to the radiation source in addition a deuterium lamp ( D2 lamp ) into the beam path or alternatively used in the graphite furnace AAS Zeeman background correction. The High - Resolution Continuum Source AAS (HR -CS AAS ) uses a high-resolution echelle spectrometer. In addition to the intensity of the line analysis, the spectral environment is recorded simultaneously. This interference can be seen immediately. The need for optimization or correction of the parameters are automatically detected in the HR-CS -AA spectrometers. Minimizing the interference is achieved at optimum line separation by the use of a powerful detector.

Non- spectral interferences occur during atomization. We distinguish:

• Transport interference are chemical interference by matrix components or by physical disturbance by the viscosity, density and surface tension of the solvent. They are especially problematic in flame AAS, since only a very small proportion of the sample enters here into the flame. The elimination of interferences is achieved by transporting the standard addition method or a matrix adjustment of the calibration standard to the sample.
• Gas interference arises when it comes not to the complete dissociation ( AB → A B) or to ionization. The addition of releasing agents, such as LaCl3 for phosphates, complete dissociation can be achieved. By the addition of alkali metal elements, a better ionization can be achieved. In both cases, the mode of action obeys the law of mass action. An excess of this tool causes a shift of the reaction equilibrium to the gaseous analytes in the atomic ground state.
• Evaporation interference play a role in the graphite tube. They occur too early or too late by evaporation of the analyte in the sample, based on the behavior of the analyte in the calibration standard. Thereby can provide a distinctly different signal form and height of the signal of a sample and calibration standards for the same analyte. A time -resolved signal (evaluation in signal area ) can still lead to correct results without having to resort to a standard addition here.

Deuterium background correction

When using a deuterium lamp for correction of the background light attenuation of the hollow cathode lamp, and a D2 lamp is detected either at the same time or alternately. D2 lamp provides, in contrast to the hollow cathode lamp, a continuous light spectrum, the intensity of which depends on the wavelength. Above about 350 nm, it provides virtually no intensity, so that elements with an absorption line above this wavelength can be measured without background correction. The selection of the wavelength (preferably the wavelength range ) to determine the background absorption takes place across the width of the Monochromatorspaltes spectrometer. In a first approximation, the light from the deuterium lamp is absorbed almost solely by the substrate. The proportion of the attenuation of the analysis wavelength is compared to the attenuation of the other, transmitted from the gap wavelength, negligible.

In the evaluation of the measured radiation from the hollow cathode lamp (total absorption from base atomic absorption ) is ( approximately only background absorption ) subtracting the absorption of the radiation of the D2 lamp. The absorption of the analyte in the sample is obtained. A fundamental error is in the measurement of the surface within a range of wavelengths, determined by the adjustment of the gap in the monochromator, and not exactly on the analysis wavelength. So if the ground especially strongly absorbed along with the actual analysis line to a large amount of background absorption is subtracted from the total absorption. This leads to a so-called " over-correction " of the measured signal with negative test results.

Zeeman background correction

The magnetic field of the Zeeman magnet may be considered as second " radiation source ". When magnetic field is off, the entire light attenuation of analyte and substrate is added. When the magnetic field, the Zeeman splitting of the absorption line, so that no longer the analyte absorbed on the wavelength emitted by the lamp, but only the matrix ( the substrate). The strength of the applied magnetic field is not sufficient for a Zeeman splitting of molecules or particles ( subsoil). The advantage of this background correction is the measurement of the ground exactly on the analysis line, so that the background signal is smaller and at a higher salt content can still be measured low interference. Disadvantage is a reduced linearity range and, depending on the element, a reduced sensitivity due partly incomplete Zeeman splitting.

Causes of spectral interferences in AAS are:

• Emission lines of the flame or black body radiation of the glowing graphite furnace (direct light)
• Unwanted absorption in the same wavelength
• Scattering from solid, high-boiling smoke particles and gases ( Rayleigh scattering)

Background correction in the HR -CS AAS

In HR-CS AAS no additional system for background correction is required. These units are equipped with a CCD line and thus in principle simultaneously and independently operating detectors. Some of the software of these detectors are selected on both sides of the analytical line and used for correction purposes. Any change in the radiation intensity that occur equally on all pixel correction are automatically corrected. These include, for example, fluctuations in the lamp emission, but also any continuous background absorption. Discontinuous background absorption, for example, direct line overlay with a matrix element or molecule with absorption fine structure, can be eliminated mathematically by means of reference spectra. It can broadband and spectral background effects to be separated. The former are automatically corrected by reference pixel and second consists become visible and thus made measurable. In most cases of spectral interferences excellent resolution is already sufficient so that the analytical line can be used to evaluate undisturbed. With this technique, the operation is extremely simple Especially with unknown and changing samples. But even with routine measurements with a known matrix, the measurement routine is facilitated, since spectral interference no longer need to be corrected consuming. Full automatic running underground routines use the available reference pixel and allow time true simultaneous correction. The background correction in the HR -CS AAS offers a wide dynamic linear range, advanced detection limits, unambiguous measurement results, eliminating artifacts and corrected when direct line overlay.

Matrix modifier in GF-AAS

Can be helpful in this context, the use of Matrixmodifizierern that in a uniform, thermally stable, chemical compound transfer the analyte ( Isoformierungshilfe ). Thereby higher temperatures are possible during pyrolysis to remove matrix before atomization without prematurely losing the analyte. For the determination of lead and cadmium a Mischmodifizierer of magnesium nitrate (Mg (NO3 ) 2) and ammonium dihydrogen phosphate ( NH4H2PO4 ) is often used. For the determination of many other elements, the use of a Mischmodifizierers of palladium (II ) nitrate (Pd (NO3) 2 ) and magnesium nitrate has been proven that allows the pyrolysis temperatures of about 1000 ° C, which is in cross- heated graphite tube systems, the temperature at which sodium chloride ( NaCl) is volatilized, a common ingredient of body fluids, waste water and products of the chemical industry.