This article outlines the procedure and techniques for Atomic Absorption Spectrometry.
1.0 Introduction
The atomic absorption spectrometry is used for the determination of various elements of the periodic table and basically consists of four techniques: flame atomic absorption, hydride generation, cold steam generation and graphite furnace. The techniques that use flame and graphite furnace as atomizers allow the determination of about 70 elements being metals the most part of these. The technique f hydride generation allows the determination of arsenic, antimony, selenium, bismuth, tellurium, lead, indium, tin, generation and thallium; the generation of cold steam is basically used for the determination of mercury.
For the determination of the analyte concentration by atomic absorption, radiation from a source with a specific wavelength in accordance with the element tested occurs under the atomic steam containing atoms free from this element in the ground state. The radiation attenuation is proportional to the analyte concentration according to the Beer-Lambert law.
The instrumentation for atomic absorption basically consists of radiation source, atomizer, monochromator, detector and data processing system. As light sources, hollow cathode lamps and discharge lamps without electrode that emit intense radiation of the same wavelength of the absorbed by the element to be determined are used. The atomizer can be composed of a flame or graphite furnace. The monochromator is responsible for separating the wave length desired. The radiation focuses on monochromator by a narrow slot; then, it is separated by its different wavelengths in a diffraction grating and, after, directed to the detector. The detector is typically a photomultiplier, which transforms the light energy into electrical current, which is amplified and then interpreted by a reading system.
1.1 Procedure
To operate the atomic absorption spectrometers, it is recommended to follow the manufacturer instructions. The determinations are made by comparison with reference solutions containing known concentrations of the analyte. The determinations may be made by the Direct calibration method (Method I) or by the Standard addition method (Method II). The Method I is recommended, unless otherwise specified.
1.1.1 Direct calibration method (Method I)
Prepare at least four reference solutions of the elements to be determined using the concentration range recommended by the equipment manufacturer for the analyte. All reagents used in the sample preparation should also be included, at the same concentrations, in preparation of the reference solutions. After equipment calibration with solvent, introduce in the atomizer three times each of the reference solutions and after the reading, record the result. Wash the sample introduction system with water after each operation.
Trace the analytical curve for the mean absorbance’s of three readings for each reference solution with the respective concentration. Prepare the sample as indicated in the monograph by adjusting its concentration; so the concentration falls within the concentration range of the reference solutions for the analyte. Introduce the sample in atomizer, record the reading and wash the sample introduction system with water. Repeat this sequence twice. Determine the element concentration by the analytical curve using the three readings.
1.1.2 Standard addition method (Method II)
At least, add four volumetric flasks with equal volumes of the substance solution to be determined prepared as indicated in the monograph. Into three volumetric flasks, add volumes determined from the reference solution specified in order to obtain a series of solutions containing increasing amounts of analyte. Fill up the volume of each flask with water. After calibrating the spectrometer with water, record three times the readings of each solution. Trace the analytical curve for the mean absorbance’s of three readings for each reference solution versus the respective quantity of analyte added to the solution. Record the analyte amount in module of the sample by extrapolation of the analytical curve on the x-axis.
2.1 Flame Atomic absorption spectrometry
The system consists of a pre-mix camera in which the fuel and oxidant are mixed and burner that receives the fuel-oxidant mixture. The solution is introduced through a pneumatic nebulizer, in which a fine aerosol is generated that is driven up to the flame. The energy amount that can be supplied by the flame for the dissociation and the sample atomization is proportional to the temperature. If a low temperature flame is used, the solution can not be converted into neutral atoms. On the other hand, if a flame with very high temperature is applied, the formation of a large amount of ions which do not absorb radiation from the source may occur. By modifying the ratio of oxidant and fuel used for each type of flame, it is possible to significantly change its temperature. The flames normally used are produced by air- acetylene (2100-2400oC) and nitrous oxide-acetylene (2650-2850oC). The air- acetylene mixture is used for elements with lower atomization temperature as Na, K,Mg,Cd, Zn, Cu, Mn, Co, etc. the flame generated by nitrous oxide-acetylene is applied to refractory elements as Al, V. Ti, Si, U, among others.
2.1 Interferences
2.1.1 Physical Interferences
The use of sample preparation with physical properties such as viscosity and surface tension different from the standard preparation may result in differences from the standard preparation may result in differences in relation to aspiration and nebulization, leading to incorrect readings. Whenever possible, use preparations with the same physical properties and matrix constituents.
2.1.2 Ionization Interference
Usually occurs in alkaline and alkaline earth elements that are easily ionized. The higher the ionization degree, the lower is the absorbance. In order to reduce the ionization interferances, it is possible to use flames with lower temperatures or use “ionization suppressors” that are elements such as cesium, which ionize easier than the analyte, thus increasing the number of atoms in the ground state.
2.1.3 Chemical Interferences
The formation of thermally stable compounds in the flame ass the oxide of some elements (Ca, Ti, V, Cr, Al, etc) reduces the population of atoms in the ground state. This can be resolved by increasing the temperature of the flame which results in dissociation of these compounds. Another possibility is to use a “suppressor or liberating agent” that has greater affinity for oxygen in relation to the analyte, avoiding the oxides formation. The solution containing cesium chloride and lanthanum chloride, “Schinkel’s solution”, is the most commonly used.
2.1.4 Spectruml Interferences
They occur by means of absorption or scattering of radiation selected for the analyte. The spectruml interferences caused by atoms are not common and can be resolved by changing the spectruml line used. The interference caused by molecular species are more serious but are normally resolved through bottom correction.
3.1 Hydride generation atomic absorption spectrometry
The hydride generation atomic spectrometry is a technique used for the determination of formative elements of volatile hydrides more usually to As, Se, Sb,Bi, Ge, Sn, Pb, and te. The process consists of three main steps : generation, transport and atomization of hydrides. The system can be built in batch or flow. The hydrides generation consists of the reaction of the analyte, usually in acid medium, with a reducer (NaBH4). The hydrides transport from the reaction flask to the quartz cell is done through an inert gas as argon or nitrogen. For elements that absorb at a wavelength lower than 200nm, before the hydrides generation step, a purge for the removal of atmospheric gases should be done in order to avoid that these gases absorb the radiation from the source. The atomization is made in an electrically heated quartz cell or with a typical burner of flame atomizing systems; the internal temperature of the cell is 850oC -1000oC. Normally, the signal obtained is the transient type; approximately
20 seconds are required for the signal full integration for almost all elements.
3.2 Interferences
Oxidation State Influence: the analytes typically have more than one oxidation state. Arsenic and antimony for example have oxidation states III and V, and selenium and tellurium have oxidation states IV and VI, respectively.
The superior oxidation states, in general are inert for conversion to volatile hydrides; therefore the pre reduction before the determination is required in these cases.
Hydride forming elements: mutual interference can occur between the hydride forming elements, for example, between arsenic and selenium. In these cases, the kinetics this process.
Transition elements: some metal ions as Cu2+ and Ni2+, if present in high concentrations, are reduced, forming precipitates that may adsorb volatile hydrides.
4.1 Atomic absorption spectrometry with cold steam generation
The atomic absorption spectrometry with cold steam generation is used for the determination of mercury. The equipment and reagents are the same used in the hydride generation system, however, the quartz cell does not need to be heated, because mercury is reduced to metallic mercury, that is volatile at ambient temperature. Therefore, water steam can be transported by gas and interfere in the determination. To solve this problem, an infrared lamp is used to heat the quartz cell, preventing the water steam condensation. In this case, the purging is not required, because the wavelength used for the determination of Hg is 253.7 nm, which the radiation absorption by atmosphere gases is rare.
5.1 Atomic absorption spectrometry with graphite furnace
The atomic absorption spectrometry with graphite furnace is a comprehensive technique that has high sensitivity. The furnace consists of a graphite tube of 3 to 5 cm long and 3 to 8 mm in diameter coated with pyrolytic graphite. The sample amount injected into the furnace varies from 5uL to 50uL and is generally introduced by an automated system. The furnace is electrically heated by the electrically heated by the electrical current in a longitudinal or transverse manner. Flow of inert gases such as argon are internally and externally maintained to prevent the combustion of the furnace. Furthermore, the internal flow expels the atmosphere from the air furnace and also the vapors generated during the drying and pyrolysis stages.
A graphite furnace offers a durability of approximately 300 cycles depending on the model.
The analysis with the graphite furnace can be divided into the following steps: drying of the sample, pyrolysis, atomization and cleaning. The transition from one step to another is marked by the temperature increase, therefore, a special heating program must be planned. First, the drying of the sample is carried out; in this phase, the solvents and residual acids are evaporated. After drying, the temperature is increased leads to the analyte atomization for subsequent quantification. Finally, the oven is cleaned at a high temperature (eg 2600oC) for few seconds. The temperature and the duration of each heating step can be controlled; this is essential for the development of analytical methodologies.
Curves of atomization and pyrolysis are used for the temperature optimization for such processes. The pyrolysis curve allows to determine the maximum temperature at which the analyte loss does not occur. The atomization curve allows to determine the atomization minimum temperature of the analyte with adequate sensitivity. It is recommended that the pyrolysis and atomization curves are made whenever an unknown sample is analysed.
The atomization process in a graphite furnace is complex and depends on several factors such a the furnace and platform material, the atmospheric inside the tube, the heating speed, the temperature and the substance nature. For best results, it is recommended to use the L’Vov platform inside the tube and transverse heating. The signal obtained is the transient type; up to 12 seconds are required for the signal integration.
5.2 Interferences
5.2.1 Spectruml interferences
Interferences caused by line overlap among atoms are very common. The attenuation of the radiation beam by species generated during the atomization process resulting from the matrix are more frequent. To solve this problem, the array must be eliminated efficiently. The use of a matrix modifier and a bottom corrector are essential to the reliability of the results.
5.2.2 Formation of volatile substances
In samples with high levels of halogens (especially Cl), there is the formation possibility of analyte volatile substances that may be lost in low temperatures causing an analysis error. In this case, the use of a chemical modifier able to form thermally stable complexes with the analyte reduces the formation of volatile substances. In addition, when the chemical modifier is combined with the L’Vov platform, the matrix interference effects are considered reduced. It is important to emphasize that a given chemical modifier can be very effective for some elements, but ineffective for others.
Atomic absorption spectrometry,direct calibration method, standard addition method,flame atomic absorption spectrometry, spectruml interferences, hydride generation, cold steam generation, graphite furnace
