atomic absorption and atomic fluorescence spectrometry wang-yingte department of chemistry 2003.9

Atomic Absorption and Atomic Fluorescence Spectrometry

Wang-yingte

Department of Chemistry

2003.9

principles of instrumental analysis 返回目录

• Sample atomization techniques

• Atomic absorption instrumentation

• Interferences in atomic absorption spectroscopy

• Atomic absorption analytical techniques

• Atomic fluorescence spectroscopy

principles of instrumental analysis 返回目录

A Sample Atomization Techniques

• Flame atomization

• Electrothermal aromizarion

• Specialized atomiztion techniques

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A-1 Flame Atomization

• In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. Undoubtedly other molecules and atoms are also produced in the flame as a result of interactions of the fuel with the oxidant and with the various species in the sample.

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A-2 Electrothermal Atomization

• Electrothermal atomizers offer the advantage of unusually high sensitivity for small volumes of sample. Typically, sample volumes between 0.5 and 10 μl are used; Under these circumstances, absolute detection limits typically lie in the range of 10-10 to 10-13g of analyte.

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A-3 Specialized Atomization Techniques

• By far, the most common sample introduction and atomization technique for atomic absorption analyses are flames or electrothemal vaporizers. Several other atomization methods find occasional use, however. Two of these are described briefly in this section. One is cold-vapor atomization and the other is hydride atomization.

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B Atomic Absorption Instrumentation

• Radiation sources

• Spectrophotometers

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B-1 Hollow Cathode Lamps

• The most common source for atomic absorption measurements is the hollow cathode lamp. The efficiency of the hollow cathode lamp depends on its geometry and the operating potential. High potentials, and thus high currents, lead to greater intensities.

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B-2 Single-beam Instruments

• A typical single-beam instrument consists of several hollow cathode sources, a chopper or a pulsed power supply, an atomizer, and a simple grating spectrophotometer with a photomultiplier transducer. The 100% T adjustment is then made while a blank is aspirated into the flame, or ignited in a nonflame atomizer, finally, the transmittance is obtained with the sample replacing the blank.

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Double-beam Instruments

• The beam from the hollow cathode source is split by a mirrored chopper, one half passing through the flame and the other half around it. The two beams are then recombined by a half-silvered mirror and passed into a Czerney-turner grating monochromator; A photomultiplier tube serves as thetransducer.

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C Interferences in Atomic Absorption Spectroscopy

• Spectral interferences

• Chemical interferences

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C-1 Spectral Interferences

• Spectral interferences arise when the absorption or emission of an interfering species either overlaps or lies so close to the analyte absorption or emission that resolution by the monochromater becomes impossible.

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C-2 Chemical Interferences

• Chemical interferences are more common than spectral ones. Their effects can frequently be minimized by a suitable choice of operating conditions.

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D Atomic absorption analytical techniques

• Sample preparation

• Organic solvents

• Calibration curves

• Standard addition method

• Applications of atomic absorption spectrometry

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D-1 Sample Preparation

• Many materials of interest, such as soils, animal tissues, plants, petroleum products, and minerals are not directly soluble in common solvents, and extensive preliminary treatment is often required to obtain a solution of the analyte in a form ready for atomization.

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D-2 Organic Solvents

• Leaner fuel/oxidant ratios must be employed with organic solvents in order to offset the presence of the added organic material.

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D-3 Calibration Curves

• In theory, atomic absorption should follow beer’s law with absorbance being directly proportional to concentration.

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D-4 Standard Addition Method

• The standard addition method is widely used in atomic absorption spectroscopy in order to partially or wholly counteract the chemical and spectral interferences introduced by the sample matrix.

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D-5 Applications of Atomic Absorption Spectrometry

• Atomic absorption spectrometry is a sensitive means for the quantitative determination of more than 60 metals or metalloid elements. The resonance lines for the nonmetallic elements are generally located below 200 nm, thus preventing their determination by convenient, nonvacuum spectrophotometers.

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E Atomic fluorescence spectroscopy

• Instrumentation

• interferences

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E-1 Instrumentation

• Sources• In the early work on atomic fluorescence,

conventional hollow cathode lamps often served as excitation sources. In order to enhance the output intensity without destroying the lamp, it was necessary to operate the lamp with short pulses of current that were greater than detector, of course, was gated to observe the fluorescent signal only during pulses.

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• Dispersive instruments

• A dispersive system for atomic fluorescence measurements is made up of a modulated source, an atomizer, a monochromator or an interference filter system, a detector, and a signal processor and readout.

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Nondispersive Instruments

• The advantages of such a system are several: • (1) simplicity an low-cost instrumentation,• (2) ready adaptability to multielement analys

is, • (3) high-energy throughput and thus high se

nsitivity, and • (4) simultaneous collection of energy from

multiple lines, also enhancing sensitivity.

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E-2 Interferences

• Interferences encountered in atomic fluorescence spectroscopy appear to be of the same type and of about the same magnitude as those found in atomic absorption spectroscopy.

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E-3 Applications

• Atomic fluorescence methods have been applied to the analysis of metals in such materials as lubricating oils, seawater, biological substances, graphite, and agricultural samples.