Chapter 28 Atomic Spectroscopy

28 A Origins of atomic spectra With gas-phase atoms or ions, there are no vibrational or rotational energy States, instead only electronic transitions occur. Thus, atomic emission, absorption, and fluorescence spectra are made up of a limited number of narrow spectral lines. In atomic emission spectroscopy, analyte atoms are excited by heat or electrical energy. A transition to or from the ground state is called a resonance transition, and the resulting spectral line is called a resonance line.

Absorption of radiation of 285, 330, and 590 nm excites the single outer electron of sodium from its ground state 3s energy level to the excited 3p, 4p, and 5p orbitals, respectively. After a few nanoseconds, the excited atoms relax to their ground state by transferring their excess energy to other atoms or molecules in the medium.

Atomic fluorescence is often measured at the same wavelength as the source radiation and is called resonance fluorescence. Atomic spectral lines have finite widths. Several factors contribute to atomic spectral line widths: Natural Broadening The natural width of an atomic spectral line is determined by the lifetime of the excited state and Heisenberg’s uncertainty principle. The shorter the lifetime, the broader the line and vice versa. Collisional Broadening Collisions between atoms and molecules in the gas-phase lead to deactivation of the excited state and thus broadening of the spectral line. The amount of broadening increases with the concentrations (partial pressures) of the collision partners.

Doppler Broadening

28 B Production of atoms and ions The atomization device must normally perform the complex task of converting analyte species in solution into gas-phase free atoms and/or elementary ions. Atomization devices fall into two classes: continuous atomizers and discrete atomizers. With continuous atomizers, such as plasmas and flames, samples are introduced in a steady, continuous stream. With discrete atomizers, individual samples are injected by means of a syringe or autosampler. The most common discrete atomizer is the electrothermal atomizer.

Plasma Sources A plasma is a hot, partially ionized gas. It contains relatively high concentrations of ions and electrons. Three power sources are used in argon plasma spectroscopy: a dc arc source capable of maintaining a current of several amperes between electrodes immersed in the argon plasma. A radio-frequency, or inductively coupled plasma (ICP), source offers the greatest advantage in terms of sensitivity and freedom from interference. microwave-frequency generator through which the argon flows.

When a nebulized sample is carried into a flame, the droplets are desolvated in the primary combustion zone, which is located just above the tip of the burner. The center of the flame called the inner cone is where particles are vaporized and converted to gaseous atoms, elementary ions, and molecular species

Types of Flames Used in Atomic Spectroscopy

Effects of Flame Temperature Flame temperature determines to a large extent the efficiency of atomization, which is the fraction of the analyte that is desolvated, vaporized, and converted to free atoms and/or ions. The flame temperature also determines the relative number of excited and unexcited atoms in a flame. Emission methods require much closer control of flame temperature than do absorption procedures. Absorption methods should show lower detection limits (DLs) than emission methods. Several other variables also influence detection limits.

Absorption and Emission Spectra in Flames Both atomic and molecular emission and absorption can be measured when a sample is atomized in a flame. Atomic emissions in this spectrum are made up of narrow lines and are called line spectra. Bands form when vibrational transitions are superimposed on electronic transitions to produce many closely spaced lines that are not completely resolved by the spectrometer. Because of this, molecular spectra are often referred to as band spectra.

Electrothermal Atomizers Electrothermal atomizers are used for atomic absorption and atomic fluorescence measurements, but they have not been generally applied for emission work. With electrothermal atomizers, a few microliters of sample are deposited in the furnace by syringe or autosampler. A programmed series of heating events occurs: drying, ashing, and atomization.

28 C Atomic emission spectrometry Atomic emission spectrometry is widely used in elemental analysis.

Wavelength Isolation Emission spectrometry is often used for multielement determinations. There are two types of instruments: The sequential spectrometer uses a monochromator and scans to different emission lines in sequence. The direct reading spectrometer uses a polychromator with as many as 64 detectors located at exit slits in the focal plane.

Radiation Transducers Single-wavelength instruments most often use photomultiplier transducers as do direct reading spectrometers. Computer Systems and Software Most of the newer ICP emission systems provide software that can assist in wavelength selection, calibration, background correction, interelement correction, spectral deconvolution, standard additions calibration, quality control charts, and report generation.

Sources of Nonlinearity in Atomic Emission Spectrometry Calibration curves that are linear or at least follow a predicted relationship are preferred. At high concentrations, the major cause of nonlinearity when resonance transitions are used is self-absorption. At low concentrations, ionization of the analyte can cause nonlinearity in calibration curves when atomic lines are used.

Interferences in Plasma and Flame Atomic Emission Spectrometry The interference effects are classified as: A blank, or additive, interference produces an effect that is independent of the analyte concentration. In emission spectroscopy, any element other than the analyte that emits radiation within the bandpass of the wavelength selection device or that causes stray light to appear within the bandpass causes a blank interference. Example, Spectral interferences produce an interference effect that is independent of the analyte concentration.

2. Analyte, or multiplicative, interferences change the magnitude of the analyte signal itself. Examples, Chemical, physical, and ionization interferences. Physical interferences can alter the aspiration, nebulization, desolvation, or volatilization processes. Chemical interferences occur in the conversion of the solid or molten particle after desolvation into free atoms or elementary ions. Substances that alter the ionization of the analyte also cause ionization interferences. Example, the presence of an easily ionized element, such as K, can alter the extent of ionization of a less easily ionized element, such as Ca. An ionization suppressant is an easily ionized species that produces a high concentration of electrons in a flame and represses ionization of the analyte.

28 D Atomic absorption spectrometry Flame atomic absorption spectroscopy (AAS) is currently the most widely used of all the atomic methods. Line-Width Effects in Atomic Absorption

Instrumentation The instrumentation for a single-beam AA spectrometer.

Line Sources The most useful radiation source for atomic absorption spectroscopy is the hollow- cathode lamp. Hollow cathode lamps have a cathode containing more than one element and thus provide spectral lines for the determination of several species.

Source Modulation It is necessary to discriminate between radiation from the hollow-cathode or electrodeless-discharge lamp and radiation from the atomizer. Most of the atomizer radiation is eliminated by the monochromator. The thermal excitation of a fraction of the analyte atoms in a flame produces radiation of the wavelength at which the monochromator is set, causing interference. The effect of analyte emission is overcome by modulating the output from the hollow-cathode lamp so that its intensity fluctuates at a constant frequency.

Modulation can be accomplished by placing a motor-driven circular chopper between the source and the flame.

Complete AA Instrument An atomic absorption instrument contains the same basic components as an instrument designed for molecular absorption measurements for a single-beam system. Photometers At a minimum, an instrument for atomic absorption spectroscopy must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for a measurement from other lines that may interfere with or diminish the sensitivity of the method. Spectrophotometers Most measurements in AAS are made with instruments equipped with an ultraviolet/visible grating monochromator.

Background Correction Absorption by the flame atomizer as well as by concomitants introduced into the flame or electrothermal atomizer can cause serious problems in atomic absorption. Molecular species can absorb the radiation and cause errors in AA measurements. The total measured absorbance, AT, in AA is the sum of the analyte absorbance, AA, plus the background absorbance, AB AT = AA + AB

Continuum source background correction uses a deuterium lamp to obtain an estimate of the background absorbance. A hollow-cathode lamp obtains the total absorbance. The corrected absorbance is then obtained calculating the difference between the two. Pulsed Hollow-cathode lamp background correction or Smith-Hieftje background correction uses a single hollow-cathode lamp pulsed with first a low current and then with a high current. The low-current mode obtains the total absorbance, while the background is estimated during the high-current pulse. In Zeeman background correction, a magnetic field splits spectral lines that are normally of the same energy (degenerate) into components with different polarization characteristics. Analyte and background absorption can be separated because of their different magnetic and polarization behaviors.

Flame Atomic Absorption Flame AA provides a sensitive means for determining some 60 to 70 elements. The optimum region of a flame must change from element to element and that the position of the flame with respect to the source must be reproduced closely during calibration and measurement. The flame position is adjusted to give a maximum absorbance reading for the element being determined.

Interferences in Atomic Absorption Molecular constituents and radiation scattering can cause interferences If the source of interference is known, an excess of the interferent (radiation buffer) can be added to both the sample and the standards. 28 E Atomic fluorescence spectrometry Atomic fluorescence spectrometry (AFS) is the newest of the optical atomic spectroscopic methods. An external source is used to excite the element of interest. The radiation emitted as a result of absorption is measured, often at right angles to avoid measuring the source radiation. It is not commercially successful partly owing to the lack of reproducibility of the high- intensity sources required and to the single-element nature of AFS.