Atomic Spectroscopy

Optical Atomic Spectroscopy 1. Introduction Atomic spectroscopy is used for the determination of elemental composition (at < ppm levels with high selectivity). [1 ppm = 10-6 g/mL] The optical atomic spectroscopy is dealing with the electromagnetic spectrum of elements. Electrons exist at energy levels within an atom. These levels have well defined energies and electrons moving between them must absorb or emit energy equal to the energy difference between the levels. In optical spectroscopy, the energy absorbed to move an electron to a more energetic level and/or the energy emitted as the electron moves to a less energetic energy level is in the form of a photon. The wavelength of the emitted radiant energy is directly related to the electronic transition which has occurred. Since every element has an unique electronic structure, the wavelength of light emitted is an unique property of each individual element. The science of atomic spectroscopy has yielded three techniques for analytical use: the atomic absorption (need ground state atoms), the atomic emission (need excited state atoms) and the atomic fluorescence. Either the energy absorbed in the excitation process, or the energy emitted in the decay process is measured and used for analytical purposes. The energy levels of atoms are quantized (very finite energy difference existing between orbitals) thus absorption and emission lines are very narrow (10-2-10-3 nm). During the atomic spectroscopy the sunstances are examined in gas phase. A substance is decomposed into gaseous atoms in a flame, furnace, or plasma. (A plasma is a gas that is hot enough to contain ions and free electrons.)

(in the atomic emission) requireing more atoms in excited state to optimize sensitivity (in the atomic absorption) requireing more atoms in ground state to optimize sensitivity

Absorption and emission by atoms in a flame In atomic spectroscopy samples are vaporized at 2000-8000 K and decompose into atoms. Concentrations of atoms in the vapour are measured by emission or absorption of characteristic wavelengths of radiation. The atomic spectroscopy is a principal tool of

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Atomic Spectroscopy

analytical chemistry, because it has high sensitivity, it is able to distinguish one element from another in a complex sample, and we can perform simultaneous multielement analyses with it. Analyte can be measured at parts per million (µg/g) to parts per trillion (pg/g) levels. So trace constituents can be measured directly without any preconcentration. To analyze major constituents, the sample must be diluted to reduce concentrations to the parts per million levels.

2. Atomic emission spectroscopy In atomic emission, a sample is subjected to a high energy, thermal environment in order to produce excited state atoms, capable of emitting light. The energy source can be an electrical arc, a flame, or more recently, a plasma. The emission spectrum of an element consists of a collection of the allowable emission wavelengths, commonly called emission lines, because of the discrete nature of the emitted wavelengths. This emission spectrum can be used as an unique characteristic for qualitative identification of the element. Emission techniques can also be used to determine how much of an element is present in a sample. For a „quantitative” analysis, the intensity of light emitted at the wavelength of the element to be determined is measured. The emission intensity at this wavelength will be greater as the number of atoms of the analyte element increases. The technique of flame photometry is an application of atomic emission for quantitative analysis. 2.1. Flame emission spectrometry (FES)

Premix burner In flame emission spectrometry, the flame will be the atom source. Most flame spectrometers use a premix burner, in which fuel, oxidant, and sample are mixed before introduction into the flame. Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it leaves the capillary. The spray is directed against a glass bead, upon which the droplets break into smaller particles. The formation of small droplets is termed nebulization. A fine suspension of liquid (or solid) particles in a gas is called an aerosol. The nebulizer

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Atomic Spectroscopy

creates an aerosol from the liquid sample. The mist, oxidant, and fuel flow past baffles that promote further mixing and block large droplets of liquid. Excess liquid is collected at the bottom of the spray chamber and flows out to a drain. Aerosol reaching the flame contains only about 5% of the initial sample. The flame The most common fuel-oxidizer combination is acetylene and air, which produces a flame temperature of 2400-2700 K (Table 21-1). If a hotter flame is required to atomize highboiling elements, acetylene and nitrous oxide are usually used.

In the flame profile in Figure 21-5b, gas entering the preheating region is heated by conduction and radiation from the primary reaction zone (the blue cone in the flame). Combustion is completed in the outer cone, where surrounding air is drawn into the flame. Flames emit light that must be subtracted from the total signal (with monochromator) to obtain the analyte signal.

Profile of flame Droplets entering the flame evaporate; then the remaining solid vaporizes and decomposes into free atoms. Depending on the energy of flame, the free atom can be in ground state, or in excited state, or it can be ionized. In the atomic emission experiments excited state atoms should be formed from the sample. Many elements form oxides and hydroxides in the outer cone. Molecules do not have the same spectra as atoms, so the intensity of atomic signal is lowered if molecules are formed. Molecules also emit broad radiation that must be subtracted from the sharp atomic signals. If the flame is relatively rich in fuel (a “rich” flame), excess carbon tends to reduce metal oxides and hydroxides and thereby increases sensitivity. A “lean” flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis.

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Atomic Spectroscopy

Flame atomic emission spectrometer Monochromators A monochromator disperses light into its component wavelengths and selects a narrow band of wavelengths to pass on to the detector (or sample in spectrophotometry). The monochromator consists of entrance and exit slits, mirrors, and a grating to disperse the light. Prisms were used instead of gratings in older instruments. Job of the monochromator is to isolate analytical lines' photons passing through the flame and to remove scattered light of other wavelengths from the flame. Detectors A detector produces an electric signal when it is struck by photons. For example, a phototube emits electrons from a photosensitive, negatively charged surface (the cathode) when struck by visible light or ultraviolet radiation. The electrons flow through a vacuum to a positively charged collector whose current is proportional to the radiation intensity.

Diagram of a photomultiplier tube with nine dynodes

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Atomic Spectroscopy

Photomultiplier tube A photomultiplier tube is a very sensitive device in which electrons emitted from the photosensitive surface strike a second surface, called a dynode, which is positive with respect to the photosensitive emitter. Electrons are accelerated and strike the dynode with more than their original kinetic energy. Each energetic electron knocks out more than one electron from the dynode. These new electrons are accelerated toward a second dynode. which is more positive than the first dynode. Upon striking the second dynode, even more electrons are knocked out and accelerated toward a third dynode. This process is repeated several times, so more than 106 electrons are finally collected for each photon striking the first surface. Extremely low light intensities are translated into measurable electric signals.

3. Atomic absorption spectroscopy If light of just the right wavelength impinges on a free, ground state atom, the atom may absorb the light as it enters an excited state in a process known as atomic absorption. Atomic absorption measures the amount of light at the resonant wavelength which is absorbed as it passes through a cloud of atoms. As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way. By measuring the amount of light absorbed, a quantitative determination of the amount of analyte element present can be made. The use of special light sources and careful selection of wavelength allow the specific quantitative determination of individual elements in the presence of others. The atom cloud required for atomic absorption measurements is produced by supplying enough thermal energy to the sample to dissociate the chemical compounds into free atoms. Aspirating a solution of the sample into a flame aligned in the light beam serves this purpose. Under the proper flame conditions, most of the atoms will remain in the ground state form and are capable of absorbing light at the analytical wavelength from a source lamp. The ease and speed at which precise and accurate determinations can be made with this technique have made atomic absorption one of the most popular methods for the determination of metals. 3.1. Flame atomic absorption spectrometry (FAAS) In atomic absorption experiment, a liquid sample is aspirated (sucked) into a flame whose temperature is 2000-3000 K. Liquid evaporates and the remaining solid is atomized (broken into atoms) in the flame, which replaces the cuvet in conventional spectrophotometry. The pathlength of the flame is typically 10 cm. The hollow-cathode lamp emits light with the same frequencies absorbed by the analyte in the flame. The detector measures the amount of the light (the power of the electromagnetic radiation) that passes through the sample and the flame.

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Atomic Spectroscopy

Flame atomic absorpcion spectrometer An important difference between atomic and molecular spectroscopy is the width of absorption or emission bands. Spectra of liquids and solids typically have bandwidths of ~100 nm. In contrast, spectra of gaseous atoms consist of sharp lines with widths of ~0.001 nm. Lines are so sharp that there is usually little overlap between the spectra of different elements in the same sample. Therefore, some instruments can measure more than 70 elements simultaneously. We will see later that sharp analyte absorption lines require that the light source also have sharp lines. Hollow-cathode lamp (light source) Monochromators generally cannot isolate lines narrower than 10-3 to 10-2 nm. To produce narrow lines of the correct frequency, we have to use a hollow-cathode lamp containing a vapour of the same element as that being analyzed. A hollow-cathode lamp is filled with Ne or Ar gas at a pressure of ~130-700 Pa. The cathode is made of the element whose emission lines are needed for the analysis of the sample. When ~500 V are applied between the anode and the cathode, gas is ionized and positive ions (Ar+or Ne+ ions) are accelerated toward the cathode. After ionization occurs, the lamp is maintained at a constant current of 2-30 mA by a lower voltage. Cations strike the cathode with enough energy to “sputter” metal atoms from the cathode into the gas phase. Gaseous atoms excited by collisions with high-energy electrons emit photons. This atomic radiation has the same frequency absorbed by analyte in the flame or furnace. Atoms in the lamp are cooler than atoms in a flame, so lamp emission is sufficiently narrower than the absorption bandwidth of atoms in the flame to be nearly “monochromatic” (Figure 21-17). The purpose of a monochromator in atomic spectroscopy is to select one line from the hollow-cathode lamp and to reject as much emission from the flame or furnace as possible. A different lamp is usually required for each element, although some lamps are made with more than one element in the cathode. Note that monochromator is placed after the flame to get rid of large background of white light from the flame itself.

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Atomic Spectroscopy

The hollow-cathode lamp

Relative bandwidths of hollow-cathode emission, atomic absorption, and a monochromator Atomization (Ways to form gas atoms) In atomic spectroscopy, analyte is atomized in a flame, or in an electrically heated furnace, or in a plasma. Flames were used for decades, but they have been replaced by the inductively coupled plasma and the graphite furnace. Flames: Heat of the flame desolvates species, creates salts of ions, and then further heating in flame decomposes these particles into gas phase atoms. Temperature of the flame is critical. For atomic absorption, cooler flame is required but it should be hot enough to get good atomization efficiency. For atomic emission, warmer flame is wanted to get more gas phase atoms in excited state. The temperature of the flame depends on the fuel and the oxidant used. Graphite Furnace: It is used exclusively for atomic absorption (flameless AA). An electrically heated graphite furnace is more sensitive than a flame and requires less sample amount. From the sample solution only 1 to 100 µL volumes are injected into the furnace through the hole at the center. To prevent oxidation of the graphite, Ar gas is passed over the furnace, and the maximum recommended temperature is 2550 K for not more than 7 s. The wall of the tube is heated up in 3 steps by flowing current through the wall (with external power supply). Low temp is used to dry the sample, then the wall is heated up to 1400 K for short period to ash/char the sample, then up to 2550 K for few seconds to produce gas phase atoms. After atomization, the furnace is heated to 2500 K for 3s to clean out any remaining residue. The furnace is purged with Ar or N2 during each step except atomization to remove

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volatile material. Liquid samples are ordinarily used in furnaces. However, in direct solid sampling, a solid can be analyzed also without sample preparation. In flame spectroscopy, the residence time of analyte in the optical path is