Atomic Spectroscopy

ATOMIC SPECTROSCOPY

Atomic spectroscopy deals with the line spectra from atoms. It can be of both types either atomic emission or absorption spectra. In atomic emission, the atoms are raised to excited state which will emit a specific line spectra when returns to ground state while in case of absorption, the atoms in gaseous state absorbs a specific wavelength of light. The line spectra generated from atoms depends on the electron transitions in atoms. It is strictly limited by availability of orbitals within an atom. The orbitals could not be overfilled. The transitions in an atom tend do occur between closer energy levels. So, the most obvious wavelength (l) are associated with minimal energy changes. Each element has their own characteristic emission and absorption lines which are generated during electronic transitions. Hence, line spectra can be helpful to deduce electronic structure of an atom.

PRINCIPLE:

For sample processing, atoms are volatilized (ashing) in a flame or electrothermally in an oven so that, the elements will readily emit or absorb monochromatic radiation at the appropriate wavelength (l). The absorption or emission energy is directly proportional to number of atoms however standard conditions are difficulties to achieve because of flame instability and variation in temperature within the system. Different atoms absorbs different wavelength so the line spectra should be selected according to the atoms to be analyzed.

During analysis some interference are encountered. The presence of alkali metals such as sodium, potassium etc. may enhance emission in the sample while the presence of certain elements like phosphate, silicate, aluminate suppress the emission. They require very high temperature to excite and emit line spectra. Therefore, repeated analysis with refined interfering substances and standards are required in atomic spectroscopy. In this technique each sample is processed with triplicate assays to overcome flame instability problems. Moreover the samples are stored in polythene bottles as metal ions are both absorbed and released by glass.

                

Absorption or emission energy   µ         Number of atoms

 

The atomic spectroscopy can be further classified to three different types on the basis the mechanism of line spectra generated from the atoms.

1. Atomic Emission Spectroscopy
1. A.       Simple Flame Emission Spectroscopy
1.  B.      Plasma Emission Spectroscopy
2. Atomic Absorption Spectroscopy
3. Atomic Fluorescence Spectroscopy

                  

1. 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, plasma (a gas becomes plasma when it is heated until it loses all electrons leaving highly electrified collection of nuclei and free electrons. The emission spectrum of an element exposed to such an energy source 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 a unique characteristic for qualitative identification of the element. Atomic emission using electrical arcs has been widely used in qualitative analysis. 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.

A. SIMPLE FLAME EMISSION SPECTROSCOPY:

It is based on the emission phenomenon when atoms are excited into the high temperature flaming. It is simple and conventional technique for analysis of alkali and alkaline earth metals.

WORKING PRINCIPLE:

Simple Flame photometry relies upon emission phenomenon. The compounds of the alkali and alkaline earth metals when dissociated in a flame, some of the atoms produced are further excited to a higher energy level. When these atoms return to the ground state they emit radiation which lies mainly in the visible region of the spectrum. Each element emits radiation at a wavelength specific for that element. The table below gives details of the measurable atomic flame emissions of the alkali and alkaline earth metals in terms of the emission wavelength and the color produced. The emitted radiation is then measured.

Element Emission            Wavelength (nm)       Flame Color

Sodium (Na)                     589                              Yellow

Potassium (K)                   766                              Violet

Barium (Ba)                      554                              Lime Green

Calcium (Ca)                     622*                            Orange

Lithium (Li)                      670                              Red

*Note: Calcium is measured by using the calcium hydroxide band emission at 622 nm as the Calcium main atomic emission occurs at 423nm.

 

INSTRUMENTATION:  

A Simple Flame Emission Spectroscopy has following different working parts.

1.  Sample delivery
2. Nebulization
3. Atomization
4. Monochromatic system (Filter-interference type)
5. Detector and data recording

Figure 1: Optical diagram of a Simple Flame Emission Spectroscopy

1. Sample Delivery:

The device that introduces the sample into the flame plays a major role in determining the accuracy of the analysis. The most popular sampling method is nebulization of a liquid sample to provide a steady flow of aerosol into a flame. Flame FES requires that the analyte be dissolved in a solution in order to undergo nebulization. An introduction system for liquid samples consists of three components:

1.      A nebulizer that breaks up the liquid into small' droplets,

2.      An aerosol modifier that removes large droplets from the stream, allowing only droplets smaller than a certain size to pass, and

3.      The flame or atomizer that converts the analyte into free atoms.

Different states of samples are processed in different ways.

Solid samples: The solid samples must be first dissolved into suitable solution and then analyzed. The examples of solid samples include metals, alloys, soil, animal tissue, plant material, fertilizers, ores, cement, bone ash etc.

Liquid samples: The liquid samples are directly analyzed. However if sample is concentrated, it is diluted and if it is very dilute solution, then it is concentrated by evaporation before analysis. E.g. Blood, urine, electroplating solutions, petroleum products, wines, polluted water etc.

Gas samples: In conventional samples, metal components are changed to gaseous form. In hydride analysis: some metal converted to a gaseous hydride form and analyzed. E.g. Arsenic, selenium, uranium etc.

2. Nebulization:

Pneumatic nebulization is the technique used in most atomic spectroscopy determinations. The sample solution is introduced through an orifice into a high- velocity gas jet, usually the oxidant gas. The sample stream may intersect the gas stream in either a parallel or perpendicular manner. Liquid is drawn through the sample capillary by the pressure differential generated by the high-velocity gas stream passing over the sample orifice. The liquid stream begins to oscillate, producing filaments. Finally, these filaments collapse to form a cloud of droplets in the aerosol modifier or spray chamber. In the spray chamber the larger droplets are removed from the sample stream by mixer paddles or broken up into smaller droplets by impact beads or wall surfaces. The final aerosol, now a fine mist, is combined with the oxidizer/fuel mixture and carried into the burner. Droplets larger than about 20 µm are trapped in the spray chamber and flow to waste. The distribution of drop sizes is a function of the solvent as well as the components of the sampling system. In AAS only a small percentage (usually 2% or 3%) of the nebulized analyte solution reaches the burner.

3. Atomization:

The atomization step must convert the analyte within the aerosol into free analyte atoms in the ground state for FES analysis. Very small sample volumes (5-100 ml) or solid samples can be handled by flameless electrothermal methods.

Flame Atomizers: The sequence of events involved in converting a metallic element, M, from a dissolved salt, MX, in the sample solution to free M atoms in the flame is carried out in flame atomizers. After the aerosol droplets containing metals enter the flame, the solvent is evaporated, leaving small particles of dry solid sample. Next, solid sample is converted to vapor form. Finally, a portion of the MX molecules are dissociated to give neutral free atoms. The efficiency with which the flame produces neutral analyte atoms is of equal importance in all the flame techniques.

4. Monochromatic system

Simple color filters (interference type): a means of isolating light of the wavelength to be measured from that of extraneous emissions.

5. Photo-detector:

A photo-detector similar to spectrophotometer is used as means of measuring the intensity of radiation emitted by the flame.

APPLICATIONS:

• —  Most applications of FES have been the determination of trace metals, especially in liquid samples.
• —  It should be remembered that FES offers a simple, inexpensive, and sensitive method for detecting common metals, including the alkali and alkaline earths, as well as several transition metals such as Fe, Mn, Cu, and Zn.
• —  FES has been extended to include a number of nonmetals: H, B, C, N, P, As, O, S, Se, Te, halogens, and noble gases.
• —  FES detectors for P and S are commercially available for use in gas chromatography.
• —  FES has found wide application in agricultural and environmental analysis, industrial analyses of ferrous metals and alloys as well as glasses and ceramic materials, and clinical analyses of body fluids.
• —  FES can be easily automated to handle a large number of samples. Array detectors interfaced to a microcomputer system permit simultaneous analyses of several elements in a single sample.
• Widely used in clinical laboratories for determination of metals in body fluids, Acid diagnosis as well as valuable in monitoring of therapeutic regimes. Elements such as Na, K, Ca, Mg, Cd, Zn etc can be directly measured. However elements such as Cu, Pb, Fe, Hg etc      require prior extraction for analysis.

B. PLASMA EMISSION SPECTROSCOPY

Atoms can be excited using the high energy levels associated with Inductively Coupled Plasma (ICP) instead of a flame such a method of excitation is for a more effective and permits the analysis of elements beyond the scope of simple flame emission techniques, such as the refractory elements of boron, phosphorous and tungsten. The high temperature eliminates many of interference effects and the instrumentation is designed with series of photo multiplier tubes set for the different emission wavelength (l) of spectroscopy elements, permitting multi element analysis of samples. The method is very sensitive and specific.

The ICP discharge is caused by the effect of a radio frequency field on argon gas flowing through a quartz tube. The high power frequency causes a changing magnetic field in the gas and this in turn result in a heating effect. Temperatures of 9000-10000°C can be produced in this way.

Inductively Coupled Plasma: ICP is a type plasma sources in which the energy is supplied by electrical current which are produced by electromagnetic induction that is by time varying magnetic field.

When a time varying electric current is passed through the coil, it creates a time varying magnetic field around it which in turn induces azimuthal (an angular measurement in spherical coordinates) electric currents in the rarefield gas leading to break down and formation of a plasma. Argon is commonly used rarefied gas. The temperature of the plasma ranges between 6000-10000°C. ICP discharges high electron density of 10¹⁵ cm^-1. It is relatively free of contamination because electrodes are completely outside the reaction chamber.

Plasma: It is a gas in which a certain proportion of its particles are ionized. The presence of a non negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic field. Plasma therefore is quite unlike those of solids, liquids or gases-considered to be a distinct states of matter. Like gaseous state, plasma doesn't have a definite shape and volume unless enclosed in a container. Unlike gas, in the influence of a magnetic field it may form structures such as filaments, beams and double layers. Some examples of plasma are fire, lightening and the corona of the sun.

2. 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.

INSTRUMENTATION:

An Atomic Absorption Spectroscopy has following different working parts.

1.  Source of Radiation
2. Modulator / Chopper
3. Atomization / Nebulization
4. Monochromatic system (Filter-interference type)
5. Detector and data recording

Figure 2: Optical diagram of an atomic absorption spectroscopy

1. Source of Radiation:

The absorption bands due to atoms are very narrow and the use of white light as the incident radiation would swamp even the best monochromating system with unabsorbed radiation on either side of the absorption band. It is fundamental, therefore to the technique of atomic absorption spectroscopy that the incident radiation is of the correct wavelength, bandwidth and intensity. This radiation is produced by a lamp in which the cathode is coated with atoms of the element under investigation and which emits radiation of precisely the same wavelength as that which will be absorbed by non-excited atoms of the same element in the flame.

A hollow cathode lamp consists of two electrodes sealed in a glass envelop filled with an inert gas, usually argon or neon. The end window of the lamp must be of an appropriate material in order to transmit the emitted radiation and is either quartz or silica. The cathode of the lamp is usually cup shaped is either made of the element whose spectrum is required or coated with the element, and the application of a potential of 300-500V is usually required to cause excitation of the atoms and discharge of the appropriate radiation. Lamps are available with cathodes which contain two or more elements with emission lines that are easily distinguishable. These are such multi-elements lamps that tend to give less satisfactory performance than single element lamps. For certain elements, electrodeless discharge lamps have been designed in which the excitation of atoms is achieved by radio frequencies that induce resonance effects and the energy liberated causes vaporization and excitation of the element.

2. Moduator:

The flame, as well as containing the unexcited atoms of the element will also emit radiation due to the thermal excitation of a small proportion of atoms, and it is essential that the detector is capable of distinguishing between the identical radiation that is transmitted by the flame and that emitted from the flame. This is achieved by introducing a characteristic signal or modulation into the incident radiation by means of a rotating segmented mirror (Chopper) or an electrically induced pulse. This pulsed beam is detected as an alternating signal, which is superimposed on the relatively constant signal generated by the emission from the flame. The differences between the two signals are automatically measured by the instrument and only the light emitted from the flame is recorded.

3. Atomization / Nebulization:

The design of the burner head and the method of atomization of the sample both influence the sensitivity which can be achieved. The burner head is designed to give a long narrow flame so that as many atoms as possible are presented in the light path. It needs to keep spotlessly clean to minimize  background emission and the position in the light path has to be adjusted to give maximum sensitivity, the precise position depending not only on the gases being burnt but also on their flow rate. The proportion of fuel to oxidant alters the characteristic of the flame, a high proportion of fuel resulting in a flame with reducing properties while as excess of oxidant gives an oxidizing flame. For each element analyzed the optimum proportion must be determined.

The basic design of an atomizer is the same as that for flame emission spectroscopy. The method of producing an aerosol involves spraying the sample in air or oxidant gas. The larger drops precipitates ion the baffles of the expansion chamber and flow to waste. The fuel gas is introduced and the components are mixed before passing to the burner.

4. Monochromatic systems:

High quality monochromatic systems are necessary to isolate the required emission line of the element from those emissions lines due to the gases that are also present in the lamp. Owing to the very narrow bandwidth of atomic emission lines, it is not adequate simply to select the required wavelength using the monochromator.

5. Detector and data recording:

The sample is introduced into the cuvette using a micropipette. The first step in the analysis is the heating of the sample (Ashing) to remove the solvents and to destroy the matrix. When ashing is complete, the temperature of the cuvette is rapidly raised to the atomizing temperature, which is usually about 1000°C higher than the ashing temperature. This causes vaporization of the sample and a resulting transient increase (2-5 seconds) in the absorbance. The detector in atomic absorption spectrophotometer then detects and records this maximum absorbance value.

Differences between Atomic absorption and Flame Emission spectroscopy

—  Excited atoms quickly emit a photon of light in Emission spectroscopy.

—  Signal is due to difference between the intensity of source without and with sample in optical path.

—  Emission intensity depends upon no. of exciting atoms, influenced by temperature variation.

—  In atomic absorption spectroscopy, relation between absorbance and concentration is linear.

—  Applications are more wide than that of simple flame emission spectroscopy.

Advantages of Atomic absorption spectroscopy:

—  Specific technique

—  Higher signal in atomic absorption

—  Detection limits are almost similar

—  More sensitive by flame emission:       Al, Ba, K, Ca, Eu, Ho, In, La

—  Equally sensitive:                                 Cr, Cu, Mo, Pd, Rh, Ni, V

—  More sensitive by absorption:              Ag, As, Au, B, Bi, Cd, Co, Fe

 

3. ATOMIC FLUORESCENCE SPECTROSCOPY

The third field of atomic spectroscopy is atomic fluorescence. This technique incorporates aspects of both atomic absorption and atomic emission. Like atomic absorption, ground state atoms created in a flame are excited by focusing a beam of light into the atomic vapor. Instead of looking at the amount of light absorbed in the process, however, the emission resulting from the decay of the atoms excited by the source light is measured. The intensity of this "fluorescence" increases with increasing atom concentration, providing the basis for quantitative determination. The source lamp for atomic fluorescence is mounted at an angle to the rest of the optical system, so that the light detector sees only the fluorescence in the flame and not the light from the lamp itself. It is advantageous to maximize lamp intensity since sensitivity is directly related to the number of excited atoms which in turn is a function of the intensity of the exciting radiation.