Further Reading
5.1 Fundamentals, Definitions and Terms
CHAPTER 5
Spectrochemistry for Trace
Analysis
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where cand λ0are the values for vacuum, and νiand λithe values for the medium i.
Refractive index is also a function of wavelength; a plot of niversusλis called a dis- persion curve; in common spectral regions where the material is transparent, refrac- tive index increases as the wavelength decreases. Dispersion curves for some common optical materials are shown in Figure 5.2.
Figure 5.1 Representation of a monochromatic, plane polarized light radiation. Only the electrical field is considered. λ, wavelength (nm); p, period and A, amplitude
Figure 5.2 Dispersion curves for some optical materials
Regarding the particle character, the term photonis used. The power of radia- tion,P, is proportional to the number of photons reaching an area per second. The sign hν is commonly used to denote a photon with a frequency of ν. Electromagnetic spectrum covers a large range of frequencies, starting from cos- mic rays at the high-energy end to the radio waves at the low-energy extreme.
Spectrochemistry for analysis involve ultraviolet (UV), visible (VIS) and infrared (IR) regions. From spectrometric point of view, UV and VIS are the same, since both of them involve electronic transitions, except that the latter corresponds to the range of human eye perception. These three regions of radiation are com- monly combined under the name of optical spectrometry (or spectroscopy); the behaviour of light in its interactions with matter and therefore spectrochemical instrumentation has great similarities. Regarding trace element analysis, only the UV and VIS radiations are important and these two regions are to be emphasized in this chapter.
In order to have a better understanding of spectrochemical techniques, it is impor- tant to consider the common ways in which the light interacts with matter. Light propagates on a line. Normally a light source will emit photons of different wave- lengths in a range, with the electronic components propagating on different planes.
A group of photons having ideally the same frequency is termed as monochromatic light; actually their frequencies in practice can never be exactly the same, but rather form a group with values gathering around a maximum, which is the nominal fre- quency or wavelength. There are instrumental as well as theoretical limitations for obtaining a truly monochromatic radiation. However, with better instrumentation it is possible to obtain a more monochromatic light beam, meaning that the statistical distribution for the frequencies around the nominal frequency has a narrower range;
such a beam is said to have a better spectral purity.
Aspectrumis a plot of light intensity versus its frequency. Spectrochemical analy- sis is based on a sensitive and selective response of a sample spectrum. Selectivity is based on the fact that atoms and molecules have their specific energy diagrams for electronic, vibrational and rotational levels; therefore, each species will interact with light at some specific wavelengths: The qualitative analysisis based on the position of signals on wavelength axis. The quantitativeanalysis, on the other hand, is related to intensity axis; which is a degree of the number of photons involved in spectro- chemical interaction, which is in turn a measure of the analyte concentration or quantity. Spectrochemical analyses are based on comparative methods; calibration plots must be used.
When light traverses through a medium, this phenomenon is called transmission;
a sample that allows the transmission of light at a certain wavelength is called trans- parent. If the light beam is not even partly transmitted, the sample is said to be opaque. During transmission, some of the photons may interact with chemical species on their path; if the energy of photons matches the difference between two energy states of these species, the energy of photon may be used to excite the parti- cle under interaction. This process is called absorption. Alternatively, some light will be reflected at the interface of air and sample surface. This reflection is at a mini- mum value for incident rays perpendicular to the interface, and is a function of the angle of incidence with the surface normal and the refractive indices of both media
at the sides of the interface. The following terms are used regarding transmission, absorption and reflection (Figure 5.3).
Transmittance T⫽P/P0 (5.3)
Percent transmittance %T⫽100(P/P0) (5.4)
Reflectance R⫽PR/P0 (5.5)
Absorbance A⫽ ⫺log T⫽log(P0/P) (5.6) A rarely used term, absorptance, is a measure of radiant energy absorbed, α⫽ [(P0⫺P⫺PR)/P0]. Ideally, in any case where monochromatic light is partially trans- mitted,R⫹α⫹T⫽1. When the incident beam is not perpendicular to the interface, refraction will take place together with the transmission; the path of light will be altered according to the Snell’s law,
n1sin θ1⫽n2sin θ2 (5.7)
where n1and n2are the refractive indices of media (1) and (2), and θ1and θ2the angle between the surface normal and the incident and the refracted beams, respectively.
Therefore, a medium with a refractive index larger than that of air will refract the oncoming beam in such a way that it will become closer to the surface normal. If, on the other hand, the beam is approaching from the denser medium, refraction occurs until a critical angle,θc, is reached for the value of θ2; at this point,θ1equals to 90°; at this point the beam is refracted along the interface. For the values of θ2exceeding θc,total internal reflectiontakes place and the incident beam is reflected back into the same medium with an angle of reflection θ2⬘⫽θ2, as shown in Figure 5.4.
In order to correct for reflection at interfaces and scattering, both at the sample container and the sample itself, absorbance Ais always measured against a proper blank, as shown in Figure 5.5. Depending on instrumentation, as will be discussed, this measurement may be performed in separate cases or simultaneously.
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Figure 5.3 Behaviour of transmitted monochromatic light through a partly absorbing sam- ple where the beam is perpendicular to all the interfaces. A, air ; B, transparent medium; P0, incident ray power; P, transmitted ray power and PR, reflected beam power
Another important kind of light-matter interaction is scattering. If a photon reaches a particle with dimensions significantly smaller than the wavelength of light, the photon is absorbed and reemitted mostly on the same axis on which the incident ray propagates. This process is called secondary emission; its half-life is about 10⫺15s. In this case, practically no net change occurs for the photon and it continues its propagation on its original axis. However, a very small part of energy is propa- gated at all angles and the energy of photon continuing on its unaltered path slightly decreases; this is called scattering. As the dimensions of the particles increase and becomes comparable to the wavelength, scattering becomes more important and reaches to a point where the particle becomes a new light source propagating the oncoming photon’s energy in all possible directions. On a reflecting surface, similar phenomenon takes place; if the imperfections on a smooth surface has dimensions much lower than the oncoming photon’s wavelength, almost total reflection takes place; angles of the incident and the reflected beams with the normal are equal; this Figure 5.4 Total internal reflection. N, normal to interface; n1, refractive index of the rare
medium n2, refractive index of the dense medium and n2⬎n1; θ2⬘ ⫽θ2
Figure 5.5 Absorbance measurement of a sample solution against a blank
is called specular reflection. If, however, the imperfections on surface are compara- ble to the wavelength or larger than it, the beam is reflected in all possible directions;
this is called diffuse reflection. Therefore, a mirror and a white paper both reflect all the visible wavelengths oncoming from a white light source; however, with the for- mer, an image is formed since specular reflection takes place, where no image is formed for the latter because of diffuse reflection. These two kinds of reflections are shown in Figure 5.6.
In practice, scattering is usually an important source of error in spectrochemical measurements; high-quality optics and an analysis environment free of dust particles will reduce this effect, proper blanks are often required to minimize the scattering errors.
Regarding the properties of electromagnetic radiation, another important phenom- enon is light polarization. A normal light source will emit photons having frequen- cies in a range; these photons will correspond to light waves propagating on different planes as viewed by the receiver. Light waves propagating on a single plane are said to be plane polarized. Plane-polarized radiation may be obtained by polarizers; these are optically transparent substances absorbing photons propagating only in one plane, allowing the radiation in the plane perpendicular to the plane of absorption to be transmitted. Phenomenon of plane polarization of light is shown in Figure 5.7; the beam in “c” is transmitted through a polarizer that selectively absorbs the components on YZplane only to obtain plane-polarized light.
Absorbance of a photon will cause the formation of excited state of the analyte species,
M⫹hν →M* (5.8)
where M and M* are the ground state and excited state of analyte species, respec- tively; in UV and VIS regions these correspond to electronic states. The excited elec- tronic state, M*, will have half-life of 10⫺8⫺10⫺9s. After the absorption of a photon, one of the following takes place:
(i) Radiationless relaxation
M*→M⫹heat (5.9)
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Figure 5.6 Diffuse (a) and specular (b) reflections. N, normal to the surface; i, incident ray and r, reflected ray
(ii) Photochemical decomposition
M*→A⫹B (5.10)
(iii) Emission of radiation
M*→M⫹hν (5.11)
or
M*→M⫹hν′ (5.12)
The last process above is called photoluminescencewhen excitation was realized by absorption of radiation. Photoluminescence is called fluorescencewhen the emission Figure 5.7 Polarization of light: (a) Several electrical vectors of radiation propagating on Z-axis perpendicular to page. (b) The resolution of the vector K in plane XY into the com- ponents of Kxand Ky. (c) The schematic resultant of all the vectors resolved as in (b), unpolarized light. (d) Light propagating in XZ plane only, plane-polarized light
is due to a transition from a singlet excited state (S1) to a singlet ground state (S0);
the half-lives are 10⫺4or shorter. On the other hand, if the species go from a singlet excited state (S1) to a triplet excited state (T1) prior to emission of a photon and relaxing to a singlet ground state (S0), this is called phosphorescence; the life times associated are 10⫺4s or longer. All of these phenomena, absorption, emission and luminescence will be discussed in more detail in the following sections. The princi- ples of absorption and emission are schematically shown in Figure 5.8. It must be noted that both emission and luminescence involve emitting of a photon from ana- lyte species. However, depending on the mode of excitation and the energy states involved, different terms are used. The term emission is reserved for the cases where the excitation is collisional in a usually high-temperature environment such as a flame, arc or plasma; the term thermal emissionis also used. Since at such high tem- peratures all molecular bonds will be virtually broken, analyte species are only atoms or atomic ions. On the other hand, luminescence methods mostly involve mol- ecules at room temperatures or at cryogenic conditions. Nevertheless, atomic fluo- rescence is also possible and is used as another spectrochemical technique for trace analysis.
Emission signal from an atomic cloud is dependent on the number of excited atoms. Population of excited-state species is related to temperature and the energy difference between two energy levels involved. This relation is given by Boltzmann equation:
N1/N0⫽(g1/g0) e⫺∆E/kT (5.13) where N1 and N0 are the number of species at excited (1) and ground (0) states, respectively; g1and g0are the associated statistical factors; ∆Ethe energy differ- ence between two levels; Tthe temperature in Kelvin and kthe Boltzmann constant that has a value of 1.38066⫻10⫺23J K⫺1. From Equation (5.13), it can be observed that the population of excited states increases with higher temperatures and decreases with increasing energy gap. Since emission signal is directly related to
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Figure 5.8 Simplified energy transition processes for absorption and emission of radiation.
(0) and (1) are the electronic energy levels. M and M* are the ground and excited states, respectively
the number of excited atoms, sensitivity increases with higher temperatures and decreases with higher energy difference between two levels or shorter wavelengths associated.
Blackbody radiation is another important characteristic of some substances related to their optical behaviour. Some solids with high melting and boiling points can be heated up to relatively high temperatures: at these elevated temperatures, these substances will glow in a continuous manner regarding light frequencies. The spectra of this continuous radiation exhibit features characteristic of temperature of the heated substance rather than its chemical properties. This radiation is called blackbody radiation. It has the following characteristics:
(i) The wavelength maximum of this radiation is inversely proportional to the absolute temperature.
(ii) The total energy emitted per unit area and time by the blackbody is propor- tional to the fourth power of temperature.
(iii) The power of emission at a given temperature is inversely proportional to the fifth power of wavelength; this behaviour is valid for the wavelengths larger than the maximum wavelength of the continuous spectrum.
Blackbody radiation is often used to obtain light sources for spectrochemical measurements. It is also important in atomic emission measurements when the hot atomization/excitation medium may emit a continuum to give a background on which the analytical lines are positioned. Some blackbody radiators and their emis- sion features are shown in Figure 5.9.
Figure 5.9 Some examples of blackbody radiation; broken line shows wavelengths at maxi- mum power versus temperature; a, Nernst glower, an infrared source at 2000 K;
b, W-lamp at 3300K; c, carbon arc at 4100 K