Further Readings
8.2 Instrumentation
as shown on the calibration plot; measurement of another sample aliquot diluted by a different factor can solve the question.
8.2.1 Excitation Sources
AF measurements have the high wavelength selectivity in a way similar to AAS when excitation is performed with a line source; the species may emit fluorescence within the spectral slit used will not be excited and thus will not interfere. When a continuum source is used for excitation, on the other hand, this advantage will be lost; however, multi-element detection may then be possible. Both the line and con- tinuum sources have been used for AFS. Hollow cathode lamps (HCL) result in higher fluorescence intensity if they are pulsed. Special HCLs operated with higher than normal current pulses were used. As compared to HCL devices, electrodeless discharge lamps (EDL) have higher emission power and thus cause better AFS sig- nals; however, their use is limited only to some elements and their radiational power drift with time. Regarding the continuum sources, high power EIMAC® Xe arc lamps were also used; multi-element detection is feasible with a high-resolution polychromator such as an Echelle system. Since the wavelength selectivity is a func- tion of the spectral slit, high resolution is compulsory with a continuum source. A low-cost monochromator as used in AAS is sufficient when a line source is used where the resolution is limited by the source emission linewidth.
In last few years, lasers became increasingly popular in AFS studies. Commonly, a tunable laser pumped by a more powerful laser is used for excitation. Saturation of fluorescence can be achieved with such a system. Working with saturation by lasers has several advantages. In addition to high sensitivity, AF signal becomes independ- ent of the source power P0, and also any fluctuations in its value. Despite their excel- lent optical properties, lasers could not be used in absorption spectrometry, because well-stabilized, drift-free source emissions could not be obtained from these devices.
In an AFS measurement with laser excitation, this disadvantage of lasers may be elim- inated since the fluctuations cannot be sensed by the analyte atoms which are already at saturation point. Better linearity is obtained as the ground-state atoms are limited in number, self-absorption is minimal. Low-detection limits obtained by lasers result in a large dynamic range, up to 108. The thin optical profile of the laser beam should be matched by the atomizer design; graphite furnace has been fairly popular for this purpose. Using frequency doubling by special crystals, excitation in UV is possible.
Another excitation source used is the inductively coupled plasma (ICP)into which the analyte species are aspirated. High emission radiated at specific wavelengths from ICP functions as a single- or multiple-element line source system.
8.2.2 Atomizers
An ideal atom cell should be considered together with the properties of the exciting source and the detection system; it should have the following properties.
● It should provide a high degree of atomization, 100% as ideal.
● The analyte species must have a fairly long residence time, so that a good S/N value can be obtained by exciting the same atoms using a pulsed source.
● The atomic cloud, the exciting beam and the area viewed by the detection sys- tem must have a good overlap. The atomizer design must have the appropriate dimensions for this condition to give the best AFS signals.
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● The environment of analyte species must have a composition that would cause the minimum quenching effect for analyte. The order of quenching for some common species are Ar⬍H2⬍H2O⬍N2⬍CO⬍O2⬍CO2.
● The atomizer should have a minimum background emission at analyte wavelength.
Flames were the most popular atomizers in the early stages of AFS research. When H2 is employed as fuel, the flame has low quenching properties and a low back- ground emission. Despite these useful properties, H2flames have relatively low tem- peratures and thus can atomize only certain elements. O2 is better than air as an oxidant, because N2absorbs the heat and lowers the atomizer temperature. Some AFS flame heads were designed in both laminar and cylindrical shapes often with a protective steam of Ar resulting in a separated flame, so that the adverse effects of air were eliminated. Air-C2H2and N2O–C2H2flames were also used.
ICP has been another atomizer for AFS. Since temperatures as high as 6000 K can easily be obtained in ICP, an effective atomization can be realized and chemical inter- ferences are minimized. At such high-temperature values, ionization is appreciable and fluorescence from some ions can also be observed. Light scattering interferences are also significantly lower as compared to flames. In addition, the inert Ar atmos- phere provides lower quenching as compared to flame environments. Background emission is generally higher than in flames. The AF signal is viewed above the coil, at a height of about 50 mm, where the background emission is minimal.
Some atomization techniques used in AAS, such as the hydride generation and cold vapour method for mercury can also be employed for AFS. As a matter of fact, a very sensitive commercial AFS system for Hg determination is based on cold vapour formation, as shown in Figure 8.4; this instrument can be used to determine Hg at sub ng mL⫺1range.1
Electrothermal atomizers offer several advantages for AFS measurements. The inert Ar atmosphere used in a graphite furnace provides low quenching. Atoms are
Figure 8.4 Cold vapour Hg determination using AFS (Adapted with permission from PS Analytical Ltd, Orpington, Kent, UK); ES, excitation source; FO, focusing optics;
AC, atom cell; CO, collimating optics; IF, interference filter; SB-PMT, solar-blind photomultiplier tube; SP, signal processor
confined into a rather small volume for a fairly high-residence time; therefore a successful optical design for both the excitation and collection of emission can be conveniently realized when pulsed laser excitation is used. Although there is yet no commercial instruments, laser-excited atomic fluorescence (LEAF) has found a wide acceptance in research laboratories.
Most commonly, a hole is bored on the body of a graphite cuvette so that the axes for excitation and emission can be made in a 90°geometry. Due to the small-volume confinement and the presence of particles, light scattering in graphite furnaces is a serious problem. Usually non-resonance fluorescence modes are selected so that the scattering background near the excitation wavelength is avoided. The detection lim- its obtained with LEAFS is as low as ng L⫺1or pg as absolute.