2 lnstrumentatlon 2.1 LIGHT SOURCES 2.1.1 Lasers The growth in the literature dealing with the application of lasers to analytical atomic spectroscopy continues to accelerate and a wider range of techniques is now being reported. An important advance has been the use of direct electrical detection of optical processes either by counting ionization events or by measurement of the ionization current, This leads to a far more efficient detection method, with single-atom monitoring being feasible through selective excitation followed by ionization and detection of the free electrons.Direct electrical detection offers many advantages, including obviating the need for an optical detection system, i.e., lenses, monochromators, filters, photomultipliers, etc. All optical laser fluorescence measurements are ultimately limited by scatter; it is claimed that electrical detection is not affected by scatter and therefore allows the full power capability of laser sources to be employed.Single-atom detection has been demonstrated by Young et al. (341) using what is termed resonance ionization spectroscopy (RZS). In this case Cs atoms were ionized by a two-photon process involving saturation of the 72P,, state by 455.5 nm photons followed by ionization by a second photon of the same energy.The Cs atoms were located in a proportional counter tube, capable of responding to single-electron events. Travis et al. (1 1 1 , 1067, 1068, 1237) have demonstrated “uptu-galvanic detection” of Na and Ba in analytical flames using a tunable laser source.The voltage drop produced between electrodes placed in a flame, when the resonance line was irradiated, was found to be linear with concen- tration over 4 orders of magnitude and yielded detection limits similar to those obtained by emission methods. The technique has also been applied to a graphite furnace atomizer (685) and it was suggested that the mechanism was optical excitation of the resonance transition followed by collisional ionization of the excited atom.Opto-galvanic detection has also been used as a means of calibrating laser wavelength and bandwidth (1603) by monitor- ing the variation in voltage drop across a standard HCL when the atomic cloud was illumin- ated by a fraction of the laser beam. Bandwidth was determined by comparison with the known line widths produced by the HCL.An alternative or perhaps additional mechanism for laser ionization has been discussed by Measures (969), who notes that the ionization produced by pumping the resonance transition of Na is unusually efficient. This author proposes that three-photon ionization occurs, creating a pool of electrons that are then accelerated by collisions of the second kind with excited Na atoms.The accelerated elec- trons have sufficient energy to ionize the excited atoms, which because of their large number behave like a pseudo-ground state of low ionization potential. The high spectral power density and narrow line output of lasers makes them obvious sources for AFS. However, the laser is not simply a super-intense conventional source, it is coherent and therefore its properties are in many ways more similar to those of a radio- or microwave beam than a light source.The combination of high intensity and coherence produces a range of effects when lasers are used for excitation that arc quite different to those encountered from conventional sources. Saturation spectroscopy i s now well known and during the past year more papers have appeared discussing its importance in analytical spectroscopy.Piepmeier et ul. (1233) have studied saturation effects in different atomizers, i.e., air/C,H, flame, Ar d.c. arc, carbon rod and laser microprobe plume. Line-width studies showed that “power broadening”, or “saturation broadening” as it is sometimes known, became evident as saturation was approached.The phenomenon of power broadening has 32Part I: Fundamentals and Instrumentation 33 been well known in microwave spectroscopy for many years, but its observation in the optical region of the spectrum is relatively recent. Power broadening occurs because the frequency of population-depopulation of the excited state is so great at or near saturation, that there is an apparent shortening of the spontaneous lifetime leading to increased spectral line-width, Sharp and Goldwasser (1 504) have presented theoretical calculations showing the power required to produce saturation under various excitation conditions; namely for homogeneous line profiles, for an inhomogeneous Doppler profile and for a completely inhomogeneous line.The latter case was included in an attempt to show the limitations of rate equations in describing high irradiance phenomena.This point has been taken further by Daily (493), who has used density-matrix formalism to show precisely the conditions under which rate equations can be used and where it is necessary to take account of the coherance of the excitation field. Winefordner et al. (810) have observed saturation effects in Na vapour using a CW dye laser and Alkemade (664) has described experiments demonstrating sane of the unusual effects associated with laser excitation, such as power broadening, two-photon absorption and intra-cavity quenching.Li (823) has investigated the effect of phase and intersection angle on the amplitude of Doppler-free two-photon absorption.The laser-excited atomic fluorescence of Pb has been studied by Bolshov et aZ. (1395). These authors showed that 40% of the Pb atoms were transferred to the metastable level within 5 ns of excitation and this process leads to an optimal excitation pulse length for maximum analytical sensitivity. New laser systems and modifications to commercial lasers are still being described and it is to be hoped that this trend will continue, with particular emphasis on producing equipment that can yield long-term reproducible performance, as required for analytical applications.Malmstadt et al. (1174, see also ARAAS, 1976, 6, Ref. 1158) have published a paper describing the design of their microprocessor controlled N,-laser-pumped tunable dye laser. The dye laser gives output from 350-650 nm with a line width of 0.06 nm and can be scanned at speeds of up to 2nms-1 with an added 5 s required each time a dye is changed.Wallenstein (16) has given details of the construction of tunable dye lasers pumped by high-power-gas and solid-state lasers. This author emphasiscs the superior spectral perform- ance of CW systems, but points out that their wavelength range is generally limited to 460-640 nm and that pulsed systems are necessary to achieve efficient frequency doubling for output in the U.V.region. The problem of Rayleigh scattering in laser excited AFS has been investigated by Yeung and Goff (1234). By the addition of a commercially available electro-optic tuning element to a CW dye laser it was possible to achieve frcquency modula- tion of the beam; lock-in detection was used to remove scatter signals.The high sensitivity obtainable by use of laser-excited AFS is demonstrated in work by Bolshov et al. (507). A frequency-doubled dye laser was used to detect sub-pg quantities of Fe and Pb atomized in a graphite tube furnace. Similarly Hohimer and Hargis (819) have determined pg quantities of Cs using ETA and direct line fluorescence detection.Gelbwachs et al. (1293, 1668) have reported detection limits in the 1-100 atoms range for Na, Pt and Ni using what they term saturated optical non-resonance emission spectroscopy (SONRES). A CW laser was used for excitation and under saturated conditions non-resonant fluxes of about 107 photons s-1 for each atom were obtained, These authors quoted detection limits of fractions of an atom; it might be considered more meaningful to report single-atom detection at a given SNR.Winefordner et d. (1420) have presented analytical data for the determination of Ba, Cu, Li, Mo, Nd, Rh, Sc, Na, U, V and Sr in either air/C,H, or N,O /C,H, flames using CW laser excitation. The limited excitation-wavelength range was offset by using lower excited states and non-resonance fluorescence processes.Other references of interest - Fluorescence excitation profile in flames: 1186.34 Analytical Atomic Spectroscopy Fluorescence of diatomic sodium: 228. Intra-cavity atomic absorption: 184, 665. Plasma inhomogeneity studied by He/Ne laser: 1312. Pulsed versus continuous wave excitation sources; 920, 989.Temperature and OH measurement by frequency-doubled CW dye laser: 1536. Tunable laser AFS: 378. 2.1.2 Continuum Sources Continuum sources have some advantages for spectroscopic analysis. They enable a con- siderable simplification of the instrumentation in that only one source has to be used and optimized, and simultaneous multi-element analysis is possible. A compact, flexible con- tinuum source operable at wavelengths from the visible to V.U.V.has been described by Norton and Wooding (1308). It consisted of a plasma produced by the ablation of a solid dielectric surface in the presence of an intense electric discharge in a vacuum of 1.3 mPa. The device operated by applying a voltage of 5-10 kV between Mo electrodes, which were mounted on a dielectric surface with a separation of 3 mm.The discharge duration could be varied between 1 and lops. Butler and Human (671) have described a pulsed flash lamp source consisting of a He-jet guided spark which was discharged on the axis of a 0.1 pF coaxial capacitor charged to 12 kV. The light pulse had a duration of 1 ps and showed an equivalent black-body temperature of 20 000 K yielding a spectral radiance of 400 W cm-2 sr-l nm-1 at 300 nm.A pulsed microwave discharge in He at pressures of about 1.5 Torr has been shown to exhibit intense emission in the V.U.V. between 30 and 70 nm (926). The He I atomic lines between 50 and 60 nm were found to dominate the emission spectrum with the He 30.38 nm line becoming evident at pressures below 1.5 Torr. The spectral characteristics and properties of a 300 W ‘Eimac’ continuum lamp have been investigated and compared with those of a conventional 150 W Xe high-pressure lamp (1183). The ‘Eimac’ lamp was found to provide up to 37 times more radiant power than the Xe lamp.However, it was noted that the SNR varied by a factor of 5 over the window of the lamp, indicating that careful consideration of its spatial properties is necessary in analytical applications (see also 1528).Tamura et al. (1049) have described a combination tungsten (halogen) lamp with a deuterium lamp, which provides continuum output between 190 and 900 nm. The application of the device as a background corrector for AAS was demonstrated, Improvements in continuum source AAS using an echelle spectrometer have been reported by O’Haver et al.(572). The improvements resulted from modifications to the spectrometer including the use of a larger order-sorting prism thereby reducing order- overlap stray light, and a refinement to the slit mounting enabling the use of narrower spectral bandpasses. Other references of interest - AAS with a continuum source: 374, 1361. Exploding wires as a continuum source: 892. 2.1.3 Hollow-Cathode and Electrodeless Discharge Lamps The established position of hollow-cathode lamps as sources for analytical spectrometry is reflected in the relatively few research papers that are now publishcd on their design and characteristics. However, some workers are still active in this field. Schearer and Rambow (553) have described a demountable, free flowing HCL in which the cathode was machined from boron nitride, lined with Ta foil, and mounted on a stainless-steel rod insulated by borosilicate glass, The cathode was mounted in a glass jacket opposite a water-cooled brassPart I : Fundamentals and Instrumentation 35 anode.The quartz window was flushed with flowing He such that the lamp pressure was maintained between 1 and 50Torr.The lamp was used as a source of resonance radiation for Ca, Ba, Zn, Mg, Sr, Yb and Eu and was operated at the unusually high current of 3 A. Mehs and Niemczyk (333) have measured the electron temperature in HCL discharges using a floating double-probe technique. The plasma electron temperature was used to discuss the operating mechanism of the source and comparisons were drawn with similarly measured temperatures in electrodeless discharges.Falk (1 393) has carried out theoretical calculations of the intensity of line emission from HCLs and has shown that these agree well with experimental rcsults. In comparing local thermodynamic equilibrium (LTE) with non-LTE sources the theory shows that line to background ratios should be higher in non- LTE sources.The determination of B isotope ratios using HCL sources has been reported by Hannaford and Lowe (1176). Three methods were described based on measurerncnt of the ratio of the intensities of the 208.89/208.96nm doublet: an accurate method using an enriched lOB source (Ne filled) with a water-cooled sputtering-cell atom reservoir, a less accurate determination using an enriched source with a N20/C,H, flame atom reservoir and an approximate technique based on a natural-boron source with the N20/C,H, flame reservoir.The eiectrodeZess discharge lamp has perhaps had an unfortunate history as a source for analytical atomic spectroscopy. Many laboratories attempted to make and use EDLs when their initial potential was realised, but owing to the lack of truly systematic study, erratic rcsults were obtained and the source fell somewhat into disrepute.In more recent times much has been done to rectify the position and EDLs at least for some elements may yet prove the most practical source for AFS. The value of tempcraturc control has been established and there is now a move toward the use of r.f. rather than microwave frequen- cies for the excitation field.This has obvious advantages in that r.f. generators of high power are easier and cheaper to construct and inductive coupling can be used rather than expensive, precision-made resonant cavities. Browner and Novak (668, 1070) have describcd the construction and operation of r.f. (35 MHz) excited lamps in both CW and pulsed mode with peak powcrs up to 1 kW.It was found that for pulsed operation source radiance increased linearity above 200W and showed no levelling off at the maximum available power of I kW. The onset of line broadening was also less severe in pulsed as compared with CW operation. A vacuum-jacketed EDL powered by an r.f. field has been designed by Baranov et al. (1331). Sources were constructed for the elements Cd, Cs, Cu, Fe, K, Na, Pb, Rb, Sb, Se, Te and T1 and yielded equivalent performance to conventional EDLs. Similar results have been reported for r.f. excited EDLs by Winefordner et at. (1418). Stephens ( I 614) has described the construction of an r.f. capacitively-coupled vapour discharge lamp, specially designed to facilitate Zeeman modulation of the emitted spectra. Parsons and Bentley (568) have continued to refine their hydride formation method for filling EDLs and have applied the “Simplex” algorithm to optimization of the construction parameters. These lamps were doped with KBr to increase the electron density, thereby facilitating Penning ionization in the lamp. Other references of interest - Line-profilc studies in EDLs: 669. Noise studies on pulsed HCLs: 1503. Studies on HCLs: 670.