1 Light Sources While a variety of light sources is available f o r absorption and fluorescence work t h e most important are the hollow cathode lamp and the vapour discharge lamp driven by d.c. low frequency a.c. or microwave power. The ideal light source for fluorescence should emit high intensity over t h e whole of t h e absorptiotl line-width of t h e atom. In atomic absorption t h e ideal source should b e highly stable and monochromatic emit- ting only a t t h e peak of t h e absorption line while intensity would not be a prime consideration. In practice most of t h e light sources used in atomic absorption have a narrower line width t h a n t h e absorption profile in the flame (445). Whatever type of light source is used it should be reliable and with long life.1.1 HOLLOW CATHODE LAMPS The present state of development of these lamps has been reviewed ( 1 9 5 659) and emphasis placed o n t h e improved stability and light o u t p u t achieved by careful selec- tion of filler gas pressure out-gassing and electrode design. Most manufacturers supply lamps for some 50 t o 6 0 elements. The range of multi-element lamps is much smaller a n d usually revolves around a combination of Fe Cu Mn Ni Co and Cr; Ca Mg and Na K may also b e satisfactorily combined. Falk and Lucht suggested ( 6 5 3 ) t h a t t h e essential process f o r excitation of atoms of t h e carrier gas i n t h e hollow cathode discharge is three b o d y re-combination with direct electron excitation while f o r other atoms it is direct electron excitation.A s t u d y of t h e line width from a B lamp indicated t h a t unusual emission occurred during t h e initial rapid movement of t h e sputtered particles away from t h e cathode surface (167). This led t o large Doppler widths and some wavelength shift. The effect depended markedly o n whether t h e lamp was filled with Ar or Ne. The line width of t h e Ca 422.673 nm line in a Ne-filled hollow cathode lamp varied from 0.00092 t o 0.001 5 4 nm ( 6 3 ) and corresponded t o t h e Doppler broadening expected f o r a cathode temperature o f 347 t o 429OK. The intensity of t h e o u t p u t of hollow cathode lamps can be increased b y pulsing t h e current through t h e lamp u p t o unusually high levels.Further studies of this mode have been carried o u t by Prugger e t a1 ( 7 3 ) and Kielkopf (309). The latter investigated t h e operating parameters of hollow cathode lamps operated with pulses between 3 0 0 and 1500 amps. The general behaviour of t h e lamps was similar f o r any cathode metal a n d geometry t h e intensity of emission increasing with filler gas pressure and being stable in t h e range 5 t o 50 torr. This extremely high current mode of operation has n o t been applied t o absorption o r fluorescence spectroscopy and may warrant further investigation if t h e light o u t p u t is reproducible.Prugger et a1 extended t h e medium current work of Dawson and Ellis* L'vovt and L o w e t b y scanning t h e line profiles of t h e emission from hollow cathode lamps using a Fabry-Perot interferometer. They confirmed t h a t t h e intensity of t h e emission pulse was u p to 200 times greater t h a n i n d.c. operation and that line broadening and self reversal were only observed with increased power dissipation. They found t h a t t h e corresponding t o t a l flux of radiation could b e increased u p t o 2 0 fold f o r t h e resonance lines of Al Ba and Ca. Human and * J. B. Dawson and D. J. Ellis Spec. Actu 1967,23A 565. t B. V. L'vov "Atomic Absorption Spectrochemical Analysis" Adam Hilger London (1970) p.5 8 ff. 5 R. M. Lowe Spec. Actu 1 9 6 9 2 4 B 191. 2 3 Part I Fundamentals and Instrumentation Butler (64 4 7 7 ) examined a n alternative method of increasing t h e intensity of light o u t p u t which employed t h e hollow cathode lamp as a n atomic vapour generator. This vapour was t h e n excited at 2450 MHz. The increase in resonance line intensity (for Cu Mg Ca and Ni) was between 5 and 1 0 fold b u t t h e stability o f t h e light o u t p u t was reduced b y a factor of 2.5. This technique would appear t o have no significant advantage despite considerable increase i n t h e complexity of t h e system. There have been some modest developments in t h e design of hollow cathode lamps.One of these incorporated a lens as t h e window of t h e t u b e (691). The advantage of placing a condenser lens close t o t h e hollow cathode is likely t o be marginal. Modific- ations of t h e high intensity hollow cathode lamp for use in atomic fluorescence have been described b y Lowe (74). The secondary discharge is confined t o t h e region of t h e negative glow of t h e hollow cathode discharge. Lamps have been made for Fe Ni Co Cr Ag Cu and Au and have a light o u t p u t several times greater than t h a t of t h e earlier high intensity lamps. In a survey of light sources attention was drawn (662) t o t h e heated cathode high intensity lamp (426 463).The o u t p u t of these lamps f o r Cu Fe Pb a n d Mg is a t best 10 times greater t h a n t h a t of a conventional hollow cathode lamp and t h e analytical sensitivity u p t o 20% greater. Undoubtedly t h e most expensive hollow cathode lamp available is t h e "Glomax" demountable model (Barnes Engineering Co. 3 0 Commerce Road Stamford Conn. 06902 USA). This is a continuously pumped lamp with water cooling o f t h e cathode mount t h e b o d y diameter is similar t o t h a t of conventional sealed hollow cathode lamps a n d t h e unit can be mounted in a n y atomic absorption instrument. The lamp can be operated in t h e pressure range 0.5 t o 4 0 t o r r and with currents as high as 1,000 mA t h e nominal power dissipation being u p t o 2 5 0 W.Gas flow rates are 2 t o 1 0 0 ml min-l. It seems probable t h a t t h e development of conventional hollow cathode lamps is reaching its limit. The problem of instability however remains and leads t o fluctuations of t h e order of 0.1% per second. In atomic fluorescence where t h e o u t p u t signal is directly proportional t o light source brightness this instability is n o t a serious problem b u t i n absorption it can be a serious limitation i n obtaining low detection limits. In addition t o all t h e major instrument companies suppliers of hollow cathode lamps include:- (1) Cathodeon Ltd Nuffield Road Cambridge England.(2) S. a n d J. Juniper a n d Co 7 Potter Street Harlow Essex England. See also t h e Analytical Chemistry 1971 -72 Laboratory Guide. 1.2 DISCHARGE TUBES The use of electrodeless discharge tubes (EDTs) in analytical spectroscopy has been reviewed (464 1034). Tubes are now available f o r approximately 3 0 elements. Multi- element tubes have been prepared f o r Li Na a n d K (464) and f o r Co and Ni (394). The former were used f o r atomic absorption and t h e latter f o r fluorescence. Consider- able effort is being directed a t improving t h e manufacture of tubes t o give more reproducible a n d stable light sources (109 419 695). A special s t u d y has been made of t h e manufacture of vacuum-jacketed tubes f o r U235 and U238 (462).Thompson ( 4 5 ) used a Ge EDT as a primary source for t h e low-sensitivity measurement of calcium b y absorption a t its 422.673 nm line. The Ge line was 0.016 nm from t h e Ca and there Part I Fundamentals and Instrumentation 4 was analytically useful overlap between the lines giving good linearity of calibration curves up t o 1 500 pg m1-l. To improve the stability of EDTs a special cavity has been designed (425) t o give smooth air flow over the tube for cooling purposes. The power supply for this lamp provided current stabilisation t o 0.5%. A simple circuit t o provide current stabilisation against changes in supply voltage has been described (823).A 5% fluctuation in line voltage produced only a 0.1% change in light output. A detailed study of factors affecting lamp performance has been carried out by Cook et a1 (399). They evaluated the performance of Hg Se and T1 EDTs in absorption and fluorescence analysis using four different types of %-wave resonant cavity and with varying conditions of lamp preparation modulation stability cooling cavity tuning and deterioration of cavity surfaces. Modulation of light sources is standard practice in absorption and fluore- scence spectroscopy. There are a number of advantages in effecting this modulation within the lamp.Several workers have described systems for modulating the output of EDTs (302 573 855 896). Frequencies as high as 150 KHz have been tried but Dagnall et a1 (855) found the optimum frequency t o be ca 20 KHz preferably using a square wave. Micro-wave excitation has also been used t o generate UV lines and continuum radi- ations. The spectra of low pressure rare gas (Ar Kr and Xe) lamps at a power input of 8 0 W was a continuum with superimposed lines (1 01 6). At high pressure (400 torr) and high power (1 kW) the spectrum of a Xe tube was a continuum with an intense band at 280-300 nm (18). A refillable low pressure xenon lamp has been described (752). At an input power of 30 watts the 147 nm line flux was 1.6 X 1013 photons s-l sr-l.Similar results were obtained (244) when gas mixtures including Ne Xe He N 0 were used. The 02concentration was 1013 atoms cm-3 and the flux of the 130 nm was 1014 photons s-l sr-l. The source temperature was 2700°K. The Xe arc with metallic halide additives and confined in narrow bore quartz tubes was investi- gated (1018) as a possible excitation source in fluorescent discharge tubes. The power dissipation ranged from 2 t o 30 watts per linear cm. A continuum emission was obtained if the vapour pressure of the metallic halide was high. The flux density of the 184.9 nm radiation in the low pressure Hg discharge has been investigated (433). The intensities nearly saturated at about 2 amps cm-2 which suggested that this discharge would be potentially useful as a radiation standard.The pressure range varied from t o 0.5 torr and the line width from 0.006 nm (2 mtorr) t o 0.1 nm (0.75 torr). An attempt was made t o increase the intensity of the Hg 253.7 nm radiation by decreasing self absorption (97). Introduction of a higher pressure of foreign gas (Ar) reduced self absorption but as the Ar reduced the electron energy fewer Hg atoms were excited and there was no gain in line intensity. Electrodeless discharge tubes may be obtained from (1 ) EM1 Electronics Ltd Hayes Middlesex England. (2) EDT Supplies Ltd 26 Grove Park Terrace London W 4 England. (3) Southern Spectral Sources Frimley Road Camberley Surrey England.(4) Ophthos Instrument Co 9600 Overlea Drive Rockville Md. 20850 USA. For use with conventional metal vapour discharge lamps a simple circuit has been described which allowed the discharge lamp t o be modulated in phase with a lock-in amplifier (264). The lamp was driven at half mains frequency and the polarity of the electrodes alternated in each cycle. This produced light and dark periods of equal duration. The output of a gas discharge lamp has been modulated at twice the mains 5 Part I Fundamentals and Instrumentation frequency by inserting an inductor in series with the lamp (693). A phase lag occurred and this had t o be corrected before the lamp could be used in a lock-in amplifier system.1.3 LASERS The laser is an attractive source for atomic fluorescence work as it has high intensity and narrow line widths. Unfortunately no means has yet been found of producing laser sources using the energy levels of the resonance transitions. Tunable lasers have been developed whose tuning ranges include the wavelength of some resonance lines but their emission band width is relatively broad and the freedom of tuning is limited. Capelle and Phillips (280) have developed dyelasers t o generate 5 X lob9 s pulses of l o 3 watts with a 0.3 nm band width over the wavelength region 414-642 nm. UV tunable lasers have been made using frequency doubling of the emission from a dye laser (248 649).The Rhodamine 6G dye-solution laser (245) was tuned from 571-615 nm (half power point) with a peak output of 0.41 J at 595 nm and a band width a t half maximum of 0.17 nm. Other workers (654) using Rhodamine and other dyes covered the range 340-1000 nm; the widths of tuning bands were from 20- 180 nm and the emission bandwidth was 4 X 1 O4 nm. One of the first analytical applications of laser excitation in atomic fluorescence (253) was the measurement of sodium vapour in the atmosphere (200 atoms cm-3 in a path length of 4 Km). Fluorescence was excited by a 0.2 J/pulse Rhodamine 6G/ ethanol dye laser at an altitude of 80 t o 120 Km. The detector was a photomultiplier attached t o a 45 cm diameter telescope and photon count rates of 0.03 counts per laser pulse were recorded.A theoretical study by Piepmeier (930) of the use of lasers in atomic fluorescence showed that a laser with sufficient power density and spectral band width could saturate the excited state of an element thereby almost eliminating the dependence of analyte fluorescence intensity upon the power density of the primary light source. Fraser and Winefordner (413 923) used a pulsed tunable dye- laser (10 kW peak power) pumped with an N2 laser t o excite atomic fluorescence in flames. Detection limits of 0.01 ppm t o 0.3 ppm have been obtained for elements such as Ca and Fe. An argon laser has been used to excite Cz molecular fluorescence (908).Peterson et a1 (229) found that introduction of a small amount of a weakly absorb- ing atom or molecule into a laser system could lead t o the extinction of the laser action owing t o atomic absorption. The effect was demonstrated by introducing Na and I into the resonant cavity of a dye-laser generating a continuum emission with a band width of 2 t o 10 nm. Detection limits for atoms and molecules were two orders of magnitude lower than by conventional methods. 1.4 MISCELLANEOUS LIGHT SOURCES There is a continuing need in absorption and fluorescence for a source suitable for multi-element analysis. Applications of Grimm glow discharge and "Glomax" lamps have been investigated (221 592 593) for this purpose and their operating parameters determined.They have a high intensity output but are less stable than hollow cathode lamps. However it appears that these sources may be more suitable for excitation in emission analysis (see section 2.3). An alternative approach t o the light source problem is the wall-stabilised metal vapour arc (301) of the modified Maecker type. When the spectrum of this arc was investigated using a Fabry-Perot interferometer it was found that the line widths were broader than those of the conventional hollow cathode lamp. Part I Fundamentals and Instrumentation 6 High resolution atomic absorption spectra of U were obtained b y combining flash photolysis and flash discharge (434).U14 vapour in a quartz cell was decomposed by a pulse from a Xe flash lamp and a discharge passed through t h e vapour. The emission spectrum f r o m this lamp was simpler than that from a Ne filled hollow cathode lamp. In atomic spectroscopy there has been little attempt t o produce absolute standards of intensity although t h e wider use of standard light sources would facilitate inter- laboratory comparison of emission sensitivities. A W ribbon lamp fitted with a sapphire window and operating at 39 amps has been suggested as a standard for t h e range 150-270 n m (240). The blackbody temperature was 2798OK.The count rate was 11 cps at 150 nm (corresponding t o an emission of 4.5 X l o 6 photons cm-2 s-l sr-l nm-l) and 3 X l o 4 cps at 270 nm (2.7 X 1013 photons cmh2 s-' s f ' nm-l). A ribbon W lamp operated at a known temperature and assumed emissivities has been used as a standard source f o r determining t h e relative efficiency of a monochromator and detector system (1 90). The developments in light sources which have taken place over t h e years have contributed a great deal t o t h e advancement of analytical atomic spectroscopy. Never- theless some improvement in stability a n increase in light output and some reduction in price would be welcome.