I
INSTR UMENTATI0 N
Advisory Panel Jonathan W. A m y Jack W. Frazer G Phillip Hicks
Donald R. Johnson Charles E Klopfenstein Marvin Margoshes
H a r r y L. Pardue Ralph E . Thiers William F. Ulrich
Radiation Sources for Optical Spectroscopy Source technology is a vital and exciting field which continues to add new dimensions to analytical spectroscopy AUGUST HELL1 Scientific Instruments Division, Beckman Instruments, Inc. 2500 Harbor Blvd., Fullerton, Calif. 92634 PTICAL SPECTROSCOPY is one of the O o l d e -a t and most useful techniques man has yet developed for probing and mensuriiig his physical environment. Essentially, two factors are involved: a p r o p e r t y characteristic of the object of study, :inti a ?neasuring s y s t e m . The propert!-, electromagnetic radiation, either einanate,s from or nets upon the objcct. The measuring system can take ninn!. forms ranging from the human eye t o complcx photometric For convenience, spectroscopy is iimdly subdi\-ided on the basis of the nie:isuriiig systcm. ThuL?,the range of the human eye sets limits for the visible range. Other ranges extend beyond the visible, beginning with ultraviolet 011 the blue side and infrared on the red. I n this discussion, a different classification is utilized, one more closely related to the specific function of the radiation source than to spectral range. Three source types are considered: sample-exciting sample sources ; soiirces; and sample-probing sources.
Spectroscopic studies on celestial sample sources represent an ancient field, hut the past decade has brought tlram:itic cliaiiges. For tlie first time, these soiirce.5 can be observed free from the attenuating filter of the earth’s atmosphere. .iltliougli the significance of these spectral studies is mainly of interest to astronomers and other ecienti.sts, it cnii be foreseen that further improwmerits in orbiting telescopes and yectronieters will bring a deeper and more accurate understanding of our iiniverse incliiding the sun and planets in the eolnr system. In the latter case,
fly-bj. probes and actual landings will provide even better data. Perhaps more exciting is tlie opportunity to exnmine the earth from outer -pace. As a large infrared source, the cnrtli can be utilized to study neather, environmentnl changes, crop growth, and natural resources. The impact of theye developments on our future life is so important that they certninly deserve mention. Similar studies can be made on snialier objects including tlie liumnn body. Here, infrared mapping can be used for detecting abnormal growths.
Sample Sources
I n this category the object itself is tlie source of radiation measured. Various examples can be given, including celestial bodies and radiating objects in the terrestrial environment. ilnother type consists of systems in which the ,?ample is brought into a radiating state by an auxiliary energy source such as a flame, arc, RF-plasma torch, or glow discharge. Even a second light, source such as a laser can be considered here, providing it doesn’t participate in the radiation measurement p e r se (Figure
LASER
ARC/SPARK
G EN E RATED
1). ~~
~
1 Present address, Agfa Gevaert Camera Werk, 8 Munchen 90, Tegernseer Landstr. 161, West
Germany.
NATURAL
Figure 1.
Sample sources
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6 , MAY 1971
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Instrumentation
With the development of Fourier spectroscopy, such measurements may become even more precise and may be extended to objects having only weak emissions. Of greater interest to analytical chemists are sample sources energized by an external means and used for atomic studies. I n recent. years, flames have received considerable attention. Mainly this has resulted from atomic absorption work, but much of the effort has brought benefits in e m iwz 'o n as well ( I ) . The premised nitrous osideacetylene flame now commonly used for emission is even more suited with the advent of the separated flame ( 2 ) . K i t h it, a curtain of argon or nitrogen forms around a normal flame which lifts the secondary combustion zone and exposes the hotter inner flame zone. This reduces the effects of background emission by molecular species and improves detection limits for certain elements. Tlie increased interest in flames also has led to the w e of a premised oxyacetylene flame ~vhichgives benefits in improved sensitivity. .hother nuance in flame-stimulated sources i s the use of modiilnted sample feeding ( 3 ) . Sormally, the flame background radiation adds significantly to the noise level and enuses zero offset i n tlie signals. With modulated snmple feeding, the background radiation remains constant while the sample signal i s pulsed. Thus, an R C light signal, which can be processed and amplified with ac electronics, is generated. Resides flames, various other energy sources are used to create sample sources. Arc and spark sources are widely used, particularly for simultaneous multielement analyses. T'arious methods are used to introduce the sample siicli that uniform and representative distribution is achieved. Y i t h solutions, this can be achieved by means of a rotating disk electrode. With solids, analysis is more difficult because differences in melting temperatures and heats of evaporation complicate generation of a representative vapor sample. Improvements in solid
sampling hive been made by the use of lasers which produce highly localized energy, penetrating deep into the sample surface. The resulting vapor cloud is subsequently excited bj- the arc discharge. Undoubtedly, this will be s n area of continued growth as more powerful lasers become available 2nd rapid sequence firing is possible. Successive measurements c x i be visualized whereby readings can be averaged by photographs or evaluated b!. computer processing. Work is also progressing with plnsmn torches (4>5 ) . These have a flame-like appenrance but are RF-excited discharges in inert gases at atmospheric pressure. The sltmple is introduced in mist form along nith a carrier gas that sustains the discharge. Major benefits are obtained in improved sensitivity for some elements and reduced chemicnl interferences by elimination of oside.:. To date, n major limitation 1ia.s been qiienching nhicli occiirs wlien snmple qiinntities are increased as in trace nnnlysis of complex organic snmples. If this problem ia overcome, plasma torches sho~ildnttnin widespread w e in analytical work. Sample-Exciting Sources
Sources of this type are used to escite tlie snmple whereby characteristic radiation i.: re-emitted and measiired. Included ;ire various sources for moleciilnr nnd atomic fliiorexencc ns well as Raman spectroscopy. Tlie spectral chnracteristics of the primary and reemitted radiation mny not coincide: ncvertlieless, spectrd rlistribiition is important and e m be used advnntageously. I n considering soiirces for molecu1:ir fluorescence, or phosphorescence, several factors are involved. Most analytical viork is done in solution, so part of the energy of the exciting photon is converted into kinetic energy. Consequently, sharp lines are not required for excitation. However, the escitation efficiency varies with wavelength. I n fact, if the wavelength of the primary radiation is varied, a characteristic
emission spectnim often can be obtained. Two general types of sources are used in molecular fluoreacence : 1 o ~ power line soiirces and pon-erful continun (Figure 2 ) . Generally, these involve uv-visible eScitatior1. K i t h filter fluorometers, low-pressure mercury discharge lnmgs ;ire common. Usually, the full escitation and fluorescence rnngcs Lire iitilizecl, so sources of lo^ to medium inten~ities are acleqiinte. Howl-er, iiltimate sensitiT-itJ- depend$, to n large extent, iipon the rntio of snmplc fliiorexwice t o that from other ioiirre.- plus scattered primnry r:idiation. Therefore? inoclifying the .peetrill outpiit of tlie source mny be more licneficinl than incrcahing the totnl source olitpilt. To this end, grenter 1i.e ia being mndc of fliiore-wnt h n p s in nliicli the prim:ir!. radintion is modified h ~ .:I phosphor momited cithcr 11-itliin or oiitsidc the lamp. In oiic t , ~ y c n, ,serie.s of pliospliora i- arrnngcd on :I t;ilxilnr i l e e w a r o w d :I mercury lnrnp. The desired blpctrnl oiitput is bclccted 1)). simply rotnting the sleeve. K i t h hpcctrofliioronietpr,s n new f x tor i; introtliicetl, namely that tlie avail:ible energy ma!. be comidernbl!- less than n-it11 fluorometers. Thi; rc;nlts I)ccniise : The J-niiniber or apertiire of n monocliromntor is iiaiinlly .mailer tlinii thnt of :I filter photometer: or, the -1icctrnl bniicln.idth~of b o ~ hcscitatioii and fluorescence lmitls arc kept ,sm:ill to incren..c ,selectivity n n t l t o groJ.itlc high resoliition. Bccniise of tlie.se factors, higli-intensity ;oiircey are u5ed almost escliisively. Thc>c are commonly known a:: liighpressure, comlpnct :arc lnmps :ind include merciiry arc Inmps, xenon arc lxnp-, or Sc-Hg I n i i i ~ , . ~ . RespectivclJ-, TliPSc normall!. proikle strongl!. brondcned lines, broad continiin, or continun with broadened lines. During operation of high-pressure lamps, the arc is compressed in the narrow z i p lietn-cen the electrodes and become3 extremely bright. Lnmps of this type are nvnilnble from 7 5 mtt.: t o several kilon-ntts of pon-er. Spectroscopic
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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7
CONTINUOUS
*
Instrumentation
ire 3. Anodefcathode configurations of discharge lamps
sources are usually operated below one kilowatt. Although intensity increases n i t h power, benefits may not increase correspondingly because arc size becomes too large for effective use. Also, t,here is danger of sample degradation if sonrce radiation becomes too intense. A serious limit,ation of the compact arc lamp is instability which changes the illumination into the monochromator. This is caused mainly hy arc wander rather than by actual change in hrightness. Arc stability is better with dc arc lamps and these are generally nreferred to ac lamm for snectrosco& work. I n a tvnical dc lamn. the cathode is much henvier than the’anode and has cooling rings (Figure 3 ) . Sturdy const,rnction is required to withstand the intense heating caused by hombardment of positive ions. During operation, the electrode glows white and intense temperatures and pressures are generated. Special quartz envelopes are utilized to handle this harsh condition. With larger lamps, the intense uv radiation generates ozone which can create a hazardous condit,ion if proper venting is not provided. I n recent years, extensive effort has been devoted to improve de lamps, originally known as Osram types. Use of mdioactive materials near the electrodes has improved both arc stability and firing characteristics. Fnrthermore, compact, better-regulated, solidstate power supplies have become available which make such lamps more attractive for analytical work. Atomic fluorescence differs in several
respeck from molecular fluorescence and instrumental needs are somewhat unique. First of all, means must be available for generating atomic species. Flames can be used but considerable attention is being given to other techniques involving demountable hollow cathode lamps, laser-powered flash-vnporiaation cells, and other devices. A second factor rcqniring attention is thc critical wavelength match required between the exciting sonrce and absorption line. Effectively, this requires a correspondence within a fraction of an angst.rom. Obviously, n line sonrce such as a compact mercury lamp is useful only in the rare instances where atomic resonance lines coincide with the broadened source lines. A high power continuous arc lampe.g., high-pressnre Xenon lamp-would appear suited for this since it could be used for nnmerous elements of interest. Horvever, two shortcomings limit its practical value: Only a small fraction of radiation is found a t the excitation wavelength; and the ratio of scattered to fluorescent radiat,ion is high. These factors are related. As the percent of effective radiation decreases, the contribution of scattering increases. Alt.hongh monochromators can he used t,o reject scattered radiation, their spectral bandwidth is large compared to the width of the atomic fluorescence line. Therefore, an appreciable quantity of nonfluorescence radiation reaches the detector limiting analytical sensitivity. Ohvionsly, the desirable sonrce is one which gives intense radiation and is spectrally matched for the analyte of
interest. Since the excitation spectrum of a ground state atom matches its ahsorption spectrum, hollow cathode lamps can be considered. Unfortunately, the typical low-pressure hollow cathode discharge is too weak for this, so attention has been given to the development of “high-intensity” lamps and vapor discharge lamps. These are useful hut not totally satisfactory. A more attractive development is the microwave-excited discharge lamp (6, 7 ) . Although it has been available for many years, improvements were needed in terms of operating characteristics, useful life, and the number of elements covered. Significant progress is being made in this respect. Sources of this type are relatively simple devices consisting of an argonfilled, sealed quartz tube about 1.0 to 1.5 in. long which contains a small quantity of an element or its volatile salt. I n operation, the lamp is placed in a microwave cavity and subjected to microwave radiation. This generates a gas discharge which in turn vaporizes the enclosed element. When the vapor pressure is sufficiently high, a direct coupling is formed between the atomic vapor and the microwave. This creates an intense atomic line emission from tlic plasma. Consequently, the excitation radiation is strong relative to background and is selective for a givcn clement. It is conceivable that these sources will also find w e in molecular fluorescence mark for applications where intense and selective radiation is desired. They can also be used for wavelength calibration, but power input needs consideratioii to avoid excessive line broadening. The initial costs for power supply, cavity, and lamp are not excessive; in fact, they are comparable to those for high power compact arc lamps. Another area where extremely highpower sonrces are required is Raman spectroscopy. This technique measnres noncoherently scattered light which has heen modulated by energy. exchange widh molecular oscillators. This provides valnable information similar to that nfforded hy infrared spectroscopy. I n fact, the two techniques are complementary since Raman observations are possible on infrared inactive vibrations and vice versa. Until recently, Raman spectroscopy was considered source-limited. Even with high-intensity mercury discharge Inmps, measnrements were made with difficulty. The advent of lasers dramatically altered this situation and significantly improved the growth potential of the technique. A preceding article in this series discusses lasers in detail (8).
ANALYTICAL CHEMISTRY, VOL. 43,
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I Figure 4.
Sample-probing sources
Figure 5. Hydrogen discharge lamps 82A
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
Sample-Probing Sources The third category of sources pertains to those used to probe samples either through absorption or reflection effects. Included are continnous-spectriim sources used in ultraviolet, visible, and infrared spectroscopy, and the various line sources employed in atomic absorption spectroscopy (Figure 4 ) . For molecular measurements, the resolution of monochromators is normally adequate to permit continuous sonrces to be used. Generally, brightness is not a problem; in fact, extremely bright soiirces sometimes canse undue complications. Three general t,ypes of continuous sources are in common use: discharge lamps, hot-filament lamps, and ceramic or carbide types. Work in the ultraviolet region is done mainly with hydrogen or deuterium discharge I m p s (Figure 5 ) operated nnder low pressure and de conditions. Heated cathodes provide the e s s e n t d function of maintaining the discharge. The discharge has negative characteristies, so a current-regulated supply is required. A vital feature of these lamps is a mechanical aperbure between t,he cathode and the anode which constricts the discharge to a narrow path. Normally, the anode is placed close to the aperture which creates a n intensely radiating ball of light nbont 0.040 to 0.060 in. in d i m on the cathode side of t,he opening. The light is imaged a t the slit of the spectrophotometer. The unique emission of the hydrogen or deuterium lamp is generated by a power input of only 20 to 30 watts. Below 350 nm, a strong continuum is provided which fulfills most needs in the ultraviolet. With quartz envelopes, work to about 160 nm is feasible. The use of deuterium in place of hydrogen slightly increases the size of the light ball but doesn't seem to enhance brightness. Imaging of these sources at the lengthy spectrometer slit depends upon the aperture plate. I n some lamps, the plate may include a small spheric or ellipsoid reflector around n circular hole. Ot,hers may have a reflector adjacent to n square opening. Lamps of t.his type can have either directly or indirectly heated cat,hodes and typically operate at a relatively low temperature. At longer wavelengths, starting approximately a t 360 nm, the hydrogen discharge has emission lines superimposed on the continuum. These can he used as a convenient means for calibration in much the same mmner as the line spectra of low-pressure mercury lamps. However, in most work, the emission lines present a nuisance. Therefore, rout,ine measurements above 360 nm and into ihe near-infrared are
Instrumentation
usually made with hot tungsten filament lamps n-hich give continua over this rnnge. Filament 1:iriips :ire rugged, low-cost iunits siifficiently bright for nearly all work. Special broad-band-type tung.sten lmips are frequently used ns stant1:irtl sources. These c ~ be n calibrated xiti verified by the Satioiinl Bureau of Stnnd:irds. The s1i:ipe of the radiating bnnd on tliese lamps al1on.s optimum iinnging a t the slit and tliiis iiniform illiiminatioii. Spectral characteristics of the tiingeten lnmp nre basicnlly those of a hlnckbody. Therefore, mensurements w r y far from the peak waveIciigtli :ire siisceptihle to stray light effects. I n tlie region above 2 pm, blackbodyt y p e mircc-: 1%-itliout envelopes are coninioiilj. i l c e d . The same spectral c Iia r:i c t exist ic. cit ecl for the tungsten lanil) :i1)ply to tlicse ns well. The 1111cvcii ,spectral distribution is particiilarly serioii: for loiia-n-:i~-elengtli mensure111ell t SI A populnr ioiircc of this type emploj.$ iiiclirome n-ire coils. These are -iinplc aut1 rligged, init less intense than otlicr infrared soiirw'. -4 hlack oxide form< on the wire n-liich gives accept:iblc cmisqivity. Temperaturra up to 1100°C can hr renched. In one configuration, the hot coil is used to heat n ccrnmic sleei-c irliicli niodifies tlie spectr:il ontput, -till hotter, nnti therefore brighter, , m i r c c i.G the Scriist glover n-hich has n n operating temperature ns high as 1500°C. This Soiirce is generally prefcrrcd if longer nxvelengtli work is involved-c.g., to about 50 pm. Sernst gloircrs :ire constriicted from yttria,~tal~ilizcrl zirconin in the form of narrow rods. Elcctrical connectioiis arc nintle with coated plntiniim vires. -4ltlioiigli niore intensc than wire fil:imcnt hoiirccs, Sernst glowers have .-cwrnl tli.~ndvant:igc~+. The rod has a ncgntivc cocfficicnt of resignnee and Inwt he prcheatcd t o be condiictirr. Therefore, aiisilinry hwters must be providcd :I' well as n hallnqt prevent overheating. Scrnst glower,: :irc frngilc, pnrtieiilarly a t the points of clcctrical connection. During use, platinum tliffiises into thc ceramic material pnrticulnrly n-hen local overhenting or arcing o c c i r p . TILS w a k e n s the glower joint. find evcntuall>-cnnses failiirc. Crmting the joints with ceramic cement rcdiice. the evaporation of plntiniim and inrrrnses life to some cstent. T'ntil recently, nnotlier disadvantage of Sernst glon-ers has been their limitation to rlinmrters of one mm or less. In tlie far infrared, where Inrge slit n-idtlis arc rcquired to increnec energy
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
in spectrometers, such narrow rods are inadequate to illuminate the full slit. Fortunately, improved ceramic technology has led to purer, more uniform materials which can be used to construct rods nit11 diameters up to two mm. Uniformity is essential to prevent current clianneling which causes nonuniform surface temperatures. It is likely that these improvements will continue and even larger diameter sources will become available. These should have significant benefits in the far-infrared. .Another infrared source, \Tit11 characteristics intermediate b e b e e n heated wire coils and the Nernst glower, is constructed from silicon carbide. Commonly known as the globar, this source has an operating temperature near 1300°C. I t has a positive temperature coefficient, and t h w can be operated with a simpler power supply than that for the Sernst glower. More recently, a source material which has aroiised appreciable interest is molybdenum disilicate. Sources of this type can be operated to about 1450GC, thiis approaching the temperature of Sernst glowrs. The surface of the glower becomes coated with silica, providing favorable spectral characteristics. rnfortiinntely, resistiJ-ity is very low, so currents u p to SO amperes are required to heat a two-mm diam source. This neccssitntes heavy electrical leads which can get extremely hot. Formation of silica at the leads results in an increase in contact, resistnnce. I n the very far-infrared, beyond 50 pm, blackbody-type sources lose effectiveness since their radiation decreases with the 4th poner of wavelength. However, high-pressure mercury arcs give intense radiation in this region with maxima near 218 and 343 I*m. Somewhat different from the compact mercury lamps described previously, these are high-yapor pressure discharge lamps which have an extra quartz jacket t o reduce thermal loss. Output is similar to that from Hackbody sources, but additional radiation emits from a plasma which enhances the long wavelength output. I n the field of atomic absorption spectroscopy, source requirements are quite different from those for molecular studies. Typically, atomic lines have widths of the order of 0.02A; therefore, incident radiation miist be of similar or smaller width t o prevent serious loss of sensitivity and linearit>-. This requirement, is beyond the capabilities of small and moderate size monochromators equipped with continuum-type sources. Therefore, work with continuum sources has been confined to research studies and has not entered the routine analytical phase. Development of aa
Instrumentation
Figure lamp
6.
Electrodeless
discharge
spectroscopy as a major technique has occurred mainly because early workers selccted a more suitable source and procccdcd to develop practical units such as hollow cathode lamps '(9). The basic design of hollow cathode lamps is now well known. Essentia.lly, they include a cathode coiitaining the particular element to be test,ed and a noble gas a t low pressure. In operabion, the gas discharge sputters off cathode material to forri a n atomic cloud. This bccomcs excited and radiates thc atomic line spectrum of the clemcnt iii qiicstion. The lines arc normally narrower than the corresponding absorpiion lincs, so both linearity and sensitivity are achieved. A monochromator may be utilized to reject. rindesircd lincs, although it has a negligiblc effcct upon the analytical line itself. Tlic simplc onc-element lamps first used have bcen followed by multielemcnt, lamps. Tliese are constrncted by including several elements rvithin the single cathode. Gcnerally, elements are selcctcd on the basis of construction compatibility and freedom from overlapping lines. Use of intermetallic compounds helps provide uniform sputtering rates as dcsircd to avoid preferential loss of one element over others. Also, consideration is given to group combinations commonly related to each other in analytical work. Another trend in the development of hollow cathode lamps has been to increase brighiness. This is particularly desirable in situations where signal-tonoise ratios are critical. Improvement in this regard has resiiltcd from the US^ of auxiliary electrodes mounted in front of the cathode. These serve to improve atomic excitation without increasing the sputtering rate. &ill another modification has been the development of selective modnlation lamix which in some instances climinate the need for a monochro-
mator (IO). Actually, two lamps are used. The first, gcnerates a line spectrum in the conventional manner. This radiation enters a second hollow cathode lamp of the same clement. The second lamp is periodically turned on and off. As n result, its atomic vapor nets as a selective and modulating absorber for the radiation from the first lamp. If the photomet,ric system is t,uned to the frequency of modulation, it responds only to the analytical line. The dc component may increase the noise level hilt this can often he reduced by using solar blind detectors. The electrodeless discharge lamps (Figure G) discussed in connection with atomic fluorescence are also useful in aa spectroscopy work. These are particularly attractive for elements where conventional hollow cathode lamps liave proved ineffective-ie., volatile elements such as arsenic and selenium. Hoivcver, great care must be taken with rlcctrodeless discharge lamps to avoid linc broadening with the resulting loss of sensitivity. Summary
This discnssion on soiirces has pro14dcd an opportunity to describe different types of sources nnd to point out various aspects in selecting sonrces for a particnlar analytical fimction. Siiccess or failure often depends upon availability of suitable sonrces; in fact, many techniqnes became practical only a f t w significant breakthroughs werc made in this area. Atomic absorption simtroscopg is one example; Raman spect.roscopy is likely to be another. Even in relatively successful fields such as infrarrd, new soiirces could still bring significant improvements. For this reason, source technology remains n vital and exciting field which will continue to add new dimensions to
(8) R. G. Smith, ANAL. CHEM.,41 (lo), 75A IlUfi!?) (9) W. G. Joncs and A . Walsh, Speetrocham. Acta, 19, 249 (1960). (10) J. V. Sullivan and A . Walsh, A p p l . Opt., 7, 1271 (1968).
Addrarr
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S U I ( L _ _ Z I L
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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