Topics in..
. Insfrumenf a f h
1
2, feature
Edited by 5. 2. LEWIN, N e w York University, New York 3, N. Y.
These articles, most of wiich are to be c u n t n ' W by pa6 nulhors, are intended to serve the readers of U~isJOURNAL bg d i n g aUnUion lo new dmelopmenls in Ute theory, design, w availabilily of chemical taboralmy instrumenlalion, or by presenting useful i'w'ghts and ezplanntions of topics lhal are of practical importance to those who use, or l w h the use of, modmn instrumenlalion and instrumental leehniques.
XXV. Instrumentation for Atomic Absorption-Part
Two
Herbert L. Kahn. Perkin-Elmer Corporation, Norwolk, Connectkut Sources for Atomic Absorption Many of the virtues of atomic absorption stem from the fact that the desired element ahsorbs only a t very narrow lines, with a half-width of the order of 0.01 A. Instrumental design is therefore greatly simplified if the emission sources emit lines no wider than this, and narrower if possible. Discharge lamps and hallow cathodes have been found to fill the requirements, and particularly the latter have been developed to a considerable level of sophistication, Other sources have been proposed and in some cases developed. I t has been suggested that a flame containing a high concentration of the element of interest would produce a simple, versatile, and inexpensive source, hut difficulty was encountered in achieving the requisite stability and freedom from background radiation. Fassel and Kniseley of Iowa State have used the bright continuum of a xenon are as a. source (7j, depending on the monochrometor to achieve a narrow wavelength range. Working with an instrument of about 0.2 1 resolution, t,hey achieved useful detection limits. Despite the promise offered by the continuous source to make possible "scanning" or multi-element atomic absorption, the severe instrumental difficulties associated with i t have thus far discouraged commercial development or analytical use. A system promised by Block as so cia be^, based an a high-resolution interferometer, may increase the usefulness of the continuous source.
Single-Element Hollow Cathodes At the present time, all t,he elements except the alkalis and possibly mercury are determined most effectively and commonly with hollow-cathode sources. Discharge lamps are best for the alkalis, though hollow-cathode lamps far them are available. For mercury, the 0 2 4 "germicidal" lamp is a popular source, C Circle No. 149 on Readen'
probably about equal to newly available hollow cathodes. A single-element hollow-cathode lamp is shown in Figure 13. The active components are a cathode made of or lined with the element of interest, generally in the shape of a. cylinder closed a t one end, about 1 em deep and 1 cm across, and a n CATHODE
\
elements and designs, from 5 to 100 me, a t 10W2-200 vo1l.s. hlanufacturers' rated maxima represent values above which the cathodes are in danger of dest,nrrtion. Below llro maximum, the lamp emission inwertses an the current is raised, impraving the signal-to-noise ratio and rednring t,he flicker in the instrument output reading. However, for many element,^ the ahsovt,ior for a given conrentrat,ion of sample is reduced as tlrc current rises over a certain value, since the emission line becomes mrnewhat self-absorbed. In self-absorption, sputtered ground slate atoms in the lamp itself shsorh the lamp ~zdiation,cawing the emission lo assume the form of Figure 14b. Recommended lamp currents represent a comprami~e between emission intensity and fiensitivity (absorption for a given ronrentrat,ion).
N E O N OR ARGpN
ANO~E Figure 13. Schematic of Hollow Cathode Lomp. The cup-like cathode is mode of or contains the element of interest. The envelope il fllled with argon or neon .t 0 low prewre.
anode which is merely a. straight metal wire. Neon or xrgan is used a t a few millimeters pressure as a filler gas. The front face of the lamp is m d e of quartz or any of several types of glass, depending an the wavelength range it must transmit and also on t h e preferenre of the manufacturer. When the current flows, metal atoms are sputtered from the cathode into the area within and in front of the cup. Collisions with the neon or argon ions cause a proportion of the metal atoms to become excited and emit their characteristic radiation. The choice of filler gas depends on the element: lead, iron, and nickel perfom far better with neon than with argon; however, neon is not suitable with some elements, mob as lithium and arsenic, because a strong neon emission line is close to the best resonance line. For many elements, there is little to choose between the two gases. Optimum lamp currents vary between
Figure 14. Resononce Line Emission of a Hollow Cathode Lomp. lo1 Normal Emisrion. (bl SelfReversed Emission, covred by excesive current, or flaws in design.
Manufacturers and Lomp Life Currently, threelamp manufacturers are of importance in North America: Westinghouse, PerkiwElmer, and the Australian firm Atomic Spectre1 Lamps Pty. (ASL). A number of other companies, inchlding Unimm and Hilger and Watts, produce hollow-cathode lamps, but these are not yet in wide use in the USA and little is known here ahout their performance. Perkin-Elmer and ASL produce lamps whose diameter is about two inches, and whose length is approximately six inches. Both rnenufwturers argue that lamp life is limited by the clean-up rate of the filler gas, and t,hxt a high lamp volume therefore co.ltributes to long life. Westinghouse, on the ot,her hand, makes considerably slimmer lamps with the claim that a larre gas volume is not required if the cathode size is reduced. Perkin-Elmer, which sells lamps of all three producers,
Seniee Card Volume 43, Number 2, February 1966
/
A103
Chemical Instrumentation guarantees them for six months or five ampere-hours (typically 500 hours). In general, there is little t o choose between the price and performance of the three makes, although a t the fast pace of lamp technology, one maker or another may produce a better lamp for a given element. Perkin-Elmer, for example, recently introduced a lamp for arsenic, the first to produce reasonable emission for an acceptable length of time, while Westinghouse makes a particularly good mercury hollow cathode. Prices for individual lamps vary between 575 and $200. Table 3 shows the lamp types suitable for the products of the different instrument manufacturers. Lamps produced during the past two years, for all but a few particularly recalcitrant elements, have shown such long life that it bas been all but impossible to gather statistical data on failure rates. For example, if one determination per minute is made on a double-beam instrument which does not expend any lamp life in warm-up, a 500-hour performance corresponds to 30,000 determinations. Until recently, the greatest problem in producing, using, or maintaining lamps was the possible presence of hydrogen within the metal cathodes. Hydrogen produces a stmng emission continuum in the ultraviolet region, and also has strong emission in the visible. The effect of its presence is to reduce the intensity of the resonance line, and to produce a pronounced flattening and bending of Lhe working curve, as shown in Figure 15. When a l m m is failine. the most com-
properly for other elements, it can be deduced that the lamp is a t fault. Particularly if the trouble occurs after the lamp has not been used far some time, it
porn 0 0
I
I
i
I
10
20
40
60
Figure 15. Effect of Background Hydrogen Emission on the Working Curve. lnrteod of orsurning (shape of the upper line), the working curve shows o lower sensitivity, has eorly curvature, and reocher an asymptote. Similar effects could b e caused by the presence of unabrorbmble lines within the possbond of the monochrommtor.
A104
/
Journal of Chemical Education
Chemical Instrumentation can sometimes be cured by operating the lamp a t maximum current overnight. I t should be noted that the permissible current varies with the lamp manufacturer and the type of instrument. Westinghouse and Perkin-Elmer lamps are rated for full-wave operation, while those of ASL are rated for an unfiltered hslf-wave supply. In practice, this means the following: Westinghouse and PerkinElmer lamps can be run a t rated current in the Jarrell-Ash and Beckman equipment and in the Perkin-Elmer >lode1 303, and a t one-half rated current in the Techtron instruments and the P-E Model 290. ASL lamps can be run at rated current in the Teehtron and the 290, and at. twice their rated current in the other equipment. Vapor Discharge Lamps
Vapor discharge lamps produce emission by passing an electric current through a vapor composed s t least partly of the element of interest. Such lamps are produced by Osrnm in Germany and Philips in Holland, The Osmm lamps, which have iound far more use in atomic absorption than have the Philips, are available in North America from a number of suppliers, including Perkin-Elmer and Edmund Scientific. Osram lamps exist for the elements mercury, thallium, zinc, cadmium, and the alkalis. Osram lamps for mercury cannot he
used in atomic absorption, because they contain vapor a t so high a pressure that the emission line is almost completely self-absorbed, and the sensitivity is very small. It is also generally agreed that hollow-osthode lamps for thallium, zinc and cadmium are superior to the discharge lamps. However, for sodium, potassium, cesium and rubidium, the Osram lamps are superior, despite the fact that they are more troublesome to use. Osmm lamps require special mounts, and a special power supply capable of delivering a l-ampere current a t a starting voltage of about 300 volts and a running vollage of about 50. Furthermore, even in double-beam equipment, they require a wait of about ten minutes for warm-up, while the time in single-beam instruments is in excess of a half hour. Perkin-Elmer and Techtron provide Osram lamp mounts in their equipment, while Jarrell-Ash does not. There appears to be no reference to vapor discharge lamps in the literature from Beekman. For the alkalis, Osram lamps are about 100 times brighter than hollow-cathode lamps. This is particularly important for potassium, rubidium, and cesium, which have their most sensitive resonance lines in the near infrared spectral region, where monochromator efficiency and detector sensitivity are a t their lowest. The brightness of Osram lamps is also important when ultimate detection limits and high precision are required. For (Continued on page A106)
Chemical Instrumentation sodium and potassium, it is often desirahle to work a t t h e secondary resonance lines, which are far less sermitive than tho primary lines, and thereby minimize the need for dilution of a roncent p i e d sample. The p o n d a r y linw (3302 A far sodium, 4044 A for potassium) are readily usable with the Orrnm lamps, but bare1.v detectable with the hollow eathudes. This sertion should ,lot e h s e without mentioning that some workers take Ihe view that t h e alkalis should he &let.milled with flame emission, rather than hy atomic nbsorption. Ilowever, the author believes that ntomir absorption is capable of hetter precision and greater freedom from interferences than emiwion, if the equipment is well designed. This is take!) up i l l more detail later on.
Multi-Element Lamps The thought of building several elements into one cathode occurred t o workers in atomic absorption almost immediately after equipment became available. Init,ial efforts were defeated mainly by the metallurgical naivete of t h e experiments. I h r i n g the past year, however, multi-element hollow-cathode lamps hersme available from Westinghuuse and f m m Perkin-Elmer, and have been promised soon by ASL. Ilepending on the metals involved, t h e cathodes w e made from alloys, inlermetallic compounds, or mixtures of powders sintered together. The importance and value of multielement lamps should he appraised realistically and not over-estimated. Many presently available romhinations can be used without disadvantage as compared to singleelement lamps, and can usually he ohtained at, n financial saving. O l l w ronihinations present problems due tr, liue selection and potential spertral inlerfercnees, but may nevertheless be worth having if the limitations are understood. All commercial atomic ahsorption equipmen1 oll'ers easy iut,errhange of lamps, usually in some sort of pre-focused mount. 11)single-beam instrumenl.s, where warmup lime delays are appreciable, a multielemen1 lamp is stlractive herause all its elcmmts :we ready when one is. In douhle-1,earn equipmalt, lamp warm-up time is not a factor. Here, t,he charm uf multi-element lamps is ror~fi~led to their lower cost, and t h e smaller requirement for storage. Cerlain multi-element lamps are exmllcnt bsrgai,tx. Tho standard PerkinElmer chmmium and manganese lamps, fur example, are boLh made with strong admixtures of copper, so cmh of them i a w goad n source of copper rudiatiou as is the rtaudnrd copper lamp. Recenlly, PerkinElmer doveloped s. zinc lamp with much hfighter and purer radiation than had been previously availnhl-with a csthode of an intermetallic rwnpound between zinc and mlrium. The new stmdard zinc lamp is thus perfectly ussllle for ralvinm. One of the earliest Westinghouse multi-element sources works well fiw calrium, msgneqium, and aluminum. (Conlinurd on page .4108)
A 106
/
Journal of Chemical Educdion
Chemical Instrumentation On the other hand, the Perkin-Elmer copper - chromium- cobalt-nickel-manganese-iron lamp falls into the limitedusefulness category. Its camplici~ted emission spectrum (a portion of which is shown in Figure 16) makes it somewhat diflicult to find t.he resonance lines. Furthermore, the energy is reduced and the detection limits are poorer than with
single-element lamps, especially for cobalt and iron. A similar problem exists for the Westinghouse zinc-copper-lead-silver lamp, in whieh the most s p i t i v e resonance line for lead (2170 A) cannot be used due to strong neighboring copper lines, and whieh gives relatively weak emission a t the other lead line (2833 A).
High-Brightness Lamps The emission intensity of ordinary hallow-cathode lamps is limited by the fact that only a fraction of the metallic atoms sputtered off the cathode are excited. In most analyses, a higher emission intensity would be very welcome, since it leads to a. better signal-to-noise ratio and thus to better precision m d detection limits. The tin determination in Figure 4 is a particulxrly poignant example of the effect of a weak lamp. Within the limits of the conventional design, manufacturers have made remarkable advances. Lead, iron, nickel, and zinc hollow-cathode Lamps are all far brighter than they were two years ago. For tin, Perkin-Elmer has recently made available a lamp containing tin in a swaged cup made of titanium, in which it is possible to obtain high emission by running the tin in its molten state. However, it was again a t the Australian CSIRO Laboratories that a definitive advance in lamp design was made (8). I n the "high-intensity" lamp design, shown schematically in Figure 17, a pair of auxiliary electrodes is mount,ed across and slightly in front of the cathode. A CATHODE
AUXILIARY ELECTRODES
/
ANODE
Figure 17. Schematic of High-Intensity Lamp. Auxiliary electrodes, supplied with high current a t o relatively low vdtage, =owethsflller gar to excite o lorge proportion of the sputtered m e l d otom~
Figure 16. Emission of Perkin-Elmer 6-Element Lamp Around Iron ResononceLine. A very strong Cv I1 line is shown only 2.5A owoy. This causes problems with iron determination when the sixelement lamp ir used. The icon war made with the Perkin-Elmer Model 303, equipped with emission occerrory.
A 108
/
journal of Chemical Education
direct current of several hundred millamps a t a low voltage flows between the auxiliary electrodes. A stream of ionized gas atoms is produced, whieh collides with the sputtered metal atoms and eacites them. A 10 to 100-fold increase in radiation intensity results. At the same time, metallic ion radiation is actually suppresed, so that for many elements the working curve becomes straighter and more sensitive. In Figure 18 are shown the emission spectra of an ordinary nickel lamp and a high-intensity lamp. In the ordinary lamp, a strong ion line a t 2319.76 A is almost superimposed on the resonance line at 2320.03 A, and cannot be resolved even by a good monochromator. The ion line does not absorb, so that the working curve (Fig. 19) is httened and bent. (Continued on page AIIO)
Chemicul Instrumentation There is another ion line a t 2316 A. The ion lines sre suppressed in the highintensity lamp while the resonance line is enhanced. A substantial improvement can he noted in the working curve. High-intensity lamps are very useful for many metals, particularly those wit,h eomplex emission spectra, and t,hose which must be determined with bright flames, like vanadium and titanium. The lamps require an rtuxilislly power supply for the secondary electrodes. The high-intensity lamps are being made in pilot quantities hy ASL and by Perkin-Elmer. Perkin-Elmer has made successful lamps for nickel, vanadium, tin, and tungsten, while ASL produces at
least nickel, iron, and cobalt in the highintensity version. The lamps of both makers are in their standard size.
Interchange of Lamps The diierent manufacturers offer varying systems for changing one lamp for another, all employing kinematic mounts. All are about equdly convenient for the rapid installation of a lamp in proper focus. Since hollow-cathode lamps do not grow uncomfortably warm to the touch, there is no problem in exchanging them by hand. 111 the Beekman system, three lamps in kinematic, "universal" mounts can be warmed up simultaneously in the source space. Without being removed from their mounts, they can he manually hterchanged.
The Jarrell-Ash 82-362 includes a. rotating turret which houses six lamps. As t,he t,urret is rotated, one lamp after another is placed a t the source position. No warm-up supply is included, however. As a lamp reaches the source position, time must be allowed for i t to reach steady emission. Perkin-Elmer mounts its lamps in prealigned holders, whirh are inserted in the source position as desired. Since lamps for different elements require different currents, the Perkin-Elmer instruments provide several current ranges, proteebing lamps from accidental burn-out (Fig. 20). In the double-beam Model 303, a warm-up supply is not required; with the Model 290, a n external %lamp supply is offered a? a n accessory. The Techtron AA-3 also provides kinematic mounts. The lamp supply is capable of warming up six lamps simultaneously.
Monochromators a n d Detectors
F i g ~ r e18. Em'sion Spectro of Perkin.Elmer hickel Lamps. lo1 Ordinory omp ihows presence of very strong ion line ov 2316.03.1, Io) Hign intenr'ty lamp . h o w ltrong ennonrement for g r o m o stove line a t 2320.03.i.. ond srpprellion for ihc ion ine of 2316.03.1.
O9
t
P
Hi BriQhtnelE Lamp 2 5 m A . 1 7 5 mA oux.
0 Ordinary 2 5 mA
H.C. Lamp
ppm NICKEL 1 , 20
0'
1 , 40
1 1 60
1 1 80
1 1 100
1 1 1 1 120 140
l 1 160
1 1 1 180 200
Figure 19. Working Curves for Nickel. The lower curve w a r achieved with on ordinary lornp. upper curve reprerenhthe result3 from o high-intensity lamp.
A1 10
/
Journal o f Chemical Education
The
The wavelength range of atomie sbsorption is the same as that of flame emission and of ultraviolet spectrophotometry; as a result, i t is possible to work successfully with atomic absorpt,ion accessories to equipment originally designed for other purposes. However, in many respects the requirements of atomic absorpt,ion are different. The specifications for bandpass, for example, are easy to write. The monochromator must he able to pass the resonance line, and yet screen out the nearest non-resonant lines, in order to permit s. straight working curve. Among the elements whose resanance lines are most closely surrounded by other lines are iron, nickel, and cobalt, and these require a monochromator of about 2 A bandpass. A larger bandpass will cause the ahsorbanee curve to flatten and bend; s. smaller handpsss will produce a reduction in the available light and therefore a degradation of the signal-to-noise ratio. The large majority of elements can be determined at slit widths corresponding to bandpasses of 7 to 40 A. As the slits are widened, more light is passed through the munochromator, and this e m be translated into a n improvement in precision and detection limit. A good monochromator for atomic absorption should therefore be capable of being operated a t wide slit widths, and have as high a light transmission as possible. The useful wavelength range of atomic absorgtion currently runs between the 1037 A arsenic line and the 8521 A cesium line. The monochromator-detector combination should have good energy over this range. To achieve the above goals with reasonable economy, certain features required of monochromators for different purposes can be sacrificed. A first-rate UV speetrophotometer, for example, has a w a v e length accuracy of about I 1,and a stmy light level of O.OOOIYo. I n atomie absorption the wavelength can be "tuned in," so that a 10 A wavelength accuracy i~ sufficient. Furthermore, stray light levels of perhaps 1% can be tolerated. I n flame emission, spectral interferences are (('ontimed on page A l l d )
Chemical Instrumentation minimized by improved r e d u t i o n , and commercial flame emission manochromstors, such ss t h e Jarrell-Ash 0.5 meter unit, havp resolution specifications as law as 0.2 A. In atomic absorption, a bsndpsss as low a s this is not needed. (However
wheu the same inacrument is required to do atomic absorption and flame emission, the low bandpbss capability is of course desirable.)
ENTRANCE ,'SLIT GRATING
Design Differences Between Instruments The two most popular monochromator designs are the Littrow and t h e Ebert-
SLIT
M2
-GRATING
LITTROW Figure 21. Simplified Schematics of Mono. chromator Designs. la) Ebert-Fastie. ~ i n o , modification of this is known as Czerny-Turner. (b) littrow.
LAMP
Figure 20. Simplified Schemotis of Perkin-Elmer Lamp Supply. intended to prevent accidental burnout of lamps.
A1 12 / Journal o f Chemical Education
The different re4stonce ranger ore
Fastie, shown sehematicltlly in Figure 21. T h e merits of the two sydems have heel, compared lor many years; lor atomic absorption purposes, there seems to be little to choose between them. The available energy passing through a. system is the product of several variables; including grating efficiency, the dispersion of t h e monochromator, and t h e cathode efficiency of t h e photomultiplier detector. T h e dispersion is generally expressed in Angstroms/mm; the lower the number, the better t h e dispersion. As the number becomes lower, i t is possible to achieve a. given bandpass with wider slits, thus admitting more light. The light transmission of a monochromat,ar also varies (Continued on page A 214)
Chemical Instrumentation dilwlly with the sulfare awn a prism orgrnli~>g, and with focal lenglh. 111 phr~tnmultipliers, the rhoire is p e ~ w : ~ l llxtween y the Hi-0-Ag and Cs-Sh iypes of cathode. The popular and widely wed RCA IP2X has a, Cs-81, : t h l : llre more expensive Hi-0-Ag malerial Itas ndvanlages ill ilto far UV (imenir a d selenium), and even greater ndvatrlagei in t h e red (potnssium, ruI,idium nud rt:sinm). At the potassium wnvelength (7665 A) the Bi-0-Ag surface has perhaps i e ! ~limes the e f i c i e n a y ~ the f Cs-Sb. Hel.ween 5200 and 5800 A, the two surfaces are nhoul equal. 113 Table 4, a numerical mmpariso~lof some of (,he published chnlncteristics of vnriuus instrumenls is give!). Some of the more important points we listed I~BIow. Ijerkman DB: 260-mm Littroy prism Resolution is 5 A iu the rnr,~~or,h~.r,ma~or. 1:V aud 1.5 A i l l the visible rsuge. 1P2X pltotomulliplier. Wavelength range iis given as 2050-7000 A. Bwkmnn 1)TJ-2: 500-mm Littrow pdirm m~moeh~x,malor.Resolution is 3 A in the UY and 10 j. it1 t h e visihle range. AC-line operated version includes IP'28, plus red-sensitive phofotnhe. Wavelength range is given as 1900-10000 A. N : Older D l & map have a much m w e restricted wavelength range, depending on age, optics, and deterlor.) J:LIPPII-AZ~R2-362 or 6 XU-mm
A1 14
/
Journal o f Chemical Education
Table 3.
Usability of Various Hollow-Cathodes with Instruments from Different Manufacturers 1J.whle in Berkmn
Lamp Typc Westinpho~~se ASL Perkin-Elmel.
Yes
YP;
Yes, with sdapl.er
Usable in Jarrell-A-h Staldard
No No
graling Eh@ monorhromator. Resolulion is 0.2 A. Standard detector is 1P3X. To extend the wavelength range toward t,he red and the TJY, i t is possible t o remove the standard grating and deterlor in order Lo substitute ot,hers. Slit assemblies u p to z bandpass of 1.6 1 ran be i~nsertedin the interest,^ of higher energy. Grating has a ruled surface 52 X .5? mm. lleciprorsl linrar dispersion is 16 A/mrn. Perkin-Elmer Mrdel 290: 267-mm grrtt,ing Liltrow mrpochromnlor. Lowest $t width gives 2 A resolut.ion. i and 20 A ilks are also supplied. Standard detwtor is 1P2X wil,h c.zthnde modified by HCA to provide improved response in the red region. Grating has ruled surface 64 X 64 mm. Reriproeal linear dispersion is 16 A/mm. Wavelenglh range is given as 2000-Xi00 A. Perkin-Elmer Model 303: 400-mm grating Ebcrt monachromntor. Nominal resolutinu a t lowesf slit widt,h is 0.2 A i n 1 . Ilesolutian is variable by swil.rh selet.l.ion froynominal 0.2 t o 20 A in the TJV, 0.4 to 40 A in the VIS region. Stand-
Usable in Perkin-Elmer
Yes, with adnpi.er alpplied hy \I,. or P-E. Yes
Yw
Usable in Techtnm
Nu Standard N
