R e v i e w A r t i c l e

Recent Trends in Analytical Atomic Absorption Spectroscopy

Walter Slavin and Sabina Slavin

Perkin-Elmer Corporation, Norwalk, Connecticut

(Received 18 April 1969; revision received 18 June 1969)

Atomic absorption spectroscopy is reviewed especially as to the effect of developments in the past :few years on analytical applications. Sources and sampling devices are given s p e c i a l attention. Atonlic absorption, flame emission, and atomic fluorescence are compared as to their detection limits and their applicability to analysis. It is shown that the three methods are in many ways complementary rather than competing. The potentialities of resonance spectroscopy are examined and instrument systems utilizing resonance methods are described. INDEX HEADINGS: Atomic absorption spectroscopy; Hollow cathode lamps; Air-acetylene

flames; Nitrous oxide-acetylene flames; rf-exeited lamps; Atomic r e s o - n a n c e radiation; Atomic fluorescence.

INTRODUCTION

In March, 1969, the Royal Society of London hon- ored Alan Walsh for his great contribution to ana- lytical chemistry and spectroscopy by electing him a Fellow. In 1953, he founded the technique of atomic absorption spectroscopy and the instrumental ap- proach he proposed in 19551 is still essentially the same technique that is in general use today. He con- tinues to the present day as monitor of the expansion of atomic absorption, chiding those who would claim too much, exhorting those who would aspire to too little.

Testimony to the extent to which atomic absorp- tion is established in analytical chemistry is found in the more than 10 000 atomic absorption spectre- photometers in use throughout the world in this year of 1969. 2 Our cumulative bibliography a indicated al- most 1200 publications on the subject by the end of 1968.

I f this review is extended to developments within the past five or six years, it will exclude only that period when some 100 instruments were in use in the world and about that many research papers had been published. Therefore, we will at tempt in this review to establish the state of the ar t at this time and fill in some detail of the recent developments in the past few years. For early work we refer the reader to several books and general reviews. The first book on the subject was by Elwell and Gidley. I t was revised in 1966. 4 Robinson, ~ Ramirez-~CIufioz, 6 and Slavin 7 have published books more recently. Kahn s has a long chapter devoted to atomic absorp- tion instrumentation and technique. The Ju ly 1968, issue of Applied Optics contains some 15 articles on newer developments in atomic absorption instru-

mental techniques. In this review we have assumed that the book by Slavin has reported the instru- mentation, technique, and applications of atomic ab- sorption through about the middle of 1967.

I. INSTRUMENTATION

An atomic absorption instrument consists of (1) a source of the desired radiation, (2) a device for converting the sample into an atomic vapor, (3) some procedure for separating the absorbable radia- tion from extraneous radiation, (4) transduction of the light signal to an electrical signal, and (5) a means to read out or record this signal. Ins t rument companies have devoted considerable attention to the last three segments of the instrumentation but most of the body of scientific publication in atomic ab- sorption technology has been devoted only to the source of radiation and the means for converting the sample to an atomic vapor.

A. Sources

In the early 1960's the chief problem with prac- tical atomic absorption methods was associated with the difficulty of making useful light sources for many elements. Walsh proposed the use of hollow cathode lamps in his original publication and, with his col- leagues, described how they could be built by the energetic chemist. Most of the early atomic absorp- tion spectrochemists were forced to become amateur glass workers and much literature space was devoted to the practice of making lamps. By now the situation has completely reversed and several commercial sup- pliers provide a complete line of hollow cathode lamps almost all of which are as bright for each

Volume 23, Number 5, 1969 APPLIED SPECTROSCOPY 421

element as an atomic absorption speetrophotometer can utilize. Since new sources are being developed, and considerable claims are being made for them, it is important to understand why the spectroscopist expects little performance improvement f rom these sources--at least in their application to conventional atomic absorption, as distinct from atomic fluores- cence.

The measurement of the atomic concentration is made by measuring the reduction of the light in- tensity when the sample atoms are introduced into the light beam. A brighter light source will improve the signal-to-noise ratio of the measurement, which is useful in several ways. I t will improve the precision of the determination and, using instrumental scale expansion techniques, the improved signal-to-noise ratio can provide a larger signal for small concen- trat ions of metal in the beam, thus improving the analytical detection limit. However, in both cases the improvement does not increase indefinitely and is eventually limited by other instrumental character- istics other than by light intensity.

Let us first consider the case of detection limits. I f the flame burns with acetylene or hydrogen as the fuel, there are very wide regions of the spectrum where the flame will be almost totally t ransparent when no analyte is being burned. In such cases the stability of the baseline is limited by instrumental conditions. ( I t is in this very important instance that the double-beam photometric technique is most help- ful) . In the detection-limited situation, a very small amount of absorbing material is present in the light beam and the effect of flame instability is usually negligible (in a well-designed burner) . TO improve the detection limit, the instrumental scale expansion is increased as much as the available signal-to-noise ratio of the light source will permit, until the limita- tion of other parts of the instrumental system is reached. In practice, instrument manufacturers find that this instrumental limit varies f rom about 10- to 100-times scale expansion, or f rom 0.1%-0.02% absorption (0.00008 absorbance), depending upon the quality of the photometric system.

Our current experience with a double-beam instru- ment is that, at wavelengths where the flame is trans- parent, the minimum useful absorbance (to improve the detection limit) is not improved by increasing the lamp intensity using modern lamps for almost all elements. The same is t rue in single-beam instrumen- tation except that some lamps show a tendency to dr i f t and this limits scale expansion in certain cases. However, the tendency to dr i f t should not be con- fused with lamp intensity because one cannot be t raded for the other; a brighter lamp with more dr i f t produces a poorer detection limit on a single- beam instrument.

At certain wavelengths the flame absorbs a portion of the resonance radiation. In these cases any insta- bility in the flow of fuel or oxidant gases will alter the absorption and produce noise, even when no ana-

lyte is being burned. This occurs at the extreme of the usable ultraviolet, near 2000 A. Thus for arsenic (1937 A), selenium (1960 2~), zinc (2139 /~), and lead (2170 A) the detection limit is set by the sta- bility of the background absorption. Br ighter lamps do not improve the scale expansion that can be used but a less absorbing flame will often reduce the back- ground noise.

At certain wavelengths the flame is very emissive, although the absorption is negligible. Some workers believe that the demodulation system of certain ac atomic absorption speetrophotometers responds to a component of this emission signal. Our experience with our own instruments has never confirmed this. However, we have explained in detail elsewhere 9 that the flame emission signal produces noise at the de- tector. This is reduced by increasing the lamp bright- ness and /o r reducing the spectral slit width of the monochromator to increase the ratio of lamp bright- ness to the flame emission. This situation is particu- lar ly severe for some of the nitrous oxide-acetylene determinations but, by now, lamps for most of these elements are so bright that flame emission no longer provides the limiting noise. Usable scale expansions are just as large for most of the re f rac tory metals as for those that utilize the air-acetylene flame. Ex- ceptions to this still occur for a few of the rare-earth elements.

Thus little is to be hoped for f rom brighter lamps for conventional atomic absorption. Atomic fluores- cence spectroscopy is a different mat ter since atomic fluorescence detection limits are usually l inearly re- lated to the intensity of the light source.

B. rf-Excited Lamps The groups at Imperial College, the University of

Flor ida and the National Bureau of Standards have been describing the use of metal lamps to which power is coupled through an r f or microwave field. These lamps are disarmingly simple. They consist of a narrow quartz tube into which is sealed a small amount of the metal of interest (sometimes as the halide) and a low pressure of an iner t gas. Their con- struction has been described by several authors. 1°,11 Usually, power is provided by using a commercially available dia thermy microwave generator with a coupling microwave cavity. The group at Imperial College has pointed out the advantage of modulating the higher-frequency energy at a low frequency to which the detector is tuned. This provides the advan- tages of ac atomic absorption to atomic fluorescence.

Perhaps the published l i terature has a tendency to oversimplify the problems associated with rf-ex- cited lamps. I t is not difficult to make lamps which provide an increase in intensity over the best hollow cathode lamps of between 10× and 100×, and some workers claim even better ratios. Our experience (un- published) over a period of about a year seems to be similar to that of the other workers. We ob- served several practical problems that we have not

422 Volume 23, Number 5, 1969

yet learned how to control although our confidence is high that we will eventually get the technique under better control. An impor tan t difficulty is tha t many of the lamps produced are very unstable with time. Some lamps will operate for short periods of time in a stable manner but will be erratic at other times. Rains 12 has put the lamp and cavity inside a heated chamber to control the effect of thermal varia- tions on the intensi ty of the lamp. The adjus tment of some lamps in some cavities is very critical and it is difficult to reproduce the same performance when a lamp is replaced in a cavity in which it had earlier worked in a sat isfactory manner. For some elements the lamps are relat ively easy to make; cadmium is one such element. F igure 1 shows that the self ab- sorption is no greater in a microwave-excited lamp that was about 50× brighter than a modern Intensi- tron T M hollow-cathode lamp for cadmium. The cri- terion for estimating the effect of self-absorption is the observation of the slope of the working curve. I t is well known tha t as self-absorption is increased (e.g., by raising the lamp current in a cadmium hollow-cathode lamp or by increasing the microwave power in an rf-excited lamp) the sensitivity of the determination decreases.

Certainly one of the impor tan t fields of current development is the taming of the rf-excited lamp and the uti l i ty of atomic fluorescence probably depends upon the success of such research.

C. Flame and Continuum Sources

Alkemade 's classical work in atomic absorption TM

used as the light source a flame burning large con- centrations of metal. Many of us have since used the flame because it provides an oppor tuni ty to deter- mine metals for which no lamp happens to be avail- able. Rann 1' has recently repor ted a very thorough study of the flame as a spectral source for atomic ab- sorption spectroscopy. He shows, both theoretically and experimentally, tha t the sensitivity (slope of the analytical curve) is about half tha t f rom a hollow cathode lamp.

Svoboda 15 has proposed an ingenious modification of the continuum source used earlier by several workers but never very popular for analytical pur- poses. He suggested that an oscillating F a b r y - P e r o t interferometer be added in series with the ac-conti- nuum source. By suitable demodulation of the result- ing signals the sensitivity of conventional atomic ab- sorption can be obtained while correcting for non- specific absorption. Unfor tuna te ly it is not s imple to provide the appropr ia te modulation of the F a b r y - Perot interferometer. The paper was a proposal and apparen t ly the experimental embodiment has not yet been completed.

D. Sampling

Almost all atomic absorption measurements are made on samples in solution which are aspirated into

0 .4

(I=50) EDT-IOW

H C L - 6 m o .

0 .3

I = 2 0 0 ) : D T - 3 0 W

0.2 co

I = 2 0 0 0 ) : D T - 4 0 W

( I = 2 0 ) H C L - I O m a

0.1

OUt r I I I I I 1.0 2.0 3.0 4 .0 5.0

ppm CADMIUM

FIG. 1. Analytical curves for cadmium using hollow-cathode lamps (ItCL) and elcctrodeless-diseharge tubes (EDT). For equal sensitivity (a measure of line broadening in the source), the electrodeless discharge tube is 50×-100× brighter than the hollow-cathode lamp.

a conventional flame. The advantages of this arrange- ment are very impor tan t :

(1) The precision available is very great, 0.2% of the amount of metal present is achievable. 16

(2) The equipment is simple, inexpensive, and easily handled.

(3) There is remarkably little memory, and it is easy to analyze, successively, solutions which va ry in concentration by 100 000 times.

(4) Interferences, while present, are remarkably few and are generally controllable.

However, there are residual problems and limita- tions to a flame and these have sparked a large num- ber of research programs of considerable interest :

I. Sensitivity

The sensitivity is not as great as is sometimes re- quired. Fo r this reason, some workers have tr ied to avoid the inefficient waste of 90% of the solution in the premix burners. One obvious method of improv- ing the efficiency of the burner is to use an ultrasonic nebulizer. This device will convert a larger fract ion of the solution to the small droplets utilized in the premix burner. Most workers who have a t tempted to adapt the ultrasonic nebulizer to atomic absorption have found tha t a smaller quant i ty of sample will produce a given signal (an improvement in effi- ciency) but that this is difficult to convert into a real improvement in detection limits. Hoare, Mostyn, and Newland 17 directly compared an ultrasonic nebulizer

APPLIED SPECTROSCOPY 423

of the West and Hume ~s design with a conventional pneumatic nebulizer on a P e r k i n - E l m e r model 303 spectrophotometer. They found tha t about one-fifth the sample-uptake rate produced about the same absorption signal for some 15 elements. However, it was not possible to increase the uptake rate of the ultrasonic nebulizer to take advantage of this im- provement in efficiency. They found that the weighted mean droplet size of the aerosol reaching the flame was about 4-5 t~ with both nebulizers, indicating tha t the baffle a r rangement on the premix burner was very efficient in removing larger droplets pr ior to emergence into the flame. In addition they found tha t the signal was less stable when the ul t ra- sonic nebulizer w a s used.

We, and several other laboratories, confirm the general findings of the Hoare paper and concur that the l imitations of the ultrasonic nebulizer for atomic absorption appear to be engineering problems. Spitz and U n y 19 found tha t an ultrasonic nebulizer im- proved the sensitivity about ten times for a group of seven elements. S tupar and Dawson 2° a t tempted to adapt an ultrasonic-nebulizer design which was more convenient to change f rom sample to sample. Never- theless, the usefulness of the ultrasonic nebulizers has remained remarkab ly disappointing.

Another approach to avoiding the inefficiency of the premix burner is to heat the aerosol to remove the solvent pr ior to its emergence into the flame. Many workers have gained a factor of two in sensi- t ivi ty by preheat ing the air used to aspirate the sample. Hell et al. 2~ and Venghiat t is 2: have provided heat ing devices between the nebulizer and the burner head and have removed the evaporated solvent by passing the dried particles and solvent vapor over a condensing coil before mixing with the acetylene. The expected gain in sensitivity of about 10x was found and Venghiat t is showed tha t in his design the detection l imit also was improved by about ten times since the signals were stable and no noisier than the signals in the " c o l d " mode of operation. Somewhat more memory is experienced with these heated chain- ber burners and they are not tolerant of the high solids content of the sample as are conventional premix burners.

I t has occurred to others to take more than fleeting analytical advantage of the atoms as they fly across the optical beam by observing along the axis of flow of the atoms. This approach has been pioneered by Fuwa, ~8 who pointed the flame f rom a burner down a ceramic tube, but has been used by many workers for increasing sensitivity al though usually at the expense of increased trouble f rom interfer- ences. Recently Rubeska and Moldan 24 have heated the F u w a tube to prevent the condensation of metal along the tube. Somewhat bet ter detection limits are provided for certain metals and potential interfer- ences are made smaller.

I f the flame tempera ture is reduced, the gases in the flame do not expand so great ly in the flame and

this provides somewhat bet ter sensit ivity in the cooler flames. (Obviously each of these ways of im- prov ing sensitivity carries with i t a compensating problem. )

2. C h e m i c a l I n t e r f e r e n c e s

There are still some residual " c h e m i c a l " inter- ferences result ing f rom the difficulty of breaking down re f rac tory compounds that fo rm as the aerosol is desolvated in the flame. The ni trous oxide-acet- ylene flame has t remendously helped this problem since the higher tempera tures available break down a larger propor t ion of the compounds formed in the aerosol. This flame was first suggested by Willis. ~ Some workers 2° propose the use of an ultrasonic gen- erator to provide a finer sp ray to the flame and thereby reduce the size of the re f rac tory compounds that must be broken down in the short t ime before the analyte passes through the optical system.

3. Low Absolute Detection Limits

Certain analytical problems require the determi- nation of small traces of metals in a very limited quant i ty of solution. This is bet ter understood in terms of absolute-detection limit, the total amount of metal which is jus t detectable in a given sample, as contrasted to the min imum concentration tha t is the subject of conventional solution detection limits. Such problems have led to the L ' v o v furnace, ~6 which requires only microli ters of solution or micrograms of solid sample. The sample is added to an electrode which is inserted in a small cylindrical electrical fur- naee purged with an inert gas such as argon. When the furnace has come to the equil ibrium tempera ture the sample is vaporized into the cylinder using a high current are. The absorption is recorded rapidly. The several years of work with this technique are reviewed by L ' v o v in a very lucid report . 2~ Detection limits of about 10 -14 g are shown for a long list of metals. This is some five orders of magni tude lower than the best conventional flame detection limits. The technique is probably more sensitive than any other procedure for a large group of metals and it is sur- pr is ing tha t the work of L ' v o v has not been taken up more widely in the eight years since it was first an- nounced.

Simpler to use is the boat technique developed at P e r k i n - E l m e r ~7 although it does not increase absolute detection limits to the same extent as the furnace. The sample is dried in a thin metal " b o a t " by plac- ing the boat, to which sample is added, close to the flame. When the sample is dried the boat is quickly inserted into the flame and the sample and its in- eluded analyte is r ap id ly volatilized producing an absorption signal which is recorded. Using the back- ground correcting technique, which will be described below, to remove non-specific absorption due to the burn ing of the organic materials, the absorption sig- nals for lead in urine are recorded in Fig. 2. This

424 Volume 23, Number 5, 1969

is a typical application of the boat technique to a part icularly practical problem.

Between the extreme sensitivity available with the method of L 'vov and the simply utilized high sensi- t ivity with our boat method, there are many com- promises. Woodriff and his colleagues 2s have devel- oped an electrically heated crucible to which the sample is added relatively easily as compared to the arrangement of L'vov. Massmann 29 also has simplified the L 'vov furnace although with some loss of the very favorable characteristics of L 'vov ' s arrange- ment.

4. Nonsolution Methods

Flame methods require the use of solutions and if solids are to be analyzed an extra, often difficult, step is required. Walsh 8° continues to at tempt to develop a sputtering chamber much like a demountable hol- low cathode lamp into which a piece of metallic sample can be mounted. This is then operated like an ordinary hollow cathode and the absorption of the metal cloud is measured. Venghiattis 3~ attacked the same problem by mixing the powder of the solid sample with a charge of solid propellant and igniting the mixture beneath the optical beam of the atomic absorption spectrophotometer.

5. Background Absorption

In certain eases, nonspeeific absorbance of the flame can cause significant errors when the analyst fails to separate such an effect from atomic absorp- tion. Koir tyohann and PicketP ~ solved this problem by taking the ratio of two measurements, one using a specific source (hollow cathode) and the second with a continuum source (e.g., hydrogen discharge lamp). The continuum source is negligibly affected by the atomic vapor, while the specific source is affected both by the nonspeciflc absorption and the atomic absorption. The ratio of the two signals can-

URINE URINE +0.04 l~glml +0.04 pglml

URINE +0.02 pg/mI

URINE

URINE +0.02 pglml

URINE

Fire 2. Measurement of lead in urine using the sampling '%oat" technique and the deuterium background correction accessory on the Perkin-Elmer model 403 spectrophotometer. The method of additions indicated that there was 0.029 ttg of l%/ml of urine. The figure is from unpublished work in our laboratory by J'. Schallis.

4 ~ K c ,

t m 4 . / . KCl

~ ~ N r O IPq/ml NOIE Pq/m 0 I ~lq/ml Ni 2 0 2 ~g/ml N i z z "0

FIG. 3. The absorption of nickel at 2320 ~ in a conventional atomic absorption speetrophotometer to the left of the vertical line and utilizing the automatic correction of background ab- sorption with a continuous source to the right of the line. With the correction arrangement the apparent absorption of a nickel-free :KC1 brine disappears and trace nickel determina- tions can readily be made in this matrix. The absorption of 0.2 ~g/ml nickel in 4% KC1 can be measured against stan- dards in water when automatic background correction is made. The figure is from unpublished work by J. Schallis in our laboratory.

eels the effect of the nonspecific absorption whether it arises from scattering of light from refractory par- ticles in the flame or from molecular absorption of the flame constituents or materials in the sample.

Since the cancellation of background effects results from taking the ratio of the signal using the specific metal source and the continuum source, it is easily and automatically applicable to a double-beam spec- trophotometer. Kahn 83 adapted the Koir tyohann and Pickett procedure along lines proposed by L'vov. 26 A typical example of the background correction tech- nique applied to the Perk in-Elmer model 403 or model 303 spectrophotometer is shown in Fig. 3. With a conventional atomic absorption spectropho- tometer a 4%-KC1 brine absorbs strongly at the nickel resonance line at 2320 .A even when no nickel is present. However, when the background correcting ratio is taken, the brine absorption disappears and it is easy to measure fractions of a t~g/ml of nickel in such solutions. Since both beams experience sig- nificant absorption, the small variations in this ab- sorption provide a noise signal which is theoretically larger than the noise of the conventional measure- ment.

E. F lames a n d Burners

While the flame has proven to be a most convenient and dependable device for atomizing (preparing an atomic vapor) samples prior to analysis by atomic absorption, emission, or fluorescence, it is still very incompletely understood. A very important study of the processes by which atoms are formed in the premixed flame has been published by Cowley, Passel, and Kniseley24 By using a combination of atomic emission and atomic absorption, they have studied the spatial distribution of various atomic molecular species in premixed oxyacetylene flames, both fuel- rich and stoichiometric. The burner that they used was described by Florin. , Kniseley, and Fasse], 35 and provided a long-slot geometry suitable for atomic- absorption spectroscopy. This arrangement provided detection limits for the elements that require a high

APPLIED SPECTROSCOPY 425

Table I. Comparison of detection limits ~g/ml) .

Atomic Flame Wavelength absorption emission

Element (£) (42) (39,40)

Ag 3280.7 0,005 0.02 A1 3092.7 0. I f

3961.5 0.01 As 1937.0 0,2

2 J49.8 50. ~,d Au 2428.0 0.02

2676.0 0.5 B 2497.7 6. r 30. ~,d Ba 5535.5 0.05 f,h 0.001 Be 2348.6 0.002 r 40. Bi 2230.6 0,05 40. a,d Ca 4226.7 0.002f, h 0.0001 Cd 2288.0 0,005

3261.1 2. Ce 5697.0 10. d Co 2407.2 0.005

3453.5 0.05 Cr 3578.7 0.005

4254.4 0.005 Cs 8521.1 0.05 e,h 0.008 b,d Cu 3247.5 0.005

3274.0 0.01 Dy 421.1.7 0.4 r,h 0.1 d Er 4008.0 0.1 f'h 0.3 d Eu 4594.0 0.2 f,h 0.003 a Fe 2483.3 0.005

3719.9 0.05 G~ 2874.2 0.1

4172.1 0.01 Gd 3684.1 4. f,h

4519.7 2. d Ge 2651.2 I. f 0.5 Hf 3072.9 15. f

3682.2 75. d Hg 2536.5 0.5 Ho 4103.8 0.3 f,h 0.1 d In 3039.4 0.05

451 ] .3 0.005 Ir 2639.7 2. f

3800.1 100. d IK 7664.9 0.005 ~ 0.003 a,d L~ 5501.3 2. f

5791.3 2. Li 6707.8 0.0006J 0.00003 Lu 3312.1 3. f 0.2 d Mg 2852.1 0.0005 0.005 l ' In 2794.8 0.003

4030.8 0.005 Mo 3132.6 0.1

3903.0 0.1 Nil 5889.9 0.005 0.0001 ~,d Nb 3343.7 5. f

4058.9 1.

Atomic Flame Wavelength absorption emission

Element (X) (42) (39,40)

Nd 4634.2 2d 'h 4924.5

Ni 2320.0 0.005 3414.8

Os 2909.0 0.6 j'f 4420.5

Pb 2833.1 0.01 4057.8

Pd 2476.4 0.02 3634.7

Pr 4939.7 4951.4 10. f,h

P t 2659.5 0.1 Rb 7800.2 0.005 °,h l~e 3460.5 1.5 f Rh 3434.9 0.03

3692.4 Ru 3498.9 0.3

3728.0 Sb 2175.9 0.2

2598.0 Sc 3911.8 0.2 f

4020.4 Se 1960.3 0.5 Si 2516.1 0.1 f Sm 4296.7 5. f'h

4883.8"] 4884.0.3

Sn 2246.0 0.06~ 2840.0

Sr 4607.3 0.01 T~ 2714.7 6. f

4812.7 Tb 4326.5 2. r'h Te 2142.7 0.3

2383.2 Th 5760.5 Ti 3642.7 0.2 f

3998.6 T1 2767.9 0.2

5350.5 T m 3717.9 0.15 f

4105.8 U 3514.6 30. f

59]5.4 V 3184.0 0.04 f

4379.2 W 4008.8 3. f Y 4077.4 0.3 f'h Yb 3988.0 0.04 r Zn 2138.6 0.002 Zr 3601.2 5. r'h

1. d

0.03

10. d

0.2

0.05 2. d

2. 0.002 b,d 0.2

0.3 d

0.3 a

20. d

0.03

5a,d

0 . 6 d

0.3 0.0001

18. a 1. d

200. a,d 150. d

0.2

0.02

0.2 d

lOft

0.01 0.5 1. 0.05 d

50.a,d 3.

a An E:~II-6255 B photomultiplier (S-13 response) was used. b An ITT-FW-118 photomultiplier (S-1 response) was operated at 125 ]K. e An EMI-9558 B photomultiplier (S-20 response) was used. a Fassel and Golightly (41). e Osram spectral lamp is used.

t e m p e r a t u r e f l a m e t h a t w e r e v e r y s i m i l a r to t h e de -

t e c t i o n l i m i t s f o u n d w i t h t h e n i t r o u s o x i d e - a c e t y l e n e f l ame .

C o w l e y e t al . , s h o w e d t h a t t h e e f f e c t i v e n e s s o f t h e

f u e l - r i c h o x y a c e t y l e n e f l a m e s r e s u l t e d f r o m t h e c om-

b i n a t i o n of t h e r e l a t i v e l y h i g h f l a m e t e m p e r a t u r e a n d

a n e n v i r o n m e n t w h i c h is r e l a t i v e l y d e f i c i e n t i n o x y -

g e n . S t o i c h i o m e t r i c o x y a c e t y l e n e f l a m e s p r o v i d e a

h i g h e r t e m p e r a t u r e b u t , f o r m a n y m e t a l s , s u f f e r a

loss o f s e n s i t i v i t y r e l a t i v e to f u e l - r i c h c o n d i t i o n s .

T h i s r e s u l t s f r o m t h e u n f a v o r a b l e e n v i r o n m e n t w h i c h

p r o m o t e s c h e m i c a l r e a c t i o n s t h a t b i n d t h e m e t a l

f A nitrous oxide-acetylene flame is required or preferred. g Using an air-hydrogen flame. h In presence of high concentrations of another ionizable metal. i G.E. OZ 4 mercury gerndcidal lamp used. i Recent unpublished data in our laboratory.

a t o m s . B y c o n t r a s t , t h e f u e l - r i c h a i r - a c e t y l e n e f l a m e

p r o v i d e s a n e n v i r o n m e n t w h i c h is v e r y e f f ec t i ve f o r

m a n y m e t a l s b u t a t e m p e r a t u r e w h i c h is too low f o r

s o m e o t h e r m e t a l s . T u r b u l e n t f l a m e s o f t h e s e m i x -

t u r e s p r o v i d e a n o n o p t i m u m e n v i r o n m e n t b e c a u s e

t h e y do n o t f o r m w e l l - d e f i n e d z o n e s i n t h e f l a m e a n d

t h e f a v o r a b l e c h e m i c a l e n v i r o n m e n t doe s n o t p r e v a i l .

T h e s u c c e s s o b t a i n e d b y t h e n i t r o u s o x i d e - a c e t y l e n e

f l a m e of W i l l i s 3~ r e s u l t s f r o m t h e h i g h t e m p e r a t u r e

a c h i e v e d , a b o u t 2 8 8 0 K , ~7 w i t h o u t t h e v e r y r a p i d

b u r n i n g v e l o c i t y a s s o c i a t e d w i t h t h e o x y g e n - s u p -

p o r t e d a c e t y l e n e f l a me . T h u s a f a v o r a b l e c h e m i c a l

426 Volume 23, Number 5, 1969

and thermal environment are provided in a burner which is easily handled. This arrangement has made it possible to determine ahnost all metals by atomic absorption with detection limits that are usually better than a ~g/ml in aqueous solutions and often approach 0.001 ~g/ml. For many elements (e.g., Ca, Sr, Ba, Mo, Cr, etc.) that are determinable in an air-acetylene flame, the nitrous oxide-acetylene flame will significantly reduce the chemical interferences. The features, successes, and remaining limitations of the nitrous oxide-acetylene flame have been reviewed very recently by Willis2 s

The advantages of the premixed nitrous oxide- acetylene flame are certainly not limited to atomic absorption alone. Flame emission results with the same long-slot burner that is used for atomic ab- sorption have been reported by Pickett and Koirtyo- hann. 39,~° The complementary nature of absorption and emission is evident by comparing the detection limits they report for atomic lines with those avail- able by atomic absorption (Table I ) . The burners used for absorption are essentially the same as those used for emission, although both air-acetylene and nitrous oxide-acetylene flames have been used for absorption. The emission spectra] slit width was sometimes as small as 0.4 A. The emission detection limits for Li, Sr, Ba, Ca, and In are significantly better than atomic absorption provides, while the absorption results for Be, Cd, Co, Pb, and Zn are considerably better than emission provides. The rare earth element data in this table have been taken from Fassel and Golightly 4~ for emission in the fuel- rich oxyacetylene flame. Their emission results are similar to those found in the nitrous oxide-acetylene flame and both sets of emission results are compared to atomic absorption32 The fact that both emission and absorption provide advantages emphasizes the uti l i ty of a system that permits the same burner as- sembly to be used for both techniques, so that the best features of both systems may be available.

F. Relative Merits of the Turbulent and Laminar Flow Burners

Atomic absorption methods have generally utilized premixed burners which provide laminar flow flames and emission methods have customarily utilized total consumption burners which provide turbulent flow flames. Controversy has existed in the l i terature as to which is preferable. Comparison has been difficult since, until recently, it was awkward to use premixed burners for flame temperatures higher than achiev- able with air-acetylene. However, with the advent of the nitrous oxide-acetylene flame, the comparison is more easily made.

Since all of the solution taken up by total-con- sumption burners is put into the flame, they could be expected to be more efficient. However, several authors 36,43,4~ have shown that not all the droplets of aerosol are dried during the transi t time through the

flame and therefore the efficiency is not as great as implied by the "total consumpt ion" title. Both types of burners typical ly vary from about 10%-20% ef- ficiency, as measured by the conversion of solution analyte to an atomic vapor.

We have shown 4~ numerous examples f rom the l i terature of interferences that are found in the total consumption ( turbulent) flame that are absent from the premixed flames. We believed that the principal reason for this was the incomplete vaporization of metal f rom the larger desolvated particles of aerosol. The work of Cowley et al., quoted above indicates that a large par t of the problem of the turbulent flame probably results f rom disruption of the suit- able chemical environment found in the well-defined zones of the premixed flame. In fact, considerable improvement in the performance of the typical total consumption burner is provided by adding a small premixing chamber between the surface where the gases emerge from the concentric tubes and the flame. 4~ Recently, Mossotti and Duggan 4~ found im- provement in the performance of the total consump- tion burner by premixing the fuel and oxidant in the two outer concentric tubes, thereby making the flame more laminar and producing well-defined zones. Fu tu re tests of this new burner should include inter- ference effects at high ratios of in terferent to analyte, since in their s tudy of the interference of phosphate on magnesium, the molar ratio is only 1/50th that which we showed 4~ produced no specific chemical interference effect with the conventional premixed air-acetylene flame.

The current l i terature indicates quite clearly that turbulent flow flames have significant disadvantages over laminar flow flames for both emission and ab- sorption. With the design of Mossotti and Duggan, there is no need for total consumption burners to be limited to turbulent flames. The useful advantage of the total consumption burner is tha t it is undoubt- edly less likely to flash back since there is no large chamber containing a potential ly explosive mixture of fuel and oxidant. Modern premixed burners for nitrous oxide-acetylene have been engineered to mini- mize such risks. The work in Fassel 's laboratory and at the University of Missouri has confirmed that the flame requirements of emission and absorption are not very different, including the usefulness of the long-slot flame in improving emission detection limits.

An advantage is often quoted for the total con- sumption burner of providing favorable absolute detection limits because the uptake rate is low. Dis- cussion under "Nebulizers" later in this review indi- cates that much lower uptake rates may be used in conventional premixed burners with an improvement in atomization efficiency. The remarkable quanti tat ive stability of premixed burners is somewhat degraded at very low uptake rates. However, it is not accurate to assume that better absolute detection limits are available f rom total consumption burners.

These considerations lead the authors to believe

APPLIED SPECTROSCOPY 427

RESONANCE

LAMP I ~ ] FLAME ~ . ~ C T O R

PHOTOMULTIPLIER

FIG. 4. Resonance detector configuration. The resonance de- tector acts as a monoehromator. The metal cloud in the de- tector fluoresces with an intensity that is proportional to the intensity of resonance lines emitted by the lamp.

that the requirements of both emission and absorp- tion are presently best served by long-slot, premixed burners equipped to utilize flames of air-acetylene and nitrous oxide-acetylene. In certain cases some- what better atomic absorption detection limits will be found for the so-called argon-hydrogen flame, although with an increase in interferences. 4s In a few cases somewhat better emission detection limits will be found with the fuel-rich premixed oxyacet- ylene flame, although with an increased tendency for flashback. In certain very specialized cases the tem- perature of the flame may be usefully adjusted by the judicious mixing of air and nitrous oxide with acetylene.49, 64

G. Instrumental Systems

Commercial instrumentation will not be described here because most such descriptions are out-of-date before the publication reaches its audience. Kahn 7 has described instrumentation from the point of view of principles. Most recent changes in instrumentation relate to features that affect sampling or to the con- venience of obtaining data from the instrument. A recently developed instrument permits data to be recorded directly on punched paper tape taking ad- vantage of the versatility of a teletypewriter. In addition this equipment permits the operator to pre- pare his report format and avoid the need to recopy data, risking error, and consuming time.

H. Detection of Resonance Radiation

Walsh and his colleagues at CSIRO, Australia, have attempted to provide simpler systems for atomic absorption by taking advantage of the special opti- cal properties of the resonance lines that are used for atomic absorption. Sullivan and Walsh ~° have reviewed this work recently.

The simple arrangement of Fig. 4 represents a complete atomic-absorption spectrophotometer for a single element. The emission of the light source con- sists of many lines, of which only the few originating at the ground state or at a low level provide the reso- nance spectrum of the element. These lines are ab- sorbed in varying degrees as they pass through the metal vapor in the flame. I f all the light which passes through the flame is incident upon a cloud of atoms of the part icular element of which the light source is made, the resonance radiation will be absorbed but the other radiation will not interact with the metal

vapor cloud. The absorbed resonance radiation will excite the metal atoms and in re turning to their un- excited state, the atoms can emit radiation at the same or longer wavelengths. A photomultiplier ex- posed to the emission of the metal cloud will receive a signal proportional to the intensity of the resonance radiation. The properties of the resulting working curves will depend more upon the state of the atoms in the fluorescing cloud than upon conditions in the hollow-cathode light source. Thus the working curves might be expected to be linear over a longer working range and to be somewhat steeper for elements where self-absorption at higher lamp currents reduces the sensitivity in conventional atomic absorption spec- t rophotometry (e.g., Cd or Zn).

The absorption of resonance radiation by a cloud of metal atoms can be utilized in another way to pro- vide a potential atomic absorption spectrophotometer. This is illustrated in Fig. 5 where an open cylindrical cathode has been interposed between the source and the flame. I f the source is driven by a d c supply, the typical spectra of the metal and fill gas are emitted. I f the open cylindrical cathode is driven by an ac supply, the space through which the source light must pass is periodically filled with a cloud of absorbing atoms of the same metal of which the source is made. The resonance lines of the source radiation will be alternately absorbed and transmitted at the fre- quency of the ac supply. Since the non-resonance lines will not be attenuated as they pass through the metal cloud they will be incident upon the photo- detector as dc signals. Thus when the detector output is tuned to the frequency used to drive the open cylindrical cathode, only resonance radiation will be recorded. ~Valsh has called this technique "selective modula t ion" of the resonance lines.

Selective modulation of resonance lines can be uti- lized in several ways. The simple system of Fig. 5 can avoid the need for a monoehromator as long as the total intensity of dc radiation incident upon the photodetector does not produce too large a noise sig- nal. Walsh ~1 has shown that a judicious choice of filters or photomultipliers sensitive only to a small spectral range can provide very useful results. The technique can also be used for conventional atomic

Un-moduLated PuLsating A.C. amplifier tight source atomic vopour Monoohromator Detector output meter

FIG. 5. Selective modulation of resongnce radiation. The light from the de source passes through the open cylindrical cathode of the same metal as that of the source. The open cathode is pulsed with an ac voltage which produces a pulsating cloud of metal. Resonance radiation from the source will be made ac by being alternately absorbed and transmitted as it passes through the pulsating cloud of metal atoms. The non-resonance radiation from the lamp will remain de and will be rejected by the tuned ac amplifier. The purpose of the monochromator is to select the useful spectral region and reduce the noise- producing dc radiation on the phototube. The figure is from Sullivan and Walsh.So

428 Volume 23, Number 5, 1969

absorption spectrophotometers to separate resonance radiation from other radiation passed by the mono- z.5 chromator. The linear dynamic range of many ana- lytical curves may be extended by using selective modulators.

I t is possible to incorporate the modulator cathode z o within the envelope of the source lamp, although we have had difficulty in separating the several signals that are present in the lamp. I t is also possible to reduce the emission of the modulator cathode by a 15 suitable design, such as the use of a wire ring or by shielding the emission with a baffle of smaller in- t~ ternal diameter than the cylindrical cathode or ring. I t is our opinion that the advantage of the wider ~ I o dynamic range of l inearity does not just i fy the added o

(1)

complication and expense for the purposes of con- < ventional atomic absorption spectrophotometry.

I. Mult i-Element Atomic Absorption without a Monochromator

In their review article, Sullivan and Walsh 5° de- scribed several instrumental arrangements they have proposed for multielement analyses utilizing reso- nance detectors or selective nlodulation, or a com- bination of both. I f a light source of the multi- element variety is used in Fig. 4 and several reso- nance detectors are aligned along the optical axis, it can be seen that each photomultiplier observing its part icular resonance detector will respond to just the element of that detector. The output from several light sources can also be utilized by combining them with semi-transparent beam splitters. We have suc- ceeded in combining more than one element in a single resonance detector observed by a single photo- multiplier by coding the input light from several sources at different frequencies and sorting the signal from the photomultiplier at the several input fre- quencies. By a suitable combination of frequency modulation and separate resonance detectors, a con- siderable number of metals can be accommodated.

Sullivan and Walsh have used other simple ap- proaches to multielement analysis with resonance de- tectors. One arrangement permits the operator to select different portions of a circular flame for the different elements according to the sensitivity that is desired. An experimental instrument is being used for the simultaneous determination of Cu, Zn, Ag, Ni, and Pb in ores.

More recently Walsh 51 has shown that selective modulation of the source radiation can be accom- plished in some cases without requiring the auxiliary open cylindrical cathode. I f an ordinary hollow- cathode lamp is driven by a dc supply upon which is superimposed a very narrow pulse of high current, an intense cloud of metal will be created periodically in f ront of the cathode. This cloud will serve the same purpose as the open cylindrical cathode and selectively absorb resonance radiation from the dc background of the source lamp without at tenuating the nonresonance lines. Thus a detector properly

0.5

®

I ®

O I I I I I 2 0 4 0 6 0 8 0 I 0 0

pglml

FiG. 6. Analytical curves for nickel and iron. Curve 3 is the analytical curve for iron using a conventional atomic absorp- tion spectrophotometcr. Curve I shows the iron curve when selective modulation is used in addition. Curve 4 is the con- ventional curve for nickel and curve 2 is the curve when a nickel selective modulator is used in addition. All curves utilized a 2 A spectral bandwith. The resonance modulation used the double :frequency technique of Sebestyen.~a

adjusted in phase and frequency will respond to the modulated-resonance signal and ignore the constant signal from the rest of the emitted radiation. In practice it is also usually necessary to block the photomultiplier by shorting its dynode supply during the short (microseconds) intense pulse.

Using this arrangement, Sebestyen in our labora- tory ~2 has studied the resonance modulation per- formance of many elements and shown that appro- priate systems can be devised for most of them. Be- cause the emission of the atomic cloud, as well as the presence of atoms in it, decays at different rates for different elements, Sebestyen has proposed a double frequency demodulation system. Using this system and an ordinary hollow cathode lamp in an ordinary single-beam atomic absorption spectrophotometer (the Perk in-Elmer model 290), he has considerably ex- tended the range of linear operation of the analytical curves for nickel and iron, as shown in Fig. 6.

J. Atomic Fluorescence

We well remember the frustrat ion of many of the early workers in atomic absorption spectroscopy as reviews appeared written by people who had never published in atomic absorption and whose claim to expertise apparent ly resulted from their work in a different discipline. For this reason we will refer the reader to several recent and enthusiastic reviews

APPLIED SPECTROSCOPY 429

of atomic fluorescence: one by Winefordner and Mansfield ~ who pioneered the technique as an ana- lytical method and by West 54 whose group at Im- perial College has contributed greatly. We will com- ment here on the relationship of atomic fluorescence and atomic absorption for analytical chemistry and of the fu ture potentiali t ies of atomic fluorescence. The reader must recognize that any review requires judgment as to relative importance and judgment is an opinion.

Atomic fluorescence utilizes the emission of atoms tha t have been excited by the process of absorption of incident resonance radiation. I t therefore utilizes and depends upon atomic absorption but has the added advantage tha t the absorption is observed as the emission of the excited atoms ra ther than the diminution of the intensi ty of the incident resonance radiation. I t was pointed out earlier in this review tha t the atome-absorption-deteetion l imit is ulti- mate ly restr icted by the instrmnental difficulties in measur ing small changes in the intensi ty of a light beam, even if the beam is very intense. By contrast, emission methods require the detection of a vanish- ingly small signal in the presence of a smaller back- ground. Improved ins t rumental technology can con- t inuously improve the emission detection limits if (but only if) the background can in fact be kept small in relation to the signal to be detected. I f the background emission becomes large by comparison with the signal to be detected, fluorescence becomes l imited by the same problem as atomic absorpt ion; that is the difficulty of measuring a small change in a large signal.

Probably the pr incipal advantage expected of atomic fluorescence is the improvement of detection Iimits over atomic absorption or flame emission. This has a l ready been demonstrated for a number of elements whose compounds can be dissociated in rela- t ively low-temperature flames. This applicat ion of atomic fluorescence, is highly dependent upon the brightness of the source, and the development of rf-excited lamps with emission intensities which are orders of magni tude br ighter than hollow cathode lamps, is expected to provide a significant boost to atomic fluorescence. As repor ted earlier in the " s o u r c e s " section of this review, such lamps are not yet as stable and predictable as is required for rou- tine analysis.

The fluorescence signal is usual ly distinguished f rom the emission of the flame by using an ac source and tuning the detector to the ae frequency. How- ever l ight scattered by solid particles or droplets in the flame is difficult to separate f rom fluorescence, and several authors have been led to believe tha t such scat tering signals were in fact fluorescence. I t seems to us that atomic fluorescence would be great ly aided by the use of Koi r tyohann and Picke t t ' s pro- posa132 mentioned earlier in conjunction with atomic absorption. By their method atomic effects are dis- t inguished f rom unspecific (broad-band) effects by

comparing the signals f rom a continuous source with tha t f rom the specific source. The use of background correction may make it possible to achieve favorable detection limits in the presence of complex and even re f rac tory matrices. However, at the present t ime atomic fluorescence does not tolerate as complex a mat r ix as can be used for atomic absorption. I t is perhaps for this reason that, up to the t ime of writ- ing, no applications papers have been repor ted uti- lizing atomic fluorescence.

A second l imitat ion of atomic fluorescence involves the need mentioned above to keep background emis- sion small if favorable detection limits are to be found. The higher tempera tures that certain metals require to provide a measureable atomic populat ion in the flame generally demand a hotter flame that is likewise more emissive. Thus the metals that became determinable by atomic absorption with the introduc- tion of the nitrous oxide-acetylene flame are gener- ally not determinable by atomic fluorescence because the ni trous oxide-acetylene flame emits intensely over most of the useful spectral range. The separated nitrous oxide-acetylene flame recently repor ted by Ki rkbr igh t ~s holds promise of alleviating this prob- Iem.

There is a large unexplored potent ial for atomic fluorescence resul t ing f rom the fact that, in principle, a monochromator is not required. Jenkins ~6 was the first to propose a system based on this advantage. A modulated-l ight source specific to the element be- ing detected i l luminates the flame. Careful ly exclud- ing the exciting light, the fluorescence of the flame is observed direct ly with a photomultip]ier . A filter is used to reduce the dc radia t ion f rom the flame that will produce a noise signal on the detector. Jenkins also points out tha t certain flame species should be excluded or minimized because their presence will provide a radiationless pa thway for the decay of ex- citation, thus reducing the fluorescence signal. F lame tempera tu re may be reduced to the lowest value prac- tical for the par t icu lar element by mixing a noble gas into the flame.

Another potential applicat ion for atomic fluores- cence is for quali tat ive or semiquanti ta t ive analysis. I f the flame is i l luminated by an intense continuous source (such as a xenon are) and the fluorescence is scanned with a monoehromator of moderate resolu- tion, the various elements present will provide a spectrum which is considerably simpler and more easily in terpre ted than the corresponding flame emis- sion spectrum. However, the advantages of such an a r rangement would have to be cri t ically compared with the per formance to be expected of an atomic absorption system for the same purpose using the s a m e s o u r e r .

II. TECHNIQUE

A. Interferences I t is beyond the scope of this review to consider

in detail the recent papers tha t have been published

430 Volume 23, Number 5, 1969

on various real or imagined interferences. The most general class of interferences in atomic absorption or emission are chemical, the fai lure to break the chemi- cal bonds tha t fo rm between the analyte and other materials in the mat r ix when the solvent is evapo- ra ted out of the aerosol droplet in the flame. This ef- fect is similar in emission and absorption since the effect is to l imit the atomization of the analyte. In general i t is minimized by using a high tempera ture flame and these authors recommend tha t flames with tempera tures lower than the air-acetylene flame be avoided except in those cases where the need for the highest sensitivity suggests the use of cooler flames. When cooler flames are used, the possibility of chemi- cal interferences must be explored. Many metals typi- cally determined in a i r -acetylene flames are probably bet ter determined in nitrous oxide-acetylene flames to minimize small residual chemical interferences. Thus it is preferable to use the hotter flame for Mg, Mo, Cr, Sr, Ca, Ba, and in certain cases, Ni, Co, and Fe.

The t rend towards the use of hotter flames in- creases ionization interference, the loss of a variable fract ion of the atoms to the singly ionized state. When using the nitrous oxide-acetylene flame we find it advisable to keep the amount of easily ionized alkali metal relat ively similar between s tandards and samples for most determinations. I t is imperat ive to match the alkali content of s tandards and samples for those elements which are s trongly ionized in the hot flame, e.g., Sr, Ca, Ba, and m a n y of the rare earths.

An enter ta ining s tudy of spectral interferences was reported by Fassel, Rasmuson, and Cowley. 5~ They took sharp issue with the atomic absorption l i terature which minimizes the effect of spectral interferences as an inherent advantage of atomic absorption over emission. They showed experimental ly tha t if copper is determined at the 3247-5 copper-resonance line in high concentrations of europium, an interference amount ing to 0.07 absorbance for 5000 t~g/ml Eu, will result. I t is useful to point out that this has not been observed experimental ly because it is customary to determine copper in the air-acetylene flame where the absorption for europium is very much smaller. Fassel, Rasmuson, and Cowley used a nitrous oxide- acetylene flame for their experiments.

The remainder of the spectral interferences re- por ted by these workers included an interference of vanadium on silicon if the 2506.9-5 silicon line is used. This silicon line is rare ly used since it has about one-third the sensitivity of the recommended 2516.1-5 line. P la t inum at concentrations of 2000 t~g/ml will provide a significant interference on the determina- tion of iron at the ra re ly used 2719.0-5 line, also about one-third the sensitivity of the recommended iron line. High concentrations of vanadium will in- terfere with aluminum if the 3082.2-5 a luminum line is used. Some workers might use this a luminum line since it is not much less sensitive than the usu- al ly recommended a luminum line at 3092.7 A.

We have shown 5s tha t high concentrations of lead absorb radiat ion at the p r i m a r y ant imony resonance line at 2175.9 A when the typical spectral slit of 7 5 is used. This appears to be due to the absorption of the 2170.23-A Sb line by lead solutions whose ab- sorbance peak is at 2169.99 5 , a spectral interval of about 0.24 A. Manning and Fernandez 59 found tha t high concentrations of cobalt absorbed at the reso- nance line of mercury at 2536.52 A, a spectral differ- ence of only 0.03 A f rom the 2536.49-5 metastable line of cobalt.

Very recently Allan 6° has shown tha t a spectral interference occurs between gall ium (at 4032.98 A) and manganese (a t 4033.07 5 ) . He has considered the effect both theoretically and experimental ly in a thorough manner .

Thus these recent examples of spectral interfer- ences force us to modify our earlier experimental observation tha t spectral interference had not been observed in atomic absorption. They have now been observed but are still ve ry few and will rare ly pro- vide a pract ical problem in analytical applications.

I t may be worth repor t ing that we were not able to confirm several spectral interferences recently re- por ted2 ~ Appa ren t spectral interferences can arise f rom the presence of impuri t ies in hollow-cathode lamps. Fo r example, small traces of copper in a lead lamp will provide an appa ren t copper interference at the 2170-5 lead line if the spectra] slit width is large enough to pass the 2165- or 2178-5 resonance lines of copper. In testing our mult ielement lamp combinations we test for such problems. Most lamp manufac tu re r s make a considerable effort to avoid accidental mult ie lement lamps by using h igh-pur i ty metals; however, there is no reason to believe tha t all manufac ture rs are always successful in this effort.

III. INDIVIDUAL ELEMENTS

A considerable wealth of informat ion is available in the recent l i terature to supplement the under- s tanding of pract ical determinations repor ted ear- lier. s Provided below are some noteworthy references to the recent l i terature.

A. Osmium

The convenient atomic absorption determinat ion of osmium has proven elusive and pr iva te ly communi- cated results of Willis in 1967 were quoted in the book by Slavin. s We have recently decided in work tha t is still unpublished tha t the pr incipal problem with the determinat ion is s imply the availabil i ty of materials in which the osmium content is correct ly known. Using a lamp which provides strong emission for the spectrum of osmium, a conventional ni trous oxide-acetylene flame and the line at 2909 5 , a de- tection l imit of 0.6 ~g/ml is easily found. An a i r - acetylene flame can also be used with some sacrifice of sensitivity. Osolinski and Knigh t 62 have recently repor ted on the determinat ion of osmium in a similar

APPLIED SPECTROSCOPY 431

environment. They found certain chemical interfer- ences which may be specific to the par t icu lar ni trous oxide-acetylene burner tha t they happened to use.

B. Strontium

I n a search for a supressant for the interference of iron on the absorption of strontium, Ful ton and But ler 6~ found tha t mixed ni trous oxide-a i r -acet - ylene flames produced higher absorption for stron- t ium than did a i r -acetylene flames. A t an op t imum rat io of about 53% air and 47% nitrous oxide, the sensit ivity was about 0.075 t~g/ml for 1% absorption in the ni trous oxide-a i r -ace ty lene flame as compared to 0.12 /~g/ml for 1% in a i r -acetylene flames and 0.25 /~g/ml for 1% in the ni trous oxide-acetylene flame.

In the a i r -acetylene flame 2000 /~g/ml Fe de- pressed Sr absorption by about 18%, while in a ni trous oxide-acetylene flame the same amount of Fe enhanced Sr absorption by about 70%, due to the suppression of Sr ionization in the hot ter flame. Un- for tunately, the ni trous oxide-a i r -ace ty lene flame did not completely suppress the Fe interference but i t did reduce it significantly. Fu l ton and But ler finally eliminated the Fe interference by reducing the tem- pera tu re of the ni t rous oxide-acetylene flame with Ar instead of air. Fo r most purposes the advantages of this par t icu lar combination do not w a r r a n t the fuss involved but the potentiali t ies of mixed suppor t gases for the acetylene flame is certainly demon- strated.

C. Sulfur

Sulfur has been determined by flame absorption using the molecular bands of su l fur dioxide in the ul traviolet by F u w a and Vallee2 ~ Two band systems are accessible to conventional ul t raviolet spectrom- eters, one centered near 2000 A and a less sensitive set centered near 2900 A.. An a i r -hydrogen flame was passed through a long (273 cm) Vycor tube which was heated on the outside. A hydrogen discharge tube was used as a source of uv radia t ion and at 2070 A the sensit ivity was 10 /~g S per ml for 1% absorp- t ion in aqueous solutions. The working curve was l inear to 80 /~g/ml.

IV. APPLICATIONS

Space does not allow the critical review of the recent extension of atomic absorption methods to analyt ical chemistry. A very useful guide to the recent applicat ions l i te ra ture can be found in the bibliographic index to some 300 papers published most ly in 1968. 8

A book devoted exclusively to the appl icat ion of atomic absorption to geochemistry has been published by Angino and Bill ings2 ~ I t should cer ta inly be consulted by atomic absorption workers in tha t field.

I n biological applications the earlier reviews by

Willis ~6 and Zettner 67 are still recommended as well as the extensive mater ia l in Slavin2 s

While the atomic absorption method for serum cal- cium seems to be well established, there stiI1 appear in the l i terature suggestions and questions. Johnson and Riechmann 69 compared several accepted proce- dures on a group of some 60 sera: (1) the direct dilution with an ttC1-LaC13 di luent using calcium s tandards in the same diluent, which the authors call the P e r k i n - E l m e r method, 7° (2) the method of Zet tner and Seligson 71 in which the diluent contains buty l alcohol and the s tandards contain protein, and (3) the original method of Willis 72 in which the dilution in a solution containing l an thanum is pre- ceded by TCA precipi ta t ion of the protein. The aver- age difference between the two dilution methods was less than 0.1 rag/100 ml indicating tha t the s impler P e r k i n - E l m e r procedure was quite adequate. The TCA precipi ta t ion procedure produced an 0.4 rag / 100 ml higher average than the dilution methods. There was no clue as to which was more near ly ac- curate.

ACKNOWLEDGMENTS

We hope tha t this pape r will help the spectros- copist or chemist through the recent extensive li tera- ture on atomic absorption spectroscopy and related fields. We recognize tha t t ime has not permi t ted our taking adequate account of several significant con- tr ibutions and we sincerely regre t these omissions. The t r ea tment of experience with individual ele- ments in pract ical applicat ions is woefully inade- quate and we hope tha t the book by Slavin will con- t inue to be a useful reference for this type of in- formation. The applicat ion of atomic absorption to individual fields of analyt ical chemistry has likewise been slighted in this review.

We thank our colleagues at P e r k i n - E l m e r for their help in p repa r ing this information. Wi thou t them the work would not have been accomplished.

1. A. Walsh, Spectrochim. Acta 7, 108 (1955). 2. A. Walsh, Appl. Opt. 7, 1259 (1968). 3. S. Slavln, Atomic Absorption Newsletter 8, 8 (1969). 4. W. T. Elwell and J. A. ~. Oidley, Atomic Absorption

Spectrophotometry (Pergamon Press, Ltd., London, 1966) 2nd ed.

5. 3". W. Robinson, Atomic Absorption Spectroscopy (Marcel Dekker, New York, 1966).

6. J. Ramirez-~c~unoz, Atomic Absorption Spectroscopy (El- sevier Publ. Co., Inc., New York, 1968).

7. W. Slavin, Atomic Absorption Spectroscopy (Wiley--In- terseience, Inc., New York, 1968).

8. It. L. Kahn, in Trace Inorganies in Water, R. A. Baker, Ed., Advances in Chemistry Series, No. 73 (American Chemical Society, Washington, D. C., 1968).

9. W. Slavin, Atomic Absorption Spectroscopy (Wiley-In- terscience, Inc., New York, 1968), p. 74.

10. R. 1Yl. Dagnall and T. S. West, Appl. Opt. 7, 1287 (1968). 11. J. lV[. ~¢Iansfield, Jr., tel. P. Bratzel, Jr., It. O. Norgordon,

D. O. Knapp, K. E. Zacha, and g. D. Winefordner, Spectrochim. Acta 23B, 389 (1968).

432 Volume 23, Number 5, 1969

12. T. C. Rains, National ]3ureau of Standards, personal com- munication.

13. C. T. J. Alkamade and J. 1VI. W. Milatz, J. 0pt. Soe. Am. 45, 583 (1955).

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