Atomic fluorescence spectrometry - ACS Publications - American

Atomic Fluorescence Spectrometry Although more research work is needed before atomic fluorescence is used a great deal for real samples, this sensitive, selective, and versatile method will undoubtedly become increasingly important in trace analysis

24A

e

TOMIC

A

FLUORESCENCE SPECTROM-

is a rather new technique of trace element analysis. I n this method, a sample solution is atomized in a flame or nonflaine cell, the resulting atoms are illuminated with a light source, and a fraction of the atomic fluorescence, resulting when a portion of these excited atoms undergo radiational deactivation and emit radiation toward the detection device, is measured. The early nonanalytical work on atomic fluorescence of metal vapors in different atmospheres in quartz containers is discussed by 3litchell and Zemansky ( 1 ) and by Pringsheim ( 2 ) . T h e first reference to atomic fluorescence of metal vapors in flames was reported in 1924 by Sichols and Howes ( 3 ) . I n 1927, Badger (4)wrote a classic paper ETRY

ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971

on the atomic fluorescence of several elements in unseparated and in beparated flames. I n 1962, Xlkemade ( 5 ) suggested the use of atomic fluorescence flame spectrometry as an analytical technique. I n 1964, Kinefordner and Tickers (6) wrote their first paper on atomic fluorescence flame spectrometry as an analytical method. Since then, numerous manuscripts ( 7 ) concerning the basis, the instrumentation, and the possible uses and advantages of atomic fluorescence flame sgcctrometry for analytical and physical studies have been published. Several comprehensive reviews 18-12) of atomic fluorescence flame spectrometry have been written, and the reader is referred to them for a more detailed discussion. I n this paper, the authors will at-

REPORT FOR ANALYTICAL CHEMISTS

J. D. WINEFORDNER R. C. ELSER Department of Chemistry University of Florida, Gainesville, Fla. 32601

tempt to review briefly, the fundamental aspects, the instrumental requirements, and the rinalytical uses of atomic fluorescence spectrometry.

Source Spectral Radiance

so&

Source Spectral Irradiance

EsX,erg set? cnf2 nm"

B,x, erg s e t ' cm2 sr4 nm'' Theoretical Considerations (5,6, 8, 11-17)

Mechanism of Atomic Fluorescence. I n Figure 1, the types of atomic fluorescence processes are indicated. Resonance fluorescence has been most used for analytical studies. Both types of direct-line fluorescence have been of some use for analytical studies, whereas both types of stepwise atomic fluorescence h a w been of little analytical use. Energy transfer atomic fluorescence--i.e., sensitized atomic fluorcscence-was not of a n y analytical use. I n energy transfer atomic fluorescence. the analyte a t o m are excited via donors t h a t were excited by a light source; in flames, energy transfer atomic fluorescence is of no analytical use because of collisional deactivation of the donors and because of relatively lo^ concentration of the donor. Radiance of Atomic Fluorescence (8, 13, 1 4 ) . Figure 2 gives a pictorial derivation of the radiance iriiost workers use the word "intensity" to signify radiance) of atomic fluorescence, BF (ergs of fluorescence per second per unit area of the cell per unit solid angle). By evaluating the integral in EA (see Figure 2) and f 9 and by simplifying, the expressions in Table I result, These expressions were obtained by assuming the sample cell, which has the shape of a parallelepiped (absorption path length, (, fluorescence path length, L , and height, .e'), is completely illuminated and the

Irradiance Absorbed by Atomic Vapor

EA ,erg sec'l

'A,

@A

= LC'E,

'"-I

@Ff YbAfS

r / J

}

Radiant Flux Absorbed by Atomic Vapor

}

Radiant Flux Fluoresced by Atomic Vapor

Irradiance Fluoresced by Atomic Vapor

,erg BF=E

I

'F

(-1 I

Radiance Fluoresced by Atomic Vapor

47T

Figure 2. Pictorial derivation of fluorescence radiance for a dilute atomic gas To obtain an expression explicit i n t e r m s of atomic concentration of analyte, it is neces. a specific analyte absorber a t a specified sary t o evaluate E, f o r t h e specific case-e.g., concentration being excited by a specific source (continuum, line, o r intermediate source). At high concentrations of analyte atomic gas, t h e fluorescence expressions m u s t also be corrected f o r t h e reabsorption of fluorescence (the f, factor) p r i o r t o emergence f r o m t h e cell. The definitions of t e r m s are: Lz = solid angle of exciting source collected and incident upon t h e analyte atomic vapor; fix = atomic absorption coefficient f o r t h e analyte erg atomic vapor a t wavelength, A; Y' = fluorescence power yield f o r analyte atoms-i.e., sec-1 o f fluorescence per e r g sec-1 of absorption; X = wavelength of absorption. All other t e r m s i n c l u d i n g cell dimensions ate defined i n t h e figure: and f is a factor t o account f o r reabsorption (self.absorption) of a t o m i c fluorescence by analytde atoms w i t h i n t h e i l l u m i nated portion of t h e cell


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Report for Analytical Chemists

Table 1.

Atomic Fluorescence Radiance (erg sec-1cm-2sr1)Expressions" Low no

C1= V % / 2 d E no units; Cz = d i / l n 2 , no units: ~4~ = modified atomic absorption coefficient for pure Doppler broadening = XX2dU/C~d/rrcAXD;X = aeZ/mc, cmzsec-1, m and e = mass, in grams, and charge in esu, of the electron: n o = concentration of atoms in the ground state, cm-3; Xd = fraction of analyte atoms in the lower state involved in the absorption transition, no units; f e u = absorption oscillator strength for transition (+ u. w h e r e 8 is the lower state and u is the upper state, no units; c = speed of light, c m sec-1; A i D = Doppler half-width of absorption line, in nm; Adu= absorption line peak for transition 8 += u, cm: B

= spectral

fluorescence emanating a t right angles to the exciting beam from the cell toward the detector is measured. [If the cells is incompletely illuminated and if all of the fluorescence emanating from the cell is not measured, then correction factors (13, 1 4 ) must be included.] Other assumptions include: the source area is larger than the absorption area ( L X e') of the cell; the exciting radiation reaching the cell is collimated and the measured fluorescence radiation is also a collimated beam perpendicular t o the exciting beam ; the continuum source has a constant spectral radiance oyer the absorption line width, and the line source has a small half-width compared to the absorption line half-width; and, the atomic concentration and temperature of the atomizer a t the height of measurement are constant across the atomizer. For a more thorough discussion and a complete derivation of the expressions in Table I, refer to Hooymayers (13) and t o Kinefordner et al. (8). From the expressions in Table I, the following analytically useful conclusions can be made: (i) If the atomic concentration, no, is low, and the cell is illuminated with the same radiant flux from the source of excitation, then the fluorescence radiance, BF, is linearly related t o no, whatever the source of excitation and whatever the cell shape. 26A

cxcu

radiance for continuum source, erg sec-1cm-zsr-lnm-1;

High no

E," = radiance of line source, erg sec-*cm-2sr-l; 0 = solid angle of exciting radiation collected and impinging upon flame, in sr; L,d,C'= absorption path length, in cm, fluorescence path length, in cm, and atomizer height, in cm, respectively; A, = total atomizer cell surface area, cm* (for parallelepiped cell, A, = 2 L d + 2U' 28&; Y' = fluorescence power yield, erg fluoresced sec-1 per erg absorbed sec-1: 8du = factor to account for finite halfwidth of line source compared to absorption line, no units; a p = damping constant for absorption of fluorescence; R, = modified absorption coefficient (same as but for reabsorption of flu. orescence; X , = fraction of atoms in lower state involved in re. absorption of fluorescence: and f R = absorption oscillator strength for reabsorption of fluorescence.

+

(ii) T h e fluorescence radiance, B F , is independent of no if no is high and a continuum source is used; B F is dependent upon l/v'K if n, is high and a line source is used. (iii) Therefore, atomic fluorescence is primarily useful for trace analysis (low atomic concentrations). (iv) The fluorescence radiance, B F , depends directly upon the fluorescence power yield, Y', whatever the source of excitation, the cell shape, and the atomic concentration. ( v ) The fluorescence radiance, BF, increases linearly with effective source radiance (essentially BcxdU~hn for a continuum source, and BS for a line source). (vi) T h e analytical curve plot of log S us. log C, where S is the instrumental signal owing to BF, and C is the analyte concentration introduced into the atomizer cell, has essentially the same shape as log Bp us. log no (Figure 3 ) . However, some deviation from the log B F vs. log no may result a t low concentrations owing to ionization of analyte atoms and a t high concentrations owing t o decreased sample introduction rate into the atomizer, reduced solute vaporization rate, and reduced aspirator yield, if one is utilized-e.g., in flame studies.


"eu)

KO equations accounting for incomplete illumination of the cell, incomplete measurement of atomic fluorescence over the cell area toward the detector, nonparallel exciting or fluorescing radiation, cell shapes other than rectangular parallelepipeds, sources intermediate between continuum and line, or the intersection region between high and low concentrations will be giyen here. The limiting expressions for the fluorescence radiance, BF, will generally be quite similar to those given in Table I even for real analytical situations where the above inadequacies may be present. Alkemade ( 1 4 ) has given a n excellent discussion and treatment of several of the above inadequacies. ,4190 refer to Hooymayers ( I S ) and Zeegers and Winefordner (15). Atomization of Analyte. I n atomic fluorescence spectrometry, the analyte must be atomized prior to radiational excitation. Atomization has been performed so far by flame and nonflame cells. These cells are discussed later in this review. Predicted Limits of Detection ( 8 ) . If the limit of detection is defined as t h a t concentration (or amount) of analyte resulting in a specified signal-to-noise ratio (two, for instance) , and if the instrumental signal [signal owing t o fluorescence radiance, B F ,modified by the instrumental system (16)] and noise [primarily a result of flame flicker and shot noise ( 1 6 , l 7 ) ] are

Report fur Analytical Chemists

evaluated for atomic emission flame spectrometry, ae, atomic absorption flame spectrometry, a:%,and atomic fluorescence flame spectrometry, af, then comparison of relative limits of detection obtained with these three methods is possihle. Assuming t h a t the same resonance line of the same atom is measured by the same instrumental system and t h a t the same flame atomizer is used for all three methods, then it can be shown ( b y using expressions for a a and ae similar t o those in Table I ) t h a t ae should result in lower limits of detection than af and aa for atoms with resonance lines above approximately 4000 A and af [and sometimes aa, especially for high temperature flames ( 8 )] should result in lower limits of detection than ae for atoms with resonance lines below approximately 3000 A. All methods should give similar results for atoms with resonance lines in the intermediate region of 3000 to 4000 A (Figure 4 ) . With nonflame cells. af should result in even lower limits of detection than a a (assuming all other factors are equal) because the noise level should be reduced more in af than in aa. With nonflame cells, the background radiation and radiation flicker often can be made negligible compared t o other noise sources. Therefore, in af, the major source of noise becomes shot noise which is often much less than flame flicker noise, whereas in a a , the major source of noise is source flicker noise with a contribution from shot noise (source flicker and shot noises are often greater than flame flicker noise, and so, nonflame cells should be more beneficial t o af than a a , in terms of reducing limits of detection). Of course, the major adrantage of nonflame cells is the possible use of small samples for analysis. Because of the vaht difference in excitation energy available and in background in RF, microwave, and dc plasma discharges, it is impossible t o compare a a and af with ne when using nonflame cells. Predicted Interferences ( 8 ) . I n Table 11, the predicted effect and relative extent of the major types of interferences in atomic flame spectrometric methods are given. T h e worst type of interference-physi-

Continuum Source Cell Source:

44

Detector

Log B, of cell

Incomplete

/

Measurement of Fluorescence from Cell

Log no

Line Source Cell Source:

44 Detector

BF

/ /

\

\

\

Log no Figure 3. Hypothetical growth curves [log(Ba) vs. log (no), where Et. is the a 6 m i c fluorescence radiance and n, is the concentration of analyte atoms in the ground state] The dashed lines indicate t h e general influence of increasing t h e extent of incomplete ill u m i n a t i o n o f t h e cell w i t h exciting radiation a n d / o r increasing t h e extent of incomplete measurement of a t o m i c fluorescence f r o m t h e cell

cal, chemical-should be (and is) identical in all atomic flame methods. Scattering and band absorption interferences should be (and are) troublesome to sonie extent in a a and a f , and the temperature variation interference should be (and is) a little worse in ae. Spectral interferences should be (and are) a little worse with ae, and a a and af with continuum sources, than with a a and af with line sources. Therefore, the total iilfluence of interferences should be the same for all atomic flame snectroas long gooc] inStrunlelltatiOn is Utilized for all Perhaps, i f "' relative magnitude of interferences was established, then atomic ad-

Figure 4. Theoretical prediction of wavelength range over which various atomic flame spectroscopic methods achieve lowest limits of detection I t is assumed t h a t t h e s a m e resonance line of any g i v e n element is measured b y t h e t h r e e m e t h o d s of a t o m i c emission (ae), a t o m i c absorption (aa), and a t o m i c fluorescence (af). spectrometry. Also, it is assumed t h a t t h e same f l a m e cell a n d basically t h e s a m e in. s t r u m e n t a l system is utilized f o r all t h r e e methods


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Table II. Comparison of Predicted Interferences in Atomic Flame Spectrometric Methods" Type

Effect

Measurement of radiation characteristic of interferent rather than or as well as analyte radiation Physical, chemical Reduction in concentration of analyte atomic concentrationd Temperature variation Change in degree of excitation and change in degree of atomization Scattering of radiation Reduction in source radiand band adsorption ance owing to nonatomic absorption and reduction in fluorescence radiance Quenching of excited Reduction in fluorescence radiance atoms by species resulting from sample matrix Spectral

Relative Extentb aac, afc, ae aflc

>

aal,

Same for aac, aal, ae, afc, and afl ae 7 aac, aal, afc, and afle aal, afl > aef

aac, afc

afc, afl 2 aac, aal, aei:

Ae = atomic emission flame spectrometry: aac = atomic absorption flame spectrometry with a continuum source: aal = atomic absorption flame spectrometry with a line source (this is t h e commonly used method, often called atomic absorption spect r o m e t r y or just aa); afc = atomic fluorescence flame spectrometry w i t h a continuum source: afl = atomic fluorescence flame spectrometry with a line source (this is the commonly used method called atomic fluorescence spectroscopy or just af o r afs). b > means specific interference is greater for methods listed t o left of > t h a n for those listed t o right of >. A sign of means greater t h a n or nearly t h e same. c The extent of spectral interference i n aac, afc. and ae depends upon t h e spectral bandwidth and t h u s t h e dispersion of the optical system. The smaller the spectral bandwidth, t h e smaller t h e spectral interferences. 1' Interferences which reduce the atomic concentration of analyte i n flames are t h e most severe type of interferences For most elements-i.e., those f o r m i n g stable in flame spectrometric methods. monoxides, monohydroxides, etc., in the flame gases-the effect of temperature change is essentially the same for all flame methods. These interferences are not present i n ae. Generally these interferences are not of great importance i n either aa o r af meth. ods. This interference is not present i n aac, aal, or ae. However, it is also negligible in afc and afl.

7

Q

f

line Sources should result in slightly lesaer oi~erallinterferences than the other iiiethods in Table 11. Interferences with noiiflame cells will he similar to those predicted for flame cells but are difficult t o compare in detail without spccifyiiig the exact nonflaiiie cell type and method of sample introduction.

Basic S y s t e m . The hasic layout of a n atomic fluorescence spectrometer is given in Figure 5 . It can be thought of as an atomic absorption .pcctrometer hcnt at a 90" angle around the absorption cell axis. Looking a t it another way, one sees that without the source, the setup is simply that of an atomic emission

Characteristic

1. High radiance over the absorption line of analyte atoms

Comments

Radiance over absorption Line generally higher line Spectral interferences Line generally results in fewer

2. Long-(little drift) and short-(little flicker) term stability

3. Operation under either continuous or pulsed conditions

Wavelength adjustment Multielement analysis

4. Simple tuning and focusing 5 . Availability for all elements

6. Long lifetime 7 . Safety of operation

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systcni. Basically, all atomic fluorescence systems have been h i l t around this arrangement. Sources. In Figure 6, the types of boiirces used in atomic fluorescence are schematically shown. As pointed out above, fluore:+cence respoiise is a function of source radiance (intensity), Consequently, sources of high radiance are desiralile. I n Table 111, the requireineiits for excitation source'. for a t oini c fluorescence sp EC t rome t ry are outlined. Both line and continuum soiirces have lieen used with success. Table IT' compares h i e and continuum excitation S O U ~ C E S . The choice of a line source or continuum aoiirce must 1x2 made on the hasis of analytical rquirementsi e . , iiuniber of different elements to lie analyzed, interferences expected, and radiance required for i*elial)le detection. The general types of line sources used in past atoiiiic fluowscence studies are depicted in Figure 6a-f : these sources a150 have been used in many atomic absorption studies. The c.!ectrode-

Table IV. Comparison of Line and Continuum Source of Excitation

Table 111. Excitation Sources Requirements for Atomic Fluorescence Spectrometry

8 . Low cost

Figure 5. Schematic diagram of basic instrumental system for atomic fluorescence spectrometry

I


Stability Tuning and focusi'ng

Line simpler Single continuum source vs. many line sources Continuum generally better Continuum generally simpler

Lifetime

Continuum generally longer


less discharge lamp (Figure 6c) has been the most-used line source in atomic fluorescence work because of its high radiance over most resonance lines. Electrodeless discharge lamps are operated in a microwave field which causes excitation of the atomic vapor contained in the discharge lamp. The field is directed on the discharge lamp by means of either antennae or waveguide cavities. Optimum performance and lifetime is a function of the power of the microwave field, the lamp temperature, and uniformity of the microwave radiation around t h e lamp. As a result, tuning of the lamp in the field is critical. Tuning requires primarily either mowment of the lamp within the field or movement of the device directing the field. Consequently, difficulties arise in tuning the lamp properly and maintaining it in the correct optical position. Xenon arc lamps of high spectral radiance are commonly available and offer the advantage of single lamp operation over multiple-line spectral lanips (one for each element). They suffer from the disadvantage of possessing low radiance in the uv below 2500 A. They can be obtained either as “point” sources (Figure 6,) or as collimated beam sources (Figure 6f). Metal yapor disrharge lanips (Figure 6d) emit rather intense resonance radiation but are plagued with self-reversal problems. Hollow cathode lamps of the normal sealed variety (Figure s a ) used in atomic absorption studies are generally of insufficient radiance for atomic fluorescence work. Boosted-output hollow cathode lamp. (Figue 6b’l emit intense resonance radiation but are expensive and not commercially available for many elements. Lamps which operate continuously a t some arbitrary radiance can provide a greater peak radiance by pulsing them a t higher current levels than is obtained a t continuous operation. The only limitation is that the average pulsing current be about the same as the average continuous operating current. Overpowering a t high current levels reduces the lifetime of the lamp. Optimally, detection in pulsed systems should he synchronous with source pulsing.

Atomizers. The requirements (Table of atomizers in atomic fluorescence spectrometry are essentially identical to those for atomic absorption and atomic emission spectrometry. T h e major difference lies in the third requirement of Table V ; because nonradiational deactivation decreases the atomic fluorescence radiance regardless of how ideal the remainder of the system m a y be, radiational quenching processes should be minimized. This is accomplished by choice of the major cell gases. For example, the major quenchers in flames are CO, COS,and K2; thus CO and CO, are ayoided by using nonhydrocarbon flames such as oxyhydrogen or hydrogen diffusion flames. Also, NScan be minimized in flames by (Continued on page 32 A )

Table V. Requirements of Atomizers Used in Atomic Fluorescence Spectrometry 1. Good atomization efficiency” 2. Low radiational background and background flicker

3. Low concentration of quenchersb 4. Long residence time of analyte atoms in optical path

5. Simplicity of operation

6. Low cost of initial purchase and operation a The atomization efficiency m e a n s t h e efficiency of converting analyte w i t h i n t h e s a m p l e m a t r i x i n t o analyte a t o m s in t h e CO, atomizer. b Molecular species-e.g., COP, N2, etc.-deactivate excited a t o m s via non radiationa I means-e.g., collisional processes.


-.

-

---

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wter for aas chrom I-

-

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Tfi:

Figure 7. Schematic diagram of flame atomizer shapes (burner shapes) for atomic fluorescence flame spectrometry a. Rectangular flame-right.angle illumination-measurement (see 15,24,46,59,80,81,87) b. Round flame-right-angle illumination-measurement (see most references between 18 and 89 except 15,24,46,59,80,81.87,90) c . Rectangular flame-front.surface illumination-measurement (see 90)

excluding air entrainment into t h e flame so far as possible by sheathing the analytical flame with either a burning sheath or an inert gas sheath. Koiiflanie cells are also sheathed with argon. Some shapes of flames (burneratomizers) used in atomic fluorescence spectronietry are shown in Figure 7. Generally, laminar-prenixed flames with or without either flame or inert gas sheaths are supported on the square (Figure 7a) or rectangular (Figure 7c) burners. Turbulent or turbulent-premixed (39, 40) flames without flame or inert gas sheaths are usually supported on round burners (Figure 7b). Nonflanie atomizers are shown in Figure 8. The l l a s s m a n n furnace (32) in Figure 8a and the West filament (30, 31, 67’) in Figure 8b were used for both atomic fluorescence and atomic absorption spectrometric studies. All nonflame cells are electrically heated. -411 argon atmosphere is also maintained in most nonflame cells by either passing a stream of argon through them or around them. However, the graphite filament and the metal loop (Figure 8d) can be sheathed either by an argon stream or by a hydrogen diffusion flame. T h e metal tube furnace (Figure 8c) of Shull and Winefordner 191) is sim32A

SLIT TO OBSERVE FLUORESCENCE

ilar t o the 1Iassniann graphite tube furnace except the sainple is introduced continuously n-ith a n efficient nebulizer. Metal tubes of platinum are simple to fabricate and are not porous compared t o graphite tubes, but the upper temperature limit for platinum elements is only 1800°K compared t o about 3000°K for graphite. Refractory metals can provide higher temperatures but are highly susceptible t o air oxidation and become hrittle after being heated. T a n t a lum tubes have been used allowing an increase in temperature of the atomizer to about 3000°K but oxidation problems, even in the presence of high flow rates of argon past the element, are appreciable a t present. Sampling for the wire loop (platinum or tungsten, Figure 8d I is much more tedious and less precise than for any of the above non-

%

SAMPLE HOLDER

-GRAPHITE

GRAPHITE CUP FOR SAMPLE

ROD

(a 1

ALUMINA TUBE PLATINUM HEATING ELEMENT ELECTRODE

*

SAMPLE INTRODUCTION

ri5 %TO

,SAMPLE

HOLDER

ELECTRODES

Figure 8. Schematic diagram of nonflame atomizers for atomic fluorescence spectrometry


a. b. c. d.

Massmann graphite furnace (see 32) West graphite filament (see 30,31,38,62) Shull m e t a l t u b e furnace (see 91) Bratzel metal loop (see 63,64)

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33A


flame atomizers. T h e sample is either applied t o the loop by dipping t h e loop into the solution t o be analyzed, or is applied by nieaiis of a hypodermic syringe. For all of the nonffame cells except the metal tube and the metal loop, discrete sampling of solutions via a hypodermic syringe is utilized. Therefore, in systems employing atomizers of these types, the detectorreadout system must respond reliably t o the transient signal which results when the free a t o m pass through the analytical detection zone of the optical system. Because of the requirement of short time constants in the detection system, the frequency response bandwidth of the measurement system is increased, and background noise may become a problem. Entrance Optics. I n Figure 9 schematic diagrams are given for various arrangements of entrance optics which have been reported for atomic fluorescence spectrometry. All of these arrangements, except the one in Figure 9e, employ the typical 90" arrangement. T h e simplest one, used in most laboratory-constructed systems, is shown as Figure 9a. I n this arrangement, the amount of light collected from both the source and the sample cell is limited by the f-number of the lenses employed. Unfortunately, the amount of radiation collected depends upon the size of the lenses (cost increases with size). An arrangement which improves the efficiency of irradiating the sample cell with source radiation and improves the efficiency of collecting fluorescence radiation from the cell is illustrated in Figure 9b. b n advantage of this arrangement over the one in Figure 9a is t h a t lowercost, more readily available mirrors rather than lenses are used. Also mirrors are better than lenses in transferring images-e.g. fewer aberrations. I n addition, since radiation from the cell and source is isotropic, placement of mirrors behind the source and the cell increases the amount of radiation collected from each-more than lenses having the same effective f-number and being arranged as in Figure 9a. The radiance of fluorescence is directly proportional t o the radiant flux of the source incident upon 34A

MIRROR

FLAME

FLAME

SLIT

LENS

(a 1

-v-

ENTRANCE SLIT

FLAME ELLIPSE SOURCE

-LENTRANCE SLIT

T 7 r

CASSEGRAIN MIRROR

MIRROR

SOURCE

SLIT Figure 9. Schematic diagram of entrance optics used in atomic fluorescence spectrometry a. All lens system-right.angle illumination-measurement (See 74) b. All m i r r o r or mirror-lens combination-right-angle illumination-measurement (see 23. 26,2938) c. Elliptical m i r r o r system-right-angle illumination-measurement (see 9 1 ) d. Systems w i t h Cassegranian mirror-right.angle illumination-measurement (see 92) illumination-measurement (see 90) e. All m i r r o r systems-front-surface

the sample cell. Therefore, the arrangement in Figure 9b-conipared to the one in Figure 9a-markedly improves the fluorescence response. The focusing ellipse illustrated in Figure 9c provides a n even more efficient means of collecting source radiation. B y placing the source a t one focus and the sample cell a t the second focus, all of the source radiation in the plane of the ellipse is focused onto the sample cell. T h e cell is viewed through a slit in the side of the ellipse. .klthough a large gain in incident radiant flux is obtained with this arrangement, care must be taken t o avoid ineasurement of scattered source radiation.


A Cassegranian system (Figure 9d) has been employcd in t h e Technicon AFS G ( 9 2 ) . This expensive mirror system has the great advantage of being capable of collecting a large, solid angle of fluorescence radiation. Front-surface illumination of t h e sample cell is employed in the arrangement depicted in Figure 9e. Here, also, mirrors with small aperture ratios having great light-gathwing power may be employed a t relatively low cost. The principal advantage to using front-surface illumination is that source radiation scatter is less. Both dispersive and nondispersive spectral isolation devices have

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Table VI. Comparison of Experimental Limits of Detection (in pg/ml) in Atomic Flame SpectrometryR Element-Wavelength

(AIb

aal'

afl'

aec

Ag-3281

0.0005 (93)

0.0001 " ( 5 7 )

0.02(94)

AI-3962

0.04(103)

0.1(69)

0.005 * (95)

AS-1 937,1937,23 50

0.1 (93)

0.1*(45)

50(96)

Au-2428,2676,2676

0.01*(103)

0.005*(33) 0.01(40,54)

4(96) 0.1(97)

0.005*(59) 0.02 (5 7 )

2(98) 0.0001" ( 9 4 )

Be-2349

0.002*(93,103)

Bi-2231

0.05(93,103)

Ca-4227 Cd-2288,2288,3261 CO-2407,2407,3454 Cr-3 579 ,3 579 ,4254 CU-3 247,3247,3274

0.0005(103) 0.0006(103) 0.005 * (93,103) 0.005 * (93) 0.003 * (1 03)

0.00000 1(5 7 ) 0.005 * ( 2 2 3 2 ) 0.05(45) 0.00 1* (1 8,24, 3 7,74) Fe-2483,2483,3720 0.005*(93,103) 0 008 * (45) Ga-2874,4172,4 172 0.07(93) 0.01*(59) Ge-2 65 2 0.1 * (1 03) 0.1* ( 6 6 ) H g-2 53 7 0.0002*(80) 0.2( 103) ln-3039,4511,4511 0.05(93,103) 0.1(57) Mg-2852 0.0003 "(93,103) 0.001*(22,30) Mt1-2795,2795,4031 0.002 * (93) 0.006 * (5 7 ) MO-3 133,3 133,3903 0.03*(93) 0.5(69) Ni-2320,2320,3415 0.003*(25) 0.005 * (93) 0.01*(93) Pb-2833,4058,4058 0.01 *(24,29, 58) Pd-2746,3405,3635 0.02 * (1 03) 0.04 * (34) Rh-3435,3692,3692 0.03 * (93) 3(19) 0.05*(48) Sb-2 175,23 11,2598 0.07 * (1 03) Si-25 16,2040,25 16 0.1 * ( 9 3 ) 0.6 * (44) 0.1*(93) 0.04 * (59) Se-1960,1960,1960 0.03 * (93,103) 0.05 * (42) Sn-2246,3034,2840 Sr4607 0.004( 1 03) 0.03(5 7) 0.1 ' ( 9 3 , 1 0 3 ) 0.005*(59) Te-2143,2143,2383 0.02 * ( 1 03) TI-2768,3 77 6,377 6 0.008 * (5 7 ) 0.02*(93) 0.0 7 (69) V-3 184,3184,4379 0.002(93,103) 0.00002 * (5 7, Zn-2138 80) I

2 (94) 0.05(94) 0.005 * (94) 0.0 1(94) 0.05 (94) 0.01*(94) 0.5 (94) 40(96) 0.005*(95) 0.005(94) 0.005 * (94) 0.1(94) 0.6(96) 0.2(94) 0.05*(94) 0.3 (96) 20(96) 5(96) N" 0.3 (94) 0.0002 * (94) 200(96) 0.0 2 * (94) 0.01*(94) 50 (96)

n L i m i t of detection is usually defined as t h a t concentration resulting in a signal-tonoise ratio of two. All l i m i t s of detection are for elements atomized i n "laminar" o r "turbulent" flames produced using commercially available burners-e.g., l i m i t s of detection obtained by increasing t h e residence t i m e of atoms i n a flame by introducing the flame into a long quartz t u b e in aal are n o t listed i n t h e above table. Only l i m i t s of detection are given for those elements which have been studied by all three methods and for those elements for which reliable l i m i t s of detection are listed. L i m i t s of detection for aal are f o r t h e PE 303 or t h e Techtron AA 5. The l i m i t s of detection f o r t h e PE 303 were taken f r o m t h e article by Kahn (93). More recent values are undoubtedly available for the PE 503 atomic absorption spectrometer b u t were n o t available t o t h e authors. The l i m i t s of detection f o r t h e Techtron AA 5 were taken f r o m recent literature (103). Also, lower l i m i t s of detection i n atomic absorption spectrome. t r y are available i n t h e literature for some elements, b u t it was considered m o r e reliable t o take all aal values f r o m the above t w o sources. It would also be more reliable t o take a l l afl and ae values f r o m one source, b u t no such tabulation is currently available. I f three wavelengths are listed, then t h e three l i m i t s of detection correspond t o wavelengths used f o r aal, afl, and ae, respectively. If just one wavelength is listed, then t h e same spectral line was measured f o r all three methods. c aal = atomic ab. sorption flame spectrometry w i t h a line source: afl = atomic fluorescence flame spec. trometry w i t h a line source: ae = atomic emission flame spectrometry. d Not detect. able by ae. * Asterisk superscript indicates t h a t the listed value is threefold or more lower than other limits of detection f o r t h e same element. If a n asterisk superscript l S o n t w o (or three) l i m i t s of detection for t h e same element, then those t w o (or three) methods give essentially the same result. A threefold range is used here because of t h e differences in defining l i m i t s of detection by some authors and t o account f o r differences i n imagination when measuring low signal.to.noise ratios.

36A


been employed. Generally, a grating inonochroiiiator of fairly large aperture is used. Several iiivestigators ('77, 80, 90) have used a conibination of interference filters and so la r- 11 1irid m u It ip 1i er phototubes . The choice as to spectral isolation device must he niade on the basis of possible iiiterfcrciices which may be present, Sondiepereive devices offer a definite cost advantage. Analytical Studies (18-92)

Livzits of Detection. In Table VI) a comparison is given of the 11 est ( b t a t e - o f - t he- ar t ) ex p erinieiital concentrational (in niicrogranis per milliliter) limits of detection for several elements measured by atomic absorption flame spectrometry with a line source (awl), atoinic flu ores c en c e flame spectrometry with a line source (afl), and atomic emission flame spectrometry (ae) , The asterisk superscripts denote the methods giving the lowest limits of detection [threefold or lower than the other method(s)]. As seen from the data in Table V I , the results compare favorably with t h o v predicted in Figure 4. I n Tahle VII, a comparison is also given of the hest (state-of-thea r t ) absolute (in nanograiiisi limits of detection for a t oiiii c spec t rom et ric methods with both flame and nonflarne cells. Sonflaiiie cells are useful for smaller amounts of analyte than flanie cells. Also, with most nonflame cells, smaller sample sizes-e.g.. one pg-are necessary. The hest available absolute limit? of detection for atomic absorption spectrometry with a, line source and a graphite cell igaal) and atomic fluorescence with a line source and either a graphite cell igafl) or a metal loop (niafl) compare favorably with the hest ahsolute limits of detection obtained with more exotic and espensivc methods such as neutron activation and spark source maw spectrometry (1021. Interferences. Studies performed Ily West's group a t Imperial College and Winefordner's group a t the Vniversity of Florida indicate that the predicted degree of interferencei: listed in Tahle I1 and discussed previously is experimentally valid. Comparison of Atomic Fhiorescence Spectrometry with Atomic

This is the way all fluorescence spectra are going to look.

I

I

I

I

I

240

280

320

360

400

WAVELENGTH (nm) True excitation spectrum of anthracene (in ethanol). 5 mp bandpass. Emission wavelength: 420 m p .

It's the true fluorescence spectrum of anthracene, (above) rec o r d e d on Perkin-Elmer's n e w Model MPF-3 Fluorescence Spectrophotometer. It looks the way an excitation fluorescence spectrum was meant to look. Identical to its UV absorption spectrum-but recorded at a concentration 1000 times lower! Up to now fluorescence excitation or emission spectra could not be used routinely between

210

280

320

,60

400

WbVELENGTH Inmi

Uncorrected excitation spectrum of anthracene (in ethanol). Emission wavelength: 420 rnp.

L-, L 1 240

LBO WAVELENGTH 320

lnm)560

400

Absorption spectrum of anthracene (in ethanol). 5 m p bandpass.

laboratories or for publication because the relative intensities of peaks were incorrect. Incorrect because source and detector interacted with spectra. Now, with our new M P F B you can record true band intensities permitting d ay-t o-day, i n s t r u ment-to-instrument, and lab-to-lab comparisons. Comparisons that are both easy and valid Circle No. 142

You can begin to realize the full power of fluorescence spectrophotometry-its ability to measure nanogram levels (for pollutant ana Iys is) , to iden t if y com ponents in mixtures, to analyze powders and solids, to probe into living cell structure. If you want to make quantum efficiency measurements (to study rare earth phosphors, for example) you're way ahead i f y o u r emission and excitation spectra are true a n d don't have to be corrected. What's more, you can make direct comparison of fluorescence excitation and UVIVIS absorption spectra published in literature and reference libraries. The new MPF-3, like the MPF2A which has become the reliability standard for fluorescence spectrophotometers, has continuously adjustable slits and high efficiency grating monochromators. This gives you greater precision in your quantitative analyses and more accurate working curves. And the wide range of accessories increases its versatility. Write for full information on the newest addition to PerkinElmer's F l u o r e s c e n c e P r o d u c t line and we'll send you our brochure and four technical reprints, If you own an MPF-2A, we can provide an accessory to give it the t r u e s p e c t r a c a p a b i l i t y of t h e MPF-3. Write to Instrument Division, Perkin-Elmer Corporation, 702 Main Avenue, Norwalk, Conn. 06852.

PERK1N-ELM ER

on Readers' Service Card ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971

37A


Table VII. Absolute Limits of Detection (in ng) for Atomic Spectrometric Methods” Element

Ag As Au

Be Bi

Ca Cd

cu Fe

Ga Hg Mg Pb TI

Zn

aalb,c

0.1

8 2 0.4 10 0.1 0.1 0.6 1 14 40 0.06 2 4 0.4

afPC

0.02 20 1 2 1 4

0.0002 0.2

2 2 0.04 0.2 2 2 0.004

aebse

4 10,000 800

200 400 0.02 400 2 10 2 8,000 1

40 4

10,000

gaal‘

0.0005 * (99) 0.2 * ( 100) 0.07(99) 0.003*(99, 100) 0.02 * (99) 0.003(100) 0.00006* (99) 0.0 0 6 * (99) 0.02 * (99) 0.02*(99) 0.4(99) 0.003 (99) 0.02 * (1 00) 0.002 ‘(99) O.OOl(99)

L i m i t of detection is usually defined as t h a t amount (in nano. grams) resulting i n a signal-to-noise ratio of two. Only limits of detection are given for elements measured by atomic absorption and atomic fluorescence flame and nonflame spectrome. try. No wavelengths are given because they often differ f o r the various techniques. Exact experimental conditions can be obtained by consulting t h e literature references. b Values are calculated using concentrations in Table V I and assuming a 0.2.ml c aal= atomic absorption flame spectrometry sample is used. with a line source; afl = atomic fluorescence flame spectrometry

Absorption and Atomic Emission Spectrometry. The three niethods with respect to limits of detection and interferences were compared above. The precision of measurements for a given sample depends primarily upon the sampling method, the source stability, and the instrumental system, but should be about the same for all three methods-e.g., the percent relative standard deviation should he about 1%. The accuracy of measurement depends primarily upon the analyte type, the sample matrix, and the chemical steps involved in the sample preparation. T h e instrumentation used in atomic emission flame spectrometry is simpler (less entrance optics and no external source needed) than the instrumentation required for atomic fluorescence or atomic absorption spectrometry. A4tornie fluorescence spectrometry should be the simplest and best method to utilize for multielement analysis with multiplex spectrometers--e.g., source alignment is difficult in atomic absorption spectrometry and background radiation and flicker are limiting in atomic emission spectrometry. 38A

gafP

0.005*(32, 38)

rfae“(l0l)

0.002

0.5 * (38)

0.01 *(38) 0.03(38) 0.01 “(31) 0.0001*(38) 0.00003*(38) 0.005 * (38) 0.01*(38) 0.05*(31)

20

4 0.2 0.00002*

6

1

20 0.02*

0.0000001*(30) 0.01*(31) 0.05(31) 0.00005*(31, 32)

4 4 2 0.02

2 2

with a line source; gaal = graphite cell atomic absorption spec. trometry with a line source: gafl = graphite cell atomic fluorescence spectrometry with a line source; m a f l = metal loop atomic f!uorescence spectrometry with a line source; rfae = RF excitation atomic emission spectrometry (assuming a 0.2-ml sample is nebulized). Only results obtained by Dickinson a n d Fassel (101) are listed here because they are generally lower t h a n measured by other workers using RF excitation. * Asterisk indicates t h e method (or methods) giving threefold or lower limits of detection t h a n t h z other methods.

Recently, Technicon Corp. announced the del-elopnient of an atomic fluorescence spectrometer for niultieleinent analysis ( 2 2 ) . This instrument involves t h e usc’ of pulsed hollow cathode lamps. a rotating interference filter wheel, a flame cell, and logic circuitry to measure the fluorescence of each metal when the proper interference filter is in place. Six elements per aaniple and 100 samples per hr can lie measured with this unique and exciting new instrument. Applications of Atomic Flziorescence Spectrometry. So far, few applications of atomic fluorescence spectrometry to real samples have resulted. Smith et a l . (56) determined trace w a r metals in jet engine lubricating oils. Sychra and RIatousek ( 3 5 ) determined Ni in gas oils and petroleum distillates. Cotton and ,Jenkins (87’) determined low concentrations of Cu, Fe, and P b in hydrocarbon fuels. Amos et a l . ( 3 8 ) determined lead in blood and urine by a unique approach involving measurement of direct-line fluorescence. Most workers have determined metals in synthetic solutions with


mafI‘(63)

added interferences rather than in real samples. Finally, the new multielernent Technicon atomic fluorezcence instrument should lie of particular use in the metallurgical industry. Certainly, more research iiir-olring the use of atomic fluoreacence spectrometry for analysis of real samples is necessary before there \Till lie a widespread use of this senbitive, selective. and versatile method of trace analysis. References

W.Zemansky, “Resonance Radiation and Excited .items," University Press, Cambridge, England, 1961. (2) P. Pringsheim. “Fluorescence and Phosphorescence.” Intersciencc, Xeiv T o r k . N . Y., 1949. (3) E. I, Tichols and H. I,. Howves, Phys. X e L ’ . 23, 472 (1924). (4) R . h1. Badger, Z . Phys. 5 5 , 56 (1929). ( 5 ) C. Th. J. Alkemade in “Proceedings of the 9 t h Colloquium Spectroscopicum International,” E . R . Lippincott and M. Marposhes, Eds., Spartan Books, Kashington? D. C., 1963. (6) J. D. Winrfordner and T. J . Vickers. . ~ N A L .C H E M36, 161 (1964). ( 7 ) J. D. Kinefordnrr and T. J. S’ickers, AISAL.CHEM.42, 207R (1970). (8) J. D. Winrfordnver, T.’ Svohoda, and I,. J. Cline, C R C Crit. ReLi., Annl. C h e m . 1, 233 (1970). (1) -4. C. G. ?rlitche11 and A I .

Problem seem unsolvable? Try Perkin-Elmer'snew kind of UVWIS Spectrophotometer: the Model 356. Combine conventional doublebeam p e r f o r m a n c e w i t h highly versatile double-wavelength capabilities and you have the Model 356. Problems usually considered too difficult to solve by UV analysis are done easily and reliably by the Model 356. This different kind of instrument will solve your analytical problems better than any other instrument if you're: a biochemist or biophysicist and your samples are highly turbid;

or a plant physiologist and your samples are plant leaves; or a physiologist whose samples are microtomed tissue sections; or a textile or paper chemist who must measure dyes or inks on paper or textiles; or an a n a l y t i c a l c h e m i s t w h o must make measurements on overlapped absorption bands; or Cytochrome in Yeast h

1

1

450

500

,

550

concerned with environmental pollution problems requiring m e w urement of pesticides, herbicides, polynuclear aromatics and other types of organic and inorganic pollutants; or required to measure the rate of change of two components in a reaction cell simultaneously; or measuring the rates of reactions that p r o d u c e a b s o r b a n c e changes as small as 0.006A/hr. The Model 356 solves such problems when other spectrophotometers can't for a number of reasons. Analyses of turbid samples are done with the large photomultiplier of the Model 356 only 3mm from the sample-light scattering losses are minim ized, Band overlap problems are minimized by direct recording of the first derivative curve of the absorption spectrum. Trace analyses are done by using the 700X scale expansion to achieve lower detection limits.

Simultaneous measurement of concentration changes of two components in a single reaction cell can be done by using the DoubleWavelength mode, Kinetics of slow reactions or reactions at high dilution are made using the low noise 700X scale expansion a n d the Double-Wavelength mode. For full details, write for our Model 356 brochure to: Instrument Division, Perkin-Elmer Cbrporation, 702 Main Avenue, Norwalk, Connecticut 06852.

PERK1N-ELM El3 Bovine Albumin 1

Derivative c i i w

!

Absorption curve

1

600

WAVELENGTH IN nm CYTOCHROMES IN YEAST - This absorption soectrum of a veast cell SusDension clearlv shows the absorption bands of the various cytdchromes T o compensate f o r high turbidity, 8 sheets of filter paper were required in the reference beam The sampling geometry and the sensitivity of the Model 356 are such that the transmitted light has been measured through fifteen sheets of Whatmans No 2 filter paper at 600 m p with 1 mp bandpass

I

LEAVES - This is the absorption spectrum of a lettuce ieaf at liquid nitrogen temperature. Note the shoulder at approximately 700 mir. This may arise from a component referred to as P700 and is quite difficult l o detect in vivo.

Readers' Service

i

350

WAVELENGTH I N nm

WAVELENGTH IN nm

Circle No. 143 on

I

300

- These

curves illustrate the relationship between the absorption spectrum of a milk sample containing added bovine albumin, and its first derivative curve. The presence of bands hidden in the absorption spectrum is clearly shown in the first derivative curve. MILK

Card


39A

New family of I?A.R.

ELECTROMETERS

Model 1 3 4 -extended voltage, current, charge and resistance ranges, $615

n

I.‘

9

W Model 1 3 5 - internal battery power supply and off-ground oprration, $675 . I

9 I

k 3

i l

Model 136 - digital d i s p l a ~and BCD output, $995

--

Zeio stability

Atomic fluorescence spectrometry - ACS Publications - American

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