Atomic Fluorescence Flame Spectrometry - ACS Publications

20 Atomic Fluorescence Flame Spectrometry DAVID

W.

ELLIS

and

DONALD

R. D E M E R S

Downloaded by CHINESE UNIV OF HONG KONG on November 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch020

University of N e w Hampshire, D u r h a m , Ν. H.

Atomic fluorescence flame spectrometry is a new flame method of analysis which is competitive with and often superior to atomic absorption and flame emission methods for the analysis of trace inorganics. Various experimental arrangements with different burners, flames, and excitation sources have been used successfully. Several advantages of atomic fluorescence as compared with the other flame meth ods are discussed. Using an intense line source, atomic fluorescence is more sensitive for most elements than other flame methods. Using a continuum source, such as the xenon arc, a qualitative and quantitative analysis can be performed simultaneously by scanning the spectrum. Chemical interferences are similar to those found in atomic absorption and can usually be eliminated by use of a suitable suppressing agent, such as strontium chloride.

À tomic fluorescence flame spectrometry has received increasing attention during the last six years, particularly since 1964, ( J , 2, 3, 4, 5, 6, 8, 9, 11, 14, 15, 16, 17, 18, 19, 20, 21, 22). This is primarily attributed to the interest generated by the work of Winefordner and co workers (8, 9, 15, 16, 17, 18, 19, 20, 21, 22). Winefordner (16, 20) has reviewed the early work in the field. ) Simply stated, the method is based upon radiational excitation of netural atoms present in a flame and the detection of their fluorescence. Atomic fluorescence differs from flame emission because in the former excitation is produced primarily by electro magnetic radiation while in the latter heat is primarily responsible. Atomic absorption, as the name implies, depends upon absorption and hence is a method dependent upon a difference between two signals, while atomic fluorescence is dependent upon the magnitude of the fluorescence signal which is measured above whatever background may be present. 326

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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327

Flame Spectrometry

Winefordner and Vickers (16) have described four basic types of atomic fluorescence: resonance fluorescence, direct-line fluorescence, step wise line fluorescence, and sensitized fluorescence. Resonance fluorescence almost always refers to the line corresponding to the transition between the ground state and the first excited state of the atom; it has received by far most of the attention to date. Direct-line fluorescence refers to the process where an atom is excited to a higher excited state and then emits a photon i n undergoing a transition to a lower excited state (not the ground state). Stepwise line fluorescence involves excitation to a higher excited state, radiationless deactivation to a lower excited state, and then emission accompanying the transition from that state to the ground state. Both direct-line and stepwise line fluorescence w i l l occur at longer wave length than the wavelength of absorption. Though no useful applications for these phenomena have as yet been published, they may have some potential usefulness i n specialized applications. Sensitized fluorescence is the phenomenon whereby an excited atom transfers its excitation to another atom which then emits radiation. The probability that this phenomenon w i l l be analytically useful seems slight, because the con centration of donor and acceptor atoms i n flames is low, and because excited atoms are primarily deactivated by collisional and radiational means. A study by Goodfellow (6) with C d , In, T l , and Zn supports this hypothesis. Winefordner and co-workers (9, 16, 20, 22) have developed the theory of atomic fluorescence flame spectrometry most extensively. The integrated intensity of atomic fluorescence, //, in w-sec./cm. -ster. for low concentrations of absorbing atoms is given by the following equation: 2

_

I Q ^A 0

A

t

where I is the excitation intensity in w-sec./cm -ster. for a continuous source, Ω is the solid angle (no units) over which excitation occurs, Φ is the total power or energy efficiency for fluorescence (no units) which is equal to the quantum efficiency φ for resonance absorption-fluorescence, c is the sample aspiration efficiency, which is related lineraly to N, the atomic concentration of the absorbing atoms, A is the fraction of the incident radiation absorbed by the atoms of interest (the total absorption factor ) i n sec" and 4 π is the number of steradians i n a sphere. A more detailed discussion of the theoretical and applied considerations has been presented by Winefordner (9, 21, 22). A t low concentrations, the theory predicts calibration curves with linear slopes; with high concentrations, self-absorption and other effects may cause a decrease i n the slope of the calibration curve. 2

0

Α

t

1


328

T R A C E

I N O R G A N I C S

I N

W A T E R

Several advantages of atomic fluorescence, as compared with thermal emission and atomic absorption methods, are apparent. ( 1 ) The intensity of the fluorescence can be increased by increasing I , the intensity of the source. Generally, the limit of detection is also decreased as I increases, because the signal to noise ratio is increased. 0


0

(2) A n y change which increases the value of Φ or c (for example changing the character of the flame or the solvent) w i l l result in an i n creased signal. (3) For atomic fluorescence, the emission profile of the source can be wider than the absorption profile of the Une—even a continuum can be used. A source with a spectral profile narrower than the absorption profile of the line is usually preferred for atomic absorption. (4) W i t h atomic fluorescence, the fluorescence signal is added to a constant small background signal which permits improvement of the fluorescence signal by electronic amplification until the system becomes noise-limited. This advantage is also inherent to flame emission; however, with atomic absorption a difference measurement is made such that, as the concentration of the ion decreases, the signals of the blank and sample approach each other. A theoretical comparison of the sensitivities of the different flame methods has been presented by Winefordner (9). While it is not immediately apparent, another advantage of atomic fluorescence is the unusually wide concentration range over which linear calibration curves are obtained, usually of the order of 10 to 10 . This represents a major improvement over atomic absorption with which the practically useful linear region of concentration may be as small as ten and seldom exceeds 100. Naturally the greater the region of linearity of a calibration curve the less time is consumed in necessary sample dilutions. As a disadvantage of atomic fluorescence, there are some cases where scat tering by liquid droplets or solid particles in the flame contributes to the background signal. This disadvantage is largely eliminated if a continuum source is used and the spectrum is scanned over the line of interest. Both atomic absorption and atomic fluorescence methods have the advantage over flame emission methods that ultraviolet Unes are useful. 2

4

The various instruments used for the measurement of atomic fluores cence have been similar to each other in principle and optical design. In most studies, the source of excitation, of whatever type, has been focused on the flame; the fluorescence, usually at a right angle, has been focused on the entrance slit of the monochromator. The detector in all studies has been a photomultiplier tube, the output of which has been amplified and recorded. Figure 1 is a block diagram of the apparatus used successfully in our laboratory (5); it is quite similar to one described by Winefordner



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(15). A n atomic absorption spectrophotometer modified for atomic fluorescence measurements has been used b y Dagnall et al. (3, 4). A wide variety of excitation sources have been used successfully. They are conveniently separated into two categories: The line sources emitting one or a few lines (such as the resonance lamps, electrodeless and arc discharge lamps, various types of hollow cathode lamps) and continuum sources such as the xenon arc. The obvious advantage of a continuum source is that only one source is needed for all elements, pro vided the source has sufficient intensity. If one wishes to analyze for more than one element, the xenon source is particularly advantageous. By scanning the spectrum, the region between the lines of interest pro vides the background signal because of scattering; the fluorescence signal is superimposed on this. However, for those applications where the best possible limit of detection must be obtained, a line source would normally be preferred; this is because there are available line sources for most ele ments which have much more intense emission at the resonance line than can be obtained practically with a continuum source. Thus, as mentioned above, use of a line source would result in a lower limit of detection, pro vided that the source is intense over the atomic absorption line. Phase-sensitive Amplifier

Recorder A. B. C. D. E. F. G. H.

Figure 1.

Continuum Source - Xe Lamp Chopper Lens Phototube Flame Light Tube P.M. Tube and cooling chamber

Block diagram of an atomic fluorescence flame spectrometer

Almost all studies have involved aqueous solutions; however, two studies (3,18) have indicated that the use of non-aqueous solvents results in an enhancement of the fluorescence signal for certain systems. The maximum improvement has been of the order of 5 to 8. The background signal caused by scattering is also markedly reduced with organic solvents. Various different types of burners have been used by different work ers. In most of the studies by Winefordner and co-workers ( 8 , 1 5 , 1 7 , 1 8 ) , a total consumption burner was used. Other workers (2, 5) have also used total consumption burners with considerable success. Veillon and co-workers (15) reported that the background signal caused b y scattering


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I N

W A T E R

from water droplets and salt particles was less with a heated-chamber aspirator-burner than with a total consumption burner. Dagnall et ah (3, 4) have also had excellent results with a chamber type aspiratorburner. In a comparison of two different total consumption burners, Ellis and Demers (5) observed only minor differences.


Table I. Limits of Detection (in p.p.m.) for Several Elements in Oxyhydrogen and in Hydrogen—Entrained-Air Flames Element Ag Co Cu Fe Mg Mn Ni Tl Zn

H /0 2

2

0.003 3.50 0.10 25.00 0.18 35.00 5.00 0.30 0.35

H

/Entrained-Air

g

0.001 0.18 0.018 1.8 0.004 0.04 1.0 0.07 0.01

Two additional variables have been shown to be extremely important: the type of flame used and the height in the flame above the burner at which excitation and emission occur. ( Naturally, the height in the flame is also dependent upon the type of burner and the flow rates of the gases. ) Both of these variables are important since the population of neutral atoms ( N ) depends directly on them. Work in this laboratory (5) has shown that the use of an hydrogen-entrained-air flame is greatly superior to an oxyhydrogen flame for those elements which tend to form refractory oxides while the improvement was much less for elements that do not tend to form refractory oxides. Table I presents some representative data. Veillon and co-workers (J5) found similar results using an argon-hydrogenentrained-air flame. Dagnall and co-workers (3, 4) have recommended that air-propane be used in preference to other flames, especially below 3200 A . In most of the studies to date, the use of a reducing flame has been generally preferred. As would be expected the concentration of free neutral atoms varies both horizontally across the flame and vertically within the flame. A t some particular height, the concentration reaches a maximum and it is at this height that one wishes to operate. Winefordner and Staab (17,18) have stressed the importance of determining the best height. Dagnall and co-workers (4) have investigated the effect caused by the height above the burner at which excitation occurred as well as measuring the effect of changing the fuel gas pressure at a constant burner height. In our studies, we have found that 8.5 cm. is optimum for silver while 5.5 cm. is optimum for calcium.



20.

ELLIS AND DEMERS

Flame Spectrometry

331

The different types of detection and readout systems have all included photomultiplier tubes as detectors. The readout systems have been based on a number of different modes of operation. D . C . systems have been used successfully (6, 8, 17) though it is then necessary to measure the fluorescence as a signal which is additive to the background emission sig nal. D . C . systems are most usable if measurements are made above the luminous tip of the flame or below 3000 A . where the background emission owing to thermal emission is much reduced. Mechanical or electrical choppers and various types of a.c. amplifiers have been used extensively as a means of eliminating any background signal caused by d.c. processes, such as thermal emission. Tuned a.c. amplifiers have been used b y Armentrout (2) and by Goodfellow (6) i n conjunction with a mechanical chopper. Dagnall and co-workers ( 3, 4 ) have used an electrically modu lated source and a tuned a.c. amplifier. Both Winefordner and co-workers (15) and Ellis and Demers (5) chopped the excitation mechanically and used phase-sensitive ( or lock-in ) amplifiers. Typical calibration curves obtained for several elements are shown i n Figures 2 and 3. The lowest concentration indicated is that for which the magnitude of the signal equals the peak-to-peak, or absolute magnitude, of the noise. This represents a point which is easily measured experi mentally. The plots shown i n Figures 2 and 3 show varying slopes which for some elements deviate appreciably from unity. If the entire flame cell is illuminated by the excitation source and all of the fluorescence radiation is measured, the absorption factors should approach unity (22) and the experimental analytical curves should have a slope of unity. A t high con centrations of absorbing atoms, the absorption factors can approach zero 10000_

Tl

-Ag •Mg Cu

01

0.001

0.01

0.1

1

10

100

1000

Concentration p.p.m.

Figure 2. Experimental analytical curves for copper (Cu 3248 Α.), magnesium (Mg 2852 Α.), silver (Ag 3281 Α.), and thallium (Tl 3776 A.)


332

T R A C E

I N O R G A N I C S

1000 _

I N

W A T E R

Ca •Co


Zn

0)

ta

1

.01

0.1

10

1

100

1000


Figure 3. Experimental analytical curves for calcium (Ca 4228 Α.), cobalt (Co 2407 Α.), iron (Fe 2481 Α.), nickel (Ni 2320 Α.), and zinc (Zn 2139 A.) and the curves w i l l level off or they may even bend back toward the abscissa in some cases, for example, with copper. A fair comparison of atomic fluorescence with atomic absorption and flame emission is difficult because there is little agreement on a standard definition for the term "detection limit. Table II is an attempt to make such a comparison for selected elements. From the data in Table II it can be concluded that for atomic fluorescence the use of a high-intensity line source improves significantly the limits of detection as compared with the use of the 450 watt xenon source. This is attributed to the increased intensity of the exciting radiation at the specific wavelength of interest, as for example with the elements C d , H g , N i , T l , and Zn. F o r these same elements, the detection limits using a line source are equal or superior to those obtained using either atomic absorption or flame emis sion methods. A comparison of the limits of detection for atomic fluores cence using the 450 watt xenon source are better than those of flame emission and are approximately equal to those of atomic absorption, except for F e and N i , for which atomic absorption is superior. Although several workers (4, 15) have used a 150 watt xenon source, as expected, the limits of detection obtained were poorer than those obtained with the 450 watt source; data obtained with the larger source is therefore listed. ,,

For routine work it is often important to know the minimum concen tration that can be reliably and conveniently included as part of a cali bration curve, rather than to know the detection limit. For this purpose, the 1 % absorption column in Table II under atomic absorption becomes extremely useful. Comparable values for atomic fluorescence can be obtained using that solution concentration which results i n a signal whose absolute magnitude is equal to the absolute magnitude of the noise. T o


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obtain such comparable values, the values listed in Table II should be multiplied by a factor of about three. (The detection limits in Table II are in terms of a signal to rms noise ratio of unity. ) Table II.

A Comparison of Detection Limits in p.p.m. for Different Flame Methods


Atomic Fluorescence Xe(450 watt)

9

Ag Ca Cd Co Cu Fe Hg Mg Mn Ni Tl Zn

0.001 0.02 —

0.18 0.018 1.8 —

0.004 0.04 1.1 0.07 0.01

1% Abs.

— — e

—

0.04

e

—

0.1

e

— —

0.1 > 0.04 0.0001 d

Flame Emission

e

Line Source

0.0002

Atomic Absorption

e

e

e

0.1 0.1 0.04 0.45 0.2 0.3 10 0.01 0.15 0.2 1.0 0.04

Det. him.

Det. Limit

0.02 0.01 0.01 0.15 0.005 0.05 0.5 0.003 0.01 0.05 0.2 0.005

0.04 0.01 0.05 0.25 0.1 0.12 6 0.02 0.01 0.12 0.1 —

" Unpublished data by Ellis and Deniers, limit of detection taken as signal-to-noise ratio of 1. b r d r

See Ref. 12,13.

See Ref. 7. See Ref. 2. See Ref. 8, 18.

Various types of interferences are found with flame methods in gen eral. Both atomic absorption and atomic fluorescence are not susceptible to excitation interference, which is caused by changes in flame tempera ture and results primarily in changes in the excited-state population; radiation interference can be largely eliminated using a modulated sys tem. Dagnall et al. (3, 4) have studied exhaustively the effect of 41 cations, 18 anions and five reagents on the atomic fluorescence of cad mium and zinc; they found no evidence of interference. Interference studies in this laboratory have been concerned with calcium which would be expected to show interferences much more readily than cadmium or zinc. The ions studied were chosen on the basis that they might be present in water. Standard solutions of calcium chloride were used for comparison. The anions studied were present as the sodium or potassium salt, and the cations as the chloride salt. The results obtained are listed in Table III and several interference curves are shown in Figure 4. The interferences found were similar to those obtained for calcium with atomic absorption (10); this agrees with what would be expected on theoretical grounds. The addition of


334

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I N O R G A N I C S

I N

W A T E R

S r C l * 6 H 0 (final concentration 0.1% ) eliminated all of the interferences with the exception of the ones attributed to sulfide and silicate anions, for which the interference was decreased. 2

2

Table III.

Interference Studies with Calcium


No Interference

0

Interference—% Signal Depression

Ba, Co, Cu, Κ La, Mn, Na, Sr Zn

Al Cr Fe Li Mg Pb .. citrate oxalate

65 75 25 70 10 50 70 100

75

CO3 2

F" HC0 ~ I" N0 " P0 S" S0 -

85 75 80 65 100 80 75

3

3

4

3

2

4

2

" CaCl2 as standard ( 4.0 p.p.m. ). Interfering ion (400 p.p.m.): anions present as Na or Κ salts, cations as CI salts. On

204L

IOOJ 0

I

I

I

I

I

I

500

I

I

1

1 1000


Figure 4.

Depression of the atomic fluorescence of calcium by selected ions

As of this time, atomic fluorescence flame spectrometry has not been reported as having been applied to any specific analytical problem. One can readily ascertain that it should be applicable i n many areas where atomic absorption is commonly used. In addition, for the analysis of multiple elements in a single sample, for example cations i n water, atomic fluorescence flame spectrometry incorporating a xenon arc source should have major advantages. Experimental A l l results listed except where specifically noted were obtained with an apparatus composed of a 450-watt Osram high pressure xenon arc



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source; the burner was a Jarrell-Ash Hetco total consumption burner operated at a hydrogen flow rate of 20 liter/min. through the central ori fice. Oxygen for combustion was provided by the entrained air (5). Aqueous samples were used; they were injected using a Sage motordriven syringe at a rate of 7.5 ml./min. (This rate or introduction of sample provided a 6 0 % enhancement of the signal as compared with a sample introduction rate of 2.0 ml./min.). Excitation and fluorescence were at a height of 8.5 cm. above the burner for all elements except calcium, which was measured at a height of 5.5 cm. above the burner. The fluorescence was detected using a Jarrell-Ash 0.5 meter scanning monochromator equipped with an E M I 6255B photomultiplier tube which was thermoelectrically cooled at —15°C. The output of the photomulti plier tube was fed to an E . M . C . phase-sensitive amplifier ( M o d e l R J B ) with a 3 second time constant and recorded on a Leeds and Northrup strip chart recorder. The limits of detection listed in Table II are opti mum values for A g and C a ; it is anticipated that the other values can be improved by further optimization of the experimental parameters, such as the flow rate of hydrogen, the rate of introduction of sample, height above the burner, slit widths, etc. Conclusion Atomic fluorescence flame spectrometry is receiving increased atten tion as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pres sure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectrurn. Only chemical interferences are of concern; they appear to be quali tatively similar for both atomic absorption and atomic fluorescence. W i t h the current development of better sources and investigations into devices other than flames for sample introduction, further improvements i n atomic fluorescence spectroscopy are to be expected. Acknowledgment The work upon which this publication is based was supported in part by funds provided by the United States Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research A c t of 1964. The authors also wish to express their apprecia tion to Michael Pleva for his assistance in part of the work.


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Literature

Cited

(1) Alkemade, C. T. J., "Xth Colloquium Spectroscopicum Internationale, Proceedings," E. R. Lippincott and M . Margoshes, Eds. p. 143. Spartan Books, Washington, D . C., 1963. (2) Armentrout, D . N., Anal. Chem. 38, 1235 (1966). (3) Dagnall, R. M., West, T. S., Young, P., Talanta 13, 803 (1966). (4) Dagnall, R. M., Thompson, K. C., West, T. S., Anal. Chim. Acta 36, 269 (1966). (5) Ellis, D . W., Demers, D . R., Anal. Chem. 38, 1943 (1966). (6) Goodfellow, G . I., Anal. Chim. Acta 36, 132 (1966). (7) Herrmann, R., Alkemade, C. T. J., (trans, by P. T. Gilbert, Jr.), "Flame Photometry," Interscience Publishers, John Wiley & Sons, Inc., New York, Ν. Y., 1963. (8) Mansfield, J. M., Winefordner, J. D . , Veillon, C., Anal. Chem. 37, 1049 (1965). (9) Parsons, M . L., McCarthy, W . J., Winefordner, J. D., J. Chem. Ed. 44, 214 (1967). (10) Ramakrishna, T. V., Robinson, J. W., West, P. W . , Anal. Chim. Acta 36, 57 (1966). (11) Robinson, J. W., Anal. Chim. Acta 24, 254 (1961). (12) Robinson, J. W., "Atomic Absorption Spectroscopy," Chap. 5, Part III, Marcel Dekker, In., New York, Ν. Y., 1966. (13) Slavin, W . R., Bio-Medical Lab. Dir. 1, 8 (1966). (14) Sullivan, J. V., Walsh, Α., Spect. Acta 21, 727 (1965). (15) Veillon, C., Mansfield, J. M., Parsons, M . L., Winefordner, J. D., Anal. Chem. 38, 204 (1966). (16) Winefordner, J. D . , Vickers, T. J., Anal. Chem. 36, 161 (1964). (17) Winefordner, J. D., Staab, R. Α., Anal. Chem. 36, 165 (1964). (18) Ibid., 36, 1367 (1964). (19) Winefordner, J. D . , Vickers, T. J., Anal. Chem. 36, 1939 (1964). (20) Winefordner, J. D . , "Conference on Trace Characterization, Chemical and Physical," Analytical Chemistry Division, Natl. Bur. of Std., Wash ington, D . C., Oct. 1966. (21) Winefordner, J. D . , Parsons, M . L., Mansfield, J. M., McCarthy, W . J., Anal. Chem. 39, 436 (1967). (22) Winefordner, J. D . , Parsons, M . L., Mansfield, J. M., McCarthy, W . J., Spectrochim. Acta 23B, 37 (1967). RECEIVED

April 24, 1967.


Atomic Fluorescence Flame Spectrometry - ACS Publications

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