ample, the data of Harrison and Wadlin (11).However, by measuring an emission or absorption profile across the flame, it is possible to establish whether a concomitant causes a lateral diffusion interference even if that concomitant produces a large overall depression.
SOME INTRIGUING DEPRESSION EFFECTS During studies involving Mo as the analyte, several concomitants were found which depressed the Mo absorption or emission signal to a surprising degree throughout the flame. Tables V and VI summarize these results, and a typical profile is shown in Figure 10. The effects of Ca, K, and Na are similar to those reported for CaC12, KCl, and NaCl by Van Loon ( 1 5 ) . Center enhancements are small or non-existent with these concomitants. Whether this is due to the absence of a lateral diffusion interference effect
or to the high stability of the analyte-concomitant mixture is uncertain. The latter seems more likely; if the solid particles were unstable, large depressions high in the flame would not be expected. Of particular interest is the very large depression of Mo absorption and emission produced by Ti(III), in contrast to the enhancement by Ti(1V). We can offer no explanation of this difference.
ACKNOWLEDGMENT The authors gratefully acknowledge the assistance of Constance C. Butler for some of the early observations on the lateral diffusion interference effect. Received for review November 30, 1972. Accepted March 15, 1973.
Theoretical and Experimental Evaluation of Resonance Monochromators for Atomic Absorption Spectrometry E. F. Palermo and S. R . Crouch' Department of Chemistry, Michigan State University, East Lansing. Mich. 48823
Theoretical expressions are developed for the use of resonance monochromators in atomic absorption flame spectrometry in order to point out the parameters which influence the output of the resonance monochromator and to compare atomic absorption sensitivities to those with conventional monochromators. Continuum source atomic absorption sensitivities are shown to be comparable to sensitivities with a line source because of the narrow spectral bandpass of the resonance monochromator. A demountable resonance monochromator has been designed and evaluated. Experimental results indicate that calibration curves with the resonance monochromator and a line source are much less dependent upon self-reversa1 in the source than are calibration curves with a conventional monochromator. Experimental growth curves for a line source and a continuum source qualitatively agree with theoretical predictions. The narrow spectral bandpass of the resonance monochromator is also shown to be useful in reducing spectral interferences.
Resonance radiation from a hollow cathode discharge tube was first observed in 1959 by Russell and Walsh ( I ) , who noted that an appreciable amount of the metal sputtered from the cathode was in the form of an atomic vapor. Since this atomic vapor is a t low temperature and pressure, the conditions are those required for the fluorescence of any absorbed radiation, i.e., parameters may be chosen to give a high quantum yield. In 1965, Sullivan and Walsh (2, 3 ) used this principle to construct their first resonance detector, in which the atomic vapor was 1
A u t h o r t o whom requests f o r r e p r i n t s s h o u l d b e addressed
(1) 8.J. Russell and A. Walsh, Spectrochim. Acta.. 15, 883 (1959). (2) J. V. Sullivan and A. Walsh. Spectrochim. Acta.. 21, 727 (1965). (3) J. V. Sullivan and A. Walsh, Spectrochirn. Acta.. 21, 721 (1965).
1594
produced by cathodic sputtering. Later articles ( 4 , 5 ) reported on the design and application of resonance detectors which utilized electrical heating to produce atomic vapor clouds for the more volatile elements. There are several reasons why the resonance monochromator is attractive for atomic absorption flame spectrometry. First, any unwanted flame background emission which reaches the photomultiplier transducer should be very low because the photomultiplier is positioned a t right angles to the incident radiation source and because the spectral bandpass of the resonance monochromator is determined by the line width of the fluorescent radiation; typically about 0.01 A (2). As a result of this low spectral bandpass. a second attractive feature is the virtual elimination of spectral interferences. Interferences due to the absorption of impurity or filler gas lines in the hollow cathode by atomic species in the flame should be virtually eliminated, while interferences due to absorption by molecular species in the flame should be reduced. Another potential use of the resonance monochromator is in conjunction with a continuum source. With conventional dispersive monochromators, continuum sources have not proved extremely useful in atomic absorption because absorption sensitivities are usually lower than with line sources and dependent on the monochromator spectral bandpass (6-9), unless techniques like selective modulation are used ( 1 0 ) . Demountable resonance monochromators, or single resonance monochromators in series, on the other hand, could prove very useful with continuum sources. (4) J. V. Sullivan and A. Walsh. Spectrochim. Acta.. 22. 1843 (1966). ( 5 ) J. V. Sullivan and A. Walsh. Spectrochim. Acta.. 236, 131 (1967). (6)V. A . Fassel and V. G. Mossotti. Anal. Chern.. 35, 252 (1963). (7) W. McGee and J. D. Winefordner. Anal. Chim. Acta.. 37. 429
(1967). (8) A . Walsh. Spectrochim. Acta.. 7,108 (1955). (9)J. D. Winefordner. Appl. Spectrosc.. 17,109 (1963). (10) V. G. Mossotti, F. N. Abercrornbie, and J. A . Eakin. Appl. Spectrosc., 25. 331 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
In this paper, the theoretical basis for the use of the resonance monochromator in atomic absorption flame spectrometry is presented. Equations for the spectral radiance output of the resonance monochromator with both an incident line source and a continuum source are derived in order to point out its unique advantages and possible limitations when compared to a conventional dispersive monochromator. A demountable resonance monochromator with provision for rapid interchange of eleme& has been designed and evaluated. Experimental data are presented for a resonance monochromator-line source combination and a resonance monochromator-continuum source combination which point out that the fluorescence signal us. atomic population curves qualitatively agree very well with theoretical growth curves. Optimum parameters for a demountable zinc resonance monochromator are discussed, and a comparison of atomic absorption calibration curves with the resonance monochromator and a conventional monochromator is made. A study of the interference of copper on the atomic absorption of lead is described, which points out the usefulness of the resonance monochromator in reducing spectral interferences. Finally, absorption sensitivities for a line source and conventional monochromator are compared to those obtained with a continuum source and a resonance monochroma tor.
RADIANCE EXPRESSIONS FOR ATOMIC ABSORPTION WITH RESONANCE MONOCHROMATORS Atomic absorption flame spectrometry with a resonance monochromator involves normal flame absorption of a line or continuum source. The transmitted radiation is then incident on the resonance monochromator in which absorption and re-emission (fluorescence) of the resonance radiation takes place. In this section, expressions for the radiance output of atomic absorption and atomic fluorescence are reviewed and then combined to give a complete expression for the radiance output of the resonance monochromator. Block diagrams for the three systems that will be considered in this discussion, Atomic Absorption Spectrometry (AAS) Atomic Fluorescence Spectrometry (AFS) and Atomic Absorption Spectrometry with a resonance monochromator (RDS) are shown in Figure 1. Atomic Absorption Spectrometry. Expressions for the radiance absorbed in atomic absorption spectrometry have been derived by Winefordner et al. ( 2 1 , 22) for a line source (AAL) and a continuum source (AAC). Equations 1 and 2 give the radiance of a line source, BTL, and a continuum source, B T C h o , after absorption by the ground state analyte atoms in the flame. All symbols are defined in Appendix I. BTI. = BL exp[-k/b(a,t')l (1) BTCh,,
=
Bcx,[s- exp(-koZ)]AX, -
S
E L
ATOMIC VAPOR
%Ao
CELL
b
BTL BTCA,
P *
A.F.S. LINE OR CONTINUUM SOURCE
E L
ATOMIC VAPOR
BCA,
CELL
BAFCBAFL
R
R.D.S. _ .
1'
&
L
LINE OR CONTINUUM SOURCE
ATOMIC VAPOR CELL
E L
=Tcxo
L--,J BRFL
~ R F C
Block diagrams for Atomic Absorption Spectrometry (AAS), Atomic Fluorescence Spectrometry (AFS), and Atomic Absorption Spectrometry using a resonance monochromator Figure 1.
(RDS)
(3) Aaac = -log [s - {I - exp(-h,l)\AAD S
]
(4)
Only when the spectral bandpass, s, approaches the Doppler line half-width in the flame, AXD, will absorption sensitivities using a continuum source approach absorption sensitivities using a line source. Of course, there will still be a dependence on the CI parameter ( 4 X s / A h ~ )of the Voigt profile in the case of the line source, so the comparison cannot be exact. Also as s approaches 4x11, the Voigt profile may no longer be neglected. Atomic Fluorescence Spectrometry. Winefordner et al. (12) have also derived expressions for atomic fluorescence spectrometry. Equations 5 and 6 give the fluorescence radiance with a line source, BAFL,and continuum source BAFC, respectively.
(2)
It is assumed in Equation 1 that the source line halfwidth, ~ X S , is much smaller than the absorption halfwidth in the flame, AXA. Therefore, D is an average over the range from zero to AXsfAX.4. In Equation 2, it is assumed that the spectral bandpass of the system, s, is Theremuch larger than the absorption half-width, SA. fore, there is no dependence on the Voigt profile, b(a,u). If the absorbance is defined in the usual manner, Equations 3 and 4 result. (11) L. de
LINE OR CONTINUUM SOURCE
G a l a n , W . W. McGee. and J. D. W i n e f o r d n e r , Anal. Chim. Acta., 37,436 (1967). (12)J. D. W i n e f o r d n e r , V . S v o b o d a , and L. J. C l i n e , CRC Crit. Rev. Anal. Chem.. 1, 233 (1970).
where [XI is the self-absorption factor (12), and all other terms are defined in Appendix I. Atomic Absorption with Resonance Monochromator. When a resonance monochromator is used, the transmitted radiance from the flame is absorbed by the ground state atoms of population no' in the resonance monochromator. The radiance absorbed when a line source is used is given by
BRAL = B T L P J [~ exp(-k,'l'd'(a, vlldh' 4
(7)
5
where the factor /3 is used to account for any transmitted
ANALYTiCAL C H E M I S T R Y , VOL. 45, NO. 9, AUGUST 1973
1595
line in the flame with 0 ranging from zero to AAD’IAAA. If Equation 11 is substituted into Equation 9, Equation 12 results for the radiance absorbed in the resonance monochromator with a continuum source.
Table I. Dependence of Resonance Monochromator Radiance Terms on no and no’ Dependence on no‘ at constant no Radiance term BRAL BRAC
[XfI BRFL BRFC
Low f l g
High no’
#
no’ no’
equal to 1 nof no’
f(no’)
1 / 6 7 1/m P f(nof)
radiation which does not reach the resonance monochromator, and any fluorescence from the flame is assumed to be negligible. Since the source half-width, AAs, is still much less than AAA, BTL from Equation 1 may be used directly in Equation 7 . However, since AAs is not much smaller than the absorption line half-width in the resonance monochromator, AAD‘, the integration must be carried out over AAs to obtain an exact expression. Equation 8 results when BTL from Equation 1 is substituted into Equation 7 : BRAL = BL exp {-kJ6(a,c..)lPS[l - exp{k,’l‘b’(a,u)}ldX’ (8) Ah,
The prime symbol (’) is used to differentiate terms in the resonance monochromator from terms in the flame. Thus, when a resonance monochromator-line source combination is used, the spectral bandpass of the system will be determined by the overlap of the emission-absorption profiles in the resonance monochromator. When a resonance monochromator-continuum source combination is used, the spectral bandpass of the system is dependent only on the Doppler half-width of the absorption line in the resonance monochromator. The radiance absorbed when a continuum source is used is given by BRA, = B,CA,PJ
[l - exp(-k,’ZWa,u)J]dX’
(9)
Ai”’
Now the integrated absorption is taken over the resonance monochromator Doppler half-width, AAD‘, to obtain an exact expression. However, B T C Aof~Equation 2 may not be used in Equation 9 because Equation 2 assumes that the monochromator spectral bandpass, s, is much larger than the absorption half-width, AAA. In fact, when a resonance monochromator is used, the spectral bandpass, AAD‘, is less than AX*. The exact expression for B T C Ais~ thus, ”
If the absorption half-width in the resonance monochromator is similar to the absorption half-width in the flame, . for then Equation 10 must be used for B T C A ~However, cases when the absorption half-width in the resonance monochromator is narrower than that in the flame Equation 10 can be simplified to where 6(a,u)‘ describes the Voigt profile for the absorption 1596
- exp(-k,’l’6’(a,u)J]dX’
AX”‘
(12)
The radiances of resonance fluorescence emitted from the resonance monochromator with a line and continuum source are given by
Dependence on no at constant no’
--_--_
BRAC = BcA,[exp{-ko16(a,@’]]PJ[l
m
In both cases, the fluorescent radiance from the resonance monochromator is dependent on both the number of ground state analyte atoms in the flame, no, and the number in the resonance monochromator, no’. Table I gives the dependence of the resonance monochromator radiance terms on both n, and no’. It is interesting to note that the fluorescence radiance with a continuum source, BRFC,is predicted to become independent of the ground state atom population in the resonance monochromator, no’,at high no’. On the other hand, when a line source is used, BRFLis predicted to pass through a maximum. Therefore, BRFLis more susceptible to changes in no’ than is BRFC. If it is assumed that the concentration of atoms in the resonance monochromator remains constant, and that, therefore, ko’ remains constant, the following expressions result for the absorbances when a resonance monochromator is used with a line source (Equation 15) and a continuum source (Equation 16).
k,16(a, D) =
2.303
k,16(a, ij)’ 2.303
(15) (16)
Equations 15 and 16 predict that if the resonance monochromator has a spectral bandpass less than the Doppler half-width of the absorption line in the flame, the absorption sensitivity for a continuum source, Equation 16, is quite comparable to the absorption sensitivity for a line source, Equation 15. If the b parameters are identical, then ARFL= ARFC.However, the emission line width of a line source will usually be narrower than the absorption line width in the resonance monochromator. Under these conditions, Equations 15 and 16 predict that a line source working curve should become nonlinear a t a lower concentration than a continuum source working curve, because the D parameter is smaller for a line source ( AAs < AD'). EXPERIMENTAL Detector Design. Early in this work, a completely demountable resonance monochromator was deemed most desirable. With a demountable unit, fundamental studies of the effect of experimental parameters would be facilitated. Also anticipated analytical applications in the area of multielement analysis and continuum source atomic absorption dictated that the metals of interest be readily interchangeable. A thermal detector was chosen because many of the elements which show high sensitivity in atomic absorption and fluorescence spectrometry (Zn, Cd, Hg, Pb, etc.,) are relatively volatile, and atomic vapors can be readily obtained thermally. Therefore, the detector described here contains a completely interchangeable heating element in which metals or alloys can be placed. The resonance monochromator shown in Figure 2 consists of two detachable sections. The first section contains the entrance
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
'
window and side window where the emitted radiation is directed to the cathode of the photomultiplier transducer, while the second section consists of the heating assembly and the attachment for connection to a vacuum line. The two parts are connected by way of a 7 50/50 ground glass joint. Alumina (Morganite, Inc., Dunn, N.C., ARR crucibles, C.O.E.) was chosen to contain the metal to be heated because of its stability at high temperatures, chemical inertness, good electrical resistivity, and high thermal conductivity. Nichrome wire (No. 30 gauge) was placed between two alumina cylinders, and castable alumina was used as a sealant. A cylinder of the metal, or metals, of interest is placed inside the alumina boat and heated by passing current through the nichrome wire. The detector is evacuated with a diffusion pump to a pressure of approximately 10-3 Torr, and current is passed through the nichrome wire for a few minutes. Filler gas is then added to the desired pressure. Optimum conditioning of the detector for analytical applications is discussed in a later section. Atomic Absorption System. A block diagram of the system used is shown in Figure 3, and the specific experimental system is described in Table 11. For comparison purposes, the resonance monochromator and associated power supply were frequently replaced by a conventional grating monochromator. Since the radiance from the source is modulated at 30 Hz,the output signal from the lock-in amplifier is due primarily to the fluorescence signal. Unmodulated radiation from stray room light, emission from the flame, or flame background does not contribute to the measured signal, but can introduce shot noise. Therefore, the resonance monochromator was housed in a light-tight black box, and a solar blind photomultiplier tube was used.
RESULTS AND DISCUSSION Resonance monochromators with Zn, Cd, or P b placed inside the alumina boat were constructed in order to verify the theoretical expressions derived previously and to compare them with conventional grating monochromators for analytical atomic absorption applications. Fluorescence Growth Curves. In order to verify qualitatively the theoretical fluorescence growth curves (Equations 13 and 14), the ground state atom population in the resonance monochromator, no', was varied by changing the power applied to the heating element. The fluorescence signal obtained with a line source incident on the monochromator was assumed to be proportional to the fluorescence radiance, BRFL, while the signal obtained with a continuum source was assumed proportional to BRFC.The temperature of the metal in the resonance monochromator was experimentally determined to be proportional to the power applied to the heating element. Since the temperature is proportional to the logarithm of the vapor pressure of the metal (13), and thus to the logarithm of the ground state atomic population, no', a plot of log fluorescence signal us. applied power should correspond to a plot of log fluorescence radiance us. log no'. Figure 4 shows a n experimental growth curve for a Zn resonance monochromator with a Zn hollow cathode as the incident source. The atomic population in the flame, no, was assumed to be zero for the growth curves. The ratio of the slope of the plot at low applied power (ml)to that at high applied power (mz) is minus 2, as predicted in Table I for a plot of log BRFLus. log no'. To verify qualitatively the predicted growth curves for a continuum source-resonance monochromator combination, a cadmium resonance monochromator was used. Plots of log fluorescence signal us. applied power are shown in Figure 5 for both a Cd hollow cathode and Xenon arc continuum source. The continuum source plot has a slope (7713) equal to the line source slope ( m l )at low applied power (low no'), but reaches a slope (m4) of zero at high applied power (high no') as predicted in Table I for a plot of log BRFCus. log no'. (13) "C.R.C. Handbood of Chemistry and Physics." T h e Chemical Rubber Company, Cleveland,Ohio, 47th ed., 1967, p D-108.
rv IL=,
I- I o a i
QUARTZ LENS
.
----_
I
2 '4"
L
QUARTZ WINDOW
QUARTZ WINDOW
SILVER SOLDER I N N E R A L U M I N A BOAT VACUUM V A L V E
TO VACUUM BALL a SOCKET JOINT
.-t
NICKEL
-4
-
SEAL
+
Figure 2. Diagram of demountable resonance monochromator
+, "1 MULTIPLIER
H.VPS
AMPLIFIER
I
~~~FFERENTIAL I SINGLE-ENOEO CONVERTER
I I LOCK- I N AMPLIFIER
I I DIGITAL READOUT
Figure 3. Block diagram of resonance monochromator atomic absorption system
Optimum Conditions for Analytical Applications. The two major parameters of the resonance monochromator which should be optimized are the applied power and the filler gas pressure. As shown in the previous section, there is an optimum applied power which maximizes the fluorescence output when a line source is incident on the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9 , AUGUST 1973
1597
Table II. Experimental System Component
Description and type
Sources
Supplier
Fisher Scientific Co., Pittsburgh, Pa. Illurnination Industries, 610 Vaqueros, Sunnyvale, Calif. Heath Co., Benton Harbor, Mich. Illumination Industries, Sunnyvale, Calif. Heath Co., Benton Harbor, Mich.
Hollow cathode discharge tubes Model 111-150A, 150-W xenon arc lamp
Source power supplies
Model EU-703-70 AA, AE, AF module hollow cathode supply Model CA-150 xenon arc supply
Chopper
30-Hz leaf shutter in Model EU-703-70 AA, AE, AF module powered by EU703-31 photometric readout module Tri-flame, 10-cm slot premixed, air,"?
Burner
Fisher Scientific Co., Pittsburgh, Pa. Heath Co., Benton Harbor, Mich.
Model EU-700, 350-mm focal length, f/6.8 aperture, 20 A/mm reciprocal linear dispersion
Monochromators
Raytheon Co., Sorenson Div., Manchester, N.H.
Resonance monochromator as described in text powered by QB6-8 low voltage supply R 166 solar blind
Photomultiplier tube Photomultiplier power supply Pre-amplif ier
Hamamatsu Corp., Lakesuccess, N.Y. Heath Co.. Benton Harbor, Mich. Teledyne PhiIbr ic k, Dedham, Mass.
Model EU-701 Photomultiplier module
Lock-in amplifier
Current-to-voltage converter constructed from SPPA premium parametric operational amplifier Model 840 Autoloc
Readout devices
Model 3000 recorder
Keithley Instruments, Cleveland, Ohio Houston Instruments, Bellaire, Texas Heath Co., Benton Harbor, Mich.
Model EU-805 Universal digital instrument
>
a a
t
m
a
a I--
3
n I-
3
0
a
5a
I-
0
a
I
V
0 z 0
a w
$1, Y
,
,
,
,
,
,
10
12
14
16
I8
W
a
.I
6
8
POWER APPLIED, W
Figure 4. Experimental growth curve for zinc resonance monochromator with incident line source: argon filler gas; pressure, 12 Torr
resonance monochromator, while w i t h a c o n t i n u u m source t h e fluorescence signal becomes independent of applied power a t high powers. With a given resonance monochromator, t h e applied power should b e varied experimentally until t h e maximum fluorescence signal i s obtained or t h e p l a t e a u i s reached. T h e o p t i m u m applied power when a 1598
% J
,
,
,
I
2
3
4
POWER APPLIED, W
Figure 5. Experimental growth curves for cadmium resonance monochromator with incident line or continuum source: argon filler gas; pressure, 2 Torr 0 Line source; 0 Continuum source
l i n e source i s used is dependent u p o n t h e filler gas pressure.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
To determine the effect of filler gas pressure, a zinc resonance monochromator was used with a zinc hollow cathode source and operated a t the maximum of the fluorescence signal applied power plot. A plot of fluorescence signal us. argon filler gas pressure is given in Figure 6. As can be seen, the fluorescence signal is greatest when there is no filler gas present, and decreases as the argon pressure increases. These results agree with those of L'vov (14) who determined the relationship of hollow cathode emission intensity to filler gas pressure for highly volatile elements. In both cases, the signal is dependent on the diffusion of ground state atoms. Experimentally, it was found that the filler gas pressure also influences slightly the optimum power applied to the resonance monochromator. For example, for the Zn monochromator used to construct Figure 4, the optimum power was 12 W a t an argon pressure of 12 Torr. At a pressure of 6 Torr, however, the maximum fluorescence signal was obtained a t a n applied power of 17 W. The filler gas pressure also influences the lifetime of the resonance monochromator. As the pressure is increased, the diffusion of atoms is decreased, and thus the deposition of vapor on the windows of the detector will be minimized. The optimum filler gas pressure, therefore, is a compromise between fluorescence signal and lifetime of the resonance monochromator. A pressure of about 8 or 10 Torr appears to be the optimum. Sullivan (15) stated that when he used a lead resonance detector with an argon filler gas pressure of 10 Torr "deposition of vapour on the windows of the detector did not occur for a t least 2000 hours." It is interesting to note the stability of the fluorescence signal while a t the optimum power. The signal was observed on a strip chart recorder for a period of seven hours during which time the relative standard deviation of the signal was approximately 0.2%. These measurements reflect both fluctuations in the primary radiation source and in the atomic concentration in the resonance monochromator. Comparison with Conventional Monochromator. Calibration Curves for Line Source. While a t the optimum power for the zinc resonance monochromator (constant no'), a calibration curve was determined for zinc. A hollow cathode lamp current of 40 mA was used for the zinc resonance monochromator calibration curve and lamp currents of 20 and 40 mA were used for the grating monochromator calibration curves. These results are shown in Figure 7. Note that when using a grating monochromator, the absorption sensitivity for the 20-mA lamp current is greater than the sensitivity for the 40-mA lamp current. The reason for this, as shown by L'vov (16) and Rann (17), is self-reversal in the hollow cathode caused by an increase in the atomic vapor at higher lamp currents, This phenomenon is observed when using a grating monochromator because the entire self-reversed line falls within the spectral bandpass of the grating monochromator. As stated by Sullivan and Walsh ( I s ) ,a calibration curve with a resonance detector should be independent of the conditions under which the light source is operated. Our experimental results have confirmed this as can be seen in Figure 7. The resonance monochromator sensitivity, which remains constant a t various lamp currents, is greater than the sensitivity with a grating monochromator because the spectral bandpass of the grating monochromator isolates (14) 6.V . L'vov, "Atomic Absorption Spectrochemical Analysis," American Elsevier Publishing Company, New York. N.Y., 1970, p 42. (15) J. V. Sullivan, C.S.I.R.O., Clayton, Victoria, Australia 3186, personal communication. March 1972. (16) 6.V. L'vov, ref. 14. p 50. (17) C. S. R a m , Spectrochim. Acta.. 238,827 (1967). (18) J. V. Sullivan and A. Walsh, Appl. Opt. 7, 1276 (1968).
1
2
1
1
l
l
I
4
6
8
IO
12
l
14
FILLER GAS PRESSURE, TORR
Figure 6. Resonance monochromator output as a function of
argon filler gas pressure
ZINC CONCENTRATION, pprn Figure 7. Atomic absorption calibration curves for zinc with hol-
low cathode source 0 resonance monochromator; lamp current 40 mA. 0 grating monochromator; lamp current 20 mA. A grating monochromator; lamp current 40 mA
that broadened portion of the line source which does not overlap the absorption line in the flame. The absorption line-width in the resonance monochromator remains constant and, therefore, is not affected by the broadening of the emission line of the line source. From Equation 3, as AXS increases, G(a,ir) decreases, which decreases the absorption sensitivity. The decrease is observed with a grating monochromator because all of the broadened emission line falls within its spectral bandpass. However, from Equation 7, as AXS increases, it will approach and eventu-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
1599 < 'a+#*
.4c
10
.30
A
A 05
.20
oc 2
4
i
i
Ib
1'2
MONOCHROMATOR SPECTRAL BANDPASS,
.oo 0
25
50
100
COPPER CONCENTRATION, ppm
Figure 8. Spectral interference of copper on lead with multielemen! hollow cathode source using 2170-A resonance line for grating monochromator. Each solution contains 5 ppm of lead 0 500-pm slit width, 0 400-pm slit width, A 300-pm slit width, V 200-
p m slit width, 0 lead resonance monochromator
ally become greater than A b ' . When this occurs, the limits of integration in Equation 7 must be changed to AAD'. Thus the broadened emission profile will not be included in the integrated absorption in the resonance monochromator and changes in the line source width will not affect the calculated absorbance of Equation 15. Therefore, calibration curves using a resonance monochromator are less affected by self-reversal in the hollow cathode lamp, and should have greater sensitivities and more reproducibility. Spectral Interferences. Another interesting feature of the resonance monochromator is the potential elimination of spectral interference because of the narrow spectral bandpass. Fassel et al. (19) have shown that spectral line interferences can occur in atomic absorption spectrometry. Joworowski and Weberling (20) reported the interference of copper on the determination of lead using the 2170-A lead line when using a hollow cathode lamp containing both elements. Hall (21) also reported interferences of copper on lead from copper impurities in a lead hollow cathode tube. This interference is due to the copper lines at 2165 and 2179 A. If a grating monochromator with a spectral bandpass of about 4 A is used, this interference will not be significant. However, if a grating monochromator which has a spectral bandpass greater than 4 A is used, copper will introduce an interference. Figure 8 shows the dependence of the absorbance of 5 ppm lead a t 2170 A on copper concentration when a Pb, Cu, Zn, Ag hollow cathode lamp is employed with various monochromator slit widths. The reciprocal linear dispersion of the (19) V . A. Fassel, J. 0. Rasmuson, and T. G. Cowley, Spectrochirn. Acta., 238,579 (1967). (20) R. Jaworowski and R. P. Weberling. At. Absorption Newslett.. 5 , 125 (1966). (21) J. M. Hall, Spectrosc. Lett., 2, 113 (1969).
1600
A
Figure 9. Dependence of lead absorbance on monochromator
spectral bandpass for multielement line source 0 2170 A; o 2833 A: 0 2170 A tor
Resonance line isolated grating monochromator grating monochromator and 2833 A; resonance monochroma-
lnterfering line ( s i isolated 2165 A and 2179 A 2824 A
......
grating monochromator was 20 A/mm. Thus, the four slit widths correspond to nominal spectral bandpasses of 4, 6, 8, and 10 A. At the 4-A bandpass, very little copper interference is observed. However, as the monochromator bandpass is increased, the absorbance increases because more of the copper line is isolated. The lead resonance monochromator virtually eliminates the copper spectral interference because of its narrow spectral bandpass as can be seen in Figure 8. If the monochromator bandpass isolates only the resonance line from the line source, absorption sensitivities should be independent of monochromator slit width as long as stray light is negligible. However, if a non-absorbing interfering line is also isolated by the monochromator, absorption sensitivities will decrease with increasing monochromator slit width as is illustrated in Figure 9 for the absorption of a 5 ppm lead solution using the Pb, Cu, Zn, Ag multielement hollow cathode. As can be seen when the 2833-A lead resonance line is employed, the observed absorbance remains constant until the monochromator bandpass is sufficiently wide to isolate a non-absorbing Cu line at 2824 A. The absorption sensitivity of the 2170A. P b line follows the same behavior, as the absorbance of 5 ppm Pb is constant a t low monochromator bandpass but decreases as the bandpass increases to isolate the nonabsorbing Cu lines a t 2165 and 2179 A. Because of the very low bandpass of the resonance monochromator, nonabsorbing lines should not cause interference. The observed absorbance for a 5 ppm lead solution is also shown in Figure 9 and falls between the two limiting absorbance values for the 2170 and the 2833 A line. Since the resonance monochromator produces fluorescent radiation at both Pb wavelengths, this behavior is expected at low atomic concentrations because the observed absorbance
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
will depend on the relative oscillator strengths of the two resonance lines and the spectral response of the detector. Calibration Curves for Continuum Source. Because of its narrow bandpass, the resonance monochromator is useful not only with an incident line source, but also with a continuum source. For a continuum source and a conventional grating monochromator, the absorbance should continue to increase as the spectral bandpass decreases to a limiting value equal to the Doppler half-width in the flame, a bandpass unapproachable with normal monochromators. Atomic absorption data for a 100 ppm cadmium solution were obtained with a 150-W xenon arc and a conventional grating monochromator a t several slit widths. A plot of observed absorbance us. monochromator spectral bandpass is shown in Figure 10. Note that if the monochromator bandpass is extrapolated to 0.01 A, (approximately the spectral bandpass of the resonance monochromator), an absorbance of 1.2 results. Atomic absorption calibration curves were obtained with a cadmium resonance monochromator with both a cadmium line source and a xenon arc continuum source. These results are shown in Figure 11 along with a calibration curve using a cadmium line source and a grating monochromator. A lamp current of 10 mA was used for the cadmium line source to avoid self-reversal. Note that the absorbance for the 100 ppm cadmium solution with the continuum source and resonance monochromator is 1.25, which agrees favorably with the value of 1.2 obtained by extrapolation of the plot in Figure 10 to a spectral bandpass of 0.01 A. Note also the improved linearity obtained when using the continuum source. This phenomenon may be explained by recalling Equations 15 and 16. As the concentration of atoms in the flame increases, the fraction absorbed increases, but the absorption line does not initially broaden. This trend continues until the fraction absorbed reaches one, a t which point the absorption line begins to broaden (22). Since the spectral bandpass of the resonance monochromator-line source combination is equal to the source line half-width, then, according to Equation 15, the absorbance should increase linearly with no until resonance broadening occurs ( 2 2 ) . At this point, bending will be observed, as indeed is the case in Figure 11 a t a Cd concentration of approximately 40 ppm. The spectral bandpass of the resonance monochromator-continuum source combination, on the other hand, is equal to the half-width of the absorption line in the resonance monochromator. In this case, the concentration may be increased further before the resonance monochromator senses any resonance broadening. For the Cd working curves of Figure 11, the line source-resonance monochromator combination has a greater sensitivity than the continuum source-resonance monochromator combination because of the narrow half-width of the line source. However, the linearity of the line source-resonance monochromator is lower than that of the continuum source-resonance monochromator. The continuum source-resonance monochromator combination, on the other hand, has a decreased sensitivity, but improved linearity because its wider spectral bandpass does not respond to resonance broadening in the flame until a higher concentration is reached. This theory could be extrapolated to its limit where we have the case of a very wide spectral bandpass which would be independent of resonance broadening in the flame. The sensitivity would be very low, but the linearity would be extremely high. This case, of course, is covered in Equation 4 where the spectral bandpass was assumed to be much larger than the absorption line width in the flame. (22) J D Winefordner. W W McGee, J' M Mansfield, M L Parsons, and K E Zacha A n a / Chim Acla 36,25 (1966)
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A Figure 10. Dependence of cadmium absorbance on monochromator spectral bandpass for continuum source. Spectral bandpass is extrapolated to 0.01 A to give an absorbance of 1 . 2 MONOCHROMATOR SPECTRAL BANDPASS,
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CADMIUM CONCENTRATION, ppm
Figure 11. Atomic absorption curves for cadmium with line and
continuum source [3 Resonance monochromator and line source. A Grating rnonochromator and line source. 0 Resonance monochromator and continuum source
CONCLUSIONS The resonance monochromator has been shown to have several advantages over a conventional dispersive monochromator for atomic absorption flame spectrometry. The narrow spectral bandpass of the resonance monochromator is perhaps the most advantageous feature. Consequently, calibration curves, when a line source is used, are relatively independent of source broadening and thus should remain constant from day to day. Also many troublesome spectral interferences can be avoided with the resonance monochromator. In addition, the resonance monochromator has been shown to be of use with continuum sources for atomic absorption analyses. Other advantages, such as the absence of tuning in to a given resonance line, the compactness and durability, and the lower cost have been previously summarized (28).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
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There are however, several disadvantages of the resonance monochromator system described here. Thermal monochromators may be constructed only for relatively volatile elements, whereas sputtering type monochromators are required for other elements. This precludes the use of one simple demountable monochromator design with a readily interchangeable element for a large number of elements. Also, resonance monochromators have a limited lifetime, and about fifteen minutes of warm-up time are required for thermal type monochromators. Absorption sensitivities have been shown to be a function of the oscillator strengths of all resonance lines viewed by the photomultiplier detector. A final disadvantage is that the resonance monochromator is specific for only one element unless multielement models are used. The resonance monochromator, however, does offer attractive possibilities in the field of multielement analysis by placing more than one element in the alumina boat. If line sources are used, the fluorescent output of each element could be isolated by adjustment of the power applied to the heating element. For example, the optimum power for the cadmium resonance monochromator was 2.5 W, the zinc resonance monochromator 12 W, and the lead resonance monochromator 28 W. If a continuum source is used, appropriate filters could be used to isolate the respective fluorescent lines. Finally, sputtering type resonance monochromators can be constructed for multielement analysis, thus widening its range of usefulness ( 2 3 ) .
APPENDIX 1. DEFINITIONS AND UNITS OF SYMBOLS USED AA~C = absorbance for atomic absorption with a continuum source. A ~ A L= absorbance for atomic absorption with a line source. ARFC = absorbance for atomic absorption with a resonance monochromator and continuum source. ARFL = absorbance for atomic absorption with a resonance monochromator and line source. AAC = atomic absorption flame spectrometry using a continuum source. AAL = atomic absorption flame spectrometry using a line source. AFC = atomic fluorescence flame spectrometry using a continuum source. AFL = atomic fluorescence flame spectrometry using a line source. BAFC= radiance of atomic fluorescence produced using a continuum source, W cm-2sr-1. BAFL= radiance of atomic fluorescence produced using a line source, W cm-2sr-1. Bcxo = spectral radiance of continuum source a t wavelength A, W c m - % - h m - 1 . BL = radiance of line source, W cm-2sr-1. BRAC= radiance of atomic absorption in resonance monochromator using a continuum source, W cm-2sr-1. BRAL= radiance of atomic absorption in resonance monochromator using a line source, W cm-%-?. BKFC= radiance of atomic fluorescence produced in resonance monochromator using a continuum source, W cm - 2sr - 1. (23) A Walsh, Pure Appl Chem 23,1 (1970)
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BRFL= radiance of atomic fluorescence produced in resonance monochromator using a line source, W c m - % - I . Bwxo = spectral radiance of continuum source a t wavelength Xo that is not absorbed by atoms in flame, W cm -2sr -1nm -1: BTL = radiance of line source that is not absorbed by atoms in flame, W cm-2sr-1. k o = atomic absorption coefficient in flame for pure Doppler broadening defined a t A,, cm - l. ko’ = atomic absorption coefficient in resonance monochromator for pure Doppler broadening defined a t A,, cm-1. 1 = path length of atoms in flame in direction of source, cm . 1’ = path length of atoms in resonance monochromator in direction of source, cm. L = path length of atoms in flame in direction of detector, cm. L’ = path length of atoms in resonance monochromator in direction of detector, cm. no = ground state concentration of analyte atoms in flame, atoms cm-3. no‘ = ground state concentration of analyte atoms in resonance monochromator, atoms ~ m - ~ . s = spectral bandpass of monochromator, A. [XI = self-adsorption factor in flame as defined by Winefordner et al. (IZ),dimensionless. [X’] = self-absorption factor in resonance monochromator, dimensionless. Y = quantum yield for resonance absorption-resonance fluorescence in flame, dimensionless. Y‘ = quantum yield for resonance fluorescence in resonance monochromator, dimensionless. /3 = collection efficiency of resonance monochromator, accounts for any radiance which does not reach the atoms in resonance monochromator, dimensionless. s ( a , ~ )= Voigt profile for absorption line in flame with 6 ranging from zero to A X s / - l L , dimensionless ( 1 1 ) . 6 ( a , ~ ) ’= Voigt profile for absorption line in flame with b ranging from zero to A b ’ / . l h , dimensionless. 6’(a,u) = Voigt profile for absorption line in resonance monochromator, dimensionless. AXA = absorption line half-width in flame, cm. AXD = Doppler half-width of resonance line in flame, cm. AAD‘ = Poppler half-width of resonance line in resonance monochromator, cm. -lXs = source line half-width, cm. Q 4 = solid angle of radiation collected from source by entrance optics in atomic fluorescence, sr. Q A f = solid angle of radiation collected from source hr\. entrance optics of resonance monochromator, sr.
ACKNOWLEDGMENT The authors wish to thank Keki Mistry and his crew a t the Michigan State University, Department of Chemistry, Glass Shop for construction of the resonance monochromator. Received for review December 18, 1972. Accepted Fehruary 27, 1973.
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O . 9, AUGUST 1973
