Lateral Diffusion Interferences in Flame Atomic Absorption and Emission Spectrometry A. C. West,’ V. A. Fassel,2 and R. N. Kniseley Ames Laboratory-USA EC and Department of Chemistry, lowa State University. Ames. lowa 50010
Enhancements of refractory metal emission and absorption by a variety of concomitants have been observed in nitrous oxide-acetylene flames supported on slot burners. This study of AI, Mo, and V confirms the existence of such enhancements and attributes them predominantly to changes in the horizontal distribution of analyte free atoms or molecules across the flame profile. This redistribution, which we classify as a lateral diffusion interference, was first observed by Koirtyohann and Pickett in 1968. It increases the analyte concentration at the center of the flame and decreases it at the edges. Because the elements studied form stable monoxides, oxide formation at the edges of the flame due to entrained air obscures edge depressions accompanying the enhancements in the center of unshielded flames. The interferences were therefore observed in an argon-shielded flame and confirmed in an unshielded flame with metals forming less stable oxides. The following mechanism is proposed to explain this type of interference: The concomitant delays the atomization of spray droplets or solid particles, thus shortening the time available for lateral diffusion in the flame by analyte atoms or molecules before they reach the optical path. The analyte free atoms or molecules are therefore concentrated in the center of the flame and depleted at the edges.
Solute vaporization interferences remain one of the major sources of inaccuracy of flame atomic absorption, emission, or fluorescence methods of analysis when the analyte is determined in a matrix of varying total chemical composition. If these interferences are to be satisfactorily eliminated, their mechanisms must be understood, and this understanding often follows from a proper choice of experiments and correct data interpretation. For example, the observation that the depression of alkaline earth absorption and emission signals by phosphate and aluminum concomitants in the nebulized sample decreases with increasing height of observation in the flame suggested that a slower rate of analyte atomization was responsible for the interference. Thus, these particular interferences have been diminished or eliminated by making absorption or emission measurements higher in hotter flames and employing nebulizers that yield smaller aerosol drop size distributions (1-4).In view of this success, it has become common practice to consider height of observation as a primary variable in solute vaporization interference studies. 1 2
In contrast to the type of solute vaporization interferences leading to a depression in analyte absorption or emission signals, there have been repeated accounts (5-20) of enhancement effects, produced by concomitants, that apparently could not be attributed to ionization suppression. All of these enhancements were observed in flames formed on linear slot burners, and all, except David’s work on Mo (12), in nitrous oxide-acetylene flames. Dagnall et al. have also reported (21) extensive observations of enhancement for the emission signals of Al, Mo, Ti, V, and Zr when a variety of concomitants was added to the sample solutions, but it is important to note that their observations were made with a circular rather than a linear slot burner. It is also significant to note that we have recently reinvestigated several of the more prominent enhancements reported by Dagnall et d . , using a circular slot burner constructed according to their description. Although our experimental conditions were closely similar to those used by Dagnall et al., we were unable to confirm their observations. Thus, historically, there has been a consistent pattern of observations of these enhancement effects in linear slot burners but not in circular slot burners, suggesting that the flame geometry plays a n important role. Among the many speculative interpretations that have been offered on the nature of this enhancement, the geometry factor has not heretofore been considered. In this communication, we report the results of detailed studies on the effect of concomitants on Al, Mo, and V absorption and emission signals in nitrous oxide-acetylene flames supported on linear slot burners. We focused our attention on these three elements because dramatic enhancements by a variety of concomitants have been reported for them by several authors (5-8, 22-25, 19, 20). Extensive unequivocal experimental evidence is presented to show that the horizontal distribution of the analyte free atoms across a linear slot burner flame may be markedly affected by the presence of concomitants. The effect of these concomitants, classified as a lateral diffusion inter-
On leave f r o m Lawrence University,Appleton, Wis. 54911. To w h o m request for reprints should b e addressed.
(1) V. Slavin, A. Venghiattis, and D. C. Manning, At. Absorption Newslett.. 5, 84 (1966). (2) M. D. Amosand J. B. Willis. Spectrochim. Acta. 22, 1325 (1966). (3) D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta. 36, 312 (1966). (4) V. A. Fassel and D. A. Becker. Anal. Chem., 41, 1522 (1969).
1586
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
J. A . Bowman and J. E. Willis. Anal. Chern.. 39, 1210 (1967). T. V. Ramakrishna, P. W. West. and J. W. Robinson, Anal. Chim. Acta. 39, 81 (1967). S. R . Koirtyohann and E. E. Pickett. Anal. Chem.. 40. 2068 (1968). J. Y . Marks and G. G. Welcher. Anal. Chern.. 42. 1033 (1970). H. D. Fleming, Specfrochfrn. Acta. 238, 207 (1967) L. Wilson.AnaL Chim. Acta. 40, 503 (1968). W. W. Harrison and W. H. Wadlin, Anal. Chem.. 41, 374 (1969). D. J. David, Analyst (London). 86, 730 (1961). T. V. Ramakrishna, P. W. West. and J. W . Robinson, Anal. Chim. Acta. 44, 437 (1969). E. N. Pollock, At. Absorption Newslett.. 9, 47 (1970). J. C. Van Loon, At. Absorption Newslett.. 11, 60 (1972). L. Capacho-Delgado and D. C. Manning, Analyst (London). 92, 553 (1967) J. E. Headridge and D. P Hubbard. Anal. Chirn. Acta. 37, 151 (1967). R. A. Mostyn and A F. Cunningham, At. Absorption Newsletf.. 6, 86 (1967) S. L. Sachdev, J. W. Robinson, and P. W. West, Anal. Chim. Acta, 37, 12 (1967). D. C. G. Pearton. J. D. Taylor, P. K. Faure. and T. W. Steele. Anal. Chim. Acta. 44,353 (1969). R. M. Dagnall. G. F. Kirkbright. T. S. West, and R. Wood, Anal. Chem.. 42, 1029 (1970).
ference, is to increase the concentration of analyte free atoms along the linear center of the flame and to decrease their concentration at the edges. Since analytical measurements of absorption or emission are made along the central axis of the flame, this effect will appear as an enhancement, and the edge depressions will be unobserved, in normal interference studies. The common occurrence of the effect in our studies suggests that it may be an important, if not primary, cause of many of the other enhancements reported in the literature.
EXPERIMENTAL
Table I. Effect of Concomitants on the Flame Absorption and Emission of 25 pg/ml AI Height above burner, m m co
4.0 5.0 7.0 10.0 15.0
+lo%
+40%
+lo%
+20%
...
+30% +30%
...
0 0
+ 10%
+10%
0
0
0
Data corrected for emission by concomitant at wavelength used Concomitant concentration = 2500 pg/ml. . . . Not investigated. (3961.5 A ) .
All observations reported here were made in nitrous oxide-acetylene flames. The two linear slot burners of similar design used in this work have been previously described (22). The burner heads were not water-cooled. Teflon spray chambers without flow spoilers were used, and variable nebulizers were operated a t a solution uptake rate of 3.2 ml/min. Vertical and horizontal positioning of the burners to 10.05 mm was achieved with the racking mechanism previously described (22). The flame shield for the burner, a rectangular water-cooled galvanized steel box, was sealed to the base of the burner head and open a t the top. Its horizontal dimensions were 11.5 X 4.4 cm, and the shield itself extended 8.6 cm above the burner tip. Quartz windows, which were 4 X 2.2 X 0.16 cm in height, width, and thickness, were mounted a t opposite ends of the shield to allow spectral observation of the flame. Entrance ports for the shielding gas were located a t the center of each long side of the shield, 7.2 cm below the burner tip. The space between the shield and burner head was packed with glass wool (at the bottom) and small steel spheres to ensure an even distribution of shielding gas around the flame; the layers of spheres extended almost to the tip of the burner. Argon was used as the shielding gas, and shielded flame measurements were made with an argon flow rate of about 38 l./rnin. The gas flow-metering system has been previously described (22). The unshielded burner was operated at p = 1.72 ( N 2 0 = 12.14 l./min, CzHz = 7.08 l./min), the shielded one a t p = 1.83 (NzO = 10.66 l./min, CzHz = 5.84 l./min). The tendency for the flame to be lifted off the burner slot by the rapid argon flow necessitated the decrease in gas flow rates for the shielded flame. When shielded, the flame displayed a continuous well-defined layer of incandescent carbon that sheathed its length on both sides a t a distance of 1.5 to 2.0 mm from its center. Within these layers, the pink interconal zone extended above the top of the quartz windows, ie., a t least 4 cm above the burner tip. (This zone was 1.0 to 1.5 cm tall without argon flowing.) The external optical system was similar to that used by Fassel et al. (23). A 1 : l image of the primary light source was focused a t the center of the flame, and this image was focused on the entrance slit of the spectrometer a t 1 : l magnification. The lens closest to the spectrometer was diaphragmed down to 3 mm; entrance and exit slits were 2 mm high and 0.1 mm wide (0.5 mm high for the calcium studies). Thus, the volume of flame gases observed by the spectrometer consisted of two slightly distorted cones with apexes a t the center of the flame and with bases 1 to 2 mm in diameter a t the ends of the flame. The spectrometer, detector, and electronic equipment have been previously described ( 2 4 ) , with the following exceptions: A Keithley Model 417 picoammeter was used for emission signal amplification; most of the data acquisition was made with an Infotronics CRS-80 Digital Readout System operated in the linear mode with 8-second signal integration. The following wavelengths were used, unless otherwide noted: Al, 3961.5 A, Ag 13280.7 A; Ca, 4226.7 A; Mo, 3132.6 A or 3903.0 A (emission), 3132.6 A (absorption); V, 4379.2 A (emission), 3184.0 A (absorption). All metal salt solutions were prepared from reagent-grade chlorides or from the metal dissolved in a minimum of HC1, except for Ti, which was dissolved in an excess of HC1 or HF, and Mo, V, and W solutions, which were prepared from ammonium rnolybdate, metavanadate, and tungstate, respectively. A small amount of HC1 was added, where necessary, to repress hydrolysis. (22) J. A. Fiorino, R. N. Kniseley, and V. A. Fassel. Spectrochirn. Acta. 2 3 8 , 4 1 3 (1968). (23) V . A . Fassel. J. 0. Rasmuson. R. N. Kniseley, and T. G. Cowley, Spectrochirn. Acta, 258, 559 (1970). (24) J. B. Willis, Spectrochirn. Acta, 258, 487 (1970).
Analyte concentrations were 25 pg/ml of Ag, Al, Mo, and V, and 12.5 pg/ml of Ca. Concomitant concentrations were 2500 pg/ml of metal and 0.2M of acid unless otherwise noted.
RESULTS AND DISCUSSION It is now common knowledge that the absolute degree to which solute vaporization interferences occur is sensitively dependent on the aerosol particle size distribution, the construction details of the nebulizer-burner system, the site of observation in the flame, and flame stoichiometry. Given these uncertainties, it was necessary to establish initially that the interferences reported elsewhere could be reproduced with the facilities described above, and to determine the effect of changing height of observation in the flame. In the linear slot burner flame, the height of maximum absorbance was about 10, 7 , and 8 mm above the burner tip for Al, Mo, and V, respectively. Emission intensities peaked lower in the flame; peak absorption and emission signals occurred slightly higher in the shielded flame than in the unshielded one. Tables 1-111 summarize the results for Al, Mo, and V, giving the percentage change in the analytical signal produced by the concomitants indicated. Most of the numbers are averages of several measurements; they include all the emission and absorption data, in both the shielded and unshielded flames. Differences between absorption and emission were small and inconsistent. These enhancements are, therefore, the result of changes in ground state free atom populations. Background changes produced by the concomitants have been noted; the data were corrected for them where necessary. The percentage changes were rounded off to the closest 10% (absolute) to emphasize their uncertainty. For example, fifteen measurements of the enhancement of Mo absorption or emission by Ni at 5 mm, made over a period of six weeks, gave results ranging from +26% to +67% (standard deviation of the mean = f 1070,absolute). Possible reasons for this variability will be discussed later in the paper. Our inability to reproduce enhancements quantitatively on a single instrument over a period of time makes large discrepancies among laboratories understandable. Viewed in this light, the observations reported here are, for the most part, more than adequately consistent with earlier work in this laboratory, with previously reported studies using nitrous oxide-acetylene flames on linear slot burners and, surprisingly, with the observations of Dagnall et al. (21) in a circular slot burner. It will be recalled that we could not confirm their results in our own studies with a circular slot burner. The short-term reproducibility of interference effects was considerably better than that achieved over a long period. Sets of percentage changes observed during a single day gave, on the average, an absolute standard deviation of about f4%. Horizontal flame profile studies of Al, Mo, and V in the presence of concomitants were made initially in an un-
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 9, A U G U S T
1973
1587
Table II. Effect of Concomitants on the Flame Absorption and Emission of 25pg/ml Mo Ht. above burner, mm
Alasb
Cr(lII)"
cu
- 10%
+ 10%
0 +20%
+lo%
+50%
7.0
+6O% +60%
+60% +6O%
10.0
+50%
15.0 20.0
+60% +6O%
+50% +50%
25.0
+50%
+10%
1.0 2.0 4.0
+20% +50%
5.0
Ti(lV)"Vb
+ 10% +10% +30%
+So%
+40%
+40% +40% +40%
+50%
... ... ...
+40%
...
...
...
V ( V p
W(V1)a.b
0 +20%
-20% -20%
+40% +6O%
0 0
+20% +40%
+6O%
...
+40%
+6O% +60% +60% +60% 3903.0 A ) .
0 -10% - 10% -10%
+40% +30%
+30% +60% +70% +70% +40%
5 +so0
v
Fe(lll)
AIa
c
Mn(ll)
Mo(VI)~
-30% -30% -10% - 10%
Na
Wa
-20%
+30% +30% +30% +30%
-10%
+lo% +20%
+lo%
+40%
+30% +30% +30%
+50% +50%
I
1
I
I
+60%
+80
I V
.S .14.I2
+50%
c
2a
.04
Figure 2. Effect of AI on M o absorption profile at 5 mm above the burner tip in the argon-shielded flame
t
I-
PAMo+A' \
t
06
m 04
a
.02 0
.06
HORIZONTAL CENTER OF DISPLACEMENT FLAME ( m 1.0m ) FROM 2.0 3.0
-80
.08
4\
2
.o02
-40
.IO
-
.IO-
O
.I2
0.
3 -80-
...
V
4.0
3.0 2.0 1.0 0 1.0 2.0 3.0 HORIZONTAL DISPLACEMENT FROMCENTER OF FLAME ( m m )
Figure 1. Effect of AI on M o absorption profile at 4 mm above t h e burner tip in the argon-shielded flame
shielded flame a t heights of 4 mm and above. Whenever a concomitant produced an enhancement a t the center of the flame, the enhancement decreased to zero a t the edges, but no depressions were observed a t the edges, in agreement with the observations of Marks and Welcher (8).It has been pointed out that entrained air a t the edges 1588
I
W
+50% +40%
z W
am
I
3 -40-
V
a
l
W
0
Y V
l
by concomitant at wave-
I
8 *40 z
a
. . . Not investigated.
c-
z
Z
* Data corrected for absorption
&
Data corrected for emission by concomitant at wavelength used (4379.2 A). Concomitant concentration = 2500 r g / m l . . . . Not investigated.
2
....
z
a
3
...
+40%
V
4.0 +60% 4.5 , . . 5.0 +EO% ... 6.0 7.0 +90% 10.0 +70% 12.0 ... 15.0 +90%
I
+20%
0 +40-
Ht. above burner, mm
t
-10%
+lo%
I-
5 k
HClOi
0
+ 10%
a Data corrected for emission by concomitant at wavelength used (3132.6 or length used (3132.6 A). Concomitant concentration = 2500 r g / m l for metals, 0.20Mfor acids.
Table 111. Effect of Concomitants on the Flame Absorption and Emission of 25 pg/ml V
Zn
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST
of the flame will lead to oxide formation by analytes that have high monoxide dissociation energies (23), as these three elements do: A10 = 4.6 eV, MOO = 5.0 eV, VO = 6.4 eV, (25). It is probable, therefore, that oxide formation a t the edges of the flame obscures observable differences in free atom populations produced by concomitants, and the inability of Marks and Welcher (8) to see the edge depressions for A1 in the presence of Cr can be attributed to this same effect. When horizontal flame profiles were measured in an argon-shielded flame, using the flame shield described earlier, the concomitants that enhanced analyte absorption or emission at the center of the flame produced definite edge depressions. These edge depressions were observed for A1 with W as concomitant, for V with Al, and for Mo with Al, Cu, Ni, V, Zn, HClOI, and H3P04, a t heights ranging from 4 to 10 mm above the burner tip. Figures 1-5 show typical plots of several horizontal profiles of Mo in absorption with the concomitants indicated. Included in the figures are plots of the percentage change produced by the concomitants at each point. Although the Mo absorbance profile in Figure 5 is noticeably flatter than those in Figures 1 and 3, repeated measurements (25) A. G. Gaydon, "Dissociation Energies and Spectra of Diatomic Molecules," Third ed., Chapman and Hall, London, England, 1968.
1673
5- 8; I-
+BO
+80
f o
I
z 8 I
+40
0
s
0
c
* -40
3 -40
0
W W
W
-
z a -80
W
5
I
V
z
8 .I2
I
.IO
.08
V
z a
-80
I
V
2
o
I
k
"
t40
z
n..'"
.08
%4,\
W
V
5
.06
.06
I
Mo+HCIO
0
a
.04
.02 0 4.0 3.0 2.0 1.0 0 1.0 2.0 3.0 4S HORIZONTAL DISPLACEMENT FROM CENTER OF FLAME ( m m )
CENTER OF FLAME (mm)
Figure 3. Effect of V ( V ) on M o absorption profile at 4 mm above the burner tip in the argon-shielded flame
Figure 5. Effect of HC104 on M o absorption profile at 4 mm above t h e burner tip in the argon-shielded flame
W (3
Mot V
. IO
I-
d
\
"
Figure 4. Effect of V ( V ) on M o absorption profile at 7 mm above the burner tip in the argon-shielded flame
have confirmed that the percentage change in absorption or emission produced by a concomitant a t any point in the flame is essentially independent of the absolute value of the absorption or emission signal. All of the horizontal profiles that were measured in the presence of concomitants that enhanced analyte absorption or emission a t the center of the flame, produced an edge depression. The effect of W on Mo (Figure 6) is equally significant; a potential substantial depression is converted to essentially no interference a t the center of the flame. The integrated areas under absorption profiles measured across the flame provide insight on the total relative
4.0
3.0 2.0 1.0 0 1.0 2.0 3.0 HORIZONTAL DISPLACEMENT FROM CENTER OF FLAME (mm)
4.0
Figure 6. Effect of W ( V I ) on Mo absorption profile at above the burner tip in the argon-shielded flame
4
mm
free atom population of the analyte at the height of measurement. Consequently, the effect of the concomitant on the horizontal redistribution of the analyte can be evaluated by comparing the percentage change in the integrated area with the percentage change at the center of the flame. Table IV summarizes this comparison for Al, V, and Mo for the concomitants indicated, in the shielded flame. It is clear that a major part of each large center enhancement was due, not to an increase in the total free atom population in the flame, but to an increase in the number density or concentration of analyte at the center
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
1589
Table IV. Effect of Concomitants on Absorption and Emission Profile Integrated Areas of AI, Mo. and V t i t . above
burner. rnm
4.0
+ 1 2 % (+22)" -3% (+4)
7.0 Ht. above burner, m m
V-Absorption effect of AI
4.0
+ 1 4 % (+56) + 3 5 % (+82)
10.0 Mo-Absorption
cu
AI
0 (+64)
4.0
5.0
+ 9 % (4-62)
7.0
...
Ht. above burner, mm 4.0 a
Ht. above burner, rnrn
At-Absorption effect of W
... ...
V
Ni
+13% (+69)
+5%
(+40) +6% (+35) + 1 6 % (+71)
...
+6% +8% +11% 4712% +22%
(+46) (+54)
W
Zn
"301
-22% (+3) -21% (-7)
0 (4-24)
- 1 3 % (+60)
... ...
...
...
...
...
Hap04
+5%
(f52)
(+47) (+59) (i-64)
, . .
...
Mo-Emission HCIOI AI
cu
...
...
Ni +13% ( f 3 3 )
V +16% (i-46)
Percentage change at center of the flame is given in parentheses
W -17% (-3)
Zn + 4 % (-I-18)
...
H3p01 , . .
Not investigated
of the flame. The effect of A1 on Mo at 4 mm is the most striking; a t that height, the total Mo free atom population was unchanged while Mo absorption a t the center was enhanced by 64%. The question may be posed as to whether the observed edge depressions are large enough to be significant. For some of the profiles, they unquestionably are. For others, such as those in Figures 3 and 4, the small changes in absorbances may not be very convincing. (The effects of Cu, Ni, and Zn on Mo gave profiles similar to Figure 3.) One answer to this question is that the percentage change in profile area is reproducible for the replicate results for Ni, V, and W given in Table IV. However, a direct evaluation of the reproducibility of profile shapes and absorbance values at points along them encounters two difficulties. There is both the uncertainty in absorbance values taken on different days under identical conditions and the day to day change in the breadths of the profiles. Despite these difficulties, the average absolute standard deviation for percentage change in absorption at corresponding points on the three sets of Mo plus V profiles at 4 mm is only 670, which is very close to the short-term reproducibility. If the edge depressions for these profiles are to be significant, the plots of percentage change us. horizontal position should have slopes that are consistent (independent of profile breadth and of the magnitude of the change at each point); that is, the overall changes from edge to center should be similar and occur at similar rates for all three profiles. For points between 1.75 mm and 0.25 mm from the center, sets of point to point slopes calculated from a variety of intervals are consistent with each other at an average confidence level of 97% using the t test of significance. The average change in percentage interference from 1.75 mm to the center is 70%, the average enhancement a t the center was +49% for these three profiles; thus, it is safe to conclude that the edge depressions are significant for all the profiles measured. Several interpretations have been proposed for enhancements such as those reported in the present work. It has been suggested that preferential combination of the concomitant with oxygen facilitates reduction of the analyte to free atoms in the flame, thus producing an enhancement (6, 8, 19), or that the reverse process produces a depression (26). If this hypothesis is accepted, it should be possible to predict the effects of concomitants purely on the basis of relative monoxide dissociation energies. How-
ever, many of the data presented herein and in earlier reports do not fit the expected pattern. A fundamental objection to the oxygen deficiency hypothesis has also been documented and discussed by Rasmuson (27). He showed that the concentration of atomic oxygen is exceedingly small (of the order of 10-8 atm) in the carbon-rich nitrous oxide-acetylene flames that are usually used for determining refractory elements, and in which many of these enhancements appear. In these flames, the oxygen concentration is fixed by equilibria involving carbon-containing species whose total concentration is in the range 10-3 to 10-5 atm (not including CO), and it is these species that really control metal-metal oxide equilibria. Since no metal monoxides approach CO in stability, and the free atom concentrations of concomitant elements are of the order of 10-6 to 10-8 atm (24, 27), it is unlikely that concomitants will change measurably the equilibria between analyte free atoms and monoxide. It may be suggested that a non-equilibrium process is occurring in the immediate vicinity of the vaporizing solid particles where the concentration of metal-containing species is much higher, and that this process continues through the optical path. However, if the concomitant forms a more stable oxide than the analyte, the concomitant will be more likely to vaporize as the oxide than the analyte, if aqueous solutions are nebulized. Under these conditions, the analyte will be reduced first, and there will be no oxygen competition. Koirtyohann and Pickett (7) have suggested that enhancements of alkaline earths by high-boiling acids and nonvolatile salts in the nitrous oxide-acetylene flame formed a t a linear slot burner could be explained by changes in the horizontal distribution of analyte free atoms. They showed that Sr emission in the presence of HClOl was enhanced at the center of the flame and depressed a t the edges. Similar results were obtained for Ca with HzS04. They hypothesized that acids less volatile than H20 left heavier, longer lived droplets that would diffuse laterally in the flame more slowly than the lighter particles remaining in the absence of concomitant. If the concomitant were a nonvolatile solid, the increased weight of the residual solid particle would have a similar effect. On subsequent atomization, the analyte would therefore be more concentrated at the center of the flame and less so a t the edges than if no concomitant were present. Since most instrumental systems focus along the central axis of
(26) B. Gandrud and J. C. Marshall, Appl. Spectrosc.. 24, 367 (1970).
(27) J. 0.Rasmuson. Acta. in press.
1590
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 9,
AUGUST 1973
V. A. Fassel, and R. N. Kniseley, Spectrochim.
a slot burner flame, such concomitants would enhance the observed analyte signal. The enhancements observed t)y lioirryohann and I'ickett seem to be a manifestation of I he -:i:ntb phenomenon observed in the present work ;ind clsewhrr!. but the hypothesis that the effect depends on ra:es oi' lateral diffusion of solid or liquid particles d r w not appear to he a tenable interpretation. Willis ( 2 . : )has :u;.ctlsted that the horizontal distribution of analyte free atoms in a slot burner flame is determined hy the initial spray pattern of droplets as they emerge from the mouth o!' the slot. and that differences among profiler: ot' disparntcl rl(>:t;cmtsare due to differences in rates of diff'lisirin ot' tb(. :itomic vapor formed. Concomitants will not chance the mass of droplets significantly, and, therefore. will nnt change the initial spray pattern unless they change the drop size distribution from the nebulizer. Once the droplets are moving on a particular trajectory in thc !Inme. their horizontal position will not be changed significantly in t h e time necessary to reach the optical path. A t t h e temperature of the nitrous oxide-acetylene flame. a n atom ot' rrlat ive atomic weight 60 has an average velocity of ahnut 1 mm/psec, while the average velocity of a typical solid or liquid particle is about 10-5 mm/psec. (.'onsequently. it is more reasonable to propose that: ( a i slower atomization in the presence of a concomitant delay): the rmtirict inn of' free atoms; (b) the atoms then have a shorter time for lateral diffusion before entering the optical path; ( c ) they will diffuse a shorter distance and will be concentrated a t the center of the flame, accounting for an enhancement at the center and a depression at the edges. This lateral diffusion hypothesis interprets enhancements in nitrous oxide-acetylene flames with the same mechanism that has been used to account for traditional solute vaporization interferences. The presence of a nonvolatile concomitant or the formation of a refractory compound containing the analyte can only produce the effect of a lateral diffusion interference and cause an enhancement in flames whose temperature and chemical composition permit sufficiently rapid atomization so that analyte free atom production is delayed, but not substantially inhibited through the optical path. This may account for the fact that enhancements have been observed in hot reducing nitrous oxide-acetylene flames but not in the cooler air-acetylene flame. It should be emphasized that a concomitant may increase the analyte concentration a t the center of the flame without producing an enhancement a t that point. For example, W decreases the Mo free atom population a t 4 mm by an amount that is almost exactly balanced at the center of the flame by a lateral diffusion interference enhancement (Figure 6). Examples will be given later of concomitants that depress Mo absorption by 5070 or more a t the center of the flame while still concentrating Mo to some extent at the flame center. Even with those systems that produce a large center enhancement. however, the observer may see an increase, no change, or a decrease in the analyte signal, depending on his instrumentation and choice of experimental variables. Some of these variables have not been considered critically with respect to interference studies. Of primary importance is the type of burner used. Lateral diffusion interferences should be most prominent (and perhaps occur only) with linear slot burners, because only such burners viewed lengthwise permit observation of the center of the flame with virtual exclusion of the edges. The data presented in this paper stress sufficiently the importance of horizontal burner alignment. A lateral displacement of 0.5 mm or an angular displacement of 1-2 degrees would sig-
I
.22
.IO
'
I
I
1
1
1
I
1
I
I
I
t 1-
A
Imm 1
0.4
1
I
0.4 OB 04 0 0.4 HORIZONTAL DISPLACEMENT FROM CENTER OF FLAME ( m m )
0
I
0.8
Figure 7. Effect of AI and Ti(IV) on Mo absorption profiles in the unshielded flame as a function of height above the burner tip. 0 = Mo. A = Mo +- concomitant
nificantly affect the measured enhancement of Mo by A1 a t 5 mm (Figure 2). The angle of acceptance of the spectrometer may also have crucial importance. For example, an observed depression of Mo by W a t 4 mm can be minimized by limiting the optical path to the smallest solid angle a t the center of the flame (Figure 6), while A1 will appear to enhance Mo less a t 4 mm, the wider the optical path through the flame. The height of the optical path through the flame has long been considered a critical variable, and it has been common practice to make observations high in the flame when solute vaporization interferences exist. However, for the burner and optical system employed in our study, it is clear, for example, that for minimal interference effects: (a) A1 should be observed high in the flame in the presence of Mo as a concomitant (Table I); (b) Mo should be observed low in the flame in the presence of Cr as a concomitant (Table II); and (c) V should be observed at an intermediate height in the presence of Fe as a concomitant (Table 111).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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.08 .8
w Y O 6
6
4
m
8 v)
m
a .04
.4
.02 .2
HORIZONTAL DISPLACEMENT FROM CENTER OF FLAME lmm)
Figure 8. Effect of concentration on t h e shape of Mo absorotion
profiles in the argon-shielded flame 10.000
10.0
D.
w
1,000
10
z
3 $a D W
I
fl
P
4
4
3
W
io0
01
D
a
8
F 01
IO 10
I00 1000 MOLYBDENUM CONCENTRATION ( p p / m l )
Figure 9. Log-log plots of peak absorbance and area under absorption profile vs. Mo concentration at 5 rnm above the burner tip in the argon-shielded flame
The flow pattern of spray droplets through the burner slot is another variable that must be considered. Turbulence in this flow may alter the distribution of analyte in the flame so as to lessen or even eliminate the effect of changes in lateral diffusion. One source of such turbulence is the pattern of air entrainment by the flame close to the burner top. The linear slot burners used in this work were designed to minimize this kind of turbulence. Other designs may behave differently. Another source of turbulence is encrustation or irregularities along the edges of the slot. In his discussion of the distribution of metal atoms across laminar slot burner flames, Willis ( 2 4 ) says, “The slot acts as a nozzle which directs the droplets in a thin, vertical stream up the center of the flame, and the momentum of the larger drops will cause them to be less readily deflected from this direction than are the smaller ones.” The fact that the center enhancements by Mo by Ni and Zn go to zero as the rate of solution nebulization decreases may be interpretable in part on this basis-i.e., as the nebulization rate and average droplet size decrease, 1592
the “thin stream” of droplets spreads out more in the flame and the effect of changes in lateral diffusion diminishes. The sensitivity of the center enhancements observed in this work to small changes in a number of experimental variables provides a reasonable explanation for the wide variation in these enhancements that was noted and discussed earlier in the paper. Manifestations of lateral diffusion interferences may appear very low in the flame as shown in Figure 7 . The series of abbreviated profiles of Mo with A1 concomitant show that a t 1 and 2 mm above the burner top, there is a small center enhancement, in spite of the fact that Mo atomization in the absence of concomitant is less complete at the center of the flame than on either side. For Ti(1V) concomitant, the center enhancement becomes clearly evident a t 3 mm above the burner tip. As indicated earlier, the edge depressions that accompany the center enhancements when the lateral diffusion interference mechanism is operating are best observed in a shielded flame. To demonstrate that the edge depressions really exist in an unshielded flame and also to confirm the work of Koirtyohann and Pickett (7) with our experimental system, measurements were made on the behavior of Ag as the analyte and W as the concomitant. Ag was chosen because its oxide is relatively unstable, so that no problem with oxide formation at the edges of the flame was expected. Horizontal flame profiles in absorption and emission of Ag and Ag plus W a t 4 mm indeed showed a lateral diffusion interference effect in the presence of W, with depressions of 20% a t the edges of the flame and an enhancement of 20% at the center. The area under the integrated emission profile was enhanced 4% by W; the corresponding integrated absorption was not changed by W.. Both HClO4 and HsP04, at 0.2M, produced a lateral diffusion interference effect with Ca, in agreement with the results of Koirtyohann and Pickett who used Sr with HC104 and Ca with H2S04. Absorption and emission profiles of Ca at 4 mm measured in our laboratory gave, with HC104, a center enhancement of 20% and an area depression of 570, and, with H3P04, a center enhancement of 5% and an area depression of 15%. The lateral diffusion hypothesis postulates a matrix effect that may be physical and nonspecific, as well as due to specific chemical interactions. If this is so, then Mo should produce the effect in a shielded flame not only on AI and V, but on itself. That is, Mo absorption (and emission) profiles should become relatively more narrow as the concentration of Mo in the nebulized solutions increases. The integrated areas under the absorption profiles should therefore increase less rapidly than peak absorbance values a t the center of the flame; the latter should increase more rapidly than concentration in solution, because Mo will be increasingly concentrated at the center of the flame as the average mass of its solid particles becomes larger. The experimental proof of this prediction is shown in Figures 8 and 9. To generate the profiles shown in Figure 8, absorbance measurements were made on the Mo 3132.6-A line for solutions containing 10 to 500 fig/ml of Mo and on the Mo 3208.8-A line for solutions in the 250 to 2500 gg/ml of Mo range. To convolute these measurements into single profiles, eight pairs of absorbance values in the 0.08 to 1.00 range for the two Mo lines on the 250 and 500 fig/ml profiles were selected for the computation of the factor for converting 3208.8-A absorbance values to equivalent values at 3132.6 A. In this way, the profiles for 500, 1000, and 2500 pg/rnl could be plotted on the same scale while maintaining precision in the absorbance measurements. That the lateral diffusion interference effect need not involve an interelement interaction is clearly
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
Table V . Effect on Concomitants on Flame Absorption and Emission of 25 pg/ml Mo Concomitants causing depressions Ht. above burner, mm
1.o 2.0 4.0 5.0 7.0 10.0 15.0 20.0
25.0
Ca
co
K
Na
-8 0 % - 90%
- 60%
-70% -70%
-90%
-60%
-70% - 60% - 60% - 40%
- 60% - 60% - 60%
-50%
- 50%
...
-40%
-60%
...
-70%
- 60%
...
...
- 40%
-70% - 70% - 70% - 70% - 60% - 70%
-80%
-70% - 60% - 60% -30% ...
-40% -40% -40% -40% -40% - 30% -30% -40%
...
...
-80%
- 60%
. . . Not investigated.
H2SO4
- 100%
- 60%
-80%
- 60%
a Data corrected for emission by concomitant at wavelength used (3132.6
(3132.6 A).
Ti( I Il)a.b
...
A ) , Data corrected for absorption by concomitant at wavelength used
Table V I . Effect of Concomitants on Absorption and Emission Profile Integrated Areas of Mo Concomitants causing depressions ~~
Absorption Ht. above burner, mm
Ca
co
4.0
-86% (-78)a
-54% (-43)
5.0
...
Na
K
-78%
...
-58% (-60) -57% (-56)
( - 73)
...
- 7 7 % (-68) Emission
Ht. above 4.0
...
...
-50% (-40)
-58% (-60)
Percentage change at center of the flame is given in parentheses. . . . Not investigated
shown in Figure 8--i.e., the profiles sharpen as the Mo concentration increases. Figure 9 shows log-log plots of Mo concentration us. peak absorbance and integrated absorption profile area. The latter has a slope close to unity, the former, as predicted, a slope greater than unity. The height of maximum absorption for 25 pg/ml Mo is between 7 and 10 mm above the burner tip in the shielded flame. Therefore, Mo is incompletely atomized from this solution a t 5 mm. Its degree of atomization ought to be less complete the higher the concentration, and, consequently, the profiles should peak less sharply as concentration increases. This, in turn, would make the slope of the peak absorbance us. concentration calibration curve fall below unity when plotted on a log-log scale. The fact that opposite results are observed, in spite of incomplete atomization, strengthens the significance of the data as evidence for a lateral diffusion interference effect. It can also be shown that ionization suppression cannot account for the slope of the log-log plot of Mo concentration us. peak absorption. Using the flame temperature of 2900 K measured by Rasmuson (27), the flame electron partial pressure of 3.5 X 10-8 atm determined by Becker (28), and the metal vapor partial pressure of 10-8 atm calculated from Willis ( 2 4 ) , Mo was estimated to be 0.3% ionized in the flame under the experimental conditions described above when a 25 pg/ml solution was nebulized. The conditions under which the above authors made their measurements were close to those used in the present work. Even if Mo is more ionized than this by a factor of 10-20, the peak absorbance for the 2500 pg/ml Mo solution is 50% greater than the value that would give a slope of unity. The results reported in this paper should make it clear that the lateral diffusion interference effect is by no means the only cause of the enhancements that were observed. However, the experimental evidence indicates that (28) D. A. Becker. Ph.D. Thesis. Iowa State University, Ames, Iowa. 1970.
-
$ -80 c
w
0
5m .06 a
.04
a .02 40
30
2.0 ID
0
1.0 2.0 3.0 4
HORIZONTAL DISPLACEMENT FROM CENTER OF FLAME ( m m )
Figure 10. Effect of K on Mo absorption profile at the burner tip in the argon-shielded flame
4
mm above
it is the primary cause, and that ionization suppression and other possible effects are of secondary importance. The tendency for analyte to concentrate a t the center of the flame probably occurs to some extent with all slot burners whenever a concomitant is present that slows the rate of vaporization of spray droplets or solid particles. In cooler or less-reducing flames than nitrous oxide-acetylene, or in the presence of severe solute vaporization depressions, a center enhancement may not appear; this may explain, for ex-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
* 1593
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
Author t o whom requests for reprints should be 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
