Anal. Chem. 1990, 62, 1233-1238
1233
Studies of Chemical Interferences in an Inductively Coupled Plasma Using Moment Analysis of Space-Resolved Emission Profiles K. P. Li Department of Chemistry, University of Lowell, Lowell, Massachusetts 01854
J. D. H w a n g and J. D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 32611
Interelement effects Induced by sodlum, aluminum, and phosphate Ion on caiclum and magneshm emission were studled wlth a two-channel spectrometer that could record spatially resolved emission profiles of an atom line and an ion line slmultaneousiy. The statlstlcai moments of the digitized emission proflles were calculated and were employed for ratetonstant estimation using our prevlously establlshed theory. Interelement effects are resuits of Interferences in the analyte atomization, Ionization, and/or recombination steps. Characteristic changes will be induced in the emlssion proflles by each of these Interferences. These changes can be characterized by the statistical moments of the corresponding profiles. Therefore, by Investigating varlatkns in the rate constants caused by the specific concomitant, one may better understand the analyte atomization-excltatlon mechanlsms. Sodhim enhances the atomic emlsslon of both calcium and magnesium but has little effect on their ionic emisslon. Based on the trends of variation in the rate constants, the atomic signal enhancement seems to be attributed more to the facllltation of analyte atomizatlon rather than to the shm of knkatkn equHlbrlum, Aiunlnun and phosphate ion appear not to Induce slgnlficant interferences on either elements.
Freedom of interelement effects has commonly been proclaimed in inductively coupled plasma atomic emission spectrometry (ICP-AES) and has been one of the major factors in its popular acceptance as the contemporary method of choice for multielement analysis. Recent diagnostic studies and measurements, however, have demonstrated that interelement effects, though significantly smaller than in any other conventional excitation sources, do exist in the ICP (1). Spectral interferences, which are outside the scope of this report, have been widely mentioned. The problem is particularly serious for determination of trace elements in real samples such as rare-earth elements (REE) in geological and related materials. Boumans and co-workers (2-4) used spectral modeling techniques and found a total of 1075 possible interfering REE lines for the 26 prominent lines of Ce, La, Nd, Pr, and Sm. Such spectral interferences can significantly increase detection limits and degrade the accuracy of the analytical determination. Line selection should be carefully exercised for these applications. The origins and mechanisms of chemical interferences, on the other hand, are not that well understood. Studying the literature of these interferences in the ICP has been remarked as and still is “a study in confusion” (5, 6). The means of measurements, Le., fixed point or fixed height observation,
* To whom all correspondence should be addressed.
may have misled the earlier investigators (7-26) to contradictory interpretation of their results; the lack of a consistent theory and a set of reduced parameters suitable for interlaboratory comparison may be the cause of the present controversies (27-29). Recently, Li et al. (30, 31) used a kinetic approach for elucidation of analyte transformation mechanisms in the ICP. They explicitly formulated the axial distribution functions for the molecular, atomic, and ionic analyte species. These functions are expressed in terms of the reaction time, i.e., the time lapse reactions such as atomization, excitation, and ionization actually taking place in the central channel of the plasma. Plume expansion due to diffusion and molecular dissociation is also included in the treatment. If the average linear flow velocity of the plasma gas is known, the theoretical functions can be directly correlated with the measured height profiles of emission of the individual species. By comparison of the statistical moments of the theoretical functions with those of the measured profiles, rate constants of the aforementioned reactions can be estimated. With this method, we have demonstrated that elements, e.g., Mg and Ca, having characteristic variations in their atomic and ionic height profiles with respect to experimental conditions, have corresponding changes in their reaction rate constants, indicating that the fate of the analyte species in the plasma is kinetic rather than fast equilibrating and that rate-determining steps (RDS) are not necessarily the same for’different elements or under different conditions. Coaspiration of large amount concomitants will more or less modify the rate-determining reaction sequences of the analyte, thus, characteristically varying the atomic or ionic emission (height) profiles inducing corresponding changes in the rate Constants. In other words, study of these variations in the rate constants may provide specific information about the chemical interferences by the concomitant. This is the scope of the present report. EXPERIMENTAL SECTION Instrumental Setup. The ICP and operational conditions were described in the previous report (31). Digitized atomic and ionic emission (height) profiles were recorded simultaneously and were corrected for background emission before moment calculations. Preparation of Solutions. Stock solutions of calcium and magnesium were prepared from reagent grade chlorides (Fisher Scientific Co.). Solutions of sodium, aluminum, and phosphate were prepared from reagent grade (Fisher Scientific Co.) NaCl, aluminum metal powder, and phosphoric acid (85%),respectively. Testing solutions were prepared by mixing various ratios of concomitant to analyte solutions and diluting to volume with distilled,deionized water. Analyte concentrationswere maintained at 50 ppm. Method of Calculation. Calculations of the statistical moments and the ionization rate constant, kI,were carried out as described in the previous report (31). However, the calculation
0003-2700/90/0362-1233$02.50/0 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
of dissociation rate constant, kD, recombination constant, kR, and vapor plume expansion constant, kx, were modified as described below. On the basis of the derivation given in the previous reports (31), the zeroth moment (area under the emission intensity vs time curve) and the normalized first moment (the center of gravity of the intensity vs time profile) of the atomic and ionic profiles are, respectively
( -S
'la
= -1 + -1+ -1- - 1 a
B
N
d
(3)
and =
1+ -1 + -1 a
a
@
(4)
where
Table I. rf Power Dependence of Rate Constants of Ca power, kw
1.3 1.2 1.1 1.0 0.9 0.8
Substitution of eqs 9 and 10 into eq 8 yields
Substitution of eq 11 into eq 4 gives eq 12 after rearrangement
- -1 = w P
kx, ms-l 0.83 f 0.04 0.89 f 0.06 0.93 f 0.03 0.98 f 0.03 1.07 f 0.02 1.18 f 0.01
ndkI - d )
=Y
(13)
Coupling eqs 12 and 13 yields
(10)
-
0.69 f 0.18 33.36 f 5.23 0.81 f 0.01 0.64 f 0.06 31.95 f 3.93 0.82 f 0.01 0.56 f 0.04 31.46 f 5.45 0.85 f 0.03 0.32 f 0.16 17.67 f 6.91 0.81 f 0.08 0.21 f 0.08 14.68 f 4.47 0.92 f 0.03 0.14 f 0.05 10.22 f 2.14 1.02 f 0.04
aff
and the ratio of eqs 1 and 2 gives
@
k,, ms-I
k~ - [(Sdi- (So)a)lP _
1 d= (SA -
a
k,, ms-'
pm would give reasonable rate constants. Too large or too small diameters would give extremely large or small or even negative rate constants. Therefore, we chose the droplet diameter 0.07 pm as the mean droplet size in the tertiary distribution for no calculation. It was assumed that the measured emission profiles can be theoretically approximated by the distribution functions generated from the reaction kinetics within the selected droplet (30). The zeroth moment of an emission profile is, by definition, a measure of the total amount of the emitting analyte species. If only one droplet is concerned, the zeroth moment will be the total analyte species within the droplet. In other words, no can be numerically chosen as the analyte molecules originally existing in the selected droplet. Substituting this value of no in eqs 1 and 2, one obtains eq 13 after rearrangement
Combination of eq 3 and 4 gives
-1 + -1 =
kD, ms-l
(12)
Because there is one unknown parameter more than the number of independent equations, a, a,and no will have to be solved iteratively. On the basis of the knowledge of M-O bond strengths, the narrow range that k, can vary, and the variations in the atomic and ionic emission profiles upon changes in the radio frequency (rf) power, we estimated (31)the value of wkx and calculated kD and kR from eqs 12 and 6. This procedure was very tedious and was not practical for processing a large collection of data. In this report, the value of no,was estimated from the analyte solution concentration and the droplet size. Theoretically, analyte dissociation, excitation, and ionization should start taking place as soon as there is a single analyte molecular species in the vapor phase. This initial time, t = 0, is unfortunately very difficult to be specified even with a single droplet in the plasma. In practice, a great ensemble of droplets with various sizes is continuously introduced into the plasma. Each droplet has its own initial moment, i.e., each droplet starts vaporization, atomization, and ionization at a height determined by its size and its path of flight in the plasma. Therefore, it is meaningful to estimate no only on an average sense. Quantitative information about the tertiary droplet size distribution in an ICP system is not available mainly because of the insurmountable mechanical and technological difficulties in these measurements. The microscopic and impaction methods are intrusive and are incapable of giving time-resolved information, whereas the optical scattering measurements using either Mie or Fraunhofer theory involve extremely expensive and sophisticated equipment and are incapable of detecting submicrometer particles. In the present report, we have used droplet sizes of 0.01-1.00 pm for calculation and compared the results with the previously reported values (31). We found that droplet diameters of O.OH.10
kx = 2/(w + Y)
(14)
kD = 47/(W2 - 7')
(15)
and The recombination rate constant kR was obtained by substituting k x into eq 6. This approach greatly simplified the calculations. Calibration. The two optical channels were not equal in sensitivity. The less sensitive one was used for ionic emission. The ionic emission profiles thus need to be calibrated with respect to the corresponding atomic profiles. Calibration was performed by setting both channels at the ion line of the element and scanning along the plasma axis. The bin-to-bin ratios of the resulting profiles were then averaged. The average ratio was 10 for calcium and 9 for magnesium. In the previous report (31), the bin-to-bin ratio at the profile maximum was used as the calibration factor, which was 7 for calcium and 4 for magnesium, respectively;the differences with calibration factors in the latter case was a result of noise. RESULTS AND DISCUSSION Power Dependence of Rate Constants. I t has been demonstrated that the ionization rate constants, kI, of calcium and magnesium vary differently with rf power (31). Calcium ionizes increasingly faster with the rf power, as reflected by the shift of its atomic emission profile maximum toward the load coil, whereas ionization of magnesium is relatively independent of the power. The power dependence of the other rate constants, unfortunately, was not previously investigated because of the tediousness of the iterative process. With the modified calculation method, a large collection of emission profiles can now be examined. Rate constants listed in Tables I and I1 were obtained from differently oriented experiments carried out a t different times. The trends were seen to be the same as previously reported (31),which were experimentally done on the same day. Fluctuations in the rate constants from the randomly accessed data were seen to be larger, as expected. Although it is not obvious, it is still noticeable that the molecular dissociation constant, k D , increases, whereas the vapor plume expansion constant, k x , decreases with rf power. The increase in k D with power is readily understandable. The physical meaning of the decrease in k x with power, however,
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
Table 11. rf Power Dependence of Rate Constants of Mg power, kw
kD, ms-l
1.3 1.2 1.1 1.0 0.9
0.27 f 0.09 0.24 f 0.10 0.25 f 0.01 0.13 f 0.06 0.08 f 0.03
kI,ms-'
kR, ms-l
Table 111. Effects of Na on Ca Rate Constants
kx, ms-l
power, kw
A
,
I
6.00
31.38 f 11.33 5.51 f 1.09 40.99 f 15.51 7.84 f 1.83 47.46 f 6.99 9.65 f 2.09 35.94 f 20.62 10.26 f 5.02 22.42 f 5.25 8.77 f 1.10
1.18 f 0.04 1.26 f.0.02 1.33 f 0.04 1.34 f 0.02 1.36 f 0.03
Na concn, ppm
1.3
-a
500 1000 2000 1.2
I
J
-a
1000 2000 1.1
5.00
-a
500 1000 2000
4.00
1.0
w
-a
lo00 2000
3.00
I-
0.9
Z
-a
500 2000
- 2.00 a
1.oo
1235
kD, ms-'
kI, ms-l
kR, ms-'
kx, ms-l
0.69 0.87 0.91 0.68
33.36 26.08 24.38 16.06
0.81 0.71 0.69 0.39
0.83 0.84 0.84 0.81
0.64 0.82 0.87
31.95 24.06 22.18
0.82 0.72 0.74
0.89 0.88 0.88
0.56 0.67 0.74 0.77
31.46 24.38 23.47 23.02
0.85 0.75 0.75 0.83
0.93 0.93 0.92 0.95
0.32 0.48 0.63
17.67 19.35 24.42
0.81 0.85 0.97
0.98 1.00 1.03
0.21 0.36 0.32
14.68 14.05 11.67
0.92 1.20 1.44
1.07 1.09 1.09
Average value from Table I.
Table IV. Effects of Na on Mg Rate Constants 0.00 0
TIME (ms) 0 6.00
4.00
c
v, Z
1
ii
Na concn, ppm
kD, ms-'
kI, ms-'
1.3
500 2000
0.33 0.42 0.59
30.46 26.50 26.57
5.55 4.99 4.42
1.16 1.23 1.23
1.2
500 1000 2000
0.32 0.41 0.45 0.50
51.40 53.05 30.61 30.55
9.63 10.46 5.58 5.62
1.23 1.28 1.29 1.31
1.1
500 1000 2000
0.26 0.32 0.36 0.39
52.40 28.55 28.46 25.71
11.13 6.47 6.57 6.03
1.29 1.36 1.37 1.36
1.0
500 1000 2000
0.18 0.29 0.31 0.35
58.78 28.54 26.37 30.77
16.06 7.30 6.98 8.76
1.34 1.42 1.44 1.46
0.9
500 1000 2000
0.09 0.14 0.17 0.22
23.83 20.99 20.26 20.61
10.01 9.28 8.99 8.41
1.35 1.45 1.45 1.47
power, kw
I
1
W I-
Z
- 2.00
Flgure 1. Interelement effect of sodium on calcium atomic (A) and ionic (8)emission: curve a, 0 ppm Na; curve b, 500 ppm Na; curve c, 1000 ppm Na; curve d, 2000 ppm Na in 50 ppm Ca solution.
is not clear even though its mathematical relation with kD has been established in our previous report (31). We showed that kx approaches l l w when k D approaches infinity and approaches 2 / w when kD approaches zero. It is also worth mentioning that the recombination rate constants, k R , of calcium are considerably smaller than those of magnesium. This was expected (31)because calcium has a lower ionization potential and a lower electron affinity than magnesium, and so, it is more difficult for a calcium ion to recombine with an electron. The sluggishness in recombination renders it more difficult for calcium to approach equilibrium (or LTE) than magnesium under the same plasma conditions. As a result, the peak separation between Mg atomic and ionic emission profiles is narrower than that of Ca, ion-to-atom intensity ratios of Mg are less spatially dependent, and the rf power dependence in atomic peak emission position of Mg is smaller than that of calcium.
kR, ms-l
kx, ms-l
Interelement Effects of a n Easily Ionized Element. Figures 1 and 2 show typical interelement effects of sodium on calcium and magnesium atomic and ionic emission, respectively. It is seen that atomic emissions of both elements are enhanced by Na, whereas their ionic emissions are relatively unaffected. The enhancement in atomic emission by an easily ionized element (EIE) such as sodium has long been attributed to the shift of analyte ionization equilibrium in flame spectroscopy. The same mechanism has also been implied by some in ICP spectrometry. Recent studies on electron number distribution in the plasma (32-36), however, have demonstrated that the spatial distribution pattern and amplitude of the electron number density, ne, in the plasma are not significantly affected by the addition of a fair amount of EIE as a concomitant. This indicates that ionization equilibrium shifting may not be the principal mechanism in the signal enhancement of analyte atomic emission. Further evidence may be obtained by examining the rate constant variations induced by sodium (see Tables I11 and IV). As
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
A 8.00
A 4.00
d
3.00
6.00
czcn
c
v, z 4.00
w
w
2.00
-I Z -
t-
z
1.oo
2.00
0.00 0.00
1.00
3.00
0.00 0.00
4.00
2.00
1.00
3.00
4
3.00
4.
TIME (ms)
TIM E”1ms) 5.00
a
3.00
4.00
4
11
- 3.00 v, Z
5w- 2.00 1.oo
0.00 0.00
2.00
1.00
3.00
0.00 0.00
4.00
TIME (ms) Figure 2. Interelement effect of sodium on magnesiumatomic (A) and ionic (B) emission: curve a, 0 ppm Na; curve b, 500 ppm Na; curve c, 1000 ppm Na; curve d, 2000 ppm Na in 50 ppm Mg solution.
Table V. Effects of A1 on Ca Rate Constants power, kw 1.3
A1 concn, ppm 500 1000 2000
1.2
-
500 1000 2000
1.1
500 1000 2000
1.0
-
500 1000 2000 0.9
-
500 1000
2000
k D , ms-’
kI, ms-I
kR, ms-’
kx, ms-’
0.69 0.57 0.51 0.62
33.36 39.28 44.17 38.03
0.81 1.02 1.16 0.95
0.83 0.89 0.89 0.86
0.64 0.50 0.50 0.47
31.95 37.71 40.90 42.10
0.82 1.12 1.22 1.29
0.89 0.92 0.93 0.91
0.56 0.42 0.42 0.38
31.46 31.76 34.18 36.39
0.85 1.14 1.22 1.53
0.93 0.96
0.32 0.34 0.31 0.30
17.67 27.39 31.20 31.00
0.81 1.23 1.59 1.68
0.98 1.03 1.04 1.01
0.21 0.24 0.24 0.23
14.68 24.90 29.19 33.78
0.92 1.57 1.94 2.76
1.07 1.08 1.07 1.07
0.95 0.97
one can see, both kI and kR are either slightly suppressed or essentially not changed in the presence of large amounts of
1.oo
Figure 3. Interelement effect of aluminum on calcium atomic (A) and ionic (B) emission at rf power 1.2 kW: curve a, 0 ppm AI; curve b, 500 ppm AI; curve c, 1000 ppm AI; curve d, 2000 ppm AI in 50 ppm Ca solution.
sodium, whereas the dissociation constants, kD, of both elements are consistently enhanced. This seems to indicate that atomic signal enhancement may be caused mainly by the increase in atomization of the molecular species. This is conceivable because analyte atoms may associate with the concomitant element to form metal-metal complexes, instead of oxides or hydroxides, during desolvation and vaporization in the plasma. Since the analyteEIE association is commonly weaker than the analyte-oxygen bonding, atomization is facilitated in the presence of large amounts of EIE. Interferences Induced by Aluminum and Phosphate Ion. At high rf power (above 0.9 kW) aluminum exerted a slight suppression on both atomic and ionic emission of calcium but essentially had no effect on magnesium emission signals (see Figure 3). At low power (below 0.8 kW), both magnesium atomic and ionic emissions were enhanced (Figure 4).
Signal suppression by aluminum has normally been attributed to the formation of less-volatile compounds and/or to the occlusion of analyte in less-volatile aluminum oxide matrices in flame spectrometry. This could also occur in ICP spectrometry. Unfortunately, no existing theory can provide quantitative proof for such ‘speculation. Some mechanistic information may be retrieved from examination of the rate constants given in Tables V and VI. As one can see from
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
A 2.00
Table VII. Effects of Phosphate on Ca Rate Constants
d
PO?power, kw 1.3
concn, ppm -
L
v, Z
W IZ
1.2
-
1.1
1.0
0.83 0.86 0.87
33.36 150.09 144.61 146.36
0.81 1.89 1.81 1.83
500 1000 2000
0.64 1.47 0.58 0.63
31.95 73.06 38.85 41.11
0.82 1.03 1.00
0.88
0.89 0.72 0.89 0.87
500 1000 2000
0.56 0.58 0.58 0.57
31.46 42.04 42.29 42.50
0.85 1.00 0.95 0.93
0.93 0.93 0.92 0.92
-
0.32 0.16 0.20 0.25
17.67 13.48 17.07 19.24
0.81 0.92 0.95 0.90
0.98 0.97 0.98 0.97
0.21 0.14 0.21 0.21
14.68 13.53 20.26 19.93
0.92 0.96 1.05 1.01
1.07 1.04 1.05 1.04
-
500 1000 2000 0.9
kI, ms-' k ~ ms-' , kx, ms-'
0.69 3.16 2.94 2.76
500 1000 2000
t
kD, ms-'
500 1000 2000
0.88
Table VIII. Effects of Phosphate on Mg Rate Constants
PO?' concn,
power, kw 1.3
ppm 500 1000 2000
1.2
500 1000 2000
Figure 4. Interelement effect of aluminum on magnesium atomic (A) and ionic (9) emission at low rf power: curve a, 0 ppm AI; curve b, 500 ppm AI; curve c, 1000 ppm Al; curve d, 2000 ppm AI in 50 ppm Mg solution.
Table VI. Effects of A1 on Mg Rate Constants
1.1
500 1000 2000
1.0
1.3
ppm
500 1000 2000
1.2
500 1000
k,, ms-'
kl, ms-'
kR, ms-'
kx, ms-'
0.16 0.16 0.17 0.35
20.54 18.13 17.21 33.31
4.41 3.97 3.85 4.89
1.16 1.19 1.20 1.28
23.17 19.85 17.42 32.02
5.97 5.29 4.68 5.08
1.25 1.28 1.26 1.30
2000
0.13 0.13 0.14 0.32
1.1
500 1000 2000
0.11 0.11 0.12
17.55 16.61 18.36
5.34 5.37 5.97
1.28 1.28 1.33
1.0
500 1000
0.07 0.09 0.09
18.71 15.99 17.72 17.69
7.25 6.58 7.17 7.61
1.32 1.34 1.36 1.35
0.05 0.05 0.05 0.06
16.50 17.46 18.43 16.81
8.36 10.08 10.88 10.95
1.34 1.34 1.33 1.35
2000 0.9
500 1000 2000
0.08
Table 5,aluminum slightly enhances kIand kR of calcium but suppresses kD Since the slower processes in a kinetic system
500 1000 2000
A1 concn,
power, kw
-
0.9
500 1000 2000
kD, ms-* k,, ms"
kR, ms-'
kx, ms-'
0.31 0.30 0.30 0.33
43.14 45.51 43.76 52.50
6.58 6.81 6.32 7.41
1.21 1.21 1.22 1.26
0.28 0.26 0.25 0.24
48.41 42.62 47.55 45.92
7.91 7.03 8.05
7.80
1.28 1.27 1.27 1.27
0.24 0.21 0.20 0.20
42.52 39.48 41.63 42.05
8.17 7.67 8.31 8.37
1.36 1.31 1.32 1.35
0.15 0.14 0.14 0.14
35.32 35.92 35.47 35.61
7.47 8.90 8.60
1.35 1.35 1.35 1.35
0.11 0.09 0.09 0.09
26.82 30.15 26.25 28.22
7.93 10.18 8.46 9.09
1.40 1.42 1.38 1.42
8.68
are the rate-determining steps (RDS), variations in k D will induce a greater influence on the analyte spatial profile than those of k , and kR. The effect of aluminum on the dissociation of calcium molecular species may be responsible for the signal suppression in calcium atomic emission. No consistent trends could be traced for the effect of aluminum on the Mg rate constants. The phosphate ion effects were not significant for both calcium and magnesium (see Tables VI1 and VIII). The extremely high values of k I of calcium induced by the phosphate ion a t 1.3 kW are most likely an artifact as reflected by the drastic change in the spatial distribution (Figure 5). The cause of this anomaly is not known.
CONCLUSION As demonstrated by spatial profiles, interelement effects
do exist in the inductively coupled plasma. The interferences
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
A 4.00
I l
a
in the excitation of a specific analyte under certain experimental conditions can be pinpointed. Studies of Ca and Mg emission profiles appear to support this approach. The precision of the rate constants needs to be improved. Registry No. Ca, 7440-70-2; Mg, 7439-95-4; Na, 7440-23-5; Al, 7429-90-5;PO4, 14265-44-2.
t
LITERATURE CITED
z
(1) Davis, J.; Snook, R. D. J. Anal. At. Spectrom. 1988, 7 , 25. (2) Boumans. P. W. J. M.; Tlelrooy, J. A.; Maessen, F. J. M. J. Spectrochim. Acta, Part B 1988. 438, 173-199. (3) Boumans, P. W. J. M.; Vrakking. J. J. A. M.; Heljms, A. H. M. Spectrochim. Acta, Part B 1988, 438, 1365-1404. (4) Boumans, P. W. J. M.; He, 2 . 2.; Vrakking. J. J. A. M.; Teikooy, J. A,; Maessen, F. J. M. J. Specfrochim. Acta, Part 8 1989, 448, 31-93. (5) Blades, M. W.; Horlick, G. Spectroch/m. Acta, Part B 1981, 368, 881-900. (6) Gunter, W.; Visser, K.; Zeeman, P. B. Specfrochim. Acta, Part 8 1985, 408, 617-629. (7) Mermet, J. M.; Robin. J. Anal. Chim. Acta 1975, 75,271. (8) Koirtyohann. S. R.; Jones, J. S.;Jester, C. P.; Yates, D. A. Spectrochim Acta, Part B 1981, 36,49. (9) Kawaguchi, H.; Ito, T.; Ota, K.; Mizuike, A. Specfrochim. Acta, Part B 1980, 35, 199. (10) Savage, R. N.; Hiftje, G. M. Anal. Chem. 1980, 35, 1267. (1 1) Horlick, G.; Furuta, N. Spectrochim. Acta, Part 8 1982, 37,999. (12) Rybarczyk, J. P.; Jester, C. P.; Yates, D. A. Koirtyohann, S. R. Anal. Chem. 1982, 54,2162. (13) Roederer, J. E.; Bastiaans, G. J.; Fernandez, M. A,; Fredeen, K. J. Appl. Specfrosc. 1982, 36,383. (14) Hoare, H. C.; Mostyn, R. A. Anal. Chem. 1987, 39, 1153. (15) Kirkbright, G. F.; Warden, A. F.; West, T. S . Anal. Chim. Acta 1974, 64, 353. (16) Boumans, P. W. J. M.; de Boer, F. J. Proc. Anal. Div. Chem. SOC.
m
t
m
z
Flgure 5. Interference of phosphate ion on calcium atomic (A) and ionic (B) emission at rf power 1.3 kW: curve a, 0 ppm phosphate: curve b, 500 ppm phosphate: curve c, 1000 ppm phosphate; curve d, 2000 ppm phosphate in 50 ppm Ca solution.
induced by a certain concomitant may be more pronounced to some elements but less severe to others. The magnitude of profile distortion also varies with experimental conditions. This seems to indicate that there is unlikely a single common “dominating” excitation process, such as ionization equilibrium shifting, Penning ionization, recombination, etc., for various elements under various conditions. A kinetic approach using rate-determining steps appears to be more logical in describing the trends of profile distortions. Since the spatial profile can be specified by its statistical moments, which, in turn, are functions of the kinetic rate constants, profile distortions, Le., interelement effects, can be quantitatively reflected by the variations in these rate constants. The rate-determining steps
1975, 12, 140.
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RECEIVED for review August 29, 1989. Accepted February 8, 1990. Research supported by NIH-5RO1-GM 38434-02.
