1472
Anal. Chem. 1988, 60, 1472-1474
Registry No. NOz-, 14797-65-0;NOz, 10102-44-0;Au, 744057-5.
LITERATURE CITED (1) Xing, X.; Scherson, D. A. Anal. Chem. 1987, 59, 962. (2) Parts, L.; Sherman, P. L.; Snyder, L. D.; Jaye, F. C. Anal. Instrum. 1972, 10, 157. (3) Plieth, W. J. I n €ncyclopedla of fhe Nectrochemlstry of fhe Nements; Marcel Dekker: New York. 1978; Vol. V I I I , pp 321-479. (4) Vetter. K. J. 2 . Phys. Chem. (Frankfurt am Main) 1950, 194, 199. (5) Tanaka, N.; Kato, K. Bull. Chem. SOC.Jpn. 1956, 2 9 , 837. (6) Raspi, G.; Guidelli, R . Chlm. Ind. (Milan) 1963, 4 5 , 1398. (7) Guidelli, R.; Pergola, F.; Raspi. G. Anal. Chem. 1972, 4 4 , 745. (8) Schmidt, G. 2 . Elekfrochem. 1959, 63, 1183. (9) Schmidt, G.; Lobeck, M. Ber. Bunsen-Ges. Phys. Chem. 1964, 6 8 , 677. (IO) Erlikh. Yu. I.: Anni, K. L.; Palm, U. V. Nekfrokhymiya 1979, 14, 925. ( 1 1 ) Erlikh, Yu. 1.; Anni, K. L.: Palm, U. V. Elekfrokhymiya 1980, 15, 1573. (12) Harris, W. E.; Kolthoff, I.M. J . Am. Chem. SOC. 1945, 6 7 , 1484. (13) Imai, H. Bull. Chem. Soc. Jpn. 1957, 3 0 , 873. (14) Oriemann, E. F.; Kern, D. M. H. J . Am. Chem. SOC. 1953, 7 5 , 3058. (15) Koutecky, J.: Koryta, J. Collect. Czech. Chem. Commun. 1954, 2 0 , 845. (16) Mastragostino, M.; Nadjo, L.; Saveant, J. M. Electrochim. Acta 1968, 13, 721. (17) Mastragostino, M.; Saveant, J. M. Nectrochim. Acta 1968, 13, 751. (18) Amatore, C.;Saveant. J. M. J . Hectroanal. Chem. 1977, 8 5 , 27.
(19) Amatore, C.; Saveant, J. M. J . Electroanal. Chem. 1980, 107, 353. (20) Amatore, C.; Gareil, M.; Saveant, J. M. J . Elecfroanal. Chem. 1983, 147, 1. (21) Compton, R . G.; Daly, P. J.; Unwin, P. R.; Waller, A. M. J . Elecfroanal. Chem. 1985, 191, 15. (22) Ulstrup, J. Necfrochlm. Acta 1968, 13, 1717. (23) Holub, K. J . Electroanal. Chem. 1971, 3 0 , 71. (24) McIntyre, J. D. E. J . fhys. Chem. 1967, 7 1 , 1196. (25) Mcintyre, J. D. E. J . fhys. Chem. 1989, 7 3 , 4102. (26) Kuta, J.; Yeager. E. J . Electroanal. Chem. 1971, 3 1 , 119. (27) Kuta, J.; Yeager, E. J . Electroanal. Chem. 1973, 46, 233. (28) Nekrasov, L. N.; Potapova. E. N. Nekfrokhimiya 1970, 6, 780. (29) Levich, V. G. Physicochemical Hydrodynamics : Prentice Hall: Englewood Cliffs, NJ, 1962. (30) Bard, A. J.; Faulkner, L. R. Nectrochemlcal Methods, Wiiey: New York, 1980. (31) Day, R. A., Jr.; Underwood, A. L. Quantitative Analysis, 5th ed.; Prentice Hail: Engiewood Cliffs, NJ, 1986. (32) Ehman, D. L.; Sawyer, D. T. J . Necfroanal. Chem. 1968, 16, 541. (33) Pourbaix, M. Afias d'Equilibres Nectrochimiques a' 25 "C; Gauthier Villars: Paris, 1963.
RECEIVED for review December 9, 1987. Accepted March 3, 1988. Support for this work was provided in part by IBM through a Faculty Development Award to one of the authors
(D.S.).
CORRESPONDENCE Nonspectroscopic I nterelement Interferences in Inductively Coupled Plasma Mass Spectrometry Sir: It has generally been reported that inductively coupled plasma mass spectrometry (ICP-MS) is more susceptible to nonspectroscopic interelement interferences ("matrix effects") by high concentrations of concomitant elements relative to inductively coupled plasma optical emission spectrometry (ICP-OES) (1-12). A recent paper by Beauchemin et al. (3) summarizes these reports. In most cases suppressions of analyte signals have been reported, but in a few cases ( I , 9) enhancements have been seen. The effects depend on both ICP operating conditions and lens settings. Different conditions have been used by various workers, which may explain reports of varied behavior. Nevertheless broadly similar effects have been reported for both SCIEX (1-6) and VG (7-12) commercial systems and for at least two "homemade" systems
Table I. Equipment and Operating Conditions SCIEX ELAN 250 ICP-MS 1.2 kW
power outer gas auxiliary flow nebulizer Nebulizer flow" sample uptake sampler/load coil separation i o n lenses voltages
sampling orifice skimming orifice
M e i n h a r d TR30-C3
0.85 L/min
0.7 mL/min 17 mm adjusted t o give equal response for 100 ppb Li and U and maximal response for 100 ppb Rh 1.1 mm
0.9 mm skimmer tip located 7 mm downstream of sampling orifice
(2, 12, 13).
The mechanisms proposed thus far to explain these effects have focused on the ICP or the supersonic expansion, although several workers have suggested that some of the observed effects may arise in the ion optics. In this communication we will outline a new mechanism which qualitatively seems to account for many of the varied observations. The mechanism attributes the apparent matrix effects to changes in the flux and composition of the ion beam. These changes arise due to space charge effects within the skimmer. (Although Olivares and Houk (14) noted that space charge could be important in the optics of their ICP mass spectrometer, no connection between space charge and matrix effects was made.) Both ion current measurements and the behavior of ion signals a t the detector provide clues as to the origin of the matrix effects observed. Ion current measurements were made with a stainless steel collector consisting of three concentric rings and a central circular stop. (The overall diameter of the collector was 4.5 cm.) The collector was situated 1 in. from the base of the skimmer. All four elements of the collector
12 L/min 1 L/min
Sensitivity a t this nebulizer gas flow was reduced ca. 5 times f r o m the m a x i m u m achieved a t 0.95 Limin.
were tied together and biased at -30 V through an electrically floating electrometer. A mesh lens between the skimmer and the collector was held at 0 V. Other equipment and operating conditions for all the measurements are listed in Table I. By use of these operating conditions, the ratios of analyte signals (Li+,Rb+, Th+) from solutions with and without a high concentration of matrix element were measured; the results are shown in Figure 1. Most striking are the mass effects. For equimolar solutions, heavy matrix elements give greater suppressions than light matrix elements and for a given matrix, light analytes are suppressed more than heavy analytes. Similar results have also been reported by Tan and Horlick ( 1 ) and Kawaguchi et al. (13). Other workers (9) have also reported that the extent of suppression depends on the settings of the ion lenses. This observation and the mass trends
0003-2700/88/0360-1472$01.50/0 C 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
1473
1.200 1.000
I + Th + m + Li
0.400
0200
1
.
o.ooo+
30
'
, 80
,
,
I
130
.
I
180
.
I
230
mlr of mitrlx Ion
Flgure 1. Ratio of analyte signal in the presence of the matrix element to that in the absence of the matrix element as a function of the matrix element mass for Li', Rb', and Th+ analyte ions. The matrix element concentrations were 0.0042 M.
discussed above suggest that the suppressions might derive from the ion optics. It is readily calculated (15) that for a nebulizer efficiency of 1%, a sample uptake rate of =1 mL/min, an ICP temperature of 5000 K, and an aerosol gas flow rate of 0.85 L/min, an element in solution at 0.04 M contributes -1 X 1013ions cm-3 to the plasma. This is on the order of 1%of the total positive ion density of 1 X 1015~ m - ~ Thus . a matrix element present at a concentration of 0.04 M should cause a negligible perturbation to the ion density and ionization equilibria in the ICP for the plasma operating conditions used here. Despite this, current measurements downstream of the skimmer show that nebulization of a 0.04 M matrix solution can cause an increase in the total ion current. This increase is greatest for heavy matrix elements. For example, when distilled water is aspirated, a current of 6 pA is measured but when 0.04 M U solution is nebulized, the current increases to 20 pA. Mass spectra show that this additional current (14 FA) consists of U+ ions. These observations can be rationalized if one assumes that a mass discrimination effect is inherent in the sampling process, with heavy ions being transmitted through the interface much more efficiently than lighter ions. As follows, a comparison of the measured ion currents to those predicted by theory supports this hypothesis and also reveals that the discrimination occurs within the skimmer. As outlined in ref 16 calculation of the gas flow through the skimmer is straightforward. For our system, the gas flow through the skimmer is estimated to be 1 X atoms s-l assuming a gas temperature of 5000 K. Since recombination in the sampling process is minimal (16) and since the degree of ionization of Ar is ==0.1%,the total Ar+ ion current through the skimmer is expected to be 1 X 10l6 s-l or 1500 pA. On comparison of this figure to the measured flux of 6 PA at the base of the skimmer, it appears that less than 1%of the incident Ar+ ions survive the skimmer. In contrast, if uranium is 1%of the positive ion density in the plasma, then the expected U+ ion flux arriving a t the skimmer is 15 pA. The observation of an additional 14 pA of current at the base of the skimmer when 0.04 M U solution is aspirated then points to the conclusion that little or no attenuation of U+ ions occurs within the skimmer. This mass discrimination can be explained by postulating that the ion beam within the skimmer is space charge limited; i.e. it has a density which is so high that mutual repulsion will not allow the ions to remain tightly focused. The extent of defocusing depends on the mass and energy of the ions, with lighter (less energetic) ions being defocused much more readily than heavier (more energetic) ions. Ions which defocus are presumed lost at the skimmer wall. It should be emphasized that the defocusing occurs because of the excessive Ar+ cur-
- n r r - - i Flgure 2. Calculated trajectories for Ar+ and U+ ions in the absence (a) and presence [b) of space charge. The ion path consists of a grounded skimmer, a triple cylinder Einzel lens, and an aperture plate. Only the upper half of the ion path is shown.
rent; no mention of a matrix ion has been made. Some support for space charge limited transmission within the skimmer comes from the following calculation: The space charge limit for ion current focused through a cylinder of diameter D and length L is given by where I,, is in pA, m/z is the mass to charge ratio of the ion, and V is the ion energy (17). Unfortunately, a corresponding formula giving the limiting current expected for a cone such as the skimmer is not available. To obtain a rough estimate, the skimmer was assumed to be a cylinder having a D / L of 0.5. (The diameter a t the base of the skimmer is equal to its length. The average diameter of the skimmer then is =L/2 and the ratio D/L is ~ 0 . 5 . )With an ion energy of 3 eV for Ar' ion (18),the calculated maximum Ar+ ion current is 7 MA, This agrees quite well with the measured current of 6 pA but is by no means rigorous proof. Further support for the space charge hypothesis comes from computer modeling work. A computer program has been developed a t SCIEX to calculate ion trajectories in the presence of space charge effects. The algorithm employed is an adaptation of that described by Weber (19). The potential distribution within a given lens array is calculated initially in the absence of space charge and ion trajectories are computed. The resulting space charge modification to the potential array is determined and the trajectories are recalculated. This procedure is iterated until convergence. Inputs to the program include the composition of the plasma at the entrance to the skimmer, the total positive ion current, the ion energies and the angular dispersion of the ions. Dissipation of the plasma as a function of axial distance from the skimmer tip must also be considered. Figure 2 shows calculated trajectories for Ar+ and U+ ions through a skimmer, a three-element cylindrical Einzel lens, and an aperture in the absence (Figure Pa) and presence (Figure 2b) of space charge. A 2-pA ion beam consisting of 80% Ar+ and 20% 0' (corresponding approximately to the ionic composition of the ICP) was used in the calculations. Ion energies were taken from ref 18. Without space charge, ions are well-focused regardless of mass; however, as discussed, it is seen that in the presence of space charge, Ar+ ions are strongly defocused within the skimmer whereas U+ ions remain on axis. It is also evident that the trajectories of ions downstream of the skimmer are dramatically altered by the inclusion of space charge effects. Although the current downstream of the base of the skimmer is on the order of A, our modeling results show that space charge effects in this region are important down to currents on the order of A. Any change then in the flux and/or composition of the ion beam at the base of the skimmer such as is observed will affect the space charge downstream, the ion trajectories, and ultimately the ion signals.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
I c0
)
Flgure 3. Calculated trajectories for Co’ analyte ion in the presence space charge for plasma compositions of (a) 80% Ar’, 20% 0’ and (b) 80% Ar’, 19% O’, 1 % U.’ of
This is illustrated in Figure 3 which presents calculated Co’ analyte ion trajectories for 2-FA ion beams containing no uranium (Figure 3a) and 1% uranium (Figure 3b). In the presence of uranium, some ions (which previously were transmitted through the Bessel box aperture in Figure 3a) are defocused both at the first lens element and a t the aperture; transmission through the aperture is subsequently reduced. To summarize thus far, we propose the following general mechanism for the “matrix effects” observed in ICP-MS: Ion transmission through the skimmer is affected by space charge repulsion, with heavy ions being transmitted most efficiently. Heavy matrix ions initially comprising only 0.1-1% of the beam arriving a t the skimmer can perturb or dominate the current leaving the skimmer due to this mass discrimination. (The more massive the ion the greater the perturbation.) This results in increased defocusing of lighter ions within the skimmer and also alters the space charge and hence the ion trajectories in the ion optics downstream of the skimmer. In this framework, the worst case would be a light analyte ion and a heavy matrix ion; this is borne out by the trends shown in Figure 1. On the other hand, the best case should be a matrix ion having a m / z less than Ar+ (e.g. Na). In this situation, little or no matrix effect would be seen because Na+ ions should be defocused to an even greater extent than Ar’ ions; little or no change in beam current down stream of the skimmer is then expected. The exact changes to analyte signals will depend on the details of the ion optics and the potentials applied to the lenses, so some variation of matrix effects is expected (and observed (9)). In addition, plasma conditions should have a marked influence on the observed effects. Any change to the ICP which reduces the density of matrix ions relative to argon ions (e.g. higher power, lower aerosol gas flow rate, or sample dilution) would be expected to decrease the change in ion current at the base of the skimmer and minimize matrix effects; this is generally what is observed. As aerosol gas flow rate is increased to the point where ion count rates are maximized
(commonly referred to as “the top of the mountain”), matrix ions become more concentrated relative to Ar+. This will magnify the effects described above. The plasma is also being cooled however and so ionization suppression effects may become significant or dominant, especially with the higher sample loading realized for ultrasonic nebulization (12). Finally this mechanism suggests that it may be possible to develop ion optics which minimize the effects of space charge or of changes in the space charge with different matrices. One such example was reported recently by us (20). This modified ion optics showed less than 10% change in analyte signal even with a 0.04 M uranium matrix. The altered ion optics, however, were found to degrade the instrument stability (particularly in the presence of 1% solution) and were not suitable for a routine instrument. Nevertheless this result suggests that in the future ICP-MS may be able to provide freedom from nonspectroscopic interferences similar to ICP-OES while retaining much lower limits of detection.
LITERATURE CITED (1) Tan, S. H.; Horlick, G. J . Anal. Atomic Spectrosc., in press. (2) Houk, R. S. Anal. Chem. 1988, 58, 97A. (3) Beauchemin, D.; McLaren, J. W.; Berman, S. S. Spectrochim. Acta, Part B 1987, 4 2 8 , 467. (4) Gregroire, D. C. Spectrochim Acta, Part B 1987, 428, 895-907. (5) Longerich, H. P.; Fryer, 6. J.; Strong, D. F.; Kantipuly, C. J. Spectrochim Acta, Part B 1987, 42B, 75. (6) Longerich, H. P.; Fryer, B. J.; Strong, D. F. Spectrochim Acta, Part B 1987. 4 2 8 , 101. (7) Pickford, C. J.; Brown, R. M. Spectrochim Acta, Part B 1988, 41B(1/ 2), 183. (8) Gray, A. L.; Date, A. R. Analyst (London) 1983, 708, 1033. (9) Hutton. R. C.; Shaw, C. J. SSC Workshop on ADDlications of ICP-MS, .. Toronto, 1985. (10) Gray, A. L.; Jarvis, K.; Williams, J. G. Paper E2.5 presented at the XXV Colloquium Spectroscopium Internationale, June 1987, Toronto. (11) Long, S.E.; Brown, R . M.;Pickford, C. J. Paper THA392 presented at the XXV Colloquium Spectroscopium International, Sept 15-20, 1985, Garmisch Partenkirchen SDectrochim Acta. (12) Olivares, J. A.; Houk, R. S . Anal. Chem. 1988, 58, 20. (13) Kawaauchi. H.: Tanaka. T.: Nakamura, T.: Morishita, M. Anal. Sci. 1987,-3. 305. (14) Olivares, J. A.; Houk, R. S.Anal. Chem. 1985, 57, 2674. (15) De Galan, L.; Winefordner, J. D. J . Quant. Spectrosc. Radiat. Transfer 1987, 7 , 251. (16) Beijerinck, H. C.; et al. Chem. Phys. 1985, 96, 153. (17) Pierce, J. R., Theory and Design of Electron Beams, 2nd ed.; Van Nortrand: New York, 1954. (18) Fulford, J. F.; Douglas, D. J. Appl. Spectrosc. 1986, 4 0 , 971. (19) Weber, C. Focussing of Charged Particles; Academic: New York, 1967; Vol. 1. (20) Boorn, A.; Gillson, G.; Fulford. J.; Douglas, D.; Quan, E. Presented at the 1987 Winter Conference on Plasma and Laser Spectrochemistry, Lyon, France, Jan 12-16, 1987. * Author to whom correspondence should be addressed
George R. Gillson* Donald J. Douglas John E. Fulford Kenneth W. Halligan Scott D. Tanner
SCIEX 55 Glen Cameron Road Thornhill, Ontario Canada L3T 1P2 RECEIVED for review October 5, 1987. Accepted February 28, 1988.
