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Non-spectroscopic matrix interferences in inductively coupled plasma-mass spectrometry
0584~8547/90 $3.00+.00 Pergamon Press plc
Speruochzmicu Acto. Vol. 458, No. 3, pp. 333 339. 1990 Pnnted m Great Britain.
Non-spectroscopic
matrix interferences in inductively coupled plasmamass spectrometry
YOUNG-SANG KIM,* HIROSHI KAWAGUCHI,~TOMOKAZUTANAKAand ATSUSHIMIZUIKE Faculty of Engineering,Nagoya University,Chikusa-ku,Furocho, Nagoya 464, Japan (Received 17 May 1989; in revised form 29 August 1989)
Abstract-Fourteen 0.01 M matrix elements with atomic weight from 7 to 207 (Li, B, Na, K, Cr, Zn, Rb, MO, Cd, Cs, Sm. Ho, Lu and Pb) have been selected to study their interference effects on analyte (Al, Co, Y, In, La and Tl) signals with a commercial ICP mass spectrometer. The parameters investigated affecting the degree of interference were: mass of matrix element and analyte, ionization potential of matrix element, voltage of the first ion lens element and sampling depth. The matrix effect could not be explained by any single mechanism, e.g. collisions of analyte ions with matrix ions in the supersonic expansion region, space charge effect in the ion optical system and shift in ionization equilibria in the plasma, but by a combination of several mechanisms.
1. INTRODUCTION SINCE
the first paper on inductively coupled plasma-mass spectrometry (ICP-MS) was published in 1980 by HOUK et al. [l], various studies have been reported on factors affecting analyte signals as well as on its applications to inorganic trace analysis. The analyte signals in ICP-MS are influenced not only by the operating parameters of the instrument [2-71 but also by the presence of concomitant elements [8-211. The effects of concomitant elements can be classified into non-spectroscopic [8-141 and spectroscopic interferences [15-211. The latter consist of isobaric interferences and spectral overlap with doubly charged ions and polyatomic ions such as oxide and hydroxide ions of concomitant elements. Non-spectroscopic interferences cause suppression of analyte signals in most cases and enhancement in a few cases in the presence of matrix elements. Generally, these non-spectroscopic interferences, i.e. matrix effects, are more severe than those in ICPAES [S, lo]. GREGOIRE [9], KAWAGUCHI et al. [lo], TANand HORLICK[l l] and BEAUCHIMIN et al. [12] independently identified mass dependence of the matrix effect. Heavy matrix elements cause more severe suppression effects and light analytes are more seriously affected [lo, 111. As a mechanism of the mass dependence of the matrix effect, KAWAGUCHI et al. [lo], BEAUCHIMINet al. [12] and CRAIN et al. [14] explained that the suppression was caused by collisions of analyte ions with heavy matrix ions which were enriched on the central axis of the supersonic expansion region behind the sampler. BEAUCHIMIN et al. [12] also proposed mechanisms for the formation of oxides which can deposit on the interface and of ionization induced by atom-lectron collisions. TAN and HORLICK [l l] and GILLSON et al. [13] reported that the ion transmission through the skimmer is affected by space charge repulsion with heavy ions being transmitted most efficiently. In addition to the above explanations, OLIVARES and HOUK [8] proposed a shift in ionization equilibria by the presence of easily ionizable matrix elements. The ambipolar diffusion effect in the plasma, having a heterogeneous structure, was proposed by GREGOIRE [9]. TAN and HORLICK [l l] also reported that non-spectroscopic interferences varied with the operating conditions of the plasma such as the nebulizer gas flow rate and sampling depth. In the present work, we have investigated the matrix effect with a commercial instrument. It is considered to be necessary to examine the matrix effect for individual instruments because specific mechanism may dominate for different types of instrument. * Present address: Department of Chemistry, College of Science and Technology, Korea University, Jochiwon 338-800, Korea. t Author to whom correspondence should be addressed. 333
334
YOUNG-SANGKIM et
al.
2. EXPERIMENTAL The ICP mass spectrometer used in this work was a Seiko SPQ 6100 A Plasma Quadrupole Mass Analyzer. Its structure is similar to the one described earlier [22], i.e. the sampling interface consists of a sampler and skimmer of pure copper and the vacuum is achieved with a three-stage differential pumping system. A schematic diagram of the ion lens system is shown in Fig. 1 together with the sampling interface. The ion lens consists of six cylindrical elements and an X-Y beam deflector. An optical baffle plate was placed on the center axis of the first ion lens element to block the optical radiation from the ICP. Other components and operating conditions are listed in Table 1. Solutions
All solutions were prepared with analytical grade reagents without further purification. Metals and oxides were dissolved in nitric acid except Cr and MO which were dissolved in hydrochloric acid. Nitrates were used for metal salts. A stock solution of six analytes was prepared to contain 1 x 10m4 M each of Co, Y, In, La and Tl and 5 x 10e4 M of Al whose sensitivity was a little lower than that of the other analytes. This solution was diluted 100-fold with a 1% nitric acid solution containing a matrix element of 0.01 M. Fourteen matrix elements, Li, B, Na, K, Cr, Zn, Rb, MO, Cd, Cs, Sm, Ho, Lu and Pb, with atomic weight from 7 to 207 were selected to investigate the interference effects. For the effect of easily ionizable elements, Na matrix was examined over a concentration range from 10F4 to 10-l M.
The isotopes measured were 27Al 99Co “Y, ‘151n, i39La and 205Tl, the most abundant isotopes of the considered elements. In order toyovercAme the instrumental drift and the memory effect of previous solutions, analyte signals for solutions without matrix elements were measured before and after the measurement of the solution with matrix elements. De-ionized water was aspirated after each
Fig. 1. Schematic diagram of sampling interfad and ion lens. 1: sampler, 2: skimmer, 3-8: ion lens, 9: X-Y beam deflector, 10: quadrupole.
Table 1. ICP mass spectrometer and operating conditions Mass spectrometer Sampler Skimmer Ion lens voltages Vacuum pumps
Ion detector RF generator Ar flow rate Sample uptake Sampling depth Multiscalor Measuring mode
Quadrupole mass analyzer, 3-250 amu, resolution (50% peak height) M/AM > 2M Copper 120” cone, orifice dia. 1.1 mm Copper 60” cone, orifice dia. 0.7 mm (see Fig. 1) 2-90, k-50, 5-115, k-35, p-10, 8:-185V 1st stage: 840 I min- 1 mechanical 2nd stage: 840 1min- ’ mech., 2000 I s- 1 oil diffusion 3rd stage: 420 I min- 1 mech., 700 1s-’ oil diffusion Channel electron multiplier Frequency 27.12 MHz, forward power 1.3 kW Outer 16, inte~ediate 1.3, carrier 1.1 Imin-’ 1.1 ml min-’ 11 mm from outer end of load coil 2048 channels (8 channels per unit m/z) Peak hopping (3 channels centered on the m/z were measured), dwell time lOOms, number of scan 10, i.e. total meas. time 3 s for each analyte peak, replicate 3
Non-spectroscopic matrix interferences
335
measurement for more than 3 min. Normalized signals were calculated by dividing the analyte ion count rate for the solution containing the matrix by the mean analyte count rate of the matrix-free solution run before and after the solution with matrix. The differences between the two analyte count rates of the matrix-free sotution were l-6% and the long persisting enhancement effect by the easily ionizable elements as described by BEAUCHIMINet a!. [12] and GREGOIRE[9] was not observed. The background was not subtracted from the analyte signals because it was negligible (ca. 250 cps for Al and less than 20 cps for other elements).
3. RESULTSANDDISCUSSION E&G of Na concentrution on an~~yte signals Sodium was chosen as representative of easily ionizable
matrix elements. As shown in Fig. 2, analyte signals for all the elements decreased similarly with increasing Na concentration indicating that the suppression effect of Na is independent of the ionization potential and atomic mass of the analytes. Several researchers [X-l 1,141 have shown a similar suppression effect from Na, although the extents differed slightly from each other. KAWAGUCHIet al. [lt] have shown with a custom-made instrument that the extent of the signal suppression by Na followed the order of analyte ionization potential. Conversely, TAN and HORLICK [l l] reported that the suppression
was not related to analyte elements.
GRAIN et al.
[14] described that the extent of the signal attenuation was changed by different combinations of the orifice diameters for the skimmer and sampler. The discrepancy between the published data for the effect of easily ionizable elements probably arose from the difference of the type of instrument and their operating conditions. Efect of the voltage of the first ion lens element
The voltage of the first ion lens element (3 in Fig. 1) placed just behind the skimmer most significantly affects analyte signals. The variation of analyte signals as a function of the voltage of this lens element is given in Fig. 3. The Al count rate changed by a factor of about 4 between - 30 and - 90 V, while In changed by a factor slightly greater than 2, and the other analytes by about 2. The difference is considered to be due to the mass discrimination effect by the ion lens system.
1.0.
0.6. 0.6.
a
0.4.
!Slo.z* iii
~:~i
10-k
10-J 10-Z Na concentration, H
lo-
Fig. 2. Effect of Na concentration on analyte signals. Analytes 8: Al, o: Co, + : Y, x : In, 0: La, concentration Al: 5 x 10e6 M, others: 1 x 10m6M.
YOUNG-SANGKIM et al.
336 60
0’
-
z
-30
-60
-70
-90
-110 -130
E
350 8 40 30 20 10 0
-30
-60
-70 -90 Voltage.
-110 -130 V
Fig. 3. Variation of analyte signals as a function of the voltage of the first ion lens element. Analyte A: Al, o: Co, + : Y, x : In D: La, 0: TI, concentration Al: 5 x 10v6 M, others: 1 x 10e6 M.
1.2
Li NaK
ZnRb
Cd CsSm Lu
Pb
1.0 0.8 0.6 0.4 T! .po.2 u)
;
0.01 0 40
80
120
$1.27..
240 I
160 200
.I
, .
. . .
. . ..
.
E 0” 1.0.
2
0.8. 0.6.
120 160 Atomic weight
200
2
Fig. 4. Mass dependence of the matrix effect. Analyte A: Al, o: Co, f : Y, x : In, Cl: La, 0: TI, analyte concentration Al: 5 x 10m6M, others: 1 x 10e6 M, matrix element 0.01 M. l@ects
of the mass and ionization
pote~tiai
5~mat~ix
elements
Normafized analyte signals in the presence of various matrix elements are plotted against the atomic mass of the matrix elements, as shown in Fig. 4. Here several data points which apparently suffered from isobaric interferences by matrix elements, e.g. *15In in the presence of Cd, were omitted. As a whole, the mass dependence of the matrix effect is similar to those obtained in earlier studies [lo, 111, i.e. the heavier the matrix elements, the more severe the
Non-spectroscopic matrix interferences
337
interference effects for equimolar (0.01 M) matrix solutions, though there are several points deviating from this pattern. Interestingly, the suppression effects are not so clearly dependent on the mass of the analytes, although it has been reported that light analytes were more seriously affected by the matrix [ 10,111. The Tl signal, for example, is suppressed most strongly by the Cd matrix, in spite of the fact that Tl is the heaviest among the analytes examined. KAWAGUCHI et al. [lo] have shown no apparent correlation between the suppression effect of matrix elements and their ionization potentials. On the contrary, TAN and HORLICK [ 1l] have observed that ion signals were suppressed more strongly by matrix elements with lower ionization potentials than those having higher ionization potentials. They obtained two smooth curves showing the mass dependence of the matrix effect, i.e. one being for the element group with high ionization potentials and the other for the group with low ionization potentials. By using the data shown in Fig. 4, normalized signals of Co and La were plotted against ionization potentials of matrix elements in Fig. 5. The data for Sm, Ho, Lu and Pb were omitted because these matrix elements have atomic masses higher than 150, and therefore, the mass effect is clearly dominant. Obviously, a correlation is observed between the degree of suppression and the ionization potential of matrix elements. This result suggests that the shift of ionization equilibria cannot be negligible in the plasma in the presence of large amounts of matrix elements. EfSect of instrumental conditions The mass dependence of the matrix effect was investigated at different voltages of the first ion lens element to find possible causes of the matrix effect. The results for Co and La measured at -20, - 57, -90 and - 130 V are given in Fig. 6. As shown in Fig. 3, the maximum ion counts are obtained at - 90 V, and the signals decrease at values above and below this. In Fig. 6, the suppression effect generally decreases; in other words, the normalized signals approach or even exceed unity, with increasing negative voltage of the first element, particularly for heavy matrix elements. As a result, the mass dependence of the matrix effect is moderated by the increase in the voltage. The variation of the suppression
1.2
Cs K Rb
Na
Cr hi0
B CdZn
Li
La “‘3 I
4
5 6 7 6 Ionization potential,
9 eV
Fig. 5. Suppression of Co and La signals as a function of the ionization potential of matrix elements. Concentration of Co and La: 1 x low6 M, matrix elements: 0.01 M.
YOUNG-SANGKIM
338 1.2LiNaK
ZnRb
et al.
CdCsSm
Lu m
E1.0. * 0.8’ 0.6 0.4
La
0.2
t
L
O.00
I 40
a0
120 160 Atomic weight
200
240
Fig. 6. Effect of the voltage of the first ion lens element on the mass dependence of the matrix effect for Co and La. Voltage + : - 20 V, d: - 57 V, o: - 90 V, q : - 130 V. con~ntration of Co and La: 1 x iW6 M, matrix element: 0.01 M
l,2LiNaK
ri3
ZnRb
Cr
?i 0.2
CO
h vI ‘.‘O @2...~..
40
CdCsSm
MO
80
LU Pb Ho
120 160 *‘,I
200
80 120 160 Atomic weight
200
..I
:
1
3 g l.O = 0.8. 0.6.
0.2
La
‘.‘O
10
2
Fig. 7. Effect of sampling depth on the mass dependence of the matrix effect for Co and La. Sampling depth o: 11 mm, 0: 14 mm, concentration of Co and La: 1 x 10m6M, matrix element: 0.01 M.
effect with the lens voltage suggests that there exists a space charge effect as GILLSONet al. [ 131 pointed out. Figure 7 shows that the suppression effects on Co and La at a sampling depth of 14 mm are less than those at 11 mm for all the matrix elements. This is presumably caused by a
Non-spectroscopic
matrix
interferences
339
decrease in the amounts of matrix ions introduced into the sampler with increasing sampling depth, which is considered to be similar to the effect of dilution of sample solutions [ll]. Discussion on the suppression mechanism Although several investigators have tried to explain the mechanism of the matrix effect in ICP-MS, there has not been a conclusive explanation. The present work shows that the suppression of analyte signals by matrix elements cannot be explained by a single mechanism but by a combination of several mechanisms. Collisions of analyte ions with matrix ions in the supersonic expansion from the sampler to the skimmer may be the most important cause of the mass dependence of the matrix effect when the orifice diameter of the skimmer is relatively small compared to that of the sampler [ 141. The variation of the suppression effect with varying voltage of the ion lens element may be explained by the change in the space charge effect in the ion optical system. The shift of ionization equilibria in the plasma cannot be neglected because the lower the ionization potential of the matrix element, the larger the suppression effect. The extent of the contribution of each of these causes to the total suppression effect, however, is considered to depend on the type of instrument employed and the operating conditions.
REREFENCES [l] [2] [3] [4] [S] [6] [7] [8] [9] [lo] [l l] [12] 1131 [14] [15] 1167 [17] 1183 1191 [20] 1211 [22]
R. S. Houk, V. A. Fassel, G. D. Flesch, H. I. Svec, A. S. Gray and C. E. Taylor, Anal. Chem. 52, 2283 (1980). G. Horlick, S. H. Tan, M. A. Vaughan and C. A. Rose, Spectrochim. Acta 4OB, 1555 (1985). G. Zhu and R. F. Browner, Appl. Spectrosc. 41, 349 (1987). H. P. Longerich, B. J. Fryer, D. F. Strong and C. J. Kantipuly, Spectrochim. Acta 42B, 75 (1987). D. C. Gregoire, Appl. Spectrosc. 41, 897 (1987). M. A. Vaughan, G. Horlick and S. H. Tan, J. Anal. Atom. Spectrom. 2, 765 (1987). B. T. G. Ting and M. Janghorbani, J. Anal. Atom. Spectrom. 3, 325 (1988). J. A. Olivares and R. S. Houk, Anal. Chem. 58, 20 (1986). D. C. Gregoire, Spectrochim. Acta 42B, 895 (1987). H. Kawaguchi, T. Tanaka, T. Nakamura, A. Morishita and A. Mizuike, Anal. Sci. 3, 305 (1987). S. H. Tan and G. Horlick, J. Anal. Atom. Spectrom. 2, 745 (1987). D. Beauchimin, J. W. McLaren and S. S. Berman, Spectrochim. Acta 42B, 467 (1987). G. R. Gillson, D. J. Douglas, F. E. Fulford, K. W. Halligan and S. D. Tanner, Anal. Chem. 60, 1472 (1988). J. S. Crain, R. S. Houk and R. G. Smith, Spectrochim. Acta 43B, 1355 (1988). A. L. Gray, Spectrochim. Acta 41B, 151 (1986). S. H. Tan and G. Horlick, Appl. Spectrosc. 40, 445 (1986). M. A. Vaughan and G. Horlick, Appl. Spectrosc. 40, 434 (1986). A. R. Date, Y. T. Cheung and M. E. Stuart, Spectrochim. Acta 42B, 3 (1987). D. A. Wilson, G. H. Vickers and G. M. Hieftje, J. Anal. Arom. Spectrom. 2, 365 (1987). H. Kawaguchi, T. Tanaka and A. Mizuike, Spectrochim. Acta 43B, 955 (1988). S. J. Jiang, R. S. Houk and M. A. Stevens, Anal. Chem. 60, 1217 (1988). A. L. Gray and A. R. Date, Analyst 108, 1033 (1983).
Speruochzmicu Acto. Vol. 458, No. 3, pp. 333 339. 1990 Pnnted m Great Britain.
Non-spectroscopic
matrix interferences in inductively coupled plasmamass spectrometry
YOUNG-SANG KIM,* HIROSHI KAWAGUCHI,~TOMOKAZUTANAKAand ATSUSHIMIZUIKE Faculty of Engineering,Nagoya University,Chikusa-ku,Furocho, Nagoya 464, Japan (Received 17 May 1989; in revised form 29 August 1989)
Abstract-Fourteen 0.01 M matrix elements with atomic weight from 7 to 207 (Li, B, Na, K, Cr, Zn, Rb, MO, Cd, Cs, Sm. Ho, Lu and Pb) have been selected to study their interference effects on analyte (Al, Co, Y, In, La and Tl) signals with a commercial ICP mass spectrometer. The parameters investigated affecting the degree of interference were: mass of matrix element and analyte, ionization potential of matrix element, voltage of the first ion lens element and sampling depth. The matrix effect could not be explained by any single mechanism, e.g. collisions of analyte ions with matrix ions in the supersonic expansion region, space charge effect in the ion optical system and shift in ionization equilibria in the plasma, but by a combination of several mechanisms.
1. INTRODUCTION SINCE
the first paper on inductively coupled plasma-mass spectrometry (ICP-MS) was published in 1980 by HOUK et al. [l], various studies have been reported on factors affecting analyte signals as well as on its applications to inorganic trace analysis. The analyte signals in ICP-MS are influenced not only by the operating parameters of the instrument [2-71 but also by the presence of concomitant elements [8-211. The effects of concomitant elements can be classified into non-spectroscopic [8-141 and spectroscopic interferences [15-211. The latter consist of isobaric interferences and spectral overlap with doubly charged ions and polyatomic ions such as oxide and hydroxide ions of concomitant elements. Non-spectroscopic interferences cause suppression of analyte signals in most cases and enhancement in a few cases in the presence of matrix elements. Generally, these non-spectroscopic interferences, i.e. matrix effects, are more severe than those in ICPAES [S, lo]. GREGOIRE [9], KAWAGUCHI et al. [lo], TANand HORLICK[l l] and BEAUCHIMIN et al. [12] independently identified mass dependence of the matrix effect. Heavy matrix elements cause more severe suppression effects and light analytes are more seriously affected [lo, 111. As a mechanism of the mass dependence of the matrix effect, KAWAGUCHI et al. [lo], BEAUCHIMINet al. [12] and CRAIN et al. [14] explained that the suppression was caused by collisions of analyte ions with heavy matrix ions which were enriched on the central axis of the supersonic expansion region behind the sampler. BEAUCHIMIN et al. [12] also proposed mechanisms for the formation of oxides which can deposit on the interface and of ionization induced by atom-lectron collisions. TAN and HORLICK [l l] and GILLSON et al. [13] reported that the ion transmission through the skimmer is affected by space charge repulsion with heavy ions being transmitted most efficiently. In addition to the above explanations, OLIVARES and HOUK [8] proposed a shift in ionization equilibria by the presence of easily ionizable matrix elements. The ambipolar diffusion effect in the plasma, having a heterogeneous structure, was proposed by GREGOIRE [9]. TAN and HORLICK [l l] also reported that non-spectroscopic interferences varied with the operating conditions of the plasma such as the nebulizer gas flow rate and sampling depth. In the present work, we have investigated the matrix effect with a commercial instrument. It is considered to be necessary to examine the matrix effect for individual instruments because specific mechanism may dominate for different types of instrument. * Present address: Department of Chemistry, College of Science and Technology, Korea University, Jochiwon 338-800, Korea. t Author to whom correspondence should be addressed. 333
334
YOUNG-SANGKIM et
al.
2. EXPERIMENTAL The ICP mass spectrometer used in this work was a Seiko SPQ 6100 A Plasma Quadrupole Mass Analyzer. Its structure is similar to the one described earlier [22], i.e. the sampling interface consists of a sampler and skimmer of pure copper and the vacuum is achieved with a three-stage differential pumping system. A schematic diagram of the ion lens system is shown in Fig. 1 together with the sampling interface. The ion lens consists of six cylindrical elements and an X-Y beam deflector. An optical baffle plate was placed on the center axis of the first ion lens element to block the optical radiation from the ICP. Other components and operating conditions are listed in Table 1. Solutions
All solutions were prepared with analytical grade reagents without further purification. Metals and oxides were dissolved in nitric acid except Cr and MO which were dissolved in hydrochloric acid. Nitrates were used for metal salts. A stock solution of six analytes was prepared to contain 1 x 10m4 M each of Co, Y, In, La and Tl and 5 x 10e4 M of Al whose sensitivity was a little lower than that of the other analytes. This solution was diluted 100-fold with a 1% nitric acid solution containing a matrix element of 0.01 M. Fourteen matrix elements, Li, B, Na, K, Cr, Zn, Rb, MO, Cd, Cs, Sm, Ho, Lu and Pb, with atomic weight from 7 to 207 were selected to investigate the interference effects. For the effect of easily ionizable elements, Na matrix was examined over a concentration range from 10F4 to 10-l M.
The isotopes measured were 27Al 99Co “Y, ‘151n, i39La and 205Tl, the most abundant isotopes of the considered elements. In order toyovercAme the instrumental drift and the memory effect of previous solutions, analyte signals for solutions without matrix elements were measured before and after the measurement of the solution with matrix elements. De-ionized water was aspirated after each
Fig. 1. Schematic diagram of sampling interfad and ion lens. 1: sampler, 2: skimmer, 3-8: ion lens, 9: X-Y beam deflector, 10: quadrupole.
Table 1. ICP mass spectrometer and operating conditions Mass spectrometer Sampler Skimmer Ion lens voltages Vacuum pumps
Ion detector RF generator Ar flow rate Sample uptake Sampling depth Multiscalor Measuring mode
Quadrupole mass analyzer, 3-250 amu, resolution (50% peak height) M/AM > 2M Copper 120” cone, orifice dia. 1.1 mm Copper 60” cone, orifice dia. 0.7 mm (see Fig. 1) 2-90, k-50, 5-115, k-35, p-10, 8:-185V 1st stage: 840 I min- 1 mechanical 2nd stage: 840 1min- ’ mech., 2000 I s- 1 oil diffusion 3rd stage: 420 I min- 1 mech., 700 1s-’ oil diffusion Channel electron multiplier Frequency 27.12 MHz, forward power 1.3 kW Outer 16, inte~ediate 1.3, carrier 1.1 Imin-’ 1.1 ml min-’ 11 mm from outer end of load coil 2048 channels (8 channels per unit m/z) Peak hopping (3 channels centered on the m/z were measured), dwell time lOOms, number of scan 10, i.e. total meas. time 3 s for each analyte peak, replicate 3
Non-spectroscopic matrix interferences
335
measurement for more than 3 min. Normalized signals were calculated by dividing the analyte ion count rate for the solution containing the matrix by the mean analyte count rate of the matrix-free solution run before and after the solution with matrix. The differences between the two analyte count rates of the matrix-free sotution were l-6% and the long persisting enhancement effect by the easily ionizable elements as described by BEAUCHIMINet a!. [12] and GREGOIRE[9] was not observed. The background was not subtracted from the analyte signals because it was negligible (ca. 250 cps for Al and less than 20 cps for other elements).
3. RESULTSANDDISCUSSION E&G of Na concentrution on an~~yte signals Sodium was chosen as representative of easily ionizable
matrix elements. As shown in Fig. 2, analyte signals for all the elements decreased similarly with increasing Na concentration indicating that the suppression effect of Na is independent of the ionization potential and atomic mass of the analytes. Several researchers [X-l 1,141 have shown a similar suppression effect from Na, although the extents differed slightly from each other. KAWAGUCHIet al. [lt] have shown with a custom-made instrument that the extent of the signal suppression by Na followed the order of analyte ionization potential. Conversely, TAN and HORLICK [l l] reported that the suppression
was not related to analyte elements.
GRAIN et al.
[14] described that the extent of the signal attenuation was changed by different combinations of the orifice diameters for the skimmer and sampler. The discrepancy between the published data for the effect of easily ionizable elements probably arose from the difference of the type of instrument and their operating conditions. Efect of the voltage of the first ion lens element
The voltage of the first ion lens element (3 in Fig. 1) placed just behind the skimmer most significantly affects analyte signals. The variation of analyte signals as a function of the voltage of this lens element is given in Fig. 3. The Al count rate changed by a factor of about 4 between - 30 and - 90 V, while In changed by a factor slightly greater than 2, and the other analytes by about 2. The difference is considered to be due to the mass discrimination effect by the ion lens system.
1.0.
0.6. 0.6.
a
0.4.
!Slo.z* iii
~:~i
10-k
10-J 10-Z Na concentration, H
lo-
Fig. 2. Effect of Na concentration on analyte signals. Analytes 8: Al, o: Co, + : Y, x : In, 0: La, concentration Al: 5 x 10e6 M, others: 1 x 10m6M.
YOUNG-SANGKIM et al.
336 60
0’
-
z
-30
-60
-70
-90
-110 -130
E
350 8 40 30 20 10 0
-30
-60
-70 -90 Voltage.
-110 -130 V
Fig. 3. Variation of analyte signals as a function of the voltage of the first ion lens element. Analyte A: Al, o: Co, + : Y, x : In D: La, 0: TI, concentration Al: 5 x 10v6 M, others: 1 x 10e6 M.
1.2
Li NaK
ZnRb
Cd CsSm Lu
Pb
1.0 0.8 0.6 0.4 T! .po.2 u)
;
0.01 0 40
80
120
$1.27..
240 I
160 200
.I
, .
. . .
. . ..
.
E 0” 1.0.
2
0.8. 0.6.
120 160 Atomic weight
200
2
Fig. 4. Mass dependence of the matrix effect. Analyte A: Al, o: Co, f : Y, x : In, Cl: La, 0: TI, analyte concentration Al: 5 x 10m6M, others: 1 x 10e6 M, matrix element 0.01 M. l@ects
of the mass and ionization
pote~tiai
5~mat~ix
elements
Normafized analyte signals in the presence of various matrix elements are plotted against the atomic mass of the matrix elements, as shown in Fig. 4. Here several data points which apparently suffered from isobaric interferences by matrix elements, e.g. *15In in the presence of Cd, were omitted. As a whole, the mass dependence of the matrix effect is similar to those obtained in earlier studies [lo, 111, i.e. the heavier the matrix elements, the more severe the
Non-spectroscopic matrix interferences
337
interference effects for equimolar (0.01 M) matrix solutions, though there are several points deviating from this pattern. Interestingly, the suppression effects are not so clearly dependent on the mass of the analytes, although it has been reported that light analytes were more seriously affected by the matrix [ 10,111. The Tl signal, for example, is suppressed most strongly by the Cd matrix, in spite of the fact that Tl is the heaviest among the analytes examined. KAWAGUCHI et al. [lo] have shown no apparent correlation between the suppression effect of matrix elements and their ionization potentials. On the contrary, TAN and HORLICK [ 1l] have observed that ion signals were suppressed more strongly by matrix elements with lower ionization potentials than those having higher ionization potentials. They obtained two smooth curves showing the mass dependence of the matrix effect, i.e. one being for the element group with high ionization potentials and the other for the group with low ionization potentials. By using the data shown in Fig. 4, normalized signals of Co and La were plotted against ionization potentials of matrix elements in Fig. 5. The data for Sm, Ho, Lu and Pb were omitted because these matrix elements have atomic masses higher than 150, and therefore, the mass effect is clearly dominant. Obviously, a correlation is observed between the degree of suppression and the ionization potential of matrix elements. This result suggests that the shift of ionization equilibria cannot be negligible in the plasma in the presence of large amounts of matrix elements. EfSect of instrumental conditions The mass dependence of the matrix effect was investigated at different voltages of the first ion lens element to find possible causes of the matrix effect. The results for Co and La measured at -20, - 57, -90 and - 130 V are given in Fig. 6. As shown in Fig. 3, the maximum ion counts are obtained at - 90 V, and the signals decrease at values above and below this. In Fig. 6, the suppression effect generally decreases; in other words, the normalized signals approach or even exceed unity, with increasing negative voltage of the first element, particularly for heavy matrix elements. As a result, the mass dependence of the matrix effect is moderated by the increase in the voltage. The variation of the suppression
1.2
Cs K Rb
Na
Cr hi0
B CdZn
Li
La “‘3 I
4
5 6 7 6 Ionization potential,
9 eV
Fig. 5. Suppression of Co and La signals as a function of the ionization potential of matrix elements. Concentration of Co and La: 1 x low6 M, matrix elements: 0.01 M.
YOUNG-SANGKIM
338 1.2LiNaK
ZnRb
et al.
CdCsSm
Lu m
E1.0. * 0.8’ 0.6 0.4
La
0.2
t
L
O.00
I 40
a0
120 160 Atomic weight
200
240
Fig. 6. Effect of the voltage of the first ion lens element on the mass dependence of the matrix effect for Co and La. Voltage + : - 20 V, d: - 57 V, o: - 90 V, q : - 130 V. con~ntration of Co and La: 1 x iW6 M, matrix element: 0.01 M
l,2LiNaK
ri3
ZnRb
Cr
?i 0.2
CO
h vI ‘.‘O @2...~..
40
CdCsSm
MO
80
LU Pb Ho
120 160 *‘,I
200
80 120 160 Atomic weight
200
..I
:
1
3 g l.O = 0.8. 0.6.
0.2
La
‘.‘O
10
2
Fig. 7. Effect of sampling depth on the mass dependence of the matrix effect for Co and La. Sampling depth o: 11 mm, 0: 14 mm, concentration of Co and La: 1 x 10m6M, matrix element: 0.01 M.
effect with the lens voltage suggests that there exists a space charge effect as GILLSONet al. [ 131 pointed out. Figure 7 shows that the suppression effects on Co and La at a sampling depth of 14 mm are less than those at 11 mm for all the matrix elements. This is presumably caused by a
Non-spectroscopic
matrix
interferences
339
decrease in the amounts of matrix ions introduced into the sampler with increasing sampling depth, which is considered to be similar to the effect of dilution of sample solutions [ll]. Discussion on the suppression mechanism Although several investigators have tried to explain the mechanism of the matrix effect in ICP-MS, there has not been a conclusive explanation. The present work shows that the suppression of analyte signals by matrix elements cannot be explained by a single mechanism but by a combination of several mechanisms. Collisions of analyte ions with matrix ions in the supersonic expansion from the sampler to the skimmer may be the most important cause of the mass dependence of the matrix effect when the orifice diameter of the skimmer is relatively small compared to that of the sampler [ 141. The variation of the suppression effect with varying voltage of the ion lens element may be explained by the change in the space charge effect in the ion optical system. The shift of ionization equilibria in the plasma cannot be neglected because the lower the ionization potential of the matrix element, the larger the suppression effect. The extent of the contribution of each of these causes to the total suppression effect, however, is considered to depend on the type of instrument employed and the operating conditions.
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