Oxide formation in electrothermal vaporization inductively coupled plasma mass spectrometry

Specnochimica Acta. Vol. 48B. No. 9. PP. 1127-1137, 1993 Printed in Great Britain.

Oxide formation

058+a547/93 $6 00 + .oa fJ 1993 Pergamon Press Ltd

in electrothermal vaporization plasma mass spectrometry

NAOKO SHIBATA, NORIKO FUDAGAWA NationalInstitute

of Materials and Chemical Research,

inductively

coupled

and MASAAKI KUBOTA

l-l, Higashi, Tsukuba-shi, Ibaraki 305, Japan

(Received 13 November 1992; accepted 24 February 1993) Ah&act-In electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) using a tungsten furnace, the effects of plasma parameters and removal of solvent on interfering monoxide ion signals were investigated in order to determine rare earth element (REE) impurities in high-purity REE oxide samples without spectral interferences. The monoxide ion to element ion ratio (MO’IM’) was dependent on the plasma parameters, showing a decreasing tendency with increasing rf power. To reduce spectral interferences, the plasma parameters were chosen so as to attain a larger analyte signal and smaller MO+/M+ for a matrix element. The effect of oxide interferences on analyte signal could be further reduced by proper selection of integration time of the ion count. Also, a theoretical calculation of MO/M, assuming the Boltxmann equilibrium of MO in the ICP, was performed to elucidate the mechanism related to oxide formation. In nebulixation ICP-MS, experimental MO+IM+ values for REEs were in good agreement with theoretical MO/M ones, which indicates that oxide ion species in ICP-MS may be derived from undissociated MO and/or MO+ in the ICP. In ETV-ICP-MS, however, experimental MO+/M+ values were two orders of magnitude larger than theoretical ones, probably owing to the air entering the ICP. Under the optimized conditions that the oxide formation was minimized, the use of the ETV technique enabled us to determine Tb and Lu impurities at a concentration level of 0.01 ug gg’ in high-purity GdlOX.

1. INTR~~U~HON ONE OF the outstanding problems associated with inductively coupled plasma mass spectrometry (ICP-MS) is that of the sample introduction method. A conventional nebulization (NEB), which is similar to that for ICP atomic emission spectrometry (AES), has been successfully used for the sample introduction into an ICP mass spectrometer because of rapid sample handling and good stability. However, there are negative aspects of this technique such as (1) poor sample transport efficiency (- 5%), (2) sample volume in ml, (3) inability to nebulize organic solvent and (4) spectral interferences from polyatomic ions, especially oxide ions, arising from solvent and/or a matrix element. In the analyses of rare earth elements (REEs), the reduction of oxide ion formation is an important aspect of ICP-MS [l, 21 because singly charged ion signals of heavy REEs are overlapped by monoxide ion signals of light REEs. The problem of oxide ion interferences in ICP-MS has been discussed by many researchers in the literature [l-lo]. It is well known that the monoxide ion to element ion ratio (MO+/M+) is dependent on plasma operating parameters such as carrier gas flow rate, radio frequency (rf) power and sampling depth [4, 5, 8, 9, 111, and diameter of sampling orifice and shape of a sampling cone [3, 91. DOUGLAS and FRENCH [3] investigated the effects of orifice diameter and angle of the sampling cone and found that CeO+/Ce+ was improved (0.03). However, the oxide levels reported up-to-date using the NEB method are too high to determine REE impurities contained in highpurity REE materials without spectral interferences. In order to minimize monoxide ion interferences, sample introduction methods that do not introduce solvent into the ICP would be required. Such techniques include desolvation [12-141, direct sample insertion [15-171 and electrothermal vaporization (ETV) [l, U-231. In our previous work using a nebulizerdesolvator system [14], it was clearly demonstrated that, for the determination of REEs, the oxide interferences were reduced on average by one order of magnitude as compard to those obtained using conventional nebulization. Since the first use of ETV with ICP-MS reported by GRAY and DATE [18], a relatively large number of papers have appeared. One of our reports on the effect of 1127

1128

N.

SHIBATA

et al.

Table 1. Typical operating conditions for ICP-MS Plasma: RF power

1.2 kW

Reflected power Carrier gas flow rate (for NEB)

<5w 1.0 I min-’

Plasma gas flow rate Auxiliary gas flow rate Sampling depth Electrothermal vaporizations: Carrier gas flow rate Ar gas H2 gas Heating cycle drying holding vaporization

15 1 min-’ 1.3 1 min-’ 15 mm

1.0 1 min-’ 0.02 I min-’ 50 s at 100°C 30 s at 200°C 9 s at 2600°C for Lu

solvent removal using an ETV device showed that MO+/M’ values for REEs were two or three orders of magnitude smaller than those obtained by NEB-ICP-MS [l]. The origin of oxide ion species has also been explored in the literature [3, %ll]. DOUGLAS and FRENCH [3] tried to fit temperature to the relation between MO+/M+ values and dissociation energies of MO+ by assuming Boltzmann distribution. LONGERICH et al. [5] also calculated a temperature using the same approach. The temperatures they obtained were 21000 K and 10000 K, respectively, which were far higher than expected for the ICP. DOUGLAS and FRENCH [3] suggested that residual oxide ions were formed in the sampling process. For the oxide formation, VAUGHAN and HORLICK [9] and LEPLA et al. [ll] proposed possible reactions in the boundary layer around the sampling cone. In addition DOUGLAS and FRENCH [3] and VAUGHAN and HORLICK [4] indicated that monoxide ion signals observed in the mass spectrometer could not originate from undissociated MO in the ICP. This paper describes the influences of plasma parameters (rf power and sampling depth) on the ETV transient signals of analyte and interfering monoxide ions. In order to make clearer the origin of the interfering oxide ion species, MO+/M+ values for REEs were measured by NEB-ICP-MS and ETV-ICP-MS and the experimental results are compared with theoretical MO/M values calculated from the dissociation equilibrium constants of monoxides in the ICP. Also, under the optimized plasma and ETV conditions, the application of the ETV technique to trace analyses of REE impurities in high-purity REE oxides (Gdz03 and Tb407) is presented.

2. EXPERIMENTAL 2.1. Apparatus

The ICP-MS instrument with an ETV device as a sample introduction system is an SPQ 6100-S (Seiko Instrument Inc., Japan). The ETV device was described in detail elsewhere [l]. The ETV device consists of a tungsten furnace with a 50 p.1 capacity hole, which is encased in a 20 ml Pyrex glass chamber. The furnace is resistively heated up to 2700°C using a power supply. The temperature of the furnace was measured with a Pyroscope IR-Q2 pyrometer (CHINO, Japan). A small amount of hydrogen gas (0.02 1 min-‘) was mixed with argon carrier gas in order to suppress oxidation of the furnace. The Ar-Hz mixed carrier gas was introduced into the chamber at a flow rate of 1.0 1 min-‘, and was used to transport the vaporized material to the ICP via a 90 cm x 6 mm i.d. PTPE tube. Both argon and hydrogen gas flow rates were regulated with precise mass flow controllers. Typical operating conditions for the ICP and the ETV device are given in Table 1. Each mode of sample introduction, NEB and ETV, required different optimization of the ion lens settings to obtain maximum signals. In this study, the settings for NEB were optimized for Be, Co, Ba and Pb by continuous solution nebulization; those for ETV were optimized for Hg by introducing HgIz gas mixed with the Ar carrier gas.

Oxide formation in ETV-ICP-MS

1129

2.2. Samples and chemicals

High-purity Gdz03 and Tb407, prepared by Sumitomo Metal Mining Co., Ltd., Japan, were analysed in this study. About 1.0 g of powder oxide was dissolved in nitric acid (1+3). The solution obtained was heated at a temperature of 150°C for 20 min on a hot-plate and then diluted to 100 ml with hydrochloric acid (l+ 100). Standard solutions were prepared for each REE by diluting 1000 pg ml-’ stock solutions with ion-exchanged distilled water. Calibration using matrix-mat&hed standard solutions was used throughout. 2.3. Measurement procedure The details of signal measurement procedure were described in a previous paper [l]. A 20 l~,l aliquot of sample solution was injected into the furnace. Samples vaporized during the vaporization cycle were carried into the ICP by a stream of Ar-Hz carrier gas. The signal measuring sequence starts at the beginning of the vaporization cycle. Transient signal profiles were recorded as time vs ion count curves. After the end of the vaporization cycle, peak areas were measured by integrating the ion count. In NEB-ICP-MS, it was impossible to determine Tb, Yb and Lu contained in Gd203 and Tb407 because of severe spectral interferences owing to hydride (GdH) and monoxide (GdO and TbO) ions arising from the matrix. Therefore, ICP-AES measurements were also performed for these elements to compare the analytical results with those obtained by the ETV method. A CTM-100 spectrometer (l-m Czerny-Turner, 3600 grooves mm-’ grating, Shimadzu Co., Japan) with an ICPS-2H generator was used. The plasma operating conditions for ICP-AES were almost the same as those for ICP-MS. The analytical wavelengths chosen for lb II, Yb II and Lu II were 367.635, 289.138 and 261.542 nm, respectively.

3. RESULTS AND DISCUSSION 3.1.

Mass spectra of Gd203 To clarify monoxide interferences specific to the NEB method, the mass spectrum of Gd203 obtained by NEB-ICP-MS was compared with that obtained by ETV-ICP-MS. Plasma operating conditions for NEB-ICP-MS and ETV-ICP-MS were the same. In ETV-ICP-MS, the heating temperature of the furnace was 26009Z: The temperature was optimized for Lu. The same plasma conditions were used for NEB-ICP-MS. Figure 1 shows the mass spectrum obtained for a 100 p.g ml-’ Gd203 solution containing 14 REEs at a concentration level of 10 ng ml-‘. The upper (A) and the lower (B) spectra are obtained by NEB-ICP-MS and ETV-ICP-MS, respectively. Since the fast transient signal produced by the ETV operation requires rapid monitoring, the intensity at one channel was measured and the dwell time for each channel was set to 10 ms. Figure 2, plot (B), presents the transient spectrum at approximately 2.5 s after the start of vaporization in the heating cycle, when ion counts for most REEs reached maximum values. Clearly, in the NEB method, it is impossible to determine Yb and Lu impurities because these signals are interfered with by the peaks of GdO. However, the use of the ETV device can suppress considerably the formation of these interfering ion species, though the plasma operating conditions employed were not necessarily optimized ones. These spectra demonstrate that the drying procedure in ETV-ICP-MS plays an important role for the reduction of spectral interferences.

3.2. Effects of rf power and sampling depth In order to minimize the formation of interfering oxide ion species, plasma parameter studies were performed. The effect of sampling depth (12-18 mm from the load coil) with different rf powers of 1.0, 1.2 and 1.4 kW on interfering MO ion to analyte ion ratios (GdO’/Yb+ and LuO’/Ib*) and MO+/M+ values (GdO+/Gd+ and TbO+/Tb+) were investigated. The results are shown in Fig. 2, plot (A), for GdO+Nb+ and in Fig. 2, plot (B), for GdO+/Gd+, and are shown in Fig. 3, plot (A), for TbO+/Lu+ and in Fig. 3, plot (B), for TbO+/Tb+. The carrier gas flow rate was fixed at 1.0 1 min-l. The heating temperatures of the furnace chosen so as to produce the largest signal for Yb and Lu were 2100°C and 26OO”C, respectively. As shown in Figs

N. SHIBATAet al.

1130 x10*

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0.0

140

145

158

155NUMBER:

%$165

m

Gd

Tb

Ho

140

145

Fig. 1. NEB-ICP-MS,

150

178

Lu

170

165

1

Pm

Tm

175

plot (A), and ETV-ICP-MS, plot (B), spectra for 100 pg ml-’ Gd203 + 10 ng ml-’ REEs.

x

0.012

10 -4

2.!5-

(8)

(A)

2

+

4 $ l.! i-

1.0 kW

\

Q

: 1

0.0051 11

12

13

14

15

15

Sampling depth

Fig. 2. Dependence

17

, mm

18

19

11

12

13

14

15

16

Sampling depth

17

18

, mm

of GdO+A’b+(A) and GdO+/Gd+(B) on sampling depth at different rf powers. Heating temperature is 2300°C.

1s

Oxide formation in EW-ICP-MS

(A)

n

0.07-

0.06

1131

6.5

-

-

1.0 kW

\

o11

’

’

’

’

’

’

’

12

13

14

15

16

17

16

Sampling

Fig. 3. Dependence

depth

, mm

19

11

12

13

14

Sampling

15

I6

depth

17

18

19

, mm

of TbO+/Lu+(A) and TbO+Rb+(B) on sampling depth at different rf powers. Heating temperature is 2600°C.

2(A) and 3(A), the behaviour of interfering monoxide ion to analyte ion ratios differ with the element under consideration. For the determination of Yb, a sampling depth of 15 mm and an rf power of 1.4 kW were employed as the optimal conditions. Also optimal conditions for Lu can be easily selected, because the signal for Lu reached a maximum value at a sampling depth of 14 mm and an rf power of 1.4 kW. On the other hand, the MO+/M’ values decreased as the rf power was increased for both REEs. This trend can be easily explained. If the rf power is increased, the initial radiation zone (IRZ), where the atomic emission intensity reaches a maximum, and the normal analytical zone (NAZ), where the plasma temperature is higher than that in the IRZ (and thus the ionic emission intensity reaches a maximum [24]), move closer to the load coil because the central channel of the ICP contracts. Increasing the sampling depth means that the sampling region in the ICP changes closer to the NAZ. Therefore, at a fixed sampling depth, increasing the rf power changes the plasma region closer to the NAZ, resulting in a larger degree of monoxide dissociation. Decreasing tendencies of MO+/M+ values with increasing rf power were also reported for NEB-ICP-MS in the literature [4]. 3.3. Signal profiles of analyte and interfering monoxide Another interesting characteristic observed in ETV-ICP-MS is the dependence of ion signals on measurement time. In this study, the ETV signal profiles of analytes were compared with those of interfering monoxides in order to examine a possibility of elimination of interferences. The results are illustrated in Fig. 4, plot (A), for Yb and GdO, and in plot (B) for Lu and TbO. These profiles were obtained with 100 pg ml-’ oxide sample solutions including 10 ng ml-l Yb and Lu, respectively. The peak time for Yb was earlier by 0.5 s than that of GdO, that of Lu being later by 0.2 s than that of TbO. In our previous work [l], it was found that the REEs having lower melting points gave signal profiles with earlier peak times and broader shapes. The relations of the melting points (m.p.) and boiling points (b.p.) of analytes and interfering elements are: Yb (824°C) < Gd (1312”C), Lu (1652°C) > Tb (1356°C) for m.p. and Yb (2230°C) < Gd (243O“C), Lu (2530°C) > Tb (2430°C) for b.p. Though m.p. and b.p. of monoxides are unknown, a close relationship exists between

IV. SHIBATAet

1132

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x 10’ 8 .

e

x loo (A)

-

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8 8 4

4 1 . 0

1

2

3

4

5

8

1

2

3

4

5

6

x lo3 ;5T;(0

0

Timc,s Fig. 4. Signal profiles of analyte and interfering oxide. (A) Yb’ and GdO’ at a heating temperature of 2300°C; (B) Lu+ and TbO+ at a heating temperature of 2600°C.

the temperatures and the peak times of signals. The difference in peak times means that the effect of interfering oxide species on analyte signals can be reduced by a proper selection of integration time. For the determination of Yb, the integration range from 1.5 to 2.5 s was employed as an optimal condition, whereas for Lu, it was rather difficult to optimize the time range because the difference between the peak time of Lu and that of TbO was not so large. Therefore a compromised time range from 2 to 3 s was selected. 3.4. Theoretical calculation of MO/M Though the ETV device enabled the removal of solvent from the sample introduction system, the monoxide interferences were still observed in the analyte signal profiles shown in Fig. 4. In order to examine the cause of the monoxide formation in ETV-ICP-MS, MO/M values calculated from dissociation equilibrium constants of monoxide in the ICP were compared with the experimental MO’/M+ values for REEs. For the dissociation equilibrium of monoxide (MO f, M+O), an equilibrium constant K,, is defined as:

where m is the species mass, Ed is the dissociation energy (eV), k is the Boltzmann constant, T is the plasma temperature and Z is the partition function. Only the lowest electronic level was taken into consideration for calculation of the partition function of MO [25]. When Eqn (1) is rearranged, we obtain: log K,, = log Z,Z,

+ 0.5 log T + log (l- 10-“.625”‘T) - 5040 Ed/T + C

(2)

where C = 20.432 + 1.5 log (M,&/A4,,)

+ log (B/g).

(3)

Here, g is the statistical weight, B (cm-‘) and w (cm-l) are the rotational and vibrational constants of a diatomic molecule, respectively, and M is the atomic or molecular weight in atomic units. The partition function of M and 0 were calculated from the data presented by DE GALAN et al. [26].

Oxide formation Be Sr III

in EW-ICP-MS

V

Sn I

Co Al

I!

I

SC II!

Zr

v

Ge

Ba

1133 La

I

I

la

O_,

_

_5_

P~=183X10-’

Pc.=2ooxlo-’

4

5

6 Dissociation

Fig. 5. Theoretical oxygen, P,=1.83

7

8

energy, eV

MO/M as a function of oxide dissociation energy: (0) partial pressure of lo-* atm, plasma temperature T=5000 K; (0) P,=1.83 T=6000 K; and (A) Po=2.00 x 10m6 atm, T=5000 K.

X

x 1O-2 atm,

Assuming that the MO/M value equals the ratio of the number densities of MO and M (nr,.&aM) in the ICP, we obtain: MO -=_-_ M

no K,,

PO (atm) 1.01325 x lo6 (erg cmp3 atm-r) K,, (cm-‘) k (erg K-l) T(K)

= 7.3389 x lozl PO K,T

(4)

where PO is the partial pressure of oxygen (atm) and no is the number density (cm-“) of oxygen atoms in the ICP. Because the spectroscopic data for most REE monoxides are lacking, calculations of MO/M values were performed for 14 elements including transition metals with a wide dissociation energy range from 4 to 8 eV. The spectroscopic data (B, g, o and E,,) of diatomic molecules required for the calculation were taken from the literature [27-291. Under the typical operating conditions of NEB-ICP-MS listed in Table 1, the amount of water aerosols introduced into the ICP was 15 mg for every litre of Ar carrier gas. Since Hz0 molecules are almost dissociated into hydrogen and oxygen atoms in the ICP at a temperature of 5000 K [30], the partial pressure of oxygen was estimated to be 1.83 x lo-* (atm). In ETV-ICP-MS, the partial pressure was assumed to be 2.00 x lo+’ (atm), which was equivalent to the sum of oxygen contained as an impurity in the Ar carrier gas and oxygen in the REE oxides. The theoretical MO/M ratios at plasma temperatures of 5000 and 6000 K are plotted in Fig. 5 as a function of the dissociation energies of MO. These temperatures were used as reasonable values of plasma temperatures, referring to those reported by DOUGLAS. and FRENCH[3] and LQNGERICHet al. [5]. Clearly, the larger the plasma temperature, the smaller the slope. It is easy to appreciate a close correlation between the logarithmic term of MO/M (log MO/M) values and dissociation energy as predicted from Eqns (2) and (3). Figure 6 compares the experimental results for REEs obtained by NEB-ICP-MS and ETV-ICP-MS (expressed by the solid lines) with the theoretical MO/M values at a plasma temperature of 5000 K (dotted lines). The operating conditions for

1134

N. SHIBATA et al.

Eu I Yb

I

Tm I, Ai

I Sm

i-ly Ho 11, Er

Lu Tb ,I,, Gd Nd

Pr I

Ce ,I

La

O*

1

-1 NE&ICP-MS

Dissociation

energy, eV

Fig. 6. MO/M for REEs obtained by NEB-ICP-MS (0) and ETV-ICP-MS (A) as a function of oxide dissociation energy. Dotted lines express calculated data for Po=1.83 x lo-* atm, T=5000 K and Po=2.00 X 1OV atm, T=SOLB K, as shown in Fig. 5.

NEB-ICP-MS and ETV-ICP-MS were the same as those presented in Table 1. The MO’/M+ values were plotted not against dissociation energies of MO+ but against those of MO. However, except for Lu, most REEs have dissociation energies of MO about the same as those of MO’ [31]. In NEB-ICP-MS, the theoretical values agreed satisfactorily with the experimental ones, which suggests that the MO’IM’ values in ICP-MS may reflect the extent to which monoxides dissociate in the ICP. In ETV-ICP-MS, the experimental values were about two orders of magnitude larger than the theoretical ones. The best fit of experimental results gave a temperature of 6000 K higher than that in NEB-ICP-MS and an oxygen partial pressure of 3.63 x lOA (atm), two orders of magnitude larger than that used for the theoretical calculation. The effect of water loading on plasma temperatures has been extensively studied using a variety of methods such as optical spectroscopy [32, 331, Langmuir probe measurement [34, 3.51and Thomson and Rayleigh scattering 1361. Most investigators have suggested that the absence of water (dry plasma) caused a decrease in excitation temperature. However, LONGet al. [37], who employed an ETV device as a dry sample introduction system, reported a higher temperature in the ICP. In our previous papers [l, 381, where the effects of hydrogen mixed with argon carrier gas on the MO’“/M+ values for REEs and on the excitation temperature (I’& of the plasma formed in the interface region were investigated, it was found that hydrogen addition caused a decrease in the MO+/M+ values and a slight increase in T,,. Thus, the excitation conditions of dry plasma generated with the ETV technique seems to be different from those generated by the conventional NEB method without water loading. The larger partial pressure of oxygen may be due to the air entering into the ICP from the atmosphere. Though it is difficult to estimate accurately the amount of oxygen entering into the sample introduction system, the larger experimental MO+/M+ values observed in ETV-ICP-MS suggest that further reduction of monoxide formation would be possible if the sample introduction system is improved.

Oxide formation in ETV-ICP-MS

1135

Table 2. Analytical results of high-purity Gd*O, (pg g-i) Sample II*

Sample I NEB-ICP-MS SC

Y La Ce Pr Nd Sm Eu Tb DY Ho Er Tm Yb Lu

El-V-ICP-MS

0.1 2.8 0.5 1.1 1.0 3.2 11 127 co.9 0.2 0.6 Cl.1 -

0.2 2.3 0.7 0.9 0.8 3.2 11 149 37 0.8 0.1 0.6 0.3 9.4 12

-

ICP-AES

21

5.8 6.6

NEB-ICP-MS

ETV-ICP-MS

co.7 7.8 co.03 co.05 co.02 co.1 0.6 78 X1.1 co.03 1.4 ~2.8 -

0.01 8.4 0.02 0.04 0.01 0.02 0.4 96 29 0.1 0.02 1.7 0.6 11 12

ICP-AES

19

5.1 6.3

* Purified sample I.

3.5. Analytical results Under the conditions that the influence of oxide formation on analyte signals was minimized based on studies in previous sections, REE impurities in two kinds of highpurity REE oxides were determined. Tables 2 and 3 list the analytical results for Gd203 and Tb407 samples. Table 4 shows the detection limits of REE impurities in Gd203 obtained by NEB-ICP-MS and ETV-ICP-MS. Sample II in Tables 2 and 3 is a high-purity material obtained by solvent extraction of Sample I. The data were obtained by both NEB-ICP-MS and ETV-ICP-MS. The heating temperatures of the furnace in ETV-ICP-MS were optimized for respective REEs to yield the maximum signals. ICP-ABS measurements were also performed for the elements interfered with by oxide signals in order to compare the results with those obtained by ETV-ICP-MS. The result for Gd203 is given in Table 2. The REE impurities contained in Sample I were of the order of 0.1 pg g-l, which were high enough to be detected by Table 3. Analytical results of high-purity Tb,O, (pg g-i) Sample II*

Sample I NEB-ICP-MS SC

Y La Ce

Pr Nd Sm Eu Gd DY Ho Er Tm Yb Lu

co.7 13 (0.01) (0.03) (0.01) (0.04) 0.1 0.05 25 1.0 0.3 0.1 <0.02 0.1 -

* Purified sample I. N.D.: not determined.

ETV-ICP-MS 0.2 15 0.06 0.05 0.03 0.1 0.3 0.07 39 1.2 0.4 0.05 0.01 0.02 26

ICP-AES

NEB-ICP-MS co.9

0.1 (KZ) 0.07 0.1 0.3 0.08 33

16

(%) (0.01) (0.01) 0.1 -

El-V-ICP-MS 0.07 0.2 0.08 0.08 0.05 0.06 0.5 0.1 45 0.1 N.D. N.D. N.D. 0.01 15

ICP-AES

15

1136

N. Table 4. Detection

Element SC

Y La Ce Pr Nd Sm EU Tb DY Ho Er Tm Yb LU

SHIBATA et al.

limits of impurities contained (CLgg-‘1 NEBICP-MS 0.03 0.007 0.01 0.01 0.01 0.06 0.03 0.01 0.02 0.01 0.01 0.01 -

-

ETV-ICP-MS 0.001 0.001 0.001 0.001 0.001 0.004 0.005 0.003 0.01 0.003 0.001 0.003 0.001 0.06 0.01

in high-purity Gd203

Interference CO,H

GdH

GdO GdOH

NEB-ICP-MS. The result for Sample II, in which the effect of purification was clearly seen, demonstrated that NEB-ICP-MS was insufficient for the determination of impurities in a concentration level of less than 0.01 kg g-l. Table 3 presents the result for Tb407. In Sample II, heavy REEs (Ho, Er, Tm and Yb) were lowered but other REEs (La, Ce, Pr, Nd, Sm and Y) were slightly condensed compared to those in Sample I. Values in parentheses are ones not certainly determined because of blank variations. For Sample II, it was impossible to determine Ce, Ho, Er and Tm with a satisfactory precision because the concentration levels were less than 10 times the detection limits shown in Table 4 (- 10 ng g-l). As seen ‘in Tables 2 and 3, even with the ETV device, it seems too difficult to eliminate the effect of monoxide ion interferences on the values for Tb, Yb and Lu perfectly, because these values were slightly larger than those obtained by ICP-AES. The detection limits for most REEs for ETV-ICP-MS were approximately one In addition, ETV-ICP-MS order of magnitude lower than those for NEB-ICP-MS. enabled the determination of Tb, Yb and Lu impurities at a concentration level of 0.01 ug g-1.

4. CONCLUSIONS In ETV-ICP-MS, the MO+/M+ values, which are strongly dependent on plasma parameters such as rf power and sampling depth, can be reduced to a lesser degree than those obtained by NEB-ICP-MS. Also a proper selection of integration time can further suppress the influence of oxide interferences on analyte signals because the peak times of oxide signals as well as analyte ones depend on the melting points of the elements. The MO+/M+ values for REEs obtained by NEB-ICP-MS were in good agreement with the theoretical MO/M ones calculated from assuming the dissociation equilibrium of monoxide in the ICP at a plasma temperature of 5000 K, which suggests that oxide ion species would originate from undissociated MO in the ICP. While in ETV-ICP-MS, the experimental MO+/M+ values were slightly larger than the theoretical MO/M ones, though the effect of solvent removal on spectral interferences was clearly presented. The larger experimental values may be due to the entrance of air into the ICP. Further work would be required to elucidate problems associated with the origin of oxide ion species and their formation mechanism. However, in the determination of REE impurities in high-purity Gd203 and Tb407, the use of the ETV technique demonstrated the excellent low detection limits in the

Oxide formation in ETV-ICP-MS

1137

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