J. Mass Spectrom. Soc. Jpn.

Vol. 52, No. 4, 2004

REGULAR PAPER

New Reduction Technique for Isobaric Interferenceson Ba Using ICP-Mass Spectrometry
Takafumi H>G6I6῎(Received December 15, 2003; Accepted May 19, 2004)
A new technique for the removal of isobaric interferences on Ba has been developed for ICP-mass spectrometer. Mass spectrometric interferences on Ba by 138Laῌ, 136Ceῌ, or 138Ceῌ isobaric signals were e#ectively
eliminated by a selective production of oxide ions. The Laῌ and Ceῌ ions could be quantitatively converted tooxide ions (LaOῌ and CeOῌ) by the ICP ion source under the low-power operation. Production e$ciency of oxidesignals for La and Ce was remarkably enhanced, i.e., ῎2,000,000 times the level obtained by the conventional ICP
power (1.35 kW). Residual signal intensity of Laῌ signal was almost the same level as the instrumental
background. The overall contribution of isobaric 138La and 138Ce to 138Ba signals were 0.02῏ and 0.1῏,
respectively, even with the presence of ῍10,000 times higher concentration of La and Ce. The data obtained
here demonstrate that the level of the isobaric interferences by 138La, 136Ce, and 138Ce signals produced by the
low-power ionisation was negligible within the analytical precision achieved in this study (ῌ1῏). It should be
noted that the analytical sensitivity for Ba obtained by the low-power ICP remains almost 85῏ of the level
achieved by the conventional ICP power. This is very promising because the present technique can be applied
to the laser ablation sampling technique which requires high sensitivity without chemical separation or
puriﬁcation procedures.

1.

Introduction

Age of rock sample is one of the key information for
the understanding of geological sequence of the Earthand the solar system. Earth scientists have been trying
to understand the underlying order and inherent linkage of the events in the world based on the chronological description of the sample. Ages of the samples havebeen determined by means of the rate of erosion or
sedimentation, fossils contained therein or nuclear decay of radioactive isotope. It is well recognised thatisotopic chronometer based on radioactive decay of
unstable nuclides, such as 87Rb, 147Sm, 238U, or 235U, canprovide most accurate age data of the samples.1), 2) Recent progress in analytical technique to measure small
variation in isotopic composition with high sensitivity
can enables us to determine precise chronological information. Atmospheric pressure inductively coupledplasmas (ICPs) are ﬂame like electrical discharge thathave revolutionised the practice of elemental and isotopic ratio analysis. The high temperatures available
at atmospheric pressure in the plasmas make themvery e$cient excitation and ionisation sources. The
ionisation e$ciency to the singly charged state formost elements is approaching ῎80῏,3), 4) indicative ofvery attractive characteristics as an ion sources.5)῍7)
Many geologists are increasingly interested in processes in rocks that operate at the microscopic scale
such as zoning or metamorphic recrystallisation. Because these processes have implications for the larger
scale behavior of the Earth, new techniques for chemical and isotopic measurements at the mm scale need to
be developed. Combination of laser ablation (LA) tech῎ Laboratory for Planetary Sciences, Tokyo Institute of Technology (O-okayama 2῍12῍1, Meguro-ku, Tokyo 152῍8551,
Japan)

nique and ICP-MS has now become a fast and accurate
method of in situ trace element and isotopic analysis.8)῍15) In order to obtain accurate isotopic data,isobaric interferences onto the isotopes of interest must
be removed. Chemical separation procedures such as
chromatography or liquid῍liquid distribution techniques have been widely used to separate the isobaric
interferences from the analyte. However, in the case oflaser ablation sample introduction technique, theseconventional techniques can not be applied because
sample aerosols produced by the laser ablation are
directly introduced into the ICP. Recently, isobaric
interference of 87Rb onto 87Sr was successfully minimised by means of collision or reaction cell technique.16)῍19) However, in most cases, elemental sensitivity of the instrument can be lowered through thisreaction process. Moreover, modiﬁcation of the instrument is needed with the standard ICP-MS instruments.
La῍Ba isotopic chronometer is based on radioactive
decay of 138La to 138Ba with half-life of 1.1῍1011 yrs.
The La῍Ba chronometer can provide precise chronological data for REE-bearing minerals or ores.20), 21) In
order to obtain accurate La-Ba age, precise isotopic
ratio data on Ba are highly desired. However, even asmall contribution of mass spectrometric interferenceson 136Ba and 138Ba by 136Ce, 138Ce, and 138La ionic signals can be a large source of analytical error (Fig. 1).
To minimise the e#ect of isobaric interferences, prior
to the isotopic analysis, Ba was chemically separated
from La and Ce. Despite the obvious success in effective separation and high recovery achieved by thechemical separation procedure, this conventional technique can not be applied to the LA technique. In orderto obtain in-situ La῍Ba isotopic data using the LA
technique, the isobaric interferences on Ba isotopes
must be separated or removed within the mass spectrometer. In this study, I would like to show a newῌ 171 ῌ

T. Hirata

Fig. 1.

Schematic diagram of isobaric interferences on Ba by La and Ce. The 136Ba and 138Ba signals were interfered by
138
La, 136Ce, and 138Ce isobaric signals. La῍Ba isotopic chronometer is based on the small isotopic growth of 138Ba
due to radio-active decay of 138La.

technique for reduction of isobaric interferences on Ba
using a preferential conversion of Laῌ and Ceῌ ions to
LaOῌ and CeOῌ signals achieved by the low-power ICP
operational condition.
The strength of ICP-MS lies with the ion source
which achieves high ionisation e$ciency for nearly all
elements.4) In order to take full advantage of the highionisatin capability of the ICP, RF incident power of
῎1.2 kW was generally accepted as an ICP operation
setting. Moreover, higher ICP operation power is very
important to minimise the oxide (MOῌ) production.
The production of oxide has been evaluated by theMOῌ/Mῌ ratio. The MOῌ/Mῌ ratios of ῍0.05 can beachieved by the ICP ion source under the conventional
power setting, whereas the ratio increased up to ῎0.1
when the lower ICP power (1,100 W) was applied.22)
Because of the lower elemental sensitivity of the instrument with higher oxide production (MOῌ/Mῌ ratio), the actual utility of the low-power ICP has beenhighly restricted. Despite these apparent drawbacks ofthe low ICP operation power, lower power (500 W) wasapplied to the ICP to enhance the oxide production
aiming at removal of isobaric interferences on Ba.
2.

Experimental

The ICP-mass spectrometer used in this study was a
Thermo Electron (Winchester, UK) VG PlasmaQuad 2
Omega. This uses quadrupole mass spectrometer with
M/DMῌ250 (10῏ height) as a mass analyser. Conventional pneumatic nebuliser MicroMist (Glass Expansion, Melbourne, Australia) was used for solution introduction into the ICP. It is well recognised that secondary discharge between vacuum interface and the ICP
becomes serious when low incident power was used.

The secondary discharge results in lowering the instrumental sensitivity. In this study, plasma shield technique was applied to minimise the formation of secondary discharge,23) and no obvious discharge could befound even with RF incident power of 400 W. The
pressure of analyser chamber (quadrupole housing) becomes high (῎10῍4 Pa) when the lower ICP incident
power (῍400 W) was applied. This is mainly due tolowering of the gas temperature at the tip of the sampling oriﬁce,4) and this results in higher gas ﬂux entering the vacuum chamber through the oriﬁce. In thisstudy, 500 W incident power was applied to test thelow-power ICP operation because there was no signiﬁcant increase in pressure of analyser housing.
Operational conditions of the ICPMS such as torch
position, Ar gas ﬂow rates, ion transfer lens biases orspray chamber temperature were tuned to maximise
the signal intensity of 138Ba. In the case of low-power
ICP, optimum nebuliser gas ﬂow rate were 0.75῍0.8 L/
min which was signiﬁcantly lower than the values(0.95 L/min) required for the conventional ICP power(1.35 kW). This can be attributed to the poorer ionisation e$ciency of the low-power ICP, and therefore thelower nebuliser gas ﬂow rate avoids the further coolingof the plasma by the water load. Details of the instrument and the operational parameters used in this study
are listed in Table 1. For comparison, operational settings used for conventional ICP operation were also
listed in this table.
3.

Results and Discussion

3.1 Production of oxide ions
It has been well recognised that oxide ions (MOῌ)
was enhanced when lower ICP power was applied.

ῌ 172 ῌ

New Reduction Technique for Isobaric Interferences on Ba Using ICP-MS

Table 1.

ICP-MS Instrumentation and Operational Settings

1. ICP-mass spectrometer
Instrument
ICP incident powerReﬂection
Vacuum interface
Lens typePressure (Pa)Expansion chamberIntermediate chamber
Analyzer chamber
2. Instrumental setting
Nebuliser
Nebuliser gas ﬂow rate
Solution uptake rate
Spray chamberSpray chamber temp.
Peristaltic pump
Sampling depth
Sensitivity for Ba
Background
3. Data acquisition
Monitoring signals
Data acquisition time
Scan mode
Dwell time
Settled time
Correction for mass discrimination: Thermo Electron VG PlasmaQuad 2 Omega: 1350 W (standard), 500 W (low-power operation): ῑ5 W: S-option: CHICANE lens
: 120
: ῑ10῍2: 2῏10῍4 (standard operation)
6 ῏10῍4 (low-power operation): Glass Expansion MicroMist: 1.02 L/min (standard operation)
0.75 L/min (low-power operation): 1 mL/min (standard operation)
0.8 mL/min (low-power operation): Scott-type double path: 4ΐ (standard operation)
12ΐ (low-power operation): not used
: 20 mm above the load coil
: 400 Mcps/mg g῍1 (standard operation)
ῌ360 Mcps/mg g῍1 (low-power operation): ῑ5 cps (at m/zῐ101.5): 134Ba, 135Ba,136(Ba῍Ce), 137Ba, 138(Ba῍La῍Ce), 139La, 140Ce: 15 s: peak jump (3 points per isotopes; M and M῎0.025 amu)
: 10 ms/point: 10 ms/isotope: not made

Note:
Standard operation: Standard power setting for ICP operation (1,350 W)
Low-power operation: low-power ICP operation (500 W)

Lichte et al.22) reported that production ratio deﬁned as
MOῌ/Mῌ ratio increased up to 10῔ with the ICP incident power of 1,100 W.
More importantly, they
pointed out that oxide production ratio is basicallycorrelated with chemical a$nity with oxygen. In thisstudy, further lower power was used to enhance theproduction e$ciency of oxide ions, aiming at separation of mass spectrometric interferences by shifting theinterferences to heavier masses as oxide molecularions.
Dependence of production ratio of oxide ions (MOῌ/Mῌ) upon ICP incident power was examined. In thisstudy, BaOῌ/Baῌ and LaOῌ/Laῌ ratios were monitored by introducing a mixed solution of Ba and La (1ng/g each), and the resulting MOῌ/Mῌ ratios wereplotted against the ICP incident power (Fig. 2). Uncertainties of the BaOῌ/Baῌ and LaOῌ/Laῌ ratios denote
2s (standard deviation) calculated from 5 times repeated analysis. The BaOῌ/Baῌ and LaOῌ/Laῌ ratios
obtained on the conventional ICP power (1,350 W) was
0.00052῎0.00008 and 0.0029῎0.0003, respectively.
The level of oxide production increased dramaticallywhen the ICP incident power was lower than 1,000 W,
and the resulting BaOῌ/Baῌ and LaOῌ/Laῌ ratios
found on the present low-power ICP (500 W) condition
were 0.0912῎0.0047 and 6301῎205, respectively. The
enhancement factors for oxide production deﬁned by(MOῌ/Mῌ)500 W/(MOῌ/Mῌ)1350 W were 200 for Ba andῒ2,000,000 for La, suggesting that Laῌ was preferentially converted to LaOῌ ions under the low-power
ionisation condition. Fig. 3 represents the resulting
mass spectrum for Ba῍La mass region obtained by the
low-power ICP operation. It should be noted that resid-ῌ 173 ῌFig. 2.

Level of oxide production (BaOῌ/Baῌ andLaOῌ/Laῌ) as a function of ICP incident power.
The resulting LaOῌ/Laῌ ratio increased
ῒ2,000,000 times when the low power (500 W)
was applied.

T. Hirata

Fig. 3.

Mass spectrum obtained by the low-power ICP operation (incident power 500 W). The Laῌ ion was quantitatively
converted to LaOῌ ion, and residual signal intensity for 139La was almost identical to the background count rate.

ual signal intensity at m/z῍139 would be originatedfrom the hydride ion (138BaHῌ) since production yieldfor hydride ions could also be increased with the low
ICP power. However, this is not the case because thesignal intensity at m/z῍139 was independent upon thesignal intensity of 138Baῌ. Although it is not clear that
residual signal at m/z῍139 was originated either from139Laῌ or other polyatomic ions, it is feasible that residual signal intensity at m/z῍139 could reﬂect the upper
limit of the residual signal intensity of 139Laῌ ions. The
resulting signal intensity at m/z῍139 was ῎10 ions/s
which was almost same level as the background countof the instrument, and therefore, the most of the Laῌ
ion was quantitatively converted to LaOῌ ions under
the low-power ICP operation. In strike contrast, only
10῍15῏ fraction of Ba was converted to BaO ions evenwith the low-power ICP condition, and the main Ba ion
produced by the ICP was still Baῌ. This indicates that
contribution of isobaric interference on 138Ba by 138La
could be remarkably reduced by the low-power ICP
Table 2.

achieved in this study. It should be noted that loss ofelemental sensitivity for Ba under the low-power ICP
conditions was only 15῏ level, and therefore, sensitive
detection of Ba was still possible. The result obtained
here reveals that contribution of isobaric interference
on 138Ba could be minimised by the preferential conversion of 138Laῌ to 154LaOῌ signals. This is identical tothat ion-molecule reaction chemistry through the collision or reaction cell techniques.16)῍19)
In the case of isotopic analysis of Ba, not only 138La
but also 136Ce and 138Ce ions can become a mass spectrometric interferences on Ba. In order to validate thee#ectiveness of the conversion technique achieved by
the low-power ICP, isotopic ratios of Ba for chemical
reagents were measured.
3.2 Isotopic analysis of Ba
In this study, 134Ba/137Ba, 135Ba/137Ba, 136Ba/137Ba,
and 138Ba/137Ba ratios for mixtures of Ba, La, and Cefrom high purity chemical reagents have been meas-

134

Ba/137Ba, 135Ba/137Ba, 136Ba/137Ba and 138Ba/137Ba Isotopic Ratios Obtained by ICP-MS with Di#erent ICP
Operation Power
No correction for the mass discrimination e#ect was made.
Ln῎1(ng/g)ICP Power(Watts)Ba(ng/g)

Hot plasma

1,350

5

0

0.1955
ῌ0.0073

0.5805
ῌ0.00880.7014
ῌ0.00556.511
ῌ0.061Cool plasma

500

5

0

0.2002
ῌ0.0050

0.5779
ῌ0.00650.7007
ῌ0.00636.455
ῌ0.0645

50.1922
ῌ0.00630.5842
ῌ0.00800.7017
ῌ0.00416.511
ῌ0.05155,000

0.1968
ῌ0.0055

0.5801
ῌ0.00890.7001
ῌ0.00526.501
ῌ0.058550,0000.1955
ῌ0.0075

0.5822
ῌ0.00910.7008
ῌ0.0066

6.498
ῌ0.078134

Ba/137Ba῎2135

῎1 Lanthanoid elements (La῍Lu).
῎2 Errors are 2-sigma standard deviation calculated by 15 repeated analysis.
ῌ 174 ῌBa/137Ba῎2

136

Ba/137Ba῎2

138

Ba/137Ba῎2

New Reduction Technique for Isobaric Interferences on Ba Using ICP-MS

Fig. 4.

Isotopic ratios of Ba (136Ba/137Ba and 138Ba/137Ba) obtained by the standard ICP operation and the low-power
operation. There were no signiﬁcant di#erence in resulting 136Ba/137Ba and 138Ba/137Ba ratios even with thepresence of La and Ce (Ln/Ba῍1῍10,000).

ured in order to test the separation e$ciency of isobaric interferences more rigorously. Four analyticalsolutions were made: ﬁrst solution was 5 ng/g Ba solution simply diluted from the Ba standard solution.
Second solution was a 5 ng/g Ba solution containing
same concentrations of La and Ce (5 ng/g each). The
third and fourth solutions was 5 ng/g Ba containing
ῌ1,000 times and ῌ10,000 times higher concentrations of La and Ce, respectively. For further comparison, Ba isotopic ratios measurement was made under
the standard ICP operation (1.35 kW). Data acquisition
parameters such as signal integration area, integration
time, mass-scan rate or number of scans were summarised in Table 1. The resulting 134Ba/137Ba, 135Ba/137Ba,
136
Ba/137Ba and 138Ba/137Ba ratios were listed in Table
2. Among them, data for 136Ba/137Ba and 138Ba/137Ba
ratios could vary due to possible isobaric interferencesby La and Ce, and therefore, resulting 136Ba/137Ba and138
Ba/137Ba ratios were plotted for clariﬁcation (Fig. 4).
All the isotopic ratio data including 136Ba/137Ba and138
Ba/ 137Ba ratios show excellent agreement to thedata obtained by the pure Ba analytical solution within
the analytical uncertainties (1ῐ, 2SD). Moreover, nosigniﬁcant di#erence was found on the resulting isotopic ratios obtained by the low-power ICP operationfrom the data for pure-Ba solution obtained with thestandard-power ICP operation. Contributions of 138La
and 138Ce signals onto 138Ba isotope were ῎0.02ῐ and῎0.1ῐ, respectively, even with the presence ofῌ10,000 times higher concentrations of La and Ce, andtherefore, level of isobaric interferences on Ba due to
La and Ce were negligible within the precision of themeasurement achieved in this study.
Allanite ((Ca, Ce, Y, La, Th)2(AlFe)3Si3O12(OH)) andmonazite ((Ce, La, Y, Th)PO4) are the typical REE (rareearth elements)-bearing minerals. Since La/Ba abundance ratio for the allanite or monazite is generally
very high, age of these minerals can be well constrained by the La῍Ba isotopic chronometer. In mostcases, the La/Ba ratio for alanites or monazites is generally 50῍5,000,20)῍21) and this is signiﬁcantly lower
than the nominal La/Ba ratio simulated in this study(ῌ10,000). This suggests that precise isotopic ratios of
Ba can be made on bulk samples without chemical
separation of Ba from La and Ce.
No correction of the mass discrimination e#ect wasmade for all the isotopic data for Ba (134Ba/137Ba, 135Ba/
137
Ba, 136Ba/137Ba, and 138Ba/137Ba ratios) given in
Table 2 and Fig. 4. This indicates that accuracy andprecision of the isotopic ratio measurements could befurther improved when the mass discrimination e#ect
is corrected internally. Moreover, further precise andaccurate isotopic ratio measurements can be achievedby the combination of the present low-power ionisation
technique and a multiple collector-ICP-mass spectrometer.
4.

Conclusion

Isobaric interferences on Ba by La and Ce isotopes
were e#ectively minimised by the preferential production of LaOῌ and CeOῌ molecular ions under the
low-power ICP operation (500 W). Since the signal
intensity of residual Laῌ ions were almost the samelevel as the instrumental background (῎10 ions/s),
Laῌ ions were quantitatively converted to LaOῌ ions
under the low-power ICP operation. Resulting LaOῌ/Laῌ ratio observed on the low-power ICP operationwas ῏2,000,000 times higher than the ratio obtainedby the conventional operation with ICP power of 1.35
kW. In contrast, the main Ba signal observed on the
low-power ICP was still Baῌ ion. Instrumental sensitivity for Ba was still ῏300 MHz/mg g῍1, which was almost 85ῐ level of that achieved by the standard ICP
power. This is very important for the isotopic analysis
of Ba with high sensitivity. No signiﬁcant changes in
136
Ba/137Ba and 138Ba/137Ba ratios was found even withthe presence of ῌ10,000 times higher concentration of
La and Ce (50,000 ng/g each), indicative of e#ective

ῌ 175 ῌ

T. Hirata

removal of isobaric interferences by La and Ce
isotopes. The removal of isobaric interferences bymeans of the selective conversion achieved with the
low-power ICP can be applied for the isotopic analyses
of various elements without any serious modiﬁcationof the ICP-MS instruments.
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Keywords: ICP-mass spectrometer, Isotopic analysis, Barium,
Isobaric interference, Low-power ionisation, Cool plasma operation

ῌ 176 ῌ