Sci. Rep. Kanazawa Univ.
Vol.52, p.27-36, 2008

27

Comparison of calibration methods to estimate
chemical concentrations using Laser AblationInductively Coupled Plasma-Mass Spectrometer(LA-ICP-MS) ; mean count rate vs integrated
signal intensity
Kazumi ITO１, Noriko HASEBE２, Akihiro TAMURA３ and Shoji ARAI１

１

Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192,
Japan

２

Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa 920-1192,
Japan

３

Department of Earth Science, Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japan

Running title : Comparison of calibration methods to estimate chemical concentrations using LA-ICPMS
Abstract : Laser Ablation - Inductively Coupled Plasma - Mass Spectrometer (LA-ICP-MS)
provides quick and simple experimental procedures for multi-elemental analyses of little amountof samples without complex pretreatments or preparation. Two methodologies, which are themean count rate method (MCRM) and the integrated signal intensity method (ISIM), arecompared to obtain the NET signal, which is then incorporated into the equation to obtainchemical concentration. The effects of repetition frequency in laser setting and choice of internalstandard, and a detection limit are investigated. The repetition frequency and the choice ofinternal standard give no significant effect on concentration obtained by LA-ICP-MS. Allconcentrations by the ISIM show good agreement with the reference values of Pearce et al.,(1997). The detection limit of ISIM is one-fifth of that of the MCRM. Therefore, to measuresmall quantity of radioisotopes, such as 234U and 230Th, the ISIM can be the effective method.

Introduction
Laser Ablation - Inductively Coupled Plasma - Mass Spectrometer (LA-ICP-MS) provides
quick and simple experimental procedures for the multi-elemental analyses of little amount ofsamples without complex pretreatments or preparation. Because of its advantages, the LA-ICPMS has been successfully applied to geochemistry of solid materials (e.g., Jarvis and Williams,
1993, Pickhardt et al., 2005, Spears, 2004, Pearce et al., 1999). The signals are acquired by peak
hopping, one point per isotope during each mass spectrometer sweep, producing the timeintensity profile for each analyte (Figure 1). In this study, two methodologies to calculate
representative signals of each isotope are compared to obtain chemical concentrations : the mean
count rate method and integrated signal intensity method. In the mean count rate method (MCRM,

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Comparison of calibration methods to estimate chemical concentrations using LA-ICP-MS

Figure 1. Time − signal intensity profile to estimate the NET signal by MCRM.

Figure 2. Time − integrated signal intensity profile to estimate the NET signal by ISIM.

Kazumi ITO, Noriko HASEBE, Akihiro TAMURA and Shoji ARAI29

Figure 3. Theoretical advantage of ISIM to MCRM. In the schematic figure for the ISIM, the color
change in lines represents the growth of signal through time. Integrated signal grows more
over the background, therefore the detection limit is expected to become less.

Table 1

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Comparison of calibration methods to estimate chemical concentrations using LA-ICP-MS

Table 2

Kazumi ITO, Noriko HASEBE, Akihiro TAMURA and Shoji ARAI

31

Figure 4. Concentrations (μg/g) of NIST 612 obtained by ISIM. The repetition frequencies are 5 Hz or4 Hz, the internal standards are 29Si or 43Ca and the error bars represent ± 2 σ. (a) 232Th (b) 238U

Figure 5. Comparison of concentrations (μg/g) of NIST 612 obtained by MCRM and ISIM. The
repetition frequency and internal standard are 5 Hz and 29Si, respectively. Solid line represents
one-to-one correlation line, (a) 232Th (b) 238U

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Comparison of calibration methods to estimate chemical concentrations using LA-ICP-MSTabel 3

Kazumi ITO, Noriko HASEBE, Akihiro TAMURA and Shoji ARAI33

Figure 6. Comparison of detection limits (μg/g) of NIST 612 obtained by MCRM and ISIM. The
repetition frequency and internal standard are 5 Hz and 29Si, respectively. (a) 232Th (b) 238U

Figure 1), the average of signal intensity, after background (blank) correction, is incorporated
into the equation to calculate the concentrations. The background is calculated as an average ofsignal intensities before laser ablation (Figure 1). The detection limit is estimated as 3σ (σ,standard deviation) of background signals. In the integrated signal intensity method (ISIM,Figure 2), signals are summed up for a given time slices during laser ablation. For the blank34

Comparison of calibration methods to estimate chemical concentrations using LA-ICP-MS

correction, the average of background intensity measured before the laser ablation is summed up
for the same duration with that to obtain an integrated signal (integrated background), andsubtracted from the integrated signal (Figure 2). The detection limit is calculated as 3σ ofintegrated background. By sliding the calculation window through the sequence of backgroundmeasurement, several integrated backgrounds are obtained and the standard deviation iscalculated. In theory, the ISIM has an advantage to reduce the detection limit (Figure 3). In thisstudy, concentration of radioisotopes and detection limits are calculated using above two methods
and compared for the purpose of radiometric age determination.
Sample and experimental setting
238
U and 232Th concentrations, which are useful for radiometric dating, are measured by LAICP-MS, Kanazawa University (Agilent 7500s equipped with GeoLas Q+). The synthetic glassfrom National Institute of Standards and Technology (NIST), NIST SRM 612, whose
concentration is well known (Pearce et al., 1997, 232Th ; 37.23 ± 0.72 μg / g, 238U ; 37.15 ± 1.23μg / g), is analyzed to compare two calibration methods and precisions are evaluated. NIST SRM
610 is used as an external standard (Pearce et al., 1997, Horn et al., 2000). 29Si and 43Ca areadapted as the internal standards, because silicate or calcium bearing minerals are often suitablefor dating. Instrumental setting is listed in Table 1 and the detail of the equipment is described inelsewhere (Ishida et al., 2004, Morishita et al., 2005). Two laser repetition frequencies, 5 Hz and4 Hz, are tested.
Results and discussion
Concentrations and detection limits of each isotope under a certain repetition frequency arelisted in Table 2 and 3, and plotted in Figure 4 - 6.
Effect of repetition frequencies and internal standard
Hasebe et al. (in press) discussed that repetition frequency in laser shot and the choice ofinternal standard have no systematic effect on the results by the MCRM. In Figure 4,
concentrations of 232Th and 238U by the ISIM are shown along with a reference value of NIST
SRM 612 (Pearce et al., 1997). The error of each ablated spot is calculated as the percentage
same with the standard deviation against the average of 16 measurements. The concentrations ofeach ablated spot, using the internal standard of 29Si or 43Ca, show little discrepancies among ± 2σ
error range. When they are compared to the reference value, they are overlapped within ± 2σerror. An average value for each experimental setting agrees very well for all settings (Table 2).
There is no clear evidence that, in the ISIM, both repetition frequencies and choice of internal
standards give a systematic effect on the obtained concentrations.
Concentration ; MCRM vs ISIM
Based on the above discussion that there is no systematic effect of both repetition
frequencies and the choice of internal standards on concentrations obtained by both MCRM andKazumi ITO, Noriko HASEBE, Akihiro TAMURA and Shoji ARAI35

ISIM, the concentrations obtained under the experimental condition of 5 Hz repetition frequency
and 29Si internal standard are compared between MCRM and ISIM. In Figure 5, concentrations ofthe ISIM are compared with those of the MCRM. All concentrations by the ISIM show good
agreement with the MCRM and the preferred values of Pearce et al. (1997). Since the ISIM /MCRM ratios show one-to-one correlation relationship, choice of calculation methods cause no
significant influence on the estimated chemical concentration.
Effect on detection limit
In Table 3, detection limits that are 3σ of background intensities or integrated background
intensities, are listed. In Figure 6, the detection limits at repetition frequency of 5 Hz with internal
standard 29Si are shown. The detection limit of the ISIM is one-third ~ one-seventh (one-fifth onaverage) of that of the MCRM. This result indicates that the ISIM is the effective method for
calculating a very small quantity (< 10-4 μg/g) of radioisotope, say 234U and 230Th, concentrations.
Conclusion
1. The ISIM provides reliable concentrations of 232Th and 238U, concordant with the reference
value of NIST SRM 612.
2. Repetition frequency in laser setting and choice of internal standard give no significant effecton concentration calculated by LA-ICP-MS.
3. Since the detection limit of ISIM is one-fifth of that of MCRM, ISIM is more effective method
for the purpose of calculating an isotope concentration with very small quantity (< 10-4 μg/g).
Acknowledgements
A part of this study is supported by the Inamori Foundation.
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Comparison of calibration methods to estimate chemical concentrations using LA-ICP-MS

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