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2003 © The Japan Society for Analytical Chemistry
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Notes
Estimation of Metal Impurities in High-Purity Nitric Acids Used for Metal
Analysis by Inductively Coupled Plasma–Mass Spectrometry
Kyue-Hyung LEE,* Koji OSHITA,** Akhmad SABARUDIN,** Mitsuko OSHIMA,**† and
Shoji MOTOMIZU**
*Department of Applied Chemistry, School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464–8603, Japan
**Department of Chemistry, Faculty of Science, Okayama University,
3-1-1, Tsushimanaka, Okayama 700–8530, Japan
A simple method to estimate the amounts of ultra-trace metal impurities in nitric acid reagents has been developed. The
determination of sixty-four metals in nitric acid was accomplished by direct measurements of 0.1 M nitric acids
accurately diluted with ultrapure water by ICP-MS. Though accurate metal concentration could not be obtained for all of
the elements, we could effectively evaluate the nitric acid quality by comparing the ion counts of the samples, ultrapure
water and standard metal solutions for a calibration prepared with UltrapurTR nitric acid.
(Received July 28, 2003; Accepted September 22, 2003)
Introduction
Experimental
It is most often encountered to use some acids, such as nitric,
hydrochloric, hydrofluoric, perchloric and sulfuric acid, for the
decomposition of solid samples, such as plants, sediments and
rocks, and for atomic spectroscopic analyses, such as atomic
absorption spectrometry (AAS), inductively coupled
plasma–atomic emission spectrometry (ICP-AES) and
inductively coupled plasma–mass spectrometry (ICP-MS). In
such practical analyses, nitric acid has often been used to utilize
its oxidizing ability. For an ICP-MS measurement, nitric acid is
used as a reagent blank solution and a washing/rinse solution,
because it shows the simplest spectra of all the acids and the
lowest background levels next to pure water. High-purity
reagents are usually required for the sensitive determination of
metals at concentrations of ppb (1 ppb = 10 –9 g ml–1) or ppt
(1 ppt = 10–12 g ml–1) levels in sample solutions. Since lower
background levels lead to lower limits of detection and higher
sensitivities, we must take into account the aims of the analyses
and the costs of reagents when selecting the grade of reagents.
However, there is little information on the amounts of metal
impurities in nitric acids, as well as other acids. An estimation
of the metal contents in nitric acid will lead to a more reliable
trace analysis, and therefore the choice of a nitric acid grade
suitable for the purpose of the analysis is very important.
The preconcentration methods, such as evaporation,1 solvent
extraction,2 precipitation,3 and ion-exchange,4 for trace metals in
high-purity reagents, are infeasible because possible
contaminations from experimental environment and vessels are
often very serious. In this work, an evaluation for metal
impurities in nitric acid was directly accomplished by
measuring diluted nitric acid by ICP-MS.
Seven kinds of nitric acids purchased from two suppliers (A and
B) were evaluated by measuring their impurity levels by ICPMS. Four kinds of reagent grades (ultrapure, for trace metal
analysis, analytical and extrapure grades) from supplier A, and
three kinds of reagent grades (ultrapure, analytical and
extrapure grades) from supplier B were examined. Each nitric
acid sample (60%, specific gravity of 1.38) was diluted with
ultrapure water to give a final acid concentration of 0.1 M (1 M
= 1 mol dm–3), and sixty-four elements in the sample solutions
were simultaneously measured under normal nebulization
conditions. Ultrapure water (18.2 MΩ cm–1 resistivity) was
prepared with an Elix3/Milli-Q ElementTR (Nihon Millipore,
† To whom correspondence should be addressed.
E-mail: [email protected]
Table 1
Operating parameters of the ICP-MS system
Plasma conditions:
RF frequency
Forward power
Reflective power
Coolant gas flow rate
Auxiliary gas flow rate
Carrier gas flow rate
Sample uptake rate
Spray chamber
Nebulizer
Sampling conditions:
Sampling depth
Sampling cone
Skimmer cone
Data acquisition:
Dwell time
Data points
No. of scans
27.12 MHz
1.1 kW
<5 W
Ar 15 L min–1
Ar 1.0 L min–1
Ar 0.45 L min–1
1.0 mL min–1
Scott type
Meinhard TR-30-C2
10 mm from load coil
Cu 1.1 mm i.d.
Cu 0.35 mm i.d.
100 ms at each m/z
3 points per peaka
60 times
a. Assuming peak center and ±0.125 u from the center.
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Table 2
Analytical results for trace elements in several nitric acids by ICP-MS
Supplier B/ng mL–1
Supplier A/ng mL–1
Element
m/z
Ultraa
Traceb
Analc
Extrad
Ultraa
Analc
Extrad
a. Ultrapure reagent grade. b. For trace metal analysis. c. Analytical reagent grade. d. Extrapure reagent grade. Concentration in 0.1 M
nitric acid solution diluted with ultrapure water from concentrated nitric acid reagents.
Tokyo, Japan). The operation conditions of ICP-MS system are
listed in Table 1.
A multielement standard stock solution (100 ng ml–1) was
prepared from single element standard solutions (1000 µg ml–1)
for atomic absorption spectrometry (Wako Pure Chemicals,
Osaka, Japan). The stock standard solution was diluted with 0.1
M nitric acid (UltrapurTR, Kanto Chemicals, Tokyo, Japan) to
match the final acid concentrations of the sample solutions. A
three-point calibration method was used to determine trace
elements: a blank (0.1 M nitric acid, prepared with UltrapurTR
nitric acid) and multielement standards, 0.1 ng ml–1 and 1 ng
ml–1, solutions were measured by ICP-MS.
Results and Discussion
The analytical results obtained for sixty-four trace elements in
seven nitric acid samples (0.1 M solution) are given in Table 2.
Since the concentrated nitric acids were diluted by 138-fold
with ultrapure water for measurements, analytical values were
obtained using a calibration graph corresponding to the
difference in ion counts between the UltrapurTR nitric acid and
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the samples. Several metal concentrations obtained from the
calibration graphs showed negative values. The concentrations
of these values were secondly corrected by subtracting each
metal concentration in ultrapure water. The results obtained for
the net metal contents between ultrapure water and each sample
(0.1 M nitric acid) are given in Table 2. The precision of the
ion counts for most metals ranged over 5 – 20%. These poor
precisions were because that the metal concentration levels
detected were around, or under, the instrumental limit of
detection for several elements. The chemical stability for some
elements in the 64 mixed metal standards in nitric acid is not
very good. For example, platinum group elements (Pt, Pd, Ir,
Os, Rh and Ru) tend to be stable in hydrochloric acidic media,
and Ta and Nb are stable in hydrofluoric acidic media.
Therefore, it is necessary to measure as soon as possible when
the standard samples are prepared. Fortunately, the impurity
levels of these elements are extremely low, and it is not
seriously problematic in practical cases.
The concentrations of Mo, Ag, Ce, Nd and Hg in the
analytical reagent grade of supplier A are considerably higher
than those of other nitric acids; at least, this nitric acid is not
adequate for the sub-ppb analysis of metal elements. In
particular, the concentrations of Mo (from 0.60 to 41.7 ng ml–1)
and B (from 0.27 to 1.72 ng ml–1) in all nitric acid samples
examined in this study were still higher compared with those of
ultrapure water. These results suggest that the removal of Mo
and B from nitric acid is very difficult in the purification
process. The impurity levels for the ultrapure reagent grade of
nitric acids from both suppliers are not very different; it has
been proved that in most cases such a reagent grade may be
suitable for ultratrace or trace metal analysis. The concentration
levels of rare earth elements (REEs) in most of the nitric acids
examined were down to the ppt level, which means that there
should be no problem in the determination of REEs in the usual
analysis (higher than ppb level), even when lower grade nitric
acids, such as for trace metal analysis, analytical reagent grade,
and extrapure reagent grade, are used.
The advantages of the present method focus on the simplicity
and rapidity for estimating entire metal impurity levels. The
best grade of nitric acid is frequently needed to obtain the
lowest background level in the mass spectrum for multielement
and ultra-trace metal analysis by ICP-MS. In practice, there is
no way to evaluate such a high-purity reagent grade without a
special enrichment method, and it can only be possible to
examine for a few especially concerned elements.
In conclusion, the metal impurity data obtained by the
proposed method are not absolutely precise values for some
elements used in this study. However, this estimation method
gives us very useful information about the entire blank level for
metal analysis prior to an ICP-MS or ICP-AES measurement.
We can easily select an adequate reagent grade in accordance
with the purpose of the metal analysis, such as ultra-trace, trace,
minor and major analysis, if we have a data-base for several
kinds of reagent grades.
Acknowledgements
This work was partially supported by Okayama University
Venture Business Laboratory and also partially by a Grant-inAid for Scientific Research (C, No. 15550138) from the
Ministry of Education, Science, Sports and Culture, Japan.
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