J. Cent. South Univ. (2012) 19: 2416-2420 

DOI: 10.1007/s11771-012-1290-0

Determination of trace elements in high purity nickel by high resolution inductively coupled plasma mass spectrometry

NIE Xi-du(聂西度)^{1, 2}, LIANG Yi-zeng(梁逸曾)¹, TANG You-gen(唐有根)¹, XIE Hua-lin(谢华林)³

1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Department of Material and Chemical Engineering, Hunan Institute of Technology, Hengyang 421002, China;

3. College of Chemistry and Chemical Engineering, Yangtze Normal University, Fuling 408100, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The contents of Mg, Al, Si, Ti, Cr, Mn, Fe, Co, Cu, Ga, As, Se, Cd, Sb, Pb and Bi in high purity nickel were determined by high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). The sample was dissolved in HNO₃ and HCl by microwave digestion. Most of the spectral interferences could be avoided by measuring in the high resolution mode. The matrix effects because of the presence of excess HCl and nickel were evaluated. Correction for matrix effects was made using Sc, Rh and Tl as internal standards. The optimum conditions for the determination were tested and discussed. The detection limits range from 0.012 to 1.76 μg/g depending on the type of elements. The applicability of the proposed method is also validated by the analysis of high purity nickel reference material (NIST SRM 671). The relative standard deviation (RSD) is less than 3.3%. Results for determination of trace elements in high purity nickel were presented.

Key words: high resolution inductively coupled plasma mass spectrometry; high purity nickel; trace element; matrix effect; internal standard

1 Introduction

High purity nickel is mainly used in producing such as catalyst for various organic syntheses and cracking processes in the petroleum industry, as material for targets of high vacuum sputtering, in the telecommunications and electronics industries and for the manufacture of chemical equipment. The trace elements in high purity nickel have serious effects on their physical functions, so it is very important to determine precisely the trace elements in high purity nickel. At present, several analytical techniques, including spectrophotometry [1], neutron activation analysis [2], atomic absorption spectrometry (AAS) [3], inductively coupled plasma optical emission spectrometry (ICP-OES) [4] and inductively coupled plasma mass spectrometry (ICP-MS) [5] have been applied for trace impurities determination in high purity nickel or nickel salts. However, AAS is not appropriate for multi-element routine analysis because AAS instrumentation is typically constructed as a mono- element method. ICP-OES and ICP-MS usually require the separation of matrix/constituent elements to minimize interferences and signal suppressions. Chemical separation of the matrix from all other minor or trace constituents is not possible in a single       step. Matrix separation depends on high selective reactions.

High resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) provided several analytical advantages over ICP-QMS, including higher sensitivity resulting from higher transmission efficiency of ions passing through a mass spectrometer, so HR-ICP-MS reduced spectral interference problems owing to high resolution up to m/Δm=10 000, and was successfully applied in determining many elements in complex matrix sample [6-12]. The contents of Mg, Al, Si, Ti, Cr, Mn, Fe, Co, Cu, Ga, As, Se, Cd, Sb, Pb and Bi in high purity nickel after being dissolved were directly determined through high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). The loss of the analyte elements could be effectively avoided by microwave digestion. Most of the spectral interferences could be overcome by using different resolution mass spectrometry mode. The effects of nickel matrix on HR-ICP-MS were discussed in detail in this work. The correction for matrix effects was made using Sc, Rh and Tl as internal standards. The trace impurities in several high purity nickel samples were determined.

2 Experimental

2.1 Instrumentation

Plasma Trace 2 high-resolution inductively coupled plasma mass spectrometry (Micromass Corporation, UK) was employed to determinate the ultra-trace elements. The device can be operated in low (LRM, m/Δm=600), medium (MRM, m/Δm=4 000) and high resolution modes (HRM, m/Δm=7 500). The instrumental parameters were as follows: RF power 1.3 kW, coolant gas flow 14.0 L/min, auxiliary gas flow 0.75 L/min, nebulizer gas flow 1.25 L/min, sampling cone nickel (1.1 mm in orifice diameter), skimmer cone nickel (0.8 mm in orifice diameter), ion sampling depth 11mm, sample uptake rate 0.25 mL/min, dwell time 20 ms, and accelerating voltage 6 kV; the voltage of ion lens was adjusted to obtain the maximum signal intensity.

A microwave oven (MDS-81D, CEM Corporation, USA) was used for sample digestion. All reagents used were prepared from analytical-grade/specpure chemicals (E. Merck). Ultrapure water, specific resistance above 18.0 MΩ·cm from a Milli-Q deionization unit (Millipore, USA.), was used throughout the experiment.

2.2 Standards and reagents

Single-element calibration solutions were prepared from 1 000 mg/L stock solutions of Mg, Al, Si, Ti, Cr, Mn, Fe, Co, Cu, Ga, As, Se, Cd, Sb, Pb and Bi in 2% HNO₃ by dilution with ultrapure water. The same procedure was followed for the preparation of the working solution to be used as the internal standard for mass spectrometric determinations. Multi-element standard solutions were prepared from single-element stock solutions. These solutions were freshly prepared. Reference material NIST SRM 671 was used to study the accuracy.

Purifications of HNO₃ and HCl were carried out by the recycled sub-boiling distillation of analytical-grade acid for three times. Glass containers were used throughout the work, and were cleaned by immersion in concentrated HNO₃ and concentrated HCl overnight, then successively steamed with HNO₃ and water vapor for 8 h. The concentration of the internal standards liquor was 50 μg/L.

2.3 Analytical procedures

Microwave digestion was used for sample preparation. About 0.100 0 g sample was accurately weighed and transferred into PTFE vessel, followed by adding 2 mL HNO₃ and 3 mL HCl. The program of microwave digestions was: 300 W, 180 s; 600 W, 480 s; 400 W, 240 s. After that, open the lid and place the sample solution in an airing cupboard for static draught until it became limpid and the sample was diluted to  100 mL with ultrapure water. HR-ICP-MS could be directly applied to the determination of the elements under the optimized experiment conditions. Process a reagent blank in the same way.

3 Results and discussion

3.1 Optimization of instrumental parameters

Through optimizing the analysis conditions of instrument, the sensitivity and the precision of analysis could be greatly improved. Increasing the high-frequency power would bring about the higher ionization efficiency. When the high-frequency power reached 1 300 W, the ionization efficiencies of all the elements to be determined could be close to their maxima. In the experiment, the high-frequency power was set to be    1 300 W. The ion peak intensities were measured with the variation of nebulization air flow ranging from 0.8 to 1.6 L/min. The result showed that when the rate of nebulization air flow was 1.25 L/min, the signal intensities of the isotopes to be determined could all get to their maxima. In addition, sampling depth had a direct effect on ionization efficiencies. In this experiment, we examined the different signal intensities of ions when the sampling depth varied from 9 to 13 mm and finally decided that the best sampling depth was 11 mm.

3.2 Spectral interferences

Even in difficult matrices, like matrices with high salt concentration, HR-ICP-MS can separate the interfering species from the isotope of interest. The majority of interferences that may potentially affect the determination of the elements under consideration in high purity nickel are presented in Table 1. Three different modes have been used to improve the accuracy in the analysis.

The determination of ²⁷Al, ²⁸Si, ⁴⁸Ti, ⁵²Cr, ⁵⁵Mn, ⁵⁶Fe, ⁶³Cu, ⁷¹Ga, ⁷⁵As and ⁷⁸Se is often difficult with conventional quadrupole ICP-MS instruments because of overlap from molecular ions. In this work, however, the higher resolution settings available on the HR-ICP-MS instrument resolved these elements from nearby polyatomic interferences. Figure 1 shows the peaks of ⁶³Cu and interfering ⁶²Ni¹H obtained in MRM and HRM. As shown in Fig. 1, ⁶³Cu peak was separated from ⁶²Ni¹H peak sufficiently in HRM. Similarly, ⁵⁶Fe, ⁵⁹Co, ⁷⁵As and ⁷⁸Se peaks were disturbed by polyatomic ion peaks. Therefore, these five elements were measured in HRM. Figure 2 shows the peaks of ⁵⁵Mn and interfering ⁴⁰Ar¹⁴N¹H，⁴⁰Ar¹⁵N and ³⁸Ar¹⁷O obtained in LRM and HRM. It can be seen that the ⁵⁵Mn peak was separated from ⁴⁰Ar¹⁴N¹H，⁴⁰Ar¹⁵N and ³⁸Ar¹⁷O peaks sufficiently  in MRM. Similarly, ²⁷Al, ²⁸Si, ⁴⁸Ti, ⁵²Cr and ⁷¹Ga were determined in the MRM. It should be noted that the trade-off for this increased resolution was a subsequent drop in signal intensity [13]. As a result, longer acquisition times were used for these elements. The other elements (²⁴Mg, ¹¹²Cd, ¹²¹Sb, ²⁰⁸Pb and ²⁰⁹Bi) were measured in LRM.

Table 1 Potential spectral interferences in HR-ICP-MS determination of high purity nickel

Fig. 1 Mass spectra for ⁶³Cu obtained by HR-ICP-MS

Fig. 2 Mass spectra for ⁵⁵Mn obtained by HR-ICP-MS

3.3 Non-spectral interferences

The effects of HCl and Ni contents on the analyte ion peak intensities must be evaluated for the analysis of the high purity nickel samples by HR-ICP-MS. The matrix effect of Cl was negligible [14]. But in the experiment, HCl content in the sample was controlled to be less than 4%. The matrix effects of high content nickel indicated that the signals of the analyte isotopes were held back. Figure 3 describes the nickel matrix effects in the relative ion signals of six elements, such as Mg, Ti, Mn, Ga, Sb and Bi (their concentrations are    1 μg/L) in MRM. As the concentration of nickel increased, the relative ion signal became weak. This study was corrected by the elements as internal standards because different elements have different correcting effects on different ranges of elements. For this reason, we respectively tried the five frequently-used internal standard elements, ⁴⁵Sc, ⁸⁹Y, ¹⁰³Rh, ¹¹⁵In and ²⁰⁴Tl. These elements were frequently used as internal standards for drift correction in mass spectrometric measurements [15]. Through comparing the relatively standard deviations of the results gotten from determining the 16 elements in  2 h six times before and after using the internal standard elements, the proper internal standard elements were decided [16]. As shown in Fig. 4, Sc should be used as the internal standard element to determine Mg, Al, Si, Ti, Cr, Mn, Fe, Co, Cu, Ga and As, Rh as the internal standard element to determine Se, Cd and Sb, and Tl as the internal standard element to determine Pb and Bi.

Fig. 3 Relative intensity of several analyte elements and internal standard elements as function of nickel concentration (1 μg/L mixed solution used)

Fig. 4 Correction of internal standards on determination of trace elements in purity nickel

3.4 Analysis of SRM 671

The detection limit was estimated as the analyte concentration corresponding to three times the standard deviation (3σ) of the blank signals obtained in the replicate measurements (n=11). The detection limits for analyte elements are listed in Table 1.

In order to validate the HR-ICP-MS method, the contents of 16 elements were determined in the SRM 671 sample. Analytical results for the SRM 671 sample together with certified values are presented in Table 2.

Table 2 Analytical results for nickel oxide reference material (SRM 671) determined by HR-ICP-MS

3.5 Analytical applications

The proposed HR-ICP-MS was applied to the determination of the 16 trace elements in two kinds of high purity nickel samples. The analytical results are given in Table 3.

Table 3 Analytical results of samples (μg·g^-1)

4 Conclusions

1) A novel and reliable method for the simultaneous determination of trace elements in high purity nickel using HR-ICP-MS after microwave dissolution is established.

2) The technique demonstrates that HR-ICP-MS is not only a powerful diagnostic tool for determining and resolving spectroscopic interferences, but also a versatile technique for quantitative determination of analytes in low-, medium-, and high-resolution modes. The measured values in good agreement with the certified values indicate that the accuracy is adequate for the determination of these elements.

3) HR-ICP-MS may provide a better alternative for measuring trace elements in high purity nickel because of shorter analytical time and less contamination.

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(Edited by YANG Bing)

Foundation item: Project(21075138) supported by the National Natural Science Foundation of China; Project(cstc2011jjA0780) supported by Natural Science Foundation of Chongqing City, China; Project(KJ121311) supported by Educational Commission of Chongqing City of China

Received date: 2011-07-19; Accepted date: 2011-10-11

Corresponding author: XIE Hua-lin, Professor; Tel: +86-731-88830886; E-mail: hualinxie@vip.163.com