2025 Volume 65 Issue 4 Pages 576-580
This report describes an improved application for quantifying trace nickel contents, which are affected by interference from co-existing chromium, cobalt, and molybdenum as matrix constituents. Numerous spectral interferences of atomic and ionic emission lines excited by argon inductively coupled plasma lead to poor application of trace nickel contents in cobalt–chromium–molybdenum alloy and high alloy steel. Reliable and non-skilled application was achieved using nitrogen microwave-induced plasma, which decreased ionic emissions, leading to negligible chemical interference. An excellent lower limit of quantification (0.0005 mass fraction % nickel in 0.20 g sample) was estimated using atomic spectrometric measurements based on the nitrogen microwave-induced plasma and no additional preconcentration.
Trace contaminants in biomedical materials such as stainless steel and cobalt–chromium–molybdenum (CCM) alloy,1,2,3,4) which consists of 26.5–30 mass fraction % chromium, 4.5–7 mass fraction % molybdenum, < 1 mass fraction % nickel, and the balance cobalt,5) must be controlled strictly. As one example, nickel-free (< 0.01 mass fraction %3,4)) CCM alloys have been developed to avoid nickel biotoxicity. Such contaminants must be quantified for quality control reliably and accurately during manufacturing. Nevertheless, no Japanese Industrial Standards (JIS) chemical analysis of cobalt alloy exists. Analysts must assay trace nickel content in a CCM sample5) while following steel analytical procedures.6,7,8) These procedures limit co-existing constituents in a sample as follows: (1) ≤ 0.2 mass fraction % cobalt, ≤ 3 mass fraction % chromium, and ≤ 1.2 mass fraction % molybdenum;6) (2) ≤ 20 mass fraction % cobalt, ≤ 35 mass fraction % chromium, and ≤ 10 mass fraction % molybdenum;7) and (3) ≤ 1 mass fraction% cobalt, ≤ 35 mass fraction % chromium, and ≤ 3 mass fraction % molybdenum.8) Furthermore, the lower limits of quantification were 0.01 mass fraction % nickel6,7) and 0.02 mass fraction % nickel.8) These methods are insufficient to assure a nickel-free CCM product. Therefore, an alternative analytical procedure with a better lower limit of quantification (at least 0.001 mass fraction %) must be developed. Additionally, the use of the developed procedure for the simultaneous cobalt-alloy and steel analyses is preferred.
Inductively coupled plasma atomic emission spectrometry (ICP-AES) is used frequently to quantify metallic elements in various alloy samples. Generally speaking, chemical interference can be overcome because of the ca. 10000 K9) temperature of argon ICP. Argon ICP generates enormous atomic and ionic emission lines of co-existing major constituents efficiently. However, because of severe spectral interference, ICP-AES application is not preferred for quantification of trace nickel contents in CCM alloy.10,11) Such ICP-AES application requires tedious and time-consuming separation pretreatment such as solvent extraction.12)
Actually, another chemical analysis can be conducted for this pourpose using atomic absorption spectrometry (AAS) for this purpose.13) In general, spectral interference appears only rarely because of confined atomic absorption. Although high contents of matrix constituents can be introduced to a robust acetylene flame, severe chemical interference is expected to be unavoidable because of the 2600–3000 K14) flame temperature, which is lower than that of argon ICP. For steel analysis, the ICP-AES application requires matrix matching of iron as a major constituent to overcome physical interference of different nebulization efficiency related to solution viscosity.6,7,8) By contrast, AAS application might require additional matrix matching of chromium and nickel contents for chemical interference during stainless steel analysis.15) Although inductively coupled plasma mass spectrometry (ICP-MS) is also applied for trace content analysis, it often necessitates minimization of co-existing matrix constituents because of the effects of element separation on an ion optics path behind ICP.
Aside from AAS, ICP-AES, and ICP-MS, this work used the application of microwave-induced plasma atomic emission spectrometry (MIP-AES). Nitrogen MIP with ca. 6000 K16) of intermediate temperature between the acetylene flame and argon ICP leads to minor excitation of ionic emission lines and short wavelength atomic emission lines. For this work, simultaneous quantification of nickel in both cobalt-based alloy and steel samples was achieved using MIP-AES.
An MIP-AES spectrometer (4210 MP-AES; Agilent Technologies Inc., Santa Clara, CA, USA) was used to quantify nickel in several samples of cobalt alloy and steel. This spectrometer comprises a Czerny–Turner monochromator, which was capable of sequential and axial measurement of emission lines of 178 nm to 780 nm wavelengths. Analytical conditions of the spectrometer are presented in Table 1. The following spectrometers were also used for this work: contrAA 800 D (Analytik Jena GmbH, Jena, Germany) for flame AAS, ARCOS FHM22 MV130 (Spectro Analytical Instruments GmbH, Kleve, Germany) for ICP-AES, and iCAP TQ (Thermo Fisher Scientific Inc., Waltham, MA, USA) for ICP-MS. Microwave ovens, produced as TOPwave (Analytik Jena GmbH, Jena, Germany) and ETHOS UP (Milestone Srl, Sorisole, Italy), were used to digest cobalt alloy and steel samples.
| Microwave frequency of generator | 2.45 GHz | |
| Power of generator | 1.0 kW | |
| Flow rate of nitrogen gas | Coolant gas | 20 dm3 min−1 |
| Auxiliary gas | 1.5 dm3 min−1 | |
| Nebulizer gas | 0.60 dm3 min−1 | |
| Nebulizer | Ethylene tetrafluoroethylene concentric type | |
| Spray chamber | Polyethylene terephthalate double-pass cyclonic type | |
| Peristaltic pump rate | 0.2 cm3 min−1 | |
| Accumulation time | 3 s, triplicate |
The following acids were used to digest samples and pure metals: 35 mass fraction % hydrochloric acid, 50 mass fraction % hydrofluoric acid, 70 mass fraction % nitric acid, and 96 mass fraction % sulfuric acid. The following materials were used for adding analyte and matrix constituents to prepare synthetic and calibrating solutions: pure chromium, cobalt chloride hexahydrate, pure iron, pure nickel, and pure molybdenum.
Validation analyses were conducted using certified reference materials (CRMs): European Certified Reference Material of ECRM 378-1 cobalt alloy (Centre technique des industries de la fonderie, Sèvres, France) and Japanese Iron and Steel Certified Reference Material (The Japanese Iron and Steel Federation, Tokyo, Japan) of JSS 610-11 high-speed steel and JSS 650-15 ferritic stainless steel. Commercially available cobalt–chromium–molybdenum alloy powder was purchased and assayed.
Each sample (0.2 g) was placed in a 100 cm3 polytetrafluoroethylene digestion vessel. For cobalt-based alloy and ferritic stainless steel samples, after an acid mixture (10 cm3) of 2.7 mol dm−3 hydrofluoric acid and 9.0 mol dm−3 sulfuric acid was poured into digestion vessels, the vessels were set in a microwave oven. The oven temperature program was set as follows: increased the temperature to 433 K in 15 min and maintained temperature for 90 min. After cooling the vessel to a temperature lower than 313 K, 7.8 mol dm−3 nitric acid (1 cm3) was added to the vessel. The resulting acid solution was placed in a 50-cm3 polypropylene volumetric flask.
A high-speed steel sample was not digestible using the procedure described above. Another acid mixture (2 cm3) of 13.5 mol dm−3 hydrofluoric acid and 3.9 mol dm−3 nitric acid was poured into the digestion vessel to decompose the iron matrix of high-speed steel.17) Then, 9.0 mol dm−3 sulfuric acid (10 cm3) was added to the digestion vessel for decomposition of the residue materials.18) This vessel was treated in the same manner as that applied for microwave digestion of the CCM alloy and stainless steel.
Synthetic solutions containing 2.5 mg cm−3 cobalt, 1.2 mg cm−3 chromium, 0.25 mg cm−3 molybdenum, and/or 0.002 mg cm−3 of nickel were prepared. This sample solution comprised 63 mass fraction % cobalt, 30 mass fraction % chromium, 6 mass fraction % molybdenum, and 0.05 mass fraction % nickel. The following solutions containing three constituents of four kinds were prepared to obtain emission spectra (Fig. 1) and atomic absorption spectra (Fig. 2): cobalt-less, chromium-less, molybdenum-less, and nickel-less.
, cobalt-less; 
, chromium-less; 
, molybdenum-less; 
, nickel-less; and ▲, distilled water. (Online version in color.)
, cobalt-less; , chromium-less; , molybdenum-less; , nickel-less; and ▲, distilled water. (Online version in color.)Lower limits of quantification (LLQ) related to 10-times standard deviation of blank measurements using nickel calibrating standard solutions with and without CCM matrix matching were evaluated for MIP-AES and ICP-AES, as presented in Table 2. An appropriate sensitive emission line of Ni I 352.454 nm (Fig. 3), which was unusable for ICP-AES because of its spectral interference (Fig. 1), was applicable to the present MIP-AES quantification. The present analyses adopted the Ni I 300.249 nm emission line for ICP-AES and Ni I 352.454 nm for MIP-AES. However, ICP-AES using Ni I 300.249 nm was inapplicable to assay steel samples because of spectral interference caused by the Fe II 300.264 nm emission line (Fig. 4(b)).
| MIP-AES | ICP-AES | |||
|---|---|---|---|---|
| Emission line | No matrix added | Matrix matched | No matrix added | Matrix matched |
| Ni II 231.604 nm* | 3.5 | ― | 0.003 | ― (cobalt interference) |
| Ni I 232.003 nm** | 0.10 | 0.17 | 0.011 | ― (chromium interference) |
| Ni I 300.249 nm | 0.006 | 0.084 | 0.008 | 0.4 |
| Ni I 352.454 nm | 0.016 | 0.019 | 0.005 | ― (molybdenum interference) |
The MIP-AES calibration curves using cobalt-matrix matching and iron-matrix matching mutually agreed at 95% confidence level. Additionally, influences of co-existing matrix constituents were evaluated using synthetic solutions containing various compositions of cobalt, chromium, iron, and molybdenum, as presented in Fig. 4(a). All emission intensities of Ni I 352.454 nm agreed with the intensity of the 64Co–30Cr–6Mo (i.e., 64 mass fraction % cobalt, 30 mass fraction % chromium, and 6 mass fraction % molybdenum) solution at the 95% confidence level. Therefore, the present MIP-AES analysis limited co-existing constituents in a sample as 0–100 mass fraction % cobalt, ≤ 50 mass fraction % chromium, 0–100 mass fraction % iron, and ≤ 20 mass fraction % molybdenum.
Table 3 presents calibration methods used for the present analyses. The LLQ of MIP-AES was 0.0005 mass fraction % nickel in a 0.2 g sample. Prepared sulfuric-acid specimen solutions might not be preferred for acetylene–air flame AAS applications because of spectral interference caused by sulfur oxide molecular absorption (Fig. 2(a)). Therefore, acetylene – nitrous oxide flame AAS was adopted for this quantification.
| Method | Analytical line | Matrix matching of calibrating standards/mg cm−3 | Calibration method | Lower limit of quantification/mass fraction % |
|---|---|---|---|---|
| MIP-AES | Ni I 352.454 nm | Co (4.0) or Fe (4.0) | Simple linear calibration | 0.0005 |
| ICP-AES | Ni I 300.249 nm | Co (2.5) and Cr (1.2) | Simple linear calibration | 0.01 |
| C2H2–N2O flame AAS | Ni I 232.003 nm | Co (2.5) and Cr (1.2) | Fe I 232.036 nm internal standard method | 0.004 |
| ICP-MS* | 60Ni+ | Co (0.1) or Fe (0.1) | 45Sc+ internal standard method | 0.0003 |
Trace nickel contents of different matrix (cobalt or iron) samples were quantified simultaneously using MIP-AES (Table 4). Analytical values of nickel in cobalt-based alloy and steel CRMs agreed with each certified value at a 95% confidence level. Analytical values of nickel in commercially available CCM alloy, which consisted of 63 mass fraction % cobalt, 28 mass fraction % chromium, and 6 mass fraction % molybdenum, agreed with results obtained using MIP-AES, AAS, and ICP-MS. The MIP-AES gave the accurate analytical values presented in Table 4 using simple linear calibration method. By contrast, internal standard methods for AAS19,20) and ICP-MS21) were necessary to obtain accurate results.
| Method | ECRM 378-1 of cobalt alloy | Commercially available Co–Cr–Mo alloy | JSS 610-11 of high-speed steel | JSS 650-15 of ferritic stainless steel | |
|---|---|---|---|---|---|
| Analytical value | MIP-AES | 0.610 ± 0.019 | 0.0076 ± 0.0007 | 0.0475 ± 0.0025 | 0.224 ± 0.014 |
| ICP-AES | 0.615 ± 0.004 | < 0.01 | ― | ― | |
| C2H2–N2O flame AAS | 0.666 ± 0.021 | 0.0085 ± 0.0008 | ― | ― | |
| C2H2–N2O flame AAS using ISM* | 0.606 ± 0.005 | 0.0082 ± 0.0006 | ― | ― | |
| ICP-MS | ― | 0.0064 ± 0.0001 | 0.0399 ± 0.0010 | ― | |
| ICP-MS using ISM | ― | 0.0069 ± 0.0002 | 0.0451 ± 0.0006 | ― | |
| Certified value | 0.617 | ― | 0.0463 | 0.215 | |
| Uncertainty | 0.011 | ― | 0.0016 | 0.002 |
However, the precision of MIP-AES was inferior to these of AAS and ICP-MS based on F test at 95% confidence level for the ECRM 378-1 results between MIP-AES and acetylene – nitrous oxide flame AAS using the iron internal standard method and the JSS 610-11 results between MIP-AES and ICP-MS using the scandium internal standard method. A palladium internal standard method18) was tested for the present MIP-AES measurements to improve their precision. However, the MIP-AES calibration curves using cobalt-matrix matching and iron-matrix matching did not mutually agree at a 95% confidence level. The sequential MIP-AES instrument of this work cannot measure emission lines of an analyte and an internal standard element simultaneously. Therefore, the palladium internal standard method might yield such an inappropriate result.
Microwave-induced nitrogen plasma atomic emission spectrometry was demonstrated as an appropriate method for robust quantification of nickel in cobalt-based alloy and steel samples, which are vulnerable to spectral and chemical interference caused by cobalt, chromium, and molybdenum in a sample-digested specimen solution. An excellent lower limit of quantification (0.0005 mass fraction %) was estimated using this nitrogen plasma, which requires no preconcentration method.
The author declares no conflicts of interest regarding this manuscript.
