Naoki Sugiyama
Agilent Technologies, Japan

Keywords

Platinum Group Elements, gold, silver, ore exploration, geochemical, environmental, catalytic converter, ammonia

Introduction

The precise determination of the noble metals, comprising the Platinum Group Elements (PGEs: Ru, Rh, Pd, Os, Ir and Pt), Au and Ag, is of great interest in areas such as ore exploration and geochemical studies, and these metals are increasingly used for industrial applications including advanced materials and alloys, medical devices, and catalysts for pharmaceutical manufacturing. Environmental monitoring is also required as some of these elements are used in automobile catalytic converters. ICP-MS is widely used for these applications due to its high sensitivity and multi-element capability. However, the analysis is challenging because the metal concentrations are often low and they are subject to severe spectral overlaps. 

Table 1 summarizes the interferences and abundance (%) of each isotope of the elements (the isotopes highlighted in yellow represent the recommended isotope for determination by ICP-MS). Several methods have been developed to resolve the interferences, such as mathematical correction, matrix removal and high-resolution magnetic sector (HR-)ICP-MS. However the mass resolution required to separate some of the interferences is beyond the capability of current commercial HR‑ICP‑MS. For example separation of ¹⁰³Rh⁺ from ⁸⁷Sr¹⁶O⁺, ¹⁰⁵Pd⁺ from ⁸⁹Y¹⁶O⁺, and ¹⁰⁹Ag⁺ from ⁹³Nb¹⁶O⁺ requires mass resolution (M/ΔM) of 102900, 27600 and 31500, respectively; commercial HR-ICP-MS instruments are limited to a maximum resolution of 10,000. To remove the multiple, complex interferences on noble elements, the Agilent 8800 ICP-QQQ was used in MS/MS mode, using ammonia as the reaction gas.

Experimental

Instrumentation: Agilent 8800 #100.

Plasma conditions: Preset plasma/Low matrix.

Ion lens tune: Soft extraction tune: Extract 1 = -3 V, Extract 2 = -200 V.

CRC conditions: NH₃ (10% NH₃ in He) was used as CRC gas in MS/MS mode. 

Following a preliminary optimization study, three different NH₃ gas flow rates (low (L), medium (M) and high (H)) were used. Cell conditions are given in Table 2. No gas mode was also applied for comparison purposes.

Method

The BECs of the noble metals were determined in a series of synthetic-matrix samples, using an external calibration method. Indium (In) internal standard (ISTD) was mixed online with the sample via the standard ISTD mixing T-connector. An integration time of 1 s per isotope was used with 3 replicates (7 replicates for blank).

Samples and sample preparation

Standards and matrix samples were prepared from single element stock solutions purchased from Kanto Chemical Co., Inc. (Saitama, Japan) and a REE mixture standard, XSTC-1 purchased from Spex certiPrep. All solutions were diluted into a final acid mix of 1% HNO₃ and
3% HCl.

Results

Matrix interference study

Tables 3 and 4 summarize the results of the spectral interference study obtained by analyzing individual synthetic matrix blank solutions. Table 3 shows the observed interferences, expressed as BEC (ppb), in each matrix blank measured using no gas mode. As expected from Table 1, the synthetic matrices caused significantly elevated BECs (>> 1 ppb) on all the primary and secondary isotopes of all the analytes except for Ru; Rh suffered a relatively minor increase in BEC of ~0.5 ppb in the 10 ppm Pb/1 ppm Hg matrix. 

Table 4 shows the results obtained using NH₃ reaction mode. The optimum gas flow rate for NH₃ for each element was investigated and three gas flow rates
(Low: 2.0, Medium: 3.0, and
High: 5.0 mL/min) were used. The best isotope and method is highlighted in bold in the Table. It can clearly be seen that NH₃ reaction mode effectively removes the interferences on all the analytes, giving BECs of << 0.1 ppb for the preferred isotope/cell mode in all the matrices. The mechanism for the removal of each interference using the MS/MS capability of the 8800 ICP-QQQ is as follows: 

• Ru: slight interferences from Zn and Mo were resolved using on-mass method with NH₃-M.

• Rh: Pb⁺⁺ interference was resolved using on-mass method with NH₃-M.

• Pd: significant interferences from SrOH⁺ and YO⁺ were seen on ¹⁰⁵Pd, the only isotope free from atomic isobar. On-mass method with NH₃-H removed the interferences.

• Ag: significant ZrO⁺ interference on both ¹⁰⁷Ag and ¹⁰⁹Ag was resolved using on-mass method with NH₃-H.

• Os: YbO⁺ interference was observed on both ¹⁸⁸Os⁺ and ¹⁸⁹Os⁺. Since Os⁺ sensitivity in NH₃ mode is low, but Os⁺ forms a product ion of OsNH⁺, NH₃-L with mass-shift gave the best result.

• Ir: LuO⁺ and HfO⁺ interfere with ¹⁹¹Ir⁺ and ¹⁹³Ir⁺ respectively. NH₃-M with mass-shift method worked for ¹⁹¹Ir⁺ as Ir⁺ forms a product ion of IrNH⁺. 

• Pt: ¹⁹⁵Pt⁺ suffers a significant interference from HfO⁺. While the overlap is less significant on ¹⁹⁸Pt⁺, ¹⁹⁸Pt⁺ suffers an atomic isobar interference from ¹⁹⁸Hg⁺. However Hg⁺ is effectively neutralized by NH₃ so ¹⁹⁸Pt⁺ can be measured free from interference. 

• Au: significant interferences by TaO⁺ and HfOH⁺ are resolved by mass-shift method with NH₃-M. Au⁺ forms a product ion of Au(NH₃)₂⁺.

Analysis of complex synthetic matrix sample using optimized NH₃ reaction mode

A complex synthetic matrix sample containing 10 ppm each of Cu, Zn, Sr, Rb, Ni, Mo, Pb, Zr, Nb, REEs, Ta, Hf, W and
1 ppm Hg was prepared, and this matrix was spiked with 1 ppb each of Ru, Rh, Pd, Ag, Os, Ir, Pt and Au as analytes. The concentration of the noble metals was determined in two modes: No gas mode and NH₃ reaction cell mode, and the spike recovery results are displayed in Figure 1 for each mode. The results demonstrate that MS/MS mode with NH₃ reaction cell gas successfully removes multiple interferences on all the noble metals, providing accurate results for these analytes even in a complex and challenging matrix.