MP-AES Determination of Gold and Precious Metals

Topics Covered

Introduction
Instrumentation and Sample Preparation
Method Detection Limits (MDLs)
Linear Range and Sample Volume
Accuracy of 4100 MP-AES
Recovery of Unknown Samples
Conclusion
About Agilent Technologies

Introduction

Accurately determining gold and other precious metals (PMs) is crucial for companies producing PMs, and the contract or service labs supporting them. Although a wide selection of elemental analytical methods are available, the selection of an appropriate technique will rely on several factors, such as the availability of the technology, the skill of the operator, cost of analysis, sample type, sample throughput prerequisites, and the objective of the analytical result.

The Agilent 4100 Microwave Plasma-Atomic Emission Spectrometer (MP- AES) (Figure 1) is an affordable, highly automated instrument that is ideally suited for the trace analysis of PMs in specimens typical in mining and exploration activities. The use of flammable gases, such as acetylene, is avoided because MP-AES depends on the production of microwave plasma with nitrogen. This improves safety and lowers operating costs in the lab.

Figure 1. Agilent 4100 MP-AES

Nitrogen supply is from either bottled gas or the Agilent 4107 Nitrogen Generator. This helps reduce the complexity and costs associated with sourcing of gases like acetylene, particularly in remote locations. This article discusses the application of the Agilent 4100 MP-AES to analyze PM samples prepared by fire assay.

Instrumentation and Sample Preparation

Being a fast, sequential multielement analytical instrument, the Agilent 4100 MP-AES has an innovative Microwave Excitation Assembly for the creation of a concentrated axial magnetic field around a traditional torch. This focuses and comprises the microwave energy where it is required, to generate a toroidal plasma with a cooler central channel, which is appropriate for the stable introduction of liquid samples by means of a traditional sample introduction system.

Fire assay was used to prepare a set of samples that are typically analyzed by flame AA. The process began with heating a 30g rock sample with flux to over 1,000°C, yielding a small silver sphere. This was then dissolved in 4mL of 25% aqua regia. The operating conditions of the 4100 MP-AES were then optimized (Table 1)

Table 1. Agilent 4100 MP-AES operating parameters

Instrument parameter Setting
Analytes (wavelength) Au (267.595), Pt (265.945), Pd (363.470)
Nebulizer pressure 140–240 kPa
Read time 3 s
No. of replicates 3
Sample uptake delay 10 s
Stabilization time 5 s
Background correction Auto

Method Detection Limits (MDLs)

MDLs for palladium, platinum and gold were identified through the measurement of two sets of ten method blanks twice, on non-consecutive days, applying the conditions prescribed in the analytical method. The MDL was estimated as the 3σ standard deviation of the 20 concentration results.

Linear Range and Sample Volume

The concentration or working range of an analytical method is the range of concentrations that are accurately measureable without recalibration or dilute the sample. The 4100 MP-AES was used to analyze the linear range for palladium, platinum, and gold. The calibration graphs obtained are illustrated in Figures 2, 3, and 4, revealing that the linearity for all three analytes was acceptable up to 120 mg/L concentration. This exceeds the prerequisites of the application.

Figure 2. Calibration graph for Au at the 267.595nm wavelength

Figure 3. Calibration graph for Pt at the 265.945nm wavelength

Figure 4. Calibration graph for Pd at the 363.469nm wavelength

A PM analysis is sensitive to sample volume. The volume of sample consumed during analysis was 1.8mL and the method cycle time (sample to sample) was 55s when an Agilent SPS 3 Sample Preparation System was used in conjunction with the 4100 MP-AES.

Accuracy of 4100 MP-AES

A set of Certified Reference Materials (CRMs) was examined to validate the potential of the 4100 MP-AES in the analysis of PMs at different concentrations. The results, summarized in Table 2, reveal that the 4100 MP-AES measured results are in good agreement with the certified values.

Table 2. Results for Au, Pt and Pd obtained using the 4100 MP-AES compared to certified reference values

Gold CRM certified value (mg/L) MP-AES result (mg/L)
CRM 1 19.8 19.3
CRM 2 7.9 7.4
CRM 3 23.1 22.7
CRM 4 5.6 5.7
CRM 5 57.8 55.3
CRM 6 3.1 3.3
CRM 7 35.9 35.4
CRM 8 8.4 8.9
Platinum CRM certified value (mg/L) MP-AES result (mg/L)
CRM 6 0.74 0.75
CRM 7 35.6 35.9
CRM 8 9.0 9.5
Palladium CRM certified value (mg/L) MP-AES result (mg/L)
CRM 6 3.21 3.4
CRM 7 44.4 44.0
CRM 8 35.0 36.5

Recovery of Unknown Samples

The 4100 MP-AES was used to analyze a set of unknown samples for gold content and the results were then compared with results of the conventional flame AA analysis (Table 3).

Table 3. Results for gold in unknown samples, comparing the 4100 MP-AES with flame AA

Sample MP-AES result (mg/L) Flame AA result (mg/L) Agreement with AA result (%)
1 0.09 0.09 100
2 0.85 0.84 101
3 5.3 5.1 104
4 13.7 14.4 95
5 20.8 21.8 95
6 4.3 4.1 105
7 1.0 1.0 100

Figure 5 shows the characteristic spectrum of a sample consisting of roughly 40ppm of gold, showing the lack of spectral interferences due to unprecedented signal to noise ratio with the flat baseline and the narrow emission peak.

Figure 5. A typical PM sample spectrum for Au at the 267.595 nm wavelength

Conclusion

The results clearly demonstrate the improved speed of analysis, an increased linear dynamic range, and superior sensitivity of the 4100 MP-AES over conventional analysis techniques like flame AA in the analysis of PMs. The 4100 MP-AES also exhibits unprecedented accuracy and has very low operating costs, owing to reduced gas costs. The use of non-combustible nitrogen enables the 4100 MP-AES to offer safer, multi-element, unattended overnight operation. The 4100 MP-AES can be installed at remote locations, thus facilitating laboratories to perform on-site sample analysis in a safer environment through the avoidance of the use of flammable gases, such as acetylene.

About Agilent Technologies

As the world's premier measurement company, Agilent is committed to providing innovative measurement solutions that enable our customers and partners-- the leaders in their fields -- to deliver products and services that make a measurable difference in the lives of people everywhere. With a singular focus on measurement, Agilent helps:

  • Advance next-generation wireless communications
  • Aid the military to become more flexible, mobile and reliable
  • Enable non-destructive subsurface electronic testing of semiconductor materials
  • Analyze the causes and cures for disease
  • Make the world more safe and secure from crime and drugs
  • Aid in the discovery and quality of medicines
  • Keep our air, water, soil and food clean and safe

This information has been sourced, reviewed and adapted from materials provided by Agilent Technologies.

For more information on this source, please visit Agilent Technologies.

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