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Methodology
A Thermo Scientific XSERIES 2 equipped with third generation collision/reaction cell technology (Thermo Fisher Scientific, Bremen, Germany) was configured with an SC2 FAST system (Elemental Scientific Inc). The schematic in Figure 1 outlines the principle of the FAST system that relies on a six-port switching valve to load and inject samples.
Table 2. QC samples.
Geological Sample Analysis
A sample turnaround of approximately 83 seconds, including uptake, analysis and wash, provided an overall batch acquisition time of 12 hours for the entire 500 samples. Five samples (an interference check solution (ICSA), the same solution with a 20 ppb spike (ICSAB), NRC SLRS-3 river water reference material with varying spike concentrations, NIST 1643c reference material and a detection limit check solution (CRI)) were classed as unknown samples and looped continuously throughout the experiment. The mean results for periodic analysis (every 20 unknowns) of the laboratory control standards (LCS) are shown in Table 3.
Figure 1: Schematic of FAST system.
This system significantly reduces sample uptake and washout times and dramatically decreases matrix load that reaches the plasma. The XSERIES 2 instrumental parameters are shown in Table 1. A universal H2 in He gas mixture was used in the collision/reaction cell for the suppression of interferences.
Table 1. System parameters. Table 3. LCS results.
Rock samples weighing 0.5 g were digested using an Aqua Regia mix (12.5ml 30%) and diluted a further 10 times prior to analysis. A sequence of 478 samples, including a calibration and integrated QC, was performed for 30 analytes. A sample turnaround, including uptake, analysis and wash, of approximately 80 seconds resulted in an overall acquisition time of 11 hours for the 478 samples. Following an SOP similar to the CLP method, a well characterised rock was used as a laboratory control standard (LCS) in the QC protocol, along with continuing calibration verification (CCV) and a continuing calibration blank (CCB). Five samples (an ICSA, the same solution with a 20 ppb spike, a GEOMS rock sample, GEOMS spiked with varying analyte concentrations and a CRI) were classed as unknown samples and looped continuously throughout the experiment. Results for the determination of analytes in the unknown samples over the 11-hour period are shown in Figure 4.
Figure 4. Measured values in samples over 11h period; top panel – major elements in ICSA, middle panel – GEOMS sample, bottom panel – CRI low concentration standard check.
Environmental Sample Analysis
The experimental methodology performed to test the high throughput setup for environmental samples was based on conditions outlined in the Multi-Media, Multi- Concentration, Inorganic Analytical Service for Superfund Method ILM05.4. This method was introduced by the US Environmental Protection Agency (EPA) within the Contract Laboratory Program (CLP) for water and soil/sediment environmental analysis.
To improve the determination of analytes with a high ionisation potential (such as Be, As and Se) a stream of methane was added to the nebuliser gas. This carbon- loading enhances sensitivity for these analytes (Figure 2) and improves long term stability by the reduction of matrix deposition on the ICP-MS interface.
A sequence of 500 samples including a calibration and integrated QC (Table 2) was performed for 23 analytes.
Figure 3. Measured values in samples over 12h period; top panel: SLRS-3 + spikes, bottom panel NIST 1643c.
DISCUSSION
The stability of the results for samples SLRS-3 + spike and NIST 1643c are shown in Figure 3.
When used with a universal gas mixture for all analytes, CRC-based ICP-MS has been demonstrated to be a multielemental technique with high throughput capabilities. With methane addition, the analysis can benefit from an increase in sensitivity for analytes with a higher ionisation potential such as Be and Se which are often at lower concentrations in environmental samples, as well as benefiting from an improvement in long term stability. Throughput and stability are further improved and necessary maintenance is reduced thanks to the configured FAST system which cuts uptake and washout time and introduces less matrix into the plasma over time.
CONCLUSION
CRC-based ICP-MS has cemented its place in routine environmental laboratories due to its wide elemental coverage, high sensitivity and rapid analysis. Using CRC- based ICP-MS, elements can be determined to DWI requirements, following NS30 analytical guidelines, using a single analytical technique. CRC-based ICP-MS is a proven multi-elemental technique capable of performing fast and accurate analyses of an extended range of common environmental and geological sample matrices. Through the use of a segmented flow sample introduction system, matrix deposition on the ICP-MS interface is significantly reduced, leading to increased first time pass QC analyses and longer periods between necessary maintenance. Spectral interferences associated with the traditional ICP- MS method are considerably reduced and sample throughput is increased.
Figure 2: Comparison of sensitivity for Be and As with and without methane addition. Spotlight
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