Overcoming the Challenges Associated with the Direct Analysis of Trace Metals in Seawater Using ICP-MS

Nov 01, 2011

The direct analysis of seawater by inductively coupled plasma–mass spectrometry (ICP-MS) is notoriously difficult because of the high matrix content of the sample that leads to both spectroscopic (for example, polyatomic interferences) and nonspectroscopic interferences (for example, signal suppression). Additionally, the low target concentration levels demand a noncontaminating, robust sample introduction technique. The latest ICP-MS techniques provide high-throughput methods that are able to process large numbers of samples presented for analysis.

The oceans cover 71% of the planet and have a combined volume of 1.4 × 1021 L (3.3 × 108 cubic miles). Any attempt to understand the chemical inputs to the planet's ecology must include a close look at seawater. Toxic substances and high concentrations of certain trace elements may present a health hazard for irrigation, swimming, fishing, boating, and industrial uses. These conditions also can affect wildlife, which use the water for drinking or a habitat.

The analysis of trace elements in seawater is one of the most challenging analytical tasks in the field of environmental monitoring, mainly because of matrix interferences from its high dissolved salt content. In recent years, methods of adequate sensitivity became available for true ultratrace-metal determinations in drinking water. However, in the case of seawater, only now has it become possible to both resolve the complex mixtures and achieve the very low detection limits required.

The most commonly employed method of analysis for trace metals in seawater is inductively coupled plasma–mass spectrometry (ICP-MS). Although the technique is very powerful by virtue of its sensitivity and selectivity, it is limited in terms of the amount of dissolved solids that can be introduced. When aspirating samples with high levels of dissolved solids, salt may deposit on the interface (cones) during the analytical run, degrading signal stability. Additionally, high levels of dissolved salts can cause spectral (polyatomic) interferences. Because these interferences originate from the seawater matrix itself, standard ICP-MS for seawater analysis can often lead to false positive results that are much higher than the actual concentrations in the solution.

To address these analytical challenges, a range of different sample preparation and introduction techniques have been coupled with ICP-MS. This article demonstrates the contamination-free, fully automated, direct, trace-metal analysis of seawater using a specialized sample introduction system in conjunction with ICP-MS.

Methods


Figure 1: Schematic of the sample introduction system.
The results in this study were obtained using an Elemental Scientific Inc. (Omaha, Nebraska) PC3 Fast sample introduction system with a Thermo Scientific (Bremen, Germany) XSERIES 2 ICP-MS system. The ICP-MS system was configured with the sample introduction system in combination with an autosampler. The schematic in Figure 1 outlines the principle of the sample introduction system, which utilizes a vacuum pump to load samples onto a PFA loop mounted across a six-port switching valve. In addition to providing direct, routine trace-metal analysis of high matrix samples including seawater, the system ensures high sample throughput and minimizes sample deposition on the ICP-MS interface for increased long-term stability. The system also ensures minimal contamination as a result of automated sample manipulation and short sample pathway consisting of high-purity components.

Experimental

The ICP-MS instrument parameters are outlined below.
Nebulizer: ESI PFA-ST nebulizer
Spray chamber: Quartz cyclonic spray chamber
Injector: Demountable torch with a 2.5-mm diameter
Interface: Ni Xs high sensitivity cone
Additional gas: 2% methane in argon at 100 mL/min
Nebulizer gas: 0.93 L/min
Forward power: 1500 W
Collision cell: 4.0 mL/min 7% hydrogen in helium at 2 V (kinetic energy discrimination [KED] mode)