Elemental mass spectrometry - a tool for monitoring trace element contaminants in the marine environment

Introduction

To understand the role and effects of trace elements and their species in marine ecosystems sensitive techniques are necessary to monitor their distribution between different environmental compartments. Since the beginning of industrialisation, anthrophogenic activities such as smelting, energy production, traffic, corrosion processes and landfill, and natural processes such as alteration, leaching or volcanism both influenced the specific distribution of trace elements within the marine environment.

Figure 1: Average matrix composition of 1 kg seawater (values taken from Grasshoff et al., 1999).

Even though the element concentrations in the water phase are relatively low, as indicated in Fig. 1, significantly increased concentrations at higher levels of the food chain can be observed due to biomagnification effects. Especially top predators such as marine mammals are influenced, and different metal related effects on their health status have been recently investigated (Kakuschke et al., 2005). Therefore, precise information on the pattern of trace elements within the ocean as well as their concentration in selected animal species is of great importance to understand the related biological effects.

Figure 2:Overview of the ICP-MS detectable elements within the periodic table.

Figure 3: Environmental samples such as seawater, biological fluids or tissues are complex mixtures. Often they contain a few highly abundant elements, which interfere with the sensitive determination of the remaining less concentrated elements. Different isobaric polyatomic ions are formed in an argon plasma, which interfere with the determination of various elements. The red font indicates interferences due to a sea water matrix.

Challenges

Environmental samples such as seawater, biological fluids or tissues are complex mixtures. Often, they contain a few highly abundant elements (g l^-1 level), which interfere with the sensitive determination of the remaining less concentrated elements (ng l^-1 level) (Fig. 3). Established methodologies often require complex separation schemes to remove the interfering matrix compo- nents. Often, they are prone to errors and contamination, which leads to inaccurate results. Furthermore, most of them do not allow the simultaneous determination of a set of elements.

Figure 4: Schematic view of a collision/reaction cell ICP-MS system used for trace element determination in the marine environment. The collision/reaction cell allows a significant reduction of polyatomic ions, which interfere with the sensitive determination of most elements due to gas phase reactions with hydrogen (H₂) or helium (He).

Methodology

To overcome these limitations, a method based on elemental mass spectrometry, namely the collision/reaction cell inductively coupled plasma mass spectrometry (CC-ICP-MS) has been developed (Fig. 4 and 5). It enables us to quantitatively determine a set of elements within a sample simultaneously (Leonhard et al., 2002). Here, an inductively coupled argon plasma is used to dry and to destroy the sample matrix as well as to generate mainly singly charged element ions, which makes them detectable by mass spectrometry.

Figure 5: Schematic view of the function of a collision/reaction cell. Polyatomic ions are reduced due to their dissociation caused by collisions with the cell gas, while the analyte ions are transferred to the quadrupole mass filter.

As shown in Fig. 2, ICP-MS allows the determination of nearly all relevant elements present in the periodic table with outstanding sensitivity and accuracy (Fig. 6 and 7).

Figure 6: The developed methodology allows the detection of element concentrations at trace levels in environmental samples such as sea water. Instrumental detection limits down to the pg l^-1 have been obtained in three different modes.

Application fields

Vertical profiles in the Baltic Sea Within an intercalibration exercise, the collision/reaction cell ICP-MS method has been compared with an analytical method for trace element determination in seawater, which is based on a complex chemical matrix separation strategy and atomic absorption spectro- scopy (AAS). For ICP-MS measurements the samples were only acidified and diluted ten times with ultra pure water. The results of both methods were in good agreement, which indicates the potential of the developed methodology for the fast and reliable multi-element analysis of seawater samples.

Figure 7: The results obtained from the analysis of a certified sea water reference material (NASS 5) clearly demonstrate the potential of collision/reaction cell ICP-MS for the sensitive, reproducible and accurate multi-element determination of complex samples.

Metal body burdens in seals

Even though the original collision/reaction cell ICP-MS was developed for trace element analysis of marine water samples, it is easily adapted to new tasks such as the determination of metal body burdens of marine mammals. As part of the monitoring of the health status of marine mammals, trace element levels in blood and tissue samples are under investigation, using ICP-MS for a reliable multi-element screening. The concentrations of selected elements were measured in fresh whole blood samples of 80 harbour seals, captured at three different locations of the German and Danish Wadden Sea.

Figure 8: Trace element determination in Baltic Sea water, sampled at the “Gotland Tief”. Method intercalibration between CC-ICP-MS and AAS with chemical matrix separation revealed comparable results.

For essential elements, such as calcium, iron or zink, low variations in the concentration level (12-25%) were observed due to their homeostatic regulation. Also no significant relation with gender, age or locality has been observed, and the levels were in the same order of magnitude as in humans.

In contrast, the level of trace elements shows a much wider variation of 30-287%.

Figure 9: The concentration of selected elements measured in blood samples of 80 seals reveals less variation in the concentration level of essential elements (RSD 12-25 %) due to their homeostatic regulation. Also, no significant relation with gender, age or locality has been observed. In contrast the level of trace elements show a much wider variation of 30-287%. Blood levels of these elements were more directly influenced by dietary sources.

Blood levels of these elements were more directly influenced by dietary sources.

Furthermore, differences between sampling sites in the North Sea have been observed and could be explained by geographical variation of differently contaminated prey. In comparison with other trace elements, especially high arsenic concentrations have been observed (Griesel et al., 2008).

Outlook

These examples show the potential of elemental mass spectrometry for the investigation of trace elements in the marine environment. Beside the amount of an element, also its chemical form (speciation) is of great importance, especially for its toxicity and, accordingly possible effects in the marine environment. CC-ICP-MS can be readily combined with chromatographic separation techniques allowing the investigation of relevant element species such as organotin, mercury or lead compounds.

See also

Wikipedia article on mass spectrometry

References

The main authors of this article are Pröfrock, Daniel, Kakuschke, Antje, Griesel, Simone and Pepelnik, Rudolf Please note that others may also have edited the contents of this article. For other articles by this author see Category:Articles by Pröfrock, Daniel For other articles by this author see Category:Articles by Kakuschke, Antje For other articles by this author see Category:Articles by Griesel, Simone For other articles by this author see Category:Articles by Pepelnik, Rudolf