Determination of Mercury Species Using ICP/MS Techniques

Holger Hintelmann, Trent University, Department of Chemistry, Peterborough, ON K9J 7B8, Canada. email: hhintelmann@trentu.ca

ABSTRACT

This paper presents an overview of different analytical methods using ICP/MS as a detector to determine mercury species in environmental samples. Total mercury is measured using either cold vapor flow injection analysis or cold vapor gold amalgamation preconcentration. Both techniques have absolute detection limits of approximately 1 pg of Mercury per isotope. Methylmercury is determined after GC separation on-line by ICP/MS. These techniques are used to carry out isotope dilution analyses and stable isotope tracing experiments. For such measurements, the achievable isotope ratio precision is of critical importance. The RSD for isotope ratio measurements was in the range of 0.5- 2 %, depending on the individual method used.

INTRODUCTION

In the last decade investigations of mercury in natural waters have established that concentrations of mercury species are in the range of 0.05 – 10 ng/L (Mierle, 1990). Levels commonly observed in pristine air are in the 1 – 2 ng/m³ range (Lindberg et al., 1995). Clearly, sensitive techniques avoiding any memory effect or carry over problems are required to analyze mercury at these low concentrations. Mercury and its species are usually very “sticky” and a number of problems were reported in the past, especially when conventional sample introduction systems in combination with ICP detection were employed (Shum et al., 1992; Bushee, 1988). On the other hand, it is very desirable to take advantage of the unique capabilities of the ICP-MS detector. Only this technique allows the measurement of individual isotopes of mercury with sufficient sensitivity, which is essential if one wants to apply enriched mercury isotopes in any kind of stable isotope tracing experiment or carry out isotope dilution analyses (Hintelmann and Evans, 1997). Our research team has developed over the past years several methods using ICP/MS as a detector to accomplish this goal. Different approaches to achieve the ultra low levels of detection necessary for determining mercury species in pristine environments will be presented.

MATERIALS AND METHODS

Isotope enriched HgO was obtained from Trace Sciences International (Richmond Hill, ON, Canada) and dissolved in acid. Isotope enriched CH₃HgCl was prepared from inorganic mercury using methylcobalamin (Hintelmann, 1999).

ICP/MS analyses were performed using either a PE/SCIEX Elan 5000 or Elan 6000 instrument. A FIA 400 from PE was used for the flow injection analysis. Methylmercury was isolated from the sample matrix using distillation (Horvat, 1993) and determined after aqueous phase ethylation using NaBEt4, purge and trap preconcentration on TENAX and gas chromatographic separation.

MMHg was measured by adjusting subsamples of the final aqueous solutions to pH 4.9 using a sodium acetate buffer (2 M). 50 µL of sodium tetraethylborate (1 %, w/v) were added and after 20 min reaction time, volatile Hg species were purged onto Tenax (20/35 Mesh). Analytes were thermodesorbed onto an packed GC column (40 cm, 4 mm ID, packed with 15 % OV3 on Chromosorb WAW-DMSC, 80/100 Mesh) coupled to the ICP/MS.

RESULTS AND DISCUSSION

Instrument tuning

Introducing a continuous stream of gaseous mercury into the nebulizer gas stream using a peristaltic pump was used to tune the ICP/MS instrument. Instrumental operating parameters like nebulizer gas flow rate, forward power, ion lens settings were optimized using the set-up as illustrated in Figure 1.

Figure 1: Set-up for tuning the ICP/MS using a dry plasma for mercury determinations

Figure 3: Signal response as a function of nebulizer flow, forward power and ion lens voltage

A regular concern is the memory effect associated with the introduction of mercury into an ICP-MS instrument. Long wash out times are commonly reported. Figure 3 demonstrates the memory we observed after shutting of the feed of elemental mercury into the nebulizer gas stream. Instantaneously, the signal dropped and after 30 sec, the signal decreased to less than 2% of its original value.

Figure 3: Wash out time for elemental mercury in a dry plasma

Sample introduction

Total mercury was measured using two different cold vapor techniques: a) after preconcentration on gold (Figure 4) and b) using a flow injection cold vapor technique (Figure 5). The two methods resulted in quite different signal shapes. The gold trap method produced rather broad peaks. High integrated count rates were obtained, which is advantageous for precise isotope ratio measurements. The flow injection technique was mainly used, when the mercury level in the sample was high enough. The main advantage is its capability of automatisation, short analysis times and the resulting high sample through put. On the down side, the flow injection method generates very short, sharp signals, which are a challenge for quadropole ICP-MS instruments. The resulting precision of the isotope ratio measurement was consequently only 2 % RSD, compared to 0.5 % for the amalgamation technique. The overall detection limits of 1 – 2 pg were comparable, but the concentration based detection limits were obviously better for the preconcentration technique, which employed a 100 fold greater sample volume.

Figure 4: Cold vapor gold amalgamation technique for total mercury determination

Figure 4: FIAS for total mercury determination

The speciation work used a packed GC column interfaced to the ICP-MS. Again, we are introducing gaseous mercury species into a dry plasma, avoiding the many problems associated with introduction of ionic mercury species into a wet plasma.

Applications

The perennial problem in methylmercury determinations, particularly in soil, sediment and water analysis, is the determination of procedural recoveries of the overall method. To isolate methylmercury from the matrix, typically distillation or extraction techniques are employed. Since such techniques do not achieve a 100% quantitative yield, the recovery must be determined for an accurate measurement. Standard addition techniques are commonly used; but to be accurate, they should be performed for each individual sample, which is often not possible due to time constrains. However, using speciated isotope dilution analysis, it is possible to determine individual recoveries by spiking each sample with an enriched isotope of methylmercury and measuring the recovery of this isotope. The importance of the individual recovery determination will be illustrated in a series of samples from different sediment cores and sediment slurries. While recoveries from water samples do not vary too much, there are occasional outliers, which would go unnoticed using the traditional standard addition method. The situation is even more challenging, when analyzing sediment cores. Individual recoveries ranged from 50 to 110%. Clearly, each sediment layer represents a slightly different matrix and the application of an average recovery factor to all samples is problematic. These examples demonstrate the usefulness and strength of applying speciated isotope dilution analysis to determine methylmercury concentrations in complex sample matrices.

REFERENCES

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