Sharon L. Goddard

Spatial inhomogeneity of metals in particulate matter on ambient air filters determined by LA-ICP-MS and comparison with acid digestion ICP-MS

Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been used to examine the spatial distribution of metals in particulate matter on ambient air filters. The implications of the inhomogeneous distribution observed for the sub-sampling of such filters for multiple analyses have been discussed. It has been shown that large biases in measured values may occur unless the position and size of the sub-sample are chosen judiciously. Additionally, LA-ICP-MS has been compared against acid digestion ICP-MS as an alternative method for the determination of metals in particulate matter. It has been shown that the majority of variation in the LA-ICP-MS sensitivity accrues from small differences in the positioning of samples.

Improvements to Standard Methodologies for the Analytical Determination of Metals in Stationary-Source Emissions Samples

ABSTRACT Deficiencies with the current European reference method for the analysis using inductively couple plasma–mass spectrometry of metals in samples from stationary emissions sources are presented based on experimental data obtained from real samples. The effect of these deficiencies on the quality and accuracy of data is highlighted with biases of up to 40% being observed in real samples. Suggestions to improve the performance of the standard method are presented. In particular, the beneficial effect of using a drift correction procedure to account for the decrease in instrument sensitivity observed during an analytical measurement series is demonstrated. It is shown that this corrective procedure results in substantial improvements to the accuracy of data produced. IMPLICATIONS This work highlights the deficiencies in the analytical and quality control specifications of documentary standard methods for the determination of metals in stationary-source emissions and the systematic biases in analysis that can occur as a result. There are implications of this work for policy-makers and enforcement officers with respect to the real possibility of mistakes being made when gauging compliance with legislative emissions limits when using current methods. The solutions presented to ensure accurate measurements will be of interest to documentary standards experts and legislators to consider when the relevant standards are next revised.

Investigation into Alternative Sample Preparation Techniques for the Determination of Heavy Metals in Stationary Source Emission Samples Collected on Quartz Filters

Monitoring stationary source emissions for heavy metals generally requires the use of quartz filters to collect samples because of the high temperature and high moisture sampling environment. The documentary standard method sample preparation technique in Europe, EN 14385, uses digestion in hydrofluoric acid and nitric acid (HF/HNO3) followed by complexing with boric acid (H3BO3) prior to analysis. However, the use of this method presents a number of problems, including significant instrumental drift during analysis caused by the matrix components, often leading to instrument breakdown and downtime for repairs, as well as posing significant health and safety risks. The aim of this work was to develop an alternative sample preparation technique for emissions samples on quartz filters. The alternative techniques considered were: (i) acid digestion in a fluoroboric acid (HBF4) and HNO3 mixture and (ii) acid extraction in an aqua regia (AR) mixture (HCl and HNO3). Assessment of the effectiveness of these options included determination of interferences and signal drift, as well as validating the different methods by measurement of matrix certified reference materials (CRMs), and comparing the results obtained from real test samples and sample blanks to determine limits of detection. The results showed that the HBF4/HNO3 mixture provides the most viable alternative to the documentary standard preparation technique.

The Effect of Isotopic Composition on the Uncertainty of Routine Metal Mass Concentration Measurements in Ambient Air

The main sources of uncertainty encountered during the analysis of the mass concentration of metals in ambient air as part of the operation of the UK Heavy Metals Monitoring Network are presented. It is observed that the uncertainty contribution from possible variation in the isotopic composition of the sample depends on the element in question, but can be significant (e.g., for Pb, Cd, and Hg). The working curve method for the ICP-MS analysis of metals in solution, with a low resolution, high throughput instrument measuring at one m/z ratio per element, relies on the relative abundance of the isotopes under consideration being the same in both the sample and the calibration solution. Calculation of the uncertainty in this analysis assumes that the isotopic composition variation within the sample and calibration solution is limited to a defined range. Therefore, in order to confirm the validity of this quantification methodology and its uncertainty budget, the isotopic composition of the calibration standards used for quantification has been determined. The results of this analysis are presented here.

Exact matching strategies for generalised sequential standard addition calibration

A novel strategy for calibration using an exact matching form of generalised sequential standard addition is presented. The method has advantages for high throughput techniques using auto-sampling where the best repeatability for removal and delivery of solutions is obtained when a single volume setting is considered throughout. The mathematical characteristics of the procedure have been fully described and the benefits of taking large sample portions, because of the ability to then perform repeat analytical measurements, has been assessed. The method has been demonstrated using synthetic samples of mercury in dichromate of the type resulting from sampling from a stationary source. When a large number of repeated measurements were performed the precision determined was observed to be poorer than predicted theoretically; this is ascribed to drift in the analytical system over extended timescales.

Observation of a physical matrix effect during cold vapour generation measurement of mercury in emissions samples.

The observation of a physical matrix effect during the cold vapour generation-atomic fluorescence measurement of mercury in emissions samples is reported. The effect is as a result of the different efficiencies of liberation of reduced mercury from solution as the matrix of the solution under test varies. The result of this is that peak area to peak height ratios decease as matrix concentration increases, passing through a minimum, before the ratio then increases as matrix concentration further increases. In the test matrices examined - acidified potassium dichromate and sodium chloride solutions - the possible biases caused by differences between the calibration standard matrix and the test sample matrix were as large as 2.8% (relative) representing peak area to peak height ratios for calibration standards and matrix samples of 45 and 43.75, respectively. For the system considered there is a good correlation between the density of the matrix and point of optimum liberation of dissolved mercury for both matrix types. Several methods employing matrix matching and mathematical correction to overcome the bias are presented and their relative merits discussed; the most promising being the use of peak area, rather than peak height, for quantification.

An investigation of calibration materials for the measurement of metals in ambient particulate matter on filters by LA-ICP-MS

Three different types of simple and low-cost calibration material for the measurement of the metals content of ambient particulate matter (PM) on filters using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) have been compared: cellulose ester filters spiked with multi-element calibration solutions, pellets of compressed ambient particulate matter certified reference material (CRM), and powdered ambient particulate matter CRM adhered to a surface. Elements determined were As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn, each at approximate levels of 1000 ng per filter. Blank filters spiked with multi-elemental standards were found to be significantly more reproducible and repeatable than materials based on powdered reference materials. However, a comparison of these spiked filters with real samples of ambient PM showed that the analytical sensitivities obtained per mass of analyte were significantly different. It is concluded that the spiked filters could act as very effective quality control standards correcting, to within 1%, drifts in LA-ICP-MS measurements of up to 10%, or as indirect calibration materials supported by additional measurements using traditional wet chemical techniques.

Using principal component analysis to detect outliers in ambient air monitoring studies

The need to determine outliers in analytical data sets is important to ensure data quality. More sophisticated techniques are required when the checking of individual results is not possible, for instance with very large data sets. This paper outlines a novel method for the detection of possible outliers in multivariate sets of air quality monitoring data, here the metals content of ambient particulate matter. Principal component analysis has been used to take advantage of the expected correlation between metals concentrations at individual monitoring sites to produce a summary statistic based on the deviation of each observation from the expected pattern, which can then be interrogated using one-dimensional robust statistical techniques to identify possible outliers. The sensitivity of this statistic to the number of principal components included in the summary statistic has been examined, and the method has been demonstrated on exemplar data from the UK Heavy Metals Monitoring Network where it has produced very accurate predictions of outlying data.

Characterising and reducing the blank response from mercury vapour sorbent tubes

An investigation into the factors contributing to the blank response of sorbent tubes used for sampling and measuring mercury vapour is presented. These contributing factors are quantified and strategies to mitigate or remove their effects have been proposed – the most effective of which on a routine operational basis is the cleaning of the sorbent tubes in air to remove surface adsorbed mercury and any organic contaminants that can be oxidised. Contributions of up to 175 pg of mercury, or mercury equivalent mass, were identified and removed. The largest contributors were deeply absorbed mercury and hydrocarbons and other organic compounds oxidised and removed by heating in air. Decreasing the blank response resulted in an improvement in detection limit of a factor of two. This estimate was corroborated by a novel technique for assessing the detection limit of analytical methods employing multiple desorptions that relies on determining when the ratio of the third desorption response was equivalent to the first desorption response.