Introduction

The accurate measurement of the metals content of ambient air is of increasing importance as the potential toxicological effects of a number of metal species are becoming better established. Additionally, UK and EU legislation is increasingly defining statutory legal limits for the concentrations of metal compounds in ambient air.

Sampling

Ambient air samples are collected either by actively sampling onto cellulose filters, or by sample deposition. Three samplers are used:

 

High Volume sampling 'Hi-vol' Sampler (67 m3 hour-1)	Low Volume sampling 'Kleinfiltergerat' (2.3 m3 hour-1)	Deposition measurement Bergerhoff Deposit Dust Gauge

 

Analytes: Ni, As, Cd, Pb, V, Cr, Mn, Fe, Cu, Zn, Pt & Hg

Robust analytical methods have been developed for the four metals of greatest legislative interest: Nickel (Ni), Arsenic (As), Cadmium (Cd) and Lead (Pb). This work supports and validates the development of a CEN European Standard Method from measurement of these four metals in the PM₁₀ fraction of ambient air. Sampled filters are microwave-digested in a matrix of concentrated nitric acid and hydrogen peroxide prior to analysis.

In addition to the four metals measured by the CEN metals Standard (above), NPL has also developed fully-validated ICP-MS methods for the analysis of Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Copper (Cu), Zinc (Zn), Platinum (Pt) and Mercury (Hg) in ambient air.

Accurate measurement of these elements is of importance due to their toxicological and /or legislative issues; they are also routinely measured for in national air quality networks. The measurements of Cr (volatile), Pt (very low levels in ambient air) and Hg (volatile; memory effects) are particularly challenging.

 

UKAS Accredited Analysis

NPL has obtained UKAS testing accreditation for the ICP-MS analysis of the metals listed above. The accreditation covers samples on cellulose filters, or in acidic matrices.

Techniques: ICP-MS & Voltammetry

The primary method for analysis is ICP-MS (Inductively Coupled Plasma - Mass Spectrometry). Robust methods have been developed incorporating rigorous internal standardisation and drift correction procedures. The methods have been validated by repeated extractions of certified reference materials and underpinned by participation in international intercomparisons. Calibration curves are fitted using XLGENLINE, a Generalised Least Squares software package, and a full uncertainty budget has been developed for the entire analytical procedure.

A table comparing routinely-achievable method (sampling and ICP-MS analysis) parameters with 'typical' and target concentrations is given below:

 

Parameter	Ni	As	Cd	Pb
Method limit of detection	0.71 ng m-3	0.05 ng m-3	0.01 ng m-3	0.10 ng m-3
'Typical' concentration in air	3.3 ng m-3	1.4 ng m-3	0.22 ng m-3	13 ng m-3
'Typical' relative experimental uncertainty (k = 2 coverage factor)	20%	21%	21%	19%
Working concentration range (in air) of draft CEN Metals Standard	2 - 100 ng m-3	0.5 - 350 ng m-3	0.1 - 50 ng m-3	1 - 4000 ng m-3

The measurements can also be carried out using Stripping Voltammetry using a hanging mercury drop electrode. The results from this technique, which uses standard addition calibration, have shown a high level of agreement with those from ICP-MS.

Defra UK Heavy Metals Monitoring Network

NPL manages the UK Heavy Metals Monitoring Network for the Department for Environment, Food and Rural Affairs (Defra).

The Network comprises 24 sites located at rural, urban and industrial locations throughout the UK. Data is required for the air concentrations of Ni, As, Cd, Pb, V, Cr, Mn, Fe, Cu, Zn, Pt & Hg in the particulate-phase, and also vapour-phase mercury. NPL carries out all of the analyses in-house, using ICP-MS (particulate phase metals) and atomic fluorescence (vapour phase mercury).

Representation on Standardisation Committees

Experts from NPL represent the UK on a number of European Working Groups within CEN Technical Committee 264 (Air Quality).

Publications

Publications arising from the metals project at NPL include:

• A practical uncertainty budget for ambient mercury vapour measurement. R J C Brown, A S Brown, R E Yardley, W T Corns and P B Stockwell, Atmos. Environ., 2008, 42, (10), 2504-2517.
• Twenty-five years of nationwide ambient metals measurement in the United Kingdom: concentration levels and trends. R J C Brown, R E Yardley, D Muhunthan, D M Butterfield, M Williams, P T Woods, A S Brown and S L Goddard, Environ. Monit. Assess., 2008, 142, (1-3), 127-140.
• On the optimum sampling time for the measurement of pollutants in ambient atmospheres. R J C Brown, D Hood and A S Brown, J. Autom. Meth. Manag. Chem., 2008, Article ID: 814715.
• Accurate calibration of mercury vapour measurements. R J C Brown and A S Brown, Analyst, 2008, 133, (11), 1611-1618.
• Establishing SI traceability for measurements of mercury vapour. A S Brown, R J C Brown, W T Corns and P B Stockwell, Analyst, 2008, 133, (7), 946-953.

Services

NPL offers a UKAS accredited service for the analysis of metals. For further information, please contact us.