Nicholas A. Martin

Studies using the sorbent Carbopack X for measuring environmental benzene with Perkin–Elmer-type pumped and diffusive samplers

Studies with the sorbent Carbopack X in pumped and diffusive samplers, of the Perkin–Elmer-type, have been carried out in a controlled atmosphere test facility (CATFAC) employed to generate volatile organic compound (VOC) atmospheres. The benzene safe sampling volume for Carbopack X has been measured, using an indirect method, and found to be (5400 ± 740) 1 g−1, suggesting that this sorbent is suitable for long-term pumped sampling. Good agreement in concentration has been obtained between calibrated benzene test atmospheres and values derived from pumped samplers, well within the expected uncertainty for this technique. The 2-week benzene diffusive uptake rate for Carbopack X has been determined to be (1.99 ± 0.18) ng ppm−1 min−1 for environmental applications, and was found to be constant over a wide range of VOC concentrations. The 2-week toluene and o-xylene diffusive uptake rates for this sorbent are also reported, over a more limited range, together with the 2-week benzene, toluene and o-xylene diffusive uptake rates for Chromosorb 106.

Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies

We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC (0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N-2, O-2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O-2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O-2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O-2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.

A metrological approach to improve accuracy and reliability of ammonia measurements in ambient air

The environmental impacts of ammonia (NH3) in ambient air have become more evident in the recent decades, leading to intensifying research in this field. A number of novel analytical techniques and monitoring instruments have been developed, and the quality and availability of reference gas mixtures used for the calibration of measuring instruments has also increased significantly. However, recent inter-comparison measurements show significant discrepancies, indicating that the majority of the newly developed devices and reference materials require further thorough validation. There is a clear need for more intensive metrological research focusing on quality assurance, intercomparability and validations. MetNH3 (Metrology for ammonia in ambient air) is a three-year project within the framework of the European Metrology Research Programme (EMRP), which aims to bring metrological traceability to ambient ammonia measurements in the 0.5–500 nmol mol−1 amount fraction range. This is addressed by working in three areas: (1) improving accuracy and stability of static and dynamic reference gas mixtures, (2) developing an optical transfer standard and (3) establishing the link between high-accuracy metrological standards and field measurements. In this article we describe the concept, aims and first results of the project.

Accurate and adjustable calibration gas flow by switching permeation and diffusion devices

A new method for generating adjustable concentrations of calibration gases from permeation and diffusion devices is described. The method achieves high accuracy by maintaining the source device at a constant temperature and pressure and in a constant gas flow. It involves switching the gas stream and subsequently smoothing it to generate dilutions of up to 100 with a relative uncertainty of ?2%. Its dynamic performance has been modelled and its performance under steady-state conditions has been validated.