Harmonisation of proficiency testing schemes

The harmonisation of proficiency testing (PT) schemes has been under debate for a long time. There are obvious reasons why harmonisation of the practices in PT would be beneficial. In many areas, there is still a belief that further harmonisation of practices in PT would improve the comparability of measurement data. In particular when two laboratories are to be compared that have not participated in a single PT, problems arise which allegedly can be overcome by further harmonisation of PT schemes. In practice, however, parties involved in PT are not always embracing the idea of harmonisation. With the results of two European projects in mind, a discussion is given on harmonisation aspects, and some considerations are given that may help to decide in practice whether harmonisation is likely to solve particular problems. The first project, the European Proficiency Testing Network (EPTN), is concerned with further harmonisation. The second European project (COEPT) aims at providing a basis to assess equivalence across proficiency tests, and explores the conditions under which such an assessment is feasible.

     

The evaluation of a tenax GR diffusive sampler for the determination of benzene and other volatile aromatics in outdoor air

The applicability of a tube-type diffusive sampler as an environmental monitor for benzene, toluene, ethylbenzene and xylene (BTEX) is reported. Uptake rates have been experimentally determined for a novel type adsorbent, Tenax GR, and compared to theoretical values. It is shown, that the uptake rates are virtually independent of environmental parameters within the experimental conditions studied. The response of the sampler to transient changes in concentrations has been determined in the laboratory. It is found that the sampler is capable of following an extreme concentration profile. Field comparisons with pumped samplers have been performed and good agreement is observed between the results of the two independent methods. The samplers have also been applied as environmental monitors at different locations.     

Reference materials at emission levels of NH3 and HCl

Due to instability in static mixtures, in the past only dynamic preparation techniques for mixtures of NH3 and HCl were maintained; however, recent developments of new passivation techniques for cylinder treatment and commercially available nitrogen with a very low content of impurities have opened up the possibility of preparing stable mixtures in cylinders by gravimetry. Mixtures of NH3 (300 microL/L and 30 microL/L) and HCI (300 microL/L) were prepared in nitrogen in cylinders with different passivation treatments. The mixtures were compared with dynamically prepared mixtures using Photo Acoustic detection (PAS) and FT-IR. The uncertainty is about 3.5% with PAS analysis for NH3 at the 300 microL/L level. For the analysis of HCI with FT-IR the initial uncertainty in the 300 microL/L mixtures is about 7%. Two of the HCI mixtures show significant instability; for the other three mixtures the uncertainty in the measurement is too large to discriminate between stability and instability. The NH3/N2 mixtures are stable within 3.5% for a one-year period. The stability of the 30 microL/L mixtures of NH3 cannot be underpinned by measurement results, but a systematic effect in the measurement procedure seems more likely than instability. Improvement in the measurement procedure should clarify this point.     

On the dielectric properties of molecular crystals: II. First-order spatial dispersion and optical activity

The general formal expression for the wavevector- and frequency-dependent dielectric susceptibility tensor of a simple molecular crystal, derived in paper I, is applied to the case of a Bravais lattice with gyrotropic molecules. The dielectric susceptibility tensor is calculated with first-order spatial dispersion and from this an expression for the gyration tensor is derived. As an example the optical rotary power is calculated for light propagating along the optical axis of a gyrotropic tetragonal crystal.     

Harmonisation of proficiency testing schemes

The harmonisation of proficiency testing (PT) schemes has been under debate for a long time. There are obvious reasons why harmonisation of the practices in PT would be beneficial. In many areas, there is still a belief that further harmonisation of practices in PT would improve the comparability of measurement data. In particular when two laboratories are to be compared that have not participated in a single PT, problems arise which allegedly can be overcome by further harmonisation of PT schemes. In practice, however, parties involved in PT are not always embracing the idea of harmonisation. With the results of two European projects in mind, a discussion is given on harmonisation aspects, and some considerations are given that may help to decide in practice whether harmonisation is likely to solve particular problems. The first project, the European Proficiency Testing Network (EPTN), is concerned with further harmonisation. The second European project (COEPT) aims at providing a basis to assess equivalence across proficiency tests, and explores the conditions under which such an assessment is feasible.     

Results from interlaboratory comparisons of aldehyde-2,4 dinitrophenylhydrazone analysis

Within the framework of the SMT "Aldehydes" project (SMT4-CT97-2910) three laboratory comparisons have been organized in which artificial aldehyde hydrazone samples have been distributed among participating laboratories. To these samples potentially interfering compounds have been added to simulate as closely as possible real air or air emission samples.In addition to the "Aldehydes" project partners external EU laboratories have participated. It may, therefore, be assumed that the results of these comparisons reflect the state-of-the-art in the analytical determination of aldehydes in air samples using the 2,4-dinitrophenylhydrazine (DNPH)-method. In this paper several aspects of the organization of these comparisons and the overall results obtained are presented. These demonstrate that results obtained using liquid chromatographic techniques are usually comparable between laboratories within +/-2 times the coefficient of variation (approximately 6-15%), indicating the robustness of the methodology. In addition, laboratory (liquid chromatography) mean results can be made to agree with preparation concentrations at the 95% confidence levels after optimization of separation and integration conditions; these are mainly related to the measurement of formaldehyde in the presence of interferents and the measurement of low concentrations of acrolein and acetone. Some problem areas still remain, however--namely the measurement of glutaraldehyde and the measurement of low levels of acetone.