B. A. Goody

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.

Microfabricated planar glass gas chromatography with photoionization detection

We report the development of a microfabricated gas chromatography system suitable for the separation of volatile organic compounds (VOCs) and compatible with use as a portable measurement device. Hydrofluoric acid etching of 95x95mm Schott B270 wafers has been used to give symmetrical hemi-spherical channels within a glass substrate. Two matching glass plates were subsequently cold bonded with the channels aligned; the flatness of the glass surfaces resulted in strong bonding through van der Waals forces. The device comprised gas fluidic interconnections, injection zone and 7.5 and 1.4m long, 320microm internal diameter capillaries. Optical microscopy confirmed the capillaries to have fully circular channel profiles. Direct column heating and cooling could be achieved using a combination of resistive heaters and Peltier devices. The low thermal conductivity of glass allowed for multiple uniform temperature zones to be achieved within a single glass chip. Temperature control over the range 10-200 degrees C was achieved with peak power demand of approximately 25W. The 7.5m capillary column was static coated with a 2microm film of non-polar dimethylpolysiloxane stationary phase. A standard FID and a modified lightweight 100mW photoionization detector (PID) were coupled to the column and performance tested with gas mixtures of monoaromatic and monoterpene species at the parts per million concentration level. The low power GC-PID device showed good performance for a small set of VOCs and sub ng detection sensitivity to monoaromatics.

Accurate measurements of water vapor transmission through high-performance barrier layers

We report a new approach to measuring very low rates of water vapor transmission through high-performance barrier layers, based on detection of the water vapor by cavity ring-down infrared spectroscopy. It provides accurate and traceable measurements with a detection limit for water vapor transmission significantly below 1 × 10(-4) g/m(2)/day. The system is underpinned by dynamic reference standards of water vapor generated between 5 and 2000 nmol∕mol with an estimated relative expanded uncertainty of ±2%. It has been compared with other methods and demonstrates good comparability.

High-accuracy stable gas flow dilution using an internally calibrated network of critical flow orifices

A network of critical flow orifices has been developed to form a gas flow dilutor capable of stable and repeatable operation, which is not influenced by environmental conditions. When used with a novel self-calibration method it achieves dilutions of up to 31:1 with a relative standard uncertainty of ±0.1%. This new approach avoids the uncertainty generated by setting and controlling the flow which is present in devices based on variable flow elements.

High-accuracy gas flow dilutor using mass flow controllers with binary weighted flows

A high-accuracy gas flow dilutor has been developed using mass flow controllers operating at flow rates ordered in terms of factors of two. It uses a novel self-referencing calibration procedure to generate variable dilutions with a typical accuracy of ±0.4%. There is no requirement for calibration to any external standard of flow. A detailed analysis of the uncertainty of configurations giving constant total gas and constant complementary gas flow is presented. Applications of the dilutor to the analysis of standard gas mixtures and to testing the linearity of gas analysers are demonstrated.

A dynamic gravimetric standard for trace water

A system for generating traceable reference standards of water vapor at trace levels between 5 and 2000 nmol/mol has been developed. It can provide different amount fractions of trace water vapor by using continuous accurate measurements of mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. An estimated relative expanded uncertainty of ±2% has been achieved for most amount fractions generated. The system has been used in an international comparison and demonstrates excellent comparability with National Metrology Institutes maintaining standards of water vapor in this range using other methods.

Accurate, Adjustable Calibration Source for Measurements of Trace Water Vapour

A number of manufacturing processes are adversely affected by the presence of trace amounts of water vapour in purge and process gases. It is important for the operators of such processes to monitor the levels of water vapour in these gases continuously. There are many analytical methods for determination of trace water vapour which all require regular, traceable calibration. In this paper, a new standard facility for generating an adjustable level of trace water vapour is described. It uses continuous accurate measurements of the mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. Details of the verification of the facility are presented along with results of comparisons with the National Physical Laboratory thermally-based dew/frost point humidity standards facility.

Calibration of DIAL and open-path systems using external gas cells

The importance of calibrating optical open-path measurements of atmospheric gases is discussed. Work carried out at NPL has involved calibrating integrated-path and range- resolved measurement instruments. The designs of the calibration gas cells used for this work are described and some results are presented. These results include the calibration of a DIAL measurement system using a calibration cell in which a stable flow of the target gas is maintained.

The development of gas standards and calibration techniques for measurements of vehicle, aircraft and industrial emissions, natural gas, occupational exposure and air quality

The National Physical Laboratory (NPL) is involved in the dissemination of nationally traceable standards to which measurements of air quality, occupational exposure and air pollution source emissions, and natural gas analyses, can be referenced. This has required the development of national primary gas standards using absolute gravimetric and other techniques, and the development of dynamic calibration techniques for gaseous species which would be unstable in high-pressure cylinders. The methodology used for preparing gas standards gravimetrically is described, together with the rigorous quality assurance measurements and consistency checks which are used to demonstrate their accuracy and stability. The uncertainty budget assigned to these standards will also be summarised. NPL primary standards are used to certify traceable ‘secondary’ gas standards which are disseminated so as to ensure the accuracy of gas analysis measurements. Examples of the applications of these secondary standards are presented. The gas standards are employed in proficiency testing of industrial stack-testing organisations, and results of the initial rounds are presented. NPL gas standards are also now being used as the basis of the United Kingdom Environment Agencys new type-approval and certification scheme for continuous industrial stack-emission analysers. A recent important international initiative, in the field of gas analyses, is the agreement by national standards laboratories across the world to demonstrate the equivalence of their calibrations, by means of key comparisons between them. These worldwide key comparisons are complemented in Europe through the EUROMET initiative which seeks to establish the equivalence and comparability of calibration standards held at national standards laboratories across Europe. Examples of these intercomparisons are presented.

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.