Filter Gravimetry Study: Introduction

Accurate weighing (gravimetry) of air filters is essential to support the growing number of International and European Standards methods being developed for a wide variety of air monitoring techniques. As the mass of sampled particulate is typically very small (masses of less than 0.1mg are not unusual), it is critical that the weight of the filter before and after sampling can be determined accurately, reproducibly and with minimal uncertainty.

NPL has recently undertaken a series of filter gravimetry studies to maximise the validity of air monitoring methods, particularly a forthcoming UK-wide series of equivalence trials for particulate monitors. All experiments were carried out in an atmosphere-controlled cabinet.

Stage One: Weighing Protocol

The initial objectives of the study were to produce an optimised weighing protocol and to define the characteristics of the balances used, thus quantifying the major sources of error in the gravimetric technique. A further aim was to produce an uncertainty budget for the process.

The following factors were investigated in depth and the results of the study were used to develop a filter weighing protocol and uncertainty budget:

• Calibration and 'exercising' the balance before weighing
• Balance repeatability and drift
• Taring the balance and/or using reference filters and weights
• Neutralisation of static charges

Stage Two: Selection of Filter Material

Once established, the weighing protocol was used in investigations to select the filter material most suitable for ambient particulate matter measurements. The following effects on the mass of a variety of filter materials (glass fibre, quartz, PTFE and PTFE-bonded glass fibre) were studied:

• Temperature and humidity
• Filter storage and transport
• Inter-laboratory weighing
• Particulate matter volatility

An example of the results of the study are given below: the graphs show the variations in the mass of filters of two materials (quartz and PTFE-bonded glass fibre) at different temperature and humidity conditions. The PTFE-bonded glass fibre filters can be seen to exhibit very little change in mass with varying temperature and humidity.

The study found that the most important factors to consider when selecting a filter material are the variation of filter mass with temperature and relative humidity; structural integrity; flow resistance; chemical effects; particle sampling efficiency and static charge effects. Thorough consideration of these factors led to the selection of PTFE-bonded glass fibre as the best filter material for subsequent air monitoring studies. Using the gravimetric method defined in the above protocol, the expanded (k = 2) uncertainty in the weighing of a 47mm diameter PTFE-coated glass fibre filter was found to be 31μg.

Report

The full findings of the study were published as NPL Report DQL-AS 015: Ambient air particulate matter: quantifying errors in gravimetric measurements.

Contact

For further details about any aspect of NPL's filter gravimetry studies, contact us via:

Customer Service tel: +44 20 8943 8637
E-mail: environmental_enquiries@npl.co.uk