Development of a reliable and cost-effective weighing chamber for aerosol sample analyses

Abstract

Reliable gravimetric analysis of particulate matter filters is of great importance in exposure and risk assessment in industrial hygiene and environmental health, especially for light weight fine particles like PM 2.5. Weighing bias may be caused by a number of environmental and operational factors. Among them, the humidity effect is most often addressed. This study proposes a cost-effective weighing chamber with humidity control by using a saturated magnesium chloride solution to meet the USEPA requirements for filter weighing. To exclude the interference of electrostatic charges, filter samples are treated with a radioactive source, Am 241, on a microbalance pan before weighing. The results of long term monitoring showed that the relative humidity inside the weighing chamber is between 31–35%, when the temperature is between 18–21°C. The air pressure inside the weighing chamber is kept slightly positive by adjusting the air supply of the humidity control unit at 15 L/min during filter conditioning and at 25 L/min during filter weighing to maintain a stable humidity condition and prevent aerosol contamination from outside. Moreover, the operator should wear gloves to prevent hand moisture evaporation. We suggest that filter samples to be stored and conditioned in a weighing chamber with the filter holder’s lid open to expedite and enhance the filter conditioning. When a hygroscopic filter, such as an MCE, is used to collect about 0.5 mg of hygroscopic NaCl particles, a mass increase of 0.5 mg results occurs due to the filter. An additional 1.2 mg mass increase occurs due to collected NaCl particles, when the weighing is performed under RH 85% rather than in the environmentally-well-controlled weighing chamber. With the use of the weighing chamber and the recommended practices, this study demonstrates excellent weighing quality. The mass determination limits are below 3 μg for 37-mm particle-laden filter samples commonly encountered in practical situations.