Thermally induced filter bias in TEOM mass measurement 

Steven J. Page, Donald P. Tuchman and Robert P. Vinson 

Researchers at the National Institute for Occupational Safety and Health (NIOSH) have long 

used stationary tapered element oscillating microbalances (TEOMS~) in laboratory settings. They 

have served to assess the mass concentration of laboratory-generated particulates in experimental 

, dust chambers and they provide a reference method for comparison with other particulatemeasuring instruments. Current NIOSH research is focused on further adapting TEOM 

technology as a wearable personal dust monitor (PDM) for coal mining occupations. This 

investigation's goal is to help identify, quantify, and provide means for resolving certain TEOMrelated error. The present research investigated bias caused by thermal effects on filter assemblies. 

New filters used in the PDM for 8 h tests show an average positive bias of 25.5 pg, while similar 

tests of equivalent filters used in two 1400A model TEOMs show an average positive bias of 

34.3 pg. The derived bias values allow correction of previously collected biased data. Also, preheating the filters for 24 h at 46 "C shows significant bias reduction, with PDM pre-heated filters 

subsequently averaging -3.3 pg and 1400A TEOM filters averaging 5.9 pg. On a single-point 

comparison to gravimetric sampling, a 25.5 pg bias is only significant at low mass loadings. At 

2.5 mg, this bias represents a negligible' 1 % of the mass measurement. If ordinary linear 

regression is used, the bias is still insignificant. However, if the more valid weighted linear 

regression is used, it gives more weight to the smaller dependent variable values, which are more 

impacted by the bias. Consequently, what is 1% bias on a single high-mass value can translate 

into a larger bias percentage at high-mass values when performing a weighted regression on data 

that include a large number of low-mass values. 

1. Introduction 

1.1 Background 

Among the various instruments developed to quantify concentrations of airborne particulate matter, those based on the 

tapered element oscillating microbalance (TEOM~') have 

gained wide acceptance in a variety of applications. Beginning 

in 1981, the TEOM technique was commercialized by Rupprecht & Patashnick Co. (R&P), Albany, NY, now part of 

Thermo Fisher Scientific, Inc. (hereafter referred to as Thermo), Waltham, MA. This class of instruments is capable of 

nearly continuous time-resolved measurements directly related 

to the particulate mass, independent of other particulate 

properties, such as composition, density, size, and color. With 

an external display, TEOM instruments can serve as directreading near-real-time particulate monitors. Varied applications' for TEOM-based monitors have included particulate 

measurement for the ambient environmentY2 industrial stack 

emissionsY3 engine exhaustY4 and as discussed below, occupational exposure assessment. 

The Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) has long 

used stationary TEOM instruments in laboratory settings to 

monitor and control experiments with airborne particulate, 

typically of coal, mineral, or diesel origin. They have been used 

to assess the mass concentration of laboratory-generated 

particulate in experimental dust chambers and they provide 

a reference method for comparison with other particulatemeasuring techniques or instruments. Further advancing this 

instrument category, recent NIOSH contract research has 

miniaturized, stabilized, and reconfigured TEOM technology 

into a worker-wearable personal dust monitor (PDM) that 

also provides a data file for dust levels over a work shift. 

Performance of this new TEOM-based PDM has been documented by NIOSH researchers through extensive testing.5 In 

this current investigation, both pre-commercial (not for general sale) Model 3600 wearable PDMs and Thermo Model 

1400A stationary TEOMs were used. The Model 1400A 

instruments have US EPA certification for continuous 

PM-10 particulate monitoring. Details on the principles6 for 

and operation7 of these instruments are available elsewhere: 

therefore only the instrurnent details most relevant to this 

research are reviewed here. 

1.2 TEOM device descriptions 

The core component of any TEOM mass sensor is a vibrating 

hollow tube called the tapered element (TE). It is secured at its 

base but free to oscillate at the opposite end. The TE is 

set into oscillation at its'resonant frequency and an electronic 

feedback system maintains the oscillation amplitude. A small