Abstract:
Relatively accurate gravimetric analysis of airborne particulate matter in a sample is achieved by making gravimetric measurements of the sample on a microbalance in a closed chamber, continuously electronically monitoring air pressure, humidity and temperature in the chamber, continuously controlling humidity and temperature in the chamber, and combining the gravimetric measurement with measurements of air pressure, humidity and temperature in the chamber to make a buoyancy corrected determination of the mass of the particulate matter.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority on U.S. Provisional Patent Application 60/591,084 filed Jul. 27, 2004 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Field of the Invention  
         [0003]     This invention relates to a system and method for determining the mass of particulate matter, and in particular airborne particulate matter.  
         [0004]     When conducting the gravimetric analysis (mass determination) of airborne particulate matter, the mass of individual filters is determined before and after the filters are exposed to an atmosphere containing such matter. Many environmental factors cause unacceptable errors in the mass determination including air density of airborne particulate matter. Measurements of very small particle masses (less than 0.1 mg) may require a correction for the influence of air density, depending on the required level of accuracy. This is called an air buoyancy correction.  
         [0005]     Buoyancy corrections can be made in two ways. The classical method is to calculate air density by measuring air temperature, air pressure and relative humidity in a balance room at the time of mass determination. Air density is then used to determine the buoyancy correction. An alternate method is to correct for air buoyancy using a mass artifact (also known as a “buoyancy standard”), in which case air density does not need to be known [Wunderli et al,  Anal. Bioanal. Chem.  376: 384-392, (2003)]. However, the artifact method appears to be inapplicable to the measurement of airborne particulate matter, due to the difficulty in estimating sample volume with sufficient accuracy.  
         [0006]     The classical method is appropriate in theory, but it requires suitable equipment for simultaneous monitoring of air pressure, temperature and humidity, in order to determine air density at the precise time of mass determination. Current US-Environmental Protection Agency (EPA) guidelines for mass determination of airborne particulate matter do not require measurement of air density, and therefore disregard buoyancy corrections. At present, there is no apparatus, product or process for effecting buoyancy-corrected gravimetric analysis of airborne particulate matter, while at the same time meeting or exceeding all other requirements of the US-EPA guidelines (stable relative humidity, stable temperature, low airborne particle concentrations, elimination of electrostatic charge, and physical stability).  
         [0007]     Recently there has been a trend towards using controlled environmental chambers to meet US-EPA guidelines, because this approach can be less costly and more effective than attempting to control environmental parameters inside an entire room. However, publications relating to existing environmental chambers designed to meet US-EPA guidelines indicate that they do not include the capacity to make buoyancy corrections [Allen et al (2001) and Carlton et al (2002), Journal of the Air and Waste Management Association, Vol. 51, pp 1650-1653 and Vol. 52, pp. 506-510, respectively]. Accordingly, a need still exists for means to make buoyancy corrected mass determination of particulate matter.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     The object of the present invention is to meet the above defined need by providing a relatively simple method and apparatus for effecting buoyancy corrected determination of the mass of particulate material.  
         [0009]     According to one aspect, the invention relates to a system for determining the mass of a sample containing particulate matter comprising: 
        a housing defining a chamber;     a microbalance in said chamber for measuring the mass of a sample;     a source of air under pressure for supplying air to said chamber;     humidifier means for humidifying air from said source of air;     heating and cooling means in said chamber for changing the ambient temperature in said chamber;     sensor means for continuously monitoring the pressure, temperature and relative humidity in said chamber; and     control means connected to said sensor means and to said source of air, said humidifier means and said heating and cooling means for controlling the flow of air to said chamber, and the relative humidity and temperature in said chamber,     whereby measurements of the pressure, temperature and humidity can be combined with gravimetric measurements made using the microblance to provide buoyancy corrected determinations of the mass of a sample containing particulate material.        
 
         [0018]     According to another aspect, the invention relates to a method of determining the mass of a sample of particulate matter comprising the steps of: 
        making gravimetric measurements of a sample containing particulate matter in a closed chamber;     continuously controlling humidity and temperature in the chamber;     continuously monitoring the air pressure, humidity and temperature in the chamber while making the gravimetric measurements, and     using the gravimetric measurements in combination with the measurements of the air pressure, humidity and temperature to make a buoyancy corrected determination of the mass of the sample.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention is described below in greater detail with reference to the accompanying drawings, which illustrate a preferred embodiment of the invention, and wherein:  
         [0024]      FIG. 1  is a block diagram of a gravimetric analysis system in accordance with the invention;  
         [0025]      FIG. 2  is a schematic front view of a clean room and a gravimetric analysis apparatus used in the system of  FIG. 1 ;  
         [0026]      FIG. 3  is a schematic, isometric view of the gravimetric apparatus of  FIG. 2 ; and  
         [0027]      FIG. 4  is a cross section taken generally along line  4 - 4  of  FIG. 2 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     Referring to  FIG. 1 , the preferred embodiment of the gravimetric analysis system includes a gravimetric analysis chamber  1  containing a microbalance  2  ( FIGS. 2 and 3 ) for making gravimetric measurements. A suitable microbalance is the Mettler-Toledo UMX2 balance. A disc-shaped air filter (not shown) is placed on the microbalance. The filters in the present case are 37 or 47 mass PFTE members available from Pall Corporation, East Hill, N.Y. The mass of the filters is determined before and exposing the filters to an atmosphere containing particulate matter. Air temperature, pressure and humidity in the chamber  1  are continuously monitored, and the air temperature and humidity are continuously controlled. For monitoring, a temperature, humidity and pressure sensor unit  3  is provided in the chamber  1 . A suitable sensor unit is a Vaisala PTB100A analog barometer available from Vaisala, Oyj, Vantaa, Finland and a DewTrac® Humidity Transmitter, EdgeTech Model 200 available from EdgeTech, Milford, Mass., which includes a sensor probe, an electronic control unit and an ambient temperature kit. The outputs of the sensor unit  3  are fed via line  4  to a central processing unit (CPU)  5 , i.e. the ambient pressure, dew point temperature, ambient temperature and equivalent relative humidity are routed to a data acquisition control system at one minute intervals. In this case, the control system is an ADAM 5000E (Advantech Co. Ltd., Carlsbad, Calif.). The ADAM control system transmits the data to a software program (Labtech Control version 12.1.2 (2001) from Measurement Computing Corporation, Middleborough, Mass.) which continuously receives, processes and records the data for storage on a hard drive.  
         [0029]     The CPU  5  is connected by line  6  to a humidifier  7 . Dry air from a compressed air tank  8  flows through a Teflon® tube  9 , a mass-flow controller  10  and a three way valve  12  to the humidifier  7 . A suitable flow controller  10  is Model GFC1715 available form Aalborg Instruments &amp; Controls Inc., Orangeburg, N.Y. The humidifier  7  and the flow control  10  are connected to the CPU  5  by lines  13  and  14 , respectively. If the humidity is low, the ADAM controller triggers the solenoid valve  12  to direct the air through the humidifier  7 , which is in the form of a closed, heated water tank, in which the air takes up water as it passes over the water surface. If the humidity in the chamber  1  is high, the solenoid vale  12  is operated to direct dry air through a tube  15  which bypasses the humidifier  7  and directs air to a humidifier outlet tube  16 . Dry or moist air in the tube  16  passes through a filter  17  and a tube  18  into the chamber  1 .  
         [0030]     The temperature in the chamber  1  is controlled by circulating hot or cold water through a heat exchanger in the form of a stainless steel coil  20  ( FIGS. 2 and 3 ) in the top of the chamber. Hot and cold water is fed from sources  21  and  22 , respectively through tubes  23  and  24  containing solenoid valves  25  and  26 , respectively. The valves  25  and  26  are connected to the CPU  5  by lines  28  and  29 , respectively. After passing through the valves  25  and  26 , the heating and cooling mediums are mixed in tube  30 , which is connected to the coil  20  in the chamber. Temperature control medium is discharged from the coil  20  via a tube  31  connected to a medium recovery vessel  32 .  
         [0031]     With reference to FIGS.  2  to  4 , the Plexiglas® walls of the housing  34  form the chamber  1 . The housing  34  is mounted on a table  35  in a soft-walled Class 100 clean room  36  equipped with a high efficiency particulate arrestance filtration system  37 , which operates twenty-four hours a day. A high voltage (7 kV) Mettler-Toledo point de-ionizer (not shown) in the chamber  1  ionizes surrounding air to create ozone, which effectively removes static charge from the filters in the system  37 . Two polonium-210 anti-static strips can also be located in the chamber  1  if an alternate de-ionizing approach is more appropriate for a given application. The table  35  weighs 700 lbs. and includes a top  39  and legs  40  all of which are formed of marble and a stainless steel crossbar  41  extending between the legs  40 .  
         [0032]     The housing  34  includes a front wall  42  with access ports  43 , side walls  44  and a rear wall  45 . Sample holding shelves  47  are provided on the interior of the side walls  44 . The top of the housing  34  is defined by a rectangular inlet manifold  48  for receiving air from the tube  18 . Air is introduced into the housing  34  via an inlet  49  in the rear wall  50  of the manifold  48 . The air entering the manifold  48  passes through openings  51  in a partition  53  extending between the rear wall  50  and a front wall  54 , and then through vertical orifices  56  in the bottom wall  57  of the manifold.  
         [0033]     In operation, the microbalance  2  is used to make mass measurements manually with a readability of 0.1 μg. The measurement data from the microbalance  2  may be transmitted electronically to the CPU  5  using BalanceLink software (Mettler-Toledo) or entered into an electronic spreadsheet from handwritten notes. The microbalance  2  is programmed to auto-calibrate at the same time (2 am) each day.  
         [0034]     The four atmospheric parameters required to calculate air density (relative humidity, pressure, temperature and dew point temperature) are recorded at one minute intervals by the Labtech software. Air density (AD) is calculated using the equation: 
 
 AD =(3.484 P− 0.80439726×10 ((7.5Tdp)/(237.3+Tdp)) )+( T+ 273.15) 
 
 where P=pressure (κPa), T=temperature (°C.) and Tdp=dew point temperature (°C.). 
 
         [0035]     By calculating air density at the precise time of measuring the mass of a sample and at the precise time of auto calibration, the buoyancy correction equation can then be applied to the measured mass of the sample. The buoyancy correction equation is: 
 
 Mp=Wp (1 −par/Pr )/(1 −pa/Pp ) 
 
 where Mp is the corrected mass of the sample, Wp is the weighing value of the sample, Pp is the density of the sample, Pr is the density of a reference weight, pa is the air density at the time of mass measurement and par is the air density at the time of the last auto-calibration. 
 
         [0036]     A custom software application was written in Microsoft Access to combine computationally the atmospheric parameters in the chamber  1  (recorded using the Labtech software) with the mass measurement data (recorded using the BalanceLink microbalance software), and to calculate the final buoyancy-corrected mass of the sample.