DRI Model 2001A OC/EC Thermal Optical Carbon Analyzer :



General Description:

The Model 2001A is the much refined commercialized version of the OC/EC analyzer used at Desert Research Institute for the U.S. EPA’s visibility assessment program “Interagency Monitoring of Protected Environments (IMPROVE)”. It was developed with key features to facilitate particulate carbon research as well as analytical production. Some of the features like dual modes of pyrolysis monitoring, automatic boat inlet, and MS ACCESS, do increase the cost of the analyzer. But the resulting product is unmatched by other analyzers. An automatic sample loader, will work in conjunction with the automatic boat inlet to process up to 18 samples sequentially unattended.

The manufacturer will warrant the instrument for a period of one year. Any parts found defective should be returned to the factory for repair or replacement.

Specific Features:  

• Analyzer is computer-controlled with voice command to perform OC/EC analysis. Protocols such as IMPROVE, NIOSH 5040, and other variants of NIOSH 5040 like USEPA-STN, Canadian-MSC1, and Hong Kong-TOT, as well as protocol for direct carbonate determination of samples can be executed after a selection is made from the list of methods in the Analysis screen.
  
• System consists of an automatic sample boat inlet for introduction of a sample into the volatilization/combustion furnace, an oxidizer oven to convert OC into CO2, and a methanator to convert CO2 to methane for detection by a flame ionization detector. The detector signal is monitored simultaneously at two ranges (10-6 and 10-8) to accommodate a wide range of filter carbon concentrations. At the more sensitive 10-8 scale, it can detect samples with less than 0.2 µg/cm2 of carbon or more than 28 µg of carbon per peak without over-ranging.
  
• The automatic boat inlet system permits much better control of the analysis. For example, after an analysis is complete, the boat will not retract to allow the operator to reload until the boat has cooled to a pre-determined temperature. Likewise, after loading, the boat will not advance to the furnace to begin the analysis until the system is ready. 
  
• Pyrolysis is continuously monitored by both reflectance and transmittance of a He-Ne laser and results are given for both modes of pyrolysis correction. 
  
• Tip is located directly at the filter disc to monitor its temperature during analysis.
  
• There is an injection port to permit analyzer calibration with liquid standards like KHP and sucrose, or gaseous standards like methane or CO2. The same port can be used for sample acidification for direct carbonate determination of particulate samples using the carbonate protocol in the Model 2001. 
  
• MnO2 oxidation oven has a bypass vent outlet for connection to another detector such as a NDIR instead of the FID. 
  
• All zones (totaling six) starting from the sample furnace to the FID are heated with temperature controllers with readouts to preset temperatures to prevent loss of sample from deposition, and to achieve long term system stability.

• Another key feature unique to the Model 2001A is the event-driven stepping of one temperature to the next. The dwell time at each temperature step in a protocol like IMPROVE is dependent on the slope of the peak and a minimum dwell time. For a large peak that lasts beyond the minimum dwell time, the analyzer will not advance to the next temperature until the peak has reached a pre-determined slope. This function allows much cleaner separation of the evolved fractions than other systems in which each temperature step lasts for a fixed duration. The OC/EC fractions are useful in source apportionment applications.
  
• The analyzer is geared for method modification and development. The protocols are stored as command tables (time-event tabulation) in Microsoft ACCESS that can easily be modified by the user, requiring little or no programming skills.
  
• The raw data are stored as ACCESS tables and ASCII files that can be exported for other purposes. 
  
• The DRI software has reprocessing features to allow replotting of thermograms or recalculation of data. Thermograms can be exported and read into MS Word for publication.
  
• The Model 2001A can be placed on a laboratory bench that is 30” deep and 40” in length. The left half of the analyzer can serve as a working platform for filter sample preparation.
  
• Carbuser@dri.edu has been set up for users of the Model 2001 to exchange research ideas, to disseminate service notes, and newly developed analytical protocols. Users are multi-national, including the Pacific Rimcountries, Europe and America .
  
• Standard warranty by the manufacturer for the Model 2001A is one year except those parts in the analyzer that are manufactured elsewhere. These include the laser, and the flame ionization detector, both of which are under the respective manufacturer’s warranty of different duration. Any parts found defective should be returned to the factory for repair or replacement. Extended warranty can be purchased for an additional cost.

Rationale for pyrolysis Monitoring by Reflectance and Transmittance:

Pyrolysis monitoring using both reflectance and transmittance is important because some method like IMPROVE uses reflectance, while others like NIOSH 5040 and the variants use transmittance, The Model 2001A will allow the comparison of results from the two modes of pyrolysis correction and provide limits for error estimation. Our current research (Chow et al 2003, Fung et al, 2003) suggests that over-correction using transmittance may be due to the pyrolysis of adsorbed gases within the filter. The reflectance and transmittance signals can also be utilized to estimate the charring of the particles and sub-surface charring.

Chow, J.C., J.G. Watson, D. Crow, D.H. Lowenthal, and T. Merrifield “Comparison of IMPROVE and NIOSH Carbon Measurements” Aerosol Sci. Technol. 34: 23-34 (2001)

Chow, J.C., J.G. Watson, L.W. A. Chen, W. P. Arnott, H. Moosmuller, K. Fung “Equivalence of Elemental Carbon by the EPA IMPROVE and Speciation Trends Network STN Thermal/Optical Methods” Submitted to Environ. Sci. &Technol.

Fung, K., J.C Chow, J.G. Watson. “Factors Contributing to the Difference in Elemental Carbon by the IMPROVE and USEPA Speciation Trends Network STN Methods” Paper No. Wed-B2, 3rd Asian Aerosol Conference, Hong Kong , Jan.6-8, 2004.


Specifications for DRI Model 2001A Carbon Analyzer:

1. Analyzer operates at 115VAC, 60 or 230VAC, 50 Hz (specify the frequency when ordering), at 15 amp.
2. The analyzer is designed to operate in typical temperature controlled air-conditioned environment of a laboratory without the presence of acid or solvent fumes.
3. Gases for operation: High pressure cylinders of helium, 10% oxygen in helium, hydrogen, air (for FID) and 5% methane in helium regulated to 15 psig operating pressure by dual stage metal diaphragm regulators. These gases should be of the highest purity (hydrocarbon-free, 99.999%) to obtain the highest performance from the analyzer.
4. The minimum detection limit (MDL) of the DRI carbon analyzers based on the analyses of 693 individual quartz-fiber filters and defined as three times the standard deviation of their measured results. They are: 

• total organic carbon 0.82 µg/ cm2 
• high-temperature organic carbon 0.81 µg/ cm2
• total elemental carbon 0.2 µg/ cm2 
• high-temperature elemental carbon 0.2 µg/ cm2 
• total carbon 0.93 µg/ cm2

MDL may vary depending on the quality of the operating gases and operating conditions. MDL improves after Initial conditioning of the analyzer.

5. Temperature Settings:

• Sample oven may be programmed from 120 oC to 900 oC. Maximum heating rate is approximately 250 oC per minute depending on supplied voltage to analyzer and other environmental conditions.      
• Temperature reproducibility - + two degrees at each step above 250oC. 
  Temperature accuracy – within 1% or 5 degrees, whichever is the greater at or above 250oC. Note that in a dynamic setting (heating or cooling), the temperature indicated by the thermocouple may be different than the actual filter temperature due to the difference in thermal mass of a filter and a thermocouple. Experiments showed that the filter temperature is within 10 oC of the thermocouple reading in this process.

6. He/Ne Laser: 

• Provides both reflectance and transmission values of sample filter.
• Maximum output signal: 2.5 volts, adjustable, operated at 1.6 to 2 volts reflectance mode, 1.0 to 1.4 volts transmission mode with approximately + 2% S/N.

7. Computer system, minimum requirements:

• 2 GHz or higher 
  
• Minimum 128 MB SDRAM 
  
• One hard drive, 40 GB <
• One CD-R/W, 48X/24X
• At least two USB ports and one serial port 
  
• Sound card (integrated) and speakers
• Mouse and keyboard 
• Mouse and keyboard 
• Colour Monitor. 
• Mouse and keyboard 
• Windows XP for compatibility with DRI’s software. MS ACCESS 2000 MS-Office Professional) is also required.

Gases:

A K-size tank holds ~291 cu ft, (8.24 m3) of helium, ~310 cu ft (8.78 m3) of air and ~261 cu ft (7.39 m3) of hydrogen. Thus under continuous operation, helium will need replacement in about two months, hydrogen, about 4 months, and air (FID), about 3 weeks. The compressed air to actuate pneumatic cylinders needs infrequent replacement (many months) due to very low consumption rate. The calibration gas, 5% methane in helium typically lasts for one year or longer.

Accessories:

The following items are used in conjunction with the analyzer to perform filter carbon analysis: 

• Stainless steel punching tool: 5/16 inch diameter, 0.516 cm2 area for removing small sample punches from quartz filters. 
• Syringes: Hamilton gas-tight 1000 ul syringes for calibration injections; 10 ul syringe for carbonate analysis and for analyzer calibration.
• Tweezers for filter handling.