Passive Air Sampling Tubes for US EPA Method 325

Jamie Brown

Reporter US Volume 33.4

Supelco's FLM™ Carbopack™ X Thermal Desorption Tubes are designed for the new US EPA Method 325. This new method is specific for monitoring the air concentration of volatile organic compounds emitted from petroleum refineries. There are approximately 140 active petroleum refineries operating in the United States. The target compounds for the method include a number of Hazardous Air Pollutants (HAPs) including benzene, toluene, and xylenes just to name a few. This method uses thermal desorption sampling tubes to passively collect air samples along the perimeter of the property. The air concentration is determined by analyzing the tubes using thermal desorption and gas chromatography. The method requires the sampling tubes be placed in a non-emitting shelter to protect the tubes from the weather elements (Figure 1). The shelters are permanently installed along the fence line (property line) of the refinery, and the sampling tubes are replaced from the shelters every 14 days. The required quantity and placement of the air sampling tubes is based on the size of the property, and prevailing winds. The specifics of the tube placement is detailed in the proposed method.

Figure 1. FLM Carbopack X Tubes Placed in a Shelter. Shelter supplied courtesy of Enthalpy Analytical Inc. Durham NC.

The FLM Carbopack X passive sampling tube is made of stainless steel and has the dimensions of 6.35 mm O.D. × 5 mm I.D. × 89 mm long (about the size of ball-point pen  as shown in Figure 2). The tube is packed with Carbopack™ X. The physical design of the stainless steel tube has been in used for well over 25 years, and is well accepted in the market place for collecting both passive and active air samples. This particular size tube fits into a number of thermal desorbers made by different manufacturers, such as PerkinElmer^® Markes International^®, DANI™, and Shimadzu™.

Figure 2. Size Comparison of the FLM Carbopack X Tube

These passive sampling tubes work by collecting the air sample without the need of an air sampling pump to physically pull the air sample through the adsorbent. The FLM Carbopack X tube relies on the natural movement (of the HAP's) across a concentration gradient inside the inlet of the tube called the "air gap". The volatile HAPs travel through the air gap and are adsorbed by the Carbopack X contained inside the tube.

Carbopack X is a granular graphitized carbon black (GCB) adsorbent with a surface area of 240 m²/g. This particular Carbopack contains pores in the meso range that allows it to retain a broader range of volatile analytes than other GCB's, or porous polymers like Tenax^® TA. Carbopack X is hydrophobic, so no appreciable amount of water vapor is retained when sampling in humid conditions. The sampling tubes are packed with 40/60 mesh material. This mesh size packs uniformly in the tube and prevents channeling along the walls of the tube from occurring during sampling. The pre-conditioned FLM Carbopack X tube comes supplied with brass endcaps on each end of the tube that creates an impermeable seal for the Carbopack X before and after sampling. To collect an air sample, the brass endcap from the inlet side is removed and is replaced with a diffusive sampling cap (Figure 3) (Cat. No. 28017-U). The brass endcap on the outlet of the tube remains attached during the 14-day sampling period.

Figure 3. Diffusive Sampling Caps

This diffusive sampling cap serves two purposes; first it defines the starting point of the “air gap” between the sampling cap and the Carbopack X adsorbent. Second it prevents abnormal air movement within the diffusive air gap when the tubes are deployed in windy locations. The rate at which the volatile HAPs are retained by the Carbopack X is called the uptake rate, or passive sampling rate. Each compound has its own uptake rate because of its unique chemical properties, and how it interacts with the particular adsorbent. The US EPA has determined the uptake rate for a number of the volatile (HAPs) specific for Carbopack X and are listed in the method. The concept of passive sampling is based on Fick’s law, which states a gas molecule will diffuse from an area of higher concentration to an area of lower concentration according the equation below:

Fick’s First Law of Diffusion:

Q = D (A/L) C T
Where:
Q = amount collected (ng)
D = diffusive uptake rate (cm²/min)
A = cross-sectional area of the air gap (cm²)
L = length of the air gap (cm)
C = airborne concentration (mg/m³)
T = sampling time (min)

During manufacturing we have several in-process inspections to assure you're receiving the highest quality sampling tubes. We batch test every lot of tubes for back pressure, which assures a consistently packed tube. The benzene emissions from the refineries are of specific concern because of its toxic properties. We perform a final QC test for each pack of tubes to assure the benzene background is below 2 nanograms per tube. The actual QC testing report is included with each box of tubes (Figure 4).

Figure 4. Example of the QC Testing Report

Since the quantity of sampling tubes required for Method 325 can be significant, an efficient way to identify the tubes is essential. Therfore, each tube includes a barcode along with its corresponding unique ID number permanently etched on the outside of each tube (Figure 5). This assures traceability of the samples and helps maintain the chain of custody throughout the sampling process. A barcode reader can automate portions of the data collection process, but reading the barcode on shinny curved surface can be challenging for common laser based readers. By using an image based reader such as, a DataMan^® 8600 from Cognex or a PowerScan™ PD9530-DPM from Datalogic, assures the barcode reader can detect the barcode on the tube quickly and efficiently. In addition we also etch the FLM Carbopack X name on the opposite side of the tube so you’re assured the correct tube is being used to collect the sample (Figure 6).

Figure 5. FLM Carbopack X Tube with a Barcode and a Unique ID Number

Figure 6. FLM Carbopack X Etching (shown with a diffusive sampling cap sold separately)

We source high quality stainless steel (grade 316) to manufacture our tubes, and also provide an additional level of protection for the FLM Carbopack X tubes by deactivating the stainless steel prior to packaging. Our Supelcoat deactivation process produces a ceramic like protective coating on the stainless steel surface. The coating covalently bonds to the steel surface and protects the surface from oxidation. The Supelcoat treatment is stable to (>400 °C), and creates a reproducible surface on the inner diameter of the tubes that withstands repeated use.

We have used the Supelcoat deactivation for more than 10 years on passive sampling tubes supplied for various US EPA air sampling projects like the Detroit Exposure Aerosol Research Study (DEARS). DEARS was a large air monitoring effort which used several different air sampling devices/techniques to collect a broad range of different pollutants. Other studies included the Dallas Traffic Related Exposures to Air Toxics (DTREAX), Detroit Children’s Health (DCHS), and Beaumont-EPA Region 6 Air Toxics Monitoring Study.

The new FLM Carbopack X tube is designed for the US EPA Method 325 method. We have incorporated several important features that make the tubes ready-to-use for most applications. The tubes are pre-conditioned, and each box of tubes is QC tested to assure low background levels. Each tube includes a unique barcode, and an ID number permanently etched on the tubes, to aid in tracking the tubes. These tubes have a long proven track record for a number of US EPA air sampling projects

Trademarks

• FLM, Carbopack is a trademark of Sigma-Aldrich Co. LLC
• Tenax is a registered trademark of Buchem B.V.
• Shimadzu is a trademark of Shimadzu Corporation.
• DANI is a trademark of Dani Instruments S.p.a.
• PerkinElmer is a registered trademark of PerkinElmer Inc.
• DataMan is a registered trademark of Cognex Corporation.
• PowerScan is a registered trademark of Datalogic ADC, Inc.