A Stepwise Approach to Complying with US EPA Method 325 for Monitoring Volatile Organic Compounds at Refinery Perimeters

Refineries need to conform to the newly updated US federal regulation CFR 40. To facilitate this process, US EPA Method 325 has been developed that stipulates tracking of volatile organic compounds (VOCs) at refinery perimeters.

This approach requires two-week passive, also known as diffusive, sampling onto sorbent tubes and this is followed by thermal desorption (TD) analysis and gas chromatography (GC), along with mass spectrometry (MS) analysis or other detection methods.

The main target compound is benzene, but the sampling and analysis approach can also be employed for establishing other VOCs such as xylenes, butadiene, ethylbenzene, toluene, and other similar hazardous air pollutants (HAPs).

This article demonstrates a stepwise method to conform to US EPA Method 325 for tracking VOCs around the perimeters of the refineries (Figure 1), and shows how advanced sampling and analytical equipment from Markes International is utilized to conform to US EPA Method 325.

Figure 1. The stepwise sampling and analytical process using equipment from Markes International, enabling compliance with Method 325. See ref. 2 for a listing of the sections of Method 325 corresponding to each of the stages.

What Do You Need to Carry Out Method 325?

Table 1 shows Markes’ analytical and sampling instrumentation to perform VOC monitoring according to the Method 325.

Table 1. Method 325 equipment available from Markes International

Description Part number
Sampling accessories
325 Field Station C-325FS
EPA 325 tube for BTEX and butadiene, with TubeTAG, pk 10 (inert-coated stainless steel tubes, conditioned & capped) C1-CCAX-5020
Caps, diffusion, axial, aluminium, pk 100 C-DF100
325 Container (an air-tight, non-emitting container for 15 capped tubes), pk 2 C-325CT
TubeTAG starter kit C-TAGKT
Thermal desorption system
TD-100 Advance, electronic re-collection, with internal standard dry-purge, TAG-ready U-TD100-221-2S
Calibration Solution Loading Rig C-CSLR
325 trap U-T18325-2S
Cap, DiffLok (one stainless steel, one inert), ¼", pk 100 pairs C-DL1P0
TC-20, compatible with TubeTAG R-TC20-TAG
Pneumatic gas controller U-GAS03
Internal standard gas Call Markes for advice

Field Sample Deployment

According to Method 325, up to 24 monitoring sites must be distributed around the refinery’s boundary, and this should be in a pattern based on the shape and size of the location (Figure 2).

Figure 2. Example of monitoring stations on a rectangular site of 750–1500 acres containing emission sources (grey). Monitoring sites () are placed just beyond the boundary at 20° intervals. Sources between two monitoring stations and within 50 m of the boundary (*) require that additional monitoring stations () are installed.

Further monitoring stations may be needed, where there are possible interfering sources like major roads or adjacent industrial plants, or the distribution of monitoring stations does not sufficiently cover specific VOC emission sources.

Figure 3. Top: Markes’ 325 Field Station, pictured with passive sampling tubes equipped with diffusion caps, storage caps and TubeTAG. Bottom: Sampling tubes in situ within the shelter.

The 325 Field Station is to be located away from large obstructions such as walls, trees, buildings, etc. which may cause wind funneling, resulting in differences in ambient air concentration (Figure 3).

This protocol is followed according to customary practice in diffusive monitoring. During the course of the sampling procedure, meteorological information should be collected from the nearest centre.

Prior to usage, the samplers are fully conditioned and forced into clips inside the shelter, without using any equipment. At the sampling end, a downward-pointing diffusive cap is fitted in individual tubes that reduces particulate ingress and prevents turbulence in the air gap.

When the sampling tubes are being shifted from the laboratory to the field and vice versa, care must be taken that clean tubes and sample tubes are not mixed together and should be kept ready for the next phase of sampling.

This issue can be prevented by keeping the sealed sampled tubes in an inert and sealable container for storage and transport under standard ambient conditions of about 20°C. These requirements can be met by using Markes’ 325 containers that are distinctly labeled to prevent any mixing of clean conditioned tubes and sample tubes.

Passive Sampling

Markes has designed a unique 325 tube that is integrated with the widely used 325 sorbent and is capable of trapping different types of compounds, such as benzene, toluene, ethylbenzene, xylenes, and butadiene; the first four compounds are commonly known as BTEX.

Sorbent tubes that are conditioned, cleaned, and capped are deployed to the field in customer-labeled 325 Containers™ supplied by Markes and made to equilibrate at ambient temperature prior to removing from the storage container to reduce condensation risk.

At the beginning of sampling, the storage cap fixed at the tube’s sampling end is taken off and its place a diffusion cap is used. At a specified monitoring station, if more than one sampler is being utilized the same should be sent in rapid succession so that the samplers exhibit the same start time.

Sampling is performed for at least two weeks at which point gaseous VOCs travel into the air gap within the tube, and are subsequently adsorbed onto the sorbent, as indicated in Figure 4.

Figure 4. Schematic showing axial diffusive sampling on sorbent tubes.

Once the sampling period is completed, the diffusion caps are taken off from the sample tubes and in their place, the durable storage caps are used ready to be transported to the analytical lab.

The speed at which a certain analyte is adsorbed onto a specific sorbent is based on the strength of the interaction between the sorbent and analyte and is called the uptake rate.

With the extensive usage of diffusive sampling over a number of years, many published uptake rates have been established, which means new users no longer have to determine the uptake rates experimentally.

According to Method 325, sorbent tubes filled with Carbopack X† (or Carbograph 5TD), Carbograph™ 1TD, or Carbopack™ B should be used. Table 2 shows the uptake rates of benzene onto these sorbents over a period of two weeks.

Table 2. Two-week diffusive sampling uptake rates for benzene on Markes’ passive sampling tubes

Sorbent Uptake rate (mL/min) Uptake rate (ng ppm–1 min–1) Ref.
Carbograph 1TD or Carbopack B 0.64 2.03 3
0.63 2.00 4
Carbopack X or Carbograph 5TD 0.61 1.93 5

A unique ID number in numerical and barcode format is etched in all Markes’ tubes. However, Markes’ patented TubeTAG™ should be used when handling huge numbers of tubes needed by Method 325 (Figure 5). This eliminates manual transcription, improves the audit trial, and prevents litigation risk.

Figure 5. The simple stepwise process involved in using Markes’ TubeTAG system to log tube- and sample-related information electronically both in the field and during laboratory analysis.

The tubes are fitted with radio-frequency identification (RFID) tags, and a mobile TAGSCRIBE™ unit is utilized for feeding data like sampling start and finish times, sampling location, the number of analytical cycles, date of packing, and sorbent type.

The thermal desorption system logs this data and the same is added to the reports. By performing this process, TubeTAG enables the development of a rugged chain of custody from the field to the lab, guarantees a certifiable method for audit trails, and improves the standard qualify check of sorbent tubes.

Sample and Data Analysis

As soon as the sampled tubes arrive at the laboratory, they are examined by means of thermal desorption–gas chromatography (TD–GC), and detected by mass spectrometry (MS) technique.

Thermal desorption instruments by Markes function by desorbing the sample from the tube and sending the same to the focusing trap, prior to desorbing it from the trap and finally administrating it onto the GC column.

This two-stage desorption process is highly efficient and leads to a narrow band of vapor that allows better concentration improvement and analytical sensitivity. The TD-100™ thermal desorber from Markes (Figure 6) can automatically run 100 sample tubes without the necessity for liquid cryogen. This not only reduces operating costs but also enables unsupervised operation over the weekends.

Figure 6. Markes’ TD-100 automated thermal desorber

Automatic addition and rigorous leak-testing of internal standards onto individual sample tubes on the TD-100 guarantee both analytical and sample integrity.

Repeat analysis of thermal desorption samples can be done, thanks to the capability of the TD-100 to quantitatively re-collect the divided part of samples onto a clean sorbent tube. This allows easy and consistent repeat analysis, with substantiation of analyte recovery.

According to Method 325, tubes must be kept closed after the sampling period to prevent contamination and loss of analyte. Durable storage caps are utilized for sealing sampled and blank tubes during transport and storage, but soon before analysis they should be substituted with Markes’ push-on DiffLok™ caps that guard the tubes while they are kept on the TD autosampler.

Difflok caps is designed with patented diffusion-locking technology allows gas to pass via the tube whenever pressure is applied and at the same time reduces the risk of contaminant ingress or sample loss (Figure 7). These measures prevent the necessity for uncapping within the system.

Figure 7. Schematic of a DiffLok cap.

Data Interpretation

The main goal of Method 325 is to measure the benzene level; however, the above-mentioned sampling and analytical methods enable current analysis of various compounds by utilizing the same workflow, without any extra cost or effort. Figure 8 shows an example of TD–GC–MS analysis of contaminated refinery fenceline air, utilizing two-week passive sampling. Here, toluene, benzene, and xylene have been detected.

Figure 8. The results of two-week diffusive sampling of contaminated air around a refinery perimeter, with analysis by TD–GC–MS, showing the detection of a number of hazardous VOCs.

In order to find out the benzene concentration, five-point calibration curves are utilized for measuring the mass on the tube from the peak abundance. The equation given below is employed for calculating the airborne concentration:

Sample Tube Cleaning

According to Method 325, sorbent tubes should be cleaned, and must demonstrate =0.2ppbv of interferences or contaminants prior to usage. With the aid of Markes’ TC-20 TAG™ (Figure 9), this process is simplified, and costs are reduced by enabling about 20 tagged and industry-standard sorbent tubes to be conditioned at increased temperatures, at the same time.

Figure 9. Markes’ TC-20 TAG tube conditioner.

Markes’ TC-20 TAG™ eliminates the instrument time needed to run the samples and not the condition tubes. This offers a quick return on investment. Using TC-20 TAG™, tubes are also conditioned using high-purity nitrogen, instead of the more costly carrier gas, helium.


This article has shown the different steps needed to fully comply with the US EPA Method 325. Markes International offers a complete portfolio of instrumentation, tubes, and accessories to fully comply with the method and also, provides comprehensive solution packages that are fully compliant with Method 325.

The company is an expert in sampling and analysis of VOCs and has in-depth experience of consulting on standard and regulatory methodology in the USA and across the globe.

This information has been sourced, reviewed and adapted from materials provided by Markes International Limited.

For more information on this source, please visit Markes International Limited.


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