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Note 103: EPA Method 325B, Novel Thermal Desorption Instrument Modification to Improve Sensitivity


Ronald E. Shomo, II, Christopher Baker, and John J. Manura
Scientific Instrument Services
Ringoes, NJ
(presented at ASMS 2016)


The new EPA method 325B method requires monitoring of petroleum refineries' fence lines for benzene and other aromatic hydrocarbons by thermal desorption GC/MS. The samples are collected via thermal desorption tubes that contain an adsorbent and placed on the perimeter of the manufacturer's property. The materials are adsorbed on the tubes via passive collection. The collected tubes are then desorbed and injected onto a GC column and analyzed via GC/MS. One of the challenges is to avoid cross-contamination or carryover from sample to sample; generally the transfer line between the GC and mass spectrometer is where this will most likely occur.

This work was done with a modified thermal desorber that incorporates a new transfer line with each sample to avoid the possible cross contamination problem. The transfer line is a short needle (35 mm) that is connected to each desorption tube. The added benefit is this short transfer line allows for more efficient transfer of compounds allowing for greater coverage of less volatile compounds and improves the sensitivity of the technique.

Materials and Methods

Standards (Table 1)

1. PAH mix (Accustandard) M-8310-QC-ATI diluted 10:1 with ACN

2. Benzene-d6 (Accustandard) M-624-SS-01 diluted 10:1 with MeOH

Desorption tubes- 4 mm ID x 4” stainless steel Silco coated, were packed with a small plug of deactivated glass wool and 100 mg of Tenax TA (60/80 Mesh) and conditioned for 4 hours at 320C with 25 mL/min of high purity nitrogen flowing through the tubes. Conditioning oven was an SIS model 781056 with a 24 tube capacity.

The loaded desorption tube is placed in a SIS model TD5 Thermal Desorption System. (Figure 1), and a 35 mm preconditioned desorption needle (transfer line) was attached (Figure 2). The TD5 was coupled to an Agilent 5973 GC/MS mated with a 6890 GC. The GC had a SIS cryotrap installed using a liquid CO2 as the cryogen. The cryotrap was cooled to -55 C during the desorption process. The TD5 desorbed the samples at 250 C for 10 minutes after a 1 minute non-heated purge to remove any residual oxygen present in the desorption tube prior to injection. During the desorption process a divert valve is actuated by the desorption software and allows the injection port helium flow to pass through the desorption tube and attached transfer line into the injection port. After the desorption process is complete the diverter valve switches the flow of helium again to allow the normal injector flow to resume. (see Figure 3)

During the desorption the cryotrap remains at -55 C, after the 10 minute desorption period the cryotrap is ballistically heated to 300C for 3 minutes and the GC/MS data acquisition is initiated. The GC/MS is scanning a mass range of 45-450 at 1 second per scan.

The GC column used was a SGE BP5MS, 0.25 mm ID x 30 m with a 0.25 μm film. The column has a 5 m Silguard column on the front end. The GC operated in the split mode with a 5:1 split ratio. All desorption parameters were controlled by the TD5 software that is integrated into the Agilent Chemstation program. Mass Spectral data was compared against the NIST14 library software for compound identification.

A blank preconditioned desorption tube was run between samples to verify no cross contamination or carry over was present.

Results & Discussions

Table 1

Analyte # 	Analyte ID 		CAS Number 		Concentration per μl
1 		Acenaphthene 		83-32-9 		100 pg/μl
2 		Acenaphthylene 		208-96-8 		200 pg/μl
3 		Anthracene 		120-12-7 		10 pg/μl
4 		Benz(a)anthracene 	56-55-3 		10 pg/μl
5 		Benzo(a)pyrene 		50-32-8 		10 pg/μl
6 		Benzo(b)fluoranthene 	205-99-2 		20 pg/μl
7 		Benzo(k)fluoranthene 	207-08-9 		10 pg/μl
8 		Benzo(g,h,i)perylene 	191-24-2 		20 pg/μl
9 		Chrysene 		218-01-9 		10 pg/μl
10 		Dibenz(a,h)anthracene 	53-70-3 		20 pg/μl
11 		Fluoranthene 		206-44-0 		20 pg/μl
12 		Fluorene 		86-73-7 		20 pg/μl
13 		Indeno(1,2,3-cd)pyrene 	93-39-5 		10 pg/μl
14 		Naphthalene 		91-20-3 		100 pg/μl
15 		Phenanthrene 		85-01-8 		10 pg/μl
16 		Pyrene 			129-00-0 		10 pg/μl
17 		1-Methylnaphthalene 	90-12-0 		100 pg/μl
18 		2-Methylnaphthalene 	91-57-6 		100 pg/μl
19 		Benzene-d6 		1076-43-3 		20 pg/μl

Figure 4 shows the chromatogram of the 1/10 diluted PAH and Benzene-d6 standards. All components in the standards are observed with the exception of #6 and #10. The benzo(b)fluoranthene and benzo(k)fluoranthene may be co-eluting and a run with the individual standards of these analytes would need to be run to confirm or disprove this hypothesis. The Dibenz(a,h)anthracene was not observed and would also require an individual standard to determine the conditions under which the analyte can be detected.


Utilizing a short transfer line coupled to each desorption tube on a modified desorption system extends the volatile range and sensitivity for the EPA 325B Method. The Short Path® system reduces the chance for carry over or cross contamination that occurs using a conventional transfer line common for other commercial Thermal Desorption Systems.

The promising results have invited more studies to be done. Presently we are working to determine the LLOD & LLOQ for the standards in full scan as well as SIM mode of operation.


1. EPA Method 325B. Volatile Organic Compounds from Fugitive and Area Sources, available from the Federal Register Vol. 79 No. 125 June 30, 2014.