by
P. Berno, V. Bonamin, and C. Gianoli,
SGS Ecologia, Padua (Italy)

Since the mid-eighties, several methods have emerged for the sampling and analysis of VOCs in both outdoor and indoor air based on the so-called canister methodology.

The U.S. Environmental Protection Agency (U.S. EPA) introduced the most complete analytical methods available to date: TO-14 (recently upgraded as TO-14A) and IP-1A^1,2 for the quantitative detection of a subset of 41 of 187 hazardous air pollutants (HAPs) listed in the CAAA. ³ Subsequently, the ASTM published its own method suitable for "ambient, indoor, or workplace atmospheres".⁴

More recently in Italy, an Environmental Ministry decree based on the canister technology mandates benzene monitoring in urban areas.⁵ A new method, known as TO-15, was recently released to address a still larger number of VOCs, including polar organic compounds, thereby covering a subgroup of 97 HAPs listed in the CAAA.⁶ The TO-14 list, incidentally, contains apolar (BTEX) or slightly polar compounds such as chloroethanes, chloroethylenes, chlorobenzenes and various freons (see Figure 1).

Figure 1. Chromatogram of a TO-14 standard mixture.

Canister Approach Superior
Because of the high dilution of the analytes of interest, both field sampling and analyses must be carried out carefully. In our experience, the collection and analysis of samples from canisters presents several advantages:

• Option of remote and unattended sampling
• Appropriate integration of the sample over a specific period of time
• Ease of storing and shipping, because the sample identity is unaffected for weeks
• Analysis of sample from multiple sites with a single analytical setup
• Repeatability of the analysis to allow, for example, assessment of measurement precision

The U.S. EPA Method TO-14 is composed of two parts: field sampling and lab analyses.

Field Sampling Using Canisters Typical problems such as sample breakthrough or recovery efficiency that can be encountered with the presently used solid sorbents, are totally eliminated. No sample preparation is needed, which markedly shortens the analysis time. Air samples are collected in stainless-steel containers known as canisters which underwent a process of electro-passivation (SUMMA® process) to largely reduce the presence of polar active sites on the inner surface. SilcoCans from Restek Corporation are even more inert, because they have a chemically bonded fused-silica layer on the internal surface that further reduces surface activity.

Table 1. Fields of application of the canister technique. Application Range Urban Air Monitoring ppt-ppb Landfill Site Gas Emissions ppb Industrial Emissions ppb-ppm Workplace Air Quality ppb-ppm Indoor Air Monitoring ppt-ppm

Eliminating Active Sites
The presence of active sites inevitably leads to undesired phenomena, such as irreversible absorption of most pollutants of interest. Canisters are therefore cleaned before any sampling campaign through a series of evacuation/filling cycles utilizing membrane and turbomolecular pumps.

It is crucial to use oilless pumps and wet ultrapure nitrogen (whose function is to saturate any residual active site still present on the surface with water molecules) to achieve very low (sub-ppb) levels of residual VOCs at the end of the cleaning cycle.

It is strongly recommended to check periodically for residual VOCs by performing blank runs after the cleaning process. When the system is properly performing, this residual content should not exceed 0.01-0.1 ppb in each component.

Controlling Passive Sampling
Passive sampling is carried out starting with a perfectly evacuated canister (internal pressure less than 50 mtorr). Air is allowed to enter through a fine metering valve with a "critical orifice" for careful control of the flow. The valve is set according to the desired sampling time and/or canister volume using a mass flow-meter and generally falls within the 3-100 sccm range. A mass flow meter is the most appropriate tool for a proper reading of the flow. With a 6-liter canister, sampling may last from a few minutes to 24 hours. When longer sampling times are required, larger canisters (up to 33 liters) or active sampling (canister pressurization) may be used.

Variety of City Pollutants
Table 2 shows our results for a replicate measurement of a sample of urban air collected in daytime and close to the rush hour. As expected, the sample contains a variety of hydrocarbons from car exhausts. Traces of other pollutants such as freons and halogenated solvents have been detected at ppb-level.

The results shown are part of a 12-day benzene monitoring carried out in Padua in collaboration with the local environmental authorities. The study was aimed at providing a verification of the data obtained from the air monitoring network located throughout the city along main roads and intersections.

We collected two samples a day, the first around the rush hour from 8 a.m. to 9 a.m., and the second averaged over 24 hours. The results reflect a single-point sample (one-hour sampling) collected twice independently. The very good reproducibility reflects optimum sample stability vs. time and the fine control of the analytical conditions the system allows. The system easily meets the most demanding standards imposed by several local and international regulations aimed at improving air quality.

Standards and Equipment Certified gaseous calibration standard mixtures of these compounds, normally at 1-ppm levels and pressurized in electro-polished aluminum cylinders, are commercially available. The analytical equipment (see photo) is composed of an Entech Model 7000 preconcentrator, which acts as an injector to the GC/MS (HP 6890/5972). Other indispensable tools are the canister cleaner (Entech 3000SL) and the dynamic diluter (Entech 4650SL) for standards preparation.

Analyte	Variation (x)/X %	Mean Conc. (x) (pg/m3)	Conc. x1 (pg/m3)	Conc. x2 (pg/m3)
1. Freon 12 (dichlorodifluoromethane)	7.4	2.7	2.6	2.8
2. Methyl chloride	0.0	1.4	1.4	1.4
3. Freon 114 (1,2-dichlorotetrafluoroethane)			ND	ND
4. Vinyl chloride			ND	ND
5. Methyl bromide			ND	ND
6. Ethyl chloride			ND	ND
7. Freon 11 (trichloromethane)	3.6	2.8	2.8	2.7
8. 1,1-dichloroethane			ND	ND
9. Methylene chloride	4.7	4.3	4.4	4.2
10. Freon, 113 (1,1,2-trichlorotrifluoroethane)	11.8	0.9	0.8	0.9
11. 1,1-dichloroethane			ND	ND
12. cis-1,2-dichloroethane			ND	ND
13. Chloroform	0.0	0.2	0.2	0.2
14. 1,2-dichloroethane			ND	ND
15. 1,1,1-trichloroethane	0.0	1.0	1.0	1.0
16. Benzene	1.9	15.7	15.5	15.8
17. Carbon tetrachloride	0.0	0.9	0.9	0.9
18. 1,2-dichloropropane	0.0	0.6	0.6	0.6
19. Trichloroethylene	0.0	0.9	0.9	0.9
20. cis- 1,3-dichloropropene			ND	ND
21. trans- 1,3-dichloropropene			ND	ND
22. 1,1,2-trichloroethane			ND	ND
23. Toluene	3.4	47.0	46.2	47.8
24. 1,2-dibromoethane			ND	ND
25. Tetrachloroethylene	0.0	1.5	1.5	1.5
26. Chlorobenzene			ND	ND
27. Ethylbenzene	0.0	10.1	10.1	10.1
28. m,p-xylene	0.9	35.0	35.1	34.8
29. Styrene	0.0	2.4	2.4	2.4
30. 1,1,2,2- tetrachloroethane			ND	ND
31. o-xylene	0.0	13.0	13.0	13.0
32. 1,3,5-trimethylbenzene	0.0	4.8	4.8	4.8
33. 1,2,4-trimethylbenzene	0.6	16.1	16.0	16.1
34. m-dichlorobenzene	0.0	0.1	0.1	0.1
35. p-dichlorobenzene	0.0	0.5	0.5	0.5
36. o-dichlorobenzene			ND	ND
37. 1,2,4-trichlorobenzene	0.0	0.1	0.1	0.1
38. Hexachloro-1,3-butadiene	0.0	0.1	0.1	0.1
Sum of VOCs	1.1	161.9	161.0	162.7
ND = Not Detected

Table 2. VOCs measured in the city of Padua during the rush hour. Two independent samples of urban air were collected in parallel.

Conditions
Preconcentrator
Configuration:	Cold Trap Dehydration
Carrier:	He
	Module 1	Module 2	Module 3
Trapping Vol. (mL)		400
Trapping Flow (mL/min)		60
Trapping Temp (°C)	-10	-70	-150
Trans. Temp (°C)	20	190	70
Trans. Flow (mL/min)	10	1.5	1.5
Trans. Vol. (mL)	10
Trans. Time (min)		3.5	2
Gas Chromatograph
Column:	HP-1; methylsilicone; 60 m X 0.32 mm X 1.0 µm film
Carrier:	He
Flow:	1.5 mL /min (constant)
Injector Temp:	250°C
Transfer Line:	280°C
Oven:	35°C X 10 min to 150°C at 7°C/min; to 220°C at 20°C/min X 6 min
Mass Spectrometer
Start Time:		3.8 min
Mass Range:		33.0 to 270 amu
Threshold:		100
Sampling:		2
Scans/sec:		3.13

The Analytical Process The system is designed so that sample transfer from the canister to the GC column is quantitative, accurate and reproducible. The larger the sample volume, the more critical to eliminate most of the water and carbon dioxide in the steps preceding the injection. The diagram shows the Entech three-module water management system. In the Microscale purge-and-trap configuration, 20 to 1000 milliliters of sample are transferred from the canister to a glass beads trap (Module 1), kept at -150°C with liquid nitrogen. Here VOCs are quantitatively blocked together with most of the carbon dioxide and water. Module 1 is then warmed up to room temperature, and volatiles are transferred by a He stream to a Tenax trap kept at -10°C. At this temperature, most of the CO2 is not retained, while VOCs remain adsorbed on the Tenax packing of Module 2. Desorption takes place by heating Module 2 to 180°C, which allows sample cryofocusing in Module 3 to be cooled to liquid-nitrogen temperature. After rapid injection and separation on a high-resolution methylsilicone column (HP-1), VOCs are analyzed by a quadrupolar mass-selective detector (see chromatographic conditions). Data are processed with an HP-Vectra PC using the SmartLabTM package developed at Entech for the control of the Model 7000 preconcentrator, the 3000SL canister cleaner and the 4650SL dynamic diluter, while the HP 6890 Series MSD software controls the GC/MS. Quantitation can be done using appropriate standards (either internal or external) or even labelled standards when very high precision is required.

SGS Ecologia in Brief SGS Ecologia srl is part of the environmental branch of the SGS Group based in Geneva (Switzerland). Founded in 1878, the Group is the world's largest organization in the field of inspection and verification. Truly worldwide and benefiting from a unique international network of affiliated companies with over 1,180 offices, 321 laboratories and 33,000 employees, the SGS Group is able to provide its international clientele with a comprehensive range of services in more than 140 countries. SGS Ecologia assists customers and experts in the field providing air quality assessments, indoor and outdoor ambient air monitoring, industrial-gas emission testing and monitoring services, and a wide range of other ecological-assessment services.

REFERENCES:

1. U.S. EPA Compendium Method TO-14A,
   Determination of Volatile Organic Compounds
   (VOCs) in Ambient Air Using Specially Prepared
   Canisters with Subsequent Analysis by Gas Chromatography (1997).

2. U.S. EPA Method IP-1A, Determination
   of Volatile Organic Compounds (VOCs) in
   Indoor Air (1989).

3. Clear Air Act Amendments of 1990, United
   States Code, Title III - Hazardous Air Pollutants.

4. ASTM Method D5466-93, Standard Test
   Method for Determination of Volatile Organic
   Chemicals in Atmospheres (Canisters Sampling
   Methodology), Annual Book of ASTM Standards,
   Vol. 11.03, p. 404 (1995).

5. D.M. n. 159, 25 Nov. 1994; G.U. Suppl. Ord.
   n. 290 del 13 Dic. (1994).

6. U.S. EPA Compendium Method TO-15,
   Determination of Volatile Oraganic Compounds
   (VOCs) in Air Collected in Specially Prepared
   Canisters and Analyzed by Gas Chromatography/
   Mass Spectrometry (GC/MS) (1997).

ACKNOWLEDGEMENT
We thank Dr. L. Baracco of the Ufficio Tutela Ambiente,
Provincia di Padova for permission to reproduce some
of the data obtained during the 1996 VOC monitoring in
the city of Padua.