VOC Monitoring by Adsorbent Tube SLEA tube study since 1985 Less costly than canister Easy to use May be prone to moisture issues, particularly with longer duration sample times Latest generation of tubes are much better with regard to moisture, break- through and artifacts
VOC Monitoring by Canister SLEA has been using 6L evacuated canisters since 2004 Easy to use / no power required Usually used with a flow controller and orifice Orifice is very small for 24-hour sampling periods – must be very diligent with regard to cleaning and leak checking.
VOC Monitoring by Canister Can be used with a battery-operated timer for unattended operation We have found that the sample train should be supported to ensure a stress- free and leak-free system Again, leak-checking is critical to getting a good sample.
EPA has been operating the PAMS (Photochemical Assessments Monitoring Stations) network for a number of years, and in the past year or so has been testing autonomous GC systems for deployment SLEA has been an “early adopter” of this technology, using it since 1994 at their Front St. station and 2001 in their Emergency Response Trailer Continuous (Real-Time or Near Real Time) Measurement by GC-FID
Two main factors made this possible – the availability of a new concentration system for ambient VOCs coupled with the knowledge gained by the installation of a continuous water analyzer in 1987. Today, there are a number of systems available, in rack-mount configurations. The most common detectors are PID and FID (with FID preferred for many VOCs). A drawback of the rack-mount analyzers is that they are usually pre-configured for certain compounds, whereas the lab equipment based system is user configurable for specialty applications. Continuous (Real-Time or Near Real Time) Measurement by GC-FID
VOC Monitoring – SLEA Mobile Monitor In operation since 2001 Has been used for emergency situations (community monitoring) Used for industrial emergency analysis of tedlar bag samples Used for industrial turn-around monitoring Used for industrial special production runs Used for industrial CAMMs monitoring Used for industrial remediation projects
We have found that the GC-FID system is ideally suited for the analysis of benzene at very low concentrations in the Sarnia area While there is always a possibility of interferences with a general detector like the FID, we have been able to eliminate or substantially reduce interference concerns by choosing appropriate column and trap combinations. Measurement of benzene in some areas may require a more specific detector (mass spectrometer, FTIR, etc.) When using a continuous analysis method such as GC-FID, periodic parallel sampling with analysis by MS (or other specific detector) is an important QA task. GC-FID and Measurement of Benzene
GC-FID and System Sensitivity MDL Run # Analyzer Run # Result (ug/m 3 ) 11936.15 21946.20 31956.23 41966.22 51976.23 61986.21 71996.19 82006.17 Standard Dev. 0.03 No. of Samples (n-1) 7 T-Table Value 2.998 MDL 0.09 The GC-FID coupled with a concentration system and capillary column offers the ability for measurement of VOCs at extremely low concentrations. Method detection limits are determined for each individual analytical system. Presented here is the mdl data for benzene (mobile unit analyzer data shown here)
Sample concentration offers a wide dynamic range The linearity curve shown here shows excellent linearity characteristics between approximately two and 130 ug/m 3. The two SLEA systems have a linear range of up to 3000 ug/m 3. GC-FID and System Linearity
We have found that the continuous VOC analyzers that SLEA operate are extremely consistent over time with regard to response and baseline stability. Part of this is due to the use of electronic flow controllers in the concentrators and GCs. Although some manufacturers design their systems to be calibrated on a “timed-on-trap” basis, we have preferred to introduce calibration gas exactly the same way as a sample would be treated – this removes any possibility of error occurring if any of the control systems of timed-on-trap have malfunctioned. The downside is that this technique does use more calibration gas, which can be expensive. Calibration and QA/QC
Typical ambient concentrations of benzene are very low (1ug/m3). To measure benzene at such low concentrations and have confidence in the data generated, it is critical to use high quality calibration standards, preferably from more than one source. SLEA uses a combination of NIST-traceable gases and permeation devices for calibration. Each time a new gas standard cylinder is purchased, it is cross-checked against the old standard as an added measure of confidence in the certified concentrations. Our preference is an accuracy certification of 2% for gases, and 5% for permeation devices. Dilution systems are necessary for the gas standards. Ideally, the use of multiple calibration systems will help ensure that the data generated is accurate.
SLEA uses three electronic dilution systems which are cross-checked routinely against each other. To provide a “root-level” check of the electronic systems, we use a custom built system incorporating rotameters (flowmeters) and a digital timer to generate known gas concentrations in tedlar bags. The analysis of the tedlar bags serves as a base check of the electronic calibration systems. Calibration and QA/QC
SLEA VOC Monitoring – Continuous and Adsorbent Tube
Agency / LocationMethod Number of Samples Average Concentration (ug/m 3 ) SLEA Front St.Continuous Monitor63621.1 SLEA Front St.Adsorbent Tube341.8 EC Centennial ParkCanister510.9 MOE AFNCanister311.1 Cross-Method Comparison of Annual Benzene Data (2012)
Method Comparison (Continuous/Canister/Tube) Test setup for comparison of methods
Voc Analyzer Analysis by ORTECH (GC-FID) Canister analyzed by ALS Environmental (GC-MS) Adsorbent tube analyzed by Maxxam Analytics (GC-MS) Expected Con’cn. @ 99:1 Dil’n. (ug/m 3 ) Actual Result % Dev. VOC Analyzer 38.7 40.1+3.6 Canister36.9-4.7 Tube42.0+8.5 Method Comparison Test Results
Lessons Learned Summary Many different methods and technology available to accurately measure benzene at low concentrations Comprehensive QA/QC program essential for achieving high confidence in data. When using continuous analyzers utilizing non- specific detectors, periodic parallel sampling is recommended Through various work done, have had opportunity for cross-method comparison, and analysis by differing techniques – results have demonstrated that all options are viable, with specific circumstances playing an important role in the final choice