1997 Progress Report: Measurements of Non-Methane Volatile Organic Compounds in the Lower Troposphere From Tethered Balloon and Kite Sampling Platforms by Internal Standard Calibration Using Ambient CFC Reference CompoundsEPA Grant Number: R825417
Title: Measurements of Non-Methane Volatile Organic Compounds in the Lower Troposphere From Tethered Balloon and Kite Sampling Platforms by Internal Standard Calibration Using Ambient CFC Reference Compounds
Investigators: Helmig, Detlev , Balsley, Ben , Birks, John , Karbiwnyk, Christine
Current Investigators: Helmig, Detlev , Balsley, Ben , Birks, John , Karbiwnyk, Christine , Mills, Craig
Institution: University of Colorado at Boulder
EPA Project Officer: Hunt, Sherri
Project Period: October 1, 1996 through September 30, 1999
Project Period Covered by this Report: October 1, 1996 through September 30, 1997
Project Amount: $436,172
RFA: Analytical and Monitoring Methods (1996) RFA Text | Recipients Lists
Research Category: Environmental Statistics , Air Quality and Air Toxics , Water , Land and Waste Management , Air , Ecological Indicators/Assessment/Restoration
Objective:The main goal of this project is to develop an analytical method for the analysis of atmospheric volatile organic compounds (VOCs) using atmospheric chlorofluorocarbons (CFCs) as internal standard compounds. This technique will allow the development of low-cost, light-weight, and battery powered sampling packages to analyze VOCs in atmospheric samples from airborne sampling platforms such as tethered balloons and light aircraft. Along the way to achieving these goals, solid adsorbent sampling methods are investigated for their reproducibility in the analysis of selected CFCs and VOCs.
Progress Summary:A Perkin Elmer Automated Thermal Desorber (ATD-400) with a Perkin Elmer Autosystem gas chromatograph (GC) utilizing flame ionization detection (FID) and electron capture detection (ECD) was installed during March and April 1997. The GC column effluents are split in a manner to direct approximately 10% to the ECD and 90% to the FID by fastening a glass "Y"- connector to the GC column outlet and restricting the flow to the detectors with pieces of uncoated, deactivated fused silica columns. This configuration allows multiple detection of each sample. The approximate 1:10 split ratio was achieved by selecting appropriate column lengths and diameters. The GC is controlled by PE Nelson Turbochrom software. This system permits method generation, oven temperature programming, modification of detector response, sequence generation, data collection and data analysis. We expect to get an upgrade to this software which will include computer control of the ATD-400 as well.
Analysis of the first test samples in April 1997 revealed the need for a stronger retaining GC column in order to achieve better separation of C3 - C4 hydrocarbons. A DB-1 column (30 m x 0.32 mm) with a 5 mm film thickness was ordered from J & W to replace a DB-1 column (30 m x 0.32 mm) with a 0.1 mm film thickness. In August 1997, graduate student Christine Karbiwnyk began working on the project. Since that time, the new column was installed in the GC and a cryofocus feature using liquid nitrogen was added to the GC oven temperature control. These two features have improved the separation of C3 - C4 hydrocarbons. The flow resistance of the sampling tubes (Air Toxics, Supelco) was tested at various flow rates to determine viable flow rates for sampling. In addition, loss of VOCs due to breakthrough of the sampling cartridge was tested at various flow rates to insure accurate sampling of target VOCs.
Much work has concentrated on reducing background signals. To that end, hydrocarbon and oxygen traps were installed on the Helium carrier gas line and oxygen traps (one high capacity trap and one indicating trap bed) were installed on the ECD make-up gas line. Thermal tube conditioning procedures were tested to insure adequate removal of all contaminants. The sample tubes may be cleaned by a sequence on the ATD-400 or cleaned in a GC oven under nitrogen purge. These two methods were compared for residual background levels and were found to be equivalent. Cleaning the tubes in the GC oven is preferable though, because it frees the ATD-400 for analysis. It is important to note, however, that because the two cleaning methods are equivalent, we will not need extra equipment in the field in order to clean any sampling tubes.
It was observed that a considerable amount of contamination of the sample tubes can occur when they are stored with their analysis caps awaiting analysis on the ATD-400 sample carousel. These Teflon caps are sealed with Teflon-coated Viton?1 O-rings and it is believed that VOCs and CFCs found in ambient air diffuse through the O-rings and are trapped by the solid adsorbent within the sample tube. Because each tube takes almost an hour to run through an entire thermal desorption/GC analysis, sample tubes late in the sequence may sit on the ATD-400 for several hours while waiting for desorption. In order to investigate contaminate uptake rates, sample tubes capped with Teflon caps were left on the ATD-400 sample rack for 24 to 72 hours. The results from the experiment clearly demonstrated a substantial uptake of certain VOCs and CFCs through the O-ring of the tube end caps during storage in lab air. For example, over a 48 hour period carbontetrachloride increased by a factor of 18 over blank levels. However, chloroform only increased by a factor of 2 over the same period. In the case of hydrocarbon uptake, butane increased by a factor of 14 over blank levels while toluene increased by a factor of 3. We have now constructed an acrylic box to house the ATD-400 and isolate it from lab air. The air inside the box will be re-circulated through a molecular sieve filter to maintain a clean headspace atmosphere. We anticipate that this will yield a substantial reduction in the contaminant uptake rate.
A Hewlett Packard (HP) flame ionization detector (FID) was added to an HP 5890 Series II gas chromatograph with mass spectrometric detection (GC/MS) in May 1998. Flow from the GC column is split in a manner to direct approximately 80% to the FID and 20% to the MS similar to the FID/ECD system and allowing multiple detection of each sample. Dual MS/FID detection will confirm compound identification and compliment the GC/FID/ECD system. Computer control, data acquisition and analysis on this system is currently being upgraded from a UNIX based software to HP MS ChemStation software.
Construction of a multi-port sampler is nearing completion. It will hold 8 tubes for sequential sampling and permit both manual and radio control options to open valves for sampling. The sampler may be used for ground based sampling or airborne measurements from a balloon platform. The sampler should increase efficiency and accuracy when taking multiple samples. Field testing will begin once the electronics are operational. Construction of a second 10 port sampler is also underway. Manual and computer control options will be available to create sampling programs. Size and weight constraints will limit the use of this sampler to ground based sampling and use aboard small aircraft. In addition, the use of this sampler in the laboratory will greatly enhance the accuracy of timed sampling procedures.
The collection of humid air on sampling cartridges, which were subsequently desorbed for analysis, caused the detector flame to extinguish. The cold trap temperature, desorption flow rate and desorption time must be optimized to accommodate humid samples. Methods to generate humidified air were evaluated. A humidity system that utilizes a micro-porous PTFE tube (International Polymer Engineering; Tempe, AZ) surrounded by water is currently being assembled. This system will create a direct technique to humidify dry standard air at low levels to better investigate and optimize the thermal desorption of humid samples.
Construction of a gas dilution system was completed and it was delivered to us in November 1997. This system allows the generation of standard gas mixtures for calibration purposes. Several aluminum cylinders were purchased from Scott Marrin for the preparation and storage of standard gas mixtures. One cylinder was filled with air from Niwot Ridge, Colorado as a "clean" ambient air standard. Another cylinder was filled with a neohexane and decane standard. Two cylinders were sent to Scott Specialty Gas Company to undergo Accu-Life? treatment to deactivate the cylinder walls. A 1 PPM TO-14 standard gas mixture containing 39 components was ordered from Supelco. This gas standard will be diluted and used to create a CFC standard in one of the deactivated cylinders. A hydrocarbon gas standard will be made in the remaining treated cylinder using an ASTM D3710 Qualitative Calibration Mix purchased from Supelco, Inc. These two standards will be sent to an independent laboratory for analysis and concentration confirmation.