Final Report: Techniques for Standardization, Validation, and Measurement of Targeted Trace Gases Which Participate in Tropospheric Ozone FormationEPA Grant Number: R825261
Title: Techniques for Standardization, Validation, and Measurement of Targeted Trace Gases Which Participate in Tropospheric Ozone Formation
Investigators: Apel, Eric C. , Fried, Alan , Gilpin, Tim , Riemer, Dan
Institution: National Center for Atmospheric Research
EPA Project Officer: Shapiro, Paul
Project Period: January 1, 1997 through December 31, 1999
Project Amount: $472,250
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
The research objectives covers three distinct areas: (1) development of necessary standards for target oxygenated volatile organic compounds (OVOCs); (2) development of accurate analytical methods, including a formaldehyde method, to verify the standards and to lay the groundwork for reliable ambient measurements; and (3) development of a calibration facility whereby university researchers, including PAMS personnel (cartridges), may come to calibrate and test their instruments under simulated ambient conditions.
The objectives of our proposal have been met. We have investigated several methods for the preparation and analysis of standards. Part of the work in this proposal was based on the premise that independent methods of standards generation and analysis that agree increase the confidence in the overall accuracy of the standards. We constructed and employed a 6-position device for a standards generation system based upon permeation tubes and employing gravimetry as the method of calibration. We have calibrated permeation tubes for methanol, acetone, methyl ethyl ketone, and toluene. Acetaldehyde and propanal were unstable in the permeation tube: the calibration curve would change after about 3 weeks of use. This system consists of six independent aluminum ovens, each housing one permeation tube. The ovens are temperature controlled to within ± 0.1°C. The tubes are contained in glass housings with high purity nitrogen continuously flowing over them. A dynamic dilution system was constructed to control, mix, and dilute the permeation tube effluent to the ppbv to pptv range of concentrations. Although the tubes are factory calibrated, we also characterized them using a Mettler balance capable of measuring to ± 10 mg.
We also have prepared a number of high-pressure ppmv and ppbv standards with two independent techniques described below:
NCAR High Pressure pptv Standard Preparation-Volume Dilution Method 1. Pure starting materials (purity verification with GC/MS analysis) are transferred into a known volume via a high vacuum line. (The known volume is calibrated to an accuracy of 0.1 percent using a mercury gravimetric method using the ideal gas law pressure-volume relationships). Upon introduction of the compounds of interest, the increased pressure of the known volume is measured with a calibrated pressure transducer (MKS-Baratron); thus, giving a quantitative amount. The pure starting material within the known volume is then transferred to a larger volume and diluted with ultra high purity zero air, which has been scrubbed and analyzed to insure purity (purity analysis by either: GC/FID, GC/ECD, GC/MS, or by any number of assorted techniques available at NCAR depending on the compound).
NCAR High Pressure pptv Standard Preparation-Volume Dilution Method 2. This relatively straightforward method uses a heated injector, built at the National Center for Atmospheric Research (NCAR), that allows introduction of known volumes of the compound(s) of interest directly into a treated, evacuated cylinder. Calibrated syringes are used for the volume measurement. The known volume is injected through a septum and into a calibrated flowing stream (Mass Flow Controller) of diluent gas. The combination of the flowing stream of gas and the heated injector insure that all of the starting material is swept into the cylinder. The cylinder is then pressurized to the desired amount (calibrated high-pressure transducer) and allowed to equilibrate over several days. Uncertainties may arise from errors in the volume measurement and the final pressure measurement. This method is used to prepare low ppm standards and must be used with a calibrated dynamic dilution device described below. There is merit in preparing ppm standards as the stability of these levels in cylinders should be good.
Verification of the Accuracy of the Standards. Through this grant, methods have been developed to verify the accuracy of the standards. A GC-FID system was used to compare the high-pressure cylinders with the permeation tubes. The top panel in Figure 1 shows our analysis of the six tubes at once (however, it should be noted that for quantification our normal procedure is to run each permeation tube separately using a highly characterized diluent flow rate). The bottom panel shows our analysis of a high-pressure cylinder that was prepared containing acetaldehyde, methanol, ethanol, propanal, acetone, butanal, and methyl ethyl ketone. Note that all compound peaks are well resolved and easily quantified.
Together with the GC-FID analysis, an independent spectroscopic (FTIR) method also has been developed and used to ascertain the concentrations in the high-pressure cylinder. Table 1 shows the results of the comparison of the high-pressure cylinder preparation results with the permeation tubes and the FTIR analysis. Good comparability was observed for the cylinder preparation and the permeation tubes for methanol and acetone. We do not have reliable permeation tube values for acetaldehyde and propanal. We found that although the initial values we obtained were close to the manufacturer's certified rate, the permeation rate of these increased as a function of time. This is despite the fact that we used N2 sweep gas and maintained the permeation tubes at 30 °C. The manufacturer (VICI) is aware that there is a problem with the aldehyde perm tubes and is working with us to help resolve it. We are presently calibrating the MEK permeation tube.
The FTIR values agree within the error of the measurement with the gravimetric
cylinder values for all compounds measured, except for MEK. The reasons for
this are unclear and we will investigate this result further. It is important
that the compounds are stable in the cylinders. Table 2 shows our analyses,
taken almost 6 months apart, of some of the compounds relevant to this proposal.
These compounds showed excellent stability. Except for methanol (-7.1%), all
compounds agreed to within 5 percent for the two analyses.
Figure 1. Top panel: Chromatogram showing analysis of permeation tubes. Bottom panel: Chromatogram showing the analysis of the high-pressure cylinder standard.
|Table 1. Quantification of Standards Results|
|Compound||High Pressure Cylinder gravimetric (ppm)||High Pressure Cylinder analyzed (ppm)*||FTIR(ppm)|
|Methanol||3.00 ± 0.03||3.0 ± 0.1||2.9 ± 0.2|
|Ethanol||3.02 ± 0.03||n.d.||2.9 ± 0.3|
|Acetone||3.03 ± 0.03||3.03 ± 0.08||3.2 ± 0.2|
|Acetaldehyde||3.03 ± 0.03||n.d||3.2 ± 0.4|
|Propanal||3.03 ± 0.03||n.d||n.|
|Butanal||3.02 ± 0.03||n.d.||n.d.|
|*analysis based on calibration factors from permeation tubesn.d.- not determined|
|Table 2. Stability of Alcohol and Carbonyl Standards in High Pressure Cylinders|
|Compound||High Pressure Cylinder gravimetric (ppb)||% Difference Analysis* 22 October 1999 versus Analysis* 4 May 1999|
|Methanol||2080 ± 30||-7.1|
|Ethanol||207 ± 3||2.0|
|Acetone||1090 ± 16||4.6|
|MEK||206 ± 3||4.8|
|Acetaldehyde||528 ± 8||-2.1|
|Propanal||212 ± 3||1.5|
|Butanal||210 ± 3||3.8|
|*analysis based on relative calibration factors from FID and represent an average of 3 chromatographic runs.|
A 2-meter all-glass manifold was built at NCAR and set up to deliver a zero air stream containing standards and possible interferent species (Figure 2). A zero air generation system (AADCO 737-12) is located next to the inlet delivering ultra pure air with less than 1 ppm water and ambient levels of CO2. This zero air generator was used in conjunction with the glass manifold to deliver synthetic air along with the standards we developed and possible interferents to help validate our methodology and, in the future, to test other analysis systems for accuracy of the particular method in question. It is possible to run ambient levels of VOC species and interferents of interest. Systems are available to deliver and measure spikes of NOx, SO2, O3, and H2O.
We refined our measurement methods by using the above-mentioned manifold. Before taking a system into the field, we recommend running a suite of tests for quantitative measurement in the presence and absence of possible interferents. Our facility may be made available to research groups for this purpose. For example, ozone can be added, from 0 to 150 ppbv, to the synthetic airstream containing trace levels of OVOCs of interest. Although this does not cover all possible contingencies encountered in the field, it goes a long way toward ensuring good measurements in the field.
To address Objective 2, A GC/MS system was developed for measuring OVOCs. The system was optimized for resolution and time response for a number of OVOCs. Figure 3 shows a GC/MS chromatogram of an NCAR-prepared OVOC standard. The specificity is excellent with this type of analysis because compounds are separated chromatographically and the ions detected are specific to the compound whose retention time is known.
A GC and mass spectrometer may be purchased from a number of vendors. In any
case, the key component of the GC/MS OVOC measurement device is the sample preconcentration
system. A number of vendors also now supply an automated preconcentration device;
however, most are optimized for NMHC analyses. We built a preconcentration device
during the course of this grant and optimized it for quantitative analysis of
OVOC compounds. The GC/MS/preconcentration system consisted of a custom-built
three-stage (trap) device and an HP 5890 Series II GC/HP 5973 mass spectrometer
system. The medium for cooling the traps was provided by passing dry nitrogen
gas through copper coils immersed in a dewar of liquid nitrogen. The cooled
nitrogen gas was then passed through the traps to effect cooling. The temperature
was maintained by controlling the flow of N2 gas through
Figure 2. Manifold system for diluting standards and testing interferences.
Figure 3. GC/MS run of a 7-component standard. An 60-m HP-624 column was used.
In a typical ambient analysis, air is drawn from a port on the high velocity glass manifold and into the preconcentration system (Figure 4). Typically, the sampling rate is held at 50 cc min-1 for 6 minutes, yielding a total air sample of 300 cc. The sample is passed through the first stage trap, a 12 cm x 0.3175 cm PFA teflon tube maintained at -20 °C, which was designed to remove water from the sample. It is then passed into the second stage, which consisted of a 12 cm x 0.3175 cm o.d. silicosteelTM tube filled with silanized glass wool and maintained near -130 °C. It is here that the compounds of interest are initially trapped. Following this, a valve is switched, the trap is heated, and helium carrier gas sweeps the compounds of interest to the third cryofocusing trap, which is a 10-cm piece of megabore (0.53 mm i.d.) tubing maintained at -186 °C. This third stage is then rapidly heated and the compounds of interest transferred to the head of a 60-m DB-624 column. The GC oven is held initially at 35 °C for 2 minutes, temperature programmed at a rate of 25 °C /min until it reaches 190 °C, and then held at 190 °C for 11 minutes, giving a total chromatographic run time of 20 minutes. Carrier gas flowrate was maintained at 6 ml/min using the Electronic Pressure Control feature of the HP GC.
Figure 4. Preconcentration system
The specificity is excellent with this type of analysis because compounds are separated chromatographically and the ions detected are specific to the compound whose retention time is known. Single ion monitoring mass spectrometry was utilized for quantitation.
After testing for quantitative measurements, the GC/MS system that we developed was deployed in the field for testing and analysis. Figure 5 shows a typical analysis of select compounds of an air sample obtained during a study in Nashville sponsored by the Southern Oxidants Study.
Figure 5. GC/MS Analysis of rural air outside of Nashville Tennessee
Detection limits are good for this instrument. For a 300 mL sample, detection limits were for the compounds shown: acetaldehyde-5 pptv, methanol-30 pptv, ethanol-25 pptv, propanal-4 pptv, acetone-4 pptv, butanal-12 pptv, and MEK-8 pptv. The system was calibrated with NCAR high-pressure standards and a rack-mountable dilution system.
This work has contributed greatly to our understanding of what is needed to quantitatively measure oxygenated VOC compounds. The importance of these compounds has almost surely been misunderstood because of the difficulty in making good measurements of them. Using the standards and measurement methodology developed through this grant, quantitative measurements may be more readily attained.
One of the objectives (Objective 3) of the proposal was the development of a calibration facility whereby university researchers and the PAMS network (cartridges) could come to calibrate and test their instruments under simulated ambient conditions. The facility was indeed built and offered to the PAMS network, although it should be noted that the offer was not taken.
Quality Assurance Requirements. Our quality assurance objectives were based on the redundant calibration approaches for the target compounds previously discussed. Our goal was to achieve agreement among the various approaches to within 10 percent. As can be seen from Tables 1 and 2, these objectives were met (except for MEK) for our calibration standards, which were prepared and tested through support from this grant. For measurements, we need to be involved directly in an intercomparison experiment to fully evaluate the accuracy of the techniques that were developed in a field measurement setting.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
|Other project views:||All 9 publications||4 publications in selected types||All 4 journal articles|
||Apel EC, Calvert JG, Greenberg JP, Riemer D, Zika R, Kleindienst TE, Lonneman WA, Fung K, Fujita E. Generation and validation of oxygenated volatile organic carbon standards for the 1995 Southern Oxidants Study Nashville Intensive. Journal of Geophysical Research-Atmospheres 1998;103(D17):22281-22294.||
||Apel EC, Calvert JG, Riemer D, Pos W, Zika R, Kleindienst TE, Lonneman WA, Fung K, Fujita E, Shepson PB, Starn TK, Roberts PT. Measurements comparison of oxygenated volatile organic compounds at a rural site during the 1995 SOS Nashville Intensive. Journal of Geophysical Research-Atmospheres 1998;103(D17):22295-22316.||
||Apel EC, Riemer DD, Hills A, Baugh W, Orlando J, Faloona I, Tan D, Brune W, Lamb B, Westberg H, Carroll MA, Thornberry T, Cooper O, Geron CD. Measurement and interpretation of isoprene fluxes and isoprene, methacrolein and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 intensive. Journal of Geophysical Research 2002;107(D3):Art.No.4034.||
||Riemer D, Pos W, Milne P, Farmer C, Zika R, Apel E, Olszyna K, Kliendienst T, Lonneman W, Bertman S, Shepson P, Starn T. Observations of nonmethane hydrocarbons and oxygenated volatile organic compounds at a rural site in the southeastern United States. Journal of Geophysical Research-Atmospheres 1998;103 (D21):28111-28128.||