JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D17, PAGES 22,375-22,386, SEPTEMBER 20, 1998

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D17, PAGES 22,375-22,386, SEPTEMBER 20, 1998 Performance characteristics of an automated gas chromatograph-ion trap mass spectrometer system used for the 1995 Southern Oxidants Study field investigation in Nashville, Tennessee E. Hunter Daughtrey Jr., Jeffrey R. Adams, Karen D. Oliver, and Keith G. Kronmiller Methods Research, ManTech Environmental Technology, Inc., Research Triangle Park, North Carolina William A. McClenny Atmospheric Methods and Monitoring Branch, Human Exposure and Atmospheric Sciences Division, National Exposure Research Laboratory U.S. Environmental Protection Agency, Research Triangle Park, North Carolina Abstract. A trailer-deployed automated gas chromatograph-masspectrometer (autogc- MS) system capable of making continuous hourly measurements was used to determine volatile organic compounds (VOCs) in ambient air at New Hendersonville, Tennessee, and Research Triangle Park, North Carolina, in The system configuration, including the autogc-ms, trailer and transfer line, siting, and sampling plan and schedule, is described. The autogc-ms system employs a pair of matched sorbent traps to allow simultaneou sampling and desorption. Desorption is followed by Stirling engine cryofocusing and subsequent GC separation and mass spectral identification and quantification. Quality control measurements described include evaluating precision and accuracy of replicate analyses of independently supplied audit and round-robin canisters and determining the completeness of the data sets taken in Tennessee. Data quality objectives for precision (_+10%) and accuracy (_+20%) of 10- to 20-ppbv audit canisters and a completeness of >75% data capture were met. Quality assurance measures used in reviewing the data set include retention time stability, calibration checks, frequency distribution checks, and checks of the mass spectra. Special procedures and tests were used to minimize sorbent trap artifacts, to verify the quality of a standard prepared in our laboratory, and to prove the integrity of the insulated, heated transfer line. A rigorous determination of total system blank concentration levels using humidified scientific air spiked with ozone allowed estimation of method detection limits, ranging from 0.01 to 1.0 ppb C, for most of the 100 target compounds, which were a composite list of the target compounds for the Photochemical Assessment Monitoring Station network, those for Environmental Protection Agency method TO-14, and selected oxygenated VOCs. 1. Introduction itoring has been with automated gas chromatographs (autogcs) using hourly sampling and analysis on a continuous In 1990 the Clean Air Act Amendments (CAAA) expanded basis. These systems provide almost complete (up to 57 min in the list of volatile organic compounds (VOCs) that must be monitored in ambient air (58, Federal Register, 8452, February 12, 1993). Compendium methods TO-15, TO-17, and the revised TO-14 (available from the National Exposure Research Laboratory, U.S. Environmental Protection Agency (EPA), Research Triangle Park (RTP), North Carolina) have provided more flexible guidelines to meet the varied monitoring requirements for VOCs. Since then, several developments in preconcentration and analysis methods have extended the range of VOCs that can be determined in ambient air, by compound class, detectable concentrations, and sheer numbers of compounds of interest that are listed by the CAAA. 1 hour) time coverage [Oliver et al., 1996a]. We have successfully used this approach in the analysis of hazardous VOCs in ambient air near industrial sources [Daughtrey et al., 1995]. This approach has also been used in a monitoring network that operates in ozone nonattainment areas of the country, the Photochemical Assessment Monitoring Stations (PAMS [Gerald, 1993]), employing an autogc system with flame ionization detection (FID) for determining ozone precursor hydrocarbons listed under Title I of the CAAA (58, Federal Register, 8452, February 12, 1993). However, the lack of specificity of the FID and the large amount of data generated by such monitoring stations make the validation of the data laborious. Some recent research on methods and techniques for mon- Most autogc systems require the use of a liquid cryogen [Yokouchi et al., 1994; Greenberg et al., 1994; Goldan et al., Copyright 1998 by the American Geophysical Union. Paper number 98JD /98/98JD ; Leibrock and Slemr, 1997], which can be problematic. In addition to the logistical problems of supplying liquid cryogens to field locations, the use of cryogenic sampling may pose 22,375

2 22,376 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM analysis problems due to ozone preconcentrated with the VOCs, if the ozone is not removed [Helmig, 1997]. description of the operation of the concentrator, cryotrap, and GC-MS system is given elsewhere [Oliver et al., 1996b]. We use an autogc system in which the FID component has Trailer. The outside dimensions of the mobile labbeen replaced with a mass spectrometer (MS). The sensitivity oratory are 2.8 m wide by 8.5 m long, including the tongue, and of the autogc-ms system (method detection limits (MDLs) of 0.01 to 1 ppb C for 60-cm 3 samples of selected VOCs in air the inside dimensions are 2.7 m by 6.7 m. The laboratory is furnished with two instrument benches, which measure 3.3 and [Oliver et al., 1996b]) allows analysis of small sample volumes, 3.7 m long, two gas bays that will accommodate a total of eight which effectively reduces the interference associated with residual water vapor [Almasi and Kirshen, 1992]. In this paper we discuss the use of this system in the Nashville study for continuous monitoring of ozone precursor VOCs, method TO-14 target compounds, and selected oxygenated VOCs, and we describe the techniques we developed for ensuring the quality of the reported data. cylinders, a computer desk, a storage closet, a small refrigerator, and storage shelves and drawers. Two heating/air conditioning units (20 A, 120 V) and two ceiling ventilation fans are available for temperature control. The laboratory is equipped with two 50-A, 240-V electrical boxes and a total of 64 outlets. A standard surface meteorological (met) station (MetOne Instruments, Grants Pass, Oregon) measuring wind speed, wind The focus of this paper is on the quality control procedures direction, temperature, and relative humidity at a 10-m height used to ensure that the data taken meet the data quality objectives for hourly measurements of compounds related to tropospheric ozone production. We use as a starting point for the data validation procedure presented here that used in the Atlanta Southern Oxidants Study (SOS) for monitoring ozone precursor VOCs in ambient air [Apel et al., 1995; Bernardo- Bricker et al., 1995]. We use selected data for compounds of special interest to illustrate the steps taken in qualifying the was employed. Solar radiation and ozone measurement instruments were added in July and August 1996, respectively. Data are recorded by a Campbell Scientific model CR10 data logger (Campbell Scientific, Inc., Logan, Utah) Siting. The trailer was used at two sites for the data presented here, the 1995 Nashville SOS site in Tennessee and a site near our laboratory in RTP. The SOS site was on a farm near New Hendersonville, Tennessee, about 20 miles (32 km) data. northeast of Nashville. The site was intended to be downwind Preliminary results from the Nashville study were presented at the Air and Waste Management Association National Meeting in Nashville in 1996 [McClenny et al., 1996; Daughtrey et al., 1996]. Further experiments were performed to qualify these data. This paper discusses the results of that process. 2. Experimental Procedures We present in this section our system configuration, sampling plan, and data analysis method System Configuration The system configuration includes the autogc-ms system, the trailer, and the siting AutoGC-MS system. A commercially available XonT½ch Model 930 organic vapor concentrator (XonT½ch, Inc., Van Nuys, California) equipped with two multisorbcnt traps was interfaced to a XonT½ch model 940 cryofocusing trap, which is also commercially available. All tubing and transfer lines were 1/16-inch fused-silica-lined stainless steel. The model 930 two-trap system allowed the collection and dry purging of sample on one trap while the other trap was simultaneously cycled through the desorb, clean, and cool modes. The multisorbcnt traps contained 0.05 g Tcnax-GR, 0.04 g Carbotrap, and 0.51 g Carbosicv½ S-III [Levaggi et al., 1992]. The collected VOCs were desorbed from the model 930 sot- bent trap and focused on the model 940 cryotrap. The VOCs were then desorbed onto the GC column by heating the model 940 trap. This trap consists of a length of small-diameter, thin-walled, stainless steel tubing (10-inch length, inch outside diameter, inch wall thickness). This tubing was wrapped around a copper coil form that was connected to the expander of a Stirling cooler (Hughes, Electron Dynamics Division, Torrance, California). An electric heater tape, wrapped around the tubing-copper coil form assembly, was used to heat the trap and desorb the collected VOCs onto the GC column. During the evaluation in the Nashville study, the model 940 cryotrap was interfaced to a Saturn II GC-MS (Varian Chromatography Systems, Walnut Creek, California). A detailed from downtown Nashville, but the prevailing winds were seldom from that direction during the June 17-27, 1995, period of our measurements. Our home site is at the rear of the parking lot of the EPA RTP Environmental Research Center Annex. Measurements at the site are potentially impacted by a stand of pines to the north and an Interstate 40 interchange to the east. This site is used only to test methods and instrumentation, not to take data for modeling, policy, or regulatory purposes. Data were taken here from August 17 through September 3, 1995, and April 17 through August 30, At the New Hendersonville site, air was sampled from a 24-m cross-linked, Teflon-coated manifold supplied by Georgia Institute of Technology researchers at the site. Air was pulled into this sampling manifold at a velocity of 10 m/s. The 11-m perfluoroalkoxy (PFA) Teflon transfer line, which was electrically heated (approximately 10øC above ambient temperature) to prevent loss of the less volatile VOCs, connected the Georgia Tech manifold to the heated glass sampling manifold inside the trailer. Air was pulled through the transfer line to the XonTech 930 at a rate of 3 L/min. Sample was collected on the sorbentrap by pulling 5 cm3/min of air from the airstream. At the RTP site, we mounted an inlet directly on the end of the heated transfer line and affixed it to the crossbar of the met tower at approximately 10 m. For certain experiments at RTP, we used a mixing box that allowed blending of ozone (from a Teco 49PS ozone generator (Teco, Waltham, Massachusetts)), a VOC standard from a cylinder, ambient air, scientific-grade air, and humidified scientific-grade air (HSA) in controlled proportions. This allowed the observation of artifact formation and VOC conversions at concentration levels above the detection limits for the com- pounds of interest Sampling Plan Our sampling plan included preparing test mixtures for the target compounds and adherence to a sampling schedule Target compounds. Test gas mixtures for the eval- uation were prepared by dynamic dilution of low-concentration (part per million by volume level) gaseou standards in

3 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM 22,377 Table 1. Target Compounds for Nashville 1995 Summer Study Ozone Precursors TO-14 (Alphagaz Standard) (Alphagaz Standard) "Aidehyde Standard" Isobutane 1-Butene n-butane trans-2-butene cis-2-butene 3-Methyl-l-butene 2-Methylbutane 1-Pentene n -Pentane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n-heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane Isoprene trans-2-pentene cis-2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-l-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane n -Octane Ethylbenzene m,p-xylene Styrene n -Nonane o-xylene Isopropylbenzene a-pinene n -Propylbenzene 1,3,5-Trimethylbenzene 3-Methylpentane 2-Methyl-l-pentene n -Hexane trans-2-hexene cis-2-hexene 2-4-Dimethylpentane Methylcyclopentane Benzene Cyclohexane 2-Methylhexane n-decane /3-Pinene 1,2,4-Trimethylbenzene Undecane dichlorodifluoromethane chloromethane 1,2-dichloro-l,l,2,2-tetrafluoroethane chloroethene bromomethane chloroethane trichlorofluoromethane 1,1-dichloroethene dichloromethane 3-chloropropene 1,1,2-trichloro- 1,2,2-trifluoroethane 1,1-dichloroethane cis- l,2-dichloroethene trichloromethane 1,2-dichloroethane 1,1,1-trichloroethane benzene carbon tetrachloride 1,2-dichloropropane trichloroethene cis- 1,3- dichloropropene trans- 1,3-dichloropropene 1,1,2-trichloroethane toluene 1,2-dibromoethane tetrachlorothene chlorobenzene ethylbenzene m,p-xylene styrene 1,1,2,2-t e trachloro ethane o-xylene 4-ethyltoluene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene benzyl chloride m -dichlorobenzene p-dichlorobenzene o-dichlorobenzene 1,2,4-trichlorobenzene hexachlorobutadiene methacrolein methyl vinyl ketone butanal 1,1,1-trichloroethane 2-pentanone pentanal trichloroethene hexanal tetrachloroethene heptanal benzaldehyde octanal nonanal decanal high-pressure cylinders in HSA (National Specialty Gases, RTP) at 70% relative humidity [Oliver and Pleil, 1985]. Summa-polished stainlessteel canisters (SIS, Inc., Moscow, Idaho; Graseby-Andersen, Smyrna, Georgia) were then pressurized to 30 pounds per square inch gauge (psig) (207 kpa) with the dilute gas mixtures for use in the Nashville SOS study. The relative humidity of the delivered gaseous standards in the canisters varied from 35% to 65%, depending on the canister pressure at the time the sample was extracted. The gas standards included the following components. 1. The first component was a mixture of 55 ozone precursor VOCs at nominal concentrations of 2 ppmv each in nitrogen (Alphagaz, Walnut Creek, California). Note that this compound list differs from the current PAMS target list, as the standard was procured before the EPA Office of Air Quality Planning and Standards changed some of the compounds in the target list. 2. The second component was a mixture of the 41 VOCs on the EPA method TO-14 target list at nominal concentrations of 1-2 ppmv each in nitrogen (Alphagaz, Walnut Creek, California). 3. The third component was an "aidehyde" mixture pre- pared by injection of an aqueous solution of the compounds of interest into an evacuated canister. Our preparation method is documented in an internal report and is available [Bowyet, 1996]. The list of compounds for which we calibrated the instrument are sorted by source in Table 1. All standards were made to a nominal 10 ppbv each in the canisters. Previous multipoint

4 22,378 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM calibrations for each of the standards had established linearity of the analytical system for each of the compounds Sampling schedule. Continuous hourly samples were collected and analyzed at the New Hendersonville site from June 17 to June 27, 1995, except for daily local (MDL), 0.1 ppbv; relative percent difference between replicates, 10%; internal audit accuracy, 20%; and completeness, 75%. In addition, specific target objectives were set for each compound, based on the previous performance history of each compound. We analyzed synthetic audit standards and roundtime blocks of time used to run calibration standards and robin samples to evaluate precision and accuracy. blanks. A few sampling periods were lost because of power interruptions. Concurrent meteorological measurements were taken. At RTP, hourly sampling was conducted for large blocks Audit standards. As a test of system accuracy during the Nashville study, an analysis was performed on a canister prepared from an independent audit cylinder traceable to a of time between August 17 through September 3, 1995, and National Institute of Standards and Technology (NIST) stan- April 17 through August 3, Concurrent meteorological dard, which was obtained from the EPA Atmospheric Remeasurements were also taken for the RTP sampling. Ozone search and Exposure Assessment Laboratory, Quality Assurmeasurements were added in August Hourly collection ance and Technical Support Division, and originally made by of the ambient samples was interrupted during the spring of Research Triangle Institute (Research Triangle Park, North 1996 to conduct experiments mixing ozone with standards, Carolina). The analytical results were submitted to the Quality ambient air, and HSA. During the summer of 1996, an ozone Assurance and Technical Support Division and compared with denuder was placed on the inlet to determine the effect of the the audit "known" values. The results of this comparison are absence of ozone on our measurements. given in Table 2. The signed mean percent difference of the measured concentration from the true value was less than 3% 2.3. Data Analysis (unsigned mean difference, 6.35%), with only trans-2-butene A commercially available statistical analysi software package, StatGraphics Plus (Manugistics, Rockville, Maryland), and 3-methyl-l-butenexceeding the accuracy data quality objective for the study. On comparison, the audit results were was used in evaluation of the data sets. closer to the "true" values than results from the QA laboratory (lab B), which averaged 13% difference from true. Internal precision of our measurements, as measured by the mean un- 3. Results and Discussion signed difference between replicates, averaged 5%. The discussion that follows describes the quality control (QC) measures we used, the results of our data set review, and results of some special system tests we conducted. We also include a discussion of blank determination, estimating detection limits, and the observed ambient ranges. A second independent audit of PAMS participants was conducted by Eastern Research Group (Morrisville, North Carolina) in the fall of We analyzed two different canisters, each with the same audit standard, twice each. Some PAMS target list compounds were not analyzed by our system, as was 3.1. QC Measurements noted above, as our standard predated the current PAMS list, and we were not analyzing for C2 and C3 hydrocarbons at the time of the audit. The results of these analyses are summarized The three standard quantitative measures of data quality are precision, accuracy, and completeness [U.S. Environmental in Table 3. The RSD of the four replicate measurements was Protection Agency, 1989]. Precision is usually given as percent 5% RSD. We compared our results to results of two independent laboratories, treating their results as the true values. Both relative standard deviation (%RSD) unless the sample quantity is a limiting factor, in which case replicate precision is used. laboratories employed Nation drying, cryogenic trapping, and Replicate precision is expressed as the mean difference be- GC-FID with a propane calibration standard. Our results tween pairs of measurements, accuracy is expressed either as a ranged from 1% to 25% higher than those of laboratory A, percent recovery or as percent difference between a measured averaging 14%. On comparison with laboratory B results, our value and a "known" value, and completeness is expressed as results ranged from -12% to +3% difference from the referpercent data capture, or the fraction of possible measurements ence laboratory results, averaging 5% lower than those of that is actually achieved. Quantitative data quality objectives laboratory B. All results indicate our accuracy was acceptable, are often set for these measures; these may be based on the especially given the differences in calibration standards and intended use of the data, but are more often based on the analysis methods. historical performance of the monitoring instrumentation. For Round-robin results. In addition to the synthetic the measurements required for ozone precursor compounds, audit standards, a 33-L canister of urban ambient air taken in particularly at background sites, the methods normally employed for evaluating these data quality objectives may give a false sense of security about the quality of the data. This misleading information comes about because the measured concentration levels are often 1-3 orders of magnitude lower than typical audit concentrations. In the following, we employ such standard methods to show system performance, demonstrate how some measures fail at low concentrations, and discuss some other techniques which assist in qualifying data Data quality objectives for this study. Our data quality objectives were set in the quality assurance (QA) project plan prepared before the Nashville SOS intensive began. The approximate overall goals for the quantitative data quality QA objectives are as follows: minimum detection limit Nashville was used as a round-robin sample among the SOS participants. The results from the analysis of that canister are given in Table 4. We have taken the results of the EPA laboratory conducting the round-robin as true. For the 28 compounds detected both in our analysis and that of the EPA laboratory using cryogenic trapping GC-FID, results varied as much as +_50% from the true value. However, half of these values were within the _+20% audit accuracy data quality objective. There also seems to be an overall low bias of our results. There may be two factors influencing this result. At the time of the round-robin analysis in the field, we had not yet discovered the effect of canister pressure on the preconcentrator trap loading, which led to the change in the canister analysis procedure described by Oliver et al. [1996b]. If there were

5 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM 22,379 Table 2. Field Audit Results From New Hendersonville, Tennessee (Canister 01628) This Lab (n = 2) "True" Concentration, Concentration, Compound ppb C ppb C Precision* 1-Butene % trans-2-butene % 3-Methyl- 1-butene % 1-Pentene % Isoprene % cis-2-pentene % 2,2-Dimethylbutane % 4-Methyl- 1-pentene % 2,3-Dimethylbutane % 3-Methylpentane % n -Hexane % cis-2-hexene % 2,4-Dimethylpentane % Cyclohexane % 2,3-Dimethylpentane % 2,2,4-Trimethylpentane % Methylcyclohexane % 2,3,4-Trimethylpentane % 2-Methylheptane % Ethylbenzene % m,p-xylene % o-xylene % Isopropylbenzene % 1,3,5-Trimethylbenzene % Lab B Concentration, Accuracy? ppb C Accuracy 4.94% % 13.60% % % % 4.88% % % % 4.90% % 2.77% % 5.52% % 7.00% % 6.33% % 7.87% % 7.86% % % % 10.45% % 5.40% % 2.66% % 1.12% % 7.02% % 8.68% % -6.92% % -5.88% % 0.06% % 1.05% % 8.27% % *Precision = (a - b) x 200/(a + b).?accuracy = (Found- "True") x 100/"True." substantial differences in the pressure of the sample canister (35-40 psig (241 kpa) initial pressure) and that of the calibration standard canister (initially psig (276 kpa), but usustandard is 0.5 ppbv in each component; this translates to 0.5 to 5.5 ppb C for the ozone precursor compounds. Most of the VOC concentrations found in this round-robin sample fell ally lower, because more aliquots are taken from a standard between this lowest calibration standard and zero. than a sample canister), a bias could be introduced. Second, system calibration is difficult at low concentrations because of Completeness. The autogc-ms system collected valid data for 191 out of a possible 200 collection periods, the greater difficulty in preparing very low concentrations, representing 94% data capture. Two interruptions were due to especially in canisters, and the difficulty in attaining measure- weather-induced electrical power outages; one was due to a ment precision at low concentration. Our lowest calibration late start after a calibration sequence. Table 3. Analysis of Independent PAMS Audit Canister Accuracy Precision of Our Lab (n = 4) Lab A Lab B Mean s.d., Compound ppb C ppb C %RSD 1-Butene % n-butane % n-hexane % Benzene % Cyclohexane % Toluene % n-octane % Ethylbenzene % m-, p-xylene % o-xylene % n -Propylbe nzene % 1,3,5-Trime thylbenzene % 1,2,4-Trimethylbenzene % n-decane % Mean value 5.17% Concentration, Our Concentration, Our ppb C %Diff* ppb C %Diff? % % % % % % % % % % % % % % % % % % % % % % % % % % % % 13.93% -5.32% *%Diff = (our results - lab A results) x 100/lab A results.?%diff = (our results - lab B results) x 100/lab B results. Here, s.d. denotes standard deviation, and %RSD denotes percent relative standard deviation.

6 22,380 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM Table 4. Results of Nashville SOS Round-Robin Canister 0002 Isobutane 1-Butene n -Butane Compound trans-2-butene cis-2-butene 3-Methyl-l-butene 2-Methylbutane 1-Pentene n -Pentane Isoprene trans-2-pentene cis-2-pentene 2-Methyl-2-butene 2,2-Dimethylbutane Cyclopentene 4-Methyl-l-pentene Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2-Methyl-l-pentene n -Hexane trans-2-hexene cis-2-hexene 2,4-Dimethylpentane Methylcyclopentane Benzene Cyclohexane 2-Methylhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane n -Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane 3-Methylheptane n -Octane Ethylbenzene m,p-xylene Styrene n -Nonane o -Xylene Isopropylbenzene a-pinene n -Propylbenzene 1,3,5-Trimethylbenzene n -decane /3-Pinene 1,2,4-Trimethylbenzene Undecane Relative This EPA Difference, Lab Lab % < % < <0.32 <0.40 < % < % % % < % % < <0.48 Values are in parts per billion C. < < % % < % < < < % % < % % % % % % % % < < % % % < % % < < < < % < % % measures include retention time stability, calibration checks, frequency distribution checks, and checks of the mass spectra Retention time stability. For either FID or mass spectral detection, GC retention time stability is critical for proper identification of the sought-for constituents. Because we were conducting these studies in a trailer in the summer in the southern United States, we were concerned about the adequacy of the air conditioner to allow the GC to cycle back to its start temperature. An efficient way to screen for problems is to evaluate the data set as a whole. A spreadsheet was constructed for the measured retention times for each com- pound, along with the associated meteorological variables for each run. We checked the mean, median, and standard deviation for each retention time and examined univariate plots (scatterplots, box-and-whisker plots, and frequency histograms) as a measure of performance and to check for significant outliers. Figure 1 gives the box-and-whisker plot for tol- uene retention time from the New Hendersonville data set. This plot reveals only a single outlier, the same run occurring for all points, indicating a late start for that GC run Calibration checks. Review of the aggregate data of a data set can reveal response calibration changes that may arise from improper integration of a calibration standard, retuning of the MS parameters, or a change in the standard's concentration. An illustration of both types of changes was found in our/3-pinene results over the course of the study. In the review of our 1995 RTP data (Figure 2), a plot of/3-pinene versus a-pinene concentrationshowed two distinct slopes to the response line. This would indicate either a separate source of/3-pinene or a shift in calibration. We examined the data both with respect to meteorological variables and in the time sequence. We could find no correlation with temperature, wind speed, wind direction, or humidity that would explain the observation of two slopes. We checked the calibration standards that bracketed the suspect data, and found that this unusual response was due to a difference in the response factor for /3-pinene in that one canister. In the review of the overall data for 1994, 1995, and 1996, we also observed a change in the relative area counts for /3-pinene versus a-pinene, this ratio being 2.24, 4.13, and 4.65, respectively. This discrepancy is attributable to the degradation of/3-pinene in our calibration cylinder, an observation consistent with the experience of other researchers (W. A. Lonneman, personal communication,, ttlier due :o late start 3.2. Data Set Review The large quantities of data collected for hourly measurements of nearly 100 target VOCs present both a problem and a potential solution for qualifying the data. Manual review of each mass spectrum for each peak in each chromatogram for a 2-week to 4-month study, while not impossible, is impractically time consuming. The large quantity of data does lend itself, once tabulated, to statistical evaluation of the aggregate. Several statistical analysis software packages are commercially available; we used StatGraphics versions 2.1 and 3.0. Review Toluene retention time, min. Figure 1. Box-and-whisker plot for toluene retention times, Tennessee, I I

7 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM 22, ). For this reason our current interpretation of/3-pinene results is only semiquantitative Frequency distribution checks. In examining a data set, one may wish to determine whether the measurements represent the air mass as a whole or may reflect perturbations from a local source or special phenomena. Statistical treatment of a large data set may allow such determinations to be made. Figure 3 demonstrates the power of applying statistical plotting methods to the validation and screening of such large data sets. Carbon tetrachloride is known to have a constant global background of approximately 0.1 ppb [Singh et al., 1982]. A scatterplot (Figure 3) of the CC14 data from RTP in 1995 shows a group of outliers at an apparent concentration of 0.2 ppb. This anomaly was also demonstrated in a normal frequency distribution plot (not shown), in which most of the data appear normally distributed except for a clear discontinuity due to the outlying data. A plot of the data by sampling date (not shown) revealed that the outliers all occurred in a 3-day span, after which there was a return to the expected value of 0.1 ppb. Review of the data revealed a misintegration for CC14 in the calibration for that 3-day period Checks of mass spectra. The mass spectrum of each GC peak is matched against the user-generated library spectra of compounds in the retention time window by the MS data processing software. Use of a selected major fragment ion is the basis for quantitation of a target analyte. However, identification of a unique ion within a retention time window that is representative of the analyte alone is not always possible. Unless one is extremely familiar with the complete composition of the sample and what interferences can occur, it is easy to be misled by coeluting species, which may have an ion in common with that selected for quantitation of the analyte. Thus at a minimum, spot checking of the mass spectra should be done. A first step is to review the "reverse fit" (the degree to which a sample mass spectrum is contained in the library mass spectrum) measurements given in the analysis report; they can be either manually scanned or statistically evaluated, as described above for retention times. In addition, the spectra of selected chromatographic peaks can be evaluated for evidence of either a poor match or "contamination" of the spectra from a coeluting compound. Such an intensive evaluation of the spectra was required for the Nashville SOS data set, as we were requested to share our methacrolein and methyl vinyl ketone results with another " = 90 o 60 o 30 h o 6o 8o alpha-pinene concentration, ppbc Figure 2. Plot of/3-pinene versus a-pinene concentrations, Research Triangle Park, Carbon tetrachloride concentration, ppb Figure 3. Scatterplot for carbon tetrachloride concentration, Research Triangle Park, researcher in the 1995 Nashville intensive. Both methacrolein and methyl vinyl ketone showed a good reverse fit, 977 and 992 out of 1000, respectively. Because the concentrations measured were very low and we were concerned about the possibility of ozone artifact formation from reaction of the sorbent with ozone during sampling or during thermal desorption, we compared spectra for a Nashville sample to some spectra from runs made later at RTP in which HSA was spiked with 100 ppb ozone. (See the following discussion of blanks and MDL determination.) For this comparison we used spectra from the MS library and a calibration standard. The mass spectra from the mass spectral library, a calibration standard, a Nashville sample, and an ozone-spiked HSA are given in Figure 4. There were peaks found within the retention time window for the ozone blank samples that appeared to match the usergenerated library spectra for methacrolein and methyl vinyl ketone. However, closer examination of the spectra for each tentatively identified methacrolein and methyl vinyl ketone peak in the ozone-spiked blank showed the presence of other ions that were neither a match for the library spectra nor structurally possible for the two sought-for constituents. The spectra for the Nashville samples did not contain these spurious ions, and therefore the peaks could be qualified as methacrolein and methyl vinyl ketone Special System Tests There are several aspects of our system and procedures that differ significantly from the well-established canister techniques. We conducted a series of special experiments to ensure that these aspects have no deleterious effects on data quality. These included sorbent tube changing and conditioning, testing of the aldehyde standard, and testing of the transfer line Sotbent tube changing and conditioning. Sorbent tubes need to be periodically changed, as the repeated cycling of sampling and thermal desorption will change the adsorbent characteristics of the sorbent trap. We have successfully used traps for up to 28 months of continuous operation. If the two sorben traps begin to diverge in their ability to collect sample, as indicated by the relative area counts of calibration standards, the traps should be changed. The traps should always be changed as pairs. New traps need to be conditioned for 24 hours of cycling before data are to be considered usable, in addition to any conditioning done by the manufacturer. On one occasion we observed initially high background levels for some compounds, such as octanal, nonanal, and decanal, which reduced to zero or a low constant level after conditioning.

8 22,382 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM Library and Calibration Standard Spectra Methyl Vinyl Ketone Methacrolein Library Calibration Standard 55 7O -,.- :-:,' Real and Synthetic Samples New Hendersonville Air Sample ,.,,. 6.0_... _7J 7.8 8_4 I ' I ' I ' I ' I ' I- ' I I ' I ' I ' I ' I ' I ' I Ozone-Spiked HSA =1...,,I I ' I I ' I I ' I ' Figure 4. Spectra used for comparison: mass spectral standard from mass spectralibrary for methacrolein and methyl vinyl ketone, calibration standards for methacrolein and methyl vinyl ketone, New Hendersonville air sample, and ozone-spiked HSA synthetic sample Test of "aidehyde standard." Because the aidehyde standard we used was made by our staff, it does not have the proven reliability of a NIST-traceable standard. We performed two special studies to evaluate the precision and accuracy of the standard. We conducted a 7-day canister stability study to evaluate the variability both in our preparation method and in short-term storage of canisters. Standard component concentrations were of the order of ppbv. We also added internal standards of 1,1,1-trichloroethane, tetrachloroethene, and trichloroethene to the aldehyde mixture, as they are well characterized in TO-14 analysis. Canisters were analyzed on days 0, 2, 4, and 7. No drift or sample degradation was observed, and the overall %RSD was within the _+10% data quality objective for precision. We also analyzed the standard with a different sorbent preconcentrator and GC with atomic emission detection (AED). The AED provides carboncounting ability, and this feature indicated that our standard exhibited a constant 100 area counts/ppb C for the homologous series of aldehydes, except for decanal, whose results we had looked at with suspicion because of known solubility and polymerization problems. Also, hexanal had a coeluting interference on the Dynatherm-GC-AED system (Dynatherm ACEM 900, Dynatherm Analytical Instruments, Inc., Kelton, Pennsylvania) not observed on either the GC-FID-ECD system, on

9 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM 22,383 which the stability study had been done, or on the autogc-ms system used in the Nashville study. Measuring the aldehyde standard on the autogc-ms system gave an average 120% recovery for the chlorinated internal standards, which is not unreasonable for a standard made by an unproven or unvalidated experimental technique Test of integrity of insulated, heated transfer line. To test the integrity of the 11-m heated, insulated Teflon transfer line, the ozone precursor standard was mixed with ambient air in a mixing chamber attached to the inlet of the transfer line. Four canister samples were collected from a sampling port after the mixing chamber, each canister sample taken concurrently with sample collection on the XonTech 930 sorbent trap. This procedure yielded pairs of measurements of canister samples taken before the air passed through the transfer line and real-time sorbent samples after the transfer line. The canister samples were then analyzed on the autogc-ms system. Results of these paired measurements and the calculated percent difference for each pair are given in Table A1. The mean percent difference shows neither compound loss nor compound gain from passage through the manifold for the target ozone precursor compounds. The apparently higher level of n-aldehyde concentrations observed for canisters taken at the top of the manifold is attributable to canister artifacts. The ozone precursor standard does not include n-aldehydes. Also as a part of the investigation of system integrity, we performed a series of runs at RTP in which a potassium iodide-coated annular denuder was placed at the inlet of the heated transfer line to remove ozone. Tests with and without the denuder showed about 70-80% removal of ozone (maximum concentration - 70 ppb O. ). No clear reduction or increase of target list compound concentrations in ambient air was observed with the denuder in place. However, the ambient VOC concentration levels were very low during this experiment, which precluded an assessment of any small differences due to the presence or absence of ozone. Future plans include experiments with synthetic sample streams Blank Determination, Estimate of Detection Limits, and Observed Ambient Ranges trations observed in "clean" canisters filled with HSA. Supporting data are available with entire article on microfiche. Order by mail from AGU, 2000 Florida Ave., NW, Washington, DC or by phone at Document 98JD01249M; $2.50. Payment must accompany order. Our first approach to addressing the problem of blank determination was to fill a set of canisters that were straight from the manufacturer and had no previous sample collection history. These canisters were evacuated and filled with HSA. A set of 23 runs was made, giving determinations on each trap. This method achieved very low blank levels, lower than all measured levels of most of the target VOCs. However, we realized that as canisters are used, they develop an increased blank level for several target compounds, so we sought an alternative source of clean air for our blank analysis. We con- structed a clean air system, which blended scientific-grade air with scientific-grade air from a dual-stage distilled water bubbler, both airstreams controlled by mass flow controllers, so that a range of humidities could be dialed in. While humidified clean air is a measure of the lowest con- centrations that can be observed, it is still not representative of air that is devoid of only VOCs. We alternated samples of 100 ppb ozone with samples of the humidified clean airstream because we were concerned about possible artifact formation by reaction of ozone in air with the sorbent. While 100 ppb is less than the maximum ozone concentrations observed for Nashville and RTP, it is a typical high value and large enough that any effect should be observable. For each VOC for which the system is calibrated, an MDL is calculated by the following formula: MDL = t(n_l,l_a=0.99) S where t(n_, _ = 0.99) is Student's t value appropriate to a 99% confidence level and a standard deviation estimate with n - 1 degrees of freedom and S is the standard deviation of the replicate analyses. The definition of which replicate analysis to use has often been problematic. The standard deviation of the lowest standard whose concentration is no greater than 5 times the com- puted detection limit has often been used. As mentioned above, it is difficult to make a true measure of the detection limit both because of the difficulty in accurately preparing very dilute standards, particularly in canisters, and because synthetic standards seldom are completely representative of the air being measured (possibly because of the absence of water vapor and ozone). With these caveats well in mind, we have calculated detection limits, based on both HSA and HSA spiked with 100-ppb ozone. This MDL can be used as a filter in determining the effects of meteorological and other measurements on the ambient air VOC concentrations to avoid the In addition to the normally applied measures of data quality (system evaluation and qualification and a statistical viewing of the whole data set), a true determination of blank concentration levels must be made. The question may then be asked, What constitutes a true blank? Some researchers simply use the carrier gas helium blank; this clearly is not representative overinterpretation of data. The chief shortcoming of this of an air sample. Others use scientific-grade nitrogen or air; method is that it does not account for closely eluting VOCs, this does not account for the sample humidity characteristic of such as a-pinene and benzaldehyde, which may have insuffimost samples taken in the eastern United States. In our labocient differences in the primary ions chosen for quantitation ratory we have for several years filled canisters with HSA. This and in their mass spectra to totally preclude misidentification. procedure had proved to be satisfactory for the TO-14 target This rarely happens with the acceptance criteria for mass speclist of VOCs collected in canisters, as TO-14 specified a blank tral identification, and the closely eluting compound pairs that level less than 0.2 ppbv for each of the target list compounds. we have identified are potential, not necessarily actual, coint- However, with the improved instrumental detection limits of erference. The detection limits for HSA and ozone-spiked the autogc-ms system and the expanded target list needed for HSA are given in Table A2 (available on microfiche). this study, this was no longer satisfactory for direct measure- Most spreadsheet and statistical analysis programs allow ments, as concentrations for some compounds could be mea- correlation of VOC measurements with each other and with sured in ambient air at levels that were lower than the concenassociated meteorological measurements and with time. At a minimum, one must use all the above tests to qualify the data for further evaluation. We blank corrected and screened out all data below the MDL, using the more conservative (i.e., higher) measure found for each compound for the HSA and HSA plus ozone blanks. Table 5 presents the selected MDL, the highest

10 22,384 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM Table 5. Method Detection Limits (MDL), Maximum Concentration Observed, and Percentage of Observations Above MDL for Measurements at Tennessee and Research Triangle Park, 1995 Tennessee (June) Research Triangle Park (August) MDL, Maximum, Percent Above Maximum, Percent Above Compound ppb C ppb C MDL ppb C MDL Notes* TO-14 Standard Dichlorodifluo rome thane Chloromethane ,2-Dichloro-1,1,2,2-tetrafluoroethane Chloroethene Bromomethane Chloroethane Trichlorofluoromethane ,1-Dichloroethene Dichloromethane Chloropropene ,1,2-Trichloro-l,2,2-trifluoroethane ,1-Dichloroe thane cis-l,2-dichloroethene Trichloromethane ,2-Dichloroethane ,1,1-Trichloroethane Benzene Carbon tetrachloride ,2-Dichloropropane Trichloroethene cis-l,3-dichloropropene trans-l,3-dichloropropene ,1,2-Trichloroethane Toluene ,2-Dibromoethane Tetrachloroethene Chlorobenzene Ethylbenzene rn,p-xylene Styrene ,1,2,2-Tetrachlo roe thane o-xylene Ethyltoluene ,3,5-Trimethylbenzene ,4-Trimethylbenzene rn-dichlorobenzene Benzyl chloride p-dichlorobenzene o-dichlorobenzene ,2,4-Trichlorobenzene Hexachlorobutadiene Ozone Precursor Standard Isobutane Butene n-butane trans-2-butene cis-2-butene Methyl-l-butene Isopent ane Pentene n -Pent ane Isoprene trans-2-pentene cis-2-pentene Methyl-2-butene ,2-Dimethylbutane Cyclopentene Methyl- 1-pentene ,3-Dimethylbutane Cyclopentane Methylpentane M ethylpentane Methyl- 1-pentene n-hexane trans-2-hexene cis-2-hexene ,2-Dimethylpentane

11 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM 22,385 Table 5. (continued) Tennessee (June) Research Triangle Park (August) MDL, Maximum, Percent Above Maximum, Percent Above Compound ppb C ppb C MDL ppb C MDL Notes* Ozone Precursor Standard (continued) Methylcyclopentane Benzene a Cyclohexane Methylhexane ,3-Dimethylpentane Methylhexane d 2,2,4-Trimethylpentane n-heptane Methylcyclohexane ,3,4-Trimethylpentane Toluene Methylheptane Methylheptane n -Octane Ethylbenzene m,p-xylene Styrene o-xylene n-nonane Isopropylbenzene a-pinene e n-propylbenzene ,3,5-Trimethylbenzene n-decane /3-Pinene f 1,2,4-Trimethylbenzene n-undecane Aldehyde Standard Methacrolein Methyl vinyl ketone Butanal ,1,1-Trichlo r o ethan e Pentanone 0.67 ß Pentanal Trichloroethene Hexanal Tetrachloroethene Heptanal Benzaldehyde Octanal Nonanal Decanal *Notes are as follows: a, constant sorbent artifact; b, styrene-heptanal close elution, but clear mass spectrometry (MS) identification; c, high background; d, 3-methylhexane-pentanal close elution, but clear MS identification; e, a-pinene-benzaldehyde close elution, but clear MS identification; f, calibration standard decay. concentration found, and the percentage of samples found above the detection limit for the 1995 New Hendersonville and RTP data sets for each compound on our target list. If any compound is subject to coeluting interferences or any other problem, it is noted. From this list it can be seen that many species were observed at concentrations at least 1 order of magnitude above the detection limits. 4. Conclusions There are three major conclusions that can be drawn from this work. We have extended our target list of compounds that can be analyzed hourly by our field-deployable autogc-ms system to 100 compounds, including biogenic and anthropogenic ozone precursors and oxygenated VOCs. We have demonstrated that through a clean air delivery system we can trans- fer low-level (often <0.1 ppb C) VOC samples through an 11-m transfer line with no analyte gain nor loss. We have also demonstrated a data review and qualification strategy that shows that the data collected in New Hendersonville were reliable. Acknowledgments. Although the research described in this paper has been funded wholly or in part by the United States Environmental Protection Agency under contracts 68-D and 68-D to ManTech Environmental Technology, Inc., it has not been subjected to agency review and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. We also acknowledge helpful technical discussions and assistance of Matthias J. Yoong of XonTech and Norman A. Kirshen and Elizabeth B. Almasi of Varian.

12 22,386 DAUGHTREY ET AL.: AUTOMATED GC-ION TRAP MASS SPECTROMETER SYSTEM References volatile organic compound analysis, in Proceedings of the 1992 U.S. EP.4/.4&WMA International Symposium on Toxic and Related.4ir Almasi, E. B., and N. A. Kitshen, The determination of volatile organic Pollutants, pp , Air and Waste Manage. Assoc., Pittsburgh, compounds (VOCs) in air by the TO-14 method using the Saturn II Pa., GC/MS, Rep. GCMS18, Varian Chromatography Syst., Walnut McClenny, W. A., E. H. Daughtrey Jr., K. G. Kronmiller, J. R. Adams, Creek, Calif., and K. D. Oliver, Hourly measurement of target VOCs by autogc Ap½l, E. C., J. G. Calvert, R. Zika, M. O. Rodgers, V. P. Ancja, J. F. at the New Hendersonville, TN, Southern Oxidants Study site: June Mcaghcr, and W. A. Lonncman, Hydrocarbon measurements dur , 1995, paper 96-RP presented at the Air and Waste ing the 1992 Southern Oxidants Study Atlanta intensive: Protocol Management Association 89th Annual Meeting and Exhibition, and quality assurance, J. `4it Waste Manage. `4ssoc., 45(7), , Nashville, Tenn., June Oliver, K. D., and J. D. Pleil, Automated cryogenic sampling and gas Bcrnardo-Brickcr, A., C. Farmer, P. Milne, D. Ricmcr, R. Zika, and chromatographic analysis of ambient vapor phase organic com- C. Stoneking, Validation of sp½ciatcd nonmethane hydrocarbon pounds: Procedures and comparison test, Rep. TN , compound data collected during the 1992 Atlanta intensive as part Northrop Serv., Inc.-Environ. Sci., Research Triangle Park, N. C., of the Southern Oxidants Study (SOS), J. `4it Waste Manage. `4ssoc., (8), , Oliver, K. D., J. R. Adams, E. H. Daughtrey Jr., W. A. McClenny, M. J. Bowyer, J. R., Procedure for preparing water-based canister standards, Yoong, and M. A. Pardee, Technique for monitoring hydrocarbons Rep. SP , ManTech Environ. Technol., Inc., Research in air at Photochemical Assessment Monitoring Stations: Sorbent Triangle Park, N. C., preconcentration, closed-cycle cooler cryofocusing, and GC-FID Daughtrey, E. H., Jr., J. R. Adams, C. R. Fortune, K. G. Kronmiller, analysis,.4tmos. Environ., 30(15), , 1996a. and K. D. Oliver, Carbon disulfide monitoring by autogc/ms at Oliver, K. D., J. R. Adams, E. H. Daughtrey Jr., W. A. McClenny, M. J. Axis, Alabama, March 15-24, 1994, in Measurement of Toxic and Yoong, M. A. Pardee, E. B. Almasi, and N. A. Kirshen, Technique Related Air Pollutants, Rep. VIP-50, pp , Air and Waste for monitoring toxic VOCs in air: Sorbent preconcentration, closed- Manage. Assoc., Pittsburgh, Pa., cycle cooler cryofocusing, and GC/MS analysis, Environ. Sci. Tech- Daughtrey, E. H., Jr., J. R. Adams, C. R. Fortune, K. G. Kronmiller, nol., 30, , 1996b. K. D. Oliver, and W. A. McClenny, Interpretation and data quality Singh, H. G., L. J. Salas, R. Stiles, and H. Shigeishi, Measurements of evaluation of hourly measurement of target VOCs by autogc/ms at hazardous organic chemicals in the ambient atmosphere, report the New Hendersonville, TN, Southern Oxidants Study Site, June prepared under Cooperative Agreement by SRI Int., Menlo 17-27, 1995, paper 96-RP presented at the Air and Waste Park, Calif., (Available from U.S. Environ. Prot. Agency, Re- Management Association 89th Annual Meeting and Exhibition, search Triangle Park, N. C.) Nashville, Tenn., June U.S. Environmental Protection Agency, Preparing perfect project Gerald, N. O., Photochemical Assessment Monitoring Station imple- plans: A pocket guide for the preparation of quality assurance mentation manual, Rep. EPA 454/B , Off. of Air Qual. Plann. project plans, Rep. EP.4/600/9-89/087 Risk Reduction Eng. Lab., and Stand., U.S. Environ. Prot. Agency, Research Triangle Park, U.S. Environ. Prot. Agency, Cincinnati, Ohio, N. C., Yokouchi, Y., H. Akimoto, L. A. Barrie, J. W. Bottenheim, K. Anlauf, Goldan, P. D., W. C. Kuster, and F. C. Fehsenfeld, Hydrocarbon and B. T. Jobson, Serial gas chromatographic/mass spectrometric measurements in the southeastern United States: The Rural Oxi- measurements of some volatile organic compounds in the Arctic dants in the Southern Environment (ROSE) Program 1990, J. Geo- atmosphere during the 1992 Polar Sunrise Experiment, J. Geophys. phys. Res., 100, 25,945-25,963, Res., 99, 25,379-25,389, Greenberg, J.P., B. Lee, D. Helmig, and P. R. Zimmerman, Fully automated gas-chromatograph-flame ionization detector system for J. R. Adams, E. H. Daughtrey Jr., K. G. Kronmiller, and K. D. the in situ determination of atmospheric nonmethane hydrocarbons Oliver, Methods Research, ManTech Environmental Technology, Inc., at low parts per trillion concentration, J. Chromatogr..4, 676, 389- P.O. Box 12313, 2 Triangle Drive, Research Triangle Park, NC , ( daughtrey.hunter@epamail.epa.gov) Helmig, D., Ozone removal techniques in the sampling of atmospheric W. A. McClenny, Atmospheric Methods and Monitoring Branch, volatile organic trace gases,.4tmos. Environ., 31(21), , Human Exposure and Atmospheric Sciences Division, National Expo sure Research Laboratory, U.S. Environmental Protection Agency, Leibrock, E., and J. Slemr, Method for measurement of volatile oxy- Research Triangle Park, NC genated hydrocarbons in ambient air,.4tmos. Environ., 3 (20), , Levaggi, D. A., W. Oyung, and R. V. Zerrudo, Noncryogeniconcen- (Received August 27, 1997; revised February 16, 1998; tration of ambient hydrocarbons for subsequent nonmethane and accepted April 10, 1998.)