EVALUATION OF A PID-BASED PORTABLE DETECTION SYSTEM FOR TVOC AS AN INDOOR POLLUTION ANALYZER

Transcription

1 EVALUATION OF A PID-BASED PORTABLE DETECTION SYSTEM FOR TVOC AS AN INDOOR POLLUTION ANALYZER KH Kim *, JD Lee 1, YJ Hong, YJ Choi Department of Earth & Environmental Sciences, Sejong University, Goon Ja Dong 98, Seoul, Korea KINSCO Technology, Seokyo-dong 372-8, Mapo-ku, Seoul, Korea ABSTRACT In this study, we conducted a preliminary study to evaluate the performance characteristics of a portable analyzer equipped with a photoionization detector (PID) for the analysis of total volatile organic compounds (TVOC). For the purpose of our study, we made a total of six side-by-side experiments to measure TVOC using both the PID-based portable detector and GC/FID system. The former system was calibrated against a gaseous toluene standard after modifying its spanning system to accommodate linearity deflection above certain concentration range (e.g., 3 ppm). To simplify the comparison of TVOC data, we operated the GC chromatographic technique by integrating and calibrating all peaks eluting between C6 and C16 by the same toluene gas standard. The sum of six comparative analysis conducted in a newly built apartment unit indicated that the results from both approaches agree fairly well with the bias of about 10 % range. It is perceived that PID detection characteristics can vary depending on chemical speciation of VOC or environmental conditions of study area (e.g, changes in RH). However, the results of our preliminary study suggest the possibility that its application to the indoor environments can be made fairly effectively under certain environments such as a newly built apartment. INDEX TERMS PID, TVOC, GC, indoor, toluene INTRODUCTION In the recognition of the environmental significance of indoor pollution phenomenon, the need for accurate diagnosis of indoor pollution is emphasized greatly as the prerequisite for its control (Lee et al., 2002). In line with a widespread demand to resolve such issues, many different types of instruments developed for the general air quality analysis have been deployed to the indoor environments for the acquisition of the data sets to explore the indoor air quality (IAQ) issues (Guo et al., 2003, 2004; Tuomainen et al., 2001). Although the use of such traditional set ups contributed noticeably to the advancement of IAQ analysis, many efforts have also been made to develop and improve more field-oriented, portable instruments (e.g., instantaneous readout and easy deployment: Karlik et al., 2002; Wolkoff, 1955). As part of a project to investigate various instruments for the IAQ analysis (Kim et al., 2004), we have been involved in examining the performance of a portable instrument targeting one of the most essential indoor pollution indices, total volatile organic compounds (TVOC) (Kim, 2004). As a means to test the performance of such portable system, we conducted a number of experiments to check for the suitability of such instrument with an aid of an on-line GC set up equipped with a FID system. In this study, based on our collateral analysis of TVOC using two different instrumental systems, we attempted to provide an in-depth analysis with respect to the confidence of portable analyzer built on the PID system. MATERIALS AND METHODS A notation for TVOC For the comparative analysis of TVOC data, it is imperative to acknowledge that there are differences in the notation for TVOC between different measurement techniques (Calogirou et al., 1996; Weschler and Shields, 2000). The TVOC analysis made by the GC/FID relies on quantifing all VOC detected in the range of C6 through C16 (i.e., n-hexane to n-hexadecane) (ECA, 1997). Hence, for the quantification of TVOC by the GC system, the experimental data were treated as follows. First, all peaks identified as C6 through C16 were integrated. These peak areas were then calibrated against the curves obtained by the toluene standard gas. By considering the volume of samples loaded to the GC/FID system, the TVOC data for GC system was derived as ppm values. On the other hand, the TVOC term for the PID analyzer differs in that it cannot specifically * Corresponding author khkim@sejong.ac.kr 3400

2 distinguish various types of VOC with different response characteristics (e.g., Karlik et al., 2002). As such, what both systems actually target to measure can be different from each other in principle. Nonetheless, those measurement data from two independent systems can share some similarities from other respects. It is because this comparative analysis is confined to the indoor environments where certain types of VOC exist more dominantly than others. For instance, our selected target site (like a newly built apartment) is generally full of several aromatic VOC including benzene, toluene, ethylbenzene, and xylene (commonly called as BTEX). Thus, it is expected that the test for compatibility between the two instrumental systems can be meaningful enough to examine the feasibility of the portable system for the application to the IAQ diagnosis. A portable PID system To make an in-depth examination of a portable analyzer for TVOC, our study was organized in the following manner. Considering the fact that this portable analyzer was basically built on as a PID system (Fig 1), our test was conducted by running a series of parallel analysis against the GC/FID system (Fig 2). All of our experiments were carried out from a newly constructed apartment unit in E city, Gyung Gi Province, Korea (1 Oct. 2004). In this selected apartment, a portable PID system was operated continuously to measure TVOC. In the meantime, the samples for the GC/FID analysis were also collected using a Tedlar bag sampling system for a total of six times. These bag samples were brought into the lab immediately and analyzed by the GC/FID system to produce data sets for the parallel analysis. In this work, we selected a portable analyzer, SNIFFER II system developed by KINSCO Inc. (Seoul, Korea) for the comparative study of TVOC. This model is built on PID with UV lamp voltage potential of 10.2 ev and configured to provide digitalized information of TVOC concentration with the following specifications. - Measuring range: 0-10 ppm - Lower detection limit: 0.01ppm - 90 % response time: 2~3 sec. - Data output: variable but normally recommended at 5 min average values Although the general response characteristics of the PID system (such as the SNIFFER system) vary among different VOC, we simplified the calibration of the SNIFFER system by spanning against a 3 ppm toluene standard gas (being diluted from a primary standard with a 10 ppm concentration). In addition, as the PID tends to lose linearity after certain concentration range (e.g., beyond 3 ppm for the PID system investigated in this study), the final read-out of this system was shown to accommodate such deflection in linearity by programming a pre-adjusted correction equation. Hence, the SNIFFER system used for our experiments is set to measure TVOC concentration up to 10 ppm with a self-calibration correcting procedure. A GC/FID system For the parallel analysis of TVOC, we used a GC/FID system (Model DS6200, Donam Instrument, Korea) combined with the Peltier cooling (PC) and the thermal desorption (TD) method (PC/TD system: UNITY air server, Markes International, Ltd., UK). The use of the PC/TD system is essential to analyze samples with significantly low VOC contents, as it can allow preconcentration without the use of liquid nitrogen (LN2) (e.g., Kim et al., 2004). To initiate the GC analysis, samples contained in Tedlar bags were analyzed by being introduced into the PC/TD system at a flow rate of 40 ml min -1 for 3 min. The VOC components transported from bag samples were then pre-concentrated in a LN2-free cold trap (packed with two types of adsorbents, Carbopack B and C) at -15 o C. Those compounds were then released thermally by heating the cold trap for 2 minutes at 320 o C. Each VOC were eventually separated on a BP-1 column (60 m x0.32 mm, SGE Corp., USA) by a temperature program (1 min holding at 50 o C, ramping at 6 o C min -1, and final holding for 5 min at 230 o C), prior to the GC-FID detection. Gases for the GC operation were supplied at flow rates of 30 (H 2 ), 30 (N 2 as make-up gas), and 300 ml min -1 (air). The TVOC data for GC analysis were calibrated using a 10 ppm toluene standard gas by loading diluted samples with three different concentration ranges (e.g., if computed by the absolute toluene content, they are 150, 300, and 450 ng (Fig 3)). The results of replicate GC analyses indicated that the precision of GC analysis was approximately 5 % with its detection limits around 0.1 ng. RESULTS AND DISCUSSION The results of the basic environmental conditions for our comparative measurements are summarized in Table 1. The TVOC data for this comparative analysis were measured in two different manners. First of all, the TVOC data for PID were collected continuously at every 2 sec for the total duration of three hours (15:00 ~ 18:00). At the same time, samples for GC analysis were also collected by the separate system for the total of six times. 3401

3 Such intermittent sampling for GC analysis was made at intervals of 20 to 30 mins. In light of this difference, we numbered each experiment based on sampling protocols of GC analysis to offer the equal basis for comparative analysis between the two measurement systems. The results of these experiments indicated that the TVOC concentration data tend to increase steadily, as the experiments proceed (Table 2). Hence, at the last experiment, when the experiment was interfered by an unexpected intrusion of the painter with a paint box, the TVOC data increased dramatically. In Table 3, the TVOC data measured by the GC/FID system are summarized. As the GC/FID allows more detailed speciation of the compounds that fell between C6 and C16, we distinguished these data into the following three classes: 1] individual speciation for BTEX, 2] contribution of non-btex components, and 3] TVOC (or sum of BTEX and non-btex). It is known that the GC/FID responses with VOC generally correlate with the number of carbon. Hence, our calibration of GC based on a toluene standard gas is expected to allow a fairly reasonable comparison with the PID results. According to the VOC speciation results summarized in Table 3, it is found that toluene alone contributed most significantly to the TVOC concentration. In addition, a simple computation of bias between the two measurement systems shows that there is approximately 10 % of bias between the two instrumental systems. As shown in Fig 4, compatibility of the two systems is also checked by the regression analysis. The results confirm that the TVOC data obtained by the portable PID analyzer were systematically lower than those of GC/FID, while they generally tend to exhibit very good correlations. CONCLUSIONS In this work, we attempted to check for the analytical applicability of the portable PID analyzer in the measurement of TVOC concentrations for IAQ investigations. For the purpose of our study, we conducted a total of six consecutive experiments from a newly built apartment unit by examining its performance in relation with the GC/FID analysis. The results of our comparative test indicated that the TVOC measurement data from the portable PID were slightly lower than those of the GC/FID; but based on these comparable data sets, the bias between the two systems were also computed as 10 % range. It further suggests the potential for the portable PID system for the application to the IAQ analysis; despite the inherent differences involved in the notation of TVOC terms and the simplicity involved in its application, its performance for TVOC analysis is fairly excellent under the selected experimental settings. To further demonstrate the feasibility of this portable system, more efforts need to be directed to accurately describing the limitations associated with its application under varying environmental settings. REFERENCES Calogirou A., Larsen BR., Brussol C., Duane M. and Kotzias D. Decomposition of terpenes by ozone during sampling on tenax. Analytical Chemistry 68, 1996, ECA, European Collaborative Action ECA-IAQ. Total volatile organic compounds TVOC in indoor air quality investigations. Report No. 19, European Commission, Brussels, Guo H., Murray F. and Lee SC. The development of low volatile organic compound emission house a case study, Building and Environment, 38, 2003, Guo H., Lee SC. and Chan LY. Indoor air quality in ice skating rinks in Hong Kong, Environ. Res., 94, 2004, Karlik JF., Mckay AH., Welch JM. and Winer AM. A survey of California plant species with a portable VOC analyzer for biogenic emission inventory development, Atmospheric Environment, 36, 2002, Kim KH. Comparison of TVOC measurement methods and evaluation of portable analyzer. Indoor Air Quality Workshop 2004, Konkuk UNniv., Seoul, Korea, May Kim KH., Oh SI. and Choi YJ. Comparative analysis of bias in the collection of airborne pollutants: Tests on major aromatic VOC using three types of sorbent-based methods. Talanta 64(2), 2004, Lee SC., Lam S. and Fai HK. Characterization of VOCs, ozone, and PM10 emissions from office equipment in an environmental chamber, Building and Environment, 36, 2002, Tran NK., S. M. Steinberg, and B. J. Johnson, Volatile aromatic hydrocarbons and dicarboxylic acid concentrations in air at an urban site in the Southwestern US, Atmospheric Environment, 34, 2000, Tuomainen, M., Pasanen AL., Tuomainen A., Liesivuori J. and Juvonen P. Usefulness of the Finnish classification of indoor climate, construction and finishing materials: comparison of indoor climate between two new blocks of flats in Finland, Atmospheric Environment, 35, 2001, Weschler CJ. and Shields HC. The influence of ventilation on reactions among indoor pollutants: dynamic modeling and experimental observations. Indoor Air 10, 2000, Wolkoff P. Volatile organic compounds sources, measurement, emissions and the impact on indoor air quality. Indoor air Supplimentary, 3, 1995,

4 Table 1. The basic experimental conditions for six collateral performance tests between Sniffer II (PID) and GC/FID system. Exp. Time Condition No. Start End Temp RH 1 15:20:01 15:21: :50:00 15:51: :10:00 16:11: :30:01 16:31: :50:03 16:51: :10:00 17:11: Table 2. A statistical summary of six consecutive IAQ experiments: results of the Sniffer II measurement data. Exp. No. 1st 2nd 3rd 4th 5th 6th Param. TVOC ppm mg/m 3 Avg SD Range (N) 4.14~4.49 (35) 15.6~16.9 (35) Avg SD Range (N) 5.17~5.33 (35) 19.5~20.1 (35) Avg SD Range (N) 5.45~5.73 (35) 20.5~21.6 (35) Avg SD Range (N) 5.69~5.85 (34) 21.4~22.0 (34) Avg SD Range (N) 5.79~6.03 (34) 21.8~22.7 (34) Avg SD Range (N) 6.39~6.64 (35) 24.1~25.0 (35) Table 3. Results of GC/FID measurements of individual VOC and TVOC* (All concentrations are in ppm unit, unless otherwise specified). Exp. VOC speciation TVOC** No. benzene toluene ethylbenzene m,p-xylene o-xylene ΣBTEX 1 ΣN-BTEX 2 ppm ppmc *All concentration terms are derived by assuming that all integrated peak areas of each identified (or unknown) compound resemble area-to-mass relationships (calibration patterns) of toluene. **TVOC terms are compared by both ppm and ppmc. It should be reminded that the former was derived by calibrating all peaks with toluene calibration curve, while the latter corresponds to integration per carbon basis. Superscripts 1 and 2 denote the sum of BTEX (so called as the sum of benzene, toluene, ethylbenzene, and xylene) and non-btex. 3403

5 Figure 1. A picture of SNIFFER II system for IAQ analysis. Figure 2. A picture showing instrumental settings for TVOC analysis by GC/FID. Volume of toluene standard analyzed Total Amt Peak area Time (min) Flow (ml/min) Total Vol. (ml) ng Toluene ,131, ,365, ,691,

6 8,000,000 7,000,000 6,000,000 5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 0 y = 14806x R 2 = Figure 3. GC-based calibration of toluene using a 100 ppb gaseous toluene standard TVOC (GC/FID) y = x r = TVOC (Sniffer II) Figure 4. Results of linear regression analysis between two different TVOC terms derived by Sniffer II and GC/FID system. 3405