ANALYTICAL SCIENCES AUGUST 1998, VOL The Japan Society for Analytical Chemistry

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1 ANALYTICAL SCIENCES AUGUST 1998, VOL The Japan Society for Analytical Chemistry 763 Multielement Analysis of Suspended Particulates Collected with a Beta-Gauge Monitoring System by ICP Atomic Emission Spectrometry and Mass Spectrometry C. F. WANG*, C. J. CHIN* and Pen-Chi CHIANG** *Department of Nuclear Science, National Tsing Hua University, Hsinchu, Taiwan, ROC **Institute of Environmental Engineering, National Taiwan University, Taipei, ROC This work presents a two-step HNO 3+HClO 4/HF(1:2:1, v/v) acid mixture microwave digestion method for determining the elemental concentrations in suspended particulates collected on glass fiber filters by a beta-gauge monitoring system. Eighteen elements (Al, As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, S, Sb, V and Zn) were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS). The interference from the glass fiber filters and the variations of the detection limits of the major elements with total suspended particulates (TSP) were thoroughly examined. According to these results, the concentrations of most of the elements in suspended particulates can be determined via the proposed method. On the other hand, determining Na is relatively difficult owing to significant interference from glass fiber filters of the beta-gauge monitoring system. In addition, real samples collected from an air-pollution episode in Taiwan during the spring of 1995 were analyzed, in which the proposed method s effectiveness was confirmed. The analysis results also indicate that Kosa aerosols, having originated in China, might have been the culprit of that episode. Keywords Multielement analysis, airborne particulate matter, beta-gauge monitoring system, microwave digestion, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry Owing to the accelerated growth in the population, economy and industrial sector, Taiwan has encountered increasing air pollution-related problems. Air-pollution episodes seriously deteriorate the air quality without any advance warning, and usually within a short period of time. To effectively monitor such episodes in real time, the Environmental Protection Administration of the Republic of China established sixty-six automatic air-quality monitoring stations, where the total suspended particulates (TSP) was determined using a betagauge monitoring system. 1 Meanwhile, a long-term air-pollution investigation was headed by an intrauniversity research group to collect and analyze the suspended particulate matter on glass fiber filters via a conventional high-volume (Hi-Vol) sampler for characterizing and identifying the major air pollution sources in Taiwan. 2 Conventional sampling techniques cannot reliably forecast episode events, due to such an event s unpredictable nature. 3 A beta-gauge monitoring system can continuously determine the amount of TSP in ambient air, having positively contributed to episode monitoring. 4,5 Chemical components regarding the air-pollution episode must be made available to identify pollution sources. Therefore, in this study, we present a means of determining the elemental concentration of To whom correspondence should be addressed. airborne particulates on filter samples collected by the beta gauge system. As generally known, mass loading of the beta-gauge sampling technique is markedly less than that of the conventional Hi-Vol sampler. For instance, if a beta-gauge monitoring system is used for atmosphere aerosol monitoring every four hours, it is estimated that (a) less than 0.5 mg/filter of suspended particulate matter can be collected on the filters under the normal condition, and (b) about 1 2 mg/filter can be collected only if a serious episode occurs. Another disadvantage of determining the elemental concentrations of airborne particulate samples collected by the beta-gauge system is the blank interference caused from the glass fiber filter media. 6 Therefore, it is necessary to use a more sophisticated method to determine the low level of elemental concentrations in the airborne particulate samples collected by the beta-gauge monitoring system. Our previous studies 3,6 9 suggested that an acid-mixture digestion method for airborne particulate collected on glass fiber filters by a Hi-Vol sampler with concentrated HNO 3 +HClO 4 under pressure, followed by adding HF after removing of the intact filters from the digestion solution, is one of the best pretreatment procedures for multielement analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS). However, the blank contributions due to the dissolution of impurities in the

2 764 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 filter media, particularly for Al, Ca, K, Mg and Na, may still significantly interfere with the spectrometric measurement. ICP-MS has several attractive features for determining elemental concentrations in airborne particles, e.g. the spectra are relatively simple to interpret and mainly free from any inter-element interference, the isotopic information is inherent, the multielemental capability allows a large corpus of chemical data throughput, and the detection limits are superior to those in most conventional techniques; in addition, most elements in the periodic table can be detected. 10 However, interference problems due to filter media- or acid-derived background ions having formed during the ion-extraction process of ICP-MS significantly degrade the analytical abilities for many elements. 11,12 For instance, in the ICP-MS measurement, the formed 12 C 12 C +, 13 C 14 N +, 16 O 16 O + and 38 Ar 1 H + may inhibit their respective detection of 24 Mg + (78.7%), 27 Al + (100.0%), 32 S + (95.0%), and 39 K + (93.1%), even if these elements are abundant in airborne particles. It is also noted that the sensitive ICP-MS method is not favored for determining highconcentration elements contributed primarily from the analytes and filter media. As a result, ICP-AES may be recommended for determining these high concentration as well as high interference elements in ICP-MS. 6 This investigation attempted to determine the elemental concentrations in suspended particulates collected on glass fiber filters by a beta-gauge monitoring system with ICP spectrometers. Notably, in this study, only a limited amount of TSP was collected by the beta-gauge monitoring system within a short period of time. The chemical interference attributed to the filter matrix was critical in affecting the proposed method s detection limits of Al, Fe, K, Mg, Na, S and Zn. Interference from the glass fiber filters and the variation in the detection limits of these elements with the amount of collected TSP are therefore thoroughly examined. Finally, samples collected from an air-pollution episode in Taiwan during the spring of 1995 were analyzed via the proposed method to verify its effectiveness in determining the potential air-pollution sources. Materials and Methods Fig. 1 Geographical locations of air-quality monitoring stations in the northern Taiwan area. Sample collection Six sampling stations in northern Taiwan area (Shih- Lin, Tao-Yuan, Lin-Kou, Dan-Shui, Da-Yuan and Kuan-Yin) were selected to monitor the TSP concentrations using a beta-gauge monitoring system during the episode event. Figure 1 depicts the geographical locations of the selected stations, indicating that the Shih- Lin, Tao-Yuan and Lin-Kou stations are located in the urban areas of Taipei City and quite a distance away from the coastal area. Meanwhile, Dan-Shui, Da-Yuan and Kuan-Yin stations are located near to the coastal area and are considered as rural areas. A Wedding & Associates TSP beta-gauge monitoring system was used. Particulate matter in the ambient air was collected through a TSP inlet at a volumetric flow rate of 18.9 l/min. The beta-gauge system was designed to continuously collect particulate matter every four hours. The collected particulates were deposited on a 3.14 cm 2 round area of the glass fiber filter tape, which could be moved at discrete time intervals by a filter tape-transport system. Sample digestion and ICP measurement This investigation focused primarily on determining the elemental concentrations in suspended particulates collected by the beta-gauge monitoring system. A twostep acid mixture (HNO 3 +HClO 4 /HF) microwave digestion procedure proposed in our previous study was therefore employed. 9 For the chemical analysis, pieces of loaded filter samples collected from the beta-gauge monitoring system were punched out, then digested with an acid mixture for the determination by ICP-AES and ICP-MS. A Jarrel-Ash ICAP-9000 multi-collector type ICP-AES was employed for determining Al, Ca, Fe, K, Mg, Na, S and Zn while a SCIEX Elan Model 5000 ICP-MS manufactured by Perkin-Elmer was employed for determining As, Ba, Cd, Cr, Cu, Mn, Ni, Pb, Sb and V. The detailed experimental procedures can be found elsewhere. 6 8 To validate the digestion method, the NIST Standard Reference Material SRM 1648 urban particulate matter was used to simulate the loaded filter samples. 6 8 Glass-fiber filters with no particulate matter were placed in decomposition vessels and followed the same digestion procedures to investigate the possible interference caused by the reagent blank and filter blank. Results and Discussion Blank interference from glass fiber filter Table 1 reveals that with a HNO 3 +HClO 4 (1:2, v/v)

3 ANALYTICAL SCIENCES AUGUST 1998, VOL Table 1 Elemental concentrations released from blank glass fiber filters with HNO 3 +HClO 4 (1:2, v/v) acid mixture during the first step digestion (n = 5) Element Digestion time/min Al (mg/cm 2 ) 0.025±0.009 a 0.033± ± ±0.001 Fe (ng/cm 2 ) 2.347± ± ± ±0.288 K (mg/cm 2 ) 0.019± ± ± ±0.003 Mg (mg/cm 2 ) 0.020± ± ± ±0.001 Na (mg/cm 2 ) 0.614± ± ± ±0.027 S (ng/cm 2 ) 1.453± ± ± ±0.325 Zn (ng/cm 2 ) 0.133± ± ± ±0.020 a. Data presented as mean±1s (SD). acid mixture in the first digestion step, the blank concentrations of various elements having dissolved from filter media generally increased with the digestion time. As is generally known, the blank elemental concentrations vary widely in glass fiber filters from type to type, and even from manufacturing lot-to-manufacturing lot for the same type. Thus, finding a compromise digestion time for the dissolution of both filters and samples is a prerequisite to reduce the background interference from the filter media. According to a subsidiary experiment described herein, a complete sample dissolution from the intact filter could be easily achieved by HNO 3 +HClO 4 digestion during the first 5 min of the digestion time. Consequently, a 6 min (75% microwave power) digestion was designated for the HNO 3 + HClO 4 digestion after a 3 min (50% microwave power) preheating process. Since the background contribution from filter media is variable for different experiments, performing the blank substraction for each batch of the digestion should not be neglected. Figure 2 confirms that the amount of acid mixture is not the major factor affecting the digestion of airborne particulates collected on a glass fiber filter. However, the cut circular filter samples must be not only completely immersed by the acid in the vessel before the microwave digestion proceeds with, but also preheated with a lower power to prevent the filter from inhomogeneous heating. Table 1 also demonstrates that a significant amount of Na was observed and attributed to the glass fiber filter, thereby making it unfeasible to determine Na in airborne particulates. On the other hand, for elements of Al, Fe, K, Mg, S and Zn, blank concentrations ranging from 0.03 ppm to 2 ppm can significantly affect the sensitivity of elemental determinations in the particulate-matter analysis by the method proposed herein. Although no data of certified values in the SRM standard are available for Ca, we still determined the concentration of Ca via the proposed method. Relationship between the detection limits of chemical elements and TSP concentrations The feasibility of determining these elements in airborne particulate matter by ICP-AES can be more Fig. 2 Variations of the elemental concentrations derived from blank glass fiber filters with an added volume of HNO 3+ HClO 4 (1:2, v/v). A 6 min digestion for the first step was selected. quantitatively described by comparing the spectra of blanks with those of simulated real samples under different air-pollution conditions. With a TSP concentration of 100 mg/m 3 in ambient air, it was estimated that a continuous four-hour sampling by the beta-gauge monitoring system with the flow rate at a 1.13 m 3 /h level can collect about 0.5 mg of suspended particulates on a filter having about a 3.14 cm 2 circular area. To simulate the ambient-air samples for TSP concentrations of 50, 100 and 200 mg/m 3, respectively, 0.25 mg, 0.5 mg and 1.0 mg of NIST SRM 1648 urban particulates were mixed with a cut circular-glass fiber filter and analyzed by the method proposed herein. Table 2 compiles information concerning various elements in the spectra of ICP-AES under different simulated air-pollution conditions. According to this table, higher signal-to-background ratios (SBR) for Zn, Al, Fe and S were obtained, even with TSP=50 mg/m 3. In general, determining Zn, Al, Fe and S should be rela-

4 766 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 tively easy, even with lower TSP concentrations, due to a lower blank interference or a higher concentration level. The level of the Zn content in airborne particulates was found to be very low in ppb. However, an extremely high instrumental sensitivity and low blank background were favored in the Zn determination. On the other hand, determining K and Mg concentrations in the airborne particulates become more reliable with increasing TSP concentrations. At low TSP concentrations it was observed that the relative insensitivity of a K element analysis could be attributed to Ar interference which occurred at the 766 nm peak in the ICP- AES. The much lower SBR of Na displayed in Table 2 Table 2 Blank and sample intensities of various elements measured by ICP-AES under different simulated air pollution circumstances Element Wavelength/ nm Total suspended particulate concentration/ mg m 3 Signal-to-background ratio (SBR) a may indicate that determining Na becomes extremely difficult whenever a glass fiber filter is utilized. It is estimated that at least a SBR value of 0.3 should be achieved to distinguish the signal of the sample from the blank if a 10% detection fluctuation is assumed. 13 Figure 3 plots the SBR values of K, Mg and Na to the TSP concentrations to assess the proposed method s effectiveness in determining various elements. The solid and dashed curves presented in this figure only depict the SBR trend of an individual element to the TSP concentrations. This finding suggests that Zn, Al, Fe and S can be accurately determined under a minor pollution condition. K and Mg could only be accurately determined at higher TSP concentrations. As Fig. 3 reveals, K and Mg in airborne particulates could be determined if the TSP concentrations exceeded 110 mg/m 3 and 150 mg/m 3, respectively. On the other hand, Na could only be determined in a seriously pollution episode. The method detection limits (MDL) of these high Al Fe K Mg Na S Zn a. SBR=(sample (counts) blank (counts))/blank (counts) for each element at specific wavelength. Fig. 3 Relationships of the signal-to-background ratios (SBR) of K( ), Mg( ) and Na( ) with the TSP concentrations. Fig. 4 Hourly TSP concentrations of six monitoring stations in northern Taiwan on March 12, 1995, collected by a beta-gauge monitoring system.

5 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 concentration as well as high blank elements in airborne particulates for the proposed method could be calculated as the triple s of the obtained background concentrations. Their MDLs were estimated as follows: Al, 0.33 mg/m 3 ; Fe, 0.26 mg/m 3 ; K, 1.92 mg/m 3 ; Mg, 0.61 mg/m 3 ; Na, 8.46 mg/m 3 ; S, 0.17 mg/m 3 and Zn, 0.02 mg/m 3. Case study Identifying pollution sources for airpollution episodes A typical air-pollution episode was observed on March 12, Figure 4 depicts the TSP concentrations of several stations in northern Taiwan during a particular episode. According to this results, the TSP concentrations appeared to significantly increase at around 10 am on March 12, reaching a maximum at between 4 and 7 pm, and gradually decreasing at all sampling stations. A markedly higher TSP was found in Kuan-Yin, Da-Yuan, and Dan-Shui stations, which are near to the seashore (Fig. 1). Kuan-Yin and Da- Yuan stations were selected in our long-term air-quality investigation in northern Taiwan as the background reference since their 24 h TSP averages are the lowest in the area. Figuer 5 displays box plots of the long-term averaged TSP concentrations of six stations over the past three years. As this figure depicts, significantly high TSP concentrations were observed for Shih-Lin, Lin-Kou and Tao-Yuan stations. The abnormally high data of Dan-Shui, Da-Yuan and Kuan-Yin stations taken during this episode strongly suggest that instead 767 of the contributions of local sources, the long range transport of air pollutants from regions may play a prominent role in this event. A chemical analysis was further performed using the Fig. 5 Long-term averaged TSP concentrations of the investigated monitoring stations during the past three years. The TSP data are illustrated using box plot displays. Table 3 Elemental concentrations of ambient air on March 12, 1995 Element Da-Yuan Dan-Shui Reference concentrationsa 6:00 10:00 16:00 20:00 6:00 10:00 16:00 20:00 Average Range Sample no. Al (mg/m 3 ) Ba (mg/m 3 ) Ca (mg/m 3 ) Cu (mg/m 3 ) Fe (mg/m 3 ) K (mg/m 3 ) Mg (mg/m 3 ) Mn (mg/m 3 ) Na (mg/m 3 ) b Ni (mg/m 3 ) Pb (mg/m 3 ) S (mg/m 3 ) Sb (mg/m 3 ) Sr (mg/m 3 ) V (mg/m 3 ) Zn (mg/m 3 ) As (ng/m 3 ) Cd (ng/m 3 ) Cr (ng/m 3 ) Se (ng/m 3 ) The samples were collected by beta-monitoring system and determined by the suggested analytical method at Da-Yuan and Dan- Shui stations. a. 24-h averaged elemental concentrations in suspended particulate matter collected by High-Volume samplers at Taiwan in last five years. b. Not determined.

6 768 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 proposed method to investigate the possible source of the event. Table 3 summarizes the results of the Da- Yuan and Dan-Shui stations, in which filter samples were collected at 6:00 10:00 hours (normal) and 14:00 18:00 hours (episode). The data were also compared with the average elemental concentrations in the air of Taiwan over the last five years. According to that comparison, the concentration levels of most elements determined before the episode are quite similar to the long-term averaged values in Taiwan. On the other hand, a significant concentration increase in Ca, Fe and Mg was observed in the afternoon of March 12. The enrichment of Ca, Fe and Mg may confirm contributions from Kosa aerosols, which originates from China s arid areas and is well known to be a potential pollution source in East Asia during the springtime Previous literature indicated that the characteristic elements in Kosa aerosol compared with the background aerosol were alkaline and alkaline earth elements, particularly Ca. Calcium in the Kosa aerosols is considered to be abundant in the substance of a carbonate formed by the lime accumulation phenomenon on the surface desert soil. On the other hand, insignificant variations in the concentrations of other elements before and after the episode might be considered as an indicator of local source contributions. Obviously, the impurity interference of the filter material during the digestion process makes it impossible to determine many trace elements under the normal air-pollution condition. However, for a specific air-pollution event, the concentrations of certain marker elements regarding the pollution source may be elevated up to 100 times in suspended particulate matter. Consequently, the proposed two-step microwave aciddigestion method, followed by ICP-AES and ICP-MS, can be successfully used for the elemental determination of suspended particulate samples to monitor an airpollution episode. The data obtained herein can provide valuable information to identify pollution sources if combined with the data of the hourly TSP concentrations, meteorological conditions and source profiles. The background interference of the filter media has limited the determination of many important elements. We therefore recommend employing filter media with less impurity, such as Teflon filters, for sampling work in a future investigation. The authors would like to thank the National Science Council, Republic of China for financially supporting this research under Contract No. NSC M Prof. M. H. Yang, Department of Nuclear Science, National Tsing Hua University is appreciated for his valuable help and discussion. Mr. W. F. Chang of the National Science Council s Regional Instrument Center in Hsinchu is also commended for taking the ICP-AES and ICP-MS measurements. References 1. G. S. Spagnolo, J. Aerosol Sci., 20, 19 (1989). 2. P. C. Chiang, C. F. Wang, S. T. Kao and M. H. Yang, Proceedings of the 2nd IUPPA Regional Conference on Air Pollution, Seoul, Korea, V2, 257 (1991). 3. C. F. Wang, E. E. Chang, P. C. Chiang and N. K. Aras, Analyst [London], 120, 2521 (1995). 4. J. M. Jaklevic, R. C. Gatti, F. S. Goulding and B. W. Loo, Environ. Sci. Technol., 15, 680 (1985). 5. E. S. Macias, Environ. Sci. Technol., 10, 904 (1976). 6. C. F. Wang, J. Y. Yang and C. H. Ke, Anal. Chim. Acta, 320, 207 (1996). 7. C. F. Wang, M. F. Huang, E. E. Chang and P. C. Chiang, Anal. Sci., 12, 201 (1996). 8. C. F. Wang, T. T. Miau, J. Y. Perng, S. J. Yeh, P. C. Chiang, H. T. Tsai and M. H. Yang, Analyst, 114, 1067 (1989). 9. C. F. Wang, W. H. Chen, M. H. Yang and P. C. Chiang, Analyst [London], 120, 1681 (1995). 10. H. -M. Kuss, Fresenius J. Anal. Chem., 343, 788 (1992). 11. H. Kawaguchi, T. Tanaka, T. Nakamura, M. Morishita and A. Mizuike, Anal. Sci., 3, 205 (1987). 12. J. A. Olivares and R. S. Houk, Anal. Chem., 58, 20 (1986). 13. L. A. Currie, Anal. Chem., 40, 586 (1968). 14. J. W. Winchester and M.-X. Wang, Tellus, 41B, 323 (1989). 15. R. A. Duce, C. K. Unni, B. J. Ray, J. M. Prospero and J. T. Merrill, Science [Washington, D. C.], 209, 1522 (1980). 16. M. Uematsu, R. A. Duce and J. M. Prospero, J. Atmos. Chem., 3, 123 (1985). (Received July 30, 1997) (Accepted December 4, 1997)