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CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 38, Issue 5, May 2010 Online English edition of the Chinese language journal

Cite this article as: Chin J Anal Chem, 2010, 38(5), 719–722.

RESEARCH PAPER

Purge-and-Trap Gas Chromatography Method for Analysis of Methyl Chloride and Methyl Bromide in Seawater YANG Gui-Peng*, LU Xiao-Lan, SONG Gui-Sheng, WANG Xiao-Meng Key Laboratory of Marine Chemistry Theory and Technology of Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China

Abstract: Purge-and-trap technique coupled with gas chromatography with electron capture detector (GC-ECD) is developed for analyzing the concentrations of methyl chloride (CH3Cl) and methyl bromide (CH3Br) in seawater. The optimum conditions were established as follows: purge time (12 min), purge gas flow rate (50 mL min–1), purge temperature (40 °C), desiccant (Mg(ClO4)2), liquid nitrogen trap, boiling water desorption (2 min). The detection limits were 2.7 and 0.3 pM, the precision was < 2% and < 8%, and the recoveries were 89%–97% and 75%–85% for CH3Cl and CH3Br, respectively. Experiments for sample storage were carried out to test the reliability of the storage procedure used in this study, and the results showed that there was no significant change in CH3Cl and CH3Br concentrations in seawater within 24 h using this procedure. Merits of this method are that exact concentrations could be estimated and the analysis could be completed within 6 h in situ. This method was also used to measure in situ concentrations of CH3Cl and CH3Br in the surface seawater of the Jiaozhou Bay, and the results showed that the concentrations of CH3Cl and CH3Br ranged from 130 to 346 pM and 3.90 to 20.7 pM, respectively, which were in good agreement with those values reported in literature in other coastal waters. Our study shows that this method meets the demands of the measurement of in situ concentrations of CH3Cl and CH3Br in seawater. Key Words:

1

Methyl chloride; Methyl bromide; Purge-and-trap technique; Gas chromatography; Jiaozhou Bay

Introduction

Methyl chloride (CH3Cl) and methyl bromide (CH3Br) are the important trace greenhouse gases in the atmosphere and contribute ozone-depleting chlorine and bromine, respectively, to the atmosphere. Moreover, chlorine and bromine participate in other atmospheric reactions[1–3]. CH3Cl and CH3Br are ubiquitous in seawater and can be released into the atmosphere easily. Emission of these two gases from sea to air will directly affect the global atmospheric budget of CH3Cl and CH3Br[4,5]. Therefore, more attention has been paid to the study of CH3Cl and CH3Br in the atmospheric and oceanic environment over the past 30 years, because of their effects on atmospheric

chemistry and global warming[6,7]. Because of their low concentration in seawater, preenrichment of CH3Cl and CH3Br is needed for the analysis by gas chromatography (GC). Methods used for sample enrichment include headspace extraction, solid-phase microextraction, liquid-liquid extraction, and purge-and-trap technique[8]. The purge-and-trap method coupled with gas chromatography with electron capture detector (GC-ECD) to analyze the concentrations of CH3Cl and CH3Br in seawater was developed in this study, with high precision and reproducibility, low detection limit and disturbance from the water sample. Furthermore, seawater samples collected from the Jiaozhou Bay were analyzed successfully using this developed technique.

Received 9 August 2009; accepted 25 January 2010 * Corresponding author. Email: [email protected] This work was supported by the National Natural Science Foundation of China (No. 40776039), the National Science Fund for Distinguished Young Scholars of China (No. 40525017), the Science and Technology Key Project of Shandong Province of China (No. 2006GG2205024), the Natural Science Foundation of Shandong Province of China (No. Z2005E01), the Changjiang Scholars Programme, Ministry of Education of China, and the Taishan Scholars Programme of Shandong Province, China. Copyright © 2010, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(09)60046-3

YANG Gui-Peng et al. / Chinese Journal of Analytical Chemistry, 2010, 38(5): 719–722

2 2.1

Experimental Instruments and reagents

Instrument used in this study include Agilent 6890N gas chromatograph, coupled with ȝECD detector and Rtx-624 silicon capillary column (60 m × 0.32 mm × 1.8 ȝm); purge-and-trap equipment (established in lab), with stainless-steel trap loop (length: 1m, ID: 1/16 inches) inside. Regents used are CH3Cl and CH3Br standard gases (CH3Cl/N2, 4 u 10–8, V/V); CH3Br/N2, 1 u 10–9, V/V) (Dalian Specific Gas Limited Company); magnesium perchlorate (Mg(ClO4)2 (analytical grade, Tianjin Guangcheng Chemical Reagent Limited Company); CO2 sorbent (Merck); high-pure nitrogen (> 99.9995%, Qingdao Tianyuan Gas Company). 2.2 Sample collection Seawater samples were collected using either 10-L plastic bucket or 12-L Niskin bottles. The samples were drawn into 100-mL all-glass syringes through a silicon tube. The diameter of the silicon tube was matched with that for the syringes, and the silicon tube was translucent to check the presence of bubbles. Before collecting the samples, the syringes were fitted with a three-way nylon valve and preflushed with samples. Then, 100 ml of sample was injected into the syringe along its handle quickly to avoid bubbles and large eddies. All samples were stored in a bucket in dark until the analysis, usually within 6 h after the collection.

3 3.1

Results and discussion Optimizing GC separation conditions

Proper choice of column and appropriate process conditions, including oven temperature and carrier gas flow rate, are the main factors for improving the GC analysis. In this study, a Rtx-624 silicon capillary column (60 m × 0.3 mm × 1.8 ȝm) was used. After many attempts, the optimum conditions for the separation were set up as follows: carrier gas (high-purity N2), split ratio (10:1), carrier gas flow rate (2.1 mL min–1), make-up gas flow rate (15 mL min–1), injector temperature (110 °C), and detector temperature (275 °C). Under these conditions, the initial temperature of oven was held at 45 °C for 10 min and was increased to 200 °C at 15 °C min–1, with final temperature held for 5 min. 3.2

designed for the glass stripper. The results of our experiments showed that 3# and 5# were too small or too large, and 4# was suitable for the gas dispersion and was used to optimize the purge gas flow rate and the purge time. Purge gas flow rate not only affected the trap effect but also the analysis time. Lower flow rates caused a longer analysis time, and a high flow rate was unfavorable for adsorbing CH3Cl and CH3Br onto the trap loop. 100-mL standard CH3Cl or CH3Br gas was injected and purged for 15 min at 40 °C and was desorbed in boiling water bath for 2 min, the effect of purge gas flow rates on peak areas are shown in Fig.1. The results showed that almost all CH3Cl and CH3Br could be extracted from the seawater samples and adsorbed onto the trap loop when the purge gas flow rate was up to 40 mL min–1. However, a higher flow rate (50 mL min–1) was used during in situ investigation to shorten the analysis time. The effect of purge time on the analysis was studied using this purge gas flow rate, injected volume of standard gases, purge temperature, and desorption conditions (Fig.2). The results showed that when the seawater samples were purged for 12 min, CH3Cl and CH3Br were stripped well. Desorption time and temperature affect the analytical results directly. If the desorption time was too short, the tested compounds could not be desorbed completely and the residue in the trap loop would affect the analysis of the next sample; on the contrary, if the desorption time was too long, the analysis time would be extended, which was not favorable for in situ measurement. If the desorption temperature was too low, the sample would not be desorbed completely, whereas if the temperature was too high, some impurities with high boiling point would be injected into the column and then

Fig.1 Effect of purge gas flow rate on analytical results

Optimizing purge-and-trap conditions

Because of low concentrations (pM) of CH3Cl and CH3Br in seawater, a pre-enrichment is necessary before injecting them into the gas chromatograph for analysis and detection. Glass grit with three different apertures (3#, 4# and 5#) was

Fig.2 Effect of purge time on analytical results


and then the calibration curves were determined according to the developed method. The calibration curves were linear over the concentration range of interest. The peak areas (y) as a function of the moles (x) of CH3Cl and CH3Br can be written as follows: for CH3Cl: y = 2.561x + 4.889, r = 0.999, n = 6; for CH3Br: y = 16.656x + 4.530, r = 0.997, n = 6. The concentrations of CH3Cl and CH3Br in seawater samples were extrapolated from the peak areas using the calibration curves. 3.4

Detection limit, precision and recovery

Fig.3 Effect of desorption time on analytical results

would affect the separation. Because boiling points of CH3Cl and CH3Br are –24 °C and 4 °C, respectively, boiling water bath (100 °C) was used during the desorption. The effect of desorption time on the analytical results was studied under the above-mentioned optimum conditions such as purge gas flow rate, purge time, injection volume, purge and desorption temperature (Fig.3). The results showed that 2 min of water bath heating would be enough to completely desorb enriched CH3Cl and CH3Br. In this study, purge temperature, trap temperature, sorbent, and desiccant were also optimized. According to the physical properties of CH3Cl and CH3Br and previously reported results[9], purge temperature was set at 40 °C, and liquid nitrogen was used as the cryogen (about –190 °C). Though the adsorption efficiency of the empty stainless steel tube is lower than that of the tubes filled with sorbents, the residue in the empty steel tube is less than that in the latter. Furthermore, when an empty tube was used, the shape of GC peaks was much better and the interference of the successive analysis was reduced dramatically. Therefore, an empty stainless steel tube was chosen for trapping. Purge gas needs to be dried before being injected into the trap loop; otherwise, the vapor entering the trap loop will block due to ice formation, and vapor entering the GC analyzer will also damage the capillary column, and then tailing peak was formed in the ECD detector. Mg(ClO4)2 was used as the desiccant and CO2 sorbent (NaOH) was added to avoid the interference of CO2 and H2S during the analysis. 3.3

Standard calibration curves

Peak areas corresponding to CH3Cl and CH3Br were calibrated as the function of gas mass, using gravimetrically produced standard containing 4 u 10–8 (V/V) of CH3Cl and 1 u 10–9 (V/V) of CH3Br in nitrogen stored in the Aculife-treated aluminum cylinders, respectively. The standards of CH3Cl and CH3Br were measured using all-glass syringes, and volumes of 0, 1, 2, 5, 10 and 15 mL for CH3Cl and 0, 2, 5, 10, 20 and 30 mL for CH3Br were injected, respectively, via a septum port by the syringes to the stripper filled with 100-mL blank natural seawater free of dissolved CH3Cl and CH3Br,

The detection limit of this method is defined as three times the standard deviation of the blank levels (the average of 7 blank concentrations). In this study, the limits of detection of CH3Cl and CH3Br were 2.7 and 0.3 pM, respectively. Under the optimized conditions for separation, concentrations of CH3Cl and CH3Br in five parallel samples in coastal seawater at Shilaoren of Qingdao were determined, and their average concentrations were 98.1 and 3.8 pM, and the relative standard deviations for the analysis of CH3Cl and CH3Br were 1.9% and 7.1%, respectively, which were in good agreement with those reported in the literature [10]. The recovery experiment was conducted by adding known amount of standards to seawater samples and then analyzed 5 times. The recovery of CH3Cl and CH3Br was 89%–97% and 75%–85%, respectively. 3.5

Experiment of sample preservation

CH3Cl and CH3Br in seawater can be produced in situ, consumed by biological and chemical degradation and removed by sea-to-air exchange. If the samples were not analyzed immediately after the collection, the accuracy of analytical results may be affected. Thus, sample preservation is very important to obtain accurate results. Seawater samples were preserved according to the described method (see Section 2.2). Duplicate analyses were conducted after collecting the samples in the time series of 0, 3, 6, 12, 24 and 48 h, and the results showed that there were no significant variations in the concentrations of CH3Cl and CH3Br within 24 h. In our study, all samples were analyzed within 6 h in situ after the collection, ensuring the accuracy of the results. 3.6

Distributions of CH3Cl and CH3Br in surface seawater of Jiaozhou Bay

To prove its applicability toward a real sample analysis, this method was used for measuring the concentrations of CH3Cl and CH3Br in surface seawater of the Jiaozhou Bay. The concentrations of CH3Cl and CH3Br in the Bay were determined on 3 June 2007, which are shown in Table 1. The sampling stations are shown in Fig.4. The cruise included 19 stations, 16 in the Bay and 3 off the mouth of the Bay.


pM, respectively (Table 1). The results fall in the range of those reported for the eastern Pacific (CH3Cl, 125–828 pM; CH3Br, 5.30–38.9 pM)[11] and the northwestern Atlantic (CH3Cl, 69–455 pM)[12].

References [1]

Lovelock J E. Nature, 1975, 256(7): 193–194

[2]

Albritton D L, Watson R T, Aucame P J. Scientific Assessment of Ozone Depletion, 1998. Geneva: World Meteorological Organization

[3]

Reifenhäuser W, Heumann K G. Chemosphere, 1992, 24(9): 1293–1300

Fig.4 Sampling stations in Jiaozhou Bay

[4]

Table 1 Concentrations of CH3Cl and CH3Br in surface seawater of the Jiaozhou Bay

[5]

Station A1 A2 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5 D1 D2 D3 D4 E1 E2 E3 Average

Research, 1999, 104(D7): 8373–8389 Singh H B, Kanakidou M. Geophysical Research Letters, 1993, 20(2): 133–136

Concentration (pM) CH3Cl 130 340 272 346 309 272 278 173 161 260 297 247 284 247 284 247 241 179 173 250

Lobert J M, Keene W C, Logan J A. Journal of Geophysical

CH3Br 8.70 8.80 3.90 20.7 17.9 9.10 13.7 8.10 5.40 10.1 13.1 12.8 10.6 9.40 16.4 16.0 8.70 7.90 7.50 11.0

[6]

Kato S, Watari M, Nagao I, Uematsu M, Kajii Y. Deep Sea Research Part II: Topical Studies in Oceanography, 2009, 56(26): 2918–2927

[7]

Hashimoto S, Toda S, Suzuki K, Kato S, Narita Y, Kurihara M K, Akatsuka Y, Oda H, Nagai T, Nagao I, Kudo I, Uematsu M. Deep Sea Research Part II: Topical Studies in Oceanography, 2009, 56(26): 2928–2935

[8]

Dewulf J, Langenhove H V. J. Chromatogr. A, 1999, 843(1-2): 163–177

[9]

Moore R M, Groszko W, Niven S. Journal of Geophysical Research, 1996, 101(C12): 28529–28538

[10]

Christof O, Seifert R, Michaelis W. Biogeochemistry, 2002, 59(1): 143–160

[11]

Singh H B, Salas L J, Stiles R E. Journal of Geophysical Research, 1983, 88(C6): 3684–3690

Concentrations of CH3Cl and CH3Br were 130–346 pM and 3.90–20.7 pM, with the average values of 250 and 11.0

[12]

Tait V K, Moore R M, Tokarczyk R. Journal of Geophysical Research, 1994, 99(C4): 7821–7833