In the Laboratory
W
Tetraglyme Trap for the Determination of Volatile Organic Compounds in Urban Air Projects for Undergraduate Analytical Chemistry Wilbert W. Hope,* Clyde Johnson, and Leon P. Johnson Department of Physical, Environmental and Computer Sciences, Medgar Evers College, Brooklyn, NY 11225; *
[email protected] Chemistry educators have heeded the call for closing the gap between real-work experience and university training in chemistry (1–4). New approaches to undergraduate analytical chemistry have been affecting pedagogy, course content, and the nature of the laboratory experiences (5–9). Cooperative learning (5–6) in its various forms seems to be the preferred pedagogical approach, and group-project work (6– 9) is gaining acceptance over the three-hour laboratory experiment. The thrust is to enhance students’ ability to think critically, communicate effectively, and work as a team to solve problems while ensuring the course content is relevant and keeping pace with the advancement of technology (10). Through project-based approaches, courses in analytical chemistry have provided students with the opportunity and the experience to use science to address environmental issues of concern to their community (11). Air pollution, a major concern for inner-city communities, is the central theme for project-based experiments at Medgar Evers College (8). Incidences of asthma (12) are above the national average in the communities of Medgar Evers College students and volatile organic compounds (VOCs), the most common pollutants in urban air, exacerbate asthma symptoms. Further, a number VOCs emitted from “on-road mobile sources” and small industries have been classified as probable or possible human carcinogens (13). Tetraglyme [CH3(OCH2CH2)4OCH3] as an impinger fluid has efficiently scrubbed a number of VOCs from heated gas streams (14). Its solubility in water permits dispersion of the trapped VOCs in water, from which the VOCs are purged, then trapped on solid sorbent material, thermally desorbed, and analyzed by gas chromatography–mass selective detector (GC–MSD). This method is very attractive because of its simplicity and its potential for use to investigate VOCs in ambient and indoor air employing a purge-andtrap concentrator connected to a GC–MSD (15). The aim of this project is to compare the levels of VOCs in the air of two Brooklyn neighborhoods, Sunset Park and Crown Heights. Sunset Park is located in southwest Brooklyn by the Gowanus canal. Originally it was a shipping and warehousing facility, now it is an urban industrial city featuring manufacturing, warehousing, distribution firms, and residential housing. A major expressway separates the industrial park from the residential community. On the other hand, Crown Heights is located in central Brooklyn and is a typical inner-city residential neighborhood. According to the EPA, small industrial sources such as gas stations, drycleaners, and landfills contribute approximately 40% of the emissions of hazardous air pollutants. Higher levels of VOCs are there-
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fore expected in urban centers such as Sunset Park where a number of small industries are in closer proximity. Another aim of this project is to expose students to the practical features of a 23-factorial experimental design, used to optimize processing conditions and analytical methods. There is potential for improvement of this method for ambient air measurements, thus providing opportunity for student involvement in method development, an important responsibility of analytical chemists. The students are asked to investigate the effects of tetraglyme volume used in the impinger and air-sampling rate on sampling efficiency. The 23-factorial design is a simple and convenient way to monitor the effects of three variables (air flow, volume of tetraglyme, and location) simultaneously. Each trial requires eight experiments that generate eight vials of tetraglyme impinger-fluid for GC–MSD analysis (16). A student group is able to complete two trials in the six-week time period allotted for this project (8). For the first part of this lab course (8), students are required to complete six laboratory experiments, one of which prepares them for this project (see the Supplemental MaterialW); they determined the percent recovery and are asked to discuss why tetraglyme traps VOCs. Future projects may require students to compare this method with the classical canister method or methods involving various solid sorbent traps. Future projects may also focus on chemically modifying tetraglyme to trap specific VOCs.
Table 1. Matrix Description of Experiments Experiments
Volume of Tetraglyme
Rate of Air Flow
Location
1
---
---
---
2
+
-- -
---
3
---
+
---
4
+
+
-- -
5
---
---
+
6
+
---
+
7
---
+
+
8
+
+
+
Legend ---
3 mL
100 mL/min
Crown Heights
+
5 mL
150 mL/min
Sunset Park
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Project Design The 23-factorial statistical design approach requires that each of the three variables (flow rate, volume of tetraglyme, and location) be investigated at two levels (16). A total of eight sampling experiments described in the 3 × 8 matrix (Table 1) must be conducted simultaneously. The rows describe the experiment in terms of the levels used for each variable; low level is indicated by “–” and the high level is indicated by “+”. For example, experiment 1 shows that the flow rate, volume of tetraglyme, and location are at the lower levels (–, –, –) while for experiment 8 the same three variables are at the higher levels (+, +, +). Two sampling units were constructed, each accommodating four midget impingers attached to a single air-sampling pump (Figure 1); the Universal XR air sampling pump was obtained from SKC Inc.1 One sampling unit representing four experiments was placed in each of the two locations. Sampling at both locations started and ended at the same time. For each experiment 45 L of air were bubbled through chilled tetraglyme. Hazards All chemicals used are common laboratory chemicals except tetraglyme (17). Tetraglyme must always be handled with gloves. Its ratings (0–4) by the National Fire Protection Association (NFPA) for health, flammability, and reactivity are 1, 1, and 0, respectively. Signs and symptoms of overexpo-
airflow adjustment screw
SKC QALFH
flow meters silicone rubber tubing
air
thermometer Thomas flask containing ice–salt bath
SKC pump
impinger with chilled tetraglyme
Figure 1. A 4-impinger sampling unit: an SKC quad-adjustable lowflow holder (QALFH) with screws to adjust airflow; four impingers immersed in ice-salt bath contained in Thomas flask. Thermometers for monitoring the temperature are also immersed in the ice–salt bath.
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sure may include slight eye or skin irritation. For first aid response, flush eyes with plenty of water for 15 minutes or remove contaminated clothing and shoes and wash affected areas with soap and water. Tetraglyme biodegenerates slowly in the environment, so for small spills, absorb on inert material for subsequent incineration in an EPA approved manner; decomposition products include carbon monoxide, carbon dioxide, and incompletely burned carbon products. Store tetraglyme in a cool, dry, well-ventilated place away from heat, ignition sources, and incompatible material. It can form peroxides after prolonged and repeated contact with air. Experimental Procedure
Construction and Calibration of Air Sampling Unit A SKC universal air sampling pump was connected to a SKC quad-adjustable low-flow holder (QALFH) and each of the four inlets of the QALFH was connected to a modified midget impinger. Silicone tube connected the pump to the QALFH and impingers. Four Thomas flasks, (10.2-cm i.d. × 13.5-cm depth) with covers contained the ice–rock salt mixture into which the bottom portion of the impingers were immersed; holes in each flask cover expose the top portion of an impinger and accommodated a thermometer for temperature monitoring. A schematic of the setup for a sampling unit of four experiments is shown in Figure 1. Two such units were constructed, one for each of the two locations. A flow rate of 5 L兾min was set on the SKC pump before connecting it to the QALFH. Screws on the QALFH were adjusted to achieve flow rates of 100 mL兾min or 150 mL兾min of air through the impinger fluid. Air Sampling The required quantities (3 mL or 5 mL) of tetraglyme were poured into the respective modified midget impingers, which were then immersed into an ice–salt bath contained in a Thomas flask. The sampling units were transported to their respective locations, where the flow rates were again checked with a DC-Lite Primary flow meter. The temperature of the ice bath was monitored every 0.5 hours. Typically, two changes in the ice–salt mixture were adequate to maintain temperatures between ᎑20 and ᎑10 ⬚C for 7.5 hours. For all experiments, a constant volume of 45 L of air was drawn through the impingers; sampling at 100 mL兾min was conducted for 7.5 hours while sampling at 150 mL兾min was conducted for 5 hours. Sampling was conducted from 10:00 a.m. to 5:30 p.m. After sampling, the impinger fluid was poured in 20-mL EPA sample bottle, sealed, and stored in a freezer for analysis by purge-and-trap followed by GC–MSD. Results and Conclusions Calibration curves obtained for benzene, toluene, and other benzene derivatives are shown in Figure 2; correlation coefficients ranged from 0.970 to 1.00. The tetraglyme from each experiment was analyzed three times. Typical chromatograms are shown in Figure 3, and a list of compounds identified with quality index >50 for library matches are shown in Table 2. In the chromatogram representing the air over
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Crown Heights (Figure 3A), the strongest peak has a retention time of ∼12.01 min (abundance ∼14,000) and is identified as dichlorobenzene. The second most abundant peak has a retention time of 13.63 min (nonanal), followed by cyclopentasiloxane at ∼14.41 min, then toluene at ∼6.2 min (abundance ∼8,000) followed by 5-octadecene at 17.85 min. The other prominent peaks between 6.0 and 14.0 min, with few exceptions are benzene derivatives. In the chromatogram representing the air over Sunset Park (Figure 3B), the strongest peak at 17.85 min (abundance ∼90,000) represents 5-
40
Response (peak areas) / 105
Table 2. List of Compounds and Properties Identified by GC–MS
benzene toluene ethyl benzene benzene, 1,2-dimethyl benzene, trimethyl 4-isopropyl toluene
35
30
4-isopropyl toluene
20
toluene 15
benzene benzene, 1,2-dimethyl benzene, trimethyl
10
5
ethyl benzene
0 5
10
15
20
25
30
35
40
45
Quantity of VOC / ng Figure 2. Calibration curves for benzene, toluene, ethyl benzene, dimethyl benzene, trimethyl benzene, and 4-isopropyl toluene.
Figure 3. Typical chromatograms of VOCs purged from tetraglyme impinger fluids from (A) Crown Heights and (B) Sunset Park locations in Brooklyn, New York.
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Retention Time/min
Compounds
25
0
octadecene. The toluene peak is the second most abundant (abundance ∼50,000) followed by cyclopentasiloxane at ∼14.4 min and cyclobutyl benzene at 14.6 min. The students explored the mass spectra related to more than 50 major and minor peaks in the chromatograms and found out that more than one compound could sometimes be associated with a single peak. They listed more than 50 possible compounds that might be extracted from the air by this method and speculated as to their origins. The students discussed the difficulties in obtaining the absolute value for
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Molecular Weight/ (g/mol)
QualityLibrary Matcha
Benzene
3.33
78
90
1,3-Hexadien-5-yne
3.33
---
74
Ethene-[2-methoxyethoxy]
5.21
102
Toluene
6.16
92
91
Phosphorous bromide, chloride, fluoride
7.38
164
74
Ethyl benzene
8.80
106
90
Benzene, 1,3-dimethyl
8.98
106
95
Benzene, 1,2-dimethyl
8.98
106
93
p-Xylene
8.98
106
94
Benzene, 1,2-dimethyl
9.54
106
94
p-Xylene
9.54
106
91
Benzene, 1-ethyl-2-methyl
11.04 11.18 11.40
120 120 120
90 80 74
Benzene ,1,2,3-trimethyl
11.66
120
91
Benzene, 1,2,4-trimethyl
11.66
120
91
Benzene ,1,4-dichloro (1,3) (1,2)
12.01
146
94, 91, 91
Nonanal
13.63
142
80
Cyclopentasiloxane, decamethyl
14.41
370
83
Benzene cyclobutyl
14.59
132
58
Decanal
15.21
156
90
Octadecanal
15.21
268
53
5-Octadecene
17.85
252
90
Undecane
17.98
156
74
a
83, 74
Only compounds with the quality of library-match > 50 are listed.
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In the Laboratory
the total VOC extracted from the air. They discussed some steps that might be followed: • Identify all the peaks in the GC (a minimum of 60 peaks) and determine peak areas. • Obtain a pure sample of all the compounds identified by the GC peaks (more problems with those peaks that showed more than one compound). • Do laboratory studies to determine percent recovery of each compound. • Do calibration curves for each compound. • Determine the quantity of each compound extracted after adjusting for percent recovery. • Sum the values to obtain the total.
A simple approximation of the above-mentioned steps was also discussed.
Figure 4. Average concentration levels of benzene, toluene, and other benzene derivatives in the ambient air of Crown Heights (experiments 1 to 4 at location 1) and Sunset Park (experiments 5 to 8 at location 2).
• Obtain the total-area response for all peaks in the sample chromatogram. • Obtain the total-area response for all peaks in the blank sample chromatogram.
The actual and coded factor levels with the corresponding total VOCs for all experiments are shown in Table 3. The effects of volume of tetraglyme (V), flow rate (R), and location (L) were calculated by averaging the change in concentration of VOC moving from the low to the high value of the factor (16). Calculation of the main effects result in:
• Subtract the total-area response of the blank from the total-area response for the sample to obtain the net total-area response. • Use the net total-area response to determine the total VOC using the toluene response factor (in this case all the compounds are treated as though they were all toluene).
Location effect = 1/4(−8 − 4 − 8 − 3 + 28 + 33 + 27 + 35) = 25
However to simplify quantitative measurements, only benzene and four derivatives of benzene were considered. These compounds eluted at retention times 3.33, 6.16, 8.80, 9.98, and 11.66 min for benzene, toluene, ethyl benzene, dimethyl benzene, and trimethyl benzene, respectively. The mean concentrations of benzene, toluene, and other benzene derivatives were determined and added to give the total VOCs. A comparison of the concentration levels of benzene, toluene, ethyl benzene, dimethyl benzene, and trimethyl benzene in the air from Crown Heights and Sunset Park from the experiments is shown in Figure 4.
Volume effect = 1/4 (−8 + 4 − 8 + 3 − 28 + 33 − 27 + 35) = 1 Flow Rate effect = 1/4 (−8 − 4 + 8 + 3 − 28 − 33 + 27 + 35) = 0 As expected the greatest effect was due to location, the level of VOCs in the ambient air from Sunset Park was four times higher than that from Crown Heights. Flow rate had no measurable effect and the effect of volume was slight. However, a closer examination of Table 3 suggests that the sampling method might be optimized in low VOC locations by using the smaller volume of tetraglyme; the opposite seems true for the high VOC location.
Table 3. Actual and Coded Factor Levels and Yields of the 23 Factorial Design of Experiments Actual Factor Level Experiment
1
Coded Factor Level
Volume of Tetraglyme, V/mL
Rate of Air Flow, R/ (mL/min)
Location
V*
R*
L*
3
100
Crown Heights
---
---
---
Total VOCs/ (µg m᎑3)
8
2
5
100
Crown Heights
+
---
---
4
3
3
150
Crown Heights
---
+
----
8
4
5
150
Crown Heights
+
+
----
3
5
3
100
Sunset Park
---
---
+
28
6
5
100
Sunset Park
+
---
+
33
7
3
150
Sunset Park
---
+
+
27
8
5
150
Sunset Park
+
+
+
35
NOTE: Total VOCs represent the sum of the concentrations of benzene and benzene derivatives shown in Figure 4.
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Summary
Literature Cited
Students were amazed at the results establishing the difference in the levels of VOCs in the ambient air from the two locations. Some elected to continue this work in the summer that followed and made presentations at a number of undergraduate research conferences. They reviewed, on their own, the factorial design of the experiments and were eager to make suggestions on how to improve the experiment and to obtain results in a shorter sampling time. Discussions were held on how to obtain hourly measurements of the VOCs in ambient air. A number of students wondered whether the VOCs of ambient air could provide a “finger print” of urban air.
1. Hodgson, S. C.; Casey, R. J.; Orbell, J. D.; Bigger, Stephen W. J. Chem. Educ. 2000, 77, 1631. 2. Houghton, T. P.; Kalivas, J. H. J. Chem. Educ. 2000, 77, 1314. 3. Davis, D. S.; Hargrove, R. J.; Hugdhal, J. D. J. Chem. Educ. 1999, 76, 1127. 4. Craig, P. A. J. Chem. Educ. 1999, 76, 1130. 5. Wenzel, T. J. Anal. Chem. 1995, 67, 470A–475A. 6. Wenzel, T. J. Anal. Chem. 1998, 70, 790A–795A. 7. Fitch, A.; Wang, Y.; Mellican, S.; Macha, S. Anal. Chem. 1996, 68, 727A–731A. 8. Hope, W. W.; Johnson, L. P. Anal. Chem. 2000, 72, 460A– 467A. 9. Werner, T. C.; Tobiessen, P.; Lou, K. Anal. Chem. 2001, 73, 84A–87A. 10. National Science Foundation Directorate of Education and Human Resources/Division of Undergraduate and Division of Chemistry. Curricular Development in the Analytical Sciences, A Report from the Workshops; Atlanta, Georgia, 1997, March 13–15, 4–9. 11. Wenzel, T. J.; Austin, R. N. Environ. Sci. Techol. 2001, 35, 327A–331A. 12. New York City Department of Health. Asthma Facts; New York City Childhood Asthma Initiative: New York, 1999. 13. U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report; 1999. http://www.epa.gov/oar/ aqtrnd99/ (accessed May 2004). 14. Troost, J. R. Anal. Chem. 1999, 71, 1474–1478. 15. Hope, W. W; Johnson L. P. In CHED Newsletter & Abstracts Fall 2001; ACS Meeting, Chicago, IL August 26–30, 2001. Division of Chemical Education, Inc. American Chemistry Society: Washington, DC, 2001; Abstract 315. 16. Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experimenters An Introduction to Design, Data Analysis, and Model Building; John Wiley & Sons: New York, 1978; pp 306–323. 17. See http://www.matheson-trigas.com/msds/MAT22965.pdf (accessed May 2004).
Acknowledgments We acknowledge support from the National Aeronautics and Space Administration (NASA) Minority University Research and Education Program: NASA Partnership awards NCC5-205, NASA PAIR at CCNY, and NASA MUSPIN grant NCC5-98. Analytical instruments were provided by funds from the Brooklyn Borough President’s Office. We also acknowledge the NSF-funded New York City–Louis Stokes Alliance for Minority Participation in Science, Engineering, and Mathematics, and the special contributions from Yvette Samuels, Delroy Burnett, Jamilla Dick, and Mohamed Bangura. W
Supplemental Material
Instructions for the students are available in this issue of JCE Online. Note 1. SKC Inc., 863 Valley View Road, Eighty Four, PA 15330; http://www.skcinc.com/ (accessed Apr 2004).
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