Method Development for Rapid Extraction of Volatile Organic Compounds from Rock Samples Using Microwave Assisted Extraction

Transcription

1 Method Development for Rapid Extraction of Volatile Organic Compounds from Rock Samples Using Microwave Assisted Extraction Supervised by Tadeusz Gorecki By Xiaoxia Zhai

2 Abstract: Microwave Assisted Extraction (MAE) method has been developed for rapid extraction of Volatile Organic Compounds (VOC) from low permeability rock samples. It has been considered a necessity that new method for extracting VOCs from low permeability media with much shorter processing time and equivalent or better results to be developed.

3 Abstract: Shake-flask solvent extraction is the standard method for extracting VOCs from low permeability media and it is the current method in use in the Department of Earth Science, University of Waterloo. It takes 6 weeks of extracting rock samples.

4 Background Information: What are VOCs? Volatile Organic Compounds (VOCs) are organic chemicals that easily vaporize at room temperature. Many VOCs have been found to have adverse effects on air quality and human health. 1 VOCs are the most common contaminants of soil. The main emission sources are industry, traffic and energy production. 2

5 Background Information: Why TCE? Trichloroethylene (TCE) was chosen to be the model compound in this research project because of its widespread contamination. TCE has been widely used as an industrial solvent in a variety of manufacturing operations.

6 Introduction: Soxhlet extraction and shake-flask extraction are the mostly used techniques for extraction of organic compounds from soils. Soxhlet extraction is good for non-volatile compounds. Shake-flask extraction is more reliable but very timeconsuming.

7 Introduction: New techniques have been developed to assess the organic contaminants in solid matrices. Supercritical Fluid Extraction (SFE) Pressurized Liquid Extraction (PLE) Microwave Assisted Extraction (MAE) Static Headspace Analysis (HS) Purge-and-Trap (PT) Sonication Extraction

8 SFE & PLE: Both SFE and PLE yield good recoveries in a short time for some semi- and non-volatile organic compounds in soil samples, but they are generally not well suited for VOC determinations. 3 Analytes are lost at the extract collection stage.

9 HS & PT: The most popular methods are Static Headspace and Purge-and-Trap technique connected either to a gas chromatograph or to a mass spectrometer. 2 These techniques work well with soil media with moderate to high permeability.

10 MAE: Microwave dielectric heating uses the ability of polar molecules to transform electromagnetic energy into heat. So which solvent to choose plays an important role in the analysis. Two categories of solvents: -Microwave absorbing solvents are called nontransparent solvents. -Non-microwave absorbing solvents are called transparent solvents.

11 Polar and non-polar solvents: Polar solvents - Elevated temperature enhances the molecular diffusion through the solid sample to solvent. Non-polar solvents - Water content within the solid sample heats up.

12 Instrumentation: Microwave Oven: Ethos SEL Microwave Labstation Extraction Rotor: MPR-600/12S Medium pressure segment rotor

13 Instrumentation: Teflon Vessel: Custom-built pressurized Teflon vessel, a spring is used to keep the vessel closed. Spring External hole Outer layer shield Safety valve bar Teflon cover

14 Instrumentation: GC: HP 6890 series equipped with Agilent 7683 series auto-injector with 100 sample capacities. J&W DB- 624 fused silica capillary column (30 m x 320 µm x 5 µm) and µ-ecd detector. The ultra high purity helium (Praxair) was used as the carrier gas in GC analysis and TAG grade nitrogen (Praxair) was used as the makeup gas in the µ-ecd detector.

15 Experimental Section: A Teflon vessel containing 20 ml of methanol and a certain amount of samples was heated in the microwave oven at 120 o C for 40 min. After heating, samples were cooled in an ice-water bath for at least 35 min. Cooling is important. Opening the lids without complete cooling will result in analyte loss.

16 Experimental Section: The standard methanol extraction method was done by putting a weighed amount of sample into a glass vial containing 20mL of methanol. These vials were sealed with Teflon tapes and were shaken once a week using a mechanical shaker.

17 Experimental Section: Direct cool on-column injection Injection volume was 1 μl. Carrier gas was helium with a constant flow rate at 3.0 ml/min. Oven temperature was increased from the initial 55 o C to 145 o C at a rate of 10 o C/min. Then it was continuously increased to 210 o C at a rate of 35 o C/min. Holding for 9 min. at this temperature. μ-ecd detector was set to have a temperature of 350 o C and nitrogen makeup gas at a flow rate of 60 ml/min.

18 Results & Discussion: Why chose methanol as the extraction solvent? Methanol was chosen as the extraction solvent because it can efficiently penetrates the inner core of the rock samples, which results in an increase in solute diffusivity and allows a reasonable detection limit to be achieved. 3

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20 Results & Discussion: Testing normality of the data set in the difference column. Data points may be any real number and the range of possible values is infinite. The data points are not proportions, rates or frequencies. The data points are not counts. Is the mean approximately the same as the median?

21 Results & Discussion: Rearrange the data set from the smallest number to the largest, we could find the median to be ppb. This is not a close estimation of the mean value 0.52 ppb. Sample size n = 396. Blom coefficient is calculated as i 8 = 3 /, for i = 1 n. n p i + 1/ 4 Convert Blom coefficients to x i, x, i = ln( 4pi (1 pi )) for i = 1 n. Calculate normal scores, = sign p 1/ 2) 1.238x (1+ z ) ( i i for i = 1 n, where sign = -1, for negative values, +1 for positive values and 0, otherwise. i x i

22 Figure 1. Plot of Normal Scores z i vs. Sample Size

23 Figure 2. Plot of logz i vs. Sample Size

24 Testing Median is Equal to a Specified Value; H o : median = d o. This test is used to determine at a level of 100(1.0 α)% confidence that the median is not equal to some pre-specified value, d o. The value, d o will often be the performance that a technology is claiming to achieve. The test presented is the Wilcoxon Signed Ranks test. 4

25 Testing Median is Equal to a Specified Value; H o : median = d o. Null hypothesis: median 0 Alternative: median < 0 Sort the data from the smallest to the largest. Calculate the vector d i = d o x i, where x i is each datum. Rank the d i from smallest to largest to obtain a vector R i. Identical d i are assigned the average of the ranks they would otherwise have received. The test statistic T + is calculated T + = Σ R i, for positive d i only. In our case, the test statistic T + was calculated to be

26 Testing Median is Equal to a Specified Value; H o : median = d o. Table 2 Lower-Tail Critical Values for the Wilcoxon Signed-Rank Test 4

27 Figure 3. Critical values w vs. sample size

28 Figure 4. Critical values w with trend lime vs. sample size

29 Testing Median is Equal to a Specified Value; H o : median = d o. At 95% confidence level w 0.05 = n n , R 2 = 1, where n is the sample size. When n = 396, w 0.05 = 33073; w = n (n + 1) / 2 w 0.05 = If T + w 0.95 we reject the null hypothesis and accept the alternative hypothesis. Since we got T + for 24459, which is less than w , we do not reject the null hypothesis. And we conclude that MAE technique has an equivalent or higher efficiency in analyte recovery than the standard method.

30 Figure 5. TCE Concentration in Rock Samples Obtained by MAE and the Standard Method

31 References: (1) Zhao, W.; Hopke, P.; Karl T. Environ. Sci. Technol. 2004, 38, (2) Ojala, M.; Mattila, I.; Tarkiainen, V.; Sarme, T.; Ketola, R.; Maattanen, A.; Kostiainen, R.; Kotiaho, T. Anal. Chem. 2001, 73, (3) Dincutoiu, I.; Gorecki, T.; Parker, B. Environ. Sci. Technol. 2003, 37, (4) Environmental Technology Verification, General Verification Protocol, Appendix B; Environment Canada, March 31, 2000.