In the Laboratory
Measurement of Organics Using Three FTIR Techniques: Absorption, Attenuated Total Reflectance, and Diffuse Reflectance
W
M. E. Gebel,† M. A. Kaleuati, and B. J. Finlayson-Pitts* Department of Chemistry, University of California, Irvine, Irvine, CA 92697-2025; *
[email protected] A recent review has illustrated the benefits and ease of adapting environmental science projects to analytical chemistry courses (1). Students across the nation have taken part in analyses ranging from the determination of lead in soil to toxics in wood smoke and methyl t-butyl ether, MTBE, in drinking water. One important aspect of many of these new experiments is the application of different instrumental methods to measure independently a species in a particular sample to illustrate the advantages and disadvantages of each analytical technique. In an earlier report (2), we described a GC–MS experiment to measure MTBE as well as some toxics such as benzene in gasoline. An alternate approach for the analysis of oxygenates in gasoline, using water extraction followed by GC analysis, has also been reported (3). We describe in this paper a third approach for the determination of MTBE in gasoline, the use of transmission Fourier transform infrared spectroscopy. This approach emphasizes the utility of FTIR as a quantitative analytical technique, rather than simply providing qualitative identification as is commonly done in undergraduate organic chemistry courses. In the first part of the experiment, students explore the effects of increased numbers of repetitive scans on the signal-to-noise ratio, as well as the relationship between length of mirror travel and instrumental resolution. There are several other infrared techniques that are not commonly taught in the undergraduate curriculum, even though they are relatively straightforward and have significant advantages over conventional absorption spectroscopy. One infrared method is attenuated total reflectance (ATR). In the ATR technique, the sample is placed on an internal reflection element and the IR beam is directed into the element. The beam strikes the internal crystal–air interface at an angle greater than the critical angle and, as a result, undergoes internal reflection inside the crystal. At each point of internal reflection an evanescent wave penetrates a small distance into the sample where infrared absorption by the surrounding sample can occur. This absorption of infrared light can then be detected and measured. ATR is particularly useful for aqueous solutions since the depth of penetration (dp) into the sample on each reflection is small. The depth of penetration depends on the wavelength (λ), the nature of the crystal material and the surrounding medium, and the angle of incidence, θ, dp =
(
λ
)
2
2 π n1 sin2 θ − n21
1
2
† Current address: Monitoring and Laboratory Division, California Air Resources Board, 9528 Telstar Ave., El Monte, CA 91731
672
where n21 = n2兾n1, and n2 is the index of refraction of the sample medium in contact with the crystal of refractive index, n1 (4). For example, the depth of penetration into water for a crystal with a refractive index of 2.8 and angle of incidence of 45⬚ is 0.3 µm at 3400 cm᎑1 and 0.9 µm at 1150 cm᎑1, much smaller path lengths than can be achieved with conventional transmission cells. As a result, the absorption of infrared radiation by the aqueous solvent is greatly reduced using ATR, allowing quantitative measurements to be carried out for species in aqueous solutions. In the second part of the experiment, the ATR method to used to measure the ethanol content in an aqueous solution (vodka). Both “single-bounce” (i.e., single reflection) and multiple reflection cells are used to demonstrate the enhanced sensitivity obtainable with multiple reflections. Previous laboratory experiments using GC–TCD (5) and visible Raman spectroscopy techniques to analyze ethanol in water have been reported (6). The third part of the experiment involves the application of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to the analysis of hydrocarbons in soils. The analysis of solids using infrared spectroscopy is usually carried out by making a pellet in a non-infrared absorbing salt such as NaCl or KBr or a mull in a viscous organic matrix such as Nujol. Students have difficulty getting quantitative and reproducible results, which may be the result of nonuniform dispersal of the analyte within the salt matrix. In addition, the students often have difficulty making sufficiently thin pellets that transmit the light beam, but sufficiently thick enough so as not to crumble on handling. The use of Nujol mulls, on the other hand, precludes the analysis of small amounts of organics in solids due to overlapping absorption peaks. The DRIFTS method has been successfully used for qualitative analysis of solids in the classroom (7–9). The experiment described here illustrates its utility for quantitative analysis. In the DRIFTS method, the beam is directed onto the surface of the powder and undergoes multiple reflections through the sample as well as from the powder surfaces before being directed to the detector (10). DRIFTS spectra are commonly described by the Kubelka–Munk function ƒ (R∞) (10–12),
f ( R∞ ) =
(1 −
R∞ ) k = 2R∞ s 2
where R∞ is the ratio of the diffuse reflectance signal from the sample to the diffuse reflectance signal from a non-absorbing reference, k is an absorption coefficient that is proportional to the analyte concentration, and s is a scattering coefficient. If the specular reflection is small and the scatter-
Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu
In the Laboratory
ing coefficient is constant, the Kubelka–Munk function should vary linearly with concentration. In practice, the analog of “absorbance”, that is, the log of the ratio of the signal from the reference to the signal from the sample, is often found to vary linearly with concentration (9, 13) and can be used for quantitative analysis. Diffuse reflectance measurements can be carried out using a reference that does absorb infrared radiation, but the sensitivity is reduced (10). The ATR and DRIFTS analyses require that these accessories be available for the infrared spectrometer. Such accessories are now readily available commercially from a number of different manufacturers. Experimental Procedure In all experiments reported here, a JASCO Model 615 FTIR spectrometer was used.
Transmission Spectroscopic Measurement of MTBE in Gasoline Standard solutions of MTBE in hexane with concentrations between 0.2–1.6% (v兾v) were prepared. The absorbances of the peaks at 1088 and 1204 cm᎑1 (C⫺O stretches) were used to generate calibration plots. The gasoline unknown was diluted to 10% (v兾v) with hexane and analyzed in a similar manner. A standard gasoline used for emissions certification (CA Cert. II) with 10.9% (v兾v) MTBE was obtained from the California Air Resources Board. Gasoline samples with different octane ratings were also obtained from local gas stations. (The MTBE content of gasoline is dependent on local regulation and supplier). In most states, a 10% byvolume-concentration of MTBE can be expected, as this is the threshold for the federal oxygenate requirement for gasoline sold in polluted areas. Attenuated Total Reflectance Spectroscopic Analysis of Aqueous Solutions Spectra were taken using a horizontal multi-pass ATR (HATR) and a single-bounce accessory (ATR) with ZnSe crystals (Pike Technologies; the multi-pass system has 10 reflections). For the calibrations, the sample trough of the multi-pass accessory was filled with aqueous ethanol solutions ranging from 1 to 5% (v兾v) and the C⫺O stretch at 1044 cm᎑1 was measured. Two brands of vodka, Absolut and Kamchatka, were tested. The vodka came from partially used bottles that had been stored for over six months. In addition, a newly purchased bottle of one brand was compared with the older bottle. The vodka was diluted by a factor of ten in water and the concentration of ethanol was determined using the absorbance at 1044 cm᎑1. The number of internal reflections that occur within the multi-pass ATR crystal can be determined from the ratio of the absorbance from the multi-pass crystal to the absorbance from the single-bounce for a given sample. This ratio can be compared to the theoretical number of internal reflections (N), N =
l cot θ 2t
where l is the crystal length, θ is the effective angle of incidence, and t is the thickness of the crystal (14). Measure-
ments were also taken using a single-bounce ATR accessory to illustrate the effect of fewer reflections; this is equivalent to a shorter path length in absorbance and thus lower sensitivity. If a multi-pass accessory is not available, the analysis of ethanol can be carried out using a single-bounce apparatus with proportionately less dilution of the sample.
DRIFTS Analysis of Hydrocarbons in Soil In this experiment, motor oil was used as a surrogate for hydrocarbons in soil. The peaks at 2925 and 2858 cm᎑1 representing the ⫺CH2⫺ asymmetric and symmetric stretch (15), respectively, were used for the measurement of total organics. The stock calibration mixture was prepared by thoroughly mixing 1 drop of motor oil with 10 g of washed sand that has been ground into a fine powder using a mortar and pestle. Although the particle sizes were not measured in this experiment, grinding of both the motor oil–sand and the soil samples gave powders that were visually much finer than the unground sand or soil and, which during handling, were clearly much lighter. This stock mixture was then diluted with additional finely ground sand to obtain standards with progressively smaller concentrations covering the range from ∼2500 to 250 ppm (w兾w). The standards were placed in the DRIFTS accessory (EasiDiff, Pike Technologies) and calibration plots of concentration versus absorbance at 2858 and 2925 cm᎑1 were generated. A sample of the pure sand was used as the background. The unknown soil samples should be finely ground to a particle size similar to the sand used in the standard solutions before recording their spectra. Students can also be asked to perform the analysis of motor oil in sand using the transmission technique for comparison to DRIFTS. A very thin pellet of motor oil–sand solution can be made using a typical device for transmission analysis of solids. This process will prove to be quite difficult and may be facilitated by mixing the motor oil–sand mixture with NaCl or KBr powder. It was difficult to make pellets that could be handled easily or to obtain reproducible results on these pellets, illustrating the sample preparation advantages of DRIFTS. Results and Discussion
MTBE in Gasoline by Transmission FTIR The MTBE absorbance at 1088 cm᎑1 for a state-supplied certification gasoline and six different gasoline samples, as well as the corresponding concentrations obtained using the calibration plots are summarized in Table 1. The concentration of the certification gasoline determined by transmission FTIR spectroscopy, 11.3 ± 0.4% (v兾v), is in good agreement with both the value provided by the supplier, 10.9% (v兾v), and with the value measured earlier using GC–MS (2), 12 ± 2%. All errors are ± 2s, where s is calculated using s =
∑ ( xi
− x )2
i
N −1 _ where xi is the ith measurement, x is the mean and N is the
JChemEd.chem.wisc.edu • Vol. 80 No. 6 June 2003 • Journal of Chemical Education
673
In the Laboratory Table 2. Multi-pass ATR Analysis of Ethanol in Vodkaa
Table 1. Results of the Analysis of MTBE in Gasolinea Gasoline Type
Average Absorbanceb (± 2 s) d
MTBE Concentrationc (% by volume, ± 2 s) 11.3 ± 0.4
Brand
Old Bottle
New Bottle
35.7 ± 4.0
39.6 ± 4.1
40
Kamchatka
39.5 ± 4.1
NDb
40
0.542
Brand A (87 octane)
0.667 ± 0.006
12.2 ± 0.5
Brand A (89 octane)
0.624 ± 0.009
11.4 ± 0.5
a
11.2 ± 0.5
b
0.612 ± 0.004
Brand U (87 octane)
0.499 ± 0.001
9.2 ± 0.5
Brand U (89 octane)
0.582 ± 0.004
10.6 ± 0.5
Brand U (92 octane)
0.585 ± 0.001
10.7 ± 0.5
a
᎑1
Absorbance at 1088 cm .
b
Unless otherwise indicated, these are the average absorbances from two trials using a 1:10 gasoline-–hexane mixture. c
The error in MTBE concentration was determined based on the error in the calibration plot as it was greater than the precision of the measured absorbances for the two trials. d
Only one trial was performed for this sample; this “certification gasoline” was provided by the California Air Resources Board and has a stated concentration of MTBE of 10.9%.
total number of measurements. MTBE was identified in all of the commercial gasoline samples in similar concentrations; there was no consistent trend in the MTBE concentration with the octane rating.
Ethanol in Vodka by ATR Spectroscopy The measured alcohol contents of the two different brands of vodka of different “ages” are shown in Table 2. The ethanol concentrations in the older bottles were measured to be 35.7 ± 4.0% and 39.5 ± 4.1% (v兾v), respectively. The new bottle had an alcohol content ∼10% greater than its older counterpart, likely owing to the evaporation of ethanol from the old bottle during use and storage. Hydrocarbons in Soil by DRIFTS The analog of absorbance (logarithm of the ratio of the signal of the clean sand to the signal from the calibration samples) was found to be linear with the concentration of motor oil added to the sand and was used for quantitative analysis. Six soil samples were measured using DRIFTS. The samples were obtained from the following locations: the center planter containing a tree at a gas station, the lot edge of a gas station, an area outside the physical sciences building on campus, a campus walkway in the dormitory area, under a tree at a residential home, and a vegetationless area near a stepping stone at a residential home. The results are summarized in Table 3. The hydrocarbon concentrations found in the soil ranged from < 120 ppm to 915 ppm. Not surprisingly, the largest value was found in a soil sample from a planter at a gas station and the smallest from the yard of a residential home.
674
Labeled Concentration
Absolut
CA Cert. II (87 octane)
Brand A (92 octane)
Ethanol Concentration of Vodka (% by volume, ± 2 s)
Absorbance at 1044 cm᎑1. Data not determined.
The detection limit of 120 ppm was determined as the concentration that would give a signal equal to three times the noise measured in the region from 2800 to 2750 cm᎑1. The determination of hydrocarbons in soil is not species-specific since the aliphatic C⫺H absorptions common to all hydrocarbons are measured. However, it is accepted in analyses of complex mixtures to express concentrations in terms of a standard with a similar structure, in this case, motor oil. For example, hydrocarbons in air, either in the form of gases or particles, are commonly measured as total hydrocarbons expressed as ppm C owing to the difficulty of complete speciation of the many compounds (16). It is likely that a portion of the total hydrocarbons is due to the presence of degraded plant material dispersed within the soil. Students were asked to discuss possible methods (e.g., GC–MS) for distinguishing between such biogenic sources of hydrocarbons and anthropogenic pollutants like motor oil as part of the lab in order to test their knowledge of the analytical techniques they had been studying. Hazards The major hazards in this experiment arise from the use of organic reagents. These reagents are all volatile, flammable liquids and irritants. They should be handled with care inside a fume hood, away from any excess heat or ignition sources, and using eye protection and gloves. Toluene, MTBE, and some of the components of gasoline are suspected carcinogens and pose an inhalation risk if not treated properly; skin contact should also be avoided. Acknowledgments We are grateful to Meitai Shu for helpful discussions and to the Dreyfus Foundation and the University of California– Irvine for support of this work. We also thank Rafael Susnowitz of the California Air Resources Board for supplying the certified gasoline for use in this experiment and Chuck Higgins for advice on the FTIR accessories. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu
In the Laboratory
Table 3. DRIFTS Analysis of Total Hydrocarbons in Soila Av Hydrocarbon Concentration Using 2925 cm᎑1 Band (ppm, w/w, ± 2 s)
Sample
Av Hydrocarbon Concentration Using 2858 cm᎑1 Band (ppm, w/w, ± 2 s)
Overall Av Hydrocarbon Concentration (ppm, w/w, ± 2 s)
Gasoline station planter
884 ± 294
946 ± 64
915 ± 88
Side of gas station lot
585 ± 280
617 ± 248
601 ± 45
Campus beside science building
773 ± 212
944 ± 48
859 ± 242
Campus beside dormitory
571 ± 90
653 ± 178
612 ± 116
Yard of residential home under tree
381 ± 208
514 ± 176
448 ± 188
Yard of residential home beside walkway
NDb
NDb
< 120
a
Measurements were made on three different samples from the same soil batch in each case. The data represent the average and standard deviation for each set of three measurements. b
Not detected.
Literature Cited 1. Wenzel, T. J.; Austin, R. N. Environ. Sci. Technol. 2001, 35, 326A. 2. Quach, D. T.; Ciszkowski, N. A.; Finlayson-Pitts, B. J. J. Chem. Educ. 1998, 75, 1595. 3. Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056. 4. Harrick, N. J. Internal Reflection Spectroscopy; Wiley Interscience: New York, 1967. 5. Leary, J. J. J. Chem. Educ. 1983, 60, 675. 6. Sanford, C. L.; Mantooth, B. A.; Jones, B. T. J. Chem. Educ. 2001, 78, 1221. 7. Scamehorn, R. J. Chem. Educ. 1995, 72, 566. 8. Raymond, K. W.; Corkill, J. A. J. Chem. Educ. 1994, 71, A204.
9. Clark, R. J. H. J. Chem. Educ. 1964, 41, 488. 10. Griffiths, P. R.; Fuller, M. P. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heydon and Sons: London, 1982; Vol. 9, Chapter 2, pp 63–129. 11. Kubelka, P; Munk, F. Z. Tech. Phys. 1931, 12, 593. 12. Kubelka, P; J. Opt. Soc. Am. 1948, 38, 448, 1067. 13. Vogt, R.; Finlayson-Pitts, B. J. J. Phys. Chem. 1994, 98, 3747. 14. Pike Technologies FTIR Accessories Pages. http:// www.piketech.com (accessed Feb 2003). 15. Pavia, D. L.; Lampman, G. M.; Kriz, G. S., Jr. Introduction to Spectroscopy: A Guide for Students of Organic Chemistry; Harcourt Brace Janovich: Orlando, FL, 1979. 16. Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments and Applications; Academic Press: San Diego, 2000.
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