Determination of Carbon Monoxide in Automobile ~xhaustby FTIR Spectroscopy An Instrumental Analysis Laboratory Experiment Maty Beth SeashoHz, Laura E. Pence, and Owen A. Moe, Jr. Lebanon Valley College, Annville, PA 17003 The advent of Fourier transform infrared (FTIR) spectroscopv has sparked a renewed interest in infrared applications, ehpecialiy in ihe area of quantitative analysis (jj. The ad\,antages of F T I R instruments over conventional dispersive instruments are well documented (2, 3): (1) high Eght throughput to the detector (Jacquinot's advantage), (2) short spectral acquisition times, allowing signal averaging (Fellgett's advantage), and (3) high precision and accuracy in wavenumber determination (Connes's advantage). The computer data station that is a required component of all FTIR instruments provides additional data processing advantages that are not availahle in instruments lacking computer interfaces. These advantages produce large gains in signal-to-noise ratios, in sensitivity, and in accuracy for the FTIR instruments. For single-component analyses by FTIR spectroscopy, Beer's law plots have been shown in some cases to remain linear beyond 3 absorbance units (4). The use of FTIR spectroscopy for the determination of trace gases has become important in the analysis of automobile emissions and other air pollutants, in upper atmosphere physics, and in astronomical studies of planetary atmospheres (5,6).The quantitative accuracy and linearity of the Beer-Lambert relationship in FTIR spectroscopy has been demonstrated for a number of analytically important gases fn,. Laboratory experiments that employ infrared spectroscopy for quantitative analysis are difficult to find in textbooks 2nd in the rhpmiral ednntinn literatnr~ Tn the nnst 15 \
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Journal of Chemical Education
years, for example, only five experiments dealing with the use of infrared spectroscopy for quantitative analysis have been reported in this Journal (8-121, and two of these (11, 12) are more suitable for physical chemistry laboratory than for analytical chemistry or instrumental analysis. This report describes an experiment for the quantitative determination of carbon monoxide in automobile exhaust bv FTIR spectroscopy. The experiment applies infrared spec-. trosco~v s . . to the analvsis of a oollutant. it e m.~ l o" vtechniaues for handling gaseous samples, it demonstrates rhe quantitative capabilities of FTIR spertroscow, and it direrts attention t o a currently important area ofkkvironmenta~concern. Although the experiment was designed for a FTIR spectrometer, it can also be carried out using a dispersive instrument.
Carbon Monoxide Standard Curve Carbon monoxide, CP grade (99.5%), was purchased from MG Scientific Gases in a lecture bottle for use as a standard. A lecture tank regulator was used to control the gas outlet pressure. A vacuum manifold system, as shown in Figure 1, was used to fill a 10-cm infrared gas cell (NaCI windows) with carbon monoxide at pressures ranging from 5 to 600 torr. Because of the known toxicity of carbon monoxide (13), the vacuum manifold system, including the pump, should be positioned and used inside of a forced-ventilation fume hnod.
I
V a c u u m Manifold
Tubing
Vacuum M a n i f o l d or
Hose C l a m p
Exhaust Pipe
&vacuum
Stopcock Stopper
Manometer
Figwe 1. Vacuum system used to fill infrared gas cell w l h carbon monoxme Standards and with exhaust samples.
Before the infrared mr " - cell ~ was filled.. the vacuum manifold svsEm, including the gas cell and the line to thecarbon monoxide~kik, was evacuated. The stopcock to the wcuum pump was then closed. and carbon monoxide was bled into the system. After the desired pressure of carbon monoxide was achieved, the manometer was read, and the cellstopcock was closed. The infraredspectrum of the filled gas cell was measured from 4600 to 400 cm-' at a resolution of 4 cm-1 on a Nieolet 5DX FTIR Spectrometer with computer data station. Each samole . soectrum .~ was orodueed hv averaeine .. .. 50 sinele " scans, and then was corrwted for background absorbanre due to the gas cell hy subtracting the spectrum of the evacuated rpll from the sample spectrum. To dlspwe of the cnrhon monoxide sample, the cell was evacuated using thevacuum manifold system in aventilated hood. ~~~~~
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Collection of Automobile Emission Samples A thick-walled 4-L glass bottle with rubber stopper, vacuum stopcock, and inlet pressure tubing (see Fig. 2) was used as the exhaust collection aonaratus. Usine the vacuum manifold and manometer, it was possibie-to demonst& that this relatively simple collect immediately upon collection. and (41 a FTlR soecrrumeter was used in olace of (he nondis. persive infrared gas analyzer. To collect the exhaust samole. . .the end of the uressure tubine inlet of the evacuated collection buttb was placed approximately 20 cm into the exhaust pipe of the car. The stopcock of the hottle a,as rlowly opened co drnw in the exhaust ample, and was closed again after the bottle had filled (-30 s). The bottle was placed on the evacuated vacuum manifold, and the rubber inlet tubing was evacuated. The ~wpcockto the vacuum pump was clored, and the stopcock to the hottle was opened, drawing the exhaust into the system to fill the cell. The final pressure of thc exhaust in the system was
Figure 2 Exhaust CollsCtlon apparatus The glass bon s was wrapped n elecnocal tape (not shown n tqure I to prolect agamsr mplasoon
Figure 3. Infrared abswption spectra (2275 cm-' to 1950 cm-'1 monoxlde al slx dllferent pressures: 94. 71.46.30, 16, and 9torr.
of
carbon
measured. the cell stoocock was closed. and the cell was removed from the &uum mantie for FTIR soec&al analvsis. . Disoosal of the exhaust sample was accomplish~dhy evacuating the IR cell and [he exhaust collection botrle using the vacuum manifuld sptem in a ventilated hood. ~
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Results Carbon Monoxide Standard Curve Reprcscntative infrared spectra I2275 t o 1950 ern-') for six different pressures of t h e carbon monoxide standard are shown in Figure 3. T h e spectrashow t h e R (>2143 cm-1) a n d P R 2 1 4 3 cm-') branches characteristic of t h e vibrational a n d rotational tiansitions i n carbon monoxide (15).T h e cleft a t 2143 cm-' corresponds to a transition a t t h e fundamental vibrationalfrequency of carbon monoxide, v = 0, J = 0 u = 1, J = 0, where t h e terms v a n d J r e f e r t o t h e vibrational a n d rotational quantum numbers, respectively. This transition is forbidden under t h e infrared selection rules for diatomic
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September 1988
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Flgure 4. A plot of A2,70. Uw abswbance of c a m n monoxide at 2170 cm-'. PCO.the pressureofthe carbon monoxide standard. The line drawn through the data was determined by least-squares analysis. VS.
Flgure 5. An inhared absorption s p e m m (2275 cm-' to 1950 cm-'1 of a sample of aulomoblle exhaust taken trom a 1983Mdan Lsmg a cold mn.The total pm66ure of exhaust In the cell was 625 torr.
molecules: Au = f 1 and A J = i l (15). The rotational fine structure can be discerned in the R and P branches, but is obscured by the 4-cm-1 resolution of the FTIR instrument used. A plot of Ax~o,the absorbance a t 2170 cm-I, which is the absorption maximum of the R branch of the carbon monoxide spectrum, vs. Pco, the pressure of carbon monoxide, is shown in Figure 4. The linearity of this standard curve is excellent in the pressure range shown, giving a linear correlation coefficient of 0.999. Analysis of these data by linear regression gives a slope of 9.9 (f0.1) X 10-4 torr-1 and an The 95%confidence interval intercept of -1.1 ( f 6.2) X for the Y-intercept absorbance is calculated to be from +0.0012'to -0.00i5. Thus, the experimental intercept does not differ significantly from the expected y-intercept absorbance value of 0 for Pco = 0. The absorptivity for carhon monoxide, aco, can be calculated from the slope of the standard curve using Beer's law applied t o gases (16): In eq 1,Pco can be either the pressure of pure carbon monoxide in torr, or the partial pressure of carbon monoxide in a mixture, and b is the path length of the cell in centimeters. torr-I em-' The value of aco was calculated to be 9.9 X from the slope of the standard curve shown in Figure 4. Linearity in the plot of A ~ ~ ~ oPCO v swas . found to continue to -300 torr, but an upward curvature was seen from 300-600 torr. Automobile ~rnissionAnalvsis A sample FTIR spectrum (2275 em-' to 1950 cm-') of an automobile exhaust sample is shown in Figure 5. Interferences are observed in the region >2225 cm-I due to spectral overlap with the tail of a strong C02 band centered a t 2330 cm-1, -and in the region
