Anal. Chem. 1995, 67, 1881-1886
FT-IR in the Quantitative Analysis of Gaseous Hydrocarbon Mixtures Arja Hakuli/'t.t Arla Kytttkivi,* Eeva-Liisa Lakomaa,* and Out! Krause* Microchemistry Ltd., P.O. Box 45, 02151 Espoo, Finland, and Helsinki University of Technology, Department of Chemical Engineering, Kemistintie 1, 02150 Espoo, Finland
An FT-IR spectrometer provided with software for multicomponent analysis of IR spectra was used for simultaneous determination of up to 12 hydrocarbons in a gaseous mixture. The hydrocarbons, ranging from methane to n-butane, were selected to simulate reaction product mixtures obtained in the dehydrogenation of n-butane. The concentrations of the hydrocarbons in the sample were determined by fitting to the sample spectrum a set of pure calibration spectra of the gases present in the sample. In principle, the entire wavenumber range of the low-resolution IR spectrum could be utilized in the fit The reliability of the results was checked from the residual spectrum, obtained by subtracting the fitted from the measured spectrum. The residual spectrum will be pure noise when the set of calibration spectra chosen for the analysis fits exactly with the sample spectrum. Precision of the results, expressed as average relative standard deviation, was ±4%. Concentrations of most of the hydrocarbons were determined with an error less than ±10%. The concentrations of isobutane, frans-2-butene, propane, and ethane deviated by more than 10% from the reference concentrations. The sum concentration of ethane and propane, however, deviated less than ±1% from the reference. Difficulties in determining ethane and propane accurately were due to the broad nature and small number of peaks in their IR spectra. IR spectrometry with multicomponent analysis of spectra provides a rapid and powerful method with potential for the study of catalytic reactions at a time resolution of seconds. The rapidity of measurement in FT-IR spectroscopy makes it invaluable analytical tool in many areas. Typical applications of FT-IR gas analysis include measurement of combustion products, pollution control, and on-line process control.1·2 FT-IR spectroscopy has also been shown to be suitable for reaction kinetic studies in which a small number of components are involved.3 A quantitative IR analysis of one or a few components in a mixture is a relatively simple task if an isolated absorption peak can be found for each component. Difficulties arise if the characteristic peaks of the components overlap, and in these cases methods such as gas chromatography, in which a large number of components can be chemically separated, have been preferred instead. More recently, the problems of spectral overlap in FTan
+
Microchemistry Ltd. * Helsinki University of Technology. (1) Doyle, W. M. Process Control Quality 1992, 2, 11—41. (2) Li-Shi, Y.; Levine, S. P. Anal Chem, 1989, 61, 677-683. (3) Gopalakrishnan, R.; Seehra, M. S. Energy Fuels 1990, 4, 226—230. 0003-2700/95/0367-1881 $9.00/0
©
1995 American Chemical Society
IR have been attacked by applying chemometric methods such as classical (CLS), partial (PLS), and inverse (ILS) least-squares methods to digitized spectral data.4·5 A number of reviews concerning the multicomponent analysis of IR spectra can be found in the literature.6·7 The aim of this work was to determine whether FT-IR spectroscopy can be applied to the study of catalytic reactions involving light hydrocarbons. For this purpose, up to 12 light hydrocarbons (C1-C4) were determined in a reference gas mixture by means of CLS, which is one multicomponent analysis method for IR spectra. The hydrocarbons in the mixture were selected to simulate reaction product mixtures obtained in the dehydrogenation of «-butane, where the similar spectral features of the light hydrocarbons make the analysis a demanding task. In the multicomponent analysis employed in this study, the FT-IR spectrometer is calibrated with pure gases diluted in nitrogen. The concentrations of the gases in the sample are then determined, according to Beer’s law, by calculating a linear combination of a set of calibration spectra that best explains the sample spectrum.8 In using the pure calibration spectra, it is assumed that no interaction occurs between the components in the sample. In principle, the entire IR spectrum from 4000 to 1000 cm™1 can be used in the fit. The goodness of the fit is checked from the residual spectrum, obtained by subtracting the fitted from the measured spectrum. Ideally, the residual spectrum is pure noise. Other residual absorbance indicates that the set of calibration spectra is not sufficient to explain the measured spectrum. Reasons for other residual absorbance may be the use of an incomplete set of calibration spectra in the fit or other errors made in developing the analysis method. In this multicomponent method, specific software was designed for fast data processing. The method has previously been applied in the analysis of stack gases with highly repeatable and accurate results.9
EXPERIMENTAL SECTION FT-IR Apparatus. The FT-IR spectrometer provided with multicomponent analysis software was manufactured by Temet Instruments Oy. The spectrometer was controlled by a 286 (4) Haaland D. M.; Thomas, E. V. Anal. Chem. 1988, 60, 1193-1202. (5) Fuller, . P.; Ritter, G. L; Draper, C. S. Appl. Spectrosc. 1988, 42, 228— 236. (6) Gillette P. C.; Lando, J. B.; Koenig, J. L. In Fourier Transform Infrared Spectroscopy, Vol. 4; Ferraro, J. R., Basile, L. J., Eds.; Academic Press, Inc.: New York, 1985; pp 1-59. (7) Haaland, D. M. Practical Fourier Transform Infrared Spectroscopy, Academic Press, Inc.: San Diego, CA, 1990; Chapter 8, pp 395-468.
(8) Saarinen, P.; Kauppinen, J. Appl. Spectrosc. 1991, 45, 953—963. (9) Jaakkola, P.; Vahlman, T.; Saarinen, P.; Kauppinen, J. 9th International Conference on Fourier Transform Spectroscopy, 23-27 August, 1993, Calgary, AB, Canada.
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Figure 1. Spectra of
(a) -butane, (b) 1-butene, (c) c/s-2-butene, (d) frans-2-butene, (e) isobutane, (f) isobutene, and (g) 1,3-butadiene
measured at a concentration level of 5%.
microcomputer, which was also used for processing the analytical data. All the spectra were recorded at a resolution of 8 cm-1 and a scan rate of 12 scans/s. The Peltier cooled detector (modified NCT) was operated in the wavenumber range from 4000 to 1000 cm-1. The volume of the continuous flow cuvette was 9 cm3, with a sample path length of 4 cm. The windows were made of BaF2. Calibration and sample gases were introduced into the cuvette with Tylan mass flow controllers. The temperature of the cuvette and the pipe lines was set to 175 °C, while the pressure varied with ambient pressure. The spectra reported in this article were measured at pressures from 100 to 101 kPa. Calibration, The FT-IR spectrometer was calibrated with 12
hydrocarbons («-butane, 1-butene, cis-2-butene, fnm-2-butene, isobutane, isobutene, 1,3-butadiene, propane, propene, ethane, ethene, and methane) typically produced in the dehydrogenation of «-butane. Also, carbon dioxide and water vapor, which could be present in samples as impurities, were included in the calibration. Calibration gases (Aldrich and Merck) were of high purity (>99%), except cw-2-butene, which contained about 3% trans2-butene as determined by GC. Calibration and background spectra were recorded with 300 and 500 scans, respectively. An example of each calibration spectrum is presented in Figures 1 and 2. Many of the calibrated hydrocarbons have closely similar chemical structures, and spectral overlap of the peaks was anticipated. The calibrations were performed at a pressure of 100 kPa using a flow rate of 300 cm3/min (STP). The calibration spectra were recorded at concentrations ranging from 0% to 5% with nitrogen (99.995%) as a diluent. The maximum error in preparing the calibration mixtures was calculated to be ±7% on the basis of the manufacturer’s statement that the mass flow controllers were accurate to within ±1% at the full scale operation flow. linearity between concentration and absorbance was checked at the two wavenumbers with largest absorbances in the spectrum. Regression coefficients and standard errors of 0.9999 ± 0.0001 and 0.0021 ± 0.0030, respectively, were observed for all hydrocarbons except methane. For methane, the 1882
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Figure 2. Spectra of (a) propane, (b) propene, (c) ethane, (d) ethene, (e) methane, (f) water vapor, and (g) carbon dioxide measured at a concentration level of 5%. Table 1. Composition of the Reference Gas Mixture analyzed by FT-IR and GC concentration, vol
%
FT-IR0
method A6
GC
method
RSD,
compound
actual
measdr
1-butene tis-2-butene fro«s-2-butene isobutane isobutene 1,3-butadiene propane propene ethane ethene methane
5.24 4.90 5.25 2.02 2.04 5.01 2.03 2.04 2.04 2.04 5.07
5.39 ± 0.16 4.98 ± 0.26 4.67 ± 0.15 1.43 ± 0.05 2.09 ± 0.13 5.23 ± 0.04 1.87 ± 0.27 1.89 ± 0.19 2.24 ± 0.33 1.85 ± 0.22 4.83 ± 0.04
%
B6
RSD, rneasd0
+2.9 5.22 ± +1.6 4.80 ± -11 4.66 ± -29 1.49 ± +2.5 2.11 ± +4.4 5.20 ± -7.9 1.65 ± -7.4 2.06 ± +9.8 2.44 ± -9.3 2.02 ± -4.7 4.89 ±
%
0.13 -0.38 0.10 -2.0 0.11 -11 0.02 -26 0.05 +3.4 0.05 +3.8 -19 0.13 0.06 +0.98 0.17 +20 0.07 -0.98 0.04 -3.6
measd 5.34 4.85 5.34 1.99 2.03 5.41 1.99 2.07 1.92 2.14 4.51
0 Results were averaged from 70 spectra measured at pressures from 100 to 101 kPa.6 The wavenumber ranges included in methods A and
B are presented in Table 1.c Expressed as median ± standard
deviation.
regression coefficient and standard error were 0.9980 and 0.0067, respectively. Reference Gas of 11 Hydrocarbons. The reference gas mixture, prepared by Praxair NV, consisted of 11 hydrocarbons in nitrogen (see Table 1). Altogether, 70 spectra of the reference gas were measured during 7 days at pressures from 100 to 101 kPa with 100 scans. In addition, 30 spectra were recorded from the reference gas mixture with 10 and 2 scans at 100 kPa. The reference gas mixture was also analyzed with a gas chromatograph equipped with a Chrompack Plot T column. Modified Reference Gas. Two modified mixtures were prepared from the reference gas mixture: the reference gas mixture diluted 1:2 with nitrogen and the reference gas to which «-butane was added. Thirty and ten spectra, respectively, were recorded with 100 scans at time intervals of 14 s at 101 kPa. The
Table 2. Wavenumber Sets Included in the Fits in Methods A-D method
sample
A
A
B
C
D
reference gas
diluted reference
reference gas and diluted reference gas 2100-1000
diluted reference
reference gas with «-butane
gas
wavenumbers, cm™1
3200-3050, 2850-2700, 2100-1000
maximum error in preparing the mixtures
3200-2700, 2100-1000
was
gas
3200-2850
4000-3050, 2850-1000
calculated to be
±7%.
Mixtures of Three Hydrocarbons. Four mixtures, each containing three hydrocarbons, were prepared by mixing calibration gases. Ten spectra of these mixtures were recorded with 100 scans at time intervals of 14 s at 100 kPa. Interpretation of Sample Spectra. The calibration points of the hydrocarbons used in the fits were chosen to correspond to the concentrations of the hydrocarbons in the sample. Single base line fitting was performed over the whole spectrum by including a constant and a cosine function in every fit as compounds. A spectrum of water vapor was also included, since water vapor absorbs, in part, at the same wavenumbers as the hydrocarbons. The four different wavenumber sets, i.e., methods A-D, that were applied to obtain quantitative results are presented in Table 2. In all four methods, wavenumbers at which the absorbance exceeded 0.6 were excluded to avoid nonlinearities between absorbance and concentration.10 In method A, wavenumbers at which hydrocarbons do not absorb, i.e., 4000-3200 and 2700-2100 cm™1, were excluded as well, to minimize the effect of noise on the data fitting. In methods B and C, the number of frequencies utilized in the fit was reduced to cover the wavenumber ranges 2100-1000 and 3200-2850 cm™1, respectively. Method D corresponded to method A but the wavenumber ranges of 4000-3200 and 2700-2100 cm™1 were included. This allowed carbon dioxide, which has a peak at 2400-2250 cm™1, to be determined in addition to the hydrocarbons.
RESULTS AND DISCUSSION Reference Gas of 11 Hydrocarbons. Methods A and B determination of the 11 hydrocarbons in the reference gas. The spectrum of the reference gas and the residual spectra are presented in Figure 3. The residual spectra were not pure noise, as is clearly seen in Figure 3b. The maximum absorbances in the residual were smaller than 0.006, however, which is only about 1% of the maximum absorbance in the sample spectra. The quantitative impact of the residual absorbance was estimated to be 0.4 vol % by assuming an absorptivity equal to that of the reference gas mixture. Accordingly, the effect of the residual on the quantitative results was not considered to be significant. Athough the most likely reason for the residual absorbances is impurities, since impurities in the reference gas mixture were independent of the impurities in the calibration gases, non-Beer’s law behavior and spectral shifts cannot be ruled out. The deviations in the concentrations resulting from the use of different methods, i.e., wavenumber ranges, were minor, as can were applied to the
(10) Griffits, P. R; deHaseth, J. A. Fourier Transform Infrared Spectrometry, John Wiley and Sons: New York, 1986.
Figure 3.
(a) Spectrum of the reference gas (Amax *= 2). (b) Residual spectrum after applying method A. (c) Residual spectrum after applying method B.
be seen in Table 1. Precision of the results, expressed as relative standard deviation (RSD), varied from ±0.75% to ±14.8% de-
pending on the hydrocarbon, and most of the hydrocarbons were determined with an error less than ±10%. The largest relative standard deviations (>7%) were in the concentrations of propane, propene, ethane, and ethene. Propane and ethane could not be determined independently of each other—a too high concentration of ethane was typically accompanied by a too small concentration of propane, and vice versa—but the sum concentration was accurate within ±1%. The difficulties in determining propane and ethane accurately were apparently due to the broad nature and small number of the peaks in their IR spectra, all located at about the same wavelengths. In addition, the absorptivity of ethane was rather low. Deviations larger than ±10% in determining isobutane and trans-2-butene were systematic, since the precisions of the concentrations of these two hydrocarbons, expressed as RSD, were ±2.0% and ±3.6%, respectively. If there had been more isobutane in the sample than in the fitted spectrum, there should have been some structure at 1200 cm™1 in the residual spectrum since only isobutane has a significant peak at 1200 cm™1. Characteristic structure was not, however, observed at that wavenumber. The calibration spectrum of isobutane was correct since isobutane was determined with good accuracy in the mixtures of three hydrocarbons. Thus, the reason for the Analytical Chemistry, Vol. 67, No.
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Table 3. Results Derived from 30 Spectra of the Diluted Reference Gas and Analyzed with Methods A-C concentration, vol method A
a
compound
actual
measdú
1-butene «s-2-butene fra«s-2-butene isobutane isobutene 1,3-butadiene propane propene ethane ethene methane
1.27 1.19 1.27 0.49
1.35 ± 0.02 1.15 ± 0.01 1.23 ± 0.06 0.37 ± 0.00 0.48 ± 0.02 1.25 ± 0.01 0.43 ± 0.02 0.45 ± 0.03 0.39 ± 0.07 0.49 ± 0.05 1.25 ± 0.01
0.49 1.21
0.49 0.49 0.49 0.49 1.23
%
method B RSD,
%
+6.3
-3.4 -3.1 -24 -2.0 +3.3
-12 -8.2 -20.4 0
+ 1.6
measd0 1.36 1.15 1.24 0.37 0.47 1.25 0.42 0.43 0.39 0.50 1.25
± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.05 0.00 0.02 0.01 0.02 0.03 0.07 0.05 0.01
method C RSD,
%
+7.1
measd0 1.24 1.09 1.24 0.39 0.53 1.49 0.43 0.28 0.42 0.22 1.29
-3.4 -2.4 -24 -4.1 +3.3
-14 -12 -20.4 +2.0 +1.6
± ± ± ± ± ± ± ± ± ± ±
RSD,
0.04 0.02 0.04 0.01 0.02 0.07 0.03 0.04 0.05 0.07 0.01
%
-2.4 -8.4 -2.4 -20 +8.2 +23
-12 -43 -14 -55 +4.9
Expressed as median ± standard deviation.
deviation in the concentration of isobutane was not found, but the possibility of incorrect calibration spectra was excluded. The systematic deviation in the concentration of trans-2-butene was probably due to the impurity in the calibration gas. The calibration gas of fra«s-2-butene contained about 3% m-2-butene, and a correspondingly lower concentration for fra«s-2-butene could be expected.
In
an
attempt to clarify the discrepancy between the actual
and measured concentrations, the reference gas mixture was also analyzed by GC (see Table 1). The GC results were highly consistent with those reported by the gas manufacturer, except
for 1,3-butadiene and methane. The fact that methane and 1,3butadiene were the first and last components eluted from the column might have caused an error in their determination by GC. Effect of the Number of Frequencies. The number of frequencies employed in the fit was not much smaller in method B than in method A when the reference gas mixture was analyzed (see Table 2). To better illustrate the effect of the number of frequencies on the results, the reference gas mixture was diluted with nitrogen. After dilution, the maximum absorbance in the gas spectra was below 0.6, and all the wavenumbers around 3000 cm™1 could be included in the fit when method A was applied. Results were similar with methods A and B, however (Table 3), which would imply that the addition of the frequencies at 32002850 cm™1 to the analysis simultaneously brought some additional sources of error to the analysis to offset the positive influence of
adding spectral information.11™14 Analysis of the diluted reference gas mixture with the spectral range 3200-2850 cm™1 (method C) showed that the set of calibration spectra did not explain the sample spectra as well at 3200-2850 cm™1 (method C) as at 2100-1000 cm™1 (method B). With method C, an average relative error of ±20% was obtained, which is more than twice the error obtained with method B. In the residual spectra, there were also higher absorbances at 32002850 cm™1 than at 2100-1000 cm™1, indicating that the set of calibration spectra fitted better with the sample spectra at 21001000 cm™1 than at 3200-2850 cm™1 (see Figure 3b,c). The (11) Dublin, T.; Thone, H. J. Fresemos Z. Anal. Chem. 1989, 279-285. (12) Maris, M. A; Brown, C. W.; Lavery, D. S. Anal. Chem. 1983, 55, 16941703. (13) Haaland, D. M.; Easterling, R. G. Appl. Spectrosc. 1980, 34, 539-548. (14) Haaland, D. M.; Easterling, R. G.; Vopicka, D. A. Appl. Spectrosc.
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spectral range.2 Effect of Scan Number. The spectra discussed above were averaged from 100 interferograms. Sampling with 100 scans took about 8 s. In applications, shorter sampling intervals might be useful. The effect of the scan number on the results was studied with 30 spectra recorded from the reference gas mixture with 100, 10, and 2 scans and analyzed with method A The reduction in the number of scans from 100 to 10 increased the average RSD only slightly, from ±5.7% to ±6.1%, but when the number of scans was further reduced from 10 to 2, the average RSD was doubled. With two scans, RSDs as high as ±37% were obtained for propane and ethane. In general, the reduction in the number of scans did not affect the accuracy of the results. The measured concentration of propane decreased when the number of scans was reduced, however. Samples of Unknown Composition. The effect of an incomplete set of calibration spectra on the quantification was studied by analyzing the reference gas plus «-butane with a set of calibration spectra from which one calibration spectrum at a time was omitted. Thus, the set of calibration spectra used in the fits consisted of the spectra of 11 hydrocarbons, water, and carbon dioxide. Some examples of such fits are given in Table 4. In the first column of Table 4, the results obtained with a complete set of calibration spectra are shown for comparison. The largest percentage error was in the concentration of isobutane, while most of the components were measured with an error less than ±10%.
1985, 39,
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residual spectrum from the diluted reference gas mixture is not shown since the same phenomenon can be seen in the residual spectra obtained from the undiluted reference gas mixture. In many investigations reported in the literature, better results have been achieved with a maximum number of frequencies included in the model.11™14 Haaland et al.15 have shown that, even in the presence of deviations from Beer’s law, high accuracy of fit can be obtained when using the whole spectral range if those regions where Beer’s law is not followed are given lower weights in the fit. In our study, the structure at 3200-2850 cm™1 might have been due to the presence of impurities in the sample or calibration gases preferentially absorbing around 3000cm™1. It is also possible that Beer’s law was not obeyed equally well at 32002850 and 2100-1000 cm™1, or that the base line fitting was not as appropriate over the whole spectral range as over a selected
(15) Haaland, D. M.; Easterling,
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R
G. Appl. Spectrosc.
1982, 36, 665-672.
Table 4. Errors in Determining the Reference Gas Containing n-Butane When One of the 12 Hydrocarbons Was Not Included in the Fit, Compared to the Case in Which All the Hydrocarbons Were Included (Method D) RSD,
fit
compound «-butane 1-butene «s-2-butene íraws-2-butene isobutane isobutene 1,3-butadiene propane propene ethane ethene methane water" carbon dioxide"
+14 +5.8 -0.44 -9.8 -26 +12
fit
1
+26 b
-6.3 +12
-38 +59
+0.41
0
-8.4 -16 +1.6
+170
+3.1
-2.3 (0.03) (0.00)
-81 -45 +14 -1.4 (-0.71) (0.00)
%
fit
2
+40 -5.4 +15 -22 b
+13 +3.1 +78
-18 -60 -2.6
fit 4
3
+21
-41
-2.6
+15
+9.2
-40 -12 +28
-5.7 -34 +7.3 +1.0 +1.4 -1.6 b
+1.5
-3.4 -1.2
(0.34) (0.01)
(0.09) (0.00)
“ Values in parentheses are expressed spectrum omitted from the fit.
as
+120 +4.3 +60 +96 +140 +15 b
(-2.61) (0.01)
vol %.6 Calibration
Figure 4. Residual spectrum obtained when the spectrum
With an incomplete set of calibration spectra used in the fit, methane and 1,3-butadiene, which have distinct spectral features relative to other hydrocarbons in the study, could be determined with good accuracy. Semiquantitative results with an average error less than ±30% were obtained for other hydrocarbons present in the sample at the 5 vol % level, as well as for ethene and isobutane. Concentrations of ethane, propane, propene, and isobutene appeared to be the most unreliable when one calibration spectrum was omitted from the fit. Predictably, the use of an incomplete set of spectra did not affect the concentration of carbon dioxide, whereas the concentration of water varied between -2.6 and 0.34 vol % in the different fits. The spectral residuals corresponding to the results in Table 4 are presented in Figure 4. Methane is a good example of a component that can easily be identified from the residual spectra, contrary to ethane, which is more difficult to identify. Besides spectra missing from the set of calibration spectra, problems may also arise if the set of calibration spectra includes components not present in the sample. Ideally, the concentrations of such components should be fitted to zero. We analyzed mixtures of three hydrocarbons with a set of calibration spectra of 12 hydrocarbons. The sample gas mixtures were prepared from calibration gases to ensure that any components erroneously concluded to be present in the sample were not impurities. In general, only small positive or negative concentrations with high RSDs were found for hydrocarbons not present in the sample, as can be seen from Table 5, in which two examples of the results are presented. The high RSDs in themselves suggest that these hydrocarbons were not actually present in the sample. Applicability of FT-IR to Catalytic Reaction Studies. The results described above confirm that FT-IR spectroscopy is a suitable technique for studying the catalytic dehydrogenation of «-butane. The rapidity of the measurement is the main benefit: in the time required for one GC run, which is about 15 min, the composition of the sample gas mixture can be determined by FTIR hundreds of times. The optimum scanning time is determined by the time constant of the phenomenon monitored and the signal
of the reference gas with n-butane was analyzed with a set of calibration spectra that did not include the spectrum of (a) 1-butene, (b) isobutane, (c) ethane, or (d) methane.
Table 5. Results for Mixtures of Three Hydrocarbons When 12 Hydrocarbons Were Included in the Fit
(Method D) concentration, vol
mixture I compound
actual
«-butane 1-butene os-2-butene fra«s-2-butene isobutane isobutene 1,3-butadiene propane propene ethane ethene methane water carbon dioxide
3.30
"
Expressed
as
3.30 3.40
measd" 3.43 0.00 3.54 3.55
-0.03 -0.05 -0.02 0.13
-0.12 -0.52 0.02 0.01
-0.02 0.00
%
mixture II
± ± ± ± ± ± ± ± ± ± ± ± ± ±
actual
measd"
2.80
2.81 ± 0.03 0.06 ± 0.02 0.14 ± 0.02 -0.12 ± 0.01 2.28 ± 0.02 3.05 ± 0.02 -0.11 ± 0.00 -0.08 ± 0.04 -0.16 ± 0.01 -0.21 • 0.02 0.02 ± 0.01 0.02 ± 0.01 -0.08 ± 0.05 0.01 0.00
0.06 0.02 0.06 0.07 0.01 0.01 0.00 0.03 0.03 0.04 0.02 0.01 0.02 0.00
2.30 2.80
median ± standard deviation.
to noise ratio needed for precise results. The use of a minimum of 10 scans corresponding to 0.8 s measurement time would seem reasonable. In this work, the calibration and sample spectra were measured at constant air pressure. In real applications, the pressure of the IR cuvette would need to be controlled to eliminate the variations in ambient pressure. Accurate results also require that all the components in the product mixtures interfering in the spectral range of interest are known and included in the calibration. The residual spectra must thus be inspected with great care for the presence of unknown components. In the dehydrogenation of «-butane, for example, some heavy hydrocarbons might be released during the reaction. The spectral features and concentration of the unknown component will determine whether it can easily be identified in the residual spectrum. The use of other
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analytical methods parallel with IR might be helpful in identifying the components in complex mixtures.
CONCLUSIONS The multicomponent analysis of spectra employed in this study provides a practical method for determining up to 12 hydrocarbons in gaseous mixtures despite the highly overlapping spectral features of the hydrocarbons. The precision of the method, expressed as RSD, was ±4%, and most of the hydrocarbons could be determined with an error less than ±10%. The main advantage of FT-IR is fast measurement time, which makes the method
highly suitable for catalysis research and many other fields.
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ACKNOWLEDGMENT The work was partly supported by the Technology Development Centre (Tekes) and the Ministry of Education. The authors thank Temet Instruments Oy and Tech. Lie. Petri Jaakkola for helping in the use of FT-IR equipment and for valuable discussions. We also thank the technical staff at Microchemistry Ltd.
for assistance. Received for review October 3, 1994. Accepted March 14, 1995.®
AC940971V ®
Abstract published in Advance ACS Abstracts, April 15, 1995.
