Anal. Chem. 1994,66, 536-542
High-Resolution NMR Process Analyzer for Oxygenates in Gasoline Timothy W. Skioss, Ae Ja Kim,+ and James F. Haw' Department of Chemistty, Texas A&M University, College Station, Texas 77843 We report a high-resolution 42-MHz 'H FT-NMR instrument that is suitable for use as a process analyzer and demonstrate its use in the determination of methyl tert-butylether (MTBE) in a flowing stream of gasoline. This spectrometer is based on a 55-kg permanent magnet with essentially no fringe field. A spectral resolution of 3 Hz was typically obtained for spinning samples, and this performance was only slightly degraded with flowing samples. We report a procedure for magnet drift compensation using a software procedure rather than a fieldfrequency lock channel. This procedure allowed signal averaging without loss of resolution. Regulatory changes to be implemented in the near future have created a need for the development of methods for the determination of MTBE and other oxygenates in reformulated gasolines. Existing methods employing gas chromatography are not fast enough for process control of a gasoline blender and suffer from other Limitations. This study demonstrates that process analysis NMR is wellsuited to the determination of MTBE in a simulated gasoline blender. The detection limit of 0.5 vol % MTBE was obtained with a measurement time of 1 min. The absolute standard deviation of independent determinations was 0.17% when the MTBE concentration was lo%,a nominal value. Preliminary results also suggest that the method may be applicable to gasolines containing mixtures of oxygenate additives as well as the measurement of aromatic and olefinic hydrogens. Process analysis and control is an important and rapidly growing area of analytical chemistry. Although some degree of control over chemical plant and refining operations can be achieved through careful monitoring of pressure, temperature, flow rate, and feedstock composition, process optimization frequently requires feedback control based on chemical analysis. Consideration of elementary control concepts implies that the time difference between the analysis and the adjustment of the process should be as brief as possible; therefore, there has been much effort to develop on-line analytical instruments based on gas or liquid chromatography, near-infrared or Fourier transform infrared spectrometry, mass spectrometry, and flow injection analysis-to name a Notably absent from the list of mature process analysis technologies is high-resolution nuclear magnetic resonance. Indeed, there was no mention of process NMR in the first comprehensive application review in the area of process
'
Present address: Department of Chemistry, Ewha Womens University, Ewha, Korea. (1) Riebe, M. T.; Eustace, D. J. Anal. Chem. 1990, 62, 65A-71A. (2) Callis, J. B.; Illman, D. L.; Kowalski, B. R. Anal. Chem. 1987, 59, 624A637A. (3) Beebe, K. R.; Blaser, W. W.; Bredeweg, R. A,; Chauvel, J. P., Jr.; Harner, R. S.;LaPack, M.; Leugcrs, A.; Martin, D. P.; Wright, L. G.; Yalvac, E. D. Anal. Chem 1993.65, 199R-216R.
536
AnalyticalChemistry, Vol. 66,No. 4, February 15, 1994
analytical chemistry, which was recently published by the j ~ u r n a l .NMR ~ spectrometry has many intrinsic characteristics that would make it well-suited for process analysis. The analytical signal is directly proportional to the number of spins (e.g., hydrogen atoms) in the receiver coil region, and this response is linear over a range of analyte concentrations from 100%to the detection limit, which can be ca. 10 ppt or lower for typical analytes, depending on field strength, line width, structure, and number of scans. Even at very low magnetic field strengths, well-resolved spectral features corresponding to different functionality can be discerned for typical mixtures of organic analytes and solvents. Method development is thus potentially simpler than other forms of spectroscopy that have greater variability in spectroscopic transition probabilities from analyte to analyte or for the same analyte in a variable matrix. Although the proton is reasonably expected to be the most important nucleus in process NMR, I9F and 31P offer reasonable sensitivity and outstanding selectivity for appropriate analyses, and even 13CNMR may prove useful in selected analyses of major constituents. Other advantages of process NMR might include the fact that the measurement does not consume supplies such as carrier gasses that must be replenished, nor does it generate waste that must be collected, provided that the analysis stream is reinjected into the process stream. NMR is also completely noninvasive in the sense that the response of the instrument does not change from repeated contact with analyte. This may be an advantage over chromatographic procedures that can be compromised by column plugging or changes in retention behavior. Against the above potential advantages of process NMR must be placed the possible problems expected with this method. Standard high-field spectrometers may be too expensive, fragile, and operator intensive for all but the most ambitious on-line applications, but they are already in use in a number of sites for selected off-line process control problems that benefit from a NMR analysis even with a lag time of 1 h. Several approaches for getting an NMR instrument online-or at least close to the line-make use of inexpensive low-resolution units that can count total protons or differentiate between rigid and mobile protons on the basis of relaxation times. Indeed, analyses based on low-resolution measurements are important in quality control, and this type of technology is starting to make the transition to on-line applications. In the past two years, several vendors have begun to develop process NMR instruments which meet some of the needs for on-line analyzers, and we have been encouraged to provide an academic base for developing instrumentation and methodology. In the course of this work we have carried out applications research on a prototype commercial instrument 0003-2700/94/0366-0536$04.50/0
0 1994 American Chemical Society
and constructed a home-built instrument largely from readily available modular components. Preliminary experiments have characterized the use of continuous-flow 'H N M R for a variety of generic process analysis problems including organics in water, soluble fats and sugars (foods and beverages), and additives of various sorts. In the present contribution, we describe the construction and operation of a high-resolution NMR process analyzer based on a lightweight permanent magnet equipped with a flow probe. In lieu of a field-frequency lock, we have revived an alternate software-based scheme for compensating for magnetic field drift. We report a procedure that recognizes a particular peak in the spectrum and then shifts the frequencies of individual acquisitions as necessary to achieve coherent signal averaging. Process analysis by NMR is demonstrated with an application to the blending of oxygenates into gasoline. Environmental concerns have led to regulatory changes mandating the reformulation of gasoline! A minimum oxygen level will be required in gasolines sold in many areas of the United States. This will be achieved by the blending in of one or more of the following oxygenates: ethers such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and methyl tert-amyl ether (TAME); or alcohols such as methanol or ethanol. Oxygenate gasoline additives contribute to a cleaner burning fuel, reducing the emissionof carbon monoxide and smog-producing hydrocarbons. Oxygenates are also octane boosters and will be increasingly important with the upcoming ban on lead in motor vehicle fuels and anticipated limits on aromatics in some localities. MTBE or mixtures of MTBE and ethanol are emerging as the oxygenate additives of choice. The anticipated standards call for minimum oxygen contents in the range of 2.0-4.0 wt %, which corresponds to 11-22 V O ~% MTBE. Blending of oxygenates into gasoline is performed either at the refinery or at terminals where gasoline is loaded on tanker trucks for local distribution. Blending at the terminal level is advantageous in that it facilitates formulation to meet local requirements, and it also avoids restrictions on the transport of alcohols through gasoline pipelines. There is an urgent need to develop analytical instruments and procedures for the determination of and/or control of oxygenate levels at the time of blending. Currently, there are two methods that have been proposed for quantifying oxygenates in gasoline, both of them employing gas chromatography. The first method, ASTM D 4815-89,5 determines c1-c4 alcohols and MTBE by a fairly complicated procedure involving column switching and back-flushing. A second method considered by the EPA makes use of an oxygen-selective flame-ionization detector (OFID).6 This device has a cracking reactor that converts gasoline components eluting from the column to elemental carbon, hydrogen, and, in the case of oxygenates, CO. The latter is then converted to methane in a second reactor which is quantified using an FID. This clever detector thus produces chromatographic peaks selectively from fuel components containing oxygen. Process analytical staffs at several petroleum companies have concerns about the use of GC methods for control and
certification of oxygenate levels at gasoline blenders. Those methods require up to 20 min per analysis; this is an unacceptably long lag time, and it creates the possibility of large errors or even control-loop oscillations. Furthermore, GC methods require periodic recalibration and servicing by technically qualified personnel, and these are undesirable characteristics for a process analytical method at a remote site. Infrared methods have been applied to a variety of process analytical problems, including the estimation of gasoline composition7 and the prediction of octane number.8 Infrared spectroscopy can be used for oxygenate determinations, but extensive calibration is required. If the hydrocarbon composition of a given gasoline were fixed, an infrared analysis for oxygenates would be straightforward; but frequent changes in the detailed hydrocarbon composition of any given gasoline are the rule, and infrared signals from the hydrocarbons will interfere to some degree with those from oxygenates. Other possible problems include spectral changes due to interactions between different oxygenate components and temperaturedependent spectral changes due to hydrogen bonding. It is believed that the latter will necessitate careful regulation of the sample compartment t e m p e r a t ~ r e . ~ In this contribution, we describe a prototype high-resolution FT-NMR spectrometer that meets many of the requirements for use in a process environment, and we describe its application to the monitoring of MTBE and ethanol levels in a simulated gasoline blender. This instrument is based on a 1-T (42MHz 'H frequency) permanent magnet weighing 5 5 kg and having negligible fringe field. The spectrometer electronics are in five small rack-mountable units, which are designed to be located 4 m or more from the magnet. The instrument is controlled by an Apple computer, which could be located at a much greater distance from the process. This instrument provides a spectral resolution of ca. 3 Hz for spinning samples and ca. 4 Hz for nonspinning samples in our flow probe. The 42-MHz NMR process analyzer reported here achieves a detection limit for MTBE in gasoline of 0.5% (all subsequent percentages are v/v) with a total analysis time of 1 min (1 2 scans). This analyzer can also quantify both MTBE and ethanol in gasolines blended with mixtures of these oxygenates. The 'H NMR spectra of flowing gasoline samples also show well-resolved signals due to aromatics and olefins, suggesting a future application to the monitoring of these fractions. A high level of automation was achieved in the MTBE analysis by using macroing capabilities of the Apple computer that runs the analyzer. This computer also controls a simulated gasoline blender based on liquid chromatography pumps, and full process control is readily achieved.
(4) Wood,A. Chem. Week 1991, 149, 35-41. ( 5 ) 1989 Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1989; Vol. 05.03. ( 6 ) Fed. Regist. 1992, 57, 44444448.
(7) Kelly, J. J.; Callis, J. B. Anal. Chem. 1990, 62, 14441451. ( 8 ) Kelly, J. J.; Barlow, C. H.; Jinguji, T.M.;Callis, J. B. Anal. Chem. 1989,61.
EXPERIMENTAL SECTION Process Analysis NMR Spectrometer. Two different instruments were used in the course of this work, although the same magnet was used in all of the experiments reported. One of the instruments was the prototype of a line of commercial instruments developed by Elbit-AT1 (Chicago, IL). This
3 13-320.
(9) Young, C. A.; Knutson, K.; Miller, J. D. Appl. Specfrosc. 1993, 47, 7-11.
AnalyricalChemisby, Vol. 66, No. 4, Februaty 15, 1994
537
%................I
PUMP A
n y
[ , .......,............
T SHIM CURRENT AMPLIFIER
F 42 MHz AT1 MAGNET
. r.
POWER SUPPLIES
55 Kg
.
TECMAG ARIES
.
. ..
TECMAG NMRKITII
*Teflon
tubing Teflon heat-shrink tubing
E E 0
Y c3
Teflon heat-shrink tubing
. ....... ................ ................ .I.......
PTS 160
-
~&/1/16’’
& ,,,,,, MATCHING NETWORK POWER
7 AMPLIFIER 1 Flgure 1. Block diagram of the home-built process control FT-NMR spectrometer and the flow system used to simulatea gasolineblender.
instrument is based on a 55-kg, 1-T rare-earth permanent magnet in which protons have a resonance frequency of 42 MHz. This magnet was encased in a thermostated, 32-cm cubical box, and the magnetic fringe field was less than 1 G outside of this box. The magnetic field was made homogeneous by the adjustment of 26 shim coils. The Elbit-AT1 prototype was run by a 486 computer, and the spectrometer components were housed in several rack-mountable units. Most of the results reported in this contribution includifig all of the work on gasoline additives were obtained on a homebuilt process analyzer that made use of the Elbit-AT1 magnet. A block diagram of this system is shown in Figure 1. This system was built around rack-mountablecomponents supplied by Tecmag (Houston, TX) in order to take advantage of the high-level NMR software package, MacNMR, as well as to provide compatibility with other, more conventional spectrometers in the laboratory that are controlled with Tecmag components. This system was controlled by a Macintosh Centris 650 computer (Apple Computer, Cupertino, CA) that communicated with the Tecmag Aries pulse programmer and acquisition unit via a NuBus board. The 42-MHz transmitter and receiver channels were provided by a Tecmag NMRKit I1 using a PTS 160 synthesizer as a frequency source. An EN1 (Rochester, NY) rf amplifier delivered pulses that were typically .set at 1.2 W. A matching network consisting of series diodes, quarter-wave cable, and diodes to ground delivered the FID signal to an Advanced Receiver Research (Burlington, CT) Model P45VDG preamplifier. The above units as well as the magnet shim current amplifiers and power supplies were mounted in a small rack unit on wheels. The process stream was simulated using two Waters Model 5 10 liquid chromatographypumps precisely controlled by the Macintosh computer via a National Instruments (Austin, TX) NB-TIO- 10 interface board. Typically, one of these pumps was used to deliver gasoline and the second an additive such as MTBE. The outputs of these pumps were connected with a mixing “tee”, and the flow stream was connected to the NMR probe using stainless steel HPLC tubing. In an actual process environment, the magnet would be located within several meters of a convenient sampling point. A small stream would be removed through a take-off valve and go to a small pump which would regulatethe pressure and flow rate delivered to the NMR probe. 531 Analytical Chemistry, Vol. 66, No. 4, February 15, 1994
Stainless steel HPLC tubing SMA RF connection
Figure 2. Schematic of the flow probe used for processanalysis NMR.
Figure 2 shows a diagram of the flow probe used in this investigation. This was developed based on previous experience with the design of NMR flow probes for application to liquid chromatography detection.lOJ The active region of the flow cell had a volume of 0.23 mL. A 90’ pulse width of 12.5 ps was achieved with 1.2 W of transmitter power. Magnet Drift Compensation. Neither process NMR instrument was equipped with a field-frequencylock channel; therefore, the magnet was free to drift. Typically the drift was as large as 6 ppm/h. Although magnet drift did not have a noticeable effect on spectral resolution achievable with single scans, it tended to preclude signal averaging even for times as short as 1 min. .This problem could have been remedied by adding a lock channel and putting an external lock sample in the probe, but instead a simpler strategy was used for magnet drift compensation in the interest of keeping the analyzer simple and less expensive. We have heard a number of anecdotal accounts of various vendors and researchers using software procedures to compensate for magnet drift, but we have been unable to find a published description of such a procedure. Our implementation of this idea is explained in the context of Figure 3. The single-scan spectra in parts a-d of Figure 3 were acquired at equal intervals over a period of 10 min; magnet drift is evidenced by the displacement of these spectra on the frequency axis. +The.procedurewas initiated by selecting a particular peak to use as a reference: in the case of Figure 3, the intense downfield signal. The procedure autoniatically transformed and phased the four FIDs and then in each spectrum located the reference peak and determined its center by fitting to a Lorentzian line shape. The four spectra were then frequency shifted to stack them at their correct chemical shift values, and then they were added to yield the signal-averaged spectrum shown in Figure 3e. Inspection of the signal and noise levels in Figure 3 indicates that the‘expected gain in signal to noise of a factor of 2 was achieved by the procedure. CYCLOPS phase cycling was performed during each block of four scans to remove quadrature artifacts.12 After each block of four scans, the transmitter frequency was updated.by the software procedure (through computer control of the PTS synthesizer) to keep the signals on resonance within the sweep width. The drift (10) Haw, J. F.; Glass, T. E.; Hausler, D. W.; Motell, E.; Dorn, H. C. Anal. Chem. 1980,52, 1135-1 140. (1 1) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1981,53, 2327-2332. (12) Homans, S. W. In A Dictionaryof ConceptsinNMR;Clarendon Press: Oxford, UK, 1989; p 22 1.
I
12.0 10.0 8.0
6.0
4.0 2.0 PPm
4x
0.0 -2.0 -4.0
Figure 3. Demonstratlon of the software procedure for magnet drlft Compensation. Four scans of 5 % ethylbenzene In CDC13 were taken 10 mln apart. The algorithm Indvldually transformed and phased the FIDs to yleld the spectra a 4 that clearly show magnet drift. The procedure described In the text then coherently added those spectra to give the slgnal-averaged spectrum e.
compensation procedure was run as a C program linked to the MacNMR operating system. Total processing time was approximately 2 s per block of four scans. Reagents. MTBE, ETBE, TAME, methanol, and hexamethyldisiloxane (HMDS) were obtained from Aldrich and used as received. Absolute ethanol was obtained from our research stockroom. The gasoline used was Exxon regular unleaded obtained directly from a pump at a local station.
RESULTS AND DISCUSSION The achievable resolution on the 55-kg, 42-MHz permanent magnet was first evaluated using standard 5-mm spinning NMR tubes as sample holders; representative spectra are shown in Figure 4. With careful adjustment of the magnet shims, it was possible to obtain line widths as narrow as 1 Hz, but the 3-Hz resolution shown for acetone in Figure 4a was more typical. As expected, the 'H spectrum of the resolution standard o-dichlorobenzene (Figure 4b) was not well resolved, reflecting the low field strength, but the ethylbenzenespectrum (Figure 4c) showed good resolution of the scalar multiplets. A signal-to-noise measurement was made for 5% ethylbenzene using one pulse and no apodization. A value of 21 was obtained for the quartet; this value is somewhat low based on extrapolation from the specifications of commercial instruments, but most of the difference appears to be due to line shape. Flow NMR has been investigated for the last 40 years;13 it has been used to investigate transient intermediates'"'' as (13) Suryan, G. Proc. Indian Acad. Sci., Sect. A 1951, 33, 107-111. (14) Fyfe, C. A.; Cocivera, M.; Damji, S. W. H.; Hostetter, T. A:; Sproat, D.; OBrien, J. J. Magn. Reson. 1976, 23, 371-384. (15) Fyfe, C. A.; Cocivera, M.; Damji, S. W. H. J. Am. Chem. SOC.1975, 97, 5707-571 3. (16) Cocivera, M.; Fyfe, C. A.; Effio, A.; Vaish, S.P.; Chen, H. E. J . Am. Chem. SOC.1976, 98, 1573-1578.
10.0
8.0
6.0
4.0 PPm
2.0
0.0
Flgure 4. The 42-MHz proton NMR spectra of several resolutlon standards measured In splnnlng 5-mm tubes: (a) acetone: (b) dichlorobenzene; (c)ethylbenzene.These samples contained HMDS as a chemical shlft reference.
well as a detector for liquid chromatography.10J1J8-21 Caveats associated with this technique include the following: First, if the spins in the coil region are replaced in the time period between pulses, an advantageous reduction in the effective spin lattice relaxation time (TI) may be realized, provided that an adequate volume in the field prior to the coil region exists such that polarization occurs. Second, the flow rate should not be set so high that the residence time of the spins in the coil region is shorter than the T2* observed without flow; failure to take this precaution will result in additional line broadening.22 Finally, since flow NMR experiments normally are done without sample spinning, adjustment of the magnetic field homogeneity is a somewhat greater challenge. These effects and the requirements necessary for quantitative 'H spectra of flowing samples are well understood.*3 Figure 5 shows the performance of the flow probe design in Figure 2 for the ethylbenzene resolution standard over the designed flow range of 0-6 mL/min. These singlescan spectra show that the magnet may be shimmed to reasonable homogeneity even without sample spinning and that no noticeable degradation in homogeneity occurs due to flow except at the high end of the range. A 200-MHz 'H spectrum of the gasoline sample (not shown) revealed essentially no intensity in the region of 3.04.6 ppm, and it has previously been noted that a standard 1H NMR experiment is suitable for an off-line determination of alcohols in gasoline.24 Thus, there are no interferences in the determination of additives with signals in this region. Figure ~
(17) Cocivera, M.; Fyfe, C. A.; Chen, H. E.; Vaish, S. P. J . Am. Chem. SOC.1974,
96, 1611-1613. (18) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1981, 53, 2332-2336. (19) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1983.55, 22-29. (20) Laude, D. A., Jr.; Wilkins, C. L. Anal. Chem. 1984, 56, 2471-247s. (21) Bayer, E.; Albert, K.; Nieder, M.; Grom, E.; Wolff, G.; Rindlisbacher, M. Anal. Chem. 1982,54, 1747-1750. (22) Jones, D. W.; Child, T. F. Adu. Magn. Reson. 1976.8, 123-148. (23) Haw, J. F.; Glass, T. E.; Dorn, H. C. J . Magn. Reson. 1983, 49, 22-31. (24) Renzoni, G. E.; Shankland, E. G., Gain-, J. A.; Callis, J. B. Anal. Chem. 1985, 57, 2864-2867.
Analytical Chemistry, Vol. 66, No. 4, February 15, 1994
539
c
0 mL/min
regulations specify the weight percent oxygen in the gasoline; thus the anticipated ranges of 2.0-4.0 wt 7% 0 correspond to 11-22 vol % MTBE. (Since the density of MTBE is similar to that of gasoline, no correction fromvolume to weight percent is applied in this contribution.) Each MTBE molecule has three equivalent protons, yielding an analytical signal at 3.1 ppm with an intensity proportional to one oxygen atom. MTBE is the oxygenate additive of choice, but it is likely that mixtures with ethanol or other compounds will be used as well. We thus consider the suitability of a 'H NMR analysis for percent 0 in gasoline if the oxygenate composition is variable. The OCH3 signal from TAME is essentially indistinguishable from that of MTBE with the resolution available on the prototype, but no error would result in the analysis of oxygen if mixtures of TAME and MTBE were used as the oxygenate additive. This results from the fact that again each TAME molecule has three equivalent OCH3 protons for every oxygen atom. A mixture of ETBE and MTBE as a mixed-oxygenate additive could present a problem since the analytical signal for ETBE is derived from an OCHz group, but this signal is ca. 0.2 ppm downfield of the OCH3 resonances from the other ethers, and it could be resolved and integrated separately at a slightly higher field strength, or the overlapping signals could easily be decomposed on the basis of characteristics of the distinct multiplicity patterns. The presence of alcohols would be indicated by a hydroxyl signal, and OCH3 and OCHz signals could be treated as described above for the ethers. It is unlikely that all of the compounds in Figure 5 would be used together in an oxygenate mixture, and the extension of the method reported here to two- or three-component mixtures seems straightforward. Indeed, this mixture will be demonstrated with a study of variable ethanol content in a gasoline with a fixed level of MTBE. A number of flow NMR runs were performed using both the AT1 prototype and our home-built system to assess the potential of high-resolution process NMR for control of a gasoline blender. These experiments were performed using both authentic gasoline samples and binary hydrocarbon mixtures. Integrated signal intensities were in excellent agreement with known composition in all cases. We report two representative studies, both carried out on the home-built process NMR instrument using 12 scans per spectrum and the magnet drift compensation procedure described in the Experimental Section. In these examples, the total analysis time per spectrum was less than or equal to 1 min-a significant improvement over GC methods for oxygenates that take 20 min or more. Figure 7 shows a study in which various concentrations of MTBE were blended with regular, unleaded gasoline and then flowed through the probe at 2.0 mL/min. The MTBE OCH3 signal is clearly visible at 3.1 ppm even at a concentration of 1%. An analytically useful signal due to the ferf-butyl protons at 1.1 ppm is also apparent at higher concentrations, although the latter resonance overlaps severely with the signals from the aliphatic hydrogens of gasoline. For the OCH3 signal, a small interference-in the form of a sloping base line-is present as a result of a broad line shape component from the gasoline due to magnetic field inhomogeneity. This sloping base line was easily fit and subtracted to facilitate accurate measurements of the MTBE OCH3integrated signal intensity.
1
2 mL/min
-/L-PJL
4 mL/min
1 ,
6 mL/min
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
PPm Figure 5. The 42-MHz proton spectra of ethylbenzene flowing at various rates obtained using the probe in Figure 1. The spectra were not sensitive to flow rates in the range shown.
1 " ' I " ' I " ' I " ' I " ' l " 10.0 8.0 6.0 4.0 2.0
0.0
PPm Figure 8. The 42-MHz proton NMR spectra of proposed oxygenate gasoline additives measured in splnnlng 5-mm tubes: (a) methanol: (b) ethanol; (c) MTBE;(d) ETBE; (e)TAME. These samples contained HMDS as a chemical shift reference.
6 shows 42-MHz IH spectra obtained with spinning of the important oxygenate additives. All of these compounds have analytically useful signals in the region of 3-4 ppm. Methanol (Figure 6a) has an OCH3 singlet at 3.3 ppm and an O H signal at 4.3 ppm. Ethanol (Figure 6b) has an OCH2 quartet at 3.6 ppm and an OH resonance at 4.9 ppm. The chemical shifts of hydroxyl protons in mixtures are sensitive to hydrogen bonding. MTBE shows an OCH3 singlet at 3.1 ppm (Figure 6c), while ETBE shows an OCHz quartet at 3.3 ppm (Figure 6d). TAME (Figure 6e) has an OCH3 singlet at 3.1 ppm. The IH spectra of the oxygenate additives are fortuitous for the determination of oxygen in gasoline. Anticipated 540
Analj-ticalChemistty, Vol. 66,No. 4, February 15, 1994
A
0% MTBE
1OX
10% EtOH 10.0
8:O
6:O
4:O PPm
2:O
0:O
-210
Figure 7. The 42-MHr proton spectra of various blends of MTBE and regular, unleaded gasollne. The hlghlighted areas show signals from MTBE or the aromatic and olefinic protons of gasollne. All spectra were acqulred wlth 12 scans on samples flowlng at 2.00 mL/mln.
$
0.3
c,
v/v%MTBE Flgure 8. Integrated signal intensity of the MTBE OCHs slgnal versus its volume percent In gasoline. These measurementswere performed uslng an experiment analogous to that in Figure 7.
Note that the concentration of oxygenate additives is likely to be 10% or more, and the measurement of the MTBE signal at that concentration is straightforward. Figure 8 shows a calibration curve constructed from a series of measurements at 11 different MTBE concentrations. This curve is linear over the entire concentration range, as one would expect for determinations using NMR. The precision of the determination of MTBE in gasoline by process NMR was assessed for a flowing 10% sample. Twelve separate measurements, each of 12 scans, were made over a 1-h period. The absolute standard deviation of the independent determinations was 0.17% MTBE. This precision is within the industry target, which is ca. 0.1-0.2% MTBE (0.02-0.04 wt % 0),and the precision of the 'H NMR method could be further improved by signal averaging more scans and other straightforward refinements. We estimate that the desired precision could easily be exceeded with a total measurement time of 2 min or less. Statistical analysis of a number of measurements of MTBE in gasoline suggested a detection limit of 0.5% for MTBE using the 12-scan protocol. As stated previously, mixed oxygenates are likely to be used in some gasoline blenders as a result of variations in cost and availability. Figure 9 reports results for a process NMR
10.0
8:O
6:O
4:O PPm
2:O
0:O
-2:O
Flgure 8. The 42-MHz proton spectra of various blends of ethanol in 10% MTBE In gasoline. All spectra were acquired with 12 scans on samples flowing at 2.00 mL/min. The concentration dependence of the hydroxyl proton chemical shift Is apparent In the expanded plots.
run in which various amounts of ethanol were co-fed into a gasoline with 10% MTBE. The presenceof ethanol (3.6 ppm) was easily recognized by inspection down to a concentration of 2% or less. As is typically observed for hydrogen-bonding compounds dissolved in nonpolar solvents, the hydroxyl resonance of ethanol exhibits a chemical shift that is concentration dependent. In the spectrum of the 5% sample, this signal is partially overlapped with the OCH2 resonance, but it is well resolved at a concentration of 10% as a result of the downfield shift due to hydrogen bonding. Note that although the position of this resonance is concentration dependent, its intensity is fixed as half that of the OCH2 resonance; thus it should always be possible to quantify the ethanol concentration. Indeed, the concentration dependence of the OH chemical shift may be analytically useful, although this was not investigated in detail. Referring to the top spectrum in both of Figures 7 and 9, one notes clear signals for olefinic protons in the region of 4.4-6.0 ppm and aromatics at 6.4-8.0 ppm. It is desirable to quantify this type of functionality in order to meet regulatory guidelines as well as performance criteria. Indeed, l H and 13C spectra of gasoline have previously been interpreted quantitatively as a measure of octane rating for gasolines.25-33 ~
~~
~
(25) Myers, M. E.,Jr.; Stollsteimer, J.; Wims,A. M. Anal. Chem. 1975,47,2010201s. (26) Myers, M. E.,Jr.; Stollsteimer, J.; Wims, A. M. Anal. Chem. 1975,47,23012304. (27) Muhl, J.; Srica, V. Fuel 1987, 66, 1146-1149. (28) Muhl, J.; Srica, V.; Jednacak, M. Fuel 1989.68, 201-203. (29) Ichikawa,M.;Nonaka,N.;Amano,H.;Takada, I.;Ishimori,S.Appl.Spectrosc. 1991, 45,637-640. (30) Ichikawa, M.; Nonaka, N.; Amano, H.; Takada, I.; Ishimori, S.; Andoh, H.; Kumamoto, K. Appl. Spectrosc. 1991, 45, 1679-1683. (31) Ichikawa, M.; Nonaka, N.; Amano, H.; Takada, I.; Ishimori, S.; Andoh, H.; Kumamoto, K.Appl. Spectrosc. 1992, 46, 498-503. (32) Ichikawa, M.; Nonaka, N.; Amano, H.; Takada, I.; Ishimori, S.;Andoh, H.; Kumamoto, K. Appl. Spectrosc. 1992, 46, 1294-1300. (33) Ichikawa, M.; Nonaka, N.; Amano, H.; Takada, I.; Ishimori, S.; Andoh, H.; Kumamoto, K. Appl. Spectrosc. 1992, 46, 1548-1551.
Analytical Chemistry, Vol. 66, No. 4, February 15, 1994
541
We have not assessed in detail the use of process NMR for the determination of these proton types, but the potential is clearly there. The MacNMR software operating system supports a highlevel macroing capability both internally and through compatibility with other software products for Apple computers. Although automation was not the major focus of this paper, we have successfully carried out automated determinations of MTBE without operator involvement. Both automation and remote operation are highly desirable characteristics for a mature process analyzer.
ACKNOWLEDGMENT This work was supported by Elbit-ATI, W. R. Grace, and the Center for Energy and Minerals Resources of Texas A&M
542
Analytical Chemistry, Vol. 66,No. 4, February 15, 1994
University. We thank Dr. Loring Weisenberger and Dr. Ken Gallaher of British Petroleum (Warrensville, OH) for extensive discussions regarding regulatory changes mandating reformulated gasoline and process analysis in general as well as its application to the determination of oxygenates in gasoline. This work was presented at the Council for Chemical Research's NICHE Conference on Process Analysis held in Summit County, CO, on May 16-19, 1993.
Received for review August 24, 1993. Accepted November 16, 1993." ~~
e
~
~~
Abstract published in Adoance ACS Abstracts, December 15, 1993.
