1896 Anal. Chem. 1982, 5 4 , 1896-1898

Quantitative Analysis of AlkeneIAlkane Mixtures by Carbon-1 3 Nuclear Magnetic Resonance Spectrometry

David A. Forsyth,” Markus Hediger, Sabri S. Mahmoud, and Bill C. Giessen

Department of Chemistry and Institute of Chemical Analysis, Northeastern Unlverslw, Boston, Massachusetts 02 1 15

A common problem in studies of catalytic hydrogenation and isomerization is the ready analysis of mixtures containing isomeric alkenes and the corresponding alkane. Gas-liquid chromatographic separation of a set of isomers can be difficult and typically requires long columns. For example, complete separation of the straight-chain hexenes has been achieved on a 15-m column of dimethylsulfolane on firebrick at 0 OC ( I ) or on a 150-m capillary column with triethylene glycol butyrate as the stationary phase (2). In the course of a study (3) of catalytic hydrogenation by metallic glasses (4), we have found that 13C NMR of hydrocarbon mixtures is often a suitable alternative to chromatographic separation.

The wide dispersion of chemical shifts in 13C NMR makes 13C spectra attractive for quaptitative analysis of mixtures. If each component in a mixture contributes at least one non- overlapping peak to the 13C spectrum, the relative intensities of such peaks have the potential of revealing the relative concentrations of the components. However, 13C spectra are seldom used for quantitative analysis of mixtures because there are several problems associated with the use of 13C NMR peak heights and peak areas for quantitative purposes (5,6). These potential difficulties can be circumvented by appro- priate instrument settings and application of empirical cor- rection factors to peak heights. Quantitative determinations are possible if 13C signal intensities are first calibrated for specific instrument conditions from spectra of the individual components, as described here for hexene/hexane mixtures.

EXPERIMENTAL SECTION n-Hexane and the n-hexenes were high-purity samples obtained

from Aldrich Co. or MCB. 1,4-Dioxane was obtained from Fisher Scientific Co. Separate 13C NMR spectra of the hydrocarbons and solvent revealed no extraneous peaks. Solutions for determination of calibration factors were prepared as either 15% (g/g) n-hexane and 15% hexene or 10% n-hexane and 20% hexene in 1,4-dioxane.

Broad-band decoupled I3C NMR spectra were recorded on a JEOL FX-6OQ Fourier transform NMR spectrometer operating at 15.0 MHz. Field stabilization was provided by an external deuterium lock. Chemical shifts were referenced to 1,4-dioxane and converted to parts per million relative to MelSi by the relation bC(l,4-dioxane) = 67.4. Spectra were obtained from 2 mL of solution in 10-mm tubes fitted with a Teflon vortex suppressor. For spectra of only the alkane region, the following parameters were used: 16 384 data points, 400 Hz spectral width, 60’ pulse width, 23 s repetition time, and an offset which placed the folded-in dioxane signal in a noninterfering position. I3C spectra were typically processed after 120 pulses, applying an exponential weighting to the accumulated free-induction decay signal which gave 0.1 Hz line broadening.

RESULTS AND DISCUSSION The 15.0-MHz 13C spectrum of the alkyl region of a five-

component mixture of n-hexanes and four straight-chain hexenes in 1,Cdioxane is shown in Figure 1. Of the possible hexene isomers, cis-3-hexene is not included because it was never observed as a component in our hydrogenation mixtures which started with 1-hexene or the 2-hexenes. The labeled peaks in Figure 1 were used for the analysis. The 13C chemical shifts are listed in Table I. The italicized resonances in Table I correspond to the labeled peaks in the figure and each does not overlap with any other resonance in the six-component

0003-2700/82/0354-1896$01.25/0

Table I. 13C NMR Chemical Shiftsa of n-Hexane and Hexenes in Dioxane

C-1 C-2 C-3 C-4 C-5 C-6 f b

n-hexane 14.2 23.3 32.2 1 1-hexene 114.4 139.2 34.0 31.7 22.7 14.1 1.94 cis-2-hexene 12 .7 124.0 130.8 29.4 23.2 13.9 1.96 trans-2-hexene 17.8 125.0 131.8 35.3 23.2 13.7 2.00 trans-3-hexene 14.3 26.0 131.3 0.90 cis-3-hexenec 14.4 20.7 131.3

a Chemical shifts are in ppm ( 6 ) from (CH,),Si. correction factors f are calculated for the italicized resonances from eq 1.

mixture. All chemical shifts had been previously reported in other solvents (7, 8).

One of the problems associated with the use of 13C NMR peak heights is having sufficient data points to precisely define the top of the peak (5). For most alkenes, sufficient data can be obtained by choosing a spectral width to cover only the alkyl resonances and by using the maximum available com- puter data table. Most modern spectrometers with quadrature detection and adjustable electronic filters will effectively fiiter out the folded-in resonances from the alkenyl carbons. We have found it advantageous to artificially broaden the peaks by application of an exponential weighting to the accumulated free induction decay signal and by slightly “detuning” the instrument (line widths of 0.8-1.0 Hz were satisfactory). Broadened signals give more reproducible peak heights be- cause more data points define the top of the peak.

Another instrumental source of error is variation in peak heights due to nonuniform decoupling or nonuniform exci- tation over the entire spectral width (5, 6). Problems asso- ciated with such variability can be avoided simply by always using the same offset and irradiation power settings for the decoupler and the same rf offset and pulse width settings for the excitation pulse.

A source of error which appears not to have been discussed in literature treatments of quantitative FT NMR measure- ments (9, I O ) is the effect of the electrical filters which are used to limit noise. A low pass filter is applied before ana- log-to-digital conversion to minimize fold-in of noise and unwanted peaks into the spectral range of interest ( I I , I 2 ) . Ideally, the filter is set to half the spectral width for instru- ments with quadrature detection, giving a band-pass equal to the spectral width. The problem is that the filter may actually act on some of the desired signals toward the edges of the spectral range, thereby weakening signal intensities relative to interior signals.

The effect of the filter is illustrated in Figure 2, wherein peak ratios are plotted as a function of spectral offset. The ratios were obtained from a four-component mixture of 1- hexene, cis- and trans-2-hexene and n-hexane, using the peaks labeled as a-d in Figure 1. The spectra were obtained with a spectral width and band-pass width of 400 Hz. The greatest effect is seen for the a/c ratio, because a is closest to the edge of the spectrum. As the offset is changed to place the peaks further from the edge, the ratios approach those found with a filter band-pass set a t double the spectral width. Since the filter setting is usually automatically linked to the spectral

The

Literature values (8).

0 1982 Amerlcan Chemical Soclety

ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 1897

separations. The correction factors are constant if the rf pulse repetition interval is kept constant in all measurements.

Analysis of hexenefhexane mixtures from the catalytic hydrogenation of 1-hexene was achieved through the use of correction factors for 13C peak intensities determined under constant settings for the instrument variables described above. Correction factors, f, for the italicized resonances in Table I were established from known binary mixtures using eg 1,

Flgure 1. 15.0-MHz 13C spectrum of a solution In 1,rklioxane of 9 % n-hexane, 7 % 1-hexene, ‘11 % trans-2-hexene, 12% ~/~-2-hexene, and 7 % frans-3-hexene. Labeled peaks correspond to the italicized resonances in Table I: (a) C-4 of trans-2-hexene; (b) C-3 of Ihexene; (c) C-3 of n-hexane; (d) C-4 of cis-2-hexene; and (e) C-2 of trans- 3-hexene.

b/c I

L - U L . d 5 15 25 35 45 55

O F F S E T , H z

Figure 2. Observed ratios between peak heights as a function of spectral offset, with the spectral width and filter band-pass set at 400 Hz. Ratios were obtalned from a four-component solution using peaks a -d corresponding to the labeled peaks in Figure 1. Offset is indicated by the distance in Hz of peak a from the downfield edge of the spectrum. The dotted lines indicate the average values for the ratios when the fllter setting was doubled to 800 Hz. Ratios were not de- pendent on offset with the wlder fllter.

width setting, it is important to always select the same spectral width and offset or to override the automatic control to obtain a constant broader filker width. Ratios between signal in- tensities were reproducible with constant instrument settings. With a filter band-pass of 800 Hz, the peak ratios in the four-component mixture were a / c = 0.749 f 0.026 (standard deviation), b/c = 1.193 :t 0.029, and d / c := 0.925 1 0.024, based on ten measurements.

13C NMR peak intensities and areas me usually not directly proportional to the number of carbon nuclei giving rise to the signal because of variation in spin-lattice relaxation times and nuclear Overhauser enhancements (5, 6). For quantitative analysis, attempts have been made to suppress such variation through use of spin-relaxation reagents and gated lH de- coupling routines (9). However, quantitative results can be achieved also if empiricd correction fadors can be determined and applied to the signal intensity data. This procedure is analogous to the use of correction factors for differential molar response of detectorEi to components in chromatographic

where IH and IX are intensities of the C-3 resonance of n- hexane and the appropriate hexene resonance, respectively, and mx/mH is the known mole ratio of hexene to hexane. This procedure automatically includes a correction for molecular symmetry. For the spectrum of a mixture, multiplication of observed intensities by the appropriate factors, f, yields corrected values which may be directly compared to determine mole ratios.

The accuracy of the 13C method is primarily dependent on the signal-bnoise (SIN) ratio that is achieved. An SIN ratio of about 501 could be attained in 30-60 min for a methylene carbon of a component present in about 1 M concentration (5-10 wt % hydrocarbon in dioxane). The 13C spectrometer used in this study has the lowest field strength of commercially available instruments (1.4 T). Higher field instruments would allow faster measurements or lower sample concentrations to achieve the same SIN ratios.

We consistently found agreement within 1 2 % for the percent of n-hexane in hexene /hexane mixtures upon com- paring the 13C results to GLC determinations which used a column that separated n-hexane from the hexenes. Similar agreement (11.5%) was seen between known and found mole percent of trans-3-hexene in binary mixtures with n-hexane when the total hydrocarbon concentration was varied from 7 to 31 w t % and the hexenelhexane ratio was varied from 1:4 to 3:2. Trials with multicomponent hexene/hexane mix- tures gave agreement with known mole percents to 12%. For example, the results for a four-component mixture in dioxane were as follows: n-hexane (known 13.5%, found 14.9%), trans-2-hexene (21.9%, 22.7%), cis-2-hexene (29.0%, 27.8%), and 1-hexene (35.5%, 34.5%).

Quantitative analysis by 13C NMR should be applicable to many complex mixtures. For example, in chemical shift data (7,8) for the sets of isomeric straight-chain alkenes from C4 through C,, a signal separated by at least 0.5 ppm from any other signal can be found for each isomer within a set. For the octenes, nonoverlapping signals can be found for all iso- mers except cis-4-odene. Even for mixtures where overlapping signals is a problem, analysis would still be possible by com- puter simulation of the entire spectrum using precalibrated spectra of each of the components.

ACKNOWLEDGMENT We gratefully acknowledge the support and advice of S.

Spangenberg of the Dow Chemical Co.

LITERATURE CITED (1) Panchenkov, G. M.; Vagin, M. F.; Aslanyan, M. V.; Timofeeva, G. V.

Russ. J . Phys. Chem. 1973, 4 7 , 988-9138. (2) West, P. B.; Haller, G. L.; Burwell, R. L. Jr. J . Catal. 1973, 29.

(3) Forsyth, D. A.; Mahmoud, S.; Hedlger, M.; Giessen, B. C. “Rapidly Solidlfied Amorphous and Crystalllne Alloys”; Giessen, B. C., Kear, B., Eds.; Elsevier: New York, 1982.

(4) Polk, D. E.; Giessen, B. C. “Metallic Glasses”; Gilman, J. J., Leamy, H. J., Eds.; ASM Materials Sclence Seminar Series, Amerlcan Society for Metals, Metals Park, OH, 1978; Chapter 1.

(5) Levy, G. C.; Lichter, R. L.; Nelson, G. L. “Carbon-13 NMR Spectroscopy”; Wiley: New York, 1980; pp 36-44.

(6) Wehrli, F. W.; Wlrthlin, T. “Interpretatlon of Carbon-I3 NMR Spectra”; Heyden: New York, 1978; pp 265-271.

(7) de Haan, J. W.; van de Ven, L. J. M. Org. Magn. Reson. 1973, 5 ,

488-493.

147-1 48.

I090 Anal. Chem. 1882, 54, 1898-1899

(8) Cooperus, P. A.; Clague, A. D. H.; van Dongen, J. P. C. M. Org. Magn.Reson.1976, 8,426-431.

(9) Shoolery, J. N. Prog. Nucl. Magn. Reson. Specfrosc. 1977, 77, 79-93.

(IO) Lindon, J. C.; Ferrige, A. G. Prog. Nucl. Magn. Reson. Spectrosc. lS80, 74, 27-66.

(1 1) Shaw, D. "Fourier Transform NMR Spectroscopy"; Elsevier: Amster-

(12) Becker, E. D. "High Resolution NMR", 2nd ed.; Academic Press: New York, 1980; p 229.

RECEIVED for review March 2, 1982. Accepted June 7, 1982. This work was carried Out under a grant from Chemical

dam, 1976; p 177. co.

Aluminum and Gold-Plated-Aluminum High-pressure Mass Spectrometer Sources

Karl Blom, Lisa J. Hllliard, Harvey S. Gold, and Burnaby Munson"

Department of Chemistry, University of Delaware, Newark, Delaware

A high-pressure mass spectrometer source constructed of aluminum and plated with gold may be a viable alternative to the widely used stainless steel construction. Source blocks which take days to machine from stainless steel can be ma- chined from aluminum in hours. This has obvious cost ad- vantages and makes systematic design modifications more feasible. Vapor deposition of gold onto the interior surfaces of the aluminum source gives a clean, nonreactive, highly conductive surface.

EXPERIMENTAL SECTION Figure 1 shows the stacked ring construction of the source used

in these experiments. The design is similar to one that has been reported previously (1). The stack is held tightly together with four 6-40 rods. This construction provides design flexibility and allows the ion path length to be varied while other source pa- rameters are held constant. The Teflon spacers make seals sufficiently tight that no problems were observed in reaching chemical ionization (CI) pressures of 0.2-1 torr. The usable temperature range of the source is limited by the Teflon which begins to soften and outgas between 200 and 250 "C. The ion path lengths were 0.5-1.5 cm in these experiments. The relatively small lateral cross section and orthogonal geometry ensure a relatively well-defined ion path length.

The source rings were constructed of 6061-T4 aluminum. The interior surfaces were cleaned with an abrasive pad and rinsed with toluene just prior to gold plating. An approximately 100-nm layer of gold was vapor deposited onto the inside surfaces of all aluminum parts from a tungsten hot zone dimple boat (R. D. Mathis Co. #SSA-.005W) using a Veeco VE-7700 vacuum de- position apparatus with a Veeco ED-200 Evapatrol Controller. The interiors of the source rings were coated by supporting the boat inside the rings on brass rods. I t was possible to coat the entire inner surface of a ring with a single deposition, perhaps because the gold crept to the outside of the tungsten boat. The nominal thickness of the gold layer was monitored with an Inficon XTM quartz crystal deposition monitor. No effort was made to study effeds of layer thickness. The gold layer adheres moderately well to the aluminum surface but cannot be cleaned by the usual methods. The gold layer has been replaced approximately every 4 months. There have been no problems with the gold migrating onto insulated source partxi. The mass spectrometer used in these experiments is a DuPont/CEC 21-llOB mass spectrometer that has been modified extensively for high-pressure operation (2) .

RESULTS AND DISCUSSION Initial experiments were performed with an unplated alu-

minum source. High-pressure mass spectra (above about 0.1 torr) obtained with this source were essentially identical with spectra obtained with other sources. Methane gave predom- inantly CH,+ and CzH6+ in a ratio of about 1 to 0.8. Methane chemical ionization (CHI CI) spectra of organic compounds obtained under these conditions were also very similar to spectra obtained with a stainless steel source. The unplated A1 source is suitable for general CI work.

1971 1

Table I . CH, CI Spectra of n-Butyl Propanoatea % abundanceb

stainless species m/z Au/AI steel

CP, ' 57 27 23 C,H,CO,H,+ 75 100 100 (M t C,H, - C,H,)+ 103 18 19

(M t C A ) + 159 6 5 (M f C,H,)+ 171 8 9 (M f C4H-3)' 187 4 5

(M t H)+ 131 65 66

(M + H t M)+ 261 22 24

a P(CH,) 2 0.4 torr, ion path lengthr 1.2 cm. Ions less than 5% of base peak not reported.

The field strength within the unplated source was not well-defined. No significant change in the extent of conversion of CI reactions (and hence in the ionic residence time) was observed when the potential difference between the ion exit plate (part d in Fig. I) and the back plate (part a in Fig. I) was changed. No meaningful kinetic data could be obtained. The detectable ion current was stable for long periods of time indicating that there was no buildup and decay of surface charges within the source. The anomalous field strength results may, however, be caused by a steady-state charging of the aluminum oxide layer covering the aluminum surface. The gold layer was applied to eliminate this effect.

With the gold-plated source, the distribution of ion inten- sities in the high-pressure mass spectrum of methane was that obtained previously. The CHI CI spectra of several simple organic compounds were essentially the same as spectra ob- tained with a stainless steel source. Table I shows typical data for n-butyl propanoate obtained with the Au/Al source and a stainless steel source with similar geometry. The abundant ion at mlz = 261 and the less abundant ion at m/z = 187 are products of sample ion/sample molecule reactions resulting from the large extent of conversion in the long path sources. Under these conditions of relatively high methane pressure and relatively low field strength within the source, the resi- dence times of reagent ions depend on their drift velocities and the length of the source. The residence time, and con- sequently the extent of the CI reactions, can be increased by increasing the source path length. However, with this simple source design the detected ion current decreases approximately as the square of the source length. A not yet optimized path length will give maximum sensitivity for analysis.

The field strength in the Au/Al source appears to be well-defined. Qualitative experiments show a consistent de- crease in the extent of reaction with increasing field strength. The rate constants for a few reactions of CHs+ using reaction

0003-2700/82/0354.1898$01.25/0 0 1982 American Chemical Society