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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
Determination of Methylcyclopentadienylmanganesetricarbonyl in Gasoline by Gas Chromatography with Interfaced Direct Current Argon Plasma Emission Detection Peter C. Uden," Ramon M. Barnes, and Frank P. DiSanzo Department of Chemistry, GRC Tower
I, University of Massachusetts, Amherst, Massachusetts 0 1003
A method is described for the determination of methylcyclopentadienylmanganesetricarbonyl (MMT) in gasoline utlliring gas chromatographic separation with interfaced specific manganese detection by means of dc argon plasma emission spectroscopy. The procedure is rapid, free of interferences, specific, and requires little sample preparation. The use of cyclopentadienylmanganesetricarbonyl (cymantrene) as an internal reference yields a precision of f0.8-3.4% relatlve standard deviation. The llmit of detection Is approximately 3 ng of manganese metal as the complex.
Increasing attention has been given in recent years to the use of manganese additives for improving combustion processes of various fuel oils. Organometallic compounds such as methylcyclopentadienylmanganesetricarbonyl (CH,C5H4Mn(C0)3) (MMT), the additive AK-33X, have found application in aircraft fuels and in the formulation of heating oils to reduce flue-gas smoke. In particular M M T and similar compounds have been used in conjunction with alkyllead compounds to provide synergistic octane number improvement ( I ) . Additives with typical manganese t o lead ratios of 0.1-0.3:l were patented ( 2 ) . Other manganese compounds, e.g., amine complexes, were also found valuable in reducing noxious particle and gas emission in burning fuels (3). However, t h e possible danger of increased atmospheric manganese concentrations arises from these sources and levels of 1 5 pg M n / L have been reported to lead to chronic manganese poisoning a n d / o r manganese pneumonia ( 4 ) . Introduction of catalytic converters to reduce automobile exhaust emissions has stimulated increased substitution of M M T for lead alkyls as a major antiknock additive in "unleaded" gasolines. Presently M M T is added to half of the unleaded gasolines marketed in the United States to improve octane ratings. M M T present at levels of approximately 0.9 g/gal (equivalent t o 0.125 g of manganese metal/gal) elevates hydrocarbon emissions and poisons catalytic converters. Lowering of M M T concentrations to around 65 mg Mn/gal (0.26 g MMT/gal) appears to improve engine and catalyst system durability although the conclusions as t o reduction in emissions are not yet clear ( 5 ) . Today's unleaded gasolines average 40 mg Mn/gal (0.16 g MMT/gal) ( 5 ) . A maximum limit of 60 mg MMT/gal has been indicated for one major gasoline producer (6). While many procedures for the determination both of lead alkyls and total lead in gasoline exist, few methods have been developed for t h e analysis of manganese additives. Indeed, procedures have been limited to the determination of total manganese levels. Kyriapoulos ( I ) measured colorimetrically 10-250 mg/L of Mn by oxidation t o permanganate. Manganese and lead additives were converted t o t h e metal and determined by hollow cathode emission spectroscopy ( 7 ) . Manganese in t h e range 0.003-1.0% was found. Generally flame photometric methods are limited by burner problems and complex sample handling ( B ) , although novel burner design has improved matters (91, 0003-2700/78/0350-0852$01.00/0
and atomic absorption procedures for total manganese are preferred. Lukasiewicz and Buell (10) have noted the behavior of various manganese compounds including M M T in nitrous oxide-hydrogen flames for direct determination in gasoline, and observed different responses for various compounds. Robbins applied heated vapor atomic absorption (AA) for direct measurement in the range 10-100 ng Mn/g of sample (11);and using carbon filament AA, Everett e t al. (12) determined M M T (as total manganese) in aircraft fuels and heating oils. Van Lehmden et al. evaluated neutron activation analysis, spark source mass spectrometry, a n d x-ray fluorescence for trace element determination including manganese (13). T h e most common methods presently employed for M M T in gasoline appear to be x-ray fluorescence and atomic absorption. Despite Turkel'taub e t al.'s report (14) on t h e successful gas chromatography of cyclopentadienylmanganesetricarbonyl (cymantrene), its relative obscurity and t h e general lack of quantitative GC metal analysis may explain the previous lack of a published analysis for M M T in gasoline based on t h e method, despite the wide use of GC for lead alkyl analysis. The similar volatility of both M M T and lead alkyls t o many of the hydrocarbon components in gasoline clearly poses severe problems of GC resolution, and the advantage of quantitative specific element detection is evident. A procedure has been developed which utilizes dc argon emission plasma spectroscopy for both qualitative a n d quantitative specific manganese detection.
EXPERIMENTAL Equipment a n d Operating Conditions. The gas chromatograph and the dc argon plasma emission spectrometer interface design was reported previously (15,16)and is considered in greater detail elsewhere (17); only basic features are noted here. A prototype Spectraspan I11 dc plasma echelle spectrometer (Spectrametrics, Inc., Andover, Mass.) was used. With entrance slit width set a t 0.1 mm, the spectrometer was optimized with a manganese hollow cathode lamp. A Varian 1200 gas chromatograph was adapted for on-column injection onto a 6 f t X in. 0.d. stainless steel column packed with 2% Dexsil 300 GC on Chromosorb 750, 100-120 mesh (Johns-Manville Corp., Denver, Colo.). Column effluent was split by an approximately 1:l ratio between the flame ionization detector of the gas chromatograph, and a heated, thermal, and electrically insulated l / ,6-in. 0.d. stainless steel transfer line to the dc plasma. Preheated argon sheath gas was required in addition to the argon supplied to sustain the plasma, in order to optimize spectral sensitivity. The column and injection port temperatures were set at 130 and 160 "C, respectively, and the interface temperature was 170 "C. Helium carrier gas flow rate was 25 mL/min. Materials. Methylcyclopentadienylmanganesetricarbonyl (MMT) was purchased from Strem Chemical Inc. (Danvers, Mass.) and cyclopentadienylmanganesetricarbonyl (cymantrene) was obtained from M. D. Rausch. 2,2,4-Trimethylpentane (isooctane) was obtained from Eastman Chemicals, Inc. Commercial gasoline samples were obtained during January 1978. Procedure, The dc argon plasma was allowed to equilibrate after ignition for 30 min to ensure stable reproducible behavior. C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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miru:es ,
Flgure 1.
DC argon plasma emission chromatogram of cyclopentadienylmanganesetricarbonyl (cymantrene) (100 ng) and methylcyclopentadienylmanganesetricarbonyl (MMT) (50 ng) in 5 pL isooctane solution. Column 2 meter, stainless steel, in. 0.d. 2% Dexsil 300 GC on 100-120 mesh Chromosorb 750, 130 OC
Standard solutions were prepared from stock solutions of MMT in isooctane and, for the most consistent results, were made up fresh daily. One thousand- and 100-ppm solutions in analytical grade isooctane remained stable if stored in the dark for several months, any decomposition being indicated by the formation of a brown flocculent precipitate. Preliminary experiments using hexane as solvent were discontinued as standards degraded after only a few hours. Cymantrene was added at 20 or 30 pg/mL levels as an internal reference for both standard MMT and unknown sample measurements. Samples were injected into the gas chromatograph by means of a SGE 5RNGP microsyringe (Scientific Glass Engineering Pty., Melbourne, Australia), and chromatograms could be repeated every 3 min. Chromatograms were recorded on an Omniscribe 5000 recorder (Houston Instruments) and peak areas were measured for quantitation.
RESULTS AND DISCUSSION In common with a certain number of reported gas chromatographic separations of volatile organometallic compounds, t h e manganese species investigated show excellent gas chromatographic elution characteristics. As with ferrocene and other iron group metallocenes (18, 19), and arene chromium and molybdenum tricarbonyls (20),cymantene and M M T both conform to the 18-electron rule for organometallics (Le., electrons donated by ligand groups together with those in metal-bonding orbitals total 18). Compounds of this type are noted for their great chemical stability and their GC properties bear out this behavior. The typical dc plasma emission spectrometer response from a 5-pL injection of a standard isooctane solution containing 10 pg/mL (Le., 50 pg) M M T and 20 pg/mL (Le., 100 ng.) cymantrene is shown in Figure 1. The small negative response directly upon injection corresponds to the passage of the solvent through t h e plasma. T h e dc plasma tolerates large volumes of eluted solvent. This is a n important advantage of this system. Baseline resolution was obtained under isothermal conditions at 130 “C, and no peak distortion was noted by comparison with flame ionization detection (although in this case the peaks were superimposed on t h e isooctane solvent tail). No degradation of GC resolution for this system imparted by the heated transfer line to the plasma is apparent. Dual detector responses (dc plasma and FID) are shown in Figure 2 for a commercial unleaded gasoline sample; 5 pL of gasoline was injected directly onto the column and effluent was split 1:l between detectors. T h e Mn 279.83 n m emission line was used for t h e plasma detection. Signals for only M M T are seen by t h e plasma detector except for the small negative solvent response for the
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minutes
Flgure 2. Dual detector chromatogram of unleaded gasollne containing MMT, column and GC Conditions as in Figure 1, 5 pL gasoline injected. Effluent split 1:l between flame ionization and dc argon plasma detectors. Mn 279.83 nm line monitored
0
1
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VindtQS
Flgure 3. Leaded gasoline with 40 pg/mL of cymantrene added as internal reference, 15 pL gasoline injected. DC argon plasma detection at 279.83 nm. Column and GC conditions as in Figure 1
gasoline solvent peak maximum. In the FID trace, the M M T peak is virtually unresolved from gasoline hydrocarbons and clearly could not be measured quantitatively from this chromatogram. Separate studies on open tubular columns (21) show quantitation was still extremely difficult even when greater resolution is possible. A blank analysis was obtained for a “Premium” grade “leaded” gasoline to which cymantrene internal reference had been added. T h e resulting chromatogram is shown in Figure 3 which indicates t h e complete absence of interference. I n no case was there any indication of interference with the GC of the organomanganese compounds from any hydrocarbon or other gasoline constituents. T h e long-term stability of the response of t h e dc argon plasma was investigated over a period of some hours. T h e results for the ratio of MMT-to-cymantrene in Table I indicate t h a t no notable variation was evident after 4 h, and the precision obtained was *3%. Experience has indicated that, for the same standard solutions, this reproducibility can be maintained throughout a working day. During long-term operation, the plasma electrodes are gradually consumed, and this slightly affects the spacial relationship of t h e discharge to the spectrometer entrance slit. However, the MMT-tocymantrene ratio was found constant for t h e concentration range of interest over a number of days.
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Table I. Reproducibility of Response of the dc Argon Plasma for Manganese Organometallics Lapsed time of operation of plasma, min
Response ratio
Gasolines
Mg/mL MMT
Unleaded gasoline 1 Unleaded gasoline 2 Unleaded gasoline 3 Unleaded gasoline 4 Unleaded gasoline 5 Premium leaded gasoline
1 2 0 (0.453)= 6 6 (0.249) 48 (0.181) 1 5 (0.057)
of MM’Ticyman-
trene standards
0 4 8 12 204 2 10 215 220 225
0. 56a 0.55 0.55 0.51 0.56 0.54 0.58 0.54 0.55
Average 0 . 5 5
*
Table 11. Measured Levels of Methylcyclopentadienylmanganesetricarbonyl(MMT) in Commercial Gasolines
Relative standard deviation * 3%.
Not detected Not detected
a Numbers in parentheses represent levels of MMT expressed in g MMT/gal.
Table 111. MMT Recovery Levels for Spiked Samples of Unleaded Commercial Gasoline Added, PPm
Found, PPm
0 28
0 28 68 95
IO 98
Recovery, %
i o o i i 31“
97 j t 3 j a 96 (*3)a
Relative standard deviation is based on five replicate injections.
C Y M A N TRENE
d
0
30
90 pgjn-i
6C
1 2 0 150 180
MMT
Figure 4. DC plasma emission detector peak height response for MMT
as a function of MMT/cymantrene ratio. Detection at 279.83 nm (Each calibration point 0 represents three replicate 2-pL injections of isooctane solution). Response ratios for commercial gasolines represented as 0
A typical response curve for MMT using the Mn 1279.83 nm line is given in Figure 4. This resonance line is among the more sensitive lines for dc argon emission of manganese (22). Linearity is observed over the full range of analytical interest, the upper limit being taken as the injection of 2 pL of a 170 wg/mL solution of M M T (Le., 340 ng MMT). All commercial gasoline samples examined fell well below this limit for MMT. The flame ionization detection response for M M T standards is also linear over the same sample size range. Typical relative standard deviation based on the repetitive injections for standards and samples ranged between =kO.8% and =k3.4% depending on the degree of the plasma stability. T o obtain good precision, all parameters which affect the plasma stability, notably the various gas flow rates, were carefully controlled. Results for M M T obtained for a number of commercial gasoline samples are summarized in Table 11. The M M T levels found ranged from 15 pg/mL (0.057 g MMT/gal) to 120 pg/mL (0.453 g MMT/gal), and are consistent with the range of M M T reported as added to commercial gasolines. T o test recoveries of MMT from gasolines and to show that the method was reliable, a commercial leaded gasoline containing no detectable amounts of M M T was “spiked” with MMT. Data obtained (Table 111) indicates good M M T recovery over a wide range. In Figure 5 a chromatogram illustrates the minimum dc plasma signal for M M T under conditions so far established,
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Figure 5. Detection limit for MMT with dc plasma emission detection, 15 pL of 0.8 kg/mL MMT solution in isooctane (ca. 3 ng of manganese)
corresponding to 1 2 ng M M T (ca. 3 ng manganese metal) injected. For trace analysis of M M T or cymantrene, larger injections can be made with no obvious reduction in chromatographic efficiency, since interference is minimal for volumes as large as 100 pL of gasoline, when, except for a somewhat larger negative solvent response, the baseline returned to zero well prior to the elution of the organometallic compounds. The overall method for gasoline requires no sample preparation other than the addition of internal references. The method is fast since samples may be run a t 3-min intervals, interference-free and specific for MMT. Analysis time comparison is favorable with respect to atomic absorption analysis which typically requires about 15 min per sample when run in batches. Further, the procedure is readily adapted for use with open tubular columns (21). Porous layer open tubular columns (PLOT) are best suited to accommodate the direct injection of gasoline samples. The improvement in analysis time for MMT determination by the latter method may outweigh the somewhat poorer limit of detection resulting from limitations on the sample size.
ACKNOWLEDGMENT We thank William G. Elliott for making available the echelle spectrometer-dc argon plasma system and for helpful dis-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
cussions regarding their operation. We also are grateful to Robert J. Lloyd and Bruce D. Quimby for help in the interface design.
LITERATURE CITED (1) G. B. Kyriapoulos, J . Inst. Pet., 54,369 (1968). (2) D. R. Bailey, F. J. Cordera, and R. G. Tuell, Belg. Patent 613 117; Chem. Abstr., 59, 7289b (1963). (3) A. T. Roife, U S Patent, 3443916; Chem. Abstr. 71,24 558e (1969). (4) J. W. Hwang, Anal. Chem., 44, (14), 20A (1972). (5) Chem. Eng. News, 55 (26),21 (1977). (6) G. Via, Exxon Corporation, personal communication. (7) S. K. Kyuregyan and M. M. Marenova, Zh. Prikl. Spekfrosk., IO, 313 (1969). ( 8 ) G. W. Smith and A. K. Palmy, Anal. Chem., 31, 1798 (1959). (9) V. A. Korovin, E. F. Kotenko, L. G. Mashiren, and Z. T. Yunusov, Zavod. Lab., 41, 1093 (1975). (10) R. J. Lukasiewicz and B. E. Buell, Appl. Spectrosc., 31, 541 (1977). (11) W. K. Robbins, Anal. Chem., 46,2177 (1974). (12) G. C. Everett, T. S. West, and R. W . Williams, Anal. Chim. Acta, 70, 204 (1974). (13) D. J. von Lehmden, R. H. Jungers. and R. E. Lee Jr., Anal. Chem., 46, 239 (1974).
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(14) G. N. Turkel'taub. B. M. Luskina, and S. V. Syavtsillo, Khim. Tekhnol. Topliv. Masel. 12, 58 (1967). (15) R. J. Lloyd. R. M. Barnes, W. G. Elliott, and P. C. Uden, 28th Pittsburgh Conference on Anatytical Chemistry and Applied Spectroscopy, Clevehnd, Ohio, March 1977, abstract 161. (16) P. C. Uden, R . M. Barnes, I. E. Bigley, W. G. Elliott, R. J. Lloyd, and B. D. Quimby. 174th National Meeting, American Chemical Society, Chicago, Ill., August 1977, abstract Anal. 73. (17) R. M. Barnes, W. G. Elliott, R. J. Lloyd, and P. C. Wen, submitted to Anal. Chem. (18) 0 . E. Ayers, T. G. Smith, J. D. Burnett, and B. W. Ponder, Anal. Cbem.. 38, 1606 (1966). (19) P. C. Wen, D. E. Henderson, F. P. DiSanzo, R. J. Lloyd, and T. Tetu. 174th National Meeting, American Chemical Society, Chicago, Ill., August 1977, abstract, Anal. 17. (20) H. Veening, N. J. Graver, D. G. Clark, and B. R. Wilieford, Anal. Chem., 41, 1655 (1969). (21) P. C. Uden and F. P. DiSanzo. unpublished observations. (22) Spectraspan 111, information bulletin, Spectrametrics Inc., Andover, Mass.
RECEIVED for review February 15, 1978. Accepted March 22, 1978. This work was supported in part by t h e National Science Foundation through Grant CHE73-05201.
Operating Conditions for the Determination of Sucrose by Capillary Gas Chromatography D. Nurok"' and T. J. Reardon' Sugar Milling Research Institute, University of Natal, King George V Avenue, Durban, South Africa
The variables affecting precision in the determination of sucrose as its trimethylsilyl derivatives on a 40 m X 0.5 mm open tubular column are described. A relative standard deviation of between 0.01% and 0.10% is attainable if a sample is injected three or four times in rapid succession before commencing a series of analyses and if the correct operating conditions are selected. Flow rate may be adjusted to give an analysis time between about 0.4 min and about 5 min without adversely affecting precision.
T h e determination of sucrose in sugar cane juice and in sugar factory products is of great importance to the sugar industry. T h e methods of analysis t h a t are currently used are polarimetry and variations of a method based on the determination of reducing sugars after t h e hydrolysis of sucrose. A systematic error exists in both of these methods due to the presence of other sugars in the products analyzed. Several other methods have been proposed (1-3), including gas chromatography (4-3, whereby sucrose has been separated on packed columns as the trimethylsilyl derivative. We have shown that open tubular columns are suitable for the analysis of sucrose in sugar factory products ( 8 ) and in a recent note (9) reported t h a t a high precision may be obtained for this analysis. T h e relative standard deviation for nine replicate chromatographic determinations of t h e same derivatized sample was under 0.17%. Five derivatives prepared from the same aqueous sample of pure sucrose and trehalose all had means within 0.1% of each other. T h e only reports t h a t we have been able to find of comparable analytical precision have been for simple gases chromatographed on packed columns 'Present address, Department of Chemistry, University of Houston, Houston, Texas 77004. 2Presentaddress, Koch-Li ht Laboratories, Analytical Department, Hollands Road, Haverhill, iuffolk, England. 0003-2700/78/0350-0855$01 0010
(10, 11). As the level of precision described here is greater than t h a t generally considered obtainable by gas chromatography, a detailed description of t h e factors t h a t effect i t are discussed. I t is expected t h a t these should be relevant t o other classes of compounds as well.
EXPERIMENTAL Chromatography. A Perkin-Elmer Model 3920 gas chromatograph fitted with an inlet splitter and flame ionization detector was used. The restrictor used gave a split of approximately 1 O : l at 20 psi. The column used was a 40 m X 0.5 mm stainless steel open tubular column (Handy & Harman Tube Company) coated by the plug method with OV 17 (Applied Science Laboratories, Inc.) using benzyltriphenylphosphonium chloride (Aldrich Chemical Company) as a wetting agent. Unless otherwise specified, the inlet pressure was set a t 20 psi, the inlet temperature at 360 "C, and the interface at 300 "C. The column temperature was set to give a retention for the trehalose peak of about 1 min. This was between 240 and 255 "C for different columns of nominally the same specification. Compressed air was used for the flame ionization detector fuel gas and hydrogen was used for both carrier and fuel gas. Flow rates were measured with a bubble flow meter at room temperature. Silylation. The method of silylation has been described elsewhere (9).
RESULTS AND DISCUSSION T h e chromatographic determination of sucrose as t h e trimethylsilyl ether, using trehalose as a n internal standard, may be rapidly performed on a 40 m x 0.5 mm open tubular column coated with OV 17. T h e analysis time is over three times shorter than the fastest of t h e reported analyses using packed columns. The precision found is high and is affected by several operating parameters t h a t are discussed below. The determination of sucrose is according to the following equation:
C 1978 American Chemical Society
