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Anal. Chem. 1084, 56,1808-1810
chromatogram. Chromatographic separations were performed by using a size exclusion column which allowed the elution of all chain lengths of the derivatized PAPI as a single peak, thus optimizing sensitivity and further simplifying the analysis.
ACKNOWLEDGMENT The aid of R. K. Swanson is gratefully acknowledged in work on field testing of this method. Also acknowledged are useful comments by J. W. Worley and helpful discussions with J. L. Barclay. Registry No. PAPI, 9016-87-9; NBPA, 62869-14-3. LITERATURE CITED (1) Woodrich, P. F. A m . Ind. Hyg. Assoc. J. 1982,4 3 , 89-97. (2) Anderson, M. et at. Scand. J. Work Envlron. Health 1980, 6 , 221-226. (3) "Criteria for Recommended Standard...Occupational Exposure to Diisocyanates"; U.S. Department of Health, Education, and Welfare,
National Institute for Occupational Safety and Health: Washington.
DC, 1978.
(4) Reilly. D. A.. Analyst (London) 7968, 93, 178 - 185. (5) Levine, S. P.; et ai. Anal. Chem. 1979,57, 1106-1109. (6) Goidberg. P. A.; et ai. J. Chromatogr. 1981,272,93-104. (7) Meddle, D. W.; Wood, R. Analysf (London) 1970,95, 402-407. (8) Graham, J. D. J. Chromatogr. Scl. lQ80,78, 384-387. (9) Sango, C. J. Llq. Chromatogr. 1979,2 , 763-774. (IO) Keiier. J.; et ai. Anal. Chem. 1974,4 6 , 1845-1846.
Anderson, K.;et al. Chemosphere 1081, 7 7 , 3-10. Tucker, S. P.; Arnold, J. E. Anal. Chem. IQ82,5 4 , 1137-1141. Hastings Vogt, C. R.; et al. J . Chfomatogf. 1977, 734, 451-458. Beasiey, R. K.; et al. Anal. Chern. 1980,52, 1110-1114. (15) Hiieman, D. G.; et al. "Documentation of the NIOSH Validation Tests"; DHEW (NIOSH) Publication No. 77-165, Cincinnati. OH, 1977. (16) Grubbs, F. E.: Beck, G. Technomefrlcs 1972, 74, 847-854. (17) Bethea, R.; et al. "Statistical Methods for Engineers and Scientists"; Marcel Dekker: New York, 1975; pp 247-251. (18) Hiieman, B. Environ. Sci. Techno/. 1981, 75, 983-986.
(1 1) (12) (13) (14)
RECEIVED for review January 16, 1984. Accepted March 26, 1984.
Determination of 1-Methyl-2-pyrrolidone in Refinery Hydrocarbons and Waters by Gas Chromatography Robyn Stephens Whiting Laboratory, Amoco Oil Company, 2831 Indianapolis Blvd., Whiting, Indiana 46394
A new procedure has been developed for the trace and percent level determination of I-methyl-2-pyrrolldone (NMP) In water and heavy oil reflnery feedstocks. By extraction of the NMP into an aqueous phase, analysls can be quickly performed on a gas chromatograph equipped wlth a flame Ionization detector and a nitrogen-phosphorus detector. Recycle and cleanup tlme Is held to a mlnlmum as heavy hydrocarbons are never directly analyzed. Qulck analysls and accurate results provlde the data necessary for optimal operation of the NMP extraction unit In the reflnery. Percent and parts-per-mllllon results are repeatable to 0.1 % and 2.0 ppm. The large repeatability for part-per-mllllon determlnatlons Is due to NPD lnstabllity and contamination of the gas chromatograph's Injector.
The use of 1-methyl-2-pyrrolidone, also known as N methylpyrrolidme (NMP), as an industrial solvent is commonplace. One of its primary uses in oil refining is for extracting aromatics (extract) from refinery heavy oil feeds to produce a stable, high-viscosity-indexlubricating oil base stock (raffinate). Its preferential use is due to its low toxicity and high efficiency. However, its cost is almost four times greater than other less safe and less selective solvents. This makes imperative the minimizing of solvent losses in the extraction process. Several techniques already exist for measuring NMP at the parts-per-million and percent level in water ( I ) . Recently Frick reported NMP determination by HPLC (2). However, the very viscous and heavy raffinates and extracts found in refinery processes create major problems in developing a method for determining parts-per-million levels of NMP. Table I lists common physical properties for the lightest and heaviest of these refinery hydrocarbons. GC methods have been reported, but they have dealt with hydrocarbon matrices
Table I. Physical Properties of Heavy Oil Feedstocks
SAE-5 SAE-40
viscosity, cSt
flash point (COC),"F
100 "F
385
24-25
505
210 "F
av mol wt 370
19-20
500
of low molecular weight ( 3 , 4 ) . IR and UV methods also have been reported ( 5 , 6 ) ,but these methods have limitations of relatively clean hydrocarbon matrices, long analysis time, or extensive sample preparation and cleanup. Miskovich has shown that trace level NMP can be quickly and accurately determined by dissolving extracts or raffinates in hydrocarbon solvents and analyzing the solutions by gas chromatography using a nitrogen-phosphorus detector (NPD) (7). However, after several analyses, detector response from the elution of the high molecular weight hydrocarbons forces purging the column at elevated temperatures for several hours to remove these hydrocarbons and stabilize the background interference. This paper reports a new, short, and accurate method of determining NMP in hydrocarbons and water a t the partsper-million and percent levels without the constraints of long sample preparation or column purging or complicated backflush systems. Known weights of raffinate or extract samples dissolved in heptane or xylenes are extracted with weak HC1 solutions and directly analyzed on a gas chromatograph that is equipped with an NPD. By use of external standards, concentrations in the aqueous phase are directly determined and concentrations in the original hydrocarbons are backcalculated. Water samples are analyzed with no sample preparation.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Sigma 2000 dual channel gas chromatograph was equipped with a flame ionization detector
0003-2700/84/0356-1608$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9,AUGUST 1984
1609
NMP
I
.*_.
I
. , . J
..
. d ,
,/.--.
____-__ I I
..
. \-
3-
I
2
_--
I
3
MIN
Figure 1. Gas chromatogram of parts per million of NMP
in water
Flgure 2.
Gas chromatogram of 5.00% NMP in water.
extract of raffinate. (FID) and nitrogen-phosphorus detector (NPD). The FID is used for percent level and the NPD for parts per million level concentrations. A routine in the microprocessor that controls the chromatograph reequilibrates the NPD after each run. Each detector was connected to a column (3 ft long, 1/8 in. o.d. nickel) packed with 80/100 mesh Chromosorb 103. The flow rates of the helium carrier gas in each column were maintained at 25 mL/min with automatic flow controllers. Both injection ports were maintained at 250 "C while both detectors were maintained at 300 OC. NPD bead current was maintained at 10 PA. Determinations were run isothermally at 230 "C. Using similar, but separate, columns under the same operating conditions eliminates system contamination problems while maintaining similar elution times for NMP. At first, a strip chart recorder was used for peak integration by triangulation, but later the chromatograph was connected to an Apple IIe computer with an Isaac 42-A chromatography system from Cyborg, Inc. This system provided report writing capability while improving integration precision and accuracy. Reagents and Materials. NMP and all hydrocarbons used as solvents were reagent grade. Deionized water was prepared by passing distilled water through a Milli-Q system from Millipore, Corp., for removal of inorganics, hydrocarbons, and particulates. The '/* in. nickel tubing and SO/lOO mesh Chromosorb 103 were purchased from Alltech Associates. Columns were packed by drawing a gentle vacuum through the tubing with gentle agitation. Helium and hydrogen of 99.999% purity were purchased from Air Products, Inc., and required no additional purification. In place of zero air, plant air was used after water, hydrocarbons, and particulates were removed by a heatless dryer from Pure Gas, Inc., Westminister, CO. Procedure. Percent and parts-per-million standards of NMP in water were prepared and used to establish response factors and detector linearity. Injection sizes were held constant at 1.0 pL. A 20.0 ppm standard of NMP in xylenes was prepared and analyzed on the chromatograph. A 10.0-g portion of the standard was extracted with 10.0 mL of 0.02 M HCl to measure the distribution ratio of NMP when extracted into an aqueous phase. Extractions were performed by weighing the hydrocarbons into a 100-mL beaker, pipetting 10.0 mL of 0.02 M HC1, and mixing the two phases vigorously with a magnetic stirrer. This method not only ensured precise quantitation of the weight of hydrocarbon sample but allowed for quick and easy cleanup. Analysis of the aqueous phase showed it to be 20.0 ppm NMP. The hydrocarbon phase was also analyzed and found to contain less than 100 ppb NMP. Samples of raffinate and extract free of NMP were obtained and dosed to various concentrations of NMP. Approximately 5.00 gm of each raffinate and extract were similarly prepared and analyzed as above, except they were first dissolved in 15 mL of heptane or xylenes respectively under gentle heat before the acid extraction. Percent level samples of NMP in water were prepared and analyzed directly using the FID.
RESULTS AND DISCUSSION Figures 1 and 2 show typical chromatograms of a partper-million and percent level determination. The integrator
Table 11. Statistical Evaluation of Parts-per-Million Determinations actual concn 0.6 5.0 10.0
20.0
40.0 100.0
determined concn 0.0, 0.0 4.9, 5.2 9.9, 10.3 20.8, 19.7 41.3, 40.0 98.9, 100.3
std dev = 0.65 repeatability = 2.0 L .
Table 111. Statistical Evaluation of Percent Determinations actual concn
determined concn
0.0
0.0, 0.0
1.00
1.04, 1.00 4.99, 5.10
5.00 10.00 15.00
9.91, 10.08 15.14, 14.99
std dev = 0.081 repeatability = 0.3
sampled data 10 times a second. Figures 1 and 2 show only a small fraction of the total number of data points. The initial fluctuation in Figure 1is from the elution of water. The NPD detector is rezeroed at 1.75 min to stabilize the detector. The nonideal peak shape in Figure 2 is due to column overloading. This was found to slightly affect precision, but results were still within acceptable limits for efficient operation of the refinery extraction process. Minimum detection was found to be below 100 ppb and 0.01%. These limits were well below what was needed for the type of samples to be routinely analyzed. Tables I1 and I11 show the actual and determined partsper-million and percent concentrations of NMP in the water and hydrocarbon samples. The repeatabilities of the methods were calculated to be 2.0 ppm and 0.3%. Several minor problems encountered in routine partsper-million operation contributed to its high repeatability. The injection port was found to accumulate a great deal of dirt and hydrocarbon residue from water samples and extractions as well as small bits of septum. Small amounts of NMP would lay down on this material and would be released in various amounts during injections giving false peaks and readings for water blanks and samples. Occasionally cleaning the injection port removed the false readings. This involved bringing down the system to room temperature to avoid damaging the NPD bead, cleaning the injector, starting the chromatograph back up, and recalibrating. This whole procedure takes 2-3 h. For several hours afterward, the NPD
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Anal. Chem. 1984, 56, 1610-1615
would be upset until it reached its original level of sensitivity. To keep errors at a minimum during this time period, the chromatograph’s calibration is checked before analysis of a sample. Syringe contamination problems c a w d by previous q d y s i s of high parts per million samples gave false peaks during blanks and samples. Disassembling the syringe and w&hing each part separately with acetone and concentrated HC1 usually removed any contamination. The most frequent problem was day to day instability in the NPD. Minor upsets would affect sensitivity and the response factors. Checking the calibration before a series of runs proved effective in resolving this problem.
Wilson, Roberta Wentz, and Carol Yates for their constant assistance and guidance in the development of this method. Registry No. NMP, 872-50-4; H20, 7732-18-5. LITERATURE CITED (1) Yakovleva, T. P.; Vall, E. I. Vopr. Tekhnol. Ylavlivanja ferefab, Produktov Koksovanlya 1979, (8), 83-87. (2) Frick, Darryl A. J . Ll9. Chromatogr. l983, 6 , 445-524. (3) Belder, T. B.; Kuz’mlnskaya, M. D. Mefody Anal. Konfrolya Kach. Plod. Khlm. fromsfi. 1978, (e), 1-3. (4) Masalova, L. S.; Kedrina, N. N.; Sstavrati, V. I . Khlm. fromsf., Ser.: Mef@‘Anal. Konholye Kach. Rod. Khlm. fromsfi. 1980, (6), 21-24. (5) Ned, J. W., Standard 0 11 Co. (Ind.), Napervllle, IL, unpubllshed work, 14 June 1982. (6) Mosescu, Nicolae; Stejaru, Deea; Dalmutchi, George Rev. Chlm. (Bucharest) 1977, 28 (3), 272-274. (7) Mlskovich, J. J., Standard 011 Co. (Ind.), Napervllle, IL, personal communlcatlon, 20 June 1983.
ACKNOWLEDGMENT The author thanks Terry Adams, Shelly Arispe, Jerry Phillips, Robert Smead, Rick Solan, Patti Stasik, Regina
RECEIVED for review March 1, 1984. Accepted April 5, 1984.
Structural Characterization of Polycyclic Aromatic Compounds by Combined Gas Chromatography/Mass Spectrometry and Gas Chromatography/Fourier Transform Infrared Spectrometry Kin
S.Chiu a n d K l a u s Biemann*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Krishnaswamy K r i s h n a n a n d Steve L. Hill
Digilab, Division of Bio-Rad, Cambridge, Massachusetts 02139
The complernentarlty of the mass spectral and Infrared spectral data for the structural characterlzatlonof polycycllc aromatic compounds (PAC) Is demonstrated by the parallel analyses of two PAC mlxtures by gas chromatography/mass spectrometry and gas chromatography/Fourler transform lnfrared spectrometry. Whlle the electron impact mass spectral data provide unlque lnformatlon regardlng the molecular welghts of the compounds of Interest, the Infrared data often afford unamblguous dtfferentlatlon between the Isomers. Speclflc examples are presented In which the comblnatlon of the two techniques resulted In Identifications which could not have been possible using elther technlque alone.
Polycyclic aromatic compounds (PAC) have become an important class of environmental pollutants due to their widespread occurrence in fossil fuel combustion products and because of their mutagenic and carcinogenic potentials. In view of the structure dependence of the biological activities exhibited by these compounds, it is very important that analytical methodologies be developed to facilitate the positive identification of the individual compounds. Although gas chromatography/mass spectrometry (GC/ MS) has been extensively used for the identification of the PAC, there are limitations to the effectiveness of the mass spectrometer as a gas chromatographic detector for the positive characterization of these compounds. Because of their aromaticity, the electron impact ionization mass spectra of the PAC show predominently the molecular ion but very 0003-2700/64/0356-1610$01.50/0
limited fragmentation. The lack of structurally specific cleavages greatly hampers the differentiation of the structural isomers and even certain isobars from the mass spectral information alone. However, it is very important to positively identify these aromatic compounds because their mutagenicities may vary widely. In some cases, this can be accomplished by using the gas chromatographic retention behavior of the compound if it is known for the particular gas chromatographic column or if an authentic sample is available for calibration. A limiting requirement is the necessity to know the retention behavior of all the isomers and that they differ sufficiently to be clearly differentiated. Needless to say, the larger the molecules and the more highly substituted they are, the more isomers are possible, and soon the point is reached where it is not feasible to acquire all the necessary reference compounds. In order to achieve a higher level of confidence in the identifications of the PAC, it is necessary to use complementary techniques which are sensitive to the position of substitution and functional groups. We have therefore investigated the utility of gas-phase infrared spectrometry in conjunction with gas chromatography. Mamantov et al. (1) and Tokousbalides et al. (2) have already developed methodologies using matrix isolation Fourier transform infrared spectrometry (MI-FTIR) to directly identify PAC in simple mixtures. However, the efficiency of MI-FTIR is limited by the level of complexity of the samples, and a more promising approach involves prior separation of the components of the mixture before detection by the FTIR (3,4). With sensitivity in the submicrogram levels, an FTIR spectrophotometer can 0 1984 American Chemlcal Society
