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Anal. Chem. 1981, 53, 475-477
Determination of Carboxylic Acids in Oil Samples by Capillary Gas ChromatographyIMass Spectrometry Jacob Shen' The Standard Oil Company (Ohio), Research Department, 4440 Warrensville Center Road, Cleveland, Ohio 44 128
A combined gas chromatography/mass spectrometrlc (GC/ MS) method for measuring Carboxylic acids in oil samples without flrst going through solvent extraction and group separation is reported. The carboxylic acids in oils are directly derivatired to their corresponding methyl esters via anion formation In tetramethylammonium hydroxide/methanol/ methyl lodide/n-butyl acetate solutlons prlor to GC/MS analysts using a glass wall coated caplliary column. The reaction is mlld, selecthe, and rapid. It can usually be carried out at room temperature and completed in 10-15 mln. Multlple Ion detectlon technlques (MID) can be readily used to further resolve methyl esters from other components if necessary.
The characterization of organic acids in heavy end petroleum products often necessitates solvent extraction and group separation steps prior to more detailed composition analysis by mass spectrometry or combined gas chromatography/mass spectrometry (GC/MS). These extraction and separation procedures are usually tedious and time consuming. The time factor is almost prohibitive if a large number of heavy end petroleum samples are to be analyzed for carboxylic acids. A method for direct analysis of carboxylic acids in petroleum products is thus highly desirable. The analysis of carboxylic acids by GC/MS is often facilitated by preparing their corresponding alkyl esters which in general give much better chromatographicproperties (1-3). For example, transesterification under either acidic or alkaline conditions has been useful for ester preparation (4-7). Alternately, saponification followed by methylation has also been reported as an effective way to prepare methyl esters (8,9). Literature reports on direct esterification of carboxylic acids in heavy end petroleum products, however, have been few. Alkaline transesterification is usually limited to high acid number samples while acid-catalyzed transesterification is often itself a long process. It is also difficult to apply anion formation procedures via saponification and methylation due to general low solutility of petroleum products in methanolic solutions. A mild and rapid method for preparing alkyl esters from acids via anion formation has been reported by Greeley (10). The method is virtually quantitative, involving no use of acidic reagents such as HCl or H2S04,and takes less than 10 min for sample preparation. Saponification first takes place in a methanolic solution of tetramethylammoniumhydroxide and methylation by methyl iodide then proceeds in a polar solvent such as N,N-dimethylformamide. The method has been further studied and extended by West (11)who found that a more polar solvent such as butyl acetate was required for alkyl esters of phthalic acids. We have found that the method can be readily applied to badly degraded petroleum products 'Current address: ARAMCO, P.O.Box 8745, Dhahran, Saudi Arabia. 0003-2700/81/0353-0475$01 .00/0
having high acid numbers. When applied to normal heavy end petroleum samples having low acid numbers, the poor sample solubility in a methanolic solution of tetramethylamonium hydroxide still presents a problem. We report that such a problem can be overcome by using an organic polar cosolvent such as n-butyl acetate and that the modified method can be easily applied to analyze acids in regular petroleum products.
EXPERIMENTAL SECTION Chemicals. Both N,N-dimethylformamide and tetramethylammoniumhydroxide were from Pfaltz and Bauer. Methyl iodide, oleic acid, and n-butyl acetate were from MC/B Co.; 24% tetramethylammoniumhydroxide in methanol was used in saponification. Procedures. Typically about 0.2 mL (approximately 200 mg) of petroleum product was weighed into a graduated test tube (15 X 120 mm). n-Butyl acetate (0.8 mL) was added to dissolve the sample. Methanolic tetramethylammoniumhydroxide 24% (1 mL) and methanol (1 mL) were added, and the mixture was shaken. n-Butyl acetate (4 mL) was then added and the solution became clear. The solution WBS warmed in a hot water bath for 5 min. methyl iodide (1 mL) was added afterward. Tetramethylammonium iodide precipitated on standing. The supernatant solution (1 pL) was injected directly into the GC/MS system. When a petroleum product sample was badly degraded and had a high acid number, it dissolved readily in a 24% solution of tetramethylammonium hydroxide in methanol. Saponification thus could take place directly in the methanolic solution without n-butyl acetate as the cosolvent. GC/MS Analysis. A Finnigan 4023 GC/MS/computer system was used in this work. A 50 m X 0.02 in glass SP-lo00wall-coated open-tubular (WCOT) column was used for GC separation. The oven temperature was programmed from 100 to 210 O C at 6 OC/min. The injector temperature was 350 O C . Typically 1p L of sample was injected in splitless mode. When multiple-ion detection (MID) technique, which moniters specific Components in a mixture by monitoring ions that are characteristic of these components, was used for quantitative analysis of methyl oleate in petroleum samples,the molecular ion at m/z = 296 was selected. The ion was virtually free of all interferences. MID analysis was carried out at a colummn temperature of 210 O C isothermally. Each time 1pL of sample solution was injected at a split ratio of 751.
RESULTS AND DISCUSSION It has been extremely time consuming to analyze acids in a large number of heavy petroleum samples. Direct analysis by GC is difficult in spite of the availability of columns for analysis of free acids in aqueous solutions. For example, Figure 1 shows a reconstructed ion chromatogram of a dewaxed petroleum oil (boilingrange 400-800 O F ) which contains about 1% (wt.%) oleic acid obtained with a 30-m glass SP-lo00 WCOT column. Oleic acid is only observed as a small shoulder. Figure 1further shows that meaningful GC quantitation of fatty acids in such a sample matrix is difficult at best. When the sample is saponified and methylated according to the procedures described above, methyl oleate can be readily identified under similar GC conditions. This is shown in 0 I981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
I
1
200
400
600
800
1000
I3 20
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b
2000 1640
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20w 26 40 3320 TIME Figure 1. Reconstructed ion chromatogram (RIC) of a dewaxed oil sample containing oleic acid. 6 40
E" = = 0 -
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Flgure 3. Quantitativeanalysis of methyl oleate in petroleum 011 by the GC/MS MID technique vh the monkoring of the m l z = 296 Ion. A total of 15 sample injections were made in 30 min.
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Flgure 4. GC/MS Mentlfication of carboxylic acids in badly degraded petroleum oil that can be directly dissolved in methanolic tetramethylammonium hydroxide solutions. Note that acMs instead of hydrocarbons have become predominant components In the sample.
Flgure 2. (a)RIC of a dewaxed oil sample that has been saponified and methylated. Methyl oleate is readily identified along wkh other methyl esters. (b) Single ion chromatogram of the Same Sample by monitoring the molecule ion of methyl oleate at m l z = 296. Note that there is no interference whatsoever from other compounds. Figure 2a. In addition, a small amount of methyl esters of undecanoic acid (I), 14-methylpentadecanoicacid (111, and 9-hexadecenoic acid (111) are also observed, indicating the presence of these esters in the dewaxed oil sample. Quantitative analysis can often be facilitated by the use of MID techniques. For example, Figure 2b is a single-ion chromatogram of the molecular ion of methyl oleate at m / z = 296. A comparison between Figure 2a and Figure 2b shows that methyl oleate is the only component in the sample that yields a significant ion at mlz = 296. Quantitation of the
amount of methyl oleate in the sample by monitoring the molecular ion at m / z = 296 is virtually free of interferences from any other components in the sample. Furthermore, rapid quantitative analysis of a whole batch of samples by MID can be accomplished by injecting the samples in immediate succession without having to wait for the completion of the first GC run. Figure 3, as an example, shows that a total of 15 sample injections can be made in 30 min and that GC/MS analysis of the entire series of sample can be completed in less than an hour. This represents an average of 2 min per sample while a regular GC or GC/MS run will usually take an hour. That alkyl esters can be quantitatively prepared from their corresponding acids by this method has been reported previously ( 1 0 , l l ) . Our modified procedures have not altered the quantitative nature of the method. Recovery efficiency of acids from heavy end petroleum samples is good. Table I shows that various amounts of oleic acid spiked into the dewaxed petroleum oil can be recovered as methyl oleate quantitatively by using the esterification procedures described. Quantitative GC/MS analysis was carried out by MID via the monitoring of the m / z = 296 ion. When badly degraded oil samples that have high acid numbers are to be analyzed, they can generally be dissolved directly in methanolic tetramethylammonium hydroxide solutions. It is therefore not necessary to first dissolve the sample in n-butyl acetate. Figure 4 shows the acids identified
477
Anal. Chem. 1981. 53,477-480
spiked into an oil sample which was then subjected to the entire esterification procedures, essentially all the alcohol was recovered.
Table I. Analysis of Oleic Acid in Dewaxed Petroleum Oil" amt of re1 int oleic acid re1 amt of areab of amt of spiked, mg oleic acid m/z = 296 oil, mg 13.5 1.o 1.o 167.7 12.1 0.9 0.9 0.6 0.6 161.7 7.6
LITERATURE CITED Drozd, J. J . Chromatogr. 1075, 113, 303-305. Sheppard, A. J.; Iverson, J. L. J . Chromtcgr. Scl. 1075, 73, 448-450. Krupcik, J.; Hrivrak, J.; Janak. J. J . Chromtcgr. S d . 1076, 14, 4-7. Luddy, F. E.; Barford, R. A.; Herb, S. F.; Megktem, P. J . Am. 011 Chem. Soc. 1068, 45, 549-552. Chrlstopherson, S. W.; Glass, R. L. J . Dslry Sc1. 1060, 52, 1289- 1290. Blank, M. L.; Verdino, 8.; Drtvett, 0. S. J. Am. 0llChern. SOC.1065, 42, 87-90. Peisker, K. V. J. Am. 011 Chem. Soc. 1064, 41, 87-92. Matcalf, L. D.; Schmitz, A. A.; Peke, J. R. Anal. Chem. 1066, 38, 5 14-51 7. Lloyd, J. 0 . F.; Roberts, B. R. 0. J . ChrOmatOgr. 1073, 77, 228-231. (;rWieY, R. H. J . Chromstwr. 1074, 88, 229-231. West, J. C. Anal. Chem. 1075, 47, 1708-1711.
1 pL of sample injected each time at a split ratio of 75:l. A 30-m glass SP-1000 WCOT column was used and the column temperature was kept at 210 "C isothermally. An average of five runs.
in a used petroleum oil sample. Clearly a series of n-carboxylic acids from C7 to C20 are the predominant components present in this used oil sample. We have also found that the esterification procedures described above are very selective. For example, other oxygenated species such as fatty alcohols and aldehydes in the same used oil sample were not altered. When octadecenol was
RECEIVED for review May 28,1980. Accepted November 6, 1980. Presented a t the 28th Annual Conference of Mass Spectrometry and Allied Topics, New York, May 2630,1980.
Identification of Macrocyclic Polyamines and Macrocyclic Dioxopolyamines by Thin-Layer Chromatography, Gas Chromatography, and Electrophoresis Takashl Yatsunaml, Atsuko Sakonaka, and Elichi Kimura' Institute of Pharmaceutical Sciences, Hiroshima University School of Medicine, Kasumi, Hiroshima, 734, Japan
Simple and rellable ldentlflcatlon methods for Synthetic macrocyclic polyamlnes and macrocycllc dloxopolyamlnes of chemical slgnlflcance have been establlshed by using thlnlayer chromatography, gas chromatography, and electrophoresis. The following TLC systems were effectlve: (a) butanol-acetic acld-water (4:1:5) on alumlna and (b) chloroform-methanol-28 % ammonia (2:2:1) on silica gel. Macrocyclic polyamlnes were also analyzed by GC on a Aplezon L or a Thermon 1000 KOH column or by electrophoresls uslng buffers contalnlng dl- or trlcarboxylates at pH -6. Anomalous electrophoretlc behavior of some macrocycllc polyamines mlgratlng to anode dlrectlon In cttrate buffer solutlon at pH 6 was dlscovered.
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The chemistry of synthetic macrocyclic polyamines and macrocyclic dioxopolyamines L1-L19has been drawing much interest (1-7). These macrocycles form much more stable and selective complexes with various transition-metal ions than do open chain analogues having the same donor arrangement. The macrocyclic complexes present great potential as models of metalloenzymes (8-11), sequestering agents for specific metal ions ( 4 , 6 ) , synthetic reagents (12), or biomedical applications (13). However, analytical methods for detection and identification of these macrocycles have not been well established, which very often hamper their further application. We report here simple and convenient methods for qualitative and quantitative analyses of various macrocyclic polyamines and macrocyclic dioxopolyamines using thin-layer (TLC) and gas chromatography (GC) and electrophoresis. 0003-2700/81/0353-0477$01.00/0
The inherent difficulties in analysis of these compounds are partly ascribed to their highly polar and basic characters caused by more than three amine and amide groups. The success of the present work relies upon the careful selection of solvents (butanol-acetic acid-water (4:1:5) or chloroformmethanol-28% ammonia (2:2:1)) in TLC, liquid phases (Apiezon L or Thermon lo00 + KOH) in GC, or electrolyte solutions (di- or tricarboxylates, pH -6) in electrophoresis, whereby the macrocycles having only slightly deviating structures are well separable from each other. The present methods can also distinguish macrocycles from closely related linear polyamines and the group of macrocyclic polyamines from those of macrocyclic dioxopolyamines,respectively. This is quite significant, since the former are very often synthesized from the latter as starting materials.
EXPERIMENTAL SECTION Materials. The macrocyclic triamines L1 (as L1.3HCl) and L2 (as L2.3HBr) were prepared by the method of Koyama and Yoshino (14). The macrocyclic tetramines L,-L8 (as 4HC1 salts), and hexamine L13 (as L1&HCI) were synthesized by the method of Martin et al. (15). The macrocyclic triamine L3 (as L3.3HBr), tetramine (as b.4HBr) and Ll0 (asLlo-4HBr),and pentamine Lll (as L11.5HCI)and L12 (as L12.5HBr)were prepared according to the methods described before (13, 16). All the macrocyclic dioxotetra- and dioxopentamines were obtained according to the slightly modified procedures by Tabushi et al. (17)and Kato (18). Their structures and purities were checked by elemental analysis, 'H NMR, and melting point. Free bases of L1-L13 were obtained as follows. The amine salts were dissolved in water, basified with 2 N NaOH, and extracted with chloroform four times. Combined chloroform layers were dried with sodium sulfate and evaporated to dryness. All the other solventsand chemicals were of analytical 0 1981 American Chemical Society
