Identification of Sulfonic Acids and Sulfonates by Mass Spectrometry Arthur Heywood, Alan Mathias, and Albert E. Williams Imperial Chemical Industries Limited, Dyestuffs Division, Blackley, Manchester, England
THESULFONIC ACID grouping HS03- occurs in many chemicals of commercial importance. It is introduced, sometimes free but often as the sulfonate, t o many species t o promote solubility. For example, a high proportion of dyestuff molecules contains one or more such groupings in its structure. Except for a few very simple derivatives, such as benzene sulfonic acid and the naphthalene monosulfonic acids, the sulfonic acids have very low volatility. Thus, one of the most informative techniques for the identification of unknown materials, namely mass spectrometry, cannot be used for their analysis. Ii' the acids occur as their sodium or potassium salts, which is very common, then the problem is magnified, since these salts have even lower volatility than the free acids. Often the problem of involatility can be overcome by converting the offending species into a more volatile derivative prior t o niass spectrometric examination. For example, mono- and di-basic carboxylic acids can be converted to their methyl esters ( I , 2) and involatile polyols can be examined as trimethylsilyl derivatives (3-5). Volatile derivatives of sulfonic acids include the esters and the sulfonyl chlorides. From a general chemical point of view, the greater stability of the esters makes them preferable and an initial examination of some methyl esters of various sulfonic acids, which were already available in these laboratories, showed that mass spectra could be obtained readily from these derivatives.
(2-3 ml) and treated with an excess of diazomethane in ether solution. The dried reaction product, the methyl ester, is ready for mass spectrometric examination. Diazomethane is prepared by the action of alcoholic KOH on an ethereal solution of N-methyl-p-toluene sulfonyl nitrosamide. (c). Preparation of the Tetramethylammonium Salt. The eluate from (a) is neutralized by the dropwise addition of a 10% aqueous solution of tetramethylammonium hydroxide. The solution is evaporated to dryness and the solid product retained for mass spectrometric examination. (d). Mass Spectrometric Examination of the Sulfonic Acid Derivatives. The derived solids from (b) or (c) above are examined using a mass spectrometer vacuum lock probe. In our experiments an A.E.I./G.E.C. MS-9 spectrometer was used, run under standard conditions. As the temperature of the sample in the probe is increased by heating the source block, the methyl esters yield spectra directly. The tetramethylammonium salts, however, first dissociate:-
RSOsN(CH3)4
(1) C. A. Genge, ANAL.CHEM., 31, 1750 (1959).
(2) K. Biernaim, J. Siebl, and F. Gapp, J. Amer. Chem. SOC.,83, 3795 (1961).
(3) A. G. Sharkey, R. A. Friedel, and S. H. Langer, ANAL.CHEM., 29, 770 (1957). (4) S. H. Langer, S . Connell, and I. Wender, J. Org. Chem., 23, 50 (1958). (5) B. T. Golding, R. W. Richards, and M. Barber, Tetrahedron Lett., 2615 (1964).
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RS03CH3
+ (CH3)aN
Thus trimethylamine is detected in the mass spectrometer when the sample temperature is high enough for dissociation to begin. At this stage, careful temperature control is required to prevent irregular dissociation. A mass spectrum of the dissociation products, Le., the required methyl ester of the sulfonic acid and trimethylamine, is now taken. RESULTS
EXPERIMENTAL
Two methods of preparation of the methyl ester were found to be generally applicable t o sulfonic acids (i) reaction with diazomethane and (ii) cia the tetramethylammonium salt. Both methods are equally effective, but the latter is more convenient. (a). Preparation of Free Sulfonic Acid. When the sample under investigation is a sulfonic acid salt, it is first converted to the free acid. Dissolve 0.1 g of sample in the minimum amount of distilled water and pour onto the top of a 50-ml ion exchange column. Amberlite IR 120-H resin, activated and washed until acid free in the usual manner, is a suitable packing material. New rather than re-activated resin should be used because of the small amount of sample involved. The column is eluted with distilled water until the eluate is acid free t o Congo Red paper. The eluate is now used for the preparation of the methyl ester or the tetramethylammonium salt of the sulfonic acid. If it is known that the sample under investigation is already a free acid, then obviously the above procedure can be omitted. However, in the examination of unknown materials, we include it as a matter of course. (b). Preparation of the Methyl Ester. The eluate from (a) is evaporated carefully to dryness, dissolved in methanol
+
Examination of Reference Sulfonic Acids. The ability to convert sulfonic acids of various types to either the methyl ester or the tetramethylammonium salt and then to obtain meaningful mass spectra using the experimental procedures described above, was tested on the following series of reference compounds. In all cases an attempt was also made to obtain a mass spectrum from the free acid. This failed invariably, although in some cases very weak spectra could be observed because of small amounts of corresponding unsulfonated materials, which are present in the sulfonic acids, presumably as trace impurities. (a) ~ D O D E C Y BENZENE L SULFONIC ACID. The methyl ester and the tetramethylammonium salt were prepared. Both gave good mass spectra of the ester, which showed a small molecular ion (m/e 340) and gave a fragmentation pattern typical of an alkyl benzene. (b) NAPHTHALENE-1,5-DISULFONIC ACID AND NAPHTHAL.ENE1, ~ , ~ - T R I S U L F O N I CACID. Both samples readily gave methyl esters and tetramethylammonium salt derivatives. The mass spectra obtained showed molecular ions at mje 316 and mje 410 and fragment ions corresponding to the successive loss of two and three S 0 3 C H 3groups, respectively. (C) 2-CHLORO-ANILINE-4-SULFONIC ACID AND 3,4-D1CHLOROANILINE-~-SULFONIC ACID. The tetramethylammonium salts were prepared from these two compounds. The mass spectra showed small molecular ions and ions corresponding to loss of OCH3from the parent. The largest peak in the spectra, however, corresponded t o the chloro-aniline nucleus and ions were also present which indicated that there had been partial monoand dimethylation of the NH2group.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
(d) ANTHRAQUINONE-1 -SULFONIC ACID AND ANTHRAQUIBoth derivatives were prepared satisfactorily. The mass spectra obtained indicated only a small molecular ion, but the ion formed by loss of OCH3 from the parent was quite abundant. The largest peak in both spectra was due to the regenerated anthraquinone nucleus. (e) 1,4-DIHYDROXY-ANTHRAQUINONE-2-SULFONICACID AND 1,2-DIHYDROXYANTHRAQUINONE-3-SULFONIC ACID. The tetramethylammonium salts were prepared from these compounds. The spectra obtained showed a general behavior similar to the anthraquinones (d) above, with additional peaks indicating that partial methylation of the OH groups also takes place. Thus all nine reference sulfonic acids above readily gave methyl ester and/or tetramethylammonium salt derivative which in turn gave meaningful mass spectra. In virtually all the examples above, the mass spectra obtained would have enabled the species to be identified (apart from the exact positions of substituents), had it been received as an unknown sample. The reference samples indicate the main type of ions and their relative intensities produced by benzene, naphthalene, and anthraquinone sulfonic acid derivatives and also show that when the species under investigation contains O H or NH2 groups, then partial methylation of these groups can be anticipated. Examination of Unknown Sulfonic Acids. As a further test of the method, five sulfonic acids were examined that had been selected by a colleague not associated with this work. Obviously the identity of these samples was withheld until the mass splectrometric examinations were completed. All five samples gave tetramethylammonium salts readily, which yielded good mass spectra. The analysis of these spectra by standard techniques, including in two cases accurate mass measurement of the parent ion, enabled the structure deiermination of the methyl esters to be carried to the following stages: Sample (a). Naphthalene with three substituent groups; OH, NHCONH2,and S03CHa. Sample (b). This specimen showed two components. The major one was benzene with four substituents; 2 X C1, ”2, and S03CH3. The minor component was 1,2-diphenylethane with eight substituents; 4 X C1,2 X ”2, and 2 X SO3CH3. Sample (c). Anthraquinone with three substituents; 2 X OH and S03CH3.
N O N E - ~,~-DISULFONIC ACID.
Sample (a). Benzene with three substituents; CHO, C1, andSOaCH8. Sample (e). Benzene with four substituents; 3 x COOH and S03CHs. Thus in all cases, mass spectrometry established the general structure of the methyl esters, apart from the positions of substituents on the various rings. In one case, (b), the mass spectrum showed that the sample was, in fact, a mixture of two components. Once the general nature of the samples was known, NMR spectroscopic examination allowed the ring substitution patterns t o be determined so that the final structures proposed for the starting sulfonic acids were:
OH (a)
C1
Ci (b) Main component
HO
0
OH
These strrictures were, in fact, all correct although sample
(b) had been supplied as the sodium salt and the presence of the impurity had not been known. The methods described above are now in use in our own laboratories and have led to the successful identification of many samples of commercial importance. Some of these samples have been of much higher molecular weight than those discussed above and materials with molecular weights up to 650 have been dealt with successfully. RECEIVED for review March 6,1970.
Accepted May 25,1970.
Catalytic Effect of Iron on Oxidation of Plutonium by Hydrogen Peroxide Claude W. Sill, Donald R . Percival, and Rodger L. Williams Health Services Laboratory, U.S. Atomic Energy Commission, Idaho Falls, Idaho
IN PREVIOUS ARTICLES (I, Z), conditions were described under which small quantities of‘ all large ter- and quadrivalent ions can be precipitated with barium sulfate in a form suitable for direct alpha counting. Uranium, neptunium, plutonium, and americium can be prevented from precipitating by oxidizing them to their highest oxidation states. When separation of uranium from the transuranium elements was desired, hydro(1) C. W. Sill, Health Phys., 17, 89 (1969). (2) C. W. Sill and R. L. Williams, ANAL.CHEM., 41, 1624 (1969).
gen peroxide was added to oxidize the uranium while keeping the other elements in the reduced state. While applying these procedures to samples of stream bottom sediments and soils, it was noticed (by R.L.W.) that the quantity of plutonium remaining unprecipitated by barium sulfate was invariably higher when hydrogen peroxide was used than in its absence. Because 2 minutes’ boiling with 1 ml of 30% hydrogen peroxide had been shown repeatedly to give better than 99.9% recovery of plutonium on barium sulfate from pure solutions, even when all the plutonium was
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