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Table 11. Maximum Quantities of Coexisting Substances, without Interference. (SbCl,(III) in the Sample Was 0.05-0.2 mmol in about 30 m L Solution) A. Alkali salts B. Organic solvents Salt/SbCl , Solvent/ DMF Salts (mol/mol) Solvent (v/v) NaCl 30 Acetone 3.0 KNO , 40 Dimethyl 2.0 sulfoxide NH,Cl 5 Benzene 3.0 Acetonitrile 2.0 Et hano 1 0.5 Water 0.1
RESULTS AND DISCUSSION The titration reaction is shown by the chemical equations written as follows. CS, + (CH,),NH t (C,H,),N = (CHz)sNCS,- i-(C,H5)$NH+ 3(CHz),NCS,- + Sb3+= Sb[ (CH,),NCS,], The reaction rate was rapid esough to use it for the titration at room temperature (10-30 "C), and 3 mol of piperidine were quantitatively consumed per 1mol of antimony up to the end point. The titration could be operated only in DMF or in dimethyl sulfoxide; not in water, ethanol, or acetonitrile. In place of piperidine, pyrrolidine was also applicable for this procedure, but other secondary amines, such as morpholine, were not useful. The results of the titration are shown in Table I. The content of antimony in each sample obtained by this method coincides with value by the chelatometric titration using CYDTA, within the range of experimental error.
As dialkylcarbamodithioates are strong chelating reagents, most of the heavy metals such as bismuth, tin, lead, cadmium, mercury, copper, nickel, and iron, as well as the alkali earths (barium, strontium, and calcium), even a small quantity of them, interfere with the titration. Therefore, these metal salts must be removed beforehand. Alkali metal salts do not react with the ligand, but if a large quantity of them coexists, the end point of the titration becomes nebulous. Maximum quantities of coexisting alkali salts in the sample are shown in Table 11,A, and those of coexisting organic solvents in Table 11, B. Although dialkyldithiocarbamates had not been used in any complexometric titration because of their instability, they have now been found to be applicable to volumetric determination by this technique.
LITERATURE CITED (1) A. A. A. Slbal and A. G. Fogg, Analyst (London), 98, 732 (1973). (2) K. Kuwada, A. Ouchl, K. Watanukl, and T. Shlmada, Bunsekl Kagaku, 26, 717 (1976). (3) N. H, Furman, Ed., "Scott's Standard Methods of Chemical Analysis", Vol. 1, 6th ed., Van Nostrand, New York, N.Y., 1962, p 92. (4) S. Takamoto, Nlppon Kagaku Zasshi, 7 8 , 1339 (1955). (5) I. M. Yurlst, Zavod. Lab., 32, 1050 (1966); Chem. Abstr., 66, 25750 (1967). (6) K. Kuwada and A. Ouchl, Anal. Chim. Acta, 8 5 , 209 (1976).
Yasuhiro Asano Mamoru Shimoi Akira Ouchi* Yukichi Yoshino Department of Chemistry College of General Education The University of Tokyo Komaba, Meguro, Tokyo 153, Japan RECEIVED for review March 2,1978. Accepted April 18, 1978.
Direct Mass Spectrometric Mixture Analysis by Negative Chemical Ionization/Mass-Analyzed Ion Kinetic Energy Spectrometry Sir; By utilizing a mass-analyzed ion kinetic energy (MIKE) spectrometer (i.e. a reversed-geometry double sector mass spectrometer), the direct (nonchromatographic) analysis of mixtures via mass spectrometry has been demonstrated for positive ions (1-4). The positioning of the magnet in front of the electric sector allows selection of an ion, followed by collision-induced dissociation and identification of fragments by their kinetic energy. Thus the MIKE technique characterizes ionic structure by fragmentation subsequent to mass analysis. I t is shown here that the analysis can also be made on mass-analyzed negative ions with the advantage that sample characterization can be accomplished by more than one kind of high energy ion/molecule reaction. It is also shown that detect,ion limits in the low picogram range are accessible. Carboxylic acids, because of their importance in food products and biological fluids, were selected for study. They produce intense (M - HI- ions upon chemical ionization and show little fragmentation in the mass spectra. The collision-induced dissociation process, in which negatively-charged fragments were monitored, allows these ions to be characterized. The aromatic acids studied yield a single major process, RC02- R- + COZ, which serves to characterize the
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functional group. Nonaromatic acids yield information which is indicative of other aspects of the structure, e.g., citric acid loses water, carbon dioxide, and acetic acid. Charge inversion (i.e., R1- R1+, R2+...),previously used to characterize the structures of negative ions (51, is useful in the characterization of the aromatic acids as demonstrated in Figure 1for p-hydroxybenzoic acid. This spectrum shows both the positively and the negatively charged fragments generated from the selected negative precursor ion. Both sets of ions provide complementary structural information. The charge stripping reactions can be rationalized in terms of the loss of stable neutral molecules from the (M - H)- precursor ion and provide structural information similar to that obtained from conventional mass spectra ( 5 ) . Charge stripping is also capable of distinguishing isomeric compounds, as exemplified by 3,4- and 2,5-dihydroxybenzoic acids, in which the reaction (M - H)- (M - H)+occurs with a relative efficiency of 13:l and the dissociative stripping process (M - HI- (M - H 84)' occurs with a relative efficiency of 3:8. The applicability of this methodology to complex mixture analysis is demonstrated in Figure 2 by the identification of hippuric acid in 2 ILLof urine. As the figure shows, the charge
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
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Figure 1. Positively- and negatively-charged fragments from the collision-induced dissociation of the (M - H)- ion of p-hydroxybenzoic acid with nitrogen as the target collision gas
chloride is simulaneously introduced with the sample (6). Using this reactant ion, a decrease of two orders of magnitude (to 10 pg) in the detection limit for glucose analysis is observed. These results lay the groundwork for the application of the direct mixture analysis methodology to negative ions and demonstrate that, by the simple reversal of potentials when needed, rapid and reliable analysis on a large variety of samples can be readily accomplished.
LITERATURE CITED (1) T. L. Kruger, J. F. Litton, R. W. Kondrat, and R. G. Cooks, Anal. Chem.,
% E 100
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Figure 2. Comparison of positively-charged fragments derived from hippuric acid (M - H)' in urine and in an authentic sample with nitrogen as the target collision gas
stripping spectrum of untreated urine matches that of the pure compound. Similar results have been obtained for other acids in a variety of beverages and urine samples. Sample treatment is minimized, requiring a t most freeze drying. Carbohydrates and phenolic compounds have also been analyzed in a variety of foods, e.g., ascorbic acid in urine and BHA in yeast. In general, the sensitivity of negative ions was found to be comparable to that of positive ions. Negative ions, however, offer a n additional advantage in that the sensitivity and specificity can be increased for compounds which attach chlorine to form the (M + C1)- species when mghylene
48,2113 (1976). (2) J. H. McReynolds and M. Anbar, Int. J . Mass Spectrom. Ion fhys., 24, 37 11977). (3) K. ievse; and H. D. Beckey, Org. Mass Spectrom., g, 570 (1974) (4) K. Levsen and H A . Schulten, Biomed. Mass Spectrom.,3,137 (1976). (5) J. H. Bowie and T. Biumenthal, J . Am. Chem. Soc., 97,2959 (1975). (6) R. C. Dougherty, J. D. Roberts, and F. J. Biros, Anal. Chem., 47,49 (1975).
R. W. Kondrat G . A. McClusky R. G . Cooks* Department of Chemistry Purdue University West Lafayette Indiana 47907
RECEIVED for review March 10, 1978. Accepted May 1, 1978. This work was supported by the National Science Foundation (MPS 77-01295).
AIDS FOR ANALYTICAL CHEMISTS Background Subtract Subroutine for Spectral Data R. P. Goehner General Electric Company, Corporate Research & Development, P.O. Box 8, Schenectady, New York 1230 1
Spectral data generally consist of peaks superimposed on a background. The background can be either flat, linear with a positive or negative slope, curved, or a combination of all three. The first two can be easily removed by an iterative least square procedure. The entire spectrum, or a reasonable subset, 0003-2700/78/0350-1223$01 .OO/O
is fitted to a linear square line (I). Then all the points that are a standard deviation above this line are rejected, and a new fit is made. This procedure continues until the line converges on the background. Once the equation is known, the background is simply calculated a t each point, and this 0 1978 American Chemical Society
