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Table I. 19FChemical Shifts of Trifluoromethanesulfonyl Derivatives of Model Compounds sample no.
compound
9 10 11 12 13 14
tert-butyl alcohol sec-butyl alcohol n-butyl alcohol isobutyl alcohol methanol benzyl alcohol a -naphthol phenol a-naphthol sec-butylamine n-butylamine N-methylaniline aniline triethylamine
15
pyridine
16
quinoline
17
acridine
18 19 20 21
pyrrole indole carbazole thiophenol
1 2 3
4 5 6 7 8
chemical shift,a PPm -8.18 -4.56 -3.72 -3.70 -3.20 -2.60 t 1.72 t2.12 t 2.66 -2.72 -1.72 -0.90 -0.36 -2.64 (in CDCl,), -2.36 (in DMF) -2.58 (in CDCl,), -2.26 (in DMF) -2.52 (in CDCl,), -2.14 (in DMF) -2.44 (in CDCl,), -2.06 (in DMF) -8.80b -8.40,b t 3.74' t 8.20 -1.94
a Both DMF and DMF-d, as well as CDCl, have been used, shifts are smaller than 0.01. For the C-2 sulfoFor the N-sulfonated product. nated product.
polarity increases. Since only one 19FNMR signal is observed in a mixture of tertiary nitrogen bases, rapid exchange of the bases for the electrophilic sulfonyl center must occur. The 19F chemical shifts for the trifluoromethanesulfonyl derivatives of aliphatic alcohols (sample no. 1-5) cover a range of -5 ppm indicating a prominent sensitivity to structural variations. This may provide an alternative method of differentiating primary, secondary, and tertiary alcohols. A distinct separation between the 19Fchemical shifts of the trifluoromethanesulfonyl derivatives of aliphatic alcohols and phenols is also noted. The IgF chemical shifts of primary and secondary amines (sample no. 10-13) follow the same trends as those of alcohols and phenols. In general, alkyl substitution increases the fluorine shielding and phenyl substitution de-
creases the fluorine shielding. The reactions of pyrrolic-ring compounds with trifluoromethanesulfonyl chloride are more complicated. For pyrrole, trifluoromethanesulfonyl chloride reacts by a C-2 sulfonation mechanism in the presence, or absence, of the tertiary nitrogen base catalyst. The reaction of indole with trifluoromethanesulfonyl chloride occurs to yield either the C-2 or the N-sulfonated product depending on the absence, or presence, of the tertiary nitrogen base. N-Sulfonation is favored in the presence of the base. For cabazole, since a C-2 position is not available, reactions occur slowly due to steric hindrance to give the normal N-sulfonated product only in the presence of the base catalyst. The unusually low fluorine shielding for the trifluoromethanesulfonyl derivatives of carbazole has yet to be explained. The limited data for the series of the pyrrolic-ring compounds do not allow us to draw any conclusions as to the trends of the fluorine shielding. Trifluoromethanesulfonyl chloride has been pointed out as being both a sulfonating and chlorinating reagent (10). Chlorination would produce trifluoromethanesulfonic acid with a 19FNMR signal at -11.5 ppm which was not observed for model compounds studied. The present work gives only the qualitative results for limited compounds. Further studies will be directed toward the quantitative aspects of the reaction of trifluoromethanesulfonyl chloride with a variety of compounds. LITERATURE CITED (1) Martin, T. F.; Snape, C. E.: Bartle, K. D. Prep. Pap.-Am. Chem. SOC.,Dlv. Fuel Chem. 1980, 25 (4),79. (2) Coleman, W. M., 111; Boyd, A. R. Anal. Chem. 1982, 54, 133. (3) Leader, G. R. Anal. Chem. 1970, 42, 16. (4) Leader, G. R. Anal. Chem. 1973, 45, 1700. (5) Ho, F. F A . Anal. Chem. 1973, 45, 603. (6) Ho, F. F.-L. Anal. Chem. 1974, 4 6 , 496. (7) Ho, F. F.-L.; Kohier, R. R. Anal. Chem. 1974, 46, 1302. (6) Manatt, S.L. J . Am. Chem. SOC. 1986, 88, 1323. (9) Sleevl, P.; Galss, T. E.; Dorn, H. C. Anal. Chem. 1979, 51, 1931. ( I O ) Haklmelakl, G. H.; Just, G., Tetrahedron Lett. 1979, 3643.
Feng F a n g S h u e Teh Fu Yen* Department of Chemical Engineering University of Southern California Los Angeles, California 90007 RECEIVED for review March 8,1982. Accepted April 29,1982. Support from U S . Department of Energy Contract No. 79EV10017.000 is acknowledged.
Room-Temperature Phosphorescence of Nitrogen Heterocycles and Aromatic Amines Sir: Room-temperature phosphorescence (RTP) has generated considerable interest over the past several years (1-10). Reviews have also appeared on the subject (11-14). Several materials have been used to induce R T P from a variety of compounds. However, there is still a need to test additional materials and experimental conditions for R T P so the maximum sensitivity and selectivity can be obtained. In this work, three solid surfaces and several experimental conditions were tested for RTP of nitrogen heterocycles and aromatic amines. Nitrogen heterocycles and aromatic amines are an important class of compounds as shown by work in areas such as environmental research and coal liquefaction research (15, 16). EXPERIMENTAL SECTION Apparatus. All RTP intensity data were obtained with a Schoeffel SD3000 spectrodensitometer equipped with a SDC 300
density computer (Schoeffel Instruments, Westwood, NJ) and a phosphoroscope accessory. Details of instrumental setup were reported previously ( I 7). Relative RTP signals were measured with the spectrodensitometer with the inlet and exit slits set at 2 mm and 3 mm, respectively. A 150-W Xe lamp (Conrad Hanovia Inc., Newark, NJ) and R928 photomultiplier tube (Hamamatsu Corp., Middlesex, NJ) were employed in the spectrodensitometer. Reagents. Ethanol was purified by distillation. All nitrogen heterocycles and aromatic amines were recrystallized from ethanol. The plastic-backed silica gel chromatoplates (EM Laboratories, Elmsford, NY) and filter paper (Whatman No. 1)were developed in distilled ethanol to concentrate impurities at one end. Polyacrylic acid (PAA)-sodium chloride mixtures (0.5%w/w) were prepared by grinding to a homogeneous powder in a ball-mill. Procedure. Stock solutions of all compounds in distilled ethanol were used to prepare four different spotting solutions (neutral, 0.1 M HC1, 0.1 M HBr, and 0.1 M NaOH) of each
0003-2700/82/0354-1642$01.25/0 0 1982 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
compound (100 ng/pL). Samples were spotted onto the chromatoplates and filter paper as described earlier (17). The PAANaCl mixtures were handled as were sodium acetate samples (18). All three solid matrices containing the sample were heated to 80 for 30 min. The RTP intensity measurements were made as described earlier (17,18). The maximum RTP excitation and emission wavelengths of some of the compounds used in this study were reported earlier (17). For other compounds, low temperature phosphorescence maximum wavelength values were determined with a Farrand MK-2 spectrofluorometer. The wavelength values were slightly changed, if necessary, when the Schoeffel opectrodensitometer was used to obtain maximum RTP signals for each compound. The RTP intensities of these compoundrr differ considerably from one another. As a result, different gain settings were used with the spectrodensitometer to obtain RTP intensity data. For purpose of comparison, the intensities of these compounds were normalized to the same ginin setting used for 5,6-benzoquinoline. This comparison method did not compenrrate for substrate to substrate variation.
RESULTSI AND DISCUSISION Table I gives the R1'P data for 20 calmpounds obtained under neutral, acidic antd basic conditions on silica gel chromatoplates that contained a polyacrylate salt (I), on filter paper, and on PAA-NaC1 mixtures. The italic numbers in Table I indicate the maximum signals obtained for a given compound. For 14 of the compounds, the IPAA-NaC1 mixtures with 0.1 M HBr yielded the strongest RTP signals. In some cases, the signal from the PAA-NaC1 mixtures was enhanced considerably compared to filter paper as a solid surface. For example, Cazafluorene gave a signal that was 2.9 times greater on PAA-NaC1 mixture (0.1 M HBr) than on filter paper (0.1 M HBr). This is an important result because filter paper has been widely used as a substrate to induce RTP from various compounds (2,12,13).In fact, filter paper only yielded the maximum RTP signall for two compounds, namely, 5,6benzoquinoline and 13E~-dibenzo[a,i]carlsazole. The silica gel chromatoplates with 0. I M HBr gave the largest RTP signal for five compounds. 7,8-Benzoquinoline and quinoline gave identical signals on silica gel chromatoplates (0.1M HBr) and PAA-NaC1 mixture (0.1 M HBr). With samples spotted from 0.1 M NaOH solutions, in most cases relatively weak RTP signals were obtained. However, there were exceptions such as dibenz[aj]acridine atdsorbed on PAA--NaC1mixture. For 2-aminoanthroquinone, the largest RTI' was obtained from an alkaline solution spotted on a PAA-NaC1 mixture. It was shown earlier that HBr enhanced the RTP of 5,6benzoquinoline adsorbibd on silica gel chromatoplates which contained a polyacrylate binder (1). Iin later work, it was proposed that the HEIr served a dual purpose with these chromatoplates (IO). It neutralized the polyacrylate salt forming carboxyl groups and the bromide ions caused an increase in the rate of intersystem crossing. For the samples in Table I adsorbed on the silica gel chromatoplates, it appears the above phenomena are also occurring. With filter paper and the PAA-NaCl mixture, most sampdes spotted from 0.1 M HBr solutions yielded the strongest signals relative to those spotted from neutral, 0.1 M HC1, and 0.1 M NaOH solutions. This could be due to the heavy-atom effect, but additional work is needed to prove this. The limits of detection and relative standard deviation were not determined for the data in Table I. However, in other experiments 5,6-benzoquinoline, carbazole, and 2-aminoanthroquinone were adsorbed onto 1%I'AA-NaBr mixtures from 0.1 M HBr ethainol solutions. The relative standard deviation for 35 ng of 5,6-benzoquinoline (10 samples) was 3.6%,for 50 ng of carbazole (9 samples) it was 5.4%,and for 10 ng of 2-aminoanthroquinone (9 samples) it was 3.8%. On the basis of a signal t o noise ratio of three, the limits of detection for 5,6-benzoquinoline, carbazole, and %amino-
NMQ, rl
IO
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Anal. Chem. 1982, 54, 1644-1646
anthroquinone were 1ng, 10 ng, and 6 ng, respectively. The previous data give a general idea of the relative standard deviation and limits of detection for the compounds in Table I. It should be mentioned that the relative standard deviation and limit of detection data were obtained from 1% PAA-NaBr mixtures and not from 0.5% PAA-NaC1 mixtures as in Table I. Carbazole and 2-aminoanthroquinone gave relatively weak signals on both 1%PAA-NaBr and 0.5% PAA-NaC1, but 5,6-benzoquinoline gave strong RTP signals on both mixtures. The 0.5% PAA-NaC1 mixture induced a R T P signal three times greater from 5,6-benzoquinoline compared to the signal obtained from the 1% PAA-NaBr mixture. Thus, the PAANaCl mixture was used to obtain the R T P data in Table I.
Hurtublse, R. J. Talanta 1981, 2 8 , 145-148. Hurtubise, R. J.; Datterio, R. A. Am. Lab. (Fairfield, Conn.) 1981, 13 (1 l),58-62. Parker, R. T.; Freedlander, R. S.:DUnlaD, R. 6. Anal. Chim. Acta 1880, 779, 189-205. Parker, R. T.; Freedlander, R. S.; Duniap, R. 6. Anal. Chim. Acta
---. --.
Ig80. 120. 1-17.
Hurtubise, R. J. “Soild Surface Luminescence Analysis: Theory, Instrumentation, Applications”; Marcel Dekker: New York, 1981;Chapters 3,5,and 7. Later, D. W.; Lee, M. L.; Wllson, B. W. Anal. Chem. 1982, 54.
117-1 23. Whttehurst, D. D.; Mlchell. T. 0.; Farcasiu, M. “Coal Liquefaction: The Chemlstry and Technology of Thermal Process”; Academic Press: New York, 1980. Ford, C. D.; Hurtublse, R. J. Anal. Chem. 1979, 5 7 , 659-663. Von Wandruszka, R. M. A,; Hurtubise, R. J. Anal. Chem. 1978, 48,
1784-1788.
LITERATURE CITED Ford, C. D.; Hurtublse, R. J. Anal. Chem. 1980, 5 2 , 656-662. Paynter, R. A.; Wellons, S. L.; Winefordner, J. D. Anal. Chem. 1974, 46, 736-738. Parker, R. T.; Freedlander, R. S.; Schulman, E. M.; Dunlap, R. 8. Anal. Chem. 1979, 5 7 , 1921-1926. Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 5 7 , 1915-1921. Meyers, M. L.; Seybold, P. G. Anal. Chem. 1979, 5 7 , 1809-1612. Schulman, E. M.; Parker, R . T. J . Phys. Chem. 1977, 87,
1932-1939. de Lima. C. G.; Nlcola, E. M. Anal. Chem. 1978, 50, 1658-1665. Bower, E. L.; Winefordner, J. D. Anal. Chlm. Acta 1878, 702, 1-13. Cline Love, L. J.: Skrllec. M.; Habarta, J. G. Anal. Chem. 1080, 5 2 ,
754-759.
S. M. Ramasamy R. J. Hurtubise* Department of Chemistry The University of Wyoming Laramie, Wyoming 82071
RECEIVED for review April 9, 1982. Accepted May 26,1982. This work was supported by the Department of Energy (Office of Basic Energy Sciences) under Contract No. DE-AC0280ER10624.
AIDS FOR ANALYTICAL CHEMISTS Magnetic Enhancement of an Ionization Source for Glow Discharge Mass Spectrometry B. L. Bentz and W. W. Harrison* Deparfment of Chemistty, Universiw of Virginia, Charlottesville, Virginia 2290 I
Magnetic field coupling to a gas discharge finds practical employment in several types of present-day low-pressure discharge assemblies, for example, in the duoplasmatron ion source and in commercial sputter atomization units serving to deposit coatings or films of sputtered material on selected substrates. To date, gas discharge ionization sources for glow discharge mass spectrometry (GDMS) (I), an analytical technique developed by researchers for inorganic solids analysis (2-5) and thin film process control (6),and which recently has seen the appearance of a commercially available ionization unit (7), have been without magnetic field superposition. This report describes the testing of a cylindrical diode ionization source with added magnet, designed for GDMS use, and presents new data showing that accommodation of a magnetic field affords enhancements in mass analyzed ion signals arising from the sputtered neutral atom fraction. The negative glow region of a weakly ionized dc glow discharge is rich in electrons with varying energies (8). In general, the effect of a magnetic field on a glow discharge is via the electrons; at the fields normally used, the ions are insignificantly influenced. The lifetime of an electron in the discharge can be increased by using a magnetic field to redirect electron motion in a manner to increase the net electron path length (9). In so doing, the probability of ion formation by electron collision can be enhanced owing to a more efficient use of the available electron supply. An externally applied magnetic field may, in principle, be superimposed on a glow discharge either parellel to the electric field or in a perpendicular manner. 0003-2700/82/0354-1844$01.25/0
Both of these arrangements are effective in causing enhanced electron activity. Another important advantage of a transversally applied magnetic field, apart from providing increased ionization, is that the discharge can be operated a t lower source pressures for a given discharge current (IO) due to its effect on electrons in the cathode dark space. This is advantageous for a plasma ion sampling system, such as our quadrupole-based solids mass spectrometer described previously (II), because lower source pressures reduce the pumping load on the vacuum system and also lessen redeposition of sputtered cathode particles in the source.
EXPERIMENTAL SECTION We studied a simple source geometry using two conducting, concentric cylinders as electrodes, a design referred to as a coaxial cathode ion source (CCIS) (12) as is evident when the electrodes are viewed head-on (see Figure 1). The geometry of this source enabled convenient modification to accommodate a magnetic field superimposed on the discharge. A variable field, solenoidal electromagnet was formed by winding 22 gauge insulated copper wire around the cylindrical stainless steel anode (1.52 cm i.d., 2.07 cm o.d., 6.7 cm long) with sufficient turns (14 turns/cm) and layers (20) to generate a calculated (13) static magnetic field strength of 325 G at the axis and center of the source for a magnet current of 1A, supplied by a Trygon Electronics (Westbury, NY) 40-V, 10-A dc power supply. The magnetic field was roughly mapped in the source interior by use of a Hall probe. Good calibration agreement (within 1-2%) was found at the source axis between the calculated field strength and the measured value. Use of the electromagnet, rather than a permanent ring magnet, allows the field strength to be varied such that conditions might be estab0 1982 Amerlcan Chemical Society
