Application of Organic Solvents as Matrixes To Detect Air-Sensitive

Anal. Chem. 1999, 71, 2901-2907

Application of Organic Solvents as Matrixes To Detect Air-Sensitive and Less Polar Compounds Using Low-Temperature Secondary Ion Mass Spectrometry Min-Wen Huang, Hsing-Long Chei, Jen-Pang Huang, and Jentaie Shiea*

Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, 80424 Taiwan

Using low-temperature secondary ion mass spectrometry (LT-SIMS), this study successfully detected air-sensitive and less polar compounds dissolved in various organic solvents. A low-temperature interface and copper probe were constructed to conduct SIMS analysis. The sample solution was frozen by applying it on the tip of the insertion probe which was maintained at -120 °C under a cold nitrogen stream. Bis((diethylamido)dimethylaluminum), an extremely air-sensitive organometallic compound, was dissolved in dry benzene, hexane, and diethyl ether, respectively. From these solutions, the analyte signals were directly obtained without using a viscous matrix. In addition, LT-SIMS was used to detect less polar compounds, including organometallic and organophosphorus compounds, porphyrins, long-chain alcohols, and amines dissolved in different organic solvents. Solvents used in this study include chloroform, chloroform-d, methylene chloride, acetone, ethanol, diethyl ether, hexane, benzene, toluene, 1-hexanal, 1-hexanoic acid, and 1-hexanol. Detecting and characterizing extremely air-sensitive organometallic compounds such as alkylaluminum or -magnesium complexes are challenging tasks. These compounds strongly react with oxygen in the air. Upon synthesis, the compounds must be dried under a vacuum or dissolved in dry organic solvents and protected under nitrogen.1,2 Characterizing such compounds by a spectroscopic instrument involves creating an oxygen-free environment, particularly during sample preparation. Owing to its simplicity in sample-handling processes, this oxygen-free environment can be easily created in the sample tubes used in NMR spectroscopy.3,4 Related investigations have analyzed such compounds via mass spectrometry using electron impact ionization or chemical ionization sources. In these methods, the sample is heated in an oxygen-free chamber (or tube) and, then, the gaseous analyte is introduced into the ion source. However, conventional techniques are limited to thermally stable compounds. Unfortu(1) Atwood, J. L.; Stucky, G. D. J. Am. Chem. Soc. 1969, 91, 2538. (2) Her, T. Y.; Chang, C. C.; Lee, G. H.; Peng, S. M.; Wang, Y. Inorg. Chem. 1994, 33, 99. (3) Veith, M.; Frand, W.; Toner, F.; Lange, H. J. Organomet. Chem. 1987, 326, 315. (4) Meese-Markscheffel, J. A.; Cramer, R. E.; Gilje, J. W. Polyhedron 1994, 13, 315. 10.1021/ac980516p CCC: $18.00 Published on Web 06/11/1999

© 1999 American Chemical Society

nately, many air-sensitive compounds decompose at an elevated temperature. Liquid secondary ion mass spectrometry (LSIMS) or fast atom bombardment (FAB) mass spectrometry has been used to characterize organometallic compounds at room temperature for nearly two decades.5-9 However, these techniques have seldom been applied to analyze extremely air-sensitive organometallic compounds. Although an oxygen-free interface can be constructed for the sample application in LSIMS, air-sensitive compounds may still react with the oxygen dissolved in the viscous matrix. As generally known, the success with LSIMS is usually achieved by using a viscous matrix. This also limits the type of analyte suitable for LSIMS to either polar or nonvolatile compounds. Therefore, applying LSIMS to detect compounds with a low polarity (such as alkylaluminum and -magnesium complexes) is extremely difficult. Although adding acid or heating can increase the solubility of a less polar analyte in the viscous matrix, such a technique is useful only for an acid-resistant or a thermally stable analyte.10-14 A viable means of solving the above problems is to use an organic solvent as the matrix. Owing to the volatility of the organic solvent, the solution’s temperature must be kept sufficiently low. The matrix molecule can then survive in the vacuum as well as in the sample inlet and ion source. Several SIMS studies were performed at subambient temperatures; however, most of these studies focused on elucidating the mechanisms of desorption ionization in SIMS.15-25 Only limited applications were reported, (5) Miller, J. M. Mass Spectrom. Rev. 1989, 9, 319. (6) Miller, J. M. Adv. Inorg. Chem. Radiochem. 1984, 28, 1. (7) Bruce, M. I.; Liddell, M. J. Appl. Organomet. Chem. 1987, 1, 191. (8) Cochran, R. L. Appl. Spectrosc. Rev. 1986, 22, 137. (9) Van Breemen, R. B.; Martin, L. B.; Schreiner, A. F. Anal. Chem. 1988, 60, 1314. (10) Musselman, B. D.; Watson, J. T.; Chang, C. K. Org. Mass Spectrom. 1986, 21, 215. (11) Shiea, J.; Sunner, J. Org. Mass Spectrom. 1991, 26, 38. (12) Todd, P. T. J. Am. Soc. Mass Spectrom. 1991, 2, 33. (13) Groenewold, G. S.; Todd, P. T. Anal. Chem. 1985, 57, 886. (14) Leibman, C. P.; Todd, P. T.; Mamantov, G. Org. Mass Spectrom. 1988, 23, 634. (15) Boryak, O. A.; Kosevich, M. V.; Shelkovsky, V. S.; Blagoy, Y. P. Rapid Commun. Mass Spectrom. 1995, 9, 978. (16) Katz, R. N.; Chaudhary, T.; Field, F. H. J. Am. Chem. Soc. 1986, 108, 3897. (17) Katz, R. N.; Chaudhary, T.; Field, F. H. Int. J. Mass Spectrom. Ion Processes 1987, 78, 85. (18) Katz, R. N.; Field, F. H. Int. J. Mass Spectrom. Ion Processes 1989, 87, 95. (19) Orth, R. J.; Jonkman, H. T.; Michl, J. J. Am. Chem. Soc. 1981, 103, 1564.

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Figure 1. Schematic diagram of the low-temperature SIMS interface and the copper probe: (A) copper probe; (B) Teflon tube; (C) acrylic plate; (D) acrylic tube; (E) Teflon tube; (F) O-ring; (G) Dewar vessel; (H) two-way valve; (I) copper tube.

with Kosevich’s research group using organic solvents as matrixes in low-temperature SIMS (LT-SIMS) and FAB.26-32 This study investigates the feasibility of using organic solvents as matrixes to characterize an air-sensitive organometallic compound at a low temperature. In addition, testing is undertaken to detect a broad range of less polar analytes dissolved in different organic solvents by LT-SIMS. EXPERIMENTAL SECTION The air-sensitive compound bis((diethylamido)dimethylaluminum) (DEDMA) was purchased from Aldrich. In the air, the compound strongly reacts with oxygen to form a polymer (white powder). The analyte was dissolved in dry organic solvents (e.g., benzene, hexane, and diethyl ether) and was sealed under (20) Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J. Am. Chem. Soc. 1979, 101, 1951. (21) Johnstone, R. A. W.; Wilby, A. H. Int. J. Mass Spectrom. Ion Processes 1989, 89, 249. (22) Jonkman, H. T.; Michl, J.; King, R. N.; Andrade, J. D. Anal. Chem. 1978, 50, 2078. (23) Shiea, J.; Sunner, J. Int. J. Mass Spectrom. Ion Processes 1990, 96, 243. (24) Sunner, J.; Ikonomou, M. G.; Kebarle, P. Int. J. Mass Spectrom. Ion Processes 1988, 82, 221. (25) Morales, A.; Kebarle, P.; Sunner, J. Int. J. Mass Spectrom. Ion Processes 1989, 87, 287. (26) Chen, Y. C.; Chei, H. L.; Shiea, J. J. Mass Spectrom. 1996, 31, 464. (27) Huang, M. W.; Chei, H. L.; Shiea, J. Proceedings of the 45th Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, 1997; p 947. (28) Wang, C. H.; Huang, M. W.; Lee, C. Y.; Huang, J. B.; Shiea, J. J. Am. Soc. Mass Spectrom. 1998, 9, 1168. (29) Kosevich, M. V.; Boryak, O. A.; Stpanov, I. O.; Shelkovsky, V. S. Eur. Mass Spectrom. 1997, 3, 11. (30) Boryak, O. A.; Kosevich, M. V.; Shelkovsky, V. S.; Blagoy, Y. P. Rapid Commun. Mass Spectrom. 1996, 10, 197. (31) Kosevich, M. V.; Boryak, O. A.; Shelkovsky, V. S.; Derrick, P. J. Eur. Mass Spectrom. 1998, 4, 31. (32) Gross, J. H. Rapid Commun. Mass Spectrom. 1998, 12, 1833.

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nitrogen. Other chemicals and the organic solvents (HPLC grade) were purchased from Sigma or Aldrich and were used without further purification. The sample solutions (1-10 mM) were prepared simply by dissolving the analyte in the appropriate organic solvent. An organic solvent for a particular analyte was selected only on the basis of solubility. Figure 1 depicts the LT-SIMS interface and a copper probe constructed in this study. The interface, made of an acrylic tube and plates, was screwed onto the probe inlet on the mass spectrometer. The copper probe consisted of three pieces. The second piece of the probe was made of a PTFE tube surrounding a copper stick which was then screwed into the first and third copper pieces. Next, the function of the PTFE on the copper probe was used to prevent the cold probe from directly contacting the O-rings in the inlet so that the O-rings would still be soft and had a sealing capability. Notably, the flow rate of a stream of cold nitrogen gas flowing into the interface decreased and controlled the probe’s temperature in the interface. The interfacial temperature was measured by a thermocouple attached to a thermal probe (Hanna, HI 93530; effective temperature ranging from -200 to +1370 °C). To apply the sample solution to the probe tip, the probe was initially inserted into the interface until the probe tip reached the sample inlet (Figure 1). After most of the air was excluded from the interface and the probe’s temperature reached approximately -120 °C, the sample solution (about 2-5 µL) was applied to the cold probe tip by a microsyringe. The sample solution froze immediately after making contact with the probe. For the solvents with a lower melting point, such as diethyl ether (-116 °C) and hexane (-95 °C), the initial temperature had to decrease to -150 °C to freeze the sample solution. The frozen sample solution was then sent into the ion source for SIMS analysis. Positive-ion mass spectra were obtained on a mass

Figure 2. Positive-ion LT-SIMS mass spectra of organic solvents: (a) 1-hexanol (102 g/mol); (b) acetone (58 g/mol); (c) toluene (92 g/mol); (d) methylene chloride (84 g/mol).

spectrometer (VG QUATTRO) equipped with a cesium ion gun which was operated at 10 keV and a constant discharge current of 1 mA. The mass spectrometer was scanned from m/z 1500, 1000, or 750 down to m/z 50 at a rate of 400 amu/s, with an interscan delay time of 0.5 s. For each sample, SIMS analysis was performed in triplicate. The mass spectra from the first three scans were averaged and reported.

RESULTS AND DISCUSSION Although most organic and inorganic compounds are soluble in organic solvents, directly applying such a solution to the direct sample probe is nearly impossible owing to its low viscosity. At room temperature, the solution is also too volatile to survive in the ion source of a mass spectrometer. To overcome these problems, an early work used Freon coolant to cool the sample Analytical Chemistry, Vol. 71, No. 14, July 15, 1999

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Figure 3. Positive-ion LT-SIMS mass spectra of DEDMA (258 g/mol) dissolved in (a) benzene, (b) hexane, and (c) diethyl ether.

probe down to -10 °C.33 Recently, Koserich and co-workers demonstrated that methanol/water, toluene, and dichloromethane could be used as FAB matrixes at a low temperature.29-32 In this study, a cold insertion probe and interface were constructed (Figure 1). Since the probe had been cooled far below the melting point of most organic solvents (e.g., -120 °C or lower) by liquid nitrogen, the sample solution would immediately freeze as it came into contact with the probe. When the sample solution was sent 2904 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999

into the ion source, the probe temperature rose to approximately -70 °C within 2 min. This rise was attributed to the high temperature in the ion source (ca. 22 °C). However, the vapor pressure of most organic solvents could still remain low during this temperature rising, thereby allowing the sample solution to remain on the ion source for a sufficient period. In general, the (33) Falick, A. M.; Walls, F. C.; Laine, R. A. Anal. Biochem. 1986, 159, 132.

Figure 4. Positive-ion LT-SIMS mass spectra of (a) tricarbonyl[(1-4-η)-2-methoxy-5-(((methyloxy)carbonyl)methylidene)-1,3-cyclohexadiene]iron (320 g/mol) dissolved in acetone, (b) triphenylcresyl phosphate (368 g/mol) dissolved in chloroform-d, (c) chloro[5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphinato]iron(III) (1028 g/mol) dissolved in hexanol, and (d) 1,5-diphenyl carbazide (242 g/mol) dissolved in methylene chloride.

analyte signal could be acquired for approximately 1 min using the solvents with melting points higher than -70 °C. Although the probe temperature was decreased to -150 °C, the ion signals of sample solutions using the solvents with lower melting points such as diethyl ether and hexane could only last for 10 s.

For pure organic solvents such as acetone, 1-hexanol, dimethyl sulfoxide, ethanol, 1-hexanal, and 1-hexanoic acid, the mass spectra were dominanted by the protonated molecular ion (MH+) or cluster ions (e.g., M2H+ and M3H+). Figure 2 depicts two of these mass spectra. This finding corresponds to previous invesAnalytical Chemistry, Vol. 71, No. 14, July 15, 1999

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Figure 5. Positive-ion LT-SIMS mass spectra of (a) 1-hexadecylamine (241 g/mol; 10 mM), (b) 1-octadecanol (270 g/mol; 10 mM), and (c) a mixture of equal concentrations (10 mM) of 1-dodecylamine (M1; 185 g/mol), 1-hexadecylamine (M2; 241 g/mol), and 1-octadecylamine (M3; 269 g/mol), dissolved in hexanol (3.3 mM each).

tigations in which the molecular ions of some volatile organic solvents containing a proton-acceptor group can be detected by low-temperature FAB.34 However, owing to the difference in their chemical structures, the mass spectra also reveal some discrepancy among the organic solvents. For example, the mass spectrum of 1-hexanol is dominated by cluster ions such as M2H+, M3H+, 2906 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999

and M4H+ (Figure 2a); however, only a dimer ion (M2H+, m/z 117) was observed for acetone (Figure 2b). Organic solvents such as methylene chloride, chloroform, chloroform-d, toluene, and hexane lack the proton-acceptor group, making it impossible to (34) Heckles, K.; Johnstone, R. A. W.; Wilby, A. H. Tetrahedron Lett. 1987, 28, 103.

use LT-SIMS to detect protonated molecular ions (Figure 2c,d). Positive-ion mass spectra of these solvents are characterized by extensive fragment ions in a low-mass region. Some of these ions may originate from background gas preadsorbed on the cold solvent surface. Bis((diethylamido)dimethylaluminum) (DEDMA), an extremely air-sensitive organometallic compound, is normally stored in a dry organic solvent under nitrogen. Once making contact with air, DEDMA reacts immediately with oxygen to form a white powder. The analyte’s polarity is also too low to allow dissolution in any viscous matrixes used in LSIMS. Therefore, no signal can be obtained by LSIMS at room temperature. In this study, cold nitrogen purging of the interface appears to remove most of the air from it. The oxidation rate also decreases at a low temperature, accounting for why the air-sensitive analyte survives even though a trace amount of oxygen may still exist in the interface. Herein, no white powder (i.e., oxidation product) is observed after the sample solution is applied to the probe tip. Figure 3 displays SIMS mass spectra of DEDMA dissolved in dry benzene, hexane, and diethyl ether, respectively. These figures indicate that, except for the difference in relative intensities of certain fragment ions, all three mass spectra closely resemble each other. The analyte molecule contains no proton-acceptor group, making it impossible to detect the MH+ ion. Instead, the ion formed by losing a methyl group from DEDMA (M - CH3)+ (m/z 243) is observed. The mass spectra are dominated by the fragment ions from DEDMA. These ions include m/z 27 (aluminum ion), 74, 114, 128, and 185. The technique was also employed to successfully detect an organometallic compound (Figure 4a), an organophosphorus compound (Figure 4b), a fluorinated porphyrin (Figure 4c), and a diphenyl carbazide (Figure 4d) dissolved in acetone, chloroformd, 1-hexanol, and methylene chloride, respectively. The mass spectra of above compounds were all characterized by the protonated molecular ion MH+. Notably, the preferential formation of MH+ over M•+ is expected since all of these compounds contain proton acceptors such as carbonyl, phosphorus, or amino groups. Despite the absence of any systematic studies, the sensitivity of LT-SIMS is comparable with that obtained by conventional LSIMS at room temperature. Detecting long-chain alkylamines and alcohols by LSIMS is difficult. For long-chain alkylamines, the related problems are attributed to their low polarities (and solubilities) in conventional LSIMS matrixes. For long-chain alcohols, the gas-phase basicities of the compounds are lower than those of any of the conventional LSIMS matrixes. Therefore, obtaining a proton from an ionmolecule reaction (IMR) between an analyte and a matrix ion (35) Ligon, W. V.; Dorn, S. B. Int. J. Mass Spectrom. Ion Processes 1984, 61, 113. (36) Ligon, W. V.; Dorn, S. B. J. Am. Chem. Soc. 1988, 110, 6684. (37) Ligon, W. V.; Dorn, S. B. Int. J. Mass Spectrom. Ion Processes 1986, 78, 99. (38) Chen, Y. C.; Shiea, J. J. Mass Spectrom. 1995, 30, 1435.

would also be difficult. The problems can be easily solved by using 1-hexanol as the matrix owing to its low gas-phase basicity and high solubility in both types of analytes. Parts a and b of Figure 5 display the mass spectra of 1-hexadecylamine and 1-octadecanol dissolved in 1-hexanol, respectively. When 1-octadecanol is dissolved in solvents with a higher gas-phase basicity, such as 1-hexanal or 1-hexanoic acid, its signal cannot be observed (data not shown). Ligon and Dorn indicated that, for a series of surfactants, the differences in ion sensitivity in the mass spectra are due almost exclusively to their surface activities.35-37 This phenomenon is attributed to the facts that the surface sites of the matrix tend to be occupied by more surface-active analytes and the molecules on the surface are then desorbed and ionized by the impact of primary ions.38 Results in our previous study have suggested that the surface activity effect among a series of quaternary amines is eliminated as the matrix is frozen.27 In this study, the same situation is also observed in LT-SIMS when 1-hexanol is used as the matrix. The temperature of the sample solution in the ion source is far below the melting point of 1-hexanol (-52.5 °C), accounting for why the analyte molecules are forced to remain in the solid hexanol matrix. Therefore, the difference in surface activity among the analytes would not appear in the mass spectra since the analytes cannot diffuse to the surface of 1-hexanol as they did in the liquid matrix. Figure 5c depicts the LT-SIMS mass spectrum of three long-chain primary aminessdodecylamine, hexadecylamine, and octadecylamine dissolved in 1-hexanol in an equal concentration (3.3 mM). Although the surface activities of the analytes differ from each other, the signals of the three alkylamines are nearly equal. CONCLUSION This study demonstrates the feasibility of obtaining the signal of an extremely air-sensitive and volatile organic compound by low-temperature SIMS using organic solvents as matrixes. The technique can also be applied to analyze general catalogues of organic and inorganic compounds. LT-SIMS using an organic solvent as the matrix has the following merits: (1) the analyte’s signal with low gas-phase basicity can be obtained using the organic solvent with even a lower gas-phase basicity; (2) the signal of the analyte with low polarity can be detected using nonpolar solvents as the matrixes; and (3) the effect of surface activity among the analytes seem to be eliminated as the solution is frozen. ACKNOWLEDGMENT The authors thank the National Science Council of Taiwan for financial supporting this research. The air-sensitive compound was kindly provided by Dr. Chung-Cheng Chang (Department of Chemistry, National Sun Yat-Sen University). Received for review May 11, 1998. Accepted April 9, 1999. AC980516P

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