Matrix-assisted laser desorption ionization using lithium-substituted mordenite surface

Chemical Physics Letters 546 (2012) 159–163

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Matrix-assisted laser desorption ionization using lithium-substituted mordenite surface Junya Suzuki a, Asami Sato a, Ryo Yamamoto a, Takashi Asano b, Taku Shimosato a, Hisashi Shima c, Junko N. Kondo c, Ken-ichi Yamashita a, Kenro Hashimoto a, Tatsuya Fujino a,⇑ a b c

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan Metropolitan Police Department, 2-1-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-8929, Japan Research Laboratory of Resource Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

a r t i c l e

i n f o

Article history: Received 11 June 2012 In final form 20 July 2012 Available online 2 August 2012

a b s t r a c t An efficient matrix for matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) has been developed. The use of 2,4,6-trihydroxyacetophenone (THAP) on lithium-substituted zeolite has enabled ionization of low-molecular-weight compounds that are not detectable by conventional MALDI-MS. IR measurements and quantum-chemical calculations have also been carried out to understand the adsorption structure of THAP on zeolite. The mechanism of Li+ adduction from zeolite to THAP is also discussed. The important role of the water molecules adsorbed on the zeolite surface for the Li+ adduction from zeolite to THAP has been clarified. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Matrix-assisted laser desorption ionization (MALDI) is one of the methods of ‘soft ionization’ which do not decompose analyte molecules during ionization process. The MALDI in combination with time-of-flight (TOF) mass spectrometry (MALDI-MS) has been widely used in many research fields, because it enables observation of analyte ions in terms of their molecular weights [1–3]. In MALDIMS, a peak of a protonated analyte, [M+H]+, is usually observed. However, the ion yield of the protonated analyte is sometimes influenced by the presence of alkali metal ions. As biological samples intrinsically contain abundant alkali metal ions, the peak intensity of the protonated analyte is suppressed by peaks of alkali metal ion adducts. In addition, MALDI is not suitable for studies of lowmolecular-weight compounds. The analyte undergoes soft ionization, but the matrix molecules that absorb photons from the excitation laser dissociate, producing many matrix-related peaks in the mass regions below 500 Da. Even in the processes of ionization and desorption, there still are many unclear problems. Much effort has been directed to understanding of the ionization processes [4–8]. A possible desorption process based on exciton migration in organic crystals has been proposed in our previous paper [9]. However, we still have not reached a consensus about the total mechanism of MALDI. This hinders the detection of many kinds of molecules by this method. To solve these problems, many attempts have been made to use various compounds as comatrices [10–15], and those attempts have produced valuable re⇑ Corresponding author. Fax: +81 42 677 2525. E-mail address: [email protected] (T. Fujino). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.07.050

sults. A matrix-free technique, such as desorption/ionization on porous silicon (DIOS) [16], and the use of a nano-structured surface have been proposed as well [17]. Recently, we used cyclodextrin [18,19] and zeolite [20] as host molecules for typical MALDI matrices and found that they suppressed the fragmentation of a guest matrix molecule and increased the peak intensities of the protonated analytes. Zeolites are crystalline aluminosilicates with nanometer-order cages and usually act as solid acid catalysts. Zeolites have high catalytic acidity due to the charge imbalance at the Si–O–Al bridging sites, and those sites are compensated by such cations as H+ and Na+. In the present Letter, we have exchanged cations on the zeolite surface with Li+ for application of analytes that are hardly ionized by proton adduction. Conventional MALDI as well as our previous method [20] using proton-type zeolite, which provides a proton to an analyte, are not applicable to such compounds as organic acids and biologically active substances. However, by using a typical organic matrix, 2,4,6-trihydroxyacetophenone (THAP), adsorbed on Li+-zeolite (zeolite matrix hereafter), we have observed the ion peaks of Li+-adducted analytes that are hardly detectable by conventional MALDI. Based on the IR measurements and quantumchemical calculations carried out to understand the adsorption of THAP on the zeolite surface, the mechanism of Li+ adduction from zeolite to THAP is discussed. 2. Experimental Proton-type mordenite zeolite (HM20) was supplied by the Catalysis Society of Japan. Zeolite, HM20, was mixed into the lithium acetate aqueous solution (2.0 mol L 1) and the mixture was

J. Suzuki et al. / Chemical Physics Letters 546 (2012) 159–163

HO

THAP/HM20(e)

OH HO

O

OH

3. Results and discussion 3.1. Ionization of low-molecular-weight compounds Mass spectrometric measurements were carried out with THAP/ LiM20(ne). Figure 2a shows the mass spectrum of maltohexaose (M). The peak of Li+-adducted maltohexaose was observed at m/ z = 998. Neither the analyte peak of a protonated species nor those of adducted species with other alkali metal ions were observed. Therefore, ionization of the analyte molecule by the cation on the zeolite surface was achieved. Figure 2b shows the mass spectrum of maltoheptaose. The peak of Li+-adducted analyte was observed at m/z = 1160. For both spectra, the peak intensity of the Li+-adducted analyte was almost half or slightly lower than the peak intensity of the THAP. In conventional MALDI-MS, the analyte peak intensity is usually quite weak (less than 1/10 of the matrix peak intensity), although it also depends on the mixing ratio of the matrix and analyte. This enhancement of the analyte peak intensity is one of the advantages of our zeolite matrix, as in the case of proton-type zeolite [20]. As for the matrix molecules, the peaks of Li+-adducted as well as protonated THAP were observed in both spectra, although no peak of the protonated analyte was observed. This implies that Li+ adduction from [THAP+Li]+ to analyte overrode H+ adduction from [THAP+H]+ to analyte. The binding energies of both Li+ and H+ to THAP were estimated to be 236 and 927 kJ mol 1, respectively, by quantum-chemical calculations at the B3LYP/6-31+G(d,p) level [21]. Thus we infer that Li+ can detach more easily than H+ from [THAP+Li]+ to produce [analyte+Li]+. Besides, we carried out FT-IR measurements for three zeolite matrices used in this Letter (Figure S1), from which we confirmed that the damages and structural changes of zeolite matrices by laser irradiation was negligible. The developed zeolite matrix is applicable to some compounds with low molecular weight which are not measurable by conventional MALDI-MS. Acetylsalicylic acid (ASA) is one of the molecules

0.8 0.6

H

H OH H

O O HO

OH

H

O HO

H

H

O OH

H

OH

H

H

H

O

OH H

H

6

998[M+Li]+

0.0 (b) Maltoheptaose/THAP/LiM20(ne)

0.8

H OH

H OH H

0.6

OH

Li

Li

0.2

O

O

0.0

zeolite

zeolite

Figure 1. Schematic structures of THAP/LiM20(ne), THAP/LiM20(w), and THAP/ LiM20(e).

H OH H

HO HO

343[2THAP+Li]+

0.2

0.4 O

H OH

175[THAP+Li]+

0.4

OH

(H2O)n

169[THAP+H]+

(a) Maltohexaose/THAP/LiM20(ne)

1.0

169[THAP+H]+

THAP/HM20(ne) THAP/HM20(w)

ions produced from the sample were accelerated with an electric field of 3.0–4.0 kV, and mass spectra were acquired with a linear TOF tube and an MCP detector.

7[Li]+

stirred at 353 K. After filtration, the residue was washed with distilled water, and the entire mixture was heated at 723 K to obtain Li+-substituted zeolite (LiM20). The substitution of zeolitic proton with an alkali cation (>95%) was confirmed by the disappearance of the OH-stretching mode in the FT-IR spectrum in vacuum. THAP (4 mg) and LiM20 (8 mg) were properly mixed in a mortar and pestle to produce zeolite matrix for mass measurements. This is named ‘THAP/LiM20(ne)’, as THAP molecules are introduced onto the non-evacuated (ne) zeolite surface where water molecules in atmosphere are pre-adsorbed on zeolitic OLi groups. In order to understand the Li+ transfer mechanism, we also used other zeolite matrices, such as ‘THAP/LiM20(e)’ and ‘THAP/ LiM20(w).’ To prepare THAP/LiM20(e), an anhydrous dichloromethane solution of THAP (1.0  10 3 mol L 1) was added to the dried LiM20 in vacuum. After filtration, the mixture was washed with dichloromethane to remove THAPs on outer surface. In this case, THAP molecules were found only in the zeolite cavity and considered to be adsorbed on the evacuated (e) zeolite surface directly. To prepare THAP/LiM20(w), a dichloromethane solution of THAP (1.0  10 3 mol L 1) was added to non-evacuated zeolite and thus-treated zeolite was washed with dichloromethane. Therefore, THAP molecules were found in THAP/LiM20(w) only in the zeolite cavity and were adsorbed on the zeolite surface via water (w) molecules. The amount of THAP incorporated into the zeolite cavity was determined by UV–vis absorption spectroscopy. By using chloroform, the solution seemed to become transparent by the index matching between zeolites and the solvent. Based on the solution volume and the absorbance of THAP in chloroform (10 5 mol L 1) at 285 nm (optical density: OD = 0.1376), the amount of THAP incorporated into the LiM20 cavity was calculated to be 8.14 lg (0.048 lmol) per mg of THAP/LiM20(e) as the OD of THAP/ LiM20(e) was 0.0667. Therefore, the weight ratio of incorporated THAP and LiM20 in THAP/LiM20(e) was determined to be 1:123 (±2). On the other hand, the amount of THAP incorporated into the LiM20 cavity was calculated to be 8.06 lg per mg of THAP/ LiM20(w), and the weight ratio of incorporated THAP and LiM20 in THAP/LiM20(w) was determined to be 1:124 (±2). Therefore, it was clarified that an almost equal amount of THAP was incorporated for THAP/LiM20(e) and THAP/LiM20(w). Schematic structures of the three zeolite matrices are shown in Figure 1. The zeolite matrices were suspended in a mixture of acetonitrile and water (7:3 v/v). One microliter each of zeolite matrix and analyte solution (0.50 mg mL 1) was pipetted onto a stainless steel plate, left in air for a few minutes to evaporate the solvent, and analyzed by a laboratory-built MALDI mass spectrometer. A Nd:YAG laser (Spectra Physics, 266 nm, 10 Hz, 2 ns) was used as the excitation light with a typical laser fluence of 5 lJ cm 2. The

intensity (a.u.)

160

0

HO HO

175[THAP+Li]+

200

400

H H

H OH H

O O HO

OH H

H H

600 800 mass (m/z)

H

O O HO H

7

O OH

H

OH H

OH H

1160[M+Li]+

1000

1200

Figure 2. Mass spectra of (a) maltohexaose and (b) maltoheptaose measured with THAP/HM20(ne). Analytes were observed as Li+-adducted species.

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169 [THAP+H]+

(a) ASA/THAP

6.0

23 [Na]+ 39 [K]+

(a) THAP/LiM20(e)

191 [THAP+Na]+ O

169 [THAP+H]+

OH

1.0

O O

163 [ASA-OH]+

(b) ASA

121 [ASA-OCOCH3]+

203 [ASA+Na]+ 219 [ASA+K]+

2.0

187 [ASA+Li]+

(c) ASA/THAP/LiM20(ne)

193 [ASA+2Li-H]+

7 [Li]+

0.5

0.0

(c) THAP/LiM20(ne)

1.5 1.0 0.5

175 [THAP+Li]+

0.0 (d) PB/THAP/LiM20(ne) 0.6

7 [Li]+

0.4 0.2

239

0.0

[PB+Li]+ O

H N

0

O

50

100 150 mass (m/z)

NH

200

250

O

300

Figure 3. Mass spectra of (a) acetylsalicylic acid (ASA; analyte) and THAP (matrix) as measured by conventional MALDI, (b) those obtained by LDI of ASA crystals, (c) ASA and (d) phenobarbital (PB) measured with THAP/LiM20(ne).

(a) Maltohexaose/LiM20(ne)

OH(2)

(3)HO O OH(1)

1630 δOH(3)

250

2640 νOH(3)

150 200 mass (m/z)

2940 νCH

100

3250 νOH(3)

50

Δ absorbance (0.1/div)

0

3620 νOH(w) 3530 νOH(w)

(d) THAP/LiM20(e) - bare LiM20

0.0

3655 νOH(1&2)

intensity (a.u.)

(b) THAP/LiM20(w) intensity (a.u.)

4.0

175 [THAP+Li]+

1.0 0.8

(b) Maltohexaose/anthracene/LiM20(ne)

0.6

4000

178[anthracene]+

intensity (a.u.)

0.4

3500

3000

2500 2000 wavenumber (cm-1)

1500

Figure 5. Mass spectra of (a) THAP/LiM20(e), (b) THAP/LiM20(w), and (c) THAP/ LiM20(ne). No analyte molecules were included in the measurements. (d) Difference FT-IR spectrum of THAP/LiM20(e) and bare LiM20(e).

0.2 0.0 (c) Maltohexaose/THAP/CH3COOLi 1.2 175[THAP+Li]+ 169[THAP+H]+

1.0 0.8 0.6 0.4

998[M+Li]+

0.2 0.0 0

200

400

600

800

1000

mass (m/z) Figure 4. Mass spectra of maltohexaose measured (a) with LiM20 only (without THAP), (b) with LiM20 and anthracene, and (c) with THAP and lithium acetate.

that are hardly measurable but can be ionized by MALDI-MS. Figure 3a shows the results of mass spectrometric measurements of ASA carried out only with THAP (conventional method, without zeolite). The exact mass number of ASA is 180.16 g mol 1. However, neither the peak of the protonated analyte nor that of the adducted species of alkali metal ions could be observed in Figure 3a. Even with ASA crystals, no peaks of ASA could be observed without decomposition. Figure 3b shows the mass spectrum obtained by la-

ser desorption ionization (LDI) of ASA crystals, in which matrix molecules were not used. Although small peaks of Na+- and K+-adducted ASA were observed, strong peaks of fragments [ASA–OH]+ and [ASA–OCOCH3]+ were observed at m/z = 163 and 121, respectively. When THAP/LiM20(ne) was used, however, the peak of ionized ASA was observed. The peaks of [ASA+Li]+ (m/z = 187) and [ASA+2Li–H]+ (m/z = 193) were clearly observed in Figure 3c. Therefore, THAP/LiM20(ne) can ionize acid compounds to which protons hardly adduct. In addition to ASA, phenobarbital (PB), a wellknown hypnotic, is very difficult to observe by conventional MALDI-MS, but the mass spectrum of PB measured with THAP/ LiM20(ne) is recorded in Figure 3d. Contrary to conventional MALDI, a strong peak of [PB+Li]+ has been observed at m/z = 239, and its intensity is comparable to that of [THAP+Li]+ at m/z = 175. These results confirm that our zeolite matrix, which is produced with Li+-substituted zeolite (LiM20), can be a powerful tool for laser desorption ionization mass spectrometry. 3.2. Mechanism of Li+ adduction To fully understand the mechanism of Li+ adduction, mass spectrometry was also carried out. Figure 4a shows the result of maltohexaose carried out with LiM20 only (i.e., without THAP). The amount of maltohexaose (505 pmol) was equal to that used in Figure 2a, but no peaks were detectable in Figure 4a. Therefore, the

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matrix molecule is necessary for ionization of the analyte, and the production of an ionized matrix, [m+Li]+, should be the precursor for the production of an ionized analyte. The LiM20 itself cannot act as a matrix, since zeolite has no absorption coefficient in the UV–vis region, so that the direct Li+ transfer from LiM20 to analyte is negligible. However, the presence of a substantial absorption coefficient in the wavelength region of the excitation laser is not sufficient for qualification as a matrix molecule, because the matrix molecule should be capable of cation adduction. Figure 4b shows the results on maltohexaose carried out with LiM20 and anthracene, in which anthracene was used as the matrix molecule instead of THAP. Anthracene and LiM20 were properly mixed in a mortar and pestle, so that anthracene was expected to be almost monodispersed as in the case of THAP. However, no analyte peak was detectable in Figure 4b; instead, a strong peak of anthracene ([anthracene]+; m/z = 178) was observed. Anthracene is a polycyclic aromatic hydrocarbon (PAH) that is easily cationized rather than forming Li+-adducted species. It is thus understood that Li+adducted anthracene cannot be produced by the photoexcitation of anthracene, which is why the peak of Li+-adducted analyte cannot be observed. The LiM20 also plays an important role as a cation source. When lithium acetate (3.6 mg mL 1) was used instead of LiM20, the mass peak of Li+-adducted maltohexaose could hardly be observed, as shown in Figure 4c. This finding implies that LiM20 has higher potential as a cation source than lithium salts. In the preparation of THAP/LiM20(ne), THAP was mixed with the non-evacuated (ne) zeolite on which water molecules in the atmosphere were adsorbed. It was thus expected that the adsorbed water molecules would affect the formation of [THAP+Li]+. To confirm this, mass spectrometry was carried out with THAP/LiM20(e) and THAP/LiM20(w). No analyte molecule was used in order to fo-

cus on the ionization of the matrix molecule. Only weak peaks of [THAP+Li]+ and [THAP+H]+ were observed for THAP/LiM20(e) in Figure 5a; this indicates that Li+ adduction to THAP from LiM20 did not occur efficiently. To the contrary, large peaks of [THAP+Li]+ and [THAP+H]+ were observed when THAP/LiM20(w) was used (Figure 5b). This finding clearly shows that water molecules on the LiM20 surface worked efficiently for the Li+ adduction to THAP molecule. The mass spectrum obtained from THAP/LiM20(ne) is shown for comparison in Figure 5c, where intense peaks by [THAP+Li]+ and [THAP+H]+ were also observed since THAP molecules were adsorbed on Li-zeolite via water molecules. One may assume that some portion of Li+ on the zeolite surface can be exchanged by a proton, since we used a mixture of acetonitrile and water (7:3 v/ v) as the solvent in the sample preparation for mass spectrometry. As described above, we designed to exchange H+ with Li+ for observing the disappearance of the OH-stretching mode of ‘zeolite’. However, we could not estimate the re-exchange ratio after the sample preparation, since the OH-stretching modes of ‘water molecules’ showed intense and broadened peaks, so that they hindered detection of the reformed OH-stretching modes of ‘zeolite’. In order to understand why THAP/LiM20(e) does not produce matrix-related peaks, the adsorption structure of THAP on LiM20 was studied by FT-IR and quantum-chemical calculations. It was difficult to assign the IR spectrum of THAP/LiM20(ne) for the same reason as above; the OH-stretching modes of ‘water molecules’ hindered the assignment. In this sense, we examined the IR spectra of THAP/LiM20(e) measured in vacuum. The difference IR spectrum shown in Figure 5d, where the contribution of LiM20(e) was removed, could be deconvoluted into several Gaussian line profiles with peak wavenumbers of 3655, 3620, 3530, 3250, 2940, and 2640 cm 1. For assignment of these bands, a binary complex

Figure 6. Illustration of four THAP isomers (top), and nine lowest-energy structures of the binary complex between THAP and AlH3SiOLiSIH3 at the B3LYP/6-31+G(d,p) level. Their relative energies (kJ mol 1) are given under their structures. The numbers in parentheses are the stretching frequencies (in cm 1) of the OH(1), OH(2) and OH(3) groups in this order.

J. Suzuki et al. / Chemical Physics Letters 546 (2012) 159–163

between THAP and AlH3SiOLiSIH3 was examined at the B3LYP/631+G(d,p) level. Four representative structures (I–IV) of THAP are shown in the top part of Figure 6. The geometries of the complex in which THAP is bound to the Li by one or two O atoms were optimized. The structures in which Li was initially placed at one of the A–D sites were examined for the THAP of I–IV. We also started the optimization with Li at the E site for the THAP I and IV. In addition, we took into account three combinations of the directions of OH(1) and OH(2), and the internal rotation of the AlH3SiOLiSIH3 about the Li–O bond. The 69 minima, which were confirmed by the vibrational analysis, were obtained, starting from 144 initial geometries. Nine lowest structures are illustrated in the middle and bottom rows of Figure 6 with their relative energies. The calculated harmonic frequencies (wavenumbers) of the OH-stretching modes are given each structure, being uniformly scaled by 0.970. Other structures are given in the Supplementary material. The complex structures derived from THAP I and II are more stable than those from III and IV. The isomers where Li is located at A, B, and E are more stable than others, where Li is located at C and D irrespective of the structures of THAP. Finally, four stable structures (I1, I2, II1, II2) are considered to contribute to the FT-IR spectrum. The wavenumbers of the free OH(1) and OH(2) stretching modes are calculated to be 3700 cm 1, which nearly agree with the observed shoulder around 3655 cm 1 in Figure 5d. The wavenumbers of the OH(3) stretching mode, computed to be 2570 and 3200 cm 1, are close to the weak bands observed at 2640 and at 3250 cm 1, respectively, in Figure 5d. The spectral shapes of the remaining bands at 3620 and 3530 cm 1 quite resemble those of water molecules on zeolite, although the wavenumbers of the OH-stretching modes of water molecules are 3698 and 3558 cm 1 for H2O on HZSM5 zeolite [22]. As the preparation of THAP/LiM20(e) was carried out at 1 Pa, only a very small amount of water molecules may have remained on the LiM20 surface prior to the introduction of THAP. Therefore, THAP/LiM20(e) can be considered as a mixture of THAP adsorbed mainly on dry LiM20 and on the LiM20 surfaces pre-adsorbed with water molecules. The reason why THAP/LiM20(e) does not produce matrix-related peaks may be understood by taking the computed stable structures into account. Our previous work on the proton-type zeolite [20] has given a clue for understanding. The intramolecular proton transfer from OH(3) to CO in THAP and the formation of an ionic complex of [THAP+H]+ and [Zeolite] are the two important processes for the proton transfer from zeolite to THAP. In the isomers I1 and I2, OH(3) breaks its intramolecular hydrogen bond with CO and faces the opposite side. The proton of OH(3) cannot migrate to CO, so that Li+ on the zeolite surface cannot move to THAP because no charge-separated structure (COH+


O (3) in THAP) is formed. In the isomers II1 and II2, OH(3) still forms an intramolecular hydrogen bond. However, OLi on zeolite adsorbs on the CO of THAP and Li+ adduction cannot take place efficiently in this case. For THAP/LiM20(ne) and THAP/LiM20(w), THAP adsorbs on the LiM20 via water molecules. In this case, THAP interacts with water molecules at the intramolecular hydrogen bonded OH(3) [20], and the Li+ transfer occurs efficiently from LiM20 to THAP.

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4. Conclusion We have developed an efficient matrix for matrix-assisted laser desorption ionization (MALDI) mass spectrometry. The zeolite matrix, THAP/LiM20(ne), has enabled us to measure the mass spectra of low-molecular-weight compounds as Li+-adducted ions. Furthermore, we have found that the zeolite matrix can be used with such compounds as acetylsalicylic acid and phenobarbital, which cannot be measured or ionized by conventional MALDI. The mechanism of Li+ adduction and the roles of THAP, zeolite, and water molecules have been investigated by mass spectrometry, FT-IR measurement, and quantum-chemical calculations. Water molecules on the zeolite surface are found to be important for the Li+ adduction from LiM20 to THAP. Acknowledgements T. F. and K. H. acknowledge a Grant-in-Aid for Scientific Research on Priority Area (No. 477) from MEXT. T. F. acknowledges a Grant-in-Aid for Scientific Research (C) (No. 24550030) from JSPS. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2012. 07.050. References [1] K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshida, Rapid Commun. Mass Spectrom. 2 (1988) 151. [2] F. Hillenkamp, M. Karas, R.C. Beavis, B.T. Chait, Anal. Chem. 63 (1991) 1193A. [3] F. Hillenkamp, M. Karas, Int. J. Mass. Spectrom. 200 (2000) 71. [4] H. Ehring, M. Karas, F. Hillenkamp, Org. Mass Spectrom. 27 (1992) 427. [5] M. Karas, M. Gluckmann, J. Schafer, J. Mass Spectrom. 35 (2000) 1. [6] R. Kruger, A. Pfenninger, I. Fournier, M. Gluckmann, M. Karas, Anal. Chem. 73 (2001) 5812. [7] M. Karas, R. Kruger, Chem. Rev. 103 (2003) 427. [8] W.C. Chang et al., Anal. Chim. Acta 582 (2007) 1. [9] Y. Minegishi, D. Morimoto, J. Matsumoto, H. Shiromaru, K. Hashimoto, T. Fujino, J. Phys. Chem. C 116 (2012) 3059. [10] C. Koester, J.A. Castoro, C.L. Wilkins, J. Am. Chem. Soc. 114 (1992) 7572. [11] J. Asara, J. Allison, J. Am. Soc. Mass Spectrom. 10 (1999) 35. [12] S. Kjellstroem, O.N. Jensen, Anal. Chem. 76 (2004) 5109. [13] T. Kobayashi, H. Kawai, T. Suzuki, T. Kawanishi, T. Hayakawa, Rapid Commun. Mass Spectrom. 18 (2004) 1156. [14] X. Yang, H. Wu, T. Kobayashi, R.J. Solaro, R.B. Breemen, Anal. Chem. 76 (2004) 1532. [15] T. Nishikaze, M. Takayama, Rapid Commun. Mass Spectrom. 21 (2007) 3345. [16] R. Arakawa, Y. Shimomae, H. Morikawa, K. Ohara, S. Okuno, J. Mass Spectrom. 39 (2004) 961. [17] H. Kawasaki, T. Yonezawa, T. Watanabe, R. Arakawa, J. Phys. Chem. C 111 (2007) 16278. [18] S. Yamaguchi, T. Fujita, T. Fujino, T. Korenaga, Anal. Aci. 24 (2008) 1497. [19] T. Fujita, T. Fujino, K. Hirabayashi, T. Korenaga, Anal. Sci. 26 (2010) 743. [20] Y. Komori, H. Shima, T. Fujino, J.N. Kondo, K. Hashimoto, T. Korenaga, J. Phys. Chem. C 114 (2010) 1593. [21] M.J. Frisch et al., GAUSSIAN 09, Revision B.01, GAUSSIAN, Inc., Wallingford CT, 2010. [22] F. wakabayashi, J.N. Kondo, K. Domen, C. Hirose, J. Phys. Chem. 100 (1996) 1442.