- Email: [email protected]
Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts
Nuclear Instruments and Methods in Physics Research B 268 (2010) 2930–2932
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts K. Hirata a,*, Y. Saitoh b, A. Chiba b, K. Yamada b, Y. Takahashi c, K. Narumi c a
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Takasaki Advanced Radiation Research Institute (TARRI), Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan c Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan b
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 22 April 2010 Available online 31 May 2010
a b s t r a c t Emission yields of secondary ions necessary for the identification of poly-tyrosine were compared for þ þ incident ion impacts of energetic cluster ions (0.8 MeV Cþ 8 , 2.4 MeV C8 , and 4.0 MeV C8 ) and swift heavy monoatomic molybdenum ions (4.0 MeV Mo+ and 14 MeV Mo4+) with similar mass to that of the cluster by time-of-flight secondary ion mass analysis combined with secondary ion electric current measurements. The comparison revealed that (1) secondary ion emission yields per Cþ 8 impact increase with increasing incident energy within the energy range examined, (2) the 4.0 MeV Cþ 8 impact provides higher emission yields than the impact of the monoatomic Mo ion with the same incident energy (4.0 MeV Mo+), and (3) the 2.4 MeV Cþ 8 impact exhibits comparable emission yields to that for the Mo ion impact with higher incident energy (14 MeV Mo4+). Energetic cluster ion impacts effectively produce the characteristic secondary ions for poly-tyrosine, which is advantageous for highly sensitive amino acid detection in proteins using time-of-flight secondary ion mass analysis. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Secondary ions (SIs) are ejected from a target surface upon ion impact, due to the energy transfer from the incident ion to the target, and the emission yields are strongly dependent on various conditions of the target and incident ion. Cluster ion impacts yield different SI emission phenomena from those for monoatomic ions [1,2] because of their peculiar irradiation effects caused by simultaneous energy transfer from the constituent atoms of the cluster to a small area of the target surface. The electric currents of SIs upon monoatomic and cluster ion impacts were measured for an organic polymer, and the impacts of cluster ions were found to provide higher electric current per incident atom than that of the corresponding monoatomic ion of the same element with the same velocity [3]. Higher emission yields of SIs by cluster ion impacts are useful for surface analysis; therefore, cluster ion beams were applied to SI mass spectrometry [4] and the emission yields of mass-analyzed SIs originating from surface organic compounds were compared with monoatomic and cluster ion impacts using a time-of-flight (TOF) mass spectrometer [5]. TOF SI mass analysis can provide chemical information of organic targets with chemical specificity and surface sensitivity. Characterization of adsorbed proteins on a material surface is * Corresponding author. E-mail address: [email protected] (K. Hirata). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.011
important for research on the biological responses upon implantation of a medical device, and TOF SI mass analysis is one of the most powerful methods available to characterize adsorbed proteins by analysis of the characteristic SI peaks necessary for identification of amino acids constituting the protein [6,7]. In this paper, we report the emission yields of characteristic SIs from a poly(amino acid) target upon impacts of energetic carbon cluster C8 ions with different incident energies. TOF SI mass analysis combined with SI electric current measurements were performed to identify the SI species and obtain their relative emission yields. The SI emission yields for the impacts of Cþ 8 and swift heavy monoatomic molybdenum ions with similar mass to C8 ions were also compared. 2. Experimental setup A poly-tyrosine film was prepared by spin-coating an ethanol solution of poly-L-tyrosine (Sigma–Aldrich Co.) on a silicon substrate, and SI emission yield measurements were performed using a TOF mass analyzer combined with pulsed ion beams produced by a 3 MV tandem accelerator at the Japan Atomic Energy Agency (JAEA)/Takasaki [8,9], which has been described elsewhere [5]. Briefly, a direct negative current ion beam produced in a conventional Cs sputter ion source was pulsed by electrostatic deflection plates triggered by a pulse generator and an aperture before injection into the tandem accelerator. The negative ions are injected
2931
K. Hirata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2930–2932
into the tandem and accelerated toward the high voltage terminal positioned in the middle of the accelerator vessel. The negative ions are stripped to positive ions upon colliding with the stripper gas in the charge exchange area and reaccelerated toward the ground level. After collimation to 1 mm in diameter using a series of collimators, pulsed ion beams were injected into a target that was set on a grounded metal sample holder. Positive SIs induced by pulsed primary ions with an incident angle of 45° to the target surface were analyzed using a linear type TOF mass spectrometer. The SI counting system of the spectrometer was improved using a fast digital storage oscilloscope in order to obtain more accurate TOF mass spectra [10]. For quantitative comparison of SI emission yields under various irradiation conditions, peak intensities of the TOF spectra should be scaled with the condition of the same number of incident primary atoms or impacts. Considering that most of the SIs emitted from the target are singly charged, the peak intensities per incident atom and impact were compared by scaling the total count of each spectrum based on qIS/nIo and qIS/Io (q: incident ion charge number, n: incident ion cluster number, Io: incident beam electric current, IS: SI electric current for each irradiation condition), respectively. Io and IS are directly measured using highly sensitive electrometers respectively connected to a Faraday cup and a movable metal plate with a grid, as described elsewhere [5]. 3. Results and discussion TOF SI mass spectrometry is a suitable technique for characterization of adsorbed proteins on material surfaces [7]. Characteristic SI peaks necessary for identification of amino acids constituting proteins are used for interpretation of complex mass spectra of the proteins. The characteristic SI peaks necessary for identification of poly-tyrosine are observed in positive SI spectra at m/z (mass/ charge ratio) = 107 and 136 for C7H7O and C8H10NO, respectively [6]. Fig. 1 shows positive SI mass spectra of the poly-tyrosine film
(a) 0.8 MeV C8+
þ in the m/z range from 100 to 150 for (a) 0.8 MeV Cþ 8 , (b) 2.4 MeV C8 , þ + 4+ (c) 4.0 MeV C8 , (d) 4.0 MeV Mo , and (e) 14 MeV Mo . The total count of each spectrum was scaled based on qIs/Io for each irradiation condition in order to quantitatively compare the SI emission yields per incident ion impact among the spectra. The SI emission yields for the 4.0 MeV Cþ 8 impact were increased by a factor of 3– 4 in comparison with those expected under the assumption that each constituent atom of the cluster behaves as a monoatomic ion with the same velocity. For ease of comparison of the emission yields of the SIs at m/z = 107 and 136 in Fig. 1. Fig. 2 shows the ratio R of the SI emission yield for each SI to the corresponding SI emission yield for the 4.0 MeV Cþ 8 impact. The relative SI emission yield per C8 impact increases with the increase in the incident energy of the cluster ion in the energy range studied. An incident ion transfers its kinetic energy to a target by electronic and nuclear energy loss processes. Emission yields of SIs, produced as a result of the energy transfer from an incident ion to a target, are influenced by the energy transfer processes and their deposited energy densities around the impactpoints. According to the TRIM [11] simulation, the respective electronic and nuclear stopping powers at the surface for C8 increases and decreases with increasing incident energy from 0.8 to 4.0 MeV, provided the stopping power for a cluster ion with n identical atoms is n times that of a monoatomic ion with the same velocity. Considering that the relative SI emission yields are higher in the orþ þ der of 4.0 MeV Cþ 8 , 2.4 MeV C8 , and 0.8 MeV C8 , the electronic energy transfer process makes a dominant contribution to the SI emission. Similarly, the higher emission yield of 14 MeV Mo4+ than that of 4.0 MeV Mo+ for both the characteristic SIs can be attributed to the higher energy that is electronically transferred to the target surface for the former than for the latter. Comparison of the SI emission yields for the C8 impacts with those for the Mo impacts, as shown in Fig. 2, indicates that the 4.0 MeV Cþ 8 impact provides 4–5 times higher SI emission yields than the impact of 4.0 MeV Mo+, and the 2.4 MeV Cþ 8 impact exhibits SI emission yields comparable to those for the 14 MeV Mo4+ impact. Therefore, the higher emission yields of the SIs necessary for identification of poly-tyrosine are obtained with lower incident ion energy using cluster ions rather than swift heavy monoatomic ion. It should be noted that the energies that were electronically transþ ferred to the target surface for 4.0 MeV Cþ 8 and 2.4 MeV C8 , which were calculated using TRIM, are higher than that for 4.0 MeV Mo+
Relative in ntensity
(b) 2.4 MeV C8+
(c) 4.0 MeV C8+
1 0.8 0.6
(d) 4.0 MeV Mo+
0.4
R
0.2 (e) 14 MeV Mo4+
m/z = 107
0
m/z = 136
100 110 120 130 140 150 m/z Fig. 1. Positive secondary ion TOF spectra of poly-tyrosine for impacts of (a) þ þ + 4+ 0.8 MeV Cþ 8 , (b) 2.4 MeV C8 , (c) 4.0 MeV C8 , (d) 4.0 MeV Mo , and (e) 14 MeV Mo . The relative intensity on the vertical axis is proportional to the SI emission yield per incident ion impact. Peaks at m/z = 107 and 136 are characteristic peaks for polytyrosine, C7H7O and C8H10NO, respectively.
Fig. 2. Comparison of the emission yields for characteristic SIs of poly-tyrosine among the spectra shown in Fig. 1. The scale of the vertical axis is defined as R; the ratio of the emission yield for each characteristic SI to that for the corresponding SI for a 4.0 MeV Cþ 8 impact.
2932
K. Hirata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2930–2932
and similar to that for 14 MeV Mo4+, respectively. Although there is currently no detailed mechanism that quantitatively explains the SI emission yields upon ion impacts including energetic cluster and swift heavy ion impacts, these results suggest that the electronically transferred energy may be one of the factors determining the SI emission yields upon C8 and Mo ion impacts.
Acknowledgement This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (Grant No. 21604016). References
4. Conclusion Positive SI TOF spectra of a poly-tyrosine film target were obþ þ + tained for 0.8 MeV Cþ 8 , 2.4 MeV C8 , 4.0 MeV C8 , 4.0 MeV Mo , and 4+ 14 MeV Mo impacts. Comparison of the relative emission yields of SIs necessary for identification of poly-tyrosine (m/z = 107 and 136) among the impact conditions revealed the following features: (1) the SI emission yields per C8 impact are higher in the order of þ þ þ 4.0 MeV Cþ 8 , 2.4 MeV C8 , and 0.8 MeV C8 , (2) the 4.0 MeV C8 impact provides 4–5 times higher SI emission yields than the 4.0 MeV Mo+ impact, (3) the 2.4 MeV Cþ 8 impact exhibits comparable SI emission yields to those for the 14 MeV Mo4+ impact. Energetic cluster ion impacts provide the SIs necessary for identification of poly-tyrosine more effectively than impacts of swift heavy monoatomic ion with similar mass to that of the cluster ion. Cluster ion impacts are advantageous for highly sensitive amino acid detection in proteins using TOF SI mass analysis.
[1] M. Benguerba, A. Brunelle, S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, M.G. Blain, E.A. Schweikert, G. Ben Assayag, P. Sudraud, Nucl. Instrum. Methods Phys. Res. B 62 (1991) 8. [2] Y. Le Beyec, Int. J. Mass Spectrom. Ion Phys. 174 (1998) 101. [3] K. Hirata, Y. Saitoh, K. Narumi, Y. Kobayashi, Appl. Phys. Lett. 81 (2002) 3669. [4] K. Hirata, Y. Saitoh, A. Chiba, K. Narumi, Y. Kobayashi, K. Arakawa, Appl. Phys. Lett. 83 (2003) 4872. [5] K. Hirata, Y. Saitoh, A. Chiba, K. Narumi, Y. Kobayashi, Y. Ohara, Appl. Phys. Lett. 86 (2005) 044105. [6] N.T. Samuel, M.S. Wagner, K.D. Dornfeld, D.G. Castner, Surf. Sci. Spectra 8 (2001) 163. [7] D.S. Mantus, B.D. Ratner, B.A. Carlson, J.F. Moulder, Anal. Chem. 65 (1993) 1431. [8] Y. Saitoh, S. Tajima, I. Takada, K. Mizuhashi, S. Uno, K. Ohkoshi, Y. Ishii, T. Kamiya, K. Yotsumoto, R. Tanaka, E. Iwamoto, Nucl. Instrum. Methods Phys. Res. B 89 (1994) 23. [9] Y. Saitoh, K. Mizuhashi, S. Tajima, Nucl. Instrum. Methods Phys. Res. A 452 (2000) 61. [10] K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, Y. Takahashi, K. Narumi, to be published. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts K. Hirata a,*, Y. Saitoh b, A. Chiba b, K. Yamada b, Y. Takahashi c, K. Narumi c a
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Takasaki Advanced Radiation Research Institute (TARRI), Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan c Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Takasaki, Gumma 370-1292, Japan b
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 22 April 2010 Available online 31 May 2010
a b s t r a c t Emission yields of secondary ions necessary for the identification of poly-tyrosine were compared for þ þ incident ion impacts of energetic cluster ions (0.8 MeV Cþ 8 , 2.4 MeV C8 , and 4.0 MeV C8 ) and swift heavy monoatomic molybdenum ions (4.0 MeV Mo+ and 14 MeV Mo4+) with similar mass to that of the cluster by time-of-flight secondary ion mass analysis combined with secondary ion electric current measurements. The comparison revealed that (1) secondary ion emission yields per Cþ 8 impact increase with increasing incident energy within the energy range examined, (2) the 4.0 MeV Cþ 8 impact provides higher emission yields than the impact of the monoatomic Mo ion with the same incident energy (4.0 MeV Mo+), and (3) the 2.4 MeV Cþ 8 impact exhibits comparable emission yields to that for the Mo ion impact with higher incident energy (14 MeV Mo4+). Energetic cluster ion impacts effectively produce the characteristic secondary ions for poly-tyrosine, which is advantageous for highly sensitive amino acid detection in proteins using time-of-flight secondary ion mass analysis. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Secondary ions (SIs) are ejected from a target surface upon ion impact, due to the energy transfer from the incident ion to the target, and the emission yields are strongly dependent on various conditions of the target and incident ion. Cluster ion impacts yield different SI emission phenomena from those for monoatomic ions [1,2] because of their peculiar irradiation effects caused by simultaneous energy transfer from the constituent atoms of the cluster to a small area of the target surface. The electric currents of SIs upon monoatomic and cluster ion impacts were measured for an organic polymer, and the impacts of cluster ions were found to provide higher electric current per incident atom than that of the corresponding monoatomic ion of the same element with the same velocity [3]. Higher emission yields of SIs by cluster ion impacts are useful for surface analysis; therefore, cluster ion beams were applied to SI mass spectrometry [4] and the emission yields of mass-analyzed SIs originating from surface organic compounds were compared with monoatomic and cluster ion impacts using a time-of-flight (TOF) mass spectrometer [5]. TOF SI mass analysis can provide chemical information of organic targets with chemical specificity and surface sensitivity. Characterization of adsorbed proteins on a material surface is * Corresponding author. E-mail address: [email protected] (K. Hirata). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.011
important for research on the biological responses upon implantation of a medical device, and TOF SI mass analysis is one of the most powerful methods available to characterize adsorbed proteins by analysis of the characteristic SI peaks necessary for identification of amino acids constituting the protein [6,7]. In this paper, we report the emission yields of characteristic SIs from a poly(amino acid) target upon impacts of energetic carbon cluster C8 ions with different incident energies. TOF SI mass analysis combined with SI electric current measurements were performed to identify the SI species and obtain their relative emission yields. The SI emission yields for the impacts of Cþ 8 and swift heavy monoatomic molybdenum ions with similar mass to C8 ions were also compared. 2. Experimental setup A poly-tyrosine film was prepared by spin-coating an ethanol solution of poly-L-tyrosine (Sigma–Aldrich Co.) on a silicon substrate, and SI emission yield measurements were performed using a TOF mass analyzer combined with pulsed ion beams produced by a 3 MV tandem accelerator at the Japan Atomic Energy Agency (JAEA)/Takasaki [8,9], which has been described elsewhere [5]. Briefly, a direct negative current ion beam produced in a conventional Cs sputter ion source was pulsed by electrostatic deflection plates triggered by a pulse generator and an aperture before injection into the tandem accelerator. The negative ions are injected
2931
K. Hirata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2930–2932
into the tandem and accelerated toward the high voltage terminal positioned in the middle of the accelerator vessel. The negative ions are stripped to positive ions upon colliding with the stripper gas in the charge exchange area and reaccelerated toward the ground level. After collimation to 1 mm in diameter using a series of collimators, pulsed ion beams were injected into a target that was set on a grounded metal sample holder. Positive SIs induced by pulsed primary ions with an incident angle of 45° to the target surface were analyzed using a linear type TOF mass spectrometer. The SI counting system of the spectrometer was improved using a fast digital storage oscilloscope in order to obtain more accurate TOF mass spectra [10]. For quantitative comparison of SI emission yields under various irradiation conditions, peak intensities of the TOF spectra should be scaled with the condition of the same number of incident primary atoms or impacts. Considering that most of the SIs emitted from the target are singly charged, the peak intensities per incident atom and impact were compared by scaling the total count of each spectrum based on qIS/nIo and qIS/Io (q: incident ion charge number, n: incident ion cluster number, Io: incident beam electric current, IS: SI electric current for each irradiation condition), respectively. Io and IS are directly measured using highly sensitive electrometers respectively connected to a Faraday cup and a movable metal plate with a grid, as described elsewhere [5]. 3. Results and discussion TOF SI mass spectrometry is a suitable technique for characterization of adsorbed proteins on material surfaces [7]. Characteristic SI peaks necessary for identification of amino acids constituting proteins are used for interpretation of complex mass spectra of the proteins. The characteristic SI peaks necessary for identification of poly-tyrosine are observed in positive SI spectra at m/z (mass/ charge ratio) = 107 and 136 for C7H7O and C8H10NO, respectively [6]. Fig. 1 shows positive SI mass spectra of the poly-tyrosine film
(a) 0.8 MeV C8+
þ in the m/z range from 100 to 150 for (a) 0.8 MeV Cþ 8 , (b) 2.4 MeV C8 , þ + 4+ (c) 4.0 MeV C8 , (d) 4.0 MeV Mo , and (e) 14 MeV Mo . The total count of each spectrum was scaled based on qIs/Io for each irradiation condition in order to quantitatively compare the SI emission yields per incident ion impact among the spectra. The SI emission yields for the 4.0 MeV Cþ 8 impact were increased by a factor of 3– 4 in comparison with those expected under the assumption that each constituent atom of the cluster behaves as a monoatomic ion with the same velocity. For ease of comparison of the emission yields of the SIs at m/z = 107 and 136 in Fig. 1. Fig. 2 shows the ratio R of the SI emission yield for each SI to the corresponding SI emission yield for the 4.0 MeV Cþ 8 impact. The relative SI emission yield per C8 impact increases with the increase in the incident energy of the cluster ion in the energy range studied. An incident ion transfers its kinetic energy to a target by electronic and nuclear energy loss processes. Emission yields of SIs, produced as a result of the energy transfer from an incident ion to a target, are influenced by the energy transfer processes and their deposited energy densities around the impactpoints. According to the TRIM [11] simulation, the respective electronic and nuclear stopping powers at the surface for C8 increases and decreases with increasing incident energy from 0.8 to 4.0 MeV, provided the stopping power for a cluster ion with n identical atoms is n times that of a monoatomic ion with the same velocity. Considering that the relative SI emission yields are higher in the orþ þ der of 4.0 MeV Cþ 8 , 2.4 MeV C8 , and 0.8 MeV C8 , the electronic energy transfer process makes a dominant contribution to the SI emission. Similarly, the higher emission yield of 14 MeV Mo4+ than that of 4.0 MeV Mo+ for both the characteristic SIs can be attributed to the higher energy that is electronically transferred to the target surface for the former than for the latter. Comparison of the SI emission yields for the C8 impacts with those for the Mo impacts, as shown in Fig. 2, indicates that the 4.0 MeV Cþ 8 impact provides 4–5 times higher SI emission yields than the impact of 4.0 MeV Mo+, and the 2.4 MeV Cþ 8 impact exhibits SI emission yields comparable to those for the 14 MeV Mo4+ impact. Therefore, the higher emission yields of the SIs necessary for identification of poly-tyrosine are obtained with lower incident ion energy using cluster ions rather than swift heavy monoatomic ion. It should be noted that the energies that were electronically transþ ferred to the target surface for 4.0 MeV Cþ 8 and 2.4 MeV C8 , which were calculated using TRIM, are higher than that for 4.0 MeV Mo+
Relative in ntensity
(b) 2.4 MeV C8+
(c) 4.0 MeV C8+
1 0.8 0.6
(d) 4.0 MeV Mo+
0.4
R
0.2 (e) 14 MeV Mo4+
m/z = 107
0
m/z = 136
100 110 120 130 140 150 m/z Fig. 1. Positive secondary ion TOF spectra of poly-tyrosine for impacts of (a) þ þ + 4+ 0.8 MeV Cþ 8 , (b) 2.4 MeV C8 , (c) 4.0 MeV C8 , (d) 4.0 MeV Mo , and (e) 14 MeV Mo . The relative intensity on the vertical axis is proportional to the SI emission yield per incident ion impact. Peaks at m/z = 107 and 136 are characteristic peaks for polytyrosine, C7H7O and C8H10NO, respectively.
Fig. 2. Comparison of the emission yields for characteristic SIs of poly-tyrosine among the spectra shown in Fig. 1. The scale of the vertical axis is defined as R; the ratio of the emission yield for each characteristic SI to that for the corresponding SI for a 4.0 MeV Cþ 8 impact.
2932
K. Hirata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2930–2932
and similar to that for 14 MeV Mo4+, respectively. Although there is currently no detailed mechanism that quantitatively explains the SI emission yields upon ion impacts including energetic cluster and swift heavy ion impacts, these results suggest that the electronically transferred energy may be one of the factors determining the SI emission yields upon C8 and Mo ion impacts.
Acknowledgement This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (Grant No. 21604016). References
4. Conclusion Positive SI TOF spectra of a poly-tyrosine film target were obþ þ + tained for 0.8 MeV Cþ 8 , 2.4 MeV C8 , 4.0 MeV C8 , 4.0 MeV Mo , and 4+ 14 MeV Mo impacts. Comparison of the relative emission yields of SIs necessary for identification of poly-tyrosine (m/z = 107 and 136) among the impact conditions revealed the following features: (1) the SI emission yields per C8 impact are higher in the order of þ þ þ 4.0 MeV Cþ 8 , 2.4 MeV C8 , and 0.8 MeV C8 , (2) the 4.0 MeV C8 impact provides 4–5 times higher SI emission yields than the 4.0 MeV Mo+ impact, (3) the 2.4 MeV Cþ 8 impact exhibits comparable SI emission yields to those for the 14 MeV Mo4+ impact. Energetic cluster ion impacts provide the SIs necessary for identification of poly-tyrosine more effectively than impacts of swift heavy monoatomic ion with similar mass to that of the cluster ion. Cluster ion impacts are advantageous for highly sensitive amino acid detection in proteins using TOF SI mass analysis.
[1] M. Benguerba, A. Brunelle, S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, M.G. Blain, E.A. Schweikert, G. Ben Assayag, P. Sudraud, Nucl. Instrum. Methods Phys. Res. B 62 (1991) 8. [2] Y. Le Beyec, Int. J. Mass Spectrom. Ion Phys. 174 (1998) 101. [3] K. Hirata, Y. Saitoh, K. Narumi, Y. Kobayashi, Appl. Phys. Lett. 81 (2002) 3669. [4] K. Hirata, Y. Saitoh, A. Chiba, K. Narumi, Y. Kobayashi, K. Arakawa, Appl. Phys. Lett. 83 (2003) 4872. [5] K. Hirata, Y. Saitoh, A. Chiba, K. Narumi, Y. Kobayashi, Y. Ohara, Appl. Phys. Lett. 86 (2005) 044105. [6] N.T. Samuel, M.S. Wagner, K.D. Dornfeld, D.G. Castner, Surf. Sci. Spectra 8 (2001) 163. [7] D.S. Mantus, B.D. Ratner, B.A. Carlson, J.F. Moulder, Anal. Chem. 65 (1993) 1431. [8] Y. Saitoh, S. Tajima, I. Takada, K. Mizuhashi, S. Uno, K. Ohkoshi, Y. Ishii, T. Kamiya, K. Yotsumoto, R. Tanaka, E. Iwamoto, Nucl. Instrum. Methods Phys. Res. B 89 (1994) 23. [9] Y. Saitoh, K. Mizuhashi, S. Tajima, Nucl. Instrum. Methods Phys. Res. A 452 (2000) 61. [10] K. Hirata, Y. Saitoh, A. Chiba, K. Yamada, Y. Takahashi, K. Narumi, to be published. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985.