Metal Salts for Molecular Ion Yield Enhancement in Organic Secondary Ion Mass Spectrometry: A Critical Assessment A. Delcorte* and P. Bertrand Unite ´ de Physico-Chimie et de Physique des Mate ´ riaux, Universite ´ catholique de Louvain, 1 Croix du Sud, B-1348, Louvain-la-Neuve, Belgium In a search for molecular ion signal enhancement in organic SIMS, the efficiency of a series of organic and inorganic salts for molecular cationization has been tested using a panel of nonvolatile molecules with very different chemical characteristics (leucine enkephalin, Irganox 1010, tetraphenylnaphthalene, polystyrene). The com- pounds used for cationization include alkali bromide and group Ib metal salts (XBr with X ) Li, Na, K; CF 3 CO 2 Ag; AgNO 3 ; [CH 3 COCHdC(O-)CH 3 ] 2 Cu; AuCl 3 ). Alkali ions, very good for polar molecule cationization, prove to be of limited interest for nonpolar molecules such as polysty- rene. Silver trifluoroacetate displays excellent results for all the considered molecules, except for leucine enkepha- lin (which might be due to the use of different solvents for the analyte and the salt). Instead, silver nitrate mixed with leucine enkephalin in an ethanol solution provides intense molecular signals. The influence of the respective concentrations of analyte and salt in solution, of the silver trifluoroacetate solution stability, and of the sample mi- crostructure on the secondary ion intensities are also investigated. The results of other combinations of analyte and salts are reported. Finally, the use of salts is critically compared to other sample preparation procedures previ- ously proposed for SIMS analysis of large organic mol- ecules. Limited sensitivity constitutes an issue in several application fields of organic secondary ion mass spectrometry (SIMS). Parentlike ions and large molecular fragments are often detected with a signal/noise ratio that is unsatisfactory, for instance, when characterizing 1000-10 000 Da molecules or performing high- resolution SIMS imaging. 1 In the past decade, new types of projectiles, mostly polyatomic, have been used to compensate the limited desorption and ionization yields of molecular and polymeric samples. 2,3 The measured molecular ion yields are oftentimes orders of magnitude larger. There are strong indications from theoretical and experimental studies that these polyatomic primary ions create a “splash” effect at the surface (collective motion in a liquidlike region), thereby ejecting large chunks of material in the vacuum. 4 Their effect on the ionization yield of molecular species has been also discussed. 5 Beside the properties of the incident projectiles, the performance of the SIMS analysis of organic samples is largely dependent on the specific routes used for sample preparation. In our previous articles on this topic, the effect of evaporating a small quantity of a noble metal (Ag, Au) on the sample surface has been thoroughly tested and yield increases of several orders of magnitudes have been reported. 6,7 The results were compared to the traditional cationization method, using a very dilute analyte solution cast on a Ag (Cu, Au) foil. 8-10 Another sample preparation procedure consists of adding salts with the analyte in order to provide a source of cationizing agents for the departing molecules. The salts can either be mixed with the analyte in a single solution or deposited independently from a separate solutionswhich should be more convenient for ap- plications involving bulk samples. Salts are routinely used for synthetic polymer characterization in matrix-assisted laser de- sorption-ionization 11 (MALDI) 12-15 but rarely in SIMS. Neverthe- less, the first few reports on the subject are found in the SIMS literature, 16-18 before their application to MALDI. In parallel with the enthusiasm for the MALDI analysis, they have reappeared in * To whom correspondence should be addressed. Phone: 32-10-473582. Fax: 32-10-473452. E-mail: delcorte@pcpm.ucl.ac.be. (1) Vickerman, J. C. In ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; SurfaceSpectra/IMPublications: Chich- ester, 2001; Chapter 1. (2) Van Stipdonk, M. In ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; SurfaceSpectra/IMPublications: Chich- ester, 2001; Chapter 12. (3) Weibel, D.; Wong, S.; Lockyer, N.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C.; Anal. Chem. 2003, 75, 1754. (4) Postawa, Z.; Czerwinski, B.; Szewczyk, M.; Smiley, E. J.; Winograd, N.; Garrison, B. J. Anal. Chem. 2003, 75, 4402. (5) Gillen, G.; Fahey, A. Appl. Surf. Sci. 2003, 203-204, 209, (6) Delcorte, A.; Me ´ dard, N.; Bertrand, P. Anal. Chem. 2002, 74, 4955. (7) Delcorte, A.; Bour, J.; Aubriet, F.; Muller, J. F.; Bertrand, P. Anal. Chem. 2003, 75, 6875. (8) Grade, H.; Winograd, N. Cooks, R. G. J. Am. Chem. Soc. 1977, 99, 7725. (9) Benninghoven, A.; Lange, W.; Jirikowsky, M.; Holtkamp, D. Surf. Sci. 1982, 123, L721. (10) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Benninghoven, A. Macro- molecules 1987, 20, 407. (11) Pasch, H.; Schrepp, W.; MALDI-TOF Mass Spectrometry of Synthetic Polymers; Springer, Berlin, 2003. (12) Rashidezadeh, H.; Guo, B. J. Am. Soc. Mass Spectrom. 1998, 9, 724. (13) Gidden, J.; Wyttenbach, Th.; Batka, J. J.; Weis, P.; Jackson, A. T.; Scrivens, J. H.; Bowers, M. T. J. Am. Soc. Mass Spectrom. 1999, 10, 883. (14) Macha, S. F.; Limbach, P.; Curr. Op. Solid State Mater. Sci. 2002, 6, 213. (15) Murgasova, R.; Hercules, D. M. Int. J. Mass Spectrom. 2003, 226, 151. (16) Grade, H.; Cooks, R. G. J. Am. Chem. Soc. 1978, 100, 5615. (17) Sichtermann, W.; Benninghoven, A. Secondary Ion Mass Spectrometry II; Benninghoven, A., Evans, C. A., Powell, R. A., Storms, H. A., Eds.; Springer- Verlag: Berlin, 1979; p 142. Anal. Chem. 2005, 77, 2107-2115 10.1021/ac040158s CCC: $30.25 © 2005 American Chemical Society Analytical Chemistry, Vol. 77, No. 7, April 1, 2005 2107 Published on Web 02/25/2005