AVS 51st International Symposium
    Applied Surface Science Monday Sessions
       Session AS-MoA

Paper AS-MoA7
Improved ToF-SIMS Ion Yields and Cationization of Water-Soluble Analytes by Polyelectrolyte Multilayers

Monday, November 15, 2004, 4:00 pm, Room 210A

Session: SIMS II - Biological and Organic
Presenter: Y.-Y. Lua, Brigham Young University
Authors: Y.-Y. Lua, Brigham Young University
C.A. Pew, Brigham Young University
A. Schnieders, ION-TOF USA, Inc.
P.B. Savage, Brigham Young University
R.C. Davis, Brigham Young University
M.R. Linford, Brigham Young University
Correspondent: Click to Email
Arguably one of the most important issues that has faced time-of-flight secondary ion mass spectrometry (ToF-SIMS) since its inception more than 30 years ago is the need for improved ion yields from analytes. Here we describe an entirely new method for improving ion yields and cationizing analytes that is particularly effective for charged, water-soluble species. This approach takes advantage of the highly charged, ionic nature of polyelectrolytes and the ease with which they can be deposited onto surfaces by the well known layer-by-layer method. In particular, we show that after an ultrathin film (ca. 0.5 nm) of a polycation (polydiallyldimethylammonium chloride, PDADMAC) spontaneously adsorbs onto a silicon (oxide) surface, a mixture of a polyanion (poly(sodium 4-styrenesulfonate)) and a water-soluble analyte, which contains one or more basic nitrogen atoms, will adsorb to form a second layer (ca. 1.5 nm thick). ToF-SIMS of this bilayer shows a significant enhancement in quasi-molecular analyte ion yield (roughly a ten-fold increase in signal), compared to that of the pure compound, or the compound dissolved in dilute HCl and dried on a surface. This phenomenon is demonstrated for two large organic macrocycles (m/z 672 and 745), and a smaller aromatic compound (acridine, m/z 179). Similarly, a significant enhancement in the ion yield of the quasi-molecular ion of 9-anthracenecarboxylic acid (m/z 222) is observed when it spontaneously deposits with PDADMAC to form a ca. 0.5 nm film on silicon.