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ANALYTICAL CURRENTS
Save the surface! Most techniques for depth profiling are destructive—they acquire information by sequentially eroding the surface and analyzing the resulting material. Hatsuo Ishida and colleagues at Case Western Reserve University avoided this surface destruction by using variable-angle attenuated total-reflectance (ATR) FT-IR, a nondestructive technique, to provide accurate depth profiles of stratified layers inaPMMAfilm. The ATR method works because above the critical angle required for internal reflection in a nonabsorbing layered system, an evanescent wave is produced that decays exponentially from the surface of the material. As the angle of incidence approaches the critical angle, the evanescent wave propagates to an increasing depth. Because this depth also depends on the wavelength of the incident radiation, a precise description of thefielddecay can be determined, and it is possible
MALDI matrix mechanisms
to obtain vibrational spectroscopic information as a function of depth within the sample. Ishida and co-workers measured a number of known stratified profiles to determine the applicability limits of the method. They found that the ATR measurements were in good agreement with those obtained using a profilometer when />-polarization was used, but the thickness values obtained using s-polarization were more prone to errors caused by back-reflection from the PMMA/nujol/ Variable-angle ATR thickness determination for a eicosane interface. As the thick- 0.58-fjm PMMA film using p-polarization. (Adapted with permission from the Society for Applied Spectroscopy.) ness became large enough to avoid back-reflection problems, the maximum thickness barrier wa^ nal reflection element and that this range approached. could be extended by considering stronThey concluded that thicknesses in ger absorptions for thinner films and the range of 0.3-1 um can be probed usweaker absorptions for thicker films. ing />-polarization and a germanium inter(Appl. Spectrosc. 1996,50,1082-88)
modified probe tips and mass analyzed by laser desorption TOFMS. The researchers tested several types of monolayers, including 6-mercaptonicotinic acid, methyl Ar-(4-mercaptophenyl) carbamate (MMPC), a-mercapto-/>-toluic acid, 2-mercapto-5-nitropyridine, and 4-acetamidothiophenol, for their ability to produce positive and negative analyte ions
and found that an analyte signal was observed only when the gold tip was derivatized with MMPC. In positive-ion mode, LHRH, insulin, and cytochrome c could be Matrix-assisted laser desorption/ionizaobserved; in negative-ion mode, the oligotion MS has revolutionized the analysis of nucleotide dT10 was detected, but the large biomolecules and synthetic polymixed-based oligonucleotide mers, but the critical role of the matrix in dCAGCDaACwasnot. the desorption/ionization process remains unclear. Although many compounds have They concluded that the chemical nabeen screened as possible mature of the monolayer "matrices, only a few have proven trix" is critical in the desorpuseful, and the chemical differtion/ionization process and ences between an effective and that the MMPC monomers ineffective matrix are often exact in a matrix-like manner tremely subtle. In an effort to when a thin interface exists elucidate the ionization/desorpbetween the monomers and tion mechanism, Lloyd M. the analyte. The researchers Smith and colleagues at the believe that the relatively hoUniversity of Wisconsin have mogeneous and defined developed a model MALDI sysmonolayer system should tem by replacing the usual maprove more amenable to trix with a self-assembled monostudy by surface analysis techlayer consisting of a UV-absorbniques than with conventional ing matrix-like compound MALDI systems and should be covalently linked to a gold suruseful in further elucidating face. the molecular mechanism underlying the MALDI process. Analytes such as proteins or (J. Am. Chem. Soc. 1996,118, oligonucleotides are directly Mass spectrum of cytochrome c obtained by LD/TOFMS from 863^45) deposited on the covalently an MMPC monolayer on gold. Analytical Chemistry News & Features, November 1, 1996 651 A
