Analytical Sciences

MALDI Mass Spectrometry of Small Molecules Using Nanometer-sized Clay

Jiawei XU,*,** Junko N. KONDO,*** and Tatsuya FUJINO*,**

*Department of Applied Chemistry, Graduate School of Science and Engineering, Toyo University, 2100 Kujirai, Kawagoe 350-8585, Japan
**Bio-Nano Electronics Research Centre, Toyo University, 2100 Kujirai, Kawagoe 350-8585, Japan
***Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan

Nanometer-sized clay, allophane, was used as the matrix for matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) and applied to the ionization of small molecules. First, the laser desorption ionization mass spectrum of cation-exchanged allophane was measured, and it was found that the cation exchange proceeded smoothly with increasing atomic number of alkali metals in the periodic table. This phenomenon was explained by considering the size of the counter anion on the allophane surface. Then, fructose was measured as the analyte using each alkali-cation-exchanged allophane as the matrix. Contrary to the measurements using allophane itself, the peak intensity of fructose decreased with increasing atomic number of alkali metals in the periodic table. This phenomenon was clarified by considering the stability of alkali cation in the presence of a surface anion, the desorption energy, and the solvation enthalpy of each alkali cation. The applicability of allophane to high molecular weight compounds was also confirmed by measuring cyclodextrin, angiotensin II, and insulin. Finally, a combination of allophane and zeolite was examined by assuming proton relay among allophane, zeolite, and analyte. As a result of proton supply from zeolite to allophane, the peak intensity of the proton sponge (1,8-bis(dimethylamino)naphthalene) was enhanced by almost 2.2 times.

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