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Analysis of Organic Thin Films by the Laser Desorption/Ionization Method Using the JMS-S3000 “SpiralTOF”

Laser Desorption/ Ionization-Time of Flight Mass Spectrometry (LDI-TOFMS) is generally used for analysis of organic compounds because this technique generates little fragmentation of molecular ions at ionization. It makes possible to obtain information on molecular weights and molecular structures in organic compounds. In particular, a technique which uses the matrix compounds for enhancing ionization efficiency is well known as Matrix-Assisted Laser Desorption/ Ionization-Time of Flight Mass Spectrometry (MALDI-TOFMS). This technique is widely used in the bio markets owing to its capability of ionizing proteins and peptides with the molecular weights of several thousands to several hundreds of thousands. The MALDI-TOFMS is also utilized for analysis of synthetic polymers. In many cases, LDI-TOFMS and MALDI-TOFMS have been used to estimate the molecular weights of organic compounds in solution. But very recently, techniques of imaging mass spectrometry, which controls the laser irradiation position by two-dimensional scan to acquire mass spectra for visualizing localization of chemical compounds with specific molecular weights, have been improved. The application of this innovative technique is increasingly spreading in the bio markets. The technology of Imaging Mass Spectrometry has been advancing for analyzing biological tissue sections, but in the future, it is expected to develop toward the material science markets. It is noted that various surface analytical techniques are already available in the material science markets. In order to study the advantages of LDI-TOFMS as one of effective surface analysis tools, it is essential to consider the complementary analysis of LDI-TOFMS with the existing surface analytical techniques. In this article, the advantages of using LDI-TOFMS for analyzing organic lightemitting diode material thin films, in accordance with comparison with Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy/Energy-Dispersive X-Ray Spectroscopy (SEM/EDS), have been studied. In addition, since LDI-TOFMS is a destructive analytical technique, the influence on the sample surface caused by LDI-TOFMS was also examined.

Introduction

The surface analytical techniques i r radiate an electron beam, an ion beam or X-ray on the surface of the sample for investigation of i t s morphology and physical characteristics based on the interactions between the beam and substances existing on the sample surface. To observe the sample morphology, an optical microscope and an electron microscope are mainly used. To study the sample characteristics, a wide range of techniques is available depending on the incident particles (beam) and the signals to be detected. They include Electron Probe Microanalysis (EPMA) , Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) . In recent years, electronic devices are frequently composed of organic compounds such as organic semiconductor, organic light-emitting diode (OLED) and organic film solar cell, and the use of them will be expected to further expand. It is increasingly important to inspect organic-compounds and their degradation mechanism in the products . Among sur f a c e analytical techniques, AES and XPS are capable of obtaining chemical bonding states or information on functional groups in chemical compounds, but those techniques have a difficulty in structural analysis of organic compounds. The TOF-SIMS is a mass spectrometry technique well known as a surface analytical technique. By using the dynamic SIMS, fragmentation of the molecular ions is likely to occur at ionization, thus making it difficult to apply SIMS to analyze organic compounds. Recently, techniques which utilize metallic clusters or gas clusters as a primary ion beam attached to TOF-SIMS have been succeeded to ionize more softly. These techniques are expected to expand the TOF-SIMS applications for organic compounds.

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