Journal of the Mass Spectrometry Society of Japan

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Measurement methods of spatial distribution of molecules such as proteins and drugs at cellular-scale are required in many fields including pathology, pharmacology, etc. Recently, sanning type imaging mass spectrometry (IMS) with matrix-assisted laser desorption/ionization (MALDI) is intensively used for biomolecular analysis. However, the spatial resolution of sanning MALDI-IMS is limited by the laser focus diameter to about 10–100 µm and inadequate for cellular-scale observation. Therefore, we are developing a stigmatic MALDI imaging mass spectrometer, in which spatial resolution of sub-micron can be achieved irrespectively to the laser focus diameter. The experimental apparatus consists of a matrix-assisted laser desorption/ionization (MALDI) ion source, a multi-turn time-of-flight mass spectrometer (MULTUM) and a time and position sensitive delay line detector. Ion distributions at the sample plate are magnified and projected with the ion optical lens system onto the detector. MULTUM which has four toroidal sector electric fields constitute a figure-eight trajectory is inserted into the ion flight path. The ion optical system of MULTUM satisfies the perfect spatial and temporal focusing condition, so that the spatial distributions of ions before entering the multi-turn circuit are conserved after circulation. A result of experiment carried out with our prototype apparatus indicate that an alignment accuracy of the ion optical system is critically important to obtain high quality ion image. Thus we designed new optical system as high rigidity block component to be assembled with high accuracy. We observed some samples for evaluating performance of our new apparatus. As a result, our evaluation experiment demonstrated that both the stigmatic observation of ion images with spatial resolution of 1 micro-meter and the mass resolving power of 10,000 was simultaneously achieved after 4 cycles in MULUM with conservation of image.

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