Mass Spectrometry Imaging (MSI) on mixed conductive/non-conductive substrate using JMS-S3000 SpiralTOF™ - MSTips - 288
Surface analysis methods such as EPMA, AES, or XPS can provide chemical information about element type, bonding states, or functional groups. However, few methods can obtain the molecular-structure information of organic compounds. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI–TOFMS) is a soft ionization technique that can determine elemental composition by accurate mass analysis and can obtain structural information using MS/MS. Recently, MALDI mass spectrometry imaging (MSI), which can map the spatial distribution of organic compounds, has become popular. In MALDI–TOFMS, high voltage is applied to a target plate, accelerating ions into the TOFMS usually set at ground potential. Therefore, conductivity is required for the target plate, and stainless steel is often used for solvent-based analysis. In MALDI–MSI, a tissue section about 10 μm thick is placed on an indium tin oxide (ITO) glass slide to provide conductivity on the sample surface.
In the industrial field, there is interest in measuring organic compounds on non-conductive substrates, such as resins a few millimeters thick. If the mass spectrum is obtained from the non-conductive surface with no pre-treatment, the mass resolution will be lower, and ultimately the ion intensity will decrease significantly due to the charge-up effect. This issue can be solved by providing conductivity to the non-conductive part via the gold deposition method. In this report, MSI is performed using a permanent red marker on a substrate with a conductive part and a non-conductive part. Previously, ions could be observed only from the conductive part. Now, with the gold deposition method, they can be observed from both the conductive and the non-conductive parts, and they can be properly mapped.
To create a model substrate, we formed conductive and non-conductive parts using metal patterns (Au 100 nm/Cr 30 nm) on a 1-mm-thick quartz glass substrate, alternating conductive with non-conductive parts at intervals of 400 μm. We used a red permanent marker to ionize the main component without applying a matrix compound. The letters "MS" were written with this marker so that they straddled the conductive and non-conductive parts on the model substrate. We then fixed the model substrate and the stainless-steel target plate with conductive tape (Fig. 2). Mass spectrometry imaging was performed without gold deposition. Thereafter, we used gold deposition on the same sample and performed mass spectrometry imaging again. All mass spectrometry imaging were performed in SpiralTOF positive-ion mode. Pixel size was 50 μm; number of laser shots was 50 per pixel.