To our knowledge, this was the first study in which caffeic acid (CA) was successfully evaluated as a matrix to enhance the in situ detection and imaging of endogenous proteins in three biological tissue sections (i.e., a rat brain and Capparis masaikai and germinating soybean seeds) by matrix-assisted laser desorption/ ionization mass spectrometry imaging (MALDI-MSI). Our results show several properties of CA, including strong ultraviolet absorption, a super-wide MS detection mass range close to 200,000 Da, micrometer-sized matrix crystals, uniform matrix deposition, and high ionization efficiency. More high-molecularweight (HMW) protein ion signals (m/z > 30,000) could be clearly detected in biological tissues with the use of CA, compared to two commonly used MALDI matrices, i.e., sinapinic acid (SA) and ferulic acid (FA). Notably, CA shows excellent performance for HMW protein in situ detection from biological tissues in the mass range m/z > 80,000, compared to the use of SA and FA. Furthermore, the use of a CA matrix also significantly enhanced the imaging of proteins on the surface of selected biological tissue sections. Three HMW protein ion signals (m/z 50,419, m/z 65,874, and m/z 191,872) from a rat brain, two sweet proteins (mabinlin-2 and mabinlin-4) from a Capparis masaikai seed, and three HMW protein ion signals (m/z 94,838, m/z 134,204, and m/z 198,738) from a germinating soybean seed were successfully imaged for the first time. Our study proves that CA has the potential to become a standard organic acid matrix for enhanced tissue imaging of HMW proteins by MALDI-MSI in both animal and plant tissues.

Michler's ethylketone (MEK, 4,4’-bis(diethylamino)benzophenone), commonly-known as an intermediate in the synthesis of dyes and pigments, was successfully screened and optimized as a novel matrix for the enhancement of lipid

Recently, mass spectrometry imaging (MSI) has been drawing more and more attention due to its continuous development and improvement as well as its vital role in the study of biological science. Many MSI-based strategies have been widely applied to the in situ qualitative and quantitative detection and imaging of endogenous/exogenous molecules in tissues. Among, matrix assisted laser desorption/ionization MSI (MALDI-MSI) and desorption electrospray ionization MSI (DESI-MSI) are generally regarded as two the most commonly used molecular imaging techniques. Although MSI, as a histology-based emerging molecular imaging technique, has significant advantages over other imaging techniques because it is a label-free technique which provides high sensitivity, high throughput, and molecular specificity derived from the use of mass spectrometers as detectors for a wide variety of ionized biomolecules in situ within a tissue section, it still has obvious shortcomings in the in situ analysis of low-polarity and neutral biomolecules. To solve this issue, the application of chemical derivatization technology in MSI is one of the most important approaches to overcome these deficiencies of MS molecular imaging. This review provides an overview of various chemical derivatization techniques, such as, esterification derivatization, acylation derivatization, addition reaction derivatization, substitution derivatization, oxidative derivatization, and other derivatizations, used in MS analysis, highlights the application of these chemical derivatization methods in the in situ detection and imaging of biomolecules in tissues by MALDI-MSI and DESI-MSI. It can be predicted that with the advance progress of chemical derivatization technology, the application scope of MSI will be further expanded in the fields of spatial metabolomics, spatial lipidomics, spatial proteomics, and spatial glycomics.

In this study, polyacrylamide gel (PAAG) was successfully used as a new embedding medium to provide more effective maintenance for biological tissues during the sectioning process, enhancing the tissue imaging...