MALDI (Matrix-Assisted Laser Desorption/Ionisation)

Matrix-assisted laser desorption/ionisation (MALDI) is one of the foundational technologies for bio-mass spectrometry and mass spectrometry imaging. The method involves co-crystallising the sample with a large amount of small-molecule ‘matrix’, which is then irradiated with a pulsed laser (typically a 337 nm nitrogen laser or a 355 nm laser). Upon absorbing the laser energy, the matrix rapidly vaporises, acting like a ‘micro-explosion’ to carry the encapsulated analytes into the gas phase, where they are ionised via a ‘soft’ process, primarily producing intact molecular ions or adduct ions.

The key advantages of MALDI are its tolerance to salts and buffers, its suitability for macromolecules (proteins, peptides, nucleic acids, polymers), and its capability for high-throughput spot imaging, with thousands of spots capable of being applied to a single target plate. It is widely used in proteomics and peptide imaging, and is the method of choice for many clinical and research imaging platforms.

Its limitations are also clear. Firstly, the low-mass region (m/z < 500) is overwhelmed by excessive matrix peaks, making it difficult to detect small molecules and metabolites; secondly, the matrix must be pre-sprayed or dispensed, introducing additional pre-treatment steps and matrix inhomogeneity (see Matrix and Matrix Spraying), and the uniformity of matrix crystallisation directly affects imaging quality; Thirdly, the laser is pulsed, with a short single-point sampling time; quantitative repeatability depends on strict control of the matrix.

Comparison with the New Ceyuan approach: MALDI is listed as one of the mass spectrometry imaging techniques, whilst New Ceyuan’s MSI LDPI can be regarded as a ‘matrix-free’ alternative. LDPI uses a 355 nm laser to directly desorb the sample, followed by post-photochemical ionisation to complete ionisation. The sample surface does not need to be coated with a matrix; there is no matrix dead zone in the low-mass region, resulting in clean spectra that make it easier to identify metabolites and lipids. For life science and agricultural research focusing primarily on the imaging of small molecules and metabolites, the matrix-free approach eliminates the spraying step and reduces human error.

In short, MALDI excels at analysing macromolecules and high-throughput applications, whilst LDPI excels at analysing small molecules and in situ analysis without pretreatment; the two methods are complementary, each suited to different analytical targets.

Specifically, the low-mass blind spot has a significant impact on metabolite research: the majority of endogenous metabolites (such as choline, creatine and short-chain lipids) fall within the m/z 100–500 range, where they are obscured by MALDI matrix peaks, making them difficult to observe directly; whereas MSI LDPI, being matrix-free, provides a clean and transparent spectrum in this range, allowing the spatial distribution of these small molecules to be observed directly. Furthermore, the uniformity of matrix deposition directly determines the quantitative comparability of MALDI imaging; both excessive and insufficient deposition introduce systematic bias. Matrix-free LDPI eliminates this variable from the process, thereby reducing sources of systematic error. For life science and agricultural research focused primarily on metabolite and lipid imaging, the matrix-free approach facilitates the attainment of more reproducible results.

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