Matrix and Matrix Spraying

The term ‘matrix’ has two entirely different meanings in mass spectrometry; discussing them together can easily lead to confusion, so let us first distinguish between them.

The first meaning relates to MALDI (matrix-assisted laser desorption/ionisation). Here, the matrix refers to a class of crystalline small-molecule organic compounds (such as DHB and CHCA) which, when co-crystallised with the analyte, strongly absorb laser energy and transfer it to the analyte, facilitating its gentle desorption and ionisation. The type and concentration of the matrix directly influence the ion yield and are one of the key variables determining the success or failure of MALDI—choosing the wrong matrix may mean the target molecule fails to elute at all.

The second meaning, in analyses such as ESI/DESI, refers to the ‘matrix effect’. This does not denote a specific substance, but rather the overall interference caused by co-existing components in the sample on the ionisation efficiency of the target molecule; a typical manifestation of this is ion suppression—where co-existing components compete for charge or surface space, thereby suppressing the response of the target molecule (this mechanism is detailed in the article ‘Ion Suppression’). To mitigate the matrix effect, common approaches include sample purification during pre-treatment, matrix-matched calibration or internal standard correction.

Matrix spraying is an indispensable step in MALDI imaging: the matrix solution is sprayed uniformly onto the surface of a tissue section to form a matrix layer that co-crystallises with the analyte, enabling the laser to desorb and ionise the sample stably on a point-by-point basis. This step requires great skill—the thickness of the spray and the uniformity of crystallisation directly determine the imaging quality; uneven thickness can lead to signal fluctuations or even result in false-positive distribution maps. At the same time, the matrix itself generates a large number of background peaks in the low-mass region (typically m/z < 500), which can mask the signals of small molecules.

Neo Source INSTRUMENT’s two imaging sources have specifically simplified this process. The MSI DPI employs the DESI approach, eliminating the need for matrix spraying; the MSI LDPI utilises transmission laser desorption combined with post-laser photoionisation, and does not rely on a MALDI-style matrix either. Both eliminate the matrix spraying step, thereby reducing sample preparation time, avoiding artefacts caused by uneven spraying, and eliminating the masking of the low-mass region by matrix peaks—the MSI LDPI can therefore detect mass-to-charge ratios as low as approximately 70. Both sources can be fitted to mass spectrometers from Agilent, AB SCIEX and Thermo.