MALDI matrix

A MALDI matrix is material combined with an analyte to assist its release and ionization during MALDI. In ultraviolet MALDI, the MALDI matrix absorbs light at the laser wavelength, coupling the pulse to a sample that need not absorb efficiently on its own. Its chemical and physical properties influence the resulting mass spectrum.

An early experiment compared amino acids at different laser wavelengths. In a mixture of alanine and tryptophan irradiated at $266\mathrm{nm}$, an alanine molecular-ion signal appeared at about one tenth of the irradiance required for alanine alone. Absorbing tryptophan therefore supplied a concrete matrix-assisted effect; the useful observation was the changed ion-production threshold.^[ref]

An idealized optical model illustrates why absorption matters without predicting ion yield. If a uniform layer has absorption coefficient $\alpha$ and thickness $d$, the Beer-Lambert law gives absorbed fraction $1-e^{-\alpha d}$, ignoring reflection and scattering. Optical depths $\alpha d=1$ and $3$ give approximately $63\%$ and $95\%$. These are absorbed-light fractions, not the fractions of analyte molecules that become detected ions; chemistry and subsequent transport still intervene.

The MALDI matrix can also affect spatial interpretation. A tissue-imaging study at $20\mu \mathrm{m}$ sampling found matrix-crystal clusters larger than the sampling scale; their distribution contributed granular structure to the ion image.^[ref] Matrix preparation is consequently part of the measurement, not an invisible supporting surface. Mass spectrometry image normalization may reduce some common intensity variation, but it cannot make every matrix-dependent response an absolute concentration measurement.