Toward Higher Sensitivity in Quantitative MALDI Imaging Mass Spectrometry of CNS Drugs Using a Nonpolar Matrix

Abstract

Tissue-specific ion suppression is an unavoidable matrix effect in MALDI mass spectrometry imaging (MSI), the negative impact of which on precision and accuracy in quantitative MALDI-MSI can be reduced to some extent by applying isotope internal standards for normalization and matrix-matched calibration routines. The detection sensitivity still suffers, however, often resulting in significant loss of signal for the investigated analytes.An MSI application considerably affected by this phenomenon is the quantitative spatial analysis of central nervous system (CNS) drugs. Most of these drugs are low molecular weight, lipophilic compounds, which exhibit inefficient desorption and ionization during MALDI using conventional polar acidic matrices (CHCA, DHB). Here, we present the application of the (2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile) matrix for high sensitivity imaging of CNS drugs in mouse brain sections. Since DCTB is usually described as electron-transfer matrix, we provide a rationale (i.e. computational calculations of gas-phase proton affinity and ionization energy) for an additional protontransfer ionization mechanism with this matrix. Furthermore, we compare the extent of signal suppression for five different CNS drugs when employing DCTB versus CHCA matrices. The results showed that the signal suppression was not only several times lower with DCTB than with CHCA, but also depended on the specific tissue investigated. Finally, we present the application of DCTB and ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry to quantitative MALDI imaging of the anesthetic drug xylazine in mouse brain sections based on a linear matrix-matched calibration curve. DCTB afforded up to 100fold signal intensity improvement over CHCA when comparing representative single MSI pixels, and > 440-fold for the averaged mass spectrum of the adjacent tissue sections.

The Space Dimension at the Micro Level: Mass Spectrometry Imaging of Drugs in Tissues

“…Rzagalinski et al (2018) presented (2‐[(2 E)‐3‐(4‐tert‐butylphenyl)‐2‐methylprop‐2‐enylidene] malononitrile) as a matrix for drug detection, specifically in the central nervous system, since it gave signal suppression in tissue several times lower than with traditional CHCA.…”

Validation of MALDI-MS imaging data of selected membrane lipids in murine brain with and without laser postionization by quantitative nano-HPLC-MS using laser microdissection

“…To address this issue, different solutions for quantification have been proposed [42,43]. In a targeted approach, different research groups have used isotopically labeled standards that are homogenously deposited on top or underneath the tissue section [44][45][46][47]. Because targeted analyte as well as matched standard experience the same micro-conditions at each pixel, signal intensity from the standard can be used for normalization.…”

Recent developments of novel matrices and on-tissue chemical derivatization reagents for MALDI-MSI

“…In 2000, Ulmer and coworkers [44] proved DCTB (2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2enylidene]malononitrile) as an electron transfer matrix for the analysis of labile compounds. Recently, Rzagalinski and coworkers [42] found the theoretical and experimental evidence that DCTB is a proton transfer matrix. DCTB was applied for imaging of central nervous system drugs in mouse brain tissue on positive ion mode.…”