Ion Yields in UV-MALDI Mass Spectrometry As a Function of Excitation Laser Wavelength and Optical and Physico-Chemical Properties of Classical and Halogen-Substituted MALDI Matrixes

Abstract

The laser wavelength constitutes a key parameter in ultraviolet-matrix-assisted laser desorption ionization-mass spectrometry (UV-MALDI-MS). Optimal analytical results are only achieved at laser wavelengths that correspond to a high optical absorption of the matrix. In the presented work, the wavelength dependence and the contribution of matrix proton affinity to the MALDI process were investigated. A tunable dye laser was used to examine the wavelength range between 280 and 355 nm. The peptide and matrix ion signals recorded as a function of these irradiation parameters are displayed in the form of heat maps, a data representation that furnishes multidimensional data interpretation. Matrixes with a range of proton affinities from 809 to 866 kJ/mol were investigated. Among those selected are the standard matrixes 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA) as well as five halogen-substituted cinnamic acid derivatives, including the recently introduced 4-chloro-α-cyanocinnamic acid (ClCCA) and α-cyano-2,4-difluorocinnamic acid (DiFCCA) matrixes. With the exception of DHB, the highest analyte ion signals were obtained toward the red side of the peak optical absorption in the solid state. A stronger decline of the molecular analyte ion signals generated from the matrixes was consistently observed at the low wavelength side of the peak absorption. This effect is mainly the result of increased fragmentation of both analyte and matrix ions. Optimal use of multiply halogenated matrixes requires adjustment of the excitation wavelength to values below that of the standard MALDI lasers emitting at 355 (Nd:YAG) or 337 nm (N(2) laser). The combined data provide new insights into the UV-MALDI desorption/ionization processes and indicate ways to improve the analytical sensitivity.

Influence of the Laser Spot Size, Focal Beam Profile, and Tissue Type on the Lipid Signals Obtained by MALDI-MS Imaging in Oversampling Mode

“…While for spot sizes with dimensions below approximately 75 μm, the TICs showed a further increase, albeit with a reduced slope, for larger spots above 75 μm in width/diameter, the TICs essentially reached a saturation plateau. For the larger spot sizes, these findings are in line with MALDI results obtained previously with drieddroplet sample preparations [23,27,28]. Essentially, they also reproduce the recent results obtained by Steven et al [29] with both coated tissue sections and thin-layer sample preparations The smallest dimensions produced in that work were about 50 × 120 μm 2 and might just have been above the transition point toward a different signal intensity-fluence behavior.…”

“…The data reflect the well-known sharp threshold behavior of ion generation in MALDI and the exponential signal increase [23,24,[26][27][28]. After the initial increase, approximately at values three to four times the threshold fluence, however, spot sizedependent effects set in.…”

“…Compared with the frequently used fiber-based approaches [23,24,27,29], the FBS is uncritical concerning the laser fluence (i.e., damage thresholds) under MALDI-typical conditions and also concerning speckle effects. However, a main prerequisite for obtaining the desired beam patterns with this modulating element is the use of a laser with high beam quality (M 2 ≤ 1.2).…”

“…Numerous fundamental studies were conducted to investigate the influence of relevant Binput parameters^on the MALDI ion yields, like the type and physicochemical properties of the MALDI matrix [21], the analyte-to-matrix ratio in the sample [22], or those of the laser irradiation parameters like wavelength [23][24][25], fluence [23,24,[26][27][28][29], and pulse width [30,31].…”

“…The effect of the laser spot size has been investigated roughly between 1.5 and 300 μm for both Gaussian [28,32] and flat-top beam profiles [23,24,26,27,29]. These studies generally revealed a strong increase of the ion detection threshold (and optimal fluence for maximizing the ion yields) with decreasing spot size that was larger than the reduction in irradiated area.…”