Principles of DIOS

A severe problem when quantifying low molecular weight compounds by means of MALDI-MS is the potential interference of matrix signals with the analyte signals in the low-mass region. Furthermore, target preparation, i.e. co-crystallization of matrix and sample, is often time-consuming: Due to inhomoge-neous distribution of analyte molecules within the matrix crystal, shot-to-shot reproducibility (generation of so-called ''hot spots'') and sample-to-sample reproducibility remain mostly poor. Several factors like matrix compound selection, pH value of sample solution, ratio of matrix to analyte molecules, target surface, and sample drying method are critical for the crystallization process and have to be carefully optimized [11]. To overcome the limitations related to the use of a matrix, direct laser desorption/ionization without the use of a matrix is no alternative for bioanalytical mass spectrometry, as significant analyte degradation is fre

DIOS source

Fig. 8.6 Schematic set-up of a DIOS-TOF-MS system. Initially, the sample is deposited on the porous silicon surface. Subsequently, a laser pulse is directed to the silicon surface, and the analytes are desorbed. Ions that are generated are transferred into a time-of-flight mass spectrometer.

DIOS source ft Ions

Fig. 8.6 Schematic set-up of a DIOS-TOF-MS system. Initially, the sample is deposited on the porous silicon surface. Subsequently, a laser pulse is directed to the silicon surface, and the analytes are desorbed. Ions that are generated are transferred into a time-of-flight mass spectrometer.

quently observed upon direct exposure to the laser beam. Therefore, Wei et al. developed a matrix-free strategy based on the pulsed laser desorption/ionization of molecules from a porous silicon surface (DIOS) [12]. In a DIOS experiment, the analytes in solution are spotted onto a porous silicon target, evaporated to dry-ness, and ionized by a laser pulse. The generated ions are then detected by a mass spectrometer. The set-up of a DIOS-MS system is schematically shown in Fig. 8.6.

Porous silicon is generated from flat crystalline silicon by using a galvanostatic or chemical etching procedure [12-14]. Thus, a thin layer in the submicrometer range is formed, which comprises a nanocrystalline structure and shows bright photoluminescence upon irradiation with UV light [12]. By modulating etching conditions and by selecting the appropriate silicon wafer precursors, characteristics of the formed silicon surface, e.g. morphology and porosity, can be controlled. Porous silicon has narrow pores (typically 50-100 nm) and a large surface area reaching up to several hundred m2 cm~3. The porous silicon surface may either be used in its metastable silicon hydride form, comprising Si-H endgroups, or in its functionalized form by covalently attaching organic groups, e.g. dodecyl, ethyl, phenyl, or ethylphenyl substituents. Actually, the more hydrophobic surfaces yield higher signals [12].

Due to the hydrophobicity of the silicon surface, samples are typically dissolved in water or mixtures of water and methanol. While samples dissolved in pure non-polar solvents tend to spread over the whole surface, aqueous/organic mixtures form droplets that stay localized to a small surface area. Additionally, mixtures also guarantee that the sample penetrates sufficiently deep into the silicon. Spotted volumes are typically in the low microliter to submicroliter range. Traditionally, sample spotting in DIOS-MS is carried out using pipettes, which mostly suffers from an inhomogeneous analyte distribution. This can be overcome by syringe pump

Was this article helpful?

0 0

Post a comment