|
Matrix-assisted laser desorption electrospray ionization (MALDESI) is an ambient ionization technique which combines the benefits of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). MALDESI was introduced in 2006 as the first hybrid ionization source combining laser ablation and electrospray post-ionization using a resonantly excited matrix (endogenous or exogenous). An infrared (IR) or ultraviolet (UV) laser can be utilized in MALDESI in order to resonantly excite an endogenous or exogenous matrix. The term ‘matrix’ refers to any molecule that is present in large excess and absorbs the energy of the laser, facilitating desorption of analyte molecules. The original MALDESI design was implemented using organic matrices, similar to those used in MALDI, along with a UV laser.〔 The more recent MALDESI source uses a thin layer of ice as the energy-absorbing matrix that is resonantly excited using a mid-infrared (IR) laser. The IR-MALDESI source can be used for mass spectrometry imaging (MSI), a technique using MS data collected over the sample area to visualize the spatial distribution of specific analyte molecules. A versatile IR-MALDESI MSI source was designed and implemented, which is currently coupled to a high resolving power hybrid Quadrupole-Orbitrap mass spectrometer. The source has single- or multi-shot capabilities with adjustable laser fluence, repetition rate, as well as the delay between the laser trigger and MS ion accumulation. The sample plate and moving components are enclosed in a nitrogen purged enclosure where ambient ions and relative humidity can be regulated. A water cooled Peltier thermoelectric plate is used to control the sample temperature (-10 °C to 80 °C). The IR-MALDESI source is currently used to investigate the fundamentals of the ionization process, in addition to being routinely employed for visualizing analyte distributions in biological, forensic, and pharmaceutical samples. ==Principles of operation== In IR-MALDESI imaging experiments, a thin layer of ice is deposited/formed on the sample as the energy-absorbing matrix. A mid-IR laser pulse (λ=2940 nm) is then absorbed by exciting the O-H stretching mode of water causing rapid phase change leading to an ablation event, which facilitates desorption of neutral material from surface. The plume of desorbed material interacts with an orthogonal electrospray plume where desorbed compounds partition into charged electrospray droplets and ions are generated by an ESI-like process that are sampled by a mass spectrometer.〔 The ESI-like ionization mechanism was experimentally demonstrated by Muddiman et al.〔Dixon, R. B., Muddiman, D.C. (2010) "Study of the ionization mechanism in hybrid laser based desorption techinques" Analyst. 135, 880-2 .〕 Using ice as the matrix has been reported in IR-MALDI experiments; however, the ion yields for such experiments have been very low. The electrospray post-ionization employed in IR-MALDESI helps alleviate issues associated with low ionization yield. Using an exogenous ice matrix was shown to improve the detected ion abundance by a factor of approximately 15 for selected small molecules. By employing the method of oversampling, spatial resolutions of ~100 micrometer can be readily achieved. In a recent study, oversampling was used to achieve cellular-level MSI of epithelial cells using a step size of 10 micrometer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Matrix-assisted laser desorption electrospray ionization」の詳細全文を読む スポンサード リンク
|