Desorption Ionization LDI

Laser desorption/ ionization LDI Photon induced desorption/ ionization Nonvolatile atomic and molecular ions Isotope ratio Trace analysis... 

Mass spectrometric measurements of ions desorbed/ionized from a surface by a laser beam was first performed in 1963 by Honig and Woolston [151], who utilized a pulsed mby laser with 50 p,s pulse length. Hillenkamp et al. used microscope optics to focus the laser beam diameter to 0.5 p,m [152], allowing for surface analysis with high spatial resolution. In 1978 Posthumus et al. [153] demonstrated that laser desorption /ionization (LDI, also commonly referred to as laser ionization or laser ablation) could produce spectra of nonvolatile compounds with mass > 1 kDa. For a detailed review of the early development of LDI, see Reference 154. There is no principal difference between an LDI source and a MALDI source, which is described in detail in Section 2.1.22 In LDI no particular sample preparation is required (contrary to... 

Laser desorption, including laser desorption/ionization (LDI) and matrix-assisted laser desorption/ionization (MALDI)... 

The introduction of matrices to assist the laser desorption ionization (LDI) process has lead to the development of the so-called matrix-assisted LDI (MALDI). This technique has been applied to identify terpenoid varnishes and their oxidized products [46], Combined with enzymatic cleavage, MALDI has also been used in the identification of animal glue. 

The Laser Desorption Ionization (LDI) was investigated by Franz Hillenkamp and Michael Karas [2, 3], LDI involved sample bombardment with short and intense pulses from a laser light to effect both desorption and ionization of the analyte molecules. It has become a soft desorption ionization method for mass spectrometric analyses of biological macromolecules and small molecular compounds. The MALDI technique was improved by Koichi Tanaka. 

The wide application of newly developed advanced Surface MALDI MS methods, such as laser desorption ionization (LDI), surface-enhanced laser desorption ionization (SELDI), direct ionization on silicon surface (DIOS), etc. have two shortcomings. First, problems with MALDI-TOF MS sample preparation for nano-sized systems have not been completely worked out. Second, the reliability and reproducibility of experimental data concerning similar bio-active systems is lacking. Sometimes molecular, associative and fragment ions cannot, be observed in the MALDI mass spectra. We are currently working on overcoming these difficulties with some encouraging results.13,14... 

Laser desorption/ionization (LDI) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MSs have become useful tools for the characterization of various conducting polymers. For instance, analysis of poly[2,5-di(thien-2-ylidene)-l,3,4,6-tetrathiapentalene] 56 (DT-TTP) showed ions above mlz = 500 corresponding to oligomers of DT-TTP with up to five monomer units <2000JMP550>. 

Multiphoton absorption leads to electrons in the irradiated molecules raised to highly excited states. The excitation may be such that the sample is ionized directly, as in the plasma which is the basis of laser desorption ionization (LDI). Generally, there is only a small excess of ions formed in this way, and a secondary ionization is necessary to obtain a better yield of charged species. 

FIGURE 4.4 Schematic diagram of the SELDI Ciphergen mass spectrometer. After sample preparation, the ProteinChip arrays are analyzed by a laser-desorption/ionization (LDI) time-of-flight mass spectrometer (TOF MS). The TOF MS measures the molecular weights of the various proteins that are retained on the array. For comparison purposes, the software associated with the SELDI Ciphergen instrument is capable of displaying the resultant data as either a spectral, map, or gel view. 

Selva and coworkers ° reported on their experiences to apply various mass spec-trometric techniques to the analysis of -carotene and carotenoids and their adducts formed in aqueous solution. El mass spectrometry and field desorption (FD) mass spectrometry were applied to aqueous mixtures of -carotene and -cyclodextrin, and the polyene was found to be detectable. Tandan mass spectrometry can be applied to identify j6-carotenone as a minor component in complex carotenoid mixtures. EI/MIKE spectrometry of the molecular ion (m/z 600) was used in this case. A previous study was focused on the characterization of ieco-carotenoids using EI/MIKE and CID spectrometry. The more recent ionization methods, viz. MALDI and its variant working without a matrix, laser desorption/ionization (LDI), as well as electrospray ionization (ESI) mass spectrometry, have also been applied to this topic. MALDI and LDI mass spectrometry were used to analyse mixtures of -carotene and y-cyclodextrin in aqueous solution. Adduct ions were not observed using these methods . ... 

Solid samples may be heated and vaporized using laser energy with the additional benefit that compounds may be ionized with the same laser, all at ambient pressure, and characterized in an IMS drift tube. While lasers were used with mobility spectrometers initially for selective photoionization of vapors only,° ° ions were observed from laser contact with solids, including metals and salts. This was not simple vaporization by heating and instead was understood as ablation and ionization. In laser desorption ionization (LDI), the laser pulse initiates a mobility spectrum with a... 

In early mass spectrometry applications of lasers, the sample was irradiated directly by a laser beam to desorb intact sample-related ions [27]. In this direct mode, termed laser desorption/ionization (LDI), the extent of energy transfer is, however, difficult to control and often leads to excessive thermal degradation. Also, not all compounds absorb radiation at the laser wavelength and thus are not amenable to LDI. Only those compounds that have mass below 1000 Da can be analyzed by LDI. Analytical sensitivity is also poor. A key contribution of LDI experiments is the observation that the desorption efficiency of amino acids and peptides that absorb the laser fight beam is greater than those without the chromophore [28]. IR lasers (e.g., an Nd YAG laser at 1.06 p m and a pulsed CO2 laser at 10.6 pm) and UV lasers (frequency-quadrapled Nd YAG laser at 266 nm) have aU been used. The detection of malaria parasites in blood by LDI with an N2 laser has been demonstrated [29]. 

Fig. 2 (A) Schematic view of the TOF mass spectrometer equipped with electrospray ionization (ESi) and iaser desorption ionization (LDI) sources. (B)The detaiis of the ESI source. ACS Pubiishing. Reproduced with permission. ...

Many different types of MS technique have been used for polymer characterization [4]. Before the introduction of matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) during the 1980s, MS was limited to the analysis of relatively low mass polymers of less than 3000 Da. Desorption techniques such as SIMS, fast atom bombardment (FAB), and laser desorption/ ionization (LDI) could ionize a polymer with mass of up to 10000 Da. However,... 

Similarly, enzymatic reactions can be performed directly on a MALDI plate, quenched, and the reaction products analyzed [43]. Various laser desorption/ionization (LDI)-based techniques facilitate such off-line measurements [44,45]. When the operations of initiating and quenching the chemical reactions are carried out manually, the temporal resolution of the LDI-MS-based methods is typically in the order of a few minutes. However, by implementing flow mixing and quenching methodology, one can perform observations of sub-second phenomena with this kind of off-line MS detection (see, e.g., [46-48]). 

Experimental design of time-resolved mass spectrometry (TRMS) systems can greatly affect temporal resolution in the analysis of dynamic samples (see also Section 4.2). The two popular MS approaches - laser desorption/ionization (LDI)-MS and electrospray ionization (ESI)-MS - are suitable for studies of enzymatic reactions [8]. 

Figure 3.11 summarizes such key experimental points. As a first point, we have to choose the appropriate ionization method for the detection of small metabolites, we have alternative choices other than MALDI, such as secondary ion mass spectrometry (SIMS) [15], nanostructure-initiator mass spectrometry (NIMS) [20,21], desorption/ionization on silicon (DIOS) [22], nanoparticle-assisted laser desorptiopn/ ionization (nano-PALDI) [23], and even laser desorption/ionization (LDI) [24,25]. We consider that MALDI is stiU the most versatile method, particularly due to the soft ionization capability of intact analyte. However, other methods each have unique advantages for example, SIMS and nano-PALDI have achieved higher spatial resolution than conventional MALDI-IMS, and above aU, these mentioned alternative methods are all matrix-free methods, and thus can exclude the interruption of the matrix cluster ion. Next, if MALDI is chosen, experimenters should choose a suitable matrix compound, solvent composition, and further matrix application method for their target analyte. All these factors are critical to obtain sufficient sensitivity because they affect efficiency of analyte extraction, condition of cocrystallization, and, above all, analyte-ionization efficiency. In addition, based on the charge state of the analyte molecule, suitable MS polarity (i.e., positive/ negative ion detection mode) should be used in MS measurement. Below, we shall describe the key experimental points for MALDI-IMS applications of representative metabolites. 

Holscher, D., et al. (2009) Matrix-free UV-laser desorption/ ionization (LDI) mass spectrometric imaging at the singlecell level distribution of secondary metabolites of... 

An IM-quadMS was the first hybrid instrument used to detect nitrotoluene compounds with a radioactive ionization source [189]. Later work uses laser desorption ionization (LDI) to detect TNT, DNT, RDX, and HMX with an IM-quadMS [190]. Recently, the formation reaction mechanism of RDX response ions in an atmospheric pressure GDI source is now better understood with the use of an IM-triple quadrupole mass spectrometer [179]. 

---