Detectors matrix-assisted laser desorption ionization

O. Vorm and P. Roepstorff. Detector Bias Gating for Improved Detector Response and Calibration in Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry. J. Mass Spectrom., 31(1996) 351-356. 

Even if relatively new, HF FIFFF has been used to separate supramicrometer particles, proteins, water-soluble polymers, and synthetic organic-soluble polymers. Particle separation in HF FIFFF has recently been improved, reaching the level of efficiency normally achieved by conventional, rectangular FIFFF channels. With these channel-optimized HF FIFFF systems, separation speed and the resolution of nanosized particles have been increased. HF FIFFF has recently been examined as a means for off-line and on-line protein characterization by using the mass spectrometry (MS) through matrix-assisted laser desorption ionization time-of-flight mass spectrometry (M ALDl-TOF MS) and electrospray ionization (ESl)-TOF MS, as specific detectors. On-line HF FIFFF and ESl-TOF MS analysis has demonstrated the viability of fractionating proteins by HF FIFFF followed by direct analysis of the protein ions in MS [38]. 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

Abbreviations AOD, Acousto-optical deflection BCB, bisbenzyocyclobutadiene CCD, indirect contact conductivity detection CL, chemiluminescence ECD, electron capture detector FCS, fluorescence correlation spectroscopy FRET, fluorescence resonance energy transfer ICCD, integrated contact conductivity detection GMR, giant magnetoresistive LED-CFD, light emitting diode confocal fluorescence detector LIF, laser-induced fluorescence LOD, limit of detection MALDI, matrix-assisted laser desorption ionization PDMS, poly(dimethylsiloxane) PMMA, poly(methylmetha-crylate) SPR, surface plasmon resonance SVD, sinusoidal voltammetric detection TLS, thermal lens spectroscopy. 

The dynamic development of mass spectrometry has had a huge impact on lipid analysis. Currently, a variety of suitable mass spectrometers is available. In principal, a mass spectrometer consists of an ion source, a mass analyzer, and an ion detector. The typical features of each instrument (Fig. 2) result mostly from the types of ion source and mass analyzer. To date, the ionization techniques apphed to lipid analysis include Electrospray Ionization (ESI or nano-ESI), Atmospheric Pressure Chemical Ionization (APCI), Matrix-Assisted Laser Desorption/Ionization... 

On-line MS detectors for SEC offer great promise for serving simultaneously as an absolute detector, concentration-sensitive detector (i.e., total ion current), and composition detector, especially for oligomers and low-MW polymers. Successful developments in the use of matrix-assisted laser desorption ionization (MALDI) MS for high-MW polymers are extremely encouraging, and on-line MALDI MS should... 

Matrix-assisted laser desorption ionization (MALDI) is a frequently used ionization technique, but it is rarely used as an on-line detector. The sample stream is applied to a target plate, and it is allowed to cocrystallize with the matrix, which is subsequently desorbed, ionized with a laser, and analyzed in the MS. There has been attempts of combining FFF and MALDI, and for biomolecules, MALDI is a good ion source, due to the soft ionization with high efficiency and simple mass spectra, even for heavier molecules because the majority carry only single charge. 

Several ionization methods have been applied for CE-MS couphng. Matrix-assisted laser desorption ionization (MALDI), continuous flow fast atom bombardment (FAB), laser vaporization ionization with UV laser, sonic spray ionization and electrospray ionization (ESI) have all been used for coupling CE to MS. However, ESI is now undoubtedly the most widely used ionization technique, employing numerous analyzers including quadrupoles, magnetic sector, Fourier transform ion cyclotron resonance, time-offlight and trapping devices. However, quad-rupole detectors have predominantly been applied in CE-MS [6-8]. 

Reducing the ID of the LC column offers several advantages, among which are most importantly (1) increased mass sensitivity for concentration sensitive detectors due to reduced chromatographic dilution, (2) easier coupling to MS interfaces [ESI and matrix-assisted laser desorption/ionization (MALDI)] due to the lower flow rates, and (3) a lower stationary phase and solvent consumption. 

Elucidation of degradation pathways and identification of transformation products (TPs) is of crucial importance in understanding their fate in the environment and requires the employment of advanced instrumental techniques. Analytical methods that can be used for this purpose include Uquid chromatography with diode array or fluorescence detector (LC-DAD/FL), nuclear magnetic resonance (NMR), infrared spectroscopy (IR), matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS), gas... 

The proof of Equations (5.49) and (5.50) can be found in the references [54,56]. Note, incidentally, that other techniques for measuring the MMD, such as MALDI (matrix-assisted laser desorption ionization mass spectrometry) will have a different relation between the signal and the MMD. Note also that the assumption that the GPC signal is proportitHial to the mass of polymer depends on what sort of detection is used for example, this will not hold if the detector is sensitive just to the end-group of a polymer chain. 

Time-of-flight mass analyzers are most useful for pulsed ion sources, such as matrix-assisted laser desorption ionization (MALDI). The ions depart the source at the same time but arrive at the detector at different times depending on mass. These instruments have exceptionally high sensitivity and a high practical mass range. 

Benner, W. H., Horn, D. M., Jaklevic, J. M., Frank, M., Mears, C., Labov, S., and Barfknecht, A. T., Simultaneous Measurement of Flight Time and Energy of Large Matrix-assisted Laser Desorption Ionization Ions with a Superconducting Tunnel Junction Detector, /. Am. Soc. Mass Spectrom., 8, 1094, 1997. 

Montaudo, G., MALDI-TOF as GPC Detector for MW MWD Measurements in Polydisperse Pol5nners and Copolymers, Polym. Preprints, 290,1996. Montaudo, G., Montaudo, M. S., Puglisi, C., and Samperi, F., Molecular Weight Determination and Structural Analysis in Polydisperse Polymers by H3q)hen-ated Gel Permeation Chromatography/Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry, Int. J. PolyrrL Anal Characterization, 3,177, 1997. 

Mass spectrometry is an analytical technique that can determine precisely the atomic or the molecular weight of atoms or molecules once they have been ionized. There are four key elements in mass spectrometer the sample introduction, the source where ionization occurs, the mass analyzer, and the detector. Mass spectrometry can analyze many different types of samples that range from solid, liquid, or gases. First, the molecules have to be ionized either under vacuum or at atmospheric pressure. Depending on the ionization technique, either molecular ions (M ) with an odd electron number or protonated ions ([M + H] with an even electron number are formed in the positive mode and M , M or ([M - H] in the negative mode. Ionization techniques are often classified into soft ionization, where little or no fragmentation occurs, and hard ionization, where fragmentation is extensive. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are... 

Axelsson, J., Scrivener, E., Haddleton, D.M., and Derrick, P.J. (1996) Mass discrimination effects in an ion detector and other causes for shifts in polymer mass distributions measured by matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry. Macromolecules, 29, 8875-8882. 

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