Delayed-extraction MALDI-TOF mass

Haff LA, Smirnov IP. Single-nucleotide polymorphism identification assays using a thermostable DNA polymerase and delayed extraction MALDI-TOF mass spectrometry. Genome Res 1997 7 378-388. 

Katta, V., Chow, D. T., and Trohde, M., Application of In-Source Fragmentation of Protein Ions for Direct Sequence Analysis by Delayed Extraction MALDI-TOF Mass Spectrometry Anal. Chem., 70, 4410, 1998. 

FIGURE 7.6 Schematic of a delayed-extraction MALDI-TOF mass spectrometer. (Reprinted with permission from reference 18). 

FIGURE I 1.22 Delayed -extraction MALDI-TOF mass spectra of the I I-oligonucleotide d(CACACGCCAGT) using (a) a pulsed nitrogen laser and 3-HPA as the matrix and (b) a frequency-quadrupled Nd YAG laser and picolinic acid as the matrix, (c) Assignment of the fragment ion peaks. (Reprinted with permission from reference 23). 

Vestal, M.L. Juhasz, R Martin, S.A. Delayed Extraction MALDI-TOF-MS. Rapid Commun. Mass Spectrom. 1995, 9, 1044-1050. 

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer with UV laser (i.e., 337-nm nitrogen laser), MALDI probe, and optional delayed extraction and postsource decay... 

Abundant molecular ions, M+, will be observed as the base peaks in the MALDI-TOF mass spectra. Delayed extraction will enhance the abundance of the molecular ions relative to background noise, and the use of postsource decay will facilitate the detection of structurally... 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

Mass spectra were collected on a Bruker Reflex II-MALDI-TOF mass spectrometer (Billerica, MA) equipped with delayed extraction and a N2 laser (337 nm). In the positive reflectron mode, an accelerating voltage of 25.0 kV and a reflectron voltage of 26.5 kV were used. Spectra are the sum of 50-300 shots. Spectra were calibrated with bradykinin (1060.6 MW) and glucagon (3483.8 MW) as external standards. 

The MALDI-TOF mass spectra of the Sephadex-water/ethanol (1 1) eluate (fraction 2) of both the spray ed cranberry juice and cranberry fruit showed a series of anthocyanins cyanidin-pentoside m/z 419.3), peonidin-pentoside m/z 433.3), cyanidin-hexoside m/z 449.3) and peonidin-hexoside m/z 463.3) (Figure 2A). MALDI-TOF MS equipped with delayed extraction provide unit mass resolution, allowing for the visualization of isotopic distribution. The reported observed masses (m/z) correspond to the monoisotope of the predicted compound. For example the predicted and observed monoisotope of cyanidin-pentoside is m/z 419.3) representing the contribution of C, H and 0 to the compound. The mass at m/z 420.3) represents one C, or one H, or one The mass at m/z 421.3) represents two or one C and one H, or one 0, or two H. Mass calculating programs such as IsoPro 3.0 [Shareware at http //members.aol.com/msmsoft] can be used to predict the isotopic distribution of compounds and allow for comparison between predicted and observed isotopic distributions. 

Today, the modern version of time-lag focusing, or delayed extraction,14 16 is used in almost all MALDI-TOF mass spectrometers. It is in fact a special case of the Wiley and McLaren method, in which the initial spatial distribution (usually a very thin sample and matrix mixture dried on a stainless steel plate) is assumed to be zero. Therefore, after the delay time the velocity and spatial distributions are correlated, i.e. ions of highest velocity have moved the greatest distance. This space velocity correlated focusing has been described by Colby and Riley,17 and provides considerably better focusing than can be obtained from two independent and uncorrelated initial distributions in space and velocity/energy that comprise the general Wiley-McLaren case. 

The high mass resolution that can be obtained by delayed extraction MAEDI-TOF has permitted the characterization of two poly(ethylene glycol) derivatives of pharmaceutical significance, PEG-bis(ephedrine) and PEG-bis(acefaminophenone). MALDI analysis confirmed that these polymers have ephedrine or acetaminophenone linked at both ends. End-group characterization, by MALDI-TOF, of derivatized and underivatized samples of octylphenol-teminated PEG revealed that the addition of carboxylic ends substantially enhances the detectability of these ethoxylated polymers. ... 

Perform MALDI-TOF mass measurements in negative linear, delayed extraction mode. The precise measurement parameters will depend on the specific instrumentation used and require some optimization. As a guidehne, the instrument settings for a Voyager DE-STR instrument (Apphed Biosystems) can be found elsewhere (Seibert et al., 2002, 2008). 

Other types of mass spectrometer have been used to record MALDI spectra magnetic sector instruments equipped with an array detector initially offered the advantage of higher resolution than was available with the early linear TOF instruments but these spectrometers have now largely been replaced with reflectron-TOF instruments using delayed extraction ion sources. Mass spectrometers operated with higher than normal ion source pressures to... 

A MALDI-TOF mass spectrometer, for example, a Microflex interfaced with Fleximaging Sofiware from Bruker Dalton-ics (Billerica, MA, USA) is operated in linear mode for m/z greater than 3,000, and in reflectron mode for m/z less than 3,000. The instrument is equipped with a standard nitrogen laser with 337 nm wavelength and a maximum repetition rate of 20 Hz. Standard instrument parameters are used, although delayed extraction times are optimized... 

Juhasz, P. Vestal, M.L. Martin, S.A. On the Initial Velocity of Ions Generated by MALDI and Its Effect on the Calibration of Delayed Extraction-TOF Mass Spectra. 

Recent developments in delayed extraction TOF allow higher resolving power and mass accuracy, and this method in the reflector mode has been used for accurate measurement of the mass of several compounds including two prenylated flavonoids. However, the performance of the MALDI-TOF instrument was not better than those of the FAB and FT-ICR MS instruments, and insufficient to give acceptable accuracy for literature reporting. 

---