MALDI source

Pribil, P. Patton, E. Black, G. Doroshenko, V. M. Fenselau, C. Rapid characterization of Bacillus spores targeting species-unique peptides produced with an atmospheric-pressure MALDI source. J. Mass Spectrom. 2005, 40,464-474. 

In contrast to the other ion sources, the MALDI source may operate under high vacuum or under atmospheric pressure. In the latter case the acronym AP-MALDI (atmospheric pressure matrix assisted laser desorption ionization) is used. 

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... 

The compatibility is excellent with continuous ion sources such as ESI, dynamic SIMS, CF-FAB, ICP, El, Cl, etc. Sector instalments are not well-suited for pulsed ionization methods, although there are examples where MALDI sources have been utilized [225-229]. Sector instruments are usually larger and more expensive than other mass analyzers, such as TOFs, quadrupole filters, and traps. 

Note Modem FT-ICR mass spectrometers offer ultrahigh resolving power (R= 10 -10 ) [193,194] and highest mass accuracy (Am = 10" -10 u, cf. examples in Chaps. 3.3.2 and 3.4.1), attomol detection limits (with nanoESI or MALDI sources), high mass range and MS capabilities. [195]... 

The use of MALDI for the analysis of small molecules was recently reported. Particularly attractive is the coupling of a MALDI source with a triple quadrupole mass analyzer for quantitative analysis in the selected reaction monitoring (SRM) mode due to very high analysis speed. 

Several commercial instrument designs have effectively overcome this inherent problem and can record high-energy CID product ion spectra. The Applied Biosystems 4700 Proteomics Analyzer was one of the two originally available commercial TOF/TOF designs76,77 (Fig. 24). When the instrument is operated to obtain a mass spectrum, it acts as a standard inline reflectron TOF instrument. In product ion experiments the precursor ion is isolated at the end of the first flight tube by the timed-ion selectors (TIS). The MALDI source uses DE to correct for... 

There are several types of ionization sources [MALDI, ESI, FAB (fast atom bombardment), PD (Cf-252 plasma desorption), El (electron ionization), Cl (chemical ionization) etc.], different types of mass analyzers [combinations of magnetic and electric sectors, quadrupolar filters (Q) and ion traps (IT), time-of-flight (TOF) and FT-ICR] and different detectors, each with its own advantages and drawbacks. We describe herein only the systems that presently have widespread use for the study of biomolecules ESI coupled to a quadrupole (or triple quadrupole, QqQ) mass analyzer or an ion trap, the MALDI source with the linear or reflectron TOF analyzer, and the FT-ICR system which can be equipped with both ESI and MALDI sources. 

The TOF works in a pulsed fashion. This is perfectly suited to the pulsed MALDI source. 

Contrary to most other ionization sources that yield a continuous ion beam, MALDI is a pulsed ionization technique that produces ions in bundles by an intermittent process. The pulsed nature of the MALDI source is well suited for the time-of-flight (TOF) analyser. In addition, the TOF analyser has the ability to analyse ions over a wide mass range and thus... 

ISD fragmentations lead to product ions that are always apparent in the MALDI spectra, whereas the observation of product ions from PSD fragmentation needs certain instrumental conditions. For example, a MALDI source coupled to a linear TOF analyser allows detection of fragment ions produced in the source at their appropriate m/z ratio. On the contrary, fragment ions produced after the source cannot be resolved from their precursor ions and are detected at the same apparent m/z ratio. This induces a broadening of the peaks with a concomitant loss of mass resolution and sensitivity. 

In 2000, various developments in the field of MALDI led to the advent of new methods such as the atmospheric pressure MALDI (AP-MALDI) source. This method combines the atmospheric pressure (AP) source and MALDI [45-47]. Indeed, this source produces ions of analytes under normal atmospheric pressure conditions from analyte-doped matrix microcrystals by irradiating these crystals with laser pulses. 

Diagram of an AP-MALDI source. Ions are transferred into the mass analyser using the atmospheric pressure interface. 

The AP-MALDI source is illustrated in Figure 1.17. It works in a similar manner to the conventional MALDI source. The same sample preparation techniques and the same matrices used for conventional vacuum MALDI can be used successfully for AP-MALDI. The main difference is the pressure conditions where ions are produced. Conventional MALDI is a vacuum ionization source where analyte ionization takes place inside the vacuum of the mass spectrometer whereas AP-MALDI is an atmospheric ionization source where ionization occurs under atmospheric pressure conditions outside of the instrument vacuum. 

The advantages of AP-MALDI include those advantages typically associated with a MALDI source but without some of the drawbacks. Indeed, AP-MALDI does not require a vacuum region and is decoupled from the mass analyser, allowing it to be coupled with any mass spectrometer equipped with API. It is also easily interchangeable with other... 

The Cl source operates at low pressure. Ion-molecule reactions occur and are needed for sample ionization. The MALDI source is under vacuum, but during the ionization process the pressure increases in the plume close to the target and ion-molecule reactions occur. The various sources operating at atmospheric pressure include ESI, APCI, APPI and AP-MALDI. All these sources operate at sufficient pressure to have numerous collisions between ions and molecules, and reactions between these species are observed. 

The oa-TOF analysers are suited for any of the different ionization techniques. An oa-TOF instrument has even been adapted with pulsed ionization techniques such as a MALDI source. The configuration of such an instrument is shown in Figure 2.47. 

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