Mass spectrometers desorption/ionization

Field desorption (FD) is perhaps the simplest technique, the sample is coated as a thin film onto a special filament placed within a very high intensity electric field. Ions created by field-induced removal of an electron from the molecule are extracted into the mass spectrometer. Field ionization (FI) is the equivalent process whereby gas-phase molecules are ionized by a high electric potential. These techniques are sometimes applied to relatively large, polymeric molecules but are not commonplace. 

There are two principal components of mass spectrometers the ionization chamber, where ionization of the sample occurs, and the mass analyzer, where ion sorting and detection occur. Mass spectrometer instruments vary in design with regard to both of these components. Thus far we have mentioned only one ionization technique, electron impact (El). In Section 9.18A we discuss El ionization in more detail, as well as discuss two other important ionization methods electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

The chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

Moyor, S. C. Marzilli, L. A. Woods, A. S. Laiko, V. V. Doroshenko, V. M. Cotter, R. J. Atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) on a quadrupole ion trap mass spectrometer. Int. I. Mass Spectrom. 2003, 226,133-150. 

Brown, R. S. Lennon, J. J. Mass resolution improvement by incorporation of pulsed ion extraction in a matrix-assisted laser desorption/ionization linear time-of-flight mass spectrometer. Anal. Chem. 1995,67,1998-2003. 

There is a recent hybrid between AP-MALDI and ESI, matrix-assisted laser desorption electrospray ionization (MALDESI) [202], where species desorbed from a MALDI target are subjected to an electrospray before entering the mass spectrometer. The method is similar to ELDI except that the analyte is embedded in a matrix as in MALDI. 

C. K. G. Piyadasa, P. Hakansson, and T. R. Ariyaratne. A Fligh Resolving Power Multiple Reflection Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometer. Rapid Commun. Mass Spectrom., 13(1999) 620-624. 

R. S. Brown and J. J. Lennon. Mass Resolution Improvement by Incorporation of Pulsed Ion Extraction in a Matrix-Assisted Laser Desorption/Ionization Linear Time-of-Flight Mass Spectrometer. Anal. Chem., 67(1995) 1998-2003. 

R. M. Whittal, L. M. Russon, S. R. Weinberger, and L. Li. Functional Wave Time-Lag Focusing Matrix-Assisted Laser Desorption/Ionization in a Linear Time-of-Flight Mass Spectrometer Improved Mass Accuracy. Anal Chem., 69(1997) 2147-2153. 

Electrospray ionization [21] is one of the most widely utilized ionization techniques employed today for the analysis of thermally fragile molecules. As such, it has assumed an important role in the analysis of biologically important molecules. ESI is a desorption ionization technique. This means that ions are formed before or during the transition from the liquid phase and need not be volatilized in advance of the ionization event (as is the case for El, Cl, etc.). Like APCI and APPI, ESI occurs at atmospheric pressure outside the vacuum chamber of the mass spectrometer (Fig. 11.5). A solution of the analyte passes through... 

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