Laser desorption/chemical ionization

Troendle F, Reddick C, Yost R (1999) Detection of pharmaceutical compounds in tissue by matrix-assisted laser desorption/ionization and laser desorption/chemical ionization tandem mass spectrometry with a quadrupole ion trap. J Am Soc Mass Spectrom 10 1315-1321... 

RJ Beuhler, LJ Greene, L Friedman. Solvated proton mass spectra of tripeptide derivative. J Am Chem Soc 93 4307,1971 JP Speir, IJ Amster. An investigation of the energetics of peptide ion dissociation by laser desorption chemical ionization Fourier transform mass spectrometry. J Am Soc Mass Spectrom 6 1069, 1995. 

Desorption of analytes by a pulsed CO2 laser from a TLC plate, followed by chemical ionization (laser desorption-chemical ionization, LD-CI), was already reported in 1983 [46]. Later, desorption by an infrared (IR) laser and two-photon ionization were... 

Cotter R J 1980 Laser desorption chemical ionization mass spectrometry. Anal Chem 52 1767-1770... 

Figure 2 Bronsted acid Cl mass spectra of H-Val-Pro-Leu-OH using (A) NH4+, (B) CjHs+and (C) NjOH as reagent ions. Reprinted with permission from Speir JP, Gorman GS, Cornett DS and Amster IJ (1991) Controlling the dissociation of peptide ions using laser desorption/chemical ionization Fourier transform mass spectrometry. Analytical Chemistry 6Z 65-69. Copyright (1991) American Chemical Society.

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

Several modifications of MALDI have been developed to couple additional sampling and reaction capabilities to this technique. Surface-enhanced laser desorption ionization (SELDI) is one type of modified MALDI and describes an ionization process that involves reacting a sample with an enhanced surface. With SELDI, the sample interacts with a surface modified with some chemical functionality prior to laser desorption ionization and mass analysis. For example, an analyte could bind with receptors or affinity media on the surface, and be selectively captured and sampled by laser desorption. A SELDI surface can be modified for chemical (hydrophobic, ionic, immunoaffinity) or biochemical (antibody, DNA, enzyme, receptor) interactions with the sample. This technique can act as another dimension of separation or sample cleanup for analytes in complex matrices. As discussed before, one disadvantage of MALDI is that the matrix (usually a substituted cinnamic acid) that is mixed with the sample can directly interfere with the analysis of small molecules. There have been several areas of research to overcome this issue.Direct ionization on silicon (DIOS) is an example of a modification of MADLI that eliminates the matrix. In this case, analytes are captured on a silicon surface prior to laser desorption and ionization. Other examples of matrix-free laser desorption techniques include the use of siloxane or carbon-based polymers. 

In both of these two cases, the heat of evaporation is greater than the energy of the thermal degradation of the molecules. Therefore, methods omitting evaporation prior to ionization had to be looked for. Several attempts were made to overcome this problem . Today, the most promising methods using ionization of the sample in the solid state directly from a probe surface are field desorption (FD), desorption chemical ionization (DCI), Cf-plasma desorption technique, and laser-induced desorption technique. 

Nyadong, L., McKenna, A.M., Hendrickson, C.L., Rodgers, R.R, and Marshall, A.G. 2011. Atmospheric pressure laser-induced acoustic desorption chemical ionization Fourier transform ion cyclotron resonance mass spectrometry for the analysis of complex mixtures, A a/. Chem., 83 1616-1623. 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

For ambient mass spectrometric approaches, techniques such as electrospray-assisted pyrolysis ionization (ESA-Py) (Hsu et al., 2005), desorption electrospray ionization (DESl) (Takats et al., 2004), easy ambient sonic-spray ionization (EASl) (Haddad et al., 2008), and atmospheric pressure laser-induced acoustic desorption chemical ionization (AP/LIAD-CI) (Nyadong et al., 2011) have been used for the direct analysis of crude oil with minimal sample pretreatment. Such approaches prevent unexpected effects on the composition of crude oil samples during preparation. Another attractive feature of performing analyses imder ambient conditions is the capacity for rapid sampling, thereby enabhng opportunities for high-throughpnit analysis. 

Atmospheric pressure laser-induced acoustic desorption chemical ionization/mass spectrometry (AP-LIAD/CI/MS) for crude oil analysis... 

ELDI is that samples can be presented to the inlet nozzle directly from the outside as compared to ESI or MALDI that require additional sanple preparation [46], ELDI has also been applied to the analysis of peptides and proteins [47] even from biological media [48], to detect chemicals on different surfaces [49], and of course, for conpound identification on TLC plates [50]. It has been found that addition of a matrix is beneficial for the laser desorption part of the method. This gave rise to matrix-assisted laser desorption electrospray ionization (MALDESI)... 

The type of ion source used in an SID experiment depends on the projectile ion under study. For instance, if volatile, small organic compounds are of interest then an electron ionization (El) and/or chemical ionization (Cl) source will be sufficient. However, if one wishes to study biological compounds then a spray or desorption ionization method will be required, such as electrospray (ESI), atmospheric pressure chemical ionization (APCI), desorption chemical ionization (DCI), or matrix-assisted laser desorption ionization (MALDI). 

However, this high variability in ionization efficiencies implies that LDl is a very selective ionization method. In some cases, this selectivity is advantageous, for example, when one wishes to observe the presence or concentration of one, known, select molecule of interest.However, in mass spectrometry, one usually is interested in detecting all molecules that are present, including unknown species. Thus, many methods were explored to ionize the molecules after they were desorbed by the laser. These methods are collectively known as laser desorption post-ionization methods, and the post-ionization techniques include electron impact (El, diagrammed in Figure 6.2), chemical ionization (Cl), photoionization (PI), resonant-enhanced multiphoton ionization (REMPI), and many others. 

Modem soft ionization techniques have overcome the sample volatility requirement by combining the first two steps in mass spectrometry sampling and ionization. The soft ionization techniques used for the analysis of surfactants include fast atom bombardment (FAB), field desorption (FD), desorption chemical ionization (DCI, also called direct chemical ionization), secondary-ion mass spectrometry (SIMS), and laser desorption methods. 

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). 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

With the identities and amounts of amino acids known, the peptide is sequenced to find out in what order the amino acids are linked together. Much peptide sequencing is now done by mass spectrometry, using either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) linked to a time-of-flight (TOF) mass analyzer, as described in Section 12.4. Also in common use is a chemical method of peptide sequencing called the Edman degradation. 

Due to the high mass, low volatility, and thermal instability of chlorophylls and derivatives, molecular weight determination by electron impact (El) MS is not recommended. Desorption-ionization MS techniques such as chemical ionization, secondary ion MS, fast-atom bombardment (FAB), field, plasma- and matrix-assisted laser desorption have been very effective for molecular ion detection in the characterization of tetrapyrroles. These techniques do not require sample vaporization prior to ionization and they are effective tools for allomerization studies. 

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. 

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