Mass spectrometric analysis desorption

Laser desorption to produce ions for mass spectrometric analysis is discussed in Chapter 2. As heating devices, lasers are convenient when much energy is needed in a very small space. A typical laser power is 10 ° W/cm. When applied to a solid, the power of a typical laser beam — a few tens of micrometers in diameter — can lead to very strong localized heating that is sufficient to vaporize the solid (ablation). Some of the factors controlling heating with lasers and laser ablation are covered in Figure 17.2. 

Soft desorption/ionisation methods are of course crucial for mass-spectrometric analysis of biological macromolecules. 

Krishnamurthy, T. Ross, P. L. Rapid identification of bacteria by direct matrix-assisted laser desorption/ionization mass spectrometric analysis of whole cells. Rapid Commun. Mass Spectrom. 1996,10,1992-1996. 

Hutchens, T. W. Yip, T. T. New desorption strategies for the mass-spectrometric analysis of macromolecules. Rapid Comm. Mass Spectrom. 1993, 7, 576-580. 

O.F. van den Brink, J.J. Boon, P.B. O Connor, M.C. Duursma, and R.M.A. Heeren, Matrix assisted laser desorption/ionization Fourier transform mass spectrometric analysis of oxyge nated triglycerides and phosphatidylcholines in egg tempera paint dosimeters used for environ mental monitoring of museum display conditions, J. Mass Spectrom., 36, 479 492 (2001). 

Brancia, F.L., Oliver, S.G., and Gaskell, S.J. (2000) Improved matrix-assisted laser desorption/ionization mass spectrometric analysis of tryptic hydrolysates of proteins following guanidination of lysine-containing peptides. Rapid Comm. Mass Spectrom. 14, 2070-2073. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

T. K. Dutta and S. Harayama. Time-of-Flight Mass Spectrometric Analysis of High-Molecular-Weight Alkanes in Crude Oil by Silver Nitrate Chemical Ionization after Laser Desorption. Anal. Chem., 73(2001) 864-869. 

T. Krishnamurthy and P. L. Ross. Rapid Identification of Bacteria by Direct Matrix-aAssisted Laser Desorption/Ionization Mass Spectrometric Analysis of Whole Cells. Rapid Commun. Mass Spectrom., 10(1996) 1992-1996. 

T. W. Hutchens and T.-T. Yip. New Desorption Strategies for the Mass Spectrometric Analysis of Macromolecules. Rapid Commun. Mass Spectrom., 7(1993) 576-580. 

Several original papers must be mentioned that deal with mass spectrometric techniques which the numerous reviews do not comprise. Kaufmann and coworkers268,288 studied the mass spectrometric analysis of carotenoids and some of their fatty acid esters using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and its post-source-decay (PSD) variant. Some advantages concerning the thermal instability and limited solubility were discussed, but the fragmentation paths of the carotenoid cations were found to be essentially the same as those observed with conventional techniques. 

Heine, C.E. Geddes, M.M. Field-Dependent [M-2H] Formation in the Field Desorption Mass Spectrometric Analysis of Hydrocarbon Samples. Org. Mass Spectrom. 1994,29, 277-284. 

The analysis for proteins present in plasma or a cell extract is a challenging task due to their complexity and the great difference between protein concentrations present in the sample. Simple mixtures of intact proteins can be analyzed by infusion with electrospray ionization and more complex ones by matrix assisted laser desorption ionization. MALDI is more suited for complex mixtures because for each protein an [M+H]+ signal is observed while for ESI multiply charged ions are observed. Surface enhanced laser desorption (SEEDI) is a technique for the screening of protein biomarkers based on the mass spectrometric analysis of intact proteins [49]. However in most cases for sensitivity reasons mass spec-... 

Laremore, T. N. et al.. Matrix-assisted laser desorption/ionization mass spectrometric analysis of uncomplexed highly sulfated oligosaccharides using ionic liquid matrices. Anal. Chem., 78,1774,2006. 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

FAB and LSIMS are matrix-mediated desorption techniques that use energetic particle bombardment to simultaneously ionize samples like carotenoids and transfer them to the gas phase for mass spectrometric analysis. Molecular ions and/or protonated molecules are usually abundant and fragmentation is minimal. Tandem mass spectrometry with collision-induced dissociation (CID) may be used to produce abundant structurally significant fragment ions from molecular ion precursors (formed using FAB or any suitable ionization technique) for additional characterization and identification of chlorophylls and their derivatives. Continuous-flow FAB/LSIMS may be interfaced to an HPLC system for high-throughput flow-injection analysis or on-line LC/MS. 

Figure A.3A.1 Flow chart illustrating the selection of a suitable ionization technique for the mass spectrometric analysis of a sample. Abbreviations APCI, atmospheric pressure chemical ionization Cl, chemical ionization El, electron impact FAB, fast atom bombardment MALDI, matrix-assisted laser desorption/ionization.

A convincing body of experimental, information, now available in the literature, serves as confirmation for the possibility to desorb molecular ions out of the condensed phase even for organic molecules which are generally considered nonvolatile and/or fragile and do therefore not lend themselves to classical mass spectrometric analysis. Here the laser-MS competes with techniques such as static SIMS or FABMS, plasma- and field-desorption. 

Rodgers et al. [85] identified soil surface-bound polycyclic aromatic hydrocarbons through the use of real-time aerosol mass spectrometry in two NIST standard research material soils (Montana SRM 2710 and Peruvian SRM 4355), each contaminated separately with three common petroleum hydrocarbons (diesel fuel, gasoline and kerosene). This method required no sample preparation. Direct laser desorption/ionisation mass spectrometric analysis of individual soil particles contaminated with each of the petroleum hydrocarbons at three different contamination levels (0.8,8, and 80 ppth (wt/wt)) yielded detectable polycyclic aromatic hydrocarbon cation distributions that ranged from m/z 128 to 234, depending on the fuel contaminant. The same analysis... 

S. F. Ren and Y. L. Guo, Oxidized carbon nanotubes as matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of biomolecules, Rapid Commun. Mass Spectrom., 19 (2005) 255-260. 

The most creative application of the secondary cathode approach was described by Schelles and Van Grieken [24], who investigated its ability to determine the elemental constituents of polymeric materials. Mass spectrometric analysis has almost exclusively been directed at the determination of molecular weights and disparity characteristics secondary ion mass spectrometry (SIMS) [53,54] and matrix assisted laser desorption ionization (MALDI) [55,56] have carried the major share of the workload. Growing concerns over the fate of polymeric materials in the environment and the leaching of heavy metals into ground waters have necessitated the development of methods that permit the elemental analysis of bulk polymers. In addition, the use of polymers as immobilization media for waste remediation is also pressing these developments. 

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