MALDI laser ablation

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

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. 

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... 

There are three types of ion production using lasers as vaporization and ionization sources, laser ablation (LA), direct laser vaporization (DLV), and matrix assisted laser desorption ionization (MALDI). 

MAI,PI was introduced in the late 1980s and is one of the most successfully developed MS soft ionization techniques that uses the matrix assists laser ablation of sample-coated target to vaporize gas-phase ions for injection into a mass spectrometer. The advantage of MALDI is its gentleness compared with ESI and Atmospheric Pressure Chemical Ionization (APCI) and its ability to analyze the polar, nonvolatile, and large molecules. It has been very successfully used for the analysis of both biopolymers compounds and small molecular organic compounds (<1,500 Da). 

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

Laser ablation-based microanalysis techniques have also become very successful, e.g., matrix-assisted laser desorption/ionization (MALDI) has revolutionized the identification and study of large molecular weight biomolecules and polymers [28, 29]. 

During the last decade, processing of polymers has become an important field of applied and fundamental research [48]. One of the most important fields is laser ablation involving various techniques and applications. Laser ablation is used as an analytical tool for MALDI (matrix-assisted laser de-sorption/ionization) [28, 29] and LIBS (laser-induced breakdown spectroscopy) [49] or as a preparative tool for PLD (pulsed laser deposition) of inorganic materials [37] and of synthetic polymer films [50, 51]. Another application is surface modification of polymers [52] if low fluences are applied, the polymer surface can be either chemically modified to improve adhesion... 

Since its discovery, laser polymer processing has become an important field of applied and fundamental research. The research can be separated into two fields, the investigation of the ablation mechanism and its modeling and the application of laser ablation to produce novel materials. Laser ablation is used as an analytical tool in matrix-assisted laser desorption/ionization (MALDI) [12,13] and laser-induced breakdown spectroscopy (LIBS) [14] or as preparative tool for pulsed laser deposition (PLD) of synthetic polymers [15,16] and of inorganic films [17,18],... 

Lasers have provided a convenient means to create gas phase ions from nonvolatile subsfances. In fhis chapter, application of lasers to FTMS for polymer analysis will be considered. The earliest combination of direct laser desorption and laser ablation techniques with FTMS will be discussed first. Moving on from that topic, the impact of matrix-assisted laser desorption/ ionization (MALDI) on FTMS will be addressed, with particular emphasis... 

As discussed above, luminescent protein protected clusters exist in solution with zero valent oxidation state of the metal (for example, Au° state for Au clusters). Cluster forms via the metal bound protein complex (Au" -Protein adduct for Au cluster formation) where metal is in its intermediate oxidation state.In this section we will discuss a new class of materials which were observed recently by Baksi et They have observed that when Au -Lyz adducts were subjected to laser ablation, some bare clusters were formed in the gas phase. When Au was mixed with Lyz, Au was uptaken by the protein and a corresponding change is seen in the mass spectrum in the form of appearance of multiple peaks separated by mIz 197 due to Au attachments (Lyz peak appears at mIz 14 300). Maximum 10 Au attachments were observed in the MALDI MS which was justified in terms of limited (eight cysteines forming 4 disulfide bonds) cysteine content. In the lower mass region (<10 kDa), multiple envelopes... 

Several other ionization methods and MS technologies have been investigated for possible use for imaging. Atmospheric pressure infrared MALDI (AP IR-MALDI) and mid-infrared laser ablation with electrospray ionization (LAESI) have been reported and use focused laser radiation for ambient sampling with no sample preparation or chemical modification required. In these... 

Four of the most commonly used desorption/ionization methods for MSI are secondary ion mass spectrometry (SIMS), desorption electrospray ionization (DESI), matrix-assisted laser desorption/ionization (MALDI), and laser ablation (LA) with post-ionization. Other desorption/ionization approaches such as laser desorption/ionization (LDI) see Chapter 9, (12)), desorption/ionization on silicon (DIOS) (13), electrospray ionization (ESI) (14), and nanostructure-initiator mass spectrometry (NIMS) (15, 16) also have great potential in MSI. Importantly, many mass spectrometers equipped with a MALDI ion source can be used with related ionization processes such as LDI, DIOS, LA, and laser-NIMS. Erequently, a specific ion source arrangement is optimized for a specific mass analyzer for example, MALDI is often interfaced to a time-of-flight (TOF) mass analyzer (described below) although it can also be used with ICR-based instruments. 

Becker, J. Su., Mounicou, S., Zoiiy, M. V., Becker, J. S., Lobinski, R (2008) Analysis of metal-binding proteins separated by non-denaturating gel electrophoresis using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Talanta, 76, 1183-1188. 

Novel ionization methods in ambient mass spectrometry (I) overcome these limitations by performing imaging under native conditions. Desorption electrospray ionization (2), atmospheric pressure (AP) mid-infrared (mid-IR) MALDI (3), laser ablation... 

Interfaces between Microfluidics and Mass Spectrometry, Figure 3 Two commercial microfluidics-MS interfaces (a) a polyimide microchannel device with integrated ESI tip formed by laser ablation. Reprinted with permission from Yin et al. [ ] and (b) schematic of MALDI CD device (see text for details). Reprinted with permission from Gustafsson et al. [15]... 

Mass analyzers. MALDI is ideally coupled to time-of-flight (TOF) for polymer analysis (17). This combination is ideal due to the pulsed nature of the laser and the theoretically unlimited mass range and high transmission efficiency of TOF. Ions are produced by laser ablation of the dried matrix/analyte mixture and are then accelerated by a fixed potential into a drift tube that does not contain an external electric field. The ions continue moving toward a detector. Since all the ions are accelerated at the same potential, the kinetic energy (KE) of a given ion can be given by eq. 1 ... 

Solvent-free separation using IMS-MS when combined with solvent-free sample preparation and appropriate ionization methods provides TSA that are independent of analyte solubility and is applicable to complex mixtures. A new ionization method, LSI, which operates at AP with laser ablation of samples prepared in a MALDI matrix, has been effectively interfaced with a SYNAPT G2 IMS-MS instrument. Early results show separation of protein mixtures and that protein ion structures from LSI and ESI are similar. Further, a new matrix allows LSI multiply charged ion formation to be extended to solvent-free matrix preparations. Thus TSA by LSI-IMS-MS is a new approach to tissue imaging at high spatial resolution on high-end mass spectrometers such as the SYNAPT G2 and Orbitrap instruments. LSI has... 

FIGURE 8.12 (a) Laserspray mass spectrum of bovine insulin obtained by laser ablation of a 2,5-dihydroxybenzoic acid/bovine insulin mixture prepared using the dried droplet MALDI sample preparation method, (b) The DriftScope presentation of the IMS/MS acquisition of the data. 

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