Laser enhanced ionization spectrometr

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. 

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-... 

Label-free methods use the intrinsic properties of proteins to report binding events on protein chips. Ciphergen ProteinChips can be incorporated directly into a matrix-assisted laser desorbtion/ionization (MALDI) source of a mass spectrometer, which can identify the proteins and provide quantitative analysis. The ionization of proteins bound to Protein-Chips is enhanced by the properties of the chip surface, leading to more uniform mass spectra than possible with standard MALDI mass spectrometric... 

Surface-enhanced laser desorption ionization (SELDI) uses so-called protein chips for the detection of peptides and proteins from complex biological fluids such as blood or urine, often for the identification of diagnostic biomarkers for specific carcinomas [156, 157]. These protein chips can contain various media for positive or negative ion exchange or reverse-phase chromatography, as well as specific antibodies or DNA. The functionaUzed surface is immobihzed on a MALDl sample plate for the selective enrichment of constituents of the complex mixture applied, whereas the not bound supernatant is removed by washing. Unfortunately, a large number of unsubstantiated claims for the detection of disease-related biomarkers has discredited this approach, mostly as a result of poor mass spectrometric performance. 

It should be noted that, upon irradiation with the laser, not all of the sample will be consumed consequently, when the initial measurements have been completed the material can be redissolved and treated chemically or enzymatically prior to further mass spectrometric measurements. In the case of glycans, the most common MALDI matrix to be used is 2,5-dihydroxybenzoic acid (DHB). The most recently used matrices have been reviewed [43], with mixtures of DHB and coumarin analogs having been reported to provide enhanced ionization conditions for dextran chains [44]. 

The use of silver nanoparticles has also been described in matrix-assisted laser desorption/ionization (MALDI), a powerful laser-based soft ionization technique for mass spectrometric analysis and the investigation of peptides, proteins, nucleic acids, pharmaceuticals, bacterial characterization and imaging studies. Here, liquid-liquid microextraction base-modified silver nanoparticles were employed for the extraction of a hydrophobic peptide (gramicidin) from biological samples through hydrophobic interactions, prior to MALDI analysis (Figure 4.1) [41]. The application of silver nanoparticles was shown to provide an excellent sample cleanup procedure, and also assisted in the enhancement of signal of peptides and proteins. 

Merkel, D., et al., Proteomic study of human bronchoalveolar lavage fluids from smokers with chronic obstructive pulmonary disease by combining surface-enhanced laser desorp-tion/ionization-mass spectrometry profiling with mass spectrometric protein identification, Proteomics. 5, 11, 2972, 2005. 

---