E-ESI-MS

In the other most widely applied method, i.e., ESI-MS, the sample solution is introduced into a metal capillary, the end of which bears an electrostatic potential, into the first vacuum stage of the mass spectrometer where it forms an aerosol. Evaporation of the solvent results in concentration of ionic species and finally in Coulomb explosion. Similarly to M ALDl-MS, ions appear as protonated molecules or adducts with Na" or K+. If desired, the sample solution may be spiked with a small amount of a salt or an acid, e.g., formic acid. 

Figure 1. Purification of rat CpnIO (101 residues) using Fmoc probe 2. (A) Analytical RP-HPLC (C4 medium) of crude underivatized rat CpnIO. (B) Addition of lipophilic probe 2 increases the retention time of the protein (labelled 2) thus facilitating purification from underivatized truncated sequences (labelled 1). (C) Purified protein derivatized with 2. (D Purified protein after treatment with 5% aqueous TEA to remove 2. (E) ESI-MS of purified rat CpnIO. (R Deconvoluted mass spectrum for purified rat CpnIO. The calculated mass of target product is 10770.57 Da (average). The found mass is 10771.0. (Q) RP-HPLC (C4 medium, gradient TFA-water into 100% TFA-AcCN, 60 min) of purified rat CpnIO. The insert shows the expanded peak. (H) CZE of purified rat CpnIO. The concentration of the major peak (protein in its native, heptameric state) is 84%. Separate size-exclusion chromatography experiments showed that the majority of the flanking peaks correspond to protein with correct sequence but having an aggregation state different from the major peak.

Lopes-Da-Silva, E. et al.. Identification of anthocyanin pigments in strawherry (cv Camarosa) hy LC using DAD and ESI-MS detection, Eur. Food Res. Technol., 214, 248, 2002. 

Applications With the current use of soft ionisation techniques in LC-MS, i.e. ESI and APCI, the application of MS/MS is almost obligatory for confirmatory purposes. However, an alternative mass-spectrometric strategy may be based on the use of oaToF-MS, which enables accurate mass determination at 5 ppm. This allows calculation of the elemental composition of an unknown analyte. In combination with retention time data, UV spectra and the isotope pattern in the mass spectrum, this should permit straightforward identification of unknown analytes. Hogenboom et al. [132] used such an approach for identification and confirmation of analytes by means of on-line SPE-LC-ESI-oaToFMS. Off-line SPE-LC-APCI-MS has been used to determine fluorescence whitening agents (FWAs) in surface waters of a Catalan industrialised area [138]. Similarly, Alonso et al. [139] used off-line SPE-LC-DAD-ISP-MS for the analysis of industrial textile waters. SPE functions here mainly as a preconcentration device. 

Electrospray has been successful for numerous azo dyes that are not ionic salts. Several anthraquinone dyes have been analysed by LC-ESI-MS [552]. Electrospray achieves the best sensitivity for compounds that are precharged in solution (e.g. ionic species or compounds that can be (de)protonated by pH adjustment). Consequently, LC-ESI-MS has focused on ionic dyes such as sulfonated azo dyes which have eluded analysis by particle-beam or thermospray LC-MS [594,617,618]. Techniques like LC-PB-MS and GC-MS, based on gas-phase ionisation, are not suitable for nonvolatile components such as sulfonated azo dyes. LC-TSP-MS on... 

Mass spectrometry can be specific in certain cases, and would even allow on-line QA in the isotope dilution mode. MS of molecular ions is seldom used in speciation analysis. API-MS allows compound-specific information to be obtained. APCI-MS offers the unique possibility of having an element- and compound-specific detector. A drawback is the limited sensitivity of APCI-MS in the element-specific detection mode. This can be overcome by use of on-line sample enrichment, e.g. SPE-HPLC-MS. The capabilities of ESI-MS for metal speciation have been critically assessed [546], Use of ESI-MS in metal speciation is growing. Houk [547] has emphasised that neither ICP-MS (elemental information) nor ESI-MS (molecular information) alone are adequate for identification of unknown elemental species at trace levels in complex mixtures. Consequently, a plea was made for simultaneous use of these two types of ion source on the same liquid chromatographic effluent. 

A similar analysis of cochineal can be performed with the use of CE with ESI MS detection. The results are similar to those obtained with HPLC MS.[20] In the lac dye extract, the signal of laccaic acid A is found in the mass spectrum as the dominant one at m/ z 536. However, a second peak is observed on the electropherogram, and the eluted substance can be identified as laccaic acid E, on account of the mass spectrum which consists of the following signals at m/z 494 [M H], 476 [M H20 H] and 450 [M C02 H]. ... 

For the characterization of compounds extracted from plants, wool and dye baths, acquisition in the NI mode is used. The main signals in the mass spectra of each colourant are attributed to deprotonated molecular ions [M H]. More detailed studies can be performed by ESI MS" with a quadrupole ion trap mass analyzer, and such a set-up was used e.g. for the investigation of photo-oxidation processes of components of weld and onion skins.[29]... 

Figure 13.5 Product ion spectra of curcuminoids (Nl HPLC ESI MS/MS). Product ion labels correspond to fragments depicted in Figure 13.6 (a) bisdemethoxycurcumin (b) demethox ycurcumin (c) curcumin. Reproduced from H. Jiang, A. Somogyl, N.E. Jacobsen, B.N. Timmermann and D.R. Gang. Rapid Commun. Mass Spectrom., 20, 1001 1012 (2006). By permission of John Wiley Sons, Ltd...

A point of interest at this stop in our tour is that fragmentation of organometallic ions in ESI-MS often proceeds via ligand dissociation (e.g., phosphane loss) to generate coordinatively unsaturated organometallic ions [1-9]. One of the strengths of this technique is that such unsaturated ions are typically proposed as reactive intermediates in catalytic reactions carried out in solution (vide infra), allowing ESI-tandem-MS systems to study directly the gas-phase reactivity of such species. 

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