Electrospray positive ion mode

Tandem mass spectrometry operated in the electrospray positive ion mode... 

Detector MS, PE Sciex API 300 tandem, heat-assisted nebulization, electrospray, positive ion mode, nebulizer gas at 60 psi, curtain gas at 40 psi, coUision gas thickness 4, auxiliary gas 7 L/min, dwell 400 ms, pause 5 ms, m/z 390-313... 

Detector MS, Micromass Quattro-LC, electrospray, positive ion mode, dr3ung gas nitrogen, nebulizing gas nitrogen, collision gas argon, m/z 792-573 UV 242... 

Detector MS, PE Sciex API 3000, electrospray, positive ion mode, ionspray needle 5000 V, turbo gas 400°, auxiliary gas 8 L/min, nebulizer gas 12 units, curtain gas 8 units, collision gas 4 units, declustering 46 V, focusing 200 V, collision energy 77 V, m/z 475-283... 

Fig. 4.2 Screening of a wastewater effluent sample by LC-QqTOF MS using the automatic screening method with the user-created database (a) total ion chromatogram (TIC) acquired in electrospray positive ion mode (-I-ESI) (b) database search results. The detected compounds with a score below 60 are marked in gray and the compounds which have no in-source fragments or characteristic isotope profile are highlighted in bold (Gomez et al. 2010, Fig. 4.1, with permission)...

Fig. 4.5 Identification workflow of ephedrine (a) peak was detected in electrospray positive ion mode (+ESI) extracting the m/z signal of ephedrine (b) MS spectrum was compared to database (c) database match with 4-hydroxymetamphetamine and ephedrine that actually having the same emparical formula. Once candidates were detected, sample was re-injected and MS/MS spectra acquired at several collision energies (d) MS/MS spectrum acquired were compared with (e) METLIN library MS/MS and the ephedrine was confirmed (Gonzdlez-Marino et al. 2012, Fig. 4.3, with permission)...

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

Electrospray is the softest mass spectrometry ionization technique and electrospray spectra therefore usually consist solely of molecular ions. Electrospray is unique, however, in that if the analyte contains more than one site at which protonation (in the positive-ion mode) or deprotonation (in the negative-ion mode) may occur, a number of molecular ions with a range of charge states is usually observed. For low-molecular-weight materials (< 1000 Da), the number of sites... 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

The typical solution present in the capillary consists of a polar solvent in which electrolytes are soluble. As an example, we can use methanol as solvent and a simple salt like NaCl or BHC1, where B is an organic base, as the solute. Low electrolyte concentrations, 10-5-10 3 mol/L (M), are typically used in electrospray mass spectrometry (ESMS). For simplicity we will consider only the positive ion mode in the subsequent discussion. 

Detection of M2D-C3-0-(E0)n-CH3 was possible by both positive ion mode atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) MS methods, with good response down to absolute injections of 0.1 ng. However, ionisation in the negative ion mode was negligible at all concentrations analysed, as the polyether-modified structure has no sites capable of adducting with anions, nor has it any moieties capable of cleavage to yield anionic species. 

TLC coupled with mass spectrometry employing desorption electrospray ionization has been used for the separation of synthetic dyes. The chemical structures of dyes included in the investigation are shown in Fig. 3.7. ODS HPTLC plates (10 X 10 cm) were used as the stationary phase the mobile phase consisted of methanol-tetrahydrofuran (60 40, v/v) containing 50-100 mM ammonium acetate for the positive-ion test and of methanol-water (70 30, v/v) for the negative-ion test. Test mixtures for negative- and positive-ion mode detection consisted of methyleneblue, crystal violet, rhodamine 6G... 

Fig. 1.19 (A) Q1 full-scan spectrum of bosentan [(M-l-H), m/z 552], its demethylated metabolite [(M-l-H), m/z 538] and its hydroxylated metabolite [(M- -H), m/z 568], (B) product ion spectrum of bosentan, (C) precursor ion spectrum, (D) neutral loss spectrum. Electrospray ionization is in positive ion mode.

Polar analytes can also form adducts with various ions. A typical adduct ion that is formed in the positive ion mode is [M + nNaJ L but many other adduct ions with K+, NHJ, CL, acetate are frequently observed (Figure 8.4). Their formation normally occurs in the bulk solution before the charge separation process, in the electrospray droplets during evaporation, or in the gas phase. If the analyte is weakly basic or polar, salts bearing cations may be intentionally added to the sample to stimulate the formation of positive ions [10,11]. 

Since the earliest applications of MS for the analysis of IL, higher-molecular ions assigned to molecular aggregates could be observed. For imidazolium-based ILs, clusters of the form [CaUAn. J can be observed in the positive ion mode in FAB [26], ESI [18,23], and APCI MSs [27], where x lies, depending on the ionization method applied, in the range between 2 and 12. The intensity of these clusters nearly exponentially decreases with n in electrospray MS [23], but for n = 5 a significantly increased signal intensity could be observed for different ILs. 

Electrospray ionization (ESI) was first employed more than 20 years ago, but it is fairly recently that it became a routine technique for the soft ionization of a wide range of polar analytes, including biomolecules. For this technique, the analyte is usually dissolved in a mixture of an organic solvent (most commonly acetonitrile or methanol) and water with a pH modifier [e.g. formic (methanoic) or acetic (ethanoic) acid for positive ion mode]. The presence of the pH modifier ensures that ionization takes place in the solution state. This is the only common case where ionization occurs before ion vaporization the exact mechanism of the vaporization (Figure 5.6) is still not clearly understood in ESI. 

Residues of TCs were quantified via the MCAC-HPLC method (26) in pork and chicken muscle tissue that had been previously screened with both a microbiological inhibition test using B. subtilis and an ELISA method. The correlation between the mean area of the inhibition zones and the DXC levels found in 28 samples by HPLC was 0.82 the correlation between the ELISA results and the DXC levels in the same samples was 0.73. The results indicated that an inhibition test was well suited to screen the mentioned samples for TCs residues. The authors found the more expensive ELISA screening test unnecessary, because only a minority of analyzed samples did not contain TCs. Confirmation with HPLC method was necessary because of the presence of some false-positive results. Moreover, the positive results from LC-fluorescence assay were confirmed using LC-MS-MS assay with electrospray ionization working in positive-ion mode (31). 

CCA and clopidogrel Column Cig-ether HPLC analytical column at 50 °C Detector LC-MS/MS operated in electrospray ionization (ESI) interface at positive ion mode MRM monitoring Clopidogrel m/z 322.0663 -> 212.0478 CCA m/z 308.0506 -> 198.0322 Mobile phase Gradient from 10% to 100% of ACN 0.1% formic acid Human plasma... 

The electrospray mass spectra for aspartame were collected in both negative and positive ion modes using a Waters Micromass ZQ single quadrupole instrument (Waters, Milford, MA, USA). Solution samples were infused at 25 pL/min. The spectra thus obtained are shown in Figures 8 and 9. 

Detection of imatinib is performed by triple quadrupole mass spectrometer with an electrospray ionization (ESI) interface operated in positive ion mode [103, 104, 106,107,109,110]. Except the methods published by Parise et al. for imatinib and its main metabolite [108], and for nilotinib [111], where a single quadrupole mass spectrometer was used, most TKIs are analyzed in plasma by atmospheric pressure ionization (electrospray or turbo ion spray) coupled to triple stage mass spectrometer. Expectedly, higher limit of quantifications for imatinib (30 ng/ml) [108], and nilotinib (5 ng/ml) [111], are obtained for the assays using single quadrupole MS (see Table 2). 

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