Electrospray positive-ion

Figure 2.20 Electrospray positive ion mass spectrum of a small molecule...

Figure 2.22 Charge attribution (a) and deconvolution (b) of the electrospray positive ion mass spectrum of cytochrome c (see Figure 2.21)...

Figure 1. The electrospray positive ion spectrum of 5, 10, 15, 20 tetraphenyl-21//,23//-porphine lead (II) showing the Na+ and K+ adducts. The expanded portion shows the resolution of the Na+ adduct ion isotope pattern.

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)...

Positive-ion electrospray mass spectrum of human hemoglobin (a) as initially obtained with all the measured masses, and (b) after calculation of true mass, as in Figure 8.3. The spectrum transforms into two main peaks representing the main alpha and beta chains of hemoglobin with accurate masses as given. This transformation is fnlly automated. The letters A, B, C refer to the three chains of hemoglobin. Thus, A13 means the alpha chain with 13 protons added. 

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... 

Figure 5.6 Positive-ion electrospray spectrum obtained from the major component in the LC-MS analysis of a purified recombinant 62 kDa protein using a Cig microbore 50 X 1 mm column and a flow rate of 50 p.lmin . The starting buffer (buffer A ) was 0.1% TEA in water, while the gradient buffer (buffer B ) consisted of 0.1% TEA in acetonitrile-water (9 1 vol/vol). The running conditions consisted of 0% B for 5 min, followed by a linear gradient of 100% B for 55 min. Reprinted from J. Chromatogr., B, 685, McAtee, C. P., Zhang, Y., Yarbough, P. O., Fuerst, T. R., Stone, K. L., Samander, S. and Williams, K. R., Purification and characterization of a recombinant hepatitis E protein vaccine candidate by liquid chromatography-mass spectrometry , 91-104, Copyright (1996), with permission from Elsevier Science.

The HPLC system comprised a 75 ftm x 15 cm PepMap column with a linear gradient of acetonitrile/0.1% aqueous formic acid (5 to 50% acetonitrile over 45 min) at a flow rate of 250 nlmin . Positive-ion electrospray ionization was employed using a nanospray interface. MS-MS specna were acquired over the range m/z 40 to 2000 at a rate of 1 s per scan. 

Figure 5.62 Product-ion MS-MS spectra of the molecular ions from 8-hydroxy-2 -deoxyguanosine, obtained by (a) positive, and (b) negative ionization. Reprinted by permission of Elsevier Science from Comparison of negative- and positive-ion electrospray tandem mass spectrometry for the liquid chromatography-tandem mass spectrometry analysis of oxidized deoxynucleosides , by Hua, Y., Wainhaus, S. B., Yang, Y., Shen, L., Xiong, Y., Xu, X., Zhang, F. Bolton, J. L. and van Breemen, R. B., Journal of the American Society for Mass Spectrometry, Vol. 12, pp. 80-87, Copyright 2000 by the American Society for Mass Spectrometry.

The HPLC system used consisted of a 30 x 2 mm Luna CN column with linear gradient elution employing two mobile phases A and B (A, 90% H2O 10% acetonitrile B, 10% H2O 90% acetonittile) with both phases containing 5 mM ammonium acetate and 0.2% formic acid. The hnear gradient commenced with 50 50 A B increasing to 100% B after 1 min of the analysis this composition was maintained for 1 min before returning to 50 50 A B after 4 min. Positive-ion ionspray (pneumatically assisted electrospray) was used to obtain mass spectra, with the spectrometer operating at a resolution of 5000. 

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. 

Optimized to provide maximum response for the (M + H)+ ion this process is usually automated. Then, the settings for the collision cell are optimized to produce maximum response of one or two characteristic daughter ions. Most modern instruments allow this to be done automatically. In general, sulfonylureas are very amenable to positive ion electrospray MS and MS/MS analysis, with excellent sensitivity compared with most other agrochemicals, and optimal responses can be easily obtained by proficient operators. 

Positive-ion single reaction monitoring Finnigan MAT API I ESI (electrospray ionization) 4.5 kV... 

Figure 11.6 Positive ion electrospray mass spectra of an equimolar mixture of five standard proteins, under different instrumental settings, showing cases where prominent signals for the different charge states of (A) insulin, (B) ubiquitin, (C) cytochrome c, (D) lysozyme, and (E) myoglobin were preferentially observed, and (F) where signals for all the proteins were more uniformly detected.

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. 

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