Positive- and Negative-Ion Modes

In ESI conditions, aside from the formation of protonated ([M + H]+) and deprotonated ([M - H] ) molecules arising from the oxidation-reduction reactions at the sprayer, some cationization and anionization reactions can take place, due to the presence, inside the solution, of cations and anions. As an example, the positive- and negative-ion spectra of secoisolariciresinol diglucoside (SDG), obtained by injecting the methanol solution (10 5M with 0.1% of formic acid) in the ESI source at a flow rate of 15pL/min, are reported in Fig. 1.6a and b, respectively. In the former case, the protonated molecule is detectable at m/z 687, but the most abundant peaks are present at m/z 704 and 709, due to cationization reactions with NHJ and Na respectively. A scarcely abundant adduct with K+ is also detectable at m/z 725. The negative-ion spectrum (Fig. 1.6b) shows an abundant peak due to a deprotonated molecule (m/z 685), together with those due to adducts with Cl and HCOO (m/z 721 and 731, respectively). 

This aspect is well described by the ESI spectra reported in Fig. 1.7, which is obtained by direct infusion of a CH30H/H20 (1 1) solution of an extract of Cynara scolymus. The positive-ion spectrum (Fig. 1.7a) is highly complex either for the complexity of the mixture under analysis, or for the possible ionization of the various molecular species by 

Electrospray is surely the ionization method most widely employed for the liquid chromatography (LC)-MS coupling (Cappiello, 2007). The possibility of performing ionization at atmospheric pressure [also obtained in the case of atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI), allows the direct analysis of analyte solutions. However, some problems arise from the intrinsically different operative conditions of the two analytical methods. First, there are the high-vacuum conditions that must be present at the mass analyzer level. Second, the mass spectrometers generally exhibit a low tolerance for the nonvolatile mobile-phase components, usually employed in LC conditions to achieve high chromatographic resolution. 

Summarizing, the difficulties in LC/MS coupling can be related to the following aspects  

The scheme of a LC/ESI/MS system is shown in Fig. 1.8. Depending of the LC solvent flow, the splitter S can be employed to reduce the 

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S. Lacorte and D. Barcelo, Determination of parts per trillion levels of organophospho-rus pesticides in groundwater by automated on-line liquid- solid extraction followed by liquid chr omatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation . Anal. Chem. 68 2464- 2470 (1996). 

As with conventional Cl, this is a very mild form of ionization leading to molecular species with little or no fragmentation, i.e. (M + H)+ and (M — H) . This is not, however, always the case. The use of chromatographic modifiers may change the composition of the Cl plasma to such a state that, as in Cl and thermospray, other ions may be formed, e.g. the presence of ammonium acetate may lead to (M - - NH4)+ and (M - - CHsCOO)" ions in the positive- and negative-ion modes, respectively. The chemistry of the analyte may also have an effect, as has been discussed for ESI, with, for example, the spectra of fullerenes extracted from soot particles yielding an M+ molecular species [16],... 

Description glycosidic cleavage with a hydrogen transfer charge retained on reducing end positive- and negative-ion modes often referred to as )S-cleavage. 

Different categories of Zonyl polymers are studied by ToF-SIMS both in the positive and negative ion mode. Studies have shown that, for each polymer, a specific fingerprint is obtained and the peaks corresponding to the specific chemical moieties of each polymer are detected (Figure 15.4). To represent this good selectivity, Principal Component Analysis is performed on the obtained spectra. The result clearly discriminates the different polymers. ToF-SIMS is then suited to the characterization of these materials. 

The aim of the study was to reveal whether or not egg tempera was mixed with oil as binding media. The cross-section was studied in both positive and negative ion modes. Lead white with linseed oil, tripalmitin, stearic acid and lead white with egg tempera were studied as reference samples. 

Blood is the most often reported component of the patina from African ritual statuettes but other materials are also used. The patina is generally a quite complex mixture of different products from various chemical families. In the study of such complex mixtures, ToF-SIMS offers another great advantage, namely its versatility. Indeed, in a unique analysis, performed in positive and negative ion mode, a great number of different compounds can be detected. 

L. Konermann and D. J. Douglas. Unfolding of Proteins Monitored by Electrospray Ionization Mass Spectrometry A Comparison of Positive and Negative Ion Modes. J. Am. Soc. Mass Spectrom., 9(1998) 1248-1254. 

IUPAC NAMES OF COMPOUNDS INVESTIGATED IN BOTH POSITIVE- AND NEGATIVE-ION MODES... 

Mass spectra of ammonium nitrate and of a series of additional inorganic oxidizers were studied in both positive- and negative-ion mode by ESI-MS and ESI-MS/MS-CID [22, 23]. Characterization of ammonium nitrate by a series of typical cluster ions was confirmed by using isotojiicaUy labeled ammonium nitrate (ammonium- N, nitrate- N and nitrate- O) and deuterated water [22]. It was found that, at heated capiUary temperatures in the range... 

Value obtained from average of triplicate analyses in both positive and negative ion modes. 

Laser ablation coupled to ion cyclotron resonance Fourier transform mass spectrometry (in both positive and negative ion modes) can be used to distinguish natural and... 

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

The LTQ-Orbitrap has resolution and mass accuracy performance close to that of the LTQ-FTICR. As shown in Table 5.3 (column 4), LTQ-Orbitrap accurate mass measurements, using external calibration, for a set of 30 pharmaceutical compounds resulted in less than 2.3 ppm error. The data were acquired with a 4-min, 1-mL/min-flow-rate, positive-mode LC-ESI-MS method where all measurements were performed within 5h from mass calibration. Mass accuracies below 2-3 ppm, and often below 1 ppm, can be routinely achieved in both the positive- and negative-ion mode (Table 5.3, columns 4 and 5). The long-term mass stability of the LTQ-Orbitrap is not as consistent as observed for the LTQ-FTICR-MS, and the Orbitrap requires more frequent mass calibration however, mass calibration is a routine procedure that can be accomplished within 5-10 min. Figure 5.7 displays a 70-h (external calibration) mass accuracy plot for three negative ions collected with a LTQ-Orbitrap where the observed accuracy is 2.5 ppm or better with little mass drift for each ion. Overall, for routine accurate mass measurements on the Orbitrap, once-a-week calibration (for the desired polarity) is required however, considering the ease of the process, more frequent external calibration is not a burden. 

Using electrospray ionization mass spectrometry in both positive and negative ion modes, the on-line scanning of the Morita-Baylis-Hillman reaction in the presence of imidazolium ionic liquids has been investigated. The interception of several supramolecular species indicated that ionic liquids co-catalyse the reactions by activating the aldehyde toward nucleophilic enolate attack and by stabilizing the zwitterionic species that act as the main intermediates.175... 

A sample size of approximately 0.1 nanomoles is generally sufficient for molecular weight determination in either the positive or negative ion mode, but does not normally allow the sequence of amino acids to be determined. Larger sample sizes, typically between 1 and 5 nanomoles, afford some sequence information. Sequence ions are observed in the positive and negative ion modes from both N- and C-termini of the peptide and this may enable the complete sequence of the peptide to be determined. 

In lipid analysis, the most common ionization technique is ESI. ESI can be applied in both positive and negative ion mode. While the positive ion mode is more universal, the negative ion mode is better to some polar lipids, such as glycolipids and cardiolipins as shown in Table 2. 

Use the sample for MALDI-MS with CHCA and record the spectra in positive and negative ion mode. 

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