Ionisation efficiency

ES ionisation can be pneumatically assisted by a nebulising gas a variant called ionspray (IS) [129]. ESI is conducted at near ambient temperature too high a temperature will cause the solvent to start evaporating before it reaches the tip of the capillary, causing decomposition of the analyte during ionisation and too low a temperature will allow excess solvent to accumulate in the sources. Table 6.20 indicates the electrospray ionisation efficiency for various solvents. 

Table 6.20 Electrospray ionisation efficiency Solution in water ...

High ionisation efficiencies of vapour-phase analytes... 

Much LC-MS work is carried out in a qualitative or semi-quantitative mode. Development of quantitative LC-MS procedures for polymer/additive analysis is gaining attention. When accurate quantitation is necessary, it is important to understand in depth the experimental factors which influence the quantitative response of the entire LC-MS system. These factors, which include solvent composition, solvent flow-rate, and the presence of co-eluting species, exert a major influence on analyte mass transport and ionisation efficiency. Analyte responses in MS procedures can be significantly affected by the nature of the organic modifier used in the RPLC... 

The differences in ionisation efficiencies, however, not only result from the use of FIA or LC but, as mentioned before, depend also on the application of the APCI or ESI interface for ionisation. Therefore the application of both API methods, APCI and ESI, is the only way to overcome discrimination problems because of interface type selection. Even the use of the ion spray technique instead of conventional ESI may influence the ionisation efficiency considerably. 

Increase or decrease in ionisation efficiency for the analytes of interest because of a change in mixture composition introduced into the source. 

For examination of these interferences during the ionisation of surfactants in the FIA-MS mode we tried to verify and quantify potential effects in the ionisation process. Ionisation efficiencies of... 

To recognise ion suppression reactions, the AE blend was mixed together either (Fig. 2.5.13(a) and (b)) with the cationic quaternary ammonium surfactant, (c, d) the alkylamido betaine compound, or (e, f) the non-ionic FADA, respectively. Then the homologues of the pure blends and the constituents of the mixtures were quantified as presented in Fig. 2.5.13. Ionisation of their methanolic solutions was performed by APCI(+) in FIA-MS mode. The concentrations of the surfactants in the mixtures were identical with the surfactant concentrations of the blends in the methanolic solutions. Repeated injections of the pure AE blend (A 0-4.0 min), the selected compounds in the form of pure blends (B 4.0—8.8 min) and their mixtures (C 8.8— 14.0 min) were ionised and compounds were recorded in MID mode. For recognition and documentation of interferences, the results obtained were plotted as selected mass traces of AE blend (A b, d, f) and as selected mass traces of surfactant blends (B a, c, e). The comparison of signal heights (B vs. C and A vs. C) provides the information if a suppression or promotion has taken place and the areas under the signals allow semi-quantitative estimations of these effects. In this way the ionisation efficiencies for the pure blends and for the mixture of blends that had been determined by selected ion mass trace analysis as reproduced in Fig. 2.5.13, could be compared and estimated quite easily. 

An industrial blend of ethylene oxide (EO) PEMS marketed as a personal care product was examined by positive ion FIA-APCI-MS and LC-APCI-MS-MS (Fig. 2.8.8) [41]. The FIA-APCI-MS spectrum without LC separation (Fig. 2.8.8(a)) is dominated by ions corresponding to unreacted PEG (m/z 520, 564, 608, 652,...), whilst the ions corresponding to the PEMS (m/z 516, 560, 604, 648,...) could only be clearly observed following LC separation (Fig. 2.8.8(b)). Comparison of the TIC chromatograms of PEMS and PEG (Fig. 2.8.8(c) and (h)) demonstrates the dominance of the PEG by-products in the commercial formulation. It is unclear whether the observed relative intensities are representative of the actual amounts or of the different ionisation efficiencies, due to the confidential nature of the product composition. However, the spectra indicate a trisiloxane surfactant structure of that shown in Fig. 2.8.2 (R = Ac) and FIA-MS analysis of another commercial formulation of this product showed good spectra dominated by the silicone surfactants [48], indicating that the PEG by-product composition can vary significantly in commercially available PEMS formulations. 

The properties, behaviour under biochemical degradation conditions in WWTPs as well as the extraction, concentration and ionisation efficiency of the fluorine-containing surfactants were described in the literature [49,51]. 

The ESI-FIA-MS(+) spectrum of a non-ionic compound mixture is presented in Fig. 2.9.44. Comparing ESI and APCI, differences in ionisation efficiency and in the ions generated could be observed [16]. While in the ESI-FIA-MS(+) ionisation mode, a series of [M + H]+ ions at m/z AQ1-1Y1 and [M + NH4]+ ions at m/z 470-866 were generated, both equally spaced by Am/z 44, APCI-FIA-MS(-I-) mainly resulted in [M + NH4]+ ions. It was observed that APCI in the same blend ionised the compounds with longer PEG chains, i.e. the more polar compounds. This is in contradiction to the behaviour of non-fluorinated AE compounds, which with an increasing number of PEG chain links became more polar and therefore easily ionisable by ESI. This result contradicted our own experiences [16] and also those reported by... 

With respect to quantification of SPC in environmental samples, one has to deal with the scarcity of available reference compounds. Of the four SPC homologues, C6, C8, Cio and Cn, analysed in coastal waters after SPE [20], the response factor of C8-SPC in the ESI—MS was found to be about three times lower than those of the longer chain species. This was attributed to the much higher water content in the eluent at the time of elution of C8-SPC, which resulted in a notable reduction of the ionisation efficiency. This fact was taken into account [17] by quantifying individual SPC based on the responses of the two... 

For comparison of the different ionisation methods and detection modes, the results obtained as FIA overview spectra are presented in Figs. 2.11.7 and 2.11.8. Reconstructed ion chromatograms (RIC) of APCI and ESI combined with selected mass traces of all LC separations and, in parallel, the selected standardised mass traces of the C42 and C14 homologues containing three ethoxy chain links recorded in the negative mode are presented in Fig. 2.11.9. These results again demonstrate the quite large variation in the ionisation efficiency of... 

Fig. 4.3.5. Calibration graphs of LAS and SPC. For each compound the slope of the fitted linear regression and the correlation coefficient are given behind the substance name. The concentrations of the IAS homologues represent the sum of the concentration of the four components, i.e. for comparison of the ionisation efficiencies the value of the slope has to be multiplied by the relative...

In Fig. 5.1.10 the evolution of C-even (a) and C-odd-SPC (b) overtime from the LAS-spiked FBBR is plotted. Their formation starts at day 5 with the entire range of examined SPC observed. The curves show a maximum at day 7 with C7-, C8-, and C9-SPC prevailing. However, it should be noted that in quantitative terms the short-chain homologues C4-C6 are underestimated, as their ionisation efficiency is lower than that of the longer-chain SPC. A relatively constant level of all SPC compounds was achieved on day 15 and maintained until the end of the experiment on day 30 (not all data shown). 

In El, it is customary to use an ionisation energy of 70 eV. This is achieved by accelerating the electron produced by the filament through a potential drop of 70 V, applied between the filament and the chamber. Ionisation efficiency in El is in the order of one ion produced for every 10000 molecules. In some cases, reducing... 

Figure 16.15—Electron ionisation (El). The collision of an electron with a sample molecule m produces ionisation that leads to formation of a parent ion and fragment ions. Ions that result from the reaction m/ and raj are also called secondary or daughter ions. Since they carry no charge, neutral fragments produced during decomposition, (ra, m[ and m ), are not detected. An illustration of electron ionisation of benzene is shown. Also shown is a schematic of the ionisation chamber (ion source). Using a parallel magnetic field can increase the effective path of an electron in the ion source, which increases ionisation efficiency.

Figure 16.16—Influence of electron energy on fragmentation. An example with benzoic acid is shown here. An ionisation efficiency curve as a function of electron energy is presented. It should be noted that the spectra are normalised that is, the most intense peak has a value of 100. Some claim that this is an application of Procruste s method (according to Greek legend, the bandit Procruste would force travellers to lie on his bed and, depending on their size, he would either cut their feet or stretch their bodies so they would fit the dimensions of the bed).

These experiments were undertaken in a similar manner to those described by Brady and Sanders for measuring relative binding affinities of metals with steroid derivatives by ESI-MS. Titrations with zinc and copper solutions were carried out on patellamides C and A by sequential addition of 0.25 equivalents of the metal solutions to the peptides (cone. 0.02 mg/mL in MeOH). These titrations were monitored by measuring the formation of the metal species as well as the loss of the uncomplexed peptides, using both the full spectrum and the selected ion mode of the instrument in parallel, as the ionisation efficiency of these species were very different. Once fully complexed zinc species were obtained for each peptide, copper solution was then titrated into these at a rate of 0.25 equivalents to again observe the competition effects. 

Quantification was based upon the method generally referred to as internal standardisation using, within each isomeric group, one C-labelled isomer as an internal standard. Inevitable differences in chromatographic retention and/or minor differences in ionisation efficiency, mass spectrometric fragmentation and ion masses monitored, lead to differences in sensitivities between the compounds to be determined and the corresponding C-labelled internal standards. These effects were accounted for in the final concentration calculation by the introduction of isomer specific relative sensitivity factors (RSF). Additional details on this procedure are described elsewhere [18,19]. 

Solution flow rates can range from microhtres to several millilitres making this ionisation method very suitable for interfacing to chromatographic separation methods. Within the last few years several microflow devices have been developed to meet the needs in protein analysis caused by the availabihty of only low amounts of sample [29, 30]. Especially nanoelectrospray has been shown to be feasible for protein analysis and also for the characterisation of non-covalent complexes [31]. The small nanospray-droplets enable a higher ionisation efficiency at significantly reduced spray potentials. The low flow rates enable enhanced experimental variation which is especially useful for MS/MS experiments and reaction monitoring [32, 33]. 

AEOs spiked into raw wastewaters were applied to elaborate an APCI or ESI-LC-MS method to determine non-ionic surfactants after SPE. Ionisation efficiencies of both interface types were compared and the more effective APCI technique then was applied for quantification [334]. Recoveries observed with standard determination methods for surfactants and MS detection techniques for different types of surfactants (e.g. alkylether carboxylates, sulfosuccinates, fatty acid polyglycol amines, quaternary carboxoalkyl ammonium compounds, modified AEOs, EO/ PO compounds, APGs, alkyl polyglucamides, betaine and sulfobetaine) in spiked wastewater samples were compared by applying APCI and/or ESI(-i-/-).Poor recoveries were obtained by standard methods but good results by MS [335]. APCI and... 

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