ESI interface

BordoU, R., Hoyes, J., Langridge, J., Chervet, J-P., Vissers, H. and van Veelen, P., Nanobore HPLC-MS and HPLC-MS/MS using a nanoflow ESI interface and a Q-Tof hybrid mass spectrometer . Micromass Technical Note 108, Micromass, Manchester, UK, 1998. 

Finnigan LCQ ion-trap mass spectrometer using ESI interface and Navigator 1.2 or XCalibur 1.0 SRI software... 

Confirmatory system Agilent Series 1100 liquid chromatograph Eclipse XDB Cig HPLC columu, 150 x 4.0-mm i.d., 3.5- im particle size MicroMass Quattro II triple-quadrupole mass spectrometer using an electrospray ionization (ESI) interface... 

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

Greek saffron Picrocrocin, safranal, crocins, kaempferol diglucoside Me0H/H20 A MeOH B H20 with AcOH 250, 308, 440 nm/ ESI (+) ESI interface 22... 

Mass spectrometric analysis was performed with a hybrid triple quadrupole/ linear ion trap Applied Biosystem MSD Sciex 4000QTRAP (Applied Biosystems, Foster City, USA) instrument equipped with a Turbospray ESI interface. For target quantitative analyses, data acquisition was performed in SRM, recording the transitions between the precursor ion and the two most abundant fragment ions. The developed instrumental method display excellent LODs in SRM mode between 0.5 and 1.2 pg (Table 2). 

Fig. 2.4.1. Process of ion generation in ESI interface. Adapted from Perkin-Elmer Sciex instruction manual of LC-MS-API 150.

Under ESI-FIA-MS(+) conditions, predominantly [M + H]+ ions besides small amounts of impurities could be observed (Fig. 2.5.6(b)). Compared with APCI ionisation, the ESI interface predominantly ionised the shorter chain homologues whereas the longer chain compounds contained in the blend were discriminated. 

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. 

Octylphenol (OP) and NP, were detected under negative ionisation (NI) conditions, using both APCI and ESI interfaces. The sensitivity of detection, using an ESI source was approximately 40-50 times higher than that obtained with an APCI source [30]. In contrast to the GC-MS analysis that reveals the presence of 22 isomers of the alkyl chain in the technical mixture of 4-NP, LC-MS analysis yields a single very broad peak. Using an ESI, APs give exclusively [M - H] ions with m/z 205 for OP and m/z 219 for NP as shown in Fig. 2.6.8. 

With regard to quantitative measurements of APG surfactants in, e.g. environmental samples, the authors stressed that it was of crucial importance to promote the formation of the desired molecular (or adduct) ion in order to obtain reproducible mass spectra. If tuning of the ESI interface parameters did not suffice to yield abundant ions of the selected species, acquisitions of the mass spectrometric detector after negative ionisation in conjunction with appropriate selection of the mobile phase composition were used as an alternative despite the lower sensitivity in this mode [1,2],... 

Mass spectrometric detection of AG (Fig. 2.7.9) was achievable in an ESI interface in positive as well as negative ionisation modes [8], Figure 2.7.10 displays the spectra of C12-AG, which are characterised by several molecular ions and fragments. The peak assignment with the corresponding masses of all ions is listed for the Ci2- and C14-homologues in... 

In (—)-ion mode besides the [M — H] the chloride adduct also appeared in the MS spectra with a similar intensity, but optimisation of the ESI interface parameters allowed the formation to significantly shift towards the [M — H] ion, while the chloride adduct was largely suppressed. The molecular ion yielded two fragments, one including the alkyl chain [CH3-N-CO-R]-, the other the sugar part [Gluc-N-CH3]. ... 

In a screening approach, non-ionic surfactants were monitored in the form of their [M + NH4]+ ions, equally spaced with Am/z 44 and identified by FIA-MS-MS(+) in combination with APCI or ESI interface [34,35]. Ci8-SPE was performed prior to selective elution by diethyl ether [35]. Ions of the non-ionics of AE type at m/z 350-570 (Am/z 44) were identified as surfactants with the general formula Ci3H27-0(CH2CH20)mH (m = 3-7). The complexity of the mixture confirmed the results using the diagnostic parent scans m/z 89 for aliphatic non-ionic surfactants of ethoxylate type necessary [35]. 

Owing to the anionic character of LAS, an electrospray ionisation (ESI) interface operated in negative ion mode is particularly attractive for the mass spectrometric detection of this surfactant type. Consequently, a great part of the atmospheric pressure ionisation-mass spectrometry (API-MS) work on LAS is devoted to the application of (— )-ESI-MS. 

For sensitive quantification in LC-MS analysis of non-ionic surfactants, selection of suitable masses for ion monitoring is important. The nonionic surfactants easily form adducts with alkaline and other impurities present in, e.g. solvents. This may result in highly complicated mass spectra, such as shown in Fig. 4.3.1(A) (obtained with an atmospheric pressure chemical ionisation (APCI) interface) and Fig. 4.3.2 (obtained with an ESI interface). 

Using the same instrument with an ESI interface NPEOs and OPEOs show a great affinity for alkali metal ions. They gave exclusively, evenly spaced sodium adducts [M + Na]+, due to the ubiquity of sodium in the solvents and surfaces (Fig. 4.3.1(B)). In another type of instrument applying ESI, however, a relatively complicated spectrum was obtained (Fig. 4.3.2(A)), with similarities to the APCI spectrum of the first instrument. 

LC-APCI-MS has been shown to very useful for the characterization of both neutral ginsenosides as well as thermolabile malonyl-ginsenosides in ginseng extracts (Ma et al., 2005). However, LC-MS with ESI interface is a highly sensitive and soft ionization technique for the LC-MS analysis of thermolabile compounds and is considered to be the best method for the analysis of ginsenosides as it can overcome most problems associated with the thermolabile malonyl-ginsenosides and low molecular ion abim-dance levels. LC-ESI-MS is characterized by abundant adduct formation... 

Modem ESI sources - also termed ESI interfaces - are constructed in many variations of this basic design. [48,49] They may have a heated transfer capillary... 

The design of a pneumatically assisted ESI interface differs from the pure electrospray interface in that it provides a pneumatic assistance for the spray process. This is achieved by admitting a concentric flow of an inert gas such as nitrogen around the electrospray plume. [56-58] Pneumatic assistance allows for higher flow rates and for a reduced influence of the surface tension of the solvent used. [59] Pneumatically assisted ESI can accommodate flow rates of 10-200 pi min ... 

For capillary zone electrophoresis (CZE) mass spectrometry coupling, another modification of an ESI interface has been developed. This interface uses a sheath flow of liquid to make the electrical contact at the CZE terminus, thus defining both the CZE and electrospray field gradients. This way, the composition of the electro sprayed liquid can be controlled independently of the CZE buffer, thereby providing operation with buffers that could not be used previously, e.g., aqueous and high ionic strength buffers. In addition, the interface operation becomes independent of the CZE flow rate. [62]... 

ESI ion sources are offered in combination with all types of mass analyzers, i.e., one is essentially free to choose any mass analyzer according to the analytical requirements or to upgrade an existing instrument with an ESI interface (Chap. 11.4.3). As fragment ions in ESI are often absent or exhibit very low intensity, MS/MS capability of the mass analyzer is beneficial. 

Smith, R.D. Barinaga, C.J. Udseth, H.R. Improved ESI Interface for Capillary Zone... 

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