Electrospray ionization interface

With the development of an atmospheric-pressure ionization (API) source, coupling of LC with MS has become a routine matter. The ESI format of API is the most appropriate interface for the LCYMS combination because (1) of its potential for the analysis of a variety of nonvolatile and thermally labile molecules of low to very high molecular mass at unprecedented low detection sensitivity, (2) ionization occurs at atmospheric pressure, (3) of its compatibility with RP-LC solvents, and (4) a range of solvent flow can be accepted. As a consequence, the LC/ESI-MS combination has gained prominence in several areas of research, such as to sequence proteins to identify mixtures of compounds, tryptic maps, and posttranslational modifications in proteins to elucidate structure of metabolic products to analyze drugs, pesticides, and toxins and to screen combinatorial libraries. The development of LC/ESI-MS has also greatly advanced the science of quantification. Several reviews of LC/ESI-MS technology have appeared in the literature [37-40]. 

The composition and flow rate of the solvent are two variables that are paramount for optimum operation of the ESI system. The flow rate determines the size as well as the size distribution of the droplets formed during ESI. A conventional ESI source operates at a flow rate of I toIO p,L/min. At higher flow rates, the spray is not stable because of the formation of larger droplets, which lead to electrical breakdown. Similarly, a fluid with high surface tension, such as pure water, is difficult to electrospray, but many polar solvents commonly used in RP-HPLC (e.g., methanol, ethanol, isopropanol, and acetonitrile) are suitable for the electrospray operation. Nonpolar solvents are difficult to disperse therefore, normal-phase HPLC is not easy to implement with the ESI process unless a polar solvent is admixed with the nonpolar mobile phase. 

HPLC is performed with various size columns that range from 0.1 to 4.6 mm in i.d. The conunon types of colunms and their characteristics are given in Table 5.1. The optimum mobile-phase flow rate is lower when the size of the column is reduced. For example, a decrease in the column i.d. from 4.6 mm to 320 p,m reduces the solvent flow rate from 1 mL/min to 4.9 p,Lmin . The 

A conventional ESI source can also serve as an interface for narrow- and wide-bore columns, provided that postcolumn flow splitting is used. Often, flow splitting is essential to allow recovery of a substantial portion of the eluting components for other experiments. A 1.0-mL min flow will require a 100 1 split. No degradation of the signal intensity owing to flow splitting is observed, because at low flow rates, the ESI source behaves as a concentration-sensitive detector. 

ESI Interface for CapiUary-LC and Nano-LC Columns Currently, the applications of capillary and nano-LC are on the upswing especially for many biochemical studies, where the sample amounts and volumes are both limited. For such samples, packed capillary columns of 50 to 300 xm i.d. are the ideal solutions. As pointed out above, the combined use of small-i.d. columns with an ES ion source has the advantage of optimal detection sensitivity because of its concentration-dependent response. Because these columns operate in the flow range nanoUters to microliters per minute, an ideal LC/MS system is realized when these columns are connected directly to nanospray or microspray sources [42,43]. The coupling of these columns to a conventional ES ion source can also be accomplished if an additional sheath liquid is added to increase the flow to a range that is acceptable by the source. 

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Smith, R. D., Olivares, J. A., Nguyen, N. T., and Udseth, H. R., Capillary zone electrophoresis-mass spectrometry using an electrospray ionization interface, Anal. Chem., 60, 436, 1988. 

Issaq, H.J., Janini, G.M., Chan, K.C., Veenstra, T.D. (2004). Sheathless electrospray ionization interfaces for capillary electrophoresis—mass spectrometric detection advantages and limitations. J. Chromatogr. A 1053, 37 42. 

Smith, R.D., Barinaga, J.C., Udseth, H.R. (1988). Improved electrospray ionization interface for capillary zone electrophoresis-mass spectrometry. Anal. Chem. 60, 1948-1952. 

A Waters Micromass triple quadrupole mass spectrometer was used with an electrospray ionization interface in positive ionization mode desolvation gas (400), cone gas (70), collision gas (2.74 x 10 3 mbar), capillary (3.0 kV), cone (14 (kV), source temperature (105°C), and desolvation temperature (300°C). The detection and quantitation of amlodipine and nimodipine were performed... 

Til. Ozaki, H., Itou, N., Terabe, S., Takada, Y., Sakairi, M., and Koizumi, H. (1995). Micellar electrokinetic chromatography mass-spectrometry using a high-molecular-mass surfactant - online coupling with an electrospray-ionization interface.. Chromatogr. A 716, 69—79. 

Fig. 1.15 Desorption electrospray ionization interface. The sample, in this case a pharmaceutical pill, is placed in front of the orifice and is hit by nebulized droplets. Desorbed ions are then sampled into the mass spectrometer.

To establish a sensitive and specific liquid chromatography-mass spectrometry (time-of-flight) [LC-MS (TOF)] method for the determination of donepezil in human plasma after an oral administration of 5 mg donepezil hydrochloride tablet [29]. Alkalized plasma was extracted with isopropa-nol-n-hexane (3 97) and loratadine was used as internal standard (IS). Solutes were separated on a Cis column with a mobile phase of metha-nokacetate buffer (pH 4.0) (80 20). Detection was performed on a TOF mass spectrometry equipped with an electrospray ionization interface and operated in positive-ionization mode. Donepezil quantitation was realized by computing the peak area ratio (donepezil-loratadine) (donepezil m/z 380 [M + H]+ and loratadine m/z 383[M + H]+) and comparing them with calibration curve (r = 0.9998). The linear calibration curve was obtained in the concentration range of 0.1-15 jUg/1. The detection limit of donepezil was 0.1 /zg/1. The average recovery was more than 90%. The intra- and inter-run precision was measured to be below 15% of RSD... 

Thurman, E.M., I. Ferrer, and D. Barcelo. 2001. Choosing between atmospheric pressure chemical ionization and electrospray ionization interfaces for the HPLC/MS analysis of pesticides. Anal. Chem. 73 5441-5449. 

A method has been proposed for the determination of cellular levels in lung cancer cell lines of the TKIs dasatinib and lapatinib [141], Cellular samples were extracted with a mixture of tert-butyl methyl ether/ACN/ammonium formate pH 3.5 (6 2 1, v/v/v). The organic layer was subjected to evaporation and the samples were reconstituted in acetonitrile, followed by chromatographic separation on a C18 column. Dasatinib and lapatinib were monitored by tandem MS equipped with a positive electrospray ionization interface in positive ion mode. These cellular experiments showed that lapatinib is not actively expelled from P-gp over-expressing cancer cells, while P-gp activity significantly decreases cellular levels of dasatinib [141],... 

The characterization of water-soluble components in slurries is one use of SPME with mixed solid-liquid samples. In one application, dried homogenized solid samples (10 mg of sewage sludge or sediment) were slurried in 4 ml of H,0 saturated with NaCl and adjusted to pH 2 with HCl for extraction for 1-15 h, which was followed by desorption into 4 1 methanol/ethanol over 2 min. The extracted compounds were either injected into a liquid chromatograph or fed directly via an electrospray ionization interface to a mass spectrometer with 1 s miz scans from 50-700 or selected-ion monitoring. The major components extracted included phthalates, fatty acids, non-ionic surfactants, chlorinated phenols and carbohydrate derivatives [235]. 

Smith, R. D. Maringa, C. J. Udseth, C. R. "Improved electrospray ionization interface for capillary zone electrophoresis/mass spectrometry" Anal. Chem. 1988, 60, pp 1948-1952. 

An electrospray ionization interface has been used with detection limits in the femtomole range. Reproducibility is a problem at the moment. See Chapter 19, p. 207, for more detail. 

The electrospray ionization interface is the most popular. It is also a soft ionization technique. The sample solution is sprayed across a high potential difference of a few thousand volts from a needle into an orifice in the interface (Figure 21.14). Heat and gas flow desolvate the charged droplets, giving charged analyte... 

Corn-based foods followed by fumonisin B2 (FB2).The problems and risks associated with fumonisin contamination have resulted in the development of precise, reliable and sensitive methods for its determination in corn and corn-based foods (Magan Olsen, 2004, as cited in Silva et al., 2009). Therefore, the quality parameters in the analysis of FBI and FB2 in corn-based products obtained with LC with fluorescence detector have been investigated (Silva et al, 2009). Furthermore, a comparison study between fluorescence detector (FD), mass spectrometry, and tandem mass spectrometry with a triple quadrupole (QqQ) analyzer using an electrospray ionization interface for the determination of fumonisin B1 and B2 in corn-based products has been performed. A comparative study of the three LC detectors, FD, single quadrupole, QqQ for the analysis of fumonisins in corn samples has been performed. The response achieved by the three detectors was sensitive enough to study the maximum contents established by the EU legislation. These LC detectors would be appropriate for quantification purposes but the acquisition of at least two transitions achieved with QqQ provided a univocal identification. 

Capillary electrochromatography-electrospray ionization-mass spectrometry (CEC-ESI-MS) is an analytical technique combining electrochroma-tographic separation and mass spectrometric detection with an electrospray ionization interface. 

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