PI-ESI

NOR, LEV, PROG Ultrasonic extraction with MeOH-acetone, SPE (C18) HPLC-MS (PI-ESI) (SIM) 0.04 [46]... 

The predecessor of PI-ESI was ultrasonication-assisted spray ionization (UASI) [107, 108]. In that early version, one end of a sample capillary was dipped in the sample vial held in an ultrasonicator (frequency 40 kHz) while the other (tapered) end was placed close ( 3 mm) to the MS inlet. Although no electric potential was applied to the tapered end of the capillary, electrospray could be observed, and mass spectra of analytes with... 

Recently, an even simpler version of PI-ESI [114] was proposed, in which the use of a capillary as sample emitter was circumvented. A sample droplet (4-10 pi) containing analytes was placed in front of the MS inlet connected to the source of electric potential ( 3 kV). The ions corresponding to the analyte molecules present in this droplet were instantly recorded by a mass spectrometer. Importantly, the sample droplet was deposited on a dielectric substrate. Polarization of electric charges on the surface of the dielectric and the sample contributes to detachment of smaller droplets which are directed toward the MS orifice (Figure 2.16). This step is followed by desolvation which may occur in a similar way to that in ESI. Because of the simplicity of the setup, Pl-ESI is suitable for use in TRMS studies (see Chapter 11). 

Figure 2.16 Schematic representation of the putative mechanism of the PI-ESI-MS [1141. [Polarization Induced Electrospray Ionization Mass Spectrometry for the Analysis of Liquid, Viscous, and Solid Samples. Meher, A.K., Chen, Y.-C. J. Mass Spectrom. 50/3. Copyright (2015) John Wiley and Sons]...

Contrary to V-EASI, in the case of polarization induced (PI)-ESI [e.g., contactless atmospheric pressure ionization (C-API) and ultrasonication-assisted spray ionization (UASl)] [39-41], one does not need to supply nebulizing gas, which makes the monitoring system even more simplistic. In C-API, a glass capillary ( 20 cm, inner diameter 50 pm) was inserted to the reaction vial to direct the reaction solution toward the mass spectrometer. The tapered end of this capillary was placed in proximity ( 1 mm) to the inlet of the mass spectrometer (Figure 11.2a). The flow of sample was mainly due to capillary action and the influence of the electric field near the mass spectrometer, which may induce some electroosmotic flow (EOF) (see Chapter 6). In this approach, the sample flow rate is low ( 100 nl min" ). The sampling time can be reduced by using a shorter capillary as the sampling tube. Similarly to V-EASI, the capillary combines... 

Yu-Chie Chen received her education from the National Sun Yat-Sen University (MSc) and Montana State University (PhD). She is Professor of Chemistry at the National Chiao Tung University. Her research interests include biological mass spectrometry, nanomedicine, and nanotechnology. She is the co-inventor of several ionization techniques for mass spectrometry (SAUDI, UASI, C-API, PI-ESI), which are useful in the monitoring of chemical reactions. 

Electrospray Ionization - Mass Spectrometry (ESI-MS). The Jacobsen s Co-salen catalysts dissolved in dichloromethane were pumped to the mass spectrometer system after dilution with methanol at a flow rate of 50 pi min and 600 scans were collected in 1 min. 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

Fig.4 Reconstructed SRM chromatograms obtained from the LC-ESI-MS/MS analysis of a 100 ng mL-1 standard mixture of estrogens (in the NI mode) and progestogens (in the PI mode). Column Purospher STAR RP-18e (125x2 mm, 5 pm, Merck). Mobile phase gradient acetonitrile/water. Flow rate 0.2 mL min-1...

We were also interested in how the parameter space (tube dimensions, overpressure, solvent) would affect the flow rate. For ESI-MS, flow rates of 1-50 piL min-1 cover the normal range, with 5 pi min-1 being typical. We used 0.005" i.d. PEEK tubing,... 

Fig. 2.6.15. Total ion LC-ESI-MS chromatogram (bottom trace) and reconstructed chromatograms of halogenated APEOs and APEOs, obtained in PI mode, found in sludge from a Barcelona drinking water treatment plant. Reprinted with permission from Ref. [1] 2001 American Chemical Society. 

The pure electrospray process of dispersing a liquid into an aerosol works best at flow rates of 1-20 pi min" Conventional unassisted ESI has also limitations as a LC-MS interface due to the solvent properties in terms of volatility and polarity which can be electrosprayed without some type of assistance. Therefore, a number of sprayer modifications including a heated sprayer [55] have been developed to expand the range of ESI applications (Fig. 11.4). 

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 ... 

Sample consumption is chiefly determined by the concentration of the analyte solution and the liquid flow. Por example, during a 2.5-min measurement conventional ESI consumes 10 pmol when a lO" M solution at a flow rate of 4 pi min is employed. For nanoESI this reduces to 100 fmol for the same solution at 40 nl min ... 

Fig. 5.1 Principle of MS-based enzyme assays. Enzyme (E) molecules react with the substrate (S) to form an enzyme-substrate complex (ES), leading, for example, to a subsequent cleavage into two products Pi and P2. Pi and P2 are monitored continuously by ESI-MS. The injection of an inhibitor, I, results in the temporary formation of an inactive enzyme-inhibitor (El) complex, resulting in a reduction of Pi and P2 and negative peaks in the corresponding mass traces.

Fig. 5.2 Analytical set-up for on-line enzyme assays based on ESI-MS. PI Carrier/HPLC pump. P2 HPLC pump delivering enzyme solution. P3 HPLC pump delivering substrate solution. 1 Mixing union. 2 Microcoil reactor. In case of on-line coupling to HPLC, the HPLC column is inserted...

Atmospheric pressure photoionization (APPI) was recently introduced in the world of atmospheric pressure ionization techniques to analyze nonpolar molecules that are not efficiently ionized either by ESI or by APCI. Photoionization (PI) was already exploited some 30 years ago as a detection method for GC and LC, but only in recent times it has been used as an ionization method for mass spectrometry [52],... 

Higashi and coworkers [28] used 2-hydrazino-l-methylpyridine (HMP) derivatization to introduce a positively charged moiety in testosterone and dihydrotestosterone (DHT) to achieve a sensitivity improvement of 70- to 1600-fold compared to underivatized molecules in ESI. However, they found the derivative was unsuitable for di-keto steroids such as androstenedione and progesterone, so this form of derivatization is unlikely to be widely accepted. Separable syn- and anti- (E and Z) stereoisomers are formed using this derivative. Preparation is as follows a solution of 10 pg HMP in 50 pi ethanol containing 25 pg TFA was added to the steroid dissolved in 30 pi ethanol. After heating for 1 h at 60 C, the solvents can be removed and sample injected. 

In their studies, Kushnir and co-workers [44, 45] found an increase in sensitivity of ESI detection of testosterone and adrenal steroids when they convert carbonyl groups to oximes. The steroid mixture is dissolved in 300 pi of an aqueous hydroxylamine solution (1.5 mol, pH 10), and following heating for 30 min at 90 C the derivatives are extracted with methyl t-butyl ether (2 ml). 

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