Pneumatically-assisted electrospray

The HPLC system used consisted of a 30 x 2 mm Luna CN column with linear gradient elution employing two mobile phases A and B (A, 90% H2O 10% acetonitrile B, 10% H2O 90% acetonittile) with both phases containing 5 mM ammonium acetate and 0.2% formic acid. The hnear gradient commenced with 50 50 A B increasing to 100% B after 1 min of the analysis this composition was maintained for 1 min before returning to 50 50 A B after 4 min. Positive-ion ionspray (pneumatically assisted electrospray) was used to obtain mass spectra, with the spectrometer operating at a resolution of 5000. 

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). 

Chiron S, Papilloud S, Haerdi W, Barcelo D (1995) Automated online liquid-solid extraction followed by liquid chromatography-high-flow pneumatically assisted electrospray mass-spectrometry for the determination of acidic herbicides in environmental waters. Anal Chem 67(9) 1637-1643... 

Ionspray Pneumatically assisted electrospray - a process in which nebulizing gas is used in conjunction with a high voltage to form droplets from a liquid stream. 

Note Pneumatically assisted electrospray is also termed ion spray (ISP). However, the term ISP is not recommended instead of pneumatically assisted ESI because ISP i) represents a mere modification of the ESI setup and ii) is a company-specific term. [63]... 

In atmospheric pressure ionization sources (API) the ions are first formed at atmospheric pressure and then transferred into the vacuum. In addition, some API sources are capable of ionizing neutral molecules in solution or in the gas phase prior to ion transfer to the mass spectrometer. Because no liquid is introduced into the mass spectrometer these sources are particularly attractive for the coupling of liquid chromatography with mass spectrometry. Pneumatically assisted electrospray (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) are the most widely used techniques. 

Fig. 1.9 Pneumatically assisted electrospray. The coaxial nitrogen gas assists the electrospray process allowing to operate at flow rates of several hundred microliters.

Electrospray ionization most commercial systems operate with pneumatically assisted electrospray (originally defined as ion spray)... 

Nebulizing gas (usually nitrogen) hows concentrically around the capillary, which shears droplets off as the liquid hows out of the end of the capillary. In the older literature, authors distinguish between pure electrospray without nebulizing gas and pneumatically assisted electrospray or ionspray. This is because of the mechanistic difference between the way the primary droplets form. Since all commercially available instruments allow the use of nebulizing gas, it is just a question of how rate as to whether it makes sense or not. 

Liquid chromatography creates a huge volume of gas when solvent vaporizes at the interface between the column and the mass spectrometer.22 Most of this gas must be removed prior to ion separation. Nonvolatile mobile-phase additives (such as phosphate buffer), which are commonly used in chromatography, need to be avoided when using mass spectrometry. Pneumatically assisted electrospray and atmospheric pressure chemical ionization are dominant methods for introducing eluate from liquid chromatography into a mass spectrometer. 

Pneumatically assisted electrospray,24 also called ion spray, is illustrated in Figure 22-17a. Liquid from the chromatography column enters the steel nebulizer capillary at the upper left, along with a coaxial flow of N2 gas. For positive ion mass spectrometry, the nebulizer is held at 0 V and the spray chamber is held at —3 500 V. For negative ion mass spectrometry, all voltages would be reversed. The strong electric field at the nebulizer outlet, combined with the coaxial flow of N2 gas, creates a fine aerosol of charged particles. 

Figure 22-17 (a) Pneumatically assisted electrospray interface for mass spectrometry. (fc>) Gas-phase ion formation. [Adapted from E. C. Huang, T. Wachs, J. J. Conboy, and J D. Henlon, "Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem. 1990, 62. 713A. and P Kebarte and L Tang, Rom Ions in Solution to Ions in the Gas Phase, Anal. Chem. 1993,65.972A ] (c) Electrospray from a silica capillary. [Courtesy R. D. Smith. Pacific Northwest Laboratory, Richland, WAJ... 

Molina, C., G. Durand, and D. Barceld (1995). Trace determination of herbicides in estuarine waters by liquid chromatography-high-flow pneumatically assisted electrospray mass spectrometry. J. Chromatogr. A, 712 113-122. 

The quadrupoles in an MS instrument serve as selective mass filters to isolate ions with m/z-values specific for the analytes of interest. The triple quadrupole MS/MS instrument is typically operated by a pneumatically assisted electrospray source with an additional heated auxiliary gas flow for higher flow rates. There is a trade-off between resolution (favored by lower flow) and sensitivity (favored by higher flow) of the quadrupole analyzers. The biological molecules can be proto-nated or deprotonated at multiple sites to produce ions of n charged states [M nH]n . The MRM-MS/MS scan mode has a high duty cycle for the detection... 

S. McSheehy, M. Marcinek, H. Chassaigne, J. Szpunar, Identification of dimethyl-arsinoyl-riboside derivatives in seaweed by pneumatically assisted electrospray tandem mass spectrometry, Anal. Chim. Acta, 410 (2000), 71-84. 

Thermospray introduced 1987 - Pneumatically assisted electrospray 1989 - First widely successful LC/MS/MS 1991 - AP LC/MS used in drug development... 

Baker, T.R. Pinkston, J.D. Development and Application of Packed-Column Supercritical Fluid Chromatography/Pneumatically Assisted Electrospray Mass Spectrometry, J. Am. Soc. Mass Spectrom. 9,498-509 (1998). 

Molina, C., Honing, M. and Barcelo, D. 1994. Determination of organophosphorus pesticides in water by solid-phase extraction followed by liquid chromato-graphy/high-flow pneumatically assisted electrospray mass spectrometry. Anal. Chem., 66 4444-4449. 

While pure electrospray nebulization is only capable of flow rates up to 20 nl/min, the development of pneumatically assisted electrospray is compatible with flow rates exceeding 200 pl/min (9). In order to successfully ionize a compound by the electrospray process the analyte should dissociate in solution to form solvated ions prior to nebulization. Ion formation from species that are not ions themselves always requires the presence of a polar atom or flmctional group to which solute cations or anions can be attached by ion-dipole forces. Biopolymers carrying multiple functional groups will form multiple charged ions, an observation that has had an outstanding impact on the analysis of peptides, proteins and oligonucleotides (7). 

Detector MS, Sciex API-Ill, lonSpray (pneumatically assisted electrospray)... 

FIGURE 3.21 Schematic diagram of a pneumatically assisted electrospray (a) effluent from the column (b) nebulizing gas (c) electrospray capillary (d) high-voltage power supply (e) skimmer cones. For a description of operation see text,... 

Pneumatically assisted electrospray is also called ion spray. 

Galceran, M. T., Mayano, E., Puignas, L., and Pais, P., Determination of heterocyclic amines by pneumatically assisted electrospray liquid chromatography-mass spectrometry, J. Chromatogr. A,... 

For mass spectral analysis of underivatised xenocoumacins soft ionization techniques are required. We have used both fast atom bombardment (FAB) and pneumatically assisted electrospray (ion-spray) with good results. High resolution mass measurements of the [M+H]+ ion obtained by FAB MS of xenocoumacin 1 and 2 indicated their molecular formulae to be C22H35N5O6 and C21H30N2O6 respectively. 

Depending on the amount of sample available, different rates are used. A low flow rate allows for long measurement times to optimise instrument parameters. With the pneumatically assisted electrosprays as shown in Fig. 4.13,... 

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