ESI Source Design

Mass Spectrometry Facility, University of Groningen, Groningen, The Netherlands 

3 Flow Rate and Sensitivity Electrospray Versus Nanospray 

5 Electrical Aspects of the Combination of Capillary Electrophoresis with ESI 

1 Free-Jet Expansion into VacuumHead ok where inserted  

7 Ion Optics Between Sampling Orifice and Mass Analyzer 

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Bruins, A.P. ESI source design and dynamic range considerations. Electrospray Ionization Mass Spectrom. 1997, 107-136. 

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... 

A number of different types of ESI sources, known as nanospray sources, have been designed that can operate at lower sample flow rates (10-200 nL min ). These generate smaller droplets and improve the signal intensity of the protein-ligand noncovalent complexes further, with the added benefit of reducing protein consumption up to 100-fold compared to standard ESI flow rates. Nanospray has also been reported to be more tolerant to nonvolatile cations in solution [37]. Recently, an automated fabricated chip nanospray source has been developed. This chip-based device has improved the ease-of-use for nanospray, while the design eliminates carryover effects as the spray is produced directly from an orifice on each sample well of the chip [38]. 

The early ESI interfaces were all optimised for flow rates between 1 and 10 fil/min. In trying to achieve direct compatibility with analytical HPLC, much development work has been done to accommodate higher flow rates and increase the efficiency of the nebulisation process. The present pneumatically assisted ESI interface is optimised around flow rates of 50-300 [lEmin. It is not the intention here to describe all the different manufacturers interfaces and source designs for API but the technology has been well documented [15]. 

The sensitivity of CE/ESI-MS has proven significantly better if the sheathless interface is coupled to a micro- or nanoelectrospray source designed for low flow rates. 

The first ESI design at the end of the 1980s proved to work properly as the HPLC interface with mobile phase flow rates between 1 and lOpL/min. Meanwhile, the development of the HPLC instrumentation and columns was oriented in the mL/min flow rate mode. In addition, the nebulization process based only on the application of an electrical field does not produce a stable spray from aqueous mobile phases. A modified ESI source, called ionspray, was then introduced [39], in which the nebulization of a liquid solution is pneumatically assisted by a coaxial flow of nitrogen (sheath gas) that allows the formation of a stable aerosol at mobile-phase flow rates between 10 and 500 pL/ min and the use of aqueous mobile phases. When working at higher flow rates (500-1000 pL/min), an additional nittogen flow rate can be used (auxiliary gas) to assist the desolvation of the droplets. This modified source is called turboionspray. 

Mass analyzer is the term given to the part of the mass spectrometer that discriminates between ions on the basis of their mass-to-charge ratios (m/z). The most common instruments currently in use with MALDI and ESI sources for the analysis of carbohydrates are quadrupole-based instruments, TOF mass spectrometers, and ion-trapping instruments (ion cyclotron resonance and quadrupole ion-trap designs). 

This source design was subsequently commercialized by Analytiea of Branford. Initially, these ESI sources were produced as retrofits for existing instruments from various instrament manufacturers. 

A heated countercurrent gas flow is applied in a number of other sources, e.g., in the initial Fenn ESI source (Figure 5.3) and in sources based on this design (Figure 5.5). The gas also assists in droplet evaporation. A cone gas is applied in a recent design of the Z-spray source from Waters (Figure 5.7). 

R.T. Gallagher, J.R. Chapman, M.Mann, Design and performance of an ESI source for a doubly-focusing magnetic sector MS, Rapid Commun. Mass Spectrom., 4 (1990) 369. 

A chromatogram of 11 steroids (20 pg each), detected as [M+Na] in LC-ESI-MS, is shown in Figure 13.3. The observations by Ma and Kim [19] are generally confirmed by others, although it must be taken in account that the source design from different instalment manufacturers appears to influence the choice between ESI and APCI. 

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