Pneumatically assisted ESI

Fig. 11.5. Diagram illustrating the components of an ESI source. A solution from a pump or the eluent from an HPLC is introduced through a narrow gage needle (approximately 150 pm i.d.). The voltage differential (4-5 kV) between the needle and the counter electrode causes the solution to form a fine spray of small charged droplets. At elevated flow rates (greater than a few pl/min up to 1 ml/min), the formation of droplets is assisted by a high velocity flow of N2 (pneumatically assisted ESI). Once formed, the droplets diminish in size due to evaporative processes and droplet fission resulting from coulombic repulsion (the so-called coulombic explosions ). The preformed ions in the droplets remain after complete evaporation of the solvent or are ejected from the droplet surface (ion evaporation) by the same forces of coulombic repulsion that cause droplet fission. The ions are transformed into the vacuum envelope of the instrument and to the mass analyzer(s) through the heated transfer tube, one or more skimmers and a series of lenses.

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 ... 

Fig. 11.4. Different sprayers for ESI. (a) Pure electrospray, (b) ESI with sheath liquid, (c) pneumatically assisted ESI, and (d) ultrasonic nebulizer. Adapted from Ref. [5] (p. 109) by permission. John Wiley Sons, Inc. 1997.

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]... 

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 present pneumatically assisted ESI interface is optimised around flow rates of 50-300 pl7min. The use of analytical columns 3. 6 mm i.d. with flow rates between 0.5 to 2 ml/min and narrow bore columns of 1-2 mm id with flow rates of 0.2-0.5 ml7min is routine in most pharmaceutical laboratories for HPLC analysis [24]. Capillary LC columns, because of their limited commercial availability and special practical considerations are used more where there is limited sample available or when sensitivity issues are present [25]. 

For most applications to CWC-related analysis, conventional and narrow bore columns have been used with APCI and pneumatically assisted ESI, using a split eluent if combined with ESI. The TNO Prins Maurits Laboratory routinely uses micro LC, particularly for biomedical analysis. 

The research efforts in the late 1960 s of the group of Dole (41-43) on electrospray sample introduction found continuation in the work of the group of Perm (86-87) in 1984 and later. An LC-MS interface based on ESI introduction into an atmospheric-pressure ion (API) source was described by Whitehouse et al. [88] in 1985. The flow-rate limitations of the latter system were to some extent removed by the introduction of a pneumatically-assisted ESI interface (ionspray ) for LC-MS by Bruins et al. (89) in 1987. This system was developed for a Seiex API instrument which in those days was the only commercially available instmment equipped with an API source. A major breaktlnough in ESI, and as a result of this also in the commercial availability of API instruments, was aehieved in 1988 by the observation of multiple-charge ions from peptides and proteins [90-91]. This made the ESI interface to one of the most popular and powerful methods for LC-MS. The development of API interfacing for LC-MS is discussed in detail in Ch. 5. 

The ionspray Merfaee, first described by Bruins et al. [14] in 1987, was introdueed in order to eombine the principles of ion evaporation (Ch. 3.2.3) and ESI. However, the prime ionization mechanisms of both approaches appeared to be similar. The main advantage of ionspray or pneumatically-assisted ESI over the eonventional ESI is the higher flow-rates (up to 200 pl/min instead of 10 pl/min) that can be aeeommodated. 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

D.L. Hiller, A.H. Brockman, L. Goulet, S. Ahmed, R.O. Cole T Covey, Application of a non-indexed dual sprayer pneumatically assisted ESI source to the high throughput quantitation of target compounds in biological fluids. Rapid Commun. Mass Spectrom., 14 (2000) 2034. 

In an ESI interface for LC-MS, the column effluent from a reversed-phase (RP) LC, i.e., a solvent mixture of methanol or acetonitrile and up to 10 nunol/1 aqueous buffer or 0.1% aqueous acid, is nebulized into an API somce. Pure ESI nebulization can only be achieved at flow-rates below 10 pl/min. Therefore, in most LC-MS applications, pneumatically-assisted ESI (Ch. 5.5.2) is performed ... 

The mobile-phase flow-rate is an important parameter, especially in ESI-MS. In early ESI interfaces, the flow-rate was restricted to 10 pl/min, sufficient or even too high in protein characterization, but rather low in LC-MS. Such a flow-rate is ideally suited for use in combination with packed microcapillaty columns (320-pm-ID columns). Current pneumatically-assisted ESI devices can be operated with flow-rates up to 1 ml/min or even higher. However, the optimum flow-rate giving the best response per injected amount is in the range of 100-400 pl/min for most commercial systems. Such a flow-rate is nicely compatible with 1-2-mm-ID LC colunms. 

Regarding MS detection, authors described methods performed in positive ion mode using full scan and selected ion monitoring for PSP analysis [42,43], The pioneering PSP research in the area of electrospray ionization-mass spectrometry (ESl-MS) detection analysis has been conducted in the group of Quilliam et al. [46, 47]. In their studies, the authors used LC separation before pneumatically assisted ESI. Detection limits as low as 30 pg were obtained for positive ions of several STX analogs [46]. 

The results of PAH analysis with different types of interfaces (e.g. ESI, APCI, PBI and TSP - were reported by Clench et al. reviewing the state of the art of various mass spectral techniques [28]. For more polar PAHs pneumatically assisted ESI-LC-MS was used to determine mixtures of hydroxy polycyclic aromatic hydrocarbons. The abundance of ions dependent on flow rates was shown. ESI inonization was found to be less sensitive compared to APCI ionisation [304]. PAH analysis with ESI-LC-MS combined with RP-LC with post-column addition of silver nitrate was applied for the determination of 10 PAHs in river water. PAHs resulted in [Mj and [M-i-Ag]. The detection limits of different PAHs in spiked samples ranged from 0.001 to 0.03 pg L [442]. 

Another instrumental development is based on the fact that the generation of smaller droplets is more favorable in terms of droplet evaporation during ESI, of sensitivity and the abihty to preserve non-covalent molecular associates. Thus, nanoelectrospray ionization (nESI) has been developed [68], where the analyte is sprayed from a gold-coated fused-silica capillary with a tip diameter of 1-5 pm rather than from capillaries with a 100-150-pm tips that are used in conventional (pneumatically assisted) ESI. In nESI, flow-rates as low as 20 nl/min can be nebulized. Thus, gentler operating conditions (temperature, gas flows, needle voltage) can be achieved. In order to more readily implement nESI in LC-MS operation, integrated chip-based nano-LC-nESI devices have also been developed [69]. 

Note These observations are equally relevant to (pneumatically assisted) ESI at standard flow rates [88]. Then, the corresponding voltages are just higher by a factor of > 2, which is mostly due to the increased gap between spray capillary and counter electrode to accommodate the larger plume. The optimization of the electrospray for temporal stability by adjustment of liquid flow, nebulizer gas pressure, and spray voltage is therefore necessary for any analytical ESI work. 

However, to limit the sensitivity loss that can occur in MS using a mobile phase flow rate above the optimal value for pneumatically assisted ESI, it is necessary to improve the interface and make it more readily compatible with elevated flow rates. In this context, the two most prominent manufacturers of UHPLC-MS systems have launched new source designs that can operate at elevated flow rates. 

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