Pneumatic-assistance

These solutions are not always practicable and HPLC flow rates of up to 2 mlmin may be accommodated directly by the use of electrospray in conjunction with pneumatically assisted nebulization (the combination is also known as lonspray ) and/or a heated source inlet. The former is accomplished experimentally by using a probe that provides a flow of gas concentrically to the mobile phase stream, as shown in Figure 4.8, which aids the formation of droplets from the bulk liquid, and will allow a flow rate of around 200 p. min to be used. 

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. 

ES ionisation can be pneumatically assisted by a nebulising gas a variant called ionspray (IS) [129]. ESI is conducted at near ambient temperature too high a temperature will cause the solvent to start evaporating before it reaches the tip of the capillary, causing decomposition of the analyte during ionisation and too low a temperature will allow excess solvent to accumulate in the sources. Table 6.20 indicates the electrospray ionisation efficiency for various solvents. 

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... 

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.

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. 

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]... 

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)... 

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]. 

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. 

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. 

The ionspray (ISP) interface is closely related to the ESP. Unlike the ESP interface, ISP allows higher flow rates by virtue of pneumatically assisted vaporization... 

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... 

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