Electrospray interface with pneumatically assisted

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). 

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 main problem encountered with PFT with a neutral selector remains the prevention of the chiral selector from entering the ionization source. This problem becomes particularly important at a high pH where EOF is important. Therefore, an acidic buffer pH or a coated capillary to minimize EOF is of the utmost importance. In addition, it has to be noted that the electrospray ionization process is pneumatically assisted when a sheath-liquid interface is used. The coaxial sheath gas induces an aspirating phenomenon in the capillary which may considerably affect the separation quality. This can be due to the decrease in interaction between analytes and the chiral selector and to a hydrodynamic flow induced by the Venturi effect at the capillary end [33, 34]. 

MS has been used with LC (moving belt interface), though not as extensively as with GC. Atmospheric pressure chemical ionization and pneumatically assisted electrospray mass spectrometry coupled to LC have been used for PAC analysis. Since it is easy to collect fractions continuously from LC, bioassays of the collected fractions can be performed and a muta-chromatogram (a plot of mutagenic potency versus fraction number) obtained. 

Figure 5.27 (a) Pneumatically assisted electrospray source and API interface designed for LC/MS applications, (b) Total ion current (m/z 200-450) chromatogram observed in LC/MS analysis of a mixture of five monosulfonated azo dyes ( 20 ng of each injected on-column), and ESI mass spectrum of the last-eluting component. Separation was achieved using a 1 x 100 mm C,g column, operated in reverse phase isocratic mode with 40 jiL.min of a 30 70 mixture of acetonitrile and aqueous ammonium acetate (10 mol.L ). Reproduced from Bruins, Anal Chem. 59, 2642 (1987), copyright (1987), with permission of the American Chemical Society. 

The design of a pneumatically assisted ESI interface differs from the sinple electrospray interface in that it provides a pneumatic assistance for the spray process. This is achieved by supplying a concentric flow of an inert gas such as nitrogen around the electrospray plume [58-60]. Assistance by a nebulizer gas stream of about 1-21 min allows for higher liquid flow and for a reduced influence of the surface tension of the solvent [61], Pneumatically assisted ESI can accommodate flows of 10-200 pi min . In fact, all modem ESI interfaces equipped with a nebulizer gas or sheath gas line enclosing the spray capillary. Thus, most routine ESI measurements are done using pneumatically assisted ESI. 

One of the major attractions of ESI is its ability to serve as an interface between liquid chromatography and mass spectrometry. There are currently a number of low-flow HPLC systems on the market that are compatible with electrospray, microspray, and nanospray sources. Capillary HPLC systems are interfaced with electrospray conducted in the pL/min flow regime, while nanoflow systems can accommodate nL/min flow rates. When electrospray is coupled with conventional HPLC, it is necessary to accommodate a higher sample flow rate ( 0.1—2mL/min) than normal electrospray can tolerate. To facilitate operation at these higher flow rates, a technique called pneumatically assisted electrospray or ion spray is employed, in which sample nebulization by aflow of gas is used to stimulate a more... 

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