Orthogonal spray

Orthogonal spray orientation and high-capacity drying gas system improve instrument performance and decrease maintenance requirements. 

The addition of buffering salts to the mobile phase often improves chromatographic separation, provides a stable pH during separation, and reduces problems associated with column disturbances produced by highly variable samples. These salts are usually volatile (examples are ammonium formate, ammonium acetate, and i-ethylammonium hydroxide) and the concentrations used are usually less than 10 mM. With the advent of orthogonal interfaces for ESI and APCI, the absolute requirement for volatile salts has disappeared. However, the prolonged use of nonvolatile salts is not recommended as the accumulation of salts in the spray chamber of the MS reduces sensitivity and increases maintenance requirements. 

If nonvolatile buffers cannot be completely avoided, their concentration should be kept at a minimum. High flow rates (due to a strong EOF) prevent rapid crystallization at the tip. Modern off-axis electrospray arrangements (orthogonal or Z-spray) tend to be more tolerant to nonvolatiles than classical on-axis interfaces. 

Figure 14.5 Modified-ESI source for the direct infusion of undiluted ILs. A stainless steel wire is placed in the spray, leading to the optimal vaporization of the IL. Additionally, an orthogonal ESI source is used. Only a part of the IL ions is transferred into the MS, thus minimizing pollution of the source. (Modified from Dyson, R J. et al.. Direct probe electrospray (and nanospray) ionization mass spectrometry of neat ionic liquids. Chem. Commun., 2204, 2004. Reproduced by permission of the Royal Society of Chemistry.)...

In all API sotrrce desigrts described so far, the spray device is in axial positiorr, or only slightly off-axis, relative to the sampling orifice or capillary. Hiraoka et al. [19] described an ESI sorace, where the sprayer is orthogonally positioned relative to the sampling orifice. This design allows higher flow-rates to be used. 

The most successful modification to reduce source contamination is the orthogonal positioning of the spray probe (Ch. 5.3.5, Figure 5.5, [20]). Orthogonal ESI was ev uated by Voyksner and Lee [23] in combination with an ion-trap MS. 

Over the years there has been some debate on the need to apply heat to the ESI ion source to assist in the evaporation of the droplets. Thermally-assisted solvent evaporation is especially important at higher flow-rates. A heated countercurrent gas is applied in the Fenn ESI source (Ch. 5.3.1, Figure 5.2) and in the orthogonal-sprayer API sources from Agilent Technologies and Bruker (Figure 5.5). A heated concurrent gas is applied as desolvation gas in the Z-spray source from Waters (Figure 5.7). The heater is typically set at 150°C. The ion-source block is also heated (typically 100°C). 

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