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Extractive electrospray (EESI) 	 is a spray-type, ambient ionization source     in mass spectrometry. Desorption electrospray ionization (DESI), desorption atmospheric pressure photoionization (DAPPI), electrospray-assisted laser desorption (ELDI), matrix-assisted laser desorption ionization electrospray (MALDI-ESI) and laser assisted electrospray ionization (LAESI) are other ambient ionization sources, which are different from EESI because EESI has two colliding aerosols. In standard EESI, syringe pumps provide the liquids for both an electrospray (ESI) and a sample spray (see Figure 1). In neutral desorption (ND) EESI (ND-EESI), the liquid for the sample aerosol is provided by a flow of nitrogen.



History
Many ambient ionization sources like EESI were developed for biological samples. Ionization sources are a part of a mass spectrometer and are used for creating ions. A main application of mass spectrometry is to provide the mass-to-charge ratio, which is used to determine the chemical formula for various applications. Electron impact (EI), chemical ionization (CI), matrix-assisted laser desorption ionization (MALDI), electrospray ionization (ESI), and APCI were used before 2004. EI was the most popular ionization source and is good for volatile organic compounds, but EI is a hard ionization source, which breaks some chemical bonds and reduces the signal for the molecular ion. When the molecular ion signal is weak, the chemist lacks confidence in the chemical formula. CI is a soft ionization source, so it is better than EI. Like EI, CI is also limited to volatile organic compounds. Like CI, MALDI is a soft ionization source and analyzes both semi-volitile organic compound and biological compounds. Unfortunately, MALDI could not be used with column chromatography to separate an analyte from a complex mixture. Both ESI and APCI provide molecular ions and were used with chromatography, and these ionization sources were used at atmospheric pressure. APCI could be used with biological samples. Unfortunately, the chromatography separation was time-consuming and complicated. In 2004, Cooks et al. invented desorption electrospray ionization (DESI), which analyzed solid samples under ambient conditions with no or minimal sample preparation and without chromatography. Following the introduction of DESI, more than 20 ionization techniques were developed for ambient sample analysis. Ambient ionization techniques are attractive for many samples for their high tolerance to complex mixtures and for fast testing. EESI has been employed for the rapid characterization of living objects, native proteins, and metabolic biomarkers.

Principle of Operation of the Standard Method
The principle operation of EESI is described. EESI has two sprays, which are aligned along certain angles with respect to the mass spectrometer inlet (Figure 1). Four EESI parameters exist. They are a, b, α and β for the distance between the intersection and inlet, the distance between the ESI tip and the sample spray tip, the ESI angle normal to the inlet and the sample spray angle normal to the inlet. The first spray is an ESI spray generating primary ions, and the other spray is the sample-infusing spray which lacks voltage. The sprays are non-medical, free jet nebulizers. The sample is nebulized towards the primary ion plume. Slightly acidified methanol/aqueous mixtures can be used in the ESI channel for EESI experiments of positive ions, and basic methanol/water solutions are used for experiments of the negative ions. After ionization, charged analytes move through the ion optics system into the mass analyzer for identification.

Mechanism
The ionization mechanism in the EESI process is described. The sample spray in EESI produces a liquid aerosol with the analyte in sample droplets. The ESI spray produces droplets with protons. The sample droplets and the proton-rich droplets bump into each other. Each droplet has properties: analyte solubility in the ESI spray solvent and surface tension of the spray solution and of the sample solution. With dissimilar properties, some collisions produce no extraction because the droplets “bounce", but with similar properties, some collisions produce coalescence and liquid-liquid extraction. The extent of the extraction depends on the similarity of the properties.

Principle of Operation of the Neutral Desorption Method
A ND-EESI experiment is simple in concept and implementation. A room temperature (20 °C) nitrogen gas stream is flowed through a narrow opening (i.d.~0.1 mm) to form a sharp jet targeted at a surface. The nitrogen molecules desorb analytes from the surface. The jet is only 2–3 mm above the surface, and the gas flow is about 200 mL/min with gas speeds around 300 m/s. The sample area is about 10 mm2. An optional enclosure, most commonly made of glass, can cover the sampling area to ensure proper positioning of the gas jet and the sample transfer line. A tube carries the neutral aerosol to the ESI spray.