There are three main atmospheric pressure ionization (API) techniques that differ in their application concerning compound molecular weight and analyte polarity. The best technique to use will depend largely on chemical nature (i.e., functional groups) of the compounds requiring analysis. Selecting the appropriate ionization method is essential for achieving optimum results. The three primary API ionization techniques have been described below, highlighting the advantages and disadvantages of each.
Electrospray Ionization (ESI): Electrospray is an atmospheric ionization technique that enables the transfer of analytes in an aqueous medium to the gas phase under the influence of a high potential electrostatic field. ESI works well for polar analytes and polar solvents are typically recommended for best results. The pH of the solvent has a major effect on efficiency of analyte ionization and the nature of the ion formed. The manipulation of the solvent pH by volatile additives is a common practice; acidic pHs are preferred for positive ion formation and basic pHs are favored for negative ions. Therefore analytes with basic functional groups, such as amino groups, are generally ionized in positive mode. The analyte will accept a proton from the more acidic solvent. In this case the molecule will be detected as the [M+H]+. Analytes with acidic functional groups, such as carboxylic acids, tend to ionize in negative mode. The analyte of interest will donate a proton to a base in solution. For negative mode ionization the resultant molecule will be detected as [M-H]-.
Due to the nature of the acid/base solution chemistry facilitating ionization, ESI is generally considered the softest API technique. Electrospray ionization is applicable to a wide range of compounds from species of biological origin such as proteins, oligonucleotides, and metabolites, to organic molecules.
Atmospheric Pressure Chemical Ionization (APCI): Atmospheric pressure chemical ionization is complementary to electrospray for less polar analytes. Unlike ESI, the ionization of analytes via the chemical ionization (CI) process is a secondary reaction. The solvent and analyte are completely aerosolized within a high-temperature vaporizer region. The solvent molecules are ionized under the influence of a corona discharge; these newly formed CI reagents in turn ionize the analyte. The major ions formed would be [M+H]+ and [M-H]-, in positive and negative modes respectively. APCI works well for analytes with a wide range of polarities including those that are non-polar. However, due to the higher heat generated by the vaporizer, APCI is less useful for thermally labile compounds. The advantage of APCI is that it is less sensitive to solution chemistry and resists adduct formation, such as [M+Na]+ or [M+K]+. Atmospheric pressure chemical ionization is commonly employed in the analysis of small molecule organics, polymers, and lipids.
Atmospheric Pressure Photo Ionization (APPI): Much like APCI, in APPI the solvent and sample are completely vaporized with the application of a heater. Photoionization (PI) is then initiated by a UV lamp. There are two main types of APPI: direct and dopant. With direct, an analyte molecule is ionized by the UV; with dopant, a photoionizable dopant is present in large concentrations that will transfer charge to the analyte in a secondary interaction. APPI, like APCI, can be used for non-polar analytes, it can provide a high linear calibration range, and it requires some compound volatility. Because of the application of heat, APPI has a disadvantage with thermally labile compounds. As an ionization technique APPI is frequently employed in the analysis of oils, polyaromatic hydrocarbons, and lipids.
Given the various advantages of each ionization technique, choosing the appropriate source for your particular analytes is the first step to achieving quality results. In general, ESI is the most sensitive of the techniques discussed here, in that it will ionize a greater variety of compounds and a higher percentage of molecules in solution. On the other hand, APCI and APPI are highly selective sources. They will ionize a select class of compounds and therefore might provide a higher signal-to-noise ratio.
It would be recommended that, due to ease of use and responsiveness to a wider range of compounds, ESI should be the first option for method development, instrument parameter tuning, and analysis. Because APPI and APCI work for a narrower range of compounds they can be explored as alternatives for improving sensitivity, increasing linear dynamic range, and possibly generating a higher signal-to-noise ratio.