Atmospheric-pressure chemical ionization

Atmospheric pressure chemical ionization chamber

Atmospheric-pressure chemical ionization (APCI) is a method used in mass spectrometry that allows the ionization of molecules in the gas phase at atmospheric pressure, a process important for the analysis of relatively low molecular weight compounds. This technique is particularly useful in the fields of analytical chemistry and biochemistry, where it is employed to identify and quantify compounds in a variety of samples.

Overview

APCI is a technique that involves the ionization of a sample by using a corona discharge at atmospheric pressure. The sample is typically introduced in a solvent which is then vaporized. The high voltage applied to a needle creates a corona discharge, ionizing the solvent molecules. These ionized solvent molecules then transfer charge to the sample molecules, resulting in the ionization of the sample. APCI is most effective for analyzing molecules that are less polar and have a moderate molecular weight, making it complementary to electrospray ionization (ESI), which is more suited for highly polar or large molecules.

Mechanism

The mechanism of APCI involves several steps: 1. Vaporization: The sample solution is vaporized, typically using a heated nebulizer, to form a mist or aerosol. 2. Ionization: A corona discharge from a high voltage needle ionizes the solvent molecules. 3. Charge Transfer: The ionized solvent molecules transfer their charge to the analyte molecules, resulting in the ionization of the analytes. 4. Mass Analysis: The ionized molecules are then analyzed by a mass spectrometer, which separates ions based on their mass-to-charge ratio (m/z).

Applications

APCI is widely used in various applications, including: - Environmental analysis, for detecting pollutants and contaminants in water and air. - Pharmaceuticals, for drug discovery and pharmacokinetics. - Food safety, for detecting pesticides and contaminants in food products. - Clinical research, for biomarker discovery and metabolite profiling.

Advantages and Limitations

Advantages: - APCI is suitable for a wide range of compounds, especially non-polar to moderately polar compounds. - It is less prone to matrix effects compared to other ionization techniques, such as ESI. - APCI can handle samples with a wide range of flow rates, making it versatile for different types of analyses.

Limitations: - It is not as effective for highly polar or ionic compounds, which are better ionized by ESI. - High molecular weight compounds may not ionize efficiently. - The presence of a corona discharge can lead to fragmentation of some analytes, complicating the analysis.

Comparison with Other Ionization Techniques

APCI and ESI are both atmospheric pressure ionization techniques, but they differ in their ionization mechanisms and the types of compounds for which they are most suitable. While APCI is ideal for less polar compounds, ESI is preferred for highly polar or large biomolecules. Another technique, Atmospheric pressure photoionization (APPI), is similar to APCI but uses ultraviolet light to ionize the sample and is useful for compounds that are difficult to ionize by APCI or ESI.

Conclusion

Atmospheric-pressure chemical ionization is a versatile and powerful technique in mass spectrometry, offering a complementary approach to other ionization methods for the analysis of a wide range of compounds. Its ability to ionize non-polar to moderately polar compounds at atmospheric pressure makes it invaluable in various fields, from environmental analysis to pharmaceuticals and beyond.