- Electrospray Ionization Theory
Electrospray Ionization (ESI) mode creates ions in solution. In the ESI mode, the liquid flow is forced through a stainless steel capillary where a high voltage (3-5 kV) and a nebulizing gas (Nitrogen) are applied. Small charged droplets are generated and undergo a process of solvent evaporation until ions leave the droplets in gas phase. Those ions enter the mass spectrometer to be detected under a specific scan mode. ESI is suitable for polar compounds such as amino acids and phosphorylated compounds.
- Atmospheric Pressure Chemical Ionization Theory
Atmospheric Pressure Chemical Ionization (APCI) mode creates ions at atmospheric pressure. In the APCI mode, the liquid flow is forced through a stainless steel capillary. Evaporation of both the analytes and mobile phase solvents occur under a heated nebulizer (note that no voltage is applied to the capillary). After nebulization, the analytes and mobile phase solvents are sprayed into a corona discharge needle. As a result, nitrogen is ionized first (most abundant in mobile phase), passes its charge to vaporized solvent, which transmits its charge to the analytes. Finally, ions enter the mass spectrometer to be detected under a specific scan mode. APCI is best suited for metabolites with low to medium polarities.
- Differential Ion Mobility Theory
After ESI or APCI ionization, ionized metabolites are pushed through a DMS cell (apparatus made of 2 parallel electrodes) using nitrogen as a carrier gas. The separation of the ions is achieved by alternating periods of: i) high voltage for a short amount of time, and ii) low voltage for a longer amount of time. This asymmetric waveform (separation voltage; SV) is continuously repeated across the DMS cell, which causes a drift of the metabolite towards one of the electrode. To compensate for this drift, a direct current voltage (compensation voltage; CoV) is applied to the electrodes in order to guide the ion of interest through the detector of the mass spectrometer. Additionally, chemical modifiers (acetone, isopropanol, acetonitrile, 1-propanol) can be used to enhance the resolving power of DMS.