Atmospheric Pressure Chemical Ionisation (APCI)
Definition
A complementary atmospheric-pressure source that vaporises the LC eluent and uses a corona discharge to ionise a reagent gas, which transfers charge to less polar analytes that ESI does not ionise efficiently. Steroids, fat-soluble vitamins and most pesticides sit in the APCI sweet spot.
- Ionisation method
- Corona discharge (transfers charge to analytes via reagent gas)
- Best for
- Less polar, thermally stable compounds like steroids, pesticides, and fat-soluble vitamins
- Typical pairing
- LC-MS (Liquid Chromatography-Mass Spectrometry)
Common questions
When would I use APCI instead of ESI for LC-MS?+
APCI works best for less polar compounds that ESI struggles to ionise, like steroids, pesticides, and fat-soluble vitamins. If your analytes are polar and ionic, ESI is the go-to. But if they are neutral or lipophilic, APCI is often the better choice.
What's the main difference between APCI and ESI ionisation?+
ESI uses a Taylor cone to spray the liquid sample into charged droplets. APCI takes a different path: it vaporises the LC eluent first, then uses a corona discharge to ionise a reagent gas, which transfers that charge to the analytes. Because APCI relies on thermal vaporisation, it suits thermally stable molecules. ESI is the preferred choice for thermally labile or highly polar compounds.
Why is APCI called an atmospheric pressure source?+
Both APCI and ESI operate at atmospheric pressure rather than under vacuum, which makes them compatible with the same LC equipment and easier to couple to liquid chromatography. The pressure matters because it determines how the ionisation happens without needing a high-vacuum chamber.
Related terms
- Electrospray Ionisation (ESI)
- A soft atmospheric-pressure ionisation source that sprays the LC eluent through a charged capillary into a heated counter-current gas, producing protonated or...
- GC-MS Interface
- The physical link between a gas chromatograph and a mass spectrometer. Older designs used jet separators and open splits to manage the...
- Hyphenated Technique
- A single workflow that couples a separation method to a spectroscopic detector, so a complex mixture is resolved and each component identified...
- Hyphenation
- Online coupling of a separation technique (GC, LC, ICP) with a spectrometric detector (MS, FTIR) so identification happens within the same run....
- ICP Plasma
- Inductively coupled argon plasma at 6,000 to 10,000 K that atomises and ionises samples in ICP-MS, generating singly charged elemental ions for...
- Interface
- The hardware bridge that transfers analyte from the separation stage to the detector while removing the unwanted mobile phase (carrier gas, solvent,...
- Internal Standard / Deuterated Standard
- A chemically similar (often perdeuterated) compound spiked into the sample to correct for matrix effects and ionisation suppression. Standard practice in forensic...
- Isotope Ratio Mass Spectrometry (IRMS)
- Specialised MS that measures precise ratios of light stable isotopes (delta values of 2H, 13C, 15N, 18O, 34S) on a continuous-flow combustion...
- Jet Separator
- Early GC-MS interface (Ryhage, 1964) that used differential pumping through a nozzle to enrich analyte and bleed helium. Replaced by direct capillary...
- MRM (Multiple Reaction Monitoring)
- A targeted MS/MS scan mode on a triple quadrupole that monitors specific precursor-to-product ion transitions, with two transitions per analyte (quantifier and...
- Vapour-Phase IR Spectrum
- The infrared spectrum recorded as a GC analyte elutes into a heated light pipe inside the FTIR cell, with no solvent or...
Explained in these topics
- Hyphenated Techniques: GC-MS, LC-MS and GC-FTIRA complementary atmospheric-pressure source that vaporises the LC eluent and uses a corona discharge to ionise a reagent gas, which transfers charge to less po...
- Hyphenated Techniques in Forensic Science: GC-MS, LC-MS, IR-MS and ICP-MSAlternate LC-MS source for less polar, thermally stable analytes (steroids, pesticides). Uses a corona discharge instead of a Taylor cone.