Atmospheric-pressure-chemical-ionization molecular-weight limit

Atmospheric pressure chemical ionization (APCI) is the most common alternative to ESI in LC/MS. Whereas the application of ESI is generally recommended in the case of polar compounds, which may have a high molecular weight (up to 1 million Da), the problem-solving domain of APCI is limited to compounds with an MW smaller than 1000 Da. However, less polar compounds are accessible than with ESI. 

Recent advances in electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), thermospray, and particle beam LC-MS have advanced the analyst toward the universal HPLC detector, but price and complexity are still the primary stumbling blocks. Thus, HPLC-MS remains expensive and the technology has only recently been described. Early commercial LC-MS uses particle beam and thermospray sources, but ESI and APCI interfaces now dominate. Liquid chromatography MS can represent a fast and reliable method for structural analyses of nonvolatile compounds such as phenolic compounds (36,37), especially for low-molecular-weight plant phenolics (38), but the limited resolving power of LC hinders the widespread use of its application for phenolics as compared to GC-MS. 

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