The quantification of quaternary ammonium compounds in environmental and biological samples is problematic using conventional chromatography techniques because the compounds are highly soluble in water. While analyzing them by liquid chromatography coupled tandem mass spectrometry it has been found that they follow an exception rule. Under standard electrospray ionization (ESI) conditions, mono- and di-quaternary ammonium compounds form molecular ions with the formula of m q / z q rather than m + z / z . [ clarification needed ] Formation of m q / 2 is observed for di-quaternary ammonium compounds (like diquat ) as precursor ion and m q / 1 as product ion due to the loss of one of the quaternary charge during CID. In di-quaternary ammonium compounds, this process can also result in the formation of fragment ions with higher mass as compared to their precursor ion. Hydrophilic interaction liquid chromatographic separation has been reported to demonstrate a successful separation of quaternary ammonium compounds for their quantification in ESI-MS/MS with higher precision. 
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to "mercury compounds", that classification is applied to all mercury compounds in the PAN Pesticide database, which are then marked with an asterisk.
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