LC–MS/MS analytical performance characteristics of chloride adduct ions [M + Cl]⁻ of thiamphenicol (TAP) and florfenicol (FFC) as precursor ions for quantification were studied. The calculations were based on a THF/Water solvent system to which dichloromethane was added as an additive at a composition of 2 % to generate chloride (Cl⁻) ions during electrospray ionization (ESI) of the solutions. The selectivity and signal to noise (S/N) ratio when the [M + Cl]⁻ adduct ions were used as precursor ions were excellent compared to deprotonated [M−H]⁻ ions that are traditionally used to determine the two analytes. The S/N ratios in the total ion chromatogram (TIC) of TAP and FFC were 577 and 3062 respectively, compared to 167 and 452 when [M−H]⁻ were used at the same spike levels of 50 and 100 μg/kg respectively. The limits of detection (LODs) for TAP and FFC were 4.0 and 3.7 μg/kg respectively using [M + Cl]⁻ as opposed to 2.1 and 4.3 μg/kg respectively when [M−H]⁻ were used. [M + Cl]⁻ adduct ions proved to be excellent substitutes for [M−H]⁻ especially where identity and selectivity would be the key drivers of the analytical method before quantification of the two analytes.

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