Protocol for the Analysis of Combined and Free Amino Acids in Seawater and Marine Aerosol Particles Using Hydrophilic Interaction LC–TOF-MS

A sensitive analytical method with a run time of 17 min is presented to detect and quantify free and combined amino acids (FAA/CAA) and nucleobases in complex marine matrices. The method combines hydrophilic interaction chromatography (HILIC) with quadrupole time-of-flight mass spectrometry (Q-TOF-MS). Retention time interday relative standard deviation (RSD) was 0.09–7.1%; peak area RSD was <6%; and individual limits of quantification (LOQs) ranged between 0.5 and 5 μg/L. The optimized solid-phase extraction (SPE) desalting protocol for salt water achieved a recovery of more than 50%, except for threonine (30%) and uracil (15%). Despite lower recoveries, compounds were consistently and reliably detected in ambient samples. Recovery RSD was ≤10% for most analytes, with uracil (23%) and cystine (13%) as exceptions. Hydrolysis in polypropylene vials to quantify CAA yielded recoveries between 61 and 102%. Matrix effects were evaluated via standard addition and isotope-labeled standards, revealing ion suppression/enhancement in desalted seawater samples, thus confirming the need for matrix-matched calibration. For aerosol particle extracts, no desalting procedure was necessary. The method was applied to polar-region samples, yielding atmospheric concentrations that ranged between 0.5 pg/m3 for adenine and 3.5 ng/m3 for glutamine. In seawater samples, concentrations ranged between 1.3 μg/L for glycine and 48 μg/L for uracil. Altogether, the optimized method enables reliable analysis, without the need for a derivatization step. Thus, it is allowing for the sensitive determination of amino acids and nucleobases that are challenging to analyze and rarely reported in environmental studies to date.