Analysis of nitrosamines in wastewater: exploring the trace level quantification capabilities of a hybrid linear ion trap/orbitrap mass spectrometer

A method was developed to determine nine N-nitrosamines in wastewater on the basis of solid-phase extraction and liquid chromatography mass spectrometry using a linear ion trap-orbitrap hybrid instrument at high mass resolution. Analytes and five deuterated internal standards were preconcentrated by solid-phase extraction. Positive electrospray ionization resulted in protonated molecular ions of all nitrosamines. One to three product ions were formed by collision-induced dissociation or higher energy C-trap dissociation. The signal intensity of the product ions differed up to a factor of 3 between the two techniques. The molecular ions were usually used for quantification, because of the better sensitivity, and the product ions for confirmation. An actual mass resolving power of 25 000-40 000 ensured a sufficient selectivity to distinguish all molecular and product ions from interfering background ions. Only for N-nitrosomorpholine was a coeluting isobaric molecular ion detected in wastewater samples, which, however, formed different product ions. The mass accuracy was between -12 ppm at m/z 55 and 0 ppm at m/z 205 and did not change for more than 5 ppm over a sample sequence of 20 h analysis time. The optimized method allowed quantifying nine N-nitrosamines in drinking water and wastewater samples down to method detection limits of 0.3-3.9 ng/L at instrumental detection limits of 2-14 pg on column. Recoveries over the whole method were between 75 and 125% for six compounds, but considerably lower for three compounds, probably due to strong matrix effects causing a signal suppression of up to 95% in wastewater samples. N-Nitrosodimethylamine and N-nitrosomorpholine were the most abundant compounds (3-22 ng/L) in samples from two wastewater treatment plants, another four nitrosamines (N-nitrosopyrrolidone, -piperidine, -diethylamine, and -dibutylamine) were also detected. Our study demonstrates that the LTQ Orbitrap is a powerful instrument to quantify low molecular weight compounds at the picogram level in complex matrixes with both a high sensitivity and selectivity.