Identification of Unknowns in Water Samples: The Diagnostic Ion Approach Using Ion-Trap MS/MS

By Imma Ferrer¹, E.T. Furlong¹, and E.M. Thurman²

¹U.S. Geological Survey, Denver, Colorado 80225
²U.S. Geological Survey, Lawrence, Kansas 66049

Introduction

One of the shortcomings in single-quadrupole liquid chromatography/mass spectrometry (LC/MS) is the lack of sensitivity when analyzing in scan mode. This disadvantage has been overcome by the use of selected-ion monitoring (SIM) conditions, which enable high sensitivity for a specific molecular ion or fragment ions. However, the ability to detect unknowns in an extract is precluded by using the SIM mode. Recently, the use of ion-trap mass spectrometry has increased the detection limits by one to two orders of magnitude. In this work, the authors use the ion-trap MS/MS as a tool for the identification of surfactants and pesticides in water samples by using a "fingerprint chromatogram" produced by extracting ion profiles from the diagnostic ions.

Methods

Liquid chromatography/electrospray ionization/ion-trap tandem mass spectrometry (LC/ESI/MS/MS), in positive and negative ion mode of operation, was used to separate and identify target compounds. The compounds were separated by using high-performance liquid chromatography (HPLC) equipped with a reversed-phase C18 analytical column of 250 by 3 mm and 5-Îm particle diameter. Column temperature was maintained at 25 ÷C. The mobile phase used for eluting the compounds from the column consisted of acetonitrile and 10 mM ammonium formate buffer, at a flow rate of 0.6 mL/min. This HPLC system was connected to an ion-trap mass spectrometer LC/MS/MS system equipped with an electrospray ionization (ESI) probe.

Preliminary data

The diagnostic ion is a characteristic fragment ion that is found in all members of a family of compounds. Diagnostic ions at 88 and 106 m/z for the coconut diethanol amides and ions at 89, 133 and 177 m/z for polyethylene glycol homologues are used for analysis. A similar spectrum can be obtained for different homologues between the same family by showing the same product ions but different parent ions. The diagnostic ions may be displayed as a fingerprint chromatogram that shows all the peaks containing the characteristic diagnostic ions. Molecular ion information could be directly achieved from each chromatographic peak; thus, the corresponding homologue could be structurally identified. Moreover, MS/MS experiments were performed to confirm the molecular ion and the characteristic fragments that belong to the specific compound. In this work, the different homologues of three families of compounds (benzalkonium chloride surfactants, coconut diethanol amides, and polyethylene glycols) were identified at the low microgram-per-liter level in environmental water samples by using this approach. Two new ethane sulfonic acid pesticide metabolites also were identified in ground-water samples by using diagnostic ions at 80 and 121 amu.

Novel aspect

Use of diagnostic ions and MS/MS to identify unknowns.

Additional information about the Organic Geochemistry Research Laboratory can be found at: http://ks.water.usgs.gov/studies/reslab/

Ferrer, Imma, Furlong, E.T., and Thurman, E.M., 2001, Identification of Unknowns in Water Samples: The Diagnostic Ion Approach Using Ion-Trap MS/MS [abst.]: 49th Conference on Mass Spectrometry and Allied Topics, ASMS, Chicago, Illinois, May 27-31, 2001.

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