Author's personal copy Analytica Chimica Acta 715 (2012) 71–79 Contents lists available at SciVerse ScienceDirect Analytica Chimica Acta j ourna l ho me page: www.elsevier.com/locate/aca Optimization of solid-phase extraction and liquid chromatography–tandem mass spectrometry for the determination of domoic acid in seawater, phytoplankton, and mammalian fluids and tissues  Zhihong Wang ∗ , Jennifer Maucher-Fuquay, Spencer E. Fire, Christina M. Mikulski, Bennie Haynes, Gregory J. Doucette, John S. Ramsdell Marine Biotoxins Program, Center for Coastal Environmental Health & Biomolecular Research, NOAA/National Ocean Service, 219 Fort Johnson Road, Charleston, SC 29412, USA a r t i c l e i n f o Article history: Received 14 July 2011 Received in revised form 7 December 2011 Accepted 8 December 2011 Available online 19 December 2011 Keywords: Domoic acid Seawater Phytoplankton Mammalian fluids and tissues Solid-phase extraction Liquid chromatography–mass spectrometry a b s t r a c t We previously reported a solid-phase extraction (SPE) method for determination of the neurotoxin domoic acid (DA) in both seawater and phytoplankton by liquid chromatography–tandem mass spec- trometry (LC–MS/MS) with the purpose of sample desalting without DA pre-concentration. In the present study, we optimized the SPE procedure with seawater and phytoplankton samples directly acidified with aqueous formic acid without addition of organic solvents, which allowed sample desalting and also 20-fold pre-concentration of DA in seawater and phytoplankton samples. In order to reduce MS con- tamination, a diverter valve was installed between LC and MS to send the LC eluant to waste, except for the 6-min elution window bracketing the DA retention time, which was sent to the MS. Reduction of the MS turbo gas temperature also helped to maintain the long-term stability of MS signal. Recov- eries exceeded 90% for the DA-negative seawater and the DA-positive cultured phytoplankton samples spiked with DA. The SPE method for DA extraction and sample clean-up in seawater was extended to mammalian fluids and tissues with modification in order to accommodate the fluid samples with limited available volumes and the tissue extracts in aqueous methanol. Recoveries of DA from DA-exposed labo- ratory mammalian samples (amniotic fluid, cerebrospinal fluid, plasma, placenta, and brain) were above 85%. Recoveries of DA from samples (urine, feces, intestinal contents, and gastric contents) collected from field stranded marine mammals showed large variations and were affected by the sample status. The optimized SPE–LC–MS method allows determination of DA at trace levels (low pg mL -1 ) in seawater with/without the presence of phytoplankton. The application of SPE clean-up to mammalian fluids and tissue extracts greatly reduced the LC column degradation and MS contamination, which allowed routine screening of marine mammalian samples for confirmation of DA exposure and determination of fluid and tissue DA concentrations in experimental laboratory animals. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Domoic acid (DA) is a neurotoxin produced by phytoplankton, primarily the diatom Pseudo-nitzschia, and can cause intoxi- cation and even mortality of humans and marine mammals [1–4]. Pseudo-nitzschia species are distributed worldwide in  NOAA disclaimer: This publication does not constitute an endorsement of any commercial product or intend to be an opinion beyond scientific or other results obtained by the National Oceanic and Atmospheric Administration (NOAA). No reference shall be made to NOAA, or this publication furnished by NOAA, to any advertising or sales promotion which would indicate or imply that NOAA recom- mends or endorses any proprietary product mentioned herein, or which has as its purpose an interest to cause the advertised product to be used or purchased because of this publication. ∗ Corresponding author. Tel.: +1 843 762 8991; fax: +1 843 762 8737. E-mail address: zhihong.wang@noaa.gov (Z. Wang). coastal waters. The concentration of DA in seawater and natural phytoplankton samples is generally low, with a reported high level of DA only about 12 ng mL -1 [5,6]. Analytical methods for quan- titation of trace levels of DA require either sensitive detectors or sample pre-concentration. The initial published analytical meth- ods for quantitation of DA in seawater employed DA derivatization followed by LC separation and fluorescence detection (FLD) [6–8]. LC–FLD methods are generally labor-intensive and sometimes show poor selectivity [9]. Later, LC–MS based methods emerged and provided unequivocal confirmation and/or quantitation of DA in seawater and phytoplankton samples [5,9–12]. The analysis of DA in shellfish by LC–MS has been well established for over 15 years; however, unlike shellfish samples, seawater with or without phytoplankton contains high levels of salts, which can cause severe MS contamination if salts are not removed before LC injection or diverted to waste before LC eluant enters MS [5]. Reversed- phase solid-phase extraction (SPE) cartridges [5] and disks [11], and 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.12.013