Approaches to the evaluation of matrix effects in the liquid chromatography-mass spectrometry (LC-MS) analysis of three regulated lipophilic toxin groups in mussel matrix (Mytilus edulis)

Current Research Status of Azaspiracids

The presence and impact of toxins have been detected in various regions worldwide ever since the discovery of azaspiracids (AZAs) in 1995. These toxins have had detrimental effects on marine resource utilization, marine environmental protection, and fishery production. Over the course of more than two decades of research and development, scientists from all over the world have conducted comprehensive studies on the in vivo metabolism, in vitro synthesis methods, pathogenic mechanisms, and toxicology of these toxins. This paper aims to provide a systematic introduction to the discovery, distribution, pathogenic mechanism, in vivo biosynthesis, and in vitro artificial synthesis of AZA toxins. Additionally, it will summarize various detection methods employed over the past 20 years, along with their advantages and disadvantages. This effort will contribute to the future development of rapid detection technologies and the invention of detection devices for AZAs in marine environmental samples.

Optimization and Validation of a High Throughput UHPLC-MS/MS Method for Determination of the EU Regulated Lipophilic Marine Toxins and Occurrence in Fresh and Processed Shellfish

Under the name of lipophilic marine toxins, there are included more than 1000 toxic secondary metabolites, produced by phytoplankton, with the common chemical property of lipophilicity. Due to toxicological effects and geographical distribution, in European legislation relevant compounds are regulated, and their determination is accomplished with the reference liquid chromatography-tandem mass spectrometry method. In this study a modified ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method has been developed for the identification and quantification of EU-regulated lipophilic toxins. The method optimization included a refinement of SPE-C18 clean-up, in order to reduce matrix interferences. Improved LC conditions and upgraded chromatographic ammonia-based gradient ensured the best separation of all analytes and, in particular, of the two structural isomers (OA and DTX2). Also, different MS parameters were tested, and confirmation criteria finally established. The validation studies confirmed that all parameters were satisfactory. The requirements for precision (RSD% < 11.8% for each compound), trueness (recoveries from 73 to 101%) and sensitivity (limits of quantification in the range 3−8 µg kg−1) were fulfilled. The matrix effect, ranging from −9 to 19%, allowed the use of a calibration curve in solvent (3−320 µg kg−1 in matrix) for quantification of real samples. Method relative uncertainty ranged from 12 to 20.3%. Additionally, a total of 1000 shellfish samples was analysed, providing a first preliminary surveillance study that may contribute to the knowledge of lipophilic marine toxins contamination. Increase in algae proliferation events and intoxication cases, EFSA suggestions for modification of maximum permitted levels and toxicity equivalency factors, and new studies of important toxic effects underline that implementation of reference methods still represents an important task for health and food safety laboratories.

Quantitation Overcoming Matrix Effects of Lipophilic Toxins in Mytilus galloprovincialis by Liquid Chromatography-Full Scan High Resolution Mass Spectrometry Analysis (LC-HR-MS)

The analysis of marine lipophilic toxins in shellfish products still represents a challenging task due to the complexity and diversity of the sample matrix. Liquid chromatography coupled with mass spectrometry (LC-MS) is the technique of choice for accurate quantitative measurements in complex samples. By combining unambiguous identification with the high selectivity of tandem MS, it provides the required high sensitivity and specificity. However, LC-MS is prone to matrix effects (ME) that need to be evaluated during the development and validation of methods. Furthermore, the large sample-to-sample variability, even between samples of the same species and geographic origin, needs a procedure to evaluate and control ME continuously. Here, we analyzed the toxins okadaic acid (OA), dinophysistoxins (DTX-1 and DTX-2), pectenotoxin (PTX-2), yessotoxin (YTX) and azaspiracid-1 (AZA-1). Samples were mussels (Mytilus galloprovincialis), both fresh and processed, and a toxin-free mussel reference material. We developed an accurate mass-extracted ion chromatogram (AM-XIC) based quantitation method using an Orbitrap instrument, evaluated the ME for different types and extracts of mussel samples, characterized the main compounds co-eluting with the targeted molecules and quantified toxins in samples by following a standard addition method (SAM). An AM-XIC based quantitation of lipophilic toxins in mussel samples using high resolution and accuracy full scan profiles (LC-HR-MS) is a good alternative to multi reaction monitoring (MRM) for instruments with HR capabilities. ME depend on the starting sample matrix and the sample preparation. ME are particularly strong for OA and related toxins, showing values below 50% for fresh mussel samples. Results for other toxins (AZA-1, YTX and PTX-2) are between 75% and 110%. ME in unknown matrices can be evaluated by comparing their full scan LC-HR-MS profiles with those of known samples with known ME. ME can be corrected by following SAM with AM-XIC quantitation if necessary.

Development of a Fast Liquid Chromatography Coupled to Mass Spectrometry Method (LC-MS/MS) to Determine Fourteen Lipophilic Shellfish Toxins Based on Fused-Core Technology: In-House Validation

Prevalence and incidence of the marine toxins (paralytic, amnesic, and lipophilic toxins) including the so-called emerging toxins (these are, gymnodimines, pinnatoxins, or spirolides among others) have increased in recent years all over the world. Climate change, which is affecting the distribution of their producing phytoplankton species, is probably one of the main causes. Early detection of the toxins present in a particular area, and linking the toxins to their causative phytoplankton species are key tools to minimize the risk they pose for human consumers. The development of both types of studies requires fast and highly sensitive analytical methods. In the present work, we have developed a highly sensitive liquid chromatography-mass spectrometry methodology (LC-MS/MS), using a column with fused-core particle technology, for the determination of fourteen lipophilic toxins in a single run of 3.6 min. The performance of the method was evaluated for specificity, linearity, precision (repeatability and reproducibility) and accuracy by analysing spiked and naturally contaminated samples. The in-house validation was successful, and the limit of detection (LOD) and quantification (LOQ) for all the toxins were far below their regulatory action limits. The method is suitable to be considered in monitoring systems of bivalves for food control.

Evaluation of different strategies to minimize the matrix effects on LC-MS/MS analysis of multiple lipophilic shellfish toxins in both acidic and alkaline chromatographic conditions

Lipophilic shellfish toxins (LSTs) accumulated by shellfish pose a potential threat to consumer health. A mandatory routine monitoring of LSTs has been adopted for seafood products by liquid chromatography-mass spectrometry (LC-MS) in many countries. In this study, two methods developed on liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) under acidic and alkaline chromatographic conditions were assessed for the determination of multiple LSTs. Different strategies including matrix solid-phase dispersion (MSPD), solid phase extraction (SPE) and sample dilution were applied and evaluated the matrix effects of mussel, scallop, clam, and oyster samples on the signal response of mass spectrometry. Results showed that the alkaline method achieved a lower limit of detection (LOD) and more robust compared to the acidic method. The obvious signal suppression of OA and DTX1 (55%-76%) and signal enhancement of PTX2 (27%-34%) occurred in the crude extracts of shellfish under acidic chromatography. In the alkaline method, no remarkable matrix effects of crude extracts were found except for the scallop matrix on the signal intensity of DTX1, AZA3 and GYM-A (121%-130%). Clean-up methods MSPD, SPE and sample dilution obviously reduced the inhibition of shellfish matrices on the signal response of OA and DTX1, however, which were still subject to signal inhibition under acidic condition. Sample dilution was more effective than SPE and MSPD in minimizing the matrix interference in both acidic and alkaline methods. Furthermore, sample dilution in combination with the alkaline chromatography was the most effective method. Bivalve mollusks harvested from Beibu Bay, South China Sea, were generally contaminated by GYM-A and SPX1 at low concentrations.

Identification of Causative Ciguatoxins in Red Snappers Lutjanus bohar Implicated in Ciguatera Fish Poisonings in Vietnam

Ciguatera fish poisoning (CFP) is a type of food poisoning caused by the consumption of a variety of toxic ciguatera fish species in the tropical and subtropical waters. Although there have been a large number of suspected CFP cases in the Southeast Asian countries, few were confirmed with causative ciguatoxins (CTXs), and reliable information on the symptoms still remains rather limited. In the present study, CTXs in red snapper Lutjanus bohar, implicated in two suspected CFP cases in Vietnam in 2014 and 2016, were determined by use of the single-quadrupole selected ion monitoring (SIM) liquid chromatography/mass spectrometry (LC/MS). Ciguatoxin-1B (CTX-1B), 54-deoxyCTX-1B, and 52-epi-54-deoxyCTX-1B were detected in the red snapper by our LC/MS method. Moreover, CTX-1B, 54-deoxyCTX-1B, and 52-epi-54-deoxyCTX-1B were further identified by the time of flight (TOF) LC/MS with the exact mass spectrum. The CTX profile of the red snapper in Vietnam is similar to those of ciguatera fish from Australia, Okinawa Islands in Japan, Kiribati, and Hong Kong. This is the first comprehensive report unambiguously identifying the causative toxins in fish implicated with reliable information on the poisoning symptoms in CFP in Vietnam and/or Southeast Asian countries.

Metabolism of the Marine Phycotoxin PTX-2 and Its Effects on Hepatic Xenobiotic Metabolism: Activation of Nuclear Receptors and Modulation of the Phase I Cytochrome P450

PTX-2 is a marine biotoxin frequently found in shellfish that can lead to food intoxication in humans. Information regarding PTX-2 metabolism is scarce, and little is known of its effect on xenobiotic-metabolizing enzymes (XME) or its molecular pathways. The aim of this study was consequently to examine PTX-2 Phase I metabolism using rat and human liver S9 fractions, and also to assess the capability of PTX-2: (i) to modulate the gene expression of a panel of Phase I (CYP450) and II (UGT, SULT, NAT, and GST) enzymes, as well as the Phase III or 0 (ABC and SLCO) transporters in the human hepatic HepaRG cell line using qPCR; (ii) to induce specific CYP450 in HepaRG cells measured by immunolabeling detection and the measurement of the cells’ activities; and (iii) to activate nuclear receptors and induce CYP promoter activities in HEK-T and HepG2 transfected cell lines using transactivation and reporter gene assay, respectively. Our results indicate that PTX-2 hydroxylation occurred with both rat and human S9 fractions. Whereas PTX-2 mostly upregulated the gene expression of CYP1A1 and 1A2, no induction of these two CYP activities was observed. Lastly, PTX-2 did not act as an agonist of CAR or PXR. Due to its effects on some key XME, more attention should be paid to possible drug–drug interactions with phycotoxins, especially as shellfish can accumulate several phycotoxins as well as other kinds of contaminants.

Passive Sampling and High Resolution Mass Spectrometry for Chemical Profiling of French Coastal Areas with a Focus on Marine Biotoxins

Passive samplers (solid phase adsorption toxin tracking: SPATT) are able to accumulate biotoxins produced by microalgae directly from seawater, thus providing useful information for monitoring of the marine environment. SPATTs containing 0.3, 3, and 10 g of resin were deployed at four different coastal areas in France and analyzed using liquid chromatography coupled to high resolution mass spectrometry. Quantitative targeted screening provided insights into toxin profiles and showed that toxin concentrations and profiles in SPATTs were dependent on the amount of resin used. Between the three amounts of resin tested, SPATTs containing 3 g of resin appeared to be the best compromise, which is consistent with the use of 3 g of resin in SPATTs by previous studies. MassHunter and Mass Profiler Professional softwares were used for data reprocessing and statistical analyses. A differential profiling approach was developed to investigate and compare the overall chemical diversity of dissolved substances in different coastal water bodies. Principal component analysis (PCA) allowed for spatial differentiation between areas. Similarly, SPATTs retrieved from the same location at early, medium, and late deployment periods were also differentiated by PCA, reflecting seasonal variations in chemical profiles and in the microalgal community. This study used an untargeted metabolomic approach for spatial and temporal differentiation of marine environmental chemical profiles using SPATTs, and we propose this approach as a step forward in the discovery of chemical markers of short- or long-term changes in the microbial community structure.

High resolution mass spectrometry for quantitative analysis and untargeted screening of algal toxins in mussels and passive samplers

Measurement of marine algal toxins has traditionally focussed on shellfish monitoring while, over the last decade, passive sampling has been introduced as a complementary tool for exploratory studies. Since 2011, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been adopted as the EU reference method (No. 15/2011) for detection and quantitation of lipophilic toxins. Traditional LC-MS approaches have been based on low-resolution mass spectrometry (LRMS), however, advances in instrument platforms have led to a heightened interest in the use of high-resolution mass spectrometry (HRMS) for toxin detection. This work describes the use of HRMS in combination with passive sampling as a progressive approach to marine algal toxin surveys. Experiments focused on comparison of LRMS and HRMS for determination of a broad range of toxins in shellfish and passive samplers. Matrix effects are an important issue to address in LC-MS; therefore, this phenomenon was evaluated for mussels (Mytilus galloprovincialis) and passive samplers using LRMS (triple quadrupole) and HRMS (quadrupole time-of-flight and Orbitrap) instruments. Matrix-matched calibration solutions containing okadaic acid and dinophysistoxins, pectenotoxin, azaspiracids, yessotoxins, domoic acid, pinnatoxins, gymnodimine A and 13-desmethyl spirolide C were prepared. Similar matrix effects were observed on all instruments types. Most notably, there was ion enhancement for pectenotoxins, okadaic acid/dinophysistoxins on one hand, and ion suppression for yessotoxins on the other. Interestingly, the ion selected for quantitation of PTX2 also influenced the magnitude of matrix effects, with the sodium adduct typically exhibiting less susceptibility to matrix effects than the ammonium adduct. As expected, mussel as a biological matrix, quantitatively produced significantly more matrix effects than passive sampler extracts, irrespective of toxin. Sample dilution was demonstrated as an effective measure to reduce matrix effects for all compounds, and was found to be particularly useful for the non-targeted approach. Limits of detection and method accuracy were comparable between the systems tested, demonstrating the applicability of HRMS as an effective tool for screening and quantitative analysis. HRMS offers the advantage of untargeted analysis, meaning that datasets can be retrospectively analyzed. HRMS (full scan) chromatograms of passive samplers yielded significantly less complex data sets than mussels, and were thus more easily screened for unknowns. Consequently, we recommend the use of HRMS in combination with passive sampling for studies investigating emerging or hitherto uncharacterized toxins.

A mussel (Mytilus edulis) tissue certified reference material for the marine biotoxins azaspiracids

Azaspiracids (AZAs) are lipophilic biotoxins produced by marine algae that can contaminate shellfish and cause human illness. The European Union (EU) regulates the level of AZAs in shellfish destined for the commercial market, with liquid chromatography-mass spectrometry (LC-MS) being used as the official reference method for regulatory analysis. Certified reference materials (CRMs) are essential tools for the development, validation, and quality control of LC-MS methods. This paper describes the work that went into the planning, preparation, characterization, and certification of CRM-AZA-Mus, a tissue matrix CRM, which was prepared as a wet homogenate from mussels (Mytilus edulis) naturally contaminated with AZAs. The homogeneity and stability of CRM-AZA-Mus were evaluated, and the CRM was found to be fit for purpose. Extraction and LC-MS/MS methods were developed to accurately certify the concentrations of AZA1 (1.16 mg/kg), AZA2 (0.27 mg/kg), and AZA3 (0.21 mg/kg) in the CRM. Quantitation methods based on standard addition and matrix-matched calibration were used to compensate for the matrix effects in LC-MS/MS. Other toxins present in this CRM at lower levels were also measured with information values reported for okadaic acid, dinophysistoxin-2, yessotoxin, and several spirolides.

Validation of a confirmatory method for lipophilic marine toxins in shellfish using UHPLC-HR-Orbitrap MS

Lipophilic marine toxins are produced by harmful microalgae and can accumulate in edible filter feeders such as shellfish, leading to an introduction of toxins into the human food chain, causing different poisoning effects. During the last years, analytical methods, based on liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), have been consolidated by interlaboratory validations. However, the main drawback of LC-MS/MS methods remains the limited number of compounds that can be analyzed in a single run. Due to the targeted nature of these methods, only known toxins, previously considered during method optimization, will be detected. Therefore in this study, a method based on ultra-high-performance liquid chromatography coupled to high-resolution Orbitrap mass spectrometry (UHPLC-HR-Orbitrap MS) was developed. Its quantitative performance was evaluated for confirmatory analysis of regulated lipophilic marine toxins in shellfish flesh according to Commission Decision 2002/657/EC. Okadaic acid (OA), dinophysistoxin-1 (DTX-1), pectenotoxin-2 (PTX-2), azaspiracid-1 (AZA-1), yessotoxin (YTX), and 13-desmethyl spirolide C (SPX-1) were quantified using matrix-matched calibration curves (MMS). For all compounds, the reproducibility ranged from 2.9 to 4.9 %, repeatability from 2.9 to 4.9 %, and recoveries from 82.9 to 113 % at the three different spiked levels. In addition, confirmatory identification of the compounds was effectively performed by the presence of a second diagnostic ion ((13)C). In conclusion, UHPLC-HR-Orbitrap MS permitted more accurate and faster detection of the target toxins than previously described LC-MS/MS methods. Furthermore, HRMS allows to retrospectively screen for many analogues and metabolites using its full-scan capabilities but also untargeted screening through the use of metabolomics software.

Optimization, in-house validation, and application of a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based method for the quantification of selected polyphenolic compounds in leaves of grapevine (Vitis vinifera L.)

Polyphenols in grapevine can be constitutive or induced, depending upon cultivar, plant organ, and environmental influences. The aim of the presented work was to develop and optimize a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to study the pattern and amount of selected polyphenols in leaves of Vitis vinifera L. The method is simple and does not require any sample cleanup. It covers representative metabolites of the structure classes cinnamic acids, flavonoids, and stilbenes and enables the simultaneous separation and quantification of 13 polyphenols within 9 min at concentration levels between 0.1 and 3 μg/g. We present the method performance characteristics and its application to the quantification of polyphenols in grapevine leaves of the cultivars Riesling and Pinot noir. A total of 7 of 13 target polyphenols were detected at concentrations above the limits of quantification. Interestingly, instead of the expected trans-resveratrol, the investigated leaf samples of both cultivars contained cis-resveratrol-3-O-glucoside. The measurements also showed that Riesling leaves tended to contain higher concentrations of the selected polyphenols than Pinot noir. In view of its intended future use, the developed method has been shown to be a powerful and fast tool to study polyphenols in grapevine leaves subjected to environmental stress conditions.

Automated on-line solid-phase extraction coupled to liquid chromatography-tandem mass spectrometry for determination of lipophilic marine toxins in shellfish

Automated on-line solid-phase extraction (SPE) coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been developed for fast determination of lipophilic marine toxins in shellfish samples. The direct coupling of an on-line SPE column to LC-MS/MS was accomplished using column switching techniques. Suitable chromatographic separation was performed on a reversed-phase C18 column under alkaline conditions (pH 11). The selected reversed-phase C18 SPE column allowed rapid and efficient on-line desalting of hydrolysed shellfish samples, avoiding signal suppression during mass spectrometry detection. Furthermore, the on-line SPE procedure allowed reducing matrix effects observed in raw and hydrolysed shellfish extracts. The proposed method was evaluated in terms of linearity, precision, accuracy and limits of detection (LODs). Quantitative recovery (97-102%) and satisfactory inter-day precision (RSD<8%) were achieved for all target compounds. LODs in the sub-μgkg^-1 level (0.37-0.68μgkg^-1) were obtained for all toxins except for okadaic acid, which showed a value of 2.75μgkg^-1. Several mussel samples from North-western Spain were finally analysed in order to demonstrate the applicability of the method. Okadaic acid was the predominant toxin in all samples, although other lipophilic toxins were also detected.

In situ passive solid-phase adsorption of micro-algal biotoxins as a monitoring tool

Laboratory and field studies of the passive solid-phase adsorption toxin tracking (SPATT) method have been carried out around the world. A wide range of marine micro-algal toxins have been detected and the potential of the method to provide reliable, sensitive, time-integrated sampling to monitor the occurrence of toxic algal bloom events has been demonstrated. The method has several important advantages over current phytoplankton and shellfish monitoring methods. Trials of various adsorption substrates have been carried out and the best candidates have been selected for the lipophilic marine biotoxin groups; however, research continues to locate suitable substrates for the more polar water-soluble compounds such as domoic acid and the saxitoxins. The technique has also been successfully applied to the detection of a range of freshwater cyanobacterial toxins.

Sensitive method for the determination of lipophilic marine biotoxins in extracts of mussels and processed shellfish by high-performance liquid chromatography-tandem mass spectrometry based on enrichment by solid-phase extraction

A solid-phase extraction (SPE) method for the enrichment and clean-up of lipophilic marine biotoxins from extracts of different species of bivalve molluscs and processed shellfish products was developed. Okadaic acid (OA), pectenotoxin2 (PTX2), azaspiracid1 (AZA1) and yessotoxin (YTX) were determined by LC-MS/MS in hydrolyzed and non-hydrolyzed extracts. Applying a concentration factor of 10 the limit of quantification for the four toxins was determined to be 1 microg/kg. An organized in-house ring trial proved transferability of the method protocol and satisfactory results for all four toxins with a relative standard deviation (RSD) of 5-12%. The precision of the whole method including LC-MS detection was determined by processing seven independent extractions analyzed in independent sequences. RSD ranged between 12% and 24%. This SPE method was tested within a concentration range corresponding to the range of the current European Union regulatory limits (up to 160 microg/kg for the OA group), but it would also be applicable to a lower microg/kg range which is important in view of a possible decrease of regulatory limits as proposed by a working group of the European Food Safety Authority. The potential of SPE as a cleaning tool to cope with matrix effects in LC-MS/MS was studied and compared to liquid-liquid portioning.

Formation of Azaspiracids-3, -4, -6, and -9 via decarboxylation of carboxyazaspiracid metabolites from shellfish

The azaspiracid (AZA) class of phycotoxins has been responsible for extended closures of shellfisheries in various locations around Europe, where levels of AZA1-3 are regulated in shellfish. Since their discovery in 1995, AZAs have been the focus of much research, resulting in the discovery of numerous analogues. During studies of procedures for processing of AZA-contaminated mussels ( Mytilus edulis ), an unusual phenomenon was observed involving AZA3. In uncooked tissues, AZA3 levels would increase significantly when heated for short periods of time in the absence of water loss. A similar increase in AZA3 concentrations occurred during storage of shellfish tissue reference materials for several months at temperatures as low as 4 degrees C. Concentrations of AZA1 and AZA2 did not change during these experiments. Several possible explanations were investigated, including an AZA3-specific matrix effect upon heating of tissues, release of AZA3 from the matrix, and formation of AZA3 from a precursor. Preliminary experiments indicated that toxin conversion was responsible, and more detailed studies focused on this possibility. LC-MS analysis of heating trials, deuterium labeling experiments, and kinetic studies demonstrated that a carboxylated AZA analogue, AZA17, undergoes rapid decarboxylation when heated to produce AZA3. Heat-induced decarboxylation of AZA19, AZA21, and AZA23 to form AZA6, AZA4, and AZA9, respectively, was also demonstrated. This finding is of great significance in terms of procedures used in the processing of shellfish for regulatory analysis, and it exemplifies the role that chemical analysis can play in understanding the contribution of metabolic processes to the toxin profiles observed in shellfish samples.

Determination of dissolved domoic acid in seawater with reversed-phase extraction disks and rapid resolution liquid chromatography tandem mass spectrometry with head-column trapping

Domoic acid (DA) is the principal neurotoxin responsible for amnesic shellfish poisoning (ASP) and is produced, among other species, by marine diatoms of the genus Pseudo-nitzschia. In this work, a method for the determination of dissolved DA and its isomers present in seawater has been developed, based on a solid-phase extraction (SPE) disks followed by rapid resolution liquid chromatography coupled with tandem mass spectrometry. SPE provided sample desalting and 20-fold concentration of dissolved DA, while complete resolution between DA and its isomers was achieved in less than 3 min with rapid resolution chromatography thus providing high sample throughput. Additionally, a simple on-column chromatographic procedure allowed head-column trapping of DA providing 15-fold higher sensitivity. The conditions developed in this work have shown appropriate quality parameters in a within-laboratory validation. The detection limit was 0.02 ng mL(-1) for the whole method, while trueness ranged between 92.1% and 110.6% recovery and precision between 8.4% and 19.0% relative standard deviation. Expanded uncertainty measured was 1.92, 0.23 and 0.03 for 10.0, 1.0 and 0.1 ng mL(-1) DA concentrations, respectively, which demonstrated the accuracy of this method for confirmation and quantification of DA present at very low concentration levels in seawater.