Towards establishing indicative values for metabolites of organophosphate ester contaminants in human urine

Review on typical organophosphate diesters (di-OPEs) requiring priority attention: Formation, occurrence, toxicological, and epidemiological studies

The impact of primary metabolites of organophosphate triesters (tri-OPEs), namely, organophosphate diesters (di-OPEs), on the ecology, environment, and humans cannot be ignored. While extensive studies have been conducted on tri-OPEs, research on the environmental occurrence, toxicity, and health risks of di-OPEs is still in the preliminary stage. Understanding the current research status of di-OPEs is crucial for directing future investigations on the production, distribution, and risks associated with environmental organophosphate esters (OPEs). This paper specifically reviews the metabolization process from tri-OPEs to di-OPEs and the occurrence of di-OPEs in environmental media and organisms, proposes typical di-OPEs in different media, and classifies their toxicological and epidemiological findings. Through a comprehensive analysis, six di-OPEs were identified as typical di-OPEs that require prioritized research. These include di-n-butyl phosphate (DNBP), bis(2-butoxyethyl) phosphate (BBOEP), bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), bis(2-chloroethyl) phosphate (BCEP), bis(1-chloro-2-propyl) phosphate (BCIPP), and diphenyl phosphate (DPHP). This review provides new insights for subsequent toxicological studies on these typical di-OPEs, aiming to improve our understanding of their current status and provide guidance and ideas for research on the toxicity and health risks of di-OPEs. Ultimately, this review aims to enhance the risk warning system of environmental OPEs.

Unveiling the Occurrence and Potential Ecological Risks of Organophosphate Esters in Municipal Wastewater Treatment Plants across China

Organophosphate esters (OPEs) discharged from wastewater treatment plants (WWTPs) have attracted increasing concerns because of their potential risks to aquatic ecosystems. The identification of the structures of OPEs is a prerequisite for subsequent assessment of their environmental impacts, which could hardly be accomplished using traditional target analytical methods. In this study, we describe the use of suspect and nontarget screening techniques for identification of organophosphate triesters and diesters (tri-OPEs and di-OPEs) in the influent and effluent samples acquired from 25 municipal WWTPs across China. There are totally 33 different OPE molecules identified, 11 of which are detected in wastewater for the first time and 4 are new to the public. In all tested samples, di-OPEs account for a significant portion (53% on average) of the total OPEs (ng/L-μg/L). More importantly, most of the OPEs could not be eliminated after treatment in these WWTPs, while some of the di-OPEs even accumulate. The research priority of OPEs in the effluent based on ecological risk was also analyzed, and the results reflected a previously unrecognized exposure risk of emerging OPEs for aquatic living organisms. These findings present a holistic understanding of the environmental relevance of OPEs in WWTPs on a country scale, which will hopefully provide guidance for the upgrade of treatment protocols in WWTPs and even for the modification of governmental regulations in the future.

Simultaneous determination of 16 urinary metabolites of organophosphate flame retardants and organophosphate pesticides by solid phase extraction and ultra performance liquid chromatography coupled to tandem mass spectrometry

Organophosphate flame retardants (OPFRs) and organophosphate pesticides (OPPs), pertaining to organophosphate esters, are ubiquitous in environment and have been verified to pose noticeable risks to human health. To evaluate human exposures to OPFRs and OPPs, a fast and sensitive approach based on a solid phase extraction (SPE) followed by the ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) detection has been developed for the simultaneous analysis of multiple organophosphorus metabolites in urine. The method allows the identification and quantification of ten metabolites of the most common OPFRs and all six dialkylphosphates (DAPs) of OPPs concerning the population exposure characteristics. The method provided good linearities (R² = 0.998-0.999), satisfactory method detection limits (MDLs) (0.030-1.129 ng/mL) and only needed a small volume (200 μL) of urine. Recovery rates ranged 73.4-127.1% at three spiking levels (2, 10 and 25 ng/mL urine), with both intra- and inter-day precision less than 14%. The good correlations for DAPs in a cross-validation test with a previous gas chromatography-mass spectrometry (GC-MS) method and a good inter-laboratory agreement for several OPFR metabolites in a standard reference material (SRM 3673) re-enforced the precision and validity of our method. Finally, the established method was successfully applied to analyze 16 organophosphorus metabolites in 35 Chinese children's urine samples. Overall, by validating the method's sensitivity, accuracy, precision, reproducibility, etc., data reliability and robustness were ensured; and the satisfactory pilot application on real urine samples demonstrated feasibility and acceptability of this method for being implemented in large population-based studies.

Critical review of analytical methods for the determination of flame retardants in human matrices

Human biomonitoring is a powerful approach in assessing exposure to environmental pollutants. Flame retardants (FRs) are of particular concern due to their wide distribution in the environment and adverse health effects. This article reviews studies published in 2009-2020 on the chemical analysis of FRs in a variety of human samples and discusses the characteristics of the analytical methods applied to different FR biomarkers of exposure, including polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD), novel halogenated flame retardants (NHFRs), bromophenols, incl. tetrabromobisphenol A (TBBPA), and organophosphorous flame retardants (PFRs). Among the extraction techniques, liquid-liquid extraction (LLE) and solid phase extraction (SPE) were used most frequently due to the good efficiencies in the isolation of the majority of the FR biomarkers, but with challenges for highly lipophilic FRs. Gas chromatography-mass spectrometry (GC-MS) is mainly applied in the instrumental analysis of PBDEs and most NHFRs, with recent inclusions of GC-MS/MS and high resolution MS techniques. Liquid chromatography-MS/MS is mainly applied to HBCD, bromophenols, incl. TBBPA, and PFRs (including metabolites), however, GC-based analysis following derivatization has also been used for phenolic compounds and PFR metabolites. Developments are noticed towards more universal analytical methods, which enable widening method scopes in the human biomonitoring of FRs. Challenges exist with regard to sensitivity required for the low concentrations of FRs in the general population and limited sample material for some human matrices. A strong focus on quality assurance/quality control (QA/QC) measures is required in the analysis of FR biomarkers in human samples, related to their variety of physical-chemical properties, low levels in most human samples and the risk of contamination.

Interlaboratory comparison investigations (ICIs) and external quality assurance schemes (EQUASs) for flame retardant analysis in biological matrices: Results from the HBM4EU project

The European Human Biomonitoring Initiative (HBM4EU) is coordinating and advancing human biomonitoring (HBM). For this purpose, a network of laboratories delivering reliable analytical data on human exposure is fundamental. The analytical comparability and accuracy of laboratories analysing flame retardants (FRs) in serum and urine were investigated by a quality assurance/quality control (QA/QC) scheme comprising interlaboratory comparison investigations (ICIs) and external quality assurance schemes (EQUASs). This paper presents the evaluation process and discusses the results of four ICI/EQUAS rounds performed from 2018 to 2020 for the determination of ten halogenated flame retardants (HFRs) represented by three congeners of polybrominated diphenyl ethers (BDE-47, BDE-153 and BDE-209), two isomers of hexabromocyclododecane (α-HBCD and γ-HBCD), two dechloranes (anti-DP and syn-DP), tetrabromobisphenol A (TBBPA), decabromodiphenylethane (DBDPE), and 2,4,6-tribromophenol (2,4,6-TBP) in serum, and four metabolites of organophosphorus flame retardants (OPFRs) in urine, at two concentration levels. The number of satisfactory results reported by laboratories increased during the four rounds. In the case of HFRs, the scope of the participating laboratories varied substantially (from two to ten) and in most cases did not cover the entire target spectrum of chemicals. The highest participation rate was reached for BDE-47 and BDE-153. The majority of participants achieved more than 70% satisfactory results for these two compounds over all rounds. For other HFRs, the percentage of successful laboratories varied from 44 to 100%. The evaluation of TBBPA, DBDPE, and 2,4,6-TBP was not possible because the number of participating laboratories was too small. Only seven laboratories participated in the ICI/EQUAS scheme for OPFR metabolites and five of them were successful for at least two biomarkers. Nevertheless, the evaluation of laboratory performance using Z-scores in the first three rounds required an alternative approach compared to HFRs because of the small number of participants and the high variability of experts' results. The obtained results within the ICI/EQUAS programme showed a significant core network of comparable European laboratories for HBM of BDE-47, BDE-153, BDE-209, α-HBCD, γ-HBCD, anti-DP, and syn-DP. On the other hand, the data revealed a critically low analytical capacity in Europe for HBM of TBBPA, DBDPE, and 2,4,6-TBP as well as for the OPFR biomarkers.

Identifying Dermal Uptake as a Significant Pathway for Human Exposure to Typical Semivolatile Organic Compounds in an E-Waste Dismantling Site: The Relationship of Contaminant Levels in Handwipes and Urine Metabolites

Dermal exposure to semivolatile organic compounds (SVOCs) has recently attracted widespread attention; understanding these exposures is particularly important for people whose skin is frequently exposed to different pollution surfaces. In this study, handwipes were collected from exposed occupational workers and local residents near a typical electronic waste (e-waste) dismantling area; urine samples were also sampled. The wipes were analyzed for three typical SVOCs: polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons (PAHs), and organophosphate flame retardants (OPFRs). The median levels of PAHs, OPFRs, and PBDEs in handwipes from e-waste dismantlers were 96.0, 183, and 238 ng, respectively. The analytes were higher in the handwipes collected from workers than those from residents, indicating that they were subjected to greater dermal exposure during primitive e-waste dismantling activities. Among the three SVOCs, the strongest correlation was found between triphenyl phosphate (TPhP) in handwipes and diphenyl phosphate (DPhP) in paired urine; the next strongest correlations were between PAHs and PBDEs and their corresponding urinary metabolites. The results showed that TPhP contributed the highest exposure to e-waste dismantlers via dermal exposure. Our research highlights the importance of dermal exposure to TPhP, which should be considered in future exposure risk assessments.

Human internal exposure to organophosphate esters: A short review of urinary monitoring on the basis of biological metabolism research

As alternatives to traditional brominated flame retardants, organophosphate flame retardants (OPFRs), especially for organophosphate esters (OPEs) -- the most widely used and investigated OPFRs, have raised people's concern on their environmental and health-related risks over the years. Considering their extensive environmental occurrence and potential adverse effects, precise estimation on the human body burden of OPEs will be conducive to the restrictions on the usage of these compounds scientifically. Biomonitoring research can provide precise information on human exposure to OPEs as it reveals the degree of external exposure from all exposure routes. Knowledge on biotransformation and metabolism of OPEs in the biosystems is of great significance for our understanding of the internal exposure to these compounds. In this study, the biological metabolic processes of nine OPEs prevalent in the environment, involving tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2-propyl) phosphate (TCIPP), tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), tripropyl phosphate (TPrP), tri-n-butyl phosphate (TnBP), tris(2-butoxyethyl) phosphate (TBOEP), triphenyl phosphate (TPhP), 2-ethylhexyl diphenyl phosphate (EHDPP), and tricresyl phosphate (TCrP), are comprehensively reviewed. Specifically, the metabolic pathway, kinetics and mechanism of OPEs are depicted in detail. Under this context, the advances and limitations on biomonitoring of OPE metabolites in human urine are summarized. The requirements of specificity, quantitative stability, high detection frequency/concentration are needed for OPE metabolites to be considered and validated as biomarkers. Thus far, deeper elucidations on the metabolic processes and identification of biomarkers of OPEs are urgently required, given that some OPEs have no suitable biomarkers in human biomonitoring. For better assessment of the body burden of OPEs in humans, reliable and effective methodologies for urine sampling and estimation on internal exposure to OPEs need to be further developed in the future.

A method for the analysis of 121 multi-class environmental chemicals in urine by high-performance liquid chromatography-tandem mass spectrometry

Biomonitoring of human exposure to environmental chemicals has gained momentum in recent years. Biomonitoring methods often include analysis of a single class of chemicals with similar chemical properties. In this study, we describe a method that involves solid-phase extraction (SPE) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) and capable of measuring 121 environmental chemicals comprising plasticizers (PMs; n = 45), environmental phenols (EPs; n = 45), and pesticides (n = 31) through a single extraction of urine. Urine samples were incubated with 20 µL of β-glucuronidase/arylsulfatase (4000 units/mL urine) (from Helix pomatia) buffered at pH 5.5 for 2 h at 37 °C for optimal deconjugation conditions. We compared two extraction methods, namely liquid-liquid extraction and SPE, and the latter with ABS Elut NEXUS® cartridges was optimized to yield best extraction efficiencies. For increased resolution and chromatographic separation, two methods involving Ultra AQ C18® and Betasil™ C18® columns were used. The MS/MS analyses were performed under both negative and positive ionization modes. The optimized method yielded excellent intra- and inter-day variabilities (relative standard deviation: 0.40-11%) and satisfactory recoveries (80-120%) for >95% of the analytes. The limits of detection were ≤ 0.1 ng/mL for 101 analytes and between 0.1 and 1.0 ng/mL for 18 analytes. The optimized SPE LC-MS/MS method was validated through the analysis of standard reference materials and proficiency test urine samples and further applied in the analysis of 21 real urine samples to demonstrate simultaneous determination of 121 environmental chemicals in urine samples.

Young infants' exposure to organophosphate esters: Breast milk as a potential source of exposure

Organophosphate esters (OPEs) are applied as both flame retardants and plasticizers to a variety of consumer items such as home furnishings, construction materials, and children's products. While some assessments have characterized exposure among toddlers and young children, little is known about the OPE exposure among infants, who are a vulnerable population due to their rapid development. Here, we collected spot urine samples from 6-week-old (n = 100) and 12-month-old infants (n = 63), with about half of the infants evaluated at both ages (n = 52), to characterize OPE exposure and determine what factors contributed to higher exposures. Five of six OPE metabolites analyzed were detected frequently (>70%). Diphenyl phosphate was detected in every urine sample, while bis(2-chloro-isopropyl) phosphate (BCIPP) was the most abundant metabolite measured overall. Concentrations of bis(1-chloro-2-propyl) 1-hydroxy-2-propyl phosphate (BCIPHIPP) and BCIPP [i.e., metabolites of tris(2-chloro-isopropyl) phosphate (TCIPP)] were significantly greater among 6-week-old infants compared to 12-month-olds, while levels of other OPE metabolites were not statistically different in the first year of life. OPE metabolites were generally correlated with one another in samples collected at each age (r_s = 0.25-0.75; p < 0.05), and except BCIPHIPP, concentrations of the same metabolite were correlated over time (r_s = 0.41-0.53; p < 0.05). Breastfeeding at 6 weeks of age and owning a larger number of children's products were associated with increased concentrations of urinary BDCIPP. Infants who were currently receiving breast milk had higher levels of TCIPP metabolites; urinary BCIPP concentrations were 6.2 times higher in infants receiving breast milk at 6 weeks of age, and BCIPHIPP levels were 2.2 times higher for 12-month-old infants receiving breast milk (10^β = 7.2; 95% CI: 1.6-32.1 and 10^β = 3.2; 95% CI: 1.2-8.1, respectively). Differences in the predominant TCIPP metabolite associated with breastfeeding may suggest differences in metabolism with age. Cumulatively, our results suggest levels of OPE exposure may be higher for infants than other age groups, including toddlers and older children.