Improving the recovery of ionic solutes from aqueous media by modified thermospray liquid-liquid extraction

Our previously reported procedure for the extraction of charged compounds from aqueous samples by thermospray liquid-liquid extraction (TSLLE) was essentially a one-step extraction involving large sample volumes. In this report, recirculative extraction, analysis of small sample volumes, the halide ion effect, and the effect of solvency or solvent modification on the extraction efficiency of benzoic acid (BA) by TSLLE were investigated. Compared to the one-step procedure that resulted in an extraction efficiency of only 28% for BA in n-hexane, recirculative TSLLE resulted in a BA recovery of 65% after five extraction cycles. When applied to sample volumes of 5-10 mL, TSLLE extracted BAwith a precision of 2.8-6.1%. NaF, NaCl, and NaBr were also used to enhance analyte recovery. NaF gave the best recovery, 104%, for BA relative to the 88% obtained by batch processing. Some improvements in the extraction efficiency was observed when solvent modifiers such as methanol, ethanol, and 2-propanol were used.     

Protocol for liquid chromatography/mass spectrometry of glutathione conjugates using postcolumn solvent modification

A novel protocol for thermospray liquid chromatography/mass spectrometry (LC/MS) analysis of mixtures of glutathione conjugates is reported. Solvent conditions for optimal high-performance liquid chromatography are not always the same as for optimal thermospray ionization mass spectrometry. Labile glutathione conjugates that give poor spectra in aqueous ammonium acetate yield more intense molecular ion signals with increased percentages of acetonitrile. Direct injection thermospray ionization using 30-60% acetonitrile in aqueous ammonium acetate produced protonated molecular ions for glutathione conjugates of menadione, styrene oxide, pentachlorophenyl methyl sulfone, chlorodinitrobenzene, and chlorambucil. Since, the high percentages of organic modifier needed for good molecular ion intensity preclude chromatographic separation of these polar compounds, successful graphic separation of these polar compounds, successful LC/MS was facilitated by postcolumn addition of organic modifiers to the mobile phase. This new methodology allowed excellent chromatographic separations and thermospray ionization mass spectra to be obtained for a mixture of haloalkane glutathione conjugates. Moreover, cleavage of the gamma-glutamyl-cysteine amide bond of glutathione results in class-characteristic fragment ions. Changes in the fragmentation pathways in spectra acquired with and without organic modifiers shed light on the importance of the desolvation process in obtaining good molecular ion sensitivity in thermospray.     

Analysis of pharmaceuticals in fish using liquid chromatography-tandem mass spectrometry

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) screening method has been developed targeting 23 pharmaceuticals and 2 metabolites with differing physicochemical properties in fish tissue. Reversed-phase separation of target compounds was achieved using a C18 column and a nonlinear gradient consisting of 0.1% (v/v) formic acid and methanol. Eluted analytes were introduced into the mass analyzer using positive or negative electrospray ionization, as appropriate. A variety of extraction solvents, differing in polarity, pH, or both, were investigated in order to assess recovery of target compounds from 1-g tissue homogenates. Among 10 solvents tested, a 1:1 mixture of 0.1 M aqueous acetic acid (pH 4) and methanol was identified as optimal, resulting in extraction recoveries for 24 of 25 compounds exceeding 60%. Tissue extracts were found to influence the LC-MS/MS response for several analytes. Consequently, matrix-matched calibration standards were employed to determine analyte concentrations in environmental samples. Statistically derived method detection limits were <6 ng/g for most analytes. The method was subsequently used to screen for target analytes in fish from an effluent-dominated stream. Diphenhydramine, diltiazem, carbamazepine, and norfluoxetine were detected in 11 of 11 environmental samples at concentrations ranging from 0.11 to 5.14 ng/g.     

气相色谱-质谱联用测定饮用水源中半挥发性有机物

以美国USEPA8270分析方法为基础,参照国内外有关SVOCs分析方法,建立二氯甲烷液液萃取、GC-MS同时定性定量饮用水源中21个组分SVOCs的方法;21种SVOCs的RSD为2.01~8.13%;回收率为40.2~91.5%;方法检出限为0.06~1.00μg/L,均满足地表水环境质量标准(GB3838-2002)中各化合物的标准限值要求;应用于中山市3个饮用水源地样品SVOCs测定,结果满意。     

Automated in-tube solid-phase microextraction coupled with liquid chromatography/electrospray ionization mass spectrometry for the determination of beta-blockers and metabolites in urine and serum samples.

The technique of automated in-tube solid-phase microextraction (SPME) coupled with liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) was evaluated for the determination of beta-blockers in urine and serum samples. In-tube SPME is an extraction technique for organic compounds in aqueous samples, in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. LC/MS analyses of beta-blockers were initially performed by liquid injection onto a LC column. Nine beta-blockers tested in this study gave very simple ESI mass spectra, and strong signals corresponding to [M + H]+ were observed for all beta-blockers. The beta-blockers were separated with a Hypersil BDS C18 column using acetonitrile/methanol/water/acetic acid (15:15:70:1) as a mobile phase. To optimize the extraction of beta-blockers, several in-tube SPME parameters were examined. The optimum extraction conditions were 15 draw/eject cycles of 30 microL of sample in 100 mM Tris-HCl (pH 8.5) at a flow rate of 100 microL/min using an Omegawax 250 capillary (Supelco, Bellefonte, PA). The beta-blockers extracted by the capillary were easily desorbed by mobile-phase flow, and carryover of beta-blockers was not observed. Using in-tube SPME/LC/ESI-MS with selected ion monitoring, the calibration curves of beta-blockers were linear in the range from 2 to 100 ng/mL with correlation coefficients above 0.9982 (n = 18) and detection limits (S/N = 3) of 0.1-1.2 ng/mL. This method was successfully applied to the analysis of biological samples without interference peaks. The recoveries of beta-blockers spiked into human urine and serum samples were above 84 and 71%, respectively. A serum sample from a patient administrated propranolol was analyzed using this method and both propranolol and its metabolites were detected.     

Subzero-temperature liquid-liquid extraction of benzodiazepines for high-performance liquid chromatography.

On the basis of the phenomenon that hydrophilic acetonitrile is separated from the aqueous phase at -20 degrees C, we employed a novel extraction method, "subzero-temperature liquid-liquid extraction", to extract benzodiazepines (estazolam and triazolam) from serum or aqueous solution for liquid chromatography. A 1:1 mixture of acetonitrile and the specimen was cooled at -20 degrees C for 20 min to separate the acetonitrile and aqueous phases. The acetonitrile phase was directly injected into a high-performance liquid chromatograph. Recovery rates of the drugs following the first subzero-temperature liquid-liquid extraction were 50.3 +/- 0.6-54.0 +/- 0.9%, which were lower than those (73.9 +/- 3.3-80.6 +/- 0.6% and 81.6 +/- 4.7-96.1 +/- 2.6%) of the first conventional liquid-liquid extraction using diethyl ether and solid-phase extraction using a Sep-Pak C18 column, respectively. However, three to four repeated subzero-temperature liquid-liquid extractions and conventional liquid-liquid extractions resulted in recovery of almost 100% of the drugs. In the chromatogram of the benzodiazepines recovered from serum by the subzero-temperature extraction, no coextracted component interfered with determination of the drugs. Detection limits of the drugs were 0.02-0.08 microgram/mL, and coefficients of variance were 1.14-2.17% suggesting high reproducibility.     

Flow-through dispersed carbon nanofiber-based microsolid-phase extraction coupled to liquid chromatography for automatic determination of trace levels of priority environmental pollutants

Handling of carbon nanoparticles as sorptive materials in a flow-through packed-bed mode has been to date hampered by undue pressure drop and deteriorated retention efficiency because of nanoparticle bundling and entanglement. To surmount this limitation, a dedicated stirred-flow sorptive microchamber integrated in a fully automated sequential injection (SI) assembly is herein proposed for expedient handling and reuse of carbon nanoparticles in microsolid-phase extraction (μSPE) procedures. The assembled setup features automatic uptake, preconcentration, and retrieval of target organic species using dispersed nanoparticles as a front-end to liquid chromatographic (LC) assays. Chlorotriazine residues (atrazine, simazine, and propazine) and dealkylated metabolites thereof (deisopropyltriazine (DIA) and deethylatrazine (DEA)) were selected as model compounds because of their electron-poor aromatic structure and potentially strong π-π interactions with electron-rich sorptive materials. The effect of several parameters on the analytical performance including the type and amount of nanoparticles (carbon nanofibers (CNFs), multiwalled carbon nanotubes (MWCNTs) and oxidized carbon nanotubes (MWCNT-COOH), the sample volume (breakthrough volume), the nature and volume of eluent, and the interface between the sample processing module and LC was explored in detail. Using dispersed CNFs at-line coupled to LC, absolute recovery percentages for 10 mL sample percolation were >94% for the overall herbicides with enrichment factors of ca. 20, limits of detection (S/N = 3) of 0.004-0.03 ng mL(-1), limits of quantification (S/N = 10) of 0.01-0.09 ng mL(-1) and repeatability within the range 0.5-1.8%. The SI-CNF-LC hyphenated system was harnessed to the analysis of not merely untreated environmental waters at concentration levels below those endorsed by the current EU Water Framework Directives but to crude soil extracts for which CNF reuse with no loss of retention efficiency was proven feasible by resorting to appropriate automatic regeneration procedures and internal standardization.     

Sample preparation of organic liquid for off-site analysis of chemical weapons convention related compounds

Off-site analysis of chemical warfare agents (CWAs) and related compounds plays a key role in the verification program of the Chemical Weapons Convention (CWC). The analysis results, aiming toward unambiguous identication of compounds, depend on the type of sample preparation method. Development of milder sample preparation methods, which offer good recoveries and do not alter the structure of analytes, is highly desirable. Organic liquid with high hydrocarbon background is a frequently encountered challenge in off-site analysis and in official proficiency tests conducted by OPCW. Sample cleanup procedures, namely, solvent exchange followed by cooling and liquid-liquid extraction were studied to eliminate the hydrocarbons from organic liquid. Acetonitrile, a polar aprotic solvent, was effectively used to remove the background in both methods, and recoveries of spiked CWAs by the two techniques were between 69 and 99%.     

Supercritical fluid extraction of natural antioxidants from rosemary: comparison with liquid solvent sonication

Supercritical fluid extraction (SFE) and liquid solvent sonication, in combination with two different sample treatments, were compared for the extraction of natural antioxidants from rosemary leaves. Dried, ground, and sieved rosemary leaves (20 mg) were subjected to SFE with CO(2) at 355 bar at 100 °C (CO(2) density 0.72 g/mL) for 20 min at a liquid flow rate of 4 mL/min. The analytes were concentrated on an ODS trap and subsequently eluted with acetone. Antioxidants in the SF and liquid solvent extract were analyzed by HPLC. Compounds of known antioxidant activity such as carnosol, carnosic acid, and methyl carnosate were identified by mass spectrometry of the HPLC fractions collected. Freezing and grinding the samples in liquid nitrogen resulted in decreased carnosic acid recoveries. Supercritical CO(2) extraction provided the highest recovery of carnosic acid from rosemary leaves (35.7 mg/g), the lowest relative standard deviation (4.4%), and the cleanest extract [Formula: see text] no cleanup prior to HPLC was required. Among the liquid solvents studies, only acetone provided comparable results (73% recovery relative to SC-CO(2) extraction); however, it required decoloration with active carbon prior to HPLC analysis.     

Adaptation of a thermospray liquid chromatography/mass spectrometry interface for use with alkaline anion exchange liquid chromatography of carbohydrates

An interface is described that allows the direct coupling of high-performance alkaline anion exchange liquid chromatography with thermospray mass spectrometry. A membrane suppressor is used to remove nonvolatile alkaline salts from the mobile phase after the chromatographic process is completed and prior to introduction into the mass spectrometer. Examples are given of both isocratic and gradient separations of a three-component test mixture of N-acetylated mono- and disaccharides, followed by on-line mass spectral data acquisition. Sensitivity studies show minimum detection limits for the test compounds to be in the microgram range.     

Determination of proanthocyanidins in grape products by liquid chromatography/mass spectrometric detection under low collision energy

A method has been established for the identification of proanthocyanidins and quantification of individual monoproanthocyanidins using liquid chromatography/electrospray ionization-mass spectrometric detection (LC/ESI-MSD) for raw grape products. The separated monoproanthocyanidins and oligoproanthocyanidins were individually analyzed and identified by their molecular ion peaks using LC/MS. Using HPLC/ESI-MSD, the proanthocyanidin monomers, (+)-catechin (C), (-)-epicatechin (EC), (-)-catechin gallate (CG), and (-)-epicatechin gallate (ECG) in grape products were successfully quantified by LC/MS/MS detection of protonated molecular ions and characteristic fragment ions for each component under the optimized low collision energy level of 20%. For the investigated concentration ranges of C (21.88-11,200 ng/mL), EC (21.10-10,800 ng/mL), CG (36.72-18,800 ng/mL), and ECG (39.84-20,400 ng/mL), good linearities (r2 > 0.99) for standard curves were obtained. Validation of this method showed an accuracy that was well below 15% and precision (RSD) within 8% for the four compounds. The method proposed here is simple, sensitive, and allows a direct sample preparation procedure. This is the first method that enables the determination of individual monoproanthocyanidins in grape products without any solid-phase extraction.     

高效液相色谱测定福龙口服液中的腺苷

本文介绍了等度反相高效液相色谱(HPLC)测定福龙口服液中腺苷的方法,采用μBondapak C_(18)色谱柱,以甲醇:磷酸二氢钠(0.01mol/L)=13:87(v/v)为流动相。流速为1 mL/分,在254nm波长处检测。在0.17～10.2mg/mL测定范围内腺苷的峰面积与质量浓度的线性关系良好,相关系数为0.9997;腺苷的回收率97.18%;日内测定相对标准偏差(n=6)在1.35%～2.01%之间,隔5天测定结果基本无变化,该法已成功应用于福龙口服液中腺苷的日常测定。     

Description of the thermospray formed at low flow rate in thermospray flame furnace atomic absorption spectrometry based on high-speed images

The mechanism of the thermospray formed at low flow rates using a peristaltic pump in thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) is described here for the first time. The study was based on magnified images of the thermospray formed inside the hot tube furnace by using a high-speed CMOS camera. For this purpose different image acquisition speeds were used (from 1000 to 18,000 frames/s), revealing that the thermospray obtained under such conditions is quite different from those already reported. The frames of the thermospray evolution indicate that the Leindenfrost effect plays an important role and allow us to propose a mechanism for its formation. The analysis of the images contributed to calculation of parameters related to thermospray formation, such as pulse incidence average (110 +/- 10, 320 +/- 50, and 1200 +/- 150 pulses per second) and pulse speed (6 +/- 1, 10 +/- 1, and 14 +/- 2 m s(-1)) for 0.1, 0.4, and 1.0 mL min(-1) flow rate, respectively, for both parameters. Additionally, the evaporation constant (lambda) of 10(-4) m2 s(-1) was estimated, and the present thermospray was correlated to the conventional sprays using the Sauter mean diameter (SMD) parameter, which ranged from ca. 2 to 44 microm. In order to correlate the information obtained through images with analytical parameters employing the thermospray, the sensitivities for cadmium determination at each condition (0.12, 0.11, and 0.069 s L microg(-1) for 0.1, 0.4, and 1.0 mL min(-1) flow rate, respectively) were taken into account.     

Strategies for bioanalysis of an oligonucleotide class macromolecule from rat plasma using liquid chromatography-tandem mass spectrometry

Electrospray ionization (ESI) liquid chromatography-tandem mass spectrometry (LC/MS/MS) assays provide high-throughput and selective methods for quantitation of small molecules. Use of LC/MS/MS assays for macromolecules, like oligonucleotides, is challenging due to lack of sensitivity and low analyte recovery from biomatrixes. Due to this fact, the method of choice for oligonucleotides quantitation remains hybridization-based ligand-binding assays. These biological assays usually possess high sensitivity but low selectivity and narrow dynamic range. They also require optimizing suitable "capture and detection" probes, which can be prohibitively time-consuming and expensive in a drug discovery lead-optimization scenario. In this paper, we present a unique LC/MS/MS assay for a model phosphorothioate backbone oligodeoxynucleotide (ODN) drug (7692 amu) from rat plasma. Multiple analytical challenges were encountered. The strategies used to solve these challenges should prove useful to scientists pursuing mass spectrometry (MS) to quantitate oligonucleotides. The challenges include analyte multiple charging and cation adduction (reduced sensitivity), oxidation of analyte on drying and high protein binding (low recovery), ODN affinity to exposed silica (low chromatographic reproducibility and high carryover), nonspecific binding of analyte to containers (low storage stability), and optimization/synthesis of an appropriate internal standard (interference and cross-talk). A buffer (7 mM triethylamine and 3 mM ammonium formate)/methanol, 50:50 (v/v), was used as an ESI-MS infusion solvent and produced a sharp multiple charge-state distribution. The sample extraction method combined a phenol/chloroform liquid-liquid extraction and solid-phase extraction steps, which improved the absolute recovery to >70%. The method was validated in the range of 5-2000 ng/mL and had precision (percent relative standard deviation)<10.1% and accuracy (percent relative error)<11.4%.     

Determination of anionic and nonionic surfactants, their degradation products, and endocrine-disrupting compounds in sewage sludge by liquid chromatography/mass spectrometry

A comprehensive analytical method based on reversed-phase liquid chromatography and mass spectrometry using both atmospheric pressure chemical ionization and electrospray ionization has been developed for the simultaneous determination of anionic and nonionic surfactants, their polar degradation products, and endocrine-disrupting compounds (EDCs) in sewage sludge. Extraction of target compounds, with recovery rates from 86% to nearly 100% for polyethoxylates and from 84 to 94% for polar degradation products, was achieved applying ultrasonic solvent extraction with a mixture of methanol/ dichloromethane (7:3, v/v). Cleanup of sample extracts was performed on octadecyl solid-phase extraction cartridges. Determination of less polar compounds: alcohol ethoxylates (AEOs), nonylphenol ethoxylates (NPEOs), coconut diethanol amides, poly(ethylene glycol)s, and phthalate esters was accomplished by reversed-phase LC-APCI-MS in positive ionization mode, while more polar compounds: nonylphenolcarboxylates, nonylphenol (NP), octylphenol, and bisphenol Awere analyzed by ion-pair LC-ESI-MS under negative ionization conditions. This protocol was successfully applied to the trace determination of anionic and nonionic surfactants, polar degradation products, and EDCs in sewage sludge collected from different sewage treatment plants. The analysis revealed the presence of NP at high concentration levels ranging from 25 to 600 mg/kg. Polyethoxylates (AEOs and NPEOs) were also found in all samples at parts-per-million levels (10-190 mg/kg AEOs and 2-135 mg/kg NPEOs, respectively).     

Hot phosphate-buffered water extraction coupled on-line with liquid chromatography/mass spectrometry for analyzing contaminants in soil

We evaluated the feasibility of analyzing rapidly traces of polar and medium polar contaminants in soil by coupling on-line a hot phosphate-buffered water extraction apparatus to a liquid chromatography/mass spectrometer system. Coupling was accomplished by using a small C-18 sorbent trap for collecting analytes and two six-port valves. The efficiency of this device was evaluated by extracting 13 selected pesticides from 200 mg of laboratory-aged soils by varying the extraction temperature, the extractant volume, and the flow rate at which the extractant passed through the extraction cell and the sorbent trap. In terms of extraction efficiency, robustness of the method, and extraction time, the best compromise was that of using 8 mL of extractant at 90 degrees C and 0.5 mL/min flow rate. Under these conditions, recoveries of 11 out of 13 analytes ranged between 82 and 103%, while those of the least hydrophilic pesticides, i.e., neburon and prochloraz, were 73 and 63%, respectively. By increasing the extractant volume to 60 mL, additional amounts of the two latter compounds could be recovered. Under this condition, however, the most hydrophilic analytes were in part no more retained by the C-18 sorbent trap. From a naturally 1.5-year aged soil, hot phosphate-buffered water removed larger amounts of three herbicides and hydroxyterbuthylazine (a terbuthylazine degradation product) than pure water and Soxhlet extraction. This result seems to confirm that hot phosphate buffer is also able to remove from soil those fractions of contaminants that, on aging, are sequestered into the humic acid framework.     

Uptake of diet resveratrol into the human low-density lipoprotein. Identification and quantification of resveratrol metabolites by liquid chromatography coupled with tandem mass spectrometry

In this paper, a sensitive, precise, and selective analytical method has been developed for the identification and quantification of resveratrol metabolites in human low-density lipoprotein (LDL) after moderate consumption of red wine, using high-performance liquid chromatography electrospray in tandem mass spectrometry (LC-ESI-MS/MS). From different extraction procedures tested, solid-phase extraction was selected to minimize matrix effects reaching the highest sensitivity. Standard calibration curves prepared in human LDL for trans-resveratrol were linear over a range of 0.44-438.59 pmol/mL. The accuracy and interassay precision of this LC-MS/MS assay for resveratrol showed a coefficient of variation of <6.0%. The method allows detection and quantification limits for resveratrol in LDL at 0.15 and 0.44 pmol/mL, respectively. Results to date indicate that resveratrol metabolites were incorporated into LDL after a moderate intake of red wine. The metabolites identified in LDL were trans-resveratrol-3-O-glucuronide, cis-resveratrol-3-O-glucuronide, and cis-resveratrol-3-O-glucoside, as well as free trans-resveratrol. To our knowledge, it is the first time that a polyphenol from red wine, specifically resveratrol, has been identified in human LDL after moderate intake of red wine. Furthermore, these findings suggest that these compounds may deliver their antioxidant effect to LDL.     

In-membrane preconcentration/membrane inlet mass spectrometry of volatile and semivolatile organic compounds

The on-line determination of volatile and semivolatile organic compounds (SVOCs) is reported using membrane inlet mass spectrometry with in-membrane preconcentration (IMP-MIMS). Semivolatile organic compounds in aqueous samples are preconcentrated in a flow-through silicone hollow-fiber membrane inlet held in a GC oven. The sample stream is replaced with air, and the SVOCs are thermally desorbed into the mass spectrometer by rapid heating of the membrane. The method is evaluated for the on-line determination of 4-fluorobenzoic acid, 3,5-difluorobenzoic acid, 2-chlorophenol, p-tert-butylphenol, and dimethyl sulfoxide (DMSO) in water. The selectivity of the IMP-MIMS technique for SVOCs in the presence of VOCs is demonstrated. Cryotrapping and a rapid gas chromatographic separation step were added between the membrane and the mass spectrometer ion source for the determination of SVOCs in complex mixtures. The procedure is demonstrated for the determination of dimethyl sulfoxide (DMSO) in equine urine, using internal standardization with DMSO-d6. Full-scan electron ionization (EI) mass spectrometric detection showed good linearity (R = 0.998) and RSDs, relative to the internal standard, of 2.2% for desorption only and 4.6% for desorption and cryotrapping.     

固相萃取-液相色谱法测定糕点中脱氢乙酸、苯甲酸、山梨酸的方法研究

建立了固相萃取-高效液相色谱法测定糕点中的脱氢乙酸、苯甲酸、山梨酸的方法。实验结果表明,在(0.02～0.15)mg/mL时有良好的线性关系(r>0.997),相对标准偏差(RSD)为1.0%～2.5%,回收率96.2%～102.1%。该方法简单、快速、灵敏度高,并具有良好的精密度与准确度,可作为糕点中检测防腐剂的有效定量方法。     

Analysis of trace levels of sulfonamide and tetracycline antimicrobials in groundwater and surface water using solid-phase extraction and liquid chromatography/mass spectrometry

A method has been developed for the trace analysis of two classes of antimicrobials consisting of six sulfonamides (SAs) and five tetracyclines (TCs), which commonly are used for veterinary purposes and agricultural feed additives and are suspected to leach into ground and surface water. The method used solid-phase extraction and liquid chromatography/mass spectrometry (LC/MS) with positive ion electrospray. The unique combination of a metal chelation agent (Na2EDTA) with a macroporous copolymer resulted in quantitative recoveries by solid-phase extraction (mean recovery, 98 +/- 12%) at submicrogramper-liter concentrations. An ammonium formate/formic acid buffer with a methanol/water gradient was used to separate the antimicrobials and to optimize the signal intensity. Mass spectral fragmentation and ionization characteristics were determined for each class of compounds for unequivocal identification. For all SAs, a characteristic m/z 156 ion representing the sulfanilyl fragment was identified. TCs exhibited neutral losses of 17 amu resulting from the loss of ammonia and 35 amu from the subsequent loss of water. Unusual matrix effects were seen only for TCs in this first survey of groundwater and surface water samples from sites around the United States, requiring that TCs be quantitated using the method of standard additions.