Sampling Of Organic Compounds Research Articles

Identifi cation of Adhesives for Repairing Ancient Ceramics: The Case of the Neolithic and Chalcolithic of Far Northeast Europe

This study focuses on the composition of the adhesives used to repair clay vessels, and on the technique of their preparation in the Late Neolithic and Chalcolithic (late 4th to early 3rd millennia BC) sites of Far Northeast Europe (the Republic of Komi and the Nenets Autonomous Okrug). Remains of adhesives were detected on 70 of 171 repaired pots. To date, five samples of ceramics from dwellings of the Chuzhyael culture have been analyzed. Gas chromatography-mass spectrometry revealed no markers of coniferous trees or bitumen; but did reveal markers of birch, suggesting that fractures and cracks on broken pots were plastered with birch tar. The composition of organic compounds in samples indicates the use of two vessels in the technological process: in one of them, birch bark was subjected to pyrolysis, while the other was a receptacle for tar. This comparatively complex technology reveals one more specialization in the domestic manufacture of the taiga hunter-gatherers, including the use of special furnaces. Analytic procedures employed by us open up new prospects for the study of the material culture of Far Northeast Europe, extend our knowledge of domestic manufacture, and offer new material for AMS dating.

An Innovative Approach to Assessing the Quality of Organoleptic Properties of Base Polymers in Orthopedic Dentistry

Modern technologies using an «electronic nose» imitating the human sense of smell open up new prospects for the objective assessment of the organoleptic properties of polymer materials. The purpose of the study was to compare the level of emissions of volatile organic compounds from modern base materials used in various technological processes for the manufacture of removable denture structures, as well as to assess the possible organoleptic properties of the finished dentures in comparison with known analogues using an «electronic nose» gas analyzer. For this purpose, samples were made from different polymer base materials in the amount of 26 pieces, which were studied using an “electronic nose” gas analyzer using the author’s method [Kuchmenko et al., 2023]. The study confirmed the effectiveness of the proposed improved technology in reducing the noise level of volatile organic compounds in samples made from the «Belakril-E GO» base material by 29 % compared to similar samples produced using standard technology. A reduction in the amount of volatile organic compounds that can cause unwanted odors confirms the higher purity and quality of the polymer and will contribute to a more comfortable adaptation of patients to removable dentures made from this polymer.

Biological Activity of Natural Formulation with Propolis, Lavender and Thyme Oil on Candida Species

Objective: Candida strains have been frequently associated with nosocomial infections in recent years. On the other hand, public prefer natural resources rather than synthetic materials. Therefore, it is important to research combine effects of natural products. This study aimed to investigate the antifungal and antibiofilm effectiveness of essential oils and propolis containing samples on Candida species and the toxicity of samples.
 Material-Method: Three different samples were prepared with propolis, lavender oil and thyme oil. Gas Chromatography-mass Spectrometry (GC/MS) GC/MS was used for the determination of organic compounds in samples. The antifungal effects of a natural product mix were tested against C. albicans, C. krusei and C. parapsilosis by disk diffusion method. MIC and MFC tests of the mixture were performed against a standard such as Nystatin. Antibiofilm tests were performed with the microplate system, taking into account the detected concentrations. Finally, in the cytotoxicity test investigated in the L929 cell line with the WST-1 kit.
 Results: C. albicans was determined as the most sensitive species (MIC50: 90 ppm, MIC90: 750 ppm MFC: 3120 ppm). In the antibiofilm tests, it was determined that the sample was more effective on C. albicans and it inhibited biofilm formation. Also it was determined the IC50 value was 5052 ppm.
 Conclusion: The natural product mixture, which has scientifically proven antifungal effect, has a feature that has high added value and contributes to the economy. But of course, it has to investigate by further studies. The findings of this study may lead to new therapeutic approaches with standardized natural products in the future.

Determination of SI-Traceable Property Values of Volatile Organic Compounds in Samples by Using Post-Column Reaction Gas Chromatography

Determination of SI-Traceable Property Values of Volatile Organic Compounds in Samples by Using Post-Column Reaction Gas Chromatography

Distribution and origin of organic compounds in the condensates from a Mexican high-temperature geothermal field

We report herein for the first time, the presence and relative composition of medium volatility organic compounds in samples from a super-hot geothermal field in Mexico. We sampled ten water/vapor condensates from active exploitation wells from Los Humeros Geothermal field. We identified 48 compounds, being alkanes, aromatics and S-bearing the predominant homologues, followed by O-bearing. Evidence suggests that the origin of the hydrocarbons is mainly thermogenic derived from hydrothermal alteration of buried organic matter, supported by the presence of aromatics, S-bearing and an alkane CPI below 1. The results of this study increase the scarce knowledge of dissolved organic fraction in hydrothermal systems.

ATR-FTIR Spectral Analysis and Soluble Components of PM10 And PM2.5 Particulate Matter over the Urban Area of Palermo (Italy) during Normal Days and Saharan Events.

Several epidemiological studies have shown a close relationship between the mass of particulate matter (PM) and its effects on human health. This study reports the identification of inorganic and organic components by attenuated total reflectance-Fourier-transform infrared spectroscopy (ATR-FTIR) analysis in PM10 and PM2.5 filters collected from three air quality monitoring stations in the city of Palermo (Sicily, Italy) during non-Saharan dust events and Saharan events. It also provides information on the abundance and types of water-soluble species. ATR-FTIR analysis identified sulfate, ammonium, nitrate, and carbonate matter characterized by vibrational frequencies at 603, 615, 670, and 1100 cm–1 (SO42–); at 1414 cm–1 (NH4+); at 825 and 1356 cm–1 (NO3–); and at 713, 730, and 877 cm–1 (CO32–) in PM10 and PM2.5 filters. Moreover, aliphatic hydrocarbons were identified in the collected spectra. Stretching frequencies at 2950 cm–1 were assigned to CH3 aliphatic carbon stretching absorptions, while frequencies at 2924 and 2850 cm–1 indicated CH2 bonds. In filters collected during Saharan dust events, the analysis also showed the presence of absorbance peaks typical of clay minerals. The measurement of soluble components confirmed the presence of a geogenic component (marine spray and local rocks) and secondary particles ((NH4)2SO4, NH4NO3) in the PM filters. ATR-FTIR characterization of solid surfaces is a powerful analytical technique for identifying inorganic and organic compounds in samples of particulate matter.

Application of dispersive liquid-liquid microextraction and gas chromatography with mass spectrometry for the determination of oxygenated volatile organic compounds in effluents from the production of petroleum bitumen.

We present a new procedure for the determination of oxygenated volatile organic compounds in samples of postoxidative effluents from the production of petroleum bitumens using dispersive liquid-liquid microextraction and gas chromatography with mass spectrometry. The eight extraction parameters were optimized for 43 oxygenated volatile organic compounds. The detection limits obtained ranged from 0.07 to 0.82 μg/mL for most of the analytes, the precision was good (relative standard deviation below 2.91% at the 5 μg/mL level and 4.75% at the limit of quantification), the recoveries for the majority of compounds varied from 70.6 to 118.9%, and the linear range was wide, which demonstrates the usefulness of the procedure. The developed procedure was used for the determination of oxygenated volatile organic compounds in samples of raw postoxidative effluents and in effluents after chemical treatment. In total, 23 compounds at concentration levels from 0.37 to 32.95 μg/mL were identified in real samples. The same samples were also analyzed in the SCAN mode, which resulted in four more phenol derivatives being identified and tentatively determined. The studies demonstrated the need for monitoring volatile organic compounds content in effluents following various treatments due to the formation of secondary oxygenated volatile organic compounds.

Monoethanolamine Degradation during Pilot-Scale Post-combustion Capture of CO2 from a Brown Coal-Fired Power Station

The use of aqueous amines, such as monoethanolamine (MEA, 2-aminoethanol), for post-combustion capture (PCC) of CO2 from fossil-fuel-fired power station flue gases leads to undesirable reactions with oxygen, SOx, and NOx. This study has used a gas chromatography with mass spectrometry detection (GC–MS) method to measure the changes in concentrations of organic compounds in samples of a 30% (w/w) aqueous MEA absorbent obtained from CSIRO’s PCC pilot plant operating at AGL’s Loy Yang brown coal-fired power station in Latrobe Valley, Victoria, Australia. This aqueous MEA absorbent was previously used for more than 700 h of PCC, and the collected samples represent a further 834 h of PCC operation. These data provide a new perspective on the close, interdependent relationships between corrosion and amine degradation reactions. Other important outcomes include confirmation that (a) organic degradation products identified during laboratory-scale trials were also produced during pilot-scale PCC and (b) N-(2-hydro...

Occurrence and suitability of sucralose as an indicator compound of wastewater loading to surface waters in urbanized regions

Urban watersheds are susceptible to numerous pollutant sources and the identification of source-specific indicators can provide a beneficial tool in the identification and control of input loads, often times needed for a water body to achieve designated beneficial uses. Differentiation of wastewater flows from other urban wet weather flows is needed in order to more adequately address such environmental concerns as water body nutrient impairment and potable source water contamination. Anthropogenic compounds previously suggested as potential wastewater indicators include caffeine, carbamazepine, N,N-diethyl-meta-toluamide (DEET), gemfibrozil, primidone, sulfamethoxazole, and TCEP. This paper compares the suitability of a variety of anthropogenic compounds to sucralose, an artificial sweetener, as wastewater indicators by examining occurrence data for 85 trace organic compounds in samples of wastewater effluents, source waters with known wastewater point source inputs, and sources without known wastewater point source inputs. The findings statistically demonstrate the superior performance of sucralose as a potential indicator of domestic wastewater input in the U.S. While several compounds were detected in all of the wastewater effluent samples, only sucralose was consistently detected in the source waters with known wastewater discharges, absent in the sources without wastewater influence, and consistently present in septic samples. All of the other compounds were prone to either false negatives or false positives in the environment.

Target and Nontarget Screening of Organic Micropollutants in Water by Solid-Phase Microextraction Combined with Gas Chromatography/High-Resolution Time-of-Flight Mass Spectrometry

The potential of gas chromatography coupled to high-resolution time-of-flight mass spectrometry (GC/TOF-MS) for screening of organic pollutants in water has been explored. After optimization of the solid-phase microextraction (SPME) step, where parameters such as fiber selection and addition of salt were studied, this extraction technique was applied to the analysis of different types of water samples. Investigation of 60 target organic pollutants, including pesticides, octyl/nonyl phenols, pentachlorobenzene, and polycyclic aromatic hydrocarbons (PAHs) was carried out by evaluating the presence of up to five representative m/z ions per analyte, measured at high mass accuracy, and the attainment of their Q/q (Q, quantitative ion; q, confirmative ion) intensity ratio. This strategy led to the detection of 4-t-octylphenol, simazine, terbuthylazine, chlorpyrifos, terbumeton, and terbutryn in several water samples at low part-per-billion levels. Full spectrum acquisition data generated by the TOF-MS analyzer also allowed subsequent investigation of the presence of polybrominated diphenyl ethers and several fungicides in samples after MS data acquisition, without the need to reanalyze the water samples. In addition, nontarget analysis was also tested by application of a deconvolution software. Several organic pollutants that did not form a part of the list of contaminants investigated were identified in the water samples, thanks to the excellent sensitivity of TOF-MS in full spectrum acquisition mode and the valuable accurate mass information provided by instrument. Bisphenol A, the antioxidant 3,5-di-tert-butyl-4-hydroxy-toluene (BHT), its metabolite 3,5-di-tert-butyl-4-hydroxybenzaldehyde (BHT-CHO), the polycyclic musk galaxolide, and the UV filter benzophenone were some of the compounds present in the water samples analyzed. SPME in combination with GC/TOF-MS has been proved to be an attractive and powerful approach for the rapid screening of multiclass organic pollutants in water, with very little sample manipulation and no solvent consumption. This combination provides to the analyst with information-rich MS data that facilitates the reliable identification of many different organic compounds in samples.

A comparison of three solvent-free techniques coupled with gas chromatography for determining trihalomethanes in urine samples

The analysis of volatile organic compounds in samples of biological fluids characterized by complex matrices is highly challenging. This paper presents a comparison of the results obtained in this field using three solvent-free techniques: thin-layer headspace with autogenous generation of liquid sorbent (TLHS) and membrane separation of the trace substances (pervaporation, PV), both of which are coupled to direct aqueous injection gas chromatography-electron capture detection (TLHS-DAI-GC-ECD and PV-DAI-GC-ECD), as well as conventional static headspace analysis followed by GC analysis with ECD detection (HS-GC-ECD). Basic validation parameters of the HS-GC-ECD, TLHS-DAI-GC-ECD and PV-DAI-GC-ECD procedures were calculated for water and urine samples. The calibration curves for all procedures were linear within the concentration range examined. The intermediate precisions of the procedures were good and reached about 10% (for all analytes) for HS-GC-ECD and TLHS-DAI-GC-ECD. The poorest results were obtained for PV-DAI-GC-ECD: about 20% for all analytes. The lowest method detection limits were obtained for the TLHS-DAI-GC-ECD procedure: below 0.0022 microg/L for all analytes. The enrichment factors did not differ significantly between water and urine samples, indicating little or no matrix effect in all procedures.

Identification, characterization and quantitation of pyrogenic polycylic aromatic hydrocarbons and other organic compounds in tire fire products

On 15 August 2001, a tire fire took place at the Pneu Lavoie Facility in Gatineau, Quebec, in which 4000 to 6000 new and recycled tires were stored along with other potentially hazardous materials. Comprehensive gas chromatography–mass spectrometry (GC–MS) analyses were performed on the tire fire samples to facilitate detailed chemical composition characterization of toxic polycyclic aromatic hydrocarbons (PAHs) and other organic compounds in samples. It is found that significant amounts of PAHs, particularly the high-ring-number PAHs, were generated during the fire. In total, 165 PAH compounds including 13 isomers of molecular weight (MW) 302, 10 isomers of MW 278, 10 isomers of MW 276, 7 isomers of MW 252, 7 isomers of MW 228, and 8 isomers of MW 216 PAHs were positively identified in the tire fire wipe samples for the first time. Numerous S-, O-, and N-containing PAH compounds were also detected. The identification and characterization of the PAH isomers was mainly based on: (1) a positive match of mass spectral data of the PAH isomers with the NIST authentic mass spectra database; (2) a positive match of the GC retention indices ( I) of PAHs with authentic standards and with those reported in the literature; (3) agreement of the PAH elution order with the NIST (US National Institute of Standards and Technology) Standard Reference Material 1597 for complex mixture of PAHs from coal tar; (4) a positive match of the distribution patterns of PAH isomers in the SIM mode between the tire fire samples and the NIST Standard Reference Materials and well-characterized reference oils. Quantitation of target PAHs was done on the GC–MS in the selected ion monitoring (SIM) mode using the internal standard method. The relative response factors (RRF) for target PAHs were obtained from analyses of authentic PAH standard compounds. Alkylated PAH homologues were quantitated using straight baseline integration of each level of alkylation.

Organic compound tracers of fine soil and sand particles during summer in the metropolitan area of Riyadh, Saudi Arabia

Soil and sand fine particles, which may be resuspended as fine dust in the atmosphere, contain a variety of anthropogenic and natural organic components. Samples of fine soil and sand particles (sieved to <125 μM) were collected from the Riyadh area in the summer of 2003 and extracted with a mixture of dichloromethane and methanol (3:1, v:v). The derivatized total extracts were analyzed by gas chromatography–mass spectrometry in order to characterize the composition and sources of the organic components. Both anthropogenic and natural biogenic inputs were the major sources of the organic compounds in these extracts. Discarded plastics and vehicular emission products were the major anthropogenic sources in the fine particles from populated areas of the city. Their tracers were plasticizers, UCM, n-alkanes, hopanes and traces of steranes. Vegetation was the major natural source of organic compounds in samples from outside Riyadh and included n-alkanols, n-alkanoic acids, n-alkanes, methyl alkanoates, sterols and triterpenoids. Carbohydrates had high concentrations (42–54%) in all samples and indicate sources from decomposition of cellulose and/or the presence of viable microbiota such as bacteria and fungi. The results were also compared with the data obtained in winter 2002 and showed that anthropogenic inputs were higher in summer than in winter, whereas the opposite trend was observed for natural inputs.

Persistent halogenated compounds in black guillemots ( Cepphus grylle) from Greenland––levels, compound patterns and spatial trends

Twenty-seven black guillemot eggs and 39 livers were analysed for polychlorinated biphenyls (PCBs), chlorinated pesticides including chlordane-related compounds and toxaphene, and polybrominated diphenylethers (PBDEs). The samples were collected at Qeqertarsuaq (Godhavn, West Greenland) and Ittoqqortoormiit (Scoresbysund, East Greenland). The concentrations of halogenated organic compounds in samples from East Greenland were somewhat higher than the corresponding concentrations from West Greenland. Differences in compound patterns were found between West and East Greenland, with higher percentages of the heavier PCB molecules, p, p ′-DDE and α-HCH in the samples from Ittoqqortoormiit. Similarly, different levels and different compositions were observed for eggs and livers. The eggs had generally higher concentrations of all compounds as well as higher percentages of CHB-50, CHB-62 and α-HCH than liver samples from the same area. Dividing the liver samples into age groups revealed increasing concentrations with age.

Extractable organic matter in PM 10 from LiWan district of Guangzhou City, PR China

PM 10 (particulate matter with aerodynamic diameter <10 μm) samples were collected at LiWan District of Guangzhou, PR China during April and July 2001 using a high volume air sampler to determine the distributions of homologous compounds and biomarkers. Polycyclic aromatic hydrocarbons (PAHs) including non-alkylated PAHs, methyl-alkylated PAHs, and some PAHs containing S/O atoms and n-alkanes were measured using gas chromatography/mass spectrometry analysis. The Σ n-alkane and ΣPAHs ranged from 26.4 to 719.2 ng/m 3 and 7.4 to 159.4 ng/m 3, respectively. A seasonal fluctuation was clearly evident with higher concentrations occurring during the colder months (April). In addition, some compositional differences are observed for the organic compounds in samples collected from different heights above ground level. Higher sites had a significant contribution from vascular plant wax. The presence of petroleum products with no carbon number preference, pristane, phytane and a significant unresolved complex mixture (UCM) with unresolved to resolved components ratio (U/R) of 6.2–13.2 confirm the petroleum component. The relative distribution of n-alkanes and the values of molecular diagnostic ratio, such as carbon preference index (CPI) values ranging from 1.0 to 1.4 (for the whole range of n-alkanes), indicated the importance of petroleum and diesel residues and gasoline emissions, as well as the minor contribution of n-alkanes emitted directly from epicuticular waxes. Indeed, the percent contribution of leaf ‘wax’ n-alkanes (5.2–19.4%) indicated a low contribution of biogenic sources. The fossil fuel biomarkers, hopanes and steranes were observed in the PM 10 samples, which indicate a petroleum origin. The distribution pattern of PAHs was characteristic of anthropogenic emissions. Coupling carbon number maximum ( C max), CPI, U/R values, molecular marker and molecular diagnostic ratios for alkanes and PAHs revealed a classification of natural biogenic and anthropogenic components of atmospheric aerosols. These analyses support the conclusion that vehicular emission was the major source of organic compounds during the study period, while the contribution of epicuticular waxes emitted by terrestrial plants was minor.

A flow method with photocatalytic oxidation of dissolved organic matter using a solid-phase (TiO2) reactor followed by amperometric detection of consumed oxygen.

A photocatalytic sensor for the determination of chemical oxygen demand (COD) using titanium dioxide, based on the use of a pair of oxygen electrodes and flow injection analysis, is described. The measuring principle is based on the direct determination of the oxygen concentration change resulting from photocataltic oxidation of organic compounds. One of the two oxygen electrodes, the reference oxygen electrode, was utilized to measure the reference signal responding only to oxygen present in the injected samples. Oxygen consumption due to the TiO2-catalyzed photochemical oxidation of organic compounds in samples was monitored with the working oxygen electrode. The COD value of this sensor was calculated as the difference of the currents at reference and working oxygen electrodes, respectively. The operation characteristics of the sensor are demonstrated using artificially treated wastewater as a substrate. The sensor was also applied to the determination of COD in real water samples from dam reservoirs (n = 20) all over Japan. The results were in good agreement with those from the conventional COD methods (i.e., permanganate and dichromate methods).

Determination of organic molecular markers in marine aerosols and sediments: one-step flash chromatography compound class fractionation and capillary gas chromatographic analysis

n-Alkanes, polycyclic aromatic hydrocarbons (PAHs), n-alkanals, n-alkan-2-ones, long-chain alkenones, oxy-PAHs, n-alkanols, sterols and free alkanoic acids have been reliably determined in various environmental matrices by using one-step flash chromatography compound class fractionation and subsequent gas chromatography (GC)–MS and/or GC–flame ionization detection analysis. The obtained recoveries, for the whole analytical procedure were 62–98% for n-alkanes, 60–100% for PAHs, 75% for n-alkanals, 96–100% for nitroarenes, 65–73% for oxy-PAHs, 88–98% for n-alkanols, 80–89% for steroidal alcohols and 98% for alkanoic acids. This analytical protocol has been applied, and proved suitable, for the determination of more than 170 organic compounds in samples, such as aerosols and marine sediments, collected during a 1 year period in a coastal environment of Eastern Mediterranean.

Occurrence and determination of organic pollutants in tap and surface waters of the Gdańsk district

The results of the determination of different classes of organic pollutants (trihalomethanes, polycyclic aromatic hydrocarbons (PAHs), pesticides, volatile hydrocarbons and phenols) in ground water and drinking water sampled at different sites in the Gdańsk District are presented. Different methods were used to determine organic compounds in water: direct aqueous injection or sorption on solid sorbent (XAD-4) and extraction with pentane followed by a gas chromatography-electron capture detection (GC-ECD) for determination of volatile organohalogen compounds; purge and trap with a gas chromatography-flame ionization detection for determination of volatile hydrocarbons; sorption on solid sorbent XAD-4 or C 18, elution with organic solvent and GC-ECD for determination of pesticides; sorption on solid sorbent C 18, elution with organic solvent and a gas chromatography-mass spectrometry for determination of PAHs and sorption on solid sorbent C 18, elution with organic solvent and high performance liquid chromatography-ultraviolet determination of phenols. The proposed methods have been used successfully for the determination of organic compounds in samples of tap, swimming pool, river and sea waters. The content of analysed organic compounds in the water delivered to the water supply system for Gdańsk from the surface water intake in Straszyn and from other selected intakes satisfies Polish, EC and WHO standards. The surface water bodies, i.e., rivers, sea and lakes in the Gdańsk District are not very polluted by anthropogenic compounds originating from municipal and industrial sewages and from agriculture.

Treatment of Recalcitrant Organic Compounds in Oil Reclaiming Wastewater by Ozone/Hydrogen Peroxide and UV/Titanium Dioxide

Refractory organic compounds in samples of a biologically treated oil reclaiming wastewater with a COD of about 300 mg/l were removed by ozonation with or without hydrogen peroxide or by irradiation with UV light in the presence of titanium dioxide in laboratory scale batch experiments. A 5 hrs UV/TiO2 treatment at pH 3 (energy consumption: about 0.9 kWh/l) resulted in a COD elimination of 70% and an enhanced biodegradability of the organic wastewater constituents. Ozonation was hardly affected by hydrogen peroxide, and a treatment for 47 min (energy consumption about 0.005 kWh/l) led to a COD reduction of about 17% and also to an improved biodegradability. Both processes are suitable for further treatment of biologically oxidized oil reclaiming wastewaters.

Cathodic stripping voltammetry of glutathione in natural waters

The electrochemistry of metal–glutathione complexes at a hanging mercury drop electrode was investigated and a procedure was optimized to determine glutathione in natural waters using cathodic stripping voltammetry (CSV). Adsorption of complexes of mercury(I) and copper(I) with glutathione can be utilized to preconcentrate the glutathione on the electrode prior to a voltammetric scan. The greatest sensitivity was obtained when the voltammetric scan was preceded by adsorptive deposition of a 1:1 complex of copper(I) with glutathione in the presence of added copper(II). The reduction current of the copper(II) in the adsorbed glutathione complex was utilized to quantify the concentration of glutathione in solution. The sensitivity of the CSV determination of glutathione in sea-water was high at pH 5 and 8.5 with optimum copper concentrations of 1 and 0.2 µmol l–1, respectively. Adsorption was carried out at –0.225 V at pH 5 and –0.250 V at pH 8.5. The limit of detection was 1.2 nmol l–1 of glutathione at pH 8.5 using a deposition time of 60 s. Interference by surface-active organic compounds in samples from the Mersey estuary was minimized by passing the samples through a Sep-Pak C18 cartridge; interference by copper complexing compounds was minimized by omitting the addition of copper to the sample. A preliminary study of the Mersey estuary using the proposed procedure indicated the presence of glutathione at levels between 20 and 300 nmol l–1; the glutathione behaved conservatively at salinities between 23 and 33 [of the practical salinity scale (psu)], decreasing with increasing salinity.