Water sample preparation

Because hydrocarbons, especially n-alkanes, in aqueous solution are susceptible to microbiological degradation, surrogate samples were used in the interlaboratory comparison. All participants were requested to prepare water samples from the solutions listed in Table 1 according to... 

Preparation of soil—sediment of water samples for herbicide analysis generally has consisted of solvent extraction of the sample, followed by cleanup of the extract through Uquid—Uquid or column chromatography, and finally, concentration through evaporation (285). This complex but necessary series of procedures is time-consuming and is responsible for the high cost of herbicide analyses. The advent of soUd-phase extraction techniques in which the sample is simultaneously cleaned up and concentrated has condensed these steps and thus gready simplified sample preparation (286). 

Color. Many water samples have a yellow to brownish-yeUow color which is caused by natural substances, eg, leaves, bark, humus, and peat material. Turbidity in a sample can make the measurement of color uncertain and is usually removed by centrifiigation prior to analysis. The color is usually measured by comparison of the sample with known concentrations of colored solutions. A platinum—cobalt solution is used as the standard, and the unit of color is that produced by 1 mg/L platinum as chloroplatinate ion. The standard is prepared from potassium chloroplatinate (K PtCl ) and cobalt chloride (C0CI26H2O). The sample may also be compared to suitably caUbrated special glass color disks. 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

A student prepares a sample of hydrogen gas by electrolyzing water at 25°C. She collects 152 mL of H2 at a total pressure of758 mm Hg. Using Appendix 1 to find the vapor pressure of water, calculate... 

The basic technology for the preparation of sample material is similar in all TLC preparations, irrespective of the origin of the hpid and specific preparation method for a variety of biological samples [43]. The most important factor is the solubihty of the sample. The lipid sample must be completely soluble in the dissolving solvent prior to the application and must be free from water. Either toluene or chloroform is commonly used as the solvent to dissolve hpid materials. The dissolving solvent should be nonpolar in namre and volatile at such a concentration that the hpid components in the sample are completely adsorbed throughout the entire thickness of the layer as quickly as possible. Although sample sizes as small as 1 to 10 pi can... 

Samples generally should be prepared or at least extracted immediately after collection or after arrival in the laboratory. If it is not possible for water samples to be prepared or extracted immediately, they should be stored at 5 °C until analysis, in order that no transformation or degradation products occur. 

Plant material. Weigh 25 g of the chopped and frozen sample into a blender jar. To confirm recoveries, prepare fortiflcation samples by spiking the matrix with the appropriate volume of metabolite standard. Add 200 mL of acetonitrile-water (4 1, v/v) solution to the jar, and blend the mixture at medium speed for 5 min. Filter the extract through a Buchner funnel fitted with a glass-fiber filter pad into a 500-mL round-bottom flask containing 10 drops of Antifoam B and 3mL of 10% aqueous Igepal CO-660 (nonionic surfactant). The flask is connected to the Buchner funnel by... 

Experience has shown a linear range from 5 to 1000 trgL with the listed instrumentation. This is equivalent to 0.05-5.0 qgL in a 200-mL water sample. The extended standards are only prepared and used when the analytes exceed approximately 6.00 qgL- (120% of 5 qgL-i). 

In order to estimate the analytical accuracy of the method within a given set of water samples, a number of control water samples should be fortified with a known amount of each metabolite. Control water samples are fortified at different analyte levels across the range of anticipated concentrations. The aqueous solutions used to fortify control water samples are prepared at 0.10, 0.25, 0.50,1.0,2.0, 5.0, 10.0, and 20pgL-i. 

Samples are generally prepared and analyzed in sets of 30 that include at least one control and one fortified control water sample. Optima-grade bottled water may be used as the matrix for the controls and the laboratory-fortified samples for all water types. Depending on the appearance of the samples, filtration may be required. 

Table 3 Summary of solid-phase extraction techniques applied to the preparation of water samples for the determination of triazine pesticides...

Supercritical fluid extraction (SFE) is generally used for the extraction of selected analytes from solid sample matrices, but applications have been reported for aqueous samples. In one study, recoveries of 87-100% were obtained for simazine, propazine, and trietazine at the 0.05 ug mL concentration level using methanol-modified CO2 (10%, v/v) to extract the analytes, previously preconcentrated on a C-18 Empore extraction disk. The analysis was performed using LC/UV detection. Freeze-dried water samples were subjected to SFE for atrazine and simazine, and the optimum recoveries were obtained using the mildest conditions studied (50 °C, 20 MPa, and 30 mL of CO2). In some cases when using LEE and LC analysis, co-extracted humic substances created interference for the more polar metabolites when compared with SFE for the preparation of the same water sample. ... 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

The extracts of plant, soil and water samples, if necessary, should be purified with the following method prior to methylation Dissolve the residue prepared as in Section 6.1.2 or 6.2.1 in 5 mL of ethyl acetate and transfer the solution into a silica gel mini column. Rinse the column with 15 mL of ethyl acetate. Allow the solution to percolate through the column and discard the eluate. Collect the bispyribac in a 50-mL round-bottom flask with 20 mL of methanol-ethyl acetate (3 7, v/v). Evaporate the eluate to dryness under pressure. 

Wheat samples should be sampled and prepared for analysis according to the general instructions provided in the Pesticide Analytical Manual , Vol. 1. Soil samples should be prepared for analysis by homogenization with a hammer mill or knife mill. Water samples are used without sample preparation. 

All crop samples should be prepared with a food chopper or Wiley mill to achieve a finely divided material. Soil and water samples should be well mixed to ensure a homogeneous sample. 

Sample preparation techniques vary depending on the analyte and the matrix. An advantage of immunoassays is that less sample preparation is often needed prior to analysis. Because the ELISA is conducted in an aqueous system, aqueous samples such as groundwater may be analyzed directly in the immunoassay or following dilution in a buffer solution. For soil, plant material or complex water samples (e.g., sewage effluent), the analyte must be extracted from the matrix. The extraction method must meet performance criteria such as recovery, reproducibility and ruggedness, and ultimately the analyte must be in a solution that is aqueous or in a water-miscible solvent. For chemical analytes such as pesticides, a simple extraction with methanol may be suitable. At the other extreme, multiple extractions, column cleanup and finally solvent exchange may be necessary to extract the analyte into a solution that is free of matrix interference. 

Preparation and instrumental analysis of agrochemical residues in water samples... 

From a technical standpoint, this article emphasizes recent advances in sample preparation and instrumentation. A brief history of modern sample preparation techniques is covered, together with the impact of modern instrumentation on water sample analysis. 

This section discusses treatment of the water samples in preparation for instrumental analysis after they have been received, archived and stored in the laboratory. Many approaches may be taken in preparation of water samples for final analysis. The techniques employed will depend upon the type of matrix, e.g., groundwater vs surface water (containing organic materials), the instrumental method and the required detection limits. 

In the last several years, on-line extraction systems have become a popular way to deal with the analysis of large numbers of water samples. Vacuum manifolds and computerized SPE stations were all considered to be off-line systems, i.e., the tubes had to be placed in the system rack and the sample eluate collected in a test-tube or other appropriate vessel. Then, the eluted sample had to be collected and the extract concentrated and eventually transferred to an autosampler vial for instrumental analyses. Robotics systems were designed to aid in these steps of sample preparation, but some manual sample manipulation was still required. Operation and programming of the robotic system could be cumbersome and time consuming when changing methods. 

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