Humic acids, sample preparation

The preparation of n-alkylammonium organocom-plexes is described elsewhere [16], The amounts of alkylammonium salts added to the humate solution were equivalent to the acidic groups (carboxyl and phenolic hydroxyl) of the humic acid sample. 

Humic acido from ooilo and ligniteo have been examined by ERR spectrometry. All samples showed a stable free organic radical content of about 1018 spins per gram. When these samples were converted to their sodium salts, a marked increase in radical content occurred. This was interpreted to indicate that a quinhydrone moiety exists in the humic acid macromolecule. Synthetic humic acid, prepared by oxidizing catechol in the presence of amino acids, also showed similar ERR spectra, as did selected quinhydrone model compounds. The radical moiety appeared to be stable to severe oxidation and hydrolytic conditions. Reduction in basic media caused an initial decrease in radical species continued reduction generated new radical species. A proposed model for humic acid based on a hydroxyquinone structure is proposed. 

Sodium Salts. The sodium salts of all samples were prepared by dissolving a given weight of humic acid in IN aqueous sodium hydroxide. Adding a tenfold excess of absolute ethanol to the basic solution precipitated the salt which was filtered, washed with ethanol, and dried at room temperature in... 

Figure 16.37. Typical examples of fluorescence spectra of humic acids from Brazilian soil (Hapludox) under different tillage systems. The samples were prepared in aqueous solutions (20 mg liter-1, pH 8). (a) Fluorescence emission (Xexc = 240nm). (b) Fluorescence synchronous-scan excitation spectra (AX = 55 nm). (c) Fluorescence excitation spectra (Xem = 517 nm). (d) Fluorescence emission spectra (Xoxc = 465 nm).

Nine different chlorophenoxyalkanoic acids and esters (Pestanal grade, > 99%, Riedel-De-Haen AG Seelze-Hanover, West Germany) (Table 1) and 10 humic acid (HA) samples of different origin and nature, have been examined in this study. Traditional procedures of extraction, fractionation, and purification were used for the isolation of the humic acids of natural origin (soil, peat, coal, anlaboratory-prepared by oxidative... 

Sample Preparation of Foam and Water Samples and Humic Substances Isolation. All foam and water samples were filtered through 0.45- Lim silver filter using stainless-steel filtration units. Silver filtration of Como Creek and Suwannee River foam samples resulted in build up of a brown extract on the filter paper, which was readily solubilized in 0.1 N sodium hydroxide. This extract was refiltered through silver filters as a sodium hydroxide solution. This fraction was believed to be colloidal in nature and was treated as a separate humic fraction, called the "foam-extract" fraction. A part of the filtered foam was freeze dried directly and considered "raw" foam. Fulvic and humic acids were isolated from foam and stream-water samples via the XAD-8 adsorption technique developed by Thurman and Malcolm (77), freeze dried, and weighed. To obtain a sufficient mass of humic substances, each entire sample was used for one extraction. As multiple samples were not extracted, calculation of the error associated with humic substances isolation cannot be made, and the contributions of humic substances to the DOC content must be regarded as estimates. 

Polyvinylpyrrolidone (PVP) is suited for the preparation of samples that contain humic acids, lignins, tannins, and azo dyes. In addition, PVP exhibits a high selectivity for phenolic compounds as well as for aromatic carboxylic acids and aldehydes. PVP is stable to all common HPLC solvents and is stable in the pH range between 1 and 10. 

Nitrogen to carbon ratios in humic acids are generally slightly higher than N/C ratios in fulvic acids. In stable residues, most of the nitrogen occurs in the hydrolyzable fraction and is lost during sample preparation. Therefore, the N/C ratios of stable residues are always considerably less than the N/C ratios of humic acids. 

All the samples of the Fenland H -exchanged humic acids were completely dissolved in the solvent systems used in Table 8. The data show that the absorbance value at 465 nm for the sodium hydroxide solution was greater than that for the same concentrations of solutes in the DMSO preparations. Although DMSO was stored over calcium hydride to prevent excessive water uptake no attempt was made to fully dry the solvent, and it is assumed that it contained some water. Additional water (up to about 4% of the total volume) did not significantly alter the solution properties of the DMSO. Addition of concentrated hydrochloric acid, even to the extent of only 0.4% of the total volume of the mixture, significantly altered the absorbances and the EjEf, ratios. 

Table II lists two values for Kads at lOOZ SW. The bracketed value is that evaluated using a humic acid extracted from estuarine sediments. The sedimentary sample was obtained from the Chesapeake Bay near the town of Bivalve (BV). Because the values of Kads were so similar for the estuarine humic acid and the commercially prepared soil humic acid, use of the estuarine sample was discontinued and the soil humic acid was used consistently thoughout the study.

Adsorption of Cd on suspended humic acids. Recognizing the property of humic acids as adsorbents for metal ions, suspensions of humic acid (HAL, HAM and commercial "Fluka" samples) were prepared at egging dm . Cd was added to that suspension and the adsorption of... 

A new selective enrichment technique was investigated for the sample preparation for GC-MS analysis of 16 acidic herbicides in water. By using a dynamic ion-exchange solid-phase extraction (DIE-SPE) combined with reverse-phase SPE, interference by humic substances could be reduced and most of the acidic herbicides were extracted with recoveries above 70%. ... 

Regardless of the source of a PCR product, e.g., human, plant, or microbe, sample preparation is identical and the final product is an MS or MS/MS spectrum. Certain complex samples, e.g., environmental samples, contain inhibitors of the PCR reaction (e.g., humic acids and metal ions) and developments in this area are continuing which are beyond the scope of this article. The examples below are only provided to illustrate what can be achieved with PCR-MS or MS/MS analysis. 

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