Concentration procedures

The original procedure from which the Formalinethyl acetate centrifugal sedimentation technique was adapted was the Formalin-ether concentration method of Ritchie. Hie Formalin-ethyl acetate procedure avoids problems with the flammability and storage of ether. This procedure can be performed on specimens which have been fixed in Formalin for a time or on specimens with Formalin added during the processing. The procedure can also be performed on material fixed in MIF. 

The procedure with Formalin-preserved specimens is as follows. 

Decant the supernatant, and wash it again with tap water, if desired. 

Add 4 ml of ethyl acetate, stopper the tube, and shake the tube vigorously in an inverted position for 30 s. Remove the stopper with care. 

Centrifuge the solution at 450 to 500 x g for 1 min. Four layers should result ethyl acetate, plug of debris, Formalin, and sediment. 

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Consideration must be given to the possibility of losses occurring during the concentration procedure for example, boric acid, halogen acids and nitric acid are lost from boiling aqueous solutions. 

Aqueous solutions may sometimes be analysed directly without any pretreatment, but it is a matter of chance that the given solution should contain the correct amount of material to give a satisfactory absorbance reading. If the existing concentration of the element to be determined is too high then the solution must be diluted quantitatively before commencing the absorption measurements. Conversely, if the concentration of the metal in the test solution is too low, then a concentration procedure must be carried out by one of the methods outlined at the end of this section. 

Sample preparation will involve extraction techniques, concentration procedures and methods for removing solid contaminants that may damage the sample valve, column or some other part of the... 

This sample preparation involved, firstly, an extraction and the elimination of the solid matrix by filtration and, secondly, a concentration procedure employing a solid phase extraction cartridge. The compounds of interest were separated solely by dispersive interactions with the reversed phase. In the example given, the corn meal was spiked with the aflatoxins. 

Determination of caffeine in soft drinks was undertaken using the aerosol alkali flame ionization detector.24 Soft drinks studied were Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew. A sample clean-up and concentration procedure is employed followed by GC separation with alkali flame ionization detection. Results showed that Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew contained 41 2, 52 2, 43 4, 35 9, 46 6, and 60 15 mg caffeine per 355-ml serving. These values compared favorably with levels reported in the literature. 

Comparison of results using concentration and normalized absorbance for the experimental data demonstrates the effectiveness of this approach. The concentration procedure results in k1 = 1.85/min and k = 0.35 A/mol/min. The normalized absorbance procedure results in k. = 1.94/min and k of 0.33 A/mol/min. This demonstrates the ability of the normalization procedure to accurately determine the rate constants from the shape of the curve. The maximum isocyanate absorbance calculated by the normalization procedure,. 712 M, agrees very well with the experimentally determined value of. 692 M. 

The three principal microscopic examinations performed on stool specimens are direct wet mount, wet mount after concentration, and permanent stain. Although each examination can contribute to diagnosis, the yield of some methods is small with certain kinds of specimens. As a minimum, formed specimens should be examined by a concentration procedure. Soft specimens should be examined by concentration and permanent stain, and, if submitted fresh, by direct wet mount. Loose and watery specimens should be examined by wet mount and permanent stain. If specimens are received in fixative and the consistency is not known, concentration and permanent stain should be performed. Other examinations may be helpful. Special procedures which may assist in the diagnosis of specific parasites are noted below in discussions of the parasites. 

Concentration procedures have been described for feces preserved in MIF, sodium acetate-Formalin, or PVA fixative. MIF- or sodium acetate-Formalin-fixed feces may be used in place of Formalin-fixed feces in the Formalin-ethyl acetate concentration procedure. Some workers feel that organisms do not concentrate as well from material... 

The following procedure is useful for staining Nocardia species as well as Cryptosporidium species. This procedure may be used on fresh or Formalinfixed material or on material from concentration procedures. If the specimen is liquid, centrifuge it, and use the sediment to prepare a smear. 

The three most widely used methods are membrane filter, saponin lysis, and Knotts concentration. Procedures for the first two methods... 

Early work was based on concentrating the radium from the seawater sample by adding barium and coprecipitating with barium sulfate. This concentration procedure has been replaced by one involving the extraction of radium from seawater on acrylic fibre coated with manganese dioxide [19,20] (Mn fibres). By use of this technique, volumes of 200-2000 litres may be sampled routinely. 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Even though a sample may be dissolved in a liquid, it may not be ready for the method chosen. There may be other sample components present that may interfere, or the solvent may not be suitable for the chosen method. Thus original liquid solutions and the extracts and solutions resulting from the procedures described in Section 2.5 may require separation or dilution-concentration procedures. 

Caution Although tert-butyl perbenzoate is one of the safest peresters/peroxides to handle, one should remain aware of the inherent shock sensitivity and instability of these compounds. Users should exercise appropriate caution during concentration procedures. 

This method assumes that the response factor is constant over a range of concentrations and it is often more acceptable to determine the response factor for a range of test concentrations. In this method, a calibration curve is produced by incorporating a fixed amount of the internal standard in samples that contain known amounts of the test compound. For each concentration the ratio of peak heights is determined and plotted against concentration (Procedure 3.2). For quantitation of a test sample, the same amount of the internal standard is introduced in its usual way and the ratio of peak heights for the standard and unknown is used to determine the concentration of the unknown from the calibration curve. 

Trace hydrocarbons that may be regarded as contaminants may be determined by the gas chromatographic methods already discussed. Heavier hydrocarbons in small amounts may not be removed completely from the column. If accurate information is required about the nature and amount of heavy ends, temperature programming or a concentration procedure may be used. 

This simple demonstration shows that coulometric measurements are often not valid when determining trace or ultra-trace amounts of analyte, and some form of pre-concentration procedure needs to be employed in order to increase Camlyte to a more reasonable level. Stripping is one means of achieving such pre-concentration. 

The extraction of technetiiun by different organic solvents is used in numerous separation and concentration procedures. Technetium is extracted as pertechnetate or, in lower oxidation states, as a complex compound. TcO can be extracted by the following main types of reactions ... 

Combine all elution buffer fractions and dialyze against several changes of PBS until the majority of the hapten has been removed (see Note 14 and Chapter 2 for concentration procedure). 

To exploit the possibility of rapid multi element determinations by modem sequential ICP instruments the pre-concentration procedure must be carried out off-line. Knapp et al. [4i] have developed a microprocessor-controlled preconcentration instrument that performs various preconcentration regimes. In order to demonstrate the options available, trace elements have been concentrated in brines and in standard reference... 

The mass spectrometer GC detector has a high degree of compatibility with the purge and trap technique, and GC/MS has been employed widely with this isolation and concentration procedure for the analysis of volatile organic compounds. The mass spectrometer is strongly recommended for samples where there is a possibility of unexpected compounds, and for broad spectrum analyses f 6J of poorly defined samples. 

The following draft guidelines are being developed by the OECD TG 433 (Acute Inhalation Toxicity-Fixed Concentration Procedure), TG 434 (Acute Dermal Toxicity-Fixed Dose Procedure),... 

FEVj 3-5 min after administration of each serial concentration procedure is complete when there is a >20% reduction in FEV, compared to baseline (positive response) or when 5 inhalations have been administered at each concentration and FEVj has been reduced by < 14% (negative response)... 

S. Saracoglu, M. Soylak, D. S. Kacar-Peker, L. Elci, W. N. L. dos-Santos, V. A. Lemos and S. L. C. Ferreira, A pre-concentration procedure using coprecipitation for determination of lead and iron in several samples... 

A. P. Lindomar, H. S. Ferreira, W. N. L. Dos-Santos and S. L. C. Fereira, Simultaneuous pre-concentration procedure for the determination of cadmium and lead in drinking water employing sequential multielement flame atomic absorption spectrometry, Microchem. J., 87(1), 2007, 77-80. 

Doehlert matrix design to study an on-line pre-concentration procedure... 

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