Sample preparation methods Aqueous samples

As well as typical sample preparation methods such as filtration and liquid-liquid extraction, newer developments are now extensively used. The first of these is solid-phase extraction (SPE). This is a rapid, economical, and sensitive technique that uses several different types of cartridges and disks, with a variety of sorbents. Sample preparation and concentration can be achieved in a single step. Interfering sugars can be eluted with aqueous methanol on reversed-phase columns prior to elution of flavonoids with methanol. 

Despite the advantages of an equilibrium, nonexhaustive extraction procedure, there are also disadvantages. Matrix effects can be a major disadvantage of a sample preparation method that is based on equilibration rather than exhaustive extraction [134], Changes in the sample matrix may affect quantitative results, due to alteration of the value of the distribution constant relative to that obtained in a pure aqueous sample [68,134],... 

Solid Phase MicroExtraction (SPME) is a solvent-free sample preparation method based on the adsorption of analytes directly from an aqueous sample onto a coated fused-silica fiber. Headspace SPME was used in combination with gas chromatography-mass spectrometry/ selective ion monitoring (GC/MS-SIM) to analyze for TCA in wine. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

In the application of atomic emission spectroscopy for quantitative analysis, samples must be prepared in liquid form of a suitable solvent unless it is already presented in that form. The exceptions are solids where samples can be analysed as received using rapid heating electro-thermal excitation sources, such as graphite furnace heating or laser ablation methods. Aqueous samples, e.g. domestic water, boiler water, natural spring, wines, beers and urines, can be analysed for toxic and non-toxic metals as received with... 

The concentration of metals that are detrimental to catalysts added can vary between 20.0 ppm for Fe to 100 ppm for Ni and lOOOppm for V. The presence of these metals necessitates the need for analysis of these metals to determine their concentrations prior to the cracking process. The best method to analyse these oil samples needs to be rapid and accurate. Careful selection of the method either from experience or by trial and error may be applied depending on the metal and the concentration. Sample dissolution in a solvent or solvent mixture is considered the easiest but may not be suitable for low limits of detection. Destructive sample preparation methods, i.e. oxygen bomb combustion, microwave acid digestion followed by pre-concentrating may be required for trace analysis and/or with the aid of a hyphenated system, e.g. ultrasonic nebuliser. Samples prepared by destmctive methods are dissolved in aqueous solutions that have very low matrix and spectral interferences. 

Alanine is an amino add, which in this example is in the form of an aqueous solution in hydrochloric acid as a result of the sample preparation method. For this reason, a TLC silica gel (O WF254S precoated plate (Merck 16485) was used for the chromatography. After development in a ninhydrin-containing solvent, alanine is converted into a compound visible in daylight. The conditions for the assay are given below ... 

Flame Photometric Detector (PFPD) or an ICP-MS. The various sample treatment steps, liberation of the compounds, their derivatization and preconcentration via headspace on to a SPME phase, all occur in the same vial, hmiting contamination and loss risks. SPME is a solvent-free sample preparation method in which a fused-sUica fiber coated with a polymeric organic stationary phase is used to extract organic compounds directly from aqueous or gaseous samples.Further GC separation of the compounds and MS, FPD, or ICP-MS detection allows very sensitive determinations. This method will be further referred to as the SPME method. 

In this section, we will introduce in general the extraction and clean-up techniques used for EDCs in aqueous, solid, and biological samples. Only those commonly used sample preparation methods will be discussed in the following. 

This high and reproducible percent recovery for the isolation and recovery of lindane from water strongly suggests that RP-SPE is very appropriate as a sample preparation method for this analyte. Lindane is of continued interest to environmental and toxicological scientists. One such study, discussed next, taken from the author s work, involves the isolation and recovery of lindane from homogenized myometrial tissue suspended in an aqueous matrix. 

Sample preparation methods without any loss of the target compounds are important, especially when a quantitative assessment of all alkaloids in a given plant sample is required. Eor many alkaloids, the sample preparation procedure is based on the difference in the solubility of the alkaloids salt in comparison with the free bases. An acidic aqueous extraction must be basified to transfer alkaloids into the free base for extracting into organic solvents nonmiscible with water. Excessive use of alkali, however, is detrimental for the ester alkaloids. 

The finishing of liquid detergents and cleansers typically employs short-chain alkylbenzene sulfonates such as toluene sulfonate or cumene sulfonate, which, because of their hydrotropic properties, ensure the solubility of other detergent components in an aqueous environment. Analysis of hydrotropic compounds is performed by ion-pair chromatography. The compounds are eluted in the order of increasing alkyl substitution. As demonstrated in Fig. 9-120, these compounds can be determined directly in the finished product without extensive sample preparation. The analyte samples are only diluted with de-ionized water and membrane filtered. As aromatic sulfonates may be detected both by their electrical conductivity and their UV absorption, the choice of suitable detection method mainly depends on the type of matrix. 

Ghosh and co-workers [42, 69] also described a dye partition method for the determination of hydroxyl end groups in PMMA samples prepared in aqueous... 

Perhaps the earliest contribution to the control of microdomain orientation via interface control was the experiments of Mansky et al. [5], who demonstrated that cylindrical microdomains could be oriented perpendicular to a film by solvent casting on an aqueous surface. It was also demonstrated here that ozone could be used to selectively remove PB microdomains from a PS matrix, thereby enabling subsequent research in lithography. However, the uniformity of this sample preparation method was highly lacking - only certain randomly distributed regions of samples prepared in this manner yielded perpendicularly orinted cylinders. 

Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used flame atomization and electrothermal atomization. A few elements are atomized using other methods. 

The spherical fine nickel powders have been prepared fiom aqueous NiCU and hydrazine hydrate at various temperatures wife ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased wife inrareasing fee reaction temperature in both fee method, but was relatively shorter in fee ultrasonic mefeod. Compared to the conventional one, the surface morphology and particle size of fee sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by fee ultrasonic mefeod was relatively higher than feat obtained by fee conventional one. 

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