Aliquot sampling technique

The ninhydrin reaction (see Basic Protocol 1), the TNBS reaction (see Alternate Protocol 1), the fluorescamine reaction (see Alternate Protocol 2), and formol titration (see Alternate Protocol 3) all evaluate released amino groups by comparing the amounts of free amino groups before and after hydrolysis. The first three methods are spectro-photometric techniques, whereas the fourth is a potentiometric technique. The first and second are chromogenic techniques, whereas the third is fluorometric. These techniques are usually performed as time-course experiments. As the hydrolysis reaction proceeds, aliquots (samples) of the reaction are taken periodically and treated with a test reagent. Products of this reaction are proportional to the amount of free amino groups at each time point. 

Experimental studies, particularly those involving the metabolism or mode of action of toxic compounds in animals (or, less often, plants), can be conducted either in vivo or in vitro. Because organisms or enzyme preparations are treated with known compounds, the question of random sampling techniques does not arise as it does with environmental samples. Enough replication is needed for statistical verification of significance, and it should always be borne in mind that repeated determinations carried out on aliquots of the same preparation do not represent replication of the experiment at best, they test the reproducibility of the analytical method. 

Once grains have been selected for analysis and measured for a ejection correction, a (U-Th)/He age determination involves He extraction and measurement, and U and Th measurement on either the same or a separate aliquot. Analytical techniques, especially for the small amounts of He obtained from most samples, have evolved rapidly in the last five years, and so are described in some detail here. 

The different headspace sampling techniques can be classified into one-step procedures, such as static headspace, where an aliquot of the vapor phase is transferred in a closed container directly to the gas chromatograph, and two-step procedures, where the volatile analytes are transferred from the matrix of the headspace to a "trap" where they are released... 

In this set of experiments track only the disappearance of cydohexyl bromide, and not the appearance of cyclohexene. As mentioned previously, the amount of cyclohexene observed in successive aliquots tends to drop below the predicted values because of the sampling techniques employed. As the reaction progresses, however, the amount of cydohexyl bromide continues to decrease at the predicted rate (see below). 

Three related methods based on the quasiisostatic method are used to measure permeability. The most commonly used technique allows the permeant gas or vapor to flow continuously through one chamber of the permeability cell. The gas or vapor permeates through the sample and is accumulated in the lower-concentration chamber. At predetermined time intervals, aliquots are withdrawn from the lower cell chamber for analysis. The total quantity of accumulated permeant is then determined and plotted as a function of time. The slope of the linear portion of the transmission-rate profile is related to the sample s permeability. 

Some preliminary laboratory work is in order, if the information is not otherwise known. First, we ask what the time scale of the reaction is surely our approach will be different if the reaction reaches completion in 10 ms, 10 s, 10 min, or 10 h. Then, one must consider what quantitative analytical techniques can be used to monitor it progress. Sometimes individual samples, either withdrawn aliquots or individual ampoules, are taken. More often a nondestructive analysis is performed, the progress of the reaction being monitored continuously or intermittently by a technique such as ultraviolet-visible spectrophotometry or nuclear magnetic resonance. The fact that both reactants and products might contribute to the instrument reading will not prove to be a problem, as explained in the next chapter. 

Radioactivity Analysis. Samples of urine, feces, and tissues were combusted to COo and analyzed for radioactivity (5). By using this method the recovery of radioactivity from samples spiked with C was 95 dt 5%. To determine the radioactivity expired as CO2, 5-ml aliquots of the solution used to trap the CO2 were added to 15 ml of a scintillation counting solution containing 4 grams 2,5-diphenyloxazole (PPO) and 0.1 grams l,4-bis-2(5-phenyloxazolyl)-benzene (POPOP) per liter of 1 1 toluene 2-methoxyethanol. Samples were counted for radioactivity in a Nuclear Chicago Mark II liquid scintillation counter. Counting eflSciency was corrected by the internal standard technique. 

This technique is used mainly for nonpolar compounds. Typically a small aliquot of soil (10-30 g) is dried by mixing with sodium sulfate prior to extraction. Next, the sample is extracted with a solvent for 10-20 min using a sonicator probe. The choice of solvent depends on the polarity of the parent compound. The ultrasonic power supply converts a 50/60-Hz voltage to high-frequency 20-kHz electric energy that is ultimately converted into mechanical vibrations. The vibrations are intensified by a sonic horn (probe) and thereby disrupt the soil matrix. The residues are released from soil and dissolved in the solvent. 

This is a relatively new technique that is used for PCBs and other nonpolar, volatile and semi-volatile organic compounds. Typically, a small aliquot of soil sample (0.5-20 g) is used for the extraction. Soil samples are extracted with one or more organic solvents using microwave energy at elevated temperature (100-115 °C) and pressure (50-175 psi). This method uses less solvent and takes significantly less time than Soxhlet extraction but is limited to thermally stable compounds. 

The most common final separation techniques used for agrochemicals are GC and LC. A variety of detection methods are used for GC such as electron capture detection (BCD), nitrogen-phosphorus detection (NPD), flame photometric detection (FPD) and mass spectrometry (MS). For LC, typical detection methods are ultraviolet (UV) detection, fluorescence detection or, increasingly, different types of MS. The excellent selectivity and sensitivity of LC/MS/MS instruments results in simplified analytical methodology (e.g., less cleanup, smaller sample weight and smaller aliquots of the extract). As a result, this state-of-the-art technique is becoming the detection method of choice in many residue analytical laboratories. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

The cloth dosimeter from the California roller technique consisted of placing a 15.2-cm x 61.0-cm piece of cotton cloth on treated turf, covering the cloth with plastic, and rolling over it with a 25-lb roller. The cloth was then placed in a sample container, which was put in a cooler and transported to the laboratory. The dosimeter was then extracted with hexane. An aliquot of... 

The relative amounts of 9-phenyl-9-fluorenol and 9-bromo-9-phenyl-fluorene were determined as follows A 13C NMR spectrum (CDCI3 solution, Bruker 400 MHz) of an authentic sample of 9-bromo-9-phenylfluorene was recorded and then doped in 1% increments with authentic 9-phenyl-9-fluorenol. A 13C NMR spectrum (CDCI3 solution, Bruker 400 MHz) was recorded after each doping, and the heights of the peaks at 120.3 ppm (bromide) and 120.0 ppm (alcohol) were monitored. These spectra were compared with a 13C NMR spectrum (CDCI3 solution, Bruker 400 MHz) of the sample in question. Application of this technique to an evaporated aliquot of the reaction mixture in Step B, indicated >97% conversion of alcohol to bromide after 24 hr. 

The samples were analysed by injecting 25 pi aliquots into an HGA 2000 Perkin-Elmer graphite furnace attached to a Jarrell-Ash 82-800 double beam atomic absorption spectrophotometer. Graphite tubes in the furnace were replaced after 75-100 analyses. Metal concentrations were determined by comparing the peak heights of the samples to the standard curve established by the determination of at least five known standards. The detection Emits of this technique for 1% absorption were 0.9 pmol/1 (Fe), and 0.2 pmol/1 (Mn). The coefficient of variation was 11% at 6.5 pmol/1 for iron and +12% at 11.8 pmol/1 for manganese. 

Bertine et al. [99] have discussed the determination of uranium in deep sea sediments and water utilising the fission track technique. In this technique a weighed aliquot (50-100 mg) of the powdered sample is made into a pel-... 

In addition to the analyte, the matrix will contain many other compounds. The method chosen must discriminate between the analyte of interest and other compounds also present in the sample. The test portion may have to pass through many analytical stages before the analyte is obtained in a form suitable for final measurement. First, the analyte may need to be separated from the bulk of the sample matrix. Further treatment may then be required to obtain an aliquot that is sufficiently clean (i.e. free from potential interferences) for the end-measurement technique. A general scheme of analysis is presented in Table 4.4 to illustrate the different approaches used depending on the nature of the analyte and of the matrix. 

A large variety of tools, utilizing both chemical and physical methods, are available to the experimentalist for rate measurements. Some can be classified as ex-situ techniques, requiring the removal and analysis of an aliquot of the reacting mixture. Other, in-situ, methods rely on instantaneous measurements of the state of the reacting system without disturbance by sample collection. 

The combustion infrared technique has been used for the analysis of diluted sludges [34], Schaffer et al. [34] have used a blender to prepare suspensions of samples of this type. After the sample was homogenized, a microlitre syringe was used to remove a 20pL aliquot from the blender. However, the method described by Schaffer et al. [34] does not necessarily allow for the isolation of a representative portion of the sample in a 20pL aliquot [35]. This was found to be particularly true if the sample contained large particles which settled rapidly, and it must be assumed that the precision... 

Much effort has been devoted to the applicaton of electrophoretic techniques for the analysis of dyes and dye decomposition products in industrial waste-waters. CZE with DAD and MS detection was employed for the determination of reactive dyes in spent dye-baths and waste-waters. The chemical structure of the dyes included in the experiments are shown in Fig. 3.149. Liquid samples were purified and preconcentrated by SPE. ODS cartridges were conditioned by washing with two bed volumes of methanol, four bed volumes of water and two bed volumes of 5 mM tetrabutylammonium phosphate. Aliquots of 10 - 30 ml were loaded and the cartridges were washed with 2 ml of water. Analytes were eluted with 2 ml of methanol-water (70 30, v/v). Untreated fused-silica capillaries (110 cm and 57 cm X 50 /im i.d.) were coupled to MS and DAD. The running... 

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