Active sampling techniques

Indirect analytical methods have a high significance for personal exposure measurements in regard to ha2ardous chemicals at workplaces. Though most indirect methods are based on active sampling techniques, passive sampling devices have also been developed. 

The collection of surface water samples generally falls into two classes of methods active or passive. Active sampling techniques involve physically taking a sample... 

The introduction of high-resolution, high-efficiency /-ray detectors composed of lithium-drifted germanium crystals has revolutionised /-measurement techniques. Thus, /-spectrometry allows the rapid measurement of relatively low-activity samples without complex analytical preparations. A technique described by Michel et al. [25] uses Ge(Li) /-ray detectors for the simultaneous measurements of 228radium and 226radium in natural waters. This method simplifies the analytical procedures and reduces the labour while improving the precision, accuracy, and detection limits. 

Although affinity chromatography has not been used directly as an analytical method, it may be modified in the future to produce a viable technique. Leucovorin has been used as an effective spacer in obtaining active samples of dihydrofolate reductase.79 If the enzyme could be immobilized without losing its activity, perhaps it could be used to separate folates. 

A comparison of active (using pumps) and passive (relying on diffusion) sampling techniques for the determination of nitrobenzene, benzene and aniline in air was mentioned in Section IV.A77. Several LLE methods for nitroaromatic compounds dissolved in water were evaluated. High recoveries were achieved with discontinuous or continuous extraction with dichloromethane, adsorption on a 1 1 1 mixture of Amberlite XAD-2, -4 and -8 resins and elution with dichloromethane445. 

For hydrogen equilibrium pressures below 100 Torr, an absorption technique was used. Thoroughly activated samples were outgassed at 500° C to a vacuum... 

This study is concerned with the comparison of data from x-ray fluorescence (2), electron microprobe (2), and thermal neutron activation (3) techniques with the metallurgical structure of the sample. The maximum precision which can be expected from all of these techniques, with normal precautions, is 5%. In all cases, the analytical parameters used in the measurements reported here were chosen to maximize the sensitivity and precision for silver-based alloys. The parameters used in each technique are listed in Table II. 

Active Sampling. These techniques use the dynamic passage of the sampled air at a specified rate through a substrate (e.g., a filter), an absorbant (e.g., Tenax (diphenylphenylene oxide) or activated charcoal), or a detector (e.g., a photometer) that measures a parameter that is proportional to detectable quantities of a contaminant. 

The post-bombardment processing of the activated sample may follow either a nondestructive assay of the radioactivity in the sample (gamma-ray scintillation spectrometry is used most often for this) or a chemical processing of the sample prior to the radioactivity assay. Techniques involving either precipitation, electrodeposition. solvent extraction, and ion exchange or some combination of these form the basts of the radio-chemical separation techniques used in activation analysis. 

One of the key steps in any isotope dilution analysis concerns the isolation and purification of the diluted activity, plus the measurement of its specific activity. Two techniques are usually preferred for the separation precipitation and solvent extraction. As a purification step, precipitation has the advantage that the precipitate can easily be weighed at the time of separation, thereby allowing a quick determination of the specific activity. The main problem with the use of precipitation techniques involves the occurrence of co-precipitation phenomena, in which unwanted materials are precipitated along with the desired substance, thus altering the sample specific activity. Precipitation techniques are used for the isolation of inorganic components. 

Neutron activation analysis techniques are frequently used for trace element analyses of coal and coal-related materials (Weaver, 1978). Precision of the method is 25%, based on all elements reported in coal and other sample matrices. Overall accuracy is estimated at 50%. Neutron activation analysis utilizing radiochemical separations (NAA-RC) is employed by investigators when the sensitivity for a particular element or group of elements is inherently low or when spectral interference for a given element in a specific matrix is too great to be detected adequately. This situation was more prevalent before the advent of Ge(Li) spectrometry when only low-resolution Nal(TI) detectors were available. 

The technique of IR-ATR spectroscopy is easy to apply in reaction analysis as no sampling or flow-through cells are required. As most organic compounds are infrared-active, the technique is useful for many reaction types. However, there are some matters that should always be kept in mind when the reaction s IR-ATR spectrum is interpreted. 

Adiabatic The temperature of the sample results from its thermal activity. This technique gives direct access to the thermal runaway curve. The results must be corrected by the adiabacity coefficient, since a part of the heat released in the sample is used to increase the temperature of the calorimetric cell. This rends the kinetic evaluation complex. 

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