Sampling methods pellet technique

The most common method for preparing samples for pretreatment and subsequent measurement of IR spectra is the self-supporting pellet technique. In this... 

Crystalline or solid samples (ca. 10 mg) are ground with 5 drops of nujol in an agate mortar to form a paste for subsequent CD or MCD measurement. Perfluoro-butadiene can also be used instead of nujol. Otherwise, the KBr pellet technique, for which KBr and sample should be completely dried in vacuo and ground into a fine powder, can be used conveniently. On using the KBr technique, one can often observe a different CD spectrum from that of the nujol-mull method. 

Other researchers have evaluated the crystallinity index by using IR spectrophotometry. This method requires the KBr pellet technique of sampling. The X-ray diffraction method was investigated for its potential as a nondestructive technique for recording changes in cellulose as it ages. 

Figure 1. Infrared spectra of natural organic materials isolated from Volo Bog obtained on solid samples by A) the KBr pellet technique and B) the diffuse reflectance method and C) in aqueous solution by CIR.

An advantage of IR spectroscopy in general, and as applied to humic substances in particular, is the small quantity of sample required. For example, the alkali halide pressed-pellet technique, as normally carried out, requires about 1-2 mg of sample, which is considerably less than that required for the other most useful spectroscopic method, NMR, in the study of humic substances (see Chapter 22). However, microsampling methods have been developed which can record IR spectra on less than 0.01 /xg of sample (Alpert et al., 1964 Parker, 1971 Price, 1972 Griffiths and Block, 1973). These special techniques may have advantage in the study of humic samples isolated only in minute quantities, or for investigating small samples obtained in the fractionation of humic substances. 

If the physical form of the sample is different from the physical form required by the analytical instrument, more elaborate sample preparation is required. Samples may need to be dissolved to form a solution or pressed into pellets or cast into thin films or cut and polished smooth. The type of sample preparation needed depends on the nature of the sample, the analytical technique chosen, the analyte to be measured, and the problem to be solved. Most samples are not homogeneous. Many samples contain components that interfere with the determination of the analyte. A wide variety of approaches to sample preparation has been developed to deal with these problems in real samples. Only a brief overview of some of the more common sample preparation techniques is presented. More details are found in the chapters on each instmmental method. 

The fissile isotopes of uranium ( U) and plutonium ( Pu, Pu) can be measured using active neutron counting techniques. This technique uses an external neutron source to induce fission in the fissile plutonium and manium content of the sample. The multiple induced fission neutrons are then measured using standard coincidence counting methods. The technique is mainly applied to determine the mass of in uranium-bearing samples (from LEU to HEU) in powder, metal, pellets, fresh fuel elements, and waste drums. It can be operated either with or without a cadmium liner (fast or thermal mode). 

At the option of the instructor, obtain the infrared spectrum using the dry film method (Technique 25, Section 25.4) or as a KBr pellet (Technique 25, Section 25.5) and the NMR spectrum in CDCI3 (Technique 26, Section 26.1). Submit the sample in a labeled vial to the instructor. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

Extraction of Sodium Channel Blockers. A review of published reports shows that methods for purification of sodium channel blockers from bacterial cultures are similar to techniques for isolation of TTX and STX from pufferfish and dinoflagellates (30, 31, 38, 39). Typically, cell pellets of bacterial cultures are extracted with hot 0.1% acetic acid, the resulting supernatant ultra-filtered, lyo-philized, and reconstituted in a minimal volume of 0.1% acetic acid. Culture media can also be extracted for TTX by a similar procedure (Ji). Both cell and supernatant extracts are analyzed further by gel filtration chromatography and other biological, chemical, and immunological methods. Few reports describe purification schemes that include extraction of control samples of bacteriological media (e.g., broths and agars) which may be derived from marine plant and animal tissues. 

The gas dispersion tube technique is much easier to assemble and use than the dialysis membrane/wire mesh basket assembly, but it gives similar release profiles for 5-FU from EHCF or copolymers of EHCF with either HA or HHA. The gas dispersion tube method only requires 0.1 gram of material for an accurate kinetic profile of these prodrugs. The small gas dispersion tubes are useful for studies involving powders, but pellets would not fit into these tubes. The large size gas dispersion tubes or the wire basket technique could be used for powders or pellets, and can accommodate larger sample sizes. In summary the gas dispersion tube technique is reproducible, easy to assemble, easy to use, can distinguish polymers with different release rates, and the results can be correlated with our earlier studies. 

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