Problems with Sample Material

Therefore, if a large quantity of sample is introduced into the flame over a short period of time, the flame temperature will fall, thus interfering with the basic processes leading to the formation and operation of the plasma. Consequently introduction of samples into a plasma flame needs to be controlled, and there is a need for special sample-introduction techniques to deal with different kinds of samples. The major problem with introducing material other than argon into the plasma flame is that the additives can interfere with the process of electron formation, a basic factor in keeping the flame self-sustaining. If electrons are removed from the plasma by... 

Table 28-3 illustrates another problem with heterogeneous materials. Nickel ore was crushed into small particles that were sieved and analyzed. Parts of the ore that are deficient in nickel are relatively resistant to fracture, so the larger particles do not have the same chemical composition as the smaller particles. It is necessary to grind the entire ore to a fine power to have any hope of obtaining a representative sample. 

Automation is one issue of paramount importance, as the biggest problem with sample preparation is time. The benefits of automation, apart from the obvious economic one, include the reduced manual operations and laboratory materials required, the increased sample throughput... 

The reasons for postponing a discussion of the preliminary steps to this point are pedagogical. Experience has shown that it is easier to introduce students to analytical techniques by having them first perform measurements on simple materials for which no method selection is required and for which problems with sampling,. sample preparation, and. sample dissolution are either nonexistent or easily solved. Thus, we have been largely concerned so far with measuring the concentration of analytes in simple aqueous solutions that have few inteifering. species. 

As with all distillation techniques, boiling a sample to achieve concentration can be a problem with reactive materials. Sufficient experiments should be done to insure that compounds of interest are neither formed nor destroyed during the process. 

There are also some practical problems connected with the instrumental issues of MS. Most of the sources used in mass spectrometers are so small that it is possible to also have, for example, a very small inlet for the gas or liquid. In this case, such TLC/MS methods may only be used if they are based on the introduction of samples via a stream of gas. The matrix, together with sample material, may be introduced directly into the MS source. Some types of instruments even allow for the insertion of a whole chromatographic plate into the MS source. 

RBS can provide absolute quantitative analysis of elemental composition with an accuracy of about 5%. It can provide depth-profile information from surface layers and thin films to a thickness of about 1 pm. In some cases, however, the high-energy beam can damage the surface. This is particularly a problem with insulating materials, such as polymers, alkali halides, and oxides. The Mars Pathfinder mission in 1997 contained an alpha proton X-ray spectrometer (APXS). In its RBS mode, the spectrometer bombarded samples with alpha particles and determined elemental composition via energy analysis of the backscattered particles. In addition to RBS, the APXS instrument was designed to carry out proton emission and particle-induced X-ray emission (PIXE) experiments. Soil and rock compositions were measured and compared to those from the earlier Viking mission. 

Raman Structural information for highly disperse phases, characterization of adsorbates Qualitative, signal superposition for complex materials, problems with sample fluorescence (yes)... 

Advances have been made in fiber-optic sampling techniques that overcome problems with sampling high temperature and high viscosity materials. As a result, in situ sampling with fiber-optic probe systems is being implemented for these types of applications. 

The complexity of petroleum products raises the question of sample validity is the sample representative of the total flow The problem becomes that much more difficult when dealing with samples of heavy materials or samples coming from separations. The diverse chemical families in a petroleum cut can have very different physical characteristics and the homogeneous nature of the cut is often due to the delicate equilibrium between its components. The equilibrium can be upset by extraction or by addition of certain materials as in the case of the precipitation of asphaltenes by light paraffins. 

The other major problem concerned with sampling is that of the sample size. The size of the sample taken from a heterogeneous material is determined by the variation in particle size, and the precision needed in the results of the analysis. 

Replication avoids the problem of sample deterioration in the instrument, but it is destructive in that reaction of the material cannot be continued after the replica has been prepared. Transitory features cannot be detected unless a series of preparations are examined corresponding to increasing progress of the reaction considered. The textures of replicas have been shown [220] to be in satisfactory agreement with those of the original surface as viewed in the scanning electron microscope. The uses and interpretations of observations made through sample replication procedures are illustrated in the studies of decomposition of metal carboxyl-ates by Brown and co-workers [97,221—223]. 

The use of formulated material (generally suspended in water) allows the researcher to work with the form of the test material that will be the most commonly encountered under field conditions. The formulated material would be found under most circumstances on field surfaces and in the air after treatment of the field with the test product. The greatest problem with the use of formulated product in water as a field fortification suspension is the maintenance of the homogeneity of the field fortification suspension. To maintain the homogeneity of the active ingredient in the field fortification suspension, one should shake the field fortification suspension vigorously for at least one minute and immediately withdraw the aliquot for the field spike from the fortification suspension just prior to fortification of the sample. 

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