Mechanical sampling

Although mechanical sampling methods are to be the focus of attention, manual sampling methods are also employed for practical sample collecdion in commerce. Techniques of mechanical sampling should be emulated as closely as possible for best results with sampling by manual procedures. 

Approved techniques for manual and mechanical sampling are often documented for various commodities handled in commerce by industiy groups. Examples are the International Standards Organization (ISO), British Standards Association (BSA), Japan Institute of Standards (JIS), American Society for Testing Materi s (ASTM), and the Fertihzer Institute. Sampling standards developed for use in specified industry applications frequently include instructions for labora-toiy work in sample preparation and analysis—steps (2) and (3) above. 

Two principal topics are considered under theory of sampling. First is theoiy accounting for physical properties of material to be sampled. Second is the process of mechanical sample extrac tion. The theoiy predicts accuracy of sample taking—how much sample to take and howto take it to meet an accuracy specification. 

As stated in Sect. 2.3, hybrid schemes suffer from the dilemma that using semiempirical methods gives a wrong description of the primary system whereas ab initio methods are so expensive that the affordable number of time steps does not allow proper statistical-mechanical sampling. For selected systems, however, there is a possibility of using very accurate methods and still executing many time steps. The requirements on the system are that the internal conformational space of the primary system is finite and that the primary system can be described by only a few active degrees of freedom. 

An automatic probe tuning and matching (ATM) accessory allows one to automatically tune the NMR probe to the desired nuclei s resonant frequency and match the resistance of the probe circuit to 50 Q [7]. Traditional NMR instruments are designed so that one must perform these adjustments manually prior to data acquisition on a new sample. The advent of the ATM accessory allows the sampling of many different NMR samples without the need for human intervention. The ATM in conjunction with a sample changer enables NMR experiments to be conducted under complete automation. The sample changers are designed so that once the samples are prepared, they are placed into the instrument s sample holders. Data are then acquired under software control of both the mechanical sample delivery system as well as the electronics of the spectrometer. 

An advantage to precolumn derivatization is that there are no limitations with respect to reaction kinetics. Of course, rapid reaction is always desirable, but need not be the principal factor when choosing a derivatizing reagent. This means that a wide variety of chemistries are available for derivatization. However, it is important that very stable derivatives be formed. The limited stability of OPA derivatives is the reason why OPA is most commonly employed for postcolumn reaction. If it is to be used in the precolumn fashion, very well-controlled and automated (mechanized) sample/reagent handling immediately prior to injection must be employed. 

Increments from a moving coal stream are often collected on a preset interval of time by a mechanical sampling device. The opening of the device must be sufficient to accommodate a full stream cut in both directions without disturbing the coal. 

Conventional design of most mechanical sampling systems for large tonnages of coal use some form of cross-stream primary cutter to divert the primary increments from the main stream of coal (Figure 2.2). A major advantage of these systems is that they sample coal from a moving stream, and most of them satisfy the principle that every particle in the entire mass has an equal opportunity to be included in the primary increments. 

Two processes of sample division and reduction are covered (1) procedure A, in which manual riffles are used for division of the sample and mechanical crushing equipment for reduction of the sample, and (2) procedure B, in which mechanical sample dividers are used for division of the sample and mechanical crushing equipment for reduction of the sample. A third process that is, in reality, a combination of procedures A and B may be used at any stage. 

Knapp, G. Development of mechanized sample preparation for plasma emission spectrometry. ICP Inf Newsl. 10, 91-104 (1984)... 

MAS is the most common technique employed in solid-state NMR to remove line broadening. It involves fast mechanical sample rotation about an axis inclined at 54°44 (the value where 3 cos G -1 = 0, Equation 5.4) to the direchon of the external magnehc field. It can remove line broadening from dipolar interactions (averaged... 

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