Amount of Sample

The rate of evaporation of ions from a heated surface is given by Equation 7.3, in which Q, is the energy of adsorption of ions on the filament surface (usually about 2-3 eV) and Cj is the surface density of ions on the surface (a complete monolayer of ions on a filament surface would have a surface density of about 10 ions/cm ). 

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The amount of sample required is quite small as little as 10 mole is typical So many peptides and proteins have been sequenced now that it is impossible to give an accurate count What was Nobel Prize winning work m 1958 is routine today Nor has the story ended Sequencing of nucleic acids has advanced so dramatically that it is possible to clone the gene that codes for a particular protein sequence its DNA and deduce the structure of the protein from the nucleotide sequence of the DNA We 11 have more to say about DNA sequencing m the next chapter... 

Designing an experimental procedure involves selecting an appropriate method of analysis based on established criteria, such as accuracy, precision, sensitivity, and detection limit the urgency with which results are needed the cost of a single analysis the number of samples to be analyzed and the amount of sample available for... 

Another way to narrow the choice of methods is to consider the scale on which the analysis must be conducted. Three limitations of particular importance are the amount of sample available for the analysis, the concentration of analyte in the sample, and the absolute amount of analyte needed to obtain a measurable signal. The first and second limitations define the scale of operations shown in Figure 3.6 the last limitation positions a method within the scale of operations. ... 

A proportional determinate error, in which the error s magnitude depends on the amount of sample, is more difficult to detect since the result of an analysis is independent of the amount of sample. Table 4.6 outlines an example showing the effect of a positive proportional error of 1.0% on the analysis of a sample that is 50.0% w/w in analyte. In terms of equations 4.4 and 4.5, the reagent blank, Sreag, is an example of a constant determinate error, and the sensitivity, k, may be affected by proportional errors. 

A determinate error whose value depends on the amount of sample analyzed. 

What is the minimum amount of sample needed for each analysis ... 

Determine Ks and the amount of sample needed to give a relative standard deviation for sampling of 2.0%. Predict the percent relative standard deviation and the absolute standard deviation if samples of 5 g are collected. 

The amount of sample needed to give a relative standard deviation of 2%, therefore, is... 

When the target population is segregated, or stratified, equation 7.5 provides a poor estimate of the amount of sample needed to achieve a desired relative standard deviation for sampling. A more appropriate relationship, which can be applied to both segregated and nonsegregated samples, has been proposed. ... 

In the previous section we considered the amount of sample needed to minimize the sampling variance. Another important consideration is the number of samples required to achieve a desired maximum sampling error. If samples drawn from the target population are normally distributed, then the following equation describes the confidence interval for the sampling error... 

Precision The relative precision of precipitation gravimetry depends on the amount of sample and precipitate involved. For smaller amounts of sample or precipitate, relative precisions of 1-2 ppt are routinely obtained. When working with larger amounts of sample or precipitate, the relative precision can be extended to several parts per million. Few quantitative techniques can achieve this level of precision. 

According to the procedure, the sample should weigh between 0.5 and 5 g. On what basis should a decision on the amount of sample be made ... 

Quantitative Calculations The result of a quantitative analysis by particulate gravimetry is just the ratio, using appropriate units, of the amount of analyte to the amount of sample. 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

As ions and neutrals evaporate from a heated filament surface, the amount of sample decreases and the surface densities (C, Cq) must decrease. Therefore, Equation 7.1 covers two effects. The first was discussed above and concerns the changing value for the ratio n+/n° as the temperature of the filament is varied, and the other concerns the change in the total number of ions desorbing as the sample is used up. The two separate effects are shown in Figure 7.8a,b. Combining the two effects (Figure 7.8c) reveals that if the temperature is increased to maintain the flow of ions, which drops naturally as the sample is used up (time), then eventually the flow of ions and neutrals becomes zero whatever the temperature of the filament because the sample has disappeared from the filament surface. 

Few of the naturally occurring elements have significant amounts of radioactive isotopes, but there are many artificially produced radioactive species. Mass spectrometry can measure both radioactive and nonradioactive isotope ratios, but there are health and safety issues for the radioactive ones. However, modem isotope instmments are becoming so sensitive that only very small amounts of sample are needed. Where radioactive isotopes are a serious issue, the radioactive hazards can be minimized by using special inlet systems and ion pumps in place of rotary pumps for maintaining a vacuum. For example, mass spectrometry is now used in the analysis of Pu/ Pu ratios. 

The flame can become unstable if too large an amount of sample is introduced or if the sample contains substances that can interfere with the basic operation of the plasma. For example, water vapor, air, and hydrogen all lead to instability of the plasma flame if their concentrations are too high. 

In some instances, the plasma flame can go out altogether if the amounts of sample or other contaminants rise too high. This possibility has led to the development of a wide variety of gas/liquid separators that treat the sample before it is introduced to the flame. 

Determination of DMAC in Air. DMAC can be measured in air by passing a known amount of sample through water in a gas-scmbbing vessel and then analyzing the solution either chemically or by gas chromatography. 

Instrumental Interfaces. The basic objective for any coupling between a gas chromatograph (gc) and a mass spectrometer (ms) is to reduce the atmospheric operating pressure of the gc effluent to the operating pressure in the ms which is about 10 kPa (10 torr). Essential interface features include the capability to transmit the maximum amount of sample from the gc without losses from condensation or active sites promoting decomposition no restrictions or compromises placed on either the ms or the gc with regard to resolution of the components and reliability. The interface should also be mechanically simple and as low in cost as possible. 

Manufacturer and system Throughput Samples per hour Tests Number of resident methods Amount of sample needed per test, p.L Incubator temperature, °C Optical a system Distinctive features... 

When a test result for a particular specimen is found to have an elevated, out of method range value, some analy2ers, eg, Beckman CX3, can automatically repeat the sampling from the same specimen. For elevated concentrations, the precision of the optical system is reduced so an automatic dilution of the sample, eg, by aspiration of a reduced amount of sample, is provided during the second sampling. 

For capillary columns fused siHca is the material of choice for the column container. It has virtually no impurities (<1 ppm metal oxides) and tends to be quite inert. In addition, fused siHca is relatively easily processed and manufacture of columns from this material is reproducible. In trace analysis, inertness of tubing is an important consideration to prevent all of the tiny amounts of sample from becoming lost through interaction with the wall during an analysis. 

Sampling. The procedures for taking a sample, reducing the particle size of the sample, and separation of a smaller portion for later analysis are given in ASTM D2234 and D2013 (18) and BS1017. The procedures describe the minimum amount of sample needed to maintain a representative sample for analysis. 

Nonvolatile. A weighed amount of sample is placed ia a dryiag oven maintained at a temperature of 105°C. After three hours, the sample is removed from the oven, placed ia a desiccator to cool, and reweighed. The weight of the residue, consisting of soHds and nonvolatiles, is calculated as a percentage of the total sample weight taken for analysis. 

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