Size of sample

Important factors which must be taken into account when selecting an appropriate method of analysis include (a) the nature of the information which is sought, (b) the size of sample available and the proportion of the constituent to be determined, and (c) the purpose for which the analytical data are required. 

In 1951Castaing8 published results to show that an electron microscope could be converted into a useful x-ray emission spectrograph for point-to-point exploration on a micron scale. The conversion consisted mainly in adding a second electrostatic lens to obtain a narrower electron beam for the excitation of an x-ray spectrum, and adding an external spectrometer for analysis of the spectrum and measurement of analytical-line intensity. Outstanding features of the technique were the small size of sample (1 g cube, or thereabouts) and the absence of pronounced absorption and enhancement effects, which, of course, is characteristic of electron excitation (7.10). Castaing8 gives remarkable quantitative results for copper alloys the results in parentheses are the quotients... 

Figure 1.2. The range R(n) for size of sample n, with n - 2. .. 40 (left). The line gives the tabulated values.range R is given as y = R/sx in units of the experimental standard deviation. A total of 8190 normally distributed values with mean 0 and standard deviation 1 was simulated. (See Section 3.5.5.) The righthand figure gives the distribution of ranges found after simulating 100 sets of n = 10 normally distributed values.

Size of sample 2 was determined using magnetic susceptibility, as described below. 

The size of sample required has been reduced by a number of technical developments including micro inverse probes and micro cells (references in Martin et al. 1998), and has been reduced even further using a newly developed 1.7-mm submicro inverse-detection gradient probe (Martin et al. 1998). The combined use of inverse detection probes with solenoid microcoils has also been developed to reduce sample volumes for NMR (Subramanian and Webb 1998). 

Microanalysis is the common name used to refer to a variety of techniques for identifying, characterizing, and evaluating minute amounts of materials. Some microanalytical techniques are scaled-down versions of well-known conventional or physical analytical techniques others are specialized techniques that can be implemented only on extremely small samples. Table 11 lists the minimum size of samples required for microanalysis and the minimum amount of substance detectable by microanalytical techniques (Janssens and Van Grieken 2004). 

TABLE 11 Analytical Techniques and Size of Sample Required... 

It is worth noting that the partide sizes of samples prepared by the urea hydrolysis method are larger than other LDHs. Such a finding can be rationalized by considering the slow hydrolysis of urea [61], since it can be classified as a very weak Bronsted base (pfCb= 13.8). Its hydrolysis mechanism consists of the formation of ammonium cyanate as the rate-determining step, followed by fast hydrolysis of the cyanate to ammonium carbonate ... 

Fig. 1.45. Schema of a differential scanning calorimetric (DSC) apparatus. Gerate DSC 821 . Temperature range with LN2 cooling -150 °C to 500 °C accuracy of temperature 0.2 °C resolution 0.7 gW in the measuring range of +/-350 mW cooling rate from +100 °C to -100 °C approx. 13 °C/min size of sample, several mg to 200 mg.

Gel filtration may be best used to analyze fractions already separated from a digest supernatant by ultrafiltration, as used in a recent study by Sandstrom, et al. (3.2). A more precise separation of complexes can be obtained with gel filtration, but the size of sample which can be applied is limited. Thus, in many situations, the sample must be concentrated before being applied to the gel column. Either pre-purification or sample concentration could introduce possible shifts in mineral binding which should be understood for proper interpretation of the results (33). 

The mass of sample taken for analysis is primarily dependent on four factors (1) the amount of material available, (2) the concentration of the analyte, (3) the heterogeneity of the sample, and (4) the method of analysis. Most conventional solvent extraction techniques currently start with more sample than is required, use more extraction solvent than is necessary, and ultimately only analyze 0.1% of the material prepared, e.g., 1 pi from 1 ml. Micro-extraction techniques [468] can be used in conjunction with on-line LC-GC or LC-MS to utilize the whole extract in the final determinations. This approach can significantly reduce the size of sample required and the volume of solvent used. Many workers have reported the use of solid phase microextraction (SPME) in different environmental matrices for various pollutants [288,342,345,469 - 477]. 

There have also been recent improvements to reduce background noise, which is caused by random thermal motions of electrons in the detection coil and in the first amplifier range.15 Cryoprobes in which the coil and first amplifier are cooled to 30 K and the size of sample coil reduced lower electrical resistance and noise. A further benefit noted by Grivet et al.15 is that "the smaller the probe, the closer the conductors are to the sample and the higher the measure of the coil s [efficiency], as felt by the nuclei."... 

A statistically valid sampling plan requires careful design emd execution so that generalizations based on mathematical probability can be drawn from a small number of test portions. Guidelines are given for estimation of the minimum number and size of sample increments needed to achieve a given level of confidence in chemical analyses. 

Given these data, what statements can we make about the number and size of samples that would have to be taken to hold the sampling standard deviation to some prescribed level Calculation of Ingamell subsampling constant is not appropriate since segregation is present. 

The ultrasonic intensity and size of sample to be irradiated can be matched fairly accurately for optimum effect. 

The explosion (ignition) temperature decreases with increasing size of sample and increases with rate of heating... 

One important restriction set by the analyst and his instruments is already mentioned, the limits size of samples. Other restrictions influencing the act of sampling are the accuracy of the method of analysis, the limit of detection and the sensitivity. These analytical parameters set an upper limit on the information that can be obtained. 

The size of sample is also restricted in the field of art conservation research. Nevertheless, the degree of heterogeneity of the object imposes an inferior limit to the sample s size, so a compromise between size of sample and size of the confidence interval acceptable should be achieved in each particular case study. 

What is the expected concentration range of the analyte(s) in the samples What size of sample is available ... 

One serious limitation of NMR measurements is the size of sample required. It is not possible to measure trace amounts or extremely dilute solutions as is possible in optical spectroscopy. When there is but a single proton in a compound to be measured, we have been able to use concentrations down to one mole per cent, and correspondingly lower concentrations if more than one equivalent proton is present. Gaseous samples, with the possible exception of such gases as methane, ethylene, benzene, etc., having several equivalent protons, usually require pressures in excess of one atmosphere. 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

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