Samples and standards

Samples for charged particle activation analysis are usually thicker than the range (thick samples). Cylindrical metal discs of 15-20 mm diameter and 1-2 mm thick are most often used. The preparation of such samples is described in Chapter III. 

Charged particles of the energies required for activation analysis can be obtained with a cyclotron or a Van de Graaff accelerator. Most of the applications described in this book can be carried out with a so-called compact cyclotron. An example is the CGR-MeV 520 machine, which allows to accelerate 2,5-24 MeV protons, 3-14.5 MeV deuterons, 6-32 MeV helium-3 and 6-29 MeV helium-4. Maximum beam intensities range from 10 to 100 pA, depending on the energy. 

The samples and the standards are in general irradiated in vacuum, placed in an appropriate sample holder, to be mounted on the accelerator beam transport system. A simple sample holder is shown in Fig. 11-19. It is watercooled and an aluminium tube is placed on the sample holder to minimize the escape of secondary electrons, which may result in inaccurate beam intensity measurements. Before the sample holder a collimator is placed as shown in Fig. II-21b. 

Powdered samples or standards are irradiated in appropriate sample holders such as the one described by Mortier et al. (95). 

When short-lived radionuclides are used, it is important that the sample is available as soon as possible after the irradiation. Moreover the radiation level at the target station directly after the irradiation may be very high, so that entering the shielded target area and manual removal of the sample holder from the beam transport system must be avoided. Therefore a pneumatic transfer system such as the one described by Strijckmans (39) can be used. This system allows samples mounted on a copper rabbit to be send from the sending station to the irradiation station situated in a room shielded with thick concrete walls. 

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The successful application of an external standardization or the method of standard additions, depends on the analyst s ability to handle samples and standards repro-ducibly. When a procedure cannot be controlled to the extent that all samples and standards are treated equally, the accuracy and precision of the standardization may suffer. For example, if an analyte is present in a volatile solvent, its concentration will increase if some solvent is lost to evaporation. Suppose that you have a sample and a standard with identical concentrations of analyte and identical signals. If both experience the same loss of solvent their concentrations of analyte and signals will continue to be identical. In effect, we can ignore changes in concentration due to evaporation provided that the samples and standards experience an equivalent loss of solvent. If an identical standard and sample experience different losses of solvent. 

A standard, whose identity is different from the analyte s, that is added to all samples and standards containing the analyte. 

A standardization is still possible if the analyte s signal is referenced to a signal generated by another species that has been added at a fixed concentration to all samples and standards. The added species, which must be different from the analyte, is called an internal standard. 

Since all samples and standards are prepared using the same volume of ammonium acetate buffer, the contribution of this source of iron is accounted for by the calibration curve s reagent blank. 

Precision For samples and standards in which the concentration of analyte exceeds the detection limit by at least a factor of 50, the relative standard deviation for both flame and plasma emission is about 1-5%. Perhaps the most important factor affecting precision is the stability of the flame s or plasma s temperature. For example, in a 2500 K flame a temperature fluctuation of +2.5 K gives a relative standard deviation of 1% in emission intensity. Significant improvements in precision may be realized when using internal standards. 

Why is it important to use the same stirring rate and time for all samples and standards ... 

Another approach to matrix matching, which does not rely on knowing the exact composition of the sample s matrix, is to add a high concentration of inert electrolyte to all samples and standards. If the concentration of added electrolyte is sufficient, any difference between the sample s matrix and that of the standards becomes trivial, and the activity coefficient remains essentially constant. The solution of inert electrolyte added to the sample and standards is called a total ionic strength adjustment buffer (TISAB). 

The concentration of Ca + in a water sample was determined by the method of external standards. The ionic strength of the samples and standards was maintained at a nearly constant level by making each solution 0.5 M in KNO3. The measured cell potentials for the external standards are shown in the following table. 

Quantitative Analysis Using the Method of Standard Additions Because of the difficulty of maintaining a constant matrix for samples and standards, many quantitative potentiometric methods use the method of standard additions. A sample of volume, Vx) and analyte concentration, Cx, is transferred to a sample cell, and the potential, (ficell)x) measured. A standard addition is made by adding a small volume, Vs) of a standard containing a known concentration of analyte, Cs, to the sample, and the potential, (ficell)s) measured. Provided that Vs is significantly smaller than Vx, the change in sample matrix is ignored, and the analyte s activity coefficient remains constant. Example 11.7 shows how a one-point standard addition can be used to determine the concentration of an analyte. 

Samples and calibration standards are prepared for analysis using a 10-mL syringe. Add 10.00 mL of each sample and standard to separate 14-mL screw-cap vials containing 2.00 mL of pentane. Shake vigorously for 1 min to effect the separation. Wait 60 s for the phases to separate. Inject 3.0-pL aliquots of the pentane layer into a GC equipped with a 2-mm internal diameter, 2-m long glass column packed with a stationary phase of 10% squalane on a packing material of 80/100 mesh Chromosorb WAW. Operate the column at 67 °C and a flow rate of 25 mL/min. 

Both the samples and standards are treated identically so their relative concentrations are unaffected by an incomplete extraction. 

As noted earlier, gamma-ray emission is measured following a cooling period in which short-lived interferents are allowed to decay away. The initial activity therefore, is determined by extrapolating a curve of activity versus time back to f = 0 (Figure 13.15). Alternatively, if the samples and standards are irradiated simultaneously, and the activities are measured at the same time, then these activities may be used in place of Aq) and (Ao)s in the preceding equations. 

The concentration of Ni in a new alloy is determined by a neutron activation analysis using the method of external standards. A 0.500-g sample of the alloy and a 1.000-g sample of a standard alloy known to contain 5.93% w/w Ni are irradiated with neutrons in a nuclear reactor. When irradiation is complete, the sample and standard are allowed to cool, and the gamma-ray activities are measured. Given that the activity is 1020 cpm for the sample and 3540 cpm for the standard, determine the %w/w Ni in the alloy. 

Bromine ttifluoride is commercially available at a minimum purity of 98% (108). Free Br2 is maintained at less than 2%. Other minor impurities are HF and BrF. Free Br2 content estimates are based on color, with material containing less than 0.5% Br2 having a straw color, and ca 2% Br2 an amber-red color. Fluoride content can be obtained by controlled hydrolysis of a sample and standard analysis for fluorine content. Bromine ttifluoride is too high boiling and reactive for gas chromatographic analysis. It is shipped as a Hquid in steel cylinders in quantities of 91 kg or less. The cylinders are fitted with either a valve or plug to faciUtate insertion of a dip tube. Bromine ttifluoride is classified as an oxidizer and poison by DOT. 

Quantitative Phase Analysis. Once the identity of the components in a sample are known, it is possible to determine the quantitative composition of the sample. There are several different methods for doing a quantitative analysis, but the most rehable method is to use mixtures of known composition as standards. The computer can determine quantitatively the relative amounts of each component in the unknown sample. For accurate calculations of relative amounts in the unknown sample, it is necessary that the sample and standards have uniform distributions of crystaUites. Often the sample and standards are rotated during data collection to provide a more even distribution of crystaUites which diffract. 

A chromatogram is produced by developing a TLC/HPTLC plate, but it may be necessary to employ one of the reagents descnbed to make the positions, structures and sizes of the chromatogram zones apparent so that they can be recorded If the Rf values are the same a companson of the sizes of the zones of the sample and standard substances gives an indication for estimating the amounts If, as a result of matnx effects, the Rf values of sample and standard are not the same then their... 

B W Woodget and D Cooper Samples and Standards, ACOL-Wiley, Chichester, 1987... 

C. Dilution method. The sample and standard solution are contained in glass tubes of the same diameter, and are observed horizontally through the tubes. The more concentrated solution is diluted until the colours are identical in intensity when observed horizontally through the same thickness of solution. The relative concentrations of the original solutions are then proportional to the heights of the matched solutions in the tubes. This is the least accurate method of all, and will not be discussed further. 

Note. It is good practice to make the fluorescence measurements for samples and standards as close together as possible to minimise any drift in instrument response. 

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

Before the pH electrode is used, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modem pH meters, such as the one shown in Figure 5-8, are microcomputer controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to 0.001 pH unit, all with few basic steps. The electrode must... 

A weighed amount of sample is dissolved in a mixture of propanone and ethanoic acid and titrated potentiometrically with standard lead nitrate solution, using glass and platinum electrodes in combination with a ferro-ferricyanide redox indicator system consisting of 1 mg lead ferrocyanide and 0.5 ml 10% potassium ferricyanide solution. The endpoint of the titration is located by graphical extrapolation of two branches of the titration plot. A standard solution of sodium sulfate is titrated in the same way and the sodium sulfate content is calculated from the amounts of titrant used for sample and standard. (d) Water. Two methods are currently available for the determination of water. 

If the sample and standard have essentially the same matrices (e.g., air particulates or river sediments), one can go through the total measurement process with both the sample and the standard in order to (a) check the accuracy of the measurement process used (compare the concentration values obtained for the standard with the certified values) and (b) obtain some confidence about the accuracy of the concentration measurements on the unknown sample since both have gone through the same chemical measurement process (except sample collection). It is not recommended, however, that pure standards be used to standardize the total chemical measurement process for natural matrix type samples chemical concentrations in the natural matrices could be seriously misread, especially since the pure PAH probably would be totally extracted in a given solvent, whereas the PAH in the matrix material probably would not be. All the parameters and matrix effects. Including extraction efficiencies, are carefully checked in the certification process leading to SRM s. 

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