[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Methodological developments and applications 615 ga y

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Methodological Developments and Applications 615 Ga, Y, Co). The liquid sample (5–50 µ l) is then pipetted onto the pre-cleaned quartz reflector and dried. The drying process is performed in a clean envi- ronment, taking only a few minutes to avoid contamination. Applying a hot plate or an infrared lamp can reduce the drying time. The residue formed has a size of 2–6 mm depending on the droplet volume and content. Furthermore, the horizontal dimensions can be reduced with surface preconditioning of the sample reflector with silicon oil to make it hydrophobic. The vertical extension of the residue is normally a few hundred nanometers, again depending largely on the actual composition, volume, and drying procedure. Uniform distribu- tion of the internal standard along with the analyte, as well as the minimum residue height ( 50 nm) is the two main prerequisites of the accurate analy- sis [312]. For calibration, a multi-element standard solution is prepared taking into account a limited number of elements in order to avoid line overlapping. It is recommended to use two standard solutions each containing five to seven elements, but one common element should be present (normally the internal standard). Spectra are acquired from the calibration samples and calculation is based on the relative sensitivities with respect to the internal standard and is determined from the calibration spectrum by the formula: S x = N x /c x N std /c std , (7.33) where S is the relative sensitivity of the elements ( x ) with respect to the internal standard (std). N and c stand for the net peak area and the concen- tration of the given element. This elegant method is widely used in TXRF quantification because it is reliable for all types of samples and matrices, and a single measurement is sufficient for the multielement calibration [313]. The calibration curve is a plot from the individual sensitivity values versus atomic number. A polynomial fit of the experimental sensitivities makes possible the determination of those sensitivity values of elements, which were not present in the standard sample. Elements to be analyzed can be divided into two groups; one for those that can be detected from the K α line and the oth- ers detected from the L α line. Sensitivity values of L α lines are about one order smaller than those values of K α lines, therefore two separate calibra- tion curves are established. Once performed, the calibration remains valid for several weeks and only a routine check is recommended before measurement. The sample to be analyzed is spiked with the internal standard as mentioned before, and measured with the same instrumental settings as the calibration. The unknown concentrations are calculated by: c x = N x /S x N std c std , (7.34) where N is the net peak area of the analyte ( x ) and the internal standard (std); S the relative sensitivity; c the concentration.
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