Detector

Fig. 6.3 Total reflection X-ray fluorescence (TXRF) geometry quartz sampie holder

Fig. 6.3 Total reflection X-ray fluorescence (TXRF) geometry

Secondary target EDXRF

Fig. 6.4 Secondary target EDXRF geometry

The following details can be identified:

XT: X-ray tube

ST: Secondary target

S: Sample

F: Filter

C: Collinators

D: Detector

Fig. 6.4 Secondary target EDXRF geometry detector (gas) can be used. However, one shortcoming of such a device is the poor energy resolution of the detector (800-1,000 eV at 5.9 keV), making the quantification of the insufficiently resolved characteristic X-ray lines in the measured spectrum quite difficult. This problem can be overcome by using semiconductor detectors such as Si (Li), Si PIN or Si drift (SDD) detectors, which have energy resolutions of around 140 eV at 5.9 keV, and so quantification is easier and more accurate. Si(Li) detectors are cooled by liquid nitrogen, although the newer Si PIN or SDD detectors are smaller and are cooled electrically. These electrically cooled detectors and low-power air-cooled X-ray tubes are combined in portable EDXRF analyzer systems, which have recently become very popular for in situ element analysis.

Fig. 6.5 Spectra of the same sample analyzed in different EDXRF modes a) EDXRF mode with Cd-109 radioisotope source excitation, b) TXRF mode with X-ray tube (Mo anode) excitation, and c) EDXRF mode with X-ray tube (Mo anode) excitation

Energy [keV]

Fig. 6.5 Spectra of the same sample analyzed in different EDXRF modes a) EDXRF mode with Cd-109 radioisotope source excitation, b) TXRF mode with X-ray tube (Mo anode) excitation, and c) EDXRF mode with X-ray tube (Mo anode) excitation

Standard EDXRF usually allows the determination of elements from Z = 13 (Al) to Z = 92 (U). In some cases, depending on the installation of better detectors and the taking of measurements in a vacuum, elements with even lower Z values than 13 (F, Mg and Na) can be determined. The concentration ranges of these analyses vary from a few percent to a few |g g-1. The limits of detection decrease at higher Z values, and for lighter elements these are a few percent or a few parts of a percent for F, Mg, Na, Al, Si, a few 100 |g g-1 for S and Cl, a few 10 |g g-1 for Fe, Cu, Ni and Zn, and a few |g g-1 for Cd, Ag and U.

Both solid and liquid samples can be analyzed by EDXRF. In the case of solid samples, no special destructive chemical treatments of the sample are necessary. Approximately 100 mg of solid sample is sufficient for analysis; however, the sample should be very well ground up and homogenized because any inhomogene-ity of the pulverized solid samples can have a large influence on the accuracy of the measurements, especially in the case of lighter elements (Necemer et al. 2008). Using the milled and homogenized sample, a pellet is pressed out by a pellet die and a hydraulic press. The pellet is then analyzed directly by the EDXRF system.

For liquid samples, 100-1,000 ml of solution is required, and the elements are extracted from the sample by precipitation. Several precipitation agents are available for this purpose. For instance, the reagent ammonium pyrrolidine dithiocar-bamate (APDC) can be used for the precipitation of Cu, Fe, Ni and Pb. Note that any specific precipitating agent can selectively precipitate only some specific elements. Therefore, only certain specific elements can be determined in liquid samples using this method of sample preparation. The precipitated elements are separated from the liquid phase by filtration, and the precipitate that is gathered on the filter is measured directly by the EDXRF system. Due to the preconcentration of the precipitated elements, the limits of detection for these elements decrease to a few 10 ng g-1, which cannot be achieved with the analysis of solid samples. This approach is especially suitable for monitoring contaminating elements in water.

The main advantages of EDXRF analysis are its multielement capability and its nondestructive nature, as well as the simple sample preparation required that does not involve time-consuming sample destruction and only requires personnel with a minimum of manual skills. This is undoubtedly the cheapest and the simplest analysis technique among the chemical instrumental analytical techniques discussed here. Therefore, EDXRF is particularly well suited to environmental and plant biology studies, where a large number of samples need to be analyzed. In addition, EDXRF allows the analysis of nonmetals (P, S and Cl), which can also play important roles in plant biological processes.

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment