Atomic
spectrometry is the most obvious choice of technique to be used for metal
determination in vegetation samples. In WD-XRF spectrometry the characteristic
radiation emitted from the sample is separated into wavelengths using a
diffraction device. Usually in WD-XRF spectrometers, the analysis of different
elements is carried out in a sequential way by scanning synchronously the
orientation of the mono chromator device and the detector (2θ). In
multi-channel spectrometers, the use of several diffraction devices set-ups
allows one to measure several elements simultaneously.
DEFINITION
Wavelength dispersive x-ray fluorescence spectroscopy (WD XRF) is a non-contact, non-destructive technique used to measure elemental composition, elemental concentration per unit area, and film thickness. Due to its acute element sensitivity, it is particularly useful for identifying trace elements.
WORKING OF WDXRF
In a WDXRF measurement, the sample is irradiated with high energy mono-chromatic x-rays. This irradiation stimulates the emission of characteristic x-rays associated with elements present in the material. WDXRF spectrometers use Bragg diffraction from crystals within the instrument to produce wavelength-separated peaks, each associated with a specific element. This yields semi-quantitative information about the elements present in the sample matrix and their atomic ratios.
WDXRF systems are based on Bragg’s law, which states that crystals will reflect x-rays of specific wavelengths and incident angles when the wavelengths of the scattered x-rays interfere constructively. While the sample position is fixed, the angles of the crystal and detector can be changed in compliance with Bragg’s law so that a particular wavelength can be measured. Only x-rays that satisfy Bragg’s law are reflected.
SAMPLE PREPARATION
Solid Samples
Solid samples can be anything from unprepared pieces of metal or electronics or plastics to cut and polished metal samples. The ideal sample for XRF analysis will have a perfectly flat surface. Irregular sample surfaces change the distance from the sample to the x-ray source and introduce error. All XRF systems are calibrated based on a fixed sample to source distance. Changing the distance can increase or decrease the intensity coming from any element contained in the sample.
Loose Powders
The analysis of loose powdered material usually requires that the sample be placed into a plastic sample cup with a plastic support film. This insures a flat surface to the X-ray analyzer and the sample to be supported over the X-ray beam. The more finely ground the sample the more likely it is to be homogeneous and have limited void spaces providing for a better analysis. Sufficient powder should be used to insure infinite thickness is obtained for all of the elements of interest.
Pressed Pellets
The process includes grinding a sample into a fine powder, ideally to a grain size of <75um, mixing it with a binding /grinding aid and then pressing the mixture in a die at between 20 and 30T to produce a homogeneous sample pellet. The binding /grinding aid is usually a cellulose wax mixture and combines with the sample in a proportion of 20%-30% binder to sample.This sample preparation approach provides better analytical results than loose powders because the grinding and compression creates a more homogeneous representation of the sample with no void spaces and little sample dilution. This leads to higher intensities for most elements than loose powders. Pressed pellets are still susceptible to particle size effects if not ground fine enough, but the biggest limitation to this approach is the mineralogical effects which most predominantly affect the analysis of major elements. Pressed pellets are excellent for the analysis of elements in the ppm range.
Liquids
Liquids are prepared by pouring them into a plastic sample cup in the same way as loose powdered samples. There are limited options for analyzing liquid samples and the main trick is to choose the correct support film that provides a balance of strength and transmission capabilities and contamination. Mylar is a good general purpose film often used for the analysis of sulfur in fuels or lubricating oils. Polypropylene has better transmission than Mylar but has a lower tensile strength. Kapton is the "bomb proof” film but dramatically attenuates your signal for lighter elements and is susceptible to strongly basic solutions.
Procedure
- Preparation
of the calibration samples: Use of standard addition method and press
pellet technique.
- Optimization
of the measuring conditions: element line, tube settings, primary filter,
analyzing crystal and collimator.
- Set
up of the background positions
- PHA
window evaluation
- Calibration
measurements
- Calibration
curve corrections
- Drift
sample selection and measurements
INSTRUMENT
The instrument used in this work had a special feature of a tube-above configuration where the X-ray tube and the optics are placed above the measured sample. The other configuration would be the other way around, tube below configuration. Placing the tube and the optics above the sample eliminates contamination falling from analyzed samples but may not be as practical for liquid and powder samples, as the sample is turned upside down during the analysis.
X-ray tube
The purpose of the X-ray tube is to generate the primary X-ray beam. Since every element have different excitation energies, the X-ray tube needs to provide a stable, high intensity X-ray beam with a wide range of energies for efficient sample excitation. There are a few different tube types, the end-window and side-window types being the most typical ones. However, all the X-ray tubes work on the same principle utilizing an electrical field for electron acceleration and suitable anode material for deceleration.
Excitation Condition
The sample excitation is based on the excitation condition which consists of the applied voltage and the current in the tube. Also the created excitation beam can be filtered with primary filters if beneficial.
Tube Voltage and Current Tube voltage is the positive charge (kV) of the target and the tube current (mA) is the electron flow from the heated filament to the target. Both of them are adjustable parameters. The total voltage and current applied regulate the overall measuring intensity. More powerful continuous X-ray is produced when the applied voltage and current is increased. To detect a specific element, efficient amount of X-ray energy is needed but excessive energy does not change the excitation effect for the specific element, but may negatively affect the peak resolution.
Generally, high voltage is recommended to be used with heavy elements and high current with light elements.
Target Material
The target material of the X-ray tube affects the excitation as well. Typically an XRF spectrometer is equipped with one X-ray tube with one selected target material but some special XRF spectrometers have dual target X-ray tubes which have two targets in one X-ray tube. For the target materials most common alternatives are rhodium (Rh), chromium (Cr) and tungsten (W).
Bremsspectrum
The X-ray tube distributes simultaneously the full range of the needed energies, referred as Bremsspectrium, which produces the desired characteristic X-rays from the sample. However, part of the Bremsspectrum can be scattered from the surface of the sample and reach the detector creating background. While most part a continuous background is produced, the characteristic element lines of the anode material and its scattering usually produces a major background peaks.
Primary Filters
The primary beam from the tube can be modified by using primary filters. The primary filters are placed between the X-ray tube and the sample and their purpose is to reduce or eliminate disturbances of the Bremsspectrum.
Collimator
The optic sensitivity and resolution are determined mainly in WDXRF by the chosen combination of collimator and analysing crystal. The secondary X-rays are scattered in all directions from the sample and the purpose of the collimator is to deliver the secondary X-rays from the sample reliably to the analysing crystal. Collimator consists of parallel soller slits. The soller slits are a set of metal plates which the secondary X-rays are set to travel through. Spacing between the plates affects the sensitivity and the resolution. The less space there is between the plates, the less secondary X-rays will make the way through but higher resolution is gained.
Detector
An X-ray detector converts X-ray photons coming from the crystals into a measurable energy form, voltage pulses. Because the full range of X-ray wavelengths is rather wide, 0.012-12 nm (100-0.1 keV), two different detectors are used: gas proportional counter and scintillation counter detector.
APPLICATIONS
Wavelength-dispersive X-ray fluorescence (WD-XRF) spectrometry involves the classification and quantification of several kinds of biological samples. Multiple applications of WD-XRF spectrometry are anticipated in the mining industries, nuclear power industry, medical diagnostics, forensics, environmental, and in agricultural studies.