XRF Principles

Electron Microprobe Analysis (µ-XRF or EDS)

Energy-Dispersive-X-ray-Fluorescence-ED-XRF-600x350

Micro x-ray fluorescence (µ-XRF) gets its name because these instruments are designed to analyze very small spot sizes.

They are fundamentally like EDXRF systems, and have seen a similar development path through the years. There are a number of differences between u-XRF and EDXRF to make it worthwhile to classify them separately.

Hardware

The typical µXRF system has the usual EDX hardware, but has several key differences, most notably, downward facing optics. Special x-ray tubes are used with a smaller spot size on the target, so that there is less beam spread after collimation. Usually direct incident radiation is used, because filters tend to make the x-ray pattern broader. Systems usually have several collimators ranging from 12 microns to a millimeter or more, and can automatically switch between them. Polycapillary optics that focus the x-rays are becoming increasing common in high end systems, and they are capable of producing spot sizes in the 50-200 micron range at much higher x-ray flux rates than with an ordinary through hole collimator. Samples are normally presented on a movable XYZ stage that can be manually operated in some low cost systems, or automatic and programmable in higher end system. The most sophisticated µXRF analyzers can even raster the sample producing a dot map of the surface. Cameras and lasers are typical devices used to aid in sample positioning. The detectors can vary from proportional counters in the low end systems to PIN diodes, Si(Li), or SDD detectors in higher end systems. There is even a µXRF system that uses WDX optics.

Applications

The biggest application by far is plating analysis, particularly for analyzing printed circuit boards. Literally thousands of instruments have been sold into this application, primarily for analyzing copper, gold, and tin-lead solder. But there are numerous other applications, and more sophisticated plating analysis systems can measure several layers at once.

High-end µXRF systems are normally sold into forensic, fine art, and archeological applications. These systems can do an excellent job of elemental fingerprinting to either match evidence taken from a crime scene or match materials from a painting or other work of art with known authentic materials.

Nondispersive X-Ray Fluorescence (NDXRF)


Non-dispersive x-ray fluorescence (NDXRF) got its start in the 1920’s when Ross and other experimenters discovered that they could isolate an x-ray line for an element by using two filters made of different elements over two detectors. One filter absorbs the elements x-rays, while the other transmits them.

The difference in counts between the two matched detectors with balanced filters is the net intensity and is related to that elements concentration. When combined with earlier work that demonstrated that elements could be measured by measuring total x-ray intensities from some simple samples, a new and powerful method was born. Unfortunately it was almost 50 years later when small microprocessor based analyzers were built in the 1970’s that NDXRF started to make a commercial impact.

Hardware

NDXRF has the least expensive hardware of any of the XRF methods, because it only requires a few low costs components. It needs an x-ray source, usually either a radioisotope such as Fe-55, Cd-109, Cm-244, Am-241 of Co-57, or a small x-ray tube. And it requires a detector such as an ionization chamber or Geiger-Mueller Counter, which does not need to be energy dispersive. While Ross used two detectors, the more common approach is to use a single detector and use a filter wheel or tray to position the filters over the detector in sequence. In addition to the Ross method a single filter (Hull Method) or no filter at all may be used to measure some elements..

In commercial devices it is most common to see a proportional counter used as the detector since it is a low resolution EDXRF detector. The advantage of a proportional counter is that it can be configured to not count the backscattered source x-rays making the overall background counts substantially lower. At the same time a proportional counter instrument may be used for EDXRF analysis, making it a hybrid EDX/NDX instrument. With x-ray tube source devices, x-ray tube filters may be used in combination with specially selected target anodes to produce optimal sources for exciting the elements in a sample. The non-dispersive XRF method is very powerful, and cases where an appropriate filter pair exists and can successful isolate an elements wavelength it is often possible to match the performance of a WDX analyzer at a tenth the cost using 100 times less source intensity. .

Applications

One of the most common applications is measuring phosphorus, sulfur and chlorine in oil. Generally either a Fe-55 radioisotope, or an x-ray tube with either a Pd, Ag, or Ti target is used to excite those elements. By looking at the absorption edges for various materials it is easily seen that chlorine has an absorption edge above chlorine in energy, sulfur has and absorption edge between chlorine and sulfur, and phosphorus has an absorption edge between sulfur and phosphorus. X-rays do not readily excite hydrogen and carbon in the base matrix and the detector windows readily absorb their x-rays, and so they aren’t measured.

In this case chlorine can be measured by using chlorine as a transmitting filter and sulfur as an absorbing filter. The difference between the counts of x-rays off the oil sample and through the filter correlates to the chlorine concentration. The filers are electronically balanced by measuring the intensity on a blank and introducing a coefficient that when multiplied by one intensity yields zero net count. Similarly a sulfur and phosphorus pair of filters can be used to measure sulfur. A single filter can be used to measure phosphorus since there are usually no measurable elements below phosphorus that would produce counts.

The matter is confused somewhat because it is difficult to produce good sulfur and phosphorus filters, so usually an element with an L absorption edge at the appropriate energy is used instead, Mo or Nb for S, and Zr for P. Since the heavy metals are denser the filters are usually much thinner than their K absorption edge counterparts.

Total Reflection X-Ray Fluorescence (T-XRF)


Total Reflection x-ray fluorescence (TXRF) and the fundamentally related Grazing emission x-ray fluorescence (GEXRF) rely on scatter properties near and below the Bragg angle to reduce background intensities and improve detections limits an order of magnitude or more over more traditional XRF instruments.

If light is directed at a smooth surface at a very small angle (typically less than 0.5 degree for x-rays) virtually 100% of the light will be reflected at an equally small angle. This is the same principle relied on for polycapillary optics. A few x-rays will excite atoms immediately at the surface, and those atoms emit their characteristic radiation in all directions. Because there is virtually no backscatter into the detector, extraordinary detections limits can be achieve.

GEXRF turns the theory around and takes advantage of the fact that when x-rays are directed at a surface they will not be scattered at an angle below the Bragg angle. A detector that only detects x-rays coming off a surface at an angle less than the Bragg angle, will only detect fluorescence x-rays and not background scatter.

Hardware

TXRF instruments are usually very sophisticated and expensive pieces of equipment with finely tuned optics. The x-ray tubes are usually very high in power, several kilowatts, and must have a small spot size on the anode. A long collimator or wave-guide is needed to restrict the angle to less than the Bragg angle. Using multilayers in the wave-guide can improve the efficiency. The sample needs to be finely and reproducibly polished and positioned precisely with respect to angle and height. A detector is positioned above the surface. Given the sophistication of these systems, Si(Li) or other high resolution detectors are used in most systems.

Some people prefer the GEXRF variation. The x-ray tube can be directed at the sample with little regard to spot size or angle. This saves on a lot of hardware expense. A detector and collimator assembly is positioned so that only x-rays coming from less than the Bragg angle are counted.

Advantages and Disadvantages

While these techniques can achieve amazing performance, they are seldom used. The principle problems are that only a few products are suitable for TXRF analysis without a substantial amount of sample preparation. The other problem is that the optical alignment is so critical that minor vibrations and temperature changes make it necessary to re-align the optics, and/or calibrate the instrument. These problems, in addition to the high cost of most existing systems, have limited the use of these techniques to date.

Wavelength Dispersive X-ray Fluorescence (WD-XRF)


Wavelength dispersive x-ray fluorescence (WDXRF) is the old timer among commercial x-ray spectrometers, since the method works without high-resolution solid-state detectors.

Instead, WDXRF instruments rely on diffractive optics to give them high spectral resolution. WDX spectrometers with simple electronic counting circuits were around well before the computer age, and are still the workhouse and leading performer for routine XRF analysis.

Hardware

WDXRF can be relatively simple and inexpensive, or complex and very expensive depending on the number of optical components. WDX instruments use a x-ray tube source to directly excite the sample. Because the overall efficiency of the WDXRF system is low, x-ray tubes in larger systems are normally rated at 1-4 kilowatts. There are some specialized low power systems that operate at 50 to 200 watts. A diffraction device, usually a crystal or multilayer, is positioned to diffract x-rays from the sample toward the detector. Diffracted wavelengths are those that satisfy the 2dnsin relationship, where d is the atomic spacing within the crystal, n is an integer, and theta is the angle between the sample and detector. Other wavelengths are scattered very inefficiently. Collimators are normally used to limit the angular spread of x-rays, to further improve the effective resolution of the WDX system. Because the detector is not relied on for the systems resolution it can be a proportional counter or other low-resolution counter capable of detecting a million or more counts per second.

All the components can be fixed to form a fixed single WDX channel that is ideal for analyzing a single element. A simultaneous WDX analyzer will have a number of fixed single channels usually formed in a circle around the sample with the x-ray tube facing upward in the middle. Other WDX analyzers use a goniometer to allow the angle to be changed, so that one element after another may be measured in sequence. This type of instrument is a sequential WDX analyzer. There are also combined sequential/simultaneous instruments as well.

Applications

WDXRF can be used for a tremendous variety of elemental analysis applications. It can be used to measure virtually every element form Na to Pu in the periodic table, and some instruments can be used for quantitative or semi-quantitative work for even lighter elements. It can measure elemental concentrations ranging from a few ppm to nearly 100 percent. It can be used for monitoring major components in a product or process or the addition of minor additives. WDXRF is extremely popular in the geological field and is often used for measuring raw minerals, and finished products composed of minerals.

Advantages and Disadvantages

See EDXRF vs WDXRF comparison: Energy Dispersive X-ray Fluorescence(ED-XRF) vs Wavelength Dispersive X-ray Fluorescence (WDXRF) Comparison

Energy Dispersive X-ray Fluorescence (ED-XRF)


Energy dispersive x-ray fluorescence (EDXRF) relies on the detector and detector electronics to resolve spectral peaks due to different energy x-rays.

It wasn’t until the 1960’s and early 1970’s that electronics had developed to the point that high-resolution detectors, like lithium drifted silicon, Si(Li), could be made and installed in commercial devices. Computers were also a necessity for the success of EDXRF even if they were often as large as the instrument itself.

Hardware

EDXRF is relatively simple and inexpensive compared to other techniques. It requires and x-ray source, which in most laboratory instruments is a 50 to 60 kV 50-300 W x-ray tube. Lower cost benchtop or handheld models may use radioisotopes such as Fe-55, Cd-109, Cm-244, Am-241 of Co-57 or a small x-ray tube. The second major component is the detector, which must be designed to produce electrical pulses that vary with the energy of the incident x-rays. Most laboratory EDXRF instruments still use liquid nitrogen or Peltier cooled Si(Li) detectors, while benchtop instruments usually have proportional counters, or newer Peltier cooled PIN diode detectors, but historically sodium iodide (NaI) detectors were common. Some handheld device use other detectors such as mercuric Iodide, CdTe, and CdZnTe in addition to PIN diode devices depending largely on the x-ray energy of the elements of interest. The most recent and fastest growing detector technology is the Peltier cooled silicon drift detector (SDD), which are available in some laboratory grade EDXRF instruments.

After the source and detector the next critical component are the x-ray tube filters, which are available in most EDXRF instrument. There function is to absorb transmit some energies of source x-rays more than other in order to reduce the counts in the region of interest while producing a peak that is well suited to exciting the elements of interest. Secondary targets are an alternative to filters. A secondary target material is excited by the primary x-rays from the x-ray tube, and then emits secondary x-rays that are characteristic of the elemental composition of the target. Where applicable secondary targets yield lower background and better excitation than filter but require approximate 100 times more primary x-ray intensity. One specialized form of secondary targets is polarizing targets. Polarizing XRF takes advantage of the principle that when x-rays are scattered off a surface they a partially polarized. The target and sample are place on orthogonal axis’ to further minimize the scatter and hence the background at the detector.

Fixed or movable detector filters, which take advantage of non-dispersive XRF principles, are sometimes added to EDXRF devices to further improve the instruments effective resolution or sensitivity forming a hybrid EDX/NDX device.

Applications

EDXRF can be used for a tremendous variety of elemental analysis applications. It can be used to measure virtually every element form Na to Pu in the periodic table, in concentrations ranging from a few ppm to nearly 100 percent. It can be used for monitoring major components in a product or process or the addition of minor additive. Because XRF’s popularity in the geological field, EDXRF instruments are often used alongside WDXRF instruments for measuring major and minor components in geological sample.

Advantages and Disadvantages

See EDXRF vs WDXRF comparison: Energy Dispersive X-ray Fluorescence(ED-XRF) vs Wavelength Dispersive X-ray Fluorescence (WDXRF) Comparison

Energy Dispersive X-ray Fluorescence(ED-XRF) vs Wavelength Dispersive X-ray Fluorescence (WDXRF) Comparison

In an effort to save money, space, sample preperation time, or simply to add an analytical instrument to their process many companies will decide to evaluate energy dispersive x-ray fluorescence (ED-XRF) analyzers as a substitute for their standard wavelength dispersive x-ray fluorescence (WD-XRF) analysis.

This is very common with geological applications where WDX is the benchmark, but it occurs with many other applications as well. What all these companies eventually discover is that ED-XRF is not the low cost drop in replacement that they thought it would be but has significant differences, some positive and some negative, that must be considered in the evaluation process or else dealt with later when it may be less convenient.

As most scientifically minded persons know, the energy of the light photon increases as the wavelength decreases, so in an EDX spectra the low atomic number elements are on the left while they are to the right of a WDX spectra. But the difference goes far beyond that.

WD-XRF

The WD-XRF analyzer uses a x-ray source to excite a sample. X-rays that have wavelengths that are characteristic to the elements within the sample are emmitted and they along with scattered source x-rays go in all directions. A crystal or other diffraction device is placed in the way of the x-rays coming off the sample. A x-ray detector is position where it can detector the x-rays that are diffracted and scattered off the crystal. Depending on the spacing between the atoms of the crystal lattice (diffractive device) and its angle in relation to the sample and detector, specific wavelengths directed at the detector can be controlled. The angle can be changed in order to measure elements sequentially, or multiple crystals and detectors may be arrayed around a sample for simultaneous analysis.

ED-XRF

The ED-XRF analyzer also uses an x-ray source to excite the sample but it may be configured in one of two ways. The first way is direct excitation where the x-ray beam is pointed directly at the sample. Filter made of various elements may be placed between the source and sample to increase the excitation of the element of interest or reduce the background in the region of interest. The second way uses a secondary target, where the source points at the target, the target element is excited and fluoresces, and then the target fluorescence is used to excite the sample. A detector is positioned to measure the fluorescent and scattered x-rays from the sample and a multichannel analyzer and software assigns each detector pulse an energy value thus producing a spectrum. Note that there is absolutely no reason why the spectra cannot be displayed in a wavelength dependant graph format.

Points of Comparison

1. Resolution: It describes the width of the spectra peaks. The lower the resolution number the more easily an elemental line is distinguished from other nearby x-ray line intensities
a. The resolution of the WDX system is dependent on the crystal and optics design, particularly collimation, spacing and positional reproducibility. The effective resolution of a WDX system may vary from 20 eV in an inexpensive benchtop to 5 eV or less in a laboratory instrument. The resolution is not detector dependant.
b. The resolution of the EDX system is dependent on the resolution of the detector. This can vary from 150 eV or less for a liquid nitrogen cooled Si(Li) detector, 150-220 eV for various solid state detectors, or 600 eV or more for gas filled proportional counter.
ADVANTAGE WD-XRF – High resolution means fewer spectral overlaps and lower background intensities.
ADVANTAGE ED-XRF – WDX crystal and optics are expensive, and are one more failure mode.

2. Spectral Overlaps:Spectral deconvolutions are necessary for determining net intensities when two spectral lines overlap because the resolution is too high for them to be measured indepedantly
a. With a WDX instrument with very high resolution (low number of eV) spectral overlap corrections are not required for a vast majority of elements and applications. The gross intensities for each element can be determined in a single acquisition.
b. The ED-XRF analyzer is designed to detect a group of eleemnts all at once. The some type of deconvolution method must be used to correct for spectral overlaps. Overlaps are less of a problem with 150+ eV resolution systems, but are significant when compared to WDXRF. Spectral overlaps become more problematic at lower resolutions.
ADVANTAGE WD-XRF – Spectral deconvolution routines introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the error.

3. Background: The background radiation is one limiting factor for determining detection limits, repeatability, and reproducibilty.
a. Since a WDX instrument usually uses direct radiation flux the background in the region of interest is directly related to the amount of continuum radiation within the region of interest the width of which is determined by the resolution.
b. The ED-XRF instrument uses filters and/or targets to reduce the amount of continuum radiation in the region of interest which is also resolution dependant, while producing a higher intensity x-ray peak to excite the element of interest.
Even – WDX has an advantage due to resolution. If a peak is one tenth as wide it has one tenth the background.
EDX counters with filters and targets that can reduce the background intensities by a factor of ten or more.

4. Source Efficiency: efficiently the source x-rays are utilized determines how much power is needed to make the system work optimally. Higher power costs much more money.
a. Every time an x-ray beam is scattered off a surface the intensity is reduced by a factor of 100 or so. For any XRF system intensity is lost in the process of exciting the sample, but a WDX analyzer also looses a factor of 100 in intensity at the diffraction device, although some modern multilayers are more efficient. The sample to detector path length is often 10 cm or more introducing huge geometrical losses.
b. With direct excitation the EDX system avoids wasting x-ray intensity. When filters are used the 3 to 10 times more energy is required, and when secondary targets are used 100 times more energy is required making the total energy budget simlar between Seconday target EDX and WDX systems before the path length is considered. An EDX system typically has sample to detector path lengths less than 1 cm.
ADVANTAGE ED-XRF – In order to achieve similar counts at the detector a WDX system needs 100-1000 times the flux of a direct excitation EDX system and 10-100 times the flux of a secondary target system. This one proinciple reason WDX systems cost more.

5. Excitation Efficiency: Usually expressed in PPM per count-per-second (cps) or similar units, this is the other main factor for determining detection limits, repeatability, and reproducibility. The relative excitation efficiency is improved by having more source x-rays closer to but above the absorption edge energy for the element of interest.
a. WD-XRF generally uses direct unaltered x-ray excitation, which contains a continuum of energies with most of them not optimal for exciting the element of interest.
b. ED-XRF analyzers may use filter to reduce the continuum energies at the elemental lines, and effectively increaseing the percentage of x-rays above the element absorption edge. Filters may also be used to give a filter fluorescence line immediately above the absorption edge, to further improve excitation efficiency. Secondary targets provide an almost monochromatic line source that can be optimized for the element of interest to achieve optimal excitation efficiency.

X-Ray Fluorescence Theory


Although X-ray fluorescence spectroscopy is no longer regarded as a new instrumental technique for elemental analysis, ongoing evolutionary developments continue to redefine the role of this important analytical tool.

From the demonstration of the first principles in the 1960’s to the development of the first commercial instruments in the 1970’s, the increasing availability of affordable computational power has a least been as important to the desirability and acceptance of the technology as innovative hardware design. With the widespread availability and use of a 32-bit microprocessor personal computer as the industry standard platform, X-ray fluorescence spectroscopy has become a useful and complimentary laboratory tool to other techniques.

X-Ray Fluorescence Theory

An electron can be ejected from its atomic orbital by the absorption of a light wave (photon) of sufficient energy. The energy of the photon (hv) must be greater than the energy with which the electron is bound to the nucleus of the atom. When an inner orbital electron is ejected from an atom, an electron from a higher energy level orbital will be transferred to the lower energy level orbital. During this transition a photon maybe emitted from the atom. This fluorescent light is called the characteristic X-ray of the element. The energy of the emitted photon will be equal to the difference in energies between the two orbitals occupied by the electron making the transition. Because the energy difference between two specific orbital shells, in a given element, is always the same (i.e. characteristic of a particular element), the photon emitted when an electron moves between these two levels, will always have the same energy. Therefore, by determining the energy (wavelength) of the X-ray light (photon) emitted by a particular element, it is possible to determine the identity of that element.

For a particular energy (wavelength) of fluorescent light emitted by an element, the number of photons per unit time (generally referred to as peak intensity or count rate) is related to the amount of that analyte in the sample. The counting rates for all detectable elements within a sample are usually calculated by counting, for a set amount of time, the number of photons that are detected for the various analytes’ characteristic X-ray energy lines. It is important to note that these fluorescent lines are actually observed as peaks with a semi-Gaussian distribution because of the imperfect resolution of modern detector technology. Therefore, by determining the energy of the X-ray peaks in a sample’s spectrum, and by calculating the count rate of the various elemental peaks, it is possible to qualitatively establish the elemental composition of the samples and to quantitatively measure the concentration of these elements.

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