Bruce Scruggs

The measurement of trace elements is important across a wide variety of materials characterization problems. When measuring small glass fragments collected from crime and accident scenes, forensics experts analyze trace strontium (Sr) and zirconium (Zr) typically unintentionally incorporated into the glass during manufacturing as one point of identification or comparison. Traces of lead (Pb) and cadmium (Cd) are measured in a wide variety of commercial materials for verification of adherence to environmental regulations such as the European Union's Restriction of Hazardous Substances (RoHS) directive. Trace chlorine (Cl) is depth profiled in concrete to quantify ion permeation from deicing agents. In these cases and many others, improvements in detector performance can lead to improvements in micro-XRF sensitivity and faster data collection, thereby providing users with faster, more accurate results. 

Up to now, the Orbis micro-XRF analyzer could be configured with two different X-ray detectors. 

•Apollo XRF-XL Silicon Drift Detector (SDD) 

The Si{Li} detector technology is the most mature XRF detector technology having been developed and commercialized about 50 years ago. Over the last decade or so, XRF detector developments have moved towards electrically cooled silicon drift detectors providing comparatively better spectral resolution at significantly higher throughput rates. The newly launched Apollo XRF-ML-50 detector is an SDD with improved active area yielding a larger solid angle of signal collection while maintaining high spectral resolution and throughput. This allows for improved overall sensitivity and measurement speed in many applications. 

Improvements in speed and sensitivity of analysis can be achieved with detectors and associated signal processing electronics by improving spectral resolution, enhancing signal collection by size and positioning of the detector or increasing signal processing speed, also referred to as "throughput." The general goal in optimizing and improving these detector parameters is to avoid augmenting one at the sacrifice of the others. For example, it would do little good to increase the detector size while making significant sacrifices in detector resolution or throughput. Such a detector could collect more signal but would be unable to provide a high resolution spectrum or process the additional signal collected. This application note will explain the benefits of the new 50 mm

 Apollo XRF-ML50 detector and describe in which analytical situations increased speed and sensitivity can be realized. 

Figure 1: Orbis Micro-XRF analyzer with optional viewport.			

Improvements in speed and sensitivity begin with the amount of sample X-ray signal collected by the detector, which is referred to as the "solid angle" by system designers. Detector solid angle is proportional to the signal collection area of the detector, known as active area, and inversely proportional to the square of the distance between sample and detector as shown in equation 1. 

In general, augmenting active area leads to larger solid angles and increased X-ray signal collection as long as the distance between sample and detector can be optimized. In Table I, a comparison of performance parameters is made between the three detectors which are available on the Orbis system. The structural and performance characteristics of the 30 mm

 SDD detectors are very similar except for the active area while the throughput of the Si{Li} detector is limited. 

Table I: Comparison of Orbis detector structural and performance parameters			

 detector yields improvements in speed and sensitivity in analytical problems where the 30 mm

 detector would be signal starved. When a detector is signal starved, the detector's processing electronics still have available capacity to process more signal. In these cases, having a detector with a greater solid angle allows for more X-ray signal to be passed to the processing electronics. As long as the larger SDD's spectral resolution and throughput are similar to that of the smaller SDD, which is the case here, improvements in speed and sensitivity can be achieved with the larger detector. Typical materials analysis problems which fall into this category include measurements of light element matrices such as glasses, plastics, and aluminum alloys; measurements of trace elements where heavier filters are needed for best sensitivity such as trace Pb and Cd in solders and plastics and measuring thin residues, corrosion, and coatings on light element matrices. Table II compares the limits of detection for several elements spanning the XRF spectral range. 

Table II: Limits of detection (ppm) for detectors available on Orbis

For the data collected in Table II, the SDD detectors were operating in the signal starved regime where the X-ray tube is run at full power for maximum excitation. Improvements in sensitivity of the 50 mm

 SDD are approximately consistent with the increase in solid angle of the 50 mm

In applications where improved sensitivity is not needed, the 50 mm

 SDD can be used for increased productivity. The proportionality between sensitivity as described by the limit of detection (LOD) and measuring time is shown in equation 2. 

Hence, the increased sensitivity provided by the 50 mm

 SDD can be traded for faster measuring time. For example, using equation 2 and the improvements in measuring sensitivity shown in Table II, the 50 mm

 SDD can achieve the same measurement sensitivity obtained with the 30 mm

 SDD with a nominal 40% reduction in measuring time. 			

 detector yields improved sensitivity and faster measuring times in materials analysis where the 30 mm

 detector would be signal starved, but not in cases where the 30 mm

 detector is saturated. Saturation refers to when the detector's rate of signal collection exceeds the throughput of the analyzer electronics. Hence, sensitivity and measurement speed are limited by throughput. In this analytical situation, the X-ray tube is generally running below maximum power limiting the input XRF signal to the analyzer throughput. Typical materials analysis problems which fall into this category include measurement of major elements in steels, heavier transition metal alloys and measuring thin residues, corrosion, and coatings on substrates comprised of heavier transition metal alloys. Having equal throughputs, both the 30 mm

 SDDs provide similar speed and sensitivity for samples which can saturate the detectors; however, the 50 mm

 SDD allows for a greater reduction in X-ray tube power, which in turn improves tube life. 

The data in Table II does show that the 80 mm

 Si{Li} has better sensitivity for Cd measured at the 23.1 keV Ka line. The Si{Li} detecting crystal is about 10 times thicker than the SDD detecting sensor allowing > three times the absorption efficiency for the Cd(Ka) X-ray signal. Hence, the better Si{Li} detection limit for Cd is the result of superior solid angle and absorption at high X-ray energies. 

Two detector operating regimes have been identified, that is,starvation and saturation. These regimes are dictated by the types of samples analyzed. When the detector is signal starved, the Apollo XRF ML-50 SDD can provide nominally 20 to 30% better sensitivity or 40% reduction in measurement time in comparison to the standard 30 mm

 SDD. In saturation, using the ML-50 SDD, tube current can be reduced to a greater extent, extending X-ray tube life. 

Articles

The measurement of trace elements is important across a wide variety of materials characterization problems. When measuring small glass fragments collected from crime and accident scenes, forensics experts analyze trace strontium (Sr) and zirconium (Zr) typically unintentionally incorporated into the glass during manufacturing as one point of identification or comparison.

Improved Sensitivity with the New Apollo XRF ML-50 Detector on the Orbis Micro-XRF Analyzer

The measurement of trace elements is important across a wide variety of materials characterization problems. When measuring small glass fragments collected from crime and accident scenes, forensics experts analyze trace strontium (Sr) and zirconium (Zr) typically unintentionally incorporated into the glass during manufacturing as one point of identification or comparison. Traces of lead (Pb) and cadmium (Cd) are measured in a wide variety of commercial materials for verification of adherence to environmental regulations such as the European Union's Restriction of Hazardous Substances (RoHS) directive. Trace chlorine (Cl) is depth profiled in concrete to quantify ion permeation from deicing agents. In these cases and many others, improvements in detector performance can lead to improvements in micro-XRF sensitivity and faster data collection, thereby providing users with faster, more accurate results. 			

				     o Liquid nitrogen cooled			

				      o High spectral resolution			

					• Apollo XRF-XL Silicon Drift Detector (SDD)				

The Si{Li} detector technology is the most mature XRF detector technology having been developed and commercialized about 50 years ago. Over the last decade or so, XRF detector developments have moved towards electrically cooled Silicon Drift Detectors providing comparatively better spectral resolution at significantly higher throughput rates. The newly launched Apollo XRF-ML-50 detector is an SDD with improved active area yielding a larger solid angle of signal collection while maintaining high spectral resolution and throughput. This allows for improved overall sensitivity and measurement speed in many applications. 

Improvements in speed and sensitivity of analysis can be achieved with detectors and associated signal processing electronics by improving spectral resolution, enhancing signal collection by size and positioning of the detector or increasing signal processing speed, also referred to as "throughput". The general goal in optimizing and improving these detector parameters is to avoid augmenting one at the sacrifice of the others. For example, it would do little good to increase the detector size while making significant sacrifices in detector resolution or throughput. Such a detector could collect more signal but would be unable to provide a high resolution spectrum or process the additional signal collected. This application note will explain the benefits of the new 50 mm

  Apollo XRF-ML50 detector and describe in which analytical situations increased speed and sensitivity can be realized.  

Detector Solid Angle α (Active Area)/(Distance)

  SDD detectors are very similar except for the active area while the throughput of the Si{Li} detector is limited.

Table I: Comparison of Orbis detector structural and performance parameters											

  detector yields improvements in speed and sensitivity in analytical problems where the 30 mm

  detector would be signal starved. When a detector is signal starved, the detector's processing electronics still have available capacity to process more signal. In these cases, having a detector with a greater solid angle allows for more X-ray signal to be passed to the processing electronics. As long as the larger SDD's spectral resolution and throughput are similar to that of the smaller SDD, which is the case here, improvements in speed and sensitivity can be achieved with the larger detector. Typical materials analysis problems which fall into this category include measurements of light element matrices such as glasses, plastics, and aluminum alloys; measurements of trace elements where heavier filters are needed for best sensitivity such as trace Pb and Cd in solders and plastics and measuring thin residues, corrosion, and coatings on light element matrices. Table II compares the limits of detection for several elements spanning the XRF spectral range. 

  SDD are approximately consistent with the increase in solid angle of the 50 mm

Figure 1: Orbis Micro-XRF Analyzer with optional Viewport.			

  SDD can be used for increased productivity. The proportionality between sensitivity as described by the limit of detection (LOD) and measuring time is shown in equation 2. 

                         [2] 

  SDD can be traded for faster measuring time. For example, using equation 2 and the improvements in measuring sensitivity shown in Table II, the 50 mm

  SDD can achieve the same measurement sensitivity obtained with the 30 mm

  SDD with a nominal 40% reduction in measuring time. 

  detector yields improved sensitivity and faster measuring times in materials analysis where the 30 mm

  detector would be signal starved, but not in cases where the 30 mm

  detector is saturated. Saturation refers to when the detector's rate of signal collection exceeds the throughput of the analyzer electronics. Hence, sensitivity and measurement speed are limited by throughput. In this analytical situation, the X-ray tube is generally running below maximum power limiting the input XRF signal to the analyzer throughput. Typical materials analysis problems which fall into this category include measurement of major elements in steels, heavier transition metal alloys and measuring thin residues, corrosion, and coatings on substrates comprised of heavier transition metal alloys. Having equal throughputs, both the 30 mm

  SDDs provide similar speed and sensitivity for samples which can saturate the detectors; however, the 50 mm

  SDD allows for a greater reduction in X-ray tube power, which in turn improves tube life.  

  Si{Li} has better sensitivity for Cd measured at the 23.1 keV Ka line. The Si{Li} detecting crystal is about 10 times thicker than the SDD detecting sensor allowing > three times the absorption efficiency for the Cd(Ka) X-ray signal. Hence, the better Si{Li} detection limit for Cd is the result of superior solid angle and absorption at high X-ray energies. 

  SDD. In saturation, using the ML-50 SDD, tube current can be reduced to a greater extent, extending X-ray tube life.  

			tel. (201) 529-4880, fax (201) 529-3156

Improved Sensitivity with the New Apollow XRF ML-50 Detector on the Orbis Micro-XRF Analyzer

Building on more than 10 years of Micro-XRF experience, the Orbis spectrometer yields a system with excellent Micro-XRF capability while setting a new standard in analytical flexibility. The Orbis incorporates a unique motorized turret integrating video and X-ray optics allowing coaxial sample view and X-ray analysis. The turret can accommodate two additional collimators along with the X-ray optic for a total of three X-ray beam sizes to expand the Orbis analytical capabilities beyond traditional Micro-XRF analysis. Primary beam filters can be used with all spot sizes available on the turret to allow true XRF analytical capabilities in a micro-spot analysis. The working distance is increased to allow analysis over rougher sample topography without sacrificing signal intensity. 

The Orbis is a table-top unit with powerful, easy-to-use analysis software. Orbis users can make elemental analyses on small samples such as particles, fragments and inclusions, or automated multipoint and elemental imaging analysis on larger samples with all the benefits and simplicity of an XRF analyzer. Benefits include: 					

Minimal sample preparation (for example, no sample  coating is necessary)

Improved sensitivity for many elements in comparison   to SEM/EDS

Inclusion and coating thickness analysis with the  penetrating power of X-rays

Small spot XRF analysis is typically used for analysis of fragments, targeted analysis on larger components, and high-resolution elemental imaging. The Orbis system can be equipped with a 30 μm ultra high intensity poly-capillary capable of providing fast, sensitive XRF analysis. Glass fragments are typically analyzed by micro-XRF for criminal and industrial forensic applications. With the Orbis poly-capillary and primary beam filter system, fast, non-destructive analysis can be done with improved sensitivity at a spot size of 30 to 60 μm (FWHM) depending on the elemental X-ray energy. 

In Figure 1, analysis of glass using the 30 μm poly-capillary and primary beam filters for comparative purposes and compositional analysis is shown. Analysis of chlorine down to a limit of detection of approximately 25 ppm was achieved using a filter to remove the overlapping Rh (L) tube  scatter line.  

Figure 1: Orbis XRF spectrum (logarithmic intensity scale) of SRM 93A glass using two primary beam filters.			

In another example (see Figure 2), the Orbis poly-capillary is used to elementally image a portion of a fading black and white photo using the Ag (L) signal to digitize the image. This was done as part of an initial effort to evaluate the suitability of micro-XRF imaging for the archival preservation of Ag-based photos.  

Figure 2: (Above) Orbis montage video composite of mapped area (27.5 mm Ã 16.5 mm) on a faded black/white photo. The image is of a horse-drawn wagon on a city street. (Below) Ag (L) elemental image of photo using 30 Î¼m poly-capillary at approximately 250 DPI pixel density.			

In a variety of analytical scenarios, XRF analysis using a larger analytical beam diameter is more advantageous than probing the sample with a small beam analysis. Analyzing inhomogeneous materials, for example, with a small probe beam yields information on the level of inhomogeneity of the material while analyzing with a large, collimated beam can give a compositional analysis more consistent with the average composition with improved high energy line sensitivity. A larger analytical beam is also useful for measuring sample inhomogeneity over a larger area. A good example of this comes from the measurement of thickness and composition on a photovoltaic panel. 

A thin layer of copper, indium, gallium, and selenium (CIGS) semiconductor material is commonly used in the production of thin film solar cells. In the following example, CIGS was sputter coated onto a large molybdenum-coated glass substrate. The molybdenum metal coating serves as an electrode and electrical contact. Hence, the coating structure analyzed was a CIGS coating over a molybdenum coating on the glass substrate. The main goal of the XRF analysis was to determine how large the edge effects are in such a sputter coating process for the CIGS layer.  

Measurements were made on a 40 mm × 40 mm section of the coated solar panel sampled from an edge of the panel located at the limits of the effective area of the sputter coating process. A 1 mm collimator was used with a contiguous matrix of analysis points to measure the homogeneity of the CIGS coating over the entire 1600 mm

  area. Orbis coating software was employed which has the capability to measure thickness and composition of up to 5 layers with up to 10 elements in alloy layers. The coating software calculations are based on a Fundamental Parameter algorithm which allows measurements to be made without standards in a full computer modeling mode and to accept the input of a minimal number of standards to provide the computer calculations with a calibration reference. In this example, one standard (CIGS layer: 2.15 μm; 18.9 At% Cu, 15.9% In, 4.9 At% Ga, 60.3 At% Se; Mo layer: 0.28 μm) was used to increase the accuracy of the thickness/composition calculations.  

Figure 3: CIGS layer thickness measured over a 40 mm Ã 40 mm area near edge of effective sputtering region.			

The results of the coating analysis for the CIGS layer are shown in Figures 3 and 4. Figure 3 shows the film thickness of the CIGS layer. A gradient in film thickness is clearly observed extending approximately 2 cm from either edge of the panel. Figure 4 shows the concentration of the Se component of the CIGS layer. The overall average measured Se concentration around 54 At% is consistent with the reported effort to enrich the CIGS layer in Ga well beyond what is typical, approximately 4 to 5 At%. The composition of the CIGS layer is highly uniform up to within roughly 2 mm of the edge of the panel. This suggests that while the overall amount of material being sputter deposited on the edge of the panel is decreased, the overall composition of that material is unchanged.  

Figure 4: Selenium atomic % composition measured over a 40 mm Ã 40 mm area near edge of effective sputtering region.			

The Orbis micro-XRF spectrometer was developed to provide a state-of-the-art micro-XRF analyzer with excellent analytical flexibility. Orbis systems provide the user with: 					

Access to both ultra-high intensity X-ray optics and larger beam collimators

Primary beam filters at all spot sizes, small and large

Multilayer coating thickness and composition analysis

With this analytical flexibility, the Orbis system is capable of providing solutions to a wide variety of problems suited to XRF analysis.  

tel. (201) 529-4880, fax (201) 529-3156  

Building on more than 10 years of Micro-XRF experience, the Orbis spectrometer yields a system with excellent Micro-XRF capability while setting a new standard in analytical flexibility. The Orbis incorporates a unique motorized turret integrating video and X-ray optics allowing coaxial sample view and X-ray analysis. The turret can accommodate two additional collimators along with the X-ray optic for a total of three X-ray beam sizes to expand the Orbis analytical capabilities beyond traditional Micro-XRF analysis.

Bruce Scruggs

Articles

Improved Sensitivity with the New Apollo XRF ML-50 Detector on the Orbis Micro-XRF Analyzer

Improved Sensitivity with the New Apollow XRF ML-50 Detector on the Orbis Micro-XRF Analyzer

The Orbis Micro-XRF Analyzer Series