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WDXRF as an Investigative and Analytical Tool Using Small Spot/Mapping and UniQuant
Process control, failure troubleshooting and in situ identification are now all possible using a standard laboratory WDXRF spectrometer with the advent of small spot/mapping capability. To narrow a sample surface analysis area down to 0.5mm and obtain chemistry of an inclusion or irregularity has helped many a scientist solve problems once only possible through more complex techniques. An analysis at 0.5mm by WDXRF is in no way a challenge to other methods such as SEM but still offers the ability to produce answers which could help resolve a process situation rapidly and easily. The test results provided in this article were performed using the Thermo Scientific™ ARL™ PERFORM’X Sequential X-Ray Fluorescence Spectrometer.
A surface stain or inclusion on a material can quickly be targeted and elementally defined, showing where it originated in the process. This answer can define the breakdown or wearing of a particular step in the process which requires attention, servicing, or replacement. Identification of larger minerals from field exploration can be both defined and chemically imaged in 2 and 3 dimension displays. Homogeneity testing of a coating over a large or small area, forensics testing on minute samples or scattered residue, the list goes on and on.
Elemental distribution for the examples mentioned above are easily displayed in an intensity format and normally this would provide the required answer to the common question "What is it?" but if concentration resolution is required the operator may run into some trouble – how do you calibrate for a stain? What reference materials are readily available for a metal inclusion in a wire? How do you quantify components on a circuit board without removing them? This is where the second step to total small spot analysis comes into play – the Thermo Scientific™ UniQuant™ Analysis Package.
Small spot/mapping description
Small spot analysis is a rather easy concept to describe. The first step is for the XRF unit to image the sample surface. From this image it is possible to click on areas of interest to designate analysis areas; for mapping the operator would select one of a number of shapes and enlarge of contract it to a size which best contains the area of interest. The sample is excited through the use of a primary X-ray beam. The secondary X-rays emitted are collimated through an aperture which allows only the 0.5mm area of interest excitation to continue on through the beam path to the detector. Obviously, the collimator and sample surface are tightly aligned. Small spot analysis is a selection of one or more unique and individual points on a sample surface, each one producing a singular analysis result whereas mapping is the joining of these individual points into a unified pattern to produce a 2- or 3-dimensional presentation along with intensity/concentration results for the selected area.
UniQuant software uses the original factory calibration to determine concentrations for completely unknown samples by subjecting the intensities acquired from the samples to comprehensive mathematical algorithms. These algorithms correct for matrix effects as well as inter-elemental effects to provide results. UniQuant is also unique in its method of intensity measurements. Unlike other semi-quantitative software programs, UniQuant uses a method known as peak hopping instead of a continuous scan technique to acquire the intensities for all measurable elements. The procedure of peak hopping allows for faster analysis by not wasting measurement time on any location that an element peak will not be found or the time to climb and descend existing peak shoulders. UniQuant software will measure every theoretical 2-theta angle for each element, including alternative lines for some heavier elements and background positions. By focusing the elemental counting times to only peak locations, UniQuant is able to provide more accurate results and lower detection limits compared to other scan based semi-quantitative methods. UniQuant uses a Primary Beam Filter device to remove spectral interference from Rhodium Tube lines. An interesting feature is that the counting time for each analytical line can be defined separately depending on the main interest of the analyst. This last point makes this analysis routine extremely useful for small spot analysis since the obtained intensities will be far lower than the standard 29mm aperture and therefore require longer count times.
Mapping example: Mapped elements in a geological sample
Mapping imaging is a very helpful way of better understanding a problem. The 2D images can be viewed as individual element distributions or overlaid to give a more comprehensive correlation of the elements as a group. The 3D images are single element display and can be rotated for a full 360 degree visualization or even a birds-eye view. While most maps are collected as intensity only images, empirical calibrations can also be used to fully quantify the result. Geological samples can offer the most interesting and informative mapping images. In this example, one can see the base material being Mn due to its uniform base and strong intensity response but also note the other elements present with a large formation of Si of to one side and the presence of possibly a Ca, S, P vein running through the center of the sampled area. The figure to the left shows a mapping of a Mn Rock:
The examples presented here are only a fraction of the applications that many analysts are using today. Investigative abilities using WDXRF have been shown to save thousands of dollars in process monitoring. With UniQuant software, small samples are no longer a hindrance due to size and lack of calibrating materials. Coating thickness across a surface is now possible to review without the need of specialized instrumentation. Quantification using empirical calibrations on undersized or irregular shaped materials is possible through the use of a standard laboratory WDXRF, even in-situ. Mapping/small spot analysis brings many benefits to the analyst which simply was not available in the past, especially when combined with a standardless routine such as UniQuant.