Measuring trace element concentrations and complex elemental composition are essential in environmental, recycling, and industrial process control applications. For instance, compliance with regulatory norms is needed to confirm that heavy metals are below acceptable levels, reclaim valuable precious metals catalysts, and ensure the highest quality control in manufacturing. Energy dispersive X-ray fluorescence (EDXRF) is a simple and powerful technique for the analysis of elemental composition. A tip to meet the challenges of trace analyses and measurement of complex elemental composition is the use of Cartesian Geometry EDXRF. Rigaku NEX CG II EDXRF benchtop analyzer brings the latest innovations with Cartesian Geometry indirect excitation.

Cartesian Geometry is indirect excitation where the X-ray tube, secondary targets, sample, and detector are positioned at 90° angles. Secondary targets are used in the excitation beam path instead of traditional tube filters used in direct excitation XRF. The X-ray tube does not irradiate the sample directly but first beams off a secondary target, which is a metal or a carbon crystal. This allows for several advantages. When the source X-rays from the tube first irradiate a secondary target, the Bremsstrahlung background X-rays are essentially eliminated and replaced by the fluorescence of the target, creating monochromatic excitation with little or no background X-rays scattering to the detector. In the case of a carbon crystal as the secondary target, polarization is achieved when in the 90° angle, further reducing background for measuring the low energy light elements sodium through chlorine.

Innovation in EDXRF combines the Cartesian Geometry indirect excitation with updated electronics and a wide area high-performance silicon drift detector (SDD). The SDD EDXRF detector collects X-rays simultaneously and gives high count rate throughput with sharp resolution, or peak width, thus minimizing the need for element peak overlap corrections. Combined, the optics of Cartesian Geometry yield very low or no background and high X-ray intensity collection of net characteristic X-rays from each element without the detector throughput being filled with background X-rays. This allows for very low detection limits and the measurement of samples containing many elements.

Cartesian Geometry EDXRF is advantageous for analyzing many types of trace and complex applications. A shortlist follows here:

Consumer goods, plastics and polymers, RoHS compliance, and manufacturing quality control of additives in polymers

Measuring ultra-low sulfur and metals in petroleum fuels such as diesel and gasoline for ASTM, U.S. EPA compliance, and other international testing norms

Trace particulate matter in air filters for air quality monitoring

Reclaim precious catalysts, including refining catalysts in the petrochemical industry and automotive catalysts in used catalytic converters

Figure 1 shows the measurement of low or trace platinum (Pt) in recycled automotive catalysts. Cartesian Geometry provides flat background and well-resolved peaks for the measurement of Pt in the presence of the other matrix elements from the converter honeycomb core, which include Al, Si, Ti, Fe, Ni, Cu, Zn, Pb, Rb, Sr, Y, Zr, Ba, La, Ce, Pr, and the other catalysts Pd and Rh.

Cartesian Geometry EDXRF is ideal for measuring ultra-low and trace element concentrations. It excels in complex applications with trace elements and variable base matrices, such as testing agricultural soils and plant materials, analyzing finished animal feeds, measuring waste oils, environmental monitoring, and many others.

Articles

Cartesian Geometry EDXRF for Trace Analysis and Measurement of Complex Elemental Composition

In a demonstration of the new cryoRaman low-temperature Raman microscope’s capabilities, a tungsten diselenide flake on Si/SiO

 was investigated and its numbers of layers were determined.

Interest in Raman microscopy at cryogenic temperatures has been increasing across a range of scientific fields over the past two decades. Though initially the primary driver of applications was the carbon nanotubes and graphene community, a recent surge in the number of published cryogenic Raman studies is dominated by research on transition metal dichalcogenides and van der Waals heterostructures.

To meet both existing and evolving market requirements, cryoRaman was jointly developed by cryogenic specialists attocube systems and Raman microscope developer WITec GmbH. It offers excitation wavelengths from vis to NIR with optimized spectrometers, 1.6 to 300 K operating temperatures, high magnetic fields, unique cryogenically compatible Raman-specific objectives, and an exceptionally precise piezoelectric scan stage.

In the following demonstration, cryoRaman was used to investigate a WSe

, characterize its numbers of layers, and to establish the system’s sub-400 nm lateral resolution (1).

To measure the tungsten diselenide flake, a 532 nm laser was used for excitation, and a RayShield coupler allowed detection of Raman bands down to ~±10 1/cm. The experimental setup also included a UHTS300 spectrometer optimized for detection in the visible range with an 1800 g/mm grating and a back-illuminated CCD camera.

The cryostat cooled the sample to 120 K. An LT-APO 532-Raman objective with a numerical aperture of 0.82 and piezoelectric positioners controlled through the Control Five software were used. The acquired data set was evaluated with the Project Plus software package.

Images of the tungsten diselenide that show different numbers of layers were acquired and combined in the false color image shown in Figure 1a. It is color-coded according to the Raman spectra shown in Figure 1b. The image is 30 × 30 μm

 in size with 400 × 400 pixels and was created with an integration time of 150 ms per spectrum. A zoomed-in image (Figure 1c) shows the low-wavenumber area with the most prominent peaks for the double and triple layers as labelled.

The cryoRaman microscopy system offers researchers a fast, user-friendly and versatile tool for examining transition metal dichalcogenide samples and determining their numbers of layers at very low temperatures. Its sub-400 nm lateral resolution together with low-wavenumber detection, high magnetic fields, and polarization control all make it unique in the marketplace. These capabilities will be of great use as research on 2D materials and TMDs continues.

(1) Extracted from the white paper: Cutting-edge Cryogenic Raman Microscope. https://www.witec.de/assets/Literature/Files/AppNote-cryoRaman.pdf

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Cryogenic Raman Imaging of a Tungsten Diselenide Flake

Here we explore the benefits of using a variable angle attenuated total reflection (ATR) accessory to generate the most informative spectral data. Example spectra of toluene are shown.

The appearance of an ATR spectrum is influenced by several variables including the refractive index of the ATR crystal (

), the number of internal reflections, the quality of contact between the sample and the angle of incidence (AOI) of the infrared (IR) beam. Having the flexibility to change some parameters, specifically the AOI and ATR crystal type, allows optimization of spectral data to obtain the most useful information for the application.

Mid-infrared spectra of toluene were collected using the variable angle, multiple reflection ARTMax II accessory (PIKE Technologies) configured with an AMTIR trough plate. With the ATRMax II, the AOI may be adjusted by turning the center mirror assembly position dial, where the resulting effective AOI is a function of the face angle of the AMTIR crystal and the accessory position dial setting. For data presented, the face angle of the AMTIR crystal was 45

. Spectra were collected at several AOI values. 30 s of data were co-added for each measurement at a resolution of 4cm

Most single reflection and multiple reflection ATR accessories have a fixed nominal AOI of 45

. When using these accessories to measure samples with a refractive index of roughly 1.5, the depth of penetration (d

 for a diamond or ZnSe ATR element and 0.6 microns for a Ge ATR element. d

 decreases at shorter wavelengths and increases at longer wavelengths.

For total internal reflection to occur, the AOI must be greater than the critical angle (θ), which is defined as θ =sin

 increases exponentially as the angle of incidence nears the 

. By probing deeper into the sample—making measurements closer 

—the magnitude of the absorbance bands increases significantly. By using a variable angle ATR accessory such as the PIKE Technologies’ VeeMAX III single reflection or the multiple reflection ATRMax II, the absorbance band strength may be optimized to provide maximum chemical information by easily changing the AOI, whether the application requires probing more deeply into the sample by making measurements closer to 

 or probing closer to the surface at grazing AOI.

Unlike a variable angle, single reflection ATR where only the d

 is changing with varying the AOI, the number of reflections in addition to the d

 is changing when using a variable angle, multiple reflection accessory. Figure 1 shows toluene spectra collected at 43

 effective AOI where the number of reflections on the sample is 7 and 5, respectively. The spectrum collected at 43

 shows absorbance bands over 50% greater than the spectrum collected at 50

. Therefore, the increase in absorbance band strength is because of two contributing factors, increased d

 and the number of reflections. In addition, changing the ATR crystal material provides added flexibility to accommodate various ATR probing depths into the sample.

Variable angle ATR accessories can optimize spectral data for a given application. By simply changing the angle of incidence, it is possible to probe more shallow or deeper into the sample, as needed.

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Optimizing Spectral Data Using a Variable Angle ATR

Raman spectroscopy monitors the kinetics and reaction completion of adhesive curing.

Epoxy resins are distinguished by durability, strong adhesion, chemical resistance, and other specialized properties, which are contingent upon proper mixing. In this application note, we monitor the curing process for a consumer-grade, two-part epoxy.

Devcon® 15 Minute Epoxy is comprised of bisphenol A diglycidyl ether resin and a polymercaptan/polyamine mixture. To monitor the curing process, we used an Ocean Insight QE 

-Raman+ spectrometer with a 300 mW, 785 nm laser, and a Raman probe suspended at a fixed distance over the epoxy. Signals were acquired immediately upon mixing and in 2–3 min intervals thereafter.

Two samples of different proportions of resin and hardener were mixed, at the recommended ratio of 1:1 and at a 2:1 hardener-to-resin mixture, then monitored. The analytical focus was on the mixture’s epoxide and thiol features, which diminish upon curing.

Raman bands corresponding to epoxide vibrational frequencies have been reported (1), with 1254 cm

 assigned to be the breathing mode of the epoxide ring (Figure 1). The intensity of this peak is linearly dependent on the concentration of epoxide groups in the resin mixture; its change over time is interpreted as consumption of the free epoxide groups during epoxy vitrification.

Raman spectroscopy is useful in monitoring epoxy curing processes in real time and can be extended to other types of reactive processes.

(1) V. Křesálek, and H. Vašková, “Raman spectroscopy of epoxy resin crosslinking,” Recent Researches in Automatic Control, Conference Paper, May 2011.

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Application Note: Raman Spectroscopy for Reaction Monitoring

Spectroscopic reaction monitoring is a proven tool in R&D and production environments, where it is used to gain insight into chemical reaction kinetics. In this application note, we showcase how dual-comb spectroscopy (DCS) can be used to monitor protein dynamics on microsecond to second timescales. Kinetics of irreversible protein reactions require an analytical technique that provides time-dependent infrared spectra in a single shot. First results have been validated with the recording of transients of the photoactivated proton pump bacteriorhodopsin with DCS and StepScan FT-IR (1).

Non-repetitive reactions with microsecond time resolution

Fourier transform infrared spectroscopy (FT-IR) with time resolutions faster than 10 ms is only possible with proteins, where the reactions can be repeated thousands of times. Many relevant protein reactions are nonrepetitive. In many cases reactions can be induced by caged compounds (2). As an example, we investigate the inhibiting Gi protein and the larger protein–protein complex of Gα

 with its cognate regulator of G-protein signaling (RGS). With DCS, we observe good data quality with 4 μs time resolution, three orders of magnitude faster than any FT-IR measurements.

Stopped-flow applications coupled with mid-IR spectroscopy—more than the sum of its parts! 

Additionally, DCS can be used to monitor fast kinetic reactions in combination with the stopped-flow technique. Mid-IR stopped flow was so far limited to timescales of RapidScan FT-IR on the order of 100 ms. The combination with DCS paves the way to new research possibilities for kinetic studies in the microsecond time regime. Simply speaking, it works by rapidly mixing reagents before a spectroscopic cell, as depicted in Figure 2.

For these applications, we partnered with TgK Scientific and BioLogic SA. It connects via an umbilical to a spectroscopic cell with CaF

The hydrolysis of methyl chloroacetate (MCA) is often carried out as a test for stopped flow systems and typically hydroxide concentrations of 0.1 M or less are used.

The results of a single shot of the hydrolysis reaction are shown in the Figure 3. Excellent signal to noise ratios are seen throughout the spectra and despite the low intensity of the MCA band, it can be fitted well with a monoexponential decay. A lifetime of 53 ms is found for this process at room temperature.

High resolution dual-comb spectroscopy with QCLs

Optical frequency comb spectroscopy has further proven a very useful tool for high resolution molecular spectroscopy of gaseous samples. Frequency combs based on quantum cascade lasers (QCL) offer the possibility to easily explore the mid-infrared spectral range (4–12 μm) but suffer from very large repetition frequencies (~10 GHz) which make them seemingly unsuitable for high resolution spectroscopy.

We have developed techniques to overcome this limitation, which we call the rapid-sweep (3) and step-sweep (4) technologies. In rapid-sweep, full spectra can be measured in only 6 ms by simultaneously sweeping both frequency combs. The step-sweep technique guarantees a very accurate knowledge of the frequency axis in a record time of several min.

Both techniques have been applied to the measurement of absorption spectra of gases with narrow absorption lines at high resolution and sensitivity. The measured spectra cover a range of more than 60 cm

 and the narrowest observed lines have a full width at half maximum of 15 MHz (0.0005 cm

The broadband coverage is useful for the analysis of complex gas mixtures. The low measurement noise makes the technique the ideal tool for studying gas concentration in environmental or industrial applications (Figure 4).

(1) https://doi.org/10.1021/acs.analchem.8b02531.

(2) https://doi.org/10.1021/acs.analchem.1c00666.

(4) Quantum cascade laser dual-comb spectroscopy (DCS) for methane gas mixture studies (spiedigitallibrary.org)

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Mid-Infrared QCL Dual-Comb Spectroscopy: From Non-Repetitive Protein Dynamics to Stopped-Flow Applications with Microsecond Time Resolutions

Optical phase-locking is a common technique for transferring the frequency and phase characteristics of one laser beam to another. It is commonly used in heterodyne metrology, free-space optical communications, and spectroscopic applications. In this technical note we discuss the implementation of offset phase locking using a digital phasemeter and characterize the stability of phase locking two lasers.

To put it simply, offset phase locking stabilizes phase difference between two lasers. This is done by a relatively simple process combining the outputs of the two lasers (via a beamsplitter or similar) and illuminating a photodetector with the combined beam. The combined power on the photodetector PPD can be described by:

 denote the phase of each laser. Note that the high order terms are generally outside the bandwidth of the photodetectors. It is important to recognize that even though the beat note contains phase information of the lasers, this information is contained in the argument of the signal and is relatively difficult to use in feedback systems in this form. To extract the phase from the beat note we use a phase detector.

Although a mixer and low pass filter are adequate for most offset phase locking system, they are not without their limitations. For one, the range of the mixer-filter combination is limited to ±π/2, and the phase output from the system is linear only when very close to zero. These range and linearity issues often make it difficult to deal with systems with large fluctuations. Moku:Pro Phasemeter uses phase-locked loops (PLL) as the phase detectors, and provides automatic phase unwrap function to extend the linear dynamic range of a standard mixer-type phase detector. The PLL can assist in initial lock acquisition, removal of nonlinear affects (such as cycle slipping), and ensure a more robust lock.

We demonstrated Moku:Pro's Phasemeter offset phase locking architecture using two nonplanar ring oscillators (NPRO) lasers. The beams of the master and slave lasers were combined by a beamsplitter and interfered at a photodiode as shown in Figure 1. The beat note was connected to Moku:Pro's input 1. The feedback signal was then connected to the frequency controller of the following laser.

To evaluate the phase stability of the lock, an additional Moku:Lab Phasemeter was connected to the photodetector to capture the frequency and phase drift as a function of time. Taking the amplitude spectral density of the frequency, we see that an improvement in the stability of over 4 orders of magnitude was measured, with relative frequency stability of 1 Hz/√Hz measured down to 0.1 Hz.

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Laser Frequency Offset Locking

Per- and polyfluoroalkyl substances (PFAS) are a large class of synthetic chemicals that present numerous analytical challenges, including their widespread presence in a variety of environmental samples. In the US, PFAS may contaminate public drinking-water systems that serve an estimated 19 million people. The U. S. Environmental Protection Agency (EPA) is continuing to aggressively implement their PFAS Action Plan—the most comprehensive cross-agency plan ever to address an emerging chemical of concern. The EPA has established a non-enforceable health advisory level of 70 parts per trillion (ppt) for the sum of pentadecafluorooctanoic acid (PFOA) and heptadecafluorooctanesulfonic acid (PFOS) for drinking water. The EPA is developing a potential rapid screening tool to identify total PFAS presence and absence in 2021. This eventual standard operating procedure will be used to quantify total organic florine (TOF).

Therefore, a sensitive, fast, and simple TOF detection method is needed for environment pollution monitoring and control. The method is based on a combination of solid-phase extraction (SPE) with high-resolution–continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS). In another Application Note, “Determination of Extractable Organically Bound Fluorine (EOF) in Surface Water”, the solid phase extraction (SPE) procedure has been fully discussed for the extraction of fluorinated com- pounds. HR-CS GF MAS was used to detect total fluorine via in-situ formation of gallium(I) fluoride (GaF) molecules. In this study, we focus on improving the performance of total florine (TF) analysis in wastewater by optimizing HR-CS GF MAS method and applying novel calibration strategies.

 A simple, fast, and sensitive detection of total fluorine in wastewater.

 With regard to the determination of a sum parameter of fluorinated compounds, a species-unspecific response of high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS) was developed.

For the quantification of fluorinated organic compounds as a sum parameter, a species-unspecific response for HR-CS GF MAS method needs to be developed and optimized. From our past research, gallium has proven to enable the most sensitive fluorine detection via diatomic GaF formation. The characteristic molecular absorption of the GaF is then detected by HR-CS-GF MAS. To achieve the best signal, the furnace tube was conditioned with the molecule forming agent (Ga) and modifiers (Pd/Mg/Zr/Ba) before the sample was injected. From the literature, it’s known that melting point/vapor pressure of the fluorinated compounds significantly impacts the recovery rates of the different compounds. Thus, the drying and pyrolysis procedure in the furnace program were optimized to keep possible losses of fluorinated compounds as low as possible. To evaluate the optimization approach, 24 different fluorine compounds were mixed for the QC sample. These 24 compounds were selected with different numbers of fluorine atoms and boiling points. Calibration strategies were developed to obtain the best recovery rate for this QC sample.

In this study, a fast, easy, and sensitive complementary method for total fluorine analysis in wastewater is presented. This study shows that the MAS method can be successfully applied using the contrAA 800 G for the measurements. The optimized furnace program and calibration strategy provide high sample throughput, sensitivity, and accuracy. Only 3.5 min/replicate is needed for each sample. A 100% recovery rate of QC sample is successfully achieved with an inorganic and organic mixture calibration strategy. The lowest LOD of 4 ppb is also achieved using the inorganic and organic mixture calibration strategy. The results of fluorine determination by HR-CS-GF MAS show good reproducibility and long-term stability. Sample dilution and spiking can be easily achieved with an AS-GF autosampler. No additional cleaning steps are required for this method.

A Novel Method for the Fast, Sensitive, and Simple Analysis of Total Fluorine in Wastewater

	Tribology–the study of friction, wear, and lubrication–is important in mechanical, electromechanical, and biological systems. Changes in materials due to abrasion, friction, lubrication, erosion, and corrosion result in failures of the moving components during use. These effects are usually minimized by careful surface finish design and lubrication. Recycling and refinishing surfaces can extend the lifetimes of components. By examining worn surfaces, better methods can be developed to prevent wear. This applications note uses infrared spectroscopy to study tribology changes in steel.

	The sample investigated was 28 mm in diameter by 8 mm thick with a carbon-based coating. The sample was lubricated, tested, and then cleaned with n-heptane, leaving chemisorbed species in the wear scratches on the surface.

	The spectra of the disc were measured on an FT-IR spectrometer equipped with the Harrick VideoMVP™ (see Figure 1). The background spectrum was collected from the clean ATR crystal. Then the sample was placed, face down, on the diamond crystal and positioned over the area of interest using the image on the display. The pressure applicator was lowered to apply ~240 N of force to compress the sample against the crystal. The spectrum and image were then recorded. All spectra were collected from 4000 to 400 cm

	Figure 2 shows spectra measured from a section with wear scratches in comparison to a shiny smooth section, along with photographs of the areas of each sample that were examined. The infrared spectrum of the wear scratches show a weak broad band in the –OH stretching region and additional bands at 1450 cm

, all of which are not present in the unworn section. This indicates that a residue from the lubricant or chemisorbed species remains in the worn area. More detailed analysis could be used to identify the chemisorbed species.

	The results presented here clearly show that diamond ATR can be used to examine tribology materials. The viewing capabilities and small sampling area of the VideoMVP allow for inspection of the wear scratches in comparison to the unworn areas.


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The Application Notebook 09-01-2021