Total Reflection X-Ray Fluorescence Spectrometer

What is the Total Reflection X-Ray Fluorescence Spectrometer?

A total reflection XRF spectrometer measures tiny sample residues on a flat carrier at grazing incidence. It is chosen for ultratrace analysis of small sample volumes in semiconductor, water and biological work.

Overview

In total reflection XRF the primary beam strikes a polished carrier at a very shallow angle, below the critical angle for total reflection. The beam does not penetrate the carrier, so scattered radiation and carrier fluorescence are greatly reduced and background is extremely low. A small aliquot of sample, often a few microlitres, is dried onto the carrier, leaving a thin residue that is measured directly. Detection limits reach picograms for many elements, and quantification is straightforward when the residue is thin enough to avoid matrix absorption and enhancement. The technique analyzes microelectronics surfaces, ultrapure water, semiconductor chemicals, biological fluids and environmental samples. Carriers must be highly polished and clean, and sample preparation takes place in a clean environment to avoid contamination at these levels.

How does it work?

A primary X-ray beam from a tube or radioisotope source ejects inner-shell electrons from sample atoms. Electrons from higher shells fall into the vacancies and emit X-ray photons whose energies equal the difference between the two levels, which is unique to each element. The intensity of each characteristic line is proportional to the amount of that element, so calibration with reference materials gives concentration. Energy-dispersive instruments use a semiconductor detector such as silicon drift to measure photon energies directly and record the whole spectrum at once. Wavelength-dispersive instruments use analyzing crystals to diffract specific wavelengths to a detector, giving better resolution and lower detection limits, especially for light elements. Matrix effects from absorption and secondary excitation are corrected by fundamental parameters or empirical influence coefficients, and sample form is matched to calibration standards.

What are the key features?

• Grazing-incidence geometry below the critical angle
• Very low background from carrier and scatter
• Sample volumes of a few microlitres
• Detection limits in the picogram range
• Simple quantification for thin residues
• Requires polished carriers and clean preparation

What are the technical specifications?

A typical total reflection x-ray fluorescence spectrometer configuration offers element range of Na to U, detection limit of picogram to nanogram range, sample volume of 1–50 µL, incidence angle of below critical angle, < 0.1°, carrier of polished quartz or silicon wafer and excitation of X-ray tube or synchrotron. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Total Reflection X-Ray Fluorescence Spectrometer is used in semiconductor wafer surface contamination, ultratrace metals in ultrapure water, biological fluids and tissue digests and reference material and nanomaterial analysis. Across the wider XRF class, laboratories also deploy it for elemental analysis of alloys, ceramics and glass, roHS and restricted substance screening and coating thickness and thin-film composition.

Which standards and certifications apply?

Relevant standards and methods include ASTM E1621 Wavelength dispersive X-ray fluorescence, ASTM D4294 Sulfur in petroleum products by EDXRF, ISO 3497 Metallic coatings — X-ray spectrometric thickness, IEC 62321 Determination of restricted substances in electrotechnical products, US EPA Method 6200 Field portable XRF for soil, ISO 13196 Soil quality — Screening of selected elements by EDXRF, IEC 61010-2-091 and ISO/IEC 17025. Certification candidates for this model are CE, Radiation safety enclosure and Cleanroom compatibility; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

XRF instruments are regulated radiation devices, so installation requires shielding, interlocks, radiation surveys and trained operators, with portable units also needing safe work procedures. Sample preparation strongly affects accuracy: surface contamination, insufficient thickness and particle size effects bias results, and powders are best pressed or fused. Calibration must match the sample matrix, and drift is monitored with check standards. Vacuum or helium systems need clean seals and gas supplies for light elements. Detector windows and X-ray tubes are wear parts with finite life. Keep records of tube hours, calibration and checks, and dispose of X-ray components according to radiation regulations.

How do I select the right Total Reflection X-Ray Fluorescence Spectrometer?

Select total reflection XRF when the sample volume is very small and detection limits must reach the picogram or low nanogram range, as in semiconductor, ultrapure water and biological analysis. Specify the elements, sample volume and carrier requirements. Confirm cleanroom capability, because contamination dominates the achievable detection limit.

How does it compare with other XRF variants?

Select an XRF configuration by element range, detection limit and sample type. Energy-dispersive instruments are compact, fast and well suited to routine screening and multi-element analysis, with moderate resolution. Wavelength-dispersive instruments cost more and are slower but excel for light elements, precise major-oxide analysis and low detection limits. Total reflection instruments reach ultratrace levels on small sample spots and serve semiconductor, water and biological analysis. Polarized energy-dispersive instruments lower background for trace work without the cost of wavelength dispersive optics. Micro-XRF adds spatial resolution for inclusions, coatings and forensic samples. Consider sample size, whether vacuum or helium purge is needed for light elements, automation level and the regulatory method your laboratory must follow before deciding.