Measurement of Residual Metal Catalysts by X-ray Fluorescence

Significance of the topic

Management and quantification of residual metal catalysts in organic products are critical for product safety, regulatory compliance and cost control. Residual metals such as palladium (Pd) are common in homogeneous catalytic processes (e.g., Suzuki-Miyaura cross-coupling) but are difficult to recover completely. Rapid, accurate screening tools that require minimal sample pretreatment help manufacturers perform risk assessments (e.g., in line with ICH Q3D) and reduce analytical turnaround time compared with traditional wet chemical techniques.

Objectives and overview of the study

The study demonstrates the capability of the ALTRACE energy dispersive X-ray fluorescence (EDXRF) spectrometer to quantify residual Pd catalyst at low mg/kg levels in organic reaction mixtures and solids. The goals were to evaluate sensitivity and repeatability, compare catalyst-removal methods (a commercial metal scavenger versus activated carbon), and show practical sample workflows with minimal pretreatment. The analysis focused on Pd resulting from a Suzuki-Miyaura fluorophore synthesis kit and tested filtrates and recovered solids after catalyst capture.

Methodology

Sample types and preparation:

• Reaction mixtures: two fluorophore syntheses catalyzed with palladium acetate (Samples 1 and 2).
• Filtrates after catalyst removal: solutions treated with SiliaMetS DMT metal scavenger or activated carbon, then centrifuged/filtered (Samples 3–6).
• Recovered solids: powders obtained by centrifugation of scavenger or carbon after capture (Samples 7–10).

Calibration and standards:

• Calibration solutions prepared at 0, 1, 5, 10, 20 and 100 mg/kg Pd (reference AAS solutions).
• Scattering X-ray internal standard (matrix correction) was used to compensate material-property effects.

Analytical approach:

• Direct measurement of liquid and powder samples placed into sample vessels sealed with 5 µm polypropylene film—no chemical dissolution required.
• EDXRF spectra analyzed quantitatively using the calibration curve and qualitatively by signal profile inspection.

Used instrumentation

The principal instrument and key settings used in the study:

• Instrument: ALTRACE energy dispersive X-ray fluorescence spectrometer (Shimadzu).
• Detector: Silicon drift detector (SDD).
• X-ray tube: Rh target, operating at 65 kV; tube current set to auto (µA).
• Primary filter: Filter #1; atmosphere: air (no vacuum).
• Integration (live) time: 300 s; dead time reference 100 s (max. 40%).

Main results and discussion

Calibration performance and sensitivity:

• The calibration curve (0–100 mg/kg) exhibited excellent linearity with a correlation coefficient of 0.9999.
• Calibration accuracy was reported around 0.12 mg/kg, and the system delivered reliable quantitation at or below ~1 mg/kg for Pd in prepared matrices.
• Repeatability: a 1 mg/kg Pd reference measured 10 times yielded mean 1.009 mg/kg, SD 0.023 mg/kg and coefficient of variation 2.3%—demonstrating high precision at trace levels.

Quantitative findings on catalyst removal:

• Initial reaction mixtures contained ~56 mg/kg Pd (Samples 1 and 2).
• After treatment, filtrates showed large reductions in Pd: measured values included 0.80 and 0.49 mg/kg for fluorophore 1 treatments and comparable low mg/kg values for fluorophore 2, demonstrating effective lowering of dissolved Pd to sub-1 mg/kg levels in some cases.
• Recovered solids (scavenger/carbon powders) contained much higher Pd concentrations (hundreds to over a thousand mg/kg) because the Pd mass was concentrated into a smaller solid phase.

Qualitative spectra and detectability:

• Clear Pd peaks were visible in EDXRF signal profiles even at concentrations below 1 mg/kg for filtrate samples, supporting both qualitative detection and quantitative capability down to ~1 mg/kg.

Practical considerations and limitations:

• EDXRF provides direct analysis of solids, powders and solutions without sample digestion, reducing sample prep time and potential contamination risks associated with wet-chemistry methods (AAS, ICP-AES, ICP-MS).
• EDXRF sensitivity depends on matrix homogeneity and appropriate calibration; matrix effects were mitigated here using an internal scattering standard and calibration in aqueous reference solutions.
• Typical EDXRF lower quantitation limits are near 1 mg/kg for heavy metals; the ALTRACE’s high-power tube and optical improvements push reliable quantitation to ~1 mg/kg or lower for Pd under the conditions described.

Benefits and practical applications

• Rapid screening: analysis times compatible with under-10-minute workflows per sample (instrument integration 300 s plus handling), enabling higher throughput than many wet methods for routine monitoring.
• Minimal pretreatment: solids and liquids measured directly in dedicated sample vessels, avoiding digestion and associated chemical hazards and time.
• Cost-efficiency: non-destructive direct measurement reduces reagent use and sample-processing labor.
• Regulatory utility: suitable for preliminary and routine measurements associated with risk assessment and control of residual catalysts (e.g., supporting ICH Q3D compliance workflows), and for process monitoring during catalyst-removal optimization.

Future trends and possibilities

• Hardware evolution: further increases in X-ray tube power, optimized optics and advanced detectors (improved SDDs) will lower detection limits and shorten acquisition times.
• Software and data corrections: enhanced matrix-correction algorithms, fundamental-parameter models and multivariate chemometric approaches can reduce calibration requirements and expand robustness across diverse matrices.
• Integration into workflows: coupling rapid EDXRF screening with targeted ICP-MS confirmatory testing can streamline QA/QC pipelines—EDXRF for high-throughput screening, ICP-MS for definitive trace-level confirmation where sub-ppm limits are required.
• Field and at-line deployment: more compact, rugged EDXRF instruments can enable near-process or at-line monitoring of catalyst removal in manufacturing environments.

Conclusion

The ALTRACE EDXRF spectrometer, with a high-power X-ray tube and optimized optics, demonstrated reliable detection and quantitation of palladium residual catalyst at approximately 1 mg/kg and below in both liquid filtrates and solid scavenger/carbon residues. The method delivers excellent linearity, high repeatability (CV ~2.3% at 1 mg/kg) and practical advantages including direct analysis of multiple sample phases and rapid turnaround. ALTRACE is therefore a useful tool for routine monitoring and process control of residual metal catalysts in industrial organic synthesis, complementing higher-sensitivity wet-chemical techniques when required.

Reference

1. Tamura Y. ALTRACE Energy Dispersive X-ray Fluorescence (EDXRF) Spectrometer: Measurement of Residual Metal Catalysts by X-ray Fluorescence. Application News, Shimadzu Corporation; First Edition: Mar. 2026.
2. Shimadzu Corporation. ALTRACE product documentation and analysis conditions. 2026.
3. International Council for Harmonisation (ICH). Q3D Guideline for Elemental Impurities. 2017 (regulatory context referenced in the application note).