Laboratory and Synchrotron Validation of $μ$-XRF for Sulfur Mapping in CTMP Paper Samples

TL;DR

The paper tackles the challenge of mapping sulfur sulfonation heterogeneity in CTMP pulp using a laboratory μ-XRF system and validates its performance against a high-resolution synchrotron benchmark. By scanning CTMP handsheets on a $32\times32$ grid, the study demonstrates that a lab XRF setup can detect sulfur at $2.31$ keV with comparable peak positions to the synchrotron reference, albeit with lower spectral resolution. Quantitative comparison using normalization, SNRs, and Wasserstein distance ($0.102$) shows overall agreement in sulfur distribution and intensity patterns, confirming the lab system as a practical tool for studying sulfonation at the fiber level. The work highlights the potential of accessible lab XRF to complement synchrotron investigations, supporting the development of energy-efficient, fiber-based packaging materials, while acknowledging acquisition-time and artifact considerations that can be mitigated with further technology improvements.

Abstract

The transition toward renewable, fiber-based packaging requires an improved understanding of chemical modifications in high-yield pulps such as chemithermomechanical pulp (CTMP). Sulfonation uniformity is essential for the energy-efficient production of high-strength CTMP pulp. However, laboratory methods only measure total sulfur and cannot illustrate its distribution at the fiber level, which can be visualized using $μ$-XRF. In this work, we present a laboratory $μ$-XRF system developed at Mid Sweden University and assess its capability to detect light elements in CTMP paper handsheets. A 32 $\times$ 32 point grid scan (1.6-1.6 mm$^2$ field of view, 50 $μ$m step, 300 s/point) successfully resolved sulfur K$α$ (2.31 keV) and calcium K$α$ (3.69 keV) fluorescence without helium flushing. Comparative measurements at the Elettra synchrotron confirmed consistency of sulfur peak position and spatial distribution, with higher spectral resolution and signal-to-noise ratio. Histogram analysis using Wasserstein distance metrics demonstrated close agreement between datasets despite differing acquisition conditions. These results demonstrate that laboratory XRF can reproducibly detect and map sulfur in CTMP fibers under ambient conditions, providing a practical tool to complement synchrotron studies and supporting the development of energy-efficient, fiber-based packaging materials.

Figures (7)

• Figure 1: Paper sample mounted on plastic holder for XRF measurement.
• Figure 2: XRF experiment setup. The left image shows the sample holder at the synchrotron. The right image shows the lab setup with the xy-stage controller, spectrometer, and source visible.
• Figure 3: Comparison of XRF spectra. Top: full spectra from the lab. Bottom: full spectra from the synchrotron.
• Figure 4: Sulfur peak fits from the laboratory (left) and synchrotron (right) datasets. Both spectra show the expected emission at ≈ 2.30, fitted with a Gaussian plus linear background model.
• Figure 5: Sulfur (S K ) maps for the lab and synchrotron datasets. The color bar indicates relative sulfur intensity.