Using Micro X-ray Fluorescence (Micro-XRF) to Analyze Absorption of Wood Preservatives

Often, wood is chemically treated to safeguard it from physical degradation and preserve its structural integrity against natural elements, like moisture, insects, and fungus. The practice and concept of lumber treatment has been utilized for millennia. Previous examples of how wood was protected include the Romans brushing wood with tar in order to shield and increase the life span of the material, and the ancient Greeks who soaked wood in olive oil.

In state-of-the-art lumber treatments there are several processes and preservative chemicals that can be utilized, which are generally defined by the solvent utilized to carry the preservative into the wood (such as, light-organic solvents, oil, or water). Copper is used as a main constituent by many preservatives, especially by those which are water-borne. The preservatives are made to dissolve in a solution through chemical reactions; however, they have also recently been used by mixing micronized copper particles in an aqueous solution.

It is now possible to apply these different preservatives to the wood by utilizing a variety of processes, such as pressure-assisted methods, brushing, steeping, or even incorporating the preservative into the sap stream of a live plant. The final goal of these processes is to develop a deep and even absorption of the preservatives into the wood, to ensure that the effectiveness is maximized.

This article shows how Micro-XRF can be effectively utilized to detect the uniformity and depth of the preservatives in wood.

The Sample

In this study, a 4” x 4” wood sample was treated with a copper-based chemical applied on its surface (Figure 1). The sample was later cross-sectioned against the grain into a slice measuring roughly one-half inch thick. This analysis aimed to examine the depth that the copper-based solution had been absorbed into the wood, using Micro-XRF to measure copper.

Example of untreated wood with “dry rot”.

Figure 1. Example of untreated wood with “dry rot”.

Spectroscopic techniques that rely on the visible light spectrum are not practical, as the treatment is not visible to the naked eye - there are no traces of discoloration. The absorption depth can be easily evaluated using Micro-XRF to perform an elemental spectrum map of the cross-section, particularly looking for copper. Micro-XRF uses a micro-focused X-ray beam to produce characteristic X-ray energy lines, identical to that used in energy dispersive spectroscopy (EDS) .

Conversely, Micro-XRF is a non-destructive measurement method with excellent sensitivity for higher-energy elements, like copper. Detection limits for copper are nominally less than 10 ppm for a one point measurement. Micro-XRF usually requires extremely minimal sample preparation and works under low vacuum, while other methods may need extensive sample preparation, especially for biological or organic materials.

The Analysis

The EDAX Orbis PC Analyzer was used for this analysis. The Orbis PC was appropriate for this sample characterization as the X-rays were focused to nominally 30 µm in diameter, using a mono-lithic hollow glass fiber bundle, also known as poly-capillary optic.

The scale of features on a 4” x 4” cross-section of wood sample cannot be completely imaged using EDS, because the beam diameter is too small to cover such a huge area. However, certain “bulk” XRF systems possess smaller apertures on the order of a few millimeters, which is extremely big and cannot produce images of higher resolution.

The 30 µm beam diameter on the Orbis was the correct size to achieve high image resolution, while also having the potential to map a larger area within a short time span. It is imperative to consider the size of the preferred beam spacing, the beam diameter, and the mapping area in order to improve the mapping collection parameters. For this sample, the area to be mapped is highlighted in a red outline in Figure 2, about 50.4 x 3.5 mm.

Montage image of wood cross-section sample, with mapping area highlighted in red. The wood sample has been treated with a copper micro-particle preservative.

Figure 2. Montage image of wood cross-section sample, with mapping area highlighted in red. The wood sample has been treated with a copper micro-particle preservative.

Extraneous mapping time is reduced by the narrow area yet will continue to provide a distinct profile of the copper signal as a function of distance. 420 x 35 points were the X/Y matrix, or the number of points collected in individual axis, providing roughly one beam space (~30µm) in between individual collection points.

Dwell time (per point) should be decided by the composition of the target elements, as trace elements need prolonged dwell times than that of major elements. It is important to note that total sensitivity degrades considerably when mapping, since the acquisition time is relatively shorter as opposed to a longer single-point analysis. The overall collection time was around 2 hours with a dwell time of 500 msec per point.

The images that were produced display the video image of the mapped area (Figure 3a), together with the imaged Cu (K) intensities exhibited with thermal color scaling (Figure 3b). As predicted, the Cu (K) was extremely intense close to the wood’s outer edge with the highest intensity of nominally 21,000 counts per second.

Video image of the mapped area and (b) the Cu (K) spectral map in thermal scaling.

Figure 3. (a) Video image of the mapped area and (b) the Cu (K) spectral map in thermal scaling.

Copper distribution was not smooth and uniform across the wood. Instead, Cu (K) “hot spots” were clearly obvious, together with streaking normal to the wood rings. The hot spots appeared to develop right before (to the left) of the subsequent tree ring, which is clearly displayed in the total counts map in Figure 4.

he total counts map does not separate maps by energy, but instead represents the total countrate at any given point. This clearly shows that the copper hot spots do not directly correlate with the tree rings.

Figure 4. The total counts map does not separate maps by energy, but instead represents the total countrate at any given point. This clearly shows that the copper hot spots do not directly correlate with the tree rings.

All the streaking patterns seemed to be against the grain, and instead of slowly reducing towards the center, there was a sudden drop in Cu (K) intensities close to the fourth and third ring from middle. Further than that point, the copper quickly drops to trace levels, and at that stage relies on the instrument’s sensitivity. On the whole, copper absorption appeared to be relatively deep, but not homogeneous. Absorption was successful via the outer eight tree rings (out of eleven), or about 2.8 mm.

Conclusion

Important data regarding the distribution of copper micro-particles absorbed into an unprocessed piece of wood was provided by the Orbis PC Micro-XRF analyzer. The copper preservative was deeply absorbed into the wood, revealing an absorption pattern that seems to rely on the wood’s structure. The Orbis PC Micro-XRF analyzer provides non-destructive measurements, which maintained the sample for measurements with several other methods, and also required minimal preparation of sample. Generally, other elemental measurement methods need intensive sample preparation processes for analysis, especially for organic and biological samples.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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