Wood Preservative Analysis of Disintegrating Wood

AME was provided wood samples from a disintegrating wood bridge and tasked with determining if the wood had been treated with a wood preservative. Since many of the organic preservatives are detectable by appearance and most other wood preservatives are based on copper compounds in the time frame in which this disintegrating bridge was built, we examined the disintegrating wood samples with XRF to determine their copper content. We compared this content with the copper content of a known preservative pressure-treated wood sample and with a cedar sample not expected to have been treated with a wood preservative. The disintegrating wood had a copper concentration similar to that of our known preservative pressure-treated wood sample, while the cedar wood sample proved to have no detectable copper. The disintegrating wood proved to have a very high silicon content.

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Wood Preservative Analysis Summary

Disintegrating Douglas fir lumber from a bridge and pieces of untreated cedar wood and of known preservative pressure treated wood in the AME facility were analyzed using XRF (X-Ray Fluorescence) to determine their elemental composition for carbon, nitrogen, fluorine, sodium, magnesium, aluminum, silicon, and all heavier elements. We were tasked with determining whether the wood had been treated with a copper-based wood preservative. There was particular interest in copper since it is found in the common non-organic wood preservatives. Some of the organic preservatives such as creosote can
be visually ruled out.

  • Two different pieces of lumber from the bridge had ~0.29 wt% Cu and ~0.69 wt% of
    copper (Cu), respectively.
  • The cedar wood, which was thought to be untreated with preservative, had no
    detectable copper in it.
  • The known pressure treated wood had ~0.36 wt% of copper in it.
  • Thus, the lumber from the bridge seems to be pressure treated wood.
  • Both of the disintegrating bridge wood samples had silicon (Si) concentrations that
    were very high. These concentrations were ten times higher than the two AME wood
    samples used as reference wood samples.

Samples and Background

Wood samples identified as from the surfaces of rotting wood from a bridge were sent for analysis for a wood preservative. See Figure 1 for a picture of the wood samples we received for analysis. Figure 2 shows the samples which were actually analyzed by XRF with the surface facing the x-ray source and detector shown.

The disintegrating wood received from the surfaces of the bridge wood.
Figure 1: The disintegrating wood received from the surfaces of the bridge wood.
Figure 2. The two samples taken from the submitted bridge wood sample material which were
analyzed by XRF are shown at the top of this picture. The cedar wood sample is at the lower left
and the known pressure treated preservative wood sample with the slightly green surface color
is shown in the lower right of the picture.

XRF Spectrometry Analysis

Our wavelength-dispersive XRF spectrometer can quantitatively measures the elemental concentrations for all elements from fluorine through uranium and when the material has a low density, as in polymers, we can also analyze carbon and nitrogen using an additional crystal. The depth of analysis depends upon the characteristic x-ray energy emitted from the detected element and the density of the material. This depth can vary from a micrometer to a millimeter. XRF analysis has very low detection limits for the elements. Wavelength-dispersive XRF systems have greater elemental sensitivity and higher energy resolution than do less expensive energy-dispersive XRF spectrometers. We can detect all but the lightest elements at concentrations as low as 10 ppm. Solid Samples, powders, and liquids can be analyzed with XRF analysis. Our spectrometer also has an unusual small spot capability to measure spots of 0.5 or 1.5-mm diameter, as well as the capability to measure areas of 10 mm and 29 mm diameter. Of course, large
area measurements offer lower detection limits and greater accuracy of measurement. For this work, the 29 mm aperture was used, and the Samples were analyzed in vacuum.

Figures 3-6 show the elemental composition analysis of lumber pieces from bridge, cedar wood and a pressure treated wood. The two different pieces of lumber from the bridge had ~0.29 wt% copper (Cu) and ~0.69 wt% Cu, respectively. The cedar wood had no Cu in it while the pressure treated wood had ~0.36 wt% Cu in it. Thus, the lumber from the bridge has a copper concentration consistent with that of wood treated with a copper based wood preservative.

There is a curious observation to be made. Both of the disintegrating bridge wood samples had silicon (Si) concentrations that were very high. These concentrations were ten times higher than the two AME wood samples used as reference wood samples. What is the cause of such a high Si concentration?

Elemental composition analysis of Lumber from bridge decline – piece 1 using WD XRF.
Figure 3: Elemental composition analysis of Lumber from bridge decline – piece 1 using WD XRF. Note the high silicon (Si) concentration.
Elemental composition analysis of Lumber from bridge decline – piece 2 using WD XRF
Figure 4: Elemental composition analysis of Lumber from bridge decline – piece 2 using WD XRF. Note the high Si concentration.
Elemental composition analysis of cedar wood using WD XRF
Figure 5: Elemental composition analysis of cedar wood using WD XRF. Note that no copper (Cu) was detected and the Si concentration is only about one-tenth that of the rotting bridge wood.
Elemental composition analysis of a pressure treated wood using WD XRF
Figure 6: Elemental composition analysis of a pressure treated wood using WD XRF. Note that the Cu concentration is 0.36 wt.% and that the Si concentration is less than one-tenth that of the rotting bridge wood.

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