J Wood Sci (2011) 57:166–169 © The Japan Wood Research Society 2011

DOI 10.1007/s10086-010-1155-9

NOTE

Ikuo Momohara · Wakako Ohmura

Spectrophotometric assay of a wood preservative, N,N-didecyl-N-methyl-

poly(oxyethyl) ammonium propionate (DMPAP), in aqueous solution

Received: July 29, 2010 / Accepted: September 10, 2010 / Published online: February 26, 2011

Abstract A spectrophotometric assay based on the color because of its colorless appearance, high penetrability, low
reaction between N,N-didecyl-N-methyl-poly(oxyethyl) odor, and zero metal content. Although didecyldimethylam-
ammonium propionate (DMPAP) and 4-[4-(dipropyl- monium chloride (DDAC) was the most common QAC in
amino)phenylazo]-benzenesulfonic acid (propyl orange) the past decade, N,N-didecyl-N-methyl-poly(oxyethyl)
was used to determine DMPAP concentrations as a wood ammonium propionate (DMPAP) has been recently devel-
preservative. The assay was carried out using a propyl oped as a new type of chlorine-free QAC. This preservative
orange solution at pH 2.9. The visible absorption spectrum is now on the list of accepted preservatives in both the Japa-
of propyl orange showed an absorption maximum at 510 nm, nese Agricultural Standard (JAS) for sawn timber6 and the
which decreased linearly with increasing DMPAP concen- Japanese Industrial Standard for Wood Preservatives.7
tration from 0 to 10 ppm. To apply this assay method to The performance of treated wood is known to be affected
determine DMPAP retention in treated wood, the influence by preservative retention.8,9 Therefore, the JAS for sawn
of wood extractives on the assay was investigated. Extrac- timber sets five retention levels and evaluation methods for
tives from Japanese cedar, hinoki cypress, and Japanese determining preservative retention.6 In the case of DMPAP,
larch were found to increase apparent DMPAP concentra- the principle of the evaluation method is similar to the
tion. However, it was also found that measuring visible DDAC evaluation method described in the American Wood
absorption at 477 nm prevented overestimation and gave Protection Association Standard (AWPAS).6,10 In a previous
precise values. This assay can be a viable alternative to the article, we discussed the demerits of the conventional
current methods for the determination of DMPAP DDAC assay method described in AWPAS and proposed a
concentrations. simple, eco-friendly, and cost-effective approach for deter-
mining DDAC retention based on a color reaction between
Key words N,N-didecyl-N-methyl-poly(oxyethyl) ammo- DDAC and 4-[4-(dipropylamino)phenylazo]-benzenesul-
nium propionate (DMPAP) · Quaternary ammonium chlo- fonic acid (propyl orange).11,12
ride · Propyl orange · Treated wood · Wood preservative It is assumed that this method will also be useful for
determining DMPAP retention in treated wood. We applied
the color reaction between propyl orange and DMPAP and
Introduction found that propyl orange can successfully determine
DMPAP concentration. The influence of wood extractives
To mitigate global warming by using wooden products, pre- on DMPAP assay using propyl orange was also examined.
servative treatments are expected to extend the service life
of wooden materials and thus delay the emission of carbon
dioxide gas.1–4 Among several types of preservative treat- Materials and methods
ments, the proportion of wood undergoing pressure treat-
ment using quaternary ammonium compounds (QACs) has Materials
gradually increased in the past few decades in Japan5
A DMPAP standard containing 71.6% DMPAP (supplier:
Lonza Japan) was kindly provided by Mr. Akira Makita,
Dainihon Wood-Preserving Co., Ltd. Propyl orange was
I. Momohara (*) · W. Ohmura synthesized from sodium sulfanilate as described in previ-
Forestry and Forest Products Research Institute, 1 Matsunosato,
Tsukuba, Ibaraki 305–8687, Japan
ous articles.12,13 Wood powder was also prepared from the
Tel. +81-29-829-8298; Fax +81-29-874-3720 sapwood and heartwood of commercial timber from
e-mail: momohara@ffpri.affrc.go.jp Japanese cedar (Cryptomeria japonica), hinoki cypress
 167

(Chamaecyparis obtusa), Japanese larch (Larix kaempferi),

and Western hemlock (Tsuga heterophylla) according to a
previous article.12 Extraction solvent was prepared by
adding formic acid to methanol until a pH meter (TOA;
HM-30S) showed pH 5.0.10

Determination of DMPAP concentration using

propyl orange

The assay solution was prepared by mixing about 7.2 mg

propyl orange with 1120 mM sodium monochloroacetate
buffer adjusted to pH 2.9 and containing 0.02% Triton
X-100.12 The DMPAP standard was diluted with the extrac-
tion solvent to make a standard solution containing
0–1000 ppm DMPAP. Then, 20 μl standard solution was
added to 1980 μl assay solution. The visible absorption spec-
trum of the mixed solution was recorded on a Shimadzu
UV-2400 spectrometer.

Fig. 1. Visible absorption spectra of N,N-didecyl-N-methyl-

Determination of DMPAP concentration in the presence poly(oxyethyl) ammonium propionate (DMPAP)–propyl orange
of wood extractives complex in an acidic buffer. DMPAP concentration in the assay solu-
tion varied from 0 to 10 ppm

Wood extractives were prepared by mixing 1 g of wood

powder with 20 ml methanol extraction solvent according
to the procedure given in a previous article.12 After the
extraction procedure, part of the supernatant solution was
filtered with a membrane filter (DISMIC-25HP, Advantec)
for use in further studies.
A sample solution was prepared by mixing filtered super-
natant containing wood extractives with the same volume
of DMPAP standard solution. A control solution was pre-
pared by mixing the extraction solvent with the DMPAP
standard solution. Then, 20 μl sample solution or control
solution was mixed with 1980 μl assay solution, and its
visible absorption was measured using the Shimadzu
UV-2400 spectrometer.

Results and discussion

Determination of DMPAP concentration with

propyl orange

Assay methods using propyl orange were developed Fig. 2. The relationship between DMPAP concentration and absor-
to determine the concentration of QACs such as the bance at 510 nm (open circles) and 477 nm (solid circles). The unbroken
distearyldimethylammonium ion14 and the nCx- line and broken line indicate the linear least-squares fit of absorbance
at 510 nm and 477 nm against DMPAP concentration in the assay solu-
trimethylammonium ion (x = 10–18).13 The compound was tion from 0 to 10 ppm, respectively. The lines were drawn using the
also revealed to be useful for determining DDAC concen- following equations: Absorbance at 510 nm = −0.042 × DMPAP conc.
trations when used as a wood preservative.12 + 0.72; Absorbance at 477 nm = −0.017 × DMPAP conc. + 0.50
However, it is not clear whether the assay method using
propyl orange can be applied to DMPAP determination.
Therefore, the first step was to investigate the color reaction about 460 nm. This result accorded with our previous find-
between propyl orange and DMPAP. The change of the ings using DDAC and propyl orange,12 and with those of
DMPAP–propyl orange complex spectra in the assay solu- Momohara et al. and Motomizu et al.13,14
tion is shown in Fig. 1. Propyl orange has a visible absorp- Absorbance at 510 nm was then plotted against DMPAP
tion maximum at 510 nm, and this maximum decreased with concentration (Fig. 2): the absorbance linearly decreased
increasing DMPAP concentration. In contrast, a new peak with increasing DMPAP concentration from 0 to 10 ppm in
appeared at 422 nm. An isosbestic point was observed at the assay solution (R2 = 0.998).
168

Table 1. Effect of wood extractives on relative absorbance in DMPAP assay at wavelengths of 510 nm and 477 nm (absorbance of control sample
at each DMPAP concentration = 100)
Measured DMPAP Control Japanese cedar Hinoki cypress Japanese larch Western hemlock
wavelength conc.a
Sapwood Heartwood Sapwood Heartwood Sapwood Heartwood Sapwood Heartwood

510 nm 0 ppm 100 (0.1) 96.0 (0.4) 95.9 (0.2) 98.9 (0.3) 96.1 (0.1) 98.8 (0.2) 98.6 (0.2) 99.5 (0.2) 99.3 (0.4)
10 ppm 100 (0.7) 100 (0.7) 85.0 (0.6) 93.7 (0.3) 89.6 (0.8) 95.1 (0.5) 97.3 (2.3) 99.0 (0.4) 100 (0.7)
477 nm 0 ppm 100 (0.1) 99.2 (0.1) 99.4 (0.2) 100 (0.1) 99.4 (0.3) 100 (0.1) 100 (0.2) 100 (0.1) 100 (0.1)
10 ppm 100 (0.2) 98.3 (0.6) 99.0 (0.5) 98.6 (0.6) 103 (0.5) 98.9 (0.7) 99.4 (0.8) 99.1 (0.2) 98.5 (0.5)
Coefficient of variations are shown in parentheses (n = 3)
DMPAP, N,N-didecyl-N-methyl-poly(oxyethyl) ammonium propionate
a
DMPAP concentration in assay solution

The reproducibility of this assay was also investigated. A

standard solution containing 0 or 1000 ppm DMPAP was
prepared, and the DMPAP concentration was measured
using the assay in triplicate. The results showed that the
coefficients of variation were 1.2% for the assay solution
containing 0 ppm DMPAP, and 2.0% for that containing
10 ppm DMPAP (data not shown). The low value of the
variation coefficient indicates that this assay procedure is
reproducible enough for DMPAP determination.

Application of propyl orange to determine DMPAP

retention in treated wood

The previous study revealed that wood extractives inter-

fered with the DDAC assay using propyl orange when the
assay was carried out using absorbance at 510 nm. It also
revealed that the influence of the wood extractives can be
excluded by determining the DDAC concentration at
477 nm.12 In this way, the influences of wood extractives and Fig. 3. Differential spectra of DMPAP–propyl orange complex in the
measurement wavelength on DMPAP determination using presence and the absence of wood extractives. The difference in the
visible absorption of the DMPAP–propyl orange complex in the pres-
propyl orange were investigated. ence and absence of wood extractives of Japanese cedar heartwood
To estimate the effect of wood extractives, a DMPAP were plotted: Unbroken line, with DMPAP 10 ppm; broken line, with
standard solution was mixed with extraction solvents con- DMPAP 0 ppm in the assay solution
taining wood extractives from the sapwood or heartwood
of four wood species, and the mixture was subjected to
DMPAP assay. Extraction solvents in this study were pre-
pared according to AWPAS to determine DDAC retention the DDAC–propyl orange complex has an isosbestic point
in treated wood.10 The AWPAS defines 0.5 g as the amount at around 477 nm, which is not affected by the presence of
of wood powder placed in 20 ml extraction solvent. Double wood extractives.12 Therefore, differential spectra of the
this amount of wood powder was used in this experiment DMPAP–propyl orange complex were also recorded in the
and mixed with the same volume of DMPAP standard solu- presence and in the absence of wood extractives. Figure 3
tion. DMPAP concentration after mixing both solutions was shows an example of differential spectra observed in the
set to 0 or 1000 ppm, which was slightly higher than that presence and in the absence of wood extractives from Japa-
expected for treated Japanese cedar timber at the K4 level.6 nese cedar heartwood, indicating the presence of an isos-
As shown in Table 1, wood extractives from Japanese bestic point at 477 nm similar to the DDAC–propyl orange
cedar heartwood and hinoki cypress heartwood significantly complex. Since there is an isosbestic point at 477 nm where
decreased absorbance at 510 nm. In addition, wood extrac- absorbance is unaffected by the presence or absence of
tives from hinoki sapwood, Japanese larch heartwood, and wood extractives from these wood species, visible absor-
Japanese larch sapwood also influenced absorbance. In bance was measured at this wavelength. Table 1 shows the
other words, DMPAP retention in these wood species is visible absorbance of the DMPAP–propyl orange complex
overestimated by the above assay method. at 477 nm. The results clearly indicate that DMPAP concen-
There was a similar problem in the previous study on tration can be determined precisely at this wavelength even
DDAC assays using propyl orange. This was solved by in the presence of wood extractives from these species.
changing the wavelength from 510 nm to 477 nm because It is also shown that there is a good correlation between
 169

absorbance at 477 nm and DMPAP concentration (R = 2

5. Japan Wood Preservers Industry Association. Transition of amount
0.997) (Fig. 2). of wood preservatives (in Japanese). http://www.jwpia.or.jp/
shinchaku/index.html. Accessed July 15, 2010
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in the JAS for sawn timber.6 It is also noteworthy that the Agriculture, Tokyo
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Acknowledgments We would like to thank Mr. Akira Makita for pro-
(DDAC) determination in treated wood, A16-08. American Wood
viding the DMPAP standard. This study was partly supported by the
Protection Association, Birmingham, AL
program “Research and Development Projects for Application in Pro-
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