Copper
Naphthenate:
An Analysis of the Materials Found in the Worldwide Marketplace
Using
a New Analytical Technique (cont)
The AWPA P-8 Standard for copper naphthenate has been reaffirmed and revised over the years to further specify only the use of unadulterated copper naphthenate that has years of efficacy data. Note the changes in the specifications between the revisions, highlighted in bold typeface:
2. COPPER NAPHTHENATE (from 1977 Book of Standards)
2.1 The naphthenic acid used in the manufacture of copper naphthenate shall be of the group of cyclopentane carboxylic acids occurring in petroleum and shall have an acid number of not less than 180, on an oil-free basis.
2.2 The copper naphthenate concentrate used to prepare wood preserving solutions shall contain not less than 6 percent nor more than 8 percent copper in the form of copper naphthenate.
2.3 All of the copper present in the concentrate shall be combined as copper naphthenate.
2.4 The copper naphthenate concentrate shall not contain more than 0.5 percent water.
2.5 The foregoing tests shall be made in accordance with the standard methods of the American Wood-Preservers’ Association.1 (See AWPA Standard A5.)
2.6 Solvents used to prepare solutions of copper naphthenate shall comply with the standards of the American Wood-Preservers’ Association. (See AWPA Standard P9.)
1Methods are being prepared for determining conformity with pars. 2.1 and 2.3.
2. COPPER NAPHTHENATE (Reaffirmed 1998)
2.1 The acid used in the manufacture of copper naphthenate shall be naphthenic acid of the group of alicyclic carboxylic acids occurring in petroleum and shall have an acid number of not less than 180 and not more than 250 on an oil-free basis.
2.2 The copper naphthenate concentrate used to prepare wood preserving solutions shall contain not less than 6 percent nor more than 8 percent copper in the form of copper naphthenate.
2.3 All of the copper present in the concentrate shall be combined as copper naphthenate.
2.4 The copper naphthenate concentrate shall not contain more than 0.5 percent water.
2.5 The foregoing tests shall be made in accordance with the standard methods of the American Wood-Preservers’ Association.1 (See AWPA Standard A5.)
2.6 Solvents used to prepare solutions of copper naphthenate shall comply with the standards of the American Wood-Preservers’ Association. (See AWPA Standard P9.)
2.7 The copper naphthenate concentrate shall not contain more than 2% (relative) of the total copper in the concentrate as being water extractable as determined by AWPA Standard A14.1.
1. A gas chromatographic method for determining conformity with part 2.1 and 2.3 was published in the 1999 AWPA Proceedings as an appendix to the Subcommittee P-5 Report.
Copper naphthenate began to be actively promoted as an alternative wood pole preservative to the major three formulations/products (pentachlorophenol, creosote, and inorganic arsenicals) in the 1980’s. Investigations were already underway into methods to better characterize naphthenic acid, which could be used to validate sections 2.1 and 2.3 of the AWPA Standard.
Gas chromatography coupled with mass spectroscopy (GC/MS) is widely used to separate and identify mixtures of organic compounds. As most analysts are probably aware, electron impact mass spectrometry causes individual compounds to be broken down into a multiple of fragments of characteristic mass, which can then be used to identify the parent compound. This is equivalent to identifying an ancient piece of pottery based on the broken shards. That is feasible with a single isomer product like 2-ethylhexanoic acid, but it gives undecipherable fragmentation patterns when analyzing a complex mixture like naphthenic acid.
Since naphthenic acid cannot be separated into individual components, only the determination of relative distribution of acids classified by carbon number and hydrogen deficiency is possible. Dzidic (1988) published a method to characterize naphthenic acid using gas chromatography coupled with ion mass spectroscopy. That method used fluoride ion chemical ionization to abstract the proton off the carboxylic acid. It also is transparent to hydrocarbons, which simplifies analyses immensely. This method was further modified by Fan (1991), utilizing fast atom bombardment with triethanolamine to likewise deprotonate the naphthenic acid while avoiding further fragmentation. The chemical ionization techniques described in the two papers employ so-called "soft" ionization that results in just one mass fragment per isomer from the deprotonated parent ion (M-1). Both the Fan and Dzidic papers show several examples.
The M-1 data resulting from these analyses can be analyzed and grouped into a distribution of molecular weights and therefore carbon number. Graphing the data also gives a clearer indication of ring distribution (acyclic, mono-, bi-, tri-cyclic, etc) based on hydrogen deficiency. The chemical formulas for the various naphthenic acid types include CnH2nO2 for acyclic (straight or branched chain) acids, CnH2n-2O2 for monocyclic acids, CnH2n-4O2 for bicyclic acids, CnH2n-6O2 for tricyclic acids, etc.
Figure 3 shows the breakdown of components from a single source of naphthenic acid based on negative ion chemical ionization MS. In this particular sample, bicyclic acids predominate along with significant amounts of mono- and tricyclic acids in the C14-C22 range. The sample also includes fatty acids and higher ring components ranging from C10 to C28.
Further GC/MS work by Hein (1992) was presented following an industry poll to determine availability of commercial instruments in the US capable of performing either the Dzidic or Fan methods. The informal poll found that only six instruments could be located in the USA to validate the naphthenic acid used to produce commercial copper naphthenate by these MS methods. Shell Chemical Company operated two, and none of the other four instruments were available for contract analyses. Work by Freeman and Wessner (1995) showed that, after springing (acidulating) copper naphthenate with sulfuric acid, the resulting naphthenic acid could be derivatized and analyzed by gas chromatography alone, although the derivatization step could be time consuming.
Archer (1990) analyzed methyl ester derivatives of carboxylic acids in a survey of commercially available products, including agar block tests with two fungi. Although many copper naphthenates found in the market place were composed of naphthenic acid, nearly all also contained non-naphthenic acid adulterants. Some of the products purported to be copper naphthenate in fact did not contain any naphthenic acid, but instead were composed entirely of synthetic acids such as 2-ethylhexanoic acid or mixed neo acids. Oil-free acid values of several samples were above the current 250 maximum specified in AWPA Standard P-8.
Formulations containing naphthenic/synthetic acid blends had higher toxic thresholds than pure naphthenic acid-based formulations. One of the formulations in the Archer study containing 100% synthetic acids actually seemed to promote decay with increasing Cu retention. Results of that study further served to encourage the proponents of copper naphthenate to define and provide a method by which commercial copper naphthenate samples could be shown to be in compliance with section 2.1 and 2.3 of the existing AWPA Standard.
The aforementioned analytical techniques rely on expensive analytical techniques with limited availability or time-consuming derivatization steps to generate samples for analysis. A simpler method employing the relatively widespread gas chromatography was needed to provide unambiguous identification of non-naphthenic adulterants in naphthenic acid recovered from copper naphthenate. Work by the Copper Naphthenate Task force chaired by Brient, under the jurisdiction of AWPA Sub-Committee P-3, re-affirmed the AWPA P-8 Standard for copper Naphthenate in 1997. Revisions included (1) rewriting the Standards to further clarify and make them even more specific as to the source of naphthenic acid used in copper naphthenate and (2) adding a footnote that a gas chromatographic analytical method would be published as an Appendix to the Annual P-5 Report (Anderson, 1997). Due to clerical errors, the final method was actually published as an appendix to the 1998 Annual P-5 report in the 1999 AWPA proceedings by Anderson (1999).
The purpose of this paper is to analyze commercially available materials found in the marketplace claiming to be copper naphthenate using the newly adopted AWPA GC method. Selected samples were also evaluated by a modification off that method using mass spectroscopy developed by the Queensland Forestry Research Institute. A further purpose was to determine if the incidence of adulterated copper naphthenate samples in the marketplace had diminished since the 1990 work by Archer.
Methods and Materials
A request for commercial samples went out to wood preservation and industrial contacts in the Europe, Australia, New Zealand, Canada, and the United States. Several samples were purchased from commercial vendors located throughout the United States and independent researchers who had commercial products in their laboratory also submitted samples. An effort was made to ensure that all samples were of a commercial nature and had been offered for sale to either industry or to retail consumers in the respective areas of purchase.
The basic gas chromatographic method is divided into two parts, (1) regeneration of the naphthenic acids from the copper naphthenate solution, and (2) determination of non-naphthenic carboxylic acids in the regenerated naphthenic acids. In the first part, the copper naphthenate is diluted with isopropyl alcohol and acidulated with excess 10% sulfuric acid to recover the naphthenic acid following the procedure described in AWPA method A13-96, section 4.0.
In the second part, the regenerated naphthenic acid is injected into a gas chromatograph to separate oils and non-naphthenic acid contaminants from the naphthenic acids. A qualitative and semi-quantitative analysis of the gas chromatogram is used to determine the presence of synthetic or other non-naphthenic acids in the regenerated naphthenic acids. For this method, the non-naphthenic acids can be represented by 2-ethylhexanoic, neo-decanoic, mixed C9-C19 neo acids and oleic/linoleic acids. The GC method provides the user with a quick and simple method for analyzing neat naphthenic acids (Figure 1) without the use of derivatization agents.
The specific gas chromatographic method is described in the 1999 AWPA Proceedings. A bonded polyethylene glycol stationary phase fused silica capillary column capable of temperature programming to 265°C is used to analyze the free carboxylic acids, such as the 30 m x 0.32 mm I.D. x 0.25 µm film thickness HP-INNOwax (Hewlett-Packard). A FFAP stationary phase is also suitable, but has a lower temperature limit of 240°C, and is not recommended for high molecular weight acids. Hydrogen carrier gas was used with a temperature program of 85°C held for 2 minutes, then programmed at 10°C/minute to a final temperature of 265°C and held until elution of all components. Injector and detector were set at 280°C and 300°C, respectively.
Standards containing approximately 20% each of 2-ethylhexanoic, neo-decanoic, and oleic acids were prepared by dilution in acetone. Approximately 0.4µl of the acid standard was injected into the chromatograph, and the retention time of each major peak present in the standard was recorded using a data acquisition system. Naphthenic acid samples recovered from copper naphthenate were also diluted 1:1 by volume with acetone. For qualitative analyses a visual comparison of the chromatograms of sample versus the acid standards was made. Any adulterant should stand out above all other components. A comparison of retention times of the standards against the sample was done to verify which adulterant was present.
Figure 1 and Figure 2, taken from the method published in AWPA, show the general areas of elution of various non-naphthenic compounds relative to the unresolved hump of naphthenic acid peaks. An adulterant will typically appear as a large peak or group of peaks that tend to dominate or otherwise stand out from the naphthenic acid hump in the chromatogram.
Four copper-naphthenate samples designated US-1, US-2, US-3, and A-1 were also analyzed by GC/MS after preparation by a modified AWPA A13-96 method. Copper naphthenate (1.00 g) was dissolved in 10 ml warm isopropyl alcohol, and after cooling, the solution was mixed well with 8 ml 10% sulfuric acid. The mixture was extracted twice, each with 5 ml petroleum spirit. A further 2 ml 10% sulfuric acid and 5 ml petroleum spirit were added to the aqueous layer, and the resultant mixture was vortex mixed. The organic layer was combined with the previous extracts and was dried over anhydrous sodium sulfate. After filtering, the solvent of the filtrate was evaporated with a stream of nitrogen at 45°C for 1 hour, then at 105°C for 2 hours. The residue was re-dissolved in acetone (10.0 ml) and analyzed by GC/MS.
GC/MS was carried out on an HP-INNOwax (crosslinked polyethylene glycol) capillary column (30 m x 0.32 mm i.d., 0.25 µm film) with helium (rather than hydrogen) as carrier gas at a constant velocity of 40 cm/sec. The temperature program started at 85°C for 2 minutes, then increased to 265°C at 10°C/min and remained at this temperature for 20 min. Samples were injected into GC by splitless mode.
Identification of individual acyclic acids was achieved by matching both the GC retention time and the MS spectrum with those of the standard material. The standard materials used included 2-ethyhexanoic acid, propionic acid, n-nonanoic acid, n-decanoic acid, abietic acid, oleic acid, linoleic acid, linolenic acid, stearic acid, and two commercial synthetic "neo" acid mixtures, Exxon Neo-913 and Exxon Neo-919.
Quantitative analysis was performed by two methods as follows:
Method 1: Standard addition quantitation, used with the sample US-3. The sample was spiked with standards at a range of concentrations. A linear curve was constructed, and the content of analyte was determined by the intercept.
Method 2: Matrix-matched external standard quantitation. The mixtures of standards and US-3 prepared for Method 1 were used as external standards for the other three samples. It was appropriate to base the external standards on US-3 acid, as this contained no detectable non-naphthenic acid components and provided a matrix as close as possible to the other samples.
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