This article was originally published in 1993, and was revised in July 2008, to provide an update on the advances made in sampling cells for FT-IR. The applications have not changed, however, the technology has made the technique more useful than before. The example spectra provided in this article were generated with older techniques such as salt plates and KBr pellets, but comparable results would be achieved today in less time with the modern sample cells such as the ATR described below.
Environmental laboratories are often requested to provide fast, cost-efficient methods for analytical screening of samples of unknown composition. These unknowns may originate from various regulatory compliance and site investigation activities, and typically consist of bulk liquids and solids collected from unlabeled drums, tanks or pits with little or no history to verify the contents. Without a systematic approach to the screening of such waste, the laboratory will often embark on a costly analytical project that might include comprehensive analyses for inorganics, metals, volatile and semivolatile organics, pesticides and herbicides.
A screening technique that can narrow the list of possible components is necessary to minimize both cost of analysis and turnaround time. Many clients request extensive waste analyses, including full TCLP on unknowns that are later found to be solvents or other organic liquids. Matrix interference on these samples can result in detection limits higher than regulatory limits, which are either interpreted as useless, or potentially may cause the waste to be designed RCRA hazardous by default. A screening analysis can be used to quickly categorize that waste as a pure product or a common commercial mixture, often ruling out TCLP.
For several years, we have used Fourier Transform Infrared (FT-IR) Spectrophotometry for identification of major components in unknown waste samples. Our results indicate that the technique is useful as both a screening tool and, in some cases, the primary analytical procedure. Infrared (IR) spectrophotometry has been widely used in industrial and academic laboratories as a powerful tool in the identification of unknown compounds, especially in combination with nuclear magnetic resonance, mass spectrometry and physical properties such as melting and boiling points.
In this article, we will provide details for sample preparation, analysis, example spectra, and the use of commercial and user-compiled libraries for both organic and inorganic liquids and solids. Our examples will illustrate specific laboratory analyses in which FT- IR was the primary tool used to identify products and waste, such as high purity industrial solvents, paint waste, antifreeze, aqueous wastes and inorganic cyanide components.
It is also a useful screening procedure for identification of hydrocarbon mixtures such as gasoline, diesel, jet fuels, lubrication oil, and solvent mixtures. IR spectra of these samples will be included along with discussion to document the analytical conclusions of such projects. The data will demonstrate the utility of FT-IR as a quick and cost-effective method for identification of major constituents in an unknown, often limiting the need for subsequent lengthy and expensive analyses.
Equipment and Supplies
Sample preparation for liquids is extremely simple, requiring only disposable pipettes and the configuration of the ATR Universal Sampling Accessory. A small drop of the liquid sample is placed in the trough insert which is mounted on the top of the ATR assembly.
The insert forms a shallow well around the crystal, which holds the liquid or ground solid samples. Solids and soft pliable films are placed directly on the ATR crystal plate, and held in place with special pressure clamps provided as accessories in the ATR kit. The insert and crystal plate are easily cleaned with cotton swabs and a solvent, such as acetone or isopropyl alcohol. Highly-concentrated solvents are analyzed utilizing their vapor phase by gas cell methodology, in which gas cell contents are evacuated and replenished with the vapor phase of the sample headspace using a gastight syringe. The complete sample preparation process takes less then a minute for most samples.
Both commercial and user-prepared spectral reference libraries are necessary to maximize information obtained for the IR spectrum of an unknown material. Some general conclusions can usually be drawn very quickly regarding the general composition for certain classes of liquid wastes. It is often readily apparent that the waste is either of aqueous or petroleum origin based on the characteristic sample spectra for water and hydrocarbons. Further investigations will often suggest the presence of ketones, alcohols, glycols or chlorinated solvents. An instrument with library search capabilities can be very helpful in identification of specific liquid unknowns when the purity is high, although the information provided by the spectra on complex mixtures must be verified by additional analytical techniques.
Results and Discussion
This colorless liquid was miscible in water and acetone, but immiscible in hexane, and produced the IR spectrum presented in Figure 1A. The library spectrum for deionized water is presented in Figure 1B and is obviously a close match. The liquid was presumed to be an aqueous waste, with no major organic constituents probable at one percent or greater. The water content was later confirmed by the Karl Fischer titration method to be greater than 99%. Based on the clients’ knowledge of the waste profiles at the base, the laboratory was instructed to limit additional testing to the total metals.
A state regulatory agency collected this white, unidentified solid while conducting an investigation at an abandoned site. Although unlabeled, history and other evidence at the site suggested the possibility that the material could be cyanide salts. The agency requested that the sample be tested for total cyanide by SW-846 Method 9010. Due to concerns for analyst safety and apparatus contamination, a screening analysis by FT-IR was proposed to determine if the solid contained high levels of cyanide. The analysts employed the KBr pellet technique and the spectrum presented in Figure 2A was generated. The absorption band at 2079cm-1 is characteristic of cyanide salts attributed to the C-N bond. The laboratory used solid potassium cyanide and a KBr pellet to produce the spectrum labeled as Figure 2B. Due to the excellent match, we presumed the white powder to be cyanide salt, and the cyanide analyst was able to proceed on that assumption. Treating the unknown like a standard, the analyst dissolved a known mass of the sample in deionized water and serially diluted the solution into the expected linear range of the calibration curve. Wet chemical analysis by Method 9010/9014 confirmed the IR data, giving a result of 440,000 mg/kg (44 percent cyanide).
This transparent liquid with a solvent-like odor was miscible in acetone and hexane, but immiscible in water. The FT-IR spectrum was very similar to the library spectrum for methyl ethyl ketone (Figure 3B). The client had requested analysis by the EPA Method 8260 to check for solvents. The preliminary FT-IR analysis provided the GC/MS analyst with enough information about the sample to prepare the proper dilution of the waste and confirm the screening data. Although the salt plate method was employed for this sample, the ATR or gas/vapor cells could be used in similar situations to identify solvents of relatively high purity.
This transparent liquid with familiar odor was received as an unknown from an Air Force base. The sample was hexane and acetone miscible, but water immiscible. A thin film was prepared between two KBr plates and the FT-IR spectrum presented in Figure 4A was generated. A library search gave several possibilities with the best match being unleaded gasoline presented in Figure 4B. Confirmation by direct injection into a GC/FID produced a fingerprint with a carbon range of C4-C10, which was indicative of gasoline range organics. No additional testing was required.
This was an amber-clear liquid with solvent-like odor that was miscible in hexane, but immiscible in water or acetone. The thin film KBr plate technique that yielded the FT-IR spectrum is typical of many petroleum distillate unknowns received at our laboratory for identification, classification, and possible candidates for fuels blending. Note the similarities of this spectrum to that of Figure 5B - a laboratory- specific entry in our library for a brush cleaner, a product similar to an oil-based paint stripper. Confirmation by GC/FID produced a fingerprint and carbon range, not unlike that of our standard brush cleaner. Although the FT-IR spectrum is typical of many petroleum distillates, the absorption band at approximately 1700 cm-1 is typical of carbonyl compounds. Because of the additional data, it is determined that analysis by GC/MS Method 8260 might be needed to identify and quantify any other major components. GC/MS analysis confirmed the presence of percent levels of 4-methyl-2-pentanone, 2-butanone, toluene, and total xylenes.
A clear, colorless liquid, this sample was miscible in both water and acetone, but immiscible in hexane. The thin-film KBr plate technique yielded the spectrum labeled as Figure 6A. The spectrum shows characteristics of –OH from water and alcohols, and the absorption band at 2900-3000cm-1 is typical of the C-H stretch. The IR spectrum was similar to that of short chain alcohols and glycols, including methanol and ethylene glycol. It was determined that further analysis by SW-846 Method 8015 (GC/FID) would be necessary to identify and quantitate the major constituents. Based on retention time data for a series of alcohols, it was determined that the sample was high purity methanol. The IR spectrum for methanol is presented as Figure 6B, however; note the similarity to ethylene glycol (Figure 7B).
Another typical waste originating from an Air Force base, this green liquid was miscible in water and acetone, but immiscible in hexane. The KBr plate technique was used to produce the FT-IR spectrum presented in Figure 7A. The bands at 3345, 2944, and 2832cm-1 are typical of neat alcohols and glycols or aqueous solutions of the same. In this example, the particular fluorescent green color often indicates that we are dealing with an ethylene glycol-based antifreeze mixture. Although confirmation may not have been necessary based on the color, sample origin, and similarity to the spectrum for ethylene glycol, the composition was confirmed both qualitatively and quantitatively by GC/FID.
A transparent liquid sample with a solvent-like odor was collected from an unknown waste drum for characterization. Solubility tests were performed that revealed the miscibility of the sample in hexane and acetone, as well as the immiscibility of the sample in water. An FT-IR spectrum, presented in Figure 8A, was obtained using the thin film KBr plate technique. The IR spectrum, when library-searched, closely resembled the laboratory Freon 113 standard that contains characteristic bands within the fingerprint region (Figure 8B). The confirmation for the halogenated solvent was performed by use of a volatile GC/MS analysis. The Freon 113 was qualified by a positive spectral identification and qualified at a concentration of greater than 99 percent.
This sample, a yellow liquid - soluble in hexane, but insoluble in water and acetone - was submitted from a vehicle maintenance facility. The FT-IR spectrum was achieved using the thin film KBr plate technique (refer to Figure 9A). The resulting spectrum, which was characteristic of hydrocarbon, was library-searched with a tentative identification of lubricating oil (See figure 9B). The hydrocarbon sample was then fingerprinted by GC using the modified SW-846 Method 8015. The hydrocarbon fingerprint in the chromatogram had a carbon range of C14-C32, which is similar to a lube of oil carbon range.
This unknown was a transparent liquid sample, which exhibited a distinct fuel odor. The FT-IR analysis using the thin film KBr plate technique gave characteristic hydrocarbon spectrum as shown in Figure 10A. The fuel library, when searched, gave a tentative match of diesel fuel (Figure 10B). The sample was then analyzed for diesel range organics by a modified SW-846 Method 8015 to confirm the carbon range and to provide a quantitative result. The diesel pattern had a carbon range of C9-C28 at a level of approximately 67%.
From these examples, we have shown that FT-IR, in combination with a modern sample cell such as the Pike MIRacle ATR, can be a fast and effective tool in the identification of unknown wastes. The sample preparation and screening procedures can be performed in just a few minutes and the resultant spectra can provide enough information to characterize wastes and aqueous, petroleum distillates, pure solvents, solvent mixtures and to identify many common commercial products. Although some samples can be positively identified by FT-IR alone, the main utility of the spectra is to guide the laboratory in the selection of subsequent confirmatory techniques armed with the information from the FT-IR analysis and, with or without knowledge of waste, several costly and time consuming analytical methods (such as TCLP) can be reduced or eliminated entirely.
The authors would like to thank the management and technical staff at Brooks Air Force Base, San Antonio, Texas, for its technical assistance in implementing these procedures. Many of the procedures described were developed at the base’s occupational health and environmental testing laboratory, formerly known as Armstrong Laboratory/OEHL. We would also like to thank Microbac employees Chad Barnes, Rodney Campbell and Erin Vandenberg for their help in preparing the article.
David L. Bumgarner
Leslie S. Bucina