Industrial and academic scientists working in the development and application of engineered materials are increasingly in need of measurement technologies that can be used closer to the source of the sample or the object requiring analysis. These measurements are often made in more demanding environments, and typically not in traditional analytical or QA/QC labs. A number of analytical technologies are available to meet this growing need, powered by the general trend in analytical instrumentation toward smaller size, more power, improved reliability, and greater ease of use.
Optical spectroscopy provides an excellent example of this new genre of technology, with portable FTIR, Raman, and NIR spectrometers leading the way in innovation. Over the past two decades, FTIR spectroscopy in particular has expanded into these more demanding out-of-lab applications in such diverse endeavors as chemical reaction monitoring, homeland defense, and hazardous chemical analysis. Additionally, portable and handheld FTIR spectrometers are currently used in QA/QC of food ingredients,1 biodiesel analysis,2 art and historical object conservation,3 and mineral analysis.4 This article focuses on some recent applications of handheld and portable FTIR spectrometers for the measurement of engineered materials.5
The need for mobile measurement in materials analysis
The materials industry encompasses a range of product segments including polymers, composites, optical components, coatings, semiconductors, and metals. In spite of the diversity and breadth of applications, all have some common requirements that must be met. The raw materials from which engineered materials are synthesized must be analyzed for quality; the relationship between composition, structure, and performance of the material must be defined; production variables that affect performance must be characterized; the quality of the finished material must be confirmed; the effect of use on the material must be measured; and recycling and reclamation efforts must be supported.
Portable and handheld FTIR analyzers support a number of these needs, resulting from the advantage of moving analysis closer to the source of the sample or the object of interest:
- Rapid, on-the-spot testing of raw materials for verifying identity and acceptability at loading docks, tanker cars, and trucks
- Ability to quickly screen materials to identify samples that need to be sent to a lab for additional analysis and minimize the number of these samples
- Nondestructively analyze large, valuable, or nonmovable objects for which excising samples is not possible
- Identify the most important areas on an object for analysis, gather more data in more critical locations, and minimize analysis of less important regions
- Scan large areas or surfaces to map distribution of specific components
- Measure the effect of aging, weathering, and other stresses that can affect the viability of an engineered material; determine how use affects engineered material performance and lifetime
- Obtain “real-time” answers that allow actionable decisions to be made on-the-spot.
Considerations in designing effective portable and handheld FTIR analyzers
Figure 1 – The diversity of advanced materials and their application necessitates that handheld and portable FTIR spectrometers have sampling capabilities that can meet the varied analysis requirements. The Agilent 4100 ExoScan FTIR system for material analysis employs permanently aligned, interchangeable sampling technology, including spherical diamond and germanium ATR, specular, grazing angle, and diffuse reflectance interfaces. A docking station enables the spectrometer to be used in a benchtop configuration as well as handheld.
To take a spectrometer out of the lab and closer to the sample or the object to be analyzed requires that the system be designed to function effectively in a more demanding environment. Out-of-lab FTIR analyzers must:
- Provide performance similar to a lab FTIR equivalent
- Have the robustness required by the ambient conditions
- Have an interferometer with stable performance in all physical orientations
- Be resistant to vibration and shocks
- Have software that is intuitive and provides answers
- Have a user interface and ergonomics that reflect the environment in which the analyzer will be used
- Have sampling technology that is easy to use, readily interchangeable, and designed to meet the measurement objectives
- Have limited maintenance needs, use as few consumables as possible, and be highly reliable
- Allow use in a lab for methods development and data analysis.
Because of these special attributes, it is necessary to design and engineer the FTIR system with the end-goal in mind—for application in more diverse environments when compared to lab-only instruments (Figure 1).
Handheld FTIR for the analysis of polymers, composites, coatings, and contaminants
In 2008, A2 Technologies, now Agilent Technologies (Danbury, CT), developed the first handheld FTIR spectrometer targeted at materials applications in the aerospace industry. The first application to be studied was a nondestructive method for analyzing thermally induced stresses in epoxy–carbon composites. This led to the development of the Exoscan FTIR spectrometer, which over the past several years has evolved into a complete system for the analysis of materials. The Agilent 4100 ExoScan FTIR system has five interchangeable sampling interfaces that can analyze infrared reflective, nonreflective, scattering, and absorbing materials. It is equally usable for both in-lab and out-of-lab material analysis applications.
In the analysis of aircraft composites, the diffuse reflectance interface is used for both unsanded and sanded composite surfaces. Sanded surfaces result from the process of repairing composite in which a damaged area is sequentially removed by sanding in preparation for patching and bonding. Unsanded composite surfaces have a higher polymer-to-carbon fiber ratio as compared to the sanded material, and produce a stronger infrared signal than the sanded surface. It is straightforward to track the effect of thermal exposure on large areas of unsanded composite surfaces by measuring the carbonyl band intensity as a function of position (Figure 2). The carbonyl moiety is indicative of oxidation of the epoxy polymer, which results from excess thermal exposure. This oxidation potentially leads to microcracks, delaminations, and weakening of the exposed area. Sanding the exposed composite reveals the inner material, which has a far higher carbon fiber-to-polymer ratio and is more absorbing of infrared radiation. For this measurement, the diffuse reflectance sampling technology can be employed with the handheld FTIR to track the change in polymer composition as a function of location during the sequential sanding process. The goal of the sanding process is to remove thermally overexposed composite to reveal pristine material. The handheld FTIR with diffuse reflectance sampling interface can guide the process by elucidating the condition of the composite as the laminate layers are exposed by the sanding process.
Figure 2 – Handheld FTIR is used to measure regions of composite-based aircraft that show potential thermal, UV, or chemical overexposure. Prior to composite repair, the handheld FTIR utilizes the diffuse reflectance sampling interface and tracks the carbonyl region of the infrared spectrum as an indicator of oxidation caused by thermal overexposure. Mapping intensity of the 1725 cm–1 carbonyl band as a function of position defines the shape and boundaries of the oxidized portions of the composite.
Carbon black-filled polymers are found in a wide variety of products. Examples include automobile tires, in which carbon black and silica are added to the rubber to provide extra wear protection; plastics used in the electronics industry, in which the carbon black acts as an antistatic agent; and rubber gaskets, O-rings, and seals, for which the addition of carbon black adds strength to the polymer material. Because carbon black absorbs and scatters infrared radiation, the mid-IR spectra of carbon black-filled polymers are often compromised and have strongly skewed baselines as well as low absorbance-to-noise for the polymer infrared absorbance bands. For this reason, the ExoScan system uses a germanium internal reflection sampling interface to obtain high-quality spectra of carbon black-filled polymers. As a result of the index of refraction of germanium, infrared radiation does not penetrate as deeply into the polymer material, and the presence of carbon black has less of an effect on the resultant polymer spectrum.
In the ExoScan system, the germanium ATR sensor is spherical in design to optimize contact with the polymer, as well as to allow flexibility of the contact angle with the polymeric material to provide quality spectra. For noncarbon black-filled polymers, the ExoScan employs a spherical diamond attenuated total reflection (ATR) sampling interface.
Figure 3 – Handheld FTIR equipped with the spherical germanium ATR was used at the 2011 Deutsche Tourenwagen-Meisterschaft (DTM) races to analyze tires. Changes in polymer composition and/or the effect of track contaminants on the tire were measured. Unintentional or intentional adulteration or contamination of the tire surface by oil, brake fluid, softening agents, etc., is quickly detected and analyzed by the FTIR system.
The analysis of automobile tires is simplified using the ExoScan equipped with the spherical germanium ATR sampling interface. For example, the handheld system has been used to determine if racecar tires have been affected by high-speed use or contaminants on the race course, or have been purposely adulterated to improve traction. Changes in the rubber composition at the surface of the tire as well as the presence of foreign substances are elucidated by measurement with the handheld FTIR system (Figure 3).
In electronic industry scrap recycling, the handheld system equipped with the germanium ATR is used to identify the composition of the carbon black-filled plastic so that it can be appropriately reclaimed and recycled.
By matching the spectra of an unknown plastic scrap with reference spectra contained in an on-board library, the identity of the unknown is determined in less than 1 min.
Correct composition, thickness, and uniformity of coatings on metal substrates are essential to the performance of the coated metal in a variety of applications (Figure 4). The handheld FTIR system equipped with external reflectance sampling interface is well-suited for verifying the quality of polymer coatings. Similarly, the system is used for the analysis of paints and primers on a variety of substrates for a range of commercial applications. The curing of polymer film on a metal surface is another important application for handheld FTIR. Furthermore, the ExoScan system equipped with specular reflectance sampling technology measures the thickness of metal oxide layers on metal substrates in anodization processes.
Figure 4 – The thickness and uniformity of coatings on metal surfaces are critical to the performance of the final coated product. The handheld FTIR system, equipped with the spherical germanium ATR, measures the thickness of an aliphatic coating on metal (a) with a limit of detection of approximately 0.05 μm. The thickness of an anodization layer on aluminum is measured to 0.05 μm (b) using the FTIR system equipped with the grazing angle sample interface.
In the manufacture of aerospace, automobile, metal, and biomedical products, as well as in other industries, surface contaminants are frequently detrimental to the production process and the performance of the final commercial product. In particular, silicone and hydrocarbon oil contamination is prevalent in manufacturing environments, and cleaning processes are employed to remove these contaminants. Since the surface coverage of contaminants can be very low, yet drastically affect the ability to bond or coat metal surfaces, the handheld FTIR system, with grazing angle sampling interface, is used to measure low-level contaminants on the surface of metals. The 82° grazing angle interface provides a longer pathlength over the surface of the reflective metal substrate and affords detection of the contaminants in the ng/cm2 range (Figure 5). This capability allows the effectiveness of the cleaning process to be monitored and a final determination to be made when a metal surface is considered uncontaminated.
Figure 5 – Silicone and hydrocarbon oils are pervasive, detrimental contaminants affecting bonding, painting, and coating processes. The 4100 ExoScan FTIR, equipped with the grazing angle sample interface, is used to measure silicone oil on metals with limits of detection in the ng/cm2 range.
Portable instrumentation for use in out-of-lab applications is one of the growing trends in analytical chemistry. In the case of FTIR, this is motivated by the need for more rapid answers about the identity and quality of raw materials and finished products, to detect and measure changes in the performance of materials in use, and to analyze objects that are impractical to analyze in a lab. Handheld FTIR makes the measurements discussed in this article more practical and useful.
In the past, these material analysis applications were performed in a lab and required excising a piece of the object in question. With the advent of handheld FTIR, these measurements can now be done in situ without removing a sample, thereby providing true, nondestructive analysis, regardless of the shape, location, or orientation of the object in question.
In addition to FTIR and other portable optical spectrometers, technologies such as GC/MS, HPLC, and GC are now available in mobile configurations. This trend will continue to grow as new applications continue to drive the need for out-of-lab analysis.
- Rein, A. FTIR Analysis Provides Rapid QA/QC and Authentication of Food Ingredients Prior to Processing; Agilent Technologies, Inc., Publ. 5991- 1246EN, Oct 1, 2012.
- Seelenbinder, J.; Higgins, F. Test Method for Low Level Detection of Biodiesel in Diesel Using the Agilent 5500t FTIR Spectrometer; Agilent Technologies, Inc., Publ. 5990-7804EN, May 1, 2011.
- Rein, A.; Higgins, F. et al. Handheld FTIR Analysis for the Conservation and Restoration of Fine Art and Historical Objects; Agilent Technologies, Inc., Publ. 5990-8739EN, July 26, 2011.
- Rein, A.; Higgins, F. At Site Rock and Mineral Analysis Measurement Using a Handheld Agilent FTIR Analyzer; Agilent Technologies, Inc., Publ. 5990-7794EN, May 1, 2011.
- Richard, S.; Leung, P.T. Identification and Evaluation of Coatings Using Handheld FTIR; Agilent Technologies, Inc., Publ. 5990-8075EN, May 1, 2011.
Alan J. Rein, Ph.D., is Market Development and Strategy Manager, Mobile FTIR, and John Seelenbinder, Ph.D., is Mobile FTIR Marketing Manager, Agilent Technologies, Mobile Measurement, Chemical Analysis Group, 14 Commerce Dr., Danbury, CT 06810, U.S.A.; tel.: 201-909-8824; e-mail: firstname.lastname@example.org. The authors wish to acknowledge Dr. Steve Donahue and Mr. Frank Higgins of Agilent Technologies for their contributions to this article.