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An Introduction to Infrared spectroscopy
Infrared spectroscopy is a technique that can be used mainly qualitatively, but occasionally quantitatively, for the identification of organic (and sometimes inorganic) substances.
Each organic compound, whether it be a simple compound, polymer or adhesive, produces a characteristic spectrum when a beam of radiation from the infrared region of the electromagnetic spectrum is passed through it. As the light source changes in wavenumber, certain molecular vibrations give rise to absorptions or peaks at definite values of wavenumber. It is the position and pattern of these peaks that produce a characteristic spectrum.
As part of a continuing programme of investment, Sandberg has acquired a spectrometer with attenuated total reflectance sample handling. This allows a direct spectrum to be obtained on dense samples with black or coloured fillers. In the simplest form, the sample might be a piece of plastic, rubber or sealant that can be placed in the sampling device. The sampling device can look at very small amounts of material down to 2mm across without further preparation.
In some cases, however, the compound of interest must be separated from a substrate such as concrete, plaster or stone and converted to a suitable form for presentation to the instrument.
The technique together with state of the art computer software allows identification of subtle product variations from the same or different manufacturers’ products of the generic substance types such as polysulphides or silicones.
The technique has been successfully used on small quantities of extracts to identify additives and bonding agents in construction materials, for example, PVA bonding agents at render interfaces.
Using Infrared Spectroscopy
Infrared spectroscopy is an excellent technique for the identification of organic materials, including sealants, paints, resins, plastics, rubbers, adhesives, oils, fats, waxes and pure or mixed solvents, whether they are in the form of a simple compound or a complex mixture of polymers. Each material, providing that it is infrared active, produces a unique infrared spectrum and it is this property of a material that allows us to identify it.
Sandberg has a state of the art infrared spectrometer coupled with an omni sampler that allows us to record an infrared spectrum with little or no sample preparation, therefore saving time and altering the sample as little as possible, resulting in a more accurate identification. We also have an extensive and ever-growing library of infrared spectra that allow us to compare the infrared spectrum produced by the sample with a library spectrum produced on the same infrared spectrometer.
The technique of infrared spectroscopy is particularly useful for the analysis of sealant samples to identify the generic type or condition of the sealant in service. It is also useful in the identification of paint samples for resin type, either for paint stripping purposes or for overpainting with a compatible paint system. Other uses for infrared spectroscopy include the identification of unknown plastic or rubber samples, the characterisation of adhesives or grouts and the identification of solvents in unmarked containers for recycling or safe disposal.
The following sections are intended to give a taste of the technique of infrared spectroscopy and the many and varied situations that it can be applied to within the built environment. The technique is highly adaptable and extremely powerful and many more situations will arise where it will prove to an invaluable technique. In the future, with the development of ever more sophisticated instruments and computer software programmes, the technique will go from strength to strength.
Producing an Infrared Spectrum
A beam of radiation from the infrared region of the electromagnetic spectrum is passed through the material. As the infrared light source changes in wavenumber, this gives rise to certain molecular movements such as bending and stretching These movements absorb energy at definite values of wavenumber giving rise to peaks on the infrared spectrum when viewed in absorbance mode. It is the position and pattern of these peaks that produces the characteristic infrared spectrum for a material.
Analysing the Sample
The process of analysing the sample can be considered in two steps.
The first step is the presentation of the sample. Modern instruments, such as the one we possess at Sandberg, allow us to analyse the sample with little or no sample preparation. The sample can be placed directly onto the crystal and pressed in place by a torque limiting device. This allows us to examine very small sample sizes, say about 2mm in size. The advantages of this direct analysis are obviously in time-saving but also in its non-destructive nature.
In addition to this direct approach, solvent extraction techniques do have their place, especially if the sample has a high quantity of inorganic fillers which can mask the nature of the organic component of interest. This process involves dissolving the material in a solvent and placing a drop on to the germanium crystal. Evaporating the solvent off leaves a cast film of the organic component.
Another technique employed is pyrolysis of the sample and examination of the resulting pyrolysate, also useful in separating the organic component from the inorganic filler.
The second step in the analytical process is spectral interpretation. This can be carried out manually by identifying the groups present and building up a picture of the molecule in a similar way to a jigsaw puzzle. However, computer software is available to do this, with the computer searching databases and coming up with a best match for the spectrum in terms of position and relative height of the peaks. The databases are commercially available and there is even an online searching facility. However, it is often more reliable to build your own library of data so that, when searching, the spectra have all been obtained under the same conditions.
Infrared Spectroscopy – Applications in the Built Environment
The range of circumstances encountered in the built environment for which analysis by infrared spectroscopy is a useful analytical technique are numerous and diverse, as the following examples demonstrate.
Infrared Spectroscopy – Analysis of Sealants
In perhaps the most straight forward cases, infrared spectroscopy can be used to analyse sealants to identify the generic type and possibly even a manufacturer. The generic types of sealant have very different and highly characteristic infrared spectra (as can be seen in Figure 1) showing infrared spectra of a silicone sealant, a polysulfide sealant and a polyurethane sealant. In cases where the project specification calls for a particular sealant type, infrared spectroscopy can easily determine if the specified product has in fact been used.
Infrared spectroscopy can also be used to undertake condition surveys of sealants already in use. Because the sample size does not have to be very large, sealant surfaces can be examined by an infrared method for degradation, and then compared with the body of the material to highlight potential sealant failures.
A further use of infrared spectroscopy in the examination of sealants is in the case of two-part sealants comprising a resin and a hardener. Infrared spectra can be obtained of both starting materials and of a range of reference laboratory mixes in different ratios. These can be compared with the spectra obtained from the site mixed sealant to ascertain that the correct materials had been used and in the correct mix ratios. Figure 2 shows the infrared spectra obtained for a two-part modified polyurethane sealant mixed correctly and also overdosed and underdosed with hardener. The relative peak heights show whether the material has been correctly mixed and, whilst these differences may not appear great to the naked eye, the computer software can differentiate them.
Infrared Analysis of Paint Samples
Infrared spectroscopy can also be used to identify generic paint types as these also have distinct infrared spectra. This information may be required for the purpose of overpainting with a compatible paint system, selecting a suitable paint stripper to remove multiple layer paint systems or for the purposes of obtaining data for historical records. Due to the requirement of a small sample size, individual layers of paint can be analysed using infrared spectroscopy within a multiple layered paint system, provided they can be separated. As in the case of sealants, the instrument is capable of detecting very slight differences between resins and Figure 3 shows the infrared spectra of alkyd resins variously modified.
Infrared spectroscopy is also sensitive enough to detect the difference between paint application methods and Figure 4 shows the infrared spectra obtained for a sprayed and a powder-coated polyester resin paint finish.
Concrete Staining
In a more investigative role, infrared spectroscopy has been used to examine a concrete sample with heavy pink staining. Due to the nature of the sample, a direct analysis was not possible so an infrared spectrum had to be obtained using a solvent extraction and cast film technique. The resultant infrared spectrum, when compared with our own library data of infrared spectra, was found to be very similar to phenolphthalein (as can be seen in Figure 5) so the staining on the concrete sample was probably the result of spillage by a careless operative.
In a similar investigative vein, a ceramic tile had surface markings which may have originated from a paint product or an adhesive material. Comparison of the infrared spectra obtained from the three materials in question soon identified the source of the markings to be the paint product, rather than the adhesive.
Using Infrared Spectroscopy to Investigate Cable Damage
Both of the previous two examples show how the technique of infrared spectroscopy can be used in a comparative way to identify unknowns. However, in a less obvious scenario, some cabling had been accidentally immersed in an inhibitor solution containing sodium nitrite. The purpose of the analysis was to determine if the inhibitor solution had penetrated the outer sheath of the cable thus posing a threat to the high purity copper conductors within.
Due to the small quantity of water, and therefore even smaller quantity of sodium nitrite that may have penetrated the cable, classical analytical techniques could not be employed. Water is, however, infrared active and so, employing the technique of infrared spectroscopy, spectra could be obtained for the outside face of the cable, inside surface of the cable and, by paring the cable at a shallow angle with a scalpel, the middle of the cable.
Lengths of cable were variously subjected to immersion over a range of time spans, heating in water and immersion with a vacuum being applied to the inside of the cable. Infrared spectra were then obtained of the three positions and compared one to another. The results did not show any evidence of water penetration into or through the outer sheath so, therefore, it was felt unlikely that a short term immersion would have had any dramatic effect.
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