When analysts are faced with the decision of selecting either mid-IR or near-IR spectroscopy for materials characterization, the decision is usually based on the analytical, sampling, and/or operational needs. For example, when the greatest analytical specificity is required or certain inorganic materials need to be measured, the choice favors mid-IR, whereas the need to keep an unstable or dangerous material inside a sealed glass container during measurement might suggest near-IR as the preferred choice. In practice, mid-IR is fairly ubiquitous in analytical laboratories, but the use of near-IR is growing faster, especially in the quality control environment. Those laboratories handling a wide range of samples often choose mid-IR because it is a more familiar and well-established technique, often passing over the benefits of near-IR because in budget-conscious times it may be difficult to justify both mid-IR and near-IR instruments. Ideally, in order to select the best method for the widest range of materials, it is desirable to have both techniques available at one’s disposal.
This situation has not been helped by the very different development histories of mid-IR and near-IR. With routine near-IR entering the marketplace much later than IR, the instrument vendors usually focused on one technique or other; consequently, users were sometimes provided less than impartial advice on the most appropriate choice of spectral region. This resulted in a split in the user community into mid-IR and near-IR groups. Even today, some laboratory functions are segregated into mid-IR and near-IR sections with astonishingly little communication between them. In addition, the useful long-wavelength near-IR region (ca 1.5–2.5 μm) is now readily accessible using modern, high-performance FTIR spectrometers that satisfy most application needs. This presents a difficult predicament for the new user: Faced with the choice of a mid-IR or near-IR purchase, a high-performance, wide-range FTIR might be a safer choice than a near-IR system, but would it be the optimum choice?
Combining recent mid-IR and near-IR developments
The concept of a multirange FTIR spectrometer is not new. Previously, instruments offered the provision for multiple sources, beamsplitters, and detectors, enabling access to the various spectral regions on a single bench. However, these instruments did not provide an acceptable level of the right near-IR performance, and were largely adopted by more advanced users in R&D laboratories requiring sampling flexibility and customizable experimental setups. On these systems, switching between ranges required a level of care in the system reconfiguration and equilibration that was not consistent with routine use in a busy laboratory. In recent years there have been a number of significant improvements in instrument stability and standardization; high-sensitivity, short-wavelength transmission sampling; and QA/QC software aimed specifically at the more routine user. Unfortunately, these developments were generally associated with dedicated mid- or near-IR instruments and were not available to the earlier multirange instruments.
Figure 1 - Spectrum 400 combined mid-/near-IR spectrometer.
The Spectrum 400 combined mid-IR/near-IR spectrometer (PerkinElmer Life and Analytical Systems, Shelton, CT) brings together these latest developments in mid-IR and near-IR (Figure 1). The system incorporates sophisticated automated range switchover and fully exploits the latest mid-IR and near-IR QA/QC sampling and imaging developments. The result is a highly versatile and fully optimized FTIR and near-IR system in a single optical bench. A variety of mid-IR and near-IR sampling options are shown in Figure 1, e.g., diamond-attenuated total reflectance (ATR) for mid-IR sampling of solids, combined transmission/reflection short-wavelength near-IR sampler for automated tablet analysis, and a liquid sipper for automated liquid sampling. The instrument can also be equipped with a range of IR microscopes and imaging systems—techniques that have become invaluable problem-solving tools in many industrial laboratories. In such a system, multiple sampling accessories can be permanently installed and selected by motorized beam switching.
Advantages of a combined system
For the busy laboratory testing a wide range of materials, a combined system offers the freedom to select the best sampling technique for the application regardless of spectral range. In methods development laboratories, use of common software and analysis platforms helps in the comparison of the relative performance of mid-IR and near-IR methods. When the most appropriate technique has been chosen it is very easy to transfer the method to the manufacturing plant, where, if required, a single-range mid-IR or near-IR version of the instrument may be used with dedicated sampling. Excellent reproducibility can be achieved between the methods development and production instruments. This is achieved by standardizing both the wavelength calibration and lineshape of instruments using high-resolution gas phase spectra as built-in reference materials.
The combined system offers the scope to address a wider range of applications than single-range systems. In laboratories handling many samples per day, the ability to rapidly switch range and always be ready for the next sample provides significant productivity improvements over time. Other benefits of using a common software platform for both mid-IR and near-IR ranges include faster learning and lower training needs.
Analyzing a wide range of solids and liquids
In most laboratories there is a requirement to use IR spectroscopy to characterize samples with a wide variety of physical forms. Near-IR diffuse reflection is often the ideal technique to use for waxy or granular powdered solids. However, some solvents (e.g., the homologous series) are homogeneous liquids that may need a high level of discriminatory power to distinguish between them. While it is possible to use special reflectors to enable a spectrum of a clear liquid to be obtained with a diffuse reflectance accessory, this arrangement is far from optimal, and errors introduced by such a sampling configuration will limit the reliability of the method. However, mid-IR ATR analysis, or even a direct transmission measurement, is more successful. A combined system with both near-IR diffuse reflectance and mid-IR ATR installed simultaneously offers a better combination of convenience and analytical performance.
Another common situation is in raw materials testing, where near-IR libraries are developed for many incoming materials. However , there are some common ingredients, such as anhydrous silica, that do not provide a useful near-IR spectrum. Fortunately, the mid-IR spectrum of this material is highly distinctive and reproducible, and in these circumstances, the same combination of near-IR diffuse reflectance with mid-IR universal attenuated total reflectance (UATR) is more appropriate than a dedicated near-IR system.
Samples that require careful handling
Figure 2 - Mid-IR ATR spectra of sodium acetate after exposure to air.
Sodium acetate is a white, hygroscopic powder with a very wide range of uses in industries such as textiles, foods, and cosmetics. Attempting to measure the anhydrous form by mid-IR UATR is difficult because the spectrum changes according to the amount of time it is exposed to moist air (Figure 2).
In this situation it makes more sense to measure the powder noninvasively in a sealed vial or jar using near-IR diffuse reflection. While mid-IR is the standard technique in the bulk chemicals and textiles industries for raw materials testing, a combination of both techniques is more useful and allows unstable materials, and those requiring special handling from a health and safety perspective, to be conveniently measured.
Materials characterization using mid-IR and near-IR imaging
Near-IR chemical imaging is becoming widely recognized in the pharmaceutical industry as an invaluable tool for studying the distribution of ingredients in solid dosage forms. A potential drawback is that the measurement is a diffuse reflection technique, and this limits the spatial resolution and level of detail that can be seen. If the domain sizes for the ingredients of interest are greater than around 50 μm, current near-IR imaging systems work well and are faster than Raman or standard mid-IR imaging. However, many ingredients have particle sizes less than 10 μm, and cannot be observed in the near-IR unless particle aggregation occurs during manufacture.
Figure 3 - Near-IR and mid-IR ATR images of pharmaceutical tablets. The ATR technique can readily reveal single particles of less than 10 μm. The near-IR technique images larger areas and whole tablets more efficiently.
This is where mid-IR ATR imaging is invaluable. In this technique, the incident beam travels through a high refractive index medium that is in contact with the sample. The beam only penetrates the sample to a depth of ca. 1–2 μm, resulting in a huge improvement in spatial resolution and image clarity compared with near-IR. Single particles of ca. 5 μm have been readily observed using ATR imaging. For troubleshooting products in which ingredient domain sizes can range from less than 10 μm to many hundreds of μm, mid-IR ATR and near-IR imaging are truly complementary (Figure 3).
Conclusion
Due to the recent advances in system performance, software, and the flexibility of sampling, it now makes sense to combine high-performance mid-IR and near-IR in a single instrument. Given the freedom to use either technique, the analyst should no longer face the difficult decision of choosing between a mid-IR and near-IR spectrometer, and can direct this effort toward finding the most suitable method for a particular sample. This will result in fewer failed methods, more effective troubleshooting, and more productive instrument use.
Dr. Sellors is IR Technology Manager, Molecular Spectroscopy, PerkinElmer Life and Analytical Sciences, Chalfont Rd., Seer Green, Beaconsfield Bucks. HP9 2FX, U.K.; tel.: 800-762-4000; fax: 203-925-4654; e-mail: [email protected].