CHAPTER 2 High-Resolution, High-Speed and Spectral Range Extension of FT-IR 2.1 High-Resolution FT-IR Spectroscopy Research-grand Nicolet FT-IR spectrometers provide a wide range of software-selectable spectral resolution Δσ, from 128 to 0.125cm-1. The high resolution capability (better than 0.5cm-1) is often needed for gas phase analysis due to the rotational fine structure present in the vibrational spectra of gaseous molecules. The highest spectral resolution of 0.125cm-1 is achieved by translating the moving mirror to extreme position at 0.5(1/Δσ) = 4cm away from the ZPD location, or the centerburst of an interferogram. However, since the data points (Ns) sampled on one side of the interferogram are power of two required by current OMNIC software in the continuous-scan mode, the spectral resolution Δσ is then defined by the interferometric data points actually collected when 0.125cm-1 is chosen on the software for the data acquisition, i.e.
where σmax is the maximum wavenumber allowed with the sample space (undersampling ratio) of 2. Therefore the actual spectral resolution is slightly higher than the software defined. Note that the average full width at half height (FWHH*) of the peaks over the entire CO band region is 0.084cm-1 (with a standard deviation of 0.002 cm-1) when spectral resolution of 0.125cm-1 is selected by OMNIC software. Thus, an actually spectral resolving capability of better than 0.09cm-1 for the natural separation of two identical lines is achieve. Figure 2.1 shows a spectrum of the standard CO sealed gas cell in the region of 2250-2000cm-1 with an expanded view of the spectrum around the peak at 2180cm-1. This spectrum was obtained with standard boxcar apodization function and a software-selected resolution of 0.125cm-1.
2.2 Rapid-scan FT-IR Spectroscopy Rapid-scan capability on the research-grade Nicolet FT-IR sepctrometers is based on continuous-scan mode. It allows the study of reactions or processes as fast as 13ms (or 77 spectra/sec) at a spectral resolution of 16cm-1 (or 8cm-1 data resolution due to a zero filling factor). The synchronization between the spectrometer measurements and the process under study can be easily achieved using an optional Remote Start Accesory (RSA). This accessory is an interface that passes a trigger signal from the spectrometer to your experiment. Thus a precisely timed signal can be provide to the experiment. The RSA further allows you to start data collection from a remote location.
Rapid-scan is applicable to many kinetic processes as long as the rate of process is under the limit of the spectrometer speed. Applications of rapid-scan FT-IR include monitoring liquid phase dispersion, gas phase mechanics and many chemical reaction kinetics such as polymer curing and so forth. Figure 2.2 shows a series of rapid scan spectra of a butane flame generated from a Weber lighter with periodic spark ignitions, collected at a speed of 77 specta/sec, and spectral resolution of 16cm-1. When a reaction process is much faster and on the scale of microsecond or nanosecond, a step-scan FT-IR with a fast TRS digitizer is required. Microsecond and nanosecond time-resolved applications will be addressed in detail in Chapter 4.
2.3 Sepctral Range Extension of FT-IR Spectroscopy Research-grade Nicolet FT-IR spectromenters facilitate spectroscopic measurements over a wide range from far-IR to near-ultraviolet. Since mdi-IR and near-IR spectroscopies have been addressed elsewhere in our literature, only the far-IR, and UV-Vis spectroscopic regions are discussed in this section.
2.3.1 FT-Far-IR Sepctroscopy The far-IR spectral region is generally considered below 600cm-1. Although many organic compounds are far-IR active, this spectral region is particularly useful for inorganic studies. Absorption due to the streching and bending vibrations of bonds between metal atoms and inorganic/organic ligands, generally occurs at frequencies lower than 600m-1 (>17um). In the basic far-IR configuration on a research-grade Nicolet FT-IR Spectrometer, a standard Ever-Glo IR source and a Solid-Substrate beamsplitter are combined with a polyethylene-windowed deuterated lanthanum doped triglycine sulfate(DLaTGS) detector. The Nicolet patented Solid-Substrate beamsplitter provides coverage for the full far-IR spectral region. This single optical element eliminates the need to use multiple beamsplitters to cover the whole far-IR region (by comparsion, three mylar beamsplitters are required to span the far-IR region). The Solid-Substrat beamsplitter is rigid and eliminates problem with beamsplitter flexing during the Michelson interferometers scan cycles, while maintaining a flatness simliar to a wire grid beamsplitter. A far-IR water vapor spectrum and a 100% line collected with the basic far-IR configuration are show in Figure 2.3. The advanced configuration uses a dedicated xenon (Xe) arc source and a highly sensitive helium-cooled silicon bolometer. A far-IR spectrum of acetylferrocene and a far-IR single-beam, collected using the advanced configurations are shown in Figure 2.4 and 2.5, respectively.
Atmospheric water vapor exhibits strong spectral features in the far-IR region, particularly below 400cm-1. The three alternative methods that are commonly used to reduce this atmospheric contribution are: 1) use of a sample shuttle to alternate background and sample collection; 2) removal of water vapor by purging the instrument with N2 or dry air, and 3) evacuating water vapor under vaccum. The sample shuttle device works well for thin film/pellet-based transmission measurements but is nor ideal for other sampling methods, such as attenuated total reflectance (ATR) and diffuse reflectance. Effectively purging the system with N2 works well for almost all sampling techniques, as partially demonstrated by Figure 2.3-2.5. Evacuating the water vapor with a vacuum system is a mechanically effective approach, but this option is costly and limits your sampling capabilities. It introduces electronic and optical components of the system. Evacuation of the system is only necessary in exceptional circumstances. Furthermore the mechanical complexity of vacuum systems limits their popularity.
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