Commercial Fourier transform spectrometers

Commercial Fourier transform spectrometers operating at moderate resolution (1cm-1) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the... 

Table 2. Survey of commercial Fourier transform spectrometers... 

More recently, we have improved the time-resolution of the system substantially. The present instrument is capable of recording high time- and frequency-resolution spectra of transients having decay times from the nanosecond to the millisecond regime. The minimum time delay between the initiation of the transient and the first spectral observation can be arbitrarily short. (Typically, the first spectrum is recorded just before the transient in order to provide a background observation. A maximum of 128 successive time-delayed spectra of a single transient can be recorded the minimum time delay between each of these is 10 ns. All operational parameters (resolution, sensitivity, etc.) of the commercial Fourier transform spectrometer with which the system is used, are unchanged by time-resolved operation. Variability in die baseline due to amplitude instabilities in the excitation source (usually a pulsed laser) are taken into account, and appropriate corrections are made. 

The introduction of commercial Fourier transform (FT) spectrometers in the early 1960 s has made it possible, in part, to overcome the limitations associated with dispersive instruments and has helped to broaden the scope of problems amenable to investigation by infrared spectroscopy. The purpose of this review is to compare the performance of FT and dispersive spectrometers and to illustrate areas of application in which FT spectroscopy has proven advantageous for the study of adsorbed species. In view of these objectives only a limited treatment of the theory underlying FT spectroscopy will be presented here. 

Data Acquisition, Storage, and Display. Fourier-transform spectrometers require access to an on-line computer in order to permit rapid transformation of interferograms to spectra, and, in fact, the development of commercial instruments was in large... 

The situation, however, is different for the infrared spectroscopic measurements with opposed anvil cells. The source beam in commercial Fourier transform infrared spectrometers is generally focused to about 1 cm diameter at the sample, whereas the diameter of the gasket hole in the high pressure cell is only about 0.3 mm. Therefore, a source beam condensing system is required in order to obtain infrared spectra with a good signal-to noise ratio. Commercial beam condensers (4X, 6X) could, in principle, be adapted for these purposes. In practice, however, the mirrors of the... 

This Section compiles information on a number of commercially available Fourier transform spectrometers. Though this compilation is not complete, the interested reader will find here descriptions and technical data of altogether thirteen instruments from six manufacturers. Especially those instruments have been included here which were recently developed or improved. Therefore, this introduction to commercial instruments in the field of Fourier transfrom spectroscopy is believed to be a useful completion of a similar one published about 4 years ago in Ref. 

Nuclear magnetic resonance has been shown to be a most effective method for the study of lipid chemistry (Chapman, 1965 1972 Henrikson, 1971). With the advent of commercially available fast Fourier transform spectrometers, high resolution natural abundance 1 3 C spectra and relaxation times of lipids have become relatively commonplace. Utilization of these 1 3C nmr techniques has yielded a considerable amount of information concerning the mobility and organization of lipids in liquid crystals and membranes (Oldfield and Chapman, 1971). 13C Chemical shifts of lipids are given in Table 21. The rest of this discussion will be devoted to the interpretation of these results. 

Infrared spectrometers have been commercially available since the 1940s. At that time the instruments relied on prisms to act as dispersive elements, but by. the. mid 1950s, = diffraction gratings had been introduced into dispersive machines. The most significant advances in infrared spectroscopy, however, have come about with the introduction of Fourier-transform Spectrometers. This type of instrument employs an interferometer and explbits the well established mathematical process of Fourier transformation. FT-IR spectroscopy has dramatically improved the quahty of infrared spectra and has minimised the time required to obtain data. Thus j with the improvements to computers achieved in recent years, infrared spectroscopy has made great strides. 

In virtually all types of experiments in which a response is analyzed as a function of frequency (e.g., a spectrum), transform techniques can significantly improve data acquisition and/or data reduction. Research-level nuclear magnetic resonance and infra-red spectra are already obtained almost exclusively by Fourier transform methods, because Fourier transform NMR and IR spectrometers have been commercially available since the late 1960 s. Similar transform techniques are equally valuable (but less well-known) for a wide range of other chemical applications for which commercial instruments are only now becoming available for example, the first commercial Fourier transform mass spectrometer was introduced this year (1981) by Nicolet Instrument Corporation. The purpose of this volume is to acquaint practicing chemists with the basis, advantages, and applications of Fourier, Hadamard, and Hilbert transforms in chemistry. For almost all chapters, the author is the investigator who was the first to apply such methods in that field. 

As was true with infrared and nuclear magnetic resonance instruments. Fourier transform mass spectrometers provide improved signal-to-noisc ratios, greater speed, and higher sensitivity and resolution." Commercial Fourier transform mass spectrometers appeared on the market in the early 1980s and are now offered by several manufacturers. 

This is not to imply that molecular structure of simple organic molecules cannot be determined by infrared spectroscopy. In fact, the information that can be deduced from an infrared spectrum is complementary to that of other methods, and infrared spectroscopy provides valuable information that is unattainable by other methods, as is shown in the remainder of the book. More important, however, a plethora of other applications became available with the advent in 1969 of the first commercial mid-infrared Fourier transform spectrometer with better than 2cm resolution. These include quantitative analysis of complex mixtures, the investigation of dynamic systems, biological and biomedical spectroscopy, microspectroscopy and hyperspectral imaging, and the study of many types of interfacial phenomena. All of these applications (and many more) are described in this book. Furthermore, because of the development of such sampling techniques... 

Data processing programs for the above-mentioned purposes are often supplied with commercial Fourier transform-infrared (FT-IR) spectrometers as integral parts of their software packages. They are also available in commercial software packages... 

Most commercial Fourier-transform (FT) spectrometers are based on the Michelson interferometer (Figure 3). A collimated beam of radiation falls on a beam splitter that divides it into two beams with approximately equal intensities. These two beams strike mirrors that return them to the beam splitter where they recombine, creating interference, and... 

The monochromator selects and passes only the radiation in a narrow bandpass of known wavelength. This can be done with prisms, or gratings, or a Fourier transform spectrometer. Commercial instruments are listed in a table by Zissis and LaRocca (1978, Tab. 20-7). The same reference discusses the various instrument types in detail. Moore et al. (2009) discuss optical dispersing instruments and provide a table comparing the various types. Shannon and Wyant (1979) also discuss spectral dispersing... 

Jeener first described a two-dimensional NMR experiment in 1971, but it was not until 1975 that the first two-dimensional spectra were published by Ernst (1975). The pioneering efforts of Ernst and co-woricers in developing the technique should be appreciated by those who glean information from two-dimensional experiments. The NMR community also owes a debt of gratitude to the laboratory of Ray Freeman. His contributions and those of his associates to two-dimensional NMR have expanded the technique and promoted its growth. The NMR spectrometer manufacturers have also been instrumental in the development and application of two-dimensional NMR. By 1980 all of the commercially available Fourier-transform spectrometer systems provided two-dimensional data processing as part of then-standard software package. 

Time-resolved Fourier transform infrared spectroscopy has been used surprisingly little considering the nuadter of commercial spectrometers that are currently in laboratories and the applicability of this technique to the difficult tine regime from a few is to a few hundred is. One problem with time-resolved Fourier transform spectroscopy and possibly one reason that it has not been more widely used is the stringent reproducibility requirement of the repetitive event in order to avoid artifacts in the spectra( ). When changes occur in the eiaissirr source over the course of a... 

The most recent advance in VCD instrumentation has been its adaptation to Fourier transform infrared (FTIR) measurement (23-25). The details of this technique involve a new method of FTIR measurement termed double-modulation FTIR spectroscopy. Thus spectra of very high quality and resolution have been obtained using a standard VCD modulator and detector, a glower source, and a commercially available FTIR spectrometer system. In fact an entire FTIR-VCD spectrometer can be assembled from a few commercially available components. It is found that the major advantages of resolution, throughput, and... 

D Commercial COTS controlled by external computer Hybrid systems such as automated dissolution workstation with high-performance liquid chromatography (HPLC) or ultraviolet-visible (UV-Vis) interface Liquid chromatographs, gas chromatographs, UV/Vis spectrophotometers, Fourier transform infrared (FTIR) spectrophotometers, near-infrared (NIR) spectrophotometers, mass spectrometers, atomic absorption spectrometers, thermal gravimetric analyzers, COTS automation workstations... 

An overview of commercial ICP mass spectrometers from different companies (quadrupole based ICP-MS with and without collision/reaction cell, double-focusing sector field instrumentation with single and multiple ion collectors, time-of-flight (ToF), ICP-ion trap-MS and non-commercial ICP-Fourier transform ion cyclotron resonance (FTICR) mass spectrometers is given in Figure 5.2. By using ion traps and FTICR mass spectrometers in ICP-MS isobaric interferences of atomic ions... 

Advantages of Fourier transform infrared spectrometers are so great that it is nearly impossible to purchase a dispersive infrared spectrometer. Fourier transform visible and ultraviolet spectrometers are not commercially available, because of the requirement to sample the interferometer at intervals of S = l/(2Av). For visible spectroscopy, Av could be 25 000 cm 1 (corresponding to 400 nm), giving S = 0.2 im and a mirror movement of 0.1 xm between data points. Such fine control over significant ranges of mirror motion is not feasible. 

---