Computers infrared spectrometer

The infrared spectra of the different samples were taken with a Fourier Transform infrared spectrometer (Digilab FTS-14) using the double beam mode vs. air as reference. 150 scans per sample and 100 scans per reference, at a resolution of 4 cm-l, were taken for every sample. All spectra were stored on tape, and a digital substraction of the after- and- before UV exposure (or any other sample treatment) spectra was performed, whenever needed, by an on-line computer, thus permitting a better visualization of the spectral changes in the polymer by UV- photooxidation. 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

Fig. 6.5. Computed structures due to the hydrogen dimer, in the quadrupole-induced (0223,2023) components near the So(0) line center at 120 K (the temperature of Jupiter s upper atmosphere). Superimposed with the smooth free — free continuum (dashes) are structures arising from bound — free (below 354 cm-1) and free - bound (above 354 cm-1) transitions of the hydrogen pair (dotted). The convolution of the spectrum with a 4.3 cm-1 slit function (approximating the instrumental profile of the Voyager infrared spectrometer) is also shown (heavy line) [282].

For qualitative analysis, two detectors that can identify compounds are the mass spectrometer (Section 22-4) and the Fourier transform infrared spectrometer (Section 20-5). A peak can be identified by comparing its spectrum with a library of spectra recorded in a computer. For mass spectral identification, sometimes two prominent peaks are selected in the electron ionization spectrum. The quantitation ion is used for quantitative analysis. The confinnation ion is used for qualitative identification. For example, the confirmation ion might be expected to be 65% as abundant as the quantitation ion. If the observed abundance is not close to 65%, then we suspect that the compound is misidentified. 

The earliest estimate of kB was by Johnston.224 On the basis of work he had performed on the N205 system,313 he computed a lower limit for k5 of 1010 Af-1 sec-1 at room temperature. Hisatsune, Crawford, and Ogg,202 using a rapid-scanning infrared spectrometer, studied the decomposition of N205 in the presence of NO. Relevant to the NO + N03 reaction is their determination of k6 and k6k5/k-6, where kg and k 6 are forward- and reverse-rate constants for the reaction... 

CAUTION Books can have a hard time keeping up with advances in computer technology. For example, here is how the sixth edition of Fundamentals of Analytical Chemistry described Fourier-transform infrared spectrometers in 1992. 

Fourier-transform infrared spectrometers offer the advantages of unusually high sensitivity, resolution, and speed. .. offsetting these advantages is their high cost, because a moderately sofisticated dedicated computer is needed to decode the output data [1]. (The sofisticated computer was a 33 MHz PC with 512 K of RAM, current market value 2.)... 

A Fourier transform infrared spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitised. A digital computer initially records and then processes the interferogram and also allows the spectral data that result to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

Remote multicomponent air samples can be drawn into a centrally located analyzer (under computer control) and then into the gas cell of an infrared spectrometer. Within the spectrometer, a system of lenses and mirrors passes an infrared beam in a predetermined path through the sample. The amount of energy absorbed by the sample is compared against a standard beam, and the difference is related to the concentration of the gas of interest. By changing the wavelength of the infrared beam, additional materials may be checked for in the gas sample and their concentration levels determined the same way. If the concentration of the compound of interest exceeds a predetermined level, an alarm is activated. 

Apparatus Use a Fourier transform infrared spectrometer (FTIR), with its associated computer and peripherals, capable of measuring from 4500 to 500 cm-1 and of acquiring data with a resolution of at least 2 cm-1. The optics of the instmment must be sealed and desiccated, or, like the sample chamber, must be under continuous dry air or nitrogen gas purge. The spectrometer is equipped with software capable of multicomponent analysis using the partial least squares method (PLS-1, or equivalent). This software is commercially available as an accessory to the spectrometer or as an external software package. 

A. R. H. Cole, Tables of Wavenumbers for the Calibration of Infrared Spectrometers, 2d ed., Pergamon, Oxford (1977). See also http //www.hitran.com/ (HITRAN is a compilation of spectroscopic parameters that a variety of computer codes use to predict and simulate the transmission and emission of infrared light in the atmosphere.)... 

Most modern infrared spectrometers are equipped with a computer or microprocessor. The computer can record and store spectra, plot either absorbance... 

Figure 1 illustrates a data path in a typical ratio-recording, dispersive infrared spectrometer. The digitization of the analogue signal produced by the detector M.A. Ford, in Computer Methods in UV, Visible and IR Spectroscopy , ed. W.O. George and... 

A modern-day petroleum refinery is a complex chemical operation that involves numerous separations and chemical processing steps. Today virtually all the chemical analysis equipment found in the research laboratory is also used in the refinery or an online basis is often coupled to a control circuit to monitor product quality and make the necessary immediate adjustment in process conditions required to meet product specifications. While the online gas chromatograph is the most widely used instrument, infrared spectrometers, mass spectrometers, pH indicators, new infrared spectrometers with chemometric capability and moisture analysis based in solid-state conductors are not found in every refinery in the country. Until the 1970s, samples of most process streams in the refinery were taken at periodic intervals during the day and adjustments were made after the research was received from the refinery s analytical lab. This process was followed by the installation of online analysis equipment that sounded alarms, and the equipment operators took appropriate action. Today most operations are on computer control and the information received from online analytical equipment is processed almost continuously and controls make the required changes. An alarm may still sound and the equipment operator still responds, but usually the problem has already been corrected. 

The last component part of an infrared spectrometer other than the evaluating computer is the amplifier. The purpose of the amplifier is to amplify the signal coining from the detector to enable the computer to evaluate the signals. 

Conventional infrared spectrometers are known as dispersive instruments. With the advent of computer- and microprocessor-based instruments, these have been largely replaced by Fourier transform infrared (Fllk) spectrometers, which possess a number of advantages. Rather than a grating monochromator, an FTIR instrument employs an interferometer to obtain a spectrum. 

A typical interferogram is shown in Figure 16.26. The tall part of the signal corresponds to when the two mirrors are equidistant from the beam splitter, when destructive interference between the two beams is zero, and is called the centerburst. The intensity drops off rapidly away from this, due to destructive interference. This is converted, using a computer, into the frequency domain via a mathematical operation known as a Fourier transformation (hence the name Fourier transform infrared spectrometer). A conventional appearing infrared spectrum 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. 

The operation of dispersive instruments is generally straightforward, with most of j he skill involved being associated with the sample preparation. If you have access to a Fourier-transform infrared spectrometer, you will probably need to spend some time becoming familiar with the computer software which drives the instrument. Some suggestions for experiments to try out yourself are listed here. However, it is not necessary to attempt all of those mentioned. You may decide to concentrate on samples and sampling techniques which are relevant to your field of interest. 

A variation on the GC-MS technique includes coupling a Fourier transform infrared spectrometer (FT-IR) to a gas chromatograph. The substances that elute from the gas chromatograph are detected by determining their infrared spectra rather than their mass spectra. A new technique that also resembles GC-MS is high-performance liquid chromatography-mass spectrometry (HPLC-MS). An HPLC instrument is coupled through a special interface to a mass spectrometer. The substances that elute from the HPLC column are detected by the mass spectrometer, and their mass spectra can be displayed, analyzed, and compared with standard spectra found in the computer library built into the instrument. 

Multivariate optical computing (MOC) is a very recent and innovative strategy to build near-infrared spectrometers. It is based on the creation of... 

It is perhaps not universally known that Raman spectroscopy preceded infrared spectroscopy as a routine analytical tool in American industry. This author used it extensively in the analysis of aviation gasoline during WWII, even though he had to employ various tedious chemical procedures to reduce fluorescence. Raman spectroscopy became dormant in industry when inexpensive infrared spectrometers became available. The resuscitation of Raman spectroscopy in carbon industry is mainly due to the invention of the laser, of very sensitive detectors, of inexpensive digital computers to improve signal-to-noise ratios. 

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