Commercial FT-IR spectrometers

Fourier Transform Infrared (FT-IR) Spectroscopy With the introduction of commercial FT-IR spectrometers, the application of oil analysis by IR became relatively commonplace for production oil analysis laboratories. The mathematically intensive infrared data analysis techniques that were difficult or impossible to perform on the earlier IR systems became easy on these systems. In addition, quantitative analysis measurement techniques such as peak height, peak area, local baselines and more sophisticated matrix methods could be easily employed in the analysis, and the automation of lubricant analysis became commercially viable. 

Commercial FT-IR spectrometers have dedicated computer systems to collect, store, transform and output the data. These computer systems range from limited-task microcomputers to versatile, powerful minicomputers. Computer systems are general to all Fourier transform techniques and will not be discussed in this chapter. 

Although dispersive dynamic IR experiments are presently more sensitive than FT experiments over limited spectral ranges, recent introductions of commercial FT-IR spectrometers with step-scanning capability should make dynamic 2D IR spectroscopy accessible to many more laboratories. 

It is outside the purpose of this book to go deeply into the details of the various optical designs of commercial FT-IR spectrometer interferometers and their peculiarities and nuances, the means of sampling data, the way inter-ferograms are processed to obtain mid-IR spectra, etc. There are many books and reference works that cover these aspects the interested reader is recommended particularly to consult reference 4. Suffice it to repeat here what was mentioned above. 

An example of a real system where these factors have been brought into consideration may be found in commercial FT-IR spectrometers that collect high-resolution... 

This multistep approach is being used in several commercial FT-IR spectrometers. Only with efficient gain ranging can the SNR of FT-IR spectra measured with a pyroelectric detector and 16-bit ADC come close to the theoretical value. 

To determine whether 0/ or 0 > should be used in Eq. 7.8, both parameters should be calculated and the smaller one used. This is fairly easily done and will be illustrated using the parameters of one commercial FT-IR spectrometer ... 

It is instructive to calculate the theoretical SNR of a typical commercial FT-IR spectrometer that operates with a 2-mm DTGS detector (D = 2 x 10 cm W ) with an optical throughput of 0.06 cm sr. We will assume... 

To achieve a SNR of 10 in the mid-infrared spectrum (Vn x = 4000 cm ), the positional error must be less than 10 cm (1 A) (i.e., about one atomic diameter ). It is a testimony to the power of laser referencing that this specification is easily met by all commercial FT-IR spectrometers. 

A good description of all the factors contributing to the efficiency of an FT-IR spectrometer has been reported by Mattson [10]. He measured the effect of several different parameters that include beamsplitter efficiency, Fresnel losses at the substrate and compensator plate, reflection losses at the mirrors, radiation obscured by the mounting hardware for the HeNe laser, the emissivity of the source, and losses caused by imperfect optical alignment. He calculated the overall efficiency ( in Eq. 7.8) as being 0.096. This value is in accord with the value of 0.10 used in Section 7.1 to estimate the SNR of a commercial FT-IR spectrometer. 

The SNR of many FT-IR spectrometers designed for operation in the mid-infrared spectmm is usually worse between 600 and 400 cm than in any other spectral region. There is also a greater difference in the performance of commercial FT-IR spectrometers in this spectral region than in any other, as some manufacturers have taken more care to optimize the performance in this region than others. Three reasons account for the poor performance of mid-infrared spectrometers below 600 cm ... 

Many commercial FT-IR spectrometers have a variety of built-in software routines used for purposes other than those described above for example, calculations of the refractive and absorption indices from a reflection spectrum by using the Kramers-Kronig relations (see Chapter 8), baseline correction, peak picking, zap, (i.e., drawing a straight line over a desired wavenumber region in a spectrum), and so on. 

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