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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers OJTHIS JOURNAL
by calling attention to new developments i n the theory, design, or availabilily of chemical laboratory instrumentation, or by presenting useful insights and explanations of topics thal are of practical importance to those who use, or leach the use of, modern instrumenlation and instrumental techniques. invites correspondence from prospective contributors.
The editor
LI. Fourier Transform SpectrometersPart One M. J. D. LOW. De~arlmenfof Chemirby, New York University, New York, ~ e ~wb r k10453 The choice of the title of this article was prompted by the ususl editorial requirements of brevity and conciseness as well as a desire to avoid long, ungainly structures arch as "Instn~mentatiou for Iteeording Spectra by hleans of Fourier Transform Methods," or "Spectrometers Using Interferometers to Measure Spectrrt," and t h e like. I t is unforl.unate that the more deseripbive titles are iia clumsy, because snappier, shorter t,itles might give t,he impression that the spectra prod~lced by Fonrier Transform speclrometers are different from those produced by the conventional dispersion spectrometers. The final data are identical. The end results produced by a Fourier Trmsform spectrometer system and a ronventiond dispersion speelrometer are the same. Each tbypeof instnmel\t yields a spectrum-a plot of intensity versus frequency. As the ways specbra are used in t,he various areas of chemistry are well know,) and widely described, the prescnt. art,icle will not colmider appliestions or uses of spectra to any ext,enl. However, there are drastic dilTerences it, the way spectra are prodncetl. Enbirely diiferent, r;oncepts and cnnseqneuily entirely difierent, hardware are involved. so that a
even thnogh conventioml spectra in t h e visible to fa-infrared ranges are pmdoeed. I n considering the current stat,us of spectrometers employing Fourier Transform methods for producing spect,ra in the visible to far-infrared mmiges, and especially in tmderstanding the design features of commercisl instrumentation, it is cansequently imefol to ontline the principles of Fourier Transform spectroscopy.
OUTLINE OF FTS I n contrast to the conventional speetrometer, the Fourier Transform spee-
t,romet,erdoes m t contain a monochromalor or dispersing elemel~t. In the COILvent,imal dispersion speelrometer the incoming polyehromxlic rsdinliou is separated into bundles of xlmosl, monov.hrumntic r n d i a t i o ~hecnune the frequencies of the mdiation are so high that x detector cannot discriminnto ammg Ihem. The detector can only respond to tho intensity of radiation, hnt caltnol. diswiminnte among frcqnencies in the S ~ I O P ~rnnge I.IL~ ill which t.he deleelor e m operate. Tho dispersed ritdiatiou is consequently swepl, pas1 n deteel,or in seqnellce. The tleleel.or then produces s simple eleeiriral signal which is proport,ior~nlto the intensity of the $most monoehromnlic radistion st,riking it. The s p e e l ~ x m produced in this fashion is thus tho result of n series of radiometric monsurements. In contrast, the optical system of the Fourier Transform spectrometer trausforms the polychromntic signal in its entirety, so that i l msy be analyzed for frequencyintensity coni.ent without dispersion. The Fowier Trausform melhod is based on the fael Ilmt, ihe signnl (lel.rned "inle~.forogrnm") produced hy the deleeial. of n l.wo-henm i~~lel.ieromeler rtnd the speclnd distribution of the ~.sdiatimel~teringthe inlel.ieromeler n1.e Fourier nwine T~.misforms. P~.eeisedescript.ic,w of the processes iinwlverl arc 1.alher complex, and are give11in rlclnil clrcwhwe (I-::). Hnwever, tho way intcricl.ogl.ams arc pmdnced and processed to restllt in spcrttx can be readily u~idcrslood if one cousidet..; the simple case of mo~wchromntierndinlion or n mixture of jml. niew waveleugths. Son~,ose il beam of mot~xhromntic rxrlintion of wnvelongth X enters s IIichelson interferometer mnd is divided into two
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moved. The s c r o ~ ~ dplane mi~.rar is slalio~inry. 1~;:trhof the lwo rays is refleeted xt .z mirror n d ihen relurned to Ihe he;nnspliller, where their nmpliludes will nrld. If the mys r, nud n arrive nf.the I,earnsplit,ler i u phme, s w h n t r a ~ ~ * f < ~ r t ~ ai:t i8l ifrequewy o~t it.sdf. the cllirienry of tlw Iwnmrplitter i s nud p r o h c w 8 , d ~ t e c l o rwnve of n~ttique c l e l c ~ r m i ~ I,? ~ n lthe nnlurc of tlw m ~ t e r i n l freq~tetwy. The ..iu,:sl or iutcrfenqmm rind how m d hnr well it i*d c p o d t d , ntrd pndw~cl I,? t h r drtertcnr is the,, the XIIIII(('onlinunl on prpr A //XI
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