4 Quantitative Analysis of Textiles by IR Spectroscopy A Comparison of Fourier Transform and Dispersive Techniques
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NANCY M. MORRIS and RALPH J. BERNI Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, LA 70179 Enhancement of the accuracy of quantitative infrared determinations by use of mathematical manipulation of the spectral data as performed by Fourier Transform (FTIR) and Computerized Dispersive (CDIS) Infrared Spectroscopy has been compared. Cotton-polyester blends and cotton treated with THPOH-NH3 and with dimethyloldihydroxyethylene urea (DMDHEU) were analyzed by FTIR and CDIS. The mathematical techniques used included direct spectral subtraction and spectral subtrac tion combined with analysis of peak areas. Advantages and disadvantages of each method were found. Determination of f i n i s h add-ons or blend ratios for t e x t i l e s containing cotton are of prime interest i n t e x t i l e f i n i s h i n g laborat o r i e s . Many approaches have been taken to acquire this information including Kjeldahl nitrogen analysis, other wet chemical determinations, x-ray fluorescence or d i f f r a c t i o n (1), and infrared spectroscopy (2-6). The use of infrared spectroscopy required that a band due to the treatment or blend component be isolated from bands due to the c e l l u l o s e so that peak height or peak area measurements could be made without interference from overlapping peaks. Peak area measurements involved counting squares on the graph paper, cutting out the peak and weighing the graph paper, or making planimeter measurements. With the advent of computerized spectroscopy, f i r s t with FTIR and then with CDIS, the question arose whether spectral subtraction routines and other mathematical manipulation of the spectra could provide better and/or easier quantitation of these blends or treatments. Comparison of CDIS and FTIR CDIS interfaces a standard grating spectrophotometer and a microcomputer. The spectrophotometer alone gives a usable infrared T h i s chapter not subject t o U . S . copyright. Published 1984, A m e r i c a n C h e m i c a l Society
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
POLYMERS FOR FIBERS A N D E L A S T O M E R S
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spectrum (a plot of intensity versus wave number or wavelength). Use of the computer adds the a b i l i t y to: 1. 2.
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3. 4. 5.
Store the spectral data on a microfloppy disk for l a t e r manipul a t i o n and r e t r i e v a l . Mathematically manipulate the spectra ( i . e . , addition, subtraction, m u l t i p l i c a t i o n , baseline flattening, smoothing, e t c . ) . Replot the spectrum on a video display unit or on paper. Perform repetitive runs. Measure the peak heights or integrated areas with the computer.
FTIR takes a completely different approach. The spectral data are acquired as an interferogram (Figure 1) which must be transformed into a plot of intensity versus wavenumber or wavelength through the application of Fourier transform equations. Thus, the computer i s an integral part of the system without which l i t t l e useful information could be obtained. FTIR has the following advantages over computerized dispersive infrared spectroscopy: 1. 2.
3.
The multiplexing, or F e l l g e t t ' s , advantage which arises because a l l of the resolution elements are observed a l l of the time. The throughput, or Jacquinot's, gain which i s produced by the large sampling area and absence of narrow s l i t s i n the i n t e r ferometer. These factors result i n a large amount of energy reaching the detector and a s i g n i f i c a n t improvement i n the signal-to-noise r a t i o for FTIR. The use of the He-Ne laser interferometer to reference the p o s i t i o n of the moving mirror increases the accuracy of frequency determination (Connes' advantage) for FTIR.
In addition to the a b i l i t i e s provided dispersive infrared spectroscopy by interfacing a computer, the FTIR adds the a b i l i t y to select the resolution through software parameters. This sets the mirror displacement which, depending on the instrument, can take on values from 0.125 cm" to 32 cm" . Good spectra are obtained with a resolution of 4 or 8 cm" . The farther the mirror travels, the greater the resolution; but, at greater resolutions, the time for c o l l e c t i o n and computation of the spectral data increases considerably. 1
1
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Experimental To study the quantitation of c e l l u l o s i c t e x t i l e s , potassium bromide disks were prepared by the method of O'Connor et. a l . C3). The infrared spectra of the disks were obtained and stored for further calculation. The Fourier transform IR spectra were obtained on a Digilab Model 15B (Company or trade names are given f o r information purposes only and their use does not imply that the USDA recommends those products over others that may also be suitable.) with a Nova 3 computer, a TGS ( t r i g l y c i n e sulfate) detector, a globar source and a KBr beam s p l i t t e r . The instrument was continuously purged with dry nitrogen. The following parameters were used to c o l l e c t the
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by UNIV OF BATH on March 17, 2016 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch004
M O R R I S A N D BERNI
Quantitative Analysis of Textiles by IR
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