Infrared Analysis Using Tissue Paper Noel L. Owen1 U. C. N. W.. Bangor, Wales, U. K. Steven G. Wood Brigham Young University, Provo, UT 84602 Infrared spectroscopy has long been recognized as being one of the most effective analytical techniques currently available. I t represents a relatively inexpensive, quick, and easy-to-operate method that can often give very definitive, analytical information. The successful development of Fourier transform infrared spectroscopy during the past decade or so as an alternative approach to conventional dispersion spectrophotometry has-risulted in many 1aborato;ies replacing old infrared spectrometers with modern FTIR equipment. What may not be fully appreciated, however, is the fact that in addition to obtaining standard infrared spectra, an FTIR instrument opens up new dimensions in spectroscopic analysis. The increased sensitivity and the powerful com~utationalfacilities (to m o t e iust two features) that accompany this new techniquebffer-the possibility of new develo~mentsand new applications for infrared work. In this short commun&ion, we wish to describe a very quick, easy, and cheap, but effective method of obtaining infrared spectra of solids and nonvolatile liquids, made possible by the high sensitivity of FTIR spectroscopy. The technique utilizes tissue paper as a support matrix t o obtain spectra. This material would normally be considered as opaque to dispersive infrared instruments. The use of this method represents an alternative to the KBr and nujol mull techniques while offering some advantages (and disadvantages) over the current methods. For example, the FTIR spectrum of facial tissue paper (Kimwipes) taken in the usual transmittance mode produces a well-defined spectral pattern (Fig. 1). This spectrum was generated using a Mattson Sirius 100 interferometer at a resolution of 4 em-' and it represents an average of 60 scans with a total scanning time of about 1%minutes. The spectrum of our unknown solid can he obtained by dissolving the material in a small volume of volatile solution and then transferring the solution dropwise onto the tissue paper. The solvent rapidly evaporates, leaving the solid resi-
' Present address: Department of Chemistry, Brigham Young Unl-
due well dispersed within the paper fibers. A new scan of the impregnated paper shows a very similar pattern t o the original spectrum, but with some new svectral features. The ''~imwipe" spectrum of acetanilide is shown in Figure 2. Using the subtraction routine that is offered on all FTIR ins~r;ments, a -differenre" spectrum can be generated within a few seconds. This differenre spectrum for acetanilide is shown in Figure 3 and illustrates some of the strengths and weaknesses of this method. The absolute intensities of the absorption peaks are fairly low, but practically every absorption observed in the spectrum of a KBr disk of acetanilide is renroduced in the tissue snectrum [see Fie. 4(a) . . and (b)l. . .. For comparison we have also inrluded a spectrum of acetanilide, run as a nuiol mull usinr NaCl dater;. The extra absorntiona due t o the h y d r ~ c a r h o ~ oand i l the cut off of NaCl atabout 600 cm-' can he clearly seen [Fig. 4(c)]. The intensity pattern is slightly different especially around 1000 cm-', where the cellulose component of the paper exhibits strong absorp-
..
Figure 2. FTIR speetrurn of aMttanilide dispemed on paper tissue.
verslty, Provo, UT 84602
wa"e"""ber
Figure 1. FTlR spemurn of "Kimwipes" paw tissue 976
Journal of chemical Education
N.V*"""bW
Figure 3. "Difference" infrared spectrum for acetanilide
Figure 5. FTlR specba of acetarnide in (a) paper tissue and (b) KBr disk
3. Increasing the sample concentration is very simply effected by adding more solution to the tissue. 4. It takes very little experience to be able to judge whether the sample concentration is sufficient, simply by inspecting the solid traces seen on the tissue. 5. Our experiences indicate that no significant difference can he detected between the infrared spectra of tissue samples taken from the same box. 6. It is sometimes very difficult to prepare a good KBr disk and the procedure may have to be repeated several times before a satisfactorv soectrum is obtained. I . KBr powder is prone to ahsorh moisture, and it is quite difficult to preserve a spectroscopically "clean" batch free from impurity peaks. 8. The tissue method eliminates the need to use a hydraulic press with its attendant difficulties. 9. As mentioned earlier, this new method does not give good results in the 0-H and N-H stretching regions, and some care must be exercised when interpreting difference spectra around 1000em-'. However. it does enable soeetra to be taken down to 400 em-' without difficulty (NaCI tits out at about 600 cm-I). 10. One interesting feature that should he noted is that in some instances significant frequency shifts occur between spectra taken on tissue and spectra taken using KBr. Figure 5(a) and (b) illustrates this point for acetamide. (See also the table.) This is understandable when one considers that the local molecular environment is quite different for the two eases. The fact that KBr itself is not an inert matrix, but represents a very polar environment, can he easily demonstrated by comparing spectra of samples run as hydrocarbon mulls with those in
..
Figure 4. Specha of acetanilide between 2000 and 400 cm-' in (a) tissue, (b) KBr dirk and (c)nujol hydrocarbon oil.
paper
tions. T h e ereatest disadvantaee of this new matrix is the fact t h a t i n i h e region between it300 a n d 3200 cm-', t h e very strong absorptions of the cellulose 0-H groups mask most other absorptions from t h e sample. Some of the advantages and disadvantaees. towther with some of t h e interesting features associatedwiti using tissue paper a s a matrix base are summarized below. I. 2.
'The method USPR di~posahlepaper tissues involving negligible rosr of sample preparation ("Kimwipes" appear to be well
11. Because of the differences in the matrix materials as outlined
suited for this purpose). The whole experimental procedure is very simple and very quick.
above, the resolution of individual absorption bands varies between tissue and KBr spectra. In some instances the former gives superior results (e.g., Fig. 5, the 1600 em-' peak of acet-
KRr.
Volume 64
Number 11 November 1987
977
A Comparlson of the Positions (in cm-') 01 Some of the Major Absorption Peaks in KBr Powder and in a Paper Tissue Matrixa
KB~
acetamide 1677 1402 1356 1151 584
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paper 1662 1391 1348 1142 571
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Figure 6. FTlR specba of 4-aminopyridinein (a)paper tissue and (b) KBr disk. amide) and in some the latter (e.g., Fig. 4, the 1650 em-' peak of acetamide). In conclusion. we would like to illustrate the usefulness of the tissue meth"d by comparing spectra of other samples (4aminoovridine and 2-adamantanol: Fie. 6 and 7 . resoectively) take; using KBr and by this diff&ence technique. T h e wavenumber values of the major peaks for all spectra taken in KBr and on tissue are listed for comparison in the table. T h e a h s o r ~ t i o npatterns in the fineerprint regions of hoth 4.. . aminopyrihine and 2.adamanmnol are reprohuced faithfully with the tissue method usinr minute quantities of solid matter (-10 mg). Indeed, for adamantanol it is quite easy to distinguish, from tissuespectra, the 2- and 1-isomeric forms for thii comnound (see. . .for examole..Aldrich's FTIR soectra collection).
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Figure 7 . FTlR spemaot 2-adamamanol in (a) paper tissueand(b)(rlghl)KBr disk. 978
Journal of Chemical Education
Obviously nonvolatile liquids may be studied using the tissue method.. simnlv . "hv" denositine a small droo onto the tissue-although there woulh appear to he no obvious advantazes in this method over using conventional salt dates. except for the reduced cost of mat&als. However, themeth: od could he used very effectively to follow the progress of a chemical reaction, by removing a drop of reactant a t fixed time intervals and scanning the liquid spectrum using tissue as a matrix material. In particular, we wish to emphasize the sheer simplicity, ease, and convenience associated with this method, and the fact that it offers a greater latitude for many different types of nroiects of relevance to teachine laboratories. The method seem; well suited for an underGaduate teaching environment where cost. cleanliness. and sim~licitvare often a t a premium. The gkneral technique is not resiricted to Kimwipe tissues, and other matrix materials may be utilized, leading to the possibility of research projects into the effect
of different matrix materials on certain characteristic absorntion bands. EXPerlmental Paper tissue was used by cutting small sections of Kimwipes and
mounting them (usingadhesive tape) onto cardboard. Any suitable volatile solvent may be used (e.g., chloroform,methylene chloride, acetone, toluene, methanol, etc.), and in some instances mixed solvents improved the solubility of samples and the distribution of the solute on the paper. (For example, methanollmethylene chloride mixtures gave improved results compared with the latter solvent alone.) Quantities of materials used ranged from 10 to 15 mg of solute dissolved in about 0.1-0.3 mL of solvent. Acknowledgment
We wish to thank Bruce Jackson for his help and support, and NLO wishes to thank Brigham Young University for its support during a sabbatical period.
Volume 64
Number II November 1987
979
