Anal. Chem. 1986, 58,2179-2184
2179
Effect of Absorbing Matrices on Diffuse Reflectance Infrared Spectra Paul J. Brimmer and Peter R. Griffiths*
Department of Chemistry, University of California, Riverside, California 92521
The effects ol absorblng matrlces on the dtffuse reflectance Infrared spectra of organlc analytes are Invostlgated. The Kubelka-MunkfuncUonforcompounds~ln absorMng matrlces varles In a nonllnear manner wlth concentratlon, maklng quantitathre analysls dmkutt. The qualltathre behavior of these systems Is also dtfferent from that of species dlspersed In nonabsorblng rnatrlces. For example Intense absorption bands may appear as reflectlon maxlma when absorbing matrlces are used. Potentlal causes of thls behavior are discussed. The results of thls study are Important In the Interpretation of spectra of adsorbates on absorblng substrates, such as TLC plates and supported catalysts, and carbon-contalnlng materlals, such as coals.
The number of applications for which diffuse reflectance (DR) infrared spectrometry has been used has grown substantially since 1978, when Fuller and Griffiths (1,2)described optics for DR measurements that were readily compatible with rapid scanning Fourier transform infrared (FT-IR) spectrometers. Since that time DR infrared spectrometry has been used in a qualitative manner for a wide variety of samples (3, 4). However DR infrared spectrometry may be described as semiquantitative a t best, and then only if characteristics of the sample and dilution matrix adhere to certain criteria. These criteria originate from the assumptions made in the derivation of the Kubelka-Munk equation (5,6),which is the diffuse reflectance analogue to Beer’s law for transmission measurements. The Kubelka-Munk (K-M) equation is often used to describe the behavior of diffuse reflectance in the mid-infrared region. The K-M function, F(R,) is given by where R, is the diffuse reflectance spectrum of an “infinitely thick” sample ratioed to that of a nonabsorbing reference, k is the absorption coefficient, and s is the scattering coefficient. If the sample is mixed with a matrix, the reference spectrum is usually that of the pure diluent. The dilution matrix should be nonabsorbing so that k is related to the absorption of the sample alone. In this case, It is related to the absorptivity, a, and the concentration, c, by (7)
k = 2.303ac
(2)
In many applications of DR infrared spectrometry, the characteristics of the sample or the matrix do not follow the assumptions that were made in the derivation of the K-M equation. For example, the K-M equation was derived for weakly absorbing samples. Thus to achieve adherence to K-M behavior for mid-infrared DR spectrometry, samples must often be diluted in a nonabsorbing matrix, such as an alkali halide powder. There are certain applications, however, where the matrix has strong absorption bands in several spectral regions. In these spectral regions, the intensity of the absorption bands of minor components dispersed in the matrix does not usually follow the behavior predicted by the K-M 0003-2700/86/0358-2179$01.50/0
equation. Components of a mixture separated by thin-layer chromatography (TLC) measured in situ (8, 9) and species adsorbed on alumina or silica supported catalysts ( 1 0 , I I ) fall into this category. The reduction in intensity of bands of separated components on silica TLC plates in the region of strong absorption by the silica adsorbent is readily apparent in the spectra reported by Zuber et al. (9). Most organic compounds must be diluted in a nonabsorbing matrix (such as KCI) to avoid the effects of anomalous dispersion (12).We have noticed, however, that the DR spectra of coals prepared at various concentrations in a nonabsorbing powdered diluent behave in a quite different manner to the DR spectra of most organic compounds. For example, the spectra of coals of low mineral content are strikingly similar whether or not they are diluted in KC1 (see Figure l),and no evidence of anomalous dispersion may be seen in the DR spectrum of the neat coal. For coals of high mineral content, the absorption bands of minerals (especially clays) are greatly enhanced relative to those of the organic components when the sample is ground with a KCl diluent as opposed to being simply mixed; see Figure 2. The reason for this behavior was initially not readily apparent to us. In this paper we report the results of several measurements that suggest reasons for behavior of the type described above for coals and species adsorbed on substrates such as silica. A strongly absorbing material, carbon black, was added to a transparent diluent, KC1, to simulate the effect of absorption by the matrix while minimizing the introduction of artifacts due to anomalous dispersion from intense discrete absorption bands in the matrix (as would been seen, for example, in the spectrum of silica). The behavior of absorption bands of organic analytes added to this matrix was then investigated quantitatively.
EXPERIMENTAL SECTION Diffuse reflectance measurementswere performed with a Model 296 interferometer (Digilab Division of Bio-Rad, Cambridge, MA) and the optical layout described by Fuller and Griffiths (I,2). All measurements were made with an intermediate range mercury cadmium telluride detector (Infrared Associates, New Brunswick, NJ). Interferograms were collected at 4 cm-’ resolution and were triangularly apodized. The amplifier gain was adjusted in each case so that the signal triggered the most significant bit of the analog-to-digitalconverter, and 1000 scans were signal averaged for each spectrum. Potassium chloride (obtained as “random cuttings”, Harshaw Chemical Co., Solon, OH) was gound for 5 min in a WigL-Bug ball and mill grinder (Crescent Manufacturing Co., Chicago, IL) and subsequently sieved to less than 100 pm diameter with a sonic sifter (ATM Corp., Milwaukee, WI) and stored in an oven at 120 “C. Carbon black was obtained from Bay Carbon Co. (Bay City, MI) and used as received. The average diameter of the carbon black particles estimated from electron micrographs was less than 10 wm and each particle appeared to be formed from an aggregate of several smaller particles. Coal samples were obtained from the Pennsylvania State University Coal Bank, and were ground for 5 min in a Wig-L-Bug. Carbazole (Eastman Kodak Co., Rochester, NY) was used as a typical organic analyte for most of the experiments reported here; it was ground in a Wig-L-Bugfor 5 min and stored in a desiccator. Dilution matrices, which ranged from 0 1986 American Chemical Society
2180
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
T
I
n
a 6 L
L 4000
e00
2400 WAVENJNBER (em-')
Figwe 1. Kubeka-Munk spectra of a low volatile bituminous coal of low mlneral content (PSOC645,4.32% mineral matter) measwed neat (top) and as a 1 % mixture (by weight) in KCI powder (bottom).
4
4000
0.c5
h 2400
800
WAVEWMBER (crn-1)
Figure 3. Kubdka-Munk spectrum of 1 % carbazole (weight percent) in KCI powder.
matrices, and coal in KC1, were mixed in a similar fashion and stored in a desiccator prior to measurement of the DR spectrum. Prepared samples were pressed into 4 mm diameter cups by using a modification of a Parr pellet press (A. H. Thomas, Philadelphia, PA). The amount of sample, as well as the amount and duration of applied pressure, was kept constant, since the manner in which the sample is packed can drastically affect the scattering coefficient in the K-M function (14).In our laboratory, band intensities of DR spectra of samples prepared in this manner have a relative standard deviation of approximately 3 % The single beam spectrum of carbazole in the different dilution matrices was always ratioed to the single beam spectrum of the matrix in which the analyte was dispersed. The resulting reflectance spectrum sometimes exceeded 100%. This occurred when using highly absorbingmatrices, where addition of carbazole causes an increase in the intensity of the reflected radiation. Therefore, before computing the K-M function, the single-beam spectrum of the reference was scaled so that the base line was just below 100% (at about 98% for the region in which the reflectance was greatest). I
/
RESULTS AND DISCUSSION
)
le00
1400
1200
WAVENUMBER (cm") Flgure 2. Kubelka-Munk spectra of a high volatile bituminous coal wlth high mineral content (PSOC 401, 13.13% mineral matter) prepared as a 1 YO mixture in KCI by simple mixing (bottom) and by grinding together (top).
"totally absorbing" (carbon black) to nonabsorbing (potassium chloride),were made by mixing carbon black and preground KCl in a Wig-L-Bug capsule with no ball bearings in the capsule to prevent further grinding. This method of mixing samples was found to be the most accurate and reproducible for quantitative DR measurements (13). Mixtures of carbazole in the prepared
When a nonabsorbing powder is used both EB a diluent and as a reference for DR measurements, the calculated K-M spectrum appears similar to the absorbance spectrum of the compound dispersed in a KBr pellet. As an example, the K-M spectrum of 1%carbazole (weight percent) in KC1 is shown in Figure 3. Quantitative analysis can be performed from plot9 of the intensity or area of an absorption band from K-M spectra of known concentrations of the compound. A typical K-M plot for a strongly absorbing band of an organic compound is shown in Figure 4,curve A, which shows the change in the intensity of the 3400-cm-' band of carbazole with its weight percent in KC1. The intensity of this band increases linearly at low concentration and exhibits a strong deviation from linearity at higher concentrations. The deviation from linearity occurs in part because absorption by the analyte at high concentration causes the effective penetration depth at the analytical wavenumber to decrease (13) and in part because of the effect of anomalous dispersion. Both effects
ANALYTICAL CHEMISTRY, VOL. 58. NO. 11. SEPTEMBER 1986
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1'
Flgure 6 . Electron micrograph of a matrix composed 01 9 5 5 carbon black/KCl (X432)
Fbwe 4. Plot 01 me change in intensky 01 the 3400cm ' oard In the specmum 01 carLmzok3 wnh the welgnt Percent Caroazole in a dilution M b i X of (A) KCI. IB) 99 1 KC I C a M black. (C) 95 5 KCllCaM black. ID, 90 10 KC lcarbon black
Flgure 7. Electron micrograph of 50% carbazole mixed in a matrix composed 01 9 5 5 carbon blackIKCI (X576).
Flgure 5. Electron
micrograph of carbon
black
powder
(X.5761
operate in concert, causing the ohserved negative deviation from linearity, The three remaining curves in Figure 4 are plots of the variation in the intensity of the 3400-cm-' hand of carbazole with concentration, for carbazole dispersed in dilution matrices composed of 991 KCl/carhon black (CB), 955 KCI/CB, and 9O:lO KCI/CB, respectively. None of the curves for adsorbing matrices follows the behavior predicted by the K-M equation. For a given concentration of the carbazole analyte, the observed intensity of each hand decreases as the absorption by the matrix increases (i.e., as the carbon black content increases). As the ahsorption by the matrix increases, the effective penetration depth decreases. This in turn causes a decrease in the number of analyte (carbazole) particles that can interact with the infrared radiation, with a resultant decrease in the hand intensity. It is also interesting that the shapes of the curves for analytes diluted in absorbing matrices are sigmoidal, which is quite different from the curves obtained for nonabsorbing
matrices. Our rationale for this behavior is that at low analyte concentrations, the surface of the sample is composed mostly of particles of the matrix, which consists of a large number of carbon black particles (d < 10 p n , see Figure 5 ) and a smaller number of larger KCI particles (d < 100 fim, see Figure 6). In Figure 6, the large KC1 particle near the middle of the electron micrograph is shown to he surrounded by a number of smaller carbon black particles. For compacted samples, the smaller carbon black particles probably coat the larger KCI particles to a greater degree. The particles of carbazole are much larger than carbon black particles, as can he seen in F i p r e 7. The large (d = 100 wm) carbazole particles are again coated with smaller carbon black particles, which strongly absorb infrared radiation and to a large degree prevent it from interacting with the carbazole analyte. This behavior results in greatly reduced intensities for the carbazole hands. Although a fraction of the infrared beam is undouhtedly transmitted through either a KCI or a carbazole particle, these particles are surrounded hy carbon black particles. There is therefore a high probability of ahsorption of the photons which have been transmitted through a given carbazole particle by the particles of carbon black on its surface and a concomitantly low probability of the beam reemerging from the sample and being measured a t the detector.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
5OX CARBAZOLE MIXED IN 95% CARBON BLFICK/U! KCL
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composed of
955
carbon black/KCI.
At higher carbazole concentrations, a greater proportion of analyte is present in the uppermost layer of the sample independent of the nature of the matrix. More carbazole particles therefore interact with the incident i n f r d radiation, and fewer of the transmitted photons are absorbed by carbon black on the surface of the analyte. Thus more intense absorption bands result. At very high carbazole concentration, the K-M curves deviate toward a horizontal asymptote for the same reason discussed earlier for nonabsorbing matrices. For high concentrations of the analyte,an unusual behavior is observed when the carbon black content exceeds 10% by weight. In this case reflectance maxima, or reststrahlen bands, are observed a t intense absorption bands of the analyte. For example, in the DR spectrum of 50% carbazole in a matrix of 955 CB/KCl, the bands near 750 cm-'appear as reflectance maxima; see Figure 8. Reststrahlen bands are observed for bands of very high absorptivity in the specular reflectance spectra of bulk samples. The reason for this behavior can be understood conceptually by using the simplest form of the Fresnel equations (namely for unpolarized incident radiation at normal incidence), which relates the front surface reflectance, R , to the index of refraction, n, and the index of absorption, K of pure samples as
R= K
(n-
+ d~~
(n+
+ n2K2
(3)
can be related to the absorptivity, a, by 4 m
a = In 10-K
x
(4)
where h is the wavelength. For bands of low absorptivity, Fresnel (front surface) reflectance is low, and reflection minima result in the DR spectrum of powdered samples as absorptivity maxima. For bands of very high absorptivity, the Fresnel reflectance is greater and reflectance maxima may be observed near absorptivity maxima. For bands of intermediate absorptivity a combination of absorption and front surface reflection gives rise to anomalous dispersion, which
can be detected as a small shift of the band maxima to longer wavelength. The intense doublet near 750 cm-' in the spectrum of carbazole has a different appearance when measured neat than when measured after dilution in KC1. The 750-cm-' bands are slightly shifted to lower wavenumber in the spectrum of the neat sample as anomalous dispersion begins to occur. This behavior is probably not a result of a change in the index of refraction of carbazole, but rather a change in the type of interface the infrared photons must encounter when impinging on a carbazole particle. For the neat sample, the interface is air-carbazole or carbazole-carbazole. For the sample diluted in KC1, the interface is KC1-carbazole. In this case the ratio of the refractive indexes is significantly less than for aircarbazole at all but the most intense carbazole absorption bands. Thus, for the neat sample there is a greater amount of Fresnel reflectance and there is usually indirect evidence of anomalous dispersion for bands of intermediate intensity. A similar situation exists in the case of the reflectance maxima that occur when using highly absorbing matrices. For example, the surface of the sample discussed earlier (Figure 81, which was composed of 50% carbazole in the matrix of 955 CB/KCl, is largely composed of carbon black and carbazole particles. That fraction of the incident radiation that impinges on carbon black particles is totally absorbed. The radiation impinging on a carbazole particle is partially transmitted and partially reflected, according to the Fresnel equations. The radiation that is transmitted through the carbazole particle will be partially absorbed, giving rise to an absorption spectrum. On emerging from the particle, this transmitted radiation has a high probability of subsequently encountering a carbon black particle and being absorbed. Thus most of that fraction of the incident radiation that has been transmitted through an anal@ particle, and therefore contains absorption information about the compound, never reaches the detector. Much of the radiation reflected from the surface of the carbazole particles, however, is collected and directed to the detector. The effect of grinding the analyte and matrix together, as opposed to mixing, was examined by preparing the sample in a manner similar to that described above, except that during mixing of the carbazole in the matrix, two small ball bearings were included in the Wig-L-Bug capsule. The DR spectrum of the same sample discussed above (Figure 8) prepared by grinding in this manner is shown in Figure 9. In this case, the bands near 750 cm-' appears as reflectance minima, with little evidence of anomalous dispersion. The electron micrograph of this sample, shown in Figure 10, indicates that the carbazole particles have been ground much smaller than when the components of the mixture are simply mixed. Thus the alkali halide appears to act as an abrasive during grinding. The sample is also quite possibly more intimately mixed, with particles composed of all of the components ground into each other. Thus, instead of having large particles of carbazole surrounded by smaller carbon black particles, we have an intimate mixture of smaller particles, with more carbazole accessible to the incident infrared radiation. The intensities of the other absorption bands in the spectrum of carbazole are increased as well, which is consistent with this interpretation, since the scattering coefficient, s, decreases when the mixture is ground. In summary, the DR infrared spectra of samples prepared in absorbing matrices by two different methods appear vastly different. This was observed not only for carbazole but also for most other organic analytes. In all the cases studied, the softer analyte particles had a tendency to agglomerate into larger particles, which were decreased in size when ground in the presence of an abrasive matrix. By preparation of the
ANA1-YTICAL CHEMISTRY. VOL. 58. NO. 11. SEPTEMBER 1986
50% CARBAZOLE GnOUNO WITH 95% CARBON BLACK / 5% KCL m
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Flgure 10. Electron
matrix composed of
micrograph of 50% carbazole ground with a carbon black1KCI (X1440).
95:5
sample in this manner, the amount of Fresnel reflectance from the surface of particles was reduced, and the appearance of reststrahlen hands was eliminated. With the knowledge gained in this study, we are in a position to offer explanations for the behavior of DR infrared spectra of material adsorbed on an infrared absorbing suhstrates and of coals. I t can he deduced from Figure 4 that the effective penetration depth near regions of absorption hy the matrix (e.g., alumina or silica) is much smaller than in the "window" regions where absorption due to the matrix is much weaker. This results in less intense absorption hands for the analyte than would be observed using a transmitting matrix. In addition, Fresnel reflectance from the substrate in the regions of absorptivity maxima is much greater than the "window" regions, so that less of the incident radiation penetrates into the adsorbent in the first place. The behavior of DR infrared spectra of coal appears to be due to a very fine dispersion of carbon particles, or another material which has a very high absorptivity across the spec-
2183
trum. within the organic matrix. The penetration of the infrared beam into the coal is therefore very small because of absorption by the carbon particles. In fact, Fuller has reported that the DR infrared spectrum of a bulk coal can be measured by creating a roughened layer on its upper surface no more than 10pm in thicknesa (25). This suggests that the effective penetration depth of the infrared beam into a coal sample is not much greater than this. The effect of the strongly absorbing carbon particles is a reduction in hand intensity well below the value that would have been measured if the organic molecules were dispersed in a transmitting matrix (see Figure 4). If a coal is simply physically mixed in a nonabsorbing powder, such as KCI, the spectrum appears very similar to the spectrum of the neat coal, but of lower intensity, as shown in Figure 1. If the coal is ground together with KCI, several spectral differences result, which may be observed in Figure 2. After the coal is ground with KCI, all absorption bands in the spectrum are of greater intensity than if the coal were only mixed with the Same quantity of the matrix. As in the case of carbazole in an absorbing matrix, this band intensification is the result of producing smaller products. As a result the wattering coefficient, s, in the K-M equation is decreased. Since a more intimate mixture also results, some of the organic molecules are released from the absorbing matrix and are made more accessible to the incident IR beam. The absorption band a t 1135 cm-', which is largely due to the mineral kaolinite present in the coal, is enhanced to an even greater degree by grinding with KCI. The absorptivity of this hand is much greater than that of typical organic compounds, and therefore a greater amount of Fresnel reflectance would be expected near 1135 cm-' (see eq 3). After the mal is ground with KCI, the amount of Fresnel reflectance from the surface of mineral particles is decreased. This hehavior may he compared to the differences observed in the 750-cm-' band of carbazole, which appear as reststrahlen hands when the analyte is simply mixed with an absorbing matrix and as absorption bands when it is ground with the absorbing matrix. With a decrease of the amount of Fresnel reflectance, the transmission through mineral particles increases, resulting in enhanced absorption hands. This enhancement, along with that gained by liberation from the carbon matrix during grinding with KCI, combines to increase the relative intensity of the mineral band a t 1135 cm-I.
CONCLUSIONS This study has shown that a quantitative description of DR infrared spectra using absorbing matrices will be very difficult unless the samples are prepared in an exceptionally reproducible manner. Qualitative analysis of such systems can be performed, hut great differences in the infrared spectrum can appear as a consequence of different methods of sample preparation. The differences that occur, specifically the appearance of anomalous dispersion or reststrahlen features, should not he confused with chemical changes in the compound of interest. Registry No. KCI, 7447-40-7; carbazole, 86-74-8. (1) Fuller.
LITERATURE CITED Michael. P.; GrHlHhr. Peter R. Anal.
Chem. 1978. 50.
1906-1910.
(2) Fuller, Mlcheel P.: Grims,Paw R. Am. Lab. ( F a M . CMn.1 1978.
lO(l0). 69-82. (3) Chalmers. J. M.; Mackenzle. M. W. Appl. Spechooc. 1985. 39(4).
634-641. (4) GrilHths. Peter R.; Fuller. Michael P. A&. InfraredRaman Specmc. 1983, 9 . 277. (5) Kubelka. P.: Munk. F. 2. Tech. phys. 1921, 12. 593-601. (6) Kubelka. P. J . Opt. SOC. Am. 1948. 38. 448-457. (7) Frei, Roland. W.: MacNeil. J. D. Diffuse Remctance Speclmscopy h Environmental Robkrn SoMng; CRC Press: Cleveland. OH. 1973. (8) Fuller. Michael P.: Griffllhs. Peter R. Appl. Spechorc. 1980. 34, 533-539.
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Anal. Chem. 1966, 58, 2184-2189
(9) Zuber, Gary E.; Warren, Richard J.; Begosh, Peter P.; O'Donnell. Ellen L. Anal. Chem. 1984, 56, 2935-2939. (10) Hamadeh, Issam M.;King, Dewey; Griffiths, Peter R. J . Catal. 1984, 88, 264-272. (11) Van Every, Kenneth W.; Griffiths, Peter R. Roc. SOC. Photo-Opt. Instrum. €no. 1885. 553. 509. (12) Brimmer, Pail J.; Grifflths,' Peter R. Appl. Spectrosc., In press. (13) Hamadeh, Issam M.;Yeboah, Samuel A.; Trumbull, Kathy A,; (~iffm, Peter R. Appl. Spectrosc. 1984, 38(4), 486-491
(14) Yeboah, S. Agyare; Wang, Shih-Hslen; Grlfflths, Peter R. Appl. SpectrOSC. 1984, 38(2), 259-264. (15) Fuller, E. L. Martln Marletta Energy Systems, Inc., Oak Ridge, TN, personal communication, 1984.
RECEIVED for review February 4, 1986. Accepted April 28, 1986.
Continuous Recording of Reflection-Absorbance Fourier Transform Infrared Spectra of the Effluent of a Microbore Liquid Chromatograph John J. Gage1 and Klaus Biemann* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A new method for the recording of the Infrared spectra of components separated by high-performance liquid chromatography is demonstrated. I n thls scheme, dmwtographlc effluent is contlnuowly sprayed onto a rotcrthg dkk that has a rellective ouface. Mer ckpoawkn the 8oMos are analyzed by again rotating tho disk In the sample compartment of a Fourier transform Infrared spectronnkr epubrped wlth a beam condenser while reflectance-absorbance spectra are contlnuously collected. By thls method, spectra that compare well wlth conventionalKBr data over the e n t h mkcwrared r e g h may be obtained at sensitlvltks In the hundred nanogram range for anthracene and lower for more strongllr absorMng compounds. The contlnuoue record of the chromatographic separath allows dlsplay d the data as Gremsdwn#1 plots, spectra, or wavelength chromatograms, whkh d ald In the Interpretatlon. Slnce th&i record is permanent, spectra may be enhanced by signal averaging many mans whlk the mkror is stopped at a position correspondlng to the component of interest.
The high scan speed and sensitivity of Fourier transform infrared (FTIR) spectroscopy have made it possible to record infrared spectra of the individual components of mixtures separated by chromatographic techniques (1-22).To date thii concept has been successful for gas chromatography coupled with FTIR (GC/FTIR) (2-6). However, many compounds of interest are not sufficiently volatile for GC separation and the sensitivity of the popular lightpipe method of GC/FTIR is reduced for less volatile compounds due to higher lightpipe temperatures (4, 5). These less volatile and/or more polar compounds can usually be separated by high-performance liquid chromatography (HPLC). The interfacing of an HPLC with FTIR is, however, more difficult because the infrared absorption of the mobile phase must be compensated for by spectral subtraction or eliminated before infrared analysis in order to obtain useful data for a solute present in low concentrations. To achieve this end, two general types of systems have been developed: (1)flow cells, which allow the recording of spectra while HPLC effluent passes by a window transparent in the infrared region (7-ll),and (2)solvent deposition systems, which involve transfer and elimination of the solvent on a medium compatible with infrared spectroscopy (12-18). 0003-2700/86/035&2184$01.50/0
The flow cell approach represents the simplest method for obtaining infrared data. Furthermore, it makes the spectrometer a true chromatographic detector by allowing continuous analysis of the chromatographic effluent. Although several applications have been reported (9,IO),the usefulness of the technique is limited because the spectral regimes where the solvent absorbs radiation are opaque result in loss of information or require such short path lengths that sensitivity is drastically decreased. A more recent innovation in the flow cell type design involves flowing the chromatographiceffluent along the outside of an attenuated total reflectance cell (ATR) (11).Because of the short path lengths inherent to ATR, even the highly absorbing aqueous solvents may be used with this technique. For the same reason, however, the sensitivity is decreased and the spectra of milligram injections were reported (11). Solvent deposition designa have utilized diffuse reflectance Fourier transform infrared spectroscopy (DR-FTIR) and transmittance spectroscopy. The DR-FTIR interface, pioneered by Kuehl and Griffiths (12),involves the depositing of concentrated portions of HPLC effluent into a series of diffuse reflectance cups filled with KBr powder. After the solvent is evaporated, the cup is brought into the beam path of the spectrometer and the diffuse reflectance spectrum is recorded. Although this system has demonstrated good sensitivity, especially with the use of microbore HPLC columns (13),it does not allow a continuous analysis of the chromatographic effluent. This is a considerable disadvantage because a component can be missed or more than one component may collect in a single cup. Furthermore, continuous recording of spectra has many advantages in the interpretation of the spectral data such as Gram-Schmidt reconstructions (19), wavelength chromatograms, or the plotting of successive spectra to identify closely eluting compounds. Continuous collection with a DR-FTIR interface has been accomplished recently using supercritical fluid chromatography (SFC) (16). In these experiments the deposition of the sample onto a strip of KBr powder was facilitated by the easy elimination of the mobile phase (CO,), which is a gas a t atmospheric pressure. However, according to the authors, the same principle appears not to be applicable to conventional HPLC using liquid phases. A solvent deposition device introduced by Jinno et al. (17, 18) utilizes transmission spectroscopy. In this system, the effluent from a microbore HPLC flows directly onto a moving, rectangular KBr crystal. After evaporation of the solvent, the 0 1986 American Chemlcal Society
