1169
Anal. Chem. 1984, 56, 1169-1 177
Comparison of Attenuated Total Reflectance and Photoacoustic Sampling for Surface Analysis of Polymer Mixtures by Fourier Transform Infrared Spectroscopy Joseph A. Gardella, Jr.,* and George
L. Grobe I11
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 Wendy L. Hopson and Edward M. Eyring*
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Two methods are compared for Improvement of the surface sensltlvlty In FT-IR analysis of polymer surfaces wlth commerclaliy avallabie biocompatlbie polymer mixtures. Attenuated total reflectance (ATR) sampling yields results dlrectiy related to absorbance and can sample over a range of depths by varying the angle of incidence. Focuslng and sample preparatlon requirements llmit the best results to smooth surfaces. Photoacoustlc (PA) methods are insensitive to surface scattering and roughness contributions from dtfferent sample morphologies, but PA suffers from low signal Intensltles and saturation which can contrlbute to photometrlc Inaccuracies. Sampilng depths in both methods are wavelength dependent, introducing problems when utlllrlng ratios of peaks to internal standards for quantitatlon of surface concentratlons. Results from the anaiysls by both methods of Blomer and Avcothane, two commerclaiiy available blocompatlble polymer mixtures, and their homopolymer constltuents allow the detectlon of surface lmpurltles and segregatlon of speclflc polymer components in a near surface reglon. The degree of surface Segregation can be estlmated by uslng the dtfferent means of varylng the sampling depth of each method.
The analysis of polymers by spectroscopic means has had great impact on the development of specific structure-property relationships in materials (1). These relationships are useful in explaining such phenomena as adhesion, degradation (corrosion), lubrication, reactivity, and more recent developments such as biocompatibility, conductivity, and adsorption capability (1,2). Much of the microstructure of importance for these material characteristics can be explained in terms of specific surface composition or structure, because chemical interactions in these systems occur at a surface or interface. Thus, more recently, much focus on the spectroscopy of polymer surface has led to the application of particle ejection-based spectroscopies such as X-ray photoelectron spectroscopy (XPS or ESCA) and secondary ion mass spectrometry (SIMS) (3-5), which are sensitive to a near surface region of 10-100 A. Optical spectroscopic methods such as IR, UVvis, and Raman methods are routinely used with special sampling methods meant to limit response to a near surface region in order to deduce the composition at the surface (6-10); however, these techniques sample much deeper than ESCA or SIMS, on the order of 1-100 pm in the best cases (9,lO). Analyses by combinations of both kinds of methods are especially important where segregation of a specific component of a complex polymer mixture to the surface or interface occurs. Where this segregated component may be responsible for particular surface properties, the degree of segregation may be deduced, or conclusions drawn from each method alone may be confirmed in a multitechnique approach (5,11,12). 0003-2700/84/0358-1189$01.50/0
Reflectance methods have been the most popular means of gaining surface sensitivity for optical spectroscopic techniques (6,8-10,12-17), and in particular, internal and external specular reflection (6, 7, lo), and diffuse reflectance (6, 17) methods have been widely used with FT-IR for surface analysis. External techniques at grazing angles of incidence are particularly useful for thin film samples a few monolayers thick, adsorbed or cast on a reflecting medium, while diffuse reflectance has been developed by Griffiths (17) for use with granular, high surface area samples. Internal reflection techniques have been used since Harrick (9) developed the theory and methodology appropriate for multiple internal reflections of attenuated total reflectance (ATR), and ATR has been coupled routinely with FT-IR (7,10,12) for surface analysis. The sampling depth (or penetration depth) for an ATR experiment has been given by Harrick (9) as described in eq 1,where X1 = l / n l (cm), 8 is the angle of incidence with depth of penetration =
A1
&&in2
8 - n212)1/z
(1)
respect to the surface normal, and nZ1is the ratio of the refractive indexes of the sample (nz)and the internal reflection element (IRE) (nl). Several consequences result from this analysis, one being that penetration is greater at longer wavelengths, for the mid-IR region (2.5-25 pm) this means a factor of 10 difference in sampling depth across the spectrum, affecting its photometry (9) as compared to transmission. A second result is that as 8 gets larger the sampling depth is smaller, to the limit of grazing incidence, the sampling depth approaches 0.11. Finally, for a fixed 8, penetration is greater as n21approaches unity, i.e., where the indexes of refraction are matched. Photoacoustic (PA) spectroscopy (18)has been suggested as a technique for examining the surfaces of solid materials (18,19). The PA effect is based on the detection of acoustic waves generated by radiationless relaxations of an absorption process initiated by a modulated light source. Most of the first PA experiments were in the UV-vis region of the spectrum, because of the dependence of PA intensity on the source intensity (18). More recently, the increased signal intensity available in the FT-IR experiment has been coupled with PA detection to provide a method which is proving to be extremely versatile (19-33) for various types of solid samples which are extremely difficult to sample otherwise. The suggestion of surface sensitivity in the PA experiment is a consequence of Rosencwaig and Gersho's (RG) theoretical treatment of the so-called gas microphone experiment (18)in which the modulation frequency affects the sampling depth (or thermal diffusion length) ( p ) as a function of the thermal and optical properties of a sample. This relationship is given in eq 2, p
= (2K/pcw)'I2
0 1984 American Chemical Society
(2)
1170
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
where K is the thermal conductivity (cal/(cm.s.OC)), P is the density (g/cm3), c is the specific heat (cal/(g°C)), and w is the modulation frequency (rad/& Fernelius (34) and Bennett and Forman (35) have introduced revised theories to take surface absorption into effect, and thus the simple treatment shown by RG theory is not exact but can serve as a useful guide in understanding the effects of various parameters in attempting to use PA as a surface technique. Vidrine (19), in his comprehensive review of FT-IR/PAS, notes the possibility of variation in the modulation frequency as a means to enhance the surface sensitivity of the gas microphone experiment. This can be accomplished by utilizing the inherent modulation of the interferometer output in a rapid scanning FT-IR, described by Griffiths (34) and given in eq 3, where
w = 4TVii
(3)
Vis the mirror velocity (cm/s) and 3 is the wavenumber (cm-l). The result of these considerations is that across the mid-IR spectrum, a fador of 10 difference in the modulation frequency is inherent to the rapid scanning experiment, which introduces a difference in p across the spectrum, in a manner similar to the ATR experiment. Additionally, since the signal intensity is also a function of w (18),there are differences in signal levels across the spectrum, resulting in a lack of true photometric accuracy. These considerations can be overcome by the use of a ratioing method introduced by Teng and Royce ( 2 3 , which produced a photometrically accurate PA spectrum for poly(methy1 methacrylate), or by the use of a step and integrate interferometer, where modulation must be introduced independently (37,38), although this approach has not been utilized for mid-IR work. A final consideration of the use of PA for surface analysis is an effect reported by Vidrine (19) and others (39) which indicates that in a gas microphone experiment, a higher surface/volume ratio increases the signal intensity. Thus powdered samples and rough surface morphologies are actually more favorable for PA sampling, whereas smooth morphologies are needed for ATR (for good contact between crystal and sample), because inefficient scattering of the excitation source does not greatly affect the resulting PA spectrum. The initial studies by Vidrine (20,21), Rockley (22), and Krishnan (23)have all addressed the application of FT-IR/PA to polymer surfaces and have suggested that important differences exist in spectral features because of different contributions to photometric inaccuracies between PA and ATR. Vidrine’s work (20,21) showed that PA techniques are very strong in analyzing variable morphologies of polymers but Krishnan (23) suggests that a close comparison of specific bands analyzed by both methods showed some differences, explained by incipient saturation in the PA spectrum. Krishnan (25) has also utilized PA methods to analyze the differences in the spectra of oriented polymers, yielding dichroic ratios which show similar trends to ATR but are lower, in general, than the ratios from ATR measurements. In the course of our work in the development of PA spectroscopy (26,28-33) and in surface analysis of polymers (5, 11, 12), investigations of the specific differences and capabilities of the two methods seemed appropriate for their optimum use, alone and in conjunction with other methods. In the present work the analysis of a complex biocompatible polymer mixture is chosen as illustrative of the capabilities for polymer surface analysis. Much previous work has already been accomplished in the characterization of commercially available biocompatible Biomer and Avcothane surfaces, because of the importance of describing the surface composition if it is different than the bulk. In a complex polymer mixture or copolymer, it is quite common for the lower surface energy component to be surface segregated. Analysis of Biomer and Avcothane by FT-IR/ATR (40), ESCA and other particle
ejection methods (41-451, and other biocompatible segmented polyurethanes (SPU’s) (46) have routinely shown that the lower surface energy polyether (PEO) or poly(dimethy1siloxane) (DMS), which are components or impurities in Biomer and Avcothane (43, 44), are segregated in a near surface region. As part of efforts to characterize the extent of segregation by FT-IR/ATR and ESCA (47),a comparison of PA and ATR with these well-known mixtures was undertaken. In this work we will compare results from each technique, contrasting also to transmission spectra of the mixtures and the pure components (DMS) and a polyurethane (Avomat) similar to that in Avcothane, in order to comment on both degree of surface sensitivity and experimental aspects which affect photometric accuracy. Assuming literature values for indexes of refraction (9,48) and thermal conductivity (18) rough calculations show that both techniques, used under the available experimental conditions, should yield sampling depths in the range of 2-15 pm depending on the wavelength. There are limitations in both theories however, recent work (49) showing that ATR may only yield absorption information from a portion of the calculated sampling depth and assumptions of RG theory may not hold up under a variety of conditions for surface analysis (34,s Thus, ).the experimental results presented will show how well the theoretical assumptions hold up for each method in the practical cases presented.
EXPERIMENTAL SECTION Biomer was obtained from Ethicon, Inc. (New Jersey), nonpigmented, as a 30% solution in dimethylacetamide. Samples of Avcothane and Avcomat (15% solutions in 2/1 THF/dioxane) were generously donated from the laboratories of D. Gregonis and J. Andrade of the Department of Bioengineering of the University of Utah. Poly(dimethylsi1oxane) (DMS) was purchased from Scientific Polymer Products (Ontario, NY) and dissolved in chloroform (approximately2%). Samples were solution cast either in an inert atmosphere desiccator connected to a vacuum line or in an inert atmosphere glovebox, over PzOs. These thick film samples were cast onto aluminum coupons or onto glass substrates, and the air facing side was analyzed. FT-IR spectra were collected with a Nicolet 7199A spectrometer. In-house modifications for gas microphone PA detection have been described previously (29). The calibration of the mirror velocities used in this study (V = 4 , 7 , and 13 corresponding to 0.124,0.186, and 0.229 cm/s, respectively) was done by sampling the output of the interferogram of the HeNe laser and using eq 3 for the laser frequency of 15800 cm-l. ATR spectra were collected with a Barnes Analytical Model 320 attachment, which is capable of collecting at 30°, 45O, and 60° angle of incidence (with respect to surface normal). The samples were pressed against a KRS-5 crystal, which was face cut at 45O for analysis, and run in each of the three sampling positions, yielding different angles of incidence within a few degrees of 4 5 O (9). Typically 100 spectra were averaged for the ATR experiments, while the number of spectra for PA experiments varied with the mirror velocity. All spectra were taken at 2 cm-’ resolution. PA spectra were ratioed vs. the output of the deuterated triglycine sulfate detector,as described previously (25);ATR spectra were referenced to the bare KRSd crystal, at the same angle. All spectral manipulation was done with standard Nicolet software.
RESULTS AND DISCUSSION This section will be organized by sequential discussion of the photometric accuracy of each technique by the comparison of the spectra of the pure components to the transmission results, followed by a presentation of the analytical results from each method for the polymer mixtures via comparison to the transmission results. Transmission spectra of all four polymers are shown in Figure 1,with a list of band assignments given in Table I. The relative intensities are given as both absorbance and absorbance relative to the NH stretch vibration. This dual method will become useful where changes in photometric accuracy must be contrasted with changes due
ANALYTICAL CHEMISTRY, VOL. 56,NO. 7, JUNE 1984
Table I. Results of Transmission FT-IRAnalysis Avcomat
bandassignment NH stretch CH aromatic stretch CH,-CH stretch (as) CH,-CH stretch (s) CH,-CH stretch (as) CH,-CH stretch (s) amide I free CO amide I H bonded CO NH bend amide I1 CH bend (SiCH,) CH bend (SiCH,) CN stretch C-0-C stretch C-0-C stretch Si-0-Si stretch Si-0-Si stretch Si-C Si-C a
band position, cm"
abs
re1 abs
3330 3123 3040 3010 2941 a 2856 2797 1732 1704 1598 1534
0.197
1,000
0.611
1223 1111 1082
Avcothane
band position, cm-l
Biomer
abs
re1 abs
0.379
1.000
3.012
3327 3123 3040 3010 2941
1.151
0.798 0.203 0.588 0.757 0.363 1.019
4.049 1.031 2.985 3.846 1.843 5.173
2856 2797 1732 1704 1598 1534
1.681 0.347 1.119 1.582 0.653 3.705
0.898 1.046 0.697
4.566 5.310 3.538
1223 1111 1082
2.047 3.601 1.357
band position, cm-'
DMS
band re1 position, abs cm-'
abs 0.751
1.000
3.049
3325 3123 3040 3010 2945
2.266
3.020
4.444 0.916 2.950 4.174 1.724 9.776
2863 2797 1732 1710 1598 1537
3.029 1.082 1.287 0.936 1.106 1.549
4.032 1.440 1.751 1.246 1.472 2.063
5.405 9.500 3.580
1222 1120 1111
1.699 3.544 3.791
1171
abs
re1 abs
2964 2906
0.575 0.315
1.000 0.548
1413 1261
0.255 0.155
0.391 1.486
1092 1019 865 800
0.892 0.255 1.103 0.394
1.381 1.550 1.919 0.685
2.262 4.719 5.048
Shoulder.
to different concentrations of functional groups, which produce changes due to Beer/Lambert considerations. Differences in absorbances due to different film thicknesses between samples will also be eliminated. It is helpful to first consider the bulk composition of Biomer and Avcothane before interpretations of the transmission FT-IR results are presented. Biomer is a polyether (poly(tetramethylene oxide)) and polyurethane mixture (toluene 2,4-diisocynate and ethylenediamine chain extender), while Avcothane is a mixture of DMS and a polyether (poly(propylene glycol))-polyurethane (toluene 2,4-diisocyanate and 1,4-butanediol chain extender) structure (45). Comparison of the band positions from the transmission (TXM) results in Table I indicate that Biomer, Avcothane, and Avcomat are all similar to the polyurethanepolyether type structure which yields characteristic bands at 3325 cm-l (NH stretch), 2941, 2856, and 2797 cm-' (CH stretch), 1732 and 1704 cm-' (free and H bonded carbonyl), 1598 cm-' (ring breathing and/or amide carbonyl), and 1534 cm-' (amide 111,besides the various fingerprint peaks in the CC and CN stretch and CH bend regions (1500-1200 cm-') and COC stretches at 1220-1000 cm-l. Examination of the absorbances and relative absorbances can provide some quantitative insight into the polymer structure. For the most part, Avcomat and Avcothane appear identical in composition. Both peak positions and relative absorbances are equivalent within error limits. Results from Biomer suggest some specific differences, in comparison to the Avcomat and Avcothane which are consistent with the suggested structures of the three polymers (45). A lower concentration of amide type functionality is evident from relative absorbances of bands between 1750 and 1500 cm-l, along with differences in the band positions and absorbances due to C-0 stretches (1150-1000 cm-l), given the different type and amount of soft polyether component and the different chain extender in the two polymer mixtures. An interesting result is the lack of any obvious evidence of DMS component in the Avcothane, although the presence of a small amount of DMS may be masked by other urethane bands. A first step in the analysis of the ATR spectra of the polymer mixtures involves the description of the differences in band position (wavenumbers) and relative absorbance for the pure materials, DMS and Avcomat. Table I1 presents
Wavenumbers
Figure 1. Transmission FT-IR spectra of (A) Avcomat, (B) DMS, (C) Avcothane, and (D) Biomer. Conditions are described in the text.
these data as a function of increasing angle of incidence (i.e., decreasing sampling depth). Qualitatively, for both samples, some shifts in band position are seen, usually within f 5 cm-l. In general, relative to the intensity of the higher wavenumber band (NH in the Avcomat and CHa asymmetric stretch in DMS) both samples show differences in relative absorbances, which may indicate, at first glance, that changes in the concentrations of specific functional groups are evident in the
1172
ANALYTICAL CHEMISTRY, VOL. 56,NO. 7, JUNE 1984
Table 11. Comparison of ATR Data for Avcomat and DMS TXM Avcomat band assignment NH stretch CH aromatic stretch CH,-CH stretch (as) CH,-CH stretch (s) CH,-CH stretch (as) CH,-CH stretch (s) amide I free CO amide I H bonded CO NH bend amide I1 CN stretch C-042 stretch C--0-C stretch
band position, cm-' abs 3330 31 23 3040 3010 2941
ATR-30" abs
band position, cm-'
abs
0.197
1.000
3334
0.100
0.611
3.012
2939 2908 2857 2796 1729 1700 1595 1526 1218 1101 1078
0.178 0.137 0.227 0.131 0.288 0.363 0.220 0.330 0.274 0.268 0.263
re1
a
2856 2797 1732 1704 1598 1534 1223 1111 1082
0.798 0.203 0.588 0.757 0.363 1.019 0.898 1.046 0.697
4.049 1.031 2.985 3.846 1.843 5.173 4.566 5.310 3.538
TXM DMS band assignment
band position, cm-'
CH,-CH stretch (as) CH,-CH stretch (s) CH bend (SiCH,) CH bend (SiCH,) Si-0-Si stretch Si-0-Si stretch Si-C Si-C
2964 2906 1413 1261 1092 1019 865 800
a
ATR-45' re1 abs
band position, cm''
abs
re1 abs
1.000 3319
0.049
1.000
3326
0.107
1.000
1.779 1.370 2.270 1.310 2.880 3.630 3.200 3.300 2.740 2.680 2.630
0.098
2.000
0.116 0.060 0.063 0.087 0.049 0.100 0.101 0.126 0.129 ATR-4 5'
2.367 1.225 1.286 1.776 1.000 2.041 2.061 2.571 2.632
2943 2909 2864 2798 1729 1702 1597 1530 1220 1104 1080
0.152 0.122 0.186 0.077 0.114 0.148 0.070 0.175 0.173 0.190 0.178
0.421 1.140 1.738 0.719 1.065 1.383 0.654 1.636 1.617 1.776 1.664
2941 a
2853 2794 1729 1700 1596 1526 1219 1103 1077
ATR-30"
abs
re1 abs
band position, cm-I
abs
0.575 0.315 0.255 0.855 0.892 0.255 1.103 0.394
1.000 0.547 0.391 1.486 1.381 1.550 1.919 0.685
2962 2906 1412 1262 1092 1018 859 792
0.731 0.252 0.310 0.549 0.313 0.235 0.074 0.146
ATR-60" band position, cm-' abs
re1 abs
band position, cm-'
1.000 2963 0.345 2906 0.424 1412 0.751 1262 0.428 1096 0.322 1015 0.101 863 791 0.200
re1 abs
ATR-60"
abs
abs
band position, cm-'
0.868 0.158 0.263 0.901 0.633 0.539 0.303 0.453
1.000 0.182 0.303 1.038 0.729 0.625 0.349 0.522
2963 2906 1413 1259 1087 1009 862 799
re1
abs
re1 abs
0.576 0.169 0.128 0.737 0.628 0.664 0.370 0.638
1.000 0.293 0.222 1.280 1.090 1.153 0.642 1.108
Shoulder.
near surface region. After examination of the individual spectra for Avcomat (Figure 2), the differences in relative absorbance can mostly be attributed to differences in background signal in each of the spectra. Linear background subtraction methods were applied at each of the angles to try to overcome the variable background, which most likely results from inefficient coupling of the evanescent wave as it leaves the IRE due to inperfect sample/IRE contact (9). Figure 2 illustrates the variable background, especially in the region around 3000 cm-l. Even linear background subtraction over a very short range does not completely flatten the base line. This results in inconsistent variations in relative absorbances, even when attempts at background subtraction are made, because the shape of the background is not known exactly. This is especially apparent in the analysis of DMS at the different angles. Coupled with the differences in absorbance expected from the ideal reflection process (9), this can account for variations in spectral response even in homogeneous samples, in both band position and relative absorbance. Thus, it is important that spectra for model compounds and materials be analyzed at specific angles and conditions in ATR, before comparisons can be made. Included in this analysis are assumptions regarding equivalence of index of refraction between sample and model compound. With this background, examination of the ATR results for Avcothane and Biomer can be done. Tables I11 and IV and Figure 3 give the data and serve as an important illustration of one of the prime capabilities of the variable angle ATR experiment. In the case of the analysis of Avcothane, Figure 3 shows the increases in DMS character as the angle of incidence increases and, thus, the sampling depth decreases. This can be seen in Table I11 where the CH stretches characteristic of DMS, at higher wavenumber grow in relative intensity to the NH stretch and the CH bend at 1258 cm-' becomes very apparent, dominating the spectrum. Addi-
d
Wavenumbers Figure 2. FT-IR/ATR analysis of Avcomat with crystal in position at (A) 30°, (B) 45', and (C) 60'. For conditions see text.
tionally, there is a shift in frequency in the C-0-C stretch region to the frequencies and band shape characteristic of the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Table 111. Analysis of Avcothane by ATR TXM band position, re1 cm-' abs abs band assignment 0.379
1.000
3326
0.041
1.157
3.049
2962 2943 2904 2858 2794 1728 1698
0.174 0.077 0.074 0.118 0.086 0.152 0.232
4.244 1.878 1.805 2.878 2.098 3.707 5.659
2962 2922 2909 2857 2800 1730 1701
0.352 0.143 0.152 0.181 0.078 0.153 0.220
5.254 2.134 2.269 2.702 1.164 2.284 3.284
2962
0.254
5.292
2906 2858 2799 1730 1702
0.097 0.102 0.057 0.084 0.111
2.021 2.125 1.188 1.750 2.313
1594 1527 1411 1251 1216
0.131 0.236 0.185 0.433 0.204
3.195 5.756 4.512 10.561 4.976
1597 1529 1413 1257 1220
0.115 0.266 0.205 0.564 0.262
1.716 3.970 3.060 8.418 3.910
1596 1531 1413 1258 1221
0.058 0.134 0.102 0.587 0.135
1.208 2.791 2.125 12.229 2.813
1076 1076 1013 862 791
0.409 0.409 0.446 0.051 0.356
9.976 9.976 10.878 1.244 8.683
1074 1074 1012 863 7 96
0.544 0.544 0.565 0.233 0.551
8.119 8.119 8.433 3.478 8.224
1080 1080 1011 86 3 793
0.564 0.564 0.617 0.179 0.624
11.750 11.750 12.854 3.729 13.000
CH,CH stretch (as) CH,CH stretch (s) CH,CH stretch (as) CH,CH stretch (s) amide I free C=O amide I H bonded
2856 2797 1732 1704
1.681 0.347 1.119 1.582
4.444 0.916 2.950 4.174
1598 1534
0.653 3.705
1.724 9.776
1223 2.047 1111 3.601 1082 1.357
5.405 9.500 3.580
c=o
NH bend amide 11 CH bend (SiCH,) CH bend (SiCH,) CN stretch C-0-C stretch C-0-C stretch Si-0-Si stretch Si-0-Si stretch Si-C Si-C a
ATR-60"
ATR-45"
band position, cm-' abs 1.000 3316 0.067
3327 3123 3040 3010 2941
NH stretch CH aromatic stretch
ATR-30" band position, cm-' abs
1173
a
re1 abs
band re1 position, abs cm-I abs 1.000 3334 0.048
re1 abs 1.000
Shoulder.
Table IV. Analysis of Biomer by ATR
band assignment NH stretch CH aromatic stretch CH,-CHstretch(as) CH,-CH stretch (s) CH,-CHstretch(as) CH,-CHstretch (5) amide I free CO amideIH bondedCO NH bend amide I1 CN stretch C-0-C stretch C-0-C stretch
a
TXM band position, cm-I abs 3325 3123 3040 3010 2945
re1 abs
0.751
1.000
2.266
3.020
3.029 1.082 1.297 0.936 1.106 1.549 1.699 3.544 3.791
4.032 1.440 1.751 1.246 1.472 2.063 2.262 4.719 5.048
ATR-30" band position, cm-' abs
re1 abs
ATR-45" band position, cm-l abs
re1 abs
ATR-60" band position, cm-' abs
re1 abs
0.150
1.000
3322
0.134
1.000
3322
0.111
1.000
3013 2928
0.310
2.120
2934
0.455
3.396
2940
0.341
3.072
2852 2795 1730 1711 1597 1536 1220 1107 1097 1080
0.333 0.224 0.189 0.138 0.086 0.120 0.100 0.115 0.120 0.11
2.220 1.493 1.260 0.920 0.573 0.080 0.667 0.767 0.800 0.740
2853 2797 1731 1710 1597 1536 1220 1104
0.546 0.228 0.339 0.226 0.251 0.368 0.379 0.456
4.075 1.702 2.530 1.687 1.873 2.746 2.828 3.403
2855 2797 1731 1710 1598 1536 1221 1106
0.441 0.159 0.242 0.513 0.190 0.299 0.390 0.584
3.973 1.432 2.180 1.378 1.712 2.694 3.514 5.261
1017
0.300
2.239
1018
0.264
2.378
3319 3124
a
2863 2797 1732 1710 1598 1527 1222 1120 1111
Shoulder,
Si-0-Si stretches at 1080 and 1012 cm-l. This analysis is particularly straightforward and even though there is some overlap of the bands, the carbonyl bands characteristic of the urethane structure are in a region clear of any DMS absorbance, and the CO stretches are far enough away from the S i 0 stretches to show easily identifiable shifts. Thus the three components can be easily identified from specific characteristic bands. The segregation of the DMS component, reported by several previous ESCA studies (42,43,47)and the earlier ATR study (401, is confirmed by our work; in addition, we have run an effective depth profile by varying the angle of incidence and the results show a gradual increase in the relative concentration of the DMS. This result is important in determining the degree of segregation of the softer component in these polymer mixtures, because the segregation is to a surface region of considerable thickness on the scale of the sampling
depth of the ATR experiment. This sampling depth is much greater than those of the particle ejection methods and indicates that a simple model of a thin surface layer of DMS may be a reasonable approximation, but further refinement of the model is necessary to get a complete picture of the morphology of a polymer mixture. This certainly is a justification of the use of several methods of different sampling depths, in order to have a complete picture. However, the ATR results show distinctively the segregation of the DMS; as with the crystal in the 60° position, the spectrum is dominated by the DMS bands, with a small, but clear contribution from the polyurethane bands. The results from the ATR analysis in Table IV of Biomer are not as spectacular but show a shift in the frequency and band shape of the CO stretches, indicative of the increased concentration of the softer poly(tetramethy1ene oxide) at the surface. Further evidence is the enhancement of the inten-
1174
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Table V. Comparison of PAS Data for Avcomat and DMS TXM
Avcomat band assignment NH stretch CH aromatic stretch CH,-CH stretch (as) CH,-CH stretch (s) CH,-CH stretch (as) CH,-CH stretch (s) amide I free C=O amide I H bonded
c=o
NH bend amide I1 CN stretch C-0-C stretch C-0-C stretch
band position, cm-' abs 3330 3123 3040 3010 2941
0.197
1.000
3337
0.699
1.000
0.611
3.012
2949
2.119
3.032
2943
0.767
2943
0.400
2856 2797 1732 1704
0.798 0.203 0.588 0.757
4.049 1.031 2.985 2.846
2858 2796 1733 1703
2.106 0.929 4.266 4.517
3.013 1.329 6.103 6.462
2863
0.809
2874
0.435
1735 1704
3.715 3.680
1733 1701
2.454 2.798
1598 1534 1223 1111 1082
0.363 1.019 0.898 1.046 0.697
1.843 5.173 4.566 5.310 3.538
1597 1541 1224 1116 1076
5.605 5.997 5.997 5.761 4.791
8.019 8.579 8.579 8.242 6.854
1598 1540 1224 1119 1088
2.637 5.096 6.003 6.039 5.089
1597 1541 1223
2.217 4.014 3.026
1085
4.832
TXM DMS band assignment
CH,-CHstretch (as)
PAS ( V = 4) PAS ( V = 7) PAS ( V = 13) band band band re1 position, re1 position, re1 position, re1 abs cm-' intens intens cm-' intens intens cm-' intens intens
band position, cm-l abs 2964
0.575
PAS ( V = 4) PAS ( V = 13) PAS ( V = 7) band band band re1 position, re1 position, re1 position, re1 abs cm-' intens intens cm-' intens intens cm-' intens intens 1.000
CH,-CHstretch (s) CH bend (Si-CH,)
2906 1413
0.315 0.255
0.548 0.391
CH bend (Si-CH,) Si-0-Si stretch Si-0-Si stretch Si-C stretch Si-C stretch
1261 1092 1019 865 800
0.155 0.892 0.255 1.103 0.394
1.486 1.381 1.550 1.919 0.695
2965 2936 2879 1472 1413 1266 1100 1034 857 827 797
2.744 1.892 1.291 0.562 0.597 1.452 1.391 1.397 1.000 1.370 1.233
1.000 0.690 0.471 0.205 0.218 0.530 6.507 0.509 0.364 0.499 0.449
2966 2934 2879 1468 1410 1266 1107 1033 864 819 796
3.370 2.551 1.879 2.970 2.246 6.404 4.526 4.338 2.136 3.305 2.878
1.000 0.756 0.558 0.881 0.667 1.900 1.343 1.287 0.634 0.981 0.854
2968 2936 2879 1471 1406 1266 1108 1039 865 821 795
1.865 1.608 1.098 2.741 1.729 5.632 3.931 3.979 1.878 2.719 2.511
1.000 0.862 0.589 1.470 0.927 3.020 2.108 2.134 0.958 1.458 1.346
M C Wavenumbers
Figure 3. FT-IR/ATR analysis of Avcothane with crystal in position at (A) 30°,(B) 4 5 O , and (C) 60'. For conditions see text.
sities due to the CH2stretch at 2856 cm-' relative to the NH stretch or with respect to the CHa stretch band. These results present a picture of the depth profile of the segregated component but do not identify any impurities in these samples.
3400
3000
2600
2200
4800
4400
4000 800
Wavenumbers Figure 4. FT-IWPA analysis of DMS at mirror velocity: (A) 0.124, (B) 0.186, (C) 0.229 cm s-'. For conditions see text.
Analysis of the PAS data for the same four samples is presented in Tables V-VII. The spectra shown in Figures
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Table VI. Analysis of Avcothane by PAS TXM
band assignment NH stretch CH aromatic stretch CH,-CH stretch (as) CH,-CH stretch (s) CH,-CH stretch (as) CH,-CH stretch (s) amide I free C=O amide I H bonded
c=o
NH bend amide I1 CH bend (Si-CH,) CH bend (Si-CH,) CN stretch C-0-C stretch C-0-C stretch Si-0-Si stretch Si-0-Si stretch Si-C stretch Si-C stretch Si-C stretch
band position, cm-'
abs
re1 abs
3327 3924 3046 3040 2941
0.379
1.000
1.157
3.049
2856 2797 1732 1704
1.681 0.347 0.588 0.757
4.444 0.916 2.985 2.846
1598 1534
0.363 1.019
1.843 5.173
1223
2.047 3.601 1.357
5.405 9.500 3.580
1111 1082
PAS ( V = 4) band position, cm" intens
1175
PAS ( V = 7) PAS ( V = 13) band band re1 position, re1 position, re1 intens cm-' intens intens cm-' intens intens
2965 2905
3.850 1.263
1.000 0.328
2966 2906
1.230 0.435
1.000 0.353
2964
0.515
1.000
1732 1704
1.210 1.200
0.314 0.312
1732 1704
0.920 0.920
0.748 0.748
1732 1704
1596 1534 1445 1414 1267 1220 1115
1.409 1.394 1.923 3.530 10.214 1.868 6.652
0.365 0.362 0.499 0.916 2.653 0.485 1.728
1596 1534 1444 1413 1266 1220 1105
1.151 1.058 1.382 2.471 8.010 1.262 5.070
0.935 0.860 1.124 2.009 6.512 1.027 4.122
1596 1529 1445 1413 1265
0.700 0.769 1.108 1.847 6.965
1.359 1.493 2.515 3.586 13.524
1115
4.808
9.335
1115 1030 863 825 795
6.652 5.796 3.097 4.203 3.445
1.728 1.505 0.804 1.092 0.894
1115 1043 860 820 795
4.803 4.707 2.197 3.620 2.668
3.905 3.827 1.786 2.943 2.169
1115 1039 864 816 805
4.808 4.357 1.988 3.153 2.992
9.335 8.4 60 3.860 6.122 5.810
Table VII. Analysis of Biomer by PAS
band assignment NH stretch CH aromatic stretch
CH,-CHstretch (as) CH,-CH stretch (s) CH,-CHstretch (as) CH,-CHstretch(s) amide I freeC=O amideIHbonded
c=o
NH bend amide I1 CN stretch C-0-C stretch C-0-C stretch
TXM band position, cm-' abs
re1 abs
PAS ( V = 4) PAS ( V = 7) PAS ( V = 13) band band band position, re1 position, re1 position, re1 intens intens cm-' intens intens cm-' intens intens cm-'
3325 3123 3040 3010 2945
0.751
1,000
3324
0.985
1.000
3325
0.390
1.000
3313
0.344
1.000
2.266
3.020
2941
3.921
3.981
2947
1.169
2.997
2935
0.468
1.361
2863 2797 1732 1710
3.029 1.082 1.287 0.936
4.032 1.440 1.751 1.246
2865 2794 1732 1710
4.046 1.840 4.171 2.906
4.108 1.868 4.235 2.950
2862 2797 1731 1709
1.246 0.739 3.848 2.289
3.195 1.895 9.867 5.869
2856 2800 1732 1710
0.589 0.350 2.435 1.752
1.712 1.017 7.078 5.093
1598 1537 1222 1120
1.106 1.549 1.699 3.544 3.791
1.472 7.063 2.262 4.719 5.048
1598 1536 1224 1119
3.814 5.057 5.508 6.120
3.872 5.134 5.592 6.213
1596 1536 1221 1124 1075
3.245 4.385 4.428 4.968 3.261
8.320 11.244 11.354 12.738 8.362
1594 1535 1224
2.564 3.665 4.098 4.511 2.896
7.453 10.654 11.913 13.113 8.419
1111
4 and 5 represent selected results from the data given in the tables. The PAS data for the homopolymers show some very interesting results. First of all, strict comparison to the transmission spectra shows several important results, which involve interpretation of the differences in the PA vs. absorption mechanisms. The first obvious difference noted in Figure 4 involves several bands in the DMS spectra which are identified in Table V at specific frequencies which are not detected or are of much greater intensity relative to neighboring bands than in either the transmission or ATR experiments. Specifially these are bands at 2936 and 1472 cm-'. This result could be the effeds of an unknown and undetected surface impurity or be due to the differences in fundamental processes for PA and absorbance. The seemingly enhanced intensity of the band a t 1472 cm-' could be explained by saturation of specific bands (25) in the PA spectrum. Two explanations can be considered for the new band at 2936 cm-l. Under exactly the same resolution conditions, either narrower bands in the CH3-CH stretch region are observed, which
1118
1072
resolves into a new band identifiable at 2936 cm-', or the band is due to an impurity. Since only a very weak band exists in the transmission IR spectrum of DMS at 1472 cm-', assigned to a CH bending vibration, the possibility of saturation enhancement can be considered, but this is unlikely given the large intensity change. No evidence in any published spectra of DMS could be found to account for the 2936-cm-' band. However, possible assignments are absorptions due to a hydrocarbon or alkoxysilane CH3-CH stretch for the 2936-cm-' band and CH bend for the 1472-cm-' band. These could be from low molecular weight alkoxysilane or hydrocarbon impurities in the DMS which segregate to the surface upon solution casting. The presence of an alkoxysilane would lead to a strong band in the region of 1100 to 1080 cm-l, and the shift of position and of relative intensities of the two bands in the SiOSi region also supports this explanation. Even without a firm assignment for this impurity, none of the ATR experiments could detect this, which suggests that the PA experiment may have a shallower sampling depth under these
1176
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
k
v
used for the PA experiment was a normal setting, and not extremely fast, so that these moderate conditions for both experiments, which may be considered “typical”, and the comparisons of sampling depths for the two techniques, while not rigorous, give a good indication that PA under typical conditions can be advantageously used for surface analysis. The second important result comes from the PA data in Table VI1 and shows that the presence of other impurities in the Biomer, detected by ESCA (441, primarily a low molecular weight silicone, cannot be seen even with the PA experiment. Thus, the sampling depths of both techniques cannot approach the particle ejection surface spectroscopies but do provide a means to getting a profile of a surface region without destruction of the sample. The addition of PA methods to ESCA, SIMS, etc. seems worthwhile even at the cost of the time it takes to analyze very smooth surfaces like these polymers. The PA experiment does provide some important information that was not gained from the ATR experiment and, for air-sensitive or highly scattering surfaces, may be a faster and easier experiment.
CONCLUSIONS
Wavenumbers Flgure 5. FT-IRIPA analysis of (A) Avcothane and (B) Biomer at mirror velocity of 0.124 cm s-’. For conditions see text.
conditions. Since previous work has shown similar impurities in Biomer (43),more work with ESCA analysis to confirm the identity of the impurity is being done (47). A second phenomenon, related to the difference in the fundamental details of the two experiments, is the loss in intensity of the high-frequency portion of the spectrum as higher and higher mirror velocities are examined. This is the result of the above-mentioned variation in w across the spectrum for the rapid scanning mirror in the FT-IR, and the signal intensity dependence on w which is inherent in the PA experiment (18) causes a loss in relative intensity at the high-frequency end of the spectrum as one tries to probe shallower depths with higher mirror velocities. Application of Teng and Royce’s (27) procedure for the correction of this analysis is only valid for the homopolymer and, thus, not useful for the polymer mixtures; so for calibration purposes, the spectra are uncorrected. This phenomena is especially problematical for the low surface area polymer films which yield low signal to noise, making the PA experiment very time consuming. It also limits the use of the mirror velocities to the relatively low values, with respect to what is possible with the Nicolet instrument. The development of a correction method which can be generalized to inhomogeneous surface layers in a solid would involve complete knowledge of the degtee of segregation or the thickness of the layers in the sample and would be extremely involved but could be useful for further use of the technique, since analyses below show that the PA method does have useful sensitivity to a near surface region. Analysis of Avcothane and Biomer by the PA method yields two important results. The data in Figure 5 and Table VI show that the spectrum of Avcothane is almost identical with that of DMS, with only a small contribution of the polyurethane-polyether spectrum identifiable in the open range around 1700-1600 cm-l. This again is evidence that under these analysis conditions, PA techniques sample a shallower depth than a typical ATR experiment. Since our ATR attachment was not continuously variable, very high angles of incidence could not be attempted to test the limits of sampling depth for ATR, nor were crystals with higher indexes of refraction (i.e., germanium) (9) utilized, which could have changed the sampling depth. However, the mirror velocity
The results from this study show that both ATR and PA methods for surface analysis of polymers by FT-IR have distinctive advantages and disadvantages which make conjunctive use a viable means to increase structural information. For the polymer mixtures studied, ATR can provide an infrared spectrum representative of surface regions of different depths, giving a nondestructive depth profile over a fairly large sample depth, in comparison to particle ejection techniques. For smooth surfaces, ATR is fast but suffers from variable background arising from imperfect sample/IRE contact, which can affect apparent photometric accuracy. Thus, for a valid comparison, it is important to utilize analysis of model compounds under specific angle and sample conditions. PA methods are slow, having low signal intensity for smooth, low surface area solids, but under specific conditions can have a shallower sampling depth than ATR. PA methods suffer from the variable signal level across the spectrum, which creates photometric inaccuracies, but PAS was more sensitive to surface impurities and segregation in this analysis. For higher surface area samples, or air-sensitive samples, PAS can provide a simple means of obtaining a vibrational spectrum, without the surface disruption which could occur when pressing a sample against an IRE for ATR analysis.
ACKNOWLEDGMENT The authors thank W. P. McKenna for many stimulating discussions, acknowledge the assistance of L. B. Lloyd in developing the PA analysis, and acknowledge the generous donation of samples, information and support, and the loan of the ATR attachment from D. Gregonis and J. D. Andrade of the Bioengineering Department at the University of Utah.
LITERATURE CITED (1) Magill, J. H. I n “Treatise on Materials Science and Technology, Volume 10, Properties of Solid Polymeric Materials”; Schuitz, J. M., Ed., Academic Press: New York, 1977; Part 10A, pp 3-368. (2) Koenig, J. L. “Chemical Mlcrostructure of Polymer Chains”; Wiley-Interscience: New York, 1980. (3) Dwlght, D. W., Fabish, T. J., Thomas, H. R., Eds. “Photon Electron and Ion Probes of Polymer Structures and Properties”; American Chemical Society: Washington, DC, 1981. (4) Clark, D. T. “Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy”: Brlggs, D., Ed., Heyden: London, 1972; pp 21 1-247. (5) Gardella, J. A,, Jr.; Hercules, D. M. Anal. Chem. lQ81, 53, 1879. (6) Wendlandt, W. W.; Hecht, H. 0.“Reflectance Spectroscopy”; WlleyInterscience: New York, 1966; pp 1-90, (7) Jakobsen, R. J. I n “Fourier Transform Infrared Spectroscopy”; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New York, 1979, Vol. 2, Chapter 5, pp 165-191. (8) Tompkins, H. G. I n “Methods of Surface Analysls”; Czanderna, A. W., Ed., Elsevier: New York, 1975; Chapter I O , pp 447-472. (9) Harrlck, N. J. Internal Reflectance Spectroscopy”; Wlley-Interscience: New York, 1967.
Anal. Chem. 1984, 56, 1177-1182 (10) Krishnan, K.; Ferraro, J. R. I n "Fourier Transform Infrared Spectroscopy"; Ferraro, J. R., Baslle, L. J., Eds.; Academic Press: New York, 1982; Vol. 3, Chapter 5, pp 149-209. (11) Hercules, D. M. Anal. Chem. 1978, 5 0 , 739A. (12) Gardeila, J. A,, Jr.; Chen, J. S.;Magiii, J. H.; Hercules, D. M. J. Am. Chem. SOC. 1983, 105, 4536. (13) Iwamoto, R.; Miya, M.; Ohta, K.; Mima, S . J . Am. Chem. SOC.1980, 102, 1212. (14) Iwamoto, R.; Miya, M.; Ohta, K.; Mima, S . Appl. Spectrosc. 1981, 35, 584. (15) Knoll, W.; Phiipon, M. R.; Golden, W. G. J. Chem. Phys. 1982, 77, 219. (16) Vanwagonen, R. A,; Rockhold, S.;Andrade, J. D. "Biomaterlai Interfacial Phenomena and Applications"; American Chemical Society: Washington, DC, 1982; Advances in Chemistry Series No. 199, p 351. (17) Fuiier, M. P.; GriffJths,P. R. Anal. Chem. 1978, 5 0 , 1906. (18) Rosencwalg, A. Photoacoustics and Photoacoustic Spectroscopy"; Wiiey-Interscience: New York, 1980. (19) Vidrine, D. W. I n "Fourier Transform Infrared Spectroscopy: Techniques in Fourier Transform Interferometry"; Ferraro, J. R., Basiie, L. J., Eds.; Academic Press: New York, 1982; Vol. 3, Chapter 4. (20) VMrine, D. W.; Lowry, S.D. "Polymer Characterization: Spectroscopic Chromatographic and Physical Instrumental Methods"; American Chemical Society: Washington, DC, 1983; Advances in Chemistry Series No. 203, p 595. (21) Vidrine, D. W. Appl. Spectrosc. 1980, 34, 314. (22) Rockley, M. G. Appl. Spectrosc. 1980, 3 4 , 405. (23) Krishnan, K. Appl. Spectrosc. 1981, 3 5 , 549. (24) Laufer, G.; Huneke, J. J.; Royce, B. S. H.; Teng, Y. C. Appl. Phys. Len. 1980, 37,517. (25) Krishnan, K.; Hili, S.;Hobbs, J. P.; Sung, C. S. P. Appl. Spectrosc. 1982, 36, 257. (26) Riseman, S.M.; Yaniger, S. I.; Eyring, E. M.; MacInnes, D.; MacDiaramid, A. G.; Heeger, A. J. Appl. Spectrosc. 1981, 3 5 , 557. (27) Teng, Y. C.; Royce, B. S. H. Appl. Opt. 1982, 2 1 , 77. (28) Riseman, S.M.; Eyring, E. M. Spectrosc. Lett. 1981, 14, 163. (29) Lloyd, L. B.; Yeates, R. C.; Eyring, E. M. Anal. Chem. 1982, 5 4 , 549. (30) Riseman, S. M.; Massoth, F. E.; Dhar, G. M.; Eyring, E. M. J. Phys. Chem. 1982, 5 4 , 1760. (31) Gardelia, J. A., Jr.; Eyring, E. M.; Klein, J. C.; Carvaiho, M. B. Appl. Spectrosc. 1982. 38, 570. (32) Gardella, J. A., Jr.; Jiang, D.-2.; Eyring, E. M. Appl. Spectrosc. 1983, 37, 131. (33) Gardella. J. A., Jr.; Jiang, D.-2.; McKenna. W. P.; Eyring, E. M. Appl. Surf. Sci. 1983, 15, 36-49. (34) Fernelius, N. C. Appl. Opt. 1982, 2 1 , 481. (35) Bennett, H. S.;Forman, R. A. J. Appl. Phys. 1978, 4 9 , 2313.
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(36) Griffiths, P. R. "Transform Techniques in Chemistry"; Griffiths, P. R., Ed. Plenum: New York, 1978; Chapter 5, pp 121-122. (37) Lloyd, L. B.; Riseman, S.M.; Burnham, R. K.; Farrow, M. M.; Eyring, E. M. Rev. Sci. Instrum. 1980, 57(11), 1488. (38) Lloyd, L. B.; Burnham, R. K.; Chandler, W. L.; Eyring, E. M.; Farrow, M. M. Anal. Chem. 1980, 5 2 , 1595. (39) Freeman, J. J.; Friedman, R. M.; Reichard, H. S. J. Phys. Chem. 1980, 84, 315. (40) Sung, C. S.P.; Hu, C. B.; Merriii, E. W. J. Biomed. Mater. Res. 1978, 72,791. (41) Sung, C. S. P.; Hu, C. B. J. Biomed. Mater. Res. 1979, 73,45. (42) Sung, C. S. P.; Hu, C. B. J. Biomed. Mater. Res. 1979, 73, 161. (43) Graham, S. W.; Hercules, D. M. J. Biomed. Mater. Res. 1981, 75, 349. (44) Graham, S. W.; Hercules, D. M. J. Biomed. Mater. Res. 1981, 75, 465. (45) Gardeiia, J. A,, Jr.; Graham, S.W.; Hercules, D. M. "Structural Analysis of Polymeric Materlals by LAMMA"; Polymer Characterization; Carver, C. D., Ed.; American Chemical Society: Washington, DC, 1983; Advances in Chemistry Series No. 203, p 635. (46) Knutson, K.; Lyman, D. J. I n "Biomedical and Dental Applications of Polymers"; Gebelein, E., Koblitz, F. G., Eds.; Plenum: New York, 1981; p 173. (47) Gardeila, J. A., Jr.; Hopson, W. L.; McKenna, W. P.; Eyring, E. M., to be submitted for publication in Macromolecules. (48) Brandup, J., Immergut, E. H., Eds. "Polymer Handbook"; Wiiey-Interscience: New York, 1975. (49) Fornalik, M. S.;Baier, R. E.; Meyer, A. E., Calspan Advanced Technology Center, Buffalo, NY, personal communication, Jan 1984.
RECEIVED for review November 3,1983. Accepted February 21,1984. This work was partially supported by a grant (Public Health Service No. RR 07092) from the Biomedical Research Support Grant Committee of the University of Utah for the work performed at Utah. W.L.H. acknowledges support from the Department of Chemistry a t the University of Utah in the form of an Undergraduate Research Participation grant for the summer of 1982. J.A.G. and G.L.G. acknowledge support by a grant (No. 150 H019 K) from the Biomedical Research Support Grant program of SUNY/Buffalo for the work performed at Buffalo.
Organic Acid Eluents for Single-Column Ion Chromatography James S. Fritz,* Dean L. DuVal, and Robert E. Barron Ames Laboratory, U.S.D.O.E. and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Various organic acids are tested for use as eluents in singlecolumn anion chromatography. Adsorption and dissociation characteristics of the acids are related to eluent strength and seiectlvity variations. Sensitivities and background conductances of these eluents are also compared to other common eluents used in single- and dual-column anion chromatography. Conclusions are drawn concerning the selection and advantages of acid eluents.
into a slightly dissociated weak acid with a low conductivity. The signal is generated by differences in the specific conductivities between the analyte and eluent anions as well as differences in the dissociation constants between the two acids that are formed. The only major drawback of the system is the broadening of analyte peaks by the second column. More recently, a hollow fiber suppressor column has been used. There is still appreciable peak broadening in the hollow fiber, although this is reduced by packing the fiber with small beads (2).
Ion chromatography was first introduced in 1975 ( 1 ) as a dual column system which permitted the separation and determination of various ions in a short period of time. The initial system utilized a low-capacity, anion-exchange column followed by a high-capacity, hydrogen-form, cation-exchange suppressor column. The separator column separates the sample anions by adsorption and ion-exchange phenomena and then the suppressor column converts the anion into completely dissociated mineral acids with a high conductivity. Meanwhile, the suppressor column is continuously converting the eluent anion
In 1979 ( 3 , 4 )a single-column system was introduced which utilized a carefully chosen ion exchanger and eluent salt to eliminate the need for a suppressor column before the conductivity detector. A low-capacity resin is used which allows the use of such a low eluent concentration that the background conductivity remains low. The eluent anion is chosen so that it has a low equivalent conductance and a high affinity for the resin. In single-column ion chromatography the signal for an eluted anion arises from the higher equivalent conductance of that anion compared to the eluent anion. While this difference provides a detection sensitivity that is quite adequate
0003-2700/84/0356-1177$01.50/00 1984 American Chemical Society
