-
lnstrumentation Peter R. G r W i i Stephen L. Penloney, Jr. Aklo Giorgelli Kemeth H. Shafer' Depanment 01 Cnemistry University 01 California R versiae. Calit 92521
The impetus for combining chromato-
M yPHENATIOW
graphs with spectrometers arm bec a w the identification capability of chromatography alone is generally insufficientfor the identificationand quantification of every component of a complex mixture. The limitations of chromatography arise largely from the anbiguity of retention data. Typically, identiiication by chromatographic means done-Guires some prior knowledge of the probable chemical structure of the components to be identified and the availability of authentic reference standards. These standards must therefore be predesignated for a particular analysis. Mixtures such as extracts of environmental samples often contain a multitude of components with a wide range of chemical diversity, the origin of which is generally unknown. The components may be present at levels from major to ultratrace, with a rapidly increasing number of compounds being observed as the detection limit is reduced. In many cases the identity of the major components is known, but the identity of minor chromatographic peaks (several of which may never have been observed previously in chromatograms of samples with a similar origin) can be of critical interest. The need for potentially thousands of standards for a conventional cbromatographic analysis of samples of this type, together with the limited availability of many of these standards, makes the time (and cost) per analysis prohibitive. A more certain identification of each peak can be achieved through the use of sensitive spectrometers to obtain a unique signature of each eluate. Ideally the spectrum should be measured in real time (i.e., in a time that
F
SPEmOMETRY ----------
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1986 American Chemical Society
is less than the width of the peak) as each component elutes, so that each measurement can be performed without trapping the sample. Mass spectrometry (MS) is the most commonly applied technique for characterizing chromatographic peaks. MS certainly merits its preeminent position in view of its high sensitivity and fast scan speed and the uniqueness of the mass spectra of many compounds. Despite its strengths, MS has certain drawbacks. For example, structural isomers often are not differentiated by MS, and library searches can sometimes lead to erroneous identifications. Under these circumstances, a complementary technique is desirable, and Fourier transform infrared (FT-IR) spectrometry has emerged as the most generally useful alternative-not only for gas chromatography (GC) but also for high-performance liquid chromatography (HPLC) and, more recently, supercritical fluid chromatography (SFC). When applied to a complex sample without a separation step, neither MS nor FT-IR spectrometry will lead to the identification or quantification of even one component. A t best FT-IR can provide a general description of the chemical class of the major components, and mass spectra of such samples are so complex that they cannot
give information. even thisOnly much when these spectrometers axe linked to a chromatograph, so that each component is separated from the others, can the full power of MS or FT-IR for mixture analysis be realized. This linkage of two complementary analytical instruments was dubbed hyphenation in a farsighted
g;;g;gY
combininr the separation power of achromatograph and the identification power of a spectrometer, it often becomes possible to identify many of the components of a complex mixture. After the various components have been identified, it becomes a relatively easy (albeit somewhat time-consuming) matter to determine the concentration of each through the use of calibration standards. The hyphenated techniques share an important characteristic in that identification is accomplished with unique spectroscopic information rather than retention time data. The purpose of the spectrometry is predominantly identification, even though quantification is often also possible, for example from peak heights or areas measured from total ion current or Gram-Schmidt reconstructed chromatograms. The hyphenated techniques consequently do not have the limitations stated pFeviously for chromatography or spectrometry applied alone, and complex mixtures often can be analyzed in detail with little or no prior chemical information
* Current address: Analect Instruments, Division of Laser Precision, 17819 Gillette Ave., Imine, Calif. 92714
ANALYTiCAL CHEMISTRY, VOL. 58, NO. 13. NOVEMBER 1986
134SA
about the sample. In this paper, the ways in which gas, liquid, and supercritical fluid chromatographs have been interfaced to FT-IR spectrometers will be described. The extent to which the performance of the chromatograph can be matched to that of the spectrometer through the interface determines the analytical capabilities of any hyphenated technique. Spectroscopic sensitivity and information content, chromatographic resolution and capacity, and the time of analysis are all interdependent. In some interfaces, chromatographic selectivity has been compromised to improve spectral sensitivity. For example, for several HPLC/ FT-IR measurements, a particular mobile phase was selected based on its spectral transmission rather than its chromatographic performance. In successful interfaces, the performance of neither the chromatograph nor the spectrometer should be compromised if their combination is to become of general utility. Thus, many of the more recent developments in HPLC/ FT-IR have centered on eliminating the mobile phase prior to the FT-IR measurement so that the interface can be applied to separations performed with any mobile phase. Until a few years ago, GC/FT-IR systems were only available commer-
cially as accessories to top-of-the-line high-resolution FT-IR spectrometers. It is now becoming generally accepted that high resolution and high sensitivity are largely independent. Indeed many low-cost FT-IR spectrometers can have equally high (and sometimes higher) signal-to-noiseratio (S/N) compared with high-resolution instruments operating at well under their maximum resolution. It is also noteworthy that several contemporary bench-top GC/FT-IR systems fit in an area smaller than the space once required for the spectrometer alone. User convenience has also improved, in part because of the implementation of compact and inexpensive microcomputers instead of the larger, yet no more powerful, minicomputers that were installed in many FT-IR spectrometers manufactured before about 1981. The software for the hyphenated techniques that is now available from most manufacturers allows data acquisition and manipulation to be performed either in a totally automated or user-interactive mode. Simultaneous display of the spectral and chromatographic data has become quite commonplace because of improvements in both computer software and hardware. Spectral searching, especially for GC/FT-IR data, has become very fast
I1 I / / I
and can give less ambiguous identifications than GC/MS data ( 2 , 3 ) .However, it should be recognized that unequivocal identification depends on the availability of good reference data. Three libraries of vapor-phase reference spectra are available for GC/FTIR. The first is the US.Environmental Protection Agency’s (EPA) data base, containing 3300 entries. This is a widely used library, because it is in the public domain, but it has a high proportion of errors. The second, which is sold by Sadtler Research Laboratories (Philadelphia, Pa.), contains more than 10,000spectra, and the third (the Aldrich vapor-phase library) contains the spectra of 5100 compounds. Several other data bases contain digital reference spectra of compounds in the condensed phase, but there is far less uniformity in condensed-phase spectral libraries than in their vaporphase analogues. For some collections, samples have been prepared as KBr disks; for others mineral oil mulls were used. Some libraries contain only hard-copy spectra, whereas other spectra can be purchased in digital form. Some digitized data bases contain high-quality spectra measured on FT-IR spectrometers, but others (in particular the large data base of digital condensed-phase spectra marketed by Sadtler Research) contain grating
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spectra that were subsequently digitized from the hard copy. Computerized identification of compounds from their IR spectra has been performed in several ways. The most common method involves comparison of the absorbance spectrum of the unknown with a library of reference spectra after scaling the absorbance of the most intense peak in the sample and reference spectra to the same value. This approach has been very successful for gas-phase spectra, but the level of success for condensed-phase spectra is reduced by the presence of sloping base lines. Condensed-phase spectra have been converted to their first derivatives to reduce the effect of base line slope before starting the search. Another way of improving search results is to compare the Fourier transform (FT) of the spectrum of each unknown with the FT of all spectra in the data base. Both the early points and late points in these arrays may be truncated to eliminate the effect of base line slope and high-frequency noise, respectively. Although the efficacy of this approach has been shown ( 4 ) ,it has still to be implemented on any commercial GC/FT-IR system. The sensitivity and data-processing capabilities of contemporary FT-IR spectrometers are at a very advanced
level. Scan speeds are sufficiently high that several interferograms can be averaged per second, allowing real-time monitoring of peaks eluting from all types of chromatographs. In the remaining sections of this paper, the present state of the art of GC/FT-IR, HPLC/FT-IR, and SFC/FT-IR interfaces is described. We hope that this treatment shows the common features of these interfaces and provides an indication of the directions that the development of these devices is taking.
Capillary GC/FT-IR The most important component of most GC/FT-IR interfaces is an internally gold-coated glass tube, or light pipe, which serves as a gas cell through which the effluent from a capillary GC column flows. ,Spectra are measured continuously a t intervals of about 1s, typically by signalaveraging blocks of 3-10 interferograms per spectral data file. The dimensions of,most light pipes designed for capillary GC/FT-IR are fairly similar as the cell volume should be approximately equal to the volume of carrier gas contained between the half-height points of the GC peaks to obtain spectra with the optimum S/N ( 5 ) .The actual peak volume depends on the internal diameter (i.d.) of the GC column and the thickness of the
stationary phase, but it is usually between about 50 and 200 pL. A suitable compromise between length and i.d. of the light pipe must be made if maximum performance is to be attained. A consensus among most instrument designers now appears to have been reached, and most light pipes for capillary GC/FT-IR are constructed from glass tubes with an i.d. of 1 mm and a length between 10 and 20 cm. With GC/FT-IR interfaces based on light pipes, identifiable spectra can usually be measured from injected quantities of strongly absorbing samples in the 5-25-ng range (6-8). For weakly absorbing compounds, however, it may be necessary to inject as much as 100 ng to obtain an identifiable spectrum. Although it is possible that the minimum identifiable quantity (MIQ) of strongly absorbing samples being characterized using light-pipe-based GC/FT-IR interfaces could be reduced to about 400 pg in a completely optimized system (9),it is quite unlikely that any further decrease can be achieved without changing the fundamental nature of the GC/FT-IR interface. One way in which the MIQ has been reduced to subnanogram levels has involved trapping each separated component in some fashion. One of the better ways of trapping GC eluates is
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Flgure 1. Gas chromatogram of l-fiL injection of a herbicide still bottoms exbact Spectra of peaks a IhroYgh d are shewn in Fiwe 2
to add a small quantity of argon to the column effluent and to pass the resulting mixture onto the surface of a metal substrate that has been cooled to about 12 K. Each solute is trapped in an argon matrix as the helium is pumped off. By slowly moving the surface on which the matrix is being formed, a record of the chromatogram is made. After deposition, the substrate is moved into the beam of an FT-IR spectrometer for the spectral measurement of each GC peak (lO-J3). A commercial GC/FT-IR interface based on this principle, known as the Cryoleet, has resulted in compounds with average absorptivities being identified for quantities in the range of 100-200 pg. For strong IR absorbers, even lower MIQs (10-50 pg) can he achieved. One of the keys to the high sensitivity of the matrix isolation approach is the very small area over which the sample is deposited. The effective thickness of a given amount of a GC . component (or any other sample) is inversely proportional to its cross-sectional area. With the Cryolect, each solute can be deposited in a spot whose diameter can he as small as 0.25 mm. Thus, all other factors being equal, the intensity of the bands due to a given quantity of a matrix-isolated GC solute trapped as a 0.25-mm-diameter spot should he 16 times greater than that of hands due to the same amount of the solute held in a 1-mm i.d. light pipe. The low temperature of the matrix also results in reduced widths and increased absorbances of the spectral bands, which leads to a further de-
crease in MIQ and an increase in specificity, relative to the light pipe interface. Because the point a t which the solutes are deposited is separated by several centimeters from the focus of the IR beam, spectral data may not be acquired in real time. Although several hours may be required for a single GC/FT-IR run made with the matrix isolation interface, each component is retained on the surface of the substrate for an indefinite period of time. The S / N of the spectra of minor GC peaks can therefore he increased by extensive signal averaging. Three factors (low matrix temperature, small-area deposition, and trapping to allow signal averaging) therefore contribute to the high sensitivity of the matrix isolation GC/FT-IR interface. Of these, we will see that the last two are also of importance in several other interfaces to be described. The suhnanogram detection limits of the matrix isolation approach are not attained cheaply, and the suhstantially higher cost of the Cryolect compared with a light pipe interface has undouhtedly limited its use. In an attempt to reap many of the benefits of matrix isolation without the cost, we have studied the feasibility of trapping GC peaks a t temperatures that are only a few degrees below ambient. In our approach the eolumn effluent is passed onto a moving thermoelectrically cooled ZnSe window, which is held a t a temperature between -10 and -45 "C. Each solute can he deposited as a spot of approximately 150-pm diameter (14). The window is then moved into the focus of an F l IR microscope, so that the beam diam-
1354 A * ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
eter can he matched to the size of the spot. Spectra are then measured a short time after deposition. MIQs for spectra measured with this interface are only slightly greater than those reported for the matrix isolation interface (15),whereas the cost is comparable to that of the light pipe interface. At this point, the biggest disadvantage of the microscope-based interface is its inability to trap suhnanogram quantities of volatile compounds hecause the lowest temperature to which the window has been cooled is -45 "C. To circumvent this problem, the interface in our laboratory is being modified for operation a t 77 K. The quality of the data that can be obtained with this interface can be illustrated by spectra obtained from the chromatogram of the chloroform extract of the still bottoms from a herbicide manufacturing plant. The sample was injected onto a 60-m-long, 0.25-mm-i.d. wall-coated open-tubular fused-silica column with a 0.25-pmthick layer of DB-5, an intermediate polarity stationary phase. The chromatogram is shown in Figure 1,and the spectra of the four peaks indicated are shown in Figure 2. Although all these peaks have not yet been unequivocally identified, it is apparent that good spectra can be obtained from q a n y of the separated components. Because of the complementary nature of IR and mass spectral data, more GC peaks can be identified when GC/FT-IR and GC/MS are used together than when either technique is applied separately (16-20). The development of capillary GC/FT-IR inter-
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fluent through a flow cell and continuously measuring spectra (21-24). In this respect, HPLC/FT-IR interfaces with this design can be considered to be analoeous to lieht nine GC/FT-IR ~~. interfaces. The key difference between GC/FT-lR and HPLC/FT-IR is that. whereas the mobile phase in GC is transparent to IR radiation, all HPLC mobile phases absorb IR radiation at certain wavelengths and some, such as water, have intense absorption across most of the spectrum. Absorption of the incident radiation by the solvent leads to several problems for the HPLC/Fl-IR interface. First, solvent absorption hands must always be compensated if identifiable solute bands are to be observed. This effect completely obscures solute bands in several spectral regions and reduces the S/N a t any wavelength where ahsorption by the solvent is appreciable. Spectral subtraction of the solvent bands is almost impossible when the composition of the mobile phase is changing (e.g., during gradient elution). Finally, the maximum path length of the cell is determined by the ahsorption spectrum of the solvent. I t is rare that the cell path length exceeds 200 pm without much of the useful IR spectrum being obscured by the solvent absorption hands. Because the diameter of the flow cell should not exceed 4 mm, the largest cell volume is about 2.5 pL, which is only about 1% of the width of a typical peak eluting from a 4.6-mn-i.d. column. For any interface between a chromatograph and an FT-IR spectrometer, the greatest sensitivity is found when the cell volume equals the full width at half height of the peak (5). Thus the limitation on path length imposed by the mobile phase places a significant limitation on the sensitivity of flow cell HPLC/FT-IR interfaces. For reversed-phase (RP) HPLC, where the mobile phase always contains water as a major constituent, the largest path length is about 25 pm. In this case,the cell volume is reduced to 0.3 pL, and the sensitivity of the measurement is degraded proportionately. Thus, few useful RP-HPLC/FT-IR measurements have been reported using a flow cell interface. i One way of making the volume of flow cells and the width of HPLC peaks more compatible is to reduce the width of the peaks through the use of microbore columns. Taylor’s group has reported some impressive HPLCI FT-IR spectra obtained using such columns (25-28).The use of microbore columns does not get around the limitations imposed by the mohile phase completely, however, and most ofthe data reported by Taylor’s group have necessitated the use of CDC4 as I
) 1 1 1 ( ( 1 ( ( 1 ( 1 Do0 1500 loo0
Wavenumber I
Flgure 2. IR spectra of four components ofthe still bottoms extract shown as peaks a d in Figure 1; note the com plete resolution of peaks b and c
faces that has taken place over the past decade has permitted detection limits to be reduced to the point a t which IR spectra can now be obtained on many components of complex mixtures for which only mass spectra were obtainable in the past. Perhaps we are approaching the type of “black box” instrument in which a sample could be injected into a gas chromatograph and the identity and quantity of each separated component would be printed out shortly afterward.
HPLCIFT-IR To a large degree, the history of GCI FT-IR development is mirrored in the development of H P L C m - I R interfaces. All early H P L C m - I R interfaces involved passing the column ef1356A
. .~
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13. NOVEMBER 1986
the mobile phase. It may be noted that the low solvent consumption of microbore columns means that expensive deuterated solvents can be used for microbore HPLC (FHPLC) without solvent costs getting completely out of hand. Thus, for samples that can be separated using chloroform or other chlorinated solvents as the mobile phase, the application of microbore columns seems like a good solution to many ofthe problems of HPLC/FTIR. Jinno et al. (29) have even reported using DzO as the mobile phase for FHPLCIFT-IR measurements. Despite the promise of these approaches, flow cells appear to impose too many limitations on the HPLC/ FT-IR interface, and a much better approach is to eliminate the solvent prior to the IR measurement. Several approaches to solvent elimination have been described. They can be separated into two categories, involving either conventional or microbore HPLC columns. The first category necessitates eliminating the mobile phase emerging from 4.6-mm4.d. columns, where the flow rate is typically on the order of 1mL/min. A two-stage processhas proved to he a useful way of eliminating organic solvents a t this flow rate. In the first stage, 80-90% of the solvent is evaporated, usually after nebulization of the column effluent. In the second stage, each solute is deposited onto a suitable substrate for measurement of its IR spectrum. During deposition, the remaining mobile phase must he completely evaporated to obtain a spectrum free of solvent absorption hands. The higher the surface area of the substrate, the more rapidly the solvent can be eliminated. The use of powdered substrates has been favored because the transfer of heat through drops on a hot flat surface can he quite slow. After deposition, the diffuse reflectance (DR) IR spectrum of each solute is measwed (30-32). Detection limits achieved using an interface based on this principle are about 2 orders of magnitude lower than can be obtained with a flow cell interface (33).Additionally all of the spectrum can be observed-not just those bands in the window regions of the solvent spectrum. This interface is not without its drawbacks, however. First, it is necessary to dispose of a considerable quantity of solvent vapor. Second, it was noted earlier that the MIQ obtainable with a GC/Fl-IR interface is strongly dependent on the area in which the sample can be deposited. This is equally true for an HPLCET-IR interface. When the solution comes into contact with a powdered substrate, it can spread rapidly by capillarity. To minimize the extent of spreading, it is usually necessary to
contain the powder in cups, and for the study of multicomponent mixtures. devices with many cups are needed. These have either been held in carousels (30-32) or linear transport devices (34,35).One of these devices held 160 cups (31).and refilling even a small number of these cups after a chromatographic run proved to be an excessively time-consuming experience. The most common powders used as substrates for solvent elimination HPLCFT-IR have been the alkali halides. Most alkali halides are very hygroscopic and can pick up water from the atmosphere while the solvent is being evaporated. Worse yet. they cannot be used at all if water is a component of the mobile phase, and a considerable amount of effort has been expended in the development of an HPLC/FT-IR interface suitable for RP-HPLC. Several approaches have been investigated for RP-HPLCPTIR interfaces, based on the principle of solvent elimination. The simplest and most obvious is direct deposition on a substrate that is insoluble in water. Several randidates have been suaaested. of which the most promiski me diamond powder (industrial grade) and sulfur (36).If alkali halides are used, water must be completely eliminated before the elu-
ate comes into contact with the substrate. One approach involves adding 2,2'-dimethoxypropane a t the end of the column, to remove water by the reaction: CH,C(OCH,),CH, i H,O = CH,COCH, i 2 CH,OH The products of the reaction, acetone and methanol, can then be readily removed in the &e fashion a~ normalphase solvents (34). In another approach, dichloromethane is added to the column effluent. The distribution ratio generally favors partition into the organic phase. Even if this is not the case, it is often possible to adjust the composition of the column effluent so that partition in the organic phase becomes favored (e.g., by addition of an acid or base). After separation and removal of the aqueous phase in the manner originally proposed by Karger e t al. (37), CHzClz can be eliminated by evaporation and the DR spectrum of the solutes measured (38). Finally, it is also poasible to separate water by means of a hydrophobic membrane (39). None of these solvent elimination approaches has proved popular enough tocatalyze the development of a commercial HPLCPT-IH interfare. Although the detection limits are
quite acceptable, the amount of time needed to fill cups is not. We believe that an alternative solvent elimination device based on the use of microbore columns will prove more acceptable in the long term, as it should prove feasible to use an easily cleaned IR window or a metal plate as the substrate. The first solvent elimination interface between a microbore liquid chromatograph and an FT-IR spectrometer was the buffer memory technique of Jinno et al. ( 4 M . Z ) .These workers deposited eluates from 0.3- and 0.5mm-i.d. packed fused-silica columns on a moving IR-transparent plate. Evaporation of the solvent was assisted by flowing nitrogen. The diameters of typical spots were apprpximately 2 mm, so that transmission spectra of the deposited solutes could be measured using a 4X beam condenser. Detection limits were about 100 ng per component. Jinno subsequently showed that it was possible to deposit solutes from RP-fiHPLC columns onto metal mesh screens and to measure their spectra in an analogous fashion (43). Bearing in mind the criteria for reducing the MIQ for GC/ FT-IR components given in the previous section, it would appear that the best way to reduce detection limits in pHPLC/FT-IR is to decrease the diameter of the deposit below 0.5 mm. (continued on D . 1360 A )
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Several methods of achieving this are being investigated in our laboratory, but none has yet allowed us to reach our goal of subnanogram detection limits. SFC/FT-IR When a substance is raised above its critical temperature and pressure, a supercritical fluid is formed. The solvent power of a supercritical fluid is a function not only of its polarity but also its density. In turn, the density is affected by changes in temperature and pressure. SFC is unique among chromatographic techniques in that temperature, pressure, and composition programming can all be accomplished. The physical properties of supercritical fluids are intermediate between those of gases and liquids: Densities are liquidlike, viscosities are gaslike, and solute diffusivities are about 10 times greater in supercritical fluids than in liquids. If SFC is performed on packed columns, these properties enable higher linear velocities to be used in comparison with the corresponding HPLC separation, without loss in separation efficiency. More important, however, these properties allow wall-coated open-tubular columns to be used for SFC. The use of these columns has led to the current surge in popularity of SFC, because of
the increase in chromatographic resolution that can be achieved in comparison with packed columns ( 4 4 ) . Like GC/FT-IR and HPLC/FT-IR, both flow cell and mobile-phase elimination approaches have been described for SFC/FT-IR. The use of flow cells for SFC/FT-IR is easier in some respects and more difficult in others than for HPLC/FT-IR. On the negative side, cells must be able to withstand pressures well in excess of 4000 psi. Few IR spectra of supercritical fluids have been reported in the past, so that the IR transparency of all mobile phases potentially applicable to SFC/FT-IR has not been well characterized. On a more positive note, by far the most popular fluid used to date for SFC has been CO2, and the transmittance of supercritical COZ is significantly greater than that of most HPLC mobile phases, allowing cells of increased path length to be used. Thus flow cell SFC/FT-IR measurements would be expected to have greater sensitivity and yield more spectral information than the corresponding HPLC/FT-IR measurements, and several reports of flow cell SFC/FTIR measurements with CO? as the mobile phase have appeared over the past four years ( 4 5 , 4 6 ) . The absorptivities of bands in the spectrum of supercritical COPhave
been observed to increase with pressure, with the intensities of some symmetry-forbidden bands being more sensitive to pressure than the intense fundamentals (where the spectrum is “blacked out” in any event). Even though the problems of solvent compensation encountered in pressureprogrammed SFC/FT-IR are not as severe as those with gradient elution HPLC/FT-IR, a single reference spectrum still cannot be used throughout the analysis to compensate for absorption by the mobile phase, so that most SFC/FT-IR measurements using flow cells have been performed isobarically. French and Novotny ( 4 7 ) have proposed substituting supercritical xenon for CO? to circumvent the problems imposed by the absorption bands of CO?. The chromatographic properties of COz and xenon are surprisingly similar, and preliminary results are promising. However, CO:! and xenon are both nonpolar and cannot be used for the separation of strongly polar analytes. In this case, polar mobile phases such as the Freons, “3, or SO? will probably have to be used. Because these compounds are strong IR absorbers, the flow cell approach for SFC/FT-IR will have the same limitations that it does for HPLC/FT-IR. Johnson et al. (48) have suggested performing separations with two different
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supercritical fluids (COz and Freon23). chosen on the basis of their IR spectra, so that IR opaque regions i q the spectrum of one fluid are windows in the spectrum of the other. The disadvantages of this approach are that not only must two chromatograms be run, but the eluent strength of the two mobile phases must be very similar. Although COz and Freon-23 have surprisingly similar chromatographic properties, this will not generally be the case, and the assignment of corresponding peaks in the two chromatograms will probably prove troublesome. Although flow cell interfaces have several disadvantages with regard to the analysis of complex samples, their use in selected applications, such as on-line process monitoring, can be beneficial (49).In this type of application, a few componnds of known structures can be monitored by selecting a mobile phase that not only affords an adequate separation but that also bas the appropriate spectral windows. For complex sample analysis, mobile-phase elimination appears to have several advantages for SFCIFT-IR. In
r I Time(min) 0 I
pressure psi
the first such interface for SFCIFTIR, the eluate was deposited on KC1 powder in a fashion similar to the earlier HPLCAT-IR interfaces, and DR spectra of the components were measured (50,51). For this work, separations were performed on a 1-mm4.d. column packed with 10-pm silica using COn with 5% CHaOH as the mobile phase. A great advantage of this approach for SFCIFT-IR results from the volatility of the mobile phase on releasing the pressure a t the restrictor. It was observed that each component could be readily deposited in a spot of approximately 1mm diameter if the substrate remained stationary. Alternatively, solutes could be deposited continuously on a moving substrate, thereby obviating the need for discrete sample cups. An investigation of the MIQ that can be expected from the approach showed that identifiable spectra could be obtained from approximately 50 ng of each component of a mixture of quinones. As a rule of thumb, the critical temperature is approximately equal to 1.5 times the boiling temperature in Kelvins. To keep the temperature of the
I
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1
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I
15
30
45
Bo
75
90
+- 1600
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m
column reasonably low, most mobile phases for SFC are gaseous a t standard temperature and pressure. Thus, on releasing the pressure at the restrictor, the mobile phase evaporates immediately. With the slow flow rates involved in capillary SFC with narrow bore columns, the rapid evaporation of the mobile phase allows solutes to be deposited as spots of about 250 pm diameter when the column effluent passes through a narrow-bore restrictor. As a result, S F C m - I R measurements have been performed under a microscope in the same manner described above for GCIFT-IR (52,53)or using a technique similar to the buffer memory method for HPLCI
FT-IR (54). To illustrate the quality of SFC/ FT-IR spectra measured in this manner, a capillary supercritical fluid chromatcgram of the same sample whose gas chromatogram was illustrated in Figure 1is shown in Figure 3.
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3400 1500
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Figure 8. Supercritical fluid chromatog ple shown in Figure 1
.fa O.O&&
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Column: 20 m long, 1 0 0 - m . d . heed ~lllka.wlth 0.40-rrm layer of oB5:oven teempwnWe: 120 OC; m u e pogam: lndkated on abrclsas. Speotraof psaka ad ssshavn In Fipvs 4
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ANALYTICAL CHEMISTRY, VOL. 58. NO. 13, NOMMBER 1986
Flgun 4. IR spectra of companents a d in Figure 3
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SFC/FT-IR spectra of four of the components are shown in Figure 4, including that of one component present as the leading edge of an unresolved doublet (Figure 4b). Most of the peaks in the supercritical fluid chromatogram do not correspond to peaks in the gas chromatogram. The early-elpting peak a characterized by SFCPTIR, however, has the identical spectrum as the late-eluting peak d in the GC/FT-IR data, so it can be deduced that most peaks in the SFC chromatogram are not attributable to the same components giving rise to peaks in the gas chromatogram.
B. Meyer, Editor
University of Washington
B.A. Kottes Andrews and R.M. Relnhardt, Editors US. Department of Agriculture Summ~rizerrcurrent information on problems relating to measuring, abating, and understandingformaldehyde emirdons from wood roducts bondad with tormaldeh& b a d adhesive resins. Ri~zubsesthe success of misston-reduction efforts, explores the problems remaining to be -, and examines lmprovements in the understandlng of mechanisms of formaldeh de generation and release. A usefurresource for textile chemists and manufacturers, wood chemists and wood product manufacturers, industrial hygienists, architects, construction engineers, consumers, public interest groups, and regulatory groups. CONTENTS
Formaldehyde Release An Overview Plywood and Wood-Based Panel Products Wood-Panel Products Bonded with Phenol Formaldehyde Adhesives Release Rate Coefficientsfrom Selected Consumer Products * Cellulose Reaction with Formaldehyde Cellulose Models for Formaldehyde Storage in Wood Urea-Formaldehyde Resins Mechanisms of Release from Bonded Wood Products Automated Flow Injection Analysis System Enzymatic Methods for Determining Formaldehyde Release * Formaldehyde Release from Building Materials Large-Scaie Test Chamber Methodology Predicting Real-Life Formaldehyde Release Tannin-Induced Formaldehyde Release Depression Effect of Diffusion Barriers on Formaldehyde Emissions European Formaldehyde Regulabonb * Occupational and Indoor Air Formaldehyde Exposure Based on a symposium sponsored by the Drvisfon of Celhhse, Pajier, end Textile Chemistry of the Amencan Chemical Society ACS Symposium Series No 316 240 pages (1986) Clothbound LC 86-14194 ISBN 0-8412-0982-0 US 8 Canada $49.95 Export $59.95 Order From: AmChemlcai Sockty DIstrlbutIon Omm Dept. 24 1155 Slxtmth Stroot, N W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and U M your credH card!
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concludon It is obvious from the preceding discussion that many significant developments have taken place recently in GC/FT-IR, HPLCPT-IR, and SFC/ FT-IR. It should be equally apparent that by no means all the problems involved in the design of an interface suitable for all types of chromatography have been solved. Practical evidence for this statement can be found in the lack of a solvent elimination interface for HPLCPT-IR and SFC/ FT-IR from any commercial source. Good designs for such interfaces are just starting to be reported, and we believe that within a year or two these devices will begin to appear in the marketplace. The recent developments in GCIFT-IR and SFCI FT-IR taking place in the authors’ laboratory were su ported in part by Cooperative Agreements C!R811730-02 and CR812258-01 between the University of California, Riverside, and the US. EPA’s Environmental Monitoring Systems Laboratory, Las Vegas, and by a rant from the University of California Toxic Sutstances Program. This paper has been reviewed in accordance with the US.EPA’s peer and administrative review policies and approved for presentation and publication. Mention of trade names does not constitute endorsement or recommendation for use. S.L.P. gratefully acknowledgessupport by an ACS Summer Analytical Fellowship, sponsored by
Calgon Corporation.
References (1) Hirschfeld, T. Anal. Chem. 1980,52, 297-312 A. (2) Shafer, K. H.; Hayes, T. L.; Brasch, J . W.; Jakobsen, R. J. A n d . Chem. 1984, 56,237-40. (3) Gurka, D. F.; Hiatt, M.; Titus, R. Anal. Chem. 1984,56,1102-10. (4) de Haseth, J. A.; Azarraga, L. V. A n d . Chem. 1981,53,2292-96. (5) Griffiths, P. R. Appl. Spectrosc. 1977, 31,284-88. (6) Azarraga, L. V.; Potter, C. A. J . High Res. Chromatogr. Chromatogr. Comm. 1981,4,60-69. ( 7 ) Kuehl, D.; Kerneny, G. J.; Griffiths, P. R. Appl. Spectrosc. 1980,34,222-24. (8)Griffiths, P. R.; de Haseth, J. A.; Azarraga, L. \’. Anal. Chem. 1983,55,136187 A. (9) Hirschfeld. T. Appl. Spectrosc. 1985, 39, 1086-87. 110) Mamantov, G.; Wehry, E. L.; Kemmerer, R. R.; Hinton, E. R. Anal. Chem. 1977,49,86-90. (111 Reedy, G. T.; Bourne, S.: Cunning-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
ham, P. T. Anal. Chem. 1979,52,153540. (12) Schneider, J. F.; Reedy, G. T.; Ettinger, D. G. J . Chromatogr. Sci. 1985, 23,49-53. (13) Reedy, G. T.; Ettinger, D. G.; Schneider, J. F. Anal. Chem. 1985,57, 1602-9. (14) Shafer, K. H.; Griffiths, P. R.; Fuoco, R. J . High Res. Chromatogr. Chromatow. Comm. 1986,9,124-26. (15) Fuoco, R.; Shafer, K. H.; Griffiths, P. R. Anal. Chem., in press. (16) Crawford, R. W.; Hirschfeld, T.; Sanford, R. H.; Wong, C. M. Anal. Chem. 1982,54,817-20. (17) Gurka, D. F.; Betowski, L. Anal. Chem. 1982,54,1819-24. (18) Wilkins, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1982,54,2260-64. (19) Laude, D. A.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984.56.1163-68. (20) Laude, D. A.’; J o h l m k , C. L.; Cooper, J. R.; Wilkins, C. L. Anal. Chem. 1985, 57,1044-49. (21) Kizer, K. L.; Mantz, A. W.; Bonar, L. C. Am. Lab. 1975.7(5). ,-,,85-97. -- (22) Vidrine, D. W.; Mattson, D.R. Appl. Spectrosc. 1978,32,502-6. (23) Vidrine, D. W. J . Chromatoar. - Sci. 1979,17,477-82. (24) Brown, R. S.;Taylor, L. T. Anal. Chem. 1983,55,723-30. (25) Brown, R. S.; Taylor, L. T. Anal. Chem. 1983,55,1492-97. (26) Johnson, C. C.; Taylor. L. T. Anal. Chem. 1983.55.436-41. (27) Amateis,’P. G.; Taylor, L. T. Anal. Chem. 1984,56,966-71. (28) Johnson, C.C.: Taylor, L. T. Anal. Chem. 1984,56,2642-47. (29) Jinno, K.; Fujimoto, C.; Vematsu, G. Am. Lab. 1984, I6(2), 39-45. (30) Kuehl, D.; Griffiths, P. R. J . Chromatogr. Sci. 1979,17,471-76. (31) Kuehl, D.; Griffiths, P. R. Anal. Chem. 1980.52.1394-99. (32) Conroy, C. M.;Griffiths, P. R.; Jinno, K. Anal. Chem. 1985,57,822-25. (33) Brown, R. H.; Knecht, J.: Witek, H. Proc. SOC. Photo-Opt. Instrum. Eng. 1981,289,51-52. (34) Kalasinsky, K. S.; Smith, J.A.S.; Kalasinsky, V. F. Anal. Chem. 1985,57, 1969-74. (35) Kalasinsky, V. F.; Smith, J.A.S.; Kalasinsky, K. S. Appl. Spectrosc. 1985,39, 552-54. (36) Brackett, J. M.; Azarraga, L. V.; Castles, M. A.; Rogers, L. B. Anal. Chem. 1984,56,2007-10. (37) Karger, B. L.; Kirby, D. P.; Vouros, P.; Foltz, R.; Hidy, B. Anal. Chem. 1984, 56,2636-42. (38) Conroy, C. M.; Griffiths, P . R.; Duff, P . J.; Azarraga, L. V. A n d . Chem. 1984, 56,2636-42. (39) Johnson, C. C.; Hellgeth, J. W.; Taylor, L. T . Anal. Chem. 1985,57,610-15. (40) Jinno, K.; Fujimoto, C. J . High Res. Chromatogr. Chromatogr. Comm. 1981, 4,532-33. (41) Jinno, K.; Fujimoto, C.; Hirata, Y. Appl. Spectrosc. 1982,36, 67-69. (42) Jinno. K.: Fuiimoto. C.: Ishii. D. J. Chromatogr. 1982,239,625-33. (43) Fujimoto, C.; Oosuka, T.; Jinno, K. Anal. Chim. Acta 1985,178, 159-67. (44) Novotny, M.; Springston, S. R.; Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1981,53,407-14 A. (45) Shafer, K. H.; Griffiths, P. R. Anal. Chem. 1983,55,1939-42. (46) Olesik, S. V.; French, S. B.; Novotny, M. Chromatographia 1984,18,489-95. (47) French, S. B.; Novotny, M. Anal. Chem. 1986,58,164-66. (continued on p. j366 A)
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(48)Johnson, C. C.; Jordan, J. W.;Taylor, L. T.; Vidrine, D. W. Chromatographin 1985.20,717-23. (49) Skelton, R. J.; Johnson, C. C.; Taylor, L. T. Chromatographin 1986,21,3-8.
(50) Shafer. K. H.; Pentoney, S. L.; Griffiths. P. R. J. High Res. Chromatogr.
Chromatogr.Comm. 1984,7,707-9. (51) Shafer, K. H.; Pentoney, S.L.; Griffiths, P. R. Anal. Chem. 1986.56.58-64.
(52) Pentoney, S. L.; Shafer, K. H.; Griffiths, P. R. J.High Res. Chromatogr. Chromatogr. Comm. 1986.9.168-71. (53) Pentoney, S. L.; Shafer, K. H.;Griffiths, P. R. J. Chromotogr. Sei. 1986.24, 230-35. (54) Fujimotn, C.; Hirata. Y. H.; Jinno, K. J. Chromatogr. 1985,332,4746.
Peter Griffiths (left)is professor of chemistry at the University of California,Riverside (UCR).His research interests include the application of FT-IR spectrometry to environmental, surface, and coal chemistry, the theory and practice of diffuse-reflectance spectrometry in the mid- and near-infrared,and the development of SFC. Stephen Pentoney (leftcenter) received his B.S. from California State University,Long Beach, and is now a fourth-year graduate student at UCR,where he is working on the development of SFC and its analytical
applicalicin~ .41 I'CR, he received a Sohw fellou~shrpin analytical ehem-
GC and SFC for the separation o/ mixtures of volatile and nonvolatile molecules.
istry in 1983 and a Shell fellowship in 1984, and this summer he was awarded an ACS Division of Analytical Chemistry fellowship.
Kenneth Shafer (right)received his B.S. and M.S.degrees from Ohio University, after which he was employed at Battelleb Columbus Laboratories for seven years. He returned to Ohio University to obtain his Ph.D. and until recently was a postdoctoral associate at UCR. His main research interest is the development of hyphenated techniques. Currently he is manager of FT-IR peripherals at Analect Instruments.
Aldo Giorgetti (right center) received his Diploma of Chemistry and Ph.D. from the University of Lausanne, Switzerland, and is a postdoctoral associate at UCR. His research interests include the study of crown ether complexes of rare earths and the development of coupled techniques such as GCIFT-IR and the combination of
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