Anal. Chem. 1980, 52, 96R-100R
Raman Spectrometry Derek J. Gardiner Newcastle upon Tyne Polytechnic, Newcastle upon Tyne, England NE1 8ST
This review covers the literature from late 1977 to late 1979. During this period around 3600 papers have appeared dealing with various aspects and applications of Raman spectroscopy. It has been necessary to be highly selective in collecting material which has particular relevance to analytical chemistry. The review therefore makes reference mainly to quantitative analytical applications with mention of qualitative work only where it is clearly analytical in nature. Also included are references to other useful reviews, new and analytically untested techniques, and novel sampling and instrument designs. Raman spectrometry is finding application in a wide variety of fields from the nondestructive analysis of drugs as solids or in aqueous solution ( 1 ) to problems in environmental science (2). The technique has found use in both food (3)and petroleum ( 4 ) industries and the presentation of Raman spectral data has come under the scrutiny of IUPAC who have made some provisional recommendations ( 5 ) . Pattern recognition techniques have been developed to interpret and distinguish characteristic frequencies of infrared and Raman spectra (6) and an a b initio method of calculating Raman intensities from force constant data has been developed ( 7 ) . Digital spectral line analysis has received some further attention ( 8 ) . Raman studies of the solid state are rarely analytical in nature. There are a few notable exceptions. A Raman method has been employed to detect As and Sb at oxidized GaAs and InAs interfaces with underlying semiconductor ( 9 ) . Measurements on suspended particulate material-aerosols ( I O ) , the detection of graphitic soot and ("&SO4 in automobile exhaust ( I I ) , and a study of the microstructure of coal (12) are further examples. In this connection, a theoretical study of Raman scattering from molecules within small dielectric particles has revealed that the morphology and optical properties of the particle need to be accounted for if quantitative estimates of molecule concentrations are to be determined (13). Unlike the last review in this series (14),a section devoted to solids is not included here but such studies as are relevant are included throughout.
INSTRUMENTATION AND SAMPLING
the sample (31). Increases in resolution and light throughput have been claimed for a spectrometer coupled to a scanning Fabrey-Perot interferometer (32);the interferometer has also been used as a prefilter with some success (33). Optical heterodyne detection in coherent Raman spectroscopy has also been used to enhance signals (34). Raman difference spectroscopy continues to attract attention but principally in the biological field (35, 36). Ramanlidar systems are being improved and allow remote quantitative analysis of atmospheric pollutants to be carried out and also provide range information by the temporal analysis of the return Raman signals. An excellent review describing the techni ues involved has appeared (37). The concentrations of Nz, HzO, and COz in a free atmosphere and CO, COz, CH4, CzH , CsH6, CHzO, HzS, NO, and NOz concentrations in automobile exhaust contaminated atmospheres have been measured (38). Further detections of lo3 ppm concentrations of SOz at 300-m distance (39)and the rotational spectrum of COz (40) have been reported. An illustration of the potential of Raman-lidar is provided by humidity sounding of the atmosphere ( 4 1 ) where a vertical resolution of 30 m has been achieved with an accuracy of 15% at a height of 1000 m. The use of such systems for the detection and characterization of oil spills at sea has been demonstrated (42) and a Raman technique for the remote sensing of subsurface water temperature is being developed (43). Rapid Raman techniques find valuable application in kinetics measurements and in the study of transient species (44). Intermediates in the photochemical cycle of bacteriorhcdopsin in the millisecond time scale have been observed using both a rotating disk system (45)and time-resolved resonance Raman spectroscopy (46). In the case of photolyzed carboxyhaemoglobin, a transient in the nanosecond range has been detected (47). Rapid Raman spectroscopy has further been used to study field-induced changes in Raman spectra (48) and the construction of a Raman temperature-jump apparatus has been described (49). Full descriptions of the laser Raman molecular microprobe (MOLE) and its applications have appeared (50) and i t has been used to analyze individual urban airborne particles (51). Also a micro-Raman spectrometer for routine analysis of microscopic particulate matter (1-5 p m ) has been developed (52).
aZ,
Computerized spectrometer systems continue to grow in usage and a review on the subject has been prepared (15). Digital control of a Raman spectrometer (16) and computer controlled high resolution Raman spectroscopy have been reported (17). Apparatus for observing and preparing samples at high and low temperatures and pressures has been further developed. A vacuum furnace operating up to 1650 K using only 200 W of power (18) and a laser beam heated mini oven for use with matrix isolation studies (19)have been described. An ultra-high vacuum chamber has been tested through a study of CO + H adsorbed on Ni surfaces (20) and a rotating vacuum cell to accommodate photosensitive samples has been designed (21). Helium cryostats (22,23) and a cell capable of maintaining samples at temperatures down to 4.2 K and pressures up to 10 kbar (24) have been reported. Experiments which link GLC and Raman spectroscopy have demonstrated the analytical possibilities of such systems (25, 26). Low sample concentrations frequently restrict the use of conventional Raman spectroscopy. Methods for overcoming this problem may lie in the use of optoacoustic spectroscopy (27) or in the use of a Cassegrain collector to increase light collection (28). Measurement of small depolarization factors in compressed gases also has been improved by the use of a Glan prism (29) and polarization data have been employed to improve resolution (30). An intracavity laser Raman spectrometer has been reported, based upon an Ar+ laser of normal output 1 W, capable of providing 160 W of power a t
It is probably true to say that the majority of Raman spectra are observed from samples either as liquids or solutions; thus the value of this section as a category of analytical work on this type of sample is questionable. Many studies of liquids and solutions appear more appropriately elsewhere in this review; those remaining are noted here. Analysis of waste and treated waters has been attempted by Raman spectroscopy and the NO3- ion has been detected down to a limit of 2 ppm in pure water samples (53). The Raman spectrum of aqueous phosphoric acid from 0.005 to 15.6 M concentrations also has been reported (54) and the analysis of species present in liquid ammonia solutions of electrolytes continues (55-57). An interesting analytical development has been the study of levels of fluorescent impurities in solvents using the relative sensitivities of the impurity fluorescence and solvent Raman wavelengths to variations in the synchronous excitation interval (58). A Raman method for the detection and estimation of dimethylnitrosamine down to concentrations of 10 mg L-' has been reported (59) and a further forensic application is illustrated by the identification of benzodiazepins in pharmaceutical preparations and in aqueous solutions (60). The possibility of measuring critical micelle concentrations is suggested by a study of ternary HzO-Na octanoate-alcohol
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1980 American Chemical Society
0003-2700/80/0352-96R$Ol.OO/O
LIQUIDS AND SOLUTIONS
R A M A N SPECTROMETRY
Derek J. Gardlner is a lecturer in the Department of Chemistry at Newcastle upon Tyne Polytechnic, England. He was born in Ipswich, Suffolk, and received his B.Sc. degree from London University in 1968. Then, as a research student at King's College, Cambridge, h e worked with James J. Turner on vibrational spectroscopic studies of the oxygen fluorides, recieving his Ph.D. degree in 1971. He then moved to York University where, as a teaching fellow, he worked with Ronald E. Hester on Raman studies of moken salt hydrates and nonaqueous solutions. Afler two years, he took up his present lectureship at Newcastle upon Tyne where he teaches physical-inorganic chemistry. His research interests include Raman studies of electrotytes in nonaqueous solutions and liquids at high pressure. He is a Fellow of The Chemical Society.
systems in which octanoate conformation was monitored as a function of concentration (61). Finally, in this section, a study of elemental sulfur in aluminum hexachloride and chloroaluminate melts (62) has shown that sulfur is present as S8 and there has been a study of aligned thin films of MBBA-liquid crystal (63).
GASES A N D MATRIX ISOLATION Measurements of species concentration and temperatures in jets (64) and flames (65,66) have been improving steadily. A description of some of the instrumentation involved has appeared (67,68),computer programs have been developed (6$71), and a useful review has been published (72). A study of water vapor in flames has been reported (73) and an analysis of Raman intensity by fast Fourier transform has shown that concentration fluctuations in unsteady flows can be measured (74). Further high resolution studies have been reported extending to 37Clz,3Hz,XeF2, SO , and Se03 (75-79). Raman studies of matrix isolated species are now accepted as fairly routine though it is doubtful if they will ever be used to the same extent as infrared. In an analytical context, such work is included because the technique frequently allows identification and characterization of novel and otherwise unstable species. A review of the application of this technique to studies of the main group metals has been prepared (80) along with a review of alkali metal atom matrix reactions (81). A new method has been devised for observing electronic and resonance Raman spectra from the same spot in a matrix (82)and the effect of pressure up to 8 kbar on NOz in Ar has been investigated (83). Low frequency bands appearing in the Raman spectra of Mg, Zn and Cd vapors in solid K r appear to indicate the presence of M2 molecular species (84, 85). Raman spectra have been obtained for NaZSO4and K2S04in Ar and N matrices (86) and FeC13, Fe2Cls (87) and zinc dihalides (88) have also been studied. The latter case has provided the first experimental evidence for the existence of mixed zinc dihalides. The species M(C0)5N2(M = Cr, Mo, or W) has been characterized (89) and HCl and DC1 monomers and dimers have been studied (90). Mercury arc photolysis of Xz + Fz mixtures (X = C1, Br, or I) results in the formation of the new species XF2, X2F, and X2Fz along with other halogen fluorides (91). Cyanogen iodide, iodine isocyanide (92), and tert-butyl alcohol (93) also have been examined.
BIOLOGICAL MOLECULES A N D POLYMERS The study of biological molecules by Raman spectroscopy continues to grow and it has been necessary to be highly selective in order to highlight the contributions to analytical chemistry in this area. General applications of the technique to biochemical science have been reviewed (94) and, although normal Raman spectroscopy has been extensively used, in many instances the resonance Raman technique has found useful application. These will be dealt with in the appropriate section. Studies of lipids in relation to membrane structure have been reported (95). Trans-gauche conformational changes as a function of temperature of gels (96) and multilayers (97-99) have been monitored and environmental chan es resulting from removal of proteins from erythrocyte memiranes have been studied (100).
The Raman spectrum from intact single muscle fibers of the giant barnacle have been reported which indicate that the myofibrillar proteins are predominantly a-helical in structure (101) and that intracellular water in the fibers is indistinguishable from pure water (102). Interactions between DNA and proteins also have been investigated (103, 104). Polymer morphology has been the subject of many Raman studies (105). Polyethylene still attracts a large amount of interest; the all-trans molecular length (106) and the distribution of straight chain segment length (107) can be determined in bulk crystallized samples. The 1170 cm-l crystalline band and the 1081 cm-I amorphous band have been used to arrive at orientation averages in highly orientated polyethylene (108) and characteristic band intensities can indicate the fraction of crystallinity in partially crystallized polyethylene (109). Raman studies on poly(viny1idene chloride) have also provided useful conformational data (110). A more directly analytical application is provided by the method devised to determine the percentage conversion of polyacrylamide to poly(N-dimethylaminomethylacrylamide)from the intensity ratio of the respective C-N stretching bands (111). The effects of stress on polymers can be examined by following gauchtrans changes (112),or in the case of polyamides (113) chan es in polarization properties of the bands can be interpretecf in terms of alterations in the amide linkage angles. A method for determining terminal mercapto groups in polythioether prepolymers (114) has been shown to be effective down to 0.5% mercapto group content with a precision of *1'70.
SURFACE ADSORPTION Zeolites continue to command significant attention in this field. Estimates of adsorbed oxygen and nitrogen on 4A molecular sieves have been made (115) along with studies of benzene (116) and cyclopropane (117)on zeolites. In the latter case, the symmetry of the adsorbate appears to be lowered from Dshto CW Surface reactions on zeolites also have been monitored (118). Polymerization of acetylene on zeolites has been shown to depend on zeolitic cation size and on surface coverage (119) and there is Raman evidence to suggest that adsorption of propylene on a zinc oxide surface results in an anionic species (120). Resonance Raman spectroscopy has been used to characterize the unstable carbonium ion intermediates formed when Ph,C:CH2 is adsorbed on Vycor glass (121) and MeOH on silica has been studied (122). As far as metal surfaces are concerned, Hz on silica supported Ni (123), CO, H2, and O2 on Ni (111)(124) and CO on evaporated Ag films (125) have all received attention. Raman spectroscopy has further been used to investigate the structural changes undergone by molybdenum catalysts after chemical treatment (126). A fairly extensive amount of work has appeared over the period of this review concerning species adsorption a t electrode surfaces. Pyridine adsorbed on a silver surface has attracted particular attention since it results in what has been called anomolous intensity enhancement of the pyridine bands. This intensity enhancement has been monitored as a function of excitation wavelength (127) and silver crystal surface type (128). Spectra of this system have also been recorded during an electrode potential shift (129). Intensity enhancement has also been observed for CN- ions a t a silver electrode (130). A potential dependent, background continuum has been reported to be present in Raman spectra from electrolyte/silver interfaces (131) which is thought to arise from radiative recombination of adsorbed ions produced by photoionization. Corrosion by NCS- ions at a silver electrode surface also has been studied (132). Work in this area has been extended to include 2-, 3-, and 4-cyanopyridine on a Ag electrode (133), p-nitrosodimethylaniline on Ag and Pt electrodes, and a dye (Rose Bengal) on a ZnO sinter electrode studied by resonance Raman spectroscopy (135). Detection of strongly adsorbed species formed during the anodic oxidation of phenylhydrazine and p-nitrophenylhydrazine has also been reported (136). The classical theory of scattering from a molecule near a solid surface (137) has been extended to accommodate adsorbed molecules in an attempt to explain intensity enhancement (138) and has been further extended to cover resonance Raman scattering (139). The possibility that intensity enhancement arises from surface plasmon mixing with molecular electronic states has been explored (140) and other calculations have shown that excited electronic states involving ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
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charge transfer from adsorbate to metallic surface are important in describing the resonance Raman scattering of adsorbed diatomics (141). Models involving electron-hole pair excitations or again charge transfer excitations have been employed to describe the particular cases of CN- or pyridine on silver (142). Intensity enhancement has also been shown t o depend upon electrode surface structure (143) and the electrode roughening procedure (144).
RESONANCE RAMAN EFFECT Although most of the resonance Raman applications are collected together in this section, because the technique is now so widely used, some references have been reviewed under other sections. The various applications of resonance Raman spectroscopy have been recently reviewed (145) along with applications to complex molecules (146),and in biochemistry and biology (147). Only work having a direct relevance to, or potential in, analytical chemistry will be mentioned here. The theoretical side of the technique is continually developing (148-151) and a useful method of calculating optimum sample concentrations in relation to cell dimensions and molar absorptivities for maximum scattered intensity has appeared (152). The intensity enhancement which occurs in the resonance Raman effect can be put to use in determining trace quantities of suitable species. Pesticides and fungicides (0.4-7 ppm), industrial dyes (100 ppb), methyl orange (7 ppb), aramanth (24 ppb), New Coccine (24 ppb), and 4-nitroaniline derivatives of phenols (20 ppb) have been detected in aqueous solutions (153-156). Quantitative analysis of carotenoids in tobacco has also been undertaken (157) and it has been demonstrated that the effect of y irradiation on poly(viny1 chloride) can also be studied by this technique (158). Resonance Raman spectroscopy also lends itself to the study of low concentrations of unstable and short lived species. I t has been shown that the 1,4-dimethoxybenzene radical cation exists in both cis and trans forms (159) and the diazabicyclooctane radical cation, prepared in a continuous flow system a t M concentrations, has also been studied (160). The resonance Raman spectrum of the SCN2- radical anion produced by pulse radiolysis of aqueous SCN- solutions has been reported (161) and time resolved resonance Raman spectroscopy has been used to studyp-terphenyl in the triplet state (162). Another example of the versatility of the resonance Raman technique is provided by a study of azo dye monolayers adsorbed a t a CC14-aqueous solution interface (163). In the biochemical field, a UV resonance Raman study of DNA has been reported (164) and a difference in polarizability has been detected between normal and sickel cell oxyhemoglobin (165).
STIMULATED AND COHERENT ANTI-STOKES RAMAN SPECTROMETRY There have been few reports involving stimulated Raman scattering, SRS, of analytical interest. Selective generation of SRS from one component in C6H6-PhN0, mixtures has been achieved (166) and SRS has now been observed using continuous wave dye lasers (167). Yet a further acronym has appeared in the literature-PARS, photoacoustic Raman spectroscopy (168). This uses acoustic methods to detect energy deposited in a molecular gas by SRS and has been demonstrated using the u1 vibration of methane. Coherent anti-Stokes Raman spectroscopy, CARS, is fast becoming an established technique and the past two years has seen a big increase in the number of publications in the literature. Some useful reviews have appeared (169-172) and the advantages of CARS over regular Raman spectroscopy have been discussed along with its applications in combustion and photochemistry (173). Where solids are concerned it has been indicated ( I 74) that refractive index, ratio of particle size to exciting and emitted wavelengths, along with scattering angle need to be specified if quantitative chemical analysis is to be undertaken. A computer program has been described for calculating CARS spectral profiles for homonuclear diatomics (175). The low frequency (50-120 cm-') CARS spectrum from air has been observed (176) and CARS spectra of water vapor in flames have been recorded (177). A computer controlled CARS system which uses pulsed dye lasers for excitation has been used to observe high resolution spectra 98R
of 0 2 , Nz, CzHz and NH, (178). Polarization properties of CARS scatter has been investigated for isotropic liquids (179) and the pressure dependence of integrated CARS power has been related to pressure broadening of Raman lines in CH4, CO, Nz, and Hz (180). Use of a crossed-triple-beam phasematching technique, BOXCARS, has been shown to result in improved spatial resolution (181). Coherent anti-Stokes resonance Raman scattering, CARRS, is yet another development of the technique and a theoretical approach to CARRS intensities has been made (182). Spectra from rhodamine 69 (183) and acridine dyes (184) have been recorded. LITERATURE CITED
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X-Ray Spectrometry G. L. Macdonald Central Materials Laboratory, Mullard Mitcham, Surrey, England
During the period under review (late 1977 to late 1979), the published literature indicates a continuing growth in the areas of application and the number of users of X-ray spectrometry. This increase is partially due to the coverage by the technique of more than 80 elements, to the multielement analysis capability for each sample, and to the extremely wide range of sample form and size that can be accommodated. These aspects have, however, been recognized for many years and it is the ease with which a Si(Li) detector can be pointed a t a sample and, via its energy-dispersive analyzer, can be made to provide almost instantaneous elemental information, that has attracted so many users to the technique. A few years ago there was a tendency for the diehard wavelength-dispersive spectrometer (WDS) users, particularly in the X-ray fluorescence (XRF) field, to dismiss the energy-dispersive spectrometer (EDS) as a useful qualitative tool but of little value for quantitative analysis. Newcomers to the field blinded by the immediacy of the information and the lower cost, blundered past the obstacles and used the built-in computer to render their information into acceptable quantitative form. In consequence, some of the obstacles were overcome and a balance is now being reached in the X R F field with EDS taking a reasonable share of the market (1). With particle excitation (usually electron or proton) the expansion has almost all been due to the Si(Li) detector and in the past two years it is the increase in use of EDS with transmission electron microscopes that has been most noticeable, although the ion beam enthusiasts have also continued to grow in number. More than half the papers published recently are concerned with direct applications and are not relevant to this review. Somewhat surprisingly, 30% of the remainder are still devoted to the conversion of X-ray intensities into elemental weights or concentrations. Instrumental developments and technique improvements account for almost 20% and the remainder is fragmented in a manner that will become apparent.
EXCITATION X-Rays and Electrons. No completely new developments have taken place in the areas of X-ray and electron excitation. Pulsed X-ray tubes continue to attract users and 3-kW power is giving way to 4 kW for the large tubes. As mentioned above, electron columns of all descriptions with applied voltages up to 1 MeV ( 2 ) are now being used for excitation. The combination of X-rays and electrons represented by cold cathode X-ray tubes, usually windowless, is having some revival of interest (3-5), probably because of the continuing inadequacy in one way or another of almost every method devised for light element analysis. The conversion of either a scanning (6, 7) or a transmission (8) electron microscope into an X-ray tube by insertion of a suitable target is also not an entirely original suggestion but the potential gain in signal-to-noise ratio is worth the reminder. Radioisotopes. On-stream analysis is probably still the most useful application for radioisotope excitation, with both characteristic and scattered radiation providing useful information (9). Otherwise radioisotopes provide the cheapest method of carrying out X-ray spectrometry, and offer reasonable flexibility for sample size and shape (10). 100 R
0003-2700/80/0352100R$01 .OO/O
A novel if not universally applicable method suggested by Ishada and Mazaki ( 1 1 ) is to mix a @-rayemitter with the s a m d e material and to measure (bv ~“ EDS) the characteristic X-rays emitted. Ions. The particle-induced X-ray emission (PIXE) school still tends to be Dartiallv isolated from the mainstream of X-ray spectrometiy and fLr that reason most of the comments about this branch of the technique have been confined to the following section. Because the average laboratory cannot purchase a cyclotron or a Van de Graaff accelerator, the isolation will continue, but it is in the interests of the analyst to be aware that a visit to his nearest accelerator center may sometimes offer, a t a reasonable cost, results which improve upon his immediately available XRF or SEM/EDS possibilities. Most PIXE workers use EDS for detection and hence the count rate limitations again come into play. Beam deflection (12, 13) offers the same solution as pulsed X-ray tubes. Mingay et al. (12) claim a factor of 10 improvement in operational count rate as well as an improvement in signal-tonoise ratio. The ease of sample handling, the reduction of charging effects and the potential increase in beam current have for some time been recognized as advantages to be achieved by bringing the ion beam out of the accelerator and into the air (or into a helium atmosphere). Shroy and his colleagues (14) use a differential pumping system to allow a windowless exit port to be used and offer an example in which the background is relatively two orders less than that obtained with an SEM/EDS system. Fou et al. ( 1 5 ) use a system with a hole down to 10-pm diameter for the beam to pass into the air. They overcome the lost information a t the light element end by using a Ge(Li) detector to analyze y-rays from the light elements. Several other methods for dealing with charge buildup on the sample have been devised, without removin it from the vacuum. The normal electron microprobe methof of evaporating a thin conducting film onto the sample is not popular because of the “contamination” and increased background produced. An electron flood-gun can be used but is also felt by Mingay and Barnard (16) to be liable to contaminate the sample and they prefer to use a transverse magnetic field of up to 0.03 T to induce the secondary electrons to strike the target before being accelerated. The effect is to reduce the bremsstrahlung end point to a low energy, outside the region of interest. An even simpler solution is that of Oona et al. (17)who insert a thin carbon foil (5-10 pg cm-;) into the ion beam about 10 cm in front of the sample. This acts as a beam diffuser but also produces electrons to neutralize the charge on the sample, without any contamination. Although protons of around 2-MeV energy are the most popular projectiles, other particles and/or energies are sometimes preferred. Moriya and his colleagues (18) give clear evidence that up to atomic number 18, 150-keV protons offer better trace element detection than 2-MeV protons. On the other hand, Al-Ghazi et al. (19) suggest that for medium atomic number elements, the cross-section maxima occur in the 20-50 MeV range. The use of argon ions up to 3 MeV is pointed out by Heitz et al. (20) to induce preferentially X-ray emission below 3.2 keV and therefore to give enhanced
0 1980 American Chemical Society
