1300 Volume 57, Number 10, 2003 APPLIED SPECTROSCOPY0003-7028 / 03 / 5710-0000$2.00 / 0q 2003 Society for Applied Spectroscopy

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High-Pressure High-TemperatureRaman Spectroscopy of Liquid andSupercritical Fluids

YURI E. GORBATY,* GALINA V.BONDARENKO, ELENIVENARDOU, EDUARDOGARCIA-VERDUGO, MAIASOKOLOVA, JIE KE, andMARTYN POLIAKOFFInstitute of Experimental Mineralogy, Russian Acade-my of Sciences, Chernogolovka, Moscow Region,142432 Russia (Y.E.G., G.V.B.); and School of Chem-istry, University of Nottingham, University Park, Not-tingham, NG7 2RD, United Kingdom (E.V., E.G.-V.,M.S., J.K., M.P.)

Index Headings: Raman spectroscopy; Supercritical � uids; Hightemperatures; High pressures.

INTRODUCTION

High-temperature high-pressure (HTHP) studies of liq-uids and supercritical � uids have been greatly stimulatedby the recent achievements in environment-friendly tech-nologies,1–3 as well as by the need to improve our fun-damental understanding of the structure and properties ofliquids and supercritical � uids.4 An advantage of Ramanspectroscopy over the complementary technique of infra-red (IR) absorption is that there are no limitations im-posed by the spectral range of transparency of the ma-terials most frequently used for optical windows (sap-phire, diamond). So far, a large number of HTHP cellshave been created for Raman spectroscopy. Here wemention only a few of these designs,5–12 some of whichprovide unique possibilities.10,12 Most cells are based onthe classic 908 geometry, where the scattered light is col-lected normal to the incident laser beam. A back-scat-tered, or 1808, geometry simpli� es the design of anHTHP cell and increases reliability because only onewindow is needed. Such a geometry is preferable formodern spectroscopic systems equipped with research-

Received 15 April 2003; accepted 30 May 2003.* Author to whom correspondence should be sent. E-mail:

hunch@issp.ac.ru.

grade microscopes. In this paper, a miniature HTHP Ra-man cell is described, which can be used either with amicroscope or directly with a Raman spectrometer.

DESCRIPTION OF THE CELL

Figure 1 shows the design of the cell, which is pro-vided with a single sapphire window, SW, 6 mm in di-ameter, with an unsupported area 3.5 mm in diameter,quite suf� cient for work with modern, high-sensitivityRaman spectrometers. All parts of the cell, except for thewindow, are made of EI437B Ni–Cr alloy. The details ofthe window assembly are shown enlarged in Fig. 1c. Theforcing screw, FS, presses the window against the � atpolished surface of the window mounting, WM; this isscrewed in place by hand, using the threads in WM, andis tightened by applying a small torque using a screw-driver. Usually, a thin GraFlex† (elastic graphite) or agold gasket is inserted between the window, SW, and thewindow mounting, WM. Because of its small dimensions,this gasket is not shown explicitly in Fig. 1. It is impor-tant that the threads in the housing, HO, forcing screw,FS, and window mounting, WM, are loose; there shouldbe some noticeable play between the parts of the assem-bly to allow the window to seat itself against the windowmounting. The assembled window is inserted into the ex-ternal cylinder of the cell, EC, and sealed with a GraFlexring, GS, compressed by the � ange, FL. The advantagesof GraFlex as a sealing material have been brie� y dis-cussed elsewhere.13,14

As can be seen in Fig. 1b, the cell has two input ports,IN1 and IN2, for 1 /16-in. capillary tubes, sealed with stan-dard ferrules. IN1 is always used for input or output ofthe substance to be studied, whereas there are two optionsfor IN2, depending on whether the experiment requiresstatic or � ow conditions. In the case of the static mode,IN2 can accommodate a 1.5-mm-o.d. thermocouple tomonitor the temperature inside the cell more accurately.However, if one wishes to work in a � ow mode, the ther-mocouple can be replaced with a 1 /16-in. capillary tube,with IN2 serving as an input or as an output. In this case,a thermocouple of 0.5-mm o.d. is inserted into the exter-nal thermocouple well, TW, via a shallow groove, SG,on the cell body, EC, so that the thermocouple does not

† We have obtained Gra� ex from the Russian supplier UniChimTek(www.unichimtek.ru), but material of apparently similar quality isavailable in the U.S. and can be obtained, for example, from MBIGRAPHITE, division of M. BRASHEM Inc. (P.O. BOX 1975, Ta-coma, WA 98401).

APPLIED SPECTROSCOPY 1301

FIG. 1. The HTHP Raman cell. (a) Front view. (b) Sectional view.(EC ) Cell body; (FL ) � ange; (GS ) GraFlex sealing; (IN1 , IN2 ) inputs;(GY ) guide yoke; (TW ) thermocouple well; and (SG ) shallow groove.(c) Enlarged view. (SW ) Sapphire window; (WM ) window mounting;(FS ) forcing screw; (HO ) housing; and (FP ) focal point.

SCHEME 1. The decarboxylation reaction of phthalic acid, to give ben-zoic acid.

FIG. 2. Raman spectra of 0.08 M aqueous solution of equimolar mix-ture of phthalic acid and sodium hydroxide heated up to 350 8C at aconstant pressure of 300 bar. The front panel shows (I ) benzoic acid,0.11 M in water, and (II ) sodium benzoate, 0.11 M in water, roomtemperature, 300 bar. Recorded on the Almega Thermo Nicolet spec-trometer, NIR diode laser, 785 nm, 17 mW.

obstruct the heating jacket. The external thermocoupleshould be calibrated with the internal thermocouple stillinside. Note that the input, IN1, is arranged in the re-movable guide yoke, GY, which facilitates having twoports on such a small cell. The yoke can be easily re-moved to put, for example, a tubular heater around thecell (however, it is even more convenient to use a hori-zontally split heater). The cell has been tested for severalhours at 580 8C and 1800 bar.

The cell has been used with two different commercialRaman spectrometers. First, it has been used with a LeikaDMLM microscope attached to a Renishaw RM1000 Ra-man System. In this case the laser beam is directed intothe cell through the standard Renishaw Angular Acces-sory, � xed onto the nosepiece of the microscope. Theworking distance of the accessory is 60 mm. The cellwas mounted on the microscope stage and, by using theXYZ adjustments of the microscope, it was very easy tomove the cell and to � nd the optimum position of focus.

On the other hand, the Thermo Nicolet Almega Ramanspectrometer has a separate and fairly spacious samplecompartment that can be used independently of the mi-croscope. The cell was installed on the XYZ motorizedstage in the compartment using a simple adapter. Al-though the cell occasionally had to be aligned manually,the use of the cell directly with the spectrometer wasquite effective.

REPRESENTATIVE RESULTS

Reactions in Near-Critical Water. One area of theresearch interests at Nottingham involves the selectivepartial oxidation of organics in near-critical and super-

critical water (scH 2O), in particular the oxidation of p-xylene to terephthalic acid.15 For this type of reaction, animportant parameter is the formation of benzoic acid asa result of decarboxylation of di-carboxylic aromatic ac-ids in scH2O.

Here we consider the decarboxylation of phthalic acid(the ortho isomer of terephthalic acid) to benzoic acid,as shown in Scheme 1. To enhance the solubility ofphthalic acid and to increase the concentration of the so-lutions, one molar equivalent of sodium hydroxide wasadded to the sample. The experiment was carried out ata constant pressure of 300 bar over the temperature rangefrom 25 to 350 8C. Above 350 8C, � uorescence becomesmuch stronger, masking weak Raman bands. This is pos-sibly the result of the onset of decomposition, althoughbenzoic acid itself has been found to show only a minordegree of decomposition after 6 h at 350 8C.16 Figure 2shows a set of spectra recorded during such an experi-ment. As can be clearly seen, decarboxylation becomesquite noticeable above 300 8C. A new band (marked b inFig. 2) corresponding to the mono-substituted benzenering17 appears at 1000 cm21, while the band a at 1049cm21, typical for the di-substituted benzene ring, decreas-es considerably in intensity. Also, a shoulder at 841 cm21

(marked c in Fig. 2) appears, which most probably is

1302 Volume 57, Number 10, 2003

FIG. 3. (a) Raman spectra of 1-butanol at a constant pressure of 500 bar. Thermo Nicolet Almega, DPSS laser, 532 nm, ;20 mW; (b) IR spectraof 1-butanol at the same pressure and in the same temperature range (from Ref. 21); (c) effect of pressure on the Raman spectrum of i-propanolat room temperature. Renishaw RM1000, Ar1 ion laser, 514 nm, 20 mW.

associated with sodium benzoate.18 The spectra are com-pared with the spectra of authentic samples of benzoicacid (front panel, spectrum I) and sodium benzoate (spec-trum II), at room temperature.

Thus, phthalic acid transformed into benzoic acid inscH2O at temperatures close to the critical temperature ofwater (374 8C), in good agreement with previous data.19

This spectroscopic result is con� rmed by high-perfor-mance liquid chromatography (HPLC), which reveals thepresence of benzoic and phthalic acid or their salts as theonly products in the cell. An intriguing feature in Fig. 2is the strange behavior of the band near 800 cm21

(marked d ), which rapidly grows as temperature is in-creased. This band should, in principle, be assigned tobenzoic acid. However, it is much stronger than the ref-erence spectrum (marked I ) and increases from the verystart of heating, before benzoic acid is produced in quan-tity. Obviously, this effect needs further investigation.

Hydrogen Bonding in Liquid and Supercritical Al-cohols. Hydrogen bonding in alcohols at high tempera-ture is currently a subject of some interest.20,21 Figure 3apresents Raman spectra of 1-butanol taken at a constantpressure of 500 bar over the temperature range 25 8C to375 8C; above this temperature the spectra show signs ofincipient decomposition of the alcohol. From Fig. 3a, onecan see that the intensity of the broad band, associatedwith H-bonded OH groups of butanol, decreases with in-creasing temperature. By contrast, the narrow band near3640 cm21, corresponding to ‘‘free’’ OH, increases, sug-gesting that only monomeric molecules of butanol existat 375 8C (weak manifestations of H-bonded OH are stillseen at 300 8C). However, this is not the case becausethe IR spectra of 1-butanol obtained under similar con-ditions,21 shown in Fig. 3b, are a more sensitive indica-tion of H-bonding. The striking difference in the shapeof IR and Raman spectra is due to the very differenteffect of H-bonding on the intensity of IR absorption andRaman scattering. The IR absorption coef� cient for then(O–H) band of H-bonded OH groups greatly exceedsthat for non-bonded OH, while the cross-section for Ra-man scattering has a much smaller dependence on theenergy of H-bonding. This example illustrates the valueof using two complementary vibrational spectroscopictechniques to avoid misleading conclusions.

However, it is interesting to note that the roles of the

two techniques are reversed at room temperature. Look-ing at the IR spectrum in Fig. 3b, one cannot detect anynon-bonded OH groups in 1-butanol at 20 8C. However,evidence for the ‘‘free’’ OH is clearly seen in the Ramanspectra in Fig. 3a. The effect is even more striking in theRaman spectrum of i-propanol (Fig. 3c), where the effectof pressure is shown. These Raman spectra con� rm theexistence of ‘‘free’’ OH at room temperature even at apressure of 2000 bar. Another interesting feature of theRaman spectra is the structure of the band of the H-bond-ed species (Fig. 3c), which is not re� ected in the corre-sponding IR spectra.

CONCLUSION

Our new miniature cell for Raman spectroscopy at hightemperatures and pressures described in this paper can beused to study a wide range of phenomena and chemicalprocesses as well as to obtain basic knowledge of thenature of liquid and supercritical states. The appealingrange covered by the cell is highly relevant to experi-ments in near-critical and supercritical regions of aqueoussystems. The cell has shown good performance underHTHP conditions, and we are currently improving thedesign and modifying it for speci� c applications.

ACKNOWLEDGMENTS

Support from the Royal Society, the EPSRC (Grant GR/N06892), andthe Russian Basic Research Foundation (Grants 03-05-64332 and 03-03-32950) is greatly appreciated. We thank Dr. W. B. Thomas and Dr.K. Whiston for their help and Dupont Textile Interiors for � nancialsupport. We are very grateful to Messrs. M. Dellar, M. Guyler, J. Whal-ley, R. Wilson, P. Fields, and K. Hind for technical assistance.

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APPLIED SPECTROSCOPY 1303

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Single Scan Cosmic Spike RemovalUsing the Upper Bound SpectrumMethod

DONGMAO ZHANG, JEANETTE D.HANNA, and DOR BEN-AMOTZ*Department of Chemistry, Purdue University, West La-fayette, Indiana 47907-1393

Index Headings: Cosmic spike; Charge-coupled device; CCD; Sur-face-enhanced Raman spectroscopy; SERS; Raman; Upper boundspectrum; UBS.

Two previous papers have reported the removal of cos-mic spikes from charge-coupled-device (CCD) spectraand images using variants of the upper bound spectrum(UBS) method.1,2 These previous UBS implementationsrequire either comparison of data derived from two ormore CCD scans,1 or analysis of a large data set com-prising many spectra.2 However, in many applications,particularly those involving chemically reacting and/orphotosensitive systems, it would be desirable to avoid theneed to multiply read-out the CCD. This work demon-strates a new variant of the UBS method in which datapoints from different pixels in a single CCD scan are usedto generate comparison spectra for spike identi� cationand removal.

This method is applicable to situations in which several

Received 3 March 2003; accepted 27 May 2003.* Author to whom correspondence should be sent.

rows of CCD pixels contain spectral data derived fromthe same sample. The required UBS comparison spectraare obtained by grouping spectra derived from spatiallyseparated CCD rows (referred to as row spectra) to createtwo or more composite spectra for UBS comparison, asdescribed below. The optimal number of comparisonspectra needed to effectively implement the UBS methoddepends on the average number of spikes in each CCDscan, as described previously.1 Implementation of thisspike removal method is demonstrated using both con-ventional and surface-enhanced Raman (SERS) spectraobtained using two different types of dispersive Ramaninstruments.

A pre-condition for implementing the original UBSmethod 1 is that the Raman spectral features in the com-parison spectra have the same shape (although not nec-essarily the same overall intensity). However, spectrafrom different CCD rows may be somewhat different inshape as a result of imperfections in the spectrometercollection/diffraction optics, which leads to non-unifor-mity in the spectrometer throughput for signals going todifferent regions of the CCD detector surface. Such dif-ferences can be minimized by grouping alternate CCDrow spectra into different comparison spectra, so eachcomparison spectrum represents the sum of non-neigh-boring row spectra. More speci� cally, the following is anoutline of the procedure we have used to generate suit-able UBS comparison spectra.

(1) De� ne the region of interest (ROI) on the CCD.Record the signal intensity in the ROI as a datamatrix D(m, n), where n corresponds to the numberof pixels in the wavelength (horizontal) axis, andm is the total number of row spectra (vertical pix-els) contained in D .

(2) Determine C, the required number of UBS com-parison spectra according to the estimated densityof cosmic spikes (required in order to reduce thechance that all the successive spectra have spikesin the same position), as described previously.1

(3) Split the m row spectra in D into C groups byalternately selecting row spectra such that rowspectrum i belongs to group k if modulo (i 2 1,C ) 5 k 2 1, where k 5 (1 . . . C ) and i 5 (1 . . .m). The C comparison spectra are obtained bysummation of the spectra in each group.

(4) Apply the UBS method to the C comparison spec-tra in order to identify and remove suspected spikeartifacts.1 The UBS method uses multiple spectrato statistically distinguish spectral features fromcosmic spike events. The summation of the result-ing spike-free comparison spectra is the UBS out-put spectrum.

Two Raman instruments were used to collect spectrafor testing the UBS method. One is a lens-coupled single-point micro-Raman instrument3 used to acquire SERSspectra of a 1023 M adenine solution deposited on Agcolloid nano-particles immobilized on a glass slide.4 Theother is a � ber-bundle coupled portable Raman imagingsystem, similar in design to a Near-IR Raman ImagingMicroscope,5 used to collect Raman spectra from a phar-maceutical Tylenol tablet. In both cases single pixel bin-ning (1 3 1) was used to read-out the CCD signal in