Design and Interfacing of an Automated Langmuir-Type Film Balance to an FT-IR Spectrometer

MIP there existed no discernible scat tered radiat ion in- terference with ei ther of the excitat ion sources (Xe-arc lamp and hollow cathode lamp) at all observation heights. Considering the above calcium-phosphate studies, the absence of scat tered signals by A1 suggests tha t the MIP is able to vaporize the majori ty of ref ractory compounds bu t is not energetic enough to cause complete dissocia- t ion of the gaseous refractory molecule into free metal atoms.

C O N C L U S I O N S

This work shows tha t the MIP has promise as an atom cell for atomic fluorescence spectrometry. I t exhibits a large dynamic range over several magni tudes of concen- t ra t ion and is relatively free f rom background interferences. Fur thermore , the MIP as an atom cell is virtually free from scat tered radiat ion interferences, even at large concentrat ions of very refractory elements. Although matr ix matching will be needed due to ionization interferences, this MIP is less prone to this type of interfer- ence than are those employed current ly in atomic emission.

ACKNOWLEDGMENTS

This work was supported by the Department of Interior-Bureau of Mines and VPI&SU core research grants.

1. A. T. Zander and G. M. Hieftje, Appl. Spectrosc. 35, 357 (1981). 2. T. H. Risby and Y. Talmi, CRC Crit. Rev. Anal. Chem. 14, 231

(1983). 3. J. P. Matousek, B. J. Orr, and M. Selby, Progr. Anal. Atom. Spec-

trosc. 7, 275 (1984). 4. D. L. Haas and J. A. Caruso, Anal. Chem. 56, 2014 (1984). 5. K. J. Mulligan, M. Zerezhgiand, and J. A. Caruso, Spectrochem.

Acta 38B, 369 (1983). 6. D. J. Douglas and J. B. French, Anal. Chem. 53, 37 (1981). 7. L. G. Matus, C. B. Boss, and A. N. Riddle, Rev. Sci. Instrum. 54,

1667 (1983). 8. G. L. Long and L. D. Perkins, Appl. Spectrosc. 41, 980 (1987). 9. R. D. Deutsch and G. M. Hieftje, Appl. Spectrosc. 39, 214 (1985).

10. L. R. Layman and F. E. Lichte, Anal. Chem. 54, 638 (1982). 11. R. M. Dagnall, J. M. Mansfield, M. D. Silvester, and T. S. West,

Spectrosc. Lett. 6, 183 (1973). 12. D. R~ Demer8, D. A. Busch, and C. D. Allemand, Am. Lab. 42, No.

3, 167 (1982). 13. D. R. Demers, Baird Corporation, personal communication. 14. D. R. Demers and C. D. Allemand, Anal. Chem. 53, 1915 (1981).

Design and Interfacing of an Automated Langmuir-Type Film Balance to an FT-IR Spectrometer

R I C H A R D A. D L U H Y , * M E L O D Y L. M I T C H E L L , T H O M A S P E T F E N S K I , a n d J E F F E R Y B E E R S Battelle-Columbus Division, 505 King Avenue Columbus, Ohio 43201-2693

We report the design of a surface analysis accessory that allows for the use of external reflection FT-IR spectroscopy to investigate the structure of insoluble monolayer films in situ at the air/water interface. The Lang- muir film balance constructed for this purpose is optimized to contain the monolayer film at the focal point of the infrared beam, as well as to generate high surface pressures within the film. The optical interfacing of the film balance to an FT-IR spectrometer is also described. Results are presented for a representative surface pressure/molecular urea isotherm and infrared spectrum for a phospholipid monolayer film. Index Headings: Infrared; Instrumentation, Langmuir film balance; Re- flectance spectroscopy; Surface analysis.

I N T R O D U C T I O N

In spite of practical exper imental difficulties, there has in recent years been a resurgence of interest in the use of monolayer techniques for the s tudy of amphiphil ic molecules at the ai r /water (A/W) phase boundary. 1,2 The

Received 11 April 1988. * Author to whom correspondence should be sent.

renewed interest in the s tudy of these so-called "Lang- mui r - type" monolayers stems from their relevance to a wide variety of scientific disciplines (e.g., interfacial chemistry, microelectronic device fabrication, and biophysical applications) as well as the ability to easily ma- nipulate the physio-chemical propert ies of these surface films. By varying exper imental parameters , the research- er is allowed a wide la t i tude in choosing the composit ion and physical state of the molecules in the monolayer, even within the same experiment . Also, the planar nature of the interface facilitates the s tudy of the interact ion of soluble ligands with the monolayer surface. Nei ther of these options is available when one is working with bulk systems.

Unfor tunate ly , while amphiphil ic monolayers at the A/W interface have been extensively s tudied as models for a variety of interfacial phenomena, there is virtually no informat ion available concerning the detai led physical s t ructure of these films. 1,2 The lack of a detailed, molecular-level unders tanding of the s t ructure and dy- namics of interfacial phenomena can historically be traced

Volume 42, Number 7, 1988 0003-7028/88/4207-128952.00/0 APPLIED SPECTROSCOPY 1289 © 1988 Society for Applied Spectroscopy

FIG. 1. Schematic diagram representing the Langmuir trough and compression barrier design described in detail in the text. Included is an exploded diagram of one of the compression barriers.

to the fact that most spectroscopic techniques cannot study flat, low-surface-area interfaces with sufficient sen- sitivity to produce spectra with reasonable signal-to-noise ratios. Lately, however, this situation appears to be changing with the publication of several in situ syncro- tron x-ray diffraction studies of amphiphilic monolayers directly at the AAV interface2 ,4

Recently, too, several papers describing the applica- tion of external reflection FT-IR spectroscopy to the in situ measurement of phospholipid monolayers at the A/W interface have appeared. ~v With the use of this reflectance FT-IR method, the IR beam is directed in a path external to the spectrometer and is specularly reflected from the A/W interface; the water surface itself acts as the reflective element.

Our goal in studying these surface films in situ has been to characterize the biophysical chemistry of mem- brane phospholipid monolayers using vibrational spectroscopy. Early in our experiments we saw the need for a low-cost, highly flexible design in a surface balance that could accommodate the constraints of the infrared external reflection optics. In addition, we wished to design the trough to enable us to generate very high surface pressures within the monolayer, unlike several commercial varieties of film balance. In the present communication, we describe a Langmuir trough design and its optical interfacing to an FT-IR spectrometer that allows the unhindered manipulation of complex surface films and the collection of infrared reflectance spectra at any point on the monolayer surface pressure-molecular area curve.

DESCRIPTION OF THE FILM BALANCE

Design of the Langmuir Trough. The Langmuir surface balance is machined from solid Teflon ® and consists of a trough and two barriers, as shown in Fig. 1. The total surface area of the trough is 28.90 × 12.90 cm, and it is

1290 Volume 42, Number 7, 1988

Fro. 2. Diagrammatic representation of the thermostatted trough baseplate assembly and driving mechanism for the compression barriers' horizontal support mounts. (1) Aluminum baseplate; (2) self- aligning ball bushings; (3) precision stainless steel shafts; (4) reflective object sensor assembly; (5) support blocks for stainless steel shafts; (6) horizontal support mount for compression barrier; (7) pulley; (8) driving belt; (9) motor; (10) optical encoder; (11) fittings for external ther- mostat control; (12) thermostatted baseplate; (13) power/data cable harness.

milled to a depth of 2.0 cm. The barriers are attached via thumbscrews to motor-driven support mounts (see below) and are placed within the trough in such a way that the edges of the barriers are in contact with the interior walls of the trough. The tension with which the barriers contact the interior trough wall is controlled by means of Teflon ® set screws (Fig. 1) and is empirically determined to prevent film leakage around the barrier edges. In this design the water surface is kept below the rim of the trough, and the dual barriers are simulta- neously driven from the edges towards the center of the trough; this allows for the generation of very high surface pressures within the monolayer film. The maximum effective separation between the barriers when they are in place is 23.0 cm; this corresponds to an effective maximum trough surface area of 296.70 cm 2.

The trough is mounted on a 2.54-cm-thick solid aluminum baseplate assembly (Fig. 2) by means of thumbscrews; this allows for the easy dismounting of the trough from the baseplate for cleaning. The area of the baseplate directly beneath the trough is milled to form a cavity of dimensions 23.8 x 13.5 x 1.9 cm deep, which is interlaced with 6.4-mm-o.d. thin-walled copper tubing to provide for temperature control of the trough subphase water. Swagelock® fittings on the exterior of the baseplate allow for connection of the copper tubing to an external water bath. The free volume of the milled cavity is filled with a 50:50 ethylene glycol : water solution that acts as a heat- exchange fluid; the cavity is covered by a 4.4-mm-thick copper plate to maximize thermal transfer to the trough subphase water. The subphase water itself is deionized

A. Side View

• , I

I ...

FIG. 3. Schematic views from (A) side, and (B) top perspectives of the optical path used to redirect and focus the infrared radiation at the A/W interface. The spectrometer is on the left side of both views. The first lens after the spectrometer is made of CaF2 and has /1= 160 ram. A polarizer is placed in the optical path before the fl focal point. Lens #2 is also made of CaF2 and has f~ = 63 mm.

and filtered through a Milli-Q purification system (Mil- lipore, Bedford, MA) and has a nominal resistivity of 18 mega-ohm/cm. A plexiglass enclosure surrounds both the trough and associated optics in order to eliminate any perturbing air currents or contaminating organic materials; in addition, the spectrometer and film balance are both mounted on a vibration isolation table in order to stabilize the water surface during data collection.

Monolayer Compression. Film compression is achieved by driving the barriers towards the center of the trough. The barriers are attached to horizontal aluminum support mounts, which are driven via a timing belt and pulley mechanism attached to the trough baseplate assembly (Fig. 2). The horizontal support mounts and Tef- lon ® barriers are guided with the use of dual self-aligning ball bushings (SUPER ball bushings, Thomson Indus- tries Inc., Manhasset, NY) which move on two precision stainless steel shafts (Thomson Industries Inc.) mounted on either side of the trough. These stainless steel shafts are then mounted to the baseplate assembly with the use of support blocks.

The timing belt to which the two horizontal support mounts are clamped is driven by means of an ironless rotor dc motor equipped with a Hewlett-Packard HEDS optical encoder and a 2970:1 reduction gearhead (Stock Drive Products, Hempstead, NY) providing a resolution of 3.66 x 105 encoder counts/cm. The compression speed of the barriers can be continuously varied throughout the range 0.05 to 5.00 cm/min. This corresponds to a change in total area of between 0.645 and 64.5 cm2/min. Typically, we run at a barrier compression rate of 0.25 cm/min or total area change of 3.23 cm2/min. The begin- travel and end-travel positions for the barriers are determined with the use of reflective object sensor assemblies (Model OPB703A, TRW Inc., Carrollton, TX) mounted on the horizontal motor mount or barrier support mount, respectively. These reflective object sensor

assemblies are a dual infrared emitting diode and Si phototransistor detector package which provides a motor interrupt voltage level when triggered.

Measurement of Surface Pressure and Trough Area. Film pressure is determined with the use of the Wilhelmy plate method. 8 A detergent-free hydrophilic filter paper (0.22 ttm, GVWP, Millipore Corp., Bedford, MA) is used as the Wilhelmy plate; it is washed successively in 6 M acetic acid and 5 M NH3OH and then exhaustively di- alyzed against 18 mega-ohm/cm H20 to remove any possible surface contaminant. A fresh plate is used for each experiment. The weight displacement of the Wilhelmy plate is measured with the use of a Cahn 27 electrobalance (Cahn Instruments Inc., Cerritos, CA), which is equipped with BCD output; the resolution of the measured surface pressure is _+0.04 dyne/cm.

The total trough area at any point in the compression cycle is measured as a function of the distance between the wipers, since the trough width is constant. This position can be accurately determined from the counts of the motor's optical encoder relative to the begin-travel position of the trough barrier, which is precisely determined with the use of the infrared object sensors (see above).

Motor Control and Data Acquisition. Trough operation and data collection are fully automated via an IBM PS/2 Model 25 personal computer. The dc motor is controlled by a Galil DMC-105 motion controller board (Ga- lil Motion Control Inc., Palo Alto, CA), which is inter- faced to the PC. The BCD output from the Cahn electrobalance is accessed via a 32-channel digital input card (Model DIM32, Industrial Computer Source, San Diego, CA). Both the optical encoder counts (i.e., trough area) and weight output of the electrobalance (i.e., surface pressure) are therefore available in digital form to the computer. A BASIC-language program was written to control the trough operations and acquire data.

APPLIED SPECTROSCOPY 1291

75.0

E t~

50.0

ID g if3

~. 25.0

,+_ r

U~

0 . 0

F I G . 4 .

&

d

C , / &

~" b &

30.0 4Ko 65.0 ' 7Ko ' 90.0

Molecular Area (square angstroms)

The surface pressure-molecular area isotherm for a monolayer film of DPPC obtained at an aqueous subphase temperature of 23°C on the film balance described in the text. The labeled areas on the plot correspond to: (a) the liquid-expanded (LE) phase, (b) the intermediate LE/LC phase transition region, (c) the liquid-condensed (LC) phase, and (d) the high-pressure phase.

A 0.005 - I ~

b 0.004 - -

S 0.003 - -

0 0.002 - -

r 0.001 - - , ~ .,J~

b a 0.000 - -

n -o.oot -

C -o.002 --

e -o.003 - -

~0 ~ 0 t~0 .bo ' Wavenumber

FIG. 5. FT-IR reflection spectrum of a monolayer film of DPPC at the A/W interface (surface pressure ~50 dyne/cm). The incoming radiation was plane parallel polarized. The spectrum is plotted in absorbance units (i.e., -log [R/Ro]) where R is the single-beam reflectance spectrum of the DPPC monolayer on H~O and Ro is the single-beam reflectance spectrum of the H20 subphase. The spectrum was recorded at 8-cm i resolution by coaddition of 1024 scans and has not been smoothed.

OPTICAL INTERFACING

Single reflectance FT-IR spectra of monomolecular films at the A/W interface are obtained with the use of the optical path diagrammed in Fig. 3. Modulated infrared radiation from the spectrometer's interferometer is directed in a path external to the spectrometer bench by means of a flat pick-off mirror (not shown). The IR beam is then brought to a focus at the A/W interface with the use of a dual-lens system (Fig. 3). The first lens is a long-focal-length optic made from CaF2 ([1 = 160 mm, d = 50.8 mm, Ealing Optical, South Natick, MA) designed to capture the IR beam from the spectrometer. The second lens is a shorter-focal-length CaF2 lens (f2 = 63 mm, d = 38.1 mm, Ealing Optical) designed to refocus the IR beam. A flat gold-coated mirror is used to redirect the output from the second lens to the Af~V surface. A second flat mirror directs the reflected radiation to the detector optics.

The incoming radiation is polarized with an IR polarizer (AI wire grid on KRS-5, Cambridge Physical Sci- ences), which is placed in the optical path just before the focal point of the first lens.

RESULTS

Figure 4 illustrates the surface pressure-molecular area isotherm at the A/W interface for a monolayer film of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, Avanti Polar Lipids Inc., Birmingham, AL). This pressure-area curve was obtained on the Langmuir film balance described in this article and is in good agreement with the published literature. 9,~° The barrier compression speed was 0.25 cm/min, and data points were taken every 30 s. The three states of the monolayer first-order ther- modynamic phase transition are clearly indicated on this isotherm: the liquid-expanded (LE) region (>~75/~2, region a in Fig. 4), the intermediate (LE/LC) phase transition region (from ~ 50-75/~2, region b), and the liquid- condensed (LC) phase (<50/~2, region c). In addition, the barrier design is capable of generating high surface

pressures within the monolayer film. For example, a surface pressure of ~67 dyne/cm is readily attained at ~30 /~2 in the DPPC monolayer (Fig. 4, region d).

The film balance/motor interface has been designed in such a way that the barrier drive can be interrupted at any point during monolayer compression in order to obtain FT-IR reflectance spectra. Figure 5 is an example of a spectrum of a DPPC monolayer in the LC phase generated with the use of the film balance and optics described in this article. The spectrum was obtained with a Digilab FTS-40 infrared spectrometer and a high-sen- sitivity (~50 × 109 D*) narrow-band, liquid-N2-cooled HgCdTe detector at 8-cm -1 resolution; 1024 scans were coadded. The presence of negative absorbance bands in this spectrum is based on the optical physics of the reflection experiment and has been explored elsewhere. 5,6 The absorbance bands in Fig. 5 correspond to the char- acteristic fundamental vibrational modes for phospholipid molecules. 11,12 For example, the bands in the region 3000-2800 cm -1 are due to the C-H stretching bands of the lipid's saturated hydrocarbon chains, while the region between 1250 and 1000 cm -1 contains vibrational modes due to the P02- stretching bands of the phosphate ester. The sharp feature at 1740 cm -1 is due to the interfacial carbonyl ester C=O stretching vibration. The sharp features between 1800 and 1400 cm -1 are water vapor absorptions, which are invariably present in this type of spectrum.

CONCLUSIONS

The Langmuir balance described in this article incor- porates several design features that allow it to be easily adapted for infrared external reflection studies. First, the dual barrier design ensures that the monolayer being compressed always remains within the focus of the infrared beam; second, the fact that the water meniscus is below the trough rim allows the attainment of higher film pressures than are possible with several commercial varieties of film balance. Several problems do remain

1292 Volume 42, Number 7, 1988

with this des ign - - fo r example, the p rob lem of monolayer s t reaming during the t ime course of spectra l da ta collection and the possibil i ty of film leakage a round the t rough barriers. However, given the in tended range of applications, we feel t h a t the design and optical interfacing of the surface balance presen ted here will allow the cost-effective use of v ibra t ional spect roscopy to probe the detailed, molecular- level s t ruc ture of complex surface films a t the A/W interface.

ACKNOWLEDGMENTS The work reported here was partially supported by Grant No. RR-

01367, Division of Research Resources, National Institutes of Health, establishing the National Center for Biomedical Infrared Spectroscopy at Battelle-Columbus Division. Additional funds were provided by the Biotechnology Department at Battelle. We would like to thank Pro- fessor K. M. W. Keough of the Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada, for several valuable discussions as well as for providing us with a copy of the plans for his own Langmuir film balance design. We would also like to thank Harry Williams of Battelle for his help and advice on the design and machining of this system.

1. J. A. Mann, Langmuir 1, 10 (1985). 2. J. D. Swalen, D. L. Allara, J. D. Andrade, E. A. Chandross, S.

Garoff, J. Israelachvilli, T. J. McCarthy, R. Murray, R. F. Pease, J. F. Rabolt, K. J. Wynne, and H. Yu, Langmuir 3, 932 (1987).

3. K. Kjaer, J. Als-Nielsen, C. A. Helm, L. A. Laxhuber, and H. Mohwald, Phys. Rev. Lett. 58, 2224 (1987).

4. P. Dutta, J. B. Peng, B. Lin, J. B. Ketterson, M. Prakash, P. Georgopoulos, and S. Ehrlich, Phys. Rev. Lett. 58, 2228 (1987).

5. R. A. Dluhy, J. Phys. Chem. 90, 1373 (1986). 6. R. A. Dluhy, N. A. Wright, and P. R. Griffiths, Appl. Spectrosc.

42, 138 (1988). 7. M. L. Mitchell and R. A. Dluhy, J. Amer. Chem. Soc. 110, 712

(1988). 8. G. L. Gaines, Insoluble Monolayers at Liquid-Gas Interfaces (Wi-

ley-Interscience, New York, 1966), p. 45. 9. M. C. Phillips and D. Chapman, Biochim. Biophy8. Acta 163, 301

(1968). 10. N. R. Pallas and B. A. Pethica, Langmuir 1, 509 (1985). 11. D.G. Cameron and R. A. Dluhy, in Spectroscopy in the Biomedical

Sciences, R. M. Gendreau, Ed. (CRC Press, Boca Raton, Florida, 1986), p. 53.

12. H. L. Casal and H. Mantsch, Biochim. Biophys. Acta 779, 381 (1984).

An Exploration Geochemical Technique for the Determination of Preconcentrated Organometallic Halides by ICP-AES

J. M. MOTOOKA U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, Colorado 80225

An atomic absorption extraction technique which is widely used in geochemical exploration for the determination of Ag, As, Au, Bi, Cd, Cu, Mo, Pb, Sb, and Zn has been modified and adapted to a simultaneous inductively coupled plasma-atomic emission instrument. The experi- mental and operating parameters are described for the preconcentration of the metals into their organometallic halides and for the determination of these metals. Lower limits of determination are equal to or improved over those for flame atomic absorption (except Au), and ICP results are very similar to the accepted AA values, with precision for the ICP data in excess of that necessary for exploration purposes. Index Headings: Emission spectroscopy; Induction coupled plasma; Analysis of geologic materials; Preconcentration as organometallic halides.

I N T R O D U C T I O N

Induct ive ly coupled p lasma-a tomic emission spectroscopy ( ICP-AES) is being increasingly relied upon for the analysis of geologic mater ia ls to de te rmine t race-e lement concentra t ions not only in the field of explorat ion geo- chemis t ry bu t also in other disciplines where low-level e lementa l abundances and minera l speciat ion are de- sired. T h e adven t of ICP-AES 1~ is proving to be an im- p o r t a n t supp l emen t to es tabl ished techniques, and when

Received 17 March 1988.

coupled with classical chemical procedures for selective digestion and extract ion, 9-12 it lends grea ter insight into orebody genesis.

When originally presented, ICP-AES appea red to offer an improved approach to the analysis of geologic ma te - rials a t or near the crustal abundance level of m a n y ele- men t s t ha t were previously very difficult to determine. Al though ICP did show i m p r o v e m e n t for some elements , the sensitivities for m a n y chalcophile e lements t ha t are i m p o r t a n t to geochemical studies were still no t adequate . I t became clear, af ter numerous a t t e m p t s to analyze rock and soil samples, t ha t s t a te -of - the -a r t ICP techniques still could not achieve de te rmina t ions t ha t were sensit ive enough to supp lan t the current ly employed procedures.

The procedure t ha t we concent ra ted on is the tr ica- p r y l m e t h y l a m m o n i u m chloride (Aliquat 336) /methyl isobutyl ke tone (MIBK) extract ion of meta l chelate com- plexes t ha t was developed by Viets 13 for f lame a tomic absorp t ion (AA) analysis. This process concent ra tes geochemical ly i m p o r t a n t t race meta ls (Ag, As, Au, Bi, Cd, Cu, Mo, Pb , Sb, and Zn) and el iminates or reduces com- monly encountered interferences f rom such major ele- men t s as a luminum, calcium, iron, magnes ium, and man- ganese. Once this procedure was modif ied and adap ted to the ICP ins t rument , the analyt ical t ime was reduced considerably. However , M I B K is difficult to introduce into the p lasma, and organic solvents usual ly require a

Volume 42, Number 7, 1988 0003-7028/88/4207-129352.00/0 APPLIED SPECTROSCOPY 1293 © 1988 Society for Applied Spectroscopy

Documents

Design and Interfacing of an Automated Langmuir-Type Film Balance to an FT-IR Spectrometer