Anal. Chem. 1994,66, 1032-1037
Quantitative Static Secondary Ion Mass Spectrometry of pH Effects on Octadecylamine Monolayer Langmuir-Blodgett Films Jlan-Xin LI and Joseph A. Gardella, Jr.’ Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214-3094
A modified method to obtain quantitative information from molecular ion signals from static secondary ion mass spectrometry analysis is reported. The technique utilizes both integration and curve-fittingprocedures. This method has been applied to study solution cast and Langmuir-Blodgett (L-B) films of octadecylamine on polycrystalline silver. The L-B films have been prepared with subphase pH values ranging from 1 to 11. The results reported test the validity of the newly developed quantitative approach and allow conclusions about the secondary ion formation mechanisms. In particular, the mechanisms involved with the formation of secondary (M + Ag)+ and (M H)+ ions are discussed. A surface titration curve is developed, and this allows the quantitative evaluation of local pH at the surface of these films. For L-B monolayers, it is found that the integrated peak intensity ratio of (M + Ag)+/Ag+ is independent of the subphase pH value while that of (M + H)+/Ag+ decreases rapidly as the pH value increases from 1 but remains relatively constant for pH values over 7.
structure and bonding can be preserved. This isvery important in analyzing organic materials.6 As a surface sensitive technique, SIMS has several advantages over electron spectroscopy for chemical analysis (ESCA) and Auger electron spectro~copy.~.~ Such benefits include the ability to detect all elements and their isotopic distribution, low detection limit, detection of molecular secondary ions which are directly related to the molecular structure, and surface compositional sensitivity because of the matrix dependence of secondary e m i ~ s i o n . ~However, ?~ major difficulties which limit routine analysis of SIMS include sample damage due to sputtering, lack of understanding of the relationship between matrix-dependent secondary emission and the surface composition, and difficulty in obtaining reproducible, accurate, quantitative molecular information. Quantitative information with a precision between 2%and 15%relative standard deviation is considered obtainable from only the elemental ions produced in dynamic SIMS.7-9 Still, the ability to provide elemental quantitative information Secondary ion mass spectrometry (SIMS) has become an through secondary ion yield is complicated by the variation important and powerful technique in surface analysis.’ In by 4 orders of magnitude across the periodic table. In addition, SIMS, a primary ion beam of 0.5-30-keV energy bombards matrix effects, instrumental parameters, and ion collection a solid surface, and the sputtered secondary ions are mass efficiencies influence secondary ion yield and must be analyzede2Roughly, SIMS experiments can be classified into considered when one is attempting to quantitate SIMS data.279 two groups, namely, “dynamic” and “static” modes. Under dynamic conditions, high current densities, greater than 1 Recently, there has been much progress in quantitative pA/cm2,are used to gain elemental information as a function study of static SIMS data of organic compounds. Tamaki, of sputter depth into the bulk.2 In addition, elemental maps Sichtermann, and Benninghoven’O investigated amino acids or imaging can also be obtained by focusing the ion beam to adsorbed on silver and found a linear relationship between the small (1-3 pm) spot sizes (ion microprobe) or by illuminating arginine molecular ion ((Arg + H)+) absolute signal intensity the sample with a large-diameter ion beam and using ion and surface concentration ranging from 1 X 1013 to 5 X 1014 optics to preserve the spatial resolution (ion micro~cope).~*~ arginine molecules/cm2. Below 1 X 1013 molecules/cm2 Because of the high current densities involved, molecular ion instrumental noise limited the detection of the secondary ion information often cannot be preserved in the dynamic mode and above 5 X 1013 molecules/cm2 the intensity became (although there are some reports of success with chemically relatively constant. The authors attributed these results to a stable ions.5) In the static mode, however, low current densities complete monolayer of arginine being formed with 5 X 1013 and ion dosages (less than 1 nA/cm2 and 1 X 1013ions/cm2, molecules/cm2 with multiple layers formed at higher conrespectively) yield a very small probability that primary ions centration. Because the number of molecules that can be will strike the same molecule or surface region twice in a ionized due to direct interaction with the substrate is limited typical analysis time. Therefore, information about molecular with increasing concentrations, the secondary ion intensity becomes constant. They concluded the quantitative method (1) Benninghoven, A . ; Rudenauer, F. G.;Werner, H. W. In Secondary Ion Mass Spectrometry: Basic Concepts,Instrumental Aspects, Applications and Trends; is limited only to submonolayer to monolayer coverage.
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Elving, P. J., Wineforder, J. D., Kolthoff, I. M., Eds.; John Wiley & Sons: New York, 1987. (2) (a) Steffens, P.; Niehuis, E.; Friese, D.; Greifendorf, D.; Benninghoven, A . J. Vac.Sci. Technol. A 1985,3, 1322.(b) Benninghoven,A.; Niehuis, E.; Heller, T.; Feld, H. J. Vac. Sci. Technol. A 1987, 5, 1243. (3) Turner, N. H.: Colton, R. J. Anal. Chem. 1982, 54, 293R. (4) Vickerman, J. C. Chem. Br. 1987, Oct, 969. (5) Gillen G.; Simons D. Anal. Chem. 1990, 62, 2122.
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(6) Benninghoven, A . Z . Phys. 1970, 230, 403. (7) Werner, H. M. SIA, S u r . Interface Anal. 1980, 2, 56. (8) Simons, D. S.; Baker, J. E.; Evans, C. A.Ana/. Chem. 1976, 48, 1341. (9) Ramseyer, G.0.;Morrison, G.H. Anal. Chem. 1983, 55, 1963. (10) Tamaki, S.; Sichtermann, W. K.; Benninghoven,A . Jpn. J. Appl. Phys. 1984, 23, 544. 0003-2700/94/0366-1032$04.50/0
0 1994 American Chemical Society
In SIMS, the absolute ion yield strongly depends on the chemical nature of the surface species under investigation and on its chemical environment.' Utilizing SIMS with a time-of-flight analyzer (TOF-SIMS), Hagenhoff et al." investigated the quantification of molecular SIMS by the use of an internal standard. Cyclosporine D (CsD) was used as standard, which differs from cyclosporine A (CsA) by only one CH3 group, but is of no physiologicalrelevancy. Mixtures of various composition were cast on Ag substrates, and the ratio of the absolute ion intensity of (CsA Ag)+ to (CsD Ag)+ is plotted against the ratio of the amounts in the mixture. A linear relationship was observed with the error of the slope at 15%. However, the slope was not 1 and the intercept was not 0. Recently, research from this laboratory introduced a methodlzof obtaining quantitativemolecular ion information using thin organic films produced from micropipet deposition and L-B techniques.'3 Biological molecules which are precursors for the biopolymer eumelanin form thin partially polymerized films at the air-water interface. The precursor molecules were studied as monolayer films produced from solution deposition. Preparations of 1-8-3,4-dihydroxyphenylalanine (L-DOPA), various indoles, and dopamine were used to investigate how the secondary molecular ion peak intensity varied with concentration and ion dosage.12 It was noted that the intensity varied with ion dosage, as expected, given the nature of molecular ion formation in SIMS. This obviously limits the ability to obtain quantitative information from static SIMS by simply taking absolute relative peak intensities."JJ The solution proposed in this work12 was to integrate the signal by summation from the molecular ion (in this case a protonated molecular ion) peak intensity as a ratio to the silver ion intensity over the ion dosage. In this method, the assumption is that the total ion signal is a better estimate of concentration than the signal at any one point during ion bombardment, and this integrated amount is related to the concentration. For the solution cast films of known amounts, this method of quantitation yielded a linear relationship, within error limits, with a good correlation coefficient (e.g., 0.998). However, the results still were related to the ion formation probability, in that this approach effectively yields a calibration curve with a slope related to the formation probability. That means that the results were still specific to the matrix. In order to overcome this specificity, an internal standard approach was developed using mixed LB films of known concentration.14 In this study, the ratio of molecular ion intensities should then be directly correlated (Le., a slope of 1 and an intercept of 0) to the molar concentration ratio (unitless, vida infra). This model is contingent upon a similar mechanism of molecular ion formation and no chemical interaction between components of interest.1° Results showed that high correlation coefficients (>0.990) and intercepts within a few percent error of zero were determined over a range of concentration ratios from 0.2 to 3.
+
+
(1 1) Hagenhoff, B.; Kock, R.; Dcimel, M.; Benninghoven, A. Proceedings of the Eighth International Conferenceon SecondaryZon Mass Spectrometry (SIMS VIII); J. Wiley: Chichester, UK, 1991; p 831. (12) Clark, M. B.; Cardella, J. A., Jr. Anal. Chem. 1990, 62, 871. (13) Cornelio P. A.; Gardella, J. A., Jr. Langmuir 1991, 7, 2454. (14) Cornelio P. A.; Gardella, J. A., Jr. J. Voc. Sci. Technol. 1990, A8, 2283.
These two studies12J4 validated the general approach of integrating signal intensities and now give the opportunity to apply the technique to a number of organic thin film problems which require quantitative analysis of mixtures. In these applications,SIMS may have unique capabilities, in particular to mixtures of small and trace amounts in the presence of larger amounts of other species. There has been much work reported on the static SIMS studies of L-B films of alkanoic acids.13-16 However, the formation of (M H)+ in such systems is not as thermodynamically favorable as the formation of (M - H)-. In this paper, we report the further development of the quantitative static SIMS while applying it to the investigation of the surface chemistry of an ampiphilic alkanoic amine, octadecylamine using L-B films and solution casting. The favorable protonation reaction of the amine should be sensitive to local pH. Thus, the L-B films of octadecylamine on Ag substrate are prepared under different subphase pH values. As a result, we are able not only to elucidate the secondary ion formation mechanism but also to quantify small changes in the surface chemistry of the L-B films.
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EXPER IMENTAL SECTION Sample Preparation. L-B samples were prepared by spreading films of octadecylamine (Aldrich, 99%, used without further purification) from 0.1 mg/mL solution in benzene (ACS reagent grade) on a triple-distilled water subphase containing 0.01 M Na2HP04 solution. The pH value of the water subphase was adjusted by adding HCl or NaOH, respectively; Le., no polyvalent ions besides the H3P04 dissociation products were present in the solution. The pH values of the subphase were measured by pH meter (Orion research digital ionalyzer 501). After solvent evaporation (about 20 min), the films was compressed to a surface tension of 15 dyn/cm, corresponding to a closed packed mon01ayer.l~ The film then was transferred (Y deposition) to polycrystalline silver foil (99.999% Alfa) at 4 mm/min. The KSV Langmuir trough used for L-B preparation has been described elsewhere.'* Prior to L-B filmdeposition and the solution casting, the silver foil was treated in an inductively coupled radio frequency glow discharge (RFGD) argon plasma to lower the concentration of surface contaminants and produce a highenergy (55-60 dyn/cm) surface and therefore increase the surface wettability and favor the attachment of L-B films. The RFGD procedure involved exposure in a rotary vacuum pumped (base pressure 5 mTorr) 4-in.-diameter inductively wound quartz plasma chamber (Harrick, Ossining, NY) where air or other gases may flow in at controlled p r e s s ~ r e .In ~~.~~ this case, clean substrates were inserted in the chamber and treated with a 100-W argon plasma for 15 min. After treatment, the substrates were retrieved, immediately immersed in distilled water, and then transferred to the trough. Solution deposition samples were made from the stock solution (15) Wandass, J. H.;Schmitt, R. L.; Gardella, J. A., Jr. Appl. Sur/. Sci. 1989.40, 85-96. (16) Wandass, J. H.; Cardella, J. A., Jr. J. Am. Chem. SOC.1985, 107, 6192. (17) Mingotaud, A.-F. HandbwkofMono1ayers;AcademicPrtss Inc.: San Diego, CA, 1993. (18) Hook, K. J.; Cardella, J. A., Jr. J . Yac. Sei. Technol. 1989, A7, 1795. (19) Wandass,J. H. Ph.D. Dissertation,SUNY Buffalo, DepartmentofChemistry, September 1986. (20) Grimm, W. Natunvissenschaften 1967, 54. 588.
Ana~icalChemistry,Vol. 66, No. 7, April 1, 1994
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with an octadecylamine concentration in benzene of 0.01 mg/ mL. Sample concentrations corresponding to 0.25,0.5,0.75, and 1 monolayerwere produced by pipeting from stock solution onto Ag foil treated by the same RFGD method as described above. The solvent was allowed to evaporate for 30 min. Instrumentation. SIMS experiments were carried out on a Leybold-Heraeus LHSlO SIMS100. This quadrupolebased instrument was modified for organic SIMS by incorporation of a special low kinetic energy pass filter into the optics. This instrument is turbomolecular pumped with a base pressure of 5 X 10-l" mbar and an operating pressure of 4 X lo-* mbar. A Balzers quadrupole mass filter (3-1000 amu) modified with a Kramer off-axis multiplier was used for mass separation and detection. Resolution of the quadrupole is nominally unity at 98 amu (as specified with molybdenum).lg Samples were mounted on a isolated sample rod with a bias voltage of 13.5 V to assist in the extraction of organic ions. A Leybold-Heraeus Model 12-38 differentially pumped ion gun (0.5-5 keV continuous) was used to ionize 99.9995% Ar+ (Airco) accelerated to 4 keV as the primary ion source. The ion beam produced was defocused and rastered over a 4 X 4 mm area, resulting in a ion current density of less than 5 nA/cm2, referenced to a Faraday cup. Data collection was accomplished with a custom interface system designed by Assmuth and Muelhoff, Inc. (Wilkensburg, PA). The interface is responsible for quadrupole control and data acquisition. With this system, rapid signal averaging was made possible, yielding organic SIMS spectra with improved signal-to-noiseratios. All data manipulations (peak areas and positions) were carried out with the SIMS software provided by Assmuth and Muelhoff, Inc. All molecular spectra were collected at a data step of 0.1D (10 data points per mass unit). Five scans were signal averaged per scan. The scanning rate for the Ag peak is 5 ms per data point, 100 ms for (M H) and (M Ag). Calculation. The peakintensities were taken as integration over several mass units, 106-1 11D for Ag, 267-2750 for M H, and 372-383DforM + Ag. Thisgave the peakintensities by counts. They were then converted to counts per seconds by dividing a time factor that involved the mass range, scan time, number of scans, and resolution. Relative peak intensities of (M H)+/Ag+ and (M Ag)+/Ag+ were plotted against the primary ion dosage. To obtain the accurate area under those original plots, SIGMA PLOT4.1 software (Jandel Scientific) was used to simulate experimental plots. An exponential equation was used to simulate the response of the relative peak intensity along with ion dosage. Accurate peak areas were then obtained by integrating the fitted curve. The coefficient of variation was expressed as a percentage (CV % = (standard deviation X 100)/mean).
+
+
+
+
+
RESULTS AND DISCUSSION Quantitative Static SLMS. To try to relate quantitative static SIMSinformation obtained from well-ordered L-B films to surface acid-base chemistry, octadecylamine films were prepared on Ag substrates. Many static SIMS quantitation methods use the ratio between the molecular ion peakintensity and the Ag peak intensity in order to correct for any variation in instrumental parameters. Thus, the Ag signal is used essentially as a internal reference. However, as pointed out 1034
Ana!YtIcalChemisty, Vol. 66, No. 7, April 1, 1994
in the introduction, the previously published quantitative methodI2 has shown that it is necessary to inregrare this ratio over the ion dosage. This integration method, in principle, should avoid both the variation in the peak intensity ratio with increased ion dosage and the nonlinearity between the peak intensity ratio and the molecular concentration. Previously, the integration was carried out over a specific ion dosage. Intuitively, the integration value should be more accurate if carried out over the whole range of the ion dosage until the molecular ion signal disappears. This is not possible if the film is bombarded until there are no more molecular ions emitted, because (i) the creation of defects to the film structurefrom theion beam even under staticconditionswould lead to the attenuated ejection of substrate ions, so that the foundation of using the Ag signal as a reference as the ion dosage becomes relatively large becomes less accurate, and (ii) the inherent baseline noise level in the SIMS instrument could lead to large error in the obtained integration value. In this paper, a theoretical model is proposed which should overcomethese two difficulties and yield more accuratevalues for the integration. Under static conditions for a given primary ion current density, the amount of the secondary molecular ions produced per unit time interval should be, intuitively, approximately proportional to the amount of molecules present on the film when the film is not yet heavily damaged. Mathematically, this corresponds to an exponential relationship between the secondary molecular ion peak area and time. The model used in this work is
Y = a exp(-bt)
(1)
where Yis the ratio between the molecular ion peak intensity and the Ag peak intensity, which is assumed to be relatively constant, r is the time of sputtering (related to ion dosage), and a and b are coefficients to be determined from the fit of nonlinear least-squares fit of the data. Figure 1 shows the typical peak intensity ratio between (M + Ag)+ and Ag as a function of sputtering time for a monolayer of octadecylamine from a subphase of pH 4.8 onto Ag substrate. The dots on the plot are experimentally obtained values; the solid line is obtained from a least-squares fit using eq 1. The fitting parameters are found to be a = 1.602 X l e 3 and b = 1.114 X 10-2/min. (CV % are less than 5% for all the calculations.) The data in Figure 1 illustrate the excellent fits obtained through the work. Similar results are obtained for (M H)+ secondary ions. The proposed fitting model between the molecular ion peak intensity and the time of sputtering under static condition in eq 1 is very good. One property of eq 1 is that the integrated ratio between the molecular ion and the Ag peak intensity can be easily obtained: it is the ratio of a / b . Such treatment avoids the necessity of sputtering the sample film until the molecular ion signal disappears and eliminates the subsequent complications of using the Ag signal as a reference. It should be also pointed out that the error from the inherent noise signal of SIMS instrument is relatively limited. In order to test the validity of our proposed quantitation method, an experiment was carried out using static SIMS on a set of four solution cast films deposited on Ag substrate. The
+
0.02
Table 1. Comparkon ol Summation and Intrgratlon MetSolution Cad Sampler ol 0ctad.Cylamine for (U H)+
+
(3
T
surface concn (monolayer) summation results E(M + H)+/Ag+ integration results
+ -
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+ H
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> t
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2
d~
lor
0.25
0.5
0.75
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2.52
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2.7
3.78
3.74
5.87
8.01
12.35
0.01
W
+
z
w
2
!-
a
1 W
E
0.00 0
200
400
600
800
1000
1200
ION DOSAGE(NA/CM’)
+
Figure 1. Relative intensity of (M H)+/Ag+ of one L-B layer of octadecylamlne on Ag as a function of primary ion dosage. Dots are experimental data: solid line Is from cwve Wing (Y = a exp(-bx), where a = 1.602 X and b = 1.114 X
s
ii U w
G U
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14
-
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I
,
0 0.0
0.2
0.4
0.6
0.8
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1.2
SUBSTRATE COVERAGE
+
Figure 2. Integrated relativeintensity of (M H)+/Ag+ of solution cast film of octadecylamine as a function of surface coverage.
molecular density (MD) of an octadecylamine L-B monolayer with a subphase of pH 7 was defined as unity. The molecular densities of the solution cast films were prepared to be MD of 0.25,0.5,0.75, and 1, respectively. The ratio of (M H)+ secondary ion peak intensity over the Ag peak intensity is fitted by using eq 1. There is generally good agreement between the fitted and the experimental values. Figure 2 shows the plot of the integrated value of this ratio as a function of the molecular density. The four experimental data are fitted by a linear function, y = cx, using the least-squares fitting procedure. This assumes that the origin should be the intercept and does not allow that as an adjustable parameter. The results of the fit are shown as the solid line. The fitting parameter c is found to be 11.85 and the least-squaresstandard deviation is found to be only 0.32. Such a linear calibration
+
-.
60
80
$00
120
140
Mean Molecular Area (A%nolecule)
Figure 3. Force area Isotherms of octadecylamine monolayers: (A) pH 4.8; (B) pH 7.3; (C) pH 11.
curve on Figure 2 is not unexpected,12J4 if the integrated molecular ion peak intensity can be directly related to the concentration of molecules present on the films. Table 1 showed a comparison between curve-fitting results and direct summation of all data points. Those data from summation do not give a linear curve. By taking summation of all data points to estimate the area under the curve, only data over a specific ion dosage range were counted. In order to relate surface concentration to integrated curve area, this curvefitting method is necessary. During preparation of octadecylamine L-B films at different subphase pH values, the packing density of a monolayer (from the isotherm) increases as the pH value increases. This general trend of fatty amine L-B monolayers had been noted earlier by Gaines.21 The effects of pH on octadecylamineL-B films have also been discussed by Petrov, Kuhn, and Mobius2*in their study of three-phase contact line motion in the deposition of spread monolayers. The reason for such a dependence is as follows. Under acidic conditions, the octadecylamine molecules are protonated, and these functional groups interact strongly by binding the divalent anion HP042-, so that the area per molecule is relatively large for a lower pH value. As the subphase pH value increases until it reaches 7.5, the deprotonation of the amine molecules begins, as the equilibrium RNH3+ = RNH2 H+ is shifted to the left side. Therefote, in a basic subphase, the area per molecule is relatively smaller. Over all, the octadecylamine molecules exist in different forms in the L-B monolayers, depending strongly on the subphase pH value. Figure 3 illustrates the variation of molecular packing density at different pH of the subphase.
+
(21) Gaines, G.L., Jr. Insoluble Monolayers at Liquid Gas Intqfaces; Wiley Interscience: New York, 1966. (22) Petrov, J. G.;Kuhn, H.; Mobius, D. J. Colloid Interface Sci. 1980, 73,66.
Ana&ticalChemistry, Vol. 66, No. 7, April 1, 1994
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0.65
formed is directly related to the subphase pH value. As the pH value increases from 1 to -7, a drastic decrease of the ratios of the integrated peak intensity of (M + H)+ from 6.64 to 1.65 was observed. For pH values above 7, the ratios of the integrated peak intensity for (M H)+ ions remains essentially a constant value of 1.5. This dependence of the ratio of the integrated peak intensity of (M + H)+ on the pH value, as compared to the independence of that of (M + Ag)+ ions, can be explained in the secondary ion formation mechanism of the (M H)+ ions. There are two possible pathways to the formation of the (M H)+ ions. First, for a pH value smaller than 7, there are (M + H)+ ions already formed during the film preparation, the amount of which decreases with the increase of the subphase pH value. These ions can be ejected directly from the film during the bombardment of the primary ions. Such a process has been discussed by Benninghoven as the precursor model.23 If a precursor of the finally emitted ion exists on the target surface, there is a high probability of this precursor being emitted as an unfragmented parentlike secondary ion. Because the amount of (M + H)+ ions on the film decreases with the increase of the subphase pH value, the amount of the finally emitted secondary (M H)+ ions decreases as the pH value increases. Second, (M + H)+ secondary ions may also be formed through a recombination process similar to (M + Ag)+. Because this process is not affected by the pH value of the subphase, the amount of the secondary (M + H)+ ions formed through the recombination mechanism should remain independent of the pH value. This is clear demonstrated in a near plateau for pH values over 7. In this region of the pH value, recombination is the dominant mechanism. So the data in Figure 5 are a result of the interplay of the precursor and the recombination mechanisms. Figure 5 shows a distinctive dependence of the integrated peak intensity ratio of the (M H)+/Ag+ ions. This suggests that the quantitative static SIMS study can be used not only to investigate the secondary ion formation mechanism but also to study the surface chemistry (local pH) of the fatty amine films. Future study will focus on applying the quantitative static SIMS method to alternating layers of fatty amine and fatty acid monolayers. Such systems can be used as ideal model systems to study the proton-transfer mechanism in the secondary ion formation process.
-
P
0.55
+
-
050
0.45 -
0
I
I
I
I
I
I
2
4
6
8
10
12
pH OF SUBPHASE Figure 4. Integrated relative intensity of (M of pH of the subphase.
+ Ag)+/Ag+ as a function
8 -
+
+
+
71 P 6
5 -
PQ
4 -
3 -
2-
+
1 0 '
I
I
I
I
I
1
0
2
4
6
8
10
12
pH OF SUBPHASEe Figure 5. Integrated relative intensity of (M of pH of the subphase.
+ H)+/Ag+ as a function
Figure 4 shows the integrated value of peak intensity ratio between the (M Ag)+ and the Ag+ secondary ions as a function of the subphase pH value, using the quantitative approach discussed in this section. The data on this plot are reproducible within -20%. One sees from Figure 4 that the amount of (M Ag)+ ions formed remains essentially the same for different pH values. This is not surprising, because the secondary ion formation mechanism of (M + Ag)+ ions during the bombardment is believed to be a recombination process, which happens within the selvedge layer near the surface.15J6 This process should not be related to the pH value during the preparation of the film. Therefore, one would expect that the number of (M + Ag)+ secondary ions emitted remains constant with respect to changes in the pH value. Figure 5 shows the integrated value of peak intensity ratio of the (M H)+ over the Ag+ secondary ions as a function of the subphase pH value. Typically the error of the ratio for each pH value is -20%. The amount of (M + H)+ ions
+
+
+
1096 Analytical Chemistry, Vol. 66, No. 7, April I, 1994
CONCLUSIONS In this paper, a modified quantitative method using static secondary ion mass spectrometry was developed. The peak intensity of the secondary molecular ions as a function of primary ion dosage is modeled by an exponential function with fitting parameters. The fitting procedure has been thoroughly investigated by using this quantitative method to study both octadecylamine solution casting and LangmuirBlodgett films.' The results for the solution cast films show that the molecular density of octadecylamine of a solution casting film on Ag substrate correlates well with the integrated peak intensity ratio of (M + H)+ over the Ag+ signal. For octadecylamine L-B films, it is found that the integrated peak intensity ratio of (M Ag)+ to Ag+ is independent of the
+
(23) Benninghoven, A. In?. J . Mass Spectrom. Ion Phys. 1983, 53, 85-99.
+ +
subphase pH value. This suggests that the secondary (M Ag)+ ions are formed through recombination processes. On the other hand, the integrated peak intensity ratio of (M H)+ to Ag+ decreases rapidly when the subphase pH value increases from 1 and reaches a plateau at a pH value of 7. This is the consequence of the interplay of the precursor mechanism and the recombination process during the formation of (M + H)+ secondary ions. The results suggest that
the quantitative static SIMS method can be used not only to investigate the secondary formation mechanism but also to relate surface pH information. Received for review September 15, 1993. Accepted January 11, ,QoAa 1""7.
* Abstract
published in Advance ACS Abstracts, February 15, 1994.
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Vol. 66,No. 7, April 1, 1994
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