Anal. Chem. 1986, 58, 1091-1097 (8) Dixon, J. B. chimi8 1984, 38, 82-86. (9) Barber, M.; Bordoli, R. s.; Elliott, G. J.; Sedgwick, R. D.;Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A-654A.
RECEIVD for review September 26,1985. Accepted December
1091
4,1985. This research was supported by a grant, PCM 8200954, from the National science ~ ~ ~ ~FABd spectra ~ t were obtained a t the Middle Atlantic Mass Spectrometry Facility, an NSF shared instrumentation facility.
Mass Spectrometric Study of Ion Adsorption on Poly(ethy1ene terephthalate) and Polypropylene Surfaces Ronald
D.Macfarlane,* Catherine J. McNeal, and Charles R. Martin
Department of Chemistry, Texas A&M University, College Station, Texas 77843
The adsorption of ions on polypropylene and poly(ethylene terephthalate) surfaces from polar solutions has been studied by use of 252Cfplasma desorption mass spectrometry. The solutes were Li and Cs salts and Rhodamine 6-6 hydrochiorlde in aqueous and ethanol soiutlons. Adsorption was studied as a function of solute concentration and pH and mixed solute solutions were used to ascertain competitive adsorption behavior. Polypropylene and poiy(ethyiene terephthalate) behave Identically functioning as cation exchange surfaces with preference for the adsorption of organic cations. Ion intensities deduced from mass spectra were found to be a function of solution concentration reaching a maximum value at full monolayer coverage. The adsorption sites appear to be small chemlsorbed strongly basic anions present as impurities and the hydroxyl ion, which is in equilibrium wlth water in the mobile phase.
In a recent study, we investigated the possibility that polymer surfaces with ion exchange properties might be useful for preparing samples for solid-state mass spectrometry (1). The concept was devised with the objective of developing a general scheme utilizing the principles of liquid chromatography and solute partitioning a t a liquid/solid interface to selectively adsorb an analyte onto a compatible surface in the presence of other components in the solution. The adsorbed layer is directly analyzed by mass spectrometry. In the first study, we demonstrated that a Mylar polyester film surface coated with a cation exchange polymer, Nafion, had a remarkable affinity for polarizable cations and that the cationic species adsorbed on the Nafion could be detected with high sensitivity by 262Cfplasma desorption mass spectrometry. It has been known for some time that all polymers acquire a surface charge when exposed to an aqueous medium (2). This property is used in coloring synthetic fibers using ionic dye molecules (3) and is also responsible for the coagulation of blood proteins on the surface of polymeric materials used in medical applications (4). The reason why polymers acquire a charge can sometimes be related to particular acidic or basic functional groups in the polymer structure, but even polymers that are pure hydrocarbons such as polystyrene and polyethylene acquire a significant negative surface charge when exposed to a polar solvent. I t has been suggested that the origin of these surface charges may be molecular remnants of the polymerization process containing negatively charged functional groups that lie on the surface of the polymer (5). In this study we investigated the ion adsorption properties of polypropylene and poly(ethy1ene terephthalate) (Mylar)
using solid-state mass spectrometry (262Cf-plasmadesorption) as a probe to elucidate the mechanism of the ion adsorption properties of these polymers. In addition we developed further the concept of the use of polymer films as a substrate and analyte ions as adsorbate as a medium for solid-state mass spectrometry and as a means for the preparation of a more well-defined and controlled matrix for the study of the dynamics of the emission of molecular ions from surfaces. EXPERIMENTAL SECTION Apparatus. Mass Spectrometer. Mass measurements were made with a z62Cfplasma desorption (=‘Cf-PDMS) time-of-flight (TOF) mass spectrometer (6). The system was operated at an acceleration voltage of f 1 0 kV with a 4-mm target-grid spacing and a 55 cm long field-free region. A 10-pCi262Cfsource (Isotope Products Corp., Burbank, CA) was positioned behind the sample on a linear motion feedthrough to within 4 mm of the sample. Details of the internal elements and geometries are given elsewhere (7). Time-of-flight measurements were made with a multistop time interval digitizer with a 78-ps resolution and a 850-11s dead time (8). The secondary ion detector at the end of the flight tube consisted of two microchannel plates in a chevron configuration coupled to a 5 0 4 impedence matched anode (Galileo Electrooptics, Sturbridge, MA, Model FTD 2003). Photoelectron Spectrometer. Photoelectron spectra of electrosprayed and adsorbed CsI were obtained from a photoelectron spectrometer (Kratos, Manchester, U.K., Model XSAM-800) operating with 1453.6-eV photons (A1 Ka) at an incident angle of 30’. Spinner. A Model AHT3A-T-54photoresist spinner (Headway Research, Inc., Garland, TX) operating at 10000 rpm was used to remove the liquid phase from the polymer surface after adsorption. Materials. Polypropylene and poly(ethy1ene terephthalate) (Mylar) films, 4 and 1.5 pm thick, respectively, and aluminized on one side, were obtained in a 3.7 cm wide roll from Atlan-To1 Industries, West Warwick, RI. Rhodamine 6-G (laser grade) was obtained from Eastman Kodak Co., Rochester, NY. Procedures. Multilayer Sample Preparation. Multilayer deposits were prepared by the electrospray method (9). Ethanol solutions of solute (5 pg/pL) were sprayed onto an area 1 cm2on the A1 side of the polymer film giving a multilayer approximately 20 pg/cm2 in thickness. Adsorbed Layer Sample Preparation. The polymer film (with the A1 side down) was stretched and mounted in the specimen holder that serves as the “target”for a 262Cf-PDMSmeasurement. This was then mounted on the head of the photoresist spinner and a 200-pL aliquot of solution was placed on the surface of the polymer with the spinner in the rest position. After a 2-min period, the liquid was rapidly spun off the surface by rapid acceleration to 10000 rpm, which was maintained for a 1-min period. The sample was then transferred to the mass spectrometer for mass analysis.
0003-2700/86/0358-1091$01.50/00 1986 American Chemical Society
i
~
~
1092
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6,MAY 1986 Cs‘( 133)
0.51
0.0
- I
L.
1. , I
1..
I
400 (M+H)* (443)
0.0
0.5
0.5 44
0.0 100
400
200
Mass ( m h ) Figure 1. 252Cf-PDMSspectra of electrosprayed solutes; counting time, 15 min. The numbers in parentheses are masses ( m l z ) . (a) CsI, positive ions, 10’ Ions, full scale (fs.); (b) CsI, negative ions, 8 X lo4 ions, f a ; (c) Rhodamine 6-G, positive ions, 4.5 X lo4 Ions, f.s.; (d) Rhodamine 6-G,negative ions, 1.5 X lo3 ions, f.s.
Calculations. Absolute yield measurements of ion intensities were made in terms of number of ions emitted per incident fission fragment. The effective fission fragment (FF) intensity was determined by measuring the intensity of the FF-e- TOF spectrum at “zero mass” in the negative ion spectrum. An average intengity s on the time digitizer) was of 1827 f 10 FF/s (for a 1 5 - ~range recorded for this study. Because of the high electron multiplicity/FF, no correction factor was required for geometry or detection efficiencies in determining the effective FF intensity. The absolute yield of emitted ions was calculated by integrating the number of detected ions in the mass peak of interest and multiplying by a detection efficiency comprised of attenuation by two 90 % transmission grids, a measured solid angle correction for beam divergence (SO%), and an assumed 50% efficiency for the microchannel plate detection system based on the estimated sensitive area of the detector. The overall detection efficiency accounting for these factors was 32%. The accuracy of the absolute yields was estimated to be 25% while the relative yields were within 10%. The reproducibilityof the measured yields for the adsorbed layers was typically -+lo%;all quoted absolute ion yield measurements were carried out in duplicate and represent the average value.
RESULTS AND DISCUSSION 252Cf-PDMSof CsI a n d Rhodamine 6-G Multilayers. Two salts were chosen in this study on the basis of particular properties of their ions. The salt CsI was chosen to represent a class of inorganic ions that are primarily sensitive to the electrostatic part of the adsorption interaction. That the cation and anion have essentially the same charge, size, and polarizability meant that both cation and anion adsorption sites could be investigated simultaneously with ions having comparable properties. Rhodamine-6G hydrochloride was selected because it readily ionizes in polar solvents to give a stable, amphiphilic organic cation sensitive to electrostatic
0.0 100
400
200
Mass (m/z) Figure 2. 252Cf-PRMSspectra of polymer surfaces; countlng time, 15 min. (a) poly(ethy1ene terephthalate), positive ions, 15 000 ions, f.s.; (b) poiy(ethy1ene terephthalate), negative ions, 17 500 ions, f s . ; (c) polypropylene, positive ions, 2500 ions, f.s.; (d) polypropylene, negatlve ions, 1750 Ions, f s .
and polarization components of the adsorption interaction. The 252Cf-PDspectrum of this molecule is dominated by the molecular ion (M + H)’ with a very weak fragmentation component, a feature that makes it possible to determine absolute ion yields directly from the molecular ion intensity without having to include contributions from a fragmentation pattern. The 252Cf-PDmass spectra of electrosprayed samples of these salts represent ion desorption under conditions of complete surface coverage and for the species in a polycrystalline, multilayer matrix. The 252Cf-PDMSspectra of these molecules are shown in Figure 1. For CsI, the positive ion spectrum (Figure la) is dominated by the Cs+ ion a t mlz 133 and contains a set of cluster ions of the type Cs (&I)+ as observed in SIMS and FAB ( 1 0 , I I ) . In the negative ion spectrum (Figure Ib), the I- ion a t m / z 127 is the most intense and cluster ions of the type I(Cs1)- and OH(Cs1)- are observed. Even though the molar ratio of Cs/I is unity, the intensities of these two ions are not the same; the Cs+ intensity is almost a factor of 2 larger, reflecting differences in the dynamics of the emission of these two ions. Figure ICshows the 252Cf-PDMSpositive ion spectrum of the electrosprayed Rhodamine 6-G. The peak at mlz 443 is the (M + H)+ ion that dominates the spectrum. The negative ion spectrum in the same mass range is very weak with an indication of an (M - H)- ion a t mlz 441 and a set of fragment ions in the m / z 100-350 region (Figure Id). The C1- counterion is present in the low mass region with an intensity 40% that of the (M H)+ ion. The absolute yields of the major ions for these two salts in terms of ions emitted per incident fission fragment (ions/FF) are given in Table I. 252Cf-PDMSof Poly(ethy1ene terephthalate) (Mylar) and Polypropylene. Figure 2 shows the 252Cf-PDMSspectra of the Mylar and polypropylene side of the aluminized polymer
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1093
Table I. Absolute Yields of Ions from Electrosprayed CsI and Rhodamine 6-G Samples ion type
cs+ I-
Rhodamine 6-G, (M + H)' Rhodamine 6-G, (C1-)
mass range
absolute yield (no./FF)
132-134 126-128 440-445 34.5-35.5
0.41 0.22 0.31 0.087
(mlz)
film in the region of interest for this study (mlz 100-500). (Uncoated polymer films could not be used because a conducting surface is required in the 262Cf-PDmeasurement to establish the electric field for ion acceleration from the surface.) The mass resolution was essentially the same as that observed from the A1 side of the film showing little evidence of surface charging as had been observed earlier in SIMS studies of polymer surfaces (12). This is probably because our polymer films were much thinner and the incident ion intensity was orders of magnitude smaller. The mass spectra are similar to what had been previously observed using SIMS (13). The overall intensities for the Mylar spectra (Figure 2a,b) were 6 times higher than for polypropylene with the monomer unit at m / z +149 clearly identified. The spectrum continues to m/z 4000 (not shown) with a periodicity in m/z consistent with the mass of the monomer. The intensities of the peaks at m/z 149 and -121 diminished with increasing surface coverage by an adsorbed layer but did not closely correlate with the degree of surface coverage deduced from the ion intensities of species associated with the adsorbed layer. Most of the peaks shown for the polypropylene surface (Figure 2c,d) were found to be due to localized regions of surface contamination in the roll of polymer film. Not all foils showed these peaks, particularily those at m / z -265 and -293, which are due to a low level of impurities introduced during the sample preparation. These two peaks were also observed in the electrosprayed Rhodamine 6-G sample (Figure Id) where the polymer surface is completely covered. The peaks at m/z -121 and -220 seemed to be a component of the polypropylene film but could not be correlated with fragmentation of the polymer structure. They may arise from components introduced in the fabrication of the polymer. The mass spectra of the adsorbed layer were readily resolved from the contribution of the polymer substrate, even at low surface coverage. General Comments on Adsorption Studies. Several methods were investigated for determining the optimum procedure for carrying out the adsorption experiments. The procedure described in the Experimental Section was selected on the basis of simplicity and reproducibility. A critical parameter in this study was the time required to establish an equilibrium condition at the solution/polymer interface. The contact time was varied from 1 min to 1 h. No differences were observed in the mass spectra of samples prepared with this range of contact times indicating that with a smooth planar polymer surface and small solution volume thermodynamic equilibrium is established very quickly. A 2-min contact time was chosen as the standard for these measurements. Adsorption of CsI on Mylar and Polypropylene. Figure 3a shows the positive ion mass spectrum of the adsorbed layer formed from M aqueous CsI on Mylar. The Cs+ ion at m/z 133 was the most intense peak in the spectrum in the mass region of interest (mlz 100-500) with an absolute yield of 0.09 ions/FF. The peak at m / z +149 was from Mylar (Figure 2a). The negative ion spectrum is shown in Figure 3a. There is no evidence of an I- ion at m / z 127. Only the ion at m / z -121 due to Mylar (Figure 2b) was observed. Not one of the Cs-containing cluster ions observed for the multilayer deposit (Figure la,b) was detected. (It was determined
0.0
400
'1 0.5
0.0
200
0.0 100
400
I
I
200
400
Mass (m/z) Figure 3. 252Cf-PDMSspectra of polymer surfaces containing ions M aqueous CsI; counting time, 15 min. (a) adsorbed from poly(ethy1ene terephthalate), positive ions, 22 000 ions, f.s.; (b) poly(ethylene terephthalate), negative ions, 10 000 ions, f.s.; (c) polypropylene, positive ions, 20 000 ions, f.s.; (d) polypropylene, negative ions, 2200 ions, f.s.
in subsequent studies to be discussed below that at this concentration and with HzO as a solvent, saturation coverage of the Mylar surface by Cs+ ions was not achieved.) The adsorption experiment was repeated using the polypropylene surface and the same aqueous CsI solution. The 262Cf-PDspectra are shown in Figure 3c,d. The results were identical with what was observed for the Mylar surface. The absolute intensity of the Cs+ ion was also 0.09 ions/FF; no I- ions were observed nor were there any Cs-containing cluster ions. In the negative ion spectrum, only ions detected in the bare polypropylene spectrum (Figure 2d) were observed. For this particular film, the surface contamination peaks at m / z -265 and -293 were absent but the peaks at m / z -121 and -220 were still present at an intensity nearly the same as for the bare foil (Figure Id). The influence of the presence of adsorbed species on the component of the mass spectrum due to the polymer substrate is a curious phenomenon. For Mylar, the intensities of the oligomer ions (trimer and larger) seem to correlate with the degree of surface coverage. The monomer ion (m/z +149) intensity is strongly attenuated at high surface coverage but the m / z +lo4 ion intensity is not attenuated to the same extent. For polypropylene, the m / z -121 and -220 ion intensities seem to be little affected by the degree of surface coverage. One possible explanation for these differences is that the smaller ions are not directly associated with an adsorption site and therefore remain as uncovered or only partially blocked surface species whereas the larger pieces of the polymer undoubtedly would be covered by adsorbate. XPS Measurements of Multilayer and Adsorbed CsI. Before any conclusions could be made regarding the absence
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
0.0
745
, 740
735
730
725
720
Electron Binding Energy (eV)
.-z
I
I
1
I
735
730
725
720
I
Electron Binding Energy (eV)
1.o
1.0
Y)
0
-
.-b
c
C
m
a,
- 0.5 4-
0.5
0.0 640
I
635
630
625
I
I
620
615
I
I
I
I
I
635
630
625
620
615
Electron Binding Energy (eV) Figure 5. X-ray photoelectron spectrum of polypropylene exposed to lom3M aqueous CsI: (a) electron binding energy region for Cs+ ion; (b) electron binding energy region for I- ion.
Electron Binding Energy (eV) Figure 4. X-ray photoelectron spectrum of an electrosprayed CsI deposit: (a) electron binding energy region for Cs+ ion: (b) electron binding energy region for I- Ion.
of I- in the mass spectra of the adsorbed layer, it had to be established by a technique other than mass spectrometry (XPS analysis) whether I- ions were present in the adsorbed layer. Figure 4 is the XPS spectrum of a multilayer deposit of CsI (the same sample used to obtain the 252Cf-PDmass spectra shown in Figure la,b). Figure 4a covers the region for Cs+ and Figure 4b covers the region for I-. The spectrum compared closely with the library XPS spectra for these species (14). Figure 5 is the XPS spectrum of CsI adsorbed on polypropylene (the same sample used to obtain the 252Cf-PDmass spectra shown in Figure 3c,d). The spectrum is much weaker because it is a submonolayer sample, but the Cs+ ion XPS spectrum is clearly indicated. There was no evidence for 1- ions (Figure 5b) and the conclusion is that the XPS measurement corroborates the 262Cf-PDMSfindings that no I- ions are present in the adsorbed layer. Adsorption of Rhodamine 6-Gon Mylar and Polypropylene. As in the case of the adsorption of Cs+ ions, Mylar and polypropylene have the same capacity for adsorbing the protonated Rhodamine 6-G cation. Positive ion spectra of the species adsorbed from a M aqueous solution on the two polymer surfaces are shown in Figure 6. The mass spectra (not including the contribution from the polymer) are identical in terms of the nature of the ions detected and intensity to what was observed for the multilayer deposit (Figure IC).The concentration-dependence study to be discussed later in the paper indicated these spectra are for samples that have a full monolayer coverage. The C1- counterion was observed for samples with submonolayer coverage but the intensity was an order of magnitude smaller that what was observed for the multilayer deposit. Influence of Solvent on the Adsorption of CsI and Rhodamine 6-G. Prior to beginning a studv of the influence of solution concentration on-adsorption, it was found that
I
0.0,
"'I"'
(443)
O.7 100
200
300
400
500
Mass (m/z)
Flgure 6. 262Cf-PDMS positive ion spectra of Rhodamine 6-G adsorbed on polymer surfaces: counting time, 15 min; (a)Mylar surface, 35 000 ions, f s . ; (b) polypropylene surface, 45 000 ions, f.s.
(.
because of the high solubility of CsI in H20, it was not possible to achieve full monolayer coverage using reasonable concentrations. With a change to ethanol where CsI has a lower solubility, and hence is more readily adsorbed from solution, it was possible to obtain saturation coverage at a much lower concentration. These limited data demonstrated nicely the connection between solubility and degree of adsorption because CsI and Rhodamine 6-G have quite different relative solubilities in water and ethanol. For M CsI in water, the absolute Cs+ ion yield of the adsorbed layer was 0.09 ions/FF. From ethanol at the same concentration, the ion yield increased to 0.24 ions/FF, which translates to a factor of 2.5 increase in surface coverage by Cs+ ions. Rhodamine 6-G is readily miscible in water and ethanol because of its amphiphilic character so that these solvent effects were not observed in the Rhodamine adsomtion experiments. Samples
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
the M sample, implying that a very significant fraction of the surface of the Mylar was covered by Rhodamine at this concentration. The adsorption curves shown in Figure 7 have a shape that is very similar to that observed in classical Langmuir adsorption studies where the amount of material adsorbed is determined by gravimetric procedures. These results show that mass spectrometry can also be used to obtain adsorption isotherms giving at the same time an analysis of the composition of the adsorbed layer. The increase in the Cs+ and Rhodamine 6-G (M H)+ion intensities at the higher concentrations to a maximum value close to that observed for the multilayer electrosprayed sample confirms previous observations using other surface-sensitive mass spectrometric methods that only the top monolayer is contributing to the mass spectrum (15). The mass spectra of the adsorbed layers prepared from dilute solutions show no evidence for cluster ions of the type observed with the multilayer samples. This means that the adsorbed molecules are well separated on the surface and that in cont'rast to the ionomeric clustering observed for Nafion films, the adsorption sites are uniformily distributed on the surface. An independent confirmation of this was obtained for Rhodamine 6-G by measuring the peak wavelength in the fluorescence spectrum of a sample adsorbed on Mylar. This parameter is sensitive to the average spacing between adsorbed Rhodamine 6-G molecules (16). For adsorbed layers prepared from solutions at concentrations just above saturation in the adsorption isotherm, cluster ions were observed in the mass spectra for both CsI and Rhodamine 6-G. For Rhodamine, ions were observed in the m / z region of the dimer ion corresponding to M2C1+and Mz+. At the onset of the appearance of these cluster ions the C1- ion also appeared with enhanced intensity that increased sharply with more concentrated solutions. Nature of the Ion Adsorption Site on Polypropylene. From the results discussed above we conclude that the ion adsorption sites on both Mylar and polypropylene are negatively charged. There is no evidence for negative ion adsorption. The surface concentration of adsorption sites is high capable of supporting full monolayer coverage. Mylar and polypropylene exhibit identical adsorption isotherms for Cs+ and the (M + H)+ion of Rhodamine, which means that they have the same concentration of cation adsorption sites and the adsorption interaction is essentially the same. It is as if the adsorption were independent of the molecular structure of the polymer and that there was a general mechanism for generating negatively charged adsorption sites on the surface of the polymer film. The literature on the subject of ion adsorption of polymers is rather limited and has mainly focused on the interaction of cationic dyes with synthetic fibers used in the textile industry (3). Although it is not clear what differences there might be in the action of a polymer as a fiber vs. film with quite a different morphology, it may be that some of the fundamental interactions might be the same. These studies clearly show that polyester fibers (Mylar is in this class) behave as cation exchange materials in aqueous media, and this is the reason for the choice of organic cation dyes in treating these materials. For acrylic fibers, it has been postulated that the ion exchange sites are anions of strong acids (sulfonates, phosphates, sulfates) remaining as impurities from the manufacture of these polymers (5). The presence of these groups on the surface of a polymer film can be detected with high sensitivity by a surface-specific mass spectrometric method such as Cf-PDMS. In our previous work with a Nafion surface, the sulfonate ion responsible for its ionomeric behavior appeared in the negative ion spectrum with high intensity; the
+
001 105
10.4
10 3
10.2
Solution Concentration (M)
Flgure 7. Desorbed ion intenslties as a function of solution Concentration (95% ethanol). The open circles (0)are for Cs' ions desorbed from a polypropylene surface and the closed circles ( 0 )are for Rhodamine 6-G desorbed from a Mylar surface.
prepared from adsorption from l W 5 M aqueous solutions gave an absolute molecular ion yield of 0.10 ions/FF, and for an ethanol solution of the same concentration, the ion yield was essentially the same, 0.11 ions/FF. Influence of Concentration on Cs+and Rhodamine 6-G Ion Adsorption. The concentration dependence for ion adsorption was carried out using ethanol solutions of these solutes in order to achieve saturation adsorption for CsI at low solution concentrations. As discussed above, we found little difference in the adsorbing abilities of polypropylene and Mylar surfaces for Cs+ or Rhodamine ion adsorption. For CsI, the adsorption studies were carried out using a polypropylene surface and for Rhodamine 6-G, a Mylar surface. The concentration range was from M to M. The results are shown in Figure 7 where the absolute ion yield from the adsorbed layer i s plotted as a function of solution concentration. For CsI adsorption, the Cs+ intensity increased almost linearly over a concentration range from M to M. A saturation of the ion yield occurred at concentrations above M. Up to a concentration of lo-* M, the only dif5X ference between the mass spectra of the adsorbed layers for increasing solute concentration was an enhancement of the Cs+ ion intensity; there was no evidence for the presence of the I- counterion. The absolute Cs+ ion yield at saturation corresponded closely (0.50 ions/FF) with the value observed for the multilayer sample, which means that full monolayer coverage of the surface by Cs+ ions has been realized at the higher concentrations. Above 2 X M, the I- ion began to appear in the negative ion spectrum and increased linearily with CsI concentration up to the maximum concentration studied. This we attribute to the formation of Cs21+cluster ions in solution a t high solute concentration, which are adsorbed along with the Cs+ ions. The adsorption data for the Rhodamine 6-G molecular ion (M + H)+are also shown in Figure 7. The absolute ion yield increases with concentration of solute reaching a maximum value of 0.48 ions/FF at M. A C1- ion was observed at a very low level in the concentration range of M to M and independent of Rhodamine concentration. At 2 x lo-' M and above, the C1- intensity increased linearily up to the highest concentration used M). The saturation value of the absolute molecular ion yield was a little higher than what was observed for the multilayer sample. Using the Mylar surface, we had an independent monitor of the degree of surface coverage using the attenuation of the intensities of the oligomer ion peaks from the Mylar. The peaks associated with the trimer ion were reduced to 26% of the bare Mylar value for a M solution and were not in the spectrum for
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986 1.01
i
0.5
0.54
I
1
OD' (18)
0
1 I
a i l I
I
I
Mass (m/z) Figure 8. 252Cf-PDnegative ion spectra for polypropylene in the low mass region showing evidence for adsorbed OH- Ions: (a) after exposure to M HCI; (b) after exposure to M DCI in D20.
mass spectra of biopolymers containing phosphate groups give a clear signature of the presence of this group. We feel confident that any of these anions on the polymer surface that would serve as ion exchange sites could be detected by 252Cf-PDMS.However, none was detected for either polymer. There were, however, low levels of F and Cl- in the spectrum and OH- was also identified and found to respond to deuterium exchange. A mass spectrum in the region of the OHion is shown in Figure 8. Figure 8a shows the spectrum of polypropylene after exposure to M HC1. Figure 8b shows the spectrum after exposure to M DC1 in DzO. The presence of the OD- ion was clearly identified proving that the OH- ion on the surface is in contact with bulk HzO. T o learn more about the cation adsorption site for these polymers we carried out some exploratory studies to verify that an ion exchange mechanism was operating a t these polymer film surfaces and to gain some insight to the nature of the adsorption interaction by studying competitive adsorption for mixed solute systems. These results are discussed below. NH4+/Li+/Cs+Ion Adsorption and the Influence of pH. The first study to learn more about the nature of the desorption site was to determine whether the negatively charge site was a strong or weak base. The criterion for this was whether NH4+ions could be adsorbed from aqueous solution. If it was a strong base, it would deprotonate the NH4+ion on adsorption, and if it was a weak base (such as the sulfonate groups in Nafion), the NH4+ion would be adsorbed on the surface (as in the case of Nafion). In the 252Cf-PDMSspectra of polypropylene films exposed to 0.2 M NH4Cl (aqueous) there was no evidence for an m / z +18 ion. This was a dominant ion in the mass spectrum of NH4+-Nafion. We conclude from this that the cation exchange site is strongly basic. Previous studies on the properties of ion exchange surfaces have established standard tests to verify the ion exchange mechanism and to determine to what extent electrostatic and dispersion forces are involved in the substrate-adsorbate interaction. For ions with low polarizability, the electrostatic force between the negatively charged site on the surface and the positive ion in solution dominates the interaction. To bring this feature out, we studied the competition of Li' and hydronium ions for the adsorption sites on polypropylene using aqueous solutions. For an adsorbed layer prepared from a M LiCl (pH 7) solution, the absolute ion yield for Li+ ions was 0.037 ions/FF. Repeating the measurement a t pH 2, the yield was reduced to 0.0032 ions/FF. This means that the hydronium ion is competing favorably with Li' ions for the adsorption site and that mass action of the hydronium
ion at the lower pH reduces the surface concentration of adsorbed Li+ ions. The influence of dispersion forces in ion exchange processes has been studied in the past by using a sequence of lyotropic ions, ions that vary only in polarizability such as the group 1A (group 1 in 1985 notation) metal ions. Li+ ions are adsorbed more strongly on small anion sites while Cs+ ions prefer sites where the negative charge is more diffuse and polarizable (18). When we compared the ion yields of adsorbed layers from M LiCl and from M CsI (aqueous, pH 7), we found that the Cs+ intensity was 2.4 times higher than for Li+ ions, which reflects the relative sensitivities for detection of these ions by 252Cf-PDMS.This was independently verified by measuring the Li/Cs ion intensities from a multilayer electrosprayed sample of known composition. We then carried out a competitive adsorption study using a Cs/Li solution in which the mole fraction of CsI was 0.5 (2 X 1L.I total concentration, pH 7). The ion intensity ratio (Cs/Li) of the adsorbed layer determined by 252Cf-PDMSwas 4.4. Correcting for the relative sensitivities for detection of these ions, this means that the mole fraction of Cs+ ions in the adsorbed layer was enriched to 0.65 from a 0.50 value in solution. This means that the adsorption of a polarizable cation is preferred and that the adsorption sites have the characteristics of a large polarizable anion. Rhodamine 6-G/Cs Ion Adsorption and the Influence of pH. The competition between Rhodamine 6-G, hydronium, and Cs+ ions for adsorption on polypropylene was studied to bring out the influence of dispersion forces when a large organic cation is adsorbed. An adsorbed layer was formed from a M aqueous solution of Rhodamine 6-G at two different pH values, 6.3 and 2.0. The absolute ion yields, (M H)+, were 0.10 and 0.09 ions/FF, respectively. The insensitivity of adsorption to pH, an effect reported earlier for cationic dye adsorption on acrylic fibers (19),was in marked contrast to Li' adsorption. This meant that either the binding of Rhodamine to polypropylene was much stronger than for the hydronium ion or that they occupied different adsorption sites. The resolution of this question was provided by studying the competitive adsorption of Rhodamine 6-G and C d ions. For the competitive adsorption study, ethanol was chosen as the solvent because Cs+ ions are more readily adsorbed from that solvent and Rhodamine 6-G adsorption is the same for water and ethanol. Concentration values were chosen that gave high yields for these solutes when adsorbed separately. In the first study, a 1:lmolar ratio was used with a total solute concentration of M. The 252Cf-PDMSof the adsorbed layer showed only the (M H)+ ion of Rhodamine 6-G. There was no evidence for Cs+ ion adsorption. The absolute yield of Rhodamine was reduced to 74% of the value previously observed for this concentration, which is in the saturation region of the adsorption isotherm (Figure 7). We repeated the adsorption experiment increasing the Cs+ ion concentration to a Cs+:Rhodamine molar ratio of 4:l and a total solute concentration of 2.5 x M (ethanol). A mass spectrometric analysis of the adsorbed layer from this solution showed the (M + H)+ ion of Rhodamine to be again dominant with the same intensity as observed with the 1:l solution and ions/FF) was also a very weak Cs+ ion intensity (4 X observed, 4 orders of magnitude smaller than what was observed when the Rhodamine was absent from solution. This means that both solute ions are competing for the same adsorption site and the very large fractionation of Rhodamine from Cs+ ions in the adsorption process confirms the findings of the Li/Cs studies that dispersion forces are an important component of the adsorption interaction. Some other details were brought out in the Rhodamine/Cs+ study that indicate that ion pairing in solution can be studied
+
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6,MAY 1986
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Mass (m/z) Flgure 9. 252Cf-PDpositive ion mass spectra of the adsorbed layer from ethanol solutions of CsI at different concentrations in the mass region for cluster ions: (a) M; (b) 5 X M; (c) 5 X M. Counting time, 15 min.
using this methodology. The 262Cf-PDmass spectrum of the adsorbed layer from the 4:l molar ratio (Cs:Rhodamine) solution contained a prominent I- ion in the negative ion spectrum at 10% the intensity for an electrosprayed CsI sample (Figure lb). (This ion also appeared weakly in the spectrum for the 1:l solution with a factor of 6 lower intensity.) In addition, dimer ions of Rhodamine were observed corresponding to M2+and MzI+. The appearance of these species in the adsorbed layer is undoubtedly due to ion pairing processes occurring in solution when the I- concentration is elevated. The pairing of Rhodamine (M + H)+ with I- would be more likely to occur than with C1- ions due to dispersion force interactions between these polarizable ions. Through the adsorption of complex cations containing I-, this ion becomes incorporated into the adsorption layer and appears in the mass spectrum. It may be that the small reduction in surface concentration of adsorbed Rhodamine when I- is present is due to the fact that some of the Rhodamine is bound as a neutral ion pair with I- in solution so that the effective Rhodamine ion concentration is reduced. Identifying the Nature of the Adsorption Site. A clue to the chemical identity of the adsorption sites came from the mass spectra of adsorbed Cs+ ions from CsI at high surface coverage. Cs-containing cluster ions appeared in the mass spectra but the most intense ion was Cs2C1+and not CszI+. These spectra are shown in Figure 9. A peak at mlz +283 due to Csz(OH)+was also observed with an intensity comparable to Cs21+. We interpret these results as follows. At high surface concentration, it is possible that two Cs+ ions are in close proximity to a negatively charged adsorption site on the surface. In the desorption event induced by the 252Cf fission fragment, a cluster comprised of two Cs+ ions and the negative ion responsible for cation adsorption are desorbed. This means that the species responsible for the cation exchange behavior of polypropylene and Mylar are tightly bound C1- and OH- ions. Perhaps when a neutral pristine polypropylene surface is exposed to an aqueous solution, the adsorption of anions and the development of a negative surface potential are necessary to establish thermodynamic equilib-
1097
rium at the interface. The OH- ion from water and Cl- at the impurity level are apparently preferred primary adsorbates even in the presence of high concentrations of I-. Perhaps once adsorbed, polarization interactions with the polymer surface give these ions the properties of a polarizable anion with preference for ion pairing to large polarizable cations and with a strong base character capable of abstracting H+ from NH4+. Application to Solid-state Mass Spectrometry. The methodology reported here has potential application to two problem areas in analytical solid-state mass spectrometry. The observation that organic cations can be preferentially adsorbed from aqueous solution in the presence of inorganic salts means that it may be possible to adsorb biomolecules (that form cations in solution) from a compatible aqueous environment onto a polymer surface leaving interfering salts, buffers, and neutral impurities in solution. The amount of sample required to obtain a mass spectrum may be reduced since monolayer coverage is all that is required to achieve maximum ion intensity. Finally, the ability to control the surface density of analyte and to change the substrate (e.g., Mylar vs. polypropylene) without significantly altering the environment of the analyte makes it possible to learn more about the role of the matrix in the emission of molecular ions from surfaces.
ACKNOWLEDGMENT We thank V. K. Mancill and E. A. Jordan for assistance in making the mass measurements, D. Cocke and D. E. Halverson for obtaining the XPS spectra, and K. Itoh for providing the fluorescence data. Reeistrv No. CsI. 7789-17-5;Rhodamine 6-G hydrochloride, 7682-83-9;polypropylene, 9003-07-0;poly(ethy1ene terephthalate), 25038-59-9. LITERATURE CITED Jordan, E. A.; Macfarlane, R D.; Martin, C. R.; McNeal, C. J. Int. J . Mass Spectrom. Ion Phys 1983, 53,345. Connor, P.; Ottewill, R. H. J . Colloid Interface Sci. 1971, 37,642. Giies, C. H. "Adsorption from Solution at the SolldlLiquid Interface"; Parfitt, G. D., Rochester, C. H., Ed.; Academic Press: New York, 1983;Chapter 7. Iordanski, A. L.; Polischuk, A. Ja.; Zaikov, G. E. JMS-Rev. Macromol. Chem. Phys 1983, C23, 33. Balmforth, D.; Bowers, C. A,; Guion, T. H. J . SOC.Dyers Colour 1984, 80, 577. Macfarlpne, R. D. Anal. Chem. 1983, 55, 1247A. Macfarlane, R. D.; Torgerson, D. F. Jnt. J . Mass Spectrom. Ion Phys. 1876, 21, 81. Turko, B.; Macfarlane, R. D.; McNeai, C. J. I n t . J . Mass Spectrom. Ion Phys 1983, 53,353. McNeai, C. J.; Macfarlane, R. D.; Thurston, E. L. Anal. Chem. 1878, 51 , 2036. Barlak, T. M.; Wyatt, J. R.; Colton, R. J.; DeCorpo, J. J.; Campana, J. E. J . Am. Chem. SOC. 1882, 104, 1212. Ens, W.; Beavis, R.; Standing, K. S. Phys. Rev. Lett. 1983, 50, 27. Gardella, J. A., Jr.; Hercules, D. M. Anal. Chem. 1980, 52,226. Briggs, D. S I A , Surf. Interface Anal. 1982, 4, 151. Wagner, C. D.; Riggs, W. M.; Davis, J. F.; Moulder, G. E.; Muilenberg, P. E. "Handbook of X-Ray Photoelectron Spectroscopy"; Perkln-Elmer: Eden Prairie, MN, 1979. Lange, W.; Jirikowsky, M.; Benninghoven, A. Surf. Sci. 1984, 136,
.
.
.
419. Garoff, S.;Stephens, C.; Hanson, C. D.; Sorenson, G. K. Opt. Commun 1982, 4 1 , 257. Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J . Am. Chem. SOC. 1984, 106, 1620. Lykiema, J. "Adsorption from Solution at the SolidlLiquid Interface"; Parfitt, G. D., Rochester, C. H., Ed.; Academic Press: New York, 1963;Chapter 5. Giies, C. H. Br. Polym. J . 1971, 3, 279.
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RECEIVED for review September 4,1985. Accepted December 20, 1985. We gratefully acknowledge the financial support of the National Science Foundation (CHE-82-06030), the National Institutes of Health (GM-26096),and the Robert A. Welch Foundation (Grant No. A-258) for this study.
