Combined XPS and SIMS study of amino acid overlayers

Surface Science 84 (1979) 235-248 0 North-Holland Publishing Company

COMBINED XI’S AND SIMS STUDY OF AMINO ACID OVERLAYERS

R.J. COLTON, J.S. MURDAY, J.R. WYATT and J.J. DeCORPO Chemistry Division, Naval Research Laboratory,

Washington, DC 20375,

USA

Received 13 October 1978; manuscript received in Fmal form 12 March 1979

Factors influencing the SIMS fragmentation patterns are studied for three simple amino acidsglycine, ar-alanine, and serinedeposited onto Ag substrates from aqueous solution. Secondary ion emissions are measured for 1 keV Ar+ ions incident at 70” from sample normal as a function of substrate preparation and solution concentration. Studies by XPS and X-ray induced AES prior to SIMS analysis show that the amino acids adsorb in a film on the Ag surface and that the film thickness increases with solution concentration. In addition, considerable amounts of amino acid can be deposited on the surface from a water film retained during extraction from a concentrated solution. On acid etched samples, positive ion fragments of mass AgM, Ag(M - 45), Ag, M + 1, and M - 45 are observed, where M is the molecular-weight of the parent amino acid. With the exception of the (M + l)+ fragment, these peak intensities behaved similarly for the different surface concentrations. When the adsorbed film grows too thick, the positive molecular ion emissions drop considerably; this substantiates the need for proximity between the Ag substrate and the amino acid molecule.

1. Introduction The application of secondary ion mass spectrometry (SIMS) to the study of the surface phenomenon of adsorption has been accelerating in recent years. Of particular note are the pioneering efforts of Benninghoven and co-workers [ 1,2] in the area of static SIMS which uses a low primary ion beam flux to minimize the sputtering of the surface allowing analysis of monolayer quantities. Some of the more surprising results have been obtained from the analysis of organics for which high secondary ion (SI) emission of parent molecular ions has been observed. Karasek [3] showed characteristic SIMS spectra of benzoic acid pressed into “KBr-type” pellets; the spectra contained parent and fragment ions similar to those observed in electron impact (EI) mass spectra. Dawson [4] monitored the slow adsorption of propane on aluminum using static SIMS and observed a characteristic “cracking pattern” from the adsorbed species. Benninghoven et al. [5-71 demonstrated how static SIMS could be used as an analytical technique for the identification of biologically important compounds, such as amino acids, peptides, drugs, vitamins, etc. supported on metal substrates. The mechanisms for the secondary ionization and fragmentation of organic mol235

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R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

ecules adsorbed on surface are difficult to identify. Whether ionization and fragmentation occur at the surface or above the surface is a subject of vigorous debate and research. The process undoubtedly depends upon various chemical and physical properties of the adsorbent and adsorbate, such as composition, electronic state, surface topography and coverage, and reactivity. Barber et al. [8] have shown how SIMS, when applied to the chemisorption of CO and CzH4 on Ni, reflected the various adsorption states. Grade et al. [9,10] have recently described a cationization process for organic SIMS in which metal ions become attached to the organic molecules. Their results were general with regard to the type of metal substrate (alkali, transition, or noble) and adsorbed organic species (polar or nonpolar). A similar process is also observed for organics utilizing a laser induced ionization technique [l l] in which ionization occurs mostly by attachment of an alkali cation to the desorbed molecules. SIMS spectra of the amino acids include the observation of high molecular weight parent-like and fragment ions of unusual high emission as compared to conventional EI mass spectra. In particular, Benninghoven et al. [S-7] observed high SI emission at (M + l)‘, (M - l)-, and (M - COOH)’ for all of the amino acids studied. The exact role that the metal support plays is little understood although Benninghoven et al. indicated that acid etched substrates of Ag, Cu, and Ni worked the best. Grade and Cooks [lo] showed that the nature of the substrate, i.e., pure, oxidized, or alloyed, influenced the degree of organic fragmentation. It is the purpose of this paper to examine more closely the properties of the surface and overlayer-surface interaction which might influence the fragmentation and/or SI emission. We have studied two model carbonaceous overlayer systems including (i) several amino acids on acid etched and nonetched Ag substrates and (ii) several fatty acids on etched Ag or polished Cr substrates (12). The analytical tools employed in this investigation consist of Auger and X-ray electron spectroscopy (AES and XPS), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS).

2. Experimental Carbonaceous overlayers are formed from the various alipathic o-amino acids listed in tabel 1. The a-amino acids all possess the general structure NH&HRCOzH where the amine group occupies a position on the carbon atom alpha to that of the carboxyl group and the R group represents H, CHa, or CH20H. The amino acid samples are Eastman white label and are used without further purification. The alanine and serine are both racemic compounds. Aqueous solutions of the amino acids are prepared in acid cleaned glassware with triply distilled water. The dilute solutions are prepared from a 5 ml aliquot of the 0.1 M solution. The reported concentrations are only approximate and do not account for the amount of acid adsorbed at the container walls.

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

231

Table 1 Aliphatic a-amino acids, NHzCHRCOzH R group

Atomic density a (cme3)

Glycine cY-Alanine

H CH,

4.7 x 1022 5.8 X 1O22

15 89

25.3 16.6

Serine

CH,OH

6.2 X 1O22

105

5.0

Name

Aminoacetic a-Aminopropionic (DL) 2-Amino-3-hydroxy-propanoic (DL)

Molecular weight (g/mol)

Solubility (g/100 cm3 Hz0 at 25°C)

a Based on bulk properties, excluding the hydrogens.

The silver substrates as prepared by the methods of Sichtermann and Benninghoven [13] in which the polycrystalline foils are immersed in a 2% solution of nitric acid long enough to physically etch the metal surface. The etch is accompanied by the evolution of gas bubbles and a change in surface texture. The substrates are then rinsed with ample amounts of distilled water and immediately immersed (without drying) into the aqueous amino acid solutions. After a few minutes, the substrates are withdrawn from the solution and allowed to air dry by gently waving the foil through air. Samples are mounted for analysis on multi-sample carrousels and placed into an ultra-high vacuum (UHV) chamber which is then evacuated to a base pressure of -2 X lo-’ Torr. The UHV chamber is designed for a multi-analytical approach employing AES, XPS, and SIMS. AES and XPS analyses are performed with a Physical Electronics, Inc. double pass cylindrical mirror analyzer (CMA) and the appropriate electron and X-ray (Al Kol at 1486.6 eV) sources. The core electron binding energies and band intensities are referenced to the Au 4f7,2 band at 83.8 eV [14]. A quadrupole mass analyzer sits at right angles to the CMA and requires the sample to be turned in its direction for SIMS analysis. The primary ion source is of the flood gun type which requires the entire vacuum chamber to the backfilled with the sputter gas (Ar in this experiment). The current of the primary ion beam is measured by a Faraday cup also mounted to the sample carrousel. We have used a static SIMS approach to study these organic overlayers. Here, we reduce the primary ion current for a 1 keV Ar’ ion beam to obtain current densities of 1 to 10 nA/cm’. The primary ion beam has an angle of incidence of 70’ from the target normal. The SIMS instrumentation consists of an Extranuclear, Model 4-1628, quadrupole mass filter and Bessel Box energy prefilter, Model 616-1. The performance characteristics of the prefilter have not been measured; however, for this experiment, the energy prefilter is set’to provide a maximum output signal for a given mass-to-charge (m/z) ratio. The potential settings permit a relatively large bandpass of a few eV.

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The samples are usually studied first by XPS and second by SIMS. Influence of the X-ray photons on the adsorbed species is checked by comparing the SI emission for irradiated and non-irradiated samples. The differences are found to be negligible.

3. Results and discussion 3.1. Formation

of the surface film

Our experimental core electron binding energies (BE’S) for a-amino acid powders of glycine, cY-alanine, and serine are in full agreement with those measured by Clark et al. [15], i.e., C 1s = 286.8 f 0.2 and 288.7 f 0.2 eV, N 1s = 402.0 * 0.1 eV, and 0 IS = 532.4 f 0.1 eV. The assignment and interpretation are consistent with the zwitterionic formulation for solid-state a-amino acids [ 15 ,161. The XPS band intensities are used to differentiate the various stoichiometries of the amino acids. The intensities are measured by the band areas estimated geometrically (+I 5%) and normalized by the appropriate relative photoionization cross sections of Scofield [17]. Since all the bands are within ?80 eV of 1040 eV and the energy dependence of the inelastic mean free path (MFP) is not too large [ 181, within the accuracy of the area estimates, the MFP is treated as a constant. The intensity ratios are in agreement with bulk stoichiometry except for a consistent -10% excess carbon attributed to bulk impurities, surface contamination, and/or measurement inaccuracies. The total XPS signal intensity from the amino acid, i.e., ZCNo, differs between the three acids by 0.90 : 1 .O : 1.05 for glycine, cu-alanine, and serine, respectively and reflects the ratio of their atomic densities 0.8 1 : 1 .O : 1.07. XPS analysis of the Ag foils used as substrates shows the presence of C and 0 in addition to Ag. The Ag 3ds12 XPS band occurs at 368.2 eV, indicative of elemental Ag [19]. If the foils are dipped in Hz0 prior to analysis, two 0 1s bands appear at -531 and -533 eV which correspond to either metal oxide or adsorbed OH and adsorbed HzO, respectively. The Ag PE bands and an independent Auger study did not indicate bulk oxide formation although it is possible that a very thin surface oxide layer escaped detection, especially since the chemical shifts of Ag are known to be small [ 191. Two C Is bands also occur at -284 and -289 eV. They are attributed to residual contaminates, such as hydrocarbons and some oxygen containing species, respectively. The amino acids are deposited onto Ag substrates that have been chemically etched or physically wiped by a metal brush. The BE’s for the Ag 3d5,2 bands are unchanged by the deposition. The N 1s band is used as an indicator for the adsorbed amino acid since other C and 0 containing species are observed on the Ag surface prior to deposition. The general trend shows that as the acid concentration in the solution decreases, the overlayer signal ZCN~ decreases (table 2). The N 1s BE’s occur at 401.7 + 0.2 eV with the amino acid on etched Ag, but at 402.0 + 0.2

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

239

Table 2 Relative peak intensities and estimated film thickness (d/h) for some amino acids deposited from aqueous solutions onto Ag substrates Molar concentration

Sample

Ag 3d,, ‘Ag

N 1s ZN

ICNO

ZA~IPA~ dlh

Glycine/Ag etched

1x 2x 4 x 8x

10-l 10-2 10-3 lo4

7.9 8.0 8.2 9.9

1.2 1.1 0.4 c

7.9 6.9 4.0 2.8

0.72 0.73 0.75 0.90

0.24 0.23 0.21 0.08

wAlanine/Ag etched

2 x 10-2 4 x 10-3 Hz0 only a

7.3 9.4 9.6

1.3 0.2 c

8.5 5.5 4.1

0.63 0.82 0.83

0.34 0.14 0.13

1.6

1.0

0.0

Ion sputtered b

11.5

c

Serine/Ag etched

1x 2x 4x 8x Ion

10-l 10” 10-3 lo4 sputtered b

5.1 7.3 9.5 12.1 12.4

2.6 2.2 0.3 c ’

21.3 17.4 9.6 6.0 0.0

0.36 0.52 0.68 0.86 0.89

0.74 0.47 0.28 0.11 0.08

or-Alanine/Ag brushed

1 x 10-l 2 x 10” 4 x 10-3 8x104 8x lo+ Hz0 only a Ion sputtered b

3.1 5.4 10.0 13.5 14.3 15.0 16.8

5.2 3.9 2.6 c c c c

37.1 30.9 22.2 13.4 11.1 11.8 3.1

0.17 0.29 0.54 0.73 0.77 0.81 0.91

1.29 0.90 0.45 0.23 0.19 (x15 0.07

a Substrate dipped in Hz0 only. b Organic covered substrated sputtered by 1 keV Ar+ ions. c Signal below measurable level.

eV when the acid is on brushed Ag, The stoichiometry of the overlayer indicates an excess of carbon and oxygen above the acid bulk composition, probably due to substrate and/or solution contamination. Expressions developed for the determination of the electron mean-free-path [ 181, h, and the XPS signal intensity [20] can be used to show for a sample in which X is constant and the atomic density is invariant, the measured intensities for the overlayer and substrate, once corrected for cross section and collection efticiency, can be expressed as IAglz:g

= (1 -

zCNO/z:NO)

.

The P terms represent intensities from the pure substrate or an infinitely thick overlayer. Plotting ZAg versus Z~NO defines a straight line as shown in fig. 1 (data from table 2) whose intercepts are pAg and Z&O. The relative slopes of the lines for

P SERINE IAg ETCHED

14.0

II.0 II.5

18.5

46.0 23.5 27.0 35.0

ItNO (I&J) (39.5)

of the lines gives values for PAP and PCNo

/Ag ETCHED

ETCHED

a -ALANINE

/Ag BRUSHED

a -ALANINE GLYCINE /Ag

I& 18.5

,--

Ag BRUSHED

,

SAMPLE

r

Fig. 1. Plot of the XPS intensities I_& versus ICNO for the various surface films on Ag. Extrapolation which are characteristic of the pure substances.

2 IO H 8

12

14

16

18

20

$ c1

R. J. Colton et al. / XPS and SIMS study of amino acid overlayers

241

Fig. 2. SEM micrographs of Ag foils that have been brushed (left column) and acid etched (right column). The micrographs show magnifications of 5000 X (top) and 20,000 X (bottom). The scale insert (20,000 X only) is equivalent to 1 pm.

the amino acids on etched Ag are constrained to reflect the observed ZC~o values for the bulk acids. All intensities (table 2) are normalized to the signal intensity of a Au 4f,,, band measured on the same day to compensate for instrumental fluctuations that occurred during the several months of data collection. Differences in the Z”s values (fig. 1) are attributed to the effects of the acid etch. Comparison of acid etched and non-etched surfaces of Ag foil is shown by the SEM micrographs in fig. 2. The Ag foils are initially prepared by brushing the surface with a wire brush (result - fig. 2, left column) to remove the oxide and contamination layers and secondly by nitric acid etching (result - fig. 2, right column). Most features caused by the acid etch are between 0.5 and 2.0 pm in size as indicated by the scale. Since the photoelectrons are collected by the CMA at an angle of -43’ from the CMA axis and the grazing incidence of the X-rays gives rise to a severe shadwomg effect, only those photoelectrons ejected from the plateau (top) region of the surface are collected. Acid etched samples therefore exhibit a smaller “XPS surface area” than non-etched samples. The relative surface areas are manifested in the Z& values of fig. 1 where they are the same for the brushed Ag foil with or

242

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

without an overlayer, but are less for the etched foils. The etched foil appears to have lost between 25 to 40% of its plateau area; the variance attributed to differences in etching time. The ZAs values are used to monitor film thickness [ 181 since Z = I0 exp(-d/h

cos 0) ,

(2)

where Z is the observed signal, Ze is the signal from the pure substance, d is an average film thickness, h is the inelastic electron MFP, and B is the CMA collection angle (0 -43’). The relation cos 0 ln(Ze/Z) gives the d/h values listed in table 2 (right column). For d/h Q 1 the measured value ofZ/Ze reflects a value of d averaged over the sampled area. For very thick islands interspersed on an otherwise clean surface, the Z/Ze value reflects the area covered by the islands. In either case a diminishing Z/Ze ratio (increasing d/h) reflects a decreased probability for the amino acid to be in direct contact with the Ag. The average film thickness d can be determined by stipulating a value for X. Recent measurement of h for organics with densities which are similar to those of the amino acids vary by a factor of 2 or more [2 1,221; for 1 keV electrons the values are in the range -20-40 A.

1.0 0.9 -

0.8 -

0.70.8-

na.ALANlNE/Ag BRUSHED . GLYClNElAgETCHED n cu.ALANlNElAgETCHED OSERlNElAg ETCHED

0.50.408 % cc"0.3-

0.2-

0.1

0.2

0.3

I i 1111 0.4 0.5 0.60.70.80.91.0

IncJllOns Fig. 3. Plot of log(RA /RI,) versus log(lA /PAP) for the various surface films on Ag. Ike straight line is plotted dor oi = 2.3 after the re ! ationships of Penn [23]. Island formation at high solution concentrations causes the data points to deviate from a straight line.

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

243

The existence of islands can be checked experimentally by observing the variation in the intensity ratio R = M sN 4,5 N4,5 (AES)/3ds,a (XPS) for Ag as a function of overlayer coverage. Following the assumptions of eq. (l), but for uniform film thickness, it can be shown that

where R” is the AES/XPS ratio for pure Ag and (Y= hXPS/X~~~ is the ratio of the electron MFP’s. Plotting log(R/RO) versus log(Z/Ze) gives a straight line whose slope is equal to 01 - 1. Values of (Yrange from 1.8 assuming a ,!?‘I* dependence for h to 2.3 after the relationships of Penn [23]. For uniform film growth, the XPS R,Q and Z*s signals will monotonically decrease in a fashion related to Q. Our data at lower coverages are consistent with the Penn value of CL If islands grow and leave relatively uncovered areas on the surface, then the Ag signal under the islands is lost altogether and the ratio will reflect only the uncovered areas where the overlayer is thin. Hence, island formation for higher solution concentrations would cause the R/R0 in fig. 3 to approach a limiting value for the relatively uncovered area. The data in fig. 3, although scattered, do show this trend with the thicker overlayers. 3.2. SIMS fragmentation pattern The SIMS spectra for the alipathic o-amino acids, glycine, cw-alanine, and serine, are shown in fig. 4. The relevant data are represented by stick mass spectra where the log (peak intensity) in counts per second (cps) is plotted against mass in amu. The general features of the mass spectra include the intense isotopic doublet of Ag at m/z 107 and 109 and a second intense high mass doublet at m/z 182 and 184 for glycine, 196 and 198 for cr-alanine, and presumably 212 and 214 for serine (data was not obtained beyond 200 amu). These high mass doublets are assigned to the corresponding silver-amino acid complex ions, i.e., (AgM)’ where M is the molecular weight of the parent amino acid. Also present in the high mass region is a number of weaker doublets (most noticeable in the serine spectrum) which have similar intensity ratios as the Ag doublet. Several other intense peaks occur at various masses below the Ag isotopes. Mass assignments are outlined in table 3. The only parent-like species found are those attached to Ag or H. A similar observation involving alkali cation stabilized parents has also been made for organics desorbed by a laser induced ionization technique [I l] and by SIMS [9,10]. Complex formation may occur in the gas phase as postulated by the model of Garrison, Winograd, and Harrison [24] or by a surface adsorption complex [25-271. It is not possible to distinguish between these two models on the basis of our data. Our observations show (fig. 5) that the secondary ion (SI) emission of Ag’ is constant at lower coverages and that the organic fragment intensities of (AgM)‘, Ag(M - 45)‘, and (M - 45)’ are also constant (table 4), in spite of the observed

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

GLYClNE

I pig ETCHED I

107

CH2

b+

I

-t-

COOH I

,310 82

I

L :”

164

i

42

L ‘0

40

_

60

Fig. 4. Positive secondary ion mass spectra of glycine, Lu-alanine, and serine deposited from aqueous solution onto acid etched Ag. The spectra are obtained by a 1 keV Ar+ion beam with a current density of 2 X lo-* A/cm2.

245

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

Table 3 Secondary ions observed in common for the amino acids on etched Ag Ion

(&M)+ Ag(M - 18)+ Ag(M - 31)+ Ag(M - 46)+ (Ag + 17)+ Ag+ (M + H)+ (M - 45)+

Observed m/z

Postulated species loss (gain)

Glycine

wAlanine

HaO, NH; CH3NHz HCOOH (OH) M Ag, (H’) A&O2 H

182,184 164,166 151,153 136,138 124,126 107,109 76 30

196,198 (179,181) 165,167 150,152 124,126 107,109 90 44

Serine (212,214) 194,196 (182,184) 166,168 124,126 107,109 106 60

b

a c

a Not observed; mass spectrum recorded to 200 amu only. b M - 17 only; possibly OH. c M - 30 only.

\ ?

A a -ALANINE/Ag

BRUSHED

l

GLYClNE/Ag

ETCHED

n

a - ALANlNE/Ag

0

SERINE /Ag

ETCHED

\

ETCHED

A\

‘0.0

0.02

0.04 XPS

0.06

0.08

0.10

INTENSITY

RATIO

IN / I&

0 12

0 14

Fig. 5. Plot of SI emission of Ag+ (I&+) versus the XPS signal intensity of the N 1s band (IN); each normalized to compensate for differing sample areas and instrument gains. The XPS intensity ratio is proportional to the overlayer coverage.

246

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

Table 4 Relative mass peak intensities a for some of the commonly observed secondary ions of amino acids on etched Ag as a function of solution concentration Sample

Ion

m/z

Glycine

(M - 45)+ (M + 1)’ Ag+ Ag(M - 45)+ (AgM)+

30 76 107 137 182

cwklanine

(M - 45)+ (M + l)+ Ag+ Ag(M - 45)+ (AgM)+

44 90 107 151 196

Serine

(M - 75)+ (M - 45)+ (M + l)+ Ag+ Ag(M - 45)+

30 60 106 107 167

Solution concentration 1 x 10-l M

2 X 10” M

4 x 1O-3 M

8 x104M

5.3 0.14 1.0 0.03 0.21

5.1 0.08 1.0 0.03 0.21

4.7 0.03 1.0 0.04 0.18

4.8 0.03 1.0 0.03 0.17

3.9 0.08 1.0 0.04 0.12

3.6 0.05 1.0 0.04 0.09

1.2 2.1 0.30 1.0 0.09

1.3 1.8 0.21 1.0 0.09

1.4 1.9 0.45 1.0 0.10

1.3 1.1 0.18 1.0 0.03

a SIMS intensities relative to m/z 107 (Ag+).

1,/&o variation from 0.0 (50.005) to 0.05. The only exception is the diminished (M - 45)’ and Ag(M - 45)’ of serine for the lowest solution concentration; this may be due to an absence of adsorbed serine for this low concentration. The observations of the SIMS organic fragments for coverages where XPS intensities were not observed indicates the higher sensitivity of the SIMS technique. The invariance of the SIMS yield of (AgM)’ and Ag(M - 45)’ in spite of a considerable change in the amount of adsorbed organic suggests that an active site may play a role in the organic ion emission and that the adsorption on this site can be saturated by a -10-j M solution. At higher coverages the ion emissions associated with Ag begin to drop rapidly when the R value suggests the formation of islands. The (M + 1)’ molecular ion, on the other hand, behaves in a different fashion. In absolute terms, the (M + 1)’ intensity is roughly constant (or slightly increasing) as a function of concentration. This observation suggests that this ion comes from a different origin than do the Ag related species, probably related to the availability of H or H’. Benninghoven and Sichtermann [7] have observed a strong dependence of the (M + 1)’ and (M - l)- SI emission on solution pH. The (M - 45)’ fragment ion cannot be directly identified as associated with either H or Ag stabilization. It corresponds to the amine fragment ion RCH+; I

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

241

these fragment ions also appear in the EI mass spectra of o-amino acid derivatives. In the EI mass spectrum, the amine fragment ion at m/z 60 for serine undergoes further rearrangement to form a fragment ion at m/z 42 by the elimination of Hz0 --Hz0

CH,-CH’ AH

NH2

-

CH2 = 7’ .

(4)

NH2

We also observe this mass peak in the SIMS spectrum. This observation raises the possibilitiy that other features in the SIMS fragmentation pattern of a-amino acids can be interpreted in a fashion similar to corresponding EI mass spectra where a complex (AgM)’ or (HM)’ molecular ion would fragment by loss of neutral species. 3.3. Effects of the acid etch Comparison of the XPS measurements on brushed and etched Ag substrates from table 2 before amino acid deposition (i.e., exposed to Hz0 only) showed that the “cleanest” surface, i.e., lowest C and 0 signal, is produced by ion sputtering, followed secondly by acid etching, and then by brushing. As mentioned in section 3.1.) the acid etched surface shows an appreciable decrease (between 25 and 40%) in the XPS signal due to a decrease in the measurable surface area. Even if the intensities of C and 0 for the etched substrates are increased by 40%, the new values are still considerably lower than the corresponding data for brushed Ag. Furthermore, there is no XI’S evidence of chemical change of the surface caused by the deposition which would substantiate an “oxide effect” as the source for enhanced organic SI emission. Rather, we believe that the surface porosity is a major factor governing the organic SI emission. When the metal foil is retracted from the aqueous solution, a thin film of solution is retained. As it evaporates, the solute molecules concentrate and eventually precipitate. For brushed samples, islands nucleate and grow, resulting in a thick film which stifles ion emission. For etched samples, the solute concentrates in the solution as it recedes into the pores. When precipitation occurs, islands form in the pores and are not observed because of shadowing. The outer surface is left with a thin layer of adsorbed material yielding good emission. For a given molar concentration, the variance in coverage for serine solutions compared to glycine solutions (table 2) is probably due to two factors: the lower solubility of serine (table 1) which thereby precipitates earlier and the lesser porosity of the Ag surfaces (fig. 1) for the serine samples. Notice that at the highest serine concentration the etched sample is beginning to show a decrease in SI emission and the R ratio suggests island formation on the observable surfaces. If this model for the effects of etching on ion emission is correct, the use of lower concentrations on smooth surfaces should have larger SI emissions. In fact, the SI emission from cu-alanine on brushed Ag does increase as the solution is diluted from 10-r to lo4 molar. The value of the etched (porous) surface is that it provides for an extended range of solution concentrations without degrading the

248

R.J. Colton et al. / XPS and SIMS study of amino acid overlayers

SIMS signal intensity. and for materials tions.

which

This would be important form

an adsorbed

for materials

of unknown

solubility

film only from more concentrated

solu-

References [l] [2] [3] [4] [S] (61 [7] [S]

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