ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2011, Vol. 47, No. 6, pp. 756–761. © Pleiades Publishing, Ltd., 2011. Original Russian Text © I.S. Pytskii, A.K. Buryak, 2011, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2011, Vol. 47, No. 6, pp. 630–635.
SEPARATION PROCESSES AT INTERFACES: CHROMATOGRAPHY
MALDI/SELDI MassSpectrometric Surface Investigation of AMg6 and Ad0 Materials I. S. Pytskii and A. K. Buryak Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia email: chrom
[email protected] Received April 20, 2011
Abstract—Matrixassisted and surfaceenhanced laser desorption/ionization massspectrometry is carried out in order to investigate the surfaces of structural materials using marker substances. The method is shown to provide estimates of the surface heterogeneity and can be used for the quantitative comparison of the het erogeneity of different surfaces. In the domains where homogeneity is distorted, a larger amount of the marker substance is observed. Properly selected marker substances and the high sensitivity of massspectrom etry make it possible to use the method for the express estimation of the surface heterogeneity. DOI: 10.1134/S2070205111060165
INTRODUCTION There are numerous physical, chemical, and physi cochemical methods of the surface analysis [1–8]. In particular, when studying the surface states of materials made of Ad0 and AMg6 alloys, optical and electron microscopy is used. The methods enable one to visually control the state of certain surface sites, determine the microscale morphology of the surface, and carry out a microprobe analysis of the elemental composition of the surface with an accuracy of 0.1 wt % [6]. Using the matrixassisted or surfaceenhanced laser desorption/ionization (MALDI or SELDI) massspec trometry, one can construct 3D diagrams of the marker substance distribution over the surface, which makes it possible to judge the surface state and compare different surfaces. To our knowledge, there is no work devoted to the construction of 3D diagrams of the substance distri bution over the surface of a structural material, though the method seems to be promising as an express tech nique that provides estimates of the surface state upon various treatments, including those under extreme con ditions. EXPERIMENTAL Equipment In order to develop a technique of surface probing with marker substances, we used Bruker Daltonics Ultraflex II matrixassisted laser desorption/ionization timeofflight massspectrometer (Bruker, Germany) equipped with a nitrogen laser. Massspectrometers of this kind are widely used to study highmolecular [9, 10] and labile inorganic compounds [11, 12]. To record and
process mass spectra, Flex Control 2.2 and Flex Analy sis 2.2 programs developed by Bruker were used. Stan dard 384 spot stainlesssteel sample targets (Bruker, Germany) were used as the reference surfaces. Experi ments were carried out in either the positive or negative ion mode. Linear and reflectron modes of ion recording were used [13]. An accelerating voltage was 25 kV. The main parameters of the device operation are listed in Table 1. Measurements were carried out in the mass range of 20–3600 Da. The instrument was calibrated using Pep tide calibration standard II for mass spectrometry pro duced by Bruker Daltonik GmbH. For the more accu rate calibration in the working range, an aqueous sus pension of silver iodide with a silver content of 10–9 mol/l was taken. Calibration was carried out with respect to peaks corresponding to the cluster ions of the known composition. The cluster ions and the peak weights used in calibration are listed in Table 2. The cal ibration is necessary for processing mass spectra of con taminated specimens that contain a large amount of admixture ions, which prevent one from distinguishing the peaks of the marker substance ions. Pretreatment of Specimens When carrying out experiments with marker sub stances, the pretreatment of specimens described in [14] was used. Specimens of AMg6 and Ad0 alloys were taken. Different parts of the specimens upon their contact with a fuel were studied [15], while the alloys contacted with nitrogen served as the reference. Parts with sizes of 1 × 1 cm cut out of tanks were used [16]. A
756
MALDI/SELDI MASSSPECTROMETRIC SURFACE INVESTIGATION
total of eight specimens of AMg6 and Ad0 alloys were studied after contact with fuel components and nitro gen. The fragments were mounted on the surface of the specially pretreated target and marker substances were applied on them from solutions or suspensions. In a number of experiments, a matrix substance, i.e., ditra nol and αcyanohydroxycinnamic acid, was added to the solutions or suspensions of marker substances. A MALDI/SELDI investigation of the materials was car ried out by scanning the surface with laser pulses and recording mass spectra at each point. The amount of the marker substance at the point was estimated from the peak intensity. The surface was scanned in steps of 100 to 1000 µm and diagrams of the substance distribution over the surface were constructed.
757
Table 1. Main characteristics of laser unit of Bruker Dal tonics Ultraflex II MALDI TOF mass spectrometer Parameter, units Working wavelength, nm
337.1
Spectral bandwidth, nm
0.1
Full width at half maximum pulse, ns
3
Pulse energy, µJ
110
Pulse power, kW
43
Repetition rate, Hz
50
Power generation stability, %
2
Angular spread of laser beam, mrad
3
RESULTS AND DISCUSSION Selecting Marker Substances for Studying the Surface Morphology Substances that can be applied to the surface from nonpolar or weakly polar solvents, which provides the uniform distribution of the reagent over the surface, and that can be ionized on the specimen surface (in the absence of the matrix substance as well) and give rise to reproducible peaks of cluster or molecular ions were selected as markers. As was shown in [14, 17], silver halides can form a broad set of cluster ions, which make it possible to determine the relative content of the sub stance on the surface. Cluster ions can also be formed at the ionization of nickel chloride, which was also taken as a marker substance in the investigations. Marker sub stances that were selected based on the preliminary model experiments, then used in the surface analysis of the specimens are listed in Table 3. Due to the presence of ions with a large weighttocharge ratio (300– 800 Da), all of the considered silver halides were shown to be applicable as marker substances in MALDI/SELDI experiments. Studying the Qualitative Composition of the Mass Spectra of Marker Substances under MALDI/SELDI Conditions The correspondence between the experimental and calculated mass spectra was checked based on the sta tistical Student criterion [14, 18] using empirical for mulas of ions and the sets of ion peaks in the actual mass spectra. The qualitative composition of the ionized spe cies judged from the main peaks of the mass spectra of different marker substances is given in Table 4. As can be seen, silver halides form ions of Ag+[AgHal]n and [AgHal]nI– composition. The forma tion of these ions indicate the successive gradual decomposition of large halide clusters. Besides that, sig
Table 2. Cluster ions used to calibrate Bruker Daltonics Ultraflex II MALDI/SELDI mass spectrometer in a weight range of 20–3600 Da Positiveionregistration mode
Negativeionregistration mode
calibration ion
calibration ion
calibration weight
calibration weight
Ag+
106.90
I–
Ag2I+
342.71
AgI 2
Ag 4I 2
+
576.52
Ag 2I 3
−
596.52
Ag 4I 3
+
812.33
Ag 3I 4
−
830.33
+
1046.14
Ag 4I 5
−
1066.14
Ag 5I 4
126.91 −
360.71
Table 3. Marker substances used in testing actual surfaces and accurate weights of ions in the mass spectra
Marker substance
Peak m/z value in determining the relative surface content of the substance, Da positiveion mode negativeion mode
Silver chloride
394.65 538.52
322.71 466.59
Silver bromide
482.55 670.37
454.56 642.39
Silver iodide
576.52 812.33
596.52 830.33
not used
129.87 164.84
Nickel chloride
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 6 2011
758
PYTSKII, BURYAK
Table 4. Empirical formulas of the main ions detected in mass spectra of the marker substances
Probable empirical Marker sub formulas of positive stance ions
Silver chloride
+
−
Ag2Cl+; Ag 3Cl 2 ; +
Probable empirical formulas of negative ions
−
AgCl 2 ; Ag 2Cl 3 ; −
+
−
Ag 3Cl 4 ; Ag 4Cl 5 ;
Ag 4Cl 3 ; Ag 5Cl 4 ; +
Ag 6Cl 5 ;
Silver bromide
Silver iodide
Nickel chloride
−
Ag2Br+;
+
−
AgBr2 ; Ag 2Br3 ;
+ + Ag 3Br2 ; Ag 4Br3 ; + + Ag 5Br4 ; Ag 6Br5 ;
− Ag 3Br4 ; − Ag 5Br6 ;
+
−
Ag 4Br5 ; −
Ag 6Br7 ;
−
−
−
Ag2I+; Ag 3I 2 ; Ag 4I 3 ; AgI 2 ; Ag 2I 3 ; Ag 3I 4 ; −
+
−
Ag 5I 4 ;
Ag 4I 5 ; Ag 5I 6 ;
no spectrum
NiCl 2 ; NiCl 3 ;
−
−
−
NiKCl 3 ; −
NiKCl *3 H 2O ; −
−
Ni 2Cl 4 ; Ni 2Cl 5
nals of the adducts formed by the clusters and potassium or sodium ions (K+[KBr] or Na+[AgBr]), salts ([AgCl•KCl]Cl– and [Ag2I2•NaI]I–), and water ([Ag5•H2O] –and Ag+[AgI•H2O]) were also found in the mass spectra. In the mass spectra of nickel chloride, peaks that correspond to cluster ions involving water and alkaline metals were also present. 3D diagrams of the distribu tion of all the considered substances over the surfaces of a number of specimens were constructed and studied. As was shown, silver halides are most suitable for this kind of surface tests. In the studies, mainly silver bro mide was used because the mass spectra of the com pound involve highintensity signals of its highmolec ular cluster ions in the isotopic distribution.
Studying Surfaces of Alloys Using Marker Substances The main difference between the mass spectra of marker substances applied to the surfaces of particular materials compared to those on a standard substrate is the substantially lower intensity of peaks because of the stronger sorption of the marker on the developed sur face and adsorption sites. These processes do not ham per the analysis of the surface morphology of actual specimens because the mass spectrometry is a very highly sensitive technique. Figure 1a shows a pseudo threedimensional diagram of the distribution of silver bromide over the AMg6 surface, which had no contact with a fuel, while Fig. 1b shows a twodimensional dia gram of the surface distribution of the substance. A sim ilar investigation was carried out for Ad0 alloys and the corresponding diagram and distribution histogram of the marker over the surface are given in Figs. 2a and 2b, respectively. This uniform distribution indicates the homogeneous surface state of the material. In the case of the alloy specimens in contact with the fuel, the diagrams look quite different. As follows from Fig. 3, the marker is not uniformly distributed over the surface, which means that the surface is noticeably het erogeneous. In the righthand part of the surface, one can see several sites where the marker is accumulated. This fact can be explained either by the formation of the sorption sites where the marker is predominantly adsorbed or the formation of cracks, pits, and hollows on the surface under the effect of aggressive fuel com ponents during the operation. The difference between the value obtained and the k factor of the surface, which had no contact with fuel, is statistically meaningful. Figure 4 shows a diagram of the Ad0 surface, which was in contact with fuel. As one can see, there are also surface parts where the marker concentration is higher, but, compared to the AMg6 surface, their total area is larger and their distribution on the surface is broader, which may mean that the surface of this material was damaged to a stronger degree during the operation. The results obtained show that the method is effi cient with regard to estimating the surface state. As fol lows from the diagrams, domains with the increased concentration of the marker substance are observed only on the surfaces, which were in contact with an aggressive environment. This fact confirms a hypothesis that these domains appear due to the adsorption of the marker on the surface sites, which were damaged by the fuel components during operation. The method considered enables one to carry out an express comparison of the surface states of alloys in a relatively broad range. The macroscale domain of 10 × 10 mm is fairly representative for studying the chemistry and morphology of the surface, which makes it possible
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 6 2011
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 6 2011
1 2 3 4 0 Surface coverage in fivepoint scale
(b)
5
1.9
3.8
5.7
7.6 Surface step, mm
Fig. 1. (a) Pseudo3D and (b) 2D distribution diagrams of silver bromide on the surface of AMg6 material, which was in contact with nitrogen. Mass spectra were recorded in the positive ion mode. The amount of the marker substance was estimated from the most intense peak at m/z = 482.55 Da corresponding to clus ter ions. The scanning step was 1.1 mm.
10
20
30
40
50
60
70
80
0.55 1.65 2.75 3.85 4.95 6.05 7.15 8.25 9.35 9.3510.4511.55 Surface step, mm
Surface part, %
9.5
(а)
0
1 2 3 4 Surface coverage in fivepoint scale
(b)
1.9
3.8
5.7
7.6 Surface step, mm
5
Intensity, arb. units
0–500
500–1000
Fig. 2. (a) Pseudo3D and (b) 2D distribution diagrams of silver bromide on the surface of Ad0 material, which was in contact with nitrogen. Mass spec tra were recorded in the positive ion mode. The amount of the marker sub stance was estimated from the most intense peak at m/z = 482.55 Da corre sponding to cluster ions. The scanning step was 1.1 mm.
10
20
30
40
50
60
70
80
90
100
Surface step, mm
0.55 1.65 2.75 3.85 4.95 6.05 7.15 8.25 9.35 10.4511.55
Surface part, %
(а)
MALDI/SELDI MASSSPECTROMETRIC SURFACE INVESTIGATION 759
Surface part, %
0
1 2 3 4 5 Surface coverage in fivepoint scale
(b)
10.45 9.50 8.55 7.60 6.65 5.70 4.75 3.80 2.85 1.90 0.90 Surface step, mm Intensity, arb. units.
1500–2000 1000–1500 500–1000 0–500
Fig. 3. (a) Pseudo3D and (b) 2D distribution diagrams of silver bromide on the surface of AMg6 alloy, which was in contact with fuel. Mass spectra were recorded in the positive ion mode. The amount of the marker substance was estimated from the most intense peak at m/z = 482.55 Da corresponding to cluster ions. The scanning step was 1.1 mm.
5
10
15
20
25
30
35
40
45
0.55 1.65 2.75 3.85 4.95 6.05 7.15 8.25 9.35 10.4511.55 Surface step, mm
(а)
0
1 2 3 4 Surface coverage in fivepoint scale
(b)
5
1.9
3.8
5.7
7.6
9.5
Surface step, mm
Intensity, arb. units.
1500–2000 1000–1500 500–1000 0–500
Fig. 4. (a) Pseudo3D and (b) 2D distribution diagrams of silver bromide on the surface of Ad0 alloy, which was in contact with fuel. Mass spectra were recorded in the positive ion mode. The amount of the marker substance was estimated from the most intense peak at m/z = 482.55 Da corresponding to cluster ions. The scanning step was 1.1 mm.
5
10
15
20
25
30
35
40
0.55 1.65 2.75 3.85 4.95 6.05 7.15 8.25 9.35 10.4511.55 Surface step, mm
Surface part, %
760 PYTSKII, BURYAK
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 6 2011
MALDI/SELDI MASSSPECTROMETRIC SURFACE INVESTIGATION
to apply the method proposed in combination with optical and electron microscopy. CONCLUSIONS The studies show that the massspectrometric scan ning of the surface using marker substances can provide valuable information about the surface state. Compar ing the distribution diagrams of the marker over the sur faces of specimens that were either brought into contact with an aggressive environment or not revealed the dif ferent distribution of the marker substance, which may mean that the surface homogeneity is broken during the operation of the material. A criterion of the uniform coverage of the surface with the marker substance (k) is proposed as the basis for comparing the surface states. ACKNOWLEDGMENTS The work was financially supported by the State sup port program of Leading Scientific Schools (project no. NSh7853.2010.3) and P 08 Program of the Presid ium of the Russian Academy of Sciences. REFERENCES 1. Whiston, C. and Prichard, F.E., XRay Methods, New York: Chichester, 1987. 2. Shaw, S.H. and Jossem, E.L., XRay Spectroscopy, Ohio: Belvoir Defense Technical Information Center, 1964. 3. Bahr, J.L., Blake, A.J., Carver, J.H., and Kumar, V., Photoelectron Spectroscopy for Some Autoionized States of Molecular Oxygen, Adelaide: Belvoir Defense Technical Information Center, 1969.
761
4. Flanagan, D.J., The Angular Resolved Photoelectron Spectroscopy of Various Polyatomic Molecular Systems, Los Angeles: Pasadena, 1985. 5. Wilson, C.L. and Wilson, D.W., Wilson and Wilson’S Comprehensive Analytical Chemistry, London: Elsevier, 1979, vol. 9. 6. Yamanaka, C.R. and Li, Zh.R., Industrial Applications of Electron Microscopy, Wilmington, USA, 2003. 7. Buyuksagis, A., Aksut, A.A., and Ozal, D., Prot. Met. Phys. Chem. Surf., 2009, vol. 45, no. 3, p. 342. 8. Zor, S. and Ozkazanc, H., Prot. Met. Phys. Chem. Surf., 2010, vol. 46, no. 6, p. 727. 9. Gies, A.P., Vergne, M.J., Orndorff, R.L., and Hercules, D.M., Macromolecules, 2007, vol. 40, no. 21, p. 7493. 10. Willemse, R.X.E. and Van Herk, A.M., J. Am. Chem. Soc., 2006, vol. 128, no. 13, p. 4471. 11. Kawasaki, H., Uota, M., Yoshimura, T., et al., Lang muir, 2007, vol. 23, no. 23, p. 11540. 12. Schnöckel, H., Chem. Rev., 2010, vol. 110, no. 7, p. 4125. 13. Mamyrin, B.A., Karataev, V.I., Shmikk, D.V., and Zagulin, V.A., Sov. Phys. JETP, 1973, vol. 37, p. 45. 14. Pytskii, I.S., Buryak, A.K., and Revel’skii, I.A., Mass Spektrometriya, 2010, vol. 7, no. 1, p. 35. 15. Pytskii, I.S. and Buryak, A.K., Fizikokhim. Poverkhn. Zashch. Mater., 2011, vol. 47, no. 1, p. 100 [Prot. Met. Phys. Chem. Surf. (Engl. Transl.), 2011, vol. 47, no. 1, p. 133]. 16. Gulidov, G.Ya., Resh, G.F., Kurbakov, A.A., et al., Pamyati B.V. Gidaspova, (In memory of B.V. Gidaspov), 2008, p. 92. 17. Pytskii, I.S., Buryak, A.K., Sukhorukov, D.O., and Revel’skii, I.A., Sorbts. Khromatograf. Prots., 2009, vol. 9, no. 6, p. 789. 18. Garmash, A.V., Metrologicheskie osnovy analiticheskoi khimii (Metrological Principles of Analytical Chemis try), Moscow, 1999.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 6 2011
