Systematization of the Mass Spectra for Speciation of Inorganic Salts

The analytical use of mass spectra from static secondary ion mass spectrometry for the molecular identification of inorganic analytes in real life sur...
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Anal. Chem. 2004, 76, 2609-2617

Systematization of the Mass Spectra for Speciation of Inorganic Salts with Static Secondary Ion Mass Spectrometry Rita Van Ham,† Luc Van Vaeck,*,† Freddy C. Adams,† and Annemie Adriaens‡

Department of Chemistry, University of Antwerp, Universiteitsplein 1, B 2610 Wilrijk, Belgium, and Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, B 9000 Ghent, Belgium

The analytical use of mass spectra from static secondary ion mass spectrometry for the molecular identification of inorganic analytes in real life surface layers and microobjects requires an empirical insight in the signals to be expected from a given compound. A comprehensive database comprising over 50 salts has been assembled to complement prior data on oxides. The present study allows the systematic trends in the relationship between the detected signals and molecular composition of the analyte to be delineated. The mass spectra provide diagnostic information by means of atomic ions, structural fragments, molecular ions, and adduct ions of the analyte neutrals. The prediction of mass spectra from a given analyte must account for the charge state of the ions in the salt, the formation of oxide-type neutrals from oxy salts, and the occurrence of oxidation-reduction processes. The development of new materials increasingly exploits chemical engineering techniques, allowing the composition of different phases (monomolecular surface layers or inclusions, sometimes as small as 10-100 nm) to be precisely tailored as a function of the applications. Chemical engineering exploits the physicochemical interactions between molecules, and therefore, elemental analysis is no longer sufficient. Apart from material research applications, the chemical composition of microobjects is of importance to, for instance. environmental studies. Assessment of the toxicological effects from inhaled aerosols and insight in the chemistry of particle formation and subsequent transformations during atmospheric transport make characterization of the molecular composition a prerequisite.1 Hence, new methodologies must be developed for molecular identification and eventually quantification of both inorganic and organic components in solids with high depth or lateral resolution. At this moment, two mass spectrometric methods have significant potential in this respect. Laser microprobe mass spectrometry (LMMS) uses a focused UV laser to generate molecular adduct * Corresponding author. Tel.: +0032 (0)3/820.23.48. Fax: +0032 (0)3/ 820.23.76. E-mail: [email protected]. † University of Antwerp. ‡ Ghent University. (1) Sipin, M. F.; Guazotti, S. A.; Prather, K. A. Anal. Chem. 2003, 75, 29292940. 10.1021/ac0400156 CCC: $27.50 Published on Web 04/07/2004

© 2004 American Chemical Society

ions and structural fragments from both inorganic and organic constituents in solids.2 Recently developed aerosol time-of-flight mass spectrometers for in-field studies allow the ambient air to be introduced directly and single particles in the (sub)micrometer size range to be ionized and mass analyzed on line.1,3-6 Automated data processing is a prerequisite, but lack of insight in the relationship between the detected ions and sample composition becomes a major bottleneck in the development of suitable algorithms. Alternatively, laser microbeam irradiation has been combined with Fourier transform mass spectrometry to exploit the analytical specificity of high mass resolution in combination with microanalytical sensitivity. The lateral resolution is ∼5 µm and the information depth is at least 10 nm, i.e., 10-100 monolayers.2 In contrast, static secondary ion mass spectrometry (S-SIMS) allows signals to be detected from only the upper monolayer whereas imaging capabilities are also available.7,8 Up to now, the literature contained only discrete studies on a few inorganic analogues achieved with different instruments and experimental conditions, e.g., with respect to the ion dose. For instance, sulfates/sulfites and nitrate/nitrites of sodium or silver have been studied in the so-called low-damage regime.9-11 Nitrates and nitrites have been analyzed with different projectiles such as (CsI)n‚Cs+ and ReO4-.12,13 Several authors have investigated alkali halides14-22 but no analogues with, for example, transition elements. Recent spectrum libraries contain data of ∼30 compounds, (2) Van Vaeck, L.; Struyf, H.; Van Roy, W.; Adams, F. Mass Spectrom. Rev. 1994, 13, 189-208. (3) Noble, C. A.; Prather, K. A. Mass Spectrom. Rev. 2000, 19, 248-274. (4) Johnston, M. V. J. Mass Spectrom. 2000, 35, 585-595. (5) Kane, D. B.; Johnston, M. V. Environ. Sci. Technol. 2000, 34, 4887-1893. (6) Lake, D. A.; Tolocka, M. P.; Johnston, M. V.; Wexler, A. S. Environ. Sci. Technol. 2003, 37, 3268-3274. (7) Van Vaeck, L.; Adriaens, A.; Gijbels, R. Mass Spectrom. Rev. 1999, 18, 1-47. (8) Vickerman, J. C., Briggs, D., Eds. ToF-SIMS: Surface Analysis by Mass Spectrometry; IM Publications: Chichester, U.K., 2001. (9) Marien, J.; De Pauw, E. Bull. Soc. Chim. Belg. 1979, 88, 115-121. (10) Marien, J.; De Pauw, E. Int. J. Mass Spectrom. Ion Phys. 1982, 43, 233247. (11) De Pauw, E.; Marien, J. Int. J. Mass Spectrom. Ion Phys. 1981, 38, 11-19. (12) Van Stipdonk, M. J.; Justus, D. R.; Santiago, V.; Schweikert, E. A. Rapid Commun. Mass Spectrom. 1998, 12, 1639-1643. (13) Groenewold, G. S.; Delmore, J. E.; Olson, J. E.; Appelhans, A. D.; Ingram, J. C.; Dahl, D. A. Int. J. Mass Spectrom. Ion Processes 1997, 163, 185-195. (14) Honda, F.; Lancaster, G. M.; Fukuda, Y.; Rabalais, J. W. J. Chem. Phys. 1978, 69, 4931-4937. (15) Honda, F.; Lancaster, G. M.; Rabalais, J. W. Surf. Sci. 1978, 76, L613-L617. (16) Taylor, J. A.; Rabalais, J. W. Surf. Sci. 1978, 74, 229-236.

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most of which are related to specific technological applications; they are not directly useful to describe the MS behavior of salts in general.23 Therefore, we started a systematic study of inorganic compounds, selected as a function of their anticipated MS behavior. To complement results on oxides,24 the database has been extended with over 50 binary and oxy salts. The purpose of this paper is to survey the main characteristics of the positive and negative ion mass spectra recorded with a state-of-the-art timeof-flight (TOF) S-SIMS instrument. The derived trends in the relationship between the detected signals and sample composition provide a basis to predict the mass spectra of unexpected inorganic compounds in applications. EXPERIMENTAL SECTION Instrumentation. The analyses were performed using an ION TOF IV SIMS instrument (Cameca). The Ga+ liquid metal ion gun was operated at 25-kV beam voltage in the bunched mode with a pulse width of 20 ns for a better than nominal mass resolution up to m/z 1000. An area of 100 × 100 or 300 × 300 µm2 was analyzed during 300 s with a total primary ion dose density below 1013 ions cm-2. Charge compensation by electron flooding was required for most pellets. Sample Preparation. Caution: Several reference products are harmful (harm), irritant (irr), oxidizing (ox), or toxic (tox) (classification according to EU norms). Commercial analytical grade products were used as received. The samples were prepared by pressing pellets. The motivation to use this method was reported elsewhere.25 As the use of Na3PO4‚12H2O pellets prevented the instrument from being pumped down within a reasonable time, aerosol sprayed samples were prepared according to the procedure described before.25 The analyzed compounds, ordered according to the supplier, include CaCl2‚2H2Oirr, CoCl2‚6H2O, SnCl2‚2H2O, BaCl2‚2H2Otox, HgCl2tox, Hg2Cl2tox, InCl3, NH4NO3, NaNO3, KNO3, Ca(NO3)2‚4H2Oox, K2CO3harm, Na2CO3, CaCO3, SrCO3harm, Na2SO4irr, K2SO4, CaSO4‚2H2O, KH2PO4, Ca(H2PO4)2, K2Cr2O7irr (Merck), NH4Cl, NaCl, KClirr, CuCl, MgCl2, CuCl2, SnCl2‚2H2O, CdCl2tox, FeCl3cor, NaBr, KBr, CaBr2‚xH2O, ZnBr2irr, BaBr2‚2H2Oharm, HgBr2tox, FeBr3irr, NaI, KI, FeSO4‚7H2Oharm, NiSO4‚6H2O, Fe2(SO4)3‚5H2Oharm, K3PO4harm, CaHPO4 (Aldrich Chemical Co.), LiClharm, ZnCl2, NaNO2tox, NaH2PO4, Na2HPO4, Na3PO4‚12H2Oirr (Janssen Chimica), K2HPO4, NaBrO3 (UCB), Mg3(PO4)2 (Ventron), Mg(NO3)2‚6H2Oox, (NH4)2SO4 (Fluka), and Pb(NO3)2harm (Acros). (17) Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J. Am. Chem. Soc. 1979, 101, 1951-1958. (18) Barlak, T. M.; Campana, J. E.; Colton, R. J.; Decorpo, J. J.; Wyatt, J. R. J. Phys. Chem. 1981, 85, 3840-3844. (19) Campana, J. E.; Barlak, T. M.; Colton, R. J.; DeCorpo, J. J.; Wyatt, J. R.; Dunlap, B. I. Phys. Rev. Lett. 1981, 47, 1046-1049. (20) Barlak, T. M.; Wyatt, J. R.; Colton, R. J.; DeCorpo, J. J.; Campana, J. E. J. Am. Chem. Soc. 1982, 104, 1212-1215. (21) Barlak, T. M.; Campana, J. E.; Wyatt, J. R.; Colton, R. J. J. Phys. Chem. 1983, 87, 3441-3445. (22) Colton, R. J.; Campana, J. E.; Kidwell, D. A.; Ross, M. M.; Wyatt, J. R. Appl. Surf. Sci. 1985, 21, 168-198. (23) Vickerman, J. C.; Briggs, D.; Henderson, A. The Static SIMS Library; Surface Spectra Ltd.: Manchester, U.K., 1998. (24) Cuynen, E.; Van Vaeck, L.; Van Espen, P. Rapid Commun. Mass Spectrom. 1999, 13, 2287-2301. (25) Van Ham, R.; Adriaens, A.; Van Vaeck, L.; Adams, F. Nucl. Instrum. Methods Phys. Res. B 2000, 161-163, 245-249.

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RESULTS AND DISCUSSION Generally speaking, speciation refers to the detection of analytes by means of more specific information than just the elemental composition. Depending on the analytical methodology used, the oxidation state of an element, the presence of specific bonds, or the entire molecule as a whole is characterized. Molecular speciation specifically refers to the detection of ions that comprise the original analyte molecules in the form of molecular ions (i.e., analyte molecule with loss or addition of an electron) or adduct ions (i.e., analyte molecule attached to a stable ion). Polymeric adducts comprise several neutral molecules. Structural fragment ions are assumed to be formed from molecular or adduct ions. Hence, the bindings between atoms in molecular ions, structural fragments, and neutrals in adduct ions exist in the sample. In contrast, cluster ions consist of atoms not bound to each other in the sample. Their formation is associated with the atomization and subsequent “random” recombination of atoms and ions or electrons. The distinction between the different ion types and the systematic representation of high-m/z ions as combinations of stable neutrals with an ionic species (instead of overall composition) is motivated by a number of formulated concepts about ion formation. Among the numerous approaches in SIMS,26 the energy isomerization concept27 and the desorption-ionization (DI) model2 seem most adequate to rationalize the formation of detected ions. Unlike the precursor approach,28-29 these models stress the initial desorption of molecules or ion pairs as neutrals while electron or adduct ionization occurs as a separate step in the selvedge. Lowm/z ions are generated through subsequent unimolecular decomposition. While the energy isomerization remains rather vague in its physical processes, the DI model specifically denotes fast thermal processes as the driving force. This idea is recently supported by simulations that take into account molecular dynamics and reaction enthalpies along the primary ion trajectory.30,31 Additionally, the DI model stresses the role of the analytedependent local pressure in the selvedge and the time domain of ion formation and mass analysis. It has been proven to provide an adequate basis to rationalize organic and inorganic mass spectra in LMMS, (matrix-assisted) laser desorption MS, and S-SIMS.32-35 (26) Adriaens, A.; Van Ham, R.; Van Vaeck, L. In ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; IM Publications: Chichester, U.K., 2001; p 195-222. (27) Cooks, R. G.; Bush, K. L. Int. J. Mass Spectrom. Ion Phys. 1983, 53, 111124. (28) Benninghoven, A. Molecular Secondary Ion Emission. In Secondary Ion Mass Spectrometry II (SIMS II); Benninghoven, A., Evans, C. A., Jr., Powell, R. A., Shimizu, R., Storms, H. A., Eds.; Springer: Berlin, 1979; pp 116-121. (29) Plog, C.; Wiedmann, L.; Benninghoven, A. Surf. Sci. 1977, 67, 565-580. (30) Krantzmann, K. D.; Postawa, Z.; Garrison, B. J.; Winograd, N.; Stuart, J. S.; Harrison, J. A. Nucl. Instrum. Methods Phys. Res. B 2001, 180, 159-163. (31) Delcorte, A.; Garrison, B. J. Nucl. Instrum. Methods Phys. Res. B 2001, 180, 27-43. (32) Van Vaeck, L.; Van Roy, W.; Gijbels, R.; Adams, F. In Laser ionisation mass spectrometry; Vertes, A., Gijbelsen, R., Adams, F., Eds.; Chemical Analysis Series 124; John Wiley & Sons: New York, 1993; pp 177-319. (33) Struyf, H.; Van Vaeck, L.; Van Grieken, R. Rapid Commun. Mass Spectrom. 1996, 10, 551-561. (34) Koellensperger, G.; Poels, K.; Van Vaeck, L.; Struyf, H.; Van Roy, W.; Grasserbauer, M. Rapid Commun. Mass Spectrom. 1996, 10, 538-550. (35) Van Ham, R. Molecular Speciation using Static Secondary Ion Mass Spectrometry (S-SIMS): Methodology and Applications. Ph.D. Thesis, University of Antwerp, 2002.

In this paper, the threshold for detection is set at a signal intensity of 0.3 counts per second of analysis time (cpsat) under the conditions described. Previous experience has shown that ions detected from a pure product with an intensity of 3 cpsat can be considered for analyte characterization in “typical” applications such as ambient aerosols.35,36 Binary Salts. A survey of the mass spectra in our database shows distinct trends for salts with cations having only one stable oxidation state, and analogues with, for example, transition elements, for which several stable oxidation states occur. Within each group, the charge state of the cation appears to govern the kind of diagnostic ions and their relative intensities. Analogues of Cations with Only One Stable Oxidation State. Alkali halides (MX) provide the best starting point as these analogues contain monovalent ions. The base peak due to atomic ions (M+ and X-) is typically accompanied by low-m/z cluster ions such as M2+, X2-, and M2O+. As a result, ∼80% of the total ion current is carried by these low-m/z ions. Of particular interest for molecular speciation are the signals due to the (polymeric) adduct ions (MX)nM+ or (MX)nX- with n ) 1, 2, .... The intensity of at least the monomeric adduct signals exceeds the application threshold of 3 cpsat in positive and negative ion mass spectra. Hence, molecular speciation is readily achieved by the m/z difference between these signals and the base peak. Additionally, molecular anions (MX-) are detected with similar intensities as the monomeric adducts. Comparison of the mass spectra from LiCl, NaCl, KCl, NaBr, KBr, NaI, and KI shows a similar total ion yield. However, the signal intensity ratio of dimeric over monomeric adduct anions is lower for LiCl than for NaCl and KCl but increases from chlorides over bromides to iodides (with the same cation). Among the binary salts, NH4Cl represents a particular case because the polyatomic cation can undergo fragmentation. The mass spectra in Figure 1 illustrate the detection of intense peaks due to NH4+ and Cl- but no signals from ions containing the intact molecules. Hence, molecular speciation is not feasible here. Unlike the positive ions, anions provide information on both structural moieties by major signals due to Cl-, NH3-, NH2-, and NH-. The detection of radical ions such as NH3+ is associated with the generation of NH3 neutrals during the sputtering process, and subsequent electron ionization is an alternative to the direct fragmentation of NH4+. This line of reasoning is consistent with LMMS data37 and avoids the transition of even-electron species to radicals during fragmentation. The absence of very large polymeric adducts in the mass spectra contrasts with earlier literature data.14-16 Adducts with up to 12 neutrals have been detected for alkali halides with magnetic sector or quadrupole MS in the low-damage regime. Cesium iodide is found to produce adducts with n up to 70.19 The relative intensities of the adducts reflect the stability of cubiclike structures at “magic numbers” of 12, 27, 37, and 62.18,20-22 We have verified the detection of such high-m/z adducts by prolonging the acquisition time from 300 to 10 000 s, under which conditions adducts with n ) 9 have been already observed in the positive ion detection mode from NaI. However, the intensity difference (36) Van Ham, R.; Adriaens, A.; Prati, P.; Zuchiatti, A.; Van Vaeck, L.; Adams, F. Atm. Environ. 2002, 36, 899-909. (37) Van Vaeck, L.; De Waele, J.; Gijbels, R. Mikrochim. Acta 1984, III, 237257.

Figure 1. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from NH4Cl.

between monomeric adducts and adducts with nine neutrals is ∼5 orders of magnitude, which make the use of these high-m/z adducts impractical in applications. In comparison to alkali halides, mass spectra of analogues with bivalent cations, e.g., CaCl2, CaBr2, MgCl2, BaCl2, and BaBr2, typically show a reduced intensity of the molecular adducts, in particular for the polymeric forms. Additional signals are due to MX+ and MX-. According to the DI model, the former ion is considered as a structural fragment while the latter one is seen as the attachment of Cl- to a neutral Ba atom. Moreover, MX+ can act as a charge carrier in monomeric adducts (e.g., BaCl2‚ BaCl+). Identification in the negative ion mode can be based on the molecular anions, the monomeric adducts, and the MX- ions, all of which yield signals in the same intensity range. The positive ion mass spectra are similar for the barium, magnesium, and calcium halides. In contrast, negative ions show an increased intensity ratio of the polymeric adducts over atomic ions for bromides in comparison to chlorides. In conclusion, it seems that the generation of polymeric adducts becomes more difficult for earth alkali than for alkali halides and this trend extends for salts with trivalent cations. This tendency in S-SIMS is also seen in LMMS. It is readily conceived that the release of an intact tri- or tetraatomic molecule into the selvedge is more delicate than that of a diatomic ion pair because an increasing number of interactions within the lattice must be disengaged without breaking intramolecular bonds. Analogues of Cations with Several Stable Oxidation States. The generation of secondary electrons during the photon- or Analytical Chemistry, Vol. 76, No. 9, May 1, 2004

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The mass spectrometric behavior of copper chlorides can be extended to a series of chlorides or bromides with transition element as cations such as cobalt, cadmium, and mercury. Figure 2 illustrates the mass spectra of HgBr2. The low intensity of the Hg+ signal makes the compound an exception within our database and contrasts with the corresponding LMMS results.38 In general, the peaks of diagnostic interest for the halides of transition elements in positive ion S-SIMS are M+, MX+, MX2‚M+ and MX2‚ MX+. The incorporation of neutral M into ions such as M2+ and M‚MX+ seems to give systematically more intense peaks for salts

with the heavier elements. Specifically in the case of HgBr2, signals of ions, which must be logically written with HgBr+ as charged carrier, are more intense than the ones with Hg+. The series of adduct ions is terminated by (HgBr)2‚HgBr+ containing reduced neutrals with Hg atoms in the reduced form (+II to +I). Again, the signal is higher than that due to the corresponding adduct with Hg+. The negative ion mass spectrum shows a familiar appearance with a base peak due to Br- ions, intense signals from monomeric adducts, and minor peaks from the dimeric forms. The presence of molecular adducts in both positive and negative ion mass spectra is an advantage for applications. As MS is destructive, there is only one polarity of the mass spectrum that can be used to identify the surface constituents of a given microobject, unless a dual TOF is used as in aerosol TOF MS.40 Unfortunately, analogues such as SnCl2, ZnBr2, ZnCl2, and CoCl2 give only structural fragments in the positive ion detection mode, and molecular identification must be achieved by the anions. Comparison of mass spectra for the anhydrous and hydrated form of SnCl2 shows little effect from the crystal water. Salts with trivalent cations primarily produce the same characteristic signals in the positive ion mode as observed before for the salts with divalent cations. The mass spectra of FeCl3 in Figure 3 show the prevalence of the Fe(III) form to rationalize the highm/z ions structurally. The same holds true for FeBr3. This illustrates the need for reference spectra to verify identification.25 Specifically, prevalent signals are due to the molecular ions FeX3and the monomeric adduct FeX3‚X-. The molecular anion is equally efficient as X- as a charge carrier in the adducts. The relative adduct ion intensities in the mass spectra of FeBr3 and FeCl3 are virtually identical. Although InCl3 is often considered to occur only in the In(III) form, the negative ion mass spectra contain signals to be explained as adducts with the neutral InCl. Also the expected species such as In, InCl, InCl3, and InOCl serve as neutral building blocks in adducts. A similar behavior is observed in LMMS.41 Species of In (+I) are known to be unstable in solutions but are assumed to be sufficiently long-lived under vacuum conditions.42 Oxy Salts. Two additional complications arise in comparison to the mass spectra of binary salts. First, the generation of oxidetype ions is a characteristic feature, also seen in, for example, LMMS. Second, not only the cation but also the anion can be readily converted by oxidation-reduction into another stable chemical form. Hence, identification becomes a task of fine speciation except for carbonates. Also for oxy salts, the adduct ion intensities depend on the charge state of the ionic groups (the number of atoms in analyte). Finally, analogues such as Na2HPO4 and NaH2PO4 provide a test case for the distinction between compounds with the same elements in the same oxidation state but a different stoechiometry. Carbonates. In the positive ion mass spectra, a number of signals appear that have already been detected in binary salts. Specifically, the M+ yields the base peak while monomeric and dimeric adducts occur for carbonates with monovalent cations. The intensity of the monomeric adducts exceeds the application threshold of 3 cpsat. A second series of ions is related to the oxide,

(38) Struyf, H.; Van Vaeck, L.; Poels, K.; Van Grieken, R. J. Am. Soc. Mass Spectrom. 1998, 9, 482-497. (39) Poels, K.; Van Vaeck, L.; Gijbels, R. Anal. Chem. 1998, 70, 504-512. (40) Suess, D. T.; Prather, K. A. Chem. Rev. 1999, 99, 3007-3035.

(41) Struyf, H.; Van Roy, W.; Van Vaeck, L.; Van Grieken, R. Rapid Commun. Mass Spectrom. 1994, 8, 32-39. (42) Cotton, F. A.; Wilkinson, G. Advanced inorganic chemistry: A comprehensive text, 3rd ed.; Interscience Publishers, John Wiley & Sons: New York, 1972.

Figure 2. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from HgBr2.

ion-solid interaction is known to cause the detection of ions referring to the oxidized or reduced form of the original analyte. Well-studied examples in the class of the binary salts are copper(I) and copper(II) chlorides. In LMMS38,39 as well as in S-SIMS,25,40 relative peak intensities reflect a preferential formation of adducts, using the original analyte as neutral. The DI model rationalizes the process according to following scheme:

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Figure 4. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from K2SO4.

Figure 3. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from FeCl3. The assignment of the negative ions is given in the included list.

e.g., M2O+, M2O‚H+, and M2O‚M+. As seen before, surface water is unlikely to cause such intense oxide peaks. Therefore, the generation of oxides is rationalized by the following scheme illustrating the formation of SrO from SrCO3

It only takes a likely polarization of the O-C bond toward the electropositive cation to form the oxide and expel the stable CO2 neutral. The same line of reasoning explains the formation of M2O species from salts with monovalent cations. The oxides are assumed to undergo ionization yielding MmO+ (m ) 1 or 2 for salts with divalent or monovalent cations, respectively) as well as (MmO)n‚H+, (MmO)n‚M+, and (MmO)n‚MmO+ adducts in which n is typically higher for carbonates with divalent cations than for those with monovalent cations (e.g., n ) 1 in Na2CO3, n ) 3 in SrCO3). The remaining signals are due to clusters such as M2+,

as were seen for halides, and peroxide-type ions, e.g., M-O-OM+ and M-O-O-M‚M+. Peroxide ions are not detected already in LMMS and therefore related to ion beam-induced damage, due to the more drastic energy regime of the DI process in S-SIMS. In the negative ion detection mode, the base peak due to Ois a distinctive feature between oxy salt and binary salts. Therefore, generation of O- is primarily related to the atomization of the oxy salt and not to water or oxygen at the surface. In the case of Na2CO3 and K2CO3, structural information is limited to a weak CO3signal (typically 3 cpsat) while analogues with divalent cations (Ca, Sr) additionally produce a series of oxide ions (e.g., MO-, O-, and OH- adducts to (MO)n(H2O)m with n ) 1, 2 and m ) 0 or 1). Molecular speciation of salts with divalent cations is feasible by the molecular ions and monomeric adducts to OH- (MCO3and MCO3‚OH-). Unfortunately, the intensity of the corresponding signals approaches the practical detection limit of 3 cpsat for applications. Sulfates. The mass spectra of K2SO4 in Figure 4 illustrate that the major signals from positive ions are due to oxide species whereas cationized molecules (K2SO4‚K+ at m/z 213) yield a peak with intensity of >3 cpsat. The generation of cationized sulfite may complicate the speciation of sulfate-sulfite mixtures. The sulfate-sulfite conversion has been associated before with ion beam-induced damage.9-11 However, it is also seen in LMMS using low-energy photons.39 In fact, the peak intensity ratio of cationized sulfites over sulfates in sulfate spectra taken with LMMS approaches the one observed at elevated ion doses in the so-called low-damage regime SIMS9-11 and is significantly higher than in Analytical Chemistry, Vol. 76, No. 9, May 1, 2004

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Figure 5. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from (NH4)2SO4.

Figure 4. The collective nature of the ionization in LMMS and low-damage SIMS is likely to stimulate the selvedge reactions in comparison to the regime of short pulses in TOF S-SIMS. Hence, we tend to relate this conversion to the inherent interactions in the selvedge. The diagnostically most useful anion is the structural fragment KSO4-, which yields an intense signal. Sulfate and sulfite adducts are seen with insufficient intensity for practical applications. A characteristic pattern of intense SOx- peaks (x ) 0-4) is accompanied by a series of peaks associated with K2O. In contrast NaOH becomes the preferred form over Na2O in the case of Na2SO4. Whereas NH4Cl yields only structural fragments, positive and negative ion mass spectra of (NH4)2SO4 in Figure 5 contain intense signals from diagnostic ions at at high m/z. As to cations, the base peak from NH4+ is accompanied by the characteristic fragments discussed before. The absence of oxide-related signals is structurally expected here. Several monomeric and dimeric adducts use H2SO4 instead of (NH4)2SO4 as neutral while also NH3 is a suitable building block. This trend is related to the easy decomposition of (NH4)2SO4 into NH3 and H2SO4. The same line of reasoning explains why H+ and NH4+ are equally important as charge carriers in adducts. The numerous signals at low m/z reflect the presence of hydrocarbons at the surface, due to either contamination of the reference compound or deposition from the gas phase (during sample preparation or analysis). Negative ion mass spectra show a trend similar to that seen for the positive ions in that H2SO4 plays an important role as a neutral and as a 2614 Analytical Chemistry, Vol. 76, No. 9, May 1, 2004

charge carrier. The relative intensity of the HSO4- signal exceeds that of SO4-. No information on the presence of a NH4+ group is available from the anions. Sulfates of other elements follow the trends seen for alkali analogues except that the molecular adducts may occur in the positive or the negative ion detection mode, depending on the salt under study. Additional complications arise when, for example, a trivalent cation is combined with a divalent anion as in Fe2(SO4)3‚ 5H2O. Whenever a neutral analyte becomes a large assembly, adduct detection seems disfavored. Specifically, positive ions give only limited information by means of the signals from Fe+, FeH+, FeO+, and FeO‚H+. In contrast, the negative ion mass spectra yield numerous peaks due to combinations of Fe2O3, FeO, FeSO3, FeSO4, and the {[HOFeIII]2+}{SO4--} ion pair as neutrals attached to charge carriers such as O-, OH-, SO4-, and HSO4-. Although most ions refer to the Fe(II) form, the original oxidation state is preserved in, for example, the Fe2O3 structural decomposition products. Nitrates and Nitrites. Alkali nitrates and nitrites largely follow the trends outlined for the sulfates. For instance, the positive ion mass spectra of nitrates contain the series of ions related to M2O or MOH (Na analogues), while both M+ and M2O+ can serve as charge carriers in the mono- and dimeric adducts. The combination of MNO3 with the M2O or MOH as neutral building block occurs as well. As expected, the cationized MNO2 yields a significant signal, which is lower than the cationized analyte MNO3‚M+. In the negative mode, a characteristic pattern of NO-, NO2-, and NO3- together with the molecular adducts is available. For nitrites, the description above can be readily adapted. The peak intensity ratios of cationized nitrate/cationized nitrite and NO3-/NO- reflect the sample composition in that both ratios are significantly lower for nitrites than nitrates. The positive ion mass spectrum of NH4NO3 in Figure 6 contains a relatively clean peak pattern with signals from monomeric and polymeric adducts with up to three and four neutrals. Most ions refer to combinations of NH4NO3 and NH4+ without incorporation of HNO3 (as opposed to H2SO4 in the case of (NH4)2SO4). In contrast to the alkali nitrates, the nitrate-nitrite conversion appears to be negligible. The polyatomic cation causes the absence of oxide-related ions but leads to specific ions such as cationized ammonia (NH3‚NH4+ at m/z 35). The negative ion mass spectra contain adducts with up to three NH4NO3 neutrals. Unlike for the cations, HNO3 and NH4NO3 serve as neutrals in polymeric adducts. The positive ion mass spectra of calcium nitrate in Figure 7 display the diagnostic information as expected with an increased contribution of the MO species in comparison to the alkali sulfates and no molecular adducts. The structural fragment CaNO3+ in combination with its adduct to CaO is most useful for identification. The S-SIMS data parallel the observations in FT LMMS.41 Looking to the negative ion mass spectra, NO2- and NO3- signals are accompanied by the corresponding adduct ion peaks, allowing molecular speciation to be achieved. In contrast to FT LMMS, no ions above m/z 250 are seen in S-SIMS. The results for Mg(NO3)2 are largely similar to the ones for Ca(NO3)2 except for a reduced abundance of the MgO-related cations while the MgNO2+ and MgNO3+ signals are more intense. As to Pb(NO3)2, the signal of PbOH+ reaches ∼5% of the Pb+

Figure 6. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from NH4NO3.

peak while the PbNO3+ fragment cationizes PbO monomers and dimers. The negative ion mode is preferred for molecular speciation because of the strong signals due to Pb(NO3)2‚NO3-. Bromates. The formation of halide ions from bromates or iodates has been taken as experimental evidence for the beaminduced damage and recombination of nonneighboring atoms in the sample during the sputtering and ionization in SIMS.14,43 The negative ion mass spectrum in Figure 8 provides intense peaks directly indicative for the analyte composition, e.g., NaBrO3.)‚Brand NaBrO3‚BrO3-. In contrast, the major signals from positive ions refer to NaBr and Na2O species. The peaks from cationized molecules and dimers are weak, and also, NaBrO or NaBrO2 can serve as a neutral building block. Looking to the positive or negative ion spectrum as a whole, the role of beam-induced decomposition as reflected by NaBr-related signals is relatively small. Fine Speciation of Alkali (Hydrogen) Phosphates. The main information for the speciation of alkali (hydrogen) phosphates is detected in the positive ion mass spectra given in Figure 9. As Na3PO4 had to be analyzed in the form of microcrystals, the absolute peak intensities are typically a factor 10 lower than for analytes in pellets but relative intensities are identical. Negative ion mass spectra (not shown) essentially contain signals from PO-, PO2-, and PO3- signals and a minute HPO4- peak. In this respect, S-SIMS differs from LMMS, where the NaPO3.PO3- signal from (43) Van Stipdonk, M. J.; Santiago, V.; Schweikert, E. A.; Chusuei, C. C.; Goodman, D. W. Int. J. Mass Spectrom. 2000, 197, 149-161.

Figure 7. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from Ca(NO3)2‚4H2O.

Na3PO4 is 10% of base peak from PO3-.38 In contrast, positive ion mass spectra from both methods largely agree. The typical ions in Figure 9 are related to NaOH and Na2O, Na2O2, and the cationized NaPO3 with or without H2O. Dimeric adducts occur with relatively low intensity. The occurrence of metaphosphate neutrals is logical because Na3PO4 forms linear or cyclic structures with NaPO3 repetition units.42 The signal of Na2CO3‚Na+ is due to contamination. Following the line of reasoning about the generation of metaphosphate by orthophosphates, NaPO3, HPO3, Na2O, NaOH, and H2O are expected to serve as structural neutrals for the various ions from Na2HPO4 and NaH2PO4. The formal representation of the ions in Figure 9 maximizes the use of Na+ (instead of H+) as charge carrier and of the original analyte as neutral. The signals of NaPO3‚Na+, Na3PO4‚Na+, and Na3PO4‚NaPO3‚Na+, which are specific for Na3PO4, are also detected from hydrogen phosphates. However, the relative signal intensity of the monomeric and dimeric adducts containing NaH2PO4‚Na+ and Na2HPO4‚Na+ (relative to Na3PO4‚Na+) reflects the original analyte composition. The intensity of Na2+ decreases according to the order Na3PO4 - Na2HPO4 - NaH2PO4. The negative ion mass spectra of hydrogen phosphates essentially contain intense signals from PO2- and PO3- ions. The results for the potassium analogues parallel the ones discussed for the sodium salt. Magnesium phosphate confirms that the combination of a divalent and trivalent ion disfavors the detection of molecular or adduct ions. In fact, diagnostic information is confined to the Mg+, MgO+, MgO.H+, PO2-, PO3-, and Analytical Chemistry, Vol. 76, No. 9, May 1, 2004

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Figure 8. Positive (top) and negative (bottom) ion mass spectrum recorded with S-SIMS from NaBrO3.

H(2)PO4- signals. The trends described for hydrogen phosphates have been checked on the Ca analogues. As expected, the distinction between the CaH2PO4 and Ca(HPO4)2 is less obvious and can be based on the relative peak intensity of CaPO2+/CaPO3+ which is about 1.3 for CaHPO4 and 0.2 for Ca(H2PO4)2. The protonated molecules allow molecular speciation to be achieved only for Ca(H2PO4)2, not for CaHPO4, in the positive ion mode. As to anions, the peak intensity ratio of Ca(PO3)2‚PO3- over HPO3/ CaPO4- can be used to distinguish Ca(H2PO4)2 and CaHPO4 with typical values of about 0.2 and 2, respectively. As a result, identification of such “unfavorable” analogues practically requires the use of reference spectra. In that case, it has been shown that also the relative intensities of low-m/z ions such as O-, OH-, PO2-, PO3-, Ca+, and CaO‚H+ and Ca+ can be used to distinguish between different calcium (hydrogen) phosphates.44,45 CONCLUSIONS A comprehensive database of inorganic salts has allowed the feasibility of molecular speciation by adducts incorporating the intact analyte to be demonstrated for all but few analogues. The mass spectra can be readily rationalized using basic chemical knowledge and the relatively simple concepts of the DI model. Key aspects to be considered in deductive identification are the (44) Chusuei, C. C.; Goodman, D. W.; Van Stipdonk, M. J.; Justes, D. R.; Schweikert, E. A. Anal. Chem. 1999, 71, 149-153. (45) Lu, H. B.; Campbell, C. T.; Graham, D. J.; Ratner, B. D. Anal. Chem. 2000, 72, 2886-2894.

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Figure 9. Positive ion mass spectrum recorded with S-SIMS from Na3PO4 (top), Na2HPO4 (middle), and NaH2PO4 (bottom).

oxidation-reduction of the analyte in the selvedge and the formation of oxides from oxy salts. The former complication does not prevent molecular speciation as the original analyte is usually the main neutral in the adducts. Transition metals in the analyte reduce this preference and reference spectra are required. The relative importance of adducts, in particular of the polymeric ones, decreases with the size of the neutral, which in turn depends on the charge state of the anion and cation. The role of the polarizability is seen in the increased adduct yield from, for example, iodides in comparison to chlorides. The very logical systematization of the ion formation in S-SIMS largely parallels that in FT LMMS. As a result, advantage can be taken from the experimental evidence about ion formation in LMMS. The deductive identification in S-SIMS is essential for application to real life

samples because the molecular environment of the analyte can affect the mass spectra. Simple effects such as surface oxidation might compromize the score of correlation in fingerprinting but are readily dealt with deductively. ACKNOWLEDGMENT This work was supported in part by the Belgian Office for

Scientific, Technical and Cultural Affairs (IUAP 5) and by FWO, Brussels, Belgium (research projects G.0172.00 and G.0080.04).

Received for review January 9, 2004. Accepted February 12, 2004. AC0400156

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