Microprobe Speciation Analysis of Inorganic Solids by Fourier

determination of the inorganic molecular composition. Compounds from different classes can be tentatively identified by deductive reasoning while thos...
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Anal. Chem. 1998, 70, 504-512

Microprobe Speciation Analysis of Inorganic Solids by Fourier Transform Laser Mass Spectrometry Katrien Poels, Luc Van Vaeck,* and Renaat Gijbels

Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk-Antwerpen, Belgium

Fourier transform (FT) laser microprobe mass spectrometry (LMMS) aims at the characterization of local constituents at the surface of solids. Signals from structural fragments specify the main building blocks of the analyte while adduct ions, consisting of one or two intact analyte molecules and a stable ion, allow specific identification of the molecule. A series of inorganic reference compounds including binary salts, oxides, and oxy salts was analyzed to assess the FT LMMS capabilities for the determination of the inorganic molecular composition. Compounds from different classes can be tentatively identified by deductive reasoning while those with the same elements in different stoichiometries require comparison with reference spectra. The characterization of the molecular composition of the chemical species at the surface of solids on a microscopic scale is essential in scientific and technological research. However, information on the elemental composition, the oxidation degree of given elements, or the presence of specific bonds in functional groups is often not sufficient. For instance, understanding of the toxicological impact of given components in a tissue or of the local surface properties of materials requires knowledge about the molecules involved. An ideal microprobe method should detect signals, directly related to the whole molecules in the sample. Laser microprobe mass spectrometry (LMMS)1-3 is reported to be a potential tool for the determination of the molecular composition of inorganic and organic compounds with minimal sample preparation and a spatial resolution of 1-5 µm.4-5 The initial instruments with time-of-flight (TOF) mass analyzer were shown to allow inorganic speciation by fingerprinting and comparison with reference spectra2,6-15 or by statistical treatment of

large data sets.16 The need for high mass resolution to unravel the sometimes complex mass spectra triggered the development of Fourier transform (FT) LMMS.17-20 Although the laser microbeam ionization is shared with TOF LMMS, the species detected in FT LMMS and their relative abundances may look different21 because of the mass analyzer characteristics1 with respect to the initial kinetic energy (Ekin) of the detected ions, their formation period and/or place. Therefore the purpose of this paper is to report on the application of FT LMMS to a comprehensive series of pure inorganic compounds. To our knowledge, this is the first systematical study of “molecular speciation” of binary and oxy salts as well as oxides by FT LMMS. Particular attention is given to the question of whether identification of a compound needs reference spectra or, alternatively, can be performed by deductive reasoning.

EXPERIMENTAL SECTION Instrumentation. Caution: The 4.7-T magnet imposes health risks for people with metallic implants and pacemakers. Safety glasses are needed to avoid eye damage from the high-power pulsed Nd:YAG laser. The FT LMMS20 with external ion source was adapted from the commercial Spectrospin CMS 47X FTMS22 with the Aspect

(1) Van Vaeck, L.; Struyf, H.; Van Roy, W.; Adams, F. Mass Spectrom. Rev. 1994, 13, 189-208. (2) Van Vaeck, L.; Gijbels, R. Fresenius’ J. Anal. Chem. 1990, 337, 743-754. (3) Van Vaeck, L.; Van Roy, W. In Encyclopedia of Analytical Sciences; Thownsend, A., Ed.; Academic Press Ltd.: London, 1995; Vol. 5, pp 29032916. (4) Van Vaeck, L.; Struyf, H.; Van Roy, W.; Adams, F. Mass Spectrom. Rev. 1994, 13, 209-232. (5) Van Vaeck, L.; Gijbels, R. Fresenius’ J. Anal. Chem. 1990, 337, 755-765. (6) Bruynseels, F. J.; Van Grieken, R. E. Spectrochim. Acta 1983, 38B, 853858. (7) Michiels, E.; Gijbels R. Mikrochim. Acta 1983, 3, 277-285. (8) Bruynseels, F. J.; Van Grieken, R. E. Anal. Chem. 1984, 56, 871-873. (9) Michiels, E.; Gijbels, R. Anal. Chem. 1984, 56, 1115-1121. (10) Dennemont, J.; Landry, J. C. In Microbeam Analysiss1985; Armstrong, J. T., Ed.; San Francisco Press: San Francisco, CA, 1985; pp 305-309.

(11) Musselman, I. H.; Linton, R. W.; Simons, D. S. In Microbeam Analysiss1985; Armstrong, J. T., Ed.; San Francisco Press: San Francisco, CA, 1985; pp 337-341. (12) Bruynseels, F.; Otten, Ph.; Van Grieken, R. J. Anal. At. Spectrom. 1988, 3, 237-240. (13) Gu ¨ c¸ er, S.; Van Vaeck, L.; Adams, F. Spectrochim. Acta 1989, 44B, 10211039. (14) Poitevin, E.; Muller, J. F.; Klein, F.; De´chelette, O. Analusis 1989, 17, 4757. (15) Hachimi, A.; Poitevin, E.; Krier, G.; Muller, J. F.; Pironon, J.; Klein, F. Analusis 1993, 21, 77-82. (16) Ro, C.-U.; Musselman, I. H.; Linton, R. W. Anal. Chim. Acta 1991, 243, 139-147. (17) Pelletier, M.; Krier, G.; Muller, J. F.; Weil, D.; Johnston, M. Rapid Commun. Mass Spectrom. 1988, 2, 146-150. (18) Brenna, J. T., Creasy, W. R., McBain, W.; Soria, C. Rev. Sci. Instrum. 1988, 59, 873-879. (19) Ghaderi, S.; Littlejohn, D. P. In Proceedings of the 33rd ASMS Conference on Mass Spectrometry and Allied Topics; San Diego, CA, 1985; pp 727-728. (20) Van Vaeck, L.; Van Roy, W.; Struyf, H.; Adams, F.; Caravatti P. Rapid Commun. Mass Spectrom. 1993, 7, 323-331. (21) Struyf, H.; Van Vaeck, L.; Kennis, P.; Gijbels, R.; Van Grieken, R. Rapid Commun. Mass Spectrom. 1996, 10, 699-706. (22) Grossmann, P.; Caravatti, P.; Allemann, M.; Kellerhals, H. P. In Proceedings of the 36th ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA, 1988; pp 616-617.

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3000 data system. An Infinity Cell23 in a 4.7-T magnet is employed, and static electrical fields transport ions from source to cell. Differential pumping permits us to maintain ∼10-8 Torr in the source and 2 × 10-10 Torr in the cell. Samples are ionized in the reflection geometry under 45° by a high-power frequencyquadrupled Nd:YAG laser (Quanta-Ray DCR 2-10) focused to a 5-µm spot. The local power density on the sample can be adjusted between 107 and 1010 W cm-2. Tuning of the Instrument. The laser power density is optimized for detection of the highest possible adduct ions with a crater size under 10 µm. Typical voltages are as follows for positive ions: sample holder and shield between -10 and 10 V, source housing 10-25 V, pusher 50 V, extractor -25 V, extraction lens -360 V, first flight tube -2100 V, focusing lenses -700 and -1100 V, second flight tube -1400 V, x,y deflectors 0 V, second focusing lens -740 V, third flight tube -1500 V, front ion injection electrode -8 V, both side-kick plates ∼-6 V, and trapping plates 1.5 V. For negative ions, the voltages are reversed and compensated for the electronic offset. The source potentials are tuned to compensate for the initial Ekin and the potential of the plane (sample or selvedge) where the ions are generated (Epot). If collisional ion cooling is neglected, the cell can only store ions with a final energy (Ekin + Epot) up to ∼1.5 eV. If the ions are directly ejected from the sample with an initial Ekin of 5 eV, the sample surface must be at -3.5 V. When ions are also formed in the selvedge at another potential line, discrimination may occur when the energy difference exceeds 1.5 eV. Hence, optimization involves iterative tuning for the highest m/z clusters, associated with selvedge ionization. In this way, the low-m/z ions direcly ejected from the sample, stay in the spectrum because of their relative abundance and usually broad energy distribution. A compromise on their signal intensity is acceptable since these ions carry less specific information. The m/z scale is externally calibrated using CsI or Bi2O3. For not well understood reasons, lower excitation is required under m/z 40 in the Infinity Cell. Time-of-Flight Effect in FT LMMS with External Ion Source. The operational procedures of an FTMS with external ion source and ion transport with static electrical fields depend on the duration of the ionization pulses. The speed of ions upon acceleration and deceleration by electrical field relates to (m/z)-0.5. Hence, low-m/z ions arrive sooner in the cell than the higher m/z ions. External ion injection into the cell requires biasing the entrance electrodes so that the trapping potential of the first plate is deactivated. Once in the cell, the ions are reflected by the second trap plate back toward the cell entrance. This means that, if the trapping potential is not restored in time, the low-m/z ions that arrived earlier escape before the high-m/z ions enter the cell. However, the optimal duration of the injection event is significantly different for the short ionization pulses of a few microseconds in LMMS and the millisecond range ionization of, for instance, classical electron ionization. When the ionization continues over periods much longer than the ions’ flight times through the transfer line, low-m/z ions made later in the source may arrive in the cell together with high-m/z ions, generated sooner. Biasing two side-kick injection electrodes for instance +6 and -6 V creates a transversal electrical field, which results in the accumulation of

ions in the cell.24 This, however, is not applicable in FT LMMS. The short period during which ions are generated does not allow a continuing supply of ions to permit the accumulation in the cell and the side-kick field reduces the transmission through the injection electrodes.20 The experimental parameter Tgate refers to the time period of ion injection enabled after the laser pulse. In principle, the optimal Tgate for an ion depends on its m/z, initial Ekin, and Epot as well as on its moment of formation, i.e., during the laser pulse or a few microseconds afterward. Therefore, the SIMION 6.0 ion trajectory simulation program (Idaho National Engineering Laboratory, Idaho Falls, ID) allowed assessment of the limits of the practical

(23) Caravatti, P.; Alleman, M. Org. Mass Spectrom. 1991, 26, 514-518.

(24) Caravatti, P. U.S. patent 4, 924, 089, May 8, 1990.

Figure 1. Positive and negative ion mass spectra of CrPO4, recorded at a Tgate of 200 (a), 300 (b), and 500 µs (c), a laser power density of 3.6 × 109 (a) and 1.9 × 109 W cm-2 (b, c), and with 20 (a, b) and 150 (c) scans.

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Figure 2. Positive and negative ion mass spectra of Na2SO4 (a, c, d) and Na2SO3 (b) recorded at a Tgate of 250 (a, b), 200 (c), and 300 µs (d), a laser power density of 2 × 109 (a, b) and 5.1 × 109 W cm-2 (c, d), and with 40 (a), 100 (b), 130 (c), and 170 (d) scans.

Tgate (in µs) as a function of m/z under the voltages used for ions formed at the same time from the sample with an initial Ekin of 1.25 eV by means of

20.84xm/z e Tgate e 31.71xm/z

The minimal Tgate corresponds to the flight time, and the maximal Tgate additionally includes the time spent on one back-and-forth movement in the cell. It can be found that the m/z range of ions which can be trapped simultaneously is a factor 2.5.25 Sample Preparation. Small quantities are ground between microscopy slides or by using a mortar. The fine-particle fraction is mounted on the sample holder by double-sided tape or adheres as such to a dry or a methanol-wetted sample holder. Caution: Several reference products are toxic (indicated by *t) and/or carcinogenic (indicated by *c). The analyzed inorganic compounds, ordered according to the supplier, include sodium, potassium, calcium, magnesium, indium, tin(II), and mercuric(I) chloride; sodium and calcium phosphate; sodium hydrogen phosphate and potassium dihydrogen phosphate; sodium and calcium carbonate; sodium, calcium and magnesium nitrate hexahydrate; sodium nitrite; sodium sulfate, sulfite, and thiosulfate pentahydrate; indium, bismuth, and manganese(IV) oxide (Merck); cuprous(I)*t, cupric(II) and mercuric(II)*t chloride, chromium(VI)*c, titanium(II)*t, manganese(II) (25) Struyf, H.; Van Vaeck, L.; Van Grieken, R. Rapid Commun. Mass Spectrom. 1996, 10, 551-561.

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and arsenic(V)*c oxide trihydrate (Aldrich Chemical Co.); titanium(IV) oxide (J. T. Baker Chemicals); chromic(III) phosphate (Johnson Matthey GmbH); silver chloride and bromide; potassium and lead*t nitrate; chromium(III) oxide (Fluka); cobalt chloride hexahydrate; arsenic(III)*t, lead*t and molybdenum*t oxide (UCB); lithium chloride, cesium iodide, and nickel(II)*t oxide (Janssen Chimica); magnesium phosphate (Ventron); tungsten, cerium, europium, and holmium oxide (BDH Chemicals). RESULTS Oxy Salts Mn(XOx)m. (a) Phosphates. The mass spectra of chromic(III) phosphate in Figure 1 show that the main information is carried by the negative ions. The detection of only Cr+ in the positive mode reveals the presence of that element in the analyte but does not specify the class. However, the low-m/z anions PO2- and PO3- are indicative of phosphates, while the signal from CrO3- occurs for a chromium oxy salt or oxide, not for a binary salt. Taking this information, the analyte could be already identified as CrPO4 but then the speciation is still indirect, i.e., only by structural fragments. The “direct” speciation requires molecular ions or clusters containing the intact analyte. Molecular ions CrPO4- are detected at m/z 147 and simple adducts such as CrPO4‚PO3- occur at m/z 226. Clusters such as CrO3‚CrPO4-, P2O5‚CrPO4-, and CrPO4‚CrO3‚PO2- additionally confirm the molecular composition of the analyte. The elemental composition of the ions is verified by narrow-band experiments, where the mass accuracy is better than 1 ppm. This will be illustrated in the next section where the assignment is less obvious from the nominal

m/z. However, the way that clusters are represented as a “logical” combination of neutrals and ions remains a question of interpretation. Figure 1 also illustrates the implications of the TOF effect on the FT LMMS data. The absence of the high-m/z clusters in Figure 1b issues from the shorter Tgate than in (c), so the highm/z ions have not yet arrived in the cell. However, the peak intensity ratio of m/z 79/63 decreases with Tgate. This may reflect that the PO2- production continues longer after the laser pulse than that of PO3-, that the Ekin distribution of PO2- extends to lower Ekin, that the ions are formed at a different place, or that a combination of these possibilities occurs. Further fundamental research on the ion formation is needed to fully elucidate the relationship between peak intensity and Tgate. However, at this moment it is clear that signals of a given m/z detected at different Tgate may refer to ions with quite different ion formation mechanisms and characteristics. Otherwise stated, these signals may arise from completely distinct fractions of the initial ion population, and hence, the combination of partial spectra by normalizing the intensities of common signals at different Tgate is not meaningful. As a result, whenever peak ratios must be used for identification, reference spectra must be taken under exactly the same experimental conditions. The importance of molecular and/or adduct ions in the positive and/or negative mode depends on the analogue. For instance, sodium phosphate yields an intense peak from Na3PO4‚Na+ at m/z 187 and dimeric adducts (Na3PO4‚Na3PO4‚Na+, m/z 351) with low abundance.25 In the negative mode, structural ions such as NaPO3‚PO3- (m/z 181) are available. The use of NaPO3 as a building block in the clusters parallels the representation of orthophosphoric acid as hydrated metaphosphoric acid. It is a general guideline that oxides can be used as suitable building blocks in the clusters from oxy salts. For instance, the positive ion mass spectra of calcium phosphate contain numerous signals from (CaO)n‚Z+ (n ) 1, 2; Z ) Ca, CaO, H). Molecular ions or adduct-type clusters are not detected. High-m/z signals are due to CaPO2+ and Ca2PO4+, and their combination with CaO while the information in the negative mode is essentially contained to PO2- and PO3- ions. Strictly speaking, FT LMMS does not allow “direct speciation” here although we consider the information from Ca2PO4+ as equally specific. (b) Sodium Sulfate, Sulfite, and Thiosulfate. Figure 2 illustrates the positive and negative mass spectra of the former two analogues, and Table 1 includes the exact mass measurements. As typical for FT LMMS on analytes with the same elements in different stoichiometry, all signals are common to both analogues, but their relative intensities show characteristic differences. The number of scans reflects that the ion yield from sulfate is higher than from sulfite. In the low-m/z region of the positive mass spectrum, the Na+ signal is intense at a Tgate below 150 µs under reduced excitation conditions while at a Tgate of 250 µs peaks related to the Na2O fragment appear. Specifically, the signal intensity ratio of the adduct ions Na2SO4‚Na+/Na2SO3‚Na+ is significantly higher for the sulfate than for the sulfite, as in TOF LMMS,8 and can be used to distinguish the two analogues. The use of relative peak intensities to characterize the analyte imposes evident limitations whenever mixtures of these analogues are involved. Depending on the difference between characteristic

Table 1. High Mass Resolution Data of the Diagnostic Signals in the Positive and Negative Mass Spectra of Sodium Sulfate, Sulfite, and Thiosulfate m/za

assignment

exp error (ppm)

61.9739 62.9817 84.9636 100.941* 109.941 132.913* 148.926 164.921 178.893* 210.865* 226.877* 242.872* 256.844* 258.849* 274.844* 274.862 288.816* 290.857 304.829* 320.824* 336.801*

Positive Ions Na2O+ Na2O‚H+ Na2O‚Na+ Na2S‚Na+ Na2SO2+ Na2S2‚Na+ Na2SO3‚Na+ Na2SO4‚Na+ (Na2S)2‚Na+ Na2S2‚Na2S‚Na+ Na2SO3‚Na2S‚Na+ Na2SO4‚Na2S‚Na+ (Na2S)3‚Na+ Na2S2O3‚Na2S‚Na+ Na2S2O3‚Na2S‚NaO+ (Na2SO3)2‚Na+ (Na2S)3‚S‚Na+ Na2SO4‚Na2SO3‚Na+ Na2SO3‚(Na2S)2‚Na+ Na2SO4‚(Na2S)2‚Na+ Na2S2O3‚(Na2S)2‚Na+

0.6 0.5 0.7 0.6 0.6 0.7 0.6 0.9 0.6 0.8 0.3 0.2 0.1 0.5 0.1 0.2 0.5 0.6 0.4 0.1 0.3

31.9727 47.9676 63.9447* 63.9625 79.9395* 79.9573 95.9168* 95.9523 118.942 150.879* 150.896* 150.914* 166.873* 166.891* 182.868* 212.871* 228.883 244.878 260.873 354.820*

Negative Ions SSOS2SO2S2OSO3S3SO4NaSO4NaS4NaS3O2NaS2O4NaS4ONaS3O3NaS4O2Na2S2O3‚NaSNa2SO3‚NaSO3Na2SO3‚NaSO4Na2SO4‚NaSO4(Na2SO3)2‚NaSO3-

1.7 0.7 0.3 0.3 0.9 0.7 0.3 0.6 0.9 0.1 0.2 0.1 0.6 0.9 0.9 0.1 0.3 0.2 0.1 0.5

a

Asterisks identify signals exclusively detected for the thiosulfate.

abundances of given ions (e.g., m/z 149/165 ∼0.6 for the sulfate and ∼1.4 for the sulfite, at a Tgate of 250 µs) and the reproducibility of the method (∼20%), it can be anticipated in a first approximation that the method is adequate for mixtures in the range of 40-80% (mol/mol) sulfite/sulfate. Preliminary experiments tend to sustain this idea, but further work has to be done. The topic will be addressed in more detail later on (see Discussion). The peak intensity ratio of the low-m/z anion fragments SO3over SO2- is in sulfate 1.3 ( 0.2 and 0.5 ( 0.1 in sulfite at the optimal Tgate of 200 µs. The signal from NaSO4- is much more intense for the sulfate in comparison to the sulfite. The maximum in the cluster abundance distribution at higher m/z shifts from Na2SO4‚NaSO4- for the sulfate to Na2SO3‚NaSO4- for the sulfite, yielding at a Tgate of 300 µs a distinctive ratio of m/z 261/245 of, respectively, 1.5 ( 0.3 and 0.6 ( 0.1. The Na2SO3‚NaSO3- signal is only seen for sulfite as expected. Analytical Chemistry, Vol. 70, No. 3, February 1, 1998

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Figure 3. Positive and negative ion mass spectra of Na2S2O3 recorded at a Tgate of 250 (a), 400 (b), 200 (c), and 300 µs (d), a laser power density of 2 × 109 (a, b) and 5.1 × 109 W cm-2 (c, d), and with 15 (a, b), 80 (c), and 200 (d) scans.

The mass spectra from thiosulfate in Figure 3 and accurate mass data in Table 1 show numerous peaks that are not found for sulfates nor sulfites and point to the presence of the sodiumsulfur binding in the analyte. While sulfate and sulfite produce Na2O-based clusters, thiosulfate yields ions such as (Na2S)n‚Na+ (n ) 1-3) and Na2SOx‚(Na2S)n‚Na+ (x ) 3, 4; n ) 1, 2). The adduct-type ions Na2S2O3‚(Na2S)n‚Z+ (n ) 1, 2; Z ) Na, NaO) mentioned in Table 1 can be detected at higher Tgate, however, with low abundance. Hence, the presence of intense signals at m/z 101 and 179 in addition to m/z 149 and 165 serves to distinguish thiosulfate from sulfate and sulfite. The negative ions are less suitable for speciation in this case. Signals at higher m/z, e.g., m/z 151 and 167, are specific for thiosulfate but their intensity is low. The peak intensity ratio m/z 64/80 comes close to that of sulfate but the signals now comprise two contributions (see Table 1) instead of only one for sulfates and sulfites. Hence, measurement of S2- or S2O- in narrow band is in principle adequate for distinction. The mass resolution of FT LMMS easily exceeds 100 000 in the m/z range below m/z 300, while the replacement of sulfur by two oxygens (0.018 amu) only requires a resolving power of ∼17 000 up to m/z 300. (c) Carbonates, Nitrates, and Nitrites. The most abundant positive ion clusters from sodium carbonate refer to Na2O+, Na2O‚Na+, and Na2CO3‚Na+. The main information from the anions comes from CO3-. As for sulfates and sulfites, sodium nitrate and nitrite can be distinguished by the relative peak intensities in the negative ion mode. Specifically, the signal intensity ratio NaN2O5- (m/z 131)/NaN2O4- (m/z 115) is 4.7 ( 0.9 times higher for the nitrate than for the nitrite at a Tgate of 325 508 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998

µs. The peak ratio NO2-/NO3- is practically the same for the two compounds. Major positive clusters correspond to (Na2O)n‚Na+ or (Na2O)n‚NaO+ (n ) 1-4) and minor peaks refer to NaNO2‚(Na2O)n‚Na+ (n ) 0, 1, 2). This illustrates again that the mass spectra readily allow distinction of oxy salts with different X by deductive reasoning only while speciation of, for instance, nitrates or nitrites requires reference spectra. Oxides MnOm. The mass spectra from oxides show, especially in the positive mode, characteristic clusters of peaks separated by 16 m/z units while the clusters themselves lie at a m/z difference corresponding to the molecular weight of the analyte. The results of tungsten oxide in Figure 4 are recorded at a Tgate for optimal trapping of the monomeric adducts between m/z 400 and 460. The continuing formation of the smaller cluster ions and/or their low inital Ekin distribution allows them to be maintained in the spectrum albeit not with optimal signal intensity. In fact, both the positive and negative ion results alone permit identification of the analyte. The series of positive clusters ends with the dimeric adducts. Trimeric adducts are seldom seen in FT LMMS and chromium(VI) and nickel oxide are the only examples within our database (cf. infra). As to the negative ions, WO3 produces the molecular ion and its adduct with the intact analyte, in the absence of signals from ions with an oxygen less. Since metal oxide species occur in the cluster ions of oxy salts, these compounds must be distinguished from oxides by the typical low-m/z anions such as PO2-/PO3-, SO2-/SO3-, etc. This imposes some limitations whenever the microvolume consists of oxy salt/ oxide mixtures (see previous discussion under the section b).

Figure 4. Positive and negative ion mass spectra of WO3 recorded at a laser power density of 3.6 × 109 W cm-2, a Tgate of 700 (a) and 500 µs (b), and with 30 (a) and 10 (b) scans.

Chromium(III) and -(VI) oxide only yield Cr+ in the positive ion mode. Hachimi et al.26 detected CrO+ from Cr2O3 and an extended series of CrnOm+ clusters (n ) 1-4; m ) 1-10) from CrO3 using laser ionization at 266 nm and TOF LMMS while only Cr+ is observed upon irradiation at 249 nm by dual-cell FTMS. As to the anions, Panels a and b of Figure 5 show a CrO2-/CrO3signal ratio of 2.8 ( 0.5 times higher for Cr2O3 than for CrO3 as expected from the stoichiometry in the analyte. However, distinction is preferentially based on adduct ions, not on fragments. The peak intensity of the higher m/z adducts at long Tgate is larger for CrO3 than for Cr2O3, and the peak intensity patterns are clearly distinct. Specifically, the clusters at m/z 184 are the major ones for Cr2O3 and the ones at m/z 200 for CrO3 while the ions at m/z 168 only occur for Cr2O3. As a result, the signal intensity ratio of m/z 200/184 at a Tgate of 400 µs can be used to distinguish the two oxides. A similar trend is observed for the ions above m/z 250. The adducts Cr2O3‚CrO3- are only seen for Cr2O3, while (CrO3)2‚CrO2- and (CrO3)2‚CrO3- only occur for CrO3. The trimeric adduct (CrO3)3‚CrO- at m/z 368 is unique for CrO3, but its abundance is too low for practical applications. Table 2 surveys the major diagnostic features of the spectra. Strong- and medium-intensity signals are readily detected in the single-shot narrow-band mode while weak signals typically require accumulation of 10 shots. The ions to be preferred for identifica(26) Hachimi, A.; Poitevin, E.; Krier, G.; Muller, J. F.; Ruiz-Lopez, M. F. Int. J. Mass Spectrom. Ion Processes 1995, 144, 23-45.

tion are represented in italics and selected according to their relative abundance and molecular specificity. Most oxides yield sufficiently intense adduct-type clusters for single-shot narrow-band identification in at least one ion mode, sometimes in both. Unlike oxides with different M, analogues with the same elements in different stoichiometries require multiple ion detection. To differentiate, for instance, manganese(II) and -(IV) oxide, the abundance ratio MnO2-/MnO3- must be monitored. Note that the detection of molecular ions, having the same composition as the neutral analyte, offers in principle the same level of information as adducts. However, MnOm typically produce low-m/z signals due to MOx- where x ) 1, ..., p and p can exceed m/n. For instance, nickel(II) oxide produces the molecular ion NiO- as well as NiO2-, which is less specific for the molecular composition. Therefore, adducts are still preferred for speciation. Exceptions are, for instance, molybdenum and tungsten oxides, which produce MO3- in the absence of MO2- and MO-. Literature data allow comparison with TOF LMMS. The positive mass spectra from titanium(IV) oxide show close agreement between TOF LMMS and FT LMMS as to the species detected and their relative abundances.27 Similar laser power densities were used but the spot was ∼1 µm in TOF LMMS. TOF LMMS data on holmium oxide showed HoxOy+ clusters up to m/z 169228 while a similar power density in FT LMMS yielded no ions above Ho2O3‚Ho+. The nickel(II) oxide mass spectra from TOF LMMS11 contain Ni2+ and NiO‚Ni+ signals, accounting for less than 0.5% of the total ion current. However, the signal from the adduct ion cluster shows a medium intensity in FT LMMS. The analysis of arsenic(III) and -(V) oxide using TOF MS and a defocused spot was recently described.29 The positive mass spectra of the trioxide agree qualitatively well with those in FT LMMS. The pentaoxide was however not hydrated, so the TOF spectrum contains less signals. In the negative mode, the As2O5‚AsO2- and As2O3‚AsO2- ions provide intense and unique signals for As2O5 respective to As2O3 in both instruments but the dimeric forms for the pentaoxide were only seen in FT LMMS. As to sensitivity, the TOF data were accumulated from typically 500 scans and a spot of 160 µm while our data issue from 20 to 100 scans with a 5-µm spot and superior mass resolution. Binary salts MYm. Figure 6 summarizes the mass spectra of copper(I) and copper(II) chloride at representative Tgate to evidence the major high-m/z clusters. At short Tgate, the Cu+ and Cl- ions yield intense peaks. As expected, the two analogues share the positive signals at high m/z but the ratio of CuCl‚Cu+/ (CuCl)2‚Cu+ is higher for CuCl2 than for CuCl. This represents a dinstinguishing feature, but these ions do not correspond to molecular adducts in the case of CuCl2. In the negative mode, the clusters (CuCl)n‚Cl- (n ) 1, 2) are again common but CuCl2 yields the additional and specific signals from CuCl2‚Cl-, and CuCl2‚CuCl‚Cl-. However, the yield of these ions is too low for single-shot narrow-band detection. (27) Michiels, E.; Mauney, T.; Adams, F.; Gijbels, R. Int. J. Mass Spectrom. Ion Processes 1984, 61, 231-246. (28) Mauney, T. Anal. Chim. Acta 1987, 195, 337-341. (29) Allen, T. M.; Bezabeh, D. Z.; Smith, C. H.; McCauley, E. M.; Jones, A. D.; Chang, D. P. Y.; Kennedy, I. M.; Kelly, P. B. Anal. Chem. 1996, 68, 40524059.

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Figure 5. Negative ion mass spectra of Cr2O3 (a, c) and CrO3 (b, d) recorded at a laser power density of 1.8 × 109 W cm-2, a Tgate of 200 (a, b) and 500 µs (c, d), and with 20 (a, d), 10 (b), and 100 (c) scans.

The distinction between mercury(I) and -(II) chloride parallels the previous results. In the positive mode, all major signals are shared, e.g., Hg+, HgCl+, Hg‚Hg+, HgCl‚Hg+, and HgCl2‚HgCl+. The peak intensity ratio of HgCl‚Hg+ over HgCl2‚HgCl+ at a Tgate of 600 µs equals 1.3 ( 0.3 for Hg2Cl2 and 0.25 ( 0.04 for HgCl2. All major negative clusters at higher m/z can be described as Cladded to one or two molecules of HgCl2. Again, this building unit is common for both forms. The relative abundances are roughly comparable. The trimer adduct is only detected for HgCl2, but its abundance is low. Comparison with the relative peak intensities in positive reference spectra is needed for identification. The characteristic features of the mass spectra of binary salts MYm include in general the low-m/z fragments M+ and Y-, accompanied by MYm‚M+ and/or MYm‚Y- adducts for molecular speciation. Bi- or trivalent halides may yield clusters of the type MY+ (e.g., InCl+ from InCl3 30 ). Depending on the analogue composition, dimeric adducts can occur, while trimeric forms are seldom seen and only with low abundance.25 DISCUSSION The described results address the speciation capabilities of FT LMMS by means of data on pure compounds while FT LMMS as a microprobe method actually aims at the application to locally heterogeneous samples. As a result, the question arises as to what extent the reported data are useful in the latter situation. In our experience, a variety of material research applications involve (30) Struyf, H.; Van Roy, W.; Van Vaeck, L.; Van Grieken, R.; Caravatti, P. Rapid Commun. Mass Spectrom. 1994, 8, 32-39.

510 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998

the characterization of microscopic inclusions and heterogeneities1,4,21,30 with relatively simple chemical composition. For instance, biological tissues are inherently complex mixtures in which the overall concentration of most components does not exceed the percent level. Nevertheless, a variety of pathological problems with, for instance, implants are due the presence of micrometer-size foreign bodies, corresponding to (chemically modified) implant wear particles.31 Due to the localized ionization of the laser microprobe, these trace constituents become microobjects with rather simple chemical composition. Furthermore, FT LMMS only generates ions from the upper 10-50 nm of the evaporated microvolume,32 so that the constituents in the subsurface layers do not appear in the mass spectrum. As a result, in quite a lot of real-life applications, the local composition corresponds to an almost pure compound and the data on pure products represent an adequate starting point. At the present state of art, the application of FT LMMS to locally more complex mixtures depends on the investigation of several fundamental aspects of the ion formation process. The main question is according to which relationship the absolute ion yield depends on which parameters. For instance, ion yield is supposed to depend on the energy deposition in the local microvolume. This can be determined primarily by the UV absorption, which is in turn a function of the local concentration of a given compound, the refractive and/or reflective properties (31) De Nollin, S.; Poels, K.; Van Vaeck, L.; De Clerck, N.; Bakker, A.; Duwel, V.; Vandevelde, D.; Van Marck, E. Pathol. Res. Pract. 1997, 193, 313-318. (32) Van Roy, W.; Struyf, H.; Kennis, P.; Van Vaeck, L.; Van Grieken, R.; Andrle C. Mikrochim. Acta 1995, 120, 121-137.

Table 2. Overview of the Signals in the Positive and Negative Mass Spectra of Oxides from Our Databasea compd

positive ions

negative ions

compd

NiO

Ni+

PbO

Pb+ (m/z 208) s PbO‚Pb+ (m/z 432) w

(m/z 74) m As2O3 NiO2- (m/z 90) s NiO‚NiO- (m/z 148) w NiO‚NiO‚OH- (m/z 165) m NiO‚NiO‚NiO- (m/z 224) w NiO‚NiO‚NiO‚NiO(m/z 298) w Pb- (m/z 208) w As2O5‚3H2O PbO- (m/z 224) w

In2O3

In+ (m/z 115) s

InO- (m/z 131) m

Bi2O3

Bi+ (m/z 209) s Bi‚Bi+ (m/z 418) w Bi‚Bi‚Bi+ (m/z 627) w Bi2O3‚BiO+ (m/z 691) m Bi2O3‚Bi2O3‚BiO+ (m/z 1157) w Ho+ (m/z 165) s HoO+ (m/z 181) w Ho2O3‚Ho+ (m/z 559) w

Bi- (m/z 209) w BiO- (m/z 225) m BiO2- (m/z 241) s

Ce+ (m/z 140) s CeO+ (m/z 156) s

CeO- (m/z 156) w CeO2- (m/z 172) s CeO3- (m/z 188) s CeO2‚OH- (m/z 189) s MoO3- (m/z 146) s MoO3‚MoO3- (m/z 288) s

(m/z 58) s NiO‚Ni+ (m/z 132) m

Ho2O3

CeO2

MoO3 Mo+ (m/z 98) s MoO+ (m/z 114) s MoO2+ (m/z 130) s MoO3‚MoO+ (m/z 258) w MnO

Mn+ (m/z 55) s

NiO-

HoO- (m/z 181) w

TiO2

a

Ti+ (m/z 48) s TiO+ (m/z 64) s TiO‚TiO+ (m/z 128) w Ti+ (m/z 48) s TiO+ (m/z 64) s TiO‚TiO+ (m/z 128) w TiO2‚TiO+ (m/z 144) m

negative ions

(m/z 91) s As2O2+ (m/z 182) m As2O3‚AsO+ (m/z 289) s As2O3‚As2O3‚AsO+ (m/z 487) s

AsO2 (m/z 107) s AsO3- (m/z 123) w As2O3‚AsO2- (m/z 305) s As2O3‚AsO3- (m/z 321) m

AsO+ (m/z 91) s As2O2‚H+ (m/z 183) s As2O3‚H+ (m/z 199) s As2O3‚AsO+ (m/z 289) s As2O3‚HAsO3‚AsO+ (m/z 413) s As2O3‚As2O3‚AsO+ (m/z 487) w As2O5‚As2O3‚As+ (m/z 503) m As2O3‚As2O3‚HAsO3‚H+ (m/z 521) w As2O3‚As2O3‚HAsO3‚AsO+ (m/z 611) m As2O5‚As2O3‚As2O3‚As+ (m/z 701) w

AsO2- (m/z 107) m AsO3- (m/z 123) s HAsO3‚AsO3- (m/z 247) s As2O3‚AsO3- (m/z 321) m As2O5‚AsO2- (m/z 337) s As2O5‚AsO3- (m/z 353) s As2O5‚HAsO3‚AsO2- (m/z 461) m As2O5‚HAsO3‚AsO3- (m/z 477) m As2O5‚As2O5‚AsO- (m/z 551) w As2O5‚As2O5‚AsO2- (m/z 567) m

Cr2O3

Cr+ (m/z 52) s

CrO2- (m/z 84) m CrO3- (m/z 100) s CrO3‚CrO- (m/z 168) s CrO3‚CrO2- (m/z 184) s CrO3‚CrO3- (m/z 200) w Cr2O3‚CrO3- (m/z 252) m CrO3‚CrO3‚CrO- (m/z 268) m

CrO3

Cr+ (m/z 52) s

WO3

W+ (m/z 184) s WO+ (m/z 200) s WO2+ (m/z 216) m WO3‚W+ (m/z 416) w WO3‚WO+ (m/z 432) s WO3‚WO2+ (m/z 448) s WO3‚WO3‚W+ (m/z 648) m WO3‚WO3‚WO+ (m/z 664) m WO3‚WO3‚WO2+ (m/z 680) m

CrO2- (m/z 84) m CrO3- (m/z 100) s CrO3‚CrO- (m/z 168) w CrO3‚CrO2- (m/z 184) m CrO3‚CrO3- (m/z 200) s CrO3‚CrO3‚CrO- (m/z 268) m CrO3‚CrO3‚CrO2- (m/z 284) m CrO3‚CrO3‚CrO3- (m/z 300) w CrO3‚CrO3‚CrO3‚CrO(m/z 368) w WO3- (m/z 232) s WO3‚WO3- (m/z 464) s

MnO2- (m/z 87) s MnO3- (m/z 103) s MnO‚MnO3- (m/z 174) w

MnO2 Mn+ (m/z 55) s MnO2- (m/z 87) s MnO2‚MnO‚Mn+ (m/z 213) w MnO3- (m/z 103) s MnO‚Mn+ (m/z 126) w MnO‚MnO3- (m/z 174) w MnO2‚Mn+ (m/z 142) w

TiO

positive ions AsO+

TiO- (m/z 64) w TiO2- (m/z 80) w

-

TiO2- (m/z 80) w TiO2‚TiO2- (m/z 160) w TiO2‚TiO2‚TiO2(m/z 240) w

For explanation of s, m, and w, see text.

of the sample, determined by the specimen’s micromorphology. Additionally, ionization can be unimolecular or occur by adduct formation, which may cause an additional higher-order dependence on the concentration. In the absence of a theoretical description, an empirical approach must be followed by constructing calibration curves for mixtures of known composition. Current investigations show, however, that sample preparation is the main bottleneck. Preliminary results show that the mass spectra from mixtures practically correspond to simple superpositions of the ones from the individual components. This could mean that the mutual influence of the sample constituents in the whole process of desorption and ionization is negligible or that the sample is not yet a perfect mixture with respect to all important aspects for FT LMMS (chemical composition, UV absorption, reflection,

refraction). The lack of independent methods to verify these aspects complicates the problem. It has been demonstrated that the composition of the detected ions from locally pure inorganic compounds in FT LMMS is systematical and structure specific. Low-m/z fragments identify the element M+ in the positive ion mode while the anions of the type Y-, XOp-, or MOp- in the absence of XOp- classify the analyte as a binary salt, oxy salt, or oxide. At higher m/z, adducts of these low-m/z ions with the intact analyte molecule are seen in the positive or negative ion detection mode, sometimes in both. As a result, inorganic compounds with different elements can be readily identified by deductive interpretation of the mass spectra. Reference data are not strictly needed but remain useful for confirmation. Note that the molecular weight of the analyte is Analytical Chemistry, Vol. 70, No. 3, February 1, 1998

511

Figure 6. Positive and negative ion mass spectra of CuCl2 (a, c) and CuCl (b, d) recorded at a laser power density of 3.6 × 109 W cm-2, a Tgate of 400 µs, and with 15 (a,b), 70 (c), and 220 (d) scans.

directly deduced from the m/z difference between the adducts and the low-m/z ions. This makes an identification by FT LMMS molecule-specific as opposed to other microprobe and surface analysis methods, characterizing the elemental composition, specific bounds, or functional groups. The high mass resolution and mass accuracy of FT LMMS still increases the specificity. A complete description of all signals including the minor ones may require some ingenuity. Specifically, clusters from phosphates may incorporate neutrals like P2O3 and P2O5 while hydrated or hygroscopic samples require consideration of H+ or OH- as a charge carrier and H2O or acids such as HNO2, HNO3, and HPO2 as neutral buiding blocks. However, a comparative approach remains mandatory to distinguish compounds with the same elements in different stoichiometries. The reason is that most of the cluster ions from,

512 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998

for example, copper(I) as well as copper(II) chloride consist of one to several molecules of CuCl combined with a Cu+ or Clion. As a result, the majority of signals is common. However, the relative peak intensities exhibit significantly different ratios that can be used for identification on the condition that reference spectra under exactly the same experimental conditions are available. ACKNOWLEDGMENT Luc Van Vaeck and Katrien Poels acknowledge support from the F.W.O., the Belgian Fund for Scientific Research as research director and assistant respectively. Received for review August 20, 1997. Accepted November 6, 1997. AC9709108