Anal. Chem. 1987, 59, 2077-2081
2077
Separation of Metal-Containing Compounds by Supercritical Fluid Chromatography Mehdi Ashraf-Khorassani, John W. Hellgeth, and Larry T. Taylor* Department of Chemistry, Virginia Polytechnic Institute a n d State University, Blacksburg, Virginia 24061 -0699
Supercrltlcal fluid chromatography using packed, analytlcalscale chromatographlc columns was applled to galn separatlons of several metal-contalnlng systems. Mlxtures of substltuted ferrocenes and metal P-dlketonates were separated by using methanoknodmed COz under moderate temperatures (50-100 "C)and pressures (2000-5000 psl) as the mobile phase. Two different types of chromatographlc behaviors were observed which were found to be dependent on whether the separated metallic specles was either substltutlonally Inert or substltutlonally lablle. For Inert specles, proper chromatographic behavlor lndlcatlve of a reversible adsorption mechanism was found. For labile specles, retentlon time varlatlon wlth Injected amount, appearance of broad component peaks, and observatlon of multlple specles for a slngle Injection gave Indication of an lrreverslble adsorptlve mechanism.
The facility of supercritical fluid chromatography (SFC) to separate mixtures on both packed and capillary columns using a variety of detectors such as flame ionization detectors (FID), ultraviolet (UV), Fourier transform infrared (FTIR), and mass spectrometry (MS) has been shown (1-5). To date, practically all investigations have been conducted on organic samples even though the first reported SFC separation dealt with organometallic compounds. In contrast, less than a dozen reports have appeared regarding separations of metal-containing species throughout the past 25 years of SFC development. Numerous methods, however, based upon gas or liquid chromatographic techniques have been reported (6-10). With gas chromatographic methods, difficulties arise from the low vapor pressure of many metallic species. The use of high temperature to increase component volatility often leads to decomposition during the GC experiment. In contrast, liquid chromatographic methods ( I 1)have been generally successful in gaining the desired separation of components. However, problems of limited resolution, ligand exchange, and irreversible adsorption of the analyte to the stationary phase have been prevalent. In light of recent progess, SFC offers yet another avenue that may easily yield improved separations of metal-containing species. Historically, the first reported (12)use of SFC (which was then termed high-pressure gas chromatography) dealt with the separation of two nickel porphyrins, etioporphyrin I1 and mesoporphyrin IX dimethyl ester. This study demonstrated the potential of different halomethanes as supercritical fluid mobile phases with a solid supported liquid stationary phase (33% Carbowax 20 M on 60/80 mesh Chromosorb W). In this initial report, pressure control and mobile phase flow rate could not be maintained adequately, and component detedion was obtained via X-ray analysis of the isolated colored fractions. In a later report, Karayannis (13) described an apparatus that maintained proper SFC operating conditions. Separations of metal salicylaldoximate and metal thenoyltrifluoroacetonate mixtures were achieved on a variety of packed gas-liquid chromatography (GLC) columns. Separations of etioporphyrin I1 metal chelates were detailed in
subsequent articles (14,15). Again, packed GLC columns and supercritical CClzF2were employed. Chromatographic behaviors related to component gas-liquid partitioning, solubility, and stability were examined as a function of liquid stationary phase. At times, the addition of a polar modifier to the supercritical mobile phase was necessary to elute the same compounds from the different stationary phases. Using similar methodology, Karayannis (16) examined the elution behaviors of 43 metal acetylacetonates with supercritical CClzF2. Thermal decomposition of many metal chelates was prevalent as only 23 compounds remained intact throughout the separation process. In contrast to the previously discussed work where supercritical dichlorodifluoromethane was used exclusively, Jentoft and Gouw (17)reported on the use of supercritical COz for organometallic compound elution. A separation of three metal-cyclopentadienyl (Cp) complexes served to demonstrate the utility of COz. Components (Fe(Cp)z,Mn(Cp)(CO),, and Ti(Cp),Clz) were eluted from a GLC column of Carbowax 400 on Porasil F in a two-step pressure program. More recently, supercritical COz with polar modifiers (methanol or ethanol) was studied (18, 19) in regard to the elution of metal diethyldithiocarbamates, acetylacetonates, and oxinates. Separations were obtained on conventional HPLC silica and RP-8 columns with a binary solvent mixture of alcohol/COz up to 30 mol % alcohol. In this paper, the use of supercritical fluid chromatography to obtain separations of metal-containing compounds is reexamined. Attention is given particularly to the study of ferrocene derivatives and acetylacetone-metal complexes. Performance was evaluated on the basis of peak retention, shape, and resolution. Through adjustments in pressure, temperature, flow rate, mobile phase composition, and bonded stationary phases, metal-containing component retention behavior was examined in a systematic fashion. Model systems were chosen to demonstrate both equilibrium and nonequilibrium adsorption phenomena with regard to stationary phase type.
EXPERIMENTAL SECTION Experiments were carried out with a Hewlett-Packard (Avondale,PA) Model 1082B liquid chromatograph modified for use with supercritical fluids. Carbon dioxide and methanolmodified C02 (Scott Specialty Gases, Plumsteadsville, PA) were used as mobile phases. A variable wavelength ultraviolet detector, Hewlett-Packard Model 79875, equipped with a high-pressure, 1cm path length (8 p L internal volume) flow cell was employed. Samples were injected onto the analytical-scale packed column through a Rheodyne (Cotati, CA) Model 7125 injection valve having a 10-pL loop. Chromatographic separations were obtained from a variety of packed HPLC columns representing seven different stationary phases. The columns incorporated in this study are described in Table I. A flow rate of 2.0 mL/min was employed throughout the study. The weight percent methanol was varied from 2 to 20%. Conditions of pressure and temperature were varied from 2000 to 5000 psi and from 40 to 100 O C , respectively. The typical pressure drop across each packed column was 100-500 psi. Chromatographic behaviors of acetylacetonates and of substituted ferrocenes were studied. Injections as either single
0003-2700/87/0359-2077$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
Table I Characteristics of Columns stationsupplier
ary phases
particle size pm
Hewlett-Packard DuPont IBM IBM Hewlett-Packard Hewlett-Packard Hamilton
Si-OH Si-CN Si-C Si-$ Si-CE Si-CIE PRP-1
7 10 10 5
10 5 5
column dimensions, mm 100 X 250 X 250 X 250 X 200 X 100 X 150 X
4.6 4.6 4.6 4.6 4.6 4.6 4.6
Table I1 Compounds Screened text reference bis(acetylacetonato)cobalt(II) his(acetylacetonato)nickel(II) bis(acety1acetonato)copper (11) bis(acetylacetonato)cadmium(II) tris(acetylacetonato)aluminum(III) tris(acetylacetonato)chromium( 111) tris(acetylacetonato)iron(III) oxobis(acetylacetonato)vanadium(IV) tris(hexafluoroacetylacetonato)chromium(III) tris(1-phenyl-1,3-butanedionato)chromium(III) difluoro(acety1acetonato) boron( 111) ferrocene
Co(acac)* Ni(acac)* Cu(acac),
I
Figure 1. SFC separation (2 mL/min) of a mixture (3 pg/3 pL) of ferrocene, acetylferrocene, and 1, l'dlbcetylferrocene with CO,/MeOH (9812) mobile phase on four columns: ODs,80 OC, 2400 psi; phenyl, 50 OC, 5000 psi; PRP-1. 75 OC, 4000 psi; silica, 75 OC, 3000 psi. 12400
2000 P S I
3200 P S I
Cd(acacjz Al(a~ac)~ Cr(acac)3 Fe(acac)g VO(acac)z Cr(hfa)3
Cr(Phacac1, BF,(a~ac)~
acetylferrocene 1,l'-diacetylferrocene components or as simple mixtures were made. The compounds employed in this study were obtained from a variety of commercial sources and are lsited in Table 11. Solutions were prepared with HPLC-grade chloroform (Fisher Scientific, Raleigh, NC) in concentrations of 1-10 wg/mL. Typically, 3-FL volumes of each solution were applied t o the chromatographic column.
RESULTS AND DISCUSSION The objectives of this research were 2-fold. First, we wished to demonstrate the performance of SFC separations for metal-containing species using commercially available equipment and bonded-phase, packed columns. Second, the chromatographic behaviors of these species, as a function of SFC conditions (temperature and pressure) and compound type (metal and associated ligands), were of interest. For the latter objective, the behaviors of a variety of metal acetylacetonates and ferrocene derivatives were examined with regard to column type, supercritical mobile phase composition, and supercritical fluid density. SFC of Ferrocene Derivatives. Figure 1 illustrates the optimum separation (in terms of elution time and resolution) of a mixture of equal amounts of ferrocene, acetylferrocene, and 1,l-diacetylferrocene dissolved in CHC13 with four different packed columns (i.e., Si-OH, Si+, Si-CIB,and PRP). Supercritical C 0 2 was incapable of removing the substituted ferrocenes from any of the four columns in a reasonable time; however, methanol-modified COz (98/2 CO,/MeOH) proved to be quite adequate for all three compounds. Optimized detection was achieved a t 230 nm after noting little or no detector signal at 254 and 200 nm. (Absorption maxima for acetylferrocene (Sadtler Research Laboratories, No. 27185 UV) are 456 nm (570), 335 (1550). 266 (7230), and 223 (16900). Numbers in parentheses are molar extinction coefficients.) Even though the elution order of the mixed components did not change as the stationary phase was changed from polar (Si-OH) to nonpolar (Si-Cls and PRP), the degree of separation of the ferrocenes did appear to be a function of column type. For example, a base-line separation of the three components was afforded with the phenyl-derivatized silica column in less than 5 min. whereas, the Si-Cls column after trying a number of pressure-temperature combinations never yielded
TIME
-
Figure 2. SFC separation (2 mL/min) on a C,, column of ferrocene mixture (3 p g / 3 pL) at 80 OC as a function of inlet pressure; mobile phase, CO,/MeOH (98/2).
a base-line separation of components. The different resolving efficiency of each column cannot be overlooked here since column length and, in certain cases, particle size are not uniform. This could be the rationale for the much better optimized separation obtained on the phenol column (25 em) as opposed to the ODS column (10 cm). The behavior of these compounds suggests a retention mechanism dependence upon solute/stationary phase interaction as opposed to a dependence on component vapor pressure since the boiling points of these ferrocenes are relatively high. Two modes of interaction can be envisioned depending upon whether the metal or ligand is involved. An adsorption mechanism between the stationary phase acidic sites (Si-OH) and the cyclopentadienyl unit appears to exist here rather than any direct column-metal interaction. Incorporation of carbonyl groups into the cyclopentadienyl unit increases retention time for each column studied which supports the idea of coordinated ligand-stationary phase interaction. A longer retention time is prompted by the expected stronger interaction of the acidic sites on the silica with the more electron-rich (relative to ferrocene alone) carbonyl functionality. The mode of separation found on the PRP-1 and the Si-@columns could be via effective molecular size or via acid-base interactions as previously mentioned or a-cloud interaction between the polystyrene and ferrocene. In order to shed additional insight into the mode of separation, the ferrocene mixture on a CIScolumn was studied as a function of changes in pressure and temperature. Figure 2 illustrates the separation as a function of pressure at a
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 70 O C
80 *c
95 o c
i3
0
P-
h!
TIME 4
Flgure 3. SFC separation (2 mL/min) on a C,, column of ferrocene mixture (3wg/3 wL) at an inlet pressure of 2400 psi as a function of temperature: mobile phase, CO,/MeOH (9812).
constant temperature of 80 "C. Increases in pressure from 2000 to 3200 psi caused the species to elute faster and reduced resolution to only one peak appearing near the solvent front. Striking chromatographic behavior is observed a t 2000 psi relative to the behavior found at higher pressures. This same behavior was found at a slightly higher pressure (2400 psi) when the temperature was varied to obtain the same density. While maintaining constant pressure, retention time was found to increase slightly with a temperature increase from 70 to 95 "C (Figure 3). At 70 "C, both acetylferrocenes coeluted and were only partially resolved from ferrocene. Base-line resolution was, however, achieved a t 95 "C for the three components. Higher temperatures at constant pressure resulted in lower solvent strength of the less-dense mobile phase which accounts for the broader eluting peaks. These observations suggest that the fluidlike properties of the supercritical fluid mobile phase are highly important in determining retention. Higher pressures a t a fixed temperature give rise to greater densities and subsequently greater solvating power. Lower temperatures a t a fixed pressure likewise result in a greater density and lower rentention times. This strong dependence of elution behavior on solvent density suggests weak solute/stationary phase interaction with this C18 packing. A similar study was conducted with the same mixture on a phenyl column. Surprisingly, very little change in retention time was found by changing the pressure and/or temperature in this case. Ferrocene gave identical retention times a t 3000 and 5000 psi. 1,l-Diacetylferrocene decreased from 5.6 to 4.4 min over the same pressure range. The monofunctional ferrocene changed less than 10%. In contrast to the results with Cls, a much stronger interaction between solutes and the phenyl stationary phase is suggested by the greater retention times observed. This interaction is believed to involve mainly *-cloud associations between the phenyl and cyclopentadienyl substituents. This different behavior with CIS and phenyl columns is similar to that observed with both 2-propanol (polar) and cyclohexane (nonpolar) modifiers in supercritical pentane wherein both caused polystyrene to elute a t lower retention times (20). 2-Propanol was believed to deactivate the column, while cyclohexane was suggested to be a stronger solvent for the polystyrene materials.
2079
2
4 6 MINUTES
8
10
Figure 4. SFC separation (2 mL/min) of a mixture of four Pdiketonates on a phenyl column at 100 "C and an inlet pressure of 4750 psi: mobile phase, CO,/MeOH (80120);detector, 280 nm, 1 = CHCI,, 2 = Fe(acac),, 3 = Cr(acac),, 4 = Co(acac),, 5 = F,B(Phacac).
SFC of Metal Acetylacetonates. In the second part of our study, the behaviors of different types of metal acetylacetonates (acac) (Table 11) were examined. To elute these species, a mobile phase of 20% (w/w) methanol/COz was necessary. T h is corresponds to a mole fraction of methanol equal to -0.26. At such a high modifier content, one must be concerned that supercritical conditions are being met. Most of the materials eluted in a short time period from the six columns (Si-OH, 4, C1, C8, C18, CN) studied under pressure approximating 4500 psi a t 99 "C. Considerable attention has been given to the prediction of critical temperatures and pressures for solvent mixtures. Henion (21) has calculated the minimum temperature and pressure necessary for supercritical conditions for various mole fractions of methanol modifier. For 0.26 mol fraction P, and T , are estimated to be -1400 psi and 370 K. On the basis of these data the separations which will be described appear to have been obtained under supercritical conditions. Wright (22) has employed the same method (23)with slightly different coefficients and estimated a critical mixture temperature of approximately 381 K. If our estimates are slightly in error, the retention behavior should not change drastically in going from supercritical to subcritical conditions as has been demonstrated with COz, NzO, and NH3 mobile phases (24). Detection of species was a t 280 nm, the position of maximum UV absorbance for acetylacetone. Two different chromatographic behaviors were observed based on the specific metal (and its oxidation state) coordinated to the acetylacetone ligand. The two behaviors, we believe, reflect reversible and irreversible interaction of the stationary phase and analyte. As might be expected substitutionally inert complexes reversibly interact with the stationary phase and substitutionally labile complexes interact irreversibly. More specifically, inert complexes probably associate through the outer coordination sphere (i.e., coordinated ligand), whereas labile complexes may bond with the stationary phase via a direct metal link after loss of one of the peripheral ligands bonded to the metal. These two behaviors are well-demonstrated by the separation shown in Figure 4. Only Fe(acac)3is labile and fails to yield a sharp chromatographic peak even though it has the lowest retention time of the four components in the mixture. Such an observation gives credence to the idea that the nature of the chromatography of metal-containing species in the supercritical fluid mode offers information regarding the identification of the material. A better demonstration of this point is shown in Figure 5 where the SFC separation of a mixture of labile Co(acac)2 and inert C o ( a ~ a c on ) ~ a C1 column is presented. The labile Co(I1) complex is found in a broad peak, whereas the inert Co(II1) material was obtained in a much sharper peak. Considering the metal complexes which have been investigated, only Cr(III), Co(III), and B(II1) appear to reversibly adsorb. Single-component elution of each analyte is observed
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
Retention T i m e ( m i n )
Figure 5. SFC separation (2 mL/min) of a mixture of Co(acac), and Co(acac), on a C, column at 99 OC and an inlet pressure of 4500 psi: detector, 280 nm, 20 pg/3 pL. mobile phase, CO,/MeOH (80/20);
1
- 1
136, , 2 0 6
I
I
Retenlion Time ( m i n
i
Figure 7. SFC separation (2 mL/min) of Fe(acac), at variable concentrations on a C, column at 99 OC and an inlet pressure of 4500 psi: mobile phase, CO,/MeOH (80/20); detector, 280 nm.
i
I
0
3 90
,
2
,
,
4
,
6
Time (minutes) Retention Time i m i n )
Flgure 6. SFC separation (2 mL/min) of a mixture of chromium(I1) Pdiketonates (see Table 11) on a phenyl column at 99 OC and an inlet pressure of 3800 psi: mobile phase, CO,/MeOH (80120); detector, 280 nm, 1 = Cr(hfacac),, 2 = Cr(acac),, 3 = Cr(Phacac),.
as a sharp peak with each of the six columns in less than 3 min with 20% (w/w) methanol/COz. Separation of mixtures which have the same metal but differ in the attached ligand can also easily be achieved. Figure 6 illustrates the base-line separation of three chromium species in approximately 2 min. Further evidence for reversible interaction of these materials in provided by the fact that repeated injections gave retention times which vary within less than 1% . Furthermore, injections varying over a 10-fold concentration range gave essentially identical peak shapes and retention times. The remaining metal-acetylacetonate systems that we have investigated appear to irreversibly adsorb to each of the six columns. Cd(acac)zwas not eluted from any of the columns with 20% methanol/COZ. Broad peaks with considerable tailing were found for single-component injection of M g ( a ~ a c ) ~ , VO(acac)z,Al(acac)3, Cu(acac)2,Ni(aca&, and Fe(acac)B. The chromatographic behavior of these materials also exhibited variability of retention time with the amount injected. This ) ~a C1 column. point is illustrated in Figure 7 with F e ( a ~ a con A 4-fold increase in amount injected keeping the same injection volume results in an approximately 33% reduction in retention time. The failure of Fe(acac), to reversibly bind, as indicated previously, may reflect the partial dissociation of the complex via loss of an acac ligand whereupon the stationary phase and Fe(II1) directly and strongly associate. The broad peak might also reflect the separation of multiply coordinated Fe(II1) species. Similar chromatoraphic behaviors were also observed with Mg(acac)2,VO(acac)z,and Al(acac),. Cu(acac),! and Ni(acac)2gave repeatedly multiple chromatographic peaks from a single injection. This fact is demonstrated with Cu(acac), in Figure 8, wherein, a sharp peak illustrative of an inert material elutes earlier than a much broader peak representative of a labile species. Obviously,
Figure 8. Three replicate SFC separations (2 mL/min) of a CHCI, solution (23 p g / 3 pL) of Cu(acac), on a C, column at 99 OC and an detector, inlet pressure of 4500 psi: mobile phase, CO,/MeOH (80120); 280 nm.
the CHC13 solution of Cu(acac), has given rise to at least two quite different species. Without a more specific detector, the exact nature of these components cannot be exactly determined. Two possibilities can be recognized. In the solid state, Cu(acac)zis known to be a dimer. In the injection medium, CHC13,the complex may yield a dimer-monomer equilibrium. The dimer is expected to have few vacant coordination sites and therefore be inert to substitution. By the nature of the peak shape the dimer would then be predicted to elute first. Alternatively, partial dissociation of the complex as promoted by either the stationary phase or the injection solvent may have released free ligand into the chromatographic system, thereby generating an inert and a labile species. Free acetylacetone ligand has been found to elute from the C1 column under similar conditions as a broad peak. This would suggest that the inert component may contain copper. In addition to these findings, retention was observed to change with both time and column history. Without some preliminary type of column cleaning, retention times for both peaks slowly grew to longer values (Figure 8) indicating permanent adsorption of metal to some of the active sites. It is also noted from this experiment that the relative peak areas of the two components did not change from one injection to another. This indicates that the concentration ratio of the two components was not appreciably changed. The pressure drop across the column also did not appear to change as the repeated injections were made. The chromatographic behavior of Ni(acac), in the supercritical fluid medium was similar to Cu(acac), in that two components were again observed in the separation. With this system it was noticed that the relative intensity of the two peaks was not constant if injections from different days were compared in contrast to the Cu(acac), case. Figure 9 shows the results of injecting equal amounts of the same solution onto the column at day 0, day 7 , and day 9. It appears that
Anal. Chem. 1987, 5 9 , 2081-2087 I
Day 0
Day 7
49
Day 9
0 46
2081
number of substitutionally inert coordination complexes which we studied behaved in a straightforward manner, the same cannot be said of the more prevalent substitutionally labile coordination complexes. As we have shown, this phenomenon can prove to be both a help and a hindrance. While speciation information for certain metal-containing species might be possible, any identification based solely on retention time comparisons is risky at best given the possibility of (a) irreversible adsorption, (b) time-dependent peak intensity, (c) retention data which depend on the amount injected, and (d) the tendency for ligand exchange in certain systems.
LITERATURE CITED
,
0
I
2
4
Retention Time (rnin )
-re 0, Three replicate SFC separations (2 mL/min) of CHCI, solution (5 mi5 p l ) of NNacac), on a C,,column at 99 O C and an inlet pressure of 5000 psi: mobile phase, CO,/MeOH (80/20); detector, 280 nm.
material in the solution which elutes in the broader peak is being converted to material which elutes as the sharp peak. As expected, the increasing amount of reversibly adsorbed material does not affect the retention time, whereas the irreversibly adsorbed species (since its concentration is decreasing) elutes at progressively longer retention times. This observation is in agreement with data earlier presented for Fe(acac)3 which eluted in a large peak volume. As was the case with Cu(acac)z,the chemistry taking place with Ni(acac)z in the injection solution can only be conjectured. In this case, the nickel compound is known to be a trimer in the solid state. The hypotheses given for the copper case may also apply here. A nickel-specific detector would be desirable in order to determine whether the early eluting peak contains nickel. In summary, bonded-phase packed-analytical columns provide an adequate separation of some metal-containing materials. The interpretation of SFC data regarding neutral inorganic systems must, however, be cautiously performed. This is especially true for coordination complexes rather than for true organometallics such as ferrocenes. While the limited
(1) Johnson, C. C.; Jordan, J. W.; Taylor, L. T.; Vidrine, D. W. Chromafographia 1985, 2 0 , 717. (2) Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1984, 5 6 , 619A. (3) Chester, T. L. J . Chromatogr. 1984, 299, 424. (4) Holzer. G.; DeLuca, S.; Voorhees, K. J. HRC CC, J . High Resoluf. Chromafogr. Chromafogr. Commun. 1985, 8 , 528. (5) Gere, D. R.; Board, R.; McManigill, D. Anal. Chem. 1983, 5 4 , 740. (6) Kobayashi. M.; Saitoh, K.; Suzuki, N. Chromafographia 1985, 20, 72. (7) Smith, R. L.; Iskandaranl, Z.; Pietrzyk, D. J. J . Liq. Chromafogr. 1984, 7, 1935. (8) Fish, R. H.; Komlenlc, J. J.; Wines, 8. K. Anal. Chem. 1984, 5 6 , 2452. (9) Dilli, S.; PatsalMes, E. J . Chromatogr. 1983, 2 7 0 , 354. (10) Estes, S. A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1982, 5 4 , 2402. (11) Willeford, B. R.; Veening, H. J . Chromatogr. 1982, 257, 61. (12) Klesper, E.; Corwin, A. H.; Turner, D. A. J . Org. Chem. 1982, 2 7 , 700. (13) Karayannis, N. M.; Corwin, A. H.; Baker, E. W.; Klesper, E.; Walter, J. A. Anal. Chem. 1968, 4 0 , 1736. (14) Karayannis, N. M.; Corwin, A. H. Anal. Blochem. 1988, 2 6 , 34. (15) Karayannis, N. M.; Corwin, A. H. J . Chromafogr. 1970, 4 7 , 247. (16) Karayannis, N. M.; Corwln, A. H. J . Chromatogr. Sci. 1970, 8 , 251. (17) Jentoft, R. E.; Gouw, T. H. Anal. Chem. 1972, 4 4 , 681. (18) Wenckwiak, B.; Blckmann, F. fresenius' 2.Anal. Chem. 1984, 379, 305. (19) Bickmann, F.; Wencbwiak, B. fresenius' 2.Anal. Chem. 1985, 257, 61. (20) Semonian, B. P.; Rogers, L. B. J . Chromatogr. Scl. 1978, 16, 49. (21) Crowther, J. B.; Henion. J. D. Anal. Chem. 1985, 5 7 , 2711. (22) Wright, R. W.; private communication. (23) Chueh, P. L; Prausnitz, J. M. AICHE J . 1967, 73, 1099. (24) Laver, H. H.; McManigill, D.; Board, R. D. Anal. Chem. 1983, 5 5 , 1370.
RECEIVED for review December 8, 1986. Accepted May 29, 1987. The financial assistance of the Commonwealth of Virginia and Department of Energy Grant DE-FG2284PC70799 is greatly appreciated.
Secondary Ion Emission from Metal Targets under CF,' 0 , ' Bombardment
and
Wilhad Reuter ZBM T. J. Watson Research Center, Yorktown Heights, New York 10598
CF,' has been extracted from a cold cathode Ion gun supplied wlth a CF,/N, gas mixture. A CF,' beam current of about 200 nA is obtained, whlch Is about a factor of 5 smaller than the - 1 pA 0,' beam currents obtalnable from conventlonai oxygen Ion sources. This decrease k partially offset by the hlgher sputter yields obtalned with CF,'. Ionlzatlon probabliltles are hlgher or equal to those found for 0,' bombardment. Composition and chemical state analyses were performed after saturation bombardment. Generally metal carbldes are found to have formed wlth a fluorlne uptake of about 10-20 atom %. The fluorlne is bonded to the metal. Matrlx effects are smaller under 0,' bombardment due to the low fluorlne concentratlon.
It has long been recognized in secondary ion mass spectrometry (SIMS) that the sensitive detection of electropositive elements in a metallic target requires bombardment with primary ions which form strong ionic bonds with the target atoms. Traditionally, ion sources are operated in an oxygen ions. Excellent ambient to produce abundant amounts of 02+ secondary ion yields are obtained for those elements that can be completely oxidized and which form strong ionic bonds with oxygen. It has been shown in a static SIMS study of positive metal ion (M+)emission that for such elements as Mg, Al, Cr, Si, and Fe, more than 10% of all sputtered particles are emitted in the M+ state ( I ) . Secondary ion yields, however, may be less by up to several
0003-2700/87/0359-2081$01.50/00 1987 American Chemical Society
