Anal. Chem. 1983, 55, 1157-1160
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Matrix Efffects in Secondary Ion Mass Spectrometry Kenneth L. Bwsch, Blh Hsuing Hsu, Ya-Xlang Xie, and R. Graham Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
An ammonium chloride matrlx used In secondary ion mass spectrometry of organlc salts leads to enhanced signals due to the Intact ion and to decreased fragmentation. Slmllar results are reported with the use of other solid and liquid matrices, but ammonlum chlorlde Is advantageous In that it does not contribute to the spectral background. As a result, data can be obtained from very dilute mixtures, and analyses of organlc salts at the Row parts-permillion level in ammonium chlorlde are presented. The results are Lnterpreted to suggest mechanisms In which large clusters of Ions and molecules are sputtered from the surface, with subsequent dlssociatlon leading to Intact anaiyte Ions. Large scale mixlng of molecules accompanles energetic Ion beam bombardment.
Ions from nonvolatile, thermally fragile, and high molecular weight compounds are created by the techniques of desorption ionization mass spectrometry (I). Particular methods include secondary ion mass spectrometry (SIMS) (2), fast atom bombardment (FAB)( 3 ) , laser desorption (LD)(4),and fission fragment or plasma desorption (PD) (5). Energetic ions, atoms, photons, and nuclear fission fragments produce similar spectra (6). These contain ions which indicate molecular weight (e.g., (M H)+, (M Na)+, and (M - H)-, or C+ and A- for ionic salt8 CA) as well as fragment ions, many of which have been shown to arise by unimoleculdlr dissociation of the sputtered parent ions (7). Matrix effects have been of recent interest in desorption ionization studies. In SIMS, the effect of elemental matrices on atomic ion yields has been investigated (8, 9). For molecular SIMS, Michl used a frozen argon gas matrix to sample small hydrocarbons (10, 11). More recently, matrix assisted SIMS using ammonium chloride at room temperature has been advocated (12,13) and extensively employed in studying organometallic and inorganic compounds (14,15). Intermolecular reactions and cluster ion formation are reduced with ammonium chloride or with a polyphenyl ether liquid matrix (16).Most significantly, the signal-to-noise ratios in spectra of samples in ammonium chloride are not reduced, even at dilutions of 1:200, and in fact, the absolute ion abundances are enhanced in some cases. The use of liquid matrices in fast atom1 bombardment mass spectrometry (17)has multiplied interest in the matrix effect in particle-induced desorption. Initially, glycerol was used for most work, but the list of solvents has expanded to include thioglycerol, dimethyl sulfoxide, diethanolamine, poly(ethy1ene glycol), various surfactants, and other substances. Comparisons have been made (18-20) with SIMS spectra obtained by using some solvents and these show no significant differences. Many investigators have simply eought solvents from which an ion characteristic of molecular weight can be desorbed, and the effects of the matrix on the spectra have not been carefully studied. In this paper, SIMS results are reported which clarify the role of the matrix with respect to its analytical effects (effects on S I N ratio and on relative and absolute ion abundances), and the mechanistic implications of these observations are discussed.
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EXPERIMENTAL SECTION The SIMSinstrument has been described (2,13).The substrate in all experiments was roughened 0.005 in. thick silver foil onto which a solid sample is burnished or a solution is syringed (with the solvent removed under vacuum). The sample is bombarded by argon ions of 4.5 keV energy at an angle of incidence of 45O. The flux of primary ions is measured at the sample by a picoammeter. Measured currenb of up to 2 X lo4 A/cm2 were used, with a spot size of 5 mm2. Scar? speeds of 5 amu/s were typically used. For high dilution (greater than 1:lOO) experiments with an ammonium chloride matrix, the weighed sample was dissolved in a water-ethanol mixture (9010 v/v). Serial dilutions reduced the sample concentration; an appropriate amount of ammonium chloride was added to the diluted solution. Several microliters of the solution were placed directly on the silver foil. To obtain physical mixtures for low dilution experiments, sample and ammonium chloride were finely ground together in a mortar and pestle. For liquid matrices, powdered sample was mixed in to form a slurry which was then placed on the silver planchet for analysis. All experiments were performed under static SIMS conditions.
RESULTS AND DISCUSSION Figure l a shows the mass spectrum of a neat quaternary salt taken under standard SIMS conditions (argon ions at a A/cm2). The intact cation a t mlz 390 is flux less than desorbed, and hydrogen rearrangement with elimination of neutral cyclohexene produces the fragment ion a t mlz 308. This fragmentation to an even electron ion is typical of the SIMS spectra of organic salts (21), and the occurrence of analogous elimination reactions when mass-analyzed quaternary ammonium ions are activated by gas-phase collisions (13,22) suggests the SIMS fragmentations occur unimolecularly. Figure lb,c shows positive ion SIMS spectra of this same pyridinium salt, physically mixed with ammonium chloride at dilutions of 1:2 and 1:20, respectively. As noted previously for other compounds (12,13), no reduction in SIN is observed. The data also show that the extent of fragmentation decreases with increased dilution. The same effect has been observed with many other organic and inorganic salts. For example, in the case of candicine chloride [ (4-hydroxyphenethy1)trimethylammonium chloride], the ratio of the intact cation a t m / z 180 to the fragment ion at mlz 121 (loss of trimethylamine) increases from about 1:l in the spectrum of the neat compound to a final value of about 5:l upon dilution in ammonium chloride by 1:lOOO. Dispersal of the sample in a liquid matrix has also been investigated. Results are given in Figure 2 which shows the positive ion SIMS spectrum of a small organic salt analyzed as the bulk organic (a) or mixed (1:2 dilution) with poly(ethylene glycol) to form a mull. The upper spectrum shows, as expected, a signal for the intact cation at m / z 137 and fragmentation by loss of a small neutral molecule (CH,CNO) to yield protonated pyridine a t mlz 80. The ratio 137'180' is increased when the sample is examined in the matrix, as seen in the lower spectrum. However, the ether itself gives rise to abundant ions a t mlz 45,73,87,89,133, and 147 (and others outside of this mass range) as well as contributing to a general increase in the noise level a t all masses. As a consequence, dilute solutions and suspensions cannot be examined. This same effect limits most FAB work with glycerol
0003-2700/83/0355-1157$01.50/00 1983 American Chemical Socletv
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
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DILUTION I: 50,000 NH,CI from solution onto silver foil 4.5 keV Art 2 x IO-’ A
MATRIX CONTROL
OF INTERNAL ENERGY
Ph
intact cotton
1390‘
m/z
Figure 3. Spectra of this quaternary ammonium salt can be obtained with good S I N ratlo even when diluted in a matrix at the 1:50000 level. The fragmentation pattern is similar to that observed at lower dilutions (compare Figure IC).
(C)
308
I 2 0 NH4CI
A 300 400 m/z Figure 1. Positiie ion SIMS spectrum of a quaternary ammonium salt: (a) neat, (b) diluted 1:2 in ammonium chloride, (c) diluted 1:20.
la1 neat ~
Figure 4. The intact cations of organic salts in an ammonium chlorlde matrix can usually be observed with good S I N at dilutions of up to several thousand, as shown here.
n/z
Figure 2. The spectrum of this pyridinium salt shows decreased fragmentation when the sample Is dispersed in a liquid matrix of poly(ethy1ene glycol) (1:2). Many ions derived from the matrix are present, however, and may cause interferences at higher levels of dilution.
to the 1:lOO to 1:lOOO concentration level (3). With an ammonium chloride matrix, previous studies (13) had shown dilutions of up to several hundred to be feasible, and work with tropylium salts (23)had shown that dilutions of up to several thousand fold in NHICl did not preclude observation of the organic cation (but did reduce the occurrence of complicating addition reactions). The present work extends these studies to even lower levels. Figure 3 is the SIMS spectrum obtained when the quaternary salt shown is diluted (via solution, see Experimental Section) 1:50000 with ammonium chloride. Both the intact cation a t mlz 390 and the fragment ion at mlz 308 are observed with good SIN ratio. There are no other ions in the spectrum in the mass range 20-500 except for Ag,+, n = 1-3, and m/z 388; the ubiquitous (M - H)+or (C - Hz)+ ion (1). Analysis of this organic salt at the 5 ppm level in the matrix (prepared from solution) can be achieved, although the signal-to-noise ratio is lower, and the signal fades after a few minutes of bombardment. The spectra obtained a t these very high dilutions (total sample size of 100 pg, that is, 0.5 ng of analyte) contain the same intact
cationlfragment ion abundance ratios as that obtained at a dilution of 1:20, indicating a leveling in the effect of dilution upon fragmentation. Positive ion SIMS spectra have also been obtained for other pyridinium salts and also pyrylium and phosphonium salts at dilutions of up to several thousand in ammonium chloride; two examples are given in Figure 4. Reductions in the amount of fragmentation seen in a SIMS spectrum have been observed to result from other sample treatments, although the mechanism is thought to be different. Derivatization reactions (24) developed to increase absolute intensities of quasi-molecular ions in SIMS and laser desorption experiments have the overall effect of decreasing fragmentation. A further example is the SIMS spectrum of MoC14(PCzH5Phz)2; fragmentation to give the diphenylphosphonium ion is decreased by a fador of 4 when the sample is treated with p-toluenesulfonic acid (25). These results with ammonium chloride suggest a model which explains matrix effects in terms of cluster ion desorption and decomposition. Three experimental findings must be accommodated: (i) gross physical mixtures of analyte and matrix are sampled as if they are homogeneous solutions, (ii) increasing matrix dilution results in decreasing ion fragmentation, and (iii) spectra can be obtained from organic salts present in vanishingly small concentrations in ammonium chloride matrices. The first observation is exemplified by reductions in the extent of intermolecular reactions observed when mixtures of ammonium chloride and sample are analyzed. The characteristic (C 14)+ion often seen in desorption ionization spectra of organic ammonium salts can be reduced in intensity or eliminated totally by admixture of the sample with the matrix (13). Since the sample and matrix are ground together in a mortar and pestle, a homogeneous solid solution at the molecular level is not achieved, but the spectrum belies this. It would seem that mixing occurs through the agency of the primary ion beam upon energization of the bulk sample. The
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
, ,(CH,),Nt
1-
mixed with
@ C+ ENERGIZATION
\
intact cations
CAC+ clusters
Cf
Figure 5. Mixed cluster ions can be formed from simultaneous irradiation of two physlcally separate and isotopically distinct quaternary ammonium salts. When the halogen counterions differ, clusters of cations with both of the anions are observed.
requirements of sample preparation are eased because solution a t the molecular level is apparently achieved during the sampling process. Atomic mixing is a well documented phenomenon in secoindary in mass spectrometry (26); apparently the process extends to the molecular level. Direct evidence for such mixing on a macroscopic scale comes from experiments in which mixed cluster ions are observed to arise from mixed but physically discrete crystalline salts CA and CIAl subjected simultaneously to ion bombardment. Figure 5 illustrates this effect for a mixture of isotopically distinct ammonium salts (CH3)4N+I-and (CH3)3CD,N+I-in which one salt was burnished directly onto a supporting planchet and crystals of the second dispersed over the first. All possible combinations of the cation and anion are observed, although the covalent bonds within the cations remain intact and the integrity of the isotopic label is retained. Similar observations have been made in fast atom bombardment in which mixing on even a larger scale could be investigated (27). The second observation to be accounted for is that control over the extent of fragmentation can be exercised, with decreased fragmentation at higher dilution. Under static SIMS conditions, thermal fragmentations are not expected to contribute to the spectrum. [Increased fragmentation can be achieved by using higher primary ion fluxes (28) and higher surface temperatures (29).] Rather, since the products observed result from unimolecular proclesses, the amount of internal energy in the gas phase ion determines the degree of fragmentation. The data suggest that the internal energies of ions desorbed from these matrices are decreased with increasing dilution. If sputtering liberates large clusters from the surface (30, 31), then this could ;account for both the second and third experimental facts. The sputtered clusters contain molecules which can act to solvate the ions. Internal energy can then be dissipated by a desolvation process illustrated in Figure 6. Excess energy is used to break bonds, and the weakeet bonds within the cluster structure are those between solvent molecules, including CA, and ions of the analyte. Jettisoned solvent molecules carry away some portion of the total energy of the complex. Repetition of this process leads to stable C+ ions, or C+ ions clustered with one or several solvent molecules, as can sometimes be observed in FAB (3). Desolvation is, for a v,wiety of systems (positively and negatively charged, metal and nonmetal containing), the favored process, giving ion intensities an order of magnitude greater than those due to fragmentations of nonsolvated ions. Examples in which this has been explicitly shown by tandem mass spectrometry include silver ion solvated by olefins (32), sodium by alcalhols of various sizes (33),and proton-bound dimers (34). A related explanation for the effect on fragmentation suggests a process of collisional relaxation arising
/
aL/ @ r
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i’
@
DES0,VAT
ON
For preformed ions C+ and solvent molecules S, the desorption/desolvation mechanlsm shown here results in relaxation of ions and reduced fragmentation. Flgure 6.
from interaction between the sputtered sample ions and the localized high pressure of gas released by dissociation of the matrix. This process must occur in the selvedge in order to account for the magnitude of the effect. The distinction between this and the process postulated here is extremely fine. Support for the ejection of large clusters from surfaces is given explicitly in recent work (31). ( A referee has suggested an alternative explanation of the decreased fragmentation on dilution. He proposes an enrichment of the quaternary salt at the phase (or grain) boundary of NH4Cl microcrystals and suggests that the high lattice energy of NH4Cl makes increased fragmentation likely in the absence of this effect.) These issues are not settled but the decluster mechanism (30, 31) is a reasonable working hypothesis. Interestingly, an ion at m / z 18 for NH4+from the ammonium chloride matrix is very seldom observed, and this particular matrix seems to be “transparent”. Ammonium halides in general have low heats of sublimation, and a mechanism of dissociation into NH3 and H X has been described (35). Desolvation may be favored because of this property; the end result is a desorbed ion buffered by an expanding gas envelope. Other compounds with similar sublimation properties should also form comparable matrices. Detailed studies have not yet been undertaken, but in the dilution range most often used (1:lO to 1:100),ammonium nitrate and tetraethylammonium chloride have been observed to behave similarly to ammonium chloride. Under conditions of high flux atom bombardment (as contrasted to these experiments under static SIMS conditions), ammonium chloride forms clusters C,+,A,+ extending past m / z 2000, with evidence for enhanced stability for ions with regular structures. However, cluster ions of ammonium chloride with organics have not been observed, although protonation is increased in its presence. Solvation with an ionic matrix such as ammonium chloride differs from the hydrogen-bonded interactions with an organic solvent such as glycerol, although the change in the degree of fragmentation is in the same direction. Mass spectra obtained directly from droplets of effluent from a liquid chromatograph resemble those generated by desorption ionization techniques (36,37),supporting a view of sputtering as a supermolecular phenomenon. The common cationized species (C + M)+ observed in these spectra are the products of the penultimate desolvation reaction and represent a particularly stable end point (38). An alternative view of surface sputtering, based on dynamical calculations, postulates that cationization results from a near-surface collision process (39), although similar studies concede an emission of large conglomerates (40). It seems probable that both cluster dissociation and association reactions are occurring in the selvedge. Experimental parameters determine their relative contributions; in addition, real surfaces encompass a varied morphology which may promote the emission of aggregates. The present data refocus attention on selvedge processes as
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Anal. Chem. 1983,
the key in establishing surface/spectrum relationships and suggest that the process of desolvative relaxation may be used to study species which would otherwise not survive the desorption process. A particularly important inference to be drawn from this study is this: even when sputtering is done under such mild conditions that molecular ions are ejected intact and there is no evidence in the mass spectra of surface damage, molecular reordering occurs over large distances. This effect may be inherent to sampling with energetic ion beams, and if so, it implies that molecular microscopy using probe ion beams (2) will have limited resolution.
ACKNOWLEDGMENT We thank A. R. Katritzky for valuable discussion and provision of some of the samples and R. J. Day and S. E. Unger for contributing to early stages of this work.
LITERATURE CITED (1) Busch, K. L.; Cooks, R. G. Science 1982, 218. 247. (2) Day, R. J.; Unger, S.E.; Cooks, R. G. Anal. Chem. 1980, 52, 557A. (3) Barber, M.; Bordoli, R. S.;Elliott, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A. (4) Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A.; LI, C. P. Anal. Chem. 1882, 5 4 , 280A. (5) Macfarlane, R. D. Acc. Chem. Res. 1982, 15, 268. (6) Ens, W.; Standing, K. G.; Chait, B. T.; Field, F. H. Anal. Chem. 1981, 5 3 , 1241. (7) Chait, B. T.; Field, F. H. Int. J. Mass Spectrom. Ion Phys. 1981, 41, 17. (8) Deline, V. R.; Katz, W.; Evans, C. A,, Jr.; Williams, P. Appl. Phys. Lett. 1878, 33, 832. (9) Willlams, P. Appl. Surf. Sci. 1982, 13, 241. (10) Jonkman, H. T.; Michl, J. J. Chem. SOC., Chem. Commun. 1978; 751. (11) . . Jonkman. H. T.: Michl. J.: Kina. R. N.; Andrade, J. D. Anal. Chem. 1978, 50, 2078. (12) Llu, L. K.; Busch, K. L.; Cooks, R. G. Anal. Chem. 1981, 5 3 , 109. (13) Unger, S. E.; Day, R. J.; Cooks, R. G. Int. J. Mass Spectrom. Ion Phys. 1981, 39, 231. (14) Pierce. J. L.; Busch, K. L.; Cooks, R. G.; Walton, R. A. Inorg. Chem. 1982, 2 1 , 2597.
55,1160-1165 (15) Pierce, J. L.; Wlgley, D. E.; Walton, R. A. Organometallics 1982, 1, 1328. (16) Busch, K. L.; Unger, S. E.; Cooks, R. G. Proceedings of the 1980 Miinster Conference on Ion Formatlon from Organic Solids, In press. (17) Surman, D. J.; Vlckerman, J. C. J. Chem. Res., Synop. 1981, 170. (18) Harada. K.; Suzukl, M.; Kambara, H. Org. Mass. Specfrom. 1982, 17, 386. (19) Kambara, H.; Ogawa, Y.; Mochizuki, K.; Hlshida S.Mass Spectrosc. 1982, 30, 169. (20) Aberth, W.; Straub, K. M.; Burlingame, A. L. Anal. Chem. 1982, 5 4 , 2029. (21) Ba-isa, A.; Busch, K. L.; Cooks, R. G.; Vlncze, A.; Granoth, I . Tefrahedron, in press. (22) Gllsh, G. L.; Todd, P. J.; Busch, K. L.; Cooks, R. G., unpublished work, Oak Rldge Natlonai Laboratory, 1982. (23) Ba-lsa, A.; Cooks, R. G., unpubllshed work, Purdue University, 1981. (24) Busch, K. L.; Unger, S. E.; Vlncze, A.; Cooks, R. G.; Keough, T. J. Am. Chem. SOC. 1982, 104, 1507. (25) Plerce, J. L. Ph.D. Thesls, Purdue University, 1982. (26) McHugh, J. A. in "Methods of Surface Analysls"; Czanderna, A., Ed.; Elsevler: New York, 1975. (27) Evans, S.,personal communicatlon, Sept 1981. (28) Unger, S.E.; Ryan, T. M.; Cooks, R. G. Anal. Chim. Acta 1980, 118, 169. .. (29) Bennlnghoven, A.; Lange, W.; Jlrikowsky, M.; Holtkamp, D. Surf. Sci. 1982. 123. L721. (30) Jonkman. J. T.; Michl, J. J. Am. Chem. SOC. 1981, 103, 733. (31) Barlak, T. M.; Wyatt, J. R.; Colton, R. J.; Decorpo, J. J.; Campana, J. E. J. Am. Chem. SOC. 1982, 104, 1212. (32) McLuckey, S. A.; Schoen, A. E.; Cooks, R. G. J. Am. Chem. SOC. 1882, 104, 848. (33) McLuckey, S. A. Ph.D. Thesis, Purdue University, 1982. (34) Wright, L. 0.; McLuckey, S. A.; Cooks, R. G.; Wood, K. V. I n f . J. Mass Spectrom. Ion Phys. 1982, 42, 115. (35) Chaiken, R. F.; Slbbett, D. J.; Sutherland, J. E.; Van de Mark, D. K.; Wheeler, A. J. Chem. Phys. 1962, 37, 2311. (36) Blakley, C. R.; Carmody, J. J.; Vestal, M. L. Anal. Chern. 1980, 5 2 , 1636. (37) Arpino. P. J.; Gulchon, G. J. Chromatogr. 1982, 251, 153. (38) Rollgen, F. W.; Glessmann, U.; Levsen, K. Adv. Mass Spectom. 1980, 8, 997. (39) Garrison, B. J. J. Am. Chem. SOC. 1982, 104, 6211. (40) Heyes, D. M.; Barber, M.; Clarke, J. H. R. Surf. Scl. 1981, 105, 225.
RECEIVED for review December 13,1982. Accepted March 2, 1983.
Trace Element Analysis of Natural Water Samples by Neutron Activation Analysis with Chelating Resin R. R. Greenberg" and H. M. Kingston Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
Procedures are descrlbed to preconcentrate the trace elements In 100, 200, or 500 mL of natural water into a soild sample of approxlmateiy half a gram by using a chelatlng resln. These procedures are appllcable to both freshwater and seawater and leave the transHion metals and other elements of Interest essentlaily free from the aikail metals, the aikallne earth metals, and the halogens. The concentratlons of 15 elements In one seawater sample have been determined by using thls separatlon procedure coupled with the neutron activation analysis technique.
Seawater and other high-salinity natural water samples are among the most difficult materials to analyze for trace elements. The extremely high concentrations of the alkali and alkaline earth metals and the halogens make direct analysis by most analytical techniques difficult or impossible, while the extremely low levels of the transition metals and other
elements of interest make the analytical and sampling blanks critical. Neutron activation analysis (NAA) has the sensitivity and accuracy to determine a number of important trace elements in seawater at their naturally occurring levels (1). Unfortunately, a salt water matrix is not well suited for activation analysis. The use of liquid samples limits both the amount of material and the length of irradiation available in most reactor facilities. The high levels of Na, C1, and Br produce an extremely high background level of radiation that totally obscures the signals of most other elements whose neutron activation products have comparable half-lives. A number of preconcentration/separation procedures have been developed for trace elements in high-salinity water samples by use of the chelating resin Chelex-100 (2-5). Earlier procedures, however, were only partially successful in eliminating the alkali and alkaline earth metals, while a more recent procedure (6) left the elements of interest in a dilute nitric acid/ammonium nitrate solution. While the acid-nitrate
This article not subject to US. Copyright. Published 1983 by the Amerlcan Chemical Society
