Progress in Molecular SIMS

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Progress in Molecular SIMS Recent studies provide a detailed picture of sputtering and molecular ion emission as new derivatization techniques result in lower detection limits Secondary ion mass spectrometry (SIMS) is a surface analysis technique in which a primary particle beam strikes a surface, resulting in the emis­ sion of secondary ions that are then detected and counted in a mass spec­ trometer. Of course, the interaction of a SIMS primary beam with a specimen surface is much more complex than this simple description of the process might suggest. Indeed, considerable effort has been expended over the past few years to study the way particle beams deposit energy into surfaces and cause sputtering, the ejection of atoms, mole­ cules, and ions from a surface. In a presentation made at a session on "Modern Methods of Thin Film and Surface Analysis" at the 1986 Eastern Analytical Symposium (EAS) in New York City, Richard J. Colton of the Na­ val Research Laboratory (NRL) point­ ed out that a SIMS primary particle, which usually has an energy in the few keV range, does not simply hit a sur­ face and bounce off like a billiard ball. Instead, it actually penetrates the sur­ face to some depth—perhaps 100 A— and deposits its energy into that sur­ face, creating what is known as a colli­ sion cascade. As this collision cascade evolves, some of the recoiling species are directed back toward the surface where they strike the top layer from beneath, imparting sufficient energy and momentum to cause surface spe­ cies to be desorbed into the ambient vacuum. Soft ionization Research by A. Benninghoven (Univer­ sity of Muenster, West Germany) has demonstrated that the energy deposit­ ed by a primary ion falls off exponen­ tially with distance from the point of impact. If surface molecules are too close to the impact site, they will ac­ quire too much internal energy and will fragment. If they are too far from an impact site, the energy at that point may be too low for desorption to occur.

However, there will be a region around the impact site where the energy avail­ able at the surface is optimal to desorb molecules as intact species. The partition of keV-range primary particle energies into smaller packets of energy (perhaps 10-100 eV) carried by great numbers of recoiling particles creates an environment in which it is even possible to desorb nonvolatile or thermally labile organic molecules without extensive fragmentation. Thus SIMS provides a "soft" ionization source in which proteins, sugars, and other molecules of biological interest can be placed on surfaces in monolayer or submonolayer coverages and then desorbed as intact species by energetic ion or neutral beams. Static (low-damage) SIMS, the tech­ nique used for soft ionization of biomolecules and other higher molecular weight organic species, is characterized by current densities of 1 nA/cm 2 or less. In dynamic SIMS, a much higher cur­ rent density primary beam (>1 μ A/ cm2) erodes the surface at a fast rate and causes extensive fragmentation of surface species. Dynamic SIMS is used to identify trace impurities in inorganic samples (e.g., semiconductors) and for concentration depth profiling, ion im­ aging, and, with instruments such as the ion microscope or ion microprobe, small-area analysis. Barbara Garrison (Pennsylvania State University) has done molecular dynamics simulations that provide a detailed picture of the way organic spe­ cies are desorbed from surfaces with static SIMS. In previously unpublished drawings shown in Figure 1, Garrison simulates the interaction of a primary particle with a system of three benzene molecules on a nickel surface just be­ fore (Figure la) and just after (Figure lb) collision. After impact, the primary particle is implanted in the lattice (bot­ tom right) and the nickel atoms are scattered as a result of numerous sec­ ondary and tertiary recoiling events.

588 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987

The central benzene molecule has sus­ tained a direct hit from the primary particle and has fragmented. The mol­ ecule on the left has been desorbed as an intact species with some additional internal vibrational energy, and the one on the right has been desorbed as a Ni2-benzene complex. The formation of cationized complexes with metal species from the surface is a commonly observed phenomenon in molecular SIMS. Of course, even with static SIMS, de­ sorption can become more of a problem as the molecular weight of the analyte increases. "When you get into proteins and higher mass species, can we ever get them off the surface?" asked Col­ ton rhetorically in his EAS presenta­ tion. "It turns out that nature is a little kind here. As the molecular weight of a species goes up, actual size on the sur­ face does not increase proportionally." For example, Catherine McNeal of Texas A&M University has calculated a molecular area increase of only about a factor of seven (from 5 to 36 A in diameter) as molecular weight in­ creases by a factor of 216 from benzene (78.11) to myoglobin (16,900). The de­ sorption of polyatomic species as high as mass 10,000 has been reported in the literature. Derivatization SIMS In recent research studies, ionization efficiencies for organic molecules have been improved by derivatizing them to form "preformed ions," a technique re­ ferred to as derivatization SIMS. Pre­ formed ions are species such as salts that are present in ionic form on the surface. Colton and co-workers David Kidwell and Mark Ross at NRL, and other investigators such as Kenneth L. Busch of the University of Indiana, have been using derivatization SIMS to detect sugars, aldehydes, and ketones; to sequence peptides; to analyze drugs in body fluids; and for other applica­ tions involving soft ionization of bio-

Primary particle

Figure 1. Molecular dynamics simulation of sputtering of benzene on a nickel surface. (a) Before impact, (b) 3 Χ 10~13 s after impact. (Published with permission of Barbara Garrison and Nicholas Winograd.)

ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987 · 589 A

FOCUS molecules and other organic species. The three most common ionization events in molecular SIMS are the for­ mation of cationized organometallic complexes (such as the nickel-benzene complex in Figure 1), the emission of radical cations, and the direct emission of preformed ions. The ionization effi­ ciencies of these three types of desorption events are highest for preformed ions, intermediate for organometallic complexes, and lowest for radical cat­ ions. "Why do the preformed ions desorb from that surface at much higher abun­ dance than other species?" asked Col­ ton. "What is shown [Figure 2] is a sur­ face that I'm going to bombard on which I've placed two kinds of mole­ cules, one that has no inherent charge and one that is a preformed cation. The first ionization potential of an un­ charged organic molecule is on the or­ der of 8-10 e V, and we probably have to add another eV to desorb it. So it might take on the order of 10 eV to lift it off the surface as an ion. With the preformed ion, we have a cation-anion pair sitting there, and it takes less energy to break that bond, on the order of 6 eV, plus 1 eV to desorb it for a total of 7 eV. "In the figure, energy falls off expo­ nentially from the impact site in the center, so we obtain these concentric rings of available energy per unit area. The neutral molecule can only be desorbed as an ion from the centermost ring. It will also desorb from the outer rings, but only as a neutral species, not as an ion. Because it takes less energy to ionize the preformed ion, it will be desorbed from the area within the two inner circles. Therefore the number of preformed ionic species available is much higher. The ionization efficiency of the preformed ion is a factor of 10 to 100 higher than that of the neutral molecule." Thus derivatization SIMS can be used to improve detection limits for or­ ganic species. In one recent experiment (Figure 3), Colton and co-workers de­ posited a dilute solution of cortisone on a silver surface. "Shown at the top," said Colton, "is the mass spectrum re­ sulting from ion bombardment of 1 μζ of cortisone. We see two species, a protonated molecular ion and a silver ca­ tionized species, in addition to some fragment ions that are characteristic of the structure of this molecule. If we reduce the concentration further [Fig­ ure 3b], we see only the silver-cation ized species, the protonated species having disappeared. "Now, in the third spectrum [Figure 3c], we've reacted cortisone with a hydrazide to form a complex hydrazone," Colton continued. "This reaction will

Path of primary Darticle

Figure 2 . Secondary ion emission. M, molecule without inherent charge. C, preformed cation. Available energies in the areas between adja­ cent rings that arise from a single collision event are shown. (Adapted with permission from Colton, R. J.; Kidwell, D. Α.; Ross, M. M. In Mass Spectrometry in the Analysis of Large Molecules; McNeal, C. J., Ed.; John Wiley & Sons: New York, 1986; pp. 13-48.)

Figure 3. SIMS spectra of cortisone (a, b) and derivatized cortisone (c, d). See text. (Adapted with permission from Ross, M. M.; Kidwell, D. Α.; Colton, R. J. Int. J. Mass Spectrom. Ion Proc. 1985, 63, 141-48.)

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FOCUS work with molecules containing a keto or aldehyde group. Because we've add­ ed exactly 134 mass units to the mole­ cule, we find the molecular ion of this preformed ion at ml ζ 494—360 plus 134—at fairly high intensity and with an order-of-magnitude decrease in its concentration on the surface. So we've

"We've picked up an order of magnitude in sensitivity by going with derivatization." Richard Colton picked up an order of magnitude in. sensitivity by going with derivatization. The last spectrum [Figure 3d] shows that we can reduce the concen­ tration another order of magnitude and still have ample signal to detect this species. This demonstrates the capa­ bility of using derivatization proce­

dures to increase detection sensitivity." Drug-screening applications Why is the NRL interested in derivatiza­ tion SIMS? The Navy currently has five drug-screening laboratories in the United States that test military per­ sonnel for drugs such as marijuana, cocaine, LSD, and opiates. The primary screening is done by ra­ dioimmunoassay. If that result is positive, it is con­ firmed with a second, in­ dependent method such as gas chromatography/ mass spectrometry (GC/ MS). According to Colton, "Some species such as LSD are used in such small quantities that, by the time your body me­ tabolizes them, there is very little left in the urine. Even GC/MS has trouble detecting LSD and some of the design­ er drugs that are now becoming popu­ lar. Therefore we undertook a course of study in which we introduced several other techniques, including derivatiza­ tion SIMS, to demonstrate better sen­

sitivity for some of these drugs of abuse." For example, the derivatization SIMS technique has been applied to the detection of a 3-ng mixture of methamphetamine, methadone, qui­ nine, and propoxyphene in urine. A small amount of CH 3 I is first added to the sample to form methylated quarternary salt derivatives with the four drugs. "We put a few microliters of this sample on a silver substrate and placed it in the SIMS instrument," Colton said. "That was the extent of our sam­ ple preparation. No extraction or cleanup was needed." Colton and co­ workers found that SIMS peak intensi­ ties for the derivatized species were im­ proved by a factor of 10 over those ob­ tained from the analysis of an underivatized mixture spiked into urine at the same concentration level. In addition, says Colton, "For LSD, SIMS provides better detection sensi­ tivity than thin-layer chromatography, liquid chromatography/MS, or direct insertion probe MS, and SIMS detec­ tion limits are well below the limits of GC/MS. This is important because, when someone is taking LSD, what you find in urine is at the subnanogram level." Stu Borman

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592 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987

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