TOF‐SIMS: Interpreting Negative Ion Spectra - mikee9265/SIMS-Wiki GitHub Wiki

With a simple setting change, a TOF-SIMS instrument can measure negatively charged ions instead of positive. It’s so easy that you would think it would be done more often. It probably should. As a practical matter, there are reasons why negative ion spectra are often omitted from an analysis.

• Much of the work needed in one mode needs to be repeated for the second. Obtaining both positive and negative ion data takes just a little short of twice the time.
• If one has a limited sample (usually because the size of the feature of interest is small), then analyzing both positive ions and negative ions means splitting the possible dose before the static SIMS limit (or if you are not so picky, before the sample is damaged beyond recognition). You may want to save your limited signal-to-noise for one data set.
• For many types of samples, (e.g., hydrocarbon-based polymers) the negative ion spectra can be quite uninformative. Instead of the hydrocarbon envelope, one may see a much weaker set of signals including atomic species and small fragments.

However, there are many electronegative elements and anionic species that can only be detected in the negative ion mode. Ions that are mysterious in positive ion mode may contain heretofore unsuspected elements that become apparent when the negative ion spectra are obtained. There are also certain materials for which the negative ion spectra are richer in information than positive ion spectra.

The information to be found in the mass excess or defect an ion has is the same for negative ions as for positive. Because a negative ion is more likely to lose a H atom than to pick one up and also because it will be more likely to produce an intense signal in negative ion mode if it has heteroatoms such as O or lots of unsaturation, organic fragments in the negative ion mode tend to have less mass excess than those generally found in the positive ion mode.

As in positive ion spectra, atomic species are, for the same reasons, only present at lower masses. In the reverse of the positive ion mode, halides will produce the strongest signals relative to their concentrations, while alkalis will not be detectable. Because of oxygen’s electronegativity, clusters of oxides are generally more pronounced in the negative ion mode.

Odd and even electron species follow the same rules in the negative ion mode as in the positive. As before, odd electron species tend to be rare (although this is less true for per fluorinated species). To the extent that molecular ion species are present, they tend to be present as M−1 even electron ions rather than as the odd electron ion with the same mass as the molecule. As in the positive ion mode, the exception is in cases where the odd electron molecular ion will be well stabilized, usually in highly unsaturated molecules.

One major reason analysts turn to negative ion spectra is to search for counter ions. Positive ion spectra may have revealed an alkali element or a tetra-alkyl ammonium cation, and for a complete picture, they take the negative ion data to determine the nature of the inevitable anion. Halide, sulfide, and oxide anions of all types (carbonate, nitrogen oxide anions, oxidized S, oxidized P, etc.) are readily detected. Unfortunately, the interpretation of the oxide anion spectra is not as straightforward as one might hope. This is because in addition to the anion actually present, fragments and collision products of the anion will also be present. So, for example, surfaces with sulfates and sulfites will display the same peaks, with the major difference being the relative ratios of those peaks.

Standard spectra may be needed to identify which is which. If the sample is amenable to X-ray photoelectron spectroscopy (XPS) analysis, this is a good situation in which a combination of both the methods of analysis may be particularly helpful. The XPS is usually quite capable of determining the compound in which the oxidized element is to be found. Another problem with the various oxide anions is that their intensities can be wildly different in the TOF-SIMS. For example, the TOF-SIMS is quite sensitive to sulfide and sulfate anions, but some of the intermediate S oxide anions produce very weak spectra. Sulfide and sulfate can be detected in the TOF-SIMS at levels beneath the detection limits of Auger and XPS, but some of the intermediate oxides have been readily detected by Auger and XPS, whereas they were not seen in the TOF-SIMS spectra.

Often the search for a counter ion is fruitful, other times not. Surfaces can themselves hold a charge, and so cations may be present without there being an obvious anion. For example, surfaces that are washed with an ionic surfactant will often retain a readily detectable trace of the surfactant that is visible in positive ion mode, but analysis does not show the anionic portion of the surfactant. It is also true that the TOF-SIMS sensitivity for alkali elements is so extreme that these may be easily detected when the anion cannot be found. Finally, OH⁻ is a common fragment in the negative ion spectra of almost any surface containing O. It would be impossible to tell if this were the anionic partner of a cation detected in the positive ion mode.

The CN⁻ ion is another example of a negative ion whose interpretation is not as direct as one might hope. While certainly one will find CN⁻ ions in the negative ion spectra of samples containing cyanide or samples with organic species containing nitrile functionality, it is also detected from a wide variety of other materials. In short, it will be found, and usually will be found to be quite intense, in the spectra of any sample of a species having N bound to C in any kind of structure. Substances such as sputtered carbon with added N and discrete organic molecules containing N will produce spectra with the CN⁻ ion. This can be useful if, for example, there is doubt as to how to interpret a positive ion spectrum with a few even mass ions. To prove that N is present in the organic species on the surface of this sample, simply look for CN⁻ in the negative ion spectrum.

Inorganic oxides can produce quite remarkable negative ion spectra in the TOF-SIMS, with the surface fragment series extending to quite a high mass. The exact masses of the lower mass fragments in these series coupled with the mass differences between the clusters allow ready identification of the formulae for the ions in these series. The example shown below has characteristic 43 amu (AlO) mass differences between the peaks in the series.

As for polymers, the ratios of peaks in these series tell about the materials from which they originate. However, the meaning of the peak ratios of inorganic oxide clusters is less clear in many cases. Polymers often consist of chains, but inorganic oxides are 3D structures. The absence of defects in these structures tends to suppress the formation of high mass inorganic oxide cluster ions. Passing the static SIMS limit and thereby damaging the structure can actually enhance inorganic cluster ion yields.