Comparison of primary monoatomic with primary polyatomic ions for the characterisation of polyesters with static secondary ion mass spectrometry

Static secondary ion mass spectrometry (S-SIMS) emerges as one of the most adequate methods for the surface characterisation of polymers with an information depth of essentially one monolayer. The continuing search for increased analytical sensitivity and specificity has led to exploring the use of polyatomic primary ions as an alternative to the traditionally applied monoatomic projectiles. As part of a systematic investigation on polyatomic bombardment of organic and inorganic solids, this paper focuses on selected polyesters. Mass spectra and ion yields are compared for layers deposited on silicon wafers by spincoating solutions with different concentrations of poly(epsilon-caprolactone) (PCL), poly(butylene adipate) (PBA) and poly(ethylene adipate) (PEA). Accurate mass measurements have been used to support the assignment of the ions and link the composition of the detected ions to the analyte structure. Use of polyatomic projectiles increases the yield of structural ions with a factor of +/-15, +/-30 and +/-10 for PCL, PBA and PEA, respectively, in comparison to bombardment with Ga+ primary ions, while the molecular specificity is improved by the detection of additional high m/z ions.     

Ion yield improvement for static secondary ion mass spectrometry by use of polyatomic primary ions

Static secondary ion mass spectrometry (S-SIMS) is one of the potentially most powerful and versatile tools for the analysis of surface components at the monolayer level. Current improvements in detection limit (LOD) and molecular specificity rely on the optimisation of the desorption-ionisation (DI) process. As an alternative to monoatomic projectiles, polyatomic primary ion (P.I.) bombardment increases ion yields non-linearly. Common P.I. sources are Ga+ (liquid metal ion gun (LMIG), SF5+ (electron ionisation) and the newer Au(n)+, Bi(n)q+ (both LMIG) and C60+ (electron ionisation) sources. In this study the ion yield improvement obtained by using the newly developed ion sources is assessed. Two dyes (zwitterionic and/or thermolabile polar functionalities on a largely conjugated backbone) were analysed as a thin layer using Ga+, SF5+, C60+, Bi+, Bi3(2+) and Bi5(2+) projectiles under static conditions. The study aims at evaluating the improvement in LOD, useful and characteristic yield and molecular specificity. The corrected total ion count values for the different P.I. sources are compared for different instruments to obtain a rough estimate of the improvements. Furthermore, tentative ionisation and fragmentation schemes are provided to describe the generation of radical and adduct ions. Characteristic ion yields are discussed for the different P.I. sources. An overview of the general appearances of the mass spectra obtained with the different P.I. sources is given to stress the major improvement provided by polyatomic P.I.s in yielding information at higher m/z values.     

Chemical characterisation of different separation media based on agarose by static time-of-flight secondary ion mass spectrometry

In this paper, the novel application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for qualitative and semi-quantitative investigation of the surface chemistry of separation media based on beaded agarose is reported. Five different media were studied: DEAE Sepharose Fast Flow, Q Sepharose Fast Flow, SP Sepharose Fast Flow, Phenyl Sepharose Fast Flow at ligand densities between 7 and 33% (w/w) and the base matrix Sepharose 6 Fast Flow. The obtained TOF-SIMS spectra reveal significant chemical information regarding the ligands (DEAE, Q, SP and Phenyl) which are covalently attached to the agarose-based matrix Sepharose 6 Fast Flow. For the anion-exchange media (DEAE and Q Sepharose Fast Flow), the positive TOF-SIMS spectra yielded several strong characteristic fragment peaks from the amine ligands. Structural information was obtained, e.g. from the peak at m/z 173.20, originating from the ion structure [(C2H5)2NCH2CH2NH(C2H5)2l+, which shows that the ligand in DEAE Sepharose Fast Flow is composed of both tertiary and quaternary amines. The positive spectrum of Phenyl Sepharose Fast Flow contained major fragments both from the base matrix and the ligand. The cation-exchanger (SP Sepharose Fast Flow) gave rise to a positive spectrum resembling that of the base matrix (Sepharose 6 Fast Flow) but with a different intensity pattern of the matrix fragments. In addition, peaks with low intensity at m/z 109.94, 125.94 and 139.95 corresponding to Na2SO2+, Na2SO3+ and Na2SO3CH2+, respectively, were observed. The positive TOF-SIMS spectrum of Sepharose 6 Fast Flow contains a large number of fragments in the mass range up to m/z 200 identified as CxHyOz and CxHy structures. The results clearly show that positive TOF-SIMS spectra of different media based on Sepharose 6 Fast Flow are strongly influenced by the ligand coupled to the matrix. The negative TOF-SIMS spectra contained several ligand-specific, characteristic peaks for the cation-exchanger, having sulphonate as the ion-exchange group. Negative fragments such as S-, SO-, SO2-, SO3-, C2H3SO3-, C3H5SO3- and OC3H5SO3- were observed. Phenyl Sepharose Fast Flow, which has an uncharged group (Phenyl) coupled to the agarose matrix yielded one ligand-related peak corresponding to the C6H5O- fragment. DEAE and Q ligands could only be identified by the appearance of the fragments CN- and CNO- in the negative spectrum. However, a strong peak corresponding to the counter ion (Cl-) was observed. TOF-SIMS analysis can also be used for the investigation of residues from the coupling procedure that bonds the ligands to the matrix. One example is the observation of bromine peaks in the negative spectrum of Q Sepharose Fast Flow. Furthermore, it has also been shown that different ligand concentrations of Phenyl Sepharose Fast Flow can easily be detected by TOF-SIMS analysis. Information regarding the difference between the ligand density on the surface of the beads and in the bulk can also be obtained. However, spectra registered on the outermost surface and on the pore surface (crushed beads) of DEAE Sepharose Fast Flow clearly show that the agarose and the DEAE groups are homogeneously distributed in the beads.     

Comparison of mono- and polyatomic primary ions for the characterization of organic dye overlayers with static secondary ion mass spectrometry

Organic carbocyanine dye coatings have been analyzed by time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) using three types of primary ions: Ga(+) operating at 25 keV, and Xe(+) and SF(5) (+) both operating at 9 keV. Secondary ion yields obtained with these three primary ions have been compared for coatings with different layer thickness, varying from (sub)-monolayer to multilayers, on different substrates (Si, Ag and AgBr cubic microcrystals). For (sub)-monolayers deposited on Ag, Xe(+) and SF(5) (+) primary ions generate similar precursor ion intensities, but with Ga(+) slightly lower precursor ion intensities were obtained. Thick coatings on Ag as well as mono- and multilayers on Si produce the highest precursor and fragment ion intensities with the polyatomic primary ion. The yield difference between SF(5) (+) and Xe(+) can reach a factor of 6. In comparison with Ga(+), yield enhancements by up to a factor of 180 are observed with SF(5) (+). For the mass spectrometric analysis of dye layers on AgBr microcrystals, SF(5) (+) again proves to be the primary ion of choice.     

Organic ion imaging beyond the limit of static secondary ion mass spectrometry

Secondary ion mass spectra and images were obtained from spikes of choline chloride, acetylcholine chloride, and methylphenylpyridinium iodide deposited onto specimens of porcine brain tissue. Samples were subsequently subjected to a dose of 10-keV Cs⁺ sufficient to suppress secondary ion emission characteristic of the targeted analytes. Following ablation of the samples by massive glycerol clusters generated by electrohydrodynamic emission, secondary ion mass spectra and images could be obtained that reflected the identity and location of the spiked analytes. The absolute intensity of secondary ion emission that followed ablation was found to be between 30 and 100% of the intensity obtained prior to exposure to the high dose of Cs’. Not all chemical noise is removed by ablation, however, so that the signal-to-noise ratios after ablation correspond to between 10 and 85% of their values observed under conditions of low primary ion dose.     

Microscopical speciation analysis with laser microprobe mass spectrometry and static secondary ion mass spectrometry

This paper presents a set of data which compares the potential and limitations of laser microprobe mass spectrometry (TOF-LMMS and FT-LMMS) and static secondary ion mass spectrometry (S-SIMS) for inorganic speciation at a microscopical level. In general LMMS yields prominent signals of adduct ions consisting of the intact molecule combined with a stable ion, which allows a direct identification of the analyte. S-SIMS also yields abundant diagnostic signals to specify the molecular composition. However, adduct ions are not always present, which means that the identification often relies on fingerprinting. Results further indicate that the potential and the application area of S-SIMS and FT-LMMS are complementary to one another.     

Speciation analysis of oxides with static secondary ion mass spectrometry

Speciation analysis of inorganic solids, without dissolution of the sample, aims at specific molecular information. Two potentially useful microanalytical techniques emerge, namely, laser microprobe mass spectrometry (LMMS) and static secondary ion mass spectrometry (S-SIMS). This paper focuses on the molecular characterisation of oxides by application of the S-SIMS method. For this purpose, mass spectra of pure oxides were acquired under static conditions. Analytical parameters such as repeatability, accuracy and resolution were assessed. Also, the peak patterns in the mass spectra are discussed in connection with the older Plog model, describing the relative ion yield as a function of the cluster size. Finally, a comparison is made with the mass spectra from a S-SIMS library and with those obtained by Fourier transform LMMS. Copyright 1999 John Wiley & Sons, Ltd.     

Molecular weight determination of bulk polymer surfaces by static secondary ion mass spectrometry

The determination of molecular weights at surfaces of bulk polymer materials can be accomplished by static secondary ion mass spectrometry (SIMS) via fragments originating from repeat units and end groups. The intensity ratio of these fragments depends on the polymer chain length as seen for bisphenol-A-polycarbonate and perfluorinated polyethers (Krytox). A kinetic model of fragment ion formation explains the molecular weight dependent fragment intensities and links them to properties of the molecular weight distribution. In the most simple case one obtains the number average molecular weight <M_n> at the surface. This technique can be used for the determination of the molecular weight at bulk polymer surfaces such as a CD-ROM made from polycarbonate by injection molding.     

Static secondary ion mass spectrometry for nanoscale analysis: surface characterisation of electrospun nanofibres

The viability of static secondary ion mass spectrometry (S-SIMS) for selected applications of nanoscale analysis has been investigated, focusing on nanofibres produced by electrospinning (ES) as a test case. The samples consist of non-woven nanowebs of which the individual fibres have diameters in the range of 100 nm. Use of solutions with functionalised polymers or polar additives potentially allows the surface composition to be tailored as a function of the application. So far nanowebs are primarily characterised by morphological examination. This paper describes the first detailed characterisation of molecular composition at the surface of nanofibres electrospun from poly(epsilon-caprolactone) (PCL) solutions in acetone containing 0-15 mol% (relative to PCL) of cetyltrimethylammonium bromide (CTAB). Application of S-SIMS to nanowebs has allowed mass spectra to be recorded containing the major diagnostic ions of both components. Their relative intensities point to surface enrichment and depletion of the polar CTAB additive relative to the PCL matrix for samples electrospun from solution containing low and high CTAB concentrations, respectively.     

Ion characterisation by comparison of ion mobility spectrometry and mass spectrometry data

The major uncertainty related to ion mobility spectrometry is the lack of knowledge about the characteristics of the ions detected. When using a radioactive atmospheric pressure ionisation source (e.g. ⁶³Ni), from theory proton bound water clusters are expected as reactant ions. When analyte ions occur, proton transfer should lead to proton-bound monomer and dimer ions. To increase the knowledge about those ionisation processes in an ion mobility spectrometer (IMS), a ß-radiation ionisation source was coupled to a mass spectrometer (MS) and an identical one to an IMS. Exemplarily, acetone, limonene and 2- and 5-nonanone were introduced into both instruments in varying concentrations. By correlating the MS and IMS spectra, conclusions about the identities of the ions detected by IMS could be drawn. Proton-bound monomer, dimer and even trimer ions (MH⁺, 2MH⁺, 3MH⁺) could be observed in the MS spectra for acetone and 5-nonanone and could be assigned to the related signals detected by IMS. The oligomers could be expected from theory for increasing concentration. Limonene and 2-nonanone yielded in a variety of different ions and fragments indicating complex gas phase ion chemistry. Those findings on the obviously different behaviour of different analytes require further research focussed on the ion chemistry in IMS including the comparison of different ionisation sources.     

Analysis of bone minerals by time-of-flight secondary ion mass spectrometry: a comparative study using monoatomic and cluster ions sources

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an important tool for the analysis of bone minerals at implant surfaces. Most studies have been performed with monoatomic primary ion sources such as Ga(+) with poor secondary molecular ion production efficiency and only elemental distributions and minor fragments of bone minerals have been reported. By using cluster ion sources, such as Au(1-3) (+) and Bi(1-3) (+), identification of larger hydroxyapatite species at m/z 485, 541, 597 and 653, identified as Ca(5)P(3)O(12), Ca(6)P(3)O(13), Ca(7)P(3)O(14) and Ca(8)P(3)O(15), respectively, became possible. The ions appear to be fragments of the hydroxyapatite unit cell Ca(10)(PO(4))(6)(OH)(2). Each ion in the series is separated by 55.9 m/z units, corresponding to CaO, and this separation might reflect the columnar nature of the unit cell.     

On the mechanism of secondary ion formation from poly(methylmethacrylate) under static secondary ion mass spectrometry conditions

The negative ion spectrum of a relatively thick layer (± 0. 5 μm) of poly(methylmethacrylate) (PMMA) with M?_w = 1890 and its positive ion spectrum of a very thin layer (± 1. 0 nm) on silver measured with a time of flight secondary ion mass spectrometer are presented. From the negative ion spectrum it is concluded that formation of enolate anions from PMMA under static secondary ion mass spectrometric conditions is an important ion formation process. From fragmentation products of the polymer, detected as silver cationized species in the positive ion spectrum, more evidence was found of a fragmentation mechanism for PMMA under static secondary ion mass spectrometric conditions recently proposed in the literature. From the relation between the information obtained from the two types of spectra an extension of this mechanism is obtained. This mechanism implies scission of the polymer chain by the primary ion bombardment with subsequent formation of enolate anions from the newly formed polymer chain-ends.     

Liquid secondary ion mass spectrometry of porphyrin dimers: reduction reactions and structural characterisation

New synthesised porphyrin dimers, with an amide or ester linkage between the two porphyrin units, were studied using liquid secondary ion mass spectrometry (LSIMS). The formation of reduced species was observed for all the compounds and it was found that the extent of reduction is dependent on the matrix used and on the structure of the porphyrin dimer. The main fragmentation pathways lead to monomer fragments resulting from cleavage of the amide or ester linkage between the two porphyrin units. The consistency of the fragmentations for all the dimers studied leads to the proposal of a common designation for the fragment ions. LSIMS, in addition to molecular weight determination, can provide important structural information for this type of compound.     

Depth profile measurement by secondary ion mass spectrometry

The possibilities of measuring depth profiles by secondary ion mass spectrometry are evaluated. The influence of different instrumental and experimental parameters on depth resolution in the profiles are studied: the effects of primary ion beam characteristics, reactive gas adsorption and mechanical aperturing in secondary ion extraction are discussed. Beam effects are studied from the point of view of surface damage. The effects of secondary processes, such as crater edge effects, element mixing, preferential sputtering, background signals, (residual) gas contamination and ion-induced topographical and compositional changes are studied for thin metal and binary materials.     

Method for improved secondary ion yields in cluster secondary ion mass spectrometry

A method to increase useful yields of organic molecules is investigated by cluster secondary ion mass spectrometry (SIMS). Glycerol drops were deposited onto various inkjet‐printed arrays and the organic molecules in the film were rapidly incorporated into the drop. The resulting glycerol/analyte drops were then probed with fullerene primary ions under dynamic SIMS conditions. High primary ion beam currents were shown to aid in the mixing of the glycerol drop, thus replenishing the probed area and sustaining high secondary ion yields. Integrated secondary ion signals for tetrabutylammonium iodide and cocaine in the glycerol drops were enhanced by more than a factor of 100 compared with an analogous area on the surface, and a factor of 1000 over the lifetime of the glycerol drop. Once the analyte of interest is incorporated into the glycerol microdrop, the solution chemistry can be tailored for enhanced secondary ion yields, with examples shown for cyclotrimethylenetrinitramine (RDX) chloride adduct formation. In addition, depositing localized glycerol drops may enhance analyte secondary ion count rates to high enough levels to allow for site‐specific chemical maps of molecules in complex matrices such as biological tissues. Published in 2010 by John Wiley & Sons, Ltd.     

Secondary ion formation of low molecular weight organic dyes in time-of-flight static secondary ion mass spectrometry

Time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) was used to characterize thin layers of oxy- and thiocarbocyanine dyes on Ag and Si. Apart from adduct ions a variety of structural fragment ions were detected for which a fragmentation pattern is proposed. Peak assignments were confirmed by comparing spectra of dyes with very similar structures. All secondary ions were assigned with a mass accuracy better than 50 ppm. The intensity of molecular ions as well as fragment ions has been studied as a function of the type of organic dye, the substrate, the layer thickness and the type of primary ion. A large yield difference of two orders of magnitude was observed between the precursor ions of cationic carbocyanine dyes and the protonated molecules of the anionic dyes. Fragment ions, on the other hand, yielded similar intensities for both types of dye. As the dye layers deposited on an Ag substrate yielded higher secondary ion intensities than those deposited on a Si substrate, the Ag metal clearly acts as a promoting agent for secondary ion formation. The effect was more pronounced for precursor signals than for fragment ions. The promoting effect decreased as the deposited layer thickness of the organic dye layer was increased.     

Comparative study of organic dyes with time-of-flight static secondary ion mass spectrometry and related techniques

A series of cationic, zwitterionic and anionic fluorinated carbocyanine dyes, spin-coated on Si substrates, were measured with time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) under Ga(+) primary ion bombardment. Detailed fragmentation patterns were developed for all dyes measured. In the positive mode, the resulting spectra showed very intense signals for the precursor ions of the cationic dyes, whereas the protonated signals of the anionic dyes were hardly detected. Differences of three orders of magnitude were repeatedly observed for the secondary ion signal intensities of cationic and anionic dyes, respectively. All measured dyes yielded mass spectra containing several characteristic fragment ions. Although the secondary ion yields were still higher for the cationic than the anionic dye fragments, the difference was reduced to a factor of < or =10. This result and the fact that M(+), [M + H](+) or [M + 2H](+) are even-electron species make it very likely that the recorded fragments were not formed directly out of the (protonated) parent ions M(+), [M + H](+) or [M + 2H](+). In the negative mode, none of the recorded spectra contained molecular information. Only signals originating from some characteristic elements of the molecules (F, Cl), the anionic counter ion signal and some low-mass organic ions were detected. A comparative study was made between TOF-S-SIMS, using Ga(+) primary ions, and other mass spectrometric techniques, namely fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). The measurements showed that MALDI, ESI and FAB all give rise to spectra containing molecular ion signals. ESI and FAB produced M(+) and [M + H](+) signals, originating from the cationic and zwitterionic dyes, in the positive mode and M(-) and [M - H](-) signals of the anionic and zwitterionic dyes in the negative mode. With MALDI, molecular ion signals were measured in both modes for all the dyes. Structural fragment ions were detected for FAB, ESI and MALDI in both the positive and negative modes. Compared with the other techniques, TOF-S-SIMS induced a higher degree of fragmentation.     

Protein denaturation detected by time-of-flight secondary ion mass spectrometry

In the present work we investigate the denaturation of a functional protein, horseradish peroxidase (HRP), under various experimental conditions using time-of-flight secondary ion mass spectrometry. HRP was immobilized on TiO(2), and the samples were stored under different conditions. The activity of the enzyme was assessed colorimetrically and compared to ToF-SIMS spectra. We show that denaturation of the protein can be monitored using the ToF-SIMS signal of the disulfide bonds, which is related to the tertiary structure of the protein. As disulfide bonds appear in a vast range of proteins, the present findings may be of wide significance; i.e., a tool is provided that can allow the investigation of the presence of an active protein structure by a comparably simple surface analytical method.