Abstract:A new approach is reported for secondary‐ion time‐of‐flight mass spectrometry (TOF‐SIMS) sample preparation. The method involves the use of halide additives or halide modification of silver substrate surfaces to promote analyte cationization and protonation. The enhancement of signal intensity has been demonstrated for neutral organic lipophilic and hydrophilic compounds including various small peptides and nucleosides. Improvement factors range from 2–30 for cationized species to 20–2000 for protonated specie… Show more
“…When the polymer monolayers were dosed with sodium ions, a dramatic enhancement in the intensity of large mass fragment peaks was observed in the positive spectra of both polymer monolayers. This cationization phenomenon has been detected on various PEG-modified surfaces, and some systematic studies on it have been published. − 4 Low mass range (0−100 m / z ) positive ToF-SIMS spectra for (a) PEG−Sil and (b) NHS−PEG−Sil.…”
Two new grafted polysiloxane polymers, a copolysiloxane and a terpolysiloxane, have been synthesized.
Both polysiloxane backbones are grafted with dialkyl disulfide chains and 600 MW methoxy-terminated
poly(ethylene glycol) (PEG) chains, while the terpolysiloxane also has 3400 MW PEG side chains terminated
with N-hydroxysuccinimide (NHS) reactive ester groups. The two polymers spontaneously form monolayers
on gold surfaces with estimated thicknesses of 23 and 31 Å, respectively. Combined analysis with angle-dependent X-ray photoelectron spectroscopy and static time-of-flight secondary ion mass spectrometry
showed that most grafted dialkyl disulfide side chains were bound to the gold surface, forming multiple
molecular linkages at the monolayer/gold interface. The PEG-grafted chains were concentrated in the
outermost exposed region of the monolayers. Protein adsorption on the two immobilized polymer monolayers
was examined with surface plasmon resonance (SPR). The copolymer-covered surface resisted protein
adsorption from buffer, while the terpolymer monolayer bound significant amounts of protein. The NHS
headgroup in the terpolymer was the primary site for protein binding. These two polymers have potential
applications for SPR biosensor surface modification as protein-resistant and protein-immobilizing surfaces,
respectively.
“…When the polymer monolayers were dosed with sodium ions, a dramatic enhancement in the intensity of large mass fragment peaks was observed in the positive spectra of both polymer monolayers. This cationization phenomenon has been detected on various PEG-modified surfaces, and some systematic studies on it have been published. − 4 Low mass range (0−100 m / z ) positive ToF-SIMS spectra for (a) PEG−Sil and (b) NHS−PEG−Sil.…”
Two new grafted polysiloxane polymers, a copolysiloxane and a terpolysiloxane, have been synthesized.
Both polysiloxane backbones are grafted with dialkyl disulfide chains and 600 MW methoxy-terminated
poly(ethylene glycol) (PEG) chains, while the terpolysiloxane also has 3400 MW PEG side chains terminated
with N-hydroxysuccinimide (NHS) reactive ester groups. The two polymers spontaneously form monolayers
on gold surfaces with estimated thicknesses of 23 and 31 Å, respectively. Combined analysis with angle-dependent X-ray photoelectron spectroscopy and static time-of-flight secondary ion mass spectrometry
showed that most grafted dialkyl disulfide side chains were bound to the gold surface, forming multiple
molecular linkages at the monolayer/gold interface. The PEG-grafted chains were concentrated in the
outermost exposed region of the monolayers. Protein adsorption on the two immobilized polymer monolayers
was examined with surface plasmon resonance (SPR). The copolymer-covered surface resisted protein
adsorption from buffer, while the terpolymer monolayer bound significant amounts of protein. The NHS
headgroup in the terpolymer was the primary site for protein binding. These two polymers have potential
applications for SPR biosensor surface modification as protein-resistant and protein-immobilizing surfaces,
respectively.
“…Time-of-Flight SIMS Analysis. Negative ToF-SIMS spectra of carboxylic acid terminated SAMs were recorded and revealed distinct peaks at m/z 287.2 (M-H)and 681.2 (Au 2 (M-H) -) with (M) describing the mercaptohexadecanoic acid molecule [HS(CH 2 ) 15 COOH]. Thiol fragments bound to gold such as AuSC 2 H 2and Au 2 SC 2 H 2were some of the major peaks in the spectra.…”
Section: Resultsmentioning
confidence: 99%
“…This makes it difficult to desorb and detect molecular ions and fragments from polymers that are of more than approximately 300 mass units. Enhanced detection of larger mass fragments can occur through cationization. − Silver, , lithium, , and sodium , are some of the metal ions (X + ) known to complex with sputtered molecules (M) to form cationized fragments (M + X) + . These larger ion complexes are more stable than their noncationized counterparts and are readily detected.…”
Section: Introductionmentioning
confidence: 99%
“…Enhanced detection of larger mass fragments can occur through cationization. [6][7][8][9][10] Silver, 7,11 lithium, 12,13 and sodium 14,15 are some of the metal ions (X + ) known to complex with sputtered molecules (M) to form cationized fragments (M + X) + . These larger ion complexes are more stable than their noncationized counterparts and are readily detected.…”
Cationization of large polymers and their fragments on novel self-assembled monolayer substrates has been studied with time-of-flight secondary ion mass spectrometry (ToF-SIMS). Low (1 kDa) and high (400 kDa) molecular weight poly(ethylene oxide) (PEO) were spin-coated on metal ion substituted, carboxylic terminated, self-assembled monolayers on gold. Both cationized fragments and additional whole molecular species were observed for the 1 kDa PEO, while for 400 kDa PEO, only chain fragments of up to 30 monomer units were detected. Fragmentation pattern dependence on polymer molecular weight and the nature of the substrate is discussed. The intensity of the cationized fragments was found to depend on the metal ion used. Complementary experiments carried out on methyl terminated self-assembled monolayers suggested that metal ions that are desorbed from the gold surface, and therefore not in close proximity to the polymer, do not cationize polymers and their fragments. The substrates used in this work are simple to prepare, reproducible, and provide a model system for studies on the cationization of both polymers and proteins. Also, these ionically terminated self-assembled monolayer substrates open up new analytical possibilities for static SIMS.
“…107 Alternatively, transition and alkali metal salts can be mixed with the sample to achieve cationization. 91,109,110 However, there is still some debate concerning the exact mechanisms and, for instance, recent ab-initio studies indicate that ionization might occur upon recombination of the molecule M with a metal atom in an excited electronic state, 111,112 as a result of energy curve-crossing upon interaction. The ionization of organometallic clusters from self-assembled monolayers of alkanethiols on metal (see Fig.…”
Section: Substrate and Matrix Effects On Ionizationmentioning
This article reviews the recent progress in the understanding of kiloelectronvolt particle interactions with organic solids, including atomic displacements in a light organic medium, vibrational excitation and desorption of fragments and entire molecules. This new insight is the result of a combination of theoretical and experimental approaches, essentially molecular dynamics (MD) simulations and secondary ion mass spectrometry (SIMS). Classical MD simulations provide us with a detailed microscopic view of the processes occurring in the bombarded target, from the collision cascade specifics to the scenarios of molecular emission. Time-of-flight SIMS measures the mass and energy distributions of sputtered ionized fragments and molecular species, a precious source of information concerning their formation, desorption, ionization and delayed unimolecular dissociation in the gas phase. The mechanisms of energy transfer and sputtering are compared for bulk molecular solids, organic overlayers on metal and large molecules embedded in a low-molecular weight matrix. These comparisons help understand some of the beneficial effects of metal substrates and matrices for the analysis of molecules by SIMS. In parallel, I briefly describe the distinct ionization channels of molecules sputtered from organic solids and overlayers. The specific processes induced by polyatomic projectile bombardment, especially fullerenes, are discussed on the basis of new measurements and calculations. Finally, the perspective addresses the state-of-the-art and potential developments in the fields of surface modification and analysis of organic materials by kiloelectronvolt ion beams.
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