Filtern
Dokumenttyp
Sprache
- Englisch (3)
Schlagworte
- Cationization (1)
- Cations (1)
- Laser-induced redox reactions (1)
- MALDI (1)
- MALDI-TOF MS (1)
- Polymer MALDI (1)
- Polystyrene (1)
- Redox-reactions (1)
- Redoxreactions (1)
- Substrate (1)
The cation addition mechanism to polystyrene and the possible interactions between the target plate material and the cationization agent have been studied. A range of different trifluoroacetate salts mixed with DCTB and polystyrene, deposited on several different target plate materials, have been systematically investigated with MALDI mass spectrometry. MALDI-MS yielded polystyrene- and DCTB-Cu adducts when sample solutions were applied on a copper substrate. For silver the same effect was observed. Control experiments showed that these species could only result from laser‐induced redoxreactions between the added salt and the substrate material. Reversed redoxreactions between added salts and target plate material are enabled by laser photon energy input during MALDI experiments.
Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is often applied to assess the dispersity and the end groups of synthetic polymers through the addition of cationizing agents. Here weaddress how these cation adducts are formed using polystyrene (PS) as a model polymer. We analyzed PSby MALDI-MS with a 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as the matrix and a range of trifluoroacetate (TFA) salts as cationizing agents on a range of different targetplate materials (copper, 1.4301 stainless steel, aluminum, Inconel 625, Ti90/Al6/V4 and chromium-, gold-and silver-plated stainless steel). It was found that on a stainless steel substrate the metal cations Al+,Li+, Na+, Cu+and Ag+formed polystyrene adducts, whereas K+, Cs+, Ba2+, Cr3+, Pd2+, In3+, or their lower oxidation states, did not. For the copper and silver substrates, PS and DCTB adduct formation with cations liberated from these target plate materials was observed upon addition of a cationizing agent, which indicates the occurrence of redox reactions between the added TFA salts and the target plate material. Judging from their standard electrode potentials, these redox reactions would not normally occur, i.e.,they require an additional energy input, strongly suggesting that the observed redox reactions are laser-induced. Furthermore, copper granules were found to successfully sequester PS from a tetrahydrofuran(THF) solution, consistent with the view complex formation with the copper target plate can take place prior to the MALDI-MS measurement.
Polymers, such as polystyrene, have been successfully analyzed with matrix-assisted laser desorption/ionization (MALDI) through the addition of e.g. copper or silver salts. This method is often used to establish the polydispersity index of polymer blends.
However, the mechanism of cation addition and the possible interactions between the added salts and the chosen target material are still points of interest. Therefore, the addition of several trifluoroacetate salts to a mixture of polystyrene and matrix
on a range of different target plate materials was systematically investigated, revealing several new interesting aspects of MALDI.
Polystyrene (Mw 1,920 Da) was mixed with a range of trifluoroacetate salts (Li, Na, K, Cs, Ba, Cr, Pd, Cu, Ag, Zn, Al and In, as well as
trifluoroacetic acid) and analyzed with MALDI using 2
-[2E-3-4-tert-butylphenyl)-
2-
methylprop-2-enylidene]ma
lononitrile DCTB) as matrix on different target plate materials (chrome, copper, silver, gold, Ti90/Al6/V4, Inconel® 625, Zinc and stain
less steel) to evaluate the occurrence of redox-reactions.
Polystyrene/salt/matrix solutions were deposited through pneumatic-assisted spraying on microscope slide
-shaped target plate insets of varying material, which, secured with copper tape, fitted a milled out structure from the original target plate. Spectra, obtained on a Bruker Autoflex I MALDI-Time
-of-Flight mass spectrometer, were processed with MATLAB to obtain polystyrene-and matrix
-adduct ion signal intensities for direct comparison between chosen conditions.
The resulting spectra shed light on the MALDI adduct formation process and the cation-polystyrene interactions. It was found
that the following cation
-polystyrene adducts were formed on stainless steel: Al, Li, Na, Cu and Ag, where the yield was found to depend on the sample layer thickness and possibly the cation’s ability to form a complex with either one or two of polystyrene’s phenyl rings, based on the ligand-field and the valence bond theory. With the exception of Al, these salts also formed adducts
and in case of Cu and Ag also sandwich adducts with DCTB. Some alkali salts (e.g. potassium) formed clusters rather than interacting with polystyrene or DCTB, which can be explained with the HSAB theory. Application of TFA salts on a copper surface
led to copper cation formation, resulting in DCTB and polystyrene copper-adduct formation. The same effect occurred for silver substrate. In the absence of copper or silver salts, it is therefore still possible to form their respective adducts by choosing the proper alternative salt (e.g. Li, Cs, Ba, Cr) in combination with either a silver or a copper substrate surface. Incubation tests with copper beads in various salt solutions, before matrix and polystyrene addition, support that copper ions are not generated during the deposition process before the MALDI experiment is carried out, except when trifluoroacetic acid, indium and aluminium
trifluoroacetate are used. For all other salts used on a copper plate, it can therefore be concluded that these copper cation forming redox-reactions are enabled by the input of laser photon energy. Furthermore, it was discovered that copper beads can successfully sequester polystyrene from the sample mixture, indicating the strong bonding of polystyrene to the copper surface.
These findings support that the redox-reactions occur (almost) instantaneously with laser pulse impact at the sample-coated substrate surface.