Mahnaz Farahani

A GC–MS study of the addition reaction of aryl amines with acrylic monomers

Previous studies have shown that the interaction of carboxylic acid groups with the amine functionalities of aryl amines, especially secondary and tertiary aryl amines, can lead to the free-radical polymerization of acrylic monomers such as methyl methacrylate. In this study, the Michael addition reaction of primary and secondary aryl amines with acrylic monomers such as acrylic acid (AA) was investigated. Equivalent amounts of either p-toluidine (PT) or N-phenylglycine (NPG) and AA were combined in polar solvents such as ethanol. The reactions were conducted at ambient (23°C) or near-ambient (37–60°C) temperatures. Samples (about 3–5 mg) of these products were then trimethylsilylated with a solution consisting of 0.4 mL of bis(trimethylsilyl)trifluoroacetamide (BSTFA) and 0.4 mL of acetonitrile by heating for 30 min at 140°C under N2. These derivatives were characterized by gas chromatography–mass spectrometry (GC–MS). The GC–MS analyses suggest that 1 mol of the primary amine PT had reacted with 2 mol of AA to yield the expected N-p-tolyliminodipropionic acid. Similarly, the secondary amine NPG added to 1 mol of AA yielded the corresponding mixed iminodiacid, N-phenyliminoacetic–propionic acid. It would appear that the Michael reaction of primary and secondary amines with acrylic monomers may offer a general, facile synthetic route to a variety of tertiary amines. Aryl amino acids of the type synthesized in this study may find use in a number of dental applications, e.g., as surface-active adhesive agents and as polymerization initiators or activators.

Unexpected peaks in tandem mass spectra due to reaction of product ions with residual water in mass spectrometer collision cells

RATIONALE Certain product ions in electrospray ionization tandem mass spectrometry are found to react with residual water in the collision cell. This reaction often leads to the formation of ions that cannot be formed directly from the precursor ions, and this complicates the mass spectra and may distort MRM (multiple reaction monitoring) results. METHODS Various drugs, pesticides, metabolites, and other compounds were dissolved in acetonitrile/water/formic acid and studied by electrospray ionization mass spectrometry to record their MS(2) and MS(n) spectra in several mass spectrometers (QqQ, QTOF, IT, and Orbitrap HCD). Certain product ions were found to react with residual water in collision cells. The reaction was confirmed by MS(n) studies and the rate of reaction was determined in the IT instrument using zero collision energy and variable activation times. RESULTS Examples of product ions reacting with water include phenyl and certain substituted phenyl cations, benzoyl-type cations formed from protonated folic acid and similar compounds by loss of the glutamate moiety, product ions formed from protonated cyclic siloxanes by loss of methane, product ions formed from organic phosphates, and certain negative ions. The reactions of product ions with residual water varied greatly in their rate constant and in the extent of reaction (due to isomerization). CONCLUSIONS Various types of product ions react with residual water in mass spectrometer collision cells. As a result, tandem mass spectra may contain unexplained peaks and MRM results may be distorted by the occurrence of such reactions. These often unavoidable reactions must be taken into account when annotating peaks in tandem mass spectra and when interpreting MRM results. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.

New method for shielding electron beams used for head and neck cancer treatment

Shields and stents of metals with high atomic number, which are custom cast in molds from the melt, are the materials most widely used to protect surrounding tissues during treatment of skin or oral lesions with therapeutic electron beams. An improved fabrication method is to mix a polysiloxane-metal composite, which is readily cast at room temperature by combining a metal-powder/polysiloxane resin mixture with a hardening catalyst. The purpose of the present study is to compare the shielding effectiveness of two different metal-polysiloxane composites with that of conventional cast Lipowitz metal (50.1% Bi, 26.6% Pb, 13.3% Sn, 10% Cd). Also, a 2(3) factorial experiment was run to investigate the effects and interactions of metal particle size (20-microns vs 100-microns diameter), the atomic weight of the metal (304 stainless steel vs 70% Ag, 30% Cu alloy), and the presence or absence of a layer of unfilled polymer added to the forward-scatter side of the shield. The composites of different thicknesses were made by blending 90% (w/w) metal powder separately with 10% polysiloxane base and catalyst. A thin GafChromic dosimeter film was placed between the shielding material and a polystyrene base to measure the radiation shielding effect of composite disc samples irradiated with a 6-MeV electron beam normal to the flat surface of the disc. The results show that composite shields with the metal of higher atomic weight and density (Ag-Cu) combined with an additional unfilled layer are more effective than the stainless-steel composite with a similar additional unfilled layer, in terms of diminishing the dose at the surface of the polystyrene backing material.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiochromic solutions for reference dosimetry

A chemical radiochromic dosimeter using hexa(hydroxyethyl)pararosaniline nitrile (HHEV-CN) dissolved in an aerated mixture of triethyl phosphate and dimethyl sulfoxide can be used over a wide absorbed dose range as a stable reference dosimeter, with useful characteristics, both for steady-state and pulsed radiation fields. Solutions of the leuco dye at 2-mM concentration, containing small amounts of acetic acid, p-nitrobenzoic acid and polyvinylbutyral, show high precision and a linear response for absorbed doses up to 4 kGy. When the leuco dye concentration is increased to 100 mM, the response is also linear, and absorbed doses as low as 0.5 Gy can be read with a precision of 1.3% (95% confidence limits). The radiation chemical yield is constant with changes in absorbed dose rate, but it increases with concentration of the leuco dye up to 10−1 molarity. The radiation chemical yields for dye formation are: for 2-mM solution, G(dye) = 0.015 μmol J−1; for 100-mM solution, G(dye) = 0.28 μmol J−1. The uncertainties for these measured values are ±2.6% (95% confidence limits). The molar linear absorption coefficient at 605–608 nm wavelength is 1.0 × 105 L mol−1 cm−1, the uncertainty (95% confidence limits) being ±2.2%. This dosimeter solution may be used in small sealed glass ampoules or plastic vials and is useful for measuring x- and γ-ray doses of interest in food irradiation and in clinical radiology. The combination of ingredients simulates water and biological tissue (muscle) in terms of radiation interaction cross sections, i.e. mass energy-absorption coefficients for photons (0.1–20 MeV) and mass collision stopping powers for electrons (0.1–50 MeV).

Radiochromic dye dosimeter solutions as reference measurement systems

Solutions of leucocyanides of triphenylmethane dyes in organic solvents are designed as stable reference dosimeters for large radiation doses, with useful characteristics, both for steady-state and pulsed radiation fields. These radiochromic solutions may be used in conventional glass ampoules to cover the absorbed dose range 102–104kGy, when analyzed spectrophotometrically at visible wavelengths at the maxima of radiation-induced absorption bands. The radiation chemical yields of dye formation (G-values) and molar linear absorption coefficients (ϵm) of the dyes in several formulations, with and without dissolved polymer and weak oxidizing agents, are established. The most stable formulation before and after irradiation consists of new fuchsin cyanide in a mixture of dimethyl sulfoxide and triethyl phosphate containing small amounts of acetic acid, p-nitrobenzoic acid and polyvinyl butyral. The useful range of doses for this solution is 102-4 x 103 Gy when measured at 557 nm wavelength. The radiation chemical yield, G-value for dye production, is 3.35 x 1015 molec J-1 (0.0055 μmol J-1) and the value of ϵm at this wavelength is 1.32 x 105 M-1 cm-1.

Shielding Small-Field High-Energy Electron Beams in Cancer Treatment

Abstract The purpose of this study was to find an effective material that can be prepared quickly and easily prior to small-field electron-beam treatments so that lesions of the head and neck can be treated with minimal irradiation of the surrounding healthy tissue. Conventional preparation of custom anatomical prosthetic radiation shields, which are usually metal alloy masks, has been time-consuming and uncomfortable for the patients. New materials, made from light-body Reprosil TM (L. L. Caulk) filled with fine metal powder consisting of 70% Ag-30% Cu alloy, can be made by blending 90% (w/w) metal powder with 10% polysiloxane base and adding the polymerization catalyst separately. These combinations were mixed to form comfortably fitted shielding composites of different thicknesses. The electron-beam attenuation properties of slabs of this material were studied by irradiating calibrated radiochromic film (GafChromic TM ) dosimeters behind different thicknesses of composite samples with small-field 13-, 15- and 18-MeV electron beams from a therapeutic linear accelerator. The results showed that this material can suitably attenuate high-energy electron beams when used in reasonable thicknesses.

A sensitive dichromate dosimeter for the dose range, 0.2–3 kGy

Abstract The response of aqueous dichromate dosimeters can be enhanced by adding small controlled amounts of organic reducing agents, however, such additives cause instabilities before and after irradiation. A choice is made of combinations of potassium dichromate, silver dichromate, perchloric acid, with the organic agent, acetic acid, to enhance appreciably the radiation chemical yield, G(-Cr 2 O 7 ) -2 and still have sufficient stability for practical use in dosimetry in the food irradiation range. The reduction yield is thereby increased from the reported value for the classic K-Ag dichromate dosimeter (G ≅ 0.04 μmol J -1 ) to G ≅ 0.17 μmol J -1 . This enhancement allows the dosimeter to be used with good precision down to the upper part of the dose range of the Fricke dosimeter (≈ 200 Gy) up to the lower part of the dose range of the conventional K-Ag dichromate dosimeter (≈ 3000 Gy). Because of instability of the solutions containing acetic acid, however, there is the limitation that spectrophotometry should not be made until at least three hours after irradiation.

A radiochromic folm dosimeter for gamma radiation in the absorbed-dose range 0.1–10 kGy

Abstract A commercially available leuco-dye film (FWT-63-02), having a thickness of 0.55 mm, has been investigated spectrophotometrically for its characteristics as a radiochromic dosimeter and for its potential use in food-irradiation applications. The γ-ray irradiation of the nearly colorless, transparent film induces blue color with an absorption maximum at 600 nm. The increase in absorbance at 600 nm per unit thickness of film (ΔA mm-1) is linear with dose in the dose range up to 8 kGy, with a slope of 0.91 mm-1·kGy-1. After a modest additional increase during the first day following irradiation, the radiation-induced color is stable when stored at room temperature at least for 5 weeks. The response slope is 16% higher when stored at 60°C, however, after the initial 1-day increase it is stable for several weeks when stored at that temperature. The response of the dosimeter is independent of dose rate in the range 0.5–170 Gy min-1.

Radiation-induced crosslinks between thymine and 2-D-deoxyerythropentose

Abstract Hydroxyl radicals generated by ionizing radiation in aqueous solutions of thymine (Thy) and 2- d -deoxyerythropentose (dR) induce crosslinking between Thy and dR. The crosslinked products were identified by gas chromatography—mass spectrometry (GC-MS) and their yields determined by GC. The mechanisms of their formation are discussed and may serve as a model for radiation-induced or free-radical induced intra-DNA crosslinks.