Dosimetry in photochemistry

Abstract— A method of dosimetry which is useful in analyzing complex photochemical reactions involving more than one photoproduct is described. This involves determining a photochemical cross section for each of the products, σ_B, σ_c…., and an absorption cross section σ. for the reacting species. The ratio σ_B/σ_a is the quantum yield for production of product B. Where a number of photoproducts are produced simultaneously, it is convenient to express ‘dose’ in terms of the average number of photons per cm²‘seen’ by each molecule in the vessel. This average can be calculated from the number of incident photons, the volume of the liquid, the absorbance and the path length. Experimental details of the irradiation facility which is convenient for such studies are presented. The method of dosimetry is illustrated using photochemical data for uracil and orotic acid in solution.     

A genipin-gelatin gel dosimeter for radiation processing

Genipin, a fruit extract from Gardenia jasminoides Ellis, forms cross-links in solutions of gelatin, to form a blue hydrogel that bleaches quantitatively upon irradiation and the colour change can be measured with a spectrophotometer. With the addition of sulphuric acid this dosimeter is sufficiently sensitive for quality assurance of radiotherapy level dosimetry. Without sulphuric acid the gel has a reduced sensitivity and responds linearly with dose between 100 and 1000 Gy, making it potentially useful as a dosimeter for radiation processing applications such as the phytosanitary irradiation treatment of food. We investigated the dose response characteristics of this new formulation and found that the darker gels are more sensitive to dose and have a reduced uncertainty.     

Preparation and characterization of RF aerogel on UV irradiation method

The UV irradiation method provides us a new way of preparing resorcinol–formaldehyde (RF) aerogel. In this paper, water soluble, neutral photo-initiator 659 was used to synthesize RF aerogel by using free radical UV initiator. Incorporation of the UV initiators during the sol–gel stage usually decreases the gel-time. On changing the performed time and initiators quantities in synthesizing RF aerogel, The results will be discussed that gel-time of the RF aerogel definitely depends on the polymerization time and initiators quantities. The paper will also be discussed the RF aerogel developed for UV irradiation method and Sol–gel method by IR spectrum, Scanning electron microscopy, N₂ adsorption/desorption to show that light initiator does not play any role in the formation of the aerogel skeleton; The RF aerogel using UV irradiation method still maintain a high pore structure, shorter gel-time and the three-dimensional size block.     

Dose distribution verification in intraoperative radiation therapy using an N-isopropyl acrylamide-based polymer gel dosimeter

The purpose of the current study was to verify the dose distribution of an Intrabeam-50 kV IORT system using polymer gel dosimetry technique. Results of dose distribution evaluation using NIPAM polymer gel dosimetry were compared with those measured using an ionization chamber and simulated using MCNPX code. Results showed the calculated gamma index was less than 1 with 2% dose-difference/2 mm distance-to-agreement for comparison between NIPAM and ionization chamber as well as between NIPAM and MCNPX simulation. It was concluded that the NIPAM polymer gel dosimetry is useful for verifying the dose distribution of low energy X-ray IORT technique.

     

A study on the role of gelatin in methacrylic-acid-based gel dosimeters

In radiotherapy treatment, polymer gel dosimetry can be used for verifying three-dimensional (3D) dose distributions. Gelatin is generally used as a gelling agent in the dosimeters. In this paper, another role of gelatin in a methacrylic-acid-based gel dosimeter (MAGAT) is investigated. Temperature increases due to exothermic polymerization in the irradiated gel are measured directly. Dose–R₂ responses are also obtained using MRI. It is shown that no appreciable increases in either temperature or R₂ are observed in MAGAT dosimeters made without gelatin, and that significant temperature and R₂ increases are observed when very low gelatin concentrations are used. These results indicate that gelatin is an important enabler for radiation-induced free-radical polymerization in methacrylic-acid-based gels. When gelatin is replaced by amino acids, changes in temperature are observed, along with small changes in R₂. The resulting dosimeter solutions remain transparent because the polymer does not precipitate as it does in regular MAGAT dosimeters containing gelatin. When the amino acids are replaced by acids without amino groups, no temperature or R₂ changes are observed, indicating that no polymer forms. These results show that amino groups (and possibly other functional groups) on the gelatin catalyze the radiation-induced free-radical polymerization that occurs in MAGAT dosimeters.     

Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH₄ mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H₂, C₂ and C₃ hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C₂ and C₃ formation dramatically decreased, and syngas (H₂ and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH₄ mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH₃, CH₂, CH, H, O and O (¹D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.     

An approach in exploring the fundamental dosimetric characteristics for a long shelf life irradiated acrylamide-based gel

In this study, a long shelf life irradiated gel made from a low-toxicity N-isopropylacrylamide (NIPAM) monomer was used to study the dosimetric characteristics associated with magnetic resonance imaging (MRI). Specifically, the Taguchi method was utilized to investigate the influence of MRI scanning parameters on the performance objectives of NIPAM gels, and the priorities of the parameters that influence the process were determined using Delta statistics. The influence of different echo spacing on the relaxation rate–dose response of the gel and the relationship among the dose resolution (D _Δ ^P ), dose accuracy, and reproducibility were also examined. The results indicated that the repetition time (TR) and echo time (TE), having Delta values of 3.81042 and 4.52752, respectively, were the most influential factors. Analysis of variance further confirmed that TE was the major factor, with a percentage contribution of 58.36 %, and TR was the second factor with a percentage contribution of 41.34 %. The maximum sensitivity was 0.1506 s^?1 Gy^?1 from 0 to 15 Gy, and the best D _Δ ^P of the NIPAM gel was 0.017 Gy. Two months after the irradiation, no obvious changes in linearity and sensitivity were observed, indicating that the NIPAM gels are highly stable. The gel dosimeters were also applied in comparing the planar dose distributions of the gel dosimeters and treatment planning system. The comparison showed good agreement of isodose lines on selected planes in the transverse plane. This result indicates the great potential of the NIPAM dosimeter has a clinical dose verification tool.     

Spectrofluorimetric Studies of the Interaction Between Quinine Sulfate and Riboflavin

The molecular interaction between quinine sulfate [QS] and riboflavin [RF] was investigated by fluorescence and UV-VIS absorption spectroscopy in aqueous solutions. The fluorescence of QS was quenched by RF. The Stern-Volmer quenching equation was successfully applied and the corresponding thermodynamic parameters enthalpy change (??H), Gibbs energy change (??G) and entropy change (??S) were calculated at different temperatures (294, 301, 307 and 314 K) according to the van??t Hoff equation. This information showed that hydrogen bonds play a major role in stabilizing the complex. The value of the critical energy transfer distance (R ₀) was found to be 46.47 Å, less than 50 Å, which proves that an efficient resonance energy transfer takes place between QS (donor) and RF (acceptor). The oxidation and reduction potentials determined from cyclic voltammetry indicated that association of QS and RF occurs in the excited state by electron transfer. The fluorescence quenching method was successfully applied for the direct determination of riboflavin (vitamin B₂) from pharmaceutical samples.     

Effect of inorganic salt on the dose sensitivity of polymer gel dosimeter

The effect of inorganic salts, non-transition metal chlorides, on the dose sensitivity of methacrylic-acid-based polymer gel dosimeter is investigated. Dose-R₂ responses are obtained from magnetic resonance imaging data. Temperature increase due to exothermic polymerization reaction in the gel is also measured directly during irradiation. As a result, substantial increases in R₂ response are observed in the polymer gel dosimeter containing inorganic salt, especially with MgCl₂. The sensitivity of the gel with 1.0 M MgCl₂ is approximately 2.8 times higher than that of without MgCl₂. As the salt concentration increases, an increase of polymerization rate is also observed via the temperature measurements. These results indicate that inorganic salt acts as an accelerator for radiation-induced free-radical polymerization in methacrylic-acid-based gel.     

A methodology for choosing parameters for ESR readout of alanine dosimeters for radiotherapy

Spectrometer settings for ESR readout of alanine dosimeters for radiotherapy have been investigated. Several ESR parameters were studied and determined. The main reason for this work is to choose the suitable parameters to increase signal-to-noise ratio and to reduce the uncertainty on ESR readout, which is one of the main components of uncertainty of alanine/ESR dosimetry system for radiotherapy. The new spectrometer settings have been applied for ESR readout of alanine dosimeters irradiated from 1 to 10 Gy. A higher signal-to-noise ratio has been achieved compared to our old spectrometer settings. The extended uncertainty (k=2) has been evaluated in the dose range 2–10 Gy (maximum uncertainty of 4.9% for 2 Gy, while minimum uncertainty of 1.4% for 10 Gy), which implies that the alanine/ESR dosimetry system can be applied to radiotherapy dose level that needs a global accuracy of 5%.     

A novel quantification strategy of transferrin and albumin in human serum by species-unspecific isotope dilution laser ablation inductively coupled plasma mass spectrometry (ICP-MS)

Species-specific (SS) isotope dilution analysis with gel electrophoresis (GE)-laser ablation (LA)-ICP-MS is a promising technique for the quantification of particular metal-binding proteins in biological samples. However, unavailable isotopically enriched spike and metal losses in GE separation are main limitations for SS-isotope dilution PAGE-LA-ICP-MS. In this study, we report for the first time the absolute quantification of transferrin (Tf) and albumin (Alb) in human serum by non-denaturing (native) GE combined with species-unspecific isotope dilution mass spectrometry (IDMS). In order to achieve a homogeneous distribution of both protein and isotope-enriched spike (simulated isotope equilibration), immersing the protein strips with ³⁴S spike solution after gel electrophoresis was demonstrated to be an effective way of spike addition. Furthermore, effects of immersion time and ³⁴S spike concentration were investigated to obtain optimal conditions of the post-electrophoresis isotope dilution method. The relative mass of spike and ablated sample (m_sp/m_sam) in IDMS equation was calculated by standard Tf and Alb proteins, which could be applied to the quantification of Tf and Alb in ERM-DA470k/IFCC for method confirmation. The results were in agreement with the certified value with good precision and small uncertainty (1.5–3%). In this method, species-specific spike protein is not necessary and the integrity of the heteroatom-protein could be maintained in sample preparation process. Moreover, the application of species-unspecific isotope dilution GE-LA-ICP-MS has the potential to offer reliable, direct and simultaneous quantification of proteins after conventional 1D and 2D gel electrophoretic separations.     

Preparation of the visible light responsive TiO2 thin film photocatalysts by the RF magnetron sputtering deposition method

TiO₂ thin film photocatalysts which could induce photoreactions under visible light irradiation were successfully developed in a single process by applying an ion engineering technique, i.e., the radio frequency (RF) magnetron sputtering deposition method. The TiO₂ thin films prepared at temperatures greater than 773 K showed the efficient absorption of visible light; on the other hand, the TiO₂ thin films prepared at around 573 K were highly transparent. This clearly means that the optical properties of TiO₂ thin films, which absorb not only UV but also visible light, can be controlled by the preparation temperatures of the RF magnetron sputtering deposition method. These visible light responsive TiO₂ thin films were found to exhibit effective photocatalytic reactivity under visible light irradiation (λ > 450 nm) at 275 K for the reductive decomposition of NO into N₂ and N₂O. From various characterizations, the orderly aligned columnar TiO₂ crystals could be observed only for the visible light responsive TiO₂ thin films. This unique structural factor is expected to modify the electronic properties of a TiO₂ semiconductor, enabling the efficient absorption of visible light.     

Lysine modification of human serum albumin and its effect on protein conformation and nalidixic acid binding

In order to investigate the involvement of lysine residues of human serum albumin (HSA) in nalidixic acid (NA) binding, various modified preparations of HSA such as 44% carbamylated (C₄₄), 83% carbamylated (C₈₃) and 85% acetylated (A₈₅) were made by treating the HSA solution with a different molar excess of potassium cyanate and acetic anhydride. The extent of modification, charge homogeneity and conformational changes of these derivatives were checked by TNBSA reaction method, polyacrylamide gel electrophoresis (PAGE) and gel filtration using Sephacryl S-200 HR column, respectively. Binding of NA to HSA and its derivatives was examined using fluorescence quenching titration method to determine the binding constant. The emergence of a single band in PAGE and single symmetrical peak in gel filtration results confirmed the charge and size homogeneity of these derivatives. Hydrodynamic properties such as Stokes radius and frictional ratio, as obtained from the analytical gel filtration results suggested molecular expansion in C₈₃ and A₈₅ HSAs while C₄₄ HSA retained the native conformation. Addition of NA to both native and modified HSA derivatives quenched the fluorescence intensity of the protein at 344 ​nm to a different extent. Whereas the values of the Stern-Volmer constant (K_SV) and bimolecular quenching rate constant (k_q) suggested, NA-HSA complex formation, binding constant (K_a) value suggested an intermediate binding affinity between NA and HSA. Furthermore, the decrease in the K_a value with the extent of modification was indicative of the involvement of lysine residues in NA-HSA interaction.     

Luminescent characteristics and microstructures of Sr<Subscript>2</Subscript>CeO<Subscript>4</Subscript> phosphors prepared via sol–gel and solid-state reaction routes

Blue-light-emitting Sr₂CeO₄ phosphors were synthesized via a sol–gel process and the conventional solid-state method in this study. The developed sol–gel process lowered the synthesis temperature of monophasic Sr₂CeO₄ to as low as 900 °C. In comparison with the solid-state derived powders, the sol–gel derived powders had more uniform morphology and smaller particle sizes. In addition, sol–gel derived Sr₂CeO₄ displayed higher luminescent intensity than that prepared via the solid-state route under the same heating conditions. This is attributed to the improved compositional homogeneity and crystallinity in the sol–gel process. During the heating processes, Sr₂CeO₄ tended to thermally decompose at elevated temperatures. This decomposition reaction resulted in the formation of an impurity phase- SrCeO₃ and thereby a decrease in the luminescent intensity. For obtaining Sr₂CeO₄ phosphors with high luminescent intensity, the heating conditions in both processes need to be well modulated.     

A basic measurement model for reference material preparation

The basic model $ x_{\text{CRM}} = x_{\text{char}} + \delta x_{\text{bb}} + \delta x_{\text{sts}} + \delta x_{\text{lts}} $ in the ISO Guide 35 for the evaluation of uncertainty is reviewed. In the basic model, the authors of the Guide assume that the homogeneity and stability studies are designed in such a way that the values of the error terms are zero, but their uncertainties are not. Based on their assumption, it is shown that the basic model in the Guide requires output quantity x _CRM to be employed as input quantities to define ??x _bb, ??x _sts, and ??x _lts, which results in circular reasoning on the value of CRM. In order to revise the basic model in the Guide, a proposed basic model Y?=?X is derived from the general model Y?=?X(1?+?b??T) in the Guide as the special case of T?=?0. The uncertainty evaluation for the proposed model is discussed. It is shown that it is not necessary to evaluate the uncertainty due to the between-bottle inhomogeneity u _bb separately, when a single method is employed in the characterization. The u _bb is shown to be one of the uncertainty components of the certified value x _char, because x _char is the mean value of at least 10 bottles.     

The Rayleigh—Schrödinger BK method applied to the lower electronic states of pyrrole

A second-order Rayleigh—Schrödinger B_K method (RS B_K) is described and applied to computing the lower electronic states of pyrrole. This method is more nearly size consistent than the previously used Brillouin—Wigner B_K (BW B_K) method. Pyrrole RS B_K excitation energies compare closely to BW B_K, second-order perturbation theory, and extrapolated CI energies. To provide insight into the origin of the bands in the experimental absorption spectrum, absorption coefficients and second moments are also reported.     

Theory of dynamic light scattering by polymers and gels

It is shown under very general conditions that the intermediate scattering function for the generalized Rouse—Zimm model always takes the simple form G(K, t) α exp[?K²(k_BT/f)t], when the scattering vector K becomes sufficiently large. (Here k_B is Boltzmann's constant, T is the absolute temperature and f is the individual bead friction factor.) A microscopic formulation for the bulk modulus and friction factor density of a gel network is incorporated into the viscoelastic continuum model of Tanaka et al. The resulting expression for the apparent long-wavelength diffusion coefficient of the gel is D_G = (k_BT/f)2(1 - 2/Φ), where Φ is the network functionality.     

Dielectric properties of tetragonal tungsten bronze films deposited by RF magnetron sputtering

Tetragonal tungsten bronze (TTB) films have been synthesised on Pt(111)/TiO₂/SiO₂/Si substrates from Ba₂LnFeNb₄O₁₅ ceramics (Ln = La, Nd, Eu) by RF magnetron sputtering. X-ray diffraction measurements evidenced the multi-oriented nature of films with some degrees of preferential orientation along (111). The dependence of the dielectric properties on temperature and frequency has been investigated. The dielectric properties of the films are similar to those of the bulk, i.e., ε ∼150 and σ ∼10⁻⁶ Ω⁻¹ cm⁻¹ at 1 MHz and room temperature. The films exhibit two dielectric anomalies which are attributed to Maxwell Wagner polarization mechanism and relaxor behaviour. Both anomalies are sensitive to post-annealing under oxygen atmosphere and their activation energies are similar E_a ∼0.30 eV. They are explained in terms of electrically heterogeneous contributions in the films.     

High Performance Liquid Chromatographic Separation of Bacitracin Methylenedisalicylate

Abstract

Our previously described isocratic RP HPLC procedure was convenient for monitoring and quality control of bacitracin production and Zn - bacitracin feed grade preparations. Separation and quantitative determination of the main active bacitracin components (A₁ B₁ and B₂) are possible and those elution is not interrupted by other ingredients in this type of samples. But when the methylenedisalicylic salt of bacitracin was tested some modification of method were necessary for the correct separation of bacitracin components. Mobile phase had to be modified and polystyrene based packing was an alternative and useful complement to octadecylated silica gel packings.     

Qualitative analysis by online nuclear magnetic resonance using Carr-Purcell-Meiboom-Gill sequence with low refocusing flip angles

The Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence has been used in many applications of magnetic resonance imaging (MRI) and low-resolution NMR (LRNMR) spectroscopy. Recently, CPMG was used in online LRNMR measurements that use long RF pulse trains, causing an increase in probe temperature and, therefore, tuning and matching maladjustments. To minimize this problem, the use of a low-power CPMG sequence based on low refocusing pulse flip angles (LRFA) was studied experimentally and theoretically. This approach has been used in several MRI protocols to reduce incident RF power and meet the specific absorption rate. The results for CPMG with LRFA of 3π/4 (CPMG₁₃₅), π/2 (CPMG₉₀) and π/4 (CPMG₄₅) were compared with conventional CPMG with refocusing π pulses. For a homogeneous field, with linewidth equal to Δυ = 15 Hz, the refocusing flip angles can be as low as π/4 to obtain the transverse relaxation time (T₂) value with errors below 5%. For a less homogeneous magnetic field, Δυ = 100 Hz, the choice of the LRFA has to take into account the reduction in the intensity of the CPMG signal and the increase in the time constant of the CPMG decay that also becomes dependent on longitudinal relaxation time (T₁). We have compared the T₂ values measured by conventional CPMG and CPMG₉₀ for 30 oilseed species, and a good correlation coefficient, r = 0.98, was obtained. Therefore, for oilseeds, the T₂ measurements performed with π/2 refocusing pulses (CPMG₉₀), with the same pulse width of conventional CPMG, use only 25% of the RF power. This reduces the heating problem in the probe and reduces the power deposition in the samples.