Three-dimensional dose verification of the clinical application of gamma knife stereotactic radiosurgery using polymer gel and MRI

This work seeks to verify multi-shot clinical applications of stereotactic radiosurgery with a Leksell Gamma Knife model C unit employing a polymer gel-MRI based experimental procedure, which has already been shown to be capable of verifying the precision and accuracy of dose delivery in single-shot gamma knife applications. The treatment plan studied in the present work resembles a clinical treatment case of pituitary adenoma using four 8 mm and one 14 mm collimator helmet shots to deliver a prescription dose of 15 Gy to the 50% isodose line (30 Gy maximum dose). For the experimental dose verification of the treatment plan, the same criteria as those used in the clinical treatment planning evaluation were employed. These included comparison of measured and GammaPlan calculated data, in terms of percentage isodose contours on axial, coronal and sagittal planes, as well as 3D plan evaluation criteria such as dose-volume histograms for the target volume, target coverage and conformity indices. Measured percentage isodose contours compared favourably with calculated ones despite individual point fluctuations at low dose contours (e.g., 20%) mainly due to the effect of T2 measurement uncertainty on dose resolution. Dose-volume histogram data were also found in a good agreement while the experimental results for the percentage target coverage and conformity index were 94% and 1.17 relative to corresponding GammaPlan calculations of 96% and 1.12, respectively. Overall, polymer gel results verified the planned dose distribution within experimental uncertainties and uncertainty related to the digitization process of selected GammaPlan output data.

Technical considerations for implementation of x-ray CT polymer gel dosimetry

Gel dosimetry is the most promising 3D dosimetry technique in current radiation therapy practice. X-ray CT has been shown to be a feasible method of reading out polymer gel dosimeters and, with the high accessibility of CT scanners to cancer hospitals, presents an exciting possibility for clinical implementation of gel dosimetry. In this study we report on technical considerations for implementation of x-ray CT polymer gel dosimetry. Specifically phantom design, CT imaging methods, imaging time requirements and gel dose response are investigated. Where possible, recommendations are made for optimizing parameters to enhance system performance. The dose resolution achievable with an optimized system is calculated given voxel size and imaging time constraints. Results are compared with MRI and optical CT polymer gel dosimetry results available in the literature.

Use of Rigid Spherical Inclusions in Young’s Moduli Determination: Application to DNA-Crosslinked Gels

Current techniques for measuring the bulk shear or elastic (E) modulus of small samples of soft materials are usually limited by materials handling issues. This paper describes a nondestructive testing method based on embedded spherical inclusions. The technique simplifies materials preparation and handling requirements and is capable of continuously monitoring changes in stiffness. Exact closed form derivations of E as functions of the inclusion force-displacement relationship are presented. Analytical and numerical analyses showed that size effects are significant for medium dimensions up to several times those of the inclusion. Application of the method to DNA-crosslinked gels showed good agreement with direct compression tests.

Phosphoproteome Analysis

Protein phosphorylation is directly or indirectly involved in all important cellular events. The understanding of its regulatory role requires the discovery of the proteins involved in these processes and how, where and when protein phosphorylation takes place. Investigation of the phosphoproteome of a cell is becoming feasible today although it still represents a very difficult task especially if quantitative comparisons have to be made. Several different experimental strategies can be employed to explore phosphoproteomes and this review will cover the most important ones such as incorporation of radiolabeled phosphate into proteins, application of specific antibodies against phosphorylated residues and direct staining of phosphorylated proteins in polyacrylamide gels. Moreover, methods to enrich phosphorylated proteins such as affinity chromatography (IMAC) and immunoprecipitation as well as mass spectrometry for identification of phosphorylated peptides and phosphorylation sites are also described.

Mechanical properties of two orthodontic adhesive materials polymerized with halogen and plasma curing

OBJECTIVES

The aim of this study is to compare the efficiency of two polymerization techniques (halogen curing--Astralis 7 and plasma curing--Flipo), with two orthodontic adhesive materials (Enlight, a composite resin, and Fuji Ortho LC, a glass ionomer cement).

METHODS

The efficiency of the polymerization techniques was shown by two mechanical tests. The hardness test was carried out on the exposed and non-exposed surfaces using 10 x 4 x 3-mm samples, polymerized either by halogen curing (40 seconds) or by plasma curing (5 seconds). The three-point bending tests were carried out on 2 x 2 x 25-mm samples polymerized as above. The samples were kept 1 hr at room temperature, then for 24 hrs in distilled water at 37 degrees C.

RESULTS

Whatever the polymerization technique used, the results are similar for hardness and flexion, with the exception of the hardness tests carried out after polymerization with the Flipo light on the surface not directly exposed.

CONCLUSION

In orthodontic practice, both polymerization techniques can be used. But a multi-bracket session can be long, and the reduction of time spent in the chair obtained by using plasma lamps seems to make this technique preferable.

Effect of three surface conditioning methods to improve bond strength of particulate filler resin composites

The use of resin-based composite materials in operative dentistry is increasing, including applications in stress-bearing areas. However, composite restorations, in common with all restorations, suffer from deterioration and degradation in clinical service. Durable repair alternatives by layering a new composite onto such failed composite restorations, will eliminate unnecessary loss of tooth tissue and repeated insults to the pulp. The objective of this study was to evaluate the effect of three surface conditioning methods on the repair bond strength of a particulate filler resin-composite (PFC) to 5 PFC substrates. The specimens were randomly assigned to one of the following surface conditioning methods: (1) Hydrofluoric (HF) acid gel (9.5%) etching, (2) Air-borne particle abrasion (50 microm Al2O3), (3) Silica coating (30 microm SiOx, CoJet-Sand). After each conditioning method, a silane coupling agent was applied. Adhesive resin was then applied in a thin layer and light polymerized. The low-viscosity diacrylate resin composite was bonded to the conditioned substrates in polyethylene molds. All specimens were tested in dry and thermocycled (6.000, 5-55 degrees C, 30 s) conditions. One-way ANOVA showed significant influence of the surface conditioning methods (p < 0.001), and the PFC types (p < 0.0001) on the shear bond strength values. Significant differences were observed in bond strength values between the acid etched specimens (5.7-14.3 MPa) and those treated with either air-borne particle abrasion (13.0-22.5 MPa) or silica coating (25.5-41.8 MPa) in dry conditions (ANOVA, p < 0.001). After thermocycling, the silica coating process resulted in the highest bond values in all material groups (17.2-30.3 MPa).

Influence of light-curing procedures and photo-initiator/co-initiator composition on the degree of conversion of light-curing resins

OBJECTIVE

The hypothesis that the degree and rate of conversion can be modified favourably by using different light-curing procedures and different photo initiator/co-initiator combinations was tested.

METHOD

A photo-initiator (0.02 mM/g resin); either camphorquinone (CQ) or 1-phenyl-1,2-propanedione (PPD), was mixed with bisGMA:TEGDMA (50:50 by weight). In addition, a co-initiator (0.04 mM/g resin); either N,N-dimethyl-p-aminobenzoic acid ethylester (DABE), N,N-cyanoethylmethylaniline (CEMA), or 2-dimethylaminoethyl methacrylate (DMAEMA), was added. These six combinations were subjected to three curing conditions (standard curing, soft-start curing or LED curing). The conversion levels (DC) were determined with differential scanning calorimetry (DSC). The DSC results were analysed using a general linear model (GLM) and Duncan's multiple range test and regular t-test.

RESULTS

The fastest conversion initially was obtained by standard curing, followed by LED curing and soft-start curing. After 40 s of curing, conventional curing and soft-start curing produced a higher DC than LED curing. However, strong interactions occurred between the different variables (curing method, initiator and co-initiator). Initially, CQ was more efficient than PPD, but after 40 s, this difference was insignificant.

CONCLUSION

By using soft-start curing and an appropriate photo initiator/co-initiator combination it is possible to achieve slow curing and a high DC at within a curing time of 40 s.

Magnetic resonance imaging of radiation dose distributions using a polymer-gel dosimeter

A new formulation of a tissue-equivalent polymer-gel dosimeter for the measurement of three-dimensional dose distributions of ionizing radiation has been developed. It is composed of aqueous gelatin infused with acrylamide and N, N'-methylene-bisacrylamide monomers, and made hypoxic by nitrogen saturation. Irradiation of the gel, referred to as BANG, causes localized polymerization of the monomers, which, in turn, reduces the transverse NMR relaxation times of water protons. The dose dependence of the NMR transverse relaxation rate, R2, is reproducible (less than 2% variation) and is linear up to about 8 Gy, with a slope of 0.25 s(-1)Gy(-1) at 1.5 T. Magnetic resonance imaging may be used to obtain accurate three-dimensional dose distributions with high spatial resolution. Since the radiation-induced polymers do not diffuse through the gelatin matrix, the dose distributions recorded by BANG gels are stable for long periods of time, and may be used to measure low-activity radioactive sources. Since the light-scattering properties of the polymerized regions are different from those of the clear, non-irradiated regions, the dose distributions are visible, and their optical densities are dependent on dose.

Ability of Polyvinylpyrrolidone and Polyacrylic Acid to Inhibit the Crystallization of Amorphous Acetaminophen

The inhibition of crystallization of amorphous acetaminophen (ACTA) by polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) was studied using amorphous solid dispersions prepared by melt quenching. Co-melting with PVP and PAA decreased the average molecular mobility, as indicated by increases in glass transition temperature and enthalpy relaxation time. The ACTA/PAA dispersion exhibited much slower crystallization than the ACTA/PVP dispersion with a similar glass transition temperature value, indicating that interaction between ACTA and polymers also contributed to the stabilizing effect of these polymers. The carboxyl group of PAA may interact with the hydroxyl group of ACTA more intensely than the carbonyl group of PVP does, resulting in the stronger stabilizing effect of PAA. Dielectric relaxation spectroscopy showed that the number of water molecules tightly binding to PVP per monomer unit was larger than that to PAA. Furthermore, a small amount of absorbed water decreased the stabilizing effect of PVP, but not that of PAA. These findings suggest that the stronger stabilizing effect of PAA is due to the stronger interaction with ACTA. The ability of PAA to decrease the molecular mobility of solid dispersion was also larger than that of PVP, as indicated by the longer enthalpy relaxation time.

Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide)

Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C.

Wastewater treatment polymers identified as the toxic component of a diamond mine effluent

The Ekati Diamond Mine, located approximately 300 km northeast of Yellowknife in Canada's Northwest Territories, uses mechanical crushing and washing processes to extract diamonds from kimberlite ore. The processing plant's effluent contains kimberlite ore particles (< or =0.5 mm), wastewater, and two wastewater treatment polymers, a cationic polydiallydimethylammonium chloride (DADMAC) polymer and an anionic sodium acrylate polyacrylamide (PAM) polymer. A series of acute (48-h) and chronic (7-d) toxicity tests determined the processed kimberlite effluent (PKE) was chronically, but not acutely, toxic to Ceriodaphnia dubia. Reproduction of C. dubia was inhibited significantly at concentrations as low as 12.5% PKE. Toxicity identification evaluations (TIE) were initiated to identify the toxic component of PKE. Ethylenediaminetetraacetic acid (EDTA), sodium thiosulfate, aeration, and solid phase extraction with C-18 manipulations failed to reduce PKE toxicity. Toxicity was reduced significantly by pH adjustments to pH 3 or 11 followed by filtration. Toxicity testing with C. dubia determined that the cationic DADMAC polymer had a 48-h median lethal concentration (LC50) of 0.32 mg/L and 7-d median effective concentration (EC50) of 0.014 mg/L. The anionic PAM polymer had a 48-h LC50 of 218 mg/L. A weight-of-evidence approach, using the data obtained from the TIE, the polymer toxicity experiments, the estimated concentration of the cationic polymer in the kimberlite effluent, and the behavior of kimberlite minerals in pH-adjusted solutions provided sufficient evidence to identify the cationic DADMAC polymer as the toxic component of the diamond mine PKE.

Characterisation of commercial ionomer glasses using magic angle nuclear magnetic resonance (MAS-NMR)

Five commercial ionomer glasses (Fuji IX, Ketac Molar, G338, G2, and G2SR) used to produce glass (ionomer) polyalkenoate dental cements were studied. 29Si, 27Al, 31P and 19F magic angle spinning nuclear magnetic resonance (MAS-NMR) Spectroscopy was used to characterise the glasses and the resulting spectra compared with previous studies of model glasses. The 29Si NMR spectra were consistent with Q4(3Al) and Q4(4Al) units being present and agreed with the low non-bridging oxygen contents calculated from the elemental composition. The 27Al NMR spectra typically exhibited three distinct sites at 45-60, 20 and 0 ppm which have been attributed to Al(IV), Al(V) and Al(VI) coordinate aluminium. The presence of Al(V) and Al(VI) are consistent with previous studies of model ionomer glasses. The 31P spectra all exhibited a chemical shift between -8 and -23 ppm with the exception of the Ketac Molar glass, which exhibited a peak at 2-3 ppm consistent with orthophosphate. The chemical shift of 31P in the range -8 to -23 ppm indicates a PO(4) tetrahedra surrounded by 1-4 Al moieties. The (19)F NMR spectra indicated the presence of Al-F-Ca(n) in the G2 and G338 glasses, Al-F-Sr(n) in the G2SR and Fuji IX glasses and crystalline CaF2, LaF3, Al-F-Ca(n) in the Ketac Molar glass. The G338 glass with a high non-bridging oxygen content showed the presence of a F-Ca(n) species. There was also present in all the glasses a peak corresponding to Al-F-Na(n). The intensity of this peak was approximately proportional to the sodium content.

Analysis of gums by capillary electrophoresis with laser induced fluorescence

Gums were derivatised with the fluorescence reagent, 9-aminopyrene-1,4,6-trisulfonic acid followed by microcentrifuge filtration. The resulting high mass fractions were analysed by capillary electrophoresis (CE) on a polyacrylamide coated capillary with laser induced fluorescence (LIF) detection. A wide pH range of electrolytes was used to study the influences on the electrophoretic mobilities and on the peak shapes of the gums. In this way, the separation of a mixture of five commercial gums, namely iota carrageenan, kappa carrageenan, alginic acid, xanthan and carboxymethyl cellulose (CMC), could be achieved at pH 3.2 with a 25 mmol/L trisodium citrate buffer. It is also shown that a mixture of Arabic gum, Karaya gum and CMC could be separated at pH 7.8 in a similar buffer.

Lactic acid-induced modifications in films of Eudragit RL and RS aqueous dispersions

Eudragit RL (ERL) and RS (ERS) are polymethacrylate co-polymers, used in film coating of sustained release dosage forms, possessing some hydrophilic properties due to the presence of quaternary ammonium groups (QAG), where ERL contains more of such groups, hence more permeable, than ERS. However, because these groups ionize in solution, they undergo electrostatic interaction with negatively charged species. This phenomenon was utilized in this study to introduce modification in the film properties of ERL and ERS by interaction with lactic acid (LA). Thermal and mechanical analyses were carried out on polymeric free films. DSC showed a shift in Tg of the film while 1H NMR spectroscopy revealed a significant deshielding in the peak of QAGs protons after interaction with LA. Stress-strain test showed an increase in three mechanical parameters of the new film (containing LA): tensile strength to modulus ratio, relative surface energy and toughness index, indicating an enhancement in the mechanical stress resistance. Tablets coated with LA-containing films showed an increase in the release rate and extent and good stability upon aging, compared to those coated with the original film.

Metalloinitiation Routes to Biocompatible Poly(lactic acid) and Poly(acrylic acid) Stars with Luminescent Ruthenium Tris(bipyridine) Cores

Poly(lactic acid) (PLA) and poly(acrylic acid) (PAA) biomaterials with luminescent ruthenium tris(bipyridine) centers couple drug delivery and imaging functions. Hydrophobic [Ru(bpyPLA2)3](PF6)2 (1) was generated from [Ru[bpy(CH2OH)2]3](PF6)2 in bulk monomer using 4-(dimethylamino)pyridine as the catalyst. The bromoesters, [Ru[bpy(CH2OR)2]3](PF6)2, [Ru[bpy(C13H27)2][bpy(CH2OR]2](PF6)2 (4), and [Ru[bpy(PLAOR)2]3]2+ (9) (R=COCBr(CH3)2), served as initiators for tert-butyl acrylate (tBA) polymerization. Conversion of PtBA to PAA via hydrolysis affords water soluble materials, [Ru(bpyPAA2)3]2+ (7) and [Ru[bpy(C13H27)2](bpyPAA2)2]2+ (8) and the amphiphilic star polymer [Ru[bpy(PLA-PAA)2]3)](PF6)2 (11), which is soluble in a H2O/CH3CN (1:1) mixture. Luminescence excitation and emission spectra of the Ru polymers were in agreement with the parent [Ru(bpy)3]2+ chromophore (lambdaex=468, lambdaem=621 nm). Lifetimes of tau approximately 700 ns in both air and nitrogen atmospheres are typical for most materials; however, the amphiphilic star block copolymer 11 is quenched by oxygen to some degree. Thermal analysis shows the expected glass transitions for the polymeric ruthenium complex materials.

EDX fluorescence analysis and SEM observations of resin composites

The purpose of this study was to evaluate the filler compositions of recently available light-cured resins. The composition of each resin paste was evaluated using an energy dispersive X-ray fluorescence spectrometer. Scanning electron microscopic observation of the polymerized resin pastes was also conducted. The main component of each resin composite was Si, while the other elements detected were Al, Ba, Sr, Zr, and K. These elementary compositions differed among the resin pastes used. Three different types of filler morphology were observed; splintered, prepolymerized and splintered, and spherical. The results of this study have thus characterized recently developed resin composites based on their filler elements and morphology.

Identification of some polymeric materials by low-temperature pyrolysis mass spectrometry

The application of low-temperature pyrolysis mass spectrometry to the identification of some commercial and synthetic polymers using a direct insertion probe and electron ionization has been studied. The polymers were analyzed directly in the solid probe of a mass spectrometer within the ion source at 70 eV electron impact. The quadrupole mass spectrometer, equipped with a gas chromatogram software and library of low molecular weight compounds, was used to characterize the polymers by measuring the chemical structure of the repeat units and the end groups, and to examine the thermal degradation pathways. The polymers investigated show different degradation pathways. Investigation of sequences and determination of composition of copolymers were studied. Total ion pyrogram and spectrum subtractions were used to separate and measure spectra of pyrolysis steps at distinctly different temperatures.

Capillary electrophoresis--electrospray ionization--mass spectrometry for the characterization of natural organic matter: an evaluation with free flow electrophoresis-off-line flow injection electrospray ionization-mass spectrometry

The separation of Suwannee River natural organic matter (NOM) with capillary zone electrophoresis hyphenated to electrospray ionization-mass spectrometry (CZE-ESI-MS) is presented. The obtained electropherograms and signal distributions are comparable to the mobility distributions obtained with more classical UV detection. A direct comparison of the results was possible with free-flow electrophoresis (FFE), which allows an upscaling of the CZE method and the analysis of the collected fractions in an off-line modus with flow-injection electrospray ionization-mass spectrometry (FI-ESI-MS). The changes of the m/z distributions with mobility are very similar with both methods and show a decrease of the m/z with increasing electrophoretic mobility in the humic hump at alkaline pH; superimposed on this hump a low-molecular-weight fraction migrates at lower mobility. The analysis of benzene carboxylic acids, glycerrhycic acid as well as oligomers of polystyrene sulfonic acid and polyacrylic acid additionally illustrates possible fragmentation, formation of adducts and multiplicity of the charges of the molecules prior to MS detection. These hardly controllable difficulties add a challenge to the interpretation of the obtained m/z distributions of NOM in terms of charge and mass distributions of molecules present in the NOM mixture.

Determination of polyacrylamide in soil waters by size exclusion chromatography

Determination of polyacrylamide (PAM) concentration in soil waters is important in improving the efficiency of PAM application and understanding the environmental fate of applied PAM. In this study, concentrations of anionic PAM with high molecular weight in soil waters containing salts and dissolved organic matter (DOM) were determined quantitatively by size exclusion chromatography (SEC) with ultraviolet (UV) absorbance detection. Polyacrylamide was separated from interferential salts and DOM on a polymeric gel column eluted with an aqueous solution of 0.05 M KH2PO4 and then detected at a short UV wavelength of 195 nm. Analysis of PAM concentrations in soil sorption supernatants, soil leachates, and water samples from irrigation furrow streams showed that SEC is an effective approach for quantifying low concentrations (0-10 mg L(-1)) of PAM in waters containing soil DOM and salts. The method has a lower detection limit of 0.02 microg and a linear response range of 0.2 to 80 mg L(-1). Precision studies gave coefficients of variation of < 1.96% (n = 4) for > 10 mg L(-1) PAM and < 12% (n = 3) for 0.2 to 3 mg L(-1) PAM.

The effect of home-use fluoride gels on glass-ionomer, compomer and composite resin restorations

The purpose of this study was to investigate the resistance to dissolution by two home-use fluoride gels on the surface integrity of glass-ionomer, resin modified glass-ionomer, compomer and composite resin restorations. Class V cavities prepared in extracted teeth were restored with a glass-ionomer (Fuji II), a resin modified glass-ionomer (Vitremenr), two compomers (Dyract and F-2000) and a composite resin (Z-100). Groups of five specimens of each material were treated for 24 h with one of the following: (i). distilled water, (ii). neutral fluoride gel and (iii). acidulated phosphate fluoride (APF) gel. Surface degradation of the restorations was studied using standard electron microscopy (SEM), rated according to specific criteria and statistically analysed by the Wilcoxon test (rank sums). Acidulated phosphate fluoride was found to have a significant effect on all examined materials, while minimal effects resulted from the neutral fluoride gel compared with the control group. The effect of home-use fluoride gels on glass-ionomer, compomer and composite resin restorations.

Influence of dentinal regions on bond strengths of different adhesive systems

This in vitro study assessed comparatively the shear bond strengths of three composite resins, 3M Valux Plus (3MVP), Herculite (H), Clearfil AP-X (CAP-X), a polyacid modified composite resin Dyract (D), and a resin modified glass-ionomer materials Vitremer (V), to cervical and buccal dentine regions of extracted human molar teeth. Four different bonding systems, 3M ScotchBond Multipurpose (SB), Clearfil Liner Bond 2 (LB2), Opti Bond (OB), and Prime & Bond 2.1 (PB 2.1) were used with the manufacturer's respective composite and compomer materials. One hundred freshly extracted mandibular molar teeth were selected for this study. Flat buccal dentine surfaces were created on 50 teeth and cylindrical rods of the five materials were bonded to the dentine surfaces. For assessment of cervical bond strengths, the materials were bonded to mesial and distal enamel bordered occlusal dentinal surfaces of the remaining 50 teeth. The five groups of restorative procedures were applied as follows; Group 1: SB + 3MVP, Group 2: LB2 + CAP-X, Group 3: OB + H, Group 4: PB2.1 + D, Group 5: Vitremer primer (VP) VP + V. Each restorative procedure thus had 20 specimens (10 buccal + 10 cervical). After 24 h of water storage (37 degrees C), the specimens were tested on a Universal Testing machine in shear with a cross head speed of 0.5 mm min-1. The bond strength values were calculated in MPa and the results were evaluated statistically using Kruskal-Wallis one-way/anova and Mann-Whitney U-tests. It was found that the bond strengths of SB + 3MVP, LB2 + CAP-X and VP + V to buccal dentine surfaces were significantly stronger (P < 0.05) than those to the occluso-cervical dentine floors. When the bond strengths to the occluso-cervical dentine and buccal dentine surface were compared, there was no significant difference between the materials (P > 0.05). Vitremer was found the least successful adhesive material in terms of shear bond strength on both buccal and occluso-cervical dentine surfaces.

Polyacrylamide distribution in columns of organic matter-removed soils following surface application

Knowledge of how polyacrylamide (PAM) penetrates and distributes in a soil profile after application in irrigation water is important for understanding PAM conditioning depth and evaluating its environmental effects. Little is known, however, about PAM distribution in soil because of the difficulty in quantifying PAM content in natural soils. By using a recently modified substrate-borne PAM quantification method, PAM distribution in columns of organic matter-removed soils was determined. Results showed that penetration of PAM into the soil was affected by salt level of irrigation water, soil texture, initial soil water content, water application method, and other factors. Polyacrylamide penetration depth was about one-eighth to one-half of the water penetration depth, with a particularly high PAM retention in the top few centimeters of the soil. Under different experimental conditions, the PAM retained in the top 0 to 2 cm of soil ranged from 16 to 95% of the total applied amount. More favorable solution-soil contact conditions, longer solution-soil contact time, and lower initial soil moisture caused much more PAM retention in the top few centimeters of the soil. High sorptive affinity of PAM on soil is the main reason for its low penetration into the soil. Although these results were not obtained from natural soils, they are still helpful in improving our understanding of PAM transport behavior in soils.

Radiation dose distribution in polymer gels by Raman spectroscopy

The Raman spectroscopy of polymer gel dosimeters has been investigated with a view to developing a novel dosimetry technique that is capable of determining radiation dose within a micrometer of spatial resolution. The polymer gel dosimeter, known as the PAG dosimeter, is typically made up of acrylamide, N,N'-methylene-bis--acrylamide, gelatin, and water. A polyacrylamide network within the gelatin matrix forms in response to an absorbed dose. The loss of monomers may be monitored by corresponding changes to the Raman spectrum. Principal component analysis offers a simple method of quantifying the absorbed radiation dose from the Raman spectrum of the polymer gel. The background luminescence in the spectrum increased significantly with dose and is shown to originate in the glass of the sample vial. The competing effects of elastic scatter, which increases with dose due to the formation of polymer, and sample absorption were quantified and found to introduce errors of up to 5% under certain conditions. Raman spectra as a function of distance from the air-surface interface have been measured for samples that were subjected to doses delivered by a clinical linear accelerator. The depth dose profile thus obtained compared favorably with "gold standard" ion-chamber measurements.

Spectrophotometric determination of substrate-borne polyacrylamide

Polyacrylamides (PAMs) have wide application in many industries and in agriculture. Scientific research and industrial applications manifested a need for a method that can quantify substrate-borne PAM. The N-bromination method (a PAM analytical technique based on N-bromination of amide groups and spectrophotometric determination of the formed starch-triiodide complex), which was originally developed for determining PAM in aqueous solutions, was modified to quantify substrate-borne PAM. In the modified method, the quantity of substrate-borne PAM was converted to a concentration of starch-triiodide complex in aqueous solution that was then measured by spectrophotometry. The method sensitivity varied with substrates due to sorption of reagents and reaction intermediates on the substrates. Therefore, separate calibration for each substrate was required. Results from PAM samples in sand, cellulose, organic matter burnt soils, and clay minerals showed that this method had good accuracy and reproducibility. The PAM recoveries ranged from 95.8% to 103.7%, and the relative standard deviations (n = 4) were <7.5% in all cases. The optimum range of PAM in each sample is 10-80 microg. The technique can serve as an effective tool in improving PAM application and facilitating PAM-related research.