Teflon Ring Research Articles

Effects of shade and composition on radiopacity of dental composite restorative materials

The aim of this study was to evaluate the effects of the shade and composition of five dental composite resins on their radiopacity. Five composite resins in various shades were used in this study. Composite specimens were prepared in triplicate for each material and shade using Teflon ring molds. In addition, 1-mm-thick enamel and dentin specimens were prepared from freshly extracted premolar teeth. Each specimen was placed on a dental film together with an aluminum step-wedge to establish radiopacity equivalents for the composite materials, dentin, and enamel. Dental films were exposed using a dental X-ray machine at 70 kVp and 8 mA, and processed using an automatic processor. The optical densities of the composite specimens, step-wedge, enamel, and dentin were evaluated using a densitometer. The data were analyzed by one-way and two-way ANOVA and a post hoc Tukey HSD test at a significance level of a = 0.05. The shades of the tested composite resins did not affect their radiopacity level (p > 0.05). The composition of fillers of the tested composite resins influenced their ability to absorb X-ray radiation. All shades of the tested composite resins met the ISO requirement for radiopacity of resin-based restorative materials. The tested shades of the composite resins were appropriate for dental restorations regarding their radiopacity level. Although there were significant differences in radiopacity among the tested composites, they may not have clinical significance.

LIGHT TRANSMISSION THROUGH POSTS TO SIMULATED REGIONAL ROOT CANAL USING DIFFERENT CURING SOURCES

Statement of problem: Light-activated luting agents have become an indispensable material for clinical dental use. However, it remains debatable as to which level, the light curing unit pro­vides sufficient intensity of light to achieve polymerization. Purpose: The purpose of this study was to assess quantitatively the light intensity transmitted from three different light sources through translucent glass fiber posts with sequential cuts at depths of 15 mm, 10 mm, 5.0 mm Materials and Methods: A 15 x 30 mm hollow Teflon ring was used. 60 specimens were prepared and assigned randomly into three groups (n=20 per each) according to the curing source. Within each group, 15 specimens were used for the experimental and 5 for the control group. For the experimental ones, a self-cure acrylic resin with black pigment was poured into the ring. A translucent glass fiber post was placed in the center of each ring and held in position till com­plete polymerization was occurred. For the control groups, the black resin blocks were filled with a lubricated dowel for its later removal. Therefore, the resin blocks with internal hollow orifices were obtained without the dowel. After preparing the blocks, readings for the passage of light will be made at the apical, middle , and cervical third level, involving dowel lengths of 15, 10, 5 mm respectively. A 40-second light exposure was used for each specimen and curing unit combina­tion .The output of light intensity was recorded. Mean light intensities were analyzed with 2-way ANOVA test followed by Kruskal-Wallis tests at α = 0.05. Results: Two-way ANOVA results of light intensities for the experimental and control groups demonstrated significant differences between the light curing units (P<0.001) and the root level (P<0.001). Moreover, their interaction was also significantly different (P<0.001). For the experi­mental groups, Kruskal Wallis test revealed the highest quantity of light intensity was recorded for the high power LED curing unit at the cervical level of the root canal. For the control groups, the highest quantity of light intensity was displayed for the HPLED curing unit at the cervical level of the root canal. Conclusions: The quantity of the light intensity transmitted from three different light sourc­es are significant different through translucent glass fiber posts with sequential cuts at depths of 15 mm, 10 mm , 5.0 mm

A new rotating-disk sorptive extraction mode, with a copolymer of divinylbenzene and N-vinylpyrrolidone trapped in the cavity of the disk, used for determination of florfenicol residues in porcine plasma

A novel extraction approach was developed based on rotating-disk sorptive extraction (RDSE). In this approach the rotating-disk extraction device consists of a Teflon disk, with a cavity that is loaded with a commercial sorbent phase selected according to the polarity of the analyte. To avoid leakage of the sorbent, the cavity is covered with a fiberglass filter and sealed with a Teflon ring. The proposed novel analytical RDSE technique was used in this study to determine florfenicol levels in plasma as a model analyte, or sample system, to describe the pharmacokinetics of a veterinary formulation. The sorbent used for this application was the copolymer of divinylbenzene and N-vinylpyrrolidone (Oasis HLB), which was selected because the florfenicol molecule contains both hydrophilic and lipophilic moieties. After the extraction, final determination of the analyte was performed by HPLC-DAD. Calibration plots and other analytical features were obtained after 90 min of extraction. The calibration plot was linear over the interval 0.4-16 μg mL(-1) (n = 6), with R (2) = 0.9999. Recovery and repeatability were determined using a blank plasma sample spiked with 4.8 μg mL(-1) florfenicol. A recovery of 91.5 %, with a relative standard deviation (RSD) of 8.8 %, was obtained when the extraction was evaluated using six different rotating-disk devices. Precision was also assessed, using the same disk (containing the same sorbent phase) for eight aliquots of the same sample. The RSD under these conditions was 10.2 %, clearly indicating that the sorptive phase could possibly be re-used. Accordingly, RDSE is a suitable sample preparation alternative to liquid-liquid extraction (LLE), solid-phase extraction (SPE), and stir-bar sorptive extraction (SBSE).

Effect of Vital Bleaching on Disintegration Tendency of Glass Ionomer Restorations

This study was designed to assess the effect of two bleaching agents on the disintegration tendency of three types of glass ionomers. A total of 90 specimens were prepared by using a split Teflon ring with an internal diameter of 5 mm and a thickness of 2 mm. The tested materials were applied and bleached according to manufacturer's instructions. Dissolution measurements were made by calculating weight loss through different periods of the test; (one week, one month and three months) and they were analyzed by using one-way analysis of variance (ANOVA), followed by Tukey's post-hoc test. All glass ionomer materials exhibited a degree of dissolution. Opalescence Xtra increased the dissolution of Photac Fil and F2000 significantly, while Opalescence Quick had no effect on dissolution of glass ionomer restoratives. Bleaching effect on dissolution of glass ionomers is material and time dependant. Care should be taken by clinicians When bleaching teeth that are restored by glass ionomer, because this dissolution may affect the physical properties of these restorations.

Effect of addition of hyaluronic acids on the osteoconductivity and biodegradability of synthetic octacalcium phosphate

The present study was designed to investigate whether three sodium hyaluronic acid (HyA) medical products, Artz®, Suvenyl® and a chemically modified derivative of sodium HyA Synvisc®, can be used as suitable vehicles for an osteoconductive octacalcium phosphate (OCP). OCP granules (300–500μm diameter) were mixed with these sodium HyAs with molecular weights of 90×104 (Artz®), 190×104 (Suvenyl®) and 600×104 (Synvisc®) (referred to as HyA90, HyA190 and HyA600, respectively). OCP–HyA composites were injected using a syringe into a polytetrafluoroethylene ring, placed on the subperiosteal region of mouse calvaria for 3 and 6weeks, and then bone formation was assessed by histomorphometry. The capacity of the HyAs for osteoclast formation from RAW264 cells with RANKL was examined by TRAP staining in vitro. Bone formation was enhanced by the OCP composites with HyA90 and HyA600, compared to OCP alone, through enhanced osteoclastic resorption of OCP. HyA90 and HyA600 facilitated in vitro osteoclast formation. The results suggest that the osteoconductive property of OCP was accelerated by the HyAs-associated osteoclastic resorption of OCP, and therefore that HyA/OCP composites are attractive bone substitutes which are injectable and bioactive materials.

Fluoride release of glass ionomer restorations after bleaching with two different bleaching materials.

Objective:This study was designed to evaluate the effect of two bleaching agents on the fluoride release of three types of glass ionomer materials.Materials and Methods:A total of 90 specimens of the tested materials (Ketac Fil, Photac Fil and F2000) were prepared by a split Teflon ring with an internal diameter of 5 mm and thickness of 2 mm. The tested materials were applied and bleached according to manufacturer instructions. Fluoride release measurements were made by using specific ion electrode.Results:Results revealed that bleaching with opalescence Xtra caused little increase in fluoride release from Ketac Fil and Photac Fil but has no effect on F2000. However, Opalescence Quick had no significant effect on the three tested materials.Conclusions:Bleaching effect on fluoride release is material dependent and time has a significant role on fluoride release.

Effect of exposure parameters on the radiopacity of root canal sealers

BackgroundThe aim of this study was to test the effect of different kilovoltage, milliampere, and exposure time settings on the radiopacity of two endodontic sealers using an image plate system. Materials and methodsMM-Seal (MicroMega, Besancon, France) and Diaket (3M Espe, Seefeld, Germany) were packed into a polytetrafluoroethylene ring mold with an internal diameter of 10mm and a depth of 1mm. Five discs of each material were imaged alongside an aluminum step-wedge. Sets of radiographs were obtained using storage phosphor plates (SPPs). The SPPs were exposed at 60, 65, or 70kVp, 7 or 8mA for 0.12 or 0.16s. Radiographic densities of the sealers and each step of the step-wedge were analyzed with Image Tool 3.0 SDK software (University of Texas Health Sciences Center, TX, USA). Three readings were made for each image. The mean was calculated to give the radiographic density expressed as mean gray values. Three-way ANOVA was used to test the differences in radiopacity of sealers with respect to kilovoltage, milliampere, exposure time, and the interaction of the three factors. ResultsNo significant difference in sealers’ radiopacity was found with change in three exposure parameters (p>0.05). However, standard deviations of the measurements for lower exposures were greater; MM-Seal was significantly more radiopaque than Diaket (p<0.0001) at any parameter. ConclusionsDifferences in kilovoltage, milliampere, and exposure time do not affect the radiopacity measurements of sealers on SPP images. However, choosing the proper exposure parameters may optimize the gray value range of the sealers allowing for better discrimination of the sealer and surrounding structures thereby providing better clinical guidance.

Porous Teflon Ring-Solid Disk Electrode Arrangement for Differential Mass Spectrometry Measurements in the Presence of Convective Flow Generated by a Jet Impinging Electrode in the Wall-Jet Configuration

A porous Teflon ring|solid disk electrode is herein described specifically designed for acquiring online mass spectrometric measurements under well-defined forced convection created by liquid emerging from a circular nozzle impinging on the disk under wall-jet conditions. Measurements were performed for the oxidation of hydrazine, N(2)H(4), in a deaerated phosphate buffer electrolyte (pH 7) on Au, a process known to yield dinitrogen as the product. The N(2)(+) ion currents, measured by the mass spectrometer, i(N(2)(+)), as well as the corresponding polarization curves recorded simultaneously displayed very similar s-like shapes when plotted as a function of the potential applied to the Au disk. In fact, the limiting currents observed both electrochemically and spectrometrically were found to be proportional to [N(2)H(4)]. However, the limiting values of i(N(2)(+)) did not increase monotonically with the flow rate, ν(f), reaching instead a maximum and then decreasing to values independent of ν(f). This behavior has been attributed in part to hindrances in the mass transport of gases through the porous materials.

Three Different Strategies to Obtain Porous Calcium Phosphate Cements: Comparison of Performance in a Rat Skull Bone Augmentation Model

Preprosthetic surgery has become a routine procedure to obtain sufficient bone quantity and quality for dental implant installation in patients with an initial inadequate bone volume. Although autologous bone onlay or inlay grafting is still the preferred bone augmentation technique, a broad range of synthetic bone substitutes have been developed, for example, calcium phosphate cement (CPC). The introduction of porosity within CPC can be used to increase CPC degradation and bone ingrowth. Therefore, three different strategies to obtain porous CPCs were evaluated in this preclinical study. Instantaneously porous CPC (CPC-IP) was compared with delayed porous CPC in vitro and in vivo. CPC-IP was obtained by the creation of CO₂ bubbles during setting, whereas delayed porous CPC was obtained after the degradation of incorporated poly(lactic-co-glycolic acid) (PLGA) microspheres. As an additional aspect, delayed porous CPC was created by the incorporation of either hollow or dense degradable PLGA microspheres (CPC-hPLGA and CPC-dPLGA). All CPC compositions showed appropriate clinical handling properties and an interconnected porous structure with a final porosity above 70% (v/v). In vitro degradation studies showed the gradual formation of pores and further CPC-matrix dissolution for CPCs containing PLGA microspheres (dPLGA microspheres > hPLGA microspheres). For in vivo evaluation of the CPCs, an augmentation model was used, allowing a CPC injection into a rigidly immobilized Teflon ring on the rat skull. Histological evaluation after 12 weeks of implantation showed bone formation using all three CPCs. Bone apposition reached volumetric amounts of up to 10% of the augmentation area and a maximum augmentation height of ∼1 mm. CPC-IP showed significantly more bone formation and resulted in a superior bone apposition height compared with both CPCs containing PLGA microspheres. No differences in biological performance were observed between the CPCs containing hPLGA and those containing dPLGA microspheres. Further research is necessary to enhance the bone appositional speed and amount of CPCs for bone augmentation procedures before them being used in a potential clinical setting.

Fourth Recurrence of Aortic Annular Dehiscence Following AVR for Aortic Regurgitation

It is a report of an unusual case of recurrent periprosthetic aortic regurgitation which occurred four times in the same patient over a span of 26 years. After pre-operative stabilization, the patient was subjected to a fifth open heart surgery to replace the 4 th prosthetic aortic valve by an upsized 5 th prosthetic valve. Since endocarditis was excluded by repeated blood culture and paucity of vegetations, a working diagnosis of Behcet's disease was arrived upon. The fifth prosthetic aortic valve was implanted using a special suturing technique as described by Azumo, et al in 2009, which involves a sub-annular reinforcement by a Teflon ring. The patient was discharged after an uneventful recovery and is well on regular follow-up.

Microhardness of nanofilled composite resin light-cured by LED or QTH units with different times

Aim: To evaluate the influence of light-curing units and light-curing time on the microhardness of a nanofilled composite resin. Methods: Forty-five composite resin (Z350 - 3M) specimens were randomly prepared using Teflon ring molds (4.0 mm internal diameter and 2 mm depth) and divided into nine experimental groups (n=5): three polymerization units (conventional - 450 mW/ cm2 ; 2nd generation LED - 1100 mW/cm2 ; and 3rd generation LED - 700 mW/cm2 ) and three lightcuring times (20 s, 40 s, and 60 s). All specimens were polymerized with the light-curing tip positioned 8 mm far from the top surface of the specimen. After 24 h, Knoop microhardness measurements were made on the top and bottom surfaces of the specimen, with a load of 10 g for 10 s. Five indentations were made on each surface. All results were analyzed statistically by subdivided parcel ANOVA (Split-Plot) and Tukey’s tests (p<0.05). Results: There were no statistically significant differences for the polymerization unit and light-curing time factors in either top or bottom surface. For all experimental conditions, the top surfaces showed greater hardness than the bottom surfaces (p<0.0001). Conclusions: The mode of polymerization and the lightcuring time did not affect the hardness of the nanofilled composite resin, and increasing the lightcuring time did not improve the hardness of the bottom surface of the composite resin.

Binary system thermodynamics to control pore architecture of PCL scaffold via temperature-driven phase separation process

The use of scaffold-aided strategies for the regeneration of biological tissues requires the fulfilment of an accurate architectural design, that is, micro and macrostructure, with the final goal of realizing architectures to adopt as guidance for those cell activities specific to the formation of novel tissues. Here, highly porous scaffolds made up of biodegradable poly(ε-caprolactone) (PCL) have been realized by thermally induced phase separation (TIPS). Two different polymer/solvent systems, derived by the dissolution of PCL in dioxane and DMSO respectively, were investigated. The aim was to demonstrate the high potential of TIPS technique, in imprinting specific pore features to the polymer matrices, by a conscious selection of polymer/solvent systems. The investigation of pore architecture by SEM/mercury intrusion porosimetry/image analyses, firstly allow to detect remarkable variations in porosity (from 92% to 78%,) and pore sizes, ranging from micro-scale (ca 10 µm) to macro-scale (greater than 100 µm) as a function of the used polymer/solvent systems. Moreover, experimental and theoretical evidences referred to scaffold shaped in custom-made molds--a thin Teflon ring between two copper plates--allow exploring how the sensitivity of polymer solution features (i.e., crystallinity, thermal inertia) to the cooling temperature can affect the alignment of polymer phases and, ultimately, scaffold pore anisotropy. Analytical results supported by preliminary biological studies demonstrate the higher ability of PCL/dioxane solution to promote the formation of aligned pores which provide a morphological guidance to cell advance during the preliminary stage of culture. These findings, taken as a whole, put the basis for a better informed regeneration of structurally complex tissues based on the modeling of scaffold micro and macro-architecture by thermodynamic forces.

Plasma etching uniformity control for making large and thick dual-focus zone plates

A typical zone plate structure is composed of concentric rings with a radially decreasing feature size such that the path of light through every second zone to the focus differs by one optical wavelength. In order to minimize the wavefront error, the etching depth has to be controlled uniformly across the whole substrate area to a phase difference of one half wavelength between adjacent zones. In this study, an optimized plasma etching process has been developed to minimize the feature size variation effect and a Teflon ring has been made to eliminate the edge effect on thick fused silica. A large phase type dual-focus zone plate with varied concentric ring sizes with good etching profile and better than 2.0% etching depth uniformity was successfully made.

Mechanical Stress-Related Calvaria Bone Augmentation by Onlayed Octacalcium Phosphate–Collagen Implant

Previous studies have suggested that the biodegradability of octacalcium phosphate-collagen (OCP/Col) composite by osteoclasts is accelerated in association with mechanical stress suffered by the host tissue around the implant. The present study was designed to investigate whether alleviation of mechanical stress restores the bone regenerative properties of OCP/Col, as previously shown in nonload-bearing sites. OCP/Col discs supported with a polytetrafluoroethylene (PTFE) ring, which has a higher modulus than OCP/Col, were implanted in a rat subperiosteal pocket for up to 12 weeks. The structural features of the implant and biological responses were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, histomorphometry, histochemistry, and tissue mRNA expression around the implants. The effect of compression was analyzed using mouse stromal ST-2 cells by deforming the cell-seeded OCP/Col discs in vitro with or without a PTFE ring. The results clearly indicated the restoration of bone formation by the alleviation of mechanical stress and the upregulation of osteoblast-related genes, such as osterix on the other hand, the implantation of OCP/Col on calvaria or in an in vitro test without PTFE support resulted in the upregulation of osteoclast-related genes, such as tartrate-resistant acid phosphatase (TRAP) and cathepsin K, in the tissues or receptor activator of the nuclear factor-kappaB ligand (RANKL) in ST-2 cells. The results confirmed that calvaria augmentation is enhanced by implanting OCP/Col if suitable conditions regarding mechanical stress are provided.

“Randomized trial comparing the use of carpentier’s ring with posterior segment annuloplasty using a C-shaped ring of teflon felt for mitral valve repair in rheumatic population”

Introduction Mitral Valve (MV) repair has gained preference over MV replacement in the past few decades. This randomized trial compares our technique of C-shaped Teflon ring annuloplasty with Carpentier’s ring for annuloplasty in rheumatic mitral valve disease.

Testing of Hygrothermally Aged E-Glass/Epoxy Cylindrical Laminates Using a Novel Fixture for Simulating Internal Pressure

This paper discusses the development of a novel test fixture for applying a uniform radial stress to cylindrical composite laminates in such a way as to eliminate bending moments and axial loads. This fixture requires the use of easily fabricated poly-tetrafluoroethylene (PTFE) ring seals. These seals, which fit just inside the composite sample, are designed to be internally pressurized in the fixture and expand outwards, applying a uniform radial load to the sample. The results of mechanics analyses of the PTFE ring seal and composite sample are presented, which estimate the radial stress applied to the composite sample as a fraction of the applied internal hydraulic pressure. Further, results are presented from a prolonged hygrothermal aging study, in which the E-glass/epoxy, cylindrically wound composite samples are aged under varying hygrothermal conditions (ambient dry; and 30, 40, and 50°C submerged in water). At various points during the aging, a subset of samples are removed and destructively test...

Liquid–liquid–liquid microextraction followed by HPLC with UV detection for quantitation of ephedrine in urine

Liquid-liquid-liquid microextraction (LLLME) in combination with HPLC and UV detection has been used as a sensitive method for the determination of ephedrine in urine samples. Extraction process was performed in a homemade total glass vial without using a Teflon ring, usually employed. Ephedrine was first extracted from 3.5 mL of urine sample (pH 12) into a microfilm of toluene/benzene (50:50). The analyte was subsequently back extracted into an acidic microdrop solution (pH 2) suspended in the organic phase. The extract was then injected into the HPLC system directly. An enrichment factor of 137 along with a good sample clean-up was obtained under the optimized conditions. The calibration curve showed linearity in the range of 0.01-50 mg/L with regression coefficient corresponding to 0.998. The LODs and LOQs, based on a S/N of 3 and 10, were 5 and 10 microg/L, respectively. The method was eventually applied for the determination of ephedrine in urine sample after oral administration of 5 mg single dose of drug.

Platelet deposition in rabbit common carotid arteries promoted by arterial stenosis and spasm

Coronary arteriograhy in patients with ischemic heart disease often shows spasm of the coronary arteries. The question is whether spasm is a triggering factor for thrombosis in a stenotic artery. If so, what are the mechanisms for this? A stenosing teflon ring was applied to the right common carotid artery of anesthetized rabbits and 1-nor-epinephrine was dripped over the outer surface of both carotid arteries, causing spasm. In control animals an indifferent solution did not cause spasm. Nineteen rabbits were killed 30 min or 24 h after treatment. Microscopically, arteries with stenosis and spasm contained thrombi nearby the stenosis significantly more often than arteries in control animals. In another 14 rabbits, killed at 30 min, the number of platelets on the intimal surface away from the stenosis was quantified. In arteries with both stenosis and spasm the counts were significantly greater than in arteries with no treatment. The intimal surface in stenotic and spastic arteries showed assumed imprints of eddying flow and endothelial injury downstream and upstream of the stenosis. Spastic arteries showed increased folding of the internal elastic membrane, altered endothelial cells, and adhering platelets. Spasm in a rabbit artery with a preformed stenosis facilitates thrombosis probably by creating increased flow disturbances. Spasm may induce endothelial injury, causing adherence of platelets.

Evaluating Pressure Integrity of Polymer Ring Seals for Threaded Connections in HP/HT Wells and Expandable Casing

Summary This paper presents new technology for evaluating high-pressure gas-seal integrity of polymer ring seals used as secondary or backup pressure seals in casing and tubing threaded connections. This new technology may also enable the further consideration of API connections with ring seals, as an alternative to premium connections, for appropriate applications. A nonlinear elasto-viscoplastic constitutive model for the behavior of polymers and elastomers has been developed and extended to the specific application of analysis of casing and tubing connections with fiberglass-filled polytetrafluoroethylene (PTFE) ring seals. Procedures for modeling makeup of a connection including a fiberglass-filled PTFE ring seal have been developed using a finite-element model (FEM) of 10¾-in. OD, 45.5 lb/ft, P-110 API buttress thread casing-seal ring groove (BTC-SRG). The results of finite-element analysis (FEA) of makeup, followed by the application of thermal, axial, and internal pressure loads are presented in this paper. In addition, based on the interest in the development of gas-tight threaded connections for expandable casing, the FEM was subjected to a radial expansion of a 20% increase in the outside diameter. In this paper, the theory of the constitutive model is summarized and calibration of the model with experimental test and published data are presented. The focus of the FEA results is on the contact pressures between the ring seal, coupling groove, and pin threads. Historical Perspective FEA of threaded connections has been used for overcoming challenging well-design problems for many years (Crose et al. 1976). FEA has become an important part of the validation and service evaluation process of API and proprietary casing and tubing threaded connection designs, along with the physical testing procedures documented in API RP 5C5 (1996) and ISO 13679: 2002 (2002). Major advances have been achieved in design of premium connections through analysis of metal-to-metal seal contact stresses computed from FEM (Hilbert and Kalil 1992). Analysis and verification of the performance of threaded connections that include polymeric or elastomeric ring seals has been limited to full-scale physical testing (Payne 1988). Until now, only costly full-scale gas pressure tests have been used to evaluate ring seal integrity. Ring-seal design has been a trial and error process, with new ring-seal or pin and coupling dimensions prescribed only after failure of the seal in a proof test. In some cases, ring design or the effects of ring dimensions have been based on analytical calculations, relying on the bulk modulus of the material. When more advanced design tools, such as FEA, have been used, the pressure generated by entrapment of the ring seal has been estimated and then these pressures have been applied to the groove and pin thread surfaces to simulate the effect of the actual ring seal. The developments in the paper were motivated by a need to reduce the cost of connection qualification by reducing the number of tests and to improve the process of ring-seal design. Properties of PTFE PTFE is a thermoplastic fluorocarbon derived from the monomer tetrafluoroethylene (TFE). PTFE is a semi-crystalline polymer composed of crystalline and amorphous regions. Its molecular structure, shown in Fig. 1, consists of long chains of carbon atoms symmetrically surrounded by fluorine atoms. This structure imbues PTFE with unique mechanical and chemical properties. The straight "backbone" of carbon atoms provides PTFE with a high degree of chemical inertness, stability, and one of the lowest coefficients of friction of any commonly used material. PTFE is more commonly known by the trade name Teflon. In a moment of pure serendipity, in 1938 Roy Plunckett of DuPont discovered TFE when he was conducting experiments to develop nonflammable, nontoxic, colorless, and odorless refrigerants (Ebnesajjad 2000).

Modified solvent microextraction with back extraction combined with liquid chromatography-fluorescence detection for the determination of citalopram in human plasma

A modified solvent microextraction with back extraction method (SME/BE) combined with high performance liquid chromatography and fluorescence detection (HPLC-FD) was developed for the determination of citalopram in human plasma. Extraction process was performed in a home-made total glass vial without using a teflon ring, usually employed in SME/BE. Citalopram was first extracted from 0.5 mL of plasma, modified with sodium hydroxide, into hexane. Back extraction step was then performed into 5.2 μL of 45 mM ammonium formate solution (pH 4) using a GC microsyringe. The extract was subsequently transferred into a liner-like vial and then injected into the HPLC system. An enrichment factor of 150 along with a good sample clean-up was obtained. The calibration curve showed linearity in the range of 1.0–130.0 ng mL −1 with regression coefficient corresponding to 0.992. This range covers therapeutic window and even lower amounts which is important in pharmacokinetic studies. Limits of detection and quantification, based on a signal to noise ratio (S/N) of 3 and 10, were 0.3 and 0.8 ng mL −1, respectively. The method was also applied for the determination of citalopram in plasma samples after oral administration of 40 mg single dose of citalopram.