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PubMed2004

Two-dimensional fatty acid retention indices.

A two-dimensional retention index system for fatty acid methyl esters (2D-FARI) is proposed. The system is based on the application of different temperature and pressure programs on a single capillary column. A calibration sample is analysed and the retention data is calibrated against a set of reference 2D-FARI values. The calibration models are then applied to predict the 2D-FARI values for compounds that are not present in the calibration sample. The two dimensions in the retention index system lead to increased selectivity and a reduced risk of retention index overlap between different compounds. The 2D-FARI system is also more robust towards differences in stationary phase properties than ordinary retention indices and is therefore convenient for comparison of retention data acquired on different columns, or at different times at the same column.

Chromatography, Gas↗
PubMed2005

Retention behavior of small polar compounds on polar stationary phases in hydrophilic interaction chromatography.

Retention characteristics of four polar stationary phases (i.e., amide, amino, silica and sulfobetaine) were studied by using a group of small polar compounds in hydrophilic interaction chromatography (HILIC). Different polar stationary phases shared certain degrees of similarity, but also exhibited differences in retentivity and selectivity for the model compounds. Among the four columns studied, HILIC Silica column had the least retention for the model compounds, but also showed different selectivity from other three columns. Experimental data also provided some evidences that functional groups on the stationary phases might have certain degrees of influence on selectivity possibly through secondary interactions with the model compounds. The retention of the acids on the amino phase decreased with increasing salt concentration in the mobile phase due to the ion-exchange effect, and the retention process was endothermic as opposed to exothermic on other phases. This study also systematically investigated the effect of various experimental factors on the retention of the polar stationary phases, such as acetonitrile content, column temperature, buffer pH, salt type and concentration in the mobile phase.

Acetonitriles↗
PubMed2005

Mechanism of retention loss when C8 and C18 HPLC columns are used with highly aqueous mobile phases.

We describe investigations into the cause of retention losses encountered when C8 and C18 HPLC columns are used with highly aqueous (> 90% water) mobile phases. A procedure for quantifying these losses is described, involving stopping and restarting the flow. This procedure was used to study the dependence of retention loss on the pore size, surface concentration, and chemical structure of the bonded phase. Experiments were also carried out to determine how to restore the original retention of the columns by changing the composition of the mobile phase, or by increasing the pressure applied to the column. The results are shown to be consistent with a mechanism based on the theory of pore filling by non-wetting liquids, as employed in Mercury Porosimetry. The retention losses are attributed to the highly aqueous mobile phase being forced out of the pores when the flow is stopped and the pressure released. Retention is lost because the mobile phase is no longer in contact with the interior surface of the particles, where most of the surface area is located. The implications of this phenomenon for maximizing the reversed phase retention of polar analytes are discussed.

Chromatography, High Pressure Liquid↗
PubMed2005

Effect of temperature on the chromatographic retention of ionizable compounds. II. Acetonitrile-water mobile phases.

The retentive behavior of weak acids and bases in reversed-phase liquid chromatography (RPLC) upon changes in column temperature has been theoretically and experimentally studied. The study focuses on examining the temperature dependence of the retention of various solutes at eluent pH close to their corresponding pKa values, and on the indirect role exerted by the buffer ionization equilibria on retention and selectivity. Retention factors of several ionizable compounds in a typical octadecylsilica column and using buffer solutions dissolved in 30% (v/v) acetonitrile as eluent at five temperatures in the range from 25 to 50 degrees C were carefully measured. Six buffer solutions were prepared from judiciously chosen conjugated pairs of different chemical nature. Their pKa values in this acetonitrile-water composition and within the range of 15-50 degrees C were determined potentiometrically. These compounds exhibit very different standard ionization enthalpies within this temperature range. Thus, whenever they are used to control mobile phase pH, the column temperature determines their final pH. Predictive equations of retention that take into account the temperature effect on both the transfer and the ionization processes are evaluated. This study demonstrates the significant role that the selected buffer would have on retention and selectivity in RPLC at temperatures higher than 25 degrees C, particularly for solutes that coelute.

Acetonitriles↗
PubMed2005

Method for reducing the ambiguity of comprehensive two-dimensional chromatography retention times.

Comprehensive two-dimensional chromatography generates a two-dimensional chromatogram from a one-dimensional signal array. This process can only be done unambiguously when the range of secondary retention times is less than the modulation period. However, complex samples often produce wider ranges of secondary retention times. Peaks with retention times that exceed the modulation period are said to be "wrapped-around". A simple algorithm has been developed that determines absolute retention times when wrap-around occurs. A sample is first analyzed under standard modulation conditions and then re-analyzed with a modulation period that is increased by an integer fraction of the original modulation period. Retention shifts along the secondary axis are used to determine absolute retention times. A theoretical analysis has been performed to optimize the implementation conditions and characterize the technique limitations. The efficacy of this algorithm has been tested through a series of isothermal GC x GC separations. This method has been found to be particularly useful during the initial stages of method development.

Algorithms↗
PubMed2006

Theoretical calculation of the retention enthalpy effect on the viscous heat dissipation band broadening in high performance liquid chromatography columns with a fixed wall temperature.

The problem of the additional band broadening arising from the trans-column temperature profile under conditions of significant viscous heat dissipation has been extended by including the retention enthalpy effect. This effect can significantly aggravate the trans-column migration velocity profile of retained species as compared to the already known zero-retention enthalpy case. An approximate analytical solution for the accompanying additional band broadening could be established. The solution is valid in nearly all practically relevant cases and shows how the additional band broadening varies as a function of the generated heat, the retention coefficient and the retention enthalpy. Surveying all possible HPLC operating conditions, it was found that the inclusion of the retention enthalpy effect leads to significantly larger viscous heating plate height contributions than those predicted by the zero-retention enthalpy solution (up to a factor of 10 or more).

Chromatography, High Pressure Liquid↗
PubMed2006

Different retention behavior of structurally diverse basic and neutral drugs in immobilized artificial membrane and reversed-phase high performance liquid chromatography: comparison with octanol-water partitioning.

The retention behavior of 43 structurally diverse neutral and basic drugs in immobilized artificial membrane chromatography was investigated and compared to the reversed-phase retention and octanol-water partitioning. IAM chromatography was performed using morpholinepropanesulfonic acid (MOPS) or phosphate buffer saline (PBS) at pH 7.4 as the aqueous component of the mobile phase. The differences in the retention factors were attributed to increased electrostatic interactions in the MOPS environment, dependent on the fraction of charged species. Electrostatic interactions were found to play a key role in the relationships with reversed-phase retention factors determined under two different mobile phase conditions as well as in the relationships with lipophilicity data. IAM retention factors correlated better with octanol-water partition coefficients log P than with log D(7.4), as a result of the contribution of electrostatic forces in IAM retention. With log D(7.4) the relationships were improved when the fraction of charged species was taken into consideration. In any case the regression coefficient of log P or log D(7.4) was considerably lower than 1 reflecting the reduced hydrophobic environment of the IAM stationary phase. The different data sets were submitted to principal component analysis for further exploration of their similarities/dissimilarities.

Chromatography, High Pressure Liquid↗
PubMed2006

Retention in gas-liquid chromatography with a polyethylene oxide stationary phase: molecular simulation and experiment.

Configurational-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble were carried out to investigate the partitioning of normal alkanes, primary and secondary alcohols, symmetric alkyl ethers and arenes between a helium vapor phase and a polyethylene oxide stationary phase (M(W)=382 g mol(-1)). The united-atom version of the transferable potentials for phase equilibria force field was used to model all solutes, polyethylene oxide and helium. The Gibbs free energies of transfer and Kovats retention indices of the solutes were calculated directly from the partition constants at two different temperatures, 353 and 393 K. Chromatographic experiments on a Carbowax 20M retentive phase were performed for the same set of solutes and temperatures ranging from 333 to 413 K. The predicted retention indices for alcohols, ethers and arenes are overestimated by about 120, 70 and 20 retention index units, respectively, pointing to an overestimation of the first-order electrostatic interactions in the model system. Molecular-level analysis shows that hydrogen-bonding and dipole-dipole interactions lead to orientational ordering for the alcohol and ether analytes, whereas the weaker dipole-quadrupole interactions for the arene solutes are not sufficient to induce orientational ordering. The retention indices of alcohols and ethers decrease with increasing temperature because of the large entropic cost of hydrogen-bonding and orientational ordering. In contrast, the retention indices for arenes increase with increasing temperature because the entropic cost of cavity formation is smaller for arenes than for comparable alkanes.

Alcohols↗
PubMed2006

Comparison of retention models for polymers 1. Poly(ethylene glycol)s.

The suitability of three different retention models to predict the retention times of poly(ethylene glycol)s (PEGs) in gradient and isocratic chromatography was investigated. The models investigated were the linear (LSSM) and the quadratic solvent strength model (QSSM). In addition, a model describing the retention behaviour of polymers was extended to account for gradient elution (PM). It was found that all models are suited to properly predict gradient retention volumes provided the extraction of the analyte specific parameters is performed from gradient experiments as well. The LSSM and QSSM on principle cannot describe retention behaviour under critical or SEC conditions. Since the PM is designed to cover all three modes of polymer chromatography, it is therefore superior to the other models. However, the determination of the analyte specific parameters, which are needed to calibrate the retention behaviour, strongly depend on the suitable selection of initial experiments. A useful strategy for a purposeful selection of these calibration experiments is proposed.

Chromatography, Gel↗
PubMed2006

Determination of retention indices in constant inlet pressure mode and conversion among different column temperature conditions in comprehensive two-dimensional gas chromatography.

A method to determine the second dimensional real retention time, dead times on both dimensions and retention indices in constant inlet pressure mode was developed in comprehensive two-dimensional gas chromatography. At the same time, the conversion of GC x GC retention indices among different column temperature conditions were also conducted based on some thermodynamics parameters. The calculation accuracies are better than 1.0 retention index unit. Furthermore, a retention index database was developed and used to identify the compounds in a cigarette essential oil sample. It showed that identification by the database was of close agreement with by time-of-flight mass spectrometry, and some isomers could also be distinguished based on the retention index database.

Allergens↗
PubMed2006

The retention properties of nucleobases in alkyl C8-/C18- and IAM-chromatographic systems in relation to log Pow.

In order to evaluate the differences in the partition properties of 35 structurally congeneric nucleobases of biological interests in octanol-water biphasic, alkyl C(8)/C(18), and IAM systems, a comparative chromatographic study was performed. Comparing with the reversed-phase C(8)/C(18) retention data, most of the purines possessed weaker IAM retention except for those with specific H-bond and/or electrostatic interactions. Quantitative correlations between the experimental log P(ow) literature values and the IAM, C(8), and C(18) log k were evaluated (R(2)=0.943, 0.794, and 0.767, respectively). Although IAM retention correlated significantly better (larger R(2) value) with the log P(ow) values statistically, the latter was revealed apparently behaving more like (slope approaching unity) alkyl C(8)/C(18) retention and hence also has the same shortcoming in under-representing analytes capable of forming short-term H-bond/electrostatic interactions with polar head-groups of phospholipids. A chemically meaningful structure-retention model (q(2)=0.824 and R(2)=0.968) was derived, in which the hydrophobic interaction is identified as the underlying factor for the retention of purines in IAM system modulated non-trivially by H-bond/electrostatic interactions.

Chromatography, Liquid↗
PubMed2004

In vitro effect of microwave irradiation on the retentive force of magnets.

STATEMENT OF PROBLEM: Few studies have addressed the possible effect(s) of microwave irradiation on the magnetic properties of "permanent" magnets during the fabrication of dental and maxillofacial prostheses. PURPOSE: The objective of this study was to investigate the influence of microwave irradiation energy on the retentive force of a magnetic attachment system used in maxillofacial prosthetic rehabilitation. MATERIAL AND METHODS: A heat-polymerized PMMA disk (6 cm in diameter) was fabricated. Seven magnets were placed around the wafer in a circumferential fashion: 1 in the center and 6 surrounding it. The 7 magnets were spaced 2 cm from their respective centers. Seven heat-polymerized PMMA cylinders were also used, and a magnet (counter-magnet) was placed in the center of each even with the cylinder's surface. Once the investment had set (45 minutes after mixing), the 7 counter-magnets in the cylinders were placed against the 7 magnets in the acrylic wafer. A second mixture of investment material was added. The flasks were separated, and the acrylic wafer was removed to accommodate the 7 counter-magnets in the base mold in the same geometric configuration and to serve as a "spacer" for the silicone material. A 1:1 mixture of medical grade elastomer (MDX4-4210) and medical adhesive silicone (type A) was packed and compressed, the molds were reclamped, and the excess silicone was removed. The elastomer/silicone wafer was packed and compressed into the test and base molds, the molds were reclamped, and excess silicone was removed. The first group of magnets, designated Group A, received microwave irradiation for 5 minutes at low power (112 W). This procedure was repeated for each group of magnets at the following polymerizing times (n=14): Group B, 10 minutes; Group C, 15 minutes; Group D, 20 minutes; Group E, 25 minutes; Group F, 30 minutes; and Group G, 35 minutes. Measurements of retentive force (N) at 10 mm/min ramp rate of speed of separation was conducted. The specimen rate read 5.0 points/second. Data were analyzed using a 1-way analysis of variance (alpha=.05); individual mean values were compared using the Tukey test (alpha=.05). RESULTS: There were no statistical differences in retentive force between groups D, E, and F (20, 25, and 30 minutes, respectively) or between groups A, B, C, D, F, and G (5, 10, 15, 20, 30, and 35 minutes, respectively). When the microwave-irradiated groups A through G were compared with the control group, there was a significant difference (P<.05) in retentive force (N). Group E (25 minutes) showed the largest reduction of retentive force (0.3 N, a reduction of 12%). CONCLUSION: If a prosthesis is processed using a microwave and contains samarium cobalt magnets, the retentive force may be reduced up to 12% under specific conditions.

Cobalt↗
PubMed2005

An in vitro comparison of retentive force and deformation of acetal resin and cobalt-chromium clasps.

STATEMENT OF PROBLEM: The use of metal clasps on anterior teeth may cause esthetic problems. Recently, acetal resins have been used as an alternative tooth-colored denture clasp material to improve esthetics. However, there are few studies to support acetal resin use. PURPOSE: The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin and cobalt-chromium clasps after 36 months of simulated clinical use. MATERIAL AND METHODS: Forty clasps each of acetal resin (1.2 or 2.0 mm thick) and cobalt-chromium (Dentorium)(1.2 mm thick) were fabricated using half-round standard prefabricated clasp patterns. The groups were further subdivided (n=10) into the type of tooth (premolar or molar metal model) and undercut (0.25 mm or 0.50 mm). The retentive force of the clasps was measured in distilled water by a specially designed insertion-removal testing apparatus with intervals corresponding to 0, 6, 12, 18, 24, 30, and 36 months of simulated clinical use of a removable partial denture. The distance between the clasp tips (mm) was measured with a microscope before and after the insertion-removal testing procedure. Comparison of the mean values of the retentive force (gram force) of the clasps and the distance (mm) between the clasp tips was conducted with 3-way analysis of variance and a Least Significant Difference (LSD) multiple range test (alpha=.05). RESULTS: The mean values of tensile load required to dislodge acetal resin clasps with 1.2-mm thickness (111.6 g or 0.11 N) and with 2.0-mm thickness (178.4 g or 1.75 N) was significantly lower than that to dislodge Co-Cr clasps (694.1 g or 6.81 N) (P<.001). The retentive force needed to dislodge all 3 types of clasps was significantly lower for the molar than premolar models and also lower for the models with 0.25-mm undercuts than for those with 0.50-mm undercuts (P<.001). After 36 months of simulated clinical use, there was evidence of deformation in the cobalt-chromium clasps but no deformation noted for the acetal resin clasps. The retentive force of cobalt-chromium clasps (297.4 g or 2.91 N) after deformation remained significantly higher (P<.001) than the retentive force of acetal resin clasps that were 1.2 mm (110.7 g or 1.08 N) and 2.0 mm thick (177.5 g or 1.74 N), respectively. CONCLUSION: Within the limitations of this study, the results suggest that both thicknesses of acetal resin clasps evaluated required less force for insertion and removal than Co-Cr clasps over a simulated 36-month period.

Analysis of Variance↗
PubMed2006

Selective serotonin reuptake inhibitor (fluoxetine) decreases the effects of ghrelin on memory retention and food intake.

Ghrelin (Ghr) is an appetite stimulating hormone that is produced peripherally, by the stomach, and centrally as well. Previous investigations show that Ghr increases food intake and memory retention in rats, and that extra-hypothalamic structures, such as the hippocampus, participate in these effects. In the present work we analyzed the effect on food intake and memory retention induced by Ghr after serotonin (5-HT) availability modification at the serotoninergic synapses. Animals only treated with a selective serotonin reuptake inhibitor (SSRI), fluoxetine (FLU) 5 mg/kg or clomipramine (CLO) 2.5 and 5 mg/kg, showed a significant reduction in both food intake and memory retention. On the contrary, Ghr administration induces a significant increase in food intake and a dose-dependent increase in short and long term memory retention. When the animals were treated with FLU prior to Ghr injection, the food intake induced, as well as the expression of short and long term memory retention, was decreased. In conclusion, evidence presented in this paper suggests that the effects of Ghr on both feeding and memory retention in extra-hypothalamic structures such as the hippocampus, could depend on the availability of 5-HT.

Animals↗
PubMed2002

Sputum retention after lung operation: prospective, randomized trial shows superiority of prophylactic minitracheostomy in high-risk patients.

BACKGROUND: Sputum retention after lung operation is a potentially life-threatening condition. The minitracheostomy (Minitrach II, SIMS Portex, Hythe, Kent, UK) is a 4-mm percutaneous cricothyroidotomy device, which allows immediate and repeated aspiration of the tracheobronchial tree by minimally trained staff, and can effectively treat sputum retention. This trial was designed to test the hypothesis that prophylactic minitracheostomy could prevent sputum retention in a high-risk group. METHODS: Between March 1997 and October 1999, 102 patients undergoing lung procedures and considered to be at high risk were prospectively randomized to postoperative, prophylactic minitracheostomy insertion in the recovery room with regular aspiration, or to standard postoperative respiratory therapy. RESULTS: Sputum retention developed in 15 patients (30%) in the standard group (n = 52) compared to 1 patient (2%) in the minitracheostomy group (n = 50) (p < 0.005). There were three deaths related to sputum retention in the standard group compared to none in minitracheostomy group during the perioperative period. CONCLUSIONS: It is possible to identify a group of patients at high risk for sputum retention who will benefit from prophylactic therapy. Minitracheostomy is effective as prophylaxis and treatment.

Adult↗
PubMed1999

Factors influencing uptake and retention of amino-containing drugs in large unilamellar vesicles exhibiting transmembrane pH gradients.

The level of uptake and retention of amino-containing drugs in large unilamellar vesicles (LUVs) following uptake in response to a transmembrane pH gradient (DeltapH) can vary dramatically depending on the drug. For example, the anticancer drugs doxorubicin and epirubicin can be readily retained, whereas the anticancer drug vincristine and the antibiotic ciprofloxacin tend to leak out rapidly. In this investigation, we examine the influence of the hydrophobicity of the entrapped amines (that induce the DeltapH) and the anionic lipid content of the LUV on drug retention. It is shown that entrapment of increasingly hydrophobic monoamines (methylamine to amylamine) all lead to an induced DeltapH of 3 units and essentially complete drug uptake under the conditions employed, but do not lead to improved retention of vincristine and ciprofloxacin. However, significantly improved retention could be achieved by substitution of the anionic lipid distearoylphosphatidylglycerol (DSPG) for distearoylphosphatidylcholine (DSPC) in the LUV bilayer. Further, it is shown that if the induced DeltapH is reduced to 1.4 units (driven by entrapped diamine) nearly 100% accumulation of doxorubicin and epirubicin could be achieved, whereas only 25% loading for vincristine and ciprofloxacin was observed. Taken together these results provide methodology for improving (weak base) drug retention in liposomes and indicate that drugs that can partition into the lipid bilayer exhibit improved uptake and retention characteristics.

Amines↗
PubMed1986

Effects of lead in the laboratory mouse. 1. Influence of pregnancy upon absorption, retention, and tissue distribution of radiolabeled lead.

Whole-body retention and excretion following a single oral dose of radiolabeled lead (203Pb) in chronically lead-exposed pregnant BK:W mice were examined over 10-13 days. This was compared with values in similarly treated nonpregnant females and in ip injected females. Whole-body and bone retention were greater in injected than in orally dosed nonpregnant females; gastrointestinal absorption was estimated from this difference. Whole-body retention in the pups was measured at birth and at the end of the experiment. Whole-body retentions in pregnant females and in their pups at birth were significantly raised after treatment at Gestational Day 17, but not after treatment at Gestational Day 11, 14, 20, 23, or 26. Pup body burdens at birth were significantly and positively correlated with maternal retention. Measurement of radioactivity in bone, kidney, brain, heart, and liver of adults at the end of the experiments showed significantly increased levels in bone and kidney from females treated at Gestational Day 17 and in bone from females treated at Gestational Day 11. Lead excretion did not differ significantly among the groups but the total lead content of the femur was significantly raised in females treated at Gestational Days 17 and 26. Thus, these experiments provide evidence that lead absorption and retention in mice are markedly increased in the latter stages of pregnancy.

Animals↗
PubMed2004

Frequency and clinical outcome of capsule retention during capsule endoscopy for GI bleeding of obscure origin.

BACKGROUND: Capsule endoscopy is now commonly performed for GI bleeding of obscure origin. Regional transit abnormality refers to slowed capsule movement during capsule endoscopy. The frequency and clinical outcome of capsule retention and regional transit abnormalities are unknown. METHODS: Initial capsule endoscopies performed at a single institution in 52 patients with GI bleeding of obscure origin were reviewed retrospectively. For patients with capsule retention or regional transit abnormality, preprocedural characteristics, and post-procedural outcomes were recorded. OBSERVATIONS: Capsule retention occurred in 7 patients, all of whom remained asymptomatic. Regional transit abnormality was noted in 3 patients. Sources of bleeding were localized in all cases. Seven patients underwent surgery. Stricture induced by non-steroidal anti-inflammatory drugs was the major cause of retention. In all patients, anemia resolved during follow-up. CONCLUSIONS: Capsule retention and regional transit abnormality occurred in almost 20% of patients who had capsule endoscopy for GI bleeding of obscure origin. These capsule movement abnormalities led to the diagnosis of bleeding sources and thereby influenced patient management. A history of non-steroidal anti-inflammatory drugs use may be associated with an increased risk of capsule retention.

Aged↗