Everything below concerns retention time. We keep the language plain, cite what the science says, and separate well-supported claims from open questions.
Last reviewed on 2025-12-27. Where a claim depends on a specific study, the study is described rather than over-claimed.
Validation establishes that a method is suitable for its intended purpose. Typical parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantification, robustness, and stability of standards and samples. Acceptance criteria are defined in advance, and results are documented in a validation report. Regulatory guidance for pharmaceuticals, foods, and environmental testing differs, so the applicable framework must be identified. Ongoing verification uses control samples and trend charts after validation. Method transfer to another laboratory may require partial revalidation.
Routine quality control includes blanks, duplicates, spiked samples, and certified reference materials. Calibration curves are prepared with standards at several concentrations, and the detector response is checked for linearity. Carryover, column aging, mobile phase evaporation, and temperature drift can shift retention times or peak areas. Maintenance such as replacing seals, filters, and columns helps prevent failures. Records of injections, integration, and deviations support traceability. Audits may request raw data and instrument logs for each batch.
Developing an HPLC test begins with defining the analytes, matrix, and required reporting limits. Chemists select a separation mode, column chemistry, mobile phase composition, flow rate, and detection wavelength or mass transition. Experiments then adjust these variables to achieve adequate retention, resolution, and peak shape. System suitability tests confirm that the instrument and method perform consistently before sample analysis. Without suitable resolution, quantitative results may be unreliable. Preliminary runs often use scouting gradients to locate retention windows.
High-performance liquid chromatography is an analytical technique that separates components in a liquid sample by passing them through a packed column under pressure. A pump delivers a mobile phase at a controlled flow rate, and an injector introduces the sample into the stream. Differences in how analytes partition between the mobile phase and the stationary phase cause them to exit the column at different times. Detection then records a signal proportional to the amount of each separated substance. The resulting chromatogram provides retention times and peak areas for identification and quantification.
Instrumentation includes a solvent delivery system, an autosampler, a column oven, and one or more detectors. Reversed-phase columns with chemically modified silica are widely used, but normal-phase, ion-exchange, size-exclusion, and affinity modes exist for specific separations. Detectors may rely on ultraviolet absorbance, fluorescence, refractive index, or mass spectrometry. Column temperature, mobile phase composition, and flow rate are adjusted to improve resolution. System pressure is monitored because rising pressure can indicate column blockage or deteriorating packing.
Separation performance depends on particle size, pore size, column length, and the chemistry of the stationary phase. Smaller particles generally improve efficiency but require higher pressure and suitable instrumentation. The mobile phase often contains buffers and organic solvents that influence retention and selectivity. Testing labs select conditions based on the analytes, sample matrix, and required sensitivity. Method development frequently involves screening several columns and solvent mixtures before a final set of conditions is chosen.
| Property | Value | Notes |
|---|---|---|
| Accuracy | Closeness to true value | Often assessed by recovery of spiked samples |
| Precision | Agreement among repeated measurements | Often reported as relative standard deviation |
| Specificity | Ability to measure analyte without interference | Must separate analyte from impurities and matrix |
| Linearity | Proportional detector response | Evaluated across a defined concentration range |
| Robustness | Resistance to small method changes | Tests flow rate, pH, temperature, and mobile phase composition |
System suitability testing is performed before and during analytical runs to confirm that the instrument and method are working as expected. Typical checks include retention time, peak area precision, resolution between critical pairs, tailing factor, and theoretical plate count. Acceptance criteria are set in the method or pharmacopeial monograph. If a suitability check fails, the run may be rejected and the instrument or sample preparation may need investigation. This practice helps prevent release of data from a system that has drifted out of control.
Quality control samples are inserted at intervals to monitor accuracy and precision throughout a batch. Blank samples detect contamination, while spiked samples assess recovery from the sample matrix. Calibration standards establish the relationship between detector response and concentration, and control samples are prepared independently from them whenever possible. Laboratories also participate in proficiency testing and maintain audit trails, instrument logs, and reagent records. Ongoing review of control charts can reveal trends before they cause out-of-specification results.
Method validation demonstrates that an HPLC procedure is suitable for its intended purpose. Common validation parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantification, and robustness. Accuracy reflects agreement with a reference value, while precision describes repeatability under defined conditions. Specificity shows whether the method can measure the analyte in the presence of impurities or matrix components. Validation documents are reviewed before a method is used for routine testing or regulatory submissions.
High-performance liquid chromatography, or HPLC, separates dissolved compounds by passing a liquid mobile phase through a packed column. Components distribute differently between the stationary phase and the moving liquid, so they travel at different speeds and exit at different times. A detector records these eluting bands as peaks, and peak area or height relates to amount. The technique supports testing in pharmaceuticals, foods, environmental samples, and industrial chemicals. Quantification usually depends on calibration with known standards.
Several separation modes exist, including reversed-phase, normal-phase, ion-exchange, size-exclusion, and hydrophilic interaction liquid chromatography. Reversed-phase uses a nonpolar stationary phase with a polar mobile phase and is widely applied to small organic molecules. Gradient elution changes mobile phase composition during the run, while isocratic elution keeps it constant. Column chemistry, particle size, temperature, flow rate, and mobile phase pH all influence retention and resolution. Method development selects conditions that separate analytes from matrix components and from each other.
Detection commonly uses ultraviolet-visible absorbance, fluorescence, refractive index, or mass spectrometry. Ultraviolet detection depends on molecular chromophores that absorb light at specific wavelengths. Mass spectrometry provides mass information and sensitive quantification, often after electrospray ionization. Before sample batches, performance checks examine resolution, elution time repeatability, peak symmetry, and plate count. Matrix effects and co-elution remain recognized uncertainties; formal validation studies and orthogonal detection help address them. Detector choice depends on analyte properties and required sensitivity.
Quality control for HPLC testing combines scheduled checks, documented procedures, and review of results. Before sample analysis, system suitability testing confirms that the instrument, column, and method meet predefined criteria. Common criteria include resolution between critical peaks, retention time precision, peak tailing, and theoretical plate count. Failure triggers investigation before results are reported. Records link raw data, calculations, instrument logs, and analyst identity to each batch, supporting audits and repeat analysis.
Method validation evaluates accuracy, precision, specificity, linearity, range, detection limit, quantitation limit, and robustness. Regulatory guidance for pharmaceuticals, foods, and environmental testing defines expected documentation and acceptance criteria. Verification confirms that a validated method works in a specific laboratory with its own instruments and reagents. Calibration curves use reference standards with known purity and traceability, while measurement uncertainty is estimated from validation data, control charts, and collaborative studies. The scope of validation depends on the method's intended use.
Routine quality control monitors retention time shifts, baseline noise, system pressure, and peak shape. Trends can reveal column aging, mobile phase preparation errors, detector drift, or sample degradation. Corrective actions may include replacing the column, preparing fresh mobile phase, or recalibrating the detector. Stability testing often uses HPLC to measure parent compound loss and degradation product formation. Open questions remain about how accelerated stability results extrapolate to long-term storage under varied conditions.
Sprigg urged Letsie I to negotiate Masopha's unconditional surrender, until a force of Cape Mounted Riflemen (CMR) could arrive to assist him. Letsie I replied that this was unrealistic, as most of the Basuto, including his sons, had rallied behind Masopha. Letsie and his armed retinue, returned to their village on 19 August after several days of negotiations, fearing that they would be ambushed if they remained outside Thaba Bosiu any longer. In a last ditch effort to prevent an uprising, Sprigg visited Letsie I in person. Letsie I then held another pitso—under Sprigg's new terms the rebel chiefs were to appear in court where they would receive a token fine, pledge to compensate those whose property they had seized, and comply with the gun regulations. Masopha remained defiant and held an assembly of his own, where he and Lerotholi began to prepare for war. Masopha believed that the Cape's troops had proved themselves to be incompetent during the suppression of Moorosi's revolt. He was further encouraged by rumors that the British would refuse to reinforce the Cape, and by the British defeat at the Battle of Isandlwana a year prior.
reverse genetics An experimental approach in molecular genetics in which a researcher starts with a known gene and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's DNA sequence or by repressing or silencing its expression and then screening the mutant organisms for changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of forward genetics, in which a known phenotype is linked to one or more unknown genes.
=== Engineering === MHD is related to engineering problems such as plasma confinement, liquid-metal cooling in nuclear reactors, and electromagnetic casting. A magnetohydrodynamic drive, or MHD propulsor, is a method for propelling seagoing vessels using only electric and magnetic fields, with no moving parts. The working principle involves electrifying the propellant (gas or water), which can then be directed by a magnetic field, thereby pushing the vehicle in the opposite direction. Although some working prototypes exist, MHD drives remain impractical. The first prototype of this type of propulsion was built and tested in 1965 by Steward Way, a professor of mechanical engineering at the University of California, Santa Barbara. While on leave from his job at Westinghouse Electric, Way assigned his senior-year undergraduate students to develop a submarine using this new propulsion system. In the early 1990s, the Ship & Ocean Foundation (Minato-ku, Tokyo) built an experimental boat, the Yamato-1, which used a magnetohydrodynamic drive incorporating a superconductor cooled by liquid helium and could travel at 15 km/h. MHD power generation fueled by potassium-seeded coal-combustion gas showed potential for more efficient energy conversion (the absence of solid moving parts allows operation at higher temperatures) but ultimately failed due to cost-prohibitive technical difficulties. One major engineering problem was the abrasion-induced failure of the wall of the primary-coal combustion chamber.
Sources: en.wikipedia.org
and associated with familial thrombophilia. In 1994, the relatively common genetic mutation that produces Factor VLeiden was noted (Bertina et al.). Two years later, Gla-domainless APC was imaged at a resolution of 2.8 Ångströms. Beginning with the PROWESS clinical trial of 2001, it was recognised that many of the symptoms of sepsis may be ameliorated by infusion of APC, and mortality rates of septic patients may be significantly decreased. Near the end of that year, Drotrecogin alfa (activated), a recombinant human activated protein C, became the first drug approved by the U.S. FDA for treating severe sepsis. In 2002, Science published an article that first showed protein C activates protease-activated receptor-1 (PAR-1) and this process accounts for the protein's modulation of the immune system.
This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate do complicate matters. The surface effects of nanoparticles also reduces the incipient melting temperature.
Evans' successor Lawrence Quincy Mumford took over in 1953. During his tenure, lasting until 1974, Mumford directed the initiation of construction of the James Madison Memorial Building, the third Library of Congress building on Capitol Hill. Mumford led the library during the government's increased educational spending. The library was able to establish new acquisition centers abroad, including in Cairo and New Delhi. In 1967, the library began experimenting with book preservation techniques through a Preservation Office. This has developed as the most extensive library research and conservation effort in the United States. During Mumford's administration, the last significant public debate occurred about the Library of Congress's role as both a legislative and national library. Asked by Joint Library Committee chairman Senator Claiborne Pell (D-RI) to assess operations and make recommendations, Douglas Bryant of Harvard University Library proposed several institutional reforms. These included expanding national activities and services and various organizational changes, all of which would emphasize the library's federal role rather than its legislative role. Bryant suggested changing the name of the Library of Congress, a recommendation rebuked by Mumford as "unspeakable violence to tradition." The debate continued within the library community for some time.
=== Adverse events === Acupuncture is generally safe when administered by an experienced, appropriately trained practitioner using clean-needle technique and sterile single-use needles. When improperly delivered it can cause adverse effects. Accidents and infections are associated with infractions of sterile technique or neglect on the part of the practitioner. To reduce the risk of serious adverse events after acupuncture, acupuncturists should be trained sufficiently. A 2009 overview of Cochrane reviews found acupuncture is not effective for a wide range of conditions. People with serious spinal disease, such as cancer or infection, are not good candidates for acupuncture. Contraindications to acupuncture (conditions that should not be treated with acupuncture) include coagulopathy disorders (e.g. hemophilia and advanced liver disease), warfarin use, severe psychiatric disorders (e.g. psychosis), and skin infections or skin trauma (e.g. burns). Further, electroacupuncture should be avoided at the spot of implanted electrical devices (such as pacemakers). A 2011 systematic review of systematic reviews (internationally and without language restrictions) found that serious complications following acupuncture continue to be reported. Between 2000 and 2009, ninety-five cases of serious adverse events, including five deaths, were reported. Many such events are not inherent to acupuncture but are due to malpractice of acupuncturists. This might be why such complications have not been reported in surveys of adequately trained acupuncturists.
Sources: en.wikipedia.org
In most single-tablet, over-the-counter, daily vitamin and mineral supplements, zinc is included in such forms as zinc oxide, zinc acetate, zinc gluconate, or zinc amino acid chelate. Generally, zinc supplement is recommended where there is high risk of zinc deficiency (such as low and middle income countries) as a preventive measure. Although zinc sulfate is a commonly used zinc form, zinc citrate, gluconate and picolinate may be valid options as well. These forms are better absorbed than zinc oxide.
Philosopher Eric Voegelin stated that Marxism–Leninism is inherently oppressive, writing that the "Marxian vision dictated the Stalinist outcome not because the communist utopia was inevitable but because it was impossible." Criticism like this has itself been criticised for philosophical determinism, i.e. that the negative events in the movement's history were predetermined by their convictions, with historian Robert Vincent Daniels stating that Marxism was used to "justify Stalinism, but it was no longer allowed to serve either as a policy directive or an explanation of reality" during Stalin's rule. In contrast, E. Van Ree wrote that Stalin considered himself to be in "general agreement" with the classical works of Marxism until his death. Graeme Gill stated that Stalinism was "not a natural flow-on of earlier developments; [it was a] sharp break resulting from conscious decisions by leading political actors." Gill added that "difficulties with the use of the term reflect problems with the concept of Stalinism itself. The major difficulty is a lack of agreement about what should constitute Stalinism." Historians such as Michael Geyer and Sheila Fitzpatrick criticised the focus upon the upper levels of society and the use of Cold War concepts, such as totalitarianism, which have obscured the reality of Marxist–Leninist systems, such as that of the Soviet Union. Mervyn Matthews criticized Marxism–Leninism for failing to solve poverty, noting that a large number of people in the Soviet Union were still in poverty despite its planned economy.
Resonance Raman spectroscopy shows that O2 is bound in a symmetric environment (ν(O-O) is not IR-allowed). OxyHc is EPR-silent indicating the absence of unpaired electrons Raman spectroscopy shows ν(O-O) of 755 cm−1 Much work has been devoted to preparing synthetic analogues of the active site of hemocyanin. One such model, which features a pair of copper centers bridged side-on by peroxo ligand, shows ν(O-O) at 741 cm−1 and a UV-Vis spectrum with absorbances at 349 and 551 nm. Both of these measurements agree with the experimental observations for oxyHc. The Cu-Cu separation in the model complex is 3.56 Å, that of oxyhemocyanin is ca. 3.6 Å (deoxyHc: ca. 4.6 Å).
Sources: en.wikipedia.org
System suitability is a set of checks performed before and during a run to confirm that the instrument, column, and method work as expected. Common checks include resolution, tailing factor, theoretical plates, and relative standard deviation of replicate injections. Failure triggers troubleshooting or method adjustment.
Validation demonstrates that a method produces reliable results for a defined purpose. It documents performance limits and acceptance criteria. Regulated industries require validation before routine testing of products or samples.
Retention time shifts can arise from changes in mobile phase composition, pH, temperature, column age, or flow rate. Contamination or worn seals may also alter pressure and delivery. Systematic checks of these factors help identify the cause.
HPLC separates and detects individual compounds in a liquid sample, producing peaks at characteristic retention times. Peak area or height can be used to estimate concentration when calibrated with known standards. It does not identify unknown compounds with certainty unless additional detectors or reference materials are used.