retention time is one of those subjects where the details matter more than the headlines. This page pulls together the background, the mechanisms, and the practical points readers ask about most.
Last reviewed on 2026-04-02. Where a claim depends on a specific study, the study is described rather than over-claimed.
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.
Most routine HPLC testing uses reversed-phase columns, where the stationary phase is nonpolar and the mobile phase is a polar mixture such as water with an organic solvent. Analytes partition between the two phases according to polarity, size, and charge. Gradients that change solvent composition over time can separate compounds with broad retention ranges. Isocratic conditions keep solvent composition constant and suit simpler mixtures. The choice of column chemistry, pH, and temperature affects selectivity and peak shape.
Detection in HPLC testing commonly relies on ultraviolet-visible absorbance, fluorescence, refractive index, or mass spectrometry. UV detection is widely used because many organic compounds absorb light, but it requires a chromophore. Mass spectrometry provides mass-based identification and high sensitivity for trace analytes. Each detector has trade-offs in selectivity, cost, and compatibility with mobile phases. Quantification typically uses calibration curves prepared from reference standards. Results are reported as concentration, purity, or presence above a limit.
| Property | Value | Notes |
|---|---|---|
| Separation principle | Differential partitioning | Analytes distribute between mobile and stationary phases. |
| Mobile phase | Liquid solvent mixture | Composition controls retention and selectivity. |
| Stationary phase | Packed column particles | Often chemically bonded silica. |
| Typical detector | UV-Vis or photodiode array | Mass spectrometry is also common. |
| Common synonym | High-performance liquid chromatography | Abbreviated as HPLC. |
High-performance liquid chromatography is an analytical technique that separates components of a liquid sample by passing it 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 interaction with the stationary phase cause analytes to migrate at different rates. Detectors record elution as peaks, and a data system converts signals into a chromatogram. The method is suited to compounds that dissolve in a liquid and are not volatile enough for gas chromatography.
Reversed-phase chromatography dominates modern HPLC testing, using a nonpolar stationary phase such as chemically bonded octadecyl groups and a polar mobile phase of water mixed with organic solvent. Analytes partition between the mobile and stationary phases according to hydrophobicity. Gradient elution changes the mobile phase composition over time to separate compounds with a wide range of retention. Isocratic elution keeps the composition constant and is simpler for routine assays. Column temperature, pH, and flow rate influence selectivity, peak shape, and retention time, so these parameters are controlled during a validated method.
Detection in HPLC testing commonly relies on ultraviolet-visible absorbance, fluorescence, refractive index, or mass spectrometry. A diode array detector records full spectra across a wavelength range, which helps identify co-eluting peaks. Mass spectrometry provides mass-to-charge ratios and can confirm molecular identity at low concentrations. The choice of detector depends on analyte structure, required sensitivity, and whether quantitation or identification is the goal. No single detector works for every compound, and method development often compares responses before selecting one.
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.
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.
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.
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.
== Function == Cytosolic and membrane-bound forms of glutathione S-transferase are encoded by two distinct supergene families. At present, eight distinct classes of the soluble cytoplasmic mammalian glutathione S-transferases have been identified: alpha, kappa, mu, omega, pi, sigma, theta and zeta. This gene encodes a cytoplasmic glutathione S-transferase that belongs to the mu class. The mu class of enzymes functions in the detoxification of electrophilic compounds, including carcinogens, therapeutic drugs, environmental toxins, and products of oxidative stress, by conjugation with glutathione. The genes encoding the mu class of enzymes are organized in a gene cluster on chromosome 1p13.3, and are known to be highly polymorphic. These genetic variations can change an individual's susceptibility to carcinogens and toxins, as well as affect the toxicity and efficacy of certain drugs. Null mutations of this class mu gene have been linked with an increase in a number of cancers, likely due to an increased susceptibility to environmental toxins and carcinogens. Multiple protein isoforms are encoded by transcript variants of this gene.
==== MeSH E05.300.120 – administration, topical ==== MeSH E05.300.120.040 – administration, buccal MeSH E05.300.120.060 – administration, cutaneous MeSH E05.300.120.080 – administration, intranasal MeSH E05.300.120.500 – administration, intravaginal MeSH E05.300.120.505 – administration, intravesical MeSH E05.300.120.610 – administration, rectal
===== Mass Spectrometry ===== Mass spectrometry methods are unable to determine the folding of nascent polypeptides. No current methods examine the folding states of nascent polypeptides globally in the cell. There are methods for examining the folding state of individual nascent polypeptides. One method uses a nonspecific protease to cleave the nascent peptide at a low temperature. The protease can cleave the unfolded, flexible regions, but cannot cut tightly folded regions. The products of the cleavage can then be separated and studied to determine the folded regions of the nascent peptide.
The American Way supermercato the year before and Supermarket USA were among the first of several instances in which American supermarkets were deployed internationally at the height of the Cold War as shock and awe instruments of American propaganda to demonstrate the supposed superiority of Western Bloc capitalism over Eastern Bloc communism. Before then, a few countries had already begun to implement supermarkets due to their proximity to or affinity for the United States. Canada, to the north, had implemented the new retail format at the same time during the 1930s. For example, Québec's first supermarket opened in 1934 in Montréal, under the banner Steinberg's. In Canada, the largest supermarket chain is Loblaw, which operates stores under a variety of banners targeted to different segments and regions, including Fortinos, Zehrs, No Frills, the Real Canadian Superstore, and Loblaws, the foundation of the company. Sobeys is Canada's second largest supermarket with locations across the country, operating under many banners (Sobeys IGA in Quebec). In the United Kingdom, self-service shopping took longer to become established, despite its Special Relationship with the United States. In 1947, there were just ten self-service shops in the country. In 1951, ex-US Navy sailor Patrick Galvani, son-in-law of Express Dairies chairman, made a pitch to the board to open a chain of supermarkets across the country.
Sources: en.wikipedia.org
As a result of protests by Akwesasne residents on their rights to cross the border unimpeded, as provided under the 1795 Jay Treaty, the Canada Border Services Agency closed its post on Cornwall Island, instead requiring travellers to proceed to the checkpoint in the city of Cornwall. As a consequence of the arrangement, residents of the island are required to clear border controls when proceeding North to the Ontario mainland, as well as when proceeding South to Akwesasne territory in New York, thereby constituting internal controls from both a Canadian and an Akwesasne perspective. Similarly, travelling between Canada and the Quebec portion of the Akwesasne nation requires driving through the state of New York, meaning individuals must clear American controls when leaving Quebec proper and Canadian border controls when entering Quebec proper. However, Canada does not impose border controls when entering the Quebec portion of the Akwesasne Nation. Nevertheless, for residents who assert a Haudenosaunee national identity distinct from Canadian or American citizenship, the intricate network of Canadian and American border controls is seen as a foreign-imposed system of internal border controls, similar to the Israeli checkpoints in Palestinian territory. The city of Hyder, Alaska, has also been subject to internal border controls since America chose to stop regulating arrivals in Hyder from British Columbia.
Petri dish A shallow, transparent plastic or glass dish, usually circular and covered with a lid, which is widely used in biology laboratories to hold solid or liquid growth media for the purpose of culturing cells. They are particularly useful for adherent cultures, where they provide a flat, sterile surface conducive to colony formation from which scientists can easily isolate and identify individual colonies.
=== Medicinal === The toxicity is useful when applied at ultra-low concentrations, such in antiseptics like thiomersal and merbromin, and fungicides such as ethylmercury chloride and phenylmercury acetate.
Sources: en.wikipedia.org
==== Research ==== In 2005, University of Miami researchers, in cooperation with the attorney representing death-row inmates from Virginia, published a research letter in the medical journal The Lancet. The article presented protocol information from Texas, Virginia, and North and South Carolina, which showed that executioners had no anesthesia training, drugs were administered remotely with no monitoring for anesthesia, data were not recorded, and no peer review was done. Their analysis of toxicology reports from Arizona, Georgia, and North and South Carolina showed that postmortem concentrations of thiopental in the blood were lower than that required for surgery in 43 of 49 executed inmates (88%), and that 21 (43%) inmates had concentrations consistent with awareness. This led the authors to conclude that a substantial probability existed that some of the inmates were aware and suffered extreme pain and distress during execution. The authors attributed the risk of unconsciousness among inmates to the lack of training and monitoring in the process but carefully made no recommendations on how to alter the protocol or how to improve the process. Indeed, the authors conclude, "because participation of doctors in protocol design or execution is ethically prohibited, adequate anesthesia cannot be certain. Therefore, to prevent unnecessary cruelty and suffering, cessation and public review of lethal injections is warranted." Paid expert consultants on both sides of the lethal-injection debate have found opportunity to criticize the 2005 Lancet article.
and hit .409 with 11 home runs, 20 walks and 1.384 OPS and collected his second consecutive Player of the Month award. He would hit another three home runs with six walks in nine August games before a pitch from Andy Benes fractured his left hand on August 10 and ended his season in the same manner the season before. Entering that game, Bagwell carried an 18-game hitting streak. Two days later, the strike ended the season for all the major leagues. His production accelerated in 26 games after the All-Star break, as he posted a .432 batting average, .530 OBP, .916 SLG, 1.446 OPS, 10 doubles, 12 home runs, 34 RBI and 30 runs scored. The Astros finished one-half game out of first place in the inaugural season of the National League Central division, a product of MLB's division realignment. Bagwell played in 110 games in the 1994 season, batting .368 with a .750 SLG, 1.201 OPS, 39 home runs, 116 RBI, 104 runs scored, 300 total bases and 213 adjusted OPS (OPS+) in 400 at bats. He led the major leagues in SLG, OPS+, RBI, and total bases, and the NL in runs scored and OPS, but fell short of winning the batting Triple Crown, finishing second for the batting title to Tony Gwynn, who, after batting .394, had the highest average in the major leagues since Ted Williams in 1941. Bagwell finished second in home runs to Matt Williams, who hit 43.
Mephedrone administration leads to increased extracellular dopamine in the nucleus accumbens, elevated serotonin levels in the frontal cortex, and alterations in brain temperature. These pharmacodynamic actions translate into various behavioral effects, including increased locomotor activity, rewarding effects (as measured by conditioned place preference), and potential for drug discrimination (similar to cocaine and MDMA).
Protactinium (91Pa) has no stable isotopes. As 231Pa occurs in usable quantity, and comprises virtually all of the element, it defines the standard atomic weight. Thirty radioisotopes of protactinium have been characterized, ranging from 210Pa to 239Pa. The most stable isotopes are 231Pa with a half-life of 32,700 years, 233Pa with a half-life of 26.975 days, and 230Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. This element also has five meta states, 217mPa (t1/2 1.15 milliseconds), 220m1Pa (t1/2 = 308 nanoseconds), 220m2Pa (t1/2 = 69 nanoseconds), 229mPa (t1/2 = 420 nanoseconds), and 234mPa (t1/2 = 1.16 minutes). The only naturally occurring isotopes are 231Pa, 233Pa, 234Pa, and 234mPa. The first occurs as an intermediate decay product of 235U, the second of (rare) 237Np, and the last two as intermediate decay products of 238U. 231Pa dominates solely because of its longer life. The primary decay mode for protactinium isotopes lighter than (and including) the most stable isotope 231Pa is alpha decay to isotopes of actinium, except 228Pa to 230Pa, which primarily decay by electron capture to isotopes of thorium. The primary mode for the heavier isotopes is beta minus (β−) decay to isotopes of uranium.
Sources: en.wikipedia.org
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.
Pressure drives the liquid mobile phase through a column packed with small particles. Without pressure, flow would be very slow or stop because the packed bed resists liquid movement. Modern pumps maintain a steady flow despite the resistance.
A chromatogram is a plot of detector signal against time after sample injection. Each peak represents a compound or group of compounds eluting from the column. Retention time and peak area are the main measurements read from the plot.
It separates components in a liquid sample and measures their amounts using a detector. Results can indicate concentration, purity, or identity based on retention time and detector response. The technique works for mixtures that can be dissolved and filtered.