retention time comes up often in conversation and rarely with the context attached. Here we lay out the basics in order, then work through the practical considerations.
Last reviewed on 2025-09-19. 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.
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.
| 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. |
Quality control laboratories use HPLC to check identity, purity, concentration, and stability of raw materials and finished products. A validated method specifies the column, mobile phase, flow rate, detection wavelength, injection volume, and run time. Samples are prepared and compared against reference standards of known concentration. The resulting chromatogram provides quantitative data, such as assay values and impurity levels. This approach is common in pharmaceutical, food, environmental, and industrial testing where consistent measurements are required.
Method validation demonstrates that an analytical procedure is suitable for its intended purpose. Typical validation characteristics include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, and robustness. Regulatory guidance from bodies such as the International Council for Harmonisation and the United States Pharmacopeia outlines expectations, though specific criteria depend on the product and method. System suitability tests are run before sample analysis to confirm resolution, peak symmetry, column efficiency, and injection repeatability. Failure of these checks can invalidate a batch of measurements.
Practical HPLC testing depends on careful sample preparation and instrument maintenance. Samples may require filtration, dilution, pH adjustment, or extraction to avoid column damage and matrix interference. Mobile phases are degassed and filtered, and columns are equilibrated before injection. Common problems include peak tailing, baseline drift, ghost peaks, carryover, and co-elution of analytes. Documentation of instrument logs, calibration records, and electronic audit trails supports data integrity and traceability. Ongoing training and routine maintenance help reduce variability between analysts and laboratories.
Separation in HPLC depends on the chemistry of the stationary phase, the composition of the mobile phase, and the physical properties of the column. Reverse-phase separations use a nonpolar stationary phase and a polar mobile phase, and they are common for many organic compounds. Ion-exchange, size-exclusion, and normal-phase modes serve other classes of analytes. Gradient elution changes solvent strength over time, while isocratic elution holds it constant. Flow rate, temperature, particle size, and column length all influence peak shape and resolution. Detection may use ultraviolet absorbance, fluorescence, refractive index, or mass spectrometry, depending on the analyte and the required sensitivity.
Routine HPLC testing compares a sample result with a calibration curve prepared from known reference standards. Peak area or peak height is plotted against concentration, and the curve is used to estimate unknown amounts. Retention time supports tentative identification when compared with a standard, though mass spectrometry or another confirmatory method may be needed for definitive identification. Pre-run checks verify repeatability, resolution, and peak symmetry before sample analysis. Limits of detection and quantification describe the smallest amounts that can be reliably observed or measured. Sample preparation, filtration, and degassing help prevent column damage and inconsistent results.
High-performance liquid chromatography is an analytical technique that separates components in a liquid sample. A pump moves a liquid mobile phase through a column packed with a solid stationary phase. Compounds interact differently with both phases and travel at different rates, leaving the column at distinct retention times. A detector records these arrivals as peaks on a chromatogram. The resulting pattern supports identification and quantification of substances in mixtures. Modern instruments use high pressure to force solvent through small particles, which improves speed and resolution compared with older low-pressure liquid chromatography methods.
Following this report, American neurologist Silas Weir Mitchell tried peyote and published his experience with it in December 1896. After reading Mitchell's article, others, including psychologist and sexologist Havelock Ellis and American psychologist William James tried peyote and described their experiences. The German chemist Arthur Heffter isolated and ingested mescaline from peyote, experiencing psychedelic effects with the pure compound, in 1897, and published his findings in 1898. Austrian chemist Ernst Späth synthesized mescaline for the first time in 1919. The German pharmaceutical company Merck then began distributing pharmaceutical mescaline in 1920. The German psychiatrist Kurt Beringer, a student of Lewin and an acquaintance of Hermann Hesse and Carl Jung, became the father of psychedelic psychiatry and conducted experiments with mescaline in more than 60 people starting in 1921. He published his monograph on the subject, Der Meskalinrausch (Mescaline Intoxication), in 1927. German–American psychologist Heinrich Klüver published his monograph, Mescal: The Divine Plant and Its Psychological Effects, in English in 1928. He is said to have been the first to attempt to provide a phenomenological description of the psychedelic experience. The French pharmacist Alexandre Rouhier also studied and published on peyote and mescaline with his book Le Peyotl: La Plante Qui Fait les Yeux Émerveillés (Peyote: The Plant That Fills the Eyes with Marvels) in 1927.
Ac-SDKP (Thymosin Beta 4 Fragment (1–4), Goralatide) is an endogenous tetrapeptide derivative that is the N-acetylated derivative of the fragment composed of amino acids 1-4 derived from cleavage of the N-terminus of the signalling factor thymosin beta-4 by the enzyme prolyl oligopeptidase. It is a selective inhibitor of haematopoietic cell proliferation and has antiinflammatory, anti-fibrotic, and pro-angiogenic properties, with particular applications in preventing the development of kidney fibrosis and cardiac fibrosis following injury to these organs.
=== Therapy === After his period of psychological transformation and his later discovery of alchemy, Jung saw analysis as more of a tool for personal growth than treatment for certain mental disorders. Whereas Freud mainly gleaned or tested his theories on a small group of upper-middle-class patients, mainly women suffering from (what was thought to be at the time) hysteria, Jung had seen patients from all walks of life and with a huge diversity of diagnoses.Jung believed psychosis (schizophrenia) and neurosis (hysteria) to be extreme expressions of the two basic attitudinal types. The psychotic patient's libido has withdrawn so far from external reality that they inhabit a private world of fantasy and archetypal imagery (the unconscious). The neurotic patient's libido has been directed so far away from internal reality that they become hugely preoccupied with their own influence on the world and social relationships (i.e., they live in their persona). In this sense, Jung saw all mental illness as forms of imbalance. He also saw mental illness as a creative act, in that it is a product of the individuation process whereby the psyche is continuing to grow and develop in abnormal psychic circumstances. Thus, the purpose of psychotherapy, for Jung, was individuation and to find a more balanced mode of existence. Influenced by Freud's psychoanalysis, Jung saw the analysis of dreams as essential to Jungian analysis. However, particularly after his separation from Freud, Jung's ideas surrounding dreams departed in significant ways from Freud's theory.
== Discovery and Research == The partial purification of 17 alpha- and 17 beta-estradiol dehydrogenase activities from chicken liver was reported by Renwick AG and Engel LL in 1967, shedding light on the enzyme's properties and functions. Subsequent research has further elucidated its physiological significance, with ongoing studies aiming to understand its regulation and potential therapeutic applications.
Sources: en.wikipedia.org
=== A balanced nervous system === In the balanced nervous system with required cognitive functions, the sympathetic (SNS) and parasympathetic nervous systems (PNS) operate in synergy while opposing each other. Stimulation of the SNS boosts body activity and attention: it raises heart rate and blood pressure. In contrast, stimulation of the PNS is the rest-and-digest state: it reduces blood pressure and heart rate. The nervous system interplays with the immune system. Through these interactions, the nervous and immune systems ensure the nervous system maintains immune homeostasis.
=== EC 1.5.98 With other, known, physiological acceptors === EC 1.5.98.1: ethylenetetrahydromethanopterin dehydrogenase EC 1.5.98.2: 5,10-methylenetetrahydromethanopterin reductase EC 1.5.98.3: coenzyme F420:methanophenazine dehydrogenase
=== Cementum and Periodontal Ligament === Surrounding the apical foramen is apical cementum, often cellular cementum embedded with cementocytes. Sharpey’s fibres from the periodontal ligament insert into this cementum, anchoring the tooth to alveolar bone.
Sources: en.wikipedia.org
Economists Alan Blinder and Mark Watson reported that budget deficits tended to be smaller under Democratic presidents, at 2.1% potential GDP versus 2.8% potential GDP for Republican presidents, a difference of about 0.7% GDP. Their study was from President Truman through President Obama's first term, which ended in January 2013.
OPD (o-phenylenediamine dihydrochloride) turns amber to detect HRP (horseradish peroxidase), which is often used to as a conjugated protein. TMB (3,3',5,5'-tetramethylbenzidine) turns blue when detecting HRP and turns yellow after the addition of sulfuric or phosphoric acid. ABTS (2,2'-Azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium salt) turns green when detecting HRP. PNPP (p-Nitrophenyl Phosphate, Disodium Salt) turns yellow when detecting alkaline phosphatase.
== Human use == Different parts of the plant are used as traditional remedies for disease and skin afflictions. Juice from the leaves is used to treat warts and snakebite, and the flower is applied to burns. This species has been used as indigenous traditional medicine in Asia for rheumatism, fractures, and other ailments. In Korean folk medicine, this impatiens species is used as a medicine called bongseonhwa dae (봉선화대) for the treatment of constipation and gastritis. Chinese people used the plant to treat those bitten by snakes or who ingested poisonous fish. Juice from the stalk, pulverised dried stalks, and pastes from the flowers were also used to treat a variety of ailments. Vietnamese wash their hair with an extract of the plant to stimulate hair growth. One in vitro study found extracts of this impatiens species, especially of the seed pod, to be active against antibiotic-resistant strains of Helicobacter pylori. It is also an inhibitor of 5α-reductases, enzymes that converts testosterone to dihydrotestosterone (active form of testosterone), thus reducing action of testosterone in our body. In Nepal, the balsam leaves are crushed to dye fingernails on the day of Shrawan Sakranti (Shrawan 1). The day is also observed as Luto Faalne Deen (Go Away-Itch Day). Similarly, in China and Korea, the flowers are crushed and mixed with alum to produce an orange dye that can be used to dye fingernails. Unlike common nail varnish, the dye is semi-permanent, requiring dyed nails to grow off over time in order to remove any traces of color.
Narrated by Eva Pope, produced by David Frank, directed by Paul Sapin; made by RDF Media and the Discovery Channel 15 September The Secret Life of the Dog, with David Paxton of the University of Papua New Guinea; Jonica Newby wrote The Pact for Survival; 75% of all dogs came from one female dog 100,000 years ago, from mitochondrial genetic evidence; British zoologist John Bradshaw; anthropologist Colin Groves of Australian National University; the domestication of dogs has been modelled by Russian zoologist Lyudmila Trut and experiments from 1952 on the domesticated silver fox, whereby only individual silver foxes that didn't bite humans could breed, at the Institute of Cytology and Genetics in Novosibirsk - by 1972, silver foxes from the silver fox Laska had become so suddenly similar to dogs, in their temperament, and would respond to their name being called; Russian zoologist Larissa Kolesnikova.
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.
System suitability is typically performed before each batch or according to the validated method and laboratory procedure. Some long runs include periodic checks during analysis. The required frequency depends on regulatory expectations and method performance.