retention time raises a handful of sensible questions. This page answers them in order, starting with the fundamentals and moving to applications.
Reviewed 2026-01-28. Anything still debated is marked as such rather than presented as settled.
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.
Method validation establishes that an HPLC procedure is suitable for its intended use. Key parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, and robustness. Accuracy measures agreement with a true or accepted value, while precision describes repeatability and intermediate precision. Specificity confirms that the method measures the analyte without interference from impurities, degradants, or excipients. Validation is documented in a protocol and report, and acceptance criteria are set before experiments begin. Regulatory guidance varies by region, but the general principles are widely harmonized.
System suitability testing is performed before and during analytical runs to confirm that the instrument and method are working as expected. Common checks include retention time, peak area, resolution between critical pairs, tailing factor, and theoretical plate count. Results are compared with predefined limits, and a failed check requires investigation before sample results are reported. Quality control samples at low, middle, and high concentrations are injected at intervals to monitor accuracy and precision. Blank injections detect carryover and contamination, while control charts track performance over time.
Data handling and documentation are central to HPLC quality control. Electronic systems should have audit trails that record changes to methods, sequences, and results. Integration parameters, such as peak baseline and threshold, can affect reported areas and must be defined in advance. Out-of-specification results trigger a structured investigation that may include reanalysis, instrument checks, and review of sample preparation. Regulatory inspections often examine raw data, audit trails, and training records to verify that reported results are traceable and reliable.
| Property | Value | Notes |
|---|---|---|
| Separation mode | Reversed-phase | Nonpolar stationary phase with polar mobile phase |
| Typical column particle size | 3–5 µm | Smaller particles improve resolution but raise pressure |
| Typical flow rate | 0.5–2.0 mL/min | Depends on column dimensions and pressure limits |
| Common detection | UV-Vis absorbance | Requires analytes with chromophores |
| Typical run time | 5–30 min | Varies with method, gradient, and sample complexity |
Method validation establishes that an HPLC procedure is suitable for its intended purpose. Typical parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, robustness, and solution stability. Accuracy reflects closeness to a reference value, while precision reflects agreement among repeated measurements. Specificity shows whether the method can measure the analyte without interference from matrix components. Validation is documented through protocols and reports, and the required extent depends on the method's use and regulatory context.
Routine quality control uses system suitability, blank injections, check standards, and control samples to detect drift or contamination. System suitability criteria may specify minimum resolution, maximum tailing factor, and a permitted range for repeated injections. Blank injections reveal carryover or solvent contamination, while check standards confirm calibration accuracy over a batch. Control samples with known analyte levels can show whether results remain within statistical limits. When a control result falls outside limits, the analyst investigates the cause and may invalidate affected results before repeating the batch.
Documentation and traceability are central to regulated HPLC testing. Records typically include instrument logs, column history, mobile-phase preparation, sample preparation, injection sequences, raw chromatograms, and audit trails. Electronic systems may require user access controls, time-stamped changes, and backup procedures. Training records show that analysts are qualified for assigned methods. Audits and inspections check whether written procedures match actual practice and whether deviations are documented. These controls support reproducibility and allow results to be reconstructed if questions arise later.
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.
The hydrogenation process was widely adopted by the food industry in the early 1900s; first for the production of margarine, a replacement for butter and shortening, and eventually for various other fats used in snack food, packaged baked goods, and deep fried products. Hydrogenation makes the fat more saturated, which has desirable properties:
=== 25 June === An RSF drone strike hit civilian and military positions in Dalang, killing four civilians and an unknown number of soldiers. The SAF claimed to have retaken the town of Baldago in Blue Nile State from the RSF and the SPLM-N (al-Hilu).
=== Children === The infant lamina propria is composed of only one layer, as compared to three in the adult, and there is no vocal ligament. The vocal ligament begins to be present in children at about four years of age. Two layers appear in the lamina propria between the ages of six and twelve, and the mature lamina propria, with the superficial, intermediate and deep layers, is only present by the conclusion of adolescence. As vocal fold vibration is a foundation for vocal formants, this presence or absence of tissue layers influences a difference in the number of formants between the adult and pediatric populations. In females, the voice is three tones lower than the child's and has five to twelve formants, as opposed to the pediatric voice with three to six. The length of the vocal fold at birth is approximately six to eight millimeters and grows to its adult length of eight to sixteen millimeters by adolescence. The infant vocal fold is half membranous or anterior glottis, and half cartilaginous or posterior glottis. The adult fold is approximately three-fifths membranous and two-fifths cartilaginous.
Sources: en.wikipedia.org
A 2014 study of 18 Latin American countries in the 20th-century study found the legislative powers of the presidency does not influence coup frequency. A 2019 study found that when a country's politics is polarized and electoral competition is low, civilian-recruited coups become more likely. A 2023 study found that civilian elites are more likely to be associated with instigating military coups while civilians embedded in social networks are more likely to be associated with consolidating military coups.
In another case, a man ingested 200 castor beans mixed with juice in a blender and survived. Victims often manifest nausea, diarrhea, fast heart rate, low blood pressure, and seizures persisting for up to a week. Blood, plasma, or urine ricin or ricinine concentrations may be measured to confirm diagnosis. The laboratory testing usually involves immunoassay or liquid chromatography-mass spectrometry.
A coating is a covering that is applied to the surface of an object, or substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids, e.g., powder coatings. Paints and lacquers are coatings that mostly have dual uses, which are protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is for identification, e.g., blue for process water, red for fire-fighting control) in addition to preventing corrosion. Along with corrosion resistance, functional coatings may also be applied to change the surface properties of the substrate, such as adhesion, wettability, or wear resistance. In other cases the coating adds a completely new property, such as a magnetic response or electrical conductivity, as in semiconductor device fabrication, where the substrate is a wafer, and forms an essential part of the finished product. A major consideration for most coating processes is controlling coating thickness. Methods of achieving this range from a simple brush to expensive precision machinery in the electronics industry. Limiting coating area is crucial in some applications, such as printing. "Roll-to-roll" or "web-based" coating is the process of applying a thin film of functional material to a substrate on a roll, such as paper, fabric, film, foil, or sheet stock.
Sources: en.wikipedia.org
=== Do–Du === George H. Dodd (c. 1942–2020). British biochemist who studied perfumes and pheromones. Edward Adelbert Doisy (1893–1986). American biochemist at St Louis University, known for discovering vitamin K. Nobel Prize in Physiology or Medicine (1943). Ford Doolittle (b. 1942). American biochemist at Dalhousie University, known for contributions to the study of cyanobacteria and of biochemical evolution in general. Member Natl. Acad. Sci. USA. Jonathan Dordick (b. 1959). American biochemical engineer at Rensselaer Polytechnic Institute, known for development of enzyme catalysis under extreme conditions. Ralph Dorfman (1911–1985). American biochemist at Stanford, known for treatments for cancer and rheumatoid arthritis Jennifer Doudna (b. 1964). American biochemist at UC Berkeley, known for CRISPR-mediated genome editing. Member Natl. Acad. Sci. USA. Nobel Prize in Chemistry, 2020. Alexander Dounce (1909–1997). American protein chemist at the University of Rochester, active in early work on the genetic code, one of the first to suggest that it was triplet-based. Gideon Dreyfuss (PhD 1978). American biochemist and biophysicist at the University of Pennsylvania, concerned with the function and biogenesis of non-coding RNA and the proteins that interact with RNA. Member Natl. Acad. Sci. USA. Jack Cecil Drummond FRS (1891–1952). British biochemist at University College London, known for the isolation of Vitamin A, and wartime advisor on nutrition. Murdered in France, with his wife and daughter. Vincent du Vigneaud (1901–1978).
=== Religion === Kardashian is a Christian and reads a daily devotional to herself and her "glam squad" every day. She is interested in theology and enjoys attending church. In April 2015, she was named godmother of her niece North West, by Kim and Kanye, as the child was baptized in the Armenian Apostolic Church at the Cathedral of St. James in Jerusalem.
Paul Berg, a Nobel laureate in chemistry, wrote to the Senate that "Quackery will always prey on the gullible and uninformed, but we should not provide it with cover from the NIH," and called the office "an embarrassment to serious scientists". Allen Bromley, then-president of the American Physical Society, similarly wrote to Congress that the OAM had "emerged as an undiscriminating advocate of unconventional medicine. It has bestowed the considerable prestige of the NIH on a variety of highly dubious practices, some of which clearly violate basic laws of physics". Leon Jaroff, writing for The New York Times in 1997, described the OAM as "Tom Harkin's folly". In 1995, Wayne Jonas, a promoter of homeopathy and political ally of Harkin, became the director of the OAM, and continued in that role until 1999. In 1997, the NCCAM budget was increased from $12 million to $20 million annually. From 1990 to 1997, use of alternative medicine in the US increased by 25%, with a corresponding 50% increase in expenditures. The OAM drew increasing criticism from eminent members of the scientific community with letters to the Senate Appropriations Committee when discussion of renewal of funding OAM came up. In 1998, the President of the North Carolina Medical Association publicly called for shutting down the OAM. In 1998, NIH director and Nobel laureate Harold Varmus came into conflict with Harkin by pushing to have more NIH control of alternative medicine research. The NIH Director placed the OAM under stricter scientific NIH control.
Sources: en.wikipedia.org
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.
It offers high resolution, reproducibility, and compatibility with many sample types. A single run can separate and quantify multiple analytes. It is common in pharmaceutical, food, environmental, and industrial laboratories.
Samples must be soluble in a suitable mobile phase and free of particles that can block the column. Detector response depends on analyte structure, so some compounds need derivatization or alternative detection. Complex matrices may require extensive sample preparation.
System suitability is a set of checks that confirm the instrument and method perform within limits before sample analysis. It typically includes resolution, tailing factor, retention time, and peak area reproducibility. If a check fails, the run is invalidated until the cause is resolved.