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.
Updated 2025-10-20. Numbers and descriptions here follow the published literature rather than marketing material.
HPLC testing is not a single fixed procedure; it is a family of separation modes. Reversed-phase, normal-phase, ion-exchange, size-exclusion, and affinity chromatography each suit different analyte properties. Reversed-phase methods dominate because they handle many neutral and moderately polar compounds. Detection can be optical, electrochemical, or mass spectrometric, and the detector dictates what information is available. Coupling with mass spectrometry increases selectivity and enables identification when standards are unavailable. The technique cannot separate every mixture without adjustment.
HPLC testing is an analytical technique used to separate, identify, and quantify components in a liquid sample. It relies on a pressurized mobile phase that carries the sample through a column packed with stationary phase. Different compounds travel at different rates because of interactions with the stationary and mobile phases. The resulting signal versus time is a chromatogram. Peak position indicates identity under specified conditions, while peak area or height relates to amount.
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.
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.
| Property | Value | Notes |
|---|---|---|
| Abbreviation | HPLC | Also called high-performance liquid chromatography |
| Separation mechanism | Differential partitioning | Compounds distribute between mobile and stationary phases |
| Typical column chemistry | C18 (octadecylsilane) | Used in reversed-phase separations |
| Typical detector | UV-Vis or photodiode array | Mass spectrometry is common for trace and confirmatory work |
| Typical particle size | 1.8–5 µm | Smaller particles require higher pressure and can improve speed |
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.
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.
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.
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.
Validation demonstrates that a method is suitable for its intended use. Typical performance characteristics include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, and robustness. Regulators and standards organizations provide frameworks, but specific requirements depend on the application and jurisdiction. System suitability tests are run before sample analysis to confirm resolution, peak symmetry, retention time repeatability, and sensitivity. A validated method is not permanently fixed; changes may require partial or full revalidation.
Routine HPLC testing depends on controlled reagents, calibrated instruments, and documented procedures. Columns degrade over time, so retention times and peak shapes are monitored for drift. Mobile phases are filtered and degassed to prevent pump damage and detector noise. Reference standards must be traceable and stored under suitable conditions. Data handling systems record injections, calculations, and audit trails. Quality control samples interspersed with unknowns help detect errors during a run.
Developing an HPLC method begins with defining the purpose, such as quantifying a main component, measuring impurities, or confirming identity. Analysts select separation mode, column, mobile phase, detection, and sample preparation based on analyte properties and matrix. Experiments vary solvent strength, pH, buffer type, and temperature to achieve resolution between critical peaks. The goal is a robust method that produces reliable results across instruments and operators. Method development often involves trial runs and statistical optimization.
Using the various properties of molecules, such as the energy required to break bonds and the dipole moments of molecules, he established a scale and an associated numerical value for most of the elements — the Pauling Electronegativity Scale — which is useful in predicting the nature of bonds between atoms in molecules. In 1936, Pauling was promoted to chairman of the division of chemistry and chemical engineering at Caltech, and to the position of director of the Gates and Crellin Laboratories of Chemistry. He would hold both positions until 1958. Pauling also spent a year in 1948 at the University of Oxford as George Eastman Visiting Professor and Fellow of Balliol.
US Patent for PCR Archived 16 October 2011 at the Wayback Machine What is PCR plateau effect? YouTube tutorial video History of the Polymerase Chain Reaction from the Smithsonian Institution Archives Veritasium (26 December 2024). The Man Who Took LSD and Changed The World. Retrieved 29 January 2025 – via YouTube.
With an area of 22,402,200 square kilometres (8,649,500 sq mi), the Soviet Union was the world's largest country, a status that is retained by the Russian Federation. Covering a sixth of Earth's land surface, its size was comparable to that of North America. Two other successor states are also very large — Kazakhstan ranks among the top 10 countries by land area, and Ukraine is the largest country entirely in Europe. The European portion accounted for a quarter of the country's area and was the cultural and economic centre. The eastern part in Asia extended to the Pacific Ocean to the east and Afghanistan to the south, and, except some areas in Central Asia, was much less populous. It spanned over 10,000 kilometres (6,200 mi) east to west across 11 time zones, and over 7,200 kilometres (4,500 mi) north to south. It had five climate zones: tundra, taiga, steppes, desert and mountains. The USSR, like Russia, had the world's longest border, measuring over 60,000 kilometres (37,000 mi), or 1+1⁄2 circumferences of Earth. Two-thirds of it was a coastline. The country bordered Afghanistan, the People's Republic of China, Czechoslovakia, Finland, Hungary, Iran, Mongolia, North Korea, Norway, Poland, Romania, and Turkey from 1945 to 1991. The Bering Strait separated the USSR from the United States, while the La Pérouse Strait separated it from Japan. The country's highest mountain was Communism Peak (now Ismoil Somoni Peak) in Tajikistan, at 7,495 metres (24,590 ft).
=== Sex pheromone === The sex pheromone of the American cockroach is the sequiterpene (1Z,5E)-1,10(14)-diepoxy-4(15),5-germacradien-9-one, which has been given the trivial name periplanone-B. This pheromone was isolated from the feces of virgin female cockroaches. Previously, 2,2-dimethyl-3-isopropylidenecyclopropyl propionate had been thought to be the structure of this pheromone, but on synthesis was shown to be inactive. The structure determination of this pheromone was an eventful chapter in the history of pheromone chemistry.
Mueller–Hinton agar is a type of growth medium used in microbiology to culture bacterial isolates and test their susceptibility to antibiotics. This medium was first developed in 1941 by John Howard Mueller and Jane Hinton, who were microbiologists working at Harvard University. However, Mueller–Hinton agar is made up of a couple of components, including beef extract, acid hydrolysate of casein, and starch, as well as agar to solidify the mixture. The composition of Mueller–Hinton agar can vary depending on the manufacturer and the intended use, but the medium is generally nutrient-rich and free of inhibitors that could interfere with bacterial growth.
Sources: en.wikipedia.org
== Background == Following the signing of the Pretoria Agreement in November 2022, the Interim Regional Administration of Tigray was established as a transitional governing body intended to stabilize the war-torn region. However, implementation of the peace deal stagnated over unresolved constitutional territories (such as Western Tigray), delays in disarming and reintegrating forces, and the failure to return nearly one million internally displaced persons (IDPs). Tensions escalated dramatically in early 2026 when the federal government under Prime Minister Abiy Ahmed unilaterally extended the mandate of the TIA by one year and suspended voting in 30 Tigrayan constituencies, excluding the region from the June 2026 national elections. Concurrently, a severe political fracture emerged within Tigray between TPLF officials and TIA leadership regarding the pace and direction of the post-war transition.
The company's earliest stores included its original site in Rittenhouse Square as well as its New York City, Chicago, and Washington, D.C. locations. Locations in New York were opened in 2007 and 2009 in Tribeca and SoHo, respectively. The first location in Chicago was opened in 2011. The second Philadelphia location was opened in Dilworth Park (near Philadelphia City Hall) in 2011. In 2013, the company operated three locations in Seoul, South Korea. Headquarters moved to a new construction in Fishtown, Philadelphia in 2015. The construction had been voted on by residents in 2013. The same year, the company announced plans to expand to 150 new locations in the next three to four years. Expansion to Boston began in September 2015. Expansion to Los Angeles was announced in early 2016, with locations in Beverly Hills and Silver Lake. New locations in Chinatown, Washington, D.C. and Lincoln Park, Chicago were announced in 2016. In San Diego, a new location opened in Westfield UTC in fall 2017. In 2021, a location was announced in Austin, Texas inside a Whole Foods Market, the company's first Texas location. In 2025, both locations in Boston closed (at 745 Atlantic Ave. in Leather District and at 29 Northern Ave. in the Seaport District), with the owners citing financial performance of the locations and other factors. As of 2025, La Colombe advertises on its website 30 locations in the United States, in Philadelphia, New York City, Washington, D.C., Boston, Chicago, Austin, Los Angeles, and San Diego.
=== Quantification in disease === Elevation of serum, plasma, or tissue SP and/or its receptor (NK1R) has been associated with many diseases: sickle cell crisis; inflammatory bowel disease; major depression and related disorders; fibromyalgia; rheumatological; and infections such as HIV/AIDS and respiratory syncytial virus, as well as in cancer. When assayed in the human, the observed variability of the SP concentrations are large, and in some cases the assay methodology is questionable. SP concentrations cannot yet be used to diagnose disease clinically or gauge disease severity. It is not yet known whether changes in concentration of SP or density of its receptors is the cause of any given disease, or an effect.
There have been few incidents of doping in football, mainly because of little unannounced testing relative to the high amount of professionals. There is also a lack of further investigation (e.g. Tony Schumacher revelations and the still unknown footballers of Eufemiano Fuentes), preservation of samples, and consequences. In 2014, the biological passport was introduced in the 2014 FIFA World Cup; blood and urine samples from all players before the competition and from two players per team and per match are analysed by the Swiss Laboratory for Doping Analyses.
Sources: en.wikipedia.org
It measures the presence and amount of one or more compounds in a liquid sample. Separation occurs in a column, and detection produces a signal proportional to concentration. Identification usually requires comparison with a known reference standard under the same conditions.
In most cases the sample is consumed or altered during analysis, though some detectors are non-destructive. Fractions can be collected after separation for further study. Repeated testing therefore requires additional sample.
Run times range from under a minute for fast methods to over an hour for complex separations. Sample preparation, equilibration, and data review add time. Throughput depends on instrument configuration and method requirements.
HPLC testing measures the presence and amount of one or more compounds in a liquid sample. It separates mixture components and records detector responses as peaks, which are compared with reference standards. Results are usually reported as concentrations or relative percentages.