If you have been reading about retention time and want a single page that covers the useful parts, this is it: definitions, context, how it is studied, and the questions that come up repeatedly.
Last reviewed on 2026-08-01. Where a claim depends on a specific study, the study is described rather than over-claimed.
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.
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.
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.
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.
| Property | Value | Notes |
|---|---|---|
| Separation mode | Reversed-phase | Most common for neutral and moderately polar analytes |
| Column particle size | 3–5 µm | Smaller particles improve resolution but raise backpressure |
| Mobile phase pH range | 2–8 | Silica-based columns may degrade outside this range |
| Typical flow rate | 1.0–2.0 mL/min | For analytical columns with 4.6 mm internal diameter |
| Common synonyms | HPLC, LC, high-pressure liquid chromatography | High-performance liquid chromatography is the standard expansion |
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.
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.
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 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.
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.
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.
=== 13 April === The BBC reported that the UK plans to send a third Westland Sea King helicopter to Ukraine in the "coming weeks", with the Ukrainian crew being trained in its maintenance at a base in "southern England".
==== Food competition ==== Food distribution, density, and accessibility can determine population density and territory size in blue tangs. Territories with low biogenic structure are larger than those of higher biogenic structure. Since the algal food resources are less dense in low-biogenic structured areas, these territories would have to be larger in order to include the necessary amount of food. This is in accordance with the Ideal free distribution model. This model states that competitors should adjust their distribution in accordance with habitat quality such that each individual will gain the same amount of resources. According to this model, there should be a lower density of blue tangs on low-biogenic structured territories compared to higher-biogenic structured territories where there is a higher abundance of food. In both territories, each individual will receive similar amounts of resources due to competition. There is no significant difference in feeding rates of blue tangs on each type of territory, meaning that those in larger, uncolonized, and low density territories can match the resources of those in smaller high density territories.
=== Natural === Natural polymeric materials such as hemp, shellac, amber, wool, silk, and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper.
Belize Electricity Limited (BEL), the nation's primary distributor of electricity, proposed a $500 million investment to support the integration of 60 MW of utility-scale solar photovoltaic (PV) generation and 40 MW of battery energy storage systems in its 2024–2028 Full Tariff Review, in an effort to stabilize electricity rates at 40 cents per kilowatt-hour and reduce reliance on imported electricity from Mexico. The Government of Belize's National Energy Policy 2023–2040 commits to achieving 75% renewable energy in the electricity generation mix by 2030. As of 2022, renewable sources accounted for 53% of total gross electricity generation. To improve grid reliability, BEL has initiated several infrastructure upgrades. In 2024, the company completed an upgrade of the West Lake Gas Turbine, increasing its capacity from 19 MW to 30 MW. Additionally, a new gas turbine was installed on Ambergris Caye, adding 20 MW of generation capacity to the national grid. The World Bank is supporting these efforts through the Belize Energy Resilience and Sustainability Project, which includes the deployment of four 10 MW battery storage systems in key districts. Belize Telemedia Limited (BTL), operating under the brand Digi, is the nation's primary telecommunications provider. As of 2025, BTL maintains approximately 32,000 fixed telephone line contracts and serves around 190,000 GSM mobile customers, with active expansion ongoing in accordance with the Government of Belize's National Digital Agenda 2022–2025.
=== Risk factors === Heavy lifting, obesity, sedentary lifestyle and lack of exercise can increase the risk of back pain. Cigarette smokers are more likely to experience back pain than are nonsmokers. Excessive weight gain in pregnancy is also a risk factor for back pain. In general, fatigue can worsen pain. A few studies suggest that psychosocial factors such as work-related stress, burnout, sleep problems, and dysfunctional family relationships may correlate more closely with back pain than do structural abnormalities revealed in X-rays and other medical imaging scans. Back pain physical effects can range from muscle aching to a shooting, burning, or stabbing sensation. Pain can radiate down the legs and can be increased by bending, twisting, lifting, standing, or walking. While the physical effects of back pain are always at the forefront, back pain also can have psychological effects. Back pain has been linked to depression, anxiety, stress, and avoidance behaviors due to mentally not being able to cope with the physical pain. Both acute and chronic back pain can be associated with psychological distress in the form of anxiety (worries, stress) or depression (sadness, discouragement). Psychological distress is a common reaction to the suffering aspects of acute back pain, even when symptoms are short-term and not medically serious.
Sources: en.wikipedia.org
In combined mass spectra obtained by summing a large number of individual ion detection events, each peak is a histogram obtained by adding up counts in each individual bin. Because the recording of the individual ion arrival with TDC yields only a single time point, the TDC eliminates the fraction of peak width determined by a limited response time of both the MCP detector and preamplifier. This propagates into better mass resolution. Modern ultra-fast 10 GSample/sec analog-to-digital converters digitize the pulsed ion current from the MCP detector at discrete time intervals (100 picoseconds). Modern 8-bit or 10-bit 10 GHz ADC has much higher dynamic range than the TDC, which allows its usage in MALDI-TOF instruments with its high peak currents. To record fast analog signals from MCP detectors one is required to carefully match the impedance of the detector anode with the input circuitry of the ADC (preamplifier) to minimize the "ringing" effect. Mass resolution in mass spectra recorded with ultra-fast ADC can be improved by using small-pore (2-5 micron) MCP detectors with shorter response times.
Adsorption refrigeration also uses a thermally driven sorption process, but the refrigerant is taken up by a solid adsorbent rather than dissolved in a liquid absorbent. Common adsorption working pairs include water with silica gel or zeolite, and methanol with activated carbon. In a basic adsorption cycle, heating the adsorbent releases refrigerant vapour, which is condensed. When the adsorbent is subsequently cooled, it adsorbs refrigerant vapour again; evaporation of the refrigerant during this part of the cycle produces the cooling effect. Adsorption systems can be driven by low-grade heat sources, including solar or waste heat. Adsorption refrigeration has been extensively studied as an alternative to conventional vapor-compression cooling. Reviews have noted advantages such as the potential use of low-grade thermal energy and relatively simple solid-sorbent systems, while also identifying limitations including comparatively low performance and continuing technical and economic challenges. The main difference from absorption cycle is that in adsorption cycle, the refrigerant (adsorbate) can be ammonia, water, methanol, etc., while the adsorbent is a solid, such as silica gel, activated carbon, or zeolite, while in the absorption cycle the absorbent is liquid. The reason adsorption refrigeration technology has been extensively researched in recent 30 years lies in that the operation of an adsorption refrigeration system is often noiseless, non-corrosive and environmentally friendly.
== Production == Chuño is made at the beginning of winter during June and July, during which time the temperatures reach around −5 °C (23 °F) at elevations of over 3,800 metres (12,500 ft). After fall harvest (April–May), potatoes are selected for the production of chuño, typically small ones for ease of processing. These small potatoes are spread closely on flat ground, and allowed to freeze with the low night temperatures and dehydrate in the daytime, for about three nights. This process results in natural freeze-drying. By the end of this process, the potatoes are taken to chuñochinapampas – flat areas where the potatoes can be laid out. The term is Aymara in origin and translates to “the place where the chuño is made”. Once they make it to the chuñochinapampas, they are trampled by foot, traditionally through the work of extended family groups and their pets. This eliminates what little water is still retained by the potatoes, and removes the skins, enabling subsequent freezing and drying. They remain as they are for over a week, depending on weather conditions. During the process of manually squeezing water out of the potatoes via stepping on them, whole families will participate. The previous freeze-drying breaks down cell walls, making it easier to remove water from the potatoes. They build a small pile of potatoes with their feet and then "dance" on the pile, removing the skins as they do so. This will not entirely remove the skins, so the remaining skin is removed by hand afterwards. Starting from this basic freeze-dry process, two varieties are obtained:
After the first two days the insurgents lost their offensive initiative, and the combined South African forces drove PLAN back across the border in a counteroffensive codenamed Operation Merlyn. Between 1 and 9 April 1989, 273 PLAN insurgents were killed in the fighting. The SADF and police suffered 23 dead. On 8 April 1989, the JMMC issued the Mount Etjo Declaration, which reiterated that the Tripartite Accord was still in effect and that South Africa, Angola, and Cuba remained committed to peace. It also ordered all PLAN insurgents remaining in Ovamboland to surrender at UNTAG-supervised assembly points. Sam Nujoma denied any incursion had taken place on 1 April, claiming that he had only ordered PLAN insurgents already inside South West Africa to begin establishing base camps. He also pointed out that SWAPO had never been a signatory to the Tripartite Accord, and therefore the cessation of hostilities as dictated by its terms was non-binding. This drew some ire from Angola, which had given guarantees to the UN that PLAN would remain north of the 16th parallel. The SADF was re-confined to its bases on 26 April, then released into Ovamboland again to verify that the insurgents had departed. By May, all but a small handful of PLAN insurgents had been relocated north of the 16th parallel under JMMC supervision, effectively ending the South African Border War. General elections under a universal franchise were held in South West Africa between 7 and 11 November 1989, returning 57% of the popular vote for SWAPO.
osmotic shock Also osmotic stress. Physiological dysfunction caused by a sudden change in the concentration of dissolved solutes in the extracellular environment surrounding a cell, which provokes the rapid movement of water across the cell membrane by osmosis, either into or out of the cell. In a severely hypertonic environment, where extracellular solute concentrations are extremely high, osmotic pressure may force large quantities of water to move out of the cell (plasmolysis), leading to its desiccation; this may also have the effect of inhibiting transport of solutes into the cell, thus denying it the substrates necessary to sustain normal cellular activities. In a severely hypotonic environment, where extracellular solute concentrations are much lower than intracellular concentrations, water is forced to move into the cell (turgescence), causing it to swell in size and potentially burst, or triggering apoptosis.
Sources: en.wikipedia.org
The post-1989 period has been characterised by the privatisation and closure of several former industrial and economic enterprises from the communist period, while corruption has been a major issue in contemporary politics. A National Anticorruption Directorate was formed in the country in 2002. During the 2000s, Romania had one of the highest economic growth rates in Europe and has been referred at times as "the Tiger of Eastern Europe". This has been accompanied by a significant improvement in living standards as the country successfully reduced domestic poverty and established a functional democratic state. However, Romania's development suffered a major setback during the late 2000s' recession leading to a large gross domestic product contraction and a budget deficit in 2009. This led to Romania borrowing from the International Monetary Fund. Worsening economic conditions led to unrest and triggered a political crisis in 2012. Since 2014, Romania launched an anti-corruption effort that led to the prosecution of medium- and high-level political, judicial and administrative offences by the National Anticorruption Directorate. In 2015, massive anti-corruption protests which developed in the wake of the Colectiv nightclub fire led to the resignation of prime minister Victor Ponta. During 2017–2019, in response to measures which were perceived to weaken the fight against corruption, some of the biggest post-1989 protests took place in Romania, with over 500,000 people protesting nationwide.
==== Cells ==== Sponges have around 6 different types of cells that can perform different functions. Sponges are a good model for studying the origin of multicellularity because the cells are capable of communicating with one another and re-aggregating. In an experiment conducted by Henry Van Peters Wilson in 1910, it was found that cells from dissociated sponges could send out signals and recognize each other to form a new individual. This suggests that the cells that compose sponges are capable of independent living, however once multicellularity was possible then aggregating together to form one organism was a more efficient way of living.
=== 2012–2018: Change in pace === He starred alongside former co-star Steve Carell in the Don Scardino-directed comedy film The Incredible Burt Wonderstone (2013). Carrey played Steve Gray, a dangerous street magician who overshadows the formerly successful magician Burt Wonderstone (played by Carell). The film was released in March 2013 to mixed reviews and underperformed significantly at the box office, grossing just over $27 million on a $30 million budget. Around the same time, he appeared in Kick-Ass 2 (also 2013) as Colonel Stars and Stripes. He retracted support for the film two months prior to its release. He issued a statement via his Twitter account that, in light of the Sandy Hook Elementary School shooting, "Now in all good conscience I cannot support that level of violence." Peter Farrelly said in April 2012 that Carrey and Jeff Daniels would return for a Dumb and Dumber sequel, Dumb and Dumber To, with the Farrelly brothers writing and directing and a planned September 2012 production start. In June, however, Carrey's representative said Carrey had left the project because the comedian felt New Line and Warner Bros. were unenthusiastic toward it. However, on October 1, 2012, Yahoo!'s The Yo Show carried the news item that the script was complete and that the original actors, Carrey and Daniels, would be reprising their roles. The plot involved one of the characters having sired a child and needing to find them to obtain a kidney. Dumb and Dumber To was released in November 2014.
=== Acquired === Methemoglobinemia may be acquired. Classical drug causes of methemoglobinemia include various antibiotics (trimethoprim, sulfonamides, and dapsone), local anesthetics (especially articaine, benzocaine, prilocaine, and lidocaine), and aniline dyes, metoclopramide, rasburicase, umbellulone, chlorates, bromates, and nitrites. Nitrates are suspected to cause methemoglobinemia. In otherwise healthy individuals, the protective enzyme systems normally present in red blood cells rapidly reduce the methemoglobin back to hemoglobin and hence maintain methemoglobin levels at less than one percent of the total hemoglobin concentration. Exposure to exogenous oxidizing drugs and their metabolites (such as benzocaine, dapsone, and nitrates) may lead to an increase of up to a thousandfold of the methemoglobin formation rate, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels. Infants under six months of age have lower levels of a key methemoglobin reduction enzyme (NADH-cytochrome b5 reductase) in their red blood cells. This results in a major risk of methemoglobinemia caused by nitrates ingested in drinking water, dehydration (usually caused by gastroenteritis with diarrhea), sepsis, or topical anesthetics containing benzocaine or prilocaine resulting in blue baby syndrome. Nitrates used in agricultural fertilizers may leak into the ground and may contaminate well water. The current EPA standard of 10 ppm nitrate-nitrogen for drinking water is specifically set to protect infants.
Sources: en.wikipedia.org
HPLC testing separates and quantifies components in a liquid sample. It is used to check identity, purity, concentration, or stability. The technique works best for compounds that dissolve and are not easily vaporized.
The pump maintains a steady flow rate and pressure, which keeps retention times reproducible. Pulsation or flow errors can shift peaks and distort quantitation. Modern pumps use feedback control to reduce these variations.
HPLC alone usually separates compounds but does not always identify them. Retention time matching with a known standard provides tentative identification. Coupling HPLC to mass spectrometry adds mass information that supports structural identification.
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.