This is a working overview of stationary phase, written for readers who want more than a one-paragraph summary but less than a textbook.
This page was last updated on 2025-11-29 and is reviewed periodically as new material appears.
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.
Separation modes differ by the chemistry of the stationary phase and the composition of the mobile phase. Reversed-phase testing uses a nonpolar column and polar solvents, making it common for pharmaceutical, environmental, and food analytes. Normal-phase testing uses a polar column and nonpolar solvents for compounds that are poorly retained in reversed-phase systems. Ion-exchange and ion-pair methods separate charged species, while size-exclusion methods sort molecules by hydrodynamic volume. Gradient elution changes solvent strength over time to resolve complex mixtures, and isocratic elution holds solvent composition constant for simpler assays.
Key performance measures include retention time, peak area, peak height, resolution, tailing factor, and plate count. Retention time helps identify a peak under fixed conditions, but confirmation often requires a second method or detector. Peak area and height relate to concentration through calibration curves, which may be linear or nonlinear depending on the detector response. Resolution describes separation between adjacent peaks, while tailing factor and plate count describe peak shape and column efficiency. Performance checks verify these values before and during a run to confirm that the instrument is performing within limits.
| Parameter | Typical acceptance criterion | Notes |
|---|---|---|
| Resolution | ≥ 1.5 | Baseline separation of adjacent peaks |
| Tailing factor | ≤ 2.0 | Peak symmetry measure |
| Theoretical plates | > 2000 | Column efficiency indicator |
| Injection repeatability | ≤ 2% RSD | Relative standard deviation for replicate injections |
| Linearity | r² ≥ 0.995 | Calibration curve over the working range |
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.
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.
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.
In the case of neutrophilic pericholangitis, neutrophils surround the portal ducts and obstruct them. Neutrophilic pericholangitis has a variety of causes including endotoxemia, Hodgkin's disease, among others. Cholangitis lenta can also cause changes to the portal tracts. This occurs during chronic cases of sepsis and results in dilation of the bile ductules. Cholangitis lenta is likely a result of a stoppage of bile secretion and bile flow through the ductules. Back pressure created from obstructive cholestasis can cause dilation of the bile duct and biliary epithelial cell proliferation, mainly in the portal tracts. Portal tract edema may also occur as a result of bile retention, as well as periductular infiltration of neutrophils. If the obstruction is left untreated, it can lead to a bacterial infection of the biliary tree. Infection is mostly caused by coliforms and enterococci and is evident from a large migration of neutrophils to the duct lumina. This can result in the formation of a cholangitic abscess. With treatment, many of the histological features of cholestasis can be corrected once the obstruction is removed. If the obstruction is not promptly resolved, portal tract fibrosis can result. Even with treatment, some fibrosis may remain.
The TCR alpha chain is generated by VJ recombination, whereas the beta chain is generated by VDJ recombination (both involving a random joining of gene segments to generate the complete TCR chain). Likewise, generation of the TCR gamma chain involves VJ recombination, whereas generation of the TCR delta chain occurs by VDJ recombination. The intersection of these specific regions (V and J for the alpha or gamma chain; V, D, and J for the beta or delta chain) corresponds to the CDR3 region that is important for peptide/MHC recognition (see above). It is the unique combination of the segments at this region, along with palindromic and random nucleotide additions (respectively termed "P-" and "N-"), which accounts for the even greater diversity of T-cell receptor specificity for processed antigenic peptides. Later during development, individual CDR loops of TCR can be re-edited in the periphery outside thymus by reactivation of recombinases using a process termed TCR revision (editing) and change its antigenic specificity.
Despite his advanced age and having stepped down as prime minister, Mahathir remains active on the international stage, frequently travelling, delivering speeches, and continuing to voice his views and influence both domestically and globally. Shortly after Malaysia entered its first movement control order in March 2020, Mahathir underwent self-quarantine at home after having close contact with Bandar Kuching MP Kelvin Yii Lee Wuen, who later tested positive for COVID-19. He formed the Homeland Fighter's Party (PEJUANG) in August 2020. Four other MPs joined the new party, including Mahathir's son Mukhriz. The party was registered in July 2021. In August 2021, Mahathir and other MPs protested in Merdeka Square, calling for Muhyiddin's resignation over the government's response to the COVID-19 pandemic, after being blocked from entering parliament by police. In April 2022, UMNO president Ahmad Zahid Hamidi filed a defamation lawsuit against Mahathir. In June 2022, Mahathir made irrendentist comments by stating that Singapore and the Riau Islands of Indonesia was once owned by Johor, and argued the state should claim them as part of Malaysia. In September 2022, Mahathir said he was open to becoming the prime minister for a third time if there were no other suitable candidates. Having previously said he would not defend his Langkawi parliamentary seat, he announced he would contest the 2022 general election. In the election on 19 November, Mahathir lost his seat and election deposit, marking his first defeat in 53 years.
=== Nanofiltration === Virus removal processes using nanofiltration techniques remove viruses specifically by size exclusion. This type of process is typically used for parvoviruses and other viruses containing a protein coat. A typical HIV virion is 180 nm and a typical parvovirus can vary between 15 and 24 nm, which is very small. One great advantage of filtration, as opposed to methods involving extremes of temperature or acidity, is that filtration will not denature the proteins in the sample. Nanofiltration is also effective for most types of proteins. Since it is not chemically selective, no matter what the surface chemistry of the viral particle is, viral removal processes using nanofiltration techniques will still be effective. Another great advantage of this technique is its ability to be performed on a lab scale and then effectively scaled up to production standards. It is important to consider, however, the fact that the level of removal of the viruses is dependent on the size of the pores of the nanofilter. In some cases, very small viruses will not be filtered out. It is also necessary to consider the possible effects of pressure and flow rate variation. Some of the filters used for to perform these types of processes are Planova 15N, Planova 20N, BioEX, VAG - 300, Viresolve 180, Viresolve 70TM, and the Virosart range.
Sources: en.wikipedia.org
After the Netherlands in World War II in 1945, Korvezee returned to Delft, initially serving as a curator and, from 1948, as a lecturer in theoretical chemistry. Although an expert in radioactivity, she had no role in the planning of the reactor center later built on the university's grounds. When the chair became vacant following Scheffer's retirement, Korvezee was passed over once more. In 1954, the Department of Chemical Technology created an extraordinary professorship for her in the emerging field of theoretical chemistry largely regarded as compensation for the chairs she had earlier missed. Her appointment on 14 April 1954, making her the first female professor at Delft, received extensive press coverage. As a full professor, however, she retained the salary of her former lectureship. As professor, Korvezee supervised doctoral and master's students and published more than forty scientific works before 1940, many co-authored with fellow female assistants. She was a member of the Association of Women with an Academic Education (VVAO), though she was never actively involved in the women's movement despite being widely regarded as a feminist figure.
On March 22, 2023, Valve announced Counter-Strike 2, which utilizes the Source 2 game engine. It was originally announced as a free upgrade to Global Offensive with a summer 2023 release date. On September 1, 2023, it was released as a limited beta, and the game replaced Global Offensive on September 27.
=== Later work === In 1965, Zuckerkandl moved back to France to direct in Montpellier, the "Centre de Recherche de Biochimie Macromoléculaire" of the Centre National de Recherche Scientifique. In 1971, he became the founding editor of the Journal of Molecular Evolution, and in the late 1970s became President of the Linus Pauling Institute (then in 1992 of its successor, the Institute of Molecular Medical Sciences). His recent work includes criticism of social constructionism and intelligent design.
The Southern Rhodesian economy grew considerably during the war despite the concurrent rise of war expenditure to pay for the expansion of the military and the air training scheme. Expenditure on the war grew from £1,793,367 in the financial year 1940–41 to £5,334,701 in 1943–44—total Southern Rhodesian expenditure on the air training scheme was £11,215,522. These sums, while tiny compared to those incurred by larger nations, were enormous when scaled against the white population of less than 70,000 that accounted for most of the colony's economic output. Annual costs for the air training scheme alone far exceeded the pre-war national budget. Southern Rhodesia was then the second largest gold producer in the world, after South Africa. The colony's gold output had expanded greatly during the 1930s, and it remained the territory's main source of income during the war, though many extracting operations were diverted towards strategic minerals, most prominently chrome and asbestos. Southern Rhodesia became one of the two main sources of chrome for the Allies (South Africa was the other) and the world's third largest producer of asbestos after Canada and the Soviet Union. By the end of the war the mines at Shabani and Mashaba were turning out 1.5 million tonnes of asbestos a year, in addition to 600,000 tonnes of chrome. Gold output reached peak levels in 1941–42 and thereafter subsided. Southern Rhodesia also exported tungsten, mica and tin, and provided coal for the copper mines of Northern Rhodesia and the Congo.
=== Clinical quantification and genotyping === Viruses can be present in humans due to direct infection or co-infections which makes diagnosis difficult using classical techniques and can result in an incorrect prognosis and treatment. The use of qPCR allows both the quantification and genotyping (characterization of the strain, carried out using melting curves) of a virus such as the hepatitis B virus. The degree of infection, quantified as the copies of the viral genome per unit of the patient's tissue, is relevant in many cases; for example, the probability that the type 1 herpes simplex virus reactivates is related to the number of infected neurons in the ganglia. This quantification is carried out either with reverse transcription or without it, as occurs if the virus becomes integrated in the human genome at any point in its cycle, such as happens in the case of HPV (human papillomavirus), where some of its variants are associated with the appearance of cervical cancer. Real-time PCR has also brought the quantization of human cytomegalovirus (CMV) which is seen in patients who are immunosuppressed following solid organ or bone marrow transplantation.
Sources: en.wikipedia.org
American kestrels feed largely on small animals such as grasshoppers, crickets, butterflies, moths, dragonflies, beetles, lizards, mice, voles, shrews, frogs, and small birds. The kestrel has also been reported to have killed scorpions, snakes, bats, and squirrels. The kestrel is able to maintain high population densities, at least in part because of the broad scope of its diet. The American kestrel's primary mode of hunting is by perching and waiting for prey to come near. The bird is characteristically seen along roadsides or fields perched on objects such as trees, overhead power lines, or fence posts. It also hunts by kiting, hovering in the air with rapid wing beats and scanning the ground for prey. Other hunting techniques include low flight over fields, or chasing insects and birds in the air. Prey is most often caught on the ground, though occasionally they take birds in flight. Before striking, the kestrel characteristically bobs its head and tail, then makes a direct flight toward the prey to grab it in its talons. Much like the red-tailed hawk, American kestrels conserve energy in a hunt and pick their attacks with care as to position and odds of success. During the breeding season, the bird will carry large prey back to its mate or young. One study found that an American kestrel pair "foraged in ways that minimized the costs of energy acquisition in its particular situation". For example, if the success rate for catching prey decreases significantly in a particular area, the bird will move to a different area.
AlphaFold has been used to predict structures of proteins of SARS-CoV-2, the causative agent of COVID-19. The structures of these proteins were pending experimental detection in early 2020. Results were reviewed by scientists at the Francis Crick Institute in the United Kingdom before being released to the broader research community. The team also confirmed accurate prediction against the experimentally determined SARS-CoV-2 spike protein that was shared in the Protein Data Bank, an international open-access database, before releasing the computationally determined structures of the under-studied protein molecules. The team acknowledged that although these protein structures might not be the subject of ongoing therapeutical research efforts, they will add to the community's understanding of the SARS-CoV-2 virus. Specifically, AlphaFold 2's prediction of the structure of the ORF3a protein was very similar to the structure determined by researchers at University of California, Berkeley using cryo-electron microscopy. This specific protein is believed to assist the virus in breaking out of the host cell once it replicates. This protein is also believed to play a role in triggering the inflammatory response to the infection.
The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 Hz, which is equal to s−1.
The energy needed to bring an alpha particle from infinity to a point near the nucleus just outside the range of the nuclear force's influence is generally in the range of about 25 MeV. An alpha particle within the nucleus can be thought of as being inside a potential barrier whose walls are 25 MeV above the potential at infinity. However, decay alpha particles only have energies of around 4 to 9 MeV above the potential at infinity, far less than the energy needed to overcome the barrier and escape.
The 2nd Black Watch, with its Rhodesian contingent, took part in the unsuccessful Allied defence of Crete in May–June 1941, then joined the garrison at Tobruk in August 1941. No. 237 (Rhodesia) Squadron was re-equipped with Hawker Hurricanes the following month.
Sources: en.wikipedia.org
Method validation is the documented process of confirming that an HPLC procedure is suitable for its intended use. It evaluates accuracy, precision, specificity, linearity, range, detection limits, and robustness. Validation criteria depend on the regulatory context and the sample type.
System suitability tests are short checks performed before or during an HPLC run to verify instrument and method performance. They often include resolution, tailing factor, theoretical plates, and injection precision. Results must meet predefined limits for sample data to be accepted.
HPLC retention time alone cannot definitively identify an unknown substance. A match with a reference standard under identical conditions provides supporting evidence. Confirmation typically requires mass spectrometry, nuclear magnetic resonance, or another orthogonal technique.
It measures the amounts and identities of compounds in liquid samples by separation and detection. Depending on the detector and reference standards, results can be qualitative or quantitative. The technique is used in fields such as pharmaceutical analysis, food safety, and environmental monitoring.