This is a working overview of stationary phase, written for readers who want more than a one-paragraph summary but less than a textbook.
Reviewed 2025-08-28. Anything still debated is marked as such rather than presented as settled.
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.
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.
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.
| 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 |
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.
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.
==== Cognitive performance ==== Caffeine is a central nervous system stimulant that may reduce fatigue and drowsiness. At normal doses, caffeine has variable effects on learning and memory, but it generally improves reaction time, wakefulness, concentration, and motor coordination. The amount of caffeine needed to produce these effects varies from person to person, depending on body size and degree of tolerance. The desired effects arise approximately one hour after consumption, and the desired effects of a moderate dose usually subside after about three or four hours. Caffeine can delay or prevent sleep and improves task performance during sleep deprivation. Shift workers who use caffeine make fewer mistakes that could result from drowsiness. Caffeine in a dose dependent manner increases alertness in both fatigued and normal individuals. A systematic review and meta-analysis from 2014 found that concurrent caffeine and L-theanine use has synergistic psychoactive effects that promote alertness, attention, and task switching; these effects are most pronounced during the first hour post-dose. A 2025 systematic review and meta-analysis found that acute caffeine intake can improve reaction time and accuracy for cognitive tasks. Increased dosages can further improve reaction time but lead to decreases in accuracy after specific intake thresholds are reached.
=== Other === Arterial stiffness Arthrofibrosis (knee, shoulder, other joints) Chronic kidney disease Crohn's disease (intestine) Dupuytren's contracture (hands, fingers) Keloid (skin) Lipedema (fat cells, typically in lower limbs) Mediastinal fibrosis (soft tissue of the mediastinum) Myelofibrosis (bone marrow) Myofibrosis (skeletal muscle) Peyronie's disease (penis) Nephrogenic systemic fibrosis (skin) Progressive massive fibrosis (lungs); a complication of pneumoconiosis Retroperitoneal fibrosis (soft tissue of the retroperitoneum) Scleroderma/systemic sclerosis (skin, lungs) Some forms of adhesive capsulitis (shoulder) Fibrosis reversal Historically, fibrosis was considered an irreversible process. However, several recent studies have demonstrated reversal in liver and lung tissue, and in cases of renal, myocardial, and oral-submucosal fibrosis.
Red Acid fuchsin, xylidine ponceau, chromotrope 2R, Biebrich scarlet, ponceau 6R, phloxine Blue and green Light green SF yellowish, Fast Green FCF, methyl blue, water blue Yellow Picric acid, orange G, Martius yellow, tartrazine, milling yellow
Indium wire is used as a vacuum seal and a thermal conductor in cryogenics and ultra-high-vacuum applications, in such manufacturing applications as gaskets that deform to fill gaps. Owing to its great plasticity and adhesion to metals, Indium sheets are sometimes used for cold-soldering in microwave circuits and waveguide joints, where direct soldering is complicated. Indium is an ingredient in the gallium–indium–tin alloy galinstan, which is liquid at room temperature and replaces mercury in some thermometers. Other alloys of indium with bismuth, cadmium, lead, and tin, which have higher but still low melting points (between 50 and 100 °C), are used in fire sprinkler systems and heat regulators. Indium is one of many substitutes for mercury in alkaline batteries to prevent the zinc from corroding and releasing hydrogen gas. Indium is added to some dental amalgam alloys to decrease the surface tension of the mercury and allow for less mercury and easier amalgamation. Indium's high neutron-capture cross-section for thermal neutrons makes it suitable for use in control rods for nuclear reactors, typically in an alloy of 80% silver, 15% indium, and 5% cadmium. In nuclear engineering, the (n,n') reactions of 113In and 115In are used to determine magnitudes of neutron fluxes.
Sources: en.wikipedia.org
Liu took an anti-Chiang stance in the Central Plains War, declaring his support for the coalition of Feng Yuxiang, Yan Xishan, and Wang Jingwei in 1930. This stood in contrast to Liu Xiang's more pro-Chiang attitude. Official Chinese sources also claim that Liu supported anti-Nanjing forces such as Hu Ruoyu in Yunnan and Wang Jialie in Guizhou. Nevertheless, this stance was a tepid one and tempered further by pragmatic opportunism; like the rest of his colleagues, Liu refrained from committing his army to the war when a representative of Feng Yuxiang arrived in Sichuan in 1929 to convince the province's warlords to side with the coalition. Once the anti-Chiang side began suffering military defeats, Liu professed his loyalty to the national government in 1931.
The research assistant's job can include some or all of the following: providing the local institutional review board (IRB) with the documentation necessary to obtain its permission to conduct the study, assisting with study start-up, identifying eligible patients, obtaining consent from them or their families, administering study treatment(s), collecting and statistically analyzing data, maintaining and updating data files during followup, and communicating with the IRB, as well as the sponsor and CRO.
Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects the reaction rate of the enzyme. In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway.
== Gene regulation == MMP3 itself can enter in nuclei of cells and regulate target gene such as CTGF/CCN2 gene. Expression of MMP3 is primarily regulated at the level of transcription, where the promoter of the gene responds to various stimuli, including growth factors, cytokines, tumor promoters, and oncogene products. A polymorphism in the promoter of the MMP3 gene was first reported in 1995. The polymorphism is caused by a variation in the number of adenosines located at position -1171 relative to the transcription start site, resulting in one allele having five adenosines (5A) and the other allele having six adenosines (6A). In vitro promoter functional analyses showed that the 5A allele had greater promoter activities as compared with the 6A allele. It has been shown in different studies that individuals carrying the 5A allele have increased susceptibility to diseases attributed to increased MMP expression, such as acute myocardial infarction and abdominal aortic aneurysm. On the other hand, the 6A allele has been found to be associated with diseases characterized by insufficient MMP-3 expression due to a lower promoter activity of the 6A allele, such as progressive coronary atherosclerosis. The -1171 5A/6A variant has also been associated with congenital anomalies such as cleft lip and palate, where individuals with cleft lip/palate presented significantly more 6A/6A genotypes than controls.
== History == GABA is the principal inhibitory neurotransmitter in mammalian brains. By the early 1970s, it was appreciated that there are two main classes of GABA receptors, GABAA and GABAB and also that baclofen was an agonist of GABAB receptors. Gabapentin was designed, synthesized, and tested in mice by researchers at the pharmaceutical company Goedecke AG in Freiburg, Germany (a subsidiary of Parke-Davis). It was meant to be a structural analog of the neurotransmitter GABA that could more easily cross the blood–brain barrier. It was first synthesized in 1974/75 and described in 1975 by Satzinger and Hartenstein. The first pharmacology findings published were sedating properties and prevention of seizures in mice evoked by the GABA antagonist, thiosemicarbazide. Shortly after, gabapentin was shown in vitro to reduce the release of the neurotransmitter dopamine from slices of rat caudate nucleus (striatum). This study provided evidence that the action of gabapentin, unlike baclofen, did not arise from the GABAB receptor. Initial clinical trials using small numbers of subjects were for treatment of spasticity and migraine but neither study had statistical power to allow conclusions. In 1987, the first positive results with gabapentin were obtained in a clinical trial using three dose groups versus pre-treatment seizure frequency for 75 days, as add-on treatment in patients who still had seizures despite taking other medications. Under the brand name Neurontin, it was first approved in the United Kingdom in May 1993, for the treatment of refractory epilepsy.
Sources: en.wikipedia.org
Nemolizumab, sold under the brand name Nemluvio, is a humanized monoclonal antibody used for the treatment of prurigo nodularis and atopic dermatitis. It is a monoclonal antibody that blocks the interleukin-31 receptor A. Nemolizumab is humanized IgG2 monoclonal antibody that inhibits interleukin-31 signaling by binding selectively to interleukin-31 receptor alpha. It is an interleukin-31 receptor antagonist. IL-31 is a cytokine involved in pruritus, inflammation, epidermal dysregulation and fibrosis. By inhibiting IL-31-induced responses, nemolizumab prevents the release of proinflammatory cytokines and chemokines. Nemolizumab was approved for medical use in the United States in August 2024, and in the European Union in February 2025. The US Food and Drug Administration (FDA) considers it to be a first-in-class medication.
The aldehyde groups of the triose sugars are oxidised, and inorganic phosphate is added to them, forming 1,3-bisphosphoglycerate. The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give NADH + H+ for each triose. Hydrogen atom and ion balance and charge balance are both maintained because the phosphate (Pi) group actually exists in the form of a hydrogen phosphate anion (HPO2−4), which dissociates to contribute the extra H+ ion and gives a net charge of −3 on both sides. Here, arsenate ([AsO4]3−), an anion akin to inorganic phosphate may replace phosphate as a substrate to form 1-arseno-3-phosphoglycerate. This, however, is unstable and readily hydrolyzes to form 3-phosphoglycerate, the intermediate in the next step of the pathway. As a consequence of bypassing this step, the molecule of ATP generated from 1–3 bisphosphoglycerate in the next reaction will not be made, even though the reaction proceeds. As a result, arsenate is an uncoupler of glycolysis.
{\displaystyle {\ce {AB + C <=> AC + B}};\quad K_{\rm {B,C}}={\frac {{\ce {[AC][B]}}}{{\ce {[AB][C]}}}}={\frac {K_{\rm {AC}}{\ce {[A][B][C]}}}{K_{\rm {AB}}{\ce {[A][B][C]}}}}={\frac {K_{\rm {AC}}}{K_{\rm {AB}}}}}
In folklore and mythology, exhumation has also been frequently associated with the performance of rites to banish undead manifestations. A historical example is the 1892 Mercy Brown vampire incident of Rhode Island.
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.
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.