reversed-phase raises a handful of sensible questions. This page answers them in order, starting with the fundamentals and moving to applications.
Reviewed 2025-11-13. Anything still debated is marked as such rather than presented as settled.
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 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.
| Property | Value | Notes |
|---|---|---|
| Column particle size | 3–5 µm for conventional HPLC; sub-2 µm for UHPLC | Smaller particles increase backpressure and efficiency. |
| Typical flow rate | 0.5–2.0 mL/min for a 4.6 mm internal diameter column | Flow scales with column diameter and particle size. |
| UV detection wavelength | 190–400 nm | Selection depends on analyte chromophore. |
| Column temperature | 25–40 °C | Temperature affects retention, selectivity, and pressure. |
| Injection volume | 1–20 µL | Larger volumes may distort early-eluting peaks. |
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.
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.
1993/654) Rent Act 1977 (Forms etc.) (Amendment) Regulations 1993 (S.I. 1993/655) Rent Book (Forms of Notice) (Amendment) Regulations 1993 (S.I. 1993/656) Assured Tenancies and Agricultural Occupancies (Rent Information) (Amendment) Order 1993 (S.I. 1993/657) Local Government Finance (Housing) (Consequential Amendments) (Scotland) Order 1993 (S.I. 1993/658) Rent Assessment Committees (Scotland) (Consequential Amendments) Regulations 1993 (S.I. 1993/659) Child Support (Amendments to Primary Legislation) (Scotland) Order 1993 (S.I. 1993/660) A66 Trunk Road (Stainmore-Banks Gate De-Trunking) Order 1993 (S.I. 1993/661) A66 Trunk Road (Stainmore-Banks Gate Improvement) Order 1993 (S.I. 1993/662) County of Wiltshire (Electoral Arrangements) Order 1993 (S.I. 1993/679) M621 To M1 Link Roads (and Connecting Roads) Scheme 1993 (S.I. 1993/681) Nuclear Installations (Application of Security Provisions) Order 1993 (S.I. 1993/687) Council Tax Benefit (General) Amendment Regulations 1993 (S.I. 1993/688) Council Tax Benefit (Permitted Total) Order 1993 (S.I. 1993/689) Adoption (Designation of Overseas Adoptions) (Variation) Order 1993 (S.I. 1993/690) Cheshire, Lancashire and Merseyside (County and Metropolitan Borough Boundaries) Order 1993 (S.I. 1993/691) War Pensions (Miscellaneous Amendments) Order 1993 (S.I. 1993/692) Guy's and St Thomas' National Health Service Trust (Establishment) Order 1993 (S.I. 1993/693) Lewisham Hospital National Health Service Trust (Establishment) Order 1993 (S.I. 1993/694) St Thomas' Hospital National Health Service Trust Dissolution Order 1993 (S.I.
The organic compound citrulline is a non-essential α-amino acid. Its name is derived from citrullus, the Latin word for watermelon. Although named and described by gastroenterologists since the late 19th century, it was first isolated from watermelon in 1914 by Japanese researchers Yatarō Koga and Ryō Ōtake, and further codified in 1930. Citrulline has the formula H2NC(O)NH(CH2)3CH(NH2)CO2H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia by converting it into urea. Citrulline is also produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase.
=== RNAs involved in protein synthesis === Messenger RNA (mRNA) carries information about a protein sequence to the ribosomes, the protein synthesis factories in the cell. It is coded so that every three nucleotides (a codon) corresponds to one amino acid. In eukaryotic cells, once precursor mRNA (pre-mRNA) has been transcribed from DNA, it is processed to mature mRNA. This removes its introns—non-coding sections of the pre-mRNA. The mRNA is then exported from the nucleus to the cytoplasm, where it is bound to ribosomes and translated into its corresponding protein form with the help of tRNA. In prokaryotic cells, which do not have nucleus and cytoplasm compartments, mRNA can bind to ribosomes while it is being transcribed from DNA. After a certain amount of time, the message degrades into its component nucleotides with the assistance of ribonucleases. Transfer RNA (tRNA) is a small RNA chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has sites for amino acid attachment and an anticodon region for codon recognition that binds to a specific sequence on the messenger RNA chain through hydrogen bonding.
Sources: en.wikipedia.org
=== Periodicals === Links (magazine), a US golf magazine The Link (newspaper), a student newspaper at Concordia University, Canada The Link, the newsletter of the organization Americans for Middle East Understanding
Other uses that have been reported for this bacterium include the production of pickled vegetables, beer or wine, some breads, and other fermented foodstuffs like soymilk kefir, buttermilk, and others. L. lactis is one of the best characterized low GC Gram positive bacteria with detailed knowledge on genetics, metabolism and biodiversity. L. lactis is mainly isolated from either the dairy environment, or plant material. Dairy isolates are suggested to have evolved from plant isolates through a process in which genes without benefit in the rich milk were lost or downregulated. This process, called genome erosion or reductive evolution, has been described in several other lactic acid bacteria. The proposed transition from the plant to the dairy environment was reproduced in the laboratory through experimental evolution of a plant isolate that was cultivated in milk for a prolonged period. Consistent with the results from comparative genomics (see references above), this resulted in L. lactis losing or downregulating genes that are dispensable in milk and the upregulation of peptide transport. Hundreds of novel small RNAs were identified by Meulen et al. in the genome of L. lactis MG1363. One of them, LLnc147, was shown to be involved in carbon uptake and metabolism.
Announced tests: The athletes know when they are tested. Unannounced tests: Athletes don't know when they are tested. These tests are the effective ones when they are conducted outside of the competition periods.
Sources: en.wikipedia.org
== J == Sophie E. Jackson (active from 1991). Biochemist at the University of Cambridge known for work on protein folding. Alec Jeffreys FRS (b. 1950). British biochemist and geneticist at Leicester University, known for inventing genetic fingerprinting. William Jencks FRS (foreign member) (1927–2007). American biochemist at Brandeis University, known for applying chemical mechanisms to enzyme-catalysed reactions and for his masterly book Catalysis in Chemistry and Enzymology. Member Natl. Acad. Sci. USA. Thomas H. Jukes (1906–1999). British-American biologist at UC Berkeley known for work in nutrition and molecular evolution. He was very active in denouncing pseudoscience. John Michael Jumper (b. 1985). American chemist and computer scientist at DeepMind Technologies. Nobel Prize in chemistry, 2024.
=== Flight-time equivalent dose === Natural background radiation typically increases with altitude above the earth's surface. Utilizing this phenomenon, dose resulting from radiological exposures can be expressed in units of flight-time. Flight-time equivalent dose is defined as the time spent in an aircraft at cruising altitude required to receive a radiological dose approximately equivalent to a radiological exposure such as a medical x-ray. One hour of flight-time is approximately equivalent to a dose of 0.004 millisieverts.
Very low bodyweight; Rapid weight loss, regardless of starting bodyweight; Medical comorbidities, such as infection or malabsorption; Excessive purging behaviours, including self-induced vomiting, laxative abuse, and diuretic abuse. Two of the most common indicators that RFS is occurring are low phosphate levels and low potassium levels. In these circumstances, it is recommended to start refeeding more slowly but to build up rapidly as long as RFS does not occur. Recommendations on energy requirements in the most medically compromised patients vary, from 5–10 kcal/kg/day to 1900 kcal/day. Treatment professionals must balance the risk of RFS with the risk of underfeeding, which can result in poor health outcomes for short- and long-term recovery.
Sources: en.wikipedia.org
HPLC separates and quantifies compounds in a liquid sample. Detectors produce a response proportional to the amount of a compound passing through the flow cell. Identification by retention time requires comparison with a known standard.
UHPLC uses columns with smaller particles and operates at higher pressures than conventional HPLC. These conditions can improve speed, resolution, and sensitivity. Both techniques use the same fundamental separation principles.
Validation shows that a method performs reliably for its intended purpose across a defined range. It assesses accuracy, precision, specificity, linearity, and robustness. Regulated testing often requires documented validation before routine use.
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.