The short version of stationary phase fits in a sentence. The long version — which is the one that helps — is below.
Reviewed 2026-06-09. Anything still debated is marked as such rather than presented as settled.
Developing an HPLC test begins with defining the analytes, matrix, and required reporting limits. Chemists select a separation mode, column chemistry, mobile phase composition, flow rate, and detection wavelength or mass transition. Experiments then adjust these variables to achieve adequate retention, resolution, and peak shape. System suitability tests confirm that the instrument and method perform consistently before sample analysis. Without suitable resolution, quantitative results may be unreliable. Preliminary runs often use scouting gradients to locate retention windows.
Validation establishes that a method is suitable for its intended purpose. Typical parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantification, robustness, and stability of standards and samples. Acceptance criteria are defined in advance, and results are documented in a validation report. Regulatory guidance for pharmaceuticals, foods, and environmental testing differs, so the applicable framework must be identified. Ongoing verification uses control samples and trend charts after validation. Method transfer to another laboratory may require partial revalidation.
Routine quality control includes blanks, duplicates, spiked samples, and certified reference materials. Calibration curves are prepared with standards at several concentrations, and the detector response is checked for linearity. Carryover, column aging, mobile phase evaporation, and temperature drift can shift retention times or peak areas. Maintenance such as replacing seals, filters, and columns helps prevent failures. Records of injections, integration, and deviations support traceability. Audits may request raw data and instrument logs for each batch.
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 |
|---|---|---|
| Accuracy | Closeness to true value | Often assessed by recovery of spiked samples |
| Precision | Agreement among repeated measurements | Often reported as relative standard deviation |
| Specificity | Ability to measure analyte without interference | Must separate analyte from impurities and matrix |
| Linearity | Proportional detector response | Evaluated across a defined concentration range |
| Robustness | Resistance to small method changes | Tests flow rate, pH, temperature, and mobile phase composition |
High-performance liquid chromatography testing separates components of a liquid sample by forcing a mobile phase through a packed column. The stationary phase inside the column interacts with analytes to different degrees, so each compound exits at a characteristic retention time. A pump delivers solvent at controlled flow and pressure, while an injector introduces a precise sample volume. Detectors such as ultraviolet-visible, fluorescence, refractive index, or mass spectrometric instruments record the separated bands. The resulting chromatogram provides qualitative and quantitative information about the mixture.
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.
Many pathways and signals lead to apoptosis, but these converge on a single mechanism that actually causes the death of the cell. After a cell receives stimulus, it undergoes organized degradation of cellular organelles by activated proteolytic caspases. In addition to the destruction of cellular organelles, mRNA is rapidly and globally degraded by a mechanism that is not yet fully characterized. mRNA decay is triggered very early in apoptosis. A cell undergoing apoptosis shows a series of characteristic morphological changes. Early alterations include:
The general detection scheme involves: Pneumatic nebulization of mobile phase from the analytical column forming an aerosol. Aerosol conditioning to remove large droplets. Evaporation of solvent from the droplets to form dried particles. Particle charging using an ion jet formed via corona discharge. Particle selection – an ion trap is used to excess ions and high mobility charged particles. Measurement of the aggregate charge of aerosol particles using a filter/electrometer. The CAD like other aerosol detectors, can only be used with volatile mobile phases. For an analyte to be detected it must be less volatile than the mobile phase. More detailed information on how CAD works can be found on the Charged Aerosol Detection for Liquid Chromatography Resource Center.
Similar structures include the 310 helix (i + 3 → i hydrogen bonding) and the π-helix (i + 5 → i hydrogen bonding). The α-helix can be described as a 3.613 helix, since the i + 4 spacing adds three more atoms to the H-bonded loop compared to the tighter 310 helix, and on average, 3.6 amino acids are involved in one ring of α-helix. The subscripts refer to the number of atoms (including the hydrogen) in the closed loop formed by the hydrogen bond.
Sources: en.wikipedia.org
Analysis of molecular variance (AMOVA), is a statistical model for the molecular algorithm in a single species, typically biological. The name and model are inspired by ANOVA. The method was developed by Laurent Excoffier, Peter Smouse and Joseph Quattro at Rutgers University in 1992. Since developing AMOVA, Excoffier has written a program for running such analyses. This program, which runs on Windows, is called Arlequin and is freely available on Excoffier's website. There are also implementations in R language in the ade4 and the pegas packages, both available on CRAN (Comprehensive R Archive Network). Another implementation is in Info-Gen, which also runs on Windows. The student version is free and fully functional. Native language of the application is Spanish but an English version is also available. An additional free statistical package, GenAlEx, is geared toward teaching as well as research and allows for complex genetic analyses to be employed and compared within the commonly used Microsoft Excel interface. This software allows for calculation of analyses such as AMOVA, as well as comparisons with other types of closely related statistics including F-statistics and Shannon's index, and more.
In over one hundred years of implementation, aviation safety has improved considerably. In modern times, two major manufacturers still produce heavy passenger aircraft for the civilian market: Boeing in the United States, and the European company Airbus. Both of these manufacturers place a huge emphasis on the use of aviation safety equipment, now a billion-dollar industry in its own right; safety is a key selling point for these companies, as they recognize that a poor safety record in the aviation industry is a threat to corporate survival. Some major safety devices now required in commercial aircraft are:
The United States Food and Drug Administration and the European Medicines Agency have approved weight loss medications for adults with either a body-mass index (BMI) of at least 30, or a body-mass index of at least 27 with at least one weight-related comorbidity. This patient population is considered to have sufficiently high baseline health risks to justify the use of anti-obesity medication. The American Academy of Pediatrics had not previously supported the use of weight loss medication in adolescents but issued new guidelines in 2023, which recommend considering the use of weight loss medication in some overweight children aged 12 or older. The European Medicines Agency has approved semaglutide for children aged 12 or older who have a BMI in the 95 percentile for their age and a weight of at least 60 kilograms (130 lb). However, GLP-1 agonists may not be cost effective in this population.
Sources: en.wikipedia.org
There are three amino acids with side chains that are cations at neutral pH: arginine (Arg, R), lysine (Lys, K) and histidine (His, H). Arginine has a charged guanidino group and lysine a charged alkyl amino group, and are fully protonated at pH 7. Histidine's imidazole group has a pKa of 6.0, and is only around 10% protonated at neutral pH. Because histidine is easily found in its basic and conjugate acid forms it often participates in catalytic proton transfers in enzyme reactions.
The main method of treatment for potential death from signaling-related diseases involves either increasing or decreasing the susceptibility of apoptosis in diseased cells, depending on whether the disease is caused by either the inhibition of or excess apoptosis. For instance, treatments aim to restore apoptosis to treat diseases with deficient cell death and to increase the apoptotic threshold to treat diseases involved with excessive cell death. To stimulate apoptosis, one can increase the number of death receptor ligands (such as TNF or TRAIL), antagonize the anti-apoptotic Bcl-2 pathway, or introduce Smac mimetics to inhibit the inhibitor (IAPs). The addition of agents such as Herceptin, Iressa, or Gleevec works to stop cells from cycling and causes apoptosis activation by blocking growth and survival signaling further upstream. Finally, adding p53-MDM2 complexes displaces p53 and activates the p53 pathway, leading to cell cycle arrest and apoptosis. Many different methods can be used either to stimulate or to inhibit apoptosis in various places along the death signaling pathway. Apoptosis is a multi-step, multi-pathway cell-death programme that is inherent in every cell of the body. In cancer, the apoptosis cell-division ratio is altered. Cancer treatment by chemotherapy and irradiation kills target cells primarily by inducing apoptosis.
Cryptic binding sites are the binding sites that are transiently formed in an apo form or that are induced by ligand binding. Considering the cryptic binding sites increases the size of the potentially "druggable" human proteome from ~40% to ~78% of disease-associated proteins. The binding sites have been investigated by: support vector machine applied to "CryptoSite" data set, Extension of "CryptoSite" data set, long timescale molecular dynamics simulation with Markov state model and with biophysical experiments, and cryptic-site index that is based on relative accessible surface area.
Therefore, Akt promotes G1 phase progression in a positive feedback loop. Akt promotes cyclin D1 translation via indirect activation of mTOR. mTOR increases translation of cyclin D1 by activating ribosomal protein S6K, and inhibiting eukaryotic translation initiation factor 4E-binding protein (4E-BP), thus increasing eIF4e activity. Akt both indirectly and directly regulates cyclin-dependent kinase (CDK) inhibitors p21Cip1 and p27Kip1, allowing cell cycle progression. Akt phosphorylates p27Kip1 at Thr157, preventing its nuclear import. In addition, Akt phosphorylates Thr145 and Ser146 of p21Cip1, preventing PCNA binding and decreasing stability. Akt phosphorylation of Foxo transcription factors also affects the cell cycle, as inhibitory phosphorylation of FoxO4 (also named AFX) prevents p27 gene expression. Akt phosphorylates many proteins involved in polymerisation and stabilisation of the actin cytoskeleton. In normal cells, this can either increase the stability of cytoskeleton components or promote migration via remodelling. Examples are listed below:
Sources: en.wikipedia.org
System suitability is a set of checks performed before and during a run to confirm that the instrument, column, and method work as expected. Common checks include resolution, tailing factor, theoretical plates, and relative standard deviation of replicate injections. Failure triggers troubleshooting or method adjustment.
Validation demonstrates that a method produces reliable results for a defined purpose. It documents performance limits and acceptance criteria. Regulated industries require validation before routine testing of products or samples.
Retention time shifts can arise from changes in mobile phase composition, pH, temperature, column age, or flow rate. Contamination or worn seals may also alter pressure and delivery. Systematic checks of these factors help identify the cause.
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.