If you have been reading about reversed-phase 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 2026-04-29. Where a claim depends on a specific study, the study is described rather than over-claimed.
Routine quality control monitors retention time shifts, baseline noise, system pressure, and peak shape. Trends can reveal column aging, mobile phase preparation errors, detector drift, or sample degradation. Corrective actions may include replacing the column, preparing fresh mobile phase, or recalibrating the detector. Stability testing often uses HPLC to measure parent compound loss and degradation product formation. Open questions remain about how accelerated stability results extrapolate to long-term storage under varied conditions.
Quality control for HPLC testing combines scheduled checks, documented procedures, and review of results. Before sample analysis, system suitability testing confirms that the instrument, column, and method meet predefined criteria. Common criteria include resolution between critical peaks, retention time precision, peak tailing, and theoretical plate count. Failure triggers investigation before results are reported. Records link raw data, calculations, instrument logs, and analyst identity to each batch, supporting audits and repeat analysis.
Method validation evaluates accuracy, precision, specificity, linearity, range, detection limit, quantitation limit, and robustness. Regulatory guidance for pharmaceuticals, foods, and environmental testing defines expected documentation and acceptance criteria. Verification confirms that a validated method works in a specific laboratory with its own instruments and reagents. Calibration curves use reference standards with known purity and traceability, while measurement uncertainty is estimated from validation data, control charts, and collaborative studies. The scope of validation depends on the method's intended use.
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.
| Property | Value | Notes |
|---|---|---|
| Retention time RSD | ≤1% for five replicate injections | Typical criterion; method-specific limits apply. |
| Resolution | ≥1.5 between critical pair | Baseline separation is generally desired. |
| Tailing factor | ≤2.0 | Measures peak symmetry. |
| Theoretical plates | ≥2000 per column | Method-dependent; higher values indicate greater efficiency. |
| Peak area RSD | ≤2% for replicate injections | Reflects autosampler and detector precision. |
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.
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.
2-Pentanol (IUPAC name: pentan-2-ol; also called sec-amyl alcohol) is an organic chemical compound. It is used as a solvent and an intermediate in the manufacturing of other chemicals. 2-Pentanol is a component of many mixtures of amyl alcohols sold industrially. 2-Pentanol is chiral and thus can be obtained as either of two stereoisomers designated as (R)-(−)-2-pentanol and (S)-(+)-2-pentanol. 2-Pentanol has been detected in fresh bananas by gas chromatography–mass spectrometry, at an abundance of 14.26±2.63 ppm. 2-Pentanol can be manufactured by hydration of pentene. sec-Amyl acetate 2-Methyl-2-butanol 3-Pentanol Hedonal 1-Octanol
A carboxylic acid has the general formula R-C(O)OH, where R is an organic radical. The carboxyl group -C(O)OH contains a carbonyl group, C=O, and a hydroxyl group, O-H. Acetic acid (CH3COOH) Citric acid (C6H8O7) Formic acid (HCOOH) Gluconic acid HOCH2-(CHOH)4-COOH Lactic acid (CH3-CHOH-COOH) Oxalic acid (HOOC-COOH) Tartaric acid (HOOC-CHOH-CHOH-COOH) Halogenation at alpha position increases acid strength, so that the following acids are all stronger than acetic acid. Fluoroacetic acid Trifluoroacetic acid Chloroacetic acid Dichloroacetic acid Trichloroacetic acid Normal carboxylic acids are the direct union of a carbonyl group and a hydroxyl group. In vinylogous carboxylic acids, a carbon-carbon double bond separates the carbonyl and hydroxyl groups. Ascorbic acid Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA) Listing of strengths of common acids and bases Zumdahl, Steven S. (1997). Chemistry (4th ed.). Boston: Houghton Mifflin. ISBN 9780669417944. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. (2004). Organic Chemistry Volume I. Mason, OH: Cengage Learning. ISBN 0759347271.
The predecessor to the CAD, termed an evaporative electrical detector, was first described by Kaufman in 2002 at TSI Inc in US patent 6,568,245 and was based on the coupling of liquid chromatographic approaches to TSI's electrical aerosol measurement (EAM) technology. At around the same time Dixon and Peterson at California State University were investigating the coupling of liquid chromatography to an earlier version of TSI's EAM technology, which they called an aerosol charge detector. Subsequent collaboration between TSI and ESA Biosciences Inc. (now part of Thermo Fisher Scientific), led to the first commercial instrument, the Corona CAD, which received both the Pittsburgh Conference Silver Pittcon Editor's Award (2005) and R&D 100 award (2005). Continued research and engineering improvements in product design resulted in CADs with ever increasing capabilities. The newest iterations of the CAD are the Thermo Scientific Corona Veo Charged Aerosol Detector, Corona Veo RS Charged Aerosol Detector and Thermo Scientific Vanquish Charged Aerosol Detectors.
The amino acids that make up a particular helix can be plotted on a helical wheel, a representation that illustrates the orientations of the constituent amino acids (see the article for leucine zipper for such a diagram). Often in globular proteins, as well as in specialized structures such as coiled-coils and leucine zippers, an α-helix will exhibit two "faces" – one containing predominantly hydrophobic amino acids oriented toward the interior of the protein, in the hydrophobic core, and one containing predominantly polar amino acids oriented toward the solvent-exposed surface of the protein. Changes in binding orientation also occur for facially-organized oligopeptides. This pattern is especially common in antimicrobial peptides, and many models have been devised to describe how this relates to their function. Common to many of them is that the hydrophobic face of the antimicrobial peptide forms pores in the plasma membrane after associating with the fatty chains at the membrane core.
Sources: en.wikipedia.org
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.
Post-crash survivability is the chance that drivers and passengers survive a crash after it occurs. Technology such as Advanced Automatic Collision Notification can automatically place calls to emergency services and send information about a vehicle collision. Cars are much more dangerous to pedestrians than they are to drivers and passengers. Two-thirds of 1.3 million yearly auto related deaths are pedestrians. Since at least the early 1970s, attention has also been given to vehicle design regarding the safety of pedestrians in car-pedestrian collisions. Proposals in Europe would require cars sold there to have a minimum/maximum hood (bonnet) height. From 2006, the use of "bull bars", a fashion on 4x4s and SUVs, became illegal in the European Union, after having been banned on all new cars in 2002. Recent research has explored the use of multimodal sensor fusion to improve pedestrian detection in challenging conditions, including low-light environments and unusual pedestrian postures such as prone or fallen individuals.
Transmembrane protein 125 is a protein that, in humans, is encoded by the TMEM125 gene. It has 4 transmembrane domains and is expressed in the lungs, thyroid, pancreas, intestines, spinal cord, and brain. Though its function is currently poorly understood by the scientific community, research indicates it may be involved in colorectal and lung cancer networks. Additionally, it was identified as a cell adhesion molecule in oligodendrocytes, suggesting it may play a role in neuron myelination. The TMEM125 gene has no aliases, except for its encoded protein’s name. Its cytogenic location is at 1p34.2 on the plus strand and it spans from bases 43,272,723 to 43,273,379. TMEM125 comprises four exons. Five TMEM125 promoters were identified by Genomatix Gene2Promoter. The primary promoter (NM_001320244) is 1881 bp in length. It consists of binding sites for fork head domain factors and zinc finger transcription factors.
Some anesthetics and chemotherapy drugs are injected intrathecally into the subarachnoid space, where they spread around CSF, meaning substances that cannot cross the blood–brain barrier can still be active throughout the central nervous system. Baricity refers to the density of a substance compared to the density of human cerebrospinal fluid and is used in regional anesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Liquorpheresis is the process of filtering the CSF in order to clear it from endogen or exogen pathogens. It can be achieved by means of fully implantable or extracorporeal devices, though the technique remains experimental today.
14-3-3 proteins play an isoform-specific role in class switch recombination. They are believed to interact with the protein activation-induced cytidine deaminase in mediating class switch recombination. Phosphorylation of Cdc25C by CDS1 and CHEK1 creates a binding site for the 14-3-3 family of phosphoserine binding proteins. Binding of 14-3-3 has little effect on Cdc25C activity, and it is believed that 14-3-3 regulates Cdc25C by sequestering it to the cytoplasm, thereby preventing the interactions with CycB-Cdk1 that are localized to the nucleus at the G2/M transition. The eta isoform, YWHAH, is reported to be a biomarker (in synovial fluid) for rheumatoid arthritis. In a systematic review, 14-3-3η has been described as a welcome addition to the rheumatology field. The authors indicate that the serum based 14-3-3η marker is additive to the armamentarium of existing tools available to clinicians, and that there is adequate clinical evidence to support its clinical benefits in the management of patients diagnosed with rheumatoid arthritis (RA). 14-3-3 proteins bind to and sequester the transcriptional coregulators YAP/TAZ to the cytoplasm, inhibiting their function.
Sources: en.wikipedia.org
RGD is the most widely used of a larger class of cell adhesive peptides. These short amino acid sequences are the minimum motif of a larger protein that is necessary for binding to a cell surface receptor that drives cell adhesion. The majority (89%) of published studies on biomaterials functionalized with cell adhesive peptides use RGD, whereas IKVAV and YIGSR are used in 6%, and 4% of those studies, respectively. Cell adhesive peptides isolated from fibronectin include RGD, RGDS, PHSRN, and REDV. YIGSR and IKVAV are isolated from laminin, whereas DGEA and GFOGER/GFPGER are isolated from collagen. Artificial amino acid sequences, which bear no biological similarity to ECM proteins, have also been synthesized, and include the α5β1-specific peptide RRETAWA.
In many organisms including Xenopus and Drosophila, the midblastula transition usually occurs after a particular number of cell divisions for a given species, and is defined by the ending of the synchronous cell division cycles of the early blastula development, and the lengthening of the cell cycles by the addition of the G1 and G2 phases. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle. The addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created. This transition begins the growth in size of the organism. The mid-blastula transition is also characterized by a marked increase in transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as SMAUG in Drosophila or by microRNA. These two processes shift the control of the embryo from the maternal mRNA to the nuclei.
Reactions of acids are often generalized in the form HA ⇌ H+ + A−, where HA represents the acid and A− is the conjugate base. This reaction is referred to as protolysis. The protonated form (HA) of an acid is also sometimes referred to as the free acid. Acid–base conjugate pairs differ by one proton, and can be interconverted by the addition or removal of a proton (protonation and deprotonation, respectively). The acid can be the charged species and the conjugate base can be neutral in which case the generalized reaction scheme could be written as HA+ ⇌ H+ + A. In solution there exists an equilibrium between the acid and its conjugate base. The equilibrium constant K is an expression of the equilibrium concentrations of the molecules or the ions in solution. Brackets indicate concentration, such that [H2O] means the concentration of H2O. The acid dissociation constant Ka is generally used in the context of acid–base reactions. The numerical value of Ka is equal to the product (multiplication) of the concentrations of the products divided by the concentration of the reactants, where the reactant is the acid (HA) and the products are the conjugate base and H+.
Announced on 8 May 2024, AlphaFold 3 was co-developed by Google DeepMind and Isomorphic Labs, both subsidiaries of Alphabet. AlphaFold 3 is not limited to proteins, as it can also predict the structures of protein complexes with DNA, RNA, post-translational modifications and selected ligands and ions. AlphaFold 3 introduces the "Pairformer," a deep learning architecture inspired by the transformer, which is considered similar to, but simpler than, the Evoformer used in AlphaFold 2. The Pairformer module's initial predictions are refined by a diffusion model. This model begins with a cloud of atoms and iteratively refines their positions, guided by the Pairformer's output, to generate a 3D representation of the molecular structure. The AlphaFold server was created to provide free access to AlphaFold 3 for non-commercial research. As of November 2025, the AlphaFold 3 research paper has been directly cited more than 9,000 times.
VIP is highly localised in lungs (70%) and binds with alveolar type II (AT II) cells via VPAC1. The biological (vasodilator) activity of vasoactive intestinal peptide (VIP) was discovered in the lungs before the peptide was isolated and chemical identity characterized from intestine. VIP levels are also considerably high in the brain and the gut. It is localized in key sites in the lung, has potent activities on its major functions, and appears to play an important role in pulmonary physiology and disease. The principal localization of VIP-containing neurons in the tracheobronchial tree is in the smooth muscle layer, around submucosal mucous glands and in the walls of pulmonary and bronchial arteries. Immunoreactive VIP is also present in neuronal cell bodies forming microglia that provide a source of intrinsic innervation of pulmonary structures.
Sources: en.wikipedia.org
System suitability is typically performed before each batch or according to the validated method and laboratory procedure. Some long runs include periodic checks during analysis. The required frequency depends on regulatory expectations and method performance.
Retention time drift can result from changes in mobile phase composition, column temperature, pump flow, or column age. A gradual shift often points to column degradation. A sudden shift may indicate a leak, mixing error, or incorrect mobile phase.
Retention time alone cannot confirm identity because different compounds may elute at similar times. Coupling HPLC with mass spectrometry or comparing against authenticated standards increases confidence. Confirmation usually requires orthogonal data.
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.