retention time comes up often in conversation and rarely with the context attached. Here we lay out the basics in order, then work through the practical considerations.
Last reviewed on 2026-05-26. Where a claim depends on a specific study, the study is described rather than over-claimed.
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.
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.
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.
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.
| 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. |
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.
Routine quality control uses system suitability, blank injections, check standards, and control samples to detect drift or contamination. System suitability criteria may specify minimum resolution, maximum tailing factor, and a permitted range for repeated injections. Blank injections reveal carryover or solvent contamination, while check standards confirm calibration accuracy over a batch. Control samples with known analyte levels can show whether results remain within statistical limits. When a control result falls outside limits, the analyst investigates the cause and may invalidate affected results before repeating the batch.
Documentation and traceability are central to regulated HPLC testing. Records typically include instrument logs, column history, mobile-phase preparation, sample preparation, injection sequences, raw chromatograms, and audit trails. Electronic systems may require user access controls, time-stamped changes, and backup procedures. Training records show that analysts are qualified for assigned methods. Audits and inspections check whether written procedures match actual practice and whether deviations are documented. These controls support reproducibility and allow results to be reconstructed if questions arise later.
=== Anorexia === Interactions between MCH and chemokines/cytokines that lead to an overall decrease in MCH release and neuron excitability has been linked with infection-induced anorexia. Chemokines and cytokines often appear as the result of inflammation or infection, and they can then damage MCH neurons, which can lead to anorexia in an individual.
=== Eclampsia === Magnesium sulfate is effective in decreasing the risk that pre-eclampsia progresses to eclampsia. Intravenous magnesium sulfate is used to prevent and treat seizures of eclampsia. It reduces the systolic blood pressure but does not alter the diastolic blood pressure, so the blood perfusion to the fetus is not compromised. It is also commonly used for eclampsia where compared to diazepam or phenytoin it results in better outcomes.
Aldehyde dehydrogenase, mitochondrial is an enzyme that in humans is encoded by the ALDH2 gene located on chromosome 12. ALDH2 belongs to the aldehyde dehydrogenase family of enzymes. Aldehyde dehydrogenase is the second enzyme of the major oxidative pathway of alcohol metabolism. ALDH2 has a low Km for acetaldehyde, and is localized in mitochondrial matrix. The other liver isozyme, ALDH1, localizes to the cytosol. Most White people have both major isozymes, while approximately 36% of East Asians have the cytosolic isozyme but not a functional mitochondrial isozyme. As of 2023, it is estimated that as many as 709 million people (8.8% of the global population) carry the ALDH2*2 (E504K) mutation. A remarkably higher frequency of acute alcohol intoxication among East Asians than among Whites could be related to this absence of a catalytically active form of ALDH2. The increased exposure to acetaldehyde in individuals with the catalytically inactive form may also confer greater susceptibility to many types of cancer.
Sources: en.wikipedia.org
== Access == The library is open for academic research to anyone with a Reader Identification Card. One may not remove library items from the reading rooms or the library buildings. Most of the library's general collection of books and journals are in the closed stacks of the Jefferson and Adams Buildings; specialized collections of books and other materials are in closed stacks in all three main library buildings, or are stored off-site. Access to the closed stacks is not permitted under any circumstances, except to authorized library staff, and occasionally, to dignitaries. Only the reading room reference collections are on open shelves. Since 1902, American libraries have been able to request books and other items through interlibrary loan from the Library of Congress if these items are not readily available elsewhere. Through this system, the Library of Congress has served as a "library of last resort", according to Herbert Putnam, the librarian of Congress from 1899 to 1939. The Library of Congress lends books to other libraries with the stipulation that they be used only inside the borrowing library. In 2017, the Library of Congress began development on a reader's card for children under the age of sixteen.
Psychiatric care may be required following recovery. A liver transplant may be required if damage to the liver becomes severe. The need for transplant is often based on low blood pH, high blood lactate, poor blood clotting, or significant hepatic encephalopathy. With early treatment, liver failure is rare. Death occurs in about 0.1% of cases. Paracetamol poisoning was first described in the 1960s. Rates of poisoning vary significantly between regions of the world. In the United States more than 100,000 cases occur a year. In the United Kingdom it is the medication responsible for the greatest number of overdoses. Young children are most commonly affected. In the United States and the United Kingdom, paracetamol is the most common cause of acute liver failure.
A norepinephrine and dopamine disinhibitor (NDDI or NDD) is a drug that acts at specific sites to disinhibit downstream norepinephrine and dopamine release in the brain. Agomelatine, an antidepressant which disinhibits norepinephrine and dopamine release in the frontal cortex by antagonizing 5-HT2C receptors, was the first drug to be described as an NDDI. While many other drugs also antagonize 5-HT2C receptors to some degree or another, they tend to be very non-specific in their actions, and as a result, the term "NDDI" has generally, though not always (for instance, fluoxetine has been called an NDDI in addition to SSRI due to its (weak) blockade of 5-HT2C), been reserved for describing newer, more selective agents in which disinhibition of norepinephrine and dopamine release is their primary mechanism of action. Another drug that has been referred to as an NDDI in the medical literature is flibanserin, which is approved as a treatment for hypoactive sexual desire disorder in premenopausal women. Flibanserin disinhibits norepinephrine and dopamine release in the prefrontal cortex by activating postsynaptic 5-HT1A receptors in this area. Aside from agomelatine, fluoxetine, flibanserin, mianserin and mirtazapine, as of present, no other drugs have been described as NDDIs in the medical literature, despite the fact that many other existing drugs possess effects consistent with those of the definition of an NDDI. In any case, more drugs labeled specifically as NDDIs may be seen in the future.
Sources: en.wikipedia.org
=== Toxicity due to capping ligands === Some of the capping ligands associated with AuNPs can be toxic while others are nontoxic. In gold nanorods (AuNRs), it has been shown that a strong cytotoxicity was associated with CTAB-stabilized AuNRs at low concentration, but it is thought that free CTAB was the culprit in toxicity . Modifications that overcoat these AuNRs reduces this toxicity in human colon cancer cells (HT-29) by preventing CTAB molecules from desorbing from the AuNRs back into the solution. Ligand toxicity can also be seen in AuNPs. Compared to the 90% toxicity of HAuCl4 at the same concentration, AuNPs with carboxylate termini were shown to be non-toxic. Large AuNPs conjugated with biotin, cysteine, citrate, and glucose were not toxic in human leukemia cells (K562) for concentrations up to 0.25 M. Also, citrate-capped gold nanospheres (AuNSs) have been proven to be compatible with human blood and did not cause platelet aggregation or an immune response. However, citrate-capped gold nanoparticles sizes 8-37 nm were found to be lethally toxic for mice, causing shorter lifespans, severe sickness, loss of appetite and weight, hair discoloration, and damage to the liver, spleen, and lungs; gold nanoparticles accumulated in the spleen and liver after traveling a section of the immune system. There are mixed-views for polyethylene glycol (PEG)-modified AuNPs. These AuNPs were found to be toxic in mouse liver by injection, causing cell death and minor inflammation.
==== Other actions ==== Phenylpiracetam binds to α4β2 nicotinic acetylcholine receptors in the mouse brain cortex with an IC50Tooltip half-maximal inhibitory concentration of 5.86 μM. Racetams generally, but including phenylpiracetam, have been described as AMPA receptor potentiators.
Indications for the surgical treatment of aortic dissection include an acute proximal aortic dissection and an acute distal aortic dissection with one or more complications. Complications include compromise of a vital organ, rupture or impending rupture of the aorta, retrograde dissection into the ascending aorta. These are more common with a history of Marfan syndrome or Ehlers–Danlos syndrome. The objective in the surgical management of aortic dissection is to resect (remove) the most severely damaged segments of the aorta and to obliterate the entry of blood into the false lumen (both at the initial intimal tear and any secondary tears along the vessel). While excision of the intimal tear may be performed, it does not significantly change mortality. The particular treatment used depends on the segment or segments of the aorta involved. Some treatments are:
Several neurotoxins, both natural and synthetic, function by blocking the action potential. Tetrodotoxin from the pufferfish and saxitoxin from the Gonyaulax (the dinoflagellate genus responsible for "red tides") block action potentials by inhibiting the voltage-sensitive sodium channel; similarly, dendrotoxin from the black mamba snake inhibits the voltage-sensitive potassium channel. Such inhibitors of ion channels serve an important research purpose, by allowing scientists to "turn off" specific channels at will, thus isolating the other channels' contributions; they can also be useful in purifying ion channels by affinity chromatography or in assaying their concentration. However, such inhibitors also make effective neurotoxins, and have been considered for use as chemical weapons. Neurotoxins aimed at the ion channels of insects have been effective insecticides; one example is the synthetic permethrin, which prolongs the activation of the sodium channels involved in action potentials. The ion channels of insects are sufficiently different from their human counterparts that there are few side effects in humans.
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.
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.