This is a working overview of retention time, written for readers who want more than a one-paragraph summary but less than a textbook.
Reviewed 2026-05-22. Anything still debated is marked as such rather than presented as settled.
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.
Key performance measures include retention time, peak area, peak height, resolution, tailing factor, and plate count. Retention time helps identify a peak under fixed conditions, but confirmation often requires a second method or detector. Peak area and height relate to concentration through calibration curves, which may be linear or nonlinear depending on the detector response. Resolution describes separation between adjacent peaks, while tailing factor and plate count describe peak shape and column efficiency. Performance checks verify these values before and during a run to confirm that the instrument is performing within limits.
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, 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.
| Property | Value | Notes |
|---|---|---|
| Separation mode | Reversed-phase | Common for polar and moderately polar analytes |
| Typical column length | 100-250 mm | Shorter columns can reduce run time |
| Particle size | 3-5 micrometers | Smaller particles improve efficiency but raise pressure |
| Flow rate | 0.5-2.0 mL/min | Depends on column dimensions and pressure limits |
| Detection | UV-Vis absorbance | Widely used for compounds with chromophores |
Instrumentation includes a solvent delivery system, an autosampler, a column oven, and one or more detectors. Reversed-phase columns with chemically modified silica are widely used, but normal-phase, ion-exchange, size-exclusion, and affinity modes exist for specific separations. Detectors may rely on ultraviolet absorbance, fluorescence, refractive index, or mass spectrometry. Column temperature, mobile phase composition, and flow rate are adjusted to improve resolution. System pressure is monitored because rising pressure can indicate column blockage or deteriorating packing.
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.
HPLC testing is an analytical technique used to separate, identify, and quantify components in a liquid sample. It relies on a pressurized mobile phase that carries the sample through a column packed with stationary phase. Different compounds travel at different rates because of interactions with the stationary and mobile phases. The resulting signal versus time is a chromatogram. Peak position indicates identity under specified conditions, while peak area or height relates to amount.
Laboratories apply HPLC testing across pharmaceutical, food, environmental, and industrial chemistry. The method can measure active ingredients, impurities, additives, preservatives, and degradation products. Sample preparation often includes dilution, filtration, and sometimes extraction or derivatization. The choice of column, mobile phase, pH, temperature, and detector depends on the analytes and matrix. Results are compared with reference standards to assign identity and concentration. Method suitability is judged by resolution, precision, and accuracy.
general studies of protein structure and properties (because it is available in large quantities). studies of serpin structure and function (the fact that ovalbumin does not inhibit proteases means that by comparing its structure with that of inhibitory serpins, the structural characteristics required for inhibition can be determined). proteomics (chicken egg ovalbumin is commonly used as a molecular weight marker for calibrating electrophoresis gels). immunology (commonly used to stimulate an allergic reaction in test subjects; e.g., established model allergen for airway hyper-responsiveness, AHR). (For in vivo and in vitro studies based on ovalbumin it is important that the endotoxin content is less than 1 EU/mg.)
== History == Myxedema was first treated successfully in 1891 when George Redmayne Murray diagnosed a 46-year-old woman with the disease. He prescribed an extract from sheep thyroid. The patient improved significantly within a few weeks and lived another 28 years while taking the sheep thyroid extract.
While there were well-known gyms throughout the country during the 1950s (such as Vince's Gym in North Hollywood, California, and Vic Tanny's chain gyms), there were still segments of the United States that had no "hardcore" bodybuilding gyms until the advent of Gold's Gym in the mid-1960s. Finally, the famed Muscle Beach in Santa Monica continued its popularity as the place to be for witnessing acrobatic acts, feats of strength, and the like. The movement grew more in the 1960s with increased TV and movie exposure, as bodybuilders were typecast in popular shows and movies.
== Method == SISCAPA is an extension of the well-known gold-standard methods of stable-isotope dilution for quantitation of small molecules by mass spectrometry (MS). Rather than measure an intact protein directly by mass spectrometry, SISCAPA makes use of proteolytic digestion (e.g., with the enzyme trypsin) to cleave sample proteins into smaller peptides ideally suited to quantitation by mass spectrometry. By selecting a target peptide whose sequence occurs only in the selected target protein (a so-called “proteotypic” peptide), the target peptide can serve as a direct quantitative surrogate for the target protein (assuming the digestion process is complete, or at least reproducible). A synthetic version of the target peptide containing a stable isotope label is added in a known amount to the digested sample to serve as an internal standard (SIS). Since the target peptide and SIS are chemically indistinguishable throughout the workflow, but can be measured separately by a mass spectrometer due to the mass difference of the stable isotope label, their ratio provides the desired quantitative estimate of the target peptide amount. The SISCAPA workflow adds a specific enrichment step to the isotope dilution method in which a selected target peptide, together with its associated SIS internal standard, is captured by a sequence-specific anti-peptide antibody.
Rete pegs (also known as rete processes, rete ridges or epidermal ridges) are the epithelial extensions that project into the underlying connective tissue in both skin and mucous membranes. In the epithelium of the mouth, the attached gingiva exhibit rete pegs, while the sulcular and junctional epithelia do not. Scar tissue lacks rete pegs and scars tend to shear off more easily than normal tissue as a result. Also known as papillae, they are downward thickenings of the epidermis between the dermal papillae.
Sources: en.wikipedia.org
While there were well-known gyms throughout the country during the 1950s (such as Vince's Gym in North Hollywood, California, and Vic Tanny's chain gyms), there were still segments of the United States that had no "hardcore" bodybuilding gyms until the advent of Gold's Gym in the mid-1960s. Finally, the famed Muscle Beach in Santa Monica continued its popularity as the place to be for witnessing acrobatic acts, feats of strength, and the like. The movement grew more in the 1960s with increased TV and movie exposure, as bodybuilders were typecast in popular shows and movies.
general studies of protein structure and properties (because it is available in large quantities). studies of serpin structure and function (the fact that ovalbumin does not inhibit proteases means that by comparing its structure with that of inhibitory serpins, the structural characteristics required for inhibition can be determined). proteomics (chicken egg ovalbumin is commonly used as a molecular weight marker for calibrating electrophoresis gels). immunology (commonly used to stimulate an allergic reaction in test subjects; e.g., established model allergen for airway hyper-responsiveness, AHR). (For in vivo and in vitro studies based on ovalbumin it is important that the endotoxin content is less than 1 EU/mg.)
The most common form of body modification related to the penis is circumcision: removal of part or all of the foreskin, and sometimes the frenulum. It is most commonly performed as an elective procedure for prophylactic, cultural, or religious reasons. Infant circumcision can employ modern devices such as the Gomco clamp, Plastibell, and Mogen clamp. The ethics of circumcision in children is a source of controversy. Among the world's major medical organizations, there is a consensus that circumcision reduces heterosexual HIV infection rates in high-risk populations during penile-vaginal sex. There are differing perspectives on the prophylactic efficacy and cost effectiveness of circumcision in developed nations. Circumcision plays a significant role in many of the world's cultures. When performed for religious reasons, it is most common among both Jews and Muslims, among whom it is near-universal.
Information published by the Department for Science, Innovation and Technology reveals that £15,000 in damages was paid to academic Kate Sand after Science Minister Michelle Donelan described her as a Hamas supporter in 2023; Donelan retracted the allegations on 5 March. University Challenge contestant Melika Gorgianeh accepts substantial damages from Conservative peer Jacqueline Foster after Foster accused her of being antisemitic following her appearance on the quiz show in November 2023. Senedd member Rhys ab Owen is to be banned from the Senedd for six weeks after an investigation by the Senedd Commission found he inappropriately touched two women during a night out in June 2021. 7 March – Blur drummer Dave Rowntree is selected as the Labour Party candidate for Mid Sussex. 8 March – Former prime minister Theresa May announces she is stepping down from Parliament at the next general election. Brenda Dacres is elected Mayor of Lewisham becoming the first black woman directly elected mayor in England. Robin Simcox, the UK government's Commissioner for Countering Extremism, warns that London has become a "no-go zone for Jews" at weekends because of pro-Palestinian protests. 9 March – In an article posted on LinkedIn, senior UK government ministers Anne-Marie Trevelyan and Tom Tugendhat urge Sunak to increase defence spending to above 2.5% of GDP, arguing that the UK needs to "lead the way" on defence spending and invest at a "much greater pace".
Sources: en.wikipedia.org
Widespread travel by naturalists in the early-to-mid-19th century resulted in a wealth of new information about the diversity and distribution of living organisms. Of particular importance was the work of Alexander von Humboldt, which analyzed the relationship between organisms and their environment (i.e., the domain of natural history) using the quantitative approaches of natural philosophy (i.e., physics and chemistry). Humboldt's work laid the foundations of biogeography and inspired several generations of scientists.
=== Labeling === For US food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value. For vitamin B6 labeling purposes 100% of the Daily Value was 2.0 mg, but as of May 27, 2016, it was revised to 1.7 mg to bring it into agreement with the adult RDA. A table of the old and new adult daily values is provided at Reference Daily Intake.
but I may mention that this last conclusion cannot be true except so long as the compressed vapours remain entirely in the elastic state; and this requires that their temperature shall be sufficiently elevated to enable them to resist the pressure which tends to make them assume the liquid state.
Sources: en.wikipedia.org
It measures the amounts and identities of compounds in liquid samples by separation and detection. Depending on the detector and reference standards, results can be qualitative or quantitative. The technique is used in fields such as pharmaceutical analysis, food safety, and environmental monitoring.
Performance checks confirm that the chromatographic system works within preset limits before results are accepted. They examine factors such as peak resolution, tailing, and repeatability. If criteria fail, the run may need correction or repetition.
Retention time alone is not definitive proof because other compounds can elute at similar times. Confirmation usually uses a second method, a different column, or a detector such as mass spectrometry. Authentic standards strengthen identification.
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.