If you have been reading about retention time 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.
Updated 2025-08-20. Numbers and descriptions here follow the published literature rather than marketing material.
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.
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.
Routine HPLC testing compares a sample result with a calibration curve prepared from known reference standards. Peak area or peak height is plotted against concentration, and the curve is used to estimate unknown amounts. Retention time supports tentative identification when compared with a standard, though mass spectrometry or another confirmatory method may be needed for definitive identification. Pre-run checks verify repeatability, resolution, and peak symmetry before sample analysis. Limits of detection and quantification describe the smallest amounts that can be reliably observed or measured. Sample preparation, filtration, and degassing help prevent column damage and inconsistent results.
| 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. |
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.
System suitability testing is performed before and during analytical runs to confirm that the instrument and method are working as expected. Typical checks include retention time, peak area precision, resolution between critical pairs, tailing factor, and theoretical plate count. Acceptance criteria are set in the method or pharmacopeial monograph. If a suitability check fails, the run may be rejected and the instrument or sample preparation may need investigation. This practice helps prevent release of data from a system that has drifted out of control.
Method validation establishes that an HPLC procedure is suitable for its intended use. Key parameters include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, and robustness. Accuracy measures agreement with a true or accepted value, while precision describes repeatability and intermediate precision. Specificity confirms that the method measures the analyte without interference from impurities, degradants, or excipients. Validation is documented in a protocol and report, and acceptance criteria are set before experiments begin. Regulatory guidance varies by region, but the general principles are widely harmonized.
System suitability testing is performed before and during analytical runs to confirm that the instrument and method are working as expected. Common checks include retention time, peak area, resolution between critical pairs, tailing factor, and theoretical plate count. Results are compared with predefined limits, and a failed check requires investigation before sample results are reported. Quality control samples at low, middle, and high concentrations are injected at intervals to monitor accuracy and precision. Blank injections detect carryover and contamination, while control charts track performance over time.
nucleic acid sequence The precise order of consecutively linked nucleotides in a nucleic acid molecule such as DNA or RNA. Long sequences of nucleotides are the principal means by which biological systems store genetic information, and therefore the accurate replication, transcription, and translation of such sequences is of the utmost importance, lest the information be lost or corrupted. Nucleic acid sequences may be equivalently referred to as sequences of nucleotides, nitrogenous bases, nucleobases, or, in duplex molecules, base pairs, and they correspond directly to sequences of codons and amino acids.
The first king of Hungary, Stephen I, who supported Western European missionaries, defeated the local chieftains and established Roman Catholic bishoprics (office of a bishop) in Transylvania and Banat in the early 11th century. Significant Pecheneg groups fled to the Byzantine Empire in the 1040s; the Oghuz Turks followed them, and the nomadic Cumans became the dominant power of the steppes in the 1060s. Cooperation between the Cumans and the Vlachs against the Byzantine Empire is well documented from the end of the 11th century. Scholars who reject the Daco-Roman continuity theory say that the first Vlach groups left their Balkan homeland for the mountain pastures of the eastern and southern Carpathians in the 11th century, establishing the Romanians' presence in the lands to the north of the Lower Danube.
=== Initial discoveries === Theories on the coagulation of blood have existed since antiquity. Physiologist Johannes Müller (1801–1858) described fibrin, the substance of a thrombus. Its soluble precursor, fibrinogen, was thus named by Rudolf Virchow (1821–1902), and isolated chemically by Prosper Sylvain Denis (1799–1863). Alexander Schmidt suggested that the conversion from fibrinogen to fibrin is the result of an enzymatic process, and labeled the hypothetical enzyme "thrombin" and its precursor "prothrombin". Arthus discovered in 1890 that calcium was essential in coagulation. Platelets were identified in 1865, and their function was elucidated by Giulio Bizzozero in 1882. The theory that thrombin is generated by the presence of tissue factor was consolidated by Paul Morawitz in 1905. At this stage, it was known that thrombokinase/thromboplastin (factor III) is released by damaged tissues, reacting with prothrombin (II), which, together with calcium (IV), forms thrombin, which converts fibrinogen into fibrin (I).
Sources: en.wikipedia.org
If, as may well happen, there should be created in our own lifetime by the banks of the Jordan a Jewish State under the protection of the British Crown which might comprise three or four millions of Jews, an event will have occurred in the history of the world which would from every point of view be beneficial. At the 22 June 1921 meeting of the Imperial Cabinet, Churchill was asked by Arthur Meighen, the Canadian Prime Minister, about the meaning of the national home. Churchill said "If in the course of many years they become a majority in the country, they naturally would take it over ... pro rata with the Arab. We made an equal pledge that we would not turn the Arab off his land or invade his political and social rights".
=== Metal sulfides === Sulfur reacts with many metals. Electropositive metals give polysulfide salts. Copper, zinc, and silver are attacked by sulfur; see tarnishing. Although many metal sulfides are known, most are prepared by high temperature reactions of the elements. Geoscientists also study the isotopes of metal sulfides in rocks and sediment to study environmental conditions in the Earth's past.
Affinity label for the tRNA binding sites on the E. coli ribosome allowed the identification of A and P site proteins most likely associated with the peptidyltransferase activity; labelled proteins are L27, L14, L15, L16, L2; at least L27 is located at the donor site, as shown by E. Collatz and A.P. Czernilofsky. Additional research has demonstrated that the S1 and S21 proteins, in association with the 3′-end of 16S ribosomal RNA, are involved in the initiation of translation.
Sources: en.wikipedia.org
=== Surgical anatomy for nasal reconstruction === The human nose is a sensory organ that is structurally composed of three types of tissue: (i) an osseo-cartilaginous support framework (nasal skeleton), (ii) a mucous membrane lining, and (iii) an external skin. The anatomic topography of the human nose is a graceful blend of convexities, curves, and depressions, the contours of which show the underlying shape of the nasal skeleton. Hence, these anatomic characteristics permit dividing the nose into nasal subunits: (i) the midline (ii) the nose-tip, (iii) the dorsum, (iv) the soft triangles, (v) the alar lobules, and (vi) the lateral walls. Surgically, the borders of the nasal subunits are ideal locations for the scars, whereby is produced a superior aesthetic outcome, a corrected nose with corresponding skin colors and skin textures.
The two substrates of this enzyme are androsterone and oxidised nicotinamide adenine dinucleotide (NAD+). Its products are 5α-androstane-3,17-dione, reduced NADH, and a proton. The alternative cofactor nicotinamide adenine dinucleotide phosphate is also used by this enzyme. The enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD+ or NADP+ as acceptor, more specifically it is part of the group of hydroxysteroid dehydrogenases. The systematic name of this enzyme class is 3alpha-hydroxysteroid:NAD(P)+ oxidoreductase (B-specific). Other names in common use include hydroxyprostaglandin dehydrogenase, 3alpha-hydroxysteroid oxidoreductase, and sterognost 3alpha. This enzyme participates in 3 metabolic pathways: bile acid biosynthesis, c21-steroid hormone metabolism, and androgen and estrogen metabolism.
During the later part of the 18th century, fundamental internal reforms were attempted in the Polish–Lithuanian Commonwealth as it slid into extinction. The reform activity, initially promoted by the magnate Czartoryski family faction known as the Familia, provoked a hostile reaction and military response from neighboring powers, but it did create conditions that fostered economic improvement. The most populous urban center, the capital city of Warsaw, replaced Danzig (Gdańsk) as the leading trade center, and the importance of the more prosperous urban social classes increased. The last decades of the independent Commonwealth's existence were characterized by aggressive reform movements and far-reaching progress in the areas of education, intellectual life, art and the evolution of the social and political system. The royal election of 1764 resulted in the elevation of Stanisław August Poniatowski, a refined and worldly aristocrat connected to the Czartoryski family, but hand-picked and imposed by Empress Catherine the Great of Russia, who expected him to be her obedient follower. Stanisław August ruled the Polish–Lithuanian state until its dissolution in 1795. The king spent his reign torn between his desire to implement reforms necessary to save the failing state and the perceived necessity of remaining in a subordinate relationship to his Russian sponsors. The Bar Confederation (1768–1772) was a rebellion of nobles directed against Russia's influence in general and Stanisław August, who was seen as its representative, in particular.
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 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.