This is a working overview of Limit of detection, written for readers who want more than a one-paragraph summary but less than a textbook.
Reviewed 2025-12-24. Anything still debated is marked as such rather than presented as settled.
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.
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.
| Property | Value | Notes |
|---|---|---|
| Common abbreviation | HPLC | High-performance liquid chromatography |
| Separation basis | Differential partitioning | Between liquid mobile phase and solid stationary phase |
| Common mode | Reverse phase | Nonpolar column, polar mobile phase |
| Typical detector | UV-Vis absorbance | Widely used for compounds with chromophores |
| Typical column particle size | 2–5 µm | Smaller particles can improve resolution |
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.
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.
Alfred Ellis Wilhelmi (1910–1994) was an American endocrinologist recognized for contributing to the understanding of anterior pituitary hormones. Born in Lakewood, Ohio, Wilhelmi attended Cleveland public schools. Wilhelmi earned a B.S. degree in premedical sciences from Western Reserve University in 1933. He then attended Oxford University as a Rhodes Scholar, where he obtained a B.A. in 1933 and Ph.D. in animal physiology in 1937. He then joined Yale University's Biochemistry Department, rising to the position of Professor in 1950. Wilhelmi chaired the Department of Biochemistry at Emory University School of Medicine from 1950 to 1977. In 1960, he was named Charles Howard Candler Professor of Biochemistry. In 1979, he received Emory's Thomas Jefferson award for service to the university and community. Wilhelmi also was President of the Endocrine Society from 1968 to 1969. During his career, he published over 80 articles in scientific journals.
The cellular response in signal transduction cascades involves alteration of the expression of effector genes or activation/inhibition of targeted proteins. Regulation of protein activity mainly involves phosphorylation/dephosphorylation events, leading to its activation or inhibition. It is the case for the vast majority of responses as a consequence of the binding of the primary messengers to membrane receptors. This response is quick, as it involves regulation of molecules that are already present in the cell. On the other hand, the induction or repression of the expression of genes requires the binding of transcriptional factors to the regulatory sequences of these genes. The transcriptional factors are activated by the primary messengers, in most cases, due to their function as nuclear receptors for these messengers. The secondary messengers like DAG or Ca2+ could also induce or repress gene expression, via transcriptional factors. This response is slower than the first because it involves more steps, like transcription of genes and then the effect of newly formed proteins in a specific target. The target could be a protein or another gene.
Caddisfly silk is silk that is secreted by the silk glands of the caddisfly (Trichoptera), similar to Lepidoptera silkworms (B. mori). The larvae use silk to hunt and defend themselves. The silk's underwater binding properties are a subject of ongoing scientific research. Trichoptera, or caddisfly, larvae use silk to hunt and protect themselves in their aquatic environment. Much like silkworms and other Lepidoptera, this silk protein is excreted by specialized silk glands. The silk’s structure is mostly conserved among many different caddisfly species, and can be used to bind debris including rocks, sticks, twigs and shells, as well as to build nets for catching prey. Caddisflies, which spend the majority of their life cycle in the larval stage, need these casings to protect their underbellies and pupate. Caddisfly silk is very strong and durable. Because their silk must be able to bind to a variety of components while completely submerged in water, it is therefore being studied for potential applications as a waterproof adhesive.
Enzymatic specificity provides useful insight into enzyme structure, which ultimately determines and plays a role in physiological functions. Specificity studies also may provide information of the catalytic mechanism. Specificity is important for novel drug discovery and the field of clinical research, with new drugs being tested for its specificity to the target molecule in various rounds of clinical trials. Drugs must contain as specific as possible structures in order to minimize the possibility of off-target affects that would produce unfavorable symptoms in the patient. Drugs depend on the specificity of the designed molecules and formulations to inhibit particular molecular targets. Novel drug discovery progresses with experiments involving highly specific compounds. For example, the basis that drugs must successfully be proven to accomplish is both the ability to bind the target receptor in the physiological environment with high specificity and also its ability to transduce a signal to produce a favorable biological effect against the sickness or disease that the drug is intended to negate.
Sources: en.wikipedia.org
Hydrogen halides and their solutions: hydrofluoric acid (HF), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI) Halogen oxoacids: hypochlorous acid (HClO), chlorous acid (HClO2), chloric acid (HClO3), perchloric acid (HClO4), and corresponding analogs for bromine and iodine Hypofluorous acid (HFO), the only known oxoacid for fluorine. Sulfuric acid (H2SO4) Fluorosulfuric acid (HSO3F) Nitric acid (HNO3) Phosphoric acid (H3PO4) Fluoroantimonic acid (HSbF6) Fluoroboric acid (HBF4) Hexafluorophosphoric acid (HPF6) Chromic acid (H2CrO4) Boric acid (H3BO3) A sulfonic acid has the general formula RS(=O)2–OH, where R is an organic radical. Methanesulfonic acid (or mesylic acid, CH3SO3H) Ethanesulfonic acid (or esylic acid, CH3CH2SO3H) Benzenesulfonic acid (or besylic acid, C6H5SO3H) p-Toluenesulfonic acid (or tosylic acid, CH3C6H4SO3H) Trifluoromethanesulfonic acid (or triflic acid, CF3SO3H) Polystyrene sulfonic acid (sulfonated polystyrene, [CH2CH(C6H4)SO3H]n)
Normally pathogenic bacteria are weakened so they are no longer pathogenic. Commensal or food-grade bacteria which are not pathogenic. Using bacterial surface display of antigens is a valuable alternative to conventional vaccine design for various reasons, one of them being that the proteins expressed on the bacterial cell surface can act favourably as an adjuvant. Conventional vaccines require the addition of adjuvants. Another advantage of generating vaccines using bacterial display systems is that the whole bacterial cell can be incorporated in the live vaccine Unlike bacteriophage display systems which are generally used in vaccine development to find unknown epitopes, bacterial display systems are used to express known epitopes and the cells act as a vaccine delivery system. Under similar conditions, selection of bacterial-displayed peptides to model protein streptavidin proved worse.
Accelerator mass spectrometry (AMS) is a form of mass spectrometry that accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the different methods of mass spectrometry is its ability to separate a rare isotope from an abundant neighboring mass ("abundance sensitivity", e.g. 14C from 12C). The method suppresses molecular isobars completely and in many cases can also separate atomic isobars (e.g. 14N from 14C). This makes possible the detection of naturally occurring, long-lived radio-isotopes such as 10Be, 36Cl, 26Al and 14C. (Their typical isotopic abundance ranges from 10−12 to 10−18.) AMS can outperform the competing technique of decay counting for all isotopes where the half-life is long enough. Other advantages of AMS include its short measuring time as well as its ability to detect atoms in extremely small samples.
A variety of natural and synthetic inhibitors of ATP synthase have been discovered. These have been used to probe the structure and mechanism of ATP synthase. Some may be of therapeutic use. There are several classes of ATP synthase inhibitors, including peptide inhibitors, polyphenolic phytochemicals, polyketides, organotin compounds, polyenic α-pyrone derivatives, cationic inhibitors, substrate analogs, amino acid modifiers, and other miscellaneous chemicals. Some of the most commonly used ATP synthase inhibitors are oligomycin and DCCD. E. coli ATP synthase is the simplest known form of ATP synthase, with 8 different subunit types. Bacterial F-ATPases can occasionally operate in reverse, turning them into an ATPase. Some bacteria have no F-ATPase, using an A/V-type ATPase bidirectionally. Yeast ATP synthase is one of the best-studied eukaryotic ATP synthases; and five F1, eight FO subunits, and seven associated proteins have been identified. Most of these proteins have homologues in other eukaryotes.
Other 1,2- and 1,3-dithiols give related 1,3-dithiolanes (five-membered) and 1,3-dithianes (six-membered rings). Diols such as ethylene glycol undergo analogous reactions to give 1,3-dioxolanes. One distinguishing feature of the dithiolanes and dithianes derived from aldehydes is that the methyne group can be deprotonated and the resulting carbanion alkylated. 1,2-Ethanedithiol has been used as a scavenger in peptide cleavage synthesis. Like 1,3-propanedithiol, 1,2-ethanedithiol readily forms metal thiolate complexes. Illustrative is the synthesis of the derivative diiron ethanedithiolate hexacarbonyl upon reaction with triiron dodecacarbonyl: Fe3(CO)12 + C2H4(SH)2 → Fe2(S2C2H4)(CO)6 + H2 + Fe(CO)5 + CO Ethane-1,1-dithiol
Sources: en.wikipedia.org
In organic chemistry, an active ester is an ester functional group that is highly susceptible toward nucleophilic attack. Activation can be imparted by modifications of the acyl or the alkoxy components of a normal ester, say ethyl acetate. Typical modifications call for electronegative substituents. Active esters are employed in both synthetic and biological chemistry. Active esters are mainly used as acylating agents. They undergo the same reactions as their unactivated analogues but do so more rapidly. They are prone to hydrolysis, for example. Of great interest is the enhanced reactivity of active esters toward amines to give amides. Active esters are prominent in biochemistry. Glutamine synthetase is an enzyme that forms an active ester from the terminal carboxylate of glutamic acid. This activation, imparted by phosphorylation, facilitates the conversion of the carboxylate to an amide called glutamine.
A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals. Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for intercell communication and engaging in coordinated multicellular behaviour. The communal benefits of multicellular cooperation include a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types. For example, bacteria in biofilms can have more than five hundred times the increased resistance to antibacterial agents than individual "planktonic" bacteria of the same species. One type of intercellular communication by a molecular signal is called quorum sensing. Quorum sensing determines whether the local population is dense enough to support investment in processes that are only successful if large numbers of similar organisms behave similarly, such as excreting digestive enzymes or emitting light. Quorum sensing enables bacteria to coordinate gene expression and to produce, release, and detect autoinducers or pheromones that accumulate with the growth in cell population.
Amino acid-based formula is a type of infant milk formula made from individual amino acids. It is hypoallergenic and intended for infants suffering from severe allergy to milk and various gastrointestinal conditions, such as food protein-induced enterocolitis syndrome and malabsorption syndromes. It is sometimes referred to as elemental formula but this is considered a misleading name. Issues with the use of amino acid-based formula include its high cost and its unpalatable taste. Intake of amino-acid formula for healthy infants shows no advantage in growth.
ALK-positive ALCL is diagnosed by histological and immunological examinations of involved tissues, typically lymph nodes. These tissues have lymphoma-like infiltrates that have variable numbers of ALCL "hallmark" cells, i.e. cells with kidney- or horseshoe-shaped nuclei that strongly express CD30 as detected by immunohistochemistry and an ALK fusion protein as detected by fluorescence in situ hybridization. These cells are scattered throughout the infiltrates. WHO classifies these infiltrates into 5 patterns: a common pattern consisting of large variably shaped cells with large nuclei that typically contain multiple nucleoli (60–70% of cases); a small-cell pattern consisting of small to medium-sized neoplastic cells with clear cytoplasm and "hallmark" cells that are concentrated around small blood vessels (5–10% of cases); a lymphohistiocytic pattern consisting of small neoplastic cells along with abundant histiocytes (10% of cases); a Hodgkin's-like pattern in which the architecture resembles the nodular sclerosis pattern of Hodgkin lymphoma (3% of cases); and a composite pattern consisting of two or more of the just described patterns (15% of cases). Detection of circulating autoantibody against ALK supports the diagnosis. Individuals with low levels of these autoantibodies are at an increase risk of relapsing after treatment.
PABA and its salts are used in nutritional epidemiological studies to assess the completeness of 24-hour urine collection for the determination of urinary sodium, potassium, or nitrogen levels. The potassium salt (potassium paraaminobenzoate, "aminobenzoate potassium") is used as a drug against fibrotic skin disorders, such as Peyronie's disease, under the brand name Potaba. There is a lack of strong evidence. It is believed to work by inhibiting fibroblast glycosaminoglycan secretion and stabilizing monoamine oxidase A activity. PABA is also occasionally used in pill form by sufferers of irritable bowel syndrome to treat its associated gastrointestinal symptoms. PABA derivatives have also been proposed to function as acetylcholinesterase inhibitors in diseases that cause deficient cholinergic systems, such as Alzheimer's disease.
Sources: en.wikipedia.org
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.
Retention time is the interval between sample injection and the detector response for a given compound. It depends on the compound's interactions with the stationary and mobile phases under set conditions. Matching a retention time to a standard supports tentative identification but is not always unique.
HPLC alone can separate unknown compounds and provide retention times, but it often cannot identify them with certainty. Coupling HPLC to mass spectrometry gives mass information that improves identification. Confirmation usually requires comparison with reference standards or complementary techniques.
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.