A practical reference on quality control: what it is, how it behaves, what the literature reports, and where the honest uncertainties sit.
This page was last updated on 2026-01-02 and is reviewed periodically as new material appears.
Quality control laboratories use HPLC to check identity, purity, concentration, and stability of raw materials and finished products. A validated method specifies the column, mobile phase, flow rate, detection wavelength, injection volume, and run time. Samples are prepared and compared against reference standards of known concentration. The resulting chromatogram provides quantitative data, such as assay values and impurity levels. This approach is common in pharmaceutical, food, environmental, and industrial testing where consistent measurements are required.
Method validation demonstrates that an analytical procedure is suitable for its intended purpose. Typical validation characteristics include accuracy, precision, specificity, linearity, range, limit of detection, limit of quantitation, and robustness. Regulatory guidance from bodies such as the International Council for Harmonisation and the United States Pharmacopeia outlines expectations, though specific criteria depend on the product and method. System suitability tests are run before sample analysis to confirm resolution, peak symmetry, column efficiency, and injection repeatability. Failure of these checks can invalidate a batch of measurements.
Practical HPLC testing depends on careful sample preparation and instrument maintenance. Samples may require filtration, dilution, pH adjustment, or extraction to avoid column damage and matrix interference. Mobile phases are degassed and filtered, and columns are equilibrated before injection. Common problems include peak tailing, baseline drift, ghost peaks, carryover, and co-elution of analytes. Documentation of instrument logs, calibration records, and electronic audit trails supports data integrity and traceability. Ongoing training and routine maintenance help reduce variability between analysts and laboratories.
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.
| Parameter | Typical acceptance criterion | Notes |
|---|---|---|
| Resolution | ≥ 1.5 | Baseline separation of adjacent peaks |
| Tailing factor | ≤ 2.0 | Peak symmetry measure |
| Theoretical plates | > 2000 | Column efficiency indicator |
| Injection repeatability | ≤ 2% RSD | Relative standard deviation for replicate injections |
| Linearity | r² ≥ 0.995 | Calibration curve over the working range |
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.
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.
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.
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.
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.
== Mechanism of action == Hydroxycarbamide decreases the production of deoxyribonucleotides via inhibition of the enzyme ribonucleotide reductase by scavenging tyrosyl free radicals as they are involved in the reduction of nucleoside diphosphates (NDPs). Additionally, hydroxycarbamide causes production of reactive oxygen species in cells, leading to disassembly of replicative DNA polymerase enzymes and arresting DNA replication. In the treatment of sickle-cell disease, hydroxycarbamide increases the concentration of fetal hemoglobin. The precise mechanism of action is not yet clear, but it appears that hydroxycarbamide increases nitric oxide levels, causing soluble guanylyl cyclase activation with a resultant rise in cyclic GMP, and the activation of gamma globin gene expression and subsequent gamma chain synthesis necessary for fetal hemoglobin (HbF) production (which does not polymerize and deform red blood cells like the mutated HbS, responsible for sickle cell disease). Adult red cells containing more than 1% HbF are termed F cells. These cells are progeny of a small pool of immature committed erythroid precursors (BFU-e) that retain the ability to produce HbF. Hydroxyurea also suppresses the production of granulocytes in the bone marrow which has a mild immunosuppressive effect particularly at vascular sites where sickle cells have occluded blood flow.
=== 25 April === Three people were killed in Russian attacks in Donetsk Oblast. Russian-installed officials claimed that four people were killed in separate Ukrainian attacks in Kherson and Zaporizhzhia Oblasts. Two Russian soldiers were arrested for fatally shooting at least seven people while intoxicated in the villages of Podo-Kalynivka and Abrykosivka in Kherson Oblast, including the head of the latter village. The Polish and Lithuanian governments offered to repatriate Ukrainian men living in their countries to Ukraine for them to be drafted into the Ukrainian military. Spain pledged to provide Patriot missiles to Ukraine. The Swiss Parliament's Security Committee approved a proposal to send five billion Swiss Francs in aid to Ukraine. Russia seized the village of Novobakhmutivka, ten kilometers northwest of Avdiivka.
== Detection of citrullinated peptides and proteins == Citrullinated peptides and proteins can be detected using antibodies targeting the citrullinated residues, or detected using mass spectrometry-based proteomics technologies. Citrullination of arginine results in a monoisotopic mass increase of +0.984016 Da, which can be measured with mass spectrometry. The mass shift is close to the mass difference between the different peptide isotopes of +1.008665 which can be mistaken for a citrullinated peptide, especially on low-resolution instruments. However, this is less of an issue with modern high resolution/high accuracy mass spectrometers. Furthermore, the mass shift is identical to the mass shift caused by deamidation of the amino acid asparagine or glutamine side chain, which are common modifications. Citrulline residues can be chemically modified with butanedione or by biotinylation prior to analysis, leading to a different mass shift, and this strategy has successfully been used to facilitate identification by mass spectrometry. Another approach is to utilize the neutral loss of isocyanic acid (HNCO) from citrulline residues when submitted to low energy collision induced dissociation fragmentation in mass spectrometers. The loss causes a mass shift of −43.0058 Da, which can be utilized by mass spectrometers to predominantly select citrullinated peptides for fragmentation (sequencing). Finally, the loss of positive charge at physiological pH caused by citrullination can be utilized. Prior to bottom-up proteomics analysis, proteins are enzymatically cleaved into peptides.
== Cannabinoid receptors == Cannabinoid receptor 1 (CBR1) and Cannabinoid receptor 2 (CBR2) are the two main binding targets in the endocannabinoid system. They share similar molecular shape and signalling mechanisms, but differ in their tissue distribution and effects. CB1R mainly presents in the central nervous system, with its activation commonly associated with the regulation of mood, pain and appetite. CB2R is abundant in the immune tissues, for example spleen, lymph nodes and bone marrow. Its activation causes immunosuppressive effects.
Sources: en.wikipedia.org
== External links == The award webpage Archived 2020-03-16 at the Wayback Machine The Franklin Institute: The John Scott Legacy Medal "The John Scott Award Recipients". Eugene Garfield at University of Pennsylvania. Archived from the original on 15 June 2021. Retrieved 28 September 2018. Medals awarded by The Franklin Institute between 1822 and 2017.
Abortive initiation, the unproductive cycling of RNA polymerase before the promoter escape transition, results in short RNA fragments of around 9 bp in a process known as abortive transcription. The extent of abortive initiation depends on the presence of transcription factors and the strength of the promoter contacts.
=== Connective tissue research === Schleip, R.; Klingler, W.; Wearing, S.; Naylor, I.; Zuegel, M.; Hoppe, K. (2016). "Functional in vitro tension measurements of fascial tissue – a novel modified superfusion approach". Musculoskeletal Neuronal Interactions. 16 (3): 256–260. PMC 5114348. PMID 27609040. Schleip, R.; Wilke, J.; Schreiner, S.; Wetterslev, M.; Klingler, W. (2018). "Needle biopsy-derived myofascial tissue samples are sufficient for quantification of myofibroblast density". Clinical Anatomy. 31 (3): 368–372. doi:10.1002/ca.23040. PMID 29314236.
Sources: en.wikipedia.org
=== Debt categories === The total federal debt is divided into "debt held by the public" and "intra-governmental debt." The debt held by the public refers to U.S. government securities or other obligations held by investors (e.g., bonds, bills, and notes), while Social Security and other federal trust funds are part of the intra-governmental debt. As of September 30, 2012, the total debt was $16.1 trillion, with debt held by the public of $11.3 trillion and intragovernmental debt of $4.8 trillion. Debt held by the public as a percentage of gross domestic product (GDP) rose from 34.7% in 2000 to 40.3% in 2008 and 70.0% in 2012. U.S. GDP was approximately $15 trillion during 2011 and an estimated $15.6 trillion for 2012 based on activity during the first two quarters. This means the total debt is roughly the size of GDP. Economists debate the level of debt relative to GDP that signals a "red line" or dangerous level, or if any such level exists. By comparison, China's budget deficit was 1.6% of its $10 trillion GDP in 2010, with a debt to GDP ratio of 16%.
A surface is prepared with a known quantity of capture antibody. Any nonspecific binding sites on the surface are blocked. The antigen-containing sample is applied to the plate, and captured by antibody. The plate is washed to remove unbound antigen. A specific antibody is added, and binds to antigen (hence the 'sandwich': the antigen is stuck between two antibodies). This primary antibody could be in the serum of a donor, to be tested for reactivity towards the antigen. Enzyme-linked secondary antibodies are applied as detection antibodies, which bind specifically to the antibody's Fc region (nonspecific). The plate is washed to remove the unbound antibody-enzyme conjugates. A chemical is added to be converted by the enzyme into a color, fluorescent, or electrochemical signal. The absorbance, fluorescence, or electrochemical signal (e.g., current) of the plate's wells is measured to determine the presence and quantity of the antigen. The image to the right includes the use of a secondary antibody conjugated to an enzyme, although, in the technical sense, this is not necessary if the primary antibody is conjugated to an enzyme (which would be direct ELISA). However, the use of a secondary-antibody conjugate avoids the expensive process of creating enzyme-linked antibodies for every antigen one might want to detect. By using an enzyme-linked antibody that binds the Fc region of other antibodies, this same enzyme-linked antibody can be used in a variety of situations.
== Synthesis == The first reported synthesis of mepindolol in 1971 used 4-hydroxy-2-methylindole (9) with epichlorohydrin and then isopropylamine to add the sidechain which was known to produce beta blockers, by analogy with drugs discovered by Imperial Chemical Industries, such as propanolol. The requisite intermediate was synthesized in a multi-step procedure from 4-benzyloxyindole-2-carboxylic acid (1) which was converted into 9 by conventional chemistry.
However, it was hypothesized that even if the putative first RNA-dependent RNA-polymerases are estimated to be longer—the smallest reported up-to-date RNA-dependent polymerase ribozyme is 165 nucleotides long—they did not have to arise in one step. It is more plausible that ligation of smaller RNA chains performed by the first RNA ligases resulted in a longer chain with the desired catalytically active polymerase domain. Forty years after the publication of Manfred Eigen's primary work dedicated to hypercycles, Nilesh Vaidya and colleagues showed experimentally that ribozymes can form catalytic cycles and networks capable of expanding their sizes by incorporating new members. However, this is not a demonstration of a hypercycle in accordance with its definition, but an example of a collectively autocatalytic set. Earlier computer simulations showed that molecular networks can arise, evolve and be resistant to parasitic RNA branches. In their experiments, Vaidya et al. used an Azoarcus group I intron ribozyme that, when fragmented, has an ability to self-assemble by catalysing recombination reactions in an autocatalytic manner. They mutated the three-nucleotide-long sequences responsible for recognition of target sequences on the opposite end of the ribozyme (namely, Internal Guide Sequences or IGSs) as well as these target sequences. Some genotypes could introduce cooperation by recognizing target sequences of the other ribozymes, promoting their covalent binding, while other selfish genotypes were only able to self-assemble.
Sources: en.wikipedia.org
Method validation is the documented process of confirming that an HPLC procedure is suitable for its intended use. It evaluates accuracy, precision, specificity, linearity, range, detection limits, and robustness. Validation criteria depend on the regulatory context and the sample type.
System suitability tests are short checks performed before or during an HPLC run to verify instrument and method performance. They often include resolution, tailing factor, theoretical plates, and injection precision. Results must meet predefined limits for sample data to be accepted.
HPLC retention time alone cannot definitively identify an unknown substance. A match with a reference standard under identical conditions provides supporting evidence. Confirmation typically requires mass spectrometry, nuclear magnetic resonance, or another orthogonal technique.
Validation establishes suitability for a new method, while verification confirms that a method works in a specific laboratory. Verification is often used when a validated method is adopted with existing equipment and staff. Both rely on documented acceptance criteria.