A modern self-contained<unk> . Schematic representation of an<unk>  unit. (1) Solvent reservoirs, (2) Solvent degasser, (3) Gradient valve, (4) Mixing vessel for delivery of the mobile phase, (5)<unk> -pressure pump, (6) Switching valve in "inject position", (6') Switching valve in "load position", (7) Sample injection loop, (8) Pre-column (guard column), (9) Analytical column, (10) Detector (i.e., IR, UV), (11) Data acquisition, (12) Waste or fraction collector.<unk>  <unk> , formerly referred to as<unk> -pressure<unk><unk> , is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pass a pressurized<unk>  solvent containing the sample mixture through a column filled with a solid adsorbent material. Each component in the sample interacts slightly differently with the adsorbent material, causing different flow rates for the different components and leading to the separation of the components as they flow out of the column.<unk>  has been used for manufacturing (e.g., during the production process of pharmaceutical and biological products), legal (e.g., detecting<unk>  enhancement drugs in urine), research (e.g., separating the components of a complex biological sample, or of similar synthetic chemicals from each other), and medical (e.g., detecting vitamin D levels in blood serum) purposes.<unk>  can be described as a mass transfer process involving adsorption.<unk>  relies on pumps to pass a pressurized<unk>  and a sample mixture through a column filled with adsorbent, leading to the separation of the sample components. The active component of the column, the adsorbent, is typically a granular material made of solid particles (e.g., silica, polymers, etc.), 2–50 μm in size. The components of the sample mixture are separated from each other due to their different degrees of interaction with the adsorbent particles. The pressurized<unk>  is typically a mixture of solvents (e.g., water, acetonitrile and/or methanol) and is referred to as a "mobile phase". Its composition and temperature play a major role in the separation process by influencing the interactions taking place between sample components and adsorbent. These interactions are physical in nature, such as hydrophobic (dispersive), dipole–dipole and ionic, most often a combination.<unk>  is distinguished from traditional ("low pressure")<unk><unk>  because operational pressures are significantly<unk> er (50–350 bar), while ordinary<unk><unk>  typically relies on the force of gravity to pass the mobile phase through the column. Due to the small sample amount separated in analytical<unk> , typical column dimensions are 2.1–4.6 mm diameter, and 30–250 mm length. Also<unk>  columns are made with smaller adsorbent particles (2–50 μm in average particle size). This gives<unk>  superior resolving power (the ability to distinguish between compounds) when separating mixtures, which makes it a popular<unk> c technique. The schematic of an<unk>  instrument typically includes a degasser, sampler, pumps, and a detector. The sampler brings the sample mixture into the mobile phase stream which carries it into the column. The pumps deliver the desired flow and composition of the mobile phase through the column. The detector generates a signal proportional to the amount of sample component emerging from the column, hence allowing for quantitative analysis of the sample components. A digital microprocessor and user software control the<unk>  instrument and provide data analysis. Some models of mechanical pumps in an<unk>  instrument can mix multiple solvents together in ratios changing in time, generating a composition gradient in the mobile phase. Various detectors are in common use, such as UV/Vis, photodiode array (PDA) or based on mass spectrometry. Most<unk>  instruments also have a column oven that allows for adjusting the temperature at which the separation is performed.
High-performance liquid chromatography
Cloned enzyme donor immunoassay
Helium dilution technique
Hydrostatic weighing
Immunoassay
Bolton analysis
Nephelometry 
Radioimmunoassay