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High-performance liquid chromatography (HPLC) is also sometimes know as high-pressure liquid chromatography and is a type of column chromatography. It is a type of chromatography that is common across a wide spectrum of forensic scientific disciplines including drug screens, forensic toxicology, the analysis of explosives and more.

HPLC is based on isolating specific testable molecules into one of two different phases. This it has in common with all forms of chromatography. Where it differs, though, is that in gas chromatography, the mobile phase is always inert, whereas with HPLC, the mobile phase can consist of non-inert gases or of entirely different [url=http://www.skoda-witzke.de/woolrichoutlet.php]woolrich[/url] solvents - sometimes even [url=http://www.burnabystorage.com/barbour.php]barbour italia[/url] multiple solvents. In high performance liquid chromatography, the mobile phase is forced under high pressure to flow below special beads that are coated with the same solvent material, except in it's solid phase.

The molecules of interest that are being forced through will interact differently with the mobile phase and with the stationary phase. Certain types of molecules will "prefer" the stationary phase, and will interact with it more fully. Others will show a "preference" for the mobile phase, and will emerge from the column more quickly. What you are doing is basically separating different types of molecules. You will see separate peaks on the graph, corresponding to the difference in reactivity with the mobile and stationary phases. The first components out of the tube will be those which interacted more with the mobile phase, with those preferring the stationary phase lagging behind. A detector (there are various types) will then test for elution of the molecules as they exit the tube. A subset of this type of experiment is to collect the eluting solvent or compounds at specific times in order to isolate a material or substance of interest. This allows the technician with a small sample of the pure solution, which can then be tested with different methods to confirm it's composition.

Now let's move on to a discussion about the detector components. There are a wide array of different detector types that differ in their sensitivity, their method of detection, and the compounds that they are capable of picking up. Probably the most high end detector is a mass spectrometer, with the most entry level being the refractive index detector. The mass spectrometer is the preferred high performance liquid chromatography detector for many lab managers, although as a result of it's cost and the technical difficulty in maintaining them, they are not the most common type.

The refractive index detector is usually consideerd fairly entry level, as it easier to operate and quite often the lowest cost of the detector types. However, due to it's low sensitivity, it is not appropriate for most labs. The most commonly used detectors fall somewhere between the RI and the MS. These are UV detectors that base their function on the differing rates of absorption of light in the UV versus the visible spectrums. While not as sensitive to low concentrations or to as many different compounds as a mass spectrometer, they do tend to have a sensitivity level that most scientists find acceptable for a wide array of applications. Finally, another somewhat common type of HPLC detector is the PDA, or photodiode array detector, although [url=http://www.rtnagel.com/louboutin.php]louboutin pas cher[/url] it's use is limited to specialized applications.

High performance liquid chromatography has a wider array of uses than traditional gas chromatography. For the analysis of explosives in particular, HPLC is better suited since, unlike with GC, the mixture or compound is not required to be converted into a gas phase. You can imagine why the need to convert to a gas phase would be inconvenient to say the least for the testing of explosive compounds!

There is a specific type of HPLC that is often used in forensic labs. It is referred to as "reversed phase HPLC". This type is used on a daily basis in labs around the world for the analysis of agents used in [url=http://www.corsodiesperanto.it/peuterey.php]peuterey outlet[/url] drug dilution such as sugars and flour, in the analysis of explosives and bomb making materials, in medical toxicology, and in the analysis of drug use. Drug use analysis [url=http://www.diecastlinks.co.uk]hollister outlet[/url] can include that done in what are typically called "drug tests" or "drug screens" (the type that you probably had to undergo to get your job) as well as for post-mortem forensic drug screening. This type of HPLC is one of the only methods of testing for certain drugs such as LSD and mescaline. RP-HPLC involves using "polar" solvents in the mobile phase. As you probably remember from college chemistry, this means that different parts of the same molecule carry different charges. One side may have a partial positive charge, with the other having a partial negative charge. Water, for instance, is a polar molecule. Other examples include ethanol and methanol. The stationary phase, of course, must consist [url=http://www.skoda-witzke.de/woolrichoutlet.php]woolrich outlet[/url] of a non-polar [url=http://park15.wakwak.com/~haruka/cgi-bin/off/yybbs.cgi]barbour outlet Planning Your [/url] solvent.

The result is that the different molecules in the compound being tested will separate based not just on their degree of affinity for the mobile versus stationary phases, but based on their affinity for the degree of polarization in the solvents [url=http://www.diecastlinks.co.uk]hollister uk[/url] used for the mobile and stationary phases.For forensic applications, one of the most commonly used types of HPLC is referred to as "reversed phase." In this type of HPLC, the mobile phase is a solvent or mix of solvents that are "polar," meaning that different parts of the individual solvent [url=http://www.rtnagel.com/louboutin.php]louboutin[/url] molecules carry a partial positive or negative charge. Water, methanol (methyl alcohol), [url=http://www.wiis.fr]louboutin pas cher[/url] ethanol (ethyl [url=http://www.burnabystorage.com/barbour.php]barbour sito ufficiale[/url] alcohol), [url=http://park10.wakwak.com/~hahha/cgi-bin/simajiri.cgi]barbour online shop Wedding I[/url] and acetone are examples of polar solvents. The stationary phase in reverse phase HPLC is a nonpolar material such [url=http://www.tagverts.com/barbour.php]www.tagverts.com/barbour.php[/url] as a long chain hydrocarbon molecule. In this type of HPLC, components in the sample will partition and separate [url=http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=140751927900]barbour pas cher soldes Elect[/url] based on their degree of interaction with the stationary phas

Yet another subtype of HPLC is called "size exclusion chromatography". This is a long phrase simply indicating that compounds separate based on the sizes of the molecules that they contain. SEC may use a variety of solvents for the mobile phase, but for the stationary phase, it utilizes a gel varying sizes of microscopic holes or pores in it's make-up. Molecules can be separated based on how large they are, because larger molecules take longer to find their way through the pores in the solvent. As a result, they take longer to reach the detector, which can analyze which molecules reach it first. SEC is most commonly used, not in forensics as most high performance liquid chromatography is used, but in the analysis of proteins and plastics.

Other types of HPLC which are not as common, but which we may cover in a future article, are Chiral chromatography and ion exchange [url=http://www.riad-marrakesh.fr]abercrombie pas cher[/url] chromatography.

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