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  #1  
Old 06-02-2004, 12:30 AM
Scupper Scupper is offline
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How do chemists determine the composition of an unknown substance?

Chemists (or just anyone who knows more about chemistry than I--not a great feat): Suppose it was necessary to determine what a particular, unidentified "thing" was made of. How is this generally accomplished? I'm writing a story of which this is a peripheral element, but I'd rather not just have someone say, "We analyzed it, and it's ______," as that seems to be a sub-comic book turn of phrase.

Please assume in this case that whatever the substance in question is, it is rather hard to identify, forcing the chemist in question to exhaust many resources in the process.
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  #2  
Old 06-02-2004, 12:36 AM
Q.E.D. Q.E.D. is offline
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There are several tools which are used to analyze samples of unknown substances, such as mass spectrometers, gas chromatographs, absorption photometers and others. If you do searches on these terms, you'll find a wealth of information to get you started.
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Old 06-02-2004, 12:50 AM
Nametag Nametag is offline
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If I had that sort of lab, I guess I'd start with determining the class of compound; in fairly short order, one can distinguish an inorganic compound from an organic compound. Identifying inorganic compounds is done with a series of simple wet chemistry techniques. For organic compounds, I'd start with GC-MS (gas chromatograph / mass spectrometer).

A GC basically carries molecules of a substance in a stream of gas, and passes them through a column containing a material that is "molecularly sticky"; the molecules stick in the column briefly, and are swept away again by the carrier gas. The exact time that molecules of a given substance come out of the column is highly characteristic, and a mixture will usually be separated in the process. A detector will register the substances as a series of "peaks" in the data stream.

The chosen detector in this case is a mass spectrometer; these instruments use a variety of techniques to ionize the incoming molecules. The MS then separates the ions according to the ratio of mass to charge. For organic molecules, this is usually close to the moelcular weight (or a simple fraction thereof). Destructive MS can resolve the elemental composition of the material.

This information, along with a library of data for known compounds, will go a long way toward identifying an unknown material. Other good techniques are infrared spectroscopy, nuclear-magnetic resonance, and crystallography.
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Old 06-02-2004, 02:10 AM
antechinus antechinus is offline
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It also depends what form the substance is in - pure liquid, solid (powder, rock, crystal, amorphous), mixture of phases - liquid in solid, solid in liquid etc

Also a range of techniques can be used together to best determine the substance.

You may want to extract certain types of compounds from the sample first and analyse these if it is a mixture.

If you have an idea what it is you are looking for then a particular instrument or analytical technique may be chosen.

Assume it is not pure, then as nametag said, chromatography would be good (after you have extracted the substance into a solvent).

GC-MS
HPLC - another chromatography
XRF for solids
Ion chromatography
UV-vis spectroscopy - can help to start with
Infra red spectroscopy - good for a start
NIR - useless
Thermal analysis - for thermophysical properties
There are many other techniques as well as these common instruments.

If you can purify the substance NMR (nuclear magnetic resonance) is very useful for determining the exact structure.

Having the analysis associated with microscopy is sexy. eg XRF on electron microscope or infrared microspectrometry or mass spectroscopy.

A good one also would be IR followed by GC-MS then NMR. With various purifications along the way.
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Old 06-02-2004, 04:53 AM
Shalmanese Shalmanese is offline
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A lot also depends on the amount of sample you have avalible. Some techniques require a certain minimum amount so they might not be applicable in all cases.
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Old 06-02-2004, 07:05 AM
Roches Roches is offline
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Antechinus, that's a good summary of the modern technique of analysis and structure elucidation. Since this is going to be used to write a story, knowing two more things might be helpful: approximately what year the story is set in (some of these techniques are relatively new), and what the substance to be analyzed is. If it's a simple mixture (such as a white powder found in an envelope), the process would be different than if it were a more complex mixture (like green slime found on a wall). If it's a biological sample, such as blood, the process would be different entirely.

The older methods of analysis (classical, as opposed to spectroscopic analysis) are much more crude than the modern ones. Classical analysis involves a lot of separation with solvents, testing with indicators ('if it turns red, it's an aldehyde'), and determination of melting point (including preparing derivatives of a compound that have different but clear and well-known melting points). It also involves burning the compound to be analyzed in order to determine its chemical formula. This method is sufficient to identify a substance and determine its composition. Determining its structure is much, much easier by modern methods (especially NMR) but it was still possible to determine structures by classical analysis.
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Old 06-02-2004, 08:26 AM
bizzwire bizzwire is online now
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Analytical chemist (mass spectrometrist) chiming in.

As everyone who has posted has pretty much said it all, I'll just say that as someone who does this for a living, you usually don't start totally from scratch. I typically start working on an unknown by trying to gather as much information as possible before going near an instrument. Where did it come from, what's it's history? Who is bringing the material to me, and why? From these, you can get an idea of what it could be and how to attack it, but more importantly, you get a good feel for what it can't be.
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Old 06-02-2004, 10:06 AM
danceswithcats danceswithcats is offline
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When I've encountered an unknown substance, the HazCat™ kit is the toolbox to reach for. With a flow chart guide, persons with little chemistry background can classify, if not identify an unknown via a series of tests using materials and reagents within the kit.
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  #9  
Old 06-02-2004, 09:32 PM
Roches Roches is offline
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HazCat looks like it would be very useful for first responders that needed to have a rough idea of how a potentially dangerous chemical should be handled. It uses qualitative tests, which can only give basic information about the broad class of compounds to which a chemical belongs. In an emergency, it could be very useful. However, it can't identify chemicals definitively (which is what analytical chemists want to do).

In some cases it appears that it can be used to identify a specific compound, but even in then it wouldn't rule out all possible similar compounds. For example, a police hazardous materials team investigating a meth lab would know that a white powder that gave a positive result to a secondary amine test and a negative result to a test for alcohols would likely be methamphetamine. But a research chemist trying to identify such a compound would not be able to rule out hundreds of other possible secondary amines that are not alcohols. These sorts of answers can only be obtained through spectroscopy/spectrometry. (Well, not only, but classical analysis by degradation and derivatization is much less efficient.) For a story set in the present day, the analytical technique will probably be GC/MS, which has already been explained.

One other thing -- the chemical will probably not be all that hard to identify for an analytical or forensic lab. Most of the compounds one might expect to find in the real world are quite simple, and an experienced analytical chemist would probably be able to produce a conclusive result using only a few techniques. Even if it was something that came from outer space, it's unlikely that it'd be anything harder to identify than, say, a protein or a complex natural product.
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  #10  
Old 06-02-2004, 10:16 PM
bizzwire bizzwire is online now
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Quote:
Originally Posted by Roches
One other thing -- the chemical will probably not be all that hard to identify for an analytical or forensic lab. Most of the compounds one might expect to find in the real world are quite simple, and an experienced analytical chemist would probably be able to produce a conclusive result using only a few techniques. Even if it was something that came from outer space, it's unlikely that it'd be anything harder to identify than, say, a protein or a complex natural product.
Protein identification is a devilish tricky business. Proving that something is a protein is relatively straightforward. Identifying which protein is whole different beast. The current state of the art is to digest it into smaller pieces and compare the mass fingerprint (MALDI) or LC-MS/MS spectra to a database of known proteins. If the protein isn't in the database, you're screwed. A good match is just that- a good match, but not definitive identification since you never get complete coverage. To try and come up with the primary amino acid sequence ab initio is only done as a last resort.
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