June 25, 2006

Analytical Chemistry 

Pretty soon during the course of civilization (I suppose), people began to ask the questions "What is this stuff?" and "How much of it is in here?". Analytical chemistry does answer these questions; specifically, its qualitative branch can tall what is that stuff, and its quantitative branch can tell its concentration. There is also structural analytical chemistry, and it task is determining the exact structure of molecules and other species.

Qualitative analysis is the oldest, and in its original form it was based on carachteristic reactions. I studied all this long ago, but don't recall much now; to make a couple of examples, silver ions produce a white, cheese-like precipitate of AgCl reacting with chloride ions; α-methyl ketones and aldehydes react with iodine in a basic environment to produce a yellow precipitate of iodoform. There is a number of rections of this kind, and in the hands of a skilled chemist they can be used to recognize with good precision a wide variety of substances.

Qualitative analysis is still very important for forensics, to find invisible traces of blood and gunshot residues; in medicine to test for pregnancy and the presence of certain bacteria, and finally on the battlefield to detect chemical and biological agents - in this latter case, the field test are designed to give more false positives than negatives.

Quantitative analysis can also be done using carachteristic reactions and measuring the amount of reagent consumed or the products produced, but it is done much better using instruments. Instrumental analysis is the most recent development, but it revolutioned the field; a very welcome feature of instrumental techniques is that they often are qualitative and quantive at the same time. I cannot describe all of them, even briefly, tho.

In the quest to answer to the questions asked at the beginning, no property of matter is left inviolated: atoms and molecules are sorted by their mass, size and shape and interaction with similar and different species (chromatography); they are irradiated with radiations from almost all of the electromagnetic spectrum (spectroscopy, X-ray diffraction and fluorescence); heated up into plasma (ICP-AES); bombarded with electrons, neutrons and ions (neutron diffraction and some methods to examine surfaces); fragmented, ionized, squeezed and accelerated through electromagnetic fields (mass spectrometry) and forced to align with powerful magnetic fields (NMR). And other perversions I do not recall or have been put together more recently.

We've become pretty good at torturing chemical species until they spill the beans, so much that the sensitivity of the best techniques is in the order of the ppb - parts per billion, or a milligram of something in one metric ton of something else. And as little as 10-2 g of some substance is enough for a complete structural analysis using NMR.

Analytical chemists do screw up from time to time (like anyone else), but most of the time their findings are accurate and reliable, no matter how surprising or even weird they may be.


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