Analytical chemistry is the field of chemistry which concerns itself with identifying the components of an unknown or a mixture and quantifying the amount of a species which is present. The analysis may be on a large scale (such as the amount of calcium in Portland Cement) or trace (trace cadmium in oyster shells). It may be low tech as in titrations and gravimetric analysis, or requiring high tech instruments including, but not restricted to, gas chromatographs, liquid chromatographs, mass spectrometry, flame atomic absorption and electrochemistry. Each technique has it's strengths and weaknesses with respect to the types of analysis it can perform.
This can probably be best demonstrated with some examples:
1) Anodic Stripping Voltammetry to find trace heavy metals in aqueous solution
2) MALDI-TOFMS to find the molecular weight of a purified protein
3) GC-MS to detect hydrocarbons in ocean sediment
4) Titration to determine iron in iron ore, copper in brass alloy, etc.
To do an accurate analysis of anything, care must be taken from sampling (make sure that the sample is a good representation of the whole), sample preparation (may involve extraction, filtration, etc.), as well as the analysis itself. Analytical chemistry techniques are widely applied in the fields of food safety, quality control, and forensic science.