CHE 232 Section 1

MWF 12-12:50pm

Chemical Identification

• Comparison of Physical Properties– Boiling Point– Melting Point– Density– Optical rotation– Appearance– Odor

• Chemical Test– Elemental Analysis

• Burn the compound and measure the amounts of CO2, H2O and other components that are produced to determine the empirical formula.

• Used today as a test of purity of compounds that have already been identified.

• Spectroscopy – measures the interaction of a compound with electromagnetic radiation of different wavelengths.– Nuclear Magnetic Resonance Spectroscopy (NMR) measures the

absorption of radio waves by C and H in a magnetic field. Different kinds of C and H absorb energy of different wavelengths.

– Infrared (IR) Spectroscopy measures the absorption of infrared (heat) radiation by organic compounds. Different functional groups (C=O, -OH) absorb energy of different wavelengths.

– Ultraviolet/Visible Spectroscopy (UV/Vis) measures the absorption of ultraviolet and visible light by bonds in an organic compound. Bonds of different types and with different extents of conjugation (C=C, C=O, C=C–C=C, aromatic) absorb energy of different wavelengths.

• Mass Spectrometry– Doesn’t involve the absorption of any type of light.– Used in determining the molecular weight and formula of a

compound.– A compound is vaporized and ionized by bombardment with a

beam of high-energy electrons. – The electron beam ionizes the molecule by causing it to eject an

electron.– When the electron bean ionizes the molecule, the species formed

is called a radical cation, and symbolized as M+•.– The radical cation M+• is called the molecular ion or parent ion.– The mass of M+• represents the molecular weight of M.

• Because M is unstable it decomposes to form fragments of radicals and cations that have a lower molecular weight than M+•.

• The mass spectrometer measures the mass of these cations.

• The mass spectrum is a plot of the amount of each cation (relative abundance) versus its mass to charge ratio (m/z, where m is mass and z is charge)

• Since z is almost always +1, m/z actually measures the mass (m) of the individual ions

Methane CH4

Though most C atoms have an atomic mass of 12, 1.1% have a mass of 13. Thus, 13CH4 is responsible for the peak at m/z = 17. This is called the M + 1 peak.

• Alkyl Halides and the M+2 peak– Most elements have one major isotope.– However some halogens have more than one. Iodine and Fluorine are

isotopically pure but….– Chlorine has two common isotopes, 35Cl and 37Cl, which occur naturally in a

3:1 ratio.– The larger peak, M, which corresponds to the compound containing 35Cl. The

smaller peak, the M+2 peak, corresponds to the compound containing 37Cl. – Thus, when the molecular ion peak consists of two peaks (M, M+2) in a 3:1

ratio, a Cl atom is present.– Bromine also has 2 isotopes, 79Br and 81Br in a 1:1 ratio. Thus when the

molecular ion consists of two peaks (M, M+2) in a 1:1 ratio, the compound contains a Br atom.

ClCl-CH3-Cl-CH3

Br-Br-CH3

Sample introduced into GC inlet vaporized at 250 °C , swept onto the column by He carrier gas & separated on column. Sample components emerge from column, flowing into the capillary column interface connecting the GC col-umn and the MS (He removed).

Gas Chromatography/Mass Spectrometry (GC/MS)

Mass SpectrometryGas Chromatography-Mass Spectrometry (GC-MS)

•To analyze a urine sample for tetrahydrocannabinol, (THC) the principle psychoactive component of marijuana, the organic compounds are extracted from urine, purified, concentrated and injected into the GC-MS.•THC appears as a GC peak, and gives a molecular ion at 314, its molecular weight.

Forensic Mass Spectrometry

J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995

•Analysis of Body Fluids for Drugs of Abuse•Analysis of Hair in Drug Testing•Sports Testing•Analysis of Accelerants in Fire Debris•Analysis of Explosives•Use of Isotope Ratios

2-chloropropane

Bromobenzene

pentane

1-pentene

1-pentyne