In this experiment, a Finnigan 4000 GC/MS was used to identify unknowns. Five unknown compounds were analyzed by 70 eV electron ionization mass spectrometry. Three of these compounds were identified using the NIST mass spectral database. The other two compounds were identified by analyzing the mass spectrum. The unknowns were identified as: methane (unknown 01), ethane (unknown 02), propane (unknown 03), butane (unknown 04), and pentane (unknown 05). Methane and ethane were identified from the analysis of their mass spectra, propane, butane and pentane were identified by library matches. Based upon this experiment it is apparent that mass spectrometry is a very powerful tool for identifying unknown compounds.
Vacuum ultraviolet photoionization by coherent 10.5 eV radiation is evaluated for soft ionization of organic molecules. Coherent 10.5 eV radiation is produced by frequency tripling the third harmonic of a Nd:YAG laser (355 nm) in a mixture of xenon and argon. A number of intermediate size, C6 to C8, aliphatic compounds are studied to determine the extent and characteristics of fragmentation. Compared to 12 eV electron impact ionization, all show higher molecular ion abundances, less fragmentation, and significant enhancement of low energy rearrangement ions. n-Alkanes, alkenes, ketones, carboxylic acids, and ethers all form predominant molecular ions. Aldehydes and amines show significant molecular ion abundances but also extensive fragmentation. Branched alkanes, dienes, alcohols, and esters show little or no molecular ion, but do have a single dominant fragment ion. Metastable broadening in these spectra gives qualitative information on the energetics of low energy rearrangements.
Photodissociation-photoionization mass spectroscopy can distinguish isomeric compounds which have very similar conventional mass spectra. This is demonstrated for 1-, 2-, and 4-octene. By varying the time-delay between photodissociation and photoionization, the kinetics of dissociation can be studied. Secondary fragmentation is evident in the mass spectra and can make spectral interpretations difficult. Ways to reduce the extent of secondary fragmentation are proposed.
Photodissociation-Photoionization Mass Spectrometry (PDPI/MS) is used to probe the neutral unimolecular photodissociation of branched alkenes and dienes. Molecules are photodissociated with an ultraviolet laser, and the resulting neutral fragments are then photoionized with coherent vacuum ultraviolet radiation. The unimolecular photodissociation of dienes and branched alkenes is dominated by -cleavage which can be used to identify alkene isomers. When two isomers produce identical -cleavage fragments, differences in the secondary fragmentation products can be used to determine double bond and branching locations. Neutral fragmentation patterns are especially useful for characterizing compounds which isomerize prior to ionic fragmentation. The -cleavage and secondary fragmentation patterns are predictable and usually occur without isomerization. As a result, PDPI/MS is a useful tool for distinguishing alkene isomers.
Please send any comments, corrections, or suggestions to svanbram@science.widener.edu.
This page has been accessed
times since 1/5 /96 .
Last Updated 1/5/96