In the ideal case, the EI spectrum of a sample compound will provide the molecular weight (as the m/z value of the [M].+ ion), in addition to the corresponding m/z values of the fragment ions. Fragment ion peaks may provide structural information about the sample compound analyzed. As a result of the high energy of the ionization process (the sample is bombarded by 70 eV electrons), the molecular ion may not be observed. Note that unknown sample compounds can sometimes be identified by searching a database. The NIST library, which contains spectra for approximately 150,000 compounds, is commonly used for database searches.
An accurate mass measurement on a compound can be useful for confirming a proposed molecular formula, particularly if for some reason the elemental composition cannot be measured. The measured mass, with some assumed accuracy (generally ± 5 ppm), is submitted into a program which returns the possible molecular formulae. Usually constraints are included on the number of possible atoms to limit the number of retrieved formulae.
GC/MS measurements are performed on the GCT instrument interfaced to a Agilent 6890 Series GC System. The Agilent 6890 has a J&W DB-XLB 30m column and an autosampler. This is particularly useful in analysis of mixtures - samples may contain either several analytes of interest or impurities which interfere with MS analysis of the compound of interest.
GC trace of a pesticide mixture
MS of each component of pesticide mixture and their respective NIST reference spectra.
Raw data of Mut AroA.
Deconvoluted spectrum of Mut AroA using the Maximum Entropy calculation. (P.Berti)
LC/MS and LC/MS/MS measurements are carried out on the Quattro Ultima instrument, which is interfaced to a Waters 2690 High Performance Liquid Chromatograph (HPLC). The Waters 2690 has a ternary pump system, column compartment temperature control and an autosampler. This is particularly useful in analysis of mixtures - samples may contain either several analytes of interest or impurities which interfere with MS analysis of the compound of interest.
High resolution ESI data is similar to that obtained on the low resolution instrument, but offers higher sensitivity, resolution and mass accuracy.
MSMS sequencing of a tryptic peptide of Cytochrome c.
Compounds are observed as [M]+, [M+H]+, [M+Na]+, etc., in positive ion mode, and [M]- or [M-H]- in negative ion mode. This is a relatively gentle ionization mode, but usually more fragmentation is observed than in ESI. Complications can arise because of fragmentation and interactions with matrix-derived moieties.
Material requirements are quite low, for example, for peptides1 ul of a 1 uM solution is sufficient.
MALDI Spectrum of PNIPAM (poly-N-isopropylacrylamide) polymer. (Stover group)