Mass spectrometry (MS) is a technique utilized to facilitate structure determination by providing accurate molecular weight measurements. Several modes of mass spectrometry are available including electron ionization(EI)-MS, chemical ionization(CI)-MS, gas chromatography(GC)-MS, and electrospray ionization(ESI)-MS.

In EI-MS, the sample is bombarded with electrons such that ionization and fragmentation occur. The molecular ion and characteristic fragments are introduced into the mass spectrometer and the charge/mass ratio is recorded.

In CI-MS the sample molecules are ionized in the gas phase by ion-molecular reactions from a reagent gas (commonly methane). The sample reacts with the ions from the reagent gas to produce protonated molecular ions. The result is a much simpler mass spectrum when compared to EI-MS.

In GC-MS, the sample mixture is first separated by a gas chromatograph. Upon entering the MS, the components in each peak are identified by their molecular ion and fragmentation pattern. In the last technique, electrospray-MS, molecular ions are continuously produced directly from solution. The samples, typically soluble in water or alcohol, are usually polar and relatively clean.

In MALDI-TOF-MS, the sample material is incorporated into a matrix that acts to absorb energy from the laser to prevent polymer decomposition and isolate the polymer molecules from one another. The sample/matrix mixture is placed on a sample probe tip. The solvent is then evaporated, leaving co-crystallized polymer molecules homogeneously dispersed within matrix molecules. When the pulsed laser beam is tuned to the appropriate frequency, the energy is transferred to the matrix which is partially vaporized, carrying intact polymer into the vapor phase and charging the polymer chains. Multiple laser shots are used to improve the signal-to-noise ratio and the peak shapes, which increases the accuracy of the molar mass determination. In the linear TOF analyzer, the distribution of molecules emanating from a sample are imparted identical translational kinetic energies after being subjected to the same electrical potential energy difference. These ions will then traverse the same distance down an evacuated field-free drift tube; the smaller ions travel down the tube more rapidly and thus reach the detector before the more massive ions. Separated ion fractions arrive at the detector, which produces a signal upon impact of each ion group. The digitized data generated from successive laser shots are summed yielding a TOF mass spectrum. MALDI-TOF-MS has become an increasingly valuable technique for the characterization of synthetic polymers and biopolymers. With a mass range up to 500,000 Da, MALDI-TOF-MS is particularly useful for identification polymer component end groups, purity, and overall molecular weight distribution. The data also provide us with the ability to calculate polydispersity values for synthetic polymer systems.

Examples
Mass spectrometry techniques are utilized for molecular characterization, mixture analysis, trace analysis and quantitative analysis. Because of its broad application, MS is used extensively in our laboratory. Some examples of projects in which MS has played a critical role include:

• Identification of reaction products of an organic reaction.
• Molecular weight determination of water soluble polymers.
• Identification of low molecular weight oligomers in polymeric products.
• Identification of volatile unknowns separated by gas chromatography.
• Identification of impurities in pharmaceutical type products.