Mass Spectrometry is used to measure
the molecular mass and molecular mass distribution of synthetic polymers,
as well as in studies of molecular architecture and chemical composition,
especially of prepolymers and copolymers.
MALDI-TOF-MS, Matrix Assisted
Laser Desorption Ionization-Time of Flight-Mass Spectrometry uses
laser light to ablate entire, unfragmented polymer molecules, from
a target surface into a time-of-flight mass spectrometer. A UV-absorbing
organic matrix is used to facititate the ablation of intact polymer
molecules.
The facility has a Bruker Reflex time-of-flight mass spectrometer
(shown in top photograph) with the capability of mass measurements
on synthetic polymers up to 250 ku. Isotope resolution may be obtained
for polymers with masses less than 5 ku. Polymers of larger mass,
depending on the repeat mass of the polymer, may be resolved at
the repeat mass level. For example, polystyrenes with masses less
than 40 ku, can be resolved by repeat units. As a consequence, copolymer
compositions or end group distributions may be obtained as a function
of molecular mass for polymers with total mass below 40 ku. Also,
at low masses, studies may be made of polymer architecture and how
it is affected by chemical synthesis.
The facility also has a Comstock/Atom Sciences RTOF-260 time-of-flight
mass spectrometer with the unique capability of a heated sample
stage. This allows for studies of crystalline polymers by heating
them above their melting point. It also allows for the study of
target temperature effects on the MALDI process. This instrument
has a gentle 5kV ion-extraction voltage and delayed ion extraction
with extraction times on the order of microseconds.
Sample preparation of synthetic polymers that will not dissolve at room
temperature presents a significant obstacle to the MALDI analysis of such
materials. Bill Blair has constructed a device in our laboratory for the
electrospray deposition of polymer samples dissolved in solvent up to 150
°C. The electrospray apparatus, the MALDI target and a small reservoir of
dissolved polymer are contained in a thermostated heated zone allowing the
dissolved polymer to be electrosprayed onto the target without
precipitation out of solution. To electrospray a sample, solvent delivered
by a syringe pump operating at room temperature displaces hot
polymer-solvent solution within the heated zone of the apparatus. We have
found that electrospray MALDI sample preparation crucial for creating
reproducible mass spectra, which is critical to our standards development
effort.
The Division is actively pursuing research in the development
of MALDI-TOF-MS as 1) a quantitative tool to obtain the overall
molecular mass distribution of synthetic polymers, 2) a method to
obtain the mass spectrum of hydrocarbon polymers, such as polyethylene
and polypropylene, that have historically not been amenable to analysis
by MALDI-MS, 3) a means to study polymers (particularly prepolymers)
where, with the help other techniques, the unique structural or
architecture properties of these polymers can be observed.