Direct Analysis of Polymer Chemical Mixtures by Field Desorption Mass Spectroscopy

Abstract

Abstract Several examples have been given in which field desorption mass spectroscopy was very effective in the analysis of polymer chemical mixtures. The method has proved to be an excellent screening technique for initial examination of complex samples. Thus FD-MS data may be used as a basis for deciding whether further characterization by other spectroscopic or chromatographic techniques is necessary. In many cases, the FD molecular weights alone can quickly provide the data necessary for solution of a particular analytical problem. The fact that normally only one piece of data (the molecular ion) is provided for each component makes FD-MS unique in its ability to characterize polymer chemical mixtures. FD-MS is a very versatile tool that can be used in a number of practical applications of interest to the polymer chemist. On the negative side, it is evident that FD-MS by itself provides only limited chemical structure information. The molecular weight is provided and often nothing else. If this along with the history of the sample is not enough to deduce the structures present, then additional information must be obtained. Accurate mass measurements were used in several of the examples cited above. Isotope peaks for certain elements (e.g. S, Cl, Br) are often helpful, and electron impact (EI) and chemical ionization (CI) mass spectra may be obtained for the same samples. Other spectroscopic techniques (infrared and magnetic resonance) can be used to provide detailed structural information (e.g. isomerism and stereochemistry). Finally, chromatographic separations (GC, LC, GPC) may be necessary to isolate individual components for spectroscopic characterization. Field desorption may in some cases have other limitations that are not so obvious. The technique generally is not a good quantitative method although some promising work in this regard has appeared in the literature. The method is also compound selective; desorption rates and ionization efficiencies are quite dependent on molecular weight, volatility, chemical structure, and other factors. Certain types of highly polar compounds (underivatized carboxylic acids, for example) can be particularly troublesome. FD-MS operation at higher masses (MW > ∼ 2000) can be very difficult. Isomers which may be present are not distinguished since they have the same molecular weight. Finally, the method is not a “trace” technique in the modern sense of the word. Submicrogram quantities of pure materials can be examined, but for mixtures useful information may be lost for components present at levels less than ∼ 1–2% of the total. Nevertheless, we estimate that over 90% of the polymer chemical samples examined in our laboratory by field desorption have given good, readily interpretable FD mass spectra. Thus FD-MS has proven to be very effective tool for general characterization of these types of samples. The novel advantages of the method normally far outweigh its negative aspects and sometimes fickle reputation. Field desorption provides a very nice complement to structural data obtained by magnetic resonance, infrared, and electron impact mass spectroscopy. In most cases FD-MS can quickly provide chemical information on complex polymer chemical mixtures never before obtainable by any technique.