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A matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) interlaboratory comparison was conducted on mixtures of synthetic polymers having the same repeat unit and closely matching molecular mass distributions but with different end groups. The interlaboratory comparison was designed to see how well the results from a group of experienced laboratories would agree on the mass fraction, and molecular mass distribution, of each polymer in a series of binary mixtures. Polystyrenes of a molecular mass near 9000 u were used. Both polystyrenes were initiated with the same butyl initiator; however, one was terminated with -H (termed PSH) and the other was terminated with -C[H.sub.2]C[H.sub.2]0H (termed PSOH). End group composition of the individual polymers was checked by MALDI-TOF MS and by nuclear magnetic resonance (NMR). Five mixtures were created gravimetrically with mass ratios between 95:5 and 10:90 PSOH/PSH. Mixture compositions where measured by NMR and by Fourier transform infrared spectrometry (VF-IR). NMR and FT-IR were used to benchmark the performance of these methods in comparison to MALDI-TOF MS. Samples of these mixtures were sent to any institution requesting it. A total of 14 institutions participated. Analysis of variance was used to examine the influences of the independent parameters (participating laboratory, MALDI matrix, instrument manufacturer, TOF mass separation mode) on the measured mass fractions and molecular mass distributions for each polymer in each mixture. Two parameters, participating laboratory and instrument manufacturer, were determined to have a statistically significant influence. MALDI matrix and TOF mass separation mode (linear or reflectron) were found not to have a significant influence. Improper mass calibration, inadequate instrument optimization with respect to high signal-to-noise ratio across the entire mass range, and poor data analysis methods (e.g., baseline subtraction and peak integration) seemed to be the greatest obstacles in the correct application of MALDI-TOF MS to this problem. Each of these problems can be addressed with proper laboratory technique.

A mathematical algorithm is presented that locates and calculates the area beneath peaks from real data using only reproducible mathematical operations and no user-selected parameters. It makes no assumptions about peak shape and requires no smoothing or preprocessing of the data. In fact, it is shown that for matrix-assisted laser desorption time-of-flight mass spectra noise exists at all frequency ranges making the smoothing of data without distortion of peak areas impossible. The algorithm is based on a time-series segmentation routine that reduces the data set to groups of three strategic points where each group defines the beginning, center, and ending of each peak located. The peak areas are found from the strategic points using a commonplace polygonal area calculation routine. Peaks with statistically insignificant height or area are then discarded. The performance of the algorithm is demonstrated on a polystyrene mass spectrum with varying degrees of noise added either mathematically or experimentally. An on-line implementation of the method, termed MassSpectator, for public use can be found at www.nist.gov/maldi.

Polyethylene is one of the most important industrial polymers and is also one of the most challenging polymers to be characterized by mass spectrometry. We have developed a substrate-assisted laser desorption/ionization (LDI) mass spectrometric method for polyethylene analysis. In this method, cobalt, copper, nickel, or iron metal powders are used as a sample substrate and silver nitrate is used as the cationization reagent. Using a conventional UV LDI time-of-flight mass spectrometer, intact oligomer ions having masses up to 5000 u can be detected. Cobalt is found to produce spectra with the highest signal-to-noise ratio and the lowest level of fragmentation. Cobalt powder size is shown to have some effect on the spectra produced. The best results are obtained with the use of cobalt powders with diameters ranging from 30 to 100 [micro]m. Fragmentation cannot be totally eliminated, but the fragment ion peaks can be readily discerned from the intact polyethylene ions in the substrate-assisted LDI spectrum. Thus, the average molecular masses of low-mass polyethylene samples can be determined by using this method. A rapid heating model is used to account for the effectiveness of using the coarse metal powders to assist the analysis of intact polyethylene molecules by LDI.

1. SummaryNon-targeted global metabolite profiles of Standard Reference Material (SRM) 1950 Metabolites in Human Plasma were obtained on five different liquid chromatography/mass spectrometry (LC/MS) instrument combinations using identically prepared samples. [...]

The method of preparation and methods of analysis of a narrow distribution polystyrene of ~7 ku used in an interlaboratory comparison of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) of synthetic polymers is described. Size exclusion chromatography was used to measure the polystyrene sample variability. Fourier transform infrared spectroscopy and MALDI-TOF-MS were used to analyze end groups on the polymer. The polystyrene was analyzed by MALDI-TOF-MS and classical methods of polymer characterization. The number ([M.sub.n]) and mass ([M.sub.w]) average of the molecular mass distribution (MMD) determined by the classical methods (light scattering and NMR) were compared with those obtained by MALDI-TOF-MS. Agreement between classical methods to obtain the moments of the MMD and the MALDI is found to be good overall. However, all the experimental values obtained by MALDI fell below the classical values. A discussion of why these values are lower is included. We discuss the statistical analysis of the data from the interlaboratory comparison conducted by NIST, which includes data from 23 different laboratories. Analysis of variance is used to examine the influences of the independent parameters (laboratory, matrix, instrument manufacturer, instrument mode) on the data. The parameters, laboratory and instrument manufacturer, were determined to have an influence on the MMD, where matrix and instrument mode were found not to have a significant influence.