Standard Reference Material 2881 - National Institute of Standards ...

were also used in this experiment: PS10k (M n of 10 000 u) from Polymer StandardsService, GmbH (Mainz, Germany), and catalog number 745 (lot 02) (M n of 8000 u)from Scientific Polymer Standards (Ontario, NY). The matrix used in theseexperiments was all-trans retinoic acid (RA) purchased from Sigma-Aldrich(Milwaukee, WI) and used as received. The retinoic acid was stored in a freezer topreserve it. Silver trifluoroacetate (AgTFA) was purchased from Sigma-Aldrich andused as received. The solvent used for all the experiments was unstabilizedtetrahydrofuran (THF) also from Sigma-Aldrich. The THF was checked before eachexperiment for presence of peroxides using Quantofix Peroxide 100 (Macherey-Nagel,Düren, Germany) test strips. No THF with more than 1 mg/L peroxides was used.7.2 MALDI-TOF Mass SpectrometryAll experiments were performed on a Bruker Daltonics (Billerica, MA) Reflex IIMALDI-TOF MS with a 2 GHz digitizer. The flight distance is nominally 1.5 m fromsource to microchannel plate detector. A Laser Science Incorporated (Franklin, MA)model LSI-337 nitrogen gas laser operating at 337 nm with an approximately 3 ns pulsewidth was utilized. The laser pulse energy was tested both before and after theexperiments on each gravimetric composition using a Laser Probe Inc. (Utica, NY)universal radiometer (model RM-3700) with a model RJP-465 energy probe. Twentyevents were averaged to compensate for the inherent shot-to-shot variation found innitrogen gas lasers. All mass spectra in this experiment were obtained in reflectronmode. Mass axis calibration was performed as described in Section 5.7.3 Instrument Optimization for Signal AxisThe instrument was tuned for quantitation by numerical optimization using astochastic gradient approximation method to find a region in instrument parameter spacewith the least mass bias. This is described in full detail in a previous publication 2 . Webriefly describe the method here so we may refer to it in our discussion of Type Buncertainties for the SRM 2881.22