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

sample using a separate method, or by comparing against a certified value measured using thesame method. We call these methods "soft quantitation".From a second point of view we can ask, how well does the method measure the "true"value of the quantity we are trying to measure? How we know what is the true value is oftenunclear, so we ask another question. What are the uncertainties in the measurement, otherthan statistical, which affect the measurement or the quantity we are trying to measure? Wewould like to be able to say something quantitative about these Type B (systematic)uncertainties in the MMS measured by MALDI-TOF MS arising from instrument attributessuch as laser energy or detector voltage, and sample preparation parameters (such as choice ofMALDI matrix, or matrix:salt:polymer ratio). In order to estimate these uncertainties, oneneeds a physical or mathematical model describing the experiment, i.e., how the experimentrelates to the measurand. This is called “hard quantitation”.In this work the ratios of gravimetric masses of distinct polymers, defined as G gi , forpolymer i are compared to G Mi , the total mass estimated from the MALDI-TOF MSmolecular mass spectrum of polymer i. We cannot obtain the exact measure of total massfrom MALDI-TOF MS but we can accurately and precisely measure mass ratios, i.e. the ratioG M1 /G M2 . Comparison of G g1 /G g2 to G M1 /G M2 is the rationale on which SRM 2881 is based.5. Mass Axis Calibration ProcedureMass axis calibration is more easily performed than signal axis calibration. Calibration ofmost time-of-flight instruments is usually done with monodisperse biopolymers of knownmolecular masses. These biopolymers are selected because they typically provide a singlemajor peak whose mass is known accurately; thus, mass axis quantification is quitestraightforward. Calibration usually can be done using three of these biopolymers. Collectingdata with 2 ns time intervals, one can get better than single mass unit accuracy on aninstrument with a 1.5 m flight tube in reflectron mode at a mass of about 7000 u.10