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

In this work calibration of the mass axis was done by combining a single biopolymer witha homopolymer calibrant. The oligomeric masses, m j , with n repeat units of mass r andmasses of the end group, m end , of the polymer calibrant are given by:m = nr + m + m[5.1]jendsaltwhere n is the number of repeat units in the n-mer of the polymer and the m salt refers to themass of the metal cation adducted to the polymer n-mer.Calibration of the mass axis was accomplished through use of the biopolymer mass asfollows. The main peak from the biopolymer bovine insulin is assigned to its mass of5 730.61 u. The biopolymer peak will either lie between the masses of two n-mers of thepolymer calibrant, or exactly correspond to the mass of an n-mer. If it is at exactly the samemass as one of the n-mers of the polymer calibrant, use equation [5.1] to find the degree ofpolymerization, n, for the n-mer. If the peak of the biopolymer lies between the masses oftwo n-mers of the polymer calibrant, use equation [5.1] to find n 1 , the mass of the n-merclosest to the mass of biopolymer. Next we find additional calibration points by selectingpolymer peaks at intervals between five and 10 repeat units both less than and greater than n 1and compute their masses from equation [5.1]. Generally, a total of four or five calibrationmasses were selected.For this work, mass accuracy of only a few mass units was necessary because the mixturesof polydisperse homopolymers we used had different end groups with masses differences onthe order of tens of mass units (discussed below). All that was required is calibration closeenough to positively identify each oligomer. Furthermore, the main uncertainty in convertingfrom the MMS to the MMD lies in the signal axis quantitation. Achieving greater mass axisaccuracy nets no increase in the accuracy of the MMD measurement.11