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

where we note Q , Q , kABoAandkoBare all functions ofoM . Notice two things are needed forthe ratios of the true masses to be obtained from the measured ratio of the MALDI-TOF MS.First, the ratiokoA/ k oBneeds to be determined. If polymers A and B are chemically theidentical with different average molecular masses then the ratio is one because we have takenthe expansion point for the Taylor’s expansion as M o and that was chosen to be the same forboth. If the repeat group is identical but the end group is different then it is possible theionization probability may be different. In this paper we have chosen end groups with similarmasses (58 u to 114 u) and with the same, minimal polarity (n-butyl and n-octyl) so thatablation and ionization should be controlled by the main polymer PS repeat. (Since we expectlittle silver charging on the butyl or octyl end group, corrections should be much less than[number of repeats units] –1 ). Thus we may assume the ratio ofkoA/ k oB= 1. The sameargument applies to Q, i.e. that it is independent of the end group and only depends on M 0 andthe other machine parameters.Thus we have from equation [6.18]GGexpTAexpTB=GGoTAoTB⎧ 1 + ( Q⎨⎩1+ ( QAo/ ko)( M/ k )( MoAWAoWBo− M− MWBo))⎫⎬⎭[6.19]Simple algebra leads us to:exp 000G⎧⎫TAGTA( Q / k0)( MwA− MwB)= ⎨1+exp 00 ⎬[6.20]GTBGTB ⎩ 1 + ( Q / k0)( MwB− M0) ⎭We shall later plotGGexpTAexpTBexpGTA0expGTAversus0 . The slope,GTB0GGTB0GTATB, of that plot is:19