ALGORITHM FOR MOLECULAR-WEIGHT PREDICTION FROM EI MASS-SPECTRA

It is very important to predict the molecular weight of a compound fro m its EI mass spectrum, because molecular weight is one of the most im portant information for its structure determination. The program for m olecular weight prediction is always an important part in a structure elucidation system. One cannot directly get the molecular weight from EI mass spectra in the case of no molecular ions for many compounds, b ut it is possible to be predicted in terms of the fragmentation rules. A program for this purpose is tire STIRS program developed by McLaffe rty's group([1]). The algorithm of predicting the molecular weight is from database search results. It is impossible to give the correct mol ecular weight when there are no related mass spectra in the database. We have established a program using neutral loss information and nitro gen rule in our ASES/MS system([2]). The algorithm predicts the molecu lar weight only using the mass spectrum of the unknown without any add itional information. It automatically infers the neutral loss spectra from primary and secondary neutral losses and estimates the molecular weight by the neutral loss spectrum and nitrogen rule. The program has successfully been used in the molecular weight prediction of organic acid methyl esters in Tuebingen University of Germany. The program pre dicts the molecular weight as follows: MW = MSP + Delta M, where MW is the molecular weight of the unknown, MSP represents the maximal singl e isotope ion in the mass spectrum and Delta M is a rational neutral l oss mass, which takes the value from -1, 0, 1, 2, 3, 15, 16,.... A rat ional molecular weight must accord with two conditions: (i) it must co rrelate with a series of rational neutral losses; (ii) it must accord with nitrogen rule. The reliability factor RF is calculated by using t he following formula RF = Sigma P(Delta Mi)A((MW-Delta Mi))(1+B), wher e P-Delta Mi represents the probability for the occurence of neutral l oss Delta M(i), A((MW-Delta Mi)) shows the weighted intensity at the m ass of MW-Delta M(i), B is a parameter related with nitrogen rule fact or PAO, PAO is a ratio of the weighted intensity at odd mass to the we ighted intensity of the total. PAO = Sigma A(odd)/Sigma A(total), wher e Sigma A(odd) represents the sum of weighted intensity at odd mass an d Sigma A(total) represents the sum of weighted intensity for all peak s. When PAO less than or equal to 50%, the molecular weight should be an odd number; when PAO > 60%, the molecular weight should be an even number; when 50% < PAO less than or equal to 60%, the molecular weight takes both even and odd numbers.