98 



M~* ion by electron capture utilizes methane as a moderating gas for the production 



of thermal electrons. Thermal electrons can then be captured by species with a 

 sufficient electron affinity. Carboxylic acids such as lactic acid have not been found 

 to successfully undergo electron capture without derivatization [74]. Lactic acid 

 analyzed under the conditions employed for this work typically exhibits an odd- 

 electron M~' (formed via electron capture) abundance which is <0.5% of the even- 

 electron deprotonated molecular species [M-H]~ intensity. The process which forms 

 the deprotonated molecular species involves abstraction of a proton from lactic acid 

 by ionic species fonned via ion-neutral reactions with methane reagent gas in the ion 

 source. Examination of negative ions for structurally informative fragments is not 

 as prevalent as the use of EI or positive CI for structure elucidation. Examination 

 of negative ions with lactic acid was found to be not only informative but facile due 

 to the presence of the carboxyl function group. 

 Characteristic fragmentations 



The deprotonated molecular species, [M-H]~ of lactic acid is formed via the 

 loss of the hydrogen on the carboxyl group, leaving the species with a net negative 

 charge. This species is generally the base peak in NCI mass spectra of lactic acid, 

 provided the sample concentration in the ion source is adequately low to prevent 

 additional reactions with neutral lactic acid (this is addressed later in this chapter). 



Selection of the [M-H]~ followed by fragmentation via CID produces the 

 daughter spectrum in figure 3-1. It should be noted that this daughter spectrum was 

 acquired using a collision energy of 9 eV and an indicated pressure of 1.97 mtorr N,. 



