27115
We have demonstrated that the complementary combination of grazing incidence X - ray fluorescence ( GIXF ) with specular X - ray reflectivity ( XRR ) can be used to quantitatively determine the density profiles of Ni ( 2 ) ( + ) ions complexed with chelator headgroups as well as S atoms in recombinant proteins anchored to lipid monolayers at the air / water interface .
 Chemical: Ni ( 2 ) ( + ); Chemical: S
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: Ni ( 2 ) ( + ); Chemical: S