Check on these:

* 5 years of nanomaterial analysis by means of ICP-MS: Guest Edited by Björn Meermann and Francisco Laborda 

http://pubs.rsc.org/en/journals/articlecollectionlanding?sercode=ja&themeid=2934e025-a882-4787-b1d9-cca9d8e83e06

** Analysis of nanomaterials by FFF- and single particle ICP-MS: Björn Meermann and Francisco Laborda

http://pubs.rsc.org/en/journals/articlecollectionlanding?sercode=ja&themeid=b5340fc4-5657-435c-bdc3-a3677bf44854

Here you have some approaches for this specific purpose:

http://pubs.acs.org/doi/abs/10.1021/ac302851d

http://pubs.acs.org/doi/abs/10.1021/ac403998e

http://www.sciencedirect.com/science/article/pii/S0584854714001979

http://pubs.acs.org/doi/abs/10.1021/es504088e

ESCA (XPS) or Auger may be a better bet for surface compositions and oxidation states. If your silver NP's are exposed to air then the top 5 - 10 or so atomic layers will have Ag in the +1 oxidation state.  Can be reduced to Ag by H2 and other routes.  Needs to be kept in a reducing environment for zerovalent Ag to be completely present - and if on a support containing oxygen (e.g. SiO2, Al2O3, TiO2 etc) then will still be oxidized on the surface.