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  Global Journal of Analytical Chemistry. Volume 2, Issue 2 (2011) pp. 50-59
  Research Article
Relationship between composition of acetonitrile-water mobile phase and retention of urea, thiourea, and formamide void markers in a C18 HPLC column
  Eric Loesera*, Zhaoxia Liub, Marilyn DelaCruza, Vinay Madeppallib  
a Pharmaceutical and Analytical Development,Novartis Pharmaceuticals, East Hanover, NJ, USA
b Chemical and Analytical Development,Novartis Pharmaceuticals, East Hanover, NJ, USA

  The purpose of this study was to determine if several commonly used void markers could accurately indicate the volume of adsorbed acetonitrile layer on C18 adsorbents in contact with acetonitrile/water mobile phases. Theoretical retention volumes were calculated based on a simple partition model. To determine how much urea or thiourea would partition into the adsorbed layer, partition coefficients were estimated based on experimental solubility measurements of urea and thiourea. Experimental retention volumes of urea were consistently smaller than calculated volumes, suggesting that urea was excluded from a volume which was even larger than the adsorbed layer. Thiourea retention was larger than expected in water-rich mobile phases, but smaller than expected in acetonitrile-rich mobile phases. A third void marker, formamide, showed the most reasonable agreement between experimental and calculated retention volumes. Possible causes for the different behavior between the three void markers are discussed. Solubility measurements of urea and thiourea in the range of 20 to 100% acetonitrile are also presented.
  Dead volume; Void volume; Void marker; Solubility; Phase ratio  

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