Endogenous program

Instrumental variables are used with nonexperimental design to control for selection bias. These variables determine program participation, but do not affect outcomes. Evaluators can often use geographic variations in program availability and program characteristics as instruments, especially when endogenous program placement seems to be a source of bias. 

Endogenous protective programs may include ischemic preconditioning and the heat shock response 571... 

The application from van der Hoeven et al. (1997) used an ADS cartridge online SPE to measure cortisol and prednisolone in plasma and arachidonic acid in urine. A precolumn packed with a C18 alkyl-diol support (LiChrosphere RP-18 ADS, 25 /an, Merck) was used. To reduce run time, column switching was programmed as heart-cut , diverting only the analyte fraction into the analytical column. Another LiChrosphere column (125 x 4 mm inner diameter, Merck) handled separation. After the injection of 100 fiL plasma, the lower limit of detection for prednisolone was 1 ng/mL while cortisol was readily quantitated at its endogenous level of 100 ng/mL. The run time was 5 min. For arachidonic acid, a Hypersil ODS column (200 x 3.0 mm inner diameter, 5 /.an) was used. The injection volume was 200 //I. and run time was 9.5 min. The detection limit was 1 ng/mL and recovery was 77%. 

---