| 
      S.D.= Standard deviation.
        | Actual 
              concentration 
            (μg/mL) | Intra-day | Inter-day |  
        |  |  |  | Measured concentration | ± S.D., | R.S.D. (%) |  
        | Measured    concentration | ±    S.D., | R.S.D. (%) |  
        | 20 | 20.157 | 0.201 | 0.999 | 20.080 | 0.130 | 0.647 |  
        | 40 | 40.150 | 0.339 | 0.844 | 39.873 | 0.202 | 0.508 |  
        | 60 | 59.997 | 0.274 | 0.457 | 59.886 | 0.188 | 0.314 |  R.S.D.=Relative standard deviation
 n = no. of replicates
 
 |