Medical brain notes 35

  Calibrations, Standardizations, and Blank   Corrections We   introduced   a   relationship   between   the   measured   signal, S meas , and the absolute amount of analyte S meas  =   kn A  +   S reag      ……… 5.1 or the relative amount of analyte in a sample S meas  =   kC A  +   S reag     ……… 5.2 where   n A   is   the   moles   of   analyte,   C A   is   the   analyte’s   concentration,   k   is the   method’s   sensitivity,   and   S reag   is   the   contribution   to   S meas   from constant   errors   introduced   by   the   reagents   used   in   the   analysis.   To  obtain an accurate value for  n A  or  C A  it is necessary to avoid determinate   errors ...

  Calibrating Signals Signals   are   measured   using   equipment   or   instruments   that   must   be   properly   cali-  brated   if   S meas   is   to   be   free   of   determinate   errors.   Calibration   is   accomplished  against   a   standard,   adjusting   S meas   until   it   agrees   with   the   standard’s   known   signal. Several   common   examples   of   calibration   are   discussed   here. When   the   signal   is   a   measurement   of   mass,   S meas   is   determined   with   an   analyti- cal   balance.   Before   a   balance   can   be   used,   it   must   be   calibrated   against   a   reference weight...

  Standardizing Methods The American Chemical Society’s Committee on Environmental Improvement de- fines standardization as the process of determining the relationship between the measured   signal   and   the   amount   of   analyte.   A   method   is   considered   standardized when   the   value   of   k   in   equation   5.1   or   5.2   is   known. In   principle,   it   should   be   possible   to   derive   the   value   of   k   for   any   method   by considering   the   chemical   and   physical   processes   responsible   for   the   signal.   Unfortu- nately, such calculations are often of limited utility due either to an   insufficiently developed theoretical model of the physical processes or to nonideal chemical   be- havior. ...

  Reagents Used as Standards The   accuracy   of   a   standardization   depends   on   the   quality   of   the   reagents   and   glass- ware   used   to   prepare   standards.   For   example,   in   an   acid–base   titration,   the   amount of   analyte   is   related   to   the   absolute   amount   of   titrant   used   in   the   analysis   by   the stoichiometry   of   the   chemical   reaction   between   the   analyte   and   the   titrant.   The amount   of   titrant   used   is   the   product   of   the   signal   (which   is   the   volume   of   titrant) and   the   titrant’s   concentration.   Thus,   the   accu...