Standard addition: Lithium in the blood serum of a manic-depressive patient treated with lithium carbonate is determined by flame emission spectrophotometry using a standard additions calibration. One hundred microliters of serum diluted to 1mL gives an emission signal of 6.7 cm on the recorder chart. A similar solution to which 10 microliters of a 0.010 M solution of LiNO3 has been added gives a signal of 14.6 cm. Assuming linearity between the emission signal and the lithium concentration, what is the concentration of lithium in serum, in parts per million?