According to law of dilution  MV = M'V'
Where M = Molarity of stock = 872.0 ppm = 872.0 mg/L
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= 872.0x10-3 g/106 μL
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= 872.0x10-9 g/μL
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= 872.0 ng/μL
V = Volume of the stock = ?
M' = Molarity of dilute solution = 13.0 ng/μL
V' = Volume of the dilute solution = 19.00 mL
Plug the values we get , V = ( M'V') / M
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= (13.0x19.0) / 872.0
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= 0.28 mL
Therefore the volume of stock solution required is 0.28 mL
