CHM 410/1410 Lecture 3

What is more important, precision or accuracy? Erin Rob Bernadette Hanin What is more important, precision or accuracy? Why?

Some Definitions Arithmetic mean Geometric mean Appropriate for normally distributed data Appropriate for log normally distributed data Standard deviation Standard error Describes uncertainty around the mean Describes scatter in the data Report your data: 100 ± 10 mg/L What does is mean?

Some Definitions 95% of the data falls in 2 standard deviations 95% confidence interval is 1.96x standard error

Signal to Noise

Calibrations!

Detector Response Analyte Detector Resonse Sample Standards Conc Analyte (ng/mL) Detector Response Analyte 1 3 5 32 25 120 100 420 External Calibration Samples Detector Resonse Sample Conc Analyte (ng/mL) 42 8.2 26 4.3 13 1.2

Internal Calibration Standards Samples Conc Analyte (ng/mL) Conc Internal Standard (ng/mL) Detector Response Analyte Detector Response Internal Standard Response Factor (Analyte/IS) 1 2 3 9.5 0.32 5 32 10.5 3.0 25 120 10.3 12 100 420 9.1 46 Internal Calibration Samples Detector Response Sample Detector Response Internal Standard Response Factor Conc analyte (ng/mL) 42 5.3 7.9 17 26 10.1 2.6 5.0 13 2 6.5 14

Standard Addition Sample alone Sample + 5 ng/mL Sample + 10 ng/mL + conc (ng/mL) Detector Response 3.2 5 6.2 10 50 32 Sample alone Sample + 5 ng/mL Sample + 10 ng/mL Sample + 50 ng/mL Choice of concentration to add is not arbitrary! Need to add 1x, 2x and 10x the concentration of the unknown

Project line onto x-axis Concentration in this sample Standard Addition + conc (ng/mL) Detector Response 3.2 5 6.2 10 50 32 Project line onto x-axis let y=0 lxl = conc in sample Concentration in this sample 6.2 ng/mL

Response of sample

If you were interested in analyzing for a novel contaminant how would you go about determining an appropriate calibration technique?