Full set of Notes with Fill-Ins - San Jose State University

Chem 155 Unit 7 Page 150 of 316Three photometric problems:1. Δλ EFF >> Δλ ATOMIC so stray light is a major issue. The maximumabsorbance that one could realistically expect with a 0.25 nm Δλ EFF isroughly 0.01 using a broadband source, and this is going to be noisy so:a. Use hollow cathode lamp with emission identically matched toabsorption. This makes maximum absorbance much higher, andless noisy. For the remaining source and flame stray light, one can:b. Modulate the source to help to distinguish it from flame and otherradiation that ‘leaks’ into detector ‘around’ atomic line.2. Flame and molecules in flame present a broadband absorptioninterference. These are direct interferants, so they must be measured insome way that is not sensitive to the analyte atoms:a. Use broadband (D 2 ) source that is approximately ‘blind’ to analyteto approximate the broadband flame absorbance.b. Use Zeeman splitting to polarization select between resonantabsorption and off-peak background absorption.c. Use Smith-Hiefje ‘splitting’ to approximate the same effect.Major Learning Objectives:#1 Understand the exponential relationship between transmitted light powerand concentration.#2 Understand how stray light can affect computed values of absorption.#3 Understand how narrow atomic bands, that are narrower than theattainable effective bandwidth, yield a difficult stray light problem.#4 Understand how the hollow cathode lamp addresses the problem ofatomic absorption lines that are narrower than the effective bandwidth ofthe monochromator by matching the spectum of the source (hollowcathode lamp) to that of the absorber (free atoms in flame).#5 Understand how the absorbance of broadband D 2 radiation actuallyapproximates the ‘blank’ absorbance of the flame even when there isanalyte present in the flame.#6 Understand how Zeeman splitting samples the flame background but notthe analyte absorbance.Page 150 of 316