Lecture 21_Instrumentation

Lecture 21_Instrumentation - Lecture 21: Instrumentation...

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1 Lecture 21: Instrumentation Required reading: FP&P Chapter 11 Atmospheric Chemistry CHEM-5151/ATOC-5151 Spring 2005 Jessica Gilman Outline of Lecture z Intro z Gases z Collection Techniques z Filters and Denuders z Spectroscopic Techniques z Absorbance vs. Emission z Measuring OH z Particles z Collection Techniques z Mass Spectrometry Techniques z Bulk, Single-particle, Morphology
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2 Introduction z Measuring atmospheric constituents presents many challenges: z Identify and quantify z Complex system z Large number of possible interferences z Very small concentrations z Sub ppt z Concentrations vary! z Diurnally, temporally, geographically, etc. Introduction z Many different ways to measure/analyze z Which technique is the best? z How well do different techniques compare? z Instrumentation requirements: z Exceptional sensitivity z Low limit of detection z Good selectivity z Good time resolution z Accurate and reproducible z Robust, portable, cheap, … From Skoog, Holler, Nieman Example “working curve”
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3 Important Distinctions z Many measurement techniques will fall into one or more of these important categories: z Gas-phase vs . Particles z Particles: ± Bulk analysis ± Single-particle analysis ± Depth Profiling/Morphology z Collected vs . In situ
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4 z Collect gases for subsequent analysis z Must avoid interferences from particles z Must collect enough to be measurable z Sampling times are usually quite long (min to day) z Be careful of time resolution of measurements z From total volume of air sampled and the amount of the analyte measured, the average concentration of the species over experimental time range is determined z Beware of sampling artifacts z Reaction, decomposition, evaporation, etc. Collection Techniques Gas-phase Collection Techniques z Filters z Saturated with a substance that takes up the species of interest z Filter material is optimized for the compound of interest z Nylon is good for HNO 3 ( g ) z Analytes are later extracted from filter prior to analysis z Performance of filter must be carefully assessed prior to use Finlayson-Pitts 1. Remove particles 2. Collect HNO 3 ( g ) 3. Collect NH 3 ( g ) 4. Everything else goes thru
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5 Gas-phase Collection Techniques z Denuders z Special kind of filter that uses diffusion properties to separate gas and particles z Gases strike wall of tube numerous times because of their high diffusivity, particles just fly through z Walls of tube are coated with a substance that takes up only the species of interest z “Wash” the walls to collect the gas prior to analysis Finlayson-Pitts Schematic diagram of a denuder. G=gas and P=particles Review of Spectroscopy z Identify and quantify species based on their interactions with energy z Energy: radiation , acoustic waves, beams of particles such as ions and electrons z The energy difference b/w states is unique for every species!
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Lecture 21_Instrumentation - Lecture 21: Instrumentation...

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