Air Sampling for Gases and Vapors. Patrick N. Breysse, PhD, CIH Peter S.J. Lees, PhD, CIH Johns Hopkins University

Transcription

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2 Air Sampling for Gases and Vapors Patrick N. Breysse, PhD, CIH Peter S.J. Lees, PhD, CIH Johns Hopkins University

3 Section A Introduction

4 Exposure Assessment Methods General air-sampling methodology Air Pump Flow Adjust Collection Media Air + Contaminant 4

5 Concentration Concentrat ion = Mass conta min ant Volume air Contaminant mass from laboratory analysis Air volume from product of calibrated air flow rate and sampling time 5

6 Source: U. S. Government 6

7 Charcoal Tube Calibration Calibrator Source: Patrick Breysse Tubing Charcoal tube Holder Sampling Pump 7

8 Personal Sampling Pumps Source: Patrick Breysse 8

9 Section B Sample Collection Methods

10 Sampling for Gases and Vapors Absorptive methods Absorption is a molecular process by which molecules are homogeneously dispersed in another substance Forming a solution (i.e., requires a liquid sampling medium) Methods now mostly obsolete Continued 10

11 Sampling for Gases and Vapors Adsorptive methods Adsorption is a molecular process by which molecules are reversibly bound to a surface Bound by intermolecular or van der Waals forces Uses solids as sampling medium Most current gas and vapor methods are adsorptive 11

12 Absorptive Methods: Impingers Contaminant bubbled through absorbent solution Solution changes according to analyte Analyzed by wet chemical methods Messy and unstable; not suited to personal sampling Used for gases (NO 2 ) 12

13 Impingers Source: U. S. Government Continued 13

14 Adsorptive Methods Gas and vapor molecules stick to solid surface Highly efficient reversible process Contaminant stable for transport Many different adsorbents available Can be affected by high humidity Subject to overload Continued 14

15 Adsorptive Methods Some common adsorbent media Activated charcoal: Polar solvents (many hydrocarbons) Silica gel: Non-polar solvents (alcohols and aldehydes) Treated diatomaceous earth Gas chromatographic supports 15

16 Adsorbent Tubes Source: Patrick Breysse 16

17 Adsorptive Methods: Charcoal Sampling ml/min drawn through tube Analysis Analyte(s) desorbed (CS2, or thermally) Typically analyzed using gas chromatography Continued 17

18 Adsorptive Methods: Charcoal Overload >25% on B section indicates breakthrough (loss) Discard sample or report as underestimate 18

19 Standard Sampling Tube Source: U.S. Government Direction of flow 19

20 Section C Passive Sampling Methods

21 Passive (Diffusive) Sampling Passive sampling relies on the natural diffusive energy of molecules to move them to sampling media Contaminant flux (mass/sampler area/unit time) is governed by Fick s Law Effective flow rates generally 5 30 ml/min A variety of designs 21

22 Example: Passive Sampler Source: Peter Lees 22

23 Section D Sample Analysis

24 Gas and Vapor Analysis Numerous methods Spectrophotometric Chromatographic Gas chromatography (GC) High-performance liquid chromatography (HPLC) GC-Mass spectrometry (GC-MS) Wet chemical 24

25 Analytical Laboratory Choose analytical laboratory before sampling Choose standard method Know limits of detection Calculate minimum sample volume to be minimally detectable Evaluate breakthrough potential Know interferences 25

26 Quality Control Blanks At least 10% of samples should be blanks Spiked samples Whenever possible, submit a known or spiked sample Continued 26

27 Quality Control Split samples If possible, submit split samples to either the same laboratory or two different laboratories 27

28 Section E Direct-Reading Instruments

29 Direct-Reading Instruments Real time measurement of contaminant Information saved to data logger Subsequent data analysis (mean, min., max., etc.) Principle of operation varies Infrared absorption Ultraviolet absorption Electro-chemical method Continued 29

30 Direct-Reading Instruments Numerous designs and manufacturers Possible uses Walk-through survey (preliminary hypothesis testing) Leak detection Over-exposure alarm Personal monitoring Emergency response Continued 30

31 Direct-Reading Instruments Limitations Specific versus non-specific response Must know principal of operation Must know positive and negative interference Must be calibrated often Subject to damage 31