Environmental scanning electron microscopy a technique that offers unique possibilities for exposure studies Martin Ebert and Stephan Weinbruch

CONTENTS 1. Individual Particle Analysis by SEM

2. Basics of Environmental Scanning Electron Microscopy (ESEM)

3. ESEM applications in exposure studies

Cabability of Scanning Electron Microscopy -Excellent resolution (~ 1nm)

-Excellent depth of field

-Chemical element identification by EDX But all samples in the SEM are exposed to high vacuum ⇒ no volatile or nonconductive samples can be investigated (without sample preparation)

Why do we need high vacuum in a SEM ?

3. Basics of Environmental Scanning Electron Microscopy (ESEM)

Gaseous Secondary Electron Detector (GSE) • gas molecules are used for amplification pÇ Ö contrast Ç • more gas molecules cause more scattering pÇ Ö resolution È best image quality ~4 Torr

Temperature working range in ESEM

Heating stage 2 Heating stage 1 Cooling stage

- 30

0

30

1000

60

temperature [°C]

1500

Druck u. Temperatur-Arbeitsbereich im ESEM Druck in Torr

Der mögliche Druck und Temperatur-Arbeitsbereich im Esem ermöglicht die Untersuchung von Wasser sowohl im flüssigen als auch im festen Zustand.

4. ESEM applications in exposure studies 4.1. Characterization of volatile and astable aerosol components

At pressures of 1- 10 mbar most of the volatile components of the aerosol can be studied in the ESEM (sulfates, nitrates, organics, liquid water).

Secondary electron picture of ammonium nitrate particles in the ESEM

Characterization of volatile aerosol components

fresh soot in the ESEM

same particle exposed to high vacuum

Investigation of pollen and spores a

b

Particles deposited on gelantine covered substrates can directly be ínvestigated in the ESEM

Investigation of pollen and spores a

b

c

Spores bursting and degassing under electron bombardement

4.2. Investigation of water containing samples and interactions with water

mites soot with solved inclusions

undried or even living biogenic material (e.g. dermal tissue)

RH = 90% Activation of unsoluble particles at high relative humidities

RH = 99% soot

RH = 100% increasing relative humidity water

Deliquescence and efflorescence

drop formation RH = 80%

recrystallization RH = 85%

RH = 60%

Deliquescence and efflorescence of a sodium sulfate particle

DRH at 25°C determined in the ESEM, compared with the values obtained by other techniques Na2SO4

DRH [%] at 25°C, this study

85

(NH4)2SO4 80

NaCl 75

70

65

NH4NO3

60 60

65

70

75

80

DRH [%] at 25°C, references*

85

*Ebert et al., 2002

Deliquescence und efflorescence of sodium chloride RH inc., ESEM, DP > 100 nm, T = 5° RH dec., ESEM, DP > 100 nm, T = 5° RH inc., Jout. et al., DP = 100 nm, T = 23°C

RH dec., Jout. et al., DP = 100 nm T = 23°C RH inc., Hämeri et al., DP = 50 nm, T = 25°C RH dec., Hämeri et al., DP = 50 nm, T = 25°C

2.5

growth factor

NaCl 2.0

crystallisation ESEM drop formation

1.5 crystallisation Joutsensaari et al.

1.0 20

30

40

50

60

70

relative humitidy [%]

80

90

100

Deliquescence behavior of Ni-containing particles b) Partial deliquescence RH 72 %

RH 95 %

4.3. Ice in the ESEM

aerosol particles and ice nucleation in the atmosphere Homogeneous ice nucleation of a supercooled cloud droplet: does not start until -38°C !!! Heterogeneous ice nucleation (induced by special aerosol particles): was observed already at -5°C !!!

aerosol particles ice nuclei

?

CCN

sample chamber of the ESEM with cooling stage pole shoe SE detector sample peltier cooling stage

CCD camera

gaseous SE detector water cooling

Heterogeneous ice nucleation in the ESEM ice

mica

ice

mica

T = - 5°C

4.4. additional devices:

Micro manipulator

micromanipulator in ESEM

scratching the surface

drop off or pick up

electrical conducting

micro injector (drop off or pick up solution)

CONTEXT z Additionally

to the capabilities of conventional SEM, ESEM enables electronmicroscopic analysis at pressures up to 50 Torr (~6700 Pa) and temperatures between –30 to 1500°C. Analysis of non conductive, wet, astable and living material becomes suitable. This enables a more complete picture of the ambient aerosols.

z

Hygroscopic behaviour (e.g. deliquescnece, efflorescence, activation ice forming prperties..) of particles can be studied in situ. z

z

No charging effects ⇒ no coating of non-conductive samples.

z

Observation of in-situ processes (SE/BSE/EDX).

The scanning electron microscope can be used as a reaction chamber, customized for different requirements (cryo-SEM, micromanipulator, microinjector, residual gas analysis).

z