Environmental scanning electron microscopya 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 foramplification

p contrast

best image quality~4 Torr

• more gas molecules cause more scattering

p resolution

- 30 0 30 60 1000 1500

temperature [°C]

Cooling stage

Heating stage 1

Heating stage 2

Temperature working range in ESEM

Druck u. Temperatur-Arbeitsbereich im ESEM

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

Druck in Torr

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 theaerosol can be studied in the ESEM (sulfates, nitrates, organics, liquid water). Secondary electron picture

of ammonium nitrate particlesin the ESEM

fresh soot in the ESEM same particleexposed to high vacuum

Characterization of volatile aerosol components

a b

Particles deposited on gelantine covered substrates can directlybe ínvestigated in the ESEM

Investigation of pollen and spores

Investigation of pollen and spores

a b c

Spores bursting and degassing under electron bombardement

4.2. Investigation of water containing samplesand interactions with water

soot with solved inclusions undried or even living biogenicmaterial (e.g. dermal tissue)

mites

increasingrelative humidity

Activation of unsoluble particlesat high relative humidities

RH = 90%

RH = 99%

RH = 100%

soot

water

drop formation recrystallization

RH = 80% RH = 85% RH = 60%

Deliquescence and efflorescence

Deliquescence and efflorescence of a sodium sulfate particle

60 65 70 75 80 85

60

65

70

75

80

85Na2SO4

(NH4)2SO4

NaCl

NH4NO3

DR

H [%

] at 2

5°C

, thi

s st

udy

DRH [%] at 25°C, references*

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

*Ebert et al., 2002

NaCl

Deliquescence und efflorescence of sodium chloride

20 30 40 50 60 70 80 90 100

1.0

1.5

2.0

2.5

crystallisationJoutsensaari et al.

crystallisation ESEM

dropformation

RH inc., ESEM, DP > 100 nm, T = 5° RH dec., Jout. et al., DP = 100 nm T = 23°C RH dec., ESEM, DP > 100 nm, T = 5° RH inc., Hämeri et al., DP = 50 nm, T = 25°C RH inc., Jout. et al., DP = 100 nm, T = 23°C RH dec., Hämeri et al., DP = 50 nm, T = 25°C

grow

th fa

ctor

relative humitidy [%]

RH 72 % RH 95 %

Deliquescence behavior of Ni-containing particles

b) Partial deliquescence

4.3. Ice in the ESEM

aerosol particles and ice nucleation in the atmosphere

Heterogeneous ice nucleation (induced by special aerosol particles):

was observed already at -5°C !!!

Homogeneous ice nucleation of a supercooled cloud droplet:

does not start until -38°C !!!

aerosol particles

? CCNicenuclei

sample chamber of the ESEM with cooling stage

samplepeltiercoolingstage

watercooling

SE detector

gaseous SE detector

pole shoe

CCD camera

mica ice

T = - 5°C

Heterogeneous ice nucleation in the ESEM

ice

mica

4.4. additional devices: Micro manipulator

scratching the surface

electrical conducting

micromanipulator in ESEM

drop off or pick up

micro injector (drop off or pick up solution)

CONTEXTAdditionally 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.

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

Analysis of non conductive, wet, astable and living material becomessuitable. This enables a more complete picture of the ambient aerosols.

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

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

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