Cover Image
close this bookAsbestos Overview and Handling Recommendations (GTZ, 1996)
close this folderPart II. Asbestos
close this folder1. Introductory part: Asbestos - Deposits, uses, types, characteristics
View the document(introduction...)
View the document1.1 Types, deposits, and uses of Asbestos, chemical structure
View the document1.2 Mineralogical and mechanical properties of Asbestos
View the document1. 3 Analytical methods of determining Asbestos fibers

1. 3 Analytical methods of determining Asbestos fibers

A number of techniques have been developed for the analytical determination of Asbestos fibers. The determination is generally performed in three steps: the sampling, fiber counting and determination of the type of fiber. These steps are presented below along with the most widely used determination procedures.

Sampling

For the determination of Asbestos in fluid media (air, water), a defined volume of the media is drawn through a filter, upon which the fibers are deposited. The typical filter materials are gold-coated track-etched membrane filters or cellulose membranes. In order to determine the number of fibers in solids (e.g. in material samples or in dust), the sample can be directly used.

Fiber Counting

Generally, microscopic techniques are applied for fiber counting. The optical counting of fibers proceeds in the simplest case under the phase contrast microscope. Since the optical resolution can at best include structures of 1 mm in size, whereas in airborne particulates the main fraction of fiber bundles has sizes ranging from 2 - 0.2 mm, phase contrast microscopy can at best be used as a screening method in cases of very high fiber concentrations (e.g. for the investigation of material samples). In the case of investigations of drinking water, phase contrast microscopy is totally unsuitable (sizes around 0.06 mm).

Sufficient resolution can be achieved with using scanning electrion microscopy (SEM) or with transmission electron microscopy (TEM). These procedures are relatively expensive, however, and require extensive measures in the preparation of the samples, in addition to well-educated operating personnel.

Asbestos fibers in particulate samples can also be determined on the basis of their characteristic absorption lines using infrared spectroscopy. However, the determination limit lies relatively high at 1-5%, depending on the type of fiber.

Table 6: Data on the Most Important Types of Asbestos


Serpentine

Amphibole


Chrysotile

Anthophyllit

Crocidolite

Actinolite

Tremolite

Amosite

Chemical formula

Mg3(OH)4Si2

(Mg,Fe2+)7

Na2(Fe2+,Mg)

(Ca,Na)2

Ca2(Mg,Fe)5

(Fe2+,Mg.Al)7


O5

(OH)2Si8O22

3Fe23+(OH)2

(Fe,Mg,Al)5

(OH,F)2Si8O22

(OH)2Si, Al)8




Si8O22

(OH, F)2


O22





(Si, Al)8O22



Chemical composition in (%)







SiO2

35 - 44

52 - 64

49 - 57

0 - 63

50 - 63

45 - 56

MgO

36 - 44

25 - 35

3 - 15

18 - 33

18 - 33

4 - 7

Al2O3, Iron oxide

0 - 9

1 - 10

20 - 40

2 - 17

2 - 17

31 - 46

CaO, Na2O

0 - 2

0 - 1

2 - 8

1 - 10

1 - 10

1 - 2

H2O

12 - 15

1 - 5

2 - 4

1 - 4

1 - 4

1 - 3

Physical properties

fine light fibers

prismatic crystal and fibers

long, brittle fibers

prismatic crystal and fibers

prismatic crystal and fibers

prismatic crystal and fibers

Color

white, grey, greenish

grey-white

blue

green

white, grey- white, greenish

ash grey

Texture

soft to rough, mostly silky

rough

soft to rough

rough

rough

rough

Flexibility

very high

low

good

low

low

good

Mohs-strenght

2,5 - 4

5,5 - 6

5,5 - 6

6

5,5 - 6

5,5 - 6

Fiber diamiter (nm)

18 - 30

60 - 90

50 - 90

0 - 90

60 - 90

60 - 90

Resistance (N/nm2)

210 - 560

< 28

280 - 420

7

7 - 56

70 - 140

Modulus of elasticity(N/nm²)

160.000

-

190.000

-

-

160.000

Melting point (C)

1.500

1.480

1.180

1.393

1.320

1.400

Specific heat (kj/kg C)

1,1

0,85

0,8

0,9

0,9

0,8

Surface (m²/g)

10 - 60

7

10



9

Thickness g/cm³

2,2 - 2,6

2,8 - 3,2

2,8 - 3,6

3,0 - 3,2

2,9 - 3,2

2,9 - 3,3

Breeking index







n (alpha)

> 1,53

> 1,59

> 1,68

> 1,61

> 1,60

> 1,64

n (gamma)

< 1,57

< 1,63

< 1,70

< 1,65

< 1,63

< 1,69

pH-Value

9,5 - 10,3

9,4

9,1

9,5

9,5

9,1

Electrical charge in aqueous suspension

+

-

-

-

-

-

Stable towards acids

unstable

very good

good

fairly good

good

good

Alkali resistant

very good

good

good

good

good

very good

capability of being spun

easily spinnable

barely spinnable

mostly spinnable

unspinnable

unspinnable

partially spinnable

Source: Umweltbundesamt (Publ.): Analysis of the Asbestos Industry, written by the Battelle-lnstitut Frankfurt e.V., Report 4/78, Berlin 1978

Determination of the Type of Fiber

The determination of the type of fiber and particularly the differentiation from other inorganic fibers can be directly performed under the transmission electron microscope using small angle electron diffraction (SAED). Another possibility is the energy dispersive X-ray analysis (EDXA). The arising diffraction or energy spectra are evaluated using numerical methods. The required infrastructure is relatively expensive and places high requirements on the operating personnel. The previously common use of phase contrast microscopy to determine the type of fiber in airborne particulates based on fiber geometry is unsuitable, due to low resolution.

The type of fiber can also be identified with infrared spectroscopy based on the characteristic absorption bands of chrysotile and amphibole types. This method is inexpensive, but only applicable if the fiber concentration in the sample exceeds about 1-5 percentage by weight.

In Germany there are two accredited American procedures for the measurment of Asbestos fiber concentrations. These VDI Guidelines are for fiber in particulates and in indoor air and replace the previously common phase contrast microscopy procedures:

VDI Guideline 3861

This guideline specifies a procedure for the determination of the fraction of Asbestos fibers in particulate mass, e.g. as they occur in air vents. The sampling proceeds through deposition of particulates onto a nitro-cellulose filter. This filter undergoes cold ashing, and the fiber concentration is then determined using infrared spectroscopy with the help of the KBr Pressure Technique. The analytical result is obtained as the weight fraction in g/kg.

VDI Guideline 3492

This guideline specifies a procedure for the determination of Asbestos fibers in indoor or outdoor air. The Asbestos fibers from a defined air volume are deposited onto a gold-coated track-etched membrane, which is then cold-ached, and subsequently the fibers are counted under the scanning electron microscope. The type of fiber is determined using energy dispersive X-ray analysis.

Due to the cost of this analytical procedure, it may be assumed that in developing countries at best the phase contrast microscopy and infrared techniques are available (cost of equipment < 50,000 DM). The electron microscopy procedures (costs >>100,000 DM) would currently be applied exclusively in industrial nations. Therefore, in developing countries it is questionable whether any existing limits, e.g. for indoor air concentrations, can be monitored.