Part II

Laboratory Studies

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159

7

LAMMA and Raman Study of
Oxidation States of Chromium
in Aerosols: Application
to Industrial Hygiene

A. Hachimi, E. Poitevin, G. Krier, and J.F. Muller

CONTENTS

Introduction 159
Study of Chromium in Polyphasic Dust 160
Materials and Methods 160
Sampling 160
Analytical Techniques 161
SEM Analysis 161
Raman Microprobe 161
X-ray Photoelectronic Spectroscopy (XPS) 161
Laser Microprobe Mass Analysis (LAMMA) 161
Analysis of Dusts from Welding Fumes 162


160

Aerosol Chemical Processes in the Environment

Experimental studies have shown that chromates and bichromates are able to induce

in vitro

and

in vivo

cancers in animals.

3-5

Epidemiologic survey have shown that the lungs represent a target
organ of hexavalent derivatives of chromium. However, only a few experimental studies exist that
have allowed for the thorough study of toxicity mechanisms. In welding and steel working, high
concentrations of fumes and gas are emitted, which contain chromium in either the hexavalent or
trivalent form; thus, health problems in relation to the presence of chromium in dust fumes can
occur. Therefore, methodology application on valency determination of chromium in environmental
dusts was desirable.
In the literature, one finds two kinds of techniques for the determination of chromium valency
in aerosols of industrial origin:
• Chemical spectrophotometric analysis techniques, also called wet chemical techniques
• Physical and chemical techniques using direct measurement and spectroscopic techniques
Wet chemical techniques, such as colorimetry, ion exchange resins, luminescence, and atomic
absorption,


in situ

element oxidation rates, mass spectrometry presents new insight on this problem, especially laser
microprobe LAMMA.
One can apply the valency determination method for chromium to a complex matrix: on the
one hand, arc welding fumes on stainless steel (MMA/SS) and, on the other hand, dust from the
steel industry. Thus, one can investigate the determination of major oxidation rates of chromium
derivatives contained in the dust that is directly inhaled by workers.

STUDY OF CHROMIUM VALENCY IN POLYPHASIC DUST

The method proposed here involves the determination of chromium valency, in aerosols less than
and greater than 10

µ

m in diameter, emitted by welding and in aerosols, 0.4 to 10

µ

m in diameter
from various steel works. This method has been improved upon by precise sampling with granu-
lometric discrimination of aerosols (using cyclone and Andersen impactors) and targeting of fume
emission sites at steel-making locations. Microprobe Raman techniques, and ESCA and SEM, have
been used as complementary techniques of confirmation.

MATERIALS AND METHODS
S


• A normal Andersen cascade impactor with an air flow rate of 28 l min

–1

for discrimination
of aerosols in air flux by inertial impaction relative to their mean aerodynamic diameter.
Size fractionation in nine stages (>9

µ

m; 9 to 5.8

µ

m; 5.8 to 4.7

µ

m; 4.7 to 3.3

µ

m; 3.3
to 2.1

µ

m; 2.1 to 1.1

µ

NALYTICAL

T

ECHNIQUES

SEM Analysis

Element analysis was achieved using a JEOL 840 electron microscope coupled to a dispersive
energy spectrometer. An electron beam current of 200 to 300 pA and accelerating voltage of 15 kV
were used.

Raman Microprobe

MicroRaman analysis of aerosols collected on the portable cascade impactor was performed on a
standard DILOR XY apparatus equipped with an argon ion laser and multichannel detector (1024
diodes). Excitation wavelength was 514.5 nm using a power of 75 mW to avoid fluorescence
emission that could mask Raman diffusion. Spectral resolution was 4 cm

–1

, and integration time
varied from 1 to 15 s.
The spectrometer was coupled to an optical microscope (Olympus), permitting a spatial reso-
lution of 2

µ

m. Spectra were obtained in reflective mode and presented in arbitrary units vs.
wavenumbers (cm

), O(1

s

), and F(1

s

). This interval is due to the charge effects from fibrous dusts
and the nitrocellulose filter.

Laser Microprobe Mass Analysis (LAMMA)

LAMMA was developed for localization and determination of elements in various samples —
either conducting or isolating ones. An interesting feature of this technique is its ability to charac-
terize the molecular composition of inorganic substances. Moreover, it allows for elementary
analysis without the traditional separation step.
Element detection limits are 10

–15

to 10

–19

g. This sensitivity allows for LAMMA analysis of
aerosols in biological and environmental studies.

17,18


). Major ions obtained by laser
ionization are representative of sulfate and sodium thiosulfates.

19

The information is useful to
complete data obtained by other techniques. Additionally, a link between morphological properties
of particles can be established by sample observation under visible light.

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162

Aerosol Chemical Processes in the Environment

Technological progress (laser, configuration of ionization chamber) has permitted increased use
of this method, and has allowed for the analysis of organic and inorganic matter ((nitro)PAH
desorption) and matrix identification.

20,21

The laser microprobe already has a privileged place and
ongoing instrumentation progress will make certain its success.
Impacted dusts were extracted from filters by superficial scraping and set by simple pressure
on a microscopic grid coated with a formvar film.
Comparisons could be made with constant instrumental parameters, and the LAMMA apparatus
had the following features:
• Wavelength: 266 nm
• Pulse width: 12 ns

and are collected in the impactor after welding operation
• Welding aerosols more than 10

µ

m in diameter that are microfibers collected onto
nitrocellulose filters

Q

UALITATIVE

A

NALYSIS

We have investigated fingerprint spectral analysis of the different elements cited above. LAMMA
spectra are presented in Figures 7.1 to 7.5. General observations of these spectra lead to to the
following remarks:
• The constitutive elements of the coatings of the rod, such as Na, K, Ca, F, Cl, Mn, P,
and Si (in aluminosilicate form), are present in all the aerosols.
• Barium, which is present in its oxide and fluoride forms and comes from coatings of the
rod, is simply present in the microspherical aerosols (>10

µ

m).
• Trimming compounds, likely microspheres, contain metals (Cr, Al, Ti, and Ni) that are
representative of the elemental composition of the rod and stainless steel sheet-metal.
• Fibrous dusts (<10


) in both
kinds of aerosols.
The entire LAMMA analysis is presented in Table 7.1 where elemental ions and combinations
present in plasma are displayed.

TABLE 7.1
Elemental Analysis of Welding Dust by LAMMA

Element Rod Coating Trimming Dust (>10

µ

m) Dust (<10

µ

m)

Si +++ +++ ++ +++ +++
Mn + + + + ++
Cr ++ – ++ +++ ++
Ni ++ – ++ ++
Al – ++ ++ ++ ++
O – ++ ++ ++ +++
F – ++ +++ +++ +++
Na – +++ ++ ++ +++
Cl – ++ ++ ++ ++
K – +++ +++ +++ +++
Ca – +++ + +++ +

++ +++ + +++
Fe

x

O

y

+++ ++ +
AlSi

x

O

y

+++ ++ + ++
CaF + + +++ +
SO

x

+++ ++ –
FeSi

x

O

CaCl –– – +
K

x

Cl

y

–– – +
Al(Na

x

Cl

x

) –– – ++
Na(K

x

F

x

) –– – ++
K(Na



ALCULATION
OF

C

HROMIUM

A

MOUNT
WITH

D

IFFERENT

S

TOECHIOMETRY

Oxidation state studies were performed on 85 and 24 individual spectra of fibrous and microspher-
ical aerosols, respectively. Investigation of 455 and 325 accumulated spectra allow for the deter-
mination of the major chromium oxidation state in the fibrous and microspherical dusts, respectively.

3
–

clusters (and sometimes CrO
4
–
) were present in
negative mode. Energy variation and length variation do not interfere on spectra and intensity ratios
of negative clusters.
Results are presented in Table 7.3 with analysis details of individual spectra. The methodology
was applied on fibrous aerosols with no ambiguity and it confers to chromium an oxidation degree
of VI (93% of individual spectra have a chromium VI fingerprint). The salt character of fibers and
a value of CrO
2
–
, CrO
3
–
that corresponds to an anhydrous chromium salt could show chromium in
chromate form. In fact, this is in agreement with other results
22,23
on similar type dusts, where
chromium has been identified in sodium or potassium form.
However, LAMMA does not detect the type of cluster K
x
CrO
y
+
, CrxO
y

Weighted mean 0.205
Standard deviation 0.03
Variance 0.0008
Microsphericals 0.754 ± 0.194 24 Individual 225.7
Dust >10 µm 0.673 100 Accumulation 225.7
0.543 100 Accumulation 225.7
0.56 25 Accumulation 225.7
0.551 25 Accumulation 225.7
0.629 50 Accumulation 225.7
Weighted mean 0.614 Total: 324
Standard deviation 0.205
Variance 0.042
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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 165
• SO
x
–
ions clusters and CrO
–
chromium are systematically present (100% of spectra), in
addition to CrO
2
–
and CrO
3
–
ions clusters.
• Cluster intensity of sulfur does not interfere with the CrO
2

mentary works would show that chromium is in its III form and in the salt and oxide mixture
form:
• The CrO
2
–
/CrO
3
–
ratio varies according to studied microsphere (from 10 to 40 µm in
diameter, with value between 0.5 and 4, which corresponds to chromium III and is
constant for a particular microsphere).
• All microspheres have the same composition and spectral fingerprint (systematic presence
of CrO
–
, SO
x
–
, and lack of CrO
4
–
).
These two results support the hypothesis of a mixture containing chromium III salts (sulfate
compounds, for instance) and chromium III oxide in variable amounts depending on microspheres.
Variation of the CrO
2
–
/CrO
3
–
ratio from 0.5 to 4 could mean that microspherical aerosols contain

4
6 Cr III
Spectrum number 6 3 93% Cr VI
7% Cr III
Accumulation R = CrO
2
/CrO
3
Valency of majority of chromium
455 spectra 0.205 Value of R confer on chromium VI valency
More than 90% chromium
Dust >10 µm R > 0.3 With CrO
4
With SO
x
24 Cr III
Spectrum calc.
Spectrum number 24 24 24 100% Cr III
Accumulation R Valency of majority of chromium
324 spectra 0.614 Value of R confer on chromium III valency
Mixture of chromium III and VI
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166 Aerosol Chemical Processes in the Environment
XPS analysis has likely been performed only on fibrous aerosols because of the deficient amount
of microspheres (>10 µm) for effective detection. Major elements already detected by LAMMA analysis
are found, and Cr(2p) and Cr(3p) bands for chromium and Ni(2p) and Ni(3p) bands for nickel are
observed. The presence of the Cr(2p) band at 579.4 eV after correction for the binding energy indicates
that chromium is present in its VI oxidation form, whereas no band is detected at 675.4 eV, which is
specific for chromium III. So, the band only at 579.4 eV reveals the presence of chromium exclusively

large dust aerosols are mainly comprised of calcium (Figure 7.3).
FIGURE 7.2 LAMMA spectrum of dust particles smaller than 3.3 µm.
FIGURE 7.3 LAMMA spectrum of dust particles smaller than 0.4 µm.
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168 Aerosol Chemical Processes in the Environment
STUDY OF THE VALENCY OF CHROMIUM IN DUSTS COLLECTED BY A PORTABLE
I
MPACTOR AND BY A NORMAL ANDERSEN IMPACTOR AT THE SAME SITE
If a comparison is made between analyses of dusts collected on the two kinds of impactors, the
same majority distribution of chromium valency is found for both aerosol sizes collected on the
portable impactor and those collected on the Andersen impactor at the same site (Figure 7.4).
This means that chromium is mainly present in the hexavalent form in the smallest and largest
aerosols from both impactors. In fact, chromium is exclusively hexavalent for aerosols smaller than
3.5 µm and larger than 6 µm and trivalent for intermediate sizes. It is noteworthy that in the context
of industrial hygiene, dust as emitted from the site has the same characteristics as that sampled on
a worker’s portable impactor.
A more detailed study of the relative ratio of chromium III/VI from all filters of the Andersen
cascade impactor revealed that, for the entire dust sample (Figure 7.5), 30% of chromium is trivalent
and 70% is hexavalent.
Furthermore, the ratio of chromium VI increases as aerosol size decreases and more than 60%
of hexavalent chromium is present in the smallest sizes (<3.3 µm).
In the context of occupational health, the similarity of distributions of the valency of chromium
as collected by the two impactors at the same site indicates an accurate simulation of the inhalation
by a worker of fumes emitted.
It is remarkable that aerosols larger than 8 µm contain such a large amount of chromium VI.
We predict that they are composed of a particular matrix that we believe is deserving of further study.
CHROMIUM ANALYSIS OF LARGE DUST AEROSOLS (8 TO 10 µµ
µµ
M) COLLECTED WITH

24
This hypothesis is
consistent with an observation ratio of cluster CrO
2
–
/CrO
3
–
intensities less than 0.8, and CrO
4
–
/CrO
3
–
FIGURE 7.5 Relative chromium concentration in relation to particle size in dust collected with an Andersen
impactor.
FIGURE 7.6 SEM spectrum of dust particle (8 to 10 µm) collected with portable cascade impactor.
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170 Aerosol Chemical Processes in the Environment
intensities less than 0.1 (Figure 7.8). The investigations by LAMMA in both modes of ionization
suggest hexavalent chromium is present in the form of calcium chromate.
Raman measurements permit the observation of the four modes of vibration of CrO
4
2–
that
belong to T
d
symmetry. This last one is highly reduced by the presence of others ions in the
crystalline structure. However, in the case of hexavalent chromates, anion vibrations are hardly

4
)
2. “Type II dichromate: spectra, which are differentiated by the presence of bands in the
range 800 to 700 cm
–1
, similar to those of the ion Cr
2
O
7
2–
(Fe-, Ni-, Zn-, Cu-, Co-, Al-,
and Cd-Cro
4
)
Raman results of standard chromium compounds and dust aerosols (8 to 10 µm) collected on
the portable impactor are summarized in Table 7.4 and in Figures 7.9 and 7.10. The presence of
calcium chromate implied by the SEM and LAMMA analyses is confirmed by this study: the
Raman spectra of dust from sample A is identical to the CaCrO
4
reference spectrum. The spectrum
of dust sample B, however, shows a shift of vibration bands toward low wave-numbers. This shift
could be explained by modifications in the environment of chromate ion or in its crystalline mode.
A similar spectrum was also obtained from dust sample C and is possibly an unidentified compound
belonging to type II dichromate (band at 707 cm
–1
). The Raman spectrum obtained for dust sample
D can probably be compared to the reference sodium chromate spectrum. The Raman study of all
dust samples collected at the given site using the portable cascade impactor confirms the hypothesis
of the presence of hexavalent chromium — essentially as calcium chromate species — and permits
us to expect either sodium chromate or type II chromates.

905 (M), 879 (H)
465 (vW), 383 (W), 302 (W)
Dust particles A 905 (M), 880 (H)
(8–10 µm) B 900 (M), 875 (H)
C 900 (M), 875 (H), 855 (M), 833 (H), 707 (W)
D 908 (H), 875 (M), 757 (W), 650 (W)
E 926 (M), 855 (H)
Note: Intensity: vH = very high, H = high, M = medium, W = weak,
vW = very weak.
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172 Aerosol Chemical Processes in the Environment
FIGURE 7.9 Raman reference spectra of type I chromates (for band positions, see Table 7.4).
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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 173
FIGURE 7.10 Raman specta of dust particles (8 to 10 µm) collected with portable cascade impactor (for
band positions, see Table 7.1).
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174 Aerosol Chemical Processes in the Environment
CONCLUSION
Systematic LAMMA analyses of oxidation states of chromium on aerosols less than 10 µm in
diameter emitted in fumes from steelworks have proved that the valency of this metal varies
according to the size of the aerosols. The smallest aerosols (<1.1 µm) and the largest ones (>5.8
µm) contain chromium in the hexavalent state, whereas dusts of intermediary sizes mainly have
trivalent chromium.
Sampling of dust with a portable cascade impactor carried by a person working in an environ-
ment containing chromium fumes has shown a composition similar to that found for dust sampled
with a fixed Andersen impactor.

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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 175
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