AEROSOL CHEMICAL PROCESSES IN THE ENVIRONMENT - CHAPTER 7 pptx

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

Qualitative Analysis 162
Calculation of Chromium Amount with Different Stoichiometry 164
Fibrous Aerosols 164
Microspherical Aerosols 164
Complementary Analysis 165
Determination of Chromium Valency in Aerosols Less than 10

µ

m in Size Emitted as
Dust from the Steel Industry 166
Elemental Analysis of Dust Collected from the Site 167
Study of the Valency of Chromium in Dusts Collected by a Portable Impactor
and by a Normal Andersen Impactor at the Same Site 168
Chromium Analysis of Large Dust Aerosols Collected with the Portable Impactor 168
Conclusion 174
References 174

INTRODUCTION

Since its discovery in 1797, chromium has been increasingly used in industry. Its effects on human
health have been gradually understood. The two most important sights of professional pathology,
due to chromium and to its derivatives, are represented by “allergogene” action and the cancer-
producing power of some products. Indeed, hexavalent chromium derivatives cause dermatosis of
contact, bronchitic asthma, perforation of the nasal septum, and bronchiopulmonary cancers.

1,2

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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,

6-8

have the ability to determine both chromium identification and correct proportioning
in its numerous oxides forms. A main drawback concerns aerosol sampling because the sample
can be physically altered (e.g., by extraction, dissolution, electrochemical reactions during lixivi-
ation operation, etc.) with the possibility of induced chemical changes, matrix effects, and differ-
ential solubility artifacts.

9

Spectroscopic techniques such as Raman, X-ray diffraction, FTIR,

10,11

XPS,

12-14

and XRF

15,16

allow

in situ

characterization of solid aerosols, but there are many instrumental limitations related
to detection, sensitivity, sampling, and data interpretation.
Among the spectroscopic techniques recently described in the literature to identify

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

AMPLING

Welding dust was collected after a welding operation on stainless sheet steel (MMA/SS) of type
304L 18-10 (lower carbon) with rods AROSTA — basic rutile electrode 304. Dusts pass in a cyclone
impactor by suction, drawing up a flow of 1.71 l min

–1

for 46 min. Dusts are finally collected onto
a nitrocellulose filter of 0.2

µ

m porosity and 32-mm diameter. Sampling selection of dust was fixed
to 10

µ

m coating element of the rods and trimming dusts (metallic projectiles) also have been
collected in the experimentation workshop for LAMMA analysis.

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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols

161

Dust sampling was performed at a site rich in fumes containing large amounts of chromium
using two kinds of impactors:

• 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

m; 1.1 to 0.7

µ

m; 0.7 to 0.4

µ

m; and <0.4

µ

m) gave information
about the distribution of chromium compounds in relation to aerosol size.
• Dusts were collected by impaction on aluminium filters during a period of 12 to 24 h;
a portable cascade impactor consisting of five stages, with aerosols size ranging from
0.4 to 9.8

µ

m and air flow rate of 1.7 l min

–1

carried by workers during an 8-h shift.
Dusts were collected on nitrocellulose or polyethylene terephthalate filters.

A

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

–1

).

X-ray Photoelectronic Spectroscopy (XPS)

The used apparatus from Leybold Heraeus

TM

had a resolution of 1 eV. The radiation source comes
from the K

α

radiation of aluminum. The deposited dusts are directly mounted on the sampling
support. An interval of 3 eV, on average, has been recorded for the characteristic lines of C(1

s

),
K(2

p

), and K(2

s

), 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

The study of ionized clusters in both positive and negative modes is correlated with the sample
chemical composition (e.g., SO

–

, SO

2
–

, SO

3
–

, SO

4
–

, NaSO

3
–

, NaSO

4
–

). 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

• Laser focus: 2–3

µ

m
• Energy on sample: 1–3.5

µ

J
• TOF voltage: ±3000 V
• Extraction lens voltage: ±1000 V
• Reflection voltage: ±790 V
• Cathode voltage: ±6000 V
All spectra were recorded on Nicolet 4094C, a recorder connected to an Apple Macintosh II
CX computer with a 40-Mbyte system. Spectra were calibrated and linearized following mass
spectrometry conventions.

ANALYSIS OF DUSTS FROM WELDING FUMES

Welding fumes can contain large amounts of chromium compounds with varying concentrations
that depend on the welding process and the rod composition. A systematic study was achieved on
these different elements for the following:
• Coating of the rod (AROSTA, rutile basis electrode type 304 L)
• Trimming dusts (metallic projectiles)
• Welding aerosols less than 10

µ

m in diameter that have metallic microspherical form

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

µ

m) with a basic nature (fluorides and chloride compounds) likely
contain an alumino- and ferrosilicate matrix in which metallic elements are scattered.
• Microspheres contain a lot of sulfate compounds, like trimming compounds, character-
ized by SO

x
–

clusters in negative ionization mode, whereas microfibrous aerosols do not.

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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols

163

• The two different spectra of fibrous aerosols in positive mode show the heterogeneity of
the matrix, one can obtain either spectra with metallic nature of spectra with alumino-
and ferrosilicates matrix.
• Chromium and nickel are systematically present in positive spectra (Cr

+

and Ni

+

) 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 – ++ +++ ++ +
S – ++ ++ –
Ti –– ++ +++ +
Mg –– ++ +
Ba – ++ – +++ –

Type of Cluster Ions Present on Positive and Negative Modes

Al

x

O

y

+++ ++ +++ +
Si

x

O

y

+++ ++ +++ +++
PO

x

++ +++ + +++
Fe

x

O

y

+++ ++ +
AlSi

x

O

y

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

x

+++ ++ –
FeSi

x

O

y

++ ––
CrO

x

– ++ ++ ++
BaOH ++ – ++ –
TiO – + +++ +
BaF –– + –
K

x

F

y

+ –– +++
Na

x

Cl

y

–– – ++

CaCl –– – +
K

x

Cl

y

–– – +
Al(Na

x

Cl

x

) –– – ++
Na(K

x

F

x

) –– – ++
K(Na

x

Cl

x

) –– – ++
Na

2

OH –– – +
K

2

OH –– – +

Note

: Absent: –; Present: +; Majority: ++; Abundant: +++

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164

Aerosol Chemical Processes in the Environment

C

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.

All results are summarized in Table 7.2. The table displays the CrO

2
–

/CrO

3
–

ratios of different
individual and mean spectra.

Fibrous Aerosols: (<10 µµ
µµ

m)

Spectral analysis of fibrous aerosols has shown that chromium was only present in its elementary
form Cr

+

in positive ions, while CrO

2
–

, CrO

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

–
(x = 2; y = 4, 5, 6);
this is due to either the lower sensitivity or other major recombinations in plasma. Therefore, the
assumption of chromium VI presence in the chromate form must be verified by other analytical
techniques.
Microspherical Aerosols (>10 µµ
µµ
m)
After analysis, the 24 individual spectra of studied microspheres (20 µm in diameter) gave the
following results:
TABLE 7.2
Ratio Calculation of CrO
2
–
/CrO
3
–
of Welding Dust
Type of Dust CrO
2
–
/CrO
3
–
N Spectra Mode Wavelength (nm)
Fibrous 0.218 ± 0,12 85 Individual 225.7
Dust <10 µm 0.155 70 Accumulation 225.7
0.228 200 Accumulation 286.5
0.181 100 Accumulation 286.5
Total: 455

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

–
/CrO
3
–
ratio, which is about
0.8 for the aerosol (arithmetic average of 0.75). A CrO
2
–
/CrO
3
–
ratio of 0.75 might
correspond either to hydrated or anhydrous chromium salt. The lack of CrO
4
–
ion and
simultaneous presence of CrO
–
and SO
x
–
for 100% of spectra would reveal a chromium
presence as anhydrous chromium sulfate, thus in the III form.
One cannot conclude definitively about the real state of chromium in microspheres because
the Cr
2
O
y
–
series has not been detected. Nevertheless, two other observations from comple-

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

an increasing amount of chromium III oxide, depending on microspheres.
COMPLEMENTARY ANALYSIS
Raman analysis was only effective on fibrous aerosols with a minimal energy to avoid absorption
or destruction of dusts. Microspheres study is revealed to be unfeasable because of the large
absorption of the aerosols, even with minimal energy application.
Results of Raman analysis are shown in Figure 7.1. Two specific bands of the CrO
4
2–
ion are
obtained at 849 and 901 cm
–1
and given hexavalent chromium in salt form in fibrous dust.
TABLE 7.3
% Chromium VI Calculated in Welding Dust
Dust <10 µm CrO
2
/CrO
3
< 0.3 With CrO
4
Without SO
x
% of Chromium
Spectrum calc.
Spectrum number 79 12 85 79 Cr VI
CrO
2
/CrO
3
> 0.3 With CrO

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

in its hexavalent form. Nickel is present in an oxidized form (a 4-eV shift from the metal binding
energy value) with two bands at 858.1 and 876.8 eV for Ni(2p) and Ni(3p), respectively.
The LAMMA microprobe turns out to be a sufficiently sensitive and rapid technique for the
determination of the oxidation state of chromium included in complex matrices like aerosols emitted
from industrial environments. However, it appears from this work that dust sampling (i.e., the
conditions of granulometric and morphological selection) and high-risk site targeting are important
factors to consider in improving the methodology. That is the reason why we have established a
sampling strategy that is more refined for the case of the steel industry, which is an important
source of dusts containing non-oxidative products.
DETERMINATION OF CHROMIUM VALENCY IN AEROSOLS
LESS THAN 10 µµ
µµ
M IN SIZE EMITTED AS DUST FROM THE
STEEL INDUSTRY
This study deals with the determination of chromium valency in aerosols less than 10 µm in diameter
emitted as dust from the steel industry, and with the development of a strategy for controlling the
hazards these aerosols represent.
Within the context of industrial hygiene, there is great interest in the way the distribution of
trivalent and hexavalent states of chromium differ among the various dust-emitting sites. Dust
sampling by cascade impactors at a given site does not necessarily reflect the conditions of inhalation
of the aerosols by a person working at that site. Sampling can be improved by fixing a portable
cascade impactor to a person in the course of his/her normal activities in an area where chromium-
rich dusts are found.
FIGURE 7.1 Raman spectrum of dust particles smaller than 10 µm.
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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 167
ELEMENTAL ANALYSIS OF DUST COLLECTED FROM THE SITE
The LAMMA analysis of aerosols in size-fractionated dust samples shows that small aerosols
mainly consist of potassium and sodium, with relatively little calcium (Figure 7.2), whereas the

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

THE PORTABLE IMPACTOR
The aerosols of greatest size collected in the cascade impactor carried by a worker have been
studied by SEM, LAMMA, and Raman microprobes.
FIGURE 7.4 Comparison of chromium valency between dusts collected in an Andersen cascade impactor
and that collected with a portable cascade impactor in relation to the particles.
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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 169
SEM microscopy permits elemental analyses of aerosols, whereas the two other analytical
techniques, which are complementary, enable detection and identification of chromium compounds
in aerosols. Thus, SEM analysis has shown that the dust contains many of the following elements:
silicon, calcium, iron, and chromium (Figure 7.6).
LAMMA analyses of these aerosols confirmed the presence of chromium characterized by
systematic formation of Ca
x
CrO
y
–
ions (m/z 108, 124, 140, 148, 180, and 196) in the positive mode
(Figure 7.7). In the negative mode, the cluster class Cr
x
O
y
–
(m/z 84, 100, 116, 168, 184, and 200)
are always present, with systematic formation of CrO
4
–
being characteristic for the solvation of the
chromium compound clusters in a hydrated or oxygen-rich environment.

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

FIGURE 7.7 LAMMA spectrum of dust particles (8 to 10 µm) collected with portable cascade impactor
(positive ionization mode).
FIGURE 7.8 LAMMA spectrum of dust particles (8 to 10 µm) collected with portable cascade impactor
(negative ionization mode).
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LAMMA and Raman Study of Oxidation States of Chromium in Aerosols 171
disrupted.
25
Fundamental stretching vibrations of Cr-O (ν
1
and ν
2
) are intense and localized between
950 and 800 cm
–1
, whereas deformation vibration intensities are weak and localized between 420
and 370 cm
–1
(ν
2
and ν
4
).
26-30
Campbell
30
had defined two types of chromates from vibration spectra:
1. “Type I chromate” spectra previously described (Na-, Ca-, Ba-, Sr-, Rb-, Li-, Ag-, and
Pb-CrO

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.

TABLE 7.4
Attribution of Raman Vibration Bands for Different
Standard Compounds and for Dust Collected with a
Portable Cascade Impactor
Sample
Wavenumber
(cm
–1
)
PbCrO
4
839 (M)
378 (vW)
Na
2
CrO
4
, xH
2
O 3340, 3250 (vH2O)
938 (M), 923 (M), 891 (H), 853 (vH), 810 (M)
438 (W), 350 (W)
K
2
CrO
4
906 (M), 877 (M), 869 (H), 853 (H)
394 (vW), 389 (W), 348 (M)
CaCrO
4

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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© 2000 by CRC Press LLC
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.

Detailed and complementary characterization by three in situ analytical techniques of inhaled
aerosols that do not penetrate the lungs (8–10 µm) have proved that hexavalent chromium is present
as calcium chromate in this kind of aerosol.
The valency and type of chromium appear to correlate with the overall composition of the
aerosol: indeed, small aerosols (<1.1 µm) are basically composed of potassium and sodium matrices,
whereas large one (>6 µm) are mainly calcium. The presence of calcium chromates in dust aerosols
8 to 10 µm collected by a portable impactor carried by a worker at the same site confirms this fact.
The small aerosols contain hexavalent chromium forms with the possible presence of sodium and/or
potassium chromates.
This study shows the capabilities of microprobe LAMMA for in situ characterization of aerosols
of different sizes (0.4–10 µm). The analytical speed and high sensitivity of chromium detection,
combined with a well-chosen dust sampling protocol, favor the LAMMA technique for this type
of study.
However, other complementary in situ techniques — the microRaman technique in particular
are desirable for the verification of chromium oxidation state and of the nature chromium com-
pounds present in aerosols.
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