WO2003027650A2

WO2003027650A2 - Method for detection and discrimination of polycyclic aromatic hydrocarbons (pahs) and monoaromatics based on laser-induced breakdown spectroscopy (libs)

Info

Publication number: WO2003027650A2
Application number: PCT/IL2002/000782
Authority: WO
Inventors: Alexander Portnov; Ilana Bar; Zamik Rosenwaks
Original assignee: Ben-Gurion University Of The Negev
Priority date: 2001-09-26
Filing date: 2002-09-23
Publication date: 2003-04-03
Also published as: AU2002337589A1; WO2003027650A3; IL145687A0

Abstract

The invention provides a method for remote real-time detection and monitoring of polycyclic aromatic hydrocarbons (PAHs) and monoaromatics in a sample based on the use of laser-induced breakdown spectroscopy (LIBS). According to the method of the invention, detection and monitoring of PAHs and monoaromatics in a sample is carried out by the steps of using the energy from a pulsed laser to ablate a small quantity of a sample containing PAHs or monoaromatics, producing the emission spectrum of the resulting plasma plume by means of a spectrograph, detecting the intensities of the optical energy of specific features of the emission spectrum, measuring the intensities, and analyzing the measurements of the intensities. Analyzing the measurements of the intensities is done by comparing the measured maximum intensities with values in a previously acquired data base, calculating the ratio of the integrated intensity of specific features of the emission spectrum, and comparing the calculated ratios with values in a previously acquired data base.

Description

METHOD FOR DETECTION AND DISCRIMINATION OF

POLYCYCLIC AROMATIC HYDROCARBONS (PAHS) AND

MONOAROMATICS BASED ON LASER-INDUCED BREAKDOWN

SPECTROSCOPY (LIBS)

Field of the Invention

The present invention relates to remote real-time detection and

monitoring of chemical substances in ambient air. More particularly it relates to a method for remote real-time detection and monitoring of

polycyclic aromatic hydrocarbons (PAHs) and monoaromatics in a sample

based on the use of laser-induced breakdown spectroscopy (LIBS).

Background of the Invention

Polycyclic aromatic hydrocarbons (PAHs) and monoaromatics [for

example, benzene, toluene, xylene, ethyl benzene (BTXE)] are chemicals

consisting of a main block unit constructed of hexagonal structures of

carbon atoms with one hydrogen atom attached to each of them.

They are manufactured and found in coal tar, crude oil, roofing tar, and

are used in medicines or the manufacture of dyes, plastics and pesticides.

They are also found in space, since stable carbon-carbon bonds held their

structure together, surviving the harsh environment between stars [Meeus, et.al., ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be

systems: Towards an understanding of dust processing, ASTRONOMY AND ASTROPHYSICS, 365(3):476-490 JAN 2001], However, the major

reason for interest in PAHs and monoaromatics is that they are formed

during the incomplete burning of coal, oil and gas, waste, and other

organic substances such as tobacco or charbroiled meat. PAHs enter the

atmosphere mostly as releases from volcanoes, forest fires, burning coal,

vehicle exhaust, and incomplete combustion of waste. PAHs can occur in

air as aerosols and also can stick to solid particles and settle on, and

migrated downward into, the soil or on the bottom of lakes and rivers.

Moreover, monoaromatics (and also, at trace levels, PAHs) are

constituents of mineral oil products. Therefore, after oil spills, they can

contaminate soil, surface water and, due to their high mobility, also

groundwater. Enviromental problems are attracting increasing attention

and, due to the recognition that they have carcinogenic and mutagenic

potential, the United States Environmental Protection Agency (US-EPA)

has declared PAHs and monoaromatics to be priority pollutants

[http://www.epa.gov/ttnatw01/34poll.html]. Hence, determination of their

occurrence is of much importance.

Excellent laboratory techniques exist for the selective analysis of PAHs

and monoaromatics. The conventional analytical methods deployed for PAH aerosols include: collection of p articulates by drawing a large volume

of air through a filter, extraction of the collected PAH with an organic solvent, ^hromatographic cleanup and separation followed by identification using one or a combination of spectroscropic methods or

mass spectrometry analysis in a high vacuum chamber. An additional

method, described by Schechter in U.S. patent No. 5,880,830, employs

collection of PAH on non-fLuorescing filter paper accompanied by

excitation of the particles and imaging of the resulting fluorescence.

Despite the existence of so many methods of carrying out the analysis in

the laboratory, techniques for remote real-time detection and monitoring

of PAHs and monoaromatics providing high sample throughput at

relatively low cost per analysis are desired. The need for field techniques

is especially important in cases requiring continuous monitoring or

analysis of many samples wherein the objective is to assess the hazard

potential at some location.

Laser-induced breakdown spectroscopy (LIBS) is a well known diagnostic

technique that, in principle, provides a simple and rapid method for online elemental analysis of solid, liquids and aerosols. Much of the use of LIBS

has been for detection and determination of concentration of metals, as

disclosed, for example, in US 5,847,825. US 5,751,416 is an example of a

patent that discloses the application of LIBS to monitoring concentrations

of toxic metals, and other materials that are dangerous or difficult to

monitor using other techniques. The same patent discloses a method of monitoring industrial processes by determining the concentration of the

various elements at the feed and product side. Other typical references disclosing the LIBS method and its applications are, for example:

Radziemski and Cremers, Laser-Induced Plasmas: Physical, Chemical and

Biological Applications (1989),pp. 295-325; Rusak et al., Critical Reviews

in Analytical Chemistry, 27(4), 257 (1997).

LIBS employs an intense spot of energy from the beam of a pulsed laser

focused onto the surface of a material. The laser energy ablates a small

qtiantity of the material. The ablated material is vaporized and then

partially atomized, excited and ionized, producing a plasma plume above

the surface. The emission spectrum from the plasma plume is detected

and used for identification and quantification of the elements present in

the plume. The advantages offered by the method include the need for

little, or no, sample preparation and the smallness of the amount of

ablated material needed.

As mentioned above, most of the work with LIBS has been for detection

and determination of concentration of metals although there also exist

many descriptions of its application to many other types of sample. To date, however, there has not been described any method of monitoring the

presence and content of the environmentally important PAHs and

monoaromatics based on the use of LIBS. It is thus a purpose of the present invention to provide a method for

detection and identification of PAH and monoaromatic traces in a sample by using LIBS.

It is another purpose of the present invention to provide a method for

determining the ablated mass of PAH and monoaromatic traces in a

sample by using LIBS.

Further purposes and advantages of this invention will appear as the

description proceeds.

Summary of the Invention

The invention is directed towards providing a method for remote real-time

detection and monitoring of polycyclic aromatic hydrocarbons (PAHs) and

monoaromatics in a sample based on the use of laser-induced breakdown

spectroscopy (LIBS).

According to the method of the invention, the detection and monitoring of

polycyclic aromatic hydrocarbons (PAHs) and monoaromatics in a sample

is carried out by the following steps:

using the energy from a pulsed laser to ablate a small quantity of a sample containing the PAHs or monoaromatics; producing the emission spectrum of the resulting plasma

plume by means of a spectrograph; detecting the intensities of the optical energy of specific

features of the emission spectrum;

measuring the intensities; and

analyzing the measurements of the intensities.

According to the method of the invention, the sample consists of

micr op articles of the substance to be monitored. The micr op articles can

cover the surface of a solid or liquid or be dispersed in a large volume of

solid or fluid.

According to the method of the invention, measuring the intensities

includes measuring the maximum intensity and measuring the integrated

intensity of each of the specific features of the emission spectrum.

According to the method of the invention, analyzing the measurements of

the intensities includes:

comparing the measured maximum intensities with values in

a previously acquired data base; calculating the ratio of the integrated intensity of specific

features of the emission spectrum; and

comparing the ratios with values in a previously acquired

data base. In the preferred embodiment of the invention, the specific features of the

emission spectrum are the (d-a, Δv=0) band of the C₂ Swan system and the

(B-X, Δv=0) band of the CN violet system.

According to the method of the invention, the detection and monitoring of

polycyclic aromatic hydrocarbons (PAHs) and monoaromatics in a sample

involves one or more of the following: verifying the presence of a known PAH or monoaromatic in

the sample;

identifying an unknown PAH or monoaromatic in the

sample; and

quantifying the amount of the PAH or monoaromatic ablated

from the sample.

The presence of a known PAH or monoaromatic is verified, or an unknown

PAH or monoaromatic is identified, in the sample by calculating the ratio

of the integrated intensities of the (d-a, Δv=0) band of the C₂ Swan system

and the (B-X, Δv=0) band of the CN violet system and comparing the

ratios with values in a previously acquired data base.

The amount of PAH or monoaromatic ablated from the sample is

determined by comparing the measured maximum intensities of the (d-a,

Δv=0) band of the C2 Swan system and the (B-X, Δv=0) band of the CN

violet system with values in a previously acquired data base. All the above and other characteristics and advantages of the invention

will be further understood through the following illustrative and non-

limitative description of preferred embodiments thereof, with reference to

the appended drawings.

Brief Description of the Drawings

- Fig. 1 is a schematic drawing showing the LIBS apparatus employed

for the remote sensing of PAH samples;

- Fig. 2 is a graph showing an example of the emission spectrum

generated as a result of the application of the laser beam on sample

targets of different compounds;

Fig. 3 shows the C2/CN intensity ratios as a result of ablation from a surface for the samples of Fig. 2;

- Fig. 4 shows obtaining the intensity ratio of Fig. 3 from the emission

spectrum of Fig. 2;

- Figs. 5A to 5F show the emission spectrum generated as a result of the

application of the laser beam on sample targets of naphthalene on

different substrates;

- Fig. 6 shows the C2/CN intensity ratio resulting from ablation of

naphthalene from the surfaces of the different substrates of Fig. 5;

- Fig. 7A shows the CN and C₂ intensities as a function of the mass of

naphthalene ablated from a surface; and - Fig. 7B shows the C2/CN intensity ratio as a function of the mass of

naphthalene ablated from a surface.

Detailed Description of Preferred Embodiments

The invention will now be further explained through the illustrative and non-limitative description of preferred embodiments.

As described briefly hereinabove, the method of LIBS is based on using

the beam from a pulsed laser to ablate a small quantity of a sample. The

emission spectrum from the resulting plasma plume is then detected and

analyzed to identify and quantify the material composing the sample. Fig.

1 is a schematic diagram showing the experimental system used for LIBS.

Numeral 1 designates a laser, which can be of any type capable of repetitively producing short, high intensity pulses, Typically, excimer or

pulsed Nd:YAG lasers are used in LIBS. Numeral 2 designates focusing

optics used to focus the laser pulses onto sample 3. The sample consists of

microp articles of the substance to be monitored (in the case of this

invention PAHs or monoaromatics). The particles can be covering the

surface of a solid or liquid or dispersed in a large volume of solid (for ex.

soil) or fluid (for ex. water or air).

As described above, the focused laser beam results in a plume 4, the

radiation from which is collected by the first end of the fiber optics 5 and focused onto the entrance slit of spectrograph 7. Both the focusing optics 2

and the collecting optics 5 can be either a single lens or mirror or

combinations of lenses, mirrors, and other optical elements including, for

example, prisms or beam-splitters. The optical arrangement employed is

determined by the conditions under which the measurements are carried

out and the choice of optical elements and their arrangement are well

known to persons of the art.

In many situations an optical fiber cable 6 is used to guide the radiation

from the plume to the spectrograph. The use of a fiber optic cable is

essential when the sample is located a great distance from the

spectrograph, especially in the field where it is not practical, or even

possible in many cases to use an arrangement of optical elements to

transfer the radiation. It is usual to use the fiber optic cable with its first end 5 close to the sample to collect the emission and guide and couple it

into the second end of the cable. At the other end of the cable is an adapter (not shown) for attaching the cable to the spectrograph and

focusing the radiation on the entrance slit of the spectrograph. These

optical and mechanical arrangements are well known in the art and

therefore will not be discussed here.

The resultant dispersed emission is detected by an intensified charge

coupled device (ICCD) 8 and processed by a data acquisition hardware and software unit 9 and transferred to personal computer (PC) 10 for

further data processing.

In order to produce transistor-transistor logic (TTL) pulses to synchronize

the laser pulse to the ICCD and to operate and gate the ICCD at a specific

delay following the laser pulse, a delay generator and an input/output box collectively designated by numeral 11 are used. The resultant emission

captured by the ICCD produces counts as a function of wavelength.

The present invention employs a nanosecond pulsed laser to ablate a

sample containing PAHs or monoaromatics. As mentioned above, the laser causes vaporization, dissociation and ionization and produces a plasma.

The laser pulse energy is kept relatively low, around 10 mJ, in order to

prevent breakdown in the surrounding air.

As the plasma cools the excited species relax and emit optical energy at

characteristic wavelengths. The time-resolved emission is spectrally

dispersed to identify the small molecules or atoms, which are present in

the sample, based on the presence of characteristic spectral lines. The

most pronounced spectral features belong to the CN violet system and the

C2 Swan system. The intensities of the observed C₂ lines, arise from the materials contained in the sample, and reflect either the ejection of C2

fragments from the aromatic rings containing double-bonded carbon or the recombination of C atoms. The CN results from the Shockwave induced by the interaction of the expanding plume (containing C₂) with N₂, which is

the major constituent of air. A clear relationship between plasma emission

and the number of aromatic rings, given by the ratio of the characteristic

emission lines C2 CN, exists. This signal/structure relationship serves as a measure of presence of the compound of interest. Comparison of the

observed spectral features and the obtained ratios to a previously acquired

data base characterizes the PAHs and monoaromatics.

The existence of the signal structure and its use in characterizing the

sample is illustrated by the following examples, which are provided

merely to illustrate the invention and are not intended to limit the scope

of the invention in any manner.

The experimental arrangement used to collect the data in the following

examples is schematically shown in Fig. 1. The apparatus and its

arrangement described hereinbelow, is that used in the laboratory. Many

other arrangements and different choices of equipment are possible to

achieve the objects of the invention. In particular skilled persons will be able to make modifications for use of the system in the field. Such

modifications could include, for example, a different type of laser and

wavelength, a longer fiber optic bundle 6, or appropriately adapted optical

components 2, including supplementing them with a fiber optic bundle, to focus the laser energy on the remote sample. The second harmonic (532 nm) of a pulsed Nd:YAG laser 1 (Continuum

Powerlite 8000) was used as the energy source for activating the PAH in

the sample target 3. The laser emits pulses of ~5 ns. The laser beam of

-10 mJ was delivered by prisms, mirrors, and their combination and

focused by lens 2 on PAH microparticles covering a surface 3. The power

of the laser beam was adjusted to be high enough to induce ablation of the PAH microparticles, while, at the same time, not causing breakdown of

the surrounding air. The light emitted from the resulting plasma of the

analysis sample 4 was collected by a quartz fiber bundle 6 (C

Technologies, 37 fibers of 100 μm diameter, 3 m length, 10 mm o.d. ferrule

termination). The end of this fiber was mounted in an adapter and

connected to the entrance slit of a spectrograph 7 (Chromex 250IS

Imaging Spectrograph equipped with three interchangeable gratings

(150/500, 1200/500, 2400/250), providing different spectral ranges and

bandwidths, depending on the employed grating. The resultant dispersed

emission was detected by a gated ICCD 8 (Andor DH-52O-18U-03) and processed by a data acquisition hardware (Andor, PCI Controller) and

software unit (Andor MCD software package) 9. The resulting signals

were then transferred to personal computer (PC) 10 for further data processing.

In order to produce transistor-transistor logic (TTL) pulses to synchronize

the laser pulse to the ICCD and to operate and gate the ICCD at a specific

delay following the laser pulse, a delay generator (Stanford Research Systems, DG535) and an input/output box (Andor) collectively designated

by numeral 11 were used.

Example 1

Using the apparatus described above, samples containing microparticles

of different molecules covering the surface of a tantalum cruicible were

ablated. Characteristic emission spectra from ablated glucose A, toluene

B, naphthalene C, and anthracene D are shown in Fig. 2. These spectra

are the average of 5 pulses and were monitored at a delay of 1.5 μs

(following the laser pulse) with a gate width of 0.5 μs.

Fig. 3 shows the C2/CN intensity ratios, calculated from the spectra of Fig.

2, for the different molecules. From Fig. 3 it can be seen that the C2/CN

ratio is unique for each compound, increases extensively with increasing

number of aromatic rings in the compound, and therefore is a sensitive

probe for the structure of the ablated compound.

The C2/CN intensity ratios were calculated from the integrated signals

corresponding to the C2(d-a, Δv=0) and CN(B-X, Δv=0) bands, after

making the required baseline corrections. The calculation process is

illustrated in Fig. 4. In this case, the sample was napthalene ablated from an aluminum substrate. The integrated intensity ratio is C2/CN =

(11100counts)/(18500counts) = 0.6. Example 2

In order to demonstrate the effect of the substrate on the C2/CN intensity

ratios, samples were prepared in which naphthalene films were uniformly

coated on different substrates. Using the experimental arrangement

described above, the naphthalene was ablated from each of the substrates.

Figs. 5A to 5F show the resulting emission spectra. The substrates used

were: aluminum (Fig. 5A), polyvinyl chloride (PNC) (Fig. 5B), sand (Fig.

5C), paper (Fig. 5D), teflon (Fig. 5E), and glass (Fig. 5F).

Using the method described in example 1, the C2/CΝ intensity ratios were

calculated for the spectra of Figs. 5A to 5F. The results are shown in Fig. 6

where the letters identify the corresponding substrate in Fig. 5A to 5F.

From the results shown in Figs. 5A to 5F and 6, it is concluded that the

spectrum measured by the method of the invention is insensitive to the

substrate and depends on the ablated material. Further this conclusion

can be extended to cover situations other than ablation from a solid

surface. For example, if grains of naphthalene are dispersed on the

surface of a body of water, the method of the experiment would result in

the same spectrum as that of Figs. 5A to 5F. This result is to be expected

because the ablation of the naphthalene molecules is much greater than

that of the water molecules. Example 3

This example demonstrates how the method of the invention can be used

to determine the quantity of PAH that is present in a sample. Solutions of

different masses of naphthalene in methanol were uniformly dispersed on

glass slides forming films of various thicknesses. After the methanol

evaporated, the naphthalene was ablated using the above-mentioned apparatus and method. Single laser pulses were applied to the

naphthalene samples in order to obtain the spectra for different amounts.

The mass of ablated material was calculated from the thickness of the

layer and the spot size of the laser.

The results of a series of these measurements are shown in Fig. 7A. The

peak intensities of the C2(d-a, Δv=0) band (stars) and of the CN(B-X,

Δv=0) bands (dots) are plotted on the vertical axis and the ablated mass,

measured in μg, is plotted on the horizontal axis. The results show that

the intensity is linearly proportional to the amount of ablated naphthalene.

The results that are shown in Fig. 7A can therefore be used to determine

the amount of material ablated from an unknown sample of naphthalene.

This is done simply by exposing the unknown sample to the laser energy,

measuring the intensity from the resultant spectrum, and comparing the

result to the plots of Fig.7 A. To complete the analysis, the C2/CN intensity ratios were calculated from

the integrated signals corresponding to the pairs of data points in Fig. 7A.

The results are shown in Fig. 7B and confirm that the C2/CN intensity

ratio is independent of the amount of ablated material. Comparison of Fig.

7B with Fig. 3 confirms that the sample is naphthalene.

The examples described hereinabove demonstrate the application of the

method of the invention to remote real-time detection and monitoring of

PAHs and monoaromatics.

The temporal and spectral information of the emission, in particular the

ratio of the characteristic emission hnes resulting from C2 and CN

molecular fragments, is determined for each PAH of interest. This data is

stored in, for example, a computer memory in the form of calibration

curves (for example Fig. 2) derived from the control samples or a data

base containing the relevant data (for example the data of Fig.3).

In trying to determine the presence and/or identification of an unknown

PAH, the laser beam can is scanned across the surface to be investigated

and the emitted light is remotely picked up and, using an optical or fiber¬

optic link, is led to the spectrograph and detector. The resultant spectrum

is analyzed to obtain the characteristic C2/CN ratio and compared with the stored data base to determine the presence and/or identity of the PAH in the unknown sample. Using the method of the invention, the results

are determined and displayed in real time.

It will be apparent to persons skilled in the art, that the process of

analyzing the data to determine the presence of a known PAH or the

identity of an unknown PAH and also the process of determining the

amount of ablated sample can be handled completely and automatically by

the electronics of the system. Thus the method of the invention overcomes

the drawbacks of the prior art and allows rapid real-time analysis that

can be carried out either under laboratory conditions or in the field.

Although embodiments of the invention have been described by way of

illustration, it will be understood that the invention may be carried out

with many variations, modifications, and adaptations, without departing

from its spirit or exceeding the scope of the claims.

Claims

1. A method for remote real-time detection and monitoring of polycychc

aromatic hydrocarbons (PAHs) and monoaromatics in a sample based on

the use of laser-induced breakdown spectroscopy (LIBS).

2. A method according to claim 1, wherein the detection and monitoring

of polycychc aromatic hydrocarbons (PAHs) and monoaromatics in a

samplers carried out by the following steps:

using the energy from a pulsed laser to ablate a small quantity of a sample containing said PAHs or monoaromatics;

producing the emission spectrum of the resulting plasma plume by

means of a spectrograph;

detecting the intensities of the optical energy of specific features of

said emission spectrum; measuring said intensities; and

analyzing said measurements of said intensities.

3. A method according to claims 1 and 2, wherein the sample consists of

microparticles of the substance to be monitored.

4. A method according to claim 3, wherein the microparticles cover the

surface of a solid or liquid.

5. A method according to claim 3, wherein the microparticles are

dispersed in a large volume of solid or fluid.

6. A method according to claim 2, wherein measuring the intensities

includes measuring the maximum intensity and measuring the integrated

intensity of each of the specific features of the emission spectrum.

7. A method according to claim 2, wherein analyzing the measurements

of the intensities includes:

comparing the measured maximum intensities with values in a

previously acquired data base; calculating the ratio of the integrated intensity of specific features

of the emission spectrum; and

comparing said ratios with values in a previously acquired data

base.

8. A method according to claims 2, 6, and 7, wherein the specific features

of the emission spectrum are the (d-a, Δv=0) band of the C2 Swan system

and the (B-X, Δv=0) band of the CN violet system.

9. A method according to claims 1 to 8, wherein the detection and

monitoring of polycyclic aromatic hydrocarbons (PAHs) and monoaromatics in a sample involves one or more of the following: verifying the presence of a known PAH or monoaromatic in said

sample;

identifying an unknown PAH or monoaromatic in said sample; and

quantifying the amount of said PAH or monoaromatic ablated from said sample.

10. A method according to claims 1 to 9, wherein the presence of a known

PAH or monoaromatic is verified, or an unknown PAH or monoaromatic is

identified, in said sample by calculating the ratio of the integrated

intensities of the (d-a, Δv=0) band of the C2 Swan system and the (B-X,

Δv=0) band of the CN violet system and comparing said ratios with values

in a previously acquired data base.

11. A method according to claims 1 to 9, wherein the amount of PAH or

monoaromatic ablated from the sample is determined by comparing the

measured maximum intensities of the (d-a, Δv=0) band of the C2 Swan

system and the (B-X, Δv=0) band of the CN violet system with values in a

previously acquired data base.