ITRM20090617A1 - METHOD AND APPARATUS FOR MEASUREMENTS OF ISOTROPIC LUMINOUS RADIATION OBTAINED FROM LASER SPECTROSCOPY TECHNIQUES, IN PARTICULAR FOR SUBMICRONIC PARTICULATE MEASURES. - Google Patents

Description

Method and apparatus for measurements of isotropic light radiation obtained by laser spectroscopy techniques, in particular for submicronic particulate measurements

The present invention relates to a method and apparatus for measurements of isotropic light radiation obtained by using laser spectroscopy techniques, in particular for measurements of submicronic particles.

More precisely, the present invention relates to a method and an apparatus for measurements of ultrafine particulate concentration (submicronic, PM1), using the laser-induced incandescence technique and an integrating sphere to increase sensitivity. This method can be used for example for the measurement of carbonaceous particulate in combustion systems and for environmental monitoring. The developed equipment can also allow the measurement of particle sizes, referring to mathematical models already developed among the scientific community. This invention is the result of the work and experiments of research funded by the Energy and Transport Department of the CNR as part of the "Clean Coal" project of the Ministry for Economic Development.

The presence of particulate matter in the environment raises great concern in the population of large cities and public administrations due to the numerous implications related to air quality and the health of citizens. The harmfulness of powders depends on their size, as well as on their chemical composition.

The initials PM10 identify the fine dust present in the atmosphere in the form of microscopic particles whose diameter is equal to or less than 10 µm (10 thousandths of a millimeter). Such powders enter the oral and nasal cavity. The finer powders (PM2.5) penetrate the bronchi while the ultrafine powders (PM1) reach the pulmonary alveoli. There are various sources of fine dust and can be both natural, such as soil erosion, forest fires, the dispersion of pollen and sea salt, and related to human activities such as the various combustion processes in engines, heating systems, in industrial activities and in thermoelectric power plants. In urban areas, vehicular traffic accounts for about 30% of PM10 production.

The legislation for the control of air quality and emissions is essentially based on the use of a gravimetric reference method. In Italy, the limit values defined by law decree no. 60 of 2 April 2002 set two acceptable limits for PM10 in the atmosphere. The first limit is 50 µg / m <3> as an average value measured over 24 hours not to be exceeded more than 35 times a year, the second is 40 µg / m <3> as an annual average. The gravimetric method on the other hand is not able to determine the size of the particles which is instead an important parameter for assessing the danger of dust. It is also of limited accuracy especially when used to measure the dust emitted by low-emission vehicles.

There is great interest in the development of methods capable of measuring the concentration of particles and their average diameter with accuracy and high spatial and temporal resolution.

Various techniques and commercial instrumentations are known which are capable of satisfying various requirements of sensitivity and particle size range. In particular, the MOUDI (“Micro-Orifice Uniform Deposit Impactor”) and ELPI (“Electrical Low Pressure Impactor”) techniques have been developed which provide the particle size as an aerodynamic diameter; then there are tools based on the measurement of electric mobility such as the DMPS ("Differential Mobility Particle Sizer"), SMPS ("Scanning Mobility Particle Sizer") and the nano-DMA ("Nano-Differential Mobility Analyzer") which measure the equivalent diameter of electric mobility.

These instruments are not able to distinguish particles of different nature and, in the case of aggregates of submicronic particles, they only provide the measurement of an equivalent diameter rather than the value of the diameter of the primary particles that make up the aggregates.

Recently, with the development of the LII technique (“Laser-Induced Incandescence”), the measurement of the concentration of nanometric carbonaceous particles has had a great impetus for studies on the mechanisms of soot formation. This technique, which is based on the use of laser light, is able to provide information on the concentration of carbonaceous particles and on the size of primary particles with excellent spatial and temporal resolution capabilities.

Laser-induced incandescence (LII) is an experimental technique mainly used in laboratories to study flames, combustion processes and the exhaust from various combustion systems. The technique consists in irradiating the soot particles ("soot") with an intense pulsed laser radiation, capable of being absorbed by the particles and therefore causing a strong heating. Each particle behaves almost like a black body and, according to Planck's principle, emits a radiation whose spectrum depends on the temperature reached by the particle itself. Since the temperature of the soot particles can easily reach 4000 K (which is the sublimation temperature of carbon), the LII signal can be easily isolated from the radiation of the surrounding environment. With simple optical arrangements, measurements with great spatial and temporal resolution are obtained. According to the principles of LII, the intensity of the emitted radiation is proportional to the volumetric concentration (or volume fraction) of the particles while the decay profile is related to the size of the primary particles.

The phenomena that come into play in this technique are however complex and occur on the nanosecond scale. They mainly concern the absorption of laser radiation and the heating of the particles, the heat exchange with the surrounding gases, the light emission and various sublimation phenomena at higher temperatures. The LII technique has been known for some time, however some experimental difficulties and the not yet complete understanding of the physical phenomena that govern it have relegated the technique mainly to research laboratories. To date, no more than 2-3 commercial realizations are present on the market.

In particular, there is the LI <2> SA ("Laser-Induced Incandescence Soot Analyzer") produced by Esytec (www.esytec.de) of Erlangen which is a spin-off of the Department of Thermodynamics of the University of Erlangen. It is based on several patents having Leipertz et al. (WO9730335, US6496258 and US2006256330), in which it is proposed to measure the thermal radiation at two different points of the decay curve to determine the diameter of the primary particles.

The experimental setup can be used for carbon black or "carbon black" and metal oxide particles. It can also be implemented with other techniques, such as light scattering, and used in a modular system also for the analysis of engine exhaust.

A difficulty inherent in the technique used lies in the fact that the measurement relies on mathematical models that describe the decay of the temperature and therefore of the LII signal. Such models are still not entirely reliable. Furthermore, given the optical arrangement used, the signals are rather weak and noisy. The laser fluence (spatial energy density) must be as uniform as possible to allow a uniform temperature distribution of the irradiated particles in the measurement volume and therefore a correct determination of the temperature.

Another commercial release is Artium's LII200 (www.artium.com) of Sunnyvale, California. This firm has acquired the patents of the National Research Council of Canada having as inventors Snelling et al. (CA2272758, US6154277, US6181419, US6809820). In these patents it is proposed to measure the absolute value of the intensity of the LII signal which can be directly related to the concentration of the particles in case you want to know the temperature of the particles themselves. The temperature can be determined with a mathematical model or with other experimental means, in particular the two-color pyrometry technique. There is also the need to use a spatial profile of the laser as uniform as possible obtained with a special optical arrangement. In particular, in the US6809820 patent the accuracy of the measurements is improved thanks to the use of a laser not at maximum fluence in order to limit the interference of the evaporation and sublimation process of the particles on the decay of the LII signal. The mathematical model that describes the LII phenomenon is an integral part of the patents.

This arrangement is also based on a series of theoretical assumptions and requires particular and complex optical arrangements. Consequently, this technique is not sufficiently reliable and still too expensive.

Disadvantages of traditional techniques for some specific measures of interest have been illustrated so far.

However, the problem can be posed in a more general way.

In fact, in various laser spectroscopy techniques, such as laser induced fluorescence (LIF) and LII, the phenomenon of "saturation" occurs in which the light signal to be measured does not further increase beyond a certain level of applied laser power. . For example, it is well known in the scientific community that the laser fluence value for which the saturation of the LII signal is obtained is about 250 mJ / cm <2> for a laser irradiation at the wavelength of 1064 nm, corresponding to the fundamental emission of a Nd: YAG laser.

The general problem therefore arises of increasing the sensitivity of laser spectroscopy techniques, so as to be able, for example, to measure very low concentrations of particulates or micropollutants as required for environmental monitoring.

For this purpose, in the prior art systems are proposed which use particular optical arrangements, which are not sufficiently adaptable and in any case bulky.

The purpose of the present invention is to provide a method for carrying out a high-sensitivity laser spectroscopy measurement, for example for the measurement of submicronic particulate, in particular carbonaceous, using laser-induced incandescence, which solves the problems and overcomes the drawbacks. of the prior art.

A further specific object of the present invention is an apparatus for the high sensitivity measurement by laser spectroscopy, for example of submicronic particulate, in particular carbonaceous, which solves the problems and overcomes the drawbacks of the prior art.

The subject of the present invention is an apparatus for measuring isotropic radiation from gases through laser spectroscopy, comprising:

- means of gas supply in

- at least one measuring chamber;

- means for evacuating the gas from the measuring cell; - at least one laser source which generates a laser beam which passes through said measurement chamber; and characterized by the fact of further understanding:

- an integrating sphere through which said measuring chamber is made to pass;

- an optical fiber connected on one side to an opening of said integrating sphere to collect a light measurement signal, and on the other to

- a measurement light signal analysis system.

Preferably according to the invention, said gas supply means comprise a sampling probe and a duct.

Preferably according to the invention, said measurement light signal analysis system comprises:

- means for the separation of the same signal into two optical beams belonging to two suitable spectral bands, as well as

- means for detecting said two optical beams, to implement the two-color incandescent measurement technique.

Preferably according to the invention, said measurement chamber consists of a transparent pyrex or quartz tube, passing diametrically through the integrating sphere.

Preferably according to the invention, for laser-induced fluorescence measurements, with particular regard to the determination of substances subject to broadband fluorescence such as polycyclic aromatic hydrocarbons, said system (50, 60, 70) for analyzing the measurement light signal comprises a spectrograph with intensified CCD detector.

Preferably according to the invention, the apparatus is suitable for the measurement of isotropic radiation from gas by means of spectroscopy on a breakdown induced by a laser, in the apparatus, before the entrance to the measurement chamber, a suitable lens being placed which allows the focusing of the beam to the center of the integrating sphere.

The object of the present invention is a method of measuring isotropic light radiation obtained through laser spectroscopy, characterized by the fact of carrying out the following steps:

A. send a gas to be analyzed in a measuring chamber;

B. sending a laser beam into said measuring chamber;

C. collecting the light scattered by said gas hit by said laser beam, by means of an integrating sphere;

D. transforming the light measurement signal integrated by said integrating sphere into an electrical signal by means of suitable photodetectors;

E. analyze said electrical signal to obtain the desired measurement.

Preferably according to the invention, phase E implements the two-color incandescent measurement technique.

Preferably according to the invention, before phase A a calibration phase A0 is performed, in which the following phases are performed:

A0_1. place a calibrated spectral irradiance lamp at a calibration distance from a special calibration port of the integrating sphere, which can assume an open configuration and a closed configuration, so that the light of the calibration lamp illuminates the inside of the sphere integrator when the calibration door is open;

A0_2. measuring the power of the output signal from the integrating sphere through the arrangement of phase A0_1;

A0_3. Calculate the calibration constant

where λ is the wavelength of the signal outgoing from a photodetector, Vcal (λ) is the signal measured on the same photodetector, S is the section of the calibration gate of the integrating sphere, Rcal (λ) the irradiance of the calibration lamp to the calibration distance.

A0_4. measuring the power of the output signal from the integrating sphere with the arrangement of phase A0_1 in which, however, the calibration gate is closed;

A0_5. Calculate the correct calibration constant:

where LII is the measurement value with the calibration door open and LIIa is the measurement value with the calibration door closed.

Preferably according to the invention, in phase E the temporal decay of the light signal of the integrating sphere (42) is taken into account, which has an e <-t / τ> type trend, where:

where Ds is the diameter of the integrating sphere (42), c is the speed of light and ρ is the average reflectivity of the walls of the integrating sphere.

Preferably according to the invention, in phase D the time constant of the photodetectors is also taken into account, the time response of the entire system including the integrating sphere and the photodetectors being given by the convolution of the temporal responses of the integrating sphere and the photodetectors.

The invention will now be described for illustrative but not limitative purposes, with particular reference to the drawings of the attached figures, in which: - Figure 1 shows a diagram of the measuring apparatus according to the invention;

Figure 2 shows in greater detail the integrating sphere as inserted in the diagram of Figure 1;

- Figure 3 shows a diagram of the arrangement necessary for the calibration of the apparatus.

In the following, the method according to the invention will be indicated for brevity with the acronym SILIIS ("Sphere-Integrated Laser-Induced Incandescence Spectroscopy").

As previously highlighted, the signal deriving from laser-induced incandescence or "LII" derives from the light emission of submicronic particles previously heated by laser pulse. Such particles can reach a maximum temperature, which, in the case of carbonaceous soot particles, is about 4000 K. By further heating the particles with more powerful laser pulses, the temperature does not increase as the particles sublimate, reducing their mass. It is therefore evident that, given a concentration of particles, the peak of the LII signal reaches a maximum and cannot grow further, exhibiting a typical phenomenon known as "saturation".

As previously mentioned, the "saturation" value is about 250 mJ / cm <2> for laser radiation at a wavelength of 1064 nm, corresponding to the fundamental emission of a Nd: YAG laser.

Therefore, to increase the sensitivity of the spectroscopy techniques, so as to be able to measure for example very low concentrations of particulate matter or micropollutants as required for environmental monitoring, the present invention realizes an increase in the measurement volume and in the solid angle of reception of the signals.

The apparatus according to the invention is essentially composed of four elements: a sampling probe, a measuring cell with integrating sphere, a pulsed laser (for example Nd: YAG) which produces a laser beam and a spectroscopic measuring system ( for example two-color) coupled with optical fiber to the integrating sphere. Each of these elements has specific characteristics to maximize the result.

In the case of nanometric particulate measurements using the LII technique, the scheme of the equipment 100 according to the invention can be made as shown in Figure 1 and shows the different elements mentioned above.

First of all, there is a sampling probe 10 of the gases produced for example by a combustion process 200. The use of a sampling probe for the gases containing the particulate to be analyzed offers numerous advantages. First of all, it is possible to carry out a controlled dilution to carry out measurements even in conditions of high concentrations of particulates where the phenomena of light absorption could affect the measurements. On the market there are numerous probes made according to regulatory standards, to be used in the various conditions of use of the equipment according to the invention. In particular, for applications in the environmental field it is possible to insert filters to cut the particulate fraction higher than PM1, while for applications in the combustion field it is possible to use isokinetic probes.

The probe 10 is connected to a measuring section 40 by means of a suitable fitting 11 and the gases, after having passed through the measuring section 40, are sucked by a pump 30 with adjustable flow rate through a duct 31. A filter (not shown) provides the removal of particles before passing through the pump 30.

An advantage of using the probe 10 lies in the fact that the flow of gas to be analyzed can be sufficiently small as all the gas containing the particulate is analyzed in the measurement section 40. Thus a small and easily transportable apparatus is obtained.

The measuring section 40 comprising an integrating sphere is shown in greater detail in Figure 2. It comprises a measuring chamber, for example a simple transparent tube 41 made of pyrex or quartz, passing through an integrating sphere 42.

A laser source 20 produces a laser beam 21 which excites the gas particles in the measuring chamber 41 (small tube). At the exit of the measurement chamber, the laser beam is intercepted by a block cavity or "beam dump" 22.

The integrating sphere is an optical component that consists mainly of a typically spherical cavity, the interior of which is covered with a highly reflective material with some small openings necessary for the entry and exit of light radiation.

Its most significant property is the ability to diffuse light, through multiple reflections, distributing it evenly in order to minimize the effects of the initial direction of the light. An integrating sphere can be thought of as a diffuser that maintains the light power but destroys spatial information. If the reflectivity of the coating at various wavelengths is high and the openings are small, the integrating sphere can provide high optical efficiency.

Integrating spheres are normally used for a variety of optical, photometric and radiometric measurements such as the measurement of all the light radiated by a lamp, the measurement of surface reflectivity, the formation of a light source of uniform intensity and the measurement of the power of laser beams regardless of the shape and direction of incidence of the beam.

Some unconventional applications of the integrating sphere are described in some international patents. US patent 4320978 of 1982 describes the realization of a high sensitivity turbidimeter. A cylindrical measuring cell is placed across an integrating sphere and a collimated beam of light is directed through the cell. When a flow of water in which particles are dispersed is made to flow through the measuring cell, the light scattered by the particles is collected by the integrating sphere and measured. The intensity of the light signal is related to the amount of suspended particles that make the water cloudy. The system can be calibrated with NTU (“Nephelometric Turbidity Unit”) turbidity standards.

US patent 4942305 of 1990 describes the realization of a particulate detector in aerosols by means of the laser light scattering technique. A thin stream of aerosols is conveyed to the center of the integrating sphere by a duct and collected a short distance from another duct. A beam of laser light is passed through the space between the two ducts. A particle passing through the measurement area scatters the laser light. Some photodetectors placed on the surface of the integrating sphere provide a signal that is proportional to the size of the particle but is independent of the shape and orientation of the particle with respect to the laser beam.

US patent 7173697 B1 of 2007 describes the realization of a nephelometer with low truncation losses. A nephelometer measures the total scattering signal, which is the diffuse component of an extinction signal. For this purpose it is necessary to integrate the diffused light in all directions. This can be done with an integrating sphere and the cited patent describes an arrangement that reduces losses due to the introduction of the sample and improves the angular response of the instrument.

It is noted that in all the applications described above, the use of the integrating sphere is substantially limited to the diffusion and measurement of light at the same wavelength as the light source.

On the contrary, in the present invention the integrating sphere is used for laser spectroscopy measurements in which the light radiation is at wavelengths different from that of the source. The phenomena that give rise to shifts in wavelength are different and well known in the literature.

Since the integrating sphere is an almost perfect diffuser, according to the invention it can be applied to measurements of phenomena which involve an isotropic diffusion of light and with life times of the order of nanoseconds, which is the characteristic time of multiple light reflections. within the integrating sphere. There are various spectroscopic techniques that can be used with the methodology proposed by the present invention. We remember the particulate incandescence, molecular fluorescence and the "breakdown spectroscopy" that arises following a discharge due to the intense focusing of a laser beam.

Returning to the example of realization of the figures, the choice of the material of the tube 41 depends on the wavelength of the signals to be measured. Pyrex is enough for LII measurements in the visible and near infrared. For UV fluorescence measurements, quartz is required. The flow of gas to be analyzed is made to flow through the small tube 41 (measuring section 40) which, in the case of measurements LII, contains the particulate matter.

Two additional inputs 48a and 48b allow the passage of a light flow of air for the possible cleaning of the windows placed at the ends of the tube in order to allow the laser beam to cross the cell.

In the case of LII measurements, the measurement volume is determined by the diameter of the laser beam and the length of the tube 41 inside the integrating sphere. A typical value of the beam diameter of a commercial laser is about 6-7 mm.

Two closing fittings 44 are connected to the ends of the small tube 41, connected in turn with two respective windows 45 for the laser beam and two respective perforated closing caps 46.

The integrating sphere has the purpose of collecting most of the signal generated by the laser inside the tube and emitted in every direction by the particles. The integrating sphere, in addition to the two circular openings necessary to pass the measuring tube, is equipped with two further circular openings 43a, 49 placed at 90 ° on a plane perpendicular to the axis of the tube. These openings serve one 43a for collecting the light signal (LII or other), and the other 49, normally closed, for the calibration procedure that will be described below (Figure 3).

As regards the laser 20 for excitation of the signals LII, the choice of the laser depends on the type of measurements to be carried out. In the case of LII measurements, a pulsed laser of the Nd: YAG type is generally used which emits in the infrared at a wavelength of 1064 nm.

As highlighted above, it is not necessary to use a high-energy laser as a laser fluence of about 250 mJ / cm <2> is required to reach the saturation of the LII signals. By using higher fluence values, sublimation phenomena occur on a nanosecond time scale. These phenomena are not yet fully known and are still the subject of scientific research. It is also known that there are commercial Nd: YAG lasers that are built with a configuration that can have a near field energy distribution very similar to a uniform distribution ("top hat").

By directly coupling the laser output to the measuring cell and selecting a section of the laser beam with a circular aperture, it is possible to obtain uniform illumination of the particles without the need to use special optical arrangements to create a uniform beam. This is all to the advantage of the simplicity and compactness of the experimental arrangement. However, other types of lasers can be used.

There is still the spectroscopic measurement system 50 according to the invention, as follows. A bundle of optical fibers 51 with a wide collection angle is connected to an outlet 43a of the integrating sphere by means of an optical fiber attachment 43. Most of the light emitted by the gas particles inside the cell, after various reflections on the walls of the integrating sphere, falls on the optical fiber bundle, which directs it to the spectroscopic measurement system 50. Generally it is a question of analyzing the intensity of light signals at various wavelengths. This can be done by means of a spectrograph with a suitable detector. In the case of particulate measurements with the two-color LII technique, the realization of the measurement system can be done in the following way (figure 1).

After the optical fiber 51, the light signal is divided into two optical beams by means of a dichroic mirror 52. Two interference filters select two suitable spectral bands and the light signals are detected with two photomultipliers 53a, 53b. Thus, the two-color incandescence technique ("Two-color LII") is achieved, widely described in the literature for absolute measurements of particulate concentration and average size of nanoparticles by detecting the time decay of LII signals. The spectral zones to be used for measurements must be far from possible spectral interference. The regions around 400 and 700 nm offer a good compromise between the sensitivity of the photomultipliers and the distance from possible interference.

Calibration

To obtain absolute measurements of the volumetric concentration of the particulate it is necessary to calibrate the measurement system.

It can be achieved with a methodology that uses a standard calibrated spectral irradiance lamp. These lamps provide the irradiance, Rcal (λ) [mW / nm cm <2>], which arrives on a surface located at a known distance from the lamp (generally 50 cm) at various wavelengths. The calibration procedure is as follows, with reference to the diagram in figure 3.

The measuring system (composed of the chamber 41, integrating sphere 42 and optical fiber 51) is placed at the calibration distance from the lamp 81 with the calibration aperture 49 of the integrating sphere 42 open so that the light of the calibration lamp 81 illuminates the inside of the integrating sphere. A perforated screen 83 allows the light from the lamp to reach the integrating sphere, preventing spurious reflections and other light sources from influencing the calibration procedure. The diameter of the calibration aperture 49 is known, so the power entering the integrating sphere at various wavelengths can be calculated. The signal measured on a photomultiplier is given by the expression:

where GPMT is the gain of the photomultiplier, Z is the measurement impedance, S is the section of the calibration aperture of the integrating sphere, η (λ) is the reflectivity of the integrating sphere including the tube inserted in it, ΘPMT (λ) is the spectral response of the photomultiplier [A / W], and τ (λ) is the optical transmissivity of the measurement filter before the photomultiplier.

If the passband of the filter is narrow enough to be able to keep the other quantities constant, the photomultiplier signal becomes:

where λ is now the measurement wavelength. The relation allows the calculation of the calibration constant Ca (λ) [V / mW] with the calibration door open:

However, SILIIS signals are measured with the calibration opening closed. The new calibration constant C (λ) is obtained by making two measurements, for example LII one with closed door, LII, and one with open door, LIIa. The correct calibration constant is given by the simple relationship:

where the LII / LII ratio takes into account the loss through the opening itself. This calibration procedure must be repeated for all the wavelengths necessary for the measurements.

Application of the methodology according to the invention to particulate concentration measurements with the LII (SILIIS) technique.

The glow signal emitted by a single soot particle, LIIp [mW / nm], is given by the known relationship:

where h is the Plank constant, c the speed of light, k the Boltzmann constant, λ the measurement wavelength, E (m) is the absorption function dependent on the complex refractive index, m, dpè the diameter of the soot particles and Tsoot is the temperature of the soot nanoparticle. Assuming that all the particles within the measurement volume have the same diameter, the total LII signal emitted inside the measurement cell is calculated as:

where np is the concentration of the particles [# / cm <3>] and V = πΦ <2> L / 4 [cm <3>] is the volume of the pyrex tube inside the integrating sphere. The volume fraction of soot, fv, is given by the relation:

From the measured LII signal, VLII (λ) [mV] at the wavelength λ, and using the calibration constant C (λ) we obtain:

The temperature of the soot can be calculated from the ratio of LII signals at two wavelengths, λ1 and λ2, using the classic formula of two-color pyrometry:

Once the temperature of the particles has been determined, the volume fraction is given by:

The laser fluence to be used for the measurements must be chosen with some further considerations.

Using low fluences (for example up to 100 mJ / cm <2> in the IR) the LII signals can be low and quite noisy, on the other hand high fluences (above 500 mJ / cm <2>) cause sublimation phenomena of nanoparticles with decrease in diameters and loss of material that can invalidate the measurement. The "best practice" suggested by the researchers is to keep to the limit of the linearity zone of the LII signal, in the 150-200 mJ / cm <2> range, or to reach the starting "saturation" zone in the 250-300 mJ / range. cm <2>, limiting the phenomena of sublimation. The final choice of the laser fluence to be used naturally depends on the concentration of the nanoparticles and therefore on the intensity of the LII signals.

In the case of using a high laser fluence, with the possibility of sublimation phenomena, it is necessary to consider the travel speed of the gases inside the measuring cell tube to prevent multiple laser pulses from exciting the same volume of gas. Calling Lt the distance between the inlet and outlet (7a and 7b of figure 2) of the measurement gases and Φthe internal diameter of the pyrex tube, the volume of gas in the measurement section is Vt = πΦt <2> Lt / 4 [cm <3>]. Using a suction pump with a flow rate Qt [cm <3> / s] to ensure that the entire gas section is affected by a single laser pulse, the frequency of the laser pulses must comply with the condition: f <4Qt / πΦt < 2> Lt [Hz]. For example, for a gas flow rate of 1 l / min, a tube diameter of 6 mm and a length Lt of 8 cm, a maximum laser pulse frequency of 7 Hz is obtained.

Up to now, this patent description has been limited to the use of the integrating sphere coupled to the LII technique for concentration measurements of the carbonaceous particulate.

There is nothing to prevent the use of the same LII technique and the proposed equipment also for the measurements of average particle sizes. For this purpose, the scientific community has already developed some theories, quite complex, which take into account various physicochemical phenomena that occur during the interaction of a laser beam with nanometric particles. The equations that describe these phenomena concern the mass and energy balances including the absorption of laser energy, the heat losses due to vaporization, the conduction of heat with the surrounding gases, radiation and internal energy variations of the particles. .

Various calculation programs have been developed by the scientific community. In particular, on the website www.liisim.com there is an interface that allows the user to choose between different general configurations to solve the mass and energy balance equations taking into account the distribution of the particle diameters and their degree of aggregation. In particular, using the experimental data of the time decay curves of the LII signal, it is possible to determine the average diameter of the particles through a "best fit" procedure, also taking into account the temporal response of the experimental apparatus.

It should be noted that an integrating sphere introduces a time constant in the time response to the input photons. This effect is due to the increase in the passage time of the photons reflected inside the sphere that travel through different paths before reaching the light detector. The time decay of the light signal of an integrating sphere is described by an exponential relationship, e <-t / τ>, where the time constant is given by the relationship:

where Ds is the diameter of the integrating sphere, c is the speed of light and ρ is the average reflectivity of the walls of the sphere. Assuming a reflectivity value of 0.98 with a sphere diameter of 5 cm, a time constant τ = 5.5 ns is obtained. Even the photomultipliers, used for the detection of light signals, have a non-negligible time constant and of the order of the nanosecond. The temporal response of the entire detection system is therefore given by the convolution of the temporal responses of the integrating sphere and of the photomultiplier.

An experimental measurement of the time constant of the entire system can be made by injecting a low-power pulsed laser beam into the measuring cell (to avoid incandescence phenomena) and measuring the temporal form of the scattering light pulses. The time constant of the LII signal measurement system is then used for the analysis of the signals according to the procedure described above.

NOTES ON THE SILS TECHNIQUE FOR FLUORESCENCE MEASUREMENTS

(SILIFS) AND BREAKDOWN SPECTROSCOPY (SILIBS)

In particular, the SILS technique can be applied to laser induced fluorescence measurements (SILIFS) with particular regard to the determination of substances subject to broadband fluorescence such as polycyclic aromatic hydrocarbons (PAHs). The applicability of the Italian industrial invention patent no. application MI97A000240 filed on 07.02.1997 with inventors F. Cignoli, S. Benecchi and G. Zizak, two of whom are also inventors of the present patent.

Through the supply means 10, 11 the gas containing the substances to be detected is conveyed into the measuring chamber 41 according to the previously described scheme.

A suitable laser 20, for example a fourth harmonic Nd: YAG emitting UV radiation at 266 nm, passes through the measurement chamber, as previously described. Substances, such as PAHs, present in the measuring chamber absorb UV radiation and emit broadband fluorescence in the UV-VIS range.

The fluorescence spectrum depends on the composition and concentration of the chemical species present in the measurement volume. Through optical fiber 51 the fluorescence signal is conveyed to the measurement equipment as in the previously described scheme.

In this case, since these are structured broadband spectra, it will be advisable to use a spectrograph with a CCD intensified detector, instead of the two-color system. The procedure for the acquisition, the analysis of the spectra and the numerical deconvolution for the recognition of the gas composition has already been described in the 1997 patent mentioned above.

Another application of the SILS technique may concern the application of the well-known technique of laser-induced breakdown spectroscopy or "laserinduced breakdown spectroscopy" for the analysis of the atomic composition of aerosols (SILIBS). This technique uses a laser, typically a pulsed Nd: YAG in the IR, focused in order to generate a discharge that breaks the molecules contained in the aerosols that pass in coincidence with the laser pulse. Given the strong electric field, the atoms are excited and emit a light radiation that allows their identification. The spectral analysis of the radiation therefore provides the atomic composition of the aerosols. The technique is particularly useful in the detection of contaminants such as heavy metals.

To create the SILIBS apparatus, it is necessary to replace the entrance window 45 in the measuring cell 41 with a suitable lens that allows the beam to be focused at the center of the integrating sphere. The sampling system 10, 11, the laser 20 are identical to that described while the measurement system 50 can be realized as for the SILIFS technique, with a spectrograph and intensified CCD camera.

In the foregoing, the preferred embodiments have been described and variants of the present invention have been suggested, but it is to be understood that those skilled in the art will be able to make modifications and changes without thereby departing from the relative scope of protection, as defined by the claims attached.

Claims (11)

CLAIMS 1. Apparatus (100) for measuring gas isotropic radiation by laser spectroscopy, comprising: - gas supply means (10,11) in - at least one measuring chamber (41); - means (30,31) for evacuating the gas from the measuring cell; - at least one laser source (20) which generates a laser beam (21) which passes through said measurement chamber (41); and characterized by the fact of further understanding: - an integrating sphere (42) through which said measuring chamber (41) is made to pass; - an optical fiber (51) connected (43) on one side to an opening (43a) of said integrating sphere (42) to collect a light measurement signal, and on the other to - a system (50, 60, 70) for analyzing the measurement light signal. 2. Apparatus according to claim 1, characterized in that said gas supply means (10) comprise a sampling probe (10) and a duct (11). 3. Apparatus according to claim 1 or 2, characterized in that said analysis system (50,60,70) of the measurement light signal comprises: - means (52) for the separation of the same signal into two optical beams belonging to two appropriate spectral bands as well - detection means (53a, 53b) of said two optical beams, to implement the two-color incandescent measurement technique. 4. Apparatus according to any one of claims 1 to 3, characterized in that said measuring chamber (41) consists of a transparent pyrex or quartz tube, passing diametrically through the integrating sphere. 5. Apparatus according to any one of claims 1 to 4, when not dependent on claim 3, characterized in that, for laser-induced fluorescence measurements, with particular regard to the determination of substances subject to broadband fluorescence such as polycyclic hydrocarbons aromatics, said system (50, 60, 70) for analyzing the measurement light signal comprises a spectrograph with a CCD intensified detector. 6. Apparatus according to claim 5, for the measurement of isotropic radiation from gas by means of laser-induced breakdown spectroscopy, in which, before the entrance to the measurement chamber (41), a suitable lens is placed which allows the beam to be focused at the center of the integrating sphere (42). 7. Method for measuring isotropic light radiation obtained by laser spectroscopy, characterized by carrying out the following steps: A. sending a gas to be analyzed into a measuring chamber (41); B. sending a laser beam (21) into said measuring chamber (41); C. collecting the light scattered by said gas hit by said laser beam (21), by means of an integrating sphere (42); D. transforming the luminous measurement signal integrated by said integrating sphere (42) into an electrical signal by means of suitable photodetectors; E. analyze said electrical signal to obtain the desired measurement. Method according to claim 7, characterized in that step E implements the two-color incandescent measurement technique. Method according to claim 7 or 8, characterized in that a calibration step A0 is performed before step A, in which the following steps are performed: A0_1. place a calibrated spectral irradiance lamp (81) at a calibration distance (83) from a suitable calibration port (49) of the integrating sphere (42), which can assume an open configuration and a closed configuration, so that the light of the calibration lamp (81) illuminates the interior of the integrating sphere (42) when the calibration door is open; A0_2. measuring the power of the output signal from the integrating sphere through the arrangement of phase A0_1; A0_3. Calculate the calibration constant where λ is the wavelength of the signal outgoing from a photodetector, Vcal (λ) is the signal measured on the same photodetector, S is the section of the calibration gate of the integrating sphere, Rcal (λ) the irradiance of the calibration lamp to the calibration distance. A0_4. measuring the power of the output signal from the integrating sphere with the arrangement of phase A0_1 in which, however, the calibration gate (81) is closed; A0_5. Calculate the correct calibration constant: where LII is the measurement value with the calibration door open and LIIa is the measurement value with the calibration door closed. Method according to any one of claims 7 to 9, characterized in that in phase E the temporal decay of the light signal of the integrating sphere (42) is taken into account, which has an e-t / τ type trend, where: where Ds is the diameter of the integrating sphere (42), c is the speed of light and ρ is the average reflectivity of the walls of the integrating sphere (42). Method according to claim 10, characterized in that in phase D the time constant of the photodetectors is also taken into account, the time response of the entire system comprising the integrating sphere and the photodetectors being given by the convolution of the time responses of the integrating sphere ( 42) and photodetectors.