An ultrasonic nebulizer

The invention concerns an ultrasonic nebulizer which constitutes a precise atomiser for analytical samples, in particular destined for applications in analytical chemistry where the precision and accuracy of sample's introduction into the analyzer are the key parameters influencing the results.

The known and used analytical-grade ultrasonic nebulizers comprise a hermetic housing with flowing liquid introduced using an inlet and let out by an outlet and equipped with a membrane and an ultrasonic transducer placed on the opposite side of the membrane. This transducer is connected to the ultrasonic generator or is used as a part of generator's electrical circuit. Ultrasonic wave which is produced by the transducer is propagated in the beam directed towards the membrane. Frequency of the generator usually ranges from 100 kHz to 3.5 MHz. Ultrasonic transducers usually have the form of a circular disc with the thickness ranging from tenths of millimetre up to few millimetres. Transducers transform electrical oscillations into mechanical vibrations delivered to the portion of solution which contains the analyte to be analyzed. A portion of analytical solution through the cavitation effect changes into aerosol and is transported with carrier gas to an analyzer of chemical composition.

Analytical atomisers differ from other atomisers mostly regarding the delivered portion of liquid sample. The usual samples used are very small sometimes less than 1 ml and the required stability of produced aerosol must be very high typically better than 10 μl/min. hi order to fulfill these requirements several means are being applied to focus the energy of ultrasonic wave at the surface of the transducer. But the increase of energy density on the transducer's surface must be performed with care since the high energy

density leads also to higher local energy losses and necessity to provide protection against overheating of the transducer.

Cooling of transducer is often realized by means of forced air flow or using water as liquid cooling agent. Difficult problems arise with protection of piezoelectric plates against corrosive action of analytical samples which comprise solution of acids, salts or alkalis. For this reason one has to use an independent protection membrane which is capable to transfer mechanical vibrations and at the same time is resistant to the chemistry of the sample. The known and used solution of this problem is gluing a glass membrane to the plate of an ultrasonic transducer while the other side is attached to an air cooled radiator. In cases when water is used as a cooling agent one can utilize the fact that water is also a good conductor of ultrasounds, the medium within which ultrasonic wave can propagate almost without losses. The energy of ultrasounds should not cause cavitation in cooling water but should rather be directed toward the chemically resistant membrane onto which one delivers the sample to be atomized. Common feature of nebulizers with intermediate layer of cooling and propagating water is the necessity of focusing the, wave beam on the membrane which is placed against the ultrasonic transducer.

Chemically resistant membrane should be made from a material which is not only chemically pure and inert but also is characterised by good ability to the ultrasonic oscillations. Usually quartz membranes are applied having thickness of one half of ultrasonic wavelength what assures high magnitude of oscillations to occur at the membrane surface. In order to focus the energy into a narrow beam a special construction of ultrasonic transducer is applied in which one of the electrodes plated on the transducer disc has much reduced diameter. This results not only in the desired narrowing of the beam but also is the cause of transducer overload as it has to generate the same power from the reduced area what frequently leads to transducer failures.

The known from commercial catalog model U-5000AT+ ultrasonic nebulizer released by CETAC (www.cetac.com/nebulizers) is equipped with a specialized transducer intensively cooled by means of the radiator bounded to the rear side of that transducer. It is worth stressing that in this model of nebulizer the medium separating analytical sample from the transducer is a thin layer of quartz glass having thickness less than one quarter of ultrasonic wavelength. This design assumed strong load of the transducer what resulted in astonishingly good parameters of nebulization. However, the lifetime of those heavy

loaded specialized and high cost transducers bounded with thin glass membranes is limited to several hundred hours only.

The essence of ultrasonic nebulizer according to the invention is that in the hermetic housing there is placed a sleeve in the form of the cone, one opening of which is directed onto the membrane while the second is directed onto the ultrasonic transducer.

Advantageously the cone sleeve is linearly convergent in the direction of membrane or it has convergence of parabolic, exponential or stepwise character.

Advantageously between external surface of the cone sleeve and side walls of the container there is placed a flow director which locally intensifies the flux of liquid coolant.

Also, it is advantageous that in the bottom of hermetic housing there is placed a typical ultrasonic module energized from a remote power supply and that the module is a regular ultrasonic mist generator or a humidifier. The module is mounted to the housing in a dismountable manner and advantageously there is sealing means provided at joint between the module and the housing.

It is advantageous when there is an optoelectronic light source placed inside the housing illuminating the oscillating membrane through the transparent dielectric barrier. It is advantageous if the hermetic housing has a cylindrical shape and the internal diameter of that housing is smaller than a tripled diameter of ultrasonic transducer while the housing height is four times longer than this diameter.

The plane in which the ultrasonic transducer is situated is parallel or at acute angle against the surface of the membrane.

The essence of ultrasonic nebulizer according to the presented invention is also such that the height of hermetic housing is at least two times bigger than its widht.

In the new nebulizer constructed in accordance with the invention, one uses the principle of self-focusing of ultrasonic beams propagating in liquids. This self-focusing is caused by heating up the channel along which the propagation takes place. Physical background of this phenomenon is in slight change of refraction index of liquids which can be heated up along the propagation path due to the loss in propagating energy and partial conversion of beam energy into heat. Unexpectedly, it turned out that the focusing system created spontaneously is almost identical in performance to the gradient optical fibre.

The experiments performed with the propagation ultrasounds along cylindrical channels filled with water and terminated with a membrane made of quartz glass plate having the thickness equal to a half of the wavelength have confirmed this effect. Moreover, at internal tunnel's diameter less than three diameters of the ultrasonic transducer plate, and having the distance between the transducer and the membrane which not exceeded three diameters of the transducer's plate, focusing was strong enough to cause cavitation of analytical samples on the membrane's surface.

In order to intensify the focusing of ultrasonic beam in the nebulizer made according to the present invention a sleeve in form of a reversed acoustic cone has been introduced to play the role of a heat-acoustic screen in the form of a truncated cone made from solid state material. Thee sleeve of the heat-acoustic screen is directed with its wider end towards the ultrasonic transducer and with the narrower end to the membrane on which the analytical sample is nebulized. The sleeve of the heat-acoustic screen plays also a protective role against the interference connected with convective fluxes within the cooling liquid. Moreover, the sleeve can additionally be cooled to assure that even without the ultrasounds the temperature of the liquid coolant at the internal surface of the sleeve was lower than the temperature at the axis of this system. But the role of a heat-acoustic screen can be also fulfilled by the device housing. Cooling liquid is normally water but it can be replaced with any liquid in which propagation of ultrasounds is feasible. The present invention is illustrated by an realization example where the fig. 1 shows a cross-section of ultrasonic nebulizer, fig 2. shows the nebulizer with flow director of liquid coolant, fig. 3 shows the ultrasonic nebuliser equipped with mist generating module, fig. 4 shows the nebulizer having the height being twice the width, and the fig. 5 shows examples of the cone sleeve design.

Example 1

Ultrasonic nebulizer has a hermetic housing OH filled with flowing liquid which is water let in using an inlet ferrule KI and let out using an outlet ferrule KO. In the top wall of the housing there is placed a membrane M at the counter side of which in the bottom wall of the housing there is placed an ultrasonic transducer PU. The transducer PU is energized from the ultrasonic power generator GEN. Plane at which the ultrasonic transducer PU is situated is parallel to the plane at which the membrane M is. The

ultrasonic wave generated by the transducer PU propagates in the direction of the membrane M through the cone sleeve TS with the first open end directed towards the membrane M while the second end is directed towards the transducer PU. In this case the cone sleeve TS is linearly convergent in direction of the membrane M which at the same time plays the role of a screen for ultrasonic waves separating the region with propagating ultrasounds against interference caused by turbulences occurring in the strong flow of cooling water. Moreover, above the membrane M means are provided allowing the delivery and transformation of sample into an aerosol AE.

Example 2

An ultrasonic nebulizer like in the previous example with such difference that in the bottom part of the hermetic housing OH there is a separable module MOD energized from an external power supply ZAS and equipped with electronic circuit of power generator EL. Between the module MOD and the housing OH sealing means USC are applied, hi this design a commercially available mist generator equipped with ultrasonic transducer PU has been applied as the module MOD. Moreover, between external surface of the cone sleeve TS and side wall of the housing OH a flow director COL was introduced as a mean for intensifying the flow of cooling liquid.

Example 3

Ultrasonic nebulizer built as in Example 2 with such difference that in the lower part of the housing OH there is a commercially available module MOD with ultrasonic transducer PU both taken from an ultrasonic humidifier. The cone sleeve TS has a parabolic convergence in the direction of the membrane M and a plane of the ultrasonic transducer PU is slanted towards the plane of the membrane M by 30 degrees angle.

Example 4

An ultrasonic nebulizer as in examples 1 or 2 with such difference that the cone sleeve TS has an exponential convergence in the direction of the membrane M and in the housing OH there is placed an optoelectronic light source ZS which through the dielectric barrier illuminates interior of the nebulizer and at the same time illuminates membrane M what enables monitoring of the nebulizer's functioning.

Example 5

An ultrasonic nebulizer as in the examples 1 or 2 with such difference that the cone sleeve TS has stepwise convergence towards the membrane M while the housing OH has cylindrical shape and internal diameter of the housing OH is equal to three diameters of the ultrasonic transducer PU and the height L of the housing OH is equal to four diameters of the transducer PU.

Example 6

An ultrasonic nebulizer has a hermetic housing OH filled with flowing liquid which is water delivered with inlet ferrule KI and let out via outlet ferrule KO. hi the top wall of the housing OH there is placed the membrane M and on the opposite side of this membrane . at the bottom wall of the housing OH there is the ultrasonic transducer PU energized from the ultrasonic power generator GEN. Ultrasonic wave excited by the ultrasonic transducer PU propagates in a beam directed towards the membrane M. The height L of the housing OH is twice its width D. Moreover, above the membrane M means are provided allowing the delivery and transformation of sample into an aerosol AE.

List of abreviation used:

OH waterproof housing

KI inlet ferrule

KO outlet ferrule

M membrane

PU ultrasonic transducer

GEN ultrasonic power generator

TS cone shaped sleeve

COL flow director of coolant

MOD mist generator

ZAS power supply

ZS optoelectronic light source

USC sealing

EW exciting electrode

AE aerosol of the sprayed sample

EL electronic circuit of the generator

L housing height

D housing width