Method and Apparatus for Detection of Extremely Small Particulate Matter and Vapors

This invention relates to the detection by surface ionization of particulate matter and more particularly to such detection in an instrument which combines dc current measurement with counting of particulates having distinguishing characteristics such as size.

U.S. Pat. Nos. 3,808,433 and 3,973,121 disclose apparatus for detection of very small particulate matter and macromolecules of a type which is referred to hereinafter as a surface ionization monitor for particulates.

A particulate introduced into a chamber strikes a heated surface located within the chamber and decomposes, giving to the surface its various constituents and impurities. Those constituents and impurities having ionization potentials (or electron affinities) comparable to the work function of the hot surface become surface ionized and a burst of positive or negative ions is evolved from the surface. Detection of the electrical charge of such ions registers the arrival of the particulate at the hot surface.

U.S. Pat. No. 3,808,433 is addressed primarily to the pulse counting circuitry for the detection of the individual bursts of ions and the manner in which the electrical pulse heights are related to the size of each individual particulate detected. However, with a large number of countable particulates, the instrument can become overloaded. Thus, a need exists of extend the dynamic range of surface ionization monitors for particulates to handle count rates in excess of what might be sufficient to overload pulse counting circuitry. Additionally, the simultaneous use of pulse counting of particulates with measurement of the total dc ion current, in ranges where particulate concentration do not overload the counting circuitry, permits obtaining of information about general particle size in an aerosol.

The instant invention is directed to the simultaneous registering of particle count rate and total dc ion current in a surface ionization monitor for particulates. It meets the problem of an overload in the pulse count rate by registering the charge carried by many pulses in sequence and measuring the total dc ion current produced. If the pulses produced are too small to be distinguished above the electrical circuit noise, the dc ion current still registers, thus indicating that the aerosol is composed of only very small particulates or vapors or both. By adjusting the pulse height discriminator in the pulse counting circuitry to a predetermined value, the presence of a dc current and the absence of pulsed signals indicates that the particulate size in the aerosol is less than that associated with the predetermined pulse discriminator level which has been set. Finally, a low count rate combined with a high dc ion current indicates the presence of an aerosol having large particulate sizes, particularly where the count rate is relatively independent of the pulse height discriminator level when set at low level values.

Other adaptations and capabilities of the invention will be appreciated by those skilled in the art from the following description with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic isometric representation of an embodiment of the invention is partial section wherein the particulates strike a hot surface to generate bursts of fragment ions directly collected by an electrode to produce discrete and continuous measureable signals;

FIG. 1A is a diagrammatic side view sectional representation of an embodiment similar to FIG. 1 which illustrates the addition of a fan and/or a vacuum pump;

FIG. 2 is a representation similar to FIG. 1 of an embodiment including a electron multiplier to collect the bursts of fragment ions and means to dilute the air sample;

FIG. 2A is a diagrammatic side view sectional representation of an embodiment similar to that shown in FIG. 2 which includes, in addition, a charge to mass analyzer between the hot surface and the electron multiplier;

FIG. 3 is a circuit diagram disclosing circuitry which may be substituted for that shown in FIGS. 1 and 1A; and

FIG. 4 is a diagram showing a night time outdoor air sampling of particulate matter by the invention.

FIG. 1 depicts a preferred embodiment of the invention. The detector devices are located within a chamber 8 which is continuously evacuated by a suitable vacuum producing mechanism (not shown) to an operating pressure of slightly less that atmospheric pressure to the high vacuum region (better than 10.sup.-4), as desired.

Particulate matter 10 is received in the container 8 through a small orifice 9 and passed in a stream to strike a surface 15 shown in the form of ribbon which is mounted on electrically insulating supports 12 and is heated by passage therethrough of an electrical current provided by a voltage source 17. As a result of the particulates 10 impinging upon the surface 15, fragment ions 20 are produced which are emitted from the hot surface 15 and drawn to a receiving electrode 21 which is mounted on an insulated support 19 via a metal rod 22 adequate to provide the necessary mechanical strength and rigidity.

The pulses of ions arriving at the electrode 21 produce pulses of currents which pass to a high gain amplifier 26 which in its feedback loop has in parallel a resistor R.sub.1 and condensor C.sub.1 which are connected in series to a resistor R.sub.2. The R.sub.1 C.sub.1 circuit integrates many individual pulses and produces a dc voltage offset at point 28 which is measured by a voltmeter 25. Superimposed on this dc voltage is a voltage generated by the voltage drop over the resistor R.sub.2 which closely follows the rapidly changing current curves by the individual ion pulses. This rapidly varying voltage pulse is transmitted through the condensor 27 to further stages of pulse amplification, pulse height discriminator 30 and a scaler count rate meter 31 or other appropriate known types of registering devices to record the arrival of particle 10 thus providing the desired pulse count information.

By placing a diode D.sub.1 in parallel with R.sub.1 and C.sub.1 by closing switch 23, the dc output becomes linear with the ion current at low ion currents. However, as the ion current increases, the response becomes logarithmic, followed again by a linear response, dependent on the value of R.sub.2.

As shown in FIG. 3, by providing a diode D.sub.2 instead of resistor R.sub.1 and condensor C.sub.2, the direct current output is a logarithm of the total direct total output which is a desirable feature for measuring total currents when changes of many orders of magnitude occur.

FIG. 1A is a diagrammatic side view of a detector similar to that shown in FIG. 1 with the same reference numerals applied to identical components. Chamber 48 is similar to chamber 8 of FIG. 1 and includes on one side a small orifice 9 for admitting particulate matter 10. It will be noted that chamber 48 includes a plurality of openings 50 which connect into a circular conduit 51 that, in turn, leads to a vacuum pump 52 via a passage tube 54. Also, a blower mechanism 55 which includes a casing 56 may be removably secured to chamber 48 to surround the inlet of orifice 9. This mechanism includes a screen 57 which is removably connected to the entrance of casing 56 and has attached thereto a fan 60 energized by a voltage source 61. Fan 60 is supported from the frame portion of screen 57 through a plurality of struts 62. The function of the fan 60 is to displace air from the outside into chamber 48. Screen 57 prevents debris such as leaves, twigs and the like and also insects from entering or clogging orifice 9. Normally, vacuum pump 52 and blower mechanism 55 are not utilized in the same operation. With blower mechanism 55, chamber 48 operates at substantially atmospheric pressure whereas vacuum pump 52 is capable of reducing the pressure within chamber 48 to a high vacuum as desired. A heated surface 15a is supported by a rod 12a composed of electrically insulating material and it is heated by a voltage source 17 via a connecting electrical circuit as shown in FIG. 1. Particulate matter impacting on heated surface 15a causes the emission of a burst of ions which are received by electrode 21a and the resulting electrical pulse is recorded as noted previously. Electrode 21a is supported by an electrical insulated support 19a affixed to the outer wall of chamber 48. A cap 48c, to which support 12a is affixed, provides means for gaining access to the interior of chamber 48 for the repair or replacement of parts or the like. A similar closure may be included in chamber 8.

The apparatus illustrated in FIG. 1A operates in essentially the same manner as the FIG. 1 detector. The primary distinction is, with use of blower mechanism 55, the apparatus operates at atmospheric pressure within chamber 48 without the necessity of employing a vacuum pump 52.

FIG. 2 illustrates an arrangement somewhat similar to FIG. 1 except that the container 8 is provided with two chambers 8a and 8b, each of which has an orifice 9a and 9b respectively. Vacuum producing means is connected to the chamber 8b to produce a high vacuum region within chamber 8 (better than 10.sup.-4 mm of mercury). A clean gas source 32 is connected via a conduit 34 including a metering valve 35 to the chamber 8a of container 8. The purpose of this gas is to dilute the gas containing the aerosol in situations where the count rate is too great.

In the apparatus shown in FIG. 2, to increase the intensity of the electrical pulse signals, the fragment ions 20 are drawn to a first dynode 43 of an electron multiplier structure 40 by an appropriate potential placed on the first dynode by a voltage source 41. The electrons collected at the output of the electron multiplier 40 with switch 36 closed and switch 37 open appear as a pulse of electrons and are then as previously described passed through a pulse height discriminator 30 to a count rate meter 31 or other appropriate device to register the arrival of the particulates 10. However, with the switch 36 open and switch 37 closed, pulses of ions which arrive from the electron multiplier 40 pass through an ammeter or other current measuring device 44 to ground or other circuit common. The inertia of the meter integrates many pulses over time and thus reads out a dc current. Such a current measuring device may be an electron ammeter with stages of gain in it. With both switches 36 and 37 closed, the simultaneous measure of pulses and current is provided.

An embodiment similar to FIG. 2 is illustrated in FIG. 2A. This embodiment has, however, a charge-to-mass analyzer 64 provided for separating the fragment ions 20 after leaving surface 15 and before receipt at the electron multiplier 40. Preferably this charge-to-mass analyzer is a quadrupole mass filter. Alternatively, however, other known devices utilized for the purpose of mass spectrometry may be substituted. Its purpose is to determine and analyze the signals produced from specific fragment ions 20. By this means, constituents and nature of particulate matter 10 may be revealed. Analyzer 64 includes the quadrupoles 65 electrically connected to a signal source and control 66 which determines the charge-to-mass filtration ratio and includes a scan mechansim and other controls well known to the art. As shown in FIG. 2A, the chamber designated generally 68 is provided with a circular conduit skirt 51 which, together with passage tube 54 connects vacuum pump 52 to the interior of chamber 68 and more specifically to the initial confined space designated 68a which contains a small orifice 9c. Vacuum pump 52 is such that it may provide an atmospheric pressure within space 68a from only slightly less than the ambient surrounding pressure to a high vacuum better than 10.sup.-4 torr. By means of a further circular conduit 51a and a further tube passageway 54a, a plurality of openings 50a within space 68b are connected to a further vacuum pump 52a. The latter pump 52a produces and maintains a vacuum in analyzer 64 and electron multiplier 40 at a proper operating pressure for these components, such vacuum being 10.sup.-5 torr or better. A second pump 52b is provided further to evacuate the analyzer 64 via a tube passageway 54b.

A screen 67 is affixed over the entrance of the small orifice 9c which serves essentially the same purpose as screen 57 in FIG. 1A. Particles 10 pass through orifices 9c and 9d to impinge on surface 15 whereby bursts of ions 20 are received and separated by analyzer 64, the filtered ions being registered as described with reference to FIG. 2.

In the apparatus shown in both FIGS. 1A, and 3, measurement of the total material in an aerosol without regard to particulate size or size distribution may be made or such measurements may be made simultaneously. However, it is to be appreciated that the pulse time-length in surface ionization particulate detectors is typically on the order of 10 micro seconds. Thus, when the count rate from an aerosol exceeds about 10.sup.4 pulses per second, individual pulses begin to overlap and accuracy is lost. But with apparatus in accordance with the invention, even though the count rates exceed this figure for very dense aerosols, such as in smoking environments, the dc current continues to record the aerosol density. Further, calibration of the dc current relative to the count rate may be readily accomplished by diluting the gas containing the aerosol with a clean gas as from source 32 whereby the aerosol is diluted to a point where counts are accurately registered and, at the same time, the corresponding dc current may be noted. By this procedure, the dynamic range of the surface ionization particulate detector is extended several orders of magnitude.

By using pulse height discrimination in a pulse registration circuitry, only particulates having a size above a specified value are recorded as counts. The dc current, on the other hand, records contributions not only from such particles but also from particles smaller than those of sufficient size to provide a count. By then counting the dc current and the rate of pulse counts, one can discern whether a given aerosol consists of larger or smaller particles.

FIG. 4 illustrates a use of the invention in recording the air in Pittsburgh, Pennsylvania, during an early morning in February. The pulse height discriminator level was set to register particulates with a size in excess of about 0.4 microns in diameter. It will be noted that a large excursion occurred about 3:30 a.m. in the count rate. This is probably due to the clearing of smoke stacks in the area. Although the count rate increased dramatically due to the presence of particulates with diameters greater than 0.4 microns, the dc current level increased only slightly, indicating that the aerosol during this period consisted primarily of larger particulates. Later, a second excursion commencing at about 7 a.m., probably due to traffic, shows both the count rate and the dc current increasing markedly. The relative increase in the dc current level as compared to the comparable increases in the count rate level indicates a second aerosol to be substantially richer in particulates of sizes less than 0.4 microns than the first aerosol.

Although we have described the preferred embodiments of our invention, it is to be understood that it is capable of other adaptations and modifications and that other circuitry to accomplish the processing and registration of both dc and pulse information will be readily apparent to those skilled in the electronic circuit arts. Thus, the appended claims should be construed to cover not only corresponding material as described in the specification but also the equivalents thereof.