The basic method in common use involves chemically collapsing a filter made of cellulose ester membrane onto a glass microscope slide to provide a clear background aginst which to count the individual fibers with the aid of an optical microscope. Until recently, a method recommended by National Institute of Occupational Safety and Health (NIOSH) for accomplishing the filter clearing is to use a stream of acetone vapor which is produced by heating liquid acetone to a temperature of about 56.degree. C. The stream of acetone vapor used for clearing must be capable of rapidly collapsing the filter structure without distorting the filter or displacing the dust on the filter. Fire hazards associated with the use of acetone vapor are substantial and have restricted general use of this clearing technique without specialized equipment designed to reduce the likelihood of fire.
In the aforementioned procedure a flask of acetone is heated to cause vapor formation by continuous boiling. Ducting a vapor stream in a downward direction allows a sampling of cellulose ester filter to be positioned under the vapor stream to effect the clearing procedure. This technique makes inefficient use of acetone and poses a substantial fire hazard thus limiting its use to a ventilated fume hood.
Based on an improvement of the above method, two acetone vaporizers designed for asbestos air sample preparations have been marketed by BGI Incorporated of Waltham, MA and ETC of Monroeville, PA. These vaporizers are stated to be usable in unventilated areas and one is accompanied by an auxiliary heater that is in a separate housing. The heater has surfaces heated to about 50.degree. C. on which a slide can be placed to complete the clearing process if necessary.
In a Revision #2 of the NIOSH Method #7400 dated Aug. 15, 1987, an aluminum "hot block" technique was recommended. This apparatus is shown in FIG. 1 of the drawings and is described in a article entitled "An Asbestos Sample Filter Clearing Procedure", by Baron, et al.
In this prior art procedure, a portion of the circular filter, usually 90.degree. segment, has been cut and placed on a glass slide. Acetone vapors directed to the filter segment cause the cellulose ester material to clear and become essentially invisible in a period of about 5 seconds. The vapors are directed as a plume rather than as a point source so condensation occurs over the entire surface of the filter, thereby minimizing shrinkage and distortion. As in the earlier NIOSH Method #7400, one or two drops of triacetin are placed on the treated filter wedge. A glass cover slide is put over it and the edges are sealed with lacquer or nail polish to create a permanent mount.
With the FIG. 1 equipment, flash vaporization of acetone is made possible and thus the need no longer exists for continuous boiling of a quantity of acetone in a flask which was an earlier prior practice. A unitary block 10 of aluminum is heated by a suitable means such as heating pads on opposite side walls to a temperature of about 70.degree. C. as detected by a surface thermometer 12. A blind hole 14 having a diameter of about 2.5 cm is drilled from an upper surface 16, which serves as a flash vaporization chamber, and a similar blind hole 18 is drilled from the bottom or lower surface 20 which serves as a condensation chamber 18. A horizontal connecting channel 22 is drilled from a side face 23 to interconnect the holes 14 and 18. Hole 22 is plugged as by a threaded set screw 24 to prevent vapor escape. A recess having walls 26 is machined across the bottom surface 20 to provide a notch to allow a slide 28 to be inserted over the upper surface of an unheated ceramic base 29. The filter segment is intended to be centrally located under the hole 18.
An insulating plug 30 of a synthetic resin material such as Teflon a polytetrafluoroethylene, with a small passageway 32 allows the insertion of the tip of an adjustable micropipette. Acetone liquid at ambient temperature is injected into the flash vaporization chamber 14 directly below plug 30. Acetone vapor is directed along horizontal channel 22 to hole 18 where it is directed downwardly to condense on the filter and slide surfaces. The ducting has no direct path for liquid to reach the filter. The vapor is allowed to expand and diffuse along duct 22 so that the filter is exposed to a uniform plume of acetone vapor.
In operation, the metal block 10 is placed on a smooth surface 29 usually of ceramic where the block 10 remains during the time when a number of filter segments that have been placed on separate glass slides 28 are sequentially inserted under the condensation chamber 18 where the vapor is directed downwardly. Upon removal of each slide, a drop of triacetin is applied to the center of the cleared filter and a glass coverslip is applied. Complete clearing of the filter sometimes requires an additional time which may be reduced to 15 minutes or less if placed on a surface heated to no more than about 50.degree. C.
One problem with using the FIG. 1 device is that the filter material is very light and when placed on a glass slide 28 is influenced by electrostatic forces. Moving the slide 28 along the surface of ceramic base 29 into the slot having walls 26 under the heated block 10 is frequently accompanied by a displacement of the filter from the desired position in the center of the chamber 18. Visual observation of the final position of the filter on slide 28 is not possible.
Another problem is that over prolonged period of operation, latent heat builds up in that portion of ceramic base 29 that underlies block 10 which decreases the rate of condensation of acetone and hence the uniformity of the action of the vapor to clear the cellulose ester filter material.