The Apex Instruments GFA-2590 Thimble Holder is constructed completely from borosilicate glass for use with Conditional Method for Ammonia (CTM-027) and accepts tapered 25 x 90mm thimbles, specifically manufactured for stack sampling. Does not require O-Rings or grease, for testing at higher temperatures. The borosilicate glass withstands temperatures up to 480°C (900°F).