Irreversible restrictor plugging is often a problem in supercritical fluid extraction (SFE) with linear restrictors. Restrictor plugging occurs when high temperature (200 degrees C) off-line SFE is performed on either the marine sediment (SRM 1941) or the urban dust (SRM 1649) standard reference material (SRM). Microscopic evaluation of the linear restrictors (50 mu m i x 40 cm long fused silica capillary) showed that a solid mass approximately 1 cm to 3 cm from the tip of the restrictor caused the plugging in the case of SRM 1941. Three distinct zones of analyte deposition were observed in the restrictor used during SFE of SRM 1649 - a thin, diffuse, granular him (3 cm to 13 cm from tip), a thick, wavy film (13 cm to 17 cm from tip), and plugs of highly viscous liquid (17 cm to 28 cm from tip). Raman microscopy and X-ray microanalysis confirmed that inorganic sulfur was the major component of the material plugging the restrictor when SRM 1941 was extracted. X-ray microanalysis and direct insertion probe mass spectrometry confirmed that dichlorophen, phthalate esters, hydrocarbon fragments, and other compounds were contained in the material plugging the restrictor when SRM 1649 was extracted. In both cases the plugging did not occur at the tip of the restrictor. Both isenthalpic and isentropic explanations for plugging in SRM 1649 and 1941 are invalid because the zones of plugging are significantly removed from the tip of the restrictor and decompression occurred over a large region of the restrictor. The analytes appeared to have condensed in the linear restrictor either immediately after exiting the oven (SRM 1649) or further down the restrictor (SRM 1941).