Supercritical fluids as spectroscopic solvents for thermooptical absorption measurements

Hundreds-fold Sensitivity Enhancement of Photothermal Microscopy in Near-Critical Xenon

“…The enhancement is the same within error for both 5 and 20 nm and is similar to the enhancements measured for CO 2 and Xe in photothermal deflection spectroscopy. 20,21 Notably, 5 nm Au NPs can be detected with SNR of 9.4 at only 1.8 μW of heating power, which is the lowest of any reported in the literature. The operating conditions for similar SNR of 5 nm Au in Xe uses 117 times less heating power and 20 times less probe power than our measurements with glycerol.…”

Photothermal Deflection Measurements of the Extinction Coefficient k

“…The signal turns out to be proportional to this term, so that the coefficient of thermal expansion plays a crucial role in the signal level: the sensitivity of the mirage approach is due to the thermal expansion in the gas above the heated surface, the use of sensing fluids in collinear measurements on solids is due to the 100x larger thermal expansion in liquids compared to solids, and the enhancement to the signal provided by supercritical fluids arises because the thermal-expansion coefficient diverges with proximity to the criti-cal point [4,6]. The coefficient of thermal expansion of liquids is /3p (liquid) ~ 10 -3 (K-l), while for fused silica, /3p has the very low value 1.6 • 10 -6 (K-l).…”

“…The deflections are small enough that we can assume that a beam travelling in some direction z will always be deflected by gradients perpendicular to z, On~Or, according to the simple expression dO -1 On dz, (4) n Or where dO is the angular deviation from the z-axis. This can be seen directly by considering the change in the velocity along the wavefront of a planewave travelling in a medium with a gradient in the refractive index along the wavefront [59].…”

“…This creates a hot spot in the material, and a laser probe-beam is deflected by gradients in the refractive index On~Or that arise from thermal expansion around the hot spot. The second feature we would like to emphasize is that photothermal deflection is highly sensitive: The high flux of a focused, laser heating-beam, the high sensitivity of photodiode-based beam-position sensors, and the use of a chopper to modulate the heating beam for lock-in detection combine to allow background levels equivalent to A = 10 -6 routinely [3], and A = 10 -9 more recently [4][5][6]. Such a measure- ment reveals the absorptance A of the material, which is the fraction of the power P that is travelling through the material which is absorbed, 1 and which is thus directly connected to the extinction coefficient k in the medium (see Section IID.…”

Applications of Supercritical Fluids in Industrial Analysis

“…Another direction for absorbance-based detection in the future may be nonconventional absorbance, such as the thermal lens principle, without the path-length dependence of conventional methods (Leach and Harris, 1984). With plain C02 as the mobile phase, the UV detector can be used down to 200 nm.…”