Minimizing UV noise in supercritical fluid chromatography. I. Improving back pressure regulator pressure noise

Pressure fluctuations and resulting refractive index changes, induced by the back pressure regulator (BPR) can be a significant source of UV detector noise in supercritical fluid chromatography (SFC). The refractive index (RI) of pure carbon dioxide (CO2) changes ≈0.2%/bar at the most commonly used conditions in supercritical fluid chromatography (SFC) (40 °C and 100 bar), compared to 0.0045%/bar for water (CO2 IS 44× worse). Changes in RI cause changes in the focal length of the detector cell which results in changes in UV intensity entering the detector. The change in RI (ΔRI/bar) of CO2 decreases 8-fold at 200 bar, compared to 100 bar. A new back pressure regulator (BPR) design representing an order of magnitude improvement in the state of the art is shown to produce peak to peak pressure noise (PNp–p) as low as 0.1 bar, at 200 bar, and 20 Hz, compared to older equipment that attempted to maintain PNp–p < 1 bar, at <5 Hz. With this lower PNp–p, changes in baseline UV offsets could be measured as a function of very small changes in pressure. A pressure change of ±1 bar at 100 bar, common with some older BPR's, produced a UV baseline offset >0.5 mAU. A pressure change of ±0.5 bar representing the previous state-of-the-art, resulted in a UV offset of 0.3 mAU. Baseline noise <0.05 is required to validate methods for trace analysis. The new BPR, with a PNp–p of 0.1 bar, demonstrated UV peak to peak noise (Np–p) < 0.02 mAU with a >0.03 min (10 Hz) electronic filter under some conditions. This new low noise level makes it possible to validate SFC methods for the first time.