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 (CO(2)) 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 (CO(2) 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 CO(2) 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 (PN(p-p)) as low as 0.1 bar, at 200 bar, and 20Hz, compared to older equipment that attempted to maintain PN(p-p)<1bar, at <5Hz. With this lower PN(p-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 0.3m AU. Baseline noise <0.05 is required to validate methods for trace analysis. The new BPR, with a PN(p-p) of 0.1 bar, demonstrated UV peak to peak noise (N(p-p))<0.02 mAU with a >0.03 min (10Hz) electronic filter under some conditions. This new low noise level makes it possible to validate SFC methods for the first time.