Vacuum membrane degassing

Commonly used reversed-phase LC solvents, including water, acetonitrile, and methanol, are ideal for LC/MS. All reversed-phase solvents need to be degassed prior to LC/MS analysis to maintain the stability of ion signals. This can be achieved by sonihcation, helium sparging, or vacuum membrane degassing. When solvents of high aqueous content are to be used, the source and probe temperatures should be raised to assist desolvation in the ion source. Normal-phase solvents such as dichloromethane, hexane, toluene, and other hydrocarbons are not suitable for ESI-MS because a polar mobile phase for ionization is needed in ESI. These normal-phase solvents and their typical solutes are sufficiently volatile to be analyzed by APCI and work well with APCI-MS. 

Figure 2.6 schematically illustrates sections of a typical semiconductor ultrapure water (UPW) production process in a semiconductor plant. The water circuit consists of two main sections (1) makeup (or central) system and (2) polishing loop, which provides water at the point of use. There are multiple locations in such a water process where membrane degassing could be needed as shown in the figure. Reverse osmosis is mostly used in makeup line as the primary purification means in such processes. In the past, large and inflexible vacuum towers were frequently used after RO to remove dissolved gases, such as O2, N2, and CO2. Membrane contactors are the norm today for replacement or supplement to vacuum towers in makeup lines, as shown in Figure 2.6. 

Three methods are commonly used to degas HPLC solvents applying a vacuum, helium sparging, or using an in-line membrane degassing device. 

Membrane permeable to gases broken in the vacuum-based degasser Large flow and pressure variations significant leaks in the online degasser Replace degassing chamber... 

Oxidant Removal The presence of oxidizers such as chlorine or ozone can degrade polyamide RO membranes, causing a drop in salt retention. Cellulosic membranes are less sensitive to attack. Addition of 1.5 to 6 mg sodium bisulfite/ppm chlorine or contacting with activated carbon will remove oxidizers. Vacuum degassing with a hydrophobic filter module is also used. 

Mobile-phase degassing is an important step in the LC/MS experiment and can be accomplished via on-line membrane or vacuum devices, sonication, helium sparging or as part of the mobile-phase filtration step. Degassing will eliminate pump cavitation, ensure reproducible retention times and minimize possible sputtering from the ion source. 

In one embodiment patented by Makrides et al., tantalum foils were electrolyti-cally etched in hydrofluoric acid, washed with acetone and, while still wet with acetone, placed in vacuum and dried by evacuation. Using an argon plasma at a pressure of about 1 mm Hg (133.3 Pa), palladium was deposited onto both sides of the membranes to recommended thicknesses between 10 and 100 nm. Membranes of niobium and vanadium were prepared in the same manner, except that, in addition, vanadium was degassed in vacuum at 1273 K (1000 °C) to remove oxygen. Unalloyed palladium as well as Pd-Ag, Pd-Au and Pd-B, were also patented as hydrogen dissociation catalysts and as protective layers for the highly reactive niobium, tantalum and vanadium. 

The PSF membrane (17.35 cm ) was weighed precisely and irradiated at the same conditions as in the peroxide-determination experiments. 20 wt. % AAc aqueous solution was degassed by nitrogen blowing for 60 min. UV-treated PSF was quickly dipped into the solution at 60 °C for 2 h in a shaking water bath. The grafted membranes were washed with deionized water at 70 °C for 24 h and dried in a vacuum oven at 40 °C for 48 h. Details on analytical equipment and methods and the solute permeation experiments can be found in the references. - ... 

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