Degassing, solvents helium sparging

Figure 5.2 A, valve, column and flow cell asseidily of a ainiaturized liquid chromatograph for use with small bore columns (Reproduced with permission from ref. 14) and B, mobile phase reservoir designed for solvent degassing by heat and helium sparging (Reproduced with permission from ref. 34. Copyright Elsevier Scientific Publishing Co.)...

Helium sparging is a method for degassing a solvent consisting of a vigorous bubbling of helium gas through the solvent. 

All solvents should be fully degassed by vacuum, helium sparging, or sonication prior to use. 

Helium Sparging—A solvent degassing technique in which helium gas is bubbled through solvents to displace dissolved gases before solvent mixing, compression, and pumping. 

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. 

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

This instrument uses a helium sparging device with four separately controlled solvent lines, ensuring optimal solvent degassing and improving check valve reliability at low flow rates, diminishing baseline disturbances at low ultraviolet (UV) wavelengths. 

Three reservoirs are shown. The silver disks are sparging lines (streaming argon or helium into the solvent to produce a continuously degassed solvent). The white cylinders are the inlet filters, in this case made from glass. 

Air is soluble in many solvents, including water, and it should be removed prior to use because it tends to be released at the low pressure end of the apparatus, the detector, where it causes noise or even a complete loss of signal. Many commercial instruments include the degassing capability in the solvent reservoir for convenience. Some possibilities are the following, used individually or in combination vacuum, heating, son-ication, and sparging with helium. The latter is probably the easiest to include in an instrument, but the most efficient is the combination of sonication under vacuum. 

Continuous helium degassing is extremely useful when performing low- or high-pressure solvent proportioning. However, like vacuum degassing, care must be exercised when sparging at high helium flow rates to avoid selective evaporation of the more volatile components of a mixture. 

HPLC solvents To make 1 liter of solvent A, add 20 ml of acetonitrile to 980 ml of high-purity water, add 1 ml of trifluoroacetic acid, and stir. To make 1 liter of solvent B, add 900 ml of acetonitrile to 100 ml of high-purity water, add 800 fA trifluoroacetic acid, and stir. Solvents should be degassed by sparging with helium or by sonication under vacuum. 

The mobile phase reservoirs are generally screw-top plastic-coated glass bottles with inlet lines to allow for degassing of the solvents for 10-15 min either by vacuum or sparging with helium or both. A positive pressure of helium should be maintained in the solvent bottles. Degassing is important to prevent gas bubbles from lodging in the pump, column, or the detector. On-line degassing... 

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