Scavenger column

The small particle size of the packing makes 3x3 columns susceptible to plugging problems. Samples and mobile phases should be filtered before use, and the column should be protected with guard and scavenger columns (see Section 5.3.2). 

A microparticulate hplc column is a very efficient filter, and if the mobile phase contains any particulate matter, or acquires it from the pump and/or the injection valve due to wear, it will collect at the top of the column. If this happens, the pressure drop across the column for a given flow will gradually increase, and the column may eventually become completely blocked. To prevent this happening, the mobile phase should always be filtered before use, preferably through a 0.5 p,m porosity filter, and guard and scavenger columns should be used as a matter of routine (see Section 5.3.2). 

Because scavenger columns are located upstream of the injection valve, they do not add to the dispersion of the chromatogram, and their size is not critical in this respect. Guard columns, on the other hand, do cause a slight loss of efficiency, and so need to have a relatively small volume. Reducing the volume, of course, reduces the life of the guard column. 

For use with 25 cm x 4.6 mm analytical columns, guard columns and scavenger columns are often 4.6 mm internal diameter and 3-10 cm in length. They can be packed with microparticulate stationary phases or with porous layer beads. Porous layer beads are cheaper than microparticulates and are easier to pack, but they have lower capacities and will require changing more often. It is usually difficult to know how long a pre-column will last before it requires changing. In routine work, precolumns are usually repacked or replaced to a fixed schedule. 

The useful life of hplc columns can be extended by proper treatment, in particular by the use of guard and scavenger columns. Pretreatment of samples is often necessary, eg very dilute samples may require concentration, or complex samples may need to be cleaned up. Some of the more important techniques are considered. 

A second (scavenger) column is prepared in the same manner as the main column except that Dowex-2 (200-400 mesh, Cl form) is used instead of AG MP-1. In addition the purified Dowex-2 resin is treated by equilibration with catechol (10mg/g resin) in saline at pH 1. The scavenger column is necessary to reduce Os-191 breakthrough to a clinically acceptable level. Both columns are then rinsed with 30 ml of pH 1 saline before use. 

Table IV. Effect of Scavenger Column Size on Yield...

Fig. 3.4. Oxygen scavenger column. The column is packed with Ridox or BTS Catalyst. The heating tape and thermocouple wire are used during regeneration. Water which collects during regeneration can be bled out through the valve at the bottom.

Table 6.9 Results of extraction using metal scavenging columns available from Polymer...

NH2 is readily oxidized, so peroxides (in diethyl ether, dioxane or tetrahydrofuran) should be strictly avoided. Ketones and aldehydes react to give Schiff bases." Amino silicas are less stable towards hydrolysis than other bonded phases and it is recommended to use a pre-column (scavenger column) to saturate the mobile phase. 

Quaternary ammonium salts in an alkaline medium are extremely damaging to silica. A scavenger column placed in front of the sample injector, in which the mobile phase is saturated with silica, is highly recommended in such instances. 

Figure 18.8 Individual stages in the determination of tobramycin. Conditions column 1, 3cm X 4.6mm i.d. with 10(xm cation exchanger column 2, 12.5cm x 4.6mm i.d. with 5ixm RP-18 scavenger column, 2.5cm x 2.2cm i.d. with 37-54iim silica mobile phase 1,1.5 ml min of 10 mM sodium phosphate buffer (pH 5.2) mobile phase 2, 1.5 ml min of 50 mM EDTA (pH 8.8).

Why use fast LC Besides the obvious advantage of faster analyses, there is 50 to 80% less solvent use (and less waste generated). Sensitivity is increased 3 to 5 times because there is less dilution of the analyte peaks with the smaller column volumes as a result of tighter packing. These densely packed columns are more prone to clogging. A scavenger column or filter is placed before the injector, in addition to the guard column placed before the column. 

Figure 3. The hydroxyl radical scaveugiiig activity of uine difiereiit spices all bclongiug to the fkinily Labiatae. The activity was measured by using electron qiin rcsonanee technique, and the indexes were calculated on the basis of top height oni (lata of refertmce (2S). A low value of the index indicates an eCGcient scavenger. Columns with different letters are dgniUcanily different at 5% level.

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