Replacement columns

Figure 14.4 Schematic diagram of the cliromatographic system used for the analysis of very low concentrations of sulfur compounds in ethene and propene CP, pressure regulator CF, flow regulator SL, sanrple loop R, restriction to replace column 2 VI, injection valve V2, tliree-way valve to direct the effluent of column 1 to either column 2 or the restriction column 1, non-polar- capillary column column 2, tliick-film capillary column SCD, sulfur chemiluminescence detector FID, flanre-ionization detector.

In Example 21.1, consider introducing a double-effect column to replace Column 2 for the backward heat integration arrangement. Assume an equal split of flowrate into the doubleeffect column. Is there any benefit from the arrangement ... 

Typical corrective measures for poor performance are resilylation, replacing injector glass insert, breaking off a few centimeters at the front end of the column, and replacing column. Serious loss of sensitivity may require cleaning the source, replacing the filaments, or cleaning the quadrupole rods. 

Remove guard column (if present) and attempt analysis. Replace guard column if necessary. If analytical column is obstructed, reverse and flush. If problem persists, column may be clogged with strongly retained contaminants. Use appropriate restoration procedure. If problem still persists, change inlet frit or replace column. 

Column leaking silica or packing material. 8. Replace column. 

Replace column with new one of same type. If new column provides symmetrical peaks, flush old column. 

Open inlet end and fill void or replace column. 

Use fresh sample or standard to confirm sample as source of problem. If some or all peaks are still smaller than expected, replace column. If new column improves analysis, try to restore the old column, following appropriate procedure. If performance does not improve, discard old column. 

J. W. Dolan, A. Maule, D. Bingley, L. Wrisley, C. C. Chan, M. Angod, C. Lunte, R. Krisko, J. M. Winston, B. A. Homeier, D. V. McCalley, and L. R. Snyder, Choosing an equivalent replacement column for a reversed-phase liquid chromatographic assay procedure, J. Chromatogr. A 1057 (2004), 59-74. 

Column voided. —> Repack top of column with stationary phase replace column. 

Column void or channeling —> Replace column, or, if possible, open top endfitting and clean and fill void with glass beads or same column packing repack column. 

Poor column efficiency due to column void or column contaminated/wom out. Repack top of column replace column. 

Column A 100 X 2 40 p,m Bond Elut C2 (Chrompack) (Before use condition with MeCN, water, mobile phase A. Replace column after 40 injections.) B 20 X 4.6 Supelcosil LC18 DB -I- 150 X 4.6 5 j,m Supelcosil LC18 DB... 

Replace column and reactor internals (such as trays/packing, liquid distributers, gas spargers, catalyst retaining and holding screens) with low pressure drop designs... 

Increased retention frequently essential for volatiles film thickness may replace column length... 

Replace column or open Inlet end and fill void (page 16). 

The void volume (Kg) pore volume (F,), elution volume (Fr), and slope of the molecular weight calibration curve (m) should remain constant in a column that does not develop mobile phase channeling or bonded phase degradation. To give results identical to those obtained with a previous column, a replacement column must have identical values of these characteristic constants. Because it is often impossible to obtain identical columns, especially over a long period of time, it is always best to determine these parameters for every new column and to make operational condition adjustments when necessary. 

The selectivity of reversed-phase liquid chromatography (RP-LC) columns is known to vary, even columns with the same ligand (e.g., Cjg). Column selectivity can also vary from batch to batch for columns claimed to be equivalent by the manufacturer. For different reasons, it is sometimes necessary to locate a replacement column for a given assay that will provide the same separation as the previous column. In other cases, as in HPLC method development, a column of very different selectivity may be needed - in order to separate peaks that overlap on the original column. For each of these situations, means for measuring and comparing column selectivity are required. Until recently, no such characterization of column selectivity was able to guarantee that two different columns can provide equivalent separation for any sample or separation conditions. 

Routine RP LC assay procedures are often carried out over periods of months or years, as well as in different laboratories and different parts of the world. During the application of such a procedure over time, many columns may be required. For various reasons, it may prove difficult or impossible to obtain a replacement column with sufficiently similar selectivity from the original source. In such cases, it is necessary to locate an equivalent replacement column from a different source - or a column of different part number from the same source. During the... 

The sample of Fig. 2 was chosen from the test solutes used to measure values ofH, S, etc. according to Eq. (3). A better test of Eq. (3) and values ofH, S, etc. for measuring column selectivity has recently been reported [18]. Twelve different routine RP LC separations were selected from three different pharmaceutical laboratories, whereupon (successful) attempts were made to select equivalent replacement columns for each separation on the basis of Eq. (3). One of these separations is illustrated in Fig. 3, where the original separation on an ACE Cg column (a) is compared with that on three other columns (b-d) with 1.3 < F < 248. The separation with the Discovery Cg column (b) is seen to be reasonably similar to that in (a), as expected from its value of Fg = 1.3. However, separation on the... 

Fig. 3. Comparative separations of a pharmaceutical sample on an original column (a) and three possible replacement columns (b-d). The sample contained strong bases and carboxylic acids. Columns (15 x 0.46 cm, 5 gm particles) identified in the figure. Conditions gradient separation with solvents...

Here, and Xq (with values between 0 and 1) represent possible correction factors that depend on sample composition. For example, if bases are absent from the sample, the term Xq 0, because values of C mainly affect the retention of ionized basic solutes. For similar reasons, if carboxylic acids are absent from the sample, x = 0. Note that if and Xq are assumed to be equal to one (equivalent to setting Fj = Fj), maximum values of F result, with a decrease in the nttmber of possible replacement columns with F < 3. When a basic compound is partly ionized, there is a reduced contribution of C to the separation. As a rough rule, weak bases such as anilines or pyridines have 01 for a mobile phase with pH < 6 and 0 for pH > 6. Similarly, strong bases (aminoalkyl derivatives) have = 0.1 for pH > 7 and Xq 1 for pH < 6. See Ref [18] for details. 

However, the ( Sphere H80 column with F = 10.1 gives a poor separation (complete overlap of the last two bands). In most of the other ten separations reported in [18], one or more successful replacement columns could be identified in the same way as in Figs. 3 and 4. Where attempts at replacing the original column were unsuccessful, this could be anticipated from the values of Ff of the different columns compared with that of the original column. 

---