Capillary Column Care and First Aid .1 Ferrule Materials and Fittings

Ferrules for capillary columns are usually fabricated from graphite and Vespel/graphite composites. Graphite ferrules are easy to use and have a higher

FIGURE 3.49 Separation of hydrocarbon gases on Rt-QPLOT column. Column:

30-m×0.32 mm-i.d., Rt-QPLOT; temperature conditions: 40^◦C at 10^◦C/min to 240^◦C (hold 10 min). Det: FID, 18 psi He, 35 cm/s, split injection. (Used with permission of the Restek Corp.)

temperature limit but are softer, more easily deformed and are not recommended for GCMS systems. Vespel/Graphite composite ferrules are harder and thus do not deform as easily and, therefore, are recommended for GCMS systems.

The characteristics of these materials are presented in Table 3.12. An alternate ferrule technology has been emerged with the SilTite metal ferrules. Graphite and graphite/vespel composites are made from different materials to the metal nuts so the connection components expand and contract at different rates as the column oven temperature changes. With a metal ferrule system, since the components are made from the same material, the components expand and contract at the same rate during changes in oven temperatures. SilTite metal ferrules have an inherently higher temperature limit and being metallic, the risk of MS contamination is eliminated. This connection arrangement is described in Figure 3.51.

It is important to select the proper ferrule inner diameter (i.d.) to be com- patible with the outer diameter (o.d.) of the capillary column, or a carrier-gas leak will result after installation. Ferrules having an i.d. of 0.4 mm are recom- mended for 0.25-mm-i.d. columns, 0.5-mm-i.d. ferrules are recommended for

FIGURE 3.50 Separation of refinery gases on Rt-alumina PLOT column. Column:

50 m×0.53 mm-i.d., Rt-QPLOT. Temperature conditions: 5^◦C at 10^◦C/min to 120^◦C (hold 5 min). Det: FID, He 37 cm/s, split injection, FID. (Used with permission of the Restek Corp.)

MS interface

FIGURE 3.51 Illustration of an all-metal SilTite connection (reproduced with permis- sion of SGE International Pty.)

0.32-mm-i.d. columns, and 0.8-mm-i.d. ferrules are recommended for 0.53-mm- i.d. capillary columns.

Outlined below are guidelines for the preparation of a capillary column for installation.

1. Slide the retaining fitting or nut over the end of a new column and then the ferrule, and position them at least 6 in. away from the column end. It will be necessary to cut several inches from the end of the column because ferrule particles may have entered the column and can cause tailing and adverse adsorptive effects.

2. With a scoring tool, gently scribe the surface of the column several inches away from the end. While holding the column on each side of the scoring point, break the end at the scoring point at a slight downward angle. Any loose chips of fused silica or polyimide will fall away and will not enter the column, as may happen if it is broken in a completely horizontally configuration. This procedure eliminates the possibility of chips of fused silica or polyimide from residing in the end of the column. Alternatively, an excellent cut can be made with a ceramic wafer (Figure 3.52).

3. Examine the end of the column with a 10–20×magnifier or an inexpen- sive light microscope. The importance of a properly made cut cannot be overstated. An improperly cut column, as illustrated in Figure 3.53, where a series of problematic scenarios are clearly evident, can generate active sites and may cause peak tailing, peak splitting, or solute adsorption.

FIGURE 3.52 Termination of an end of a fused-silica capillary column with a ceramic wafer.

(a)

(b)

FIGURE 3.53 Scenarios that can occur in cutting an end of a capillary column:

(a) jagged protrusion; (b) jagged inner diameter; (c) particles of fused silica accumulated at inlet; (d) piece of polyimide attached to column end; (e) an acceptable cut (courtesy of Agilent Technologies).

(c )

(d )

FIGURE 3.53 (Continued )

4. A supply of spare ferrules and related tools (Figures 3.54 and 3.55) are convenient to have in the laboratory when removal or changing columns is required. Ferrules, ferrule-handling accessories, toolkits, and other handy gadgets are available from many column manufacturers.

3.11.7.2 Column Installation

Define the injector and detector ends of the column. Align the end to be inserted into the injector with a ruler, and mark the recommended distance of insertion as specified in the instrument manual with typewriter correction fluid. Then slide

(e )

FIGURE 3.53 (Continued )

FIGURE 3.54 Photograph of a capillary-column ferrule kit containing an assortment of ferrules and a pin vise drill with bits for drilling or enlarging the bore of ferrules (reprinted with permission of Supelco, Bellefonte, PA).

the ferrule and nut closer to the point of application of the correction fluid and mount the column cage on the hanger in the column oven. Alleviate any stress, sharp bends, or contact with sharp objects along the ends of the column. Insert the measured end into the injector and tighten the fitting. If the column is to be conditioned, leave the detector end of the column disconnected; otherwise, insert this end into the jet tip of the FID at the specified recommended distance, usually 2 mm down from the top of the jet. In Figure 3.56 a photograph of the

FIGURE 3.55 Photograph of a capillary-column toolkit containing tweezers, needle files, scoring tool, pin vise drill kit, flow calculator, pocket mirror, miniflashlight, labels, septum puller, stainless-steel ruler, and pipe cleaners (reprinted with permission of Supelco, Bellefonte, PA).

(a)

FIGURE 3.56 Illustration of the quick-connect fitting for installation of capillary columns: (a) internal sealing mechanism and (b) sealing mechanism inserted into fitting and locked into place (photographs courtesy of the Quadrex Corp.)

(b)

FIGURE 3.56 (Continued )

quick-connect fitting is shown. This device facilitates column installation in most gas chromatographs without the use of wrenches and extends ferrule lifetime.

3.11.7.3 Column Conditioning

Conditioning of a capillary column removes residual volatiles from the column.

There are three essential rules for conditioning a capillary column, the first two of which also apply to the conditioning of a packed column (Section 3.7.5):

1. Carrier-gas flow must be maintained at all times when the column temper- ature is above ambient temperature, and there should be no gas leaks.

2. Do not exceed the maximum allowable temperature limit of the stationary phase, or permanent damage to the column can result.

3. As opposed to the conditioning of a packed column, overnight conditioning of a capillary column is usually unnecessary. Instead, purge the column with normal carrier-gas flow for 30 min at room temperature, then temperature- program the column oven at 4^◦C/min to a temperature 20^◦C above the anticipated highest temperature at which the column will be subjected with- out exceeding the maximum allowable temperature limit. Usually after a column has been maintained at this elevated temperature for several hours, a steady baseline is obtained and the column is ready for analyses to be con- ducted. Use of high-purity carrier gas, a leak-free chromatographic system, and following the guidance of Chapter 10 will greatly extend the lifetime

of any column, packed or capillary. Other details pertaining to conditioning and column care can be found in Section 3.7.5.

3.11.7.4 Column Bleed

Column bleed is a term used to describe the rise in baseline during a blank temperature programming run and is the inevitable consequence of increasing vapor pressure and thermal degradation of a polymer with an increase in column temperature as well as that due to the accumulation of nonvolatiles in the col- umn, as shown in Figure 3.57. One should expect some degree of bleeding with every column; some phases just generate more bleed than others. Always try to select a stationary phase of high thermal stability. For example, a nonpolar phase bleeds less than a polar phase because the former typically has a higher temper- ature limit and thus is more thermally stable. Moreover, in comparing capillary columns of different dimensions, the level of bleed will increase with increasing amount of stationary phase in the column. Therefore, longer and wider-diameter columns yield more bleed than do shorter or narrower columns. Likewise, col- umn bleed increases with increasing film thickness of the stationary phase and with increasing column length. Reducing film thickness and using a shorter or narrower column will result in less bleeding.

The rate of temperature programming or ramp rate can influence the bleed profile from a column. As the rate of temperature programming increases, column bleed also increases. Finally, the more sensitive element-specific detectors (e.g., an ECD or NPD) will generate a more pronounced bleed profile if the stationary phase contains a heteroatom or functional group (–CN or –F) to which a detector responds in a sensitive fashion.

3.11.7.5 Retention Gap and Guard Columns

A 0.5–5.0 m length of deactivated fused-silica tubing installed between the injec- tor and analytical column is often referred to as a retention gap or guard column (Figure 3.58). The term, retention gap, is used to describe this segment for on- column injection where the condensed solvent resides after injection, but both solvent and solutes are not retained once vaporization occurs via temperature programming. As a guard column, this short length of deactivated tubing pre- serves the lifetime of an analytical column by collecting nonvolatile components and particulate matter in dirty samples that would otherwise accumulate at the inlet of the analytical column. As such, its latter role in capillary GC parallels the function of the guard column in HPLC. A guard column is considered to be a consumable item, requiring replacement from time to time, usually when the detector response of active analytes begins to decrease substantially. It elim- inates the need for the repetitive removal of small sections at the inlet end of an analytical column with the buildup of contamination.

Proper implementation of the connection between the guard and analytical columns is essential for the preservation of the chromatographic integrity of the system. The generation of active sites within the fitting can cause adsorptive losses and peak tailing. Commercially available fittings for this purpose include

1 2

3 5

4 6

8 7

9

1011 12 Low Bleed Column

Higher Column Bleed Level

(a)

(b)

1 2 4 6

7

8 9

10 11 12

3 5

2 4 6 8 10 12 14

retention time (min)

FIGURE 3.57 Comparison of column bleed from (a) a relatively new capillary column with a bleed profile from (b) a frequently used column of identical dimensions and film thickness under the same gas chromatographic conditions.

the metal butt connector of low dead volume, press-tight connectors, and a capil- lary Vu-Union. An illustration of the primary and secondary sealing mechanisms in the capillary Vu-Union is shown in Figure 3.59, where the two column ends are positioned into ferrules located inside a deactivated tapered glass insert. This type of connector combines the benefits of a low dead volume connection with

FIGURE 3.58 Schematic diagram of a guard column/retention gap (illustration courtesy of Walter Jennings, consultant).

the sturdiness of a ferrule seal. Furthermore, the glass window permits visual confirmation of the connection.

3.11.7.6 Column Fatigue and Regeneration

Deterioration in column performance can occur by contamination of the column with the accumulation of nonvolatiles and particulate matter, usually in the injec- tor liner and column inlet. Column contamination is manifested by adsorption and peak tailing of active analytes, excessively high column bleed levels, and changes in the retention characteristics of the column. Rejuvenation of the column can be attempted by several paths. First, remove one or two meters of column from the inlet end. If the column still exhibits poor chromatographic performance, try turning the column around and reconditioning it overnight disconnected from the

(a)

(b)

FIGURE 3.59 Photographs of a (a) disassembled capillary/micro-bore Vu-Union show- ing the primary and secondary sealing mechanisms and (b) assembled (photographs cour- tesy of the Restek Corp.)

FIGURE 3.60 Schematic diagram of a solvent rinse kit (illustration courtesy of Walter Jennings, consultant).

detector. A third approach is solvent rinsing, an extreme measure that should be attempted only with crosslinked or chemically bonded phases. Solvent-rinse kits, such as the one schematically described in Figure 3.60, are commercially available and enable a column to be backrinsed of contamination by slowly intro- ducing 10–30 mL of an appropriate solvent into the detector end of the column by nitrogen gas pressure. The results of this procedure appears in Figure 3.61.

This approach is worthwhile for heavily contaminated columns, but in all cases the recommendations for rinsing outlined by the column manufacturer should be followed.

Fatigued Column After rinsing with 30 mL of n-pentane

(a)

(b)

FIGURE 3.61 Illustration of regeneration of a capillary column by solvent rinsing with n-pentane.