COLUMN SELECTION
The fi ve critical parameters for capillary columns are: (1) internal diameter;
(2) column length; (3) fi lm thickness; (4) stationary phase composition; and (5) fl ow rate. Each will be discussed briefl y.
Internal Column Diameter, i.d.
Internal column diameters for fused silica range from 100 to 530 μ m (0.10 – 0.53 mm). Some glass capillaries have even larger internal diameters.
One - hundred - micrometer columns, row one of Table 6.2 , have limited sample capacity and are not well suited for trace analysis. Ease of operation is also limited because of the very limited sample capacity. These small - i.d. columns have very good effi ciency and produce fast analyses, but special sampling techniques and high - speed data systems are required to realize their full potential.
Many capillary columns have internal diameters of 250 or 320 μ m row two of Table 6.2 . These i.d. ’ s represent the best compromise between resolution, speed, sample capacity, and ease of operation. These are the reference columns against which all other internal diameters are measured.
Five - hundred - and - thirty - micrometer or “ widebore ” columns, seen in row three of Table 6.2 , show loss in resolution compared to analytical capillary columns. This limitation is offset in most applications by their increased capac-ity and ease of operation. For example, direct on - column syringe injection is possible, often providing better quantitative results than packed columns.
These widebore or “ megabore ” columns also show good speed of analysis.
Column Length
Plate number, N , is directly proportional to column length, L ; the longer the column, the more theoretical plates, the better the separation. Resolution, R s ,
TABLE 6.2 Effects of Column Diameter
Inside Diameter Resolution Speed Capacity Ease
100 μ m Very good Very good Fair Fair
250 μ m 320 μ m
Good Good Good Good
530 μ m Fair Good Very good Very good
however, is only proportional to the square root of column length. This means that if column length is doubled, plate number is doubled, but resolution only increases by the square root of two, or 41%.
Retention time, t R , is also proportional to column length, so long columns can lead to slow analysis times. But when high resolution is critical, long columns are required. Referring to Table 6.3 , columns that are 60 m long are suggested for natural products such as fl avors and fragrances — in fact, for any sample with more than 50 components. Remember, however, that analysis times will be long.
For fast analysis of simpler samples, short columns should be used. Only moderate resolution is possible, but speed of analysis can be impressive.
Interest in fast GC has become so popular that it is a special topic included in Chapter 13 .
Medium column lengths of 25 or 30 m are recommended for most applica-tions. They provide a good compromise between resolution and speed of analysis.
Film Thickness
A standard fi lm thickness of 0.25 μ m is a reasonable starting point. It repre-sents a compromise between the high resolution attainable with thin fi lms and the high capacity available with thick fi lms. High capacity means that not only can larger sample quantities be accommodated, but usually the injection tech-nique is also simpler.
With 0.25 - μ m fi lms, practical operating temperatures can be used with minimal concern for column bleed, since column bleed is proportional to the amount of liquid phase in the column. Finally, with this fi lm thickness, the column can be optimized for high speed using fast fl ow rates or high resolution using slower fl ow rates.
Thick fi lms (1.0 μ m or greater) are made possible today due to improved techniques in cross - linking liquid phases and also due to the more inert fused silica surface. Cross - linking techniques will be discussed later in this chapter.
Such thick fi lms show increased retention of sample components — essential for volatile compounds. In addition, their high capacity allows injection of
TABLE 6.3 Column Length Recommendations
COLUMN SELECTION 93
larger samples; this can he important when mass spectrometers or Fourier transform - infrared spectrometers are to be used for subsequent analysis.
Decreased effi ciency is one disadvantage of thick fi lms. Thus, greater lengths may be required to compensate for their lower plate numbers. Also, higher operating temperatures are required to elute compounds from thick fi lms.
Higher temperatures, in turn, produce higher bleed rates and/or more noise.
Also, since column bleed is proportional to the amount of liquid phase in the column, thick fi lms do bleed more.
Figure 6.7 is a typical thick fi lm application, the separation of natural gas components using a 50 - m column. The fi lm thickness is 5 μ m of polydimethyl-siloxane, chemically bonded. Note the excellent resolution of methane, ethane, propane, and n - butane: peaks one, two, three, and four. This column is well suited for volatile compounds but should not be used for high molecular weight samples, because it would require excessively high temperatures and long analysis times. Note, for example, that benzene (peak 14) takes 20 min to elute even at 140 ° C.
The primary advantage of thin fi lms, defi ned as less than 0.2 μ m, is high effi ciency and, therefore, higher resolution. Thus shorter columns can be used for many applications (refer to Fast GC in Chapter 13 ). In addition, lower operating temperatures may be used, giving less column bleed.
Stationary Liquid Phases
Liquid phases for capillary columns are very similar to those used for packed columns. In both cases the liquid phase must show high selectivity, α , for the
1 2
compounds of interest. In addition, they should be capable of operation at high temperatures with minimal column bleed. This is particularly important for sensitive detectors like FID, ECD, and MS, which are used for trace analysis.
Table 6.4 lists the most commonly used liquid phases for both packed and capillary columns. Basically, there are two types of liquid phases in use today.
One is siloxane polymers, of which OV - 1, SE - 30, DB - 1 (100% methyl polysi-loxane) and OV - 17, OV - 275, DB - 1701, DB - 710 (mixtures of methyl, phenyl, and cyano) polysiloxanes are the most popular. The other common liquid phase is a polyethylene glycol (Carbowax 20M, Superox ® , and DB - wax ® ).
Schematic structures of both a dimethyl polysiloxane and a polyethylene glycol liquid phase were given in Chapter 4 (Figs. 4.6 and 4.7 ). There is, however, one difference between packed column and capillary column liquid phases: Capillary column phases are extensively cross - linked. By heating the freshly prepared capillary column at high temperatures (without column fl ow), the methyl groups form free radicals that readily cross - link to form a more stable, higher - molecular - weight gum phase. There is even some chemical bonding with the silanol groups on the fused silica surface. These cross - linked and chemically bonded phases are more temperature stable, last longer and can be cleaned by rinsing with solvents when cold. Most commercial capillary columns are cross - linked.
Column Conditioning. In earlier times, all columns had to be conditioned by baking out at high temperatures for long periods of time, often overnight.
All commercial capillary columns today have been conditioned in the factory, so minimal conditioning should be necessary. A good practice for all new columns is as follows: Make sure that carrier gas is fl owing for several minutes to eliminate any air in the column before heating; then program slowly (3 – 5 ° C/min) to slightly above your operating temperature. Do not bake out at temperatures exceeding the manufacturer ’ s maximum column temperature!
Observe the baseline; when it stabilizes, you may begin to use the column.
Maximum operating temperatures for some common liquid phases are listed in Table 6.5 .
Carrier Gas and Flow Rate
Van Deemter plots were shown in Chapter 3 , and they illustrate the effect of column fl ow rate on band broadening, H . There is an optimal fl ow rate for a minimum of band broadening. With packed columns, and also with thick fi lm megabore columns, nitrogen is the carrier gas of choice since the van Deemter B term (longitudinal diffusion in the gas phase) dominates. Nitrogen being heavier than helium minimizes this B term and produces more effi ciency.
In capillary columns, however, particularly those with thin fi lms, hydrogen is the best carrier gas (refer to Fig. 3.13 ). With capillary columns the effi ciency ( N ) is usually more than suffi cient and the emphasis is on speed. Thus, capillary
95 TABLE 6.4 Equivalent Stationary Phases a Supelco Agilent/ J & W Alltech Varian (Chrompack) Macherey - Nagel Quadrex Restek SGE
Packed Column Equivalent USP Code SPB - Octyl CP - Sil 2 CB Squalane Equity - 1 DB - 1, HP - 1, HP - Ultra 1 AT - 1, EC - 1 CP - Sil 5 CB Optima 1, Permabond SE - 30
007 - 1 Rtx - 1 BP1 SE - 30, SP - 2100 G1, G2, G9 Equity - 5 DB - 5, DB - 5.625, HP - 5, HP - PAS5, HP - Ultra 2
AT - 5, EC - 5 CP - Sil 8 CB Optima 5, Permabond SE - 52 007 - 2 Rtx - 5, XTI - 5 BP5 SE - 54, SE - 52, OV - 73
G27, G36 SPB - 20 AT - 20, EC - 20 007 - 20 Rtx - 20 OV - 7 G28, G32 Equity - 1701 DB - 1701, DB - 1701P AT - 1701 CP - Sil 19 CB Optima 1701 007 - 1701 Rtx - 1701 BP10 OV - 1701 G46 SPB - 35 DB - 35, HP - 35 AT - 35 007 - 11 Rtx - 35 OV - 11 G42 SPB - 50 DB - 17, HP - 17, HP - 50+
AT - 50 CP - Sil 24 CB Rtx - 50 OV - 17, SP - 2250 G3 SPB - 17 DB - 17, HP - 17, HP - 50+
AT - 50 CP - Sil 24 CB Optima 17 007 - 17 Rtx - 50 OV - 17, SP - 2250 G3
96
Supelco Agilent/ J & W Alltech Varian (Chrompack) Macherey - Nagel Quadrex Restek SGE
Packed Column Equivalent USP Code SP - 2250 DB - 17, HP - 17, HP - 50+
AT - 50 CP - Sil 24 CB Optima 17 007 - 17 Rtx - 50 OV - 17 G3 SPB - 225 DB - 225, HP - 225 AT - 225 CP - Sil 43 CB Optima 225 007 - 225 Rtx - 225 BP225 G7, G19 PAG Pluronics F68 G18 SUPELCOWAX 10 DB - WAX, HP - WAX, DB - WAXetr, HP - INNOWax AT - WAX, EC - WAX, AT - AquaWax
CP - Wax 52 CB Permabond CW 20M 007 - CW Rtx - WAX, Stabilwax BP20 Carbowax 20M G16 SPB - 1000/NUKOL DB - FFAP, HP - FFAP AT - 1000, EC - 1000, AT - AquaWax - DA CP - Wax 58 (FFAP) CB, CP - FFAP CB
Permabond FFAP 007 - FFAP Stabilwax - DA BP21 SP - 1000, OV - 351 G25, G35 SP - 2330 DB - 23, HP - 88 AT - Silar CP - Sil 84 007 - 23 Rtx - 2330 SP - 2330 G8 SP - 2380 Rtx - 2330 G48 SP - 2340 CP - Sil 88 SP - 2340 G5 a Comparable general - purpose column chart (by increasing phase polarity). Source : Reprinted with permission of Supelco, Bellefonte, PA 16823 from their 2007 – 2008 catalog.
TABLE 6.4 Continued