Coating PLOT columns

De Zeeuw, J., Reinkofer, R., van Zee, H. Increased Temperature Stability for Styrene-Divinylbenzene Porous Polymer Coated PLOT Columns. Ibid. 406 P. 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency. Fused silica capillary tubes are used which have internal diameters ranging from 0.1 mm (small bore) to 0.53 mm (large bore) with typical lengths in excess of 20 m. The wall-coated open tubular (WCOT) columns have the internal surface of the tube coated with the liquid (stationary) phase and no particulate supporting medium is required. An alternative form of column is the porous-layer open tubular (PLOT) column, which has an internal coating of an adsorbent such as alumina (aluminium oxide) and various coatings. Microlitre sample volumes are used with these capillary columns and the injection port usually incorporates a stream splitter. 

FIGURE 9.4 Chromatograms of a reversed-phase test mixture. Plot A is the chromatogram at 30°C and ImL/min, and plot B is the chromatogram at 100°C and 5mL/min. Solutes 1, uracil 2, p-nitroaniline 3, methyl benzoate 4, phenetole 5, toluene. 2.1% (w/w) poly-butadiene coated zirconia column, mobile phase 20% ACN, flow rate was l.OmL/min at 30°C and 5mL/min at 100°C. detection 254 nm. (Reprinted from Li, J. et al., Anal. Chem., 69, 3884, 1997. Copyright 1997, American Chemical Society. With permission.)... 

PLOT Columns. Porous-layer open tubular columns wherein the internal wall is coated with a layer of adsorbent support. If the support is then coated with a liquid phase it is referred to as a SCOT Column (i.e., support-coated open tubular column). 

Capillary columns coated with aluminum oxide and molecular sieves can be described as absorption phases [22]. These columns are also known as porous-layer open tubular (PLOT) columns. Aluminum oxide PLOT columns deactivated with potassium chloride have been very effective at the separation of C,.,0 hydrocarbons. 

For a period of time, OT columns that had characteristics intermediate between those of WCOT and packed columns were popular. There were two types, but they were similar. Support coated open tubular (SCOT) columns had a thin layer of solid support coated on the inside wall of a capillary tube of larger diameter than that used for WCOT columns. This layer was coated with stationary liquid similar to packed columns. Porous layer open tubular (PLOT) columns were similar but made differently for example, the solid support was added while the capillary tube was being drawn. With a few exceptions, SCOT and PLOT columns are no longer popular because wide diameter WCOT fused silica columns are as good, more stable (no layer to flake off), and easier to use. Subsequent discussion will be restricted to WCOT columns. 

A range of open tubular GC columns were used. A 150 ft X 0.01 in. i.d. stainless steel wall-coated open tubular (WCOT) column (Perkin Elmer Corp., Norwalk, CT), a 50 ft X 0.02 in. i.d. support coated open tubular (SCOT) column (Perkin Elmer), and 33 ft and 100 ft X 0.03 in. i.d. porous layer open tubular (PLOT) columns. The latter were pre-... 

Several types of surface modification have been employed [37,38] such as etching or deposition of very fine inert supports before application of the phase. The latter are referred to as support coated open tubular (SCOT) or by others as porous layer open tubular (PLOT) columns. They have a higher sample capacity than liquid coated columns due to the greater amount of phase per unit length. Several recent papers have described modified methods of column preparation [39-41] using Silanox 101 as the support. 

Open tubular columns are simply capillary tubes in which the inside of the column wall is used as the support for the liquid phase. These wall-coated open tubular columns (WCOT) have the stationary phase distributed in the form of a thin film on the inside surface of the open capillary tube, the walls thus serving as the support. In order to reduce the thickness of the liquid phase film, a porous layer may be formed on the inside wall of the capillary tubing and then coated with the liquid phase to produce a support-coated open tubular column (SCOT). Porous-layer open tubular colunms (PLOT) are similar to SCOT colunms, the difference being that in the former, the stationary phase is deposited on fine crystalline particles or glass powder which is adsorbed onto the walls of the tube. In both cases, the available surface area of the wall is increased, and allows an increased amount of liquid phase to be accommodated in the same length and diameter of tubing. The whisker-walled (WW) colunm consists of whiskers chemically etched on the surface of the wall, which also result in a significant increase in the available surface area. Wall-coated, porous-layer, and support-coated capillary columns are all available as whisker-walled, i.e., WWCOT, WWPLOT, and WWSCOT, respectively. 

There are two basic disadvantages to the coated capillary column. First, the limited solute retention that results from the small quantity of stationary phase in the column. Second, if a thick film is coated on the column to compensate for this low retention, the film becomes unstable resulting in rapid column deterioration. Initially, attempts were made to increase the stationary-phase loading by increasing the internal surface area of the column. Attempts were first made to etch the internal column surface, which produced very little increase in surface area and very scant improvement. Attempts were then made to coat the internal surface with di-atomaceous earth, to form a hybrid between a packed column and coated capillary. None of the techniques were particularly successful and the work was suddenly eclipsed by the production of immobilize films firmly attached to the tube walls. This solved both the problem of loading, because thick films could be immobilized on the tube surface, and that of phase stability. As a consequence, porous-layer open-tubular (PLOT) columns are not extensively used. The PLOT column, however, has been found to be an attractive alternative to the packed column for gas-solid chromatography (GSC) and effective methods for depositing adsorbents on the tube surface have been developed. 

According to the tjT)e and form of the coating, capillary column may be subdivided into wall-coated open tubular (WCOT) type, porous-layer open tubular (PLOT) type, and support coated open tubular type. The most... 

High-resolution analysis of the entire suite of C2-C12 NMHCs by conventional one-dimensional gas chromatography is typically performed with various siloxane-coated fused-silica capillaries or PLOT columns. Designing a single GC analysis which will adequately separate the entire complex mixture is very difficult to achieve, because ethene, acetylene, and ethane are difficult to separate, even with a 100 m siloxane-coated column. Furthermore, separations with a column of this length take approximately 70 min. The PLOT columns can completely resolve the C2-C3 NMHCs, but resolution of the Cg-Ci2 NMHCs is poor. If the sample is analyzed separately in a 60 m siloxane column, mounted in one GC and a PLOT column mounted in another, the analyses can be completed in approximately 45 min. The preconcentrated sample is transferred to a 60 m X 0.32 mm i.d. fused-silica capillary column coated with a 1 /rm-thick film of poly-dimethylsiloxane (PDMS) and a 30 m X 0.53 mm i.d., PLOT column coated with alumina to resolve the C4-C12 and C2-C3 NMHCs, respectively. The oven temperature for the PDMS column is held at — 50°C for 2 min, then increased to 210°C at 8°Cmin and thereafter to 250°C at... 

WCOT columns. The stationary phase coating was susceptible to column bleed and damage by solvents and high boiling contaminants. PLOT columns with alumina, molecular sieve or copolymer surface coatings have been developed particularly for the separation of Ci-Q alkanes and alkene and atmospheric gases [22-24]. 

Recent developments in coating technology have enabled cross-linked styrene divinylbenzene porous copolymers to be successfully immobilised on the silica wall of a capillary column. These PLOT columns have similar retention properties to the packed column equivalents but with higher resolution. PoraPLOT columns with Porapak Q, S or U stationary phases are general purpose columns suitable for the analysis of C) -C4 hydrocarbons, chlorofluorocarbons and atmospheric pollutants [35,36]. 

Gas chromatography columns for interplanetary exploration should be very efficient, due to the stringent demands imposed on payload. Porous-layer open tubular (PLOT) columns loaded with a styrene-divinyl benzene copolymer showed better performance than Cromosorb 103 or Porapak Q columns in the analysis of low molecular weight hydrocarbons and nitriles. Potassium chloride-deactivated alumina PLOT columns, on the other hand, were not good for the intended purpose, because low molecular weight nitriles were difficult to elute. Wall coated open tubular (WCOT) columns with a stationary chemically bonded phase of dimethyl siloxane have the mechanical resilience to endure the conditions of extraterrestrial exploration and separated efficiently C1-C4 nitriles. Permanent gases were analysed in the presence of hydrocarbons and nitriles with a PLOT capillary column. The equilibrium constant for the interaction of nitrile groups with various solutes was determined by GLC. ... 

Platelet retention in the polymer-coated bead columns was quantified by noting the percentage of platelets removed from aliquots 2 through 5 of 5 successive 1 ml effluent fractions. (Aliquot 1 was diluted with saline and was not counted.) Retention values obtained for the 14 polymers tested are shown in Table 1. These data are plotted in Fig, 2 as a function of both the alkyl side chain length and the glass transition temperatures (Tg s) of the polymers. 

FIGURE 4.7 Plot of log P against the retention facto (log k) from MEEKC on a dynamically coated capillary column for compounds of varied structure. (From Poole S.K., et al., J. Chromatogr. B, 793, 265-274, 2003.)... 

Bonded-phase wide-bore capillary columns permit relatively large-volume septum injections and can offer advantages of improved efficiency, reproducibility, and reliability. A 60 m x 0.53 mm i.d. fused silica capillary coated with the dimethylpolysiloxane SPB-1 (5 pm film thickness) programmed from 40°C to 200°C offers many advantages over packed column and PLOT systems. Improved resolution of very volatile compounds is obtained and, even with an initial temperature of 40°C, the total analysis time can be reduced to 26 min, and good peakshapes can be obtained even for alcohols (Figure 1). Moreover, splitless septum injections of up to 300 pi headspace can be performed with no noticeable effect on column efficiency hence, sensitivity is at least as good as that attainable with a packed column. Indeed, nowadays packed and PLOT columns are only used in specific applications or if a capillary GC is not available. 

Wall-coated open tubular columns (WCOT columns), or simply capillary columns, and classical packed columns dominate the practice of gas-liquid chromatography today. Porous layer open tubular columns (PLOT columns) and classical packed columns dominate the practice of gas-solid chromatography. WCOT columns are typically up to 100 m... 

Micropacked, packed capillary, and support coated open tubular (SCOT) columns are rarely used today, having been superceded by developments in WCOT and PLOT column technology. Micropacked columns have diameters less than 1 mm and a similar packing density to classical packed columns. Packed capillary columns have an internal diameter less than 0.6 mm and are packed with particles of 5-20 pm diameter. SCOT columns are capillary columns containing a liquid phase coated on a surface covered... 

Modern PLOT columns are prepared either by in situ polymerization or by addition of a chemical binder to the coating solution resulting in immobilization and bonding of particles to the inside column wall. PLOT columns containing immobilized layers of inorganic oxides, carbon, molecular sieves, cyclo-dextrins, and porous polymers are available in lengths up to 100 m with internal diameters from 0.25 to 0.53 mm and layers of 5-50 pm thickness. 

Fig. 63. Comparison of suppon coated open tubular (SCOT) column and porous layer open tubular (PLOT) column.

Several gas-solid adsorption capillary columns are available. They are commonly called porous-layer open tubular or PLOT columns. These columns contain a layer of adsorbent particles coated on the irmer wall of the fused silica tubing. Phases of aluminum oxide (alumina), molecular sieves, and porous polymers (Poraplot-like) are commercially available. Gas-solid adsorption rather than a gas-liquid partition is the separation mechanism involved. PLOT columns are well suited for the analysis of light hydrocarbons, sulfur gases, permanent gases, or other very volatile solutes at or... 

Three different types of open tubular columns are in use the wall-coated open tubular (WCOT) (Figure 2.6), porous-layer open tubular (PLOT), and support-coated open tubular (SCOT) columns. WCOT columns are by far the most used. In these columns the liquid stationary phase is coated as a thin film, 0.1-0.5 pm (typically 0.25 pm), on the inner wall of the capillary. The types of stationary phase are described in Section 2.6.2. These columns are used for partition chromatography (GLC). PLOT columns contain a porous layer (or a layer of porous particles) on the inner wall of the capillary and are used for gas adsorption chromatography (CSC), where the porous layer constitutes the stationary phase. The third type is the SCOT columns, where a liquid stationary phase is coated on the porous layer/porous particles at the inner wall of the capillary and hence GLC separations can be carried out. The advantage of the SCOT column compared to the WCOT column is its higher sample capacity. Its disadvantage is lower efficiency. 

---