Film thickness stationary phase

The smaller the internal diameter of a capillary column the more efficient the column is for a given stationary phase film thickness on the capillary wall. This is because the mass transfer characteristics of the column are improved with the analyte being able to diffuse in and out of the mobile phase more frequently because of the shorter distance for transverse diffusion (Ch. 10 p. 201). 

Different capillary columns are available for organic acid separation and analysis. In our laboratory, the gas chromatography column in all GC-MS applications is crosslinked 5% phenyl (poly)methyl silicone, 25 m internal diameter 0.20 mm stationary phase film thickness 0.33 pm (Agilent HP-5, DB-5, or equivalent). Several instrument configurations are commercially available, which allow for positive identification of compounds by their mass spectra obtained in the electron impact ionization mode. A commercially available bench-top GC-MS system with autosampler (Agilent 6890/5973, or equivalent) is suitable. Software for data analysis is available and recommended. The use of a computer library of mass spectra for comparison and visualization of the printed spectra is required for definitive identification and interpretation of each patient specimen. 

SFC chromatographs represent hybrids between GC and HPLC instruments (Fig. 6.4). In order to deliver the supercritical fluid, syringe pumps or reciprocal pumps are used and maintained above the critical temperature using a cryostat regulated at around 0 "C. In instances where an organic modifier is used, a tandem pump is employed which has two chambers, one for the critical fluid and one for the modifier. The liquid then passes through a coil maintained above the critical temperature so that it is converted into a supercritical fluid. Stainless steel packed columns like those used in HPLC (1 to 4 mm in diameter) or fused silica capillary columns like those used in capillary GC (2 to 20 m in length, internal diameters as low as 50 pm and stationary phase film thickness of at least 1 pm) are used in SFC. 

For cold trapping, stationary phase film thickness must be 2 j.m. 

The medium-film thickness is about 0.3-0.6 pm and generally offers the best compromise of sample capacity, retentivity, and phase stability. The phase ratio determines the capacity of the column and influences its retentivity of solutes. The phase ratio (j8) can be defined as the ratio of the inner column radius to that of the product of twice the stationary-phase film thickness or 0 = r/2df. We can now also use phase ratios to group film thicknesses and now say that thick-film columns have phase ratios of less than about 80. (In capillary SFC the typical stationary-phase film thicknesses are 0.1-0.3 pm.) The effective phase ratio can change in capillary SFC, depending on the characteristics of the stationary phase and the operating density [57]. The change in phase ratio can be attributable to a swelling of the stationary phase under certain SFC conditions. 

The effect of temperature on retention was studies using n-hexadecane on a 20 m, 50ji I.D. fused silica capillary column coated with an OV-17 phase using FID detection. The OV-17 was cross-linked in-situ to decrease its solubility in the supercritical fluid. The stationary phase film thickness was calculated to be 0.25pm. The... 

In the second place, the parameters listed in table 4.2 cannot always be varied independently and, moreover, will have side effects on yet other parameters. All the capacity parameters affect the phase ratio (V/ V. If all other parameters are kept constant, then the film thickness and the surface area will affect Ks, the porosity will affect Vm and the diameter of open columns will affect both Vm and Vs. However, it is often impossible to keep all other parameters constant. For instance, it would be very difficult to vary the porosity without changing the surface area. An example of the effect of variations in the capacity parameters on other parameters is the decrease in the number of theoretical plates in the column that usually accompanies an increase in the stationary phase film thickness in GLC. 

In packed columns, there are two parameters which may be varied independently in order to optimize the column characteristics, i.e. the diameter of the column and the diameter of the particles. In open columns, only the column diameter may be varied. Additionally, the phase ratio may be varied by changing one of the capacity parameters (see section 3.5). For packed columns these parameters include the surface area of the packing material, the column porosity and the stationary phase film thickness. For open columns only the latter parameter is relevant. 

Stationary Phase Film Thickness Column Dimensions Helium Linear Velocity Initial Temperature Initial Time Temperature Program Hold Time Total Time... 

The stationary phase film thickness of capillary columns range from about 0.1-10 pm and can be divided into three film thickness ranges. Thin-film columns are usually 0.1-0.2 pm and offer the greatest stability. They have smaller sample capacity as compared to the thicker films but are the best for use with high temperatures. 

The separation of essential oil components is usually carried out by GC with fused-silica capillary columns. The properties and conditions of columns used are variable, depending on the polarity of the components to be separated. The most used columns include stationary phases such as DB-1, Carbowax, OV-1, OV-101, PEG 20M, BPS, and DB-5, which cover a wide range of polarities. Column lengths normally range from 25 to 100 m, and stationary phase film thickness ranges from 0.2 to 0.7 pm. Elution of components is usually performed with a temperature gradient ranging from 50°C to 280°C. 

What is the effect of stationary-phase film thickness on gas chromatograms ... 

Another point of contention relates to column bleed in general, thick film columns have higher bleed rates, and some argue that the bleed rate is directly proportional to the stationary phase film thickness (11). This generalization was once correct, but columns prepared with high purity polymers that are extremely clean, and bonded to properly deactivated tubing, do not exhibit this limitation unless the columns are abused. Some bleed problems can be associated with residues from "dirty" samples that remain on the column, while others are attributable to... 

Figure 5. On-column headspace injections (500 uL) of a California Muscat wine effect of stationary phase film thickness. Top, 30 m x 250 urn column, 0.25 um film of DB-1. Bottom, identical column, but with a 1.0 um film of DB-1. The columns were run under Identical conditions 30°C (2 min), 2°/min to 40°C,...

Presented in Table I are the dimensions and properties of the several columns prepared in this work. The internal diameter was varied as shown from 0.278 mm to 0.0345 mm, while the stationary-phase film thickness was held constant for all but the last of the columns. Thus, the phase ratio (V] /Vc) decreases on passing from column 1 to 6 over the range 231.5 to 54.25. The solute capacity factors increase accordingly from 6.83 to 29.47. The number of theoretical plates per meter leng N/m for all columns except the first is therefore very nearly equal to the number of effective theoretical plates per meter, since the capacity factors are close to or exceed 10. 

The relationship between capillary column inside diameter, column efficiency, expressed as the number of theoretical plates per meter (N/L), and film thickness is illustrated in Figure 7. These curves represent the theoretical limits of column efficiency for a given column inside diameter and stationary phase coating. The very thin-film, narrow-bore columns will be the most efficient and their efficiency changes markedly as the film thickness increases. The efficiency of a 0.25-mm i.d. column with a stationary phase film thickness of less than 1 pm is shown to be limited by mass transfer in the mobile phase (designated by Cg from the van Deemter equation), whereas mass transfer in the stationary phase (CL) becomes predominate in thick-film columns. The point at which Cg = CL is shown for narrow- and wide-bore capillary columns in Figure 7. 

Figure 7. Effect of the stationary phase film thickness on the number of theoretical plates per meter (N/L, column efficiency) as a function of the film thickness of the stationary phase for capillary columns of different...

Figure 9. Analysis of triglycerides in whole butter on a 20 m X 0.30 mm SE-52 column with a stationary phase film thickness of 0.05 pm and the on-column injection technique. (12)...

TaMe 5.6 Phase ratios for various stationary phase film thickness and column i.d. 

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