Capillary columns are gas chromatography (GC) columns in which the stationary phase is not packed into the cavity but instead coats the inner surfaces. Samples are analyzed for the particular chemical components they contain using capillary GC columns. The capillary column is used in clinical laboratories to assist identify the chemical composition of a sample and in the pharmaceutical and petroleum sectors to test for contaminants. Compared to a packed column, a capillary CG column separates the sample more effectively; yet, it is more susceptible to overloading from the introduction of excessive amounts of the sample. The primary distinction between capillary GC column types is the stationary phase’s polarity, while there are several other variations as well, such as size and whether or not the capillary column contains an end cap.
The Best Way to Select a Capillary Column The column is where an optimal chromatographic separation starts. Four important criteria should be taken into consideration while choosing the right capillary column for any given application: stationary phase, column I.D., film thickness, and column length. In this section, the practical consequences of these parameters on the column’s performance are briefly reviewed in order of significance. Keep in mind that this is generic information. There may be some circumstances in which these rules should be broken.
Step 1 – Stationary phase The most crucial stage in choosing a column is deciding on a stationary phase. The film applied to a capillary column’s inner wall is known as a stationary phase, and it should be chosen according to the intended use. The separation process is based on the variations in the physical and chemical characteristics of the organic molecules that are injected and how they interact with the stationary phase. One molecule is kept longer than the other when there is a considerable difference in the strength of the analyte-phase interactions between the two. One indicator of these analyte-phase interactions is the retention time, or how long they are kept in the column.
The stationary phase’s physical characteristics change when its chemical characteristics are changed. If there is a substantial difference in the analyte-phase interactions, two compounds that co-elute (do not separate) on one stationary phase may separate on another phase with a different chemistry. This explains why there are so many different capillary column phases available. For every chemical class of analytes, each phase offers a unique set of interactions.
Step 2: I.D. Column.
Efficiency (the number of theoretical plates) and sample capacity (the amount of any one sample component that can be applied to the column without causing the desired sharp peak to overload) can be balanced thanks to the current range of commercially available capillary column internal diameters. One of these factors must be sacrificed in order to maximize the other. The analytical requirements determine the best I.D. for a particular application.
How efficiency and sample capacity are affected by column I.D.
As demonstrated, 0.25 mm I.D. columns provide for a respectable sample capacity while offering enough plates/meter for the majority of applications. Due to this trade-off between efficiency and sample capacity, the most widely used I.D. for capillary GC columns is 0.25 mm. Depending on the needs of their application, the user can optimize either efficiency or sample capacity by using columns with a smaller or bigger I.D.
High Efficiency: Shown as well-defined, narrow peaks on chromatography.
A capillary column’s efficiency, expressed in plates (N) or plates per meter (N/m), rises as the column’s I.D. falls. This is one of the fundamental ideas of Fast GC (for more information, read the “Fast GC Brochure”). The most practicable narrow I.D. capillary column should be used if the material to be examined has a large number of analytes or analytes that elute closely together. Note that specific equipment, like a GC with a pressure regulator that permits a higher column head pressure, may be needed for very narrow bore columns, such as those with an I.D. of 0.10 or 0.18 mm.
Step 3: Thickness of the Film The film thickness of the majority of 0.25 mm I.D. columns is 0.25 or 0.50 μm.
The ideal film thickness may vary depending on the application. Reducing the thickness of the film: The advantages include decreased column bleed, which raises signal-to-noise, and sharper peaks, which could improve resolution. The maximum operating temperature of the column will also be raised. Reduced analyte capacity and higher analyte contact with the tube wall are the disadvantages. Analytes can elute at lower temperatures and with shorter retention durations when film thickness is reduced; depending on the application, this may or may not be desired. Thinner film columns are best suited for trace studies or analytes with high boiling points (>300 °C), such as pesticides, PCBs, FAMEs, phthalate esters, and other semivolatile chemicals.
Increasing Film Thickness: This increases sample capacity and decreases analyte-tubing interaction.
The disadvantages include a lower maximum working temperature for the column, higher column bleed, and wider peak widths (which could lower resolution). In addition to increasing elution temperature, increasing film thickness also increases analyte retention (and may potentially boost resolution, particularly for low k molecules). These final results can be either good or unwanted, depending on the application. Analytes having low boiling points, like volatile organic chemicals and gasses, are best suited for thicker film columns. The longer retention of these analytes on the thicker film may make subambient oven temperatures unnecessary. In addition to increasing capacity, a thicker film will make the column more suitable for samples with higher concentrations than one with a thinner film.
Step 4: Length of Column The ideal ratio of resolution, analysis duration, and necessary column head pressure is often achieved with a 30 m column. Different column lengths may be necessary for particular applications. Longer columns increase back pressure but offer better resolution. It should be emphasized that resolution only rises in proportion to the square root of column length; doubling column length will not double resolution. Double column length will not bring a critical pair to baseline (resolution value of at least 1.5) if the resolution between them is less than 1. A last resort should be to lengthen the columns in order to improve resolution. Reducing column I.D. is a more efficient way to increase resolution.
Shorter Columns: Used for simple samples with chemically different components or for screening applications where high resolution is not necessary. Resolution can be preserved, or in certain situations, even improved, if column I.D. is reduced in tandem with length.
