GAS CHROMATOGRAPHY Mobile and Stationary Phases

1. GAS CHROMATOGRAPHY Mobile and Stationary Phases

2. Capillary Columns The geometry of capillary columns is fairly simple, consisting of length, internal diameter, and stationary phase thickness. Nevertheless, there are endless possible combinations of these three factors that could be used for optimizing chromatography.

3. Column Length Doubling the column length effectively doubles the number of theoretical plates but the resolution between any two compounds is proportional to the square root of the plate number so doubling the column length only increases resolution by about 40%

4. Column Diameter Smaller Diameter: Faster Chromatography Higher Number of Theoretical Plates Lower Sample Capacity Higher Detection Limit (lower mass injected)

5. Linear Gas Velocity (cm/sec) Column Diameter

6. Film Thickness Thinner Films Higher N Used for High BP Compounds Low Sample Capacity Thicker Films Used for Low BP compounds Less Peak Tailing for Polar Compounds

7. Theoretical Plate Height Linear Gas Velocity (cm/sec) Film Thickness

8. Stationary Phases

9. Mobile Phase Lighter gasses are best for fast analysis (hydrogen) Fast analysis produces narrow peaks with better detectibility Heavier gasses have slightly higher N (but only at low velocity) (not used too much)

10. Mobile Phase Effect of Mobile Phase Gas Type and Velocity

11. Mobile Phase There is a drop in pressure as the gas moves through the column. This drop is pressure causes the gas to expand which can result in peak broadening. It also causes the gas velocity to increase as it moves through the column pi/po is the ratio of the inlet velocity(pi) to the outlet velocity (po).

12. Mobile Phase As the column oven is heated, the viscosity of the mobile phase increases. Therefore as the column is heated during the temperature ramp, theflow rate goes down. In order to keep a constant flow ( and reduce peak spreading of later eluting peaks) a process of pressure programming is used. The constant flow mode increases the pressure at the head end of the column, and keeps the mobile phase velocity constant. Pressure programming can also be used to increase mobile phase velocity as the temperature increases, further decreasing analysis time and increasing peak height.

13. Kovats Retention Index The retention time of an analyte provides some information on the chemistry of the compound. However, retention time is dependant on many operational factors such as temperature, column length, column diameter, coating thickness, etc. The use of a relative retention value compensates for many of these variations. The Kovats retention index is used to calculate relative retention values based on a scale defined by the elution of a series of n-alkanes. An index value calculated for an analytes should be the same for any chromatographic run as long as the same stationary phase is used. Information on the Kovats Index for many compounds can be found in the literature.

14. Kovats Retention Index where X refers to the adjusted retention volumes or times, z is the numberof carbon atoms of the n-alkane eluting before and (z + 1) is the numberof carbon atoms of the n-alkene eluting after the peak of interest:

15. Kovats Retention Index The classical Kováts retention index is measured under isothermal conditions. However, in the case of temperature-programmed gas chromatography a similar value can be calculated utilizing direct numbers instead of their logarithm. In other words, an equation for the Kovats index can be developed for a polynomial regression of a series of alkanes vs. their retention times.

16. Using a series of alkane standards, the retention time can be converted to a retention index which can be compared to literature values. 1000 1100 Kovats Retention Index The output from the GC is a series of peaks. While the output does not tell you what the peaks, the retention time is useful information. Detector Response Minutes

17. Kovats Retention Index