Gas Chromatography. Introduction

Transcription

1 Gas Chromatography Introduction 1.) Gas Chromatography Mobile phase (carrier gas) is a gas - Usually N 2, He, Ar and maybe H 2 - Mobile phase in liquid chromatography is a liquid Requires analyte to be either naturally volatile or can be converted to a volatile derivative - GC useful in the separation of small organic and inorganic compounds Stationary phase: - Gas-liquid partition chromatography nonvolatile liquid bonded to solid support - Gas-solid chromatography underivatized solid particles - Bonded phase gas chromatography chemical layer chemically bonded to solid support Bonded phase Magnified Pores in activated carbon Zeolite molecular sieve

2

3 Gas Chromatography Introduction 2.) Instrumentation Process: - Volatile liquid or gas injected through septum into heated port - Sample rapidly evaporates and is pulled through the column with carrier gas - Column is heated to provide sufficient vapor pressure to elute analytes - Separated analytes flow through a heated detector for observation

4 Capillary Tube GAS CHROMATOGRAPHY Liquid Stationary Phase A B A A B B A B B He Carrier gas A A B A BA B He Carrier gas A A A A A A A A B B B B B B B B 0 B Time A Immediately after injection After several minutes Resulting chromatogram Compounds A and B interact with the stationary phase through intermolecular forces: (van der Waals or dipole-dipole forces, including hydrogen bonding). A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A. Typically, components with similar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes.

5 Sample Injection System For quantitative work, more reproducible sample sizes for both liquids and gases are obtained by means of a rotary sample valve. Errors due to sample size can be reduced to 0.5% to 2% relative. The sampling loop is filled by injection of an excess of sample. Rotation of the valve by 45 deg then introduces the reproducible volume ACB into the mobile phase.

6

7 Column Configurations Two general types of columns are encountered in gas chromatography, packed and open tubular, or capillary. Chromatographic columns vary in length from less than 2 m to 50 m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven for thermostating, they are usually formed as coils having diameters of 10 to 30 cm. Column temperature is an important variable that must be controlled to a few tenths of a degree for precise work. Thus, the column is ordinarily housed in a thermostated oven. The optimum column temperature depends upon the boiling point of the sample and the degree of separation required.

8 Gas Chromatography Instrumentation 1.) Open Tubular Columns Commonly used in GC Higher resolution, shorter analysis time, and greater sensitivity Low sample capacity Increasing Resolution - Narrow columns Increase resolution - Resolution depends directly from column length Easy to generate long (10s of meters) lengths of narrow columns to maximize resolution

9 Gas Chromatography Instrumentation 1.) Open Tubular Columns Increasing Resolution Decrease tube diameter Increase resolution Increase Column Length Increase resolution

10 Gas Chromatography Instrumentation 1.) Open Tubular Columns Increasing Resolution Increase Stationary Phase Thickness Increase resolution of early eluting compounds Also, increase in capacity factor and reduce peak tailing But also decreases stability of stationary phase

11 Gas Chromatography Instrumentation 2.) Choice of liquid stationary phase: Based on like dissolves like Nonpolar columns for nonpolar solutes Strongly polar columns for strongly polar compounds

12 GC Stationary Phase Experimental Organic Chemistry D. R. Palleros, Wiley, NY, 2000

13 Gas Chromatography Instrumentation 3.) Packed Columns Greater sample capacity Broader peaks, longer retention times and less resolution - Improve resolution by using small, uniform particle sizes Open tubular column Packed column

14 Gas Chromatography Retention Index 1.) Retention Time Order of elution is mainly determined by volatility - Least volatile = most retained - Polar compounds (ex: alcohols) are the least volatile and will be the most retained on the GC system Second factor is similarity in polarity between compound and stationary phase

15 Gas Chromatography Retention Index 2.) Describing Column Performance Can manipulate or adjust retention time by changing polarity of stationary phase Can use these retention time differences to classify or rate column performance - Compare relative retention times between compounds and how they change between columns Can be used to identify unknowns

16 Gas Chromatography Temperature and Pressure Programming 1.) Improving Column Efficiency Temperature programming: - Temperature is raised during the separation (gradient) - increases solute vapor pressure and decrease retention time Temperature gradient improves resolution while also decreasing retention time

17

18

19

20 Gas Chromatography Temperature and Pressure Programming 1.) Improving Column Efficiency Pressure Programming: - Increase pressure increases flow of mobile phase (carrier gas) - Increase flow decrease retention time Van Deemter curves indicate that column efficiency is related to flow rate Flow rate increases N 2 < He < H 2 Pressure is rapidly reduced at the end of the run - Time is not wasted waiting for the column to cool - Useful for analytes that decompose at high temperatures

21 Detection Systems Characteristics of the Ideal Detector: The ideal detector for gas chromatography has the following characteristics: 1. Adequate sensitivity 2. Good stability and reproducibility. 3. A linear response to solutes that extends over several orders of magnitude. 4. A temperature range from room temperature to at least 400 o C. 5. A short response time that is independent of flow rate. 6. High reliability and ease of use. 7. Similarity in response toward all solutes or a highly selective response toward one or more classes of solutes. 8. Nondestructive of sample.

22

23 Flame Ionization Detectors (FID) The flame ionization detector is the most widely used and generally applicable detector for gas chromatography. The effluent from the column is mixed with hydrogen and air and then ignited electrically. Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air flame, produce ions and electrons that can conduct electricity through the flame. A potential of a few hundred volts is applied. The resulting current (~10-12 A) is then measured. The flame ionization detector exhibits a high sensitivity (~10-13 g/s), large linear response range (~10 7 ), and low noise. A disadvantage of the flame ionization detector is that it is destructive of sample.

24

25 GC-MS

26 GC-MS

27 Peak Area Gas Chromatography 1.) Qualitative and Quantitative Analysis Compare retention times between reference sample and unknown - Use multiple columns with different stationary phases - Co-elute the known and unknown and measure changes in peak area The area of a peak is proportional to the quantity of that compound Area of Gaussian peak peak height w 1 2 Peak area increases proportional to concentration of standard if unknown/standard have the identical retention time same compound Concentration of Standard

28 GC Peak Areas and Resolution

29 Method Development Basic parameteres Sample injection (split- splitless) Column selection (packed or capillary column) Selection of gradient or isothermal program Detector selection Inlet temperature, detector temperature, column temperature and temperature program, carrier gas and carrier gas flow rates, the column's stationary phase, diameter and length, inlet type and flow rates, sample size and injection technique

30

31 GC - Derivatization Why is chemical derivatization needed? GC is best for separation of volatile compounds which are thermally stable. Not always applicable for compounds of high molecular weight or containing polar functional groups. These groups are difficult to analyze by GC either because they are not sufficiently volatile, tail badly, are too strongly attracted to the stationary phase, thermally unstable or even decomposed. Chemical derivatization prior to analysis is generally done to: increase the volatility and decrease the polarity of compounds; reduce thermal degradation of samples by increasing their thermal stability; increase detector response improve separation and reduce tailing Derivatizing Reagents Common derivatization methods can be classified into 4 groups depending on the type of reaction applied: Silylation Acylation Alkylation Esterification

32 The Retention Index / Kovats index The retention index I was first proposed by Kovats for identifying solutes from chromatograms. The retention index for any given solute can be derived from a chromatogram of a mixture of that solute with at least two normal alkanes having retention times that bracket that of the solute. That is, normal alkanes are the standards upon which the retention index scale is based. The retention index for a normal alkane is equal to 100 times the number of carbons in the compound regardless of the column packing, the temperature, or other chromatographic conditions. Within a homologous series, a plot of the logarithm of adjusted retention time t`r (t`r = t R - t`m) versus the number of carbon atoms is linear. I = Kovats retention index, n = the number of carbon atoms in the smaller alkane, N = the number of carbon atoms in the larger alkane, t r ' = the adjusted retention time.

33 The Retention Index / Kovats index

34 CHROMATOGRAPHY Preparative vs Resolution vs Speed vs Expense

35