INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 12 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

CHAPTER 12 GAS CHROMATOGRAPHY (GC)

GAS CHROMATOGRAPHY - Mobile phase is a gas - Column is open tubular - Sample is injected through a thin rubber disc (septum) - Sample is vaporized upon entering a heated glass port - A carrier gas (He, N 2, H 2 ) carries vaporized sample to a detector - Detector temperature is higher than column temperature to keep solutes in the gaseous state

GAS CHROMATOGRAPHY Liquid Sample Size 0.1 to 2 µL for analytical chromatography 20 to 1000 µL for perspective chromatography Gas Sample Size 0.5 to 10 mL (gas tight syringe should be used)

INSTRUMENTATION - Injectors (syringes, autosamplers) - The column - Detectors

SAMPLE INJECTION - Sample is injected through a thin rubber disc (septum) to a heated glass port where it is vaporized - Injection is commonly done with syringes - Syringe needle must be very narrow - Wide needle could cut out a plug of the septum (coring) Autosamplers - Can analyze 100 or more liquid samples without manual injection - Samples are put in autosampler vials which are capped with septa

Split Injection - Good for open tubular columns - Complete injection may be too much for an open tubular column to 10% of the injected sample reaches the column - Not good for quantitative analysis (higher boiling point components may not be vaporized) SAMPLE INJECTION

Splitless Injection - Suitable for quantitative analysis - Suitable for analysis of low concentrations of solutes (trace componenets) - Dilute solution with low-boiling solvent is used - About 80% of the injected sample reaches the column SAMPLE INJECTION

Solvent Trapping - Sample is initially injected at about 40 o below boiling point of sample - A thin band of solute is trapped - Column temperature is later raised SAMPLE INJECTION

Cold Trapping - Used for high-boiling solutes - Sample is initially injected at about 150 o below boiling point of solutes of interest - Solvent and low-boiling solutes are eluted - High-boiling solutes are trapped in a narrow band - Column temperature is later raised SAMPLE INJECTION

On-column Injection - Sample is injected directly into the column - Used for compounds that decompose at temperatures above their boiling points - Solvent trapping or cold trapping is employed to trap a narrow band of analyte - Column temperature is increased afterwards to initiate chromatography SAMPLE INJECTION

OPEN TUBULAR COLUMN - Stationary phase is usually made of fused silica (SiO 2 ) (silicone polymers) - Liquid or solid stationary phase is coated on the inner wall - Stationary phase may be porous carbon - Another type is porous layer open tubular (PLOT) for relatively small molecules

Molecular Sieves - Included in the stationary phase - Has cavities and made of inorganic materials - Used to dry gaseous solutes - Strongly retains H 2 O - Separates other small molecules (CH 4, H 2, O 2, N 2, CO 2 ) OPEN TUBULAR COLUMN

- Polar column is usually used for polar solutes - Nonpolar column is usually used for nonpolar solutes - Tailing is seen when stationary phase bakes and SiOH groups (silanol) forms on the silica surface OPEN TUBULAR COLUMN

Nonpolar Stationary Phase - Solutes are eluted in order of increasing boiling point - Solutes with higher vapor pressure are eluted faster Polar Stationary Phase - Solutes are separated based on polarity - Less polar solutes are eluted faster than strongly polar solutes OPEN TUBULAR COLUMN

GUARD COLUMN - Collects nonvolatile solutes that are not eluted - Attached to the front of a chromatography column 5 to 10 meters long - Has no stationary phase and is silanized - Ends are cut off with time to discard nonvolatile solute buildup

OPEN TUBULAR COLUMN VS PACKED COLUMN - Gives better separation - Narrower peaks - Handles smaller samples (analytical chromatography)

COLUMN TEMPERATURE Increase in Column Temperature - Increases solute vapor pressure - Decreases retention time - Results in sharp peaks Temperature Programming - Used to separate compounds with a wide range of boiling points and polarities

ELUTION AND RESOLUTION Elution and resolution behavior depends on - The composition of the stationary phase and mobile phase gas - Column dimensions (length) - Column phase ratio (internal diameter and film thickness) - Column gas flow (pressure) - Column temperature

DETECTORS Mass Spectrometer (GC-MS) - Very sensitive and the most versatile - Major problem is interfacing Flame Ionization Detector (FID) - Destructive, mass-flow detector - For almost all organic compounds (not for CO, CO 2, HCN) - Low detection limits Thermal Conductivity Detectors (TCD) - Nondestructive, concentration detector - Not sensitive to narrow columns (diameter < 0.53 mm)

Electron Capture Detector (ECD) - Very sensitive to halogen-containing compounds - Insensitive to ketones, alcohols, and HCs - Nondestructive, mass-flow detector Electrolytic Conductivity Detector (ELCD) - For organic compounds with halogen, N, or S substituent - Destructive, Mass-flow detector Sulfur Chemiluminescence Detector (SCD) - For organic compounds with S atoms only - Destructive, mass-flow detector DETECTORS

Sulfur – Phosphorus Flame Photometric Detector (SP-FPD) - For organic compounds with S or P atoms - Destructive, mass-flow detector Nitrogen – Phosphorus Detector (NPD) - Selectively sensitive to nitrogen and phosphorus - Used for analysis of drugs - Destructive, mass-flow detector Photoionization Detector (PID) - For organic compounds with more easily ionizable π-electrons - Nondestructive, mass-flow detector DETECTORS

Helium Ionization Detector (HID) - Universal detector for everything except neon - Nondestructive, mass-flow detector Atomic Emission Detector (AED) - Selective for compounds containing many atoms - Tunable - Destructive, mass-flow detector DETECTORS

GC-MS GC-IR GC-GC or GC 2 (2D-Gas Chromatography) HYPHENATED TECHNIQUES

- Environmental pollution monitoring - Analysis of contaminants - Biochemical, medical, and pharmaceutical research APPLICATIONS

- Complex molecules (DNA, RNA) are too large to be volatilized and may decompose - HPLC is preferred for analyzing such molecules - Not suitable for solutions in aqueous media LIMITATIONS