Headspace Extraction of Volatile Oils

PHCH 137 (Lab) – Sec 3 Grp 4

Contents of the Report

I. Basic Principles of Headspace AnalysisII. InstrumentationIII. Headspace Application

Basic Principles of Headspace Extraction

Definition

Volatile components from complex sample mixtures can be extracted from non-volatile sample components and isolated in the headspace or vapor portion of a sample vial.

1. Static Headspace– Uses a gas-tight syringe and injection of a portion of

the headspace gas above a sample in a sealed container.

2. Purge and Trap– The sample is sparged w/ a stream of gas which

carries analytes to the solid trap. The trap is quickly heated and the analyte is released w. the stream of gas to the GC port.

Types of Headspace Sampling

Volatile analytes

Sample, dilution sample, & matrix

modifiers

Gas phase

Sample phase

Phases of the headspace vial

Partition Coefficient and Phase Ratio

Partition Coefficient (K)

• The equilibrium distribution of an analyte between the sample phase and the gas phase

K = Cs/Cg

Where: Cs=concentration of analyte in sample phaseCg=concentration of analyte in gas phase

Partition Coefficient and Phase Ratio

Phase Ratio (β)

• The relative volume of the headspace compared to volume of the sample in the sample vial

β = Vg/Vs

Where: Vs=volume of sample phaseVg=volume of gas phase

Partition Coefficient and Phase Ratio

• Partition coefficients and phase ratios work together to determine the final concentration of volatile compounds in the headspace of sample vials.

Cg = Co / (K + β)

Where: Cg = Concentration of volatile analytes in the gas phaseCo = Original concentration of volatile analytes

Partition Coefficient and Phase Ratio

• Lowest values for both K and β will result in higher concentrations of volatile analytes in the gas phase.

• Lowering of K:– Change in temperature– Introducing salt in the aqueous matrix

• Common salts used: NH4Cl, (NH4)2SO4, NaCl, Na3C6H5O7, (Na)2SO4, K2CO3

Low Partition Coefficient and Phase Ratio

Instrumentation

Sample Vials

• Should match the type and size of the sample being analyzed.

• Always use pre-cleaned vials for sample preparation and storage.

• Some septa are available with a Teflon® face to eliminate bleed from the rubber portion of the septa. These septa should not be re-used.

Heater and Mixer

• Temperature, time, and mixing can be used to improve the transfer of volatile analytes from the sample into the headspace of the vial.

Sampling System

Types of Sampling:

1. Manual sampling2. Automated sampling:

a.Gas-tight Syringe Injectionb. Balanced-pressure Systemc. Pressure-loop System

Sampling System

Sampling System

Sampling System

Applications

Organic Volatile Impurities

USP General Chapters <467>

References:RESTEK CORP. 2000. A Technical Guide for Static

Headspace Analysis Using GC. Restek Corp. www.restek.com [14 July 2011].

UNITED STATES PHARMACOPOEIAL CONVENTION INC. 2007. The United States Pharmacopoeia 30th Revision and National Formulary 25th Edition [CD-ROM]. USPCI: Rockville, MD. Chapter <467>.