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Food analysis using Gas chromatography

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Azzedine Dabo

on 5 March 2013

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Transcript of Food analysis using Gas chromatography

Koulia Dalmira and Azzedine Dabo Food Analysis using Gas chromatography Goals of this lecture
To make aware of importance of food analysis
To give an overview of Gas Chromatography
To explain the components of GC
To give an overview of flame ionization detector (FID)
To show you real life application Why Food and Drinks Analysis The truth is “We are what we eat”

The food we eat has to be incorporated, transformed and excreted from our bodies.
Food Analysis arises from nutrition and health concerns.

Questions we need to ask ourselves as scientist:
What is the natural composition of the food(s)?
What chemicals appear in food as an additive or by-product from intentional treatment, unintended exposure, or spoilage (and how much is there)?
What changes occur in the food from natural or human induced processes?

These question can be answered by performing analysis to determine:
The composition
Additives and contaminants
Transformation products Why Gas Chromatography (GC) Gas Chromatography has a number of advantages:
Precision: Provides a good separation
Time: Analysis is short
Sample: only l is required.
Detection: has a good sensitivity
Analysis: Data can be quantified.
Concentration: The area under the peak is proportional to concentration
Cost: The price of GC is relatively low
GC is a primary tool for the analysis of important compounds such as fatty acids, alcohols, steriols and oils.
This a great technique to measure the chemical contaminants and additives Sample Preparation Injection Method Column Mainly two types

Packed Columns
Made of fine particles of solid coated with non volatile liquid (stationary phase)
Properties: Lower resolution, Number of theoretical plates:10,000, length from 1-5m and diameters 3-6mm.

Capillary Columns
Made of thin fused silica capillary and has stationary phase coated on the inner surface
Properties: Higher resolution, Number of plates:200000, length 10-100m and diameter 50-200μm,higher sensitivity. Carrier Gas System GC Detectors Typical Gas Chromatography Detectors Flame Ionisation Detector (FID) Schematic Diagram of FID Isolation and Identification of Volatiles from Catawba Wine Isolation and Identification of Volatiles from Catawba Wine Isolation and Identification of Volatiles from Catawba Wine 19 compounds were identified in the white wine and 21 compounds in the other two rose wine extracts

methyl anthranilate and gamma-butyrolactone absent from white wine

Acetate esters: isoamyl acetate and 2-phenylethyl acetate are the most abundant in rose (TV)

In the final test for aroma differences, highly significant difference (99%) was found in each case- 3 model solutions were poor imitations of authentic wine Taking everything into account… GC is a powerful tool in analytical chemistry – versatile technique for separation, identification and determination of closely related chemical components from a mixture

Main components of a GC instrument: injection, carrier gas system, column and detector

By optimizing column dimensions and operating conditions, resolution of the analysis can be improved

Conclusions from Catawba Wine Analysis:

Major volatile components detected and identified in the three Catawba wines do account for their aromas

They also contribute little or nothing to Catawba character as influenced by enological technique

Model solutions easily distinguished from the authentic wine – poor imitations of authentic wine

Thus, the unidentified trace components are of critical importance to the aroma of Catawba wine References Please Feel Free to ask Any Questions Wine Preparation.

Catawba grapes divided into three 20-Kg lots for fermentation: a) white wine by pressing the crushed grapes b) rose wine by fermentation
[rose (FS)] c) rose wine by thermally vinified [rose (TV)]

Volatile Isolation:

Wine volatiles were isolated using solvent extraction with Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane)

Equal volumes of wine and Freon were stirred for 1h -> Freon phase drawn off, dried over anhydrous MgSO4 -> rotary evaporator with water bath at 20C The mobile-phase gas in GC is called the carrier gas.

It is typically required to flow through injector and push the gaseous components of the sample onto the GC column which then leads to the detector

Must be dry, free of oxygen and chemically inert

Helium is the most commonly used, although argon, nitrogen and hydrogen are also used

Flame Ionisation (FID), Thermal conductivity (TCD) and Electron Capture (ECD) detectors involve the use of both hydrogen and helium

Hydrogen and helium provide a shorter analysis time and lower elution temperatures of the sample due to higher flow rates and low molecular weight

All carrier gasses are available in pressurized tanks

A two-stage pressure regulator mounted onto both tank and chromatograph gas inlet is required to control flow rates

The carrier gas is preheated and filtered with a molecular sieve for removal of any impurities and water before being introduced to the vaporization chamber Sample is directed to an air-hydrogen flame (H2 + O2 + N2) after exiting the column where it undergoes pyrolysis or chemical decomposition

Ions and electrons which carry electric current are released and collected by a collector or electrode

A high-impedance picoammeter is used to measure this current to monitor the sample’s elution

Electric current of a gas is roughly proportional to the concentration of charged particles within the gas

FID responds to most organic/hydrocarbon containing compounds

Little or no response to air, water, inert gases, CO, CO2, CS2, NO, SO2 and H2S

High sensitivity and low noise

Mass-sensitive – unaffected by flow rate

Excellent detector for quantitative trace analysis with a wide linear range

However, it does require flammable gas and destroys the sample

Low detection limits – few picograms per second (10^-12 gs^-1) Catawba is a “created grape” developed in the 1800s: made out of cross of vitis vinifera along with vitis labrusca grapes

Catawba grapes can be used in the production of either white or rose wines

Amerine et. al. (1979) noted that the distinctive wine aroma was not due to methyl anthranilate but some other more important compounds are present

Aim: examine the volatile composition of Catawba wines prepared by three different enological techniques

Volatiles were isolated by solvent extraction, separated and quantified by gas chromatography and identified by combined gas chromatography-mass spectrometry

Wine Preparation. Catawba grapes divided into three 20-Kg lots for fermentation: a) white wine by pressing the crushed grapes b) rose wine by fermentation, rose (FS) c) rose wine by thermally vinified, rose (TV)
Volatile Isolation.

Using solvent extraction with Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane) Instrumental Analysis: combined Gas Chromatography-Mass Spectrometry using 3 macroL injections and spectra were taken at 70 eV
Varian Series 1400 gas chromatograph with 4 m x 2 mm i.d. glass column packed with 10% SP-1000 on 100-120 mesh Chromosorb W

Temperature program from 60 to 200C at 4C/min

Gas chromatograph was interfaced to a Time-of-Flight mass spectrometer equipped with CVC solid state electronics and with a computerised data collection system

Quantification of volatile components was done using Hewlet-Packard 5830 A gas chromatograph with a stainless steel column 4 m x 2 mm

Sensory Evaluation: sniffing device attached to the effluent port of a Packard Model 800 gas chromatograph

50 mL model wine solutions with Catawba odour: 12% v/v ethanol-1% w/w tartaric acid, purified Concord grape anthocyanin pigment and the identified volatiles in distilled water

Volatiles were added to white, rose (TV) and rose (FS) in a specific concentration

Each wine was judged against its corresponding model solution for aroma differences Any device located at the end of the column which provides quantitative information , i.e. concentration measurements, for the components of the mixture being eluted by generating an electric signal

The choice of detector is determined by the general class being analysed and by the sensitivity required

An ideal GC detector is distinguished by several characteristics:

adequate sensitivity – provides a high resolution signal for all components in the mixture

universal response – response for all sample components apart from carrier gas itself

selective response – response for certain types of compounds

rapid response – easy to operate

linear range – useful for quantitative analysis of both major and minor components in the same sample

high reliability and easy of use

predictable

good stability and reproducibility

non-destructive Steven J.Lehotay, Application of gas chromatography in food analysis, anal chem, vol 21, 9-10,2002
http://www.shsu.edu/~chm_tgc/primers/GC.html
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm
http://community.asdlib.org/analyticalimageandvideoexchange/2011/06/21/gc-columns/
Dr. Nikola Chmel, CH911 Chromatography, University of Warwick, 2012-2013.
Douglas A. Skoog, F. James Holler and Stanley R. Crouch, Principles of Instrumental Analysis, 6th ed.
Richard R. Nelson, Terry E. Acree and Robert M. Butts, Vol. 26, No. 5, 1978
http://www.sepscience.com/images//articles/gcsol/04/table-1.jpg
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