experiment 10
TRANSCRIPT
EXPERIMENT 10
GAS CHROMATOGRAPHYGroup 4: Lopez, Palmario, Sibug
What is gas chromatography?
Analytical technique for separating compounds based on their volatilities For
compounds that can be vaporised without decomposition
Compounds move through a GC column as gases, partition between a stationary phase and a mobile phase The
differential partitioning into the stationary phase allows the compounds to be separated in time and space
Uses of gas chromatography..
To test the purity of a particular substance To separate different components of a mixture and determine relative amounts of such components Can be used to prepare pure compounds from a mixture in preparative chromatography
Components of a gas chromatograph1.
2.3. 4. 5.
Gas supply Injector / column inlet Column Detector Data recorder
Gas Supply
Carrier gas must be inert Commonly used gases: N, He, Ar, CO2 Gas depends on detector type
Sample Injection Port = Injector
provides the means to introduce a sample into a continuous flow of carrier gas Types include: Split/Splitless
type- most common On-column inlet PTV injector Gas source inlet Purge-and-Trap system
Column
Houses the stationary phase composed of the high-boiling liquid General types: Packed-
inert solid material that is coated w/ liquid stationary phase Capillary- more efficient Wall-coated
open tubular (WCOT)- wall ang coated ng
SP Support-coated open tubular (SCOT)- more efficient
Thin layer of support material then diatomaceous earth where SP adsorbed
Detector
Different detectors give different types of selectivity non-selective
detector responds to all compounds except the carrier gas selective detector responds to a range of compounds with a common physical or chemical property specific detector responds to a single chemical compound
concentration dependant detectors vs mass flow dependant detectors
Data Recorder
Plots the signal from the detector over time the chromatogram Retention time - qualitatively indicative of the type of compound
Gas Chromatography
Theoretical Plate Model of Chromatography
Theoretical plates- analytes move down the column by transfer of equilibrated mobile phase Separate
layers in column Separate equilibrations of the sample between the stationary and mobile phase occur in these "plates
Plates serve as a way of measuring column efficiency by stating the number of theoretical plates in a column
Column Efficiency
Related to the number of compounds that can be separated by the column Measured by determining number of theoretical plates (N) and resolution (Rs)
Capacity Factor
Rate of migration of solutes in the column ratio of the mass of the compound in the stationary phase relative to the mass of the compound in the mobile phase
Factors Affecting GC Separation
Volatility of compound- more volatile mas una Polarity of compounds and column Column temperature- increase T -> faster movement -> lesser separation Flow rate of the gas through the columnhigh flow rate -> higher speed Length of the column- greater separation using longer column
Quantitative Analysis in GC
Quantitative Analysis Using GC
Peak area is directly proportional to the amount of respective component in sample BUT: response of a detector varies from one component to anotherPeak A and B have about the same peak areas BUT: actual concentration of A > B due to response factor Peak area alone does not indicate concentration of sampleB
A
Component B
Component A
The report of peak area % not indicative of concentration ratios, but more useful for
Calibration
Calibration involves correlation between known concentration of a component and resultant detector responseA = kCA = peak area, k=response factor, C = concentration
Single-level vs. Multi-level calibration
Standard Methods of Calibration
External Standard Calibration Mixtures
of the sample components at varying concentrations are analyzed and peak areas recorded
Plot
Area vs. Concentration to determine response factor k Requires very precise sample injection Prone to error due to varying injected sample
Standard Methods of Calibration
Internal Standard Calibration Errors
arising from varying injected sample volume (or any other variations) eliminated by use of internal standard Procedure: Internal
standard added to the standard mixtures prior to analysis Determine response factor (k) for each component by plotting area ratios vs. concentration ratios (or Cx only if Cint is constant for all standard mixtures):
Standard Methods of Calibration
The ideal internal standard: Should
give completely resolved peak but that which is close to components of the sampleseparate! Peak should be similar in magnitude to those of the components Should be chemically similar but not present in the original sample
Sample could then be analyzed by adding internal standard to the unknown
Determination of Peak Areas
Cut and Weigh Triangulation Integration
Peak Area Determination: Cut and Weigh Method
Cut-and-Weigh Method Area
under the peaks is cut and weigh together with a reference paper with known weight Economical Disadvantages incorrect
drawing of the base line, incorrect cutting of the peaks lack of homogeneity of the paper tedious
Peak Area Determination: Triangulation Method
A geometric method that approximates the peak area Assumes that the peaks are perfect Gaussian-curve shaped Errors can arise if peak shape is irregular
height (h)
width at h (w1/2)
Area = hw1/2
Peak Area Determination: Integration Method Automated
method less to human error measurements of relative peak areas are given by the electronic integrators or detectors Limitations distinguish
peaks from noise, correctly identifying the underlying baseline maintaining correct peak and baseline detection throughout a sequence of chromatograms correctly handling rider peaks and other unresolved peaks
Objectives-To successfully perform GC -To separate and quantify components of a mixture -To test different calibration method
Methodology
General Procedure for Quantitative GC Analysis1.
2.
3.
Determine the retention times for each component in the sample mixture Establish calibration curve using standards of each component Analyzed sample in GC and correlate peak areas obtained to concentration using the calibration curves for each component
Preparation of Standard SolutionsProcedures Rationale
Gas chromatograms of pure benzene, toluene, xylene and ethylbenzene in diethylether were obtained Standard solutions were prepared and GC were obtained
To know the Rt of each solvents To have standard calibration curves for the unknown analysis
Preparation of Standard SolutionsProcedures Areas under the peaks were determined by: -Triangulation -Cut and Weigh (not done) -Integration Plot Area vs. Concentration Rationale - for standard calibration curve - to test the different methods
Unknown Mixture AnalysisProcedures GC of unknown mixture of the used hydrocarbons Identify and quantify each component of the mixture Determine the capacity factor, resolution and average number of theoretical plates of the column
Results and Discussion
Determination of Retention Times
Determination of Retention TimesBUT WAIT!Whats this peak?? tR = 1.21 (same as benzene) Benzene contamination?! Possible implications to be discussed later
Order of ElutionCH3 CH3 CH3 CH3