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Gas Chromatography (GC)!Environmental Organic Chemistry
CEE-PUBH 5730-6730 Analysis Topic 5
Chromatography!
Group of separation techniques based on partitioning (mobile phase/stationary phase).
Two immiscible phases are brought into contact, one stationary and one mobile.
The sample is introduced into the mobile phase, partitions between the stationary & mobile phase, as it is carried by the mobile phase.
The component with the least interaction with the stationary phase elutes first.
Mobile phase (gas or liquid)
Stationary phase (liquid or solid)
Solid support
Mobile phase (gas or liquid)
Stationary phase (liquid or solid)
Solid support
Conc. of solute in stationary phase
Conc of solute in mobile phase KD =
Partition Coefficients in Chromatography!
Partition Coefficients in Chromatography!
The larger the value of the partition coefficient for a sample component the higher the solubility and the longer the retention of the component in the stationary phase.
Types of Chromatography! Partitioning
Liquid-liquid chromatography (partitioning)-High Performance Liquid Chromatography (HPLC)
Gas-liquid chromatography (partitioning)-Gas chromatography (GLC or GC)
Adsorption Liquid-solid chromatography (adsorption)-silica gel,
alumina, florisil cleanup methods Size exclusion chromatography or Gel permeation
chromatography (GPC) Ion exchange
Partition Coefficients in Gas Chromatography!
Temperature dependence higher temperatures increases
concentration in gaseous phase. Temperature is the separation variable
most often changed for a particular column.
Gas Chromatography (GC)!
GC is limited to compounds which can be volatilized either directly or after derivatization (only about 5 to 10% of all organic material in natural systems).
Derivatization: reaction that converts polar or ionizable functional group to non-ionizable or less polar functional group (e.g. acid to ester, phenol to anisole)
Derivatization example!
R C OH CH3OH H2SO4
O
R C O CH3
O
CH2 O C R
CH O C R
CH2 O C R
O
O
O
CH3OH
O
R C O CH3
CH3ONa
Fatty Acids Methylester
Reflux
+ 3
Volatile in Gas Chromatography
Volatile in Gas Chromatography
+ +
Retention time (RT)!
The amount of time that elapsed from injection of the sample to the recording of the peak maximum.
Adjusted retention time!
The solute retention time minus the retention time for an unretained peak, expressed as: tr' =tr - tm
where tm is the time necessary for the carrier gas to travel from the point of injection to the detector
Retention time!
Chromatographic separation!
Gas chromatogram schematic!
Carrier gas Inlet system Column Oven Detector Data Acquisition
Mobile phase/Carrier gas! N2 and He most common, Ar and H2 also used Sources of gases
Commercial cylinders, in-lab generators (N2, H2 & air) Purifiers: Used to remove H20, O2, oil vapors Pressure and/or Flow controls Pressure-measuring devices
Inlet/injection systems!
The inlet system is designed to receive sample, vaporize it (if necessary), and deliver it to column.
Type of inlet system used depends on column type and method of sample introduction.
Injection port!
Gas samples!
Gas sampling valves or direct injection (small vol.) Position A: sample loop filled Position B, the sample swept into the column.
Headspace
Headspace SPME
Liquid samples!
Sampling of liquid mixtures usually done by injecting sample through a self-sealing silicone septum with a microliter syringe.
Sample is introduced into a flash vaporizer or directly onto the head of the column (on-column).
Split/splitless injection!
Split reduces volume of sample-use for high concentration samples.
Splitless used for trace analysis.
Columns!
Types of columns Packed (not used much anymore) Capillary
Band broadening!
Theoretical plate number (n) !
Defines the efficiency of the column or sharpness of peaks.
Plate theory assumes that the column is divided into a number of zones called theoretical plates. Theoretical plate: distance along column necessary
for equilibrium between mobile and stationary phase. n = 16 (peak retention time/peak width) 2
Height equivalent to a theoretical plate (HETP)!
HETP = height equivalent to a theoretical plate (h) obtained by dividing the column length by the theoretical plate number: h = L/n = HETP
Packed vs capillary!
Packed vs capillary!
Stationary phase! General criteria: Non-volatile, chemically
inert, thermally stable, chemical bonded to solid support
Choice of stationary phase Standard methods generally specify a particular
stationary phase. Match polarity of the stationary phase to that of
the sample components of interest (like dissolves like).
Oven! General criteria: Free from influence of
changing ambient temperature (maintain temperature within ±1ºC), uniform and rapid circulation of heated air within the oven, and rapid cooling when desired isothermal - constant temperature temperature programming
Isothermal vs. temperature programming!
Factors important in choosing a detection system! Sensitivity: response per unit concentration of analyte Stability: extent to which the output signal remains
constant with time, given a constant input. Linearity: extent of the range over which the signal is
truly proportional to the concentration of amount of analyte.
Universality: detector's ability to detect all components in a mixture.
Selectivity: opposite of universality. Ease of use/Cost
Types of detectors!
Thermal conductivity detector (TCD) Flame ionization detector (FID) Nitrogen-phosphorous detector (NPD) Electron capture detector (ECD) Electrolytic Conductivity (Hall0 detector Nitrogen-phosphorous detector (NPD) Photoionization detector (PID)
Thermal conductivity detector (TCD)!
Earliest successful detector-good general purpose detector.
Bulk property detector sensitive to overall property of the effluent. Responds to all types of inorganic and organic compounds.
Non-destructive. Used for general analysis of organic liquids and often
used for permanent gas analysis.
TCD!
Flame ionization detector (FID)! One of most popular detectors because of its
high sensitivity, wide linear range, and great reliability.
FID responds only to substances that produce charged ions when burned in a hydrogen/air flame. For organic compounds, the response is proportional
to the number or oxidizable carbons atoms. Insensitive to most inorganic compounds, water
and permanent gases.
FID!
Nitrogen-phosphorous detector (NPD) !
Also referred to as an Alkali flame ionization detector.
This detector is selective for monitoring nitrogen or phosphorous (fifty times more sensitive for nitrogen and 500 times more sensitive for phosphorus than FID).
NPD!
Electron capture detector (ECD)!• Detector (63Ni) ionizes the carrier gas (usually argon)
and collects the free electrons produced. An electron-capturing solute will capture the electrons and cause a decrease in the detector current.
• Selective, highly sensitive to halogenated compounds, anhydrides, peroxides, conjugated carbonyls, nitriles and nitrates, organometallics, and sulfur-containing compounds.
• Insensitive to hydrocarbons. • Small linear range.
ECD!
Electrolytic Conductivity (Hall detector)!
Operates in halogen, sulfur and nitrogen modes In halogen mode, a furnace pyrolytically
reduces halocarbons to HCl in the presence of hydrogen gas and nickel catalyst. The HCl is then swept into a cell where a change in conductivity is measured.
Photoionization detector (PID) ! This is a detector in which detector photons of
suitable energy cause complete ionization of solutes in the inert mobile phase.
UV radiation is the most common source of these photons. Ionization of the solute produces an increase in current from the detector, and this is amplified and passed onto the recorder.
Non-destructive, often used in line with other detectors. Highly sensitive to aromatic compounds.
PID!
R + hν = R+ + e-
Olfactory detector"
Range and response of GC detectors!
GC/MS!
Identification of Compounds/Qualitative analysis!
Compare retention time (RT) of sample with that of standards. Analysis conditions must be same for both standard and sample.
RT of sample peak = RT of standard compound A unknown might be compound A.
If RT of sample peak is not = to RT of standard compound A, unknown peak definitely not compound A.
Multiple Columns (confirmation column)!
The use of two or more columns improves the probability that the identity of an unknown compound is the same as that of a compound with identical retention times
Relative Detector Response!
Comparison of relative detector response from two or more detectors can aid in the identification of an unknown component.
Usually the component is chromatographed on one column and the effluent split and fed to two or more detectors.
Gas Chromatograph/Mass Spectrometer (GC/MS)!
Structural information which provides means for identification
Quantitative Analysis by GC!
The size of a chromatographic peak is proportional to the amount of material contributing to that peak.
Response of detector proportional to mass of compound.
Measure the size of the peak area
Standardization! Introduce known amounts of the compounds to be
analyzed and measuring their peak areas. The quantity of the unknown is determined by
relating the size of the unknown peak to the size of the known peak.
External standard method! For each analyte (compound of interest), calibration
standards are prepared at a minimum of five concentration levels. The concentrations should correspond to the expected range of concentrations found in real samples or should define the linear working range of the detector.
Inject each calibration standard using the technique that will be used to introduce the actual samples into the gas chromatograph.
Plot peak area vs mass injected.
Internal standard method!
In the internal standard method, the detector response given by the analyte (compound of interest) is compared with that given by another compound of known concentration (the internal standard) which is also present in the sample when the analysis.
Internal standard procedure! Select one or more internal standards that are similar in
analytical behavior to the compounds of interest. Prepare calibration standards at a minimum of five
concentration levels for each analyte of interest. To each calibration standard, add a known amount of
one or more internal standards. Inject each calibration standard using the technique
that will be used to introduce the actual samples into the gas chromatograph.
Internal standard Procedure (cont.)! Tabulate the peak height or area responses against the
concentration of each compound and internal standard. Calculate the response factors for each compound as follows:
RF = (As C is)/(A is Cs) – where: – As = Response for the analyte to be measured – A is = Response for the internal standard – C is = Concentration of the internal standard, ug/L – Cs = Concentration of the analyte to be measured, ug/L
Use average RF or calibration curve of response ratios, As/A is versus RF.
