Basic Gas Chromatography. Prepared by: Mina S. Buenafe. Gas Chromatography Chromatography – A Very Brief History Definitions / Terminologies in GC Instrumentation Overview System Modules Mobile Phase (Carrier Gas) Inlets Stationary Phase(s) Columns (Packed and Capillary) Detector(s)
Prepared by: Mina S. Buenafe
IN THE EARLY 1900’S
M. Tswett published his work on separation of plant pigments. He coined the term chromatography (literally translated as color writing) and scientifically described the process – earning him the title “Father of Chromatography”
W. Ramsey published his work on separation of mixture of gases and vapors on adsorbents like charcoal.
IN THE EARLY 1940’s
A. Martin and R. Synge first suggested the possibilities of gas chromatography in a paper published in Biochem. J., v.35, 1358, (1941). Martin won a Nobel Prize for his work in Partition chromatography.
IN THE EARLY 1950’s
A. Martin and A. James published the epic paper describing the first gas chromatograph
A physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary while the other moves in a definite direction
“Official” IUPAC definition
It is the output signal from the detector of the instrument.
Distribution Constant (KC)
It is the tendency of a given component to be attracted to the stationary phase. This can be expressed in chemical terms as an equilibrium constant. Also called the partition coefficient (KP) or the distribution coefficient (KD)
KC = [A]S/[A]M
Mathematically, it is defined as the concentration of solute A in the stationary phase divided by its concentration in the mobile phase.
The attraction to the stationary phase can also be classified according to the type of sorption by the solute.
Adsorption: sorption on the surface of the stationary phase
Absorption: sorption into the bulk of the stationary phase (usually called ‘partition’ by chromatographers)
Retention Volume (VR)
It is usually defined as the distance between the point of injection to the peak maximum. It is the volume of the carrier gas necessary to elute the solute of interest.
Mathematically: VR = FC x tR
Where FC is the constant flow rate
tR is the retention time
Phase Ratio (b)
For packed columns:
Retention Factor (k)
It is the ratio of the amount of the solute (NOT concentration) in the stationary phase to the amount in the mobile phase. It is also called capacity factor (k’), capacity ratio, or partition ratio
k = (WA)S/(WA)M = KC/b
Also k = (tR - t0) = time in stationary phase
t0 time in carrier gas
k is temperature and flow dependent. Best separations occur when k is between 5 and 7
Theoretical Plates (N)
This is the most common measure of column efficiency in chromatography
N = 16(tR/Wb)2 = 5.54(tR/ Wh)2
Where Wb is thepeak width at the base
Wh is the peak width at half-height
Height Equivalent to a Theoretical Plate (H)
This is a related parameter that also defines column efficiency. Also identified as HETP
H = L/N
Where L is the Column Length
(An efficient column will have a large N and a small H)
Separation Factor (a)
It is a measure of relative distribution constants. Also known as selectivity and/or solvent efficiency.
Mathematically: a = k2/k1 = (KC)2/(KC)1
It is dependent on:
It is the degree to which adjacent peaks are separated.
Rs = (tR)B – (tR)A
[(Wb)B + (Wb)A]/2
Also Rs =—L/H x k/(k+1) x a-1/a
Schematic of a Typical GasChromatograph
Main purpose: carries the sample through the column
Secondary purpose: provides a suitable matrix for the detector to measure the sample component.
Carrier gases should be of high purity (minimum of 99.995%).
Flow Measurements and Control:
Average linear flow velocity (ū) in OT columns:
ū = L/tm
where L is column length in cm
tm is the retention time of an unretained peak (e.g. methane) in sec
To convert linear flow velocity to flow rate (Fc) in mL/min:
Fc = ū x Pr2 x 60sec/min
Effect of mobile phase (carrier gas) density on column efficiency. Van Deemter plots for the 3 common carrier gases for a column of capacity factor k’ = 7.90. The low density gases (H2 & He) have optimum efficiency at slightly higher flow rates than N2. The much lower slopes of H2 and He curves allow them to be used at higher flow rates (compared to N2) with very little loss of separation efficiency.
Inlets are the points of sample introduction
Ideal Sample Inlets for Column Type:
The oldest, simplest, and easiest injection technique.
Cross section of a typical split injector
Advantages to Split Injection:
Samples have to be diluted in a volatile solvent and 1-5mL is injected in the heated injection port. Septum purge is essential in splitless injections.
Cross section of a typical splitless injector
Advantages to Splitless Injection:
Other Types of Inlets:
Sub-classification of GC Techniques
Gas Solid Chromatography (GSC)
Some of these solids have been coated on the inside walls of capillary columns and are called “Support Coated Open Tubular” or SCOT columns.
One major application of Packed Column GSC is in Gas Analysis.
Packed Columns also provide the flexibility of allowing mixed packings for special applications (e.g. 5% Fluorcol on Carbopack B® for analysis of Freons)
Gas Liquid Chromatography (GLC)
To use liquid as stationary phase,techniques were applied to hold the liquid in a column.
Schematic representation of (a) packed column and (b) capillary column
Gas Liquid Chromatography (GLC)
Requirements for the stationary liquid phase:
Gas LiquidChromatography (GLC)
Types of Capillary Columns (OT)
WCOT: Wall-coated open tubular column (provides the highest resolution of all OT’s – i.d.’s range from 0.1mm to 0.53mm and film thickness from 0.1 – 5.0m)
PLOT: Porous layer open tubular column (less than 5% of all GC use these days)
SCOT: Surface-coated open tubular column (no longer available in fused silica)
The part of the system that ‘senses’ the effluents from the column and provides a record of the analysis in the form of a chromatogram. The signals are proportional to the quantity of each analyte.
FID: Flame Ionization Detector
The most common GC detector used.
The column effluent is burned in a small oxy-hydrogen flame producing some ions in the process. These ions are collected and form a small current that becomes the signals. When no sample is being burned, there should be little ionization, the small current is produced from impurities from the from the hydrogen and air supplies.
Hydrogen flow rate is commonly set to 40 – 45mL/min, Air, 350- 450mL/min, and for OT columns (with flows of about 1 mL/min), Make-Up gases is added to carrier gas (to make up the flow to 30mL/min)
TCD: Thermal Conductivity Detector
This is a differential detector that measures the thermal conductivity of the analyte in the carrier gas compared to the thermal conductivity of the pure gas. At least two cavities are required. These cavities are drilled into a metal block and each contain a hot wire or filament. The filaments are incorporated into a Wheatstone Bridge Circuit (for resistance measurements).
The choice of carrier gas will depend on the thermal conductivity of the analyte (H2 and He have highest TC’s, N2 gives rise to unusual peak shapes)
NPD: Nitrogen Phosphorus Detector
A bead of Rb or Cs is electrically heated when flame ionization occurs. The detector shows enhanced detectability for nitrogen-, phosphorus-, or halogen- containing samples.
MSD: Mass Spectrometric Detector
Analyte molecules are first ionized in order to be attracted or repelled by the proper magnetic or electrical fields.
Common GC Problems:
Retention Time Problems
Decrease in separation
Loss of Resolution
Increase in peak width
Loss of Resolution
Excessive Column Bleed