Eshwar Udho
CHE331
Dr. Rahni
Lab VI
May 7th 2003
Title: Gas Chromatography-Electron Capture Detector
Purpose: To Summarize the Concept of the Gas Chromatography-Electron Capture
Detector System.
Theory: Chromatography refers to the family of techniques used to separate complex
mixtures of often related compounds. This allows qualitative and quantitative analysis to
be performed on individual components of the mixture. Gas Chromatography is a welldeveloped technique particularly suited to the separation of organic compounds and is
frequently used in performing environmental analysis.
The primary components of a Gas Chromatograph system are, the injector, the
column in an oven, the detector and the data acquisition and readout mechanisms. In the
GC instrument, the sample is injected into a column via the injection port. Only
microliters of the sample are required. Such a small sample size is a great advantage to
any analysis technique. In the column, the sample is carried by an inert gas called the
mobile phase. This mobile phase transports the sample through the long, coiled column
packed with a material termed the stationary phase. Only the stationary phase interacts
chemically with the sample and it is the basis of this interaction that allows the separation
of the constituents of a mixture. The column can be packed with a viscous liquid or a
solid material which will adsorb the solutes. The former is called gas-liquid
chromatography, the latter is known as gas-solid chromatography.
The stationary phase is chosen so that the components of the sample differentially
interact with and hence distribute themselves between the mobile and stationary phases to
varying degrees. Those components that are strongly retained by the stationary phase
move slowly relative to the flow of the mobile phase. In contrast, components that have a
lower affinity for the stationary phase travel through the column at a faster rate. As
consequence of the differences in mobility, sample components separate into discrete
bands that can be analyzed qualitatively and quantitatively. These bands form peaks of
some kind on the data readout instrument.
Retention time, peak broadening and resolution are all determined by column
properties. As the sample is transported through the column by the carrier gas, the
components will dissolve to some extent into the stationary phase liquid that is present
throughout the length of the column. These solutes will maintain an equilibrium between
the stationary phase and the gas phase (carrier gas). This equilibrium is governed by
linear partitioning. Here, the ratio of the concentration of a solute in the stationary phase
(Cs) to the concentration in the mobile phase (Cm) is a constant, known as the stationary
phase partition coefficient, distribution constant or partition ratio; KS.
KS = CS/CM
If KS is constant over a wide range of solute concentrations, then Cs is directly
proportional to Cm. When this holds true, the chromatographic peaks are symmetrical.
Gaussian distributions and retention times are then independent of the amount of analyte
injected.
Retention time of an analyte is defined as the time it takes after sample injection
for the analyte to elute and reach the detector. The time for unretained species to reach
the detector is defined as the dead time. This "rate of migration" of an unretained species
is the same as the rate of motion of the mobile phase molecule. The linear rate (v) of a
solute molecule is defined as the column length divided by its retention time and is
mathematically expressed v = L/tR . This is analagous to the general speed formula where
L is the column length and tR is the retention time of the solute. The linear rate (u) of a
mobile phase molecule is calculated by dividing the column length by the dead time (tM).
The faster the solute is to move through the column, the more time it would have
to spend in the mobile phase. Therefore, The average rate at which a solute migrates
along a column, v, is directly proportional to the fraction of time that it spends in the
mobile phase. This is, as seen above, dependent on the partition coefficient.
v = u*x where x is the fraction of time spent in the mobile phase.
The fraction can also be expressed as the ratio of moles of solute in the mobile
phase to the total moles of solute. Total moles of solute in the mobile phase is equivalent
to the product of the concentration of solute and the volume of mobile phase. Likewise,
the total moles of solute is the sum of moles in the mobile and stationary phase. This is
expressed as
v = u x (CMVM) /{(CMVM)+(CSVS)} OR
v = u x /{1 +(CSVS)/CSV S}
Substituting the equilibrium constant for the ratio of solute concentrations results in
v = u x 1/{1+(KVS/VM)}
an equation expressing a solute migration rate as a function of its distribution constant.
Another way of describing migration rates of solutes on gas chromatographic
columns is the retention factor, also referred to as the capacity factor. Mathematically, the
retention factor is expressed as
k'=KVS/VM
By substituting this expression for capacity into the equation for the migration rate, a
relationship between column capacity and migration rates is established. The relationship
is mathematically expressed as
v = u x {1/(1+k')}
When the migration rate, for a solute and mobile phase molecule u are substituted with
values measured from a chromatogram, an equation to determine the retention factor can
be derived. The equation is
k'=(tR-tM)/TM
The ability of a column to retain one analyte more strongly than a second is a
function of the column's selectivity and the basis of separation. The column's selectivity
factor for two species, A and B, is defined as α(alpha) = KB/KA. By definition the more
strongly retained species is B and therefore alpha is always greater than one. Substitution
for the distribution constants yields an equation that allows the selectivity factor alpha to
be experimentally determined from a chromatogram. The equation is alpha = (tR)B TM/(tR)A TM
While movements of solutes through a gas chromatographic column are described
by distribution constants, retention times, retention (capacity) factors, and selectivity
factors, column efficiency (performance) is described by a quantitative measure labeled
theoretical plates. The number of theoretical plates (N) is calculated by dividing the
column length (L) by the height equivalent theoretical plate (H). Plate height is
experimentally calculated by dividing the variance of a Gaussian shaped chromatographic
peak divided by the column length. A separate equation that provides the number of
theoretical plates (N) is
N = 16(tR/W)2 where W is the peak width at half height.
A chromatographic column's ability to separate a mixture of compounds is
defined as its resolution. The mathematical equation for resolution is,
RS =Δ Z/(WA+WB)
Where ΔZ is the difference in time between two chromatographic peaks, WA is peak
width (in time units) of Compound A andWB is peak width (in time units) of Compound
B. Resolution can also be calculated using retention factor k' for two solutes, the
selectivity factor, and the number of theoretical plates. The following equation is used to
calculate resolution, or with simple rearrangement, to calculate the number of theoretical
plates required to achieve a desired resolution.
RS = N1/2/4{(-1)/}{k'B/1+k'B}
N = 16Rs2{/(-1)}2{(1+k'B)/k'}2
The previous discussions all assume Gaussian distributions of analytes as they
elute from the column. However, non-gaussian shaped peaks do occur and the peak
shapes provide information relative to chromatographic variables. Two frequent
phenomena related to non-gaussian peak shapes that occur are fronting and tailing. With
fronting, the front side of a peak is drawn out while the tail (or backside) on the right is
steep. The most frequent cause of fronting is too large of a sample introduced into the
column. In contract, the more frequent phenomenon of tailing results in the right side of a
peak (the tail) is drawn out. This usually occurs when the solute has c concentration
dependent (non-linear) distribution coefficient. This can also be a cause of fronting. The
result of fronting and tailing is poor separation (and thus resolution) and less accurate
quantitative analysis.
Capillary columns vary in length from 15 to 100 meters is a coiled configuration
to fit in the instrument oven. We are used a 30m tube with our GC/MS lab. Shortening
the length of the column can shorten the analysis time; however, resolution (separation)
will be compromised since the equilibration of each solute with the stationaryis now not
quite as long. Diameters of open tubular capillary columns are typically between 0.32 and
0.25 millimeter, with high resolution columns having diameters of 0.20 to 0.15
millimeter. The smaller diameter produces better resolution and greater selectivity, but
can handle only a small volume of sample (1 to 2 microliters).
The above theory is for separations by all gas chromatographs. However, the
analysis part is varied depending on the detector. When coupled to an electron capture
detector, we are usually looking for highly electronegative molecules are such as
halogens or nitrogen containing compounds. The ECD contains a small piece of
radioactive foil contining 63Ni, a beta emmitter (sometimes 3H is used). These highenergy electrons will ionize the carrier gas, and produce low-energy electrons and
positive ions. The secondary electrons are collected at the anode, which is typically
operated in a pulsed mode to avoid polarization and collection of large anionic fragments.
When an electrophilic constituent passes out of the column and into the ECD, it will pick
up some of these low-energy electrons and thereby reduced the anode current. The more
electronegative a compound is, the more sensitive the ECD is to it. The most strongly
recorded compounds are the halogenated organics, peroxides and nitro-organics. The
ECD shows almost no response to amines, alcohols and hydrocarbons. This detector is
exceptionally sensitive to halogenated pesticides and chlorination byproducts. It is also
highly selective, which means it will not be troubled by interference from nonhalogenated compounds. It will, however, respond to oxygen, so care must be taken to
avoid any oxygen contamination of the carrier flow.