Download The foundations of gas chromatography

Gas Chromatography
Gas chromatography is a type of chromatography (a class of laboratory
techniques that separates substances into individual chemical
components) used in analytical chemistry to analyze chemicals that can
be vaporized without decomposition. Gas chromatography (GC) is
also known as vapor-phase chromatography (VPC) and gas-liquid
partition chromatography (GLPC). GLPC is the the most correct term
[5].
The History of Chromatography.
The foundations of gas chromatography technology began in the
beginning of the 20th century when Mikhail Semenovic Tsvett
developed the first chromatography method as a technique to separate
plant pigments. Later in the 1940’s liquid-liquid and solid-state gas
chromatography methods were devised. The inventor of liquid-liquid
chromatography- Archer John Porter Martin, developed gas-liquid
chromatography in the year 1950 [1].
Applications
The GC is most often used to test the purity of a particular substance
and to separate two liquids and quantify their relative amounts. The
GC is useful in identifying certain compounds and to prepare pure
compounds. Creating pure compounds is known as preparative
chromatography [1].
GC In the Industry
GC is a standard instrument in analytical labs and is used extensively
in the following industries: Environmental, Industrial Hygiene,
petroleum, Biofuel, Chemical, Agricultural, Food and Beverage,
Cosmetic/Cleaning Product, Pharmaceutical, Clinical, Forensic and
Life Science [3].
Perhaps the most notable use of GC is in forensic science. The GC is
used to identify substances in crime scene samples. It can be used to
accurately test for drugs and poisons concentrations found in body
fluids. And GC plays a vital role in Arson investigations. Arson is
often committed using mixtures of hundreds of different compounds.
GC-DMS is differential mobility spectrometry is specifically designed
for on-site analysis of flammable liquids is low-cost and yields accurate
results [4].
How it is used
Retention Time
Retention time is the period between sample injection and it emerging from
the column. If the conditions of the instrument (including the carrier gas flow
rate, temperature, and column set up) are consistent than an unknown
compound can be identified by comparing the results to a known pure
compound.
Pure samples of all suspected components must be run to calibrate the
column. The column must be calibrated each time it is used. It is also
possible to collect the gas after it passes through the column and identify it
through other techniques.
The analyst must interpret the chromatogram (the visual output from
chromatography) and compare the peaks to the known substances. [3] A
good graph will have well resolved peaks that are symmetrical. Too much
sample, a poorly chosen liquid phase, too high of a temperature or an
inadequately sized column can result in poor resolution and tailing (an
unsymmetrical peak) [5].
Chromatogram: Normal Resolution vs. Problem Resolution
Above is a chromatogram graph with good resolution and a chromatogram with
exhibiting tailing.
Source: http://www.chromacademy.com/chromatography-GC-analysis-problems.html
GC Compared to Other Types of Chromatography
Gas chromatography is like other forms of chromatography such as
column, TLC, HPLC the most notable difference is that GC is a liquid
and a gas. It is also unlike the other forms due to that the GC
concentration of a compound in the gas phase is a function of vapor
pressure. It is specifically different from column chromatography in
that the reaction in the GC is temperature controlled. [1] GC has
certain advantages when compared to other methods of
chromatography. The GC has a significant advantage when working
on a microscale because it can separate very small sample sizes, as small
as one to ten microliters. The GC also has a larger range of
compounds that can be separated. As long as the chemical is thermally
stable at its boiling point and has significant vapor pressure it can be
analyzed by GC. Compounds that can be analyzed by GC include
gases and organic compounds with boiling points over 400 degrees
Celsius [5].
The main components of the GC
Carrier gas
The carrier gas must pure (>= 99.995%) and chemically inert. Out of
the inert noble gases Helium and hydrogen are the most common
followed by Nitrogen and Argon. Helium is often used over hydrogen
because it poses less of a safety risk. The linear velocity of carrier gas
is an important factor since it affects the run time and the level of
separation. The higher the linear velocity the [2].
Stationary Phase
The stationary phase is a fixed substance that the solvents and analyte
travel and bind to. [3] The stationary phase is either a liquid, wax or a
low-melting solid. Many different materials are used including silicone
oils and polymeric esters. This material must have a low vapor
pressure and a high boiling point [5].
Detectors
The thermal conductivity detector (TCD) and the flame-ionization
detector (FID) are the two most frequently used detectors. The
detector is located at the end of the column and allow the analyst to
quantify the components of the sample as they emerge with the carrier
gas [2].
The TCD actually consist of two detectors. One is in contact with the
effluent gas and the other acts as a control exposed to only the pure
carrier gas. When the gas passing over the detectors is not equivalent
an electrical signal is sent. These signals are amplified and creates what
is called a chromatogram. The FID works by burning the emerging gas
and analyzing an amplified electrical signal from the resulting ion
fragments. This detector is more sensitive than TCD but since it
destroys the sample it cannot be used in preparative chromatography
[5].
An illustration of the main GC components
Above is a simplified outline of the main GC components.
Source: http://machinerylubrication.com/Read/352/gas-chromatography
Column
The column is usually made of stainless steel or copper. These
columns vary in size but are most commonly either 3mm or 6 mm in
diameter and 4-12 feet. The column is filled with the stationary phase,
in a specific process called a packing a column. This is a very intensive
process and the column must be filled as evenly as possible so most
analyst buy commercially packed columns.
An alternative to packed columns are open tubular columns also
known as capillary columns since they use capillary action. Most of
these columns are made of glass, but other materials have been used
[2]. These use similar liquid phases but are longer in length (usually 10
to 50 feet) and thinner in diameter (0.1-0.2 mm). The size of glass
columns results in an increase interaction between the sample and
stationary phase making these columns superior to packed columns
[5].
Oven
The oven in the GC encases the column. The temperature of the
reaction is controlled within a few tenths of a degree. The oven can
be controlled in two ways. First it can be set to hold a constant
temperature in the middle of the samples boiling range. This is known
as isothermal programming and work best with pure substances which
have a narrow boiling point range. The second method is temperature
programming method. In this method the oven starts at a low
temperature and then increases over a specified range.
If the sample has a wide boiling point range the second method- the
temperature programming method, must be used. The temperature in
the column is either increased in increments as the separation occurs
or it is increased at a continuous rate. The temperature programming
method is necessary for analyzing samples with a broad range of
boiling points because in isothermal programming either the low
boiling or the high boiling components will not be properly resolved.
The temperature affects the analysis similar to the linear velocities of
the carrier gas. At the low isothermal temperatures the low boiling
components will have broad instead of sharp peaks and at high
isothermal temperatures there will be no separation because the lower
boiling components will move too quickly to interact with the
stationary phase. The peaks at the higher end will be conjoined and
the lower peaks non-existent [2].
REFERENCES
1. Chromatography - New World Encyclopedia.
(2017). Newworldencyclopedia.org. Retrieved 19 April 2017,
from
http://www.newworldencyclopedia.org/entry/Chromatography
2. Gas Chromatography. (2017). Chemistry LibreTexts. Retrieved
22 April 2017, from
https://chem.libretexts.org/Core/Analytical_Chemistry/Instru
mental_Analysis/Chromatography/Gas_Chromatography
3. GC Applications - Applications | Sigma-Aldrich. (2017). SigmaAldrich. Retrieved 26 April 2017, from
http://www.sigmaaldrich.com/analyticalchromatography/analyticalproducts.html?TablePage=22607759
4. How is Gas Chromatography Used in Forensics?
Chromatography Today. (2017). Chromatographytoday.com.
Retrieved 19 April 2017, from
https://www.chromatographytoday.com/news/gcmdgc/32/breaking-news/how-is-gas-chromatography-used-inforensics/30185
5. Pavia, D. (2005). Introduction to organic laboratory
techniques (1st ed., pp. 837-856). Belmont, Calif. [u.a.]:
Thomson Brooks/Cole.