GAS CHROMATOGRAPHY (GC)

Gas chromatography (GC) also sometimes known as vapor-phase chromatography (VPC), or gas–liquid partition chromatography (GLPC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. It is a term used to describe the group of analytical separation techniques used to analyze volatile substances in the gas phase.

Typical uses of GC include:

• Testing the purity of a particular substance
• Separating the different components of a mixture.
• It can also be used to prepare pure compounds from a mixture.

In GC, components of a sample are dissolved in a solvent and vaporized in order to separate the analytes by distributing the samples between a stationary phase and a mobile phase.

• Mobile phase: This is where a chemically inert gas or an unreactive gas such as helium, argon, nitrogen or hydrogen serves to carry the molecules of the analyte through the heated column. GC is one of the sole forms of chromatography that does not utilize the mobile phase for interacting with the analyte.
• Stationary phase: This is a microscopic layer of viscous liquid on a surface of solid particles on an inert solid support inside a piece of glass or metal tubing called a column. The stationary phase is either a solid adsorbent, termed gas-solid chromatography (GSC), or a liquid on an inert support, termed gas-liquid chromatography (GLC)

Advantages of using GC

1. Short Analysis Time
2. Wide Choice of Stationary Phase
3. Wide Choice of Detectors
4. Ease of Operation
5. High sensitivity
6. Good separation efficiency
7. Suitable for trace analysis
8. Both qualitative and quantitative analysis possible

Limitations of GC

1. Mainly suitable for volatile and thermally stable compounds
2. Some samples require preparation or derivatization
3. Non-volatile compounds are difficult to analyze directly

Types of GC

• Gas-solid chromatography (GSC): It based upon a stationary phase on which retention of analysis consequence of physical adsorption
• Gas-liquid chromatography (GLC): Is useful for separating ions or molecules that are dissolved at absolvent.

Main Components of a GC

Source: MSc. Yassen .H.jassim & MSc. Elham Faisa - Analytical Chemistry Lecture 6

i) Carrier gas reservoir

Inert gases like argon, helium, nitrogen may be used as a carrier gas. Hydrogen gas is less preferred because of it poses explosion hazards. Selection of carrier gas depends on the nature of the mixture to be separated, purity required and detector used for the analysis.

The main purpose of the gas in GC is to move the solutes along the column thus mobile phase is often referred to as carrier gas.

Carrier gas should be:

• Inert, Free from fire and explosion hazard
• Suitable for detector
• Easily available
• Have good flow rate

ii) Injector

Liquid sample is injected by means of a calibrated micro syringe and is injected through a rubber septum at the head of the column.

If the sample is gaseous then 1 to 10ml is injected while for liquid sample 0.1 to 10 micro liter is injected.

At the temperature of the injection port liquid sample is readily converted to vapors without decomposition.

iii) Column

This is the backbone of chromatography. Column is made up of stainless steel or glass and is 2 to 3 meter long and has an internal diameter 2 to 4 mm.

Types of columns

Packed column: It is made up of Teflon having internal diameter 2 to 4 mm and length 5 meter. Column is packed with finely divided solid as absorbent in gas solid chromatography.

Capillary column: These columns are 15 meters to 100 meter long and have internal diameter less than 1 mm (i.e., 0.25 to 0.30 mm). This column does not contain packing but contain stationary phase coated on their inner wall.

iv) Detectors

The most commonly used detectors in a GC machine are:

Flame Ionization Detector (FID): This detector has high sensitivity and selectivity for carbon containing compounds. FID has the following characteristics:

• This detector is 1000 times more sensitive than Thermal conductivity detector (TCD)
• It can detect component at ppb (parts per billion) level
• Fast sensitive and give response to almost all organic compound
• Not sensitive to inorganic compound
• It is widely used detector of Gas chromatography

Thermal Conductivity Detector (TCD): It works under the principles of:

• As the composition of gas changes the thermal conductivity also changes.
• Resistance of wire is the measure of it’s temperature.

Characteristics of TCD include:

• Simple and accurate
• Response is reproducible
• Give response to both organic and inorganic species
• Nondestructive (i.e., effluent can be collected and reused)

Electron Capture Detector (ECD): Detector consists of metal box acting as cathode (negative electrode). Inside this box there is beta (β) emitting source (i.e., ³H or ⁶³Ni). Collector electrode act as anode (Positive electrode).

Characteristics of ECD include:

• Very good detector for electronegative elements.
• Nitrogen gas can be used as carrier gas.
• Give very little response to electropositive elements.
• Can be used up to 350°C
• Less electronegative compounds can be detected by preparing their derivatives.

Suitable and good detector must have following properties:

• Good sensitivity
• Stability
• Selectivity
• Linearity
• Easy to use

v) Software / Data system

The software being used analyzes and displays a chromatograph for analysis and interpretation.

How a Gas Chromatography Works

Step 1: Sample Injection

A small amount of sample is injected into the system. If sample is solid, it is converted to liquid form by dissolving in suitable solvent.

Step 2: Vaporization

The sample is heated and converted into vapor inside the injector.

Step 3:  Carrier Gas Transport

An inert carrier gas (commonly helium, nitrogen, or hydrogen) carries the vaporized sample through the column.

Step 4: Separation inside the Column

Inside the column, compounds separate because they interact differently with the stationary phase based on:

• Volatility (boiling point)
• Polarity
• Molecular interactions

Step 5: Detection

Separated compounds reach a detector (such as FID, TCD, ECD, or MS) at different times.

Step 6: Chromatogram Generation

The detector response is converted into peaks called a chromatogram. Each peak corresponds to a compound, and the retention time helps identify it.

Chromatogram

This is a plot of the detector response against time. The number of peaks represents the number of components presents in the sample (mixture).

Separation of component is based on their partition coefficient. The separated component exit along with the mobile phase at the end of the column. These components are then passed through the detector and detector give response to read out device. Magnitude of the response depends upon the concentration of the component.

Example: 

Mixture of four components is analyzed by gas chromatography. Peak area corresponding to component A, B, C, D is 30cm², 15cm², 20cm², 25cm² respectively. Calculate % of each component.

Solution

Total peak area = 30 +15 +20 +25 = 90cm²

% of component A = (Peak area / Total area) x 100 = (30/90) x 100 = 33.33%

% of component B = (15 / 90) x 100 = 16.66%

% of component C = (20 / 90) x 100 = 22.22%

% of component D = (25 / 90) x 100 = 27.77%

Retention time (t_R)

This is the time taken by the sample to come out from the column after it’s injection

t_R = t₂ - t₁

t₂ = time of elution

t₁ = time of injection

Common Applications

• Solvent analysis
• Petrochemicals
• Food flavor compounds
• Environmental pollutants
• Pharmaceutical impurities
• Textile auxiliaries and finishing chemicals
• Forensic investigations

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Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) is an extremely useful technique used to obtain high-resolution images and detailed information of a samples’ surface. The typical resolution of SEM instruments ranges from < 1 nanometer to several nanometers. SEM uses a focused beam of electrons to examine the surface of a sample and analyzes the different signals emitted in a detector.

The following is an illustration of how an SEM instrument works:

Image Source: https://www.technologynetworks.com/analysis/articles/sem-vs-tem-331262

1) Electron generation. 

A high-voltage electron gun (typically 1 keV – 30 keV) generates and accelerates a beam of electrons, typically using thermionic or field emission.

• Thermionic emission involves heating a thin tungsten filament to a high temperature (approximately 2,500°C), causing electrons to be emitted. 
• Field emission is an advanced technology used to capture the microstructure image of a material. It is usually performed in a high vacuum because gas molecules tend to disturb the electron beam and the emitted secondary and backscattered electrons used for imaging. Field emission produce electron beams with the highest brightness and coherence which allows for the highest resolutions to be obtained.

2) Electron beam focusing

The electron beam generated is then focused on the sample using a series of electromagnetic lenses. These lenses are similar to optical lenses, but instead of glass lenses, they use magnetic fields to bend the electron beam.

Condenser lenses are used for this and are carefully aligned to ensure that the beam is focused to a small spot on the sample surface.

3) Scanning

The focused electron beam is scanned across the sample surface in a raster pattern, similar to how a television screen is scanned. 

This is achieved by using scanning coils that deflect the beam in the x and y directions.

4) Interaction of electron beam and the sample

As the electron beam interacts with the sample, it results in a variety of signals being emitted. These signals include:

• Secondary electrons: Low-energy electrons that are emitted from the sample as a result of the primary electron beam striking the surface.

• Backscattered electrons: High-energy electrons that are reflected back from the sample. These are high-energy electrons used to obtain high-resolution images that show the distribution of various elements that make up a sample.

• Characteristic X-rays: X-rays that are emitted when the primary electrons knock out inner-shell electrons from the atoms in the sample. The energy difference between two electron shells is equal to the characteristic X-ray energy. Characteristic X-rays that result from inelastic scattering allow for exact identification of elements present in the sample.

• Auger electrons: These are emitted at discrete energies that are characteristic of the elements present on the sample surface.  The characteristic energies of the Auger electrons are such that only the electrons from the outer 0.5 to 5 nm of the sample can escape and be detected.

5) Detection of emitted signal

The emitted signals are detected using various detectors, such as:

• Secondary electron detector: Detects secondary electrons, which are used to create an image of the sample's surface topography.

• Backscattered electron detector: Detects elastically scattered electrons, which can provide information about the sample's composition.

• X-ray detector: Detects characteristic X-rays, which can be used to determine the elemental composition of the sample.

• Auger electron spectrometer: Detects Auger electrons, which can be used to analyze the surface composition of the sample.

6) Formation of image

The detected signals are processed by a computer to create an image. The brightness of each pixel in the image corresponds to the intensity of the detected signal at that point on the sample. 

SEM images can be displayed in various modes, such as bright field, dark field, and elemental mapping.

7) Analysis of image

The SEM images can be analyzed to deduce the quantitative information about the sample, like size, shape, and distribution of features. 

This can be done using image analysis software or by manually measuring features on the image.

Application of SEM

a) Materials Science

• Microstructure analysis: SEM is widely used to study the microstructure of materials like metals, ceramics and polymers. By examining the arrangement of grains, defects, and inclusions, researchers can gain insights into the material's properties and behavior. 
• Failure analysis: When a material or component fails, SEM can be used to investigate the root cause of the failure. By examining the fracture surface, researchers can identify the type of fracture (e.g., brittle, ductile, fatigue) and identify potential defects or flaws that may have contributed to the failure.

b) Biology and Medicine

• Cell imaging: SEM can provide high-resolution images of cells and their organelles, allowing researchers to study their structure, function, and interactions. For example, SEM can be used to examine the surface morphology of bacteria, the structure of viruses, and the ultrastructure of tissues.
• Tissue analysis: SEM can be used to analyze the structure and composition of tissues, such as skin, bone, and muscle. This can be helpful for diagnosing diseases, studying the effects of treatments, and understanding biological processes.

c) Nanotechnology

• Characterization of nanomaterials: SEM is a valuable tool for characterizing nanomaterials, such as nanoparticles, nanotubes, and nanowires. It can be used to measure the size, shape, and distribution of nanomaterials, as well as to study their surface morphology and internal structure.

d) Other Fields

• Forensic science: SEM can be used for various forensic applications, such as analyzing fingerprints, examining crime scene evidence, and identifying fibers or hairs.

• Semiconductor manufacturing: SEM is essential for quality control in the semiconductor industry, as it can be used to inspect the surface features of wafers and devices.

• Environmental science: SEM can be used to study the morphology and distribution of pollutants in the environment, such as particulate matter and microplastics.

Advantages of SEM

• High Resolution: SEM can provide images with extremely high resolution, allowing for detailed examination of even the smallest structures.

• Depth of Field: SEM has a large depth of field thus it can focus on a wide range of depths simultaneously, making it ideal for imaging rough or uneven surfaces.

• Three-Dimensional Imaging: SEM can create 3D images of samples, providing valuable insights into their structure and morphology.

• Versatile Applications: SEM can be used to study a wide range of materials and samples, from metals and ceramics to biological tissues and nanomaterials.

• Quantitative Analysis: SEM can be used to obtain quantitative information about samples, such as the size, shape, and distribution of features.

Disadvantages of SEM

• Sample Preparation: SEM requires careful sample preparation, often involving coating the sample with a conductive material to prevent charging. This can be time-consuming, expensive and may introduce artifacts.
• Vacuum Environment: SEM operates in a vacuum environment, which can limit the types of samples that can be examined. Some samples may degrade or be altered under vacuum conditions.
• Cost: SEM equipment can be expensive to purchase and maintain.
• Image Interpretation: Interpreting SEM images can be challenging, especially for complex samples or when dealing with unfamiliar materials.
• Limited Penetration Depth: SEM is primarily a surface imaging technique, with limited penetration depth. This can be a limitation for studying the internal structure of thick or opaque samples.

FURTHER READING

• Goldstein, D., & Newbury, D. (2012). Scanning electron microscopy: A beginner's guide. Springer Science & Business Media.
• Scott, J. F., & Joy, D. N. (2017). Scanning electron microscopy: Techniques and applications. Springer Science & Business Media.
• Scott, J. F., & Joy, D. N. (2019). Practical scanning electron microscopy. Springer Science & Business Media.