Question

Answer the questions below

1. Which method would you use for analysis of alkanes? Explain their principles and draw diagrams of instruments

2. List and describe all advantages of XRF and NAA techniques

Answer 1

analysis of alkanes is done by gas chromatography

THEORY

A Gas Chromatograph is used to detect the components based on the selective affinity of components towards the adsorbent materials. The sample is introduced in the liquid/gas form with the help of GC syringe into the injection port, it gets vaporized at injection port then passes through column with the help of continuously flowing carrier stream (mobile phase), mainly H2 (for TCD), and gets separated/detected at the detection port with suitable temperature programming. We visualize this on computer in the form of peaks. Carrier medium can be liquid (e.g. HPLC) or gas (e.g. GC) for the ease of separation/detection, if it is gas then called gas chromatography otherwise called liquid chromatography. Different chemical constituents of the sample travel through the column at different rates depending upon, 1. Physical properties 2. Chemical properties, and 3. Interaction with a specific column filling (stationary phase). As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, and the temperature. Physical Components involve inlet port, Adsorption column, detector port, flow controller (to control the flow of carrier gas), etc.  

Two types of columns are used in GC: Packed columns are 1.5 - 10 m in length and have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass and contains a packing of finely divided, inert, solid support material (eg. diatomaceous earth) that is coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed.

Capillary columns

have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters are common. The inner column walls are coated with the active materials (WCOT columns). Some columns are quasi solid filled with many parallel micro pores (PLOT columns). Most capillary columns are made of fused silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil. Temperature dependence of molecular adsorption and of the rate of progression along the column necessitates a careful control of the column temperature to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times.

The choice of carrier gas (mobile phase) is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used.

Detectors

A number of detectors are used in gas chromatography. The most common are the Flame ionization detector (FID) and the thermal conductivity detector (TCD). While TCDs are essentially universal and can be used to detect any component other than the carrier gas (as long as their thermal conductivities are different than that of the carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD. Both detectors are also quite robust. Since TCD is nondestructive, it can be operated in-series before an FID (destructive), thus providing complementary detection of the same eluents.

The primary components to a GC system

1. Carrier Gas System (including Gas Clean Filters) • The concept of theoretical plates and van Deemter curves • Selection of proper carrier gas

2. Sample Introduction System • Split & splitless injection

3. Column (most critical component) • Column configurations: packed vs. open tubular/capillary • Stationary phase 4. Detection System/GC Detectors • Types of detectors and their specific applications

5. Computer ChemStation/Integrator

Analysis of n-alkanes

1 Weigh out sample with internal standard

2 Saponify by heating with ethanolic KOH solution Add water & extract non-polar lipids into heptane

3 Purify hydrocarbon fraction with silica-gel columns

4 Analyse individual compounds by gas chromatography

advantages of XRF

X-ray fluorescence spectrometry is a relatively quick and effective way to measure major oxide and trace element abundances in powdered whole rock samples. The main principle behind XRF spectrometry is that X-rays of characteristic wavelength (and energy) are emitted from a sample when the sample is ionised by a stream of X-rays; this process is known as X-ray fluorescence.

Most elements between atomic number 11 and 92 (e.g. Na to U) are routinely measured. There are two main sample preparation methods used for analysis; light elements and major oxides are very susceptible to mineralogical (matrix) effects and must be fused into a glass disk (fused bead); heavier elements are more straight-forward and samples can be pressed into pellets.

Advantages

Relatively simple, cheap and quick analyses

Accurate analyses of a range of elements

"Dry" method and therefore requires minimal sample preparation (for trace element analysis).

Few consumables.

NAA

Neutron Activation Analysis (NAA) is one of the most sensitive analytical techniques used for multi-element analysis available today. The NAA procedure is capable of providing both quantitative and qualitative results for individual elements, with sensitivities that can be superior to those possible by any other analytical technique. Elemental Analysis Incorporated (EAI), as an innovator in the development and application of radio-nuclear chemistry analytical techniques, now offers its clients the ability to analyze some 75 individual elements (including certain organic elements) by NAA at trace and ultra-trace concentrations.

NAA is a physical technique that is based on nuclear reactions whereby the elemental content is determined by irradiating the subject sample with neutrons, creating radioactive forms of the desired element in the sample. As the sample becomes radioactive from the interaction of the neutron particle source and the nuclei of the element’s atoms, radioisotopes are formed that subsequently decay, emitting gamma rays unique in half-life and energy. These distinct energysignatures provide positive identification of the targeted element(s) present in the sample, while quantification is achieved by measuring the intensity of the emitted gamma rays that are directly proportionate to the concentration of the respective element(s) in the sample.

Advantages of NAA: Unlike traditional multi-element analysis techniques, pre-treatment of samples is normally not required. In most cases, the only requirement is that the sample may need to be reduced to a more suitable size for packaging and the irradiation process. Other advantages of NAA include:

• No sample digestion, extraction, volume loss, or dilution required.
• No potential for contamination due to handling or laboratory chemicals.
• One simple procedure can analyze 30+ elements simultaneously.
• High sample volume capabilities
• Cost effective & affordable for small and large sample quantities.
• Quick turnaround – standard results available within 5-10 working days.                                                                             o Faster turnarounds possible – 24 hrs to 3 days.
• Limited sample volume capability – gram, milligram and microgram in many cases.
• High precision – reproducibility of quality controls over long periods of time (years) is often better than 2% relative standard deviation (RSD).

• Primary analytical method used by the National Institute of Standards & Technology (NIST) to certify elemental concentrations in Standard Reference Materials (SRMs).

Applications for NAA: Neutron activation is an established analytical technique for determining trace elements in a wide variety of materials in solid, liquid, or gaseous states. Among the potential applications for which NAA has been utilized are:

• Polymers, resins, adhesives, and organic solids to measure for trace metals, trace catalyst residues, or to verify known inorganic elemental constituents.
• Pharmaceutical materials to measure for ultra trace-impurities, catalyst residues, and for determining methods for reducing or eliminating impurities from final products.
• Semiconductor, and high-purity analysis to measure ultra trace-element impurities and to verify procedures for reducing or elimination of impurities in final products.
• Fine chemicals, solvents, and organic liquids to determine the presence of trace catalyst residues, ultra trace-impurities, or to verify known inorganic constituents.
• Catalysts – such as automotive, oil refining, and commodity chemical catalysts to determine trace-impurities, and to verify known inorganic elemental constituents, particularly precious metals.
• Environmental applications and studies to characterize pollutants, determine their sources and methods of reduction.
• Development of performance standards for other analytical techniques.
• Forensic studies as a non-destructive method to analyze evidence as an aid to the investigation and prosecution of criminal cases.
• Archeological studies to fingerprint artifacts and to determine places of origin as a means to understand activities of plants, animals, and humans from our past.
• Nutritional & epidemiological studies to investigate the contribution of diet, occupation, and lifestyle on chronic diseases.