Criteria for using resources of chemistry department at GMU: 

We all know that our chemistry department is enriched with various instruments which are frequently used by our undergrad and grad students. These instruments serve a great deal in our research projects. We have many kinds of spectrophotometers covering from atomic absorption to UV-Vis. We have High Performance liquid chromatograph and Gas Chromatograph including GC/Mass Spec. These instruments caught a great deal of attention of companies who do research on analytical chemistry as well as other educational institutes who do not have these resources. Even researchers and scientists within our university often use our instruments. Our department faculty and stuff have been dedicated to serve these people giving away their valuable time out of their busy schedule just to promote the field of research and development.

As a technology specialist of the chemistry department one of my responsibility is to take care of these instruments and keep them at their best condition. Usage of inappropriate solvent and solutions and improper handling of these instruments make them non-functional. Sometimes fixing these problems are cumbersome and time consuming. Sometimes we need the assistance of the manufacturer to fix these problems which costs the department a lot of money. Theses problems impair the researches of the department which is not pleasant either for the student or the faculty. My sole purpose of writing this paper is to educate the users of these instruments. Each instrument has its own criteria which have to be followed by each user. I urge all of you to read these instructions very carefully and prepare yourself accordingly before coming to perform the experiments. You will not be allowed to use our instruments if you fail to comply with the described rules and regulations.

General Instructions

All standards and test solutions have to be made before performing the experiments. Do not expect our department to give you supply of your desired standards or any other chemicals. It is your responsibility to buy the standards and all other chemicals ahead of time and prepare them before coming here. Having a thorough knowledge and visibility study on your own research project is expected. We may not have any knowledge about your research project and may not be able to help you. Our responsibility is to perform the experiment only.

Nature of the solutions

All solutions must be clear. Absolutely no suspension, emulsion or any kind of particles or sediments should be present in the solutions. Please filter the solutions using Millipore micro filters if you have any suspended particles. Try to dissolve the particles using acidic solutions if possible.

Atomic Absorption Spectrophotometer

Atomic absorption spectroscopy is a spectrophotometric technique based on the absorption of radiant energy by atoms. To carry out the atomic absorption process we need a monochromatic radiation source. A source called hollow cathode lamp is required to investigate each element. Thus, only one element is measured at a time. We have the lamps of the following elements available in our department.

Na

Fe

Cr

Co

Au

           

Ca

Mg

B

V

Mo

           

P

Cd

Ti

Mn

Pb

           

Cu

Si

Ba

Ni

 

           

Zn

Ta

Sn

As

 

           

The hollow cathode lamp looks like this:

If the lamp is not listed above for the element that you want to analyze you have to buy your own lamp. The contact information of the vendors who sell these lamps is listed below. You just have to make sure that it is compatible with Perkin Elmer 5100 AAS.

Perkin Elmer: 1-800-762-4000

Fisher Scientific: 1-800-766-7000

Figure1. Elements detectable by atomic absorption are highlighted in pink in this periodic table.

Range of concentration:

Elements can be determined by AA in concentrations ranging from 10 ppm to sub ppm level.

Detection limits of some elements by Atomic Absorption Spectroscopy:

Element

Detection Limit(ppm)

Aluminum

1

Antimony

0.5

Arsenic

5

Barium

8

Bismuth

1

Cadmium

0.03

Calcium

0.08

Chromium

0.05

Copper

0.1

Iron

0.1

Lead

0.3

Magnesium

0.01

Mercury

10

Nickel

0.13

Potassium

0.03

Silicon

3

Silver

0.1

Sodium

0.03

Tin

5

Zinc

0.03

Atomic Emission:

Atomic emission is a spectrophotometric technique where the atoms are excited by a source of energy (flame) and these excited atoms emit a characteristic radiation and return to their ground state. A hollow cathode lamp is not required for this method. The alkali and alkaline earth metals tend to be the easiest elemental families to undergo excitation and emission.

Alkaline earth metals:

Alkali metals:

Detection limit:

It lies in a range from few tenths of a part per billion to less than a part per million.

Most elements may be detected by atomic absorption and atomic emission except the noble gases, halogens, C, H, N, O, S and P.

Other sampling methods:

Replacing the flame with a graphite furnace and using electro thermal vaporization lowers detection limits and is a preferred method for analysis of ppm-levels of low melting point metals and heavy metals (ie. As, Bi, Sb, Se, Sn, Te, Hg, Pb, Cr, Tl, Cr, Cd, etc.). The sample must be acid soluble.

The graphite furnace is not functional at this moment.

Calibration:

Run a series of standards of the element under analysis and construct a calibration curve by plotting the concentrations of the standards against the absorbance. Most elements at higher concentrations become nonlinear in their absorption or response on a spectrophotometer. This loss of linearity can be caused by several factors, one of which is a saturation of the detector as the concentration increases. A calibration curve should be determined for each analysis to identify the optimum concentration range.

HPLC

The fundamental basis for HPLC consists of passing a sample (analyte mixture) in a high pressure solvent (called the mobile phase) through a steel tube (called a column) packed with sorbents (called the stationary phase). As the analytes pass through the column they interact between the two phases--mobile and stationary--at different rates. The difference in rates is primarily due to different polarities for the analytes. The analytes that have the least amount of interaction with the stationary phase or the most amount of interaction with the mobile phase will exit the column faster. Repeated interactions along the length of the column effect a separation of the analytes. Various mixtures of analytes can be analyzed by changing the polarities of the stationary phase and the mobile phase.

Mobile phase:

The mobile phase has to be prepared by you prior coming to perform the experiment. Thus, selection of the proper mobile phase is important for efficient separation of the analytes. Please do a research in advance on the matrix, pH and polarity of the mobile phase that you are going to prepare which will ensure proper separation of the analytes.

Mobile phase/sample preparation:

•  HPLC grade solvents are recommended to ensure purity. If an aqueous mobile phase is used the water should be doubly distilled and deionized.

•  It is strongly recommended that all mobile phases have to be filtered through .45 micrometer filters. Particles in the mobile phase can damage or plug the columns or cause excessive high operating pressure.

•  All mobile phase has to be degassed using any one of the following method.

•  Subjecting a liquid to source of vacuum.

•  Placing the solvent reservoir in an ultrasonic bath.

•  Bubbling a fine stream of helium through the mobile phase.

•  In addition all samples should be pre-filtered before injecting to the HPLC system.

Column: packed with 3 micron micro porous silica particles bonded with a hydrocarbon(octyl or octadecyl) group.

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