Determination the elemental composition of soil samples

Transcription

1 4. Experiment Determination the elemental composition of soil samples Objectives On this practice you will determine the elemental composition of soil samples by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). For the analysis samples will be prepared by microwave digestion. You will get dry, homogenized soil samples for the measurements. Microwave digestion To perform an atomic absorption or atomic emission measurement sample must be dissolved prior to analysis. It is often the case that the sampe is not easily dissolved. In such situations fusion or acid digestion may be used and microwave digestion is an effective form of acid digestion. Microwave ovens began to find widespread use in chemical laboratories in the late 1980 s. The use of laboratory microwave units has become increasingly popular because of the significant improvement in chemical reaction rates that are possible using microwave radiation. A typical microwave acid digestion can be completed in a matter of minutes, whereas the same conventional hot plate digestion can take hours. Microwave digestion usually involves placing a sample in an acid solution and heating to high temperatures and pressures. These extreme conditions will dissolve most materials, but it is potentially quite dangerous. The microwave oven has many safety features, all of which must be paid attention to when in use. It is imperative that safety rules be followed when performing a microwave digestion. Microwave digestion rotor Soil samples will be digested in concentrated HNO 3 and 30% H 2 O 2. The metals which are extracted by HNO 3 are the ones biologically available and important. Attention: If you are performing a soil analysis by ICP-OES you must separate the undissolved solid from the solution after digestion! Any particulate matter can clog the nebulizer, so filter the particulates prior to analysis! 1

2 The following instructions for use of the microwave must be followed very carefully. Do not perform any steps until you are absolutely sure that step is understood. Microwave sample preparation imposes a unique set of safety considerations beyond the basics of good laboratory practice. 1. All vessel components must be dry and free of particulate matter. Drops of liquid or particles will absorb microwave energy, causing localized heating which may char and damage vessel components, leading to possible vessel failure. 2. Ensure that the vessels are completely vented of built-up reaction gases and completely depressurized before removing liner and internal components from the polypropylene support module. 3. Do not heat concentrated base or salt solutions inside the vessel. Microwave heating of such solutions causes precipitation of salts and formation of crystal deposits which absorb microwave energy and may char into the vessel or electrically arc, leading to possible vessel failure. 4. Do not heat high boiling point acids (concentrated sulfuric or phosphoric) inside the vessels without using temperature control lmited to 200 C. 5. The vessels have a maximum recommended hold time. Hold times longer than 30 minutes at maximum temperatures or higher may result in softening of the bottom of the support module and may damage the vessel. You can digest up to six samples at a time, but pay attention: if you digest too much sample in one vessel excessive pressure can result in an explosion! Attention: You need to know the exact mass of sample which you are digested so don t forget to record the mass of your sample before the digestion procedure using analytical balance. Microwave digestion vessel with the safety equipment 2

3 Experiment outline 1. Weight 500 mg sample into the teflon vessel (tall white baker ) using analytical balance. The sample must be placed in the bottom of the vessel so that it will be completely covered by acid. The side walls of the vessel must not have sample deposits on them. 2. Put the vessel into the safety shield. 3. Add 4cm 3 of concentrated HNO 3 and then 1 cm 3 of 30% H 2 O 2 to your samples slowly, in drops. Use dispenser for measuring the HNO 3 acid! Visually look for a reaction. If a reaction occurs, allow the reaction to subside completely before capping the vessel. 4. Place TFM cover on the top of the vessel. On top of the vessel cover place a round brown, plastic load disk. The load disk is smooth on one side and has a circular depression on the other. Place the side with the circular depression facing up. 5. Assemble the device with the help of the teacher. With the vessel support modul screw at the top centered in the recess area of the brown support disk, tighten the support screw finger tight. Use the torque wrench to tighten the screw until you hear/feel a click. Do not tighten any further as this can damage the vessel. 6. Repeat steps for the remaining vessels. 7. When all of the reaction vessels have been assembled, it is time to place them in the turntable and put them inside the microwave for digestion. 8. Press START on the front keypad and the digestion will begin. Current temperatures and pressures will be displayed as the digestion proceeds. Programe 1 is used for the digestion. Digestion profile is illustrated in picture 1. Step 1: 250 W unpilsed power to start smooth oxidation of organic matter. Step 2: ZERO power. Reactions must proceed without addition of energy to avoid potentially dangerous run-away temperatures and overpressures. Step 3/Step 5: Higher power is applied to complete the oxidation, according to the type of sample. 3

4 Power (Watt) Time (minutes) Step Power (W) Time (min.) Picture 1.: Microwave digestion profile There is a 15 minute cool down time after digestion is complete. Be sure that the vessels have cooled sufficiently prior to handling. Attention: Always wear gloves, lab coat and eye protection when handling/venting the vessels!! 1. Remove the turntable with the vessels and place them in the hood! 2. Vent vessels by grasping the support module with one hand and loosening the teflon vent fitting by slowly turning it. A series of slow, partial turns of the fitting will permit decomposition gases and acid vapor to escape. Upon venting you will see brown gas rush from the fitting. These are NOx decomposition products of HNO3. This is a big reson why you should be wearing gloves!! 3. Disassemble the apparatus. 4. Quantitatively transfer your solutions to an appropriate vessel. For this filtrate your solution into 100cm 3 flasks and fill them up until the mark with distilled water. Don t forget writing the number of your samples on the flasks! 5. For the ICP-OES analysis ask the help of the teacher. Determination of the element composition of soil samples by ICP- OES Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is a technique ideally suited to the analysis of naturally occuring materials, including water, sediments, soils, rocks, minerals and biological matter. The inductively coupled argon plasma is a spectroscopic source, which can be used for the determination of a wide range of major and trace elements in a single, short 4

5 integration period. Detection limits are low for most elements and multielement determinations can be achieved in a short time. An ICP works by injecting a nebulized mist from a liquid into the center of an argon plasma. A plasma is created from a flow of gas within a high energy filed which ionizes the gas and causes intense heating. Temperatures inside an ICP plasma reach 10000K. When the mist of the sample enters the plasma, the intense heat causes the dissociation of most chemical compounds, and the energy that the component atoms absorb causes them to undergo excitation and ionization energy transitions. These transitions produce spectral emissions characteristic of the elements being excited. The spectra produced by the plasma is broken down into individual spectral lines by the ICP s spectrometer, and the ICP s computer translates the spectral lines into concentrations for a specified suite of elements. In contrast to flame emission spectroscopy, the plasma is not an oxidizing environment (like a flame) and there are fewer chemical reactions to confound the analysis. Also ICP has a large linear range around 4-6 orders of magnitude for most elements. This means that fewer dilutions are required to accomodate samples with a wild range of concentrations. The evaluation of analytical results take place in one step by a computer program called Smart Analzer. Results are given in unit mg/dm 3 (ppm). Calculate the mg/kg concentartions. From the samples element concentrations are determined 3 times successively by ICP-OES. Calculate mean, standard deviation (SD) and relative standard deviation (RSD%) of the paralell results. 5