Where do we start?

To get prepared for a visit to the local drinking water tratment plant students had a lesson at school about the water supply in our area. A delegate from our Local Company for the water supply came to school to give lesson on the topic.

Afterwards students went to a Purification plant to see how it works. This serve a very large area and new implants were recently built. They also went to the spring of Santa Maria in Sarno, where they could understand how the process ot water supply works.

Santa Maria in Sarno's spring

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Visit to the Purification Plant

LESSONS AT A.R.P.A.C

According to the main theme of the Erasmus+ project, ‘Trans European Water sustainability’, two classes of Liceo Scientifico ‘Alfred Nobel’ in Torre del Greco, III A and III F, visited Arpac Laboratories in Naples,on March 14th and 28th. Arpac is the National Agency for Environmental Protection.

It has been a great experience for us: young students, 15-16 years old, become chemists for a day, analyzing water pollution by inorganic and organic agents.

First, we learnt how using basic instruments, such as a pH-meter: it measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity, expressed as pH. It’s defined as the difference in electrical potential between a pH electrode and a reference electrode.

Also we learnt to prepare a sample for different tests, such as a calorimeter test.

We have analyzed water pollution by inorganic agents using a particular technique: inductively coupled plasma mass spectrometry. ICP-MS is a type of mass spectrometry, capable of detecting metals and several non-metals at low concentrations. This is achieved by ionizing the sample with inductively coupled plasma and then using a mass spectometer to separate and quantify those ions.

An inductively coupled plasma (ICP) for spectrometry is sustained in a torch that consists of three concentric tubes, usually made of quartz. The end of this torch is placed inside an induction coil supplied with a radio-frequency electric current. A flow of argon gas is introduced between the two outermost tubes of the torch and an electric spark is applied for a short time to introduce free electrons into the gas stream. These electrons interact with the radio-frequency magnetic field of the induction coil and are accelerated first in one direction, then the other, as the field changes at high frequency.

The accelerated electrons collide with argon atoms, and sometimes a collision causes an argon atom to part with one of its electrons. The released electron is in turn accelerated by the rapidly changing magnetic field. The process continues until the rate of release of new electrons in collisions is balanced by the rate of recombination of electrons with argon ions (atoms that have lost an electron). This produces a ‘fireball’ that consists mostly of argon atoms with a rather small fraction of free electrons and argon ions. The temperature of the plasma is very high, of the order of 10,000 K. The plasma also produces ultraviolet light, so for safety should not be viewed directly.

For coupling to mass spectrometry, the ions from the plasma are extracted through a series of cones into a mass spectrometer, usually a quadrupole. The ions are separated on the basis of their mass-to-charge ratio. Moving in a magnetic field, they are subjected to Lorentz force and divert from linear trajectory, showing a centripetat acceleration:

where m is the mass, q is the electrical charge, v is ion speed (it doesn’t change), B is the magnetic field, r is the radius of circular trajectory on which ion diverts.

A detector receives an ion signal proportional to the concentration. The concentration of a sample can be determined through calibration with certified refernce material such as single or multi-element reference standards.

In these photos, some of the data we collected analyzing water samples. We also have learnt how analyzing water pollution from organic agents. Gas chromatographyis a chemical analysis applied to a cpmplex sample for separating and analyzing compounds, that can be vaporized without decomposition. So it helps in identifying a compound.

A gas chromatograph uses a flow-through narrow tube, known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, tipically an inert gas, for example helium: this is the so-calledmobile phase) at different rates, depending on their various chemical and physical properties and their interaction with a specific column filling, called stationary phase (a microscopic layer of liquid or polymer on an inert solid support).

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, the retention time.The comparison among retention times is what gives GC its analytical usefulness.

Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, column length and the temperature.

In a GC analysis, a known volume of gaseous or liquid analyte is injected into the head of the column, usually using a microsyringe. As the carrier gas weeps the analyte molecules through the column, this motion is inhibited by the adsorption of the analyte molecules either onto the column walls or onto packing materials in the column. The rate at which the molecules progress along the column depends on the strength of adsorption, which in turn depends on the type of molecule and on the stationary phase materials. Since each type of molecule has a different rate of progression, the various components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times. A detector is used to monitor the outlet stream from the column; thus, the time at which each component reaches the outlet and the amount of that component can be determined. Generally, substances are identified (qualitatively) by the order in which they elute from the column and by the retention time of the analyte in the column.