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2.1 GENERAL INFORMATION <br /> TIP I is an analytical instrument which has been designed to sense certain <br /> important impurities in air and other gases, the name TIP* stands for "Total <br /> Ionizables Present"; this implies that the instrument senses any chemical in <br /> the air (or other gas matrix) which is "ionizable". Obviously, virtually any <br /> ni1trogen emical can <br /> etc,be ioniInd , the this <br /> casencloofes the TIP nI* actually <br /> ma"ionizable" soxygen, <br /> means <br /> photoionizable; we must begin by defining this. <br /> TIP I* contains a miniature lamp which emits very short wavelength ultraviolet <br /> (UV) radiation, the energy of this radiation is enough to bring <br /> about "photoionization" if it strikes the molecules of certain chemicals. <br /> Whether or not photoionization takes place depends upon a factor called the <br /> "Ionization Potential " (IP) of the molecule in question. Ionization Potential <br /> is normally measured in energy units known as Electron Volts (eV). <br /> Most of the light permanent gases (such as the air gases, hydrogen, helium, <br /> etc. ) have ionization Potentials at 12 eV or more. On the other hand, a huge <br /> number of organic chemicals which enter the air as gases or vapors, have <br /> ionization potentials below 10.5 eV. Included amongst this latter group are <br /> the vast majority of those compounds which we describe as "Pollutants". <br /> The standard ultraviolet lamp used in TIP I* has an energy of about 10.6eV and <br /> any molecule having an ionization potential (IP) below this value can be <br /> sensed. <br /> Let us now assume that we have an air sample which is "polluted" with a great <br /> number of gases and vapors (perhaps it could have been taken from the vicinity <br /> of a gas station at a busy time of day). If this air sample is exposed to <br /> bediation from theUV photoionized but. <br /> the lPollutants willamp in TIP I* tandaarwholesrange of(andtionsaofrall�ll shapnot <br /> es <br /> and sizes will be formed. We have thus used a carefully selected lamp energy <br /> to "pick out" the pollutants and ignore the clean air; this is the essence of <br /> the principle used in TIP I*. <br /> TIP I* uses a small pump which continuously draws air into a tiny ionization <br /> chamber which is also flooded with UV light. Inside this chamber are two <br /> electrodes; an electric voltage is applied across these electrodes and one of <br /> them is connected to a very sensitive current measuring circuit <br /> (electrometer). When ions are formed, and these will have both positive and <br /> negative charges, the negative ions will travel to the Positive electrode and <br /> the positive ions to the negative electrode. This will result in an electric <br /> current which is measured by the electrometer and can than be used in a number <br /> Of ways to express the "Total Ionizables Present". <br /> It is important to realize that TIP I* does not distinguish between different <br /> Pollutants; the signal produced represents a composite of all different <br /> ionizable pollutants. Where the "pollutant burden" in a given sample of air <br /> becomes high, TIP I* will register this fact. The correct implication to be <br /> drawn is that there is a potentially serious situation which requires further <br /> investigation using more specific detection equipment (such as Photovac's 105 <br /> gas chromatograph). <br /> N ' <br /> Y <br />