The two types of portable XRF analyzers are direct readers and spectrum analyzers. Most XRF analyzers are portable, weighing less than about two kilograms (approximately 4.5 pounds). Each instrument has a digital display, operates from a battery source, and comes with a battery charger.
Direct readers display only the calculated lead concentration; most spectrum analyzers also produce a graph, called an energy spectrum, to help resolve discrepancies in the readings due to interference of substrate materials below the paint surface (for example, wood or plaster). A direct reader is like a radio with only one station (frequency). The spectrum analyzer offers a range of stations (or frequencies).
Another difference between different types of lead-based paint XRF analyzers lies in the internal correction of the reading for interference from the substrate. All XRF analyzers attempt to correct for substrate effects. The spectrum analyzer is more sophisticated than the direct reader, since it utilizes information from the complete energy spectrum to make the internal correction. The internal correction of both types of analyzers can be imperfect, and additional correction for both substrate interference may be necessary for some instruments on some substrates.
Portable XRF instruments expose the painted surface to X rays or gamma rays, which causes the lead atoms to emit X rays with a characteristic energy. The intensity of this radiation is measure by the instrument’s detector and is then converted into a number that represents the amount of lead in the paint per unit area, usually in milligrams per squared centimeter (mg/cm²). The result will appear on the display area of the instrument and is called an XRF reading.
An XRF analyzer works by exposing a paint surface to radiation emitted from a sealed source inside the instrument. The source of this radiation in all the XRF analyzers currently used for lead-based paint inspections is the cobalt-57 isotope or the cadmium-109 isotope. Both of these radioactive materials spontaneously emit energy in the form of X rays and gamma rays.
When these rays are released from an XRF analyzer and hit a painted surface, the elements in the paint matrix – which can include lead – are ‘excited’ and respond by emitting energy in the form of X rays characteristic of each of the elements. This response is known as fluorescence.
An example of one type of fluorescence is what occurs with a ‘black light’. When a black light shines on certain paint surfaces (just like the rays emitted by the XRF), the paint absorbs the black light and then glows or gives off a visible light (i.e., fluoresces) and is seen by the viewer’s eyes.
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