There are Two Methods for
Screening for Lead paint

X-Ray Florescence Spectrum Analyzer and Sodium Sulfide

VS.
X-RAY METHOD
 
SODIUM-SULFIDE
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WHAT IS THE SODIUM SULFIDE METHOD OF SCREENING FOR LEAD PAINT? The sodium Sulfide method requires an inspector to break through and expose all the layers of paint on a surface; this is done by scoring (cutting through the surface) in a v-shape. He/She then applies a sodium sulfide solution to the newly exposed surfaces. A grey to black coloration will appear on the paint layers containing lead. This method is not foolproof, however, and is subject to chemical or visual interference. For example, cadmium can give false positive reaction; this chemical is commonly used as a pigment in paints. Varnish also can give a grayish or black appearance.

WHAT IS THE X-RAY FLUORESCENCE (XRF) METHOD OF SCREENING FOR LEAD PAINT? The X-ray fluorescence method is the most widely accepted screening method. An inspector uses a special instrument to take readings of x-rays emitted from the lead in the surface. The instrument's radioactive source excites the lead in the paint to emit x-ray radiation at a certain frequency. The detector portion of the instrument then measures the amount of x-radiation. This type of analysis is preferred by the federal government because it is nondestructive (will not damage the surface being tested). The results of this instrument are given immediately in milligrams of lead per square centimeter.

WHICH IS THE BEST METHOD TO DETERMINE THE PRESENCE OR ABSENCE OF LEAD PAINT? There are several important differences between the XRF and sodium sulfide screening methods. There is a tendency for the sodium sulfide screening method to give positive results more often than when the same material is examined by the x-ray method. This may occur for several reasons: 1.)Interpertation of the color of the chemical reaction may vary from inspector to inspector, due to differences in eyesight and/or judgement. 2.) Some state laws require that any gray to black test results be reported as positive; (not all levels of lead paint are considered dangerous & sodium sulfide can not determine the levels of lead in paint, the XRF analyzer does) The XRF is not subject to the same unreliability and bias because the instrument does all the calculations. Its results do not depend on the eyesight or opinion of the operator. Another reason the XRF method is favored is that the XRF is nondestructive, whereas the sodium sulfide method requires actual cutting into the painted surface. The nondestructive nature of the XRF makes it feasible to examine aesthetically pleasing and fragile surfaces (such as interior woodwork and window frames) without permanent damage. In contrast, the sodium sulfide method requires making a deep cut into the painted surface and performing a chemical reaction on the cut surface. The cut is permanent and the products of the reaction may stain the tested surface.