Older paints (pre-1977) can be as much as 5% lead by weight (50,000 ppm), and a single 1 cm2 paint chip might contain 175 µg of lead. Current standards for paint are a lot more prohibitive, allowing only 600 ppm lead in paint, resulting in the same size paint chip containing only 6 µg of lead.
There are three main lead testing methods for determining the lead content in paint:
1) Analyze the paint for lead in situ by using an inexpensive chemical lead testing kit.
2) Determine the quantity of lead in paint in situ using a portable X-ray fluorescence (XRF)
device. This is a non-destructive test.
3) Obtain a paint sample and determine its lead content in a laboratory using various
analytical techniques. This is a destructive test.
These lead testing techniques are explained in the next sections,
Chemical lead testing kits
You will find a selection of chemical lead testing kits available, from simple qualitative tests to more advanced
semiquantitative tests. Many rely upon a colour change to indicate the presence of lead
over a specific lead concentration. In the simplest lead testing kits, the result is either positive (i.e. lead is
present and above a particular concentration) or negative (i.e. lead is not above a certain
concentration). The tolerance content level for the colour transformation depends upon the lead test kit that is used and could be affected by the health and safety regulations of the country or region in which the test is sold. In the USA, for instance, lead test kits should be able to detect concentrations above 0.5% lead by weight (5000 mg/kg).
The less complicated lead test kits either test the paint by utilizing a swab soaked in a chemical that is rubbed on the painted surface or require the removal of a chip of paint that is then mixed with chemicals in a test tube. More sophisticated spot test kits use fluorimetric or photometric techniques on the paint chips.
Chemical lead test kits are relatively inexpensive and do not require specific training, although training
of the individual will give more reliable results. The final results are instant although the speed of colour change can be dependent on the amount of lead present. These kits possess a variety of limitations though. The kits may be used to test uncovered layers only. This means to test layers below the surface which may be more likely to contain lead, you have to score the paint surface or to take off a chip of paint (depending on the method).
For some lead test kits the colour change may be hard to observe, especially when you are examining dark paint colours. Typically, these kits cannot measure the amount of lead contained in the paint. This includes the semiquantitative
techniques that only give lead concentration ranges.
Finally, chemical lead test kits may possibly indicate that lead is below a certain concentration when it actually is above (false negative result). The USEPA, for example, currently acknowledges only three lead test kits are compliant with the false negative qualifying criterion of the United States 2008 Renovation, Repair and Painting rule. All of these 3 tests are recommended to be used only by a licensed renovator for paint existing on specific substrates, and none of them is acknowledged by the USEPA for the false positive qualifying criterion of the Renovation, Repair and Painting rule.
Portable X-ray fluorescence (XRF) spectrometry
When lead is subjected to high-energy radiation it emits X-rays of a characteristic frequency. The strength of the X-rays corresponds to the amount of lead per unit area (usually expressed in units of milligrams per square centimetre). Usually regulatory standards for lead in paint are expressed in different units such as parts per million or per cent concentration by weight so the XRF results may need to be converted. Portable XRF instruments can measure the total amount of lead in a painted surface in situ without damaging the paint or the substrate material.
The accuracy of portable XRF devices is generally good, even though they have a larger error in contrast to laboratory analysis. Depending on their dimensions and characteristics, XRF systems do require a comparatively large and flat surface to carry out a measurement.
This technique is therefore especially designed to measuring flat paint surfaces such as walls, but is less appropriate to testing highly rounded or intricate surfaces ie most toys. Some new XRF instruments do get over these limitations though.
Portable XRF products are manufactured by several companies and are comparatively expensive, ranging in price from roughly US$ 10 000 to US$ 50 000, but need little maintenance and no consumables. The lead testing is extremely rapid (about 1 minute per test)
This technique needs a paint sample to be collected.A minimum sample size of about 300 mg is generally required to conduct an analysis, although this is dependent upon the concentration of lead, sample preparation method and the analysis method. ASTM E 1729 standard recommends 4 square inches of paint be sent to the laboratory for analysis. The different methods of that are used in the laboratory to carry out lead paint analysis are described below.
Flame atomic absorption spectrometry (FAAS)
Atomic absorption spectrometry (AAS) is based on the principle that free atoms absorb light at wavelengths that are characteristic of the elements that are present. In the case of lead the wavelength is 283.3 nm. The quantity of light that is absorbed is directly related to the concentration of the particular element that is present within the test sample.
FAAS detection limits are adequate for the majority of instances. FAAS measurements are subject to some interference from light dispersing and molecular absorption by matrix elements, which can be effectively corrected by various techniques. FAAS devices, which need some laboratory expertise to operate, are widely available with or without autosamplers. The primary instrument cost is relatively low, and consumables, for example argon gas, are comparatively cheap. Maintenance requirements are fairly low, and sample lead paint testing rate may be several tests per minute.
Graphite furnace atomic absorption spectrometry (GFAAS)
GFAAS is an AAS method that makes use of an electrically heated graphite tube to vaporize and atomize the lead paint at temperatures up to 3000 °C prior to its detection. GFAAS instruments have very low detection limits and require only very minimal digest volumes (about 20 µl). GFAAS measurements are often subject to significant interference from light scattering and molecular absorption by matrix elements, but this may be adequately remedied using many approaches, including the use of matrix modifiers. GFAAS devices need to be run by properly trained laboratory technicians.
GFAAS devices are widely available and need autosamplers to increase precision and throughput. The initial equipment cost is intermediate, and maintenance and consumable costs can be expensive. Lead paint sample throughput is approximately one sample every 2 to 3 minutes.
Inductively coupled plasma atomic emission spectrometry (ICP-AES)
ICP-AES uses an inductively coupled plasma (an ionized gas consisting of electrons and protons) source to dissociate the sample being tested into its component atoms or ions. Under these high-energy conditions, lead along with other elements emits light at its characteristic wavelengths. The amount of light emitted can be analyzed and corresponds to the amount of lead in the sample. ICP-AES instruments provide the advantage of being capable of determining several elements concurrently.
The detection limit for lead is intermediate, but still adequate to quantify lead accurately in paint at typically observed concentrations. Sample volume requirements are moderate. Some spectral interferences are typical, but can be corrected. ICP-AES instruments must be operated by trained laboratory personnel. The initial piece of equipment cost is high, but the major consumable is only argon gas. Repairs and maintenance costs are relatively high due to the
complex design of ICP-AES instruments. Sample throughput is intermediate, typically about one test per minute.
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