The instant invention is directed to an instrument and method of measuring the concentration of a target element in a multi-layer, thin coating and, more particularly, to such an instrument and method that is capable of measuring non-destructively the lead concentration on a surface coating.
Lead paint was used widely around 1920 due to its durability. Later, lead pigments were found to be a health hazard. It was banned from use since 1978. Old buildings need to be checked for lead paint to make sure it is safe for kids, thus it is necessary to have a non-destructive method for detecting the lead concentration from a surface.
A lead K line based instrument can be used to measure the lead concentration. K lines have very high energy (75 keV and 85 keV). They can travel through many layers of covering material with very little loss. Thus, by simply measuring the K line x-ray intensity, lead concentration can be determined. However, many drawbacks are associated with K line based instruments. Thus:
The present disclosure provides a solution that targets the needs of and meets the following primary objectives:
The foregoing and other objects of the invention are realized via a method of measuring an areal concentration of a target element coated by more than one layer near the surface of a substrate, where the target element is capable of emitting Lα, Lβ and Lγ when suitably excited. In accordance with a preferred embodiment, the method comprises the step of:
Preferably, the method of the invention includes using a filter to reduce low energy x-rays associated with excitation radiation and, further preferably, the filter is selected from a plurality of filters provided on a filter wheel. Also, if in the method of the invention it is initially determined that m1 is substantially equal to m2, then the areal concentration is reported based on m1. The invention also concerns the instrument that implements the aforedescribed method of detecting a target element in a multi-layer thin coating.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.
During a test, the instrument is placed against a substrate having the lead paint. The analyzer 11 sets the voltage and current of the power supply 22. The x-ray tube 20 then generates x-rays when the power supply is turned on. In the meantime, the analyzer 11 controls component 13 to select the proper filter from filter wheel 15. The generated x-rays are filtered by the x-ray filter 15. The filtered x-rays reach the lead paint on the substrate.
Lead L lines (Lα, Lβ, Lγ) will be then induced by the incident x-rays. The lead L lines pass though overlying material and some of them will reach the detector 10 and be detected. The x-rays going into the detector 10 contain lead L lines.
Referring to
Because Lα, Lβ and Lγ have different energies, they have different mean free path and absorption coefficients. Thus the measured Lα/Lβ or Lβ/Lγ ratios will be different depending on how deeply the lead is buried. In other words, Lα/Lβ or Lβ/Lγ is a function of the total absorption of the layer between the lead and the surface. By mapping out the relationship between Lα/Lβ and the total absorption during calibration, one can find the absorption of the covering layer based on the Lα/Lβ ratio. Suppose the calculated absorption is A1 based on Lα/Lβ ratio, then the lead concentration, according to existing practice, is:
m1=ILβ/A1/ILβS Eq. 1
Similarly, one can calculate the lead concentration m2 based on Lβ and Lγ.
m2=ILγ/A2/ILγS Eq. 2
If the lead paint is single layered, m1 and m2 should be equal to each other regardless of whether the lead paint is buried or not. However, according to the observation of the present inventor, when there is more than one layer of lead paint, this model breaks down. m1 and m2 then significantly underreport the lead concentration.
Noticing that m1 or m2 fails to provide accurate lead concentration when more than one layer of lead is encountered, the following steps are employed to provide a method that guides accurate lead concentration values or measurements.
To calculate the lead concentration for multi-layered lead paint, one treats the plain paint in the middle of lead paint layers as a perturbation to the single layered model. The plain paint within a multi-layer lead paint sandwich changes the amount of lead detected. To a first order approximation, this change is proportional to the amount of plain paint inserted
Δm1≡mz−m1=C1f(paint) Eq. 3
Δm2≡mz−m2=C2f(paint) Eq. 4
From equation 2 and 3, one obtains: f(paint)=m1−m2/C1−C2. Thus
mz=m2+C2f(paint)=m2+C3(m2−m1) Eq. 5
Where
is a constant that can be determined by the following steps: 1) Making a multi-layer lead paint with known concentration mz; 2) Acquiring an x-ray spectrum from this multi-layer lead paint; and 3) calculating m1 and m2 using Eq. 1 and Eq. 2; and 4) calculating C3 from Eq. 5.
Once C3 is determined, it can be used on all instruments because it is independent of any given instrument. Then we can calculate the lead concentration based on the above equations, with mz being the lead concentration calculated from the multi-layer model, m1 is the calculated lead concentration based on single layer model using Lα and Lβ intensities; and m2 is the calculated lead concentration based on single layer model using Lβ and Lγ.
Reference is now made to
To check the performances in the critical concentration range of 0 to 2 mg/cm2, single layer lead paint standards were used as test samples. The following table shows the comparison of L line results using the prior single layer lead paint algorithm, K line results and the improved L line results. “Prior L Line Algorithm Reading” and “K Line Reading” are from an isotope instrument based on prior art (see U.S. Pat. No. 5,274,688).
One should notice that the error of K line based reading is very large, making it almost useless for lead measurement below 1.5 mg/cm2.
The following data is part of the record taken during lead inspection of an old building:
The paint from this building has multiple layers of lead paint and they are deeply buried. L line reading based on prior art could hardly detect any lead. K line reading detected lead, but its error is 0.5 mg/cm2 or higher. If the lead concentration is from 0.5 to 1.5 mg/cm2, it will have a hard time classifying whether or not lead is present. With the improved algorithm and optimized instrument settings, we can detect lead from 0 to 2 mg/cm2 accurately, the invention method and instrument provide reliable lead positive/negative indications in the high concentration region.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.
This application claims the benefit and priority of U.S. Provisional patent application Ser. No. 61/727,350 filed Nov. 16, 2012 entitled AN INSTRUMENT WITH IMPROVEMENT IN DETECTION OF MULTI-LAYER THIN COATING, the entire disclosure of which is incorporated herein by reference.
Number | Name | Date | Kind |
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5274688 | Grodzins | Dec 1993 | A |
5396529 | Grodzins | Mar 1995 | A |
20090067572 | Grodzins et al. | Mar 2009 | A1 |
Number | Date | Country | |
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20140140473 A1 | May 2014 | US |
Number | Date | Country | |
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61727350 | Nov 2012 | US |