Abstract:
An instrument and a method of detecting a target element in a multi-layer thin coating. Lα, Lβ and Lγ x-rays are caused to be emitted from the target element (preferably lead paint) with excitation radiation. Upon detecting the emitted x-rays, an areal concentration of the target element is calculated using Lα and Lβ intensities once, and then using the Lβ and Lγ intensities once, by reference to a single layer model; By combining the two concentrations calculated using single layer model, a more accurate concentration can be calculated for the target element in the multi-layered surface coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     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. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     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:
         (a) First, K line x-ray intensity is very weak and the measurement error can be large (when the test time is fixed). When the lead concentration is low (around 1 mg/cm 2 ), the large statistical error makes the reading unreliable.   (b) Even if the lead paint is on the opposite side of the wall or is covered by another layer of dry wall (after deleading), lead can still be detected, erroneously reporting a lead hazard when it is totally safe.   (c) A radioactive isotope source is present (needed to generate high energy x-rays to excit lead K shell electrons).   (d) L line x-rays have much less energy. Its escape depth is much shorter. Ten layers of regular paints on top of lead paint will reduce the lead L line intensities by more than 90%. Thus an L line based instrument is a surface lead detector. For single-layered lead paint, there is a well established method for measuring the surface lead concentration from lead Lα and Lβ lines (as described in U.S. Pat. No. 5,274,688 and U.S. Pat. No. 5,396,529). The lead paint can be on top of the surface or buried by non-lead containing material, and this method works accurately. However, when the paint contains multiple layers of lead paint, the method fails badly.   (e) The current action level for deleading is 1.0 mg/cm 2  and the trend is moving lower. Thus the most critical lead range is from 0 to 1.5 mg/cm 2 . In this range, the large statistic error of K line based reading is a real problem. On the other hand, buildings that are 100 years old are likely to have multiple layers of lead paint, the limitation of the lead L line based prior art makes the instrument based on the technology useless for those buildings.       

     The present disclosure provides a solution that targets the needs of and meets the following primary objectives:
         (a) Reliably and non-destructively determining whether or not the amount of lead is less than the action level (current action level is 1.0 mg/cm 2 );   (b) Handling both single layer and multi-layer lead paint coatings; and   (c) Operate without using a radioactive source.       

     SUMMARY BACKGROUND OF THE DISCLOSURE 
     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:
         a) inducing the Lα and Lβ and Lγ x-rays to be emitted from said target element with excitation radiation;   b) detecting said Lα, Lβ and Lγ x-rays and determine the intensity of the Lα, Lβ and Lγ x-rays separately;   c) calculating the areal concentration of said target element by means of the following equation:
 
 m   1   =I   Lβ   /A 1 /I   LβS   Eq. 1
           wherein m 1  is the calculated target element concentration based on single layer model using Lα and L β  intensities; A1 is the absorption factor computed from the Lα, Lβ intensity ratio (during calibration, A1 as a function of Lα, Lβ intensity ratio is fitted as a curve; and this curve is used later to calculate the A1 from measured Lα,Lβ intensity ratio), I Lβ  is the measured L β  line intensity of the target element from sample under testing; I LβS  is the Lβ intensity of the target element from a NIST standard without any shielding layers.
 
 m   2   =I   Lγ   /A 2/ I   LγS   Eq. 2
   wherein m 2  is the calculated target element concentration based on single layer model using Lβ and Lγ. A2 is the absorption factor computed from the L β , Lγ intensity ratio (during calibration, A2 as a function of L β , Lγ intensity ratio is fitted as a curve; and this curve is used later to calculate the A1 from measured Lα, Lβ intensity ratio), I Lγ  is the measured Lγ line intensity from sample under testing; I LγS  is the Lγ intensity of the target element from a NIST standard without any shielding layers; and   
           d) calculating the combine areal concentration of said target element according to:
 
 m   z   =m   2   +C   3 ( m   2   −m   1 )  Eq. 5
           wherein C 3  is a constant determined during an instrument calibration, and m z  is the lead concentration emitted from the at least one layer of coating containing the target element.   
               

     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 m 1  is substantially equal to m 2 , then the areal concentration is reported based on m 1 . 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of the invention, both diagrammatically and pictorially. 
         FIG. 2  is a chart showing aspects of the invention. 
         FIG. 3  is a block diagram of the invention. 
         FIG. 4  is a plot of measured lead levels against expected lead levels under different conditions. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of an XRF instrument according to the present invention. As can be seen, the XRF instrument includes a power supply  22 ; an x-ray tube  20 ; an x-ray detector  10 ; a filter assembly  14 ; a filter wheel  15  with a few different x-ray filters; and an analyzer  11  which includes an improved lead paint module  12  according to the presently disclosed invention. In addition, the instrument preferably includes a component  13  which is controlled by analyzer  11  to select a filter from the filter wheel  15 . During an operation, x-rays are generated by x-ray tube  20 , to energize a substrate with one or more layers of coating containing material which is the subject of detection. The x-rays are sensed by detector  10 , which generates data which it provides to analyzer  11 . 
     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.  FIG. 2  shows a typical x-ray spectrum from lead paint. It has lead lines Lα, Lβ and Lγ. 
     Referring to  FIG. 2 , the X-axis represents the energy of the x-rays in keV, while the Y-axis represents the intensity of x-rays in counts per second. The graph shows the energy distribution of x-rays detected by detector  10 . Lα, Lβ and Lγ are x-ray fluorescence lines from lead. Lα has an energy of 10.5 keV; Lβ has an energy of 12.6 keV and Lγ has an energy of 14.8 keV. The tube voltage (around 40 kV) and current are supplied by power supply  22 . They are optimized to maximize the generation of lead L lines. x-ray filter  15  is used to cut down the low energy x-rays from the tube  20  because they do not help creating lead L line, and instead introduce noise in the lead L line regions. Proper selection of the filter significantly improves the signal noise ratio in lead L line regions. In  FIG. 2 , the lead L line peaks have almost no background due to the proper use of x-ray filter  15 . 
     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:
 
 m   1   =I   Lβ   /A 1 /I   LβS   Eq. 1
         wherein I Lβ  is the measured Lβ line intensity from the sample under testing; I LβS  is the lead Lβ intensity from NIST (National Institute of Standards and Technology) standard with 1.0 mg/cm 2  lead concentration (recorded during instrument calibration), herein the “Lβ NIST intensity”.       

     Similarly, one can calculate the lead concentration m 2  based on Lβ and Lγ.
 
 m   2   =I   Lγ   /A 2/ I   LγS   Eq. 2
         wherein I Lγ  is the measured Lγ line intensity from the sample under testing; I LγS  is the lead Lγ intensity from NIST standard with 1.0 mg/cm 2  lead concentration), herein the “Lγ NIST intensity”.       

     If the lead paint is single layered, m 1  and m 2  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. m 1  and m 2  then significantly underreport the lead concentration. 
     Noticing that m 1  or m 2  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
 
Δ m   1   ≡m   z   −m   1   =C   1   f (paint)  Eq. 3
 
Δ m   2   ≡m   z   −m   2   =C   2   f (paint)  Eq. 4
         wherein Δm 1  is the difference between the expected lead concentration m z  and the calculated lead concentration m 1  based on single layer model using Lα and Lβ lines;   Δm 2  is the concentration difference based on Lβ and Lγ calculation; C1 and C2 are unknown constants;   f(paint) is an unknown function related to lead distribution within the multi-layer lead paint.       

     From equation 2 and 3, one obtains: f(paint)=m 1 −m 2 /C 1 −C 2 . Thus
 
 m   z   =m   2   +C   2   f (paint)= m   2   +C   3 ( m   2   −m   1 )  Eq. 5
 
Where
 
               C   3     =       C   2         C   1     -     C   2               
is a constant that can be determined by the following steps: 1) Making a multi-layer lead paint with known concentration m z ; 2) Acquiring an x-ray spectrum from this multi-layer lead paint; and 3) calculating m 1  and m 2  using Eq. 1 and Eq. 2; and 4) calculating C 3  from Eq. 5.
 
     Once C 3  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 m z  being the lead concentration calculated from the multi-layer model, m 1  is the calculated lead concentration based on single layer model using Lα and Lβ intensities; and m 2  is the calculated lead concentration based on single layer model using Lβ and Lγ. 
     Reference is now made to  FIG. 3 , which shows a flow chart of the steps employed by the present disclosure leading to a more accurate lead detection, particularly when multiple layers of lead paint are involved. During a test, any x-ray spectrum from lead paint is acquired in step  30 ; Lead concentration m1 is calculated based on Lα and Lβ using Eq. 1 as shown in step  32 ; Lead concentration m 2  is calculated based on Lβ and Lγ using Eq. 2 in step  34 ; If m 1  and m 2  are equal to each other, step  36  concludes that the lead paint is single-layered and proceeds to step  38  to report lead concentration m1 based on Eq. 1; If m 1  and m 2  are different from each other of step  36 , it is concluded that the lead paint is multi-layered and the method takes step  40  to report lead concentration based on Eq. 5. 
       FIG. 4  shows how the correction improves the measured lead result: the instrument was calibrated using single layer lead paint. The line in the picture shows the calibration curve of the instrument. Multi-layer samples were made from single layer lead paint standards with blank layers in between. 
     To check the performances in the critical concentration range of 0 to 2 mg/cm 2 , 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). 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Prior L 
                   
                   
                   
                   
                   
               
               
                   
                 Line 
                   
                 K Line 
                   
                 Improved L 
               
               
                 Standard 
                 Reading 
                 Precision 
                 reading 
                 Precision 
                 Line Reading 
                 Precision 
               
               
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
                 (mg/cm 2 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.00 
                 0.00 
                 0.02 
                 −0.37 
                 0.46 
                 0.00 
                 0.00 
               
               
                 0.71 
                 0.70 
                 0.10 
                 0.23 
                 0.45 
                 0.67 
                 0.02 
               
               
                 1.04 
                 1.10 
                 0.10 
                 0.60 
                 0.40 
                 1.04 
                 0.02 
               
               
                 1.53 
                 1.50 
                 0.10 
                 1.10 
                 0.50 
                 1.56 
                 0.03 
               
               
                   
               
             
          
         
       
     
     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/cm 2 . 
     The following data is part of the record taken during lead inspection of an old building: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                 Improved 
                   
               
               
                   
                   
                   
                   
                 Algorithm 
               
               
                   
                   
                 Prior Art L 
                 K Lines 
                 L Lines 
               
               
                 Unit 
                 Surfaces Tested 
                 Lines (mg/cm2) 
                 (mg/cm2) 
                 (mg/cm 2 ) 
                 Comments 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 143 B 
                 Kitchen N wall, plaster 
                 0.26 
                 2.30 
                 2.23 
                   
               
               
                 143 B 
                 Kitchen E wall, plaster 
                 0.3 
                 1.90 
                 1.49 
               
               
                 143 B 
                 Kitchen W wall, plaster 
                 0.4 
                 2.00 
                 1.58 
               
               
                 143 B 
                 Bath ceiling, plaster 
                 0.13 
                 1.40 
                 0.35 
                 Large K error 
               
               
                   
                 Bath ceiling 
                 0.13 
                 1.3/Prec 0.5 
                 0.9 
                 Large K error 
               
               
                   
                 Bath ceiling 
                 0.16 
                 1.1/Prec1.4  
                 0.52 
               
               
                   
                   
                   
                   
                 0.65 
               
               
                 327 
                 Kitchen N wall, plaster 
                 0.00 
                 1.80 
                 0.00 
                 Covered 
               
               
                   
                 ¼″ sheetrock laminate 
                   
                   
                   
                 by ¼ sheetrock 
               
               
                 327 
                 LR window casing, wood, 
                 0.40 
                 7.10 
                 2.89 
                 Covered 
               
               
                   
                 E wall - 1/″ luan laminate 
                   
                   
                   
                 by ¼ wood 
               
               
                 327 
                 BR 1 window casing, wood N 
                 0.50 
                 8.30 
                 8.97 
                 Covered 
               
               
                   
                 wall - ¼″ wood laminate 
                   
                   
                   
                 by ¼ wood 
               
               
                 347 
                 Kitchen N wall, plaster 
                 0.08 
                 8.90 
                 3.61 
               
               
                 347 
                 Kitchen E wall, plaster 
                 0.60 
                 8.10 
                 4.60 
               
               
                 347 
                 Kitchen W wall, plaster 
                 0.80 
                 8.70 
                 2.79 
               
               
                 347 
                 Kitchen door casing to Den 
                 0.50 
                 13.00 
                 4.28 
               
               
                 347 
                 LR baseboard, wood, W wall 
                 0.50 
                 12.80 
                 2.69 
               
               
                 347 
                 LR window sill, wood 
                 1.40 
                 8.20 
                 10.83 
               
               
                 347 
                 LR window sash, metal 
                 0.00 
                 −0.50 
                 0.00 
               
               
                 347 
                 LR closet door, wood, int 
                 0.90 
                 11.80 
                 3.90 
               
               
                 347 
                 LR patio door casing - chip 
                 0.04 
                 17.40 
                 2.36 
               
               
                   
                 top 
               
               
                   
                 LR patio door casing - chip 
                 10.10 
                 19.10 
                 15.00 
               
               
                   
                 bottom 
               
               
                 347 
                 Bath S upper &amp; lower wall, 
                 0.40 
                 9.90 
                 2.21 
               
               
                   
                 plaster 
               
               
                   
               
             
          
         
       
     
     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/cm 2  or higher. If the lead concentration is from 0.5 to 1.5 mg/cm 2 , 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/cm 2  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.