SAP3: a computer program for x-ray fluorescence data reduction for environmental samples [report]

K.K. Nielson
1977 unpublished
A FORTRAN computer program i s presented f o r q u a n t i t a t i v e m u l t i e l e m e n t a n a l y s i s o f s p e c t r a l d a t a f r o m b o t h i s o t o p e -and x -r a y t u b e -e x c i t e d x -r a y f 1 uorescence systems. The program i s designed f o r a n a l y s i s o f p e l 1 e t i z e d environmental samples (20-200 mg/cm2) u s i n g t h i n f i l m spectrometer c a l i brat i o n s and subsequent mathematical m a t r i x c o r r e c t i o n s f o r s e l f a b s o r p t i
more » ... l f a b s o r p t i o n , p a r t i c l e s i z e e f f e c t s and enhancement. A background-independent d i r e c t peak a n a l y s i s method p e r m i t s r a p i d d e t e r m i n a t i o n o f n e t peak areas and peak overl a p c o r r e c t i o n s . M a t r i x -c o r r e c t e d q u a n t i t i e s of 24 elements can be d e t e rmined from 1024 channels o f raw s p e c t r a l data i n 20-40 sec w i t h r e l a t i v e accuracies o f a few percent. Options f o r a p p l y i n g t h e program t o s p e c t r a from i n -s i t u seabed analyses a r e a l s o i n c l u d e d . A computer program, SAP3, has been developed t o automate spectral analysi s and quantitative interpretation of x-ray fluorescence data from envi ronmental samples. SAP3 was primarily developed f o r use with pelletized solid samples, b u t recent application has also been made t o in-situ seabed analysis. SAP3 smooths spectral data, computes net peak areas, and computes elemental concentrations from a thin film spectrometer calibration. I t then performs peak overlap corrections and three matrix corrections f o r ( a ) self absorption, (b) p a r t i c l e size and (c) enhancement. Spectral analysis and corrections require no operator attention beyond entry of sample parameters such as weight, analysis livetime, and i f p a r t i c l e s i z e corrections are desired, the estimated mean p a r t i c l e size and density. The PDP-15/20 computer used f o r data analysis has 24000 words memory and the input/output devices l i s t e d in Table I . The computer a l s o has seventeen sense switches which permit operator interaction with t h e program during execution. Table I1 l i s t s the options provided by t h e switches in SAP3. Sample Preparation. Samples t o be analyzed by e i t h e r of the laboratory systems are usually pelletized d i r e c t l y from 180 t o 1500 mg of dry sample in a 3.2 cm diameter, 27000 kg laboratory press. Geological and certain environmental samples such as flyash are similarly pelletized a f t e r f i r s t mixing with equal weights of cellulose (TLC reagent grade, J . T. Baker, Phillipsburg, NJ) in a tungsten carbide mixer mil 1 (Model 5100, Spec, Inc., Metuchen, N . J . ) . Coarse solids requiring powdering are ground several minutes in the mixer mil 1 before mixing with cellulose. N o preparation i s required f o r the seabed analyzer since i t r e s t s d i r e c t l y on the sediment surface under fresh or s a l t water. Cal i brati on. Cal i brati on for SAP3 uses thin uniform deposits (50-1 20 pg/cm2) of single metals or binary compounds evaporated onto 0.013 mm mylar backings (Micromatter, Inc., S e a t t l e , WA). S e n s i t i v i t i e s determined from the f o i l s are plotted as a function of x-ray energy and the resulting curves are smoothed t o establish the mu1 tielement thin-sample s e n s i t i v i t y curves shown in Figure 2. The thin standards are analyzed under identical conditions to those used f o r the samples except in the case of the seabed analyzer. Peak overlap coefficients are also determined from these standards. Calibration of the s c a t t e r peaks r e l i e s on analysis of a standard reference material. The spectrum from t h i s analysis i s then analyzed with various s c a t t e r peak calibration constants (using switch 13) until satisfactory r e s u l t s are obtained. The constants are then entered in the library f o r use in future analyses. The constants depend only on the instrumental geometry and excitation conditions. PROGRAM DESCRIPTION Main Program. The controlling routine, SAP3, i s always resident in core and controls data analysis by calling various subroutines as i l l u s t r a t e d by the flowchart in Figure 3 . SAP3 also uses several sense switches ( 0 , 1 and 15) to give the operator control over input/output options as indicated in Table 11 . The array EXT i s also defined by SAP3 f o r use as hollerith sample numbers in accessing input data f i l e s stored on disk or DECTAPE. BEG. Subroutine BEG i s f i r s t called t o read l i b r a r y data and i n i t i a l i z e analysis parameters. The desired 1 ibrary name (FYLIB) i s read from the computer terminal, a f t e r which the l i b r a r y i s read from Disk 2 . The various library parameters are l i s t e d in Table 111 . Figure 4 shows a typical l i b r a r y f o r the zirconium secondary source on the Kevex 890 system. The computer terminal i s next used t o enter the following sample information: date analyzed, analysis livetime, particle size and density, sample series name, sample thickness (g/cm2) and beginning run number. The peak energy, c a l ibration, peak overlap and blank data are then l i s t e d on the l i n e printer as shown in Figure 5 . Parameters from log-log f i t s t o x-ray cross sections as compiled by McMaster e t a1 . ( 5 ) are next used to compute mass absorttion coefficients f o r a l l the heavy and l i g h t elements in the library. They are computed as the sum of the photoelectric, coherent and incoherent scattering cross sections, and are stored as cm2/pg i n the arrays XE, XH, XEL and XL in the form J J where u . ( i ) i s the mass absorption coefficient of element j f o r x-rays from J element i , pi(k) i s the mass absorption coefficient f o r element j f o r the J exciting radiation, (k) and p!(i ) are the corresponding coefficients for J the l i g h t element s e t , and o and + are the mean angles formed by the exciting and fluorescent radiation w i t h the specimen surface. The summary data f i l e , FYL, i s then established with a l l zero values and control returns t o SAP3. RED/MR51. If switch 0 i s o f f , data are t o be read in from paper tape using subroutine RED. Actual reading of the tape i s done in single 8-bit character increments using subroutine MR51. Each value returned by MR51 i s examined by RED until a non-zero number i s encountered. Successsive groups of three characters are then read and combined f o r individual channel counts until the preset number of channels (NOCH) have been read. Sense switch 10 i s checked by RED t o ha1 t the program until the operator has the paper tape ready. I ACKNOWLEDGMENTS I t h e MR51 subroutine w r i t t e n by J . R. Kosorok,
doi:10.2172/7213361 fatcat:dnv4cjjpcza57coorgehfuyqam