the case with the 1.93-wm band. Here, because of its relatively large extinction coefficient, trace amounts of water can Le noticed. Regardless of how quickly we manipulated our extractions, etc., the sample always picked up some water from the atmosphere. We could have forced the curve through zero by keeping the reference beam DMSO exposed to the atmosphere for an equal length of time, but did not think it worthwhile. Our main precaution in keeping the DMSO dry was the absorption tube arrangement shown in Figure 2. This arrangement prevented the DMSO source (Le., the gallon container) from picking up additional water with each sampling as would have happened without the absorption tubes. Although we did not do any azeotropic distillation of the wood molasses samples shown in Table 111, other molasses samples showed the futility of determining water in molasses by this technique. Wood molasses sometimes contains acetic acid which will distill with water (e.g., it forms an azeotrope with water) and is soluble in water. I t is therefore not possible to determine the water content of molasses by azeotropic distillation. However, this does point to an easy method for determining acetic acid content of wood molasses: water content can be determined by the near infrared method; the water plus acetic acid can be determined by azeotropic distillation; hence, the acetic acid content is known. This combination of techniques can be generalized to determine any water soluble azeotrope in any type sample if only one water soluble azeotrope is present. We first tried to use the 1.77-pm band to monitor the water content of black liquor, but results were consistently low. We hypothesized that the large amount of solids which precipitated in the DMSO occluded some water. When we used the 1.93-pm band, much less black liquor was needed, very little solids precipitated in the DMSO, and no occlusion problems were noted. If only a trace of water is present and/or there is a large amount of precipitation in DMSO, the 1.93-wrn band should be used. Under all other circumstances, either band can be used. Although we did not use a temperature thermostated cell compartment, it is doubtful that this could cause our rela-
tively poor precision since it has been reported that small changes in temperature (5 O C or less) have a negligible effect on peak height ( 1 8 ) . All glassware was oven dried and blown with dry nitrogen before use. A likely cause of our precision difficulties was the small sample size itself. Black liquor especially is very inhomogeneous after it cools. Since the work described here was done in a laboratory, the black liquor samples were a t room temperature. We did investigate the possibility of dissolved salts in the black liquor interfering with our analysis. Although black liquor dissolved salts did extract into the DMSO, they had no noticeable effect on our precision.
ACKNOWLEDGMENT The author expresses his thanks and appreciation to the management of International Paper Co. for permission to publish this paper.
LITERATURE CITED TAPPI Standard T650 su-7 1, B. B. Edmonds. Jr., Anal. Chem., 19, 820 (1947). P. E. Borlew and S. J. Lancaster, South. Pulp Pap. Manuf., 16 (6). 44 (1953). P. B. Borlew and S.J. Lancaster, Tappl, 36(11), 504 (1953). J. L. Parker, R. P. Hensel, and C. L. Wagoner, Tappl, 53 (5),874 (1970). W. R. Fegzer, Anal. Chem., 23, 1062 (1951). TAPPI Standard T484 m-58. TAPPI Standard T208 m-60. J. H. Phillips, Tappi, 34 (10). l O l A (1951). J. H. Phillips and M. M. Rubright, Tappi, 36 (9), 392 (1953). R. M. West, Ind. Eng. Chem., 6, 31 (1916). D. M. Smith and W. M. 0.Bryant, J. Am. Chem. SOC.,57,841 (1935). John Ross, J. SOC.Chem. lnd., 51, 121-122T (1932). D. Chapman and J. F. Nacey, Analyst (London),83,377 (1958). J. D. S.Goulden and D. J. Manning, Analyst(London), 95, 308 (1970). H. F. Cordes and C.W. Tait, Anal. Chem., 29 (4), 485 (1957). E. R. Rader, J. Assoc. Offic.Anal. Chem., 50 (3), 701 (1967). D. W. Vomhof and J. H. Thomas, Anal. Chem., 42 ( l l ) , 1230 (1970). P. F. Vornheder and W. J. Brabbs, Anal. Chem., 42 (12), 1454 (1970). M. Violante, Ph.D. Thesis, The Florida State University, Tallahassee FL, 1970. H. Yamatera, E. Fitzpatrick, and G. Gordon, J. Mol. Spectrosc., 14, 268 (1964).
RECEIVEDfor review October 14, 1974. Accepted January 9, 1975.
Total-Reflection X-Ray Fluorescence Spectrometric Determination of Elements in Nanogram Amounts Peter Wobrauschek and Hannes Aiginger Atominstitut der Oestereichischen Hochschulen, A 1020 Wien, Schuettelstrasse 1 15. Austria
Energy dispersive fluorescence analysis enables a quick and simultaneous determination of different elements in a sample. The method presented here utilizes X-ray total reflection on the polished surface of the sample substrate and a special detector-sample geometry. That improves the signal to background ratio significantly and consequently the sensitivity too. The glass substrate combines its good mechanical strength, chemical resistance, and physical definiteness with the low background properties of thin foils. The effective thickness of some hundred Angstroms of the glass substrate is defined by the penetration depth of the X-rays in the case of total reflection. For one run, taking a 120-sec 852
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
counting period, 5 pl of sample volume are required. The application of the method seems to be advantageous where only small quantities of sample volume are available and concentrations ranging down to the parts per million level have to be measured.
Photon induced X-ray fluorescence spectra from a sample are used for nondestructive analysis. Compared with wavelength dispersive techniques, the energy dispersive spectrometer system provides simultaneous detection of different elements in a short time. Using mainly K-series
air
matter
Figure 1. X-Ray refraction-reflection phenomena for ( a ) cp > pcrit (b)cp =
X-rays and a Si(Li) detector together with a multichannel pulse height analyzer, the resolution of the detector (190 eV FWHM a t 5.9 keV) allows one to distinguish between adjacent elements up from 2 = 11 (Na). The use of energy dispersive systems for all elements is restricted in general for two reasons. The limit for low energy fluorescence radiation is given by absorption in the entrance window. Using a Be-foil of 0.025-mm thickness as window material, a full energy detection efficiency of approximately 8% for NaKa energy is obtained ( 1 ) . The efficiency increases rapidly to approximately 100% with increasing energy of the fluorescence radiation. The high energy limit results from incomplete absorption due to the finite thickness of the crystal, respectively its sensitive volume. In the case of the Ge(L,i) detector which is favorable for higher 2,the K shell absorption jump a t 11.1keV must be considered. The problem in detecting quantities in the nanogram region is to get a signal which can be clearly distinguished from the background. The detection limit is given, if the number of counts from the net signal equals three times the standard deviation of the background counts for a given unit of time (2) which can be written
From this, one can see the necessity of background reduction to improve the detection limit. Using X-rays as the excitation source of a sample, there are four competing processes in this energy region. Photoeffect, elastic and Compton scattering, and Auger effect. These effects, except the Auger effect, cause background, where the main part is due t o an intense flux of elastic and Compton scattered photons. The Auger effect does not contribute to an increased background, as the emitted electrons of different but low energy are absorbed in the Be foil of the detector entrance window. The method used in this work to reduce the background is the application of ‘X-ray total reflection on the polished surface of an optical reflector and a special choice of the detection geometry ( 3 ) .X-Ray total reflection occurs if an X-ray beam enters from vacuum or air into matter. The critical angle of total reflection is very small. Deriving from classical dispersion theory and neglecting quantum effects and resonance phenomena, the critical angle pcrit may be written ( 4 )
where q is expressed in radians; 2 is the atomic number of the reflecting substance; A , the atomic weight of the reflecting substance; p , the density of the reflecting substance (g/cm3));A, the wave length of reflected X-rays (cm); and where p is the angle between the direction of the incident beam and the reflecting surface as can be seen in Figure 1. The critical angle of total reflection for quartz and CuKa radiation is 0.004 rad. The penetration depth defined as that thickness where the intensity of the primary X-rays Io is reduced to I = Ioexp(-l), can be written
Pcrit ( C ) (0
< pcrit (3)
where X is the wavelength of reflected X-rays (cm); and T is the linear mass absorption coefficient (cm-l). A typical value for X, in the case of CuKa radiation and quartz as reflecting material is 0.04 pm. This ultra thin effective thickness of the substrate reduces the contribution from coherent scattering. Another advantage of X-ray total reflection is the fact that there is nearly no excitation of the sample carrier as photoeffect occurs only in the penetration depth. The intensity of characteristic radiation from the substrate is therefore decreased compared with measurements a t an angle above pcrit. A further advantage of the method is the efficient excitation due to the long primary path length in the sample: excitation is effected by both the incident and reflected beams. X-Ray total reflection is not only restricted to crystalline matter but can also be observed using amorphous substances. As the values of pcrit are small, the reflector has to show a high degree of flatness to assure undisturbed propagation of the X-ray beam. A perfect polished surface is required to obtain total reflection. Polishing also reduces background as remaining microgrooves from insufficient treatment act as scattering centers.
EXPERIMENTAL An optical reflector made of quartz-glass (Suprasil) with dimensions 25 X 10 X 1 mm with a guaranteed flatness of X/l5, where X = 589 nm, was used. Samples were salts dissolved in deionized water of highest purity. Solutions of one-element standards were made. Different concentrations were obtained by dilution. The optical reflector serves as sample carrier. The surface of this substrate was carefully cleaned and a thin layer of “Insulin Novo” (Novo Industri A/S, Kopenhagen, Denmark) was put on it, to get a homogeneous distribution of the 5-pl sample solution which was deposited on it and dried afterwards using an infrared heating lamp. The covered area is about 0.5 cm2. The whole procedure to prepare the sample takes about 5 minutes. The construction of an X-ray optical bench was necessary. This basic arrangement is described in ( 5 ) .A heavy I-profile steel girder (200 X 100 X 8 mm) of 2.6-m length is the base of the X-ray optical bench. An X-ray tube with power supply from a 100-kV stabilized generator is provided. The line focus of the X-ray tube is used. A collimator system of two slits each 0.05 mm defines a parallel beam. A vacuum chamber is mounted on a goniometer and can be rotated and moved transversally, so that the sample and substrate mounted inside the vacuum chamber can be placed in any desired position t o the X-ray beam. The rotation of the vacuum chamber is performed by a micrometer which acts on a lever of defined length. The X-ray beam enters the vacuum chamber through a Hostaphan window, irradiates the sample, and is reflected totally on the surface of the optical reflector. The beam leaves the chamber through a second Hostaphan window collinear to the first. A G-M counter mounted on a cross slide controlled by a micrometer screw enables measurement of the actual reflecting angle and also allows determination of the intensity of the X-ray beam by an electronic counting system. The Si(Li) detector is situated inside the vacuum chamber to detect also the elements emitting low energy fluorescence radiation. Its entrance window is parallel to the incident beam, so that only fluorescence radiation emitted orthogonal t o the direction of the incident beam can be seen by the detecANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
053
1
140
'
350
1
Flgure 2. Experimental arrangement for X-ray total reflection and Xray fluorescence spectrometer system (all measures in mm) tor. The schematic arrangement ( 6 ) and the necessary equipment can be seen in Figure 2. The reduction of Compton scattering background was performed using a 90° geometry between the incident X-ray beam and the detector. The differential cross section for Compton scattering is given in the non relativistic limit as 2
-
1. (moc2 L )(1 +
dn - 2
cos* .9)
where 9 is the scattering angle. For 9 = 90' as in our arrangement, this expression becomes a minimum. 500
RESULTS AND DISCUSSIONS
Concentration
Usually the X-ray fluorescence intensity is not a linear function of the concentration but, by neglecting matrix effects, the intensity may be calculated using the formula
(5) where I is the intensity of fluorescence radiation (cps); l o , the intensity of the incident radiation (cps); G, the geometry factor including solid angle; T, the photoelectric mass attenuation coefficient for energy of incident radiation and the respective sample material (cm2/g);s k , the absorption jump a t the K-absorption edge; Wk, the fluorescence yield of the K-shell; t, the detector efficient including the Be window absorption; f , the attenuation factor due to the path from sample to detector; p a , the percentage of K a emission from total K-shell emission; A, the absorption correction; and m, the areal density of sample (g/cm2). The absorption correction is described by
Figure 3. Calibration curve for Cr and Mn
Table I. Experimental Results after 120-Second Counting Period Using a Cu-Anode Operated at 40 kV/16 mA Concentration,
Element
ngI5 u l
PPm
Intensity, cps
Cr
673 336 192 112 61 5.5 1021 511 340 202 170 108
134.5 67.2 38.4 22.4 12.2 1.1 204.2 102.2 68 40.4 34 21.6 2.2
85.2 i 1.4 41.3 i 1.1 26.1 i 0.3 13.1 i 0.8 9.0 i 0.4 0.84 f 0.1 150.0 i 2.1 74.8 i 1.1 50.4 i 1.9 29.8 i 0.9 25.9 i 0.3 15.0 i 0.7 1.7 i 0.2
Mn
11
where pl is the total mass absorption coefficient for the energy of the incident radiation and sample material (cm2/g); p z , the total mass absorption coefficient for the fluorescence energy and sample material (cm2/g);m, the mass per unit area (g/cm2); p, the angle between the incident radiation and the surface of substrate; and $, the angle of observation. The expression for A may be written more generally (1 e - X ) / x , with x = [(pl/sin p) + ( & , i n $)] m which conver0 to 1. In our case, m is of magnitude ges in the limit x g/cm2, p representing the critical angle of X-ray total reflection or values below is in the range 10-3 rad and sin $ = 1. Let us assume values for p1 and 12 to be 102 cm2/g, one obtains for the exponent where the term p2/sin $ is neglected. In this case A becomes 1. The attenuation factor f also equals 1, as we used a vacuum chamber. For two different elements of the same concentration, using the same geometry and primary intensity, l o , the relation between the fluorescence intensity from element 1 and 2 is expressed by -+
854
*
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
Values for the photon cross sections may be found in ( 7 ) . The absorption jump may be also calculated from values in ( 7 ) .For data on K fluorescence yield, the review paper ( 8 ) should be consulted. Using this ratio, a comparison of intensity vs. concentration between different elements is possible. Starting with standards of known concentration of one element, expected values for the intensity of other elements in various concentrations can be calculated. To determine linearity of the technique, the elements Cr and Mn were investigated and the experimental results are given in Figure 3. The liquid concentration is given in ng/5 p1 as this represents the quantity investigated after evaporation in each run. The counting period for each concentration was 120 sec. Table I shows detailed values for the in-
tensity which are given in the form f & 5. The confidence limits were calculated using
where a t least four measurements were taken. Taking, for example, the peak to background ratio 113/ 800 from the 5.5-ng Cr probe, the minimum detectable quantity (MDQ) can he extrapolated to be 4 ng (0.8 ppm). CONCLUSIONS The work presented here shows results of two elements. Multielement standards are in preparation to investigate the detection limit in this case. The precision of the measurements depends 011 the weighing accuracy, the exact deposition of the probe on the substrate, and the instrumental precision of substrate alignment. The attainable sensitivity may be improved by the use of a semiconductor de-
tector with larger sensitive volume, slightly modified experimental arrangement for more exact positioning of the substrate, improved surface treatment of the substrate to reduce remaining surface defects, and a semiautomatic sample preparation for better reproducibility. LITERATURE CITED (1) D. A. Gedcke, X-Ray Spectrom., 1 136 (1972). (2) IUPAC Comission V4 on Spectrochemical and other optical procedures of Analysis, Appendices on Tentative Nomenclature, Symbols, Units and Standards, No. 26 (Nov. 72) p 17. (3) Y. Yoneda and T. Horiuchi, Rev. Sci. Instrum., 42, 1069 (1971). (4) M. A. Biochin, "Physik der Rontgenstrahien". VeB Verlag Technik, Berlin, 1957. (5) W. W. Weiss, Messtechnik (Braunschweig),80 (5). 127 (1972). (6) H. Aiginger and P. Wobrauschek, Nucl. Instrum. Methods, 114, 157-158 (1974). (7) E. Storm and H. I. Israel, Nucl. Data Tables, A7, 565-681 (1970). (8)R. W. Fink, R. C. Jopson, H. Mark, and C. D. Swift, Rev. Mod. Phys., 38, 513 (1966).
RECEIVEDfor review September 17, 1974. Accepted January 13, 1975.
Elemental Trace Analysis of Small Samples by Proton Induced X-Ray Emission Thomas B. Johansson,' Rene E. Van Grieken,2 J. William Nelson, and John W. Winchester Depadments of Oceanography and Physics, Florida State University, Tallahassee, FL 32306
Quantitative analysis of 12 to 15 elements simultaneously by proton-induced X-ray emission, PIXE, has been tested for thin small inhomogeneous samples such as impactor collected aerosol size fractions using a low energy, 3.7MeV, proton beam from a Van de Graaff accelerator. The procedure and a number of tests of the performance of the system and the analytical methodology are presented. Detection limits for multielement samples in a 10-min bombardment are in the nanogram range, and standard deviations are as low as 6 % for analyses of homogeneous samples and 15% for heterogeneous samples. The method is calibrated to be absolute in the sense that standards are needed only to check the stability of the system.
Elemental analysis requirements of present environmental quality investigations have indicated the need for ultrasensitive, multielement procedures, capable of ready application to air, water, soil, and biological samples. The recent renewed expansion of scientific literature on X-ray emission spectrometry illustrates that this technique has become an increasingly important tool for elemental analysis, especially since Si(1,i) detectors were introduced a few years ago. Although energy-dispersive X-ray fluorescence, like any other available technique, cannot measure all elements of interest in the concentration ranges of environmental samples, it has the potential of providing a rapid, nondestructive, low-cost, multielement analysis capability needed to investigate and monitor the behavior of many interesting elements in various matrices. X-Ray emission by charged particle excitation offers an attractive alternative to the X-ray tube ( I ) or radioisotope Present address, Department of Environmental Health, University of Lund, S-220 02 Lund 2, Sweden. Present address, Department of Chemistry, ,Antwerp University (UIA), B-2610 Wilrijk, Belgium.
source excited techniques. In 1970, the potential of heavy charged particles as an excitation source was recognized, and an interference-free sensitivity down to the picogram range was demonstrated (2). The excellent capability of accelerator beams for X-ray emission analysis is partially due to the relatively low background radiation associated with the excitation. The main contribution is Bremsstrahlung from secondary electrons in the sample. The very high intensity of particle fluxes obtainable from accelerators also contributes substantially to the power of the method. In a feasibility study, Walter et al. ( 3 ) have reviewed the development of PIXE and give references to the literature. Recently, Gilfrich et al. ( 4 ) have compared several approaches to X-ray elemental analysis. Folkman e t al. (5, 6) and Herman et al. ( 7 ) discussed the optimum choice of heavy particle and beam energy. They concluded that protons of a few MeV energy provide a preferred combination for high sensitivity analysis. As there are a large number of accelerators available that are capable of delivering such beams, the prospects for widespread use are bright. Proton induced X-ray emission analysis (PIXE) has been used in feasibility studies by Johansson et al. ( 8 ) , Campbell e t al. (9),Mandler and Semmler (10) and others and is currently applied to the analysis of aerosols by Cahill and Feeney ( I I ) , Flocchini et al. (12) and Johansson e t al. (13),to malaria infected blood by Barnes et al. (14),and to thick samples such as steel slabs or teeth by Ahlberg et al. (15). I t has also recently been applied to studies of aerosol trace metal transport in the St. Louis Regional Air Pollution Study by Winchester et al. (16) and Akselsson et al. (17). The present work describes the experimental apparatus designed for routine proton induced X-ray emission analysis of large numbers of samples and discusses the methodology of the analytical procedure. I t presents a thorough ANALYTICAL CHEMISTRY, VOL. 47, NO.
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