INSTRUMENTATI0 N
Isotope Excited X-Ray Fluorescence Major parameters in the development of isotope source-X-ray fluorescence systems are considered, including chemical and physical properties of the sample, isotopic source characteristics, detector resolution and efficiency, and source-sample-detector geometry. Possibilities for application t o sub-ppm simultaneous multielement analysis of blood appear promising
THEO. J. KNEIP and GERARD R. LAURER New York University Medical Center 550 First Avenue, New York, N.Y. 10016 of the energies of fluorescent X-rays has long been used for the identification and determination of the elemental composition of a sample. It is theoretically possible with the X-ray fluorescence technique to identify all of t h e elements in the substance without altering its composition or characteristics. Generally, it is difficult to determine elements below atomic number 12, on-ing to absorption effects. Two methods are in general use in X-ray fluorescence analysis-wavelength dispersion and energy dispersion. Wavelength dispersion by use of a diffraction crystal provides the highest possible resolution. However, because of the stringent collimator requirements, this method has rather poor efficiency, the overall result depending on the characteristics of the crystal, colliniators, and detector used. The energy dispersive method provides a n efficiency gain of a factor of 100 or more by eliminating the diffraction crystal and collimators 2nd bringing the detector (one n-hose output is a function of the energy deposited) close to the sample being analyzed. K i t h this method the entire spectrum of X-
IMI
EASUREMEST
ray energies may be measured simultaneously by electronic sorting of the detector pulses. The excitation flux available &li X-ray tubes has afforded greater sensitivity with this source than niay be obtained with isotopic sources of y- or X-rays. However, the bremsstrahlung continuum from the tube creates a problem of high background, and more complex tubes Ivith secondary fluorercent radiation from selected target elements are now coming into use. The isotopic sources generally offer simplicity and often portability in compensation for the loss of sensitivity. The ability to make a determination 11-ithout time-consuming chemical manipulations in preparing a sample for analysis is critical in many applications. This aspect, plus instrument portability through the use of a radioisotopic source for excitation, are the basic stimuli for the contiiiuiiig development of isotope excited applications of X-ray fluorescence analysis. The rapid development of silicon and germanium detectors with energy dispersion capable of resolving K X-rays from adjacent elements down t o sodium has contributed to the increase in the number of appli-
cations. K i t h the use of these detectors, it is often possible to make nondestructive, multielement determinations in the ppm range in a single sample counting period on the original sample material. .Isis the case of any such method, a period of instrumental development is followed by exploitation in applied fields and oft,en by a slowing in basic discoveries. A return to the examination of the fundamental variables often yields further improvements and added capabilities. The interaction of a number of fundamental factors in isotope source X-ray fluorescence is the basis for this discussion. The esploitatioii of these interactions is considered in describing three recently developed systems with capabilities exceeding those of currently available commercial equipment. The major parameters to be conqidered in the development of isotope source-X-ray fluorescence systems are the following: Cheinical and physical propertics of the saniple Isotopic source characteristics Detector resolution and efficiency Source-sample-detector geometry
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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four controller models three flow cells
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Instrumentation
be determined, absorption coefficient< must be considered, and fluorescence
for exciting photons, and the resulting fluorescence Isotope Sources
Fluorescent' X-rays can be excited by particles or photons emitted by the many radioactive isotopes now readily available. For most applications tlie penetrating pon-er of y-rays aiid the available activities of the isotope3 which emit such photons make these the isotopes of choice. S o effort n-ill be made to describe the wide variety of isotopes available or to coileider escitation by eit'her a or p particles. Rather. the discussion n-ill be related to the important coilsiderations which result in an optimum design of a n excitation source using y- or X-ray-emitting isotopes. The isotopes to be considered must be evaluated for the physical aiid chemical forms available, purity, half-life, yecific activity, tosicity, and cost. The problems of radiation safety generally limit source size to tens of niillicuries with gross photon outputs in the range of persion of tlie isotope, the source is normally built in a n integral container and collimator. In thi. arrangement the relation of desired flux and photon energy must be considered with regard to shielding material, beam collimation, and secondary X-rays from structural .materials. Of primary importance, of course, is the selection of the photon energy used for excitation of the fluorescent X-ra+. For multielemeiit use the energ be sufficient to escite the line elements of interest. The overlap of the L lilies of heal-y elements iyitli the K lines of the lighter elements generally makes it advantageous to use e x i t i n g
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
photons of energy no greater than the K absorption edge of the highest atomic number element sought. I n single element applications, optimization brings additional factors into consideration. Reduction of background and enhancement of signal can be achieved by careful selection of the isotope. The selection of an isotope with a phot'on energy only slightly higher than the absorption edge will result, in t,he maximum probability of excitation. If a line with high fluorescence yield has been chosen, the only further independent determinant of signal strength is the source flus. Background reduct'ion is always import,aiit and depends on several factors. The presence of interfering lines from the source may arise through scatter of emitted photons other than that used for escitation, from scatter and energy degradation of the exciting photon, and from secondary photon. from shielding or collimator. Source-emitted interfering photons may be from the principal isotope, a n impurity isotope, or a daughter product of either of these. Source isotopes with stable daughter products are desirable. Even in tlie best qystems] scattered primary phot,ons remain a major portion of tile background. The energy and intencity of the scatter are related to .scatter angle, and it may be possible to take ndvantage of this fact. For example, a t primary energies go%. Geometric efficiency, of course, increases with detector size, whereas resolution decreases with detector size. For example, Si(Li) detectors with areas of 300 mmz may be obtained with resolutions of
