INSTRUMENTATION
ANALYSIS 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 to 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
MEASUREMENT
of the energies of
fluorescent X-rays has long been used for the identification and deter mination of the elemental composition of a sample. It is theoretically possible with the X-ray fluorescence technique to identify all of the elements in the substance without altering its compo sition or characteristics. Generally, it is difficult to determine elements below atomic number 12, owing to absorption effects. Two methods are in general use in X-ray fluorescence analysis—wave length dispersion and energy dispersion. Wavelength dispersion by use of a dif fraction 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, collima tors, and detector used. The energy dispersive method pro vides an efficiency gain of a factor of 100 or more by eliminating the diffrac tion crystal and collimators and bring ing the detector (one whose output is a function of the energy deposited) close to the sample being analyzed. With this method the entire spectrum of X-
ray energies may be measured simul taneously by electronic sorting of the detector pulses. The excitation flux available with X-ray tubes has afforded greater sensi tivity with this source than may be ob tained 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 with secondary fluores cent 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 without time-consuming chemical ma nipulations 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 continuing development of isotope excited applications of X-ray fluores cence analysis. The rapid develop ment of silicon and germanium detec tors with energy dispersion capable of resolving Κ X-rays from adjacent ele ments down to sodium has contributed to the increase in the number of appli
cations. With the use of these de tectors, it is often possible to make nondestructive, multielement determi nations in the ppm range in a single sample counting period on the original sample material. As is the case of any such method, a period of instrumental development is followed by exploitation in applied fields and often by a slowing in basic discoveries. A return to the examina tion of the fundamental variables often yields further improvements and added capabilities. The interaction of a num ber of fundamental factors in isotope source X-ray fluorescence is the basis for this discussion. The exploitation 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 con sidered in the development of isotope source X-ray fluorescence systems are the following: Chemical and physical properties of the sample Isotopic source characteristics Detector resolution and efficiency Source-sample-detector geometry
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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Instrumentation This latter parameter has received major emphasis in this discussion. Sample Properties
Six absorbance ranges five optical systems four controller models three flow cells two monitoring channels one Peak Separator And a partridge in a pear tree
ISCO UV absorbance monitors offer you all this (except the partridge, etc.) and more. Linear absorbance r e c o r d i n g . Stable circuitry. Built in recorder. Nar row bandwidth. And by far the lowest cost of all high performance monitors. These instruments can monitor two col umns at once (or one c o l u m n at t w o wavelengths). They operate at 254 nm, 280 nm, or other wavelengths to 950 nm. The ISCO Peak Separator will put each peak in a different test tube, and an ISCO integrator will print out peak areas. These are some of the reasons ISCO UV absorbance monitors have been so popu lar for years. Thousands are in use throughout the world. Write or call for our catalog describing these monitors, as well as ISCO fraction collectors and many other instruments for b i o c h e m i c a l research.
ISCO
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T h e chemical composition is the major determinant of absorption and enhancement effects in t h e q u a n t i t a t i v e determination of t h e element or ele ments sought. Selection of the char acteristic X-rays to be measured m u s t account for these effects as well as t h e possible interferences from adjacent, possible overlapping lines of t h e other elements present. F o r each element to be determined, absorption coefficients must be considered, and fluorescence yields also must be intercompared to estimate probable maximum sensitivity for each possible line choice. T h u s , a compromise choice m a y be dictated by t h e interaction of the avail ability of lines sufficiently free of inter ference effects, t h e absorption factors (both total sample and elements sought) for exciting photons, and t h e resulting fluorescence efficiencies. Isotope Sources Fluorescent X-rays can be excited by particles or photons emitted by the m a n y radioactive isotopes now readily available. F o r most applications the penetrating power of -y-rays and the available activities of t h e isotopes which emit such photons m a k e these t h e iso topes of choice. No effort will be made to describe t h e wide variety of isotopes available or t o consider excita tion b y either a or β particles. R a t h e r , t h e discussion will be related to the im p o r t a n t considerations which result in an o p t i m u m design of an excitation source using 7- or X - r a y - e m i t t i n g isotopes. T h e isotopes to be considered must be evaluated for t h e physical and chemical forms available, purity, half-life, specific activity, toxicity, and cost. T h e prob lems of radiation safety generally limit source size to tens of millicuries with gross photon o u t p u t s in t h e range of ] 07 to 10 s photons/sec. T o avoid contamination through dis persion of t h e isotope, t h e source is normally built in an integral container and collimator. I n this arrangement t h e relation of desired flux and photon energy m u s t 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 t h e photon energy used for excitation of t h e fluorescent X-rays. For multielement use t h e energy m u s t be sufficient to excite t h e lines of all elements of interest. T h e overlap of t h e L lines of heavy elements with t h e Κ lines of t h e lighter elements generally makes it advantageous to use exciting
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
photons of energy no greater t h a n the Κ absorption edge of t h e highest atomic number element sought. I n single element applications, op timization brings additional factors into consideration. Reduction of back ground and enhancement of signal can be achieved by careful selection of t h e isotope. T h e selection of an isotope with a photon energy only slightly higher t h a n t h e absorption edge will result in t h e maximum probability of excitation. If a line with high fluores cence yield has been chosen, t h e only further independent determinant of signal strength is t h e source flux. Background reduction is always im p o r t a n t and depends on several factors. T h e presence of interfering lines from the source m a y arise through scatter of emitted photons other t h a n t h a t used for excitation, from scatter and energy degradation of the exciting photon, and from secondary photons from shielding or collimator. Source-emitted inter fering photons m a y be from the princi pal isotope, an i m p u r i t y isotope, or a d a u g h t e r product of either of these. Source isotopes with stable d a u g h t e r products are desirable. E v e n in the best systems, scattered primary photons remain a major portion of t h e background. T h e energy and intensity of t h e scatter are related t o scatter angle, and it m a y be possible to take a d v a n t a g e of this fact. F o r ex ample, a t primary energies < ~ 5 0 keV, the minimum backscatter energy occurs a t a scatter angle of 180°, whereas t h e minimum flux occurs a t a 90° angle. T h e o p t i m u m use of this phenomenon will be discussed under geometrical factors. Detectors For most applications the detector choice m u s t be made between scintilla tion crystals and semiconductor de tectors. T h e detection efficiency of a gas proportional tube generally is n o t high enough for use in energy dispersive analysis. Although scintillation detec tors afford high efficiency and ruggedness and are readily built into portable sys tems, t h e resolution of these detectors is poor, and most systems using t h e m incorporate so-called "balanced filters" to reduce background counts. W i t h this technique, background is measured in an energy region slightly higher t h a n t h a t of the line of the ele ment being measured with a filter in place t h a t has a high absorption co efficient for t h e desired line. T h e first filter is then replaced by one which passes t h e line b u t cuts off all lower energies. Any increased counts with the second filter in place are dvie to fluorescent X-rays of t h e element sought. This system is effective in
FTCA50, for light-scattering measurements of uncommon accuracy, automatically.
Because our light-scattering photometer has a uniquely designed index vat which keeps stray light at a negligible level. For example, pure solvent or low molecular weight solutions will give a dissymmetry lower than 1% in the angular range 30-150. And you get unmatched stability, because of a true double-beam system. Operates over the temperature range from - 10°C. to 300°C. Fully automatic operation gives calculation corrections of the rough scattering values. No errors. Saves on labor. Over 10 years of improvement and modification has made the FICA 50 line the recognized leader among lightscattering photometers. For more information, call Duane Harmon collect at (716) 385-1000, or complete the coupon below.
It all adds up. Let us demonstrate. Bausch & Lomb, Analytical Systems Division, Dept. AC-1250, 820 Linden Avenue, Rochester, MY. 14625 Name Title Firm/Institution Address State
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Instrumentation
cases where samples are identical. The increased resolution available with semiconductor detectors may offer an advantage in selected cases. The latter detectors are based on lithium-drifted silicon [Si(Li)] or germanium [Ge(Li) ] single crystals. Resolution of 100-300 eV is readily available with these detectors, but only when the semiconductor is held at liquid nitrogen temperature, resulting in a definite decrease in ruggedness and portability. The choice between Si (Li) and Ge(Li) detectors is dependent on the energies of interest. Below 20 keV the Si (Li) full energy peak efficiency exceeds 95% for a 3-mm thick detector. Above ^ 3 0 keV, Ge(Li) would be the choice for most efficient absorption, >90%. Geometric efficiency, of course, increases with detector size, whereas resolution decreases with detector size. For example, Si (Li) detectors with areas of 300 mm2 may be obtained with resolutions of
