Anal. Chem. 1982, 5 4 , 419-422 Jaklevlc, J. M.; Qattl, FI. C.; Gouldlng, F. s.; Loos, B. w. Environ. Sci. Techno/. 1981, 15, 600-686. Evans, R. D. “The Atc~rnlcNucleus”; McGraw-Hill: New York, 1955; Chapter 28. Hayes, J. M.; Schoeller, D. A. Anal. Chem. 1977, 49, 306-311. Feller, W. I n “Studies and Essays” (R. Courant Anniversary Volume); Interscience: New Yorlk, 1948; pp 105-115.
419
(8) Plell, J. D.; Drane, E. A. “Beta Gauge Response Dependence on Detector System Pulse Pile-up Effects”; Technical Note No. ES-TN-8 105; Northrop Servlces, Inc.: Research Trlangle Park, NC, 1981.
RECEIVED for review August 28, 1981. Accepted November
25, 1981.
Gelatin Calibration Standards for Quantitative Ion Microprobe Analysis of Biological Tissues Dachang Zhu,‘ Willlaim C. Harris, Jr., and George H. Morrison* Department of Chemistty, Cornell University, Ithaca, New York 14853
Secondary ion mass spectrometry, as applied to soft blologlcal tissue, has suffered from an inabllity to provide quantltative concentration data. Use of gelatln as calibratlon standards has been proposed as a solutlon to thls problem. This work was undertaken to improve and extend the scope and utility of these standards. By use of computer-controlled data acqulsttlon technlques, detection llmlts for six trace elements, B, Ba, Cu, Mn, Rb, and Sr, were reduced to 2-15 ppm and the precislon of the data improved to 5 % relative standard deviation. Mass Interferlence by matrlx molecular species, A problem common to the determlnation of many elements, was ellmlnated by the use of energy dlscrimlnatlon and hlgh mass resolution In the determinatlon of magneslum.
Since Galle, Blaise, and Slodzian (1)first used secondary ion mass spectrometry (SIMS) to determine elemental distributions in biological samples, the technique has been used to study a variety of tissues (2-8). While quantitative analysis soft tissue research has been achieved with hard tissue (6,7), has remained mainly qualitative in nature (4). Recent efforta to provide quantitative results have centered on using doped gelatin or Epon plastic as calibration standards for ion microprobe analysis (9). Gelatin was originally rielected due to the general similarity of its chemical composition and secondary ion mass spectrum to those of soft tissue, leading to the assumption that matrix effects would also be similar. It has the further advantages of being readily available, compatible with the high-vacuum requirements of SIMS instruments, and easily doped with a variety of trace elements of interest in biological systems. Experimental data published to date has shown this approach is successful in yielding linear calibration curves over a large range of concentrations for several elements with a precision of 15-20% a t best (9). However, a number of serious experimental problems encountered in the SIMS analysis of biological tissues presently limit the applicability of gelatin and other standards to quantification. Moleculw ion mass interferences obscure the entire region from mass 12 to approximately mass 90, rendering the determination of trace concentrations of biologically important elements within this mass range very difficult without resorting to techniques which reduce these interferences. Two methods used to reduce molecular interference in SIMS analysis are high-resolution mass spectrometry (10) Permanent address: Zhu, Dachang, Department of Chemistry, Fudan University, Shanghai, The People’s Republic of China. 0003-2700/82/0354-0419$01.25/0
and energy discrimination against molecular ions (1I ) . The energy discrimination technique involves moving the ion energy acceptance band-pass of the spectrometer to higher energies. This will discriminate against molecular ions since their energy distribution shows a peak a t low energy followed by a very rapid decay as opposed to elemental ions which generally have an appreciable high energy component. However, both of these techniques reduce the signal intensity of the element of interest, thereby contributing to another limitation, that of sensitivity. Low sensitivity to the trace concentrations of various elements found in soft tissue is a problem for many biologically important elements. Due t o the inherently insulating nature of biological samples, thin sections (
