The venting system has been in use for over 18 months, under a wide variety of conditions. Data have been gathered with column temperatures ranging from 50 to 325 "C, column flows from 15 to 60 ml/min, and column lengths that varied from 2 to 12 feet. No adverse effects of the
performance of the GC or the efficiency of packed columns have been observed. Received for review December 21, 1972. Accepted November 23, 1973. Manufacturers' names are used for product identificationonly.
Internal Standard Addition Method for Preparation of Homogeneous Powder Mixes J. F. Jaworski, R. A. Burdo,l and G. H. Morrison D e p a r t m e n t of Chemistry, Cornell University. Ithaca, N. Y . 14850
These solution doping procedures fall into two categories. An internal standard solution is added to the powone of the major difficulties of calibration in spark source mass spectrometry (SSMS)( 1 ) . Because of the lack of homogeneity resulting from the addition of internal standard in solid form followed by successive powder dilutions, a number of solution doping procedures have been proposed (2-8). These solution doping procedures fall into two categories. An internal standard solution is added to the powdered sample, the solvent is evaporated by heating or freeze drying. and the resulting doped sample is blended with graphite or other conducting medium to form the mix from which electrodes are pressed (2-4). The internal standard and sample, both in the same solution, are added to graphite, the solvent is evaporated, and the mix is blended with the powdered sample (3, 5-8). Neither of these two approaches was found by this laboratory t o be universally applicable to the preparation of conducting electrodes from complex samples such that the electrodes contained a homogeneously dispersed internal standard. The first of the above approaches is subject to inhomogeneities resulting from specific interactions occurring between the sample and the ions of the internal standard-ie., ion exchange. The second approach results in poorer detection limits because of the limited solubility of many samples in solution. If not completely in solution, sample precipitates will cause selective adsorption with resultant inhomogeneity. It also fails to utilize the main advantage of SSMS,the direct analysis of solid samples. A relatively rapid method is described here which allows the preparation of homogeneous internal standardsample mixes in SSMS without the limitations of previous methods. The method involves solution doping of graphite with internal standard(s) followed by freeze drying and blending. In this way, a large amount of graphite may be processed into a stable doped conducting medium which may be tested for homogeneity as well as contamination. This stock graphite may then be used to provide exactly the same amount of internal standard in the electrodes of both sample and calibration standard. Present address, Department of Chemistry, University of Rhode Island, Kingston, R.I. 02882. (1) (2) (3) (4)
G . D.Nicholsetai.. Anal Chem.. 39, 584 (1967). D. A Griffith eta/.. Talanta. 18,665 (1971). J. P. Yurachecketai..Anal. Chem , 41, 1666 (1969). P. F . S. Jackson and J. Whitehead, Analyst (London), 91, 418
(5) (6) (7) (8)
( 1968), A. W . Fitchettand R. P. Buck, Anai. Chem.. 45, 1027 (1973). G . H . Morrison and B. N . Colby. Anal. Chem . 44. 1206 (1972). S. S. C. Tongetai.. Anal Chem.. 41, 1872 (1969). I . H . Crocker and W . F. Meritt. Water Res.. 6, 285 (1971).
The precision of the internal standard lines measured within a single photoplate is approximately *5% RSD while the precision of analysis using a standard for sensitivity calibration is approximately *lo% RSD or better for 80% of 42 elements analyzed in geological samples. In addition to the preparation of internal standard mixes, the procedure can be used to prepare homogeneous solid synthetic samples for SSMS and emission spectrometry. EXPERIMElNTAL Ten grams of high purity graphit.e, suitable for making pellets (Spex Industries, Metuchen, N.J.), were weighed into a clean 60-ml polyethylene bottle. High purity In metal and high purity Re03 were each dissolved in a minimum of reagent grade nitric acid and diluted with double distilled water to yield solutions containing 365 ppm In and 19.6 ppm Re. Four ml of the In solution, 3 ml of the Re solution, as well as 3 ml double distilled water and 5 ml spectral grade methanol were mixed in a polyethylene beaker and then added to the graphite in the polyethylene bottle. Several polyethylene balls were added and the mixture was shaken in a Spex mixer mill for 10 t o 15 minutes. Tlie resulting slurry was inspected for even wetting as well as for the absence of a liquid/graphite partition ( i . e . , phase separation) and was then quick-frozen in liquid nitrogen and freeze-dried in a conventional acetone-Dry Ice freeze-drying apparatus for 18 to 24 hours. The dried powder containing 146 ppm In, 5.88 ppm Re was placed in a vacuum oven for 3 hours a t 60 "C to drive off any residual moisture, then tumbled for a n hour to break up lumps. Three such batches of graphite 'were prepared simultaneously from the same internal standard stock solutions. One gram of graphite from each batch was added to 1 gram of LSGS standard rock W-1 and blended as reported earlier (9) to yield thrte mixes from which electrodes were pressed. Triplicate analyses were made on each batch, the results being recorded on Ilford Q2 photoplates and read using a Jarrell-Ash Model 23-100 scanning microdensitometer interfaced with a PDP-11/20 computer ( 1 0 ) .
RESULTS AND IIISCUSSION Indium and Re were chosen as internal standards because they are normally present in very low concentration in most complex samples. Other criteria for the selection of suitable internal standards have been discussed by Taylor (11). Their concentrations were chosen to permit analysis of higher level elements by comparison with Il5In, while low level elements were compared with 11%, IS5Re, and IS7Re. Use of two or more widely spaced internal standards makes it possible to determine whether variations in results are due to changes in plate sensitivity or sample homogeneity. (9) G . H. Morrison and A . M . Rothenberg, A n a l . Chem , 44, 515 (1972). (10) R. A. Burdo, J. R. Roth and G . H. Morrison, Anal. Chem , 46, 701 (1974). (11) S. R. Taylor, Geochim. Cosmochim. Acta. 29, 1243 (1965) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 6 , M A Y 1974
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Nitric acid was used in the dissolution step since other acids contribute F, C1, P, or S blanks which precludes their determination in the sample. The In and Re were probably present as nitrates in the dried doped graphite. Polyethylene vessels were employed instead of Teflon to avoid a fluorine blank. Methanol was added to lower surface tension of the solution, permitting thorough wetting of the graphite. The proportion of liquid to graphite was kept in the range 1.5 to 2 ml of internal standard solution per gram of powdered graphite in order that a slurry be formed with no liquid left over. Excess liquid, when evaporated, would leave a thin layer of crystals on the walls of the container and on the surface of the bulk of the graphite ( 5 ) resulting in inhomogeneities. Graphite has the desirable property of being inert and, hence, not behaving as an inorganic ion exchanger. Therefore, provided that even wetting of the graphite has been achieved, the graphite slurry contained homogeneously dispersed internal standard. Quick freezing followed by freeze-drying was used to prevent the formation of crystalline layers of internal standard and to eliminate any chromatographic effects resulting from the drying step as reported earlier ( 5 ) . The doped graphite, when removed from the freeze-drying apparatus contained approximately 0.1 t o 0.5% moisture which was removed by heating a t 60 “ C for 3 hours in a vacuum oven. The relative standard deviation of photoplate exposures from the linear relationship between exposure value and calculated ion density (10) was used as an indicator of homogeneity. Typical values of that RSD for the internal standard lines lI3In, I1~In,le5Re, and IS7Re in the blend of W-1 basalt and doped graphite were approximately 5% based on experience with over 300 photoplates. Since the sensitivity of the Q2 emulsion has been quoted to fluctu-
ate from 5 to 60% within the same plate (12), the homogeneity of the internal standard is expected to be much better than that measured by photoplate detection. Morrison and Colby (6) have shown that electrode sampling precisions on the order of 3% are possible by using electrical detection and an electrode pressed from a mixture of pure graphite and an optimized synthetic sample prepared by solution doping of graphite. Since it is possible to achieve precisions on the order of 3% using optimized synthetic samples, it can be concluded that the main problem in preparing a homogeneous final mix is rendering both real samples and doped conducting medium homogeneous before they are blended together. The running of replicate analyses on different batches of doped graphite-sample mixtures constitutes a test of the reproducibility of the method. Comparison of sensitivity factors (SF) obtained in this laboratory from triplicate analyses of each of three of W-1-doped graphite mixtures showed that, for most elements, the SF agreed to better than &8% RSD (range 3 to 20%) within each triplicate run and to better than & l o % RSD (range 4 to 15%) among the averages for the three batches. Of necessity, these precisions reflect in part an uncertainty due to the unpredictable variation of sensitivity known to exist for the Q2 emulsion. Hence, the reproducibility of doping levels in different batches of doped graphite is expected at least to be better than 10% and probably lies in the 3 to 5% range. Received for review July 30, 1973. Accepted December 21, 1973. Ahearn, in ’ Recent Developments in Mass Spectroscopy K Ogata and T Haykawa, Ed , University Park Press, Tokyo, 1970,
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Split Crystal Ion Selective Membrane Electrodes Ronald Wawro and G. A. Rechnitz Department of Chemistry, State University of N e w York, Buffaio, N. Y 14214
Just as double beam techniques have greatly extended the usefulness of spectrophotometry, true differential measurements should offer advantages over conventional potentiometry with ion selective membrane electrodes. In a previous paper, we described ( I ) the necessary electronic instrumentation for differential potentiometry but relied upon separate membrane electrodes to serve as indicator and reference electrodes. Such an arrangement already offers certain advantages but does not allow realization of the full potentialities of the differential technique. We now describe a novel split crystal membrane electrode (Figure 1) which permits almost perfect matching of electrode characteristics and allows “double beam” measurements to be made by simultaneously operating one side of the split electrode as a reference and the other as the indicating electrode. Aside from the convenience of having matched electrodes, this new system has the important advantage of minimizing the need for restandardizntion with changes in conditions or in prolonged use. Tihis desirable feature results from the fact that the split sensor is formed from a single membrane which undergoes (1) M J D
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uniform aging with time, specific use, or temperature changes. Although the general principle of the split membrane approach should be applicable to many membrane electrodes, we selected the AgZS/AgI type iodide sensor for detailed study because of our previous experience with this electrode (2, 3) and its known reliability. When used in the differential mode, the split membrane sensor eliminates the need for an external reference electrode and should be useful for continuous monitoring, automated analysis, and process control.
EXPERIMENTAL Apparatus. Potential measurements were made using an Orion Model 801 digital meter. For some evaluation experiments an Orion Model 90-01 reference electrode was also used. Differential measurements were made with our dual high input impedance amplifier (1) connected to a Varicord Model 43 potentiometric recorder. Sample and reference flow streams were introduced using a Model 975 Syringe Infusion Pump (Harvard Apparatus Co., Millis, Mass.) (2) J. D . Czaban and G . A . Rechnitz. Ana/. Chem.. 45, 471 (1973). (3) H . Thompson and G . A . Rechnitz. Chem. instrum., 4, 239 (1972)
