installed which is typically 10 to 12 pairs for practical reasons. It is convenient, therefore, to check for possible mass interferences with the on-line computer system which might be missed during the course of an analysis using a manual switcher. The computer can be programmed to calculate and set a given electrostatic analyzer voltage so that a given mass can be analyzed by the computer even though the peak was not large enough to be observed on the oscilloscope display during the computer scan of the electrostatic analyzer voltage. In addition? considerable savings in time results from the use of the computer in locating and centering of the peaks. It is in the acquisition and reduction of data that the on-line computer approach is particularly appreciated since the speed of these operations permits almost immediate evaluation of the results. This allows for convenient accumulation of additional data or a change of experimental parameters if necessary while the experiment is in progress. A typical computer teletype readout is shown in Figure 3. The computer was programmed to start data collection and calculate mean, standard deviation, and relative standard deviation after a specified number of data points had been taken. In this case, using a beam monitor charge collection of 0.063 nC previously set by the counter, ten sets of ten measurements were made.
The time taken for the collection and reduction of this set of measurements was one and a half minutes. To test the measurement precision of the electrical detection system, a sample of 99.9 % pure iron was analyzed for its mass 58 isotope. By determining this minor matrix isotope, the measurement precision is uninfluenced by electrode sampling errors due to heterogeneity and therefore represents only the instrumental error. For ten sets of ten measurements the mean precision for the %Fe isotope was 2.3% RSD. This represents the present limit of the described system although attempts are being made to improve it further. More serious are the errors introduced by the samples themselves. An evaluation of precision in the analysis of a variety of materials using this system has been described elsewhere (10). RECEIVED for review September 27,1971. Accepted February 18, 1972. Financial support was provided by the National Science Foundation under Grant No. GP-6471X and the Advanced Research Projects Agency (DAHC 15-67-C-0214) through the Cornell Materials Science Center. (10) G. H. Morrison and B. N. Colby, ANAL. CHEM.,44, 1206 (1972).
Precision of Electrical Detection Measurements of Powdered Samples in Spark Source Mass Spectrometry G . H. Morrison and B. N. Colbyl Department of Chemistry, Cornell University, Ithaca, N . Y. 14850 The precision of spark source mass spectrometry for the analysis of powdered samples has been extensively examined using electrical detection. A variety of sample powders have been analyzed and indicate that analytical precision on the order of 3-6% can be expected. Electrostatic peak switching and computer on-line data acquisition and evaluation helped keep measurement time to a minimum. Several methods of sample pretreatment were used and the resulting electrode homogeneity was evaluated.
THE MAIN ADVANTAGES generally attributed to, the use of electrical detection over the traditional photoplate detection in spark source mass spectrometry are increased precision and speed of measurement. Regarding the evaluation of precision, most of the recent studies using electrical detection in the more precise peak switching mode ( I , 2 ) have been performed using fairly homogeneous metal samples containing a limited number of well-characterized impurities. While the accuracy of these studies is somewhat in doubt, because of the lack of high quality standards, the analytical precision has been determined to be 2-5 % relative standard deviation 1 Present address. Materials Research Laboratory, University of Illinois, Urbana, 111.
(1) C. A. Evans, R. J. Guidoboni, and F. D. Leipziger, Appl. Specfrosc.,24, 85 (1970). (2) R. A. Bingham and R. M. Elliott, ANAL. CHEM., 43,43 (1971). 1206
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7,JUNE 1972
for several metals. This is in contrast to 10-20% with photographic detection (3). Many materials currently of interest in spark source mass spectrometric analysis, however, are nonconducting powders of a complex nature. An analysis of rare earth doped scandium oxide ( 4 ) is the only mention of powder analysis by electrical detection other than semiquantitative scans (5, 6). Because of the large number of important trace elemental problems involving nonconductive materials ranging from geological to biological samples, this study will explore the role of electrical detection as a means of improving mass spectrometric precision in the analysis of graphite blended powdered samples. Nonconducting powders are generally blended with graphite or another suitable conducting matrix to sustain the R F spark. Several methods of sample pretreatment are used on a variety of materials in an effort to improve electrode homogeneity. EXPERIMENTAL
The graphite used in all samples was Spex Industries 1-200 USP Lot 600-0. All powdered samples were formed into (3) D. W. Oblas, J. J. Bracco, and D. Y . Yee, “Symposium on
Trace Characterization-Chemicaland Physical,” National Bureau of Standards, 1966, p 486. (4) R. J. Conzemius and H. J. Svec, Talunfa, 16, 365 (1969). ( 5 ) R. Brown and P. G. T. Vossen, ANAL. CHEM., 42, 1820 (1970). (6) R. Brown, P. Powers, and W. A. Wolstenholme, ibid., 43, 1079 (1971).
rods alaz inch in diameter and a/s inch long using an Associated Electrical Industries die. These were cut in half with a stainless steel blade and loaded into the instrument such that the analytical gap was located 4 mm from the accelerating plate with sparking along 2 mm of the electrode surface. All data were obtained using a Nuclide GRAF-2 spark source mass spectrometer using the computer on-line electrical detection system already described (7). A list of instrumental operating conditions is given in Table I. After initiating the spark and setting the spark gap, the spark gap controller (8) was activated and no further electrodeadjustments were made. Each sample was presparked for at least five minutes before any measurements were taken. With the prespark completed, the electron multiplier gain was adjusted such that the input to the voltage-to-frequency converter did not exceed 0.1 V and the computer was programmed to start data collection and calculate mean, standard deviation, and relative standard deviation (RSD) after the specified number of data points had been taken. The preset counter was set to 5000 corresponding to a beam monitor exposure of 0.063 nanocoulombs (nC). Using this charge collection, ten measurements were made before another isotope was selected. The procedure was repeated twice for each isotope in each electrode pair in a rotary fashion (mass 65-63-56-55-65-63- etc.) and from two to four electrode pairs were run for each sample. A spark breakdown voltage of 30 kV was maintained by the automatic spark gap controller. Powdered samples of both real and synthetic nature were prepared. The synthetic samples were of two types; the first of these was solution doped graphite and the second was solid doped graphite. Both of these sample types were designed to avoid the possibility of spectral interferences which was, of course, not possible with the complex real samples. A correspondence between the elements in the real and synthetic samples was adhered to as much as possible. Synthetic Samples. SOLUTIONDOPEDGRAPHITENo. 1. A series of stock solutions was used to prepare a solution containing six elements such that 300 pl of the solution added to 600 mg of graphite would produce an electrode with approximately 2900 ppm Ti, 80 ppm V, 70 ppm Sr, 35 ppm Zr, 15 ppm Y , and 15 ppm Rb by weight. The solution was added to 300 mg of graphite in a 1-ml Teflon (Du Pont) beaker and wetting achieved by stirring with a Teflon rod until a paste-like consistency was obtained. The water was then evaporated in a vacuum oven at 40 "C, the dried powder added to an additional 300 mg of graphite, and the mixture mechanically shaken in a Spex Model No. 8000 mixer/mill for one hour. SOLUTION DOPEDGRAPHITE No. 2. A solution of ferric nitrate and cupric nitrate was made such that 50 pl of solution added to a total of 50 mg of graphite would produce an electrode containing 50 ppm of and 56Fe. The solution was added to 25 mg of graphite in a 1-mlTeflon beaker, mixed to a paste-like consistency using a Teflon rod, and dried in a vacuum oven at 60 "C. This dried powder was then added to an equal weight of graphite and mechanically shaken for one hour. SOLIDDOPEDGRAPHITE No. 1. Approximately 5 mg of the Spex Industres oxide or carbonate of Sr, Ti, V, Y , and Zr was added to 1 gram of graphite. This was mechanically shaken for one more hour. The resulting sample electrodes contained approximately 50 ppm of each of the elements mentioned. SOLIDDOPED GRAPHITE No. 2. Approximately 50 mg of the Spex Industries oxide or carbonate of fourteen elements were weighed out and ground in a Geoscience Pulverite Type RP-0 micromill for three hours. The elements were As, Ca (as CaF2), Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Se, (7) G. H. Morrison, B. N. Colby, and J. R. Roth, ANAL.CHEM.,44,
1203 (1972). (8) B. N. Colby and G. 1%.Morrison, ibid., p 1263.
Table I. Instrumental Operating Parameters Vacuum system Magnetic analyzer 2 X 10-8 Torr Electrostatic analyzer 2 X lo-* Torr Source 5 X lo-? Torr (using cryosorption pump during sparking) Instrument parameters Charge collection 0.063 nanocoulombs (preset 5000) Spark gap voltage 30 kV Pulse repetition rate 100 Hz Pulse duration 100 psec Resolution 460 (10 valley definition)
Sc, and Zr. After the initial grinding period, 1 gram of graphite was stirred in manually and grinding resumed for three more hours. This was followed by two dilutions by a factor of five with three hours of grinding after each one. The final grind was diluted 50/50 with graphite and mechanically shaken for one hour. The final concentration of the elements was approximately 50 ppm each. SOLIDDOPEDGRAPHITE No. 3. About 5 mg of the Spex Industries oxide of carbonate of Rb (as RbCl), Sr, Ti, V, Y , and Zr was added to 1 gram of high purity sodium carbonate. After manually stirring, the sample was fused in a platinum crucible. This fused mass was ground for six hours in the Geoscience Pulverit with an interrupt time each hour to manually stir the sample. To 0.29 gram of this mixture, 0.8 gram of graphite was added, and the resulting mixture ground as before for three hours. From this final grind 0.25 gram was taken and added to 0.75 gram of graphite. The final mixture was mechanically shaken for one hour and produced electrodes containing approximately 50 ppm of each element listed. Real Samples. ALUMINUMMETAL. Disks 0.040-inch thick were cut from the Johnson-Matthey Spectrographic Aluminum standard AA2 with a spark cutter. These were sliced into half-moon shaped pieces and polished in 75z phosphoric acid-25z nitric acid solution at 70 "C for two minutes to remove surface contaminants. USGS DIABASEW-1. Approximatelv 1 gram of USGS standard Diabase W-1 was ground in the Geoscience Pulverit for six hours, stopping to mix manually each hour. The resulting rock powder was sieved and 50 mg 01the -400 mesh powder mixed with 50 mg of graphite and mechanically shaken for one hour. NBS ORCHARD LEAF. A 0.5-gram sample of NBS Standard Reference Material 1571 Orchard Leaf was ashed in the atmosphere at 230 "C for six hours followed by ten hours at 450 "C to produce 0.043 gram of ash. Then 0.04 gram of the ash was mechanically shaken for one hour with 0.1 gram of graphite.
RESULTS AND DISCUSSION
The precision of an analysis using electrical detection is determined by the sample homogeneity, the ability to reproducibly position an electrode pair in the sample chamber, and the reproducibility of the electrical measurement on a given electrode pair. Starting with the last step of the analysis, i.e., the measurement step, the precision of the mean of a number of measurements is dependent upon the number of replications, the length of each charge collection, the source operating conditions, and the homogeneity of the electrode pair used. The effect of the first of these factors, the number of replications, on the precision of the measurement is shown in ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1207
II IO 12
9-
%RSD
Preset = 5000
0 l o - O
e-
I
0
Preset = 15000 4/
O/O
I 5
L
I
1
10
20
50
Number of
Measurements
Spark Gap Voltage, hV
Figure 1. Measurement precision of blV in Solid Doped Graphite No. 1 as a function of the number of measurements taken
\ 4t
Figure 3. Measurement precision of STin Solution Doped Graphite No. 1 as a function of spark gap voltage
for the samples studied. Control of this parameter would not have been possible without the use of the automatic spark gap controller. A quantitative measurement of the effect of electrode homogeneity on precision is not readily determined by any direct method. It is possible, however, to calculate the sampling precision of the R F spark for any sample electrode if the measurement precision independent of electrode sampling error is available. Using the additivity of variance, the instrumental contribution to measurement error, previously determined to be 2.3 RSD for this system and set of operating conditions (7) may be separated from the electrode sampling precision. The electrode sampling precision obtained for an electrode pair is an indication of the relative homogeneity of the species in that electrode, and should not be confused with the overall sampling error of the analysis which also involves the removal of the original sample from its substrate. Tables I1 and I11 give the measurement precision for two groups of elements in the respective powdered samples. They are based on ten measurements per isotope at a preset count of 5000 each. In Table 11, the results of measurements of a typical geological material, W-I, are reported along with appropriate synthetic powdered samples to assess the effect of sample preparation on electrode homogeneity. Table I11 represents the corresponding situation for a biological sample, orchard leaf ash. In Table XI, as might be expected, the best measurement precision was obtained for the solution doped graphite sample indicating maximum homogeneity of the electrode pair. The poorer precision for 85Rbhere and also in the solid doped graphite sample No. 3 may be explained as the result of thermal ionization as well as a possible carbon polymer interference. The mean measurement precision for the six isotopes in the solution doped graphite (excluding 85Rb)is 3.8% RSD. When the measurement precision is corrected for the instrument precision the calculated mean electrode sampling precision is 3.0 % RSD as shown in Table IV. Solid doped graphite sample No. 3 and the USGS diabase W-1 show essentially the same mean electrode sampling precision except for 50Zr. Zircon, the primary source of zirconium in rocks, has been reported to show sampling errors as high as 10% even for I-g samples (9). The similarity in electrode sampling precision between the W-I, 10% RSD, and the solid doped graphite sample No. 3, 9.0% RSD, neglecting 50Zrand ssRb, is not
o J Preset Counts
10,000
I5.000
Figure 2. Measurement precision for 56Fe in Solid Doped Graphite sample No. 2 as a function of beam monitor preset Figure 1 for 51Vin the Solid Doped Graphite No. 1. Since no appreciable increase in precision was achieved by taking a large number of measurements, all calculations were based on ten measurements in the interest of keeping the analysis time short. The value chosen for the beam monitor preset, or length of measurement, however, did affect the precision significantly as shown in Figure 2 for 56Fein the Solid Doped Graphite No. 2. The use of preset values less than 5000, corresponding to a charge collection of 0.063 nC, resulted in a rapid decrease in precision. Preset values greater than 5000 produced an increase in precision; however, the longer time required to obtain this increase tends to reduce electrical detection’s practical advantage of rapid analysis. A short analysis time could also be obtained by making a smaller number of large charge collections, but, by doing this, the calculation of measurement precision would be impaired. With a preset of 5000, precision is acceptable and a set of ten measurements generally takes only 60-100 seconds. The third factor affecting the overall measurement precision, the source operating conditions, had a large influence. The commonly thought of parameters such as duty cycle and accelerating voltage, however, had only minor effects over their normal operating range compared to the effect of the electrical breakdown voltage at the R F spark gap. The extent of this is shown in Figures 3 and 4 for 51Vand 5*Crin the samples indicated. A gap voltage near 30 kV produced the best precision 1208
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
(9) A. W. Kleeman, J. Geol. SOC.Aust., 14,43 (1967).
Table 111. Measurement Precision for “Biological” Sample Group Solid doped graphite No. 2 NBS Orchard Leaf Isotope 7.5 14.0 26Mg 8.7 14.5 62Cr ... 13.9 66Mn ... 14.5 KBFe 4.4 ... &IFe 3.8 25.2 w u ... 12.5 6 4 Z r 3.7 27.4 66Cu
%RSD
‘t I
~~~~~
n
16.2
I
213
26.9
32.3
327
43.1
48.5
Spark Gap Voltage, kV
Figure 4. Measurement precision of 52Crin the NBS Orchard Leaf sample as a function of spark gap voltage ~
Table 11. Measurement Precision for “Geological” Sample Group Solution doped Solid doped USGS Solid doped graphite graphite diabase graphite Isotope No. 1 No. 3 w-1 No. 1 ‘#Ti 3.3 8.8 10.1 13.9 47Ti 3.7 9.5 12.5 15.5 5 ‘V 3.3 9.2 11.0 4.7 85Rb 6.5 11.0 11.3 ... 88Sr 4.1 8.9 10.0 10.8 8QY 4.5 9.9 7.0 6.0 WZr 5.0 9.6 29.5 14.2
surprising if the rock is considered a form of naturally fused material. The mean electrode sampling precision for the solid doped graphite sample No. 1, 10.7% RSD, is also very close to that of the W-1 and the graphite sample No. 3 but there is a much wider distribution in the values making up the mean. For the biological sample powders (Table 111), the NBS Orchard Leaf ash gives a better electrode sampling precision, 5.1 % RSD, than does the solid doped graphite sample No. 2, with 15.6% RSD. Both 55Mnand 56Fehad to be eliminated in the ash due to known interference with KO+ and CaO+, respectively (10). Both P6Mg and 5*Cr, which show worse than average precision, are suspect due to possible interference with CN+ and CaC+, respectively. In all of the powdered samples listed in Table IV, the measurement precision appears to consist almost entirely of the electrode sampling precision. By using electrode sampling precision as a measure of homogeneity, the solution doped graphite and the NBS Orchard Leaf ash are the best if one omits the isotopes with interferences. The most heterogeneous electrodes are the solid blended graphite synthetic samples with electrode sampling errors for individual isotopes ranging from 4.1 % RSD for 51V in the solid doped graphite sample No. 1 to 27.3 RSD for W r in the solid doped graphite sample No. 2. There are two possible explanations for this. The first is that the blending steps work reasonably well as evidenced by good precision for the leaf ash and solution (10) C . A. Evans and G. H. Morrison, ANAL. CHEM.,40, 809 (1968).
Table IV. Mean Precision for the Powdered Samples Mean Expected Mean electrode mean error measuresampling of an ment precision analysis precision Sample (% RSD) ( % RSD) ( %) Solution doped Graphite No. 1 3.8 3.0 3.1 NBS Orchard Leaf 5.6 5.1 3.4 Solid doped Graphite No. 3 9.3 9 .o 4.3 USGS Diabase W-1 10.3 10.0 4.6 Solid doped Graphite No. 1 10.9 10.7 4.8 Solid doped Graphite No. 2 15.8 15.6 6.3 ~
Table V. Measurement Precision for Johnson-Matthey AA2 Spectroscopic Aluminum Standard Measurement precision Isotope ( % RSD) 6sMn 46 56Fe 4 3 “CU 42 YZn 39
doped sample, but that the heterogeneity of the original solid compounds mixture before blending was greater than that of the leaf ash and solution doped sample. The second possibility is that the leaf ash and the solution doped samples blend much better with the graphite because of their similarity with respect to particle size, density, and shape, whereas the crystalline samples do not. It appears, however, that both explanations have some validity. This is apparent from the intermediate precision obtained for the W-1 and solid doped graphite No. 3 sample. These samples are expected to be more homogeneous than the solid doped graphite samples Nos. 1 and 2 and yet are physically dissimilar to the solution dope and leaf ash. To allow a comparison of electrode sampling precision between the powdered samples analyzed and a metal sample, where impurities are generally more homogeneously distributed, the Johnson-Matthey AA2 spectroscopic aluminum standard was run. The measurement precision obtained for the elements Mn, Fe, Cu, and Zr is given in Table V. Using the mean measurement precision for these elements. the electrode sampling precision is calculated to be 3.6 RSD. This falls between that of the solution doped graphite sample No. 1 and the Orchard Leaf ash. Although the mean measurement precision for the powdered samples runs from 3.8 to 15.8% RSD, the precision expected for an analysis is significantly different. In addition to measurement precision, one must account for the reproducibility ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1209
where S, is the mean value of the individual determinations, S l is the loading precision, ni is the number of electrode pairs loaded, S , is the measurement precision, and n, is the number of measurements per loading. This in turn is used to calculate the expected mean error in the absolute value of concentration determined by comparative analysis of standard and unknown. This expected mean error is given by Equation 2:
and standard. The mean sample loading precision for all of the powdered samples, 4.2 % RSD, was used in the calculation, The results of this calculation, shown in Table IV, indicate that, if only random errors are present, electrical detection is capable of mean errors in concentration on the order of 3 to 6% compared to 10 to 20% for photoplate detection. To check the validity of these calculations in the absence of any possible spectral interference, the solution doped graphite sample No. 2 was analyzed four times. The first two of these were used as the standard and the second two as an unknown. The results in Table VI show a mean error of 3.55 % for Cu and Fe. This is just within the 3.6% mean error calculated for this analysis. Finally, some comment is necessary regarding the accuracy of electrical detection peak switching techniques for the analysis of complex materials,such as geological or biological samples. In both W-1 and the NBS Orchard Leaf ash, interference-free peaks were not readily identifiable. This was due in part to the low resolution used to examine these highly complex spectra. The resolution was 460 using the 10% valley definition which is comparable to that mentioned in other studies. An abundance of interferences in these spectra was readily apparent when photographic detection with considerably higher resolution was employed. Unless the resolution used in electrical detection measurements can be increased significantly, the accuracy of analyses of complex materials will be poor. Comparative analysis using standards helps minimize the effects of some interferences ; however, the dearth of complex material standards is obvious.
For the powdered samples, the expected mean error in concentration was calculated assuming the measurements were taken on each of three pairs of electrodes for both unknown
RECEIVED for review September 27,1971, Accepted February 18, 1972. Financial support was provided by the National Science Foundation under Grant No. GF6471X and the Advanced Research Projects Agency through the Cornel1 Materials Science Center,
Table VI. Determination of Iron and Copper in the Solution Doped Graphite Sample No. 2 Standard No. 1 Standard No. 2 Mean Unknown No. 1 Unknown No. 2 Mean
Iron, ppm 49.17 f 3.2% RSD 50.83 f 3.2% RSD
Copper, ppm 48.98 f 3.2% RSD 51.03 + 3.2% RSD
50.00
50.00
50.61 i 3.6% RSD 52.80 & 3.5% RSD 51.74
52.89 f 3.4% RSD 50.74 i 3.4% RSD 51.81
of positioning an electrode pair in the source chamber. Also in an analysis, both the number of electrode pairs used and the number of measurements made on each electrode pair help to influence the precision according to l l d n r u l e . Using an approach similar to that of Bingham and Elliott (2) the expected precision of an analysis can be calculated according to Equation 1 :
1210
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