used was 0.00184 mg of gold for Table I and 0.01012 mg of gold for Table 11. The computer program determined both the qualitative and quantitative composition of the sample in both instances, The reference library contained spectra for the following elements : aluminum, antimony, argon, arsenic, bromine, chlorine, cobalt, copper, gold, iodine, manganese, molybdenum, nickel, potassium, sodium, tungsten, vanadium, and zinc. Most of these reference spectra were obtained about two years prior to the analysis of these samples.
always larger than the theoretical because of other errors, Typically the variance is 1.5-2.5 times larger. The principal use of this program is to predict the accuracy attainable for a sample of any composition if it were to be analyzed using some specific set of irradiation and counting conditions without the necessity of having to analyze a known sample under these conditions, Also, the effect of irradiation and counting conditions on accuracy can be easily determined so that the analyst can use near optimum conditions when the best results are required for a complicated sample.
CALCULATION OF PREDICTED ERRORS
COMPUTER PROGRAMS
A separate program has been developed which predicts the errors to be expected in the determination of each element in a sample by use of the analysis program described above. Any desired sample composition and irradiation and counting conditions may be specified in the input to the program. The spectral data required for these calculations are taken from the same library tape as used by the analysis program. These error calculations are based on the assumption that the standard errors are due solely to counting statistics. The expected error in the determination of each element is then equal to the product of this overall standard error and the square root of the corresponding diagonal term in the matrix inverse. In practice, of course, the actual variance in an analysis is
The computer programs are written mainly in FORTRAN IV for use on an UNIVAC 1108. Arrangements can be made with the first author for obtaining any of the programs. ACKNOWLEDGMENT
The authors acknowledge the contributions of R. A. Carr and R. A. Johnson for consultations regarding many aspects of neutron activation analysis and G. A. Harlow for preparing the gold quaternary ammonium cyanide solution used as the internal standard. RECEIVED for review September 16,1968. 21, 1970. Accepted May 29, 1970.
Resubmitted May
Fixed-Time Digital Counting System for Reaction Rate Methods J. D.Ingle, Jr., and S . R. Crouch Department of Chemistry, Michigan State University, East Lansing, Mich. 48823 A new method is presented for computing the initial rate of a reaction by a digital integration procedure. The instrument can be used in a continuous- or singlemeasurement mode over a wide range of input slopes. Results are presented for synthetic si nals with and without noise which show typical standaard deviations and relative errors of less than 0.3%. Results are presented for the determination of phosphate by a spectrophotometric rate method to demonstrate a practical application of the fixed-time rate computer. SEVERALRECENT AUTOMATED measurement systems for reaction rate methods have demonstrated the increased speed, accuracy, and reliability which result when digital computation methods are used in place of analog computation methods (1-3). The two integral methods for the measurement and computation of initial reaction rates, the fixed-time and the variable-time methods (4), are most amenable to digital computation techniques. The third general approach to reaction rate measurements, the slope method (5,6), is a true (1) S. R. Crouch, ANAL.CHEM., 41, 880 (1969). (2) G. E. James and H. L. Pardue, ibid., p 1618. (3) R. A. Parker, H. L. Pardue, and B. G. Willis, ibid., 42, 56 (1970). (4) W. J. Blaedel and G . P. Hicks, “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, Wiley, New York, N. Y., 1964, pp 105-142. (5) H. V. Malmstadt and S. R. Crouch, J. Chem. Educ., 43, 340 (1966). (6) H. L. Pardue, “Advances in Analytical Chemistry and Instrumentation,” Vol. 7, Wiley, New York, N. Y.,1969, pp 141-207.
differential method which must necessarily involve analog instrumentation. The variable-time method has received the most attention for use in automated reaction rate methods. A hybrid analog-digital computer has been described and shown to be capable of high accuracy and precision (1). An all digital system has recently been described (3) which eliminates all analog circuit elements in the computation system. Both of these systems have resulted in significant improvements in the reliability of automated reaction rate measurements. The fixed-time method has not previously been implemented with digital circuitry. The most elegant of the fixedtime measurement systems (7) utilized operational amplifiers to integrate over two equal time intervals of the transducer voltage us. time curve. The two integrals were subtracted and the resulting voltage difference was shown to be directly proportional to the reaction rate (7). Although the fixedtime method has been shown to be more limited in dynamic range than the variable-time method (2), the high noise immunity, which is a result of the integration procedure, and the simplicity of the computation step makes the fixed-time method attractive for many reactions. In addition, because the measurement time is independent of concentration, the fixed-time method is easily incorporated into a completely automated sampling system. The instrument described here computes the voltage change (7) E. M. Cordos, S. R. Crouch, and H. V. Malmstadt, ANAL. CHEM.,40, 1812 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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I
CONVERTER
I Figure 1. Block diagram of reaction-rate measurement system Fixed-time rate computer shown in dotted lines
phate to demonstrate the applicability of the digital system to practical analytical problems. INSTRUMENTATION
+I
'2
'3
TIME
Figure 2. Expanded section of initial part of a typical reaction rate curve
over a fixed time interval by a digital integration procedure. The output voltage from a suitable reaction monitor, such as a spectrophotometer, is converted to a frequency by a voltage to frequency (V to F) converter. Precise timing and gating circuits are used to direct the V to F converter output to a seven decade up-down counter. The number of counts accumulated over a fixed-time interval is subtracted from the number of counts obtained over a previous identical time interval by the up-down counter. This difference in counts is a digital number proportional to the reaction rate. The complete rate measuring system described here consists of a few analog circuits, to convert the reaction monitor output to a frequency, and the all-digital computation system. Drifts and nonlinearities in the computation system are eliminated. The digital computation system can measure the rates of both positive- and negative-going signals with a dynamic range of over four orders of magnitude. The reaction rate instrument has provision for an initial variable time delay before the measurement interval begins and can be used in either a single-measurement mode or a continuousmeasurement mode. Results are shown for synthetic signals with and without noise present and for the analysis of phos1056
A block diagram of a complete rate measuring system using the fixed-time digital counting system is shown in Figure 1. The chemical reaction is monitored in a reaction cell by a suitable transducer, whose signal is modified to obtain a voltage input of the proper level for the voltage to frequency converter. A spectrophotometer and an operational amplifier current to voltage converter are used in the specific rate measurement system described later. The voltage to frequency converter acts as an interface between the reaction monitor and the fixed-time computer by changing the analog reaction monitor signal to a train of pulses that can be processed by the digital circuits. The fixed-time computer is shown in dotted lines in Figure 1. The up-down counter, which is controlled by the timing and gating circuits, computes the difference in counts between two equal time intervals on the reaction rate curve. This difference in counts, appearing at the up-down counter output, is decoded from binary coded decimal to decimal and displayed on a Nixie tube readout. The mathematical relationship between the digital readout and the output slope of the reaction monitor-signal modifier system is discussed in the following section. Relationship between Readout and Rate. An expanded section of the initial portion of a typical reaction rate curve is shown in Figure 2. During the time represented in Figure 2, the rate limiting species has undergone less than 5 % reaction and for practical purposes the reaction can be considered to follow pseudo zero-order kinetics (6). In fixedtime methods, measurements must be made in this interval to ensure that the measured voltage change is directly proportional to the concentration of the rate limiting species. It is also assumed in Figure 2 that the reaction monitor follows solution concentration in a linear manner. It has been previously shown (7) that the slope of such a plot can be expressed as S
=
tan a = a/At
=
AA/(At)z
(1)
where At is the total integration period, A A is the difference in the areas ABand A , in Figure 2 , and S is the slope in V/sec. The slope can also be expressed as S
=
(E
-'
V,)/At
where E and are the average voltages of intervals 2 and 1, respectively. In the described instrument, the output of the reaction monitor-signal modifier system, V , is converted to a
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
F R O M V TO F CONVERTER
SCALER
CONT
ONTINUOUS MEASUREMENT
Figure 3. Circuit diagram of fixed-time rate computer Up-Down Counter. 7 cascaded National Semiconductor Corporation DM8560 up/down decade counters mounted on Heath Dual Mine IC cards (EU-SO-MC) NAND gates. Heath Nand Gate Cards (EU-SOOJC) DCU. Heath Quad DCU Card (EU-SOODB) Decoder and Decimal Readout. 2 Heath Decimal Readout Modules (EU-801-15) Scaler. Heath Scaler Card (EU-IOWA) Hip-Flop. Heath J-K Flip-Flop Card (EU-BOOCB) S2. Heath Relay Card (EU-800JD) SIand Monostable. Reset button and display time monostable on Heath Decimal Readout Module Crystal oscillator. Heath Oscillator Card (EU-800 KA)
frequency, f,by a V to F converter according to Equation 3 f=kV (3) where k is the conversion rate in Hz/V. Solving Equation 3 for V and substituting the result in Equation 2 f o r y l and Eyields
(5
(4) S = - fT,/kAt wherex and are the average frequencies over the first and second intervals, respectively. The readout, R, is the number of counts accumulated during the second interval minus the number of counts accumulated in interval one, and is expressed by
5
R
=
(5- fi>
If Equation 4 is solved for tion 5 the result is
R
cfi - >)
S . k
*
Af
(5)
and substituted into EquaAt2
(6) Range and Limitations. The range and limitations of the instrument can be predicted from Equation 6 . The readout is directly proportional to the slope, the conversion factor, and the square of one half the integration period. The computing circuit has an error of i1 count because the timing and sequencing circuits are not synchronized with the input signal pulses from the V to F converter (8). For 0.1 accuracy, the slope, conversion factor, and integration period must be =
*
(8) H. V. Malmstadt and C . G. Enke, “Digital Electronics for Scientists,” Benjamin, New York, N. Y . 1969.
manipulated to ensure a readout of at least 1000 counts. The degree of manipulation is dictated by the particular reaction of interest and the noise characteristics of the reaction monitor-signal modifier system. For slower reactions, where S is small, a large integration period is used to accumulate at least 1000 counts and to average noise; since the signal to noise ratio is small because of the small change in concentration during the measurement. For faster reactions, where S is large, the integration period must be shortened to ensure that only the initial reaction rate is measured. Equation 6 predicts that decreasing the integration period by a factor of 10 will decrease the readout by a factor of 100. The V to F converter chosen should have the highest possible conversion factor and linearity since both affect the accuracy. The accuracy of‘ the instrument also depends on the accuracy of the integration period. Use of a crystal oscillator produces such stable and accurate timing that the accuracy of the integration period is not the limiting factor, The V to F converter imposes another limitation in the range of input voltages it can accept. The measurement must be made when the signal from the reaction monitor-signal modifier system is within the input range of the V to F converter. The maximum counting rate is determined by the propagation delays in the gating and counting circuits and the tracking rate of the V to F converter. Practically, propagation delays are not limiting in comparison to available V to F conversion rates. The maximum readout is constrained by the number of decades in the up-down counter.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1057
yr
CLOCK
~
1
I
Q
I I I
A
I
I
I I
I
I
B
r
II Delay IIIMearurtmenl
I
t
I !Delay 1 Measurement I I I I
Hold I
I
Hold
II I
Figure 4. Waveforms of sequencing signals for timing circuit
In this instrument, the V to F converter on the Heath Universal Digital Instrument (Model EU-805A) was used. It has a conversion rate of 100 KHz/V and a linearity of 0.05%. Thus Equation 6 becomes
R
S
At2 (7) The input range is 1 to - 1 volts, so that the measurement must be made in a 1 to 0 or - 1 to 0 voltage interval depending on the polarity of the specific reaction monitor-signal modifier system. The counting frequency of the gating and counting circuits is greater than 10 MHz and, because a seven decade up-down counter is used, the maximum readout is 9,999,999. Timing, Gating, and Counting Circuits. A circuit diagram of the fixed-time counting system is shown in Figure 3. Switch Sz is the mode switch and determines whether the instrument operates continuously or takes a single measurement. The instrument is triggered by switch SI,which can be manually operated, or by a pulse derived from the sample introduction system. Changing SI to the +5 volt position starts a monostable time delay which is adjustable to delay the measurement until mixing is complete or any induction period in the reaction has passed. Figure 4 shows the timing sequence which begins when the monostable returns to a logical 1 level. The 1-MHz crystal oscillator output is divided by the scaler to obtain clock periods from to 10 seconds in decade steps. Gates 1, 3, 4, 5, and 6 direct the V to F converter output to the up-down counter at the proper time intervals. After an initial delay time of 1 clock period, the measurement begins. From the trailing edge of clock pulse 1 until the trailing edge of clock pulse 2, the counter counts up. Down counting occurs from the trailing edge of pulse 2 to the trailing edge of pulse 3. The result of up-down counting is then held until the fifth 1-0 clock transition. This pulse stops the sequence when the single measurement mode has been chosen or, if the continuous mode has been chosen, resets all the circuits and begins the sequence again. For 1- and 10-second clock periods, the initial time delay of one period plus the monostable delay might be inconveniently long, even if the minimum monostable delay time is chosen. The scaler has provision for presetting the lo6 and
+
1058
=
106
9
107 outputs initially to 1 instead of 0. This can be accomplished by connecting the output of gate 2 to the preset input of the scaler. Thus, the initial delay time after the monostable returns to 1, or after the fifth clock pulse, will be 100 msec for 1- and 10-second clock periods. Longer clock periods can be obtained by further division of the 10-second period with flip-flops if desired. Decoder and Readout. The readout is displayed on Decimal Readout Modules, which receive the binary coded decimal outputs from each decade of the up-down counter. The readout module decodes the BCD outputs and displays the final result on Nixie tubes. Two Decimal Readout Modules are used in this instrument, which allows 7 decades to be displayed. PROCEDURES
Circuit Layout and Construction. The circuit cards listed in Figure 3 were inserted into two Heath Decimal Readout Modules (EU-801-15). Power was supplied by the Heath Digital Power Module (EU-801-11). Connections between cards were made with No. 22 patch wires. A mother board with all the necessary connections is being prepared. Preparation. Before rate measurements are made, the appropriate integration period and monostable delay time must be chosen. The delay time is chosen such that the measurement starts after mixing is complete and any induction period has passed. The clock period is chosen to ensure that no more than 5% of the total reaction occurs by the end of the measurement time for the fastest reaction to be measured. This integration period should be long compared to noise fluctuations for the best noise averaging. Spectrophotometric Measurements. A Heath single beam spectrophotometer (EU-701) was used for all spectrophotometric measurements. The photomultiplier current was converted to a voltage by a Heath Photometric Readout Module (EU-703-31). The Photometric Readout Module was A range. used in the % Transmittance Mode on the The % Transmittance Mode was used because of the high T sensitivity possible in the range of 100 to 95 T, where changes are directly related to concentration changes. For
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
measurements outside this XT range, the Absorbance mode of the Photometric Readout Module should be used to ensure linear response. The photomultiplier power supply voltage and the monochromator slit width were adjusted to give a 1.00 V output for 100% Transmittance. The 1-V output of the Photometric Readout Module was connected directly to the V to F converter in the Universal Digital Instrument, The reaction cell was a standard 1.OO-cm spectrophotometric cell surrounded by a brass jacket through which water at 23.0 f 0.01 "C was circulated. A magnetic stirrer was positioned below the cell. Analog Measurements. To follow the progress of reactions on a recorder, the current output of the Photometric Readout Module was connected to a Heath Log/Linear Current module (EU-20-28). This allowed visual monitoring of reaction rate curves to ensure that no abnormalities occurred during the run and to confirm that measurements by the fixed-time rate computer were made within the linear portion of the voltage us. time curve. Phosphate Determinations. Reagents for the determination of phosphate, except for the ascorbic acid solution, were prepared as previously described ( I ) . Ascorbic acid, 0.2265M) was prepared by dissolving 3.990 grams of L-ascorbic acid in water and diluting to 100 ml. The reaction was followed spectrophotometrically at 650 nm. Two milliliters of the acid-molybdate solution and one milliliter of the phosphate sample were added to the cell and allowed to mix and equilibrate. The reaction was initiated by injecting 100 ~1 of 0.2265M ascorbic acid with a hypodermic syringe.
RESULTS AND DISCUSSION Two methods were used in the testing of the complete reaction rate measurement system shown in Figure 1. First, the fixed-time rate computer and the V to F converter were evaluated with synthetic slopes to simulate a signal from a reaction monitor-signal modifier system. This permits an evaluation of the instrument which is independent of the reaction monitor and signal modifier characteristics. Second, a spectrophotometric determination of phosphate is presented to illustrate the performance of the rate measurement system with a real reaction monitor and signal modifier. A high quality operational amplifier (Philbrick Model SP2A) integrator with an approximate R C time constant of 1 second was used as the synthetic slope generator. A high quality operational amplifier is needed because input voltage offset drifts and input current offsets affect the integrator output. Specifications of the Philbrick operational amplifier include a 10 MV/%Hr input voltage offset drift and a maximum input current offset of 1 PA. The integrator input voltage was varied to provide a wide variety of input slopes. The integrator was calibrated by varying the input resistor until a 50-mV input provided a 50 mV/sec output slope. Table I illustrates typical results for synthetic input slopes from 0.1 mV/sec to 2 V/sec. The high degree of accuracy and precision of the fixed-time rate computer is evident from the data in Table I. Larger relative errors and standard deviations are evident at the extremes of the slopes tested. For slower slopes, errors are largely attributable to the synthetic slope generator. At these slower rates, current and voltage drifts in the integrator become significant. Unidirectional voltage offset drifts as low as 1 pV/sec and current offsets of 1 pA would cause 1 errors in the 0.1 mV/sec slope. For faster rates, the tracking speed of the V to F converter becomes the limiting factor in determining the accuracy. In addition,
Table I. Fixed-Time Rate Measurements of Synthetic Slopes
Input slope, mV/sec 0.1 0.5 1 2 5 7 10 20 50 70 100 200 500 700 1000 2000
Digital readout,. counts
Re1 std dev,
1,0OSc 5 , 024c 9,9746 19,982c 50,050c 702d 1,OOOd 2, OOOd
4.00 0.94 0.16 0.19 0.16 0.28 0.13 0.09
d
6 99gd 10,001d 20,003d 5018 7018 1,003. 2,0158
Re1 error: +0.80
x
+0.48 -0.26 -0.09 +O. 16 +0.29 0.00 0.00
..*
*..
0.02 0.02 0.01 0.27 0.26 0.26 0.21
-0.01 +0.01 +0.02 $0.20 +O. 14 +O. 30 +O. 75
Averages of 10 results.
* Based on calibration with 50 mV/sec input slope. Integration time of 20 sec. Integration time of 2 sec. 8 Integration time of 0.2 sec. c
d
Table II. ' Fixed-Time Rate Measurements of Synthetic Slopes with Superimposed Sine Wave
Added noise Noise frequency, amplitude, Hz volts p-p
Digital readout,a counts
Re1 std dev,
z
Re1 error,*
z
10001 0.01 +0.01 10021 0.23 f0.21 10001 0.05 $0.01 9993 0.12 -0.07 9998 0.04 -0.02 10002 0.02 +0.02 10007 0.09 +0.07 0.04 +0.03 10003 Averages of 10 results with 100 mV/sec input slope and 2-sec 0 20 20 60 60 60 1,000 100,000
0 1 0.1 1 0.1 0.01 0.1 0.1
integration time. * Based on calibration with 50 mV/sec input slope.
as a result of the shorter integration times for faster slopes, averaging of noise and drifts is not as effective and the signal to noise ratio is less. To test the noise immunity of the fixed-time rate computer, sine waves of different frequencies and amplitudes were superimposed on a 100 mV/sec input ramp. The results obtained are shown in Table 11. The artificial noise is seen to have little effect on the accuracy and precision of the readout. Even a 60-Hz, 1-V peak-to-peak sine wave, which has an amplitude 5 times larger than the measured voltage change, had minimal effect on the results, indicating excellent 60-Hz rejection. With the 1-V peak-to-peak sine wave, measurements were made in the middle of the voltage to frequency converter range to avoid overloading. Noise levels which are so large that the input signal reverses polarity will not be correctly averaged by the integration procedure. Hence, input voltage levels to the V to F converter should be large compared to noise fluctuations, The complete fixed-time rate measurement system of Figure 1 was applied to the determination of phosphate. A spectrophotometer was used as the reaction monitor and an operational amplifier as the current to voltage signal modifier as detailed in the procedure section. The results presented in
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9,AUGUST 1970
1059
Table III. Fixed-Time Reaction Rate Determinations of Phosphate
Phosphorus ~igiul concentration in ppm Re] readouta Taken Foundb error, 6730 3 3.01 +o. 33 11167 5 ... ... 8 7.97 -0.38 17800 22249 10 10.01 $0.10 Average of 5 results. b Based on 5-ppm standard; integration time of 40 measurement time 40 s ~ .
Re1 std dev, 1.48 2.05 0.66 1.05
5
sec; pre-
Table I11 show considerably higher relative standard deviations than with synthetic slopes. This imprecision can be attributed to errors in sample introduction and preparation, and also to drifts in the spectrophotometric system and analog circuits. These drifts were made evident by rate measurements made on distilled water blanks containing no phosphate. The blanks gave an average readout of zero. However, the absolute value of the standard deviation of the blank was approximately equal to the standard deviations obtained when phosphate was present. Such drifts are small and normally
negligible when measurements are made over relatively large absorbance changes, but become highly significant when small concentration changes are measured. The effect of drifts in the spectrophotometer and analog circuits is presently under study. Although the digital counting system described here contains no analog circuits, the reaction rate measurement system is limited by drifts and nonlinearities in the reaction monitorsignal modifier system and the V to F converter. At present, work is being done to replace these analog circuits with a photon counting system. This change involves replacing the current to voltage and V to F converters with a pulse amplifier and discriminator. Thus the photomultiplier tube will be directly interfaced to the fixed-time rate computer by photon counting circuits, and the entire rate measurement system will be completely digital in measurement and computation. This should lead to significant improvements in reaction rate measurements on chemical systems. RECEIVED for review March 18, 1970. Accepted May 28, 1970. Presented at 21st Mid-America Symposium on Spectroscopy, Chicago, Ill., June 2-5, 1970. One of us (J. D. Ingle, Jr.) gratefully acknowledges a National Science Foundation Traineeship. This work was partially supported by NSF Grant No. GP-18123.
Analysis of Thin Films by Ion Microprobe Mass Spectrometry C. A. Evans, Jr., and J. P. Pemsler Ledgemont Laboratory, Kennecott Copper Corp., Lexington, Mass. 02173
The ion microprobe mass spectrometer was used to investigate isotopic and compositional gradients in thin films of both oxides and metals. Continuous recording of intensities, and sputter rates as low as 0.5 &sec enabled depth resolutions of the order of 20 A. Oxygen isotope mixing in duplexTaz1BOs/Taz1~Os films was shown to vary with the Ta2180sthickness added. Phosphorus gradients in Ta20sanodized in H3P04varied with film thickness and H3P04concentration. Homogeneities in thin films of Ag-Cu and AI-Ge-Nb depended on their modes of preparation. The ion microprobe is concluded to be a powerful tool for examining thin films. T H E CoNcePT of an ion microprobe mass spectrometer was
first described by Herzog and Viehbock (1). A sample was bombarded with ions of energy in the KeV range causing surface atoms of the target to be sputtered, a small fraction of them being ionized. These secondary or sputtered ions were extracted into a mass spectrometer and analyzed. A number of investigators have explored the application of this technique. Anderson (2) used positive and negative ions for bombarding the target, and Benninghoven (3)investigated the use of positive and negative secondary ions. Castaing and Slodzian (4) and Robinson, Liebl, and Andersen (5) demon(1) R. F. K.Herzog and F. P. Viehbock, Phys. Rev., 76,855 (1949). (2) C. A. Anderson, Int. J. Mass Spectrom. Ion Phys., 2 , 61 (1969). (3) A. Benninghoven, 2.Physik, 199,141(1967). (4) R. Castaing and G . Slodzian, J. Microsc. (Paris), 1,395 (1962). (5) C. F.Robinson, H. Liebl, and C. A. Andersen, Third National
Electron Microprobe Conference, Chicago, 1968. 1060
strated that surface spatial resolution of the order of 1 p can be obtained. Satkiewicz applied the technique to a variety of materials including metals, minerals (6), organic materials (7),and thin films (8). Relative ionization efficiencies were found to vary by several orders of magnitude from element to element (2, 3) so that detection limits vary widely. In favorable cases detectabilities in the ppm range are attainable. The removal rateeof atoms from the sample is variable over a wide range-0.5 A/sec for low ion current densities to 100 A/ sec for high current densities and high energy primary ions. Sputtering rates and ion yields are further influenced by parameters such as the nature of the bombarding ion, ion energy, sample and surface characteristics, and residual gas pressure, so that conditions must be optimized for a particular objective. THIN FILMS
In addition to their great technological significance, thin films are of fundamental interest because they offer an opportunity to study an almost two-dimensional solid. Many analytical techniques have been used to study the physical, (6) R. F. K. Herzog, W. P. Poschenrieder, and F. G.Satkiewicz,
Final Report NASA Contract No. NAS 5-9254(1967). (7) F. G. Satkiewicz, GCA Technology Div., Bedford, Mass., Personal Communication,1969. (8) F. G . Satkiewicz, Air Force Avionics Laboratory Technical Report TR-69-332,Jan. 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
