T a b l e I. C a l c i u m Activities or C o n c e n t r a t i o n s (in mol 1.-1) in S a t u r a t e d S o l u t i o n s of C a l c i u m Molybdate a t Various T e m p e r a t u r e s Ca-selective electrode [activity]
Atomic absorption [concentration] ( l t O . 1 X 10-5M)a ( l t 0 . 3 X 10-5M)a
3C
EDTA titration [concentration]
All values given below should be multiplied by 10-5
20
4.2
21 25 28 30 32 35
4.4
...
4.8 5.1
7 .O
5.4
...
11.1
...
... 13.0
40
6.9
5 .cs 6.3
...
8.4 9.2
... 9.3 ...
14.6
Estimated errors are given in parentheses.
DISCUSSION The results of atomic absorption spectrometry and EDTA titration are notably higher than those obtained with the calcium-selective electrode, and their reliability is in doubt. Possibly the saturated solution samples may have contained a small quantity of colloidally dispersed molybdate. This is suggested by the fact that the Eriochrome Black T indicator used in the titrations reverted to its preend-point color when the titrated solutions were allowed to stand for a short period. Such behavior was not observed with the calcium carbonate standard solutions of similar concentrations. Roth in titration and in atomic-absorption work, the total dissolved calcium would be measured, including that present as ion pairs. Any colloidal molybdate would further increase the results. The analytical procedures of previous workers are also prone to such errors.
Zhidikova and Khodakovskii (13) measured calcium by flame photometry and molybdenum colorimetrically using ammonium thiocyanate. At 25 "C, they obtained an activity solubility product approximately in the range 2-5 X 10-9, and they report that the solubility a t that temperamol L-'. Our calcium-selective electrode ture is 0.9 X and atomic absorption measurements also give Ks values within this range. Our solubility result obtained by titration is close to that reported by Zhidikova and Khodakovskii, but the agreement is probably fortuitous. Ramana Rao (12) proposed a value of 1.24 X 10-5 for K,; however, neither the temperature nor the method of determination were stated. Using a calcium molybdate prepared by a method similar to ours, Spitsyn and Savich (10) found the solubility to be mol 1.-l a t 22 "C, while Bashilov and Kin2.3-2.6 X dyakov (11) reported it to be about 18 X mol I.-' a t 18 "C. Zelikman and Prosenkova (9) obtained a value of 6.25 x at 20 "C for a molybdate prepared by heating a mixture of calcium oxide and molybdenum trioxide to 600 "C for several hours. In a critical review of past work, Zhidikova and Khodakovskii ascribe discrepancies between results of various workers to differences in particle size and method of preparation of the molybdate. Their own material was subjected to a calcination above 1200 "C before use (17).However, no clear relation between solubility and preparation method emerges from the results quoted above. It would seem more likely t h a t the disagreement is attributable to differences in analytical procedure.
RECEIVEDfor review March 15, 1974. Accepted June 26, 1974. (17) E. S. Khristoforov L. I. Grosman, and S. N. Kalashnikova, Zap. Vses. Mineral. Obshchest. 81, 205 (1952).
Quantitative Analysis by Dynamic Single Ion Detection Stephen R. Bareles and Joseph D. Rosen' Department of Food Science, Rutgers University, New Brunswick, N.J. 08903
The major drawback to the more widespread use of mass fragmentography ( I ) is that changes in magnetic field intensity and accelerating voltage alters the focus of specific m/e values (2), making it difficult to obtain reproducible analytical data. This problem was particularly severe on our Du Pont 21-490 mass spectrometer and has led us to build a circuit to overcome the problem. This low-cost circuit incorporates a low amplitude sweep across a specific m/e value instead of static focusing for reasons explained by Klein et al. (3).In the present application, the fluctuating electron multiplier output is demodulated by a gated analog integrator-sample/hold system for chart recorder presentation.
T o whom inquiries should be addressed. (1) A . E. Gordon and A . Frigerio, J. Chromatogr., 73, 401 (1972). (2) J. F. Holland, C . C. Sweely. R. E. Thrush, R. E. Teets, and M. A. Bieber, Anal. Chem., 45, 308 (1973). (3) P. D. Klein, J. R. Haumann. and W. J. Eisler, Anal. Chem., 44, 490 (1972).
2056
EXPERIMENTAL Instrumentation. A Du Pont Model 21-490 Mass Spectrometer equipped with a digital mass marker and interfaced to a Varian Model 2740 Gas Chromatograph (equipped with a flame ionization detector) via a glass single-stage jet separator was used. In addition we employed a dual pen Hewlett-Packard Model 7128A Strip Chart Recorder and an RCA Model 158 Cathode Ray Oscilloscope. A Tektronix Type 541 Oscilloscope with Model 53/54 K single trace plug-in could also be used. Dynamic Single Ion Detection (DSID) Circuit Description. A block diagram of the DSID system is shown in Figure 1. The Timing and Control Logic, driven by a 500-Hz Clock, performs all sequencing functions. The Accelerating Voltage Sweep/Offset section produces a 10Hz triangle function derived by integration of a *lO-volt zenerclamped square wave generated by an operational amplifier comparator. optically coupled with the Slope Polarity logic pulse train. To this triangle function are added, by a summing amplifier, one or more adjustable offset voltages. Maximum offset obtainable is 15% of the mle set on the mass spectrometer control panel. The summing amplifier is followed by a 741 type buffer operational amplifier and is connected t o the (low impedance) scan sweep inputs of the mass spectrometer through a 1-kohm resistor. The sim-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
5WHZ
SLOPE POLARITY
'
FOCUS GUIDES
INTEGRATOR CLEAR
i
SAMPLE COMMAND
I
I I,-, 1 I
OFFSET SELECl
1
ISOLATOR
TRLMUE W E
REFERENCE
~1 ,
0
SCOPE ZERO
1
I
WCDR ZERO
I
i
,
RECORDER SIGNAL PROCESSOR
SCOPE SIGNAL PROCESSOR
4 v
I
WdNGE
I
ELECTRON MULTIPLIER OUTPUT
SCIIIHNp%EEP
MASS SPECTROYETER
RECORDER
OSCILLOSCOPE
Figure 1. Dynamic Single Ion Detection System (DSID). Thin lines denote digital timing paths; bold lines, analog paths
XH)Hz
,+I
SLOPE POLARITY
,
1 VI:::"
I I
1
ISOLATOR
1-E WM GEWRATOR
CHANNEL SELECT
FOCUS GUIDES
OFFSET SELECT
I
L
,
SAMPLE COMMAND (S.1)
I
-0
bMP 200 x
i
i
CWLER
-
FILTER 100 H I
' ACCELERATING VOLTAGE SWEEPIOFFSET
I
CROSSTALK SUPPRESSION
t
f
REFERENCE VOLTAGES
INTEGRATOR CLEAR
WCDW ZERO
RANGE
,
i
SCOPE SIGNAL PROCESSOR,
RECORDER SIGNAL PROCESSOR
: S C A N SWEEP
ELECTRON MULTIPLIER MONITORED ION INPUT
MASS SPECTROYETER
OSCILLOSCOPE
RECORDER
Figure 2. Dynamic Dual Ion Detection System (DDID). This circuit is an expansion of Figure 1
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
2057
I Figure 3. Triangular function imposed on the accelerating voltage of the mass spectrometer
plicity of this Sweep Offset section is an important feature of the present design. The Scope Signal Processor amplifies and filters the electron multiplier output signal, couples it with a digital reference spike or Focus Guide for oscilloscope display of the repetitively scanned peak ( 4 ) . The Recorder Signal Processor repeatedly integrates the output of the electron multiplier. The fixed analog integrator period is the same as the period of one triangle cycle (plus an instrument specific delay) or two passes across the peak of interest. This provides compensation for possible scan asymmetry and a degree of signal averaging. After each cycle, the integrator is cleared. A sample/hold amplifier (S/H) is gated to capture the value of the integral just before the integrator is cleared. The value is held on the recorder until the next integral is ready. The S/H is followed by an R/2R ladder network voltage divider, whose advantages have been previously discussed ( 5 ) , and a low-pass, non-inverting filter (6). Dynamic Dual Ion Detection (DDID) Circuit Description. Modifications that allow automatic simultaneous dual ion focus and quantitation are shown in Figure 2. These include two additional timing lines from the Timing and Control Logic Section; a S/H Control Section which interrupts the former Sample Command line; an additional S/H section in the Recorder Signal Processor, an additional chart recorder, and a trigger line to the oscilloscope for synchronizing the display to either or both channels. The Channel Select logic line alternately selects offset A and S/ H A, or offset B and S/H B. The S/Hamplifier selected receives the same fundamental Sample Command pulse train as in the DSID circuit, with the exception that a short delay to the start of the sampling pulse train is imposed after each S/H selection change by the Crosstalk Suppression logic line. This ensures signal separation between recorder pens for accurate isotopic ratios. The noninverting filter in the added S/H section is provided with a null control ( 6 ) to adjust the base line of Pen B relative to Pen A. Both pens can be positioned together with the zero control associated with the integrator.
RESULTS AND DISCUSSION T h e described circuit circumvents the troublesome focus instability and uncertainty accompanying static SID mass fragmentography on the Du Pont 21-490 Mass Spectrometer by repeatedly scanning back and forth across a chosen peak by manipulation of the ion accelerating voltage. It plots the discrete area value of each scan cycle as a compressed, and therefore smoothly traced, function of time on a chart recorder. This plot is the conventional SID mass fragmentogram wherein chart peak area or, conveniently, peak height, is linearly related to the amount of the material injected into the gas chromatograph. T h e validity of this algorithm for the operation of the mass spectrometer and demodulation of its output signal derives from the fact t h a t the resolution of the Du Pont 21-490 is great enough t o provide better than 0.5 amu of dead space between peaks. Therefore, considerable magnet current and/or accelerating voltage drift can still be tolerated when the width of the scans is greater than the maximum spread of the chosen peak. Peak detection was considered as a n alternate t o integra(4) S. R. Pareles, Electronics, 47, 123 (1974). (5) S.R. Pareles. Anal. Chern., 46,464 (1974). (6) J. D. Rosen and S. R. Pareles, “Mass Spectrometry and NMR Spectroscopy in Pesticide Chemistry,” R. Haque and F. J. Biros, Ed., Plenum Press, New York, N.Y.. 1974, p 91.
2058
MILLISECONDS
Figure 4. Filtered electron multiplier output of the mass spectrometer. Focus Guides intercept the centers of alternate peaks
tion of the electron multiplier output signal during each scan cycle. However, peak detection requires substantial RC filtering t o prevent noise spikes from causing unrepresentative pulse amplitude readings. Such filtering significantly decreases the magnitude of the signal whose amplitude is measured ( 7 ) and thus the overall sensitivity of the method. Nevertheless, peak detection might be a preferred technique under conditions of changing scan cycle time and width in more sophisticated applications under computer control (3, 8). Since we intended t o hold our scan cycle width and time constant throughout our experiments, we prejudged that the integration approach would give high sensitivity, with adequate and perhaps superior insensitivity t o white noise or unrepresentative spikes or transients. T h e triangular scan wave form is shown in Figure 3. T h e center of scan conforms to “0” offset of the accelerating voltage (in the 1200 to 1400 volt range) as shown. Ignoring instrument-specific time lags, the centroid of the scanned peak would be expected t o occur a t “0” displacement within any scan between (accelerating voltage - x ) and (accelerating voltage + x ). Figure 4 shows the appearance on a n oscilloscope of the electron multiplier output of the mass spectrometer conditioned for ease of viewing by amplification and filtering of the actual statistical ion intensity pattern. The bars imposed on the display are the timing marks produced by the display section of the accessory as Focus Guides. Drift in prime accelerating voltage (or other parameter) after which manual accelerating voltage adjustment might easily be made, causes these peaks to stray away from these focus guides asymmetrically. As much as a 25-50% displacement in position can nevertheless be tolerated without changing peak height more than a few per cent. Moreover, substantial wandering of peak centers from these guides has not been observed. T h e stability of the system was determined by measuring the peak heights obtained from eight injections of 7.8 ng of hexadecane over a two-hour period. T h e results, obtained by repetitive scanning of the parent ion (mle 226), gave a mean value of 36.8% for the peak height. From the data, a sample standard deviation (0)of 2.25 was calculated. T o make a rather crude comparison, a 0 of 1.3 was calculated (9) for 10-ng injections of 3,6,17-tris(trifluoroacetyl)-2,4dideuterioestriol on a Finnegan Model 1015 Mass Spectrometer equipped with programmable multiple ion monitoring (PROMIM). The stability of the system was further demonstrated by the fact that 7.8-ng injections of hexadecane gave values falling within 95% confidence limits 5 days later. Sensitivity and precision differences between static and dynamic focusing were determined by making five successive 39-ng injections of hexadecane and determining the peak heights by each method. The results (Table I) clearly demonstrate the precision advantages of DSID over static (7) W. E. Reynolds, V. A. Bacon, J. C. Bridges, T. C. Coburn, 6 .Helpern, J. Lederberg, E. C. Levinthal, E. Steed, and R. B. Tucker, Anal. Chern.. 42, 1122(1970). (8) W. F. Holmes, W. H. Holland, B. L. Shore, D. M. Bier, and W. R. Sherman, Anal. Chern.,45, 2063 (1973). (9) R. C. Murphy, Finnigan Spectra, 3, No. 3 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
single ion detection and that, a t most, the sensitivity loss is only 22%.
Table I. Comparisonbetween Dynamic and Static Single Ion Detectiona
CONCLUSIONS T h e described circuit is successful in eliminating the focus instability problems associated with the Du Pont 21490 Mass Spectrometer and makes it possible for users of this instrument to achieve mass fragmentography capability. T h e DSID circuit costs approximately $350 in parts (plus the cost of a n inexpensive oscilloscope) to manufacture. An additional $100 in parts is needed for the DDID circuit. Furthermore, the circuit can be simply plugged into the scan sweep of the mass spectrometer. No modification of the mass spectrometer, digital processing equipment, or analog-digital interconversion is necessary.
Injection No.
RECEIVEDfor review March 13, 1974. Accepted June 20,
the Journal Series, New Jersey Agricultural Experiment Station, Cook College, Rutgers University-The State University of New Jersey, New Brunswick, N.J. 08903.
1974. This work was supported in part by Regional Project NE-83, United States Department of Agriculture. Paper of
Dynamic
Static
39.5t 39.5 39.5 37.5 40.0 Mean s t d dev a
39.2 0.975
50.5 47.5 33.5 41.0 27.5 s t d dev
40.0 9.57
For 39-ng on-column injections of hexadecane. Expressed as
% scale deflection at recorder = 50 mV, attenuation = 128.
Solubility Product Constants of Some Divalent Metal Ions with Ammonium Pyrrolidine Dithiocarbamate Michel Arnac and Gilles Verboom Universite du Quebec a Rimouski, Depatiement des Sciences Pures. 300 Avenue des Ursulines, Rimouski, P.Q., Canada, G5L 3A 1
It has been demonstrated t h a t a number of heavy metals can be satisfactorily extracted from an aqueous solution into organic solvents like methyl isobutyl ketone (MIBK) with the use of ammonium pyrrolidine dithiocarbamate (APDC) as a chelating agent ( 1 ) . Determination of trace elements in the aquatic ecosystem has become an important area of pollution studies. APDC in MIBK has been widely used for concentration of metals from sea-water for X-ray fluorescence analysis (P),and by atomic absorption workers (3.4 ) . Precipitations were noted on reacting some metals with APDC and the precipitates were extractable into organic solvents ( 5 ) . Only the stability constants of complexes of some divalent metal ions with a variety of N-substituted dithiocarbamic acids have been done recently (6). Further investigations involving APDC were considered desirable for several metals. A careful study of the literature shows a lack of information on the solubility product constants of metal ions with APDC. T h e experimental conditions on this subject that are found in the literature vary from one author to the other. Used concentrations for the APDC complexing solutions are in the range 1-10%. The objective of the present study is to investigate this field of analytical chemistry. EXPERIMENTAL Polarography (amperometry) was used to determine the stoichiometry of the precipitating reaction. Solubility measurements of equilibrium-saturated systems were made using atomic absorption spectrophotometry. W. Slavin. Af. Absorption Newslett.. 4, 273 (1965). A. W. Morris, Anal. Cbim. Acta, 42, 797 (1968). R . R . Brooks, B. J. Presley, and I. R . Kaplan, Talanta, 14, 809 (1967). P. G. Brewer, D. W. Spencer. and C. L. Smith, Amer. SOC.Test. Mater., Spec. Tech. Pub/., 443, 70 (1969). (5) H. Malissa and E. Schoffman, Mikrocbim. Acta, 1, 167 (1955). (6) R. R . Scharfe, V. S. Sastri, and C. L. Chakrabarti,'AnaL Chem., 45, 413 (1973).
(1) (2) (3) (4)
Apparatus. Dc polarograms were recorded using a classical three-electrode technique. The apparatus composed of several Tacussel units has been described in an earlier paper (7). Measurements were done a t constant temperature in a Tacussel waterjacketed cell, Model RM 06. Atomic absorption measurements were carried out on a double-beam Perkin-Elmer Model 306 Atomic Absorption Spectrophotometer equipped with a four-inch single-slot standard burner for use with air-acetylene. Standard Perkin-Elmer Intensitron hollow cathode lamps were used for all elements. The outputs of the instrument were recorded with a Perkin-Elmer Model 56 recorder. The spectrophotometer is capable of producing readings integrated over three and ten seconds, either on the digital or on the laboratory recorder. Electrodes. All potentials were measured with reference to the saturated calomel electrode. The auxiliary electrode was a bright platinum foil of large area. The capillary tube used in this work was described previously (7). Reagents. All reagents used were of analytical grade purity, without further purification. Commercially obtained reagents (Fisher Certified or J. T. Baker Analyzed Reagent) were employed as the nitrate salts. The pyrrolidinecarbodithioic acid (J. T. Baker) was used as the ammonium salt (APDC). Standard solutions for atomic absorption calibration curves were bought from Harleco. Doubly distilled mercury which has been tested for impurities was utilized throughout the polarographic studies. Before preparing solutions, water was primarily deionized, distilled in borosilicate glass, then deionized again, and finally fed at low flow rate through a mixed-bed ion exchange column. Average specific conductance of the purified water was between 0.4 and 1.2 X ohm-' cm-' a t 25 "C. Procedure. Polarography. Fifty milliliters of aqueous solutions containing m grams of a nitrate salt [Cu(N03)24H20; Pb(NO:
