Repetitive Scan Unit for Multichannel Analyzer Signal Averaging in Spark Source Mass Spectrometry W. A. Mattson and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville. VA 22903
Signal averaging has been shown t o improve precision and sensitivity in spark source mass spectrometry (SSMS), using manual mass scan repetitions ( I ) . Described in this report is a versatile asymmetric triangular wave form generator, designed to permit automatic multiple scan accumulations. A range of scan rates is provided by the unit, which in the repetitive mode minimizes operator intervention and error. At fast scan rates, this is particularly important. T h e repetitive scan unit (RSU) supplies a cyclic sawtooth type magnet reference voltage to a MS-702 spark source mass spectrometer, and can be set to scan all or a selected portion of the full mass range; it also supplies a trigger pulse, which synchronizes the scan of t h e magnetic field to repetitive spectra accumulation, using the multiscale mode of the rnultichannel analyzer (MCA). Scan rate and mass region. as well as mass range, are all continuously variable.
EXPERIMENTAL A p p a r a t u s . T h e RSU schematic is shown in Figure 1. Operational amplifiers A I , A2, and A3 comprise a triangular waveform generator ( Z ) , modified t o produce variable asymmetric triangular waves. A4, is a high .*.oltage operational amplifier which steps u p t h e triangular wave voltage t o the required magnitude t o drive t h e magnet reference tube-type circuit. An asymmetric wave allows variable magnet scan times, which may be a few seconds u p t o several minutes, a n d also variable magnet reset times t o reinitiate t h e subsequent scan. T h e output of comparator A1 is a t +13 or -13 V, depending upon t h e relative levels of t h e two input voltages from A3 a n d P1. A2 serves as a stepdoivn inverter to provide integrator A3 with t h e proper voltage and polarity to eventually flip t h e A1 output. T h i s then produces an opposite polarity input t o A3 which integrates until A1 again flips a n d starts a new repetitive cycle. Steering diode D4 directs t h e positive-going voltage through an S3 selected resistance, which with P3 and C 1 sets the mass scan rate. A negative going voltage acts through D3 as a reset signal, adjustable by S2 a n d P2. T h e magnitude of the mass scan is s e t by P1, which determines t o what voltage A3 must drive before reversing t h e comparator output. R 1 and zener diodes Z1,22 maintain a +6 or -6-V input t o R2 and R4. D2 prevents t h e negative output cycle of A1 from reaching t h e MCA address reset. T h e positive A1 output permits synchronization of t h e hlCA accumulation with the magnet scan by resetting t h e register address control of the MCA t o register (channel) No. 1 and holding it there until t h e reset portion of t h e cycle is complete. At 1:hat point, t h e MCA channel advance and t h e scan of t h e magnet are concurrently initiated. Because MCA channel advance is possible in one direction only, mass scans must be unidirectional. A mercury battery develops across R21 a voltage which is adjusted (P4) to set t h e proper input bias for A4. I n a full mass scan, a magnet reference voltage of about 0 t o 60 volts is required. P1 is set t o a 3-volt level, requiring the integrator to range from +3 t o - 3 V during t h e cycle. P4 is adjusted t o convert this t o a 0 t o -6-V input to A4. yielding a 0 to +60-V output from t h e gain of 10 a m plifier during the accumulation period. T h e subsequent -6 t o 0-V input reduces A4 t o 0 during t h e reset period. Our unit was wired t o take advantage of the high voltage operational amplifier already present in a magnetic peak selector ( 3 ) . However. the A4 amplifier can be wired directly into t h e R S U with a switch at the A4 output t o allow for magnet reference control by either t h e standard AEI scan control unit ( 4 ) or the RSU. T h e circuit was designed for rapid repetitive scans. T h e slowest full mass range scan :rate is approximately llh minutes. If a slower scan is desired, R,5 can be made variable to yield a lower potential
a t A3. Alternatively, t h e A3 time constant may be increased by making C1 or t h e resistance a t S3 larger. A different set of scan rates could also be produced by changing other key parameters, if desired, such as t h e zener regulated A1 output, t h e input voltage a t PI, a n d t h e gain of A4. Our parameters have worked well, b u t we do not suggest these t o be t h e only or necessarily t h e optimum conditions. Proper grounding t o protect t h e circuit from noise (e.g., t h e 60-Hz and impulse noise occurring on t h e ac lines and t h e interference rf spark noise) is essential for the circuit t o maintain adequate synchronization. Two important conditions for stability are: t h e circuit ground for t h e R S U is t h e same as t h e ac ground of t h e f l 5 - v o l t power supply; and t h e signal ground as well as t h e rest of t h e circuit is shielded from rf spark noise. T h e circuit is enclosed in a n aluminum chassis, properly ventilated and isolated from variable heat sources which can cause critical temperature coefficient effects. O p e r a t i o n of t h e RSU. T o scan a specific spectral region, it is necessary to know the approximate magnet current (or Hall voltage) t h a t corresponds to t h e mass a t t h e center of t h e chosen region. This is accomplished by closing S4, which zeros t h e output of A3 a n d yields a reference voltage a t t h e output of A4 proportional t o the mass a t t h e scan center. P4 (mass region) is then adjusted until t h e desired magnet current or Hall voltage is achieved. After
Figure 1. Schematic diagram of repetitive scan unit.
A1 A2, A3 A4 z1,22
23 D1, D2 D3,D4 M1 B1
c1 P1, P2, P3 P4
350B (Analog Devices) 233K (Analog Devices) 1022 (Teledyne Philbrick) IN3496,5.96.5 V ECG5150 (Sylvania), 62 V M2.5A (Mal lory) IN34A 150 pA Hg, 8.4 V 10 pF 100 K o h m s (ten -turn) 10 K o h m s (tenturn)
R1
1.5 K o h m s
R2
680 ohms
R3
200 K o h m s
R4
6.8 Kohms
R5
1 Kohm
R6-RlO
100 K o h m s
R11 R12-R20 R21 R22 R23
10 Mohms 100 K o h m s 1 Kohm 10 K o h m s 100 K o h m s
MPS
Magnetic peak selector (see text)
ANALYTICAL CHEMISTRY, VOL. 47.
NO. 6,
M A Y 1975
967
S" MO lliotopeb
92
9 4
..
..
9,5
96
,
..
.
97
..
....... ,....... ..'.... .'..' '..."....
.
i$otopes
(500 S c a n s 1
I18
,
MCA
Chonnels
100
,
MCA
,20
1100 S c a n s )
9.8
............."
'.
.,,,,l,,
Chonnair
Flgure 2. Molybdenum isotopes obtained during 100 RSU full range mass scans of 23-second duration each
Figure 3. 500 averaged 3-second scans of the Sn is0 Same sample as in Figure 2, Sn at 0.022%
NBS 461 steel sample, Moat 0.30%
S4 is opened, P1 (mass range), S3-P3 (scan range), a n d S2-P2 (reset r a t e ) are adjusted as desired. The m a g n e t i s n o r m a l l y put t h r o u g h several p r e l i m i n a r y cycles before d a t a a c c u m u l a t i o n is begun.
RESULTS AND DISCUSSION T h e RSU was designed to allow control and automatic accumulation of rapid repetitive mass spectral scans, providing synchronization of the MCA accumulator with the mass spectrometer magnet scan. Of critical importance to the RSU operation are the severe stability requirements imposed on the cyclic reference voltage. To deposit each element in the same MCA channels during each scan, low drift amplifiers are require&. to prevent significant resolution deterioration. Figures 2 and 3 illustrate that adequate resolution for elemental analysis can be maintained for many multiple scans. Figure 2 shows the molybdenum isot o p a resulting from 100 successive 225- to 10-cmu full range scans (each of 23-second duration). Figure 3 is the net averaged spectrum for five hundred 3-second scans over just the Sn isotopes. In general, precision in spectral
CORRECTION
averaging improves with the square root of the number of scans ( 4 ) . The normal SSMS precision of f20-40% attained by electrical scanning can be improved to f 5 % or better by multiple scan averaging. This unit is currently in use for a study of isotopic ratios and improvement of precision in quantitative analyses by spark source mass spectrometry, as previously shown by manual methods ( I ) . Its use is not necessarily restricted to the spark ion source nor to the MCA as an accumulator. Computer interfacing could also be useful in terms of data accumulation and reduction.
LITERATURE CITED (1) W. W. Harrison and W. A. Mattson. Anal. Chern., 46, 1979 (1974). (2) H. V. Malmstadt, C. G. Enke, and S. R. Crouch, "Control of Electrical Quantities in Instrumentation," W . A. Benjamin, Inc., Menlo Park, CA, 1973, p 184. (3) C. W. Magee and W. W. Harrison, Anal. Chern., 46, 474 (1974). (4) R . A . Bingham and R. M. Elliot, Anal. Chern., 43, 43 (1971).
RECEIVEDfor review October 10, 1974. Accepted January 8, 1975.
Atomic Absorption of Rhenium Using a Neon Analysis Line
CORRECTION Identification of Dicarboxylic Anhydrides in Oxidized Asphalts
In the paper on the utilization of a Ne line for the analysis of Re by R. J. Lovett and M. I,. Parsons, Anal. Chern., 46, 2241 (1974), the 346.053-nm Ne line and the 346.046nm Re line were erroneously listed in the text and brief as the 364.053-nm Ne and 364.046-nm Re lines, respectively.
An error occurs on line 9, p 107, in the Experimental section of this paper by J. C. Petersen, F. A. Barbour, and S. M. Dorrence, Anal. Chern., 47, 107 (1975). The latter part of the sentence should read "20-40 mesh quartzite mineral aggregate" rather than "Fluoropak 80".
968
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
