A Solid State Digitizer for Mass Spectrometers E. M. THOMASON Instrument Department, Hydrocarbons Division, Monsanfo Chemical Co
b A solid state digitixer is described for collecting data from mass spectrometers. The digitixer accepts ion peaks and acceleration voltage signals, converts the analog signals to digital, and presents the data on punched paper tape and printlsd output. The punched paper tape may be fed directly into a compiiter for sample calculations. Minimum detectability is 1 mv. with a background noise level of less than 0.5 mv. Mass number reproducibility is *O.l mass numbers from mass 2 to mass 125 and *0.25 mass numbers from mcrss 125 to mass 250. Precision of sample calculations is improved by a factor of 4. This is due primarily to elimination of human error. The digitizer is of particular value if mass spectrometers are used to analyze large numbers of samples. On a three-shift opera tion, payout can be as short as 6 months.
T
for a reli,ible digital data collection system for mass spectrometers has existed f x several years. A large number of control laboratories operate their mass spectrometers on a round-the-clock, 7-day-a-week basis. An operation like this requires a t least two people per shift; one to run the samples on the mass spectrometer and one to pick the peaks from the oscillographic chart and to 'og this data on HE NEED
.,
Texas City, Texas
calculation charts or to mark sense cards for the computer. The justification for having an automatic data collection system is fourfold: a reduction in number of personnel; increased accuracy from the data; elimination of the operational expense of the oscillograph; and an increase in the number of samples which can be run through the mass spectrometer. An intensive study of available digital data collection systems indicated that the equipment needed was not available on the market and it would be necessary to have the equipment designed for this particular application. The problem of collecting mass spectrometer data did not appear to be too difficult from an engineering standpoint. The output signals from the mass spectrometer are two analog voltage signals; one represents the amount of ion current produced by a particular mass and the other identifies which mass is being measured. h data collection system must accept these analog signals, convert them to digital form and present them in a format acceptable for calculations. To describe the problem as clearly as possible, the specifications included photographs of the analog output signals under various operating conditions. The data showed the effect of varying the response and damping circuits on the mass spectrometer amplifier as well
as an analysis of the types of noise present. The data also gave the slopes in volts per second of the various peaks encountered from the sharp peaks a t low magnetic fields to the broad peaks a t high magnetic fields as well as metastable peaks. Photographs of the acceleration voltage a t different decay rates were included to illustrate voltper-second decay rate throughout the decay range and the types of noise that modulated this signal. A large manufacturer of digital systems accepted the specifications and produced the all solid state digitizer in 7 months. DESCRIPTION OF SYSTEM
The digitizer accepts the analog signals from the mass spectrometer and converts these signals immediately to digital form. All control functions and subsequent handling of the information is carried out by a unique digital logic system. Figure 1 is a block diagram of the digitizer and Figure 2 is an illustration of the logic sequence that must be carried out in measuring and identifying each peak. Assume the digitizer is in logic state I. This means that the output signal from the mass spectrometer is the baseline and the logic circuit is "looking for" a signal that is greater than the signal stored in register (which is zero since each new reading is compared against the last reading). The digital voltmeter is reading the ion input signal a t 60c.p.s. rate. As soon as an ion signal appears, the logic system now has measured and compared a signal that is greater than the previous signal and thuj the control logic shifts to state 11.
m f-l
L, CLOCKS
-....-.L-
PRINTER PUNCH
/
/
J
4
0 BASELINE 2 M V BIAS
-
LEVEL LOOKING FOR > S I G N A L II LOOKING FOfi < S I G N A L I l l PEAK I D E N T I F I C A T I O N Ip R E A D A S C E L E R A T I O N VOLTAGE I.P R I N T C O M M A N D m . L O O K ' N G FOR 2 SIGNAL PLT.RESET
I
COMP. COUNTER
PROGRAM REGISTER
Figure 1 .
'3lock diagram of digitizer
Figure 2.
Signal identification-sequence
of logic control
VOL. 35, NO. 13, DECEMBER 1963
2155
Table I. Acceleration Voltage to Mars Number Correlation
Acceleration voltage
number
4.9744.990 4.9224.937 4.871-4.886 4.820-4.835 4.7714.786 4.7234.738 4.676-4.690
94 95 96 97 98 99 100
' iR 2 L L cnn
Mass
2156
Y
5 5 5 6 6 6 7 7 8 8 9 0 0 4
3 7 8 3 5 9 1 8 3 5 3 1 9 8
2 0 8 1 4 6 9 8 4 9 6 4 6 7
0 2 3 6
0 1 2 8 6 0 0 0 2 0 0 1 0 0 0 0
5 0 8 0 1 1 0 1 1 1 0 1
0 2 1 4 1 7 6 3 8 0 0 4
2 3 7 6 8 6 1 0 9 2 9 0
In,
i n %ate 11, the input signal 1s increasing in amplitude and the logic circuit is "looking for" a reading that is less than the previous reading. Satisfying this criteria identifies the maximum peak amplitude ( i l mv.) and shifts the logic into state 111. To prevent the system from falsely identifying spurious noise signals as ion signals, the logic system makes 5 additional comparisons before a decision is made that the maximum signal is a true ion peak. No signal error is encountered since the maximum signal has been held in storage. The 5 consecutive readings actually provide a digital filter that rejects all noise peaks that are less than 80 milliseconds in duration. This particular feature makes the dieitiaer extremely precise in gatherinelmass
N
2 2 2 2 2 2 2 2 2 2 2 3 3 3
9
ANALYTICAL CHEMISTRY
__
....
__ ..-.
.l."l-_""..
.-I
"""..-"
this reading is complete, the control circuit shifts to logic state V and the stored digital signal is Drinted and punched out. Acthe end*of the print cvcle the control circuit. shift,s in lnpir
9
9
9
....
"
"" ""
_"~." ""_"_
where all registers are reset and the system shifts to logic state I, reading for another Deak. The abovi! sequence of events occurs within milliseconds. the actual time is
tigure 4. digitizer
Wont view ot the solid state
the digitizer from taking any action on a positive peak. The entire digitizer ,Y controlled by two clocks, a 60-cycle clock and a 100-kc. clock. The 100-kc, clock controls the digitizing rate, the digital voltmeter, operates the digital comparator, the greater-than and less-than loglc gates, and the register. The 60-cycle clock is used to control the logic circuitry of the bystem and is the fundamental time base for decision making. Due to the widths of the ion peaks (minimum of 70 milliseconds within 2% of the top) no error in measurement occurs. The 60-cycle time base also provides inherent rejection to 60-cycle interference. If desired this clock speed can be increased to several thousands cycles per second. Installation and Startup. The mass spectrometer digitizer is quite easy t o install. There are no modifications to make t o the mass spectrometer amplifier. The amplifier output signal and a 0- t o IO-volt tap of the acceleration voltage must he supplied t o the digitizer input. Both of these signals are filtered by a pi RC filter which reduces the t o t d noise level to less than 1 mv. peak-to-peak. -1fter the digitizer has been connected and is operating properly, the next step
is to determine K factors for each magnet range that will be used. The equation for mass number is: M / e =
Rz
which can be simplified to J l l e = E where K = R2B2 and B is fised for a 2 particular range. Therefore, K = E X mass number. Since the digitizer always reads only the acceleration voltage from 9.999 down to a lesser value, it is necessary for the operator.to indicate which magnetic range is in use. The different K factors can be determined by averaging the K factors from known sample runs. Figure 3 illustrates the output data format and Table I illustrates tabular values of mass numbers us. voltage. AI?,
K
PERFORMANCE
The solid state digitizer has been in use for 11 months. In that time, the operation yielded the following results; per cent onstream, 98; per cent utility, 99+. A labor saving of one operator per shift and a reduction in cost per sample by 570/, were realized. The capacity of the lab, in samples per month, increased by 66y0 to 2000 samples. The
estimated annual savings, based upon the results thus far, should be $50,000. As shown by Table 11, the precision in analysis was improved by a factor of 4. Mass number reproducibilities were hO.1 mass number up to mass 125 and 3 ~ 0 . 2 5mass number up to mass 250. The instrument has a dynamic range of 104. Input signals may vary from 1to 9999 mv. Minimum detectability is 1 mv., and the system is linear across the complete span. The digitizer, a photograph of which is shown in Figure 4, is unaffected by large voltage swings from the mass spectrometer signals and can accept ranges of over 600y0. ACKNOWLEDGMENT
The author gratefully acknowledges the assistance of L. Y. Saunders and S. P. Scarcella, and full credit for the design of the digitizer goes to Henry Reinecke of Non-Linear Systems, Inc., Del Mar, Calif. RECEIVEDfor review June 24, 1963. Accepted August 29, 1963. Presented at the ASTM-E14 Meeting, San Francisco, Calif., May 1963.
UIt ra rnic roldete rmin at io n of Iodine by a Rapid Automatic Reaction-Rate Method H. V. MALMSTADT and T. P. HADJIIOANNOU’ Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111.
b An automatic spectrophotometric reaction rate method for the ultramicrodetermination of iodide i s based on the Sandell-Kolthoff reaction, in which a trace of iodine or iodide acts as catalyst for the reduction of Ce(lV) in the presence of As(lll). The time required for the reaction to consume a small fixed amount of ceric ions, and therefore for the absxbance to decrease by a preselecteld amount (about 0.06 unit), is measured automatically and related directly to the iodide concentration. Ultramicro amounts of iodide in the range of 0.015 to 0.45 pg. were determined with relative errors of about 1 to 25% and measurement times of only about 10 to 100 seconds. The method s’1ould be adaptable for the determination of proteinbound iodine (PBI).
T
HE method presented here utilizes the Sandell-Kolthoff reaction (15), in which a trace of either iodine or iodide acts as a catalyst for the reduction of Ce(1V) in the presence of As(II1). This reaction has b3en widely investigated because of its great impor-
tance for the determination of proteinbound iodine (PBI) in serum (3). The overall reaction is summarized in Equation 1.
iodine
2 Ce(1V)
+ As(II1) catalyst 2 Ce(II1)
+ As(V)
(1)
There have been many studies (11) to determine the reaction mechanism. Kolthoff and Sandell developed a quantitative procedure for iodide by measuring the time required to reduce all of the Ce(IV), as indicated by a visual color indicator. I t was found that the iodide concentration was inversely proportional to the measured time for total reduction. In subsequent procedures the time for the measurement was decreased by not waiting for complete reduction of cerium(IV1. In one basic procedure ( I , 2, 5, 9) the Ce(1V) remaining after a fixed time (usually 20 minutes) is determined by an absorbance measurement a t a selected wavelength characteristic of Ce(IV), usually between 350 and 420 mp. Since the absorbance changes continuously, it is necessary to
read the absorbance value a t the exact preset time. A plot of absorbance us. iodide concentration provides a suitable working curve. In another procedure (7, 8, 10) the reaction is stopped by adding a reagent that reacts immediately with Ce(1V) to reduce the remaining Ce(1V) quantitatively to Ce(II1). One suitable reagent is brucine, which is oxidized to a colored reaction product. A plot of absorbance of the colored reaction product against iodide concentration provides a working curve. In another procedure (4) the change of absorbance of Ce(1V) was recorded continuously and the slope of the recorded curve was related to the iodine concentration. To simplify and automate the iodide determination, a new procedure provides a digital readout value within a few seconds after start of the reaction. For example, extremely low concentrations of iodide in the range of 0.015 to 0.45 pg. per 4.25 ml. of solution were Present address, Department of Chemistry, College of Pharmacy, University of Illinois, Chicago, Ill. VOL. 35, NO. 13, DECEMBER 1963
e
2157
