Microphotometer modification for automatic recording of spark source

controlled manually. Measured data and operator input are displayed visually and stored with a remote video terminal. The stored information is transf...
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Microphotometer Modification for Automatic Recording of Spark Source Mass Spectra R. J. Conzemius,* D. J. Adduci, G. 0. Foss, and H. J. Svec Ames Laboratory, Iowa State University, Ames, Iowa 5001 1

Computer based schemes for interpreting mass spectra require accurate input of intensity and mass. When the spectra are recorded on a photographic plate and remote large centralized computers are available for handling complex interpretive programs, i t becomes desirable to get the pertinent spectral information (i.e., line transmittance, background, line location, and exposure) quickly and accurately to the computer. This paper describes a moderately low cost modification to a commercial microphotometer which permits automatic measurement of spectral line position, exposure, and transmittance on spark source mass spectrometer photoplates. Spectral lines are located and scanning functions are controlled manually. Measured data and operator input are displayed visually and stored with a remote video terminal. The stored information is transferred subsequently via telephone lines to a large central computer for data processing. The system described here has a number of advantages over other reported systems (1-5). First, an on-line computer is not required. Instead, the advantages of a large centralized computer such as large area data storage, high level program coding, shared costs of maintaining and upgrading the computer are used. Second, costs are kept low by making minimal modifications to a commercial microphotometer and by utilizing conventional electronic circuitry for data reading and transfer control. Third, the system is relatively fast. A photoplate containing 300 lines can be read in -40 min. The system reads accurately at scan rates of up to 25 mm/min and easily keeps up with manual rates required for line location and scan control functions. Fourth, the system is not dependent upon reproducibility of scan rates. Fifth, maximum use is made of operator capability for decisions without the burden of actual data recording. Five primary items of photoplate information are recorded. 1)Exposure identification is measured by coupling a 4-inch linear potentiometer which acts as a linear encoder for the Y-axis of the microphotometer photoplate carriage. 2) Spectral line position or mass identification is accomplished by coupling the X-axis of the microphotometer carriage to a commercial bidirectional linear motion encoder. The actual line position is located a t the point of maximum line intensity by a n electronic circuit. 3) Spectral line transmittance is recorded as the minimum in the ratio of analytical-to-reference phototube signals. Transmittance minima are recorded instead of integrated areas for several reasons. They are technically easier to record. Unless standardization techniques are used the area/minima difference will generally introduce much smaller errors than other associated uncertainties such as assumed elemental sensitivities. For quantitative work, line intensity profiles should be constant (6) and if they are constant then transmission minima are accurate indicators of line intensity (1).4) Background (fog leuel) reading is recorded as the level of photoplate transmission in a region of the plate selected by the operator. This region would normally be just preceding a line being measured where the operator decides to initiate a scan. 5 ) Control data such as comments, line identification, mass-position reference points, etc. can be included by the operator with a keyboard input along with the electronically accessed data. For example, mass-position reference points are selected about 20 mass units apart and are coded by the operator in the same data stream as the analytical data when the reference masses are encountered on the plate.

EXPERIMENTAL Apparatus. The photographic piates were exposed with a spark source mass spectrometer (Nuclide Corp. model Graf 2-2). Table I gives the operating conditions. A block diagram of the modified microphotometer (Jarrell-Ash model 23-100) is shown in Figure l. Commercially available circuits, as identified in Figure 1, were used in some cases because they were on hand and in other cases to save in design and construction time. Logic functions for instrument control in the Control Logic block are initiated by the operator via the remote Control Panel. The outputs of the photomultiplier tubes are ratioed using a precision analog divider in the Ratio Circuit. The ratioed signal is sent to a Peak Detector and to a Background digital voltmeter (DVM). Detected peak and background readings are stored in the Peak DVM and Background DVM. A t the time a peak is detected, the exposure number and the spectral line position are also stored. For the purposes of this report, a "peak" is a Ratio Circuit output giving a lower transmittance minimum than any previously stored output in a given set of readings. Data are transferred to the Data Storage Buffer by the Data Coupler and Sterilizer and are transmitted to Central Computers via telephone lines. Procedure. Operation of the data acquisition system is accomplished by function switches shown in the Control Panel block. Figure 2 is a flow chart showing sequences resulting from switch actuation. Starting at point A, the operator has a choice of recording a new background level by depressing the Background button (or foot switch) or to retain the previous background reading by depressing the Start button. Movement of the photoplate carriage across the line may be manual or with the scan motor. The operator predetermines the Scan Mode with a switch setting. If a new background is requested, the system will sample the Ratio Circuit output (Le.,measure the fog level) and then switch to the Peak Detect Mode. The operation of the Peak Detector circuit is based upon analog detection followed by analog-to-digitalconversion with a maximum rate of 200 Hz. When a 3% drop in intensity from the peak level is sensed (or when the operator depresses the Record button), a line is assumed to have been measured and the stored data for exposure, position, line intensity, and background intensity are transferred to the data buffer. The scan motor is also turned off and the system is reset to the starting point,

Table I. Experimental Operating Conditions Mass spectrometer Analyzer vacuum 10 Torr 3 x 10-6Torr Source vacuum Electrostatic potential 2400 V Accelerating potential 24 K V Mass range covered 7-240 Primary slit width 0.0005 inch Detector Ilford Q2 Plate Dark room conditions Bleach 4 min Solution A at 20 C Develop 6 min MK9 at 20 " C Fix 1 min 1 / 2 strength Ilford Ilfofix at 20 "C Wash 2 min Tap water Microphotometer conditions Slit width 17 p m Slit height 0.5 mm see text Other Sample preparation . . . mg Chemical Dry blend the listed chemicals 1.0 CrO, Pellet with polyethylene die 1.2 CUO Spark as self electrodes 11.1 MOO, 1.s SmO, 1.0 BaO 9.5 HfO, 570.1 graphite

-

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PHOTO REFERENCE

PEAK DETECTOR P H O T OP L A T E C ARR lAGE

1

BACKGND OV M

I

I I

BACKGND

SAMPLE

BACKGNO

START

STORE

DATA

RECORD

C O U P L E R AND

CLEAR

SERIALIZER

1 K E Y B O A R D E N T R Y A N D M T A STORAGE B U F F E R

TELEPHONE L I N E S

1 C E N T R A L COMPUTERS

1

Figure 1. Microphotometer data acquisition system Identification of commercial units and equipment used includes Microphotometer, Jarrell-Ash, NO. 23-100: Exposure No. DVM. TYCO, No.401; Peak and Background DVM, Datel, No. DM2100: Linear Motion Encoder, Wayne-George Co., No. L1-M10K/100s; Data Coupler and Serializer, Hewlett-Packard, No. 2019; Keyboard Entry and Data Storage Buffer, Video Systems, No. VST-7000; Central Computers, IBM, Nos. 360/65 and 370/165. The Ratio Circuit, Peak Detector, Control Logic, and Control Panel were constructed in Ames Laboratory.

RESULTS AND DISCUSSION

Table 11. Comparison of Automated and Manual Systems Apparent mass

Average minimum transmittancec (10 readings)

58.0 58.5 58.67 59.0 59.33 59.5 59.67

0.647 0.785 0.537 0.166 0.102 0.794 0.264

Manual systema

Automated systemb

SD

% RSD

SD

% RSD

0.0022 0.0018 0.0021 0.0014 0.0007 0.0024 0.0019

0.34 0.24 0.39 0.84 0.66 0.30 0.72

0.0021 0.0015 0.0018 0.0008 0.0008 0.0017 0.0010

0.33 0.19 0.34 0.51 0.82 0.22 0.39

Av.

0.50

0.40

astrip chart recording at scan rate of 1.5 mm min-'. b Scan rate of 25 mm min-'. CFor the automated system. This average agreed with the manual system within total error indicated in Table VII.

A. Data can be recorded manually by depressing alternately the Store and Record buttons. The microphotometer output is scaled by setting the Ratio Circuit output to 0 with an opaque interruption to the Analytical Phototube input and to 100% by adjusting the gain of the phototube with a clear plate set at the microphotometer slit. 1648

Several parametric studies were made in order to demonstrate the general utility of the acquisition system. These studies were confined to demonstration of precision in reading minimum line transmittance, background transmittance, line position, and to comparison of the automated system with manual methods for reading the same data. Seven different mass lines in the mass range 58 M/z to 59.67 M/z were each measured ten times. Table I1 gives the M/z and average minimum transmittance for each line in columns one and two. The standard deviation (SD) and relative standard deviation (%RSD) are also shown for data taken both by manual and by automated methods. The manual method consisted of very careful reading of a ratio strip chart recording (the equipment originally supplied for the microphotometer output) using a scan rate of 1.5 mm min-l. T h e automated system was operated as described above with a scan rate of 25 mm min-1. The data indicate that there is no significant difference in the imprecision of reading between the two methods. Precision of automatically recorded data was only slightly dependent upon microphotometer slit width. A least squares fit of a second-order polynomial to a plot of precision vs. slit width gave a minimum imprecision a t a slit width of 17 wm. This slit width was chosen as the optimum width since it is about the widest possible slit which still remains narrower

ANALYTICAL CHEMISTRY, VOL. 48, NO. 1 1 , SEPTEMBER 1976

i START

S T A R T SCAN MOTOR

S C A N MOTOR

I-

RECORD BACKGROUND FOR N E X T N SECONDS GREATER THAN

f\ STORE

STORE

INTENSITY

OUTPUT

STORED DATA

Flgure 2. Flow chart for user control panel

than the narrowest lines observed on a normal photographic plate. The slit is made as wide as possible to minimize statistical noise in the background (6). No deleterious effects of scan rate upon precision are observed even a t rates of 25 mm min-l, the fastest rates provided with the original equipment of the microphotometer. If the microphotometer carriage is moved manually across the line in order to record the peak (Manual Scan Mode in Figure 2), the operator must assure that rate of movement is less than 25 mm min-l. Variation in background readings with the automated system is illustrated in Table 111.Four different regions of the photoplate with different fog levels were measured. The standard deviations are considerably greater than those observed for line minima and reflect the statistical variation in the background level. Line position is obtained with the recording of six digits with the least significant digit being 1 gm. The standard deviation for sequential measurement of line location with the automated system is less than 3 gm.A more meaningful illustration of position accuracy is a comparison of masses computed from automatically recorded data with known masses. Table IV shows results obtained for all lines of a mass spectrum of a silicon sample recorded automatically using routine procedures. Masses shown in column 1were computed using linear extrapolation between nearest mass-position references with the assumption that spectral line position is proportional to the square root of the mass. The known mass

Table 111. Precision of Background Readings Background transmittancea

0.914 0.860

0.760 0.568

SD

% RSD

0.011, 0.017, 0.016, 0.017,

1.2 2.0 2.9 3.1

a Average of 1 0 readings.

for the line is given in column 2 and the error between computed and known masses is given in column 3. Column 4 gives the identification of the ion species and column 5 gives the relative intensity of the line. The relative error in the computed mass is listed next followed by the number of the exposure(s) on the photoplate which was read for the given line. Exposure levels ranged between 2 X 10-l' C for exposure 3 to 3X C for exposure 11. The error in the mass includes errors in corrections applied by computer techniques for misalignment of exposures on the photographic plate or to nonvertical movement of the plate carriage as it is moved back and forth through exposures. The overall median relative error is 0.00012 or a median mass uncertainty of l/SOOO. This is approximately five times better than the observed mass resolution which is -1/1500. An indication of the error in the recording of intensity can be made by comparing intensities of isotopes which were re-

ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976

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Table IV. Precision of Masses Computed from Line Positionsa Computed mas7 a m u

6.501 6.745 6.994 7.240 7.490 7.746 8.001 8.993 9.322 9.652 9.984 10.189 10.326 10.489

Known mass, amu

=

=

156.911 157.932 159.924 167.849 168.839 169.832 170.841 179.948 180.961 181.965 182.959 183.972 185.970 232.038

--=

6.501 6.745 6.994 7.244 7.493 7.743 7.997 8.992 9.325 9.659 9.991 10.189 10.324 10.488

156.924 157.924 159.927 167.862 168.862 169.862 170.862 179.944 180.946 181.946 182.948 183.949 185.951 232.038

Rel. mass error X Abs. error, m m u

Identification

Line intensity, p p m a

io3

13C + 2

0.154 0.032 2 710. 257. 195. 0.139 8.33 69.7 44 500. 1960. 1360. 0.054 1.50 0.044

0

0

Reference

4 2sSi + 4 29Si+ 4 30Si+ 4 3lP + 4 1 6 0 + 2 2 7 ~ 1 +3 '*Si + 3 29Si+ 3 30Si+ 3 5IV + 5 3lP + 3 63Cu+ 6 2 7 ~ 1 +

0 -4 -3 +3 +4

+1 -3 -7 -5

Reference +2

+1

Gd + 1

-1 3 +8 -3 -13 -23 -30 -21 +4 +15 +19 +11 +23 +19

0.562 0.882 0.692 10.6 3.36 2.47 .a44 1.97

Gd + 1 Gd + 1 Si, + 1 Si, + 1 Si, + 1 Si, + 1

w+1 Ta + 1 w+1 w+1 w+l w+l

2.11 337. 271. 566. 432. 11.7

Th + 1

Reference

Exposure no.

11 11 4 5 5

-

0 0.6 0.4 0.4 0.5

10,11 8, 9 6, 7 3 3, 4 3, 4 11 10,11 11

0.1 1 0.3 0 .I 0.5

...

0.19 0.10

11 11 11 11 11 11

0.08 0.05 0.02 0.08 0.14 0.18 0.12 0.02 0.08 0.10 0.06 0.13 0.10

11

11 11

8 9 8, 9 8 ... 11 Median rel. mass error = 0.1 2

aTotal number of lines = 203. Table abbreviated t o conserve space.

Table V. Isotopic Abundance Data C o m p u t e d data (From Table VI) Natural abundance, %a

Mass

Intensityb

Normalized ibundance, %

91.7 5.82 1.93 7.28 5.51 5.34 15.8 24.2 69.2 30.8 26.4 14.4 30.6 28.4 26.4 14.4 30.6 28.4 14.7 20.5 15.7 24.9 21.9

11.185 10.787 45.950 46.954 48.942 49.947 17.484 18.485 62.933 64.929 91.003 91.502 92.000 92.999 36.384 36.590 36.790 37.190 154.933 155.929 156.911 157.932 159.924

0.908 0.065 52.4 45.1 33.6 33.1 0.399 0.127 2850. 1160. 136. 60.3 144. 141. 0.772 0.423 1.05 1.02 0.525 0.816 0.562 0.882 0.692

91.0 6.5 8.30 7.20 5.33 5.28 75.9 24.1 71.7 28.3 28.3 12.6 30.0 29.3 23.6 13.0 32.2 31.2 14.8 23.0 15.8 24.9 19.5

a Nuclear Data Tables. National Academy b ppma uncorrected for isotopic abundance,

-0.8

Relative error, %

+0.37 -0.08 -0.17 -0.16

0.9 14. 4.7 0.1 3.1 3.0

+0.1

0.1

-0.1 +2.5 -2.5 +1.9 -1.8 -0.6 +0.9 -2.8 -1.4 C1.6 C2.8 co.1 +2.5 +0.1 0 -2.4

0.4 3.6 8.1 7.2 12.5 2.0 3.2 10.8 9.8 5.2 10.1 0.7 12.5 0.6

+0.7

0

11.0 Av re1 error = 5.4 Median re1 error = 3.6

of Sciences, National Research Council, Washington 20025, D.C., April 1959. charge state, or relative elemental sensitivity.

corded for the same exposure. A comparison of selected data from Table IV is given in Table V where the ion species and generally accepted natural abundances are given a t the left. Computed mass and intensity for a given charge state are 1650

Error

listed in columns 3 and 4. The computed normalized abundance data are listed next followed by the error and percent relative error between computed and reported abundance data a t the far right of the table. The average relative error of 5.4%

ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976

and median relative error of 3.6% indicate that relative line intensities are being determined with accuracies very close to limitations imposed by emulsion variations and calibration techniques (1,7). The time required for data recording is of considerable interest. Table VI provides a brief comparison of the time required for the manual and the automated systems to read a typical photographic plate containing 300 lines. The first item listed, align photoplate and zero microphotometer, requires a few more minutes for the automated system in order to set the ancillary equipment. Line location is given the same time requirement although, with the automated system, the operator can consider subsequent line locations immediately after initiating a scan for a given line without having to read and record data in between lines. Scan time requirements are reduced considerably with the automated system. Of course, the biggest advantage is in the elimination of reading and recording time. The insertion of ancillary spectral information for the computer is different for the two systems. The automated system requires a code letter preceding mass-position reference data. The amount of information required with manual data recording will normally be much greater than the 100-s value given in the table because each line may have to be identified. The final item, resetting the linear motion encoder, requires approximately 3 min with the new system. Although the listed total time requirement will vary from plate to plate, the time advantage for the automated system of a factor of 3 to 4 is realized in practice.

CONCLUSIONS The system described here has been found to have distinct advantages over previously used manual techniques for recording microphotometer data. Precision of recording line transmittance minima is at least as good as carefully read strip chart recordings and will not include mental errors incurred in reading and logging large quantities of data. Background readings reflect averaged transmittance adjacent to spectral lines. Time requirements are decreased by a factor of 3 to 4. The recorded data contain mass information shown to be accurate to approximately five times the mass spectral resolution. These data are especially compatible with large computer techniques for interpretation of mass spectra. The cost of the system described here is moderate. Materials cost for the Ratio Circuit, Peak Detector, Peak DVM, Background DVM, Control Logic, and Control Panel circuits (see Figure 1)was

Table VI. Comparison of Time Requirements to Read 300 lines Manual Rate, s/line

Operation

Align photoplate and zero microphotometer Line location Scan Read and record data Special spectral information Reset position encoder

...

3 10 10

Total s

Automated Rate, s/line

240 900 3000

3000

. . . 100 ... ... Total 7440 s 126 min

...

Total s

2.5

360 900 7 50

...

100 180

3

... ...

2290 s - 3 8 min

$1000 with $1000 additional labor costs. All other equipment was commercially available or on hand. Finally, the total operator time required to read a photoplate is comparable to some systems where an integrated computer is used to control the microphotometer and to record data automatically. This is due to the fact that the time required for eliminating false lines, locating mass-position reference points, and providing the additionally required operator contribution to limited small computer interpretive schemes will approach the total time required for the system described here. Details about the system are available from the authors.

ACKNOWLEDGMENT The authors gratefully acknowledge the help of Clarence Ness who operated the mass spectrograph.

LITERATURE CITED R. A. Burdo, J. R. Roth, and G. H . Morrison, Anal. Chem., 46, 701 (1974). E. J. Millett, J. A. Morice, and J. B.Ciegg, lnt. J. Mass Spectrom. /on Phys., 13, l(1973). J. R. Woolston, Twenty-First Annual Conference on Mass Spectrometry and Allied Topics, San Francisco, Calif., 1973, p 514, Paper V 7. M. A. Frisch and W . Reuter, Anal. Chem., 45, 1889 (1973). C. A. Bailey et al., UCRL-70898 (1968). J. Franzen and K . D. Schuy, Fresenius'Z. Anal. Chem., 225, 295 (1967). K . D. Schuy and J . Franzen, Fresenius' 2.Anal. Chem., 225, 260 (1967).

RECEIVEDfor review January 23, 1976. Accepted May 13, 1976.

Tube Cracker for Opening Glass-Sealed Ampoules under Vacuum David J. DesMarais' and J. M. Hayes* Department of Chemistry, Indiana University, Bloomington, Ind. 4740 1

This report describes a device which can open glass-sealed vessels in a vacuum system or other confined atmosphere. The design permits the use of standard glass tubing for the sample vessel, thus permitting the vessel to be sturdy, inexpensive, and versatile. The present application involves the transfer of gas samples to a vacuum system in 10-cm segments of 6-mm Pyrex tubing which has been sealed at both ends. Figure 1is a schematic diagram depicting the construction of the sample tube cracker. The sealed end of a 6-mm quartz or Pyrex sample tube (H) is shown inserted into the cracker 1 Present address, Chemical Evolution Branch, Ames Research Center N239-9, National Aeronautics and Space Administration, Moffett Field, Calif. 94035.

and fastened by the Yd-in. Viton O-ring vacuum fitting of a Cajon Ultra Torr reducing union (G) (Cajon Company, 32550 Old South Miles Road, Cleveland, Ohio 44139). The commercially available fitting has been drilled out to permit a %-in. 0.d. tube to pass completely through the union. The 3/s-in. fitting of the Ultra Torr reducing union seals onto one end of a 2.5-in. long by %-in. diameter Cajon flexible stainless steel tube (C). The flexible tubing contains upper (D) and lower (F) cone-shaped stainless steel supports which cause the notched (E) sample tube to crack when the enclosed stainless steel tubing is flexed. The upper end of the flexible tubing is connected to the vacuum system (A) using a %-in. Cajon Ultra Torr union (B).

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