Multichannel Image Detectors - American Chemical Society

14 Electro-Optical Ion Detectors in Mass Spectrometry Simultaneous Monitoring of All Ions over W i d e Mass

Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 21, 1979 | doi: 10.1021/bk-1979-0102.ch014

Ranges HEINZ G. BOETTGER, C. E. GIFFIN, and D. D. NORRIS Earth and Space Sciences Division, California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91103 Efficient signal collection has been a major problem in the development of observational instrumentation systems since their beginnings. One of the earliest approaches to this problem involves the utilization of photographic techniques. Light sensi tivie emulsions (photographic plates) have been used for recording images at the focal point of a telescope since the middle of the 18th century. Similarly, they have been used for recording the spectra produced by various spectrographs for nearly 75 years. The ion-sensitive photo-plate has been the traditional method for recording mass spectra since Thomson (1) and, particularly, Aston (2) built their first mass spectrographs . Unfortunately, it is not free of a number of problems, as will be shown below. The efforts made to solve some of the shortcomings attending its general use have been discussed by Ahearn (3) and Honig (4). In p r i n c i p l e , the photographic p l a t e i s i d e a l l y s u i t e d f o r simultaneously d e t e c t i n g and i n t e g r a t i n g the s i g n a l from a l l i o n s p e c i e s over an extended mass range, l i m i t e d only by the i o n o p t i c s of the mass a n a l y z e r , with good r e s o l u t i o n ( t y p i c a l l y > 100 lines/mm). However, i t s s e n s i t i v i t y i s l i m i t e d , i t s use awkward, and the conversion of the image t o numerical data i s time consuming and c o s t l y . The l i m i t e d s e n s i t i v i t y ( i t takes from 103 t o 10^ ions t o produce a measurable l i n e ) and i t s a s s o c i a t e d l a c k of dynamic range ( t y p i c a l l y i n the order of 30:1) have s e v e r e l y l i m i t e d the use o f photographic p l a t e s i n r o u t i n e a p p l i c a t i o n s of mass spectrometry. The search f o r a l t e r n a t e i o n d e t e c t i o n systems has r e s u l t e d i n the wide-spread use o f e l e c t r i c a l d e t e c t i o n systems. Modern implementation of these devices d r a s t i c a l l y shortens the time r e q u i r e d t o c o l l e c t both q u a n t i t a t i v e and q u a l i t a t i v e a n a l y t i c a l r e s u l t s and g r e a t l y s i m p l i f y the a s s o c i a t e d data r e d u c t i o n problems. Conventional e l e c t r i c a l d e t e c t i o n devices are based upon sweeping the r e s o l v e d i o n beam 0-8412-0504-3/79/47-102-291$07.00/0 © 1979 American Chemical Society In Multichannel Image Detectors; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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across an image s l i t which i s l o c a t e d a t the f o c a l point of the mass r e s o l v i n g system. The d e t e c t o r , u s u a l l y some form of e l e c t r o n m u l t i p l i e r , i s placed behind the s l i t . In s p i t e of the inherent s e n s i t i v i t y of the e l e c t r o n m u l t i p l i e r , and the populari t y of e l e c t r i c a l d e t e c t i o n systems, there are a l s o a number o f s e r i o u s l i m i t a t i o n s C5,6). The most s i g n i f i c a n t o f these i s the l o s s o f s e n s i t i v i t y by 3 to 6 orders o f magnitude due to the l i m i t e d observation time f o r each i o n species ( t y p i c a l l y 1 i o n i n 10-* to 10^) i n a spectrum as w e l l as the sample as a whole ( u s u a l l y l e s s than 1/10 of the sample i n the i o n source i s u t i l i zed). Consequently, today's e l e c t r i c a l d e t e c t i o n methods are no more s e n s i t i v e f o r r e c o r d i n g complete mass s p e c t r a than the t r a d i t i o n a l photographic methods. Attempts to overcome the l o s s of s e n s i t i v i t y have r e s u l t e d i n a technique where only a l i m i t e d number of mass values ( g e n e r a l l y not more than 4 or 5) are monitored e i t h e r continuously o r c y c l i c l y . However, due to the l o s s of u s u a l l y more than 99% o f the information content of the spectrum, t h i s approach i s r e s t r i c t e d to the d e t e c t i o n and q u a n t i t a t i o n of h i g h l y p u r i f i e d known substances f o r the m a j o r i t y of p r a c t i c a l a n a l y t i c a l problems. Thus, one can conclude that p r i o r to the developments described i n t h i s paper, there was no comp l e t e l y s a t i s f a c t o r y d e t e c t i o n system f o r mass spectrometers which combined s e n s i t i v i t y of the e l e c t r o n m u l t i p l i e r with the simultaneous c o l l e c t i o n c h a r a c t e r i s t i c s o f the photographic p l a t e . Outside the f i e l d of mass spectroscopy, the search f o r a more e f f i c i e n t method f o r observation o f astronomic and s p e c t r o g r a p h ^ images had l e d t o the use of a v a r i e t y o f image convert e r s i n c l u d i n g t e l e v i s i o n systems. The a p p l i c a t i o n of these types of devices to the r e c o r d i n g of i o n images at the f o c a l plane of a mass spectrograph appeared to be a n a t u r a l extension of t h i s technology. The general f e a s i b i l i t y o f the approach had been demonstrated by the use o f "Ion Image Converters" f o r the v i s u a l i n s p e c t i o n of complete mass s p e c t r a by von Ardenne (7,8) and, f o r more e f f i c i e n t i o n c o l l e c t i o n s o f r e s o l v e d s i n g l e i o n beams, by Daly (9,10,11) and s e v e r a l other i n v e s t i g a t o r s . These devices are a l l based upon some form o f i o n e l e c t r o n converter ranging i n gain from as l i t t l e as 5 f o r a simple CuBe wire mesh o r p l a t e to as high as 10^ f o r a Chevron type Multi-channel E l e c t r o n M u l t i p l i e r Array (MCA). In the Daly type detectors the low gain i o n electron-photon conversion stage i s followed by a high gain p h o t o m u l t i p l i e r r e s u l t i n g i n t o t a l gain o f 10^ - 10** p e r m i t t i n g counting of s i n g l e i o n events. While the work of e a r l i e r i n v e s t i g a t o r s proved the o v e r a l l f e a s i b i l i t y of the approach, a number of major problems had to be s o l v e d before a detector monitoring a s u i t a b l e mass range simultaneously could be implemented. F i g u r e 1 shows the schematic r e p r e s e n t a t i o n of the two most f r e q u e n t l y encountered i o n o p t i c a l arrangements i n commerc i a l magnetically f o c u s i n g mass spectrometers. F o r the present, only the case o f the Mattauch-Herzog type Double Focusing Mass

In Multichannel Image Detectors; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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DOUBLE FOCUSING MASS SPECTROMETER

ION SOURCE

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Figure 1.

Schematic representation of the ion optical configuration of commercial magnetically focusing mass spectrometers



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Spectrometer w i l l be considered. In t h i s geometry the r e s o l v e d i o n beams are focused simultaneously i n a plane which can range from 5 cm upwards (e.g., 36 cm f o r the CEC-21-110 s e r i e s mass spectrometers). The commercial v e r s i o n of a Robinson (13) modi f i e d Mattauch-Herzog mass spectrograph p l a c e s the f o c a l plane approximately one-half gap width e x t e r n a l to the a n a l y z e r magnet pole p i e c e s . F i g u r e 2 shows a schematic r e p r e s e n t a t i o n of the E l e c t r o - O p t i c a l Ion Detector. An i o n e x i t i n g from the magnetic f i e l d penetrates i n t o one of the channels of the micro-channel e l e c t r o n m u l t i p l i e r array (MCA) and, upon c o l l i s i o n w i t h i t s w a l l , i n i t i a t e s a cascade of secondary e l e c t r o n s . The e l e c t r o n s emerging from the MCA are a c c e l e r a t e d and proximity focused onto an aluminized phosphor screen where each e l e c t r o n generates a number of photons. The photons are conducted v i a f i b e r o p t i c s to the t a r g e t of a s o l i d s t a t e imaging device (camera). The f i r s t of the aforementioned problems concerned the f a c t that the MCA must operate i n the s t r o n g t r a n s v e r s e magnetic f r i n g e f i e l d of the spectrometer. In the case of the 30.5 cm maximum radius Mattauch-Herzog-Robinson geometry mass s p e c t r o meter, the f r i n g e f i e l d at the f o c a l plane i s approximately oneh a l f of the homogenous f i e l d i n the analyzer magnetic gap. Under t y p i c a l o p e r a t i n g c o n d i t i o n s of 10-12 K-Gauss i n the gap, the f i e l d at the f o c a l plane i s 5-6 K-Gauss. A study of the e f f e c t of such a f i e l d on the o p e r a t i o n of the MCA has shown (14) that the the c o l l e c t e d output current i s v i r t u a l l y unattenuated at a l l angles w i t h respect to the magnetic f i e l d except i n the v i c i n i t y of an angle of 0 ° . At 0° the MCA s t i l l f u n c t i o n s p r o p e r l y , but the e l e c t r o n s emerging from the MCA are unable to reach the c o l l e c t o r p l a t e . Thus, i t was concluded that indeed the MCA e l e c t r o n m u l t i p l i e r s work w e l l i n a s t r o n g magnetic f i e l d and e f f i c i e n t secondary e l e c t r o n c o l l e c t i o n e f f i c i e n c i e s can be achieved by a n g l i n g the MCA c o l l e c t o r plane w i t h respect to the magnetic f i e l d v e c t o r at an angle> 10°. The next problem concerned the f a c t that h i g h q u a l i t y images were obtained only up to approximately 4 K-Gauss i n the a n a l y z e r magnet. (^2 K-Gauss f r i n g e f i e l d ) . However, severe d i s t o r t i o n and f o r e s h o r t e n i n g of the images took p l a c e at h i g h f i e l d s . This was presumed to be due to the curvature of the magnetic f r i n g e field. A s o f t i r o n magnetic shunt, as shown i n F i g u r e 3, was placed above the EOID, which was angled at approximately 20°, to f o r c e common f i e l d l i n e s to i n t e r s e c t both the a c t i v e area of the MCA and the phosphored c o l l e c t o r plane. The d i s t o r t i o n d i s appeared. Another problem to be s o l v e d , i n v o l v e d the development of an e l e c t r o n m u l t i p l i e r array of s u f f i c i e n t length to cover the e n t i r e f o c a l plane. Commercially a v a i l a b l e MCA's were nowhere near the r e q u i r e d dimensions of approximately 1 mm high χ 361


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mm long. E x i s t i n g manufacturing techniques as w e l l as cost f a c t o r s precluded the production of s i n g l e arrays o f these dimensions. Futhermore, conventional u n i t s , using round or p o l y gonal channels could not be used i n segment form to generate a long array without l o s s e s due to end-effects created by c u t t i n g through channels (these f r a c t i o n a l channels a l s o tend towards "auto-emission", i . e . , secondary e l e c t r o n emission i n the ab sence o f incoming i o n s ) . The problem was s o l v e d , e v e n t u a l l y , by means of a novel development (ITT E l e c t r o - O p t i c a l Products D i v i s i o n ) u t i l i z i n g square e l e c t r o n m u l t i p l i e r channels (16 urn on a s i d e placed on 20 urn center to c e n t e r ) . This c o n f i g u r a t i o n allows the a c t i v e area o f the MCA p l a t e to be continuous from end to end with no dead o r broken areas a t the ends, as shown i n F i g . 4. Hence, MCA s 26 mm long can be p l a c e d end to end to make up the r e q u i r e d l e n g t h . There are s e v e r a l d i s t i n c t advant ages to t h i s approach. (1) Each MCA i s inexpensive on a product i o n b a s i s ; (2) MCA's can be s e l e c t e d f o r gain u n i f o r m i t y ; (3) arrays of v a r i a b l e , t h e o r e t i c a l l y u n l i m i t e d , length can be b u i l t up i n 26 mm increments; (4) damaged MCA's can be replaced i n d i v i d u a l l y ; (5) the s m a l l MCA's are l e s s f r a g i l e than longer ones. 1

Thus, with the development o f the primary detector (conversion from ions e l e c t r o n s to photons) complete, the remaining problem i n v o l v e d the coupling of the detector with a s o l i d s t a t e imaging device. Three types of devices were i n v e s t i g a t e d : (1) V i d i c o n camera system; (2) Photodiode arrays (Reticon); (3) Charge Coupl ed Devices (CCD's). Based upon c o n s i d e r a t i o n s o f the s t a t e - o f t h e - a r t at the time, as w e l l as c o s t , the f i r s t generationEOID was implemented with a t e l e v i s i o n type camera ( V i d i c o n ) , as shown i n F i g . 5. This d e c i s i o n r e q u i r e d one a d d i t i o n a l problem to be solvedjnamely, matching the format of the primary image (1 mm χ 361 mm) to that o f the v i d i c o n (19 mm χ 19 mm). This task was accomplished by means o f a f i b e r - o p t i c image d i s e c t o r (14) which segments the h o r i z o n t a l input f i b e r - o p t i c face i n t o nineteen 1 mm χ 19 mm segments and s t a c k i n g these segments v e r t i c a l l y at the output f a c e , as shown i n F i g . 6. Each i n d i v i d u a l f i b e r i s 15 urn i n diameter, thus g i v i n g 30 l i n e pairs/mm r e s o l u t i o n . The d i s s e c t o r has a numerical aperture of *Ό.55 and a transmission of **60%. The image of the mass spectrum produced a t the output end of the d i s s e c t o r was focused onto the t a r g e t of the V i d i c o n Camera v i a r e l a y o p t i c s , as i l l u s t r a t e d i n F i g . 5. The camera used f o r the conversion from o p t i c a l data to e l e c t r o n i c d i s p l a y o f i n f o r mation was a modified v e r s i o n o f the camera system used on the V i k i n g s p a c e c r a f t . I t was capable o f i n t e g r a t i n g the l i g h t out put from the primary d e t e c t o r f o r up to two seconds. F i g . 7 d e p i c t s the image format at the v i d i c o n .


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ANALYZER Figure 5.

Integrating electro-optical ion detector mass spectrometer with TV read-out


In Multichannel Image Detectors; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. MULTIPLIER

POLE P I E C E

First generation detector with folded fiber optics image converter

FORMATTED FOR VIDICON CAMERA READOUT

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UPPER MAGNET

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Figure 7.

Converted spectral image as seen by the vidicon camera system



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The r e s u l t s obtained w i t h t h i s f i r s t generation f o c a l plane M.S.-EOID system as w e l l as s t u d i e s by Beynon and others at Purdue U n i v e r s i t y (15) demonstrated the t e c h n i c a l f e a s i b i l i t y of such a system. Furthermore, these s t u d i e s l e d the way to s o l u t i o n s f o r the v a r i e t y of fundamental problems, which were encountered during the development and helped p o i n t out the d i r e c t i o n s towards f u t u r e changes necessary on the road towards a commercially p r a c t i c a l design f o r use of the concept i n rou t i n e a p p l i c a t i o n s of mass-spectrometry. I t became obvious that the v i d i c o n based camera system was not the b e s t approach. Some of the reasons f o r t h i s are: (1) Loss of s e n s i t i v i t y due to l i g h t l o s s e s i n the d i s s e c t o r and the t r a n s f e r o p t i c s ; (2) cost of the image d i s s e c t o r ; (3) lower dynamic range and s e n s i t i v i t y , slower read-out r a t e , e t c . , of the v i d i c o n compared to a l t e r n a t e devices. T h e o r e t i c a l l y , CCD's o f f e r e d the most a t t r a c t i v e f e a t u r e s of the remaining c h o i c e s , photodiodes and CCD's. However, at the time when the d e c i s i o n had to be made, CCD technology was, and s t i l l i s , too much i n f l u x , f o r t h e i r use i n a mass s p e c t r o meter system and too high i n cost to be a reasonable choice. The cost f a c t o r would be a m p l i f i e d even f u r t h e r when one considers the i n c r e a s e d requirements on the data a c q u i s i t i o n system due to the 6 0 - f o l d i n c r e a s e i n data r a t e ( 860,00 vs 14,300*) data p o i n t s per spectrum. These and other c o n s i d e r a t i o n s l e d to the d e c i s i o n to implement the second generation d e t e c t o r with a photodiode (Reticon) based camera. This system i s now i n oper a t i o n producing e x c e l l e n t data and w i l l be d e s c r i b e d i n d e t a i l i n the f o l l o w i n g s e c t i o n . In p a r a l l e l with the development of the EOID f o r f o c a l plane mass spectrometers of the Mattauch-Herzog type, s i m i l a r devices were developed f o r use with conventional s e c t o r - t y p e mass s p e c t r o meters (15, 16, 17, 18). A schematic r e p r e s e n t a t i v e of t h i s type d e t e c t o r , versus that implemented on a CEC type 21-490 s i n g l e focusing mass spectrometer, i s shown i n F i g . 8. The main d i f f e r e n c e s between these two a p p l i c a t i o n s of the EOID are a r e s u l t of the d i f f e r e n c e s i n the i o n o p t i c s of the two types of mass a n a l y z e r s , as shown i n F i g . 1. F i r s t , the d e t e c t o r of a s e c t o r type instrument r e s i d e s o u t s i d e the magnetic f r i n g e f i e l d , thus e l i m i n a t i n g the need f o r a n g l i n g the primary i o n sensors. T h i s s i m p l i f i e s the o v e r a l l d e t e c t o r design g r e a t l y . Second, the number of r e s o l v e d i o n beams that i s i n simultaneous focus i s g r e a t l y reduced, t h e r e f o r e , only a l i m i t e d mass range can be d i s p l a y e d simultaneously, t y p i c a l l y 10-20% on e i t h e r s i d e of the c e n t r a l mass which i s i n optimum focus. However, t h i s range of masses as w e l l as the l i n e shape can be improved through the use of a u x i l i a r y lenses (17, 18). Due to the s i m p l i c i t y of the d e t e c t o r design, such systems have been i n r o u t i n e use f o r sever a l years at JPL and at the FOM I n s t i t u t e i n Amsterdam (19). *14" long f o c a l plane χ 1024 photodiodes per i n c h .


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Figure 8.

Schematic presentation of an electro-optical ion detector for spector type mass spectrometers


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DESCRIPTION OF THE FOCAL PLANE M.S.

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The mechanical concept of the f o c a l plane MS/EOID can be seen i n F i g . 9. The MCA p l a t e s are placed at a 20° angle with r e s pect to the magnet pole p i e c e s . Secondary e l e c t r o n s produced by the MCA are a c c e l e r a t e d to the phosphor coated f i b e r o p t i c s v i a the magnetic f r i n g e f i e l d l i n e s as noted above. The phosphor coated f i b e r o p t i c s i s , i n a c t u a l i t y vacuum window whereby the image of the i o n beam made v i s i b l e at the phosphor i s transmitted with high e f f i c i e n c y outside the mass spectrometer vacuum. Since the primary i o n beams do not s t r i k e the angled MCA's at normal i n c i d e n c e , the s p e c t r a l l i n e s form a s e t of non-orthoganal (but p a r a l l e l ) images as shown i n F i g u r e 10. The " t i l t " angle of the spectrum i s not 20°, however, due to the e f f e c t s from the magnetic f r i n g e f i e l d g r a d i e n t . To q u a n t i f y the i n s e n s i t y and p o s i t i o n of the i o n images (as a measure of t h e i r abundance and mass, r e s p e c t i v e l y ) photodiode arrays (PDA's) were s e l e c t e d as the imaging elements. Table I gives the performance s p e c i f i c a t i o n s of these devices. An orthoganal format f o r the array of 25 cm wide 2 mm high s e n s i t i v e elements was s e l e c t e d so as to develop a g e n e r a l l y u s e f u l spectro s c o p i c sensor r a t h e r than one s u i t e d only f o r t h i s p a r t i c u l a r MS/EOID a p p l i c a t i o n . T h i s , and the c o n s i d e r a t i o n that each PDA was p h y s i c a l l y 1.6" long with an a c t i v e area of only 1.0" long, meant that an a d d i t i o n a l f i b e r o p t i c device was needed to couple the images presented at the vacuum window to the PDA's and i n the process r o t a t e them such that they are a l i g n e d with PDA p i c t u r e elements ( p i x e l s ) . The f i b e r o p t i c couplers are p i c t o r i a l l y explained i n F i g u r e 11. These devices are constructed from forty-two 320 χ 320 square m u l t i - f i b e r elements (each f i b e r being 6 urn on a s i d e ) and one coupler i s used to transmit the images on each 1" long p o r t i o n of the f o c a l plane to a PDA. The couplers are butted s i d e by s i d e a l t e r n a t e l y above and below h o r i z o n t a l so as to avoid p h y s i c a l i n t e r f e r e n c e between PDA's. This can be seen i n F i g u r e 9. In the case of the CEC, 21-110B MS, there are seven detectors above h o r i z o n t a l and seven below. F i g u r e 12 i s a photograph of a PDA with the f i b e r o p t i c coupler attached. A l s o seen i n the photograph i s the p r e a m p l i f i e r board d i r e c t l y attached to the PDA. T h i s i s done f o r s i g n a l - t o - n o i s e c o n s i d e r ations . F i g u r e 13 i s an o v e r a l l b l o c k diagram of the MS/EOID system being developed under a grant from the N a t i o n a l I n s t i t u t e of General Medical Sciences, N a t i o n a l I n s t i t u t e s of Health. It i s reasonably s e l f explanatory. The s h u t t e r c o n t r o l operates from a sampling of the i n t e g r a t e d t o t a l i o n current and acts to prevent s a t u r a t i o n of the EOID. The mass s h i f t c o n t r o l allows



Figure 9.

Schematic representation of double focusing of focal plane mass spectrometer/electrooptical ion detector system


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Spectral image as seen by the photo diode array camera

APERTURE WIDTH

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Table 1. Photodiode Array Specifications


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IMAGES OF SPECTROGRAPH PRESENTED TO OPTIC FIBERS IN THIS ORIENTATION

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Schematic representation of fiber optic image rotator


BOETTGER

E T A L .



14.

Figure 12.

Fiber optic image rotator and photo diode array assembly


307


D/A

MASS SHIFT CONTROL

Γ

o U

r K

MULTIPLEXER

SPARE

SPARE

PFK O V E N TEMP

SOURCE TEMP

M A G FIELO

TOTAL I O N CURRENT

RETICON TEMP

HOUSEKEEPING SENSORS SAMPLE TEMP

HIGH VACUUM! WINDOW j

FIBER OPTICS t I

a A

t I

o U

r W

Figure 13.

system

14

c = c >

DRIVERS

. 1

RETICON

CLOCK A N D CONTROL LOGIC

DRIVERS

RETICON

Block diagram of the NIH EOID/MS

ADC

TEMPERATURE CONTROL

OPTO-ISOLATED COMPUTER INTERFACE

HV

HV

SHUTTER CONTROL

PHOSPHOR

MCA

ELECTRO-OPTICAL I O N IMAGE GENERATION SUBSYSTEM

MASS ANALYZER SUBSYSTEM

PRE-AMPS


T.I 990/10 MINI COMPUTER SUBSYSTEM

ANALOG/DIGITAL CONVERTER (12 BITS)

SAMPLE A N D HOLD

MULTIPLEXER

Ο M

>

1-4

r

25 W

>

ο Χ

S

c

00

14.

BOETTGER E T A L .


309

a known spectrum (e.g., perfluorokerosene) to be " s l i d " along the f o c a l plane by scanning the magnetic f i e l d i n order to c a l i b r a t e each one of the 14,336 d e t e c t o r p o s i t i o n s on the f o c a l plane.


An o p t o - i s o l a t e d i n t e r f a c e i s used to couple the MS/EOID to a mini-computer where a new complete spectrum can be taken as o f t e n as every 50 m i l l i s e c o n d s . With r e a l time d e c a l i b r a t i o n " o n - t h e - f l y " complete s p e c t r a can be processed every 4.5 seconds. Since the EOID i s t r u l y an i n t e g r a t i n g d e t e c t o r , the c o l l e c t e d s p e c t r a are i n s e n s i t i v e to sample t r a n s i e n t s as might be ex perienced, f o r example, during GCMS analyses using c a p i l l a r y columns. This i s because a l l p o s i t i o n s along the f o c a l plane are a c t i v e simultaneously. The c y c l e time between s p e c t r a i s purely a f u n c t i o n of diode readout r a t e and the degree of r e a l time d e c a l i b r a t i o n d e s i r e d . The l e s s e r the amount of c a l i b r a t i o n , the greater the amount of data that must be s t o r e d with the advantage of more frequent s p e c t r a - t a k i n g but with the disadvant age that l e s s s p e c t r a can be s t o r e d f o r post-run a n a l y s i s . On the other hand, s o p h i s t i c a t e d d e c a l i b r a t i o n on the f l y r e s u l t s i n fewer data b i t s per spectrum being s t o r e d , but the c y c l e time i s lengthened. Obviously, the p a r t i c u l a r a p p l i c a t i o n w i l l d i c t a t e the data handling format. Figures 14-18 are examples of the computer p l o t s of s e l e c t e d data. These s p e c t r a have had dark current and f i x e d p a t t e r n n o i s e subtracted " o n - t h e - f l y " . (Both of these items are inherent i n PDA s, the c o r r e c t i o n f o r which represents t r i v i a l implement a t i o n i n terms of software and hardware). In some of the p r i n t outs, the i n d i v i d u a l diode l e v e l s can be seen r e p r e s e n t i n g 25 um s p a t i a l r e s o l u t i o n across the f o c a l plane. The o r d i n a t e i s q u a n t i f i e d to only 12 b i t s while the PDA's themselves have a dynamic range of 14 b i t s . This was done f o r expediencey only. f

DESCRIPTION OF THE

EOID FOR

SECTOR TYPE MASS SPECTROMETERS

Figure 8 gives an o v e r a l l view of the EOID. The o v e r a l l design i s s e l f - e x p l a n a t o r y . B r i e f l y , the primary d e t e c t o r , c o n s i s t i n g of the channel e l e c t r o n m u l t i p l i e r array (14 mm diam. 17 mm center-to-center channels i n a 1" diam. array) and the aluminized phosphor (P-35) coating are mounted upon a 1" diameter χ 2" long f i b e r o p t i c vacuum window. The c o a t i n g i s a p p l i e d d i r e c t l y to the face of the window. A s e t of mounting r i n g - e l e c t r o d e s and ceramic spacers are provided to p o s i t i o n the CEMA r e l a t i v e to the phosphor and to f u r n i s h the necessary p o t e n t i a l s . The window i s adjustable l o n g i t u d i n a l l y v i a a micrometer screw mechanism i n order to l o c a t e experimentally the optimum plane of focus. The photodiode array (1024 diodes χ 24 micro m wide χ 2.5 mm high) i s provided with a f i b e r o p t i c



Figure 15.

Computer plot of a partial spectrum of Tetrabromothiophene Perfluorokerosene from mass 271 to 420


plus


14.

BOETTGER E T A L .


Figure 16. Expanded computer plot of the mass range from 316 to 326, showing partial resolution of the —Br/—F doublets at m/e 317 and 319. The steps on the peaks represent the signal level in individual diodes.

Figure 17. Expanded computer plot of the molecular ion region of Tetrabromothiophene from mass 388 to 407. Peaks at 391, 393, and 405 are caused by PFK. The peak at m/e 400 is slightly saturated.


311

312


window which i s i n d i r e c t contact with the 2.5 mm diameter vacuum face p l a t e . The p r e a m p l i f i e r and the d r i v e r e l e c t r o n i c s f o r the PDA are mounted at the d e t e c t o r t o keep extraneous n o i s e at a minimum. The e n t i r e camera assembly, o u t s i d e the vacuum, i s cooled to -30 t o -50° to reduce the dark current and thus i n c r e a s i n g the dynamic range of the system, s i n c e the maximum usable s i g n a l i s l i m i t e d by the s a t u r a t i o n current of the de vice.


RESULTS I n i t i a l tests, as shown i n Figures 14-19, demonstrate c l e a r l y that the performance goals have been achieved. These goals are shown i n Table 2. While F i g . 19, a composite of photographic r e c o r d i n g and a photometric scan o f the l i n e spectrum at the f i b e r o p t i c i n t e r f a c e , demonstrates the r e s u l t of these t e s t s . As implemented p r e s e n t l y on the CEC 21-110B, the r e s o l v i n g power of the EOID-MS system i s g r e a t e r than 1000 and good s p e c t r a are recorded at t o t a l ions c u r r e n t s o f l e s s than ΙΟ-*--* amp using i n t e g r a t i o n times o f 100 m i l l i s e c o n d s . These r e s u l t s are i l l u s t r a t e d i n F i g u r e s 14 t o 18. F i g u r e 14 shows a photograph of the v i s u a l image a t the f i b e r - o p t i c window, of a p a r t i a l PFK spec trum c o v e r i n g the mass range from approximately m/e 500 t o 800. Figures 15 and 16 represents the high mass p o r t i o n of the spec trum o f tetrabromo-thiophene (TBT) w i t h and without PFK present as a mass marker, showing c l e a r s e p a r a t i o n o f the TBT/PFK doublet at m/e 317, 319, 320 and 322. T h i s f a c t i s demonstrated more c l e a r l y i n F i g u r e 17 which shows an expanded d i s p l a y o f the mass range from m/e 317 t o 325. F i g u r e 18 represents the r e g i o n of TBT from m/e 393 t o 405, demonstrating the proper i s o t o p i c d i s t r i b u t i o n of a four-bromine i o n . Peaks at 393 and 405 are due t o PFK. F i n a l l y , F i g u r e 19 shows a p a r t i a l spectrum o f c h o l e s t e r o l plus PFK from m/e 271 t o approximately 400. The spectrum represents the i n t e g r a t e d i o n currents from l e s s than 100 femto-gram of c h o l e s t e r o l deposited on the d i r e c t probe o f the mass spectrometer. APPLICATIONS The eventual a p p l i c a t i o n towards which t h i s work i s p r o g r e s s i n g i s b i o m e d i c a l mass spectrometry i n the form o f s o p h i s t i c a t e d research instruments and, u l t i m a t e l y , a f u l l y automated " C l i n i c a l Mass Spectrometer" (22). T h i s instrument w i l l be capable of c a r r y i n g out s o p h i s t i c a t e d analyses o f p h y s i o l o g i c a l f l u i d s and t i s s u e i n as r o u t i n e a f a s h i o n i n the c l i n i c a l l a b o r a t o r y as c o n v e n t i o n a l automated wet chemical procedure are employed today. In a d d i t i o n , many other a p p l i c a t i o n s of mass s p e c t r o metry are expected to b e n e f i t from the development of the EOID, e.g., spark source mass spectrometry, o f mass spectrometers i n s p a c e c r a f t and on the s u r f a c e of the p l a n e t s , e t c .



14.

BOETTGER

E T A L .

313


Figure 18. Computer plot of a partial spectrum of a mixture of PFK and approxi mately 100 femtogram of cholesterol from mass 271 to mass 410 using an integra tion time of 100 milliseconds. Parent peak at m/e 386 is saturated.

PHOTO-PLATE CAPABILITY

PARAMETER

THRESHOLD OF ION DETECTION DYNAMIC RANGE TIME REQUIRED TO OBTAIN SPEC TRAL PLOTS SPATIAL RESOLUTION AT FOCAL PLANE Table 2.

ELECTRO-OPTICAL SENSING CAPABILITY VIDICON IMAGING

SOLID STATE IMAGING

10 το 100

1

^ 30:1

>1000:1

10

> 1 HOUR

< 1 MINUTE

3

Ι Ο το 1 0

5

ι

CO

14.

BOETTGER

E T A L .


315


At present, EOID's have been implemented on s e v e r a l experimenta l mass spectrometers beyond the ones d e s c r i b e d above: (1) a m i n i a t u r i z e d (4.5" f o c a l plane) Mattauch-Herzog type mass spectrometer w i t h a mass range o f 25-500 and r e s o l u t i o n 1/500, which has been b u i l t (Perkin-Elmer Corp., Pomona, C a l i f o r n i a ) f o r JPL as a prototype f o r f u t u r e space and/or c l i n i c a l a p p l i c a t i o n s ; and (2) an u l t r a - m i n i a t u r e (2" f o c a l plane) MattauchHerzog type mass spectrometer f o r outer-space use. A d d i t i o n a l uses o f the EOID are p r e s e n t l y i n the planning and/or design stages. A n a l y t i c a l research a p p l i c a t i o n s o f mass spectrometers f i t t e d with e l e c t r o - o p t i c a l i o n d e t e c t o r s are c a r r i e d out at the present time at JPL and the FOM I n s t i t u t e . These i n v o l v e " (1) at JPL a 1" EOID on s e c t o r type mass spectrometer (CEC type 21-490) i n c o n j u n c t i o n w i t h an API (atmospheric pressure i o n i z a t i o n ) source f o r the development o f an automated assay procedure f o r amino a c i d d e r i v a t i v e s produced by a n o v e l Edman type sequenator (23), and (2) at the FOM I n s t i t u t e a 1" EOID on a s e c t o r mass s p e c t r o m e t e r , f i t t e d with magnetic and e l e c t r o s t a t i c quadruple "Zoom" l e n s e s , p e r m i t t i n g v a r i a t i o n of d i s p e r s i o n f o r focusing a g r e a t e r number o f masses on the 1" d e t e c t o r (24,25). T h i s instrument has been used e x t e n s i v e l y over the past s e v e r a l years f o r s t u d i e s i n the f i e l d o f p y r o l y s i s mass spectrometry and l a s e r d e s o r p t i o n . Proposed nearterm a p p l i c a t i o n s f o r new Mattauch-Herzog type (14" and 4.5" f o c a l plane) EOID mass spectrometers i n c l u d e t h e i r use i n f i e l d d e s o r p t i o n mass spectrometry (FDMS), l a s e r desorption mass spectrometry (LDMS), h i g h performance l i q u i d chromatography mass spectrometry (HPLCMS) and p y r o l y s i s mass s p e c t r o metry (PyMS). A l l of these a p p l i c a t i o n s have one f e a t u r e i n common, the measurement of r a p i d l y changing sample p r o f i l e s a t , u s u a l l y , low c o n c e n t r a t i o n l e v e l s . F u r t h e r complications a r i s e i n some cases, e.g., PyMS and GC o r LCMS, due t o the f a c t that the sample i n the mass s p e c t r o m e t r y s i o n source represents not a s i n g l e component but a more o r l e s s complex mixture. The most d i f f i c u l t example o f t h i s type i s found i n p y r o l y s i s mass spectrometry when a complex sample, e.g., micro-organism, bio-polymer, t i s s u e , e t c . , i s broken down i n t o a multitude of s m a l l e r compounds. The r e a c t i o n s l e a d i n g t o these p y r o l y s i s products, w h i l e extremely f a s t , proceed at d i f f e r e n t r a t e s . As a r e s u l t , the o v e r a l l spectrum of the p y r o l y s a t e changes r a p i d l y w i t h time. Thus, i n order t o get meaningful data, which can be c o r r e l a t e d with the sample, i n t e g r a t i o n o f the i o n i n s e n s i t i e s o f i n t e r e s t over the e n t i r e sample i s r e q u i r e d . F o r reasons o u t l i n e d prev i o u s l y , c o n v e n t i o n a l i n t e g r a t i o n methods, such as photographic r e c o r d i n g or e l e c t r o n i c i n t e g r a t i o n of m u l t i p l e f a s t scans are e i t h e r not s e n s i t i v e enough o r too cumbersome o r both. The same 1



316


p r i n c i p l e s apply t o the mass spectrometry of unresolved peaks i n chromatographic a p p l i c a t i o n s . However, the problem i s u s u a l l y l e s s complicated due t o the f a c t t h a t , t y p i c a l l y , unresolved chromatographic peaks seldom c o n t a i n more than two or three components. A d i f f e r e n t , but no l e s s severe problem, i s encountered w i t h the a n a l y s i s o f unknown b i o l o g i c a l substances by f i e l d desorpti o n , l a s e r d e s o r p t i o n , and/or l a s e r a s s i s t e d f i e l d d e s o r p t i o n mass spectrometry. A p r i o r i , i t i s not known at what point i n time a component of i n t e r e s t i s desorbed. T h i s makes i t d i f f i c u l t , i f not impossible t o time a scan or s e v e r a l scans i n such a way that a r e p r e s e n t a t i v e spectrum i s obtained. Again the answer t o the problem i s found by i n t e g r a t i n g the i o n output over the e n t i r e sample o r repeatedly over p o r t i o n s of the sample p r o f i l e as i t emerges from the i o n source. The f o r e g o i n g a p p l i c a t i o n s represents but a s m a l l sampling of a l l the p o t e n t i a l uses of an EOID-MS system. I t can be expected that many others w i l l m a t e r i a l i z e as instruments of t h i s type become more widely a v a i l a b l e . CONCLUSIONS The h i s t o r y , development and implementation of h i g h s e n s i t i v i t y e l e c t r o - o p t i c a l i o n d e t e c t o r f o r simultaneous d e t e c t i o n o f a l l ions a r r i v i n g at the f o c a l plane o f Mattauch-Herzog-Robinson geometry as w e l l as the adaptation o f the same p r i n c i p l e t o the q u a s i f o c a l plane s e c t o r type mass spectrometers has been d i s cussed. Experiments were performed which proved the f e a s i b i l i t y of the d e t e c t o r concept. The c r i t i c a l aspect of the d e t e c t o r design f o r mass spectrometer geometries, r e q u i r i n g l o c a t i o n of the d e t e c t o r i n s i d e the f r i n g i n g magnetic f i e l d , i n v o l v e the r o t a t i o n of the primary d e t e c t o r (MCA and phosphor) at an angle greater than 10° w i t h respect to the magnetic f r i n g e f i e l d v e c t o r and shaping of the f r i n g e f i e l d i n such a way as t o p r o p e r l y c o l l i m a t e the e q u i v a l e n t e l e c t r o n beams between the MCA and the phosphor. E l e c t r o - o p t i c a l i o n d e t e c t o r s have been b u i l t and operated f o r s e v e r a l f o c a l plane mass spectrographs and s e c t o r type mass spectrometers. One o f these, a CEC 21-110B, has been implemented with dedicated computer systems f o r data a c q u i s i t i o n and a n a l y s i s . Results t o date have shown that a l l design parameters have been a t t a i n e d and t h a t , indeed, the EOID-MS system provides a powerful t o o l f o r the s o l u t i o n of a wide v a r i e t y of a n a l y t i c a l problems. ACKNOWLEDGMENTS The authors wish t o thank P r o f . W. J . Dreyer and h i s s t a f f at C a l t e c h f o r h i s support o f p a r t of t h i s work. We a l s o wish t o


14.

BOETTGER

E T A L .


317


acknowledge D. Helprey o f C a l t e c h and Dr. J . Yinon o f the Weizmann I n s t i t u t e o f Science ( I s r a e l ) who m a t e r i a l l y c o n t r i b u t e d to the design, c o n s t r u c t i o n and i n i t i a l t e s t i n g o f the s e c t o r type EOID-MS, V. T a y l o r and H. Mohan f o r t h e i r c o n t r i b u t i o n t o the mechanical design and f a b r i c a t i o n o f the detector assemblies, and R. Johansen and h i s s t a f f f o r the design implementation of the data system. This work was funded i n part by the N a t i o n a l I n s t i t u t e o f Health (NIGMS Grant #GM-20850 and D i v i s i o n o f Research Re sources Grant #RR-00922), by the N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n under NASA Contract 7-100, and the Caltech P r e s i dents Fund.

BIBLIOGRAPHY 1. J. J. Thomson, Phil. Mag. 21 (1911) 225 2. T. W. Aston, Phil. Mag. 37 (1919) 707 3.

A. J. Ahearn, Mass Spectrometric Analysis of Solids, Elsevier, Amsterdam, 1966, Ch.1.

4. R. E. Honig, Rev. Sci. Instrum., 40 (1969) 1364 5.

C. R. McKinney and Heinz Boettger, 13th Annual Conference of Mass Spectrom and Allied Topics, 1965, p. 345

6. Κ. H. Maurer and K. Habfast, 14th Annual Conference on Mass Spectrometry and Allied Topics, 1966, p. 271 7. M. Von Ardenne, Elektronenanlagerungsmassenspectrographie Organischer Substanzen, Springer, Berlin, 1971, pp. 54-56 8. M. Von Ardenne, Tab. Zur. Angew. Physik, Band 1, Springer, Berlin, 1962 9.

M. R. Daly, Rev. Sci. Instrum., 31 (1960) 720

10.

M. R. Daly, Rev. Sci. Instrum., 31 (1960) 264

11.

M. R. Daly, Rev. Sci., Instrum., 34 (1963) 1116

12.

M. R. Daly, R. E. Powell and R. G. Ridley, Nucl. Instrum. Methods, 36 (1965) 226

13.

C. F. Robinson, Rev. Sci. Instrum., 28 (1957)

14.

C. E. Giffin, H. G. Boettger and D. D. Norris, Intern, J. of Mass Spectr. and Ion Physics, 15 (1974) 437



318

MULTICHANNEL

IMAGE

DETECTORS

15.

J. H. Beynon, D. O. Jones, and R. G. Cooks, Anal. Chem., 47 (1975) 1734

16.

D. D. Norris and C. E. Giffin, Proceedings of SPIE, 77 (1976) 103

17.

H. G. Boettger, et a l . , Advances in Mass Spectrometry in Biochemistry and Medicine, Vol. II, (1976) 513

18.

J. Yinon and H. G. Boettger, 25th Ann. Conf. on Mass Spectrometry and Allied Topics, Washington, D. C. (1977)

19.

H. H. Tuithoff and A. J . H. Boerboom, Int. J. Mass Spectrom Ion Phys. 15 (1974) 105

20.

J. H. Tuithoff, A. J. H. Boerboom and H. C. L. Meuzelaar, Inst. J. Mass Spectrom. Ion Phys., 17 (1975) 229

21

H. L. C. Meuzelaar, et al and P. J. Kistemaker, et a l . , 26th Ann. Conf. on Mass Spectrometry and Allied Topics, St. Louis, Missouri (1978)

22.

H. G. Boettger, et a l . , U. S. Patent No. 4,084,090 (11 April 1978)

23.

J. Yinon and H. G. Boettger, 25th Ann. Conf. on Mass Spectrometry and Allied Topics, Washington, D. C. (1977)

24.

H. H. Tuithoff and A. J . H. Boerboom, Int. J . Mass Spectrom. Ion Phys., 18 (1976) 111-115

25.

H. H. Tuithoff and A. J. H. Boerboom, Int. J . Mass Spectrom. Ion Phys. 15 (1974) 105

RECEIVED

January 8, 1979.


Multichannel Image Detectors - American Chemical Society

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