A facility for chemical reaction research & mass spectrometric analysis

A facility for chemical reaction research & mass spectrometric analysis at cryogenic temperatures S

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J. K. Holzhauer received his Diploma of Engineering f r o m t h e University of Stuttgart, Germany, in 1966, a n d his M.S. in Chemical Engineering f r o m t h e Georgia Institute of Technology in 1968. He is currently w o r k i n g on his doctorate in t h e School of Chemical Engineering at Georgia Tech

Henry A. McGee, Jr., is Professor of Chemical Engineering at t h e Georgia Instit u t e of Technology, and he was before t h a t associated with NASA. His educational background is in chemical engineering a n d theoretical chemistry at Georgia Tech and t h e University of Wisconsin, respectively, and his scientific interests continue t o encompass selected areas of both pure and applied chemistry. His current research interests center upon t h e use of cryogenic techniques in t h e lucidation of physical, chemical, a n d biological phenomena. Dr. McGee and his students and associates have been particularly interested over the past several years in all phases of chemical reaction research a t cryogenic temperatures. He is presently Visiting Professor of Chemical Engineering a t Caltech. 24 A

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ANALYTICAL CHEMISTRY

IR JAMES DEWAR was evidently

the first investigator t o be concerned with t h e effect upon chemistry of t h e then newly available very low temperatures (1). Dewar (who was also the world's first cryobiologist) reported in 1885 t h a t P , N a , K, H 2 S , a n d H I seemed not to react with liquid oxygen. B u t it was nearly 80 years before techniques of instrumental analysis were sufficiently developed t o permit quantitative and broad-ranging studies of cryochemical reaction systems {2—4) • We should emphasize t h e chemical reaction orientation of the present discussion for, particularly since I960, a wide variety of physicochemical characteristics of molecules and particularly free radicals have been observed from cryogenic adaptations of E S R and I R . Such studies have been highly informative. B u t the essential heart of any synthetic or reactivity investigation is, of course, analysis, a n d here E S R techniques are limited to free radicals or nonsinglet species, while both E S R and I R demand a highly specialized and sophisticated analyst. B y contrast, mass spectrometry is applicable t o all species, it requires only routine techniques and simple interpretations, and it is immediately and simply adaptable to any reactor or process configuration. Typically, in a cryogenic reaction sequence, a reactive species produced b y some high energy process is contacted with a second reactant within some sort of hard quench configuration. A variety of m e chanical arrangements have been used — quenched diffusion flames, submerged arcs, quenched gaseous

REPORT FOR ANALYTICAL CHEMISTS J. K. Holzhauer and H. A. McGee, Jr., School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 >

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discharges, etc. T h e objective m a y bo stabilization of a species n o r m a l ly an intermediate—e.g., the p r e p a ration of ethylene oxide or oxirene in t h e reaction of atomic oxygen with ethylene or acetylene, respec tively. Also, t h e stabilization of the high t e m p e r a t u r e species itself m a y be desirable, as in t h e p r e p a r a tion of borane, B H 3 , from the prod ucts of t h e pyrolysis of borane carbonyl. Characteristically one searches for new or unusual mole cules which perhaps exist as purifiable reagents only below some very low critical t e m p e r a t u r e , TC) such t h a t kTc is small relative t o the activation energy of the mini m u m energy-loss process. This de scribes t h e stable existence of any species, of course, and the essential characterization of cryogenic r e agents is the exceedingly low value of Tc. These typically very low critical temperatures demand complete t h e r m a l control, since inadvertent warming can result in the loss of the compound of interest. T h e t h e r m a l requirement is always a nuisance from the view of experimental de sign, b u t it is especially trouble some in analysis, the essential r e quirement of all chemical reaction investigations. Sample w a r m - u p followed b y analysis b y conven tional techniques is unsatisfactory a t best and meaningless a t worst, for here the cryogenic species have been lost and their presence and be havior m a y be only indirectly and usually inaccurately inferred. Cryochcmical techniques form a new dimension of preparative a n d interpretive chemistry. B u t in order t o do a n y chemistry under such extreme conditions, one m u s t

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first a d a p t common chemical tech niques t o cryogenic t e m p e r a t u r e s ; thus cryogenic design becomes a prerequisite t o chemistry. Under s t a n d a b l y , most chemists have little interest in this nonchemical cryo genic technology which is clearly an albatross which has prevented more rapid progress. Design Considerations Compositional analysis is crucial in a n y chemical investigation. I n strumental analysis in cryochemis t r y must be performed without as sociated w a r m - u p , and instrument modifications to allow this conve nient cryogenic operation should be relatively simple. T h e d a t a should be unambiguous and easy t o inter pret. T h e instrumentation should also permit the development of in sights into the molecular physics of the unusual product molecules. N o instrumentation meets perfectly all these criteria, b u t t h e mass spec trometer is a most promising com promise. However, the usual m e t h od of admitting a gaseous sample into a room t e m p e r a t u r e or (more usually) much hotter source is of little value in cryogenic applica tions since decomposition of u n stable compounds would be inevita ble. T w o possible solutions are ei ther t o cool t h e entire ionization chamber, a n d this h a s been done (5), or t o introduce t h e sample through a cooled inlet tube, injecting it directly into t h e ionizing beam and thus avoiding a n y contact with the h o t source prior t o ionization (β). F o r m a x i m u m number density of these incoming cold species within the ionizing electron beam, t h e cryogenic inlet port must b e t a n

gent t o the beam itself. Hence a final micrometer adjustment of the inlet within the source iis necessary. A number of commercial instru ments with a n open or readily ac cessible source structure are avail able. T h u s the cryogenic accesso ries can be readily assembled around and even within the source, and special designs are unnecessary. T h e high energy electron impact fragmentation or ionization is a major disadvantage, for highly un stable molecules m a y not yield ions of sufficient variety and intensity to permit unequivocal assignments. However, unusual cryochemical re agents exist a t very low temper atures because of the absence of a n activation energy for their lowest energy-loss process. This loss is not so often an intramolecular phe nomenon as it is rather a n intermolecular phenomenon—that is, cryogenic species are generally best construed as highly reactive rather t h a n inherently unstable. T h e idea of a "cold l a b o r a t o r y " m a y b e approached b y immersing the entire reaction a n d processing system in a well-stirred cryogenic refrigerant. This is somewhat awkward, but this or an equivalent technique seems necessary, for even simple fluid transfer operations must involve no w a r m - u p in transit. W i t h this "immersed" design, t h e entire reaction, processing, a n d mass spectrometric inlet system m a y be isothermal, automatically stabilized, and adjusted for any de sired temperature-time history. Various types of reactors and asso ciated processes should also be readily interchangeable. T h e mass spectrometer inlet should incorpo r a t e a control valve iso t h a t a reac-

VOL. 4 1 , NO. 1 1 , SEPTEMBER 1969

·

25 A

Report for Analytical Chemists

tor at any pressure may be under continuous compositional analysis. Finally, and obviously, the entire system must be mechanically and operationally as simple as possible.

Figure 1. Cryochemical facility, with schematic of reactor and mass spectrometer inlet system showing its refrigerant recirculation arrangement

TEMPERATURE INDICATOR

CONTROLLER LIQUID NITROGEN AMPLIFIER

HEATER-

-COOLING COIL •THERMOCOUPLE

Figure 2. Cryochemical facility, with schematic of automatic temperature control system 26A

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Facility General. The schematic of the cryogenic facility, including its reaction and analytical and processing capabilities, is presented in Figures 1 and 2, and a photograph appears in Figure 3. The several components of the facility are suspended below a 9mm brass plate which is mounted just below the ionization chamber of a Bendix Model 14-107 time-offlight mass spectrometer. The refrigerant bath, in which all components are immersed, is contained in a 21-cm i.d. stainless steel Dewar vessel which may be pulled up and sealed (O-ring) below the plate. A 6-cm thick styrofoam plug under the plate reduces the heat leak from the top. Each of the control and access connections to the entire apparatus pass through O-ring-type quick-disconnect vacuum couplings mounted on the top of the main plate. This arrangement permits interchange of reactors in a minimum of time since the only soldered connection is from the inlet valve to the new point of analytical monitoring. All other connections are made with quick-disconnect O-ring fittings. The inlet tube leads the sample from the valve directly up into the ionizing beam of the mass spectrometer. To maintain the isothermal character of the system, refrigerant from the Dewar is circulated through a cooling jacket around the entire length of the inlet tube by means of a pump which is mounted below the plate and immersed in the liquid bath. This circulation also stirs the refrigerant, thus producing more uniform temperature control and improved heat transfer to the submerged components. A small ECO laboratory pump (model PP 1M) with a nominal delivery of 14 l./min at 1750 rpm is used. The pump housing is made of bronze, with Teflon impellers and bushings and a stainless steel shaft. No lubrication is required. All bearings and the center holes in the impellers

Report

Air Products AWARD WINNING

HELIUM CRYO-TIP® REFRIGERATOR . . . selected as one of the 100 most significant new technical products of 1965 Small, compact and lightweight, this CRYO-TIP Refrigerator proves ideal for cryogenic experiments at helium temperatures —yet operates from economical cylinder gas with out the need for liquid helium. CRYO-TIPS are presently being used in: Spectroscopy · Field Ion Mi croscopy · X-Ray Diffraction • Lasers • 1R Detectors * Mossbauer Effects · Semi-Conductor Meas urements · Cryopumping * EPR, ESR, NMR Air Products CRYO-TIP Refriger ator, (Model AC-3L-110) has al ready gained international recogni tion for outstanding performance and operating simplicity. For our new brochure write:

Air Products and Chemicals INC.

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(215) 395-4911 Circle No. 1 on Readers' Service Card

28 A ·


Figure 3. cility

Photograph of cryochemical reaction and analytical fa

werc reamed out to account for the large thermal contraction of Teflon. A glass wool filter prevents the seemingly inevitable solid contami nants in the refrigerant from depos iting in the pump. The pump is driven by a Vs-h.p. motor with a controller t h a t permits continuous speed variation between 0 and 2000 rpm. Power is transmitted through a flexible shaft connected to the pump by a flexible coupling with a Teflon spider. T h e stainless steel shaft is guided by a Teflon bearing in the main brass plate. The temperature of any of the several convenient refrigerants t h a t might be selected is measured b y a copper-constantan thermocouple. T h e E M F is read from a Leeds & Northrup Speedomax H instrument which has a full scale reading of 1 mV at maximum sensitivity. T h e maximum error is within 0.3% of full scale, corresponding to ± 0 . 0 8 °K at the ice point and to ± 0 . 2 °K at liquid nitrogen temperature. The system is cooled by passing liq

uid nitrogen through a coil im mersed in t h e refrigerant bath. This nitrogen flow rate is indicated by a rotameter and is adjusted with a valve, both of which are on the vapor side. To prevent frosting of the rotameter, the cold nitrogen vapor is first warmed b y exchange with the diffusion p u m p cooling water from the mass spectrometer. T h e system is also provided with a 25-Ω electric heater powered by a dc magnetic amplifier having a maximum power output of 250 W. T h e temperature of the system can be maintained automatically by using a Leeds & Northrup (Se ries 60) control unit in connection with the Speedomax instrument, the magnetic amplifier, and the roughly adjusted liquid nitrogen influx. T h e amount of liquid nitrogen flow ing through t h e cooling coil must be slightly higher than necessary t o balance the various heat leaks. T h e controller then regulates the energy input to the heater to just balance those heat and refrigeration

Report

Is a

still still the best source of high-purity water for the laboratory? When a laboratory needs high-purity w a t e r to use as a s o l v e n t , d i l u e n t , reagent or cleaning agent, there has been no good reason to question that distilled water is the logical choice. Now there is a good reason. The Millipore Super-Q System does about everything a still can do and, in many cases, does it more effectively and more efficiently,

^ FILTRATION ®|

t

f DEIONIZATION

4 ORGANIC ADSORPTION

enough to fit comfortably in the laboratory, under laboratory control. A still with this capability would take at least five times the space. Because of its high output rate, the Super-Q System usually eliminates the need to store water. Storage is often necessary with stills, introducing the problem of contamination by containers and lines. Maintenance is negligible with the Super-Q System because it uses disp o s a b l e c a r t r i d g e s . Stills must be meticulously maintained; and when water is supplied from a central still, maintenance (and water quality) is beyond the control of the laboratory. Millipore filtration, the final Super-Q cartridge, removes all particles and microorganisms larger than filter pore size (typically 0.45 micron). Stills can only reduce particulate contamination. Good reasons why you should inv e s t i g a t e the p o s s i b i l i t y that the Super-Q System might be a better source of high-purity water for your laboratory.

^ PREFILTRATION

The Super-Q System consists essentially of disposable cartridges for prefiltration, organic adsorption, deionization and Millipore filtration. It can produce 18 megohm water from tap water. A still ordinarily produces V2 megohm water. 20 successive distillations in quartz would be r e q u i r e d to e q u a l the i o n i c p u r i t y achieved by the Super-Q mixed-bed ion exchange cartridge. (It's easier to remove one part impurity from 10 million parts of water rather than the reverse.) The Super-Q System produces 20 gallons of water an hour and is small

Write tor detailed information. Millipore Corporation, Bedford, Massachusetts 01730.

MILLIPORE systems tor analyzing and processing fluids Circle No. 48 on Readers' Service Card

30 A ·


fluxes and thus keep the temperature at a constant value. Experiments showed that the temperature can be kept to within ±0.2 °K at all temperature levels. The amount of refrigerant varies somewhat with the size of the reactor and its attached processing and control devices, but on the average, about 6 1. are needed. In a typical experiment, the system was filled with 6 1. of 2-methylpentane, and 2.5 l./hr of liquid nitrogen were required to keep the system at a constant temperature of 130 °K. At higher temperatures, the liquid nitrogen consumption decreases correspondingly. When the automatic control is used, the nitrogen flow rate has to be increased by about 25%. Of course, a reaction with continuous power input, like an electric discharge, also increases the liquid nitrogen consumption. Refrigerants. The ideal refrigerant should be liquid over a wide temperature range and have a low viscosity for easy pumping and good heat transfer. It should not be flammable, explosive, toxic, or expensive. Unfortunately, there is no known liquid that combines all these properties. Thus, a trade-off and compromise is always necessary. Liquid nitrogen is a useful refrigerant from its triple point (63.2 °K) to about 90 °K. The lower temperature is obtained by pumping over the liquid, while the upper temperature is reached by pressurizing the system to 3.5 atm. Mechanical limitations prohibit higher pressures. Propane has a wide liquid range (triple point, 86 °K; normal boiling point, 231 °K), low viscosity over the whole range, and is inexpensive. A 1:1 mixture of propane and propylene freezes below the normal boiling point of nitrogen. However, both substances are flammable and form explosive mixtures with air. Since their normal boiling points are well below room temperature, any spilling of the liquid (as in the event of a dewar breakage) would be extremely dangerous. Isopentane (triple point, 113 °K; normal boiling point, 301 °K) and 2-methylpentane (triple point, 119 °K; normal boiling point, 333 °K) Circle No. 202 on Readers' Service Card

>-

Separate amines easily, efficiently with

Report

have wide liquid ranges and have been used extensively with the facility described here. A mixture of 65% isopentane and 35% 2-methylpentane (by weight) freezes a t 105 °K and boils at 308 °K, thus being considerably safer t h a n propane. However, the relatively high freezing point leaves a gap of about 15° to the pressurized liquid nitrogen t e m p e r a t u r e . T h e Fréons are neither flammable, explosive, toxic, nor overly viscous a t low temperatures. Their liquid range is somewhat smaller than t h a t of propane, but it can be extended by using solutions. A mixture of 14% Freon-12 (CC1 2 F 2 ), 72% Freon-13 (CC1F 3 ), and 14% Freon-22 (CHC1F 2 ) has a freezing point of about 86 °K and a normal boiling point of about 197 °K, and it is therefore especially suitable for the requirements of this facility. A major disadvantage is the high price which necessitates a recovery system. T h e vapor pressure of the mixture is about 26 atm. at 25°. To fill the reactor system, the Freon mixture is liquefied by passing it through a coil immersed in liquid nitrogen. T o return the Freon back to storage, the refrigerant system is pressurized—e. g., with dry nitrogen. Before receiving the refrigerant solution, the 30-1. stainless steel storage container, which is insulated with pliable plastic foam sheeting, must be prccooled by passing liquid nitrogen through a coil which is soft soldered to its outer surface. Without this precooling, vaporization of the Freon would cause a pressure buildup in the t a n k and prevent refrigerant backflow.

New Chromosorb* 103

Chromosorb 103 is a polyaromatic porous resin developed by Johns-Manville as a solid-type support for the separation of amines and basic compounds. Until now, amine separation has been both laborious and difficult. With new highly selective Chromosorb 103, it's fast and easy. For more specific information, write for our bulletin FF-181. Johns-Manville, Box 290-C, Trenton, New Jersey.

Johns-Manville

*Chro,Tiosorb is a J o h n s - M a n v i l l e registered t r a d e m a r k f o r its b r a n d of p r o d u c t s deve o p e d for use as s u p p o r t m a t e r i a l or a d s o r b e n t s for gas c h r o m o t o g r a p h y . Circle No. 38 on Readers' Service Card

32 A .


The useful ranges of these several convenient cryogens are evident from Figure 4 as are the technical difficulties, should an experiment require a temperature range exceeding t h a t of a single refrigerant. An alternative design imposes less restrictions from this perspective (β). Mass Spectrometer Inlet. The valve between the reactor and the inlet tube has very demanding pre requisites. It must be chemically inert, and it must reliably close off 1 atm from 10-° tort· at 77 °K. After m a n y design tests, a N u p r o bellows-type valve (Model B4BKSÏ with a brass body and a stainless Circle No. 126 on Readers' Service Card

>•


steel stem with Kel-F tip was found to satisfactorily function as required. The valve is not designed for metering purposes, however, and it has to be opened very carefully when there is a large pressure differential between reactor and mass spectrometer. The reactor pressure gauge than becomes an important experimental control monitor. The 2.5-mm i.d. monel inlet tube extends vertically from the valve directly into the ionization chamber of the ion source. The tube is surrounded by a cooling jacket which is nickel-plated on the outside to reduce radiative heat transfer. This residual heat influx to the inlet jacket corresponds to an evaporation rate of 0.018 l./hr of liquid nitrogen, which is insignificant compared to the 100 l./hr of refrigerant normally circulated through the cooling jacket. The cooling jacket is enclosed within a vacuum insulation space which is open to the mass spectrometer system. A flat, nickel-plated copper extension piece is fitted to the top of the cooling jacket. Its shape permits the sample gas to be injected directly into the edge of the electron beam (perpendicular to the plane of Figure 1) without interfering with the ion grids of the source. The thermal conductivity of the copper

is sufficiently high to prevent any significant warming of the sample due to radiation from the surroundings—e. g., the temperature difference between top and bottom of the extension piece was computed to be less than 0.02 °K. The relative position of the inlet port to the electron beam of the ion source is very important if one is to obtain maximum ion current and signal-to-noise ratio. Extensive calculations predict that the sensitivity should decrease by a factor of 4 if the inlet port is only 2 mm away from the near edge of the rectangularly collimated electron beam. Because there is always an appreciable background of molecules which have already collided with the walls, the sensitivity actually decreases by a factor of about 2.5. This critical vertical adjustment is accomplished with a threaded nut and sleeve arrangement mounted on the inlet tube vacuum jacket. Horizontal adjustment of the inlet port can be done with three set screws that fix the position of the top of the inlet tube. This is important since the extension piece must not touch the ion grids, and the center lines of the inlet port and the electron beam must intersect for maximum ion intensity. The set

With a Mettler Preventive Maintenance Agreement, one of our 40 full-time service specialists w i l l help you prevent inaccurate weighings. You may not k n o w it, but your balance could be y i e l d ing incorrect results because of misuse or poor operating c o n d i t i o n s . To g u a r d a g a i n s t these weighing inaccuracies and to prevent future problems, a Mettler service specialist w i l l check and adjust your balance in y o u r lab once a year under a Mettler Preventive Maintenance Agreement. This f a c t o r y - t r a i n e d s p e c i a l i s t w i l l diagnostic-test, clean, lubricate, adjust and final-test your Mettler balance to maintain its accuracy, reliability and precision of performance. The time and money you could save by e l i m i n a t i n g i n c o r r e c t weighings could more than pay for the Mettler Preventive M a i n tenance Agreement. As a bonus, you'll receive many extra years of useful service f r o m your Mettler. For further information, write to Mettler Instrument Corporation, Box 7 1 , H i g h t s t o w n , N.J. 08520.

IT'S ALMOST AS IF HE CAME WITH THE

BALANCE

Figure 4. pressure

Convenient thermal ranges of several cryogens at atmospheric

Mettler

®

see ACS Laboratory Guile for All Products/Sales Office Circle No. 47 on Readers' Service Card VOL. 4 1 , NO. 1 1 , SEPTEMBER 1 9 6 9

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37 A

Report

The Kodak Scintillation Committee

REACTANT INLET AND PUMPOUT

-ELECTRODE TEFLON " INSULATOR * TO MASS Τ SPECTROMETER

Figure 5. Low pressure gaseous dis charge reactor

screws are not accessible from the outside, but the adjustment was easily done during the assembly, and no readjustment has been nec essary. Reactors. Several reactors were designed for use with the inlet sys tem. Most of them generate plas mas in a low temperature environ ment, thus permitting rapid quench of the reaction products. Plasma activation is not, however, unique, for any technique of active species generation may be employed equal ly well in this facility. A low pressure glow discharge is suitable for very low temperatures, even below the triple point of the reactants, provided only that the reactants exert a vapor pressure of a few torr. Typically, an electrode gap of 2—6 cm is used. Power is furnished by a variable high volt age transformer (neon sign type) with a 15,000-Ω power resistor in series. The potential across the electrodes is 500-2000 V, with a current of 20—30 mA. Depending on the nature of the reactants, glass or metal reactors may be more fa38 A

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Membership: Assistant Department Head, Analytical Services, Kodak Research Laboratories . . . Research Associate, Kodak Laboratory of Industrial Medicine, where animal studies trace metabolic fates of industrially significant compounds . . . Specialist in instrumentation to measure quantum yields . . . Specialist in radiochemical technique, the field in which he has counseled his Kodak Research Laboratories colleagues since the 1940's . . . Chemist assigned operational responsibility for the preparation, purification, and quality control of Eastman scintillation-grade chemicals . . . Chemist from marketing department, assigned to learn views of outside laboratories.

Report

Liquid scintillation-counting supplies:

Demand for them and insistence on quality are growing from Kodak's own labora tories, and growing still faster from the laboratory world at large. Wide-ranging use and importance of liquid scintillation-counting techniques justify the work of this committee. It strives to tune our quality standards to the cost of time wasted when standards are not what they should be. To share the benefit of this study with all users of liquid scintil lation techniques, a program has been initiated with five of the most fundamental necessities. Detailed scintillation specifications are available for the asking. Use coupon below to request Infor mational JJ-59, "Reagents for Liquid Scintillation Counting." List Price

p-Dioxane

(EASTMAN

l p t . $ 3.75 1 2 x 1 pt. 41.05 1 gal. 22.20 4 χ 1 gal. 80.70

13011)

(Scintillation Grade)

Toluene

(EASTMAN

13016)


Naphthalene

(EASTMAN

13007)

(Scintillation Grade) "PPO"

1.60 17.70 7.37 26.80

1 kg. 6 χ 1 kg.

16.55 90.40

100 g. 28.70 12 χ 100 g. 313.35

(EASTMAN 13000)


Phenethylamine

lpt. 12 χ 1 pt. 1 gal. 4 x 1 gal.

vorable. I n the metal reactor shown in Figure 5, the discharge is established between the center elec trode and the grounded cold walls of the reactor body. A spark discharge reactor is con veniently energized b y a commer cial auto ignition system to produce an intermittent spark of about 25,000 V. T h e reactor (Figure 6) is made of glass to eliminate the prob lem of arcing to the walls. Sparks can be generated in gases or liquids, but the latter is more interesting because of the more favorable quenching situation. Only electri cally nonconducting liquids, such as nitrogen, argon, or fluorine, can be used, and the electrode distance must be k e p t smaller t h a n 0.5 mm. Like sparks, electric arcs can be run in liquid or gaseous reactants. ac, dc, and rf arcs have been studied, but dc arcs were preferred because here the cathode gets considerably hotter t h a n the anode. This is im p o r t a n t if dissimilar electrodes are

(EASTMAN

13021)


l p t . 19.15 12 χ l p t . 208.80 ELECTRODE (ADJUSTABLE)

List prices shown are suggested prices only and subject to change without notice.

These "scintillation grade" reagents are available from any of the following nearby suppliers of all the other Eastman laboratory chemicals : B&A CURTIN FISHER HOWE & FRENCH

V//±O-RING %^~-COUPLING

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Eastman Kodak Company Eastman Organic Chemicals Dept. 412-L Rochester, N . Y . 14650 Send "Reagents for Liquid Scintillation Counting" JJ-59 to: 2CM OD BULB

NAME

GLASS TO-METAL SEAL ELECTRODE (FIXED)

Kodak

Figure 6. Cryochemical reactor utiliz ing submerged spark of submerged arc activation

VOL. 41, NO. 11, SEPTEMBER 1969 · 39 A

Report Number 2 of a Series

Signal Averaging... Principles and Practices 30 foot spectrum ? Usually, in this series, we will deal with useful facts about the signal averaging process. For example, in the first of this series we pointed out why, in high resolution spectroscopy, it is much better to use an integrating analog-to-digital converter with the averager. But once in a while we will describe some feature of our instruments that is so exceptional that we think it just plain newsworthy. One of these is the greatly improved data display for our averagers used in high resolution NME spectroscopy. The model 1074-N 4096-address signal averager, with the SW-74 NMR mag net sweep control plug-in unit provides a cathode ray tube dis play that operates like this : 1. The display is continuous, even during measurements involving the lowest sweep rates. In earlier systems, the display during measurements consisted of a single very slowly moving spot on the screen. Now you view the whole spectrum. 2. You may select any data point position, and may expand the portion of the spectrum following to any extent you wish. For example, 4 points per centimeter. And because this may be done during or after measurements, you can examine fine structure clearly without interrupting the measurement. This much expansion is equivalent to spread ing a 4096 point spectrum over 30 feet. 3. After t h e measurement you may use the unexpanded display, which shows t h e s p e c t r u m w i t h one p o i n t c l e a r l y i n t e n s i f i e d . That point may be moved right or left an address at a time, or at a slow or fast continuous rate. This marker shows the s t a r t i n g p o i n t of t h e ex panded display. 4. At the touch of a button, the numerical values of the ordinate and address number for that point are displayed. No more doubts about the exact position of a spectral feature. Whether your spectrometer requires sweep field stabiliza tion or not, you will find the 1074-N, SW-74 combination by far the easiest to use even if it didn't include the fine new data display. Write or phone to discuss your specific application.

FABRI-TEK

Instruments, Inc.

5225 Verona Rd., Madison, Wis., 53711. Phone: 608/271-3333 Sales and Service: In Canada by Ahearn & Soper Limited, 844 Caledonia Road, To ronto 19, Ont. Tel. (416) 789-4325 — Telex 02-2757 Branches in Montreal and Vancouver . . . In Europe by Bruker-Physik, 7501 Karlsruhe-Forchheim, Postfach 40, 10, Karlsruhe, West Germany Circle No. 28 on Readers' Service Card

40 A

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used and only one electrode materi al is to be vaporized. Although the potential across the arc averages only about 20 V, a high voltage power supply (0-4000 V) with a power resistor (3000-40,000 Ω) in series is required to stabilize the arc. The same reactor (see Figure 6) as for the spark discharge can be used. The exploding wire phenomenon is of special interest since it permits a large energy input in a very short time, resulting in very high temper atures. The reactor body (Figure 7) can be made of glass when the wires are exploded in a gas, whereas the shock wave produced by an ex plosion in a liquid requires the use of a metal reactor. Eight wires can be exploded consecutively during one reaction run for product mass accumulation. Energy is supplied from a 32 ^F capacitor bank which can be charged to 5000 V, corre sponding to a maximum energy of 400 Joules. The high voltage cir cuit is ungrounded to reduce arcing to the walls of the metal reactor. Typical Experimental Results

The reactor facility is being used in a research program seeking new, highly endothermic molecules which may be synthesized by eryochemical techniques. Part of this program concerns noble gas com pounds, and KrF 2 seemed a con venient prototype molecule for studies of possible compound forma tion with the lighter noble gases. KrF 2 was synthesized using a low-pressure discharge in the metal reactor of Figure 5 (7). The dis charge was operated at 500-1000 V ac and 20-30 mA for about 3 hr. Optimum yield was obtained with an average reactant composi tion of Kr:F 2 = 1 : 5 at a total pressure of 4-20 torr. Since KrF 2 does not exert a significant vapor pressure below 120 °K, 2-methylpentane was used for controlled warmup. The mass spectrum ap peared at about 190 C K with maxi mum intensity occurring between 200 and 220 °~K. KrF+, Kr+, and F+ ions were observed, and appear ance potential measurements yielded A(Kr+, KrF 2 ) = 13.21 ± 6.25 eV, and A(KrF + , KrF 2 ) =


Long-term transport of sterile or corrosive fluids

•POWER INPUT REACTANT INLET AND PUMPOUT

EPOXY INSULATOR

TO MASS SPECTOMETER

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Figure 7.

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Exploding wire reactor

13.71 ± 0.20 eV. This reactor has also been used for the routine synthesis of 0 2 F 2 from a similar discharge in an F 2 , 0 2 reactant mixture at 90 °K (8). The syntheses of argon fluorides and oxides and krypton oxides have been attempted with the low-pressure discharge reactor. Also an electric spark has been run submerged in a liquid mixture of Ar and F 2 . The latter method was considered favorable since rapid quenching can better occur in a liquid than in a low-pressure gas. However, no experiment has yielded any evidence of compound formation. Either no such compounds formed, or they decomposed before exerting a detectable vapor pressure. Related experiments, both in this laboratory and elsewhere, have been more successful. The very elusive cyclobutadiene appears to survive a quench to 77 ° Κ and sub sequent reevaporation (9), and cyclopropanone is produced in good yield from the solvent-free liquidliquid reaction between diazomethane and ketene near 120 °K (10). The isoelectronic species NH and CH 2 add across the doublebond of carbon dioxide at tempera

tures near 50 °K to produce the in teresting new molecules (11, 12) Η Η2 Ν C / \ / \ Ο C = 0 and Ο C=0 but similar additions of the iso electronic species CH 2 , NH, and Ο across triple bonds have not yet been successfully conducted. Methyleneimine is produced from the photolysis of methyl azide, and the new imine is evidently reasonably stable (12). SiF 2 and BF which may be readily produced at high temper atures have been shown to react in very interesting ways at low temperatures (13). The reactive boron containing compounds BH 3 , HBNH, and H 2 BNH 2 all seem to be stable and isolatable at very low temperatures. Diimide, N 2 H 2 , may be similarly produced. Applications

At this point one may only guess at the future applications of cryo chemistry. Certainly the synthesis of very high energy compounds suggests applications wherever chemical energy storage is impor-

Report

tant. New materials seem possible as do new insights into chemical kinetics. Only those elementary processes of lowest activation energy will occur at very low temperatures, and consequently annoying reactions may be effectively frozen out, resulting in a much cleaner and more readily interprétable rate phenomenon. A perhaps rather esoteric application is in the area of astrophysics. Efforts of astronomers to explain the observed behavior of comets and the atmospheres of the Jovian planets (and in particular Jupiter itself) have often required postulation of high-energy chemical reactions which must occur at very low temperatures by terrestrial standards. The involvement of compounds such as described herein can explain the astrophysical data. Although results are still somewhat meager, the now successful merging of cryogenics and chemistry through the necessary vehicle of cold-chemical analysis seems sure to have a powerful influence upon both pure and applied chemistry. Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under grant AF-AFOSR-1308-67. Pigment analysis of an old master painting with Kevex-ray spectrometer

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Literature Cited (1) "Collected Works of Sir James Dewar," Lady Dewar, Ed., Cambridge University Press, Vols. I and II, 1927. (2) A. M. Bass and H. P. Broida, Eds., "Formation and Stabilization of Free Radicals," Academic Press, New York, Ν . Υ., 1960. (3) H. A. McGee, Jr., "Advances in Cryo genic Engineering," Vol. 9, K. D. Timmerhaus, Ed., Plenum Press, New York, N . Y., 1964, p. 1. (4) D. E. Milligan and M. E . Jacox, J. Chem. Phys., 48, 4811 (1968), and nu merous other contributions on a variety of low-temperature systems. (5) L. P. Blanchard and P. LcGoff, Can. J. Chem., 37,515 (1959). (6) H. A. McGee, Jr., T. J. Malone, and W. J. Martin, Rev. Sci. Instr., 37, 561 (1966). (7) P. A. Sessa and H. A. McGee, Jr., J. Phys. Chem., 73 (1969), in press. (8) T. J. Malone and H. A. McGee, Jr., J. Phys. Chem., 69, 4338 (1965). (9) P. H. Li and H. A. McGee, Jr., Chem. Comm., 1969, 592. (10) R. J. Holt, Ph.D. thesis in chemical engineering, Georgia Institute of Tech nology, 1969. (11) D. E. Milligan, M. E. Jacox, S. W. Charles, and G. C. Pimentel, J. Chem. Phys., 37, 2302 (1962); D . E. Milligan and M. E. Jacox, ibid., 36, 2911 (1962). (12) D. E. Milligan, Λ Chem. Phys., 35, 1491 (1961). (13) P. L. Timms, Endeavor, 27, 133 (1968).

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