s
. .DEWAR. was evidently the first investigator to be concerned with the effect upon chemistry of the then newly available very low temperatures (1). Dewar (who was also the world's first cryobiologist) reported in 1885 t h a t P, Na, K, H2S, and HI seemed not t o 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 the chernical reaction orientation of the present discussion for, particularly since 1960, a wide variety of physicochemical characteristics of molecules and particularly free radicals have been observed from cryogenic adaptations of ESR and IR. Such studies have been highly informative. But the essential heart of any synthetic or reactivity investigation is, of course, analysis, and here ESR techniques are limited to free radicals or nonsinglet species, while both ESR and IR demand a highly specialized and sophisticated analyst. By contrast, mass spectrometry is applicable to 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 by some high energy process is contacted with a second reactant within some sort of hard quench configuration. A variety of mechanical arrangements have been used - quenched diffusion flames, submerged arcs, quenched gaseous IR JAMES
J. K. Holzhauer received his Diploma of Engineering from the University of
StUttgart, Germany, in 1966, and his M.S. in Chemical Engineering from the Georgia Institute of Technology in 1968. He is currently working on his doctorate in the School of Chemical Engineering a t Georgia Tech
Henry A. McGee,, Jr.. is Professor of Chemical Engineering a t the Georgia Institute of Technology, and he was before that associated with NASA. His educational background is in chemical engineering and theoretical chemistry a t Georgia Tech and the University of Wisconsin, respectively, and his scientific interests continue to encompass selected areas of both pure and applied chemistry. His current research interests center upon the use of cryogenic techniques in the lucidation of physical, chemical, and biological phenom~ena. Dr. McGee and his students and associates have been particularly interested over the past several years in all phases of chemical reaction research at cryogenic temperatures. He is presently Visiting Professor of Chemical Engineering a t Caltech. 24A
ANALYTICAL CHEMISTRY
J. K. H o l z h a u e r a n d H. A. McGee, Jr., School of C h e m i c a l Engineering, Georgia I n s t i t u t e of Technology, A t l a n t a , Georgia 30332
discharges, etc. The objective may be stabilization of a species normally a n intermediate-e.g., the preparation of ethylene oxide or oxirene in the reaction of atomic oxygen with ethylene or acetylene, respectively. Also, the stabilization of the high temperature species itself may be desirable, as in the preparation of borane, BH3, from the products of the pyrolysis of borane carbonyl. Characteristically one searches for new or unusual molecules nhich perhaps exist as purifiable reagents only below some very low critical temperature, T,, such that k T , is small relative t o the activation energy of the minimum energy-loss process. This describes the stable existence of any species, of course, and the essential characterization of cryogenic reagents is the exceedingly low value of T,. These typically very low critical temperatures demand complete thermal control, since inadvertent warming can result in the loss of the compound of interest. The thermal requirement is always a nuisance from the view of experimental design, but it is especially troublesome in analysis, the essential requirement of all chemical reaction investigations. Sample warm-up followed by analysis by conventional techniques is unsatisfactory a t beet and meaningless a t worst, for here the cryogenic species have heen lost and their presence and behavior may be only indirectly and usually inaccurately inferred. Cryochemical techniques form a iiew dimension of preparative and interpretive chemistry. B u t in order to do any chemistry under such extreme conditions. one must
first adapt common chemical techniques to cryogenic temperatures ; thus cryogenic design becomes a prerequisite to chemistry. Understandably, most chemists have little interest in this nonchemical cryogenic technology which is clearly an albatross which has prevented more rapid progress. Design Considerations
Compositional analysis is crucial in any chemical investigation. Instrumental analysis in cryochemistry must be performed without associated warm-up, and instrument modifications to allow this convenient cryogenic operation should be relatively simple. The data should be unambiguous and easy to interpret. The instrumentation should also permit the development of insights into the molecular physics of the unusual product molecules. N o instrumentation meets perfectly all these criteria, but the mass spectrometer is a most promising compromise. However, the usual method of admitting a gaseous sample into a room temperature or (more usually) much hotter source is of little value in cryogenic applications since decomposition of unstable compounds would be inevitable. Two possible solutions are either t o cool the entire ionization chamber, and this has been done 16), or t o introduce the sample through a cooled inlet tube, injecting it directly into the ionizing beam and thus avoiding any contact with the hot source prior to ionization (6). For maximum number density of these incoming cold species within the ionizing electron beam, the cryogenic inlet port must be tan-
gent to the beam itself. Hence a final micrometer adjustment of the inlet within the source ils necessary. ,4 number of commercial instruments with an open or readily accessible source structure are available. Thus the cryogenic accessories can be readily assembled around and even within the source, and special designs are unnecessary. The high energy electron impact fragmentation or ionization is a major disadvantage, for highly unstable molecules may not yield ions of sufficient variety and intensity to permit unequivocal assignments. However, unusual cryochemical reagents exist a t very low temperatures because of the absence of an activation energy for their lowest energy-loss process. This loss is not so often an intramolecular phenomenon as it is rather an intermolecular phenomenon-that is, cryogenic species are generally best construed as highly reactive rather than inherently unstable. The idea of a “cold laboratory” may be approached by immersing the entire reaction and processing system in a well-stirred cryogenic refrigerant. This is somewhat awkward, but this or a n equivalent technique iseerns necessary, for even simple fluid transfer operations must involve no warm-up in transit. With this “immersed” design, the entire reaction, processing, and mass spectrometric inlet system may be isothermal, automatically stabilized, and adjusted for any desired temperature-time history. Various types of reactors and associated processes should also be readily interchangeable. The mass spectrometer inlet should incorporate a control valve is0 that a reac-
VOL. 41, NO. 11, SEPTEMBER 1969
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Report for Analytical Chemists
tor a t any pressure may be under continuous compositional analysis. Finally, and obviously, the entire system must be mechanically and operationally as simple as possible. Facility
Figure 1. Cryochemical facility, with schematic of reactor and mass spectrometer inlet system showing its refrigerant recirculation arrangement
-il CONTROLLER
AMPLIFIER
HEATER /
L LIQUID
NITROGEN
‘COOLING
COIL
\THERMOCOUPLE
Figure 2. Cryochemical facility, with schematic of automatic temperature control system 26A
ANALYTICAL CHEMISTRY
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 0-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 t o 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 1 R I ) with a nominal delivery of 14 l./min a t 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
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VOL. 41, NO. 11, SEPTEMBER 1969
288
27A
W -ti& 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 without the need for liquid helium. CRYO-TIPS are presently being used in: Spectroscopy Field Ion Microscopy * X-Ray Diffraction * Lasers * 1R Detectors Mossbauer Effects Semi-Conductor Measurements Cryopumping EPR, ESR, NMR Air Products CRYO-TIP Refrigerator, (Model AC-3L-110) has already gained international recognition for outstanding performance and operating simplicity. For our new brochure write:
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ADVANCED PRODUCTS DEPARTMENT ALLENTOWN, PA. 18105 (215) 395-4911 Circle NO. 1 on Readers' Service Card
28A
ANALYTICAL CHEMISTRY
Figure 3. Photograph of cryochemical reaction and analytical facility
were reamed out to account for the large thermal contraction of Teflon. A glass wool filter prevents t h e seemingly inevitable oolid contaminants in the refrigerant from depositing in the pump. The pump is driven by a I/s-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. The 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 by a copper-constantan thermocouple. The EMF is read from a Leeds & Northrup Speedomax H instrument which has a full scale reading of 1 mV a t maximum sensitivity. The maximum error is within 0.3% of full scale, corresponding t o k0.08 "K a t the ice point and to &0.2 "K a t liquid nitrogen temperature. The system is cooled by passing liq-
uid nitrogen through a coil immersed in the 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 by exchange with t h e diffusion pump cooling water from the mass spectrometer. The system is also provided with a 25-n electric heater powered by a dc magnetic amplifier having a maximum power output of 250 W. The temperature of the system can he maintained automatically by using a Leeds & Northrup (Series 60) control unit in connection with the Speedomax instrument, the magnetic amplifier, and the roughly adjusted liquid nitrogen influx. The amount of liquid nitrogen flowing through the cooling coil must he slightly higher than necessary to balance the various heat leaks. The controller then regulates the energy input to the heater t o just balance these heat and refrigeration
We weren’t reallv sure the world was r6adv for our Model 900 Gas Chromatograph.
But we took the chance. We put the most experienced staff of GC development engineers in the business to work on the first chromatograph designed the way we knew it should be. No compromises. No cost-cutting. And no easy ways out. We knew it would have to sell at a price above the general level of the competition. And, frankly, we had our fingers crossed. But now you’ve proved that we shouldn’t
have worried. It didn’t take you long to check out the specs. And if you’re one of the many who called or wrote to tell us we’ve been too modest, or stood up at meetings and sung the Model 900’s praises, or even if you’ve just passed the good word along-we thank you. You probably know all about our unique toroidal oven design. Our all new injector and detector systems, The remarkable parametric
ionization amplifier. And the many other features that have made the Model 900 the choice of the professionals. But we’ve just come out with a new &page brochure that tells it all. Whydon’t you writeinforafewand pass them along to your friends? Instrument Division, Perkin-Elmer Corporation, 702 Main Avenue, Norwalk, Connecticut 06852.
PERKIN-ELMER
Circle NO. 158 on Readers’ Service Card
VOL. 41, NO. 11, SEPTEMBER 1969
29A
Report
Isa still still the best source of high-puritywater for the laboratory? When a laboratory needs high-purity water to use as a solvent, diluent, 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.
4 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 S u p e r 4 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 disposable cartridges. 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 ail particles and microorganisms larger than filter pore size (typically 0.45 micron). Stills can only reduce particulate Contamination. Good reasons why you should investigate the possibility that the Super-Q System might be a better source of high-purity water for your laboratory.
PRtillTRAllON
The Super-Q System consists essentially of disposable cartridges for prefiltration, organic adsorption, deionization and Millipore flltration. It can produce 18 megohm water from tap water. A still ordinarily produces Yz megohm water. 20 successive distillations in quartz would be required to equal the ionic purity 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 for detailed information. Millipore Corporation, Bedford, Massachusetts 01 730.
mILLIPORE systems for analyzing and processing fluids
Circle No. 48 on Readers' Service Card
30A
ANALYTICAL CHEMISTRY
fluxes and thus keep the temperature a t a constant value. Experiments showed that the temperature can be kept to within 2 0 . 2 O K a t 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 a t a constant temperature of 130 OK. 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 255%. 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 OK) to about 90 OK. 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 OK; normal boiling point, 231 O 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 OK; normal boiling point, 301 OK) and 2-methylpentane (triple point, 119 OK; normal boiling point, 333 OK) Circle NO. 202 on Readers' Service Card
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“Endorse your newPACE@III d y t i d - d a bsystem.?” “Never! Never! NeGer!” “Oh no, EAI. Just because you’ve come out with another improved, workable and deliverable data-processing system for lab instrumentation doesn’t mean I’m going to let you blab away in your ads about the edge it gives us. “Let our competitors know how PACE 111 handles up to 64 instruments on a time-sharing basis? It’s third-generation software? The fast turnaround and quick payout we’ve gotten? “Oh no, EAI. In fact if anyone asks me, I’ll tell them we’re using an (bleep]. That’ll keep ’em three years behind us.’’ OK, Dr. M, You won’t tell. But, we will. We’ll tell how we got your system on the air 24 hours after it was delivered. We’ll tell how we’re the only company able and willing to deliver working data-processing systems to automate GCs,
auto analyzers, mass specs, physical testing machines, IR systems, and the like. We’ll talk about the only field-proven, year-upon-year suecess in the business. And we’ll really blab about the basic-system price of $63 K, or a commensurate rental rate. so there, D ~M. , See the PACE 1 I I system at ACS and ISA
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Ctiromosorb 103 is a polyaromatic porous resin develol:led by Johns-Manville as a solid-type support for the separation of amines and basic compounds. IJntil now, amine separation has been both laborious and difficult. With new highly selective Chromosorb 103, it's fast and easy. Fur m o r e s D e c i f i c i n f o r m a t i o n , w r i t e f o r o u r b u l l e t i n FF-181. Johns-Manville, Box 29O-C, Trenton, New Jersey.
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*Chronosorb is a Johns-Manville registered trademark for i t s brand of products deve oped for use as support material or adscrbents for gar; chromctcgraphy. Circle NO. 38 on Readers' Service Card
32 A
ANALYTICAL CHEMISTRY
Report
have wide liquid range> and have been used extensively .\>;it11the facility described here. A. riiixture of 65 "i;L isopentanc and 35 "/;. 2-niethylpenttlne (by weight) ireezes a t 105 "Ii and boils a t 308 OK, thus being considerably safer than propane. However, the relat'ively high freezing point' leaves a gap of about 15' t o tlw pressurized liquid nitrogen temper,rrture. The Freons are neither flammable, exploziJ,e, toxic, not' overly \-iscous at' low temper:itures. Their liquid range is somew3iat smaller t l i m that oji prop:mc, Lat it' can be extended hy u i n g ,solutioiis. A mixture of 14% Freon-:l2 (CC,l,F,), 72% Freon-13 (CC1F3), aiid 14y0 Freon-22 (CHC'lF,) ha:; a freezing point of about 86 'I< and a normal boiling point of about 197 "I
