INSTRUMENTATION by Ralph H. Müller
A n e w low-cost i n f r a r e d spectrophotometer has b e e n designed for the chemist
Figure 1
IT1 OR some Ιό years infrared spectrosF copy has been a valuable tool in analytical chemistry, as many impor tant papers in this and other journals have shown. It is estimated that about 1000 modern infrared spectrophotome ters are in use, whereas ten times that number of ultraviolet instruments are used. Intrinsically there is more in formation in the infrared than in the ul traviolet, but heretofore such informa tion has been more costly to come by. The average first class infrared spectro photometer costs about $15,000 and while it may well pay for itself in a short time, this is a figure which is prohibitive for modest research budgets. Con versely a simple, reliable, and reasonably priced instrument to record infrared spectra would make this indispensable tool available to many more analysts. Beckman Instruments, Inc., 2500 Fullerton Road, Fullerton, Calif., manu facturers of the famous DU ultraviolet spectrophotometer, are about to fill this need with the Beckman IR6 infrared spectrophotometer. I t will be sold for under $5000, about one third the cost of
Figure 2
the more elaborate instruments. We are indebted to Lee Cahn for advance information on this development. An interesting aspect of the Beckman IR6 is the story of its design. I t was commissioned by Beckman and designed by John U. White and his associates of the White Development Corp. A general view of the IR6 is shown in Figure 1. The complete instrument is in one console 30 inches long, 25 inches deep, and 14 inches high, weighing less than 100 pounds. The recorder is of the flat-bed X-Y type used in the Beck man DK2, which shows the entire re corded spectrum at all times. I t re cords linearly in transmittance and wave length, on preprinted small filesize charts. An optical diagram of the I R 6 is shown in Figure 2. The source is a coiled Nichrome wire. While less lumi nous at short wave lengths than the con ventional Nernst glower, it appears to have virtually unlimited life, an impor tant consideration where complex main tenance must be avoided. Source radi ation is chopped at 5 cycles per second,
and goes into the monochromator at the entrance slit. I t is reflected at plane mirror M2 and collimated by spherical mirror Ms. I t is dispersed by the so dium chloride prism, reflected by Littrow mirror Mi, dispersed again by the prism, and imaged on the exit slit by the collimator. I t passes through the sample space, enters the thermocouple compartment, and is imaged on the thermocouple by mirror M-0. Thus the optics are very simple—only 5 mirrors, all plane or spherical surfaces. The electronics are correspondingly simple, with only 15 tubes. Most tubes are premium, ruggedized, long-life types, but standard television tubes may be used throughout if necessary. According to the designers, there is a novel patented chopper for the elimina tion of stray radiation. All previous systems have attempted to eliminate stray radiation by the use of filters. This system eliminates the effect of stray radiation by balancing the stray signal with an equal and opposite stray light signal on the dark half of the chop per cycle. The entrance slit is made
Figure 3 VOL. 29, NO. 1, JANUARY 1957
·
55 A
newITspeed
BROOKFIELD VISCOMETER
Figure 4
^^^taiiiiPllisllilllïBllË!ï*^i!i?v3iiïiiiiiP^
foods toothpaste resins
With the new portable Brookfield eight-speed viscometer y o u can, at the flick of a switch, easily measure the viscosity of any fluid . . . readings down to 1 / 5 of 1 per cent with materials having viscosity of from 1 to 32 million centipoises. If s p e e d , a c c u r a c y , a n d v e r s a t i l i t y count with y o u , demand the new Brookfield eight-speed viscometer. A complete line of accessories makes the Brookfield adaptable to any v i s c o s i t y m e a s u r e m e n t p r o b l e m . Y o u r prompt inquiry will bring complete information. Technical assistance available without obligation. Write,
wire or
coatings paint sizmgs molten glass asphalt solvents greases inks latex oils
phone
Brook field ENGINEERING
LABORATORIES, INC.
STOUGHTON 212, MASSACHUSETTS Send the facts to:.
56 A
·
For further information, circle number 56 A on Readers' Service Card, page 73 A ANALYTICAL CHEMISTRY
twice as high as would normally be needed. T h e central portion is used during t h e "Signal O n " portion of the cycle. During t h e dark p a r t of the cycle, the ends of the slit are illuminated and light from t h e m enters the m o n o chromator. This light is imaged beyond the ends of the exit slit and does not reach t h e thermocouple. However, a n y light scattered from the ends of the slit will pass through the exit slit a n d will essentially be equal and opposite to the stray radiation generated during the "Signal O n " p a r t of cycle. An example of the performance of this instrument is shown in Figure 3. I t is a double scan of the polystyrene spectrum in t h e 2- t o 16-micron range t a k e n in 18 minutes, a performance closely comparable to big instruments a t t h a t speed. T h e larger instruments can be slowed down, of course, to increase resolution, b u t this is not often done in chemical analysis. T h e manufacturers of this instrument h a v e also undertaken the design of accessories such as liquid cells, a K B r pellet holder, film, mull and plate holder, a minimum-volume gas cell, and a reflectance a t t a c h m e n t . A lens-type beam condensing system and a multiple t r a v ersal gas cell h a v e also been designed. Figure 4 is a holder for accommodating plates, films, mulls, demountable cells. Figure 6 is the liquid cell designed for t h e I R 6 . Figure 5 shows t h e rear view of t h e K B r pellet holder designed for minimum air p a t h . T h e designers of the I R 6 emphasize the extreme ease of operation. Performance is not as high as its more "sophisticated brothers," b u t is closely comparable to most of the spectrophotometers in use today, and appears t o be satisfactory for the uses to which infrared is p u t in chemical laboratories. T h e large saving in cost was achieved b y
INSTRUMENTATION
Instrument abstracts
Cary Applied Physics At E a s t m a n
Corporation/Pasadena/California
Kodak
Cary Model 14 Spectrophotometer measures absorbance to 7.1 without correction for stray light.
Figure 5 t h e e l i m i n a t i o n of m a n y f e a t u r e s w h i c h a r e nice b u t n o t essential. T h e r e are o n l y fixed c h a r t e x p a n s i o n , o n e fixed slit p r o g r a m , o n e fixed s c a n n i n g s p e e d . T i m e constant and source current are fixed. T h e s c a n n i n g s p e e d p r o v i d e d is t h e one m o s t c o m m o n l y selected on t h e adjustable-speed instruments. The o t h e r p a r a m e t e r s w e r e c h o s e n for o p t i m u m performance a t t h a t speed. While t h e I R 6 h a s been designed prim a r i l y for t h e q u a l i t a t i v e a n a l y s i s t h a t f o r m s t h e m a j o r i t y of i n f r a r e d w o r k t o d a y , i t is c a p a b l e of a c c u r a t e q u a n t i t a tive analysis, whether scanning or with cell-in, c e l l - o u t . A s p e c i a l d e n s i t o m e t e r a c c e s s o r y h a s b e e n d e s i g n e d for w o r k of t h e h i g h e s t a c c u r a c y . This development would seem to prov i d e a p o i n t of d e p a r t u r e for t h e m o r e w i d e s p r e a d u s e of i n f r a r e d . T h e latt e r h a s a l r e a d y f o r m e d o n e of t h e m o s t p r e c i s e m e a n s of c h a r a c t e r i z a t i o n . W h e n such techniques can be m a d e available a t v e r y m o d e r a t e cost, t h e y a r e b o u n d t o be used m o r e widely.
Figure 6
Run A represents the normal check of a Fabry-Perot interference filrer similar to those used in densitometry of color films where the passband and shape of the filter curve is important up to densities of 7.0. This particular filter curve sho-wed a nonsymmetrical peak ar 6 3 9 mu. For a closer examination of this peak a second expanded curve of this -wavelength region, Run B, was made in super-position on the first curve.
In t h e Laboratories of the Eastman Kodak Company interest in absorbance values over seven —less than . 0 0 0 0 1 % transmission—is m o r e than idle curiosity. Recently, Kodak physicists, using one of their Cary Model 14 Spectrophotometers, were pleased to find they could measure densities to 7.1 without correction for stray light. In contrast, there are n u m e r o u s instances where months of hard work were wasted because unsuspected stray light of single m o n o c h r o m a t o r instruments caused large e r r o r s s o m e t i m e s e v e n b e l o w 1.0 a b sorbance. D o u b l e monochromators cost m o r e to design and build. But they provide advantages that can be had in no other way. Besides low stray light, the double monochromator adds the dispersion of its sepa r a t e sections a n d is a r r a n g e d to cancel severe optical aberrations, giving increased resolution. In the Model 14, a silica prism and a 1 5 , 0 0 0 line diffraction g r a t i n g add t h e h i g h e r ultraviolet dispersion and t h e low stray light of the prism to the excellent visible and infrared dispersion of the grating. Each complements the other to produce exceptional performance from I 8 6 0 to 2 6 , 5 0 0 Angstroms. All Cary i n s t r u m e n t s are truly direct reading. Freedom from sttay light provides one of t h e most dramatic examples of w h a t this can mean to a user, but there are others. S o m e of t h e u n u s u a l f e a t u r e s of C a r y Recording Spectrophotometers which contribute to accuracy by helping avoid corrections are listed at right.
COUNTER-DIAL WAVELENGTH
SCALES-Easily
read, no interpolation, corrections negligible for most wotk. MULTIPOT-CORRECTED 100% LINES-A11 Cary S p e c t r o p h o t o m e t e r s h a v e M u l t i p o t s for c o m p e n s a t i n g s a m p l e and reference cell differences, and for compensating m i r r o r unbalances which inevitably occur in time. SPECIAL RECORDER FUNCTIONS AVAILABLE -
Log absorbance recording for qualitative — quantitative analyses. Kubelka—Munk function recording for p a p e r and textile dye work. Expanded absorbance or transmission scales for weak a b s o r b e r s o r differential photometty. ACCURATE PHOTOMETERS-Slide wires accurate to within limits of recorder readability. P h o t o m e t e r accuracy rigidly tested w i t h standard niters. H i g h power p e n m o t o r and l o w friction p e n c a r r i a g e . C o n s t a n t controlled pen d a m p i n g and fast response over entire absorbance range. HIGH ACCURACY ELECTRICAL ZEROING-Spurious electrical pickup reduced by careful design and testing. PHOTOMETRIC PRECISION TO STATISTICAL.
LIMITS—Statistically efficient p h o t o m e t e r systems, r e a c h i n g t h e t h e o t e t i c a l l i m i t s achievable with the best m o d e r n multiplier phototubes and semiconductor photocells. C o m p l e t e specifications are a v a i l a b l e o n all Cary R e c o r d i n g S p e c t r o p h o t o m e t e r s . W r i t e to Applied Physics Corp., 362 West Colorado Street. Pasadena, California, for Bulletin AC-21.
Far further information, circle number 57 A on Readers' Service Card, page 73 A VOL. 2 9 , N O . 1 , JANUARY 1 9 5 7
·
57
A
