INSTRUMENTATION Relatively simple a n a l o g computer facilitates r a p i d calculation of complete analysis of v a p o r phase c h r o m a t o g r a m E can expect t o see a more wideO Nspread use of analog computers in connection with analytical i n s t r u m e n t s or incorporated directly in t h e instruments. T h e advisability of doing this does n o t arise from a desire t o m a k e things still more complicated a n d expensive b u t from t h e simple fact t h a t in m a n y of our instruments it requires more time a n d effort t o calculate t h e final results t h a n t o collect t h e data. F o r a fine example of this sort of thing we a r e indebted t o Wladzia G. Podbielniak, executive vice president, a n d S. T . Preston, technical director, of Podbielniak I n c . , 341 E a s t Ohio St., Chicago 1 1 , 111. T h e Podbielniak C o m p u t a g r a m is a relatively simple analog computer which makes it possible t o calculate a complete analysis of a v a p o r phase chromatogram in less t h a n 3 minutes. T h e component fractions are accurate t o 0 . 1 % . Peak areas or peak heights are corrected b y the computer for differences in t h e r e sponse of t h e detector a n d t h e comp u t e r normalizes t h e corrected areas or heights t o total 100, thereby giving t h e per cent (mole or weight according t o correction factors inserted) of each individual component. T h e C o m p u t a g r a m is shown in Figure 1 as t h e smaller cabinet t o t h e right of a Podbielniak Chromacon Series 9580. W h e n a n analysis is m a d e b y gas c h r o m a t o g r a p h y , t h e information is presented in t h e form of a chromatogram. T o compute a q u a n t i t a t i v e analysis from a chromatogram, t h e d a t a presented on t h e chromatogram must be converted t o mole, weight, or volume per cent. Quantitative results m a y be based on t h e use of peak areas or peak heights. Compositions based on peak areas a r e generally preferred because peak heights are markedly influenced by operating conditions. P e a k areas m a y b e obtained graphically o r t h r o u g h t h e use of mechanical or electronic integrators. Rosie and Grob have reported [ A N A L .
by Ralph H. Müller
for t h e variation of t h e response of t h e detector t o t h e individual components. A comprehensive list of relative r e sponse factors (calibration constants) has been published b y Messner et al. [ A N A L . C H E M . 3 1 , 230 ( 1 9 5 9 ) ] .
The
calculation of a precise quantitative analysis is accomplished in the following s t e p s : 1. Determination of the areas under each peak of t h e chromatogram. 2. Multiplication or division of each peak area b y a calibration constant which is unique for t h e component causing t h e peak. 3. Normalizing t h e corrected peak areas t o give t h e p e r cent of each component (mole, weight, or volume per cent, according t o t y p e of calibration constants u s e d ) . M a n y of t h e gas c h r o m a t o g r a p h y instruments n o w in use a r e equipped with integrators which give a rapid direct reading of t h e observed peak area for each component. T h e P o d bielniak C o m p u t a g r a m is designed t o accomplish t h e last t w o steps in t h e calculation of quantitative results— namely, t o correct peak areas for t h e variation in response of t h e detector to t h e individual components a n d t o normalize t h e results t o total 1 0 0 % .
Three Models Available C o m p u t a g r a m s are available in three models, differing only in t h e n u m b e r of components which they can handle. T h u s t h e Series 4075-4 (Figure 2) is designed for simple mixtures having four or less components such as liquefied petroleum gases. Models 4075-8 and 4075-12 are designed t o handle u p to 8 or 12 components. The principle of operation is best described in t e r m s of Figures 2 a n d 3. The schematic diagram of Figure 3 shows a n a r r a y of calibration constant potentiometers Clt C2 t o C 1 2 (broken line), each of which is shunted b y a n area potentiometer. Neglecting loading factors (which are corrected for) such a n a r r a y provides a n o u t p u t which is t h e product of t h e respective potentiometers—i.e., G^
X
^ 1 ? ^2
X
2
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These product o u t p u t s a r e fed collectively or separately t o a summing a m plifier. T h e latter presumably is a series of cathode followers with a common resistor. If t h e s u m of all outputs is adjusted t o 1 0 0 % , t h e comp a r e n t selector switch can then b e switched t o read t h e percentage of each component. I n t h e four-component ins t r u m e n t Model 4075-4, t h e digital
C H E M . 29, 1263 (1957)] t h a t t h e r e -
sponse of a thermal conductivity d e tector is unique for each individual component of a mixture. This finding h a s been confirmed in a n u m b e r of laboratories a n d the calculation of p r e cise quantitative results requires t h e correction of t h e observed peak areas
Figure I VOL. 3 1 , NO. 7, JULY 1959
·
67A
INSTRUMENTATION
Transmission Type Fluorimeter Model
500
read-out potentiometer is balanced manually until t h e galvanometer reads zero. I n t h e eight- a n d twelve-com ponent instruments, this operation is entirely automatic and as each com ponent is interrogated a servo-system effects a balance and the percentage appears directly on the digital read-out potentiometer. Experience h a s shown t h a t the compositions calculated by means of this computer agree t o within 0 . 1 % with the composition calculated b y means of a - d e s k calculator. T h e potentiometers used for peak areas must have a linearity of 0.05%. T h e effect of ''loading" on t h e potentiom
eters used for calibration constants is eliminated b y t h e use of t h e read-out potentiometer for setting the calibra tion constants into t h e instrument. Because of the effect of loading on these potentiometers, no digital dials are used on the calibrating potentiom eters. I n repetitive analyses, t h e cali brating pots are set once and presum ably need only an occasional check. Although t h e word " c o m p u t e r " is a p t to suggest a multimillion-dollar installa tion, this example shows t h a t relatively simple analog devices are i m p o r t a n t a n d useful a n d they find increasing use b y the modern analyst.
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Applications Geochemical prospecting Uranium ore assay Control in processing and milling Uranium waste disposal investigations Urinalysis for uranium Distinctive Direct
Features
reading
Digital dial
Figure 2. Computagram Model 4075-4. ( A ) Digital area dial. (B) Calibra tion constants. ( C ) Meter sensitivity. (D) Calibrate-run switch. (E) Meter galvanometer. (F) Digital readout dial. ( G ) Component selector switch. ( H ) Power supply
read-out
High sensitivity Excellent High
linearity
resolution
Self-contained a n d compact Low
background
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Range: From less than 1 0 - ' grams to 10~ 5 grams of Uranium.
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BALL BROTHERS RESEARCH CORP. COLORADO
Circle No. 139 on Readers' Service Card
68 A
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ANALYTICAL CHEMISTRY
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