Gas chromatographs (concluded) - Journal of Chemical Education

Examines the read-out, zero suppression, read-out of peak areas, ball-and-disc ... digital integrators, and various commercial, laboratory gas chromat...
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Chemical Instrumentation

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5. Z. LEWIN, N e w York University, Washington Square, New Yo& 3, N. Y.

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The bucking-out of a known part of an input signal by means of a. calibrated reference voltage is what is involved in the electrienl sera adjustment referred to above; this is also sometimes called "zero suppression." Zero suppression circuitry is incorporated in some recorder models, and in the detector output circuitry of some gas chromatographs. For those cases where this type of circuit is not present in the equipment, i t may be added hy the user as a separate stage between the dotertor and the recorder.

his series of articles presents a survev of the basic principles, characteristics, and limitations of those instrtrments whieh lind imwortnnt applications i n chemical work. The emphasis is on ~ o m m e r ~ i aavailable ll~ equipment, and approrimate prices are rluoted to show the order of magnitude of cost of the various types of design andconstruction.

15. Gas Chromatographs (Concluded) The Read-Out The outpubs of the detect,ors employrd in gas chromatographs are suitable for rend-out an standard self-halancing potentiometric recorders. Most comm~rcial gas chromnt,agraphs include a recorder as part of the device, hut the rerorder is not in general any different in design from that. of other servo-balanced recorders. Sevcral recorder manufacturers offrr models of their instruments that are designated as "chromatography rrcorders." Thus, e.g., the MinneapolisHoneywell Regulator Ca., Philadelphia 44, Pennsylvania, offers their Chromatography Recorders Models 143X and 153X, m-hich are simply 1-mv full-scale instruments sibh pen speeds of 1 or 2 sec for R111-scale travel, and chart speeds from 20 to 120 in: per hour. Leeds and Northrup, Philadelphia 44, Pennsylvania, offers hoth its Model G and H recorders for gas chromatograph use; the latter has a I-mv span and 1-sec pen speed, and the former has in addition a continuom adjustment of electrical zero between the limits of -50 and +50 mv. The Wheelco Instruments Division of Barher-Colman Co., Rorkford, Illinois, makes recorders suited for gas chromatographic application that are designated the 8000 Series recorders (e.g., Model 27,000 1-see pen speed, span from -2.5 to +50 mv, S1040). I t i~ very common for s. gas ehromatographic run to involve the recording of hoth small and large component peaks. If t,he recorder sensitivity is such ss to yield reasonable pen deflections for the small peaks, the large onos ma." then markedly exreed the full-scale range and drive the pen off-scale. If the recorder span were chosen to keep the large peaks on-scale, then the minor peaks would correspond to such small pen deflertions that they could not he read from the chart with adequate precision. Hence all gas chromatographs provide means for adjusting either (1) the magnitude of t,he detector output signal by the use of shunting resistors, (t)the sensitivity (i.e., span) of the recorder, or (3) the electrical rera of the recorder. With most GC instruments, these adjustments are made discontinuously, by the operator, as he watches the chromatogram being traced; in some instruments, the circuitry is de-

feature

signed so that the recorder sensitivity automatically adjusts itself in aceordanrc mith the magnitude of the input signal. Lho.lt*m.h

Zero Suppression The use of an electrical-zero sdjnstment permits the maximum recorder sensitivity to he employed with all magnitudes of input signals; hence, the relative precision with which large pealis can he read is much greater than is the case ii the recorder sensitivit,y is reduced to whatever value is required to keep the peak on-scale. This point may perhaps be better appreciated in terms of the fallowing numerical illustration. Suppose a 17-mv signal is to be measured on u recorder having ranges of 0-1, 0-10, and 0-25 mv. If it is introduced directly into the recorder, the reduccd sensitivity range, 0-25 mv, would have to he used. On any range, the precisionw ith which the pen deflection can be read, assuming a somewhat noisy system, is spprox. i l % of full-scale, i.e., zt1 box (or ruled line) on chart paper having 100 bones (lines) per full span. Hence, the recorder would show 8, defletcion 17/25th of full-scale, or 68 hams (lines) of the chart paper (if full-scale deflection corresponds to 100 boxes or lines of t,he chart oaoer). . The precisian of the measurement is 1 part in 68, or 11.5%. However, if the 17-mv ~ i g n ais l opposed, or "bucked" by a constant reference voltage of. e.g., 10 mv, then only the excess over this reference value is to be measured by the recorder. The 0-10 mv span can be used, and a. deflection of 70 boxes (lines) would be observed. Since there is negligible uncertainty in the knowledge of the 10 mv that has been "bucked-out? hy the calihrated voltage reference, the only uncertainty in the total 17-mv signal is that of 1 part in 70 involved in reading the last 7 mv. That is, t,hc effective precision of the knowledge of the total signal is 1 part in 170, or 10.6'%. Similarly, if the first 16 mv of the signnl is bucked-out by a reference voltage, and the remainder is fed into the recorder nsing the 0-1 mv span, the effective precision of the determination of t,he total signal would he 1part in 1700, or ztO.OG%.

..

w~dfhat hall-

Figure 45. Appeoronce of the chart record obtoined with the Qvintran Model F Range Extender. When the recorder signal reaches the end of the 1 mv recorder span, the selector switch ir mmualiy speroted. This bucks out 1 rnv of the input signal ond reverses the polarity of the input, so that the next 1 mv of the signal driver the pen down-scale. When the downscale limit of the recorder is reached, the switch is odranced again, ond the pen once more reverses its direction. In this manner, a 3.4 mv xignal is fully recorded on o 1 mv span recorder.

A commerciallyavailable unit that serves this function is theRangeEntender(587.50) manufactured hy Quintron Instrument Co., Milwaukee 18, Wisconsin. This device contains a mercury battery, precision resistors. and a selector switch to provide several known hias voltages. The bias voltage is applied in increments to the input signal, the increments being of such magnitudes as to suppress the input voltage in stops exactly equal to the span of the recorder. Whenever the input signal has inrreaaed enough to bring the recorder pen to bhe end of the chart, the operator advances the selector switch one step. The Model F instrument of this supplier is specifically designed for use with a gas chromatograph. In this model, each increment in the bias voltage is accompanied by a reversal of the input connections, so that the direction in whieh the pen is driven by an increase in the signal reverses. This causes the recorder trace t,o he folded back and forth on the chart, as is illustrated in Figure 45, and permits the pen to give a n uninterrupted record of the chromatographic peak.

Volume 39, Number 3, Morch 1962

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Read-Out of Peak Areas -4lthough peak heights may bc cn~playcd for the estimation of the amounts of tho individual components resolv~d in the effluent stream, the best quantitative 8eeurac.y and precision are at,tained if the parameter used is the total area comprised within the envelope of the recorder pen response curve during the t,ime n given component is breaking through. Pcak areas are commonly determined by (1) estimation, based on the assumption t h a t the area under the peak is approximately equal to that of n hounding t,riangle, and hence is given by: (peak height) X (half-width); ( d ) manual rounting of the number of bores of erosssection (graph) paper contained within the

designatctl portion of the recordcr pcn tracing; (3) cutting out and weighing t,he chart paper enclosed by the chromatographic peak; (4) mechanical measurement, hased upon a. manually-operated planimeter; (5) analog computation, based upon a. rnechrtnicsl or electrical dcvire t h ~ generates t a, function which is proport.ionn1 t o the desired area, and (6) digital computation, based upon eonv~rsionof pcn displacements into digital information, which is then operated upon hy computer techniques to give the required read-out. The first three methods cited above are simple and straightforward, but of limited accuracy [(I) and ($)I, 01 very tedious [($)I. The planimeter is a geometric device consisting of n rod, the tracing arm,

an one end of which is mounted thc tmcing pen nhich follows the curve. The ot,her md of this rod is constrained by a mechanism to move on a straight line (in the rase of a lineav planimeter.), or in n circle (the polar planimeter; the motion o f this end of the tracing arm is controlled hg a second rod, the pole a m , which is fixed in position s t one end, the pole, and is joined a t its other end to the tracing pen holder. For a, description of the theory of the planimeter, and the various designs available, see Willem, F. A,, "Pmct~ieal Analysis," Dover Puhlications, Inc., N. Y., 1848, pp. 191-204. .4 typical polar planimeter is available, e.g., from the Burrell Corp., Pittsburgh 18, Pennsylvnnia. (8105). An electronic planimctrr, in which the traring arm is linked t o tho sliding contact of a vnrinhle r~sistar,so that the tracing of a rurve with this nnn generates a n electricnl sign81 whirh is int,egrat,ed electronically, is available from the Royson Engineering Ca.. Hathoro. Pennsvlvania (Planimeter for 4-in. charts, with variable s p d motor and foot pedal control of chart feed, $5'15).

Ball-and-Disc Integrator A widdy-uscd type of analog romputor t h a t is now incorporated in a number of gas chromatograph read-out systems is the hall-and-r1i.q~integmtor, the principle of nhich is illustri~ted in Figure 46. h smooth hall rides on the surface of a rotating disc. The hall is held a t a given

Figure 46. lllvrtroting the principle of the balland-disc integrator. A ball held ot different places on the diameter of a rototing disc maker o number of revolvtionr per unit lime thot is proportional to the dirtonce of the ball from the center of rotation of the disc.

point on the diameter of the disc, constrained to remain st %hatplace. As the disc rotates in t,he horizontal plane (in t,he figure), the sphere is caused by the frietional or adhesional force between i t and the disc to rotate in a plane perpendicular to the plane of the disc. The number o f revolutions per second exeeubed l,y the hall is proportional t o the distance of its position from the center of rotation of the disc, a s shoan hy tho porimrtcrs of the (Continued on page A164)

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circles in the figure, which trace the total distance traveled by a point on the disc in each revolution. In a. hall-and-disc integrator, the disc is rotated a t a. constant rate, thus generating t,he time function, and a hall is positioned on the disc by means of a mechanical linkage to the signal (c.g., the position of a. recorder pen) which is t,o he integrated. The rotation of t,he hall is communicated mechanically to a cylinder, as shown in Figure 47. The totnl number of revolutions made by this output cylinder is then proportional to the integral of the signal as a. function of time. Hence, if the rate of chart feed of the recorder is linear with time (as it is in gas chromatographs), the total number of revolutions will give the integrated men under any desired portion of tho curve traced out by tho recorder pen. A mcthod of coupling a ball-snd-disc intogrator to a re&& is shown in schematic dirtgram in ~i~~~~ 48. ~h~ of a output cylinder turns tho switch which reverses tho dircction of

Figure 47. Construction principle of the Reflectone boll-md-disc integrator, in which the output is token from the retotion of a cylindrid shaft riding on a pair of captive rpherer

mowment of a "pipping" pen after rnrh complete revolution of the hall. Thlts, (Continued on page A I f i 6 )

Figure 48. Schematic diagram of the linkage of a recorder pen to a ball-and-disc integrotor, the output of which operates 0 pipping pen on the recorder. This design ir employed b y Disclnrtrumentr. Inc

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t,he total number of revolutions of the ball is recorded in terms of the total number of "pips" made by the auxiliary pen along tlrr hase of the recorder chart.

Figure 49. Schematic diagram of the design of the Model 26 Square Root Integrator mode b y Libraxope, inc., division of General Precision Equipment Carp., Chicago 11, Illinois. The input shaft positions the ball on the lower disc in proportion to the rquore root of the input signal. The upper disc is the output component. ond its revolutions ore used both to give an indication on a digitel register, and to operate o magnetic puke witch.

Rather sophisticated types of integribtions can be performed with a hall-snddisc integrator. For example, Figure 49 shows a device for obtaining the integral of the square root of the recorder signal as a function of time. In this case, the input linkage positions the ball along bhe disc diameter in proportion to the square root of the signal. In this integrator, a second disc rides on the hall, serving as the output component. Its rotation turns the shaft of a digital indicator t o show total counts, and it also rotates a magnetized disc which causes a pair of switch contacts to close once each rotation, ta give an electrical 'pulse signal which can be used to operate a pipping pen. A square root integrator is useful in flowmetering a p plications, for with certain types of Bowmeters the total amount of material passed through the meter is given by the time integral of the square root of the signal. The ball-and-disc integrators made by Reflectone Electronics, Inc., Stamford, Connecticut; Disc Instrument Co., Santn. Ana, California; and Lihrascope, Inc., Uurbank, California, are available in kit form, for attachment to existing e q u i p ment, or are included in the original equip ment available from GC manufacturers when this type of signal integration is specified. The advantage of the hall-and-disc integrator is that it is a relatively simple and inexpensive device for performing this type of computation. The accuracy may be of the order of 10.1%. However, since i t is ~rimarilva. mechanical device.

excessive shock or vibration, etc. Eleclromechanical Integrators

An approach t o anallog computation of areas that is less dependent upon precke mechanical construction and maintenance of t h function ~ generator is the w e of an (Continued on page A168)

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elertromeehanical integrator. This involves bnsicrtlly a low-inertia dr motor, so designed that the rate of rot,atian of its armat,uro is linearly proportional to the input voltage applied t,o tho drvirc. If the motor shaft is ronnrctrd to a d i & d register, tho countz pcr rninuto reistrred is proportional to tho input volt,nge, and tho tot,al count over a period of timc is th? time integral of the counting rate. A variable rosist,or, in a pot,entiomcter or \XThentst,onchridgo circuit, is mrrhnnically linked to the self-halxneing slidewire contsat of the recorder, so that the posibion of the rerordrr pen a t any inntnnt is trnnsl~tedinto n potentiometer or hridgc urnhalanee voltage that serves as the input to the motor. For a. description of the rhararteristios and application of motor-counter units made by Elertro Methods, Ikd., Stevmage, Herts, England, see Dal Sagnre, S., Bennett, C. E., and Harden, J. C., in "Gas Chromatography,'' Ae~clemiePress, N. Y., 1958, pp. 117129. A more sophisticated approach to thp utilization of n voltage-smsit,ive mot,"? in analogue computation of arms is embodied in the Jackman-Vnn Winkle Integrator manufactured hy Rpsearrh Appliance Go., Allison Park, Pennsylvania. A block disarsm of the design principle is shown in Figure 50. Thc input signal is a voltage derived from the poait,ion of a recorder pen, as dcsrrihed ahow. This signal is amplifird in n difference amplifier (see helax), and the out,pllt of the amplifirr causes a servo-motor t,o rotate. This serva-mot,or drives a dr

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4

mwc* rn

SERVO

COY*,EIl

AWUF,E"

7.C"O"ETE"

Figure 50. Block diagram of the design of the Model 458 lntegrotor made b y Rerearch Appliance Co. The speed of the servo-motcr is c~ntrolledby the amplifier so or to cause the vcltage output of the tachometer to b e olwoyr equal to the input signal.

arhomcter, which is essentially also a volt,apc-sensitive mot,or in design, but iu which the meehnnicul rotation of the armature is the input, and a voltage is taken from across the brush contacts as the output (i.e., this is t,he inverse of motor operation, in which n voltage is the input, and t,h? armature rotation is t,he output). The output voltage of t,he tachometer is fed to the amplifier, where it is eampared with the input signal, and the vokage difference is applied to the servo-motor to adjust its s p e d to such a magnitude as to rrducr this voltage difference to zero. Thus, the spwd of t h serva-motor ~ is cantinuously compared to and made linearly proportional to the input signal. The rot.ation of the servo-motor is used as one (Continued on page A I 7 0 )

input to a mechanical differential, tho other input of which oomos from a synchronous motor. The output of the differential is coupled to a counter or impulse generator, and the total number of counts recorded gives the time integral of the input signal. Counting rates up to 1000 per min can he recorded with an accuracy of +0.15%.

A signal is obtained from the pen position in a recorder as described above. This signal drives a voltage-sensitive motor of the type used in a watt-hour meter, and in nrhirh the motor shaft turns a metal disc. Suitable orifices are cut in the disc, with a light source mounted on one side, and a cadmium sulfide photocell on the other side of the disc. Each time an

Figure 51. Block diagram of the Model PX-590 Electronic Integrator of the Ridgefleld Instrument Group. A voltoge rignoi from the recorder i, trmnsformed b y meonr of the omplifierr into o proportional current which serves to charge the memory condenser, 4. The total stored charge, which gives the time integrd of the input signal, is remd out upon command. [For an explanation of the types of ampliners represented in this and rubsequent diagrams, see Lewin, S. 2.. Anal. Chem., 34, 25A (1962).]

Another approach to elcctromeehanical integration is exemplified in the Wheelco Electronic Integrator, made by Barber-Colman Co., Rockford, Illinois (Model A, without digital register, $700).

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orifice in the watt-hour disc comes into line with the light source and photocell, a pulse is created in the photocell circuit. The orifices are sized to produce pulses (Continued on page A172)

of three different lengths, representing units, 10's, and 100's of counts. These pulses are amplified, and may be used to drivo a pipping pen, or for other purposes. This unit is capable of recording counting rates up to 2400 counts per min with an accuracy of I I '%.

All-Electronic Integrators Several instruments are now available in which there are no mechanical components involved in the integration, all functions being performed by electronic circuitry. The Electronic Integrator made by Ridgefield Instrument Group, Rehlumborgor Corp., Ridgefield, Connecticut (Model PX590, $450) is based upon the fact that the total charge stored up on the plates of a condenser is proportionxl to the time integral of tho current flawing into the condenser, Figure 51 shows a block diagram of the basic circuit. When the switch is in the "integrate" position, a voltage signal is being takcn from the recorder that is proportional to the pen position (just as in the cases troated in the preceding sections). This voltage is converted by means of the amplifiers into a proportional current which charges the "memory" capacitor, C , . When the desired portion of t,he recorder chart record has been scanned in this way, the switch is thrown to the "read" position, and the total charge on the condenser is applied as an input t,o the recorder, causing t,he pen to deflect to a. point ~.hich measures the integral of the preceding portion of the chart record. Thus, the

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integral appears as a bar graph on thr samo chart as the direct signal, as is illu~trntedin Figure 52. The accuracy of the integration is I0250jo of full scale. This mnnufacturer also offers an integrator of this type which sutomatically provides for the recording and integrxt,ion of signal amplitudes that would normally execccl tho full-scale span of the recorder. Tho Modcl PS593 Atten-U-Matic Integrator has an electronic sensing device that co~nrnandsthe recordcr attenuator t,o reduce the recorder sensitivity and eirnultaneously increases the integrator scnsitivity, whenever the recorder pen rearhes

Figure 52. Appearonce of chart recard when the inlegrotor of Figure 51 ir connected to Q standard recorder. The pips dong the upper edge of the chart indicate the paints a t which the "reset" witch has been closed to return the copatitcr to its state of zero charge.

a pre-selected point ncar the upper l m i t of the sealc. .4n electronic integrator hased upon a (Continued on page 9177)

Chemical Instrumentation different principle is the Lectracount ($845) made by Royson Engineering Co. A block diagram of the design principle of this instrument is shown in Figure 53. The difference between s n input voltage and a feedback voltage is impressed upon the input of a, high-gain ehopper-type amplifier. The output of the amplifier is converted into a current proportional to the original input signal, and this current is caused to charge up a capacitor in the storage circuit. When the charge accumuleted on the capacitor plates reaches a pre-selected value, it causes a thyratran tube in the discharge circuit to conduct, thus generating an electrical pulse which is then shaped in the squsrewave generator oircuit and is registered on the counter. The rate of pulsing of the thyratron is proportional to the input signal; hence, the total number of counts recorded gives the time integral of the signal. Digital Integrators Chll~lnrd lnsrrumtrlt Cw, t , i k l , d I, Califorma, produrrs nu Tuttmator t hIodrl I, l 9 , I M l ) ~ntended for rnc repetitive scanning of large numbers of chromatograms with the read-outs tabulated and punched into IBM data-processing cards. The operator draws on the recorder chart which has been removed from the chromatograph the base lines under each of the peaks of interest. This chart is then inserted into the Integrator. A light source illuminates the chart, and by means of a rotating spiral mask in front of a fixed aperture, a photomultiplier tube scans the chart from base line to fullacde and back again, continuously. Each time the scanning spot crosses the base line, and each time it crosses the recorder pen trace, pulses are generated in the phototube circuit. These signals are used to gate a 20-kc clock pulse on 6nd off. A register records the number of clock pulses in each traverse of the chart, and the total number of pulses so recorded as the chart advances a t a constant rate gives the area. between the starting and stopping points. The chart drive speed is 13.3 in./min, the phototube makes 600 scans per mi", and the over-all accuracy is 2 ~ 3 % . The Infotronics Corp., Houston 27, Texas, makes a. digital integrator in which the input signal is converted into s. train of pulses, but not involving optical scanning as in the instrument just described. In the Model CRS-1($4400; digital printer is an additiona1$800), the frequency of an oscillator circuit is controlled by the input voltage signal, so that the number of pulses put out in afixed time interval by the oscillator is proportional to the input signal. These pulse trains are counted by a sixdigit electronic totalizing counter; the total count is proportionel to the time integral of the input signal. Logic eircuits are provided for storage, retrieval, encoding, and resd-out of the digital information. Instrumentation for the conversion of (Continued on page A178)

an analog variable (such as recorder pen position) to digital data is also available from Datex Corporation, Monrovia, Cslifornia. A digits1 printing integrator is made by Perkin-Elmer Corp. (Alodel 194, $1545).

Square Wave

Generator

Filter

Current Rerrulatar

Amplifier

Counter

Laboratory Gas Chromatographs At the present writing there are over thirty different instrument firms each of which offers several models of gas chromatographs. Since the preceding discussion has treated the general principles of gas chromatographic instrumentation, the paragraphs that follow will round out the survey of this topic hy summarizing the main features of the various commercially available instruments, considered as complete systems. The ehnr-

H

Discriminator

Feedback Resistor

Storage & Discharge

Feedback Current

Flgvre 53. Block diagram of the design principle of the Royron Lectrocount. A condenser in the storage section charges up to a preselected value, whereupon it murer a thyratron tube to pulse. rate is linearly proportional to the magnitude of the input rignol. The

scteristics of individual com~onents of t h e v:,riou.i insrru~.s.nrsIIHW ielwsdy ~ W C I . trwtvd m I I W prmwlmg stmitms, :$mi w i l l not be described again. Beckman Inslrumnts. This manufacturer offere four different systems, designated as the Model GC-1, GC-2A, ThermotraC, and Megachrom. The IModel GC-1 ($935, without recorder) is a s i m ~ l eunit. based on s thermal con-

Figure 54. Flow diogrom of the Beckmon Model GC-?A Gas Chmmotograph (right hand ride) with Hydrogen Flame Detector unit anoched (left hand ride, plus burner in right hond lidel.

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corder) shown in schematic diagram in Figure 54, provides a variable heater with proportional control to allow the selection of any operating column temperature between 40" and 240°C. A thermal conductivity detector is supplied (Continued on page A180)

in this instmnent; if desired a hydrogen flame ionization detector ($1385) can he inserted in series with the T/C cell, as shown in Figure 54. In this rhromatograph, the carrier gas, after passing through s, pressure regulator valve, is divided into two streams. Each stream passes through a capillary orifice; one stream then goes to the reference side of the detector cell block, and the other stream goes to the column. This arrangement orevents anv chanees in messure

detector. The pressure regulator is upstrcsm from the capillaries, vhere the gas pressure is higher than i t is beyond the capillaries; the regulator operates more efficiently at higher pressures and this arrangement permits flow rate eontral to 1 0 . 2 7 0 from 10 to 200 cc per mi". A transistorized, chopper stabilized dc power supply with a vokage drift of less than 2 mv per hr is employed to energize the T / C cell bridge circuit. The ThemotraC ($1425) is a temperature-programmed gas chromatograph column. The instrument requires the attachment of a regulated carrier gas flow, a detector, and a. recorder in order to permit i t to function as s, complete GC system. The ThemotraC contains two columns; a reference stream Rows through one and into the reference side of the T/C detector, and a sample stream flows through the other. The use of balanced columns eliminates drift in the base line

due t o the change in column temperstore created during the run by the temperature program. The programming principle employed has already been described. The Meachram, shown in diagram in Figure 55, is designed for the resolution and collection of large samples (up t o 100 times larger than are appropriate for the instruments just described). The column s y s t ~ mconsists of eight U-shapd

6-ft long, 0.63-in. id columns, connected in parallel, or series-parallel. This arrangement maintains a column diameter that is small enough to give good resolutian, yet it provides the large total crosssectional area needed to handle large samples. The carrier gas is recycled after being cleaned by passage through two cold traps and an ahsarbentrfilled trap.

Figure 55. Flow diagram of the Beckman Megochrom gor chromatograph for preparative scale leporationr.

Perkin-Elmer Carp. One of the first, and most popular of the commercial gas chromatographs was the Perkin-Elmer Model 154. This was a simple system, involving a. direct How path from the helium tank through s. pressure regulator and How meter t o the reference side of the T/C detector block, to a sample injection valve, to the column, and thence

Figure 56. Physic01 l o y w t of the Perkin-Elmer Model 154 rerier of gar chromotographr. The right hond half of the main unit contoinr the gas Row and eiectricol mntrolr; the left hond half contains the column, oven and detector aaem. blier. Both a pocked column (long, U-$hoped column with a jacket heater), and o Goloy column are rhown mounted in the oven. The packed column is onached la a lhermol conductivity detector [lowert cell block); lhe G o b y cdumn to a Rome ionization detector [upper cell blackl. The amplifier for the Rome ionization cell is rhown in right foreground.

(Continued on page A185)

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Chemical Instrumentation t o the snmplc sido of a thermist,or T/C detector cell. Several models of this basis instrument have been brought out, and thc ourront versions are t,he 154C and 154D. Figure 56 shows the physioal appcaranco of these gas ehromatopraphs. The column is mounted vertically in a well-lagged air bath. The detector hlock is a t the base of the column, and the liquid ssmple injection port is in front of the detector block. T h e fipuro shows both n packed column and a Golay column in tho oven, n i t h a flame ionization detector block above the T / C detertor. The Model 154D (51050, without recorder, $4545 complctc wit,h flame ianizntion detoctor and rccordor) is specifically designcd for the Golay type column and flame or beta-ray/argon ionizat,ian detector. A flow diagram of this instrument is prcscnted in Figure 57. I n CIPILWI1" COLUUN

P ~ E ~ S U ~ E RCTUUTO'I

Figure 57. Flow diagram of the Perkin-Elmer Model 154-D gas chromatograph with Rome conductivity detector.

order t o adapt canventioni~lfiyringo torhniqucn t o the small snmple size requircments of tho Golay column, x T-joint is introduced in the flow path beyond the sample injection hlork, and R. pre-selected fraet,ion of the total sample is dlolved t o escape through a. needle orifice, so that only s suitahly small portion of t,he original sample goes t o the column. Oven temperature is thernmstatienlly controlled from ambient t o 225°C. The Model 154-I. instrument ($1150, without recorder) is intended t o be a. simple, low-cost gas chromatograph. It, emplayn a thermistor T / C detector; thc oven tcmpcraturc is not thrrmostaticnlly canbrollod. Other mod& of this manufacturer are the Model 208 (designed for maximum resolution or speed n i t h Golay columns and flame ionization detector): Model 22ti (linear temperature programmed, Golay or packcd columns): Model 188 (triplo stage instrument, in which three columns, detcctars, and ssmple systems can bc combined in series or paallel for special problems, 8i450). Also available is the Model 222 Linear Temperature Programmer, which sends a controlled currcnt directly through t,hemetal surfaces of the chromatographic column t o give rapid and precise hoat,ing and programming of the column temperat,ure. This (Continued on pare A186)

Programmer can be attached to sny of the o n e or two-meter stainless steel calumns made by Perkin-Elmer. The available heating rates range from 2O to 2fi°C per min. Loe Engineering Co. A complete line of gas chromatographic equipment is manufactured by Loe Engineering Co., Altadena, California. The simplest instrument is the Model 2 (5545, without recorder), which is designed for ambient temperature use. Model 1A (51175, without recorder) h a manual temperature control of the column up to 225%; Model 11 ($1495, without recorder) has nutomat,ir proportional control over the

Figure 58. Flaw diagram of the Loenco Chr0mat-O-Flex, Models 1 A or 11, set up for dual column operation and back-flushing of one column while an onolyrir is being performed on the omer

same range. The latter two instruments may employ single or dual eolumna; one arrangement with dual columns is illustrrtted in Figure 58. The advantage of dual columns is the flexibility made possible for the user. They may be used in series, and may have different packing8 and different lengths, as dictated by the characteristics of specific samples to be analyzed. Or, they may he used alternately, as indioated in the figure, so that one column is being used for an analysis while carrier gas is Bushing out residual substances from the ofher column by being sent through it in t,he opposite direction to t,he direction of an analysis. A high-temperature, programmed gas chromatograph is the Model 17 ($5200, without recorder), shown in Figure 59. This inst,rument contains two equivalent columns damped to the same metal heating plate. When column temperature is changing steadily, as is the case in programmed temperature gas chromatography, the vapor pressure of the liquid phase present on the solid packing varies markedly, and this affects the output of the detector, particularly a t temperatures in excess of 200'C. The use of balanced oolumns permits these effects in the reference and sample channels of t h ~ instrument to he made equal, and hence t,he base line drawn by the recorder remains constant throughout the program. Thp Model 17 is designed for operation up (Continued on page A190)

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Chemical Instrumentation to 500°C, and utilizes either tungsten or iridium filament t,hermal conductivity d~tectors. This manufacturer also offers a line of T I C detector cell blocks, a. proport,ional temperature controller based upon control of a thyratron power supply by thp unbalance signal from R. Wheatstone bridge, with thermistor sonsor (Model 60, $235), a programmed temperat,ure column control unit (Model 22, %1500),n flame ionization rhronlatograph (Model 15F, $1612, h-it,hout recorder), and other models, including a preparative scale instrument. F. and A[. Scienlij% Corp. One of the major manufacturers in t,his field is the F. and 11. Scientific Corp., Avondnle, Pennsylvania. This company numbers among its earlier models several isothermal chromatographs; viz., Model 119 (column temperature from ambient t,o 250°C; $895 without recorder); Model

17A (all-glass, to 100DC); and lModel 124 (stainless steel, to 500°C). Its principal production now consists of linear pro-

300°C, tungsten filament detectors, 61j00 without recorder); and Model 500 (to 500°C, tungsten filament detectors. $2300 without reoorder). The column, detector, and sample injection port sect,ions in these instrument are- hoated by individual heater elements, and hence can be independently controlled. Figure 60 shows the Model 500 instrument equipped wibh the Model 1609 Flame Ionization Detector Attachment, which places the lstttter detector in series nith thc T / C detector. An instrument specifically designed around a flame ionization detector is the Model 609 ($1950 without recorder) which may he operated either isothcrmslly or programmed from ambient t,o 300°C. This lnanufacturer also offers R linear programmed temperature controller (Model

Figure 59. Physical layout of the Loenco Model 1 7 Programmed Temperoture Gar Chromotograph. The corn-programmed temperature controller is a t the bottom, and the duo1 chromotogrophic columns ore mounted in the top comportment.

40, 18 linear heating rates from 0.29' to 42°C pw minute over the range &5OOPC, 9795); an automatic attenuator for recorder span adjustment to keep peaks anscale- ($250); and other accessories including sample splitters and fraction oollectors.

Figure 60. Phyricol layout of the F. and M. Scientific Corp. Model 5 0 0 progrornmed high temperature gas chromatograph with the Model 1 6 0 9 Rome ionirotion detector attachment loutlined in white).

Fishel Scientific Co. The Fisher-Gulf Partitionera are simple, unprogrammed, thermal conductivity detector instruments similar in design to several alreedy described. Model 160 (81350, with recorder) (Conlinued on page A I 9 2 )

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has a thermostated range from 35' to 160DC, is equipped to use two columns in parallel, and has thermistor sensors. .Madel 300 (83i50, with recorder and integrator) has a thermoststerl range- up ta 300DC. A preparative-scale instrument, the Prep/Partitioner (82i50, without recorder or gas regulator) is shown in Figure 61. The thermoststed tempern-

Figure 61. Portitioner.

Phyricd layout of the Fisher Prep/

ture range is from ambient to 150°C. The column is a single one, and is eompasod of a numher of I-in. id glass tubes connected in series by means of lengths of '/An. od Teflon tubing. Provision is not made for recirculation of the carrier gas; hence, nitrogen is employed as the carrier in the usual applications of this instrument. A collecting manifold provided with five spiral condensers is located a t the e h a u s t end of the gas line. This n~snufacturer also offers several Gas Partibianers, which are portable, batterypowered instruments specifically designed for the rapid analysis of selected gas mixtures. For example, the Model 25M ($470) contains two different columns in series, and a microammeter for readout. A mixture of the fixed gases (CO, CO?, N., H., 0 ~ and ) hydrocarbons can bo analyzed simply by noting the sequence of meter deflections as a. function of time. A Clinical Model ($625) is designed for tho analysis of blood gases, respiratory gases, ete. Witkens Instrument and Research. This company (Walnut Creek, California) manufactures t,he line of compactly designed Aerograph GC instruments, which include examples of all the types of Bow, control and detection circuits described previously. Among these are the: Model A-90-P ($1085, without recorder) with separate injector, detector, and column heater controls, column oven range from ambient to 4009C, tungsten filament T / C detectors; Model A-350-B ($2000, without recorder) with dual columnr, injectors, and detector blocks, and linear temperature programmer; Model A-600B ($895, without recorder) with flame ionization detector. T h e latter instrument is illustrated in Figure 62. This (Continued on page A198)

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Chemical instrumentation

Figure 62. Sectional view of the Aerogroph Hy-Fi Model 600.

manufacturer also offers an extensive line of accessories, including a hydrogen generator (Model A-650, 5225), a, linear temperature program control unit (Model A-325, $685), a flame ionization detector kit (Model A-500-B, $625). Precision Scientific Co. One of the most inexpensive, simple instruments currently available is the Model VP-1 Chranofrnc ($250, without recorder) made by Prccision Scientific Co., Chicago 47, Illinois. This uses an unheated column, so i t is suitable only for substances that boil lhclow about 105'C. A thermal conductivity type detector cell black is r n played, with coiled platinum wires as the senson. Using a bridge voltage of 2.5 v and helium or anot.her inert gas as the carrier, components are detected by thermal conductivity. If the bridge voltagc is increesed to 4.5 v, and air is the carrier, the components are detected .by catalytic combustion. The latter is ahout t,en times more sensitive than the fonner. Barher-Colman Co. This manufacturer h : pioneered ~ in the engineering devdoplmcnt of gas chromatographs designed around ionization type detectors. The U o d d 10 ($4350, single column with recorder; $7825, dual column with two recorders) utilizes an tnrggon/beta-my or flame ionization detector, and inrludes automatic proportional temperature control of column oven within =tO.2' from ambient to 300% Model 20 ($3450, with recorder) is designed for capillary columns, and contains it linear sample splitter in addition to the other features just cited. Model 15 ($4075, with recorder) is a mobile unit similar to the Model 10. Model 23C ($785, sithout recorder) is a. conventional T/C gas chromatograph for column operation up to 300°C. Model 61C is s. cornmet,. wiih recorder, depending upon system). Figure 63 shows the design prinriple of thc Model 20 instrument.

Other Manufacturers The instruments that have b w n de(Contintred a page A200)

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scribed above encompass substsntially all of the design features found in commercially available gas chromatographs. Mnny other manufacturers offerversions of gas chromatographs that represent various combinations and permutations of these design features. The following sun,marizes the remaining principal sources of supply, and cites special mpects of their instruments that are different from the models already treated. American Instrument Co., Silver Spring, Afargland. Offers option oi a rsdiafrequency glow detector system. Analytic Syrtems Co., Pasadena, California. Flame ionization detector. Burrell Corp., Pittsbwgh 19, Pennsylvania. Kromo-Tog series of isothermal and programmed temperature chramatagraphs; also oNers option of argon/bets,. ray, flame ionization, and thermionic emission detector systems. Carad Corp., Palo Alto, California. Flalnc ionization a n d y ~ e rfor total hydrocarbon content of a gas stream. Carlo Erba, Milan, Ztoly (1'. S. Distributor, Schaeler and co., New I'wk 38, N . Y.). All types of GC instrumentat.ian. Consolidated Electrodynamics Corp., P a s adena, California. High temperature ( t o 500°C) regulat,ed to *0.5°C, with T / C detector ($1855, without recorder). Central Scientific Co., Chtiogo I S , Illinois. Isothermal, column tomper* tures to 200°C, regulated to +0.5'C, T / C detector ($1030). Also, automatic fraction rollectar ($475); other aecessories. Daviv Instrumenls, Newark 4, S e w Jelaey. Instruments based on catalytic combustion detecton, &8 well a8 T/C detectors. Dohrn~onn,Inst~uments Co., Palo Alto, California. Specifically for halogen-eontaining compounds; after passage through GC column, components are pyrolyzed, and halogen is detected coulometric~lly ($7500). Dwabonic Instruments Corp., Cliicago $Z, Illineis. Column temperature to 40OSC, T / C detertor ($445, without recorder). Glowall Co~p.,Glenside, Pennsylvania. Argonbeteray detector and programmed temperature rhromatograph. Gwnbrier Instruments, Ronceverte, West Virginia. Instruments based on catalytic combustion and thermal conductivity

detectors. Hallikainen Instruments, Be~kelel, 10, California. A flame ionization chramstograph with column temperatures to 150% (U50, without recorder); also T/C-based instruments. Jwrcll-Ash Co., NewtaviUe 60, M a s sachusetls. Capillary column instrument wit,h beta-ray or flame ionization detector ($2160, without recorder). Kensington Scientific Cwp., Berkeley 10, Cal yoiornia. T'crsatile T/C ($695, vithout recorder) and flame ionization ($1005, aithout recorder) instruments. Mime-7'ek Instntments, Baton Rouge, Louisiana. Sampling systems, gas hxndling aeeessorirs, and complete GC instruments. Mine Safety Appliances Co., P i t t s burgh 8, Pennsylvania. Unique ioniza-

(Continued on page A202)

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.Heater

V Figure 63. Schematic diagram of the design of the Barber-Coimon Model 20 C h r o m o t o g r ~ ~with h argon ionization detector.

tion-type detector for trace oonstitnents in a gas stream. Nester and Fausl, Newark, Delatonre. Offers an inexpensive T / C unit with temperature range to 300DC(Model Anakro, $345, without recorder); also s. prepamtive-scale unit (Mod01 Prepkro, $1675, with temperature cont,rollcr, but wit,hout recorder). Podbielniak, Ine., Chicago 11, Illinois. Offers versatile instrument with choice of dctectors and oantrols; :dm nn analog computer for calculating results of G C runs (Compntagram, 4-unknowns, $1000; 8-unknowns, $1500; 12-unknowns, $2000). TV. G . P:t,e and Co., Ltd., Camh~idge, England. Arxoniheta-ray det,ectur-based

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chromatographs, as well a8 TIC instrnments. ffa(1iation Eqi~ipntent and Acressori~s Corp., Lynhrook, N . Y . Gas chromntograph with ionization chamber for detrrtion of radioactively lnbcllrd cornponmts ($'3995). Research Specialties Co., Richmond, California. Modular building t h r k units comprising all types of controls and drtectors. Shanrlon Scientific Co., Ltd., London (1'. S . Di~tfl~btdor, Consolidated Laborotorim, Chicago Heights, Illinois.) All t,vprs of CC instrumentation.