identification of components than is possible by taking the spectrum of a trapped peak of unknown composition. The foregoing examples also demonstrate the quantitative aspects of the powerful analytical combination of a gas chromatograph closely coupled to a Model 21-103 mass spectrometer. The time involved in collecting the mass
spectrometer data is the same as that involved in running a temperatureprogramed chromatograph. The wealth of data obtained currently takes about a week of calculating time to get the most out of them. As this work continues, faster data reduction techniques will undoubtedly be developed.
LITERATURE CITED
(1) Eggertsen, F. T., GroennisgR, S., ANAL.CHEW30,20 (1958). (2) Gohllre, R. S., Ibid., 31, 535 (1959) (3) Martin, C. C., Kurtz, S. S., Jr., et al., Ibid., 28, 490 (1936). RECEIVEDfor review March 22, 1960. Accepted August 5, 1960. Division of Analytical Chemistry, 137Lh Meeting, ACS, Cleveland, Ohio, April 1960.
Reducti o n f Noise in Thermal Conductivity for Gas Chromatography RICHARD KIESELBACH Engineering Research laborafory, Engineering Deparfmenf, E. 1. do Ponf de Nemours & Co., Inc., Wilrningfon, Bel.
b The signal from conventional thermistor thermal conductivity detectors in gas chromatographs frequently shows a noise level on the order of 10 to 20 pv., while the electrical noise level inherent in the (8000-ohm) thermistors i s usually about 0.3 pv. peakto-peak. The higher observed noise results from convective heat transfer from the thermistor, from ambienttemperature changes, and from bridge power-supply variations. Techniques for the elimination of noise from these sources are described. Application of these techniques results in a detector whose noise level (0.3 pv.) corresponds to about 2 X lo-* mole of organic vapor per mole of helium (pQo = 8.7). A brief description of the electrical measurement circuit i s given.
T
or product quality of chemical manufacturing processes is frequently critically dependent upon the concentration of trace levels of impurities in the process streams. Because of the broad applicability of gas chromatography to process stream analysis, the development of a high-sensitivity process chromatograph was undertaken. To achieve high sensitivity, i t mas necessary to identify and eliminate the various sources of noise which commonly exist in thermal conductivity detectors. Much of the noise evident in most gas chromatographs using thermistor detectors can be eliminated by proper instrument design. Application of the techniques described produces an approximate 50-fold improvement in signal-to-noise ratio over thermistor detectors of conventional design. While the major effort in this work was devoted to thermistor detectors, theory and preliminary experiments indicate that the observations made apply at least qualitatively to hot-wire detectors. The observed sources of noise in a HE YIELD
Figure 1. Noise level left, measuring circuit Right, thermistor detector
wise, temperature was coiltrolled at about 40' C, Helium was used as a carrier gas. All work reported was performed with Fenwal G-112 (8000-ohm) thermistors. [According to the manufacturer, the material used in the fabrication of the 80QC-ohm bend thermistor is the most stable of the several formulations arailable for different thermistor characteristics. Experiments with 2000-, 8000-, and 100,000-ohm beads (all of the same nominal dimensions) showed that both noise level and response to gas composition change were proportional to the square root of thermistor resistance, so that none offered a specific advantage in signal-to-noise ratio.] The detector output signal wab meosured with a self-balancing potentlnmeter having a full-scale response t h e of 0.5 second. The noise level of $he measuring equipment, as shown in Figure 1, was about 0.1 hcv. peak-topeak. The circuit is described in m m detail below. FLOW SENSITIVITY
thermistor thermal conductivity detector are: flow variations, shock or vibration, ambient-temperature variations, bridge-current variations, and electrical noise generated within the thermistor. These subjects are treated in detail below, EXPERIMENTAL EQUIPMENT
All experiments were made with a stainless steel detector block 1.5 X 2.5 X 3 inches in which the 0.25-inch diameter bores containing the thermistors were parallel and approximately 0.06 inch apart. The block was mounted on 0.1-inch diameter stainless steel posts 1 inch long, in an aluminumwalled oven. All walls of the oven were uniformly heated electrically, wall temperature being controlled with an electronic proportioning temperature controller. Except where noted other-
One of the major so'mes of n o k in 8 thermal-conductivity detector of the type ordinarily employed in gas chromatography is flow noise. 3ift'usioi: and convection-dibusion type cel: desi8r.s have been employed (3) to mini -nize this noise, a t the expanse of IO- to 20fold reduction in speed of response. It was reasoned that the advantage of the diffusion cell might be retained without the limitation of its slow response if the length of the diffusim path cou!d be made very short, T o achieve this end, a mounted thermistor was fitted with a surrounding openended cylindrical baffle made of perforated nickel sheet (Pyramid Screen Co. Type 125-T, 0.0035 inch). The clearance between thermistor and Ecraeri was about 0.Q25 inch. The s c r e e d thermistor was mounted in a 0.25-inch diameter cell, the thermistor being centered and t k x cylindricdl screen being parallel LO the axis of the eel!. Gas flowed at right angles to the
2
axis of the cell through a 0.19-inch diameter cross bore. The cell was connected to the output of a &foot gas chromatographic column. The column input helium pressure was 30 p.s.i., supplied through a 500-cc. surge tank. The thermistor was connected in a bridge, the reference thermistor of which received helium from an entirely separate source. A 1-cc. gas sample was introduced into the column with a gas-sampling valve. The transient output of the detector at the instant of sample introduction was 20 pv. The output of an unscreened thermistor tested under the same conditions was about 2 mv. The response time of the screened thermistor was compared semiquantitatively with that of a bare thermistor, by observation of a very rapid chromatographic peak produced a t a high helium flow rate. Propane was eluted from the column in 10 seconds. The width of the peak at half-height was 1.9 seconds, using an unscreened thermistor. The width of the peak produced by a screened thermistor under the same conditions was 2.0 seconds. Thus, the response time degradation resulting from the screen is not significant for peaks of ordinary speed. To improve the performance still further, a second screen was mounted around the first, with a clearance of about 0.025 inch, This additional screen reduced the transient resulting from sample introduction under the above test conditions to less than 1 pv. The width of the 10-second peak was increased to 2.1 seconds, still a n insignificant degradation for most practical applications. Tests were made in which the open ends of the cylindrical screens were plugged with glass wool. No significant difference in performance could be detected. Tests made with various cell port configurations-small bore, offset, and tangential-all showed significantly (5- to 10-fold) inferior performance as compared with the straightthrough bore originally tested. presumably because of turbulence introduced by changes in flow direction and velocity. As mentioned above, the reference thermistor in all of the above tests was supplied with a separate source of helium. It was mounted in a diffusion cell connected to a tee in a flowing helium stream by a 0.1-inch bore in the detector block. When this diffusion cell was connected to a tee in the helium line a t the input end of the chromatographic column, the detector outpht transient on sample introduction was 400 times as great as before, even though there was no steady-state flow through the cell. Performance obtained with a separate helium reference source vvas equaled by bleeding a small (5 to 10 cc. per minute) flow of helium from the
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,Thermistor
Section A A‘
T
p ‘
A
m
5
Thermistor
i Gloss - t o - M e t o l Seal,
Figure 2. Screened thermistor
column supply through a capillary restriction and a tee connected to the diffusion reference cell. Entirely equivalent performance was obtained with a reference thermistor sealed in a metal capsule containing helium a t atmospheric pressure. The capsule has shown no loss of helium in 6 months, as determined by measurement of the current-voltage characteristic of the thermistor. An additional source of flow disturbance is a fluctuation in barometric pressure, which frequently will produce noise in the detector signal, even with the reduced flow sensitivity attained by the measures described above. The opening and closing of doors in a ventilated room or a breeze blowing into the carrier gas vent will produce relatively large flow variations in the detector and noise signals on the order of 10 pv. or more. This source of noise is easily eliminated by venting the carrier gas into a surge tank of about 500-CC. capacity, which, in turn, discharges through a small restriction or needle valve to atmosphere. SHOCK SENSITIVITY
Shock oyp vibration sensitivity is obviously undesirable in an instrument intended for plant installation. Some of the screened thermistors were considerably less shock-sensitive than the bare elements. It was also noted that the detector was position-sensitiveLe., the direct current level of the output signal changed on changing the angular position of the detector block. It was demonstrated that the shock and position sensitivity did not result from motion of the thermistor, by short-
ening and rigidly supporting the thermistor leads with relatively massive lumps of solder. The shock sensitivity of a thermistor disappeared when its cell was evacuated. The shock sensitivity of the detector actually results from changes in the flow path of the heated column of gas rising from the thermistor. This was demonstrated by enclosing a thermistor with a U-shaped baffle of brass shim stock. When the assembly was mounted with the open end of the U pointing downward, shock sensitivity completely disappeared. With the open end upward, it returned. It was found that by reducing the clearance between the thermistor and the screen used for a flow barrier from 0.025 to about 0.008 inch, shock sensitivity was effectively eliminated. A severe blow of the fist on the chromatograph cabinet produces no signal at BO-,UV. full-scale sensitivity. Figure 2 shows the design of the screened thermistor currently used (patent applied for). Screened thermistors will be commercially available from Lockwood & hIcLorie, Inc., P. 0. Box 113, Hatboro, Pa. The current-voltage characteristic of a thermistor in helium is not measurably affected by the presence of these closely spaced screens. Neither is the sensitivity to gas composition changes. The screens are at essentially cell vall temperature, because of the high thermal conductivity of the metal and the low heat flow from the thermistor. Since their presence does not affect the thermistor characteristic, the major temperature drop from the thermistor must occur in an extremely thin film of gas immediately surrounding the thermistor, and the major heat sink must be the bulk of surrounding gas. It seems probable that the situation i s different with hot-wire detectors, in which the wattage dissipation of the element is several orders of magnitude greater than in the thermistors used here. AMBIENT TEMPERATURE AND BRIDGE CURRENT
I n principle, the effects of ambient temperature and bridge-current variations would be cancelled if the detector and associated bridge were symmetrical in all respects. This ideal, of course, is never realized in practice. The major asymmetry lies in the thermistors themselves, which, because of the method of their manufacture, cannot be precisely duplicated. They differ in “cold resistance,” in temperature coefficient, in dissipation constant, and in time constant. All of these factors can be compensated for, as discussed below, except for time constant. Thus, a steady-state symmetry can be achieved, but any variations in ambient
115 v.
6AQ5
Figure 3.
temperature or bridge current ail1 produce a transient output from the detector if their periods are short conipared to the time constant of the thermistors. Rapid ambient-temperature changes are eliminated easily by the simple expedient of supporting the detector block in an air bath free from convection currents. No ambient-temperature effects have been observed with the equipment described. (Experiments with a hot-wire detector in a brass block showed that the relatively high heat dissipation created conrection currents in the air bath, producing noise. This effect was greatly reduced by wrapping the detector block with an inch thickness of glass-wool batting.) An electronically regulated bridge power supply was designed having a short-time variation in output of about 10 p.p.m. maximum. As discussed below, this power supply contributes no noise to the detector signal. I t s circuit is shown in Figure 3. (An incidental feature of practical importance in this circuit is the rheostat in the cathode circuit of the 0 8 2 tube. This rheostat is adjusted in operation to bring the common mode detector bridge output level to ground potential, so as to eliminate the effect of any leakage to
Bridge power supply
ground in the associated wiring and circuit components.) Having eliminated rapid variations in ambient temperature and bridge current, it remains to compensate the steadg-state asymmetry of the detector. The approach taken to achieve this compensation is best understood by reference to the characteristic curves of two similar but mismatched therniistors (Figure 4). I n the current range in which self-heating lowers the thermistor resistance-the normal operating range for thermal-conductivity applicationsthe two curves diverge, and then become almost parallel. I n a conventional bridge circuit (upper diagram, Figure 4), a reeistance is. in effect, added in series with the thermistor of lower resistance to bring about a balance. As shown in the dashed curve, this changes the slope of the thermistor characteristic and produces a balance a t the current value where the dashed and solid curves intersect. Because of the change in slope, however, any variation in bridge current will produce a voltage change in the bridge output. It was suggested by Jones (6) that compensation might better be achieved by operating the two thermistors a t the same temperature, rather than a t the same current level. This is accomplished by the lower bridge circuit in
Figure 4 (patent applied for). In this circuit, the upper potentiometer is used to adjust the currents flowing through the two thermistors until a deliberate variation in bridge supply voltage produces no permanent outputsignal change. The tn7o thermistors are then operating on portions of their characteristics of equal slope, and the bridge output becomes current-insensitive. The lower potentiometer is then used as a voltage divider to produce a null bridge output. I n practice, this circuit achieves perfect current compensation over a reasonable (10%) range of supply variations, and reduces the detector response to ambient-temperature changes by a factor of about 20. Preliminary experiments have indicated that this bridge circuit can also be used to advantage with hot-wire detectors. SELECTION OF THERMISTOR LOAD RESISTANCE
The choice of the resistance value of the fixed arms of the bridge (and of the supply voltage) can best be discussed with reference to Figure 5. This figure shows the characteristic of a single thermistor in helium and in air, at a detector block temperature of 23' and 150' 6. The output signal of a detector on changing the gas from VOL. 32, NO. 13, DECEMBER 1960
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Figure 4. Compensation of mismatched thermistors helium to air equals the difference between the voltage levels of the helium and air curves a t the intersections of these curves with the straight load lines. The three straight lines represent different values of load resistance. The slope of a load line represents the reciprocal of resistance, while X-intercept represents bridge supply voltage. The 500-ohm load is typical of conventional bridge circuits. As has been pointed out (1, a), such a circuit shows a definite optimum in sensitivity, at room temperature, as the bridge current is raised from zero. It has also been shown (1, 8) that, a t higher temperatures, increasing bridge current finally results in a constant maximum sensitivity. The reasons for these effects are obvious from the shape of the curves shown. (A change in bridge current changes the intercepts of the load line, but not its slope.) The 500-ohm load and %volt supply shown in Figure 5 produce nearly the maximum possible sensitivity with the particular thermistor illustrated, with the detector a t room temperature. From the viewpoint of signal-to-noise ratio, however, it suffers two distinct disadvahtages. First, because of the rapid change in slope of the characteristic near the peak, a small change in either current or ambient temperature can produce a large change in the voltage level at the intersection. Second, &he slopes of the characteristics in 8
ANALYTICAL CHEMISTRY
Figure -
helium and in air are drastically different a t the two intersections with the load line as shown. Consequently, even if the sample and reference thermistors were perfectly matched in helium, the detector would become subject to noise from bridge-current variations when the gas composition in the sample cell changed. For these reasons, it is preferable to use higher resistance loads, to make a large gain in stability a t the expense of a small loss in sensitivity. The slopes of the helium and air characteristic curves are nearly parallel at their intersections with the 5000- and 10,000ohm loads plotted in Figure 5. At 150' C., the 5000-ohm load produces about twice the output signal of the 10,000-ohm load, while only slightly less signal a t room temperature. For temperatures up to 100' C., the 10,000ohm load provides sIightly higher signal levels. The high-resistance load offers an additional advantage in that it minimizes the effects of stray resistance and thermal e.m.f.'s in cables and connectors between the detector and the remainder of the bridge circuit. It suffers two minor disadvantages: An appreciable amount of power is dissipated in the resistor, resulting in a warm-up drift. The period of the drift (15 minutes), however, is less than the warm-up drift of the detector itself. Also, i t is possible to destroy a thermistor by abruptly switching on the high-voltage
5. Selection of thermistor load Fenwal G-112 (8000ohms)
supply. This can result from the relatively high power instantaneously dissipated in the thermistor before it has time to warm up and uniformly lower its resistance. This possibility has been eliminated in the present equipment by means of a time constant built into the bridge power supply. THERMISTOR NOISE
When all external sources of noise have been eliminated, that remaining must originate in the thermistor itself. To prove that the observed noise did in fact originate in the thermistor, the following experiments were made. A pair of wire-wound resistors of resistance equal to the thermistor resiatance at operating temperature (500 ohms) was connected in place of the thermistors in the detector. With bridge power on, the noise level averaged about 0.1 pv. peak-to-peak (Figure 1). Most of this noise originated in the servoamplifier. A pair of screened thernlistors was installed in the detector, which was then thermostated in its oven at 40' C, A flow of helium was maintained through the detector. The noise level (Figure 1) was about 0.3 pv. peak-to-peak. Applying the square-law relationship, it is seen that the contribution of the amplifier noise is negligible compared to this total noise. The period of the noise-about 1 to 2 seconds-was orders of magnitude
m
100 v.
Servo Motor,
I!,
y\
Shielded Cable
5.4 v. Mercury Battery Figure 6.
shorter than the thermal time constant of the detector block, so that it could not originate in ambient-temperature changes. The helium flow was stopped, leaving the detector under static helium pressure, without effect on the noise. Thus flow variations did not contribute to the noise. The contribution of the bridge power supply was measured indirectly. The output of the supply was monitored with a 10-mv. full-scale recorder, with 90-volt zero suppression supplied by a battery. The short-time noise was in the range of 0.5 to 1 mv. ( 5 to 10 p.p.m.), and showed no apparent correlation with the simultaneously observed noise in the detector. The transient output of the detector in response to a sudden deliberate change of 6 volts in bridge supply was 300 pv. From this correspondence, the 1-mv. noise level in the bridge power supply is estimated to contribute 0.05 pv. noise to the detector output. Thus the observed detector noise is six times as great as that which could be attributed to the power supply. It must, therefore, arise in the thermistors or in their electrical connections. The calculated Johnson noise for the pair of thermistors is 0.014 pv. r.m.6.
Electrical circuit
or about 0.05 pv. peak-to-peak. For periods of several seconds’ duration, the noise level shown in Figure 1 actually appears to be of this order. The discontinuous nature of the larger noise peaks lends credence to the manufacturer’s statement that the noise arises from intermittent point contacts between the thermistor material and its lead wires. It was suggested by the manufacturer that this contact noise might be reduced by cycling the temperature of the thermistors for an extended period. A pair of Fenwal G-112 thermistors was cycled between room temperature and approximately 300’ C. for 8 days, a t a cycle period of 15 seconds. Heating was accomplished by passing a current through the thermistors. No significant reduction in noise level resulted from this treatment. A further reduction in noise level may be possible by improved thermistor manufacturing techniques, and might be expected to yield a &fold improvement in the signal-to-noise ratio of thermal-conductivity detectors. This measured thermistor noise level is considerably below that reported by Cowan and Stirling (B), who used a measuring instrument having a basic
noise level of 1 to 1.5 p v . Their tests were made on a single thermistor immersed in silicone oil, rather than in helium. It seems possible that convection currents and the relatively high heat capacity of the oil contributed to their noise figure. [Stirling (6) recently indicated that his noise measurements were made over a wider frequency spectrum than that studied here. This difference could account for the discrepancy in measured noise level.] Preliminary tests with hot-wire detectors suggest that higher signal-tonoise ratio can be achieved than with thermistor detectors. The high filament temperature required for this performance may introduce other problems, particularly in convection effects, which have not been investigated here. ELECTRICAL CIRCUITS
Figure 6 depicts the electrical circuit for measuring the detector signal developed in the course of this work. The bridge circuit includes the separate current and voltage adjustments described above, plus an additional “fine” voltage adjustment. The normally closed switch and parallel resisVOL. 32, NO. 13, DECEMBER 1960
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tor in the 90-volt supply to the bridge are used to produce a change in bridge current while making the compensating adjustments. Decade attenuation of the bridge output signal is accomplished with the switches marked x 10, x 100, and x 1000. It will be noted that these are the only moving contacts so located that thermal e.m.f.’s or contact resistance might affect the signal. No noise from this source has been observed, using Automatic Electric Type BQA relays. The potentiometer circuit, suggested by Hoe11 ( d ) , comprises a 1000-ohm Helipot and two 4G-ohm fixed resistors. The current source for the potentiometer is of relatively high impedance, so that noise generated a t the Helipot slider appears across the source impedance, rather than the potentiometer output. The potentiometer span is changed by the 50,000-ohm rheostat and the tap switch a t the bottom of the diagram. When operating a t high sensitivity levels, the bridge output does not stabilize until several seconds after a bridge balance adjustment is made. This lag is undesirable in an automatic process chromatograph, where rapid zero adjustment may be required. This difficulty was overcome in the following manner : The 1000-ohm potentiometer Helipot and a 10,000-ohm transmitting Helipot are driven by a 162-r.p.m. Brown balancing motor, at a maximum speed of about 0.5 second per revolution. One turn of the transmitting Helipot drives the recorder full scale. The output of the transmitting Helipot is fed through a capacitor to an electrometer cathode follower, which, in turn, drives the recorder. Momentary grounding of the electrometer grid provides practically instantaneous zeroing of the recorder,
Pi Determinatio ALFRED
M. VOGEL
so that the manual bridge-balancing adjustments need not be manipulated unless a positive or negative zero drift of more than about four times full scale occurs. A 100-volt supply to the transmitting Helipot provides a sufficiently high signal level to eliminate problems of stability in the cathode follower circuit. A finer bridge balance adjustment can, of course, be provided in place of this feature, where speed is not required. A standard Brown 40X servoamplifier was modified for use in this servopotentiometer. The necessary modifications include substitution of a 7500ohm input transformer for that furnished with the amplifier, and the incorporation of a feedback velocitydamping circuit. The latter circuit is that ordinarily used in the Brown 0.5-second recorder. An external amplifier gain adjustment mas added for use in the automatic process chromatograph. SENSITIVITY MEASUREMENT
The sensitivity of the detector was measured in the following manner: A flow sysLem was constructed equivalent to the conventional bypass sampling system, whereby a stream of helium could flow either directly through the detector or via a column packed with naphthalene crystals. The bridge m-as balanced with helium flowing through the detector. Then, after flowing the helium over the naphthalene until a steady reading was attained, the rise in detector output signal was noted. This test Fas repeated a t progressively lower flow rates, until further flow reduction made no change in the measured signal. It was assumed that, a t this point, the helium was saturated with naphthalene vapor. From the known vapor pressure of
naphthalene a t the operating temperature (approximately 25’ C.), the sensitivity of the detector was calculated. By extrapolation, the measured noise level of 0.3 pv. corresponded to a naphthalene concentration of 2 x lo-* mole per mole of helium. In the units proposed by Young (7), pQo = 8.7. For comparison, typical commercial thermistor detectors tested at this laboratory show a pQo of about 7.0, CONCLUSIONS
The major part of the noise evident in most gas chromatographs using thermistor detectors arises from sources external to the thermistors and can be eliminated by proper instrument design. Application of the techniques discussed in this paper produces an approximate 50-fold improvement in signal-to-noise ratio over thermistor detectors of conventional design. LITERATURE CITED
(1) Bennett, C. E., dal Kogare, S., Safranski, L. W., Lewis, C. D., A~YAL.
CHEY.30,898 (1958).
( 2 ) Cowan, C. B., Stirling, P. H., Intern.
Gas Chromatography Symp., Instrument Society of America, East Lansing, Mich., August 28-30, 1957. (3) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEM.28, 290 (1966). (4) Hoell, P. C., E. I. du Pont de Nemours & Co., Inc., private communication. ( 5 ) Jones, W. L., E. I. du Pont de Kemours & Co., Inc., private communication. ( 6 ) Stirling, P. H., Second Biannual Intern. Gas Chromatography Symp., Instrument Society of America, Eaet Lansing, Mich.. June 10-12. 1959, privatecommunication. ( 7 ) Young, I. G., Second Biannual Intern. Gas Chromatography Symp., Instrument Society of America, East Lansing, Mich., June 10-12, 1959. I
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RECEIVEDfor review May 19, 1960. Accepted August 15, 1960.
graphic Metha rbon and Hydrogen
and JOSEPH J. QWATTRONE, Jr.
Chemistry Department, Adelphi College, Garden City, N. Y .
A rapid, easy method for the determination of carbon and hydrogen in organic compounds involves a bomb combustion of an 8- to 1 1 - W . Sample in an Oxygen atmosphere8 sampling the products of combustion, subjecting the sample to gas chromatography, evaluating carbon and hydrogen in terms of the planimeter-mgasured integrated ureas for water vapor and
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carbon dioxide. Based on the results obtained from five organic compounds, the average deviation from the known values was 5.0 parts per thousand for carbon and 8.4 for hydrogen. The total time for a single run is 17 minutes. Triplicate results from a single combustion can be obtained in about 40 minutes.
HE standard method used for carbon and hydrogen determination in organic compounds, that of Pregl (W), requires 135 to 145 minutes for a duplicate determination. The method requires training of a high order and meticulous technique. Some combustion procedures of 15 to 20 minutes’ duration per determination have been described,
