Table IV. Effects of Limits of Integration on the Mean Value and Precision of the Statistical Moment Calculationsa Integration limits Moment
PO PI,
sec
1 1 2 , sec2 ~ 3 sec3 , ~
4
sec4 ,
Skew Excess
f2.0%
*
2145 f 0.15% 0.5132 f 0.08% 1.388X IO-' f 0.07% 1 . 1 0 1 x 10-3 f 0.15% 0.6273 X f 0.16% 0.7144f 0.12% 3.502f 0.08%
f0.2%
*0.5%
2171 0.16% 0.5189f 0.08% 1.656X lo-'* 0.30% 2.687 X f 1.22% 1.672 X f 1.52% 1.261 f 0.79% 6.097f 0.95%
(I Data was taken with the hybrid-fluidic valve at a vaive gate pulse width of 100 msec. same conditions as reported in Table I I .
amplitude is used here and has been previously justified ( 3 4 ) . I t is of particular interest to note t h a t the precision of the peak area and mean are about constant for the limits tested. This is most encouraging because these are the moments of general interest. As expected, the precision then decreases by as much as a factor of 50 as the order of the moment increases and as the limits are extended to lower amplitudes. The peak area naturally increases as the limits are extended to include more of the peak area. At &0.2% limits, the peak area is 2% larger than a t limits of f2.0%. This error is nontrivial and not readily apparent from the valve profile in Figure 8. The high order moments also increase in a regular manner by as much as 1.9 x IO3% for the fourth moment) with the extended limits while the reproducibility decreases by a factor of about 50 for the same reasons. These data clearly show the importance of specifying the limits of integration as they affect both the absolute value of the moments and their precision.
2189 f 0.16% 0.5273 f 0.12% 2.538 X IO-* f 1.95% 11.99X f 5.52% 12.63X f 7.44% 2.996f 2.76% 19.48 f 3.86%
All calculations were made on the same data set and under the
ACKNOWLEDGMENT The authors gratefully acknowledge the construction of the valves by Art Grant, Frank Ottinger, and Chester Eastman of the University of Florida. Received for review July 17, 1972. Accepted June 12, 1973. Paper presented in part a t the 1973 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 5-8, 1973, Paper No. 126. Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. The financial support of the National Science Foundation under Grant No. GP-14'754 and the National Institutes of Health under Grant No. GM-17203 is gratefully acknowledged.
Performance of a Reduced Volume Thermal Conductivity Detector R. E. Pecsar, R . B. DeLew, and K. R. lwao Varian Aerograph, 2700 Mitchell Drive, Walnut Creek, Calif. 94598
By reducing the internal volume of the thermal conductivity detector, the range of applicability can be significantly increased. The minimum detectable quantity is reduced by an order of magnitude from that normally achievable. Such a detector is compatible with l/a-in. diameter and capillary columns, which are intrinsically more efficient, because of the decreased contribution to extra-column band-broadening. This increased sensitivity can only be realized by careful control Of the detector temperature and flow rate. Dual differential flow control, as well as fully proportional temperature control, are employed to achieve the maximum signal t0 noise ratio. The efficacy of such a proportionally controlled detector is demonstrated by the low thermal gradients across the cell and the highly stable output signal. When operated with an %-in. column, hydrocarbons in the 1-5 ppm range can readily be quantitated. Comparison is made of programmed temperature capillary column separations with
both the thermal conductivity and flame ionization detectors. Resolution in both cases is equivalent, the sensitivity of the thermal conductivity detector being within 100fold of the ionization detector.
The thermal conductivity detector (TCD) has been the principal detection means in gas chromatography since the inception in 1956, Prior to the introduction of the ionization detectors in 1959-60, it was the only practical device, since that time, this predominant position has in large measure been reduced but even today approximately 40% of all gas chromatographs soid are equipped with the
TCD. A number of reasons exist why the popularity of this detector continues. The setup and operation is very straightforward requiring no auxiliary gas flows. The operating parameters which need tuning or exact control are usually preset by the instrument design and do not neces-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
2191
sitate operator interaction. In addition, because of the long established usage, a high degree of customer familiarity has been developed. The largest contributing factor, however, is the universal response. The ionization detectors, while responding to many different classes of compounds, are still in reality largely specific detectors. The flame ionization detector is sensitive to methylenic groups while the electron capture detector responds to compounds with unpaired electrons. Only the helium ionization detector generates a response for all substances but the operation of this unit is significantly more complex than the TCD. Many practical analyses require the determination of fixed gases and light hydrocarbons for which the TCD is ideally suited. In fact, for the analysis of inorganic materials, utilization of the TCD is nearly mandatory. Unfortunately, the TCD is limited with respect to the minimum detectable quantity of a substance as compared to the ionization detectors. Typical detection limits are 10-30 ppm, or approximately the microgram level, which is a t least a factor of 1000 higher than that attainable with the ionization detectors. However, numerous applications now beyond the capability of the TCD could be achieved with only an order of magnitude improvement in sensitivity. In terms of detector classifications, the TCD is considered a concentration dependent detector. Therefore, the response is a function of the degree of dilution of the sample in the carrier gas as it passes through the detector. By focusing design attention on this characteristic, the desired sensitivity improvement can be gained.
CELL VOLUME EFFECT The flame ionization detector is a mass sensitive detector and as such responds to the mass rate of material passing through it. The performance is by and large independent of the volume of the detector. The TCD, conversely, depends on the concentration of the component passing through the detector a t the particular time. Therefore, if the internal volume of the detector is reduced, the mass of the compound is diluted by less carrier gas and the effective concentration is higher. In this manner, the net apparent minimum detectable quantity has decreased although the same actual amount of material has flowed through the detector. Practically the concentration has increased and the chromatographic band is sharper. The influence of the detector volume on the extra-column band-broadening contribution has been treated in depth by Sternberg ( I , 2). In order to gain the beneficial results of a reduction in volume, care must be paid to the total geometry of the detector. The improvement achieved by reducing the volume can easily be lost by creating unswept passages into which the sample components may diffuse and thus broaden the elution band. As a general rule for sharp narrow peaks, the detector volume should be no greater than 20% of the half width of the early eluting substances. The realization of this effective concentration increase is only possible with the narrow diameter columns. These columns are inherently more efficient with less band-broadening. Likewise, the optimum flow rate for operation of the narrow columns is lower and so less carrier gas dilution is to be expected. A t present, the typical TCD has a large internal volume and is compatible with I/4-jn. diameter columns. The optimum flow associated with such a column is 50-60 ml/min (1) J. C. Sternberg, Advan. Chrornatogr., 2, 205 (1966). (2) T. Johns and J. C. Sternberg, "Instrumentation in Gas Chromatography," J. Krugers, Ed., Centrex Publishing Co., Eindhoven. The Netherlands, 1968.
2192
of helium with a minimum HETP of 0.6 mm. Using a reduced volume TCD with this type of column negates all the potential benefits. If instead a ys-in. diameter column is employed, the column flow rate is near 30 ml/min for a n optimum H E T P of 0.4 mm. In this case, not only is the lower flow more compatible with the reduced volume cell but the higher intrinsic efficiency of the column provides sharper bands and better resolution. Again, it should be recalled that these benefits are possible only if the sample peak half width is a t least five times the detector volume. The benefits to be gained from reducing the cell volume have been appreciated by other investigators. Most particularly, these people were interested in capillary column separations where the preservation of resolution was of prime importance. The smallest detector volume commonly obtained employs a thermistor bead as the sensing element (3). This device has good sensitivity and preserves peak shape. Unfortunately, thermistor characteristics limit the thermal range of operation to about 300 "C. Above this value, the sensitivity markedly decays. For broad applicability, hot wire sensing elements are to be preferred. Camin has investigated a small hot wire TCD fabricated by Gow-Mac with a 250-p1 cavity volume ( 4 ) . When a special fitting modification was used, the peak shape was not degraded. No systematic study of the cell was conducted. The most sophisticated TCD is embodied in the design of Wilhite ( 5 ) . This cell has an internal volume of 0.1 p1 with a hot wire element. The detector was part of a micro-chromatograph for analyzing lunar samples in situ. Specific requirements for space flight are not applicable in general and as such the cost of such a system is relatively high. A further interesting approach was that of Pr6au in which the detector is constructed from two flat pieces in which the flow channels are milled (6). The parts are then bolted together with a cell volume of 40 pl. The sensitivity of such a device did not exceed the more conventional techniques and commercial versions of this approach have had difficulty in successfully sealing the unit. In addition to the advantages of reduced cell volume considered thus far, another effect exists, as demonstrated in Figure 1, which is favorable. As the cell volume is decreased, the proximity of the wall to the heated filament is increased and better heat transfer is possible as the conduction path through the gas is reduced. Therefore, for a given detector cell temperature and filament current, the filament temperature will be lower the closer the cell cavity wall. Shown for comparison are the standard Aerograph TCD and two special detector blocks, one having a larger cavity diameter with the other being undersized. The same filaments were used throughout the study. A linear relation exists between the detector and filament temperatures over the range studied with the various cell cavity diameters generating a uniform positive displacement from smaller to larger values. As the current increases, a further displacement occurs as anticipated due to the larger amount of power dissipated in the filaments. With a smaller cavity and a cooler filament temperature, the susceptibility to oxidation is reduced and a more stable output results. If the detector signal is sufficiently stable, the reduced volume may be advantageously employed to permit operation a t a higher filament current, and as the sensitivity is proportional to the cube of the current, a reduced minimum detectable quantity can result. (3) R. Teranishi and T. R. Mon, Anal. Chern., 36, 1490 (1964). (4) D. L. Camin, R . W. King, and S. D.Shawhan, Anal. Chem., 36, 1175 (1964), (5) W. F. Wilhite, J. GasChrornatogr., 4, 47 (1966). (6) G. Preau and G. Guiochon, J. Gas Chromatogr., 4, 343 (1966)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
Having deduced t h a t cell volume reduction is beneficial, we investigated the most practical means for achieving this gain. Because of the need to analyze wide boiling range mixtures, it is required t h a t the detector operate as high as 400 "C without condensation of the eluting species. For this reason a hot wire type filament was preferred. A variety of detectors were evaluated with varying cell volumes. For the practical flow rates of interest, the cell shown in Figure 2 produced sharp peaks with minimal band-broadening and had a rugged construction. The detector, manufactured by Gow-Mac, has a volume of 135 pl per flow channel. This is achieved by incorporating a continuous filament through the cavity with no support post. In this manner, the cavity diameter could be significantly reduced. While detectors of even smaller volume were evaluated, the other operating requirements could not be fulfilled.
DETECTOR OVEN CONFIGURATION The potential of the reduced volume TCD can only be realized if the environment of the detector is carefully controlled. With this type of detector, the actual property physically being determined is a temperature differential, and a t the detection limit this difference may be as small as 10-6 "C. Therefore, the temperature of the cell must be closely regulated. In isothermal analyses, no problem exists as the injector, column, and detector are each maintained a t a constant temperature and their interaction reaches an equilibrium condition. However, for broad boiling range samples, the separations are much improved if the column temperature is varied in a programmed manner. As the column physically terminates a t the detector, care must be taken to thermally decouple the column oven from the detector. If the detector temperature varies as the column oven is programmed from low to high temperatures, base-line drift will result making quantitation of the chromatogram difficult. To eliminate this effect, a crossover of narrow diameter is provided between the column oven and the detector oven. During passage of the column effluent through this crossover, the flow equilibrates to the detector temperature minimizing heat transfer between the two thermal zones. In designing a stable detector oven, a trade off must be made between response time and degree of control. Naturally, to achieve low gradient conditions, a fully proportional temperature controller must be employed. The tightness of control is determined by the gain of the control circuit and the proximity of the feedback sensor to the actual heat source. With the sensor and heater in close proximity and a high gain setting in the controller, a very narrow thermal band width is obtained with a low amplitude differential from the control point. However, the cycling frequency is high and actual control is being made on the temperature of the heater. In fact, if the cycling frequency is excessively high, a cycling will be induced in the base line such t h a t 0.01 "C fluctuations in temperature generate 1'70 oscillations in the output. Such a system is oblivious to the influence of external heat sources. Now as the detector oven is always located adjacent to the column oven to minimize flow volumes, the control mechanism should be cognizant of the column oven as a potential heat source. By separating the probe from the heat source, the oven is better buffered from external inputs but the amplitude deviation from the set point will be higher. This is partially offset by reducing the gain of the controller. The cycling frequency in this mode is reduced such t h a t 0.1-0.2 "C thermal perturbations are reflected as 1% gradual cycling in the base line. Of course, if the control bandwidth
v I------
100 100
//
Lrg Std
0.281" 0.218"
Lrg Std
0.281" 0.218"
91-11
0.156"
150 ma
I
t
i
,046" D I A
I
4 k 093"DIA
Figure 2. Reduced volume thermal conductivity cell
is too broad, the effect of line voltage fluctuations and ambient temperature variations becomes pronounced. In practice, the sensor is located a reasonable distance away from the detector oven heater but in a manner such that the influence of the column oven is not neglected. For good stability, the gain of the controller is then reduced to about 30% of the maximum level. With such a configuration, base-line perturbations are minimized. An efficient means for thermal communication between the detector cell and oven is mandatory for gradient free operation. As the filaments are dissipating about two watts of power, this self-heating must be transmitted to the surroundings or the temperature will monotonically increase and unstable operation will result. In the present instance, it was determined that air coupling of the cell and oven provided the best approach. As a n alternate, the space between the detector and the oven could be filled with a heat conducting medium. While such an approach is feasible, the response time with the air medium appeared adequately rapid, and the convenience in performing maintenance with such a configuration is a decided advantage. The reduced volume detector, mounted in the oven which evolved from these design considerations, is
ANALYTICAL CHEMISTRY, VOL. 4 5 , NO. 13, NOVEMBER 1973
2193
Oven Heater Configuration
X Gradients OC
Y Gradients
oc
Z Gradients
Baseline D r i f t
OC
MV.
e
0.05
0.I
0.1
0.065
0.05
0.1
0.0
0.075
- -X
..
Conditions: Det. Oven .2OO0C; In). -225012 Filament Current .ZOO ma; Program; 600.2000C @ 1O0C/min with a 3% SE30 column. Carrier: 30 ml/min He.
Figure 4. Temperature programming effect on detector cell 3 60 '
Figure 3. Thermal conductivity detector oven
pictured in Figure 3. Two 45-watt cartridge heaters diagonally positioned were employed with the 100-ohm Pt sensor located on the forward face as t h a t is the side adjacent to the column oven. The oven is symmetrical about the plane shown. The two threaded fittings are the exhaust tips. By placing these tips mainly in the oven body itself, condensation of the sample is prevented with the resulting flow disturbance. Internal t o these fittings are contoured tubes t o permit microcollection of the eluting fractions, as the TCD is a nondestructive detector.
DETECTOR CELL BEHAVIOR In the design of the detector oven, the minimization of gradients was of paramount importance for stable operation. Although two heaters diagonally opposed were chosen for the final configuration, a number of alternate locations were investigated. The chief external influence is the column oven. Initially a single heater was placed on the face of the detector oven adjacent to the column oven. By locating the heater on either the left or right side, large gradients in the back to front direction were produced on the opposite side. In addition, during programmed column temperature operation (PTGC), the base line drifted as much as 0.2 mV.These results confirm the inadequacy of a single heater. Options incorporating three and four heaters were also evaluated with a definite reduction in thermal gradients but the base-line shift with PTGC was still between 0.1 and 0.2 mV. As the base-line stability was only slightly improved and the complexity markedly increased, these configurations were abandoned. Four geometric options exist for placing two heaters in the detector oven. The heaters may both be placed on the front face, both on the back face, or in the two diagonal orientations. With the heaters on the front face, large back to front gradients were produced as the entire heat input was made from one direction. Concurrently, the base-line drift remained in the 0.1-0.2 mV range. N o particular advantage could be ascertained for one diagonal placement over the other. The comparison of the axial gradients and base-line shift during PTGC for the two preferred heater orientations is given in Figure 4. The gradients listed are those generated in programming a 3% SE 30 column from 60 to 200 "C a t lO"/min. As can be seen, very little difference in gradients exists between the two configurations and both are minimal. Of the three axial gradients, the area of primary concern lies in the X direction. The measuring filaments are located in the left side of the block with the reference filaments being on the right side. Therefore, if the gradients are uniform only the X gradient will upset the output bridge balance and these are the lowest. The choice was 2194
then made of the diagonally opposite heater configuration based on the lowest base-line drift. The drift of 0.065 mV was the total shift from the initiation of the program until stability was again reached about 30 minutes after the termination of the program a t 200 "C. If, in fact, the column is cooled to the initial temperature after reaching 200 "C instead of holding a t this condition, a lower base-line shift will result. The stability of the detector cell and oven were investigated for a prolonged operating period with the column held isothermally a t 100 "C. The results of this study are presented in Figure 5 . The detector temperature was monitored with a platinum sensor located on the cell near the forward face. Over the 24-hour period, the total baseline deflection was 0.035 mV and no definite trend could be discerned. In general the temperature variation of 0.2 "C was cyclic and could be correlated with the variation in ambient temperature. The net shift over the 24 hours was about 0.05 "C. These results indicate that the long term stability is definitely acceptable. A further stability test was run to validate the gradient studies conducted earlier when determining the optimum heater configuration. The platinum sensor was used to monitor the detector temperature. The temperature trace and the bridge output during a column oven temperature programming cycle are depicted in Figure 6. Starting with a stable base line and cell temperature, the column is programmed from 60 t o 200 "C a t 10"/min. When the column reaches 200 "C, the detector cell has only partially adjusted and approximately another 15 minutes are required before the cell re-equilibrates to the new condition. The output is monitored for another half hour during which no further perceptible change occurs. The oven is then cooled back to 60 "C and restabilized in preparation for another analysis. The bridge output showed a maximum displacement of 0.060 mV and returned to the original position at the completion of the cycle. The cell temperature was displaced about 0.15 "C, recovering 0.1 "C of this upon reequilibration. The stability was reconfirmed with repetitive tests over the same cycle. In addition to careful control of the TCD temperature, the carrier gas flow rate must also be closely regulated. As the TCD is a concentration-dependent detector, any nonuniformity of flow is reflected in a n irregular signal output. Small perturbations in the flow rate are manifest as noise on the recorder. To maintain a constant flow rate independent of downstream conditions, a dual differential flow controller is utilized. While this is important with isothermal column operation, it is mandatory with PTGC. When the cell volume effect was considered earlier, it was pointed out that with reduced volume a higher filament current could be employed while generating a n equivalent filament temperature. In terms of TCD life-
* ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, N O V E M B E R 1973
DETECTOR OVEN: 220% FILAMENT CURRENT. 145 ma COLUMN. 10' I 118'' 2% SE 30 at 100°C INJECTOR 220% -
__
~
-
~~
~
-_
/
-
-_
-
~ _
-
~~~
TEMP
-~
--
EACH DIVISION
BRIDGE-_
-
~
.. .
0 1 mV 0 1 %
_
_.
_ _ _ ~ ~ -
-
~~
_ _ ~ ~
_________ __~ ~ ~~
6 3 0 p
8 30
10 30
-T - - - - - T I 2 30
~2 30
-~
7 -
~~~~~~
~~
-
~
~-~~~ 1 -
6 30
4 30
~
10 30
8 30
12 30
2 30
6 30
4 30
Figure 5. Long term TC detector stability with isothermal column operation DETECTOR OVEN 220°C FILAMENT CURRENT 200ma INJECTOR 235'C
1
COLUMN IO' X 1/8" 2% S E X ) PROGRAM 6O0-2OO0C at IO'C/min
TEMP
each division = 0 I'C,
0 I mv
I
Program start
Program end
Cool down start End of Cooling
Figure 6. Effect of column temperature programming on TC detector stability
time, the filament temperature is the actual controlling variable. As the filament temperature is increased, the susceptibility of the wire to oxidation greatly increases. DeLew ( 7 ) has demonstrated that if slight leaks exist in a cell and the temperature of the filaments exceeds 400 "C, a unidirectional continuous drift of the base line occurs. Reducing the temperature to 350 "C produced a stable base line. In Figure 7 the relation between filament current, filament temperature, and cell temperature is given. As anticipated, for a fixed cell temperature, higher currents cause increased filament temperature. The above delineated operating guidelines are used to suggest recommended and absolute filament temperature limits. Naturally if a good leak-free system is developed, sensible increases in these limits may produce beneficial results with respect to the minimum detectable quantity. By employing these data, a clearly discernible operating envelope may be defined.
CPRRlER GAS-HELIUM
I =FILAMENT CURRENT-MA
450
ABSOLUTE MAX FILAMENT OPERATING TEMPERATURE
400
(SHORT TERM OPERATION-0 5 HRSI RECOMMENDED
350
MAX FILAMENT OPERATING TEMPERATURE (LONG TERM OPERATION1
300
2 50
200
150
100
CHROMATOGRAPHIC PERFORMANCE Now t h a t the operating characteristics of the reduced volume TCD have been established, let us turn our attention to the chromatographic performance, in particular to the detection of small quantities of the sample components. The ASTM subcommittee on TCD performance specifications has recently recommended t h a t the sensitivity of the detector be defined as (8) Sensitivity (mV.ml/mg) = (peak height)(peak half width)/mass of component This expression is essentially equivalent to the former DPS number. Additionally the minimum detectable quantity is defined as the amount of material which produces a signal whose magnitude is twice t h a t of the noise. By using a sensitive amplifier on the bridge output, the noise with the reduced volume TCD was assessed to be 3 p V . This noise is independent of the instrument in which the detector was installed and is due to random filament vibration resulting from the passage of flow through the (7) R. B. DeLew, J. Chrornatogr. Sci., I O , 600 (1972). (8) R. Villalobos, Recommended Detector Practices, presented at the ASTM-E19 Committee Meeting, Denver, Colo., October 1970.
0
I00
200
300
TDETECTOR ("C)
Figure 7. TC detector characteristics
cavity and also to the Johnson noise of the resistive elements themselves. With a 3-pV noise level, the minimum detectable quantity need generate only a 6-pV signal to produce a measurable response. In order to determine the lower detection limit, the trace analysis of light hydrocarbons, as shown in Figure 8, was studied. The substances investigated were ethane and propane. As the system checked leak free, the filament current and cell temperature were chosen to permit functioning outside the previously discussed operating envelope. In this manner, increased response t o the sample could be obtained. The 1000-ppm sample produced a response of nearly 6 mV so that the minimum detectable quantity of ethane and propane is about 1.3 ppm. For this sample, the maximum sensitivity was 8300 mV.ml/mg. Higher sensitivities can be achieved but this is accomplished to the detriment of cell life. If the filament current is continuously increased, failure of the wire material will occur prematurely as any system has some oxygen present. Also to be noted is the fact t h a t higher sensitivity
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
2195
0
2
4
6
8
10
Minuter
Figure 8. T r a c e h y d r o c a r b o n analysis
Figure 9. Programmed t e m p e r a t u r e
Detector: 245 "C; filament current: 300 mA; column: 200 " C , ':E-in. X 5-ft Porapak Q ; carrier: 20 cm3/min He; sample: 2 ml of 0.1% Cz and C3 in helium: att: 8
Detector: 250 "C: filament current: 250 mA; column: 10-ft X 'Ie-in,, 2% SE 30; program: 50"-125 "C at 10 OC/min: carrier: 30 cm3/min helium; att: 8
does not necessarily mean a lower detection limit. As the filament current is increased beyond a certain point, the noise level also increases and the minimum detectable quantity is determined by the signal to noise ratio. Typically high sensitivities and low detection limits are obtained with low molecular weight compounds which elute rapidly from the column with very sharp peaks. The reduced volume TCD is now compatible with the high efficiency columns and does not broaden the bands so that higher molecular weight components which elute sharply but reside in the column longer are not degraded in passing through the detector. An example of this is the chromatogram in Figure 9. Here hydrocarbons from octane to tetradecane are resolved on a n 10-ft x yg-in. column containing 2% SE 30. The detector was operated within the suggested envelope for prolonged operation. From ancillary measurements, the noise level was determined to be 0.75 pv. The concentration of each component was in the 3-15 p g range with the output level varying from 5-7.5 mV, respectively. From these data, the minimum detectable quantity (MDQ) for each substance could be determined.
detectability. With the present dual column flow regulated system in which the detector is well isolated from the column oven, programmed temperature separations can be accomplished while maintaining a stable base line. These results are sufficiently encouraging to permit the examination of capillary columns with the reduced volume TCD. In a system designed for capillary columns, very careful attention must be paid to the entire flow system. The elimination of any unswept volumes is imperative. The column efficiency is exceptionally high with plate heights near 0.3 mm, and the resolution achievable is extremely sensitive t o additional band-broadening in the injector and detector. With a capillary column, the flow rate is typically 1-5 ml/min and as such the full potential of the reduced volume TCD can be realized. Minimum dilution of the eluting species will occur, thus giving rise to higher effective concentrations. Because of the low flow rates and long columns employed, the capillary flow system is actually less susceptible to flow perturbations than the conventional column configuration. In addition as the sample is split in a ratio of 50 or 100 to 1 prior to delivery to the column, the overall back pressure is markedly increased. Because of these factors, the dual differential flow controller is usually removed from the pneumatic system. The effect of detector band-broadening, even with a reduced volume cell, is very pointedly demonstrated in Figure 10. During the evaluation of a variety of cells with decreased volume, a comparison was made of the bandbroadening effect using a capillary column under isothermal conditions. The detector previously discussed in this work generated chromatogram B while another cell having only 2.6 pl per flow channel produced chromatogram A. Here the volume ratio between the two detectors is 52 and the influence on peak shape is rather evident. The center chromatogram is a n overlay of the two results which demonstrates the additional peak spreading due to the increase of 132 p1 in detector volume. While the smaller volume cell looks superior in this respect, the hot wire elements were extremely fragile and susceptible to fatigue after short periods of operation. Nevertheless these chromatograms stress even more the need for minimum detector volume when working with capillary columns.
Compound
MDQ 1.4 ppm '2.7 ppm 3.5 ppm 4.1 ppm
As these results indicate, each of these compounds was detectable below 5 ppm even for molecular weights as high as 198. A good example exists here of a point made earlier. At the detection limit of 1.3 ppm for ethane, a sensitivity of 8300 mV.ml/mg was achieved. In the present case 1.4 ppm of octane could be determined and yet the sensitivity was only 5000 mV.ml/mg. Therefore, maximum sensitivity does not mean minimum detectability, which is the desired goal.
CAPILLARY COLUMN COMPATIBILITY The previous chromatographic performance not only demonstrates the compatibility of the reduced volume TCD with ?b-in. columns, but also indicates the improved 2196
0
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
analysis
1 2
A
1 2
1 2
,i3
B
2.6 yI detector volume 135 pI
'
Ir
X64
Figure 10. I n f l u e n c e of d e t e c t o r v o l u m e on b a n d b r o a d e n i n g 1) cC14; 2) acetone. Detector: 250 "C; filament current: 175 mA; injector: 210 "C; column; 200-ft x 0.01-in. DC 550 at 50 "C; sample: 0.5 pI CCla and acetone. split 1OO:l; carrier, 3 cm3/min He: att: 30
The largest application of capillary columns is in the area of complex natural samples as exists in petroleum processing or the foods and flavors industry. A typical separation from the latter area is shown in the analysis of oil of marjoram in Figure 11. Marjoram is a commercially available spice used as a flavor additive. The chromatogram was obtained using a flame ionization detector with a programmed temperature separation. T h e sample was split in a ratio of 1 O O : l . As can be seen, a large number of peaks are resolved with the efficient column and the flame ionization detector adds negligible band-broadening. T h e early portion of the analysis is recorded a t a n a t tenuation of 128. but after 25 minutes this was reduced to 16 so as to adequately display the remaining peaks. Many partially resolved components exist, indicating the complexity of flavoring materials. For comparison, the same analysis is depicted in Figure 12 with only the detector changed. The same capillary column, t e m p e r a h r e program, and sample split ratio were used. The carrier flow was the same but the gas was changed from nitrogen to helium for compatibility with the TCD. This chromatogram dramatically demonstrates the full performance capability of the reduced volume TCD. When comparing the initial 20 minutes of the analysis, the resolution equals or exceeds t h a t obtained when the ionization detector was employed. Due to varying response factors the peak height ratios are not always in direct proportion. During the later portions of the analysis, when the concentrations of the species are reduced, the superior sensitivity of the ionization detector is evident. The TCD is about 1.00-fold less sensitive than the ionization detector. However, all the major peaks are still clearly discernible. Throughout the programmed temperature analysis, the stability of the base line with the T C D was excellent.
CONCLUSIONS The effect of cell volume on detector performance was investigated. Reductions in the minimum detectable quantity could be related to decreased cell volume because of the increased effective concentration achieved. Likewise, the reduced volume permitted operation a t increased filament currents without increasing the filament temperature. As the response increases with the cube of the current, this is definitely a desirable result. In order t o achieve the more sensitive detection with a reduced volume cell, the temperature and flow rate must be carefully
0
5
IO
15
20
25
30
Figure 11. Oil of m a r j o r a m ( F I D ) Conditions: Flame ionization detector, l o - ' * afs; 200-ft X 0.01-in. i.d. column with Dow 550; programmed from 50' to 200 "Cat 6 " C / m i n ; carrier gas N1 at 3 mi/min; split ratio 1OO:l
0
5
1 0
I5
20
25
I
30
35
M N
Figure 12. Oil of m a r j o r a m ( T C D ) Thermal conductivity, filament current 250 mA. 200-ft X 0 01-in i d column with Dow 550 programmed from 50"-200 "C at 6' min carrier He at 3 c m 3 / m i n split ratio 100 1 att 1
controlled. Utilizing a fully proportional temperature controller with proper consideration for the placement of heaters and feedback sensor, produced a detector environment with minimal axial gradients and a n extremely stable base line. The detector stability was evaluated with prolonged operation of the column oven isothermally and also while programming the column over a n 140 "C interval followed by re-equilibration a t the initial column temperature. T h e maximum detector temperature shift was 0.2 "C, accompanied by a n 0.06-mV displacement of the base line. Because of the reduced volume. this detector was evaluated with 1h-in. diameter columns and capillary columns under programmed temperature and isothermal conditions. Since the component concentrations in the detector were increased, minimum detectable quantities of 1.4 ppm could be quantitated for hydrocarbons as large as octane. Tetradecane was detectable a t the 4-ppm level. The maximum sensitivity was 8300 mV.ml/mg for ethane but it was demonstrated t h a t high sensitivities do not necessarily yield the minimum detectable quantity because noise must also be considered. As the detector contribu-
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O . 1 3 , N O V E M B E R 1 9 7 3
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tion to band-broadening appeared to be very small, separations with capillary columns were also attempted. The resolution attainable on an oil of marjoram sample with programmed temperature and a capillary column using the thermal conductivity detector was equivalent to that achieved with a flame ionization detector but a t a 100-fold decrease in sensitivity. With this type of performance, nu-
merous applications previously restricted to the flame ionization or helium ionization detectors can now be successfully analyzed utilizing the reduced volume thermal conductivity detector. Received for review January 16, 1973. Accepted June 4, 1973.
Enhancement of the Sensitivity and Selectivity of the Coulson Electrolytic Conductivity Detector to Chlorinated Hydrocarbon Pesticides John W. Dolan’ and Randall C. Hall2 Department of Entomology Purdue University, West Lafayette, lnd 47907
Factors which influence the sensitivity and selectivity of the Coulson electrolytic conductivity detector to chlorinated hydrocarbon pesticides were determined and optimized. The most influential factors which affect sensitivity are absorptive surfaces, electrode polarization, system stability. and furnace temperature. Replacement of the standard 4-mm i.d. quartz reaction tube with one of 0.5m m i d . , replacement of the silicone rubber septum at the furnace exit with a Teflon fitting, and increasing the maximum cell voltage to 44 V dc resulted in a minimum detectability of 0.1 ng for heptachlor and a useable sensitivity of 0.4 ng a s compared to 2 ng and 5 ng, respectively, for the unmodified detector. The most influential factors which affect selectivity are furnace temperature, reaction gas composition, and reaction gas flow rate. Optimization of these parameters enables most chlorinated hydrocarbon pesticides to be selectively determined in the presence of other halogenated materials such as PCB with selectivities >103:1.
Gas chromatographic detectors such as the Coulson electrolytic conductivity ( I , 2 ) and microcoulometric (3, 4 ) , which rely upon thermal degradation of the analyte for the creation of detectable compounds, are a t present the only detectors which can be used routinely for the selective detection between nanogram levels of compounds containing the same elements. Specificity between compounds containing the same elements can be achieved from differences in thermal stability, whereas specificity between groups of compounds containing different elements can be achieved by scrubbing interfering degradation products and/or selective titration. The authors ( 5 ) recently reported the application of controlled thermal degradation for the selective determi1 Present address, Department of Environmental Toxicology, University of California, Davis, Calif. 95616. 2 To whom all correspondance should be sent.
(1) D. M Coulson. J. GasChrornatogr.. 3, 134 (1965) (2) D. M . Coulson, Advan. Chrornatogr, 3, 197 (1966). (3) D. M Coulson and L A . Cavanagh. Anal. Chern.. 32, 1245 (1960). ( 4 ) D hil Coulson. L. A Cavanagh. J. E de Vries, and B . Walther. J . A g r FoodChem.. 8, 399 (1960) ( 5 ) J. W. Dolan. R C. Hall. and T M . Todd, J. Ass. Oftic. Ana/. Chern.. 55. 537 (1972).
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nation of several chlorinated hydrocarbon pesticides in the presence of polychlorinated biphenyls (PCB), The method employed the Coulson electrolytic conductivity detector and was based on the greater ease of thermal degradation of the pesticides to form HC1, which was monitored. Depending upon conditions, detector response to chlorinated pesticides such as aldrin, dieldrin, heptachlor, and heptachlor epoxide can exceed the response to PCB by 105. Controlled thermal degradation has also been utilized by Rhoades and Johnson (6) for the selective determination of N-nitrosamines in the presence of other nitrogen-containing compounds. The selective detection of chlorinated hydrocarbon pesticides in the presence of PCB greatly facilitates their determination in environmental samples. The lengthy column cleanup procedures which were required by previous methods (7-10) can be eliminated. This decreases analysis time, expense, and the possibility of introducing interfering substances from absorbents and reagents. Realization of the full potential of controlled thermal degradation for the selective detection of pesticides in the presence of chemically similar substances is presently precluded by the decreased response of the Coulson electrolytic conductivity detector under selective conditions (response is decreased to approximately 20% of its original value) and lack of knowledge of the applicability of this method to other compounds. In an effort to enhance the utility of this method, the factors which influence the response of the Coulson electrolytic conductivity detector were investigated with two objectives in mind-enhancement of sensitivity and determination of what types of chlorinated compounds can or cannot be detected under a given set of operating conditions. The influence of such factors as reaction gas composition, furnace temperature, residence time, and conductivity solvent on detector response to 13 pesticides, several PCB formulations, and a variety of model compounds is discussed. (6) J . W . Rhoades and D E. Johnson, J . Chromatogr. Sci. 8, 616 (1970) (7) 0 . W. Berg, P. L. Diosady, and G . A V. Rees, Bull. Enwron. Contarn. roxicoi., 7 . 338 (1972). ( 8 ) V. E. McClure,J. Chrornatogr.. 70. 168 (1972) (9) C. E. Collins. D. C Holmes, and F J. Jackson. J . Chromatogr.. 71. 443 (1972). (10) D . Snyder and R. Reinert. Buli. Environ. Contarn. Toxicol.. 6. 385 (1971)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
