Selective Gas-ChromatographicDetector Utilizing Emitted Radiation from a Sensitized Flame A. V. Nowak and H. V. Malmstadt Department of Chemistry, University of Illinois, Urbana, 111. 61801
The design features and performance capabilities of a simple, sensitive, and selective gas chromatographic detector are described. The operation of the detector i s based on the measurement of emitted radiation from the flame of a modified “sodium thermionic” detector. Measurement of emitted radiation rather than ionization currents results in a selective detector sensitive to halogen-containing compounds in the subnanogram range. Indirect electrical heating of the salt-coated probe and geometrical considerations have provided stable response, so that good quantitative results are possible.
RECENTGAS CHROMATOGRAPHIC detector research has been primarily directed toward the development of detector systems which are both highly sensitive and selective. Systems which rely on the measurement of emitted radiation have been particularly successful. Because of the wavelength dependency of emitted radiation from specific constituents and because of sensitive instrumentation now available for the measurement of emitted radiation, highly sensitive and selective systems are possible. Zado and Juvet (1) described a selective-nonselective photometric detector which used an oxyhydrogen flame as an excitation source. Brody and Chaney ( 2 ) described a photometric detector which employed a hydrogen-air flame as an excitation source and utilized interference filters and a flame shield to obtain a sensitive and selective system for phosphorus- and sulfur-containing compounds. Braman (3) described a photometric detection system which had a hydrogen-air flame as the excitation source and also contained electrodes for the simultaneous recording of ionization currents. The photometric detection mode was found to be one tenth to one hundredth as sensitive as the ionization current mode. Because of the relatively low excitation energy available in the ordinary hydrogen flame and the practical importance of trace determinations of halogen, phosphorus, and sulfur compounds and permanent gases, extensive work has gone into the development of detector systems capable of sensitive and selective response toward these compounds. McCormack, Tong, and Cooke ( 4 ) described a selective and sensitive emission detector which used a 2450-Mc electrodeless discharge as an excitation source. Bache and Lisk (5) used this system for determining nanogram levels of iodinated herbicides, Moye (6later described improved performance with this device by the use of argon-helium mixtures for the carrier and microwave discharge gases. Recently, Braman (7) has described a direct current discharge emission detector capable of a high degree of sensitivity and selectivity.
(1) F. M. Zado and R. S. Juvet, ANAL.CHEM., 38, 569 (1966). (2) S. S. Brody and J. E. Chaney, J . Gas Chromatog., 4, 42 (1966). (3) R. S. Braman, ANAL.CHEM., 38, 735 (1966). (4) A. J. McCormack, S. C. Tong, and W. D. Cooke, ibid., 37, 1470 (1965). (5) C. A. Bache, D. J. Lisk, ibid., 39, 786 (1967). (6) H. A. Moye, ibid., p 1441 (1967). (7) R. S. Braman and A. Dynako, ibid., 40,95 (1968).
1108
rn
ANALYTICAL CHEMISTRY
A detector system capable of high sensitivity toward halogen and phosphorus compounds is the so-called “sodium thermionic” detector (8). An alkali metal salt heated in the flame of an ordinary hydrogen flame ionization detector produces current enhancements for halogen and phosphorus compounds which can range from tens to thousands of times the response obtained from the ordinary flame-ionization mode. Response enhancements for halogen-containing compounds can be suppressed by the use of an alkali halide salt, whereas the response enhancement for phosphorus is unaffected (9, IO). No analogous effect for phosphate salts occurs (11). The only selectivity obtained with this system is from response enhancements, since the ionization of carbon compounds remains the same in the presence of the probe (12). Increases in sodium emission upon the introduction of halogen-containing compounds to the flame of a sodium thermionic detector were visually observed by Karmen (12). He suggested that the measurement of emitted radiation should result in a more selective detector system than measurement of ionization currents because only halogen- or phosphoruscontaining compounds appreciably affect the emission processes, whereas the ionization of carbon compounds remains unaffected by the presence of the salt-coated probe. On the basis of this visual observation Karmen suggested that the combustion products of halogen- or phosphorus-containing compounds acted to increase the rate of volatilization of sodium from the probe. This mechanism has been criticized by Aue et al. (13) and by Saturno and Cooke (14), who reported seeing only decreases in sodium emission upon the introduction of a halogen- or phosphorus-containing compound. Aue and Gehrke (15) reported on the use of this emission depression phenomenon for the spectrometric determination of organic compounds. Depressions in sodium emission were found to be proportional to the amount of material introduced into the flame. Hydrogen flow rates and methods of sodium introduction into the flame were found to be critical factors affecting the performance of this system. In view of the large number of variables which can affect the performance of the sodium thermionic detector, it is not surprising that conflicting data exist. Investigations in this laboratory have shown that either increases or decreases in
(8) A. Karmen and L. Giuffrida, Nature, 201,1204 (1964). (9) L. GiufTrida, N. F. Ives, and D. C. Bostwick,J. Assoc. Ofic.Anal. Chemists, 49, 8 (1966). (10) N. F. Ives and L. Giuffrida, ibid., 50, 3 (1967). (11) Zbid., p 4. 36,1416 (1964). (12) A. Karmen, ANAL.CHEM., (13) W. A. Aue, D. L. Stalling, C. W. Gehrke, R. C. Tindle, and S. R. Koirtyohann, Fifth National Meeting, Society for Applied Spectroscopy, June 1966, Chicago, 111. (14) J. J. Saturno, and W. D. Cooke, Division of Analytical Chemistry, 152nd Meeting, American Chemical Society, September 1966, New York, N. Y. (15) W. A. Aue and C. W. Gehrke, Second Midwest Regional Meeting, American Chemical Society, Lawrence, Kan., October 1966.
CONNECTION BOLTS
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Figure 2. Diagram of detector probe and heating circuit
INLET SWAGELOK
1~611
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Figure 1. Diagram of sensitized flame photometric detector
sodium emission can be observed, depending on the experimental conditions. The designation “sensitized flame” detector (16) rather than “sodium thermionic” detector is preferred here because of mechanistic considerations. The nature of halogen- and phosphorus-induced enhancements of sodium emission is not entirely known, but the designation “sodium thermionic” detector implies that the detector is operating because of surface effects such as those described by Moesta and Schuff (17) for the “thermionic halogen” detector. Recent investigations (16, 18-20) have indicated that although surface effects might contribute to the observed enhancements, the reactions occurring in the flame are primarily responsible for producing the effect, and so the more general designation “sensitized flame” detector is preferred by the authors. The detector system described herein is capable of detecting nanogram and subnanogram quantities of halogen compounds, yet utilizes only a hydrogen-air flame as an excitation source. The operation of the detector is based on the measurement of emitted radiation from the flame of the sodium thermionic detector. The detector is operated under conditions such that increases in sodium emission are obtained upon the introduction of the halogen- or phosphorus-containing compound. Data are presented demonstrating the sensitivity, selectivity, and stability characteristics of the detector. INSTRUMENTATION
The detector system requires only a few easily machined parts for construction. It should also be possible to modify some commercial flame ionization detectors for photometric detection by the addition of an interference filter holder and photomultiplier housing. The detector can be adapted to
(16) J. M. Baldwin, doctoral thesis, University of Illinois, February 1968. (17) H. Moesta and P. Schuff, Ber. Deut. Bunsenges., 69,895 (1965). (18) E. Cremer, J . Gus Chromafog., 5, 329 (1967). (19) A. V. Nowak, doctoral thesis, University of Illinois, February 1968. (20) F. M. Page and D. E. Woolley, ANAL.CHEM., 40, 210 (1968).
any gas chromatograph possessing the necessary flow control valves and space required for placement of the multiplier phototube. Detector Design. A cross-sectional view of the detector is shown in Figure 1 . The effluent from the column and the hydrogen are introduced into the flame by means of a Swagelok tee, a. The straight part of the tee is drilled through with a S/32-inch bit to allow the 1/16-inchconnection tubing from the column to extend up to the burner base. The hydrogen gas surrounds this tubing and helps to sweep the effluent from the column into the flame. Air for combustion is introduced into the base of the detector, b, by means of a Swagelok union and is directed around the flame by means of a simple sheath arrangement provided by a Teflon ring, c, which also provides the necessary electrical insulation for mounting the salt-coated probe. The burner tip consists of a 21-gauge syringe needle ground flat and silver-soldered to a Swagelok nut. Probe Heating Circuit. For operation in the sensitized flame mode an electrically heated Na9SO4-coatedplatinum spiral is used as a sodium source. Electrical heating allows the probe to be mounted lower, as shown in Figure 2, than when the flame is used to heat the probe. This decreases the effect of flame-shape changes on response and produces a more sensitive photometric system, because more light will reach the phototube when the probe is not acting as a light shield. The probe, which was constructed by wrapping 21-gauge platinum wire around a 1/4-inch diameter rod to form a two-loop helix, was mounted such that the top of the probe was just even with the burner tip. The probe was coated by applying a saturated solution of Na2S04 with a syringe and then gently heating the probe to evaporate to dryness. After sufficient salt was applied, the current was increased until the probe glowed red and the salt fused. Detector Housing. Two types of detector housings can be used. For measuring only emitted radiation, the housing shown in Figure 1 can be used. A quartz disk placed over the port in the housing isolates combustion products from the interference filters. For simultaneous measurement of ionization and photocurrents, the detector housing shown in Figure 3 is used. This housing contains a set of parallel stainless steel electrodes (interelectrode distance 7 mm) connected by means of 1/8-inch copper tubing through Teflon insulators to BNC connectors. The interference filter holder which also serves as the connection between the detector housing and the multiplier phototube housing is shown in Figure 4. A 589-mp interference filter was used to isolate the sodium emission line. Data Readout. A schematic diagram of the arrangement used for measuring both ionization and photocurrents is shown in Figure 5. Two Heath EUW-301 recording pH electrometers equipped with photometer modules (21) were VOL 40, NO. 7, JUNE 1968
11 0 9
7/16" OD.
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STAINLESS STEEL ELECTRODES
CONNECTOR
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Figure 3. Diagram of detector housing for simultaneous recording of photocurrents and ionization currents used for measuring ionization and photocurrents. It is also possible, by combining two of these units, to record both the chromatographic peak and the integral of the peak. The results in Tables I and I1 were obtained by electronic integration of the chromatographic peaks (21). Zener-regulated power supplies were used to provide the polarizing voltages for both the 1P28 multiplier phototube and the electrode system. A number of factors affect the sensitivity of the detector system, the most important of these variables being the hydro(21) H. V. Malmstadt, R. M. Barnes, and P. A. Rodriquez, J. Chem. Educ., 41,263 (1964).
Table I. Response Selectivity of Sensitized Flame Photometric Detector HI. 62 ml/min Variac setting. 25 He, 13.7 ml/min Air. 400 ml/min Coulombs/ mole Coulomb Sample (0.5 pl) 0.128 5.54 x 10-7 n-Pentane 0.154 5.88 X lW7 n-Hexane 0.135 4.58 X lW7 n-Heptane 0.090 2.78 X 10-7 n-Octane 3.49 x 10-7 0.051 Acetone 1.50 x 10-7 0.012 Methanol 0.032 2.74 X lW7 Ethanol 0.037 2.44 x 10-7 1-Propanol Sample (5 p10.001%) 3.07 x 10-7 460 Ethyl bromide 4.22 X lO-1 681 Ethyl iodide 4.87 x 10-7 Methyl iodide 606 1.35 x 10-7 172 Methylene chloride 2.01 x 10-7 339 Chloroform 492 Carbon tetrachloride 2.45 x 10-7 Sample (5 p l ) 4.66 X 10-0 0.542 0.01% carbon disulfide 3 . 4 4 x 10-8 0.383 0.10 carbon disulfide 0.173 7.74 x 10-0 0.10% ethyl ether 0.032 1.56 X 10-8 1.0% ethyl ether 0.362 2.94 X 0.10% methyl formate 0.133 1.08 X 10-7 1.0 % methyl formate 0.031 1.52 x 10-7 10% diethylamine 5.39 2.30 x 10-7 0.10% trimethyl phosphate
1 1 10
ANALYTICAL CHEMISTRY
FRONT VIEW
SIDE VIEW -TEFLON
Figure 4. Diagram of interference filter holder
gen flow rate. Increases in sodium emission are observed at hydrogen flow rates of less than 100 ml per minute. At rates greater than 100 ml per minute decreases in sodium emission can be observed. Hydrogen flow rates of about 50 ml per minute were found to give optimum sensitivity with good signal-to-noise ratios and were used throughout this investigation. Helium carrier gas flow rates also affected sensitivity, although not as significantly as hydrogen, with decreasing sensitivity being obtained with increasing flow rates. Air flow rates were noncritical and were kept at about 400 ml per minute. A simple gas chromatograph was constructed for testing the detector. It was equipped with a heated column oven chamber, dual injection ports, and needle valves for control of gas flow rates. The column used for all separations except the phosphorus compound consisted of a 4-foot, l/s-inch 0.d. length of copper tubing packed with 60/80-mesh Chromosorb W (Johns-Manville) coated with 15% by weight Carbowax 600. The column was operated at 30 "C. For investigations with trimethyl phosphate, the column consisted of a 2-foot lI8-inch0.d. length of copper tubing packed with 60180mesh Chromosorb W coated with 5 % Apiezon M andwas o p erated at 200 "C. All gases used were passed over Linde 4A Molecular Sieve. For all investigations, the Variac was kept
IP28 PHOTOMULTIPLIER TUBE
300v
-
SALT-COATED PROBE
COLUMN EFFLUENT
CURRENT RECORDERS (21)
Figure 5. Schematic diagram showing arrangement for simultaneous recording of photocurrents and ionization currents
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QUANTITY OF CH31, GRAMS
Figure 6. Sensitivity and dynamic range of sensitized %me photometric detector at a setting of 25 or 30. The probe temperature obtained at these settings provided good sensitivity and stability. At these Variac settings, background photocurrents were of the order of 10-8 ampere. Noise levels were less than the recorder pen width at the readout sensitivities used. DETECTOR CHARACTERISTICS Sensitivity and Dynamic Ranges. The sensitivity and dynamic range of the detector are demonstrated by measuring the response to CH31 in ethanol over the range to IO-lO gram, as illustrated in Figure 6. Subnanogram levels of CHJ are determined at a signal-to-noise ratio of 5.4. The response is linear in the middle range, but above about lo-' gram and below 10-9 gram of CH31the nonlinear response becomes significant. Under the experimental conditions used, the lower limit of detection for CH31was about 50 picograms determined at a signal-to-noise ratio of 2.6. Typical chromatograms for solutions of CHII in ethanol are shown in Figure 7, and these illustrate the sensitivity and selectivity of the detector. As shown in Figure 7, 57 picograms of C H d can be easily measured above the noise. The response to the ethanol is approximately one 20,000th of the response to the CH31. Selectivity. The main advantage of measuring emitted radiation rather than ionization currents is the selectivity which can be obtained. The response selectivity of the photometric detector is illustrated in Figure 7 by comparing the response obtained for a trace amount of CH3I in the large amount of ethanol solvent. Selectivity characteristics for the photometric and ionization-current modes can be compared by simultaneously measuring the photocurrents and ionization currents for the same sample. This is done for a sample of CH31 in ethanol, and the resulting chromatograms are shown in Figure 8. Since the measured ionization current is relatively unaffected by the presence of the probe, a large ionization current is obtained for the ethanol, but the 589-mp emission enhancement for ethanol is small. The selectivity of the photometric detector to various classes of compounds was measured and some typical results are shown in Table I. Response is given in terms of coulombs per mole. Only halogen-containing compounds give significant increases in 5 8 9 - m ~emission and response ratios for halogen-containing compounds as compared to non-halogen-
Table 11. Response Stability of Photometric Detector H2 flow at beginning. 63 ml/min 57 ml/min After 21 hours. Air. 400 ml/min He. lSml/min Sample. 5 pl CHaI in ethanol Variac setting. 25 Integral divisions log(^ x 109) Sample Time 34.0 0.919 0.00001 %" Start 34.2 0.921 29.7 0.860 0.00010 %* 28.5 1.844 28.5 1.844 27.2 1.824 0,0010 %' 34.0 2.919 34.8 2.929 2.932 35.0 31.8 0.890 0.0OOol~ After 5 hours 0.897 32.3 0.893 32.0 27.0 1.819 0.000 10% 27.3 1.824 24.8 1.782 32.3 2.896 0.0010~ 34.5 2.925 32.5 2.899 0.0OOol~ 33.5 0.912 After 9 hours 31.0 0.878 31.5 0.886 0.00010% 25.6 1.797 26.3 1.809 26.3 1.814 0.0010% 31.4 2.884 32.9 2.905 31.3 2.883 29.0 0.850 After 21 hours O.ooOol% 27.0 0.819 29.5 0.857 0.00010 22.5 1.741 23.0 1.751 24.0 1.769 0.0010% 29.0 2.850 29.7 2.860 29.6 2.858 a Integral sensitivity 2.44 X 10-'0 Q/div. b Integral sensitivity 2.45 X 10-0 Q/div. Integral sensitivity 2.44 X Q/div.
z
containing compounds vary from hundreds to tens of thousands. This represents a considerable improvement over the 4- to 30-times response enhancements for halogen compounds which have been reported in the literature (22, 23). Response ratios will vary depending on experimental conditions, but these data give some idea of the degree of selectivity which can be expected with this system. Experiments with the chlorinated-methane series show that sensitivity increases with increasing halogen substitution. The response for different halogens also varies in the order CI < Br < I. The phosphorus-containing compound affected the emitted radiation to a greater extent than did the other non-halogencontaining compounds. The response was, however, two orders of magnitude lower than that obtained for the halo(22) L. Davies, reported by A. B. Littlewood, J. Gas Chromafog., 4, 32 (1966). (23) N. F. Ives and L. Giuffrida, J. Assoc. Ofic.Anal. Chemists, 50, 1 (1967). VOL. 40, NO. 7,JUNE 1968
0
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Figure 7. Chromatograms illustrating sensitivity and selectivity capabilities of the sensitized flame photometric detector Figure 8. Comparison of photometricand ionization current response gens. These results are unusual, in that response enhancements for phosphorus-containing compounds obtained by measuring ionization currents have been reported to be much larger than those obtained for halogen-containing compounds (23). Further work is required to show the effect of phosphorus-containing compounds on both the emitted radiation and the ionization currents. Stability. The main disadvantage of the sodium thermionic detector is the decrease in sensitivity as the salt is evaporated from the probe. In some commonly used versions of this detector, the collector, electrode is coated with the salt and acts as the probe. This arrangement has the disadvantage that the electrode cannot be positioned for maximum stability or maximum enhancements independently of the optimum geometry for maximum ion-collection efficiency. With the photometric detection mode or with independent electrodes, the probe serves only as a medium to hold the salt and can be positioned for maximum stability or response enhancements. Jentzsch et al. (24), in describing the operating characteristics of a dual stacked flame sodium thermionic detector, presented calibration curves for various compounds made over a 24-hour period which showed the gradual loss in sensitivity with time. The response sensitivity could decrease by as much as an order of magnitude over the 24-hour period. Davies (22) was not able to obtain good quantitative results from the sodium thermionic detector but stated that this disadvantage was outweighed by its qualitative behavior. The stability requirements of a detector system depend on the individual analysis requirements. For strictly qualitative investigations the response stability is not a very important factor. However, for quantitative analysis the response stability must be satisfactory for the desired precision and accuracy of the measurement. The stability must hold, at a minimum, for the time it takes to prepare a calibration curve and run the unknown samples. With proper positioning of the probe and electrical heating, high stability can be maintained for relatively long periods. To determine the stability of the photometric detection mode, calibration curves for samples of CHJ in ethanol were prepared at various time intervals. During this entire time no adjustments of any
kind were made on the system. The results of these experiments are shown in Table I1 and Figure 9. Table I1 contains the data used to obtain the calibration curves shown in Figure 9. These data illustrate the precision obtained for multiple injections. The sensitivity decreases with time, but so gradually, that other sources of error such as those due to injection of samples and variations in hydrogen flow rate limit the accuracy and precision during the time intervals it would normally take to complete an analysis. Under the experimental conditions used the detection limits were also excellent, since the data obtained for the lowest concentration sample were determined at a signal-to-noise ratio of more than 15. SUMMARY
The sensitivity, selectivity, and stability characteristics of a sensitized flame photometric detector are demonstrated. The measurement of emitted radiation rather than ionization currents has advantages in terms of both sensitivity and selectivity. Although Karmen predicted that measurement of emitted radiation should result in a detector system comparable in sensitivity to one obtained from the measurement of 3.00
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(24) D. Jentzsch, H. G. Zimmerman, and I. Wehling, Z . Anal. Client, 221, 377 (1966).
11 12
ANALYTICAL CHEMISTRY
OF CH31,
GRAMS
Figure 9. Stability characteristics of the sensitized flame photometric detector
ionization currents, the photometric mode seems capable of greater sensitivity. This might be the result of geometrical considerations, because electrode geometry can be a rather critical factor in determining the sensitivity limits of a detector system. The results of this investigation have shown that the measurement of sodium emission provides a detector system which is highly sensitive to halogen-containing compounds. Since photometric detection does not affect the measurement
of ionization currents, it is easy to record the enhanced ionization currents and photocurrents simultaneously. The system can also be easily converted to the ordinary flame ionization or flame photometric modes by removal of the salt-coated probe. RECEIVED
for review December 11, 1967. Accepted March
19,1968.
Selection of Optimum Position for Measuring Widths of Chromatographic Peaks with Minimum Error in Area D. L. Ball' and W. E. Harris Department of Chemistry, Uniuersity of Alberta, Edmonton, Alberta, Canada
H. W. Habgood Research Council of Alberta, Edmonton, Alberta, Canada
Relations previously developed relating the four individual measurement uncertainties in height-width measurements to the resultant error in area were used to examine the effect of variations in the value of r , the fractional height at which the width is measured, on the precision of the determination of area. The optimum fractional height is shown to depend on peak shape. I n general, sharp Gaussian peaks should be measured for width close to the base line, and flat, broad peaks should be measured close to the commonly used half height. I n practice, a single fractional height is desirable. If a full range of smooth Gaussian peaks is normally encountered, then the best single value of r to choose is 0.25.
choice of fractional height on the errors resulting from each of the four measurement steps involved in the height-width method. The area A of a Gaussian peak is calculated from the measured values of height h and width wr by the formula
where C, is a constant for a given fractional height r and is given by
c, = ! 2
MEASURING THE AREA under a Gaussian or near Gaussian peak is a common operation in gas chromatographic analysis. For a given method of integration, many factors affect the precision of the area determination. In this paper the most commonly used method of integration, the height-width technique, is considered. In particular, the problem of choice of optimum position at which to measure the width is examined in detail. As shown in previous studies (I, 2 ) there are four independent sources of indeterminate error arising out of the four operations in the height-width method. The choice of fractional height at which the peak width is measured enters into and contributes to the final error in a complex manner. Our object is to consider the optimum position for measuring peak width and how this optimum position is affected by peak shape and peak area. This question has been partly treated by Said and Robinson (3). Their analysis was concerned primarily with the effects of uncertainties in measuring peak height. The present study evaluates the effect of Present address, Selkirk College, Castlegar, British Columbia, Canada
(1) D. L. Ball, W. E. Harris, and H. W. Habgood, Separation Sci., 2, 8 1 (1967). (2) D. L. Ball, W. E. Harris, and H. W. Habgood, ANAL.CHEM., 40, 129 (1968). (3) A. S. Said and M. S . Robinson, J. Gas Chromatog., 1(9), 7 (1963).
(1)
A = C,hwr
In ( l l r )
(See Reference 3 for a development of this expression.) The intermediate height y at which the peak width is to be measured is chosen to be some fraction r of the total peak height so that r is equal to y/h. As previously explained ( 2 ) , the calculation of peak area according to Equation 1 depends on h and w,, but to obtain values for these two parameters requires four separate operations. Each of these operations introduces an error directly or indirectly into the computed result. First a base line must be located and drawn under the peak. The standard deviation of this measurement we have designated AB. The peak height h is then measured from this established base line and the associated standard deviation is Ah. Next, the position of the intermediate height y (equal to rh), is located. The standard deviation in locating y is A y . Finally, the peak width wr is measured at position y with a standard deviation in this measurement of Aw. In a previous study (2) numerical values obtained by a particular group of observers for AB, Ah, and A y were about 0.010, 0.012, and 0.021 cm and the values for A w were found to depend on the angle a between the peak sides and the scale used to measure width according to the relation Aw
=
Am(1
+C O ~ ~ C ~ ) ~ * ~
(3)
with Am equal to 0.008 cm. These four errors occur independently, hence their total effect is obtained by adding them in a statistical sense according to the following generalized error expression (2)) VOL. 40, NO. 7, JUNE 1968
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