Specific Detection of Halogens and Phosphor1 Flame Ionization ARTHUR KARMEN Johns Hopkins University School o f Medicine, Baltimore, Md.
b A hydrogen flame ionization detector has been developed that is sensitive only to compounds containing chlorine, bromine, iodine, and phosphorus. A wire mesh that has been treated with sodium hydroxide, or one of several other hydroxides or salts, is heated in a hydrogen flame. The presence of a compound containing halogens or phosphorus in the flame gores increases the rate of volatilizotion of sodium or other metal vapor from the screen. This metal vapor i s then detected b y flame ionization in a second hydrogen flame. Nanogram quantities of hologen-contoining compounds in the presence of much larger quantities of compounds not containing these elements were specifically detected.
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ORGANIC CuMPomns are burned m a hydrogen flame, the electrical Conductivity of the flame is increased. This is the basis of operation of the hydrogen flame ionization detector (9). Recently it was noted that compounds containing halogens or phosphorus increased the electrical conductivity more when a wire probe containing a sodium compound was heated in the flame. The presence of the probe thus caused the responsiveness of the detector to these compounds to be enhanced (5). Further study revealed that adding a halogen-containing compound to the flame gases could increase the rate of volatilization of a number of different metals from a wire heated in a flame. The increased rate of volatilization of these metals was manifested both by an increase in the emission of the light characteristic of the metal, and, in the case of metals with sufficiently low ionization potentials, by an increase in the electrical conductivity of the flame. The mechanism of the enhanced sensitivity of the detector to compounds containing halogens and phosphorus thus appeared to he that the products of combustion of these compounds reacted with the probe to increase the rate of release of sodium from it and the sodium vapor was then excited and ionized in the flame. This effect has been utilized in the design of a hydrogen flame ionization detector that is sensitive only to com-
1416
HEN
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ANALYTICAL CHEMISTRY
pounds containing chlorine, bromine, iodine, or phosphorus. Hydrogen gas was added to the effluent of the column and the mixture was burned in an air atmosphere. The record of the electrical conductivity of this flame was the same as that of the usual hydrogen flame ionization detertor. A wire mesh screen previously treated with a solution of an alkali metal hydroxide or salt was mounted above the flame. This screen served two purposes: it defined the volume of ga.7 of which the electrical conductivity was to he measured and it acted as a potential source of alkali metal vapor. When a compound containing halogen or phosphorus was burned in the flame, the rate of release of alkali metal vapor from the heated screen was increased. Since this vapor was carried upward in the flowing gas, it did not enter the flame and did not change its electrical conductivity. A second hydrogen flame was mounted just above the screen. The metal vapor released from the screen entered this flame and was excited or ionized by it. The electrical properties of the upper flame were therefore very sensitive to the presenrr of halogen or phosphorus in the
..
lower flame. On the other hand, since very little unburned material reached the upper flame, compounds not containing halogens or phosphorus were not detected. EXPERIMENTAL
Apparatus. The effect of halogencontaining compounds on solids heated in a flame was studied using conventional Bunsen burners (Figure 1). A wire mesh screen was mounted above one burner. The flame of the second burner was directed horizontally above the screen so t h a t the products of combustion of the first burner mixed with the gases burning in the second. Volatile materials could be added to either flame through the air intake of each burner. DETECTOR CELLS. Two different kinds of detectors were used in this study. The first was a hydrogen flame ionization detector essentially as described by McWilliam and Dewar (9) (Figure 2). Hydrogen gas was added to the column effluent and the mixture was burned a t a jet consisting of a 2% gauge hypodermic needle. A platinum wire ring electrode, approximately 8 mm. in diameter and insulated electrically from the remainder of the detector, was suspended concentric with
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detector. Viol contoining chloroform m>y b, seen near air intoke of the lower burner
was burned at the Ion er jet A separate flon of hydiogen and nitrogen mas buined a t the upper jet. The electrical conductirit? of each flame n a s recoided .eparatel> The wire mesh screen belou the upper jet divided the detector into t a o sections and provided the electrostatic -hielding necessary to permit the electrical conductivity of each flame to be measured nithout one measurement interfering with the other. The screen was constructed by spot welding stainless steel mesh (50 X 50 SurgaIGy mesh, Davis and Geck Co., Danbury, Conn.) or platinum wire gauze 52 mesh (Fibher Scientific c0.) around the circumference of a flat. stainless steel ring. The screen was cleaned by heating it in a Bunsen flame until it no longer impart'ed the characteristic vellow sodium color to the flame. i t was then dipped into a dilute aqueous solution (approximately 1%) of one of the alkali metal chlorides or hydroxides, following which it was again heated in the Bunsen flame. Responsiveness to halogens \vas checked by placing an open vial containing chloroform near the air intake of the burner and observing the color of the flame. The screen was then installed in the detector. CoLuaws. The columns used in these experiments were 5 feet long, straight glass tubes 5-mm. i.d. They were packed with ethylene glycol adipate polyester, 147,, on GasChrom P, 86%, (.ipplied Science Corp.) : or with Carbowax 400, 570 (L-nion Carbide Corp.), on Chromoborb-W, 95% (Johns Manville Corp.) or with SE-30 qilicone gum, 1% (General Electric Co.), on Chromosorb-W. 99%.
AIR Figure 2. Schematic of hydrogen flame ionization detector. The electrical conductivity of the gas between the ring and the body of the detector was recorded
the jet approximately 5 mm. above it. The electrical conductivity of the gas between the electrode and its environment was recorded. Probes were constructed by threading 25-gauge platinum wire through 3-mm. selections of unglazed alumina tubing. The alumina t'ubing could be suspended directly above the flame and could be removed without disturbing the ring electrode or materially altering the electric fields d h i n t,he detector cell. Each probe was dipped into a solut,ion of one of several metal salts or hydroxides and then dried in a 13unsen flame. Identical aliquots of mixtures of compounds were analyzed by the gas chromatography column first with the detector operated in the usual fashion and then with the treated probe in place above the flame jet. The second detector cell had two flame jets, one above the other (Figure 3). .in electrode \vas mounted above each jet. Hydrogen gas was added to the column effluent and the mixture
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d' i
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Figure
3. Schematic
of
hydrogen
flame ionization detector specific for halogens and Phosphorus
Synthetic mixture5 were analyzed under different conditions to determine the specificity of the response of the upper flame, its ultimate sensitivity, and the factors that influence its sensitivity. RESULTS
Bunsen Flames. When sodium hydrovide was applied to the wire mesh (Figure l ) , the size of the lower Bunsen flame could be adjusted so t h a t it volatilized the sodium from the mesh and delivered it to the upper flame without itself becoming yelloB-. After continuous heating for 15 to 20 minutes the rate of volatilization of the sodium slowed sufficiently so t h a t yellow was hardly detected in the upper flame. Adding chloroform vapor to the lower flame then caused a large increase in the intensity of the yellou light in the upper flame. Xdding chloroform to the upper flame had no
s
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Figure 4. Analysis of a l - p l . sample of a diethyl ether solution containing v./v.: (1 ) 0.1 % carbon tetrachloride, (2) 1 .O% fluorobenzene, (3)1 .O% toluene, (4) 1 .O% chlorobenzene
Figure 5. Analysis of a 1 -PI. sample of a diethyl ether soluchlorotion containing v./v.: ( 1 ) 1 .O% acetone, (2) 0.1 form, (3) 1 .O% toluene, (4)1 .O% chlorobenzene
The upper graph is the record of the electrlcal conductivity of the lower flame, the lower graph that of the upper flame o f the detector shown in Figure 3
The upper graph i s the record of the electrical conductivity of the lower flame, the lower graph that of the upper flame o f the detector shown in Figure 3
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Figure 6. Analysis of a 1-PI. sample of diethyl ether solution containing v./v.: ( 1 ) 0.1 butyl chloride, (2) 0.1 % butyl bromide, (3) 0.1% butyl iodide
70
The upper graph is the record of the electrical conductivity of the lower flame, the lower graph that of the upper flame of the detector shown in Figure 3. The screen used h a d been treated with sodium nitrate-nitric acid
Figure 7* Analysis Of a as in Figure 6
'-dsample Of
the same solution
The screen used hod been treated with potassium hydroxide. response to butyl chloride ( p e a k 1 ) was increased
such effect. When the screen was replaced by a copper mesh, adding chloroform vapor to the lower flame caused the upper flame to become green in color, and adding chloroform vapor to the upper flame had no such effect. Similar reactions were observed with screens treated with hydroxides, nitrates, sulfates, and chlorides of lithium, potassium, rubidium, cesium, barium, and strontium. Single Flame. K h e n the ceramic tubing was placed in the flame of the detector s h o a n in Figure 2 , the baseline current usually increased to ampere or higher, depending on the temperature of the tubing, and the color of the flame changed to the color characteristic of the metal salt with which the tubing had been treated. 1418
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ANALYTICAL CHEMISTRY
With a sodium hydroxide-treated probe in place, the detector response was increased to compounds containing chlorine, bromine, iodine, or phosphorus. KO change was observed in the response to compounds containing sulfur, fluorine, or nitrogen, or to straight chain, cyclic, or aromatic hydrocarbons, esters, ethers, acids, or ketones. The response to compounds containing halogens or phosphorus increased , with increasing temperature of the probe, but increasing the temperature increased the base-line current and background noise as well. The response to chloroform could be increased up t o tenfold by heating the ceramic tubing to red heat. As much as a two-hundred-fold increase in
The relative
response to tributyl phosphate was obtained under the same conditions. Double Flame Detector. T h e upper flame of the detector cell in Figure 3 responded only to compounds containing chlorine, bromine, iodine, and phosphorus. Its sensitivity to fluorinecontaining compounds was much less, but a definite response was obtained, particularly when a screen treated with a cesium salt was used. T h e detector did not respond to compounds containing sulfur or nitrogen, or to a variety of compounds containing carbon, oxygen, and hydrogen (Figures 4-7). To determine the effects produced by each component of the detector, a
mixture of halogen-containing and nonhalogen-containing compounds was analyzed. Then, the upper flame was turned off, the voltage to its electrode was disconnected, and the screen was removed. Repeat analysis of the same sample revealed no change in the response of the lower flame. Thus there was no appreciable backflow of sodium vapor from the screen to the lower flame. T h e screen was then reinstalled in the detector and the voltage to the upper cell was reconnected. There was no response in the upper cell to any of the components of the mixture when 150 volts were impressed between t,he lower electrode and the body of the detector. There was 3 small response to each of the compounds when only 90 volts were impressed indicating that a sufficient voltage to collect all the ions that are formed in the lower flame should be used. There was a response to the solvent peak in the upper detector, even with the flame off, indicating that the capacity for ion collection in the lower detector was overloaded. Relighting the upper flame restored the response of the upper flame to halogen-containing compounds. Increasing the size of the upper flame by increasing the hydrogen flow increased the response to these compounds. Similarly, placing the upper flame jet close to the heated area of the screen resulted in a larger response than when the upper flame jet was moved further away. Increasing the size of the lower flame, and thus increasing the temperature of the screen above it, increased the responsiveness of the upper flame up to a masimum. Further increase in the size of the lower flame increased the base-line current of the upper detector but did not increase its responsiveness. The lower flame was usually operated a t a size just sufficient to heat the screen to red heat. When the screen was operated a t a lower temperature, not only was the sensitivity less but the response time of the upper detector was somewhat prolonged leading to some distortion (tailing) of the shapes of the peaks emerging from the column soon after the solvent. Quantification and Sensitivity. T h e response of the double flame detector to graded amountz of chloroform, other halogen-containing compounds, and phosphorus-containing compounds increases progressively with increasing rates of delivery of the sample, permitting quantit,ative analyses t o he obtained without difficulty. However, the response is linearly related to the rate of delivery only a t a very low rate. As would be Iiredicted from the reported decrease in the fraction of alkali metal that is ionized in a flame as its concentration in the flame gases is increased (I)? the
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Figure 8. Analysis of 1 -PI. samples of diethyl ether solutions each containing 1 % v./v. toluene and graded amounts of chloroform One microliter of solution (A) contained 1.0 pg. o f chloroform; (6) 0.3 g g , ; (C)0.1 ; (D)0.03; (E) 0.01; (F) 0.003. The electrical conductivity of the lower flame in Figure 3 is shown on the right, the conductivity of the upper flame on the left. N o t e that p e a k 1 on the records on the right is not reduced as the chloroform is reduced below 0.3 pg., indicating a p r o b a b l e contaminant in the other reagents. That this contaminant did not contain halogen is shown 011 the records on the left
sensitivity of the upper flame to the rates of delivery of halogen to the lower flame decreases as the rate increases. Samples containing methylene dichloride, chloroform, and carbon tetrachloride were prepared so that the peak heights were grossly similar. The relative response of the detector to each of these compounds was in direct proportion to the amount of chlorine present. The relative sensitivity of the detector to butyl bromide, chloride, and iodide varied somewhat when screens treated with different cations were used. The sensitivity to butyl bromide was greater
than that to butyl chloride or butyl iodide with each of the screens. The relative sensitivity to butyl chloride was higher when a potassium-treated screen was used than with any of the others (Figures 6, 7 ) . The sensitivity of the detector to phosphorus was from seven to 14 times t h a t to an equal weight of chlorine, depending on the screen used. The usable sensitivity of the detector to chloroform was assessed by preparing solutions containing graded amounts of chloroform in a solvent consisting of 1% toluene in diethyl ether. Three nanograms of chloroform could be VOL. 36, NO. 8, JULY 1964
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distinguished from base-line noise. At the same sensitivity setting there was no response to 10 pg. of toluene (Figure 8). Leas than 0.3 nanogram of chloroform could be distinguished from noise of the same band width using the same screen. The sensitivity of the detector to phosphorus was determined by analyzing graded amounts of triethyl phosphate. One nanogram of triethyl phosphate, emerging from a column so that a peak with a base width of thirt'y seconds was recorded, could be distinguished from base-line fluctuations with a 5:l signal to noise ratio. DISCUSSION
Quantification. Because the responsiveness of the detector changes with change in the temperature of the screen, and, therefore, with changes in the size and temperature of the lower flame, and with change in the size and temperature of the upper flame as well, it was necessary to calibrate the response of the detector to obtain quantitatively reliable results. hnalysis of a sample containing different quantities of two or three halogen-containing substances and enough of a nonhalogen-containing compound to be detected easily in the lower flame readily permitted a response curve to be constructed. Khile the sensitivity of the detector decreases gradually as the alkali metal is evaporated from the screen by the flame, this process takes many days. The detector was constructed so that removal and replacement of the screen was easily performed. Mechanism. When a copper wire is heated in a flame? adding compounds that contain halogens to the flame gases imparts a characteristic green color to the flame. This reaction is the basis of t'he familiar Beilstein flame test for halogens. Flame photometric methods for halogens have also been reported, based on the reaction of halogens with copper, in which a solution of a copper salt is delivered to the flame at a constant rate (4), and halogen is detected by the intensity of the copper halide band spectra. The experiments with two Bunsen burners reported here indicated that in reacting with a wire heated in a flame, in addition to reacting with the copper vapor to yield copper halide band spectra, halogen increases the rate of volatilization of the copper from the wire as well. Evidence for this was provided by the change in color of the upper flame when halogen was added to the lower flame so that it impinged on the heated copper, and the absence of color change when the halogen was added directly to the upper flame. Similar reactions were observed when wire probes dipped into solutions of salts of each of the 1420 *
ANALYTICAL CHEMISTRY
different alkali metals were heated in the loaer flame. Thus when a probe treated with a sodium salt was heated, there was no discernible change in the intensity of the sodium emission of the upper flame when chloroform vapor was added to the upper flame, while adding the chloroform to the lower flame caused a large increase. I t is possible that the increase in the ionization that has been reported to occur in a flame containing alkali metals when a halogen is added may play a part in the mechanism of action of the detector described in this paper ( 1 1 ) . However, it appeared from these experiments that the primary effect of the halogen was to increase the rate of volatilization of the various metals from the heated wire rather than to make them more easily excited or ionized. Since the rates of volatilizaton of a number of different metals are similarly increased, various compounds can be used to treat the screen. Alkali and alkaline earth metals are all ionized to some extent in a flame. Other metals can be used as well, if flame photometry is used to detect them. The primary criterion for choosing a material with which the screen is to be treated is that its rate of volatilization be markedly increased in the presence of halogens. Both the rate of volatilization and the change in the rate are functions of the temperature of the probe and the material of the probe as well as the material with which it is treated. The materials tested so far include borosilicate glass, platinum, nichrome, and nickel wire. When these materials, untreated, were heated in a flame, adding chloroform vapor often caused a visible increase in sodium emission. I t was somptimes difficult to demonstrate the reaction with other metals without using a spectroscope because of the strength of the sodium emission. A platinum wire dipped into a solution of any sodium salt, including sodium chloride, when heated in the flame, showed the same greater release of sodium in the presence of halogen as did a wire dipped into sodium hydroxide. When the screen was treated with an acid salt rather than a hydroxide, less tailing of both halogen and phosphorus peaks was observed. This seemed reasonable in view of the acid properties of the products of combustion of these compounds. How the halogens and phosphorus cause the increase in volatilization of alkali and alkaline earth metals from a heated metal and why one is more effective in so doing than another is in doubt. If the size of the lower flame is increased, or if the screen or wire probe is placed within the flame so that metals released from it can be excited or ionized in it, the response of the lower
flame is altered. The base line current is increased and the response to halogens is enhanced. Although it has not been possible to detect halogens or phosphorus specifically by flame ionization, using a single flame in this way, because of the ionization of most organic materials in the flame, it should be possible to do so if flame photometry is used instead of flame ionization. Because of the low ionization potential of the alkali metals, their release above the screen of the detector can conveniently be detected by ionization. Several of the familiar methods of ionization detection in gases may reasonably be expected to be useful in detecting these metal vapors, including use of a source of thermal electrons, or photoelectrons, or a radioactive source. While ionization of these metals in a hydrogen flame, as described here, is not linearly related to their concentration, except a t very low levels, the hydrogen flame offers the advantages of less sensitivity to water, air, and the fixed gases than the other methods of ionization detection. The specificity of the detector reported here is also a direct result of the properties of the hydrogen flame ionization detector, particularly the complete insensitivity of the detector to organif compounds once they have passed through a hydrogen flame and have been combusted. The hydrogen flame ionization detector cell described by McWilliam and Dewar (9) has a platinum electrode suspended directly above the flame. The probable reason why enhanced sensitivity to halogens and phosphoruscontaining compounds has not been widely observed before is that the electrode is usually sufficiently far above the flame so that metal vaporized from it does not enter the flame. Care is usually taken to avoid heating any other part of the detector, arid alkali metal salts present on the flame jet, which may become quite hot, are probably volatilized in time. Other detectors for halogens in compounds eluted from a gas chromatography column include: the I3eilstein flame test itself, which has been used quantitatively (10) and qualitatively (3); the electron affinity detector, widely used for detecting halogen-containing compounds as well as other electronegative materials (8); and a halogen sensitive leak detector element ( 2 ) . Halogen-containing compounds can also be detected by subjecting the effluent to combustion followed by coulometric titration. Such selectively sensitive detectors are particularly useful for analyzing complex biological mistures. .1 detector selectively sensitive to halogen-containing compounds facilitates the analyses of trace quantities of these compounds, by minimizing the amount of prelim-
inary preparation of the sample that is necessary. A specific detector also makes it possible to analyze for a group of compounds having a common chemical functional group. For example, it is possible to detect alcohols by preparing labeled acetates with carbon-14-labeled acetic anhydride, analyzing the mixture by gas chromatography, and detecting the acetates with a radioactivity detector (6). I t is also possible to prepare chloroacetates of the alcohols and detect these using a detector sensitive to halogens ( 7 ) . Both of these procedures thus facilitate detecting compounds with a reactive functional group in the presence of large amounts of other compounds that do not have this group.
ACKNOWLEDGMENT
The author expresses his appreciation to Maurice Lofters for expert assistance in performing this work.
LITERATURE CITED
(1) Dean, J . A , , “Flame Photometry,”
( 7 ) Landowne, R., Lipsky, S.R., ANAL. CHEM.35, 532 (1963). (8) Lovelock, J. E., Ibid., 33, 162 (1961). (9) Pvlc\Villiam, I., Dewar, R. A,, ,Yature 181, 760 (1958). (IO) Monkman, J. L., DuBois, L., “Gas Chromatography,” Noebels, Wall, Brenner, eds., p. 333, Academic Press, Xew York. 1961. (11) Padley, P. J., Page, F. M., Sugden, T. XI., Trans. Faraday SOC.57, 1552 (1961).
p. 43, XcGraw-Hill, Kew York, 1960.
12) Goulden, R.. Goodwin. E. S..’ Davies. L., Analyst 88, 951 (1963). ( 3 ) Gunther, F. A , , Blinn, R. C., Ott, ~
D. E., ASAL. CHEM.34, 303 (1962). (4) Honma, XI., AKAL.CHEM.27, 1656 (19j5). (5) harmen, A,, Giuffrida, L., Sature 201, 1204 (1964). (6) Karmen, A., PIlcCaffrey, I., Kliman, B., Anal. Biochem. 6, 31 (1963).
RECEIVED for review February 14, 1964. Accepted May 4, 1964. 2nd International Symposium on A4dvances in Gas Chromatography, University of Houston, Houston, Texas, March 23-26, 1964. This study was supported in part by United States Public Health Service-National Institutes of Health Grants GM-11535-01 and 1-S.O. 1-FR-OJ145-01.
Performance and Characteristics of an Ultrasonic Gas Chromatograph Effluent Detector F. W. NOBLE, KENNETH ABEL, and P. W. COOK laboratory o f Technical Development, National Heart Institute, National Institutes of Health, Bethesda, Md. The theory and instrumentation for a detector based on the measurement of the velocity of ultrasound in column effluents are discussed. The characteristics of binary gas mixtures and the propagation of sound through these mixtures allow the quantitative prediction of response when hydrogen and, to a lesser extent, helium are used as carrier gases. The response ( a t constant mole fraction) i s directly proportional to molecular weight up to a molecular weight of about 400 for 4-mc. operation and i s linear from a mole fraction of about 1% to the minimum detectable sample which presently i s of the order of 1 O-I4 mole for molecular weight 100. The detector cells, with internal volumes of 5 to 50 PI., can b e used with packed or capillary columns, corrosive samples, and at tempera ures to 270” C. Factors affecting the attainment of high sensitivity levels are discussed and experimental verification of theory i s presented.
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that measure, directly or indirectly, the velocity of sound in gas mixtures have been utilized for a t least 70 years. A brief review of the various applications of sound velocity measurement techniques as apiilied to specific gas analysis problems up to 1948 was included in a paper by Crouthamel and Diehl ( 2 ) . A number of other applications (6, 7 , 9-11, 13, 16) and one commercial instrument (Sational Instrument Laboratory, AS ANALYSIS METHODS
Washington, D. C.) have been developed since 1948. With the exception of the ultrasonic whistle system of Testerman and JlcLeod (16) and the phase change measuring system of Noble ( I I ) , these methods have not been applied to gas chromatographic effluent detection. Robinson’s patent ( I S ) specifies application to gas chromatography, but it is not clear whether or not the method was successfully applied. The methods of Testerman and hIcLeod and of Robinson indirectly determined sound velocity by measuring frequency changes. The beat frequency occurring between a whistle operated by the column effluent and a second whistle operated by a flow of pure carrier gas was measured in the first case while Robinson used two resonant cavities (one for pure carrier and one for column effluent) and measured the difference in resonant frequency. The method described briefly by ?;oble IS an eltension of a concept used by Lawley (9) and later by Kniazuk and Prediger ( 7 ) . With this method the frequency is maintained constant and the change in wavelength accompanying the velocity change is measured by determining the change in phase of the sine wave received by a transducer a t one end of a gas-filled tube as compared with the phase of the sine wave transmitted from a second transducer a t the other end of the tube. The first part of this paper presents the theoretical considerations involved in binary gas miyture analysis utilizing phase change measurements. Partic-
ularly it is concerned with factors affecting the use of this method as applied to gas chromatographic effluent detection. The second part of the paper is concerned with the development of suitable instrumentation t o perform this type of analysis a t high sensitivity levels and with experimental verification of theory. THEORY
The time required for a sound wave of velocity, V , to travel a distance, s, is
t = -S V The phase delay in electrical degrees corresponding to t is
where f is the frequency of the wave. For pure ideal gases a t low frequency
where M is the molecular weight of the gas, y is the ratio of specific heat a t constant pressure to the specific heat a t constant volume, R is the gas constant (8314 sq. meters gram mole-] OK.-’), and T is the absolute temperature. Combining Equations 2 and 3 we h a r e
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