The presence of the anodic isopotential point on scans 2-6 demonstrates the occurrence of surface processes involving two different electrode regions whose total area is constant. At least one of the couples involved did not exist on the electrode surface during scan 1, since curve 1 does not pass through (or even close to) the isopotential point. These results suggest that an oxidized form of glutathione (species A ) is adsorbed o n the electrode during scan 1 , and that on each of the subsequent scans some A is further oxidized t o soluble products, baring more and more platinum until all of the A is removed from the electrode by the sixth potential cycle. Thus it seems likely to us that the isopotential point in this figure is due to the oxidation of A and Pt. Our last example comes from a paper by T. Biegler (5) in which he examines the activation of platinum electrodes in 1M H2S04. His Figure 2, which shows the 2nd, 5th, and 200th repetitive scans after his pretreatment method, yields 2 isopotential points on the anodic scans and 4 or 5 (?) IP’s on the cathodic sweeps. Four of the isopotential points o n the cathodic sweeps are in the hydrogen adsorption region and the fifth is in the oxide reduction region. The latter I P is not easily distinguishable because of the scale of the figure, and may not be an ideal isopotential point. Biegler interprets his experiments by assuming that, initially, there is a mixture of very active and “normal” platinum sites, and that upon potential cycling the active sites are gradually replaced by the “normal” sites. The existence of a n isopotential point supports Biegler’s view that the electrode is divided into two regions during his
experiments. However, it does not demonstrate that an activation mechanism is responsible for the results reported. Our isopotential point theory would require that the nature of the active sites be independent of their number, and that every active site be transformed to one “normal” site upon potential cycling. CONCLUSIONS
Valuable information about the nature of a n electrode surface on which there is a deposited or an adsorbed species can be obtained from the isopotential point theory. For example, the disappearance of a n isopotential point with increasing coverage of adsorbate can be useful in establishing the occurrence and stoichiometry of surface reactions. Also, as in the glutathione example, the existence of an unsuspected adsorbed intermediate can be established. Third, the presence of adsorbable impurities can be verified, as in the case of SO, on Pt. Changes in the surface coverages of impurities as low as a few per cent are quite obvious. The potential of an isopotential point is characteristic of the couples involved. Hence, the identity of unknown adsorbates may be learned by adding the suspected species to the electrochemical cell. RECEIVED for review August 18, 1971. Accepted January 7, 1972. Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research USAF, under Grant No. AFOSR-70-1832.
Radio Frequency Spectral Emission Discharge Detector for Fixed Gases L. E. BOOS,Jr.,’ and J. D. Winefordner, Depurtment of Chemistry, University o/’ Florida, Guinescille, Fla. 32601
A radio frequency spectral emission detector for detection of fixed gases was designed and evaluated. A description of the overall system used in this work is given along with information regarding the response of the detector to air, CO, CO,, SO,, NHa, NO, NOn, N2, and CHa. The radio frequency (8 MHz) plasma was initiated and sustained by a low power-modified Clapp oscillator. The background spectrum (2000-5000 A) resulted from N,, NH, NO, and OH molecular band emission due to impurities in the helium carrier gas. Characteristic lines of helium and of the platinum electrodes were also identified. Limits of detection ranged from 0.3 ppm for carbon monoxide to 2.4 ppm for methane.
siderably more selective than most other types of detectors and should have considerable use for gas analysis as long as they also have adequate sensitivity, linear range of response, speed of response, and versatility of use. In this paper, a new sensitive, selective, simple radio frequency spectral emission detector for fixed gases is described. Analytical characteristics of this detector are presented. Although the radio frequency spectral emission detector is not evaluated for the analysis of fixed gases under gas chromatographic conditions, it should have use as a selective gas chromatographic detector of fixed gases. EXPERIMENTAL SYSTEM
DETECTORS IN GAS ANALYZERS and in gas chromatographic detectors are based on a great number of physical and chemical properties of gases, e.g., thermal conductivity, excitation and emission of radiation, ionization, flame temperature, absorption of radiation, heat of adsorption, velocity of sound, and many others. Gas detectors based upon absorption and emission of electromagnetic radiations should be con1 Present address Texas Neurological Development. Inc., 310 Medical Center Professional Building, Houston, Texas 77025. Author to ujhom requests for reprints should be sent.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
A block diagram of the instrumental system in the configuration for use with a gas chromatograph is shown in Figure 1. The specific components except for the gas flow system and the rf detector cell with associated circuitry are described in Table I. Helium was used as the carrier gas for all studies. Reagent grade helium (Airco Rare and Specialty Gases, New York, N.Y.) resulted in a lower background than ultra-pure helium (Matheson, Inc., Morrow, Ga.). However, essentially the same signal-to-noise ratios were obtained with either grade of helium, and so reagent grade helium was used in all subsequent studies. However,
Table I.
Gas flow system Rf cell Power supply for cell oscillator (load and line regulation to at least 0.1 %) Monochromator Phototube Phototube power supply Electrometer Recorder
Specific Components of Instrumental System
See text and Figure 1 See text and Figures 2 and 3 Model 407-D
John Fluke Co. Seattle, Wash. Model EU;700/E f/7, 0.35-m focal length, 1180 line/min grating blazed Heath Co. Benton Harbor, Mich. at 2500 A , I-mm slit height, variable slit width RCA 1P28 Multiplier Phototube Heath Co. Model EU-701-30 Photomultiplier Module Benton Harbor, Mich. Keithley Instruments Model GIOC with laboratory constructed zero suppression circuit Cleveland, Ohio E. H. Sargent Model SR Chicago, Ill.
p ,y, OSCILLATOR
U
HELIUM SUPPLY
Figure 1. Block diagram of radio frequency detector used in a gas chromatograph system
the helium was passed through a gas purifier (Model 450, Union Carbide, New York, N.Y.) prior t o entrance into the rf cell; the gas purifier removed particles larger than about 5-12 p m which caused a poorer signal-to-noise ratio if not removed. The response of the detector was evaluated by using a n exponential dilution flask ( I ) , rather than by discrete injections into a gas chromatograph. A 21i2-ft section of copper capillary tubing (0.02-in. i.d.) was inserted between the exponential dilution flask and the rf cell to assist in flow rate regulation. Swagelok fittings (Crawford Fitting Co., Solon, Ohio) were utilized throughout the gas flow system. The rf cell acts as a capacitor in the simple modified Clapp oscillator circuit (see Figure 2) used to maintain the rf plasma. The main considerations involved in designing the rf emission cell were : volume of cell which had t o be as small as possible for maximal sensitivity; cell material which had to be electrically nonconducting, inert, and sealed so no ambient air could leak into the rf plasma; electrode design which was found to be optimal when the electrodes with spherical tips were aligned parallel to the gas flow; and type and placement of the optical wind:w which had to transmit in the wavelength of 2000-5000 A (synthetic quartz was a suitable material) and had t o be placed a suitable distance from the electrode gap to minimize deposition of sputtered electrode upon the window. Of course, as the distance of the window from the electrodes is increased, the cell volume increases, and so a compromise (about 1 cm between theelectrodesand window) was necessary. (The present cell could be used for six months with no significant decrease in emission signals, whereas the previous cell with no side window had to be cleaned after several days of constant operation.) The above cell requirements were fulfilled with the cell design shown in Figure 3. Although both platinum a n d
-
(1) H. P. Williams and J. D. Winefordner. J. Gas Chromatogr., 4, 271 (19hG).
. c4
Detector Cel I
Figure 2. Schematic diagram of oscillator for radio frequency emission detector
Values of components are: Ri = 2,500 D RS = 4,700 0, 1 watt minimum R 3 = 250 D, 1 watt minimum Rq = 25,000 Q Ci = 108 pf Hammarlund APC air dielectric condenser; Range 7.8-140 pF C S ,Ca = 50 pF C4 = detector cell Cj, c6 = 2,700 pF Ci = 5 pF L1 = 70-ph B & W "Miniductor" 1-in. diameter air wound coil 213/16inches long, 32 turns per inch. LS = 7-ph Ohmite 2-50 choke L3 = 44-ph Ohmite 2-14 T = 6AU6 pentode
tungsten could be readily sealed into glass, platinum was chosen because it was simpler to fabricate spherically-tipped electrodes. The cell was mounted in front of and in a vertical manner with respect to the slit length of the monochromator. The carrier gas entered through port 1 (see Figure 3). One platinum electrode (item 6 in Figure 3) was sealed into the borosilicate glass tubing near the gas outlet (item 7); this electrode (0.5-mm 0.d.) was connected to the ground side of the oscillator circuit. The other electrode (item 4) was sealed into a 6-mm 0.d. glass tube, mounted in a Teflon (Du Pont) gland, and connected to the coil side (4) of the oscillator circuit (Figure 2); the stability of the discharge was improved (maximal signal-to-noise ratio of about 50 and negligible drift) if this electrode had a slightly smaller diameter than the grounded electrode. (No. 26 wire0.016 in. as compared to No. 24 wire-0.020 in.), and if the electrode gap distance was about 2 mm. The electronic circuit of the Clapp oscillator was constructed o n an 8-in. square circuit board. The coil L1was ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1021
\ Figure 3. Cross-sectional diagram of radio frequency detector cell 1. Gas inlet: 12/5 O-ring joint 2. 24/40 standard taper borosilicate glass female joint 3. Teflon gland(ASC0T Teflon thermometer seal for 6-mm shaft, Arthur Smith, Pompano Beach, Fla.). 4. Electrode from coil LI. No. 26 platinum wire (American Wire or Brown and Sharpe gauge) with 0.02-inch diameter spherical tip set in 6-mm borosilicate glass tube 5. Quartz window epoxied onto borosilicate glass sidearm 6. Electrode from ground side of oscillator circuit. No. 24 platinum wire (American Wire or Brown and Sharpe gauge) with 0.03-inch diameter spherical tip 7. Gas exhaust to flow meter
mounted horizontally in the center of the circuit board, and the pentode T was mounted vertically in one of the corners. No tube shield for the pentode was used to minimize tube failure. To minimize temperature change in the electronic components, all components were thermally shielded from the pentode by mounting between the tube and the remaining in. thick) of Styrofoam components a piece (3 in. high by covered with aluminum foil o n the tube side. Also to avoid rf losses within the circuit, the lead from L1 to the cell electrode (item 4 of Figure 3) was made as short as possible. All electric components were optimized to achieve a maximum emission signal-to-noise ratio (about 50); the optimum values of all components are given in the caption of Figure 2. All analytical curves were obtained with a spherical borosilicate glass exponential dilution flask with a volume of 1092 cm3. This flask is similar t o the one described by Williams and Winefordner ( I ) , except that the flask volume is considerably larger to facilitate better mixing at high concentrations. Air samples were directly injected into the exponential dilution flask with gas tight hypodermic syringes (Hamilton Co., Whittier, Calif., or Precision Sampling, Baton Rouge, La.). Other gases were introduced into the exponential dilution flask by means of hypodermic syringes and “Baggies” in the manner described by Williams, Overfield, and Winefordner (2). Complete details of sampling can be found in the thesis of one of the authors (L. E. B.). Analytical curves, limits of detection, and spectra were obtained using a helium gas flow rate of 154 cm3/min (see Figure 4) which resulted in maximal signal-to-noise ratios. All other experimental conditions are given in the table and figure captions. RESULTS AND DISCUSSION Mechanism of Detector. The radio frequency spectral emission type detector can be used in either of two modes utilizing production or enhancement of radiation or quench(2) H. P. Williams, C . V. Overfield, and J. D. Winefordner, J . Gas Cliromatogr., 5,
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511 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
0
n
0
0‘
;,
k
A
,A A
> A ,I, ,a,
HELIUM FLOW
RATE
A, A
,;,
w.
-
__,ow
(cc/min)
Figure 4. Relative intensity as a function of helium flow rate Spectral !and width, 7A 0 3910A A 38004 3570A V 3310A 0 3100A
ing of radiation present in the background spectrum. In the first case, detection is based upon selective monitoring of radiation produced by a species excited in the glow discharge. Here, the wavelength of the radiation is dependent upon the nature of the molecular species present, and the emission signal is dependent upon the concentration of molecules undergoing radiational deactivation from an excited state. The concentration of species produced in the rf plasma is dependent upon the sample size, exponential dilution, flask size, carrier gas flow rate, time from point of injection of sample into the dilution flask, and structure of the molecules introduced into the plasma. In the second mode, detection is based upon loss of signal because the species introduced into the glow plasma quenches the emission due to a particular impurity in the carrier gas. Because some impurity band systems are quenched while others are not (until the discharge is almost totally extinguished), the wavelengths of background radiation quenched depend upon both the nature of the species added to the discharge and upon the nature of the impurity species emitting radiation. The discharge is most likely initiated by collision of thermal electrons with helium atoms. As in the case of the direct current gas chromatographic discharge detector reported by Braman and Dynako (3), “the mechanism of molecular degradation and excitation of the observed species may be attributed to (either) ionized or metastable helium atoms, to electron bombardment of molecules, and possibly to thermal excitation processes.” Also fragmentation-excitation may be brought about by high energy photons from the activated carrier gas. It is known that enough energy is present in the discharge to ionize chlorine atoms which requires 28.3 eV ( 4 ) ; however, because of the low power of the oscillator (E 1 watt), the upper limit of detection for homologs increases as the molecular size increases (e.g., observed if cases of y system of NO for nitrogen oxides and the 4300 A system of C H for alkanes) because more energy is required for fragmentation of the larger molecules. (3) R. S. Braman and A. Dynako, ANAL.CHEM.,40,95 (1968). (4) G. R. Harrison, “M.I.T. Wavelength Tables,” The M.I.T. Press, Cambridge, Mass., 1963, p XVIII.
WAVELENGTHS
IN
A
Figure 5. Scan of background spectrum between 2000 and 3000 A Experimental conditions: Voltage applied to oscillator 411 V Current to oscillator 44 mA 154 cm3 per min Helium flow rate Slit width 40r Spectrum uncorrected for phototube response
t
t
z
WAVELENGTHS
IN
z
w w Y
I4
d
0
m
0
L 9
w 9
w L 0
w Q 9
OD
L
OH
Figure 6. Scan of background spectrum between 3000 and 4000 Experimental conditions (same as in Figure 5) Background Spectrum of RF Plasma. The spectrum obtained when the discharge is sustained only in the helium carrier gas vented t o the atmosphere (Figures 5-7) shall be referred to as the background spectrum in this work. The spectrum consists of the stronger bands of the following systems ( 5 ) : ( 5 ) R. W. B. Pearse and A. G. Gaydon, “Identification of
Spectra,” Chapman and Hall, London, 1950.
Molecular
The system of NO (Third Positive System of Nitrogen) 1956.1 A to 3458.5 A degraded to shorter wavelengths; Second Positive System of the neutral nitrogen molecule 2814.3 A to 4976.4 A degraded to shorter wavelengths; First Negative System of the ionized nitrogen molecule 3923.4 A to 5864.7 A degraded to shorter wavelengths; The 3064 A System of OH 2444 A to 3472.1 A degraded to longer wavelengths ; ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1023
*Z+ 5
? WAVELENCTHS IN
1
Figure 7. Scan of background spectrum between 4000 and 5000 A Experimental conditions (same as in Figure 5)
Table 11. Spectroscopic Effects on Radio Frequency Discharge of Various Gases Investigated5
Band systems present in background spectrum 2nd Pos Nz
3064 A SYs OH
3360A Sysb NH
CO+
-
E
e
e
E
E
e
-
9
9
9
;
Q Q Q Q
-
9
-
9
e
E
4 -
-
-
e e E
-
-
E
E
-
Q
Q
E
e
Q Q Q Q e e e
a
1st Neg Nz+
9
Q Q Q Q
; Q
Fox Duffendack & Barker's
Q Q Q Q Q Q Q Q
9 9
1st Neg
CO'
4300 A Cyanogen Sys Viol2 CZ CH CN Swan
2800 A SYs CC1
2570 A SYS
Cd
e e
9
E 9
-
e e
Q Q Q Q Q E
e e
e
E E
e
E e
E
e e E e
E E
E
e e e
E E
E E E
9
Complete spectra with indication of sQectralband species and wavelengths can be obtained from one of the authors (J. D. W.).
* Only a double headed band at 3360 A appears in the background spectrum. KEY TO THE ABOVE TABLE E -+ enhancement of band system e -,slight enhancement effect Q quench of band system q -,slight quench effect - --* no apparent effect on existing band blank no enhancement of band system not in background spectrum -f
-f
The 3360 A System of NH 3023.3 degraded.
A to 3676.3 A scarcely
Also present are some lines of helium (carrier gas) and platinurn (electrodes): He 5015.67 A, 4921.93 and 3888.65 A. 1024
A, 4713.14 A, 4471.48 A, 4437.55 A
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Pt 3408.13 A, 3064.71 A, 3042.64 A, 2997.97 A, 2929.79 A, 2897.87 A, 2893.86 A, 2830.29 A, 2659.45 A, and 2646.89
A.
The relative emission signals of several band groupings is shown in Figure 4 as a function of carrier gas flow rate. The curve shapes in Figure 4 give some indication of the sensitivity
8 DISCRETE
INJECTIONS
w
2
2
w v) 111
w
> -
5 W
L Y
-
INCREASING CONCENTRATION
yo
i
3-
8-
@ EXPONENTIAL OlLUTlON OF SAMPLE
E
3m
9999
.I
,o V
w m 0 W0
B
Q
B
z
m 0
INCREASING TIME
Figure 8. Recorder response of the radio frequency spectral emission type detector a. Relative response of successively larger injections of air using a hollow copper tube column b. Response of a sample of gas which is large enough to quench the discharge using the exponential dilution flask U
of the detector t o flow rate changes. However, it should be noted that the signal-to-noise ratios for all the bands listed in Figure 4 are essentially the same, i.e., the higher the signal, the greater the noise. Spectra and Analytical Curves of Various Analyte Gases. Spectra of various gaseous analytes were made using the exponential dilution flask in conjunction with the radio frequency spectral emission detector. A large volume of the analyte gas (500 t o 1000 p1) was injected into the helium flow stream (approximately 150 cm3/rnin flow rate) using the exponential dilution flask. Because the concentration of analyte gas would decrease exponentially with time into the detector, additional injections at specified times were made when relatively Jarge scans covering the whole region from 2000 to 5000 A were recorded. I n Table 11, the spectroscopic characteristics of various analyte gases excited in the rf spectral emission detector are listed. By comparing the differences in the analyte and background spectra, several of the more intense band regions were selected; 10 or 50 p1 injections were made at each wavelength t o determine the optimum wave-
.-m
E
I;
c
.-
m
.-c
c
v ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1025
length and monochromator slit width by comparison of the signal-to-noise ratios obtained. Spectra can be obtained by contacting one of the authors (J. D. W.) or by consulting the thesis of the other author (L. E. B.). In Figure 8, the recorder response is shown for a series of discrete air injections made using a hollow copper tubing (no column packing) instead of the exponential dilution flask to illustrate the response of the detector when used in a gas chromatographic situation. In the linear region of the analytical curve, the peaks have a gaussian shape; however, when the concentration of sample reaching the detector exceeds a certain upper limit, the results are more complicated. As the leading edge of the analyte slug passes into the discharge, the emission signal increases as expected. However, the concentration of sample soon reaches such proportions that the efficiency of energy transfer is reduced, either because of lower helium concentration or insufficient power generated by the oscillator. The resulting effect is a quenching of the emission signal when the analyte concentration in the rf cell is highest. As the slug emerges from the discharge, the response again increases, but this time it is delayed and consequently the tailing side of the response peak does not reach the same signal level of the leading spike. No attempt was made to quantitate the response in this split peak region as was done by MacDonald ( 6 ) in a gas chromatographic determination of hydrogen in helium carrier stream with a thermal conductivity detector. A similar effect was observed with the exponential dilution flask when the concentration of the sample was above that which would result in maximum positive response. When the sample entered the detector, the same sharp spike would be observed due to the small but finite amount of time required for the limiting concentration to be established in the detector. This would correspond to points ABC for a given discrete injection (refer to Figure 8b). Because the concentration of sample reaching the detector was exponentially reduced, the signal would gradually increase (for times greater than C in Figure 86) until the concentration in the detector corresponded to that concentration which would again result in the maximum positive response. After that, the normal exponential curve would be displayed (above point D in Figure 8b). The (6) J. C. MacDonald, ANAL.CHEM., 40,461 (1968).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
quenching process at high concentrations manifests itself as an abrupt bending over of the analytical curve. The limits of detection and useful linear ranges (the concentration range over which the analytical curve is linear) are given in Table I11 along with the sensitivities, noise levels, optimum wavelengths, and optimum monochromator spectral band widths for several gases using the radio frequency spectral emission detector. Analytical curves were determined by plotting the average response of three injections at each concentration used. The limit of detection was defined to be that concentration which resulted in a signal five times the rootmean-square noise for three measurements of sample background (Le., a signal equal to the peak-to-peak noise) which corresponds to about 99.9 confidence level. Comparison with Related Detectors. In Table 111, limits and useful dynamic ranges for several gas chromatographic detectors reported in the literature are listed for comparison with the radio frequency spectral emission detector proposed here. With regard to limit of detection, all of the detectors have comparable detection limit values except those of Bourke et al. (11) and of Hartmann and Dimmick (12) who obtained decidedly lower values. Even so, the radio frequency spectral emission detector could have considerable use as a gas chromatographic detector because of its greater specificity than any of the other gas chromatographic detectors listed in Table 111, except for the microwave detector of McCormack, Tong, and Cooke (7). RECEIVED for review August 13, 1971. Accepted December 27, 1971. Work was taken in part from the thesis of L. E. Boos, Jr., submitted in partial fulfillment for Ph.D. degree at the University of Florida. Research sponsored by AFSOR (AFSC), U.S.A.F. Grant No. 70-1880B. (7) A. J. McCormack, S. C. Tong, and W. D. Cooke, ANAL.CHEM., 37,1470 (1965). (8) W. C. Hampton, J . Gas Chromatogr., 3,217 (1965). (9) H. P. Williams and J. D. Winefordner, ibid.,6 , l l (1968). (10) V. N. Smith and J. F. Fidiam, ANAL.CHEM.,36,1739 (1964). (11) P. J. Bourke, R. W. Dawson, and W. H. Denton, J. Chromatogr., 14,387 (1964). (12) C. H. Hartmann and K. P. Dimmick, J. Gas Chromatogr., 4, 163 (1966).
