Equation 2 to calculate K’. The experimental and calculated values are compared in Figure 4. Calculations and numerical analyses were run on a CDC 3600 computer. In general, K’ is underestimated but the trend in values is predicted reasonably well considering the wide range of ionic strengths and solution compositions covered. The pattern of selectivity isotherms, including the onset of heterofunctional behavior above 2M, is fairly accurately reproduced. The equilibrium process may involve a number of reactions and our simple treatment gives only an average behavior of the system. When y = 0.5 then, from Equation 2 acs/aNeZ= K’
This value of K’ is frequently used as an average value of the corrected selectivity coefficient at a particular ionic strength (4, p 235) and can be estimated from Figures 1 and 4-e.g., at 2M, KtaVhas the value 0.039 and at 0.75M the value 0.011. At low ionic strength the value becomes relatively small (about 10-3. KIav corresponds to the selectivity ratio determined by the standard titration procedure (6, p 304; 9, p 296) and has proved a useful parameter for glass electrodes; it is even possible to determine B of Equation 5 from the form of the titration curve (6, p 298). However, for the liquid
ion-exchanger used here, this parameter is inadequate, particularly for the more concentrated solutions. A limiting value of K’ could be defined by comparing the value of K’ at y = 1 with the value at y = 0. Recently (15) an investigation of the selectivity characteristics of a number of Orion selective electrodes, of the liquid membrane type, has been made over an ionic strength range 0.1 to 1 0 - 4 ~ . Three useful methods of evaluating K’ were described. The first two were based on comparisons of pure solutions of the salts of the two counter ions. The K’ value from the first method roughly corresponds to the limiting value of K’ and that of the second to KIaV. In the third method K’ was measured over a range of solution compositions. Although the measurements were not made on the basis of constant ionic strengths the data could be interpreted uia the simple ion-exchange theory outlined in this paper, whence some of the apparent anomalies might be resolved. RECEIVED for review October 6, 1969. Accepted December 29, 1969. (15) K. Srinivasan and G. A. Rechnitz, ANAL.CHEM.,41, 1203 ( 1969).
Analysis of Inorganic Sulfur Compounds by Flame Ionization Detector B. A. Schaefer Department of Chemistry, Royal Australian Air Force Academy (University of Melbourne), Point Cook, Victoria, Australia
By selecting appropriate conditions of operation, certain inorganic sulfur compounds could be determined using the normal flame ionization detector in gas chromatography. Carbon disulfide gave positive peaks of optimum response under conditions similar to those for hydrocarbons, requiring 20% equivalent hydrogen in the flame gases. These positive peaks inverted to negative with a high background current of hydrocarbon origin and nonoptimum conditions of 24% equivalent H P . Sulfur dioxide, H S , and SCO gave large negative peaks in the ratio of l i 3 i 3 approximately with high hydrogen, and l i l i l with low hydrogen (or high oxygen) in the flame. These negative responses were superior to responses from the katharometer, especiallywhen hydrocarbon at about10 p.p.m. was present in the flame. The response was proportional to the carbon added, and optimum responses were obtained with about 24% equivalent Hz. Positive peaks were observed from all four compounds when the burner jet became heated to redness, caused by high Hzor low Nzflow rates. The results were applied to the analysis of partially oxidized C S z . A mechanism is proposed to account for the negative peaks, involving the reaction of sulfur radical and sulfur oxide with flame radicals, particularly oxygen atom.
COLUMNS FOR SEPARATING sulfur compounds have been described (1-6), but the hydrogen flame ionization detector (FID)is generally regarded as unsuitable for inorganic substances (7). Condon, Scholly, and Averill (8) reported no response to CS2 with cold electrodes, and little or no response 448
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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to SOn,H2S, or SCO. Perkins et ai. (9), and Andreatch and Feinland (10) reported no response for CS2. Sternberg, Gallaway, and Jones ( I I ) , Phillips (12), McWilliam (13), and Walker (14) have separately examined CS2, obtaining limited (1) C. T. Hodges and R. F. Matson, ANAL.CHEM., 37, 1065 (1965). (2) E. L. Obermiller and G. 0. Charlier, J. Gas Chromntogr., 6 , 446 (1968). (3) S. Pennington and C. E. Meloan, ANAL.CHEM., 39, 119 (1967). (4) H. L. Hall, ibid., 34,61 (1962). (5) C. N. Jones, ibid., 39, 1858 (1967). (6) R. J. Liebrand, J. Gus Chromntogr.,5,518 (1967). (7) “Manufacturer’s Handbook of the F.I.D. Instrument,” Beck-
man Instruments Inc., Fullerton, Calif. (8) R. D. Condon, P. R. Scholly, and W. Averill, “Gas Chromatography,” R. P. W. Scott, Ed., Butterworth, Bethesda, Md., 1960, p 31. (9) G. Perkins, G. M. Rouayheb, L. D. Lively, and W. C . Hamilton in “Gas Chromatography Third International Symposium,” N. Brenner, J. E. Callen, and M. D. Weiss, Eds., Academic Press, New York, 1962, pp 269-83. (10) A. J. Andreatch and R. Feinland, ANAL.CHEM.,32, 1021 (1960). (11) J. C. Sternberg, W. S. Gallaway, and D. T. L. Jones, “Gas Chromatography Third International Symposium,” N. Brenner, J. E. Callen and M. D. Weiss, Eds., Academic Press, New York, 1962, p 231. (12) T. R. Phillips, “Gas Chromatography,” R. P. W. Scott, Ed., Butterworth, Bethesda, Md., 1960, pp 132-4, 317. (13) I. G. McWilliam, J. Chromntogr.,6, 110 (1961). (14) B. L. Walker, J. Gus Chromntogr.,4, 384 (1966).
responses, mostly under high hydrogen flow conditions. Dressler and Janfik (15) recently obtained responses with organic sulfur compounds, as well as with CSZand HzS, using an Alkali Flame Ionization Detector (AFID), but found no response for the latter two compounds with the normal FID. Dressler (16) then reported an anomalous response of the FID to CS2,showing dependence on the Hnflow rate and the position and condition of the collector electrode. Other work on the response to CSz, in the absence of hydrocarbons in the flame, has been reported (17). All peaks obtained from the F I D are a variation imposed on the base line, which is itself in part a measure of the hydrocarbon background current. Positive peaks have so far not appeared to depend to any great extent, on the level of this background, but Dressler and Janfik (15) noted the positive to negative inversion of the thiophene peak as the alkali salt background current was increased in the AFID. Negative peaks appear to be a complex function of such background currents, and depend on their mode of formation. The current in the normal F I D is mainly due to traces of hydrocarbon entering the flame with the fuel or the carrier gas. When hot, the burner also contributes to the background current, probably because of some form of emission (18). According to the views of Calcote (Z9), the hydrocarbon gives rise to ions through the reaction. CH'+O'-CHO++e
(1)
A negative peak could thus arise during the passage of the sample through the flame, by decreasing the concentrations of either the CH. or the 0 . radicals or by ion capture, thus momentarily depressing the background current. Depending on the conditions of operation of the flame. CSZ,SCO, H2S, and SOz are able to produce negative peaks, some of which are useful for analysis. EXPERIMENTAL The gas chromatograph was a standard Beckman GC2 fitted with a Flame Ionization Detector Accessory (FID), and thermal conductivity detector (TCD). The output from the F I D was passed to a 1-mV Sargent Recorder. Hydrogen, air, and dry nitrogen were supplied from cylinders, either without treatment, or after passage through a molecular sieve trap immersed in liquid nitrogen. The fuel-air ratio for the burner was held at rates approximately equivalent to H z : 0 2 = 1 :2 stoichiometric except where stated otherwise. The fuel and carrier gas lines were arranged so that a controlled flow of methane or ethylene could be admitted to either as required. This was most readily achieved by supplying a mixture of 1 methane in nitrogen to a Negretti & Zambra precision pressure control valve, and then through a needle valve to the high pressure side of the fuel or carrier gas line connecting to the chromatograph. Air could be supplied as the carrier gas when required, thus producing a premixed hydrogen-oxygen flame. The column was silicone oil (50%) on fire brick (60-80 mesh) in 6 feet of 1i4-inch stainless steel tubing. Column temperatures were selected at 80 or 110 "C approximately, (15) M. Dressler and J. Janhk, J. Chromatogr. Sci., 7,451 (1969). (16) M. Dressler, J. Chromatogr., 42,408 (1969). (17) D. M. Douglas and B. A. khaefer, J . Chromafogr. sei,, 7 , 433 (1969). (18) B:E. Hudson, W. H. King, and W. W. Brandt, "Gas Chromatography Third International Symposium," N. Brenner, J. E Callen and M. D. Weiss, Eds., Academic Press, New York, 1962, p 207. (19) H. F. Calcote, "Ninth International Symposium on Cornbustion," Academic Press, New York, 1963, p 622.
to give suitable elution times. T o determine the effect of column evaporation on background, a few experiments were made with a Porapak Q column, and the background was also measured with an empty column. The effect of temperature on background was determined by operating the silicone column over the range 40 to 160 "C. Special attention was paid to maintaining a high purity column by daily purging at 220 "C and 100 ml min-l Nn, and by flushing with large samples of SOz or H 2 S ; the latter in particular was very effective in removing from the column traces of absorbed hydrocarbons. In addition the burner was heated to redness for 5 minutes to burn off any deposits, and note taken of any resulting change in the background current. The sulfur compounds used were prepared according to standard methods-e.g., HnS (20)-or obtained from cylinders-e.g., SOn, SCO. Purification was effected by chemical absorption-e.g., Ascarite for SCO (20, 2Z)-drying over PzOs,and isothermal distillation. The product was checked for purity by vapor pressure, and on the chromatograph until a single maximum peak was obtained. The samples were manipulated in a standard vacuum line connected to the gas chromatograph inlet system, and measured on a mercury manometer at pressures from 1.0 to 20.0 cm of Hg in a heated stainless steel gas sampling valve, with loops calibrated to be 1.24 cma. Thus 1-cm Hg pressure at 45 "C was equivalent to 0.63 X 10-6 mole of sample admitted to the column. Care was taken to keep the loops of the sampling valve free from grease to prevent loss especially of CS2 by absorption. Elsewhere greaseless Teflon-in-glass valves were used whenever possible. Sample supply lines were heated to about 50 "C to prevent vapor condensation. The variation of response of the F I D to the additives as a function of the flow rates of carrier nitrogen and fuel hydrogen was examined, and the background current in all cases was recorded. The responses were also determined as a function of the hydrocarbon added to the gas supplies, and of oxygen in the carrier stream. The effects of several other operating conditions were also examined. The peaks were measured by manual triangulation without correction (22), and the areas reported in coulombs. RESULTS Effect of Flow Rates of Carrier and Fuel. To find the optimum conditions for a fixed sample size, the responses were determined with varying nitrogen and hydrogen flow rates as shown in Figures 1 and 2. Figure 1 shows the responses obtained using a 6-ft silicone column at 114 "C for SOZ, and 83 "Cfor H2S and SCO. In this case methane was added to the hydrogen to a concentration of 7.7 ppm of total flame gases, giving a background current of 1.00 X 10-IOA. The flow rates were H2,45 and burner air, 235 ml mini', the carrier nitrogen being varied. Figure 2 gives the responses as a function of hydrogen flow rate; the conditions were the same as for Figure 1 except that the methane was added to the carrier held at 52 ml min-' Nf. Increasing the hydrogen content of the flame by either decreasing the nitrogen or increasing the hydrogen flow rates, caused the negative response to increase by factors of 5 to 10 over a limited linear range. Further increase of hydrogen or decrease of nitrogen caused the burner to become hot, the peaks then became less negative and eventually inverted to positive; these trends are shown in the figures and are attributed to emission effects. The sensitivities for near optimum flow conditions of Hz 45, air 235, and Nz38 ml min-l, and a background current of 5.0 X lo-" (20) H. Melville and B. G. Gowenlock, "Experimental Methods in Gas Reactions," Macmillan, London, 1964, pp 197, 205, 206. (21) R. E. Snyder and R. 0. Clark, ANAL.CHEM., 27, 1167 (1955). (22) D. L. Ball and W. E. Harris, J. Gas Chrornatogr., 5,613 (1967) ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
0
449
sco. r-
H, S
I'
2,O 40 60 80 190 FLOWRATE HYDROGEN MLMIN-' Figure 1. Responses as a function of carrier flow rate, in the presence of hydrocarbon in hydrogen fuel A due to the hydrocarbon present in the hydrogen as an impurity, are given in Table I, column (a). These conditions are however nonoptimum for CSz, which requires a flow rate of 66 ml min-' Nz. When the response data from Figures 1 and 2 were plotted as a function of the variation of the equivalent hydrogen percent in the unburnt flame gas (17,23), the relationship was nearly linear, and the data from different experiments with different variables of carrier and fuel flow could be correlated in this way. For example, the responses for SCO were determined with Nz fixed at 34, 38, 52, and 67 ml min-1 and the Hzvaried from 20 to 50 ml min-l, and also with the Nz varied from 35 to 90 while the hydrogen was held at 45 ml min-l; over this wide range, the plot was linear to within + l o % . The optimum responses were observed for CS2 with flow conditions corresponding to about 20% hydrogen. For SOz, His, and SCO, the conditions corresponded to about 24% for optimum response; under these conditions the ratio of the responses was approximately 1:3:3. (23) B. A. Schaefer, Combust. Flame, in press, 1970.
Figure 2. Responses as a function of hydrogen flow rate, hydrocarbon added to carrier Effect of Burner Air Flow Rate. The burner air serves two purposes, it provides the oxidant for the hydrogen and any combustible additive, and is also a scavenger to remove combustion products. Small changes in flow rate did not cause significant changes in the responses but high flow rates caused instability of the flame, and low rates a peculiar behavior of the response. At high Hzand low air, excessive noise appeared. At low Hz and corresponding low air to preserve the ratio, consecutive samples of H2S gave increasing peak depth and width with excessive tailing, the peaks became larger and were difficult to measure. This abnormality was removed by passing an excess of Hs through the burner between sample additions, causing it to heat up for about 5 minutes. Merely increasing the air without the hydrogen worsened the condition. After this treatment with H2, the next sample behaved normally, but subsequent samples again showed the abnormal peak formation. If the responses for low H2 were taken with high air, the response diminished slightly but the successive samples were not very different from the first sample. When the burner air was increased from 235 to 300 ml min-1 with Hz at 45 ml min-l the background did not change but the response to SO2 and H S increased by about 10%. It was also noted that the non-
Table I. Selected Results Substance (a) (b) (C) Carbon disulfide" +i.6 x 10-3 +3.2 x 10-4 550 Carbon disulfideb $1.0 x 10-3 fN O ... -2.7 X lo-' -2.0 x 10-4 29.2 Sulfur dioxide -7.6 X lo+ -5.2 x 10-4 63.5 Hydrogen sulfide Carbonyl sulfide -9.8 X -6.4 x 10-4 45.5 a Optimum conditions, * nonoptimum conditions for CS2. Peak polarity indicated by (a) Sensitivity coulomb mole-' at background 5.0 X 10-llA. (b) Sensitivity coulomb mole-' at background 3.0 X 10-IOA. (c) Ratio of responses FID/TCD. (4Approximate limits of detection in moles. (e) Sensitivity coulomb mole-' with air as carrier, other conditions as (6).
450
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
(4 1
+ or - .
x
10-10
..*
1 x 10-8 1 x 10-8 1 x 10-8
(e) 4
...
-1.1 x 10-4 -1.2 x 10-4 -1.5 x 10-4
linearity, presumed due to emission effects of the hot burner, was slightly diminished at elevated air flow rates. Effect of Added Hydrocarbon. Operating with untreated gases at flow rates of air 235, hydrogen 45, and nitrogen 52 ml min-1 the background current was within the range 2.0 to 6.0 X 10-I1A and the noise variation was k5.0 X 10-14A. The range of the background current depended on the hydrocarbon impurity in the cylinder gases. This could be decreased by trapping the supply lines, or increased by adding a hydrocarbon gas to the carrier or fuel gas. Background currents of 5.0 + 0.05 X l0-lo could readily be maintained. When methane or ethylene was added to the carrier stream or to the fuel gas, the negative response increased proportionately with the background in the case of SOz, H2S, and SCO. Methane was preferred instead of ethylene for increasing the background since it was not so strongly absorbed on the column, nor did it cause the burner to become as dirty. The response increase was independent of the nature or source of the hydrocarbon and depended only on the carbon content of the flame. Methane and other hydrocarbons were present in the cylinder hydrogen used and were sufficient to give a reasonable response for SO2 etc. without further addition of CH4. The effect of the background on response is illustrated in Table I, column (b), which gives the sensitivity in the presence of 24 ppm CHI producing a current of 3.00 X 10-lOA. The linearity of the effect is seen by comparing columns ( a ) and (b) in Table I, and from the observation that mole sample increased for SCO the response to a 3.15 X from 1.50 to 4.00 coulomb X loeQwhen the CHI was raised from 16 to 40 ppm in either the Nz or H2, and the flow conditions corresponded to 21.8 hydrogen equivalent. Trapping the hydrogen line with molecular sieve immersed in liquid Nn diminished the response of SO, etc. to a very low value which could be accounted for by the small amounts of hydrocarbon present in the carrier stream and evaporating from the column. Purification of the carrier stream then slightly decreased the responses. The background current under standard flow conditions can be taken as a measure of the hydrocarbon in the flame, and this was calibrated to be 0.0775 ppm = 1.0 X 10-I2A for standard flow rates of 45 ml min-l H,, 235 ml min-’ air, and 52 ml min-’ N2 carrier. When carbon disulfide (3 X 10-6 mole) was added to the flame under conditions optimum for SOz, but nonoptimum for CS2, the response was positive if the hydrocarbon content of the flame was low, of the order of 2 to 6 ppm. However, as the background current was raised by the admission of CH, to the fuel gas the positive CS2 response diminished to zero at about 24 ppm (a background current of 3.0 X 10-IOA) and then inverted and became increasingly negative, behaving very like SOn. The area of the negative peak from CSz was approximately twice the area of the same quantity of S02, under the same conditions. Effect of Column. In order to obtain reproducible negative peaks, the column had to be very clean and especially free from absorbed hydrocarbons, which were found to co-elute (24) with the inorganic sample resulting in smaller responses. The addition of consecutive samples of H2S or SO, to a partially clean column under constant conditions, gave a series of peaks that decreased from positive, and ultimately became negative as the column cleaned up. The order of effectiveness in producing co-elution was HzS > SO, > (24) J. C. Sternberg, “Gas Chromatography Fourth International Symposium,” L. Fowler, Ed., Academic Press, New York, 1963, p 164.
SCO, and use was made of this fact in the flushing procedure. The cleanliness of the column was checked by allowing the instrument to stand without samples for intervals of 10, 30, and 60 min, and several hours and noting any return of positive peaks or decrease in size of negative peaks on addition of a standard 0.63 X 10W mole H2Ssample. A column was ready for quantitative use if the background current at a standard fuel, air, and nitrogen flow setting was reproducible with a =!=l,O x 10-IzA variation, and no change of response to a standard sample was observed over an 8-hour working period. To achieve this high purity condition, daily purging at 220 OC with 80 to 100 ml min-l flow of N,, and frequent flushing by mole) of SO2 or H2S was relatively large samples (6.3 X found necessary. The co-elution problem was the main cause of uncertainties in the results. Besides co-elution and diminished negative peaks, dirty columns caused a general shift of the base line resulting in nonreproducible responses. Effect of Column Temperature. Increasing the column temperature decreased or increased the response to SO2,HzS, and SCO, depending on the history of the column. When it had been contaminated with hydrocarbons and had not been effectively purged, increasing the temperature resulted in coelution of sample and contaminant, and thence a decrease of negative peak area. With a nearly clean column, raising the temperature sometimes caused a temporary increase in the background current due to an increased continuous elution of absorbed hydrocarbon or an increased evaporation of the liquid phase from the column, and this produced larger negative peak areas, provided little or no co-elution occurred. The background current fell as the column cleaned up, and the increased response then diminished with time to its original value. Effect of Premixed Oxygen. When oxygen was added to the carrier stream, by replacing the carrier nitrogen with air, the background was raised but this was shown not to be due to an increase of hydrocarbon in the flame. It was probably due to the increased response of the system to the existing hydrocarbon (ZI, 25) plus an increased contribution from the burner due to a rise in temperature and emission (12, 18). The positive response to CSZ under these circumstances decreased to nearly zero (17), while in the case of SOZ,H2S, and SCO, the negative responses diminished to 80.5, 29.4, and 31.4%, respectively, of the response observed with N? carrier. However, the negative response remained proportional to the hydrocarbon in the flame. Minor deviations occurred with low hydrocarbon backgrounds of the order of 3.0 X 10-llA, where added oxygen appeared to increase slightly the negative response in the case of SO,. At background currents of 6.0 to 7.0 X 10-I1A there was practically no effect either way. The hyperoxygenated flame (25) had a small effect on the SO2 response, but markedly diminished the response of H2S and SCO to a value approximating that of the SOi, the ratio of the responses SO, :H2S:SCO becoming 1 :1 :1 instead of 1 :3 :3 : A similar effect was observed at low Hz equivalent in the flame, and extrapolation to 16 hydrogen equivalent indicated an approximately zero response to all three substances. Effect of Burner and Its Temperature. By trapping the supply gases and operating with and without column packing, it was possible to assess the various contributions to the background. At standard flow conditions, the hydrogen fuel gas contribution was 1.5 to 5.0 X 10-11A depending on the individual cylinder, mostly due to CHabut other hydrocarbons (25) R. L. Hoffmann and C. D. Evans, J. Gas Chromafogr.,4, 318
(1966). ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Table 11. Comparison of Conditions for SO2 and H2S" Case te tl C R B Conditions 1 0.85 26.5 11.3 304 62.2 HIS, Porapak 15-inch column. New NZ supply, NP40 ml min-l, 80 OC 620 75.4 HPS,silicone 12-ft, Nz40, 80 "C 2 2.7 31 .O 19.9 3 1.3 415 71.4 HS, silicone 6-ft, Nz40, 80 "C 16.5 25.0 4 1.25 210 46.9 HPS,silicone 6-ft, new Ht, NP,air, Nt40, 80 "C 17.5 12.0 240 46.8 HPS,silicone 6-ft, Nz40, 80 "C 5 1.3 17.5 13.7 262 61.5 SOP,Porapak 15-inch, NZ 41.5, 80 "C 6 2.45 80.0 3.3 123 49.7 SOP,Silicone 6-ft, NZ41.5, 80 "C 7 1.85 27.5 4.5 120 55.7 SOP,Silicone 6-ft, NZflow 40, 110 "C 8 1.2 17.0 7.1 90 57.2 SOP,Silicone, new supply Nz40, 110 " c 9 1.3 17.0 5.3 10 1.0 14.5 4.3 62.6 55.0 SO2, Silicone, NZ56.5, 110 "C Sample size 5 cm Hg = 3.15 X mole HPSor SOZ. t s = elution or retention time in minutes. tl = flame residence time in seconds. C = Current = Average decrease in current in 10-12A. R = Responses in units of 10-lP coulomb. B = Background in units of 10-l2 A from natural hydrocarbon. Flow rates HP = 45, Air = 235 ml min-l, NPas given. Q
up to Cs were detected (26). The burner air supply contributed about 2 X 10-12A and so also did the nitrogen and air carrier gases. The column contributed about 2 to 4 X 10-12A. A residual background current of about 1.0 X 10-11A remained when the above sources were accounted for. With all gases purified and no column packing, the background observed was about the same. This small residual current did not give rise to any significant responses and was approximately inversely proportional to the nitrogen flow rate. When the hydrogen flow rate was increased, the current increased approximately exponentially. Simultaneous visual observation of the burner showed that the tip of the jet was becoming hotter as the background increased. This residual background was ascribed t o some thermionic effect not completely understood (18). The background current always returned to its original value when the flow rates were returned to original. When the nitrogen flow rate was low, of the order of 20-30 ml min-1, and the burner near red hot, the response pattern changed. In this region SOZ,H2S, and SCO showed a decreasing negative response eventually inverting to positive as the flow rate of N, was further decreased and burner temperature increased. These trends are shown in Figures 1 and 2. When the nitrogen was decreased or the hydrogen flow rate increased, the positive response to CS2 decreased t o a minimum at about 30 ml min-l Ni, Hz 45, ( = 23 % HZequivalent); but as the emission region was entered, the positive response t o CS2 began t o increase again (12). This emission region was not fully explored; most of the results reported were obtained for flow rates of carrier gas between 30 and 70 ml min-l, with HZa t 45 ml min-l, where the responses were more linear, Figure 1. Considerable deviations from linearity occurred outside this range, and for analytical purposes this emission region is best avoided. It is of interest, however, that most inorganic compounds examined showed the inversion to positive, but hydrocarbons showed no changes in response within this region, and COPgave a n even stronger negative response. At high flow rates of Nz, the thermionic effect made no apparent contribution t o the background, but with low Nzthe emission became quite large and
(26) D. M. Douglas, unpublished work, R.A.A.F. Academy, Point Cook, Victoria, Australia, 1968. 452
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
was the main contributor, exercising a marked effect on the response. In view of the findings of Dressler and Jan6k (13, it seemed possible that the burner used in this work may have become contaminated with alkali salts. No flame coloration due t o sodium was visually observed. The burner was dismantled, mechanically cleaned with alumina, boiled in dilute acid, water, dried, and its parts gently flamed. The jet was heated to redness in a gas torch and when reassembled was heated in an excess hydrogen flow. No change in the response to SO? was observed. There was no effect on shifting the apparatus to a different location with newly installed copper supply gas lines and no significant changes over a 2-year period of operation, or related to the interchange of burner parts. Other than the above-mentioned residual background current of 10 X 10-12A, no alterations in 3perating performance were introduced by trapping the supply lines. The only change noticed during use was the formation of a small white deposit on the upper electrode, but this had no effect on the responses. Effect of Flame Residence Time. The influence of the rate of admission of the sample into the flame, represented by the flame residence time (tf), was considered. This factor, measured by the time base of the peak in seconds, depended o n the retention properties of the column packing for the substance under examination, the flow rate of the carrier gas, and the temperature of the column. It became especially important when the carrier gas flow rate was low. The longer retention time caused a greater spread of the sample in the column, the sample being admitted more slowly to the flame, and this was associated with larger responses from such retentive columns. Table I1 contains relevant data selected t o illustrate the effects. For constant samples of SO, in cases 6 and 7, it was found that in two columns of different retention values, and differing only by about 1.0 X 10-l1A in background current, the average decrease in the background current was not wry different, so that the response area was controlled by th :tf values (80 and 27.5 sec.) giving a greater response for the more retentive Porapak Q column than the silicone column, where the flame residence time was about 0.3 of the Porapak column. It is instructive to compare the responses of H,S, SOz, and SCO under various tf conditions. For values of nitrogen carrier gas in the region of 70 to 80 ml min-l where SOs and SCO have similar tf values at 110 and 83 "C,respectively, the response ratio was found to be about 1
280r
r6000
H/C background
7
X
/
Y
v,
m
MOLES(x IO6) MOLES(x IO6) Figure 3. Calibration under nonoptimum conditions, in the presence of impurity hydrocarbon alone
to 3.5. In Table 11, considering cases 1 and 7 with ~ J ( H S= ) 26.5 sec and I j (SOz) = 27.5 sec, about the same despite differences in other conditions, the ratio of responses was about 2.5 to 1. In cases 4 and 9 with t~ = 17 sec approximately for both substances, the ratio was about 2.3 to 1, whereas under near identical conditions but differing substantially in rf as in cases 1 and 6, the ratio was about 1.2 to 1. If responses are to be compared quantitatively, it is evident that the flame residence times for the substances under comparison should be selected to be approximately the same, by controlling conditions so that in effect the peak depth at constant ff becomes a measure of the response. For constant HZflow and hydrocarbon content in the flame and the nitrogen flow varied, the peak depths as well as the peak widths were altered, so that the areas depended on both variables. When the nitrogen and hydrocarbon were held constant and the H2 flow was varied, the peak widths did not vary significantly and the area depended mainly on the peak depth. Interference with chromatographic parameters may be undesirable; however for most analytical purposes where calibrations are made, it would be unimportant. Effect of Polarizing Voltage. The response to the sulfur compounds did not depend critically on the polarizing voltage of the detector. The response followed the pattern of hydrocarbons (8), with a plateau in the 100- to 400-volt range. Outside this range some deviation was observed, so for most of this work 216 V was selected. At low background currents a higher sensitivity of the detector could be used, and it was desirable under these circumstances to maintain a constant voltage, or some nonreproducibility became evident. The normal commercial instrument was therefore slightly modified to allow the use of an external high capacity polarizing battery with tappings, so that the voltage could be measured and maintained constant within limits. To cover the range of output at high background, and allow the use of high sensitivity, an external 90-V source supplemented the usual 30-V suppression battery.
Figure 4. Calibration of carbonyl sulfide, showing effect of hydrocarbon background
Calibration and Linearity of Response. With untreated supply gases--i.e., no CHI added and no gas lines trapped-the background current was about 4.0 X 10-llA due mainly to hydrocarbons naturally present in the hydrogen fuel, and the response was linear over the range 0 to 4 X 10+ mole for SOZ, H2S, and SCO with sensitivities and limits of detection approximately as in Table I. This range was extended to about 10 x 10-6 mole with about the same sensitivity for SO?, by increasing the hydrogen and decreasing the nitrogen flow rate which increased both the hydrogen and hydrocarbon content of the flame indicated by a background current of 2.75 X 10-1OA. Even though the response with large samples and nonoptimum conditions tends to be nonlinear, it is reproducible and can be readily calibrated for standard conditions of flow rate. Figure 3 shows the calibration of the negative responses for SOz,H S , and SCO using the 6-ft silicone column at 110 "C and flow rates of Hz = 45, Nz = 56, air = 235 ml min-l. CH4was present in the hydrogen as impurity giving a background current of about 4.0 X 10-11A. These conditions were by no means the best and in fact were approximately optimum for hydrocarbon, yet the responses were satisfactory. The linearity was generally better and the sensitivity greater for high backgrounds, provided this was due to the presence of hydrocarbon in the flame, whether from the column, the hydrogen fuel, or the carrier gas, but not due to thermionic emission. The effect of the hydrocarbon background on the calibration of SCO is illustrated in Figure 4, the responses are shown for the silicone 6-ft column at 83 O C with Hz = 45, N P = 52, burner air = 235 ml min-1, at several hydrocarbon background currents. The linearity and sensitivity improved as the Hz equivalent was increased, except when the emission region was reached. Figure 5 shows the calibration for SCO under conditions as for Figure 4, but with a fixed hydrocarbon background of 2.00 X 10-lOA produced by adding CHI to the HS, and the carrier nitrogen flow at 65, 52, and 38 ml min-1. Figure 6 shows the combined influence of background and H 2 Z equivalent on the calibration of SCO. The silicone column at 83 "C was used and the burner air held at 235 ml min-l. Curve A was prepared from data at a background of ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
0
453
-r3000
FLOW
X u
N, m l min-'
Ln-
m
Table 111. CS2 -t Oz Unreacted Mixtures below 200 "C Mixture" % O2 :ound % CS2foundb Total 1 22.0 f 2 82.0 f 2 104.0 f 4 80.1 f 1 100.2 i 2 2 20.1 f 1 82.0 i 1 103.4 i 2 3 21.4f 1 79.6 f 2 101.6 i 3 4 22.0 i 1 a FID Air 235, H i 45, Nz60 ml min-l. Each mixture, average of 8 determinations in silicone 6-ft column at 110 "C. *Calibrations with CS2 samples 0.7 to 7 X 10-6M. Oxygen determined by parariagnetic analyzer calibrated against air.
+
Table IV. CSz Oz Mixtures Partly Reacted, 300 "C Approximately Mixturem
% SCO % so2 % cs, Total 95.03s 2 101.4 1.3 f 0.1 5.1 i 2 3.0 ic 0.3 95.0 i 3 100.0 2 1 . 0 i 0.1 87.0 f 2 99.3 3 2.3 i 0 . 2 10.0 f 0 . 5 102.9 4 4.4 f 0 . 5 12.1 f 0 . 7 86.5 3~ 2 6.0 f 1 . 0 92.0 f 2 100.3 5 2.3 f 0 . 5 5.3 f 1 . 0 92.5 f 1 100.1 6 2.3 f 0 . 4 9.4 f 1 . 0 86.0 i 1 99.3 7 3.9 f 0.3 100.9 8 5.0 f 0.4 11.5 f 1 . 0 84.4 f 2 a Column: silicone 6-ft at 110 "C. Conditions of analysis as in Table 111. 1
MOLES(x 10') Figure 5. Calibration of carbonyl sulfide showing effect of nitrogen carrier flow rate
9oooy
8000-
I
/A
still compared favorably with the TCD responses. Even better ratios can be obtained by increasing the hydrocarbon background and H n Z in the flame. Carbon disulfide calimole, brates very linearly with the FID up to about 7.0 X and deviates slightly above this level, but gives no difficulty if treated as a hydrocarbon, provided there is only a little impurity hydrocarbon in the flame. Application to Analysis. Sample analyses were performed on unreacted and reacted mixtures of CSLand O2 using the FID. Mixtures were prepared to be 80% i 1 in CSZ,and were analyzed using the gas chromatograph and FID. The results were checked by the TCD for CSZ,and by a paramagnetic analyzer for oxygen, and found to differ by not more than 5 2 in either case. Table I11 gives the results of the analysis of four different mixtures, each sampled and analyzed in eight separate runs, each analysis sample being approximately 5 X 10-6 mole. When such mixtures were held a t 300 "C for to 2 hours, partial reaction occurred and the products were mainly SOn and SCO. Eight such experiments were performed with slightly differing temperatures and reaction times. The partly reacted mixture was condensed in a liquid nitrogen trap and the excess oxygen pumped off. For each run the contents of the trap were vaporized, allowed to intermix, and submitted to 6 separate analysis runs, using about 3 to 4 X 10-6 mole samples for each run. From the results shown in Table IV, it is evident that the reliability of the SOz analysis is poorer than that of SCO. This is partly due to the lower sensitivity of SOz(see Table I) and partly due to the interference of absorbed hydrocarbon on the column which is displaced by the SOzrather more than by SCO, and adversely affects its negative response. The method was however found to be satisfactory in such applications.
z
MOLES (~10') Figure 6. Calibration of carbonyl sulfide showing combined effect of hydrocarbon and HzZin flame 5.00 X 10-l0A due to 40 ppm CHd in the HP, indicated by points 8 ,or in the Nz indicated by X. The HPflow was set at 35, the N z at 38 ml min-l, equivalent to 21.8 H?. Curve B was taken under the same conditions except that the background was 2.00 X 10-IOAdue t o CHI in the Nz. For curve C and D,the background was the same as for B, but the gas flow rates were adjusted to correspond with 20 and 19.1 HZ equivalent, respectively, in the flame. For conditions of untreated supply gases with the hydrocarbon in the hydrogen at about 5 ppm, the response ratios of FID to TCD are given in Table I, column (c). The conditions were optimum for CS2, but nonoptimum for SOr, HzS, and SCO. When calculated for helium carrier, the FID responses
z
z
454
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
DISCUSSION
Negative Peaks and Background. With low background currents of the order of 1 X 10-'2A, SCO, SOZ,and HzS have been reported (8) to give practically no response. It has been shown here that background currents of 3.0 to 7.0 X 10-l1A arising from hydrocarbons naturally present in the hydrogen,
enable these compounds to give adequate negative peaks with the normal FID. With higher backgrounds (up to 5 X 10-l0A) due to additional hydrocarbon, the negative peaks increased in area proportionately to the carbon present in the flame. Without a background current, negative peaks would not be observed. Apart from the thermionic contribution arising from the hot burner, the background comes from the same processes that give the positive responses for hydrocarbons, Reaction 1, (19). A possible origin of the response would appear to be electron capture. The electron concentration in the flame would be proportional to the hydrocarbon concentration, so that the capture efficiency would depend on the hydrocarbon background. This relationship would also account for the nonlinearity of the calibration curves, the capture process reaching saturation at low hydrocarbon and high sample concentration. The improvement in linearity with increased hydrocarbon background would follow. Electron capture detectors are approximately l o 4 times more sensitive to SO, than the normal FID examined here, so it is possible that the low order response reported here could arise from this process. On the other hand, preliminary experiments show that the response to oxygen is of that to SOs, so another process might be suspected. The extremely large responses with organic sulfur compounds, 50 times that of hydrocarbons, but of negative polarity, observed by Dressler and Janhk (15) raised the question whether the burner used in the present experiments might have become contaminated with alkali salts, and was acting as an AFID, but no evidence was found to support this view. Hydrocarbons were not added to the AFID to increase the background ion current, which was controlled by adjusting the hydrogen flow rate and, hence, apparently the flame temperature which determined the ion concentration in the flame (15). Garzd and Fritz (27) observed a negative signal from organosilicon compounds in the presence of CHI in the FID, and ascribed it to a recombination process connected with the presence of silicon. Page and Woolley (28) suggest that the positive response of the AFID with phosphorus, is due to an increase of ionization of the alkali metal caused by a recombination of the hydrogen atom in excess of the equilibrium value. To explain negative peaks on this basis would require the recombination of ion and electron with an increase of hydrogen atom. Sulfur dioxide has been shown to inhibit the second explosion limit of the Hz-02 reaction (29) and this was explained by the reaction, H’
+ SO, + M-HSOz + M
(2)
The recombination of atoms in hydrogen flames in the presence of SO, appears to involve particularly the removal of H ’ atom (30) by processes including Reaction 2. Since H ’ atom is essential for the formation of oxygen atom, H’
+ O z - t H O ’ + 0’
(3)
the result will be a decrease in the concentration of 0’ atom, which is required in Reaction 1 for the production of flame ions from hydrocarbons. The importance of the oxygen atom is supported by the relationship between the maximum responses for SO,, H2S,SCO, approximately 1 :3 :3. A tenta(27) G. Garz6 and D. Fritz, “Gas Chromatography 1966,” A. B. Littlewood, Ed., Institute of Petroleum, London, 1967, p 150. (28) F. M. Page and D. E. Woolley, ANAL.CHEM., 40,210 (1968). (29) P. Webster and A. D. Walsh, “Tenth Symposium (International) on Combustion,” The Combustion Institute,Pittsburgh, Pa., 1965, p 463. (30) A. S. Kallend, Trans. Furuduy Soc., 63,2442 (1967).
tive mechanism involving Reactions 4 to 8 is proposed to account for this ratio. H2S, SCO + S ’ (pyrolysis in H2 rich flame)
S’
+ 2 0’
+
SO, [in chemi-ionization zone)
SO:!
+ H’
+
SO.OH + SO’
(4) (5)
SO.OH
(2)
+ OH’
(6)
+ 0’ +SO* so, + 0’ so3 SO’
+
(7) (8)
HIS and SCO would take up 3 atoms of oxygen compared with SO, taking up one. The SOz reaction with oxygen may be indirect as in Reactions 6 and 7 involving a transfer of 0’ to H ’ radical, or direct as in Reaction 8. That SO2 reacts in the flame can be observed visually, the flame color becomes a distinct blue-white, and although the color with H2Sand SCO is somewhat like SOn, it is certainly not the same. The hydrocarbon background current acts as an indicator of the decrease of oxygen atoms but does not take a direct part in the flame reactions of the sulfur compounds. The negative peak can be regarded as a hole in the background current caused by competitive removal of oxygen atom from the background forming process. The negative responses from organo-sulfur compounds with the AFID (15) might arise in a similar way, by first forming SO?which may then alter the flame temperature by H ’ atom removal so that the subsequent decrease in ion concentration would give a negative response against the alkali background current. The effect of silicon compounds (27) may be to produce silicon atom which like sulfur may remove oxygen atom and hence diminish the hydrocarbon background current. Negative Peaks and Hydrogen Percent. When the Hn% in the flame was increased, the peak areas generaily increased. The maximum responses for Son,HB, and SCO are obtained with values of HZ% higher than that for the optimum response to hydrocarbons, which does not support the electron capture hypothesis. Though the background current increased by about 100 % with the hyperoxygenated flame at elevated hydrocarbon content, the response to SOz decreased by about 20%, and this also does not support the electron capture explanation. The influence of oxygen in the flame is more evident with H2Sand SCO than with Son. The two former compounds are probably partially preoxidized in the presence of premixed O,, thus decreasing the quantities that ultimately reach the ion-forming region, and ensuring that only the SOn produced would remain to compete for 0’ atom with the hydrocarbon. The ratio of the responses SO, :HzS:SCO only approaches 1 :3 :3 when the H2 content is relatively high and the On content negligible, the presence of premixed oxygen brings the ratio to 1 :1:1 approximately. Increasing the Hz has not always the same effect as decreasing the N, flow rate and vice versa. There is a general correspondence related to the Hz% in the flame, but interference by emission effects limits the advantage of increasing the hydrogen flow. The role of “blow-by,’’ noncapture of charges due to rapid gas flow, is evident, but not alone when responses are increased by decreasing the nitrogen flow rate. Factors such as the flame shape and position and their effects on the burner and flame temperatures cannot be assessed, but clearly a simple explanation based on Hn alone does not suffice. Negative Peak from Carbon Disulfide. Carbon disulfide was observed to give a negative peak when the conditions were optimum for the observation of SO,. It seems likely ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
455
that CS2oxidizes to SO2before reaching the ion forming region so that under these conditions it behaves like an equivalent quantity of SOp. A similar effect was observed by Dressler and Jansik whose experiments were made with a very high air flow rate (15). Preoxidation of the CS1is an important factor in determining its response (171, and the SO, produced is able to oppose the formation of a positive peak in the presence of CH4. The negative responses for SOZ,H S , and SCO with the FID have apparently not been reported, probably because they are only observed with a very pure column, and with hydrocarbon in the supply gases, whereas normal operation of the
FID usually reverses these specifications; most operators avoid high backgrounds and use hydrocarbon contaminated columns. ACKNOWLEDGMENT
The assistance of D. M. Douglas and M. Kecskemeti is gratefully acknowledged. RECEIVED for review October 31, 1969. Accepted December 29,1969.
Digitization of Time-of-Flight Mass Spectra M. A. Grayson and R. J. Conrads’ McDonnell Research Laboratories, McDonnell Douglas Corp., S t . Louis, Mo. 63166
A means of digitizing the mass spectra from a time-offlight mass spectrometer has been devised. The output of the mass spectrometer is recorded on FM magnetic tape along with a time base taken from the analog scanner. The magnetic tape is played back into an analog-to-digital converter operating at a sampling rate of 2000 samples per second. The digitized data are searched by means of a computer program which locates mass peaks, determines their intensity, and measures the value of the time base at which the mass peak occurred. With the aid of the time base, mass numbers are calculated and rounded off to integral values. The output of the program lists mass numbers in ascending order with their corresponding peak heights. An option provides the peak heights in raw or normalized form. The digitized spectrum is also available on digital magnetic tape as input to other data-handling programs. The signal-conditioning circuitry for data acquisition is described.
MASSSPECTRA are most easily interpreted and identified when they are in the digital form of mass number us. peak height. Digitized mass spectra enable the spectroscopist to subtract background accurately and quickly, to compare mass spectra against known standards rapidly, and to facilitate the use of the computer as a tool for the various tasks involved in interpretation. The digitized form is also a convenient and permanent means of storing mass spectra. The most widely used means of recording mass spectra-the oscillographic record-lacks all of these important datahandling features. As a result, many spectra are digitized by hand from oscillographic records. The task of assigning mass ncmbers to all of the peaks in a mass spectrum, reading the peak heights, and recording these data manually is tedious and prone to errors due to fatigue on the part of the person performing the work. Digitized mass spectra are a necessity when the mass spectrometer is used in conjunction with a gas chromatograph. A chromatogram may contain as many as 200 individual peaks. Since it is the usual practice to take two or more mass spectra per G C peak, several hundred mass spectra may be taken during one GC run. Techniques for digitizing low resolution mass spectra from a magnetic mass spectrometer have been described by Hites and 1 Present address, Georgia Institute of Technology, P. 0. Box 31872, Atlanta, Ga. 30332
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
Biemann. Analog to digital conversion is performed in the laboratory and the data are written onto digital magnetic tape for reduction at a later time (1) or fed directly into a digital computer for reduction in real time (2). One of the major problems in mass spectrum digitizing routines is determining the mass number correctly. Hites and Biemann solve this problem by matching peaks in the unknown spectrum against known peaks in a standard spectrum. The mass of peaks which have no match in the standard spectrum can be assigned by interpolation or extrapolation. This approach requires a standard spectrum for each scan speed the operator desires to use and assumes that the scan is fairly reproducible. Commercial mass spectrum digitizers are available and have the advantage of producing the digitized mass spectra in real time. However, at high scan speeds-1 to 2 seconds per mass decade-the digitized mass spectrum may be incomplete because of the slow response of the printer. A technique for digitizing mass spectra from a time-offlight mass spectrometer is given below. The mass spectra and a time base taken from the analog scanner are recorded on F M magnetic tape (3). The recorded data are later digitized and reduced at an analog hybrid computer center. The method by which mass numbers are determined differentiates this from previous work. Mass numbers are calculated directly from the equations governing the time-of-flight of ions in the mass spectrometer rather than by comparison with a known spectrum. The equation used for calculating the mass of an ion is ( 4 ) :
m=[d,+-] t p - tl k
(1)
Since the mass numbers are being determined for a recorded spectrum, physical time is not used for tl and f2. Rather, the value of a variable which is linearly related to physical timeLe., a time base-is used. (1) R. A. Hites and K. Biemann, ANAL.CHEM., 39,965 (1967). (2) Zbid., 40, 1217 (1968). (3) P. Issenberg, M. L. Bazinet, and C. Merritt, Jr., Zbid., 37, 1074 (1965). (4) R. W. Kiser, “Introduction to Mass Spectrometry and Its Applications,” Prentice Hall, Englewood Cliffs, N. J., 1965.
