Automatic methods for the simultaneous determination of carbon

ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979

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Automatic Methods for the Simultaneous Determination of Carbon, Hydrogen, Nitrogen, and Sulfur, and for Sulfur Alone in Organic and Inorganic Materials Wolfgang J. Kirsten Department of Chemistry, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden

Automatic methods for the simullaneous determination of carbon, hydrogen, nitrogen, and sulfur, and for sulfur alone in organic and inorganic materials are described. They are based upon combustion of the samples in oxygen-helium, reduction over copper or copper(1) oxide at 700-800 'C, and gas chromatographic separation and measurement of the reaction products. The tube fillings have a long lifetime and can furthermore be regenerated many times. The carrier gas is doped with sulfur dioxide to provide for a better chromatographic separation. A very simple method for the preparation of reliable gas permeation tubes is described.

Multielement determination methods are gaining increased importance in elemental analysis, not only because they eliminate one or another weighing operation, but also because they make it possible to use one element as an internal standard and thus to eliminate the weighing step completely. This is particularly important in the analysis of hygroscopic, volatile, and otherwise unstable materials, which are difficult to weigh, and of technical, industrial, and agricultural products and raw materials with a reasonably constant content of one main element. Methods for the simultaneous determination of carbon, hydrogen, nitrogen, and sulfur (1-3) and for sulfur alone ( 4 ) based upon combustion, reduction with metallic copper, and gas chromatographic separation and measurement of t h e reaction products have recently been described. Improved methods are reported below.

EXPERIMENTAL Apparatus. We used the oxygen/sulfur channel of our Carlo Erba Elemental Analyzer 1106 for our work. Its function is shown in Figure 1. We widened the bottom opening of the furnace with a grinding wheel mounted on a dentist's drill, so that the reactor tube, shown in Figure 2, could be inserted from below, together with the Kanthal heater. The thermocouple sheathing tube fell out during this operation. We put it back over the thermocouple and fixed it with a Kanthal wire a t the bottom of the furnace. Electrical contact between the Kanthal heater and the thermocouple must be avoided. For the determination of sulfur alone we used the 80-cm Teflon column, inner diameter 4 mm, supplied by the manufacturer, and for the CHNS determination, a 2.7-m glass or a 2.5-m steel column, inner diameter 5 mm. The columns were connected to the reactor tube with Teflon tubing, inner diameter 1 mm, heated to 120 "C. Column packing was acetone-washed Porapak QS, 50-80 mesh. We load the carrier gas with sulfur dioxide with a gas permeation tube ( 6 )placed into the sampler of the instrument. We had some difficulties to obtain tight permeation tubes according to the procedure described in (6). We prepared them as described in Figure 3. They are kept in a test tube in the deep-freezer when not in use. Reagents. Tungsten(V1) oxide was prepared according to Pella and Colombo ( 3 ) . Copper oxide from wire, Merck, Darmstadt. Copper, prepared by reduction of the same copper oxide with hydrogen. Copper wire gauze for elemental analysis was an old specimen, Merck, Darmstadt. Such copper wire gauze is now available from Engelsmann, 6700 Ludwigshafen a. Rh., West 0003-2700/79/0351-1173$01 OO/O

Germany. Copper, copper oxide, and wire gauze must be free from metals which can retain sulfur oxides. The copper must be free from organic material. Apparent nitrogen blanks and low results of the other elements can otherwise be obtained caused by the water gas reaction. Small amounts of organic material can be eliminated by loading the carrier gas with water vapor overnight. The water vapor will remove the organic material through the water gas reaction. Adjustment of Apparatus. Set up and connect the apparatus as described by the manufacturer. Fill the reactor tube and attach the Kanthal heater. Check that the thermocouple is correctly in its groove at the upper end of the furnace, and introduce an empty CHN combustion tube from above to prevent the thermocouple from leaving its position. Introduce the reactor tube from below, so that the lower end of the combustion tube is inside the reactor tube. Move reactor tube and combustion tube upward until the reactor tube is in place. Take away the combustion tube and fix the reactor tube in position. Connect the heater to a variable transformer. Insert a thermocouple from below so that its top is situated 50 mm from the bottom end of the reduction zone filling. Tighten the bottom hole of the furnace with a refractory fiber material to avoid air draft through it. If the copper(1) oxide reduction zone filling is used fill the tube with copper(I1) oxide and decompose it as described under Regeneration of the Reduction Zone Filling. Measure the temperature of the flash compartment with a thermocouple inserted from above. It should be about 1050 O C . The temperature of the flash compartment and that of the reduction zone can be regulated with the regulator of the furnace temperature of the instrument, and with the voltage of the Kanthal heater. The instrument regulator increases or decreases all temperature in the furnace. Increasing the voltage of the Kanthal heater increases the temperature of the flash compartment and decreases the temperature of the reduction zone, and vice versa. Adjust the temperature of the reduction zone to 72G760 O C . When the temperatures are stable and a stable base line is obtained, the apparatus is ready to work. The reactor tube with a copper filling is adjusted to the same temperatures. When the working temperatures have been reached, run a few analyses with an easily combustible compound like nitrobenzoic acid. Use a good excess of oxygen, for example, an oxygen pressure of 1.2 kg with a 5-mL oxygen loop. Use different times of sample inlet delay and observe the flashes to find out when the oxygen arrives to the flash compartment, and when the last oxygen leaves it. Adjust the inlet delay so that the sample falls about 2 to 3 s after the arrival of the first oxygen. Do not use sulfur-containing compounds and do not use sulfur dioxide in the carrier gas in these analyses. When all combustion conditions and all chromatographic conditions are satisfactory, the apparatus is ready to work. Procedure. Weigh out the samples in tin capsules, No. 313, available from Labor-Service, H. Reinhardt, CH 4125 Riehen, 136 Rainallee, Switzerland, or in aluminum capsules No. 113. Add 3 to 5 mg of tungsten(V1) oxide to each sample. Crimp the containers and place them into the drum together with a sulfur dioxide permeation tube. Wait until a stable base line is obtained and start analyzing according to the manual. Typical running conditions are reported in the tables below. When an old, contaminated separation column is used, the water peak may tail into the sulfur dioxide peak. In this case, the integrator can be instructed to stop integrating before the arrival of the sulfur dioxide peak, and to find a new base line for the latter. It will then draw the tangent to the bottom branches F 1979 American Chemical Society

1174


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Figure 1. Automatic CHN + CHNS analyzer. (1) Oxygen injection valve, CHN. (2) Oxygen injection valve, S. (3) Samplers. (4) Combustion tube, CHN. (5) Reduction tube, CHN. (6) Reactor tube, S, compare Figure 2. (7) Combustion furnace, S. (8)Reduction furnace, CHN. (9) Combustion furnace, C H N . (10) Oven for separation columns. ( 1 1 ) Separation column, C H N . (12) Separation column, S. (13) Hot wire detector. (14) Recorder. (15) Integrator. Carbon-hydrogen-nitrogen-sulfur determination: The oxygen injection valve (2) pumps a volume of oxygen into the carrier gas stream. A few seconds hter the automatic sampler (3) drops a sample into t h e reactor tube (6). The sample is oxidized and in the reduction zone of the tube the excess oxygen is removed, nitrogen oxides are reduced to nitrogen, and all sulfur is converted to sulfur dioxide. The resulting gases are separated in the column (12),and detected, recorded and integrated with detector (13), recorder (14), and integrator (15)

of the sulfur dioxide peak and obtain a correct sulfur result. The small amount of water lost from the tail of the water peak is negligible. When the flash compartment of the reactor tube is filled with residue, which happens after about 5-7 drums, draw up the inner tube with a hook and replace it with a new one. The residue adheres to its inner walls. Since no aggressive metal oxides are in contact with the quartz reactor tube at temperatures above 800 "C, the lifetime of the tube is very long, and its copper or copper(1) oxide filling can be regenerated many times. When the reactor tube must be replaced, the Kanthal heater can usually be left in place. Remove the sampler and the inner tube of the reactor tube and place an empty combustion tube into the upper opening of the latter. Then draw out the reactor tube from below. The combustion tube follows the reactor tube down and keeps the Kanthal heater in place. Then introduce the new reactor tube from below, as described above. Regeneration of Reduction Zone Filling. If the copper(1) oxide f i i n g is used, decrease the temperatures with the instrument regulator and turn off the Kanthal heater. Disconnect the exit end of the reactor tube. Adjust the temperature of the furnace to 1OOO-1010 "C. It should not become higher, because this could cause sintering. With a helium flow rate of about 20 mL/min, all copper oxide has become copper(1) oxide within about 6-7 h. Some copper halide might have distilled out from the tube filling and condensed in the outlet capillary. Place a dish below the tube and inject a few milliliters of water up into the tube with a syringe and brush with a pipe cleaner to wash out any salts. Reconnect the tube and readjust the furnace to the working temperature. If the copper filling is used, push the Standby button of the instrument. This will lower the temperature of the reduction zone to about 500-600 O C . Disconnect the exit end of the tube and insert a stainless steel capillary up into the copper filling and let a slow flow of hydrogen pass into it. When no more water vapor but only hydrogen comes out from the tube, remove the capillary. Inject water and brush out to remove any salts. Reconnect the tube and restore the temperature.

RESULTS Table I shows the results of a series of sulfur determinations r u n a t a n early stage of this investigation with aluminum capsules, without tungsten(V1) oxide and without sulfur dioxide doping. Copper(1) oxide was used as the reduction filling. T h e table does not really d o justice to t h e method, because samples and weights were chosen with the purpose t o investigate t h e influence of foreign elements and to check t h e linearity of the method, rather than t o obtain a very high

Figure 2. Reactor tubes with metallic copper resp. with copper(1)oxide. (A) Quartz tube, outer diameter 10.5 mm. (B) Inner tube fitting snugly into tube (A). (C)Quartz tube, outer diameter 13 mm. (D) Quartz tube outer diameter 6, inner 3 mm. (E) Quartz wool. (F) Copper(1)oxide

prepared by decomposition in situ of copper(I1)oxide. (G) Copper(I1) oxide. (H) Flakes of copper foil, which prevent hydrogen from passing up into the copper oxide when the copper filling (I) is reduced. (K) Tube rolled of copper wire gauze, inner diameter 2 mm. It makes it possible to introduce a steel tube from below into the copper filling and to regenerate the oxidized filling with hydrogen in the furnace. (L) Point where the top of the thermocouple is situated for measurement of the temperature of the reduction zone. (M) Tungsten(V1)oxide. (N) Pythagoras double channel tube, outer diameter 5.8, inner diameter 2 X 1.8 mm. (P) Pythagoras tube, outer diameter 3, inner 2 mm. (Q) Kanthal A 1 wire, diameter 0.8 mm, two wires wound together in the Pythagoras tubes, 35 turns of single wire round the flash combustion chamber (R). The Kanthal heating wire system is introduced into the reactor furnace from below together with the reactor tube. It can be transferred from one tube to another. It is used both with t h e copper type reactor and the copper(1)oxide type reactor. (S) Bottom end of inner tube (B) with slots ground into it, length about 50 mm, width about 1 mm. (T) Hole in inner tube which makes it possible to draw up t h e tube with a hook accuracy in percents. The calibration curve obtained is quite straight. I t passes through t h e abscissa, Fg S, a t 0.5 Fg S. Sulfanilic acid forms graphitic residues on pyrolysis, and it can be seen from t h e table that it is difficult to obtain complete combustion with such compounds with aluminum capsules and without tungsten(V1) oxide. A series of different test compounds run with t h e final CHNS method is reported in Table 11. All analyzed substances are microanalytical standards, except 7-iodo-&oxyquinoline-5-sulfonic acid, which is an ordinary laboratory reagent. Analyses of inorganic and ash-containing compounds are reported in Table 111. Unfortunately congo red, Fluka puriss., was not pure. Calcd: C, 55.17; H, 3.18; N, 12.06; S, 9.21. Obtained averages: C, 51.28; H, 3.10; N, 11.31;, S, 8.81. T h e deviations reported for this compound are therefore from the obtained averages. All others are from theory.

DISCUSSION The methods are based upon t h e instruments and procedures earlier described by Pella and Colombo ( 3 ) and by Biandrate and Colombo ( 4 ) . The designs of their reactor tubes and furnaces were based upon investigations of Dugan ( 1 , 2 )


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Table I. Sulfur Determnations with Copper(1) Oxide. One Drum" weight of sulfur, 7% sample, calcd. found counts factor dev., % no substance !Jg 25.71 0.079 9 3 dibenzyl disulfide 216 880 26.03 -0.32 1 666.0 25.95 0.079 1 9 -0.08 2 494.7 dibenzyl disulfide 1 6 2 623 26.03 0.00 0.06 3 448.6 benzoic acid 364 + 0.06 0.21 787 4 0.44 -0.23 299.5 substance -4, 0.44% S 16.47 sulfanilic acid 8 346 18.51 -2.04 5 40.0 1.07 0.44 substance 4 ,0.44% S 785.1 1 0 681 + 0.63 6 15.82 7 15.82 S-henzylthiuronium chloride 116 026 0.00 578.9 15.53 15.48 + 0.05 3-bromo-2-thiophenic acid 1 0 3 907 528.0 8 8.92 9.13 -0.21 744.4 9 7-iodo-8-oxyquinoline-5-sulfonic acid 84 182 dibenzyl disulfide 10 26.93 +0.90 152759 0.076 31 26.03 447.8 262.4 trifluoroacetanilide dibenzyl disulfide 11 26.38 + 0.35 114 015 0.077 9 1 26.03 341.3 260.3 triphenyl phosphine 12 2 402 0.44 0.34 -0.10 556.0 substance A , 0.44% S 15.99 13 S-henzylthiuronium chloride + 0.17 120 057 15.82 592.8 8.98 14 7-iodo-S-oxyquinoline-5-sulfonic acid -0.15 9.13 68 601 602.9 3-bromo-2-thiophenic acid 15.48 15.56 1 3 1 050 15 + 0.08 665.0 0.00 0.00 0.00 p-hromobenzoic acid 626.4 0 16 dibenzyl disulfide 547.2 1 8 0 810 0.078 78 26.03 26.09 17 +0.06 sulfanilic acid 15.34 100.3 18 1 9 487 18.51 -3.17 thiourea 42.28 42.12 599.2 19 -0.16 320 874 pheny!thiourea 21.18 20 139 083 21.07 +0.11 518.4 ti I i o urea 42.64 21 253 562 42.12 f0.54 469.5 16.33 22 sulfanilic acid -2.18 18.51 362.9 75 0 5 3 22.46 23 dibenzyl disulfide -3.57 177 164 26.03 0.091 49 622.7 Running conditions: Reducing filling Cu,O, Column Carlo Erba, Teflon, 80 cm Porapak QS. Temperatures: flash chamber 1030 " C , point L, Figure 2, 796 'C, oven 87.5 "C. Gas flow rate 1 0 m L / l 9 s. Inlet delay 20.8 s. Oxygen pressure 1 . 0 kg, 5-1nL loop. lntegrator Infotronics CRS 309. Capsules aluminum, Reinhardt 113. No tungsten(V1) oxide and n o sulfur dioxide doping, Calibration factors are from analyses 1, 2, 11,and 17. The percentages of sulfur in analyses 10 and 11 are calculated for the dibenzyl disulfide alone.

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Figure 3. Preparation of gas permeation tubes. (a) Shrink-Tefion tubing WTF-1241 from Tenntub Plastics Go. Inc., Madison Ave and Holley Street, Clifton Heights, Pa., outer diameter 4.1 mm, is heated at one end with a microburner, and the end is pressed together with a forceps. A small beaker is charged with dry-ice and acetone and clamped in an inclined position as shown in (b). The tube is connected to a SO2 tank with a tapered join! 5/20 and a pressure of about 1 kg is applied. SO, is now condensed in the tube. The tank is closed and the joint is removed and the tube is heated with a microburner as shown in inset (c). When the tube melts it is compressed with a forceps and, after cooling, the open end is cut off. Inset (d) shows the resulting permeation tube. With this method, a dozen permeation tubes can easily be prepared in an hour. The method also works with other brands of shrink-Teflon and probably also with other thermoplastics, but we have not tried any others in our analytical work. The tubes are very reliable, because there are no joints. All is one piece

concerning the reactions of copper sulfate and of copper and copper oxide a t different temperatures in t h e sulfur determination method. From his analytical experiments and his thermogravimetric curves, Dugan had drawn the conclusion that the copper-filled reduction zone of the reactor tube must be held a t a temperature "somewhere between about 800 "C a n d 1075 "C". Our investigations showed that lower temperatures are not

only feasible but also preferable, and that copper(1) oxide can be used instead of metallic copper. Oxygen Absorption Capacity of Reduction Filling. A drawback of the sulfur, respectively, carbon, hydrogen, nitrogen, and sulfur determination methods is the fact that the combustion of t h e samples and t h e reduction of t h e combustion products must be carried out in the same quartz tube. This restricts the volume of each filling considerably. T h e complete combustion of sulfur compounds requires rather large volumes of oxygen-more than is needed in the determination of carbon, hydrogen, and nitrogen only-and the filling of the reduction zone is, therefore, rather quickly consumed. When metallic copper is used a t temperatures above 800 "C, it is very difficult to avoid sintering, and when the filling is consumed it is usually necessary t o discard the tube. When the temperature is lower there is almost no sintering, and with the arrangement shown in Figure 2, the tube filling can be regenerated many times. At temperatures above about 800 "C metallic copper reacts with oxygen to form copper(1) oxide, Cu20. At lower temperatures, it forms copper(I1) oxide, CuO. Hence, a t the lower temperatures, the reduction tube filling absorbs twice as much oxygen before it is consumed. Copper(1) oxide at the lower temperature is oxidized t o copper(I1) oxide. Hence, its absorption capacity is 89% of t h a t of t h e same weight of metallic copper a t t h e higher temperature and 44% of t h a t of copper a t t h e lower temperature. However, in t h e reactor tube, copper(1) oxide functions both as a n oxygen absorbent and as a combustion catalyst. This means t h a t a longer filling can be used in the same size of tube. So, in practice t h e oxygen absorption capacity of the copper(1) oxide filling is at least equal to that of t h e copper filling at t h e higher temperature. A further advantage is t h a t there is practically no risk for incomplete combustion even when samples with high oxygen demand are analyzed, because of its high oxidation capacity. Like the

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copper filling used a t low temperature, the copper(1) oxide filling can easily be regenerated many times. Concerning the boundary between the "higher" and the "lower" temperatures, Ruer and Nakamoto (5) report that copper(I1) oxide is decomposed to copper(1) oxide in the absence of oxygen a t 775 "C, possibly also somewhat below this temperature. Dugan and Aluise ( 2 ) in a programmed thermogravimetric experiment stated that the decomposition starts at 830 "C. These results must not necessarily contradict each other. Ruer and Nakamoto worked with static heating of the samples, where even an extremely low decomposition pressure of the copper(I1j oxide results in a detectable decomposition, which would not be discovered in a reasonably rapid programmed thermogravimetric curve. In practical analytical work with the method, we found that the copper(1) oxide filling works well when the point L, Figure 2, is kept between 720 and 800 OC. The interval of 720-760 "C was most satisfactory both for the metallic copper filling and the copper(1) oxide filling. Risk of Copper Sulfate Formation. According to a thermogravimetric curve by Dugan and Aluise (2) the decomposition of copper sulfate in the absence of oxygen starts slightly above 570 "C. Hence, above this temperature copper sulfate can be formed only if the gas pressures of the entering SO,,SOz, and Oz exceed the corresponding decomposition pressures of copper sulfate-with some modification, of course, due to the mass action law. Under the conditions of the analyses, this can certainly happen only for short moments and only a t temperatures far below 800 "C, and any formed copper sulfate is quickly decomposed again. I t appears, therefore, that from the viewpoint of copper sulfate formation, the reduction zone could be held a t temperatures down to about 650 "C. Risk of Sulfide Formation. The first step of a flash combustion is a rapid pyrolysis of the sample, which produces a cloud of strongly reducing carbonaceous particles and gases. If a metal oxide, reducible by the atmosphere of this cloud, is present, it is reduced, and if the metal forms a sulfide, some sulfide is formed. T h e next step is the oxidation of the cloud with gaseous oxygen and with oxygen from combustion catalysts. The sulfides of copper, tin, and aluminum are not decomposed by heat, and they cannot be oxidized by the oxygen of combustion catalysts. It is, therefore, important to add a sufficient amount of gaseous oxygen in such a manner that an excess still is present when all the organic material has been burned. It is, however, also important to have oxygen already present a t the start of the pyrolysis. If this is not the case, the reducing cloud can penetrate deeply into the copper oxide layer, reduce it, and form sulfide there. Such sulfide is more difficult to oxidize, because the reduced copper above it prevents gaseous oxygen from reaching it. If sulfide remains unoxidized in the reactor tube in one analysis, such sulfide is oxidized by oxygen of the next analysis and memory effects are obtained. Both the quantity of the added oxygen and the accurate timing of the sample inlet are, therefore, extremely important. Tin or Aluminum Capsules? Tin capsules ignite and burn brightly, generating a very high temperature, at which also very stable sulfates are decomposed and the sulfur is liberated. However, when a series of analyses has been run, a layer of tin oxide ash is accumulated in the flash compartment. The tin oxide is easily reduced by the cloud of carbonaceous particles and gases formed in the first step of the flash combustion, and tin sulfide is formed. If there is not a very good excess of gaseous oxygen and a sufficiently high temperature-preferably 1050 "C-to reoxidize everything, tin sulfide will remain and cause low sulfur results and memory effects.

Aluminum capsules ignite slowly and do not generate an equally high temperature, and they d.o not disintegrate to loose ashes, but the residues of the sample pyrolysis remain enclosed in the oxidized capsule. This has the disadvantage that oxygen cannot penetrate freely into the capsule, and if the sample forms large amounts of carbon residues in the pyrolysis, there is a considerable risk of incomplete combustion. Also, aluminum capsules give a small nitrogen blank, which probably originates from aluminum nitride in the metal. On the other hand, aluminum capsules have the advantage that their accumulated ashes are not reduced by the carbonaceous cloud from the pyrolysis of the sample, which considerably decreases the risk of sulfide formation. Tungsten(V1) Oxide. Quite pure tin capsules are not commercially available. They contain traces of other metals, some of which can form stable sulfates in an oxidizing atmosphere at the temperature of the flash compartment. Also the samples can contain such metals, which can cause low results and memory effects in the analyses. This is avoided by the addition of tungsten(V1) oxide powder to all samples. The tungsten(V1) oxide forms tungstates with the interfering metals and prevents the formation of sulfates. It also displaces sulfur from such metals in the samples and provides for a complete recovery in the analyses. Correct sulfur results are obtained also when the ashes of several drums are accumulated in the flash compartment. Risk of Halogen Interference. Pella and Colombo ( 3 ) , holding the reduction zone of the reactor tube at 840 "C, state that chlorine from sampies passes through the reactor tube and interferes with the analyses. With the reduction zoce a t the lower temperature, we have never observed this. When the copper(1) oxide filling is regenerated with the separation column connected, something comes out and contaminates the first few centimeters of the column. Since we have analyzed fluorine-, chlorine-, bromine-, iodine-, and phosphorus-containing compounds with the tube, we do not know what comes out. Chromatography. Before we tried the addition of S O z , we obtained quite correct results for carbon, hydrogen, and nitrogen without any corrections of the integrator readings. The sulfur results were slightly low, but when a constant number of counts was added to each inkgratm reading, correct results were obtained both for samples with high and low sulfur contents. This means that a constant amount of sulfur was lost in every analysis. Dugan (2j says that the negative error should not exceed 0.5 pg, and Pella and Colombo (3) report a constant negative error of 0.6 pg. In experiments with the methods ( 3 )and (4), in which we used the same reactor tube in the same apparatus and only different columns, we found that the errors were about proportional to the volumes of the columns. With the 80-cm column of method ( 4 ) ,we obtained errors of about 1 to 2 pg, and with a 2.7-m column, which we used for CHNS determination, we obtained errors of about 6 fig. We also obtained a lower precision of the sulfur results with the CHNS method than with the sulfur determination method (4).In these experiments the Porapak filling was untreated and unwashed with acetone ( 7 ) . Washing with acetone for only 2 to 3 min (8) and heat treatment ( 7 ) reduced the negative errors by about 50%. When an electron capture detector with a much higher sensitivity for sulfur dioxide (9) was introduced after the hot-wire detector of t.he instrument, it bccame quite clear that the negative errors in our case were caused hy the combined effect of tailing of the sulfur dioxide peak and a t,oo small maximum peak width of our Infotronics CRS 309 integrator. T h e integrator cut off a part of the tail and lost it and, when the peak was very low, the integrator considered it as a false trip and lost it completely. The absolute amount


of sulfur dioxide lost was about t h e same in all cases, which explains the straight calibration curve. The tailing increases, of course, with the volume of the column. We could eliminate the negative errors completely and obtain a higher precision of the sulfur results by placing a gas permeation tube ( 6 ) with sulfur dioxide into the sampler of t h e analyzer. T h e tube liberates slowly a small amount of sulfur dioxide, which diffuses into the carrier gas. The active sites in the apparatus, which can retain sulfur dioxide, are thus continuously occupied, and the correct amount of sulfur dioxide from t h e sample passes through t h e column on top of t h e constantly present sulfur dioxide and is measured accurately without losses. The added sulfur dioxide also reduces t h e tailing of t h e other peaks in the chromatogram. From the displacement of the recorder base line on addition of t h e gas permeation tube. the amount of sulfur dioxide liberated from the tube could be roughly estimated to be about 6 pg/min. Regeneration of the Reduction Zone. The regeneration of the copper(1) oxide requires a very accurate regulation of the furnace temperature. A too high temperature can cause sintering of the filling and, with a too low temperature, the decomposition of the copper(I1) oxide takes a very long time. With our old Carlo Erba instrument, the temperature cannot

1179

be just set, but it must be carefully followed and adjusted manually, and afterward it must be carefully readjusted again to the working state. We use, therefore, t h e copper filling, which can be brought to its regeneration temperature with a pushbutton, and which lasts longer. With an instrument with a more modern temperature regulation a copper(1) oxide filling would probably be more convenient than the copper filling. The analytical results are the same with both fi, ings.

LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)

Dugan, G. U.S. Patent 3 838 969, 1974. Dugan, G. Anal. Lett. 1977, 10, 639-657. Pella, E.;Colombo, B. Mikrochim. Acta 1978 I. 271-286. Biandrate. P.;Colombo, B. Carlo Erba Elemental Analyzer, mod. 1106, manual. Ruer, R.; Nakamto, M. Recl. Tray. Chim. fays-Bas 1923, XLII, 675-682. O’Keeffe, A. E.: Ortman, G. C. Anal. Chem. 1966, 38, 760-763. De Souza. T. L. C.; Bhatia, S. P. Anal. Chem. 1975, 4 7 , 543-545. Pelia, E.;Colombo, B. Carlo Erba Strumentazione, Milano, Italy, private communication. Bethea, R. M.: Meador, M. C. J . Chromatogr. Sci. 1969, 7, 655-664.

RECEKEDfor revie January 18,1979. Accepted April 2 , 1979. The work was ma..e possible by grants from the Swedish Council for Forestry and Agricultural Research and from she Faculty of Agriculture of the Swedish University of Agricultural Sciences.

Evaluation of Several Semi-Theoretical Methods for Quantitative Secondary Ion Mass Spectrometric Analysis after Discrimination-Correction of Data M. A. Rudat’ and G. H. Morrison” Department of Chemistry, Cornell University, Ithaca, New York 14853

Four methods of quantitative analysis are evaluated using steel, copper, and aluminum standards. The mass spectra are corrected for instrumental discrimination effects in order that the models themselves rather than the instrument are tested. The effects of this correction and normalization of the calculated concentrationsto 100 % are described. The analytical results of the various approaches are presented in terms of error factors. None were found to be analytically quantitative.

Two basic approaches to the conversion of qualitative secondary ion mass spectrometric (SIMS) data into semiquantitative or “quantitative“ concentration values have been the use of sensitivity factors and the use of theoretical models combined with raw data for the determination of some “fundamental” parameter values. ‘The latter approach will be referred t o as “semi-theoretical“, since the values of the parameters are determined empirically. The former approach has been the most successful and accurate to date, particularly when some sophistication is introduced into the choice of the sensitivity factors ( I ) . However, for routine analyses the semi-theoretical approach is most desirable if it can be applied Present address: Central Research & Development Department. Experimental Station, E. I. du Pont Kemours & Company, LVilmingon, Del. 19898. 0003-2700/79/0351-1179$01 O O i O

to all samples, and several methods have been claimed to be nearly universal in applicability (2-13). These semi-theoretical methods are the ones to be discussed here. A shortcoming of previous examinations of the accuracy of these computational methods has been the failure to include the effects of instrumental discrimination on the relative ion intensities. Furthermore, no direct comparison of these methods by using a single data set has previously been made, making comparisons of claims of accuracy difficult to perform. T h e present work is designed to serve several functions. Most importantly, the effects of instrumental discrimination (14-1 7 ) are virtually eliminated so that the unencumbered accuracy of the investigated methods can be clearly seen. If the overall accuracy is not improved by removing these effects, then the analytical usefulness of the methods studied here must be brought into question. Secondly, the results of the different semi-theoretical methods will be directly compared using the same data sets, so that no confusion regarding different analysis conditions in different laboratories can exist. These methods can also be compared to the simplistic approach of considering the relative intensities of the corrected signals to be directly related to the concentration. Finally, the universality of applicability of the methods to metal alloy matrices will be explored. In considering some preliminary results obtained using Simons et al.’s local thermal equilibrium (LTE) program ( 4 ) and Andersen and Hinthorne’s published accounts of t h e F 1979 American Chemical Society

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