Analyst, March, 1968, Vol. 93, $$. 153-157 153 The Nitrous Oxide - Hydrogen Flame in Spectroscopic Analysis BY R. M. DAGNALL, K. C. THOMPSON AND T. S. WEST (Chemistry Department, Imperial College, London, S. W.7) A high temperature flame, burning hydrogen supported by nitrous oxide, is described as an atom reservoir for thermal emission spectroscopy of elements such as aluminium. The flame has a low burning velocity and can be sup- ported on most conventional burners supplied for use with air - acetylene in emission or absorption studies. Emission from aluminium a t 3962 and 3944 shows a 2 : 1 intensity ratio and extends throughout the entire length of the flame. was obtained with a 1-5 x 10-2-mm slit and a linear calibration graph between 20 and 200 p.p.m. Zinc, cadmium and lead, which show negligible emission in the air - hydrogen flame, emit strongly in nitrous oxide supported hydrogen.Calcium shows strong emission, even in the ionic doublet at 3934 and 3965 A, particularly a t low concentrations. A detection limit of 4 p.p.m. a t 3962 RECENT trends in spectroscopic analysis have been directed towards producing pre-mixed high temperature flames, which have the effect of increasing sensitivity in thermal-emission studies and of eliminating the depressive matrix effects produced by elements that form refractory oxides in atomic-absorption spectroscopy. The nitrous oxide - acetylene flame finds considerable application in this technique,l while oxy-acetylene is perhaps one of the most versatile flames in emission spectroscopy.2 One of the greatest general disadvantages of these flames is that they nearly all exhibit high background emission (the oxy-hydrogen flame being an exception, except over the OH band region).The background is not usually stable and hence leads to less precision and high limits of detection in both emission and absorption measurements, regardless of the method of amplifying the photomultiplier signal. In addition to this general disadvantage, most high temperature flames present other problems, e.g., the high burning velocity of the nitrous oxide-acetylene flame makes burner design difficult, and the explosive nature of some mixtures such as oxy-cyanogen and oxy-acetylene renders them unsatisfactory for routine use. An equally important consideration is the anomalously high electronic excitation that occurs in some pre-mixed flames when metal salts are present,3 and is particularly strong in pre-mixed flames of hydrocarbons with air or oxygen and especially so with acetylene.As the oxy- and air - hydrogen flames that have favourable background radiations unfortunately seem to be completely free from this anomalous behaviour there appears to be no one flame at present that is completely acceptable as an atom reservoir for either thermal-emission or atomic-absorption studies. The present interest in atomic-fluorescence and thermal-emission spectroscopy makes the need for alternative high temperature, low background flames even more pressing. I t is the purpose of this paper to illustrate some of the advantages to be gained from the use of a pre-mixed nitrous oxide - hydrogen flame in the above areas.Firstly, this flame has a low burning velocity, unlike oxy-acetylene or oxy-hydrogen, and thus the burner design and gas mixtures are not critical. In fact, we have found that this flame can be burned on practically any commercially available burner head whether it is designed for emission or absorption measurements. In addition, the risk of explosion is no greater with this flame than with any other commonly in use. 0 SAC and the authors.154 DAGNALL, THOMPSON AND WEST: THE NITROUS OXIDE - [Auta&!yst, VOl. 93 The background radiation is also much lower with the nitrous oxide - hydrogen flame than with any hydrocarbon flame, because there is no possibility of emission from carbon species.The only background emission obtained under normal operating conditions results from OH bands at about 3000 to 3200 A (present in all hydrogen or hydrocarbon-based flames), the NH band at about 3360 A, and from the NH, radical or the reaction between 0 and NO in the visible r e g i ~ n . ~ In spite of this, the temperature of this flame is not thought to be any lower than the usually recognised high temperature flames, such as nitrous oxide - acetylene, ox y-hydrogen, et c . Finally, the anomalously high electronic excitation that has been found in pre-mixed, oxygen-supported hydrocarbon flames is also marked in the nitrous oxide - hydrogen flame.3 In theory it would seem that this flame is well suited to the requirements of thermal- emission, atomic-absorption and atomic-fluorescence4 measurements.At this time evidence is presented to support its use in thermal-emission studies. EXPERIMENTAL BURNER UNIT- Initially the flame was burned on the 7 x 1 6 c m long, air - acetylene emission burner head supplied as a standard item with the Unicam SP9OOA thermal-emission - atomic- absorption flame spectrophotometer. This burner has a 1-cm square series of 13 holes near one end, and is particularly suitable for use with this instrument as a steady flame condition could be maintained by nebulising on 15 p.s.i. of nitrous oxide and supplying between 1-5 to 2.5 p.s.i. of hydrogen. The nitrous oxide was supplied to the burner via the normal instru- mental nebulising system, but the hydrogen was supplied via an external gauge and was led into the burner through the jet fitted in the base of the burner stem.Below 1.5 p.s.i. of hydrogen the flame struck back, and above 2.5 p.s.i. the primary reaction cones became unstable. This flame gave good emission signals for zinc (2139 A) and cadmium (2288 A) solutions, but relatively poor signals for aluminium (3944 and 3962 A), which required a more fuel-rich flame (about 3 p.s.i.). Good aluminium emission signals were obtained by using the circular air - propane burner head supplied with the Unicam SPSOOA, and steady flame conditions were achieved with 15 p.s.i. of nitrous oxide and between 2.5 and 3.5 p.s.i. of hydrogen. In the early stages of this work, the flame was lit with a small volume of nitrogen flowing through an auxiliary jet in the burner base5 to eliminate any danger of flash-back.When the required pressures of nitrous oxide and hydrogen had been supplied, the nitrogen flow was turned off. The reverse procedure was carried out when the flame was extinguished but this precaution is not essential, however, as only a comparatively weak “pop” is experienced when a flash-back occurs. FLAME SPECTRUM- The flame background of the nitrous oxide - hydrogen flame burning on the circular air - propane Unicam emission head showed above the yellow primary cones only strong OH band emission between about 3000 to 3200 A, weak NH band at about 3360 A and a steady continuum from 3500 to 6000 A, which is partly caused by the reaction3 between NO and 0. The primary cones, in addition, exhibit in the visible region the ammonia a-band system, which is now known to be caused by the NH, r a d i ~ a l .~ The air - acetylene flame background, measured under conditions as similar as possible and also above the primary cones, was about four to five times greater than that of the nitrous oxide - hydrogen flame at about 4000 A. Measurements in the primary cones showed an even greater difference in background radiation. APPLICATION TO THERMAL EMISSION OF ALUMINIUM- Thermal emission from aqueous solutions of aluminium at the resonance lines 3944 and 3962 A was observed throughout the entire length of the nitrous oxide - hydrogen flame, but reached a maximum about 3 to 4 cm above the top of the burner head. Optimum emission was obtained by using the Unicam circular air - propane burner head with a slightly fuel-rich flame.In agreement with the observations of other workers,6 who used different flames, the emission at 3962 A was found to be about twice that at 3944A. A typical spectral plot is shown in Fig. 1.March, 19681 HYDROGEN FLAME I N SPECTROSCOPIC ANALYSIS 155 1 385 400 425 450 Wavelength, mp Fig. 1. Flame-emission spectrum of aluminium, obtained by nebulising a 1000 p.p.m. aluminium solution with a slit width of 0.008 mm, gain 3,5, band width 2, 15 p s i . nitrous oxide and 3.5 p.s.i. hydrogen The limit of detection (signal-to-noise ratio of 1) measured at 3962 A with a slit width of 0.015 mm was about 4 p.p.m. of aluminium. The calibration graphs plotted over the range 20 to 500 p.p.m. showed that between 20 to 200 p.p.m.they were linear, but above this a slight curvature developed and above 1000 p.p.m. of aluminium the log - log slope was about 0.56, indicating appreciable self-absorption. The above limit of detection could be lowered by diluting the aqueous phase with organic solvents, for example, a solution in 20 per cent. isopropyl alcohol doubled the response of an equivalent concentration of aluminium in aqueous solution. This illustrates further advantages of this flame, i.e., the ability to handle solutions containing high concentrations of inflammable organic solvents without risk or without need to modify the operating conditions. Solutions containing 10 and 20 per cent. of glycerol, on the other hand, caused a signal decrease of 12 and 21 per cent., respectively. The differences in the nebulisation rates largely account for these effects.PREPARATION OF CALIBRATION GRAPH FOR ALUMINIUM REAGENTS- Standard aluminium solution-Dissolve 1 g of aluminium foil in 40 ml of 1 + 1 hydro- chloric acid (analytical-reagent grade) and dilute to 1 litre with distilled water to give a solution containing 1000 p.p.m. of aluminium. Nitrous oxide and hydrogen from cylinders. APPARATUS- The equipment used in this work was a Unicam SP9OOA thermal-emission - atomic- absorption spectrophotometer (coupled to a Servoscribe 0 to 100-mV recorder), with an air - propane burner head. The normal E.M.I. 9529B photomultiplier supplied with the instrument was replaced by an E.M.I. 9601B photomultiplier, which is more sensitive in the ultraviolet region.156 DAGNALL, THOMPSON AND WEST: THE NITROUS OXIDE - [Artahst, VOl.93 PROCEDURE (FOR 20 TO 2000 P.P.M. OF ALUMINIUM)- Pipette 2 to 20ml of the standard aluminium solution into 100-ml conical flasks and dilute to volume with distilled water. Nebulise aliquots of the solutions with 15 p.s.i. of nitrous oxide and measure the emission at 3962A with a hydrogen pressure of 3.5 p.s.i., slit width 0.015 mm, gain 3, 10, band width 3, and with the top of the burner head 3 to 4 cm below the centre of the monochromator slit. OTHER EMITTING SPECIES- Preliminary measurements indicate that this flame can also be used with advantage in thermal-emission studies of elements that do not form refractory oxides. For example, zinc, cadmium and lead, which normally show negligible emission in air - hydrogen flames,6 are readily excited in the nitrous oxide - hydrogen flame.The emission from these elements extends throughout the flame and is usually strongest about 2cm above the top of the burner head. Comparable emission signals for zinc and cadmium in an air - acetylene flame can only be obtained by taking measurements from the primary reaction zone about 2 cm above the top of the burner head. The emission from lead atoms in the nitrous oxide - hydrogen flame is about twice that obtained in the primary reaction zone of an air - acetylene flame. TABLE I THERMAL-EMISSION STUDIES IN THE NITROUS OXIDE - HYDROGEN FLAME Emission signals Concen- tration, Element p.p.m. Zn 2000 Cd 1000 Pb 200 Ca 40 40 A1 1000 Wave- Slit length, width, A mm 2139 0.1 2288 0.05 4057 0.01 3934 0.01 3968 0.01 3962 0.012 Air - hydrogen Air - acetylene r- - Gain u b U b 3,lO 0 0 28 0 3,lO 4 0 168 3 3.5 4 4 42 23 2,O 0 0 Background 0 2.0 0 0 too high 0 3 J 5 0 0 4 0 Nitrous oxide - hydrogen A U b 25 17 92 60 62 78 86 20 46 10 45 90 u corresponds to the emission in primary reaction zone. b corresponds to the emission 3 cm above the top of the burner head.Easily excited elements such as calcium can also be measured with advantage in the nitrous oxide - hydrogen flame. The ionic calcium doublet lines at 3934 and 3968 A are quite strong and exhibit an intensity ratio of 2 : 1. Emission signals at these lines cannot normally be obtained either in air - hydrogen or air - acetylene flames. Table I shows a com- parison of the emission signals of the above elements in normal nitrous oxide - hydrogen, air - hydrogen and air - acetylene flames on the Unicam SP9OOA spectrophotometer.Measurements were made in the primary reaction zone and about 3 cm above the top of the burner head. The calcium emission from the ionic doublet at 3934 and 3968 A was quite strong and at low calcium concentrations (less than 0.05 p.p.m.) was more intense than that from the 4277 A resonance line. Table I1 shows the 4227 : 3934 A intensity ratio for various calcium concentrations. TABLE I1 CALCIUM INTENSITY RATIO MEASUREMENTS Calcium concentration, p.p.m. 40 16 4 2 0.4 0.1 0.05 Emission intensity ratio, 4227 : 3934 29 21 8 6 3.3 1-4 1March, 19681 HYDROGEN FLAME IN SPECTROSCOPIC ANALYSIS 157 Hence at low calcium concentrations appreciable ionisation must occur in this flame.The extent of the ionisation at these wavelengths is known to be sensitive to traces of the other easily ionised elements, e.g., potassium, rubidium and caesium, and hence emission at these wavelengths is not normally used analytically. However, in their absence, e.g., in distilled water, a more sensitive determination of calcium becomes possible in the nitrous oxide - hydrogen flame. CONCLUSIONS The nitrous oxide - hydrogen flame shows considerable promise as a strongly reducing, high temperature flame of low background for use in thermal emission or atomic absorption, even for elements such as aluminium that form refractory oxides. Other elements, such as cadmium, lead and zinc, which do not form such oxides, but which are not easily excited in an air - hydrogen flame, show strong atomic emission and absorption throughout the body of the flame. Calcium shows strong ionic emission, particularly at low concentrations. The above experiments show that the nitrous oxide supported hydrogen flame acts as a most efficient atom reservoir for a wide range of metals and is worthy of further examination. REFERENCES 1. 2. 3. 4. 6. 6. Amos, RI. D., and Willis, J. B., Spectrochim. Ada, 1966, 22, 1325. Herrmann, R., and Alkemade, C . T. J., “Chemical Analysis in Flame Photometry,” Interscience Gaydon, A. G., “The Spectroscopy of Flames,” Chapman and Hall Ltd., London, 1957. Dagnall, R. M., Thompson, K. C., and West, T. S., Analytica Chim. Acta, 1966, 36, 269. Mackison, Ii., Aqaalyst, 1964, 89, 745. Dean, J. A., “Flame Photometry,” McGraw-Hill Book Co. Inc., New York and London, 1960, Received August 21st, 1967 Publishers Inc., New York and London, 1963. p. 230.