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Anal. Chem. 1981, 53, 1447-1450
Automated Determination of Ammonia by Gas-Phase Molecular Absorption Prem N. Vijan” and George R. Wood Laboratory Services Branch, Ontario Ministly of the Environment, 125 Resources Road, Rexdale, Ontario, Canada M9W 5L I
Strong absorption at 187.2 nm by traces of ammonia in a continuous stream of nltrogen forms the basis of this method. Automatlon is achleved by Interfacing an automatic sampler, a multlchannel perlstaltlc pump, an adjustable heating bath, and a gas-liquid separator to an atomlc absorption spectrophotometer. Ammonium ions react with a strong base and release ammonla which Is purged out of solution and passed through a windowless heated absorption cell. The absorbance Is recorded. The method is relatively free from interferences and has a sensitivity of 0.26 and a detection llmit of 0.10 pg NH,/mL, respeetlvely. The calibration curve Is h e a r between 0 and 40 ppm. Chemical conversion into ammonium form Is a prerequislte far determining total organic nltrogen. Typical results obtained on some environmental samples for ammonium and total organlc nitrogen are presented. Twenty samples can be analyzed In 1 h.
The most widely used routine methods for determining ammonia are based on the formation of colored complex compounds with hypochlorite-phenate (I) and Nessler’s reagent (2). More recently, gas-sensing electrodes (3),ion chromatography (4),and gas diffusion (5) methods have been used for this purpose. The colorimetric methods are susceptible to interference by heavy metals and colored organic species present in sample solutions. A pretreatment step in the procedure is required to eliminate such interferences. The other two methods require strict pH control and can tolerate only a low concentration of heavy metal cations and organic compounds. The gas diffusion method is time-consuming and not quite so sensitive. Cresser (6-8) utilized the well-known characteristic UV absorption bands of gaseous ammonia for its determination in solutions. His method is manual and involves considerable manipulation, including a 30-min thermostating of the prepared sample solution prior to the measurement step. The conversion/ evolution efficiency of ammonium ions to ammonia gas is poor at the near ambient temperature used. His method is highly dependent on variables such as bubbler design, bubbler depth, and slit width. Muroski and Syty (9) confirmed the findings of Cresser whereas Takahashi et al. (IO)improved the sensitivity by using a 1 m long absorption cell, a higher temperature for generating ammonia gas, and a nitrogen trap for separating ammonia from water vapor. This paper concentrates on automation and further refinement of Cresser’s approach and also its application to the determination of ammonia in environmental samples including heavily contaminated waste waters and effluents. EXPERIMENTAL SECTION Apparatus. A Varian Techtron Model AA-5 atomic absorption spectrophotometer (Varian Associates, Palo Alto, CA) equipped with a Corning 840 recorder was used for absorption measurements. An arsenic hollow cathode lamp was used as a source of radiation. A hydrogen lamp was used as a continuum light source for scanning the absorption spectrum of ammonia. A windowless quartz “T” cell, 10 cm >: 0.6 cm i.d. with a 16 cm X 0.3 cm i.d. inlet tube, was used as an absorption cell. The cell was wound with 4 m of No. 7 Chrome1 “C” wire (Fisher scientific) to which approximately 10 V were applied by means of a variac transformer
Table I. Operating Conditions Spectrophotometer wavelength setting, 197.2 nm mode, absorbance response time, 5 s lamp current, 7 mA spectral bandwidth, 1nm Recorder sensitivity, 1 and 1 0 mV full scale chart speed, 30 cm/h Proportioning Pump and Automatic Sampler sampling cycle, 1 min wash cycle, 2 min Heating Bath temDerature. 98 “C to maintain a temperature of 160 “C. A 100-W heating tape was used to heat the inlet tube of the “T” cell. The latter was fastened to the burner head and aligned with the hollow cathode lamp and the monochromator using the burner controls. The Technicon autoanalyzer (Terrytown, N.Y.) proportioning pump, sampler, and heating bath (20 ft glass coil) modules were used to automate the chemical procedure. The gas-liquid separator used is described elsewhere (11). A precision flow meter was used to control the carrier gas flow rates. The schematic of the instrumental setup is shown in Figure 1. Reagents. All chemicals used were of A.R. grade. Sodium hydroxide used was a 50% w/w solution supplied by Fisher Scientific Co. A 1000 yg NH3/mL stock standard was prepared by dissolving 3.1573 g of NH&1 (Baker Analyzed Reagent 99.5%) in water and making up to 1L. Working standards were prepared by serial dilution. Purified compressed argon and nitrogen were used as carrier gases. Procedure. The operating parameters for the various parts of the instrument are shown in Table I. The hollow cathode lamps, the “T” cell, and the heating bath are allowed sufficient time to warm up and reach equilibrium temperature conditions. The manifold tubes of the proportioning pump are lowered into the appropriate reagent bottles and the solutions are allowed to flow until a stable base line is established on the recorder. Sample Preparation. For the determination of free ammonia in sewage samples, a 10-mL aliquot of the clear supernatant is acidified with a few drops of concentrated nitric acid, boiled, and brought back to the same volume. The samples thus prepared are transferred to the autoanalyzer cups for analysis. For determining total organic nitrogen in sewage, sediment, and vegetation samples, aliquot portions are digested according to Kjeldahls’ procedure. This procedure is essentially the same as described by Muroski and Syty (9). Hi-Vol filter aliquots are extracted with hot 1% sulfuric acid solution prior to analysis. The handling and treatment of these air particulate filters are adequately covered in ref 11. Precipitation samples require no preparation at all. A set of calibration standards is run along with each set of samples, and the concentrations of ammonia are read from a standard curve of peak heights vs. concentrations. RESULTS AND DISCUSSION The Autoanalyzer AAS System. The reagent and sampling part of the system consists of Technicon autoanalyzer sampler and proportioning pump modules. Sequencing and timing of the sample and wash cycles are controlled by the two-lobe cam. The proportioning pump is a seven-channel
0003-2700/81/0353-1447$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
I Nit‘Qpqn
i
Figure 1. Schematic of autoanalyzer-AAS system for ammonia.
model and works by forcing liquid through flexible tubes pinched between a pressure plate and a movable roller system. Six identical channels are used, each capable of pumping 3.90 mL/min of solution. The sample stream (channel 5) and the reagent stream (channel 4) merge on leaving the pump. The resulting mixture at this point contains 37.4% WV NaOH. Nitrogen is next introduced through channels 1,2, and 3. The nitrogen segments the liquid stream and helps to push it through the heating bath. The 98 “C temperature of the bath aids in the conversion of ammonium ion to ammonia gas as well as its subsequent evolution from solution. Downstream from the heating bath a flow controlled nitrogen stream (145 mL/min) is introduced which sweeps the mixed gas-liquid stream into a separator. The sweep gas containing ammonia is directed into the heated “T” cell while the liquid portion is emptied into a waste contaiper. During the wash phase the cam activates the probe causing it to enter the wash reservoir for 2 min. This establishes a base line between sampling cycles. Condensation of water vapor in the inlet tube of the “T” ceU reabsorbs ammonia and obstructs the smooth flow of gases, thus causing base line drift, peak distortion, and tailing. The optimum temperature of the horizontal limb of the “T” cell was found to be 160 “C in order to overcome this problem. If fully automatic operation is not desired, the user may manually present the solution to the sample channel and, thus, do away with the automatic sampler. Figure 2 represents the absorption spectrum of ammonia between 190 and 215 nm using a hydrogen hollow cathode lamp as the source of radiation. Sharp absorption bands, 3.8 nm apart, are seen at 193.4,197.2,201.0,204.8, 208.6, and 212.4 nm. The scans appear to be somewhat different in appearance from those reported by Cresser (6)as well as Muroski and Syty (9). These authors recorded their spectra with rrV spectrophotometers equipped with compensating reference chanpels. The scans shown in Figure 2 are seen by the single beam measuring instrument. Curve “a” is the characteristic absorption spectrum of ammonia in air, uncorrected €or background absorption. Curve “b” shows absorption characteristic of the carrier gas which is largely air, since the light beam travels a long distance through air before reaching the detector. The split peaks shown by Cresser are not seen in curve “a”. The first two absorption peaks happen to coincide with the two principal resonance lines of arsenic. Therefore, an arsenic hollow cathode lamp can be used as a suitable light source at 193.7 and 197.2 nm. This was also demonstrated by Cresser (7). However, contrary to his observation, we found that the radiant output of the arsenic lamp at a given lamp current
WAVELENGTd i n m l
Figure 2. Scan of the absorption spectra: (a) with ammonia, (b) without ammonia.
and slit width was greater than the hydrogen lamp. Also, it performed equally well at both wavelengths. This may be attributed to the differences in the condition and characteristics of the lamps used. The lower curve in Figure 2 was obtained with the exclusion of ammonia from the gas stream. The half-widths of the absorption peaks of ammonia were found to be very similar to those of the emission lines from the arsenic lamp, recorded under identical instrument conditions. Gases other than ammonia absorb strongly at wavelengths shorter than 193.7 nm (curve ‘‘b”).On the longer wavelength side, an absorption minimum is reached at 215 nm. The differences between on-peak and off-peak absorptions for all absorption bands in Figure 2 are large enough to be of analytical value. The sensitivities of ammonia measurement, at the above stated absorption maxima, expressed as ratio of the sensitivity at 193.4 nm are 1.0,0.80,0.60,0.34, and 0.16 in the order of increasing wavelengths. For this work 197.2 nm was chosen because it satisfies the sensitivity requirements and yet is favorably removed from an equally sensitive and usable 193.4-nm line. The absorption spectrum allows a wide choice of wavelengths to suit the concentration range of different types of samples. A maximum available spectral band width (SBW) of 1nm was used in this work to allow for the use of a lower instrument gain setting. Cresser (7) stated that the slit width was very critical. We found that, using a hydrogen lamp, the sensitivities at SBW of 1.0,0.75, and 0.35 are only 20, 10, and 5% lower than at 0.15 nm. Figure 3 shows a few typical absorption peaks of calibration standards. The standard curve is linear up to 40 kg NH3/mL and then starts bending toward the concentration axis, The base line is stable enough to allow 10-fold expansion of the signal for better accuracy at low concentrations. The Effect of Sodium Hydroxide Concentration. The absorption at 197.2 nm by ammonia molecules increases linearly with the increase in concentration of sodium hydroxide solution from 10% to 50%. The absorbance signal obtained by the use of 10% sodium hydroxide was approximately 2.5 times less than the one obtained by 50%. Further increases beyond this concentration were not attempted as the solution becomes too viscous to be handled conveniently by the proportioning pump. The sodium hydroxide solution is mixed
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Table 11. Free Ammonia in Sewage Effluents (pg NHJmL) GMPAa method raw boiled Conway method ~~
34.0 38.9 11.6 8.9 8.7 43.0 10.9 13.9 20.0
32.4 24.8
35.0 27.8
11.0
11.8
8.2 8.2 45.6 10.9 13.8 19.7
9.0 10.7
42.0 10.8 13.0 17.7
Gas-phase molecular absorption. TIME (MINUTES)
Figure 3. Absorbance peaks of calibration standards at 10 mV full scale recorder setting.
in a 1:l ratio with the sample solution, and therefore its effective concentration is 37.4% compared with 40% used by Cresser (6), in his manual method. Such a comparison, however, is unfair because of the basic differences between the two approaches and the fact that the absorption maximum was the combined result of reaction temperature, carrier gas flow rate, and sodium hydroxide concentration. Because of the lubricating nature of the sodium hydroxide solution, it is important that the various connections between glass parts and manifold tubings are well secured by twisting pieces of wire around them. Effect of Temperature. The temperature of the sodium hydroxide and sample solution has a substantial effect on the rate of evolution of ammlonia and, hence, on its absorbance signal. Cresser (6, 7) has adequately demonstrated this fact. However, because of the condensation problem in his method, he was unable to use the optimum temperature. Muroski and Syty (9) used no heat at all which explains an order of magnitude lower sensitivity of their method (3.92 vs. 0.37 pg/mL) compared with Cresser's improved method (7). The proposed method is approximately twice as sensitive as Cresser's after accounting for the use of a 33% longer absorption cell. An optimum temperature of 98 "C was used in our method. Higher temperatures resulted in blowouts, thus making smooth operation of the system difficult. The absorbance of a 10 pg NH3/mL solution decreased almost linearly as the temperature was reduced to 50 "C. Similar observations were made by Cresser (6). Carrier Gas. Nitrogen was used as the carrier gas in this work. Substitution of nitrogen by more expensive argon did not offer any advantage. Ammonia-free air may also be used for this purpose. The optimum flow rate for maximum signal was found to be 145 mL/min. An accurate control of carrier gas flow is necessary for precise results. An increase in nitrogen flow from 145 to 580 mL/min causes a 25% reduction in the absorbance signal. Lower flow rates result in larger base line drift. Introduction alf the carrier gas upstream from the heater module tends to plush the reagents too rapidly and is not desirable. Nitrogen is also drawn in at the rate of 11.7 mL/min by the proportioning pump which serves to segment the liquid stream and carries liberated ammonia through the heater coil. Three nitrogen lines furnish smoother absorption peaks as opposed to one or two lines. Interference. Various amounts of a composite heavy metal solution were added to a 20 pg NH,/mL standard solution to give final concentrations of 5 ppm each of zinc, copper, nickel, lead, cobalt, manganese, chromium; 20 ppm of nickel and iron; and 1ppm of cadmium and 1000 ppm of potassium. These levels of the metals, specified produced no interference. Volatile organic compounds, such as methanol and acetone,
Table 111. Spike Recovery of Ammonia (pg/mL) sample present added found % recovery 1
2 3 4 5 6 av recovery
1.13 0.57
0.49 1.16
0.95 0.49
2.00 2.00 2.00 2.00 2.00 2.00
3.20 2.72 2.59 3.23 3.08 2.51
103.5 107.5 105.0 103.5 106.5 101.0
103.7
interfere by absorbing the UV radiation. Boiling the acidified sample solution prior to analysis completely removed this interference. This is demonstrated by the results obtained on boiled and unboiled samples (Table 11). The unboiled samples obviously contain volatile organic compounds which absorb at the same wavelength as ammonia. Hydrazine (250 pg/mL) did not interfere. However high concentrations of nitrogenous compounds such as plasma proteins which may evolve ammonia on reaction with strong NaOH at high temperatures will likely interfere. Table I1 also demonstrates good agreement between gasphase molecular absorptiori (GPMA) and Conway methods. The method described in ref 5 is hn improved version of the Conway method. This method is based on the slow diffusion of gaseous ammonia from an alkylated ammonium solution into a standard acid solution. The excess acid is titrated with standard alkali by means of a microburet. The whole operation takes place inside a container called the "Conway dish" (Fisher Scientific) with three concentric chambers. Table I11 contains the results of a recovery study. The data shout a slight bias toward the higher side. A mean recovery of 103.7% is quite satisfactory in view of the fact that the concentrations of ammonium ions present in the samples and those added from outside were low. A study was carried out to establish the extent of conversion and evolution of ammonia gas from ammonium ions in solution under the optimized experimental conditions. Ammonia evolved from a standard solution was absorbed in a known volume of standard hydrochloric acid, and the excess acid was back-titrated with a stahdard alkali solution. A conversion efficiency of 62.5% was obtained. Table IV contains data obtained on sediments digested by the Kjeldahl method. Ammonia was determined by the present method and the indophenol blue method by two different laboratories. The agreement is satisfactory and does not show bias. Table V contains some typical results obtained on Hi-Vol air filters and precipitation samples. Precision, Sensitivity and Detection Limit. The relative standard deviation of 10 measurements taken at 20 pg NH3/mL and 4 pg NH3/mL were 1.23% and 0.91 %, respectively. The practical as well as the calculated detection limits
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Anal. Chem. 1981, 53, 1450-1453
Table IV. Nitrogen in Kjeldahl Digested Sediments (gg/g) present method
indophenol blue method
present method
indophenol blue method
1.96 6.21 1.67 3.31 2.47
2.20 6.50 1.70 3.40 2.40
2.78 0.27 2.30 0.25 0.21
2.70 0.30 2.30 0.30 0.20
Table V. Ammonia in Environmental Samples amt of NH, as amt of NH, in particulatea in air, rainwater, Pg/m3 MmL 0.44 5.80 0.70 6.55 0.50 5.63 0.35 0.65 5.79 0.49 0.54 a
0.59 0.50 0.38 0.57 0.15 0.82 0.39 1.77 0.14 0.10 0.10
Collected on Hi-vol filters.
(twice the standard deviation at or near zero concentration) are close to 0.10 pg/mL). The sensitivity as calculated from the slope of the standard curve is 0.26 pg NH,/mL. It may be further increased by using a longer absorption cell. Comparison of Proposed Method with Manual Methods. The sensitivity of the automated method is significantly greater than that of Muroski and Syty's manual method and
27% greater than Cresser's improved method. The base line stability is superior. The ammonia absorption signals can be recorded with equal ease and sensitivity at the 193.7- and 197.3-nm arsenic lines. The proposed method is faster and more sensitive than Cresser's improved method which requires 30 min of thermostating and is subject to the effect of variables such as configuration, depth and dead volume of the bubbler, slit width, heat of neutralization, etc. Although Muroski and Syty's manual method (9) is fast, it is 15-fold less sensitive, As opposed to the transient signals generated by the manual methods, the proposed method produces steady-state signals and provides complete recording of sequential events in a continuous manner. This feature is very useful in assessing the performance of the system. The appearance of signal peaks and base line assists in diagnosing problems, if any should arise. The most desirable feature of the automated method is its manipulation-free unattended operation. The system is very rugged, and up to 140 determinations can be performed in 1 man day. The sampler module can be eliminated if fully automatic operation is not desired. In this mode, the sample can be presented manually to the sample probe. LITERATURE CITED (1) Crowther, J.; Evans, J. Analyst(London) 1980, 705,841. (2) Vogel, A. I. "A Textbook of Quantitative Inorganlc Analysis", 3rd ed.; London W-1,1961;p 783.
(3) Orion Research "Analytical Methods Guide", 9th ed.; 1978;p 33. (4) Bouyoucos, S. A. Anal. Chem. 1977, 49, 409. (5) Obrink, K. J. Biochem. J . 1955, 59, 134. (6) Cresser, M. S. Anal. Chlm. Acta 1976, 85, 253. (7) Cresser, M. S. Lab. fract. 1977, 19. ( 8 ) Cresser, M. S. Analyst (London) 1977, 702,99. (9) Muroski, C. C.; Syty,A. Anal. Chem. 1980, 52, 143. (10) Takahashi, M.; Tanabe, K.; Saito, A.; Matsumoto, K.; Haraguchi, H.; Fuwa, K. Can. J. Spectrosc. 1980, 25, 25. (11) Vijan, P. N.; Wood, G. R. At. Absorpt. News/. 1974, 73,33.
RECEIVED for review January 30, 1981. Accepted April 20, 1981.
Determination of Mercury at the Ultratrace Level by Atmospheric Pressure Helium Microwave-Induced Plasma Emission Spectrometry Kiyoshl Tanabe, Koichi Chiba, Hlrokl Haraguchi, * and Kellchiro Fuwa DepaHment of Chemistty, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Atmospheric pressure heilum microwave-induced plasma emission spectrometry has been applied to the determlnation of mercury, where the cold vapor generation technique was employed for generation of mercury from the solutlon. The detectlon limit was 4 pg/mL or 8 pg, and the dynamic range of the calibration curve was 2 X lo5. The relative standard devlation of 10 replicate measurements using 100 pg/mL standard solution was about 2%. The Interferences of some catlons with the mercury determlnatlon were negligible except for Pt2+ and Pd2+. The present method was applied to the determination of mercury In bovine liver (SRM 1577 from NBS), and anaiytlcai data were consistent with the certifled value.
Current environmental concern with the danger of mercury pollution has accelerated progress of analytical methods for
mercury. Especially in the field of atomic absorption spectrometry, the cold vapor generation technique has been developed to determine mercury a t the sub-part-per-billion (ng/mL) level (1-4), and such a method has been extensively applied to mercury analysis in various samples. Furthermore, such a technique has also been used for atomic fluorescence spectrometric determination of mercury, and has also provided great detection capability of mercury a t the sub-part-perbillion level (5,6).In spite of extensive studies, the cold vapor generation technique utilizing the amalgamation trap for preconcentration has been required for the determination of mercury in some natural samples (e.g., seawater), which contain mercury only at a few parts-per-trillion (pg/mL) level (7-9). However, this kind of preconcentration methods has many chances of loss or/and contamination of mercury. Therefore, improved methods are required for the direct determination of mercury at such low-level concentrations without resorting to tedious preconcentration procedures.
0003-2700/81/0353-1450$01.25/00 1981 American Chemical Society
