However, a few samples did exhibit an absorption. Since molecular interferences due to high concentrations of alkali metal halides have been reported elsewhere (13), and since some wastes often have relatively high sodium and potassium levels, the absorption of high concentrations of the chlorides of these metals at the 198.1 nm line were investigated. These results are summarized in Table V where it is apparent that 1000 mg/l. concentrations of each of the two metal halides cause significant background absorption. These high levels of sodium (in the presence of chloride) are sometimes observed in samples which have been concentrated during the digestion step, while potassium levels are usually only about 5 % of the sodium concentration. This problem may be overcome using several techniques (I3), but the most convenient in this case is to take the difference between the absorption peak at the 196.0 nm resonance line and the absorption peak at the non-resonance line measured under the same conditions. This technique is possible because sodium chloride exhibits nearly identical absorption at both the 196.0 nm and 198.1 nm lines. The ease of injecting samples also makes this type of compensation fairly convenient. The effects of insoluble materials present in wastewaters on the recovery of selenium were investigated to determine the feasibility of analyzing wastewaters for selenium by carbon rod techniques without prior digestion. The results shown in Table VI exhibit very poor recovery for raw sewage and eWuent from a primary treatment, while selenium recovery from waters subjected to secondary activated sludge treatment (samples extremely low in suspended material) is significantly greater. This loss may be due to adsorption of selenium compounds onto suspended organic matter, resulting in a heterogeneous mixture in which selenium-laden particles are not physically capable of being pipetted with the Hamilton syringe. It was noted earlier that 3.0 ml of concentrated nitric acid was added to each sample after digestion. Matousek (8) (13) B. V. L‘vov, “Atomic Absorption Spectrochemical Analysis,” translated from Russian by J. H. Dixon, Adam Hilger Ltd., London, 1970, p 230 ff.
Table V. Absorption of NaCl and KCl at a Non-Resonance Line of S e Peak m V Sample A, nm reading 1 mg/l. Se 196.0 21.5 200 rng/l. K 500 mg/l. Na
lo00 mg/l. K lo00 rng/l. Na
198.1 198.1 198.1 198.1
2.0 3.5 65.0 24.5 ~~
Table VI. Suspended Solids Effects in Spiked, Undigested Samples, Duplicate Analyses Concentrations, pg/ml Recovery, Sample Theoretical Analyzed I. Raw sewage 1.006 0.115 15 I. Raw sewage 1.006 0.168 17 11. Primary effluent 1.006 0.281 28 11. Primary effluent 1.006 0.275 27 111. Secondary effluent 1.006 0.849 84 111. Secondary effluent 1.006 0.801 80 IV. Secondary effluent 1.006 0.900 90 IV. Secondary effluent 1.006 0.900 90
z
reported that a 1000-fold excess of perchloric acid caused a 25 reduction of peak height, which he attributed to chemical interference. This phenomenon has been observed here in the selenium analysis, accompanied by an intense perchloric acid peak. This addition of nitric acid after digestion prevented the metal peak reduction while suppressing the perchloric acid peak. Also, this “buffering” action of nitric acid on the perchloric acid gives each carbon rod a longer analytical life. The addition of nitric acid, therefore, is a recommended step prior to carbon rod analyses when perchloric acid is employed in sample pre-treatment.
RECEIVED for review January 12, 1972. Accepted May 12, 1972.
Improved Gas Chromatographic Method for Field Measurement of Nitrous Oxide in Air and Water Using a 5A Molecular Sieve Trap Jurgen Hahn Max-Planck-Institut fir Chemie (Otto-Hahn-Institut), Mainz, W . Germany
ALTHOUGHSOIL BACTERIA have been suspected to be the major source for the atmospheric NzO and photochemical destruction in the troposphere and stratosphere the major sink, there were not sufficient reliable and systematic data available to give certainty about the cycle of N20. Some years ago, therefore, Junge and his coworkers began a comprehensive field program to study the cycle of atmospheric Nz0. In two recent papers by Schuetz et a f . ( I ) and Junge el af. (2),N?O data were discussed which were obtained during
this program using the analytical method described by Bock and Schuetz (3) and by Junge et al. ( 2 ) . In this method, N20 from air and water samples is adsorbed on 5A molecular sieves and then transferred for analysis to a gas chromatographic column by means of a Toepler pump. From water samples the N 2 0is expelled by heating and passing through a stream of clean Nz. The standard deviation of this method was + 5 z for air and *lox for water samples. In an analytical method published by Leithe and Hofer (4, N20
(1) K. Schuetz, C. Junge, R. Beck, and B. Albrecht, J. Geophys. Res., 75,2230 (1970). (2) C. Junge, B. Bockholt, K. Schuetz, and R. Beck, Meteor Forschungsergeb. Reihe B, 6, 1 (1971).
(3) R. Bock and K. Schuetz’ Fresenius’ 2. Anal. Chem., 237, 321 (1968). (4) W. Leithe and A. Hofer, Allg. Prakt. Chem., 19,78 (1968)
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11. SEPTEMBER 1972
1889
6
--E-
U
-
Figure 1. Processing system for measuring N 2 0 in water samples A. B.
C. D. E.
F. G.
H. 1.
K.
Glass U-tube filled with 5A molecular sieves Gas inlet tube 6-titer glass bulb Sample inlet tube Water-cooled condenser Glass U-tube filled with NaOH pellets Glass U-tube filled with sodium asbestos Glass U-tube filled with P2Oj N t 0 :idsorptiontube filled with 5A molecular sieves Heating mantle
from air and water samples is trapped on silica gel which is submerged in a dry ice-acetone bath. The need to use a cold bath, however, renders this method unsuitable for field measurements. Besides, the use of an azotometer for the collection of N20 from water samples appears to be problematic because of the relatively good solubility of N20 in most of the liquids utilized in the operation of this instrument. The operation of a Toepler pump in connection with a gas chromatograph as described by Bock and Schuetz (3) is not without problems, especially when it is done on board ship. In 1970, continuing the NzO measurements, we have, therefore, modified the method of Bock and Schuetz (3) and Junge et ai. ( 2 ) to eliminate the Toepler pump. The modified method is described below. EXPERIMENTAL
Sampling System. By means of a membrane pump, 15 to 20 liters of air are pulled through a series of three U-shaped adsorption tubes with a sampling rate of 30-40 liters per hour, The first tube is filled with sodium asbestos to remove most of the atmospheric C02, the second with Pz06to remove water vapor, and the third with 5A molecular sieves to adsorb quantitatively the NzO. After sampling, the N,O adsorption tube is carefully closed and stored for gas chromatographic analysis. Before use, the N 2 0 adsorption tubes are heated to 350°C under vacuum for half an hour, and for another half hour with 50 ml/min helium passing through. The molecular sieves were obtained from Dr. Virus KG, Bonn, W. Germany, and have a particle size of 0.6-0.8 mm. Water samples are transferred into 5-1. glass stoppered bottles of known volume. The bottles are filled completely, so that the dissolved NzO cannot escape prior to processing. Processing of Water Samples. The processing of water samples is carried out as soon as possible after sampling. The processing system is shown in Figure 1. A 6-liter glass bulb is fitted with a gas inlet tube and a condenser. The condenser is connected to an adsorption train similar to that used for air sampling. This adsorption train consists of four U-shaped tubes. The first tube is filled with pellets of sodium hydroxide 1890
to remove water vapor and prevent obstruction in the second tube which is filled with sodium asbestos. The third tube contains P205and the fourth 5A molecular sieves. The processing system is evacuated up to the N20 adsorption tube. Then clean nitrogen carrier gas is admitted through the gas inlet tube into the system to raise the pressure to one atmosphere. After evacuating the processing system once more, a water sample is drawn from a 5-liter storage bottle into the 6-liter glass bulb taking extreme care that no outside air enters with it. Then the pressure in the system is raised again to one atmosphere by a stream of clean nitrogen passing through the gas inlet tube with a flow rate of 150 ml/min. To exclude any N 2 0from the bulb, the nitrogen before entering the gas inlet tube passes through a tube filled with 5A molecular sieves. After pressure equalization, the bulb is heated with a heating mantle so that the water boils slowly. The nitrogen carrier gas effects a gentle stirring of the water sample and carries the N 2 0 and other gases released from the sample through the adsorption tract to the molecular sieves tube. After 1 hour of operation, all N 2 0 from the sample is transferred to the molecular sieves tube, which is then carefully closed and stored for gas chromatographic analysis. Gas Chromatographic Analysis. For the gas chromatographic analysis, a molecular sieves tube loaded with N 2 0 is directly connected to the gas sampling valve of a HewlettPackard 5750 gas chromatograph equipped with a thermal conductivity detector. After evacuating the gas sampling valve and the molecular sieves tube for 1 minute, the gas adsorbed on the molecular sieves is transferred to the gas chromatographic column by heating the molecular sieves tube to 350°C and passing through the helium carrier gas for 10 minutes with a flow rate of 50 ml/min. At 65"C, adsorption of the N 2 0takes place again on the 1/4-in. X 3-ft stainless steel column filled with 35/70 mesh 5A molecular sieves. The gas chromatographic analysis is started by raising the temperature of the column oven at a rate of 20°C per minute to 250°C. The amount of N 2 0 is obtained from the area of the N 2 0 peak and a calibration factor. The gas chromatograph is calibrated with pure N20 and a gas transfer valve obtained from Strohlein & Co., Dusseldorf, W. Germany. RESULTS
The method described above was used to continue the N 2 0 measurements in air and in sea water over the Atlantic Ocean begun by Junge and coworkers ( 2 ) in 1969. N 2 0 measurements were made every day in June 1970, during a cruise with the German research vessel Meteor in an area southeast of Iceland, and in June 1971, during another cruise with RV Meteor in the Northeast Atlantic Ocean. During these cruises the measurements in surface air over the Atlantic Ocean gave the following N 2 0 mixing ratios in air: 1970 Expedition area southeast of Iceland, average from 60 values, 0.252 ppmv; range of average values of 30 measurements (2 parallel samples each), 0.243-0.262 ppmv. 1971 Expedition Northeast Atlantic Ocean, average from 34 values, 0.277 ppmv; range of average values of 17 measurements (2 parallel samples each), 0.272-0.288 ppmv. For a sample size of 15-20 liters, the standard deviation was =t1.4z. The results of N 2 0 measurements in sea water show a marked N 2 0 supersaturation of the sea water with respect to air. To illustrate the vertical NzO distribution, as it was found by the measurements in Northeast Atlantic sea water (1971), an average is given from three vertical N 2 0 profiles with stations at 47.2 ON 26.0 O W , 42.8 "N 34.4 OW, and 43.8 ON 42.9 "W in Figure 2. For 5-liter water samples with an average N 2 0 content of about 3 ,ug/l., the standard deviation
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
was
-f
1.7 %. For more details refer to the expedition reports
O(
DISCUSSION
Sampling Procedure. In N2O field measurements, a membrane pump to pull air through an adsorption tract combined with a gas meter to measure the sample volume was the most suitable system for air sampling. As reported by Schuetz (7), 50 liters air per hour should be the maximum flow rate for sampling with this system. Because new shaped NzO adsorption tubes were used, the N2O collection efficiency was checked once more with two of these tubes placed in series, and it was found that 60 liters per hour should not be exceeded. To make sure that all N20 from the sample is trapped in the adsorption tube, the sampling system is operated with not more than 30-40 liters per hour. The N 2 0 loaded adsorption tubes can be stored for several weeks prior to gas chromatographic analysis with the ground glass joints locked by springs. During the 1969 Meteor expedition, the water samples were stored in 5-liter glass stoppered bottles prior to processing. The sample size was determined by weighing the bottles. Weighing on board ship, however, is not always without problems and can increase the error of the measurements. Before the second expedition, therefore, the volume of all storage bottles was determined. In the processing system used during the 1969 expedition, the water samples in the 6-liter glass bulb were stirred by means of a motor driven stirrer. Aboard ship, the stirrer was another possible source of error. After making the gas inlet tube more efficient, stirring the water samples by the admitted nitrogen was sufficient. Accordingly, the stirrer was removed. NzO Transfer to the Gas Chromatograph. The critical point in the method by Bock and Schuetz (3) is the Toepler pump. By using temperature programmed gas chromatography and suitable N 2 0 adsorption tubes, this source of error could be eliminated. Schuetz reported thermal decomposition of N 2 0 on the molecular sieves above 300 "C. To check this, calibration runs were made with 15 p1 of N20 injected both immediately into the gas chromatograph through a Strohlein gas transfer valve and after trapping in an N20 adsorption tube which was subsequently heated to 350 "C for 10 minutes. All peak areas obtained from the 15-pl N20 injections were within the accuracy range of a normal calibration. Besides, further calibration runs were made using a 0.306 ppmv N,O/Nz mixture. This calibration mixture was prepared with a gas dosing apparatus obtained from Telab, Homberg NDRRH, W. Germany. To simulate sampling conditions, different volumes of the calibration mixture were passed through molecular sieves tubes at a flow rate similar to that used in the sampling procedures. The N 2 0 loaded molecular sieves tubes were handled for gas chromatographic analysis as described above. For a sample size down to 8 liters, the NzO mixing ratios (0.300-0.310 ppmv) were within the accuracy range of the method. That means that if there is any thermal decomposition of NsO on the molecular sieves used, it must be so small that it cannot be detected by this method. Accuracy and Reliability of the Method. Because no supplier was able to provide a suitable standard gas mixture of the requested analysis accuracy, the accuracy of the method was examined with a series of air samples taken and analyzed ( 5 ) J. Hahn, Meteor Forschrrtzgsergeb, Reihe B, manuscript in prep-
aration (1972). (6) Ibid. (1972). (7) K. Schuetz, Ph.D. Thesis, University of Mainz, Germany, 1966.
-
Mixing Ratio of N20 in Air (ppmv) 0.4 0.5 ( 01 0.2
(596).
I
500
-E
1000
Air-
5 0 n
1500
2000
2500
Figure 2. Average vertical N 2 0 profile in sea water from measurements in the NE Atlantic (June 1971). N 2 0 concentrations in sea water are given in terms of equilibrium mixing ratios in air to show saturation conditions. Average NzO mixing ratio in air is 0.28 PPmv
within one day. Three samples of different size were analyzed according to the previous procedures. Three further samples (3 ml each) were analyzed directly by means of a gas chromatograph with helium ionization detector from Carlo Erba, Milan, Italy, without any preceding trapping of samples. The gas chromatographic analysis was carried out at room temperature using a 1/4-in. x 14-ft stainless steel column filled with 80/100 mesh Porapak Q and a helium flow rate of 45 mlirnin. Most of the atmospheric C 0 2 was removed from the samples by sodium asbestos prior to analysis. The gas chromatograph was calibrated with N20/N2mixtures prepared with the gas dosing apparatus mentioned above, The following results were obtained by the analytical method described above, 0.295,0.296, and 0.306 ppmv N20. The direct gas chromatographic analysis (helium detector) gave 0.290, 0.300, and 0.301 ppmv N20. Comparison of the values obtained from the method described above and from analysis by direct gas chromatography at room temperature shows that the method in question gives reliable data in the accuracy range given previously. The analysis of atmospheric NzO by direct gas chromatography with a helium ionization detector is, as a laboratory method, superior to the procedure described above, because sources of error due to trapping on the molecular sieves and transfer of the trapped gas to the gas chromatographic column are omitted. For field work, however, a gas chromatograph with a helium detector is too sensitive. The measurements of the atmospheric N 2 0 during the 1969 Meteor expedition ( 2 ) and the results of earlier measurements in Germany, Tenerife, and South Africa ( I ) indicate a rather uniform distribution of the atmospheric N 2 0 . Thus one should be able to check the accuracy of the analytical method described above aboard ship by calculating a quasi standard deviation from results of parallel measurements during one
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1891
expedition. For this purpose, the average N20 value was determined from each pair of parallel measurements. The deviations of the corresponding single values from this average were used to calculate a quasi standard deviation, whose percentage from the average of all measurements of one expedition was *1.7% for the 1970 expedition and f1.45 for the 1971 expedition. As mentioned previously, for 15- to 20-liter air samples the standard deviation from laboratory experiments was ?C 1.4%. Accordingly, the ac ual accuracy aboard ship is comparable with that from laboratory experiments. The slightly increased value from the 1970 expedition may be due to unfavorable weather conditions. At some stations in the Atlantic Ocean, parallel samples were taken from the sea surface water. The N20 values from these parallel samples suggest an actual accuracy for N 2 0 measurements in sea water aboard ship better than + 3 %. In a recent paper, LaHue et al. (8) described an analytical method for measurements of the atmospheric NzO. This
method, a combination of the methods of Bock and Schuetz (3) and of Leithe and Hofer (4,also works without a Toepler pump. LaHue et ai. remove the N20 from the molecular sieves trap by water treatment. The released N2O is adsorbed once more on cooled silica gel and then transferred to the gas chromatographic column by heating the silica gel to 140 “C. A technique which works at relatively low temperatures and without a Toepler pump is certainly of advantage. However, in the method of LaHue et al., this advantage is almost lost by the comparatively complicated procedure. The standard deviation which results from the data given in the paper by LaHue et al. is *1.75%. In contrast to the method by LaHue et al., the procedure described above is uncomplicated and easy to handle, especially under field conditions, and its accuracy is at least comparable.
(8) M. D. LaHue, H. D. Axelrod, and J. P. Lodge, Jr., ANAL. CHEM., 43, 1113 (1971).
RECEIVED for review January 25, 1972. Accepted May 8, 1972.
Principles of Hot Plate Chromatography Sredko Turina Institute for Material Investigations, University of Zagreb, D. Saleja I , Zagreb, Yugoslavia
Vjera Jamnicki Pharmaceutical and Chemical Works “PLIVA,” Zagreb, Yugoslavia
WARMING A CHROMATOGRAPHIC PLATE during the TLC process causes the solvents to evaporate from the plate and the chromatograms obtained show the following advantages (1): (1) better resolution of the spots and (2) possibility of detection of trace components which are undetectable under usual TLC conditions (2). These effects, obtained under special conditions, can be explained by the discontinuous counter-current model of the chromatographic process given by S. W . Mayer and E. R. Tompkins (3). Heating of the plate during the chromatographic process results in evaporation of the solvents from the plate, as is shown in Figure 1. If the temperature of the plate is uniform, the evaporation of solvents over the whole area is uniform, too, and the following equation applies. mdx)
=
- Im,(O)/O X
(1)
The value of I is a function of temperature T,solvent volatility and viscosity, thickness of the layer, and particle size of the adsorbents. Under ideal conditions, without evaporation of solvents, the velocity ut of each component of a mixture during the chromatographic process is constant. The position X’ that component i would reach on the chromatographic plate in the given time is :
X’(r)
= us
*
RF
7
(2)
where us = a constant R F =
a value deduced from the chromatogram obtained at normal conditions
If evaporation of solvents from the plate occurs, the velocity of the components decreases with distance Xfrom the start:
where m,(X> = the flow of the mobile phase per unit width of the plate at the distance Xfrom the start m,(O) = the flow of the mobile phase per unit width of the plate at the start 1 = the distance between the start and the line where the static front is established
(3) where ut = the velocity of the component i at the distance X from the start. The time dependence of the position X of a component i can be obtained from the differential Equation 3 by integration :
( 1 ) S . Turina, 2. ioljii, and V. MarjanoviE, J . Chromafogr.,39,
T(X>
81 (1969). (2) E. Stahl, “Thin-Layer Chromatography,” Springer-Verlag, Berlin, 1962, p 100. (3) S . W. Mayer and E. R . Tompkins, J. Amer. Cliem. Soc., 69, 2866 (1947). 1892
= (l/us
. RF)l
r(X) = ( - 1 h
*
L 1
R F ) In
- (X/I)I-’dX
(4)
- (X/Ol
(5)
By transforming the equation, it follows:
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
