I AIDS FOR ANALYTICAL CHEMI§TS Apparatus for Preparing Nitrogen from Ammonium Chloride for Nitrogen- 15 Determinations Lynn K. Porter* and William A. O'Deen Agricultural Research Service, USDA, P.O. Box E, Fort Collins, Colo. 80522
Traditionally, isotopic analyses for nitrogen have been determined on nitrogen gas (Nz).Two methods have been used to convert all kinds of compounds and materials to Nz. These methods are the Dumas direct combustion technique and the Kjeldahl-Rittenberg technique. A review of Fiedler and Proksch ( I ) describes both techniques. In the KjeldahlRittenberg procedure ( 2 ) ,the sample is digested in acid and the ammonium formed is subsequently recovered and oxidized with alkaline hypobromite in an evacuated Rittenberg Y tube to Nz.Finally, the NZ gas is analyzed by either mass spectrometry or emission spectrometry to determine its isotopic composition. Several papers ( I , 3-5) have outlined these techniques and described difficulties likely to be encountered. The hypobromite oxidation in the Rittenberg procedure is quite time consuming. It requires transferring of liquid ammonium samples and hypobromite to the Rittenberg Y tube, lubricating the vacuum stopcock, evacuating and degassing the solutions, reacting the solutions, and, after emitting the Nz sample into the mass spectrometer, degreasing and cleaning the Rittenberg tubes. In 1970, Ross and Martin ( 5 ) developed an apparatus that completely eliminated the classical Rittenberg tube, thus eliminating most of these time-consuming steps. The conversion system described in this paper operates on the same principle as the Ross and Martin ( 5 )apparatus, Le., the dropping of lithium hypobromite (LiOBr) onto dry ammonium salt. The conversion system described differs from the Ross and Martin system in design, materials, and construction, and offers the experimenter or analyst an alternative system that we believe is easier t o construct and repair, and uses inexpensive, throw-away shell vials. However, this paper is not intended to compare or discuss the relative merits of the two systems, since we have not built or used a Ross and Martin conversion apparatus.
CONVERSION SYSTEM Figure 1 shows the conversion system. The apparatus utilizes a stainless steel, Cajon Ultra-Torr reducing union (SS10-4UT-6). This union has a 0.64-cm (Y4-inch) vacuum, O-ring fitting on one end and a 1.59-cm (5/g-inch)vacuum, O-ring fitting on the other end. The union is made into a T by tapping it with a 0.64-cm drill, and silver soldering it to a 0.64-cm Ultra-Torr adaptor. (Initially, we attempted to have the Cajon Company machine this T for us; however, their fitting was about twice the length and contained about twice the internal volume.) The 0.64-cm Ultra-Torr adaptor fitting, or side-arm, provides a vacuum connection to any 0.64-cm metal or glass system. It also provides a means for firmly strapping or clamping the apparatus in place. The reducing T requires a firm mounting, since the O-ring seals are obtained by fingertightening metal nuts, which applies some torque to the apparatus. The Cajon Ultra-Torr fittings are designed so that when the metal nut is finger-tightened, pressure is applied to rubber O-rings, which are firmly squeezed to any glass or metal connectors, forming vacuum-tight seals. These fittings 514
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
provide vacuum connections to the hypobromite reservoir and the sample vial. The 100-mL hypobromite reservoir is constructed utilizing high vacuum stopcocks with Teflon plugs and viton O-ring seals. A 0.64-cm, o.d., 2-mm, i.d. capillary tube is glass blown to the bottom stopcock, and a small bubble (5.5 to 6 cm from the tip) is made in the tubing, so that the tube only extends a fixed distance through the Ultra-Torr fitting. The stopcock a t the top of the reservoir is a T arrangement with standard 10/30 joints, that can be readily attached to an oil-diffusionmechanical pump vacuum system and also connected to a mercury manometer and helium (He)-gas cylinder. Lithium hypobromite (LiOBr), prepared according to the method of Ross and Martin ( 5 ) ,is drawn into the evacuated reservoir by dipping the capillary tubing into a beaker of the LiOBr, and slowly opening the bottom stopcock. When the reservoir is slightly over half full (-60 ml), the bottom stopcock is closed and the LiOBr degassed. There may be some bumping and splashing of LiOBr, so a trap is used between the reservoir and the vacuum system. After degassing, the vacuum system is isolated and the reservoir brought to atmospheric pressure with He. The evacuation and degassing of the LiOBr, and emitting of He into the reservoir, is repeated several times to ensure that soluble NP in the hypobromite has been removed. Finally, the LiOBr reservoir is brought to atmospheric pressure or slightly above with He and sealed. The LiOBr reservoir is then returned to the Ultra-Torr reducing T and sealed. Ammonium samples are taken to dryness in 6-mL throwaway shell vials, 16 mm by 50 mm long. The vials are placed on a steam plate in 1.9 cm X 10.2 cm X 10.2 cm aluminum blocks, drilled with 1.59-cm diameter holes (56 per block). The A1 block conducts heat which causes a faster evaporation rate. Hydrochloric acid (0.5 mL of 1 N) is used as the trapping agent The excess HC1 is evaporated, leaving only for distilled "3. ammonium chloride salts. If sulfuric acid is used, excess acid remains and reacts with the hypobromite, generating some undesirable bromine. After the sample is dried, it is covered with parafilm and stored for later analysis. Rather than evacuate the sampling vial and conversion apparatus through the high-vacuum inlet pumping system on the mass spectrometer, an auxiliary high-vacuum, oil diffusion, mechanical pump (and the thermocouple gauge vacuum system) is used, which is positioned directly below the dual inlet system of an AEI isotope-ratio mass spectrometer. By using the auxiliary vacuum system, the conversion system, sample vial, and line leading to the mass spectrometer can be evacuated while an isotope-ratio analysis is being performed on an Nz sample already in the mass spectrometer inlet system. After the conversion system is evacuated, a high-vacuum glass T stopcock is used to isolate it from the auxiliary vacuum system. Originally, we built a dual conversion system with a four-way valve; however, one conversion system is all that is needed to use the mass spectrometer to its full capacity. The sample vial is then immersed in dry ice-isopropanol mixture. Any water vapor, ammonia, or NO, compounds generated during the hypobromite reaction are thus frozen
I0/30
Table I. Atom % 15Nof a 5% Sample Followed by Natural Abundance Sample Immersed before and after Admitting LiOBr
F JOINT
I 0 / 3 0 S JOINT
Atom % I5N Treatment.
p I\
41
LiOBr ( 6 0 m l l
loom'\
j,
HIGH VACUUM STOPCOCKS WITH TEFLON P L U G S 8 O - R I N G SEALS
TO M A S S S Z C T R O M E T E R rPI
5%
Natural
Immersed before LiOBr addition 5.0308 0.3715 Meana Standard deviation 10.0134 f0.00065 Immersed after LiOBr addition MeanQ 5.0222 0.3728 Standard deviation 10.0056 f0.00096 Sequence of 5%sample followed by natural abundance sample repeated 10 times, so mean represents 10 separate samples ( n = 10).
2 m m TUBING MODIFIED S T A I N L E S S S T E E L REDUCING UNION W I T H U L T R A TORR,, F I T T I N G S 1/4" to 5 1 8
SHELL VIAL 6 m l 16rnm 0. D. 50rnrn LONG
A-
T-VALVE
II ii ii
1r.w
1 1 I1uI1 Il I
Table 11. Conversion of Various Amounts of NHd+-N to Nz and Resulting Pressures Observed on Mercury Manometer of AEI MS-20 Viscous Inlet System
VALVE
Amount of NH4+-N converted to Nz, mg
I1I
Pressures developed in inlet system, mm Hg
UUM
LNz VAPOR MASS
DRY ICE FOR FREEZING H e 0 EL NOX BEFORE ADMITTING TO S P E C T R O M E T E R INLET.
OR
0.1 0.2 0.4
0.6 0.8 1.0
Figure 1. Illustration of the ammonium to dinitrogen conversion apparatus showing the vital components of the alkaline hypobromite reservoir and the constructed Ultra Torr T that provides vacuum O-ring seals for the system
out. Ross and Martin ( 5 ) stated that the vial must be immersed in the dry ice bath before admitting the hypobromite in order to avoid ammonia contamination of the conversion system, which could lead to cross contamination of the following sample. In order to test the effect of immersing before or after admitting the hypobromite on measured 15N abundance, we reacted a 5% 15N sample and followed it with a natural abundance sample. This sequence was repeated 10 times; first, for samples frozen before admitting LiOBr, and then 10 times for samples where LiOBr was admitted to the evacuate vial before freezing. The results are shown in Table I. For the samples where LiOBr was admitted before freezing, the 15N content of the 5% sample is slightly lower and the natural abundance sample is slightly higher in I5N content than the I5N content of samples frozen before admitting LiOBr. The data indicate there is some cross contamination if LiOBr is admitted to a sample vial before freezing, and this agrees with conclusions of Ross and Martin ( 5 ) . Ross and Martin (5)also indicated that nitrous oxide (Nz0) can be produced during the hypobromite oxidation of ammonium salts. Nitrous oxide when bombarded with electrons in the ion source of a mass spectrometer can fragment into nitric oxide (NO) which has an atomic mass of 30 and interferes in the measurement of nitrogen isotopes. However, the measurement of the mass 30 ions is only important when analyzing highly enriched 15N samples. When low-abundance samples are being analyzed by an isotope-ratio instrument (simultaneous measurement of mass 28 and 29), the NO interference is not important. Ross and Martin ( 5 )indicated if N20 removal is necessary, a highly efficient liquid Nz trap should be placed in the line leading to the mass spectrometer inlet system.
2.0 3.0
4.5
7.2 12.0 20.0 26.5 35.0 69.0 103.5
After a sample has been reacted and the N2 let into the mass spectrometer, and before the spent vial is removed, vacuum is again momentarily applied to the system to remove any LiOBr which may remain in the delivery capillary. The reacted sample vial is removed and the tip of the delivery capillary is wiped to remove any LiOBr or reactants from the previous reaction. These steps help to ensure that the next sample does not react prematurely from LiOBr remaining from the last reaction. Inlet and vacuum system differ widely for various mass spectrometers, so an analysis time per sample would probably have little meaning except for an AEI mass spectrometer similar to the AEI MS-20 isotope-ratio mass spectrometer used in these studies. The AEI MS-20 mass spectrometer used is equipped with a dual viscous-flow system incorporating matched capillary leaks. A sample is introduced into one side of the inlet system and a standard Nn gas (of known isotopic ratio) is introduced into the other. Alternate analyses of the sample and standard are performed by cycling between the two systems with an autovalve. Direct comparisons between the standard and the sample eliminate vacuum and electronic errors. Using this system, 60-70 unknown samples are analyzed in a normal 8-h workday, or an average of about one unknown every 8 min. This is about three times the number of unknown samples that could be analyzed in a workday using Rittenberg tubes. Introduction of Nz samples into the viscous inlet system is monitored by a mercury manometer, and pressure in the inlet system is adjusted with a variable reservoir with a normal volume of 7 mL, and the total volume of the reservoir-manometer system is probably about 10 mL. On mass spectrometers, using these viscous inlet systems of limited expansion volume, it is essential to minimize the volume in the ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
515
line leading from the conversion system to the inlet system of the spectrometer, in order to ensure adequate sample pressure in the inlet system. Table I1 shows pressures observed in our viscous inlet conversion system when various amounts of NH4+-Nwere converted to Nz. Manufacturers of the viscous inlet systems recommend crimping the dual capillaries of the inlet system to provide matched flows at 100 mm Hg pressure in the inlet system, and thus restricting the NZflow to the mass spectrometer so it can still be operated a t maximum sensitivity. Table I1 shows that in order to generate 100 mm Hg pressure of N2,the sample vial must contain 3 mg of NH4+-N. There is the alternative of crimping the capillaries when the inlet system is set a t lower pressures. Capillaries crimped at lower pressures provide greater Nz flow rates to the detector and still permit the mass spectrometer to be operated at maximum sensitivity. On our inlet system, we elected to crimp the capillaries at 35 mm Hg, and thus, we routinely try to
prepare our N samples so that the sample vials contain at least 1 mg NH4+-N. LITERATURE CITED ( 1 ) R. Fiedler and G. Proksch, Anal. Chim. Acta, 78, 1-62 (1975). (2) D Rittenberg in "Preparation and Measurement of Isotopic Tracers", A. 0. C. Nier and S.P. Reimann, Ed., J. W. Edwards, Ann Arbor, Mich. 1948, pp
31-42.
(3) J. M. Bremner in "Methods of Soil Analysis", C. A . Black, Ed., American Society of Agronomy,Madison, Wis., 1965, pp 1256-1286. (4) J. M. Bremner, H. H. Cheng, and A. P. Edwards in "The Use of Isotopes in Soil Organic Matter Studies", Report of FAO/IAEA Technical Meeting in cooperation with the International Soil Science Society (Brunswick-Volkenrode),Pergamon Press, New York, N.Y., 1963, pp 429-442. (5) P. J. Ross and A. E. Martin, Analysf, (London), 95, 817 (1970).
RECEIVEDfor review March 18, 1976. Accepted December 13,1976. Reference to a company or product name does not imply approval or recommendation by the USDA.
Breakdown of Alkaline Complex Cyanide by Ion-Exchange lrith Gilath Soreq Nuclear Research Centre, Yavne, Israel
The term complex cyanide refers here to complexes formed from cyanides of alkali metals combined with cyanides of heavy or transition metals. In these compounds, the cyanide is part of the complex anion (i.e., Me11(CN)42-)and cannot be determined directly. The classical method for the determination of cyanide from complexes is based on its release from an acidified solution as cyanhydric acid and distillation into an alkaline solution ( 1 ) . The Serfass reflux and tartaric acid distillation procedures are most widely used. The apparatus and the methods are described in detail in the Standard Methods (1).The reflux procedure is somewhat more complicated than the tartaric acid distillation but is preferable since it results in a higher recovery of the cyanide. In the distillation of the stable cornpiexed salts, the release of hydrogen cyanide and absorption in alkaline solution is not always complete and depends on the heating rate of the distillation, carrier velocity, and alkaline absorption facilities. A few hours of distillation may be required to break down some stable complex cyanides by both methods (2). A new, quick and reliable method was developed by us for complex cyanide breakdown for application in the plating
Table I. Composition of Complex Cyanide Solutions
Free CN-, Complex K2Zn(CNh KzCd(CN)4 K~CU(CN)~ KAg(CN)e
gll.
0.59 1.5
1.5 7.39
Complexed CN-, gll.
Total CN-,
44.25 41.50 20 11.64
44.84 43.00 21.50 19.03
gll.
industry. The cyanide anion is absorbed on a strong anionexchange column and released by elution with sulfuric acid. The strong base characteristics of the anion exchanger permit the absorption and exchange of anions of weak acids. In the hydroxyl cycle, the strong anion exchanger shows remarkable salt-splitting properties. As compared with the time-consuming acid distillation of the cyanides ( I ) , this procedure is rapid and easy to perform. EXPERIMENTAL Preparation of Ion-Exchange Column. About 5 g of dry Amberlite IRA-400 (Fluka) 20-50 mesh resin in hydroxyl form was swollen with distilled water and transferred to a burette. The burette was filled to a height which corresponded to seven times the diameter of the column. Preparation of Simple and Complex Cyanides. Simple cyanides were prepared by dissolving KCN or NaCN in distilled water and determining the concentration by argentometric titration. The cyanide complexes were prepared in solution by accurately weighing simple insoluble cyanides and dissolving them in known concentrations of NaCN or KCN solutions. The compositions of the concentrated complex cyanide solutions tested by us are summarized in Table I. Absorption of Cyanides. A 2-ml sample (containing up to 100 mg CN-/sample) of the cyanide solution and a few milliliters of distilled water were filtered through the resin bed a t the rate of 1drop/s. The resin was then washed with 15 ml of distilled water. Elution. The cyanide was released by two consecutive acid elutions: 15 m12 N HzS04 and 15 m14.5 N HzS04. The first acid elution was performed quickly to avoid the formation and escape of HCN bubbles. The second acid wash was performed slowly, to complete the removal of the cyanide from the resin. After the two acid elutions, the resin was washed with 15 ml of distilled water. The acid effluents and the water were delivered to the bottom of a magnetically stirred beaker, containing 80 ml of 2 N NaOH.
Table 11. Material Balance of Cyanide Filtered through Ion Exchangers
Complex
KzCd(CN14 KzZn(CN)4 K~CU(CN)~ KAg(CN)z 516
Total cyanide in sample,
Cyanide eluted in first wash,
Cyanide eluted in second acid
Total cyanide recovered,
mg
mg
wash, mg
mg
29.38 34.12 52.60 95.16
13.44 10.92 11.91 8.74
42.82 45.04 64.51 103.90
43 45 64.5 105
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
