The gas chromatograph had been previously modified (Figure 1) to allow flow of helium to the Watson-Biemann separator to be regulated independently of flow through the column (2). This was done because the optimum gas flow through a capillary column is usually less than the flow needed to make the separator work properly. The makeup helium is added to the column flow after the column and is controlled by the flow valve which would normally be used for the reference column, if the chromatograph were operated in the dual column mode. The modification described in this paper consists of the additions to the makeup line enclosed by the dotted lines in Figure 1 . A Nupro Cross Pattern Very Fine metering valve (Nupro SS-lSX) was installed in the makeup line after the makeup flow control valve. The Nupro valve was installed so that the makeup helium flow was not affected by the valve setting. About 25 inches of 1/16-in.o.d.-O.OlO-in. i.d. stainless steel tubing was coiled and attached to the variable orifice port of the Nupro valve by a Swagelok nut (Swagelok No. 101-1-316). The other end of the tubing was attached to a Swagelok 1/16-1/8-in.reducing union (Swagelok No. 200-6-1 316) and closed by a septum held on the reducing union by a Swagelok nut (Swagelok No. 102-316). The reducing union was attached to a panel of the gas chromatograph to hold the coiled tubing in place. A material to be used as the mass calibrant (5 to 50 microliters) is injected into the coiled tubing. It is introduced into the mass spectrometer by opening the Nupro valve, with
helium flowing through the makeup line, until calibrant peaks are observed. The coiled tubing is not heated so the calibrant must be volatile. However, the makeup line is heated and it would be easy to also heat the calibrant tubing. The calibration compound can be changed by opening the Nupro valve, closing the makeup control valve, and pumping out the calibrant. Because Swagelok fittings are used through the assembly, however, it is easier to simply change the tubing containing the calibration material. This technique is used primarily for samples introduced with the probe (Figure 2) but it can also be used when the gas chromatograph is used as the sample introduction system. The calibrant is not introduced onto the column and, because of the location of the point where the makeup helium line joins the column helium line, it does not interfere with the flame ionization detector. It does interfere with the total ionization detector in the mass spectrometer but because the partial pressure of the calibration material is nearly constant, the constant background current contributed by the calibrant can be removed by the bucking voltage control. It is also possible to introduce bursts of calibration material during blank spots in the G C run by opening the Nupro valve only momentarily. The calibrant introduction system described was installed within the gas chromatograph instrument for reasons of convenience only. It can be installed in any mass spectrometer equipped with a G C inlet as long as it is installed after the GC column.
(2) Perkin-Elmer Co., Norwalk, Conn., personal communication, 1970.
RECEIVED for review July 19, 1971. Accepted October 26, 1971.
-
Electrolytic CaIibrat ion Technique for Dissolved Determination in Seawater by On-Stream Stripping Gas Chromatography
N itrogen
L. P. Atkinson Department of Oceanography, Dalhousie Uniuersity, Halifax, Nova Scotia, Canada
PRECISE AND
ACCURATE determination of the dissolved nitrogen content of seawater Nz has proved to be difficult, and various analytical techniques have been used. Riley ( I ) reviewed the various methods used (see also 2). Gas chromatography holds great promise for high precision, accuracy, and fast analysis time. In this aid, an improvement of the on-stream stripping technique of Williams and Miller (3), using the stripping chamber of Swinnerton, Linnenbom, and Cheek (4, is presented. An electrolytic calibration for nitrogen is also outlined. This method is quite precise, accurate, and relatively fast. It also lends itself to automation.
(1) J. P. Riley, “Chemical Oceanography: Vol. 2,” J. P. Riley and G. Skirrow, Ed., Academic Press, New York, 1965, p 312. (2) H. Craig, R. F. Weiss, and W. B. Clarke, J. Geophys. Res., 72,
6165 (1967). (3) D. D. Williams and R. R. Miller, ANAL. CHEM., 34,657 (1962). (4) J. W. Swinnerton, V. J. Linnenbom, and C. H. Cheek, ibid., p 483.
EXPERIMENTAL
Apparatus. The gas flow and essential eiements of the analytical system are shown schematically in Figure 1. Gas cylinder A supplies helium through a low diffusion pressure regulator to helium purifier A ’ (Electron Tech. Inc. Model SCM-1). The manifold B (actually part of the gas chromatograph) supplies helium to both the gas chromatograph (“carrier gas”) and the stripping chamber (“stripping gas”). Flowmeters C control the flow to each system. Carrier gas flow rate is 80 ml/min and stripping gas flow rate is set at 20 ml/min. The gas chromatograph L (Bendix Model 2100) is equipped with a helium ionization detector (Ionics Inc., Model ZOO). A 6-ft X 1/4-in. 0.d. stainless steel column packed with Molecular Sieve SA is in line with a 12 X l/An. activated charcoal column. The activated charcoal chemisorbs the oxygen (5), allowing the determination of nitrogen and argon. The column temperature is 100 “C. A Carle Instrument Co. Model 2014 8-way sample valve I with 5-mI ( 5 ) R. C. Cooke, Limnol. Oceanogr., in press.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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solution held in the stripper. The current is measured by observing the voltage drop across a precision 1000-ohm resistor Q with a Hewlett-Packard Model 3430A digital voltmeter R. Calibration, The electrolytic decomposition of hydrazine sulfate produces NZand Hz. Page and Lingane (6)demonstrated that under their experimental conditions theoretically predicted amounts of NP and Hz are produced (3.486 ml of NzSTP/mA-min). Since the apparatus described in this paper is essentially identical to that of Page and Lingane, it is assumed that the theoretical yield is achieved. The stripping chamber is cleaned thoroughly and filled with 0.1M hydrazine sulfate and the pumps are sealed. A standard curve is prepared by plotting milliamperes VS. integrated nitrogen peak area (NPA) (Figure 2). The curve, or a statistical approximation of it, f(NPA), can be used to calculate the milliamperes of current equivalent to the nitrogen content of the sample. NZis calculated using the following : a
Nz =
a
Figure 1. Apparatus schematic
matched loops is mounted inside the column oven. For readability a 6-way valve I is shown with one loop .Tin Figure 1. K is the carrier gas line to and from the sample valve. A digital integrator M' (Hewlett-Packard Model 3370-A) determined the peak areas. Recorder N indicates the stage of the analysis and base-line quality. A water sample D' is pumped into the glass stripping chamber G (40 cm X 12 mm 0.d.) with a peristaltic pump D (Masterflex Model 7013). An identical pump E removes water from the stripping chamber. Silicone tubing is used in the pumps and is occasionally flushed with HODAG ML-44 (HODAG Chemical Corp., Skokie, Ill.) to reduce bubbling. The water flow rate WFR is determined by either of two methods. Water from pump E is pumped into a tared beaker for a measured time or the time to fill a volumetric flask is determined. This provides WFR in g/min or ml/min. Stripping gas passes up through the coarse frit F, removes the dissolved gas, and exits near the top. It passes through dryer H which is filled with activated Molecular Sieve 5A ( l / d n . pellets). The sample valve I removes a precise amount of the stripper gas extracted gas for analysis. For calibration a current supplied by a Hewlett-Packard 6218A DC power supply P is passed (cia Pt electrodes 0 mounted on the chamber) through a 0.1M hydrazine sulfate
+
-
10
3.486 X f ( N P A ) WFR
(1)
Other gases can easily be determined by experimentally determining the relative sensitivity of the detector to Nz and other gases. For example the concentration of argon would be: Ar
SA SN
APA NPA
= -X -
x
Nz
where SN and SA are the detector sensitivities (volume/peak area) for nitrogen and argon, NPA and APA are the nitrogen and argon peak areas, and N2is the nitrogen concentration. RESULTS AND DISCUSSION
Pump Stability. Experiments demonstrated the pumps to be stable to better than 0.01 ml/min over 19 hours. Stripping Gas Flow. The stripping gas flow must stay constant. It is sometimes necessary to flush the frit with distilled water to remove accumulated salts. This can be done by installing a Swagelok T connector at the base of the stripping chamber. One arm of the connector is used for the stripping gas, the other for a syringe filled with distilled water. Duplicate samples run before and after all analysis have seldom detected drift. (6) J. A. Page and J. J. Lingane, Anal. Chim. Acta, 16, 175 (1957).
-
0
, E'
9-
0
Figure 2. Calibration curve. The nonlinear response is due to the detector
2 k a
8-
Y
9 n 7 -
I
I
I
I
6
8
10
12
MlLLlAMPS
886
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NO. 4, APRIL 1972
Stripping Efficiency. The water flow rate was varied while holding the stripping gas flow constant at 20 ml/min. The nitrogen peak areas increased linearly with increasing WFR to about 12 ml/min indicating complete stripping. The pumps are normally run at 4 ml/min. Effect of Hydrazine Concentrations. Experiments demonstrated that reducing the hydrazine sulfate concentration by one half had no effect on the yield of NO. The yield was linear over the current densities used. Precision and Accuracy. The relative standard deviation is typically 0.5 for 4 samples. The accuracy is more difficult to assess: The resistor was calibrated to 1000.01 ohms. The digital integrator is accurate to 0.1 (ignoring false base-line tripping due to signal noise). The digital voltmeter is calibrated to 0.2%. The flow measurement is certainly
accurate to 0.1 %. Therefore, the total error in accuracy is at most 0.5 %, Seawater Itesults. About 300 samples were successfully analyzed for Nz and Ar in this laboratory and aboard ship during the pa st year. ACKNOWLEDGMENT
The author thanks P. J. Wangersky, R. C. Cooke, and T. Yoshinari for advice in developing this method. RECEIVED for review July 16, 1971. Accepted October 27, 1971. This work was supported by the Fisheries Research Board of Canada, The National Research Council of Canada, and the Department of Energy, Mines, and Resources.
Rapid Technique for Dissolution of Nitrides Dean H. Bollman US. Department of the Interior, Bureau of Mines, Albany Metallurgy Research Center, Albany, Ore. 97321 YUTAKAYAMAZAKI (I) used tin(I1)-strong phosphoric acid reagent to dissolve A1203 when determining small amounts of S in A1203. He prepared the reagent by heating 300 grams of H3P04 with 30 grams of SnC12.2Hz0 to 300 "C to expel all the water and HCl. This reagent was found to be a remarkable medium for dissolving many transition metal nitrides without loss of nitrogen. Furthermore, the decomposition of the nitrides takes place within a few minutes. The reagent will work with TiN, ZrN, HfN, UN, AlN, and niobium oxynitride. Vanadium nitrides and boron nitride would not completely decompose. Because of lack of a sample, analysis of silicon nitride was not attempted. PROCEDURE
The most satisfactory procedure evolved was to add the reagent directly as follows : Weigh approximately 0.1 gram of nitride by difference into a 50-ml Erlenmeyer flask. Add about 1.5 grams of SnC12.2H20 and 10 ml of H3P04. Heat on a hot plate until most of the water (free water) has been driven off. Then heat over a burner, swirling if necessary, until gas evolution nearly stops. This takes only about 2 to 3 minutes, usually. (Continued heating after the sample has reacted should be avoided because phosphates are formed which become fused to the glass. Milky phosphates may be formed, but they do not adhere.) Swirl the sample while cooling to help keep the phosphates suspended. After the sample has cooled, add about 15 ml of H 2 0 . Then with a thick stirring rod mix the water and acid. This may be quite difficult because the stannous-phosphoric melt cools to a thick, glue-like glass. With persistence, it can be mixed. Then transfer the sample to a distillation apparatus. Any phosphates adhering to the glass should be scraped out if possible. Also gelatinous particles should be flushed out. The 40 NaOH used for distilling is helpful for flushing out the flasks. To avoid the difficult transfer of material after dissolution, an alternate method may be used. A 300-ml, long-necked (1) Yutaka Yamazaki, Bunseki Kagaku, 19, 187-90 1970.
Table I. Nitrogen Values for Several Nitrides Nitroeen. T, Kieldahl Sn(I$HaPOa Other methods Sample reagent 20.0 20.0 11.6 11.8
TiN ZrN
.. 11.I
HfN
5.94 5.97 5.80
UN Niobium oxynitride (4.2 Z oxygen) AlN NHaCl MoiC-AlN
13.4 13.4 13.3 32.6 32.6 26.7 26.2 5.01
20.6 19.6 11.0 11.5
19.7 19.9 (a) (6) 10.8 10.7 (a) 11.2 (b) (b)
5.73 5.76 5.78 5.78 (av 28 detn) 1 3 . 3 13.6 13.6 13.6
(a)
30.7
(4
30.6
(c)
(a)
(4
26.2 theoretical,
calculated 5.22 5.15
(4 (b)
Ignition-chromatography. Kjeldahl, HF H202solvent. c Kjeldahl, H3P04 only. d Kjeldahl, HF, Teflon-lined bomb. e Kjeldahl, HF-HsP04-K2Cr207. 0
+
Kjeldahl flask may be used for dissolution and distillation in the same vessel. For this purpose, a stopper used on the Kjeldahl flask should be provided with a funnel for adding the alkali after the apparatus is assembled, and with a long glass tube that will extend to the bottom of the Kjeldahl flask for the steam inlet, Also a splash bulb should be provided to prevent traces of caustic from being carried to the acid solution in the receiver. For either procedure, the distillation and titration method used has been described in a previous Note (2). (2) D. H. Bollman and D. M. Mortimore, ANAL.CHEM., 43, 154155 (1971). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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