DISCUSSION The apparatus is similar to that described earlier for the determination of sulfur (3, 4 ) . Modifications can be seen in the figures. The system for the introduction of ultramicro samples was described by Walisch ( 6 ) . Larger samples are burned in the conventional manner with split type furance (N1) slowly moving along a motor-driven spindle over the sample, which is placed in front of furnace (N2). In the ultramicrodetermination of sulfur ( 4 ) , the combustion furnaces must be kept a t about 700 "C. Higher temperatures will cause small blanks. The hydrogenation furnace needs 1200 "C to give a quantitative hydrogenation of sulfur. In the determination of fluorine alone, good results were also obtained with the combustion furnaces a t 900 "C and the hydrogenation furnace a t temperatures down to 800 "C. Phosphoric acid ( 1 0 ) should be added to all samples in the ultramicro determination of halogen and sulfur by dry combustion. It decomposes inorganic compounds and expels sulfate and halide and desorbs them also from any alkaline spots which can occur in the tube as contamination of the quartz itself br from samples. Accurate ultramicroanalyses are still obtained when thousands of samples including sulfate and sulfite liquor from paper mills have been analyzed without cleaning of the tube or other precautions. Alkali fluorides were quantitatively decomposed by phosphoric acid, but low results were obtained with calcium and barium fluorides, probably because the phosphoric acid was volatilized already before the salts had been completely decomposed. The flux mixture was therefore introduced. The method for the simultaneous determination of fluorine and sulfur is based on the fact that hydrogen sulfide
passes right through a slightly acid water solution where fluoride is quantitatively absorbed. Tank hydrogen contains frequently small amounts of sulfur compounds. They are decomposed in furnace (N3) and the hydrogen sulfide formed is retained in the soda lime tube. When the apparatus is started, furnace (N3) is cold, and the sulfur compounds are not decomposed. They wouid be partially ad.sorbed on the soda lime and slowly desorbed in the course of the day and cause small, steadily decreasing sulfur blanks. Stopcock (D)was therefore introduced. I t is turned so that the hydrogen, which has passed through the cold furnace, passes out into free air, and first when the furnace is hot, and the sulfur compounds are decomposed, the hydrogen is allowed to pass through the soda lime tube.
LITERATURE CITED Ehrenberger and S. Gorbach, "Methoden der organiscnen Elementar und Spurenanalyse," Verlag Chemie, Weinheim, Western Germany,
(1) F.
1973, pp 319-57. (2) W. J. Kirsten, Mikrochemie, 35, 2 (1950). (3) W. J. Kirsten, Proc. lnt. Symp. Microchem., 7958, 132 (1959). (4) W. J. Kirsten, Proc. 7967-Symp. Microchem. Techno/., 479 (1962). (5) R. Belcher, "Submicro Methods of Organic Analysis." Elsevier, Amsterdam 1966, p 62. (6) W. Walisch, Chem. Ber., 94, 2314 (1961). (7) L. Gustafsson, Talanta, 4, 227 (1960). (8)W. J. Kirsten and V . J. Patel. Microchem.J.. 17. 277 11972). (9) T . D. Rees, A 8. Gyllenspetz, and A. C. Doche;ty, Analyst, (London1..96.201 119711 (10) W J Kbsten, B Danielsson, and E Ohren, Mcrochem J , 12, 177 ( 1967)
RECEIVED for review June 10, 1974. Accepted July 30, 1974. The work was made possible by grants from The International Seminar in Chemistry of the University of Uppsala and from the Swedish Medical and Natural Science Research Councils.
CORRESPONDENCE Flame Photometric Detector for Liquid Chromatography Sir: Recent advances in the chemistry of stationary phases for liquid chromatography have reduced drastically the time necessary for separations. In particular, the pellicular ion exchangers, which offer extremely good mass transfer and flow characteristics, can be used to great advantage for inorganic separations. The sample capacity of these materials is low, however, necessitating the use of highly sensitive detectors. Several detectors suitable for metallic ions have been described (1-3); none, however, are commercially available and no reports on the use of pellicular ion exchangers for metal ion separations have appeared to date. Flame emission spectrometry, with its inherently high sensitivity, appears to be an ideal choice for coupling to a chromatographic column but, although many workers have used a combination of ion exchange and flame spectrometry (4-6), no on-line system has been described. We have constructed an integral flame photometric-ion exchange system which enables high-speed separations to be performed with detection limits in the low parts-per-million range. Troublesome matrix effects are eliminated and quantitative accuracy is better than 2% in the 1-10 partsper-million range. The following report describes some characteristics of this system. 186
EXPERIMENTAL Chromatographic System. A Perkin-Elmer Model 1240 Liquid Chromatograph was used, except that the pump supplied was replaced with a Waters Associates Model 6000 Solvent Delivery System. All materials of construction were either Teflon or Type 316 stainless steel. Columns were 2.6-mm i.d. (6.25-mm 0.d.) by 50 cm in length. On-colunn injections were performed with a Hamilton H P 305 syringe. The column packing used was Zipax SCX (DuPont Instruments, Inc.) and columns were dry-packed by the method of Kirkland (7). All separations were performed a t 75 O C . Flame Photometer. A Beckman Total Consumption burner was used with hydrogen as fuel and air as oxidant. Air and hydrogen flow were regulated with Matheson Model 701 Rotameters after the primary reducing valves. The outlet of the column was connected directly to the aspirating capillary of the burner by means of a 1-meter length of capillary tubing and a Swagelok capillary union. The flame is viewed by a Schoeffel Quartz Prism Monochromator (Model GM 100) which, together with an RCA 1P21 photomultiplier tube and an American Instrument, Inc. BlankSubtract Photometer, forms the readout system. Chromatograms were recorded on a Hewlett-Packard Model 680 recorder. Reagents and Chemicals. All chemicals were ACS reagent grade with the exception of the nitric acid which was J. T . Baker electronic grade. Double distilled, deionized water was used throughout. Metal ion solutions were prepared by appropriate dilution from 1000-ppm standard solutions supplied by Aztec Instruments, Inc.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1 , JANUARY 1975
RESULTS AND DISCUSSION Detection System. The detection system is designed to be used in either of two modes: wavelength selective, in which narrow slits are used, and nonselective, in which no slits are used. In the former mode. a slit width of 0.1 mm results in a bandpass of 0.8 nm and, in the latter mode, the bandpass is approximately 50 nm. The bandpass was determined from the dispersion relationship of our monochromator and represents the full peak width a t half-height maximum. In the wavelength selective mode, the sensitivity for sodium ion a t 589 nm was determined to be 0.1 partper-million by replicate injections of standard solutions (S/N = 2:l). The nonselective mode of detection is primarily used for preliminary screening runs, since even though an air hydrogen flame is used to lower background, noise is still appreciable. The effect of flow rate on the performance of the detection system is of great importance. With the column uncoupled from the burner, the aspiration rate was found to be 0.7-0.9 ml/min, and depended slightly on the solution head maintained. Thus, when the flow rate with the column coupled is kept a t approximately that of aspirated samples, no severe loss of response is encountered. As expected, the response is severely diminished at faster flow rates. This is probably due to excessive cooling of the flame. However, even though the system flow rate is somewhat restricted, the advantages gained from a steady throughput into the flame are considerable. Burner deposits are minimal, as are particulates, and flame fluctuations due to variations in aspiration rates are greatly reduced. Separations on Pellicular Resins. Figure 1 shows a chromatogram of some alkaline earths obtained on Zipax SCX with the detector operating in the nonselective mode. The elution order is typical of that found for strong cation although the required eluant concenexchange resins (8), trations are somewhat lower. It should be noted that the time required for complete elution on SCX is greatly diminished when compared to that on solid cation exchange resins ( I ) . This can be seen clearly in Figures l b and IC where several of the rare earths elements are separated. Many excellent separations of these elements have been reported (9-11 ) since the pioneering work of Boyd and Ketelle ( 1 2 ) ;all of them suffer, however, from the same disadvantage, that of long experimental times. In contrast, the use of pellicular cation exchange resins enables the separations to be performed in minutes. The use of a chromatographic column as a separating device eliminates many of the common interferences found in flame photometry such as spectral overlap, interelement radiation effects, and anionic matrix effects. For example, determination of strontium and europium in the same sample could be complicated by the closeness of their resonance lines (4607 and 4594 Angstrom units, respectively) unless a high resolution monochromator is used. However, as Figure IC shows, in citrate media, europium is eluted well ahead of strontium. The use of the cation exchange column also eliminates interferences from anions such as PO4 (which emerge at the solvent front) which cause strong depression of the response obtained for many elements. Finally, the need for radiation buffers is eliminated. As a result of these factors. the precision of a particular determi-
1
LU
I\
Trn
ho
K
C
I
!
2
4
6 8 Time, mi?
1
0
Figure 1. ( a ) Eluant = 0.01M HN03. Wavelength, 435 nm. Slits, none. Temp, 75 OC. Flow rate, 0.9 mllmin. (b) Eluant = 0.2M citric acid, pH 3.0. Wavelength, 535 nm. Slits, none. Temp, 75 OC. Flow rate, 0.7 ml/min. ( c ) Eluant = 0.2M citric acid, pH 3.25. Wavelength, 460 nm. Slits, 0.1 mm. Temp, 75 OC. Flow rate, 0.7 nil/min
nation is enhanced. Using standards prepared from Aztec atomic absorption solutions, the precision for sodium (at 589 nm) and calcium (at 554 nm) was found to be 1.2 and 1.4%, respectively, and plots of concentration us. response were linear in the 2-10 ppm range.
CONCLUSION In summation, the combination of flame photometry with ion exchange chromatography in an on-line manner results in high sensitivity and specificity for the determination of metal ions, while a t the same time minimizing sample handling. The sensitivity of detection can be enhanced by substitution of a hotter flame and a more sensitive photomultiplier tube. Finally, the use of pellicular ion exchange resins results in greatly improved separations, and a great saving in time. LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12)
T. Yoshinori and G. Muto, Anal. Chem., 45, 1864 (1973). A. MacDonald and P. D. Duke, J. Chromatogr., 83, 331 (1973). P. L. Joynes and R. S. Maggs, J. Chromatogr. Sci, 7, 427 (1970). J. A. Brabson and W. D. Wilhide, Anal. Chem., 26, 1060 (1954). C. W. Gerhke, H. E. Affsprung, and E. L. Wood. J. Agr. Food Chem.. 3, 48 (1955). D. W. Ellis in "Flame Emission and Atomic Absorption Spectrometry," Vol. 2, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N.Y., 1971, Chapter 10 and references therein. J. J. Kirkland in "Modern Practice of Liquid Chromatography," J. J. Kirkland, Ed., Wiley Interscience, New York, N.Y., 1971, Chapter 5 and references therein. K. Dorfner. "Ion Exchangers: Properties and Applications," Ann Arbor Science, Ann Arbor, 1972, Chapter 2 and references therein. K. E. Zeyb, Fresenius'Z. Anal. Chem., 226, 159 (1967). G. Brunishoiz and R. Roulet. Chhia, 21, 188 (1967). T. Yamabe, J. Chromatogr., 83, 59 (1973). B. H.Ketelle and G. E. Boyd, J. Amer. Chem. Soc., 69, 2800 (1947).
D. J. Freed Bell Laboratories Murray Hill, N.J. 07974 RECEIVED for review July 17,1974, Accepted September 3,
1974.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1 . JANUARY 1975
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