10% of its original value, as shown in Figure 6. These data seem to demonstrate that the L D H activity is much more stable when entrapped in the acrylamide gel than in free solution. Other interpretations, however, are possible. The rabbit muscle enzyme used in the preparation of the enzyme gel was composed of more than a single isoenzyme of LDH. On an electrophoretic separation as previously described (fa), it was demonstrated that the preparation was primarily the slowest moving, less stable M-type isoenzyme (8), but a small amount of a faster migrating, more stable H-type isoenzyme (8) was also present. An equally probable interpretation of the data in Figure 6 is that the labile L D H isoenzyme was inactivated during the gel preparation, and the entrapped enzyme activity reflected only the more stable LDH fraction which was able to survive the polymerization and lyophilization procedures. In a more severe test of stability, the activities of a series of identical glucose oxidase columns, after heating for 10 minutes at room temperature from 37’ to 70” C., were compared with activities of a series of glucose oxidase solutions treated in the same manner. The stability of columns containing gels with 100 mg. and 10 mg. of GO per 100 ml. were identical, and when compared with glucose oxidase solutions (200 mg. GO per 100 ml.), demonstrated no significant increase in stability. About half of the activity was destroyed in 10 minutes a t 60” C. and all of the activity at 70” C. in both the gel and solutions. Analytical Implications. The use of immobilized activity should extend the applications of enzymes in analy-
sis. Aside from the advantage of economy, another major advantage of the immobilized system, which should permit the development of many new analytical applications, is the fact that the products of the enzyme reaction are easily separated from enzyme catalyst with high efficiency. Thus, an analytical method could be based on a series of enzymic reactions by utilizing a series of enzyme gel columns, even though the individual enzymes were not mutually compatible in a single solution. This “automatic” separation of enzyme and products should also greatly facilitate the automation of more complex analytical procedures such as enzymic amplification with cyclic reactions (16). Furthermore, the exclusion of molecules from the enzyme reaction system trapped in the gels on the basis of molecular size in a manner similar to that for “gel filtration” techniques may help to reduce the number of interferences, which is a major problem with many enzymic methods of analysis. It is possible, in principle, to develop a compact “reagentless” continuous analyzer for determinations where the enzyme is the only “reagent” required in addition to the sample for the analysis. For example, glucose oxidase has been used with an oxygen electrode to continuously monitor blood glucose (14). An immobilized glucose oxidase system coupled to an oxygen electrode should provide a “reagentless” method of continuous glucose analysis and is currently being developed in this laboratory. ACKNOWLEDGMENT
The encouragement and informative discussions with Dr. C. E. Reed are gratefully acknowledged. The assistance of Mrs. Martha White in
making measurements on the enzymes in free solution and of Mr. Pankonin in modifying the photometer system were greatly appreciated. LITERATURE CITED
(1) Barnett, L. B., Bull, H. B., Biochim. Biophys. Acta 36, 244 (1959). (2) Baunian, E. K., Goodson, L. H.,
Guilbault, G. G., Kramer, D. N., ANAL.CHEM.37, 1378 (1965). (3) Bernfeld, P., Wan, J., Science 142, 678 (1964). (4) Blaedel, W. J., Haas, R., Department of Chemistry, University of Wisconsin, personal communication, November, 1965. (5) Blaedel, W. J., Hicks, G. P., Advan. Anal. Chem. Instr. 3, 105 (1964). (6) Chang, T. 31. S.,Science 146, 524 I 1- 864). - - -,.
(7) Eli, A. B., Katchalski, E. J., J. Biol. Chem. 238, 1690 (1963). (8) Fondy, T. P., Kaplan, ?J. O., Trans. N . Y . Acad. Sci. 119. 888 11965).
wissenschajten 40, 508 (1953). (11) Guilbault, G. G., Kramer, D. N., ANAL.CHEY.37. 1675 (1965). (12) Hicks, G. P.,’Nalevac, G’. N., Anal. Biochem. 13, 199 (1965). (13) Hicks, G. P., Updike, S. J., Zbid., 10, 290 (1965). (14) Kadish, -4.H., Hall, D. A., CZin. Chem. 11, 869 (1965). (15) Lowry, 0. H., Passonneau, J. V., Schulz, D. W., and Rock, 31. K., J. Biol. Chem. 236, 2746 (1961). (16) Mitz, M. A., Summaria, L. J., .Vatwe 189, 576 (1961). (17) Vasta, B., Usdin, T., Aldrich, F., U. S. Army Rept. DA-18-108-405CML-828, (1963).
RECEIVED for review November 22, 1965. Accepted March 9, 1966. One of the authors (SJU) was supported as a postdoctoral fellow by the Badger State Civics Fund. I n part, Division of Analytical Chemistry, 150th meeting, ACS, Atlantic City, N. J., September 1965.
Solution Technique for Analysis of Silicates N. H. SUHR
and C.
0.INGAMELLS
Mineral Constitution Laboratories, The Pennsylvania State University, University Park, Pa,
A new solution technique makes possible the rapid and precise determination of most major and minor constituents of silicates in a single sample. Solution is effected by adding the melt of sample fused with lithium metaborate directly to cold dilute nitric acid that contains the internal standard cobalt. The solution is analyzed with an emission spectrometer using a rotating disk technique. Data for precision are given for the oxides of Si, AI, Mg, Ca, Sr, Ba, Ti, Mn, Fe, Cr, Cu, Zn, Zr, and Ni. Accuracy of the method is evaluated by comparison with chemical analyses of four rock samples. N a and K are de-
730
ANALYTICAL CHEMISTRY
termined on the same sample solution by flame photometry. The same solution can also be used to determine various elements colorimetrically, spectrographically, and by x-ray fluorescence spectrometry.
T
growing need for reasonably accurate analyses of large numbers of geologic samples has led during recent years to numerous developments, both chemical and instrumental. Rapid chemical methods have proved very useful, but they are intrinsically slow, and even when simplified to the utmost, demand skills which are not always available. X-ray fluorescence is widely HE
used, but requires special equipment for the determination of sodium and magnesium. I n the method here described, in which the sample is treated with dilute nitric acid after lithium metaborate (LiB02) fusion, almost all rock-forming minerals yield a clear solution which can be examined spectroscopically, flame photometrically, and chemically. A wide range of sample compositions can be tolerated. If a constituent falls outside the range of one method, another can be used without preparing a new sample. With a direct-reading spectrometer, very rapid analyses are possible. Precision compares favorably
with that obtained using rapid chemical and x-ray fluorescence techniques. Although the method is designed primarily for silicate rocks and minerals, it may be adapted to other types of materials-for example, carbonates and phosphates. An important advantage is that cross checking by different procedures when interferences are suspected does not require additional sample. Colorimetric methods using the same solution have been developed for SiOz, A1203, PzOs,NiO, FezOa,and other constituents, and will be reported in another paper. A useful peculiarity of silicate solutions obtained in the manner described, is that certain ion combinations may be present in amounts exceeding their solubility products, without immediate precipitation. Thus, samples containing barium and sulfur, or zirconium and phosphate, usually yield clear solutions which may be analyzed before precipitation occurs. Rock powders should be ground to pass a 200-mesh screen, and then thoroughly mixed by hand rolling on smooth paper. Fine grinding is particularly necessary for complete solution of highly siliceous samples. Fusion may be accomplished in graphite or in platinum. The former is preferred for speed and convenience. Analyses are often possible with 50 mg. of material or even less, with no loss of sensitivity or accuracy except that occasioned by the sampling problem. Among constituents not covered by the new combined procedure, reasonably rapid methods are available for Li (S), c (a), B (4),F (51, and s ( 7 ) . Various techniques for the spectrographic excitation of solutions have been compared and evaluated by Baer and Hodge (1). They clearly showed that the rotating disk electrode with a high-voltage a x . spark gives the best combination of sensitivity, reproducibility, and convenience. We have not investigated other techniques in detail. Xumerous solution procedures have been tried and recommended. A technique described by Baksay and Anderson (2) gives remarkable precision and good accuracy. However, solution preparation is fairly complicated, and a special modification of the rotating disk assembly is required. The sample is highly diluted, with an adverse effect on sensitivity and increased possibility of contamination. The use of hydrochloric acid makes it desirable to protect the interior of the arc-spark stand from its corrosive action. Wilkinson (9) uses a hydrochloric acid solution of a sodium hydroxide fusion of iron ores which is considerably more concentrated, but the solution procedure is lengthy, The use of sodium hydroxide precludes the determination of sodium, which is a serious disadvantage.
The lithium metaborate-nitric acid procedure overcomes all the disadvantages of these and other methods. It is rapid; the solution requires no heating during its preparation, is relatively stable and concentrated, and permits determination of sodium by flame photometry. Corrosion of equipment is not a problem. Metallic crucibles are not required, so that there is no chance of picking up metallic contaminants during preparation. EXPERIMENTAL
Preparation of Sample Solution. REAGENTS REQUIRED. Lithium metaborate, anhydrous, LiBOZ. The reagent should be examined spectrographically for impurities. Lithium metaborate may be purified as follows: Dissolve 100 grams of LiBOz in 1 liter of hot water. Filter while hot, and allow the filtrate to cool. Filter the precipitated octahydrate on a glass frit, and heat at 50' C. for 24 hours or more in a ventilated oven to produce the dihydrate. Transfer the dihydrate to a platinum dish and bring slowly up to 400' C. The dehydration is accompanied by a large volume increase, so that a capacious dish is necessary. Crush the product gently in an agate mortar and mix well before using. Alternatively, equivalent amounts of reagent grade lithium carbonate and boric acid (both obtainable in powder form) may be thoroughly mixed and heated slowly. At about 180' to 200' C. reaction occurs, resulting in an increase in volume. Further heating to 400' C. yields a product which is essentially anhydrous LiBOz. It is desirable to put the mixed reagent in a cold furnace and bring up to temperature over a period of some hours; otherwise boric acid may be lost, and the final product will contain too much alkali. Cobalt Nitrate Stock Solution. Dissolve 113 grams of Co(N08)t.6Hz0 in 1 liter of water. Filter, and store in a tightly sealed bottle. Preparation of this concentrated solution avoids frequent restandardization. Dilute Cobalt Nitrate. Dilute 50.00 ml. of cobalt nitrate stock solution to exactly 1 liter with water. Dissolving Solution (with cobalt internal standard). Dilute 50.00 ml. of dilute cobalt nitrate with 30 ml. of concentrated nitric acid to exactly 1 liter with water. SOLUTION PREPARATION. Mix 0.1000 gram of -200-mesh sample with 0.500 gram of lithium metaborate, transfer to a pre-ignited high-purity graphite crucible, and place in e muffle furnace at 950' C. for 10 to 15 minutes. Using a transfer pipet, put 50.0 ml. of dissolving solution in a flat-bottomed 200-ml. Teflon or polypropylene beaker. Add a Teflon-covered stirring bar. Remove the crucible from the furnace, swirl to gather uncoalesced beads of molten material, and pour the melt into the beaker. Cover to limit evaporation loss, and stir gently over a magnetic
stirring unit solution is solution to a tightly fitted
(without heating) until complete. Transfer the clean glass bottle with a plastic stopper (Kimble
60975-L) .
Loss by spattering, either during fusion or in the addition of the molten material to the dilute acid, does not occur. The fusion does not wet the graphite, and none of the sample remains in the crucible if the operation is properly performed. Crucibles may be used many times without contamination. Flux and sample must be well mixed before fusion; otherwise, local concentrations of silica in the melt may lead to slow or incomplete solution. Glass beakers should not be used. Solutions of a few samples high in silica-e.g., glass sand-are apt to show a small residue of undissolved polysilicic acid; otherwise, almost all rocks are easily dissolved. Care is necessary to prevent sodium contamination. Other sample weights and solution volumes may be used, but the recommended proportions of sample, flux, and nitric acid are nearly optimum, and should be approximately maintained. DILUTE SOLUTIONFOR KA A N D K. Dilute 5.00 ml. of the sample solution to 25 ml. with water. During dilution, remove specks of abraded Teflon, polypropylene, or graphite by filtration, to avoid difficulties with the flame photometer atomizer. Other portions for colorimetric determinations may conveniently be removed a t the same time. For rubidium, transfer 20 ml. of undiluted rock solution to a 25-ml. flask and add sufficient standard potassium solution [containing LiBO,, "01, and Co(NO&] to increase the KLO content of the rock solution to a standard value (conveniently 100 p.p.m. of KzO). Determine RbnO by differential flame spectrophotometry (6). Presumably a similar procedure will give results for cesium. Equipment. illthough the sparksolution technique may be used with a spectrograph, a spectrometer yields more precise and more rapid results. Table I lists the equipment and operating conditions. Special attention should be given the rotating disk assembly. The assembly furnished with the arc-spark stand rotated nearly perpendicularly to the optic axis and frequently had a jerky motion. This proved unsatisfactory because alignment was more critical and introduction of sample to the analytical gap was not uniform. The assembly was replaced by one in which the plane of rotation is parallel to the optic axis. Graphite mandrels wore rapidly, causing variation in the spark gap due to poor seating of the disk, and a permanent tantalum mandrel was substituted. This has proved very satisVOL 38, NO. 6, MAY 1966
a
731
factory. A stainless steel mandrel resulted in iron and chromium contamination. Parameters chosen for the a.c. spark discharge were somewhat arbitrary, the nisin objectives being to obtain a reproducible discharge with as much sensitivity as possible, and without boiling away the solution. The use of 200-micron exit slits seemed advisable for Ca 3179 and A1 3961 because of their inherent line broadening. A wider slit for Co 2286 was employed to obtain better stability and sensitivity. Most of the other lines are sharp, and wide exit slits are undesirable because of nearly coincident lines. Analytical line pairs are listed in Table 11. Among the factors considered in choosing the most favorable line were: freedom from common elemental line interferences, sensitivity, and proximity to other analytical lines. The spectrometer geometry makes difficult the use of lines separated by less than about 25 A. Exit slit holders are 8 mm. wide and offset slits can be mounted about 6 mm. or 24 A. apart. With lines much closer together, double slits with splitting mirrors might be used, or one line might be employed in the second order. Our particular spectrometer does not easily permit either of these alternatives and lines more than 25 A. apart were consequently chosen-for example, Fe 2755 rather than Fe 2599 was used because of the latter's proximity to M n 2576. In the analysis of any particular suite of samples, the multiplier phototube
Table 1.
14-
z
86 -
181614-
z
8-
0
4
8
8
IO
12
Multiplier phototubes Entrance slit, p Exit slits, p
,
,
,
,
,
,
18
20
22
24
26
28
Digital voltmeter readings ore proportional to capacitor voltoge ratios (analytical line/internol standard line). Proportionality constant determined b y photomultiplier sensitivity setting
gain for each analytical line is adjusted so that the voltage ratios fall within the range of the readout device. Thus, the full scale setting for CaO in granitic rocks may be 2%, whereas for basic
Consolidated Electrodynamics Gorp. maximum versatility spectrometer with reciprocal linear dispersion of 4A./mm. and sequential t y p e writer readout of voltage ratios (Vunknown/ Vintlt) for up to 20 elements. Wavelength range coverage of 2770 A. in 1st order RCA 1P28 40 75, with exception of 200-p slits for Go 2288, Ca 3179, and A1 3961
Spex Industries, Inc. Jarrell-Ash, tantalum shaft
Capacitance, pfarads Primary resistance, o h m Secondary resistance, ohms Inductance, microhenries R.F., amperes Primary amperes Breaks/cycle Spark gap, mm. Electrodes Counter Disk
0.005 24 2.5 25 5.7 7.0 12 3
ANALYTICAL CHEMISTRY
I
16
Figure 1. Typical relationship between readout and weight per cent at two sensitivity settings
Arc-spark stand Rotating disk assembly Revolutions per minute Analytical discharge
Exposure, sec. Prespark, sec.
,
14
DIGITAL VOLTMETER READING
Equipment and Excitation Conditions
Spectrometer
732
2-
rocks a 0 to 20% range may be required to include all the samples. It is sometimes convenient to dilute solutions which give off-scale readings for a particular element, rather than make a
Table II. Analytical Line Pairs Internal Analytical line standard Go I 3453.5 ZnI 2138.6 Si I 2881.6 Cu I 3273.9 A1 I 3961.5 Cr I 4254.4 Xi I1 2394.9 Co I1 2286.2 Mn I1 2576.1 Fe I1 2755.7 Mg I1 2795.5 V I1 3102.3 Ca I1 3179.3
Ti I1 Zr I1 Sr I1 B s I1
10
High Voltage a.c. spark (University of Michigan type)
0.180 inch performed (Natl. Carbon G3955) 0.200 inch performed (Natl. Carbon L-4075) ASTM D-2 30 30
Table 111.
A1208 MgO
CaO
Ti02 Fez03
MnO SrO BaO ZrOz
3349.4 3392.0 4077.7 4554.0
Lower Limits of Determination
%
0.00 0.02 0.02 0.02 0.1 0.003 0.005 0.005 0.05
7%
v20s
NiO CUO ZnO CrzOa Na2O Kz0
0.05 0.05 0.05 0.05 0.05 0.01 0.01
RbzO
0.002
sensitivity adjustment; if this is done, the diluent should be a prepared rock solution, or its equivalent. Typical analytical curves for MgO are shown in Figure 1. Flame photometric measurements were made using a low temperature flame described elsewhere (6). Preliminary work using an Instrumentation Laboratories flame photometer (Model 143) has produced extremely rapid results of high precision for sodium and A dilution of 1 to 10 potassium. instead of 1 to 5 is desirable when this instrument is used. Lithium, from the metaborate, provides an internal standard, the advantages of which are well known. Table I11 lists the useful limits of detection for 17 elements. These values are the approximate lower limits of analytical usefulness and are higher than what is commonly referred to as the limit of detection.
Table IV.
Si02
A1203 hlg0
CaO
SrO BaO Ti02 MnO Fez01
DISCUSSION
Table IV illustrates the effect of time on the silica determination. The decrease in silica is due to polymerization, which continues a t a slow rate until
11.oo
5.79 0.215 0.200 0.200 0.235
Cr~03 CUO ZnO Zr.02
Ni0
0.308 Table V.
PRECISION AND ACCURACY
Precision data are listed in Table IV. These data were collected on various days, on various samples, and are considered to be representative. The values listed are for one determination. Obviously, it is not difficult to double the precision by making four determinations per sample. With the exception of the elements below 0.5% in concentration, all relative standard deviations are in the 1 to 3y0range. Data are presented for some elements in various concentrations; there is not much change in the relative standard deviations, which remain in the 1 to 3y0 range. Although data a t present are not available, it is not unreasonable to presume that iron and magnesium would have about the same precision as calcium and aluminum a t lower concentrations. Accuracy is difficult to determine, but some approximation may be attained by comparing spectrochemical values with chemical and colorimetric values (Table V). For AlzO~the chemical values are too high, since they are difference figures and include other undetermined oxides. The spectrochemical values are undoubtedly more nearly correct. The spectrochemical value of 0.033% Ti02 in the peridotite is probably wrong and is attributed to the commercial lithium metaborate, which had not been purified. The relatively large discrepancy between values for BaO in the basalt is noted but no explanation is offered.
Concn., yo 63.0 49.2 16.35 5.16 0.631 4.70 10.88 5.12 0.550 0.045 0.061 0.560 1.10 0.100
Si02 Chem. Spec. AlzOa Chem.. Spec. Colorb Ti02
Chem. Spec. Colorb Fez02 Chem. Spec. Color 6 M nO Chem. Spec. Colorb SrO Chem. Spec. BaO Chem. Spec.
Precision of Method
Std. dev. 1.2 1.4 0.25 0.15 0.015 0.05 0.14 0.07 0.012 0.002 0.003 0.007 0.03
0.002 0.15 0.07 0.013 0.011 0.010 0.017 0.009
Rel. std. dev.7 % 1.9 2.8 1.5 2.9 2.4
No. of detns. 21 10 21 10
10 21 12 21 10 21 21 21 12 21 12 21
1.1
1.3 1.4 2.3 4.7 4.9 1.3 2.7 2.0 1.4 1.2 6.0 5.5 5.0 7.2 2.9
10 10 10
10 10
Chemical and Spectrochemical Data on Various Rocks
Granite
Peridotite
Andesite
Basalt
69.26 68.7
41.87 41.8
59.09 60.3
38.10 40.6
15.26 14.7 14.5
0.69 0.64 0.73
16.97 16.4 16.3
10.90 10.65
0.47 0.47 0.45
0.01 0.033 0.00
1.03 1.04 1.04
2.64 2.57 2.61
2.72 2.73 2.72
8.16 8.16 8.31
6.75 6.75 6.84
12.88 12.78 12.77
0.03 0.034 0.03
0.11 0.125 0.11
0.09 0.097 0.09
0.21 0.207 0.21
0.06 0.062
0.00 0.00
0.08 0.082
0.16 0.16
0.18 0.18
0.00
0.13 0.125
0.15 0.12
0.00
K20
Chem. 4.53 0.00 2.87 1.38 Flameb 4.70 0.00 3.01 1.40 Nan0 Chem. 4.03 0.00 4.19 2.94 Flameb 4.13 0.00 4.17 2.85 RbzO Chem. 0.0176 0.000 0.0068 0.0046 Flameb 0.017 0.000 0.007 0.005 M go Chem. 0.72 43.22 1.46 13.30 Spec. 0.73 43.20 1.49 13.50 CaO Chem. 2.03 0.56 4.92 13.32 Spec. 2.05 0.55 4.94 13.26 Cr203 Chem. 0.32 Spec. 0.36 NiO Chem. 0.34 Spec. 0.30 Includes R.E.’s, Zr02, S ~ O ZV206, , ThO2, and other undetermined oxides. b Performed on same solution prepared for emission spectroscopy.
VOL. 36, NO. 6, MAY 1966
733
eventually a gel forms on the walls of the container. Other elements behave differently. Iron is almost unaffected by time, whereas high barium or zirconium (>1%) shows marked decreases in concentration after several weeks or even days. A good rule to follow is to prepare several standards a t the same time as the samples to be analyzed, and to run the samples within a day or so of preparation. Use of the same sample for emission spectrometry, flame photometry, and other determinative techniques lends considerable flexibility to the proposed approach t o rapid silicate analysis. An obvious extension of the method is the use of flame absorption spectrometry for minor and trace elements. The solution technique adds the possibility of preparing standards simply by adding measured amounts of metal nitrate solutions to the dissolving acid. However, unnecessarily large additions of foreign anions may give rise to matrix effects-e.g., standards prepared by adding sulfuric acid solutions
of titanium to a rock solution are not satisfactory. The proposed method gives results generally as good as or superior in precision and accuracy to rapid colorimetric methods, and the time per analysis is in general much less. Twenty to 40 samples per day can be examined for 14 or more elements. The I L flame photometer with digital readout offers almost instant NazO and KzO determination. ACKNOWLEDGMENT
Lithium metaborate (LiB02) was first suggested to us by M. L. Keith of The Pennsylvania State University, who has made an extensive study of the fluxing properties of the alkali borates. LITERATURE CITED
( 1 ) Baer, W. K., Hodge, E. S., A p p l . Spectry. 14, 141 (1960). (2) Baksay, I., Anderson, C., Pittsburgh
Conference on Analytical Chemistry and Applied Spectroscopy, 1965. ( 3 ) Ellestad, R. B., Horstman, E. L., ANAL.CHEM.27, 1229 (1955).
Table VI.
Effect of Solution Aging on Si02 Concentration Test sample. Diabase, W-1 Age, of Age of
solution, solution, days % SiOz days 3 4 84
52.3 52.7 50.5
98 363 440
% Si02 48.3
44.8
44.5
(4) Heyes, M. R., Metcalfe, J., U,K.
Atomic Energy Authority Production Group Rept. 251 (S)(1963). (5) Ingamells, C. O., Talanta 9, 507
(1962). (6) Zbid., p. 781. ( 7 ) Sen Gupta, J. G., ANAL.CHEW35, 1971 (1963). ( 8 ) ShaGro, La, Brannock, W. W., U. S. Geol. Survey Bull. 1036C (1956), 11 14A (1962). (9)Wilkinson, L. P., A p p l . Spectry. 16, 185 (1962).
RECEIVED for review January 7, 1966. Accepted March 15, 1966. Financial support for this study came from NSF Grant GP3853.
Flame Emission and Dual Flame Emission-Flame Ionization Detectors for Gas Chromatography ROBERT S. BRAMAN IIT Research Insfifufe, Chicago, I / / . b A hydrogen-air flame emission detector was constructed employing interference filters and standard gas chromatography instrumentation. Instrumentation variables were studied. Detection sensitivity was in the microgram range, wavelength dependent and generally greatest for heterocontaining compounds. The atom study of emission response at 589, 515, and 415 mp indicates that the emission intensity attributed to CZ or CH molecules in the flame plasma are dependent upon the structure of the chromatographed compounds. The design and operation of a dual flame emission-flame ionization (FE/FI) detector for gas chromatography i s also described. The influence of structure on response ratios was studied on a chlorinated methane series of compounds, an aromatic series of compounds, and a three-carbon series of compounds. The influence of structure on response ratios was demonstrated thus establishing the potential use of the dual detector in qualitative identification of peaks.
-
of gas chromatognumerous techniques for
IKCE THE ADVENT
734
ANALYTICAL CHEMISTRY
detecting the vapors in the column effluents have been described. Selectivity of response has ranged from highly unselective for thermal conductivity and flame ionization detectors t o partially selective for electron capture, beta-ray absorption, and infrared absorption detectors. Insufficient selectivity of response of current detectors and the use of increasingly smaller sample sizes have combined to make the identification of eluted compounds difficult. Emission-type detectors, because of their dependence on wavelength and high inherent sensitivity, show considerable promise for component identification. Nevertheless, despite their potentially high selectivity of detection, until recently, emission methods have been given scant attention. Microwave-stimulated plasmas (8), high-frequency discharges (9) , and flame plasmas have been reported and are being investigated. A flame emission detector reported by Grant (3) was based upon the increase in the total emission of a hydrocarbon air flame, but no wavelength discrimination was attempted. Sensitivity of the device was restricted by the high emission of the
flame background. Response was linear with sample weight, and aromatic compounds exhibited a higher emission intensity per weight than aliphatic compounds. Juvet and Durbin reported the detection of metal chelates and organic compounds (4, 6) in a hydrogen-air flame. The potential advantages of a hydrogen-air flame emission detector were first realized by this author during the development of a portable flame emission instrument for monitoring pentaborane in air (1). Comparatively high sensitivity and wavelength dependency were observed when comparing the response of various volatile compounds. Detection sensitivity for heteroatomcontaining compounds was in the part per million range. These observations led to the conclusion that a flame emission detector would be suitable for gas chromatographic instrumentation and, in addition, would have a high degree of selectivity of response if selected band or line emission wavelengths were found for each different functional group or heteroatom. This eventually led to the construction of a hydrogen-air flame emission detector and the study of its operation.
