unvaporized particles (Mie scattering) in the flame keeps increasing with the source intensity. For resonance fluorescence work, noise related to the scattered source radiation (shot and flicker) is expected to be the limiting noise in the system, and according to Equation 2, a deterioration in the ultimate S/N is easily predicted. When nonresonance fluorescence lines cannot be used, scattering can still be minimized by time resolution, i.e., by exciting with a pulse (much) shorter than the fluorescence emission pulse and delaying the aperture of the gated electronics. It is worth noting here that, even in the absence of Mie scattering, with the high photon rates characteristic of laser excitation, Rayleigh scatter of radiation from atoms and molecules represents a fundamental limit for resonance fluorescence measurements in spectral regions where atomizer noise is negligible. Finally, shot and flicker noise related to molecular fluorescence from flame species must be taken into account even when nonresonance fluorescence transitions are considered, the more so since such molecular fluorescence has been reported in flames for pulsed xenon continuum sources (8). For the case of laser excited nonresonance atomic fluorescence of elements with strong nonresonance transition probabilities, the detection limits should be superior to those obtained by excitation with conventional line sources. Experimental results have not shown this nor have satisfactory explanations been given to explain the discrepancies.
CONCLUSIONS The simple considerations reported here show that pulsed fluorescence spectroscopy is experimentally plagued with certain drawbacks which have certainly limited its analytical usefulness. We stress here that our discussion did not take into account the detection system and the associated difficulty of processing very short pulses. However, these problems can be overcome with properly designed electronics. Finally, we
I
are not claiming that pulsed fluorescence spectroscopy cannot be superior to conventional cw fluorescence spectroscopy, but we have rather indicated, on the basis of the experimental results reported in the literature, the parameters that must be properly investigated in order to achieve such a goal.
LITERATURE CITED (1) N. Omenetto, L. M. Fraser, and J. D. Winefordner, in “Applied Spectroscopy Reviews”, vol. 7, E. G. Brame, Ed., Marcel Dekker, N.Y., 1973 p 147. (2) N. Omenetto, Anal. Chem., 48, 75A (1976). (3) N. Omenetto, P. Benetti, L. P. Hart, J. D. Winefordner, and C. Th. J. Alkemade, Spectrochim. Acta, Part 6 ,28, 289 (1973). (4) H. G. C. Human, Specfrosc. Lett., 6, 719 (1973). (5) G. J. DeJong and E. H. Piepmeier, Anal. Chem., 46, 318 (1974). ( 8 ) G. J. DeJong and E. H. Piepmeier, Specfrochim. Acta, Part 6 ,29, 159 (1974). (7) E. H. Piepmeier and L. de Galan, Specfrochim. Acta, Part 8 , 30, 263 (1975). (8) W. K. Fowler, Ph.D. Thesis, University of Florida, Gainesville,Fla., 1978. (9) J. M. Mansfleld,M. P. Bratzel, H. 0. Norgordon, D. 0. Knapp, K. E. Zacha, and J. D. Winefordner, Specfrochim. Acta, Part 6 ,23, 389 (1968). (10) M. W. P. Cann, Appl. Opt., 8, 1645 (1969). (1 1) R. N. a r e , paper given at the “Second Laser Spectroscopy Conference”, Megeve, France, June 1975. (12) J. H. Richardson, B. W. Wallin, D. C. Johnson, and L. W. Hrubesh, Anal. Chim. Acta, 86, 263 (1976).
N. Omenetto’ G. D. Boutilier S. J. Weeks B. W. Smith J. D. Winefordner* Department of Chemistry University of Florida Gainesville, Florida 32611 Present address, Institute of Inorganic & General Chemistry, University of Pavia, Pavia, Italy. RECEIVED for review January 26, 1977. Accepted March 14, 1977. One of the authors (G.D.B.) wishes to acknowledge the support of a fellowship sponsored by the Procter & Gamble Company.
AIDS FOR ANALYTICAL CHEMISTS
Simple Microreactor for Subtractive Gas Chromatography R. G. McKeag’ and F. W. Hougen” Department of Plant Science, University of Manitoba, Winnipeg, Canada R3T 2N2
Subtractive gas chromatography provides a rapid method for functional group analysis and can serve as a useful aid in the identification of chromatographic peaks. A popular form of subtractive gas chromatography is precolumn reaction involving the formation of nonvolatile derivatives. Many reagents have been reported for subtraction of compounds containing various functional groups (1,2). Different reactor designs have been used, such as precolumns of metal tubing placed in the chromatographic oven (3-5), glass tubes (6) and spring-loaded metal tubes (7) placed in the injection port, and a “carbon skeletal” apparatus (8) placed in the injection port. The present report describes a microreactor, placed in the injection port, which has the combined advantages of some of the earlier reported reactors. It allows operation a t any desired optimum temperature of the reactor, independent of the temperature of the gas chromatographic column; it is ‘Present address, The Griffith Laboratories Ltd., 757 Pharmacy
Ave., Scarborough, Ontario, Canada M1L 358. 1078
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
simple in construction; and it can easily be removed for refilling or replacement. Its application for the subtraction of alcohols, aldehydes, and ketones is documented.
EXPERIMENTAL Construction. The microreactor (Figure 1) consists of a modified union and a reactor tube. The modified union is made of a standard Swagelok union (1/8 in.) joined to a Swagelok nut (l/~ in.) by silver soldering; it is bored through to allow insertion of the reactor tube. The reactor tube is made from a straight piece of stainless steel tubing (‘/a in. 0.d.). Diameters other than ‘/a in. may be used. A notch is made at one end of the reactor tube to facilitate the flow of carrier gas into the tube when it is in position. The tube is inserted from the columm oven into the injection port until its notched end touches the septum. The tube is fastened in this position with the modified union and a set of ferrules. With the ferrules thus affixed to the tube, the tube is removed and cut to the appropriate length (6.13 in. in the present work) so that its other end will butt against the analytical column ( l / ? in. 0.d.) when the parts are assembled. The reactor tube is filled with an appropriate reactor material by gentle tapping
MODIFIED UNION
I!
I
Figure 1.
J
Ill 1
I
ANALYTICAL COLUMN
REACTOR TUBE
51 LVER
-I N S E R T I O N
SOLDER
Table 1. Microreactor Subtraction Efficiency
-
% Subtracted
J
’i
Schematic diagram of microreactor assembly
and plugged with silanized glass wool at both ends. After insertion of the reactor tube in the injection port, the union is tightened and the analytical column is joined to the other end of the union. Reactor Materials. Boric acid (Fisher Certified Reagent), finely ground, was mixed with Chromosorb W AW DMCS, 6CMO mesh, 8:lOO by wt. o-Dianisidine (Baker Practical Grade) was coated from chloroform solution on “Wilkens” Firebrick (Varian Aerograph), and on Chromosorb W AW, both 60-80 mesh and 10100. Benzidine (Baker Grade) was coated from chloroform solution on Firebrick (60-80 mesh, 25:100), on Chromosorb W AW (60-80 mesh, 30:100),and on Chromosorb W AW DMCS (60-80 mesh, 20:lOO). Sodium bisulfite (freshly prepared aqueous solution of sodium metabisulfite, BDH reagent) was coated on Chromosorb W AW, and on Chromosorb W AW DMCS, both 60-80 mesh and 15.4:lOO. Dry sodium bisulfite (by evaporation of aqueous sodium metabisulfite) was mixed in the ratios 5:lOO and 1O:lOO with a coated support material (Carbowax 20M on Anakrom ABS, 70-80 mesh, 1 : l O ) . CAUTION. Benzidine is carcinogenic (8). The solid and vapor are readily absorbed through the skin. o-Dianisidine should also be treated with caution. Test Compounds. The compounds used for evaluating the microreactor (Table I) were selected because of their structural differences, range of boiling points, and availability. Chromatography. A Varian Aerograph Series 1800 gas chromatograph was used with a flame ionization detector. The microreactor tube was coupled to an analytical column (stainless steel, 8 ft X */*in. 0.d.) packed with Carbowax 20M on Chromosorb W AW DMCS, 6&80 mesh, 1:lO. The injection port, Le., the microreactor tube, was held at a constant temperature which differed with the experiments. The detector temperature was 225 O C . The flow of nitrogen carrier gas was 23 mL/min. After injection of each test sample, the column temperature was increased from 60 to 200 OC at 4 OC/min and held at 200 OC until the analysis was complete. Peak areas were measured with an Infotronics Model CRS-100 integrator. The test compounds (0.1 pL of each), individually or as mixtures, were injected admixed with dodecane (0.1 p L ) as an inert internal standard. The amount of a compound subtracted was determined by comparing itr, peak areas (normalized relative to the internal standard) obtained with and without the microreactor inserted in the instrument.
RESULTS AND DISCUSSION The microreactor was evaluated for efficiency in subtracting alcohols, aldehydes, and ketones, using the reactor reagents boric acid, o-dianisidine, benzidine, and sodium bisulfite. Boric acid on Chromosorb W AW DMCS was sufficiently reactive at 147 “C to quantitatively remove the alcohols tested (Table I). The aldehydes and ketones passed through the reactor largely unaffected, with retention times increased by about 5% and peak areas reduced by about 2%. At higher reactor temperatures, however, 2-propanol produced a broad, tailing peak, probably from the volatile triisopropyl borate ester formed in the microreactor or from a decomposition product. o-Dianisidine on Firebrick almost completely removed the aldehydes (Table I). The alcohols and ketones passed through the reactor with retention times increased by 5% and peak areas reduced by less than 6%. o-Dianisidine on Chromosorb W AW, a t 165 OC, removed the aliphatic aldehydes but only 74% of the benzaldehyde. Benzidine on Firebrick almost completely removed the aldehydes and ketones (Table I). The alcohols passed through
Compound Alcohols 2-Propanol Butanol Hexanol 2-Octanol Decanol Aldehydes 2-Methylpropanal Pentanal Hexanal Octanal N onanal Benzaldehyde Ketones Propanone 3-Pentanone 2-Heptanone 3-Heptanone Acetophenone 2,4-Pentanedione
Boric o-Diani- Benzibp(”C) acida sidineb dineC 82.4 117.5 158 179 229 63.5
102.5 128 168 185 178.1
100 100 100 100 100
97.4 100 100 100 100 100
56.5 102.7 151 150 202 134
a On Chromosorb W AW DMCS, 147 “C. brick, 135 “C. On Firebrick, 154 “C.
100 100 100 100 100 100 96.2 100 100 100 93.6 100 On Fire-
the reactor with retention times increased by 5% and peak areas reduced by about 8%. Benzidine on Chromosorb W AW, and on Chromosorb W AW DMCS, removed the aldehydes but failed to remove the ketones efficiently. Boric acid was first reported as a subtractive agent for alcohols by Ikeda et al. (9). Bierl et al. (4) first reported the use of o-dianisidine and benzidine, both on Chromosorb P, for subtraction of aldehydes, and aldehydes plus ketones, respectively. Haken et al. (8), however, found that benzidine on Chromosorb P (and on Celite and a silanized support) was effective for removing aldehydes but not for ketones. Haken et al. (8)reported quantitative removal of aldehydes and ketones by sodium bisulfite on Celite. In the present work, sodium bisulfite on silanized and on untreated Chromosorb W AW failed to subtract the aldehydes and ketones effectively. Sodium bisulfite mixed with coated support material was equally inefficient; the amounts subtracted generally decreased with increasing reactor temperature from 65 to 165 “C. In summary, the microreactor described has been successfully used for the selective removal of alcohols with boric acid, aldehydes with o-dianisidine, and aldehydes plus ketones with benzidine. No significant broadening or distortion of the unsubtracted peaks occurred. The reactor capacities were generally sufficient to allow a week of continual daily use before the reagents had to be replaced.
LITERATURE CITED (1)
M. Beroza and M. N. Inscoe, in “Ancillary Techniques of Gas Chromatography”, L. S.Ettre and H. M. McFadden, Ed., Wiley-Intetsclence, New York, N.Y., 1969, p 89.
Leathard and B. C. Shurluck, “Identification Techniques in Gas chromatography”, Wiley-Interscience, New York, N.Y., 1970, p 66.
(2) D. A.
R. R. Allen, Anal. Chem., 38, 1287 (1966). B. A. Bierl, M. Beroza, and W. T. Ashton, Microchim. Acta, 637 (1967). J. FryEka and J. Posp%iI, J . Chromatogr., 67, 366 (1972). D. A. Cronin, J . Chromatogr., 64, 25 (1972). A. S.Ladas and T. S.Ma, Microchim. Acta, 853 (1973). J. K. Haken, D. K. M. Ho, and M. K. Withers, J. Chromatogr. Sci., 10, 566 (1972). (9) R. M. Ikeda, D. E. Simmons, and J. D. Grossman, Ami. Chem., 36, 2188 (1964).
(3) (4) (5) (6) (7) (8)
RECEIVED for review October 7, 1976. Accepted February 3, 1977. The work was supported by the National Research Council of Canada. Contribution No. 451, Department of Plant Science, The University of Manitoba. ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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