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Anal. Chem. 1987, 59,2822-2827
for a single level has already been given. It should be noted that a working technique based on the spiking of two sample levels and solving the resulting MOSA functions configured with W,/ units simultaneouslycould be used as an assay procedure but reflection will indicate that there would be no real time or effort advantages.
SUMMATION The articles on the CEC technique have all utilized the MOSA configured with W , units and it has been shown that the W , abscissa units can be a concentration, mass, or dimensionless term. Also, the ordinate response can be uncorrected for constant error, partially corrected, or fully corrected. Proper calculation based on the published concepts can then be made for each respective situation. Ferrus makes no mistakes in his circuitous and unnecesary mathematical treatments but he unfortunately erroneously concludes that the only correct MOSA must be configured with W,/Wz,u units. Also, he loses sight of the CEC objectives and again erroneously concludes that the P factor is either questionable or of no value. It has been shown that a MOSA configured with W,/ Wz,u units is just another MOSA that could be useful in specific situations. Any stand-alone MOSA techniques such as those discussed by Bader, properly corrected for the TYB constant error, are acceptable and valid. However, for the purposes of the CEC technique, the treatment in the CEC articles stands. Using a MOSA configured with W,/ Wz,uunits, the writer has developed a fundamental proof that the Youden one-sample curve is a valid procedure for the determination of the true blank, defined in the CEC as the total Youden
blank, TYB. This proof was an omission in the CEC presentations, as pointed out by Ferrus, that the writer had heretofore been unable to resolve. With this proof in place, the true functionality of the MOSA, as shown in either Figure 5B of ref 4 or Figure 1, is firmly established.
ACKNOWLEDGMENT I am grateful to Ricard Ferrfis for his dedicated analysis of the CEC articles and for this communication which has required a strengthening and extension of the CEC concepts. In particular, for the idea that permitted the proof of a vexing problem. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
(8) (9)
Ferriis, Ricard Anal. Chem., preceding paper In this issue. Cardone, M. J. J . Assoc. Off. Anal. Chem. 1983, 66, 1257-1282. Cardone, M. J. J . Assoc. Off. Anal. Chem. 1983, 66, 1283-1294. Cardone, M. J. Anal. Chem. 1988, 58, 438-445. Cardone, M. J.; Lehman, J. G. J . Assoc. Off. Anal. Chem. 1985, 68, 199-202. Cardone, M. J. Anal. Chem. 1988, 58, 433-438. Horwitz, W. Nomenclature of Sampling in Analytical Chemistry; IUPAC Commission, Vol. 3, Analytical Nomenclature, Provisional Proposal, 14th Draft, 1986, 16 May. Larsen, I. L.: Hartmann, N. A.; Wagner, J. J. Anal. Chem. 1973, 45. 1511-1513. Rothe, C. F.; Sapirstein, L. A. Am. J . Clin. Pathol. 1955, 25, 1076- 1089.
Mario J. Cardone Chemistry Department State University of New York Binghamton, New York 13901 RECEIVED for review December 1, 1986. Accepted July 22, 1987.
Room-Temperature Phosphorimetry of Carbaryl in Low-Background Paper Sir: Carbaryl (1-naphthyl N-methylcarbamate) is widely used as an insecticide throughout the world and is frequently determined by use of a molecular absorption spectrophotometric method, based on the reaction of 1-naphthol-the carbaryl hydrolysis product when in alkaline medium-with p-nitrobenzenediazonium tetrafluoroborate (1). Room-temperature spectrofuorometry has also been suggested and applied to "real-life" samples by several authors (2-9). Lowtemperature (77 K) phosphorimetry (LTP) was first proposed by Moye and Winefordner (IO),and very recently solid-surface room-temperature phosphorimetry (SSRTP) using either common filter paper (11, 12) or ion-exchange filter papers (Whatman DE-81 and P-81) was also examined (13). Kirkbright and Shaw (11)examined the SSRTP of carbaryl in Titer paper (Whatman No. 30) using carbaryl solutions in 1 M NaI-1 M NaOH, in aqueous ethanol (1:l). A 3.8-fold enhancement was observed after the heavy atom treatment. There is no information in the paper if the addition of NaOH produced modification in the spectrum as a result of carbaryl hydrolysis. The limit of detection and the lifetime of carbaryl were not determined. More recently, Vanelli and Schulman (12) investigated the SSRTP of several pesticides, including carbaryl, using different solvents (such as acetonitrile, 0.1 M NaOH in aqueous methanol, and 0.1 M HC1 in aqueous methanol). Also, the presence of sodium acetate and heavy atom salts (NaI, lead acetate, thallium fluoride) in Whatman No. 42 filter paper 0003-2700/87/0359-2822$01.50/0
was examined. The best result was found with acetonitrile and 1M NaI + 1 M sodium acetate. A limit of quantifkation (defined as the spotted mass equivalent to a signal which is twice the blank signal) of 1000 pg was found (12). Su and co-workers (13) determined the SSRTP analytical figures of merit of six pesticides, including carbaryl, using ion-exchange filter papers. The combination of DE-81 (an anionic exchange type) and iodide produced the best limit of detection (800 pg) and a linear dynamic range larger than 200. SSRTP is an analytical technique that has been growing in the recent years as a promising and simple tool for the trace level detection and determination of several organic compounds such as pharmaceuticalproducts, pesticides, polycyclic aromatic hydrocarbons, etc. Several reviews and books have been recently written dealing with the theoretical and practical aspects of this interesting analytical technique (14-1 7). Although much simpler to work with than low-temperature phosphorimetry,the broad background signal, which is usually present in the substrates (in the wavelength range of ca. 400-600 nm), has been one of the limitations of the SSRTP technique for trace analysis application. To avoid this problem to a certain degree, a series of paper substrates with low background signal was examined by Paynter, Wellons, and Winefordner (18). Lignin, hemicellulose, or even transition metals traces have been suggested as possible sources of the background signal present in cellulose substrates (19). Several attempts have been made to decrease this signal, to some 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
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Table I. Phosphorescent Background Signal Reduction with Extracted Whatman No. 1 Chromatograph Paper after Ultraviolet Treatment air irradiation time, h 4
8 12
%
R"
88.2 f 3.0 91.6 k 3.0 93.4 f 3.8
oxygen %
RSDb
ozone
%R
% RSD
%R
3.4 3.2
90.0 f 2.3 90.4 f 3.0
2.5
4.1
91.5 f 2.8
87.6 f 2.7 86.8 f 1.9 86.0 f 2.6
3.3 3.1
%
RSD 3.1 2.2 3.0
*
% R, phosphorescence signal reduction, in percentage (nine determinations). % RSD, relative standard deviation in percentage (nine
determinations). extent, using solvent extraction or chromatography, chemical or heat treatment, or signal background subtraction, with the help of a computer-assisted spectrophosphorimeter, by Winefordner and co-workers (20). An ultraviolet irradiation treatment (at 285 nm) of filter paper was recently and successfully employed by McAleese and Dunlap (21),when reductions of 78% (after 3 h of exposure) or 96% (after 57 h) were attained. When the SSRTP determination of carbaryl in analytical papers available was attempted in our laboratory, high limits of detection were found because of the large background signal present. As a result, we examined the SSRTP behavior of carbaryl, 1-naphthol, and p-aminobenzoic acid in paper substrates which had been previously treated for reduction of the background by using a two-step procedure-water extraction followed by ultraviolet exposure. With this low background substrate, the influence of irradiation, heavy atoms, and drying temperature in the analyte phosphorescent signal were investigated. Analytical figures of merit were also determined.
EXPERIMENTAL SECTION Apparatus. The SSRTP intensity measurements and the excitation and emission spectra were determined by using an Aminco-Bowman spectrophotofluorometer (SLM Instruments, Urbana, IL) equipped with an Aminco-Keirs rotating-can phosphoroscope attachment. A 150-W xenon arc lamp (Canrad-Hanovia, Newark, NJ) was used as an excitation source,and a potted 1P21 photomultiplier (Hamamatsu Corp., Middlesex, NJ) with an 54 spectral response was employed as a detector. An SLMAMINCO microphotometer was used to amplify the signal. The amplified signal was fed to an X-Y recorder (Model 7010B, Hewlett-Packard, Palo Alto, CA). A laboratory-constructed sample holder was used. For the daily intensity calibration of the instrument we used the room-temperature phosphorescent signal from a commercial "phosphor" pigment (zincand cadmium sulfide-Lumilux-Green, Riedel-De-Ha&, Seelze-Hanover,FRG), embedded in a film of poly(methylmetacrylate)as a reference (22). A flow of nitrogen or argon dried by being passed through a silica gel bed was introduced between the measurements in the phosphoroscope chamber. During the measurements, the flow was redirected to the front of the paper surface by changing the position of a gas valve. For the lifetime measurements, the output signal for the microphotometer was fed to a storage osciloscope (Model R 503 N, Tektronic,Beaverton, OR). The paper treatment in the ultraviolet range was carried out in a Rayonet photochemical reactor (The Southern N. E. Ultraviolet Co., Middletown,CT) using five lamps emission at 254 nm and seven with A, at 300 nm. with A, Ozone used in the paper treatments was generated in a laboratory ozonator (Model T-408, Welsbach Ozone Systems, Co., Philadelphia, PA). Chlorine was provided from a cylinder (White Martins, Brasilia, Brazil). Reagents. Standard carbaryl from Union Carbide (Union Carbide Agricultural Products, Research Triangle Park, NC) and a secondary standard (prepared from technical carbaryl after several crystalizations in acetone) were used. The secondary standard was compared with the Union Carbide standard by using infrared spectroscopy, melting point (140-142 "C), and the SSRTP phosphorescent characteristics. 1-Naphthol (Aldrich Chemical Co., Milwaukee, WI) was further purified by using vacuum sublimation, yielding a product with a melting point of 94-95 "C (lit.
95-96 "C) (23). p-Aminobenzoicacid (Sigma Chemical Co., St. Louis, MO) and other reagents were used as received. For the heavy atom effect study, the salt solution concentrations were determined by using iodatometry (for Tl(1)) and EDTA complexometry (for lead). Distilled, demineralized, and further bidistilled (in glass) water was used throughout the experiments. Ethanol (E. Merck, Darmstadt, FRG) was refluxed for 24 h with zinc-KOH and later distilled. Whatman (Whatman Laboratory Products, Clifton, NJ) No. 1chromatograph paper and Whatman No. 40 filter paper were employed in the experiments. Background Reduction Treatment Procedure. Pieces of the chromatograph or filter paper were water extracted in a Soxhlet apparatus for 8 h. After extraction, the strips (17 X 50 mm) were introduced into quartz tubes (15 X 150 mm) and exposed to ultraviolet irradiation for 8 h. Procedure. Two microliters of the analyte solution (in ethanol-water, 10/90 (v/v)) were spotted in the center of a paper substrate (cut in 10 x 17 mm rectangular pieces) which had been in some cases previously spotted with 2 p L of the heavy atom solution (also in ethanol-water, 1O/W (v/v)). The paper was then vacuum dried at room temperature for 1.5-2.0 h. After being dried, the spotted paper was stored in a glass desiccator with Pz05 protected from light until it was introduced in the instrument.
RESULTS AND DISCUSSION By use of Whatman No. 1chromatograph paper, preliminary attempts were carried out in the direction of the decrease of the background signal, using either water extraction or chemical treatments with aqueous solutions of hydroxilamine hydrochloride (5%, w/v), HzS04(0.2, 2, and 4 M), H20z (7 and 30%) or ozone saturated water. The best results obtained were in the treatment with water or with ozone saturated water or 2 M H2S04(ca. 60% reduction). The results obtained with ozone were unexpected, as, according to literature, lignin is degraded in the presence of ozone (24). We later examined the effect of light and it was found that ultraviolet exposure was extremely beneficial for the reduction of background signal as described recently by McAleese and Dunlap (21). In a further experiment, we associated the water extraction step with the ultraviolet (UV) irradiation step. As a result, an overall reduction of ca. 97% (using 8 h of water extraction and 12 h of irradiation (in a photochemical reator) was attained. As the chromatograph paper used was from an old batch, we also examined these procedures with a new batch of Whatman No. 40 filter paper and similar results were obtained, ca. 57% reduction after water extraction and 94% reduction after 1 2 h of ultraviolet exposure, resulting in an overall reduction of 97.4%. The effect of irradiation time and the presence of air, oxygen, ozone, and chlorine were also investigated. Table I shows the results obtained by using Whatman No. 1chromatograph paper. In the presence of air the largest reductions were after 8 and 12 h of irradiation, although these results were statistically similar ( P = 0.05; Nl = N z = 9). With oxygen, the reductions a t 4, 8, or 12 h were also statistically similar. Statistical comparison of the results obtained with oxygen and air showed no significant differences. With ozone, the results after 4,8 and 12 h were smaller than the results obtained with air or oxygen and a small increase in the background signal
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
ScQIe ID X
A
300
coo
A
