454
Anal. Chem. 1904, 56,454-457
t (e.g., 2.5 X 10” m), and A , (e.g., 0.0084 m2), and (2) the FSEM, including L (0.023 m) and A (0.032 m2),the quantity (1 + PL)is 1.06. A small dependence upon the separation of the sorbent and permeation source is indicated. Registry No. CH20,50-00-0;urea-formaldehyde, 9011-05-6. LITERATURE CITED (1) Calvert, J. G., Chairman ”Formaldehyde and Other Aldehydes”; Committee on Aldehydes, Board of Toxlcology and Environmental Health Hazards, Natlonal Research Council; Natlonal Academy Press: Washington, DC, 1981. (2) Spengler, John D., Chairman “Indoor Pollutants”; Committee on Indoor Pollutants, Board of Toxicology and Environmental Health Hazards, National Research Council; National Academy Press: Washington. ---.. DC. - - , 1981. -. (3) Matthews, T. G.; Hawthorne, A. R.; Howell, T. C.; Metcalf, C. E.; Gammage, R. B. Envlron. I n t . 1982, 8 , 143. (4) Taylor, D. G. “Manual of Analytical Methods”; Natlonal Institute for Occupational Safety and Health: Washington, DC, 1974 (Vol. I), 1961 (Vol. 7). (5) Wallace, L. A.; Ott, W. R. “Personnel Monitors: A State-of-the-Art Survey,“ JAPCA 1982, 3 2 , 601. (6) Bowen, R. P.; Shlrtliffe, C. J.; Chown, G. A. “Urea Formaldehyde Foam Insulatlon: Problem Identification and Remedlal Measures for Wood Frame Construction”; Building Practice Note No. 23, Division of Bulldlng Research, National Research Councll, Canada. (7) “Tentative Small Scale Test Method for Determining Formaldehyde Emission from Wood Products, 2-hr Desiccator Test FTM-1”; National Particleboard Assoclatlon, Hardwood Plywood Manufacturers Associatlon, 1982. (8) AATCC Test Method 112-1976 “Formaldehyde Odor in Resin Treated Fabric, Determination of: Sealed Jar Method”; AATCC Technical Manual, 1978; pp 89-90, (9) Matthews, T. G.; Hawthorne, A. R.; Daffron, C. R.; Reed, T. J.; Corey, M. D. “Formaldehyde Release from Pressed-Wood Products”; Proceedings; Washington State University, Proceedings, International Particleboard Symposlum, Pullman, WA, 1983.
(IO) Hawthorne, A. R.; Gammage, R. 6.; Dudney, C. S.;Womack, D. R.; Morris, S. A.; Westley, R. R.; Gupta, K. C. “Preliminary Results of a
(11) (12) (13) (14) (15) (18) (17) (16) (19) (20) (21) (22)
Forty Homes Indoor Alr Pollutant Monitoring Study”; Proceedings, International Speciality Conference on the Measurement and Monitoring of Non-Criterla Contaminants in Air, Chicago, IL, 1983. Miksch, R. R.; Anthon, D. W.; Fanning, L. 2; Hollowell, C. D.; Fevzan, K.; Glanvllle, J. Anal. Chern. 1981, 5 3 , 2118. Matthews. T. G.; Daffron, C. R.; Corey, M. D. “Formaldehyde Surface Emission Monitor Protocol I: Pressed-Wood Products”; ORNL/TM8658; Oak Ridge Natlonal Laboratory, Oak Ridge, TN, 1983. Bulletin, General Electrlc Co. “Permaselectlve Membranes”; 1968; No. 10. Matthews, T. G. “Evaluation of a Modifled CEA Instruments, Inc., Model 555 Analyzer for the Monitoring of Formaldehyde Vapor in Domestic Environments,” AIHAJ 1982, 4 3 , 547. Hawthorne, A. R.; Gammage, R. B. JAPCA 1982, 3 2 , 1126. Osborn, S. W.; Lee, L. A.; Heller, H. L.; Hiilman, E. E.; Coiburn, G.; Landau, P.; Thorne, E. J.; Krevitz, K. Technical Report F-C5316-01; The Franklin Research Instltute: Philadelphia, PA, 1981. Matthews, T. G.;Howell, T. C. Anal. Chem. 1982, 54, 1495. Palmes, E. D.; Gunnlson, A. R.; DIMattio, J.; Tomczyk, C. Am. Ind. Hyg. Assoc. J . 1978, 3 7 , 570. American Society for Heating, Refrigeration, and Air Conditloning Engineers, 1961, Standard 62-1981. Bevington, P. R. “Data Reduction and Error Analysis for the Physical Sciences”; McGraw-HIII: New York, 1989. Crank, J. “The Mathematics of Diffusion”; Clarendon Press: Oxford, 1975. Myers, G. E., submitted for publication in For. Prod. J .
RECEIVED for review April 25,1983. Accepted October 6,1983. Research sponsored jointly by the U S . Consumer Product Safety Commission under Interagency Agreement CPSCIAG-82-1297 and the Office of Health and Environmental Research, U.S. Department of Energy, under Contract No. W-7405-eng-26 with the Union Carbide Corp.
Extraction Procedure for the Determination of Trace Chromium in Plasma by Proton-Induced X-ray Emission Spectrometry Monique Simonoff,* Yvan Llabador, Charles Hamon, and G . N. Simonoff Chimie NuclBaire, ERA 144, Centre d’Etudes Nuclgaires de Bordeaux- Gradignan, 331 70 Gradignan, France
A method Is descrlbed for the determination of Cr3+ In blood serum by proton-Induced X-ray emission (PIXE) after wet ashlng and an extractive enrichment procedure. Wet ashlng of 5 to 10 mL of human plasma Is carried out wlth nltrlc, perchloric, and sulfuric acids. Chromlum(V1) Is complexed with ammonium pyrrolldlnecarbodlthloate-methylIsobutyl ketone (APDC-MIBK) at a pH of 2.4. After evaporatlon of the organic phase the metallic residue Is dissolved In nltrlc and hydrochloric acids. A known volume of a 50 mg/L nickel solutlon Is added as an Internal standard and the soiutlon reconcentrated. The few resldual mlcroliters are deposlted on a polycarbonate foil. Recovery for thls preconcentratlon step Is measured with the use of carrier-free “Cr. Satlsfactory values are found for chromlum In three NBS Standard Reference Materlals. The detection llmlt of thls analytical procedure is about 0.3 ng/mL.
Determination of chromium in biological samples would appear to be of great importance, because of the involvement of Cr in the glucose tolerance factor (GTF) and in the etiology of cardiovascular diseases ( 1 ) . There is general, but not universal, agreement that concentrations of chromium in blood and urine are about several nanograms per gram. However,
large variations in published results suggest that continuous improvement of analytical techniques is taking place. The rapid development of analytical methods in the past decade has created many powerful tools for trace-element research. These tools need to be adapted to the biomedical field. This paper describes an original method for the measurement of chromium in plasma or other biological media based on proton-induced X-ray emmission (PIXE), after wet ashing and an extractive enrichment procedure. The method described has been employed (2) for the determination of chromium levels in plasma from 150 human subjects, of whom 100 suffered from coronary artery disease.
EXPERIMENTAL SECTION Reagents. Ammonia solution and nitric, hydrochloric, sulfuric and perchloric acids were redistilled Merck “Suprapur” reagents containing less than 2 x lo-’% of chromium. The 2 % solution of ammonium pyrrolidinecarbodithioate (APDC) in water and the organic phase, methyl isobutyl ketone (M1BK)-xylene (3:1), w e r e Fluka reagent grade, used without further purification. “High-purity” water was obtained after three distillations of doubly deionized water (two in a quartz still). This water was kept in quartz bottles or polyethylene flasks only and never for m o r e than 2 or 3 days. Such water, purified by subboiling distillation in quartz, has a chromium concentration of 0.01 ng/g.
0003-2700/84/0356-0454$01.50/00 1964 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
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X RAY DETECTOR
Flgure 1. A view of the PIXE apparatus In Bordeaux.
Radiochromium 'Wr in the form CrC13at pH 2.5 was obtained from the "Commissariat 5 1'Energie Atomique" (C.E.A., Gifsur-Yvette). The solution contained 0.5 pg of stable chromium for 250 pCi/mL. Fifty microliters of a solution (1/1OOO) was employed as tracer, i.e., less than 0.1 ng was added to the sample to determine the enrichment yield. Chromium Decontamination and Glass Cleaning. Contamination-free sampling and storage before analysis is essential for ng/g determinations (3). All manipulations were performed in a specially designed dust-free hood, located in a "clean" room. Prior to use, all Pyrex containers were cleaned as follows: decontaminated by soaking overnight in 2% RBS (Traitement Chimique de Surfaces, France) solution, and then rinsed twice with doubly deionized water and three times with "high-purity" water. All Pyrex material (tubes and flasks) was then dried in a Pyrex oven at 450 "C and then boiled with Suprapur H2S04 (1%):HN03 (1%) for several hours in a large Pyrex beaker, followed by three successive rinses with "high-purity" water. Contact with the hands was avoided at all times. Sampling and Storage. The frozen plasma samples were kept in water-washed polyethylene tubes, as described by Anand and Ducharme (4). Polyethylene was used for storage of acid solutions and quartz for standard solutions (Ni, 50 mg/L). Blood Collection. Blood sample consisted of 10 to 20 mL, collected in a heparinized polyethylene tube. The samples were centrifuged (20 min, -300 g) and the plasma was retained and frozen in a polyethylene tube. Instruments and Apparatus: PIXE. The system of analysis of trace elements by proton-induced X-ray emission was entirely contructed in our laboratory and is shown in Figure 1. A beam of 2.5-MeV protons from a 4-MV Van de Graaff accelerator is diffused by 2 p thick aluminum foil, situated 0.3 m from the target. A collimation system of carbon apertures defies the diffused beam on the target, which is positioned at 45" to the incoming beam. This arrangement ensures that a constant area of the target is irradiated with a beam of uniform intensity. The surface area of the beam is 10 mm2. After passing through the thin targets, the beam impinges on a Faraday cup which is employed with an electrometer to measure the beam current. The carbon collimator situated before the target has a triple function. It is the last diaphragm for the beam before the target, an electron suppressor when the current is directly measured on the target (thick targets), and it is a collimator for the detectors. X-rays emitted at 90" to the beam direction emerge from the vacuum chamber through a 25-pm aluminized Mylar window and traverse an air space before detection. Appropriate graphite X-ray absorbers are positioned in this air-space to reduce the intensity of the low-energy X-rays associated with biological samples. For chromium detection, a thickness of about 18-20 mg/cm2 is sufficient to reduce the K, Ca X-rays. The targets (ten on the holder) are irradiated under vacuum (10-6-104 mbar). The inside of the chamber is entirely covered with plastic foil, overlaid with a very thin evaporated carbon film. By use of an electrostatic, one-demand, beam deflection device for the X-ray detector system, pulse pile-up is kept low and no explicit dead-time corrections had to be made.
Photon spectroscopy is carried out by means of a 100 mm2 intrinsic germanium X-ray detector, with a resolution of 180 eV (fwhm) for 5.9-keV =Fe X-rays. The use of the Ge detector rather than Si-Li avoids the problem of the Si K a "escape peak" which could otherwise arise from the Fe KO line. Inside the chamber, a surface barrier (Au-Si) detector, for particle elastic scattering measurements and for monitoring purposes, is situated at 135" to the beam direction. The detector signals are fed through conventional amplifying and pulse processing circuits and the spectra are registered on magnetic tapes, using PDP 11 minicomputer systems. For a chromium peak measurement, the results are, in general, statistically satisfactory after 10 to 20 min of irradiation (integrated current, 5 to 10 pC). The sensitivity of the system is of the order of to lo-'(' g for all elements with 2 1 14. Chemical Procedures. The destruction of plasma and the chromium oxidation is an extension of the method of Chao and Pickett (5), adapted for about 10 mL of plasma. Destruction of Plasma: Wet Digestion. Five to ten milliliters of plasma is exactly weighed and placed in a 100-mL volumetric flask. The polyethylene vial is ultrasonicated with 1mL of ultrapure water for 2 min, and this water is added to the plasma with 50 pL of a radiochromium solution (containing less than 0.1 ng of CrC13) to determine the yield of enrichment. The flask is allowed to stand overnight at 100 "C in a Pyrex oven. Two milliliters of HNOBand 500 pL of 96% HzSO4 are added and heated to 120 "C for about 1 h on a ceramic hot plate. About 0.5 mL of a black solution is obtained when most of the nitric acid has been removed by evaporation. To complete the digestion, 500 pL of perchloric acid and 1mL of nitric acid were added and the mixture is heated to 240 "C (to evaporate the perchloric acid) until dense fumes of sulfur trioxide are evolved. The mixture is colorless and about 0.5 mL of liquid remains. Chromium Oxidation. To the colorless solution are added 2 mL of water, 1drop of HzS04 and 1drop of KMn04 (1%). The oxidation from Cr(II1) to Cr(V1) is completed by heating gently for 5 min. After the mixture is cooled 100 pL of 2 mol/L HCl is added to destroy excess permanganate. To ensure complete oxidation to &(VI), 50 pL of 0.1% KMnO, solution is added at a pH of about 2.4. The solution is then heated ( 100 "C) and becomes turbid with brown MnOz, which is removed by centrifuging. The pH is adjusted by adding 1 drop of bromocresol green indicator and then concentrated ammonia solution, 1 mol/L HzS04,diluted ammonia (1/30), and 0.1 mol/L H,SO,, as needed, until the yellow color of the indicator is just attained (pH about 3.8). The indicator is sometimes destroyed by oxidation, especially when adding ammonia, and it is then necessary to add one drop more. One milliliter of 0.1 mol/L H#04 is added to give the fiial pH of 2.4 for the extraction step. Extraction. The solution at pH -2.4 (about 8 mL) is transferred to a conical tube with glass stopper and vigorously shaken for 2 min with 250 pL of 2 % ammonium pyrrolidinecarbodithioate (APDC) in water plus 500 p L of methyl isobutyl ketone-xylene (31). After elimination of the aqueous phase, the N
456
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984 ,
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Table I. Determination of Chromium in NBS Standard Reference Materials samples
procedure pure NBS sample then digestion, extraction, and concn, on target
tomato leaves (1573) -50mg 4.4
t
Pg/g
oyster tissue water (1566) (1643a) -50mg 5mL
0.2 0.75 t 0.12 17.4 t 2.2 Pglg nglg
NBS sample and 5 3.9 t 1.1 0.54 f 0.31 19.8 i 5.6 mL plasma then Pglg Pglg nglg digestion, extraction, and concn on target 16.2 i 1.5 pure NBS sample evaporated without nglg chemical treatment value certified by 4.5 i 0.5 0.69 i: 0.27 17 ?- 2 NBS wglg Pglg nglg the chromium to be determined by means of a simple ratio and eliminates the complex calculations involving the proton flux, the X-ray production cross sections, the detector efficiency, etc. Results f o r S t a n d a r d Reference Materials. Three National Bureau of Standards Standard Reference Materials (tomato leaves, oyster tissue, and water) were analyzed repeatedly during the development of the method (Table I). Fairly good agreement with the certified values was found for tomato leaves, oyster tissue, and water, with or without plasma. To check for loss of plasma chromium in acid-insoluble residues, we treated some plasmas with hydrofluoric acid after the nitric-sulfuric-perchloric digestion step. The H F treatment did not release more chromium and the chromium plasma levels obtained were the same with these two wet ashing methods. The limitation of the digestion (wet ashing) method results from chromium contamination of the acids. We cannot obtain purified acids with less than 2 ng of Cr/g, so that dry ashing would be preferable. Some authors have shown (9) that there is no loss of chromium by volatilization in dry ashing. However, our results for dry and wet ashing methods showed some differences which were not very reproducible. We found a loss of chromium from about 0 to about 30% for an aliquot of the solution in "OB after ashing at 500 OC. Perhaps the dry ashing increases the retention of chromium in acid-insoluble residues. We did not test this possibility. Thus we chose a wet digestion method and substracted the chromium
Anal. Chem. 1004, 56, 457-462
due to contamination from acids and chemical reagents by the use of many blanks. These were found to contain Cr contents in the 5 to 10 ng range. PIXE Analysis. For elements such as V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, the PIXE method can detect quantities of less than 1ng OII the target. The limit of detection for a given element depends on the X-ray emission (counting) rate of the next element on the spectrum. In the case of chromium, a high manganese level on the target is a serious problem. However, the main limitation of the present method for obtaining 1ng in a suitable form on the target is found in the preliminary sample preparation. With plasma volumes of 10 or 20 mL, we can easily measure 1 ng/mL, and even 0.3 ng/mL. Targets can be prepared and kept for 1or 2 months in a desiccator covered with aluminum foil to avoid damage to the polycarbonate from light.
ACKNOWLEDGMENT We gratefully acknowledge the cooperation of Magendie’s Internal Medicine Center. Special thanks are due to C. Conri’s department for the collection of many blood samples. The
457
authors also thank A. MacKenzie Peers for help with the manuscript. Registry No. Chromium, 7440-47-3.
LITERATURE CITED (1) Mertz, Walter I n “Chromium Nutrition and Metabolism”; Shapcott, D., Hubert, J., Eds.; Eisevier/North-Holland: Amsterdam, 1979: pp 1-14. (2) Slmonoff, M.; Llabador, Y.; Slmonoff, G. N.; Besse, P.; Conrl, C. Nucl. Instrum. Methods in press. (3) Speecke, A.; Hoste, J.; Versieck, J. “Sampling of Blologlcal Materials”; Bureau of Standards: washington, DC, 1976; NBS Special publication 422 (Proc. 7th IMR Symp., Gaithersburg, Oct 7-11, 1974), p 299. (4) Anand, V. D.; Ducharme, D. M. “Accuracy in Trace Analysis”; Bureau of Standards: Washington, DC, 1976; NBS Special Publicatlon 422, pp 611-619. (5) Chao, S. S.; Plckett, E. E. Anal. Chern. 1980, 52, 335. (6) Jones, G. B.; Buckley, R. A,; Chandler, C. S. Anal. Chlm. Acta 1975, 80, 369-392. ( 7 ) Mien, M. R.; Fishman, M. J. At. Absorpt. News/. 1967, 6 , 128-131. (8) Subramian. K. S.; MOranger, J. C. Int. J . Envlron. Anal. Chem. 1979, 7 , 25-40. (9) Versleck, J.; Hoste, J.; DeRudder, J.; Barbier, F.; Vanballenberghe, L. Anal. Left. 197% 12, 555-562.
RECEIVED for review July 11,1983. Accepted November 22, 1983.
Characterization of Peak Shape Parameters with Normal and Derivative Chromatograms Kyung-Hoon Jung,*Sun Jin Yun,and Sung Hoon Kang
Department of Chemistry, Korea Advanced Institute of Science and Technology, P.O.Box 150, Chongyangni, Seoul 131, Korea
The objective of the present study is to develop and assess a new method to extract peak shape parameters from the exponentially modified Gaussian peak model. This method requires both normal and derivative peak heights at four or five different time points with the same time Interval. The essential peak shape parameters can be readily evaluated by solving the cubic or quartic equation of T. Computer simulation studies showed that in the case of a peak-to-peak noise value of 1.0 %, the peak parameters were reproducible wtlhln 1.5% by the method developed In this study. The experimental works with the real chromatograms have shown that the parameters were recovered with the standard deviations no more than 1.57%.
The chromatographic peak shape is governed by several factors such as the characters of the column material and the external operation conditions of the column. The simplest input profile of 6 distribution is broadened asymmetrically and appears to be a skewed Gaussian form owing to the nonequilibrium mass transfer in column ( l ) dead , volume (2), nonhomogeneity of tube connection (3), and time lag of the detector-amplifier system ( 4 5 ) . The output peak profile has been well described, by adopting an exponentially modified Gaussian peak (EMGP) model, by several workers (4,6, 7). In the present study we report a new method to extract peak shape parameters from the EMGP model and the validity of the method by computer simulation study and experimental observations. 0003-2700/84/0356-0457$01.50/0
EMGP can be expressed as the convolution of a normal Gaussian with an exponential decay function
h(t) =
where A is the peak area, a is the standard deviation of the Gaussian component, t R is the retention time of the Gaussian component, and T is the time constant of the exponential modifier. In eq 1the EMGP model is valid over the whole range of time t and characterized fully by four peak parameters, Le., A, T , a, and t p These peak shape parameters reflect the process occurring in column and contain all the information necessary to optimize the column resolution (6, 7),the efficiency ( I ) , and the deconvolution of the overlapped peaks. A and tR determine the scale and the position of chromatographic peaks, respectively, while the asymmetricity of the peak depends upon the ratio of T to a. Some intensive studies have been made to find these parameters including the least-squares curve fitting technique (6,8),an alternative approach of the least-squares curve fitting technique (9, l o ) ,the moment analysis (2, 11-13), a modified moment analysis technique (14), and the graphical method (15). These techniques, however, suffer some degree of cumbersome extensive iterative search calculation and computerized data acquisition. In these regards our new technique has focused its attention on eliminating the cumbersome iterative procedure utilizing the normal and derivative chromatograms. The derivative chromatogram can be obtained 0 1984 American Chemlcal Society
