Anal. Chem. 1004, 56,439-442 Hanekamp, H. B.; BOS,P.; Frei, R. W. Trends Anal. Chem. 1982, 1 , 135- 140. Levlch, V. 0."Physiochemicai Hydrodynamics": Prentice-Hall: Englewood Cllffs, NJ, 1962 pp 112-1 16. Weber, S. G. J . Electroanal. Chem. 1983, 145, 1-7. Ruzlcka, J.; Hansen, E. H. "Flow InJectlon Analysis"; Wlley: New York, 1961. Meschi, P. L.; Johnson, D. C. Anal. Chlm. Acta 1881, 124, 303-314.
439
(9) Blaedel, W. J.; Olson, C. L.; Sharma, L. R. Anal. Chem. 1883, 35, 2100-21 03. (10) Lown, J. A.; Koile, R.; Johnson, D. C. Anal. Chlm. Acta 1980, 116, 33-39.
RECEIVED for review October 3,1983. Accepted November 28, 1983.
Nonaqueous Solvents as Carrier or Sample Solvent in Flow Injection Analysis/Atomic Absorption Spectrometry Abdulrahman S. Attiyat Chemistry Department, Yarmouk University, Irbid, Jordan
Gary D. Christian* Department of Chemistry, University of Washington, Seattle, Washington 98195
The use of methanol, ethanol, acetone, and methyl Isobutyl ketone as carriers In flow Injection analysls, wlth atomlc absorption detectlon (FIA/AA), was Investlgated. The determlnatlon of copper was demonstrated as an example. The effect of the sample solvent (acetone or ethanol wlth water) on the FIA/AA signal was also studled by using these carrlers, as well as water carrier. An &fold enhancement of the slgnal is achleved, compared to aqueous systems, when employing the optimum sample-carrler solvent comblnatlon. Acetone was found to be the best sample solvent while MIBK was the best carrier. Preclslon of better than 4 % relatlve standard devlation was readlly obtained. The effect of the length of the dlsperslon coil and of the sample volume on the FIA/AA slgnals by use of the flve different llquld carrlers Is presented.
The determination of the levels of many metals in human body fluids and tissues is increasingly of clinical and diagnostic significance (1-3). Atomic absorption spectrometry enjoys great popularity in the determination of metals in biological materials (I,2 , 4 ) . Some modifications have been introduced to increase the sensitivity and selectivity, such as the use of electrothermal atomization and the use of wavelength modulation (3). Inductively coupled plasma atomic emission spectrometry introduced a new dimension of trace metal determination in analytical chemistry (5,6). Sample preparation is another alternative to bring the level of trace metals in biological materials to the level of detection of atomic absorption. Chelation followed by solvent extraction into a nonaqueous solvent is used to determine some metals in blood, urine, and other biological materials (I,2 , 4 ) . Aspiration of aqueous solutions reduces the flame temperature approximately 40 O C (7).This causes a decrease in the vaporization efficiency of the metal. Organic solvents burn efficiently in the flame and are more efficiently vaporized due to lower viscosity and surface tension. The type of organic solvent selected can significantly influence the magnitude of the atomic absorption signal (8). Flow injection analysis (FIA) has the general advantages of high frequency of sample analysis using microliter volume samples, simplicity, and the capability of accommodating 0003-2700/84/0356-0439$01.50/0
Table I. Experimental Conditions for the FIA/AA System wavelength lamp current slit flame gas flow
burner observation height recorder output
324.7 nrn 1 0 mA 0.1 mm air-acetylene acetylene, 1 L/min air, 8 L/min for water, methanol, and ethanol carriers, 9 L/min for acetone, cyclohexane, and MIBK carriers premix nublizer-burner 7 mm above burner 1 0 mV
different types of detectors (9-11). In this technique, the sample is usually introduced in the form of a plug into a moving stream (carrier) which carries it to the detector. The carrier either acts as a means to simply carry the sample to the detector or includes a reagent, a mixture of reagents, or a catalyst that makes the sample react and carries the plug which contains the products of the reaction to the detector (9,10,12). The use of FIA, with atomic absorption or atomic emission types of detection, has been frequently demonstrated for the determination of metals (12-16). The carriers generally used are either water or water solutions of reagents (12). When introduced to the flame, cooling of the flame and decrease in atomization efficiency are to be expected. The use of organic solvents as carriers and as sample solvents is expected to increase the atomization efficiency and enhance the FIAJAA signal, resulting in lower detection limits. In this paper, the effect of using five different organic solvents compared with water as carriers is described for the FIAJAA determination of copper. The sensitivity is improved %fold with the use of the optimum sample solvent and optimum carrier.
EXPERIMENTAL SECTION Reagents. Copper stock solution (certified atomic absorption standard loo0 ppm concentration from Fisher Scientific Co.) was used to prepare copper standards. Methanol and cyclohexane (distilled in glass) were from Burdick and Jackson Laboratories, Inc., ethanol (dehydrated) was from U.S.Industrial Chemicals Co., acetone (spectrophotometric grade solvent) was from Mallinckrodt, and methyl isobutyl ketone (MIBK) was from Aldrich 0 1984 American Chemical Society
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e
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
70 WATER 100 80 60 0% 40 20 ETHANOL 0 20 40 6 0 80 ! O O % C O M P O S I T I O N OF S A M P L E SOLVENT ( V / V )
Cu ( p p r n )
Figure 1. Calibration curves for the determination of copper standards in water by FIAlAA with (A) water, (B) methanol, (C) ethanol, (D) acetone, and (E) MIBK carriers.
Chemical Co. Aqueous solutions were prepared in distilleddeionized water. Apparatus. A Perkin-Elmer Model 403 atomic absorption spectrophotometer with a strip-chart recorder was used for measurements. A Rheodyne loop injection valve was used for sample introduction in FIA. Teflon tubing of 1 mm internal diameter from Alltech Associates was used in the flow system and for sampling loops. A glass syringe of 3 mL volume was used for filling sampling loops. The loops were 100 pL volume unless otherwise stated. The sample injector was connected to the capillary of the atomic absorption (AA)instrument. The negative pressure of the nebulizer was utilized to draw the liquid carrier from the reservoir to the burner. A dispersion coil of 15 cm length and 1 mm diameter was placed between the injector and the nebulizer. Atomic absorption operating conditions are listed in Table I. Procedure. Standards of copper in appropriate solvents or solvent mixtures were prepared by dilution of the stock solution. A few drops of ethanol were added to the solution of copper in MIBK to dissolve the water drops from the stock solution. The FIA signals with different carriers were obtained by immersing the pumping tube of the flow injection system into the appropriate liquid carrier and injecting the sample solution into the carrier stream using the injector port. The flowing liquid carrier carries the sample plug into the atomic absorption spectrophotometer port. Copper standards in water, acetone, methanol, ethanol, cyclohexane, MIBK, water-ethanol mixtures, and water-acetone mixtures were injected. The flow rate was 2.5 mL/min. The conditions of the FIA/AA system were.the same for all carriers except the air flow rate, which was 8 L/min for water, ethanol, and methanol carriers and 9 L/min for acetone and MIBK carriers.
RESULTS AND DISCUSSION Figure 1depicts the calibration curves for the determination of copper in aqueous solution by using as carriers distilled water, methanol, ethanol, acetone, and methyl isobutyl ketone. The slopes of the calibration curves with different carriers, relative to that using water carrier, were 1.5 for methanol, 1.6 for acetone, 2.0 for ethanol, and 2.4 for MIBK. Precisions were 3.0%, 3.6%, 2.0%, 2.3%, and 3.0% relative standard deviation for water, methanol, ethanol, acetone, and MIBK carriers, respectively. The results with cyclohexane as a carrier liquid were unreproducible and consequently no calibration curve using this carrier was established. The enhancing effect of the water-miscible solvents is due to the favorable organic solvent effect as the sample plug mixes with the carriers, and acetone is the most favorable solvent (8). With MIBK, the solubility of water is small, although the solvents are not totally
Flgure 2. The effect of sample solvent composition on the FIA/AA signal of a 10 ppm copper solution in water-ethanol (v/v) mixtures, uslng (A) water, (B) methanol, (C) ethanol, (D) acetone, and (E) MIBK carriers.
- ' 8C u)
z
3 70
a
2
60
Q
50
cm
40 f
30 20 I
I
I
I
i
0% 80 60 40 20 WATER 100 40 6 0 8 0 100% ACETONE 0 20 COMPOSITION OF SAMPLE SOLVENT ( V / V )
Flgure 3. The effect of sample solvent composition (v/v) on the FIA/AA signal of 10 ppm copper in water-acetone mixtures, using (A) water, (B) methanol, (C) ethanol, (D) acetone, and (E) MIBK carriers.
immiscible, and the large enhancement is probably due to a combination of an organic solvent effect and limited dispersion due to the limited solubility. Without the limited dispersion, and would expect acetone to provide the greater enhancement (8).
Figure 2 depicts FIA/AA signals of 10 ppm copper solution in water-ethanol mixtures of compositions ranging from 100% water to 100% ethanol (V/V), using the different carriers. It is clear that the signal is enhanced when the ratio of ethanol in the same solvent mixture is increased. This is the case with all carriers and was most appreciable above 80% ethanol. The relative effect of the carriers on the maximum FIA/AA signal enhancement, when the water-alcohol mixtures were used as sample solvents, was the same as when water was used as a sample solvent, i.e., MIBK > ethanol > acetone > methanol. Acetone gave greater enhancement than ethanol, though, when the sample composition was 20% or more water. Figure 3 shows the results of a similar experiment, but with the copper sample in water-acetone mixtures. The same trend of signal enhancement, with increase in the fraction of the organic solvent in the sample solution, was observed. The relative
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
441
Table 11. Relative FIA/AA Signals for Different Sample Solvent-Carrier Combinations a carrier sample solvent
H,O
water methanol ethanol acetone MIBK
1
MeOH EtOH acetone
5.8
1.8 6.3
5.4 7.2 5.6
5.6 8.0 5.0
2.5
6.3 5.5 8.2 6.2
1.9 7.4 6.4 6.9 5.1
MIBK 2.9 7.1 6.9 8.9 5.9
a The signal for the aqueous-aqueous system is taken as unitv.
"
0
50 100 S A M P L E V O L U M E (PI)
150
Flgure 5. The effect of sample volume on the FIA/AA signal of 8 ppm copper solution in water, using (A) water, (B) methanol, (C) ethanol, (D) acetone, and (E) MIBK carrlers.
B
30 8P P ~ half
scale w
m e 15 X
0
0
m
l !
IO
a
5
15 35 55 75 95 L E N G T H O F D I S P E R S I O N COIL ( c m )
Flgure 8. The effect of the length of the dispersion coil on the FIA/AA signal of a 10 ppm copper solution in water, using (A) water, (B) methanol, (C) ethanol, (D) acetone, and (E) MIBK carriers.
TIME-
H Imin
Flgure 4. FIA/AA signals of copper in acetone using MIBK carrier (A) and of copper in water using water carrier (B). All the measurements were taken at 0.25 A full scale, except the last standard in A where 0.5 A full scale was used.
effects of the carriers on FIA/AA signal enhancement were preserved, except that the roles of acetone and ethanol were reversed a t greater than 60% water. Obviously, a greater initial effect occurs with a water-miscible carrier when the solvent added to the sample is the same as the carrier solvent. Table I1 shows the relative values of the FIA/AA signals of a 10 ppm copper solution using all solvent-carrier combinations of water, methanol, ethanol, acetone, and MIBK. The values are listed to the signal of the aqueous sample using water carrier taken as unity. It is obvious that acetone is the best sample solvent while methyl isobutyl ketone is the best carrier. A glass piston syringe was necessary to inject samples dissolved in nonaqueous solvents, especially acetone and MIBK, since rubber pistons are affected by these solvents. Figure 4 shows the FIA/AA signals of copper standards in acetone using MIBK carrier (A) and of copper standards in water using water carrier (B). The measurements were taken under the same conditions, listed in Table I. All the signals were taken by using 0.25 A units full scale sensitivity, except the last standard in A (8 ppm) was taken by using 0.5 A units full scale. It is clear that the signals using MIBK as carrier are much sharper than those obtained with water as carrier. A plot of calibration curves for each set of data is linear, passing through the origin. The relative response for the acetone-MIBK measurements was eight times that for the aqueous measurements. Precision was 2.3% relative standard
deviation for the determination of copper in acetone using MIBK carrier. The use of the aspiration power of the nebulizer circumvented technical difficulties of pumping organic solvents. However, the precision utilizing negative pressure drive is not as good as can be achieved by positive propelling or by air pressure. The latter should result in precision of 1%or better. Effect of Sample Volume. Figure 5 shows the effect of the sample volume on the FIA/AA signal of 8 ppm copper solution in water with all the carriers used in this study. The sample volume has the least relative effect when MIBK carrier is used as the volume is increased beyond 10 pL. This is probably due to limited solubility of water in MIBK. Note that the enhancing effect with increased sample volume is initially greater when the water-miscible carriers are used, being the most significant up to about 30 p L volume. With these carriers, the effect of the increasing sample volume is not very significant beyond 100-pL samples. The enhancement is due to increased sample presented to the flame, approaching steady state, counterbalanced by diminished organic solvent effect as dilution of the carrier is increased. With acetone carrier, the best FIA signal is obtained when sample volumes between 50 and 70 pL are used. It was observed that large sample volumes (>150 pL) caused broadening of the FIA/AA signal, with double or triple peaks for each injection, when the organic carriers were used, undoubtedly due to regions of higher and lower organic solvent. This effect is currently being investigated in more detail. Effect of t h e Length of t h e Dispersion Coil. Figure 6 shows the effect of the length of the dispersion coil on the FIA/AA signals of 10 ppm copper solution in water using the different carrier solvents. Although it is clear from all the curves that the shorter the coil the higher the signal, it is also
442
Anal. C h m . 1904, 56,442-448
clear that with MIBK, acetone, and methanol carriers, the length of the coil has relatively little effect on the signal. With water and ethanol the effect is more significant, although the signal with ethanol levels off after 55 cm.In the case of MIBK, the dispersion would be limited by limited water solubility. In the case of totally water-miscible solvents, the organic solvent effect probably counters the increased spreading of the sample plug. Time of Analysis. The FIA/AA peak emerges 4 s after injection. The peak width of the base line was 12 s for water, methanol, ethanol, and acetone carriers. It was only 6 s when MIBK carrier was used. Hence, it is possible to achieve as high as 200 measurementa/h when water, methanol, ethanol, or acetone is used as carrier and in excess of 300 measurements/h when MIBK is used as carrier. It is demonstrated in this study that it is possible to increase the FIA/AA sensitivity by 8-fold if the optimum carriersample solvent combination is used, allowing the analyst to determine trace metals at lower concentration levels, and should be particularly useful when dealing with biological samples. More dilution of the sample with the appropriate solvent is possible. This method is reproducible and does not require extra addition to the equipment or chemicals usually available to the analyst. Registry No. Methanol, 67-56-1; ethanol, 64-17-5; acetone, 67-64-1; methyl isobutyl ketone, 108-10-1; copper, 7440-50-8.
LITERATURE CITED Henry, R. J.; Cannon, D. C.; Wlnkeiman, J. W. “Clinical Chemistry, Principles and Technics”; Harper and Row: Hagerstown, MD, 1974. Tietz, N.; “Fundamentals of Clinical Chemistry”; W. B. Saunders Co.: Philadelphia, PA, 1970. Marshall, J.; Ottaway, M. Talenta 1983, 30, 571. Christian, G. D.;Feidman, F. J. “Atomic Absorption Spectroscopy. Applications in Agriculture, Biology and Medicine”; R. E. Krieger Publishing Co.: New York, 1979. Kniseiey, R. N.; Fassei, V. A.; Butler, C. C. Clin. Chem. (Winston-Sa/em, N . C . ) 1973, 19, 807. Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim.Acta, Part8 1981, 3 6 8 , 251. Bauer, H. H.; Christian, G. D.; O’Reiiiy, J. E. “Instrumental Analysis”; Aiiyn and Bacon: Boston, MA, 1978. Christian, G.0.; Feldman, F. J. Can. Spectrosc. 1989, 14, 1 . Ruzicka, J.; Hansen, E. H. “Flow InJection Analysis”: Wiley: New York, 1981. Ruzicka, J. Philos. Trans. R . SOC.London, Ser. A 1982, A305, 645. Stewart, K. K. Anal. Chem. 1983, 55, 931A. Rocks, B. F.; Riley, C. CXn. Chem. (Winston-Selem, N . C . ) 1982, 28, 409. Rocks, E. F.; Sherwood, R. A.; Riley, C. Clln. Chem. (Wlnston-Salem, N . C . ) 1982, 28, 440. Jacintho, A. 0.; Zaggatto, E. A. G.; Bergamin. H.; Krug, F. J.; Reis, B. F.; Bruns, R. E.; Kowaiski, E. R. Anel. Chim. Acta 1981, 130, 243. Rocks, B. F.; Sherwood, R. A.; Bayford, L. M.; Riley, C. Ann. Clin. Blochem. 1982, 19, 338. Attiyat, A. S.;Christian, G.D. Clin. Chim. Acta, in press.
RECEIVED for review September 29,1983. Accepted December 8, 1983. The financial assistance of Yarmouk University to A.S.A. for this research is gratefully acknowledged.
Isomer-Specific Separation of 2378-Substituted Polychlorinated Dibenzo-p -dioxins by High-Resolution Gas Chromatography/Mass Spectrometry Hans Rudolf Buser* Swiss Federal Research Station, CH-8820 Wtidenswil, Switzerland Christoffer Rappe Department of Organic Chemistry, University of UmeP, S-901 87 UmeP, Sweden
All poiychlorlnated dlbenzo-p-dioxln (PCDD) Isomers containing four and more chlorine substituents were prepared by mlcropyrolysls of chlorophenates. The syntheses included the preparation of all 22 tetra-, 14 penta-, 10 hexa-, 2 hepta-, and octachiorinated species (tetra- to octa-CDD). The gas chromatographlc and mass spectrometrlc properties of these Isomers were studled. Hlgh-resoiutlon gas chromatography (HRGC)on a 55-m Sllar 1Oc glass caplllary column allowed the separation of many of these Isomers and allowed the unambiguous asslgnment of the toxic and environmentally hazardous 2378-substituted isomers (2378-tetra-, 12378penta-, 123478-, 123678-, and 123789-hexa-CDD). Analyses were carried out to determine the occurrence of these lsomers in environmental samples and In fly ash from municipal incinerators.
Table I. Number of Isomers and Substitution Pattern of PCDDs substitution no. of total no. of compounds type ( x : y ) ‘ isomers isomers mono-CDDs di-CDDs tri-CDDs tetra-CDDs pen ta-CDDs
hexa-CDDs hepta-CDDs octa-CDD
l:o 1:l 2: 0 2:l 3:O 2: 2 3:1 4: 0 3: 2 4:1 3: 3 4 :2 4:3 4: 4
2 6 4
12 2 13
2
10
14
8
1 12 2 6 4 2 1
22 14
10 2
1 75
Polychlorinated dibenzo-p-dioxins (PCDDs) are a group of toxic and environmentally hazardous compounds. The tricyclic aromatic compounds are substituted with up to eight chlorine atoms. There is a total of 75 PCDD isomers ranging from the mono- to the octachloro compounds (mono- to octa-CDD) (see Table I).
a x and y are the number of chlorine substitutents in each carbon ring of the dioxin molecule.
Some PCDDs have extraordinary toxic properties (1-4). Toxicity depends on the number and position of the chlorine substituents and is highest for the tetra-, penta-, and hexa0 1984 American Chemical Society
