2012
Anal. Chem. 1982, 5 4 , 2012-2015
Molecular Emission Spectrometry with Hydride Generation for Determination of Subnanogram Amounts of Arsenic Kazuko Matsumoto" and Kellchlro Fuwa Department of Chemistty, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Arsenlc was generated as hydride gas and Introduced Into an alr-hydrogen flame. Molecular emlsslon In the flame was detected nondlspersively for arsenlc determlnatlon. The method was free from Interference and the detectlon llmlt was 0.2 ng of As. I t was successfully applled to the analyses of rlver water and NBS orchard leaves and coal fly ash standard reference materlals.
It is well-known that the hydride generation technique has greatly improved the detection limits of many hydride-forming elements in atomic absorption, emission, or fluorescence spectrometry (1). The advantages of the technique are as follows: it eliminates matrix elements by selectively generating the gaseous hydride of analyte from sample solution and enhances the sensitivity by concentrating the hydride in a trap. The detection limit of arsenic, for instance, in atomic absorption spectrometry was 0.63 pg/mL when the solution was directly nebulized into a flame (2). However, owing to the development of the hydride generation technique, the detection limit was improved to 8 ng (0.8 ng/mL) by introducing the hydride into an argon-hydrogen flame (3). Molecular emission or absorption spectrometry at high temperature has been generally considered as less sensitive and less selective as a tool for elemental analysis, compared with atomic absorption or emission spectrometry. Although there are some exceptions where molecular emission or absorption is effectively used for the determination of nonmetallic elements (4-9, molecular emission or absorption has not generally attracted much interest of analytical spectroscopists. In the present study, hydride generation is combined with molecular emission in an air-hydrogen flame. The usual drawback of molecular emission, that is, the low sensitivity, is circumvented by adopting a nondispersive detection system. In spite of the nondispersive detection, the system is very selective, free from interference, owing to the hydride generation. Although the method would be applicable to various hydride-forming elements, the feasibility is tested for the determination of arsenic in the present study. Henden et al. examined the molecular emission spectra observed when various hydrides are introduced into a hydrogen diffusion flame (8), but their analytical feasibilities have not yet been fully studied. Braman et al. reported a method, in which hydride generation is utilized for SnH emission spectrometry. They determined alkylated tin compounds as well as inorganic tin in natural waters (9). They measured the emission nondispersively and obtained a detection limit as low as 0.01 ng of Sn. The SnH emission band a t 609.5 nm is farily strong with a linelike shape and, in this respect, far more favorable for trace elemental analysis than any other hydride-forming elements. Other hydrides give, in most cases, only very broad continuous emission spectra in the visible and ultraviolet region (8). Our attempt is to demonstrate that even such continuous broad bands can be utilized for a very sensitive and selective method when it is combined with hydride generation and a nondispersive detection system. The method 0003-2700/82/0354-2012$01.25/0
has been successfully applied to the determination of arsenic in river water and NBS coal fly ash and orchard leaf standard reference materials. Although the present report is concerned only with arsenic determination, the interference study discussed later in this paper shows that other hydrides can be determined in a similar way without any interference from each other. EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the hydride generation molecular emission system is presented in Figure 1. The hydride generated in the reaction vessel is carried on He gas flow into an air-hydrogen flame. The quartz burner is constructed basically according to the design reported by Braman et al. (9). A band-pass interference filter with 3 cm diameter, maximum transmission of ca. 33% at 500 nm, and half width of 10 nm was used for wavelength selectivity. The connecting section between the burner housing and the photomultiplier tube housing was cooled with an electric fan in order to protect the filter from the heat. The photomultiplier tube used was R-456 from Hamamatsu T V Co., Ltd., and the photocurrent was measured with an electrometer TR-8651 from Takeda Riken Co., Ltd. The dc power supply for the photomultiplier tube was HTV C448A from Hamamatsu T V Co., Ltd. The top and the bottom parts of the quartz burner out of the housing were covered with a piece of black cloth or painted black in order to prevent the room light from being detected. The room light caused slight photocurrent if no protective measures were taken. Many investigators have used a heater around the hydride trap in order to vaporize the hydride rapidly. However, no such heating system was adopted in the present system, since there was no severe broadening of the signal peak observed without the heater. Moreover, the use of the heater diminished the separation effect based on the difference of the boiling points. If the heater was used, some of the emission signals from other hydrides, which were also trapped together with ASH,, overlapped on the As signal peak as mentioned later in the interference study. A dry ice-acetone bath was used as a water trap in order to remove the water vapor from the reaction solution. Although the boiling point of ASH, (-55 OC) is higher than the trap temperature (-78 "C), there was no significant decrease of the signal observed, probably due to the fact that the carrier gas flow was fast enough that ASH, cannot be cooled down and be trapped while it passed through the trap. Helium was used as carrier gas, since its boiling point (-268.9 OC) is lower than that of N2(-195.82 "C) and, therefore, the ASH, trap cannot be clogged when it is immersed in liquid Nz with the carrier gas flowing in it. The NaOH trap is also inserted in the system in order to remove COz,which is from the impurity in NaBH, reagent and causes the emission signal. Chemicals. All the chemicals used were of analytical reagent grade. The stock solution of arsenic (1mg/mL) was prepared by dissolving As203 in a minimum amount of 2 M NaOH and diluting the solution with a small amount of concentrated HCl to the final HC1 concentration of 0.1 M. A 4% NaBH4 solution was prepared by dissolving weighed N&H4 in 2% NaOH solution. Reagent grade HCl contains small amounts of As or any other impurity that causes a slight blank signal. Accordingly, HCl was treated with a small amount of NaBH4and was bubbled with He gas for 20 min before use. This procedure completely removes the blank signal. Procedure. Ten milliliters of sample solution acidified to 0.1 M with HCl was placed in a 25-mL reaction vessel. After the solution was bubbled for 30 s with He gas for degassing, a 4 % NaBH4 solution was allowed to flow into the reaction vessel for 0 1982 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
2013
He+
liq.
NaBH4
solution
bath
Perlstaltlc pump
Flgure 1. A schematic diagram of arsine generation-molecular emission measurement system.
I
Flgure 2. A microcomputer-controlled monochromator system for emission measurement. 3 min at a flow rate of 1.5 mL/min. After switching off the peristaltic pump, the reaction was continued for 30 s more in order to completely trap the Ihydride. Following the completion of trapping, the liquid N2 bath was removed from the trap and the hydride evaporated was introduced into the flame. Measurement of the Emission Spectra. In order to select the best wavelength for analysis, the emission spectrum was measured with continuously generating ASH* Arsenic (10 pg/mL) in 1M HCl solution and 1% NaBH4 in 1% NaOH solution were continuously injected into the reaction vessel by two peristaltic pumps. With this generation system, the emission intensity was very s m d and accordingly a monochromator with high luminosity was necessary for measurement. The microcomputer-controlled monochromator system used is shown in Figure 2. The monochromator was CT-10 from Japan Spectroscopic Co., Ltd., with the focal length of 10 cm. 'The measurement was carried out with the slit width of 100 pm, !which corresponded to the resolution of ca. 1.5 nm. The microcomputer PC-8001and 1/0 unit PC-8012 were from Nippon Electric Co., Ltd. The line printer, GP 80 M, was from Seiko. The stepping motor, 2CPH034, for driving the grating was from Oriental Motor Co., LM. Each step of the motor rotation corresponded to 1.8O (0.5 nm). The current amplifier was 427, Keithley. The emission intensity was measured every 0.5 nm from 300 nm to 650 nm. The measurement period at each point was 500 ms.
RESULTS1 AND DISCUSSION Selection of the Wavelength. The emission spectra of the flame background and that obtained with ASH3 gas introduced are shown in the Figure 3. In the flame background emission spectrum, there is a strong OH band near 310 nm and in the region above ,350nm, no other emission band was observed, although slight photocurrent was detected. This negligibly low backgrouind emission is very favorable to obtaining a high S I N ratio in emission spectrometry. In the spectrum observed when ASH3 was introduced, there was a broad continuous molecular band, maximum at 420-440 nm and 480-520 nm, which was very similar to that reported by Henden et al. (8) when they introduced ASH3 into the hydrogen diffusion flame arid also by Fujiwara et al. (IO),who introduced AsHS into a flow-type furnace-hydrogen diffusion flame. The broad peak seems to be ascribed to one or more arsenic molecular speciee such as As0 or As2, but definite
uc
&n
Icc
I*
W4VELENGTU(nml
Figure 3. Emission spectra from (a) air-hydrogen flame background and (b) ASH,. assignment was impossible. The wavelength used for analysis was determined to 500 nm, after comparing the signal peak height by changing the interference filter by 10 nm from 400 nm to 540 nm. Optimization of the Experimental Conditions. The dependence of analytical sensitivity on the concentration of HC1 was examined. The analytical sensitivity increases linearly with the HCl concentration from 0.01 M to 0.04 M, above which no increase was observed. The concentration of HCl was determined to 0.1 M. It is reported that reduction of arsenate (As6+) or arsenite (As3+) to ASH3 with NaBH, is largely affected by the pH of the solution (11, 12). As3+is reduced with maximum efficiency in the pH range of 0 to 5, whereas As6+is reduced a t 0 to 1. As6+ cannot be reduced a t pH above 3. In the present experiment, the pH of the solution, previously acidified to 0.1 M with HC1, was about 1after injection of the NaBH4 solution. Therefore, with the present experimental condition, total arsenic including As3+ and As5+ is measured. The trapping period was varied from 1 to 5 min. The increase of the signal peak was observed from 1to 3 min, but no further increase was observed above 3 min. Accordingly, the following experiments were carried out with 3-min trapping. The analytical sensitivity was largely dependent on the flow rate of He carrier gas as shown in Figure 4a. As the flow rate was increased from 100 to 700 mL/min, which was the upper limit of the flowmeter used, the signal peak height was increased 10 times. However, at a flow rate of 700 mL/min, the flame was unstable and sometimes extinguished on introduction of the hydride gas. Hence, the flow rate was usually fixed at 600 mL/min. The H2 flow rate is expected to affect the structure and the temperature of the flame, and, as a result, affect the optimum observation height for analysis. Therefore, true optimization must include multiple optimization of these factors. However, this situation extremely complicates the optimization procedure and increases the number of variables to be examined. The signal dependence of Hz flow rate was, for the moment, studied only with the observation height fixed at 1.5 cm above the burner tip and the air flow rate fixed at 400 mL/min. The result is shown in Figure 4b. Since the sensitivity is higher as the flow rate is smaller, the Hz flow rate was fixed at 100 mL/min in the following experiments. The effect of the air flow rate was also studied. The result is shown in Figure 4c. The sensitivity was maximum at the flow rate of 450 mL/min. Although the signal peak heights were compared with the inlets of Hz and air exchanged, the signal was about 30% higher for the present system. In order to determine the optimum observation height, it was varied from 0 cm to 1.8 cm above the burner tip. The maximum sensitivity was found at 1.4 cm as shown in Figure 4d.
2014
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Table I. Interferences of Inorganic Ions and Organic Compounds with Determination of Arsenic by Hydride Generation-Molecular Emission Spectrometry a concomitant
concn, &/mL
none Cr3+ Cr6+
100 100 100
Mn2+ Fe3+
100
~ 1 3 +
0
100 200 300 400 H, f l o w rate (rn,rnin-') ib )
Nil+
1 10 1 1
cuz+
10
co2+
Zn2+ Ti4+ Ge4 Sn4 Pb2+ Sbs+ Bi5+ Se4+ Te6+ CH,OH CH,C1 +
0
200
400
60
a i r flow rate crnl.rnin-18 (C)
Figure 4. The dependence of the emission intensity and the flame temperature on He, H,, and air flow rates and on the observation height: filled circles indicate relative emission intensity and open circles flame temperature; (a) H, 100 mL/min and air 450 mL/min, observation height 1.5 cm; (b) air 400 mL/min and He 400 mL/mln, observation height 1.5 cm; (c) H2 100 mLlmin and He 480 mL/min, observatlon height 1.5 cm; (d) H, 100 mL/min, air 400 mL/min, and He 500 mL/min. The arrows indicate the point of the maximum sensitivity and its corresponding flame temperature.
The temperature of the flame was measured as a function of the flow rates of He, Hz, and air and also of the burner height by using an alumel-chrome1 thermocouple. The resulta are shown in Figure 4. Among the four variables, He flow rate proved to rarely affect the temperature. The temperture was always maximum at 0.5 cm above the tip at any given gas flow rate. The error of the temperature was estimated to 1 1 5 "C, which was mainly due to the error in the reading of the height of the thermocouple placed in the burner. The highest possible temperature of the flame was nearly 900 "C. However, as the arrows in Figure 4 show, the temperature that gives the best sensitivity was around 610 "C. Although hydrogen flames have also been used for the detection of arsenic atoms by atomic absorption or atomic fluorescence spectrometry, the conditions which lead to the best sensitivity in these atomic spectrometries would be different from those of the present molecular emission spectrometry. However, no comparative studies regarding the best conditions for the present molecular emission spectrometry and other atomic spectrometries were carried out. Detection Limit and Dynamic Range. The detection limit obtained under optimum conditions was 0.2 ng, which corresponded to 20 pg/mL when a 10 mL sample solution was used. The calibration curve was linear up to lo4 ng and the relative standard deviation of 10 repetitive measurements was 0.052% a t the 10-fold concentration of the detection limit. Interference Study. The effects of various inorganic cations, anions, and organic compounds were investigated with 10 ng/mL As solution. The result is shown in Table I. As Table I shows, relatively large negative interference occurs when Co2+, Ni2+,or Cu2+exists at the 10 pg/mL level. These interferences are also reported for the hydride generationatomic absorption method (13)and are due to the reactions
10
cs,
re1
intens 100 96 95
101 98 83
102 24 98 57 64
1 100
91
100
118
10
105 208 98 97
0.01
10
10
96
10
101
10
95 92 96 92 98 > 200
10 0.8 1.5 0.8 1.2
Interferences were examined with a 10 ng/mL As solution. a
of these elements with NaBH4. Although Ge also gives an emission signal, there is no interference observed from this element, since GeH4 can be separately vaporized. Other hydride-forming elements, Sb, Pb, Bi, Se, or Te, give no significant signal at the present experimental conditions. However, slight negative interferences are observed for Se and Te at the concentration level of 10 pg/mL. Tin is the only element that causes severe positive interference. The boiling point of SnHl (-52 "C) is close to that of ASH, (-55 "C); however, the two hydrides would be separated by gas chromatographic methods as reported in ref 9. The effects of several typical organic compounds were also examined. No severe interferences were observed except for CS2,which gave a large positive interference probably due to the emission of Sz molecule in the flame. Determination of Inorganic Arsenic in River Water and Total Arsenic in NBS Standard Reference Material Samples. In order to evaluate the applicability of the present method to real samples, we determined inorganic arsenic, that is As3+ and As5+, in river water and total arsenic in NBS standard reference materials, coal fly ash and orchard leaves. The river water was taken from Tama River, which runs through the southwest part of Tokyo. The water was acidified to 0.1 M with HC1 immediately after sampling and was brought to the laboratory. As pointed out by Braman et al., water-soluble alkylated arsenic compounds such as methylarsonic acid, CH3AsO(OH)2,or dimethylarsenic acid, (CHJ2AsO(OH), exist in natural waters (12). These compounds are also reduced to corresponding hydrides and trapped together with AsH3 in a liquid Nz bath. The boiling points of these hydrides are -55 OC for AsH3, 2 "C for CH3AsHZ,and 35.6 "C for (CH3)zAsH. Therefore, these hydrides can be separated at the vaporization step and do not interfere with the determination of inorganic arsenic in natural waters. Although these alkylated hydrides were also detected in the present system, further detailed study was not carried out. Although the trapping period was determined to 3 min using simple arsenic solution, it proved that the trapping period had
2015
Anal. Chem. 1982, 54, 2015-2017
Table 11. Analytical Results of Inorganic Arsenic in River Water samplea
concn, ng/mL
1 2
0.E5 IT 0.03 0.48 = 0.02 0.42 t 0.02
3
% recovery
92 98 95
a Samples 1, 2, and 3 were taken from different points 3,ecoveries were determined with on the Tama River. the addition of 1 ng/mI. As solution.
highly sensitive and selective method for elemental analysis, when it is combined with hydride generation and nondispersive detection system. An attempt to apply the present method to other elements is now in progress.
ACKNOWLEDGMENT The authors are indebted to K. Fujiwara of our department for his help in the construction of the computer-controlled monochromator system. Thanks are also due to J. Takahashi of Seiko for the valuable discussion and also to M. Kurosawa of our department for his help in the analyses of real samples.
Table 111. Determination of Arsenic in NBS Coal Fly Ash and Orchard Leaf Standard Reference Materials amt certified found, value, sample rg/g Pglg coal fly ash, NBS SRM 1633a 145s 15 141 I 8 1 2 I 0.6 14 t 2 orchard leaves, NBS S R M 1571
to be extended to 5 min when real samples were analyzed. The reduction reaction seems to be retarded, when real samples are used, due to the matrix effect. However, if the reaction period is long enough, there is no problem in analyzing real samples. The analyses were carried out by the standard addition method and the results obtained were in good agreement with the certified values as shown in Tables I1 and 111. The present study demonstrates the possibility that even broad continuous molecular emission bands can be used as
LITERATURE CITED Robbins, W. B.; Caruso, J. A. Anal. Chem. 1979, 51, 889A-899A. Perkin-Eimer Corp. Reprint AA-332G “Technlque and Application of Atomic Absorption,” 1978. Thompson, K. C., Thomerson, D. R. Analyst (London) 1974, 99, 595-601. Tsunoda, K.; Fujiwara, K.; Fuwa, K. Anal. Chem. 1977, 49, 2035-2039. Tsunoda, K.; Fujiwara, K.;Fuwa, K. Anal. Chern. 1978, 50, 861-865. Everett, G. L.; West, T. S.; Williams, R. W. Anal. Chim. Acta 1974, 68, 387-394. Haraguchi, H.; Fuwa, K. Anal. Chem. 1976, 48, 784-786. Henden, E.; Pourreza, N.; Townshend, A. Prog. Anal. At. Spectrosc. 1979, 2 , 355-372. Braman, R. S.;Tompkins, M. A. Anal. Chern. 1979, 57, 12-19. Fujiwara, K.; Bower, J. N.; Bredshaw, J. D.; Winefordner, J. D. Anal. Chim. Acta 1979, 109, 229-239. Feldman, C. Anal. Chem. 1979, 50, 664-669. Braman, R. S.; Johnson, D. L.; Foreback, C. C.; Ammons, J. M.; Bricker, J. L. Anal. Chem. 1977, 49, 621-625. Smith, A. E. Analyst (London) 1975, 700, 300-306.
RECEIVED for review May 14,1982. Accepted July 16,1982.
Elucidation of Metanephrine to Normetanephrine and Epinephrine to Norepinephrine Ratios by Fluorescence Derivative Spect romet ry Robert H. Christenson * and C. Davlci McGlothlln* Department of Chemistry, Eiorlda State University, Tallahassee, Florida 32306
Flrst and second derlvatlve fluorescence spectroscopy was carrled out over the 325-380-nm excltatlon reglon on fluorescent products of the metanephrlnss and catecholamlnes. A constant 505-nm emission wavelength was used. Determlnatlon of methanephrlnle to normetanephrine and eplnephrlne to norplnephrlne ratlos was done by calculatlons using flrst and second derlvatlvra fluorometrlc spectra. A catlonexchange material was used to separate splked aqueous and urine solutlons of the metanephrlnes and catecholamlnes; subsequent spectra analysis yielded ratlos wlthln 5 % experlmental error.
Urinary determinations of epinephrine and norepinephrine (the catecholamines), and their 0-methylated metabolites metanephrine and normetanephrine (the metanephrines), are Present address: Departments of Laboratory Services, Duke University, and Durham Veterans Administration Medical Centers, 508 Fulton Street, Durham, bTC 27705. 2Present address: P.O. BOK13193, Tallahassee, FL 32308. 0003-2700/82/0354-2015$01.25/0
of considerable interest in the laboratory-assisted diagnosis of pheochromocytoma: a catecholamine secreting tumor of the adrenal medulla. “High-pressure” liquid chromatographic methods are available to separate the individual metanephrines (1) and catecholamines (2). However, the clinical information offered by differing proportions of metanephrine to normetanephrine and epinephrine to norepinephrine is incompletely understood. Success in isolating the catecholamines and methanephrines from urine was demonstrated by Sandhu and Freed (3)using a weakly acidic cation-exchange material. A modification of their method (3),using prepacked ion-exchange columns, is available through Bio-Rad laboratories (Richmond, CA) (4). Fluorescence spectroscopy has been used for detection and quantitation of total metanephrines (5-7) and total catecholamines (4). Two of these methods (4, 7) use the Bio-Rad ion-exchange columns to accomplish separation of the catecholamines and methanephrines from a urine matrix. Derivative spectroscopy was used as early as 1955 for resolving spectral lines of nearly equal wavelength (8). The technique has since found use in diverse applications including detection of erbium in the presence of cerium (9) and mea0 1982 American Chemical Society
