Anal. Chem. 1985, 57. 1706-1709
1706
Table 11. Recovery of Selenium(1V)after Decomposition of the Sample with Sulfuric Acid-Hydrogen Peroxide and Reduction with Boiling Hydrochloric Acid with and without a Stream of Nitrogen" recovery of Se (%)
decomposition nitrogen no.
flow
1 2
yes
3 4
no
no no
after eiapsed time 0.3 h 3 h 20 h 45 h 93 h 142 h 95 92 94 95
96 102 89 56 80 59 89 64
98 35 38 48
100 18
99 9
23 34
12 22
" Influence
of the time between decomposition reduction and determination bv hvdride-generation AAS.
to 6570, reached after about 5 days. The elimination of chlorine by flushing the decomposition apparatus with nitrogen during boiling with hydrochloric acid proved to be the simplest and most effective way of preventing the back oxidation of Se(1V). For this purpose, a nitrogen flow of 5 L f h was applied in the modified decomposition apparatus (see Experimental Section). With a boiling time of 15 min, an equilibrium concentration of 97 f 1%for Se(1V) was obtained. It could only insignifficantly be improved (98 f 1%) when boiling was prolonged to 1h. These recoveries remained the same within experimental error for an extended delay between the reduction and the hydration of up to 3 weeks, for wastewater samples as well as for reference solutions. These recoveries can be considered as high and reproducible enough for the purpose of the determination of trace elements in environmental analysis. We were not successful in our efforts to obtain Se(1V) recoveries greater than 99%. In a final set of experiments, the procedure elucidated by the radiotracer technique was applied to the determination of selenium by hydride-generation atomic absorption spectrometry. The results obtained without and with using a stream of nitrogen during boiling of the decomposed sample solution with hydrochloric acid are given in Table 11. Aliquots of the decomposed model solutions 2,3, and 4 were taken after
45 and 142 h, respectively, and were heated to boiling with hydrochloric acid in an open flask, whereby recoveries of 98 f 2% of selenium were obtained by using hydride-generation AAS. This demonstrates that the above described modification of the hydride-generation method for the determination of selenium is essential for achieving accurate results. It can also be seen that the strongly time-dependent back oxidation of tetravalent to hexavalent selenium by residual chlorine has been the actual reason for erroneous results obtained by hydride-generation AAS for selenium and not losses during the decomposition or hydration or other factors as often supposed. CONCLUSIONS
When hydrogen peroxide is used as one of the reagents for sample decomposition in the determination of selenium, chlorine is formed during a successive reduction of Se(V1) to Se(1V) by boiling the sample solution with hydrochloric acid. The residual chlorine remaining in the sample solution oxidizes Se(1V) back to Se(V1) at room temperatures within hours to days. This effect can cause considerable errors because hexavalent selenium cannot be converted to selenium hydride under the conditions used. This effect can be eliminated by the removal of chlorine with a stream of nitrogen during the boiling step with hydrochloric acid. Registry No. Se, 7782-49-2; HzO, 7732-18-5.
LITERATURE CITED (1) Verlinden, M.; Deelstra, H.; Adriaenssens, E. Taianta 1981, 2 8 , 637-646. (2) Slnemus, H. W.; Melcher, M.; Welz, B. At. Spectrosc. 1981, 2 , 81-86. (3) Welz, B.; Melcher, M. Analyst (London) 1984, 109, 569-572. (4) German Standard Methods for the Examination of Water, Waste Water
(5) (6) (7) (8) (9) (10)
and Sludge; Anious (group D); Determination of Selenlum by Atomic Absorption Spectrometry; Draft, Beuth Verlag GmbH, Berlin, 1984. Welz, B.; Melcher, M. Vom Wasser 1984, 62, 137-148. Welz, B.; Melcher, M. Spectrochim. Acta 1981, 36, 439-462. Welz, B.; Melcher, M. Anal. Chim. Acta 1981, 131, 17-25. Trapmann, H.;Devani, M. B. Naturwlssenschaffen 1981, 48, 405. Shumb, W. C. I n d . Eng. Chem. 1949, 4 1 , 992-1003. Bretschger, M. E.; Shanley, E. S. Trans. Electrochem. SOC.1948, 92, 67-76.
RECEIVED for review December 6,1984. Accepted February 25, 1985.
Automatic Determination of Aluminum in Biological Samples by Inductively Coupled Plasma Emission Spectrometry Yves Mauras and Pierre Allah*
Laboratoire de Pharnacologie, C.H. U., 49040 Angers Cedex, France An automatic method for aluminum determlnation In human blood, dlaiysis fiuld, and water by inductlveiy coupled plasma emission spectrometry is described. Background correction Is obtalned by automatlc displacement of the entrance slit. Variations In emisslon signal Intensity In matrix of dlfferent composltlons are cancelled by addltion of ceslum as a matrix modlfier and by using gallium as an internal standard. A detectlon ilmit of 0.3 pg/L in pure solution, large range ilnearlty (more than IO'), good reproduclbliity, automatlclty, and rapidity make the method partlcularly convenient for routine aluminum determlnatlon. Comparison wlth graphite furnace atomic absorption spectrometry showed very good correlations In blood and water samples. Plasma blood AI In 23 healthy subjects was 7.0 f 3.1 (standard deviation) pg/L In agreement wlth recent ilterature values. 0003-2700/85/0367-1706$01.50/0
Aluminum levels in blood, in dialysis fluids, and in water for patients under hemodialysis are regularly monitored to avoid risks of encephalopathy and osteopathy. Aluminum determinations are generally carried out by using the graphite furnace atomic absorption technique which is known to be fairly sensitive. Parallel to this method and for improved efficiency, in our laboratory we use a technique developed in 1979 based on emission spectrometry with a high-frequency inductively coupled plasma source (ICP) (I). The present article describes improvements of this technique based on a computerized instrument capable of simultaneous multielement assays. EXPERIMENTAL SECTION Apparatus. The equipment used consists of a multielement JOBIN-YVON JY 48 spectroanalyzer with a 1.5-kW high-fre0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 1707 Table I. Standard I1 Solution Composition blood Na, mg/L (NaCl)" K, mg/L (KC1)" Ca, mg/L (CaCl,)" Mg, mg/L (MgCW S, mg/L (H2S04)" P, mg/L (HJ'Od' C, g/L (glycerine)b
2000
1600 60 40
1500 300 100
10000
plasma dialysis fluid tap water 3300 150 100 20 1000 150 40
3300 150
50
100
25 10 10
20 50 50 5
I
20
10 0.5
o---o
quency (27.12 MHz) inductively coupled Plasma-Therm source. The 1 m focal length vacuum polychromator including a 2160 grooves/mm holographic grating has a dispersion of 4.6 A/mm. The electronic measuring instrument is connected to a PDP 11 computer. Samples on an automatic sampler are pumped with a Gilson Minipuls 2 pump and nebulized by means of a concentric pneumatic nebulizer. The displacement of the entrance slit, hand or computer operated, allows analysis of spectra over about 10 A on either side of the analytical line. Working Conditions. The analytical line and the working conditions giving the best sensitivity for aluminum assays are as follows: wavelength, 3961.5 A; power, 1.3 k W argon coolant flow gas, 15 L/min; nebulization pressure, 30 psi with argon flow of 2 L/min; observation height, 15 mm above the induction coil. Under these conditions, a 1mg/L solution of aluminum gives a signal to background ratio of 70. Reagents, The 1g/L aluminum reference solution used was obtained from Merck (aluminum chloride Titrisol). Cesium chloride and hydrochloric acid were Merck Suprapur. Gallium solutions were prepared with pure gallium metal from Prolabo. All the dilutions were made with osmosed demineralized water in plastic vials washed with Ultrapur nitric acid from Prolabo. Procedure. Samples of blood, plasma, and dialysis fluids are diluted to 1/10 in demineralized water and acidified using hydrochloric acid to obtain a final acid concentration of 1% . Water for dialysis is assayed without dilution after similar acidification. Gallium and cesium are added to each sample so as to obtain a final concentration of 0.5 mg/L of gallium and 2.5 g/L of cesium. Calibration is carried out with the same procedure using two solutions: the first is composed of demineralized water and the second is composed of a synthetic matrix whose composition is close to that of blood, typical dialysis fluid, or ordinary tap water. Aluminum is added daily to this second solution to obtain concentrations of 200 pg/L for blood, plasma, and dialysis fluid calibration and 40 pg/L for water calibration (Table I). For each sample, the emission signal is measured at the maximum and at the base of the aluminum peak.
Cs and 81
m....... cs c--. 61
" Suprapur from Merck. RP Normapur from Prolabo.
wlthnut 6 1 nor Cs
&----A
0 ' ' 0.02
0.05
0:2
0.15
0:1
MATRIX CONCENTRLllON
Flgure 1. Signal varhtion for a constant quantity of aluminum (20 MIL) with and without addition of gallium and cesium as a function of the matrix composition which varies gradually (0 in pure water; 0.2 In
dialysis fluid 1/5). WATER 400000
t
BlOOO
/
40000
I
O---O
WLTER
r
=
0.99191
c/
0
100
200
1000
500
AI
CONCENTRATION
pg/L
Flgure 2. Calibration graph for aluminum determination in blood and in water samples (blood scale is 10-fold lower than water scale due
RESULTS Background a n d Signal Correction. As previously described ( I ) , for aluminum determination there is no line
to the 1/10 dilution factor).
overlap but significant background modification from the matrix elements, especially from calcium which is present in high levels in blood, dialysis fluids, or water. This effect is corrected for each sample by subtracting the continuous background measured a t the base of the aluminum peak by automatic displacement of the entrance slit. Under the operating conditions described, the intensity of the signal given by a constant quantity of aluminum varies as a function of the composition of the matrix, the signal intensity being most increased by alkali and alkaline-earth elements. This interference is corrected by the use of a standard solution whose composition is close to that of the solution being tested and by the addition of cesium and gallium to each sample. The cesium increases the signal emission and reduces variation due to different matrix composition while gallium is used as an internal standard. Figure 1shows that the signal given by a constant amount of aluminum varies by the ratio of 1to 3 as a function of the matrix composition. The addition of cesium, increasing the signal, reduces the variation in the ratio of 1to 1.2, and gallium
as internal standard with cesium makes the relative signal become practically independent of the matrix composition. Calibration Curve. The aluminum calibration graphs for the different fluids under assay are remarkably linear for concentrations in a range much larger than that used for the actual studies (Figure 2). Detection Limit. The detection limit calculated as equivalent in concentration to twice the standard deviation of the background signal (2,3)is 0.3 pg/L in pure solutions (water samples) and 3 bg/L in solutions diluted to 1/10 (blood, plasma, and dialysis fluid samples). Reproducibility. The within-day reproducibility was obtained by ten successive assays of the same sample of water or blood containing around 80 pg/L of aluminum. The coefficients of variation were 0.6% for water samples and 3% for blood samples. The day to day reproducibility was obtained for water and blood samples by carrying out an assay each day for 10 days under routine conditions. The coefficients of variation were 2 to 3 % for water samples and 3 to 30% for blood samples,
1708
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Table 11. Recovery of A1 Added to Water Samples
water sample no.
Na, mg/L
K, mg/L
Ca, mg/L
Mg, mg/L
