Anal. Chem. 1987, 59, 2728-2730
2728
as determined by X-ray powder diffraction that may be contributing to the inaccuracy.
% AI
% Ai P
A
Erion
20(84)
ACKNOWLEDGMENT R. D. Shannon, E. F. Moran, and C. Bonifaz are acknowledged for supplying zeolites, C. R. Ginnard, L. Abrams, G. D. Stucky (Du Pont), and C. A. Fyfe (Guelph) for helpful discussions, and S. Bloom and L. Firment for their contributions to this work. Also, the collaborations with K. W. Jones and A. L. Hanson at Brookhaven National Laboratory and M. D. Glascock at University of Missouri are acknowledged. The software used for line-shape fitting of NMR spectra (DEPEAK) was written by M. K. Hanafey (Du Pont). Registry No. Si, 7440-21-3; Al, 7429-90-5.
19(17)
ZSM-5
LITERATURE CITED 52(64)
Ferr.
n
4.3164)
A I
150
#
l
100
~
50
097 (090)
ZSM-5
h l
#
0
I
8
L
I
,
I
-50 150 100 Chemical Shift ( p p m )
,
I
50
,
I
0
~
L
-50
Figwe 4. 27Ai MASNMR spectra of 11 zeolite samples, hydrated over saturated ammonium chloride solutions. The spectra were obtained with 15’ excitation pulses. The total integrated intensities, converted to wt % AI content, are indicated next to the spectra and in parentheses ( %AI),, from the analytical techniques. The intenslty of spinning side bands and of octahedral AI, observed near 0 ppm in the spectra of Y and erionite, are included in the integration.
are significantly lower, showing that even framework A1 positions are highly distorted in these zeolites. Furthermore, the large widths of the peaks in these two spectra show an overall increase of the QCC’s, consistent with framework distortions. The 27Alresults for ferrierite, clinoptilolite, and chabazite show deviations between 19% and 33%. These three samples though do contain significant amounts of Fe(II1) impurity. The effect of the paramagnetic impurity is a broadening of the NMR signal and, hence, an apparent decrease in the aluminum content as measured by this technique. A listing of the Fe contents of the samples is given in Table VII. In the ferrierite case, the sample also contains some microcline,
(1) Breck, D. W. Zeollte Molecular Skves; Wiley: New York, 1974. (2) Seff, K.; Mellum, M. D. J . phys. Chem. 1984, 88, 3560. (3) Keane, M.; Sonnichsen, G. C.; Abrams, L.; Corbin, D. R.; Gier, T. E.; Shannon, R. D. Appl. Cafal. 1987, 32, 316. (4) Samoson, A.; Lippmaa, E. Chem. phys. Leff. 1983, 100, 205. (5) Samoson, A.; Lippmaa. E. phys. Rev. 6 1983. 28. 6567. (6) Thomas, J. M.; Klinowski. J. A&. Catal. 1985, 33, 199. (7) Vega, A. J.; Luz, 2. J. phys. Chem. 1987, 9 1 , 365. ( 8 ) Vega, A. J. ACS Symp. Ser. 1983, No. 218, 217. (9) Robson, H. E. US. Patent 3720753, 1973. (10) Roliman, L. D.; Voiyocsik, E. W. Inorganic Synfhesls; Wiley: New York, 1983; Vol. 22, pp 61-68. (11) Hanson. A. L.; Jones, K. W.; Corbin, D. R. Nucl. Instrm. Mefhods Phys. Res., Sect. 6 1985, 9, 301. (12) Hanna, A. G.; Brugger, R. M.; Glascock, M. D. Nucl. Insfrum. Methods 1981, 188, 619. (13) Farlee, R. D.; Corbin, D. R.; Vega, A. J., presented in part at the 25th Rocky Mountain Conference, Denver, CO, Aug 1983. (14) Farlee, R. D.; Corbin, D. R., presented in part at the International Chemical Congress of Paclfic Basin Societies, Honolulu, HI, Dec 1984. (15) Debras, G.; Derouane, E. G.; Gibson, J. P.;Gabelica, Z.; Demortier, G. Zeolites 1983, 3 , 37. (16) Jones, K. W.; Hanson, A. L.; Kraner, H. W. Trans. Am. Nucl. SOC. 1982. 250. ..- - , 4. ., . .. (17) Lippmaa, E.; Wgi, M.; Samoson, A.; Engelhardt, 0.; Grimmer, A.-R. J. Am. Chem. Sac. 1980. 102. 4889. (18) Melchior. M. T.; Vaughan, D.’E.~W.;Jarman, R. H.; Jacobson, A. J. Nature (London) 1982, 298, 455. (19) Fyfe, C. A.; Gobbl, G. C.; Kennedy, G. J.; DeSchutter, C. T.; Murphy, W. J.; Ozubko, R. S.; Slack, D. A. Chem. Leff. 1984, 163. (20) Fyfe, C. A.; Gobbl, G. C.; Murphy, W. J.; Ozubko, R. S.; Slack, D. J . Am. Chem. Soc. 1984, 106, 4435. (21) Thomas. J. M.; Klinowski, J.; Ramdas, S.; Hunter, 8. K.; Tennakoon, D. T. B. Chem. Phys. Len. 1983, 102, 158. (22) Ghose, S.; Tsang, T. Am. Mlneral, 1973, 58, 748.
RECEIVED for review March 16,1987. Accepted July 22,1987. Contribution number 3431 from the Central Research and Development Department.
CORRESPONDENCE Phosphoric Acid, Nitric Acid, and Hydrogen Peroxide Digestion of Soil and Plant Materials for Selenium Determination Sir: A mixture of phosphoric acid, nitric acid, and hydrogen peroxide has been proposed as an alternative to the use of the nitric/perchloric acid mixture to digest biological fluids to determine their selenium (Se) content (I). The purpose of the studies reported here was to test the applicability of this digestion method for the determination of Se in soil and plant materials. In the initial tests of its use for soil analysis, it was observed that manganese (Mn) might serve as an indicator of com0003-2700/87/0359-2728$01.50/0
pleteness of digestion. The results of additional studies confirmed this, and the addition of Mn as a manganous salt was incorporated into a modified procedure. Completeness of oxidation is indicated by the purple coloration of the digest due to the formation of permanganate (Mn04-) ion. The addition of Mn as a redox status indicator also identifies samples that have not received sufficient oxidant at the end of the digestion and avoids Se losses from charring of samples (2). 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
Table I. Determination of Selenium in NBS Standard Reference Materials and Recovery of Added Selenite sample
added Se
Se measd, pg/g
wheat flour SRM no. 1567 rice flour SRM no. 1568
0 0.33 0 0.33 0
f 0.033” (6)* f 0 (2) f 0.030 (5) f 0.019 (2) 0.61 f 0.018 (3) 1.06 f 0.007 (3)
estaurine sediment SRM no. 1646
0.50
1.04 1.38 0.39 0.72
certified value
Table 11. Comparison of Selenium Content in Soil and Plant Samples Using H8PO4/HNO3/H2O2vs H,P04/HN03/HC104 Acid Digestion Mixtures
1.1
sample
0.4
Staten peaty muck Nahrub clay Sites clay Yo10 fine sandy loam tomato leaves
0.6c
” Standard deviation. Values in parentheses are the number of subsamples analyzed. Reported but not certified value. EXPERIMENTAL SECTION Air-dried soil or plant samples (up to 0.5 g) are placed in digestion tubes (90 mL) followed by concentrated HN03 (4 mL for soil samples, 7 mL for plant tissue) and Hap04 (1 mL). Small funnels with stems inserted into the necks of the tubes decrease evaporative losses. The tubes are placed in an aluminum-block digester equipped with an Orion Scientific Model AD4020 temperature controller and heated at 50 “C for l/z h. Refluxing is continued at 145-150 “C for 4-12 h, depending on the digestibility of the organic matter. The HN03 refluxing is terminated when there are no brown NOz fumes in the digestion tube and the digest boils with large bubbles arising from the bottom of the digest rather than small bubbles arising from the upper layer of the digest. The tubes are removed from the digester and allowed to cool for 10 min before adding H2O2(2 mL of 30% HzOzin 1-mL increments). The samples are allowed to react for 10 min, the funnels are removed, and 1drop of 2.5 g/L Mn as MnSOl is added to each tube. The tubes are returned to the aluminum-block digester and heated at 145 OC for -4 h until the volume is reduced to l-ll/zmL, and the color of the digest turns purple. If no purple coloration appears, 1-2 mL of nitric acid-hydrogen peroxide mixture (3:l) may be added to the tube and digestion continued. After digestion is completed, glacial acetic acid (1mL of 1:l acid/water) and 6 M HC1 (3 mL) are added, and the refluxing funnels are replaced onto the tops of the digestion tubes. Heating of the samples is resumed and continued at 130 OC for h. The tubes are then removed from the digester and cooled and a stabilizing solution is added (8 mL of 0.04 M EDTA in 10% NH2OHSHCl (w/v) for soils, 2.5 mL for plants). If oils or waxes are present, they may be removed from the samples by extraction with chloroform (5-8 mL) ( I ) . The fluorometric procedure for the determination of Se in the digests was that described by Levesque and Vendette (3).
-
RESULTS AND DISCUSSION Selenium contents of NBS reference materials were determined following acid digestion with H3P04/HN03/Hz02 (Table I). The values for all NBS reference materials ranged from 95% to 101% of their certified values. When the NBS reference materials were spiked with selenite, the Se values ranged from 96% to 98% of expected. The spike experiment showed that prior to the addition of 2,3-diaminonaphthalene (DAN), the added selenite was quantitatively present in the IV form, and was not converted to either a higher or lower oxidative state. For fluorometric determination, Se must be in the IV oxidation state so it can complex with DAN ( 4 , 5). The presence of residual Hz02 during the reduction of Se042-to Se032-by HC1 leads to the formation of a large volume of Clz and subsequently the back oxidation of SeOaZ-to Se042- (5). Acetic acid is added to facilitate the decomposition of residual Hz02 Results of Se determination following wet digestion of soil samples by using H3PO4/HNO3/H2O2were compared with those obtained by using a HN03/HC104 method described by Levesque and Vendette (3), modified by adding 1mL of H3P04to the digest (Table 11). The two digestion methods provided similar Se values, although the appearance of the
2729
Se measd, p g / g HBPOI/”OB/ H3PO4/HNO3/HzO2 HC104 0.91 f 0.032” (3)b 1.21 f 0.076 (3) 0.71 f 0.043 (3) 0.32 f 0.025 (3) 1.13 f 0.048 (8)
0.86 f 0.011 (3) 1.16 f 0.044 (3) 0.64 f 0.012 (3) 0.32 f 0.016 (3) 1.05 f 0.055 (9)
“ Standard deviation. Values in parentheses are the number of subsamules analvzed. H3P04/HN03/HC104digests suggested that organic matter was more completely oxidized. The total digestion time required for the peaty muck soil was 16 h when using H3PO4/HNO3/HZOz,after which the digest was still light brown, indicating imcomplete oxidation of the organic material. The brown color effectively masked the purple color of MnO,, which was visible after the particulates were allowed to settle overnight. The H3P04/HN03/HC104digestion time for the peaty muck soil was 10 h and the color of the digest was light yellow, indicating that organic matter was more completely oxidized. The results reported here indicate H3PO4/HNO3/HzO2is a satisfactory alternative to the use of HN03/HC104for digesting soil and plant samples for Se determination. Our experience indicates that reducing conditions can occur even prior to the formation of black charred particles in the digest. Thus it is important to identify and avoid this “precharring” stage during H3PO4/HNO3/HzO2digestion. The redox indicator MnS04 was added for this purpose. During the initial stage of the digestion, the color of the solution is yellow-brown due to the presence of NOz gas. After the addition of HzOz, and when the digest is heated to evaporate excess HN03, the phosphate in the digest condenses to form pyrophosphate. Pyrophosphate reacts with HzOzor phosphate hydroperoxidate to form peroxophosphate, which in turn reacts with colorless Mn(I1) to form purple Mn04- (6). In the presence of phosphate, the purple color of MnOc usually forms when the volume of the digest is reduced to -ll/z mL. In the presence of phosphoric x i d , e.g. when digesting a Se standard, the purple color may form when the volume is reduced at -4 mL. Samples that are more difficult to digest may not achieve the purple color even when the volume of the digest is reduced to 1 mL because of insufficient oxidant. Samples with high lipid content may have lipids floating on top of the digest. For both situations, the digest may be terminated before the volume reaches 1 mL, when the color of the digest is still yellow-brown, and the remaining dissolved organics or lipids may be removed with chloroform ( I ) . If a more complete digestion of organic material is desired, then additional HN03 + HzOzmay be added before the volume of the digest reaches 1mL. Upon the addition of HN03 + HzOzto the digest, the purple color will change back to colorless due to the initial destruction of Mn0; by free Hz02 The destruction of HzO2 by Mn is diminished in the presence of phosphate (6). With prolonged heating, it is possible to evaporate, decompose, or consume the remaining oxidants in the digest. The change in color from the purple Mn0; back to colorless Mn(I1) readily identifies reducing conditions in the digest that occur prior to the formation of charred particles. Thus continued heating may result in Se loss from precipitation or volatilization. The addition of Mn(I1) to the digest did not interfere with the subsequent fluorometric determination of Se, in agreement
2730
Anal. Chern. 1887, 59,2730-2732
with the findings of Brown and Watkinson (7) and Lott et al. (8). Excess HNOBcan be removed from the digest by evaporation, provided that reducing conditions in the digest do not occur. A convenient stage to stop the evaporation of excess HN03 is when the color of the digest changes from a light yellow-brown to purple, or when the volume of the digest reaches 1mL. If complete removal of HN03 is necessary, then formic acid may be substituted for acetic acid. After formic acid is added, the tube should be heated at 130 "C for 'I2 h to remove Hz02and excess HN03 before adding HCl to reduce S e 0 2 - b SeO$-, as described in Reamer and Veillon ( I ) . Our experience indicates that acetic acid tends to provide more reproducible Se values than formic acid, and acetic acid can be added with HCl in the reduction step, rather than adding the two reagents in sequence. Acetic acid heated at 130 "C does not destroy "OB, however. The presence of up to 1/2 mL of HN03 does not interfere with the fluorometric analysis of Se. The NH,OH.HCl in the stabilizing solution helps minimize oxidation of the DAN by HN03 (9).
ACKNOWLEDGMENT The authors thank D. Scherer and A. Jacobson for providing the tomato leaf sample and the value for its Se content obtained by using the HN03/HC104 digestion method.
Registry No. Se, 7782-49-2; HBP04,7664-38-2; "Os,
7697-
37-2; H20z,7722-84-1; Mn, 7439-96-5.
LITERATURE CITED (1) Reamer,
D.C.; Veillon, C. Anel. Chem. 1983, 55, 1606-1608.
(2) Gorsuch, T. T. Analyst (London) 1959, 84, 135-173. (3) Levesque, M.; Vendette, E. D. Can. J. Sol/ Scl. 1971, 51, 85-93. (4) Koh, T. S.;Benson, T. H. J. Assoc. Off. Anal. Chem. 1983, 66,
918-925. (5) Krivan, V., Petrick, K.; Welz, 8.; Melcher, M. Anal. Chem. 1985, 57, 1703-1706. (6) Creaser, I. I.; Edward, J. 0. Top. Phosphorus Chem. 1972, 7, 379-432. (7) Brown, M. W.; Watkinson, J. H. Anal. Chim. Acta 1977, 89, 29-35. (8)LOR, P. F.; Cukor, P.; Moriber, 0.; Solga, J. Anal. Chem. 1963, 35; 1 159-1 163. (9) Aliaway, W. H.; Cary, E. E. Anal. Chem. 1964, 36, 1359-1362.
Allen Dong V. V. Rendig* R. G . Burau G . S. Besga Department of Land, Air and Water Resources University of California Davis, California 95616
RECEIVED for review April 20, 1987. Accepted July 28, 1987. Funds for this research were provided by the UC Salinity/ Drainage Task Force and the Kearney Foundation of Soil Science.
Existence of Self Chemical Ionization in the Ion Trap Detector Sir: In a recent paper in Analytical Chemistry (I), Olson and Diehl made several references to self chemical ionization (self-CI) while discussing the results of their work with the ion trap m m spectrometer system (2). The concept of self-CI was invoked to explain the observation of intense (M + 1)' ions in the spectra of compounds measured n... at high concentrations of GC eluents ..." with the ion trap detector. Self-CI has been defined (3) as chemical ionization in which the reagent ions are fragment ions from the analyte neutral molecule. The process was observed in a Fourier transform mass spectrometer, which used a trapped ion cell and an appropriate time delay between ion formation and ion detection. Thii method of chemical ionization was most efficient for molecules which produce high levels of low-mass fragment ions, such as fatty acid esters. The abundant low-mass fragment ions serve as proton-transfer reagents during a time delay of 20 ms or longer in the trapped ion cell. Olson and Diehl noted that "the ion trap spectra of the phenol, aniline, alcohols, and nonanal did not show (M + 1)+ ions due to CI effects; however the dicyclohexylamine and esters did. The initial spectra obtained during the elution of the dicyclohexylamine showed a small molecular ion at 181 and no (M + 1)+ion at 182. As the concentration of dicyclohexylamine reached a maximum, self-CI effects resulted in the formation of the 182 ion; however, the rest of the spectrum did not change significantly. The (M + 1)+ions were also found in the spectra of the three methyl esters ...". Unfortunately the authors did not give the quantities of the various substances injected into the chromatograph or the concentrations corresponding to the onset of CI effects. Also, although not stated, we assume that the ions were observed in the bar graph spectra from the ion trap data system and not by direct observation of the mass peak profiles. We have observed, in bar graphs from the ion trap data system, the frequent occurrence of unusually abundant (M
+ 1)+ions in the spectra of many classes of compounds. The occurrence of these intense (M 1)+ions is concentration dependent. Below about 50-ng injected into the gas chromatograph, which is the only method of sample introduction provided with the ion trap, (M + 1)+ ions due to naturally occurring isotopes occur at normal abundances when the M+ ion itself is observed. Above about 50 ng, and this varies somewhat with the compound, when M+ ions are observed, the relative abundances of the (M + l)+ ions are sharply higher than expected from isotopic contributions and often exceed the abundances of the M+ ions. All of our observations were made with helium as the carrier gas, with none of the usual chemical ionization reagent gases present, and with no other compounds in the ion trap concurrent with the substance being measured. The ion trap is designed to operate with a relatively high background pressure of helium, that is 0.001 Torr, which, as described by the designers (2), "... is used as a medium to damp the motion of ions trapped in the device, and has been found to enhance the mass resolution, sensitivity and detection limit". The conditions present in the ion trap during our experiments do not seem likely to promote self-CI, although ion storage times of the order of tens of milliseconds may indeed facilitate processes at higher concentrations similar to those observed in the Fourier transform instrument. An alternative explanation (4) for the elevated abundances of (M + 1)+ions in data system bar graphs at higher concentrations is the algorithm used by the ion trap data system to interpret ion peak profiles and its failure to correctly interpret profiles broadened by saturation and space charging. The ion peak profile in the region of the M+ in the ion trap spectrum of 50 ng of pyrene ( 5 ) is considerably broadened, saturated, and possibly shifted to higher mass. Also, doubly charged molecular ions, M2+, from polycyclic aromatic hydrocarbons (PAH) appear as broadened ion peaks even when
0003-2700/87/0359-2730$01.50/0 0 1987 American Chemical Society
+
