Anal. Chem'. 1982, 5 4 , 149-151
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Dry Ashing Vegedtables for thie Determination of Sodium and Potassium by Atomic Absorption Spectrometry Carol A. Rowan" Department of Environmental Quality Engineering, 1 Winter Street, Boston, Massachusetts 02 108
0. 1.Zajicek Department of Chemistty, IJniversity of Massachusetts, Amherst, Massachusetts 0 1003
Edward J. Calabrese Division of Public Health, lhiversity of Massac:husetts, Amherst, Massachusetts 0 1003
Studies on the accuracy of dry ashing for the destruction of organic matter in inoirganic analysis have shown conflicting results (1, 2). Most of the inconsistencies are due to the analysis of different types of matrices; thus, it is essential t o treat samples individually rather than tio accept a standard procedure for different matrices (3). Other variables to consider include contamination and losses due to volatilization of the element a t the specified ashing temperature as well as retention losses when the element of interest reacts with the ashing vessel or solids within the ash (3). To improve the accuracy of inorganic analysis, ashing aids have been found to be useful in reducing volatilization losses by conversion of elemental forms within a matrix, and they are also used to facilitate the decomposition of organic materials ( 4 ) . Other considerations are that during dry ashing, increased oven temperature and time of ashing have resulted in increased losses of alkali salts such as sodium and potassium (5). As part of a larger public health study on sodium (6), the objective of this study was to determine the efficiency and accuracy of a dry ashing technique for samples in which sodium and potassium were t o be determined, taking into consideration the above-mentioned factors. EXPERIMENTAL SECTION Preliminary Testing:. Reagents. Reagent grade chemicals were used throughout. A sodium stock solution of 1000 ppm was prepared by dissolving 2.542 g of sodium chloride in 1 L of deionized, distilled water. The potassium stock solution of 1000 ppm was made by dissolving 1.907 g of potassium chloride in 1 L of deionized, distilled w&r. All working solutionswere prepared from the stock standards. Standard curve solutions were generally acidified with 5% hydrclnloric acid but were consistently mixed to equal the pH of the sample. Methods and Equipment. Since sodium and potassium may exist at trace levels in glassware, laboratory apparatus, an,d reagents, an assessment of background contamination was made. Occurrence of a gain or loss of sodium and potassium was examined at each step of the analysis, requiring modification of the technique when necessary. Assessment of equipment which might cause a deviation of results was conducted. Glassware was calibrated, and drying ovens and muffle furna.ces were checked for consistency. Accuracy of Dry Ashing Method. To assess the most appropriate ashing vessel for sodium measurement, recovery experiments were conducted which compared potential retention or volatilization losses among porcelain, fused quartz, and nickel crucibles. Sodium and potassium working standards of 100 ppin were made from the stock solutions. Recovery of 1 and 2 mg of sodium was tested by adding 10 and 20 mL respectively, of the sodium working standard t o each type of crucible. Recovery of 1 and 2 mg of potassium was determined by adding 10 and 20 mL of the 100 ppm potassium working standard to nickel crucibles. The solutions were slowly evaporated in a drying oven at 80 "C, and the samples1were then placed :in the muffle furnace a t 500 "C for 3 h. The effect of time and ismperature upon sodium and potassium (as chlorides) recovery will also evaluated. Working solutions of 100 ppm of sodium and potassium were prepared from the stock
solutions. Ten milliliters of each solution was added to separate nickel crucibles and were slowly evaporated at 80 "C. The crucibles were then placed in the muffle furnace and recovery was measured after varying the temperature of the furnace as well as the duration of time in the furnace. Three temperatures were tested, 475 "C, 500 "C, and 550 "C, and at each temperature, the samples were heated for 2, 3, 4, or 24 h. The effect of time and temperature upon the measurement of sodium and potassium in each vegetable matrix was then measured. Previously dried and ground samples were redried in the oven at 80 "C for 24 h. One gram of the redried sample was added to the nickel crucible. The crucibles were partially covered with nickel lids to prevent any potential splattering losses. The samples were heated for 2,3, or 4 h at the same temperatures used on the nonbiological samples. A recovery experiment was also conducted by adding sodium and potassium t o the sample. One gram of each of the redried vegetables was added to nickel crucibles. Two milliliters of the 1000 ppm sodium chloride stock was added t o the crucible and wetted the sample. Four milliliters of a 5000 ppm sodium chloride stock was added to another set of samples. This procedure was also carried out on each vegetable using potassium chloride solutions of the same concentration and volumes. The samples were dried at 80 "C in the drying oven followed by heating at 500 "C for 3 h. The use of an ashing aid was also tested on each vegetable matrix. One gram of the redried vegetable sample was placed in a crucible. Two milliliters of a 20% sulfuric acid solution was added to each sample. Samples were dried at 80 "C and were heated at 500 "(2 for 3 h. Analytical Testing. A Perkin-Elmer Model 103 atomic absorption spectrophotometer was used. Optimum running conditions were evaluated and were consistently established throughout the experiment. The method of standard additions was conducted on each matrix for both a5odium and potassium. Since ionization effects are commonly relported for alkali metals during flame absorption, attention was focused upon determining if these effects were present. Standard curves were measured for sodium when excess potassium (100,1000,5000, and 10000 ppm potassium) was added to find if sodium absorption was altered. An additional experiment was conducted to determine not only possible ionization interferences but other types of interferences. A low sodium sample with a potassium concentration of 600 ppm was used. Three separate aliquots of this sample were taken t o which sodium from the stock was added. Final sodium concentrations were 80 ppm, 120 ppm, and 160 ppm. Two sets of aqueous sodium standards of 80 ppm, 120 ppm, and 160 ppm were also made to which 600 ppm potassium (from the stock) was added to one set. The slopes of all the standard curves were measured and compared. Destruction of Organic Matter. Vegetable samples were weighed and dried to a constant weight in a drying oven at 80 "C. The dried weights of the samples were recorded, and the samples were ground with an agate mortar and pestle to a fine powder. Samples were stored in polyethylene bottles in a desiccator. Samples to be analyzed were redried at 80 "C for 24 h. One gram of the finely ground sample was weighed on the Mettler balance and placed into a nickel crucible. Nickel lids half-covered
0003-2700/82/0354-0 149$01.25/0 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
Table I. Effect of Different Types of Crucibles on the Recovery of Sodium (as Chloride) Heated at 500 " C a % recovery
Table 11. Effect of Time and Temperature on the Recovery of Potassium (as Chloride) in Nickel CruciblesQ % recovery
ashing vessel
100 ppm Na added
200 ppm
Na added
temp,"C
2h
3h
4h
24 h
porcelain fused quartz nickel
83 92 96
83 92 96
475 500 550
96 96 94
96 98 94
96 98 94
90 78 80
Time, 3 h. Values in table represent average of 1 0 measurements.
Results in table are the average of triplicate 100 ppm potassium samples.
the crucibles to prevent any splattering losses which could occur during the charring and ignition stage of dry ashing. The crucibles were placed in the middle of the muffle furnace at a temperature of 300 "C. After 10-15 min, the temperature was increased to 400 "C at a rate of 2-3 deg/min. The furnace door was kept open allowing the smoking materials to escape. After smoking had subsided, the furnace door was closed, the temperature was increased to 500 "C, and the samples were heated to their optimum time of 3 h as previously determined. The white ash was slowly dissolved in 5.0 mL of 20% HC1 and warmed on a hot plate for 30 min. The crucible lid was rinsed with deionized distilled water which was added to the sample. The sample was then filtered and diluted to a concentration suitable for the working range of the instrument. Standards and samples were matched for pH. Controls (blanks) were processed throughout the experimental procedure to detect any contamination problems.
Table 111. Effect of Time and Temperature on the Recovery of Sodium and Potassium from Vegetables Ashed in Nickel Crucibles a
RESULTS AND DISCUSSION Experiments to determine losses or gains of sodium and potassium from the surfaces of equipment and reagents were the initial tests conducted. Contamination was not detectable from the reagents nor glassware that had been acid washed. While sodium contamination from filter paper has been reported (7), no sodium or potassium contamination was detected from Whatman No. 40 filter paper. The water from rinsing the nickel lids was checked for sodium and potassium and was found to be negative. However, lids were still used to reduce any potential splattering losses which could occur during ignition and ashing of the samples. The recovery for sodium which had been added to the different ashing vessels is reported in Table I. As shown, the losses of sodium during dry ashing with porcelain, silica, and nickel crucibles were found to occur to different extents. Changes in the technique for redissolving sodium chloride by using a stronger acid solution, larger volumes of solvent, and warming the solution had no significant effect upon recovery. The losses were consistent with reports in the literature with recovery of only 95% in silica and 85% in porcelain (8). Silica surfaces on the porcelain and fused quartz crucibles may have been weakened by the high temperature of ashing, resulting in losses of sodium in the silica surface of the crucibles (9). The highest recoveries were achieved with nickel crucibles. Potassium loss was low, 2.0%, when the nickel crucibles were used and was not improved by changing the method of redissolving the potassium chloride. Since these results for sodium and potassium demonstrate nickel to be the most efficient and reproducible ashing vessel, nickel crucibles were used for the destruction of organic matter. The effect of various ashing temperatures within the range specified for ashing plant material (475-500 "C) and above (550 "C) in conjunction with the use of different heating times of 2, 3, 4, or 24 h did not result in different recoveries of sodium which had been added to the crucible. In fact, the mean recovery of sodium was 96%, regardless of the time and temperature of ashing. In contrast, however, Table I1 illustrates the effect that time and temperature of ashing had on potassium recovery from nickel crucibles. As shown, at all temperatures, potassium recovery was consistently lower at
recovery vegetable
temp, Na "C
4h
3h
2h
K
Na
K
Na
K
cabbage 415 0.320 33.6 0.320 36.8 0.320 36.8 squash potato
500 550 475 500 550 415 500 550
0.320 0.320 0.053 0.053 0.053 0.040 14.4 0.040 0.040 14.4 0.040
0.320 0.320 0.053 0.053 0.053 0.040 0.040
33.6 32.5 43.2 52.8 43.2 14.8
36.8 32.5 53.3 53.9 52.0 16.0 16.0 14.4
0.320 0.320 0.053 0.053 0.053 0.040 0.040 0.040
36.8 33.6 53.3 53.9 53.2 16.0 16.0 14.4
Results in table are the average of triplicate samples. Results are expressed as the total milligrams of sodium (or potassium) per 1.0 g of dried vegetable. 24 h of heating. When considering the temperatures, ashing
at 550 "C produced lower percentage recovery than 475 "C and 500 "C. Highest recoveries occurred at 500 OC for 3 and 4 h. The preliminary tests illustrate the importance of determining those experimental parameters necessary for achieving an accurate analytical procedure for sodium and potassium. While recovery of sodium appears unaffected by time and temperature of ashing, it is possible that high recovery of potassium occurs at 500 "C in a 3-4 h heating period. The next step, therefore, was to measure the best analytical dry ashing parameters for determining the sodium and potassium concentrations in the various matrices; thus, the effect of time and temperature of ashing upon sodium and potassium recovery from each matrix was measured in nickel crucibles. As shown in Table 111,the results were consistent with the previous nonbiological samples with highest potassium concentrations after heating a t 475 "C and 500 "C for 3 and 4 h. Sodium concentrations did not vary. While it is unknown if a 100% recovery of all the plant sodium and potassium is being obtained, these experiments suggest within the tested boundaries a best ashing time and temperature of 475 and 500 "C for 3-4 h. Since dry ashing at 500 "C could result in volatilization losses of certain chemical forms of sodium and potassium such as the nitrate form which has a melting point below 500 OC, use of an ashing aid was tested. Each sample was wetted with sulfuric acid to convert all sodium and potassium to the sulfate form. Results of this experiment were consistent with the results when no ashing aid was used and suggest that no volatilization losses were occurring. Hence, no further use of an ashing aid was made. Results of the recovery experiments by adding known amounts of sodium and potassium to each matrix are shown in Tables IV and V. The results clearly show good recovery values for sodium and potassium from each matrix. The addition of 20 mg of sodium and potassium required that an
Anal. Chem. 1982, 5 4 , 151-152 ~~~
Table IV. Recovery O P Sodium (as Chloride) Added to the Vegetable Matrix and Measured by Dry Ashing in Nickel Crucibles %
matrix
initial level
cabbage squash potato
0.320 0.053 0.040
%
recovery reccivery of 2 mg of 20 mg of Na of Na % mean added added recovery 100.6
100.1 98.8
100.3 99.9 100.6
100.4
100.0 99.7
a Heated at 500 "C for 3 h. Results aire mean values of triplicate results, Tcital milligrams of sodium per 1.0 g of dried vegetable.
Table V. Recovery of ]Potassium(as Chloride) Added to the Vegetable Matrix and Measured by Dry Ashing in Nickel Crucibles a % recov- % recovery of ery of
matrix cabbage squash potato
initial level 36.8
53.9 16.0
2mg of K added
20mg of K added
99.3 99.2 99.5
101.9 9'3.9 9'9.8
% mean recovery
100.6
99.5 99.7
a Heated at 500 "C f o r 3 h. Results are mean values of triplicate results. Total milligrams of potassium per 1.0 g of dried vegetables.
additional dilution of 1 2 0 be used for thle extracted sample. As shown, the dilutions did not effect the recovery. Since these experiments included the lowest to the highest levels of the elements to be measured during the public health study (6),the method of dry ashing WELSshown to be consistently accurate over the whole range of concentrations. Of interest is the different recovery of sodium when a matrix is present (99.7-100.6%) vs. when it is not (96%). Most likely the physical presence of the matrix to hold the elements off the crucible surface may account for th.e difference. Since sodium chloride was consistently being used in both experiments, higher recoveries were probably not related to any different chemical form of sodium in the plant being measured (initial plant sodium was accounted for in addition to sodium added during the recovery experiment, Table IV). However, in retrospect, one could hypothesize that the recovery of
151
nonmatrixed sodium in various crucibles could be different than measured in this experiment when another sodium salt was used; thus, the descrepancy may not be true with different sodium salts arid ashing vessels. Additional testing of the vegetable samples for sodium and potassium in different crucibles would aid in interpreting these results. Intraassay reproducibility for the measurement of sodium and potassium from plant tissue (based upon dry weight) was satisfactory: the coefficient of variation for 10 samples was 0.2% for sodium and 1.0% for potassium. Dry ashing was conducted at 500 "C for 3 h, following partial oxidation without the use of an ashing aid. Interferences from sodium and potassium were measured by the method of standard additions by adding known amounts of concentrated elements to the sample solutions just prior to aspiration through the atomic absorption spectrophotometer. No interferences were detected for any of the matrices. Excess potassium of 100, 1000,5000 and 10 000 ppm did not affect the sodium signal of the spectrophotometer. Experiments showed no interferences. The results of this study mainly demonstrated the importance of preliminary testing for increasing the accuracy of inorganic analysis. Of particular importance were the findings of the effect of time and temperature upon potassium measurements. Good recoveries, consistency, and an efficient dry ashing methodology for sodium and potassium were developed. Comparison of these results with a wet digestion method would greatly support the degree of accuracy of this study. While this was beyond the present scope of this work, a confirmation of results would be very beneficial to analysts who must choose between the methods. ACKNOWLEDGMENT Carol Sacco and Thomas Sieger provided technical assistance. LITERATURE CITED Mlddleton, G.; Stuckey, R. E. Analyst (London) 1953, 78,532-541. Mlddleton, G.; Stuckey, R. E. Analyst (London) 1954, 79, 138-142. Basson, W. D.; Bohmer, R. G. Analyst (London) 1972, 97,482-489. Gorsuch, T. T."The Destruction of Organic Matter". 1st ed.: Pergamon Press: New York, 1970; Chapter 8; pp 55-60. Grove, E. L.; Jones, R. A.; Mathews, W. Anal. Blochem. 1961, 2 (3), 221-228. Rowan, C. A.; Calabrese, E. J. J. Envlron. Scl. Health, in press. Hamilton, E. I. J . Assoc. Off. Anal. Chem. 1976, 3 4 , 836-840. Joyet, C. Nudeonlc. 1951, 9 ,42-47. Gorsuch, T. T. Analyst (London) 1959, 8 4 , 135-173.
RECEIVEDfor review September 18, 1980. Resubmitted April 15, 1981. Accepted September 1, 1981.
Format Conversilon for Laboratory Data Transfer Donald D. Burgess Department of Chemlstry, McMaster lJniversi& Hamilton, Ontario, Canada
Modern analytical iriritruments frequently have provision for the transmission of data to printers, computers, or other instruments (I). While ithe electronic signtds used are generally standard (e.g., EIA RSS232C), the encoding of information within the sequence of chmacters exchanged is usually peculiar to a particular instrument. Consequently, it is often impossible to transfer data directly from one device to another or to enter data into a computer from several instruments without a large
number of device handling programs. In this laboratory, two multichannel analyzers are in use for neutron activation analysis. One of these has distinct advantages for manual data reduction. The two analyzers use different data formats (even though manufactured by the same company) and therefore will not permit exchange of spectra. This paper outlines the solution adopted to overcome this problem.
0003-2700/82/0354-0151$01.25/00 1981 American Chemical Society
