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Low-temperature ashing of organic materials for determination of chloride
MICROCHEMICAL
35, 201-205 (1987)
JOURNAL
Low-Temperature Ashing Determination TAKAKO NISHIKAWA Kyoto
Pharmaceutical
of Organic Materials of Chloride
for
AND KEI HOZUMI~
University,
Kyoto
607, Japun
Received August 13, 1986; accepted November 5, 1986 The low-temperature plasma ashing technique was applied to organic materials containing sodium chloride and residual ash was analyzed for chloride using ion chromatography. Recovery of the chloride was approximately 50’S, while 20-30s of the chloride had been converted to chlorate. The remaining fraction of the chloride could have been volatilized due to electron and ion bombardments onto the sample surface within the glow discharge region. Pretreatment of the sample by impregnating with potassium carbonate improved the recovery of the chloride to 90% with practically no production of chlorate. i‘ 1~87 Academic Pre5s. Inc.
INTRODUCTION
Since a glow discharge of oxygen under low pressure produces electronically dissociated atomic oxygen by which organic materials are gently oxidized to remaining ash residues, the plasma ashing technique has been integrated with various analytical methods for the purpose of sample pretreatment (2-6, 8). It has been experienced that higher recovery of inorganic constituents was generally obtained because of a much lower temperature during the oxidation of organic materials (loo-150°C) than with the conventional crucible ashing. An attempt was therefore made to determine chloride after the plasma ashing of organic samples by which quantitative recovery was anticipated and the study was primarily aimed at assessing the sodium chloride content in foodstuffs and other biological specimens. Cellulose powder impregnated with a known quantity of sodium chloride was used as a model organic sample and the ash residue was subjected to argentometric titration. However, an unexpectedly low recovery of the chloride was obtained and, at the same time, thin-layer chromatography indicated the presence of a considerable amount of chloride formed during the plasma oxidation. Reduction of chlorate to chloride using sodium arsenite was not enough to restore the quantitative recovery of total chloride so that some loss of initial chloride due to the contact with energetic plasma gas was suggested (7). This paper reports a more precise quantitative analysis of the ash components with ion chromatography. Eventually it is found that addition of potassium carbonate to the sample before the plasma ashing is highly effective in preserving the chloride in the ash residue, probably because of fixation of the volatile chlorine atoms by
’ To whom correspondence
should be addressed. 201 0026-265X187 $ I SO Copyright All right\
0 1987 by Academic Press, Inc. of reproduction in any form reserved.
202
NISHIKAWA
alkalinity and/or functioning bardments.
AND HOZUMI
as a protective
MATERIALS
layer against electron and ion bom-
AND METHODS
Plasma apparatus. The plasma reactor and its peripheral system are illustrated in Fig. 1. The reactor was made of a Pyrex glass cylinder having an inside diameter of 7 cm and a length of 25 cm to which oxygen was introduced at one end with a flow rate of 10 ml (1 atm)/min via a flow meter and a needle valve, while the gas was pumped out of the reactor from the other end to hold the pressure at approximately 1.5 torr. A sample boat of Pyrex glass (5 x 9 x 1 cm3) carrying an organic sample was located at the center of the radiofrequency coil to which 13.56 MHz, 60 W electric power was supplied to excite the oxygen. Zon chromatoanalyzer. Yokogawa-Hokushin Electric Model IC 100 ion chromatoanalyzer, provided with a 25-cm column (i.d. 4.6 mm) packed with a low-capacity anion-exchange resin (9), was employed to analyze aqueous solutions of the ash residues. Elution was carried out in 4 mM Na,COs-4 mM NaHCO, at a flow rate of 2.2 ml/min in which a sample volume of 100 ~1 was injected. Procedure. One gram of cellulose powder (Toy0 Roshi Co. Ltd., 200-300 mesh) was uniformly spread on the flat bottom of the sample boat, and a 4-ml solution of sodium chloride or another salt of known concentration was added dropwise over the sample surface using a microburet. The wet sample was dried in vacua and was subjected to plasma ashing. The ashing process was continued for 4 hr. The ash residue was dissolved in 100 ml of distilled water and the solution was centrifuged to remove the sediment. The supernatant was then analyzed as described below. RESULTS AND DISCUSSION Analysis of Ash Components
Ion chromatography components. As was raphy reported in the peaks of chloride and tive determination of
was used to determine the chemical species of the ash predicted from a simple test of the thin-layer chromatogprevious paper (7), the ion chromatogram showed only two chlorate and no other oxychloride was observed. Quantitathe two components was therefore carried out with the inNeedle
Plasma
valve
R.F.
power
reactor
Rotary
J pump 13.56 MHz FIG. 1. Schematic diagram of plasma reactor.
t
Oxygen
ASHING
FOR DETERMINATION
OF CHLORIDE
203
TABLE 1 Recovery of Chlorine Species after Plasma Ashing of Cellulose Powder Recovery (%)
Chloride impregnated in I g cellulose powder
Cl-
0.05 M NaCl 4 ml (11.7 mg) 0.10 M NaCl 4 ml (23.4 mg) 0.05 M KC1 4 ml (14.9 mg) 0.10 M KC1 4 ml (29.8 mg) 0.025 M MgCI, 4 ml (9.52 mg) 0.05M MgCI, 4 ml (19.0 mg) 0.025 M CaCl, 4 ml (11.1 mg) 0.05M CaC1, 4 ml (22.2 mg)
45 53 50 60 38 47 51 62
cm22 17 40 35 9 3
Total 67 70 90 95 38 47 60 65
ternal standard method. Sodium bromide was found favorable as the internal standard material. A 2-ml portion was taken from the sample solution and was mixed with 2 ml of 0.005 M sodium bromide solution (lop5 mol Br-). The mixture was then subjected to ion chromatography. Peak height ratios of Cl-/Brand ClO,-/Brwere transformed to the mole ratios in the sample solution and the percentage recoveries of the two chlorine species were calculated. The results are tabulated in Table 1. Some replacements of the counter cations from the sodium chloride were also examined to determine their behaviors against the atomic oxygen. Potassium chloride, magnesium chloride, and calcium chloride were impregnated on the cellulose powder and processed the same way as the sodium chloride sample. The results are listed in Table 1. Table 1 indicates that the recovery of chloride from the sodium chloride samples was about 50% while that of chlorate was 20%. Total recovery of the chlorine species was therefore approximately 70%. The result suggested a considerable loss of chlorine species during the plasma ashing. The loss of chlorine species also raised a question of whether the sodium counterion was preserved. A simple test was therefore carried out to determine the sodium ion concentration of the loo-ml test solution which had been made up at pH 8 using tris(hydroxymethyl)aminomethane buffer solution. A sodium ion selective glass electrode (Orion Research Inc., Model 97-11) was employed to measure the concentration of the sodium ion potentiometrically with reference to a standard solution of sodium chloride (I). The result showed full recovery of sodium ion in the test solution. Only the chlorine component was volatilized, leaving the sodium component in the ash residue probably forming sodium hydroxide and/or carbonate. Potassium salt yielded a little higher recovery of chloride than did the sodium salt, while a markedly higher recovery of chlorate was obtained. The total recovery of the chlorine species came to 90-95%. Magnesium and calcium salts did not differ much from sodium and potassium salts in regard to the recovery of chloride, but a noticeably small quantity of or practically no chlorate was pro-
204
NISHIKAWA
duced. Although such characteristic not well elucidated, bond energies weight of the metals likely correlated chlorides and the oxygen at a highly Improvement
of Recovery
AND
HOZUMI
behaviors due to the counter cations were of metal to chlorine atoms and the atomic to the chemical reaction between the metal excited state.
by Sample Pretreatment
To prevent the loss of chlorine species during plasma oxidation, sample pretreatment involving an addition of alkaline medium was attempted. The cellulose powder sample containing 11.7 mg sodium chloride was further impregnated with sodium carbonate or potassium carbonate solutions of various concentrations, the wet samples were dried, and the plasma was ashed. Figure 2 shows the results of ion chromatographic determinations of the ash residues. The addition of increasing quantities of sodium carbonate increased the recovery of chloride, and the recovery reached 65% when 400 mg of sodium carbonate was added. To achieve a recovery of 75%, an addition of 1000 mg of sodium carbonate was needed. On the other hand, production of chlorate was significantly increased when a small amount of sodium carbonate was added, and the total recovery of chlorine species reached 90% if 400 mg of sodium carbonate was added. The addition of potassium carbonate showed a different behavior. The chloride was found mostly in the ion chromatogram when 100 mg was added and the recovery of chloride came to nearly 90% if 400 mg was added. It was of interest that conventional crucible ashing at high temperature yielded a recovery of chloride slightly less than 80% while no chlorate was produced. In addition, some skill was needed for the crucible ashing to avoid splashing of organic sample during the ignition. Table 2 illustrates the recovery data of chlorine species in different matrices. Potato starch and corn starch samples without addition of the alkali carbonates produced large quantities of chlorate, while agar and gelatin samples produced nothing. When alkali carbonates were added to those matrices, the recovery of
loo-
_______---- .A --
8o
0
;,>
1
l
f
60-
P J 0 40Y oz
0
Jf--
FIG.
2. Effect
Cl-,
Na2C03
0
Cl-+
CIO-j,
Na2C03
A Cl; K2CO3
zoOI
0
0 1 0
of alkaline
200
Crucible
I 400 ml Alkali carbonate,
fixatives
on recovery
800 mg
ashing
1000
of chlorine
species.
ASHING
FOR DETERMINATION
205
OF CHLORIDE
TABLE 2 Recovery of Chlorine Species after Plasma Ashing of Alkalinized
Organic Samples
l-g sample matrix containing 11.7 mg NaCl + alkaline fixative
Cl-
CIO, -
Total
Cellulose powder + 400 mg Na,CO, + 400 mg K&O, Potato starch powder + 400 mg Na,CO, + 400 mg K,CO, Corn starch powder + 400 mg Na,CO, +400 mg K&O, Agar powder +400 mg Na,CO, +400 mg K,CO, Gelatin powder +400 mg Na,CO, + 400 mg K,CO,
45 67 87 49 88 98 36 67 83 70 71 85 75 64 93
22 25 33 8 37 21 5 14 2 -
67 92 87 82 96 98 73 88 88 70 85 87 75 64 93
Recovery (%)
chloride was generally improved, and eventually potassium carbonate was found to be a favorable fixative, yielding high total recovery of chlorine species with a minute or with no production of chlorate. The reason for such advantageous behavior using potassium carbonate was not evident, but the heavier atomic weight of potassium compared to sodium would have effectively protected the chloride in the sample from the electron and ion bombardments during the plasma ashing and would have suppressed the oxidation of chloride to chlorate. REFERENCES 1. Akimoto, N., and Hozumi, K., Working property of quick-response sodium selective glass electrode and its application to microdetermination of sodium. Japan Analyst 21, 1490-1497 (1972). 2. Gleit, C. E., and Holland, W. D., Use of electrically excited oxygen for the low-temperature decomposition of organic substances. Anal. Chem. 34, 1454-1457 (1962). 3. Hollahan, J. R., Analytical applications of electrodelessly discharged gases. J. Ckem. E&c,. 43, A401-A416 (1966). 4. Hollahan, J. R., Applications of low-temperature plasmas to chemical and physical analysis. In “Techniques and Applications of Plasma Chemistry” (J. R. Hollahan and A. T. Bell, Eds.), pp. 229-253, Wiley-Interscience, New York, 1974. 5. Hozumi, K., Chemistry of low-temperature oxygen plasma and its applications. Kagaku-NoRyoiki 25, 713-723 (1971). 6. Hozumi, K., “Low-Temperature Plasma Chemistry,” pp. 97- 111. Nankodo, Tokyo, 1976. 7. Hozumi, K., Kitamura, K., Nishikawa, T., Aoki, K., and Watanabe, M., Chemical reaction of atomic oxygen on sodium chloride during low-temperature ashing. Bunseki Kagaku 32, 525-530 (1983). 8. Hozumi, K., and Matsumoto, M., Parameters for oxidation rate of organic substances in low-temperature oxygen plasma. Japan Analyst 21, 206-214 (1972). 9. Small, H., Stevens, T. S., and Bauman, W. C., Novel ion exchange chromatographic method using conductometric detection. Anal. Ckem. 47, 1801-1975 (1975).
35, 201-205 (1987)
JOURNAL
Low-Temperature Ashing Determination TAKAKO NISHIKAWA Kyoto
Pharmaceutical
of Organic Materials of Chloride
for
AND KEI HOZUMI~
University,
Kyoto
607, Japun
Received August 13, 1986; accepted November 5, 1986 The low-temperature plasma ashing technique was applied to organic materials containing sodium chloride and residual ash was analyzed for chloride using ion chromatography. Recovery of the chloride was approximately 50’S, while 20-30s of the chloride had been converted to chlorate. The remaining fraction of the chloride could have been volatilized due to electron and ion bombardments onto the sample surface within the glow discharge region. Pretreatment of the sample by impregnating with potassium carbonate improved the recovery of the chloride to 90% with practically no production of chlorate. i‘ 1~87 Academic Pre5s. Inc.
INTRODUCTION
Since a glow discharge of oxygen under low pressure produces electronically dissociated atomic oxygen by which organic materials are gently oxidized to remaining ash residues, the plasma ashing technique has been integrated with various analytical methods for the purpose of sample pretreatment (2-6, 8). It has been experienced that higher recovery of inorganic constituents was generally obtained because of a much lower temperature during the oxidation of organic materials (loo-150°C) than with the conventional crucible ashing. An attempt was therefore made to determine chloride after the plasma ashing of organic samples by which quantitative recovery was anticipated and the study was primarily aimed at assessing the sodium chloride content in foodstuffs and other biological specimens. Cellulose powder impregnated with a known quantity of sodium chloride was used as a model organic sample and the ash residue was subjected to argentometric titration. However, an unexpectedly low recovery of the chloride was obtained and, at the same time, thin-layer chromatography indicated the presence of a considerable amount of chloride formed during the plasma oxidation. Reduction of chlorate to chloride using sodium arsenite was not enough to restore the quantitative recovery of total chloride so that some loss of initial chloride due to the contact with energetic plasma gas was suggested (7). This paper reports a more precise quantitative analysis of the ash components with ion chromatography. Eventually it is found that addition of potassium carbonate to the sample before the plasma ashing is highly effective in preserving the chloride in the ash residue, probably because of fixation of the volatile chlorine atoms by
’ To whom correspondence
should be addressed. 201 0026-265X187 $ I SO Copyright All right\
0 1987 by Academic Press, Inc. of reproduction in any form reserved.
202
NISHIKAWA
alkalinity and/or functioning bardments.
AND HOZUMI
as a protective
MATERIALS
layer against electron and ion bom-
AND METHODS
Plasma apparatus. The plasma reactor and its peripheral system are illustrated in Fig. 1. The reactor was made of a Pyrex glass cylinder having an inside diameter of 7 cm and a length of 25 cm to which oxygen was introduced at one end with a flow rate of 10 ml (1 atm)/min via a flow meter and a needle valve, while the gas was pumped out of the reactor from the other end to hold the pressure at approximately 1.5 torr. A sample boat of Pyrex glass (5 x 9 x 1 cm3) carrying an organic sample was located at the center of the radiofrequency coil to which 13.56 MHz, 60 W electric power was supplied to excite the oxygen. Zon chromatoanalyzer. Yokogawa-Hokushin Electric Model IC 100 ion chromatoanalyzer, provided with a 25-cm column (i.d. 4.6 mm) packed with a low-capacity anion-exchange resin (9), was employed to analyze aqueous solutions of the ash residues. Elution was carried out in 4 mM Na,COs-4 mM NaHCO, at a flow rate of 2.2 ml/min in which a sample volume of 100 ~1 was injected. Procedure. One gram of cellulose powder (Toy0 Roshi Co. Ltd., 200-300 mesh) was uniformly spread on the flat bottom of the sample boat, and a 4-ml solution of sodium chloride or another salt of known concentration was added dropwise over the sample surface using a microburet. The wet sample was dried in vacua and was subjected to plasma ashing. The ashing process was continued for 4 hr. The ash residue was dissolved in 100 ml of distilled water and the solution was centrifuged to remove the sediment. The supernatant was then analyzed as described below. RESULTS AND DISCUSSION Analysis of Ash Components
Ion chromatography components. As was raphy reported in the peaks of chloride and tive determination of
was used to determine the chemical species of the ash predicted from a simple test of the thin-layer chromatogprevious paper (7), the ion chromatogram showed only two chlorate and no other oxychloride was observed. Quantitathe two components was therefore carried out with the inNeedle
Plasma
valve
R.F.
power
reactor
Rotary
J pump 13.56 MHz FIG. 1. Schematic diagram of plasma reactor.
t
Oxygen
ASHING
FOR DETERMINATION
OF CHLORIDE
203
TABLE 1 Recovery of Chlorine Species after Plasma Ashing of Cellulose Powder Recovery (%)
Chloride impregnated in I g cellulose powder
Cl-
0.05 M NaCl 4 ml (11.7 mg) 0.10 M NaCl 4 ml (23.4 mg) 0.05 M KC1 4 ml (14.9 mg) 0.10 M KC1 4 ml (29.8 mg) 0.025 M MgCI, 4 ml (9.52 mg) 0.05M MgCI, 4 ml (19.0 mg) 0.025 M CaCl, 4 ml (11.1 mg) 0.05M CaC1, 4 ml (22.2 mg)
45 53 50 60 38 47 51 62
cm22 17 40 35 9 3
Total 67 70 90 95 38 47 60 65
ternal standard method. Sodium bromide was found favorable as the internal standard material. A 2-ml portion was taken from the sample solution and was mixed with 2 ml of 0.005 M sodium bromide solution (lop5 mol Br-). The mixture was then subjected to ion chromatography. Peak height ratios of Cl-/Brand ClO,-/Brwere transformed to the mole ratios in the sample solution and the percentage recoveries of the two chlorine species were calculated. The results are tabulated in Table 1. Some replacements of the counter cations from the sodium chloride were also examined to determine their behaviors against the atomic oxygen. Potassium chloride, magnesium chloride, and calcium chloride were impregnated on the cellulose powder and processed the same way as the sodium chloride sample. The results are listed in Table 1. Table 1 indicates that the recovery of chloride from the sodium chloride samples was about 50% while that of chlorate was 20%. Total recovery of the chlorine species was therefore approximately 70%. The result suggested a considerable loss of chlorine species during the plasma ashing. The loss of chlorine species also raised a question of whether the sodium counterion was preserved. A simple test was therefore carried out to determine the sodium ion concentration of the loo-ml test solution which had been made up at pH 8 using tris(hydroxymethyl)aminomethane buffer solution. A sodium ion selective glass electrode (Orion Research Inc., Model 97-11) was employed to measure the concentration of the sodium ion potentiometrically with reference to a standard solution of sodium chloride (I). The result showed full recovery of sodium ion in the test solution. Only the chlorine component was volatilized, leaving the sodium component in the ash residue probably forming sodium hydroxide and/or carbonate. Potassium salt yielded a little higher recovery of chloride than did the sodium salt, while a markedly higher recovery of chlorate was obtained. The total recovery of the chlorine species came to 90-95%. Magnesium and calcium salts did not differ much from sodium and potassium salts in regard to the recovery of chloride, but a noticeably small quantity of or practically no chlorate was pro-
204
NISHIKAWA
duced. Although such characteristic not well elucidated, bond energies weight of the metals likely correlated chlorides and the oxygen at a highly Improvement
of Recovery
AND
HOZUMI
behaviors due to the counter cations were of metal to chlorine atoms and the atomic to the chemical reaction between the metal excited state.
by Sample Pretreatment
To prevent the loss of chlorine species during plasma oxidation, sample pretreatment involving an addition of alkaline medium was attempted. The cellulose powder sample containing 11.7 mg sodium chloride was further impregnated with sodium carbonate or potassium carbonate solutions of various concentrations, the wet samples were dried, and the plasma was ashed. Figure 2 shows the results of ion chromatographic determinations of the ash residues. The addition of increasing quantities of sodium carbonate increased the recovery of chloride, and the recovery reached 65% when 400 mg of sodium carbonate was added. To achieve a recovery of 75%, an addition of 1000 mg of sodium carbonate was needed. On the other hand, production of chlorate was significantly increased when a small amount of sodium carbonate was added, and the total recovery of chlorine species reached 90% if 400 mg of sodium carbonate was added. The addition of potassium carbonate showed a different behavior. The chloride was found mostly in the ion chromatogram when 100 mg was added and the recovery of chloride came to nearly 90% if 400 mg was added. It was of interest that conventional crucible ashing at high temperature yielded a recovery of chloride slightly less than 80% while no chlorate was produced. In addition, some skill was needed for the crucible ashing to avoid splashing of organic sample during the ignition. Table 2 illustrates the recovery data of chlorine species in different matrices. Potato starch and corn starch samples without addition of the alkali carbonates produced large quantities of chlorate, while agar and gelatin samples produced nothing. When alkali carbonates were added to those matrices, the recovery of
loo-
_______---- .A --
8o
0
;,>
1
l
f
60-
P J 0 40Y oz
0
Jf--
FIG.
2. Effect
Cl-,
Na2C03
0
Cl-+
CIO-j,
Na2C03
A Cl; K2CO3
zoOI
0
0 1 0
of alkaline
200
Crucible
I 400 ml Alkali carbonate,
fixatives
on recovery
800 mg
ashing
1000
of chlorine
species.
ASHING
FOR DETERMINATION
205
OF CHLORIDE
TABLE 2 Recovery of Chlorine Species after Plasma Ashing of Alkalinized
Organic Samples
l-g sample matrix containing 11.7 mg NaCl + alkaline fixative
Cl-
CIO, -
Total
Cellulose powder + 400 mg Na,CO, + 400 mg K&O, Potato starch powder + 400 mg Na,CO, + 400 mg K,CO, Corn starch powder + 400 mg Na,CO, +400 mg K&O, Agar powder +400 mg Na,CO, +400 mg K,CO, Gelatin powder +400 mg Na,CO, + 400 mg K,CO,
45 67 87 49 88 98 36 67 83 70 71 85 75 64 93
22 25 33 8 37 21 5 14 2 -
67 92 87 82 96 98 73 88 88 70 85 87 75 64 93
Recovery (%)
chloride was generally improved, and eventually potassium carbonate was found to be a favorable fixative, yielding high total recovery of chlorine species with a minute or with no production of chlorate. The reason for such advantageous behavior using potassium carbonate was not evident, but the heavier atomic weight of potassium compared to sodium would have effectively protected the chloride in the sample from the electron and ion bombardments during the plasma ashing and would have suppressed the oxidation of chloride to chlorate. REFERENCES 1. Akimoto, N., and Hozumi, K., Working property of quick-response sodium selective glass electrode and its application to microdetermination of sodium. Japan Analyst 21, 1490-1497 (1972). 2. Gleit, C. E., and Holland, W. D., Use of electrically excited oxygen for the low-temperature decomposition of organic substances. Anal. Chem. 34, 1454-1457 (1962). 3. Hollahan, J. R., Analytical applications of electrodelessly discharged gases. J. Ckem. E&c,. 43, A401-A416 (1966). 4. Hollahan, J. R., Applications of low-temperature plasmas to chemical and physical analysis. In “Techniques and Applications of Plasma Chemistry” (J. R. Hollahan and A. T. Bell, Eds.), pp. 229-253, Wiley-Interscience, New York, 1974. 5. Hozumi, K., Chemistry of low-temperature oxygen plasma and its applications. Kagaku-NoRyoiki 25, 713-723 (1971). 6. Hozumi, K., “Low-Temperature Plasma Chemistry,” pp. 97- 111. Nankodo, Tokyo, 1976. 7. Hozumi, K., Kitamura, K., Nishikawa, T., Aoki, K., and Watanabe, M., Chemical reaction of atomic oxygen on sodium chloride during low-temperature ashing. Bunseki Kagaku 32, 525-530 (1983). 8. Hozumi, K., and Matsumoto, M., Parameters for oxidation rate of organic substances in low-temperature oxygen plasma. Japan Analyst 21, 206-214 (1972). 9. Small, H., Stevens, T. S., and Bauman, W. C., Novel ion exchange chromatographic method using conductometric detection. Anal. Ckem. 47, 1801-1975 (1975).