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A novel method for the determination of iodide in table salt by X-ray fluorescence
MICROCHEMICAL
JOURNAL
A Novel Method
J. F. LAWRENCE,
31, 237-240 (1985)
for the Determination of Iodide in Table Salt by X-Ray Fluorescence R. K. CHADHA, R. O’BRIEN,
Food Research Division,
Health Protection
Branch,
AND H. B. S. CONACHER
Ottawa,
Ontario,
Canada KlA OL2
Received February 12, 1983 A novel HI generation technique was developed and evaluated for the determination of iodide in table salt. HI was generated from the samples by the addition of concentrated sulfuric acid and adsorbed onto an ion-exchange resin loaded paper disk. The disk was then measured by X-ray fluorescence. The method compared favorably to the cellulose pellet technique, being more sensitive and reproducible and requiring less time per analysis. The method was applied to six samples of table salt and one of sea salt. D 1985 Academic PESS, 1~.
INTRODUCTION
It has been reported that in recent years the U.S. Recommended Daily Allowance of 150 kg of iodine is exceeded 4-7 times in most American diets (I). This finding has created some concern since high dietary levels of iodine have been implicated in thyrotoxicosis (2, 5). As a result a need has arisen to monitor iodine content of the diet to determine the major sources of the substance. X-Ray fluorescence, being a very selective technique for the determination of a number of elements, is well suited to monitoring iodine with a minimum of sample preparation or purification compared to a number of other techniques. It has been used for the determination of iodine in milk by a pellet method (3) and by using an ion-exchange filter (4). In this work we have developed a novel HI generation technique which employs ion-exchange resin disks, and compared it to a pellet method for the determination of potassium iodide in table salt. MATERIALS
AND METHODS
Chemicals Sodium chloride (Fisher Scientific) was ACS reagent grade. Potassium iodide (Analabs) was analytical grade. Cellulose powder (Mandel Scientific) was Type CC3 1. Samples of iodized table salt and untreated sea salt were purchased locally. Solutions Potassium iodide (KI) stock solution was prepared at 1 mg KI/ml in distilled deionized water. The KI-NaCl standard solutions were prepared by adding 0, 2.0, 4.0, 6.0, and 8.0 ml of the KI stock solution to 40-g samples of NaCl in individual 200-ml volumetric flasks, and diluting to the mark with distilled deionized water. Standard addition solutions were prepared as above with 40 g of the table salt and sea salt samples. 237 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
238
LAWRENCE ET AL.
Apparatus A Phillips PW 1410 vacuum X-ray spectrometer was operated under the following conditions: tungsten target tube, 50 kV and 50 mA; PET analyzing crystal (2d spacing, 8.742 A); flow counter, 1610 V (dc); vacuum and spinner on; iodide peak intensity measured at 42.21” (28) for 1 min. Background was determined as the average of readings at 0.5” on either side of 20. The ion-exchange disks (5.0 cm diam.) were cut from SB-2 ion-exchange paper (Reeve-Angel). For HI generation a disk was inserted between the vertical mouth (29/42 joint) of a two-necked conical flask and an inverted reducing adaptor (291 42 to 24/40) as shown in Fig. 1. A 125-ml separatory funnel with a 24/40-ground glass spout was inserted into the side arm (24/40 joint) of the flask for H,SO, addition. Procedure (1) Pellet method. A 40-ml aliquot of a standard solution was transferred to a 150-ml beaker containing 4 g cellulose powder and mixed with a stirring rod. The beaker was placed in an oven at 115°C until the contents were completely dry (about 4 hr). The contents were scraped from the beaker into a mortar and pestle to break the lumps. The powder was then placed in a Spex mixer and homogenized with plastic-ball bearings. A 3-g quantity of the resulting powder was compressed into a 3-cm-diameter pellet under a pressure of 20 tons. The pellet was mounted on the X-ray unit and counted. This procedure was used for both standards and samples. (2) HZ generation method. A S-ml volume of a standard solution was pipetted into a two-neck conical flask and the solution evaporated to dryness in an oven at 115°C. A bar magnet was placed in the flask and an ion-exchange filter disk secured to the vertical mouth of the flask with the inverted reducing adaptor and a clamp, as shown in Fig. 1. The apparatus was then placed in a 90°C glycerin bath on a hot plate with a magnetic stirrer. A 6.5-ml volume of concentrated
DISK-
FIG. 1. HI generation apparatus.
239
IODINE IN TABLE SALT
sulfuric acid was added to the separatory funnel which was then attached to the side neck. Six milliliters of the acid was added dropwise over a period of 8 min while the sample mixture was stirred. The contents were heated with stirring for a further 2 min after which time the disk was removed and left in fumehood for 5-10 min to allow HCl vapors (produced from the NaCl) to dissipate. The disk was then mounted in the X-ray spectrometer and counted. This was repeated for both standards and samples. The table salt solutions were shaken immediately prior to taking an aliquot for HI generation analysis. RESULTS AND DISCUSSION
In a comparison of the HI generation and the cellulose pellet method, it was found that the former generated about 70 times the number of counts as the latter at concentrations of KI between 10 and 200 ppm in the presence of NaCl even though the HI generation technique used only one-half the sample size. The detection limit for the pellet method was about 8 ppm while that for the HI method was about 0.1 ppm. At concentrations of 0.1-10 ppm the standard curve for the HI generation was exponential in nature while above 10 ppm straight lines were obtained for both methods. The reproducibility was found to be superior for the HI generation method. For example, at 100 ppm KI (the regulatory level for table salt, according to the Canadian Food and Drug Regulations) results of four replicate analyses yielded a coefficient of variation of 6.1% for the pellet method while only 2.1% was obtained with HI generation. The poorer reproducibility for the pellet method likely arises from difficulty in obtaining a homogeneous cellulose mixture for pellet-making. Quantitation was performed by two methods, one by comparing the samples to known synthetic standards carried through each procedure and the other by standard addition to the samples. Table 1 compares the results for the two analytical techniques for the salt samples. It can be observed that the variation is much less with the HI generation method and that quantitation using the standard curve or standard addition are comparable unlike the results of the pellet method. TABLE 1 Determination of KI by Pellet and HI Generation Pellet method” Standard Salt sample 1. Table salt 2. Table salt 3. Table salt 4. Table salt 5. Table salt 6. Table salt 7. Sea salt u Parts per million.
curve
104 105 123 104 102 95 Trace
HI generation”
Standard addition
Standard curve
Standard addition
94 110 144 115 90 70 -
91 100 94 85 86 91 2.5
87 110 99 89 90 95 -
240
LAWRENCE
ET AL.
The slightly lower levels found (on the average 4%) employing the standard curve for the HI generation compared to the method of standard additions is possibly due to some adsorption of iodide onto the surface of the sodium silica aluminate which is added to the table salt to improve its free-flowing nature. In fact the aqueous solutions of the table salt samples had to be shaken immediately before transferring an aliquot to the HI generation flask so that any settled sodium silica aluminate particles would be resuspended. If this was not done, low values would invariably result. In general, the HI generation is to be preferred in terms of sensitivity, reproducibility, and ease of sample preparation. While the technique was successfully applied to the determination of KI in table salt and sea salt, it has the potential, with modification, for application to a wide variety of iodide-containing substances including some foods. The generation of HI and subsequent adsorption on ion-exchange paper disks offers an interesting means of isolating iodide from complex sample matrices. The success of the approach will likely depend upon how certain sample components such as the organic constituents in some foods interact with iodide under the strongly acidic conditions used for HI generation. Research along these lines is being explored. REFERENCES 1. Bruhn, J., Feeds are major source of iodine in dairy products. Food Chem. News 22, 50-51 (1980). 2. Connolly, R. J., Seasonal variation in thyroid function. Med. J. Aust. March, 633-636 (1971). 3. Crecelius, E. A., Determination of total iodine in milk by X-ray fluorescence spectrometry and iodide electrode. Anal. Chem. 47, 2034-2035 (1975). 4. Lawrence, J. F., Chadha, R. K., and Conacher, H. B. S., The use of ion-exchange filters for the determination of iodide in milk by X-ray fluorescence spectrometry. Int. J. Environ. Anal. Chem. 15, 303-308 (1983). 5. Wolff, J., Iodide goiter and the pharmacologic effects of excess iodide. Amer. J. Med. 47, IOl124 (1969).
JOURNAL
A Novel Method
J. F. LAWRENCE,
31, 237-240 (1985)
for the Determination of Iodide in Table Salt by X-Ray Fluorescence R. K. CHADHA, R. O’BRIEN,
Food Research Division,
Health Protection
Branch,
AND H. B. S. CONACHER
Ottawa,
Ontario,
Canada KlA OL2
Received February 12, 1983 A novel HI generation technique was developed and evaluated for the determination of iodide in table salt. HI was generated from the samples by the addition of concentrated sulfuric acid and adsorbed onto an ion-exchange resin loaded paper disk. The disk was then measured by X-ray fluorescence. The method compared favorably to the cellulose pellet technique, being more sensitive and reproducible and requiring less time per analysis. The method was applied to six samples of table salt and one of sea salt. D 1985 Academic PESS, 1~.
INTRODUCTION
It has been reported that in recent years the U.S. Recommended Daily Allowance of 150 kg of iodine is exceeded 4-7 times in most American diets (I). This finding has created some concern since high dietary levels of iodine have been implicated in thyrotoxicosis (2, 5). As a result a need has arisen to monitor iodine content of the diet to determine the major sources of the substance. X-Ray fluorescence, being a very selective technique for the determination of a number of elements, is well suited to monitoring iodine with a minimum of sample preparation or purification compared to a number of other techniques. It has been used for the determination of iodine in milk by a pellet method (3) and by using an ion-exchange filter (4). In this work we have developed a novel HI generation technique which employs ion-exchange resin disks, and compared it to a pellet method for the determination of potassium iodide in table salt. MATERIALS
AND METHODS
Chemicals Sodium chloride (Fisher Scientific) was ACS reagent grade. Potassium iodide (Analabs) was analytical grade. Cellulose powder (Mandel Scientific) was Type CC3 1. Samples of iodized table salt and untreated sea salt were purchased locally. Solutions Potassium iodide (KI) stock solution was prepared at 1 mg KI/ml in distilled deionized water. The KI-NaCl standard solutions were prepared by adding 0, 2.0, 4.0, 6.0, and 8.0 ml of the KI stock solution to 40-g samples of NaCl in individual 200-ml volumetric flasks, and diluting to the mark with distilled deionized water. Standard addition solutions were prepared as above with 40 g of the table salt and sea salt samples. 237 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
238
LAWRENCE ET AL.
Apparatus A Phillips PW 1410 vacuum X-ray spectrometer was operated under the following conditions: tungsten target tube, 50 kV and 50 mA; PET analyzing crystal (2d spacing, 8.742 A); flow counter, 1610 V (dc); vacuum and spinner on; iodide peak intensity measured at 42.21” (28) for 1 min. Background was determined as the average of readings at 0.5” on either side of 20. The ion-exchange disks (5.0 cm diam.) were cut from SB-2 ion-exchange paper (Reeve-Angel). For HI generation a disk was inserted between the vertical mouth (29/42 joint) of a two-necked conical flask and an inverted reducing adaptor (291 42 to 24/40) as shown in Fig. 1. A 125-ml separatory funnel with a 24/40-ground glass spout was inserted into the side arm (24/40 joint) of the flask for H,SO, addition. Procedure (1) Pellet method. A 40-ml aliquot of a standard solution was transferred to a 150-ml beaker containing 4 g cellulose powder and mixed with a stirring rod. The beaker was placed in an oven at 115°C until the contents were completely dry (about 4 hr). The contents were scraped from the beaker into a mortar and pestle to break the lumps. The powder was then placed in a Spex mixer and homogenized with plastic-ball bearings. A 3-g quantity of the resulting powder was compressed into a 3-cm-diameter pellet under a pressure of 20 tons. The pellet was mounted on the X-ray unit and counted. This procedure was used for both standards and samples. (2) HZ generation method. A S-ml volume of a standard solution was pipetted into a two-neck conical flask and the solution evaporated to dryness in an oven at 115°C. A bar magnet was placed in the flask and an ion-exchange filter disk secured to the vertical mouth of the flask with the inverted reducing adaptor and a clamp, as shown in Fig. 1. The apparatus was then placed in a 90°C glycerin bath on a hot plate with a magnetic stirrer. A 6.5-ml volume of concentrated
DISK-
FIG. 1. HI generation apparatus.
239
IODINE IN TABLE SALT
sulfuric acid was added to the separatory funnel which was then attached to the side neck. Six milliliters of the acid was added dropwise over a period of 8 min while the sample mixture was stirred. The contents were heated with stirring for a further 2 min after which time the disk was removed and left in fumehood for 5-10 min to allow HCl vapors (produced from the NaCl) to dissipate. The disk was then mounted in the X-ray spectrometer and counted. This was repeated for both standards and samples. The table salt solutions were shaken immediately prior to taking an aliquot for HI generation analysis. RESULTS AND DISCUSSION
In a comparison of the HI generation and the cellulose pellet method, it was found that the former generated about 70 times the number of counts as the latter at concentrations of KI between 10 and 200 ppm in the presence of NaCl even though the HI generation technique used only one-half the sample size. The detection limit for the pellet method was about 8 ppm while that for the HI method was about 0.1 ppm. At concentrations of 0.1-10 ppm the standard curve for the HI generation was exponential in nature while above 10 ppm straight lines were obtained for both methods. The reproducibility was found to be superior for the HI generation method. For example, at 100 ppm KI (the regulatory level for table salt, according to the Canadian Food and Drug Regulations) results of four replicate analyses yielded a coefficient of variation of 6.1% for the pellet method while only 2.1% was obtained with HI generation. The poorer reproducibility for the pellet method likely arises from difficulty in obtaining a homogeneous cellulose mixture for pellet-making. Quantitation was performed by two methods, one by comparing the samples to known synthetic standards carried through each procedure and the other by standard addition to the samples. Table 1 compares the results for the two analytical techniques for the salt samples. It can be observed that the variation is much less with the HI generation method and that quantitation using the standard curve or standard addition are comparable unlike the results of the pellet method. TABLE 1 Determination of KI by Pellet and HI Generation Pellet method” Standard Salt sample 1. Table salt 2. Table salt 3. Table salt 4. Table salt 5. Table salt 6. Table salt 7. Sea salt u Parts per million.
curve
104 105 123 104 102 95 Trace
HI generation”
Standard addition
Standard curve
Standard addition
94 110 144 115 90 70 -
91 100 94 85 86 91 2.5
87 110 99 89 90 95 -
240
LAWRENCE
ET AL.
The slightly lower levels found (on the average 4%) employing the standard curve for the HI generation compared to the method of standard additions is possibly due to some adsorption of iodide onto the surface of the sodium silica aluminate which is added to the table salt to improve its free-flowing nature. In fact the aqueous solutions of the table salt samples had to be shaken immediately before transferring an aliquot to the HI generation flask so that any settled sodium silica aluminate particles would be resuspended. If this was not done, low values would invariably result. In general, the HI generation is to be preferred in terms of sensitivity, reproducibility, and ease of sample preparation. While the technique was successfully applied to the determination of KI in table salt and sea salt, it has the potential, with modification, for application to a wide variety of iodide-containing substances including some foods. The generation of HI and subsequent adsorption on ion-exchange paper disks offers an interesting means of isolating iodide from complex sample matrices. The success of the approach will likely depend upon how certain sample components such as the organic constituents in some foods interact with iodide under the strongly acidic conditions used for HI generation. Research along these lines is being explored. REFERENCES 1. Bruhn, J., Feeds are major source of iodine in dairy products. Food Chem. News 22, 50-51 (1980). 2. Connolly, R. J., Seasonal variation in thyroid function. Med. J. Aust. March, 633-636 (1971). 3. Crecelius, E. A., Determination of total iodine in milk by X-ray fluorescence spectrometry and iodide electrode. Anal. Chem. 47, 2034-2035 (1975). 4. Lawrence, J. F., Chadha, R. K., and Conacher, H. B. S., The use of ion-exchange filters for the determination of iodide in milk by X-ray fluorescence spectrometry. Int. J. Environ. Anal. Chem. 15, 303-308 (1983). 5. Wolff, J., Iodide goiter and the pharmacologic effects of excess iodide. Amer. J. Med. 47, IOl124 (1969).