Anal. Chem. 1987. 59. 2218-2221
2218
Another experiment was run in which the 200-ms time interval was split into trains of smaller pulses, Le., with tz,n 20 ms. The energizing pulses, tl,n,were either constant, increasing, or decreasing in width. In the last two cases, as shown in Figure 2, the current grows in two distinct regions if the pulse is long enough. Obviously, a more elaborate hydrodynamic model is required to explain this current-time behavior. On the other hand, with the present electrode, a new drop is more rapidly stabilized. The waiting time, normally 1-2 s for an SMDE, can be shortened to as little as 80 ms for the smallest drop size, thus reducing the overall time of an experiment significantly if multiple drops are required. Performance depends on the internal diameter of the capillary. The classical DME has a bore of ca. 50 pm, whereas the SMDE has a bore of ca. 150 pm. Although drops of the same size can be formed more rapidly with a larger capillary, the latter offers less fine control of drop size. In the present design ringing occurred with a 300-pm capillary. This may be due to resonance with the mechanical motion of the valve. Of the four capillaries tested (100, 150, 200, 300 pm i.d.), the 150-pm capillary performed best with respect to stability and smooth current-time transient. “Capillary noise” due to trapping of a film of solution inside the capillary can be minimized by siliconizing (2, 10, 11). Compared with the commercially available SMDE, this electrode offers a wider operating range of drop size, shorter drop stabilization time, and easier maintanence. Together with its simple construction and relatively miniature size, it should provide an alternative to the former, especially in on-line monitoring or field work applications. In electrochemical investigations in our laboratory these electrodes have been operating reliably for months and are valued for flexibility of control and very low resistance.
-
I
I
I
I
I
io0
200
300
400
Time
(ms)
Flgure 2. Current-time transient obtained with different energizing pulse sequences: (a) f l , l = 200 ms, f l , n + l = f2,” = 0; (b) f = f2,” = 20 ms, 6 = 6‘ = 0 (n = 1, ..., 10); (c) f l , l = 20 ms, 6 = 20 ms, f2,, = 20 ms, 6’ = 0 (n = 1, ..., 4); (d) fl,l = 80 ms, 6 = -20 ms, f2,” = 20 ms, 6‘ = 0 (n = 1, ..., 4); 0.1 mM WII)in 1 M NaNO,, electrode held at -0.7 V vs. SCE.
,,”
the growth of a drop as it is operating in a “DME“ mode. Preliminary studies showed that a high Faradaic to charging current ratio was obtained if the current was sampled some time after the valve was closed in between successive actuating pulses. However, because the drop is formed rapidly in an interrupted flow, the classical expanding plane model is not applicable to describe the current-time behavior of the electrode. The situation becomes more complicated during the growth of a large drop and multiple peaks are observed. These phenonema may be traced back to the physical design of the valve system and the morphological changes of the mercury drop when it is first formed. Since the electrode described here has a different valve system and is capable of delivering the drop under different valve-actuating conditions, it serves as a better tool to study these effects. To simulate the SMDE, a mercury drop was formed by passing a single 200-ms energizing pulse. During the entire drop life, the electrode was held at a potential on the diffusion plateau of the reduction of Cd(I1); the resultant current-time transient is shown as curve a in Figure 2. As with the SMDE, there was a delay period during which no current flowed, followed by a sharp and then a slow increase in current, and finally a decrease in current due to reactant depletion after the valve was closed. The delay can be accounted for by the response time of the electromechanical valve and the inertia of the mercury to flow when the valve is opened. This is on the order of 15 ms. Although the multiple peak feature displayed by the SMDE ( 4 ) is not observed, the simple expanding plane model is inappropriate. In other experiments we have seen that at two-thirds of the maximum size the mercury drop was no longer spherical. This agrees with the observation that the current-time behavior with the SMDE is more complicated when the large drop size is used.
Registry No. Mercury, 7439-97-6.
LITERATURE CITED (1) Kissinger. P. T.; Heineman, W. R. Laboratory Techniques in ElectroenalyticalChemistry;Marcel Dekker: New York and Basel, 1984; p 268. (2) EG&G PAR Model 303 A Static Mercury Drop Electrode Operating and Service Manual. (3) Bond, A. M.; Jones, R. D. Anal. Chim. Acta 1980, 121, 1. (4) Anderson, J. E.; Bond, A. M.; Jones, R . D. J. Necfroanal. Cbem. 1981, 130, 113. (5) Osteryoung, J. Science 1982, 15, 261. (6) Kowalskl, 2 . Reviews on Analytical Chemistry; Akademia: Budapest, 1982 (Euroanalysis I V : Helsinki, 1981); p 183. (7) Foresti, M. L.; Guidelli, R . J. Nectroanal. Chem. 1986, 197, 159. (8) Guidelli, R.; Foresti, M. L. US Patent 4548679. (9) Brumieve, T.R.; O’Dea, J. J.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. ie81, 53,702. (10) Barker, G. C. Anal. Chlm. Acta 1958, 18, 118. (11) Christie, J. H.; Jackson, L. L.; Osteryoung, R. A. Anal. Cbem. 1978, 48. 561.
RECEIVED for review October 2,1986. Accepted May 11,1987. This work was supported in part by the National Science Foundation under Grants CHE8305748 and CHE 8521200.
Determination of Iodine in Foods by Cathodic Stripping Voltammetry Walter Holak Food and Drug Administration, Department of Health and H u m a n Services, New York Regional Laboratory, 850 Third Avenue, Brooklyn, N e w York 11232-1593 In its elemental or iodide forms, iodine is an essential micronutrient which is utilized by the thyroid gland for the production of triiodothyronine and thyroxine, two hormones known to regulate the basal metabolic rate. Diet is crucial
as a source of iodine and therefore as a contributor in the formation of thyroid hormones. In the United States the recommended daily allowance (RDA) of iodine for adults is 150 pg/day. It has been estimated that the average iodine
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
intake in the United States greatly exceeds the RDA, an intake that in the long run may result in adverse health-related effects (1). Methods widely used for the determination of iodine in foods include colorimetry/catalysis (2),ion selective electrode (3),gas chromatography ( 3 ) ,liquid chromatography ( 4 ) ,differential pulse polarography (5), and neutron activation analysis (6, 7). However, the reliable determination of iodine in foods is a very difficult task because of the generally very low (0.1 ppm) levels of iodine and its compounds, as determined by the collaborative study conducted by Heckman (8). Due to the complexity of the matrix, food samples must be first digested to destroy organic matter, and the liberated iodine must be isolated to eliminate interferences. The destruction of organic matter is accomplished by either a “dry ashing” in the presence of an alkali or a “wet digestion” with acids (9). The isolation is performed by distillation after reduction with phosphorous acid (10) or extraction with organic solvents (11). Recoveries of iodine are sometimes low because of volatility-related losses during the ashing/digestion steps or because of incomplete isolation/trapping when some of the aforementioned analytical methods are used on certain foods. In spite of the numerous methods available for the assay of iodine in foods, there is still a pressing need for a general, simple, and reliable one. An approach that has not been used for this purpose is cathodic stripping voltammetry (CSV) (12), undoubtedly because of the susceptibility of this technique to interference by other food components, including trace metals and other halogens. Yet, CSV is a highly attractive alternative technique for iodine analysis in foods because of its simplicity and excellent sensitivity, provided that the problems associated with the sample preparation are overcome. This goal has been achieved through the development of novel sample digestion and isolation techniques, which will ensure both an interference-free sample matrix suitable for CSV work and a quantitative recovery of the iodine.
EXPERIMENTAL SECTION Apparatus. A Model 264-A polarographic analyzer equipped with a Model 310 static mercury drop electrode (SMDE), a platinum wire auxiliary electrode, and an Ag-AgC1 reference electrode (EG & G Princeton Applied Research, Electrochemistry Products Group, Princeton, NJ) was used for all cathodic stripping work. Chemicals. All chemicals were of reagent grade. HN03, HzS04,and HC104 were Baker Instra Analyzed reagents (J. T. Baker Chemical Co., Phillipsburg, NJ). Iodine stock standard (1000 pg/mL) was prepared in water from NaI03. An iodine working standard (1pg/mL) was prepared freshly by diluting an aliquot of iodine stock standard with water. Ascorbic acid solution (5%) was prepared freshly in water. Supporting electrolyte/ trapping solution was 0.01 N HN03-0.05% ascorbic acid and was prepared by diluting HN03 with water and adding the required volume of 5% ascorbic acid. The reductant for the distillation procedure was 3% hydrazine sulfate in water. Procedure. Digestion. A quantity of sample of up to 2 g was digested with 10 mL of HN03, 10 mL of H2S04,and 30 mL of HC104containing 10 mg of K2Cr207,in a 250-mL flat-bottom flask that was attached to the apparatus described by Gorsuch (13). The digestion mixture was heated, gently at first, gradually to boiling, and eventually to refluxing for 10-15 min, until the digestion mixture turned green due to the reduction of Cr(V1) to Cr(II1). The stopcock was turned to allow the digestion mixture to distill into the collecting reservoir, until the digestion mixture in the refluxing flask became orange. The stopcock was turned once to allow the distillate in the collecting reservoir to return to the refluxing flask, and again to allow the digestion mixture to distill for a second time. The distillation and returning steps were repeated until the digestion mixture in the refluxing flask no longer turned green but remained orange throughout. This result was usually obtained after two to three cycles, after which the distillation was continued until only sulfuric acid (or a volume
2219
Figure 1. Distillationltrappingapparatus for iodine: (1) heating block, 100 OC;(2) purge tube, 20 X 160 mm, 3 24/u, with sample digest; (3) trap tube, 20 X 160 mm, 5 24/40 with trapping solution; (4) flowmeter; (5) Teflon tubing, 0.04-in. i.d.; (6) thermometer. of ca. 10 mL) remained in the refluxing flask, at which time the digestion was stopped (further heating of the acidic solution will result in the color reverting to green and in possible losses of iodine). Isolation. The digested sample, consisting essentially of 10 mL of HzS04, was aspirated into a 20 X 160 mm tube with a J 24/40 ground glass joint as shown in Figure 1. Air was drawn through the sample by means of vacuum, at a rate of 1.5 L/min for 3-5 min, in order to expel volatile substances such as chlorine. Then 10.0 mL of 0.01 M “03-0.05% ascorbic acid mixture was added to the trap tube. A 2-mL volume of 3% hydrazine sulfate was aspirated into the sample tube, and the purging of the iodine was carried out for 5 min. The sample tube was disconnected, and the flow of air was continued through the trap tube for another 3-5 min to expel sulfides and sulfites. A blank was treated in the same manner as the sample. Cathodic Stripping. The trap solution (or, if necessary, an aliquot is diluted to 10 mL with 0.01 N “03-0.05% ascorbic acid) was transferred to the electrolysiscell, deaerated by purging with nitrogen for 4 min, and subjected to CSV under the following conditions: deposition potential, 0.05 V vs. Ag-AgC1; deposition time, 240 s; equilibration time, 30 s; scan rate, 5 mV/s; mode, differential pulse; modulation amplitude, 100 mV; pulse frequency, 0.5 s; drop size, medium. A calibration curve was prepared under identical conditions over the concentration range 0-25 ng/mL of iodine, by adding 50-pL aliquots of a 1 pg/mL iodine standard to 10 mL of 0.01 N “03-0.05% ascorbic acid contained in the cell.
RESULTS AND DISCUSSION Sample Preparation. The digestion procedure is often a source of trouble in iodine determinations since this element can become reduced to the Io elemental state by sample components and, thus, be cause for losses by volatilization. Relatively large quantities of acids are required to maintain the iodine in the nonvolatile HI03 state, and the limits on sample size need to be specified. In spite of these precautions, iodine losses can still occur if sample type is not taken into account. For this reason, the use of K2Cr207as an indicator of the oxidation potential is critical to the success of the digestion. In the presence of a small amount of Crz072-in the digestion mixture, an orange color indicates that oxidizing conditions exist and that iodine is in the nonvolatile HI03 state. Therefore, by use of the proposed procedure virtually any sample can be digested without incurring losses of iodine by simply refluxing and distilling the mixture for a sufficient number of cycles until it no longer turns green but remains orange throughout the distillation cycle. The separation of iodine from interfering substances is often accomplished by purging the iodine vapors with H3P03right after reduction. This acid, however, is sometimes inadequate for completely recovering the iodine (5). Hence, the addition of chemicals like NaN03 (5)or H202(8)may become necessary to improve the recovery. These additions were also found inadequate because interfering substances were both codistilled with the iodine and became collected in the trap solution.
2220
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 r .4
It
5
0
'5
20
25
Concertration, ng/trL
Flgure 3. Calibration curve for I- derived from CSV scans shown in Figure 2. +.^I5
_..______~ -)if
'ic .CI'j
-13C c
L;
O K
-15-
3iC
Table I. Results ( p g / g ) of the Analysis of Food Samples and Spikes by the Proposed CSV Method
CQ'I
Flgure 2. Typical CSV scans of serial (0-25 ng/mL) 1- dilutions in 0.01 N HN03-0.05% ascorbic acid: 1, supporting electrolyte (0 ng/mL); 2-6, I- solutions (5-25 ng/mL). The experimental conditions are given
in the text
A more successful alternative was to add a small amount of hydrazine sulfate to the sample digest, which had been first brought to the state of a H2S04solution. In this medium the reduction of HIO, to iodine was very rapid at 100 "C, and the iodine could be flushed out and trapped in less than 5 min. The reduction process was studied by using milligram quantities of KIO, so that the formation of iodine could be visually ascertained. The addition of a greater than specified volume of hydrazine sulfate solution will both adversely dilute the H2S04mixture and allow some of the HIO, to become reduced to I-. Furthermore, if solid hydrazine sulfate were to be added, relatively large amounts of H2S and SO, would be generated from the reduction of H2S04. If these byproducts are present, they must be flushed out of the solution to prevent their depositing and stripping at potentials that are close to those of iodide. Ascorbic acid was added to the trapping solution to reduce iodine to nonvolatile I-. Voltammetry. Propst (12) has described experimental conditions for the normal pulse CSV analysis of iodine. According to this author, under the experimental conditions employed, the combined use of 0.01 N "OB and a deposition time of 4 min was optimum for the submonolayer deposition of iodine as Hg,I,, and for minimizing anomalies resulting from the formation of Hg20. The addition of ascorbic acid, on the other hand, prevented the loss of I- by oxidation to iodine. At variance with Propst, the proposed method uses the differential pulse mode for stripping and a somewhat higher concentration of ascorbic acid in the 0.01 N nitric acid supporting electrolyte, so that the same solution could be used for trapping the iodine during the isolation step. Figure 2 shows typical differential pulse CSV scans for concentrations of I- in the range 0-25 ng/mL. The reason for the curvature of the stripping current vs. iodine concentration could be due to the solubility of the Hg,12 deposit as well as the lower efficiency of the deposition at low halogen levels (14). These scans were subsequently used to plot the calibration curve shown in Figure 3. The applicability of the proposed method was demonstrated by analyzing a number of food samples, before and after suitable spikes (Table I). On the basis of replicate analysis of a 0.1 pg/g I standard sample, the precision of the method showed a relative standard
?&
sample
found
added total found recovered
strained apple sauceo
0.010 0.010 0.000 0.170 0.108 0.000 0.000
0.200 0.100 0.100 0.400 0.200 0.100 0.200
sugar
ketchup cream substituteb canned tuna in water instant coffee fresh scallops
'Baby food.
0.215 0.099 0.090 0.550 0.325 0.950 0.195
102.5 89.0 90.0 95.0 108.5 95.0 97.5
Half & Half.
Table 11. Results (wg/g) of the Analysis of NBS Standard Reference Materials
orchard method NBS value" ref 'ib ref 8 proposed
leaves SRM 1571 0.17 0.183b 0.220' 0.172 f 0.012
a Uncertified values. colorimetry.
bovine liver SRM 1577
spinach SRM 1570
0.18 0.249 f 0.12b 0.283*
1.325 f 0.005b
0.236 + 0.021
1.227 f 0.064
Neutron activation analysis.
Ce-As-I
deviation of 6.0% ( n = 6). The detection limit was found to be governed by the contribution of the reagent blank, which typically gave average f standard deviation readings of 0.015 f 0.002 pg of iodine ( n = 6). Defining the detection limit as twice the standard deviation of the blank, this value was 0.002 pg/g for a 2-g sample. A number of NBS standard reference materials were analyzed by the proposed method, and the results were compared to those reported in the literature (Table 11). ACKNOWLEDGMENT
The author thanks Cesar A. Lau-Cam, Science Advisor, New York Regional Laboratory, Food and Drug Administration, Brooklyn, NY, and Professor of Pharmaceutical Sciences, St. John's University, College of Pharmacy & Allied Health Professions, Jamaica, NY, for his help and suggestions in the preparation of this paper. Registry No. I,, 7553-56-2; sugar, 57-50-1. LITERATURE CITED (1) Connelly, R . J. Lancet 1970, 1 , 500. (2) Sandell, E. B.: Kolthoff. I. M. Mikrochim. Acta 1937, 7 , 9.
Anal. Chem. 1907, 59. 2221-2224 MUes, P. J . ASSOC.Off. A M . Chem. 1978, 6 1 , 1388. Hurst, W. J.; Snyder, K. P., Martin, R. A., Jr. J. Liq. Chromatogr. 1983, 6 , 2067. Thompson, D.;Lee, S.; Allen, R. J. Assoc. Off. Anal. Chem. 1983, 6 6 , 1380. Melvano, R.; Buzzlgoll, G.; Scarlatlni, M.; Cenderelli, G.; Gondolfi, C.; Grasso, P. Anal. Chem. 1972, 201. Allegrini, M.; Boyer, K. W.; Tanner, J. T. J. Assoc. Off. Anal. Chem. 1881. 6 4 . 1111. Heckman; M. M. J. Assoc. Off. Anal. Chem. 1979, 62, 1045. Fisher, P. W. F.; L’Abbe, R. J. Assoc. Off. Anal. Chem. 1981, 6 4 , 71.
2221
Ahmad, N. Aust. J. Dairy Techno/. 1977. 32, 102. Johansen, 0.; Stelnnes, E. Analyst (London) 1978, 707, 455. Propst, R. C. Anal. Chem. 1977, 49, 1198. Gorsuch, T. T. The Destruction of Organic Matter; Pergamon: New York, 1970; pp 137-138. (14) Colovos, G.;Wilson, G. S.; Moyers, J. L. Anal. Chem. 1974, 46, 1045. (10) (1 1) (12) (13)
for review December 22* lg8&Accepted May 7,
1987.
Construction and Comparison of Open Tubular Reactors for Postcolumn Reaction Detection in Liquid Chromatography Carl M. Selavka, Kai-Sheng Jiao, and Ira S. Krull* Barnett Institute of Chemical Analysis and Department of Chemistry, Northeastern University, Boston, Massachusetts 02115 There has been a great deal of interest in the past 10 years surrounding the use of derivatization in high-performance liquid chromatography (LC) for improving detection selectivity or sensitivity. On-line, postcolumn derivatization offers a number of advantages over other possible derivatization approaches, but the possibility of band broadening in the postcolumn reactor (PCR) must always be a concern of the analyst. The choice of PCR design is generally dictated by the reaction time (trx) required: segmented flow systems are best for t,, > 5 min, packed bed reactors are useful for 1 min < t,, < 5 min, and knitted open tubular (KOT) reactors are best incorporated for 0 min < t,, < 5 min ( I ) . There have been a number of reports regarding the theoretical basis for minimizing band broadening when open tubular reactors are geometrically deformed by helically coiling the tubing (2-5). Recently, this concept has been extended to, and the hydrodynamic equations explained and solved for, capillary supercritical fluid chromatography (6). The principles for improvements in radial mixing in coiled tubes have been exploited through the introduction of the KOT, also known as a three-dimensional coiled tube ( I , 7, 8), but the equations for these reactors have not been solved to date. Our interest in the development of a postcolumn on-line photochemical reactor for improved electrochemical (EC) detection in LC first led us to the incorporation of coiled Teflon tubing as the PCR, in which the tubing was wound about a Hg lamp (9). While this PCR design enabled the demonstration of the detection principle, there were problems with band broadening, resulting in loss of resolution and poor detection limits. T o improve upon the initial LC-photolysis-EC (LC-hv-EC) system, we wished to incorporate a KOT PCR, but the only literature report that described the actual construction process for such a reactor (7) utilized a “French knitting” children’s toy (called “Strickliesel” in Germany), which we were unable to locate in area stores. Only after consultation (10) with one of the authors of the previous work were we able to satisfactorily construct our own KOT. Shortly after we performed the work described in this paper, a report appeared regarding the construction of KOTs using crocheting tools and techniques (11). The crocheting techniques appeared relatively straightforward, except for the authors’ admonition to fill the tubing with water before construction, in order to avoid kinking of the tubing during the construction process. This kinking would reduce the lifetime of the finished KOT by increasing its back pressure. The methods we describe do not involve the use of tools of any kind. The impetus for this paper is the large number of requests we have received for instruction in constructing a KOT, as
a result of some publications on LC-hv-EC which incorporated KOT PCRs (12). In this short report, the design of three different KOT reactors is described. The performances of these reactors are then compared to coiled and straight open tubes, using plate height vs. linear velocity (h vs. u) plots and calculations of relative band broadening. The effect of tubing diameter on the performance of the PCRs is also discussed. Finally, the criteria for the use of these KOT designs in chemical and photochemical postcolumn reaction detection schemes are briefly delineated.
EXPERIMENTAL SECTION Apparatus. The flow injection analysis (FIA) system used has been described elsewhere (12) and consisted of a pump, injector, and UV (214 nm) detector. The time constant on the UV detector was set at 0.5 s. The PCRs were constructed from Teflon tubing (Rainin Instrument Co., Woburn, MA) having 0.8-, 0.5or 0.3-mm i.d., in lengths chosen to give constant reactor volumes of 1.7 mL (3.38, 8.66, and 24.05 m, respectively). Reagents. HPLC-grade methanol (MeOH) and water used in the mobile phase were obtained as the Omnisolv grade (MCB, Cherry Hill, NJ) and were degassed before use. The test analyte, sodium benzoate, was obtained as the food grade (J. T. Baker Chemical Co., Phillipsburg, NJ) and was used as received. Procedure. In general, the best performance from a KOT will be realized if the tubing is turned in alternating 180° arcs in three dimensions, keeping the diameter of the arc as small as possible without pinching the cross section of the tube into an elliptical geometry. This maximizes the aspect ratio (A = d / D , where d is the internal diameter of the tube and D is the diameter of the arc through which the tubing is turned), which yields better radial mixing in a KOT. Pinching the tubing leads to an undue amount of back pressure (increases AP),which shortens the lifetime of the KOT PCR and limits the range of flow rates available for a separation. Three distinct KOT geometries were studied, labeled KOT1, KOT2, and KOT3 for reference. The construction of KOT1, which was designed by Uwe Neue of Waters Chromatography Division, Millipore Corp., is shown in Figure 1. The analytical tubing is knitted around a backbone tube which, depending on the eventual use of the PCR, can be made of steel or Teflon. However, it should be noted that the use of a Teflon backbone tube having a larger internal diameter than the analytical tube tends to limit the chance of deforming the cross section of the analytical tube, because the backbone deforms first if too much tension is put on the analytical tube during knitting. In the design of KOT1, the analytical tube turns back against itself on each loop, which repeatedly changes the direction of flow in three dimensions. The construction of KOT2, which was designed by one of the authors (C.M.S.) in our laboratory, is demonstrated in Figures 2 and 3. Once again, a backbone tube is used in the design of
0003-2700/87/0359-2221$01.50/0 0 1987 American Chemical Society
