Leak in evaporated milk by anodic stripping voltammetry and atomic absorption spectrophotometry: cooperative interlaboratory study

The results of a cooperative study on the determination of lead in evaporated milk, using a double blind referee technique, are reported. This study was designed to determine the normal variability of methods currently used for lead analysis by canned food industry laboratories. Twenty-three laboratories participated in this study. Each laboratory was instructed to use atomic absorption spectrophotometry (AOAC 25.065), anodic stripping voltammetry, or carbon rod atomic absorption spectrophotometry. Overall, the results appear to be in close agreement with the spiking levels. The coefficient of variation for all laboratories was 36.0% at the 0.15 ppm lead level and 16.8% at the 0.40 ppm lead level.     

Optimized Monier-Williams method for determination of sulfites in foods: collaborative study

A collaborative study was conducted of the Food and Drug Administration (FDA)-optimized Monier-Williams method for determining sulfites in foods. Twenty-one industry and government laboratories participated in the study, which was jointly sponsored by the National Food Processors Association and FDA. Familiarization samples were shipped to each collaborator. Collaborators were permitted to proceed to the main study only after they demonstrated ability to perform the method to ensure that the study tested the performance of the method itself and not that of the individual laboratories. The study design involved 3 food matrixes (hominy, fruit juice, and protein [seafood]). Each matrix was prepared at 3 sulfite levels--the regulatory level, half the regulatory level, twice the regulatory level--and as a blank. All test samples were analyzed as blind duplicates, which gave each collaborator a total of 24 test portions. Collaborative recoveries gave a reproducibility (among-laboratories) coefficient of variation that ranged from 15.5 to 26.6% for sulfite determined as SO2 by weight in the 3 foods at the 10 ppm level. The optimized Monier-Williams method has been approved interim official first action to replace the AOAC modified Monier-Williams method, 20.123-20.125.     

Analysis for total sulfite in foods by using rapid distillation followed by redox titration

A rapid and accurate analysis for total sulfite as sulfur dioxide has been developed for foods and food products. The method, which combines a selective distillation cleanup procedure with the selective redox titration of sulfite ion by iodine, has been applied to a variety of foods and food products over a period of time with no significant interference encountered in any matrixes other than garlic and leeks. For the foods analyzed, the method typically shows a detection limit of 10 ppm, a relative standard deviation of 7.5% (compared with 10.4% for similar matrixes by the Monier-Williams method), and recoveries of 97.9 +/- 6.4%. Comparison of results for this method with those obtained using the Monier-Williams method showed a mean value for the distillation/titration method of 241 ppm compared with 242 ppm for the Monier-Williams method. A correlation of 0.991 and odds of a difference between methods of 10.7% (Student's paired t-test (1-alpha) X 100) were obtained for those matrixes where no interferences were encountered with either method.     

Differential pulse polarographic determination of iodine in foods and nutritional products

A differential pulse polarographic procedure is described for the analysis of iodine in foods and nutritional products. Samples with iodine concentrations ranging from 5000 to 0.2 ppm have been successfully analyzed using this procedure. Precision averaged between 2 and 10% relative to the iodine level measured. Recoveries of added iodine ranged from 89 to 108% with external standards, and 97-100% by an analyte additions technique. Samples analyzed include dried almonds, whey protein concentrate, nonfat dried milk, sea kelp, vitamin-mineral nutritional supplements, diet meal replacement products, dried green beans, dried mushrooms, and wheat germ.     

Interlaboratory comparison of analyses for heavy metals in clam tissue

Results are reported for an interlaboratory study conducted to assess the reproducibility of analyses for lead, copper, cadmium, and zinc. The 10 participating laboratories analyzed 2 samples of freeze-dried clam tissue and 1 disguised sample of NBS Oyster Tissue. Interlaboratory variations were observed for all metals, although the methods yielded reproducible data for Cu and Zn with average interlaboratory coefficients of variation of 15 and 11%, respectively. The performance of methods used for Cd and, more so, for Pb was less than satisfactory. Cadmium levels in the 3 samples ranged from about 0.7 to 3.7 ppm with an average interlaboratory coefficient of variation (CV) of 24%. Lead levels in the 3 samples were about 0.5 ppm with an interlaboratory CV of 68%. Some laboratories' results were consistently high or low but data were insufficient to relate these trends to one particular variable. Results from this study were compared with 5 other studies reported in the literature since 1980. Coefficients of variation from all studies were comparable for samples with similar metal concentrations.     

Determination of sulfite in foods and beverages by ion exclusion chromatography with electrochemical detection: collaborative study

A liquid chromatographic (LC) method for determination of total sulfite in foods and beverages by alkali extraction followed by ion exclusion chromatographic separation and electrochemical detection (IEC-EC) was collaboratively studied by 9 laboratories. Blind duplicate samples of starch, diluted lemon juice, wine cooler, dehydrated seafood, and instant mashed potatoes were analyzed without spiking and with added sulfite at 2 levels. The initial sulfite levels varied from 0 to 384 ppm SO2, and the levels added varied from 10 to 400 ppm. The initial sulfite levels determined by the IEC-EC method and the Monier-Williams method were in good agreement. Recovery of added sulfite by the IEC-EC method was generally higher than that by the Monier-Williams method. Within-laboratory repeatability (RSDr) for the IEC-EC method varied from 4.4 to 26.0%, and overall reproducibility (RSDR) varied from 8.5 to 39.3%. The collaborators found the method to be fast, sensitive, and easy to use, which makes it a useful alternative to the Monier-Williams method. The method has been adopted official first action.     

Nitrate determination in baby food, using the nitrate ion selective electrode.

The nitrate electrode has been utilized in the determination of nitrate content in food products. The AOAC xylenol method was employed for comparative results. A reasonable correlation (r=0.91) was found between the 2 methods in the analysis of 49 samples containing 30-350 ppm nitrate. At the average nitrate content (100 ppm) of these foods, the standard error was 4.3 ppm. The electrode responds directly to the ionic activity of the nitrate ion. It has a linear concentration range of 1-6000 ppm nitrate and can be used over a wide pH range. The electrode does respond to some extent to anions other than nitrate, and some interferences do occur. These interferences are easily controlled by the use of cation exchange resins. The Corning known addition (spiking) method is used on all samples to insure correct electrode response in solutions containing variable background ionic composition. The electrode has the advantage of simplicity, speed, reproducibility, and accuracy. Work time saved using the electrode as opposed to the xylenol method is about 7 hr for the analysis of 20 samples. Error, and the need for repeating analysis, is much less frequent.     

Determination of 2-alkylcyclobutanones with electronic impact and chemical ionization gas chromatography/mass spectrometry (GC/MS) in irradiated foods

The use of a column containing 60 g of silica gel for cleanup and the use of isobutane as a reactant reagent for chemical ionization-mass spectrometric analysis of the saturated and monounsaturated alkyl side-chain 2-alkylcyclobutanones (2-ACBs; specifically induced by irradiation from fat in foods until the proof of contrary) has improved both the sensibility and the selectivity of the method when applied for the detection of irradiated foods. The quality of the chromatograms obtained was improved, allowing the detection of food samples (avocados) irradiated at low doses (0.1 kGy) or irradiated ingredients included in low proportions (less than 5%, wt/wt) in nonirradiated culinary foods. These analytical modifications for the detection of 2-ACBs on the official EN 1785 method enable an extension of its current field of application using common equipment of food quality control laboratories.     

Mass spectrometric identification and gas-liquid chromatographic determination of 2-chloroethyl esters of fatty acids in spices and foods.

The 2-chloroethyl esters of 5 fatty acids have been identified in spice and food samples by gas-liquid chromatography-mass spectrometry (GLC/MS). Twenty-four spice samples were analyzed for the 2-chloroethyl esters of fatty acids by AOAC official multiple residues pesticide procedure using GLC with microcoulometric detection. The esters of capric, lauric, myristic, palmitic, and linoleic acids have been identified at levels up to 1400 ppm. 2-Chloroethyl linoleate was the most abundant ester in all samples. Several foods analyzed by the same procedures showed levels of 2-chloroethyl linoleate as high as 35 ppm. Recoveries from fortified samples ranged from 84 to 98% for the various esters. A method using an acid-catalyzed esterification reaction was developed to rapidly determine the fatty acid content of these spices. GLC analysis with microcoulometric detection was used. Recoveries from fortified samples ranged from 92 to 110%. After 2 spice samples found to be free of 2-chloroethyl esters were fumigated with ethylene oxide, the level of 2-chloroethyl linoleate reached 77 ppm. All levels of 2-chloroethyl esters were confirmed by GLC/MS.     

Gas chromatographic-thermal energy analysis method for determination of volatile N-nitrosamines in baby bottle rubber nipples: collaborative study

A collaborative study was conducted on the U.S. Food and Drug Administration (FDA) dichloromethane extraction method for determining volatile N-nitrosamines in baby bottle rubber nipples. Following dichloromethane extraction, N-nitrosamines were determined by gas chromatography-thermal energy analysis. Six pairs of blind duplicate rubber nipple samples representing 6 lots were analyzed by 11 collaborating laboratories. All samples were portions taken from equilibrated composites of cut-up rubber nipples obtained from manufacturers in the United States. Recoveries of the internal standard (N-nitrosodipropylamine) at approximately 20 ppb ranged from 10 to 120%. Reproducibility relative standard deviations (RSDx) were between 35 and 45% for N-nitrosamine levels from 10 to 20 ppb. However, when data from laboratories with recoveries less than 75% were excluded (this is now specified in the method), RSDx values were between 11 and 32% for N-nitrosamine levels from 6 to 26 ppb. Values were consistent with or better than those reported for other analytical techniques designed to quantitate trace contaminants at the low ppb level, e.g., aflatoxin in foods. The method has been adopted official first action for the quantitation of volatile N-nitrosamines in baby bottle rubber nipples.     

Colorimetric determination of total iodine in foods by iodide-catalyzed reduction of Ce+4

An earlier acid digestion determination of iodine in foods was modified to provide an improved detection limit and to allow for the analysis of a greater variety and larger amounts of foods. The organic material in the sample was oxidized overnight by concentrated nitric acid, followed by digestion in a mixture of concentrated sulfuric and 70% perchloric acid. The iodine was determined by an automated colorimetric method based on the iodide-catalyzed reduction of Ce+4 by As+3. The method had an average relative standard deviation of 3.1% for the samples analyzed, and a detection limit of 0.1 ng/mL in the digested solution and 5 ng/g in a 2 g sample prior to digestion. The recovery of added iodine ranged from 90.3 to 101.3%, using external standards. Samples analyzed included NBS Standard Reference Material 1549, and composites of a variety of dairy products, meat, eggs and fish, cereals, and potatoes. The iodine detected in these samples ranged from 9 ng/g for the potato group to 3360 ng/g for the standard reference material.     

Determination of total sulfite in shrimp, potatoes, dried pineapple, and white wine by flow injection analysis: collaborative study

A method for the determination of total sulfite in shrimp, potatoes, dried pineapple, and white wine by flow injection analysis (FIA) was collaboratively studied by 8 laboratories. In the method, the sample solution is reacted with sodium hydroxide to liberate aldehyde-bound sulfite. The sample stream is acidified to produce SO2 gas, which diffuses across a Teflon membrane in the gas diffusion cell into a flowing stream of malachite green. The degree of discoloration of the malachite green is proportional to the amount of sulfite in the sample solution. Red wine was included in the study but interlaboratory precision for these samples was not satisfactory and correlation with Monier-Williams results was poor. The present method is not recommended for use with these samples. For shrimp, potatoes, dried pineapple, and white wine, average reproducibility (RSDR) of results was 25% for samples at 10 ppm SO2 and 10% for samples at greater than 50 ppm. Overall average reproducibility was 14%. Recoveries of sulfite added to samples averaged 80%. Comparison of FIA with the Monier-Williams method indicated comparable results by the 2 methods. The FIA method has been adopted official first action for determination of greater than or equal to 5 ppm total sulfite in shrimp, potatoes, dried pineapple, and white wine.     

Gas chromatographic determination of captan, folpet, and captafol residues in tomatoes, cucumbers, and apples using a wide-bore capillary column: interlaboratory study.

An interlaboratory study of the determination of captan, folpet, and captafol in tomatoes, cucumbers, and apples was conducted by 4 laboratories using wide-bore capillary column gas chromatography with electron capture detection. The 3 fungicides were determined using the Luke et al. multiresidue method modified to include additional solvent elution in the optional Florisil column cleanup step used with this method. The crops were fortified with each fungicide at 3 levels per crop. Mean recoveries ranged from 86.2% for a 25.1 ppm level of captan in apples to 115.4% for a 0.288 ppm level of captafol in apples. Interlaboratory coefficients of variation ranged from 3.4% (24.7 ppm folpet) to 9.7% (0.243 ppm captafol) for tomatoes; from 2.8% (2.0 ppm captafol) to 8.2% (24.8 ppm captan) for cucumbers; and from 1.5% (0.234 ppm folpet) to 22.1% (0.266 ppm captafol) for apples.     

Gas chromatographic method for analysis of 2,4-D in wheat: interlaboratory study

A procedure is described for the determination of 2,4-D (2,4-dichlorophenoxyacetic acid) in dried green plant material. Samples are first extracted with dilute sodium hydroxide, and then after acidification and solvent extraction, the residues are methylated using boron trifluoride-methanol reagent. The methyl ester of 2,4-D is cleaned up on a Florisil column and quantitated using a gas chromatograph equipped with an electron capture detector. Six laboratories made quadruplicate determinations on control, dried green wheat check samples, on 4 similar samples fortified at the 0.50 ppm level, and on 4 samples fortified at the 1.00 ppm level with 2,4-D. Based on the data from 5 laboratories, the plant fortifications of 0.50 and 1.00 ppm yielded average interlaboratory recoveries of 2,4-D of 83.3 and 88.2%, respectively. The procedure also has potential for the determination of 2,4-D in wheat straw and wheat grain.     

Headspace gas chromatographic method for determination of ethanol in canned salmon: collaborative study

Six laboratories collaboratively studied a headspace gas chromatographic method for determination of ethanol in the aqueous phase of canned salmon. Ethanol is determined by a headspace sampling technique with tert-butanol as the internal standard, using a gas chromatograph equipped with a Super Q column and a flame ionization detector. With outliers excluded, the mean recoveries from samples spiked with 25.1 and 78.4 ppm ethanol were 112 and 110%, respectively. For the 4 sample pairs quantitated, repeatability coefficients of variation ranged from 1.42 to 4.25% and reproducibility coefficients of variation from 2.55 to 8.09%, with 3 of the 4 reported values less than 5%. The method has been adopted official first action.     

Enzyme immunoassay for determination of gluten in foods: collaborative study

A collaborative study was performed in 15 laboratories to validate a monoclonal antibody-based enzyme immunoassay (EIA) for determination of gluten in foods. The study included 13 samples: maize starch, "gluten-free" baking mixes, wheat flours, cookies, cooked meats, and a soup. Gluten was present in these samples at either zero or 0.02 to 10% by weight, i.e., over almost 3 orders of magnitude. The mean assay values for the foods varied from 88 to 105% of the actual amounts. The assay was quantitative for cereal products and the soup with repeatability (RDS-r, relative standard deviation) and reproducibility (RSD-R) of 16-22% and 24-33%, respectively. The assay was semiquantitative for the processed meat products (RSD-r 14 and 26% and RSD-R 46 and 56%), probably because gluten was unevenly distributed in the small (1 g) samples that were analyzed. The ELISA method produced no false positive results, and false negatives obtained with tannin-containing foods could be avoided by use of a modified sample extractant. None of the collaborators reported problems in following the protocol. The method has been adopted official first action by AOAC for determination of wheat gluten in foods.     

Determination of pentachlorophenol in animal tissues: a Canadian perspective

Analytical methods for pentachlorophenol (PCP) residues in edible animal tissue have been reviewed, with particular reference to gas chromatographic methods of analysis. Results of analyses demonstrate that significant residues of PCP can persist for several weeks in animals exposed to contaminated bedding. National surveys in Canada have found that the incidence of PCP residues in pork in excess of 0.1 ppm was reduced from 32% of survey samples in 1981-1982 to 6.6% of samples tested in 1987-1988. An interlaboratory sample exchange among Canadian laboratories demonstrated that the PCP analytical method currently used by Agriculture Canada could be successfully transferred to other laboratories. An exchange of samples between regulatory laboratories of Agriculture Canada and the Food Safety and Inspection Service of the U.S. Department of Agriculture (USDA) demonstrated equivalency of results for the 2 methods currently used in the respective laboratories, with relative standard deviations for analytical results ranging from 4.4 to 22.2%.     

Determination of naptalam and its metabolite in foods, as 1-naphthylamine, using liquid chromatography with oxidative electrochemical detection

A new method is described for the determination of the herbicide naptalam and its metabolite 1-naphthylamine in several foods. The method is sensitive, selective, and extremely rapid compared with previously reported methods. Liquid chromatography with electrochemical detection (LC/ECD) is used to determine 1-naphthylamine produced from the metabolism or base hydrolysis of naptalam in asparagus, peaches, and cranberries. These foods were spiked with naptalam at 0.05 and 0.11 ppm and hydrolyzed with 30% NaOH with concomitant distillation of 1-naphthylamine. Aliquots of the distillate were injected onto a reverse-phase PRP-1 LC column for separation of 1-naphthylamine from coextractives near the solvent front and detection at an applied potential of +0.83 V using an amperometric electrochemical detector in the oxidation mode. Recoveries ranged from 89% +/- 2% to 97% +/- 8% for all foods at both spiking levels. Accuracy of these recoveries was confirmed by use of 14C-radiolabeled naptalam and radioassay by liquid scintillation spectrometry of the 14C-1-naphthylamine released.     

Determination of sulfur dioxide in foods by modified Monier-Williams distillation and polarographic detection

A rapid, sensitive polarographic method is presented for determining sulfiting agents in foods and beverages. The method is based on the modified Monier-Williams distillation followed by polarographic detection by differential pulse polarography or square wave voltammetry. A clearly defined wave is obtained by both techniques, with a current maximum at a potential (E) of about -600 mV vs an Ag/AgCl reference electrode. The reaction is based on the reduction of sulfur dioxide at a dropping mercury electrode. Peak current was linear over the range 0-20 micrograms/mL. Quantitation is done by linear regression analysis of standard addition data or by using a standard calibration graph. Screening levels of less than 1 ppm total SO2 were easily achieved in the foods analyzed, which had levels from less than 1 ppm (cereals) to thousands of parts per million (dried fruit). Recoveries from fortified samples ranged from 70 to 108% at fortification levels of 20, 100, and 1000 micrograms/g.     

Liquid chromatographic determination of N-methylcarbamate insecticides and metabolites in crops. II. Analytical characteristics and residue findings

Four laboratories obtained 177 carbamate recovery values using a liquid chromatographic method. The average recovery of 11 carbamates (aldicarb, aldicarb sulfone, bufencarb, carbaryl, carbofuran, 3-hydroxy carbofuran, 3-keto carbofuran, methiocarb, methiocarb sulfoxide, methomyl, and oxamyl) from 14 crops was 99% with a coefficient of variation of 8% (0.03-1.8 ppm fortification levels). No statistical difference in recovery was found between oxime and phenyl carbamates, or between parent and metabolite carbamates. Average recovery of aldicarb sulfoxide was 59% due to loss in the liquid-liquid partitioning because of the polarity of this compound. A fifth laboratory contributed 34 carbamate recoveries (average 99%) on table-ready food products for 4 carbamates. Bendiocarb, dioxacarb, isoprocarb, and propoxur are also quantitatively recovered through the method. Previously reported carbamate and noncarbamate recovery data are also discussed. In the Food and Drug Administration's (FDA) analysis of 319 samples (mainly crops), 86 (27%) were found to contain residues of carbamate insecticides and/or toxic carbamate metabolites. Carbaryl and methomyl were the most common carbamate residues found on the food products excluding the aldicarb sulfone and sulfoxide residues found on potatoes. In one FDA Total Diet Program "market basket", 11 of 69 table-ready food commodities contained from 0.005 to 0.094 ppm carbamate residues. Carbaryl was the most prevalent residue. Several laboratories reported adverse effects on the determinative system when inadequately purified reagents were used.