Determination of nitrate in forages by using selective ion electrode: collaborative study

Each of 10 collaborating laboratories analyzed 4 blind duplicate pairs of forage samples for nitrate, by using a potentiometric method. Two forage controls and a 100 000 mg KNO3/L standard were also provided. Nitrate was extracted into an aqueous Al2(SO4)3 solution containing 70 mg KNO3/L and quantitated with a nitrate-selective electrode. Standards were prepared using extracting solution as diluent. Nitrate concentrations in forage samples ranged from less than 0.50 to 4.35% KNO3. Repeatability coefficients of variation (CVo) ranged from 1.74 to 3.61%, and reproducibility coefficients of variation (CVx) ranged from 6.92 to 7.66%. Mean recovery of a 0.55% KNO3 spike was 94.5%. The method has been adopted official first action.     

Liquid chromatographic determination of diazepam in tablets: collaborative study

A stability indicating liquid chromatographic method for the determination of diazepam in tablets was collaboratively studied by 6 laboratories. The method uses a C18 reverse phase column, a methanol-water mobile phase, p-tolualdehyde as the internal standard, and photometric detection at 254 nm. The collaborators were supplied with a synthetic tablet powder and 3 commercial tablet samples. The mean recovery of diazepam from the synthetic tablet powder was 100.2%. For all samples analyzed, the coefficient of variation was less than 1.5%. The method has been adopted official first action.     

Liquid chromatographic determination of acetaminophen in tablets: collaborative study

A reverse phase liquid chromatographic method previously reported for the determination of acetaminophen in tablets was collaboratively studied by 5 laboratories. Each collaborator received duplicate samples of a synthetic tablet formulation and 3 powdered commercial tablet composites. The composites represented single-component and multi-component proprietary products and a single-component generic product. The pooled repeatability (CVDo) and reproducibility (CVDx) values for the proprietary tablets were 0.89 and 1.34%, respectively. For the generic tablets, these values were 0.66 and 0.74%, respectively. The pooled recovery value for acetaminophen added to the synthetic formulation was 98.9 +/- 0.7% (n = 10) with a CV of 0.75%, CVDo of 0.37%, and CVDx of 0.78%. The overall repeatability of the method was 0.64%, and the overall reproducibility was 0.95%. The method has been adopted official first action.     

Liquid chromatographic determination of primidone in tablets: collaborative study

Six laboratories collaboratively studied a liquid chromatographic (LC) method for the quantitative determination of primidone in tablets. Two lots each of commercially prepared 50 and 250 mg tablets and 2 authentic mixtures, at 50 and 250 mg levels, were sent to each collaborator. Samples were dissolved in the mobile phase, filtered, and injected into the chromatograph. Average recoveries for the 8 samples ranged from 97.5 to 101.2%, and coefficients of variation ranged from 0.53 to 3.01%. The LC method has been adopted interim official first action.     

Liquid chromatographic determination of coumarin anticoagulants in tablets: collaborative study

A liquid chromatographic method for the determination of coumarin anticoagulants in tablets was collaboratively studied by 7 laboratories. The method uses an octadecylsilane-bonded microparticulate column, tetrahydrofuran-methanol-water-acetic acid mobile phase, and photometric detection at 311 nm. Each collaborator received samples of warfarin sodium, phenprocoumon, and dicumarol as a synthetic composite and as commercial individual and composited tablets. Pooled average assay values for synthetic and commercial tablet samples of warfarin sodium were 101.6 and 99.5%, respectively, with a combined reproducibility SD of 2.38% (CV = 2.37%) and combined repeatability SD of 1.49% (CV = 1.49%). Pooled average (SD) assay values for dicumarol and phenprocoumon commercial samples were 98.0 (2.27) and 101.3% (4.00), respectively. The content uniformity determinations of 2 mg warfarin sodium and 25 mg dicumarol tablets indicated average tablet contents (range) of 99.5% (91.0-116.0) and 98.0% (89.8-108.8), respectively. The method has been approved interim official first action.     

UV spectrophotometric determination of hydralazine hydrochloride in tablets: collaborative study

A UV spectrophotometric method for the determination of hydralazine hydrochloride in tablets was collaboratively studied by 5 laboratories. The method is based on conversion of hydralazine to a tetrazolo [5,1-alpha]phthalazine derivative which shows an absorption maximum at about 274 nm. Each collaborator received blind duplicate samples of 2 commercial powdered composites from 10 and 100 mg tablets, and 1 synthetic tablet formulation. Each collaborator also received a set of 10 tablets for determination of content uniformity. The pooled mean recovery of hydralazine hydrochloride from the synthetic formulation was 101.2 +/- 0.94%. The mean assay values for 10 and 100 mg tablets were 95.6 +/- 0.98 and 101.0 +/- 0.73% of the declared amounts, respectively, with corresponding CV values of 1.02 and 0.73%. The pooled mean for individual tablet assay was 99.8 +/- 3.26% of the declared value, with a CV of 3.29%. The method has been adopted official first action.     

Liquid chromatographic determination of methyldopa and methyldopa-thiazide combinations in tablets: collaborative study

A reverse phase liquid chromatographic method for the determination of methyldopa, methyldopa-hydrochlorothiazide, and methyldopachlorothiazide in tablets was collaboratively studied by 8 laboratories. Each collaborator received 20 samples that included drug substance, synthetic and commercial tablet compositions. The overall repeatability and reproducibility standard deviations for commercial tablets were 1.11 and 1.75% for methyldopa, 0.96 and 1.62% for chlorothiazide, and 1.21 and 2.15% for hydrochlorothiazide, respectively. The overall recoveries of methyldopa, chlorothiazide, and hydrochlorothiazide added to synthetic tablets were 100.78, 100.70, and 101.34%, respectively. The method has been adopted official first action.     

Liquid chromatographic determination of penicillin V potassium in tablets: collaborative study

A liquid chromatographic method for the determination of penicillin V potassium in tablets was collaboratively studied by 7 laboratories. The method uses an octadecyl silane reverse-phase column, a mobile phase of acetonitrile-methanol-0.01 M potassium phosphate monobasic (21 + 4 + 75 v/v/v), photometric detection at 225 nm, and sulfadimethoxine as an internal standard. Each collaborator received 6 samples: powdered composites of 2 commercial tablet preparations and 1 synthetic tablet powder mixture, each with blind duplicates. The mean repeatability and reproducibility relative standard deviations for commercial samples were, respectively, 1.10 and 1.46% (250 mg dosage), and 0.84 and 2.82% (500 mg dosage). The average standard recovery from the synthetic formulation was 99.1%, with repeatability and reproducibility relative standard deviations of 1.30 and 3.66%, respectively. The method has been adopted official first action.     

Normal phase liquid chromatographic determination of sulfamethoxazole in tablets: collaborative study

A stability-indicating liquid chromatographic method is presented for determining sulfamethoxazole in tablets. The method uses a 10 micron silica column, an isooctane-methylene chloride-2-propanol-acetonitrile-glacial acetic acid (70 + 25 + 5 + 5 + 0.5) mobile phase, and photometric detection at 254 nm. Seven laboratories collaboratively studied this method on powdered composite samples prepared from commercial 500 and 1000 mg tablets and on an authentic tablet mixture containing 83.32% added sulfamethoxazole. Mean assay results for the 500 and 1000 mg tablets were 102.2 and 97.9% of declared, respectively (n = 4). The mean recovery value for the synthetic sample was 99.4% (n = 4). The pooled reproducibility standard deviation (SD) (coefficient of variation (CV)) and pooled repeatability SD (CV) were +/- 1.01 (1.01%) and +/- 0.96 (0.96%), respectively. These results were in good agreement with those obtained by the Associate Referee for the titration method of USP XX. The proposed method can also be used for monitoring the presence of sulfanilamide in sulfamethoxazole by increasing the proportions of both acetonitrile and 2-propanol in the mobile phase. The method has been adopted official first action.     

Extraction of light filth from spirulina powders and tablets: collaborative study

Results are reported for a collaborative study of a method for the extraction of light filth from spirulina (a blue-green alga) powder and tablets. A 50 g portion of either powder or tablets is dispersed in water, and then boiled with dilute HCI solution. Hairs and insect fragments are isolated by wet sieving on a No. 230 sieve, flotation with mineral oil, and washings of the mineral oil in a percolator. Average recoveries by 12 collaborators for tablets and powders were 70.6 and 70.2%, respectively, for 10 rat hair spikes and 68.3 and 84.4%, respectively, for 20 insect fragment spikes. The method has been approved interim official first action.     

Determination of alkaline phosphatase in casein: collaborative study

A collaborative study was made to determine alkaline phosphatase in casein samples by the rapid colorimetric test. Six to eight collaborators tested 10 unknown casein samples containing various amounts of residual phosphatase with and without the addition of magnesium acetate. Results indicated that magnesium acetate significantly increased phosphatase activity. The collaborators correctly analyzed 95% of the samples with the added magnesium acetate. The method has been adopted official first action.     

Determination of phosphorus in processed cheese: collaborative study

A second interlaboratory collaborative study of the determination of phosphorus in processed cheese products by the molybdenum blue method verifies that this method is prone to producing a laboratory-induced systematic error. It would be useless to continue to make minor modifications in the details of the method, which will improve only the within-laboratory precision, until an accuracy control of the final measurement step is incorporated into the method.     

Determination of zearalenone in corn: collaborative study.

Corn samples spiked at levels of 100, 300, 1000, and 2000 mug zearalenone/kg were sent to 22 collaborators for analysis by the Eppley method. All samples were yellow corn except one white corn sample spiked at 2000 mug/kg. Results from 16 collaborators were statistically analyzed. Only 4 of 16 collaborators detected zearalenone in the sample containing 100 mu/kg, but 11 detected the toxin in the sample containing 300 mug/kg. Average recoveries from all samples were 129% at 300 mug/kg, 101% at 1000 mug/kg, and 88% at 2000 mug/kg. The between-laboratory coefficients of variation were 53.0% at 300 mug/kg, 38.2% at 1000 mug/kg, and 27.0% at 2000 mug/kg. Five naturally contaminated corn samples, one in triplicate, were also provided. The mean level of zearalenone in the naturally contaminated samples ranged from 431 to 7622 mug/kg. The mean coefficient of variation for all samples was 40.5%. Two collaborators measured quantities of zearalenone on thin layer chromatographic plates densitometrically. Their results were not included in the statistical analysis, but the results indicated that densitometric measurement, given proper dilutions of solutions, could be used. The method has been adopted as official first action.     

Determination of amprolium in feeds: collaborative study.

The official final action method, 42.028--42.032, for determining amprolium in feeds was modified by a change in the preparation of aluminum oxide for chromatography. A premix containing 0.5% amprolium was collaboratively studied by the modified and the official methods. Compared with the modified method, 87.7% of the drug was recovered from the premix by using the official method. The modification makes possible the assay of premixes as well as finished feeds. The official final action method has been modified to incorporate this change.     

Determination of ethanol in wine by titrimetric and spectrophotometric dichromate methods: collaborative study

A dichromate-spectrophotometric method for the determination of ethanol in wine was compared in a collaborative, matched pair study with the AOAC dichromate-titrimetric method, 11.008-11.011. Both methods require distillation of the sample into dichromate. The titrimetric method measures ethanol by titrating the excess dichromate with ferrous ammonium sulfate after conversion of ethanol to acetic acid; the spectrophotometric method directly measures the reduced dichromate formed after oxidation. In addition to comparing the 2 methods, the collaborative study also compared the use of 2 types of assemblies for obtaining the ethanol distillate: the Scott-type, which is used in 11.008-11.011, and the electric Kirk-type. Results of the collaborative study indicated that the repeatability and reproducibility of the official titrimetric method were generally far superior to those of the spectrophotometric method; therefore, adoption of the spectrophotometric method is not recommended. Comparison of titrimetric method results obtained using the 2 types of stills indicated that repeatability and reproducibility were somewhat better when Scott apparatus was used, but measurements using Kirk-type compared well in the range of ethanol concentrations found in table and fortified wines. The Kirk-type distillation apparatus has been adopted official first action as an alternative to Scott apparatus in the dichromate oxidation method for ethanol in wine, 11.008-11.011.     

Determination of N-nitrosodimethylamine in nonfat dry milk: collaborative study

Ten laboratories participated in a collaborative study of a method for the determination of N-nitrosodimethylamine (NDMA) in nonfat dry milk. NDMA is eluted with dichloromethane from a mixture of Celite, acidic sulfamic acid, and nonfat dry milk (all packed in a chromatography column), concentrated in a Kuderna -Danish concentrator, and finally analyzed by a GC-thermal energy analyzer technique. Ten samples were studied: 6 were naturally contaminated (NDMA levels 0.38-3.56 ppb) and 4 were spiked with known levels (0.96 and 3.2 ppb) of NDMA. The coefficients of variation (CV) of the complete data for the naturally contaminated samples (excluding the 2 samples containing the lowest levels) were 8.5% and 22.5% for repeatability and reproducibility, respectively. The corresponding CVs for the spiked samples were 14.4% and 20.4%, respectively. The percent recoveries of the added NDMA in the spiked samples (at the 2 levels indicated above) were 101.6 +/- 3.2 (omitting 1 outlier) and 95 +/- 2.1, respectively. The method has been adopted official first action.     

Determination of natamycin in cheese and cheese rind: interlaboratory collaborative study

A collaborative test on the determination of natamycin in cheese and cheese rind was conducted. Participants were from 37 laboratories in 13 countries. Eight samples, consisting of 4 duplicates, were investigated by a spectrometric method and a liquid chromatographic (LC) method. The spectrometric method gave good results (coefficient of variation [CV] = 12%) and the LC method with ultraviolet detection gave reasonable results (CV = 25%) for levels down to 15 mg/kg (0.9 mg/dm2). For very low levels, a preconcentration step is necessary, but even then quantitation is poor (CV = 35-37%) for both methods at 1.7 mg/kg, although the presence of natamycin can be detected qualitatively. For a level of 0.3 mg/kg, quantitation is poor (CV = 39%) for the LC method and impossible (CV = 60%) for the spectrometric method.     

Determination of acid-detergent fiber and crude protein in forages by near-infrared reflectance spectroscopy: collaborative study

A collaborative study was conducted to determine the standard error of difference among laboratories for near-infrared reflectance spectroscopic (NIRS) determination of acid-detergent fiber (ADF) and crude protein in forages. The 6 participating laboratories were members of the USDA/ARS National Near-Infrared Reflectance Spectroscopy Forage Research Project. The NIRS calibration equations were developed in the Associate Referee's laboratory for crude protein and ADF and were transferred to the instrument in each of the other collaborating laboratories. The calibration set included over 650 diverse forage samples with crude protein and ADF calibration data; the validation set included 94 samples of bermudagrass. Among-laboratory reproducibility for the NIRS method, calculated as the relative standard deviation for reproducibility (RSDR), was 1.14% for ADF and 0.42% for crude protein. The variance component for among-laboratory variation (coefficient of variation) was 2.54% for ADF and 2.89% for crude protein. These results confirm that it is possible to calibrate, validate, and transfer (NIRS) equations and data among laboratories for the accurate determination of ADF and crude protein, and thereby demonstrate that NIRS can be used as a standard method for the analysis of forages. The method has been adopted official first action.     

Determination of soil nitrate by means of specific ion electrode: Comparison among different extracting solutions

Abstract

Four different soil nitrate extracting solutions were compared. With the exception of the solution suggested by ORION Manual⁸ which gave the worst results, the efficacy of other solutions does not depend on the employed type but the presence or absence of particles in solution during the assay by means of the electrode.