Soy Protein Isolate

This Standard Reference Material (SRM) is intended primarily for validation of methods for determining isoflavones in soy protein isolates and similar materials. This SRM can also be used for quality assurance when assigning values to in-house reference materials. The SRM is a soy protein isolate prepared by a manufacturer of food and agricultural products. A unit of SRM 3236 consists of 5 packets, each containing approximately 10 g of material.


NOTICE AND WARNING TO USERS
SRM 3236 IS INTENDED FOR LABORATORY USE ONLY, NOT FOR HUMAN CONSUMPTION.

INSTRUCTIONS FOR STORAGE AND USE
Storage: The original unopened packets of SRM 3236 should be stored at controlled room temperature (20 °C to 25 °C). Once the packet is opened, the long-term stability of all analytes in SRM 3236 is unknown. Therefore, the certification only applies to the initial use, and the same results are not guaranteed if the remaining powder is used longer than two months after opening.
Use: Prior to removal of a test portion for analysis, the contents of a packet of material should be mixed thoroughly.
For certified values to be valid, a test portion of at least 100 mg should be used; the homogeneity of test portions less than 100 mg has not been evaluated. Results obtained in analyses should include their own estimates of uncertainty and can be compared to the certified values using procedures described in reference 6. Test portions should be analyzed as received and results converted to a dry-mass basis by determining moisture content (described below) on a separate test portion. (1) Source and Preparation: The SRM is a soy protein isolate prepared by a manufacturer of food and agricultural products. The product was packaged into single-use, nitrogen-flushed pouches, each containing 10 g of powder.

SOURCE, PREPARATION, AND ANALYSIS
NIST Analyses for Isoflavones Using ID-LC/MS: Daidzein, daidzin, genistein, genistin, glycitein, and glycitin were measured at NIST using isotope dilution liquid chromatography with mass spectrometric detection (ID-LC/MS). Calibrants were prepared gravimetrically, at levels intended to approximate the levels of the isoflavones in the SRM. Internal standards were employed; a single solution was used for the calibrants and samples. Duplicate 150 mg test portions of powder from each of 12 packets were accurately weighed into 15 mL polyethylene centrifuge tubes. An aliquot of a mixed internal standard solution containing 13 C6-daidzin, 13 C6-daidzein, 13 C6-genistein, 13 C6-genistin, 13 C6-glycitein, and 13 C6-glycitin was added. Analytes were extracted from the sample, then hydrolyzed to convert acetyl-and malonyl-glycosides to free glycosides, neutralized, diluted, and centrifuged prior to injection. Details of the separation and a typical chromatogram are provided in Figure 1. The separation was monitored using an absorbance detector at 260 nm, but MS was used for quantitation. Daidzein and 13 C6-daidzein were monitored at m/z 255 and m/z 261, respectively. Daidzin and 13 C6-daidzin were monitored at m/z 417 and m/z 423, respectively. Genistein and 13 C6-genistein were monitored at m/z 271 and m/z 277, respectively. Genistin and 13 C6-genistin were monitored at m/z 433 and m/z 439, respectively. Glycitein and 13 C6-glycitein were monitored at m/z 285 and m/z 291, respectively. Glycitin and 13 C6-glycitin were monitored at m/z 447 and m/z 453, respectively.
NIST Analyses for Isoflavones Using LC/absorbance: Daidzein, daidzin, genistein, genistin, glycitein, and glycitin were measured at NIST using liquid chromatography with absorbance detection (LC/absorbance). Calibrants were prepared gravimetrically, at levels intended to approximate the levels of the isoflavones in the SRM. An internal standard approach was utilized with a single solution used for the calibrants and samples. Duplicate 100 mg test portions of powder from each of 12 packets were accurately weighed into 15 mL polyethylene centrifuge tubes. An aliquot of an internal standard solution containing sissotrin was added. Analytes were extracted from the sample, then hydrolyzed to convert acetyl-and malonyl-glycosides to free glycosides, neutralized, diluted, and centrifuged prior to injection. Details of the separation and a typical chromatogram are provided in Figure 2. The separation was monitored and quantitation performed using an absorbance detector at 254 nm.
Determination of Moisture: Moisture content of SRM 3236 was determined at NIST (see "Instructions for Storage and Use") by (1) freeze drying to constant mass over 7 d; (2) drying over magnesium perchlorate in a desiccator at room temperature for 26 d; and (3) drying for 2 h in a forced air oven at 80 °C. The results obtained using all three techniques were averaged to determine a conversion factor of (0.951 ± 0.003) gram dry-mass per gram as-received mass, which was used to convert data from an as-received to a dry-mass basis; the uncertainty shown on this value is an expanded uncertainty. An uncertainty component for the conversion factor (0.17 %) obtained from the moisture measurements is incorporated in the uncertainties of the certified values, reported on a dry-mass basis, that are provided in this certificate. (1) Certain commercial equipment, instruments or materials are identified in this certificate to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Homogeneity Assessment: The homogeneity of isoflavones in the SRM was assessed at NIST using the methods and test portion sizes described above; analysis of variance did not show statistically significant heterogeneity.

Value Assignment:
The equally weighted mean of NIST results provided by LC/absorbance and ID-LC/MS were used to calculate assigned values.

Certified Mass Fraction Values for Isoflavones:
Each certified mass fraction value is the mean from the combination of the mean results provided by LC/absorbance and ID-LC/MS by NIST. The uncertainty provided with each value is an expanded uncertainty about the mean to cover the measurand with approximately 95 % confidence. The expanded uncertainty is calculated as U = kuc, where uc incorporates the observed difference between the results from the methods and their respective uncertainties, and an uncertainty for moisture correction, consistent with the ISO/JCGM Guide and with its Supplement 1, and k is a coverage factor corresponding to approximately 95 % confidence [2][3][4]. The measurands are the total mass fractions of isoflavones in soy protein isolate. The certified values are metrologically traceable to the SI unit of milligrams per kilogram.  (a) 31.4 ± 0.5 2.00 (a) Value was determined using a hydrolysis approach, and therefore represents total glycosides (sum of glycoside, malonyl-glycoside, and acetyl-glycoside present in the material).