This Standard Reference Material (SRM) is intended primarily for use in the standardization of procedures employed in clinical analyses for total serum protein, for critical evaluation of daily working standards used in these procedures, and as a reference standard for assays of total protein by colorimetric methods. This SRM is a solution (mass fraction 7 %) of known protein concentration and purity. It conforms to the specification for standardized protein solution approved by the Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS) [1]. The protein content of this SRM was determined using the biuret reference method [2] that is recommended for use in standardizing laboratory-prepared protein solutions and “normal” serum pools. Such standardized “normal” sera could then be used to calibrate refractometers or other instruments for serum protein estimations. SRM 927d may also be used for other procedures, such as gel diffusion, amino acid analysis, electrophoresis, nitrogen assays, or other tests that require well-characterized protein for calibration or evaluation. A unit of 927d

consists of 10 ampoules, each containing 2.2 mL of solution.

In addition to the measurements using the biuret method, NIST made measurements of the bovine serum albumin (BSA) concentration using amino acid analysis. There is a discrepancy between the results from the two approaches. As the two different approaches are not determining the same measurand, it was decided to separately

the report results from the two approaches as follows: 1) certified BSA concentration by amino acid analysis and 2)

reference total protein concentration by the biuret method.

Certified Bovine Serum Albumin (BSA) Concentration and Uncertainty from Amino Acid Analysis: Two different methods of amino acid analysis were used to measure the concentration of bovine serum albumin (see “Analytical Methods” section). The certified results are shown in Table 1. A NIST certified value is a value for which NIST has the highest confidence in its accuracy in that all known or suspected sources of bias have been investigated or accounted for by NIST.

Reference Total Protein Concentration and Uncertainty as Determined Using the Biuret Method: The biuret reference method [2] was employed to determine protein concentration in SRM 927d using SRM 927c as an external standard. The reference protein concentration and uncertainty are shown in Table 2. Reference values are noncertified values that are the best estimates of the true values; however, the values do not meet NIST criteria for certification and

are provided with associated uncertainties that may reflect only measurement precision, may not include all sources of uncertainty, or may reflect a lack of sufficient statistical agreement among multiple analytical methods.

Additional Reference Values and Uncertainties: Reference values are provided in Table 3 for additional properties including fill volume, pH, density, absorbances at various wavelengths, and relative molecular mass as determined using electrospray ionization mass spectrometry.