Fresenius' Journal of Fresenius J Anal Chem (1990) 338:466-468

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@ Springer-Verlag 1990

Recently developed NIST food related standard reference materials Robert Alvarez

National Institute of Standards and Technology, Standard Reference Materials Program, Gaithersburg, MD 20899, USA

Summary. NIST issues food related, chemical composition standard reference materials for validating food analyses. SRMs certified for inorganic constituents are: Non-Fat Milk Powder (SRM 1549), Oyster Tissue (SRM 1566a), Bovine Liver (SRM 1577a), Wheat Flour (SRM 1567a), Rice Flour (SRM 1568a), and Total Diet (SRM 1548). The certificate of analysis for the total diet SRM also provides a certified concentration for cholesterol. Oyster tissue, a renewal SRM, is certified for 25 elements including 6 (A1, C1, I, P, S, and V), that had not been certified in the previously issued SRM 1566. The elemental certified concentrations are based on concordant results of two or more independent analytical methods. The chemical compositions of the six food matrix SRMs are tabulated. Three food matrix SRMs certified for organic constituents are: Cholesterol and Fat-Soluble Vit- amins in Coconut Oil (SRM 1563), Cholesterol in Whole Egg Powder (SRM 1845) and Organics in Cod Liver Oil (SRM 1588). Serum and urine matrix SRMs are also avail- able that may be useful for metabolic and bioavailability studies.

Introduction

In recent years, the chemical composition of foods has attracted attention, not only from scientists in the food in- dustry, government agencies, and academia, but also from the general populations of many countries. Probably one of the more important reasons for this increased interest is the much publicized relationship of food constituents and health.

For example, certain fat soluble vitamins and selenium are reported to be important in the chemopre~)ention of cancer.

In many countries, there is concern with the contamina- tion of foods by pesticides and organic chemicals.

In the United States, the Food and Nutrition Board of the National Academy of Sciences issues a publication listing recommended dietary allowances of protein, vitamins, and minerals for infants, children, males, and females [1]. These RDAs are, "designed for the maintenance of good nutrition

of practically all healthy people in the United States". Be- cause there is less information on which to base allowances of two vitamins (biotin and pantothenic acid) and of five trace elements (copper, manganese, fluoride, chromium, and molybdenum) RDAs are not reported for these constituents. Instead, they are reported as estimated safe and adequate daily dietary intakes with a warning that the upper levels for these five trace elements should not be habitually exceeded because the toxic levels for many trace elements may be only several times usual intakes.

Food matrix standard reference materials

Inorganic constituents

Table 1 lists the chemical compositions of food matrix SRMs, certified for inorganic constituents, that are available from the National Institute of Standards and Technology. An element is listed when at least one SRM is certified for that element.

When a previously issued SRM has been reissued because it was sold out, the renewal is designated by a letter following the renewal. Thus, SRM 1566 a is the renewal of SRM 1566.

The Certificate of Analysis for Oyster Tissue, SRM 1566 a, provides certified concentrations for 25 elements in- cluding 6 that had not been certifed in the original SRM 1566. The 6 elements are: A1, C1, 2, P, S, and V.

Organic constituents

The importance of decreasing dietary cholesterol has led to the development of two food SRMs certified for cholesterol. They are: SRM 1563, Cholesterol and Fat-Soluble Vitamins in Coconut Oil (Fortified and Natural), and SRM 1845, Cholesterol in Whole Egg Powder.

SRM 1563 consists of samples of the natural coconut oil and oil to which known amounts of cholesterol and fat -~ soluble vitamins have been added. The Certificate of Analy- sis for SRM 1563 provides certified concentrations, in/~g/g, of cholesterol (642__+ 6), retinyl acetate (12.2_+ 0.8), ergocalciferol (10.9_+0.8), and dl-~-tocopheryl acetate (158 _+ 6) in the fortified coconut oil. The certified concentra- tion of cholesterol is based on the combined results from

Table 1. Composition of food matrix SRM's (Concentration in gg/g, or where noted, in percent by weight)

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Element Non-fat Oyster Bovine Wheat Rice Total milk powder tissue liver flour flour diet SRM 1549 SRM 1566 a SRM 1577 a SRM 1567 a SRM 1568 a SRM 1548

Aluminum (2) 202.5 (2) 5.7 Arsenic (0.0019) 14.0 0.047 (0.006) Cadmium 0.0005 4.15 0.44 0.026 Calcium 1.30% 0.196% 120 0.0191% Chlorine 1.09% 0.829% 0.28% (565) Chromium 0.0026 1.43 - - Cobalt (0.0041) 0.57 0.21 (0.006) Copper 0.7 66.3 158 2.1 Iodine 3.38 4.46 - (0.0009) Iron 1.78 539 194 14.1 Lead 0.019 0.371 0.135 (<0.020) Magnesium 0.120% 0.118% 600 0.040% Manganese 0.26 12.3 9.9 9.4 Mercury 0.0003 0.0642 0.004 (0.0005 Molybdenum (0.34) - 3.5 0.48 Nickel - 2.25 - - Nitrogen . . . . Phosphorus 1.06% 0.623% 1.11% 0.134% Potassium 1.69% 0.790% 0.996% 0.133% Rubidium 11 (3) 12.5 0.68 Selenium O. 11 2.21 0.71 1.1 Silver ( < 0.0003) 1.68 0.04 - Sodium 0.497% 0.417% 0.243% 6.1 Strontium - 11.1 0.138 - Sulfur 0.351% 0.862% 0.78% 0.165% Uranium - 0.132 0.00071 (0.0003) Vanadium - 4.68 0.099 (0.011) Zinc 46.1 830 123 11.6

4.4 0.29 0.022 0.0118

(0.018) 2.4

(O.OO9 7.4

(<O.OLO) 0.056%

20.0 0.0058 1.46

0.153% 0.1280% 6.14 0.38

6.6

0.120% (0.0003) (0.007) 19.4

0.028 0.174% 0.87%

2.6

32.6

556 5.2

3.44% 0.324% 0.606%

0.245

0.625%

0.258%

30.8

Notes: 1. Certificates of Analysis provide estimated uncertainties for the certified concentrations 2. Values in parentheses are not certified. 3. Certificate of Analysis for SRM 1548 also provides certified values for cholesterol and caloric content

the calculated gravimetric concentrat ion and a modif icat ion of the combined isotope dilut ion and gas chromatography / mass spectrometry method for serum [2]. F o r the fat-soluble vitamins, the certified concentrat ions are based on the combined results f rom the calculated gravimetric concentra- t ion and liquid chromatography [3]. The concentrat ions of the same consti tuents in the natura l oil are listed for informa- tion only. The concentrat ion of cholesterol in the natura l oil is approximate ly 3.4 #g/g and the concentrat ion of each of the three fat-soluble vitamins is less than I #/g.

The concentrat ion of cholesterol in S R M 1845, a dried egg powder, is listed in the Certificate o f Analysis as 19.0 _+ 0.2 mg/g, where the uncertainty represents two stan- dard deviat ions of the certified concentrat ion. The certified concentrat ion and associated uncertainty is based on the high accuracy method, isotope dilut ion gas chromato- graphy/mass spectrometry.

F o r years, food scientists have been concerned with the presence of contaminants in foods. This concern led to the development of SRM 1588, Organics in Cod Liver Oil, which is certified for ten chlor inated pesticides, and five individual polychlor inated biphenyl congeners. In addit ion, the Certifi- cate provides a certified concentra t ion of a lpha- tocopherol and noncertified concentrat ions of six dioxins.

The certified concentrat ions of the pesticides and PCBs are based on concordant results by two gas chromatography

methods, one with electron capture detection and, the other, with mass spectrometric detection. The ten chlorinated pesticides are: hexachlorobenzene, a lpha-HCH, trans- chlordane, cis-chlordane, t rans-nonachlor , dieldrin, 4,4'- D D D , 4 ,4 ' -DDE, 2 ,4 ' -DDT and 4,4 ' -DDT. The concentra- tions, in ng/g, range from 50 _+ 13 for t rans-chlordane to 529 ± 45 for 4 ,4 ' -DDT. The five PCBs, identified according to the system proposed by Ballschmiter [4], are 101, 138, 153, 170, and 180. The concentrations, in ng/g, range from 45 +_ 5 for PCB 170 to 276 _+ 40 for PCB 153.

The concentrat ion of a lpha- tocopherol is certified based on agreement of results by two liquid chromatography meth- ods, one using a normal phase technique, and the other, a reversed phase technique.

High-purity organic and inorganic S R M s

Compounds of known high puri ty are necessary for pre- paring s tandard solutions. These solutions, which have accu- rate concentrat ions of analytes, can then be diluted to pre- pare l abora to ry reference materials with a suitable concen- t rat ion range for the analytes of interest. The solutions can also be used to add known amounts of the analytes to samples for recovery studies. Approx imate ly 70 spectromet- ric s tandard solutions, certified, with the exception of B and

468

V, to contain 10.00 mg/ml of the element, are available. The B and V solutions are certified to contain 5.00 mg/ml.

A number of high purity, organic compound SRMs, such as cholesterol and glucose, are available.

SRMs for metabolic and bioavailability studies

Several SRMs that may be useful to those undertaking meta- bolic and bioavailability investigations are listed.

SRM 1598, Inorganic Constituents in Bovine Serum, is certified for 13 trace elements in the frozen serum. Six of the elements, Cu, Fe, Mg, K, Rb, and Zn, are at a concentra- tion level of gg/g, and seven, A1, Cd, Cr, Co, Mn, Mo, and Se are at ng/g levels. The concentrations of these elements in bovine serum are similar to those found in human serum.

SRM 909, Human Serum, is a freeze-dried material, certified for cholesterol, creatinine, glucose, urea, uric acid, Ca, C1, Li, Mg, K, and Na. The concentrations of these constituents were determined by high accuracy methods, referred to as definitive methods by the clinical chemistry community. Except for Na, a mononuclidic element, the constituents were determined by stable isotope dilution mass spectrometry. Na was determined gravimetrically after ion exchange separation of the Na and the concentration was confirmed by atomic absorption spectrometry.

SRM 968, Fat-Soluble Vitamins in Human Serum, is freeze-dried, human serum certified for three concentration

levels of retinol, et-tocopherol, and total [3-carotene. The concentrations in the reconstituted serum range from 0.313 to 1.15 gg/ml for retinol, from 4.94 to 12.4 gg/ml for ct- tocopherol, and from 0.123 to 1.49 lxg/ml for total [3-carotene.

SRM 2670, Toxic Metals in Freeze-Dried Urine, was developed primarily to validate determinations of toxic elements in human urine. However, a normal human urine is also included in addition to the elevated level. The concen- trations of certified elements in the normal urine may be of more interest to investigators making metabolic studies. These certified concentrations are: Ca, 0.105 + 0.005 mg/ml; Cu, 0.13 _+ 0.02 gg/ml; Mg, 0.063 + 0.003; Na, 2.62 _+ 0.14 mg/ml; and Se, 0.030 _+ 0.008 lag/ml.

References

1. Recommended Dietary Allowances, Revised 1989, Food and Nutrition Board, National Academy of Sciences, National Re- search Council, Washington, D.C.

2. Cohen A, Hertz HS, Mandel J, Paule RC, Schaffer R, Sniegoski LT, Sun T, Welch MJ, White VE (1980) Clin Chem 26:854-860

3. Brown-Thomas JM, Moustafa AA, Wise SA, May W E (1988) Anal Chem 60:1929-1933

4. Ballschmiter K, Zell M (1980) Fresenius Z Anal Chem 302:20

Received July 17, 1990