Anal. Chem. 1986, 58,3225-3230
As the compilation of spectral libraries proliferates in chemical laboratories and as they continue to grow in size, more attention is focused on the execution time required for searches. The logarithmic relationship between the size of a hierarchically ordered spectral data base and the average path length through the tree has already been noted. Thus, the logarithmic relationship between tree size and execution time that follows, offers a significant advantage in the processing of queries involving the data bases of 10K-100K entries and more, that can be expected to be commonplace in the near future (13). The adaptability of the feedback search to computers capable of parallel processing is likewise advantageous. As each descendent node decision is made, the first-pass and subpath searches could be executed concurrently. LITERATURE CITED Clerc, J. T.; Szekeiy, G. Trends Anal. Chem. 1983, 2 , 50-53. Zupan, J. Fresenlus‘ 2.Anal. Chem. 1982, 313, 466-472. Varmuza, K. fartern Recognition in Chemistry; Springer Verlag: Berlin, 1982; pp 157-160. Woodruff, H. B.; Smith, G. M. Anal. Chem. 1980, 5 2 , 2321-2327. Hippe, 2.Anai. Chlm. Acta 1983, 150, 11-21.
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(6) aibov, L. A.; Elyashberg, M. E.; Koldashov, V. N.; Pletnjov, I . V. Anal. Chim. Acta 1983, 748, 159-170. (7) Zupan, J.; Munk, M. E. Anal. Chem. 1985, 57, 1609-1616. (8) Munk, M. E.; Shelley, C. A,; Woodruff, H. B.; Trulson, M. 0. Fresenius’ 2.Anal. Chem. 1982, 313, 473-479. (9) Zupan, J. Anal. Chim. Acta 1980, 122, 337-345. (10) Delaney, M. F. Anal. Chem. 1981, 53, 2354-2356. (1 1) Zupan, J. Clustering of Large Data Sets; Research Studies Press (Wiley): Chichester, 1982. (12) Zupan, J. Anai. Chim. Acta 1982, 739, 143-153. (13) Zupan, J.; Novic, M. Computer Supported Spectroscopic Data Bases; Zupan, J.; Ed.; Ellis Horwood International Publishing Co.: Chichester, in press.
RECEIVED for review February 3,1986. Resubmitted July 1, 1986. Accepted July 1, 1986. Presented in part at the VI1 International Conference on Computers in Chemical Research and Education (ICCCRE), Garmisch-Partenkirchen, June, 1985. The authors acknowledge with gratitude the financial support of the US-Yugoslav Board for Scientific Research (Project Number 479), the Research Community of Slovenia, the National Institute of General Medical Sciences (USA, NIH Grant GM21703), and The Upjohn Company.
Characterization of a Diet Reference Material for 17 Elements Nancy J. Miller-Ihli* and Wayne R. Wolf
Nutrient Composition Laboratory, U.S. Department of Agriculture, Beltsuille, Maryland 20705
A freeze-drled diet reference material was prepared from commonly consumed everyday foods. Concentrations of major (Mg, Ca, Na, K, and P), mlnor (Mn,Zn, Fe, Cu, and Ai), and trace elements (Cr, NI, Co, Mo, As, Se, and Cd) In this dlet material were determined by the authors and 10 collaborating laboratories using a total of 10 different analytlcal technlques. Good agreement between concentratlon values determined by the dmerent laboratories enabled the authors to compute “recommended values” and uncertalntles for these 17 elements. Values for proximates as well as several other nutrients were also reported by collaborators. Thls diet material Is available to the sclentlfk community as Reference Material 8431 through the National Bureau of Standard’s Office of Standard Reference Materials.
The usefulness of reference materials (RM’s) for accuracy transfer, method validation, technology transfer, and quality control monitoring has been well established (1-3). Several agencies routinely produce R M s including the National Bureau of Standards (NBS), Community Bureau of Reference (BCR), the International Atomic Energy Agency (IAEA), Agriculture Canada, the National Research Council of Canada, and the National Institute for Environmental Studies (NIES). In 1981 the Nutrient Composition Laboratory (NCL) of the U.S. Department of Agriculture devloped a mixed diet RM because no mixed diet control material was commercially available from any of the RM manufacturers at that time and previous research had shown that existing biological reference materials were not very suitable when analyzing foods and diets (4). The NCL, which is involved in developing methods for the determination of a wide variety of nutrients in individual foods and diets, recognized the essentiality of a mixed
diet RM for this work. A mixed diet material made from commonly consumed everyday foods was selected because a good RM must provide similar matrix effects and analyte concentrations and contain the same chemical form of the analyte as the real world samples for which it will serve as a control. A mixed diet RM was prepared ( 4 ) by using approximately 75 kg of mixed diet material containing commonly consumed foods such as fruit, cereal, bread, noodles, vegetables, salad, fish, and poultry. Extra sources of fat were excluded resulting in a diet that was -9% fat by weight (20% fat calories). The prepared foods were blended, freeze-dried, and reblended. Care was taken to avoid possible trace element contamination since this material was being developed as an inorganic RM. The final powdered material provided 50% recovery for a 30/60 mesh cut sieved through polyethylene sieves and was packaged into 280 30-g units. The final diet material was then characterized with regard to homogeneity for eight elements, Ca, Cu, Fe, K, Mg, Mn, Na, and Zn, to assure that this material was sufficiently homogeneous to serve as a RM. Aliquots of this diet were then distributed to a range of experts in trace element analyses for the final overall characterization of this material for 17 elements. The results from the characterization are presented here along with the final recommended values for these 17 elements. This diet material (previously known as TDD-1D) has been accepted as a Reference Material by the National Bureau of Standards and is available to the scientific community as Reference Material 8431.
EXPERIMENTAL SECTION Diet RM Preparation. A typical diet menu was selected from Nutrition a human condu&d at the Belbville H-* Research Center (Table I). The diet menu included commonly consumed foods for the three meals consumed in a day. A detailed
This article not subject to US. Copyright. Published 1986 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I. Menu for Diet Reference Material
8431"
weight (g) Breakfast orange juice, frozen, unsweetened grapefruit segments, canned cereal, LIFE milk, whole muffins, English, with raisins, toasted jelly sugar
384 160 44
305 62 27 11
Lunch chicken, breast, roasted noodles, egg, steamed carrots, cooked, without salt asparagus, canned, without salt egg yolk, cooked rolls, Brown n' Serve cookies, shortbread pear nectar, canned
106 200
194 152 6.3 65 69 312
Dinner fish, haddock, baked lemon juice, bottled tomatoes, canned, stewed sugar potatoes, boiled, without salt parsley, flakes bread, rye carrots, shredded cucumbers, chopped brownies, with pecans and coconut milk, whole
106 6 151 12
171 0.4
62 35 35 100 305
Total
3080.7
Reference 4. description of the preliminary preparation of this material has been published ( 4 ) . Forty identical daily diets were prepared by weighing out prepared foods from the selected menu and cutting them into small pieces. Two of these daily diets were then mixed together with beverages into a container for storage. The calculated weight per container was 6161 g (3080.70 X 2). The mean weight for 20 containers of diet was 6218 f 31 g, which was within 1.0% of the weight called for in the diet menu, and the variation between containers was only 0.5% relative standard deviation (RSD) suggesting excellent reproducibility of the diet preparation from container to container. The 20 containers were stored a t -40 "C prior to processing. The final diet material was processed from 12 containers (75 kg) of diet. The thawed material was blended in 24 batches using a 6-qt industrial food processor (R6, Robot Coupe USA, Inc., Jackson, MS) with a plastic bowl equipped with titanium blades
to avoid possible trace element contamination from stainless steel apparatus. The blended diet was poured into six polyethylene trays, covered with filter paper, frozen (-40 "C), and then freeze-dried resulting in a loss of 76% moisture by weight. The freeze-dried material was cut into pieces with a Ti-blade and the pieces from the six trays were mixed into a large plastic bag. The dried material was then reblended in the food processor and placed into a large plastic-lined drum. The final freeze-dried material was sieved in a class-100 clean room by using 30- and 60-mesh polyethylene sieves with the 30/60 fraction (60 mesh < material < 30 mesh) serving as the final material. This represented approximately 60% of the total material. The diet RM was then packaged in clean 4-02 polypropylene sample containers (- 30 g) providing a total of 280 containers. Sample containers were labeled and sealed in plastic bags which were evacuated and purged with nitrogen. One "control" sample was removed from each group of 30 samples (total of nine controls) for preliminary in-house characterization and homogeneity studies. The packaged diet material was then radiation sterilized with a 5-7 Mrad dose of y radiation (Neutron Products, Dickerson, MD) to eliminate growth of microorganisms. Radiation sterilization was not expected to affect trace element concentrations but irradiated and nonirradiated samples were both analyzed to verify that there was no difference. The final irradiated material was stored in a walk-in refrigerator at 4 "C until it was sent to those laboratories collaborating in the characterization study. Diet Reference Material Characterization. Characterization of Diet RM 8431 was carried out by the authors and several expert collaborators using the methods listed in Table II. A total of 10 different analytical techniques were used in the characterization (Table 11) (5-30). Where unpublished results are indicated either unpublished methods were used or the collaborator did not supply a specific literature reference. Specific questions regarding any particular methodology should be directed to the individual(s) referenced.
RESULTS AND DISCUSSION The elements determined in this study fell into three categories: (1) major (concentrations of 100 pg/g or more including Mg, Ca, Na, K, and P), (2) minor (concentrations of 1-100 pg/g including Mn, Cu, Al, Rb, Zn, and Fe) and (3) trace (concentrations less than 1 pg/g including Cd, Co, Cr, Pb, Ni, Mo, As, and Se). Homogeneity studies were based on several major and minor elements including Mn, Zn, Fe, Cu, Mg, Ca, Na, and K. These eight elements were selected because the authors are able to routinely determine them accurately and precisely using single-element atomic absorption spectrometry (AAS) (line source atomic absorption methods in their laboratory and because spectrometry) (AAL) they felt that these elements would be good indicators of the overall homogeneity of this RM.
Table 11. Methods Used in the Characterization of Diet Reference Material method
8431
elements
reference
Ca, Cu, Fe, Mg, Mn, Zn
5, 6, 1, 8, 9
Na, K
graphite furnance atomic absorption spectrometry (GFAAS)
As, Cr, Se
5, 6, 7, 8, 9 10, 11, 12, 13
continuum source GFAAS (SIMAAC)
Al, Cu, Cr, Fe, Mn, Mo, Ni, Zn
14, 15
inductively coupled plasma emission spectrometry (ICP-AES)
Al, Ca, Cd, Co, Cu, Fe, K , Mg, Mn, Mo, Na, Ni, P, Pb, V, Zn 10, 11, 16
colorimetry
P
6, 17, 18
isotope dilution mass spectrometry (IDMS)
Cr, Se
19
neutron activation analysis (NAA)
Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Se, V, Zn 20, 21, 22, 23, 24, 25, 26
voltammetry fluorometry
Cd, Co, Cu, Ni, Pb, Zn
21, 28
Se
29, 30
atomic absorption spectrometry (AAS)-flame atomic emission spectrometry (AES)-flame
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table 111. Moisture Determinations of Freeze-Dried Diet Reference Material 8431 moisture, %" analysis 1 (2/84) analysis 2 (3/84)
sample no.
mean
1.79 2.25 1.74 2.16 2.12 2.01 2.59 1.19 1.97
2.62, 2.26 2.04, 2.44 2.11, 1.82 -, 2.50 2.33, 2.40 -, 2.35 1.63, 2.49, 2.42, 2.05 2.59, 1.94
2.0%
2.2%
"Analysis by microwave method (samples -3-4
9).
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Table V. Homogeneity of Diet Reference Material 8431 re1 std dev, 70 (n = 9)" analysis 1 analysis 2
element
cu
1.6
Zn Mn Na K Mg
1.8 1.5 1.8
2.8 2.6 1.8
1.7 1.9 1.4 2.4 4.1
1.2 1.8
Fe Ca
3.6 4.8
" n = 9 "control" samples (samples -0.75 8).
c"
0
660
Table IV. Moisture Content of Freeze-Dried Diet Reference Material 8431 after Storage for 1 Year at 4 O C sample no.
analysis 1"
analysis 2b
1
2.74, 2.80, 2.80 2.88, 2.60, 2.77 3.08, 2.88, 2.98
2.82, 2.83, 2.86 3.17, 3.16, 3.11 3.44, 3.53, 3.41
2.84 f 0.14%
3.15 f 0.27%
2 3 mean
"Analyses by microwave method (samples -3-4 bv vacuum oven method (samules -1.5 a).
g).
-
610 0 0 6
0.1 0.3 0.5 0.7 0.9 Weight
(9)
*Analyses
The details of the characterization of Diet RM 8431 as well as the establishment of minimum sample size requirements are presented here with the final recommended values. Moisture. The moisture content of Diet RM 8431 was characterized by analyzing the nine "control" samples, with a microwave method (31)shortly after it was prepared. The moisture data are summarized in Table 111. Moisture determinations were done on two days and the overall mean moisture value was 2.1% for the freeze-dried diet materal. After Diet RM8431 was stored a t 4 "C for 1year, moisture determinations were repeated to see if the moisture content of this diet RM had changed significantly (Table IV). Moisture determinations performed by using two different methods provided a mean moisture value of 3.0%, indicating that the moisture content had increased by only about 1% over the course of the year. Homogeneity. Preliminary homogeneity studies were performed using the nine "control" samples that were set aside during the packaging of the material. These "control" samples were systematically selected to be representative of the entire batch of diet material. Eight elements (Cu, Zn, Mn, Na, K, Mg, Fe, and Ca) were determined in each of three preparations of each of the nine "control" samples by using line source atomic absorption spectrometry (AAL), and this experiment was done twice. Mean concentrations (n = 3) were computed for each "control" sample and the homogeneity was evaluated by examining the percent relative standard deviations (RSD's) (n = 9) for each element for each experiment (Table V). In most cases, these between sample RSD's were better than 2.0% and were comparable to the average within sample RSD's. Since these RSD's include analytical variability as well as heterogeneity of the material, RSD's less than or equal to 2.0% were considered indicative of a very homogeneous material. The RSD for Ca was higher (4-5%) but was still considered acceptable. Once the homogeneity of the material was established, it was necessary to determine what sample sizes would provide representative subsamples of the whole lot of diet material. Analysts are aware that a very small sample may not be representative and is prone to contamination and that a very
I
\
/
1. o i 0.'1 ' 0.'3 0:5 ' 0:7 ' 0,'Q' Weight
(9)
Figure 1. Magnesium: (A) plot of concentration determined vs. sample weight digested; (6) plot of precision (RSD, % ) vs. sample weight digested.
large sample may not be completely digested during sample preparation, both producing erroneous results. Because different sample preparation procedures and different analytical methods have different sample size requirements, sample sizes ranging from 0.1 to 1.0 g were evaluated. Samples were prepared by using a wet ash digestion procedure (32) and analyzed by AAL for Cu, Zn, Mn, Na, K, Mg, Fe, and Ca. Results for triplicate preparations of samples weighing 0.10, 0.25,0.50,0.75, and 1.0 g were analyzed statistically by using regression methods (33,34). Analytical results for each weight group provided a mean f standard deviation (SD) for each element determined. Weighted regression analyses (weighted by the inverse of the variance) were performed by using SAS (34) to determine if measured elemental concentrations (rg/g) were dependent on sample size. First, second, third, and fourth degree polynomial fits were evaluated and plots of concentration (rg/g) vs. weight as well as precision (RSD, %) vs. weight were generated for inspection. Plots for magnesium and copper appear in Figures 1and 2, respectively. Regression statistics showed no correlation between the analyzed sample concentration and the sample weight for Na, K, Fe, or Mg (see Figure lA), suggesting that sample weights from 0.1 to 1.0 g of diet material could be successfully digested and analyzed for these elements. Cu, Zn, Mn, and Ca, however, showed correlations between concentration and sample weight. Figure 2A illustrates the sample concentration (rg/g) variation with sample weight for copper. Regression statistics indicate a
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table VI. Summary of Reported Concentrations for Diet Reference Material 8431-Major Elements (wg/g, dry weight)
method
Mg
AAS
Ca
702 f 11 617 f 15
AES
1814 f 18 1861 f 48 1966 f 238
635 f 7 652 f 13 5653 665 f 14
NAA
K
3298 f 57 (4492) 3077 f 83 (3840 f 310) 3165 f 26 2928 A 64 3258 f 130 2982 f 182
7987 f 1 2 1 8658 7641 f 114 7180 f 1020 8134 f 67 7461 f 140 8478 f 496 7675 f 397
2244 f 151 1888 1807 f 31 2030 f 60
617
ICP-AES
P
Nan
colorimetry
a
3216 f 20 3147 f 74 3478 f 55 3678 3101 f 10
Values in parentheses have been statistically identified as outliers.
Table VII. Summary of Reported Concentrations for Diet Reference Material 8431-Minor Elements (wg/g, dry weight)
method AAS
SIMAAC ICP-AES NAA
Mn
Zn
Fe
cu
Al"
8.40 f 0.14 8.1 8.6 f 0.2 8.61 f 0.43 8.2 7.76 f 0.08 7.7 f 0.2 8.11 f 0.35 7.28 f 0.13 8.78 f 0.36 7.81 f 0.42
16.4 f 0.3 17.4 16.2 f 0.2 16.5 f 0.2 17.9 f 2.9 16.3 f 0.1 18.4 f 1.1 16.2 f 0.2 18.6 f 0.2 16.5 f 0.8
31.6 f 0.9 39.8 36.1 f 0.3 38.2 f 1.7 39.3 f 4.1 35.7 f 0.7 33.5 f 1.9 36.1 f 1.0 42.6 f 1.1
3.69 f 0.08 4.0 3.07 f 0.10 2.94 f 0.16 3.9 3.32 f 0.04
4.68 f 0.25 4.6 f 0.1
(6.37 f 0.73) 17.1 f 0.9
voltammetry
3.9 f 1.1
*
3.10 0.09 3.43 f 0.10 2.79 f 0.07
Values in parentheses have been statistically identified as outliers.
Table VIII. Summary of Reported Concentrations for Diet Reference Material 8431-Trace Elements (ng/g, dry weight)
method
Cr
Ni
CO
Mo
voltammetry IDMS fluorometrv
Se
Cd
226 f 9 269 f 5
37.1 f 7.2 5825 42.2 f 3.6
783 f 62
GFAAS SIMAAC ICP-AES NAA
As
102 f 8 108 f 7 107 f 13
608 f 63 610 f 41
93 f 13 97 f 11 714 f 34
