Elemental characterization of the National Bureau of Standards milk

heated until 0.5-1.0 mL of HC104 remained. At thispoint the samples were covered and set aside until the chromatographic columns of hydrated manganese...
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Anal. Chem. lS86, 58,2511-2516

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Elemental Characterization of the National Bureau of Standards Milk Powder Standard Reference Material by Instrumental and Radiochemical Neutron Activation Analysis Robert R. Greenburg Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899

The Milk Powder Standard Reference Materlai, SRM 1549, recently prepared by the National Bureau of Standards has been analyzed by Instrumental and radiochemlcal neutron activatlon analyds. The extremely low levels of many of the elements of interest, comblned wlth the high levels of the matrix elements, necessitated improvements In many of the exlstlng analytical procedures. Special attention has been given to reducing and evaluating the analytlcal uncertainties. Bovine Llver, SRM 1577, was analyzed as a control and the results obtained were compared wlth the literature and the NBS certified values.

Well-characterized reference materials have proven to be extremely useful in verifying the accuracy of analytical procedures. Various analytical techniques require attention to chemical blank, sample contamination, matrix effects, interferences, and losses during sample dissolution or chemical manipulation. Additional confidence in an analysis can be gained when the same procedures provide the ucorrectnconcentrations in a reference material similar to the sample being analyzed. The need for suitable reference materials is particularly acute for quality assurance in the biological and medical fields. It is generally recognized that a number of trace elements, such as Cd and Hg, produce toxic effects a t very low levels. Other elements, such as Se and Cr, in trace quantities, are essential for human nutrition. Therefore, accurate determinations of the concentrations of these elements in foods, animal tissues, and other biologically important materials are of great importance. Unfortunately, accurate trace and ultratrace level analyses of such materials are often extremely difficult. These problems were illustrated by the results of a round-robin intercomparison of milk powder (1)conducted by the International Atomic Energy Agency (IAEA). The reported results for a number of biologically important elements varied over ranges of 3 or more orders of magnitude; specifically the results for Cr varied by nearly 5 orders of magnitude. The need for suitable biological and food-related certified reference materials clearly exists. The National Bureau of Standards (NBS) has recently prepared a Non-Fat Milk Powder Standard Reference Material (SRM 1549) to provide assistance in overcoming the difficulties in accurate trace and ultratrace level analyses of food and other biologically important materials. Elemental concentrations in this material are typically at or below those found in the IAEA Milk Powder (1). Neutron activation analysis (NAA) was used extensively in the certification of the NBS Milk Powder. Both instrumental (INAA) and radiochemical (RNAA) procedures were used. In addition to possessing the inherent sensitivities required for a number of important elements in milk powder a t the naturally occurring levels, NAA is normally free from direct interferences (line overlap) and other matrix-related effects. Sample dis-

solution and chemical separations, if needed, can be performed after irradiation, thus eliminating the reagent blank. The use of carriers (uninadiated elements of interest) usually produces quantitative recoveries and excellent separations of the elements of interest from other radioactive materials that reduce sensitivity by elevating the background level of radiation. This paper describes the instrumental and radiochemical procedures used for the analysis of the Milk Powder by NAA. The extremely low levels of many of the elements of interest, combined with the high levels of matrix elements (alkali metals, alkaline earths, halogens, and phosphorus), necessitated improvements in the existing radiochemical procedures. In view of the difficulty of milk analysis, as typified by the IAEA intercomparison ( I ) ,the radiochemical procedures used at NBS will be described in sufficient detail to allow other analysts to perform similar measurements on this SRM, or on other samples of milk powder. These procedures should also be effective for other matrices, such as blood serum, which also contain high levels of the alkalis, alkaline earths, halogens, and phosphorus and low levels of many of the other elements of interest. These procedures are somewhat sensitive to changes in experimental parameters, and so they should be followed exactly as described in this paper. Critical areas in these procedures will be identified, and the analytical results for the Milk Powder SRM will be presented.

EXPERIMENTAL SECTION Reagents. The hydrated manganese dioxide (HMD) inorganic ion exchanger was obtained from Carlo Erba, Milan, Italy. Bismuth, nickel, and zinc diethyldithiocarbamates(DDC) were each prepared by mixing an aqueous solution of NaDDC with an aqueous solution of the appropriate metal nitrate. The M(DDC), compound formed was insoluble in water and precipitated. This precipitate was filtered, washed with water, and dissolved in chloroform. An equal volume of ethanol was added to the solution, which was then set aside to allow the chloroform to evaporate at room temperature. After the chloroform evaporated, the M(DCC), compound crystallized in the remaining ethanol. The crystals were filtered and allowed to dry at room temperature. Chromatographic Columns, The polyethylene chromatographic columns (7 mm i.d.) used were also obtained from Carlo Erba. Three milliliters of the inorganic ion exchanger could be packed into the columns with the reservoir on top of the columns holding up to 15 mL of liquid. Dissolution Bombs. The bombs used for sample dissolution were Autoclave 3 digestion bombs obtained from Perkin-Elmer. They consisted of an aluminum shell over an inner Teflon container. The internal volume was approximately 125 mL, which allowed a relatively large sample (0.5-1.0 g) to be dissolved. Samples could be dissolved at temperatures of up to 160 O C and pressures up to 50 atm, at which point a safety valve would open releasing excess pressure. Irradiation Facility. All irrdiations were performed in the RT-3 pneumatic tube facility of the NBS Research Reactor. In this position the thermal neutron flux was 5 x 1013n cm-* s-l at 10 MW reactor power. Counting Equipment. A number of different Ge(Li) and Ge(HP) detectors were used in this study. Some of the samples analyzed for Cd and Sn were counted using a Ge(Li) detector

This article not subject to U.S. Copyrlght. Publlshed 1986 by the American Chemical Society

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equipped with a Compton-suppression system. The detectors were connected either directly to a Nuclear Data ND-6620 computer-based analyzer system or indirectly through an ND-66 analyzer system. Data reduction was accomplished by using the ND-6620 computer. In most cases two different peak integration methods were used: a computer-run peak search routine, and a channel by channel peak summation using peak and background regions specified by the analyst. Standards. Multielement standards were used for the INAA determinations. Stock solutions for each element were prepared by dissolving high-purity metals or compounds in high-purity acids or water (2). A small amount of each stock solution was irradiated and counted to check for activatable impurities. Multielement solutions were next prepared by combining appropriate amounts of the stock solutions. These multielement solutions were then pipetted onto Whatman 41 filters (5.5 cm), or into small polyethylene bags. The standards were allowed to air-dry and were doubly sealed in polyethylene bags. Particular care was taken in preparing these multielement standards and evaluating the errors associated with their production. More details can be found in ref 2. The standards used for the RNAA determinations were prepared from high-purity metals dissolved in high-purity acids in a manner similar to those used for INAA. Standard solutions were either pipetted onto Whatman 41 filters and dried or sealed directly in quartz vials as a liquid. Quality Assurance Samples. At least one sample of Bovine Liver (SRM 1577) was analyzed with each set of Milk Powder samples. Although the concentrations of a number of the elements investigated were significantly higher in the Bovine Liver than in the Milk Powder, most sources of error for NAA are not concentration dependent. An error in the standards' concentrations, or a counting geometry difference between samples and standards, would affect the Milk Powder and Bovine Liver in an identical manner. In addition, Bovine Liver appears to be the closest to the Milk Powder of the currently available, well-characterized, certified reference materials. Instrumental Procedure for Br, Ca, C1, I, K, Mg, Mn, and Na. Ten 500-mg samples of vacuum-dried Milk Powder were pressed into pellets and doubly sealed in acid-washed polyethylene bags. The samples, standards, and two control samples of Bovine Liver (SRM 1577) were individually irradiated for 30 s. Both bags were removed and discarded, and the samples (standards) were counted for a clock (real) time of 10 min at a distance of 30 cm from a 60 cm3Ge(Li) detector which was coupled to 4096 channels of computer memory. Dead time corrections were made as described by De Soete et al. ( 3 ) . Since no significant changes in analyzer dead times were observed, corrections for "short-lived nuclides in the presence of one or more long-lived radioactivities" were applied. Approximately 30 min after irradiation the samples were recounted for 15 min (clock time) a t a distance of 30 cm. After most of the activity from 38Clhad decayed (approximately 2-3 h), the samples were counted twice more for 1-2 h, 12 cm from a second 60 cm3 detector, to optimize conditions for Mn determination. Instrumental Procedure for Co, Cs, Fe, Rb, Se, and Zn. Twelve 500-mg samples of vacuum-dried Milk Powder and two 500-mg samples of Bovine Liver (SRM 1577) were pressed into pellets and doubly sealed in acid-washed polyethylene bags. The samples, standards, and empty (blank) bags were irradiated for 4 h in RT-3. Midway through the irradiation the polyethylene irradiation container (rabbit) was removed from the reactor, flipped end over end, and reinserted into the reactor to compensate for the linear neutron flux drop-off in this facility. Three weeks after irradiation, the outer bag of each sample was removed and discarded, and the samples and blanks were each counted for at least 48 h a t a distance of 5 cm from a Gamma-X detector. An ADC conversion gain of 8192 channels was used. Radiochemical Procedure for Ag, As, Cr, Mo, Sb, and Se. A modified version of an earlier radiochemical separation procedure ( 4 ) was used to separate these elements from the matrix onto a HMD (hydrated manganese dioxide) column. The modification involved both pre- and posttreatment of the column to minimize retention of ,*P. In addition, the behavior of Ag and Mo on an HMD column was subsequently investigated, and the conditions necessary to retain these additional elements on the

column were identified. Two different sample packaging procedures were used for these elements. Five 500-mg samples of vaccum-dried Milk Powder were pressed into pellets and doubly sealed in polyethylene bags, and five, 500-mg samples were sealed in Suprasil AN (ultra pure) quartz vials. A t least two samples of Bovine Liver were prepared using each procedure. An empty bag and quartz vial were prepared as blanks. Samples, standards, and blanks were irradiated together for 4 h in RT-3. Three days after irradiation, the samples were transferred to Teflon (TFE) dissolution vessels, and carriers of the six elements (10-25 fig each) were added. The samples were slowly dissolved using a t least 20 mL of HNO, and 5 mL of HCIO1. A few drops of HF were added to destroy any siliceous material present. An additional 10 mL of HNO, and 5 mL of HC104 were added and the samples heated until 0.5-1.0 mL of HC104 remained. At this point the samples were covered and set aside until the chromatographic columns of hydrated manganese dioxide (HMD) were ready. Each of the chromatographic columns was prepared by using 3.0 mL of HMD that had previously been equilibrated for several hours in 1 mol/L "0,. The columns were sequentially washed with 15 mL of 1 mol/L HN03, 15 mL of a solution containing 1 mol/L each of "03 and H3P0,, and a t least three times with 15 mL of 1 mol/L HNO,. The purpose of this preconditioning procedure was to fill up all available sites on the resin with phosphate and then remove the excess. Flow rates were adjusted to 0.25-0.5 mL/min by pressing down on the resin with a Teflon stirring rod, and any column with a flow rate slower than 0.25 mL/min was rejected. Fifteen milliliters of 1 mol/L HNO, was then added to each of the dissolved samples previously set aside, and the samples were passed through the HMD columns. After the solutions has passed completely through the columns, the HMD beds were washed twice with 15 mL of a solution containing both 0.0025 mol/L of H3P04and 1 mol/L of HNO,. The dilute H3P04 solution was used to replace most of the radioactive phosphorus on the column with the nonradioactive phosphorus of the washing solution. The HMD fractions were then transferred to scintillation vials, which were centrifuged. The supernatant was removed from each sample and discarded. The samples were then counted directly on top of a high efficiency Ge(Li) or Ge(HP) detector to determine As, Sb, and Mo. Samples counted less than 48 h after separation were later recounted to determine Mo after the equilibrium between =Mo and its daughter, mTc, had been achieved. The samples were counted approximately 1 month after irradiation to determine Ag, Cr, and Se. The standards were processed in three different ways. One set of standards was transferred to dissolution vessels along with carriers and 500 mg of unirradiated Milk Powder. These standards were dissolved and processed in a manner identical with that used for the samples. A second set of standards was prepared in the same way except that no Milk Powder was added. A third set was prepared by pipetting irradiated standard solution directly onto 3.0 mL of HMD already inside of a scintillation vial. By comparison of the dissolved (and chemically separated) standards to the pipetted ones, the possibility of problems due to bad reagents, or to unsuspected matrix effects on the separation procedure, could be virtually eliminated. The blank bag and vial were opened, filled with an aqueous solution containing 25% (by volume) of both "OB and HCl, and allowed to sit for 1 h. The solutions were then transferred to scintillation vials and water was added until the height of the solution was the same as for the samples. These blanks were then counted. Except for Cr in the bag blank, none of the elements of interest could be observed in the blanks. Radiochemical Procedure for Cu and Cd. A modification of an earlier procedure ( 4 ) was also necessary to determine Cd in the Milk Powder. Due to the very low level of Cd, it was necessary to further reduce the (small) fraction of 65Znwhich accompanies Cd through the separation procedure as a result of exchange between the irradiated Zn of the sample and the Zn of the Zn(DDC)2/chloroform solution used to extract Cd. In addition, the HMD and DDC procedures were not combined, as in ref 4, to separate Cd and Cu from the eluted fraction of the HMD radiochemical procedure. Although combining these procedures was still possible, the additional decay of "Td during the HMD procedure would have significantly worsened the measurement error due to the decreased count rate for Cd.

ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986

Five 500-mg samples of Milk Powder were vacuum-dried, pressed into pellets, and individually irradiated for 4 h in RT-3 with standards, and a control sample of Bovine Liver. Two days after irradiation, 500 pg each of Cu and Cd carriers were added to each sample, and the samples were dissolved in Teflon dissolution vessels with HNO,, HC104, and a few drops of HF. Approximately 20 mL of water and 100 mg of Zn hold back carrier were added, and the pH of the sample was adjusted to 1.5 with NH40H. The sample was transferred to a 125-mL separatory funnel and Cu was extracted into 25 mL of 0.003 mol/L Bi(DDC)3 in chloroform. A shaking time of 30 rnin was used to ensure quantitative recovery (>99.5%). The organic fraction containing Cu was drained into a linear polyethylene (LPE) bottle and the aqueous fraction was washed with 5 mL of chloroform. The wash was then combined with the organic fraction. Linear polyethylene bottles were used in view of the much slower diffusion rate of chloroform through the walls of this type of bottle compared to one made of the conventional type. Cadmium was then extracted from the original aqueous solution remaining in the separatory funnel, into 25 mL of 0.005 mol/L of Zn(DDC)2in chloforom. A shaking time of 2 min was sufficient for complete extraction. The organic fraction containing Cd was transferred to a second 125-mL separatory funnel and washed twice (shaken for 5 min) with 50-mL aqueous solutions (pH 1.5) containing 100 mg of Zn carrier. The purpose of this washing procedure was to exchange the radioactive Zn of the sample with the nonradioactive Zn of the carrier solutions, and thus virtually all radioactive Zn was eliminated from the organic fraction. Cadmium was finally back-extracted (30 s) from the organic fraction into 20 mL of 2 mol/L HC1. The HC1 (aqueous) solution was drained into a polyethylene bottle and allowed to decay for 24 h before counting, in order to establish the equilibrium between l15Cd and its daughter 115mIn,which was used for Cd quantification. This Cd separation, although not quantitative, has a high yield and is very reproducible, 95.2 & 0.4% (1 s, for five determinations). Radiochemical Procedure for Hg. As with the previous radiochemical procedures, a modification of an earlier Hg procedure (5) was required to reduce the level of 75Sein the separated Hg fractions of the samples. This was extremely important as 7%ehas a y-ray that overlaps the 279-keV y-ray of mHg and thus is a direct interference. Eight 500-mg samples of Milk Powder were sealed in cleaned quartz vials and were irradiated for 4 h in RT-3 along with standards and two samples of Bovine Liver. Four of the Milk Powder samples were vacuum-dried and four were packaged "as received". One to two weeks after irradiation, the quartz vials were washed in hot aqua regia to remove surface contamination, cooled in liquid nitrogen, and opened. Each sample was transferred to the Teflon container of the dissolution bomb along with 1 mg of Hg and 100 pg of Se carriers. The inside of the quartz vial was washed with concentrated HN03to remove any residual sample material and/or Hg remaining on the walls of the vial, and this wash was added to the sample. The sample was dissolved in the bomb using approximately 10 mL of a 3:l mixture of HN03 and HzSO,. A temperature of 110-140 "C was maintained for several hours, after which the bomb was cooled to -15 "C and opened. A previous study (5)has shown that sample dissolution could be accomplished in this manner with no loss of Hg. The sample was allowed to come to room temperature in a fume hood and about 2 mL of deionized water was added dropwise to drive off NO2. Two milliliters of 30% Hz02was added dropwise to destroy any remaining reduced nitrogen compounds. At this point the solution became clear with no visible particulate matter. Approximately 20 mL of water was added and the pH of the solution was adjusted to 1.5 with ",OH. The sample was then transferred to a 125-mL separatory funnel and was shaken for 2 rnin with 25 mL of chloroform to remove any organically soluble Se compounds. The chloroform was drained and discarded, and Hg was extracted into 25 mL of 0.005 mol/L Ni(DCC)zin chloroform. A 2-min shaking time was used. The Ni(DDC), (organic) fraction was transferred to a second 125-mL separatory funnel and washed (shaken) for 30 s with a solution containing 5% NazEDTA and 1%NaOH. The organic fraction, containing Hg, was drained into a polyethylene (LPE) bottle and counted. Mercury was quantified using the 203Hgisotope. A moisture correction (2.7%) for the "as received" samples was made from

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a moisture content determination on separate samples of Milk Powder from the same bottles. These samples were vacuum-dried for 48 h at room temperature. Radiochemical Procedure for Sn. Pellets weighing approximately 500 mg were prepared, doubly sealed in LPE bags, and irradiated, with standards, for 4 h. After the sample decayed for approximately 1 month, the bags were discarded, and the pellets were combined to form 1-5 g samples of the Milk Powder. They were then transferred to Teflon TFE dissolution vessels with 100 pug of Sn carrier, 20-30 mL of HNO,, 10 mL of H2S04, and 5 mL of HF. The samples were covered, placed on a hot plate, and heated at low heat overnight. The covers were then removed and the heat level was gradually increased to about 200 "C. The samples were then heated for a least 1h at this temperature. At this point charring was usually quite evident. This charring helps volatilize Se, which is advantageous since a small fraction of the Se present can accompany Sn through the separation procedure. The samples were removed from the hot plate and cooled, and 10 mL of HNO, was added to each. The samples were heated until the HN03 evaporated and were again cooled. Five milliliters of 30% H202was added dropwise to each sample followed by 10 mL of HF. Note that extreme caution must be used when adding HzOzto concentrated H2S04. An ice bath can be used to remove excess heat. The samples were placed on a hot plate and again slowly heated to 220 "C to volatilize additional Se. The samples were then cooled, and 10 mL of HN03 plus 5 mL of HClO, were added to each. The samples were transferred to a perchloric acid hood and heated until approximately 5 mL of H2S04(50% of the original amount) remained. The samples were cooled, and concentrated HzSO, was added to bring the volume of each solution to approximately 10 mL. The samples were transferred to 125-mL separatory funnels and carefully diluted to 40 mL with water. The samples were allowed to cool and were then washed (2 min) with chloroform to remove any organic compounds containing Se. The chloroform (lower) fractions were discarded, and 10 mL of a 5 mol/L solution of KI along with 20 mL of toluene was added to each sample. The samples were shaken for 2 min to extract SnI, into the organic (upper) phase. The aqueous fractions were discarded and the organic fractions were each washed twice with 20 mL of a freshly prepared solution containing 1mol/L KI and 3.6 mol/L H2S04. The wash solutions were prepared by first mixing the H$04 and the water, and allowing them to cool before adding the KI, in order to minimize formation of 12. The wash solutions were discarded, and Sn was back-extracted (2 min) into 25 mL of an aqueous solution containing 5% (w/v) NazEDTA and 1% NaOH. The aqueous (lower) fractions containing Sn were drained into polyethylene bottles and counted at least 12 h after separation to allow the equilibrium between l13Snand its daughter, 113mIn, to be attained. In view of the very low Sn concentration in the Milk Powder, the samples were each counted for at least 1 week.

RESULTS AND DISCUSSION The results obtained for the Milk Powder (SRM 1549) are listed in Table I. The NBS certified values (6) are also listed in this table. Agreement should be expected since the NAA values were used, in conjunction with results from other analytical techniques, for the "certified values". However, agreement between the NAA results for Cu and Hg, and the certified values is not optimal, indicating some differences between the NAA results and those of the other analytical techniques(s) used for certification. I t should be noted that other NAA data by Byrne et al. (7) for Cu and Hg in the NBS Milk Powder SRM are much closer to the NBS NAA values (this work) than to the values reported by other NBS analytical techniques. A value of 1 3 pg/g (less than or equal to) is given in Table I for Al in the Milk Powder (and for the Bovine Liver in Table 11). Although an apparent AI concentration could be measured in both materials, the determination of A1 in biological materials is difficult due to the fast (high energy) neutron interference from phosphorus which produces the same 28A1 isotope as does Al. Although the P concentration is quite high (1.06%), the well-thermalized neutron spectrum of the NBS

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Table I. Elemental Concentrations Observed in Milk Powder, SRM 1549, with Estimated 9 5 % Confidence Intervals NAA results 99.5%) were obtained for all elements, except Cr and Se, whose recoveries were 99.0 f 0.5% (Is for five determinations) and 99.0 f 0.3%, respectively. In general, the observed standard deviations for both the Milk Powder and the Bovine Liver were at least a factor of 2 better with the new procedure than could be obtained with the old one ( 4 ) , due to the 80-90% reduction in the 32Plevels. The concentrations of As, Cr, and Sb in the Milk Powder could only be determined with the new procedure. Chromium could only be determined in the samples irradiated in the high-purity quartz vials, as the polyethylene bags introduced a significant blank to the samples. A comparison of the results obtained from the two sets of Milk Powder samples indicated that 8 ng, corresponding to approximately 20% of the total Cr in each of the LPE bags, was transferred to the samples during irradiation. This was equivalent to a concentration of 15 ng of Cr per gram of the Milk Powder. Thus the Cr blank from the bag was 6 times greater than the Cr concentration (2.5 ng/g) in the Milk Powder itself. The transfer probably occurred as a result of neutron-induced recoil during the irradiation and appears to indicate that the Cr in the bags is located at or near the surface. A similar transfer was observed for other Milk Powder samples irradiated in different, conventional polyethylene (CPE) bags, indicating the possible migration of Cr to the surface during the production of (at least) some types of polyethylene films. Good agreement was observed between the results obtained for the other elements in the two sets of Milk Powder samples. It should be pointed out that the behavior of the HMD resin is changed in two ways with the new procedure and, to obtain good results, the procedure given here must be followed exactly. First, the capacity of the resin for the elements of interest is greatly reduced. No more than 25 pg of each of the elements of interest retained on the column should be used as carriers. When 50 pg of each element was used, 10-30% of the Cr, Mo, Se, and Sb passed through the column. Using the old procedure, 500 pg of each element could be retained by the column. In addition, Cr and Se move down the column during the wash procedure. Both elements begin to break through the HMD after 45-50 mL of the 1 mol/L "OB0.0025 mol/L H3P04wash solution pass through the column. I t is thus important that no more than 30 mL of the wash solution be used in order to maintain a margin of safety. The DDC extraction procedure for Cd and Cu produced individually separated solutions of these elements with extremely good radiochemical purity. Although with the modifications to the extractions procedure the recovery for Cd was no longer quantitative, it was high and very reproducible, 95.2 f 0.4% (1s for five determinations). More than a million counts were typically observed for Cu in the Milk Powder, even though its concentration was below 1 ppm. Cadmium was much more difficult to determine as its concentration was

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below 1 ppb. However, the radiochemical purity of the separated sample was such that the Cd detection limit was influenced mainly by the content of naturally occurring radioactive species (U and Th) in the P b shielding of the detector and not by residual radioactivity from the samples analyzed. The amount of the 228Ac(232Thdecay chain) in particular was critical since its 338.4-keV y-ray was within 2 keV of the 336.6-keV line of 115mIn, which was used for Cd quantification. Mercury was quantified by using the 279.2-keV y-ray of 203Hg.Separation from 75Sewas therefore critical in view of its direct interference at this energy. Although inorganic Se is readily separated from Hg by the original procedure (4), some Se-containing organic compounds are extremely difficult to destroy via acid dissolution, and these can accompany Hg through the separation procedure as they are typically soluble in the chloroform used to extract Hg(DDC)2. Approximately 0.1 % of the Se present in the Milk Powder or Bovine Liver was observed in the Hg fractions of samples separated by using the original procedure. The modifications to this procedure reduced the Se levels by an additional factor of 10-100. Although a small 265-keV peak from 75Secould be observed in some of the Milk Powder samples, the interference was small enough that it could be accurately subtracted from the 203Hg peak. This was accomplished by irradiating and counting some Se, to determine the 279/265-keV peak ratio of 75Se,and then using this ratio to determine and subtract the interference (typically 10%) from each sample. The observed recovery for Hg with the modified procedure was 99.4 f 0.3% (Is for five determinations), and most of the loss occurs during the initial chloroform wash. In addition to the small fraction of 75Se,mentioned above, the only other radionuclides that accompany Hg through the separation procedure were small fractions of the 82Br,64Cu,and lg8Au,and these decayed away before the samples were counted for Hg. The major sources of background radiation were the naturally occurring radionuclides in the detector shield, and the 40K from the walls of the counting room. Separation of Sn from other radionuclides was also very good. As with Hg, some organically bound Se was observed to accompany Sn. Although not a direct interference, 75Se can elevate the background level of radiation. Many of the steps used in the separation procedure were designed to reduce the fraction of 75Seaccompanying Sn. Typically, the level of 75Sewas reduced by a factor of 1 X lo4 using this procedure, and if necessary, additional reduction can be achieved by recycling the Na2EDTA solution through the dissolution and separation procedures. As with the other single element separations, the predominant source of background radiation, and thus the major determinator of the Sn detection limit, was the natural radiation from the P b shield of the detector and from the walls of the counting room.

CONCLUSION The concentration of 22 elements have been determined in the Milk Powder SRM, including most of the biologically important ones. Particular care was taken to minimize analytical errors, and the 95% confidence interval for each element has been estimated. Although the biological matrix elements (alkalis, alkaline earths, halogens, phosphorus, etc.) are present in the Milk Powder at relatively high levels, most other elements are present at very low concentrations, making this material particularly susceptible to contamination during sample preparation and analysis. The Milk Powder is relatively difficult to mineralize, due to its high content of complex, organic compounds, and can easily be charred during acid dissolution. These properties make it ideal for use as a quality assurance material for low-level determinations in biological samples. The results presented in this work, as well as the description of the procedures used to obtain them,

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should provide a useful supplement to the NBS Certificate of Analysis. Registry No. Ag, 7440-22-4;Al, 7429-90-5;As, 7440-38-2; Br2, 7726-95-6; Ca, 7440-70-2; Cd, 7440-43-9; Clp, 7782-50-5; Co, 7440-48-4; Cr, 7440-47-3; Cs, 7440-46-2; Cu, 7440-50-8; Fe, 7439-89-6; Hg, 7439-97-6; 12, 7553-56-2; K, 7440-09-7; Mg, 743995-4; Mn, 7439-96-5;Mo, 7439-98-1;Na, 1440-23-5;Rb, 7440-17-7; Sb, 7440-36-0; Se, 7782-49-2; Sn, 7440-31-5; Zn, 7440-66-6.

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(6) U.S. National Bureau of Standards, Certificate of Analysis Standard Reference Material 1549, Non-Fat Milk Powder, July 29, 1985. (7) Byrne, A. R.; Dermelj, M.; Kosta, L.; Tusek-Znidaric, M. “Radiochemical Neutron Actlvation Analysis in Standardization of Trace Elements in Biological Reference Materials at the Nanogram Level”; presented at the 9th International Symposium on Microchemical Techniques, Amsterdam, The Netherlands, August 1963. (8) US. Natlonal Bureau of Standards, Certificate of Analysis - Standard Reference Material 1577, Bovine Liver, June 14, 1977. (9) Gladney, E. S.;Burns, C. E.; Perrin, D. R.; Roelandts, I . ; Gills, T. E. NBS Spec. Publ. ( U . S . )1984, 260-288.

LITERATURE CITED Dybczynski. R.; Veglia, A,; Suschny, 0. “Report on the Intercomparison Run A-1 1 for the Determlnation of Inorganic Constituents of Milk Powder”; 1980; IAEA Report RL/68. Greenberg, R. R. Anal. Chem. 1979, 51, 2004-2006. De Soete, D.: Gijbels, R.; Hoste, J. Neutron Activation Ana/ysjs ; WileyIntersclence: London, 1972; pp 490-496. Gallorlnl, M.; Greenberg, R. R.; Gills, T. E. Anal. Chern. 1978, 50, 1479- 1481, Greenberg. R. R. Anal. Chern. 1980, 52,676-679.

March 13, 1986. Accepted May 21, Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedures. Such identification does not irpply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. RECEIVED for review

Nonelectric Gas Chromatograph with Direct Optical-to-Pneumatic and Pneumatic-to-Optical Conversion for Transmission and Control Raymond Amino,* Charles Caffert, and E. L. Lewis The Foxboro Company, Corporate Research, Foxboro, Massachusetts 02035

A nonelectrlc pneumatically powered process stream chromatograph Is described that Is controlled vla an optical fiber link from a remote location and that also transmits a chromatogram as a frequency modulated optlcal signal to the remote locatton where It Is transduced and decoded to produce the commonly observed chromatographic record. The detector/transmHter, conslstkrg of an orlfice/capillary primary sensor In comblnatlon with a resonant hobw beam/optlcal transducer, has a minhnum detectable quantity of 25 ppm and a dynamlc range of 2.5 X lo4 and operates on an optical source power of 50 pW. The opticactopneunatk transducer, whkh Is at the chromatograph end of the control Ink, utlllzes the photoacoustlc effect produced by 0.70 mW of optical power and fleurlc technology to amplify pressure to sufflclent power levels to do useful work, such as sequence a multlplexer, Inject a sample, etc.

A nonelectric pneumatic based process stream gas chromatographic analyzer was described in the literature some time ago (1-3). The design was directed toward those process control applications that could be satisfied by highly reliable measurements of one or two components at concentrations exceeding 0.5 mol %. As such, it was a true composition transmitter capable of being located in hostile or hazardous process environments, generating continuous pneumatic trend signals proportional to the concentration of the selected process stream components. As a stand-alone “smart” transmitter, however, no provisions were made to enable one to, in effect, “tune” the device from a remote location. In terms of chromatography, the “tuning” might involve changing the peak selection parameters, changing gain settings, transmitting the chromatogram 0003-2700/88/0358-2516$01.50/0

for total analysis by a computer situated elsewhere, putting the unit on standby, injecting standard, etc. Assuming that the remote location is too distant for accurate transmission in the pneumatic domain, the aforementioned tasks require, a t the instrument in the field, (1)pneumaticsignal to transmission-signal conversion in order to transmit information from the device to the remote location and (2) control-signal to pneumatic-signal conversion in order to act upon control signals transmitted from the remote location to the device. Again, recalling the restraints imposed upon the design by the hazardous environment where the device is to be located, all domain conversions must preclude any that might lead to an explosion in the presence of gases such as hydrogen, acetylene, ethylene oxide, etc. (that is, presuming one wishes to avoid explosion-proof housings and the like). This paper reports the results of our research using optical transmission and direct pneumatic conversion schemes to satisfy the above described requirements and to access the large dynamic range and parts-per-million sensitivity of the orifice/ capillary detector system. Although the conversion schemes are demonstrated using the Pneumatic Composition Transmitter as a vehicle, we believe they have much more general application in the field of process and laboratory instrumentation.

EXPERIMENTAL SECTION Conversion of Pneumatic Detector Signal to a Frequency-ModulatedLight Signal. The pneumatic chromatographic detector has been described previously ( 2 ) . Briefly, its output is the differential pressure signal that is developed across an 0.05 mm orifice/capilIary combination which is then amplified to span the 20.7-103.4 kPa (gauge pressure) range. In the optical version of this detector, the line carrying the signal to the on-board pneumatic logic and signal processor is teed and also connected to a resonant element pneumatic to light transducer 0 1986 American Chemical Society