AOAC Official Method Selenium in Feeds and Premixes

Transcription

1 AOAC Officil Method Selenium in Feeds nd Premixes First Action 1996 Finl Action 1997 [Applicble to determintion of totl Se in rnge of µg/g or greter in ll types of feed ingredients, finished feeds, nd premixes.] Cution: See Appendix B, sfety notes on sfe hndling of cids nitric nd perchloric cids. 2,3-Diminonphthlene is highly toxic nd crcinogenic. It cuses eye nd skin irrittion nd is irritting to mucous membrnes nd upper respirtory trct; my be ftl if swllowed. When hndling powder form, wer gloves nd protective clothing nd pproprite NIOSH/MSHA pproved respirtor, nd crry out the opertions in fume hood. Wsh hnds thoroughly fter hndling. Keep 2,3-diminonphthlene tightly closed in cool, dry storge re. In cse of contct, immeditely flush eyes or skin with copious mounts of wter nd discrd contminted clothing. If inhled, remove to fresh ir. If not brething give rtificil respirtion. If brething is difficult, give oxygen. If swllowed, wsh out mouth with wter provided person is conscious nd cll physicin. Sodium borohydride is flmmble, toxic solid which rects with wter or cid to relese flmmble H. Avoid contct with skin nd store in cool dry plce. Destroy regent by cidifying in hood. Fluorometric Method Results of Interlbortory Study: See Tbles A nd B for the results of the interlbortory study supporting cceptnce of the method. Accurcy of method ws substntited by in-house nlyses of NIST Stndrd Reference Mterils (SRM). Results of nlyzing 5 replictes of 3 different SRMs were s follows (numbers in prenthesis re the certified vlues): NIST-1567-Whet Flour ± (1.1 ± 0.2) NIST-1577-Bovine Liver ± (0.71± 0.07) NIST-1643c-Trce Elements in Wter ± ( ± ) Tble A Results of interlbortory study for determintion of selenium in feeds nd premixes by fluorometric method No. of lbs No. of fter removl outlier Men, RSD R, Smple of outliers b lbs N c µg/g s r RSD r,% s R % r d R e Dry dog food Moist ct food Medicted swine feed Swine feed supplement Swine concentrte Clf pellets Swine strter premix Swine supplement Beef minerl/vitmin premix Vitmin E selenium fortifier (ll livestock) Selenium pk * (0.2%) clcium crbonte* (0.06%) * (0.176%) * (0.6%) Different Se concentrtion rnges re seprted by line. In scending order they re: µg/g; µg/g; µg/g; nd µg/g. b Outliers determined by Cochrn s/grubbs test (1% level). c No. of test smples. d r = 2.8 s r. e R = 2.8 s r.

2 Tble B Summry of results of interlbortory study for determintion of selenium in feeds nd premixes by fluorometry Selenium, µg/g s r RSD r,% s R RSD R,% A. Principle All forms of Se re converted by oxidtive digestion to inorgnic form, presumbly Se 4+ or Se 6+. HClO 4 in oxidtion mixture prevents loss of Se. Selenium must be in Se 4+ form to rect with 2,3-diminonphthlene. All forms of Se in digestion mixture re converted to Se 4+ by reduction with HCl. Digestion reduction procedure is lso suitble for quntittion of Se by continuous hydride genertion tomic bsorption (HGAA) method. Mtrices contining high mounts of soluble Fe ( 1 mg Fe/liquot nlyzed) my cuse difficulty with fluorometric procedure, but problem cn usully be detected by the ppernce of blck precipitte during the derivtiztion procedure, E(d)(3), nd corrected by dilution or by incresing mount of chelting gent. B. Apprtus () Fluorometer. With excittion t 375 nm nd emission t 525 nm. If possible, djust fluorometer to 1 scle unit=1ng. (b) Fume hood. Suitble for hndling HClO 4. (c) Digestion system cm luminum block with 80 holes (22 mm dimeter) set on cm hot plte. (Any commercilly vilble heting block is suitble, if tests nd stndrd solutions cn be heted simultneously.) Alterntively, microkjeldhl digestion system cpble of holding 30 ml flsks or stright-wlled tubes my be used. (d) Digestion vessels. For digestion system. Screw cpped (Teflon lined) mm test tubes; microkjeldhl flsks, 30 ml, or stright-wlled tubes re cceptble. (e) Extrctor mechnized rottion unit. Mintining rpm/min. Hnd-held continer llowing mixing of rck (4 rows of 10 tubes) of tubes is suitble. (f) Pipettor. Delivering 5 ml ± 1%. (g) H 2 O bths. (1) Mintining 60 ±2 C, nd (2) boiling H 2 O. (h) Vortex mixer. (i) Volumetric flsks. 100 nd 1000 ml. (j) Erlenmeyer flsks ml nd 2 L. (k) Filter pper. Qulittive pper, 11 µm retention. C. Regents All regents should be t lest nlyticl grde. Use deionized H 2 O distilled in glss for preprtion of solutions nd dilutions. () Cyclohexne. (b) Hydrochloric cid solution. 0.1M. Pipet 8.3 ml concentrted HCl into 1 L volumetric flsk nd dilute to volume with H 2 O. Proportionte mounts for ny convenient volume my be used. (c) Nitric cid. 70%. (d) Perchloric cid. 70%. (e) 2,3-Diminonphthlene (DAN) regent. Weigh 1.0 g DAN powder (97% purity) nd trnsfer to 2 L Erlenmeyer flsk. Add 500 ml 0.1M HCl nd wrm 15 min in 60 C wter bth. Stir to help dissolve powder. Dilute to 1 L with 0.1M HCl. Extrct solution 3 5 min with ml cyclohexne nd discrd cyclohexne lyer. Repet extrction 3. Filter DAN regent through filter pper pre-wet with 0.1M HCl. DAN regent is stble t lest 2 weeks, when protected from light in refrigertor. (f) (Ethylenedinitrilo)tetrcetic cid (EDTA) stndrd solutions. (1) EDTA stndrd stock solution. 0.1M. Plce 37.2 g (ethylenedinitrilo)tetrcetic cid, disodium slt, into 1 L volumetric flsk nd dilute to volume with H 2 O. (2) EDTA working stndrd solution. 0.01M. Depending of number of tubes to be nlyzed, dilute sufficient volume EDTA stndrd stock solution (1 + 9) with H 2 O to provide 15 ml/tube (e.g., 600 ml for 40 tubes). (g) Selenite stndrd solutions. (1) Selenite stndrd stock solution. 0.4 µg Se/mL. Pipet 1.00 ml selenite stndrd solution (1000 µg Se/mL in 1% HNO 3 ; commercilly vilble tomic bsorption stndrd solution is suitble) into 1 L volumetric flsk nd dilute to volume with 0.1M HCl. From this solution, pipet 40 ml into 100 ml volumetric flsk nd dilute to volume with 0.1M HCl. (Note: As lterntive, dissolve g Se in HNO 3 in 1 L volumetric flsk nd dilute to volume with 0.1M HCl; dilute 10.0 ml of this solution to 1 L with 0.1M HCl in volumetric flsk. Finlly dilute 10 ml of this solution to 100 ml with 0.1M HCl in volumetric flsk nd use directly.) (2) Selenite clibrting stndrd solutions. Pipet 0.00 (regent blnk), 0.200, 0.500, 1.00, nd 1.50 ml selenite stndrd stock solution into seprte digestion vessels to obtin 0.00, 0.08, 0.200, 0.400, nd µg Se/vessel. (h) Sodium selenite stndrd solution. 0.4 µg Se/mL. Trnsfer g nhydrous N 2 SeO 4 into 1 L volumetric flsk, dilute to volume with H 2 O. Mix well. From this solution, pipet 5.00 ml into 1 L volumetric flsk nd dilute to volume with H 2 O. D. Qulity Assurnce Strting with digestion, with ech set of test solutions run 2 regent blnks nd t lest 4 selenite stndrd solutions, C(g)(2), (e.g., 0.080, 0.200, 0.400, nd µg Se/vessel); nd one tube contining ml sodium selente solution, C(h), (0.2 µg Se/vessel) to check dequcy of reduction step, since selente does not rect with DAN. Recoveries of % re expected. Otherwise, renlyze the entire set. Approprite NIST Stndrd Reference Mterils (SRMs) cn be included in nlysis, e.g., NIST 1643c, trce elements in wter (most convenient to use), NIST 1567, whet flour; nd NIST 1577b, bovine liver. Predigestion steps for SRMs my be omitted. Trnsfer or weigh pproprite mounts of SRMs directly into digestion tubes. E. Determintion () Pre-digestion. Weigh c 10 g premix or feed into ml Erlenmeyer flsk nd record weight to the nerest 10 mg (W s ). (Use the lrgest flsk fesible to minimize foming problems.) Add slowly nd with cre 75 ml HNO 3 nd boiling chip (or severl glss beds). (Cution: Mtrices with lrge mounts of limestone or esily oxidizble mterils my cuse foming when HNO 3 is dded.) Het on hot plte until s much of mteril is in solution s possible nd nitric oxide fumes subside (usully 15 min re dequte).

3 Cool solution nd dilute quntittively with H 2 O so tht Se content flls between µg/ml. Record finl volume of diluted predigest solution to the nerest ml (V t ). (b) Digestion. If block digestion system is not vilble, perform digestion nd reduction procedures in microkjeldhl flsks using lterntive digestion nd reduction procedure, G. Otherwise, proceed s follows: (1) Mix thoroughly predigest solution from () to suspend ll undissolved mterils. Pipet 1.00 ml liquots into test tubes (digestion vessels). If Se content of predigest solution is low (<0.02 µg/ml), liquot up to 10 ml cn be used. Record volume to nerest 0.01 ml (V ). (2) Add porous boiling bed to ech tube, including blnks, selenite clibrting stndrd solutions, nd N 2 SeO 4 stndrd solution. If glss beds re used, dd 2 3 beds. (3) Add4mLHNO 3 nd 1 ml HClO 4 [or 5 ml HClO 4 HNO 3 mixture (1 + 4, v/v)] to ech tube. (4) Plce tubes in luminum heting block. Rise temperture slowly to 210 C (c 2 h). White fumes of HCLO 4 should be visible in tubes t completion of digestion. After reching white fume stte, het dditionl 15 min. (5) Remove tubes from heting block. Cool tubes to room temperture nd heting block to C. (c) Reduction. Add 0.5 ml concentrted HCl to tubes from (c)(5). Plce tubes gin in heting block nd het 30 min. Ensure tht temperture is mintined between C for the entire period. (d) Derivtiztion nd quntittion. (1) Remove tubes from heting block nd let cool. It is criticl tht t this step tubes re t room temperture. (Note: Procedure my be interrupted t ny time, up to nd including this step.) (2) Add 15 ml EDTA working stndrd solution, C(f)(2), nd 2 ml DAN regent, C(e), to test tube. (Note: Both solutions my be dded simultneously; however, they should not be mixed together more thn 10 min immeditely before use or precipitte will form.) Mix ech tube well on Vortex mixer, tking Vortex to the bottom of tube t lest twice. (3) Plce rck of tubes in 60 C H 2 O bth nd mintin 30 min. Ensure tht H 2 O level is bove level of rection mixture. (4) Remove rck from H 2 O bth nd cool tubes 5 min in running tp H 2 O. (5) Add 5 ml cyclohexne to ech tube. Cp tubes with Teflon lined cp. Extrct 5 10 min in rotting extrction unit (60 70 rpm/min). [Note: Extrction cn be performed mnully by shking (inverting) rck of tubes for period of time tht gives mximum extrction.] (6) Trnsfer cyclohexne lyer into fluorometer cuvettes. Ensure tht solution is free of ny suspended H 2 O droplets tht might dhere to wll of cuvette in light pth. (7) Set excittion wvelength of fluorometer t 375 nm nd emission t 525 nm. Zero fluorometer with cyclohexne nd red blnk to judge qulity of DAN regent. If reding is greter thn 2 3 fluorescence units, DAN regent should be extrcted gin with cylcohexne. Zero fluorometer ginst blnk. (8) Determine fluorescence (F) of selenite clibrting stndrd solutions nd clculte regression eqution for stndrd curve. Use slope (k) in clculting Se concentrtions in test solutions. Depending on equipment vilble, this my be utomticlly done by built-in clibrtion procedure. [Note: Fluorescence response is liner when using selenite clibrting stndrd solutions t concentrtions described in C(g)(2). Stndrds contining s high s 2 µg Se/vessel my mintin liner reltionship.] (9) Determine fluorescence of test solution. F. Clcultions Depending on support softwre of fluorometer used, clibrtion dt, dilution fctors, nd test portion weights my be stored in computer nd finl content of Se [µg/g (ppm)] my be printed out. Report µg Se/g to 3 significnt digits. When using mnul system, clculte Se content in test smple s follows: µg Se/g = F Vt 1 k V W where F = fluorescence reding; k = slope of regression line obtined from plotting µg Se(x-xis) vs fluorescence reding (y-xis) (k = y/x); W s = weight of test portion, g; V t = totl volume of diluted predigest solution, ml; V = liquot of predigest solution used in nlysis, ml. G. Alterntive Digestion nd Reduction Procedure When block digestion system is not vilble, perform digestion nd reduction procedures in microkjeldhl flsks s follows: (1) Pipet 1.00 ml predigest solution from E() into 30 ml microkjeldhl flsk. If Se content of predigest solution is low (<0.02 µg/ml), liquot up to 10 ml cn be used. Record volume to nerest 0.01 ml (V ). (2) Add boiling bed to ech flsk. Add 4 ml HNO 3 nd1ml HClO 4 into ech flsk [or 5 ml HClO 4 HNO 3 mixture (1 + 4, v/v)], including blnks, selenite clibrting stndrd solutions, nd N 2 SeO 4 stndrd solution. (3) Plce flsks on microkjeldhl digestion rck with fume collecting mnifold connected to spirtion system tht collects most of the fumes from flsks. (Note: Mnifold nd spirtion system my be omitted if perchloric cid hood is vilble.) Adjust heters to gently boil contents of flsks. (4) Het flsks until HNO 3 hs evported nd white fumes of HClO 4 re clerly visible in neck of flsk. Het flsks 15 min fter white fumes of HClO 4 re observed. (5) Remove flsks from digestion rck, cool to room temperture, nd dd 0.5 ml concentrted HCl. (6) Plce flsks in boiling H 2 O bth nd mintin 30 min. Remove flsks from H 2 O bth nd cool to room temperture. (7) Quntittively trnsfer digest to test tubes fitted with Teflon lined cps using 15 ml EDTA working stndrd solution, C(f)(2). Add 2 ml DAN regent, C(e). (8) Proceed to E(d), Derivtiztion nd quntittion. Continuous Hydride Genertion Atomic Absorption (HGAA) Method Results of Interlbortory Study: See Tbles C nd D for the results of the interlbortory study supporting cceptnce of the method. s

4 Accurcy of method ws substntited by in-house nlyses of NIST Stndrd Reference Mterils (SRM). Results of nlyzing 5 replictes of 3 different SRMs were s follows (numbers in prenthesis re the certified vlues): H. Principle NIST-1567-Whet Flour ± (1.1 ± 0.2) NIST-1577-Bovine Liver ± (0.71± 0.07) NIST-1643c-Trce Elements in Wter ± ( ± ) Wet digestion procedure is identicl to tht used in fluorometric method. Digestion-oxidtion procedure converts ll forms of Se to inorgnic form, presumbly Se 4+ or Se 6+. Presence of HClO 4 in oxidtion mixture prevents loss of Se. Selenium hs to be in Se 4+ form to be converted to voltile hydride. All forms of Se in digestion mixture re converted to Se 4+ by reduction with HCl. I. Apprtus () Atomic bsorption (AA) spectrometer. Double bem, with deuterium lmp bckground corrector, Se hollow cthode lmp, nd continuous hydride genertion ccessory. Operting conditions: wrm up time, 20 min; cetylene ir flme: cetylene, 12 psi; ir, 60 psi; wvelength, 196 nm; lmp current, 10 ma; slit width, 1 nm; equilibrtion time, 30 s; integrtion time, 5 s; replictes, 3. Continuous hydride genertion ccessory: N gs purge, 50 psi; flow rte of test solution, 8 ml/min; flow rte of 0.6% NBH % NOH, 1.1 ml/min; flow rte of 10M HCl, 1.1 ml/min. (b) Fume hood. See B(b). (c) Digestion system. See B(c). (d) Digestion vessels. See B(d). Clibrte test tubes to 20 ml. (e) Vortex mixer. (f) Dispensing pipet. To deliver 19.0 ml. Equivlent pipet is suitble. (g) Volumetric flsks. 100 nd 1000 ml. (h) Erlenmeyer flsk ml. J. Regents All regents should be t lest nlyticl grde. Deionized wter distilled in glss should be used for preprtion of solutions nd dilutions. Tble C Results of interlbortory study for determintion of selenium in feeds nd premixes by HGAA method Smple No. of lbs fter removl of outliers b No. of outlier Men, lbs N c g/g s r RSD r,% s R RSD R,% r d R e Dry dog food Moist ct food Medicted swine feed Swine feed supplement Swine concentrte Clf pellets Swine strter premix Swine supplement Beef minerl/vitmin, premix Vitmin E selenium fortifier, (ll livestock) Selenium pk (0.2%), clcium crbonte (0.06%) (0.176%) (0.6%) Different Se concentrtion rnges re seprted by line. In scending order they re: µg/g; µg/g; µg/g; nd µg/g. b Outliers determined by Cochrn s/grubbs test (1% level). c No. of test smples. d r = 2.8 s r. e R = 2.8 s R.

5 Tble D Summry of results of interlbortory study for determintion of selenium in feeds nd premixes by HGAA Selenium, µg/g s r RSD r,% s R RSD R,% () Hydrochloric cid solutions. (1) (v/v). For dilution. Dilute concentrted HCl with equl volume of H 2 O nd plce in dispensing pipet, I(f). (2) 10M. For hydride genertion. Trnsfer 400 ml concentrted HCl into 500 ml volumetric flsk nd dilute to volume with H 2 O. (b) Nitric cid. 70%. (c) Perchloric cid. 70%. (d) Sodium hydroxide. (e) Sodium borohydride solution. For hydride genertion. Dissolve 2.5 g NOH nd 3.0 g NBH 4 in 500 ml H 2 O. (Cution: Sodium borohydride is flmmble, toxic solid which rects with wter or cid to relese flmmble H gs. Avoid contct with skin nd store in cool dry plce. If necessry, the regent my be destroyed by cidifying under hood.) (f) Selenite stndrd solutions. (1) Selenite stndrd stock solution. 1 µg/ml. Pipet 1.00 ml Se stndrd solution (1000 µg Se/mL in 1% HNO 3 ; commercilly vilble AA stndrd solution is suitble) into 1 L volumetric flsk nd dilute to volume with 0.1M HCl. (Note: As lterntive, dissolve g Se in HNO 3 in 1 L volumetric flsk nd dilute to volume with 0.1M HCl. Dilute 10.0 ml of this solution to 1 L with 0.1M HCl in volumetric flsk; finlly dilute 10.0 ml of this soluiton with 0.1M HCl to 100 ml in volumetric flsk.) (2) Selenite clibrting stndrd solutions. Pipet 0.500, 1.00, nd 2.00 ml selenite stndrd stock solution into seprte 100 ml volumetric flsks nd dilute to volume with HCl solution (1 + 1, v/v), J()(1), to obtin Se concentrtion of 5.00, 10.0, nd 20.0 ng/ml. (g) Sodium selente (N 2 SeO 4 ) stndrd solution. 0.4 µg Se/mL. See C(h). (h) Qulity control (QC) stndrd solution ng/ml. Pipet 40 ml selenite stndrd stock solution, (f)(1), into 100 ml volumetric flsk nd dilute to volume with 0.1M HCl. From this solution, pipet 2.00 ml into 100 ml volumetric flsk nd dilute to volume with HCl solution (1 + 1, v/v). K. Qulity Assurnce Strting with digestion, with ech set of test solutions run 2 regent blnks, nd 1 tube contining ml N 2 SeO 4 solution, J(g), (0.2 µg Se/vessel) to check dequcy of reduction step, since selente is not reduced to Se hydride by NBH 4. Recoveries of % re expected. Otherwise, renlyze the entire set. Approprite SRMs cn be included in nlysis, e.g., NIST 1643c, trce elements in wter (most convenient to use), NIST 1567, whet flour; nd NIST 1577b, bovine liver. Predigestion steps for SRMs my be omitted. Trnsfer or weigh pproprite mounts of SRMs directly into digestion tubes. L. Determintion () Predigestion. See E(). (b) Digestion. See E(b). (Note: If block digestion is not vilble, perform digestion nd reduction procedures in microkjeldhl flsks using lternte digestion nd reduction procedure, N.) (c) Reduction. After tubes from (b) hve cooled to room temperture, dd 0.5 ml concentrted HCl. Plce tubes gin in heting block nd het 30 min. Ensure tht temperture is mintined between C for the entire period. Remove tubes from heting block nd let cool to room temperture. Dilute to 20 ml with HCl solution (1 + 1, v/v), J()(1), nd mix well using Vortex mixer or cp nd mix by inverting severl times. (d) HGAA determintion. (1) Clibrte instrument using HCl solution (1 + 1, v/v) s regent blnk nd selenite clibrting stndrd solutions, J(f)(2). At the beginning of nlysis, clibrtion procedure should be repeted until bsorbnce vlues (prticulrly for 5 ng/ml Se working stndrd solution) remin constnt. AA spectrometer response is not necessrily liner for rnge of Se stndrd working solutions used in nlysis. Concentrtion of unknowns must be determined from stndrd curve. (2) Red QC stndrd solution. Concentrtion of 8.0 ± 0.4 ng Se/mL should be obtined. Otherwise, reclibrte instrument. (3) Determine content of Se (C; ng/ml) in test solution. (4) Check Se content in QC stndrd solution every tests. If Se content flls outside cceptble rnge (i.e., 8.0 ± 0.4 ng/ml), reslope clibrtion curve or reclibrte instrument. (5) Red ech test twice, but not consecutively. Averge the 2 vlues. Redings should gree within 10%. Greter devition indictes mixing or stndrdiztion problem nd series should be re-nlyzed. If test reding is outside of clibrtion curve, dilute portion of test solution with HCl solution (1 + 1, v/v) nd repet nlysis. Use pproprite dilution fctor in clcultions. M. Clcultions It is ssumed tht ll test solutions re diluted to 20 ml (finl dilution) nd tht AA spectrometer is clibrted in ng Se/mL. Clculte Se content, µg/ml, s follows: µg Se/mL = C Vt V W where C=Se content in ech tube determined by HGAA, ng/ml; V t = totl volume of diluted predigest solution, ml; V = liquot of predigest solution nlyzed, ml; W s = weight of test portion, g. N. Alterntive Digestion nd Reduction Procedure If block digestion system is not vilble, perform digestion nd reduction procedures in microkjeldhl flsks s follows: (1) Pipet 1.00 ml predigest solution from L() into 30 ml microkjeldhl flsk. If Se content of predigest solution is low (µg/ml), liquot up to 10 ml cn be used. Record volume to nerest 0.01 ml (V ). (2) Perform steps G(2) (6). (3) After flsks hve cooled to room temperture, dilute to 20 ml with HCl solution (1 + 1, v/v) nd mix thoroughly. Trnsfer diluted digest to test tubes. (4) Proceed to L(d), HGAA determintion. Reference: J. AOAC Int. 80, 469(1997). CAS (2,3-diminonphthlene) Revised: Mrch 1998 s