Agilent Low level measurement of cadmium in foods Application Note
Contents
1. 9 0 006 mg g Cd 1o Detection Limit Based on ten readings for both a blank solution and 0 0005 mg mL cadmium standard the detection limit is better than 0 0005 mg mL Cd in solution 5s For grains this equates to better than 0 01 mg mL Cd in the sample Reporting limit is therefore 0 02 mg Cd g for grains 100 The detection limit may be improved by using up to 20 uL injected volume into the graphite furnace and as little as 5 mL of 3 HNO in the back stripping step Precision 3 RSD at 0 032 ug g Cd level and typical precision measurements have shown better than 13 at 0 015 ug g Cd Quality Assurance Every tenth sample is analyzed in duplicate The thirtieth sam ple is analyzed in triplicate with 1 mL of 0 0065 ug g Cd fruit vegetables or 1 mL of 0 0125 ug g Cd grains egg dairy oil seeds added to the third replicate A recovery calculation is made on those samples as follows ug g Cd in spiked sample ug g Cd in sample ug g Cd in sample due to spike x 100 The recovery should be between 80 and 120 A correction may be made for recovery Corrected result Apparent result x 100 recovery In addition a primary standard is analyzed in each batch eg NBS 1567 wheat flour The certified value for NBS 1567 is 0 032 0 007 ug g Cd The value for NBS 1567a is 0 026 0 002 ug g Cd Results Maximum recommended levels set by the Australian National Health and Medical Research Council NH amp MRC2. Author Bruce Shelley Low Level Measurement of Cadmium in Foods Application Note Atomic Absorption Abstract This method is applicable to a wide range of animal and plant tissues including dairy products grains fruit and vegetables and oil seeds such as poppy seeds It is suit able for cadmium concentration in the range 0 01 0 3 ug g Sample is digested by wet ashing and cadmium is then complexed and extracted into an organic solvent After removal of the spent aqueous phase the organo metallic complex is destroyed by the addition of dilute HNO This aqueous phase containing the stripped cadmium is then analyzed by Graphite Furnace Atomic Absorption Spectroscopy GFAAS The additional stripping step is employed to improve precision and facilitate the use of aqueous standards Introduction Of the naturally occurring elements only about 26 are known to be essential A large number of elements occur in foodstuffs as environmental contaminants some times at levels which are considered toxic Cadmium is a toxic element whose levels in food needs to be carefully monitored Agilent Technologies Reagents and Chemicals e Ammonium acetate BDH AnalaR grade Ammonium dihydrogen orthophosphate UNIVAR analyti cal reagent grade e Ammonium pyrrolidine dithiocarbamate APDC BDH spectrosol grade Di isobutyl ketone DIBK UNILAB laboratory grade Milli Q Water 18 mQ cm e Nitric acid MALLINCKRODT Transistar grad
3. are as fol lows Cadmium in Wheat Germ 0 2 ug g Cadmium in all other foods 0 5 ug g Typical levels found from this study are shown below Grains lt 0 02 0 07 ug g Cd Vegetables lt 0 02 0 07 ug g Cd Fruit lt 0 02 ug g Cd Eggs lt 0 02 ug g Cd Milk lt 0 02 0 05 ug g Cd Cheese lt 0 02 0 05 ug g Cd Yoghurt lt 0 02 0 05 ug g Cd Butter lt 0 02 0 05 ug g Cd Oil seeds lt 0 05 0 2 ug g Cd Appendix 1 Typical calibration graph Abs scale 1 div Conclusion Conc scale IE 03 div The method has shown that cadmium can be measured at low levels in a variety of food stuffs by GFAAS Samples are digested cadmium is complexed and extracted into an organic solvent then back extracted into an aqueous phase Appendix 2 Preparation Methods for Analysis of Chemical Residues Samples for analysis should contain tissue from a number of items of the one commodity If not already established a moisture determination must be made for each commodity Vegetables Beans Beans broad Bean Sprouts Cabbage Capsicums Carrot Cauliflower Celery Leek Lettuce Wash prior to analysis Remove seed thick stems and use sections of floral heads Remove outer leaves and wash for analysis Remove outer leaves and wash radial sections of the heart for analysis Wash and use sections of flesh for analysis Peel top and tail and wash root sections for analysis R
4. e CAUTION CORROSIVE Perchloric acid MALLINCKRODT AR Select grade 70 CAUTION CORROSIVE e Titrisol cadmium standard solution MERCK 50 w v Ammonium Acetate 500 g of CH COONH dissolved in Milli Q water and diluted to 1 liter in a standard flask Possible cadmium contamination is removed by the following procedure Add approximately 0 5 g APDC mix thoroughly and add 10 mL DIBK After shaking for 60 seconds the mixture is allowed to settle before removal of the DIBK fraction Successive 10 mL portions of DIBK are added shaken and removed until the aqueous layer gives an absorbance lt 0 04 Abs when using the graphite furnace program listed This buffer solution is stable for 1 month 0 25 Ammonium Phosphate 0 75 HNO 5 g of NH H PO is dissolved in Milli Q water 15 mL concen trated HNO is added and made up to 2 litres Cadmium cont amination is removed using the same procedure as for the 50 ammonium acetate This solution is stable for 1 year 2 Ammonium Pyrolidine Dithiocarbamate 1 g of APDC is dissolved in 50 mL of Milli Q water The solu tion does not need to be filtered but must be made up fresh each time it is used Standard Cadmium Solution One liter of a 1000 pg mL Cd solution is made up using Titrisol cadmium standard solution This stock solution is diluted to 10 pg mL with subsequent dilutions to 0 10 pg mL 0 0125 g mL and 0 0063 pg mL The latter three solutions should be made up weekly Calib
5. emove thicker stems and wash curd sections for analysis Wash stem sections for analysis Wash sections of leaves for analysis Remove outer leaves and wash radial sections of the heart for analysis Melon Onion Potatoes Silverbeet Sweetcorn Tomato Zucchini Fruit Apples Apricots Avocados Bananas Berries Cherries Citrus Grapes Kiwifruit Mango Pawpaw Olive Peach Nectarine Plum Pear Persimmon Pineapple Strawberries Section radially remove skin and seeds and analyze the flesh Remove outer skin and analyze the flesh Peel wash and analyze flesh Use section of washed stem for analysis Remove and wash the seeds for analysis Wash and use sections of flesh with skin attached for analysis Wash and use mid sections for analysis Wash remove core and analyze sections of flesh with skin attached Wash remove stone and analyze sections of flesh with skin attached Remove skin and stone and analyze sections of flesh only Remove skin and analyze flesh Wash remove the small green leaves at the calyx and analyze the berries Wash remove stalk and stones and analyze the flesh with the skin attached Peel and use segments for analysis Remove berries from the stem and wash Remove peel and analyze sections of flesh Remove skin and stone and analyze sections of flesh Remove skin and seeds and analyze sections of flesh Wash remove stones and analyze the flesh with t
6. he skin attached Wash remove stone and analyze the flesh with the skin attached Wash remove stone and analyze the flesh with the skin attached Wash remove stone and analyze the flesh with the skin attached Wash remove core and analyze the flesh with the skin attached Wash and analyze flesh with the skin attached Remove skin and analyze flesh Remove leaves and calyx and wash lightly Appendix 3 The tool for removing aqueous phase consists of a 25 mL pipette pump with a cut down 50 mL pipette A piece of tygon tubing is used to attach disposable glass pasteur pipettes A new pasteur pipette is used for each sample If the pasteur pipette is found to contaminate the sample then it should be acid washed before use we ba TaN nan mig n a Sana References iF B E Limbek and C J Rowe 1986 Curve Correction in Atomic Absorption Varian Instruments at Work No AA 58 April 1989 Varian Techtron Pty Ltd Mulgrave Victoria Australia K M Bone and W D Hibbert 1979 Analytica Chimica Acta 107 219 229 S R Koirtyohann and J W Wen 1979 Anal Chem 45 1986 89 M P C De Vries and K G Tiller and R S Beckwith 1975 Comm Soil Science amp Plant Analysis 6 629 640 l Okuno et al 1978 J Assoc Off Anal Chem 61 664 667 R W Dabeka 1979 Anal Chem 51 902 907 Vollard et al 1978 Anal Chem 61 664 For More Infor
7. ins Grains are analyzed as received but require fine grinding using a Makla mill Egg A representative sample is taken from each packet of eggs and the contents blended together to form a homogeneous mixture using a kitchen blender or similar mixer Homogenized egg samples must be kept refrigerated Dairy Products Dairy products are analyzed as received If possible the sample for example milk yoghurt butter should be blended with a hand mixer to ensure homogeneity Where this may not be pos sible for example mature tasty cheese the sample should be grated and mixed thoroughly before subsampling Dairy products must be kept refrigerated Oil Seeds Oil seeds are analyzed as received Small seeds such as poppy seeds can be analyzed intact or may be freeze dried and ground as in fruits and vegetables Procedure Before proceeding with the analysis ensure adherance to the Quality Assurance procedure 1 Weigh 0 5 g grain egg dairy product oil seed or 0 25 g vegetable fruit into clean dry digestion tube 2 Add 2 mL conc HCIO and 4 mL conc HNO 3 Leave overnight to cold digest Place in digestion block Allow 1 1 2 hrs to reach 210 C from room temperature 5 After 1 hour check samples for fumes of HCIO continue checking until fumes appear Warning HCIO is a hazardous chemical and extreme care must be taken in handling this reagent Perchloric acid digests particularly of organic materials are al
8. it samples cap tube and shake for 1 minute 13 After 1 hour remove the sample of aqueous phase and analyze e Time is not critical after 1 hour e Extra back stripping step improves precision of the determination Samples may be kept and analyzed up to three days later provided the spent organic phase is removed Failure to do so will produce erratic results 14 Set up GFAAS as per manufacturer s instructions Use 0 pg mL Cd standard in Blank position e Use 0 pg mL to 0 0075 pg mL Cd calibration standards e Calibrate instrument zero before the run During run calibrate zero on 0 pg mL Cd in the Blank position thus the zero calibration can then be rechecked during the calibration e Operate in peak height mode with two replicate read ings for each sample e Reslope after every 10 samples using 0 0025 pg mL Cd standard e Reagent Blank is run as a sample and the result sub tracted from the subsequent samples An Agilent DS15 computer is used in conjunction with the Agilent Rational Algorithim to calculate results Due to the curve linear response linear quadratic and higher polynomials are unsuitable Results must have dry matter corrections made before reporting Accuracy The accuracy of the procedure was checked by analyzing an NIST standard reference material wheat flour The results are presented here for 18 replicate measurements SRM1567 wheat flour specified 0 032 0 007 mg g Cd obtained 0 02
9. mation For more information on our products and services visit our Web site at www agilent com chem www agilent com chem Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing performance or use of this material Information descriptions and specifications in this publication are subject to change without notice Agilent Technologies Inc 1990 Printed in the USA November 1 2010 AA097 s Agilent Technologies
10. ration Standards See Appendix 1 The calibrating standards are prepared according to the following dilution scheme Vol of 0 1 mg mL Cd pg mL Typical Abs mL 0 0 0 000 1 0 001 0 140 2 5 0 0025 0 330 5 0 0 005 0 560 15 0 0075 0 750 Standards made up to 100 mL in 3 HNO The calibration standards and 0 1 mg mL Cd solution should be made up weekly Instrumentation An Agilent GTA 95 graphite furnace atomizer with autosam pler was used with an Agilent AA 875 Furnace parameters are shown Editor s Note This program will be directly transferrable to a SpectrAA instrument with GTA 96 Flow Read Temp C Time secs L min Gas command 75 5 0 3 0 Ar 90 60 0 3 0 Ar 600 10 0 3 0 Ar 600 2 0 0 2100 0 9 0 2100 2 0 0 2400 1 0 3 0 Ar Sample injection volume10 uL Modifier injection volume 15 pL Sample Preparation Rubber is a major potential source of contamination Rubber gloves should not be used for sample preparation digestion or subsequent manipulations including acid washing of vessels Vinyl examination gloves may be satisfactory Fruit and Vegetables Following the procedure laid out in Appendix 2 samples are dried at 80 C for 3 5 days The higher the sugar content the longer this takes Once dry the sample is fine ground in a non metallic ring mill for 30 90 seconds according to quantity and hardness of sample Oily samples for example avocados should be freeze dried prior to grinding Gra
11. so haz ardous Their use can cause a fire or explosion resulting in injury or death ALWAYS evaporate digests in a suitable fume cupboard NEVER evaporate to dryness 6 Remove to cool 7 Add 18 mL of Milli Q water 8 Add 3 mL of the 50 ammonium acetate solution 9 Add 1 mL of the 2 APDC swirl tube to mix 10 Add 10 mL of DIBK into tube and shake for 1 minute 11 After 15 minutes remove aqueous layer using a suitable pipette pump See Appendix 3 e Precision deteriorates with increasing time at this step It is best to have two operators at this point As one removes the aqueous phase the other adds DIBK and shakes the solution A new disposable glass pasteur pipette is used for each sample When using the modified pi pump raise the plunger a couple of millimeters before inserting the pasteur pipette When the pipette tip has entered the aqueous phase push the plunger home to expel any organic phase that may have entered the pipette The aqueous phase is removed by withdrawing the plunger Keep pipette tip at the lowest point of the tube and note the rate of reduction in the meniscus to avoid over shooting Keep discarded aqueous phases collective to check the pH range is between 3 0 4 0 It has been found that precision deteriorates with decreasing pH and sensitivity deteriorates with increasing pH 12 Add 10 mL of 3 HNO for grain egg dairy and oilseed samples or 20 mL of HNO for vegetable and fru
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Agilent Low level measurement of cadmium in foods Application Note