The determination of lead by MD-HG-AFS in APMP*-QM-P3 comparison Lu Xiaohua, Lu Hai, Ma Liandi, Cui Yanjie National Research Center for Certified Reference Materials China * Asia/pacific Metrology Programme

Introduction Lead poisoning can cause lack of muscle control, nausea, seizures and death. The content of lead in milk products needs extra attention. Main methods for the determination of lead in milk powder include GFAAS, ICP-MS and HG- AFS.

The advantages and disadvantage of HG-AFS Using much more simple and cheap equipment. Lower detection limits and wider linear range than GFAAS. Low matrix influences because of the application of hydride generation technique. High demand to the acidity of sample solutions during hydride generation stage.

Our aims in this study Establish an effective microwave digestion-hydride generation-atomic fluorescence method used for the determination of trace lead in milk powder through which the samples could be digested completely and the influence of acid, reagent and environmental blank could be eliminated effectively. Participate in APMP-QM-P3 pilot study to verify the reliability of this method.

Sample digestion Drying the sample at 80 ℃ for 4h according to APMP-QM- -P3’s instruction for sample preparation. Sampling amount: 1g. Digestion: pre-digestion→ microwave digestion → removal of acid matrix. Volume of sample solution: 10mL. Measurement Matrix: 2% HCl, 2% potassium ferricyanide and 0.4% oxalic acid*. * Potassium ferricyanide and oxalic acid were added to promote the formation o f PbH 4 and eliminate interference during measurement stage respectively.

Measurement Segmented flow - hydride generation – non disperse atomic fluorescence method

Optimized Measurement conditions Lamp current （ mA ） 90 （ main cathode ）， 40 （ supplemental cathode ） Voltage of PMT300 Temperature of atomizer( ℃ ) 200 Type of gasArgon Flow rate of carrier gas （ mL/min ） 400 Flow rate of shield gas （ mL/min ） 900 Integral time （ s ） 12 Delay time （ s ） 0.5 carrier solution2 % HCl Reducing agent10g/L NaBH 4 in 0.2% NaOH Standard solutions0.5, 1.0, 3.0, 5.0ng/mL calibration modeLinear regression

Discussion  Method selection  Digestion  Acidity control  Blank influence

Method selection GFAAS Detection limits*: 0.5ng·mL -1 at 283.3nm and 0.3ng·mL -1 at 217.0nm (sampling volume: 40μL, peak area mode) High background Poor precision when using peak height mode HG-AFS Detection limit* : 0.008ng·mL -1 (sampling volume: 1mL) Better precision with smooth peak shape because of low matrix influences * All of the detection limits are expressed as 3×SD 0, where SD 0 is the value of the standard deviation (n=11) as the concentration of the analyte approaches 0.

Digestion.  High contents of protein and fat in milk powder samples.  Three Optional digestion methods: Dry ashing : needs high temperature and long time, can result in the potential loss of lead during digestion; Acid digestion: often influenced by contaminations and result in the high blank of lead in sample solutions; Microwave digestion: more effective and can avoid the environmental contamination and loss of lead because of the close pressurized digestion system and low operating temperature controlled under 200 ℃.

Selection of digestion systems HNO 3 -HClO 4 system: - High efficiency with shorter digestion time; - High reagent blank of HClO 4 ; - Easy to form the precipitate of perchlorate which could influence the measurement. HNO 3 system with long digestion time.

acidity control The acidities of sample and carrier solutions are vital in the determination of lead by HG-AFS method. The concentration of HCl in sample solutions and carrier solutions （ % ） Value of fluorescence intensity Influence of acidity in the determination of lead

Measures for acidity control The acidities of samples and standard solutions should be the same of 2% HCl to ensure the high and same sensitivities of lead in them. The large amount of nitric acid in sample solutions after microwave digestion must be removed. This process was done by transferring the sample solution into a quartz beaker and heated under infrared lamp until approximately dried.

Blank influence Several measures had to be taken: Closed microwave system decreasing the environmental influence during digestion. Avoid the use of HClO 4. Fresh prepared sub-boiling distilled nitric acid. Clean environment during removal of acid matrix. Through above efforts, the average blank of lead decreased from the first more than 10ng/mL to ng/mL.

Apparatus for the removal of acid matrix —a substitute for super clean room

CRM measurement for method evaluation The method was evaluated by analyzing lead content in GBW08509 certified reference material for non fat milk powder. The certified value for Pb is 34ng/g with expanded uncertainty of 10ng/g (k=2). A desirable result of 33ng/g was obtained in our measurement.

Application in APMP-QM-P3 pilot study The method was applied to the measurement of trace lead in skim milk powder in APMP.QM-P3 comparison organized by APMP. Mean result (ng/g)24.5 Standard deviation (ng/g)2.83 Standard deviation of the mean (ng/g)1.16 Combined standard uncertainty (u c, ng/g)1.79 Coverage factor (k)2 Expanded uncertainty (U) at 95% confidence limit (ng/g) 3.6

Uncertainty evaluation Source of uncertainty Relative standard uncertainty Type (A/B) Standard deviation of the mean (n=6) A Weighing 0.001B Sample digestion 0.05B Effect of blank 0.022B Uncertainty of standard solution 0.001B Correlation of the linear curve 0.01B Combined standard uncertainty u c,rel =7.3%

Comparability between different methods in APMP-QM-P3 MethodsHG-AFSICP-MSDouble ID–ICP-MS Average results （ µg/Kg ） Expanded uncertainty （ µg/Kg, k=2)

Conclusion The results of the three methods show comparability. Compared with ICP-MS methods, MD-HG-AFS use simpler and less expensive instrumentation. The precision of HG-AFS is poorer than Double ID – ICP- MS. But considering the practicability and popularization for routine analysis, it is still recommended for the determination of trace and ultra trace lead in complicated matrix.