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Sample Preparation Techniques in Trace Element Analysis by X-Ray Emission Spectroscopy

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Sample Preparation Techniques in Trace Element Analysis by X-Ray Emission Spectroscopy lAEA-TECDOC- 300 SAMPLE PREPARATION TECHNIQUES IN TRACE ELEMENT ANALYSIS BY X-RAY EMISSION SPECTROSCOPY . VALKOVIV C INSTITUTE RUDER BOSKOVIC ZAGREB, CROATIA YUGOSLAVIA TECHNICAA L DOCUMENT ISSUE THY DB E INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1983 SAMPLE PREPARATION TECHNIQUES IN TRACE ELEMENT ANALYSIS X-RAY B Y EMISSION SPECTROSCOPY IAEA, VIENNA, 1983 IAEA-TECDOC-300 Printed by the IAEA in Austria November 1983 FOREWORD This text on sample preparation technique in trace element analysi y x-rab s y emission spectroscop s beeha y n written for I.A.E.A. under provisions of research contract 2947/RB and 3^0/TC. The text has been written with the aim of assisting laboratories introducing x-ray emission spectroscopn a s a y analytical tool wite mosth ht difficult proble- ac n i m complishing this: mastering sample preparation techniques. Many of the preparation steps have been used in the author's laboratory for over a decade. Many more have been used by researchers all over the world, and some of them have been described in detail, some only mentioned in the reference list. Many have been omitted, not because the author wanted that to do but because of being pressed into producing this manuscript (version 1.0) e authoTh . r wile b l glad to accept any suggestions for addition, revisions or improvements of this text. Let us help those laboratories who want to introduce x-ray emission spectroscop word an yk wit. it h Note: Mention of commerical products or company names does not constitute endorsement by the author or publisher. Pleas aware eb Missine th tha l al t g Pages in this document were originally blank pages CONTENTS Preface 1. SAMPLING ............................................. ~........... 7 1.1 Introduction .............................................. 7 1.1.1 Aerosol sampling ................................1 1 . 1.1.2 Water sampling ..................................9 1 . 1.1.3 Soil sampling ....................................1 2 . 1.1.4 Samplin f biologicao g l materials .............1 2 . 1.1.5 Samplin f petroleuo g s productit d an m s .......6 2 . 1.2 Sample storage ............................................9 2 . 1.2.1 Loses from water by sorption on surfaces .... 33 1.3 Sample fragmentation, powdering and homogenization . 38 1.4 Contaminatio f samplo n e ................................9 3 . 2. SAMPLE PRETREATMENTS .......................................... 43 2.1 Preconcentration ......................................... 43 2.2 Dry ashing ................................................. "45 ashint 2.We 3 g ................................................6 4 . temperatur- w Lo 4 2. e ashing ...............................9 4 . 2.5 Loss of elements during sample pretreatments ........ 52 2.6 Chelation and Solvent extraction ....................... 56 n exchangIo 7 2. e ..............................................1 6 . 2.8 Electrodeposition ......................................... 63 . 3 SAMPLE PREPARATION PIXR FO SE ...............................6 6 . 3.1 Backing'materials ........................................7 6 . 3.2 Target uniformity and homogenity ....................... 74 3.3 Reproducibility ..........................................4 7 . 3.4 Effect irradiatiof o s n ..................................6 7 . 3.5 Internal standards ....................................... 80 3.6 Examples of sample preparation for PIXE .............. 85 3.6.1 General .,...................................'......5 8 . 3.6.2 Aqueous samples ..................................7 8 . 3.6.3 Biological samples ..............................1 9 . 3.6.4 Blood serum samples .............................. 93 3.6.5 PIXE targets preparatio r solifo n d sample6 9 s . 4 SAMPLE PREPARATION R EXCITATIOFO S N WITH RADIOACTIV9 E9 .... SOURCES OR TUBE EXCITATION ................................... 99 4.1 Water ....................................................... 99 4.2 Liquid samples ............................................ 107 4.3 Solid samples ............................................9 .10 4.4 Soil ........................................................ 112 4.5 Geological samples ......................................2 .11 6 Atmospheri4. c particulate .................................7 .11 4.7 Plants ..................................................... 119 4.8 Tissues ...................................................9 .11 5. STANDARDS ....................................................... 124 5.1 Standard solutions ........................................ 125 5.2 Reference materials ...................................... 133 5.3 Other standards ........................................... 146 5.4 Intercomparisons .........................................6 .15 6. LITERATURE ON SAMPLE PREPARATION METHODS ................. 164 1. SAMPLING 1.1 Introduction Samplin s oftei g n discussed becaus e greatesth e t sources of error in many studies are usually in the sampling steps. As a general rule analysth e t himself shoul e directlb d y involved in the sampling procedure. In such a way it is easier to ensure that the samples are representative and that no significant change n compositioi s n occur during sampling. The major concern in sampling must be that the sample accurately reflect e variationth s e materiath n i s l being sampled Samples for laboratory assays may be selected on the bases of the capabilitie e analyticath f o s l methods used: precisiod an n accuracy, sensitivity, time considerations, costs, single versus multielement analyses - but special consideration must be given to specific media sampled r exampleFo . , greater homogeneits i y often encountere n naturai d l watern bodi yr o s fluids than i n soil r tissueo s f organismso s . Such characteristics alway- de s termine the sample sizes necessary to measure the variation ac- curately. The specific technique employed must be one capable of handling the analytical sensitivity required. Great diversit n techniquei y d samplinan s g procedures exist . For some cores general sampling designs have been de- veloped. For example general sampling designs have been devel- oper environmentafo d l n servsurveyca s model a ed r an s fo s future programs (U.S. Geological Survey, 1972). Sampling plans involving large regions begin with a decision concerning the materials to be investigated from the lithosphère, hydrosphere, biosphere, or atmosphere. Increased detail can be ascertained from successive stage f samplino s a poin o t gt determiney b d th ee economic demandth e problee situation th y th b f f r o so o s m . General and theoretical considerations of sampling are discussed in details by Sansoni and lyengar (1980). In the process of chemical analysis only a small part of the total material is generally used to provide the analytical signal from which the concentrations of the components of interest are calculated n generalI . a ,larg e sampl s takei e n froe th m bulk materia d transportean l e laboratorth o t d y (laboratory sample). Subsequentl n aliquoa y s takei t o providt n a muce h smaller analytical sample f thisO . , very often onl a smaly l fraction is actually used to produce the analytical signal. The sample and its subsamples must satisfy several requirements which can be listed as (Sansoni and lyengar, 1980): 1. The mean composition of the laboratory and analytical samples should, in principle, be exactly the same as that of the bulk materia e evaluateb o t l d (representative mean compo- sition). However, this is an ideal condition which cannot n practicei usuallt me e b y. 'Compromise o increast e e ar s th e number of random aliquots to be analysed and to homogenize the bulk material before sampling. 2. The variance of the concentration levels within the laborator d analyticaan y l samples e samshoul s th thaa e e tb d e bul f materiath o k f o l (representative variance). 3. The total error introduced during the entire sampling operations shoul e lesb d s thane sam th r onle o , f o ordey f o r magnitude subsequen e erroth th f o , r as e t analytical procedure. Ther e severaar e l sampling methods used n randoI . m repre- sentative samples cannot be obtained by random sampling of inhomogeneous material unless the number of random samples taken is quite large. The overall error in this case largely depends •on the degree of inhomogeneity. For large samples that cannot be homogenized, it is necessary to use a sampling approach. This involves systemati- cally sampling several constituent parts of a given material, e.g. by collecting samples from a population covering different age groups, sexes, geographical regions and nutritional habits. n somI e case e bulth sk materia f well-define o s madi lp u e d parts whic e individuallar h y fairly homogeneous, e.g. different e humaorganth nf o sbody e compositioTh . e wholth ef o nhuma n body may be computed by analysing the individual homogeneous sections. This method is known as differential sampling. However, it is normally applied only to bulk materials that have clearly identifiable subdivisions and that cannot easily be homogenized in a single batch, as in the case of total body analysis (San- soni and lyengar, 1980). Ther e manar e y papers writte n thio n s subject somf o e them are listed in the reference list. One of the important effect n samplini s s causei g y randob d m particle distribution. This effect is based on the number of particles extracted per sample, i.e., the greater the number, the smaller the effect. Thereforee generallb n ca t yi , stated that random particle distribution
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