S~~PllI\L. 3000 . ~~-\1,

~

- -a.<ii fll J. .-._~--a..e1 a,q 5- METALS (3000)

3030 C.

,Treatment for Actd-Extractable Metais

',; '"

~~ ,rJ~ffJrtJ. ü, ' v.V

3030.0.,;: Digestion for Metais

, -, ~Extractable .metàls are lightly adsorbed on particulate mate-

rial: Because somesample digestion may be unavoidable userigidly. controlled conditions .to obtain meaningful and repro-ducible results. Maintain constant sample volume, acid volume,and contact time. !Express results as extractable metalsand spec-ify extractíorrrconditions. ' !'

At collection, acidify entire sample with 5 mL cone HN03/Lsample: To prepare sample, mix well, transfer 100 mLto a-beaker

. i

To reduce interference by organic matter and to convert metalassociated with particulates to a form (usually-the fr~~]:netal),that can be determined by atomic absorption spectrornetry orinductively-coupled plasma spectroscopy, use one of the diges-tion techniques presented below. Use the least rigorous digestionmethod required to provi de complete and consistent, recoverycompatible with the analytical method and the metal beíngan-alyzed.

Nitric acidwill digestmost samplesadequately (Section 3030E).Nitrate is an acceptable matrix for both flarne and electrothermalatomic absorption. Some 'samples may require additionfof per-chloric, hydrochloric, or sulfuric acid for complete' digestion.These acids may interfere in the analysis of some metais and allprovide a poorer 'matrix. for electrothermal 'analysis. Confirmmetal recoveryfor each digestion and analytical procedure used.Use Table 3030:1 às' a guide in 'determining .which acids (in: ad-dition to HN03) to use for complete digestión. As a general rule,HN03 alone islfadéquate for clean samplesor easily oxidizedmateriais; HN03-HzS04·.or HN03-HCl digestion is adequate forreadily oxidizable organicrnatter; HN03~HC104 ar HN03-HC104-

HF digestion is'necessary for difficult-to-oxidize organic matteror minerais. Dry ashing ishelpful if large amounts of organicmatter are present." F

Dilute samples with Ag concentrations greater than 1 mg/L tocontain less than 1 mg/L Ag for flame atomic.absorption methodsand 25 f.Lg/L or less for electrothermal analysis, To address prob-lems with silver halide solubility in HN03, digest usingmethod3030F.3b.

Report digestion technique used.Acid digestion techniques (Section 3030E-1) generally yield

comparable precision and bias for most sample typesthãt aretotally digested by the technique. Because acids used in digestionwill add metais to the samples and blanks, minimize the volumeof acids used.

. "TABLE 3030:1. .ACIDSUSED INCONJUNCfION WITH HN03 FOR

" . o.' • '. SAMPLE PREPARATION

MayBeHelpful for

Not Recommendedfor "

·'HCI - Sb, Ru, SnH2~O:C ",. '1,)TiHCI04 Organic materiaisH.r> ), '.~'.'\',L;-1.''''''',''b" ",:.Siliceous materiais

Th,Pb

Ag, Pb, Ba

cç '; ~';" ._, .. ,. I ' " ti"'

or flask, and add 5 mL 1 + 1 high-purity I:ICL Heat 15 min ona steam bath, .Filter .through .a .membrane filter and -carefully

. transfer filtrate to a tared volumetricflask. Adjust volume to100 rnL-with water , mix, and analyze.lf volume is greater than

dOO mL, determine volume to nearest O.I-mL by weight, analyze,and correct final concentration measurement by multiplying by

.the dilution, factor (final volume -7- IOOk ': ~1 ;" '

-ç, ,ffO : )-'f ~ I ~ :1 fO., cy

tô3. n'I'"'---'

'Ü,oo 1 )'I'\t

Because the acid digestion techniques (3030E and F) normallyare not total digestions, themicrowave digestion procedure (3030K)may be used as an alternate to them. The microwave method isa closed-vessel procedure and thus is expected to provide im-proved precision when compared with hot-plate techniques. ThemiErowav~.digestion method ~. ecommended for the analysis ofAg, AI; As, Ba Ca, Cd, C0, ,Cu,~, K, Mg, , Mo,Na, N', Pb, Sb, S I, an n.

Suggested samp e .volumes are indicated below. Ifthe rec-ommended volume exceeds digestion vessel capacity, add sample:as' evaporation proceeds. For samples containing particulates,wide-bore pipets may be useful for. volume measurement andtransfer.

When samples are concentrated during -digestion (e.g., >100mL sampleused) Jdetermine metal recovery for each matrix di-gested, to verify method validity. Using larger samples 'will re-quire additional acid, which also would increase the concentra-tion of -impurities.

r-

Estirnated MetalConcentfation "

mg/L ::= Wltl..

Sample Volume'mL

<11-10

10-100100-1000

1000100101

'For flame atomic absorption spectrometry. For graphite furnace or ICP,smaller volumes are appropriate.

Report results as follows:

B.- Metal concentration, rng/L = A x C

where; .,4 .,;,co~centration of metal in digestedsol~tion, mg/L,B = final volume of digested solution, mL, andC = sample size, mL. . .

Prepare solid samples ar liquid sludges with high solids con-tents on a weight basis. Mix sample and transfera suitable amount(typically 1 g of a sludge with 15% total solids) to a preweígheddigestion vessel. Reweigh and calcula te weight of sample. Pro-

••••••••••••••~~~~--------------------------------------~--------------~~=---------~~~=-~~~~-~-~---------------~

'REUMJI'J,Af!Y..lREATMENT (3030)/Digestion

ceed with one of the digestion techniques presented Q~lilw..Re- ...;port results on wer-or dry-weight basisas follows: ,',.. ,

..; ,-,- -."- ':' ..

Metal concentration, rng/kg (dry-weight baS;S) = A x 's ,x"~OOr,'_. g sa~B~e~-'! D,

, .','. ,!

I ~ ~~~~r~~:.":(-:0- ( .:-,.;~; .: - iF:. A = concentration of metal in digested solution, mgIL,

B = final volume of digested solution, mL, andD = total solids, % (see Section 2540G) .

Always prepare acid blanks for each type of digestionper-formed.Experience indicates that a blank made with the sameacids and subjected to the same digestion procedure as the sam-pIe can correct for impurities present in acids, in reagent water,and on glassware.

I

3030 E., .Nitric Acid Digestion

1. ÁppM~tus.J

a. Hot plate.b, Conical (erlenmeyer) flasks, 125-mL, ar Griffin beakers,

150 mL, acid-washed and rinsed with water.c. Volumetric flasks, 100-mL.

2. Reagents j ',.H!

Nitric, acid, HN03, cone,

3. Procedure·;a '''1.,'' !". _ ..

Transfer a .measured volume of..well-mixed, acid-preservedsample appropriate for theexpected ;n~tá!s concentrations t~ a.

,.'"

! -r

3030 F, .Nitrlc Acid-,Hydrochloric Acid Digestion.' ~,

1: Apparatus.i; .

.See 3030E.1. The following also are. needed:

.z; 'Watch glasses. .., ;,b. Steam bath. ..'

" , i.:

_ .' {

2. Re?g.ents ,• r ' f ~

. '.. - -.. - .. ,-il, Nitric ac'id,.:HN.?:, cone; ,

. p: Hydrochloric ,a~{d, ~;.~ 1.'

3. Procedure

a. Total HN03/HCI: Transfer a measured volume of well-mixed, acid-preserved sample appropriate for the expected met-aIs concentrations to a flask or beaker (see 3030D for samplevolume). Add 3 mL cone HN03. Place flask ar beaker on a hotplate and cautiously evaporate to less than 5 mL, making certainthat sample does not boil and that no areaof the bottom of the.container isallowed to go dry. Cool and add 5 mL cone HN03.

.• "f

. ' I'

flaskor beakei (see 303'OD'for'sam le ltime)'. Àdd 5 ml, 'coneHNO;'ancla few boiling '.hips, glass beads, or Hengargrariules .Bring to a slow boil and evaporate on a hot plate to the lowestvolume possible (about 10 to 20 mL) before precipitation occurs.Continue heating and adding conc HN03 as necessary until diges-tion is complete as shown by a light-colored, clear solution. Donot let sample dry during digestion.~.Wash dowil flask ar beake~waIls with water and then filter ifnecessary (see Section 3030B). Transfer filtrate to a 100-mLvolumetric flask with two 5-mL portions of water, adding theserinsings to the volumetric flask. Cool, dilute to mark, and mixthoroughly. Take: portions of this solution for required metaldeterminations.

,.,. ,

,I'

Cover container with a watch glass and return to hot plate. In-crease ternperature of hót plate 'so that agentle reflux ~ctionoccurs, Continue heating, adding additional acidas necessary,until digestion is complete (generally indicated when the diges-tate is light in color or does not change in appearance withcontinued refluxing). Evaporate to less than 5 mL and coo!. Add10 rriL 1 + 1 HCr and 15 mL water per 100mL aritícipated finalvolume. Heatfor ari additional 15 min te>dissolveàny precipitateor residue: Cool, wash down beaker 'walls and watch glasswithwater ,-filter to re'move insoluble 'material that ' could "clog' thenebulizer ,'ànd transfer filtra te' to a lUO-mLvolumetric flask withrinsings. Alternatively centrifugeor let settle overnight. Adjustto volume and mixthoroughly -.

h. Recoverable HN03/HCI:'-For this Iessrigorous digestionprócedure, transfer a measured volume of well-rnixed, acid-pre-served sample to a flask or beaker. Add 2 mL 1 + 1 HN03 and10 mL 1 + 1 HeI and heat on a steam bath or hot plate .untilvolume has been reduced to near 25 mL, making certain sampledoes not boi!. Cool and filter to remove insoluble material ar

. alternatively centrifuge -or Iét settlé' évemight. Transfer sampleto volumetric flask, adjust volume to 100 mL, and mix .

. r"

METALS (3000)

3030 G. Nitric Acid-Sulfuríc Acid Digestio~

1. Apparatus

,'See '3030E.l. 'i '', ,i.si i ; ".- 'j • i"

2. Beaqents.

a. Nitric acid, HN03, cone.b. Sulfuric acid, H2S04, cone.

3. Procedure

,:I'~;;tnsfyra>ipe~sur~çI.v.0~up~ of:-V,e,lI~mixed?,a,c,id-pres,er~e,q,samplé appropriate forthe expected metals concentrations.toa

. 1.1':" ;!' II I . :'1';-'

L ,I

1-':-1 /.. . .,.

"

flask or beaker (see 3030D for sample volume). Add 5 mL coneHN03 and a few boiling ehips, glass beads, or Hengar granules.Bring toslow boil on hot pia te ,and evaporateto 15 to 20 mL.Add 5,mL cone HN03 and 10 mL cone H2S04. Evaporate on ahot plate until dense white fumes of S03 just appear. If solutiondoes not c\ear, add 10 mL cone HN03 and repeat evaporationto fumes of S03' Heat to remove ali HN03 before eontinuingtreatment. Ali HN03 will be removed when the solution is c\earand no brownish fumes are evident. Do not let sample dry duringdigestion.

Cool and dilute to about 50 mL with water. Heat to almostboiling to dissolve slowly solublesalts. Filter if neeessary, thencomplete proeedure as direeted in Seetion 3030E.3.J:>eginningwith, "Transfer filtrate ... " ,

t "1. c. r.f

, i '1 ( I

3Q3({H., .Nitriq Acid-Perchloric Acid Digestion

1. Apparatus

See 3030E.1. The followingare also, needed: ,a. Safety shield.b. Safety goggles.c. Watch glasses.

2. Reagents

a. Nitric acid, HN03, cone.b. Perchloric acid, HCI04." , , _ "

c. Ammonium acetate solutidh: Dissolve 500'g NH4C2H302 in1 :> i ,~.... ' • i '. '

600 mL water,<,

',o

3. Procedure (1.' i, \' _.';:~'..f, • ! r • •

CAUT}ON: Heated mixtures of/iCl.04 and orgq,,!icmattermayexplode violently. A void this.hazard bytaking .th.e[ollowing.pre-cautions: (a) do not add HCL04 to. a hot solution containing or-ganic matter; (b) always pretreat samples containingorganic mat-ter with HN03 before adding f!CI04; {c)avoiq repeated fumingwith H CIO4 in ordinary hoods (For routine operations, use a.waterpump attached to a glass fume eradicator. Sttiinless steelf~Ínehoods with adequate water washdown facilities are available corn-

. ..'

mercially and are acceptable for use with HCL04.); and (d) neverlet samples being digested witn HCl04 evaporateto dryness.

Transfer a measured volume of well-rnixed, acid-preservedsample appropriate fortheexpected metais eoneentrations to aflask ar beaker (see 3030D for sarnple volume); Add 5 mL coneHN03 and a few boiling ehips, glass beads, or Hengar granules,and evaporate on a hot plate to 15 to 20 mL. Add 10 mL eaehof cone HN03 and HCI04, eooling flask or beaker between ad-

" ditions. Evaporate gently on ~ hot plate until dense white fumesof HCI04 just appear. If solution is not c\ear, eover eontainerwith a watch glass and keep solution just boiling until it c\ears.If necessary, add 10 mL cone HN03 to complete digestion. Cool,dilute to about 50 mL with-water, and. boil to expel any ehlorineor oxides of nitrogen. Filter, then complete proeedure as directedin 3030E.3 beginning with, "Transfer filtrate .. .."

If lead is to be determined in the presenee of high amountsof sulfate (e.g., determination of Pb in power plant fly ash sam-pies), dissolve PbS04 preeipitate as follows: Add ,50 mL am-monium aeetate solution to flask or beaker in whieh digestionwas earried out and heat to ineipient boíling. Rotate containeroeeasionally to wet ali interior surfaces and 'dissolve any depos-ited residue. Reeonneet filter and slowly draw solution throughit. Transfer filtrate to a 100-mL volumetrie flask, cool, dilute tomark, mix thoroughly, and set aside for determination of lead .

3030 L Nitric Acld-Perchloríc Acid-Hydrofluoric Acid Digestion

1. Apparatusa. Hot plate.b . .TFE beakers, 250-mL, aeid-washed and rinsed with water.c. Volumetric flasks, 100-mL, polypropylene or other suitable

plastie.

2. Reagents

a. Nitric acid, HN03, cone and 1 + 1.b. Perchloric acid, HCI'04'c. Hydrofluoric acid, HF, 48 to 51%.

riELlMINARY TREATMENT (3030)/Microwave-Assisted Digestion

3. Procedure

CAUTION: See precautions for using HCIO 4 in 3030H; handleHF with extreme care and provide adequate ventilation, .especiallyfor the heated solution. A void all contact with exposed skin. -Pro-vide medical attention for HF burns. s , ',' 1-. i ."

Transfer a measured volume~f well-mixed, acid-preservedsample appropriate for the expected metais concentrations into

3-7

a 250-mL Tl-E'beaker (see 3030D for sample volume). 'Add a:few boiling chips and bring to a slow boi!. Evapoíate 'on 'a hofplate to 15 to 20 mL. Add 12 ml.rconc HN03 arid evaporate tonear dryness, Repeat HN03. addition and evaporation. LeI 50-lution cool, add 20 mL HCI04 and L'rnl, HF, and boi I untilsolution is clear and white fumes ofHCI04 have appéared. -Cool,'add about 50 mL water, filier, and proceed asdirected in 3030E.3beginning with, "Transfer filtrate .. '."

',', J

3030J. Dry Ashihg

The procedure appearing in previous editions of StandqrdMethods has been deleted from this editíon. ",' " -e

/ ~3030 K. Mlcrowave-Asslsted Digestion (PROPOSED)

1. Apparatus

a. Microwave unit withprogrammable power '(minimum 545 .W) to within ± 10 W of required power, having á cbrrd~ion-'resistant; well-ventilated cavity and having ali electronics .p'ro~.tected against corrosion for safe operation. Use a unithavlng arotating turntable with a minimum speed of 3 rpm to i~stÍ~e 'homogeneous distribution of microwave radiatiori. Only lábor~-tory-grade microwave equipment and ciosed digestion contaíners'with pressure relief that are specifically designed for hot 'acidmay be used.' " . ,'::'.<: d

b.: Vessels constructed of perfluoroalkoxy (PFArteflÓn~,:* 'capable of withstanding pressures of at least 760 ± '70 kPa (110'1± 10 psi) , and capable of contrólled pressure relief ai the rnan- I

ufacturer's maximum pressure rating.Acid wash ali digestion vessels and rinse with reagent water.

For new PFA Teflon@;* vessels or when changing betweenhigh- 'and low-concentration samples, clean by leaching with hot hy-drochloric acid (1:1) for a minimum of 2 h and then with hotnitric acid (1:1) for a minimum of 2 h; rins e with-reagent waterand dry in a clean environment. Use this procedure whenéverthe previous use of digestion vessels is unknownor cross-con-tamination from vessels is suspected.

c. Plastic container with cover, I-L, preferably made of PFATeflon@. * LI) ~

d. Bottles, polyethylene, 125-mL, with caps.e. Thermometer, accurate to ± 0.1°e.f. Balance, large-capacity (1500 g), accurate to 0.1 g.g. Filtration or centrifuge equipment (optional).

2. Reagents

a. Reagent water: .See Section 1080.b. Nitric acid, HN03, cone, sub-boiling distilled. Non-sub-

boiling acids can be used if they are shown not to contribute 'blanks.

• Or equivalent.

3. Calibration of Microwave Unit

For cavity-type microwave equipment, evaluate absolute power(watts) by measuring the temperature rise in 1 kg water exposedto microwave radiation for a fixed time. With thismeasurement,the relationship between available power '(W) 'andthe partialpower setting (%) of the unit can be estimated, and àriy absolutepower in watts may be transferred from one unit to another. T\1ecalibration format required dependson 'type of electronic systemused by manufacturer to provi de partial microwave power. Fewunits have an accurate and precise linear relationship betweenpercent power settings and absorbed power. Where linear circuitshave been used, determine'calibratio~" curve by a three-pointcalibration method; otherwise, use the multiple-point calibrationmethod. '..' ,

a. Three-point calibration ;ne!hod: Measur,e pow~r at 1.00~.and 50% power using the procedure described iri ~ 3c and cal-culate power setting corresponding to required power ·iirwáJt~'.as specified in the procedure from the two-point line. Méasure'absorbed power at the calculated partial power setting. If themeasured absorbed power does not correspond to the ealculated-powerwithin ± 10 W, use the multiple-poi';lt calibration method,~ 3b. Use thispoirit péríodicàlly to verify·infegrity'ófêa:\ib'rati9,n.

b. Multiple-point calibration method: Foi each micro\v'ave'üni!;'measure the 'following power settings: 100, 99/98;-9;7-;~95,'90;\80, 70, 60, 50, and 40% using the procedure describedin-fSc.iThese data are clustéred about the customary workingtpowerranges, Nonlinéarity commonly ís encountered ãt thé.upper endof the calibration curve. If theunít'setecrronics 'are'knowntonave nonllnear deviations in any region 'oí: pr0por'tiohal powercontrol, make a 'set of measuremerits that brãcketthe pówêr-tobe used. The final calibration point should bé àt the 'partial powersetting that will be usedin the test.'Check this setting p~riodicallyto evaluate the integrity of the calibration.: If a significarit change(± 10 W)is detected, re-evaluate entire' calibration.": ",

c. Equilibrate a large volume of water to 'room ternperature(23 ± 2°C). Weigh 1 kg reagent water' ~1000g ± 1 g) or measure(1000 mL ± 1 mL) into a plastic, riot glass, container, andmeasure the tempeiature to ± 0.1°e. Condition microwave unitby heating a glass beaker with 500 to 1000 mL tap water at full

~"""""""""""""""---qt\- ~ di7 'm1iot (~ ~Jl-).?~LB - ~ +h.Ct ao: ~ O-ME~ (30~;

G-~k~,facturer of the microwave equipment.Í'The change in power,time, and temperature profile is not directly proportional ta thechange in the number of samples; thus, different power programswill be required for different numbersofvessels containing the 'appropriate amount of sample and acid. L

Weigh entire digestion vessel assembl ta 0.1 g before use andreca~ Accurately transfer 45 mL of well-shakçj; sarnplei.!U9.the~estian vesse!. Pipe ' mL oríc HN03'intõ each vesse~Insure that pressure-cap relief disks are inserted according tamanufacturer's directions, Tighten cap ta manufacturer's ~ec-ifications, Weigh each capped vessel ta the nearest 0.1 g ®.

Place appropriate number of vessels evenly distributed m thecarousel. Treat sample blanks, known additions, and duplicatesin the same manner as samples. When fewer samples than theappropriate numbér are digested, fillremáining vessels with 45mL reagent water and 5 mL cone HN03 ta 'abtain full comple-ment of vessels for the particular program being used.

Place carousel in unit and seat it carefully on turntable. Pro-gram microwave unit ta heat samples ta 160 ± 4°C in 10 minand then, for the second stage, ta permit a slow rise ta 165 ta170°C for 10 mino Start rnicrowave generator, making sure thatturntable is turning and that exhaust fan is on. .

At cornpletion of the microwave program, let vessels cool forat least 5 min in the unit before removal, Samples then may becooled furth~~ aiitsid~ the unit by removing the cara usei andletting th'em-caal on a bench ar in a water bath. Whe'n cooledta room 'temperature, weigh (ta 0.1 g) each vessel and recordweigh~@. . " ,,'

If the net weight of sample plus acid decreased by more than ,10%, discard sample, ' r

.Cornplete sample preparation by carefully uncapping and vent-ing each vessel in a' fume haad. Transfer ta acid-cleaned non-contaminating plastic bottles. If the digested sample containsparticulates, theneither centrifuge at 2000 ta 3000 rpm for 10min and filter, ar Jet settle overnight.

power for ~. min ,-;,,ith.tpe, exhaust, fan .on..Loosely cover plasticcontainer ta reduce heat.loss, and placein normal sample path(at a~t~r edge~f'r~t~ting't~~i~~le,); circulare continuously throughthe microwaye field lar,gg s ,,!tdesired pawe.r setting \."ith,ex-haust fan a~ as it will be during,p.armal operation, Remove plasticcontainer and stir water vígorously. Use a magnetic stirríng barinserted immédiately after mícrowave..irradiation; record rnaxi-mum temperature WithÚl thefirstJü s ta ;±: 0.1-c. ~~e. a newsample for each additional measurement. If the water is reused,return both water and beaker ta 23 ± 2°e. Make three meas- •urements at each power setting. When any part of the high-voltage circuit, power source, or control cornponents in the unithave been serviced ar replaced, recheck calibration pówer. Ifpower output has changed by more than ± 10 W, re-evaluateentire calibration.

Compute absorbed power by the following relationship:r / 'p = (K) (Cp) (m) (H)

t

where:P = apparent power absorbed by sample, W;K = conversion factor for thermochemicalcalories sect ' to watts .

(4.184),Cp = heat capacity, thermal capacity, orspecific heat (cal g-::'\ °C-l)

,', of water,m = mass, ef. water sample, g, :: ",..,;:l,' -s r . • '

IlT = final temperature minus initial temper')tl,lre, °C, andt = time, s; , ;,

For the experimentalconditíons í?f.J:~O:s ã.~d 1, k& distilledwater (Cp at 25°C = 0.9997), the calibptia.~<:9uatian simplifiesta: ',' '

t· J~

.i ,i__,l.;

, • ' • ,_o f, ". <"L:;! . 1':!;1:.. ,Stable line volt age is necessary for accurate and repro.d';lRible

calibration and operation, It shau\q b~,wi~~if1,t~e,.\ll<~I?-ulact~lfer's'specification , During measurement and .operation .it must notvary by more than ± 2 V. A cónsJ!!~Úpo;,vú supply, may benecessa,r~ if line voltage .is unst,able.' J' ' ,< ;',

4. Procedure

ÇAUTION: This method 4 designed jor";'i~row9ve'digestion ofwaters only. It is not intended for the digestion pf.~oliçl~"jn whichhigh concentrations of organic compounds.may result in .high.pressures and possibly unsafe conditions.r '~,,', J.. ; e"~;

. The.following procedureis based an h~ating,~cicl\fi~.d,"samp,l~s!in two stages where the first stage is to reach 1.60.±(~OC in 10minand the second stage is ta permit a slow Xise to Vi5,t.o110°C,during the second 10 mino f:. verified program úal meets thisternperature-time .profile is 545 W f()r)O'min followed by A44W for ilP min using five single-wall I,>FATeflon®t digestion,vessels.ê, The usable number of vessels is deterrnined by vesseldesign ~d power output. If more vessels are used in a high~r~,wattage unit, vérifytime-temperature profile to conform to. thegiven it.wo~stage.profile.j-This may be done by laboratory per-sonnel if i>l,li.t,ablet~st,equipment is available.. a,r by the manu-

.:. '.

"

t Or ,equivalent.

"

,--i

5. Calculations

á. Dilution correction: Multiply results by 50/45 ar 1.11 toaccount for the dilution caused by the addition of 5 mL acid to45.mL sample.

b. Discarding of sample: To determine if the net weight ofsample plus acid decreased by more than 10% during the diges-tion process, use the following calculation:

[(B - A) - (C - A)] x 100> 10%(B - A)

6. Quality Control

Preferably include a quality-control sample in each loadedcarouse!. Prepare samples in batches including preparation blanks,sample duplicates, and pre-digestion known additions. Deter-mine size of batch and frequency of quality-control samples bymethod of analysis andIaboratory practice. The power of themicrowave unit and batch size may prevent including one or moreof the quality-control samples in each caro useI. Da not graupquality-control samples together but distribute them throughoutthe various carousels ta give the best monitoríng of digestion.