It was later utilized while employed by EPL in 1995 and in 1999 for OGC - as a consultant, to be used as educational material for training their laboratory and refinery staff. Since then, based on feedback and questions I received, I felt it needed to be revisited to address some intricacies of certain areas. I am in the process of introducing part II and illustrative charts and demographics to develop a complete reference manual on this topic of As, Se, Sb analysis..

Analytical systems / procedures { S O P } This literature is copyright protected.

Page "1" of "29" Reproduction or any use without the written

Atomic Absorption / Hydride generation technique. consent of the author is prohibited.

A complete reference paper covering the administration &

technical operation aspects of the technique.

By: Dr. Paul Gouda,

1.0- Instrumentation:

1.1 Description:

a} Hardware configuration: Thermo-Jarrell Ash , Smith Heifta 22 AAS.

b} Technical software: PC Data system - TJA Thermospec S.W. by Thermo-Jarrel Ash.

2.0- Test codes:

2.1 Se-20-SO {Sample type: Soil . Parameter: Selenium}

2.2 AsSb-20-SO {Sample type: Soil. Parameter: Arsenic & Antimony}

2.3 Preservation code: D.

2.4 Storage method: Refrigeration of non-aqueous samples and HNO3 treatment of aqueous samples to pH < 2.

2.5 Container: 8

2.6 Target hold time: 1 month.

3.0- Method:

Se: EPA-7741 / modification

As/Sb: EPA-7061 & 7042 / modification.

4.0- Principal / theory:

Metalloid elements { Se / As / Sb } prepared in acidic medium to convert all forms of arsenic, selenium and antimony to arsenate { AsO₄^3- }, selenate { SeO₄^2-}, and antimonate { SbO₄^3-} react with sodium borohydride and are reduced to arsine, hydrogen selenide and stibine. The volatile hydrides in the reaction tube are carried by an inert gas [argon] into a cell [quartz tube] that is heated by air / acetylene flame and is situated in the optical path of AAS where the gaseous hydrides are reduced to atomic species and are then determined by conventional atomic absorption.

5.0- Applications:

5.1 Samples are digested in oxidizing acid mixture and are treated to:

1. reduce selenium in the digestate selenium IV as selenite SeO⁼₃ . Selenite is reduced to hydrogen selenide H₂Se by sodium borohydride "NaBH₄" and HCL. The hydrogen selenide is reduced in the flame [cell] to selenium atoms.

b) convert all forms of arsenic to arsenate ion AsO₄^3- [ The arsenate in the acidic solution is reduced by sodium borohydride to AsH₃ "arsine".]

Atomic arsenic is then determined by conventional atomic absorption technique.

5.2 Hydride forming elements must be converted to the required oxidation state. e.g. Sb "V" and As "V" are reduced to Sb "III" and As "III' with KI {after HCL treatment}. While Sb reduction is spontaneous, As reduction takes approximately 1 hour at room temperature or 5 minutes at 50  ⁰C [water bath].

6.0- Digestion scheme:

1. A soil / sludge sample aliquot of 0.25 - 1.0 gm is treated with < 1 ml of H₂O and is left for 5 minutes in the 50 cc digestion tube. The sample is then treated with an acid mixture of 6:3:1 HNO₃ / H₂SO₄ / HCLO₄ and is allowed to sit for 10 minutes. 2-5 ml of acid mixture is the common practice depending on sample nature and need. 3 ml is a practical guideline. Solution samples are shaken and sampled by weight or volume as necessary.

 

6.2 The sample is digested on a heat block with a starting temperature of 70 ⁰C and a maximum temperature of 250 ⁰C. The temperature must be monitored and controlled based on sample reaction / nature / matrix to obtain optimum digestion. Depending on the digestion temperature and the sample nature, the digestion would take 4 - 16 hours on the hot plate [ heat block ]. A good guide line would be a 10 hours digestion at 120 ⁰C. Special attention must be given to individual samples during digestion especially when the analyzing for Se or a parameter in elemental / volatile state.

All oxides of nitrogen / NOx must be expelled prior to instrumentation. An indication that traces of nitric are removed and the digestion is complete, is the reduction of the solution volume down to 1 cc and the production of floating - suspended white fumes of SO₃ is clearly observed.

 

2. The sample is removed off the heat block and let cool down to room temperature. 1 ml of H₂O is added followed with 5 ml of HCL and the sample is let sit for 10 minutes and is then heated in a water bath at 80 ⁰C for 60 minutes.

6.4 The sample is removed from the water bath, let cool to room temperature and H₂O is added to make up volume to 50 cc. Shake well and divide the sample into 2 portions of 25 cc each by pouring 25 ml into a plastic centrifuge tube.

6.5One set of the two 25 cc samples is for Se determination. The sample requires no further treatment and is ready for AAS determination with no further dilution. The other set of digested 25 cc sample is to be used for As / Sb determination {dual elements / cathode lamps - AAS setting }. Add 0.5 ml of 50 % KI w/v solution and heat in a water bath at 80 ⁰C for 15 minutes.

6.6 Dilute samples 1:10 in graduated 12 cc plastic test tube as follows:

1 ml sample { after 6.6 treatment above } + 1 ml HCL + 0.2 ml KI solution 50% w/v and add 7.8 ml of H₂O to bring final volume to 10 cc. Shake well.

This portion of the sample is ready for AAS - As/Sb instrumentation. Prepare when ready for AAS operation.

 7.0- Calibration:

7.1 lower range operation: Standard solutions / ppb: 0 (blank), 1, 3, 5 ug / L.

7.2 Higher range operation: Standard solutions / ppb: 0 (blank), 1, 5, 10 ug / L.

7.3 Correlation coefficiency: 0.995 minimum.

8.0 Instrumental accessories:

8.1: Varian VGA-76 peristaltic pump: a] Rate {cc per min.}: Hydrochloric acid : 1 Borohydride solution : 1

Sample: 8

b] Insert / purge gas : Ar { pressure : 50 psi }.

{ Air supply: compressed - cylinder or filtered air supplied by a pump }

8.2 Burner: conventional air / acetylene

8.3 Neublizer suction rate { H₂O }: 5 cc / min.

8.4 Cell: open ended quartz tube placed in the optical scope of AAS.

8.5 Other apparatus:

- Aluminum heating block with 40 holes for test tubes. - 50 cc digestion test tubes

- Temperature controlled hot plate

- Other necessary glassware

9.0 Reagents:

9.1 H₂O: Distilled & Deionized

9.2 HCL: neat - conc. analytical grade

9.3 NaBH₄: Analytical grade

9.4 NaOH: Analytical grade

9.5 H₂SO₄ / HNO₃ / HCLO₄: neat - conc. analytical grade

9.6 K.I : Analytical grade

10- Standards & special solutions:

10.1 Borohydride solution: {to be pumped into the reaction tube} 0.9 % NaBH₄ + 0.8 % NaOH

{4.5 gm sodium borohydride + 4 gm sodium hydroxide to 500 ml with H₂O. The solution is filtered with #42 paper.}

Two more concentrations of borohydride solutions may be needed for the rare & special sample:

10.2 Hydrochloric:

a} to be pumped into the reaction tube:

50 ml H²O + 300 ml HCL conc. {both 10 M & 5 M HCL may be needed as explained in this SOP}

b} conc./neat is used for other digestion as indicated.

10.3 Blank:

a] Arbitrary zero: 5 ml HCL - conc. + 45 ml H₂O {10% HCL v/v}.

b] Analytical zero: 1 ml H₂O + 3 ml of 6:3:1 nitric / sulfuric / perochloric acid mixture digested with the sample and treated in an identical manner {i.e. addition of 5 ml HCL and water bath treatment, dilution with H₂O, addition of KI for As / Sb determination...etc.}

10.4 Stock standard solution: 1000 mg/L [ppm] certified solution; e.g. Plazmachem.or Customer grade.

10.5 Working solution:

1. ug/L [ppb] solution from certified standard prepared in 5 % HCL from 2 different sources, e.g. Plazmachem& BDH.

10.6 Calibration standards:

- Blank - see above.

10.7 Standard addition:

A spiking solution of 150 ug/L Se & 1500 ug/L As/Sb in 5% HCl. 1 ml is added to the spiked sample {Q3} prior to digestion so that it would undergo the same conditions & treatment of the samples. The recovery would then reflect on both digestion and matrix affect factors.

Since the final volume of the initial digestion is 50 cc, the concentration of the standard addition = 3 ppb Se & 30 ppb As/Sb. As/Sb digestion involves a further 1/10 dilution, resulting in a final spike value of +3 ppb in solution.

11- Quality control:

11.1 Digested Q.C.:

a} Q1 : A digested blank with identical treatment.

b} Q2 : A digested 3 ug/l standard with identical treatment.

c} Q3 : A spiked sample. A replicant of 1st sample "D" with an ideal concentration / recovery of sample "D" value + 3 ppb spike conc.

d} Q4 : A replicant of "D" sample with no standard addition.

11.2 References :

a} Standard from a second / separate source for cross-check confirmation; e.g. BDH or Fisher Vs. Plazamachem or CGS.

b} Control samples ; i.e. certified soil material - e.g. "Sewage sludge" & "Pacs-1" each with a known mg/kg conc. The QC sample is digested & treated under conditions identical to the tested sample's.

c} In-house control std. from certified material such as As₂O₃, H₂SeO₃, Fe(OH)SeO₃, Sb₂O₃ and other certified salts or metals. {Checked against references "a" & "b" above for confirmation.)

12- Precautions:

12.1 use a perochloric acid fume hood.

12.2 observe the safety practice of handling acids and harmful / toxic substances.

12.3 neutralize end-solutions with CaCO₃ or limestone chips prior to disposal.

13- Analytical notes / synopsis:

The analyst must be familiar with certain facts and factors that play a major role in the successful hydride operation. A discriminating - educated approach - as opposed to a blind routine application - makes the critical difference. The following factors require special attention:

- The matrix ,e.g. soil / vegetation / alloy / fish ...etc.

- The evaluated major compound forms, e.g. salt / inorganic metallic / organic volatile / complex compounds .. etc.

- Special - recognized presence [e.g. an oxide] easily determined by colour / simple test / common or obvious reaction.

The analyst's adjusted approach to an individual sample includes these areas:

- Digestion treatment: temperature / duration / acid addition.

• Flame stoichiometry. The type of the sample and its major composition play a major role in controlling several chemical interference and atomization complications as well as several cationic and anionic related side-effects that results in serious spectral problems.

• AAS setting .This includes adjusting the spectral band width / hallow cathode current/ gain voltage to best deal with a certain matrix. The objective is to deal with:

- baseline noise

- nonlinear curve

- reversal absorption

- non-absorbable radiation

- background problems

- basic chemical & spectral interference

14. Traces of nitric acid remaining after digestion - especially after warming up the sample - is likely to result in considerable analytical interference especially with Se determination. The digestion must therefore be completed until SO₃ fumes are observed. However, it's also essential that over-digestion be avoided otherwise the sample will be subject to losses of selenium { elemental or compound form } in volatile state.

15- Oxidation states:

The addition of HCL & KI serves to reduce As/Sb/Se to valence states most favorable as follows:

Arsenic: preferred "As III" - alternate "As V". Reduction with KI. Heating needed.

Antimony: preferred "Sb III" - alternate "Sb III". Reduction with KI - spontaneous.

Selenium: preferred as "Se IV" - alternate "Se VI". Reduction with 6-7 M HCL {or a higher conc.} with possible need for heating.

16- Selenium, special notes:

1. Selenium is commonly found in soil samples as metal in association with sulfide ores, basic ferric selenite Fe(OH)SeO₃, calcium selenate and organo compounds. Selenium occurs in alkaline soils mainly as selenates. In acid soils it exists mainly as selenides. In slimes it exists mainly as CuAgSe and Ag₂Se . Based on sample matrix, the following reactions are found to be common:

The oxidation of selenides and its conversion to selenite:

Se(selenide) + Na₂CO₃ + O₂ ® Na₂SeO₃ + CO₂

Se(selenide) + Na₂CO₃ + O₂ ® Na₂SeO₄ + CO₂

Ferrous sulfate accelerates the reaction as follows:

H₂SO₄ + HCL + FeSO₄ ® H₂SeO₃ + FeCL₃ + Fe₂(SO₄)₃ + H₂O

Selenium usually exists in nature as metal selenides in association with sulfide ores. Elemental selenium, basic ferric selenite Fe(OH)SeO₃ , calcium selenate and organoselenium compounds are also found in soil samples.

 

2. Acid digestion is necessary to ensure that Se in the sample is in the inorganic form. SeVI is not quantitatively recovered by hydride generation and must be reduced to Se IV. This requires sample preparation in HCL {5-10 M} in 80-90 ⁰C for 10-15 minutes.

16-3 Other common selenium digestion reactions / complications / side effects:

With sodium sulfite : Se₂ + Na₂SO₃ ® Na₂SeO₃

With sulfuric acid : Na₂SeSO₃ + H₂SO₄ ® Na₂SO₄ + Se + SO₂ + H₂O

4. Trace level of potassium iodide will interfere severely with the determination

of Se. Special Se tubes / cell / glass separator must be assigned strictly for Se

analysis.

Selenium forms halides by reacting with fluorine and chlorine and to a lesser

degree with terhalogen compounds and bromine . It decomposes hydrogen

iodide to liberate iodine. Selenium dissolves in alkali-metal sulfites forming

selenosulfates, M₂SSeO₃.

Selenium is oxidized by solutions of alkali-metal dichromates, permanganates, chlorates and calcium hypochlorite. It dissolves in strong alkaline solutions yielding selenides and selenites. It forms selenocyanates, MSeCN with many inorganic and organic derivatives of HSeCN.

The role of KI is also evident in its use in the thiosulfate titration as follows:

H₂SeO₃ + HCL + KI ® Se + I₂ + KCL + H₂O

H₂SeO₃ + Na₂S₂O₃ + HCL ® Na₂SeS₄O₆ + Na₂S₄O6 + NaCL + H₂O

5. Selenium combines directly with fluorine, chlorine and bromine, but not

iodine, and forms the monohaliodes Se₂X₂, the dihalides SeX₂, the tetrahalides

SeX₄ and the hexafluoride SeF₆. The compounds are covalent and volatile.

 

6. Selenium oxyhalides SeOX2 dissolves many metal chlorides and chalcogenides.

Common selenium acids and salts often present and active in the test tube include selenious acid H₂SeO₃, selenic acid H₂SeO₄, selenium oxyacids such as permonoselenic acid H₂SeO₅, perdiselenic acid H₂Se₂O₈, and Pyroselenic acid H₂Se₂O₇. Other commonly interfering inorganic selenium compounds include sodium selenocyanate NaSeCN , selenocyanogen SeCN / (SeCN)₂, and selenium selenosulfate Na₂SeSO₃.

17 Arsenic, special notes:

1. Arsenic is common in soil samples in such mineral format as arsenopyrite AsFeS. Arsenic may be detected as a yellow sulfide As₂S₃ by precipitation from HCL solution.

The most common oxidation states of arsenic are -3, +3 and +5. However, recorded data indicates that in the majority of arsenic compounds the arsenic atom is in the tetrahedral valence state. Compounds in which the arsenic atom is three coordinate are assumed to contain the tetrahedrally hybridized arsenic atom with a lone pair of electrons in one of the hybird orbitals(1). Since the valence shell of the arsenic atom contains "d" orbital, compounds are known in which the arsenic atom adopts trigonal-bipyramidal or octahedral valence states.

2. Metallic As is introduced in soil samples by carbon reduction of arsenious oxide As₄O₆ or by thermal decomposition to arsine and FeS of naturally occurring arsenical pyrite FeAsS. Arsine AsH₃ is not very stable to air oxidation . In the +3 state arsenic forms arsenious oxide As4O6 producing a slightly acid solution which is believed to contain hydroxide As(OH)₃ or H₃AsO₃, [HAsO₂]. The hydroxide is amphoteric ; it neutralizes acids to give solutions containing As(OH)₂+, and it neutralizes bases to give solutions containing arsenite ions [ H₂AsO₃-, AsO₂-, or As(OH)₄^- ]:

As(OH)₃ + H₃O+ ® As(OH)₂+ + H₂O

As(OH)₃ + OH- ® H₂AsO₃^- + H₂O

 

3. Any reaction in the sample that would produce H₂S into the arsenious solution, results in a yellow precipitate forming [ commonly suspected to be solely As₂S₃ but experiments indicate that it probably has the molecular formula As₄S₆.] Arsenious sulfide has great tendency to form colloids stabilized by adsorption of negative ions. These colloids can be coagulated by addition of H₃O+ or other positive ions. If the H₂S was introduced into an arsenic acid [instead of arsenious solution] the yellow precipitation still appears, however in this case it is believed to be As₂S₅ [many chemists believe it to be As₄S₁₀].

 

4. In the +5 state, the principal compounds of arsenic are arsenic acid and its derivatives, the arsenates. Arsenic acid is primarily orthoarsenic acid, H₃AsO₄
5. The common arsenic hydrides are As₂H₄ , As₂H₂ [or AsH] and As₄H₂, however, the only well-characterized binary compound of arsenic ands hydrogen is arsine.

Arsine [hydrogen arsenide, arsenic trihydride], AsH₃ is a highly toxic colorless gas with an unpleasant garlic-like odor.

6. Arsine is soluble in water and accepts a proton from water to form an onium ion as does ammonia. At temperature below -10 ⁰C or under pressure arsine hexahydrate AsH₃.6H₂O is formed.
7. Chlorine reacts with arsine to give hydrogen chloride and arsenic. However, at low temperature the action of chlorine upon arsine produces chloroarsines AsH₂CL and AsHCL₂ {unstable yellow solids}.

The reduction of arsenic (III) compounds by stannous chloride in hydrochloric acid yields a brown amorphous powder corresponding to the formula of arsenic monohydride As₂H₂ or AsH.

17-8 Arsenic halides:

Arsenic forms a complete series of trihalides , but arsenic pentafluoride is the only simple pentahalide known. Chlorine reacts with very cold arsenic trifluoride to produce a hygroscopic solid compound, arsenic dichloride trifluride AsCL2F3 consisting of AsCL₄+ and AsF₆^- ions.

Arsenic trichloride [arsenic III chloride] AsCL₃ , the common halide of arsenic, is formed by spontaneous combination of the elements and, in addition, by reactions of chlorine with arsenic trioxide; or sulfur monochloride S₂CL₂ {or a mixture of S₂CL₂ and chlorine} with arsenic trioxide; or arsenic trioxide with concentrated hydrochloric acid.

Arsenic diiodide , a red solid , with water causes a disproportionation with the formation of arsenic and arsenic trioxide:

3 As₂I₄ ® 4 AsI₃ + 2 As

17-9 Arsenic oxides and acids:

Arsenic pentoxide As₂O₅ is thermally unstable. Commercial arsenic acid corresponds to the composition, one mole arsenic pentoxide to four moles of water { arsenic acid hemihydrate H₃AsO₄.0.5H₂O }.

Arsenates are oxidizing agents and are reduced by conc. HCL.

Hydrolysis of haloarsines gives arsinous acids, R₂AsOH , or their anhydrides (R₂As)₂O.

17-10 Hetrocyclic organoarsines:

Organoarsenic compounds are derived from arsine by replacing one , two or three hydrogens by an alkyl , cycloalkyl , aryl or even heterrocyclic group b. Examples are Tetrachlorophenylarsorane C₆H₅AsCL₄ and dichlorophenylarsine C₆H₅AsCL₂.

The two primary arsines methylarsine CH₃AsH₂ and trifluoromethylarsine CF₃AsH₂ and the secondary arsine bis(trifuoromethyl) arsine (CF₃)₂AsH are converted to gases at room temperature. All other arsines are liquids or solids. This must be kept in mind when treating such sample.

 18 Antimony, special notes:

1. Antimony is common in soil samples as stibnite {antimony trisulfide), as well as other complex sulfide ores containing lead, copper, mercury and silver.

The two allotropes of antimony are a black amorphous and a yellow covalent formed by oxidation of stibine with oxygen or chloride. The main oxide minerals are stibiconite Sb₃O₆(OH)x, cervantite Sb₂O₄ or Sb₂O₃ / Sb₂O₅, valentenite and senarmonite Sb₂O₃, and kermesite Sb₂S₃.

Antimony is oxidized by nitric acid forming a gelatinous precipitate of a hydrated antimony pentoxide. Sulfuric acid forms oxysulfate, while hydrofluoric forms fluorides or fluocomplexes - insoluble compounds.

In the -3 state antimony forms the very unstable compound SbH₃. In the +3 state antimony forms Sb₂O₃ [trioxide or sesquioxide], which at least in one crystal modification, exists as Sb₄O₆ molecules. It is an amphoteric oxide , dissolving in acid to give Sb(OH)₂⁺ [ or SbO⁺ ] ion and dissolves in base to give antimonite anions SbO₂^- or Sb(OH)₄^-. When the sample containing antimonites such as NaSbO₂ is acidified, a white precipitate which has the composition Sb₂O₃.xH₂O is formed. It appears that no simple Sb(OH)₃ is formed. The sulfide Sb₂S₃ is orange only when freshly precipitated.

In the +5 state antimony forms pentoxide Sb₂O₅ which is a slightly stronger oxidizing agent than H₃AsO₄.

 

2. Stibine SbH₃, colorless toxic gas with a disagreeable odor. It is produced when metal antimonides are treated with acid , chemical reduction of antimony compounds , and electrolysis of acid or alkaline solutions using a metallic antimony cathode.

Zn₃Sb₂ +6 H₃O⁺ ® 3 Zn²⁺ + 2 SbH₃ + 6 H₂O

SbO^3-₃ +9 H₃O⁺ + 3 Zn ® SbH₃ + 3 Zn²⁺ + 12 H₂O

3. Alkali metal borohydrides are used to reduce antimony III in acidic aqueous solution to produce stibine. Several metallic antimonides, antimony trioxides, tetraoxides, pentoxides, trifluorides, trichlorides, tribromides, triiodides, trisulfides, pentafluorides, pentachlorides, pentabromides, pentaiodides, pentasulfides play important role in the process of Sb determination by hydrides technique.

Because of the lengthy nature of covering this area, this SOP will only refer to these references:

"Brock University - 1979 paper by Dr. M. Ramzi",

"European / Dutch Encyclopedia of chemistry- volume 12",

"Cairo / Egypt University research 1970-1980 encyclopedia - v. 23", and;

"English Sctt. Ref. v. 4" for details on these compounds.

The analyst needs to be familiar with ways of identifying the presence of these compounds - whether visually or by simples tests - so that he may approach his sample accordingly.

 

4. The chemistry of heterocyclic antimony compounds and several organoantimony compounds as well as a focus on a few aliphatic primary {RSbH₂} and secondary {R₂SbH} / stibines is a major key in the area of antimony recovery. The reduction of dimethylbromostibine with sodium borohydride produces both methylstibine and dimethystibine. Both are unstable and decompose spontaneously at room temperature.

By contrast, dicyclohexylstibine is stable and is produced as a result of chlorodicyclohexylstibine reduction with lithium aluminum hydride.

 

5. Both phenylstibine and diphenylstibine are common in this type of samples and are easily oxidized. Diphenylstibine is a strong reducing agent and reacts with acids {in this case - hydride technique , hydrochloric acid} and liberates hydrogen:

(C₆H₅)₂SbH + HCL ® (C₆H₅)₂SbCL₃ + H₂

 

6. Antimony III fluoride SbF₃ is a white crystalline or thorhombic solid.It molecule shows a very distorted octahedral arrangement and very soluble in water. Antimony III iodide SbI₃ forms red rhombohedral crystals. Antimony pentafluoride reacts with iodine to form bis(antimony pentafluoride) iodide Sb₂F₁₀I. Antimony III sulfide [sesquisulfide] Sb₂S₃ is a black crystalline solid, stibnite . Antimony pentasulfide appears in the bottom of the test tube as a yellow-orange/redish amorphous solid.

 

7. The hydrolysis of halo-and dihalostibines leads to the formation of compounds of two types RSbO and (R₂Sb)₂O. The aromatic compounds undergo an unusual rearrangements when heated:

ArSbO ® [Ar₂Sb]₂O + Sb₂O₃

ArSbO ® Ar₃Sb + Sb₂O₃

 

8. A few dialkylstibinic acids exist in soil samples. They are a result of hydrolysis of the corresponding dialkyltrichloroantimony compounds:

(CH₃)₂SbCL₃ ® (CH₃)₂SbO(OH)

Aromatic stibonic acids can be produced during sample digestion by the famous diazo reaction:

ArSbCL₄ + H₂O ® ArSbO(OH)₂ + HCL

ArN₂CL + SbCL₃ ® ArSbCL₄ + N₂

9. When a diazonium salt is present in the sample and is then allowed to react with antimony pentachloride or with an aryltetrachloroantimony compound, the onium salts [ArN₂][SbCL₆] or [ArN₂][ArSbCL₅] are formed. They decompose in organic solvents with formation of diarylantimony trichloride:

2[ArN₂][SbCL₆] + 3Fe ® Ar₂SbCL₃ + 2N₂ + SbCL₃ + 3FeCL₂

10. Antimony trichloride SbCL₃, a colorless crystalline solid soluble in hydrochloric acid and in water when heated, is introduced into the sample as a result of metal chlorination or Sb₂O₃ reaction with HCL conc. It hydrolyzes giving hydrous Sb₂O₃ with excess water but with limited quantities of water a large number of partially hydrolyzed compounds were reported, e.g.. SbOCL, Sb₂OCL₄, Sb₄O₅CL₂, Sb₄O₃(OH)₃CL₂ , Sb₈O₁₁ and Sb₈OCL₂₂. The hydrolysis precipitation obtained best characterized is tetraantimony dichloride pentoxide Sb₄O₅CL₂. It is initially precipitated as a thick white solid, changing to well-defined colourless crystals. SbOCL produced changes upon further dilution with water to Sb₄O₅CL₂.

 The sample and the HCL are allowed to merge first before the sodium borohydride enters the stream to meet the sample in the reaction tube.

19. Fresh borohydride solution must be prepared on the analysis day to ensure its Stability. Sodium hydroxide is added to stabalize the solution. Allow solution to reach room temperature prior to application.

When determining As / Se in soils samples containing high concentration of metals such as Co, Fe, Ni, fewer interference have been observed when 0.3 % w/v NaBH₄ solution is used as reductant { rather than 0.7 - 1.0 % }. This was achieved at the cost of reducing sensitivity by lowering the concentration of the borohydride solution. The analyst need to consider his option according to the sample nature .  

20. Varian study was based on AsIII in 7 M HCL , SbIII in 7 M HCL & SeIV in 7 M HCL. The MOE study involved the use of 1 - 10 M HCL. The experiments indicated the following:

a) With 10 M HCL acid channel / As at 193.7 nm wavelength:

Solutions were prepared in 1 M hydrochloric acid and the analyte was reduced to As III by the action of potassium iodide at a concentration of 1% w/v.

Reduction required about 50 minutes at room temperature or about 4 minutes at 70 ⁰C . When the analyte was retained as As V by omitting the reduction step, the analytical response was only 20% of that observed as As III. At 10% KI concentration and samples prepared in 7 M hydrochloric AsV responded the same as As III

b) With 10 M HCL / Sb at 217.6 nm wavelength:

Sb III solutions were prepared in 1 M HCL and gave good recovery up to 40 ug/L. With a 10% KI solution in 7 M HCL and a second set in 10 M HCL, good recovery was secured .

c) Se VI was reduced to Se IV by warming with concentrated HCL. In the IV oxidation state in 1 M HCL , Se showed good response. In the VI oxidation state no response was detected.

 

21. AAS optimization is imperative . Allow the hallow cathode lamp to warm up for some 30 minute prior to calibration - depending on lamp condition . Ensure that the lamp is producing sufficient voltage impressed across the electrodes , i.e. its ability to ionize sufficient Ar atoms enough to bombard the cathode and thus produce an effective electromagnetic radiation [energy beam]. At times, this may necessitate alteration to AAS setting to compensate for such problem.

It should be noted that intensity of light source is proportional to the square of lamp current. The baseline noise level - lamp current relation must be also be monitored for the possible absorbance of other elements present in the sample that fell within the width of the monitored element's absorbance line. The lamp can be affected by impurities in the cathode itself or infected from the "W" anode.

It should also be noted that an increase in the cathode lamp current results in an increase in the kinetic energy of the ionized fill gas "Ar" causing more atoms to be sputtered. As the population of the sputtered atoms increases , the residual unexcited atoms cool and a cloud of neutral atoms in front of the cathode is formed. These neutral atoms absorb some of the lamp light which results in an attenuation of the resonance radiation resulting in a classic case of self reversal or self absorbance.

  

22. Adjust the wavelength at one slit bandwidth narrower than operation setting then open the bandwidth wider. Spectral bandwidth must be adjusted to suite the operation needs. A large width will generate a good signal-to-noise ratio, however the resonance line may not be isolated from other lines and as a result the analysis curve may not be as linear. At the same time, the good resolution of a too narrow spectral band width will not compensate for the poor signal-to-noise ratio due to the reduction of light. The attached method summary provides a good guideline; but , at times - with certain samples - it must altered to suit.

 

23. Allow the absorbance cell to stablize with the flame for sufficient time. This is followed by conditioning of the cell . Auto-zero , then read the blank [fresh - undigested 5% HCL] to monitor the baseline. Run a high [fresh-undigested] standard (e.g. 20 , 50 or max. 100 ug/L) and secure an obvious signal. Run the blank again to ensure a representative response.

Repeat the blank & standard runs 2 or 3 times to confirm the conditioning of the cell. This is followed by auto-zero with digested - analytical blank.

  

24. Ensure that the computer interface corresponding to hydride data is connected. Also ensure the connection of the hydride argon line. Confirm the stability of the argon, acetylene and compressed air pressure as stated herein . Any change in the acetylene or air pressure will affect the flame type and temperature. Recheck the flow rate of the DD water to the nebulizer, borohydride suction , hydrochloric suction & sample suction. Adjust the pump and the tubes to achieve the rates stated earlier in this reference.

26 - Sequence of analysis:

1]-Auto-zero, 5% HCL solution

2]- blank, 5% HCL solution

3]- cell conditioning with repeated runs of blank and standard

4]- auto zero with analytical - digested / matrix match blank

5]- calibration : digested blank , digested standards : 1, 3, 5 ug/l.

6]- blank , digested / matrix - match {Q1}

7]- digested std "3 ug/l" {Q2}

8]- digested matrix-match QC to confirm the acceptance of the standard solution, the

calibration and the digestion . {certified / digested QC soil}.

9]- first sample "D"

10]- samples, "average of a 10 samples set"

11]- replicant of "D" sample , Q4

12]- Spiked "D" sample, Q3

13]- repeat of blank - Q1

14]- repeat of Q2

15]- programmed auto-zero and auto-calib.

16]- monitor low and high point on calib. curve {e.g. std. 1 & 5 ppb }.

27 -Interference:

- High concentrations of chromium, nickel, copper, mercury, molybdenum , silver, cobalt or tin cause depression of the signal.

- Traces of nitric results in analytical interference.

- Confirm that the reagents used are contamination free {via blank}.

28- Optimization:

Part -2: [Operation]

1] after lamp warm-up [at the specified lamp current], fine-adjust the wavelength {at 1 setting lower than operation setting as indicated. At times ,at 2 settings lower} and at single beam.

2] adjust the cathode lamp horizontal & vertical to maximize signal

3] open the slit to operational setting

4] switch to double beam

5] adjust the gain voltage to obtain the ideal setting

6] switch back to single beam and place the quartz cell in the AAS optical path and monitor and correct / maximize the signal. Visually monitoring the beam for initial alignment -e.g. with aid of card- can be helpful.

7] Switch back to double beam if you have not done so already

8] background correction : no programmed method in this case.

9] Flame mixture adjusted as stated in this literature. Flame / cell are allowed to stabilize.

10] confirm the optimization based on response with final touches.

Notes: - The method must first be initiated. From "analysis mode" enter its name {e.g. PGSEHG), "Se - Hydride Generation by Paul Gouda"

The autosampler table is programmed to correspond to the method.

29- Routine maintenance:

1. The quartz cell must be cleaned regularly and soaked between runs. Se cell must be kept separately from As/Sb cell to avoid any KI contamination.
2. The pump VG-76 unit must be checked and maintained regularly. Calibration of suction rate is imperative.
3. Two separate sets of tubes and the glass separators for "Se" & "As/Sb" must be cleaned and replaced as often as needed.

4- General & basic maintenance of the AAS at large.

 30- computer programmed method:

The EPL AAS software is programmed to perform 4 methods; two for normal operational level "0-5 ug/L in solution" and two methods for a high level of up to 10 ppb as follows:

The following set-up was designed by the writer while employed as Analytical Chemist for EPL "Environment Protection Laboratories" – acquired by "MDS Environmental" (now Philips Analytical services):

 Method name: PGASSBHG - Method Report

Atomizer: Flame

Matrix: ULTRAPURE WATER

METHOD INFORMATION **

Default Setup:

Number of Repeats : 1

Flush Time (sec) : 45.0

Auto-Increment Sample Names? No

Auto-print Calibration Curves? Yes

Analysis Graphics Display: absorbance

Auto-print Analysis Graphics: Yes

Auto-save Analysis Graphics: No

Default File Names:

Analysis Data File : RESULTS Sample Limits Table: LCTAB

Autosampler Table : ELEMENT Blank (AZ) Limits Table : BLANKLCT QC Check Table : QCTAB

Recovery Check Table: RQCTAB

OUTPUT INFORMATION**

Output Mode: Concentration

Override Print Limits? Yes

Override Significant Figures? No

Limits Table : LCTAB Check? No

Correction Factor: 1

Auto-print data? Yes

Condensed report format? Yes

Auto-store data? Yes

Store individual repeats? No

Report to:

Screen Avgs, Stats, Units

Printer Avgs, Stats, Units

Method: PGASSBHG part -2-

Elements : As Sb

Mode : Double Beam Absorption

Integ Type : Automatic Integ Time : 3.0 Sec. Delay Time: 0.0 Sec.

Comment : ARSENIC/ANTIMONY (HYDRIDE GENERATION) designed by

Paul Gouda.

Flame Information:

Flame Type : Air/Acetylene

Oxidant Flow (SCFH) : 5.0 - 2.5

Wavelength : 193.70

Bandpass : 2.0

High Voltage : 620

BKG Method : None

Lamp Current : 6.0

Signif Figs : 4

Print Units : ug/L

Print Limits Low : .2

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additions

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: Spike1 = 10.00

#2: ASSESB-1 = 1.00 = 0.0582 #2: Spike2 = 20.00

A/C#3: ASSESB-3 = 3.00 = 0.1519 #3: Spike3 = 30.00

#4: ASSESB-5 = 5.00 = 0.2429

Wavelength : 217.60

Bandpass : 0.50

High Voltage : 530

BKG Method : None

Lamp Current : 10.0

Signif Figs : 4

Print Units : ug/L

Print Limits Low : .2

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additions

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: Spike1 = 10.00

#2: ASSESB-1 = 1.00 = 0.0504 #2: Spide2 = 20.00

A/C#3: ASSESB-3 = 3.00 = 0.1414 #3: Spike3 = 30.00

#4: ASSESB-5 = 5.00 = 0.2352

Method: PGSEHG Method Report

Atomizer: Flame

Matrix: ULTRAPURE WATER

METHOD INFORMATION **

Default Setup:

Number of Repeats : 1

Flush Time (sec) : 40.0

Auto-Increment Sample Names? No

Auto-print Calibration Curves? : Yes

Analysis Graphics Display : Absorbances

Auto-print Analysis Graphics? : Yes

Auto-save Analysis Graphics? : No

Default File Names:

Analysis Data File : RESULTS Sample Limits Table : LCTAB

Autosampler Table : HYDRIDE Blank (AZ) Limits Table : BLANKLCT

QC Check Table : QCTAB

Recovery Check Table : RQCTAB

OUTPUT INFORMATION **

Output Mode: Concentration

Override Print Limits? Yes

Override Signif Figs? No

Limits Table: LCTAB Check? No

Correction Factor: 1

Auto-print data? Yes

Condensed report format? Yes

Auto-store data? Yes

Store individual repeats? No

Report to:

Screen Avgs, Stats, Units

Printer Avgs, Stats, Units

Method: PGSEHG part -2-

Elements : Se

Mode : Double Beam Absorption

Integ Type : Automatic Integ Time : 3.0 Sec. Delay Time : 0.0 Sec.

Comment : SELENIUM BY HYDRIDE GENERATION.

Flame Information:

Flame Type : Air / Acetylene

Oxidant Flow (SCFH) : 5.0

Oxidant Flow (SCFH) : 2.5

Flame Type : Air/Acetylene

Element : Se

Element Name : Se

Wavelength : 196.00

Bandpass : 2.0

High Voltage : 700

BKG : None

Lamp Current : 4.0

Signif Figs : 4

Print Units : ug/L

Print Limits Low : 0

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additions

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: Spike1 = 10.00

#2: ASSESB-1 = 1.00 = 0.0274 #2: Spike2 = 20.00

A/C#3: ASSESB-3 = 3.00 = 0.0851 #3: Spike3 = 30.00

#4: ASSESB-5 = 5.00 = 0.1401

 Method: ASSBHGA Method Report

Atomizer: Flame

Matrix: ULTRAPURE WATER

METHOD INFORMATION **

Default Setup:

Number of Repeats : 1

Flush Time (sec) : 45.0

Auto-Increment Sample Names? No

Auto-print Calibration Curves? : Yes

Analysis Graphics Display : Absorbances

Auto-print Analysis Graphics? : Yes

Auto-save Analysis Graphics? : No

Default File Names:

Analysis Data File : RESULTS Sample Limits Table : LCTAB

Autosampler Table : HYDRIDE Blank (AZ) Limits Table : BLANKLCT

QC Check Table : QCTAB

Recovery Check Table : RQCTAB

OUTPUT INFORMATION **

Output Mode: Concentration

Override Print Limits? No

Override Sidnif Figs No

Limits Table: LCTAB Check? No

Correction Factor: 1

Auto-print data? Yes

Condensed report format? Yes

Auto-store data? Yes

Store individual repeats? No

Report to:

Screen Avgs, Stats, Units

Printer Avgs, Stats, Units

 Method: ASSBHGA part-2

Elements : As Sb

Mode : Double Beam Absorption

Integ Type : Automatic Integ Time : 3.0 Sec. Delay Time : 0.0 Sec.

Comment : ARSENIC/ ANTIMONY BY HYDRIDE GENERATION

Flame Information:

Flame Type : Air/Acetylene

Oxidant Flow (SCFH) : 10.0

Oxidant Flow (SCFH) : 4.5

Flame Type : Air/Acetylene

Wavelength : 193.70

Bandpass : 2.0

High Voltage : 620

BKG Method : None

Lamp Current : 6.0

Signig Figs : 4

Print Units : ug/L

Print Limits Low : 0

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additons

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: ADD1 = 10.00

#2: ASSESB-1 = 1.00 = 0.0341 #2: ADD2 = 20.00

A/C#3: ASSESB-5 = 5.00 = 0.1610 #3: ADD3 = 30.00

#4: ASSESB-10 = 10.00 = 0.2621

Wavelength : 217.60

Bandpass : 0.50

High Voltage : 620

BKG Method : None

Lamp Current : 10.0

Signif Figs : 4

Print Units : ug/L

Print Limits Low : 0

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additions

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: ADD1 = !0.00

#2: ASSESB-1 = 1.00 = 0.0259 #2: ADD2 = 20.00

A/Z#3: ASSESB-5 = 5.00 = 0.1328 #3: ADD3 = 30.00

#4: ASSESB-10 = 10.00 = 0.3393

Method: SEHGA Method Report

Atomizer: Flame

Matrix: ULTRAPURE WATER

METHOD INFORMATION **

Default Setup:

Number of Repeats : 1

Flush Time (sec) : 45.0

Auto-Increment Sample Names? No

Auto-print Calibration Curves? : Yes

Analysis Graphics Display : Absorbances

Auto-print Analysis Graphics? : Yes

Auto-save Analysis Graphics? : No

Default File Names:

Analysis Data File : RESULTS Sample Limits Table : LCTAB

Autosampler Table: HYDRIDE Blank (AZ) Limits Table : BLANKLCT

QC Check Table : QCTAB

Recovery Check Table : RQCTAB

OUTPUT INFORMATION **

Output Mode: Concentration

Override Print Limits? No

Override Signif Figs No

Limits Table: LCTAB Check? No

Correction Factor: 1

Auto-print data? Yes

Condensed report format? Yes

Auto-store data? Yes

Store individual repeats? No

Report to:

Screen Avgs, Units

Printer Avgs, Units

Method: SEHGA part-2

Mode : Double Beam Absorption

Integ Type : Automatic Integ Time : 3.0 Sec. Delay Time : 0.0 Sec.

Comment :

Flame Information:

Flame Type : Air/Acetylene

Oxidant Flow (SCFH) : 10.0

Oxidant Flow (SCFH) : 4.5

Flame Type : Air/Acetylene

Element : Se

Element Name : Se

Wavelength : 196.00

Bandpass : 2.0

High Voltage : 700

BKG Mehtod : None

Lamp Current : 5.0

Signif Figs : 4

Print Units : ppb

Print Limits Low : 0

Print Limits High : 0

Stdzn Method: Multipoint Stdn. Stdzn Method: Standard Additions

Std Names Std Conc Abs. Addn Name Std Addn

A/Z#1: BlankSTD = 0.00 = 0.0000 #1: ADD1 = 10.00

#2: ASSESB-1 = 1.00 = 0.0059 #2: ADD2 = 20.00

A/C#3: ASSESB-5 = 5.00 = 0.0428 #3: ADD3 = 30.00

#4: ASSESB-10 = 10.00 = 0.0916

 Data documentation & communication "LIMS":

a} Electronic :

1. hard copy: C drive of the AAS, under "file: results", via "enable" (Illustration sample of common software).Hard disk back-up maintained regularly.
2. software : Data transferred to a floppy via "enable". Files are named to indicate element and date, e.g. Se042295.ASC. The data is edited after taking into account such factors as background correction, and the data is then transferred into the LIMS "network" system under "AA/data" {the DCI part of the reporting}.

b) Conventional:

1) Project sheet: report indicating results; conc./ unit / dilution & volume ..etc.

2) Computer printout: attach to the main project of the run {reference number}

3. AAS record binder, including performance record (e.g. calib. correlation coefficiency, SRM/QC record , production record , .. etc).

The preceding 26 pages dealt with As / Se / Sb analysis within the scope of EPL / EPA methodology and range . It dealt solely - and as briefly as I could - with the areas related directly or significantly with the ultrace level determination of these elements by the hydride generation technique.

The sudden and positive change in the approach and attitude taken towards the environment in the past recent years necessitated the development of instrumental analytical techniques capable of determining concentration < 1 ppb . That is way below what used to be reported just a few years ago as BDL { Below Detection Limit } by conventional flame AAS.

In fact , Until a few years ago , reporting As / Se / Sb concentration in %, g/L, or in Fns. was very common. Such gravimetric and volumetric wet methods can still be utilized with samples containing high level of As / Se / Sb as a confirmation test. Such unexpected level of contamination involves ultra-trace methods such as ug/L - hydride generation in extremely high dilutions. As examples of such tests one would have to refer to the permanganate method and the potassium bromate titration.

The titration is based on the fact:

1 ml N/10 KMNO₄ {or N/10 KBrO₃ } = 6.088 mg Sb

 The principal of the potassium permanganate method is as follows:

2 KMnO₄ + 16 HCL + 5 SbCL₃ = 5 SbCL₅ + 2 KCL + 2 MnCL₂ + 8 H₂O

2 KMnO₄ + 16 HCL = 2KCL + 2 MnCL₂ + 5 CL₂ + 8 H₂O

The potassium bromate method is based on the fact that:

KBrO₃ + 3 SbCL₃ + 6 HCL = KBr + 3 SbCL₅ + 3 H₂O

The Analyst need to examine all possible and related chemical scenarios, complications and side-effects and be able to approach each sample accordingly. This applies to performing the bench - wet work as well as instrumentation. This necessitates:

• refreshing oneself with the theory part of wet chemistry
• examining EPL's methodology carefully

- elucidating the intricacies of the topic through research literature and EPA methodologies

The hydride generation technique, when applied correctly, is a very accurate ultra trace method.

EPL's recorded recovery of certified QC soil samples , spike/Q3 samples, and Q2/standards attest to this fact.

Paul Gouda, Ph.D.

Chartered Analytical Chemist

BA, B.Sc., MA, Ph.D.

Mailing: Mailing: 1-5765 Turner Rd. Suite 220 Nanaimo, BC. V9T-6M4 Canada

Telephone: 01-250-716-6543

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