U d t Update on New and Modified N d M difi d Extraction Methods - - PowerPoint PPT Presentation

▶
u d t update on new and modified n d m difi d extraction
SMART_READER_LITE
LIVE PREVIEW

U d t Update on New and Modified N d M difi d Extraction Methods - - PowerPoint PPT Presentation

Sep 06, 2022 152 likes •325 views

U d t Update on New and Modified N d M difi d Extraction Methods for Cr(VI) Determination in Dusts, Ashes, and Soils Ruth E. Wolf and Stephen A. Wilson U S Geological Survey MS 964D Denver Federal Center Denver CO U.S. Geological Survey, MS

slide-1
SLIDE 1

U d t N d M difi d Update on New and Modified Extraction Methods for Cr(VI) Determination in Dusts, Ashes, and Soils

Ruth E. Wolf and Stephen A. Wilson U S Geological Survey MS 964D Denver Federal Center Denver CO U.S. Geological Survey, MS 964D, Denver Federal Center, Denver, CO 80225

U.S. Department of the Interior U.S. Geological Survey

slide-2
SLIDE 2

Introduction Introduction

  • One of the difficulties in speciation

analyses of solid samples is analyses of solid samples is finding a suitable extraction method for the species of interest

  • The extraction method must be
  • The extraction method must be

compatible with the analytes of interest and be compatible with the analytical determination method(s) ( )

  • The extraction method must not

cause species interconversion

Cr(VI) reference materials in preparation by USGS (NIST 2701 and 2700)

  • The extraction method must also

be able to accommodate a limited amount of sample and not generate an excessive amount of ge e ate a e cess e a

  • u t o

waste

slide-3
SLIDE 3

Analytical Method† y

  • HPLC – reversed phase, ion-pairing

– Column: Brownlee C8

  • Column Oven 35 ºC

Column Oven 35 C

– Mobile Phase:

  • 2mM tetrabutylammonium hydroxide (TBAOH)

+ 0.5mM K2EDTA, pH = 7.4 – 7.6

EDTA converts Cr(III) to anionic EDTA Cr(III) – EDTA converts Cr(III) to anionic EDTA-Cr(III)

  • 5% MeOH added on-line

– Autosampler 10 ºC – Samples: Diluted minimally 1:1 in mobile- phase, 50 µL injection

  • DRC-ICP-MS

– Reaction gas: N2 or NH3 S l I t d ti

The use of company, trade, and/or product names is for identification purposes only and does not imply endorsement by the United States Government.

– Sample Introduction:

  • Baffled quartz cyclonic spray chamber
  • Meinhard TQ-30-A3 nebulizer

† Complete description in J. Anal. At. Spectrom., 2007, 22, 1051-1060

* Stated column working range to pH=7; however, have run up to pH=8 without significant column life issues

slide-4
SLIDE 4

Analytical Figures of Merit:

Parameter NH3 Reaction Gas Flow 0.75, RPq=0.65 N2 Reaction Gas Flow 1.0, RPq=0.50 Cr(III) Cr(VI) Cr(III) Cr(VI) Instrument Detection Limit (IDL), µg/L 0.09 0.06 0.1 0.1 Practical Quantitation Limit (PQL), µg/L 0.2 0.2 0.2 0.2

Chromatogram showing low calibration standard at 0.5 µg/L and standard at PQL of 0.2 µg/L

slide-5
SLIDE 5

Available Cr(VI) Extraction Methods

  • U.S. EPA Method 3060A – Alkaline Digestion for Hexavalent

Chromium Chromium

– 0.28 M Na2CO3 / 0.5 M NaOH with phosphate buffer and MgCl2 precipitation agent, pH >11.5, hotplate Applicable to soils and solid wastes – Applicable to soils and solid wastes – Requires 2.5 g sample and generates 100 mL of extract

D i i d t b ff d d b ff d t ti

  • De-ionized water, buffered and un-buffered extraction

– Applicable to soils, ashes, other solid materials OSHA ID 215 H l t Ch i i Ai Filt

  • OSHA ID-215 – Hexavalent Chromium in Air Filters

– 10% Na2CO3 / 2% NaHCO3 with phosphate buffer and Mg sulfate precipitation agent, pH 8, hotplate – Applicable to quartz or PVC air filters Applicable to quartz or PVC air filters

slide-6
SLIDE 6

Performance of Cr(VI) Extraction Methods with HPLC ICP MS Detection: with HPLC-ICP-MS Detection:

  • Preliminary investigations using various buffer solutions were
  • Preliminary investigations using various buffer solutions were

carried out using spikes of Cr(III) and Cr(VI) in blank extraction solutions and comparing chromatograms to spikes in the HPLC mobile phase mobile phase

  • Phosphate and sulfate extraction solutions were incompatible

with HPLC-ICP-MS determination method used

Buffer Solution Tested Stock Concentrations Tested Concentration pH Phosphate – pH 8 1 M K2HPO4 – 0.06M KH2PO4 10mM, 20mM 8.4 Carbonate – pH 9 0.1 M Na2CO3 – 0.9 M NaHCO3 10mM, 20mM 9.2 Sulfate – pH 10 1M (NH4)2SO4 – 1 M NH4OH 20mM, 50mM 10.3

slide-7
SLIDE 7

Soil Extraction Study:

  • Method 3060A Digestion as written is incompatible with HPLC-ICP-

MS detection method

– Phosphate buffer disrupts ion-pairing chromatography

  • Requires 50x to 100x dilution prior to analysis

– 2.5 g sample requirement and strong basic (pH ~13) extraction solution results in over 100 mL of digestate for disposal results in over 100 mL of digestate for disposal

  • Note: less than 1 mL required for analysis!

– Mg precipitation agent forms a gelatinous Mg(OH)2 or Mg(CO3)2 precipitate that is difficult to filter and wash, requires disposal precipitate that is difficult to filter and wash, requires disposal

  • Similar to co-precipitation method used to prepare microanalytical carbonate

standards for LA-ICP-MS

  • Could result in analyte loss (need to verify by SEM)

D i bl t fi d t ti th d f il th t i tibl ith

  • Desirable to find an extraction method for soils that is compatible with

the detection method of choice and easier to use and results in less waste

slide-8
SLIDE 8

Evaluation of Cr(VI) soil extractions

  • Modified 3060A

– 0.28 M Na2CO3 / 0.5 M NaOH, pH > 11.5 – Omit phosphate buffer and Mg, reduce sample size to 0.5 g and use 50 mL extraction fluid, final volume after pH adjustment 100 mL

  • BiCarb Hotplate

– 1mM Na2CO3/9mM NaHCO3, pH~9.2 extraction fluid, 0.5 g sample, 50 mL extraction fluid, 95 °C for 1 hour, centrifuge and filter ~10 mL with 0.45 µm syringe filters into 15 mL tube

  • BiCarb Microwave

BiCarb Microwave

– 1mM Na2CO3/9mM NaHCO3, pH~9.2 extraction fluid, 0.5 g sample, 50 mL extraction fluid in 100mL Teflon vessels (Anton Paar MW3000, Rotor 16), temperature program ramp to 95 °C (20 min), hold at 95 °C for 1 hour, centrifuge d filt ith 0 45 i filt and filter with 0.45 µm syringe filters

  • Evaluate effect of particle size on extraction efficiency

– Extract SRM materials as provided (mean particle size 200 µm) – Micronize SRM materials to mean particle size 30 µm Micronize SRM materials to mean particle size 30 µm

slide-9
SLIDE 9

Results – NIST 2701 Results NIST 2701

700.0

NIST 2701

551.2 420.7 487.6 469.6 408.7 376.7 449.8 397.9 379.3 400.0 500.0 600.0 µg/g 200.0 300.0 400.0 Cr(VI) µ 0.0 100.0

slide-10
SLIDE 10

Results – NIST 2700 Results NIST 2700

NIST 2700

20.0 25.0 Target Value = NIST 2701 Certified Value/ Approx. Dilution Factor = 551.2/40 = 13.8 13.8 13.4 16.3 17.2 12 0 13.6 15.0 20.0 µg/g 12.0 9.1 10.0 Cr(VI) µ 0 0 5.0 0.0 Target Value 3060A Modified 3060A Micronized - Mod 3060A BiCarb Micronized - BiCarb BiCarb Microwave

slide-11
SLIDE 11

Effect of Reduced Matrix

40.0

Diluted NIST 2701

Effect of Reduced Matrix

27.6 31.4 29.2 30.0 35.0 0 0 Target Value = NIST 2701 Certified Value/ Approx. Dilution Factor 13.8 16.3 15 0 20.0 25.0 Cr(VI) µg/g 13.8 12.0 5.5 5.1 4.9 5.0 10.0 15.0 C 0.0 20 X- Target Value 20X - Mod 3060A 20X - BiCarb 40X - Target Value 40X - Mod 3060A 40X - BiCarb 100x - Target Value 100X - Mod 3060A 100X - BiCarb Value 3060A Value 3060A Value 3060A

slide-12
SLIDE 12

Use of Isotopically Enriched Spikes

  • Prepared from Cr metal starting material
  • Dissolved

5 mg in 4 mL 6M HCl in 30 mL PFTE vial

  • Dissolved ~ 5 mg in 4 mL 6M HCl in 30 mL PFTE vial
  • Cr(III): Transfer to precleaned, tared 500 mL bottle
  • Dilute to 500 g with deionized water (pH=1)

g (p )

  • Cr(VI): transfer to 100 mL PFTE beaker
  • Add 50 µL 30% H2O2 and 4.5 mL conc. NH4OH (turns yellow), heat

to 150 °C to gentle boil boil 15 minutes (per Method 6800) to 150 C to gentle boil, boil 15 minutes (per Method 6800)

  • Transfer to 500 mL bottle and dilute to 500 g with DI water

Cr Isotope Natural Abundance

50Cr(III) spike 53Cr(VI) spike

Cr Isotope Natural Abundance Cr(III) spike Cr(VI) spike 50 4.345 96.05 0.25 52 82.789 3.66 3.73 53 9 501 0 24 95 74 53 9.501 0.24 95.74 54 2.365 0.05 0.27

slide-13
SLIDE 13

Isotopic Spikes on NIST 2701 (1:100) p p ( )

60000

Cr 53

35000

Cr 50

53Cr(VI) spiked 2701 50Cr(III) spiking

30000 40000 50000 nsity (cps) 20000 25000 30000 nsity (cps)

( ) p 9.8 ppb 108% Recovery

S ik d 2701

( ) p g solution 8.9 ppb

Cr VI

10000 20000 30000 Inten

Cr VI

5000 10000 15000 Inten

Unspiked 2701 Unspiked 2701 Spiked 2701

0.5 1.0 1.5 2.0 2.5 Time (min) 0.5 1.0 1.5 2.0 2.5 Time (min)

Using BiCarb Microwave Digestion

  • 53Cr(VI) Spike is reco ered
  • 53Cr(VI) Spike is recovered
  • No sign of reduction to Cr(III)
  • 50Cr(III) Spike is NOT recovered
  • C

fi ti C (III) d d i filt ti t

  • Confirmation Cr(III) removed during filtration step
slide-14
SLIDE 14

Isotopic Spikes on NIST 2700 (1:10)

50000

Cr 53

35000

Cr 50

p p ( )

53Cr(VI) spiked 2700 50Cr(III) spiking

30000 40000 nsity (cps) 20000 25000 30000 nsity (cps)

( ) p 9.8 ppb 89% Recovery

S ik d 2700

( ) p g solution 8.9 ppb

Cr VI

10000 20000 Inten

Cr VI

5000 10000 15000 Inten

Unspiked 2700 Unspiked 2700 Spiked 2700

0.5 1.0 1.5 2.0 2.5 Time (min)

C

0.5 1.0 1.5 2.0 2.5 Time (min)

Using BiCarb Microwave Digestion

  • 53Cr(VI) Spike is reco ered
  • 53Cr(VI) Spike is recovered
  • No sign of reduction to Cr(III)
  • 50Cr(III) Spike is NOT recovered
  • C

fi ti C (III) d d i filt ti t

  • Confirmation Cr(III) removed during filtration step
slide-15
SLIDE 15

Conclusions

  • Modified EPA 3060A extraction achieved best recoveries for

NIST 2701 and 2700

  • 1mM Na CO /9mM NaHCO

pH~9 2 extraction fluid (BiCarb)

  • 1mM Na2CO3/9mM NaHCO3, pH~9.2 extraction fluid (BiCarb)

extraction achieved lower recoveries for Cr(VI) in both hot plate and microwave digestion methods

– pH might need to be higher (10-11) pH might need to be higher (10 11) – Lowest recoveries via microwave digestion might be a function of lack of contact via stirring

  • Recoveries obtained for NIST 2701 when diluted with ground quartz

were close to “target” values by both Modified 3060A extraction and BiCarb extraction

– Indicates that low recoveries on undiluted 2701 might be a function of the sample matrix matrix

  • The use of isotopically enriched spikes of Cr(VI) and Cr(III) with the

BiCarb microwave digestion do not indicate any reduction of Cr(VI) to Cr(III)

– Verifies that Cr(III) is removed during filtration step

slide-16
SLIDE 16

Next Steps Next Steps

  • Perform extractions using sodium bicarbonate extraction fluid

adjusted to a higher pH (10-11)

  • Evaluate effects of possible oxidation/reduction reactions during
  • Evaluate effects of possible oxidation/reduction reactions during

Modified 3060A extraction using isotopically enriched spikes

  • Contact: Ruth E. Wolf (303) 236-2470, rwolf@usgs.gov