Cr(VI) Measurements: Feasibility, Fate, and Stability Ruth E. Wolf, - - PowerPoint PPT Presentation

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Cr(VI) Measurements: Feasibility, Fate, and Stability Ruth E. Wolf, - - PowerPoint PPT Presentation

Apr 17, 2023 444 likes •652 views

Cr(VI) Measurements: Feasibility, Fate, and Stability Ruth E. Wolf, Suzette A. Morman, and Geoffrey S. Plumlee USGS Denver CO 80225 USGS, Denver, CO 80225 U.S. Department of the Interior U.S. Geological Survey Recent Issues Concerning Cr(VI)

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SLIDE 1

Cr(VI) Measurements: Feasibility, Fate, and Stability

Ruth E. Wolf, Suzette A. Morman, and Geoffrey S. Plumlee USGS Denver CO 80225 USGS, Denver, CO 80225

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

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SLIDE 2

Recent Issues Concerning Cr(VI) M t Measurement

  • State of California proposal for Cr(VI)

State of California proposal for Cr(VI) regulatory limit from 0.05 µg/L to 0.02 µg/L in drinking water in drinking water

– Requires MDLs in 0.002 – 0.004 µg/L

Recent study finds Cr(VI) in numerous

  • Recent study finds Cr(VI) in numerous

municipal water sources C ( )

  • No recent studies on Cr(VI) stability

– Particularly in “naturally occurring matrices”

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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

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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 Using 50 µL injection volume

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SLIDE 5

Method Modifications to Improve IDLs Method Modifications to Improve IDLs

  • Use NH3 Reaction Gas

3

  • Investigate use of larger

Cr VI

5000 5500

Cr 52

200 ppt spex: Cr 52: 1 100 ppt spex: Cr 52: 1 50 ppt spex: Cr 52: 1

injection volume Careful optimization of

Cr III

3500 4000 4500 ntensity (cps)

  • Careful optimization of

instrument parameters

2500 3000 3500 I 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 6

50 ppt at different injection volumes 50 ppt at different injection volumes

Cr 52

50 ppt spex: Cr 52: 1 50 ppt spex 100: Cr 52: 1 6500 7000 50 ppt spex 150: Cr 52: 1 50 ppt spex 200: Cr 52: 1

  • Inj. Vol.

( L) Area cps

5000 5500 6000 ps)

(µL) cps 50 5249 100 7394

Cr VI

3500 4000 4500 Intensity (c

150 10755 200 16559

2500 3000 3500 2000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 7

IDL Determination – 200µL injections

12000

Cr 52

200 ppt: Cr 52: 1 100 ppt: Cr 52: 1 50 ppt: Cr 52: 1 25 ppt: Cr 52: 1

IDL Determination 200µL injections

Cr VI

10000 11000 12000

Cr(III) ppb Cr(VI) ppb 0.106 0.055

  • Larger injection

volume did not drastically improve

8000 9000 (cps)

0.071 0.069 0.086 0.091 0.069 0.078 0.066 0.064 0.08 0.072

drastically improve IDL for Cr(VI)

  • More improvement

seen for Cr(III)

Cr III

5000 6000 7000 Intensity

0.104 0.05 0.076 0.039 MEAN 0.082 0.065 SD 0.015 0.017

IDL=3*SD 0 046 0 050

3000 4000

IDL=3*SD 0.046 0.050 ppb ppb

2000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 8

Stability of Cr(III) and Cr(VI) y ( ) ( )

  • Cr(VI) standards stable in mobile phase at 10 °C

St t 21 °C d t i t l if th i t

  • Storage at -21 °C detrimental if other species are present

– As(III), As(V), Se(IV), Se(VI) – Reduction of Cr(VI) to Cr(III) ( ) ( )

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SLIDE 9

Stability of Cr(III) and Cr(VI)

  • Standards spiked into USGS reference water M-172 showed varying

stability

  • Elapsed time in hours from t=0 to t=120
  • Elapsed time in hours from t=0 to t=120
  • M-172 preserved with NaOCl – causes oxidation of Cr(VI)!
  • Addition of 10 mM K2EDTA preservative (E-172) showed limited

improvement if As and Se species present p p p

– Eliminates oxidation of Cr(III) to Cr(VI) – More info in Analytical & Bioanalytical Chemistry, in press

  • http://dx.doi.org/10.1007/s00216-011-5275-x

Cr(III)

  • xidized

to Cr(VI)! ( )

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SLIDE 10

In Vitro Bioaccessibility Methods - C (VI) S bili i h H B d Cr(VI) Stability in the Human Body

  • Ingestion Pathway

g y

– Simulated gastric leach fluid (based on Drexler and Brattin, 2007)

  • pH 1-2 glycine/HCl solution to simulate stomach environment
  • 1 part solid sample to 100 parts leachate fluid, placed in orbital shaker

incubator at 37 ºC for 1 hour, filtered and analyzed for pH and metals

– Simulated intestinal fluid (modified from Basta, 2007)

  • Gastric leach fluid titrated to a pH of 5.5 ± 0.1 with Na2CO3 prior to addition

f i ti d bil

  • f porcine pancreatin and bile
  • Inhalation Pathway

– Simulated lung fluid (SLF) (based on Mattson, 1994)

  • Simple proxy for a near neutral pH solution encountered along an inhalation

pathway

  • 1 part solid sample to 100 parts SLF for 24 hours at 37 ºC
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SLIDE 11

Gastric Fluid: Converts Cr(VI) to Cr(III) ≤ 200 ppb

Cr VI

35000

5 ppb standard Time = 0 pH = 1.5

Cr III

25000 30000 ps) 20000 ensity (c

200 ppb Cr(VI) – Gastric 1:50 100 ppb Cr(VI) – Gastric 1:50

10000 15000 Inte

50 ppb Cr(VI) – Gastric 1:50 100 ppb Cr(III) – Gastric 1:50

5000 10000

Blank Gastric 1:50

0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 12

Gastric: Partial conversion to Cr(III) ≥ 500 ppb

50000 55000

1000 ppb Cr(VI) – Gastric 1:50

Time = 0 pH = 1 5

Cr VI

35000 40000 45000 ps)

5 ppb standard

1000 ppb Cr(VI) Gastric 1:50 % Conversion ~ 80%

pH 1.5

Cr III C

25000 30000 35000 ensity (cp

500 ppb Cr(VI) – Gastric 1:50 % Conversion ~ 93%

15000 20000 25000 Inte

100 ppb Cr(III) – Gastric 1:50

5000 10000 15000

Blank Gastric 1:50 Blank Gastric 1:50

0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 13

Intestinal: No immediate conversion

120K

Time = 0 pH = 4 8

100K s)

1000 ppb Cr(VI) – Intestinal 1:50

pH 4.8

60K 80K nsity (cps

500 ppb Cr(VI) – Intestinal 1:50 200 ppb Cr(VI) – Intestinal 1:50

II Cr VI

40K 60K Inten

5 ppb standard

50 ppb Cr(VI) – Intestinal 1:50 100 ppb Cr(VI) – Intestinal 1:50

Cr I C

20K

pp ( ) 100 ppb Cr(III) – Intestinal 1:50

0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 14

Lung Fluid: No immediate conversion

120K 140K

Time = 0 pH = 7 4

100K ps)

1000 ppb Cr(VI) – Intestinal 1:50

pH 7.4

60K 80K ensity (cp

500 ppb Cr(VI) – Intestinal 1:50 200 ppb Cr(VI) – Intestinal 1:50

I Cr VI

40K 60K Inte

5 ppb standard

50 ppb Cr(VI) – Intestinal 1:50 100 ppb Cr(VI) – Intestinal 1:50

Cr II C

20K

pp ( ) 100 ppb Cr(III) – Intestinal 1:50

0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 15

Cr(VI) Conversion vs. Time ( )

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SLIDE 16

Removal of Cr(VI) in Tap Water

70000

  • Addition of lemon juice to tap water

spiked with 100 ppb Cr(VI) causes immediate reduction to Cr(III)

Cr(VI) – 100 ppb (tap water) 50000 60000

immediate reduction to Cr(III)

  • 200µL to 10 mL (equivalent of 1 tsp

in 8 oz glass) causes complete conversion to Cr(III)

40000 50000 ensity (cps)

conversion to Cr(III)

20000 30000 Inte + 100 µL lemon juice 87% Cr(VI) converted to Cr(III) 100 b C (III) 100 L l j i + 200 µL lemon juice pH = 3.2 10000 20000 to Cr(III) pH = 3.5 100 ppb Cr(III) + 100 µL lemon juice 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 17

Lemon Juice Converts Cr(VI) to Cr(III) Lemon Juice Converts Cr(VI) to Cr(III)

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SLIDE 18

Effect of Acids on Cr(VI) Stability ( ) y

  • 100 ppb Cr(VI) in tap water
  • Added 100 µL of 2% acid solution to 10 mL

Cr VI

70000

100 ppb Cr(VI) + 100 µL Citric acid (pH=4 1)

50000 60000 cps)

+ 100 µL Citric acid (pH 4.1) + 100 µL Nitric acid (pH=2.7) + 100 µL Hydrochloric acid (pH=3.0)

30000 40000 Intensity (c

+ 100 µL Ascorbic acid (pH=4.7)

10000 20000 10000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 19

NaOCl can cause oxidation of Cr(III)

  • Added 100 µL 5% NaOCl to 100 ppb Cr(III) in tap water

Cr VI

70000

100 ppb Cr(VI)

50000 60000 cps) 30000 40000 Intensity (

100 ppb Cr(III) + 100 µL NaOCl pH = 9.3

10000 20000

100 ppb Cr(III) ~10% conversion

  • f Cr(III) to Cr(VI)

10000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)

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SLIDE 20

Conclusions

  • Larger injection volumes did not dramatically improve IDL for

Cr(VI) in our method

  • Studies on standards shows Cr(VI) stability highly variable and
  • Studies on standards shows Cr(VI) stability highly variable and

can be affected by other elements in the sample

  • Sample preservation and holding times need to address this
  • Stability studies with just Cr(VI) present may not be valid

Stability studies with just Cr(VI) present may not be valid

  • Studies done on effects of stomach acid on Cr(VI) show

immediate conversion to Cr(III) up to 200 µg/L

  • Potential impact on Drinking Water limit for Cr(VI)
  • Potential impact on Drinking Water limit for Cr(VI)
  • Study shows addition of lemon juice or ascorbic acid can

convert > 90% of Cr(VI) to Cr(III) “in the glass” for concentrations p to 200 g/L Cr(VI) concentrations up to 200 µg/L Cr(VI)

  • Addition of NaOCl can oxidize Cr(III) to Cr(VI)
  • Agrees with studies that water chlorination can oxidize Cr(III)