Development of a field method for measuring manganese in welding fume

A. Dale Marcyab and Pamela L. Drake*a

Received 11th April 2007, Accepted 14th August 2007

First published as an Advance Article on the web 30th August 2007

DOI: 10.1039/b705252a

Workers who perform routine welding tasks are potentially exposed to fume that may contain

manganese. Manganese may cause respiratory problems and is implicated in causing the

occurrence of Parkinson-like symptoms. In this study, a field colorimetric method for extracting

and measuring manganese in welding fume was developed. The method uses ultrasonic extraction

with an acidic hydrogen peroxide solution to extract welding fume collected on polyvinyl chloride

filters. Commercially available pre-packaged reagents are used to produce a colored solution,

created by a reaction of manganese(II) with 1-(2-pyridylazo)-2-naphthol. Absorbance

measurements are then made using a portable spectrophotometer. The method detection limit and

limit of quantification (LOQ) were 5.2 mg filter1 and 17 mg filter1

, respectively, with a dynamic

range up to 400 mg filter1

. When the results are above the LOQ for the colorimetric method, the

manganese masses are equivalent to those measured by the International Organization for

Standardization Method 15202-2, which employs a strong acid digestion and analysis using

inductively coupled plasma-optical emission spectrometry.

Introduction

Welders may be exposed to a wide variety of hazardous

substances while performing routine welding tasks.1,2 The

fume that is generated during welding is a complex mixture

of metal oxides that condense from the gas phase to form very

small particles.3 The composition of the fume depends upon

the type of welding process used and the chemical composition

of the metals being joined. A wide variety of methods are used

to join metals together. Some of the more common types are

shielded manual metal arc welding (SMAW), gas metal arc

welding (GMAW), and flux-cored arc welding (FCAW). In

FCAW and SMAW, fluxes are incorporated into the consumable electrodes to shield against atmospheric gases.1,2 Manganese may be present in both the flux material and the metal

electrodes.

Welding fume may contain irritants and can be toxic to the

respiratory system, possibly causing pulmonary edema,

chronic bronchitis, emphysema, pneumonia, asthma, welder’s

lung, and metal fume fever.4 Manganese is one of the components of welding fume that may have serious effects on

health.1,5 Some studies have shown that welders exposed to

fumes that contains manganese may have an increased risk of

developing Parkinson-like symptoms,6–8 while other studies

have found no linkage.9,10 These symptoms include a fixed

gaze, bradykinesia, postural difficulties, rigidity, tremor, dystonia, and decreased mental status. Other studies have shown

that chronic exposure to elevated manganese levels in the

workplace results in increased respiratory problems.11–13

In the United Stated, there are currently three workplace

exposure limits for manganese. The National Institute for

Occupational Safety and Health (NIOSH) recommended

exposure limit (REL) for manganese is 1 mg m3 as an 8 h

time-weighted average (TWA), and 3 mg m3 as a short-term

exposure limit.14 The Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) is

5 mg m3 (ceiling limit).14 The American Conference of

Governmental Industrial Hygienists (ACGIH) has established

a threshold limit value (TLV) of 0.2 mg m3 for manganese.15

In the current analytical methods for determining the

quantity of manganese in welding fume, a sample of the

particles in workplace air is collected on polyvinyl chloride

(PVC) filters or mixed cellulose ester (MCE) filters mounted in

plastic cassettes. Samples are collected using a personal sampling pump at a flow rate of 2 L min1

, over a period of 8 h.

The sample is sent to an accredited laboratory for an analysis

of the total manganese content using strong acid digestion and

measurement by inductively coupled plasma-optical emission

spectrometry (ICP-OES).16,17 The process typically requires

4–6 weeks from the time the samples are collected until the

results of the analysis are reported. With this turnaround time,

it is difficult to relate work practices to exposure levels in a

timely fashion. The ability to measure manganese in welding

fume within 24 h would allow managers, supervisors, and/or

safety personnel to communicate exposure levels to workers

and take corrective actions to reduce exposure.

The goal of this study was to develop a rapid field method

for measuring manganese in welding fume. HACHs method

814918 is a colorimetric method for determining manganese in

water and wastewater. By adapting this method, the manganese is extracted from the filters and converted to the soluble

manganese(II) form. An ultrasonic bath is used to extract the

a US Department of Health and Human Services, Centers for Disease

Control and Prevention, National Institute for Occupational Safety

and Health, Spokane Research Laboratory, 315 E. Montgomery

Ave, Spokane, WA 99207, USA. E-mail: pdrake@cdc.gov; Fax:

+1-509-354-8099

b North Idaho College, 1000 W. Garden Ave, Coeur d’Alene, ID

83814, USA.
fume into an acidic solution of hydrogen peroxide. After

dissolution, 1-(2-pyridylazo)-2-naphthol (PAN) indicator is

added to the solution; it combines with manganese to form

an orange-colored complex. The absorbance is then measured

on a spectrophotometer. In order to ensure that the new

extraction procedure was accurate, precise, and compatible

with the HACHs method, a series of experiments were

conducted to evaluate analytical recovery, stability, method

detection limit, and interferences.

Experimental

Air sample generation and collection

Welding fume samples were collected on PVC filters (37 mm,

5.0 mm pore size, with cellulose backup pad, SKC, Inc., Eighty

Four, Pennsylvania, USA) using a custom-made multiport

sampler. The multiport sampler was connected to a highvolume (15 L min1

) air sampling pump (Gilian AirCon2,

West Caldwell, New Jersey, USA) and it collected eight

samples simultaneously with a measured flow rate of

1.7 (0.1) L min1 for each individual port. All welding was

performed on soft steel using SMAW. Three types of electrodes were used: UTP 711B (o1% Mn; Bohler Thyssen Welding, Stafford, Texas, USA); E11018-M (o5% Mn; Hobart

Welding Products, Appleton, Wisconsin, USA); and ArcTec

41 (16% Mn; ArcTec Alloys Ltd, Calgary, Alberta, Canada).

Materials and equipment

For some experiments, the PVC filters were placed into 15 mL

centrifuge tubes (Falcon, Becton-Dickinson, Franklin Lakes,

New Jersey, USA). The addition of reagents and deionized

water was performed with a mechanical pipette (Rainin Instrument Company, Inc., Franklin Lakes, New Jersey, USA).

A vortex mixer (Barnstead International, Dubuque, Iowa,

USA) was used to mix the samples prior to extraction in the

ultrasonic bath (117-V, 50 Hz, Crest Ultrasonics, Trenton,

New Jersey, USA). An aliquot of the extraction solution was

transferred to a HACHs 25 mL sample cell (2.0 cm path

length) with cap (Loveland, Colorado, USA) and diluted to

10 mL. After the HACHs reagents were added, the absorbance was measured using a DR/2400 spectrophotometer

(HACHs Company, Loveland, Colorado, USA).

For other experiments, the PVC filters were placed into

25 mL beakers (Fisher Scientific, Hampton, New Hampshire,

USA) and sequentially digested with concentrated nitric and

hydrochloric acids on a hot-plate (VWR International, Inc.,

West Chester, Pennsylvania, USA). These samples were analyzed using an ICP-OES (Applied Research Laboratories

34 000, Sunland, California, USA). The ICP-OES is a simultaneous side-view instrument with a pneumatic nebulizer for

sample introduction. The ICP-OES method detection limit for

manganese is 0.5 mg L1 for the conditions and instrument

used for this study. An analytical balance (Mettler Toledo,

model XS105, Columbus, Ohio, USA) was used for weighing.

Reagents and solutions

A 1000 mg L1 multielement standard solution (Mn, Fe, Zn,

Mg, Ca, Cu, Pb, Ti, Co, Ni, Cr) and a 1000 mg L1 Mn

solution (SPEX CertiPrep standard solution in 2% nitric acid,

Metuchen, New Jersey, USA) were used to prepare spiking,

standard, and calibration check solutions. Iron solutions at

1000 mg L1 were prepared by dissolving analytical grade

ferrous ammonium sulfate (EM Science, Cherry Hill, New

Jersey, USA) and ferric nitrate nonahydrate (JT Baker,

Mallinckrodt Baker, Inc., Phillipsburg, New Jersey, USA) in

deionized water (18 MO, Millipore Milli-Q Plus, Molsheim,

France). Analytical grade manganese(IV) oxide was obtained

from JT Baker (Mallinckrodt Baker, Inc., Phillipsburg, New

Jersey, USA). Nitric, hydrochloric, and sulfuric acids were

obtained from OPTIMA (Seastar Chemicals Inc., Pittsburgh,

Pennsylvania, USA). The 50% hydrogen peroxide solution,

ascorbic acid powder pillows, alkaline cyanide reagent, and

PAN indicator solution were obtained from HACHs

(Loveland, Colorado, USA).

Experimental methods

Acidic hydrogen peroxide extraction and colorimetric analysis

The field method for measuring manganese in welding fume

was evaluated by measuring the recovery of manganese that

was added to PVC filters in one of three ways. In the first

procedure, the PVC filters were placed into 15 mL centrifuge

tubes and solid manganese(IV) oxide was weighed directly onto

the filters. In the second method, known concentrations of

manganese(II) in solution were pipetted onto the PVC filters in

the 15 mL centrifuge tubes. In the third technique, welding

fumes were collected on PVC filters in cassettes using a

multiport sampler with a high-volume pump.

The filters were then analyzed using the colorimetric method

as follows. The cassettes were opened and the filters were

transferred to 15 mL centrifuge tubes. Two millilitres of

deionized water were added, followed by 0.5 mL of 6 M

sulfuric acid and 3.0 mL of 3% hydrogen peroxide. The tubes

were capped and the contents were mixed using a vortex mixer,

ensuring that the filters were completely submerged in the

extraction solution. The centrifuge tubes were placed in an

ultrasonic bath and the filters were extracted for 30 min. In

order to allow for complete dissolution of the manganese, the

samples were allowed to set in the acidic hydrogen peroxide

extraction solution for a minimum of 2 h before analysis. After

extraction, 4.5 mL of deionized water was added to the

centrifuge tubes and the contents were mixed using the vortex

mixer. An aliquot of the extraction solution in each of the

centrifuge tubes (100 mL to 250 mL depending on the expected

manganese concentration) was transferred to a HACHs

25 mL sample cell with cap, and diluted to 10.0 mL with

deionized water. Ascorbic acid was added from a powder

pillow, followed by 15 drops of alkaline-cyanide reagent,

and 21 drops of PAN indicator solution. After the addition

of each reagent, the sample cells were capped and inverted to

mix. The color was allowed to develop for twelve minutes

before the absorbance was measured at 560 nm using a

spectrophotometer. The absorbance was compared with five

matrix-matched calibration solutions that were prepared daily,

ranging from 0 mg L1 to 0.70 mg L1

.
ISO method 15202-2—strong acid digestion and ICP-OES

analysis

The ISO method 15202-2 was the standard reference method

against which the accuracy of the colorimetric method was

compared.17 Total manganese in the welding fume collected

on the filters was measured according to the sequential digestion procedure in nitric and hydrochloric acids described in the

ISO method. The solution was analyzed using an ICP-OES.

An ICP-OES blank solution was prepared in 5% hydrochloric acid and the matrix-matched standard solution was

prepared at 10 mg L1 of manganese. Calibration check

solutions were independently prepared at 1.0 mg L1 and

5.0 mg L1 in 5% hydrochloric acid. For all measurements, the

calibration check solutions were within 5% of the standard

value. During the evaluation of potential interferences from

other constituents in welding fume, a standard solution was

prepared in 5% hydrochloric acid at 10 mg L1 of manganese,

iron, zinc, magnesium, calcium, copper, lead, titanium, cobalt,

nickel, and chromium. A calibration check solution was

independently prepared at 1.0 mg L1 in 5% hydrochloric

acid. For all measurements, the calibration check solution was

within 10% of the standard value for all elements.

Results and discussion

NIOSH guidelines for the evaluation of workplace air sampling and analysis methods recommend that a suite of experiments be conducted to validate the method under

investigation.19 The recovery of an analyte from the sampling

medium is evaluated by adding known quantities of an analyte

to a filter and measuring the quantity that can be recovered.

With the data from the recovery experiment, the bias (B) can

be calculated from:

B = [(m/T)  1],

where m is the measured mean and T is the true concentration

of the spike.

The bias must be less than 10% at all spiking levels. For a

method to meet the NIOSH accuracy criteria, the recovery of

the analyte from the filter media should be greater than or

equal to 75% of the true concentration at least 95% of the

time.

In the high temperature, oxidizing environment that is

present during particle formation in welding fume, the manganese is expected to be present as mixed manganese oxide

compounds (manganese(II) manganese(III)oxide and manganese(IV)oxide).20 These compounds must be dissolved before

analysis by colorimetry. The standard methods for analysis

of welding fume use strong acids and high temperatures to

reduce and dissolve the manganese compounds that may be

present. The vapors generated during dissolution require the

use of chemical fume hoods. To avoid using strong acid

digestion, a field method requires a different dissolution

process. An evaluation of standard reduction potentials indicates that hydrogen peroxide in acidic solution is an adequate reducing agent according to the equation:21

MnO2(s)+2H+ + H2O2(aq) # Mn2+ + O2(g) + 2H2O(l)

E1 = 0.53 V

Preliminary experiments that were performed to evaluate the

kinetics of the reaction gave quantitative recoveries (93–105%)

for manganese(IV) oxide on PVC filters. These results demonstrated that an acidic hydrogen peroxide solution is capable of

reducing insoluble manganese(IV) oxide to soluble manganese(II) that is available to react with PAN.

The method published by HACHs18 uses two minutes for

color development. A stability experiment showed that in the

presence of the acidic hydrogen peroxide extraction solution,

the length of time for stable color development needed to be

increased to twelve minutes. The residual hydrogen peroxide

present in the extraction solution depresses the absorbance of

the orange-colored complex that is produced from the reaction

of manganese with PAN; therefore, the calibration solutions

must be matrix-matched to the samples.

To measure the accuracy and precision of the method, six

replicates of manganese(II) ion in solution were pipetted onto

PVC filters at 0.1, 0.5, 1.0, and 2.0 times the TLV for

manganese based on an 8 h TWA and a sampling flow rate

of 2 L min1

.

19 Under these conditions for filter collection, the

loading of a filter at the TLV is 190 mg filter1

. These samples

were extracted and analyzed using the colorimetric method,

after which the percent recovery of the manganese was calculated. The results show that the new colorimetric method

satisfies the desired performance criteria (Table 1). In all

samples, manganese recovery was greater than 97% and the

absolute bias at all levels was o3%.

A group of PVC filters, loaded with fume from the UTP

711B electrode (o1% Mn), were prepared for the limit of

detection (LOD) evaluation19 by spiking aliquots of manganese(II) solution onto the prepared fume samples. The reported

LOD for HACHs method 8149 is 0.007 mg L1

;

18 therefore,

the concentrations of the filter spikes were 0.007 mg L1

,

0.035 mg L1

, and 0.070 mg L1. The filters were extracted

and analyzed according to the colorimetric method. Using the

standard error of the regression and the slope of the response

curve, while correcting for the dilution factors, the LOD was

5.2 mg filter1

. The limit of quantification (LOQ; 3.33 times the

LOD19) was 17 mg filter1

. Under the standard conditions for

filter collection (8 h at 2.0 L min1

), the loading of a filter at

0.1 times the TLV is 19 mg filter1

. Because the new method

has a LOQ lower than 0.1 times the TLV of the

most conservative work place exposure limit (ACGIH,

0.2 mg m3

), the method can satisfy analytical needs.

The accepted methods for determining the amount

of manganese in airborne particulate matter16,17 describe

Table 1 Analysis of manganese(II) nitrate spikes on PVC filters at 0.1,

0.5, 1.0, and 2.0 times the TLV of 0.2 mg m3 using the colorimetric

methoda

Spike quantity/

mg filter1

Mean/mg

filter1  SD % Recovery % RSD % Bias

19 18.4  0.4 97.1 2.3 2.9

95 96.7  2 101.8 2.2 1.8

190 187  5 98.4 2.6 1.6

380 389  4 102.4 2.4 2.4

a N = 6 for all concentration levels. SD = standard deviation.

RSD = relative standard deviation.
collecting the aerosol on PVC or MCE filters, dissolving the

aerosol using nitric and hydrochloric acids, and analyzing the

resulting solution for total manganese with an ICP-OES.

These methods were compared with the colorimetric method

using a multiport sampler to collect air filter samples.

Cassettes prepared for this study using the multiport sampler

contained uniform loadings of manganese in the welding fume,

as demonstrated by the results with the ISO method

(RSD o3%, average manganese content = 26.7 mg filter1

,

N = 8). If the assumption is made that the filters in the eight

cassettes were loaded with equivalent masses of manganese,

the averages of the data produced by the different methods

may be compared statistically.

The multiport sampler was used to collect fume while

welding with ArcTec 41 electrodes (16% Mn). The number

of electrodes used and the sample collection times were varied

in order to obtain a variety of loading levels. Each set of eight

cassettes was split into two groups: four were analyzed using

the ISO method and four using the colorimetric method (Table

2). The means of the manganese masses measured with the

colorimetric method are not statistically different (P 4 0.05)

from the means measured with the ISO method. Additional

samples of fume were collected while welding with another

type of electrode (E11018-M electrodes, o5% Mn). These

fume samples were analyzed to evaluate the performance of

the colorimetric method for analysis of fume containing lower

concentrations of manganese and a different flux composition

(Table 2). The amount of manganese measured is near the

LOQ for the method; the results from the colorimetric method

are statistically different (P o 0.05) from the manganese

measured by the ISO method.

To evaluate the cause of the low recovery of the manganese

in some samples, solutions produced from the dissolution of

welding fume from both types of electrodes and both dissolution techniques were analyzed by ICP-OES (Table 3). The

results illustrated that not all of the compounds that contain

metals in the welding fume were dissolved by the acidic

hydrogen peroxide extraction. The largest concentrations of

ions other than manganese were iron and calcium. The color

development of manganese(II) might have been impacted by

the presence of iron,18 so an interference study was conducted

to evaluate the effect of elevated concentrations of iron(III) or

iron(II) in the extraction solution. Duplicate spike solutions,

equivalent to 4000 mg filter1

of either iron(II) or iron(III), and

190 mg filter1 manganese(II), were prepared. The average

recovery of manganese was 99.6%. This indicated that elevated concentrations of iron in solution did not impact the

color development of manganese with PAN.

To determine if time was a factor that impacted manganese

recovery during the dissolution of the welding fume, samples

were analyzed after the 2 h extraction period and also after 24 h

using an ICP-OES and the colorimetric method (Table 4). The

quantity of manganese measured in the fume from the ArcTec

41 electrode remained constant in the acidic hydrogen peroxide extraction solution; no additional time was required to

dissolve the manganese compounds. The manganese measured

by the acidic hydrogen peroxide extraction and the colorimetric method was equivalent to the amount of manganese

measured by the ISO method (P 4 0.05). The results produced

from analysis of the acidic hydrogen peroxide extraction

solution using an ICP-OES agreed with the manganese measured by the colorimetric method. The iron results revealed

that approximately half of the iron was dissolved by the acidic

hydrogen peroxide extraction when compared to the amount

measured using the ISO method. The results for the E11018-M

electrode show that the quantity of manganese measured by

acidic hydrogen peroxide extraction and the colorimetric

method increases slightly over 24 h. The results of the analysis

of the acidic hydrogen peroxide extraction solution with ICPOES also indicated that both the measured manganese and

iron concentrations increase over time. The difference in the

extraction efficiency appears to be correlated with the ratio of

iron to manganese. In the ArcTec 41 electrode fume, the ratio

of iron to manganese was approximately 1 : 1 and the acidic

hydrogen peroxide extraction dissolved the manganese compounds rapidly. As the ratio of iron to manganese in the fume

Table 2 A comparison of manganese in welding fume produced from

the ArcTec 41 and E11018-M electrodes collected by a multiport

sampler measured using ISO Method 15202-2 or the colorimetric

methoda

Electrode

ISO

method/

mg filter1  SD

Colorimetric

method/

mg filter1  SD

Difference

(%)b

ArcTec 41 104  6 105  7 0.1

186  5 175  6 5.9

E11018-M 18  1 12  1 33

27  1 21  2 22

a N = 4 for all sample groups. SD = standard deviation. b Difference

= [(colorimetric value  ISO value)/ ISO value]  100.

Table 3 A comparison of metals measured by ICP-OES in replicates of welding fume collected with a multiport sampler dissolved using either the

ISO Method 15202-2 or the acidic hydrogen peroxide extractiona

ArcTec 41 electrode E11018-M electrode

Metal/mg filter1  SD ISO method Acidic H2O2

b ISO method Acidic H2O2b

Mn 178  6 175  8 26  1 24  2

Fe 166  9 84  7 74  4 69  6

Ca 21  5 o1 34  10 22  2

Cr 8  0.7 5  3 0.3  0.2 0.4  0.06

Cu 0.4  0.2 o0.1 0.7  0.08 0.3  0.03

Ni 18  0.9 8.6  0.8 2.5  0.4 1.6  0.2

a N = 4 for all sample groups. SD = standard deviation. b H2O2 = hydrogen peroxide
increased (approximately 3 : 1 in the E11018-M electrode

fume), the acidic hydrogen peroxide extraction required additional time to dissolve the manganese compounds. The data

indicated that a portion of the manganese may have combined

with iron in complex oxide structures, such as those described

by Zimmer and Biswas.3 No other metal concentrations that

were included in the multielement analysis using an ICP-OES

in the welding fume correlated with the increases in manganese

concentrations over time.

Based upon the differences in manganese measured in the

extraction solution after 24 h, an experiment was designed to

determine the optimum length of time for dissolution of

manganese into the acidic hydrogen peroxide extraction solution. Using the multiport sampler, additional sets of welding

fumes were collected from the E11018-M and ArcTec 41

electrodes. The mass of welding fume collected from the

E11018-M electrodes was increased to ensure the mass of

manganese would be above the LOQ.

The amount of manganese on four of the filters from each

type of electrode was measured following the ISO method. The

amount of manganese on the four remaining filters from each

type of electrode was measured after 2, 4, 8, 16, 24, and 48 h.

At the end of each time interval an aliquot was removed from

the extraction solution and analyzed using the colorimetric

method. The manganese reported was calculated as the total

manganese extracted after correction for solution volumes.

For the fume from the ArcTec 41 electrodes, the average

amount of manganese measured by the ISO method (293

10 mg filter1

) and the colorimetric method after a 2 h

extraction period (290  27 mg filter1

), was equivalent

(P 4 0.05). For the fume from the E11018-M electrodes, the

average amount of manganese measured by the ISO method

(62  0.8 mg filter1

) and the colorimetric method after a 2 h

extraction period (56  6 mg filter1

) was statistically different

(P o 0.05). After an 8 h extraction, the average amount of

manganese measured by the colorimetric method (64  2 mg

filter1

) was equivalent to the ISO method (P 4 0.05). This

indicates that an extended extraction time may be required to

recover the total amount of manganese when the iron to

manganese ratio is high in the welding fume.

In conclusion, the results presented in this paper demonstrate that the new colorimetric method can be used to make

quantitative measurements of manganese in welding fume.

The workplace air filter samples can be collected according

to standard sampling methods. At the end of the work day, the

welding fume samples can be extracted into an acidic hydrogen

peroxide solution using sonication. Following the extraction,

the samples can be analyzed using commercially available

reagents on a spectrophotometer. If high amounts of iron

are expected in the samples, the extraction time can be

extended overnight and the samples analyzed the following

day. The method can provide accurate and precise values when

compared to values produced by the ISO method. Using

sonication and the colorimetric method, measurements of

manganese exposure in the workplace from aerosols can be

made and reported to workers and managers within 24 h. This

could prove to be a valuable tool for improving work practices

and reducing exposure to manganese from welding fume.

Acknowledgements

The authors gratefully acknowledge Paul Pierce (CDC/

NIOSH) for the precise welding fume samples generated at

the Spokane Research Laboratory.

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Table 4 A comparison of manganese and iron measured in welding fume collected with a multiport sampler using strong acid digestion followed

by ICP-OES analysis (ISO Method 15202-2) or acidic hydrogen peroxide extraction followed by colorimetric method or ICP-OES analysisa

Electrode Extraction method/analysis method Extraction time before analysis Mn/mg filter1  SD Fe/mg filter1  SD

ArcTec 41 Acidic H2O2/colorimetricb 2 h 175  6

Acidic H2O2/colorimetricb 24 h 176  8

Strong acid digestion/ICP-OESc 0 h 178  6 166  9

Acidic H2O2/ICP-OESd 2 h 175  8 84  7

E11018-M Acidic H2O2/colorimetricb 2 h 13  2

Acidic H2O2/colorimetricb 24 h 16  0.4

Strong acid digestion/ICP-OESb 0 h 14  0.4 41  1

Acidic H2O2/ICP-OESd 2 h 13  0.6 26  2

Acidic H2O2/ICP-OESd 24 h 15  0.4 38  1

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