NIOSH Manual of Analytical Methods (NMAM) 5th Edition 

BACKUP DATA REPORT

NIOSH Method No. 6011 

Title: Bromine and Chlorine

Analytes: Bromine and Chlorine

Author/developer: Mary Ellen Cassinelli (Issues 1 & 2), Method revised by

Dawn Farwick (Issue 3) 

Date: July 14, 2022

Disclaimer: Mention of any company or product does not constitute endorsement by the National 

Institute for Occupational Safety and Health, Centers for Disease Control and Prevention. In addition, 

citations to websites external to NIOSH do not constitute NIOSH endorsement of the sponsoring 

organizations or their programs or products. Furthermore, NIOSH is not responsible for the content of 

these websites. All web addresses referenced in this document were accessible as of the publication 

date. 
NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Backup Data Report for NMAM 6011 (Issue 3), Bromine and Chlorine, Dated 14 July 2022 - Page 2 of 18 

Note on NIOSH 6011, Issue 3

In this updated version of NIOSH 6011, Issue 3, the Chlorine and Bromine methods, found separately in

the NMAM 4th edition, were combined. The original data used to develop and evaluate the method may

be found in this report and additionally published in Applied Occupational and Environmental Hygiene

[1].

Introduction

The goal of this research was to develop a reliable sampling and analytical method that uses a solid

sorbent to collect halogen gases and is free from most interferences. One such sorbent is silver. Halogen

gases (X2) react with metallic silver (Ag) to form silver halides (AgX) according to Equation 1 [2]. 

(1) 2Ag + X2 = 2AgX

Silver halides are soluble in ammonium hydroxide (NH4OH), sodium thiosulfate (Na2S2O3), and potassium

cyanide (KCN), forming silver complexes [3] and releasing the halogen to be measured as the halide ion

(X-

) (Equations 2-4). 

(2) AgX + 2NH4OH = Ag(NH3)2

+ + X- + 2H2O

(3) AgX + 2Na2S2O3 = 4Na+ + Ag(S2O3)2

3- + X-

(4) AgX + 2KCN = 2K+ + Ag(CN)2

- + XAll the silver halides are photosensitive. Although in pure form, they are almost unaffected by light, 

many other substances act as sensitizers [4]. Because of photolysis, the samples must be protected 

from light during sampling, storage, and desorption. This is easily done by using opaque samplers and 

vials and by transferring the samples in relative darkness.

The use of metallic silver is a unique approach to sampling for halogen gases in workplace air. Silvered 

zeolite has been used for scavenging radioactive iodine from radioactive noble gases [5]. The use of 

silver films, silver wool, and silver membranes for the adsorption of mercury vapor has been reported in 

literature [6,7]. Silver membrane filters have been used in industrial hygiene for the analysis of silica [8]. 

However, their use as a reactive sorbent for monitoring gases and vapors has not been reported. The

reaction of halogen gases with silver offers a selective means of collecting Cl2 and Br2. Although silver 

may react with compounds that are interferences in other methods, most will produce anions that will 

be separated in the analysis. Coupled with ion chromatography (IC) analysis, the method offers 

convenience and specificity not found in most of the present monitoring methods.

The approach taken in this study was to develop a preliminary analytical procedure, and then evaluate 

commercially available silver sorbents, e.g., silver zeolite and silver membrane filters, for capacity, 

recovery, and compatibility with the analytical technique. The sorbent of choice was then evaluated for 

Cl2 and Br2 by monitoring known quantities of the halogen gases in air, following the guidelines for 

laboratory evaluation stated in the "Development and Evaluation of Methods" section of the NIOSH 

Manual of Analytical Methods, 3rd ed. [9] and the NIOSH statistical protocol [10]. Finally, the chosen 

sampling and analytical method was field tested in a chlorine-containing industrial environment by 

collecting pairs of air samples using the test method and a previously published method.
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Reagents

All chemicals were reagent grade. Deionized water was used throughout for the preparation of 

solutions, eluents, and regenerants. Chloride and bromide working standards were prepared from NBStraceable, 1000 μg/mL solutions (Alltech Assoc., Inc., Deerfield, IL).

Development of preliminary analytical procedure

Determination of Silver Complexing Solution

As shown in equations 1-4 above, silver halides are solubilized by complexing the Ag+ with NH4OH, KCN,

or Na2S2O3. Each of these matrices, in a concentration sufficient to ensure complete dissolution of the

silver halide (6 mM) but dilute enough to avoid overloading the chromatographic column, were tested

for compatibility with IC.

The hydroxide ion (OH-

) matrix produced a broad negative peak that interfered with the quantitation of

Cl-

. The cyanide ion (CN-) caused no interference because the hydrocyanic acid (HCN), which is produced

in the suppressor, is weakly dissociated. However, the bicarbonate added to stabilize the KCN solution

was in sufficient concentration to affect the quantitation of Cl-

. The Na2S2O3 matrix was the most 

compatible with IC.

A 6 mM Na2S2O3 solution was found sufficient to complex the silver and release the halide ion. When

the sample was diluted to 10 mL, the resulting matrix (~2 mM) was sufficiently dilute to avoid

overloading the analytical column. Ruggedization experiments showed that a minimum of 10 min was

required for complete desorption of the sample, although the silver filters may remain in the 6 mM

desorbing solution for up to 24 h. Although the thiosulfate ion (S2O3

2-) had a retention time of 

approximately 20 min when a standard eluent was used, no interfering peaks were produced.

Occasional swirling of the samples during desorption was adequate; no other means of agitation was

necessary. 

Analytical technique

Analyses were performed on a Dionex 4000i Ion Chromatograph with conductivity detector and

AutoRegen accessory (Dionex Corp., Sunnyvale, CA), a WISP 710B autosampler (Waters Div. of Millipore,

Milford, MA), and interfaced with a Hewlett-Packard 3357 Laboratory Data System (Hewlett- Packard,

Avondale, PA).

Ion chromatography was chosen as the analytical technique for its ability to separate the ions of interest

from those that constitute interferences in other techniques. Several buffer combinations were tested

as eluents with the purpose of achieving good resolution of the early-eluting peaks, the peaks of

interest, while shortening the retention time of the late eluting peaks. The eluent developed for use 

with the thiosulfate matrix was a buffer composed of 0.25 mM NaHCO3/4 mM Na2CO3/0.78 mM pcyanophenol. The weak concentration of NaHCO3 allows the separation of the peaks of interest (Cl-

,

ClO2

-

, BrO2

-

, Br-), while the higher concentration of Na2CO3 shortens the retention time of the late

eluting S2O3

2-

. The presence of the p-cyanophenol helps reduce peak tailing, which is ascribed mainly to

adsorption processes rather than ion exchange. Figure 1 illustrates the separation of chloride and

oxychloride peaks that may be achieved when using the above-mentioned eluent, and Figure 2 

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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

illustrates the separation of bromide and oxybromide species. The sulfate peak is the result of partial 

decomposition of the thiosulfate matrix.

Figure 1. IC separation of chloride (2), chlorite (1), chlorate (3), sulfate (4), and thiosulfate (5).
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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Figure 2. IC separation of bromate (1), bromide (2), sulfate (3) and thiosulfate (4).

Silver Sorbent Selection

Silvered zeolite and silver membrane filters were evaluated for background and for the ability to collect 

Cl2 and Br2. Blank values were determined by carrying unexposed samplers through the analytical 

procedure.

Silvered zeolite (Hi-Q Environmental Products Co., La Jolla, CA) tubes were prepared and tested. 

Weights of 150 and 300 mg of the sorbent were packed into 4-mm ID glass tubes and held in place with 

glass wool plugs. The tubes were opaqued by wrapping with tape. While silvered zeolite readily 

collected Cl2, the sorbent contained a large amount of NO3

- and had an extremely high background in the 

early portion of the chromatogram where Cl- and Br- elute. Cleaning the silvered zeolite with deionized 

water in a Soxhlet extractor reduced the background of water extracted blanks. However, when blanks 

were extracted with a complexing agent, much higher levels of interfering substances were found. 

Several attempts were made to clean the sorbent with complexing agents, e.g., NH4OH, KCN, Na2S2O3, 

but each successive treatment produced more interferences. Therefore, silvered zeolites tubes were 

excluded from further evaluation.

The silver filter sampler tested was a 25-mm, carbon-filled (opaque) polypropylene cassette with a 50-

mm (2-inch) extension (Nuclepore Corp., Pleasanton, CA) containing a thiosulfate cleaned, 25-mm silver 

membrane filter, 0.45-μm pore size (Nuclepore or Poretics Corp., Livermore, CA), a prefilter (described 

later), and porous plastic support pads (Nuclepore Corp.) (Figure 3). 
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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Figure 3. Silver filter sampler

Chromatograms of silver filters showed a large interfering Cl- peak, which required removal by cleaning. 

The cleaning procedure was optimized with respect to the concentration of the complexing agent and 

desorption time. The final cleaning procedure used 6 mM Na2S2O3 as a complexing agent to remove the 

Cl-

. The filters were desorbed for a minimum of 10 minutes with occasional swirling. The filters were 

rinsed thoroughly with deionized water and allowed to dry before use.

The need for the exclusion of light during transfer of the silver filters from the cassettes to the 

desorption vials was tested by exposing silver filter samples to laboratory fluorescent light for periods of 

15 min and 1 h. No loss of sample was found for the filters exposed for 15 min; however, samples 

exposed to light for 1 h showed a loss of ~11%. To avoid any losses due to photodecomposition, 

desorption under dim or red light is recommended. 

Pre-filter

To collect any potentially interfering particulate matter, a prefilter was needed that would allow Cl2 and 

Br2 to pass through freely. Ten filter types (eight materials) were tested for use as prefilters for retention 

of Cl2 at both high and low RH: Teflon (Teflon, 0.2-μm, Gelman and Zefluor, 0.5-μm, Gelman); polyester 

(0.4-μm, Nuclepore); polypropylene (0.2-μm, Gelman); mixed cellulose ester (MCE) (Type AA, 0.8-μm, 

Millipore); polycarbonate (0.4-μm, Poretics); polyvinylchloride (PVC, 5-μm, Nuclepore and VM-1, 5-μm, 

Gelman); nylon (Nylaflo, 0.45-μm, Gelman); and glass fiber (Type A/E, Gelman). Filters were tested for 

use as prefilters by placing the filter in the front stage of a 3-piece cassette and a silver filter in the last 

stage (Figure 3). An atmosphere of Cl2 (~2 mg/m3

) was sampled at 1 L/min for 15 min. After sampling, 
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the prefilters were desorbed in deionized water and analyzed by IC for any adsorbed Cl2. The silver

filters were analyzed for Cl2 according to the analytical procedure, and the results compared with those

of samplers without a prefilter (Table 1). Prefilters found acceptable for Cl2 were also tested for Br2 by

sampling an atmosphere of ~6 mg/m3 at 0.3 L/min for 30 min.

Table 1. Summary of prefilter study

Prefilter RHa Cl2 on 

prefilter 

(%)

Total Cl2

foundb (%)

Acceptance 

Criteria: 2% or 

less on 

prefilterc 

Acceptance 

Criteria: 

recovery of 

100 ± 5%c 

Zefluor Low 0 99 P P

High 0 105 P P

Polyester Low 0 101 P P

High 0 96 P P

Polypropylene Low 2 88 P F

High 0 109 P F

Teflon Low 0.5 104 P P

High 0.2 113 P F

MCE Low 3 92 F F

High 1.7 136 F F

Polycarbonate Low 2.2 65 F F

High 1.2 87 P F

Gelman VM-1 Low 12 98 F P

High 7 147 F F

Nuclepore PVC Low 10 20 F F

High 16 34 F F

Nylaflod High 16 114 F F

Glass Fiberd High 27 115 F F a

Relative humidity b

Based on Cl2 atmosphere of 1.8 mg/m3

c P = pass; F = fail

d Prefilter not evaluated at low humidity

Silver filter evaluation

Test atmosphere generator

All tests on the silver filters were done using active generation. The halogen gas generator was a 

dynamic flow system shown schematically in Figure 4. All components exposed to the test atmosphere 

were constructed of Teflon or glass. 
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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Figure 4. Halogen gas generation system

The source of Cl2 was a compressed gas cylinder (Scott Specialty Gases, Troy, MI) containing a mixture of 

49 ppm Cl2 in nitrogen (in a few preliminary experiments the Cl2 source was a permeation tube). The 

Br2 source was a permeation tube (VICI Metronics, Santa Clara, CA) maintained at 30 °C in a 

Dynacalibrator Model 340 permeation tube oven (VICI Metronics, Santa Clara, CA). The permeation 

tube had an emission rate of 6.06 μg/min at 30 °C in a carrier flow of 0.299 L/min. The emission rates of 

Br2 source permeation tubes were verified by determining their weight loss over time. The resulting 

output (20.3 mg/m3

) entered the mixing chamber where it was diluted with air to concentrations of 0.07 

to 1.4 mg/m3

. 

The dilution air and the test gas, which were regulated by mass flow controllers (Tylan Corp., Torrence, 

CA), passed through two mixing chambers to ensure homogeneity. The test gas mixture then passed 

through an eleven-port sampling manifold at flow rates of roughly 10 to 15 L/min, depending on the 

volume of dilution air required to produce a given concentration. Sampling rates were controlled by 

critical orifices at nominal rates of 0.3 and 1 L/min. Halogen samples were collected at flow rates of 0.3 

L/min for TWA exposures and at 1 L/min for STEL exposures. The effluent from the manifold was 

continuously monitored for relative humidity (RH) with a Mason-type wet bulb/dry bulb hygrometer, 

and for concentration with a chlorine meter (Model 1340, Interscan Corp., Chatsworth, CA) interfaced 

with a strip chart recorder (Model 056, Hitachi, Ltd., Tokyo, Japan). 

The Interscan Chlorine Meter was calibrated by means of a Cl2 permeation tube with an output of 19.43 

μg/min in a carrier gas flow of 0.299 L/min. Dilutions of the Cl2 output corresponding to 0.1 to 10 ppm 

at low RH were used to calibrate the Cl2 meter (meter scale is in ppm). Meter readings at other dilutions 

then were checked against the calculated values. The concentration of the Cl2 mixture in the 

compressed gas cylinder was verified by setting the mass flow controllers for the cylinder gas and 

dilution air to produce a meter reading of 1 ppm. From the gas volumes corresponding to the mass flow 

controller settings, the concentration of the cylinder gas was calculated at 49.8 ± 1.0 ppm (147 ± 3 

mg/m3

) (n=4). The supplier reported 49 ppm (145 mg/m3).

The Cl2 meter responded to Br2 at approximately one tenth of the response to Cl2. 
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Except for the electronic components and the compressed gas cylinder, the generator was placed in a

fume hood for reasons of safety.

Independent methods

The test atmosphere concentration as calculated from the analyte gas input and dilution flows was used

as the reference concentration throughout the laboratory evaluation. A modification of OSHA Method

ID-108, [11] which uses a midget bubbler containing a carbonate/bicarbonate buffer solution, was used

in the laboratory as an additional independent determination of concentration. The continuous monitor 

was used chiefly as a tracking mechanism to verify constant concentration. For the Cl2 field study, the

independent method was OSHA Method ID-101 [12], which uses a midget bubbler containing a 0.1%

sulfamic acid solution.

Shelf life of cleaned silver filters

The shelf life, i.e., storage time before deterioration, was examined for cleaned silver filters. As received

from the manufacturer, silver filters have a high AgCl background that is removed by treatment with a

Na2S2O3 solution (see silver sorbent selection). Silver filters were cleaned and then stored under three 

separate conditions: (1) assembled in the cassettes, where light is excluded; (2) separated by paper disks

and returned to the manufacturer's package, where air and light are excluded; and (3) placed in a plastic

bag, exposed to light but not to air. Each month three filters from each group were observed and

analyzed. Over a period of 8 months, there were no changes in the appearance of the filters or in the

analyzed blank values. At the end of this time, the filters were used to sample a Cl2 atmosphere (~0.35

mg/m3

, low RH at 1 L/min). The filters stored in the manufacturer's package gave the best recovery at 

101%. Filters stored in the plastic bag had a 94.1% recovery, but those stored in the cassettes had the

poorest recovery at 88.1%. The reason for this was unclear. There may be sufficient leakage around the 

cassette plugs to allow airborne contaminants to enter and react with the filter, or the cassettes may

off-gas during the long storage period. Thus, the stability of cleaned silver filters appears to be more

affected by exposure to air than to light. Although the differences in recovery for the three storage

conditions are not statistically significant at the 95% confidence level, the recommended long-term

storage mode for cleaned silver filters is to repack them between paper disks in the manufacturer’s

container, because recovery from this mode of storage was essentially complete.

Desorption and sample analysis

The desorption procedure was conducted under dim or red light because silver halides are

photosensitive. For each sample, the silver sorbent was transferred to an amber or opaque, widemouth desorption vial (30 mL). A 3-mL aliquot of 6 mM Na2S2O3 was added. The vial was capped and

allowed to stand, with occasional swirling, for a minimum of 10 min. Once desorbed, the samples are no

longer photosensitive. The sample volume was then brought to 10 mL by the addition of 7 mL of

deionized water (more concentrated samples may require further dilution.) The sample was then

poured into an autosampler vial or a 10-mL plastic syringe for manual injection into the IC. The IC

analytical conditions are listed in Table 2. 

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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Table 2. IC Conditions

Columns Dionex AS4A, AG4A, MFC-1 

Suppressor AMMS Anion Micromembrane

Detector Conductivity

Sensitivity 10 μS full scale

Eluent 0.25 mM NaHCO3/4 mM Na2CO3/0.78 mM p-Cyanophenol at 2 mL/min

Injection Vol. 50 μL

Column Maintenance.

The silver thiosulfate anion, Ag(S203)2

3-, produced in the desorption process has a strong affinity for the 

stationary phase of the exchange resin and can only be eluted using very strong eluents. The result is a 

reduction of ion exchange sites, degradation of peak resolution, and shortening of retention time.

Although Ag(S203 )2

3- is somewhat weakly complexed (log β2=12.78) and is likely to dissociate under the

chromatographic conditions used, the presence of excessive amounts of silver also produces a 

deleterious effect on column performance. Normal column cleaning procedures were ineffectual. The 

problem was partially solved by the addition of a metal-free precolumn (Dionex MFC-1) , which is 

designed to extract trace metals from the eluent stream. The most effective column reconditioning 

procedure was to pump through only the columns at 2 mL/min: 30 mL of deionized water to rinse, 60 

mL of 1 M HN03 to remove the contaminants, 30 mL of 0.1 M Na2C03 to remove the N03

-

, then eluent to 

equilibrate. Reconditioning is recommended after 100 to 150 analyses.

Breakthrough and capacity of silver filters

Breakthrough volume is defined as the volume at which the analyte concentration in the effluent 

compared to the concentration in the influent is significant (usually 5%). Capacity is defined as the mass 

of analyte collected by the sampler at the breakthrough volume. Breakthrough and capacity of the silver 

filter were investigated at both high and low RH. Generated halogen atmospheres were sampled with 

silver filters, which had either a Draeger Short-Term Detector Tube (Chlorine 0.2/a, 0.2 - 3 ppm) or a 

second silver filter as a backup sampler. Br2 reacts with the chlorine detector tubes with about the same 

sensitivity as Cl2. Breakthrough was indicated by the Draeger tubes and quantitated with the silver filter 

backup samplers.

In these experiments, Cl2 was sampled at a rate of 1 L/min. The concentration of Cl2 at high RH was 4.5 

mg/m3 (1.5 x OSHA PEL), at low RH 3 mg/m3 (1 x OSHA PEL). At low RH (~20%), the breakthrough volume 

of the silver filter for Cl2 was approximately 67 L resulting in a mean capacity of approximately 198 μg of 

Cl2 (Table 3). At high RH (~80%), however, a dramatic increase was seen. The breakthrough volume was 

reported to be 1150 L and the capacity reported to be 2681 μg. These values do not match each other 

for a reported challenge concentration of 4.5 mg/m3

. However, the breakthrough volume is clearly 

increased at the higher RH. For Br2 sampled at 0.3 L/min, a similar variation was seen. At low RH, the 

breakthrough volume was 70 L with a capacity of approximately 360 μg Br2. The breakthrough 

experiment for Br2 at high RH was terminated when no breakthrough had occurred after 24 h of 

sampling. The maximum sample volume is calculated as: maximum volume = 0.667 x observed mass 

capacity ÷ PEL. This calculation was done using the capacity of the silver filter at low RH, as that was the 

more limiting condition. Thus, for Cl2: Maximum Sampling Volume (L) = 0.667 x (198 μg) ÷ PEL (2.9 

μg/L) = 46 L (rounded to 45 L); and for Br2: Maximum Sampling Volume (L) = 0.667 x (355 μg) ÷ PEL 

(0.653 μg/L) = 363 L (rounded to 360 L). Therefore, the maximum sample volume is 45 L for Cl2 and 360

L for Br2. 
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The upper limit for the working range of air concentrations for a given sampling volume can be

calculated be rearrangement of this same equation:

Upper limit of the working range (mg/m3

) = (0.667 x observed mass capacity)/sampling volume

For Cl2: Upper limit of the working range for 45 L sampling volume = (0.667 x 198 μg)/45 L = 2.93 mg/m3

For Br2: Upper limit of the working range for 360 L sampling volume = (0.667 x 355 μg)/360 L = 0.658 

mg/m3

Table 3. Breakthrough volume and capacity of silver filters

Brominea Chlorineb

Humidity High Low High Low

Concentration 

(mg/m3)

1.4 5.6 4.5 3

Concentration 

level (x OSHA PEL)

2 8 1.5 1

Number of tests 5 3 3 3

Breakthrough 

Volume (± s) (L)

>430 70±7 1150±199 67±22

Capacity (± s) (μg) >596 355±76 2681±14 198±37 a

Sampling rate ~0.3 L/min; bSampling rate ~1 L/min

The data in Table 3 illustrate the extraordinary effect that high humidity had on the capacity of the silver 

filter for both Cl2 and Br2. This increase may be the combined result of two phenomena, long residence 

time in the filter and dissociation. Gas molecules have a remarkably long residence time in membrane 

filters with an estimated 1010 collisions with the walls before exiting the filter [13]. With an increase in 

the number of water molecules present, some of the halogen molecules will collide with the water and 

dissociate. Since disproportionation occurs very rapidly [3], the resulting hypohalous acid may then 

oxidize the Ag, as illustrated by the following reactions for Cl2. 

Cl2 + H2O = HOCl + HCl (5)

2Ag + HOCl = Ag2O + HCl (6)

Ag2O + 2HCl = 2AgCl + H2O (7)

The kinetics of the above reactions are probably more favorable than the direct reaction with silver (the 

predominant mechanism at low RH), particularly since the HCl produced by disproportionation would 

react with any oxidized silver already present on the filter. 

Storage Stability

A storage stability study was done to determine the maximum length of time between collection and 

analysis during which a sample may be stored without deterioration. Generated Cl2 samples were 

stored at both ambient (25 °C) and refrigerated (5 °C) temperatures. Br2 samples were stored at room 
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temperature only. Groups of samples were analyzed on the day following generation (day 1), and then

weekly for at least 4 weeks. Samples for the storage stability study were generated in five sets of six to

seven samplers each and stored in the opaque cassettes. Samples analyzed for each storage time period

were randomly drawn from the pooled sets. Cl2 samples were collected at 1 L/min for 15 min. Br2

samples were collected at 0.3 L/min for 30 min. 

Recoveries are expressed as percentage of the mean concentration for samples analyzed on day 1.

Chlorine recoveries were 103 ± 4% for 30-day ambient storage (n=4), 101 ± 3% for 64-day refrigerated

storage (n=4), and for samples refrigerated for 30 days and then stored at ambient for an additional 34

days, 103 ± 3% (n=3). Bromine recovery after 30 days was 99.4 ± 5.5% (n=5) and after 60 days was 99.2 ±

10.1% (n=6). Therefore, samples are stable at ambient temperature for at least 30 days (Cl2) or 60 days

(Br2) and probably longer.

Interferences

The possible interference of particulate halides or oxyhalides was determined by spiking approximately

100 μg of the appropriate salt in solution onto a prefilter, drying overnight, then sampling a halogen

atmosphere. Testing was done with both Zefluor and polyester prefilters. The data show that chlorinecontaining particulates (chlorides, chlorites, chlorates, and perchlorates) are not an interference in the

collection of Cl2, as shown by recoveries of 94.2 to 98.2%. Likewise, bromine-containing particulates

(bromides and bromates) do not interfere with Br2, as shown by recoveries of 93.9 to 100.5%.

The interference of hydrogen sulfide (H2S) was determined by sampling a generated atmosphere. The

source of H2S was a permeation tube with an output of 4.2 μg/min in a carrier stream of 0.299 L/min.

Because the resulting concentration was 14 mg/m3

(1 x OSHA PEL), H2S samples were collected directly

from the output of the permeation tube oven either before or after sampling an atmosphere of Cl2. The 

results show that H2S sampled before Cl2 diminished the collection of Cl2 by 11% (Table 4). 

Table 4. Effect of Hydrogen Sulfide on chlorine recovery

Sampling order RH n Cl2 Concentration (mg/m3) Cl2

a Recovery (%)

1st 2nd

H2Sb Cl2 Low 3 0.315±0.006 89

Cl2 H2S Low 3 0.225±0.104 64

Cl2 H2S High 2 0.136±0.007 39 a

Based on Cl2 concentration of 0.354 mg/m3

b

H2S concentration = 14 mg/m3

However, when H2S was sampled after Cl2, losses were from 36% at low RH to 61% at high RH. These 

data suggest that the sulfide ion (S2-) displaces Cl- from AgCl and that the process is facilitated by the 

presence of water.

2AgCl + H2S = Ag2S + 2HCl (8) 

This hypothesis is supported by the fact that the solubility product constants for AgCl and AgBr are on 

the order of 10-10 and 10-13, respectively, as compared with 10-49 for Ag2S [14]. Accordingly, H2S is a 

negative interference.
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Chlorine dioxide (ClO2) is used with Cl2 as a bleaching agent in the pulp and paper industry. The ability of

the silver filter to react with ClO2 to form silver chlorite (AgClO2) was investigated, although it has been

shown that the chlorite ion (ClO2

-

) can be separated from Cl

-

in the analysis (Figure 1). ClO2 was

generated by passing Cl2 through a concentrated solution of sodium chlorite (4.42 mM NaClO2) in a

bubbler [15]. 

Cl2 + 2NaClO2 = 2ClO2 + 2NaCl (9) 

Samples were collected on silver filters and reference bubblers for time periods of 15, 30, and 60 min at

concentrations equivalent to 3 and 4.5 times the OSHA PEL for ClO2. The bubblers contained 15 mL of a

buffer solution (0.25 mM NaHCO3/4 mM Na2CO3/0.78 mM p-cyanophenol). The silver filters contained

no ClO2; however, the backup bubblers that were used with the 30-min samples contained

approximately 86% of the ClO2 in the generated atmosphere (86% recovery is within the bias found for

bubblers in this study). Therefore, ClO2 is not an interference in the test method for Cl2. 

Hydrogen chloride and hydrogen bromide were also tested as interferences. Theoretically, the hydrogen

halides (HX) will not react with metallic silver because they are not oxidizing gases. Nonetheless, HCl and

HBr were collected by the silver filter. The plot in Figure 5 shows that the interference from HCl is

minor, but HBr poses a more significant interference problem.

Figure 5. Effect of sampling time on mass of HBr and HCl collected by silver filters
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Approximately 15 μg of HCl were collected by the filter within the first 15 min of sampling and this mass

remained constant for up to 16 h. The uptake of HBr, however, increased with time at a rate of

approximately 0.4 μg/min. While collection was incomplete for HBr, the increase in uptake presents a

positive interference that is not readily correctable. The presence of HCl or HBr in an atmosphere may

be determined using NIOSH Method 7907 [16]. 

If HCl or HBr were physically adsorbed on the surface of the filter, it could be removed with water. A

water wash of the filter produced no Cl

- or Brin the resulting solution. A review of the literature 

revealed that oxygen is chemisorbed on metallic silver forming a definite stoichiometric compound [4]. 

Therefore, it was postulated that HX had reacted with oxidized silver (presumably Ag2O) on the surface

of the filter to produce silver halide (AgX), according to the following reactions [3]. 

Ag2O + HX = AgX + AgOH (10) 

AgOH + HX = AgX + HOH (11) 

If the hypothesis holds, the degree of filter oxidation would be the determining factor in the 

interference from HCl and HBr. Thiosulfate-cleaned silver filters, which were oxidized by aging for 

periods of 1, 14, and 30 days, were used to sample an atmosphere of HBr. Recoveries were determined 

based on silica gel tube data (NIOSH Method 7903) (This method is no longer recommended for use but

is referenced here as it was used in this method development) [17]. (Note: When silica gel tubes were 

used to sample atmospheres of Cl2 and Br2, they collected approximately 30% of the free halogen in the 

atmosphere. Thus, a correction for the presence of hydrogen halides in an atmosphere containing a 

halogen gas is not a simple subtraction.) Recovery of HBr was 46% for filters aged 1 day, 68% for 14 

days, and 89% for 30 days aging. These data suggest that oxidized silver was the basis for the reaction 

with hydrogen halides. To remove the oxidized silver, the silver filters were treated with aqueous HBr 

or HCl. Overnight soaking in acid did not effectively remove the oxidized silver since filters so treated 

collected 36% of the HBr atmosphere. Hence, it was assumed that Ag2O was being formed continuously

during sampling. The uptake of HX, particularly the continued uptake of HBr with time, may be 

explained partially by the ability of hydrogen halides to form complexes. HX reacts initially with Ag2O to 

form AgX (Eqs. 12 & 13), and then it reacts with AgX [3]. 

AgX + HX = H+ + (AgX2)

-

 (12) 

(AgX2)

- + HX = H+ + (AgX3)2- (13) 

When the ligand is Cl-

, the reaction will proceed to the complex (AgCl3)

2-, and when the ligand is Br-

 

(AgBr4)

3- will be formed [18]. The limited uptake of HCl versus the continuing uptake of HBr may be due, 

in part, to kinetics, or it may be a function of the greater stability of Br- complexes (log β4 = 9.5) over Clcomplexes (log β3 = 6.0). Although we have no confirming experimental evidence, the formation of 

silver halide complexes seems a reasonable explanation for the positive interference from HBr, and 

possibly HCl. Therefore, there appears to be no simple correction for these positively interfering 

compounds. 
NIOSH Manual of Analytical Methods (NMAM) 5th Edition

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Method validation

Accuracy and precision

Samples were generated at concentration levels of 0.1, 0.5, 1, and 2 x PEL (6/level) and at both high and

low humidity. The test atmosphere concentration was determined from the analyte gas source and

dilution flows. The mean recovery of Cl2, sampled for 15 min at 1 L/min, was 98.6% (96.2% at high RH

and 101% at low RH). The sampling and analytical precision, the pooled relative standard deviation (sr),

was 6.7% for both high and low RH data (Tables 5 & 6). The estimate of the total coefficient of variation

(CVT), including 5% pump error, was 7.5%.

Table 5. Method validation for chlorine at high humidity (~80%)

Concentration 

Level (x PEL)

n Concentration takena

(mg/m3)

Concentration found 

(mg/m3)

sr Recovery (%)

0.1 6 0.473 0.478 0.103 101

0.5 5 1.45 1.37 0.041 94.5

1 6 2.78 2.58 0.066 92.7

2 6 6.77 6.53 0.030 96.4

Mean recovery = 96.2%; Precision (pooled sr) = 0.067 a

Concentration determined from generator source and dilution flows where sampling rate=1 L/min and sampling 

time=15 min.

Table 6. Method validation for chlorine at low humidity (~20%)

Concentration 

Level (x PEL)

n Concentration takena

(mg/m3)

Concentration found 

(mg/m3)

sr Recovery (%)

0.1 6 0.354 0.347 0.086 98.0

0.5 6 1.45 1.55 0.094 107

1 6 2.78 2.70 0.024 97.1

2 6 6.12 6.24 0.031 102

Mean recovery = 101%; Precision (pooled sr) = 0.067 a

Concentration determined from generator source and dilution flows where sampling rate=1 L/min and sampling 

time=15 min.

The mean recovery for Br2, sampled for 4 hr at 0.3 L/min, was 98.8% (96.1% and 102% for high and low 

RH, respectively). The sampling and analytical precision was slightly better for low RH at 3.2% over 5.8% 

at high RH (Tables 7 & 8), and the CVT estimate was 6.9%. 

Table 7. Method validation for bromine at high humidity (~80%)

Concentration 

Level (x PEL)

n Concentration takena

(mg/m3)

Concentration found 

(mg/m3)

sr Recovery (%)

0.1 6 0.069 0.067 0.097 97.1

0.5 5 0.346 0.336 0.044 97.1

1 6 0.639 0.582 0.027 91.1

2 6 1.21 1.20 0.035 99.2

Mean recovery = 96.1%; Precision (pooled sr) = 0.058 a

Concentration determined from generator source and dilution flows where sampling rate=0.3 L/min and sampling 

time=4 hr.
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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Table 8. Method validation for bromine at low humidity (~20%)

Concentration 

Level (x PEL)

n Concentration takena

(mg/m3)

Concentration found 

(mg/m3)

sr Recovery (%)

0.1 6 0.076 0.077 0.039 101

0.5 6 0.351 0.358 0.033 102

1 6 0.696 0.673 0.026 96.7

2 6 1.42 1.50 0.028 106

Mean recovery = 102%; Precision (pooled sr) = 0.032 a

Concentration determined from generator source and dilution flows where sampling rate=0.3 L/min and sampling 

time=4 hr.

All data were within the accuracy criterion of ±25% at the 95% confidence level and coefficient of 

variation (precision) recommended for a valid NIOSH method [10]. Bartlett's test for homogeneity was 

applied to the data before pooling. The mean recovery for the overall method (Cl2 and Br2) was 98.7% (-

1.3% bias) with a sampling and analytical precision of 5.2%, and a total coefficient of variation estimate 

(CVT) of 7.2%.

Instrument LOD/LOQ and linear range

The limit of detection (LOD), the smallest amount of analyte that can be distinguished from background, 

was defined in this study as three times the standard error of the calibration graph divided by its slope. 

The limit of quantitation (LOQ) was defined as ten times the standard error of the calibration graph 

divided by its slope [9]. The LOD of Cl- in the thiosulfate matrix was 0.06 μg/mL with a limit of 

quantitation (LOQ) of 0.19 μg/mL. For Br-

, the LOD and LOQ were 0.16 and 0.54 μg/mL, respectively. 

The linear range for Cl- at the sensitivity setting used in this work (10 μS) was 0.05 to 5 μg/mL, and for 

Br-

, 0.20 to 15 μg/mL. The LOD and LOQ may be reduced and linear range proportionally extended at the 

lower end by using a more sensitive instrumental setting, e.g., 3 μS or 1 μS full scale. Similarly, the linear 

range can be extended at the upper end by using a less sensitive instrumental setting or by diluting the 

sample.

The lower limit for the working range of air concentrations for a given sampling volume can be 

calculated based on the limit of quantitation (LOQ) of the analytical method and the liquid sample 

volume. The lower limit for the working range of air concentrations (mg/m3

) for a given sampling 

volume = LOQ (μg/mL) x liquid sample volume (mL) ÷ air sample volume (L): 

For Cl2: Lower limit of the working range for 45 L sampling volume = 0.19 μg/mL x 10 mL ÷ 45 L = 0.04

mg/m3

. 

For Br2: Lower limit of the working range for 360 L sampling volume = 0.54 μg/mL x 10 mL ÷ 360 L = 

0.015 mg/m3

. 

Cl2 field study

Field samples for Cl2 were collected at a plant that produces trichloroisocyanuric acid. The compound is 

prepared by treating cyanuric acid with Cl2. Paired silver filter samples and sulfamic acid bubblers (OSHA 

Method ID-101) [12] were collected at rates of 0.5 and 1.0 L/min, respectively. The results are presented 

in Table 9. 
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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

Table 9. Chlorine field study results using paired silver filters and bubblera samples

Pair number Sampler 

numbers

Air volume (L) Mass Cl2 (μg) Concentration 

(mg/m3)

Differenceb

(mg/m3)

1 Filter – 1 189 134 0.71 0.10

1 Bubbler – 1 377 230 0.61

2 Filter – 3 211 122 0.578 0.12

2 Bubbler – 4 305 140 0.459

3 Filter – 13 167 368 2.20 -0.02

3 Bubbler – 11 239 530 2.22

4 Filter – 11 157 257 1.64 -0.10

4 Bubbler – 13 270 470 1.74

5 Filter – 10 152 41.4 0.272 0.14

5 Bubbler – 12 313 43.0 0.137

6 Filter – 4 218 460 2.11c

6 Bubbler – 2 361 1700 4.71

7d Filter – 2 185 11.5 0.062

7d Bubbler – 3 342 4.8 0.014

8d Filter – 12 158 NDe NDe

8d Bubbler - 14 293 1.6 0.006 aOSHA Method ID-101 bDifferences not statistically significant at 95% confidence level

cProbable breakthrough of silver filter

dBackground levels in plant

e None detected, detection limit = 0.6 μg/sample

Pairs 1 through 6 were area samples collected at the production site. Pairs 7 and 8 were collected in the 

plant but away from the chlorination operation to ascertain background levels of Cl2. In pair 6, possible 

breakthrough of the silver filter occurred, as evidenced by the large mass of Cl2 found in the bubbler. 

The mean difference for pairs 1 to 5 was 0.048 mg/m3

. When a paired t-test for determining differences 

in paired observations [19] was applied to these data, the results showed that the difference between 

the two monitoring methods was not statistically significant at the 95% confidence level. However, since 

the number of samples taken at this field site was limited, more field studies must be done to verify the 

soundness of the significance statement.

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NIOSH Manual of Analytical Methods (NMAM) 5th Edition

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