Claire C.
Austin
*a,
Brigitte
Roberge
b and
Nicole
Goyer
b
aUniversité du Québec (UQTR), correspondence address at the National Research Council (NRC) of Canada, Fire Research Program, Bldg M59 1200 Montreal Road, Ottawa, Canada K1A 0R6. E-mail: claire.austin@nrc.gc.ca; Tel: +1 514 736 5266
bIRSST – Institut de recherche Robert-Sauvé en santé et sécurité du travail, Montréal, Québec, Canada
First published on 23rd November 2005
Ideally, the response of electrochemical detectors is proportional to the concentration of targeted airborne chemicals and is not be affected by concomitantly present substances. Manufacturers provide a limited list of cross-sensitivities but end-users have anecdotally reported unexpected interferences by other substances. Electrochemical detectors designed to measure airborne levels of CO, H2S, NO, NO2, or SO2, were challenged with potentially interfering substances in the absence of target analytes. Cross-sensitivities undocumented by the manufacturers were observed and were found to vary between different models of instruments for the same challenge chemical.
Manufacturers of electrochemical detectors normally supply a limited list of cross-sensitivities known to affect a particular model of instrument. Cross sensitivities documented by manufacturers of various carbon monoxide (CO), hydrogen sulfide (H2S), nitric oxide (NO), nitrous oxide (NO2), or sulfur dioxide (SO2) detectors include: acetylene, carbon monoxide, chlorine, ethylene, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen sulfide, nitric oxide, nitrogen dioxide, sulfur dioxide, ethylene, hydrogen, nitric oxide, nitrous oxide, and hydrogen sulfide. Presentation of this information in the product literature is generally confusing and open to interpretation. There is little or no information concerning test conditions, and it is not specified whether or not the challenge gas was present alone or together with the instrument’s target chemical.
Two previous studies found that Draeger’s Datalogger 190 carbon monoxide detector gave false readings in the presence of ammonia, but the ammonia concentration was not specified.1,2 It was found that the detector was unaffected by the presence of a mixture of 2.5% methane and 0.6% propane, or by 18 ppm NO2, 9 ppm H2S, or 21 ppm SO2.3 The authors did, however, find that it responded strongly to the presence of 40 ppm NO, 500 ppm H2, or 1% ethylene, giving readings of 77 ppm, 33 ppm and 200 ppm, respectively. Two studies evaluated H2S electrochemical detectors but they did not include the Biosystems, BW, or ISC instruments.4,5 There is little or no other information to be found in the scientific literature concerning electrochemical detector cross-sensitivities.
Field technicians and industrial hygienists using direct-reading electrochemical detectors have anecdotally reported unexpected interferences by substances other than those documented by manufacturers, notably alcohols and solvents.6 Examples where false positive readings were thought to occur included the following industries: printing where alcohols are used; plastics and printing where ketones are used; chemical factories where aliphatic and aromatic hydrocarbons are used; food factories where alcohol is used; pulp and paper plants and composting facilities where sulfur compounds are present; and in agricultural and waste management and establishments where ammonia, carbon dioxide, and nitrous oxide are found.
The objective of this study was to identify possible undocumented cross-sensitivities of amperometric electrochemical detectors commonly used to evaluate worker exposures to airborne contaminants. The aim is to provide occupational hygienists with information that will allow them to better select direct-reading instruments for a particular application or to modify their workplace evaluation strategy.
Target chemical | Biosystems Toxilog | Biosystems ToxiUltra | BW Gas Alert | Draeger Datalogger | ISC TX418 |
---|---|---|---|---|---|
Carbon monoxide (CO) | 2 | 2 | 1 | 2 | 2 |
Hydrogen sulfide (H2S) | 1 | 1 | 2 | ||
Nitric oxide (NO) | 2 | ||||
Nitrogen dioxide (NO2) | 2 | 2 | 1 | ||
Sulfur dioxide (SO2) | 2 | 1 | 2 |
Challenge chemical | TWA (ppm)a | Challenge concentration (ppm) | Detectors tested |
---|---|---|---|
a 8-hour time weighted average.14 b See Table 1. | |||
Alcohols | |||
Ethanol | 1000 | 100, 400, 1000 | Allb |
Isopropanol | 400 | 100, 250 | All |
Methanol | 200 | 150, 250 | All |
n-Propanol | 200 | 100, 260 | All |
Hydrocarbons | |||
Isopropyl acetate | 250 | 100, 200 | All |
n-Hexane | 50 | 29 300 | Toxilog, ToxiUltra, Draeger (except H2S) |
Methane | Simple asphyxiant | 22![]() |
Toxilog, ToxiUltra, Draeger |
Propane | 1000 | 12![]() |
Toxilog, ToxiUltra, Draeger |
Styrene | 50 | 10, 50 | All |
Toluene | 50 | 150, 350 | All |
Trichloroethylene | 50 | 50, 200 | All (except Toxilog SO2) |
Xylene | 100 | 50, 100, 150 | All |
Mixture | Methanol: 200 | Methanol: 150 | All (except Toxilog SO2, Toxilog H2S, and ToxiUltra H2S) |
Toluene: 50 | Toluene: 150 | ||
Xylene: 100 | Xylene: 100 | ||
Ketones | |||
Acetone | 750 | 200, 300 | All |
Cyclohexanone | 25 | 25, 37, 100 | All |
Methylethylketone | 50 | 100 | All |
Inorganic gases | |||
Ammonia | 25 | 18 | Toxilog, ToxiUltra (except H2S Ultra), Draeger |
Carbon dioxide | 5000 | 50![]() |
Toxilog, ToxiUltra, Draeger |
Chlorine | 0.5 | 1, 4 | Toxilog, ToxiUltra; Draeger |
Hydrogen sulfide | 10 | 8.9 | All (except H2S) |
Nitrous oxide | 50 | 100 | Toxilog, ToxiUltra (except H2S Ultra), Draeger |
In the case of solvents, instruments were exposed to challenge compounds in a hermetically sealed, 0.01 m3 glass test chamber, built at the IRSST and placed inside a fumehood.12 An experiment consisted of three successive 120 minute test challenges to a single compound or mixture with the ambient concentration of these substances generally bracketing the threshold limit value, or TLV (approximately 0.5, 1.0 and 3.0 times the TLV). The detector was placed in the bottom of the test chamber which was purged with zero air. It remained in this clean atmosphere for at least 24 hours prior to the beginning of the test. Accurately known volumes of pure solvents were injected into the test chamber via the septum using a microlitre syringe. The volume of challenge compound required to achieve the desired chamber concentration (C) was calculated using the following equation:
C = (V × d × 1000 mg g−1 ÷ VT) × 24.45/MW |
The chamber lid was equipped with a small fan underneath the top, two valve-controlled gas inlets, one valve-controlled outlet, and a septum. The fan was operated for approximately 10 minutes at the beginning of the test to ensure homogenous mixing of the gases. It was determined from replicate testing that similar results were obtained whether or not the fan was left on or off during the remainder of the test. The results reported here were obtained with the fan turned off during the test. Temperature and relative humidity were controlled to 24 °C and 34%, respectively. Pressure was measured both inside and outside of the test chamber. Detector readings were recorded every 5–10 minutes for 2 hours. The assumption that the chamber gas or vapor concentration remained constant over the test period was checked at the end of the test using a Gastec™ colorimetric detector tube inserted into the test chamber via a sampling port. Detector recovery was not systematically monitored following exposure to challenge substances that elicited false positive responses, but it was observed that some of the instruments required up to 24 hours of exposure to zero air to return to a stable baseline. In the case of gases, certified concentrations were introduced into the chamber with gas and air flow rates calculated to result in the desired chamber concentration after 15–20 minutes.
In the case of the highly toxic chlorine tests, detectors were challenged for only 10 minutes via a perforated cap fitted over the detector and connected to a gas generator from Advanced Calibration Designs Inc. Model F100 via a short length of tubing.13 Detectors were challenged in a similar fashion from a cylinder of compressed gas certified to contain an accurately known concentration of 8.9 ppm H2S.
More than 1200 individual tests were performed. Some of the detector-challenge combinations were not completed because the instrument was unavailable at the time that the tests were run (Table 2).
No cross-sensitivities were observed for any of the instruments tested for the following substances, individually: ammonia, carbon dioxide, n-hexane, methane, nitrous oxide, propane, toluene, trichloroethylene, or xylene. Cross-sensitivities were observed for some or all of the instruments exposed to the other challenge chemicals, individually: alcohols (ethanol, isopropanol, methanol, n-propanol), hydrocarbons (isopropyl acetate, styrene, or a mixture of toluene, xylene and methanol), ketones (acetone, cyclohexanone, methylethylketone (or inorganic gases (chlorine, and hydrogen sulfide). The most noteworthy results are summarized in Tables 3 to 7.
Challenge compound concentrationc | Biosystems Toxilog | Biosystems ToxiUltra | BW Gas Alert | Draeger Datalogger | ISC TX418 |
---|---|---|---|---|---|
a Reading in ppm CO. b TWACO = 35 ppm. c Test chamber nominal concentration ±40%, verified using Gastec™ indicator tubes after 2 hours of exposure. d No effect. | |||||
Alcohols | |||||
Ethanol (TWA = 1000 ppm) | |||||
100 | d | 7 | |||
400 | 3 | 60 | 28 | 139 | 9 |
1000 | 11 | 150 | 66 | 283 | 50 |
Isopropanol (TWA = 400 ppm) | |||||
100 | d | 31 | d | 11 | d |
250 | d | 60 | d | 60 | d |
Methanol (TWA = 200 ppm) | |||||
150 | 23 | 38 | 24 | 220 | 22 |
250 | 36 | 66 | 27 | 39 | |
n-Propanol (TWA = 200 ppm) | |||||
100 | d | 42 | d | 15 | d |
260 | d | 106 | d | 74 | d |
Hydrocarbons | |||||
Isopropyl acetate (TWA = 250 ppm) | |||||
100 | d | d | d | 4 | d |
200 | d | d | d | 12 | d |
Styrene (TWA = 50 ppm) | |||||
10 | d | 3 | d | 2 | d |
50 | d | 21 | d | 29 | d |
Mixture: | |||||
150 methanol + 150 toluene + 100 xylene | 22 | 27 | 25 | 81 | 20 |
Inorganic gases | |||||
Chlorine (TWA = 0.5 ppm) | |||||
1 | d | d | d | ||
4 | d | d | d | ||
Hydrogen sulfide (TWA = 10 ppm) | |||||
9 | d | d | d | d | d |
Ketones | |||||
Acetone (TWA = 750 ppm) | |||||
200 | d | d | d | 18 | d |
300 | d | d | d | 32 | d |
Cyclohexanone (TWA = 25 ppm) | |||||
25 | d | 5 | d | 2 | d |
37 | d | 7 | d | 3 | d |
100 | d | 27 | d | 15 | d |
Methylethylketone (TWA = 50 ppm) | |||||
100 | d | d | d | 10 | d |
Challenge compound concentrationc | Biosystems Toxilog | Biosystems ToxiUltra | ISC TX418 d |
---|---|---|---|
a Reading in ppm H2S. b TWAH2S = 10 ppm. c Test chamber nominal concentration ±40%, verified using Gastec™ indicator tubes after 2 hours of exposure. d The instrument displayed negative readings (−15 to −20 ppm) after the first 15 to 20 minutes of exposure. It stabilized at the levels indicated 1 to 2 hours following initial exposure to the challenge compound. e No effect. | |||
Alcohols | |||
Ethanol (TWA = 1000 ppm) | |||
100 | e | e | −1 |
400 | e | e | −1 |
1000 | e | −1 | |
Isopropanol (TWA = 400 ppm) | |||
100 | e | e | −2 |
300 | e | e | −6 |
500 | e | e | −6 |
Methanol (TWA = 200 ppm) | |||
100 | e | e | −1 |
150 | e | e | −2 |
n-Propanol (TWA = 200 ppm) | |||
250 | e | e | −6 |
400 | e | e | −14 |
Hydrocarbons | |||
Isopropyl acetate (TWA = 250 ppm) | |||
100 | e | e | e |
250 | e | e | e |
Styrene (TWA = 50 ppm) | |||
10 | e | e | e |
30 | e | e | e |
Mixture: | |||
150 methanol + 150 toluene + 100 xylene | e | ||
Inorganic gases | |||
Chlorine (TWA = 0.5 ppm) | |||
1 | e | ||
4 | e | ||
Ketones | |||
Acetone (TWA = 750 ppm) | |||
200 | e | e | e |
300 | e | e | e |
Cyclohexanone (TWA = 25 ppm) | |||
25 | e | e | e |
37 | e | e | e |
100 | e | e | e |
Methylethylketone (TWA = 50 ppm) | |||
100 | e | e | e |
Challenge compound concentrationd | Biosystems ToxiUltra |
---|---|
a Reading in ppm NO. b TWANO = 25 ppm. c No effect observed when challenged with the following: isopropanol, methanol, n-propanol, hydrocarbons (isopropyl acetate, styrene), ketones (acetone, cyclohexanone, methylethylketone), inorganic gases (chlorine, hydrogen sulfide), or a mixture of methanol, toluene and xylene. d Test chamber concentration ±40%, verified using Gastec™ indicator tubes after 2 hours of exposure. e No effect. | |
Ethanol (TWA = 1000 ppm) | |
100 | e |
400 | −4 |
1000 | −23 |
Challenge compound concentrationd | Biosystems Toxilog | Biosystems ToxiUltra | BW Gas Alert |
---|---|---|---|
a Reading in ppm NO2. b TWANO2 = 3 ppm. c No effect observed when challenged with the following: alcohols (ethanol, isopropanol, methanol, n-propanol), hydrocarbons (isopropyl acetate, styrene), ketones (acetone, cyclohexanone, methylethylketone), or ammonia. d Gas concentration passing over the detector directly from a certified cylinder of compressed gas using a similar setup to that used for instrument calibration. e No effect. | |||
Inorganic gases | |||
Chlorine (TWA = 0.5 ppm) | |||
1 | 1.4 | 1.4 | |
4 | 6.7 | 6.1 | |
Hydrogen sulfide (TWA = 10 ppm) | |||
8.9 | −1.9 | −9.7 | e |
Challenge compound concentrationd | Biosystems Toxilog | Biosystems Toxiultra | BW Gas Alert | ISC TX418 |
---|---|---|---|---|
a Reading in ppm SO2. b TWASO2 = 2 ppm.c No effect observed when challenged with the following: alcohols (ethanol, isopropanol, methanol, n-propanol), isopropyl acetate, ketones (acetone, cyclohexanone, methylethylketone), or inorganic gases (ammonia, hydrogen sulfide). Note: The BW Gas Alert and the ISC TX418 were not tested with chlorine.d Test chamber nominal concentration ±40%, verified using Gastec™ indicator tubes after 2 hours of exposure.e No effect. | ||||
Hydrocarbons | ||||
Styrene (TWA = 50 ppm) | ||||
10 | 0.2 | 2 | 0.2 | |
50 | 6 | 0.2 | 5.6 | |
Mixture: | ||||
150 methanol + 150 toluene + 100 xylene | e | e | −6 |
All five models of CO detector were subject to false positives in the presence of ethanol or methanol (Table 3). In general, the Draeger and ToxiUltra CO detectors did not perform as well as the others, being subject also to false positives in the presence of cyclohexanone, isopropanol, n-propanol, and styrene. In addition, the Draeger CO detector responded positively to isopropyl acetate, acetone, and methylethylketone.
The ISC instrument was the only H2S detector subject to negative responses in the presence of alcohols, exhibiting an important decrease after 15 to 20 minutes and a long period of stabilization (Table 4).
The only false positive response observed for the ToxiUltra NO detector was for ethanol at a concentration higher than 100 ppm (Table 5). It responded initially to n-propanol, but returned to baseline after approximately 15 minutes.
The Toxilog and ToxiUltra NO2 detectors exhibited false positives in the presence of chlorine, and negative responses in the presence of hydrogen sulfide (Table 6). Both the Toxilog and ToxiUltra NO2 detectors initially responded to carbon dioxide, methane and nitrous oxide challenges, but the responses returned to baseline levels after approximately 15 minutes. A similar, short period of instability was observed when the ToxiUltra NO2 detector was challenged with n-hexane.
The Toxilog and BW Gas Alert SO2 detectors exhibited false positives in the presence of styrene (Table 7). The ISC SO2 detector responded negatively when challenged with 30 ppm of styrene and with a mixture of hydrocarbons and alcohol, decreasing to a minimum of −23 ppm after 10 minutes before stabilizing at −6 ppm after 95 minutes.
There are two cases where none of the electrochemical detectors tested should be used: (1) the measurement of CO in the presence of methanol or ethanol; and (2) the measurement of NO in the presence of ethanol. In other cases, where false responses were observed, an alternative instrument can be selected that would not affected by the cross-sensitivity.
A more comprehensive study is required to understand the underlying processes resulting in the observed cross-sensitivities and to quantify their effects. Differences between instruments may be due to differences in electronic circuitry design or in the sensors used in the instruments by the different manufacturers. Most instrument manufacturers purchase the electrochemical cell from another supplier and build the electronics to process the signal. For example, detectors manufactured by BW Technologies, Industrial Scientific Corporation and Biosystems Incorporated all use the same electrochemical cell, Citicel, manufactured by City Technology Incorporated.15,16 Other manufacturers, such as Draeger, manufacture their own electrochemical cells. Manufacturers claim electrochemical cell detection limits of 1 ppm for the CO, H2S, and NO cells, and 0.5 ppm for the NO2 and SO2 cells. Performance between instruments using the same sensor may vary as a result of differences in the instrument manufacturers’ designs. This may include differences in different models of the same sensor from the same manufacturer, the electronics used to process the signal, the membranes and the filters used, the flow, the software, etc. The manufacturers’ product literature describes response times to target analytes of 20–60 seconds, operating temperatures of −40 to 0 °C at the lower end of the range to 40–50 °C at the upper end of the range, and operating relative humidities of 0–15% at the lower end of the range to 90–99% at the upper end of the range.7–11
Electrochemical detectors can be very useful for monitoring exposures to toxic vapors and gases. Manufacturers claim a linear response and, when used properly under appropriate conditions, they may offer good selectivity, repeatability and accuracy. However, a survey of potentially interfering substances present at levels below their TLVs, including substances undocumented by manufacturers, must precede exposure assessments performed using electrochemical detectors. Potential bias can be avoided by careful selection of instrument type and model.
This journal is © The Royal Society of Chemistry 2006 |