No analytical method is completely specific. Gases present in the environment, other than the “target” gas of measurement, may affect instrument response. Interferences (aka “cross-sensitivities” are not necessarily linear, and may also exhibit time dependent characteristics.
The charts that follow detail the approximate concentration in parts per million of interfering gas required to cause a 1 ppm deflection in the chosen analyzer. In many cases, specificity can be improved. Please note that the response values given are not absolute, and may vary depending on sensor formulation.
The special case of how alcohols affect electrochemical sensors is discussed in this Knowledge Base article.
For further information on the effects of interfering gases, please contact the factory.
The charts follow the format, and grouping of gases, that was originally established in early Interscan print brochures.
Click on the links below to view each chart.
Chart 1: Br2, CO, Cl2, ClO2, H2, H2S, NO, NO2, O3, SO2 analyzers
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INTERFERING GAS |
A N
A
L
Y
Z
E
R | | Br2 | C2H5SH | Cl2 | CO | H2 | HCl | HCN | hydrazine | H2S | NH3 | NO | N2O | NO2 | O3 | Saturated
HC‡ | SO2 | Unsat
HC‡ |
| Br2 | — | 3.5
[N] | 1.3 | 450
[N] | 6200
[N] | 4.5
[N] | 2
[N] | 0.7
[N] | 1.0
[N] | 17
[N] | 180 | >104 | 1.3 | 1.3 | >104 | 1.3
[N] | >500 |
| CO | >103
[N] | >103 | >103
[N] | — | 8¤ | >103 | >103 | >103 | >103 | >103 | >103 | >104 | >103
[N] | >103
[N] | >104 | >103 | 17 |
| Cl2 | 0.65 | 3
[N] | — | 400
[N] | 6000
[N] | 4
[N] | 1.5
[N] | 0.5
[N] | 0.3
[N] | 14
[N] | 150 | >104 | 1 | 1 | >104 | 1
[N] | >500 |
| ClO2 | 2.1 | 9
[N] | 2.8 | 1200
[N] | >104
[N] | 12¤
[N] | 4.5
[N] | 1.5
[N] | 1.0
[N] | 42
[N] | 450 | >104 | 3 | 3 | >104 | 3
[N] | >1500 |
| H2 | >103
[N] | >103 | >103
[N] | 0.1 | — | >103 | >103 | >103 | >103 | >103 | >103 | >104 | >103
[N] | >103
[N] | >104 | >103 | 3 |
| H2S (1) | 9
[N] | 3 | 11
[N] | 40 | 400 | 11 | 10¤ | 4 | — | 220 | 4 | >104 | 65
[N] | 15
[N] | >104 | 4 | 15 |
| H2S (2) | 40
[N] | 1 | 50
[N] | 8000 | 7000 | 15 | 15 | 6 | — | 300 | 15 | >104 | 60
[N] | 20
[N] | >104 | 6 | >500 |
| NO | >103 | >103 | >103 | >103 | >104 | >103 | >103 | >103 | >103 | >103 | — | >104 | >103 | >103 | >104 | >103 | >500 |
| NO2 | 0.6 | 3
[N] | 0.7 | 350
[N] | 6000
[N] | 4¤
[N] | 2
[N] | 0.7
[N] | 0.2
[N] | 15
[N] | 150 | >104 | — | 1 | >104 | 1
[N] | >500 |
| O3 | 0.65 | 3
[N] | 1 | 400
[N] | 6000
[N] | 4
[N] | 2
[N] | 0.5
[N] | 0.3 | 15
[N] | 150 | >104 | 1 | — | >104 | 1
[N] | >500 |
| SO2 | 2.5
[N] | 1 | 3
[N] | 700 | 5000 | 2 | 2 | 1 | 0.3 | 68 | 6¤ | >104 | 24
[N] | 2
[N] | >104 | — | >500 |
(1) Data shown for H2S models with ranges higher than 0-1999 ppb
(2) Data shown for H2S models with ranges of 0-1999 ppb and lower
[N] = Negative interference
‡ = Hydrocarbons
¤ = Rejection ratio can be improved electronically
Chart 2: Ethylene oxide (EtO) (C2H4O), Propylene oxide (PrO) (C3H6O) analyzers
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INTERFERING GAS |
| EtO ANALYZER | Cl2 | CO | CO2 | ethyl alcohol | Freon | glutaraldehyde | isopropyl alcohol | N2O | NH3 |
25
[N] | 6 | >104 | 380 | >104 | 5 | 1.1 ¶ | >104 | 300 |
| PrO ANALYZER | 26
[N] | 5.5 | >104 | 400 | >104 | 6 | 1 ¶ | >104 | 320 |
¶ Isopropyl alcohol represents the most significant interference on the ethylene oxide sensor, but in nearly all cases, potential problems can be overcome. Typical remedial actions include:
a. Point shutdown/automatic restart, which allows the operator to temporarily interrupt monitoring at points that could be affected when isopropyl alcohol is used. Monitoring restarts automatically on a time-adjustable basis.
b. Selection of monitoring points away from those areas that may be unduly affected by isopropyl alcohol.
c. Using alternative germicides, which do not contain isopropyl alcohol.
The EtO sensor may also respond to strong odors of colognes and perfumes, and to certain floor strippers and waxes. Refer to guidelines above covering isopropyl alcohol. Remember that you are attempting to monitor parts per million levels of ethylene oxide in an environment that may contain percent (10,000 ppm = 1 percent) levels of these potentially interfering compounds.
[N] = Negative interference
Chart 3: Formaldehyde (HCHO) analyzers
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INTERFERING GAS |
HCHO
ANALYZER | CH3CHO | acetone | Cl2 | CO | ethanol | glutaraldehyde | H2 | H2S | HCl | SO2 |
| 17 | >103 | 7
[N] | 5600 | 127 | 200 | >104 | 3 § | 35 | 3 § |
|
| isopropanol | CH3OH | methyl
ethyl
ketone | n-butanol | n-propanol | NH3 | NO | NO2 | phenol | propion-
aldehyde |
| 1000 | 625 | >103 | 3200 | 2000 | 300 | 500 | 35
[N] | >103 | 160 |
[N] = Negative interference
§ = Scrubber available
Chart 4: HBr, HCl, HCN, hydrazine analyzers
Click here to download PDF of this chart.
INTERFERING GAS |
A
N
A
L
Y
Z
E
R | | C2H5SH | Cl2 | CO | H2 | H2S | HCl | HCN | hydrazine | N2O | NH3 | NO | NO2 | SO2 | SO3 | Saturated
HC | Unsat
HC |
| HBr | 0.3 | 20
[N] | 1000 | >104 | 0.2 | 0.7 | 0.7 | 6 | >104 | 25 | 1 | 18
[N] | 0.7 | >104 | >104 | >500 |
| HCl | 0.2 | 17
[N] | 1000 | >104 | 0.1 | — | 1 | 5 | >104 | 21 | 0.6 | 15
[N] | 0.5 | >104 | >104 | >500 |
| HCN | 0.2 | 20
[N] | 1000 | >104 | 0.1 | 1 | — | 6 | >104 | 21 | 1 | 15
[N] | 0.5 | >104 | >104 | >500 |
Hydra–
zine | 10 | 5
[N] | 1000 | >104 | 0.1 | 45 | 8 | — | >104 | 1500 | 1300 | 30
[N] | 3.5 | >104 | >104 | >500 |
[N] = Negative interference
Chart 5: C2H4 (ethylene) analyzers
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INTERFERING GAS |
C2H4 (ethylene)
ANALYZER | acetone | acetylene | Cl2 | CO | CO2 | ethyl alcohol | ethylene glycol | EtO | Freon | glutar-
aldehyde | H2 | isopropyl alcohol | N2O | NH3 |
| 300 | 4 | 15
[N] | 8 | >104 | 150 | 0.3 | 0.2 | >104 | 1 | 500 | 0.4 | >105 | 200 |
[N] = Negative interference
Chart 6: H2O2 (hydrogen peroxide) analyzers
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INTERFERING GAS |
H2O2
ANALYZER | H2 | Cl2 | CO | CO2 | ethyl alcohol | EtO | H2S | SO2 | glutar-
aldehyde | isopropyl alcohol | N2O | NH3 |
| >8000 | 6
[N] | >1000 | >104 | >500 | >500 | 0.6 | 2 | >500 | >500 | >104 | 150 |
Chart 7: PAA (peracetic acid) analyzers
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INTERFERING GAS |
PAA
ANALYZER | NH3 | CO | CO2 | ethyl alcohol | EtO | Cl2 | isopropyl alcohol |
| 150 | >1000 | >104 | >500 | >500 | 6
[N] | >500 |
[N] = Negative interference
Chart 8: NH3 (ammonia) analyzers
