M E A S U R E M E N T OF S U L P H U R G A S E S IN A M B I E N T A I R YANK. LAU Alberta Environment, Pollution Control Division, 9820-106 Street, Edmonton, Alberta, Canada, T5K 2J6
(Received December 1988) Abstract. A gas chromatograph with a flame photometric detector is set up for the direct analysis of COS, H2S, CS2, SO 2, CH3SH, C2HsSH in ambient air. Logarithmic transformation is used to counterbalance the non-linear nature of the detector response for the individual sulphur gases. A quality assurance procedure is described to compensate any variation of response during field measurement. The use of Tedlar bags for sampling COS, CS2, CH3SH, C2HsSH is acceptable despite the general conception that sulphur gases are unstable in Tedlar bags.
Introduction
Sulphur-containing gases are occasionally emitted to the atmosphere by the sour gas processing plants and pulp and paper mills, and their odors are often objectionable to the nearby communities. The measurement of the sulphur compounds in the ambient air has to overcome two obstacles: low concentration and high reactivity of the target compounds. The low odor threshold of hydrogen sulphide (H2S) and mercaptans (RSH) necessitates the detection of these compounds in the low parts per billion (ppb) concentration range. The high reactive nature of the compounds favors direct analysis over any preconcentration methods. Dedicated continuous analyzers are commercially available for the measurement of sulphur dioxide (SO2) and H2S. No similar instrumentation is available for the other sulphur-containing compounds. A gas chromatograph (GC) was set up for the measurement of the common sulphur-containing air pollutants: H2S, SO2, carbonyl sulphide (COS), carbon disulphide (CS2), methylmercaptan (CH3SH) and ethylmercaptan (C2HsSH) in the low ppb concentration. This paper deals with the technique used for the calibration and measurement, and the storage stability of these sulphur gases in the Tedlar sampling bags.
Instrumentation
The basic chromatographic system consisted of a Hewlett Packard HP5880A GC fitted with a flame photometric detector. The column used was a 6' x 1/8" o.d. Teflon column packed with 60/80 mesh Chromosil 310 which is a specially treated silica gel prepared by Supelco (Supina, 1979). To enhance the lower detection limit, a 10 mL sample loop was used. A relatively high flow (60 mL m i n - l ) of the carrier gas (nitrogen) was necessary to retain the required resolution for the large sample size. Other chromatographic conditions include: detector gas supplies, hydrogen (110 mL Environmental Monitoring and Assessment 13., 69-74, 1989. 9 1989 Kluwer Academic Publishers. Printed in the Netherlands.
70
YAN K. LAU
COLUMN
~o E .a
o U
..~ c o.
FLOW
SAMPLE
~a. ~
E,~
,~c6
DETECTOR
95"~/ / 20~ Z ~1~
~'r
c,l~
9 C h r o m o s i l 310
6 ft., I18" OD 6 0 roll rain. N 2 I0 ml air FPD
20 min
30~ r
b.
m 0 9~
o.
'~ I min.
"" Z
N m
~r'
r
,,a~
._=.a
~
Eo
-,Q Q.
-,
IW Z
Fig. 1. A gas chromatogram of sulphur gases in air. m i n - l ) , air (60 m L m i n - 1), oxygen (10 mL m i n - 1), detec'or temperature, 200 ~ oven temperature, 30 ~ for 1 min, programmed at 20 ~ m i n - 1 to 95 ~ and held for 7 min or longer. The GC system was mounted in a mobile vehicle equipped with a power generator for field operation. The air sample was drawn continuously through the 10 mL sample loop which was put in the path of the carrier gas flow at the start of the analysis. The sample injection can be done manually or automatically at a pre-set time interval. The typical cycling time is 15 min. High sensitivity and good resolution were achieved for the measurement o f COS, HES, CS2, SO2, CH3SH, and C2HsSH in 10 min as shown in Figure 1. Dimethyl sulphide ((CH3)2S) can also be detected at a retention time of 21.0 rain. Calibration The use of a flame photometric detector for the measurement of the sulphur gases has the inherent advantages of specificity and sensitivity (McGaughey and Gangwal, 1980), making it possible to perform the ultra-trace ambient air analysis without sample preparation. However, the non-linear nature o f the detector response requires careful calibration techniques for obtaining quantitative results. The observation that the response to sulphur compounds varies exponentially (approximately 2) was
MEASUREMENT
OF SULPHUR
G A S E S IN A M B I E N T
AIR
71
reported when the flame photometric detector was first introduced by Brody and Chaney (1966). Response ~ Concentration n n = 2.
(1)
To provide proper quantitation for different sulphur compounds, the values of n for various target compounds have to be determined. The logarithmic transformation of Equation (1) yields a linear relationship. log (response) = constant + n log (concentration).
(2)
A plot of log (Response) versus log (Concentration) gives a slope of n, which is the response characteristics of the instrument for the compound. As the values of the constant and n in Equation (2) can be determined in a multipoint calibration, the quantitative relationship between the response and concentration is established for all target sulphur gases. Permeation devices were used to generate the known concentrations of the standard gases for qualitative and quantitative calibration. Cross-checking of standards was used to verify the certification of the individual permeation devices (Lau, 1988). A multipoint calibration was performed prior to the field measurement. An example of the resulting calibration curves for COS, lIES , CS2, SO2, CH3SH , and C2HsSH is shown in Figure 2. The value of the slopes varies from 1.77 to 2.03, giving good agreement to the approximate value of 2. The high response of CS2 reflects the presence of two sulphur atoms in the molecule. For sample analysis in the field, a single concentration of the standards mixture is run in the beginning of the day, after every five sample runs and at the end of the day. The results are used to confirm the qualitative calibration (retention time) and to compensate for any variation in the sensitivity of the detector. The quantitative compensation is achieved by keeping the value of n (slope) unchanged but revising the value of the constant in Equation (2) based on the single concentration checks. Compensation within a sampling day is rarely required as the response variation is typically within 1007o.The slopes indicated in Figure 2 are quite characteristics of the GC system. Based on the calibration results obtained in the last three years, the relative standard deviation of the individual slopes is between 2-5~
Sample Storage Stability Sample collection systems were examined as an alternative to taking the GC to the field. Although the use of solid adsorbent preconcentration has been reported for sulphur gases (Black et al., 1978), the whole-air sampling with Tedlar bags was chosen for evaluation because of its simplicity. The sample stability study was conducted by preparing a known ppb concentration mixture of the six sulphur gases and storing it in a 5 L Tedlar air sampling bag (from SKC). The concentrations were followed every 20 min for the first 3 h, and then periodically for the following 21 days. The results are shown graphically in Figure 3
72
YAN K. LAU
10 4
.F
LEGEND
'SO 2 SLOPE
CS 2
~o~
II
I .77
I /
2
103
mO~
COS
I .84
--e--
SO 2
I .93
---Zl--- CH3SH
I .83
._A_.
I .87
~i
C2HsSH
- - H2S
///CHaSH
SH
/
2.03
8
t'
/
CS 2
4
r
Iz ~ o
2 AO I
w
02
'~
8
z
6
w z o o. m w
/
4
///,
010
A~,O
2
I0
?.
!
8 6 /
4
/ /
2
i0 ~ Io o
i
I
i
L
I
l
I
l
I
I
I
2
4
6
8 I0 !
2
4
6
8 I02
2
CONCENTRATION Fig. 2.
IN PART
PER
BILLION
(ppbv)
Log-log calibration curves for the flame photometric detector.
i I 4
6
8 I0 ~
73
M E A S U R E M E N T OF S U L P H U R GASES IN A M B I E N T AIR
70
60
cos r
d
"='" ~
---=,.
9- - - . .
,......
.-..,.,.
z40J0
,o Z 0
....
Z~~
'
~
CS 2 . ~ .
............
~
"-A--
9 -_
:,o
(Received December 1988) Abstract. A gas chromatograph with a flame photometric detector is set up for the direct analysis of COS, H2S, CS2, SO 2, CH3SH, C2HsSH in ambient air. Logarithmic transformation is used to counterbalance the non-linear nature of the detector response for the individual sulphur gases. A quality assurance procedure is described to compensate any variation of response during field measurement. The use of Tedlar bags for sampling COS, CS2, CH3SH, C2HsSH is acceptable despite the general conception that sulphur gases are unstable in Tedlar bags.
Introduction
Sulphur-containing gases are occasionally emitted to the atmosphere by the sour gas processing plants and pulp and paper mills, and their odors are often objectionable to the nearby communities. The measurement of the sulphur compounds in the ambient air has to overcome two obstacles: low concentration and high reactivity of the target compounds. The low odor threshold of hydrogen sulphide (H2S) and mercaptans (RSH) necessitates the detection of these compounds in the low parts per billion (ppb) concentration range. The high reactive nature of the compounds favors direct analysis over any preconcentration methods. Dedicated continuous analyzers are commercially available for the measurement of sulphur dioxide (SO2) and H2S. No similar instrumentation is available for the other sulphur-containing compounds. A gas chromatograph (GC) was set up for the measurement of the common sulphur-containing air pollutants: H2S, SO2, carbonyl sulphide (COS), carbon disulphide (CS2), methylmercaptan (CH3SH) and ethylmercaptan (C2HsSH) in the low ppb concentration. This paper deals with the technique used for the calibration and measurement, and the storage stability of these sulphur gases in the Tedlar sampling bags.
Instrumentation
The basic chromatographic system consisted of a Hewlett Packard HP5880A GC fitted with a flame photometric detector. The column used was a 6' x 1/8" o.d. Teflon column packed with 60/80 mesh Chromosil 310 which is a specially treated silica gel prepared by Supelco (Supina, 1979). To enhance the lower detection limit, a 10 mL sample loop was used. A relatively high flow (60 mL m i n - l ) of the carrier gas (nitrogen) was necessary to retain the required resolution for the large sample size. Other chromatographic conditions include: detector gas supplies, hydrogen (110 mL Environmental Monitoring and Assessment 13., 69-74, 1989. 9 1989 Kluwer Academic Publishers. Printed in the Netherlands.
70
YAN K. LAU
COLUMN
~o E .a
o U
..~ c o.
FLOW
SAMPLE
~a. ~
E,~
,~c6
DETECTOR
95"~/ / 20~ Z ~1~
~'r
c,l~
9 C h r o m o s i l 310
6 ft., I18" OD 6 0 roll rain. N 2 I0 ml air FPD
20 min
30~ r
b.
m 0 9~
o.
'~ I min.
"" Z
N m
~r'
r
,,a~
._=.a
~
Eo
-,Q Q.
-,
IW Z
Fig. 1. A gas chromatogram of sulphur gases in air. m i n - l ) , air (60 m L m i n - 1), oxygen (10 mL m i n - 1), detec'or temperature, 200 ~ oven temperature, 30 ~ for 1 min, programmed at 20 ~ m i n - 1 to 95 ~ and held for 7 min or longer. The GC system was mounted in a mobile vehicle equipped with a power generator for field operation. The air sample was drawn continuously through the 10 mL sample loop which was put in the path of the carrier gas flow at the start of the analysis. The sample injection can be done manually or automatically at a pre-set time interval. The typical cycling time is 15 min. High sensitivity and good resolution were achieved for the measurement o f COS, HES, CS2, SO2, CH3SH, and C2HsSH in 10 min as shown in Figure 1. Dimethyl sulphide ((CH3)2S) can also be detected at a retention time of 21.0 rain. Calibration The use of a flame photometric detector for the measurement of the sulphur gases has the inherent advantages of specificity and sensitivity (McGaughey and Gangwal, 1980), making it possible to perform the ultra-trace ambient air analysis without sample preparation. However, the non-linear nature o f the detector response requires careful calibration techniques for obtaining quantitative results. The observation that the response to sulphur compounds varies exponentially (approximately 2) was
MEASUREMENT
OF SULPHUR
G A S E S IN A M B I E N T
AIR
71
reported when the flame photometric detector was first introduced by Brody and Chaney (1966). Response ~ Concentration n n = 2.
(1)
To provide proper quantitation for different sulphur compounds, the values of n for various target compounds have to be determined. The logarithmic transformation of Equation (1) yields a linear relationship. log (response) = constant + n log (concentration).
(2)
A plot of log (Response) versus log (Concentration) gives a slope of n, which is the response characteristics of the instrument for the compound. As the values of the constant and n in Equation (2) can be determined in a multipoint calibration, the quantitative relationship between the response and concentration is established for all target sulphur gases. Permeation devices were used to generate the known concentrations of the standard gases for qualitative and quantitative calibration. Cross-checking of standards was used to verify the certification of the individual permeation devices (Lau, 1988). A multipoint calibration was performed prior to the field measurement. An example of the resulting calibration curves for COS, lIES , CS2, SO2, CH3SH , and C2HsSH is shown in Figure 2. The value of the slopes varies from 1.77 to 2.03, giving good agreement to the approximate value of 2. The high response of CS2 reflects the presence of two sulphur atoms in the molecule. For sample analysis in the field, a single concentration of the standards mixture is run in the beginning of the day, after every five sample runs and at the end of the day. The results are used to confirm the qualitative calibration (retention time) and to compensate for any variation in the sensitivity of the detector. The quantitative compensation is achieved by keeping the value of n (slope) unchanged but revising the value of the constant in Equation (2) based on the single concentration checks. Compensation within a sampling day is rarely required as the response variation is typically within 1007o.The slopes indicated in Figure 2 are quite characteristics of the GC system. Based on the calibration results obtained in the last three years, the relative standard deviation of the individual slopes is between 2-5~
Sample Storage Stability Sample collection systems were examined as an alternative to taking the GC to the field. Although the use of solid adsorbent preconcentration has been reported for sulphur gases (Black et al., 1978), the whole-air sampling with Tedlar bags was chosen for evaluation because of its simplicity. The sample stability study was conducted by preparing a known ppb concentration mixture of the six sulphur gases and storing it in a 5 L Tedlar air sampling bag (from SKC). The concentrations were followed every 20 min for the first 3 h, and then periodically for the following 21 days. The results are shown graphically in Figure 3
72
YAN K. LAU
10 4
.F
LEGEND
'SO 2 SLOPE
CS 2
~o~
II
I .77
I /
2
103
mO~
COS
I .84
--e--
SO 2
I .93
---Zl--- CH3SH
I .83
._A_.
I .87
~i
C2HsSH
- - H2S
///CHaSH
SH
/
2.03
8
t'
/
CS 2
4
r
Iz ~ o
2 AO I
w
02
'~
8
z
6
w z o o. m w
/
4
///,
010
A~,O
2
I0
?.
!
8 6 /
4
/ /
2
i0 ~ Io o
i
I
i
L
I
l
I
l
I
I
I
2
4
6
8 I0 !
2
4
6
8 I02
2
CONCENTRATION Fig. 2.
IN PART
PER
BILLION
(ppbv)
Log-log calibration curves for the flame photometric detector.
i I 4
6
8 I0 ~
73
M E A S U R E M E N T OF S U L P H U R GASES IN A M B I E N T AIR
70
60
cos r
d
"='" ~
---=,.
9- - - . .
,......
.-..,.,.
z40J0
,o Z 0
....
Z~~
'
~
CS 2 . ~ .
............
~
"-A--
9 -_
:,o
