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Ethylene Oxide in Air Canisters – Method Performance

par
  • Jason Hoisington
Tags
  • #COV
  • #Colonnes GC Rxi
  • #Air
  • #Optimisation de méthodes
  • #Colonnes Capillaires en silice fondue
  • #Echantillonnage d'air et de gaz
  • #Échantillonnage des composés organiques volatils (COV)
  • #Blogs
  • #Environnement et hygiène industrielle
  • #Air
  • #Échantillonnage d'air
  • #GC
  • #MS
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In previous posts on ethylene oxide (EtO) I've talked about the following:

 

In this post we’ll cap it all off with the method performance data I was able to obtain, including injection volume, linearity, calibration curves, detection limits, and precision.

First, let’s have a brief refresher on the method parameters used. Table 1 shows the GC-MS and preconcentrator settings used for the method.

Preconcentrator Markes Unity 2 + Kori + CIA
Unity 2 Settings  
Unity Trap Low −30 °C
Desorb temp 300 °C
Desorb flow 6 mL/min
Desorb time 180 s
Desorb Split Flow 3 mL/min
Flow Path Temperature 120 °C
Internal Standard  
Purge flow 50 mL/min
Purge time 60 s
Volume 50 mL
ISTD flow 50 mL/min
CIA Advantage Settings  
Volume 400 mL
Purge flow 50 mL/min
Purge time 60 s
Sample flow 100 mL/min
Kori-xr Settings  
Kori Trap Low −5 °C
Kori Trap High 300 °C
GC Agilent 7890B
Injection type On-column
Column 624Sil MS 60 m × 0.25 mm × 1.4 µm
Carrier gas He, constant flow
Flow rate 2 mL/min
Oven temp

0 °C (hold 5 min) to 60 °C at 3.5 °C/min
(hold 0 min) to 260 °C at 24 °C/min
(hold 5 min)

Detector MS Agilent 5977A
Acquisition mode SIM/Scan
Scan parameters  
Scan range (amu) 29-226
Scan rate (scans/s) 3.7
SIM parameters  
SIM ions 15, 29, 43, 44, 56
Dwell time 50
Transfer line 250 °C
Analyzer type Quadrupole
Source type Extractor
Source temp 350 °C
Quad temp 200 °C
Electron energy 70 eV
Solvent delay time 0 min
Tune type BFB
Ionization mode EI

Table 1: Instrument settings for combined EtO and TO-15A analysis. The sample is split at the preconcentrator during desorb.

All standards and blanks used in these studies were made in 50% relative humidity (%RH) zero air using 6 L SilcoCans. My previous post on canister storage for EtO shows the importance of using humid air when testing for EtO, showing that dry air can potentially hide EtO interferences in real world humid samples.

Once the initial settings were determined, we had to ensure that we wouldn’t be overloading the trap at higher concentrations. To do this we took two concentrations of standard and increased the volume loaded onto the preconcentrator to see if there was any fall off at larger volumes. We found that up to 600 mL at 2,688 pptv the system was linear, as shown in Figure 1. As a best balance between sensitivity and sample load time, 400 mL was used as the standard injection volume for our study.

blog-ethylene-oxide-in-air-canisters–method-performance-01.PNG

Figure 1: EtO linearity with preconcentrator volume

Once it was established that we were in the linear range for EtO, a calibration from 34 pptv to 2,688 pptv was made using bromochloromethane as an internal standard. TO-15A states that a calibration should have less than 30% RSD between the relative response factors of the calibration points, and each point must be within 20% of the true value. As shown in Figure 2, the %RSD was 12.8%, and Table 2 shows the calibration points are all within ±20% of their true value.

blog-ethylene-oxide-in-air-canisters–method-performance-02.png

Figure 2: EtO calibration results

True (pptv)

34

67

134

269

672

1344

2688

Calculated (pptv)

40

58

150

255

727

1196

2446

% from true

119%

86%

112%

95%

108%

89%

91%

Table 2: Calculated recovery of EtO calibration standards

With the calibration established it was time to determine the method detection limit (MDL) of the method. Seven replicates at the low calibration point were analyzed, and the standard deviation was multiplied by the student T value of 3.143 to calculate the MDL. The limit of quantitation (LOQ) was taken as 3 times the MDL. As shown in Table 3 we were able to get relatively low into the pptv range for our detection limits.

Replicate

1

2

3

4

5

6

7

Standard Deviation

MDL (pptv)

LOQ (pptv)

EtO (pptv)

41

43

38

34

45

33

50

5.6

18

55

Table 3: EtO MDL and LOQ results

Finally, the precision and accuracy of the method was determined by analyzing 4 replicates at 500 pptv. In addition, a stability study was done by analyzing 4 replicates over the course of over 2 weeks to determine how long EtO can be kept in canisters. Tables 4 and 5 show these results.

Replicate

1

2

3

4

Average

SD

RSD

EtO (pptv)

514

417

588

560

520

65

13%

% recovery

103%

83%

118%

112%

104%

   

Table 4: EtO precision and accuracy

Replicate

1

2

3

4

Average

Day 1 (% recovery)

100%

122%

82%

88%

98%

Day 2 (% recovery)

109%

129%

101%

100%

110%

Day 5 (% recovery)

97%

87%

106%

100%

108%

Day 8 (% recovery)

96%

119%

130%

110%

114%

Day 12 (% recovery)

137% *

138%*

134% *

122%

133% *

Day 16 (% recovery)

107%

132%*

132% *

116%

126%

Table 5: EtO stability. Results flagged with * fail the TO-15A ±30% recovery

To sum everything up, by using cryogenic cooling EtO can be separated from interferences and combined with TO-15A with detection limits down to 18 pptv, with good accuracy and precision, and with stability of a least 1 week in SilcoCans. Given what we’ve found with EtO growth in canisters any lab testing for EtO should test their own canisters to determine a realistic holding time for their samples.

For a more in depth look at EtO analysis, check out the article by myself and Jason Herrington that was recently published in Separations.

https://www.mdpi.com/2297-8739/8/3/35

I also gave a presentation at NEMC 2020 on EtO stability in air canisters.

https://apps.nelac-institute.org/nemc/2020/docs/presentations/pdf/8-4-20-Air%20Methods,%20Monitoring,%20and%20Technology-4.02-Hoisington.pdf