Ethylene Oxide in Air Canisters – Method Performance

 

In previous posts on ethylene oxide (EtO) I've talked about the following:

• Resolving interferences with cryogenic cooling
• How to resolve EtO and the TO-15A suite of compounds
• Canister storage issues with EtO

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 ppt_v 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.

Figure 1: EtO linearity with preconcentrator volume

Once it was established that we were in the linear range for EtO, a calibration from 34 ppt_v to 2,688 ppt_v 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.

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 ppt_v 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 ppt_v. 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 ppt_v, 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