Cryogenic Cooling for Air Analysis Part 2 – Combining TO-15A and Ethylene Oxide

In my previous blog on cryogenic cooling (https://blog.restek.com/cyrogenic-cooling-for-air-analysis-interferences-from-n2-co2-and-o2/) I touched briefly on ethylene oxide (EtO) and why it may be of interest in ambient air analysis. While OSHA has a time weighted average (TWA) limit for EtO at 1ppm for an 8 hour exposure, recent work by the US EPA has shown that even low doses of ethylene oxide can increase cancer risks over a person’s lifetime (https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/1025tr.pdf), which explains the interest in testing for EtO at sub ppb levels. Since this brings EtO testing to similar levels as TO-15A, why not combine the two?

While my previous blog covered the need to separate EtO from the air peaks introduced into the instrument, there are several other potential interferences that have to be managed. Acetaldehyde has an almost identical structure and mass spectrum. It is very common in nature and produced in a wide variety of industries, and it is possible to be present in both lab blanks and samples. Methanol also shares several ions with EtO, and as a common solvent for volatile standards (e.g., 8260 and TO-15A internal standards) it’s likely present in most air labs.

Fig. 1 – Comparison of EtO, Acetaldehyde, and methanol mass spectrum

Fortunately the cryo cooling helps with these separations as well. In addition, I also found that using selected ion monitoring (SIM) produced a cleaner baseline and better signal to noise ratio, allowing for detection of EtO down to 0.05ppb or lower (Fig.2 lower trace).

 

Fig. 2 – Comparison of Scan (top) and SIM (bottom) signals for EtO (RT ~8.72) with acetaldehyde (RT ~7.50) and MeOH (RT ~9.00) interferences. EtO at 0.05ppb

Once the troublesome EtO/acetaldehyde/methanol separations are solved with cryo cooling, I was able to use EZGC to get a working oven program to separate the TO-15A compound list. Without the need for extra sensitivity on the TO-15A compounds I found it helpful to use the combined SIM/Scan capabilities of the Agilent 5977A mass spec, using the SIM data for EtO and the scan data for the TO-15A list. This meant I didn’t have to optimize the SIM parameters for nearly 80 compounds, keeping the method much simpler.

Fig. 3 – Combined TO-15A and EtO chromatogram with EIC for compounds 1-7 (top), SIM for EtO ( compound 8, middle), and TIC (compounds 9-79, bottom). TO-15A compounds at 0.2ppb, EtO at 0.05ppb.

# Name Ret Time
1 Propylene 4.17
2 Dichlorodifluoromethane 4.43
3 1,2-Dichlorotetrafluoroethane 5.45
4 Chloromethane 5.62
5 n-Butane 6.52
6 Vinyl chloride 6.54
7 1,3-Butadiene 6.87
8 Ethylene Oxide 8.72
9 Bromomethane 8.75
10 Chloroethane 9.64
11 Vinyl bromide 10.71
12 Trichlorofluoromethane 11.21
13 n-Pentane 11.85
14 Ethanol 13.29
15 Acrolein 13.74
16 1,1-Dichloroethene 13.94
17 1,1,2-Trichlorotrifluoroethane 14.3
18 Carbon disulfide 14.49
19 Acetone 14.55
20 Acetonitrile 15.83
21 Isopropyl alcohol 15.92
22 Methylene chloride 16.5
23 trans-1,2-Dichloroethene 17.6
24 Tertiary butanol 17.67
25 Methyl tert-butyl ether (MTBE) 17.73
26 Hexane 18.8
27 1,1-Dichloroethane 19.35
28 Vinyl acetate 19.65
29 cis-1,2-Dichloroethene 21.49
30 2-Butanone (MEK) 21.62
31 Ethyl acetate 21.9
32 Bromochloromethane 22.29
33 Tetrahydrofuran 22.35
34 Chloroform 22.74
35 1,1,1-Trichloroethane 23
36 Cyclohexane 23.12
37 Carbon tetrachloride 23.35
38 Benzene 23.8
39 1,2-Dichloroethane 23.96
40 Isooctane 24.09
41 Heptane 24.45
42 1,4-Difluorobenzene 24.66
43 Trichloroethylene 24.98
44 1,1,2-Trichloroethane 24.98
45 1,2-Dichloropropane 25.36
46 Methyl methacrylate 25.49
47 1,4-Dioxane 25.49
48 Bromodichloromethane 25.75
49 cis-1,3-Dichloropropene 26.28
50 4-Methyl-2-2pentanone (MIBK) 26.46
51 Toluene 26.64
52 trans-1,3-Dichloropropene 26.91
53 Tetrachloroethene 27.18
54 2-Hexanone (MBK) 27.32
55 Dibromochloromethane 27.49
56 1,2-Dibromoethane 27.6
57 Chlorobenzene-d5 28.02
58 Chlorobenzene 28.04
59 Ethylbenzene 28.11
60 n-Nonane 28.2
61 m- & p-Xylene 28.22
62 o-Xylene 28.55
63 Styrene 28.56
64 Bromoform 28.74
65 Cumene 28.83
66 4-Bromofluorobenzene 28.99
67 1,1,2,2-Tetrachloroethane 29.08
68 n-Propyl benzene 29.16
69 4-Ethyltoluene 29.24
70 2-Chlorotoluene 29.25
71 1,3,5-Trimethylbenzene 29.28
72 1,2,4-Trimethylbenzene 29.57
73 1,3-Dichlorobenzene 29.82
74 1,4-Dichlorobenzene 29.89
75 Benzyl chloride 29.97
76 1,2-Dichlorobenzene 30.17
77 1,2,4-Trichlorobenzene 31.36
78 Hexachlorobutadiene 31.39
79 Naphthalene 31.6

Table 1 – RT for TO-15A and EtO.

GC Agilent 7890B
Injection type On-column
Column 624Sil MS 60m x 0.25mm x 1.4um
Carrier gas He , constant flow
Flow rate 2mL/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/sec) 3.7
SIM parameters
SIM ions 15, 29, 43, 44, 56
Dwell time 50
Transfer line 250°C
Analyzer type Quadruple
Source type Extractor
Source temp 230°C
Quad temp 150°C
Electron energy 70eV
Solvent delay time 0 min
Tune type BFB
Ionization mode EI
Preconcentrator Markes Unity 1+ CIA
Trap 1 settings
Cooling temp 5°C
Desorb temp 300°C
Desorb flow 6 mL/min
Desorb time 180 sec
Internal Standard
Purge flow 50 mL/min
Purge time 60 sec
Volume 50mL
ISTD  flow 50mL/min
Sample
Volume 400mL
Purge flow 50mL/min
Purge time 60 sec
Sample flow 100mL/min

Table 2 – GC/MS and preconcentrator settings

While many labs may be reluctant to use cryogenic cooling due to costs and safety issues, it can be a powerful tool to separate out very volatile compounds. Here it was critical in the separation of EtO from methanol and acetaldehyde. In addition, the ability to acquire both SIM and scan MS data allowed for the increased signal to noise ratio for EtO in SIM mode, while maintaining the simplicity full scan for the TO-15A compounds. Together, cyro cooling and SIM/Scan can allow for the relatively simple addition of EtO down to 50ppt to TO-15A analysis.

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