Meeting NJ Low Level TO-15 Air Testing Method Requirements

Figure 2: Good peak shape was obtained for challenging polar compounds using the Markes Unity with CIA Advantage and a 30 m Rtx-VMS column with (extracted ion chromatogram of ion 45 (m/z).

GC_AR1166

Peaks

	Peaks	tR (min)
1.	Ethanol	2.68
2.	Isopropyl alcohol	3.45

Conditions

Column	Rtx-VMS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 19915)
	with MXT low-dead-volume connector kit (cat.# 20536)
Standard/Sample
	TO-14A internal standard/tuning mix (cat.# 34408)
	75 comp TO15 + NJ mix (cat.# 34396)
Diluent:	Nitrogen
Conc.:	10.0 ppbv 250 mL injection
Injection	on-column
Oven
Oven Temp.:	32 °C (hold 5 min) to 150 °C at 8 °C/min to 230 °C at 33 °C/min
Carrier Gas	He, constant flow
Flow Rate:	2.0 mL/min
Linear Velocity:	51.15 cm/sec @ 35 °C

Detector	MS
Mode:	Scan
Scan Program:
Group Start Time (min) Scan Range (amu) Scan Rate (scans/sec) 1 0.00 35.0 – 226.00 3.8
Transfer Line Temp.:	250 °C
Analyzer Type:	Quadrupole
Source Type:	Extractor
Extractor Lens:	6 mm ID
Source Temp.:	230 °C
Quad Temp.:	150 °C
Electron Energy:	70.0 eV
Tune Type:	BFB
Ionization Mode:	EI
Preconcentrator	Markes CIA Advantage
Instrument	Agilent 7890B GC & 5977A MSD

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Internal Standards

The NJ Low Level TO-15 method stipulates that internal standards (IS) must accompany each sample run at a concentration of 10 ppbv. Since the calibration is based on a relative response factor (RRF), which is dependent on the performance of the IS, it is a prerequisite to have low relative standard deviations (RSDs) on the IS injections. It is outside the scope of this application note to show all of the optimization experiments conducted in order to optimize method parameters such as the “pre sampling loop fill time”; however, the parameters outlined in Table I worked best on the current system. It is important to note that no two preconcentration systems are identical and they all have slightly different optimized parameters; however, the parameters in Table I represent an excellent starting point.

Markes delivered the current Unity with CIA Advantage with a 1.0 mL sampling loop, which is responsible for delivering the IS to the Unity’s trap. In order to obtain the best IS RSDs and consequently achieve the desired RSDs on the calibration RRFs, the 1.0 mL sampling loop was replaced with a 5.0 mL sampling loop (cat.# 22850). This modification accomplished the following:

1. With the default injection volume of 250 mL (to be discussed in the calibration section), one would have to inject 2.5 ppmv of IS on the 1.0 mL sampling loop in order to attain the required 10 ppbv concentration. This would require a custom IS mix, as most standards are offered at 1.0 ppmv. The 5.0 mL sampling loop avoids the costs and delays associated with acquiring custom IS.
2. It is preferential to take a 1.0 ppmv IS mix and dilute it in a 6 L canister down to 500 ppbv, which gives 10 ppbv on a 5.0 mL sampling loop. This helps to ensure that a leak will not result in the loss of an entire bottle of expensive IS.
3. The use of diluted IS in a 6 L canister allows for the use of a regulator (cat.# 27215) to run the IS at 4 psig. It is outside the scope of this application note to explain why the pressure matters; however, in short, the sample loop cannot be under pressure at the time of injection (unless the pressure is extremely consistent, which is difficult to attain). Therefore, using a regulator to maintain the IS pressure at 4 psig is preferred.
4. As with any analytical instrument, some variability exists. For example, a ±10 µL difference from injection to injection on a 1.0 mL sample loop (remembering that the pressure plays a role in this) would represent 1.0 % in run-to-run variability. However, the same 10-µL fluctuation on a 5.0 mL sample loop would only represent 0.2% variability. So the 5.0 mL sample loop minimizes the RSDs across IS injections.

Calibration Curve

The NJ Low Level TO-15 air testing method requires that the preconcentrator–GC-MS system be calibrated with a minimum of five points (more allowed) and that the range must be from 0.2 to 40 ppbv for the majority of compounds (exceptions to be discussed later). Having to span greater than two orders of magnitude with a preconcentrator–GC-MS system represents the single biggest hurdle in the NJ Low Level TO-15 method. An informal customer survey of laboratories attempting NJ LL TO-15 confirmed that the calibration criterion is the biggest challenge, especially for all 75 compounds. It is a fine balancing act between injection volume, breakthrough volume, detection limits, saturation limits, etc. However, the aim of this application note is to eliminate these obstacles.

Of the possible variables that can affect the calibration curves, injection volume was found to be almost as important as the IS injection precision. The current system had an ideal injection volume around 250 mL. This volume was largely dictated by chloromethane because it would start to break through at larger injection volumes. Had it not been for chloromethane, larger injection volumes would have been suitable. Regardless, 250 mL provides sufficient mass to the detector to have legitimate peaks at 0.2 ppbv, meaning peak signal-to-noise ratios were 10:1 or better at this concentration. This was achieved on a dirty source, which was used in order to represent a real-world scenario. Utilizing the IS parameters outlined here and a 250 mL sample volume of 0.2, 0.8, 2.0, 10, 20, and 40 ppbv of the target analytes, the calibrations curves shown in Table III, were easily achieved. It is important to note that 250 mL was injected at each calibration point, as opposed to injecting different volumes of the same calibration standard.

NJ Low Level TO-15 air testing method stipulates that the calculated RSD for RRF for each compound in the calibration must be less than 30%, with at most two exceptions up to a limit of 40%. As shown in the Table III, the %RSD for the average RRF was 14.8%. The only two compounds that do not meet the method criteria are ethanol and acetone. However, NJ LL TO-15 provides an exception that the lowest calibration point may be higher than 0.2 ppbv; however, the upper limit of 40 ppbv remains the same. This exception is applicable to the following list of compounds: acetone, chloroethane, chloromethane, carbon disulfide, dichlorodifluoromethane, 1,4-dioxane , ethanol, isopropanol, methylene chloride, methyl ethyl ketone, methyl methacrylate, tertiary butyl alcohol, tetrahydrofuran, and 1,2,4-trimethylbenzene. The authors of NJ Low Level TO-15 method realized that these compounds could present a challenge. In particular, ethanol and acetone are difficult at the lower calibration levels due to background contamination. If the two lowest calibration points are removed for both of these compounds due to blank cleanliness, then the %RSDs become 7.41% and 6.31% for ethanol and acetone, respectively.

Method Detection Limits

The NJ Low Level TO-15 air testing method identified MDLs as being one of the items modified from U.S. EPA Method TO-15. However, the only modification is that NJ LL TO-15 has outlined “specific criteria that the laboratory must meet regarding the Method Detection Limit (MDL)” and “An MDL study must be conducted annually.” Other than that, NJ LL TO-15 actually states that “the laboratory shall calculate all Method Detection Limits (MDLs), in accordance Method TO-15. Section 11.2 of US EPA Method TO-15 requires the use of the procedures stated in Appendix B of 40 CFR 136 for performing the MDL study.” This means that laboratories shall continue as is normally done for U.S. EPA Method TO-15; however, with the stipulation that they conduct the MDL determination using a spiking solution at 0.20 ppbv, and that the derived MDL must be less than the clean canister certification level of 0.20 ppbv. NJ Low Level TO-15 method does allow for some compounds to be spiked at higher concentrations. Using the parameters outlined above, seven injections of a 0.20 ppbv standard resulted in average scan method detection limits (MDLs) of 0.101 ppbv and all but two of the 75 compounds had an MDL less than 0.20 ppbv (Table III). Acetone and tertiary butyl alcohol (TBA) were marginally above 0.20 ppbv. Again, the authors of the NJ Low Level TO-15 method accounted for this and provide leeway by setting higher MDLs for acetone (1.00 ppbv) and TBA (3.00 ppbv). The aforementioned results were all achieved in full scan mode, with only a 250 mL injection, with a dirty source, and without manual integrations (except for gross errors), meaning they are realistic representations of real-world samples.

Precision

The NJ Low Level TO-15 method follows the exact same precision guidelines set forth in U.S. EPA Method TO-15. The precision experiment data (Table III) demonstrate that the Markes Unity with CIA Advantage and a 30 m Rtx-VMS column easily meet the NJ Low Level TO-15 method requirement (i.e., replicate precision within 25%) for all 75 target analytes. The average precision is 13.3 %RSD.

Accuracy

Results from the analytical accuracy experiment shown in Table III demonstrate the Markes Unity with CIA Advantage and a 30 m Rtx-VMS column meet the NJ Low Level TO-15 method requirement (i.e., audit accuracy within 30%) for all 75 target analytes, except for ethanol. The failing result for ethanol is directly related to the canister blank concentration and resulting poor calibration curve. However, if the two lowest calibration points are removed for ethanol due to blank cleanliness, then the accuracy becomes 27.7% for ethanol. The average analytical accuracy is 7.60%.

Carryover

The Markes Unity with CIA Advantage contains a proprietary triple-sorbent trap. Although the trap is a trade secret, it is clear that it contains a carbon sorbent. Carbon sorbents are so strong that they represent a concern for sample carryover. Results from the carryover evaluation demonstrate an average carryover of 0.03 and 0.17% for experiments 1 and 2, respectively. Note that these same carryovers correspond to 0.03 and 1.14 ppbv in the carryover runs for experiments 1 and 2, respectively. Results from carryover experiment 1 indicate that routine samples do not result in carryover of any concern. Concern was defined as any concentration close to the blank cleanliness level (0.20 ppbv) by NJ LL TO-15. However, results from carryover experiment 2 suggest that when analyzing high concentration samples, they should be run at the end of the sequence and with blanks/conditioning runs in between sample injections.

Table III: Results from calibration, MDL, precision, and accuracy experiments demonstrate the NJ Low Level TO-15 method criteria were met by the Markes Unity with CIA Advantage and 30 m Rtx-VMS analytical column.

¹ Six-point calibration curve in scan mode.
² Calculated as the standard deviation of seven replicate analyses of a 0.20 ppbv standard and the Student’s t-test value for 99% confidence.
³ The %RSD obtained from seven replicate analyses of a 0.20 ppbv standard in scan mode.
⁴ Determined from a 10.0 ppbv audit standard.
* Listed in NJ LL TO-15, but not required. Ethanol omitted from averages.

Conclusions

This investigation was conducted to evaluate the efficacy of pairing Restek products with the Markes Unity with CIA Advantage to meet the most critical analytical requirements of the NJ Low Level TO-15 air testing method. Results demonstrate that the combination of the Markes Unity with CIA Advantage and a 30 m Rtx-VMS column affords end users the ability to conduct the NJ Low Level TO-15 method. It is important to note that all of the aforementioned results were obtained on a “close to real world” analytical system (i.e., although the instrument was tuned, the source had not just been freshly cleaned). Furthermore, except for instances in of gross error (very infrequent), all chromatographic peaks were auto-integrated (i.e., the peaks were not manually adjusted).

Acknowledgments

Markes

References

1. U.S. Environmental Protection Agency, Compendium Method TO-15, Determination of volatile organic compounds [VOCs] in air collected in specially-prepared canisters and analyzed by gas chromatography/mass spectrometry (GC/MS), January 1999. http://www3.epa.gov/ttnamti1/files/ambient/airtox/to-15r.pdf
2. J.S. Herrington, Rapid determination of TO-15 volatile organic compounds (VOCs) in air, Application note, EVAN1725A-UNV, Restek Corporation, 2016. https://www.restek.com/articles/rapid-determination-of-TO-15-volatile-organic-compounds-VOCs-in-air/
3. New Jersey (NJ) Department of Environmental Protection (DEP), Site Remediation Program (SRP), NJDEP-SRWM Low Level USEPA Method TO-15, March 2007. http://www.nj.gov/dep/srp/guidance/vaporintrusion/newmethod2007/
4. M.S Klee, L.M. Blumberg, Theoretical and practical aspects of fast gas chromatography and method translation, J. Chromatogr. Sci., 40 (5) (2002) 234-47. https://www.ncbi.nlm.nih.gov/pubmed/12049153