This study from Markes International describes the sampling and analysis of a combined list of PAMS ozone precursors, TO-15 air toxics and OVOCs at high humidity, without the use of liquid nitrogen or other cryogen.
The method described complies with the detection limit and data-quality requirements of Chinese EPA Method HJ 759 and the Chinese Environmental Air Volatile Organic Compound Monitoring Program. The use of Markes’ groundbreaking Dry-Focus3 preconcentration and water management technology results in excellent chromatographic peak shape at 100% humidity, while maintaining sample-to-sample cycle times below 60 minutes. The use of robust, field-proven dual-column/Deans switch technology in the GC oven allows FID analysis of highly volatile C2 and C3 hydrocarbons, with the remainder detected using a single quadrupole mass spectrometer.
The Chinese Ministry of Environmental Protection has issued a document relating to the Environmental Air Volatile Organic Compound Monitoring Program (December 2017).1 This document requires the monitoring of 117 compounds comprising three main categories of hazardous airborne volatile pollutants:
- Ozone precursors
- 'Air toxics'
- Oxygenated volatile organic compounds (OVOCs).
Obtaining good peak shape and chromatographic separation for this combined compound list typically requires cryogenic cooling of the GC column, with the associated cost and inconvenience (in addition, many thermal desorption (TD) systems also require cryogen).
In this study, the quantitative analysis of this challenging 117-compound target list without the use of liquid nitrogen or other cryogen, and with cycle times of less than 60 minutes per sample is described. The analytical system comprises a canister autosampler, water removal device, thermal desorber and dual-column GC–MS/FID. Together, these enable the monitoring of samples at 100% relative humidity, offer optimum responses for the three C2 and two C3 hydrocarbons monitored using FID, as well as confident compound identification and high sensitivity for the remaining compounds monitored using MS.
The analytical system used for this study was a CIA Advantage-xr canister autosampler and UNITY-xr thermal desorber with a Kori-xr water removal device, coupled to a GC–MS system (Figure 1). This system harnesses Dry Focus3 technology – a unique, three-stage focusing and water management mechanism that operates entirely without liquid cryogen.
The experimental parameters are listed in the application note. The highly efficient water removal of Markes’ cryogen-free Dry-Focus3 approach allows the GC oven to start at the relatively high temperature of 35 °C. This allows more efficient operation without compromising analyte peak shape, and reducing the cost per sample.
Results and Discussion
1. Deans switch method optimisation
Using a double-cut Deans switch method, optimum sensitivity together with excellent peak shape, retention time stability and reproducibility were obtained for this complex target list in a single 52-minute chromatographic run.
2. Chromatography and peak shape
Good peak shape is obtained across the analyte range, including the least volatile compounds in the list. In addition, the expansion of the 30.5–31.2 min range demonstrates identification of seven closely-eluting compounds using their extracted ions.
3. Relative response factors and linearities
System linearity was assessed by sampling 50, 100, 200, 300, 400 and 600 mL of the 100% RH, 10 ppb mixed standard. This represents the equivalent mass of each compound that would be sampled from 400 mL of samples with concentrations of 1.25, 2.5, 5, 7.5, 10 and 15 ppb, respectively.
The nature of the two-column setup means that retention times can be affected by the pressure balance in the system. However, electronic carrier gas control between the GC and the CIA Advantage–UNITY-xr, and the efficient removal of water using Dry-Focus3, means that stable retention times are achieved on both columns.
5. Carryover and blank levels
It is important that the instrumentation used for analysing trace-level samples has minimal memory effects (‘carryover’), from previous samples – even if they are at a higher concentration than those typically analysed. High levels of carryover affect recovery results and also require additional blanks to be built into the sequences to prevent any compounds interfering with subsequent samples.
6. Method detection limits
In this study, method detection limits (MDLs) were determined using a 0.5 ppb standard, with the resulting concentrations for each measurement being multiplied by 3.14 (the Student’s t-value for 99% confidence for seven values) to determine MDL values in ppb. Data for the 13 duplicate compounds was generated using a single PAMS standard. The average MDL was 0.052 ppb.
7. BFB tune
According to the quality requirements of both HJ 7593 and EA-VOC-MP,1 the GC–MS instrument must be tuned so that 4-bromofluorobenzene (BFB) meets specific criteria for ion abundance (and compliance should be checked before starting a sequence of samples). Table 1 (see application note) demonstrates that the system used in this study passes the stated criteria for all ions.
In summary, the CIA Advantage–UNITY-xr preconcentration system with Dry-Focus3 technology is shown to allow simultaneous, cryogen-free analysis of PAMS ozone precursors, TO-15 air toxics and OVOCs listed in the Chinese Environmental Air Volatile Organic Compound Monitoring Program. The dual-column/Deans switch GC–MS/FID strategy employed here provides confident identification and quantitation, with maximum sensitivity achieved in this challenging application by using the optimum
detector for the various compound types.
1. EA-VOC-MP: 2018 年重点地区环境空气挥发性有机物监测方案 [Environmental Air Volatile Organic Compound Monitoring Program in Key Areas in 2018], Chinese Ministry of Environmental Protection, 2017.