off line OC/EC analysis (Sunset vs. DRI)

Inter-Instrument (Sunset vs. DRI Model 2001 Thermal/Optical Carbon Analyzer) comparison

DRI2001

Sunset

Organic carbon (OC) and elemental carbon (EC) are operationally defined due to the lack of definitive standards. Consequently, their quantification is protocol-dependent. IMPROVE and NIOSH are the two widely used thermal/optical protocols for OCEC analysis, differing in temperature programs and in the optical method for charring correction. The IMPROVE protocol is often implemented on a DRI analyzer while the NIOSH protocol is often implemented on a Sunset Laboratory Analyzer. Evaluation of the implementation of the IMPROVE protocol on the Sunset Laboratory analyzer or implementation of the NIOSH or NIOSH-derived protocols on the DRI analyzer has rarely been reported. We analyzed OC and EC in about 100 ambient samples collected in the Pearl River Delta in China by implementing the IMPROVE protocol and a NIOSH-derived (ACE-Asia) protocol on both a DRI Model 2001 analyzer and a Sunset Laboratory analyzer. The total carbon (TC) and EC filter loading as determined by the ACE-Asia protocol on the Sunset analyzer varied from 2.6-67.0 and 0.2-7.4 µg cm^-2, respectively. Inter-instrument comparison indicates that the implementation of the IMPROVE protocol on the Sunset analyzer reports TC, EC, and OC measurements in good agreement with those made on the DRI analyzer. EC and OC analyzed using the ACE-Asia protocol are also in good agreements for measurements implemented on the Sunset and the DRI analyzers. Inter-protocol comparison indicates consistency in TC determination but discrepancies in OC and EC, with the IMPROVE protocol reporting much higher EC than the ACE-Asia protocol. An analysis of different comparison scenarios reveals that the cause of the EC difference could be quantitatively attributed to temperature protocol (thermal effect) and optical pyrolysis correction method (reflectance vs. transmittance). The variation in EC concentrations was more pronounced in samples that produced more charred OC during thermal analysis.

Table 1. Temperature programs of the IMPROVE and ACE-Asia protocols.

Comparison of instrument design between Sunset and DRI ECOC analyzer

1 Oven design

The structure is more or less the same. The major difference is that DRI front port is connect to a "auto sampler" and the movement of sample boat is controlled by linear actuator while sample feeding in Sunset is manually. Sunset has one port for thermocouple while the in DRI's design the thermocouple is combined with sample boat. DRI has one port for acid injection but sunset didn't provide this feature.

2 Sample feeding

DRI analyzer

The unique feature of DRI analyzer is that it allows sample boat moves after front port sealed. These making acid injection become possible. Since movement is done by linear actuator, the positioning and alignment with laser is expected to be more precise. However, the trade-off is that leaking happens frequently at the fitting part, the user have to tighten the nut every a few days and need to replace the ferrule when tightening didn't help.

Sunset analyzer

There are no moving parts in Sunset's front port so it's more robust. The trade-off is that the positioning of sample to the laser may varied by user operations. And also, there's no port for acid injection.

3 Laser configuration

DRI laser configuration

DRI use quartz light pipe to guide the travel of laser, which minimize the effect of mirage.

4 Miscellaneous

Punch size. Sunset is 1/1.5 cm² DRI is 0.5 cm²

The DRI heating coil for sample oven embed modular design, it's not only easier to replace, but also minimize the variability of heating efficiency induced during heating coil installation.