Specifying a multi-gas detector on an emissions-sampling RFQ is a line-by-line engineering exercise, not a brand preference: the buyer must name the target analytes with explicit concentration ranges, the sensing technology per gas, sample-conditioning hardware, calibration-gas certification, and data-acquisition interface, because every omitted field tends to come back as a vendor assumption or a change order [S5][S1].
For an emissions-sampling system the underlying test plan typically references EPA methods under 40 CFR 60, with FTIR (MKS MultiGas 2030 class) or equivalent extractive analyzers reporting concentrations in ppm or percent by volume after zero/span calibration against EPA Protocol gases [S1]. Continuous Emissions Monitoring Systems (CEMS) for criteria pollutants (NO, NO2, SO2, CO, O2, CO2) commonly pair electrochemical cells for low-ppm toxic species with non-dispersive infrared (NDIR) for combustible and IR-active species, e.g. NOVA 7200 stack-ranges of 0-2000 ppm NO, 0-800 ppm NO2, 0-2000 ppm SO2, 0-10% CO, 0-25% O2, 0-20% CO2 [S2].
Target Analytes and Concentration Ranges
Every RFQ line must list each gas as a discrete row with range, units, accuracy, and resolution, because stack concentrations vary by orders of magnitude between species: O2 routinely runs 0-25% while NO/NO2 sit in 0-2000 ppm and SO2 in 0-2000 ppm, so range and resolution must be quoted per channel rather than a generic "multi-gas" [S2][S3]. Combustible species such as methane are typically 0-100% LEL, while acid gases and VOCs (H2S, NH3, HCl) often need sub-ppm resolution that only electrochemical or PID cells can deliver [S3][S5].
For compliance-grade stacks under 40 CFR 60 Subpart JJJJ, the test protocol requires introduction of zero gas to the analyzer and setting of span before each run, so the RFQ should call out the zero-gas source (zero-air generator or certified cylinder), the span-gas concentration (typically 80-100% of range), and the calibration sequence [S1]. Buyers should also declare the expected operating temperature and pressure of the flue gas; dilution probes, chilled mirrors, and nafion dryers are common conditioning steps that vendors will not include unless specified [S2][S7].
Sensing Technology Selection by Gas
IR (NDIR/FTIR) is preferred for hydrocarbons, CO2, CO, and SO2 because it is non-consuming, immune to poisoning, and stable in wet streams; the same is true of PIDs for VOCs at sub-ppm level, while electrochemical cells dominate for O2, NO, NO2, H2S, and NH3 in the 0-2000 ppm window [S3][S2]. A practical multi-gas detector in emissions service is therefore a hybrid: IR for 3-4 channels plus 2-4 electrochemical cells, with cross-interference (e.g. NO response on SO2 cell) checked against the manufacturer's published interference table, not assumed away [S3].
The table below maps the common RFQ columns to the technology that fits, based on the working ranges published for stack analyzers [S2][S3][S5]:
Target gas vs sensing technology (emissions-sampling multi-gas detector, typical stack ranges). O2 (0-25%) by electrochemical or zirconia; CO (0-1000 ppm low, 0-10% high) by electrochemical low range or NDIR high range; CO2 (0-20%) by NDIR; NO (0-2000 ppm) by electrochemical or chemiluminescence; NO2 (0-800 ppm) by electrochemical; SO2 (0-2000 ppm) by NDIR or UV; CH4/HC (0-100% LEL) by NDIR or catalytic bead; H2S (0-100 ppm) by electrochemical; NH3 (0-100 ppm) by electrochemical or PID; VOCs (ppb-ppm) by PID or FTIR. FTIR (e.g. MultiGas 2030) covers the full suite simultaneously with a single optical bench, which is the EPA-method reference for many Subpart JJJJ campaigns [S1][S2][S3][S5].
Sample Conditioning and Stream Preparation
Raw flue gas at 200-600 deg C with 5-20% moisture will destroy electrochemical cells and condense acid aerosols in sample lines, so the RFQ must call out a sample-conditioning train: heated filter, heated line (typically 180 deg C), nafion or chiller dryer, pump, and flow switch with bypass [S2][S7]. Multi-stream systems for CEMS use gas-actuated stainless diaphragm valves to asynchronously switch between sample points, calibration gas, and zero gas, which is the architecture referenced in continuous stack monitoring [S7].
Materials matter: 316L stainless or PTFE-lined sample lines are mandatory for SO2 and HCl service to avoid adsorption losses, and any elastomer seals must be specified as Viton or FFKM rather than generic nitrile [S7]. Without these line items, vendors will quote an ambient air multi-gas detector and the unit will be unsuitable for the stack application, requiring requote and a 4-8 week project slip [S2][S7].
Calibration Gas and Quality Assurance
For EPA Method compliant work the RFQ must require EPA Protocol Gas (or NIST-traceable equivalent) with a stated cylinder certification, expiration date, and a calibration-gas shelf life in months; span values typically run 80-100% of the analyzer range and must bracket the expected stack concentration [S1]. Zero air must be specified as <0.1 ppm total hydrocarbon, CO, and CH4 with a stated generator or cylinder source, and the protocol document should be appended to the RFQ so the bidder quotes against the same test plan [S1].
Relative Accuracy Test Audit (RATA) requirements mean the multi-gas detector should support automatic zero/span cycling every 24 hours and an adjacent calibration-gas port, otherwise the operator pays for mobile CEMS validation annually and the installed analyzer is classified non-compliant per EPA framing [S2]. Auditable data logging with timestamped raw counts, cell temperature, and cell pressure is part of the deliverable, not an optional extra, and should be itemized as such [S2].
Interface, Outputs, and Hazardous Area
Typical signal outputs on a stack-mounted multi-gas detector are 4-20 mA per channel plus Modbus RTU over RS-485 or Ethernet, with relay outputs for fault, high alarm, and high-high alarm, which must be enumerated on the RFQ line so the I&E team can wire to the existing DAS [S8]. Wireless options (ISA100, WirelessHART) exist for remote stacks but require the buyer to declare the existing gateway or accept the vendor's as a separate line item [S8].
For combustor or compressor exhaust service near a gas handling train, hazardous-area classification (Class I Div 1/2 or ATEX Zone 1/2) drives the enclosure spec; purge kits are available for analyzer enclosures and should be requested explicitly when the sample point sits inside a classified area, otherwise the supplier will default to a safe-area wall-mount that fails the plant's electrical classification [S2]. IP66 or NEMA 4X is appropriate for outdoor rooftop or offshore service and should be written on the RFQ line, not left to the bidder's discretion [S2][S8].
RFQ Line-by-Line Spec Template
The following field set is what separates a usable emissions-sampling RFQ from a generic multi-gas detector quote: 1) target gas list with range, units, accuracy % of reading or % of full scale; 2) sensing technology per gas; 3) response time T90 in seconds; 4) sample conditioning train (filter, dryer, pump); 5) line material and heat-tracing requirement; 6) zero and span gas type, certification, and shelf life; 7) calibration interval and RATA compatibility; 8) output signals (4-20 mA count, Modbus, relay count); 9) hazardous area classification and enclosure rating; 10) data logging and remote access; 11) MTBF and warranty terms; 12) training and startup support [S1][S2][S5][S7][S8].
Common requote triggers in real emissions-sampling procurement: forgetting to specify heated lines, omitting acid-gas line material, defaulting to ambient ranges, leaving hazardous area blank, or not declaring whether the unit is for compliance reporting or non-compliant combustion trim, which determines whether the supplier quotes a CEMS-grade instrument or a portable flue gas analyzer like the NOVA 5300FG [S2]. A clean RFQ mirrors the test-protocol document the operator already has on file, and that protocol under 40 CFR 60 is the single best template for what fields must be filled in [S1].
Applicable Standards and Sourcing
EPA Reference Methods under 40 CFR 60 govern criteria pollutant measurement on stacks, while EPA Protocol Gas standards (40 CFR 60, Appendix F) define the calibration-gas certification that the RFQ must cite [S1]. For hazardous-area electrical, IEC 60079 or NEC Class I Div 1/2 nomenclature applies, and the RFQ should not invent a standard number, but rather state the area classification and let the supplier match the certification [S2]. NFPA 72 governs heat and fire detection, not stack emissions, so it should not be cited for the multi-gas detector portion of an emissions RFQ; for related electrical-room gas-detection routing the buyer can reference the fixed gas detector architecture as a parallel selection pattern [S3].
Sample-handling schematics from established CEMS component suppliers (gas-actuated diaphragm manifolds, multi-stream sequencers, FTIR benches) are the cleanest technical reference to attach to the RFQ, and they illustrate why an analyzer is rarely a single instrument: it is a system of valves, lines, dryers, and a multi-gas detector that must be specified together to avoid requotes [S7][S8]. For buyers who need a general gas detector overview, the technology taxonomy (electrochemical, NDIR, PID, catalytic bead, MOS) is the cross-reference for stack vs ambient choices [S3][S5].
Trackable signals to watch on a 2026-09-14 procurement cycle: confirmation that bidders quoted heated sample lines and acid-gas rated wetted parts, that zero/span EPA Protocol gases were priced as a recurring consumable line rather than buried, and that the hazardous area certification matches the plant classification document exactly [S1][S2]. A secondary check is whether the supplier's published interference table covers NO-on-SO2 and CO-on-H2S, because that single line drives the accuracy spec in the field more than any other [S3][S5].
Background reading: Casting Mold RFQ: Line-by-Line Spec for Metal Treatment Builds.