HCL by FTIR 

 Marty Panek at Midac Sales 

 to: 

 Keith Barnett 

 06/02/2010 10:08 AM 

 Please respond to mpanek 

 Show Details 

Keith, here is what my apps dept came up with on your HCL application. 

Please do not hesitate to contact me if you should have any questions. 

 

Regards 

 

Marty 

 

 
1 June 2010 PPM Measurement of HCl MIDAC Corporation

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PPM-Level HCl Measurements by FTIR Spectroscopy 

Peter Zemek, PhD, MIDAC Corporation 

Joe Van Gompel, PhD, MIDAC Corporation 

Steven Plowman, MIDAC corporation 

Introduction 

Fourier Transform Infra-Red (FTIR) spectroscopy is an accepted and well-known 

technique by the EPA to measure hydrogen chloride (HCl). Over the past 10 years, HCl 

by FTIR has been utilized at many cement kilns and other incineration technologies 

successfully by EPA Method 321 or 301. HCl is a common byproduct from cement kilns 

and municipal waste incinerators. Commonly, kilns will receive hazardous waste and 

other chlorinated compounds, if a boiler and incineration furnace (BIF) source. Chlorine 

also enters the cement kiln in with the raw materials. In a municipal waste incinerator 

many poly-chlorinated materials such old PVC piping are burned and HCl is produced, 

along with CO2 and water vapor. HCl is considered a hazardous air pollutant (HAP) by 

the US EPA. A significant source is characterized as emitting HCl at greater than 10 

tons / year. 

FTIR spectroscopy is a powerful tool for analysis of HCl at sub-ppm levels. There have 

been many comparisons to HCl by manual methods such as EPA Method 26 and 26A 

to FTIR. FTIR has also been utilized as a CEM by Performance Specification 15 (PS15) and as a primary method when performing RATA analysis. The advantages of HCl 

by FTIR are the immediate near-real-time field results, reduction of laboratory error, and 

contamination. The FTIR Methods are self-validating when used with a spiking regimen 

and proven in the field and in the laboratory. This method requires the use of a high 

resolution FTIR instrument to obtain sub-ppm level detection limits. A resolution of 0.5 

cm-1 to 1 cm-1 instrumentation is usually required. 

The only drawback of HCl by FTIR in cement kilns is the possibility of kinetic reactions 

of the HCl with high ammonia levels in the stack, heated filter, or sampling system 

causing the formation of ammonium chloride salts on the filter or in the heated line. 

This causes a certain amount of lag time in the spiking regimen but is usually overcome 

by the additional wait time while spiking to reach steady-state, need for a higher spiking 

concentration, or the need to place the FTIR instrument closer to the sample port. 

These issues can be overcome in the field with a review of the sampling system and 

field modifications. However, once a spike regimen is passed, the method, instrument, 

and sampling system has been validated and concentrations may be considered 

accurate and precise. 

The work described here was performed over many years, at a cement kiln and a 

municipal waste combustor. The major goal of the study was to validate extractive 

Fourier transform infrared (FTIR) spectrometry for measurements of gaseous hydrogen 

chloride (HCl) as described in Environmental Protection Agency (EPA) Test Methods 

3011

 , 320, and 3212. Three different validation procedures were successfully 

performed; they were direct comparisons with EPA’s manual Test Method 263, “single 

instrument” dynamic spiking, and “dual instrument” dynamic spiking. 
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HCl by FTIR

The infrared spectrum of HCl is distinctive and readily observed, even in the presence 

of water vapor interference. HCl appears as a series of doublet peaks that rise, fall to 

zero, then rise and fall to zero again. These regular peaks are observed between 2650 

cm-1 and 3100 cm-1. Each peak is split into a doublet with the ratio of 3:1 based on the 

ratio of 35Cl :37Cl isotopes. The mid-infrared spectrum of HCl is shown in Figure 1. 

Figure 1. Hydrogen Chloride, 148.6 ppm-m, 25o C. 

The ability to analyze HCl in the presence of water is imperative as water vapor 

concentrations are very high from combustion sources such as incinerators. Spectra 

incorporating these high concentrations of water are shown in Figures 2 and 3. 
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Figure 2. 42 ppm-m HCl in baseline of 14% water vapor. 

Figure 3. Figure 2 expanded to see HCl clearly in presence of high levels of 

water vapor. 
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Figure 4. HCl in the presence of water vapor. HCl peaks are indicated which 

can be used for quantitation. 

The regular pattern of peaks for HCl is common for diatomic molecules such as HF, 

HBr, CO, and NO as well. It is due to rotational transitions between the ground and first 

excited vibrational states of the molecules. Known as the P-branch (lower energy) and 

the R-branch (higher energy), these fan-like spectral features surround the Q-branch, 

which is forbidden by quantum mechanics in the primary band of the heteronuclear 

diatomics. More complex materials, like NH3 and CH4, show prominent Q-branches. 

Quantitative Analysis of HCl by FTIR

Test Facility

The test facility is a typical municipal waste combustor/generating station using refuse 

derived fuel. The plant consists of several independent “units,” two of which (referred to 

as “Unit 1” and “Unit 2”) were sampled during this work. Each unit includes a boiler and 

a set of emissions control devices, notably a lime-slurry injection system for reducing 

the HCl emissions. All validation testing was performed at one of the two control-device 

outlet locations; the sampling ports were located in rectangular cross-section ducts 

connecting the outlets of the induced-draft fans to the base of the stack. Inlet testing 

was performed at a set of ports located just downstream of the Unit 1 boiler. 

Sampling Systems 

The sample streams passed through heated probe assemblies, heat-traced (~350F, 

181o

C) PTFE sample lines, and the absorption cells of each of two FTIR spectrometers; 

a pump connected to the cell’s outlet ports continuously drew sample gases through the 

cells at approximately 10 liters per minute (lpm). The only materials used in the 

sampling systems were Pyrex, Type 316 stainless steel, and Teflon. All Method 26 
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sampling was performed using equipment and techniques described in Reference 3; the 

resulting samples were submitted to Testar, Inc. (Raleigh, NC) for HCl analysis. 

FTIR Analytical Systems 

The two FTIR spectrometers used in this work are MIDAC Corporation (Costa Mesa, 

CA) Model I-2000 instruments (see Figure 1) with nominal one half wavenumber (0.5 

cm-1) spectral resolution; they are designated below as Instruments “A” and “B.” Both 

systems employ Michelson interferometers, beam splitters and cell windows of zinc 

selenide (ZnSe), hot-wire infrared radiation sources, front-surface optical transfer 

mirrors, and multi-pass absorption cells. Mercury cadmium telluride (MCT) detectors 

were used for the majority of the testing described here; for the diluted inlet samples 

(only), an indium antimonide (InSb) detector was installed in Instrument B. In all cases, 

the detectors were cooled with liquid N2 and their temperatures maintained at 77 K. The 

10-meter path length “White” absorption cells use gold-surface internal mirrors, and 

their interior cell walls are made of polished nickel to minimize chemical interactions 

with the sample gas. Transducers and thermocouples connected directly to the 

insulated sample cells provide the pressures and temperatures of the sample streams. 

During testing, the temperature of the absorption cells was 181o C; the elevated 

temperature prevents gas condensation within the cell and minimizes HCl retention by 

the cell walls and mirrors. The volumes of the absorption cells are 2.0 liters, so at a flow 

of 10 lpm the sample gas in each cell is refreshed approximately five times per minute. 

Interferograms consisting of 50 co-added scans were recorded nearly continuously 

during the test periods, and provided one-minute average concentrations. 

Heated Cell

FTIR

Spectrometer

Heated Line

Outlet to pump

Temp, Pres.

Controller

FIGURE 5. Typical FTIR spectrometer system during testing. 

FTIR Spectral Analyses 
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The program AutoquantPro (hereafter “AQ,” version 4.0.0.111, Midac Corporation, 

2004) was used to collect and analyze all the infrared field data. AQ allows the 

development and storage of analytical “methods” for analysis of spectral data 

(absorbance) files. Various methods were generated for the instrument-detector 

combinations described above and used to for determine the absorption path lengths 

and the concentrations of the gaseous analytes. On every test day, at least one 128-

scan background spectrum was recorded with dry N2. Before and after each test run, 

spectra of a calibration transfer standard (CTS) gas were obtained and used to 

determine the effective absorption path lengths of the gas sample cells. A cylinder of 

ethylene C2H4 in N2 (103 ppm) served as the CTS, and the absorption path lengths 

were determined by comparing the field CTS spectra to a well-characterized laboratory 

ethylene spectrum. All the absorption path lengths (for each instrument-detector 

combination) varied by less than 5% over the entire test period. The analytical results 

presented in this work are based on the average path length determined for each 

instrument-detector combination over the course of the field tests. 

The HCl concentrations in the extracted FTIR samples were determined by 

mathematically comparing the samples’ absorbance spectra to a number of HCl 

reference spectra; these reference spectra were recorded during the test period using 

the two field instruments and a cylinder standard of 86.2 ppm HCl and 5.03 ppm SF6

(balance N2). Using both a NIST-calibrated mass flow controller and standard 

barometric techniques, the reference samples were made by diluting the HCl/SF6

standard gas to four lower volumetric concentrations. The absolute HCl concentration of 

the cylinder standard was determined in the laboratory according to the sampling and 

analytical procedures of EPA Method 26; only the relative SF6 concentrations are 

required for the validation procedures described below, so the SF6 cylinder 

concentration was not independently confirmed. 

Preliminary concentrations of three other analytes – carbon monoxide (CO), sulfur 

dioxide (SO2), and nitric oxide (NO) – were also determined using quantitative FTIR 

reference libraries prepared in the laboratory from dilution of certified calibration gas 

standards. In some cases the observed concentrations of these compounds exceeded 

the calibration ranges of the spectrometers. 

Table 1 presents the infrared wavenumber (cm-1) analytical ranges and concentrationpath length (ppm-meter) limits of the reference libraries employed to date. 

HCl Method Validations 

EPA’s Test Methods 301 (Reference 1) and 320 (Reference 2) both provide statistical 

techniques for the validation of emissions test procedures. Three validation procedures 

for FTIR measurements of HCl were performed and are described below. Note that 1 

ppm of a gas in a 10-m gas cell is 10 ppm-m. 
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TABLE 1. FTIR REFERENCE SPECTRA AND ANALYTICAL REGIONS 

ANALYTE WAVENUMBER 

RANGE 

NUMBER OF 

SPECTRA PPM-M RANGE 

HCl 2768.93 – 2849.00 cm-1 5 45 – 763 ppm-m 

SF6 909.02 – 967.51 cm-1 5 2.67 – 44.57 ppm-m 

CO 2168.18 – 2180.62 cm-1 3 64 – 263 ppm-m 

NO 1872.50 – 1877.30 cm-1 3 563 – 2285 ppm-m 

SO2 1068.96 – 1252.55 cm-1 2 217 – 441 ppm-m 

 

Method 301: Reference Method Comparison 

EPA Method 301 (Sections 5.2 and 6.2) allows the validation of a proposed test method 

(here, FTIR measurements of HCl) by a pair-wise comparison between the results of a 

simultaneously-performed EPA reference method (here, EPA Method 26). See Figure 

6. A complete description of the validation requirements and calculations is given in 

Reference 1; only a summary is provided in this work. Briefly, the results SR of nine test 

runs using the reference method are compared with results SP of the proposed method 

and the relative standard deviation SDP of the set of nine differences (SR – SP) is 

compared via an F-test to the published standard deviation of the reference method 

(here, SDR = 3.2% at 15.3 ppm and SDR = 6.2% at 3.9 ppm). Next, the bias B and a 

correction factor CF for the proposed method are calculated and a t-test is applied to 

determine whether the bias of proposed method is statistically significant. The criteria 

required for validation of the proposed method are a) SDP  SDR and, when the bias is 

found to be significant b) 0.9  CF  0.1. Table 2 presents the nine pairs of 

concentration results used in these calculations. 

Figure 6. Validation of proposed test method using FTIR and EPA Method 26. 
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TABLE 2. METHOD 301 VALIDATION RESULTS (PPM) 

SAMPLE 

PAIR M 320 RESULTS SR FTIR RESULTS SP

1 8.54 8.22 

2 9.42 10.96 

3 11.20 12.96 

4 22.10 20.04 

5 22.60 21.30 

6 22.90 22.81 

7 23.10 23.20 

8 38.60 36.69 

9 68.80 64.29 

The paired results of Table 2 yield relative standard deviation SDP = 4.3%, which lies 

between the two values (3.9% and 6.2%) published by EPA for the reference method. 

Although the t-test shows that the proposed method bias (2.0 ppm) is significant (t = 

2.148), the F-test (F = 0.108) indicates good precision, and the correction factor (CF = 

1.031) falls in the acceptable range 0.9  CF  1.1. 

METHOD 320: Dual Instrument Spiking Validation 

EPA Method 320 (Reference 2, Section 13) allows validation of FTIR-based 

measurements by a pair-wise comparison between the results of two independent FTIR 

systems. One of these systems must provide the “native” (or “un-spiked”) 

concentrations in 12 independent samples, and the other must provide simultaneous 

measurements of samples which have been dynamically “spiked” to provide a 

calculable change in the analyte concentrations. The means of the spiked results and 

calculated spiked levels provide a bias estimate for the FTIR measurements, and a ttest is applied to the 12 differences in these values measurements to determine whether 

the bias is significant. If the bias is found significant, a correction factor is calculated 

and must lie in the range 0.7  CF  1.3. 

In this work, one of the two FTIR spectrometers (Instrument A) was used to collect and 

analyze 12 unspiked gas samples. Simultaneously, Instrument B analyzed spiked 

samples collected from the same outlet sample port. Spike gas was introduced into the 

Instrument B sampling system by injecting gas from a cylinder standard of 86.2 ppm 

HCl and 5.03 ppm SF6 (balance nitrogen) directly into the sample probe; the spike gas 

flow rate was adjusted (by monitoring the observed SF6 concentration) so that the spike 

gas represented 10% to 20% of the total Instrument B sample flow rate. Table 3 

presents the HCl and SF6 results of these measurements for the spiked and unspiked 

samples. 
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Table 3. Method 320 Dual-Instrument Spiking Results (ppm) 

SAMPLE PAIR SS (HCL) SU (HCL) SS (SF6) 

1 13.1 7.26 0.517 

2 13.4 7.45 0.516 

3 13.4 7.43 0.519 

4 13.4 7.24 0.518 

5 13.2 7.10 0.516 

6 13.4 7.41 0.519 

7 13.4 7.53 0.521 

8 13.4 9.00 0.521 

9 13.7 8.92 0.521 

10 13.5 7.81 0.522 

11 13.0 7.12 0.521 

12 13.6 7.36 0.516 

MEAN 13.4 7.62 0.520 

STDEV 0.18 0.62 

From the mean of the SS (SF6) values and the cylinder SF6 value concentration, the 

expected mean spiked concentration is CS = 16.5 ppm; the actual mean spiked 

concentration is 13.4, so the bias in the proposed method is B = 3.1 ppm. The t-test 

using the standard deviations of the values SS – SU yields t = 5.76, indicating that a 

correction factor is required; the correction factor is 

CF = [1 +B/CS]-1 = 1.236 (1) 

This value falls within the allowed range (0.7  CF  1.3). 

Method 320: Single Instrument Spiking Validation 

EPA Method 320 (Reference 2, Section 13) also allows validation of FTIR-based 

measurements by a pair-wise comparison between the results of a single FTIR system. 

The procedures and analyses are identical to those described above for the dualinstrument validation, except that the spiked-unspiked sample pairs are the results of 

sequential measurements on a single system (rather than simultaneous measurements 

on a pair of system). Table 4 presents the HCl and SF6 results of these measurements 

for the spiked and unspiked samples. 
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Table 4. Method 320 Single-instrument Spiking Results (ppm)

SAMPLE PAIR SS (HCL) SU (HCL) SS (SF6) 

1 14.2 8.83 0.517 

2 15.3 8.73 0.531 

3 15.1 8.91 0.523 

4 14.6 9.86 0.517 

5 13.5 9.45 0.517 

6 13.8 9.06 0.523 

7 13.8 8.51 0.525 

8 13.9 7.88 0.525 

9 13.9 8.79 0.523 

10 12.9 9.46 0.520 

11 13.5 8.89 0.523 

12 13.9 8.40 0.523 

 

MEAN 14.0 8.90 0.522 

STDEV 0.66 0.51 

From the mean of the SS (SF6) values and the cylinder SF6 value concentration, the 

expected mean spiked concentration is CS = 17.6 ppm; the actual mean spiked 

concentration is 14.0, so the bias in the proposed method is B = 3.6 ppm. The t-test 

using the standard deviations of the values SS – SU yields t = 4.19, indicating that a 

correction factor is required; the correction factor is (see Equation 1) 1.257, which falls 

within the allowed range (0.7  CF  1.3). 

Minimum Detectable Limits 

The MDL is a critical parameter for any type of low-concentration analysis. A study 

according to ASTM D6348-98 was performed looking at the noise levels in the baseline 

of the MIDAC I-2000 FTIR spectrometer using a heated 10-meter pathlength gas cell in 

the presence of up to 20% water vapor. Under these conditions, the MDL for HCl was 

0.150 ppm. Though not required by the ASTM method, a bias (average of the standard 

deviations of each measurement) was added to yield a more conservative 0.263 ppm 

MDL. Detection limits may be improved by using indium arsenide or indium antimonide 

detectors, though this would rule out analysis of compounds with showing absorbance 

only below 2000cm-1. 

Conclusion 

FTIR spectroscopy has been validated by direct comparison with accepted EPA 

analytical Methods for the analysis of HCl at levels of less than 1 ppm in the presence of 

moisture, making this a suitable approach for use with incinerators and cement kilns. 
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Using the principles established in EPA method 320 EPA method 321 has been 

developed specifically for the analysis of HCl in Portland cement kilns. 

References 

1. EPA Test Method 301, “Validation of Pollutant Measurement Methods from 

Various Waste Media”, U.S. Code of Federal Regulations (40 CFR 61, Appendix 

B). 

2. EPA Test Method 320, “Measurement of Vapor Phase Organic and Inorganic 

Emissions By Extractive FTIR Spectroscopy”, U.S. Code of Federal Regulations 

(40 CFR 61, Appendix B). 

3. EPA Test Method 26, “Determination of Hydrogen Chloride Emissions from 

Stationary Sources”, U.S. Code of Federal Regulations (40 CFR 61, Appendix 

B).