Real Time Detection Systems I

Podium Session 115 

Tuesday, June 3, 2014, 2:00 PM - 3:30 PM 

CS-115-01 Field-Portable Fourier Transform Infrared Spectroscopy for Simultaneous Detection and Quantitation of an Unexpected Airborne Contaminant 

P. Smith, J. Hill, U.S. Department of Labor - OSHA, Sandy, UT 

Situation/Problem: An industrial hygienist typically relies on observations, documentation, and professional judgment to assess hazards in a workplace. However, subtle airborne hazards may exist and without a field detection system that simultaneously provides identification and quantitative measurement information for both expected and unexpected analytes, these could be overlooked. 

Resolution: Bulk air samples at various locations in a plant were obtained in 5-layer aluminized bags, and a portion of the contents from each bag was subjected to analysis with a field-portable Fourier transform infrared spectroscopy (FTIR) instrument to obtain near real-time feedback on the presence of airborne hydrogen peroxide and ammonia. While these were not detected at levels that would be a cause for concern, evidence for other analytes was also monitored by the FTIR system software, and unexpectedly high levels of carbon dioxide (CO2, 0.4 to 1.2 percent by volume) were detected based on absorbance from 2000 to 2250 × cm-1. Enough air remained in each bag following the field analyses for laboratory analysis using adsorption gas chromatography. 

Results: A linear correlation was noted by plotting FTIR CO2 concentrations against the laboratory results (R2 = 0.995, slope = 1.21), although an exponential fit was suggested by visual inspection of the data. This was likely due to the range of concentrations represented in the data files used for calibration which did not extend to values seen in higher concentration field samples. When the spectra were re-analyzed using preexisting calibration files (0.03 to 2.0 percent CO2) the linear fit was improved (R2 greater than 0.999, slope = 0.952). As the OSHA permissible exposure level for CO2 is 0.5 percent (8-hr TWA), the field results indicated that overexposures were possible. 

Lessons Learned: The FTIR data led to the identification of an unexpected airborne CO2 hazard. The quantitation capability of the FTIR instrument was dependent on the range of concentrations covered in the calibration files used by the instrumentation software. Although confidence in the field data was subsequently shown to be warranted as the actual CO2 concentrations did not exceed the calibration concentration range by a large factor, the availability of calibration files that covered several concentration ranges allowed quick re-analysis. The wider calibration concentration range produced FTIR data that were highly predictive of laboratory results, differing from them by no more than 9 percent. 

SR-115-02 A Rapid and Accurate Field Method for Reduced Sulfur Compounds by GC/FPD 

J. Maclachlan, PID Analyzers, LLC, Sandwich, MA 

Objective: The OSHA method for hydrogen sulfide (H2S) is complex and involves precipitation, oxidation and ion chromatographic analysis. The dimethyl sulfide and carbon disulfide are collected, desorbed with a solvent and analyzed by a gas chromatograph (GC) configured with a flame photometric detector (FPD). These methods are complex, time consuming and do not provide a fast response for the toxic sulfur compounds. A portable GC with an FPD should have sufficient sensitivity at part per billion (ppb) measurements to detect these sulfur compounds rapidly in the field without any need for sample concentration. 

Methods: The Model GC312 portable gas chromatograph with an FPD was used for these tests. The PeakWorks™ four point calibration software was used to linearize the FPD sulfur (normal output is a square root response). A 30 meter thick film capillary column provides an accurate and reproducible separation for the three sulfur compounds at ppb to ppm levels in under four minutes. Samples were injected automatically using a 10 port valve. Low level samples were prepared using permeation tubes. 

Results: Utilizing the portable GC with FPD allows for the rapid detection of sulfur compounds at ppb levels in the air. The sulfur compounds of interest include hydrogen sulfide (PEL =10 ppm), dimethyl sulfide (PEL= 10 ppm) and carbon disulfide (PEL = 20 ppm).The detection limits for the GC/FPD were found to be < 1 percent of the PEL. 

Conclusions: The GC/FPD method provides a very rapid and sensitive technique for the analysis of sulfur compounds in the field in real-time. The precision of H2S for 10 samples at one tenth of the PEL was approximately 1 percent. Although, we have analyzed three of the problematic sulfur compounds, the same method could be applied to other compounds such as methyl mercaptan, carbonyl sulfide, etc. 

CS-115-03 Real-time Field Assessment Methods for Quantifying Carbon Dioxide Levels Present in Workplace Atmospheres 

J. Hill, U.S. Department of Labor - OSHA, Sandy, UT; P. Smith, U.S. Department of Labor - OSHA, Sandy, UT 

Situation/Problem: Workers in various industries may be exposed to carbon dioxide and current sampling methods for laboratory analysis of this analyte are cumbersome. The ability to rapidly obtain an exposure potential snapshot while in the field is valuable to indicate when a validated sampling and analysis protocol should be used, yet the various methods available have different capabilities and limitations that should be understood. 

Resolution: To assess the usefulness of various methods available for carbon dioxide measurement in the field, colorimetric detector tube, non-dispersive infrared (NDIR) absorbance, and Fourier transform infrared (FTIR) absorbance spectroscopy methods were used in the laboratory to analyze dynamically-generated standards (0.5 to >10 percent), with up to 80 percent relative humidity. In the field, multiple assessments were conducted in workplaces where carbon dioxide was present using the same instrumentation and methods, and bulk air samples were also collected in 5-layer aluminized bags for chromatographic analysis. 

Results: The laboratory results for the various carbon dioxide detection methods were highly correlated with the standard concentration sampled (R-squared>0.99 in all cases, even at 80 percent humidity), although the detector response slopes ranged from 0.71 (detector tube, dry air) to 0.94 (FTIR, humid air) meaning that all of the field detection methods tended to underestimate actual concentrations often by as much as about 20 percent. The use of NDIR absorbance in the field allowed real-time detection of unexpectedly high levels of carbon dioxide (up to about 10 percent by volume) with additional field confirmation provided by detector tubes with results found to be near NDIR-determined concentrations. The OSHA permissible exposure level for carbon dioxide is 0.5 percent (8-hr TWA) and the IDLH concentration is 4 percent, and when field results indicated that overexposures were possible this guided the collection of both grab and TWA bulk air compliance samples for laboratory analysis. 

Lessons Learned: By verifying the capabilities and limitations of field exposure assessment tools, industrial hygienists are better positioned to classify operations where potential carbon dioxide exposures are present, identify specific operations where higher exposures are more likely, identify potential overexposures, and provide feedback to improve design and reduce worker exposure.