Use of a Virtual Instrument to Measure the Mass Concentration of Nitrogen Dioxide

Scientists and engineers around the world use computer software for research, design, and production. Analytical equipment manufacturers are developing the latest tools that make possible proper use of available resources. Increasingly powerful hardware is used, and advanced solutions are offered for programming and user interface development in an effort to improve measurement and automation.

New technological advancements are associated with the quality and efficiency of scientific research in biomedicine. In 2007-8, the National Technical University of Ukraine (Kyiv), in cooperation with the Institute of Hygiene in Vilnius, Lithuania, completed a two-party scientific project entitled “Development of Data Collection and Application of Statistical Biomedical Methods for Research of Asbestos and Heavy Metals.” The project was carried out in accordance with the Treaty on Cooperation in the Fields of Education, Science and Culture signed by the Lithuanian Government and the Government of Ukraine.1 The work was conducted using the LabVIEW virtual instrument from National Instruments (Austin, TX).

Significant concentrations of nitrogen oxides, including nitrogen dioxide, are present in ambient and indoor air. The main sources of nitrogen oxide are combustion processes. Fossil fuel power stations, motor vehicles, and domestic combustion appliances emit nitrogen oxides, mostly in the form of NO, but sometimes in the form of NO2 (usually less than 10%).2 To be more precise, a single maximum permissible concentration of NO2 in residential air is equal to 0.085 mg/m3 and 0.040 mg/m3 per day.3 When inhaled, nitrogen dioxide irritates the respiratory tract and causes inflammation and oxidal stress in the lung cells and alveollae macrophages; in combination with hemoglobin, nitrogen dioxide produces methemoglobin. Due to long-term exposure to nitrogen dioxide present in ambient air at low concentrations, not exceeding the maximum permissible concentration within the 0.0126 mg/m3 to 0.0267 mg/m3 range, the risk of myocardial infarction is increased.4 A meta-analysis showed consistent association between mortality and NO2.5

In Lithuania, according to Institute of Hygiene data for the year 2007, nitrogen dioxide in workplace air was most often found in the processing industry; in transport and storage services; as well as in industries engaged in the supply of gas, steam, and air conditioning devices. The highest exposure to nitrogen dioxide was noted among welders, locksmiths, and operators, although the limit value was not exceeded (maximum concentration is equal to 2.43 mg/m3, average concentration is 0.35 mg/m3, and minimal concentration is 0.05 mg/m3; overall, 241 measurements were made). According to the regulations of the Hygiene Norm of Lithuania HN 23:2007, the limit value of long-term occupational exposure can be as high as 4.0 mg/m3, but if the pollution source is motor vehicle exhaust, then the limit value is as low as 2.0 mg/m3. A special limit value takes into account the general effect of substances present in the exhaust, including carcinogens. In this case, nitrogen dioxide is used as a pollution indicator of the exhaust fumes released from the engine's burning of diesel fuel.6

Different analytical methods are used to determine the concentration of nitrogen dioxide in air, including manual (spectrophotometric) methods and automated (chemiluminescence) methods. Historically, NO2 has been the most difficult compound to measure.7 The accurate and reliable measurement of NO2 concentration in air is of the utmost importance.

The study discussed in this article was conducted to assess the performance of the LabVIEW virtual instrument for analyzing the experimental measurements of the mass concentration of nitrogen dioxide at the Laboratory of Chemical Hazards Investigations of the Institute of Hygiene in Lithuania.

Methods

For virtual instruments modeling, National Instruments tools and technologies were selected, according to the following criteria:

  • Ability to adapt to new challenges and hardware drivers
  • Synchronization between multiple devices
  • Interactive help
  • Availability of an open-source management system
  • Compatibility with equipment from different manufacturers.

The virtual instrument combines a powerful computer and applicable software, such as LabVIEW 8.2 and other technical equipment as integrated computer boards, and additional drivers. Management and data entry to the measuring device, the spectrophotometer, were performed with the National Instruments data acquisition support card NI USB-6008. Using this model, the instrument executed the function of a traditional spectrophotometer, i.e., the absorbance measurement.8

In the study, measurements were performed using the SF-46 automatic spectrophotometer (LOMO, Leningrad, Russia). Data were used from the measurements carried out manually in 2007, prior to upgrade of the spectrophotometer. The Laboratory of Chemical Hazards Investigation of the Institute of Hygiene has been accredited for conformity with International Standard LT EN ISO/IEC 17025:2005 since 2005.9 In 2008, following modernization, the accreditation scope was extended for the measurements of nitrogen dioxide concentration in workplace air using the spectrophotometric method. The concentration of nitrogen dioxide was determined in accordance with the requirements of International Standard ISO 6768:1998, Ambient Air-Determination of Mass Concentration of Nitrogen Dioxide-Modified Griess- Saltzman Method.10 The principle of the method is as follows: The nitrogen dioxide present in air is absorbed by passage through an azo-dye-forming reagent within a specified period, resulting in formation of a pink color in the sampling probe. Color intensity, which is determined by measuring the absorbance, is directly proportional to the quantity of nitrogen dioxide in the sample. During the analysis, reagents and water of recognized analytical grade and calibrated equipment were used. The virtual instrument was tested by measuring the concentrations of standard solutions under different conditions.

The concentration of nitrogen dioxide in the air sample was calculated using Eq. (1):

Where CNO2 is the mass concentration of nitrogen dioxide, expressed in micrograms per cubic meter; ƒNO2 is the reciprocal of the slope, expressed in micrograms per milliliter; As is the absorbance of the sample solution; b is the optical pathlength, in millimeters, of the matched optical cells used; V1 is the volume, in milliliters, of the absorption solution transferred to the bubbler; and V2 is the volume, in cubic meters, of the air sample. (Note: 1) 1/ƒNO2 should be 0.992 ± 0.030 mL/µg with an optical range of 10 mm, and 2) the optical pathlength of b = 10 mm in the laboratory; thus the values of b and the coefficient 10 are missing in the equation.10

Primary measurement data were transferred from LabVIEW into a Microsoft® (Redmond, WA) Excel™ MS program and statistical processing was done. The following validation characteristics of this standardized method were estimated: detection limit, precision, repeatability, and trueness. Results were assessed according to the criteria specified for detection limit, repeatability, precision, and trueness.

Detection limit

The limit of detection is the minimum content that can be measured with a reasonable statistical certainty. The detection limit (DL) of nitrogen dioxide was determined by measuring 10 blank samples under repeatability conditions and calculated using Eq. (2):11

Where Cblank is the concentration of blank sample (µg/mL), and k is a factor that is multiplied by the standard deviation (SD) to calculate the uncertainty. In this instance, 5 was used.

Data of the blank sample measurements were obtained automatically using LabVIEW. In order to change the dimension of the DL from µg/mL to mg/m3 where Eq. (1) was used under the prescribed experimental conditions, DL was evaluated according to the criterion that the mass concentration of nitrogen dioxide could not exceed 0.003 mg/m3.10

Precision

Precision is the closeness of agreement between the independent test results obtained under the prescribed conditions.12 Precision was determined after measuring the control sample and the certified reference material.

The control sample was the nitrite solution, the concentration of which was equal to 0.5 µg/mL, and was measured by the same operator on different days. Concentration of the control sample corresponds to the average point of the calibration graph. Measurement recordings were checked by the Grubb's test and graphical expression made by the Shewhart chart, where the parameter under control was the concentration of nitrogen dioxide (µg/mL).12

The Certified Reference Material (CRM) is the nitrite solution whose concentration is 1001 mg/L. On the day of analysis, a standard solution was prepared from the CRM; the concentration was 0.5005 µg/mL. Repeated CRM measurements were performed by two independent operators with the same equipment on the same day. Measurement recordings were checked by the Grubb's test.12 Precision was checked by the X2 test.13 The value of X2 obtained in the laboratory was calculated using Eq. (3):

Where SDLab. is the within-laboratory standard deviation under repeatability conditions, and σLab.O is the required value of the within-laboratory standard deviation. (Note: The standard deviation of the CRM measurement was taken from the manufacturer's certificate [Merck, Darmstadt, Germany]).

The measurement results checked by the X2 test must satisfy the condition shown in Eq. (4):

Where X2Lab. is the laboratory value, and X2table–X2a(n) is the criterion value taken from Ref. 14 and divided from (n – 1). (Note: the value of criterion X2a(n) was taken at the chosen significance level α and the number of measurements, n.)

Repeatability

Repeatability is the precision under repeatability conditions. Repeatability conditions are those in which independent test results are obtained with the same method on identical material in the same laboratory by the same operator using the same equipment within short periods of time. Using the data of the CRM measurements, repeatability of results was assessed according to the criterion that relative standard deviation of the measurement of nitrogen dioxide mass concentration is not higher than 5%.10

Trueness

Trueness is the closeness of agreement between a test result and the accepted reference value.13 Eq. (5) is used as the criterion for acceptance:

Where x¯ is the laboratory average concentration of CRM in µg/mL, µ is the certified concentration of CRM in µg/mL, ucCRM is the standard uncertainty of CRM, and ucLab is the standard uncertainty of the laboratory measurement. (Note: Standard uncertainty of the laboratory measurement ucLab was expressed by standard deviation of the measurement.)

Trueness was also checked by the student’s tα(n) test. The student’s criterion obtained at the laboratory was calculated using Eq. (6):

Where tLab. is the laboratory value of the student’s test, C¯Lab. is the laboratory average concentration of CRM in µg/mL, CCRM is the certified concentration of CRM in µg/mL, SNLab. is the standard deviation of the laboratory measurement in µg/mL, and n is the number of measurements.

The measurement results checked by the tα(n) test must satisfy the condition shown in Eq. (7):

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