ANALYSIS AND VALIDATION OF AIRBORNE INORGANIC ACIDS BY ION CHROMATOGRAPHY

INTRODUCTION

Acid gases/vapours and aerosols are highly corrosive and can irritate the eyes and mucous membranes of the nose, pharynx, and respiratory tract, even at low airborne concentrations. Hydrofluoric acid (HF), hydrochloric acid (HCl), nitric acid (HNO3), hydrobromic acid (HBr), sulphuric acid (H2SO4), and phosphoric acid (H3PO4) are used widely in industrial processes such as ore extraction, metal processing, pickling, electroplating and a number of other chemical processes. It is common for several of these acids to occur simultaneously in the air of occupational settings. Applicable occupational exposure limits (OELs) for these substances in air generally range between 0.1–10 mg/m3 for the various acids.

The physical states of the different acids in workplace atmospheres may vary from liquid aerosols (mists) for non-volatile acids like H2SO4 to gases/vapours for volatile acids such as HCl or HNO3.

Procedures for sampling and analyzing inorganic acids in workplace air must take into account the physical state(s) of the target analyte(s).

Hence new National Institute for Occupational Safety and Health (NIOSH) methods for sampling and analysis of inorganic acids in workplace atmospheres, Methods 7906, 7907 and 7908,have been promulgated to account for the collection of inorganic acid gases/vapours and aerosols. The technical contents of these new NIOSH methods are based on related recently developed International Organization for Standardization (ISO) consensus standards.

PRINCIPLE

Anion analysis may simply identify which anions are present in a sample (qualitative analysis) or also determine the quantity of anions present (quantitative analysis). Traditional wet chemistry uses colorimetric methods to identify and quantify the anion composition. Modern separation techniques, such as anion exchange chromatography or ion chromatography (IC) for anions, not only separate the anions present in the samples, but also quantify each individual anion, providing analytical results for multiple anions in a single run in 25–30 minutes.

METERIALS AND METHODS

The acid mist in the air was collected with microporous membrane, eluted with water, separated by the column chromatography, detected by conductivity detector, qualitative by the retention time and quantitative by the peak area.

Microporous membrane filter; small plastic sampling lip, filter diameter 25 mm; tarson tube; 50 mL; thermostatic water bath, nylon 0.45 μm microporous membrane filter; ion chromatography:

ICS-1100 ion chromatography, Chromeleon V7.0 chromatogram workstation.

Anionic analysis column: Dionex IonPac AS23 (4 mm ×250 mm)
Anionic protection column: Dionex IonPac AG23(4 mm × 50 mm)
Suppressor: ASRS 4 mm
Detector:  Conductivity detector
Eluent: 4.5mM Sodium bicarbonate and 0.8mM sodium bicarbonate
Flow rate of eluent: 1.00 mL/min
Suppressor current: 30 mA
Injection volume: 25 μL
Column temperature: 30 ºC

Standard preparation

From 1000 mg/L of each reference standard intermediate and working mixture standards were prepared by serial dilution. The mixture standards were used for method validation.

Preparation of mobile phase

4.5mM Sodium carbonate and 0.8mM sodium bi-carbonate buffer: 0.476g of sodium bicarbonate and 0.0672g of sodium bicarbonate was taken and final volume was made up to 1000ml with type I water. Furthermore the solution was sonicated for 15 minutes in an ultrasonic bath.

Preparation of calibration curve from mixtures of standard

A mixture stock solution (concentration of 100 mg/L) of 6 anions was first prepared. Then the following dilutions (Table-1) ranging from 0.5 mg/L to 10mg/L was prepared with type I water as diluent.

Table-1. Details for preparation of working mixture standards

Stock Conc.

(mg/L)

Volume of Stock

(mL)

Volume of Diluent

(mL)

Final Volume

(mL)

Final Conc.

(mg/L)

Name
5 0.500 4.500 5 0.5 CC-1
10 0.500 4.500 5 1.0 CC-2
10 1.000 4.000 5 2.0 CC-3
10 2.500 2.500 5 5.0 CC-4
100 0.500 4.500 5 10.0 CC-5

Sample preparation

Sampling filter paper was placed in a 50ml plastic screw cap vessel and 10ml of HPLC water was added to it. The tube was then sonicated for 15 minutes in an ultrasonic bath. Using a 5ml or 10ml syringe the extract was filtered through PTFE filter and injected to Ion Chromatograph.

Instrumental Condition

IC system: DIONEX ICS-1100
Column: Dionex IonPac AS23 RFIC 4x250mm
Injection Volume: 10 µL
Flow rate: 1mL/minute
Suppressor current: 30mA

RESULTS AND DISCUSSION

All the 6 anions viz. fluoride, chloride, bromide, nitrate, phosphate, sulphate were validated in acid mist at two different spiking levels. Various Performance characteristics like Specificity, Limit of Detection, Linearity, Matrix Effect, Accuracy, Precision, Limit of detection, Limit of Quantification and Robustness were assessed. The method was found to be specific for the analytes and matrix combinations. The response of the detector was found to be linear within 0.5-10 mg/L (Figures. 1-7). Spiked recoveries were performed by two analysts on two different days at two different concentration levels to evaluate the accuracy and precision of the method.

In these investigations, mean analytical recoveries determined from the analysis of spiked filters were found to be in the range of 99–114% for the all 6 anions Br, Cl, NO3, F, SO42-, and PO43-.

At LOQ level (1 mg/Kg) the mean recoveries obtained were between 107.0 – 115.59 % with relative standard deviation ranging between 1.796 – 9.50 % whereas at the higher fortification level (5 mg/kg) the recovery ranged between a 100.00 – 115.0 % with relative standard deviation of  up to 8.248%.  LOQ was fixed at the level of 1 mg/kg.

CONCLUSIONS

The analytical method described in this study was optimised for determination of 6 anions viz. fluoride, chloride, bromide, nitrate, phosphate, sulphate by IC. The detection and quantification after the separation of the anions through IC method provides the specificity of the analysis. This method can be used to determine the analytes in acid mist as mentioned above at extremely low concentration (~1.0 mg/Kg). The Limit of Quantification of the method is found to fall within the range of 1.0 mg/Kg to 10.0 mg/Kg. The calibration curve was found to be linear. This was apparent from the square of the correlation coefficient (r2) value which ranged between 0.9996 to 0.9999. All the method validation parameters were within acceptable limits. This method found to be cheap, easy, effective , quick and quite rugged for the analytes and matrix combination. In conclusion the method was found to be fit for its intended purpose.

 

 

Figure 1: CC for Fluoride

Figure 2: CC for Chloride

Figure 3: CC for Bromide

Figure 4: CC for Nitrate

Figure 5: CC for Phosphate

Figure 6: CC for Sulphate

Figure 7: Chromatogram of Reference Standard mixture

Figure 8: Chromatogram of reagent Blank

Figure 9: Chromatogram of sample Blank

Figure 10: Chromatogram of Recovery sample

 

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