GC-PID method for the measurement of arsenic in food and juice


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Numerous methods have been described in the literature for the determination of arsenic both in water and in particulate matrices. Many of these methods essentially employ the same principles, but apply different reagents, concentrations or instrumentation. The most popular techniques for arsenic analyses include hydride generation and AA, ICP-OES or ICP-MS. Although many water labs have these types of spectrometers already, many labs do not. The cost of these types of spectrometers is in the $100-200K price range.
Many labs would have to choose the older colorimetric methods but we have developed and modified the PID and GC-PID methods for arsenic in water analysis at ppb levels (1) to work with food and juice. The system cost is a fraction of the $100K spectrometer price.

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GC-PID method for the measurement of arsenic in food and juice

  1. RV Knorr docked at Woods Hole, MA (WHOI)Jennifer, Jack, Greg and Carrie on board RV Knorr RV Knorr is known for supporting the WHOI andFrench researchers who discovered the wreck of the Driscoll & Cutter with PID-based arsenic GC’s on 1 RMS Titanic in 1985 board the RV Knorr
  2. Introduction• Over one third of the arsenic in the atmosphere comes from man-made sources, notably fossil fuel combustion and copper smelting, while the rest is from natural sources such as volcanoes• Arsenic can cause bladder, lung, and skin cancer in humans (National Academy of Science 1999) PID Analyzers, LLC 2
  3. Arsenic Rule• Although EPA regulations are set for total As concentrations in drinking water; the standard is 10 parts per billion ppb (http://water.epa.gov/lawsregs/rulesregs/sdwa/arsenic/regulations.cfm), the element can exist in different oxidation states (+V, +III, -III) and inorganic and organic forms within a given oxidation state (e.g., As+V as arsenate, AsO43- or dimethyl arsinate, (CH3)2AsO2 -); these all have different bioavailability and toxicity (e.g., Sanders, 1979).• Therefore, the chemical speciation of As in natural waters can be just as relevant as its total concentration. PID Analyzers, LLC 3
  4. Arsenic contamination in food & juice• Arsenic contamination in foods or juices can occur from As contaminated water or soil• A number of techniques have been used for the determination of As; some of these are quite complex and expensive as shown in the next slide• A relatively new technique, photoionization (PID) has been used since the 1980’s for the analysis of As compounds in sea water; we will discuss that technique in this paper PID Analyzers, LLC 4
  5. Instrumentation for the Analysis of Arsenic Type Detection Limit (ug/L) Cost ($)ICP-MS 1.4 200,000ICP-OES 8 80,000GFAAS 8 80,000HGAAS 0.5 60,000 ASV 1 30,000 PID 1 12,000GC-PID 0.075 20,000 PID Analyzers, LLC 5
  6. Hydride Generation• The simplest and most common method for converting As in solution to a measurable species is hydride generation (formation of AsH3 (g) with Na BH4 as a reducing agent) and using pH adjustment for speciation.• As III is converted to arsine using a TRIS-HCl buffer-pH 6.2 with NaBH4 reducing agent in the presence of sulfanilamide (0.5 mL of a 2% solution) to remove nitrite interference. For the PID only method and AsH3,as As III, the gas from hydride generation flows through the PID. The PID has enough sensitivity so that it does not have to be concentrated. PID Analyzers, LLC 6
  7. Hydride generation continued• The AsH3 is cryogenically trapped, and then determined via GC with a 2 m Carbopak B-HT packed column (1/8” FEP Teflon tubing) at 25 °C and PID.• The results for Hydride generation at 0.5 M HCl and GC/PID detection produces the concentration of As(III + V) As(V) is then the difference between the two separate determinations. The detection limit for inorganic As is 0.075 ug/L and precision at 40 ug/L is better than 5% (relative standard deviation, RSD) (12). The standard additions method of calibration can be used to improve the accuracy.• A chromatogram of a low level (ppb) arsenic sample is shown in the next slide PID Analyzers, LLC 7
  8. Low ppb Levels of AsH3 by GC PID PID Analyzers, LLC 8
  9. Photoionization• The PID uses a 10.6 eV lamp to ionize arsine (g) generated by reduction with NaBH4. When AsH3 absorbs a photon, it forms a positive ion AsH3+ plus an electron (e-). PID Analyzers, LLC 9
  10. Photoionization continued• The positive ions are collected in an ionization chamber where the bias electrode pushes the ions to the collection electrode and the current collected is proportional to the concentration over a range of 107 from ppb to %. The schematic of the PID is shown in the next slide. PID Analyzers, LLC 10
  11. PID for Arsenic SpeciesSchematic of PID Photo PID PID Analyzers, LLC 11
  12. Calibration Curve for AsH3- PID Calibration Curve As in Water- PID 2000 y = 29.427x - 25.018 1500 AsH3 Reading R2 = 0.9983 1000 500 0 0 10 20 30 40 50 60 -500 ug/L As in water PID Analyzers, LLC 12
  13. Comparison of the Precision of Various Instrumental Techniques for Arsenic Technique DL (ug/L) RSD %PN-T-MIP 500 4.8GF-3F-MP 50 1.8HG-CT-3F-MIP 0.8 6.7HG-GFT-3F-MIP 0.4 4.5HG-PID Driscoll (2006) 1 10HG-GC-PID Cutter (1991) >0.075 <5% ICP-MIP- Inductively Coupled Plasma-Microwave Induced Plasma PID- Photoionization Detection GC- Gas Chromatography HG- Hydride Generation HG-CT-3F-MIP PN-T-MIP GF-3F-MIP- Graphite Furnace PID Analyzers, LLC 13
  14. GC PID System for Arsenic PID Analyzers, LLC 14
  15. ppb Levels of Organoarsenic Compounds by GC PID PID Analyzers, LLC 15
  16. GC-PID of Multiple Hydrides PID Analyzers, LLC 16
  17. As in Food & Juices• Consumer Reports (1/12) tested 5 brands of juices and found that 10% had total arsenic levels that exceeded federal drinking- water standards (10 ppb).• Dr. Brian Jackson & co-workers at Dartmouth College tested products sweetened with organic brown rice syrup, including cereal bars and baby formulas, and found ppb levels of arsenic that exceeded U.S. standards for bottled water• The method described in this paper is easily adapted to the analysis of As in food or juices.• The advantage of the PID method is that it is easy to use, does not require a high level of training and has a low capital cost (see Slide #3) PID Analyzers, LLC 17
  18. METHOD FOR ARSENIC IN FOOD AND WATER•The most dangerous form of arsenic is As III.•This can be determined using the following procedure:•An aliquot of the juice or a known weight of a solid is added to the buffer solution.•As III is converted to arsine using a TRIS-HCl buffer-pH 6.2 with NaBH4 (reducingagent) in the presence of sulfanilamide (0.5 mL of a 2% solution) to remove nitriteinterference. As III is reduced to AsH3 (g) during the hydride generation .Nitrogen is used to sweep the AsH3 through the PID. The PID has sufficientsensitivity so that it does not need to be pre concentrated. As V can also bedetermined using the procedure in slide #4. PID Analyzers, LLC 18
  19. Summary• As is an impurity in many soils and water supplies used to grow fruits in the US and foreign countries. As a result, low ppb levels of As are often found in many fruit juices and rice sweeteners.• PID/hydride generation (HD-PID) is an extremely sensitive, simple and inexpensive method for analysis of arsenic in solution with detection limits of 1 ppb without preconcentration.• Total As, As III, As V/or organic arsenic compounds can be detected by varying the pH of the solution as shown in slide #4.• The PID method is easily adapted to the measurement of As in fruit juices or food.• Other metals that form hydrides can also be determined if the GC-PID method is used PID Analyzers, LLC 19
  20. Arsenic in Food~a thematic program at #ACSNewOrleans~245th ACS National Meeting & Exposition Spring 2013 Chemistry of Energy & Food Organizer: James N. Seiber April 7-11, 2013, New Orleans, Louisiana Abstracts Submission Opens August 20, 2012 Arsenic in Food Symposium sponsored by the Small Chemical Businesses Division of the American Chemical Society and co-sponsored by the Environmental Chemistry Division of the American Chemical Society PID Analyzers, LLC 20