Deuterium Background Correction and Effectively Handling Digested Rice Sample Matrix

Environmental contamination is leading to toxic elements such as lead (Pb) and cadmium (Cd), entering the food chain.

Being the most widely consumed cereal grain in Asia/China, rice can quickly pick up Pb and Cd from toxins, pesticides, and fertilizers in the soil, thereby jeopardizing the health of millions of people through their diet.

Therefore, it is of utmost importance to develop a simple, reliable method for the control of Pb and Cd levels in rice.

Maximum levels of Pb and Cd are stringently regulated in Asian countries, especially in China; therefore, it is vitally important to develop a simple, reliable method for trace levels of Pb and Cd in rice.

According to the Chinese national standard, GB 2715-2016 Hygienic Standard for Grain, the maximum concentrations of Pb or Cd allowed in grains must be lower than 0.2 mg/kg; the permissible level in the European Union is the same [EC 1881/2006].

The official technique for the identification of heavy metals in both cases utilizes graphite furnace atomic absorption spectroscopy (GFAAS, GB/T 5009. 12-2017, GB/T 5009-2017. 15 and EN 14083:2003).

Food samples are typically pretreated prior to GFAAS analysis using various methods: hot block digestion, microwave digestion, dry ashing, and hot plate digestion.

These traditional digestion procedures tend to be complex and time-consuming (2-4 hours or longer). Plus, they demand vast quantities of corrosive and oxidizing reagents, raising the chance for further contamination which could produce inaccurate results.

However, fast digestion (introduced and validated in another application note3) can accelerate the sample preparation procedure while limiting the use of corrosive reagents and thus lowering the chance for contamination.

In this study, rice powder was pretreated first by fast digestion, followed by analysis with the PerkinElmer PinAAcle™ 900H graphite furnace atomic absorption spectrometer. A rapid, accurate, and cost-effective method for trace level analysis of Pb and Cd in rice grains was established and verified.

Experimental Conditions

Sample Preparation

Fast digestion facilitated the convenient and rapid pretreatment of these samples: approximately 0.5 g of each rice flour sample was weighed out in duplicate and placed into 50 mL polypropylene autosampler tubes for fast digestion.

Following these steps, 1.5 mL concentrated nitric acid was introduced, and the vial was loosely capped vial and subsequently heated in the sample preparation block digestion system (SPB series, PerkinElmer) for 30 minutes at 120 °C.

This sequence of the process produced a clear solution that did not necessitate filtering as demonstrated in Figure 1. The digested samples were then brought up to 25 mL with dilution using DI water. The overall time to create an instrument-ready solution was reduced down to just 30 minutes.

PerkinElmer Food Safety and Quality

Figure 1. Transparent solution (green caps) yielded from rice flour and acid slurry (yellow cap) by fast digestion. Image Credit: PerkinElmer Food Safety and Quality

Instrumentation

The measurements were conducted using a PinAAcle 900H atomic absorption (AA) spectrometer (PerkinElmer, Inc., Shelton, Connecticut, USA). PinAAcle 900T and 900Z spectrometers can also be employed for this application.3

The PinAAcle 900H was fitted with Syngystix™ for AA software to conduct sample analysis, data reporting, and archiving results – it is also possible to use Syngistix™ for AA Express™ software.

The PinAAcle 900H was also kitted out with HGA graphite furnace and deuterium continuum source background correction, water re-circulator system, AS900 autosampler, automatic lamp selection, high-speed automatic wavelength drive, and Electrodeless Discharge Lamp (EDL) power supply.

The use of innovative fiber optics in the PinAAcle 900 spectrometers optimizes light throughput for enhanced detection levels.

Unknown Sample A and proficiency test Sample B were measured during this experiment. Analysis of the certified reference material (CRM) NIST 1568 Rice Flour was also conducted to confirm the digestion and analysis methods. Each sample was exposed to two replicates by fast digestion.

Cd determination in the CRM was also twice replicated following the same procedural instructions. The GFAAS analytical conditions remained constant throughout the whole process. The conditions of the instrument are provided in Table 1, and the optimized graphite furnace temperature programs are detailed in Tables 2 and 3.

Table 1. Instrumental conditions for analyzing Pb and Cd in rice grains on the PinAAcle 900H spectrometer. Source: PerkinElmer Food Safety and Quality

Parameter Lead (Pb) Cadmium (Cd)
Wavelength (nm) 283.31 228.80
Slit Width (nm) 0.7 0.7
Lamp Type* HCL EDL
Measurement Type Peak Area Peak Area
Read Time (sec) 5 5
Sample Volume (μL) 12 12
Diluent Volume (μL) 12 12
Matrix Modifier 0.05% Pd(NO3)2 0.3% Pd(NO3)2
Matrix Modifier Volume (μL) 5 5
Calibration Equation Standard Addition Standard Addition
Standard Concentration (μg/L) 0, 5, 10, 15, 20 0, 0.5, 1.0, 1.5, 2.0

* The determination of Cd level in rice can be obtained by both HCL and EDL. However, Cd EDL was used for better detection limit and sensitivity

Table 2. Optimized temperature program for Pb analysis in rice grains on the PinAAcle 900H spectrometer. Source: PerkinElmer Food Safety and Quality

Lead (Pb)
Temp. (°C) Ramp (s) Hold (s) Internal Gas
Flow (mL/min)
Gas Type
120 5 30 250 Normal
150 30 30 250 Normal
700 10 20 250 Normal
1800 0 5 0 Normal
2600 1 5 250 Normal

 

Table 3. Optimized temperature program for Cd analysis in rice grains on the PinAAcle 900H spectrometer. Source: PerkinElmer Food Safety and Quality

Cadmium (Cd)
Temp. (°C) Ramp (s) Hold (s) Internal Gas
Flow (mL/min)
Gas Type
120 5 30 250 Normal
150 30 30 250 Normal
650 10 20 250 Normal
1900 0 5 0 Normal
2600 1 5 250 Normal

 

Results and Discussion

Figure 2 shows the standard addition calibration curves of Pb and Cd, which were fabricated respectfully in Sample A1 and B1 as representative samples, demonstrating an R2 value ≥ 0.999 in both cases, which indicated good linearity of the analysis.

The peaks for all calibration standards and the samples are exhibited in Figures 3 and 4. The same appearance time and peak shape show that matrix effects were successfully eliminated harnessing the standard addition method.

Standard addition curves of Pb (top) and Cd (bottom) in rice.

Standard addition curves of Pb (top) and Cd (bottom) in rice.

Figure 2. Standard addition curves of Pb (top) and Cd (bottom) in rice. Image Credit: PerkinElmer Food Safety and Quality

Overlay of spectral profiles of Pb (top) and Cd (bottom) in standard addition calibration.

Overlay of spectral profiles of Pb (top) and Cd (bottom) in standard addition calibration.

Figure 3. Overlay of spectral profiles of Pb (top) and Cd (bottom) in standard addition calibration. Image Credit: PerkinElmer Food Safety and Quality

Overlay of spectral profiles of Pb (top) and Cd (bottom) in two samples and NIST 1568 and their duplicates.

Overlay of spectral profiles of Pb (top) and Cd (bottom) in two samples and NIST 1568 and their duplicates.

Figure 4. Overlay of spectral profiles of Pb (top) and Cd (bottom) in two samples and NIST 1568 and their duplicates. Image Credit: PerkinElmer Food Safety and Quality

The method detection limit (MDL) was quantified (Table 4) as predicated on the standard deviation of eleven replicates of the reagent blank while accounting for the dilution factor used during sample preparation.

The method detection limit was not impeded by the smaller sample volume of 12 μL and is well within the regulated levels, which showcases the PinAAcle 900H’s capacity to measure low concentrations in complex matrices.

Table 4. MDL using the PinAAcle 900H spectrometer. Source: PerkinElmer Food Safety and Quality

Analyte MDL (3σ, μg/kg)
Calculated Regulated
Pb 13 20a 20c
Cd 0.8 1.0b 20c

(0.5 grams sample diluted to 25 mL; a: regulated in GB 5009.12-2017; b: regulated in (EC) No 1881/2006; c: regulated in GB 5009.15-2014.)

Table 5. Analysis of NIST 1568 pretreated by fast digestion on the PinAAcle 900H spectrometer. Source: PerkinElmer Food Safety and Quality

Analyte Certified Value
(mg/kg)
Measured Value
(mg/kg)
Pb NA NA
Cd 0.029 ± 0.004 0.027

 

Table 6. Results for the detection of Pb (top) and Cd (bottom) in rice grains (mg/kg). Source: PerkinElmer Food Safety and Quality

Sample Pb (mg/kg)
FD1 RSD (%) FD2 RSD (%)
A 0.083 <1 0.081 <1
B* 0.394 <1 0.399 <1

* Result from proficiency test is 0.390 mg/kg.

Sample Cd (mg/kg)
FD1 RSD (%) FD2 RSD (%)
A 0.077 <1 0.082 <1
B* 0.378 <1 0.371 <1

* Result from proficiency test is 0.370 mg/kg.

Conclusions

Fast digestion is believed to be a straightforward effective and precision-based technique for rice grain pretreatment.

The PinAAcle 900H AA spectrometer, fitted with an HGA graphite furnace and deuterium background correction, has proved its ability to successfully control digested rice sample matrix – the system offers first-rate sensitivity needed for heavy-metal testing in food matrices, with superb accuracy and precision, as exhibited in the CRM results and low RSDs.

There is the potential to extend this application to heavy metal analysis in a range of other food types, including corn, cereal grains, flour, beans and milk powder.

References

  1. IARC: http://monographs.iarc.fr/ENG/Classification/latest_ classif.php.
  2. U.S. ATSDR: https://www.atsdr.cdc.gov/csem/csem. asp?csem=6&po=12.
  3. Fast Digestion Analysis of Lead and Cadmium in Rice Using GFAAS with Longitudinal Zeeman Background Correction, S. Wei, R-K. Yang, Q-L. Liu, PerkinElmer, Shanghai, China.

Consumables Used

Table 7. Source: PerkinElmer Food Safety and Quality

Component Description Part Number
Sample Preparation
Block
SPB 50-24, 24-position 50 mL 115/230 V
SPB 50-48, 48-position 50 mL 115/230 V
N9300802
N9300803
Cd Lamp Hollow Cathode Lamp (HCL)
Electrodeless Discharge Lamp (EDL)
N3050115
N3050615
Pb Lamp Hollow Cathode Lamp (HCL)
Electrodeless Discharge Lamp (EDL)
N3050157
N3050657
HGA Graphite Tubes Pyrocoated Graphite Tubes
with Integrated Platform
NB3001262 (5-pack)
B3001264 (20-pack)
N9300651 (40-pack)
Cd Standard 1000 ppm, matrix 2% HNO3 N9300176 (125 mL)
N9300107 (500 mL)
Modifier Pd(NO3)2 1% Pd, 50 mL B0190635
Conical Centrifuge Tube 50 mL - Qty 500 B0193234

 

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer Food Safety and Quality.

For more information on this source, please visit PerkinElmer Food Safety and Quality.

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