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Hitachi

Scientific Instrument NEWSHitachi High-Technologies GLOBAL

Kazuko Yamamoto*1

Introduction

The hydride formation system is an instrument that connects to a flame atomic absorption spectrometer to enable high-sensitivity measurements of arsenic(As), selenium(Se), and antimony(Sb) in aqueous solutions. These instruments are widely used to analyze As, Se, and Sb present in environmental water, waste water, foodstuffs, and other substances.

Our newly developed HFS-4 hydride formation system(Figure 1) retains the key performance feature of its predecessor instrument (the HFS-3), namely, that it uses flame Zeeman atomic absorption spectrometry with no baseline drift to enable highly stable measurements. In addition, this new hydride formation system significantly reduces the volume required for samples and reagents while increasing the sensitivity and analytical throughput. Moreover, the instrument is equipped with a new technology, a 4-stage peristaltic pump. As discussed in this note, this allows for the automated addition of preliminary reducing agents such as potassium iodide(KI) when analyzing As.

The HFS-4 hydride formation system
Fig.1 The HFS-4 hydride formation system.

Principles of measurement

In the hydride formation-atomic absorption method, As, Se, Sb, and similar elements in a sample aqueous solution react with sodium borohydride(NaBH4) and hydrochloric acid(HCl) to yield gaseous hydrides. These are atomized using argon gas and introduced into a heated quartz cell. The elements that can be treated using the hydride-formation method are limited, but it allows highly sensitive measurements.

Caution is required when using the hydride formation method because the efficiency of hydride roduction depends on the chemical state of the element. Inorganic As exists in trivalent and pentavalent states. Because the production of AsH3 is lower for the pentavalent state than the trivalent state, KI must be introduced into the sample in advance to reduce the pentavalent state to the trivalent state.

In foodstuffs, large quantities of As are present in the form of organic As. Because the majority of organic As does not form hydrides, organic As must be decomposed into inorganic As in advance.

Key product features

The HFS-4 flow pathways

The HFS-4 is equipped with (1) a three-liquid-blending pathway in which the sample, HCl, and NaBH4 are lended; and (2) a four-liquid-blending pathway in which KI for As analysis is blended by pumping.
Figure 2 shows the HFS-4 flow pathways. In the four-liquid-blending pathway, the preliminary reducing agent is automatically added via pumping, ensuring that pentavalent As is reduced to trivalent As; this reduces the effort required to prepare samples.

Three-liquid-blending pathway
Four-liquid-blending pathway
Fig.2 The HFS-4 flow pathways.

Stability through flame Zeeman atomic absorption spectrometry

We connected the SSC-230 autosampler to the HFS-4 to perform consecutive measurements of As. We prepared calibration curves for As concentrations of 0, 1, 5, and 10 µg/L, and then performed 50 onsecutive measurements of the 5 µg/L solution. We did not perform autozeroing or resloping of the calibration curves during sample measurements. The measurement conditions are listed in Table 1, and the measurement results are summarized in Table 2. Figure 3 shows the atomic absorption signal after the 50 consecutive easurements. The results indicate that stable, accurate, and quantitative measurements were obtained, with a mean value of 5.02 µg/L and an RSD of 1.22%.


Fig.3 Atomic absorption signal for 5 µg/L As.

Table 1 Conditions for measurement of 5 µg/L As.
Element As Atomizer STD Burner Meas. Mode Working Curve
Instrument ZA3000 Flame Air-C2H2 Signal Mode BKG Correction
Atomization Flame/Auto-sampler Fuel (C2H2) 1.2 L/min Curve Order Linear
Wavelength 193.7 nm Oxidant (Air) 160 kPa Calculation Integration
Lamp Current 12.0 mA 15.0 L/min Delay Time 70.0 sec
Slit Width 1.3 nm Burner Height 7.5 mm Calculation Time 5.0 sec
Table 2 Results of measurements of 5 µg/L As.
  Concentration Absorptivity
Mean value 5.02 0.0980
SD 0.06 0.0012
RSD 1.22% 1.23%

Applications

Measurement of Se in a standard sample of river water

As specified by the JIS K 0102 standard, KI should not be added for Se measurements. Therefore, we performed measurements using the three-liquid-blending pathway with the sample, HCl, and NaBH4. Here, we present measurements of Se in a standard sample of river water. Figure 4 shows the Se atomic absorption signal, while Figure 5 shows the calibration curve used. Table 3 lists the measurement conditions, while Table 4 summarizes the measurement results. The measured values lie within the accepted range of values.

Atomic absorption signal for Se
Fig.4 Atomic absorption signal for Se.

Se calibration curve
Fig.5 Se calibration curve.

Table 3 Se measurement conditions.
ZA3000 conditions
Element Se Atomizer STD Burner
Instrument ZA3000 Flame Air-C2H2
Atomization Flame Fuel (C2H2) 1.0 L/min
Wavelength 196.0 nm Oxidant (Air) 160 kPa
Lamp Current 12.5 mA 15.0 L/min
Slit Width 1.3 nm Burner Height 7.5 mm
HFS-4 reagent Sample preparation
Reagent Flow Rate Sample 10 mL
1mol/L HCl 1 mL/min HCl 4 mL
1% NaBH4 1 mL/min Total 1.2 L/min
Table 4 Measurement results for Se in a standard sample of river water
Sample name Measured value(µg/L) Accepted range(µg/L)
JSAC 0302-3 5.11 5.2±0.1

Measured values reported here are values converted to the concentration of the original liquid

Summary

The HFS-4 hydride formation system retains the high stability of previous-generation models while offering significant reductions in the quantities of samples and reagents consumed. We expect that these struments will find an increasingly broad array of applications, primarily in environmental science and food analysis.

Author

*1
Kazuko Yamamoto
Application Engineering Development Group
Optical Instruments Design Department

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