The legislated limits for the trace element content of food are embodied in law by the government of any given country and there are demands placed on exporters of foodstuff to adhere to international regulations. Most laboratories aim to deliver a level of quantification lower than the required value and utilise both inductively couple plasma optical emission spectroscopy (ICP-OES) and graphite furnace atomic absorption spectroscopy (GFAAS) for measurements. Fast sample throughput is essential in food analysis.
The latest evolution in measurement technology has resulted in the replacement of GFAAS and ICP-OES instruments with inductively coupled plasma mass spectrometry (ICP-MS) in order to maintain profitability in an increasingly competitive marketplace. The GFAAS technique grew in popularity in the 1970s primarily as a result of its high sensitivity. However, GFAAS requires long analysis times and is applicable only to single element analyses. The analytical working range is limited to 2-3 orders of magnitude and cost per analysis is relatively high.
The development of ICP-OES in the late 1970s and early 1980s gave the analyst the ability to perform rapid, multi-element analyses over a wider linear concentration range. However, relative to GFAAS, the detection limits for ICP-OES are orders of magnitude worse, and the amount of sample required is on the order of mLs rather than µLs. Matrix-based spectral interferences are troublesome in ICP-OES, so careful method development is required when analysing new matrices.
Figure 1: Operating principle of a modern ICP-MS.
The development of ICP-MS was driven by the need for rapid, flexible, multi-element analysis combining GFAAS detection capability with the linear working range of ICP-OES. The high sensitivity and low background obtained using ICP-MS results in ppt - ppq detection limits. Despite its extreme sensitivity, ICP-MS possesses an extremely wide linear dynamic range of typically 8 or 9 orders of magnitude. Along with speed and sensitivity, the ICP-MS provides unsurpassed selectivity.
Modern ICP-MS instrumentation has become much more tolerant of heavy matrices. Sample introduction systems specifically designed for ICP-MS can prevent overloading of the plasma. The placement of conical extraction lenses outside the main vacuum chamber, in combination with an optimised interface design, can provide for optimum ion extraction while protecting critical spectrometer components from matrix deposition. In addition, new detector technologies allow for the routine determination of analyte concentrations in the ppm range. Determining the element content for all samples on a single ICP-MS instrument, rather than dividing them into separate batches for hydride, ICP-OES or GFAAS, helps achieve improved productivity.
Figures 2a, 2b and 2c illustrate calibrations for Cd, Hg and Pb obtained in the same measurement cycle. Note the low concentration range of the calibrations and the excellent linearity, %RSD and fit.
Speciation analysis using ICP-MS coupled to chromatographic devices
IC and LC ICP-MS
ICP-MS also offers extra capabilities over the older optical techniques most notably the ability to be easily coupled to chromatographic separation techniques. Using ion chromatography or liquid chromatography, IC or LC-ICP-MS provides users with the ability to separately measure oxidation states of elements in food or differing species. The toxicity of an element is often dependent on its oxidation state (for example Cr(III) and Cr(VI) or organic molecular form (for example organotin and organomercury species).
Figure 4 is a chromatogram of a mixed arsenic species standard containing As(III), As(V), monomethylarsonic acid (MMAA); dimethylarsinic acid (DMAA) and trimetylarsenic oxide (TMAO) each at 10 ppb.
The analysis was undertaken using an Agilent 1100 HPLC coupled to an Agilent 7500a ICP-MS as part of a study of arsenic species in edible Chinese seaweed. Table 1 summarises the prelinary data from this study (see table).
The unknown peak 1 is cationic and the calibration curve of TMAO was used for its quantification. Unknown peak 2 was a different species from DMAA, since the retention time was different from one of DMAA. It was checked with spiking of DMAA to the sample and proved not be this species. In order to obtain a quantitative result the calibration curve of DMAA was used for its calibration, since the sensitivity is most likely to be similar.
GC-ICP-MS
Coupling GC and ICP-MS, combining the separation capabilities of GC with the selectivity and sensitivity of ICP-MS, offers real benefits in the measurement of ultra trace levels of organically bound metals. Agilent Technologies has introduced a novel GC interface that allows easy coupling of the 6890 series GC to the 7500 Series ICP-MS [image] instruments and makes full use of the powerful features of both devices.
Organotin and organomercury compounds as well as being toxic demonstrate substantial biological activity within the environment. GC-ICP-MS will allow the relatively easy analysis of these species at low levels that are almost impossible by alternative techniques.
Summary
Industively Coupled Plasma Mass Spectrometry offers busy laboratories a fast and flexible replacement for their current inorganic analysis systems. Combining the speed and multi-element capabilities of ICP-OES and the sensitivity of GFAAS, ICP-MS can improve productivity in a busy analytical laboratory. However, ICP-MS offers considerably more capability, and with use in combination with a chromatographic instrument provides low level speciation measurements.
