Are you Measuring a Few Elements at Moderately Low Concentrations (10’s ppb to low ppm) in a Small Number of Samples?
In part 1 of this series, we looked at the relative advantages and disadvantages of FAAS, GFAAS, ICP-OES and ICPMS in determining your choice of which of these four instrumental techniques is right for your elemental analysis. In part 2, we will be looking at what laboratories which measure just a few elements at relatively low concentrations should consider in determining which elemental technique is most suitable for them.
Which Elemental Analysis Technique?
For a lab measuring a handful of elements at moderately low levels (more than 10 ppb) in a small number of samples per day, there are three elemental analysis equipment options: Graphite furnace analysis (referred to as GFAAS or graphite furnace AAS) and the two inductively coupled plasma elemental analysis techniques of inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectroscopy (ICP-OES)
Choice of Elements
The choice of elemental analysis equipment may depend on the elements you need to measure. ICP-MS can measure 86 elements, ICP-OES can measure 74, and graphite furnace analysis can measure 48. The common elements that GFAAS cannot measure include Br, C, Ce, Cl, F, Gd, Hf, Ho, I, La, Lu, Nb, Nd, Os, Pr, Re, S, Sc, Sm, Ta, Tb, Th, Tm, U, W, Y, and Zr. Similarly, ICP-OES can’t measure C, F, Cl, Br, or I, so, if any of these elements is required, you will need ICP-MS. Even with ICP-MS, detection limits for C are poor, and F requires a specialized ICP-QQQ approach.
Purchase Price and Cost of Operations
GFAAS has the lowest purchase price and running costs of the three techniques, but GFAAS measures each element separately, with each single-element measurement taking around 2 or 3 minutes depending on the element measured. This means that measuring five elements with three replicates of each sample will take approximately 30-40 minutes per sample (5 elements x 3 replicate measurements x 2 minutes per replicate) using GFAAS. One GFAAS instrument can therefore perform approximately 240 triplicate measurements in 24 hours. This could be 1 element in 240 samples, 4 elements in 60 samples or 10 elements in 24 samples.
An ICP-MS instrument is more expensive to purchase and run, but delivers fast, multielement analysis, much wider linear dynamic range, and fewer interferences. ICP-MS measures all elements in a 2 to 3 minute acquisition for each sample, so it is much faster than graphite furnace analysis. If the number of samples and/or the number of elements is likely to increase, or the concentrations you need to measure may decrease (because of regulatory changes for example) then ICP-MS is the best option.
Of the two inductively coupled plasma elemental analysis techniques, an ICP-OES instrument costs less than an ICP-MS. ICP-OES also has the fastest sample measurement time, being able to measure multiple elements in up to 2500 samples per day. ICP-OES is a good choice if your sample load is likely to increase a lot or the number of elements will increase but the concentrations you need to measure will not decrease. ICP-OES cannot measure down to the same sub-ppb concentration levels as GFAAS or ICP-MS.
Both GFAAS and ICP-MS need a reasonable level of knowledge and skill to operate although standard, regulated methods can be automated. ICP-OES is easier to use. All techniques can run unattended, so you can load up an autosampler and leave the instrument to complete the analysis. A QA/QC lab that needs to measure the same elements in a few samples per day often selects GFAAS. However, if the lab is expanding and needs to ensure they can cope with higher sample loads in the future, either of the inductively coupled plasma elemental analysis techniques can provide the extra element coverage and higher sample throughput required for growth.
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