11 Electrothermal Atomisation
The Delves Cup approach eventually got phased out with the commercialisation of electrothermal atomisation (ETA) or graphite furnace AA in the early-1970s. This new breakthrough technique offered a detection capability for lead of ~ 0.1 ppb - approximately 200x better than FAA. However, its major benefi t for the analysis of blood samples was the ability to dilute and inject the sample automatically into the graphite tube with very little off-line sample preparation. This result was that blood lead determinations could now be carried out in an automated fashion with relative ease, even at very low levels.
Zeeman cCorrection GFAA
Figure 2: Comparison of detection capability of AS techniques (ppb) for lead and the approximate year they were developed, or improvements were made
However, the long-term effects of lead poisoning have not always been well- understood. In the early-mid 1960s, remedial action would be taken if a blood lead level (BLL) threshold value was in excess of 60 µg/dL. As investigators discovered more sensitive detection systems and designed better studies, the generally recognised level for lead toxicity has progressively shifted downward. In 1970 it was lowered to 40 µg/ dL and by 1978 the level had been reduced to 30 µg/dL. In 1985 the CDC published a threshold level of 25 µg/dL, which they eventually lowered to 10 µg/dL in 1991. It stayed at this level until it was reduced to 5 µg/dL in 2012 and eventually ended up at 3.5 µg/ dL in 2021. However, as our understanding of disease improves and measurement technology gets more refi ned, this level could be pushed even lower in the future [10]. Figure 1 shows the trend in blood lead levels considered elevated by the Centers for Disease Control (CDC), since the mid-1960s.
Major source of lead
Currently the major source of lead poisoning among children comes from lead-based household paints, which were used up until they were banned in 1978 by the Consumer Product Safety Commission. Prior to this, leaded gasoline was the largest pollutant before it was completely removed from the pumps in 1995. Other potential sources include lead pipes used in drinking water systems, airborne lead from smelters, clay pots, pottery glazes, lead batteries, household dust and some processed foods made from natural plants and crops. However, awareness of the problem combined with preventative care and regular monitoring, have reduced the percentage of children aged 1-5 years with elevated blood levels (≥3.5 μg/dL) in the US from 26% in the early-mid 1990s to less than 2.5% today. These data were taken from a recent National Health and Nutrition Examination Survey (NHANES) report [8].
Routine monitoring of lead using Atomic Spectroscopic Technique
There is no question that the routine monitoring of lead has had a huge impact in reducing the number of children with elevated blood levels. Lead assays were initially carried out using the dithizone colorimetric method, which was sensitive enough, but very slow and labour intensive. It became a little more automated when anodic stripping voltammetry was developed [9], but blood-lead analysis was not considered a truly routine method until AS techniques became available. Let’s take a more detailed look at how improvements in atomic spectroscopy instrumentation detection capability have helped to lower the number of children with elevated blood lead levels, since atomic absorption was fi rst commercialised in the early 1960s.
Flame AA
When FAA was fi rst developed, the BLRV was 60 µg/dL (600 ppb). Even though this is well above the detection limit of ~20 ppb at the time, it struggled to meet this level when sample preparation and dilution of the blood samples was taken into consideration, which typically involved acid digestion followed by dilution and centrifuging/fi ltering. When sample preparation was factored, the concentration of lead in the sample was in the order of 20 ppb - virtually the same as the FAA detection limit.
Delves Cup AA
To get around this limitation, an accessory called the Delves Cup was developed in the late 1960s to improve the detection limit of FAA [10]. The Delves Cup approach uses a metal crucible, which was positioned over the fl ame. The sample is pipetted into the crucible, where the heated sample vapour is passed into a quartz tube, which was also heated by the fl ame. The ground state atoms are concentrated in the tube and therefore resident in the optical path for a longer period of time, resulting in much higher sensitivity and about 100x lower detection limits. The Delves Cup became the standard method for carrying out blood lead determinations for many years, because of its relative simplicity and low cost of operation.
Figure 3: The improvement in real-world detection capability (in µg/dL) offered by AS techniques for blood-lead determinations compared to the trend in blood-lead levels set by the CDC
The next major milestone in AA was the development of Zeeman background correction (ZBGC) in 1981, which compensated for non-specifi c absorption and structured background produced by complex biological matrices, like blood and urine [11]. This, in conjunction with the STPF (stabilised temperature platform furnace) concept, allowed for virtually interference-free analysis of blood samples, using aqueous calibrations and as a result became the recognised way of analysing most types of complex matrices by ETA [12].
ICP-MS
Even though ETA had been the accepted way of doing blood lead determinations for over 15 years, the commercialisation of quadrupole-based ICP-MS in 1983 gave analysts a tool that was not only 100x more sensitive but suffered from less severe matrix-induced interferences. In addition, ICP-MS offered multielement capability and much higher sample throughput.
These features made ICP-MS very attractive to the clinical community, such that many labs converted to ICP-MS as their main technique for trace element analysis. Then as the technique matured, utilising advanced mass separation devices, performance enhancing tools, powerful interference reduction techniques and more fl exible sampling accessories, detection limits in real-word samples improved dramatically for some elements.
Figure 2 shows the improvement in lead detection capability (in ppb) of ICP-MS compared the other AS techniques.
It should also be emphasised these are instrument detection limits (IDLs), which are based on simplistic calculations of aqueous blanks carried out by manufacturers and not realistic method detection limit (MDL) which takes into consideration the sample preparation procedure, dilution steps and multiple analytical measurements [13]. For that reason, a 10-50 x degradation in IDL is quite common for a real-world method limit of quantitation (LOQ).
Figure 3 is a combination of Figures 1 and 2 and shows the improvement in detection capability (in µg/dL) offered by AS techniques for blood-lead determinations compared to the trend in blood-lead levels set by the CDC. Both plots are shown in log scale, so they can be viewed on the same graph.
WWW.LABMATE-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
