How to Improve the Sensitivity, Long-Term Stability and Throughput of your ICP-MS
Mon 20 Jan, 2020
More Performance for your ICP-MS
ICP-MS (inductively coupled plasma mass spectrometry) is a widely used technology for analytical chemistry. Their multi-element detection capability, the lowest detection limits in the ppb-ppq area, and their versatility make ICP-MS systems the ideal analytical instruments for an incredible range of applications. Whether in the environmental sector, food inspection, or for clinical studies, the areas of application for ICP-MS technology are highly diverse.
There are a number of factors, however, which can impair and limit the performance of an ICP-MS. In the following, we provide several tips for how to optimize your ICP-MS for three major performance criteria: sensitivity, throughput and long-term stability. These criteria are of interest primarily for laboratories that prescribe high standards of efficiency and performance for themselves, such as contract laboratories or process-adjacent industrial laboratories. These tips (solutions) can help ensure optimal performance from your ICP-MS.
Increasing ICP-MS sensitivity
Optimal trace analysis with ICP-MS systems often depends on how effectively users can prevent contamination. Each step of sample handling must be planned, and the strictest cleanliness conditions must be observed for reagents and ultrapure water. Blind tests should be performed regularly in order to detect any contamination of reagents or laboratory devices.
The possible sources of contamination are highly varied and sometimes unexpected. For example, dissolved pigments from colorful lids of sample bottles often contaminate the samples themselves. Various auxiliary chemicals can also have an adverse effect on trace detection. The use of HCl for sample digestion can often lead to the under-measurement of dissolved silver due to AgCl precipitation.
There are several factors that can accelerate the contamination of the ICP-MS device. Caution is particularly necessary for samples with high matrix concentrations. Not only can these impair ionization and influence the sensitivity of analysis, but can also lead to the formation of deposits on the cones, which cause drift effects over time by impairing performance. Make use of internal standards to identify and counteract such effects on your ICP-MS.
As soon as sample material has collected on the torch and/or the cones, however, cleaning must be carried out immediately. The ICP torch and the ICP-MS cones can be cleaned in an ultrasonic bath with detergents or with citric or nitric acid. Be careful during this process, since the glass components of an ICP-MS are extremely delicate. Glass atomizers should generally be cleaned with a special tool and not in an ultrasound bath.
The presence of carbon in a sample can strengthen the signal quality of elements with low ionization potential like arsenic or selenium. For this reason, it is important to ensure that the carbon content of the calibration solutions is similar to that of the samples. In cases of samples with low carbon content, it is often helpful to add carbon (e.g. in the form of isopropanol) in order to take advantage of this strengthening effect. The compromise here is in regard to potential disruptions, such as in the case of ArC+ with chromium. Fortunately, the PlasmaQuant MS ICP-MS is capable of adding nitrogen to the auxiliary gas, which offers the same strengthening effect but without creating disruptions.
Increase throughput of the ICP-MS
Another key performance factor for an ICP-MS, particularly in contract laboratories and process-adjacent laboratories, is sample throughput. Analysis time is typically limited by the sample supply. Image 1 compares two possible configurations: In a standard arrangement, it can take up to 80 seconds for the sample to be loaded and the signal to be stabilized. After roughly 20 seconds of data collection / measurement, up to 120 seconds of rinsing may be necessary. A total time of two minutes for only 20 seconds of measurement time is not uncommon. The second example shows a substantially higher-performing alternative with a faster sample supply system. The configuration of a PlasmaQuant MS shown here with discrete sample supply as a supplement to the autosampler enables rapid sample input as well as the rinsing out of samples in less than 10 seconds each. The total analysis time of the ICP-MS is reduced to 50 seconds while also employing a measurement time of 20 seconds.
The downtime of an ICP-MS also has an effect on total throughput. For this reason, maintenance and cleaning times should be kept as low as possible. In this regard, the high sensitivity of modern ICP-MS devices, such as the PlasmaQuant MS series from Analytik Jena, works to your advantage. Samples can easily be diluted before being supplied to the ICP-MS. This serves to reduce matrix effects, substantially decreasing the need for cleaning. Measurement times can also be reduced in this manner without impairing accuracy. This further improves sample throughput.
Optimal long-term stability and robustness of the ICP-MS through optimal plasma performance
For all types of samples, particularly high-matrix samples, the reliable and reproducible generation of ions by the plasma is crucial. In general, the robustness of the plasma is determined by the output and the stable coupling of the high-frequency generator (RF). High output results in a more reliable plasma. The ICP-MS devices of the PlasmaQuant MS series from Analytik Jena are able to employ high-frequency output with low cooling gas flows. Relatively pure and diluted samples such as drinking water and surface water typically require output of only 1.2-1.3 kW and can be analyzed with a gas throughput of 7.5 L/min. More complex matrices such as wastewater or food require a higher output and throughput quantities of 9.0 L/min. Samples with higher matrix proportions, such as salt water, geochemical samples or organic matrices, on the other hand, require a higher plasma output of up to 1.5 kW. Metal and enamel samples constitute an extreme case. These require throughput rates of up to 10.5 L/min and outputs in the range of 1.4-1.6 kW in order to ensure reliable analysis with an ICP-MS. With total consumption of 10-12 L/min argon, the PlasmaQuant MS models offer unbeatable economy with outstanding performance.
Application examples for optimized ICP-MS performance
ICP-MS systems can be used to analyze a wide range of samples, including drinking water, whole blood and foods, which are examined in greater detail below.
Water analysis: Water samples can be supplied directly into the ICP-MS, possibly following a perfunctory filtration. In the area of water analysis, laboratories often face a challenge in regard to sample throughput. The laboratories in question are often contract laboratories that need to analyze a large number of samples within a very tight timeframe. For this reason, rapid sample supply and analysis are decisive.
Whole blood: Whole blood is a complex matrix that is difficult to analyze due to its tendency to interact with hoses and other sample supply components. Analysis is further complicated by polyatomic interferences such as 40Ar12C, which overlaps 52Cr. In addition, analyte concentrations are often below 1 µg/L. Dilution is necessary in order to ensure smooth sample supply. For this reason, the detection limits of the method must stand at 0.005 µg/L.
A quick example measurement with an ICP-MS from the PlasmaQuant MS series demonstrates how this challenge can be overcome. First, the whole blood samples to be analyzed were diluted into a 20X solution. The solution contained 2% ammonia, 2% isopropanol and 0.1% TritonX100. Hydrogen was employed as the reaction gas, and the skimmer-cone was given a positive voltage in order to prevent the ion beam from defocusing (Image 2). Calibration was carried out from 0.02 to 2.5 µg/L. The detection limit for this method stood at 0.007 µg/L, corresponding to a concentration of 0.14 µg/L in undiluted whole blood. The procedure proved stable over a full eight-hour shift.
Foods: Whenever foods or agricultural products are analyzed with an ICP-MS, sample digestion is typically required. It is often necessary to measure multiple elements. The concentration range is also very broad. Arsenic is a common analysis target since it is a poisonous, naturally-occurring element and frequently appears in a wide range of foodstuffs. Analysis of total arsenic is not sufficient, however, since not all species of arsenic are toxic. Detection and quantification of the different species of arsenic are required. As such, an HPLC is coupled with the ICP-MS in order to separate the individual species. In one analysis by Analytik Jena, fish and rice samples were prepared in two different types: For mechanical sample preparation, a SpeedMill PLUS and an optional lysis buffer were used for additional breakdown of the matrix. The second preparation variant used a microwave-assisted digestion procedure for the extraction / full digestion of the samples (in this case with the help of a TOPwave from Analytik Jena). The results with certified reference materials showed that the homogenization using the SpeedMill PLUS with the addition of a lysis buffer was ideal for achieving a recovery rate of more than 99.5% for all arsenic types tested (Image 3). The results for the recovery rate without the lysis buffer were less ideal. The microwave digestion yielded the best results for the measurement of total arsenic. If speciation is required, however, there is a risk of oxidation or transformation of the arsenic species by the microwave. This can lead to a change of +/- 20% for individual species. A spike recovery test for the entire sample preparation procedure with the SpeedMill PLUS and with the addition of a lysis buffer showed excellent results for monomethylarsonic acid, dimethylarsinic acid and arsenic (V) in the range of 2–200 ppb additions.
Small optimizations make a difference for your ICP-MS
ICP-MS is a highly flexible technology that is suitable for numerous applications. The most recent advances in sample supply and device design have substantially improved the performance, reliability and throughput of ICP-MMS systems. A thorough analysis of the requirements and limitations of every analysis or experiment helps to identify the optimal performance-increasing options, such as achieving improved plasma output by adding nitrogen to improve analysis speed and ionization or selecting the right digestion method for the respective sample matrix. Modern ICP-MS devices, such as the PlasmaQuant MS family from Analytik Jena, are tuned for performance from the moment they leave the factory and can also be configured for the requirements of special and niche applications easily with the help of accessories. Small details of the approach or the analytical methodology, however, can often pose a great deal of potential for optimization. The tips above can help you to get even more out of these modern ICP-MS devices for your laboratory.
Analysis of Food and Agricultural Samples using the PlasmaQuant MS (English)
Application NoteOpen PDF
Speciation of Arsenic in Rice by LC-ICP-MS on PlasmaQuant MS Elite (English)
Application NoteOpen PDF