A Random Talk on ICP-MS Matrix Effects---Nanjing Binglab

Various chemical and physical effects can affect the accuracy of ICP-MS analysis results. Interferences encountered in the ICP-MS analysis are the main contribution. Generally, we divide interference into two categories: mass spectral interference and non-mass spectral interference. Among them, non-mass interference is divided into: matrix effect and physical effect. These interferences can obviously suppress or enhance the ion flow, and also affect the stability of the signal and the accuracy of the analysis. This article, as the final article of the “Matrix Effect” series of tweets, takes seawater analysis as an example to systematically sort out the ideas to deal with the matrix effect, so that users can deeply understand the mechanism and choose the appropriate analysis method.

Matrix effect

High concentrations of matrix components (elements) can suppress or enhance the ion flow being analyzed. The nature of such interference is not necessarily unique and is not limited to a single element. But in any case, the inhibition or enhancement effect of matrix elements with larger mass on elements with smaller mass will be more serious. The degree of matrix effect depends on the absolute amount of matrix elements rather than the relative ratio of matrix elements to analyte elements.

Thus, by reducing the absolute concentration of matrix components (eg, dilution), inhibitory or potentiating effects can be minimized. There are many theoretical supports for the mechanism of the matrix suppression effect, and the mainstream view tends to combine ionization suppression and space charge effects to explain it.

In addition to dilution, there are many ways to overcome the influence of matrix effects. Beauchemin et al. discussed in detail the mechanism and solution of ICP-MS matrix effect.

Examples include Flow Injection (FI), Matrix Separation, Injection System Upgrades, Robust Plasma, Quantitative Methods, and Ar-N2 Plasma. The above measures to deal with the matrix effect can actually be summarized into three solutions. First, reduce the absolute amount introduced by matrix components; second, maintain plasma robustness; finally, choose an appropriate quantitative method for correction.

Physical effect

As the total solids content (TDS) in the sample solution increases, the signal of the analyte ion will drift. The nature of this phenomenon in high-salt samples is a physical disturbance, and the most typical manifestation is the deposition of salt in the interface cone. This results in fewer ions being transmitted to the mass spectrometer, resulting in a lower analyte ion signal. If the concentration of TDS exceeds 0.2%, the problem of signal instability will easily occur. Similar to the idea of dealing with the matrix effect, under the premise of ensuring sufficient analytical sensitivity, the concentration of the matrix can be reduced by simple sample dilution, which can reduce the deposition of salt at the cone hole. Of course, this depends on the high sensitivity of the ICP-MS method.

Second, the use of an internal standard also corrects for a decrease in signal intensity due to salt deposition. There is no doubt that the combination of internal standard calibration and sample dilution can achieve better results. In addition, the standard addition method can achieve a complete match between the quantitative curve and the sample matrix composition, but in order to ensure the accuracy of the analysis results, it is necessary to pay attention to the negative effects of external pollution and mass spectrometry interference.

Memory effects are also a common physical disturbance. Different from high blanks caused by impure reagents or unclean utensils, the memory effect is caused by contamination of the instrument by high-concentration analytes in the sample, especially when analyzing samples in batches, the residual analytes will reduce the accuracy of the analysis results. The memory effect is related to the chemical characteristics of the analyte, for example boron is very easy to carry over in the spray chamber. In addition, the material used in the sampling system (quartz, PFA, etc.) also plays an important role in the carryover of the analyte.

Seawater Analysis

Due to the high salt content in seawater (about 3%), direct measurement by ICP-MS often leads to many problems such as nebulizer blockage, high background, serious interference, etc., resulting in a decrease in instrument sensitivity and poor accuracy in the determination of low-content elements. At present, there are two main ideas to solve the high salinity of seawater. One is to dilute the seawater to reduce the content of the salt matrix; the other is to pre-enrich the elements to be measured and separate them from the salt matrix.

After diluting seawater 10 times, the standard addition method is used for ICP-MS determination, which is the mainstream method for seawater analysis at present. The disadvantage is that offline dilution is time-consuming and labor-intensive, and secondary pollution is prone to occur. With the advancement of instrument technology, the appearance of on-line gas dilution and flow injection-discrete sampling technology makes it possible to directly determine seawater by ICP-MS.

Especially for near-shore marine environmental monitoring, the seawater direct measurement method has become the preferred method for testing large quantities of samples in a short period of time. Agilent ICP-MS can directly analyze seawater by using URK technology and ISIS discrete sampling technology. Taking URK technology as an example, it integrates online internal standard dilution (2 times), online gas dilution (HMI/UHMI), high-salt resistant nebulizer (MiraMist) and argon humidifier, using internal standard calibration combined with standard addition method , by adjusting the parameters of the instrument to maintain the robustness of the plasma, it can stably measure seawater for up to 4 hours, and the recovery rate of the internal standard has been maintained at a level above 80%. Pinheiro et al. used a technique similar to URK to dilute seawater 2 times off-line and combined with HMI, and the recovery rate of the spiked standard was between 84% (Mn) and 112% (Se).

ISIS is compatible with URK technology, but the difference lies in the way of changing traditional ICP-MS continuous sampling to discrete sampling. In this way, sample analysis and pipeline cleaning can be performed at the same time, and the sample analysis time can be shortened by 1/3. The injection volume of a single sample is greatly reduced, reducing the frequency of maintenance. For the Agilent ISIS seawater analysis case, please refer to the two application documents 5991-7936EN and 5994-4467EN.

Finally, let’s introduce the pre-enrichment separation technology, which includes chelating resin method and co-precipitation method. Co-precipitation method has complex operation steps and is easy to introduce pollution during operation. The chelating resin method has the advantages of good selectivity, material reusability and automation, and has received extensive attention. With the maturity of a resin named NOBIAS, the automatic seawater pre-enrichment and separation pre-treatment device (ESI seaFAST) developed based on this resin material came out. Compared with the seawater direct measurement method, the chelating resin method has problems such as expensive equipment, complicated method development, slow analysis speed and high daily operation cost, and is more suitable for scientific research applications such as ocean water and seawater isotope analysis.

Samanta et al. combined seaFAST with Agilent 7900 ICP-MS and compared online and offline pre-enrichment methods to detect trace and trace elements (Mn, Fe, Ni, Cu, Zn, Co, Cd, Pb). The article mentions that for low concentrations of Fe and Cd in seawater, the contribution of ArO and MoO to mass spectrometry interference cannot be underestimated. Jackson et al. combined seaFAST with Agilent 8800 ICP-MS/MS, especially using H2 (56-56) and O2 (111-111) in situ mass reaction modes to measure Fe and Cd in seawater respectively, and the interference removal effect was better than single four Extreme ICP-MS is even better.

To sum up, as the most common and important source of non-mass spectrometry interference that affects the accuracy of ICP-MS test results, the matrix effect should be given priority by operators. The three ideas summarized in this paper to deal with the matrix effect actually point out the direction for the application of analytical techniques such as URK, ISIS and seaFAST. In actual work, you can make beneficial attempts for different types of high-salt samples based on the above ideas.