An Evaluation of Material Composition Analysis and Chemical Analysis

The inquiries and commissions that MA-tek receives from our clients are numerous and varied, but they can generally be divided into two categories. One category would be inquiries about physical and chemical properties. The other would be requests to conduct qualitative or quantitative analysis of samples. The former includes information on parameters such as hardness (H), viscosity, Young’s Modulus, the refractive index, film thickness, thermal glass transition points (Tg), (linear) thermal expansion coefficients (CTE), dielectric constants, transmittance, absorptivity, reflectivity, particle size distribution, melting points, curing rates and other requirements. The latter includes the testing of designated substances in raw materials and products, as well as the identification or quantitative measurement of unknown, foreign substances that have appeared unexpectedly in processes or production lines.

The analysis of a material’s composition can usually be divided into two parts: the acquisition of qualitative information and the quantitative measurement of concentrations. However, no matter what the end goal is, it is necessary to first clarify what kind of substance the target is. Generally, the target substances to be analyzed can be simply divided into three types: elements, organic compounds and ionic substances. After narrowing down the scope of the discussion and available testing solutions, we recommend the instruments, systems or methods to be use based on the questions, information and needs provided by the client.

In terms of the analysis and testing of metals, MA-tek can provide a wide range of options, including materials analysis instruments such as the TEM/EDX and SEM/EDX, surface analysis instruments such as the XPS, XRD, XRF and SIMS, chemical analysis instruments such as the ICP-MS, GD-MS and ICP-OES, and even atomic absorption spectroscopy systems such as the GF-AAS.

Each of the instruments mentioned above provides different analytical information. Their precision, accuracy, and linear range of concentration are all significantly different. Among them, those with the best detection capabilities are SIMS (ppb), GDMS (ppb), and ICP-MS (ppt-ppb). In addition to the ability to provide the lateral distribution of elements at different locations on a sample surface, XPS and SIMS can use ion beams to conduct longitudinal Depth Profiling. Furthermore, XPS analysis results can be compared with energy spectrum information in the database to provide element bonding information. On the other hand, XRF can provide alloy metal grade information via comparison with the database and can be used as a portable inspection instrument for rapid screening once miniaturized. Most surface analysis and materials analysis instruments do not have cumbersome sample preparation requirements. Testing can be conducted directly on solid samples, thus providing fixed-point and micro-area analysis. In contrast, most chemical analysis instruments are limited to liquid samples and often require different types of pre-processing. For example, a sample may have to be prepared as a liquid sample after microwave digestion or hot plate digestion before it can be tested by the machine. Though chemical analysis instruments cannot provide fixed-point analysis, they can provide the overall average or Bulk concentration of a sample.

The ICP-MS is currently considered the most sensitive system in the field of chemical analysis when it comes to metal detection. It is easily able to meet detection requirements at ppb concentration levels. With the exception of C, H, O, N, halogens and gases, it usually demonstrates good sensitivity even for non-metallic and metalloid elements that are not easily freed. When facing the influence of the matrix of complex samples, in addition to using high temperature plasma ionization sources to destroy the sample matrix as thoroughly as possible, most current ICP-MS are designed with a Reaction Cell or Collision Cell integrated with ion lenses and paired with collision or reaction gases with different functions, such as hydrogen, helium or ammonia, to more effectively eliminate interference problems. Additionally, since the ICP-MS is capable of performing rapid mass spectrometry scans quickly, it can perform multi-element detection and analysis in a very short time. It also has technical features such as a linear range of about 7-8 levels. All of this has led it to become widely used in a variety of fields for metal concentration detection.

Generally, ICP-MS systems have a Qualitative Analysis/Semi-Quantitative Analysis (SQ) mode and a Quantitative Analysis mode, enabling it to scan for all the elements in a sample or provide accurate concentration information on specific elements. They also have less commonly used analytical functions, including Isotope Determination and access to the Isotope Dilution Method, which can obtain accurate concentration information without setting a calibration line after adding concentrated isotopes. At present, these two applications are rarely mentioned or applied except in geological analyses. In SQ mode, concentration calculations are based on internal information from the system used in conjunction with the calibration of single-point concentration value standards. Therefore, the concentration provided is considered information on the Order. The system usually selects only one isotope as the detection mass for the target element. Therefore, when there is Polyatomic Interference or Isobaric Interference, results may become skewed. The main purpose of the SQ mode is to obtain information on the concentration of a large number of elements in a single measurement for use as reference. To obtain more accurate results, it is recommended that the quantitative mode be used. In quantitative analysis, concentration detection will be kept within the linear range of the calibration line. It will also be matched with different verification tests, such as the matrix effect assessment, repeated sample testing, retesting of inter-batch verification standards, blank sample retesting, or the use of second source standards to check samples for the standard curve, etc., to ensure that the results obtained are more correct. However, experience has taught us that some samples may still exceed the interference removal capabilities of the typical ICP-MS when the matrix concentration is too high. In these situations, you will need to look for other analysis instruments or a high chemical-resolution ICP-MS with a three-section quadrupole column system for comparison and confirmation.

ICP-MS is limited to use with liquid samples, so solid samples need to be pre-processed into liquid samples before they can be tested with it. The most commonly used pre-processing methods are simple hot plate digestion and microwave digestion, which use different acids or oxidants assisted by high temperatures to destroy the sample’s matrix and convert it into a liquid sample. Therefore, samples with contents that cannot be digested this way, such as ceramic materials, sintered carbides, acid and alkali-resistant plastics or polymers, etc., cannot be effectively analyzed. In these cases, it is usually necessary to find a high temperature furnace or an alkali melting furnace to process the sample.

In contrast to metals and organic substances, the composition analysis of ionic species is relatively simple. They can generally be divided into halogens, which are anions and include fluoride ions, chloride ions, bromide ions, iodide ions, and acid ions. Acid ions can be divided simply into inorganic acid ions, including nitrate, nitride, sulfate, and phosphate, and organic acid ions, including formate, acetate, and weak organic acid ions. This type of analysis is usually required for testing the residue on the surface of objects. Sometimes the IPC TM650- 2.3.28 is used for the sample pre-processing. The pre-processed sample is then tested using an anion or cation chromatography system.

In regards to the detection and analysis of organic materials, commonly used instruments include the FTIR, Raman, GCMS, TOF-SIMS, and LCMS, etc.. When dealing with sample’s with high concentrations or when analyzing an object’s surface, it is generally recommended to use the FTIR or Raman first. In addition to having advantages such as being simple, cheap and fast, they can provide information on functional groups that cannot be provided by other instruments as well as propose possible qualitative information via comparison with the database. When the sample’s composition is more complex or the concentration is low, however, a mass spectrometry system with better sensitivity will still be needed. TOF-SIMS mainly provides spatial resolution and high quality resolution data and can be applied directly to analyze materials or finished product surfaces.

In traditional GCMS testing, solid and powder samples are first dissolved using appropriate solvents before they are extracted and injected into the GCMS for testing. It is used mainly to analyze volatile organic compounds with non-polar small molecules (MW<550). Its samples generally need to have a certain level of thermal stability. Otherwise, the sample may crack during analysis, causing the signal to split. The main problem for this method is that, when facing unknown substances, it is often difficult to determine whether the solvent chosen will be able to fully dissolve the sample. Therefore, most choose to use polar, non-polar and mixed solvents for testing to increase the completeness of experiments. However, this also increases the time and cost of the experiments. In addition, the solvent used usually forms a very obvious solvent peak in the chromatogram relative to the solute, resulting in interference between solutes and solvents with similar retention times/elution times. The example in Figure 1 shows fruit peel and pulp samples that were diluted 50 times with THF before being tested. The graphs show the results of the two samples and the THF background signal respectively. It can be seen in the THF background signal that there is an obvious peak at 4 to 7 minutes, and it can be mapped to the spectra of the two samples, though there are still discernable signals before and after the spectra of the two samples. Therefore, though it is possible to obtain some of the composition information, the signal at 4 to 7 minutes may be buried in the huge background signal, causing misjudgment.

Figure 1　Effect of THF Solvent Peak in GCMS: (a) THF Solvent Blank; (b) Fruit Peel Sample; (c) Pulp Sample

Therefore, one commonly considered alternative method is based on making use of the volatile characteristics of organic materials when heated. Direct heating or IR lamps are used to provide thermal energy to the sample, causing the organic material in it to volatilize, vaporize, crack or desorb away from the sample in gaseous form. Then an inert gas is used to carry the Outgassed results into the GCMS for analysis. In the case of analytes with lower concentrations or slower volatilization rates, the analyte can be supplemented in-line with adsorbent or liquid nitrogen. It is measured after concentration. The desorption temperature applied can be selected based on the temperature when the client’s problem actually occurred or according to the temperature conditions of the actual process or reaction. It is also possible to simply use the fact that most organic compounds will become volatile and desorb into the outgasing substance at temperatures of about 300 degrees C and set the desorption temperature at around 250-300 degrees C. A lower desorption temperature will usually moderately lengthen the sample collection time due to reaction rate considerations. Higher temperatures can increase desorption rates and shorten sample collection times. However, temperatures above 300-350 degrees C can cause organic samples to begin to crack. Therefore, it is not possible to simply set a higher temperature for the experiment. Some laboratories that already have GC-MS equipment can even set up their own front-end thermal desorption systems by applying a thermal desorption device of 300-450 degrees C to a sample and connecting it in series with the GCMS to achieve functions similar to those of commercially available thermal desorption gas chromatography mass spectrometry (TD-GC-MS) systems.

It can be anticipated that polar substances, ionic substances, and organic substances with greater molecular weights, such as polymers and resins, will be less suitable for this method. Solutions containing inorganic acids and inorganic bases are also unsuitable because they will corrode the metal parts in the pipelines and equipment at high temperatures. Additionally, if the metal content of a solution is too high, such as in the case of electroplating solutions, there is a concern that metal may be deposited on the interfaces of the metal cones, pipelines, or capillaries at high temperatures. As such, GCMS labs will often decline requests to analyze such substances. It is possible to use liquid-liquid extraction to obtain organic phase extracts from these substances then test them. However, usually, the organic compounds that will stay in aqueous solution samples will be relatively polar. Under the premise of mutual miscibility between polarities, its distribution coefficient in the organic phase is often relatively low, so the proportion that can be extracted into non-polar solvents is limited, meaning it is likely that the result will be not detectable (ND). Some organic substances with greater molecular weights, such as polymers and resins, have stronger, cross-linked structures, making them difficult/impossible to measure under normal conditions. In these cases, it is necessary to apply a higher temperature to the sample so that it has the opportunity to break bonds and form fragments with specific structures. These can then be compared with the polymer database, potentially enabling them to be identified.

This type of Hyphenation System heats samples then uses a mass spectrometer as a detection system. Using components with slightly different functions can enable the collection of different information on the sample. Analysis systems used can be divided into two categories: those used to provide qualitative and quantitative information on the analytes in samples, such as Thermal Desorption Gas Chromatography Mass Spectrometry (TD-GCMS) and Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) systems, and those that mainly study thermal desorption behavior over time, such as the Thermal Desorption Spectrometry (TDS) system, the Temperature Programmed Desorption System (TPD) and the Thermal Gravity Mass Spectrometry (TG-MS) system, etc.. Figure 2 is a schematic diagram of this type of system. The main difference between the TD-GCMS and the Py-GCMS is the heating temperature. This enables the identification and measurement of small molecule organic additives and the identification of larger molecules. Also, since most GCMS reference parameters are the same or similar, the results obtained via GCMS by different laboratories will be fairly comparable. For example, most GC columns used are DB-5-related columns. By raising the temperature of the GC-MS column, the TPD can be used as a by-pass communication pipeline. However, in actual experiments, the results obtained still indicate that, when measuring water vapor and hydrogen, it is still possible for them to be repeatedly absorbed/desorbed at the GC column, so the results obtained do not meet the client’s expectations. However, since the heavy losses at each time point can be recorded in the TGA section for TG-MS connected in series with TGA and mass spectrometer systems, using this makes it possible to perform concentration/desorption weight calculations. Unfortunately, most general commercial TGA system sample pans are ceramic crucibles with a diameter of 4mm, which limits the amount/volume of sample that can be placed in the machine at one time. On the other hand, TDS places the sample in a vacuum and uses an IR lamp to provide it with thermal energy, so the analysis time is lengthy and commissions are expensive. Even so, it may be the best choice among the three for accuracy when it comes to the abovementioned hydrogen and water gas signal measurements.

Figure 2. Schematic Diagram of Different Heat Supply Device and Mass Spectrometer Hyphenation Systems

The liquid chromatography mass spectrometer can measure polar small molecules or macromolecules in solutions. Because the ion source used is mild and low temperature compared to those used in inductively coupled plasma mass spectrometers, it is called a soft ionization system, and the ions measured will reflect their original charge states in the solution.

The instrument’s parameter settings and the wide variety of LC analysis columns used in the separation of analytes often leads to significant differences in the results obtained by different laboratories. Sometimes, even different members of the same laboratory may get very different results because they used different parameter references. For these reasons, there are currently very few ready database systems for LC-MS equipment. Databases for pesticide and drug-related testing laboratories are among the more common. Therefore, it is difficult to simply use this instrument as a qualitative analysis device. Common organic mass spectrometers include the Single Quadrupole Mass Spectrometer (LC-MS), Triple Quadrupole Mass Spectrometer (LC-MS/MS), and Quadrupole-Time of Flight Mass Spectrometer (q-TOF), etc.. Among them, the q-TOF is considered to have excellent quality and resolution. When coupled with appropriate experiment designs, statistical software, and online databases （such as MASCOT） for comparison, it can be used to analyze the structures of analytes. It has been widely used in the past for different omics analyses, such as the structural analysis of biological macromolecules and proteins in proteomics. Most Tripe-Q systems offer only Unit Resolution. However, when used with Select Ion Monitoring (SIM) or Multipole Reaction Mode (MRM), it can achieve better sensitivity and accurate quantification of organic small molecules. Therefore, it is widely used in pharmaceutical and metabolite analysis-related fields. At present, MA-tek mainly uses the Electro-Spray ionization (ESI) LC-MS/MS system with high temperature gas assistance. Typically, when used with a Reverse Phase Chromatography (RP) separation column, it can detect polar organic small molecules, and the MS2 scan mode coupled with the LC-MS/MS can handle the full mass spectrum of the sample (m/z < 4000 ). It can also provide the corresponding information if you only want to compare the differences between samples. When the specific information about the mass fragments of the sample to be tested are known, you can try to perform quantitative analysis of the analyte in the SIM or MRM modes mentioned above.

In contrast to GCMS analysis of macromolecular substances such as polymers and resins, where the solid sample is heated then the fragmented molecules are measured, LC-MS can only perform solution injection analysis. For the detection of macromolecules, we will take proteins, nucleic acid or amino acid molecules as examples. For proteins, the functional groups on the amino acids can be disassociated and charged in an appropriate buffer. Therefore, the same molecule may have multiple charges, becoming a Multiple Charged Ion. It can then be used to deduce the number of charges carried at different signal positions. Then you can back calculate possible molecular weights using the system’s deconvolution function. Figure 3 uses a metallothionein protein with a mass of about 6700 Da as an example. As you can see, the result obtained will be approximately equal to the actual molecular weight. Another method commonly used in molecular biology and biochemistry is to perform enzymatic hydrolysis of the protein using Trypsin. This degrades the original larger, longer structure so that the resulting amino acid fragments can be used to deduce the possible original amino acid structure and sequence. Because the assessment of enzymatic hydrolysis and multiple charges are difficult and require additional equipment, only biochemistry and omics-related laboratories currently provide such testing services.

Figure 3.　Using LCMS to estimate the molecular weight of metallothionein based on its multiple charge ions

At present, LCMS does not have a database for direct signal comparisons. Therefore, it is recommended to consider the GCMS system first for the detection and analysis of organic substances. Although LCMS can provide information that is different from that which is provided by GCMS, the current scope of MA-tek’s analysis services will still be limited to: (a) analysis of the differences and similarities between samples and the comparison between OK and NG components, (b) provision of compound specific analytical testing and standards and reference materials, (c) residue analysis of water soluble contaminants and foreign matter, and (d) provision of mass spectrometry data for research and discussion even if the sample cannot be analyzed using GCMS.

For the testing of provided samples, the services that MA-tek can provide usually include testing and analysis via customer-specified instrument systems as well as the analysis and discussion of unknown components and foreign materials in samples.