What was collected in the sample for chemical analysis

Basically, if there is a residue left behind after a digestion process, then there is a likelihood that your element of interest is partially or totally in that residue.

Table I shows the certified mass fractions with uncertainties for four elements and also the mean, minimum, and maximum range values obtained when using a partial digestion method.

Additional information from the certificate is available on the NIST website. For rock, soil, and other geological samples, sample preparation often means reducing the sample size further using appropriate splitting methods. Note that care must be taken to keep the sample representative in terms of particle size so as to avoid biasing the values obtained.

The best way to remove these biases is to take a representative sample and dry, grind, and sieve it so that all the particles are of a similar size Figures 4—6.

The sample should then be mixed well before taking the subsample for analytical work. It is important to be aware that the element of interest might preferentially be found associated with one particle size rather than another. In rock samples, this is because of the different physical properties of minerals such as hardness, cleavage, and crystal habit , which have been shown to cause preferential concentration at different particle sizes.

In other words, if a rock is crushed for sampling purposes, then some analytes might concentrate in either the larger or smaller particle sizes.

In soil samples, this is caused by differences in chemical or physical binding adsorption of the soil constituents. Colloidal particles such as clay show a far greater tendency to adsorb ions and molecules than the noncolloidal constituents such as silt and sand.

If more fine material is weighed and dissolved, then the readings will be higher than the "true" result. If a higher percentage of coarser particles is taken, then the reading will be lower than the "true" result.

As mentioned earlier, grinding and sieving a sample to a consistent particle size and then mixing the powder thoroughly reduces these errors. Sample preparation for XRF analysis requires making the particle size of the sample as small and homogeneous as possible. This might mean the relatively simple process of drying, grinding, sieving, and cupping the specimen before analysis.

However, ground samples also might be mixed with a binding agent and pressed into a pellet. Alternatively, the sample might undergo a fusion with sodium or lithium tetraborate and form a glass bead that can be analyzed subsequently 4. The actual chemical analysis also consists of several steps: method creation, method validation, and sample analysis.

In spectrochemical analysis, method creation is the process by which suitable spectral lines or regions are selected for analytical, background, interference, and reference measurements. The line selection process itself involves choosing lines that have the appropriate sensitivity for the concentration range required. The spectral line must not be so sensitive so that it becomes excessively nonlinear or self-absorbs. Nor must the spectral line be so weak that the element cannot be detected at the concentration of interest.

It is preferable to select spectral lines that are interference free; however, practically speaking, this is often difficult. Therefore, interfering elements must be identified and subsequently, the readings corrected 5. Reference lines are spectral lines that will be used as internal standards. The analyte signal is ratioed to the internal standard line to improve the precision 6. The internal standard method minimizes the problems caused by variations in sample transport and nebulization in the cases of liquid samples.

With X-ray spectrometry, it is not unusual to use the Compton scatter region as an internal standard to compensate for sample density variations. The calibration curves can be derived in essentially two ways, empirically or theoretically. They can be generated empirically by running multiple standards. Or they might be liquid or solid multielement standards containing all or most of the elements of interest. The other option is a theoretical calibration, often termed the "Fundamental Parameters" approach in XRF analysis.

In this case, analyte concentrations might be calculated directly from the physical constants of X-rays and their interaction with matter 7. This sometimes is termed a "standardless" calibration, although this is a serious misnomer. No quantitative results have been demonstrated to date without the use of at least several standards to refine the calibration.

In terms of solutions, acids normality should be matched between calibration standards and the samples, quality control QC standards, reference materials, and blanks to minimize nebulization variations. In essence, matrix matching standards and samples will give more accurate values. Note that serial dilution of a multielement solution should be avoided due to complications in determining if interelement corrections are suitable.

This calibration curve is critical to the success of the analysis. Therefore, the analyst must have a good grasp of how the instrument is calibrated and what type of sample can be read against a particular calibration curve. Example 1: If an organic solvent extraction is read against an aqueous ICP calibration, then the values obtained cannot be correct. This is assuming that the plasma is not extinguished, as organic solvents generally require more power to ensure that the plasma remains stable.

Typically, this is how the matrix element of the sample is calculated. We measure all analytes of interest except for the matrix element , add these together, and subtract from to obtain the matrix element concentration. Example 3: Even if an instrument is calibrated with the appropriate elements and the correct matrix, a particular sample might have a very high concentration of a particular element that is above the linear or determined portion of the calibration curve and will therefore still give an erroneous result.

After an analytical method is created, it must be validated or checked for analytical integrity. This typically is performed by running certified reference materials or other well-characterized materials, preferably ones that were not used in the original calibration process. These standards are read against the calibration curve and the values with the associated error standard deviation recorded and checked against the certified values.

If the standards are within expected ranges, the calibration curve is acceptable. If one or all of the standards falls outside of the allowable ranges, then the calibration curve should be revisited. Several check standards should be run during this method validation process. See, for example, reference 8. The analysis of QC samples is essential in testing out how well an instrument is performing. These samples should be taken through the identical process as the unknown samples.

For example, reading a certified reference material that was dissolved last week and getting the "right" number only proves that the calibration on the spectrometer is reading appropriately. It does not tell you that a bad batch of acid or an incorrect dilution was made on the samples being prepared and analyzed today. The instrument is now ready for the analysis of the unknown samples. It is advisable to run QC standards blanks, reference standards at the beginning and end of each analytical run, and depending upon how many samples are within a batch, maybe within the series, too.

This is to verify the performance of the instrument through a run and to confirm the ongoing validity of the calibration curve. How do we evaluate the results obtained from our chemical analysis? The two most important criteria from the analytical point of view are precision and accuracy. Precision is defined as the reproducibility of repeat measurements.

Specifically, it is a measure of the "scatter" or "dispersion" of individual measurements about the average or mean value. Accuracy, on the other hand, is defined as the relationship or correspondence of the measured value with the "true value. The average or arithmetic mean of a set of measurements is obtained by adding up the individual observations or measurements and dividing by the number of these observations. This is expressed mathematically in the following formula:.

It is a measure of the "spread" of measurements from the mean value. The standard deviation generally is calculated from a series of 10 or more independent measurements using the following formula:. What is the significance of the standard deviation of a set of measurements? As already noted, the standard deviation is a measure of the "spread" or "dispersion" of the measured values about the mean or average value.

Statistically, we can say that with repeated measurements of this same sample, the following is true:. The way individual measurements are spread about a mean value also is shown in the familiar "bell curve," more technically called the "Gaussian Distribution Curve.

Given a series of several measurements, take the difference of the maximum and minimum values the range , and divide by the square root of the number of measurements. It is important to note that the standard deviation is NOT a reference to accuracy, but rather an indication of the random errors encountered in the measurement process.

The relative standard deviation RSD is the standard deviation referenced to the mean measured value and expressed as a percentage:. The RSD is significant because, by reference to the mean value, it provides a measure of the dispersion of the observed values independent of their size or magnitude.

It is sometimes called the "coefficient of variation" or CV. The RSD is a very useful tool for the analyst because it gives an immediate idea of the relationship of the measured values to the instrument detection limits. This level of precision, however, typically is not found until the result is at least times the detection limit. We can now introduce a "third law of spectrochemistry" to supplement the two introduced in a previous tutorial 1. This holds regardless of the element, the matrix, and the instrumental technique used.

The corollaries have been explained in a previous tutorial 9. Further information about the relationship between RSD and concentration level is provided in two recent articles in this journal 10, Qualitative: This attempts to answer the question, "Is a certain element present or not in the sample?

Semiquantitative: Here the question is, "Approximately how much of this element is present? She has been practicing chemistry for 31 years on both sides of the pond, and holds the distinction of being the only woman to have held the title of President of the US Section of the Royal Society of Chemistry RSC. He can be reached at vbet uol. Eckschlager and K.

Gerlach, D. Dobb, G. Raab, and J. Nocerino, J. Burhke, R. Jenkins, and D. Smith, Eds. Thomsen, D. Schatzlein, and D. Mercuro, Spectroscopy 21 7 , 32—40 Mercuro, Spectroscopy 18 12 , — The Chemical Analysis Process. October 1, Volker Thomsen , Debbie Schatzlein Spectroscopy. Spectroscopy , Spectroscopy, Volume 23, Issue Figure 1 The flowchart shown in Figure 1 is quite general and all steps are of equal importance.

To minimize this determinate sampling error, we must collect the right sample. In a qualitative analysis, a sample need not be identical to the original substance if there is sufficient analyte present to ensure its detection.

For example, a quantitative analysis for glucose in honey is relatively easy to accomplish if the method is selective for glucose, even in the presence of other reducing sugars, such as fructose. Unfortunately, few analytical methods are selective toward a single species; thus, we must separate analytes from interferents. A significant difference in properties, however, is not sufficient to effect a separation if the conditions that favor the extraction of interferent from the sample also removes a small amount of analyte.

In a simple liquid—liquid extraction the solute partitions itself between two immiscible phases. One phase usually is an aqueous solvent and the other phase is an organic solvent, such as the pentane used to extract trihalomethanes from water.

As we have learned in this chapter, we can use a separation to solve the first problem.