iCap RQ:
Most routine analysis (e.g., sea water, fresh water, biological/organic-rich samples) are performed on our iCap RQ. Sample preparation is ideally performed by users in order to limit laboratory use charges, but we are happy to consult on how best to approach this.
Sample volumes and amounts
Our autosampler can hold a maximum of 240 vials (15 mL conical Falcon tubes are ideal). One analysis consumes ~3 mL. Please allow 10 mL for replicate analysis.
Sample preparation
Aqueous samples must be filtered and free of particulate matter. Samples are introduced in 2% HNO3 (2% of concentrated HNO3; ~0.3 M) and nearly always require dilution to ensure analytical stability and instrument longevity.
Matrix-matching is essential for accurate and precise quantification. This entails matching the concentrations of major cations (e.g., Na+, K+, Ca2+) between samples, as well as standards. Total dissolved solid (TDS) concentrations should be <500 ppm and each sample suite should have a relatively consistent matrix concentration.
Example: A user wishes to analyze a suite of saltwater (TDS ~35,000 ppm) and freshwater (TDS <500 ppm) samples. Both sets of samples must be free of particulates before analysis (e.g., by filtration with a 0.25 um filter). Saltwater samples must be diluted at least 100x (e.g., 0.1 mL sample + 9.9 mL 2% HNO3), while filtered freshwater samples may be acidified to 2% HNO3 (e.g., 9.8 mL sample + 0.2 mL concentrated HNO3). Each suite of samples must have its own set of standards and blanks (see below).
While new Falcon tubes are relatively clean, most users elect to acid-wash their plasticware to reduce analytical blanks (e.g., overnight soak in 10% HNO3 followed by 3-5 rinses in ultra-pure deionized water).
Standards and blanks
Sample concentrations are typically calculated from a 5-point standard calibration curve, where sample concentrations fall within the upper and lower concentration limits for all analytes of interest. For example, if samples typically have 10-75 ppb Zn, Cu and Mn, a standard curve may include 0.5, 5, 20, 50 and 100 ppb of each element (in 2% HNO3). A variety of mono-elemental and multi-elemental ICP-MS standards may be used for this purpose (e.g., see Inorganic Ventures) and are available at TraceLab upon request.
For high-matrix samples (e.g., sea water), we recommend the standard addition method, where a known low-concentration sample is spiked with increasing amounts of concentrated standard. Here, it is necessary to have a “0” standard (i.e., matrix with no standard added).
Blanks must be run in order to accurately calculate the limit of detection (LOD), and must be also be matrix-matched and in 2% HNO3. Instrument blanks track the analytical background of the instrument, while procedural blanks track the level of contamination introduced during sample preparation/digestion (e.g., if digesting biological samples, the same reagents and vials are used to “digest” an aliquot of ultra-clean acid, thereby picking up any contamination introduced via pipette tips, sample vials, dust, etc.). For high-matrix samples, we encourage including 5-10 blanks at the beginning of the sequence to condition the instrument to the typical matrix load.
Verification standards are highly encouraged to track data quality. This typically includes ~2 samples of known concentration (ideally at the high and low ends of the concentration range) which are run as unknowns and are not part of the calibration curve.
Analytical sequence:
Every sequence is different, but must include a minimum of 15 analyses of calibration standards, verification standards, conditioning blanks and analytical blanks. Thereafter, add 3 analyses (2 verification standards and a blank) for every 10-20 samples. Below is an example sequence and a rough guide for determining the final length of the analytical sequence.
Example sequence:
- 1-5: Conditioning blanks
- 6-10: Standard calibration curve (arranged low to high concentration)
- 11-12: Blanks
- 13-30: Samples
- 31-32: Verification standards
- 33-50: Samples
- 51-52: Verification standards
- 53-70: Samples
- 71: Blank
- 72-76: Replicate calibration curve
| Number of Samples | Length of analytical sequence |
| 5 | 20 |
| 20 | 38 |
| 40 | 61 |
| 60 | 84 |
| 80 | 107 |
| 100 | 130 |
| 150 | 186 |
| 200 | 240 |