Trace Elements

Flame atomic absorption spectrometry, graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma mass spectrometry (ICP-MS) are the established quantitative method families for the listed elements. GFAAS has the sensitivity for a single analyte at very low concentration, and it processes one specimen at a time. ICP-MS combines high sensitivity with broad multielement coverage and is used routinely for serum and whole-blood trace-element panels in United States reference laboratories. One ARUP validation describes a seven-element serum panel, chromium, cobalt, copper, manganese, nickel, selenium, and zinc, on a single ICP-MS with a helium collision cell and an iridium internal standard.^{22 Johnson-Davis KL, et al. Method validation for a multi-element panel in serum by… 2020.} Inductively coupled plasma optical emission spectrometry measures several elements per run from the same plasma source. Its higher detection limits restrict its use for ultratrace clinical work. The instrument engineering behind these methods is explained in the Quantitative Instrument Measurement Principles lesson.^{33 Yang HS, et al. Elemental testing using inductively coupled plasma mass… 2021.}

At these concentrations, ICP-MS-specific interferences and quality practices matter. Isobaric and polyatomic ions can share the mass of an analyte. Elemental isobars share one mass number: ⁴⁰Ca⁺ and ⁴⁰Ar⁺ both arrive at mass-to-charge 40, so calcium measurement by ordinary quadrupole ICP-MS needs a cell gas or an alternative isotope. Polyatomic ions create the same problem: ⁴⁰Ar¹⁶O⁺ interferes with ⁵⁶Fe⁺ and ⁴⁰Ar¹²C⁺ on ⁵²Cr⁺, formed when plasma argon combines with matrix-derived oxygen or carbon. A helium collision cell reduces many polyatomic interferences through kinetic-energy discrimination: larger interfering ions generally undergo more collisions, lose more energy, and are rejected at an energy barrier. Reaction-cell gases use analyte- and interference-specific chemistry. Validate the selected cell mode and isotope for the specimen matrix.^{21,321 McCurdy E, et al. Applications of helium cell gas with kinetic energy… 2021.3 Yang HS, et al. Elemental testing using inductively coupled plasma mass… 2021.}

Matrix effects from viscosity, ionization balance, and specimen dissolution are handled by matrix-matched calibrators and controls, internal standards, and validated dilution. Standard addition is used for a specimen whose matrix cannot be matched externally. An internal standard element near the analyte mass, ⁷²Ge, ¹⁰³Rh, and ¹¹⁵In are examples, added identically to every calibrator, control, blank, and specimen, corrects drift and matrix suppression that external calibration cannot detect. Biological specimens are diluted or digested before introduction, often 10-fold to 50-fold, partly to keep the dissolved-solids load tolerable to the plasma and interface.^{33 Yang HS, et al. Elemental testing using inductively coupled plasma mass… 2021.} The Clinical Laboratory Improvement Amendments (CLIA) require calibration verification at least every 6 months across a low value, a midpoint, and a value near the upper reportable limit, and again after specified reagent, maintenance, or quality-control events.^{44 Centers for Medicare & Medicaid Services. CLIA proficiency testing regulations and calibration…}

Standard addition and dilution recovery

Standard addition estimates an analyte using the response to known additions in the specimen's own matrix. Equal specimen aliquots and equal final volumes keep the matrix comparable across the series. The calculation assumes a linear response and the same multiplicative matrix effect on native and added analyte. A separately measured additive blank must be accounted for. Standard addition does not correct every interference. The added concentrations must describe the final measured solution.^{55 Slayton JL, et al. Method of Standard Additions and Effects of Dilution. 1980.}

For a blank-corrected line, the zero-addition signal divided by the slope gives the native analyte concentration in that solution. Recover the original specimen concentration by multiplying by final volume divided by specimen aliquot volume.^{55 Slayton JL, et al. Method of Standard Additions and Effects of Dilution. 1980.}

Suppose a verification series uses 1.50 mL serum in each 6.00-mL final solution. Additions of 0, 3 and 6 µg/L produce signals of 55, 100 and 145 units. The independently established blank is 10 units.

• Slope: (145 − 55) ÷ 6 = 15 signal units per µg/L.
• Native analyte in the measured solution: (55 − 10) ÷ 15 = 3 µg/L.
• Original serum: 3 × (6.00 ÷ 1.50) = 12 µg/L.

The reported result uses the original serum concentration. Omitting blank subtraction overestimates it. Omitting dilution recovery reports only the diluted-solution concentration. Nonlinear response or changing matrix effects require investigation before this calculation can support reporting.

The distribution of each element between cells and plasma determines the preanalytic rules. Packed-cell-to-plasma concentration ratios run roughly 26 for manganese, 10 for zinc, and 1.2 for selenium. Copper is about 1.6-fold more concentrated in plasma.^{11 Versieck J. The collection and preparation of human blood plasma or serum… 1977.} The method's validated separation time therefore matters. Hemolysis adds cellular content directly: about 1 g of hemoglobin per liter, roughly 1% of red cells lysed, raises serum or plasma zinc by about 5%. Reject or recollect hemolyzed specimens for zinc, manganese, and iron according to method-specific criteria. Do not correct them mathematically.^{66 Killilea DW, et al. Identification of a hemolysis threshold that increases plasma… 2017.} Because red cells contain much more manganese than plasma, even limited hemolysis can raise the measured concentration.^{11 Versieck J. The collection and preparation of human blood plasma or serum… 1977.}

The collection device is one source of contamination. In Versieck's 1986 experiments with neutron-irradiated disposable steel needles, chromium, nickel, and manganese additions were largest in the first 20-mL aliquot and smaller, but sometimes still present, in later aliquots. Those experimental conditions do not establish a universal discard volume.^{11 Versieck J. The collection and preparation of human blood plasma or serum… 1977.} A 2012 study of 10 healthy volunteers found no significant difference in cobalt, chromium, manganese, or nickel results among a standard steel needle, a butterfly needle, and a plastic cannula. It did not support routine replacement of standard needles with plastic cannulas. For an unexpected high result, low-contamination recollection may be appropriate when the performing laboratory's procedure requires it.^{77 Hodnett D, et al. A healthy volunteer study to investigate trace element… 2012.}

Stopper or additive carryover is a second source. ARUP requires its trace-element tube to be drawn first overall because puncturing another tube's stopper may contaminate the needle.^{88 ARUP Laboratories. Trace elements.} Mayo permits a regular phlebotomy needle or butterfly and places the royal-blue tube first within its additive group.^{99 Mayo Clinic Laboratories. Metals analysis specimen collection and transport.} The whole-blood chromium and cobalt method of the CDC's National Health and Nutrition Examination Survey (NHANES) lot-screens its collection devices and recommends the assay vial be the second vial collected to limit chromium from the needle.^{1010 Centers for Disease Control and Prevention. NHANES 2017-2018 laboratory methods.} The general order-of-draw guidance of the Clinical and Laboratory Standards Institute (CLSI) places listed additives in sequence and says unlisted tubes must be positioned by the risk that their additive would alter the next tube's results.^{1111 Clinical and Laboratory Standards Institute. Order of blood draw tubes and additives. 2019.} These instructions apply to different procedures and are not interchangeable.

Handling, transfer, skin preparation, and the collection environment are a third source of contamination. Stoppers, transfer utensils, ordinary pipette tips, iodine-containing disinfectants, dust, and residue on gloves or containers can add trace elements after or around the draw.^{1,81 Versieck J. The collection and preparation of human blood plasma or serum… 1977.8 ARUP Laboratories. Trace elements.} Use the collection device, matrix, tube, draw order, and discard volume that the performing laboratory's current analyte-specific instructions specify, and separate and transfer the specimen the same way. Cap color alone does not certify a tube for an analyte. The laboratory's blanks monitor reagent and environmental contamination, and carryover checks bracket a high specimen with low ones. CLIA distinguishes recollection from true confirmatory testing, which uses an independent analytical procedure to substantiate or question an initial result.^{44 Centers for Medicare & Medicaid Services. CLIA proficiency testing regulations and calibration…}

Serum and plasma measurements describe circulating concentrations. Whole-blood and red-cell measurements also reflect what cells carry or store. For example, serum selenium reflects recent intake, and whole-blood selenium reflects longer-term status. Serum and whole-blood manganese also have different reference intervals. For urine collection, ARUP specifies a clean, uncolored, metal-free container without metal caps or glued inserts, a recorded 24-hour volume when that interval is requested, and no preservative. Check the performing laboratory's catalog for the procedure that applies to the specimen in hand.^{88 ARUP Laboratories. Trace elements.} Every result must state its matrix, because serum, plasma, and whole-blood intervals are not interchangeable.

The table lists representative adult catalog intervals with their matrices, and none of them is a universal decision limit. An interpretive threshold appears only in the toxicity column, with the laboratory that applies it. Every laboratory verifies its own intervals for its population and instruments, so interpret a result against the performing laboratory's reported interval and comments.

Commercial hair and nail panels are marketed as nutritional assessments. External contamination from hair products, dyes, dust, and sweat cannot reliably be separated from the element content that incorporated during growth, reference data are inadequate for most substances, and correlation with blood or tissue concentrations is weak. An expert panel of the Agency for Toxic Substances and Disease Registry (ATSDR) identified methylmercury as the principal exception where hair can contribute a useful exposure timeline.^{1919 Agency for Toxic Substances and Disease… Hair analysis panel discussion.} Federal guidance states that hair and nails are generally not adequate for quantitative exposure estimates.^{2020 US Consumer Product Safety Commission. Guidance document for use of human biomonitoring data for…} A hair result by itself establishes neither deficiency nor toxicity. Base a clinical report on blood, timed urine, or another standard matrix.

A result above the interval can mean exposure, accumulation, recent intake, inflammation, hemolysis, stopper or skin contamination, or occupational exposure. A result below the interval can mean deficiency, hemodilution, or loss during handling. Age, sex, inflammation, estrogen and pregnancy states, and dialysis all shift the expected range, and biological variation is large for some analytes.^{12,1312 National Institutes of Health Office of… Copper, Zinc, Selenium, Chromium, Manganese, and Molybdenum…13 Mayo Clinic Laboratories. Copper, serum; Zinc, serum; Selenium, whole blood; Selenium…}

Practical bench rules:

1. Treat an isolated unexpected concentration, especially a marginal one, as unconfirmed. If the performing laboratory's procedure calls for a repeat specimen, recollect with its specified device, material, matrix, tube order, and discard volume before interpreting the result as exposure.
2. Compare the collection details with the performing laboratory's analyte-specific procedure before accepting a high value. Needle type, tube order, skin preparation, handling delay, hemolysis, and stopper contact can affect results, and most contamination raises results.
3. Distinguish recollection from true confirmatory testing, which uses an independent analytical procedure.^{44 Centers for Medicare & Medicaid Services. CLIA proficiency testing regulations and calibration…}
4. For metal-on-metal hip implant monitoring, FDA recommends keeping the same matrix, method, and laboratory so a between-method shift does not look like a biological change.^{1717 US Food and Drug Administration. Information about soft tissue, imaging, and metal ion testing…}

Testing for arsenic, cadmium, lead, and mercury exposure is covered in the Toxicology Principles and Exposure Testing lesson, with specimen choice by compound and exposure route. Atomic absorption and ICP instrumentation itself is covered in the Laboratory Operations area. Reference intervals for iron, TIBC, transferrin saturation, and ferritin, and the TIBC-from-transferrin calculation, are listed in the appendix lessons.