Ion Chromatography and Hyphenated Methods

IC with conductivity detection was first used to measure inorganic anions and cations by Small et al. in 1975. Surface and waste stream waters were analyzed in 15-20 minutes, measuring anions (F^-, Cl^-, Br^-, I^-, NO₃^-, NO₂^-, SO₄^2-, SO₃^2- and PO₄^3-) and cations (Li+, Na+, K+, Rb+, Cs+, NH₄+, Ca²+, Mg²+). The weak acidic character and the complex dissociation equilibrium of phosphate ions make IC analysis more challenging compared to other strong acidic anions. Although phosphate ions were not considered, Gjerde et al. decreased the analysis time and improved sensitivity by using a macroporous anion-exchange column with very low exchange capacity and low conductivity eluents (benzoate, phthalate, sulfobenzoate, perchlorate, and malonate).

Using IC with conductivity detection, Rosset et al. successfully separated a mixture of cyclic and condensed phosphates in <30 minutes. By comparing phosphate measurements by suppressed IC and colorimetry (molybdenum blue complex), Xie et al. found that organic phosphorus compounds were less susceptible to hydrolysis when analyzed by IC and the presence of polyvalent metal ions (e.g. Al³+ and Fe³+) caused lower phosphate concentrations when measured by IC. US EPA (method 300.1) and Standard Methods (section 4110) published IC analysis procedures for phosphate.

Samples were analyzed after a filtration through 0.45 pm filter and separated on an anion-exchange column. Continuous suppression ensures significantly decreased conductivity of the eluent and conversion of the analyzed anions into their acid forms, which have higher conductivity than that of the eluent. Using a carbonate-bicarbonate eluent, separation of all major anions was achieved in 10 min with a method detection limit of orthophosphate at 14 µg L^-1 as P.

McDowell et al. used atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) to analyze hypophosphite (m/z = 65 for H₂PO₂^-), phosphite (m/z = 81 for H₂PO₃^-), and phosphate (m/z = 97 for H₂PO₄^-) in water. Detection limits for hypophosphite was 26 µg L^-1 as P, for phosphite 12 µg L^-1 as P, and for phosphate 11 µg L^-1 as P. Development of high-capacity IC columns allowed the detection and quantification of ultra-low concentrations of phosphorus anions in environmental samples. Ivey and Foster used IC separation coupled to electrospray ionization (ESI) MS for the detection of phosphorus oxyanions.

Large volume injection (800 µL) and sample pretreatment (Dionex Ag OnGuard II cartridges) allowed a decrease in the detection limit for hypophosphite (0.34 µg L^-1 as P) and phosphite (0.62 µg L^-1 as P), but the Ag cartridge also removed the phosphate ions from the sample. Using a high-capacity IC column (Dionex AS-11HC), large injection volume (500 µL), and suppressed conductivity, Han et al. lowered the detection limit for phosphites to 0.062 µg L^-1 as P.

The results were also confirmed using a two-dimensional IC separation with suppressed conductivity and confirmed ion identities with IC-ESI-MS. In an ESI-MS study of dehydration reactions from inorganic oxyanions with aminopolycarboxylates (e.g. iminodiacetic acid, nitrilotriacetic acid), Kojima et al. found that a direct measurement (m/z = 97 for H₂PO₄^-) is the only option available for phosphate ions. With a combination of two IC columns and a third concentrator column located between the two separation columns, Wang et al. determined low-level anions in seawater, including phosphate at a detection limit of 23 µg L^-1 as P.

Inductively Coupled Plasma with Atomic Emission and Mass Spectrometry Methods

Inductively coupled plasma (ICP) either combined with atomic emission spectrometry (AES) or MS has been used to determine TP in water samples. In 1980, Sugimae analyzed phosphorus in river, sewer, and domestic wastewaters by ICP-AES. Positive interferences produced by argon and NO were minimized by source parameter optimization.

The spectral level increase from 1000 mgL^-1 Mg was equivalent to 2 mgL^-1 P, but using a preliminary cation-exchange procedure eliminated the interference. A detection limit of 130 µg L^-1 as P was achieved, and the results were in a good agreement with the colorimetrically obtained results. Fujino et al. reported difficulties in direct determination of low-level phosphorus in natural water, due to Si and Fe spectral interferences. Solvent extraction in 1-octanol with HCl/MoO₄^2- complexing agent removed interfering elements and preconcentrated phosphorus, achieving a detection limit of 50 µg L^-1 as P. Samples analyzed were from Lake Biwa (Japan) and ranged in concentrations between 12 and 32 mg L^-1. Various modes of ICP-AES sampling and influence of anions and metal species in determination of phosphorus were investigated by Wennrich et al.

Using electrothermal vaporization (ETV), the method achieved a phosphorus detection limit of 44 µg L^-1 as P at 178.27 nm. Pneumatic nebulization and addition of methanol-ethanol mixture into aqueous samples improved detection limits of the studied nonmetals. Using axial determination at 213 nm with Ga as an internal standard and matrix-matched calibration standards (added K, Mg and Ca) reduced the matrix effect of Na and Ca in ground water samples from 20% to 30% to about 3%.

The detection limit improved from 30 to 12 µgL^-1 as P by measuring at 213 nm instead of 178 nm, due to a decreased matrix effect and a negligible Cu interference. Hydride generation technique was tried to determine phosphorus by ICP-AES, by reducing phosphate to calcium phosphide by aluminum powder, followed by reaction with HCl in a graphite furnace atomizer. The argon plasma generated phosphine (PH₃) was measured at 213.6 nm achieving an order of magnitude improvement in the detection limit comparing with the direct nebulization approach [100]. Introducing the sample by ETV, Okamoto et al. lowered the detection limit to 2.6 µg L^-1 as P measured by ICP-AES. The loss of phosphorus during drying and ashing was suppressed by using a tungsten boat furnace, with phosphate forming stable tungsten phosphate species.

Much lower detection limits than with ICP-AES were achieved with ICP-MS methodology. The advantages of the ICP-MS include a wide dynamic range, specificity, sensitivity, and multielement detection capability. A decrease in excitation and ionization efficiencies by the water mist introduced into the nebulizer is responsible for poor sensitivities. Polyatomic interferences on phosphorus (³¹P⁺) include ¹⁴N¹⁶OH⁺, ¹⁵N¹⁶O⁺, and ¹²C¹H₃¹⁶O⁺. Cantarero et al. measured TP and dissolved phosphorus (DP, filtered through 0.45 pm membrane filter) in agricultural runoffs by ICP-MS, reaching a detection limit of 5pgL^-1 as P. Samples were digested with ammonium persulfate and sulfuric acid. The colorimetry and ICP-MS TP results were in good agreement.

Speciation techniques require separation, identification, and characterization of the phosphorus species. Separation techniques coupled to ICP-MS for phosphorus speciation include chromatographic methods (e.g. liquid chromatography (LC), gas chromatography (GC), and supercritical fluid chromatography (SFC)) and electrophoretic methods (e.g. CE and gel electrophoresis). Hyphenated ICP-MS techniques are effectively used for phosphorus speciation analysis.

Phospholipids, a group of organic phosphorus species, were extracted in organic solvents LC separated and measured using ICP-MS. The optimized method used mobile-phase splitting prior to nebulization for reduced solvent load to plasma, spray chamber cooling to -5 °C, and carrier gas flow adjusted to maximize condensation of the organic vapors. Helium was used as collision gas to reduce polyatomic interferences, and the absolute detection limits ranged between 0.2 and 1.2 ng as P for the analyzed phospholipids. Bisphosphonates, previously known as biphosphonates (HO₃P-CR₁R₂-PO₃H), are a group of analogues to pyrophosphate.

They were initially used as complexing agents in water treatment, detergent additives, and treatment of osteoporosis. Being water-soluble ionic compounds, ion-exchange chromatography is the most suitable separation technique, although challenges still exist due to chelation with metal ions and a loss of column performance. Kovacevic et al. separated bisphosphonates on a Dionex AS-7 IC column using diluted nitric acid as eluent and ICP-MS for detection. Polyatomic interferences at m/z 31, influence of plasma parameters on phosphorus, and background signal were investigated. Detection limits achieved for two bisphosphonates, alendronic acid (0.2 mg L^-1) and etidronic acid (0.5 mg L^-1), were higher than those achieved using pre- or postcolumn derivatization methods.

After banning the toxic and persistent organochlorine pesticides in the 1970s, the organophosphorus pesticides were introduced as less environmentally persistent species, although more acutely toxic. Fidalgo-Used et al. developed a method for the determination of organophosphorus pesticides in spiked river samples by solid-phase microextraction (SPME) coupled to GC-MS and GC-ICP-MS for separation and detection. Their experimental setup proved that SPME is an effective technique for the extraction and preconcentration of trace-level organophosphorus pesticides in river water samples. Better detection limits were obtained by SPME-ICP-MS (0.09-143 ng L^-1) than by SPME-GC-MS (0.8-504 ng L^-1).

The use of GC-ICP-MS offered a higher potential for monitoring organophosphorus pesticides in complex environmental samples. Glyphosate (N-(phosphonomethyl)glycine) is a phosphonate compound used as a broad spectrum herbicide with low toxicity to mammals, but its overall environmental impact is not fully understood yet. Trace levels of glyphosate and phosphate in water were measured by IC coupled to ICP-MS, using an anion-exchange column (Dionex AS16) and 20 mM citric acid eluent. The glyphosate and phosphate were detected separately by ICP-MS at m/z = 31. This sensitive method had a wide dynamic range and a low detection limit (0.7 µg L^-1) for both analytes, without using preconcentration or chemical derivatization. Similar detection limits were reported by Wigfield and Lanouette.