Complementary Mass Spectrometry Methods

Molecular detection via API-MS and ESD via ICP-MS can be considered as having ionization processes at opposite ends of the spectrum. Both techniques use sources at atmospheric pressure, however API is considered to be a soft- ionization technique, effectively converting the charged species present in the liquid phase into an ion in the gas phase, whereas ICP very effectively con- verts chemical species in the liquid phase into their constituent elemental ions.

Time (s) Inorganic Methyl Ethyl Unknown Phenyl 2000 4000 6000 8000 10000 0 201 401 602 803 1004 (a) 2 3 1 4 3:20 6:40 10:00 13:20 16:40 0 20 40 60 80 100 Response Time (min) (b) Response

HPLC-API-MS provides structural information, but without an authentic standard, quantitation is not possible because ionization is molecule specific. HPLC-ICP-MS can give accurate and precise quantification with an ele- mental standard even at trace concentrations, but identification is only pos- sible with a retention time standard and even in this situation mistakes can be made. By using these techniques in combination it is possible to generate a diverse range of information for a particular analytical problem. Figure 2 shows the results obtained for the speciation analysis of Hg using the same HPLC separation system, coupled to ICP-MS (Figure 2a) and APCI-MS (Figure 2 b,c). More recent work in this area has used the same column coupled in parallel to both detectors, which can provide quantitative and structural data simultaneously [68]. However, it is not possible to always

m/z 200 250 300 350 400 20 40 60 80 100 Intensity 371 295 293 291 (c)

Figure 2. (a) Separation of different mono substituted mercury species by HPLC coupled to ICP MS. The system used a reversed phase column (250 4.6 mm i.d., 5 mm), an eluent of MeOH (50%), water (50%), containing 0.05% 2 mercaptoethanol (v/v) at a flow rate of 1 mL min1. The spraychamber was cooled to 10 1C and oxygen was added post nebulization. The concentration of each component of the standard was 10 ng g1. (b) Separation of different mercury species by HPLC coupled to APCI MS. The same HPLC conditions as in (a) were used. 1¼ inorganic, 2 ¼ methyl, 3¼ ethyl, 4 ¼ phenyl. Standard concentration was 10 ng g1 _{for each com} ponent. (c) Mass spectrum for a 10 ng g1standard of methylmercury chloride. The most abundant ion at m/z 295 corresponds to a methylmercury/2 mercaptoethanol adduct, whereas the cluster at m/z 371 corresponds to a methylmercury/2 mercap toethanol adduct containing two 2 mercaptoethanol groups and loss of two protons.

achieve this aim because of the differences in sensitivity of the two detectors for some analyte-matrix combinations and often it is necessary to split the flow so that more reaches the API detector.

For GC separations there are more options because of the potential to use VG as an interface mechanism and so ICP-MS, microwave-induced plasma (MIP), and AFS can provide elemental analysis and conventional MS based on EI, in various mass analyzer configurations, can be used for structural analysis. CE applications have more niche applications and this chromatographic technique can be interfaced both to ICP-MS and ESI-MS, however a suitable device to enable coupling to both detectors simultaneously awaits development.

3.5.

Methods Based on Vapor Generation

Vapor generation has been widely used as a gas-phase sample introduction technique for species of As, Hg, Sb, and Sn that can be readily converted into stable hydrides or the elemental form and Table 2 presents LODs for a selection of VG systems. There are several recent general reviews of spe- ciation analysis by VG coupled to various detectors [69–74].

The basic design of a VG system has three or four stages: generation of the hydride or elemental form; vapor collection (optional); transfer of vapor to atomizer or spectroscopic excitation source; and atomization. Very high transport efficiencies, approaching 100%, can be achieved, whilst separating the analytes from undesirable matrix components. Because only a vapor is passed to the detector, chemical and spectral interferences are essentially eliminated, as is the need for a nebulizer, which improves transport effi- ciency. These factors help to lower the achievable LODs and VG is a technique that offers high sensitivity. Moreover, for VG operated in batch mode, relatively large sample volumes (e.g., 100 mL for batch versus 0.1 mL for HPLC flow) can be applied, further lowering the LODs achievable.

Hydride generation (HG) using sodium tetrahydroborate (STBH; NaBH4) is by far the most common means of forming hydrides. The reaction

for element E with an oxidation state m+ may be described:

NaBH4þ 3H2Oþ HCl ! H3BO3þ NaCl þ 8H ð1:Þ

Emþþ 8H ! EHmþ H2ðexcessÞ ð2:Þ

HG occurs very rapidly when an alkaline solution of STHB is mixed with an acidified sample solution. Post-reaction, hydrides, and other gases (mainly H2) are transported via an inert carrier gas to a gas-liquid separator

HG using STBH can be operated as a batch, continuous-flow or flow- injection system. Problems can occur through inadequate control of reaction conditions and separation of by-products, especially H2, which then enters

the atomizer. Such problems are mainly associated with batch systems, and

Table 2. Selection of HG based analytical systems with detection limits for deter mination of organometal(loid)s.

Analytical system Sample

Organometal(oid) species

(detection limit)a References

HG pre-separation HG CT GC ICP MS (pH gradient HG)

Soil As: MeAs (0.098)b, Me2As (0.011)b, Me3As (0.015)b [112] Sb: MeSb (0.007)b_{, Me} 2Sb (0.005)b, Me3Sb (0.001)b [112] Sn: MeSn (0.093)b_{, Me} 2Sn (0.07)b, Me3Sn (0.01)b [112]

HG SPME GC MSa Sediments Hg: MeHg (20 pg) [113]

HG CT GC AFSd Sediments Hg: mono MeHg (0.03)c,

mono EtHg (0.03)c

[114] HG CT GC ICP MSd _{Sediments Hg: mono MeHg (0.02)}c_,

mono EtHg (0.01)c

[114] HG post-separation

HPLC HG ICP MS (IP RP column)

Spring water As: MeAs (5.6), Me2As (3.6)

[115] HPLC HG AAS

(IP RP column)

Groundwater As: MeAs (110), Me2As (150)

[116] HPLC HG ICP AES

(AEx column)

Spiked water As: MeAs (380), Me2As (2,130)

[117] HPLC UV HG AFS

(AEx column)

Standards As: MeAs (14), Me2As (11), AB (15), AC (9), TMAO (17)

[118]

HPLC HG ETAAS (silica based ion exchange) Sediment, mussel tissue Sn: MBT, DBT, TBT (135 942) [119]

Flow CE HG AFS Human urine As: MeAs (11,200)c, Me2As (8,900)c

[120] Lake & river

water

Hg: MeHg (16,600)c, EtHg (15,900)c_{, PhHg (13,300)}c

[121]

a

Detection limits are given as pg of elemental form, unless otherwise stated. b

mg kg1dry weight

c_{ng L}1_{; HG was with TBH unless otherwise stated} d

phenylation derivatization

are largely eliminated in flow systems. Transition and noble metals can cause severe signal suppression and such chemical interferences are considered to be the most serious form of interference in HG [71]. Considerable effort has been made to reduce or eliminate interferences through addition of chemical agents which complex the interfering metal ions, e.g., L-cysteine, L-histidine, EDTA, tartaric acid, KI [72,75]. For multi-elemental analysis a universal method for minimising chemical interferences has not been found because of the great variety of operating conditions of the HG reaction reported in the literature, although L-cysteine and thiourea are generally regarded as the most promising masking reagents for severe interference metals such as Co(II), Cr(III), Cu(II), Ni(II), and Fe(III).

The reaction between STBH and the analyte in solution is markedly dependent upon pH, which influences both the level of protonation of the analyte and the hydrolysis of STBH. Selective batch mode methods have been used to speciate inorganic and methylated forms of As in the absence of a chromatographic separation [73]. Sample pre-treatment, the dependency of HG on pH and control of STBH and HCl concentrations, allows the non- chromatographic determination of methylated As(III) species and methy- lated As(V) species [76]. Although selective HG in batch mode operation is a simple and inexpensive approach to As speciation, it is limited to inorganic and simple methylated species and has the disadvantage of long reaction times, slow sample throughput and reliance on strict control of reaction conditions. This approach to the speciation of As, Sb, Se, and Te has recently been reviewed [73].

For speciation analysis of organometal(loid)s a chromatographic separation is almost invariably required, although as described above, che- mical parameters can be used. For example, Me3SbCl2has a derivatization

optimum near to neutral pH, while MeAsO(OH)2and MeAsO(OH) require

acidic conditions for derivatization [77]. A pH gradient procedure designed to overcome differences in pH optima for derivatization of different methyl species has been used for As, Bi, and Sb in a single run [78]. This involved adjusting the pH from 7 to 1 using citrate buffer during the HG stage, with coupling to GC-ICP-MS [78]. Anderson et al. [79,80] incorporated mer- captoacetic acid into the STBH/HCl reaction mixture and reported similar response profiles for As(III), As(V), MMA, and DMA. Incorporation of L- cysteine into reaction mixtures as a pre-reductant has been used widely in HG As speciation analysis. Not only does it minimize interferences from transition metals, it also reduces the concentration of acid required and improves the stability of the hydrides [75,81]. A further consideration is that increased demethylation occurs with decreasing pH during HG of methy- lated forms of As and other elements, including Bi, Sb, and Hg. Hirner [82] has described the artefacts that arise in speciation analysis from the appli- cation of derivatization techniques. Various acids, buffers and redox media

have been utilized successfully for HG speciation analysis of inorganic and methylated forms of As [71,73], although a universal HG method has not emerged.

Electrochemical VG in atomic spectrometry is an alternative sample- introduction technique to chemical VG. Several advantages of this approach have been reported, including: the use of similar reaction media for analysis of all HG elements; the possibility of reduced interference from related species; and independence of HG efficiency from oxidation state of analyte. Avoidance of STBH as a derivatizing agent is also an advantage because it is expensive, must be prepared daily and can introduce contaminants. Although electrochemical HG has been widely used for total element determination, there is as yet little information on its application in spe- ciation analysis. Denkhaus et al. [83] present a detailed summary of mechanistic electrolytic HG-AAS for the determination of As, Sb, Se, and Sn. The fundamentals, interferences, and application of electrochemical HG have been recently reviewed [70].

Cryogenic trapping (CT) of volatile hydrides is a useful approach for the determination of methylated forms of metal(loid)s, including those of As, Sb, Bi, Hg, and Se. The approach has also been used for focusing the hydrides formed, leading to efficient species separation and improved LODs. Columns packed with glass beads, glass wool or a suitable chromatographic material are immersed in liquid nitrogen. Removal of the liquid nitrogen alone or com- bined with subsequent electrothermal heating, releases and separates the hydrides according to their boiling points, which are then detected [84]. Generally, traps filled with chromatographic material show improved separation and species recovery compared with glass bead or wool filled traps [73]. For analysis of environmental gases for methylmetal(loid) species, sam- ples have been passed directly to a series of cryogenic traps by a vacuum pump, or collected into gas bags (Tedlar bags) prior to cryogenic trapping [85]. Low temperature GC-ICP-MS has been used to analyze loaded cryogenic gas traps, with thermal desorption within the temperature range100 to 165 1C [85].

A major disadvantage of the VG approach is that it does not differentiate between species with the same level of methylation. For example, dimethy- larsinic acid (DMA) and dimethylarsinous acid (DMAIII) both form dimethylarsine, so all three species present in a sample are indis- tinguishable. A further issue with pre-column derivatization is that deme- thylation and transalkylation can occur, which may give rise to several species from a single organometal(loid) analyte [82,86]. For As speciation, a fully automated flow-injection-HG-CT-AAS has been reported using a poly(tetrafluoroethane) (PTFE) trap heated by microwave radiation [87]. Duester et al. [78] used a multi-organometal(loid) standard for determina- tion of methylated As, Sb, and Sn species in soils, by HG-CT-GC-ICP-MS. The multi-standard comprised: MeAs(ONa)2, Me2AsO(OH), Me3AsO,

MeSnCl3, Me2SnCl, Me3SnCl, (C4H9)SnCl3, and Me3SbBr2. Other workers

have reported on improved LODs for As species with novel cryogenic traps, such as replacing a conventional glass U-trap with a chromatographic packed cold finger trap [88]. Such improvements have led to better perfor- mance in terms of species separation. Terlecka [71] has reviewed As spe- ciation in water samples by hyphenated techniques, including those involving HG.

Continuous-flow and flow-injection HG systems are more widely used than batch systems as they offer the advantages of higher volatilization efficiency with STBH, more effective transport of analytes to the atomizer; improved detector sensitivity and precision, and increased tolerance to interferences. Because not all OMCs form stable hydrides, an on-line degradation stage, such as microwave digestion or UV photolysis, may be required for speciation analysis by flow HG. This applies particularly to As- containing compounds such as AB, arsenocholine (AC), arsenosugars, and the tetramethylarsonium ion that do not form stable hydrides under normal conditions. With such degradative treatment, the organic counter-ion species would be destroyed and only the methylmetal(loid) portions detected, so that full molecular speciation is not provided.

Most flow HG systems utilize HPLC as a liquid separation stage inter- faced with an ESD: HPLC-HG-AAS; HPLC-HG-ICP-AES; HPLC-HG- AFS; HPLC-HG-ICP-MS. Figure 3 illustrates the sequential stages of a HPLC-UV-HG-detector system. Detection limits and sensitivity to inter- ferences depend on the detector used (Table 2). AFS as a flow-through detector couples well with on-line HG and has been extensively used. Advantages of AFS include high sensitivity for most of the hydride forming elements, high sampling frequency, ease of operation, and low cost [89]. HG eliminates light scattering and background interferences from the matrix, resulting in increased sensitivity for AFS [89]. In continuous flow systems (e.g., HPLC-HG), separation of matrix components such as transition metal species prior to HG also helps to minimize interferences in environmental sample analysis; hyphenation of flow injection with HG-AFS has been reviewed [89,90]. HPLC pump Injector HPLC column HCl NaBH₄ Sample Reaction coil Argon Liquid waste GLS Water trap or dryer Detector Mobile phase UV

In As speciation studies, incorporation of HG between HPLC and ICP- AES has been shown to significantly reduce the severe spectral interference and enhance sensitivity [91]; HG hyphenated with different AES sources (e.g., ICP, MIP) has been reviewed [72]. Similarly, for As speciation using HPLC-ICP-MS, incorporation of HG eliminates spectral interferences that may occur due to the formation of ArCl ions and reduces the detection limit to around 1ng L 1[71,72]. AAS offers high sensitivity, selectivity, and low LODs with different separation techniques, when combined with HG, e.g., HPLC-HG-AAS. The mechanism of hydride formation and atomization in HG-AAS has been reviewed [69]. The main advantages of HPLC-ICP-MS over HPLC-HG-AAS for speciation studies are the lower LODs and cap- ability to detect non-hydride forming species without the requirement for an additional mineralization step.