eV Lamp Research Articles

Low-cost photoionization sensors as detectors in GC × GC systems designed for ambient VOC measurements

Conventional volatile organic compound (VOC) monitoring based on thermal desorption - gas chromatography–mass spectrometry (TD-GC–MS) or gas chromatography-flame ionization detector (TD-GC-FID) is relatively cumbersome and expensive. In this study commercial off the shelf low-cost and low-power photo-ionization detector (PID) sensors are used as simple detectors in VOC analysis systems based on GC, including a miniaturised GC × GC device with portable, low-cost, and low-energy-consumption features. PID sensors produce a voltage signal positively proportional to VOC concentration, which when incorporated into a TD-GC system gave limit of detection of 0.02 ppbV for isoprene. To test PID performance in real-world applications, PID sensors were deployed as (i) a second alternative detector in a GC-Quadruple Time Of Flight Mass spectrometry (GC-Q-TOF-MS), and (ii) the main detector in a compact two-dimensional gas chromatograph (GC × GC). PID sensors with 10.6 eV and 11.7 eV lamps were used to measure eight toxic chemicals including organic sulfide and organic phosphonates via GC; two species were ionized by a 10.6 eV lamp and four species by the 11.7 eV lamp. Commercially available low-cost PIDs designed for standalone could be straightforwardly and effectively re-used as detectors in compact GC × GC systems, in this work showing excellent VOC sensitivity, fast response and low operational demands compared to comparable field instruments based on GC-FID or MS.

METHODS TO MONITOR MITC FOR PHYTOTOXICITY AND IN THE ENVIRONMENT

Soil fumigation with the methyl isothiocyanate (MITC) generators metam-sodium, metam-potassium and dazomet gives rise to residual gasses that need to be checked prior to sowing or planting. Different factors determine the residual amount: dose rate, soil type, soil organic matter content, soil humidity, soil temperature, soil porosity and tarping. Under practical conditions this safety check is made after soil tarp removal or, if not covered by plastic film, prior or after soil aeration e.g. by rotovation. Forced aeration is often standard practice and will speed up the removal of potential phytotoxic gasses. MITC monitoring will help to decide on the necessity of forced aeration or even repetition of it. Classic test for phytotoxic risk evaluation only is the cress seed germination test. Alternative seeds, such as lettuce, are also in use. This is the slowest method, needing at least 2 days. Another possibility of monitoring is the use of specific gas detection tubes giving sensitive response within 3 minutes and especially of interest in the fumigated environment, e.g. operator and bystander exposure or risk for neighbouring residents. A hand held pump is needed and the tubes are consumables. A specific measuring principle is by quartz micro balance (QMB). This is also known as the ‘electronic nose’ and was beyond other applications also developed for MITC. The instrument can become operated in the field with a car power adaptor. The whole procedure takes 45 min. Photo ionization detection is possible with hand held instruments. It is not very specific but among MITC and other volatile and active degradation compounds majority of the instrument response is for MITC with a 10.6 eV lamp. It is also very useful for environmental monitoring. Measuring and response time is a few seconds. Other possibilities need to become explored or are under consideration, e.g. infrared analyzer allowing specific detection.

GC/MS on an LC/MS instrument using atmospheric pressure photoionization

Atmospheric pressure (AP) GC/MS was first introduced by Horning et al. [E.C. Horning, M.G. Horning, D.I. Carroll, I. Dzidic, R.N. Stillwell, Anal. Chem. 45 (1973) 936] using 63Ni as a beta-emitter for ionization. Because, at the time special instrumentation was required, the technique was only applied with consistency to negative ion environmental studies where high sensitivity was required [T. Kinouchi, A.T.L. Miranda, L.G. Rushing, F.A. Beland, W.A. Korfmacher, J. High Resolut. Chromatogr., Chromatogr. Commun. 13 (1990) 281]. Currently, AP ion sources are commonly available on LC/MS instruments and recently a method was reported for converting an AP-LC/MS ion source to a combination AP-LC/MS:GC/MS source [C.N. McEwen, R.G. McKay, J. Am. Soc. Mass Spectrom. 16 (2005) 1730]. Here, we report the use of atmospheric pressure photoionization (APPI) with GC/MS and compare this to AP chemical ionization (APCI) GC/MS and electron ionization (EI) GC/MS. Using a nitrogen purge gas, we observe excellent chromatographic resolution and abundant molecular M + and MH + ions as well as structurally significant fragment ions. Comparison of a 9.8 eV UV lamp with a 10.6 eV lamp, as expected, shows that the higher energy lamp gives more universal ionization and more fragment ions than the lower energy lamp. While there are clear differences in the fragment ions observed by APPI-MS versus EI-MS, there are also similarities. As might be expected from the ionization mechanism, APPI ionization is similar to low energy EI. These odd electron fragment ions are useful in identifying unknown compounds by comparison to mass spectra in computer libraries.

Fluid inclusion volatile analysis by gas chromatography with photoionization micro-thermal conductivity detectors: Applications to magmatic MoS 2 and other H 2O-CO 2 and H 2O-CH 4 fluids

Eighteen fluid inclusion volatile peaks have been detected and identified from 1–2 g samples (quartz) by gas chromatography using heated (~105°C) on-line crushing, helium carrier gas, a single porous polymer column (HayeSep R; 10′ × 1 8 ″: 100 120 #; Ni alloy tubing), two temperature programme conditions for separate sample aliquots, micro-thermal conductivity (TCD) and photoionization detectors (PID; 11.7 eV lamp), and off-line digital peak processing. In order of retention time these volatile peaks are: N 2, Ar, CO, CH 4, CO 2, C 2H 4, C 2H 6, C 2H 2, COS, C 3H 6, C 3H 8, C 3H 4 (propyne), H 2O (22.7 min at 80°C), SO 2, ± iso- C 4 H 10 ± C 4 H 8 (1-butene) ± CH 3 SH, C 4H 8 (iso-butylene), (?) C 4H 6 (1,3 butadiene) and ± n- C 4 H 10 ± C 4 H 8 (trans-2-butene) (80 and −70°C temperature programme conditions combined). H 2O is analysed directly. O 2 can be analysed cryogenically between N 2 and Ar, but has not been detected in natural samples to date in this study. H 2S, SO 2, NH 3, HCl, HCN, and H 2 ca nnot be analysed at present. Blanks determined by crushing heat-treated Brazilian quartz (800–900°C/4 h) are zero for 80°C temperature programme conditions, except for a large, unidentified peak at ~64 min, but contain H 2O, CO 2, and some low molecular weight hydrocarbons at −70°C temperature conditions due to cryogenic accumulation from the carrier gas and subsequent elution. TCD detection limits are ~30 ppm molar in inclusions; PID detection limits are ~ 1 ppm molar in inclusions and lower for unsaturated hydrocarbons (e.g., ~0.2 ppm for C 2H 4; ~ 1 ppb for C 2H 2; ~0.3 ppb for C 3H 6). Precisions (1σ) are ~ ±1–2% and ~ ± 13% for H 2O in terms of total moles detected; the latter value is equivalent to ±0.6 mol% at the 95 mol% H 2O level. Major fluid inclusion volatile species have been successfully analysed on a ~50 mg fluid inclusion section chip (~7 mm × ~10 mm × ~100 μm). Initial inclusion volatile analyses of fluids of interpreted magmatic origin from the Cretaceous Boss Mtn. monzogranite stock-related MoS 2 deposit, central British Columbia of ~97 mol% H 2O, ~3% CO 2, ~ 140–150 ppm N 2, and ~16–39 ppm CH 4 (~300–350°C) are reasonable in comparison with high temperature (~400–900°C) volcanic gas analyses from four, active calc-alkaline volcanoes; e.g., the H 2O contents of volcanic gases from the White Island (New Zealand), Mount St. Helens (Washington, USA), Merapi (Bali, Indonesia), and Momotombo (Nicaragua) volcanoes are 88–95%, >90% (often >95%), 88–95% and ~93%, respectively; CO 2 contents are ~3–10%, 1–10%, 3–8%, and ~3.5%. CO 2/N 2 ratios for the Boss Mtn. MoS 2 fluids of ~ 190–220 are in the range for known volcanic gas ratios (e.g., ~ 150– 240; White Island). The ∑S content of the Boss Mtn. MoS 2 fluid prior to S loss by sulphide precipitation may have been ~2 mol% since CO 2/∑S molar ratios of analysed high-temperature volcanic gases are ~ 1.5. This estimate is supported by ∑S contents for White Island, Merapi and Momotombo volcanic gases of ~2%, ~0.5–2.5%, and ~2%. COS has been determined in H 2O-CO 2 fluid inclusions of interpreted magmatic origin from the Boss Mtn. MoS 2 deposit and the Tanco zoned granitic pegmatite, S.E. Manitoba at ~50–100 ppm molar levels, which are consistent with levels in volcanic gases. It appears that low, but significant, concentrations of C 2-C 4 alkanes (~ 1–20 ppm), C 2-C 4 alkenes (~ 1–480 ppb) and alkynes (e.g., C 3H 4) have been detected in magmatically derived fluids (Boss Mtn. MoS 2 deposit; Tanco granitic pegmatite). Significantly higher, low molecular weight hydrocarbon concentrations have been determined in a CH 4-rich (~ 2%), externally derived fluid of possible metamorphic or deep crustal origin trapped as inclusions in metasomatic wall-rock tourmaline adjacent to the Tanco pegmatite (e.g., 300/470 ppm C 2H 6; 50 90 ppm C 3H 8; 3–60 ppm C 2H 4 C 3H 6 n-C 4H 10 ).

Fluid inclusion volatile analysis by heated crushing, on-line gas chromatography; applications to Archean fluids

Eighteen fluid inclusion volatile peaks have been detected and identified from 1–2-g samples (quartz) by gas chromatography using heated (∼ 105°C) on-line crushing, helium carrier gas, a single porous polymer column (HayeSep R; 10′ × 1/8″; 100/120#; Ni alloy tubing), two temperature programme conditions for separate sample aliquots, micro-thermal conductivity (TCD) and photoionization detectors (PID; 11.7 eV lamp) and off-line digital peak processing. In order of retention time these volatile peaks are: N 2, Ar, CO, CH 4, CO 2, C 2H 4, C 2H 6, C 2H 2, COS, C 3H 6, C 3H 6, C 3H 4 (propyne), H 2O (22.7 mins at 80°C), SO 2, ±iso-C 4H 10±C 4H 8 (1-butene) ± CH 3SH, C 4H 8 (iso-butylene), (?) C 4H 6 (1,3 butadiene), and ± n-C 4H 10 ±C 4H 8 (trans-2-butene) (80°C and −70°C temperature programme conditions combined). H 2O is analysed directly. O 2 can be analysed cryogenically between N 2 and Ar, but has not been detected in natural samples to date. H 2S, SO 2, NH 3, HCl, HCN and H 2 cannot be analysed at present. Blanks determined by crushing heat-treated Brazilian quartz (800–900°C/4hrs) are zero for 80°C temperature programme conditions, except for a large, unidentified peak at ∼ 64 mins, but contain H 2O, CO 2 and some low molecular weight hydrocarbons at −70°C temperature conditions due to cryogenic accumulation from the carrier gas and subsequent elution. TCD detection limits are ∼30 ppm molar in inclusions; PID detection limits are ∼ 1 ppm molar in inclusions and lower for unsaturated hydrocarbons (e.g. ∼ 0.2 ppm for C 2H 2; ∼ 0.3 ppb for C 3H 6). Precisions are ∼±1–2%, except for H 2O (∼±13%). Major fluid inclusion volatile species have been successfully analysed on a ∼50 mg fluid inclusion section chip (∼ 7 m × ∼ 10 m × ∼ 100 μm). Two distinct end-member Archean fluids, one internal and one external, have been found related to the Tanco zoned, granitic pegmatite, SE Manitoba. The former is an H 2O (∼96%)-CO 2 (∼40%)-CH 4-N 2 fluid (S species not included) with a moderate salinity of 6.6 ± 1.3 eq. wt.% NaCl which is interpreted to be magmatic in origin, whereas the latter is an H 2O (∼97%)-CH 4 (∼2%)-CO 2 (∼0.4%)-C 2H 6-N 2 fluid with a distinctly higher salinity of ∼10–20 eq. wt% NaCl which is interpreted to be of metamorphic/lower crustal (∼2,900 bar/∼10km) origin. The volatile compositions of H 2O (∼87–94%)-CO 2 (∼6–13%)-CH 4-N 2 fluids with ∼5–6 eq. wt.% NaCl from primary inclusions from three structurally controlled, mafic-ultramafic rock hosted Archean Au-quartz vein deposits in the Barberton greenstone belt, southern Africa ( n=9) are distinctly different from the composition of the Tanco external fluid, but similar to the composition of primary fluids of interpreted

An evaluation of the response of some portable, direct-reading 10.2 eV and 11.8 eV photoionization detectors, and a flame ionization gas chromatograph for organic vapors in high humidity atmospheres.

Three portable, direct-reading instruments were tested to evaluate their ability to detect organic vapors in high humidity atmospheres. The instruments included two photoionization detectors (10.2 eV and 11.8 eV), and an organic vapor analyzer with a flame ionization detector. The organic vapors used were hexanes, 1,1-dichloroethane, chloroform, trichloroethylene, perchloroethylene, benzene, toluene and chlorobenzene. Relatively impervious gas bags were used to contain atmospheres of low (zero) and high (90 percent) relative humidities for comparisons. The response of the organic vapor analyzer was practically independent of humidity. The responses of the photoionization detectors were influenced by high humidity even though the 10.2 eV lamp did not detect water vapor itself. The photoionization detectors were adjudged to be unsuitable for quantitative sampling of humid confined spaces since the 11.8 eV lamp was sensitive to water vapor and the 10.2 eV lamp showed unpredictable response factors in high humidity.